Neural Regeneration

Neural Regeneration provides an overview of cutting-edge knowledge on a broad spectrum of neural regeneration, including:

1. Neural regeneration in lower vertebrates
2. Neural regeneration in the peripheral nervous system
3. Neural regeneration in the central nervous system
4. Transplantation-mediated neural regeneration
5. Clinical and translational research on neural regeneration

The contributors to this book are experts in their fields and work at distinguished institutions in the United States, Canada, Australia, and China.

Nervous system injuries, including peripheral nerve injuries, brain and spinal cord injuries, and stroke affect millions of people worldwide every year. As a result of this high incidence of neurological injuries, neural regeneration and repair is becoming a rapidly growing field dedicated to the new discoveries to promote structural and functional recoveries based on neural regeneration. The ultimate goal is to translate the most optimal regenerative strategies to treatments of human nervous system injuries.

This valuable reference book is useful for students, postdoctors, and basic and clinical scientists who are interested in neural regeneration research.

• Provides an overview of cutting-edge knowledge on a broad spectrum of neural regeneration
• Highly translational and clinically-relevance
• International authors who are leaders in their respective fields
• Vivid art work making the chapters easily understood

• Language: English
• Publisher: Academic Press
• Release date: Feb 3, 2015
• ISBN: 9780128018347

Book preview

Neural Regeneration

Editors

Kwok-Fai So

Xiao-Ming Xu

Table of Contents

Cover image

Title page

Copyright

List of Contributors

Foreword

Preface

Section I. Introduction

Chapter 1. Advances and Challenges for Neural Regeneration Research

1. Nervous System, Nerve Injury, and Neural Regeneration

2. Technological Advances in Neural Regeneration Research

3. PNS Regeneration

4. CNS Regeneration

5. Challenges and Opportunities

Section II. Neural Regeneration in Lower Vertebrates

Chapter 2. Functional Regeneration and Remyelination in the Zebrafish Optic Nerve

1. Introduction to Zebrafish Optic Nerve Injury (ONI) as a Model for Studying Regeneration and Remyelination

2. Time Course of Optic Nerve Regeneration in Adult Zebrafish

3. Intrinsic and Extrinsic Mechanisms of Axon Regeneration

4. Genetic Manipulation Tools Are Very Useful for Studying Regeneration in Zebrafish

5. In vivo Imaging of Nerve Regeneration in Zebrafish Larvae

6. Imaging Remyelination in Zebrafish

7. Behavior Tests Indexed the Visual Functional Recovery after Optic Nerve Injury

8. Future Directions

9. Conclusions

Chapter 3. Central Nerve Regeneration in Reptiles

1. Introduction

2. Timing of Regeneration—General Compared to Fish and Frogs, Including Geckos, etc

3. Retinal Ganglion Cell Survival

4. Glia: Friend or Foe?

5. Growth Promotion: Neurotrophins, Extracellular Matrix, Gefiltin, Polysialic Acid–Neural Cell Adhesion Molecule, Comparison with Development

6. Topography and Refinement: NMDA/AMPA/GABA and Synaptic Proteins

7. Conclusion: Relevance to Mammals

8. Conclusions

Chapter 4. Axon Regeneration in the Lamprey Spinal Cord

1. The Need for Lower Vertebrate Models of Axon Regeneration in the Central Nervous System

2. General Biology of the Lamprey

3. The Central Nervous System of the Sea Lamprey

4. Anatomical and Functional Evidence for Axon Regeneration in the Lamprey Spinal Cord

5. Is Axon Regeneration a Recapitulation of Axon Development?

6. Neuron-Intrinsic Determinants of Axon Regeneration

7. Conclusions

Section III. Neural Regeneration in the Peripheral Nervous System

Chapter 5. Tissue Engineering in Peripheral Nerve Regeneration

1. Introduction

2. Nerve Scaffolds

3. Support Cells

4. Growth Factors

5. Clinical Applications

6. Concluding Remarks

Chapter 6. Brachial Plexus Avulsion: A Model for Axonal Regeneration Study

1. Anatomy of Human and Rodent Brachial Plexus

2. Brachial Plexus Avulsion

3. Potential Strategies for the Treatment of Brachial Plexus Avulsion

Chapter 7. Conditions Affecting Accuracy of Peripheral Nerve Reinnervation and Functional Recovery

1. The Clinical Problem

2. Processes for Axonal Regeneration

3. Failure of Peripheral Nerve Regeneration

4. Improving Axon Targeting after Injury

5. Conclusions

Chapter 8. Gonadal Steroids in Regeneration and Repair of Neuromuscular Systems

1. Introduction

2. Gonadal Steroid Overview

3. Motoneuron Survival

4. Androgens and Axonal Regeneration

5. Androgens and Dendritic Morphology

6. Androgens and Peripheral Maintenance

7. Conclusion

Section IV. Neural Regeneration in the CNS

Chapter 9. Myelin-Associated Inhibitors in Axonal Growth after Central Nervous System Injury

1. Introduction

2. Brief History of Myelin Inhibition Research

3. Multiple Myelin-Associated Inhibitors

4. Multiple Receptors for Myelin-Associated Inhibitors

5. Other Inhibitory Molecules

6. Myelin-Associated Inhibitors and Axon Growth after Central Nervous System Injury in vivo

7. Concluding Remarks

Chapter 10. The Nogo Receptor Pathway in Central Nervous System Axon Regeneration and Therapeutic Opportunities

1. Introduction

2. The Nogo and Nogo Receptor Pathway of CNS Axon Regeneration

3. Technical Criteria for Selecting a CNS Axon Regeneration Drug Candidate

4. Drugs Targeting Myelin Molecules

5. Drug Candidates Targeting NgR1

6. Drugs Targeting NgR1 Signaling

7. Conclusion

Chapter 11. Astrogliosis and Axonal Regeneration

1. Astrogliosis After CNS Injury

2. Permissive and Nonpermissive Properties of Glial Scar

3. Molecular Mechanisms for CSPG Inhibition on Axonal Growth

4. Scar Tissue as a Target for Neuronal Repair and Regeneration

5. Conclusions and Prospects

Chapter 12. The Intrinsic Determinants of Axon Regeneration in the Central Nervous System

1. Introduction

2. Developmental Loss of Axon Regenerative Capacity

3. Role of cAMP in Neurite Growth and Regeneration

4. Mammalian Target of Rapamycin as a Central Player in Promoting Central Nervous System Axon Regeneration

5. KLFs and Their Role in Neuron Development and Axon Regeneration

6. Conclusion

Chapter 13. Optic Nerve Regeneration in Lower Vertebrates and Mammals: Bridging the Gap

1. Introduction

2. Cell Viability

3. Glial-Derived Inhibitors of Axon Growth

4. Control of the Regenerative State by Cell-Intrinsic and Cell-Extrinsic Factors

5. Intraocular Inflammation and Oncomodulin

6. Cell Signaling Pathways for Axon Regeneration of Retinal Ganglion Cells

7. Axon Guidance

8. Restoration of Central Visual Circuits and Partial Recovery of Visual Responses in Mammals

9. Future Prospects

Chapter 14. Self-Assembling Peptides Mediate Neural Regeneration

1. Tissue Engineering Is Essential for Neural Regeneration After Tissue Loss

2. SAP Is a Kind of Novel Nanomaterial for Tissue Engineering

3. SAP Facilitates Neural Regeneration

Chapter 15. Wnt Signaling in Spinal Cord Injury

1. Introduction

2. Wnt Signaling in Axon Guidance

3. Wnt Signaling in Axon Responses to Spinal Cord Injury

Chapter 16. Inflammation and Secondary Damage after Spinal Cord Injury

1. Introduction

2. Early Injury Responses and Factors That Trigger Inflammation

3. Cellular Response to Spinal Cord Injury

4. Identification of Novel Targets That Mediate Inflammation and Secondary Damage after SCI

5. Critical Points in the Inflammatory Pathway in SCI Amenable to Therapeutic Intervention

6. Concluding Remarks

Chapter 17. Neuroprotection of Retinal Ganglion Cells in Glaucoma by Blocking LINGO-1 Function or Using a Nogo-66 Receptor Antagonist

