Development of astrocytes in the mouse hippocampus as tracked by tenascin-C gene expression

The Role of Tenascin-C on the Structural Plasticity of Perineuronal Nets and Synaptic Expression in the Hippocampus of Male Mice

Neuronal plasticity is a crucial mechanism for an adapting nervous system to change. It is shown to be regulated by perineuronal nets (PNNs), the condensed forms of the extracellular matrix (ECM) around neuronal bodies. By assessing the changes in the number, intensity, and structure of PNNs, the ultrastructure of the PNN mesh, and the expression of inhibitory and excitatory synaptic inputs on these neurons, we aimed to clarify the role of an ECM glycoprotein, tenascin-C (TnC), in the dorsal hippocampus. To enhance neuronal plasticity, TnC-deficient (TnC-/-) and wild-type (TnC+/+) young adult male mice were reared in an enriched environment (EE) for 8 weeks. Deletion of TnC in TnC-/- mice showed an ultrastructural reduction of the PNN mesh and an increased inhibitory input in the dentate gyrus (DG), and an increase in the number of PNNs with a rise in the inhibitory input in the CA2 region. EE induced an increased inhibitory input in the CA2, CA3, and DG regions; in DG, the change was also followed by an increased intensity of PNNs. No changes in PNNs or synaptic expression were found in the CA1 region. We conclude that the DG and CA2 regions emerged as focal points of alterations in PNNs and synaptogenesis with EE as mediated by TnC.

Substrate-bound and soluble domains of tenascin-C regulate differentiation, proliferation and migration of neural stem and progenitor cells

Introduction

The lack of regenerative capacity of the central nervous system is one of the major challenges nowadays. The knowledge of guidance cues that trigger differentiation, proliferation, and migration of neural stem and progenitor cells is one key element in regenerative medicine. The extracellular matrix protein tenascin-C (Tnc) is a promising candidate to regulate cell fate due to its expression in the developing central nervous system and in the adult neural stem cell niches. Of special interest are the alternatively spliced fibronectin type III (FnIII) domains of Tnc whose combinatorial diversity could theoretically generate up to 64 isoforms in the mouse. A total of 27 isoforms have already been discovered in the developing brain, among others the domain combinations A1D, CD, and A124BCD.

Methods

In the present study, these domains as well as the combination of the constitutively expressed FnIII domains 7 and 8 (78) were expressed in Chinese hamster ovary cells as pseudo-antibodies fused to the Fc-fragment of a human immunoglobulin G antibody. The fusion proteins were presented to primary mouse neural stem/progenitor cells (NSPCs) grown as neurospheres, either as coated culture substrates or as soluble additives in vitro. The influence of the domains on the differentiation, proliferation and migration of NSPCs was analyzed.

Results

We observed that the domain combination A124BCD promoted the differentiation of neurons and oligodendrocytes, whereas the domain A1D supported astrocyte differentiation. The constitutively expressed domain 78 had a proliferation and migration stimulating impact. Moreover, most effects were seen only in one of the presentation modes but not in both, suggesting different effects of the Tnc domains in two- and three-dimensional cultures.

Discussion

This knowledge about the different effect of the Tnc domains might be used to create artificial three-dimensional environments for cell transplantation. Hydrogels spiked with Tnc-domains might represent a promising tool in regenerative medicine.

Serum tenascin-C predicts severity and outcome of acute intracerebral hemorrhage

BACKGROUND

Tenascin-C is a matricellular protein related to brain injury. We studied serum tenascin-C in acute intracerebral hemorrhage (ICH) and examined the associations with severity and outcome following the acute event.

METHODS

Tenascin-C samples were obtained from 162 patients with acute hemorrhagic stroke and 162 healthy controls. Poor 90-day functional outcome was defined as modified Rankin Scale score > 2. Early neurological deterioration (END) and hematoma growth (HG) were recorded at 24 h.

