Noradrenergic Signaling and Astroglia

Noradrenergic Signaling and Astroglia integrates what is known about the active role of astroglia in the locus coeruleus-noradrenergic system and outlines the most recent advances in the field. It discusses the molecular mechanisms underlying norepinephrine-induced receptor activation in astroglia, cellular metabolism and CNS energy provision, in vitro, ex vivo, and in vivo models, gliosignalling and neuronal activity, and astroglial networks, gap junctions, and morphological plasticity. The book also addresses the role of astroglial adrenergic receptor activation in memory formation, cognition, regulation of sleep homeostasis, and lastly in neurological disorders, including trauma (cellular edema), neurodegeneration (Alzheimer’s disease), and neuroinflammation (multiple sclerosis). Noradrenergic Signaling and Astroglia is a valuable source of new knowledge for a wide audience, including graduate students, post-doctoral fellows, and researchers in neuroscience, life sciences, and the biological and biomedical sciences.

• Covers what is currently known about the role of astroglia in the noradrenergic system
• Provides biochemical and physiological mechanistic data to understand how noradrenergic signals acting on astroglia produce observed effects
• Includes figures and tables of structures, mechanisms and processes related to astroglia and noradrenergic signaling in CNS

• Language: English
• Publisher: Academic Press
• Release date: Jul 13, 2017
• ISBN: 9780128134269

Book preview

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Chapter 1

Locus Coeruleus Noradrenergic Neurons and Astroglia in Health and Disease

Robert ZorecCorrespondence address  E-mail: [e-mail]; [e-mail]; [e-mail]

Abstract

The diffuse canvas of neuronal projections, arising from the brainstem locus coeruleus (LC) nucleus, is the primary source of noradrenaline (NA) in the central nervous system (CNS). During development the network of LC neurons develop in parallel with the neocortex, while projections of these neurons innervate virtually all areas in the CNS. The dense vascularization of the LC, and its proximity to the ventricles indicates high metabolic activity and vulnerability of these cells, which, in pathologic conditions can lead to the LC cell death, an early event in Parkinson’s and Alzheimer’s diseases. Reduced availability of NA affects astrocytes, the key homeostasis-providing cells in the CNS. Thus maintenance in the LC nucleus and/or enhancement of mechanisms that mimic the action of NA, delays the onset of clinical signs in neurodegeneration, a new strategy to mitigate neurodegeneration.

Keywords

Locus coeruleus; noradrenaline; astrocytes; neurodegeneration; gliocrine system; regulated exocytosis; gliosignaling molecules; vesicle dynamics; cAMP; cytosolic Ca²+; signaling; glucose metabolism; aerobic glycolysis

Chapter Outline

Locus Coeruleus: Anatomy and Pathophysiology 2

Astroglia and Neurodegeneration 6

Presymptomatic Stage of Neurodegeneration Involves Astrogliosis 10

Dysregulation of Astrocytic Vesicle Dynamics in Neurodegeneration 12

Conclusions 15

Abbreviations 16

Acknowledgements 16

References 16

Locus Coeruleus: Anatomy and Pathophysiology

The history of locus coeruleus (LC) goes back to the times of French revolution: the LC was discovered in 1784 by Félix Vicq-d’Azyr (1748–94), a French physician and neuroanatomist¹,² (Fig. 1.1). Later the LC was redescribed by Johann Christian Reil in 1809³ and named locus coeruleus by the Wenzel brothers in 1812.⁴,⁵ This small nucleus is located in the posterior area of the rostral pons in the lateral floor of the fourth ventricle. It is composed of neurons containing neuromelanin granules. Because of its color, LC is also known as the nucleus pigmentosus pontis⁶ meaning heavily pigmented nucleus of the pons. The neuromelanin, that colors the structure, is formed by the polymerization of noradrenaline (NA) and is analoguous to the black dopamine-based neuromelanin in the substantia nigra. In adult humans (ageing 19–78 years) the LC has around 50,000 pigmented neurons with mean cell volume of 35,000–49,000 μm³.³,⁷ High monoamine oxidase activity in the rodent LC was found in 1959, monoamines were identified in 1964 and noradrenergic ubiquitous projections to the central nervous system (CNS) in the 1970s.⁴ It is generally acknowledged that LC is the prime source of NA in the CNS,⁸–¹⁰ being the source of ~70% of all NA in the brain.

