Frontiers in Stem Cell and Regenerative Medicine Research: Volume 7

By Atta-ur Rahman and Shazia Anjum

Stem cell and regenerative medicine research is a hot area of research which promises to change the face of medicine as it will be practiced in the years to come. Challenges in the 21st century to combat diseases such as cancer, Alzheimer and related diseases may well be addressed employing stem cell therapies and tissue regeneration. Frontiers in Stem Cell and Regenerative Medicine Research is essential reading for researchers seeking updates in stem cell therapeutics and regenerative medicine.

The seventh volume of this series features reviews on roles of mesenchymal stem cells and biomaterials in cartilage regeneration in vivo, liver regeneration, cardiogenesis and magnetic nanoparticles for regenerative therapy.

• Language: English
• Publisher: Bentham Science Publishers.
• Release date: Oct 17, 2017
• ISBN: 9781681085524

Atta-ur Rahman

Atta-ur-Rahman, Professor Emeritus, International Center for Chemical and Biological Sciences (H. E. J. Research Institute of Chemistry and Dr. Panjwani Center for Molecular Medicine and Drug Research), University of Karachi, Pakistan, was the Pakistan Federal Minister for Science and Technology (2000-2002), Federal Minister of Education (2002), and Chairman of the Higher Education Commission with the status of a Federal Minister from 2002-2008. He is a Fellow of the Royal Society of London (FRS) and an UNESCO Science Laureate. He is a leading scientist with more than 1283 publications in several fields of organic chemistry.

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Mesenchymal Stem Cells and Biomaterials for Cartilage Repair in Vivo with a Focus on Gene Therapy

Ana Rey-Rico, Janina Frisch, Magali Cucchiarini*

Center of Experimental Orthopaedics, Saarland Medical Center, Homburg/Saar, Germany

Abstract

Articular cartilage lesions that may be limited (focal defects) or generalized like in osteoarthritis (OA) constitute a key, unsolved clinical problem as a result of the inadequate ability of this tissue to self-repair. Thus far, none of the pharmacological treatments and surgical options allow to reproduce the original cartilage integrity in patients, resulting instead in the formation of a fibrocartilaginous reparative tissue with poor mechanical function that is unable to withstand natural loading and stresses throughout life. Approaches based on the administration of mesenchymal stem cells (MSCs) provide attractive tools to enhance the repair of cartilage lesions as such cells are easy to acquire, expand, and can specifically commit towards the chondrocyte phenotype. This chapter aims at providing an overview of the most current and innovative strategies based on the implantation of MSCs as a means to enhance cartilage repair both in focal defects and in OA lesions in vivo. These approaches include scaffold-free and scaffold-guided procedures as well as gene-based strategies to stimulate the cell chondrogenic activities as a means to restore the natural structure and mechanical integrity in sites of cartilage damage.

Keywords: Focal defect, Osteoarthritis, Mesenchymal stem cells, Chondrocyte phenotype, Scaffolds, Hydrogels, Solid scaffolds, Multiphasic scaffolds, Gene therapy, Cartilage repair.

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* Corresponding author Magali Cucchiarini: Center of Experimental Orthopaedics, Saarland University and Saarland Medical Center, Kirrbergerstr. Bldg 37, D-66421 Homburg/Saar, Germany; Tel: ++496841-1624-987; Fax: ++496841-1624-988; E-mail: [e-mail]

Introduction

Articular cartilage lesions that can be limited (focal defects) or generalized like in osteoarthritis (OA) remain a critical, unsolved clinical problem in light of the low ability of this tissue to adequately repair on itself [1]. In spite of the availability of various clinical options developed to stimulate cartilage repair, none can lead to

the full reproduction of the natural integrity of the hyaline cartilage, rather promoting the formation of a poorly mechanically functional, fibrocartilaginous tissue with a restricted capability to withstand mechanical stresses in physical activities throughout life. Approaches based on applying mesenchymal stem cells (MSCs) are attractive tools for enhanced cartilage repair since these cells can be easily isolated and expanded, while having a specific potency to develop the chondrocyte phenotype [2]. Our goal here is to provide an overview of the most current and innovative approaches based on the implantation of MSCs as a means to activate cartilage repair both in focal defects and in OA lesions in vivo using scaffold-free and scaffold-guided procedures in addition to gene-based strategies that may stimulate the cell chondrogenic activities in order to restore the natural structure and mechanical integrity in sites of cartilage injury.