RESULTS

Patients had higher tenascin-C levels than controls. Tenascin-C levels were positively correlated with hematoma volume or National Institutes of Health Stroke Scale score at baseline. Elevated tenascin-C levels were independently associated with END, HG, 90-day mortality and poor functional outcome. Moreover, tenascin-C levels significantly predicted END, HG and 90-day outcomes under receiver operating characteristic curves.

CONCLUSIONS

An increase in serum tenascin-C level is associated with an adverse outcome in ICH patients, supporting the potential role of serum tenascin-C as a prognostic biomarker for hemorrhagic stroke.

Dentate granule progenitor cell properties are rapidly altered soon after birth

Neurogenesis occurs during the embryonic period and ceases soon after birth in the neocortex, but continues to occur in the hippocampus even in the adult. The embryonic neocortex has radial glia or progenitor cells expressing brain lipid-binding protein (BLBP), whereas the adult hippocampus has radial granule progenitor cells expressing BLBP and glial fibrillary acidic protein (GFAP) in the subgranular zone. We previously found that embryonic hippocampal granule progenitor cells express GFAP, but not BLBP, indicating that these cells are different from both embryonic neocortical and adult granule progenitor cells. In the present study, as the first step towards understanding the mechanism of persistent hippocampal neurogenesis, we aimed to determine the stage at which embryonic-type granule progenitors become adult-type progenitors using mouse Gfap-GFP transgenic mice. During the embryonic stages, Gfap-GFP-positive (Gfap-GFP+) cells were distributed in the entire developing dentate gyrus (DG), whereas BLBP-positive (BLBP+) cells were mainly present in the fimbria and subpial region, and to some extent in the DG. Up to postnatal day 0 (P0), double-positive cells were scarcely detected. However, at P1, one-third of the Gfap-GFP+ cells in the DG suddenly began to weakly express BLBP. Thereafter, Gfap-GFP+/BLBP+ cells rapidly increased in number, and extended their radial processes in the inner granular cell layer. At P14 and in the adult, two-thirds of the Gfap-GFP+ cells in the subgranular zone showed BLBP immunoreactivity. These results suggest that the properties of hippocampal granule progenitor cells are rapidly altered from an embryonic to adult type soon after birth.

Correlation of tenascin-C concentrations in serum with outcome of traumatic brain injury in humans

BACKGROUND

Tenascin-C, a matricellular protein, is involved in brain injury. However, change of tenascin-C concentrations in peripheral blood remains unknown after traumatic brain injury (TBI).

METHODS

Serum tenascin-C concentrations were measured in 100 healthy controls, 108 severe TBI patients, 79 moderate TBI patients and 32 mild TBI patients.

RESULTS

Serum tenascin-C concentrations of patients were significantly higher than those of controls. Tenascin-C concentrations negatively correlated with Glasgow Coma Scale (GCS) scores in all patients (r=-0.658, P<0.001). In severe TBI patients, tenascin-C in serum significantly discriminated patients at risk of 6-month mortality (area under curve, 0.821; 95% confidence interval, 0.735-0.888) and poor outcome (Glasgow Outcome Scale score of 1-3) (area under curve, 0.833; 95% confidence interval, 0.749-0.898) and emerged as an independent predictor for 6-month mortality (odds ratio, 1.114; 95% confidence interval, 1.008-1.233; P=0.005), overall survival (hazard ratio, 1.085; 95% confidence interval, 1.010-1.166; P=0.003) and unfavorable outcome (odds ratio, 1.049; 95% confidence interval, 1.014-1.076; P=0.001). By receiver-operating characteristic analysis, serum tenascin-C concentrations had similar prognostic value compared with GCS scores.

CONCLUSIONS

Enhanced serum tenascin-C concentrations are closely related to trauma severity and clinical outcomes, substantializing tenascin-C as a potential prognostic biomarker after TBI.