Figure 1.1 Cover page of the book by Felix VICQ D’AZYR Traité d’anatomie et de physiologie, avec des planches coloriées représentant au naturel les divers organes de l’homme et des animaux. Tome Premiere. Paris, Franç. Amb. Didot, 1786. https://hagstromerlibrary.ki.se/books/110

Although the LC contains a relatively small number of neurons, the axons of these project and ramify widely⁹,¹¹,¹² to the spinal cord, the brainstem, the cerebellum, the hypothalamus, the thalamic relay nuclei, the amygdala, the basal telencephalon, and the cortex, although some cortical areas receive more abundant innervation.¹² In principle, such a canvas of neuronal contacts (Fig. 1.2) may synchronously activate a wide array of neural networks in several brain and spinal cord regions. This possibility may be regarded as an anatomical basis for a functional reset for many brain networks.¹³,¹⁴ Indeed, in all these structures, synchronous activation of LC projections¹⁴ leads to synchronized electrical activity, possibly reflected by γ waves on an electroencephalogram.¹³ This mode of activity is linked to the most fundamental LC-mediated functions including arousal and the sleep–wake cycle, attention and memory, behavioral flexibility, behavioral inhibition and stress, cognitive control, emotions, neuroplasticity, posture, and balance.¹¹

Figure 1.2 The nucleus locus coeruleus in the brain is located in the posterior area of the rostral pons in the lateral floor of the fourth ventricle.

Neurons from this nucleus project axons to most, if not all, areas of the brain and into the spinal cord as denoted by the red arrows. Adapted from Feinstein DL, Kalinin S, Braun D. Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system. J Neurochem. 2016;139(Suppl. 2):154–178.

This nucleus develops before birth and it has been postulated that LC efferents are critical for the development of various parts of the brain, especially for neocortex.⁹ In rats this nucleus appears to start differentiating during 10–13 days of gestation,¹⁵ which means that LC neurons are born well in advance of many neurons in the LC target brain areas.⁹ In humans LC nucleus is present during 9–12 weeks of gestation with efferent fibers projecting to the neocortex.⁹,¹⁶ Based on the morphologic evidence of the presence of LC nerve endings in the neocortex, it has been suggested that NA is involved in the development of the neocortex.¹⁷ Axons with NA are first located in the lower part of the cortical marginal zone, which is the site where the tangential axons of Cajal-Retzius cells extend and give their numerous collaterals. Cajal-Retzius cells, the principal cells of the marginal zone, provide signals for the migration of cells born later in development and for lamination during neocortical development.¹⁸,¹⁹ It has been proposed that Cajal-Retzius neurons are the targets of the early NA input,²⁰ since removal of the NA system after birth resulted in an altered number of Cajal-Retzius cells, revealing a more direct role for NA in development and regulation of neuronal migration and laminar formation in the cerebral cortex.²¹

The growing mass of the developing brain poses a problem for cell-to-cell signaling, since distances between cells are far greater than those reachable by diffusion-mediated signal propagation.²² Thus two mechanisms appeared to have bypassed this hindrance. First, convection-based signaling, where substances in the extracellular solution are delivered from the source to the targets by the bulk flow. This flow varies diurnally: during the night the glymphatic tunnels²³ between cells get enlarged, which increases the flux of cerebrospinal fluid, thus helping to remove the extracellular debris during sleep.²⁴ Changes in the flux of cerebrospinal fluid are regulated by adrenergic receptors (ARs),²⁴ which also regulate astroglial morphological plasticity.²⁵,²⁶ Second, a conductive mechanism mediates communication between cells, which consists of action potential propagation along the ensembles of branching neurons, such as those originating from the LC,⁹ innervating brain structures.¹¹

Developing tissues, where cells divide and where cells are morphologically plastic, are highly energy demanding, requiring a special form of metabolic adaptation, the aerobic glycolysis. This nonoxidative metabolism of glucose utilization exists in such tissues despite the presence of adequate levels of oxygen, a phenomenon termed the Warburg effect.²⁷ Thus in neurodevelopment, cells increase aerobic glycolysis to generate intermediates used, most likely, for biosynthetic pathways. While this form of metabolism is an inefficient way to generate adenosine triphosphate (ATP), the advantage of this process appears to be in providing intermediates for the biosynthesis of lipids, nucleic acids, and amino acids,²⁸ which are all needed for cell division as well as for their morphological reshaping, both processes defining the developing CNS.²⁹ This metabolic strategy is considered to generate glycolytic intermediates essential for rapid biomass generation also in cancer cells³⁰ and thus appears a universal feature of tissue development. Moreover, aerobic glycolysis, the hallmark of which is the production of L-lactate, appears to be regulated in the brain. During alerting, sensory stimulation, exercise, and pathophysiological conditions, L-lactate production and release are up-regulated. Although it is still unclear, how this appears at the cellular level, the process likely depends on LC neurons.³¹