1. Glaucoma

2. The Expression of Nogo, NgR, and LINGO-1

3. Neuroprotection of LINGO-1 Antagonists on RGCs in Glaucoma and the Possible Mechanism

4. Neuroprotection of NgR1 Antagonists on RGCs in Glaucoma

5. Summary

Chapter 18. Axonal Regeneration in the Sensory Dorsal Column Pathway

1. Historical Perspective

2. Conditioning Lesion

3. Sensory Axonal Regeneration

4. Technological Considerations

5. Future Directions

Section V. Transplantation-Mediated Neural Regeneration

Chapter 19. Peripheral Nerve Graft-Mediated Axonal Regeneration

1. Introduction

2. Historical Perspective on PN Transplantation

3. Axonal Regeneration into a PNG

4. Grafting PNs to Promote Regeneration of Chronically Injured Axons

5. Effects of Exogenous Neurotrophins on Axon Regeneration beyond a PNG

Chapter 20. Transplantation of Olfactory Ensheathing Cells for Neural Repair

1. Introduction

2. Contribution of OECs to Olfactory Axon Regeneration

3. Basic Biology of the Regeneration-Promoting Properties of OECs

4. OEC-Mediated Transplantation for Neural Repair in Animal Studies

5. OEC-Mediated Transplantation in Clinical Trials

6. Concluding Remarks

Chapter 21. Glial Precursor Cell Transplantation-Mediated Regeneration after Spinal Cord Injury Repair

1. Therapeutic Potential of GPCs After SCI

2. Resources of GPCs for Potential Clinical Translation

3. The Challenges of Translating Stem Cell Therapies to the Clinic

4. Conclusions

Chapter 22. Schwann Cell-Mediated Axonal Regeneration in the Central Nervous System

1. The Origin, Development, and Functions of Schwann Cells

2. The Application of SCs to Axonal Regeneration Following SCI

3. Potential Sources of SCs for Transplantation

4. The Survival and Migration of Grafted SCs

5. Reactions of Various Neuronal Tracts to Transplanted SCs

6. Combinatory Strategies to Conquer Limitations of Grafted SCs in Axon Regeneration

7. Clinical Application

8. Conclusion

Chapter 23. Fetal Spinal Cord Transplantation after Spinal Cord Injury: Around and Back Again

1. Introduction and History

2. Alternative Cell Sources for Spinal Tissue Repair

3. Revisiting FSC and Neural Progenitor Transplantation to Establish Functional Relays

4. Summary

Section VI. Clinical and Translational Research on Neural Regeneration

Chapter 24. Spinal Cord Injury: Exercise and Clinical Trials

1. Spinal Cord Injury

2. Research Effort into Spinal Cord Injury and Exercise

3. Quality in Studies and Clinical Trials

4. Clinical Trials

5. Comparative Effectiveness Research (CER)

6. Summary and Conclusion

Chapter 25. Spinal Cord Regeneration

1. Introduction

2. Axon Growth Inhibitor Theory

3. Glial Scar Theory

4. Growth Limitation Theory

5. Regenerative Therapies

6. Cell Transplants

7. Combination Therapies

8. Recovery Mechanisms

9. Rehabilitation

Chapter 26. Biomarkers for CNS Injury and Regeneration

1. Introduction

2. Acute Phase CNS Injury Protein Biomarkers

3. Need for Biochemical Markers of Subacute and Chronic CNS Injury

4. Subacute, Chronic CNS Injury Biomarkers

5. Post-TBI Neurodegeneration Markers

6. Neuroinflammatory Markers

7. Neuroregeneration Markers

8. Systems Biology-Assisted Biomarker Integration

Chapter 27. High-Content Screening Applied to Nervous System Injury: Advantages, Challenges, and Proof of Principle

1. Introduction

2. High Content Analysis of Neuronal Functions

Index

Copyright

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List of Contributors

Larry I. Benowitz

Laboratories for Neuroscience Research in Neurosurgery and F.M. Kirby Neurobiology Center, Children’s Hospital Boston, Boston, MA, USA

Departments of Surgery and Ophthalmology and Program in Neuroscience, Harvard Medical School, Boston, MA, USA

John L. Bixby

The Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, Miami, FL, USA

Center for Computational Sciences, University of Miami Miller School of Medicine, Miami, FL, USA

Department of Neurological Surgery, University of Miami Miller School of Medicine, Miami, FL, USA

Department of Molecular & Cellular Pharmacology, University of Miami Miller School of Medicine, Miami, FL, USA

Todd J. Brown

Research and Development Service, Richard L. Roudebush VA Medical Center, Indianapolis, IN, USA

Department of Anatomy & Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA

Qi Lin Cao

The Vivian L Smith Department of Neurosurgery, University of Texas Health Science Center at Houston, Houston, TX, USA

Center for Stem Cell and Regenerative Medicine, The Brown Foundation Institution of Molecular Medicine, University of Texas Health Science Center at Houston, Houston, TX, USA

Dong Feng Chen

Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA

VA Boston Healthcare System, Boston, MA, USA

Justin Chew,     Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA

Kin-Sang Cho,     Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA

Samuel David,     Centre for Research in Neuroscience, The Research Institute of the McGill University Health Centre, Montreal, Quebec, Canada

Lingxiao Deng

Spinal Cord and Brain Injury Research Group, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Neurological Surgery and Goodman and Campbell Brain and Spine, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA

Fei Ding,     Jiangsu Key Laboratory, Nantong University, Nantong, Jiangsu, P.R. China

Sarah A. Dunlop,     Experimental and Regenerative Neurosciences, School of Animal Biology, University of Western Australia, Crawley, Australia

Sarah Alison Dunlop,     Experimental & Regenerative Neurosciences, School of Animal Biology, The University of Western Australia, Nedlands, WA, Australia

Keith N. Fargo

Research and Development Service, Edward Hines, Jr. Department of Veterans Affairs Hospital, Hines, IL, USA

Department of Molecular Pharmacology and Therapeutics, Loyola University Chicago Stritch School of Medicine, Maywood, IL, USA

Alzheimer’s Association, National Office, Chicago, IL.