Articular Cartilage

Structure

Human diarthrodial joints need to withstand important mechanical loads and under frequent movement, smooth gliding of the articulating surfaces has to be ensured. The cartilage is responsible for these challenges, consisting of only one cell type (the chondrocytes) surrounded by a dense extracellular matrix (ECM) that mainly contains proteoglycans bound to water and type-II collagen [2]. Further ECM molecules include type-VI, -IX, -XI, and -XIV collagen and noncollagenous proteins such as the cartilage oligomeric matrix protein (COMP), link protein, fibromodulin, fibronectin, decorin, and tenascin [2].

The adult hyaline cartilage is divided into four zones (Fig. 1): the superficial, middle, deep, and calcified zones confining the subchondral bone plate [2]. Each zone is characterized by a distinct shape and maturation state of the chondrocytes and by a specific distribution of the cells and of their matrix molecules. This particular organization is reflected in the histological appearance of the tissue and is crucial for its unique mechanical properties.

Injuries

Injuries to the cartilage are either restricted (focal defects) or generalized (osteoarthritis - OA). Focal defects mostly result from acute trauma, likely progressing to OA if let untreated [1]. OA is a multifactorial, chronic pathology that results from degenerative procedures, genetic disorders, or from a long-term consequence of injuries of the adjacent tissue like menisci, tendons, and ligaments [1, 2]. It is accompanied by the activation of inflammatory and catabolic cascades, leading to chondrocyte apoptosis and matrix degradation [2]. These processes finally cause deteriorations of the cartilage surface but can additionally affect the surrounding tissue (subchondral bone, synovial lining, tendons, ligaments, and muscles) [2].

Fig. (1))

Organization of the adult hyaline articular cartilage (modified from [12]).

Due to the avascular and aneural nature of the cartilage tissue, there is no access to reparative cells that normally migrate to sites of injury in order to support the reconstruction of injured areas [1, 2]. Hence, the intrinsic repair capacity of the cartilage is restricted to cells of the adjacent tissue that migrate from the synovial membrane in the sites of damage, only leading to insufficient filling of the defects and creating a low-quality repair tissue with poor mechanical properties and inadequate integration [1, 2]. Defects that extend into the subchondral bone plate (osteochondral defects) have the advantage of the migration of bone marrow-derived mesenchymal stem cells (MSCs) from the subchondral bone in the injured area that can finally undergo chondrogenic differentiation and contribute to the healing processes [1, 2]. Still, the quality of the naturally repaired tissue is not comparable with the original structure of hyaline cartilage, displaying mostly type-I instead of type-II collagen and proteoglycans and having poor mechanical resistance [1, 2].

Therapeutic Options

Several options are currently available to treat cartilage injuries, depending on the type and severity of the damage. Subchondral drilling, microfracturing, or abrasion arthroplasty are used to treat focal chondral defects in order to trigger stem cell migration from the subchondral bone in the defect area [1, 2]. More profound injuries such as large chondral or deep osteochondral defects can additionally be filled with chondrocytes (autologous chondrocyte implantation - ACI) with or without biodegradable biomaterials, subchondral bone grafts, or osteochondral cylinders. Therapeutic options for patients with OA depend both on the source and the stage of the disease (Osteoarthritis Research Society International - OARSI stage 1-6). They include conservative approaches such as weight reduction or physical therapy, pharmacological care (e.g. anti-inflammatory drugs), and operative techniques like osteotomies to shift weight loads and relieve affected parts of the joint [1, 2]. Despite the variety of options to treat cartilage injuries, none is capable of producing a repair tissue with similar qualities to those of the natural hyaline cartilage tissue, again leading to the generation of a mechanically poor, fibrocartilaginous repair tissue that can not withstand prolonged, important loads.

