Astroglial cells are truly multipotent and serve a surprizingly large and diverse variety of functions. These functions are absolutely vital for brain development, physiology and pathology.
Conceptually astroglial functions can be divided into several important groups:
Functions of astroglia 1. Developmental
• Regulation of neuro and gliogenesis – astroglia are stem elements of the CNS.
• Neuronal path finding.
• Regulation of synaptogenesis.
2. Structural
• Astroglia form the scaffold of the nervous system, thus defining the func- tional architecture of the brain and spinal cord.
• Astrocytes form a continuous syncytium and integrate other neural cells into this syncytium.
3. Vascular – formation and regulation of the blood–brain barrier
• Formation of the glial–vascular interface.
• Regulation of cerebral microcirculation.
4. Metabolic
• Providing energy substrates for neurones.
• Collecting neuronal waste.
5. Control of the CNS microenvironment
• Regulation of extracellular ion concentrations; in particular sequestration and redistribution of K+ following fluctuations associated with neuronal activity.
• Regulation of extracellular pH.
Glial Neurobiology: A Textbook Alexei Verkhratsky and Arthur Butt
© 2007 John Wiley & Sons, Ltd ISBN 978-0-470-01564-3 (HB); 978-0-470-51740-6 (PB)
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• Removal of neurotransmitters from the extracellular space.
• Brain water homeostasis.
6. Signalling
• Modulation of synaptic transmission.
• Release of neurotransmitters.
• Long-range signalling within the glial syncytium.
• Integration of neuronal–glial networks.
These functions are explained in more detail below.
7.1 Developmental function – producing new neural cells
7.1.1 Neurogenesis in the adult brain
In many vertebrates, neurogenesis persists throughout adulthood throughout the CNS. For example, new neurones are continuously born in all brain regions in birds, whilst lizards can very effectively regenerate the retina and spinal cord. In primates, including humans, neurogenesis in the adult is restricted to the hippocampus and subventricular zone. In both locations, the stem elements that produce neurones are astroglia. These ‘stem’ astrocytes have the morphology, physiology and biochem- ical/immunological markers characteristic for astrocytes: they express GFAP, form vascular endfeet, have negative resting membrane potentials, are nonexcitable, and predominantly express K+ channels. ‘Stem’ astrocytes differ from ‘classical’
mature astrocytes by specific expression of the protein nestin (a marker for neural stem cells), and some of them form cilia. Neurones born in the subventricular zone migrate to the olfactory bulb, whereas those produced in the hippocampus remain there and integrate themselves into existing neuronal networks.
‘Stem’ astrocytes residing in the hippocampus and subventricular zone are multipotent, as they give birth to both neurones and glia; the production of glia or neurones is under control of numerous chemical factors (Figure 7.1).
7.1.2 Gliogenesis in adult brain
In the adult brain, in contrast to neurogenesis, gliogenesis occurs everywhere. New glial cells are born locally; and the locality also mainly determines the type of glial cell produced. In the subcortical white matter most of the newly produced glial cells are oligodendrocytes, whereas in the spinal cord astrocytes and oligodendrocytes are produced roughly in the same quantities.
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Figure 7.1 Astrocytes as stem elements in the nervous system. In the adult CNS, ‘stem’
astrocytes can produce both neurones and glia. So far, it is almost impossible to distinguish between differentiated astrocytes and ‘stem’ astrocytes, and both retain mitotic potential. The precise pathways of transition of ‘stem’ astrocytes towards a glial or neuronal lineage are yet to be uncovered; however gliogenesis or neurogenesis can be promoted by various factors, some of which are listed
7.2 Developmental function – neuronal guidance
The vertebrate brain develops from the embryonic neuroectoderm that lies above the notochord and gives rise to the entire nervous system. The notochord induces neuroectodermal cells to generate neural stem cells and form the neural plate, which in turn forms the neural tube, from which the brain and spinal cord are derived. The neural precursor cells of the neural tube give rise to both neurones and glia in response to multiple inductive signals produced by the notochord, floor plate, roof plate, dorsal ectoderm and somites; for example, retinoic acid, fibrob- last growth factor, bone morphogenetic proteins and sonic hedgehog. Inductive signals regulate transcription factors and gene expression, including the home- obox (Hox) genes, which influence the development of the neural tube into the major brain regions; forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon). The first neural cells to develop are radial glia.
