Role of radial glia in cytogenesis, patterning and boundary formation in the developing spinal cord

From neural tube to spinal cord: The dynamic journey of the dorsal neuroepithelium

In a developing embryo, formation of tissues and organs is remarkably precise in both time and space. Through cell-cell interactions, neighboring progenitors coordinate their activities, sequentially generating distinct types of cells. At present, we only have limited knowledge, rather than a systematic understanding, of the underlying logic and mechanisms responsible for cell fate transitions. The formation of the dorsal aspect of the spinal cord is an outstanding model to tackle these dynamics, as it first generates the peripheral nervous system and is later responsible for transmitting sensory information from the periphery to the brain and for coordinating local reflexes. This is reflected first by the ontogeny of neural crest cells, progenitors of the peripheral nervous system, followed by formation of the definitive roof plate of the central nervous system and specification of adjacent interneurons, then a transformation of roof plate into dorsal radial glia and ependyma lining the forming central canal. How do these peripheral and central neural branches segregate from common progenitors? How are dorsal radial glia established concomitant with transformation of the neural tube lumen into a central canal? How do the dorsal radial glia influence neighboring cells? This is only a partial list of questions whose clarification requires the implementation of experimental paradigms in which precise control of timing is crucial. Here, we outline some available answers and still open issues, while highlighting the contributions of avian models and their potential to address mechanisms of neural patterning and function.

SASH1 contributes to glial cell migration in the early development of the central nervous system

SAM and SH3 domain-containing 1 (SASH1), a member of the SLy protein family, is a tumor suppressor gene that has been studied for its association with various cancers. SASH1 is highly expressed in the mammalian central nervous system, particularly in glial cells, and is expressed in the central nervous system during zebrafish embryo development. However, SASH1's role in brain development has rarely been investigated. In this study, Morpholino oligonucleotides (MO) were used to down-regulate sash1a expression in zebrafish to observe morphological changes in the brain. Three transgenic zebrafish lines, Tg(gfap:eGFP), Tg(hb9:eGFP), and Tg(coro1a:eGFP) were selected to observe changes in glial cells, neurons, and immune cells after sash1a knockdown. Our results showed that the number of microglia residing in the developmental brain was reduced, whereas the axonal growth of caudal primary motor neurons was unaffected by sash1a downregulation. And more significantly, the gfap + glia presented abnormal arrangements and disordered orientations in sash1a morphants. The similar phenotype was verified in the mutation induced by the injection of cas9 mRNA and sash1a sgRNA. We further performed behavioral experiments in zebrafish larvae that had been injected with sash1a MO at one-cell stage, and found them exhibiting abnormal behavior trajectories. Moreover, injecting the human SASH1 mRNA rescued these phenomena in sash1a MO zebrafish. In summary, our study revealed that the downregulation of SASH1 leads to malformations in the embryonic brain and disorganization of glial cell marshalling, suggesting that SASH1 plays an important role in the migration of glial cells during embryonic brain development.

HOP protein expression in the hippocampal dentate gyrus is acutely downregulated in a status epilepticus mouse model

Status epilepticus (SE) is a neurological emergency, and delayed management can lead to higher morbidity and mortality. It is thought that prolonged seizures stimulate stem cells in the hippocampus and that epileptogenesis may arise from aberrant connections formed by newly born cells, while others have suggested that the acute neuroinflammation and gliosis often seen in epileptic hippocampi contribute to hyperexcitability and epilepsy development. Previous studies have identified the expression of homeodomain-only protein (HOP) in the hippocampal dentate gyrus (HDG) and the heart. HOP was found to be a regulator of cell proliferation and differentiation during heart development, while it maintains the 'heart conduction system' in adulthood. However, little is known about HOP function in the adult HDG, particularly in the SE setting. Here, a HOP immunohistochemical profile in an SE mouse model was established. A total of 24 adult mice were analyzed 3-10 days following the SE episode, the 'acute phase'. Our findings demonstrate a significant downregulation of HOP and BLBP protein expression in the SE group following SE episodes, while HOP/Ki67 coexpression did not remarkably differ. Furthermore, coexpression of HOP/S100β and HOP/Prox1 was not observed, although we noticed insignificant HOP/DCX coexpression level. The findings of this study show no compelling evidence of proliferation, and newly added neurons were not identified during the acute phase following SE, although HOP protein expression was significantly decreased in the HDG. Similar to its counterpart in the adult heart, this suggests that HOP seems to play a key role in regulating signal conduction in adult hippocampus. Moreover, acute changes in HOP expression following SE could be part of an inflammatory response that could subsequently influence epileptogenicity.

Single-cell analysis reveals dynamic changes of neural cells in developing human spinal cord

During central nervous system development, neurogenesis and gliogenesis occur in an orderly manner to create precise neural circuitry. However, no systematic dataset of neural lineage development that covers both neurogenesis and gliogenesis for the human spinal cord is available. We here perform single-cell RNA sequencing of human spinal cord cells during embryonic and fetal stages that cover neuron generation as well as astrocytes and oligodendrocyte differentiation. We also map the timeline of sensory neurogenesis and gliogenesis in the spinal cord. We further identify a group of EGFR-expressing transitional glial cells with radial morphology at the onset of gliogenesis, which progressively acquires differentiated glial cell characteristics. These EGFR-expressing transitional glial cells exhibited a unique position-specific feature during spinal cord development. Cell crosstalk analysis using CellPhoneDB indicated that EGFR glial cells can persistently interact with other neural cells during development through Delta-Notch and EGFR signaling. Together, our results reveal stage-specific profiles and dynamics of neural cells during human spinal cord development.

Spinal cord precursors utilize neural crest cell mechanisms to generate hybrid peripheral myelinating glia

During development, oligodendrocytes and Schwann cells myelinate central and peripheral nervous system axons, respectively, while motor exit point (MEP) glia are neural tube-derived, peripheral glia that myelinate axonal territory between these populations at MEP transition zones. From which specific neural tube precursors MEP glia are specified, and how they exit the neural tube to migrate onto peripheral motor axons, remain largely unknown. Here, using zebrafish, we found that MEP glia arise from lateral floor plate precursors and require foxd3 to delaminate and exit the spinal cord. Additionally, we show that similar to Schwann cells, MEP glial development depends on axonally derived neuregulin1. Finally, our data demonstrate that overexpressing axonal cues is sufficient to generate additional MEP glia in the spinal cord. Overall, these studies provide new insight into how a novel population of hybrid, peripheral myelinating glia are generated from neural tube precursors and migrate into the periphery.

An Improved in vitro Model of Cortical Tissue

Intracortical electrodes for brain-machine interfaces rely on intimate contact with tissues for recording signals and stimulating neurons. However, the long-term viability of intracortical electrodes in vivo is poor, with a major contributing factor being the development of a glial scar. In vivo approaches for evaluating responses to intracortical devices are resource intensive and complex, making statistically significant, high throughput data difficult to obtain. In vitro models provide an alternative to in vivo studies; however, existing approaches have limitations which restrict the translation of the cellular reactions to the implant scenario. Notably, there is no current robust model that includes astrocytes, microglia, oligodendrocytes and neurons, the four principle cell types, critical to the health, function and wound responses of the central nervous system (CNS). In previous research a co-culture of primary mouse mature mixed glial cells and immature neural precursor cells were shown to mimic several key properties of the CNS response to implanted electrode materials. However, the method was not robust and took up to 63 days, significantly affecting reproducibility and widespread use for assessing brain-material interactions. In the current research a new co-culture approach has been developed and evaluated using immunocytochemistry and quantitative polymerase chain reaction (qPCR). The resulting method reduced the time in culture significantly and the culture model was shown to have a genetic signature similar to that of healthy adult mouse brain. This new robust CNS culture model has the potential to significantly improve the capacity to translate in vitro data to the in vivo responses.