Neuroimmunological Implications of AQP4 in Astrocytes

The brain has high-order functions and is composed of several kinds of cells, such as neurons and glial cells. It is becoming clear that many kinds of neurodegenerative diseases are more-or-less influenced by astrocytes, which are a type of glial cell. Aquaporin-4 (AQP4), a membrane-bound protein that regulates water permeability is a member of the aquaporin family of water channel proteins that is expressed in the endfeet of astrocytes in the central nervous system (CNS). Recently, AQP4 has been shown to function, not only as a water channel protein, but also as an adhesion molecule that is involved in cell migration and neuroexcitation, synaptic plasticity, and learning/memory through mechanisms involved in long-term potentiation or long-term depression. The most extensively examined role of AQP4 is its ability to act as a neuroimmunological inducer. Previously, we showed that AQP4 plays an important role in neuroimmunological functions in injured mouse brain in concert with the proinflammatory inducer osteopontin (OPN). The aim of this review is to summarize the functional implication of AQP4, focusing especially on its neuroimmunological roles. This review is a good opportunity to compile recent knowledge and could contribute to the therapeutic treatment of autoimmune diseases through strategies targeting AQP4. Finally, the author would like to hypothesize on AQP4’s role in interaction between reactive astrocytes and reactive microglial cells, which might occur in neurodegenerative diseases. Furthermore, a therapeutic strategy for AQP4-related neurodegenerative diseases is proposed.

Neural stem cells and neuro/gliogenesis in the central nervous system: understanding the structural and functional plasticity of the developing, mature, and diseased brain

Neurons and glia in the central nervous system (CNS) originate from neural stem cells (NSCs). Knowledge of the mechanisms of neuro/gliogenesis from NSCs is fundamental to our understanding of how complex brain architecture and function develop. NSCs are present not only in the developing brain but also in the mature brain in adults. Adult neurogenesis likely provides remarkable plasticity to the mature brain. In addition, recent progress in basic research in mental disorders suggests an etiological link with impaired neuro/gliogenesis in particular brain regions. Here, we review the recent progress and discuss future directions in stem cell and neuro/gliogenesis biology by introducing several topics presented at a joint meeting of the Japanese Association of Anatomists and the Physiological Society of Japan in 2015. Collectively, these topics indicated that neuro/gliogenesis from NSCs is a common event occurring in many brain regions at various ages in animals. Given that significant structural and functional changes in cells and neural networks are accompanied by neuro/gliogenesis from NSCs and the integration of newly generated cells into the network, stem cell and neuro/gliogenesis biology provides a good platform from which to develop an integrated understanding of the structural and functional plasticity that underlies the development of the CNS, its remodeling in adulthood, and the recovery from diseases that affect it.

Structural organization of glial cells at the border between the neurotransplant and recipient brain

Integration of fetal hippocampal dentate fascia neurotransplants with the neocortical somatosensory region in adult rats was studied by electron microscopy. The growth of nerve fiber through the neurotransplant/brain border formed by the glial cells was studied. The interface zone was organized by various astrocyte subpopulations and ependymocytes forming multilamellar accumulations in some sites of the interface. These conglomerations of the glial cells and their processes did not prevent the growth of axonal and axodendritic bundles; moreover, fibrous astrocyte axons accompanied them. Under conditions of immature nervous tissue transplantation to the focus of mature brain damage, the glial cells created a substrate in the interface permeable for nerve fibers, thus promoting the functional integration of the neurotransplant.

