The origin and development of retinal astrocytes in the mouse

A novel GFAP frameshift variant identified in a family with optico-retinal dysplasia and vision impairment

Gain-of-function variants in GFAP leads to protein aggregation and is the cause of the severe neurodegenerative disorder Alexander Disease (AxD), while loss of GFAP function has been considered benign. Here, we investigated a six-generation family, where multiple individuals presented with gliosis of the optic nerve head and visual impairment. Whole genome sequencing (WGS) revealed a frameshift variant in GFAP (c.928dup, p.(Met310Asnfs*113)) segregating with disease. Analysis of human embryonic tissues revealed strong expression of GFAP in retinal neural progenitors. A zebrafish model verified that c.928dup does not result in extensive GFAP protein aggregation and zebrafish gfap loss-of-function mutants showed vision impairment and retinal dysplasia, characterized by a significant loss of Müller glia cells and photoreceptor cells. Our findings show how different mutational mechanisms can cause diverging phenotypes and reveal a novel function of GFAP in vertebrate eye development.

Retinal ganglion cell circuits and glial interactions in humans and mice

Retinal ganglion cells (RGCs) are the brain's gateway for vision, and their degeneration underlies several blinding diseases. RGCs interact with other neuronal cell types, microglia, and astrocytes in the retina and in the brain. Much knowledge has been gained about RGCs and glia from mice and other model organisms, often with the assumption that certain aspects of their biology may be conserved in humans. However, RGCs vary considerably between species, which could affect how they interact with their neuronal and glial partners. This review details which RGC and glial features are conserved between mice, humans, and primates, and which differ. We also discuss experimental approaches for studying human and primate RGCs. These strategies will help to bridge the gap between rodent and human RGC studies and increase study translatability to guide future therapeutic strategies.

Microglial morphometric analysis: so many options, so little consistency

Quantification of microglial activation through morphometric analysis has long been a staple of the neuroimmunologist's toolkit. Microglial morphological phenomics can be conducted through either manual classification or constructing a digital skeleton and extracting morphometric data from it. Multiple open-access and paid software packages are available to generate these skeletons via semi-automated and/or fully automated methods with varying degrees of accuracy. Despite advancements in methods to generate morphometrics (quantitative measures of cellular morphology), there has been limited development of tools to analyze the datasets they generate, in particular those containing parameters from tens of thousands of cells analyzed by fully automated pipelines. In this review, we compare and critique the approaches using cluster analysis and machine learning driven predictive algorithms that have been developed to tackle these large datasets, and propose improvements for these methods. In particular, we highlight the need for a commitment to open science from groups developing these classifiers. Furthermore, we call attention to a need for communication between those with a strong software engineering/computer science background and neuroimmunologists to produce effective analytical tools with simplified operability if we are to see their wide-spread adoption by the glia biology community.

Activation of retinal glial cells contributes to the degeneration of ganglion cells in experimental glaucoma

Elevation of intraocular pressure (IOP) is a major risk factor for neurodegeneration in glaucoma. Glial cells, which play an important role in normal functioning of retinal neurons, are well involved into retinal ganglion cell (RGC) degeneration in experimental glaucoma animal models generated by elevated IOP. In response to elevated IOP, mGluR I is first activated and Kir4.1 channels are subsequently inhibited, which leads to the activation of Müller cells. Müller cell activation is followed by a complex process, including proliferation, release of inflammatory and growth factors (gliosis). Gliosis is further regulated by several factors. Activated Müller cells contribute to RGC degeneration through generating glutamate receptor-mediated excitotoxicity, releasing cytotoxic factors and inducing microglia activation. Elevated IOP activates microglia, and following morphological and functional changes, these cells, as resident immune cells in the retina, show adaptive immune responses, including an enhanced release of pro-inflammatory factors (tumor neurosis factor-α, interleukins, etc.). These ATP and Toll-like receptor-mediated responses are further regulated by heat shock proteins, CD200R, chemokine receptors, and metabotropic purinergic receptors, may aggravate RGC loss. In the optic nerve head, astrogliosis is initiated and regulated by a complex reaction process, including purines, transmitters, chemokines, growth factors and cytokines, which contributes to RGC axon injury through releasing pro-inflammatory factors and changing extracellular matrix in glaucoma. The effects of activated glial cells on RGCs are further modified by the interplay among different types of glial cells. This review is concluded by presenting an in-depth discussion of possible research directions in this field in the future.

Glial Bmal1 role in mammalian retina daily changes

Visual information processing in the retina requires the rhythmic expression of clock genes. The intrinsic retinal circadian clock is independent of the master clock located in the hypothalamic suprachiasmatic nucleus and emerges from retinal cells, including glia. Less clear is how glial oscillators influence the daily regulation of visual information processing in the mouse retina. Here, we demonstrate that the adult conditional deletion of the gene Bmal1 in GLAST-positive glial cells alters retinal physiology. Specifically, such deletion was sufficient to lower the amplitude of the electroretinogram b-wave recorded under light-adapted conditions. Furthermore, recordings from > 20,000 retinal ganglion cells (RGCs), the retina output, showed a non-uniform effect on RGCs activity in response to light across different cell types and over a 24-h period. Overall, our results suggest a new role of a glial circadian gene in adjusting mammalian retinal output throughout the night-day cycle.

Expression of Otx Genes in Müller Cells Using an In Vitro Experimental Model of Retinal Hypoxia

Introduction

Müller glial cells typically activate to react to hypoxic tissue damage in several retinal diseases. We evaluated the in vitro response to a hypoxia-mimicking stimulus on the expression of a set of genes, known to contribute to eye morphogenesis and cell differentiation.

Materials and Methods

A MIO-M1 Müller cell line was cultured in a hypoxia-mimicking environment by the addition of cobalt chloride to the culture medium, followed by a recovery time in which we mimic restoration from the hypoxic insult. The HIF-1α protein and VEGF-A gene expression were quantified to verify the induction of a hypoxia-like state.

Results

Among the genes under study, we did not observe any difference in the expression levels of Otx1 and Otx2 during treatment; conversely, Otx1 was overexpressed during recovery steps. The VEGF-A gene was strongly upregulated at both the CoCl₂ and recovery time points. The transactivated isoform (TA) of the TP73 gene showed an overexpression in long-term exposure to the hypoxic stimulus with a further increase after recovery. Discussion. Our molecular analysis is able to describe the activation of a set of genes, never before described, that can drive the response to a hypoxia-like status. The improved comprehension of these cellular events will be useful for designing new therapeutical approaches for retinal pathologies.