Mesenchymal stem cells for cartilage repair

Definition

The use of MSCs is an attractive strategy to improve the qualitative properties of the repair tissue due to the stem cell-typical regenerative activities including the potential for osteochondrogenic differentiation and ability for self-renewal. MSCs are defined by the Mesenchymal and Tissue Stem Cell Committee of the International Society for Cellular Therapy as plastic-adherent when kept under standard culture conditions that are capable of differentiating in cells of the mesodermal lineage (chondrocytes, osteoblasts, adipocytes), displaying positive surface expression of CD105, CD73, CD90, but negative expression of CD45, CD34, CD14, CD79α, and HLA-DR [3]. MSCs display homing, trophic, and immunomodulatory activities, thus playing important roles in regenerative medicine [4]. Several tissues can be used as sources of MSCs, with a main representation in the bone marrow [5, 6], adipose tissue [7, 8], and synovial membrane [9, 10] but also in the periosteum/perichondrium, muscle, amniotic fluid, and peripheral blood [3, 11].

Endochondral Ossification

MSCs participate in a complex procedure named endochondral ossification, a process leading to the formation of the articular cartilage and long bones during the embryonic development (Fig. 2). MSCs naturally undergo chondrogenic differentiation, a process that may occur during cartilage reconstruction when implanting MSCs in sites of cartilage injury [12]. While this process may slightly differ at the adult stage, a number of steps are being recapitulated from the embryonic stage.

Fig. (2))

Endochondral ossification of MSCs during embryonic development. Arrows: major stimulating factors. Yellow boxes: major matrix molecules. IGF-I: insulin-like growth factor I; IHH: indian hedgehog; MSCs: mesenchymal stem cells; PTHrP: parathyroid hormone related protein; RUNX2: runt-related transcription factor 2; SOX9: sex-determining region Y-type high-mobility group box 9; TGF-β: transforming growth factor beta (modified from [12]).

Each step of the endochondral ossification is stimulated by distinct factors (major participants: Fig. 2, purple arrows) and accompanied by the expression of typical ECM proteins (major participants: Fig. 2, yellow boxes). MSCs first condensate and undergo chondrogenic differentiation to form cartilage as an intermediate. MSC condensation is mostly initiated by the transforming growth factor beta (TGF-β) and is characterized by cell clustering according to cell-cell and cell-matrix-interactions due to the activation of cell adhesion molecules (N-cadherin and neural cell adhesion molecule - N-CAM). The factors TGF-β and insulin-like growth factor I (IGF-I) and the cartilage-specific transcription factor sex-determining region Y-type high-mobility group box 9 (SOX9) induce chondrocyte differentiation, maturation, and proliferation as key components in the following 2-step process to generate mature, rounded chondrocytes mainly expressing type-II collagen and aggrecan. Chondrocytes either stay in their mature state to form cartilage tissue or undergo hypertrophic changes characterized by a volume increase up to 20-fold and stimulated by the parathyroid-hormone related protein/indian hedgehog pathway (PTHrP/IHH) with the expression of the hypertrophic marker type-X collagen. Influenced by signaling from the runt-related transcription factor 2 (RUNX2), hypertrophic chondrocytes are ultimately replaced by invading osteoblasts to form long bones.

Hypertrophic events occurring during the process of endochondral ossification (chondrocyte proliferation and differentiation with depletion of SOX9 and type-II collagen, with progressive ECM mineralization as well as chondrocyte hypertrophy and blood invasion) are also noted in OA pathology [11].

In vivo Applications of MSCs for Cartilage Repair

General Aspects

Due to their reliable chondrogenic differentiation potential and regenerative activities, the use of MSCs to enhance cartilage repair in vivo gained increasing attention in the past years. To achieve this goal, animal models relevant of the clinical situation have been developed to test treatments against both experimental focal defects and OA lesions. Pridie drilling, microfracturing, and abrasion techniques are mostly used to artificially create focal defects while OA can be induced via meniscectomy, anterior crucial ligament transection, or using chemical induction with degenerative and pro-inflammatory agents [2, 13, 14]. The choice of an adequate animal model is crucial, depending on several important aspects including its analogy with humans, the availability of previous comparable data, the practicability of the surgery and the tolerance to anesthesia and rehabilitation [14]. Small and large models include mice, rats, rabbits, pigs, sheep, goats, and horses, with large orthotopic models being more adapted than small subcutaneous implantation systems that do not reflect the actual microenvironment of the joint.