Astrocyte-Derived Paracrine Signals: Relevance for Neurogenic Niche Regulation and Blood-Brain Barrier Integrity

Astrocytes are essential for proper regulation of the central nervous system (CNS). Importantly, these cells are highly secretory in nature. Indeed they can release hundreds of molecules which play pivotal physiological roles in nervous tissues and whose abnormal regulation has been associated with several CNS disorders. In agreement with these findings, recent studies have provided exciting insights into the key contribution of astrocyte-derived signals in the pleiotropic functions of these cells in brain health and diseases. In the future, deeper analysis of the astrocyte secretome is likely to further increase our current knowledge on the full potential of these cells and their secreted molecules not only as active participants in pathophysiological events, but as pharmacological targets or even as therapeutics for neurological and psychiatric diseases. Herein we will highlight recent findings in our and other laboratories on selected molecules that are actively secreted by astrocytes and contribute in two distinct functions with pathophysiological relevance for the astroglial population: i) regulation of neural stem cells (NSCs) and their progeny within adult neurogenic niches; ii) modulation of the blood–brain barrier (BBB) integrity and function.

Astrocytes are direct cellular targets of lithium treatment: novel roles for lysyl oxidase and peroxisome-proliferator activated receptor-γ as astroglial targets of lithium

Astrocytes are multifunctional glial cells that play essential roles in supporting synaptic signalling and white matter-associated connectivity. There is increasing evidence that astrocyte dysfunction is involved in several brain disorders, including bipolar disorder (BD), depression and schizophrenia. The mood stabiliser lithium is a frontline treatment for BD, but the mechanisms of action remain unclear. Here, we demonstrate that astrocytes are direct targets of lithium and identify unique astroglial transcriptional networks that regulate specific molecular changes in astrocytes associated with BD and schizophrenia, together with Alzheimer's disease (AD). Using pharmacogenomic analyses, we identified novel roles for the extracellular matrix (ECM) regulatory enzyme lysyl oxidase (LOX) and peroxisome proliferator-activated receptor gamma (PPAR-γ) as profound regulators of astrocyte morphogenesis. This study unravels new pathophysiological mechanisms in astrocytes that have potential as novel biomarkers and potential therapeutic targets for regulating astroglial responses in diverse neurological disorders.

Wnt/β-catenin signaling regulates ependymal cell development and adult homeostasis

In the adult mouse spinal cord, the ependymal cell population that surrounds the central canal is thought to be a promising source of quiescent stem cells to treat spinal cord injury. Relatively little is known about the cellular origin of ependymal cells during spinal cord development, or the molecular mechanisms that regulate ependymal cells during adult homeostasis. Using genetic lineage tracing based on the Wnt target gene Axin2, we have characterized Wnt-responsive cells during spinal cord development. Our results revealed that Wnt-responsive progenitor cells are restricted to the dorsal midline throughout spinal cord development, which gives rise to dorsal ependymal cells in a spatially restricted pattern. This is contrary to previous reports that suggested an exclusively ventral origin of ependymal cells, suggesting that ependymal cells may retain positional identities in relation to their neural progenitors. Our results further demonstrated that in the postnatal and adult spinal cord, all ependymal cells express the Wnt/β-catenin signaling target gene Axin2, as well as Wnt ligands. Genetic elimination of β-catenin or inhibition of Wnt secretion in Axin2-expressing ependymal cells in vivo both resulted in impaired proliferation, indicating that Wnt/β-catenin signaling promotes ependymal cell proliferation. These results demonstrate the continued importance of Wnt/β-catenin signaling for both ependymal cell formation and regulation. By uncovering the molecular signals underlying the formation and regulation of spinal cord ependymal cells, our findings thus enable further targeting and manipulation of this promising source of quiescent stem cells for therapeutic interventions.

Cellular composition and organization of the spinal cord central canal during metamorphosis of the frog Xenopus laevis

Studying the cellular composition and morphological changes of cells lining the central canal during Xenopus laevis metamorphosis could contribute to understand postnatal development and spinal cord regeneration. Here we report the analysis of central canal cells at different stages during metamorphosis using immunofluorescence for protein markers expression, transmission and scanning electron microscopy and cell proliferation assays. The central canal was regionalized according to expression of glial markers, ultrastructure, and proliferation in dorsal, lateral, and ventral domains with differences between larvae and froglets. In regenerative larvae, all cell types were uniciliated, have a radial morphology, and elongated nuclei with lax chromatin, resembling radial glial cells. Important differences in cells of nonregenerative froglets were observed, although uniciliated cells were found, the most abundant cells had multicilia and revealed extensive changes in the maturation and differentiation state. The majority of dividing cells in larvae corresponded to uniciliated cells at dorsal and lateral domains in a cervical-lumbar gradient, correlating with undifferentiated features. Neurons contacting the lumen of the central canal were detected in both stages and revealed extensive changes in the maturation and differentiation state. However, in froglets a very low proportion of cells incorporate 5-ethynyl-2'-deoxyuridine (EdU), associated with the differentiated profile and with the increase of multiciliated cells. Our work showed progressive changes in the cell types lining the central canal of Xenopus laevis spinal cord which are correlated with the regenerative capacities.

Spatial and Cellular Expression Patterns of Erythropoietin-Receptor and Erythropoietin during a 42-Day Post-Lesional Time Course after Graded Thoracic Spinal Cord Impact Lesions in the Rat

Erythropoietin (Epo) exhibits promising neuroregenerative potential for spinal cord injury (SCI), and might be involved in other long-term sequelae, such as neuropathic pain development. The current studies investigated the time courses and spatial and cellular patterns of Epo and erythropoietin receptor (EpoR) expression along the spinal axis after graded SCI. Male Long Evans rats received 100 kdyn, 150 kdyn, and 200 kdyn thoracic (T9) contusions from an Infinite Horizon impactor. Sham controls received laminectomies. Anatomical and quantitative immunohistochemical analyses of the EpoR/Epo expression along the whole spinal axis were performed 7, 15, and 42 postoperative days (DPO) after the lesioning. Cellular expression was investigated by double- and triple-labeling for EpoR/Epo with cellular markers and proliferating cells in subgroups of 5-bromo-2-deoxyuridine pre-treated animals. Prolonged EpoR/Epo-expression was confirmed by real-time reverse transcriptase polymerase chain reaction (RT-PCR). Quantified EpoR/Epo immunoreactivities in pain-related spinal cord regions and ventrolateral white matter (VLWM) were correlated with the mechanical sensitivity thresholds and locomotor function of the respective animals. EpoR and Epo were constitutively expressed in the ventral horn neurons and vascular and glial cells in the dorsal columns (DC) and the VLWM. After SCI, in addition to expression in the lesion core, EpoR/Epo immunoreactivities exhibited significant time- and lesion grade-dependent induction in the DC and VLWM along the spinal axis. EpoR and Epo immunoreactive cells were co-stained with markers for astroglial, neural precursor cell and vascular markers. In the VLWM, EpoR- and Epo-positive proliferating cells were co-stained with glial fibrillary acidic protein (GFAP) and nestin. The DC EpoR/Epo immunoreactivities exhibited linear relationships with the behavioral correlates of post-lesional chronic pain development at DPO 42. SCI leads to long-lasting multicellular EpoR/Epo induction beyond the lesion core in the spinal cord regions that are involved in central pain development and regenerative processes. Our studies provide a time frame to investigate the effects of Epo application on motor function or pain development, especially in the later time course after lesioning.