Toby A. Ferguson,     Shriners Hospitals Pediatric Research Center, Temple University School of Medicine, Philadelphia, PA, USA

Eileen M. Foecking

Department of Otolaryngology-Head and Neck Surgery, Loyola University Chicago Stritch School of Medicine, Maywood, IL, USA

Research and Development Service, Edward Hines, Jr. Department of Veterans Affairs Hospital, Hines, IL, USA

Qing-Ling Fu,     Otorhinolaryngology Hospital, The First Affiliated Hospital, Sun Yat-sen University, Guangzhou, Guangdong, China

Mary Pauline Galea,     Department of Medicine, The University of Melbourne, VIC, Australia

Cédric C. Geoffroy,     Department of Neurosciences, University of California San Diego, La Jolla, California, USA

Shu-chao Ge,     National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, China

Chenying Guo,     Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA

Jiasong Guo

Department of Histology and Embryology, Southern Medical University, Guangzhou, China

Key Laboratory of Tissue Construction and Detection of Guangdong Province, Guangzhou, China

Institute of Bone Biology, Academy of Orthopedics, Guangdong Province, Guangzhou, China

Xiaosong Gu,     Jiangsu Key Laboratory, Nantong University, Nantong, Jiangsu, P.R. China

Theo Hagg,     Department of Biomedical Sciences, East Tennessee State University, Johnson City, TN, USA

Cheng He,     Institute of Neuroscience and Key Laboratory of Molecular Neurobiology of Ministry of Education, Neuroscience Research Center of Changzheng Hospital, Second Military Medical University, Shanghai, China

Zhigang He,     F.M. Kirby Neurobiology Center, Children’s Hospital; Department of Neurology, Harvard Medical School, Boston, MA, USA

John D. Houle,     Department of Neurobiology and Anatomy and Drexel Spinal Cord Research Center, Drexel University College of Medicine, Philadelphia, PA, USA

Yu-bin Huang,     National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, China

Bing Hu,     National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, China

Lyn B. Jakeman,     Department of Physiology and Cell Biology, Center for Brain and Spinal Cord Repair, The Ohio State University College of Medicine, Columbus, OH, USA

Kathryn J. Jones

Research and Development Service, Richard L. Roudebush VA Medical Center, Indianapolis, IN, USA

Department of Anatomy & Cell Biology, Indiana University School of Medicine, Indianopolis, IN, USA

Yoshiki Koriyama

Graduate School and Faculty of Pharmaceutical Sciences, Suzuka University of Medical Science, Suzuka, Japan

Laboratories for Neuroscience Research in Neurosurgery and F.M. Kirby Neurobiology Center, Children’s Hospital Boston, Boston, MA, USA

Departments of Surgery and Ophthalmology and Program in Neuroscience, Harvard Medical School, Boston, MA, USA

Antje Kroner,     Centre for Research in Neuroscience, The Research Institute of the McGill University Health Centre, Montreal, Quebec, Canada

Daniel H.S. Lee,     Department of Anatomy, Faculty Medicine, University of Hong Kong, Hong Kong, China

Vance P. Lemmon

The Miami Project to Cure Paralysis, University of Miami Miller School of Medicine, Miami, FL, USA

Center for Computational Sciences, University of Miami Miller School of Medicine, Miami, FL, USA

Department of Neurological Surgery, University of Miami Miller School of Medicine, Miami, FL, USA

Shuxin Li,     Shriners Hospitals Pediatric Research Center, Temple University School of Medicine, Philadelphia, PA, USA

Jie Liu,     Jiangsu Key Laboratory, Nantong University, Nantong, Jiangsu, P.R. China

Ahmed Moghieb

Center for Neuroproteomics & Biomarkers Research, Department of Psychiatry, University of Florida, Gainesville, FL, USA

Department of Chemistry, University of Florida, Gainesville, FL, USA

Paul J. Reier,     Department of Neuroscience, University of Florida College of Medicine, Gainesville, FL, USA

Jennifer Rodger,     Experimental and Regenerative Neurosciences, School of Animal Biology, University of Western Australia, Crawley, Australia

Michael E. Selzer

Shriners Hospitals Pediatric Research Center, Temple University School of Medicine, Philadelphia, PA, USA

Department of Neurology, Temple University School of Medicine, Philadelphia, PA, USA

Dale R. Sengelaub,     Department of Psychological and Brain Sciences and Program in Neuroscience, Indiana University, Bloomington, IN, USA

Kartavya Sharma,     Department of Neurology, UT Southwestern Medical Center, Dallas, TX, USA

Michael I. Shifman,     Shriners Hospitals Pediatric Research Center, Temple University School of Medicine, Philadelphia, PA, USA

George M. Smith,     Shriners Hospitals Pediatric Research Center, Temple University School of Medicine, Philadelphia, PA, USA

Kwok-Fai So

GHM Institute of CNS Regeneration, Jinan University, Guangzhou, P.R. China

Department of Ophthalmology, and State Key Laboratory of Cognitive and Brain Sciences, The University of Hong Kong, Hong Kong, P.R. China

Huanxing Su

Department of Anatomy, LKS Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, China

State Key Laboratory of Quality Research in Chinese Medicine, Institute of Chinese Medical Sciences, University of Macau, Macao SAR, China

Zhida Su,     Institute of Neuroscience and Key Laboratory of Molecular Neurobiology of Ministry of Education, Neuroscience Research Center of Changzheng Hospital, Second Military Medical University, Shanghai, China

Chen Tian,     National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, China

Veronica J. Tom,     Department of Neurobiology and Anatomy and Drexel Spinal Cord Research Center, Drexel University College of Medicine, Philadelphia, PA, USA

Chandler Walker

Spinal Cord and Brain Injury Research Group, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Neurological Surgery and Goodman and Campbell Brain and Spine, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA

Kevin K.W. Wang

Center for Neuroproteomics & Biomarkers Research, Department of Psychiatry, University of Florida, Gainesville, FL, USA

Department of Neuroscience, University of Florida, Gainesville, FL, USA

Taipei Medical University, Taipei, Taiwan

Xiaofei Wang

Spinal Cord and Brain Injury Research Group, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Neurological Surgery and Goodman and Campbell Brain and Spine, Indiana University School of Medicine, Indianapolis, IN, USA

Wutian Wu

Department of Anatomy, LKS Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong, China

State Key Laboratory of Quality Research in Chinese Medicine, University of Macau, Macao, China

Institute of Chinese Medical Sciences, University of Macau, Macao SAR, China

Institute of CNS Regeneration, Jinan University and The University of Hong Kong, Guangzhou, China

Xiao-Ming Xu

Spinal Cord and Brain Injury Research Group, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Neurological Surgery and Goodman and Campbell Brain and Spine, Indiana University School of Medicine, Indianapolis, IN, USA

Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA

Yumin Yang,     Jiangsu Key Laboratory, Nantong University, Nantong, Jiangsu, P.R. China

Wu Yin,     National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, China

Wise Young,     Department of Cell Biology & Neuroscience, W.M. Keck Center for Collaborative Neuroscience, Piscataway, NJ, USA

Juliet C. Yuan,     Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA

Qiuju Yuan,     Department of Anatomy, LKS Faculty of Medicine, The University of Hong Kong, Pokfulam, Hong Kong SAR, China

Guixin Zhang,     Shriners Hospitals Pediatric Research Center, Temple University School of Medicine, Philadelphia, PA, USA

Zhiqun Zhang,     Center for Neuroproteomics & Biomarkers Research, Department of Psychiatry, University of Florida, Gainesville, FL, USA

Binhai Zheng,     Department of Neurosciences, University of California San Diego, La Jolla, California, USA

Lihua Zhou,     Department of Anatomy, Zhong Shan School of Medicine, Sun Yat-Sen University, Guangzhou, China

Ruilin Zhu

Schepens Eye Research Institute, Massachusetts Eye and Ear, Department of Ophthalmology, Harvard Medical School, Boston, MA, USA

Department of Ophthalmology, Peking University First Hospital, Beijing, China

Su-qi Zou,     National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, China

Yimin Zou,     Neurobiology Section, Biological Sciences Division, University of California, San Diego, CA, USA

Foreword

"An ailment not to be treated...". It was not until the early 1900s that the validity of these words inscribed on the Edwin Smith papyrus, an Egyptian medical document from the 17th Century BCE, could be challenged. The ancient statement reflects a prevailing attitude towards the management of different kinds of neural injury to humans that persisted over millennia. It was written, however to address particularly the unavoidably tragic outcome of spinal cord injuries, where serious disability, much suffering and an early death were the rule. The impressive advances in medical care that have taken place during the last 100 years have drastically changed that somber counsel. Indeed, the introduction of anesthesia, new surgical techniques, effective drugs to combat infection, novel rehabilitation methods and tools, etc. have lessened life threatening complications, reduced pain and facilitated movement and bladder control. Together, they have enabled millions of brain and spinal cord injured individuals to return to an active life, often aided by devices and public resources that were difficult to imagine only a few years ago.