Aquaporin 4-dependent expression of glial fibrillary acidic protein and tenascin-C in activated astrocytes in stab wound mouse brain and in primary culture

We previously reported that aquaporin 4 (AQP4) has a neuroimmunological function via astrocytes and microglial cells involving osteopontin. AQP4 is a water channel localized in the endofoot of astrocytes in the brain, and its expression is upregulated after a stab wound to the mouse brain or the injection of methylmercury in common marmosets. In this study, the correlation between the expression of AQP4 and the expression of glial fibrillary acidic protein (GFAP) or tenascin-C (TN-C) in reactive astrocytes was examined in primary cultures and brain tissues of AQP4-deficient mice (AQP4/KO). In the absence of a stab wound to the brain or of any stimulation of the cells, the expressions of both GFAP and TN-C were lower in astrocytes from AQP4/KO mice than in those from wild-type (WT) mice. High levels of GFAP and TN-C expression were observed in activated astrocytes after a stab wound to the brain in WT mice; however, the expressions of GFAP and TN-C were insignificant in AQP4/KO mice. Furthermore, lipopolysaccharide (LPS) stimulation activated primary culture of astrocytes and upregulated GFAP and TN-C expression in cells from WT mice, whereas the expressions of GFAP and TN-C were slightly upregulated in cells from AQP4/KO mice. Moreover, the stimulation of primary culture of astrocytes with LPS also upregulated inflammatory cytokines in cells from WT mice, whereas modest increases were observed in cells from AQP4/KO mice. These results suggest that AQP4 expression accelerates GFAP and TN-C expression in activated astrocytes induced by a stab wound in the mouse brain and LPS-stimulated primary culture of astrocytes.

Distinctive population of Gfap-expressing neural progenitors arising around the dentate notch migrate and form the granule cell layer in the developing hippocampus

In the adult hippocampus, granule cells continue to be generated from astrocyte-like progenitors expressing glial fibrillary acidic protein (GFAP) that differ from embryonic neocortical progenitors. However, during the embryonic period, dentate granule neurons and neocortical pyramidal neurons are derived from the ventricular zone (VZ) of the pallium. Our question is when do GFAP+ progenitors of granule neurons appear in the developing hippocampus during the embryonic period, and how do they form the granule cell layer. The present analysis using Gfap-GFP transgenic mice shows that the GFP+ distinct cell population first appears in the VZ of the medial pallium at the dorsal edge of the fimbria on embryonic day 13.5. During the perinatal period, they form a migratory stream from the VZ to the developing dentate gyrus, and establish the germinal zones in the migratory stream, and the marginal and hilar regions in the developing dentate gyrus. GFP+ cells in these regions were positive for Sox2 and Ki67, but negative for BLBP. GFP+ cells with Neurogenin2 expression were largely distributed in the VZ, whereas GFP+ cells with Tbr2 and NeuroD expressions were seen in the migratory stream and developing dentate gyrus. Prox1-expressing GFP+ cells were restricted to the developing dentate gyrus. These results suggest that distinctive Gfap-expressing progenitors arising around the dentate notch form germinal regions in the migratory stream and the developing dentate gyrus where they differentiate into granule neurons, indicating that distinct astrocyte-like neural progenitors continue to generate granule neurons, from the beginning of dentate development and throughout life. J. Comp. Neurol. 522:261-283, 2014. © 2013 Wiley Periodicals, Inc.

Specific glial populations regulate hippocampal morphogenesis

The hippocampus plays an integral role in spatial navigation, learning and memory, and is a major site for adult neurogenesis. Critical to these functions is the proper organization of the hippocampus during development. Radial glia are known to regulate hippocampal formation, but their precise function in this process is yet to be defined. We find that in Nuclear Factor I b (Nfib)-deficient mice, a subpopulation of glia from the ammonic neuroepithelium of the hippocampus fail to develop. This results in severe morphological defects, including a failure of the hippocampal fissure, and subsequently the dentate gyrus, to form. As in wild-type mice, immature nestin-positive glia, which encompass all types of radial glia, populate the hippocampus in Nfib-deficient mice at embryonic day 15. However, these fail to mature into GLAST- and GFAP-positive glia, and the supragranular glial bundle is absent. In contrast, the fimbrial glial bundle forms, but alone is insufficient for proper hippocampal morphogenesis. Dentate granule neurons are present in the mutant hippocampus but their migration is aberrant, likely resulting from the lack of the complete radial glial scaffold usually provided by both glial bundles. These data demonstrate a role for Nfib in hippocampal fissure and dentate gyrus formation, and that distinct glial bundles are critical for correct hippocampal morphogenesis.