Axonal Injuries Cast Long Shadows: Long Term Glial Activation in Injured and Contralateral Retinas after Unilateral Axotomy

Background: To analyze the course of microglial and macroglial activation in injured and contralateral retinas after unilateral optic nerve crush (ONC). Methods: The left optic nerve of adult pigmented C57Bl/6 female mice was intraorbitally crushed and injured, and contralateral retinas were analyzed from 1 to 45 days post-lesion (dpl) in cross-sections and flat mounts. As controls, intact retinas were studied. Iba1⁺ microglial cells (MCs), activated phagocytic CD68⁺MCs and M2 CD206⁺MCs were quantified. Macroglial cell changes were analyzed by GFAP and vimentin signal intensity. Results: After ONC, MC density increased significantly from 5 to 21 dpl in the inner layers of injured retinas, remaining within intact values in the contralateral ones. However, in both retinas there was a significant and long-lasting increase of CD68⁺MCs. Constitutive CD206⁺MCs were rare and mostly found in the ciliary body and around the optic-nerve head. While in the injured retinas their number increased in the retina and ciliary body, in the contralateral retinas decreased. Astrocytes and Müller cells transiently hypertrophied in the injured retinas and to a lesser extent in the contralateral ones. Conclusions: Unilateral ONC triggers a bilateral and persistent activation of MCs and an opposed response of M2 MCs between both retinas. Macroglial hypertrophy is transient.

Reduced mTORC1-signaling in retinal ganglion cells leads to vascular retinopathy

BACKGROUND

The coordinated wiring of neurons, glia and endothelial cells into neurovascular units is critical for central nervous system development. This is best exemplified in the mammalian retina where interneurons, astrocytes and retinal ganglion cells sculpt their vascular environment to meet the metabolic demands of visual function. Identifying the molecular networks that underlie neurovascular unit formation is an important step towards a deeper understanding of nervous system development and function.

RESULTS

Here, we report that cell-to-cell mTORC1-signaling is essential for neurovascular unit formation during mouse retinal development. Using a conditional knockout approach we demonstrate that reduced mTORC1 activity in asymmetrically positioned retinal ganglion cells induces a delay in postnatal vascular network formation in addition to the production of rudimentary and tortuous vessel networks in adult animals. The severity of this vascular phenotype is directly correlated to the degree of mTORC1 down regulation within the neighboring retinal ganglion cell population.

CONCLUSIONS

This study establishes a cell nonautonomous role for mTORC1-signaling during retinal development. These findings contribute to our current understanding of neurovascular unit formation and demonstrate how ganglion cells actively sculpt their local environment to ensure that the retina is perfused with an appropriate supply of oxygen and nutrients.

Harnessing Astrocytes and Müller Glial Cells in the Retina for Survival and Regeneration of Retinal Ganglion Cells

Astrocytes have been associated with the failure of axon regeneration in the central nervous system (CNS), as it undergoes reactive gliosis in response to damages to the CNS and functions as a chemical and physical barrier to axon regeneration. However, beneficial roles of astrocytes have been extensively studied in the spinal cord over the years, and a growing body of evidence now suggests that inducing astrocytes to become more growth-supportive can promote axon regeneration after spinal cord injury (SCI). In retina, astrocytes and Müller cells are known to undergo reactive gliosis after damage to retina and/or optic nerve and are hypothesized to be either detrimental or beneficial to survival and axon regeneration of retinal ganglion cells (RGCs). Whether they can be induced to become more growth-supportive after retinal and optic nerve injury has yet to be determined. In this review, we pinpoint the potential molecular pathways involved in the induction of growth-supportive astrocytes in the spinal cord and suggest that stimulating the activation of these pathways in the retina could represent a new therapeutic approach to promoting survival and axon regeneration of RGCs in retinal degenerative diseases.

Gabor domain optical coherence microscopy combined with laser scanning confocal fluorescence microscopy

We report on the development of fluorescence Gabor domain optical coherence microscopy (Fluo GD-OCM), a combination of GD-OCM with laser scanning confocal fluorescence microscopy (LSCFM) for synchronous micro-structural and fluorescence imaging. The dynamic focusing capability of GD-OCM provided the adaptive illumination environment for both modalities without any mechanical movement. Using Fluo GD-OCM, we imaged ex vivo DsRed-expressing cells in the brain of a transgenic mouse, as well as Cy3-labeled ganglion cells and Cy3-labeled astrocytes from a mouse retina. The self-registration of images taken by the two different imaging modalities showed the potential for a correlative study of subjects and double identification of the target.

Developmental vascular pruning in neonatal mouse retinas is programmed by the astrocytic oxygen-sensing mechanism

Vascular pruning is crucial for normal development, but its underlying mechanisms are poorly understood. Here, we report that retinal vascular pruning is controlled by the oxygen-sensing mechanism in local astrocytes. Oxygen sensing is mediated by prolyl hydroxylase domain proteins (PHDs), which use O₂ as a substrate to hydroxylate specific prolyl residues on hypoxia inducible factor (HIF)-α proteins, labeling them for polyubiquitylation and proteasomal degradation. In neonatal mice, astrocytic PHD2 deficiency led to elevated HIF-2α protein levels, expanded retinal astrocyte population and defective vascular pruning. Although astrocytic VEGF-A was also increased, anti-VEGF failed to rescue vascular pruning. However, stimulation of retinal astrocytic growth by intravitreal delivery of PDGF-A was sufficient to block retinal vascular pruning in wild-type mice. We propose that in normal development, oxygen from nascent retinal vasculature triggers PHD2-dependent HIF-2α degradation in nearby astrocytic precursors, thus limiting their further growth by driving them to differentiate into non-proliferative mature astrocytes. The physiological limit of retinal capillary density may be set by astrocytes available to support their survival, with excess capillaries destined for regression.This article has an associated 'The people behind the papers' interview.