In 1940, relatively abundant vascularization of LC nucleus was described,³² indicating that LC neurons are metabolically demanding. Indeed, high metabolic rate of LC neurons is mirrored by their relatively high autonomous spiking rate even when glutamate and GABA transmission is blocked.³³ Because of the high exposure to blood circulation, LC neurons are likely to be affected by circulating toxic substances more than neurons in other brain regions. The projections are tightly connected to the brain vascularization; on average, a single LC neuron, with a soma diameter of 45 µm, innervates a 20 m of capillary wall, a length close to that of a tennis court.³⁴ This relatively large exposure of LC neurons to circulating blood makes them vulnerable to toxin accumulation. Even when present at low concentrations with limited blood-brain barrier (BBB) penetration, toxins could be taken up insufficient quantities by LC terminal axons to be subsequently transported retrogradely to the cell body.³⁴,³⁵ The close proximity of the LC to the fourth ventricle may additionally expose LC neurons to toxins and viruses present in the cerebrospinal fluid.³⁶ All this makes LC nucleus highly vulnerable to environmental stress, which may lead to the development of neurological disorders, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and other diseases.⁸,³⁵

A deficit in LC nucleus was at first proposed to be associated in the etiology of idiopathic PD, but it was later widened into a more general theory viewing PD as member of a family of diseases which are neurological in nature, neurodegenerative in their progressive anatomopathological and functional characteristics and typically associated with the second half of the normal life span.¹⁰ This concept appears to have now gained a much wider acceptance.⁸,³⁵ However, the cell-based mechanisms that are instrumental in mediating the loss of LC neurons in neurodegeneration are unclear.

Astroglia and Neurodegeneration

While the term neurodegeneration generally associates with neurons, it is likely that significant part of the pathophysiology in these neurological diseases is mediated by nonneuronal cells, which in some parts of the brain, such as the neocortex, outnumber neurons.³⁷ Among these nonneuronal cells is neuroglia, represented by astroglia, oligodendroglia, and NG2 glia, which all are of neural origin, and microglia, myeloid cells, which invade CNS early in development. The pathological potential of neuroglia was highlighted by the early neuroanatomists and pathologists, including Rudolph Virchow, Alois Alzheimer, Santiago Ramon-y-Cajal, and many of their colleagues.³⁸,³⁹ The neuroglial cells provide for multiple functions being, for example the secretory cells of the CNS.⁴⁰–⁴²

In this chapter we are focusing into astrocytes, which populate gray and white matter of the CNS and are, arguably, the most heterogeneous (in form and function) type of neuroglia⁴³,⁴⁴; in particular we shall overview how these cells integrate into the noradrenergic signaling system in health and disease. Astrocytes are responsible for regulating a myriad of processes including synaptogenesis, synapse maturation, neurotransmitter removal from the synaptic cleft, brain microcirculation, brain metabolism and control the formation and maintenance of the BBB.²³,⁴⁵–⁵⁸ Moreover, astrocytes provide homeostatic support to the brain and hence these cells are involved in every kind of neuropathology.⁵⁹–⁶¹ In fact, diseases of the CNS result from homeostatic insufficiency triggered in particular by the disruption of the balance between cell damage and repair. Importantly, astrocytes contribute to various aspects of CNS defense through (1) existing homeostatic mechanisms (which, for example provide for regulation of glutamate and ion movements and hence contain excitotoxicity⁶² or protect against reactive oxygen species by astroglia derived glutathione⁶³) or (2) by mounting an evolutionary conserved defensive response known as reactive astrogliosis. Reactive astrogliosis protects the CNS, by isolating the damaged area, reconstructing the BBB, and by facilitating remodeling of the neural circuitry after the resolution of pathology.⁴⁴,⁶⁴–⁶⁷ Astrocytes, however, may also contribute to neuronal damage through failure or reversal of various homeostatic cascades that assume neurotoxic proportions.⁶⁰,⁶⁸

Considering that the LC-dependent deficit drives neurodegeneration the astrocytic contribution to this pathology is expected to depend on the loss of NA. Thus we need to address how NA operates in normal astrocyte function first. The effects of NA are mediated through α- and β-ARs, which are expressed in neurons, microglia, and astrocytes. When astrocytic α-/β-ARs are simultaneously exposed to NA, a multitude of cytoplasmic second messengers is generated,⁶⁹ which regulate a host of downstream cytoplasmic effects including cell shape changes²⁵ and aerobic glycolysis.⁷⁰