The T-box transcription factor Eomes/Tbr2 regulates neurogenesis in the cortical subventricular zone

The embryonic subventricular zone (SVZ) is a critical site for generating cortical projection neurons; however, molecular mechanisms regulating neurogenesis specifically in the SVZ are largely unknown. The transcription factor Eomes/Tbr2 is transiently expressed in cortical SVZ progenitor cells. Here we demonstrate that conditional inactivation of Tbr2 during early brain development causes microcephaly and severe behavioral deficits. In Tbr2 mutants the number of SVZ progenitor cells is reduced and the differentiation of upper cortical layer neurons is disturbed. Neurogenesis in the adult dentate gyrus but not the subependymal zone is abolished. These studies establish Tbr2 as a key regulator of neurogenesis in the SVZ.

Selective ablation of an astroglial subset by toxic gene expression driven by tenascin promoter

OBJECTIVE

In vitro culture, one of the astroglia-derived extracellular matrix proteins, tenascin, expressed highly in fibrous astrocytes, whereas it expressed only low levels in protoplasmic astrocytes. We devised a method of selectively altering the population of astroglial subsets in primary culture of astrocytes derived from embryonic mouse brains using toxic gene expression driven by the tenascin promoter.

METHODS

We have identified that the segment of 512-bases of 5'-flanking plus 243-bases leader sequences of the mouse tenascin gene contains maximum promoter activity in primary culture of astrocytes by deletion analysis of 5'-upstream region. This promoter element was used to specifically express the herpes simplex virus thymidine kinase (HSV-TK) gene in tenascin-positive astrocytes.

RESULTS

This strategy allowed us to selectively decrease tenascin-positive astrocytes at the optimal concentration of ganciclovir, which is cytotoxic in HSV-TK-expressing cells.

DISCUSSION

This approach should be useful for examining the role of the tenascin-negative astroglial subset in the development and regeneration of the central nervous system.

Aggrecan is expressed by embryonic brain glia and regulates astrocyte development

Determination of the molecules that regulate astrocyte development has been hindered by the paucity of markers that identify astrocytic precursors in vivo. Here we report that the chondroitin sulfate proteoglycan aggrecan both regulates astrocyte development and is expressed by embryonic glial precursors. During chick brain development, the onset of aggrecan expression precedes that of the astrocytic marker GFAP and is concomitant with detection of the early glial markers GLAST and glutamine synthetase. In co-expression studies, we established that aggrecan-rich cells contain the radial glial markers nestin, BLBP and GLAST and later in embryogenesis, the astroglial marker GFAP. Parallel in vitro studies showed that ventricular zone cultures, enriched in aggrecan-expressing cells, could be directed to a GFAP-positive fate in G5-supplemented differentiation media. Analysis of the chick aggrecan mutant nanomelia revealed marked increases in the expression of the astrocyte differentiation genes GFAP, GLAST and GS in the absence of extracellular aggrecan. These increases in astrocytic marker gene expression could not be accounted for by changes in precursor proliferation or cell death, suggesting that aggrecan regulates the rate of astrocyte differentiation. Taken together, these results indicate a major role for aggrecan in the control of glial cell maturation during brain development.

Tenascin C in stem cell niches: redundant, permissive or instructive?

