SC1, a brain extracellular matrix glycoprotein related to SPARC and follistatin, is expressed by rat cerebellar astrocytes following injury and during development

In the nervous system, extracellular matrix components are believed to influence cell shape, proliferation and migration during development and following injury. SC1 is a secreted glycoprotein expressed during neural development and in the adult brain. The molecule shows partial sequence homology to the anti-adhesive extracellular matrix molecule SPARC/osteonectin and to follistatin. We have made a surgical lesion in the adult rat cerebellum and examined changes in SC1 expression at 1 to 14 days after injury. Dual in situ hybridization/immunohistochemistry demonstrated that SC1 mRNA was induced in astrocytes surrounding the wound, reaching maximal levels at 10 days post-lesion. Immunohistochemistry revealed changes in the deposition of SC1 protein in radial fibres of Bergmann glia. SC1 protein was also detected at the border of the lesion, suggesting an association with the glial scar. Double immunohistochemistry with the astrocytic marker GFAP demonstrated that astrocytes also express SC1 during postnatal development.     

Excitotoxic brain injury in adult zebrafish stimulates neurogenesis and long‐distance neuronal integration

Zebrafish maintain a greater capacity than mammals for central nervous system repair after injury. Understanding differences in regenerative responses between different vertebrate species may shed light on mechanisms to improve repair in humans. Quinolinic acid is an excitotoxin that has been used to induce brain injury in rodents for modeling Huntington's disease and stroke. When injected into the adult rodent striatum, this toxin stimulates subventricular zone neurogenesis and neuroblast migration to injury. However, most new neurons fail to survive and lesion repair is minimal. We used quinolinic acid to lesion the adult zebrafish telencephalon to study reparative processes. We also used conditional transgenic lineage mapping of adult radial glial stem cells to explore survival and integration of neurons generated after injury. Telencephalic lesioning with quinolinic acid, and to a lesser extent vehicle injection, produced cell death, microglial infiltration, increased cell proliferation, and enhanced neurogenesis in the injured hemisphere. Lesion repair was more complete with quinolinic acid injection than after vehicle injection. Fate mapping of her4‐expressing radial glia showed injury‐induced expansion of radial glial stem cells that gave rise to neurons which migrated to injury, survived at least 8 weeks and formed long‐distance projections that crossed the anterior commissure and synapsed in the contralateral hemisphere. These findings suggest that quinolinic acid lesioning of the zebrafish brain stimulates adult neural stem cells to produce robust regeneration with long‐distance integration of new neurons. This model should prove useful for elucidating reparative mechanisms that can be applied to restorative therapies for mammalian brain injury. GLIA 2014;62:2061–2079     

Distant microglial and astroglial activation secondary to experimental spinal cord lesion

This paper analysed whether glial responses following a spinal cord lesion is restricted to a scar formation close to the wound or they might be also related to widespread paracrine trophic events in the entire cord. Spinal cord hemitransection was performed in adult rats at the thoracic level. Seven days and three months later the spinal cords were removed and submitted to immunohistochemistry of glial fibrillary acidic protein (GFAP) and OX42, markers for astrocytes and microglia, as well as of basic fibroblast growth factor (bFGF), an astroglial neurotrophic factor. Computer assisted image analysis was employed in the quantification of the immunoreactivity changes. At the lesion site an increased number of GFAP positive astrocytes and OX42 positive phagocytic cells characterized a dense scar formation by seven days, which was further augmented after three months. Morphometric analysis of the area and microdensitometric analysis of the intensity of the GFAP and OX42 immunoreactivities showed reactive astrocytes and microglia in the entire spinal cord white and gray matters 7 days and 3 months after surgery. Double immunofluorescence demonstrated increased bFGF immunostaining in reactive astrocytes. The results indicated that glial reaction close to an injury site of the spinal cord is related to wounding and repair events. Although gliosis constitutes a barrier to axonal regeneration, glial activation far from the lesion may contribute to neuronal trophism and plasticity in the lesioned spinal cord favoring neuronal maintenance and fiber outgrowth.     