Astroglial β-ARs regulate cell morphology.⁷¹ Stimulation of β-AR with subsequent increase in intracellular cyclic adenosine monophosphate (cAMP) induces in astrocytes stellation, i.e., transformation from a flattened irregular morphology to a stellate process-bearing morphology.²⁵,⁷²,⁷³ The β-ARs are abundantly expressed by astrocytes in both white and grey matter⁷⁴–⁷⁸ and these receptors are likely to be involved through shape changes in memory formation.⁷⁹

Since the early 1960s the hippocampus was recognized as a fundamental region for memory formation.⁸⁰ Subsequently, two distinct memory systems, declarative (explicit) memory for facts and events, for people, places, and objects (knowing that) and nondeclarative (implicit) memory, the memory for perceptual and motor skills (knowing how), have been defined.⁸¹ Both systems rely on similar, if not identical, mechanisms associated with reinforcement of synaptic transmission, which involves morphological changes at the synapse that outlast memory stabilization.⁸² This morphology-based mechanism was considered already by Santiago Ramon-y-Cajal, who linked cerebral gymnastics with morphological alterations of dendrites and terminals of neurons.⁸³

Besides morphological adjustments of neuronal synaptic elements, synaptic transmission may also be directly affected by changes in shape and volume of astroglial processes that tightly enwrap most of CNS synapses.⁴⁷,⁸⁴,⁸⁵ Local retractions or expansions of astrocytic processes modify the geometry of the extracellular space, affecting neuron–glia interactions.⁸⁶ The apposition of astrocyte membrane to the synaptic cleft is an important determinant for the efficient glutamate removal, which defines the properties of synaptic signals.⁸⁷ Removal of glutamate from the synaptic cleft consists of diffusion of glutamate in the synaptic cleft and flux into the astrocyte via membrane glutamate transporters; glutamate then diffuses in the cytoplasm to sites where it is metabolized.⁸⁵

Astrocytes display a remarkable structural plasticity under physiologic and pathologic conditions, including reproduction, sensory stimulation, and learning.²⁵,²⁶,⁸⁶,⁸⁸ Distal astrocytic processes can undergo morphological changes in a matter of minutes, thus modifying the geometry and diffusion properties of the extracellular space and relationships with adjacent neuronal elements, especially with synapses. This type of astroglial plasticity has important functional consequences because it modifies extracellular homeostasis of ions and neurotransmitters, thus ultimately modulating neuronal function at the cellular and system levels.⁸⁶,⁸⁸ The mechanisms responsible for morphological changes in astrocytes are not known, but these likely involve ARs, generation of second messenger cAMP²⁵,²⁶ and cytoskeletal remodeling.⁸⁹

Memory consolidation during the Pavlovian threat conditioning is associated with astrocytic processes to retract from synapses, allowing these synapses to enlarge, suggesting that contact with astroglial processes opposes synapse growth during memory consolidation.⁹⁰ In other words, if astrocytic processes enwrap synapses and the latter need to expand during memory formation, astrocytes may hinder this remodeling, which demonstrates how astrocytic structural plasticity enables morphological changes of synapses associated with memory formation. Fig. 1.3 shows a synapse partially unwrapped by astrocyte, which occurred during memory formation.⁹⁰ Hebbian memory formation is strongly regulated by NA acting through β-ARs⁹¹ and changes in astrocytic shape have indeed been observed.⁹⁰ Moreover, the existence of structural-functional changes of the astrocyte-neuron interactions during memory processes have been detected.⁹²–⁹⁴

Figure 1.3 Reconstruction of a tripartite synapse.

Blue: axon; gray: dendrite; orange: astrocyte; red: synaptic cleft; green: mitochondria; yellow: smooth endoplasmic reticulum; black: ribosomes; white: glycogen granules, a marker of astrocytes. Diameter of the presynaptic terminal, adjacent to the synaptic cleft is 5,500 nm. With permission by Dr. Linneae Ostroff: http://www.cns.nyu.edu/ledoux/SFN2011/L.%20OSTROFF.htm

Tight association between the synaptic membranes and astrocytes is considered essential for homeostatic control in the synaptic cleft, including rapid removal of glutamate⁹⁵ and K+ from the extracellular space.⁵⁷,⁹⁶ Thus retraction of astrocytic membrane from the synapse during memory formation⁹⁰ may facilitate the spillover of neurotransmitter and thus affect synaptic strength.⁹⁷ At the same time, memory formation is associated with morphological growth of synaptic elements together with enhanced protein synthesis and rearrangement of receptor proteins, all of which increase the energy consumption.⁹⁸