The stem cell niche provides the specialized environment that is able to sustain the lifelong maintenance of stem cells in their discrete locations within organs. The niche is usually composed of several different cell types and a specialized extracellular matrix consisting of many different constituents. Additionally, a variety of growth factors are secreted into the extracellular space and contribute to the functional organization of the niche. Here, I will concentrate on the multimodular extracellular matrix glycoprotein tenascin C (Tnc) and discuss it as an exemplary molecule that is present in several stem cell niches. In spite of its intuitively suggestive presence, it has been difficult to provide functional evidence for the importance of Tnc in the context of stem cells. In the nervous system, the careful analysis of Tnc-deficient mice has revealed that the developmental program neural stem cell pass-through is delayed due to changes in growth factor responsiveness. To gain further insight, we have employed the gene trap technology in neural stem cells to identify potential Tnc target genes. This approach has surfaced 2 interesting candidates that may contribute to a better understanding of the signal(s) elicited by Tnc in neural stem/progenitor cells in the niche.

Developmental expression of FMRP in the astrocyte lineage: implications for fragile X syndrome

One of the most common causes of mental retardation in humans, Fragile X syndrome, results from the absence of FMRP, the protein product of the FMR1 gene. In the nervous system, expression of FMRP has been thought to be confined mainly to neurons as little research has examined FMRP expression in non-neuronal lineages. We present evidence that, in addition to neuronal expression, FMRP is expressed in developing CNS glial cells in vitro and in vivo. The neurosphere assay was used to establish cultures of stem and progenitor cells from the brains of wildtype and FMRP knockout (B6.129.FMR1/FvBn) mouse pups. When the neurospheres were differentiated in vitro, approximately 50% of the FMRP positive cells also expressed GFAP. Immunocytochemical studies of the embryonic and postnatal mouse brain revealed coexpression of FMRP and GFAP in the developing hippocampus. Prominent coexpression was also observed in ependymal cells surrounding the third ventricle and astrocytes of the glia limitans. No double-labeled cells were evident in the brains of young adult mice. Cells coexpressing FMRP and the oligodendrocyte precursor marker NG2 were also identified in the hippocampus and corpus callosum of the early postnatal brain. Our results suggest that FMRP is expressed in cells of non-neuronal lineage(s) during development. This represents potential involvement of glial cells in the neural development of fragile X syndrome.

Plexin-B2, but not Plexin-B1, critically modulates neuronal migration and patterning of the developing nervous system in vivo

Semaphorins and their receptors, plexins, have emerged as important cellular cues regulating key developmental processes. B-type plexins directly regulate the actin cytoskeleton in a variety of cell types. Recently, B-type plexins have been shown to be expressed in striking patterns in the nervous system over critical developmental windows. However, in contrast to the well characterized plexin-A family, the functional role of plexin-B proteins in neural development and organogenesis in vertebrates in vivo is not known. Here, we have elucidated the functional contribution of the two neuronally expressed plexin-B proteins, Plexin-B1 or Plexin-B2, toward the development of the peripheral nervous system and the CNS by generating and analyzing constitutive knock-out mice. The development of the nervous system was found to be normal in mice lacking Plexin-B1, whereas mice lacking Plexin-B2 demonstrated defects in closure of the neural tube and a conspicuous disorganization of the embryonic brain. After analyzing mutant mice, which bypassed neural tube defects, we observed a key requirement for Plexin-B2 in proliferation and migration of granule cell precursors in the developing dentate gyrus, olfactory bulb, and cerebellum. Furthermore, we identified semaphorin 4C as a high-affinity ligand for Plexin-B2 in binding and functional assays. Semaphorin 4C stimulated activation of ErbB-2 and RhoA via Plexin-B2 and enhanced proliferation and migration of granule cell precursors. Semaphorin 4C-induced proliferation of ventricular zone neuroblasts was abrogated in mice lacking Plexin-B2. These genetic and functional analyses reveal a key requirement for Plexin-B2, but not Plexin-B1, in patterning of the vertebrate nervous system in vivo.