The main engine behind this and other medical success stories has been the explosion in knowledge generated by the application of the scientific method and new research technologies to the solution of problems in medical biology and related disciplines. It is hard to believe today that Golgi and Cajal provided the first detailed description of the structure of a nerve cell just over a century ago. Moreover, it has only been possible to explore the cellular and molecular events associated with neural development, maturation and function for a fraction of this time. As a result of such rapid advances, it is now possible to ask questions and provide answers in the laboratory and the clinic on issues that concern the nature of neural injury and the possibility of developing therapies that could help recover lost functions through neural repair, replacement and regeneration.

The editors of Neural Regeneration have enlisted a number of experts in the field to present a timely update of much of what has been learned in recent years. A special attribute of this publication is that it includes multidisciplinary contributions by scientists from many countries around the world. Together, they have made available to the readers a balanced overview of the various and complex issues that still limit the understanding and management of the biologic events triggered by trauma to or diseases of the nervous system. These brief comments can only acknowledge some of the many topics covered by this worthwhile publication.

Early research efforts were summarized in Cajal’s Degeneration and Regeneration of the Nervous System, published in 1914. Limited by the information and techniques available at that time, this monumental work focused on the description and interpretation of anatomical events associated with the interruption of axons in the vertebrate peripheral and central nervous systems (PNS and CNS). Throughout their studies, Cajal and his colleagues were puzzled by the marked contrast between the robust regrowth that follows the cutting of a peripheral nerve and the failure of axons to regenerate after CNS injury. It was recognized even then that the site of the axonal injury, rather than the location of the nerve cell somata within or outside the CNS, best correlated with the success or failure of nerve fibres to regenerate. A well-known example of this assertion is the difference in regrowth observed after an injury to either the spinal or the peripheral axonal projection that arises from a dorsal root ganglion neuron. A related case in point is the response that follows a transection of a motor axon along its intra or extra spinal course. In both situations, regrowth fails to occur when fibers that originate from the same nerve cell are injured within the CNS, while lesions in the peripheral portion of the axon are usually followed by regeneration. These early observations were interpreted as an indication that interactions between the axonal growth cone, originally described by Cajal, and its injured PNS or CNS environment play an important role in triggering nerve cell responses that either sustain or curtail axonal extension. Various chapters in the present publication offer updated accounts of the many studies that have since then addressed the nature and role of the various growth-facilitating and growth-inhibiting molecules that reside in the Schwann cell-populated PNS or the glial milieu of the CNS. Furthermore, as pointed out by some of the contributors, molecules that block growth inhibition or provide trophic support may also influence mechanisms involved in a wide range of other repair functions, including synaptic strength and the sprouting of uninjured neurons and terminal arbors.

Advances in cell and molecular biology also continue to yield new insights into ways to assess and influence intrinsic neuronal mechanisms responsible for axonal growth. These processes are active early on during fetal development when the distances to be covered by the growing axons are relatively short but must be navigated within a precise time period. However, axonal extension and plasticity persist post-natally, growth rates being greater during the first year and peaking again through adolescence when body and limb extension accelerate and neuronal remodeling is enhanced. The magnitude of the post- natal growth of certain fiber tracts is exemplified by the 10-fold increase in the length of the human spinal cord that can take place between birth and age 20. It is therefore intriguing that spinal cord injuries in children and young individuals are not followed by even a limited replication of what nerve cells are clearly capable of accomplishing in the undamaged spinal cord. Axons continue to lengthen with body growth but cannot bridge a damaged site.

Nonetheless, the regenerative capacity of CNS-injured mammalian neurons is not entirely lost in the mature organism. Indeed, it is now well established in laboratory animals that at least some nerve cells in the retina, brain and spinal cord whose axons are damaged in the CNS retain their ability to regrow extensively along a propitiously modified environment. Furthermore, when guided to the proximity of their targets they can form functional synapses. Excitingly, recent studies in adult rodents, clearly described in this volume, show that it is also possible to boost intrinsic neural mechanisms of growth to the point where injured axons overcome their CNS barriers and extend along damaged white matter tracts.

Finally, increasing experimental evidence, some of it discussed in this volume, for activity-dependent plasticity following damage to the CNS, particularly in groups of nerve cells located below the level of a spinal cord injury, have lead to the development of successful new rehabilitation strategies in humans that exploit the capacity of local neuronal circuits to reorganize, re-learn and regulate complex lost functions such as those involved in limb movement and even gait.

It seems possible, therefore, to look forward with some confidence to a further elucidation of the molecular mechanisms within neurons that propel growth and those that modify the environment of regenerating axons to enhance or inhibit their extension. Creative ways of combining these and other approaches, such as training and the use of device-dependent aids, should further propel forward the field of neural repair. Perhaps, as suggested in this book, it may be possible one day to replicate in humans some of the conditions that lead to functional recovery in other vertebrates such as amphibians, fish and some reptiles that are inherently capable of regenerating injured CNS fiber tracts to their targets. A needed and challenging stage along such a path of discovery will be to understand how regenerating mammalian axons can be guided to make appropriate connections and restore useful functions. This may well be the focus of a follow-up, future publication.

Albert J. Aguayo

Preface

Neural regeneration is a rapidly advancing field of neuroscience. Neural regeneration refers to the regrowth or repair of nervous tissues or cells. Broadly, it may include generation of new neurons, growth of damaged axons, and formation of new synapses after damage to a particular region of the nervous system. It may also include the generation of new glial cells including their progenitor cells and the formation of new myelin. Notably, mechanisms and outcomes of neural regeneration could be quite different between the lower and higher vertebrates, including mammals, between the developing and adult nervous system, and between the peripheral nervous system (PNS) and central nervous system (CNS). For example, PNS axons could regenerate spontaneously through and beyond a nerve injury, whereas, CNS axons failed to do so due to combined intrinsic and extrinsic mechanisms that have just, at least in part, revealed in the past two decades.

Nervous system injuries, including peripheral nerve injuries, brain and spinal cord injuries, and stroke affect millions of people worldwide every year. As a result of this high incidence of neurological injuries, neural regeneration and repair is becoming a rapidly growing field dedicated to the new discoveries to promote structural and functional recoveries based on neural regeneration. The ultimate goal is to translate most optimal regenerative strategies to treatments of human nervous system injuries. The purpose of this book is to provide an overview of cutting-edge knowledge on a broad spectrum of neural regeneration, including (1) neural regeneration in lower vertebrates, (2) neural regeneration in the peripheral nervous system, (3) neural regeneration in the central nervous system, (4) transplantation-mediated neural regeneration, and (5) clinical and translational research on neural regeneration. The contributors of this book are experts in each of their respective fields and are highly regarded internationally including, scientists working at different institutions in the United States, Canada, Australia, and China. We hope that this book will be useful for students, postdoctors, and basic and clinical scientists who are interested in neural regeneration research.

The book would not have been possible without the efforts of a great many people. We are grateful for all authors who sacrificed their time and effort to contribute each of their chapters. A special thanks goes to Ms. Patti L. Raley who helped to communicate with contributing authors and to collect, edit, and format the chapters. A special thanks also goes to Elyssa Siegel who has designed an astonishing cover for the book. Finally, we thank Ms. Jie Min and Mr. Zhijiang Pan from Science Press, China, for their publishing guidance and the opportunity to have this book published by the Science Press, China.

