A glial scar formation (gliosis) is a reactive cellular process involving astrogliosis that occurs after injury to the central nervous system. As with scarring in other organs and tissues, the glial scar is the body's mechanism to protect and begin the healing process in the nervous system.
In the context of neurodegeneration, formation of the glial scar has been shown to have both beneficial and detrimental effects. Particularly, many neuro-developmental inhibitor molecules are secreted by the cells within the scar that prevent complete physical and functional recovery of the central nervous system after injury or disease. On the other hand, absence of the glial scar has been associated with impairments in the repair of the blood brain barrier.
Scar components The glial scar is composed of several components briefly discussed below.
Reactive astrocytes Reactive astrocytes are the main cellular component of the glial scar. After injury, astrocytes undergo morphological changes, extend their processes, and increase synthesis of glial fibrillary acidic protein (GFAP). GFAP is an important intermediate filament protein that allows the astrocytes to begin synthesizing more cytoskeletal supportive structures and extend pseudopodia. Ultimately, the astrocytes form a dense web of their plasma membrane extensions that fills the empty space generated by the dead or dying neuronal cells (a process called astrogliosis). The heavy proliferation of astrocytes also modifies the extracellular matrix surrounding the damaged region by secreting many molecules including laminin, fibronectin, tenascin C, and proteoglycans.14561854 These molecules are important modulators of neuronal outgrowth. Accordingly, their presence after injury contributes to inhibition of regeneration.
Another important caveat of the astrocytic response to CNS injuries is its heterogeneity. Particularly, the response of the astrocytes to the injury varies depending on factors such as the nature of the injury and the microenvironment at the injury location.David S, Ness R. (1993). "Heterogeneity of reactive astrocytes." In: Fedoroff S (ed) Biology and pathology of astrocyte-neuron interactions. Plenum Press, New York, pp. 303-312.Fernaud-Espinosa I, Nieto-Sampedro N, Bovolenta P. (1993). "Differential activation of microglia and astrocytes in aniso- and isomorphic gliotic tissue." Glia 8: 277-291. Further, the reactive astrocytes in the immediate vicinity of the injury increase gene expression, thus compounding the response of other astrocytes and contributing to the heterogeneity. Particularly, astrocytes closest to the lesion generally secrete more inhibitory molecules into the extracellular matrix.
Microglia Microglia are the second most prominent cell type present within the glial scar. They are the nervous system analog of immune system macrophages. Microglia rapidly activate near the injury and secrete several cytokines, bioactive lipids, coagulation factors, reactive oxygen intermediates, and neurotrophic factors.Elkabes S, DiCicco-Bloom EM, Black IB (1996). "Brain microglia/ macrophages express neurotrophins that selectively regulate microglial proliferation and function", Journal of Neuroscience 16: 2508–2521 The expression of these molecules depends on the location of the microglial cells relative to the injury, with the cells closest to the injury secreting the largest amount of such biologically active molecules.
Endothelial cells and fibroblasts The various biologically active molecules secreted by microglia stimulate and recruit endothelial cells and fibroblasts. These cells help stimulate angiogenesis and collagen secretion into the injured area. Ultimately, the amount of capillaries extended into the injured area is twice that of uninjured central nervous system regions.Jaeger CB, Blight AR (1997). "Spinal compression injury in guinea pigs: structural changes of endothelium and its perivascular cell associations after blood–brain barrier breakdown and repair." Experimental Neurology 144: 381-399.
Basal membrane The basal membrane is a histopathological extracellular matrix feature that forms at the center of injury and partially covers the astrocytic processes. It is composed of three layers with the basal lamina as the prominent layer. Molecularly, the basal membrane is created by glycoprotein and proteoglycan protomers. Further, two independent networks are formed within the basal membrane by collagen IV and laminin for structural support. Other molecular components of the basal membrane include fibulin-1, fibronectin, entactin, and heparin sulfate proteoglycan perlecan. Ultimately, the astrocytes attach to the basal membrane, and the complex surrounds the blood vessels and nervous tissue to form the initial wound covering.
Beneficial effects of the scar The ultimate function of the glial scar is to reestablish the physical and chemical integrity of the CNS. This is done by generating a barrier across the injured area that seals the nervous/non-nervous tissue boundary. This also allows for the regeneration of the selective barrier to prevent further microbial infections and spread of cellular damage. Moreover, the glial scar stimulates revascularization of blood capillaries to increase the nutritional, trophic, and metabolic support of the nervous tissue.
Detrimental effects of the scar The glial scar also prevents neuronal regrowth. Following trauma to the CNS, axons begin to sprout and attempt to extend across the injury site in order to repair the damaged regions. However, the scar prevents axonal extensions via physical and chemical means. Astrocytes form a dense network of gap junctions that generates a physical barrier to axonal regrowth. Further, the astrocytes secrete several growth-inhibitory molecules that chemically prevent axonal extensions. Moreover, the basal membrane component is expected to generate an additional physical and chemical barrier to axonal extensions.