GFAP-positive progenitor cells produce neurons and oligodendrocytes throughout the CNS

Once thought to merely act as scaffolds in neuronal migration, recent evidence suggests that radial glia may serve as progenitors for the majority of neurons in the CNS. Cre/loxP fate-mapping experiments were carried out using a fragment of a glial-specific promoter (glial fibrillary acidic protein; GFAP) to drive expression of Cre recombinase. We show that GFAP+ progenitor cells give rise to neurons and oligodendrocytes throughout the CNS. We find very little regional heterogeneity in the neurogenic potential of radial glia between dorsal and ventral telencephalon. Additionally, radial glia serve as precursors for subpopulations of interneurons in the ventral telencephalon. Interestingly, the human GFAP promoter but not the mouse GFAP promoter is active in oligodendrocyte progenitor cells. We also demonstrate that the most commonly used Cre reporter lines are very inefficient in detecting Cre-dependent recombination in astrocytes and describe a new Cre reporter line for assessing recombination in astrocytes.     

Evolutionary changes of astroglia in Elasmobranchii comparing to amniotes: a study based on three immunohistochemical markers (GFAP, S-100, and glutamine synthetase)

This paper supplements former studies on elasmobranch species with an immunohistochemical investigation into glutamine synthetase and S-100 protein, in addition to GFAP, and extends its scope to the representatives of almost every group of Elasmobranchii: squalomorph sharks, galeomorph sharks, skates (Rajiformes) and rays (Torpediniformes and Myliobatifomes). More glial elements were labeled by S-100 protein, and even more so by using glutamine synthetase immunostaining than by GFAP: more astrocytes (mainly non-perivascular ones) were detected in the telencephalon of sharks, skates and rays. Only the markers S-100 and glutamine synthetase, but not GFAP, characterized the Bergmann-glia of skates and rays and astrocyte-like non-ependymal cells in Squalus acanthias. Another squalomorph shark species, Pristiophorus cirratus, however, had GFAP immunopositive astrocytes. Of all the species studied, the greatest number of GFAP positive astrocytes could be observed in Mobula japanica (order Myliobatiformes), in each major brain part. According to anatomical location, perivascular glia comprised varied types, including even a location in Mobula, which can also be found in mammals. Remnants of radial glia were found in confined areas of skates, less so in rays. In the rhombencephalon and in the spinal cord modified ependymoglia predominated in every group. In conclusion, there was no meaningful difference between the astroglial architectures of squalomorph and galeomorph sharks. The difference in the astroglial structure between sharks and batoids, however, was confined to the telencephalon and mesencephalon, and did not take place in the rhombencephalon, the latter structure being quite similar in all the species studied. The appearance of astrocytes in the relatively thin-walled shark telencephalon, however, indicates that the brain thickening promoted the preponderance of astrocytes rather than their appearance itself. Although the evolutionary changes of astroglia had some similarities in Elasmobranchii and Amniota, there was one meaningful difference: in Elasmobranchii astrocytes did not prevail in conservative brain regions as they did in the progressive brain regions.     

Adult oligodendrocyte progenitor cells – Multifaceted regulators of the CNS in health and disease

Oligodendrocyte progenitor cells (OPCs) are the often-overlooked fourth glial cell type in the central nervous system (CNS), comprising about 5% of the CNS. For a long time, our vision of OPC function was limited to the generation of mature oligodendrocytes. However, new studies have highlighted the multifaceted nature of OPCs. During homeostatic and pathological conditions, OPCs are the most proliferative cell type in the CNS, a property not consistent with the need to generate new oligodendrocytes. Indeed, OPCs modulate neuronal activity and OPC depletion in the brain can trigger depressive-like behavior. More importantly, OPCs are actively recruited to injury sites, where they orchestrate glial scar formation and contribute to the immune response. The following is a comprehensive analysis of the literature on OPC function beyond myelination, in the context of the healthy and diseased adult CNS.     

