Changes in Epidermal Growth Factor Receptor Expression and Competence to Generate Glia Regulate Timing and Choice of Differentiation in the Retina

Interleukin-4 activates divergent cell-intrinsic signals to regulate retinal cell proliferation induced by classical growth factors

In the developing retina, precise coordination of cell proliferation, differentiation, and survival is essential for proper retinal maturation and function. We have previously reported evidence that interleukin-4 (IL-4) plays critical roles in neuronal differentiation and survival during retinal development. However, little is known about the role of IL-4 on retinal cell proliferation. In the current study, we investigated if IL-4 regulates cell proliferation induced by epidermal growth factor (EGF) and by fibroblast growth factor 2 (FGF2) in primary retinal cell cultures obtained from newborn rats. First, we show that EGF and FGF2 act as mitogens for glial cells, increasing proliferation of these cells in the retina. EGF- and FGF2-induced mitogenesis requires activation of distinct cell-intrinsic signals. In retinal cells exposed to FGF2, IL-4 downregulates p53 levels (a protein whose activation induces cell-cycle arrest) and increases mitogenic responsiveness to FGF2 through activation of protein kinase A (PKA) pathway. Conversely, in retinal cells exposed to EGF, IL-4 downregulates cyclin D1 levels (a protein required for cell-cycle progression), upregulates p53 levels, and decreases mitogenic responsiveness to EGF. The inhibitory effect induced by IL-4 on retinal cells exposed to EGF requires activation of Janus kinase 3 (JAK3), but not activation of PKA. Based on previous and current findings, we propose that IL-4 serves as a node of signal divergence, modulating multiple cell-intrinsic signals (e.g., cyclin D1, p53, JAK3, and PKA) and mitogenic responsiveness to cell-extrinsic signals (e.g., FGF2 and EGF) to control cell proliferation, differentiation, and survival during retinal development.

Müller Glia in Retinal Development: From Specification to Circuit Integration

Müller glia of the retina share many features with astroglia located throughout the brain including maintenance of homeostasis, modulation of neurotransmitter spillover, and robust response to injury. Here we present the molecular factors and signaling events that govern Müller glial specification, patterning, and differentiation. Next, we discuss the various roles of Müller glia in retinal development, which include maintaining retinal organization and integrity as well as promoting neuronal survival, synaptogenesis, and phagocytosis of debris. Finally, we review the mechanisms by which Müller glia integrate into retinal circuits and actively participate in neuronal signaling during development.

Low availability of choline in utero disrupts development and function of the retina

Adequate supply of choline, an essential nutrient, is necessary to support proper brain development. Whether prenatal choline availability plays a role in development of the visual system is currently unknown. In this study, we addressed the role of in utero choline supply for the development and later function of the retina in a mouse model. We lowered choline availability in the maternal diet during pregnancy and assessed proliferative and differentiation properties of retinal progenitor cells (RPCs) in the developing prenatal retina, as well as visual function in adult offspring. We report that low choline availability during retinogenesis leads to persistent retinal cytoarchitectural defects, ranging from focal lesions with displacement of retinal neurons into subretinal space to severe hypocellularity and ultrastructural defects in photoreceptor organization. We further show that low choline availability impairs timely differentiation of retinal neuronal cells, such that the densities of early-born retinal ganglion cells, amacrine and horizontal cells, as well as cone photoreceptor precursors, are reduced in low choline embryonic d 17.5 retinas. Maintenance of higher proportions of RPCs that fail to exit the cell cycle underlies aberrant neuronal differentiation in low choline embryos. Increased RPC cell cycle length, and associated reduction in neurofibromin 2/Merlin protein, an upstream regulator of the Hippo signaling pathway, at least in part, explain aberrant neurogenesis in low choline retinas. Furthermore, we find that animals exposed to low choline diet in utero exhibit a significant degree of intraindividual variation in vision, characterized by marked functional discrepancy between the 2 eyes in individual animals. Together, our findings demonstrate, for the first time, that choline availability plays an essential role in the regulation of temporal progression of retinogenesis and provide evidence for the importance of adequate supply of choline for proper development of the visual system.-Trujillo-Gonzalez, I., Friday, W. B., Munson, C. A., Bachleda, A., Weiss, E. R., Alam, N. M., Sha, W., Zeisel, S. H., Surzenko, N. Low availability of choline in utero disrupts development and function of the retina.