MSC Transplantation

Administration of MSCs (bone marrow, adipose tissue, synovium, peri- osteum/perichondrium, peripheral blood) as isolated or concentrated cells to treat focal defects or OA has been tested in a large number of in vivo studies [15-29], with the first trial performed by Wakitani et al. [15] by implantation of bone marrow-derived MSCs to repair large, full-thickness cartilage defects in rabbit knee joints. In the majority of these approaches, the animals showed improved repair upon treatment compared with control conditions in the absence of stem cell therapy. Of note, focal defects may be precisely filled during arthrotomies while intra-articular injections may be more adapted to treat diffuse OA. MSC transplantation reached clinical level to treat patients with cartilage injuries in a number of trials for both focal lesions and OA in conjunction with operative procedures (microfracture, debridement, abrasion) [30-36]. With such techniques, the production of a cartilaginous repair tissue was noted, sometimes integrating with the adjacent cartilage and leading to reduced pain levels for the patients, but without full restoration of the original hyaline-like cartilage nor of the entire osteochondral unit.

Administration of MSCs via Biodegradable Scaffolds

The lack of complete osteochondro regeneration in cartilage lesions using the various current clinical options described above highlights the need for alternative, improved therapies. Tissue engineering has emerged as a means to develop biocompatible substitutes or scaffolds acting as biological cues to enhance the quality of the cartilage reparative processes. Scaffolds may be engineered to act as cell-supportive matrixes, allowing for them to attach, proliferate, and synthesize their ECM. An important advantage of using scaffolds for cell delivery, apart from their ability to retain the implanted cells in a site of injury, is the possibility to prevent invasion of fibroblasts in the graft that may otherwise induce fibrous repair [37, 38]. Different polymers of both natural and synthetic origin have been explored to deliver MSCs in experimental models in vivo and in clinical protocols in patients to treat both focal cartilage defects and OA lesions.

ECM-derived Compounds

Compounds derived from the ECM have been extensively used to design scaffolds with optimal properties for cartilage regeneration as they contain bioactive molecules able to drive tissue homeostasis and regeneration [38].

Collagen, a major component of the ECM, has been used as an integrant of scaffolds to transplant MSCs (bone marrow, peripheral blood) and bone marrow concentrates (BMC) in focal cartilage defects in a number of animal models in vivo (pigs, sheep) [39, 40] and in patients [35, 41-44] to produce hyaline-like cartilage. Yet, while clinical improvements were reported with this compound, its use is still limited by the fibrocartilaginous nature of the newly formed tissue, exhibiting lesser mechanical and biochemical properties than hyaline cartilage as mostly type-I (and not type-II) collagen is used to produce such scaffolds.

Hyaluronic acid (HA) and HA-based scaffolds have been also employed to provide MSCs (bone marrow, peripheral blood) and BMC in experimental focal lesions (goats, horses) [45, 46] and in OA models (rabbits, sheep, goats, donkeys) [47-50] as well as in patients [51-54], revealing hyaline-like cartilage features and reduced inflammation upon treatment.

Natural Compounds

Platelet-rich plasma (PRP), an autologous concentrated cocktail of growth factors and inflammatory mediators emerged in the early 90’s as a non-operative treatment modality for cartilage injuries [55, 56]. Activation of fibrinogen in PRP may result on the formation of a fibrin matrix capable of filling cartilage lesions as occurring during wound healing. While supportive effects of PRP on MSC proliferation and chondrogenesis have been reported in vitro [57-59], contradictory results have been published in translational setups. Even though a reduction in the extent of cartilage lesions was noted in defects in rabbits via PRP-augmented MSC implantation [60], no or only marginal improvements in cartilage repair were observed in horses [61, 62] or in patients [63], showing the necessity for improved knowledge on the basic science and therapeutic potential of PRP.