Proliferation of a subpopulation of reactive astrocytes following needle-insertion lesion in rat

It is well known that traumatic injuries of the CNS induce a gliotic reaction, characterized by the presence of reactive astrocytes. Reactive astrocytes exhibit enhanced expression of the astrocyte-specific intermediate filament, glial fibrillary acidic protein (GFAP), hypertrophy, and thickened processes. Recently, we have demonstrated that injuries of the CNS induce a re-expression of an embryonic intermediate filament-associated protein, IFAP-70/280 kDa. Based on IFAP-70/280 kDa immunolabeling, we have shown that reactive astrocytes, activated by stab-wound injury, can be divided into two major groups: 1. persistent IFAP+/GFAP+ cells which are close to the wound in the area of glial scar, and 2. transient IFAP-/GFAP+ cells which are farther from the wound. In this study, we use BrdU incorporation to examine proliferation in these two groups of reactive astrocytes induced by stab injury of the rat cerebrum. Triple/double-label immunofluorescence microscopy was performed using antibodies to IFAP-70/280 kDa, GFAP, and BrdU. The results showed that BrdU+ reactive astrocytes (GFAP+) were always IFAB-70/280 kDa+ as well. However, not all IFAP+ reactive astrocytes are BrdU+. BrdU+ signal was not observed in any IFAP- reactive astrocytes. At five days post-lesion, IFAP+ reactive astrocytes were increasing in the area of the wound (0-50 micrograms from the wound edge), but had reached a peak in the proximal area (50-800 micrograms away from the wound edge). At eight days post-lesion, IFAP+ reactive astrocytes achieved the highest percentage in the wound area. At the same time, BrdU-containing reactive astrocytes occupied an area closer to the wound. By 20 days post-lesion, following the formation of the gliotic scar at the stab-wound, a few IFAP+/GFAP+ cells still persisted. BrdU-containing reactive astrocytes were only observed in the scar. These results indicate that many IFAP+ reactive astrocytes close to the wound, in contrast to the IFAP- ones farther from the wound, appear to regain their proliferative potential to increase in number and participate in the formation of the gliotic scar.     

Increased radial glia quiescence,decreased reactivation upon injury and unaltered neuroblast behavior underlie decreased neurogenesis in the aging zebrafish telencephalon

The zebrafish has recently become a source of new data on the mechanisms of neural stem cell (NSC) maintenance and ongoing neurogenesis in adult brains. In this vertebrate, neurogenesis occurs at high levels in all ventricular regions of the brain, and brain injuries recover successfully, owing to the recruitment of radial glia, which function as NSCs. This new vertebrate model of adult neurogenesis is thus advancing our knowledge of the molecular cues in use for the activation of NSCs and fate of their progeny. Because the regenerative potential of somatic stem cells generally weakens with increasing age, it is important to assess the extent to which zebrafish NSC potential decreases or remains unaltered with age. We found that neurogenesis in the ventricular zone, in the olfactory bulb, and in a newly identified parenchymal zone of the telencephalon indeed declines as the fish ages and that oligodendrogenesis also declines. In the ventricular zone, the radial glial cell population remains largely unaltered morphologically but enters less frequently into the cell cycle and hence produces fewer neuroblasts. The neuroblasts themselves do not change their behavior with age and produce the same number of postmitotic neurons. Thus, decreased neurogenesis in the physiologically aging zebrafish brain is correlated with an increasing quiescence of radial glia. After injuries, radial glia in aged brains are reactivated, and the percentage of cell cycle entry is increased in the radial glia population. However, this reaction is far less pronounced than in younger animals, pointing to irreversible changes in aging zebrafish radial glia. J. Comp. Neurol. 521: 3099–3115, 2013. © 2013 Wiley Periodicals, Inc.     

Reactive astrocytes in neonate brain upregulate intermediate filament gene expression in response to axonal injury