We hope the readers find this book to be informative and useful. Most importantly, we hope that knowledge conveyed in this book will facilitate translation from basic research to patient care, which could benefit people suffering from nervous system injuries.

Kwok-Fai So,     Guangzhou and Hong Kong, China

Xiao-Ming Xu,     Indianapolis, USA

Section I

Introduction

Outline

Chapter 1. Advances and Challenges for Neural Regeneration Research

Chapter 1

Advances and Challenges for Neural Regeneration Research

Xiaofei Wang¹,², Kwok-Fai So⁴,⁵,  and Xiao-Ming Xu¹,²,³     ¹Spinal Cord and Brain Injury Research Group, Stark Neurosciences Research Institute, Indiana University School of Medicine, Indianapolis, IN, USA     ²Department of Neurological Surgery and Goodman and Campbell Brain and Spine, Indiana University School of Medicine, Indianapolis, IN, USA     ³Department of Anatomy and Cell Biology, Indiana University School of Medicine, Indianapolis, IN, USA     ⁴GHM Institute of CNS Regeneration, Jinan University, Guangzhou, P.R. China     ⁵Department of Ophthalmology, and State Key Laboratory of Cognitive and Brain Sciences, The University of Hong Kong, Hong Kong, P.R. China

Abstract

Injuries to the nervous system, including the central and peripheral nervous systems, are devastating events that affect the full spectrum of human life. Understanding the mechanisms by which axons fail to regenerate and developing novel strategies and therapeutic interventions to ameliorate or even prevent damage to the nervous system are ultimate goals of the scientists working in this field. Since the mid-1990s, numerous studies have been conducted and many exciting new discoveries have been made that have significantly advanced our knowledge in neural regeneration. However, only limited treatments are available today for the victims of neurological injuries, and clinicians and scientists still face significant challenges. In this chapter, we introduce basic principles of neural regeneration, highlight major advances and challenges in neural regeneration, and discuss how these challenges are met to advance science and to translate novel discoveries to clinical applications.

Keywords

Axons; Glial response; Neural regeneration; Peripheral nerve injury; Spinal cord injury; Transplantation

1. Nervous System, Nerve Injury, and Neural Regeneration

The nervous system is divided into two parts: the central nervous system (CNS), which consists of the brain and spinal cord, and the peripheral nervous system (PNS), which consists of cranial and spinal nerves along with their associated ganglia. The function of the CNS and PNS is to relay information to and from all parts of the body. This communication is made possible through an extensive network of neurons and supporting cells called glia, including astrocytes, oligodendrocytes, and microglia.

Nerve injury, whether traumatic or degenerative, disrupts the normal flow of information and can, depending on the location and mechanism of injury, lead to deleterious effects. Injury or sudden trauma, such as from automobile accidents, falls, sports-related activities, etc., can cause nerve fibers or axons to be partially or completely severed, crushed, compressed, or stretched. When an axon is damaged, the distal segment undergoes Wallerian degeneration, losing its myelin sheath [1]. The axotomized neurons either die by necrosis or apoptosis or undergo a chromatolytic reaction, which is an attempt to repair. Injury to the nervous system also triggers the responses of glial cells, including oligodendrocytes, astrocytes, and microglia in the CNS; Schwann cells (SCs) in the PNS; and blood-derived macrophages that participate in both CNS and PNS injury processes. The responses of these cells to injury include cell death, proliferation, migration, and production of inflammatory mediators and growth factors, thus influencing processes of axonal degeneration and regeneration. Thus, nervous system injuries affect not only neurons and their processes but also glial cells.

Neural regeneration refers to the regrowth or repair of nervous tissues, cells, or cell products. Such mechanisms may include generation of new tissues, neurons, glia, axons, myelin, or synapses. Beyond the common knowledge of neurogenesis, a wider concept of neural regeneration may comprise endogenous neuroprotection leading to neuroplasticity and neurorestoration. Neural regeneration can also be promoted by implantation of viable tissues or cells. Neural regeneration differs between the PNS and the CNS owing to different neuronal and glia responses to injury as well as the different environments that the regenerative axons and cells encounter.

2. Technological Advances in Neural Regeneration Research

2.1. Models

Preclinical animal models are critical for understanding regenerative neurobiology and for testing treatment strategies prior to implementation in clinical practice. For regeneration research, in vitro, ex vivo, or in vivo models, described below, have been used extensively and complementarily.

2.1.1. In vitro Model

The flexibility and ease of control offered by the in vitro model make it a useful tool for the study of neural regeneration. Glass micropipettes can be used to sever processes from cultured neurons or tissue explants to study axonal and dendritic regrowth in vitro [2–4]. Although this method can cut many axon segments simultaneously, it cannot be used to isolate axon and dendritic segments. Fine knife cutting is another localized physical injury, which can precisely cut neurites [5,6]; this method, however, damages the coated substrate and sets up an artificial sulcus, which may prevent the truncated neurite from regrowing. Microdissection of a neurite with a laser beam offers more precise control [7,8] that provides a unique platform for regeneration research [9]. A nanocutting device with a cutting edge of less than 20  nm radius of curvature was developed that enables high-precision microdissection and subcellular isolation of neuronal structures [10]. With these devices, not only can a single-axon transection model be established, but also regeneration-related functional components of neurons, such as segments of axons, dendrites, dendritic spines, and nodes of Ranvier, can be isolated in culture.

2.1.2. Ex vivo Model

An ex vivo model is ethically advantageous, requires no postsurgical animal care, enables more reproducibility between lesions, and provides a tightly controlled artificial environment for regeneration studies. Published ex vivo spinal cord models include the culture of several hundred micrometers-thick transverse slices maintained for up to three weeks [11], unfixed longitudinal cryostat sections of spinal cord maintained for one week [12], and a novel ex vivo model that enables the culture of intact postnatal spinal cord segments for up to five days and the assessment of peripheral nerve grafting repair [13].

2.1.3. In vivo Model

Although invertebrates and lower vertebrates, such as Caenorhabditis elegans [14–16], lamprey [17–19], zebrafish [20–23], and lizard [24,25], have long been applied for neural regeneration research, the rat sciatic nerve, brain, and spinal cord injury models have been the most commonly used for studies of neural regeneration. Rodent models, such as rats and mice, are economical compared to large-animal models and primates, simple to handle and care for, very resistant to surgical infections, and can be investigated in large groups. Rodent models can be used for electrophysiology, functional recovery, muscle and nerve morphology, and other assessments of nerve regeneration [26,27]. The major value of the mouse model is the ability to answer mechanistic neural regeneration questions [28,29]. The rabbit, dog, and cat are large-animal species more frequently used for peripheral and central nervous system injury research. Large mammals such as sheep [30,31], pigs [32,33], and monkeys [34–37] have increasingly been employed to study neural regeneration. These large species are limited by extremely high costs related to animal care, the narrow range of assessments available, and the complexity of training for functional testing. Transgenic animals, particularly mice, that express fluorescent proteins in specific neuronal subsets provide potentially powerful tools for the study of neural regeneration. One strategy involves expressing fluorescent proteins under the control of neuron-type-specific promoters [38]. Another approach involves the use of bacterial artificial chromosome (BAC) mice [39,40]. Genetic labels can provide specificity in axonal labeling that is hypothetically independent of tracer transport [41]. Moreover, BAC mice bearing green fluorescent protein-tagged polyribosomes (BAC-TRAP mice) provide an exceptional opportunity to identify potential regeneration-associated transcriptional events in a cell-type-specific manner [40].

A book entitled Animal Models of Acute Neurological Injuries [42] has provided a wide array of animal models currently used for assessing acute neurological injuries, providing valuable resource for neural regeneration research.