After this, neural precursors in the ventricular zone (VZ) and subventricular zone
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(SVZ) immediately surrounding the lumen of the neural tube migrate to their final destinations and give rise to the enormously diverse range of neurones and glia found in the adult brain (Figure 7.2).
An important function of foetal radial glial cells is to provide the scaffolding along which neural precursors migrate (Figure 7.2). Not all neurones migrate along radial glia, but it is always the case where neurones are organized in layers, such as the cerebellum, hippocampus, cerebral cortex and spinal cord. In the cerebral cortex, for example, bipolar postmitotic neurones migrate several millimetres from the ventricular zone to the pia along the processes of radial glia; the cerebral cortex is formed inside out, whereby the innermost layers are formed first, and the superficial layers are formed later by neurones that migrate through the older cells.
In the cerebellum, granule cells migrate along Bergmann glia, which are derived from radial glia. Migration depends on recognition, adhesion and neurone–glial interactions, which are under the influence of cell membrane bound molecules,
Figure 7.2 Radial glial cells form a scaffold that assists neuronal migration in the developing nervous system. Radial glial cells extend their processes from the ventricular zone (VZ) and subventricular zone (SVZ), where neural progenitors reside, towards the pia. Neuronal precursors attach to the radial glial cells and migrate along their processes towards their final destination.
Numerous reciprocal factors released by both neurones and glia regulate the processes of mutual recognition, attraction, adhesion, migration and final repulsion
7.3 REGULATION OF SYNAPTOGENESIS 99 and diffusible and extracellular matrix molecules. Although the specific signals are not fully resolved, they include laminin–integrin interactions and neuregulin, which is expressed by migrating neurones and interacts with glial ErbB receptors.
Subsequently, foetal radial glia disappear and transform into astrocytes; remnants of radial glia persist in the adult brain where they can generate olfactory and hippocampal neurones.
After neurones reach their final sites, they extend axons, which in some cases grow for considerable distances and have to cross the brain midline (decussate) to reach their synaptic targets. Channels formed by astrocytes provide a mechanical and guidance substrate for axon growth. In the corpus callosum, for example, astrocytes form a bridge (the glial sling) that connects left and right sides of the developing telencephalon. The ability of astrocytes to support axon growth decreases with age; embryonic astrocytes strongly support axon growth, whereas mature astrocytes inhibit axon growth – hence, the astroglial scar that forms following damage to the adult CNS is a major barrier to axon regeneration. Astro- cytes produce a number of membrane bound and extracellular matrix molecules that serve as molecular cues for axon growth. These are generally considered to act by activating receptors on axonal growth cones to regulate process outgrowth;
for example, N-cadherins and fibroblast growth factor receptors mediate neurite outgrowth by increased intracellular calcium in the growth cone. Astroglial laminin-1 is an excellent growth substrate for axons, and decussation of axons at the optic chiasm is dependent on laminin-1 and chondroitin sulphate proteoglycans produced at the glial boundary. Growth inhibitory molecules such as sempaphorins and ephrins also play important roles as guidance cues by regulating growth cone collapse.
7.3 Regulation of synaptogenesis and control of synaptic maintenance and elimination
The living brain constantly remodels and modifies its cellular networks.
Throughout life, synapses continuously appear, strengthen, weaken or die. These processes underlie the adaptation of the brain to the constantly changing external environment and, in particular, represent what we know as learning and memory.
For many years the process of synaptogenesis, maintenance and elimination of the synaptic contacts was considered to be solely neuronal responsibility; only very recently it has become apparent that glial cells (astrocytes in the CNS and Schwann cells in the PNS) control the birth, life and death of synapses formed in neuronal networks.
In general, the life cycle of the synapse proceeds through several stages:
(1) formation of an initial contact between presynaptic terminal and postsynaptic neurone; (2) maturation of the synapse, when it acquires its specific properties, in particular the neurotransmitter modality; (3) stabilization and maintenance, which
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preserve the strong connections; and (4) elimination. In fact, the last stage may follow each of the preceding ones, and many synapses are eliminated before entering the stabilization phase.
The major wave of synaptogenesis in the mammalian brain starts shortly after birth, and lasts for several weeks in rodents and for a much longer period in humans.
This wave of massive (as hundreds of billions of synapses have to occur within a relatively short time span) synaptogenesis precisely follows the massive generation of mature astrocytes, which happens during the perinatal period. This sequence of events is not coincidental as indeed astrocytes assist synapse appearance.