We have examined the injury response of astrocytes in the immature hamster brain in this study, focusing on alterations in the expression of glial fibrillary acidic protein (GFAP) and vimentin. In the adult CNS these two type III intermediate filament (IF) proteins have been shown to undergo robust increases in expression in response to axonal injury. Since injury to the immature CNS reportedly elicits less glial scar formation than adult brain injury, we examined the possibility that immature astrocytes respond differently than adult astrocytes to CNS injury with respect to IF gene expression. In situ hybridization using a ³⁵S-labeled cDNA GFAP probe was done on brainstem sections obtained 2,7 and 14 days after unilateral transection of the corticospinal tract in P8 hamster pups. The results indicated that substantial increases in GFAP mRNA were associated with the degenerating portion of the corticospinal tract by 2 days after axotomy and that the levels remained elevated for at least 14 days. Double-label immunofluorescence studies of this material suggested that GFAP as well as vimentin protein levels were also increased in many astrocytes in and around the degenerating corticospinal tract 2–14 days after axotomy. Most of the reactive astocytes in the degenerating regions exhibited increases in GFAP and vimentin immunostaining but some vimentin-negative GFAP-positive reactive astrocytes were also observed, particularly in regions surrounding the actual degenerative zones. The results from these experiments revealed that immature astrocytes have the potential for altering their normal developmental program of GFAP and vimentin expression after injury and mount a response that is qualitatively similar to that of astrocytes after CNS injury in the adult animal.     

Development of vimentin and glial fibrillary acidic protein immunoreactivities in the brain of gray mullet (Chelon labrosus), an advanced teleost

Previous studies in teleosts have revealed the presence of the intermediate filaments vimentin (Vim) and glial fibrillary acidic protein (GFAP) in glial cells of the spinal cord and/or some brain regions, but there is no comprehensive study of their distribution and developmental changes in fishes. Here, the distribution of Vim and GFAP immunoreactivities was studied in the brain of larvae, juveniles, and adults of an advanced teleost, the gray mullet (Chelon labrosus). A different sequence of appearance was observed for expression of these proteins: Vim levels decreased with age, whereas GFAP increased. In general, both immunoreactivities were expressed early in perikarya and endfeet of ependymocytes (tanycytes), whereas expression in radial processes appeared later. In large larvae, the similar expression patterns of Vim and GFAP suggest that some of these glial cells contain both proteins. Subependymal radial glia cells were observed mainly in the optic tectum, exhibiting Vim and GFAP immunoreactivity. The only immunoreactive cells with astrocyte-like morphology were observed in the optic chiasm of the adult, and they were positive for both GFAP and Vim. The perivascular processes of glial cells showed a different distribution of Vim and GFAP during development and had a caudorostral sequence of appearance of immunoreactivities similar to that observed for ependymal and radial glia cells. Several circumventricular organs (the organon vasculosum hypothalami, saccus vasculosus, and area postrema) exhibited highly specialized Vim- and/or GFAP-expressing glial cells. The glial cells of the midline septa of several brain regions were also Vim and/or GFAP immunoreactive. In the adult brain, tanycytes retain Vim expression in several brain regions. As in other vertebrates, the regions with Vim-immunoreactive ventricular and midline glia may represent areas with the capability of plasticity and regeneration in adult brain.     

Neuronal regeneration in the cerebellum of adult teleost fish, Apteronotus leptorhynchus: guidance of migrating young cells by radial glia

In contrast to mammals, adult fish exhibit an enormous potential to replace injured brain neurons by newly generated ones. In the present study, the role of radial glia, identified by immunostaining against fibrillary acidic protein (GFAP), was examined in this process of neuronal regeneration. Approximately 8 days after application of a mechanical lesion to the corpus cerebelli in the teleost fish Apteronotus leptorhynchus, the areal density of radial glial fibers increased markedly in the ipsilateral dorsal molecular layer compared to shorter survival times, or to the densities found in the intact brain or in the hemisphere contralateral to the lesion. This density remained elevated throughout the time period of up to 100 days examined. The increase in fiber density was followed approximately 2 days later by a rise in the areal density of young cells, characterized by labeling with the nuclear dye DAPI, in the ipsilateral dorsal molecular layer. Based on this remarkable spatio-temporal correlation, and the frequently observed close apposition of elongated young cells to radial glial fibers, we hypothesize that radial glia play an important role in the guidance of migrating young cells from their proliferation zones to the site of lesion where regeneration takes place.     