Notch transactivates Rheb to maintain the multipotency of TSC-null cells

Differentiation abnormalities are a hallmark of tuberous sclerosis complex (TSC) manifestations; however, the genesis of these abnormalities remains unclear. Here we report on mechanisms controlling the multi-lineage, early neuronal progenitor and neural stem-like cell characteristics of lymphangioleiomyomatosis (LAM) and angiomyolipoma cells. These mechanisms include the activation of a previously unreported Rheb-Notch-Rheb regulatory loop, in which the cyclic binding of Notch1 to the Notch-responsive elements (NREs) on the Rheb promoter is a key event. This binding induces the transactivation of Rheb. The identified NRE2 and NRE3 on the Rheb promoter are important to Notch-dependent promoter activity. Notch cooperates with Rheb to block cell differentiation via similar mechanisms in mouse models of TSC. Cell-specific loss of Tsc1 within nestin-expressing cells in adult mice leads to the formation of kidney cysts, renal intraepithelial neoplasia, and invasive papillary renal carcinoma.

Tuberous sclerosis complex (TSC) is a rare genetic condition causing tumours with differentiation abnormalities; however the molecular mechanisms causing these defects are unclear. Here the authors show that Notch cooperates with Rheb to block cell differentiation forming a regulatory loop that could underlie TSC tumorigenesis.

Suppression of Inflammatory Demyelinaton and Axon Degeneration through Inhibiting Kv3 Channels

The development of neuroprotective and repair strategies for treating progressive multiple sclerosis (MS) requires new insights into axonal injury. 4-aminopyridine (4-AP), a blocker of voltage-gated K⁺ (Kv) channels, is used in symptomatic treatment of progressive MS, but the underlying mechanism remains unclear. Here we report that deleting Kv3.1—the channel with the highest 4-AP sensitivity—reduces clinical signs in experimental autoimmune encephalomyelitis (EAE), a mouse model for MS. In Kv3.1 knockout (KO) mice, EAE lesions in sensory and motor tracts of spinal cord were markedly reduced, and radial astroglia were activated with increased expression of brain derived neurotrophic factor (BDNF). Kv3.3/Kv3.1 and activated BDNF receptors were upregulated in demyelinating axons in EAE and MS lesions. In spinal cord myelin coculture, BDNF treatment promoted myelination, and neuronal firing via altering channel expression. Therefore, suppressing Kv3.1 alters neural circuit activity, which may enhance BNDF signaling and hence protect axons from inflammatory insults.

Spinal RacGAP α-Chimaerin Is Required to Establish the Midline Barrier for Proper Corticospinal Axon Guidance

In the developing CNS, the midline barrier, which comprises guidance molecule-expressing midline glial somata and processes, plays a pivotal role in midline axon guidance. Accumulating evidence has revealed the molecular mechanisms by which the midline barrier ensures proper midline guidance for axons. In contrast, the mechanisms for establishing the midline barrier remain obscure. Here, we report that Rac-specific GTPase-activating protein (RacGAP) α-chimaerin is required for both axonal repulsion at and establishment of the midline barrier in the spinal cord. We generated cortex-specific and spinal-cord-specific α-chimaerin gene (Chn1) knock-out mice (Cx-Chn1KO and Sp-Chn1KO mice, respectively) and found that both showed aberrant corticospinal tract (CST) axon midline crossing in the spinal cord. Strikingly, Sp-Chn1KO mice had breaks (holes) in the ephrinB3(+) spinal midline barrier and EphA4(+) CST axons aberrantly crossed the midline through these holes. During normal embryonic development, EphA4(+) spinal cells are located in juxta-midline areas but are excluded from the midline. In contrast, in Chn1KO embryos, several EphA4(+) cells were aberrantly relocated into the midline and the midline barrier was broken around these cells. Similarly, the spinal cord midline of Epha4KO mice was invaded by juxta-midline EphA4 cells (i.e., Epha4 promoter-active cells) during the embryonic stage and holes were formed in the midline barrier. Juxta-midline EphA4 cells in the spinal cord expressed α-chimaerin. We propose that spinal α-chimaerin aids in establishing an intact spinal midline barrier by mediating juxta-midline EphA4(+) cell repulsion, thus preventing these cells from breaking into the ephrinB3(+) midline barrier.SIGNIFICANCE STATEMENT The midline barrier plays a critical role in midline axon guidance, which is fundamental to the formation of neural circuits that are responsible for proper left-right coordination of the body. Studies have revealed some of the mechanisms underlying how the midline barrier navigates axons. In contrast, the establishment of the midline barrier during embryonic development remains unclear. In this study, we determined that α-chimaerin is required for the formation of an intact midline barrier. Spinal-cord-specific α-chimaerin knock-out mice had spinal midline barriers with numerous breaks (holes), through which corticospinal axons aberrantly crossed the midline. We propose that α-chimaerin protects the midline barrier by mediating cell-repulsive signaling in juxta-midline cells, which prevents these cells from invading the midline.

Quantitative analyses of cellularity and proliferative activity reveals the dynamics of the central canal lining during postnatal development of the rat

According to previous opinion, the derivation of neurons and glia from the central canal (CC) lining of the spinal cord in rodents should occur in the embryonic period. Reports of the mitotic activity observed in the lining during postnatal development have often been contradictory, and proliferation was ascribed to the generation of ependymocytes, which are necessary for the elongation of CC walls. Our study quantifies the intensity of proliferation and determines the cellularity of the CC lining in reference to lumbar spinal segment L4 during the postnatal development of rats. The presence of dividing cells peaks in the CC lining on postnatal day 8 (P8), with division occurring in 19.2% ± 3.2% of cells. In adult rats, 3.6% ± 0.9% of cells still proliferate, whereas, in mice, 10.3% ± 2.3% of cells at P8 and only 0.6% ± 0.2% of cells in the CC lining in adulthood are proliferating. In the rat, the length of the cell cycle increases from 100.3 ± 35.7 hours at P1 to 401.4 ± 80.6 hours at P43, with a sudden extension between P15 and P22. Despite the intensive proliferation, the total cellularity of the CC lining at the L4 spinal segment significantly descended in from P8 to P15. According to our calculations, the estimated cellularity was significantly higher compared with the measured cellularity of the CC lining at P15. Our results indicate that CC lining serves as a source of cells beyond ependymal cells during the first postnatal weeks of the rat. J. Comp. Neurol. 525:693-707, 2017. © 2016 Wiley Periodicals, Inc.

HSPB3 protein is expressed in motoneurons and induces their survival after lesion-induced degeneration

The human small heat shock proteins (HSPBs) form a family of molecular chaperones comprising ten members (HSPB1-HSPB10), whose functions span from protein quality control to cytoskeletal dynamics and cell death control. Mutations in HSPBs can lead to human disease and particularly point mutations in HSPB1 and HSPB8 are known to lead to peripheral neuropathies. Recently, a missense mutation (R7S) in yet another member of this family, HSPB3, was found to cause an axonal motor neuropathy (distal hereditary motor neuropathy type 2C, dHMN2C). Until now, HSPB3 protein localization and function in motoneurons (MNs) have not yet been characterized. Therefore, we studied the endogenous HSPB3 protein distribution in the spinal cords of chicken and mouse embryos and in the postnatal nervous system (central and peripheral) of chicken, mouse and human. We further investigated the impact of wild-type and mutated HSPB3 on MN cell death via overexpressing these genes in ovo in an avian model of MN degeneration, the limb-bud removal. Altogether, our findings represent a first step for a better understanding of the cellular and molecular mechanisms leading to dHMN2C.

A new role of hindbrain boundaries as pools of neural stem/progenitor cells regulated by Sox2

Background

Compartment boundaries are an essential developmental mechanism throughout evolution, designated to act as organizing centers and to regulate and localize differently fated cells. The hindbrain serves as a fascinating example for this phenomenon as its early development is devoted to the formation of repetitive rhombomeres and their well-defined boundaries in all vertebrates. Yet, the actual role of hindbrain boundaries remains unresolved, especially in amniotes.