Hydrogels

Fibrin gels are attractive, long time employed materials for three-dimensional (3D) cartilage tissue engineering as they contain adhesive and proteolytic sequences enabling entrapped cells to remodel their environment in the measure as the new tissue is formed [64, 65]. MSC viability is generally high in gels prepared from a wide range of fibrinogen and thrombin concentrations (5-50 mg/ml fibrinogen and 1-250 U/ml thrombin). Fibrin glue-based biomaterials have been successfully employed to deliver MSCs (bone marrow, peripheral blood) in sites of cartilage damage in animals (rabbits, sheep) [66, 67] and in patients with both focal defects and OA [68-72]. Chitosan-based hydrogels have already been used to deliver MSCs in rats via subcutaneous injection, leading to higher amounts of GAG retention compared with the use of fibrin or of alginate hydrogels [73]. Synthetic materials have been also manipulated to produce hydrogels for the application of MSCs in various animal models. For instance, a thermosensitive poly(N-vinylcaprolactam) injectable hydrogel has been recently tested to carry MSCs in nude rats upon subcutaneous injection asa means to enhance the synthesis and regular distribution of cartilage components (GAGs, type-II collagen) [74].

Solid Scaffolds

An advantage of using solid matrices for cartilage repair is their ability to support cell adhesion and the mechanical stability immediately upon implantation, making them potentially interesting to treat large cartilage defects [75].

Solid porous scaffolds such as ceramics may confer mechanical stability immediately upon implantation, providing a temporary matrix to support new tissue formation and filling of the lesions. A bioceramic scaffold-beta-tricalcium phosphate (beta-TCP) scaffold was used to deliver bone marrow-derived MSCs in focal cartilage defects in sheep [76], leading to a rearrangement of the cartilage surface with increased glycosaminoglycan contents. The increasing capacity to design and synthesize materials with molecular resolution scaled across organizational levels of the cartilage opened new avenues in scaffolding for tissue regeneration. Polyesters such as poly(lactic acid) (PLA), polyglycolic acid (PGA), and poly(lactide-co-glycolide) (PLGA) are among the most studied synthetic biodegradable polymers for cartilage tissue engineering. PGA scaffolds were employed to deliver MSCs (muscle) or BMC in focal defects in rabbits [77, 78], promoting the formation of a hyaline-like cartilage layer within the defects. A resorbable PGA-HA matrix augmented with BMC was tested to successfully treat focal lesions in patients [79], leading to the formation of hyaline-like cartilage and defect filling for about 12 months. Of further note, BioSeed-C®, a synthetic polymers based on PGA/PLA and polydioxanone, is currently in clinical use for autologous chondrocytes implantation [80]. Such scaffolds have been further manipulated to coat bioactive molecules capable activating the chondrogenic potential of MSCs (TGF-β and the bone morphogenetic proteins - BMPs, basic fibroblast growth factor - FGF-2, IGF-I) [75, 81-84]. A PLGA-gelatin/chondroitin sulfate/HA (PLGA-GCH) hybrid scaffold was manipulated to immobilize TGF-β3 and further deliver bone marrow-derived MSCs in focal defects in rabbits [85], allowing for cartilage repair with enhanced production of ECM from cells with a typical chondrocyte morphology.

Multiphasic Scaffolds

Recent advances in biomaterial science focused on the design of scaffolds mimicking the 3D environment of the ECM, providing structural support to the repaired and surrounding tissues with an increased surface area-to-volume ratio for cellular infiltration and colonization [86]. As cartilage and bone have different requirements for optimal repair [87], the design of multiphasic scaffolds mimicking the native organization of the osteochondral unit may provide attractive tools to heal both potentially affected tissues. A biphasic collagen/glycosaminoglycan osteochondral scaffold was used to implant BMC in focal defects in sheep [40], leading to the formation of a hyaline cartilage-like repair tissue versus with control (scaffold/PRP) treatment. A biphasic scaffold consisting of PLGA, calcium sulfate, and PGA fibers was evaluated to deliver BMC in the presence of erythropoietin (EPO) within focal defects in pigs [88], allowing for improved cartilage repair relative to independent, control treatments. Porous poly(ε-caprolactone) (PCL)/F127 scaffolds encapsulating TGF-β2 and BMP-7 were created to provide adipose-derived MSCs in focal defects in rabbits [89], with improved gross appearance of the lesions over time.