Oxygen-Induced Retinopathy from Recurrent Intermittent Hypoxia Is Not Dependent on Resolution with Room Air or Oxygen, in Neonatal Rats

Preterm infants often experience intermittent hypoxia (IH) with resolution in room air (RA) or hyperoxia (Hx) between events. Hypoxia is a major inducer of vascular endothelial growth factor, which plays a key role in normal and aberrant retinal angiogenesis. This study tested the hypothesis that neonatal IH which resolved with RA is less injurious to the immature retina than IH resolved by Hx between events. Newborn rats were exposed to: (1) Hx (50% O₂) with brief hypoxia (12% O₂); (2) RA with 12% O₂; (3) Hx with RA; (4) Hx only; or (5) RA only, from P0 to P14. Pups were examined at P14 or placed in RA until P21. Retinal vascular and astrocyte integrity; retinal layer thickness; ocular and systemic biomarkers of angiogenesis; and somatic growth were determined at P14 and P21. All IH paradigms resulted in significant retinal vascular defects, disturbances in retinal astrocyte template, retinal thickening, and photoreceptor damage concurrent with elevations in angiogenesis biomarkers. These data suggest that the susceptibility of the immature retina to changes in oxygen render no differences in the outcomes between RA or O₂ resolution. Interventions and initiatives to curtail O₂ variations should remain a high priority to prevent severe retinopathy.

Retinal Angiogenesis Regulates Astrocytic Differentiation in Neonatal Mouse Retinas by Oxygen Dependent Mechanisms

In mice, retinal vascular and astrocyte networks begin to develop at birth, expanding radially from the optic nerve head (ONH) towards the retinal periphery. The retinal vasculature grows towards the periphery ahead of differentiated astrocytes, but behind astrocytic progenitor cells (APCs) and immature astrocytes. Endothelial cell specific Vegfr-2 disruption in newborn mice not only blocked retinal vascular development but also suppressed astrocytic differentiation, reducing the abundance of differentiated astrocytes while causing the accumulation of precursors. By contrast, retinal astrocytic differentiation was accelerated by the exposure of wild-type newborn mice to hyperoxia for 24 hours, or by APC specific deficiency in hypoxia inducible factor (HIF)−2α, an oxygen labile transcription factor. These findings reveal a novel function of the retinal vasculature, and imply that in normal neonatal mice, oxygen from the retinal circulation may promote astrocytic differentiation, in part by triggering oxygen dependent HIF-2α degradation in astrocytic precursors.

Astrocytes follow ganglion cell axons to establish an angiogenic template during retinal development

Immature astrocytes and blood vessels enter the developing mammalian retina at the optic nerve head and migrate peripherally to colonize the entire retinal nerve fiber layer (RNFL). Retinal vascularization is arrested in retinopathy of prematurity (ROP), a major cause of bilateral blindness in children. Despite their importance in normal development and ROP, the factors that control vascularization of the retina remain poorly understood. Because astrocytes form a reticular network that appears to provide a substrate for migrating endothelial cells, they have long been proposed to guide angiogenesis. However, whether astrocytes do in fact impose a spatial pattern on developing vessels remains unclear, and how astrocytes themselves are guided is unknown. Here we explore the cellular mechanisms that ensure complete retinal coverage by astrocytes and blood vessels in mouse. We find that migrating astrocytes associate closely with the axons of retinal ganglion cells (RGCs), their neighbors in the RNFL. Analysis of Robo1; Robo2 mutants, in which RGC axon guidance is disrupted, and Math5 (Atoh7) mutants, which lack RGCs, reveals that RGCs provide directional information to migrating astrocytes that sets them on a centrifugal trajectory. Without this guidance, astrocytes exhibit polarization defects, fail to colonize the peripheral retina, and display abnormal fine-scale spatial patterning. Furthermore, using cell type-specific chemical-genetic tools to selectively ablate astrocytes, we show that the astrocyte template is required for angiogenesis and vessel patterning. Our results are consistent with a model whereby RGC axons guide formation of an astrocytic network that subsequently directs vessel development.

Altered astrocyte morphology and vascular development in dystrophin-Dp71-null mice

Understanding retinal vascular development is crucial because many retinal vascular diseases such as diabetic retinopathy (in adults) or retinopathy of prematurity (in children) are among the leading causes of blindness. Given the localization of the protein Dp71 around the retinal vessels in adult mice and its role in maintaining retinal homeostasis, the aim of this study was to determine if Dp71 was involved in astrocyte and vascular development regulation. An experimental study in mouse retinas was conducted. Using a dual immunolabeling with antibodies to Dp71 and anti-GFAP for astrocytes on retinal sections and isolated astrocytes, it was found that Dp71 was expressed in wild-type (WT) mouse astrocytes from early developmental stages to adult stage. In Dp71-null mice, a reduction in GFAP-immunopositive astrocytes was observed as early as postnatal day 6 (P6) compared with WT mice. Using real-time PCR, it was showed that Dp71 mRNA was stable between P1 and P6, in parallel with post-natal vascular development. Regarding morphology in Dp71-null and WT mice, a significant decrease in overall astrocyte process number in Dp71-null retinas at P6 to adult age was found. Using fluorescence-conjugated isolectin Griffonia simplicifolia on whole mount retinas, subsequent delay of developing vascular network at the same age in Dp71-null mice was found. An evidence that the Dystrophin Dp71, a membrane-associated cytoskeletal protein and one of the smaller Duchenne muscular dystrophy gene products, regulates astrocyte morphology and density and is associated with subsequent normal blood vessel development was provided.

Development of astrocytes in the vertebrate eye

Astrocytes represent the earliest glial population in the embryonic optic nerve, contributing critically to retinal angiogenesis and formation of brain-retinal-barrier. Despite of many developmental and clinical implications of astrocytes, answers to some of the most fundamental questions of this unique type of glial cells remain elusive. This review provides an overview of the current knowledge about the origination, proliferation, and differentiation of astrocytes, their journey from the optic nerve toward the neuroretina, and their involvement in physiological and pathological development of the visual system.