Results

Here, we report that hindbrain boundaries in the chick embryo consist of a subset of cells expressing the key neural stem cell (NSC) gene Sox2. These cells co-express other neural progenitor markers such as Transitin (the avian Nestin), GFAP, Pax6 and chondroitin sulfate proteoglycan. The majority of the Sox2⁺ cells that reside within the boundary core are slow-dividing, whereas nearer to and within rhombomeres Sox2⁺ cells are largely proliferating. In vivo analyses and cell tracing experiments revealed the contribution of boundary Sox2⁺ cells to neurons in a ventricular-to-mantle manner within the boundaries, as well as their lateral contribution to proliferating Sox2⁺ cells in rhombomeres. The generation of boundary-derived neurospheres from hindbrain cultures confirmed the typical NSC behavior of boundary cells as a multipotent and self-renewing Sox2⁺ cell population. Inhibition of Sox2 in boundaries led to enhanced and aberrant neural differentiation together with inhibition in cell-proliferation, whereas Sox2 mis-expression attenuated neurogenesis, confirming its significant function in hindbrain neuronal organization.

Conclusions

Data obtained in this study deciphers a novel role of hindbrain boundaries as repetitive pools of neural stem/progenitor cells, which provide proliferating progenitors and differentiating neurons in a Sox2-dependent regulation.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0277-y) contains supplementary material, which is available to authorized users.

Gfap-positive radial glial cells are an essential progenitor population for later-born neurons and glia in the zebrafish spinal cord

Radial glial cells are presumptive neural stem cells (NSCs) in the developing nervous system. The direct requirement of radial glia for the generation of a diverse array of neuronal and glial subtypes, however, has not been tested. We employed two novel transgenic zebrafish lines and endogenous markers of NSCs and radial glia to show for the first time that radial glia are essential for neurogenesis during development. By using the gfap promoter to drive expression of nuclear localized mCherry we discerned two distinct radial glial-derived cell types: a major nestin+/Sox2+ subtype with strong gfap promoter activity and a minor Sox2+ subtype lacking this activity. Fate mapping studies in this line indicate that gfap+ radial glia generate later-born CoSA interneurons, secondary motorneurons, and oligodendroglia. In another transgenic line using the gfap promoter-driven expression of the nitroreductase enzyme, we induced cell autonomous ablation of gfap+ radial glia and observed a reduction in their specific derived lineages, but not Blbp+ and Sox2+/gfap-negative NSCs, which were retained and expanded at later larval stages. Moreover, we provide evidence supporting classical roles of radial glial in axon patterning, blood-brain barrier formation, and locomotion. Our results suggest that gfap+ radial glia represent the major NSC during late neurogenesis for specific lineages, and possess diverse roles to sustain the structure and function of the spinal cord. These new tools will both corroborate the predicted roles of astroglia and reveal novel roles related to development, physiology, and regeneration in the vertebrate nervous system. GLIA 2016;64:1170-1189.

Chemokine Signaling Controls Integrity of Radial Glial Scaffold in Developing Spinal Cord and Consequential Proper Position of Boundary Cap Cells

Radial glial cells are the neural progenitors of the developing CNS and have long radial processes that guide radially migrating neurons. The integrity of the radial glial scaffold, in particular proper adhesion between the endfeet of radial processes and the pial basement membrane (BM), is important for the cellular organization of the CNS, as indicated by evidence emerging from the developing cortex. However, the mechanisms underlying the maintenance of radial glial scaffold integrity during development, when the neuroepithelium rapidly expands, are still poorly understood. Here, we addressed this issue in the developing mouse spinal cord. We show that CXCR4, a receptor of chemokine CXCL12, is expressed in spinal cord radial glia. Conditional knock-out of Cxcr4 in radial glia caused disrupted radial glial scaffold with gaps at the pial endfeet layer and consequentially led to an invasion of boundary cap (BC) cells into the spinal cord. Because BC cells are PNS cells normally positioned at the incoming and outgoing axonal roots, their invasion into the spinal cord suggests a compromised CNS/PNS boundary in the absence of CXCL12/CXCR4 signaling. Both disrupted radial glial scaffold and invasion of BC cells into the CNS were also present in mice deficient in CXCR7, a second receptor of CXCL12. We further show that CXCL12 signaling promotes the radial glia adhesion to BM components and activates integrin β1 avidity. Our study unravels a novel molecular mechanism that deploys CXCL12/CXCR4/CXCR7 for the maintenance of radial glial scaffold integrity, which in turn safeguards the CNS/PNS boundary during spinal cord development.

Reactive astrocytes as neural stem or progenitor cells: In vivo lineage, In vitro potential, and Genome-wide expression analysis

Here, we review the stem cell hallmarks of endogenous neural stem cells (NSCs) during development and in some niches of the adult mammalian brain to then compare these with reactive astrocytes acquiring stem cell hallmarks after traumatic and ischemic brain injury. Notably, even endogenous NSCs including the earliest NSCs, the neuroepithelial cells, generate in most cases only a single type of progeny and self‐renew only for a rather short time in vivo. In vitro, however, especially cells cultured under neurosphere conditions reveal a larger potential and long‐term self‐renewal under the influence of growth factors. This is rather well comparable to reactive astrocytes in the traumatic or ischemic brain some of which acquire neurosphere‐forming capacity including multipotency and long‐term self‐renewal in vitro, while they remain within their astrocyte lineage in vivo. Both reactive astrocytes and endogenous NSCs exhibit stem cell hallmarks largely in vitro, but their lineage differs in vivo. Both populations generate largely a single cell type in vivo, but endogenous NSCs generate neurons and reactive astrocytes remain in the astrocyte lineage. However, at some early postnatal stages or in some brain regions reactive astrocytes can be released from this fate restriction, demonstrating that they can also enact neurogenesis. Thus, reactive astrocytes and NSCs share many characteristic hallmarks, but also exhibit key differences. This conclusion is further substantiated by genome‐wide expression analysis comparing NSCs at different stages with astrocytes from the intact and injured brain parenchyma. GLIA 2015;63:1452–1468

Chemokine-ligands/receptors: multiplayers in traumatic spinal cord injury

Spinal cord injury (SCI) results in complex posttraumatic sequelae affecting the whole neuraxis. Due to its involvement in varied neuromodulatory processes, the chemokine-ligand/receptor-network is a key element of secondary lesion cascades induced by SCI. This review will provide a synopsis of chemokine-ligand/receptor-expression along the whole neuraxis after traumatic spinal cord (sc) insults on basis of recent in vivo and in vitro findings in a SCI paradigm of thoracic force-defined impact lesions (Infinite Horizon Impactor) in adult rats. Analyses of chemokine-ligand/receptor-expression at defined time points after sc lesion of different severity grades or sham operation revealed that these inflammatory mediators are induced in distinct anatomical sc regions and in thalamic nuclei, periaqueductal grey, and hippocampal structures in the brain. Cellular and anatomical expression profiles together with colocalization/expression of neural stem/progenitor cell markers in adult sc stem cells niches or with pain-related receptors and mediators in dorsal horns, dorsal columns, and pain-processing brain areas support the notion that chemokines are involved in distinct cascades underlying clinical posttraumatic impairments and syndromes. These aspects and their implication in concepts of tailored SCI treatment are reviewed in the context of the recent literature on chemokine-ligand/receptor involvement in complex secondary lesion cascades.