Tissue Engineering and Gene Therapy as Combined Option for MSC Implantation

Genetic Modification of MSCs

Even though MSC transplantation in conjunction with biomaterials represents a milestone in the development of treatments for cartilage injuries, the analysis of the repair tissue in relevant in vivo studies still did not reflect thus far the natural structure of adult hyaline cartilage and only poor mechanical resistance was observed both in animal studies and clinical trials, showing the need for improvements of the approach.

A promising strategy is to genetically manipulate regenerative stem cells prior to implantation to further enhance their differentiation potency upon overexpression of chondrogenic factors. Administration of selected genes may allow to prolong the therapeutic effects compared with the use of recombinant factors with short pharmacological half-lives (in the order of 30 minutes in vivo) [90].

Gene transfer in MSCs can be performed via several techniques, including nonviral and viral techniques [3, 91, 92]. Nonviral methods such as microinjection, electroporation, or chemical systems (e.g. liposomes) are safe but only allow for short-term transgene expression. Viral vectors that use natural entry pathways in their targets offer instead improved gene transfer efficiencies but with specific features that add a note of caution to their use in vivo. Adenoviral vectors for instance are very effective but on the other side highly immunogenic and toxic and only allowing for transgene expression over short periods of time (about a week). Retro- and lentiviral vectors offer longer transgene expression periods due to their integration into the host genome but may lead to insertional mutagenesis. Recombinant adeno-associated viral (rAAV) vectors have moderate toxic and immunogenic activities compared with adenoviral vectors as they do not contain any of viral coding sequences, offering high and long-term transgene expression by maintenance of the transgenes in the form of safe, stable episomes, but are not capable of carrying long therapeutic sequences (less than 4.7 kb).

While no data are available thus far on the benefits of providing genetically modified MSCs in patients with cartilage damage, proof-of-principle of vivo implantation of such cellular constructs has been reported in a variety of animal models of focal defects and OA in vivo to investigate potential therapeutic effects on the quality of the repair tissue using both nonviral and viral gene transfer systems. For instance, improvements in cartilage repair was achieved upon administration of MSCs (bone marrow, periosteum/perichondrium, adipose tissue, muscle) and BMC overexpressing TGF-β [93-95], BMPs [96, 97], IGF-I [98], SOX9 [99], the zinc finger protein 145 (ZNF 145) [100], B-cell lymphoma-extra large (Bcl-xL) [101], and IHH [102] using nonviral [95, 101], adenoviral [94, 96, 102], retro-/lentiviral [97, 99, 100], and rAAV vectors [93] to treat focal defects in rats, rabbits, and sheep [93, 94, 96, 97, 99-102] and experimental OA in rats [95]. Yet, implantation of genetically modified MSCs in the absence of a support or scaffold guiding cell differentiation and promoting the secretion of a structurally consistent cell ECM may not be sufficient to achieve the goal of cartilage repair as this approach may fail to reproduce the structure and mechanical properties of the newly formed tissue [87].

Administration of Genetically Modified MSCs via Biodegradable Scaffolds

Tailored strategies based on the administration of genetically modified MSCs in cartilage lesions via delivery in adapted scaffolds such as those involved in the controlled release of pharmaceutical drugs and other recombinant factors [103] may provide optimal strategies to address such issues in vivo. A variety of biomaterials from both natural [104-107] and synthetic origin [108-114] have been tested to transplant genetically modified MSCs from the bone marrow [104, 106, 107, 109, 112, 114], periosteum [108, 110], and adipose tissue [105, 111, 113] in different animal models of focal and OA cartilage lesions.