IDH1 regulates phospholipid metabolism in developing astrocytes

Isocitrate dehydrogenase 1 (IDH1) is an evolutionarily conserved enzyme that catalyzes the interconversion of isocitrate to α-ketoglutarate with the concomitant reduction of NADP(+) to NADPH. IDH1 has previously been shown to participate in lipid biosynthesis in various tissues such as the liver and adipose tissue. We examined the potential role of IDH1 in phospholipid metabolism in the brain. Here we show that IDH1 is highly expressed in the brain and astrocytes during embryonic development and the postnatal period and subsequently declines in adulthood. Silencing of IDH1 expression using siRNA in astrocytes isolated from E18.5 mouse cortices led to increased incorporation of [(3)H]-palmitate into the phosphatidylcholines (PCs) and decreased the incorporation of [(3)H]-palmitate into sphingomyelin and the phosphatidylethanolamines (PEs). In pulse-chase experiments, knock-down of IDH1 expression impaired the turnover of PCs and decreased the synthesis of PEs. The decrease in [(3)H]-palmitate incorporation into PEs when IDH1 was knocked-down in astrocytes was not due to impairments within the CDP-ethanolamine pathway or in the rate of decarboxylation of phosphatidylserine (PS). In conclusion, our results reveal a role for IDH1 in the synthesis/turnover of phospholipids in developing astrocytes and highlight the lipid alterations resulting from the loss of wild-type IDH1 activity.

The development of blood-retinal barrier during the interaction of astrocytes with vascular wall cells

Astrocytes are intimately involved in the formation and development of retinal vessels. Astrocyte dysfunction is a major cause of blood-retinal barrier injury and other retinal vascular diseases. In this study, the development of the retinal vascular system and the formation of the blood-retinal barrier in mice were investigated using immunofluorescence staining, gelatin-ink perfusion, and transmission electron microscopy. The results showed that the retinal vascular system of mice develops from the optic disc after birth, and radiates out gradually to cover the entire retina, taking the papilla optica as the center. First, the superficial vasculature is formed on the inner retinal layer; then, the vasculature extends into the inner and outer edges of the retinal inner nuclear layer, forming the deep vasculature that is parallel to the superficial vasculature. The blood-retinal barrier is mainly composed of endothelium, basal lamina and the end-feet of astrocytes, which become mature during mouse development. Initially, the naive endothelial cells were immature with few organelles and many microvilli. The basal lamina was uniform in thickness, and the glial end-feet surrounded the outer basal lamina incompletely. In the end, the blood-retinal barrier matures with smooth endothelia connected through tight junctions, relatively thin and even basal lamina, and relatively thin glial cell end-feet. These findings indicate that the development of the vasculature in the retina follows the rules of "center to periphery" and "superficial layer to deep layers". Its development and maturation are spatially and temporally consistent with the functional performance of retinal neurons and photosensitivity. The blood-retinal barrier gradually becomes mature via the process of interactions between astrocytes and blood vessel cells.

Rax-CreERT2 knock-in mice: a tool for selective and conditional gene deletion in progenitor cells and radial glia of the retina and hypothalamus

To study gene function in neural progenitors and radial glia of the retina and hypothalamus, we developed a Rax-CreERT2 mouse line in which a tamoxifen-inducible Cre recombinase is inserted into the endogenous Rax locus. By crossing Rax-CreER(T2) with the Cre-dependent Ai9 reporter line, we demonstrate that tamoxifen-induced Cre activity recapitulates the endogenous Rax mRNA expression pattern. During embryonic development, Cre recombinase activity in Rax-CreER(T2) is confined to retinal and hypothalamic progenitor cells, as well as progenitor cells of the posterior pituitary. At postnatal time points, selective Cre recombinase activity is seen in radial glial-like cell types in these organs--specifically Müller glia and tanycytes--as well as pituicytes. We anticipate that this line will prove useful for cell lineage analysis and investigation of gene function in the developing and mature retina, hypothalamus and pituitary.

The neurovascular unit and its growth factors: coordinated response in the vascular and nervous systems

The nervous and vascular systems contain many common organizational features and develop similarly in terms of anatomical patterning. During embryogenesis and in regions of the brain undergoing postnatal neurogenesis, neural stem cells and endothelial cells are found in close proximity, or within a so-called vascular niche. The similarities in patterning and proximity may reflect coordinated development based on responsiveness to similar growth factors such as vascular endothelial growth factor, semaphorin, and ephrins/Ephs: molecules involved in the development and maintenance of both the nervous and vascular systems. Despite the blatant similarities between the vascular and nervous systems, little is still known about the co-dependence and/or interactions between the two systems during development and following alterations in metabolic demand as seen during aging, exercise, and disease processes. The interactions between the two systems involving common growth factors suggest these two systems have evolved in an interconnected way.

Hyperoxia causes reduced density of retinal astrocytes in the central avascular zone in the mouse model of oxygen-induced retinopathy

The mouse model of oxygen-induced retinopathy (OIR) is commonly used to investigate various aspects of the pathogenesis of the retinopathy of prematurity (ROP) as well as angiogenesis in general. Retinal astrocytes were suggested to be involved in retinal angiogenesis. This study aimed to describe their localization and cell density during the course of physiological vascularization and pathological revascularization. Mice expressing H2B-GFP (green fluorescent protein fused to histone 2B) from the endogenous Pdgfra promoter were kept in 75% oxygen from P7 (post natal day 7) to P12 (mouse model of OIR). Retinal flatmounts or cryosections were immunostained for glial fibrillary acidic protein (Gfap), glutamine synthetase (Glul), collagen IV (Col IV), desmin (Des), caspase 3 (Casp3), paired box 2 (Pax2), or Ki67. Astrocytic nuclei were counted with the ImageJ macro AuTOCellQuant. The hypoxic state of the retina was investigated by Hypoxyprobe. The GFP signal of the Pdgfra reporter mice co-localized with Pax2, a nuclear marker for retinal astrocytes. This bright label was much easier to quantify than Gfap or Pax2 staining. Quantification of the cell density of astrocytes during physiological development specified the spreading of astrocytes in a concentrical wave from the optic nerve head towards the periphery. Astrocyte density was reduced during the remodelling of the primary vascular plexus into a hierarchical vascular tree (maximal astrocyte density at P1: 2800 astrocytes/mm2, final astrocyte density: 800 astrocytes/mm2). In the OIR model, cell density of astrocytes was elevated in the peripheral vascularized zone. In contrast, astrocyte density dropped to a half (400 astrocytes/mm2) of the normal value in the central avascular zone during the hyperoxic phase between P8 and P10 by apoptosis and rose only after P17 as the retinal network normalized. An additional drop of astrocyte density was observed within the angles between the large vessels of the central avascular zone during hypoxia between P12 and P17. Astrocyte density was not altered at vascular tufts. The hyperoxia effect on astrocytes including the reduced astrocyte density is not the reason for vascular tuft formation. Hypoxia-affected astrocytes in combination with a reduced astrocytic network in the central avascular zone during the hypoxic phase are important determinants in the formation of pathological features during retinal revascularization.