Palisading pattern of subpial astroglial processes in the adult rodent brain: relationship between the glial palisading pattern and the axonal and astroglial organization

The architectural organization of the subpial astrocyte processes was examined near the brain surface by single immunostaining methods. The astroglial processes were stained on brain sections made parallel to the pial surface. The astroglial glial fibrillary acid protein (GFAP) antigen was used as a specific marker. We show that these subpial astrocyte processes present a well organized palisading pattern in the adult mouse and rat spinal cord, medulla and pons. This adult astrocyte palisading pattern is compared to the palisading radial glia organization we previously demonstrated in the fetal mouse brain. The observed analogies afford a new and strong argument in favor of a derivation of the subpial astrocytes from radial glia. Double immunostaining methods, using GFAP and neurofilament antigens as glial and neuronal markers respectively, show the close relationship existing between the trajectories of axonal and glial processes. Beside the colinearity already observed between the axon trajectories and the glial palisades we demonstrate a new kind of axon/glia relationship. Axons are closely intermingled, within the palisading glial tufts, with the peripheral processes of the subpial astrocytes progressing to the pial surface. The findings suggest that fetal radial glia organization has a direct and indirect influence on the adult astroglial and perhaps the axonal pattern.     

Expression of hairy/enhancer of split genes in neural progenitors and neurogenesis domains of the adult zebrafish brain

All subdivisions of the adult zebrafish brain maintain niches of constitutive neurogenesis, sustained by quiescent and multipotent progenitor populations. In the telencephalon, the latter potential neural stem cells take the shape of radial glia aligned along the ventricle and are controlled by Notch signalling. With the aim of identifying new markers of this cell type and of comparing the effectors of embryonic and adult neurogenesis, we focused on the family of hairy/enhancer of split [E(spl)] genes. We report the expression of seven hairy/E(spl) (her) genes and the new helt gene in three neurogenic areas of the adult zebrafish brain (telencephalon, hypothalamus, and midbrain) in relation to radial glia, proliferation, and neurogenesis. We show that the expression of most her genes in the adult brain characterizes quiescent radial glia, whereas only few are expressed in progenitor domains engaged in active proliferation or neurogenesis. The low proliferation status of most her-positive progenitors contrasts with the embryonic nervous system, in which her genes are expressed in actively dividing progenitors. Likewise, we demonstrate largely overlapping expression domains of a set of her genes in the adult brain, which is in striking contrast to their distinct embryonic expression profiles. Overall, our data provide a consolidated map of her expression, quiescent glia, proliferation, and neurogenesis in these various subdivisions of the adult brain and suggest distinct regulation and function of Her factors in the embryonic and adult contexts.     

RESPONSE OF FETAL AND NEONATAL RAT BRAIN TO INJURY

Previous observations have suggested that a reactive astrocytic response to damage does not occur in fetal brain. In this study the time course of the astrocytic response to injury in fetal and neonatal rat brains has been assessed using the immunoperoxidase technique for glial fibrillary acidic protein (GFAP). Cold lesions were induced in utero to the forebrain and brain stem of rat fetuses at 16-18 days of gestation. The inflammatory response and the presence of GFAP in the processes of reactive astrocytes were studied in the brains of animals killed from 4 days (20-22 days of fetal life) to 12 days (9 days of post natal life) after the injury. Reactive astrocytes containing GFAP were present at the site of injury in all fetal and neonatal rat brains. Astrocyte processes were thin and short but stained strongly for GFAP. There was a greater amount of astrocytic scar tissue in animals killed 12 days after injury than in those killed after 4 days. In contrast to adults, little mesenchymal component was observed in newly formed scar tissue on the meningeal surfaces of the fetal and newborn rat brain.     

Fetal cortical transplants in adult rats subjected to experimental brain injury.