Primary scar molecular inducers The formation of the glial scar is a complex process. Several main classes of molecular mediators of gliosis have been identified and are briefly discussed below.
Transforming growth factor β Two neuronally-important subclasses of transforming growth factor family of molecules are TGFβ-1 and TGFβ-2 that directly stimulate astrocytes, endothelial cells, and macrophages. TGFβ-1 has been observed to increase immediately after injury to the central nervous system, whereas TGFβ-2 expression occurs more slowly near the injury site. Further, TGFβ-2 has been shown to stimulate growth-inhibitory proteoglycans by astrocytes.Asher RA, et al. (2000). "Neurocan is upregulated in injured brain and in cytokine-treated astrocytes." Journal of Neurosciemce 20, 2427–2438. Experimental reduction of TGFβ-1 and TGFβ-2 has been shown to partially reduce glial scarring.Moon LDF, Fawcett JW. (2001). "Reduction in CNS scar formation without concomitant increase in axon regeneration following treatment of adult rat brain with a combination of antibodies to TGFβ1 and β2." European Journal of Neuroscience 14, 1667–1677.
Interleukins Interleukins are another potential family of scar-inducing cellular messengers. Particularly, interleukin-1, a protein produced by mononuclear phagocytes, helps to initiate the inflammatory response in astrocytes, leading to reactive astrogliosis and the formation of the glial scar.Giulian D, et al. (1988). "Interleukin-1 injected into mammalian brain stimulates astrogliosis and neovascularization." Journal of Neuroscience 8, 2485–2490.Silver J, Miller J. (2004). "Regeneration beyond the glial scar." Nature Reviews Neuroscience. 5(2): 146-156.
Cytokines The cytokine family of glial scar inducers include interferon-γ (IFNγ) and fibroblast growth factor 2 (FGF2). IFNγ has been shown to induce astrocyte proliferation and increase the extent of glial scarring in injured brain models.Yong VW et al. (1991). "γ-Interferon promotes proliferation of adult human astrocytes in vitro and reactive gliosis in the adult mouse brain in vivo." PNAS USA 88, 7016–7020. Further, FGF2 production increases after injury to the brain and spinal cord. FGF2 has also been shown to increase astrocyte proliferation in vitro.Lander C, et al. (1997). "A family of activity-dependent neuronal cell-surface chondroitin sulfate proteoglycans in cat visual cortex." Journal of Neuroscience 17, 1928–1939.Mocchetti I, et al. (1996). "Increased basic fibroblast growth factor expression following contusive spinal cord injury." Experimental Neurology 141, 154–164.
Ciliary neurotrophic factor Ciliary neurotrophic factor (CNTF) is a cytosolic protein that is not secreted. CNTF has been shown to promote the survival of neuronal cultures in vitro, and it can also act as a differentiator and trophic factor on glial cells. Further, CNTF has been previously shown to affect the differentiation of glial precursor cells in vitro; however, the influence of CNTF in the in vivo setting has only recently been determined. Winter et al. used CNTF over-expressing transgenic mice as well as wildtype controls that had CNTF levels artificially elevated via injection, were subjected to neuronal damage using ZnSO4 (a known neuronal degenerative factor), which was injected intranasally in the olfactory epithelium. The olfactory bulb was then assessed for the expression of GFAP mRNA- a common marker for the glial scar. It was determined that mice with elevated levels of CNTF increased their GFAP mRNA expression two-fold. This data suggests that CNTF may mediate glial scar formation following CNS damage.Winger, CG, et al. (1995). "A role for ciliary neurotrophic factor as an inducer of reactive gliosis, the glial response to central nervous system injury", Proc. Natl. Acad. Sci, USA, 92, 5865 - 5869.
Upregulation of nestin intermediate filament protein
Nestin is an intermediate filament (IF) protein that assists with IF polymerization and macromolecule stability. Intermediate filaments are an integral part of cell motility, a requirement for any large migration or cellular reaction. Nestin is normally present during (CNS) development and reactivates after minor stresses to the nervous system. However, Frisen et al. determined that nestin is also upregulated during severe stresses such as lesions which involve the formation of the glial scar. Mid-thoracic spinal cord lesions, optic nerve lesions, but not lesions to the sciatic nerve, have shown marked increases in nestin expression within the first 48 hours after trauma. Further, nestin upregulation was shown to last for up to 13 months post-injury. This data suggests that nestin upregulation may be associated with CNS glial scarring.Frisen, J. (1995). "Rapid, widespread, and long lasting induction of nestin contributes to the generation of glial scar tissue after CNS injury", The Journal of Cell Biology 131(2): 453-464.
Suppression of glial scar formation Several techniques have been devised to impede scar formation. Such techniques can be combined with other neuroregeneration techniques to help with functional recovery.
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