In focal defects, fibrin gels alone [105] or combined with a PGA matrix [108, 110] have been used as scaffolds for transplantation of genetically modified MSCs overexpressing the SOX trio (SOX5, SOX6, SOX9) [105], BMP-2 [108, 110], and IGF-I [110] via adenoviral [108, 110], retroviral [105], and rAAV vectors [108, 110] in minipig [108, 110] and rat models [105]. Results from these studies revealed an increased in the healing potential of the lesions with formation of hyaline-like cartilage. Transplantation of MSCs genetically modified to overexpress cartilage-derived morphogenetic protein 1 (CDMP-1) using nonviral vectors in a collagen gel was also explored to treat focal defects in rabbits [104], promoting the formation of hyaline cartilage with remodelling of the subchondral bone. Polyester scaffolds (PLA, PGA, PLGA) have also been widely used biomaterials for transplantation of genetically modified cells for cartilage regeneration. Overexpression of TGF-β [109, 111] or of SOX9 [112] in MSCs seeded in PLA [109], PGA [112], and PLGA [111] using nonviral [109] and adenoviral vectors [111, 112] has been reported to produce hyaline-like cartilage filling in focal defects in rabbits. To address the problems associated with the use of synthetic biomaterials such as PLGA (lack of mimicry of the original tissue), Zhu et al. treated PLGA with sodium hydroxide as a means to increase the rate of cell adhesion [114]. Transplantation of genetically modified MSCs overexpressing the connective tissue growth factor (CTGF) via adenoviral vectors using these scaffolds allowed to promote superior defect healing in rabbits compared with the administration of untreated material. Recent technologies involving the use of nonviral vectors for genetically modification of MSCs in cartilage regeneration focused on the manipulation of PLGA nanoparticles. Shi et al. transfected adipose-derived MSCs using PLGA systems delivering a construct encoding BMP-4 for implantation within focal cartilage defects in rabbits [113], reporting improved chondrogenesis after 12 weeks relative to scaffolds containing untreated cells and with empty scaffolds. Another innovative approach for transplantation of genetically modified MSCs in cartilage lesions has been described using pullulan-spermine/nonviral TGF-β1 gene complexes immobilized in a gelatin sponge for seeding of MSCs [106], leading to enhanced cartilage repair upon delivery in rat cartilage defects. The use of demineralized bone matrix (DBM) to deliver genetically modified MSCs has also been attempted to treat focal lesions in pigs using dual chondrogenic adenoviral gene transfer of TGF-β3 and BMP-2 [107], producing a hyaline-like cartilage in the defects.

The manipulation of scaffolds for injection of genetically modified MSCs to treat OA lesions has been much less explored compared with the treatment of focal defects. Retroviral-mediated gene transfer of the SOX trio in adipose-derived MSCs encapsulated in a fibrin gel has been explored in a surgically-induced rat OA model [105]. Eight weeks after injection, treatment with the SOX trio was capable of preventing the progression of degenerative changes in the animals.

Summary and conclusion

The effective treatment of articular cartilage lesions remains a challenge in the clinics as none of the currently available options are capable of restoring the native cartilage surface with an adapted mechanical performance. According to their participation in endochondral ossification, MSCs are promising candidates to support the generation of a natural hyaline repair tissue with improved structural features and mechanical quality. Even though transplantation of unmodified MSCs (Fig. 3, Part 1) may reduce pain and produce better repair compared with classical surgical options, the newly formed tissue still is of fibrocartilaginous nature, with mainly type-I instead of type type-II collagen and showing poor mechanical resistance. Combined implantation of MSCs with biodegradable scaffolds (Fig. 3, Part 2) improves the structural integrity of the transplants as well as the quality of the repair tissue to a higher extent, yet again full production of the original hyaline cartilage in treated lesions has not been demonstrated to date. Extensive in vivo studies have been conducted to further enhance the chondrogenic properties of the MSCs via genetic modification prior to transplantation in the presence (Fig. 3, Part 4) or absence of a biocompatible material (Fig. 3, Part 3). Promising results have been reported based on such approaches, getting closer to the biological situation in healthy individuals. Nevertheless, work is still necessary to define the most efficient transgene(s) to be delivered, optimal gene transfer system, and most appropriate scaffold prior to implement the strategy into the clinics. As no clinical protocol has been initiated thus far combining the implantation of genetically modified MSCs in patients, more experimental work in needed in relevant (orthotopic) animal models of cartilage defects in vivo, bearing in mind that approval has to be given by regulatory organizations for safe translation in affected individuals.

Fig. (3))

Approaches for MSC transplantation in cartilage repair. MSCs: mesenchymal stem cells; OA: osteoarthritis.

CONFLICT OF INTEREST

The author (editor) declares no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

Declared none.

References