Postnatal development of entorhinodentate projection of the Reeler mutant mouse

We anterogradely labeled entorhinodentate axons by the injection of biotin dextran amine into the entorhinal cortex of adult wildtype and reeler mice to clarify whether the course and terminal endings of the reeler entorhinal projection are normal or not. We found that in the reeler mouse, biotin dextran amine-labeled entorhinodentate fibers arising from the entorhinal cortex curved around the hippocampal fissure instead of crossing it, whereas in the wildtype mouse, they crossed the fissure as a perforant pathway. Next, we examined carbocyanine dye (DiI) labeling of the immature entorhinodentate projection and the developmental changes of the hippocampal fissure during early postnatal days based on the laminin and glial fibrillary acidic protein (GFAP) immunohistochemistry. Injection of DiI into the entorhinal area of the wildtype and reeler mice at postnatal day 1 resulted in anterograde labeling of pioneer axons passing through the hippocampal fissure. However, follower axons could not penetrate through the hippocampal fissure in reeler mice, whereas in the normal controls, many DiI-labeled axons continued to pass through the fissure. GFAP immunohistochemistry demonstrated that GFAP-immunopositive astrocytes were abundant along the hippocampal fissure both in the wildtype and reeler mice at birth. In the wildtype mouse, GFAP-positive neurons nearby the fissure were decreasing in number during the early postnatal days, whereas in the reeler mouse, many GFAP-positive astrocytes were continuing to accumulate there. This barrier made of astrocytes in the reeler mouse may obstruct the ingrowth of the follower axons arising from the entorhinal cortex through the hippocampal fissure, resulting in the abnormal course of the entorhinodentate axons in this mutant.

Emx2 in the developing hippocampal fissure region

Mice deficient in transcription factor gene Emx2 show developmental alterations in the hippocampal dentate gyrus. Emx2, however, is also expressed in the region around the developing hippocampal fissure. The developing fissure contains a radial glial scaffolding, and is surrounded by the outer marginal zone and the dentate marginal zone, which become specifically colonized by neurons and differentiate into stratum lacunosum-moleculare and molecular layer of the dentate, respectively. In this study we show that the Emx2 mutant lacks the glial scaffolding of the fissure and has an outer marginal zone (precursor of the stratum lacunosum-moleculare), as well as a dentate marginal zone severely reduced in size while most of the reelin (Reln)-expressing cells that should occupy it fail to be generated. We have also identified a subpopulation of hippocampal Reln-expressing cells of the marginal zone, probably born in the hem, expressing a specific combination of markers, and for which Emx2 is not essentially required. Additionally, we show differential mutant phenotypes of both Emx2 and Pax6 in neocortical vs. hippocampal Reln-expressing cells, indicating differential development of both subpopulations.

The fate of neural progenitor cells expressing astrocytic and radial glial markers in the postnatal rat dentate gyrus

In the dentate gyrus neurons continue to be generated from late embryonic to adult stage. Recent extensive studies have unveiled several key aspects of the adult neurogenesis, but only few attempts have so far been made on the analysis of the early postnatal neurogenenesis, a transition state between the embryonic and adult neurogenesis. Here, we focus on the early postnatal neurogenesis and examine the nature and development of neural progenitor cells in Wistar rats. Immunohistochemistry for Ki67, a cell cycle marker, and 5-bromo-2-deoxyuridine (BrdU) labelling show that cell proliferation occurs mainly in the hilus and partly in the subgranular zone. A majority of the proliferating cells express S100beta and astrocyte-specific glutamate transporter (GLAST) and the subpopulation are also positive for glial fibrillary acidic protein (GFAP) and nestin. Tracing with BrdU and our modified retrovirus vector carrying enhanced green fluorescent protein (GFP) indicate that a substantial population of the proliferating cells differentiate into proliferative neuroblasts and immature neurons in the hilus, which then migrate to the granule cell layer (66.8%), leaving a long axon-like process behind in the hilus, and the others mainly become star-shaped astrocytes (12.0%) and radial glia-like cells (4.7%) in the subgranular zone. These results suggest that the progenitors of the granule cells expressing astrocytic and radial glial markers, proliferate and differentiate into neurons mainly in the hilus during the early postnatal period.