Transplantation of stem cell-derived astrocytes for the treatment of amyotrophic lateral sclerosis and spinal cord injury

Neglected for years, astrocytes are now recognized to fulfill and support many, if not all, homeostatic functions of the healthy central nervous system (CNS). During neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS) and spinal cord injury (SCI), astrocytes in the vicinity of degenerating areas undergo both morphological and functional changes that might compromise their intrinsic properties. Evidence from human and animal studies show that deficient astrocyte functions or loss-of-astrocytes largely contribute to increased susceptibility to cell death for neurons, oligodendrocytes and axons during ALS and SCI disease progression. Despite exciting advances in experimental CNS repair, most of current approaches that are translated into clinical trials focus on the replacement or support of spinal neurons through stem cell transplantation, while none focus on the specific replacement of astroglial populations. Knowing the important functions carried out by astrocytes in the CNS, astrocyte replacement-based therapies might be a promising approach to alleviate overall astrocyte dysfunction, deliver neurotrophic support to degenerating spinal tissue and stimulate endogenous CNS repair abilities. Enclosed in this review, we gathered experimental evidence that argue in favor of astrocyte transplantation during ALS and SCI. Based on their intrinsic properties and according to the cell type transplanted, astrocyte precursors or stem cell-derived astrocytes promote axonal growth, support mechanisms and cells involved in myelination, are able to modulate the host immune response, deliver neurotrophic factors and provide protective molecules against oxidative or excitotoxic insults, amongst many possible benefits. Embryonic or adult stem cells can even be genetically engineered in order to deliver missing gene products and therefore maximize the chance of neuroprotection and functional recovery. However, before broad clinical translation, further preclinical data on safety, reliability and therapeutic efficiency should be collected. Although several technical challenges need to be overcome, we discuss the major hurdles that have already been met or solved by targeting the astrocyte population in experimental ALS and SCI models and we discuss avenues for future directions based on latest molecular findings regarding astrocyte biology.

Glial cells as progenitors and stem cells: new roles in the healthy and diseased brain

The diverse functions of glial cells prompt the question to which extent specific subtypes may be devoted to a specific function. We discuss this by reviewing one of the most recently discovered roles of glial cells, their function as neural stem cells (NSCs) and progenitor cells. First we give an overview of glial stem and progenitor cells during development; these are the radial glial cells that act as NSCs and other glial progenitors, highlighting the distinction between the lineage of cells in vivo and their potential when exposed to a different environment, e.g., in vitro. We then proceed to the adult stage and discuss the glial cells that continue to act as NSCs across vertebrates and others that are more lineage-restricted, such as the adult NG2-glia, the most frequent progenitor type in the adult mammalian brain, that remain within the oligodendrocyte lineage. Upon certain injury conditions, a distinct subset of quiescent astrocytes reactivates proliferation and a larger potential, clearly demonstrating the concept of heterogeneity with distinct subtypes of, e.g., astrocytes or NG2-glia performing rather different roles after brain injury. These new insights not only highlight the importance of glial cells for brain repair but also their great potential in various aspects of regeneration.

Astrocyte transplantation for spinal cord injury: current status and perspective

Spinal cord injury (SCI) often causes incurable neurological dysfunction because axonal regeneration in adult spinal cord is rare. Astrocytes are gradually recognized as being necessary for the regeneration after SCI as they promote axonal growth under both physiological and pathophysiological conditions. Heterogeneous populations of astrocytes have been explored for structural and functional restoration. The results range from the early variable and modest effects of immature astrocyte transplantation to the later significant, but controversial, outcomes of glial-restricted precursor (GRP)-derived astrocyte (GDA) transplantation. However, the traditional neuron-centric view and the concerns about the inhibitory roles of astrocytes after SCI, along with the sporadic studies and the lack of a comprehensive review, have led to some confusion over the usefulness of astrocytes in SCI. It is the purpose of the review to discuss the current status of astrocyte transplantation for SCI based on a dialectical view of the context-dependent manner of astrocyte behavior and the time-associated characteristics of glial scarring. Critical issues are then analyzed to reveal the potential direction of future research.

Repurposing Reelin: the new role of radial glia, Reelin and Notch in motor neuron migration

The role of Reelin during cerebral cortical neuron migration has long been studied, but the Reelin signaling pathway and its possible interactions are just beginning to be unraveled. Reelin is not only important in cerebral cortical migration, but has recently been shown to interact with the Notch signaling pathway and to be critical for radial glial cell number and morphology. Lee and Song (2013) show a new Notch- and Reelin-dependent role for radial glia in the mouse spinal cord: to act as a fine filter that allows somatic motor neuron axons but not cell bodies to traverse out of the CNS. Here, the types of neuronal migration are discussed, focusing on motor neurons and cues for proper localization. The interaction of Reelin signaling with the Notch pathway is reviewed, which dictates the proper formation of radial glia in the spinal cord in order to prevent ectopic motor neuron migration (Lee and Song, 2013). Future studies may reveal novel interactions and further insights as to how Reelin functions throughout the developing nervous system.

Expression of proteolipid protein gene in spinal cord stem cells and early oligodendrocyte progenitor cells is dispensable for normal cell migration and myelination

Plp1 gene expression occurs very early in development, well before the onset of myelination, creating a conundrum with regard to the function of myelin proteolipid protein (PLP), one of the major proteins in compact myelin. Using PLP-EGFP mice to investigate Plp1 promoter activity, we found that, at very early time points, PLP-EGFP was expressed in Sox2+ undifferentiated precursors in the spinal cord ventricular zone (VZ), as well as in the progenitors of both neuronal and glial lineages. As development progressed, most PLP-EGFP-expressing cells gave rise to oligodendrocyte progenitor cells (OPCs). The expression of PLP-EGFP in the spinal cord was quite dynamic during development. PLP-EGFP was highly expressed as cells delaminated from the VZ. Expression was downregulated as cells moved laterally through the cord, and then robustly upregulated as OPCs differentiated into mature myelinating oligodendrocytes. The presence of PLP-EGFP expression in OPCs raises the question of its role in this migratory population. We crossed PLP-EGFP reporter mice into a Plp1-null background to investigate the role of PLP in early OPC development. In the absence of PLP, normal numbers of OPCs were generated and their distribution throughout the spinal cord was unaffected. However, the orientation and length of OPC processes during migration was abnormal in Plp1-null mice, suggesting that PLP plays a role either in the structural integrity of OPC processes or in their response to extracellular cues that orient process outgrowth.

Evolution of oxytocin pathways in the brain of vertebrates

The central oxytocin system transformed tremendously during the evolution, thereby adapting to the expanding properties of species. In more basal vertebrates (paraphyletic taxon Anamnia, which includes agnathans, fish and amphibians), magnocellular neurosecretory neurons producing homologs of oxytocin reside in the wall of the third ventricle of the hypothalamus composing a single hypothalamic structure, the preoptic nucleus. This nucleus further diverged in advanced vertebrates (monophyletic taxon Amniota, which includes reptiles, birds, and mammals) into the paraventricular and supraoptic nuclei with accessory nuclei (AN) between them. The individual magnocellular neurons underwent a process of transformation from primitive uni- or bipolar neurons into highly differentiated neurons. Due to these microanatomical and cytological changes, the ancient release modes of oxytocin into the cerebrospinal fluid were largely replaced by vascular release. However, the most fascinating feature of the progressive transformations of the oxytocin system has been the expansion of oxytocin axonal projections to forebrain regions. In the present review we provide a background on these evolutionary advancements. Furthermore, we draw attention to the non-synaptic axonal release in small and defined brain regions with the aim to clearly distinguish this way of oxytocin action from the classical synaptic transmission on one side and from dendritic release followed by a global diffusion on the other side. Finally, we will summarize the effects of oxytocin and its homologs on pro-social reproductive behaviors in representatives of the phylogenetic tree and will propose anatomically plausible pathways of oxytocin release contributing to these behaviors in basal vertebrates and amniots.