Synaptogenesis may occur in purified neuronal cultures, albeit at a relatively low rate; addition of astrocytes into this culture system dramatically (about seven times) increases the number of synapses formed. This increase in synaptic formation strictly depends on cholesterol, produced and secreted by astrocytes; cholesterol serves most likely as a building material for new membranes, which appear during synaptogenesis; in addition, cholesterol may be locally converted into steroid hormones, which in turn can act as synaptogenic signals. Glial cells also affect synaptogenesis through signals influencing the expression of a specific protein, agrin, essential for synapse formation.
After new synapses are formed, astrocytes control their maturation through several signalling systems affecting the postsynaptic density. In particular, intro- duction of astrocytes into neuronal cell cultures boosts the size of post-synaptic responses by increasing the number of post-synaptic receptors and facilitating their clustering. In contrast, removal of astroglial cells from neuronal cultures decreases the number of synapses. In part, these effects are mediated by several soluble factors released by astrocytes, although direct contact between glial and neuronal membranes also exerts a clear influence (of yet unidentified nature) on synapse maturation. Several distinct soluble factors have been identified that are released by glial cells and affect synapse maturation. One of them is tumour necrosis factor (TNF), which regulates the insertion of gluta- mate receptors into post-synaptic membranes; another one is activity-dependent neurotrophic factor (ADNF), which, after being secreted by astrocytes, increases the density of NMDA receptors in the membrane of neighbouring postsy- naptic neurones. In chick retina, Müller glial cells control the expression of M2 muscarinic ACh receptors in retinal neurones through a hitherto unidentified protein.
Astrocytes may also limit the number of synapses that appear on a given neurone, as astroglial membranes ensheathing the neurolemma prevent the forma- tion of new synaptic contacts. Astroglial cells can also be involved in the elimination of synapses in the CNS, the process which underlies the final tuning and plasticity of the neuronal inputs. This may be achieved by secretion of certain factors or proteolytic enzymes, which demolish the extracellular matrix and reduce the stability of the synaptic contact. Subsequently, astroglial processes may enter the synaptic cleft and literally close and substitute the synapse.
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7.4 Structural function – creation of the functional microarchitecture of the brain
Protoplasmic astrocytes in the grey matter are organized in a very particular way, with each astrocyte controlling its own three-dimensional anatomical territory (Figure 7.3). The overlap between territories of neighbouring astroglial cells is minimal and it does not exceed five per cent, i.e. astrocytes contact each other only by the most distal processes. Individual astrocytes establish contacts with blood vessels, neurones and synapses residing within their anatomical domain.
Astrocytic processes show a very high degree of morphological plasticity; many of these processes send very fine expansions, the lamellopodia and filopodia, which contact synaptic regions. These lamellopodia and filopodia are in fact motile, and may expand or shrink at a speed of several m per minute. The lamellopodia show gliding movements along neuronal surfaces and filopodia are able to rapidly protrude towards or retract from the adjacent neuronal membranes or synaptic structures.
Using clearly delineated anatomical territories, astrocytes divide the whole of grey matter (both in the brain and in the spinal cord) into separate domains, the elements of which (neurones, synaptic terminals and blood vessels) are integrated via the processes of protoplasmic astrocytes; the membranes of a single astrocyte
Figure 7.3 Astrocytic domains form the micro-architecture of grey matter. Each single astro- cyte occupies a well-defined territory; astroglial contacts occur only through distal processes and overall overlap between astrocyte territories does not exceed three to five per cent. The astrocytic domains are organized in rows along the vessels, which are typically positioned in the narrow interface between astrocytes as shown on the scheme. (Modified from Nedergaard M, Ransom B, Goldman SA (2003) New roles for astrocytes: redefining the functional architecture of the brain.Trends Neurosci26,523–530)
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may cover about 100 000 to 2 millions (in humans) synapses present in its domain.
The astrocytic processes provide for local signalling within the domain, as their membranes that contact neurones, synapses and blood vessels are packed with receptors, which sense the ongoing activity. Signals activated by glial receptors may propagate through the astrocyte cytoplasm, thus integrating distant parts of the domain. Importantly, the processes of the same astrocyte are often directly coupled via gap junctions, which establish diffusion shortcuts, allowing the local metabolic signals to rapidly spread through these processes, bypassing the soma.