Nerve growth factor-mediated vascular endothelial growth factor expression of astrocyte in retinal vascular development

The angiogenic aspect of neurotrophins and their receptors rather than the neuroscientific aspect has been focused. However, their role in retinal vascular development is underdiscovered. The purpose of this study is to understand the role of neurotrophin receptors in retinal vascular development and the mechanisms of their action. To identify the expression of tropomyosin receptor kinase receptor (Trk) in developing retina, tissues of 4, 8, 12, 16 and 26 day-old mice were prepared for experiments. Immunohistochemistry and immunofluorescence double staining against glial fibrillary acidic protein and type IV collagen were performed. TrkA was expressed mainly along the vessel structure in inner part of retina, especially in retinal astrocyte. In cultured primary astrocyte, recombinant nerve growth factor (NGF) was used to activate TrkA. NGF induced the phosphorylation of TrkA, and it also enhanced the level of activated Akt and vascular endothelial growth factor (VEGF) mRNA. Inhibition of phosphoinositide 3-kinase (PI3K) reversed the NGF-induced activation of these two molecules. This study demonstrated that TrkA activation on NGF leads to VEGF elevation by PI3K-Akt pathway and therefore suggested that TrkA could be a stimulator of retinal vascular development.

Vascular endothelial growth factor and its high-affinity receptor (VEGFR-2) are highly expressed in the human forebrain and cerebellum during development

Vascular endothelial growth factor (VEGF) is an angiogenic and neurotrophic factor in both adult and neonatal animals, but its expression and role have been incompletely studied in the developing human brain. We analyzed the distribution of VEGF and its high-affinity receptor VEGFR-2 in the human forebrain and cerebellum at developmental stages from 14 weeks' gestation (WG) to the13th postnatal month. Tissue samples free of detectable neuropathologic abnormalities were assessed by immunohistochemistry and confocal microscopy using anti-human VEGF and VEGFR-2 antibodies. The VEGFR-2 was first expressed in the whole cerebral mantle and in migrating cells in the intermediate zone, whereas VEGFwas found in superficial layers of the cortical plate, in radial glia, and in the cerebellar external germinal cell layer. From 23 WG, temporospatial VEGFR-2 expression was superimposable on that ofVEGF in the cortical plate, intermediate zone, basal ganglia, limbicstructures, and external germinal cell layer. The VEGF/VEGFR-2-positive astrocytes were observed during their generation and migration from 23 WG to the first postnatal month. The VEGF-positive mature oligodendrocytes were observed in myelinating structures in the forebrain from birth and in the cerebellum from 24WG. These data suggest that VEGF and VEGFR-2 are likely involved in several aspects of human brain development.

Role and therapeutic potential of VEGF in the nervous system

The development of the nervous and vascular systems constitutes primary events in the evolution of the animal kingdom; the former provides electrical stimuli and coordination, while the latter supplies oxygen and nutrients. Both systems have more in common than originally anticipated. Perhaps the most striking observation is that angiogenic factors, when deregulated, contribute to various neurological disorders, such as neurodegeneration, and might be useful for the treatment of some of these pathologies. The prototypic example of this cross-talk between nerves and vessels is the vascular endothelial growth factor or VEGF. Although originally described as a key angiogenic factor, it is now well established that VEGF also plays a crucial role in the nervous system. We describe the molecular properties of VEGF and its receptors and review the current knowledge of its different functions and therapeutic potential in the nervous system during development, health, disease and in medicine.

Developmental control of synaptic receptivity

Are neurons born with the ability to form and receive synapses or do they acquire these abilities during development? We have previously found that purified postnatal retinal ganglion cells (RGCs) require soluble astrocyte-derived signals to form synapses in vitro and in vivo. Here we show that newly generated embryonic day 17 (E17) RGCs are able to form but not receive synapses under these conditions. Dendrite growth is not sufficient to trigger receptivity; rather, the ability of newly generated RGCs to receive synapses is acquired at E19 in response to direct contact by neighboring cell types. Direct contact with astrocytes, which are not present at E17 but are normally generated by E19, is sufficient to induce synaptic receptivity in E17 RGCs. In contrast, amacrine contact does not induce synaptic receptivity. Interestingly, astrocyte contact alters the localization of the synaptic adhesion molecule neurexin away from dendrites. In addition, dendritic expression of neurexin is sufficient to prevent astrocyte contact-mediated increases in synapse number, suggesting a molecular mechanism by which astrocyte contact regulates neuronal synaptic receptivity. Thus, synaptic receptivity is not induced simply by dendritic elaboration but must be signaled by both contact-mediated signaling from astrocytes and a shift in the dendritic localization of neurexin.

Aging-related changes in astrocytes in the rat retina: imbalance between cell proliferation and cell death reduces astrocyte availability

The aim of this study was to investigate changes in astrocyte density, morphology, proliferation and apoptosis occurring in the central nervous system during physiological aging. Astrocytes in retinal whole-mount preparations from Wistar rats aged 3 (young adult) to 25 months (aged) were investigated qualitatively and quantitatively following immunofluorohistochemistry. Glial fibrillary acidic protein (GFAP), S100 and Pax2 were used to identify astrocytes, and blood vessels were localized using Griffonia simplicifolia isolectin B4. Cell proliferation was assessed by bromodeoxyuridine incorporation and cell death by TUNEL-labelling and immunolocalization of the apoptosis markers active caspase 3 and endonuclease G. The density and total number of parenchymal astrocytes in the retina increased between 3 and 9 months of age but decreased markedly between 9 and 12 months. Proliferation of astrocytes was detected at 3 months but virtually ceased beyond that age, whereas the proportion of astrocytes that were TUNEL positive and relative expression of active caspase 3 and endonuclease G increased progressively with aging. In addition, in aged retinas astrocytes exhibited gliosis-like morphology and loss of Pax2 reactivity. A small population of Pax2(+)/GFAP(-) cells was detected in both young adult and aged retinas. The reduction in the availability of astrocytes in aged retinas and other aging-related changes reported here may have a significant impact on the ability of astrocytes to maintain homeostasis and support neuronal function in old age.