Enigmatic cerebrospinal fluid-contacting neurons arise even after the termination of neurogenesis in the rat spinal cord during embryonic development and retain their immature-like characteristics until adulthood

Despite the abundance of cerebrospinal fluid-contacting neurons (CSF-cNs) lining the central canal of the spinal cord of mammals, little information is known regarding the phenotype and fate of these cells during development and in adulthood. Using immunofluorescence of spinal cord tissue of rats from the first postnatal day (P1) until the end of the 5th postnatal week (P36), we observed that these neurons show both immature (doublecortin+, β-III-tubulin+, neurofilament 200 kDa-) and more mature (weak NeuN+, P2X2+, GAD65+) characteristics during the first postnatal weeks. Because of the gradually decreasing number of CSF-cNs in the central canal lining during development, we were also interested in the migration potential of these cells. However, the assessment of the number of CSF-cNs in the lining of the central canal during postnatal development revealed that this decline is most likely associated with the growth of the spinal cord. Lastly, to reveal the birth date of CSF-cNs, we performed 5-bromo-2-deoxyuridine administration and colocalization analyses. We found that production of these cells appears from day 12 of embryonic development (E12) until E22. The vast majority of CSF-contacting neurons arise on E14 and E15. In contrast with other types of spinal neurons, the production of CSF-cNs is not restricted to a particular neuroepithelial region and occurs even after what is thought to be the termination of neurogenesis.

Extension of the Caenorhabditis elegans Pharyngeal M1 neuron axon is regulated by multiple mechanisms

The guidance of axons to their correct targets is a critical step in development. The C. elegans pharynx presents an attractive system to study neuronal pathfinding in the context of a developing organ. The worm pharynx contains relatively few cells and cell types, but each cell has a known lineage and stereotyped developmental patterns. We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases. The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth. Only unc-119 showed fully penetrant defects for the distal phase. Mutations affecting classical neuronal guidance cues (Netrin, Semaphorin, Slit/Robo, Ephrin) or adhesion molecules (cadherin, IgCAM) had, at best, weak effects on the M1 axon. None of the mutations we tested affected the proximal phase of M1 elongation. In a forward genetic screen, we isolated nine mutations in five genes, three of which are novel, showing defects in M1, including axon overextension, truncation, or ectopic branching. One of these mutations appeared to affect the generation or differentiation of the M1 neuron. We conclude that M1 axon extension is a robust process that is not completely dependent on any single guidance mechanism.

The structural role of radial glial endfeet in confining spinal motor neuron somata is controlled by the Reelin and Notch pathways

Neuronal migration is a fundamental biological process that enables the precise positioning of neurons to form functional circuits. Cortical neurons migrate along glial scaffolds formed by radial glia guided by Reelin ligand. However, it is unclear whether the Reelin-directed behavior of radial glia is also critical for positioning the spinal neurons. Here we demonstrate a novel role of radial glia that confines motor neurons within the neural tube and is promoted by Reelin and Notch signaling. Spinal radial glia express the Dab1 adaptor for Reelin signaling and are surrounded by Reelin. In reeler mice, in which Reelin is absent, ectopic motor neurons are found outside the neural tube, although they appear to maintain their identity. Boundary cap (BC) cells, Schwann cell precursors and the basal lamina at motor exit points are intact, whereas the glia limitans of radial glia are disorganized and detached from the basement membrane. The sparse and irregular radial scaffold is wide enough to allow motor somata to pass. Forced activation of Notch signaling rescued the structural defects in radial glia in reeler mice and the appearance of extraspinal neurons. In the absence of Reelin, Notch intracellular domain (NICD) protein level was reduced. In addition, disrupting the radial glia scaffold by destroying its polarity induced ectopic motor neurons in chick embryos. These findings suggest that activation of the Notch pathways by Reelin is required to establish the radial glial scaffold, a structure that actively constrains motor neuron somata and specifies the CNS-PNS boundary.

The spatial and temporal arrangement of the radial glial scaffold suggests a role in axon tract formation in the developing spinal cord

Radial glial cells serve diverse roles during the development of the central nervous system (CNS). In the embryonic brain, they are recognised as guidance conduits for migrating neuroblasts and as multipotent stem cells, generating both neurons and glia. While their stem cell capacities in the developing spinal cord are as yet not fully clarified, they are classically seen as a population of astrocytes precursors, before gradually disappearing as the spinal cord matures. Although the origins and lineages of CNS radial glial cells are being more clearly understood, the relationships between radial glial cells and growing white matter (WM) tracts are largely unknown. Here, we provide an in-depth description of the distribution and organisation of radial glial cell processes during the peak periods of axonogenesis in the rat spinal cord. We show that radial glial cell distribution is highly ordered in the WM from E14 to E18, when the initial patterning of axon tracts is taking place. We report that the density of radial glial cell processes is tightly conserved throughout development in the dorsal, lateral and ventral WM funiculi along the rostrocaudal axis of the spinal cord. We provide evidence that from E16 the dorsal funiculi grow within and are segregated by fascicles of processes emanating from the dorsomedial septum. The density of radial glial cells declines with the maturation of axon tracts and coincides with the onset of the radial glial cell-astrocyte transformation. As such, we propose that radial glial cells act as structural scaffolds by compartmentalising and supporting WM patterning in the spinal cord during embryonic development.

A bio-inspired neural environment to control neurons comprising radial glia, substrate chemistry and topography

Achieving alignment of cells is key to the success of regenerative strategies of neural tissue. We report a high-throughput method to investigate neural cell response to surface chemistry overlaid orthogonally onto a gradient of gradually changing groove widths. Using a bio-inspired approach wherein radial glial cells, which naturally guide neurons in the developing brain, enhance the attachment and directional outgrowth of neurons, we show the differences in the interaction and cellular response of glia, neurons and co-cultured cells. Radial glia were found to preferentially reside in grooves of width 6-35 μm with greater alignment to grooves <10 μm on the hydrophobic and hydrophilic extremes of chemistry. When neurons were sequentially cultured after radial glia, they showed enhanced alignment compared to when they were cultured alone, for all chemistries and groove widths. This is not dependent on co-localisation of the neurons with glia suggesting the radial glial cells pre-condition the substrate giving rise to enhanced attachment and alignment of subsequently cultured neurons. The results indicate a dependence of both primary radial glia and neuron responses on surface chemistry and micro-groove width. Grooved surfaces (width 5-10 μm) of mid-range wettability show the greatest potential to significantly enhance axonal alignment and, therefore, potential regeneration, when pre-conditioned by radial glia, highlighting the importance of surface engineering for neural scaffolds.

Transforming growth factor α transforms astrocytes to a growth-supportive phenotype after spinal cord injury

Astrocytes are both detrimental and beneficial for repair and recovery after spinal cord injury (SCI). These dynamic cells are primary contributors to the growth-inhibitory glial scar, yet they are also neuroprotective and can form growth-supportive bridges on which axons traverse. We have shown that intrathecal administration of transforming growth factor α (TGFα) to the contused mouse spinal cord can enhance astrocyte infiltration and axonal growth within the injury site, but the mechanisms of these effects are not well understood. The present studies demonstrate that the epidermal growth factor receptor (EGFR) is upregulated primarily by astrocytes and glial progenitors early after SCI. TGFα directly activates the EGFR on these cells in vitro, inducing their proliferation, migration, and transformation to a phenotype that supports robust neurite outgrowth. Overexpression of TGFα in vivo by intraparenchymal adeno-associated virus injection adjacent to the injury site enhances cell proliferation, alters astrocyte distribution, and facilitates increased axonal penetration at the rostral lesion border. To determine whether endogenous EGFR activation is required after injury, SCI was also performed on Velvet (C57BL/6J-Egfr(Vel)/J) mice, a mutant strain with defective EGFR activity. The affected mice exhibited malformed glial borders, larger lesions, and impaired recovery of function, indicating that intrinsic EGFR activation is necessary for neuroprotection and normal glial scar formation after SCI. By further stimulating precursor proliferation and modifying glial activation to promote a growth-permissive environment, controlled stimulation of EGFR at the lesion border may be considered in the context of future strategies to enhance endogenous cellular repair after injury.