Characterization of Progenitor-Cell-Specific Genes Identified by Subtractive Suppression Hybridization

We have utilized subtractive suppression hybridization (SSH) to identify differentially expressed genes present in either neuroepithelial (NEP) cells or glial restricted precursor (GRP) cells. Eighteen clones enriched in GRP cells and 28 in NEP cells were identified. Five of the GRP-specific clones (tenascin C, cystatin C, GABA transporter 3, extracellular matrix molecule 2 and H2-4) were characterized further, and their glial specificity was confirmed by RT-PCR, in situ hybridization and immunocytochemistry. H2-4 (an expressed sequence tag) was shown to be part of chondroitin sulfate proteoglycan 3. Overall, our results show that SSH can be used to identify lineage- and stage-specific markers and that extracellular matrix molecules likely play important roles in the migration and differentiation of GRPs.

Maturation of astrocyte morphology and the establishment of astrocyte domains during postnatal hippocampal development

Mature protoplasmic astrocytes exhibit an extremely dense ramification of fine processes, yielding a 'spongiform' morphology. This complex morphology enables protoplasmic astrocytes to maintain intimate relationships with many elements of the brain parenchyma, most notably synapses. Recently, it has been demonstrated that astrocytes establish individual cellular-level domains within the neuropil, with limited overlap occurring between the extents of neighboring astrocytes. The highly ramified nature of protoplasmic astrocytes is closely associated with their ability to create such domains. This study was an attempt to characterize the development of spongiform processes and the establishment of astrocyte domains. A combination of immunolabeling for the astrocyte-specific markers glial fibrillary acidic protein and S100beta with intracellular dye labeling in fixed tissue slices allowed for the identification of immature astrocytes and the elucidation of their complete, well-preserved morphologies. We find that during the first two postnatal weeks astrocytes extend stringy, filopodial processes. Fine, spongiform processes appear during the third week. Protoplasmic astrocytes are quite heterogeneous in morphology at 1-week postnatum, but there is a remarkable consistency in morphology by 2 weeks of age. Finally, protoplasmic astrocytes initially extend long, overlapping processes during the first two postnatal weeks. The subsequent elaboration of spongiform processes results in the development of boundaries between neighboring astrocyte domains. Stray processes that encroach on neighboring domains are eventually pruned by 1 month of age. These observations suggest that domain formation is largely the consequence of competition between astrocyte processes, similar to the well-studied competitive interactions between certain neuronal dendritic fields.

ADAM7 (a disintegrin and metalloprotease 7) mRNA is suppressed in mouse epididymis by neonatal exposure to Diethylstilbestrol

We investigated whether neonatal exposure to diethylstilbestrol (DES) induces the alteration of mRNA expression in adult mouse epididymis, which plays an important role in sperm maturation. Using a cDNA subtraction method, we isolated 15 changed gene clones in neonatally DES-treated mouse epididymides, and we found a clone homologous with a disintegrin and metalloprotease (ADAM) 7 in the epididymis, as a suppressed gene, by means of neonatal DES treatment in 8-week-old mice. Indeed, it was confirmed by Northern blot analysis that the ADAM7 mRNA expression in the epididymis was at a lower level in neonatally DES-treated mice than in non-treated mice. Moreover, in situ hybridization analysis and real-time reverse transcription and polymerase chain reaction (real-time RT-PCR) revealed that ADAM7 expression was markedly reduced in the corpus region of the epididymis of DES-treated mice as compared with non-treated mice. Our results suggest that neonatal exposure to DES leads to the suppression of ADAM7 expression in the epididymis in the long term. ADAM7 gene expression might be a biological marker of fetal or neonatal exposure to estrogenic compounds, including endocrine disruptors.