Control of glial precursor cell development in the mouse optic nerve by sonic hedgehog from retinal ganglion cells

The development of glial precursor cells in the mammalian optic nerve depends on retinal ganglion cell (RGC) axons, but the signals that mediate this neuron-to-glia interaction have not been fully characterized. Sonic hedgehog (Shh) is expressed by RGCs, and we showed previously that it is required for the specification of astrocyte lineage cells at the optic disc. To study the role of RGC-derived Shh on astrocyte development at later developmental stages, we generated mice with a conditional ablation of Shh in the peripheral retina and analyzed gene expression and glial cell development in the optic nerve. Astrocyte development was initiated in the optic nerves of these mutant mice; however, the expression of Hedgehog (Hh) target genes, Gli1 and Ptch1 and cell cycle genes, Ccnd1 and Cdc25b in the optic nerves were downregulated. Astrocyte proliferation was markedly reduced. Oligodendrocyte precursor cells were fewer in the optic nerves of mutant mice, possibly as a consequence of reduced secretion of growth factors by astrocytes. At a later developmental stage, optic nerve axons displayed signs of Wallerian degeneration, including reduction of astrocyte processes, degenerating glial cells and formation of distended axons. We also demonstrate that the Hh pathway can be activated in optic nerve-derived astrocytes in vitro, but fails to induce cell cycle gene expression and proliferation. RGC-derived Shh signalling isthus necessary in vivo for maintenance of astrocyte proliferation, affecting both axo-glial and normal glial cell development in the optic nerve.

Development of the retinal vasculature

Blood vessels that supply the inner portion of the retina are extensively reorganized during development. The vessel regression, sprouting angiogenesis, vascular remodelling and vessel differentiation events involved critically depend on cell-cell signalling between different cellular components such as neurons, glia, endothelial cells, pericytes and immune cells. Studies in mice using transgenic and gene deletion approaches have started to unravel the genetic basis of some of these signalling pathways and have lead to a much improved understanding of the molecular mechanisms controlling retinal blood vessel behaviour both during development and under pathological conditions. Such insight will provide the basis of future therapeutic approaches aimed at manipulating retinal blood vessels.

Immunohistochemical localization of CNTFRalpha in adult mouse retina and optic nerve following intraorbital nerve crush: evidence for the axonal loss of a trophic factor receptor after injury

Ciliary neurotrophic factor (CNTF) is important for the survival and outgrowth of retinal ganglion cells (RGCs) in vitro. However, in vivo adult RGCs fail to regenerate and subsequently die following axotomy, even though there are high levels of CNTF in the optic nerve. To address this discrepancy, we used immunohistochemistry to analyze the expression of CNTF receptor alpha (CNTFRalpha) in mouse retina and optic nerve following intraorbital nerve crush. In normal mice, RGC perikarya and axons were intensely labeled for CNTFRalpha. At 24 hours after crush, the immunoreactivity normally seen on axons in the nerve was lost near the lesion. This loss radiated from the crush site with time. At 2 days postlesion, labeled axons were not detected in the proximal nerve, and at 2 weeks were barely detectable in the retina. In the distal nerve, loss of axonal staining progressed to the optic chiasm by 7 days and remained undetectable at 2 weeks. Interfascicular glia in the normal optic nerve were faintly labeled, but by 24 hours after crush they became intensely labeled near the lesion. Double labeling showed these to be both astrocytes and oligodendrocytes. At 7 days postlesion, darkly labeled glia were seen throughout the optic nerve, but at 14 days labeling returned to normal. It is suggested that the loss of CNTFRalpha from axons renders RGCs unresponsive to CNTF, thereby contributing to regenerative failure and death, while its appearance on glia may promote glial scarring.

Temporal regulation of EphA4 in astroglia during murine retinal and optic nerve development

Eph receptors and their ephrin ligands play important roles in many aspects of visual system development. In this study, we characterized the spatial and temporal expression pattern of EphA4 in astrocyte precursor cell (APC) and astrocyte populations in the murine retina and optic nerve. EphA4 is expressed by immotile optic disc astrocyte precursor cells (ODAPS), but EphA4 is downregulated as these cells migrate into the retina. Surprisingly, mature astrocytes in the adult retina re-express EphA4. Within the optic nerve, EphA4 is expressed in specialized astrocytes that form a meshwork at the optic nerve head (ONH). Our in vitro and in vivo data indicate that EphA4 is dispensable for retinal ganglion cell (RGC) axon growth and projections through the chiasm. While optic stalk structure, APC proliferation and migration, retinal vascularization, and oligodendrocyte migration appear normal in EphA4 mutants, the expression of EphA4 in APCs and in the astrocyte meshwork at the ONH has implications for optic nerve pathologies.

Tlx, an orphan nuclear receptor, regulates cell numbers and astrocyte development in the developing retina

Tlx belongs to a class of orphan nuclear receptors that underlies many aspects of neural development in the CNS. However, the fundamental roles played by Tlx in the control of eye developmental programs remain elusive. By using Tlx knock-out (KO) mice, we show here that Tlx is expressed by retinal progenitor cells in the neuroblastic layer during the period of retinal layer formation, and it is critical for controlling the generation of appropriate numbers of retinal progenies through the activities of cell cycle-related molecules, cyclin D1 and p27Kip1. Tlx expression is restricted to Müller cells in the mature retina and appears to control their proper development. Furthermore, we show that Tlx is expressed by immature astrocytes that migrate from the optic nerve onto the inner surface of the retina and is required for their generation and maturation, as assessed by honeycomb network formation and expression of R-cadherin, a critical component for vasculogenesis. The impaired astrocyte network formation on the inner retinal surface is accompanied by the loss of vasculogenesis in Tlx KO retinas. Our studies thus indicate that Tlx underlies a fundamental developmental program of retinal organization and controls the generation of the proper numbers of retinal progenies and development of glial cells during the protracted period of retinogenesis.