Adult spinal cord radial glia display a unique progenitor phenotype

Radial glia (RG) are primarily embryonic neuroglial progenitors that express Brain Lipid Binding Protein (Blbp a.k.a. Fabp7) and Glial Fibrillary Acidic Protein (Gfap). We used these transcripts to demarcate the distribution of spinal cord radial glia (SCRG) and screen for SCRG gene expression in the Allen Spinal Cord Atlas (ASCA). We reveal that neonatal and adult SCRG are anchored in a non-ventricular niche at the spinal cord (SC) pial boundary, and express a “signature” subset of 122 genes, many of which are shared with “classic” neural stem cells (NSCs) of the subventricular zone (SVZ) and SC central canal (CC). A core expressed gene set shared between SCRG and progenitors of the SVZ and CC is particularly enriched in genes associated with human disease. Visualizing SCRG in a Fabp7-EGFP reporter mouse reveals an extensive population of SCRG that extend processes around the SC boundary and inwardly (through) the SC white matter (WM), whose abundance increases in a gradient from cervical to lumbar SC. Confocal analysis of multiple NSC-enriched proteins reveals that postnatal SCRG are a discrete and heterogeneous potential progenitor population that become activated by multiple SC lesions, and that CC progenitors are also more heterogeneous than previously appreciated. Gene ontology analysis highlights potentially unique regulatory pathways that may be further manipulated in SCRG to enhance repair in the context of injury and SC disease.

ErbB2 activation contributes to de-differentiation of astrocytes into radial glial cells following induction of scratch-insulted astrocyte conditioned medium

Radial glial cells play a significant role in the repair of spinal cord injuries as they exert critical role in the neurogenesis and act as a scaffold for neuronal migration. Our previous study showed that mature astrocytes of spinal cord can undergo a de-differentiation process and further transform into pluripotential neural precursors; the occurrence of these complex events arise directly from the induction of diffusible factors released from scratch-insulted astrocytes. However, it is unclear whether astrocytes can also undergo rejuvenation to revert to a radial glial progenitor phenotype after the induction of scratch-insulted astrocytes conditioned medium (ACM). Furthermore, the mechanism of astrocyte de-differentiation to the progenitor cells is still unclear. Here we demonstrate that upon treating mature astrocytes with ACM for 10 days, the astrocytes exhibit progressive morphological and functional conversion to radial glial cells. These changes include the appearance of radial glial progenitor cells, changes in the immunophenotypical profiles, characterized by the co-expression of nestin, paired homeobox protein (Pax6) and RC2 as well as enhanced capability of multipotential differentiation. Concomitantly, ErbB2 protein level was progressively up-regulated. Thereby these results provide a potential mechanism by which ACM could induce mature astrocytes to regain the profile of radial glial progenitors due to activating the ErbB2 signaling pathways.

Chemokine expression in the white matter spinal cord precursor niche after force-defined spinal cord contusion injuries in adult rats

Inflammatory cascades induced by spinal cord injuries (SCI) are localized in the white matter, a recognized neural stem- and progenitor-cell (NSPC) niche of the adult spinal cord. Chemokines, as integrators of these processes, might also be important determinants of this NSPC niche. CCL3/CCR1, CCL2/CCR2, and SDF-1alpha/CXCR4 were analyzed in the ventrolateral white matter after force defined thoracic SCI: Immunoreactivity (IR) density levels were measured 2 d, 7 d, 14 d, and 42 d on cervical (C 5), thoracic (T 5), and lumbar (L 5) levels. On day post operation (DPO) 42, chemokine inductions were further evaluated by real-time RT-PCR and Western blot analyses. Cellular phenotypes were confirmed by double labeling with markers for major cell types and NSPCs (nestin, Musashi-1, NG2, 3CB2, BLBP). Mitotic profiles were investigated in parallel by BrdU labeling. After lesion, chemokines were induced in the ventrolateral white matter on IR-, mRNA-, and protein-level. IR was generally more pronounced after severe lesions, with soaring increases of CCL2/CCR2 and continuous elevations of CCL3/CCR1. SDF-1alpha and CXCR4 IR induction was focused on thoracic levels. Chemokines/-receptors were co-expressed with astroglial, oligodendroglial markers, nestin, 3CB2 and BLBP by cells morphologically resembling radial glia on DPO 7 to DPO 42, and NG2 or Musashi-1 on DPO 2 and 7. In the white matter BrdU positive cells were significantly elevated after lesion compared with sham controls on all investigated time points peaking in the early time course on thoracic level: Here, chemokines were co-expressed by subsets of BrdU-labeled cells. These findings suggest an important role of chemokines/-receptors in the subpial white matter NSPC niche after SCI.

Roles of glial cells in synapse development

Brain function relies on communication among neurons via highly specialized contacts, the synapses, and synaptic dysfunction lies at the heart of age-, disease-, and injury-induced defects of the nervous system. For these reasons, the formation-and repair-of synaptic connections is a major focus of neuroscience research. In this review, I summarize recent evidence that synapse development is not a cell-autonomous process and that its distinct phases depend on assistance from the so-called glial cells. The results supporting this view concern synapses in the central nervous system as well as neuromuscular junctions and originate from experimental models ranging from cell cultures to living flies, worms, and mice. Peeking at the future, I will highlight recent technical advances that are likely to revolutionize our views on synapse-glia interactions in the developing, adult and diseased brain.

Role of radial glia in transformation of the primitive lumen to the central canal in the developing rat spinal cord

In the last quarter of the embryonic development of rat and shortly after a termination of neurogenesis, the transformation of the spinal cord primitive lumen (pL) to the central canal (CC) occurs. In this work, we show that this phenomenon is not an insignificant event but it is directly associated with the processes of gliogenesis. Using a light microscopy and immunohistochemistry, we monitored the development of the rat embryonic spinal cord from the end of the neurogenesis on the embryonic day 17 until the maturation of the spinal cord during the first postnatal weeks. Our observations demonstrate the importance of the transformation of the pL to the CC and its connection with gliogenesis, and the mechanism of this transformation is proposed. It is found that a segregation of the glutamate transporter (GLAST) immunopositive cells from the alar plates and transformation of the radial glial cells to the fibrous and protoplasmic astrocytes play presumably a key role in the diminution of the ventricular zone. Results indicate that the very transformation and migration of the radial glial cells during gliogenesis could result in a transformation of the pL to the CC.

The role of glial cells in axon guidance, fasciculation and targeting

Axons navigate step-wise, from one intermediate target to the next, until they reach their final destination target. In the central nervous system, intermediate targets are often glial cells, and final targeting is also aided by glia. In the peripheral nervous system, however, glial cells most often follow axons, which therefore navigate following other, nonglial clues. Even in the central nervous system, interactions between axons and glia are dynamic and reciprocal, as the neurons regulate migration, survival and proliferation of the glia cells they need for guidance. We review here the experimental evidence investigating roles of glia in axon guidance. Some molecules are known to influence either the neurons or the glia, but the molecular mechanisms underlying axon-glia interactions during pathfinding are only beginning to emerge.