2.2. Labeling and Imaging Technology

How to exquisitely label nerve fibers within the nerve system and their connections to their target continues to be an important concern for neural regeneration research. Transgenic animals that express fluorescent proteins in specific neuronal subsets provide potentially useful tools for the regeneration study of these neurons [38].

Axonal tract tracing technologies are also powerful tools for identifying axonal connections. With appropriate injury models and tracing techniques, the status of axons—sparing, die-back, sprouting, regeneration, or synaptogenesis—can be readily identified [43]. Based on axonal transport, a long series of tracers has been developed as anterograde tracing or retrograde tracing according to the preferential direction of their transport in the axon.

Viruses have been developed for tract-tracing studies. Compared to conventional tracers, viruses have the ability to traverse multisynaptic pathways and replicate to amplify signals at each step in the process [44]. Depending on the species and strain of the virus, viruses can travel preferentially in the anterograde or the retrograde direction or both [45,46]. For example, Wang et al. found that a recombinant adenovirus carrying a green fluorescent protein reporter gene (Adv-GFP) can preferentially, intensely, and bidirectionally label the rat rubrospinal tract [46]. More recently, genetic modifications have allowed for many improvements. These include reduced pathogenicity, control of synaptic spread, addition of marker genes, pseudotyping for infection of selected cells, and addition of ancillary genetic elements for combining circuit tracing with manipulation of activity or functional assays.

Imaging plays an essential role in the diagnosis, treatment, and rehabilitation of nerve injury patients. Traditionally, imaging modalities have consisted of plain radiography, computed tomography, and magnetic resonance imaging (MRI). Despite their critical importance, these modalities offer comparatively less information regarding the microstructural changes after injury or regeneration. This has led to the development of novel imaging techniques that are principally focused on the microstructural and/or biochemical function of the nerve. These novel techniques include diffusion tensor imaging [47,48], MR spectroscopy [49,50], positron emission tomography [51,52], single-photon emission computed tomography [53,54], two-photon imaging [55,56], and functional MRI [57,58]. These techniques are currently in various development stages, including some whose applications are primarily limited to laboratory investigation, whereas others are being actively utilized in clinical practice. In 2011, a longitudinal coherent anti-Stokes Raman scattering imaging technique was reported to clearly monitor demyelination and remyelination of axons in live rats after spinal cord injury (SCI) [59]. A year later, a tetrahydrofuran-based clearing procedure that renders fixed and unsectioned adult CNS tissue transparent and fully penetrable for optical three-dimensional imaging was reported [60]. This procedure can be readily used to study neural regeneration.

2.3. Nanotechnology

The rapid expansion of nanotechnology during the past decade has led to new perspectives and advances in the neural regeneration field. As nanotechnology is defined by the size of a material or manipulation on the molecular level, it involves a broad range of nanoscaled materials used in various fields of regenerative medicine, including diagnosis, drug and gene delivery, tissue engineering (TE), and cell therapy. For example, to allow cells to be detected in vivo, superparamagnetic iron oxide nanoparticles have been successfully used to label transplanted cells for in vivo noninvasive MRI monitoring [61]. The basic strategy of TE is the construction of a biocompatible scaffold that, in combination with living cells and/or bioactive molecules, replaces or repairs damaged cells or tissue [62]. The large surface of nanostructured materials, such as two-dimensional (2D) electrospun nanofibers [63], 3D electrospun nanofibers [64,65], and self-assembling nanofibers [66,67], enhances the adsorption of adhesive proteins, such as fibronectin, which mediate cell-surface interactions through integrin cell-surface receptors [68]. For example, the self-assembling peptide RADA16-I supported the growth of PC12 cells and the formation of functional synapses of rat primary hippocampal neurons [69]. Modification of the RADA16 peptide by the immobilization of bone marrow homing protein motifs significantly enhanced the survival of mouse neural cells [70]. In vivo, RADA16-I repairs the disrupted optic tract [71], bridges the injured spinal cord of rats after transplantation [72], and helps to reconstruct lost tissue in the acutely injured brain [73]. Injured spinal tissue incubated with self-assembled monomethoxy poly(ethylene glycol)-poly(D,L-lactic acid) diblock copolymer micelles (60  nm diameter) showed rapid restoration of compound action potential and reduced calcium influx into axons for micelle concentrations much lower than the concentrations of polyethylene glycol, a known sealing agent for early-stage SCI. Intravenously injected micelles effectively recovered locomotor function and reduced the volume and inflammatory response of the lesion in injured rats [74]. Trends in TE include scaffold functionalization that is tailored to each specific application and cell response. Improving the cellular response and the loading and delivery of drugs or bioactive molecules as well as enhancing the scaffolds’ bioactivity can lead to the optimization of nanofibrous materials for transplantation and clinical application.

3. PNS Regeneration

The PNS comprises axons of motor neurons, which stem from the brain/spinal cord and convey information from the CNS to muscle cells, and sensory neurons, whose cell bodies reside in ganglia and transmit information to the CNS. After peripheral nerve injury, axons can readily regenerate. PNS regeneration is remarkably efficient in mammals and closely mimics neurodevelopment. When a peripheral axon is severed, the tip of the proximal segment develops a growth cone, which then samples its environment for growth signals emitted by its target cells and extends toward them. PNS axons can regrow several centimeters in this fashion [75].

Neuronal intrinsic pathways are critical for PNS regeneration. Dorsal root ganglia (DRG) neurons show a strong regenerative capability when their peripheral branches, but not their central ones, are damaged [76,77]. Interestingly, the limited regenerative capacity of the central branches can be enhanced when their peripheral axons are damaged prior to, at the time of, or following the injury of their central axon, a phenomenon defined as the conditioning effect [76–78]. The first molecule to be implicated in this phenomenon was cyclic adenosine monophosphate (cAMP) [79–81]. These studies strongly suggest that PNS neurons have an intrinsic regenerative capacity, and the DRG model can be used to investigate the molecular and genetic mechanisms driving PNS axonal regeneration. Costigan et al. compared gene expression profiles of DRGs after axotomy of the sciatic nerve to naive conditions, identifying 240 genes involved in immunity, inflammation, and neurotransmission that were associated with DRG axonal regeneration [82]. In the past few years, a number of regeneration-associated genes, such as growth-associated protein-43 [83], small proline-repeat protein 1A [84], KLF4, p53, signal transducer and activator of transcription 3 (STAT3), NFAT, RARβ, c-Jun [85], activating transcription factor-3 [86], and Sox11, have been identified as critical factors associated with PNS axonal regeneration [87,88]. Furthermore, a number of neurotrophins, including nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin 3 and 4, have been shown to initiate and contribute to the prosurvival and progrowth response of axotomized PNS neurons [89,90]. Delivery of these neurotrophic factors alone or combined with other strategies [91–94] promotes PNS regeneration.

An important extrinsic player in PNS regeneration is a type of PNS-supporting cell named the Schwann cell. SCs execute the combined functions of astrocytes and oligodendrocytes, myelinating axons and encasing synapses in the PNS [95]. After injury, the SCs become activated, assume a more primitive phenotype, and stimulate axonal growth, with upregulation of growth-related genes, including those that encode intrinsic neurotrophic factors and key transcription factors [96,97]. Activated SCs produce collagen and laminin, creating a tunnel of extracellular matrix, and express cell adhesion molecules and receptors, including interleukin-1, N-cadherin, γ-integrins, and the neural cell adhesion molecule [98]. The resulting supportive environment yields SC proliferation, formation of bands of Büngner, and, finally, supporting axonal growth. This growth occurs at a rate of 1–4  mm per day, with progressive myelination of the fibers by the neighboring SCs. The final repaired nerve usually presents thinner myelin sheets with shorter nodal lengths, less functional than the original nerve [99]. Based on their potential benefits, SCs have been extensively applied in peripheral nerve regeneration research [100–102].