Neural differentiation of mouse embryonic stem cells in vitro and after transplantation into eyes of mutant mice with rapid retinal degeneration

Embryonic stem (ES) cells can differentiate into many specialized cell types, including those of the nervous system. We evaluated the differentiation of enhanced green fluorescent protein (EGFP)-expressing B5 mouse ES cells in vitro and in vivo after transplantation into the eyes of mice with hereditary retinal degeneration. After neural induction with retinoic acid, the majority of cells in embryoid bodies expressed markers for neural progenitors as well as for immature and mature neurons and glial cells. When induced ES cells were plated in vitro, further differentiation was observed and the majority of cells expressed beta-III Tubulin, a marker for immature neurons. In addition, many plated cells expressed markers for mature neurons or glial cells. Four days after intravitreal transplantation into the eyes of rd1 mice (a model of rapid retinal degeneration), donor cells appeared attached to the vitreal surface of the retina. After 6 weeks in vivo, most transplanted cells remained adherent to the inner retinal surface, and some donor cells had integrated into the retina. Transplanted cells exhibited some properties typical of neurons, including extensive process outgrowth with numerous varicosities and expression of neuronal and synaptic markers. Therefore, after induction B5 ES cells can acquire the morphologies of neural cells and display markers for neuronal and glial cells in vitro and in vivo. Furthermore, when placed in the proper microenvironment ES-derived neural precursors can associate closely with or migrate into nervous tissue where differentiation appears to be determined by cues provided by the local environment, in this case the degenerating neural retina.

Morphological development of microglia in the postnatal rat brain

Morphological transformation of lectin-positive microglia/macrophages in the developing rat cerebral hemisphere was analysed using quantitative methods. During the first postnatal month, the cells showed increases in their size and fractal dimension accompanied by a simultaneous decrease in their solidity. Regional variations in dynamics of the process indicated the existence of spatio-temporal developmental gradients within the cerebral hemisphere wall which might correspond with regional patterns of neuronal differentiation. Results of the present study prove that the quantitative methods can be the source of reliable data replacing subjective cell typologies.

Morphological transformations of cells immunopositive for GFAP, TrkA or p75 in the CA1 hippocampal area following transient global ischemia in the rat. A quantitative study

Transient global ischemia induces intensive neuronal degeneration in the hippocampal CA1 pyramidal layer, accompanied by reactive transformation of glial cells. Previously, we have shown using the double immunostaining method that the NGF receptors (NGFR) p75 and TrkA are expressed mainly on subpopulations of GFAP+ astrocytes, and this expression increases progressively after ischemia. In the presented study, we analyzed quantitatively the morphological transformations of cells immunopositive for GFAP or NGF receptors in the stratum radiatum of the CA1 hippocampal area in different survival periods after ischemia, evoked by 10-min cardiac arrest in adult rats. In control brains, NGF receptors were expressed only on small cells with poorly ramified processes. After ischemia, the NGFR+ cells increased in size and morphological complexity (measured using fractal analysis). However, even 2 weeks after ischemia these cells did not reach the size and value of the fractal dimension typical of the largest GFAP+ astrocytes. Moreover, the reaction of NGFR+ cells was significantly delayed in comparison with the total astrocyte population. The obtained results suggest that NGF receptors are expressed mainly by immature astrocytes and ischemia induces the maturation of these cells.

Blood vessels and nerves: common signals, pathways and diseases

Both blood vessels and nerves are vital channels to and from tissues. Recent genetic insights show that they have much more in common than was originally anticipated. They use similar signals and principles to differentiate, grow and navigate towards their targets. Moreover, the vascular and nervous systems cross-talk and, when dysregulated, this contributes to medically important diseases. The realization that both systems use common genetic pathways should not only form links between vascular biology and neuroscience, but also promises to accelerate the discovery of new mechanistic insights and therapeutic opportunities.

Retinal ganglion cell-derived sonic hedgehog signaling is required for optic disc and stalk neuroepithelial cell development

The development of optic stalk neuroepithelial cells depends on Hedgehog (Hh) signaling, yet the source(s) of Hh protein in the optic stalk is unknown. We provide genetic evidence that sonic hedgehog (Shh) from retinal ganglion cells (RGCs) promotes the development of optic disc and stalk neuroepithelial cells. We demonstrate that RGCs express Shh soon after differentiation, and cells at the optic disc in close proximity to the Shh-expressing RGCs upregulate Hh target genes, which suggests they are responding to RGC-derived Shh signaling. Conditional ablation of Shh in RGCs caused a complete loss of optic disc astrocyte precursor cells, resulting in defective axon guidance in the retina, as well as conversion of the neuroepithelial cells in the optic stalk to pigmented cells. We further show that Shh signaling modulates the size of the Pax2(+) astrocyte precursor cell population at the optic disc in vitro. Together, these data provide a novel insight into the source of Hh that promotes neuroepithelial cell development in the mammalian optic disc and stalk.

Exogenous growth factors stimulate the regeneration of ganglion cells in the chicken retina

Recent reports have found that the posthatch chicken retina has the capacity for neuronal regeneration. The purpose of this study was to test whether the types of cells destroyed by neurotoxic lesions influence the types of cells that are regenerated, and whether exogenous growth factors stimulate neural regeneration in the chicken retina. N-methyl-D-aspartate (NMDA) was used to destroy amacrine and bipolar cells; kainate was used to destroy bipolar, amacrine, and ganglion cells; colchicine was used to selectively destroy ganglion cells. Following toxin-induced damage, bromo-deoxyuridine was used to label proliferating cells. In some animals, growth factors were injected into the vitreous chamber of the eye. We found that the proliferation of cells within the retina was stimulated by toxin-induced cell loss, and by insulin and FGF2. After either kainate- or colchicine-induced retinal damage, some of the newly generated cells expressed markers and had the morphology of ganglion cells. The combination of insulin and FGF2 stimulated the regeneration of ganglion cells in kainate- and colchicine-treated retinas. We conclude that exogenous growth factors can be used to stimulate neural regeneration in the retina. We propose that the type of neuron destroyed in the retina may allow or promote the regeneration of that neuronal type.