Fetal cortical tissue was injected into injured adult rat brains following concussive fluid percussion (FP) brain injury. Rats subjected to moderate FP injury received E16 cortex transplant injections into lesioned motor cortex 2 days, 1 week, 2 weeks, and 4 weeks post injury. Histological assessment of transplant survival and integration was based upon Nissl staining, glial fibrillary acidic protein (GFAP) immunocytochemistry, and staining for acetylcholinesterase. In addition to histological analysis, the ability of the transplants to attenuate neurological motor deficits associated with concussive FP brain injury was also tested. Three subgroups of rats receiving transplant 1 week, 2 weeks, and 4 weeks post injury were chosen for evaluation of neurological motor function. Fetal cortical tissue injected into the injury site 4 weeks post injury failed to incorporate with injured host brain, did not affect glial scar formation, and exhibited extensive GFAP immunoreactivity. No improvement in neurological motor function was observed in animals receiving transplants 4 weeks post injury. Conversely, transplants injected 2 days, 1 week, or 2 weeks post injury survived, incorporated with host brain, exhibited little GFAP immunoreactivity, and successfully attenuated glial scarring. However, no significant improvement in motor function was observed at the one week or two week time points. The inability of the transplants to attenuate motor function may indicate inappropriate host/transplant interaction. Our results demonstrate that there exists a temporal window in which fetal cortical transplants can attenuate glial scarring as well as be successfully incorporated into host brains following FP injury.     

Heterogeneity and Fgf dependence of adult neural progenitors in the zebrafish telencephalon

Adult telencephalic neurogenesis is a conserved trait of all vertebrates studied. It has been investigated in detail in rodents, but very little is known about the composition of neurogenic niches and the cellular nature of progenitors in nonmammalian vertebrates. To understand the components of the progenitor zones in the adult zebrafish telencephalon and the link between glial characteristics and progenitor state, we examined whether canonical glial markers are colocalized with proliferation markers. In the adult zebrafish telencephalon, we identify heterogeneous progenitors that reside in two distinct glial domains. We find that the glial composition of the progenitor zone is linked to its proliferative behavior. Analyzing both fast‐cycling proliferating cells as well as slowly cycling progenitors, we find four distinct progenitor types characterized by differential expression of glial markers. Importantly, a significant proportion of progenitors do not display typical radial glia characteristics. By blocking or activating Fgf signaling by misexpression of a dominant negative Fgf‐receptor 1 or Fgf8a, respectively, we find that ventral and dorsal progenitors in the telencephalon also differ in their requirement for Fgf signaling. Together with data on the expression of Fgf signaling components in the ventricular zone of the telencephalon, this suggests that Fgf signaling directly regulates proliferation of specific subsets of adult telencephalic progenitors in vivo. Taken together our results show that adult neural progenitor cells are heterogeneous with their respect to distribution into two distinct glial domains and their dependence upon Fgf signaling as a proliferative cue in the zebrafish telencephalon. © 2010 Wiley‐Liss, Inc.     

CD36 is involved in astrocyte activation and astroglial scar formation

Inflammation is an essential component for glial scar formation. However, the upstream mediator(s) that triggers the process has not been identified. Previously, we showed that the expression of CD36, an inflammatory mediator, occurs in a subset of astcotyes in the peri-infarct area where the glial scar forms. This study investigates a role for CD36 in astrocyte activation and glial scar formation in stroke. We observed that the expression of CD36 and glial fibrillary acidic protein (GFAP) coincided in control and injured astrocytes and in the brain. Furthermore, GFAP expression was attenuated in CD36 small interfering RNA transfected astrocytes or in the brain of CD36 knockout (KO) mice, suggesting its involvement in GFAP expression. Using an in-vitro model of wound healing, we found that CD36 deficiency attenuated the proliferation of astrocytes and delayed closure of the wound gap. Furthermore, stroke-induced GFAP expression and scar formation were significantly attenuated in the CD36 KO mice compared with wild type. These findings identify CD36 as a novel mediator for injury-induced astrogliosis and scar formation. Targeting CD36 may serve as a potential strategy to reduce glial scar formation in stroke.     