PNS axonal regeneration requires a complex interaction of a scaffold for axonal elongation, supportive cells such as the Schwann cells, growth factors, and extracellular matrix [97,103]. When end-to-end suture of the nerve is not possible, the interposition of a nerve conduit becomes necessary. Autologous nerve grafts are considered the gold standard for repairing peripheral nerve gaps [104]. Autologous grafts are often harvested from the sural, or sensory, nerves [105]; however, sometimes there are limitations such as tissue availability, size incompatibilities, and deformities. Less frequently, allografts can be used, with the disadvantages of requiring immunosuppression and of producing worse outcomes than autologous nerve grafts [106]. More recently, TE has provided nerve conduits, which function as guides for axonal regrowth. Depending on the materials used for their construction, nerve conduits can be classified as natural, based on laminin, collagen, or even vessels and decellularized nerves [107], or as artificial, usually made of polymers [108]. Although artificial, nonbiodegradable scaffolds help nerve growth and provide beneficial results, they may lead to chronic inflammation and tissue compression and, therefore, must be surgically removed once the neural connection has been concluded. To avoid the hurdles of a second surgery on the injury site, biodegradable scaffolds are preferred [109]. The major limitation for use of nerve conduits is the low rate of axonal growth, which may not yield meaningful repair within the available time. Studies have shown that the conduits are effective in promoting repair of peripheral nerve gaps measuring up to 3  cm [108], whereas nerve autografts are required for bridging nerve gaps of larger distances.

4. CNS Regeneration

In contrast to the PNS, in which severed axons often will regenerate, injured CNS neurons exhibit a burst of stymied growth but ultimately fail, with their axons stalling out and forming distinctive large endings dubbed retraction bulbs that fail to transverse the injury site. Numerous comparative studies have indicated phylogenetic differences in the regeneration capacity of various species. Whereas axons in the CNS of warm-blooded vertebrates (mammals and birds) do not regenerate, those in many lower vertebrates such as newts [110] can regenerate after injury. Young mammals are also capable of substantial CNS neural regeneration [111]. These studies demonstrate that the lack of CNS regeneration in warm-blooded vertebrates may be the result of evolutionary changes, although it is still unclear whether these varied responses are caused by differences in the expression of genes that are conserved across these organisms or by the presence of proteins that are specific to warm-blooded vertebrates. In the field’s effort to define the failure of axonal regeneration after CNS injury in mammals, the neuron’s intrinsic growth state, the glial scar, myelin inhibitors, and invading cells from the periphery have all been investigated as likely suspects involved in inhibiting CNS regeneration. Accordingly, numerous experimental research efforts aiming at these theories have been conducted and some exciting and promising interventions have been summarized below.

4.1. Intrinsic Growth Capability of CNS Neurons

As mentioned above, cAMP has been identified as the first molecule to be implicated in the conditioning effect [77,81]. Manipulation of signaling pathways by elevating the level of cAMP can similarly change a neuron’s propensity to regenerate [77]. CNS neural regeneration can be enhanced in vivo by delivering a cAMP analog or by administering rolipram, which inhibits an enzyme that blocks the breakdown of cAMP [112,113]. Studies have also indicated that conditional knockout of PTEN (phosphatase and tensin homolog) or tuberous sclerosis complex 1, both negative regulators of the mammalian target of rapamycin (mTOR) pathway in adult retinal ganglion cells, promotes robust axon regeneration after optic nerve injury [114] and adult corticospinal tract [29], demonstrating that modulating neuronal intrinsic PTEN/mTOR activity represents a potential therapeutic strategy for promoting axon regeneration and functional repair after adult spinal cord injury [115]. More recently, the suppressor of cytokine signaling 3, a negative regulator of the Janus kinase/STAT pathway, was identified as another independent pathway that can act synergistically with PTEN/mTOR to promote enhanced axon regeneration [28,116].

4.2. The Glial Scar

Glial reaction is a hallmark of CNS injury. After CNS injury, astrocytes hyperproliferate and become reactive, releasing extracellular matrix molecules, such as laminin, heparan, and especially chondroitin sulfate proteoglycans (CSPGs), which are considered to be major candidates for mediating the inhibitory activity of the scar [117]. Consistent with this possibility, therapeutic dissolution of the CSPG-rich matrix with chondroitinase ABC, an enzyme that selectively degrades CSPGs, has proven to be beneficial to axonal regeneration and functional recovery after SCI in preclinical studies in rodents [117–121]. In recent years this line of research has been further advanced with promising results [122–125]. Although it is well characterized that astrocytes produce several different CSPG family members that are differentially expressed after SCI [126–128], the molecular mechanisms through which CSPGs activate growth cone collapse are not fully understood. A transmembrane tyrosine phosphatase receptor, PTPσ, has been identified as one specific and high-binding-affinity receptor for CSPGs [129]. Subsequently, another member of the PTPR subfamily, LAR, was shown to bind to CSPGs with high affinity. Intervention with a LAR-targeting peptide improved axonal regeneration and motor functional recovery after SCI in rodents [130]. In addition, provocative reports have demonstrated that stromal cells derived from pericytes, which control the vasculature in the CNS, also constitute a substantial portion of the cells found at the glial scar. Genetically modified animals with severely reduced populations of pericytes failed to insulate spinal cord lesions with glial scar tissue [131].

4.3. Myelin-Associated Inhibitors

The clearance of myelin debris is extremely slow within the adult mammalian CNS. As these remnants stay for weeks and months after lesion, the possibility was raised that residual myelin may contain factors that can actively prevent injured neurons from regenerating. In vitro, cultured neurons are prevented from extending axons when plated on purified myelin extracts [132,133]. In vivo, animals that received irradiation to impair the formation of myelin-producing oligodendrocytes, or were immunized with myelin extracts, showed some regeneration [133]. Since 2000, three prominent myelin-associated inhibitors (MAIs) have been identified: Nogo-A [134–137], myelin-associated glycoprotein (MAG) [138,139], and oligodendrocyte myelin glycoprotein (OMgp) [140–142]. The inhibitory properties of Nogo, MAG, and OMgp have been tested in vitro and in vivo in different CNS injury models [138,141,143–146], indicating Nogo-A as the major actor in myelin-dependent CNS repair failure. An anti-Nogo-A antibody has advanced to clinical trials for SCI (http://www.research-projects.uzh.ch/p9471.htm). The potential synergistic inhibitory effect of these three proteins on axonal regeneration in injured adult CNS has been tested. Triple-knockout (TKO) mice for Nogo, MAG, and OMgp were independently generated in two laboratories, and the axonal regenerative capacity of the corticospinal tract (CST) and 5-hydroxytryptamine (5-HT), as well as the motor functional recovery of the TKO mice, compared to wild-type and single-mutant mice, was evaluated after SCI. The Strittmatter lab found that loss of Nogo-A allows corticospinal and raphe-spinal axon growth above and below the injury, as well as greater behavioral recovery than in wild-type or heterozygous mutant mice. In contrast, deletion of MAG and OMgp stimulates neither axonal growth nor enhanced locomotion. The triple-mutant mice exhibit greater axonal growth and improved locomotion, consistent with a principal role for Nogo-A and synergistic actions for MAG and OMgp, presumably through shared receptors, which provide the optimal chance for overcoming myelin inhibition and improving neurological function [142]. The Zheng lab, however, found that, whereas deleting any one inhibitor in mice enhanced sprouting of corticospinal or raphe-spinal serotonergic axons, there was neither associated behavioral improvement nor a synergistic effect of deleting all three inhibitors. Furthermore, they found that triple-mutant mice failed to exhibit enhanced regeneration of either axonal tract after SCI, indicating that although Nogo, MAG, and OMgp may modulate axon sprouting, they do not play a central role in CNS axon regeneration failure [147].