Further preliminary assessment of three human glioma cell lines as models of human astrocytic toxicity in vitro

Three human astroglioma lines U251-MG, U373-MG and CCF-STTG1 have been evaluated further as possible models for astrocytotoxicity (GFAP and IL-6 release). The effects of bacterial lipopolysaccharide, chloroquine diphosphate and acrylamide were studied on GFAP expression and LPS, chloroquine diphosphate, ethanol, trimethyltin chloride (TMTC) and acrylamide were examined on interleukin-6 (IL-6) release in the U373-MG line only. At 4-h LPS elevated GFAP (17.0±5.0% P<0.05) above control in the U251-MG cell line only. Chloroquine diphosphate over 4h in the U251-MG line resulted in an increase in GFAP-IR to 20.3±4.2% and 21.1±4.1% above control levels 0.1μM (P<0.05) and 1μM (P<0.05) respectively. CQD was associated with decreases in MTT turnover, particularly after 24h incubation. With the U373-MG line, LPS (0.5μg/ml) increased IL-6 expression 640% above control (P<0.001), whilst chloroquine diphosphate (100μM), ethanol (10mM) and TMTC chloride (1μM) also increased IL-6. It is possible that batteries of astrocytic human glioma cell lines may be applicable to the sensitive evaluation of toxicants on astrogliotic expression markers such as GFAP and IL-6.

Don't fence me in: harnessing the beneficial roles of astrocytes for spinal cord repair

Astrocytes comprise a heterogeneous cell population that plays a complex role in repair after spinal cord injury. Reactive astrocytes are major contributors to the glial scar that is a physical and chemical barrier to axonal regeneration. Yet, consistent with a supportive role in development, astrocytes secrete neurotrophic factors and protect neurons and glia spared by the injury. In development and after injury, local cues are modulators of astrocyte phenotype and function. When multipotent cells are transplanted into the injured spinal cord, they differentiate into astrocytes and other glial cells as opposed to neurons, which is commonly viewed as a challenge to be overcome in developing stem cell technology. However, several examples show that astrocytes provide support and guidance for axonal growth and aid in improving functional recovery after spinal cord injury. Notably, transplantation of astrocytes of a developmentally immature phenotype promotes tissue sparing and axonal regeneration. Furthermore, interventions that enhance endogenous astrocyte migration or reinvasion of the injury site result in greater axonal growth. These studies demonstrate that astrocytes are dynamic, diverse cells that have the capacity to promote axon growth after injury. The ability of astrocytes to be supportive of recovery should be exploited in devising regenerative strategies.

Reactive changes of retinal astrocytes and Müller glial cells in kainate-induced neuroexcitotoxicity

The aim of this study was to investigate reactive changes of astrocytes and Müller glial cells in rats subjected to kainate treatment, which leads to neuronal degeneration in the ganglion cell layer and the inner border of the inner nuclear layer as confirmed by labelling with Fluoro-Jade B, a marker for degenerating neurons and fibres. Both the astrocytes and the Müller glial cells reacted vigorously to kainate injection as shown by their up-regulated expression of nestin, glial fibrillary acidic protein and glutamine synthetase. A major finding was the induced expression of nestin together with glial fibrillary acidic protein beginning at 1 day post-injection of kainate. The marked nestin expression appeared to be most intense at 1 day and was sustained till 2 weeks as compared with the untreated/normal retina. Western blotting analysis confirmed a marked increase in expression of nestin, glial fibrillary acidic protein and glutamine synthetase as compared with untreated/normal retina. Double labelling study revealed that astrocytes and Müller glial cells expressed the radial glia marker nestin, and incorporated bromodeoxyuridine to re-enter into their cell cycle. The induced expression of these proteins in astrocytes and Müller glial cells indicated an induction of gliotic responses and de-differentiation that may be associated with regenerative efforts after kainate-induced injury. Indeed, with the acquisition of an immature molecular profile as manifested by the induced expression of brain lipid-binding protein and doublecortin in astrocytes and Müller glial cells, the potential of these cells to de-differentiate in retinal neurodegeneration is greatly amplified.

Revisiting the astrocyte–oligodendrocyte relationship in the adult CNS

The lineages of both astrocytes and oligodendrocytes have been popular areas of research in the last decade. The source of these cells in the mature CNS is relevant to the study of the cellular response to CNS injury. A significant amount of evidence exists to suggest that resident precursor cells proliferate and differentiate into mature glial cells that facilitate tissue repair and recovery. Additionally, the re-entry of mature astrocytes into the cell cycle can also contribute to the pool of new astrocytes that are observed following CNS injury. In order to better understand the glial response to injury in the adult CNS we must revisit the astrocyte-oligodendrocyte relationship. Specifically, we argue that there is a common glial precursor cell from which astrocytes and oligodendrocytes differentiate and that the microenvironment surrounding the injury determines the fate of the stimulated precursor cell. Ideally, better understanding the origin of new glial cells in the injured CNS will facilitate the development of therapeutics targeted to alter the glial response in a beneficial way.

EAAT1 and D-serine expression are early features of human retinal development

In the developing central nervous system (CNS), the activation of N-methyl-D-aspartate (NMDA) receptors is probably an important regulator of processes such as synaptogenesis and neurite growth. NMDA receptor activation is dependent upon the homeostasis of glutamate and the presence of co-agonists such as D-serine. We have investigated the expression of the glutamate transporter excitatory amino acid transporter-1 (EAAT1 or GLAST) as the key regulator of retinal extracellular glutamate levels, and the ontogeny of D-serine expression in the developing human retina. The expression of EAAT1 and D-serine was compared to the temporal and spatial distribution of the synaptic vesicle marker synaptophysin and the synaptic vesicle glutamate transporter vGLUT1. We also examined the co-expression of EAAT1 and cellular retinaldehyde-binding protein (CRALBP), and the co-expression of EAAT1 and D-serine. Human retinae aged 10-20 weeks' gestation (WG) were prepared for immunocytochemistry or for Western blotting. Expression of EAAT1 was evident at 10 WG in cell bodies, processes and end-feet of radial glia-like cells at all retinal eccentricities. D-serine immunolabelling was also evident in radial glia-like cells by 12 WG. In contrast, immunoreactivity for synaptophysin only started to appear in the central retina at 12 WG whilst immunoreactivity for vGLUT was slightly later. EAAT1 and d-serine were co-localised to the same cell population. In addition, EAAT1 and CRALBP were also co-localised to the same cell population of radial glia-like cells, suggesting that the EAAT1 and D-serine-positive cells may be Müller cells. This study shows that key potential modifiers of NMDA receptor activity are present before synaptic vesicle proteins are evident and may thus play a role in shaping synaptogenesis in the developing human retina.

Optimized and efficient preparation of astrocyte cultures from rat spinal cord

Astrocytes constitute a major class of glial cells in the CNS, and play crucial roles in physiological functioning, performance and maintenance of the CNS, as well as promotion of neuronal migration and maturation. Astrocytes have also been directly and indirectly implicated in the pathophysiology of various trauma occurrences, development of neurodegenerative diseases and nerve regeneration. To further understand mechanisms by which astrocytes elicit these effects, the first critical step in the study of astrocytes is the preparation of purified astrocytes cultures. Here we describe a simple and convenient procedure for producing rat primary astrocyte cultures of high purity, viability and proliferation. For astrocyte culture, we have optimized the isolation procedures and cultivation conditions including coating substrates, enzyme digestion, seeding density and composition of the culture medium. Using immunofluorescent antibodies against GFAP and OX-42 in combination of Hoechst 33342 fluorescent staining, we found that the purity of the astrocyte cultures was >99%. Astrocytes had high viability as measured by 3-(4, 5-dimethyl-2-yl)-2, 5-diphenyl-2H-tetrazolium bromide (MTT) assay. In addition, flow cytometric analysis was used to measure and observe variations in the cell cycle after 1-2 passages and proliferation of astrocytes was detected with a high percentage of cells stand in S+G(2)/M phase. Therefore, the method described here is ideal for experiments, which require highly pure astrocyte cultures.