Glial fibrillary acidic protein-immunopositive structures in the brain of a Crocodilian, Caiman crocodilus, and its bearing on the evolution of astroglia

Caiman crocodilus, as a representative of the order Crocodilia, was used in immunohistochemical studies. Immunohistochemical procedures were performed on free-floating sections using a monoclonal antibody against porcine glial fibrillary acidic protein (GFAP) and employing standard avidin-biotin complex methodology. The astroglia of Caiman exhibited robust immunoreactivity to the antibodies raised against mammalian GFAP. In Caiman, the predominant GFAP-immunopositive elements are the radial ependymoglia, similar to other reptiles. The regional variability of glial architecture in Caiman, however, seems greater than in other reptiles so far examined, although it is less compared with chickens. We suggest that this finding corresponds to a more advanced "regional adaptation" of the glial structure in Caiman compared with other reptiles. The main feature that distinguishes the astroglia of Caiman from those of other reptiles is the widespread occurrence of GFAP-immunopositive astrocytes. These cells are limited in lizards and snakes, are not present in turtles, but are found in every major brain area in Caiman. However, even in Caiman, astrocytes are only intermingled with radial glia and are not the predominant glial element of any brain area. The occurrence of astrocytes does not correlate with brain wall thickness. Despite their origin from different ancestral groups of stem reptiles (synapsid or diapsid), mammals and birds exhibit some common general features in their glial architecture and GFAP distribution: 1) predominance of astrocytes and 2) absent or limited GFAP immunopositivity of several brain areas. The present study demonstrates that, even in Caiman, a representative of the reptilian group most closely related to birds, these features are present only in part, suggesting that, in mammals and birds, they have evolved independently.     

Glial responses to neonatal hypoxic-ischemic injury in the rat cerebral cortex.

Neurogenesis is nearly completed after birth, whereas gliogenic activities remain intense during the postnatal period in the developing rat cortex. These include involution of radial glia, proliferation of astrocytes and oligodendrocytes and myelin formation. Little is known about the effects of hypoxic-ischemic (HI) injury on these critical postnatal processes. Here we explored the glial reactions to mild HI injury of the neonatal rat cerebral cortex at P3. We show that the HI lesion results in disruption of the normal radial glia architecture, which was paralleled by an increase in GFAP immunopositive reactive astrocytes. The morphology of these latter cells and the fact that they were immunolabelled for both nestin and GFAP suggest an accelerated transformation of radial glia into astrocytes. In addition, BrdU/GFAP immunostaining revealed a significant increase of double-labelled cells indicating an acute proliferation of astrocytes after HI. This enhanced proliferative activity of astrocytes persisted for several weeks. We found an elevated number and increased mitotic activity of both NG2-positive oligodendrocyte progenitors and RIP-positive oligodendrocytes after injury. These findings imply that glial responses are central to cortical tissue remodelling following neonatal ischemia and represent a potential target for therapeutic approaches.     

Developmental and post‐injury cortical gliogenesis: A Genetic fate‐mapping study with Nestin‐CreER mice

The primary sources of cortical gliogenesis, either during development or after adult brain injury, remain uncertain. We previously generated Nestin‐CreER mice to fate‐map the progeny of radial glial cells (RG), a source of astrocytes and oligodendrocytes in the nervous system. Here, we show that Nestin‐CreER mice label another population of glial progenitors, namely the perinatal subventricular zone (SVZ) glioblasts, if they are crossed with stop‐floxed EGFP mice and receive tamoxifen in late embryogenesis (E16–E18). Quantification showed E18 tamoxifen‐induction labeled more perinatal SVZ glioblasts than RG and transitional RG combined in the newborn brain (54% vs. 22%). Time‐lapse microscopy showed SVZ‐glioblasts underwent complex metamorphosis and often‐reciprocal transformation into transitional RG. Surprisingly, the E10‐dosed RG progenitors produced astrocytes, but no oligodendrocytes, whereas E18‐induction fate‐mapped both astrocytes and NG2+ oligodendrocyte precursors in the postnatal brain. These results suggest that cortical oligodendrocytes mostly derive from perinatal SVZ glioblast progenitors. Further, by combining genetic fate‐mapping and BrdU‐labeling, we showed that cortical astrocytes cease proliferation soon after birth (<P10) and only undergo nonproliferative gliosis (i.e., increased GFAP expression without cell‐division) after stab‐wound injury in adult brains. By contrast, 9.7% of cortical NG2+ progenitors remained mitotic at P29, and the ratio rose to 13.8% after stab‐wound injury. Together, these results suggest NG2+ progenitors, rather than GFAP+ astrocytes, are the primary source of proliferative gliosis after adult brain injury. © 2008 Wiley‐Liss, Inc.