Although the three MAIs are structurally distinct, evidence suggests that all three bind a receptor complex containing the Nogo receptor (NgR1) [148]. NgR1 function can be blocked by a soluble form of extracellular NgR1 fused to human Fc (NgR(310)ecto-Fc). NgR(310)ecto-Fc promotes corticospinal and raphe-spinal growth and functional recovery after SCI in rats [149]. Transgenic mice that secrete NgR(310)ecto under control of the glial fibrillary acidic protein promoter show enhanced functional recovery after SCI [150]. A competitive NgR1 antagonist, Nogo-extracellular peptide, residues 1-40 (NEP1-40), binds to but does not activate NgR1, attenuating inhibition of neurite outgrowth by Nogo-A and CNS myelin. After SCI, NEP1-40 promotes corticospinal and raphe-spinal regeneration and functional recovery, even when the initiation of treatment is delayed for one week [151,152].

The Nogo receptor complex also contains the low-affinity neurotrophin receptor (p75NTR) [153,154] LINGO-1 [155,156] and/or the p75NTR relative TROY [157,158]. Signaling through this receptor complex is thought to inhibit neurite outgrowth by activating a small GTPase Ras homolog gene family member A (RhoA) [159,160]. Activated RhoA, in turn, activates Rho-associated coiled-coil containing protein kinase 2 (ROCK2), a kinase that regulates actin cytoskeletal dynamics [161]. Ibuprofen, which inhibits RhoA, promotes corticospinal and raphe-spinal sprouting as well as long-distance raphe-spinal axon regeneration after spinal cord transection or contusive injury [162]. Tissue sparing at the lesion site is also enhanced by ibuprofen and thus contributes to functional recovery [163]. The ROCK2 inhibitor Y27632 promotes CST sprouting and locomotor recovery after spinal dorsal hemisection in rats [160]. In addition, ROCK2-knockout mice show enhanced functional recovery after SCI [164].

Axon regeneration inhibitors found in the CNS that are not present in myelin or the glial scar include repulsive guidance molecules (RGMs) such as protein kinase C [119] and semaphorin 3A (Sema3A) [165,166]. Evidence that these molecules limit CNS regeneration includes studies demonstrating that an anti-RGMa antibody [167] or a small-molecule inhibitor of Sema3A [166] promotes functional recovery after SCI.

4.4. Neurotrophic Factors

Neurotrophic factors have been shown to be an alternative and potent mechanism to increase the number and range of regenerating axons, to guide regenerating axons across a lesion site, and to augment regenerative cell body responses to injury. Ample evidence suggests that neurotrophic factors such as BDNF [168–170], ciliary neurotrophic factor [171], neurotrophin-3 [168,169,172–174], glial cell line-derived neurotrophic factor [175–178], and NGF are all beneficial for CNS neural regeneration. Although intracerebroventricular [179], intrathecal [180], and local [172] protein delivery of neurotrophic factors to the injured site has resulted in enhanced survival and regeneration of injured neurons, there are several drawbacks to these methods [181]. Viral vector-mediated transfer of neurotrophic factor genes to the injured tissue is emerging as a novel and effective strategy to express neurotrophic factors in the injured nervous system. Ex vivo transfer of neurotrophic factor genes, followed by transplantation of transduced tissues or cells, is being explored as a way to bridge lesion cavities for axonal regeneration. Several viral vector systems, based on herpes simplex virus [182], adenovirus [183–185], adeno-associated virus [186], and lentivirus [175,187], have been employed [188]. The genetic modification of fibroblasts [189–191], SCs [175], olfactory ensheathing glia (OEG) cells [183], and stem cells [192,193], prior to implantation into the injured nervous system, has resulted in improved neural regeneration.

4.5. Transplantation-Mediated CNS Regeneration

Since the mid-1990s, neural and nonneural tissue and cell transplantation have been used extensively to study mechanisms of nerve injury and neural regeneration. The availability of potential donor tissues and cells for transplantation and the methods developed to obtain them provide opportunity and flexibility for strategies to treat nerve injuries and diseases. A transplant of neural tissue and cells may replace particular populations of neurons lost by injury and restore levels of neurotransmitters, neurotrophic factors, or neural circuitry. These transplants may also provide a population of neurons at the injury site, which may serve as a relay to convey sensory and/or motor control to levels proximal and/or distal to the injury. Finally, neural and nonneural tissue and cell transplants, alone or in combination with other strategies, may serve as a bridge that supports axonal growth across the injury site to reach targets proximal and/or distal to the lesion [98]. Strategies that have been applied to neural regeneration research include peripheral nerve [194], fetal spinal cord tissue [195–197], fetal brain stem tissue [198], SCs [168,175,199], OEG [200], stimulated macrophages [201], embryonic and adult neural stem/progenitor cells (NSCs) [202,203], induced pluripotent stem cells (iPSCs) [204,205], bone marrow stromal cells [206], and oligodendrocyte progenitor cells [207]. Very impressively, studies have demonstrated that transplantation of NSCs or NSCs derived from human stem cell lines (566RSC and HUES7) embedded in growth-factor-containing fibrin exhibited long-distance growth and enhanced formation of new relay circuits that significantly improve functional recovery after SCI [203]. Several reviews systematically introduced the source, isolation, culture, and delivery methods of these cells and their survival, functional integration, and advantages and disadvantages following transplantation [98,208,209]. In addition, cotransplantation of various cells [210–212] and transplantation of engineered cells [112,213] to promote neural regeneration have also been extensively studied in recent years.

4.6. Combinatory Strategies

To date, it is clear from experimental and clinical evidence that no single factor accounts for the lack of neural regeneration after nerve injuries. Failure of successful neural regeneration is particularly attributed to the diminished intrinsic capacity of neurons to regenerate, the presence of physical and chemical barriers associated with the glial scar, and the existence of myelin-associated growth inhibitors in the injured CNS. Therefore, successful functional recovery in patients suffering from CNS injuries will not rely simply on a single therapeutic strategy. The possible injury mechanism and corresponding repair strategies discussed above may one day be developed to individually bring about certain degrees of anatomical regeneration and functional improvement. However, the extent of the repair response will not be enough to guarantee optimal biologically significant recovery of neurological function. These individual strategies, however, can be combined to achieve a greater or maximal regeneration and functional recovery. The most promising repair strategies that may ultimately be combined should address a specific problem associated with increasing the intrinsic capacity of neurons to regenerate, reducing the physical and chemical barriers associated with the glial scar, minimizing the inhibition of myelin-associated growth inhibitors, bridging the lesion gap, providing growth-promoting pathways, and enhancing synaptic connections and target reinnervation [98].

5. Challenges and Opportunities

Nervous system injuries affect over 90,000 people every year [214]. It is estimated that spinal cord injuries alone affect 12,000 in the United States each year [215]. As a result of this high incidence of neurological injuries, nerve regeneration and repair is becoming a rapidly growing field dedicated to the discovery of new methods to recover nerve functionality after injury. Although great efforts and ample significant advances have been made in this field during the past decades, huge challenges still remain.

5.1. Bridging the Knowledge Gap between Basic Research and Clinical Application

It is clear that our ultimate goal is to find effective intervention strategies for patients with nervous system degenerative disorders, including traumatic peripheral and central nervous system injuries. For neural regeneration research, there is still a huge gap between basic research and clinical application, which will take a long time to fill. For example, currently, the vast majority of neural regeneration studies