How biosynthetic intermediates and energy substrates, needed for ATP synthesis, are delivered to synapses where synaptic plasticity takes place remains an open question. A possibility is that pyruvate is provided to the mitochondria by glycolysis within the neuron. However, the morphology of astrocytes, with extensive end feet plastering blood vessels, is well suited to take up glucose from blood and distribute either glucose itself, or pyruvate or lactate derived from glucose, to astrocytic processes surrounding synapses, possibly by diffusion through gap junctions integrating astroglial syncytia.⁹⁹ In support of this mechanism, diffusion of glucose within astrocytes is relatively rapid¹⁰⁰ and may well support glucose delivery via interconnected astrocytes in situ. Although synapses are the main energy consumers in the brain, glycogen, the only CNS energy storage system, is present mainly, if not exclusively, in astrocytes (see Fig. 1.3; orange particles in astrocytes). Memory consolidation in young chickens requires glycogenolysis.¹⁰¹,¹⁰² Moreover, the successful consolidation of memory from short-term to long-term memory requires neuronal NA release.¹⁰³ Interestingly, noradrenergic signaling regulates the breakdown of glycogen with a rather short time-constant of about 100 s.⁷⁰ Therefore it appears that NA, released from neurons, such as those from LC, initiates astrocytic morphological changes and activates astroglial energy metabolism. Thus NA-transmission operating via astrocytes integrates neuronal activity. This requires excitation of astrocytes to initiate morphological changes and at the same time triggers an increase in aerobic metabolism. In support of a key role of NA and LC in excitation-energy coupling, NA is in the adult awake mice brain the main neurotransmitter that triggers a wide synchronous astroglial Ca²+ signaling,⁷⁸ which represents the universal form of glial excitability.¹⁰⁴

Thus NA-mediated metabolism support for morphological changes in astrocytes plays an important role in physiologic and in pathologic conditions. Astrocytes may swell under pathologic conditions and contribute to the development of brain edema. Our recent data indicates that in vivo astrocytes swelling could be inhibited by NA.²⁶ Hypertrophic astrocytes are associated with reactive astrogliosis.⁶⁴ At the early stages of neurodegeneration, reduced adrenergic innervation of the brain due to LC degeneration,¹⁰⁵ which may likely result in decreased cAMP signaling and facilitate astrocyte atrophy, as observed in the triple transgenic animal model of AD (3×Tg-AD).¹⁰⁶ Such astrocytic atrophy may lead to synaptic loss due to insufficient metabolic support for synapses by astrocytes and likely occurs during the presymptomatic stage of neurodegeneration.

Presymptomatic Stage of Neurodegeneration Involves Astrogliosis

Aging is the main risk factor for the development of neurodegeneration, including that occurring in AD-type pathologies.¹⁰⁷ While there are several stages of AD, the most common one, manifested at a late stage, is linked to the histopathological presence of extracellular deposits of fibrillar β-amyloid peptide (Aβ), and intraneuronal accumulation of aggregates of hyper-phosphorylated Tau protein,¹⁰⁸ which in the healthy brain stabilizes microtubules that transport nutrients and other substances within nerve cells. These proteins have positively charged binding domains that allow them to bind to negatively charged microtubules. Neurodegeneration occurs gradually and dementia may reflect the end stage of an accumulation of pathological changes that start to develop decade(s) before the onset of the clinical symptoms.¹⁰⁹,¹¹⁰

Mechanisms of the preclinical stages of AD remain obscure. While the current view holds that neurodegeneration in AD reflects neuron-specific deficits, such as is the loss of synapses preceding neuronal death,¹¹¹,¹¹² it is likely that preceding or concomitant changes in neuroglia also contribute to this process.¹⁰⁷,¹¹⁰,¹¹³–¹¹⁵ The role of neuroglia in dementia was already recognized by Alois Alzheimer, who found pathologically modified glial cells in close contact with damaged neurons.¹¹⁶–¹¹⁸ In postmortem tissue from patients with AD, astroglial hypertrophy (reflecting reactive astrogliosis accompanied by increased levels of glial fibrillary acidic protein (GFAP) and a calcium binding protein S100B) is often observed, particularly in astrocytes associated with senile plaques.¹¹⁹–¹²³ Consistent with a recent study in patients with AD,¹¹⁰ distinct time- and brain-specific morphologic alterations were reported in an animal mouse model of familial AD, i.e., astroglial atrophy was detected in certain brain areas.¹⁰⁶,¹²⁴–¹²⁶ Astroglial asthenia preceded the appearance of senile plaques and astroglial reactive hypertrophy and it appeared first in the entorhinal cortex, the region affected at the earliest stage of AD