Differential lineage restriction of rat retinal progenitor cells in vitro and in vivo

To identify and characterize the lineage potential of rat neural retina progenitor cells (NRPCs) in vitro and engrafted into rats with retinal degeneration, NRPCs were isolated from neural retinas of embryonic day 17 Long Evans rats and cultured in serum-free or serum-containing media with fibroblast growth factor 2 and neurotrophin 3. After expansion, cellular differentiation was initiated by the withdrawal of these growth factors. Despite forming primary neurospheres, NRPCs cultured in serum-free medium survived poorly after passage. In contrast, NRPCs cultured in serum-containing medium could be expanded for up to 12 passages and differentiated into glial fibrillary acidic protein-positive glial cells and retina-specific neurons expressing rhodopsin, S-antigen, calbindin, recoverin, and calretinin. For in vivo analysis, passage 1 (P1) undifferentiated NRPCs were labeled with bromodeoxyuridine (BrdU), implanted into the subretinal space of Royal College of Surgeons (RCS) rats, and analyzed immunohistochemically 4 weeks postgrafting. The grafted NRPCs showed extensive glial differentiation, irrespective of their topographic localization. A few BrdU-labeled grafted NRPCs expressed protein kinase C, a marker for bipolar and amacrine interneuron-specific differentiation. Other retina-specific or oligodendrocytic differentiation was not detected in the grafted cells. Although NRPCs are capable of self-renewal and multilineage differentiation in vitro, they developed mostly into glial cells following engraftment into the adult retina. These data suggest that the adult retina retains epigenetic signals that are either restrictive for neuronal differentiation or instructive for glial differentiation. Induction of lineage-specific cell differentiation of engrafted NRPCs to facilitate retinal repair will likely require initiation of specific differentiation in vitro prior to grafting and/or modification of the host environment concomitantly with NRPC grafting.

Differentiation and migration of astrocyte precursor cells and astrocytes in human fetal retina: relevance to optic nerve coloboma

The presence of astrocyte precursor cells (APCs) and time course and topography of astrocyte differentiation during development were investigated by triple-label immunohistochemistry with intact fetal and adult human retinas. Throughout retinal development and adulthood, expression of Pax2 was restricted to cells of the astrocytic lineage. Three distinct stages of astrocytic differentiation were identified during development: i) Pax2+/vimentin+/GFAP- APCs; ii) Pax2+/vimentin+/GFAP+ immature perinatal astrocytes; and iii) Pax2+/vimentin-/GFAP+ mature perinatal astrocytes. In adult, cells with the antigenic phenotype of mature perinatal astrocytes were restricted to a region surrounding the optic nerve head (ONH), whereas cells at a fourth stage of differentiation, adult astrocytes (Pax2-/vimentin-/GFAP+), were apparent throughout the vascularized retina. APC appearance was centered around the ONH and preceded the appearance of perinatal astrocytes. A cluster of Pax2+ somas was also present in a small region surrounding the ONH at the ventricular surface of the developing retina, which suggests the existence of two distinct sites of astrocytic differentiation. The coincidence in the location of APCs and perinatal astrocytes at the ventricular zone with that of optic nerve colobomas, together with the association of Pax2 gene mutations with this condition, suggests that coloboma formation may result from impaired astrocyte differentiation during development.

Neuroligin 3 is a vertebrate gliotactin expressed in the olfactory ensheathing glia, a growth-promoting class of macroglia

The molecular mechanisms that drive glia-glial interactions and glia-neuronal interactions during the development of the nervous system are poorly understood. A number of membrane-bound cell adhesion molecules have been shown to play a role, although the precise nature of their involvement is unknown. One class of molecules with cell adhesive properties used in the nervous system is the serine-esterase-like family of transmembrane proteins. A member of this class, a glia-specific protein called gliotactin, has been shown to be necessary for the development of the glial sheath in the peripheral nervous system of Drosophila melanogaster. Gliotactin is essential for the development of septate junctions in the glial sheath of individual and neighboring glia. Mutations that remove this protein result in paralysis and eventually death due to a breakdown in the glial-based blood-nerve barrier. To study the role of gliotactin during vertebrate nervous system development, we have isolated a potential vertebrate gliotactin homologue from mice and rat and found that it corresponds to neuroligin 3. Using a combination of RT-PCR and immunohistochemistry, we have found that neuroligin 3 is expressed during the development of the nervous system in many classes of glia. In particular neuroligin 3 is expressed in the olfactory ensheathing glia, retinal astrocytes, Schwann cells, and spinal cord astrocytes in the developing embryo. This expression is developmentally controlled such that in postnatal and adult stages, neuroligin 3 continues to be expressed at high levels in the olfactory ensheathing glia, a highly plastic class of glia that retain many of their developmental characteristics throughout life.

Transient expression of SorCS in developing telencephalic and mesencephalic structures of the mouse

Here we describe the expression of a third member of the VPS10 domain containing receptor family, SorCS, during mouse embryonal and early postnatal nervous system development. SorCS is expressed in a unique transient and dynamic pattern in regions where cells proliferate, as well as in areas where already differentiated cells reside, including the cerebral cortex, the ventral tegmental area, and the globus pallidus. Transcripts were absent from fiber tracts hinting at a neuronal expression. The only exception was hybridization signals on the developing optic nerve correlating with the appearance of astrocytes migrating into the retina.

A different retinal glia response to optic nerve injury/lipopolysaccharide administration in hooded and albino rats

Despite a massive degeneration of retinal ganglion cells (RGC) after optic nerve crush (ONC) in hooded rats only a minor increase in retinal glial fibrillary acidic protein (GFAP)-immunoreactivity was found in the inner retina. Interestingly, a combination of ONC with the administration of the proinflammatory agent lipopolysaccharide (LPS) but not LPS alone induces increased GFAP-immunoreactivity. In contrast albino rats showed elevated GFAP-immunoreactivity in response to both, LPS-administration and ONC with no further increase after a combination of both. These data demonstrate significant differences in retinal glia responsiveness between hooded and albino rats after optic nerve lesions.

Morphology of reactive microglia in the injured cerebral cortex. Fractal analysis and complementary quantitative methods

The present study focuses on application of quantitative methods measuring differences between particular morphological types of microglial cells as well as between their proliferating and non-proliferating examples. On the basis of subjective classification, microglial cells of three morphological types (ramified, hypertrophied and bushy) were selected from the neocortex of injured rat brain. Thereafter, the morphological complexity of each cell was assessed by calculation its fractal dimension as well as its form factor, convexity, ramification factor and solidity. The fractal dimension seemed a good parameter for detecting small changes in the space-filing capacity of cells, for example, it shows differences between ramified cells from control and injured brains. This measure seemed insensitive to some aspects of cell morphology. To obtain precise quantification of observed changes other morphological parameters had to be applied. Proliferating and non-proliferating microglial cells displayed significant differences in their solidity and ramification factors, but not in fractal dimension and convexity. The results indicated that proliferating microglia were more massive and less-ramified but they did not reduce their spatial complexity.