MicroRNA-129-5p is regulated by choline availability and controls EGF receptor synthesis and neurogenesis in the cerebral cortex

Choline availability modulates neurogenesis and cerebral cortex development through the regulation of neural progenitor cell (NPC) proliferative and differentiation capacity. In this study, we demonstrated that cortical NPC self-renewal is controlled by choline via the expression of a microRNA (miR-129-5p), whose role in the developing brain has not been examined, and which, in turn, inhibits synthesis of the epidermal growth factor receptor (EGFR) protein. Specifically, we found that low choline (LC) availability led to the upregulation of miR-129-5p expression in cortical NPCs in vitro and in vivo, causing the downregulation of EGFR and thereby disrupting NPC self-renewal and cortical neurogenesis. Furthermore, in response to LC availability, methylation potential (the S-adenosylmethionine: S-adenosylhomocysteine ratio) in the developing brain was reduced. Restoring methylation potential in LC cortical NPCs led to the re-establishment of normal miR-129-5p expression. We concluded that inhibiting miR-129-5p function and restoring EGFR protein levels in vivo is sufficient to reverse LC-induced defects in cortical NPC self-renewal. For the first time, to our knowledge, we have identified the molecular links that explain how a change in the availability of the diet metabolite choline impacts the essential cellular processes underlying brain development.-Trujillo-Gonzalez, I., Wang, Y., Friday, W. B., Vickers, K. C., Toth, C. L., Molina-Torres, L., Surzenko, N., Zeisel, S. H. MicroRNA-129-5p is regulated by choline availability and controls EGF receptor synthesis and neurogenesis in the cerebral cortex.

A Chimeric Egfr Protein Reporter Mouse Reveals Egfr Localization and Trafficking In Vivo

EGF receptor (EGFR) is a critical signaling node throughout life. However, it has not been possible to directly visualize endogenous Egfr in mice. Using CRISPR/Cas9 genome editing, we appended a fluorescent reporter to the C terminus of the Egfr. Homozygous reporter mice appear normal and EGFR signaling is intact in vitro and in vivo. We detect distinct patterns of Egfr expression in progenitor and differentiated compartments in embryonic and adult mice. Systemic delivery of EGF or amphiregulin results in markedly different patterns of Egfr internalization and trafficking in hepatocytes. In the normal intestine, Egfr localizes to the crypt rather than villus compartment, expression is higher in adjacent epithelium than in intestinal tumors, and following colonic injury expression appears in distinct cell populations in the stroma. This reporter, under control of its endogenous regulatory elements, enables in vivo monitoring of the dynamics of Egfr localization and trafficking in normal and disease states.

Arap1 Deficiency Causes Photoreceptor Degeneration in Mice

Purpose

Small guanosine triphosphatase (GTPase) ADP-ribosylation factors (Arfs) regulate membrane traffic and actin reorganization under the control of GTPase-activating proteins (GAPs). Arap1 is an Arf-directed GAP that inhibits the trafficking of epidermal growth factor receptor (EGFR) to the early endosome, but the diversity of its functions is incompletely understood. The aim of this study was to determine the role of Arap1 in the mammalian retina.

Methods

Genetically engineered Arap1 knockout mice were screened for ocular abnormalities in the National Institutes of Health Knockout Mouse Production and Phenotyping (KOMP2) Project. Arap1 knockout and wild-type eyes were imaged using optical coherence tomography and fundus photography, and analyzed by immunohistochemistry.

Results

Arap1-/- mice develop a normal appearing retina, but undergo photoreceptor degeneration starting at 4 weeks postnatal age. The fundus appearance of mutants is notable for pigmentary changes, optic nerve pallor, vascular attenuation, and outer retinal thinning, reminiscent of retinitis pigmentosa in humans. Immunohistochemical studies suggest the cell death is predominantly in the outer nuclear layer. Functional evaluation of the retina by electroretinography reveals amplitudes are reduced. Arap1 is detected most notably in Müller glia, and not in photoreceptors, implicating a role for Müller glia in photoreceptor survival.

Conclusions

Arap1 is necessary for normal photoreceptor survival in mice, and may be a novel gene relevant to human retinal degenerative processes, although its mechanism is unknown. Further studies in this mouse model of retinal degeneration will give insights into the cellular functions and signaling pathways in which Arap1 participates.

The guanine nucleotide exchange factor Vav3 regulates differentiation of progenitor cells in the developing mouse retina

The seven main cell types in the mammalian retina arise from multipotent retinal progenitor cells, a process that is tightly regulated by intrinsic and extrinsic signals. However, the molecular mechanisms that control proliferation, differentiation and cell-fate decisions of retinal progenitor cells are not fully understood yet. Here, we report that the guanine nucleotide exchange factor Vav3, a regulator of Rho-GTPases, is involved in retinal development. We demonstrate that Vav3 is expressed in the mouse retina during the embryonic period. In order to study the role of Vav3 in the developing retina, we generate Vav3-deficient mice. The loss of Vav3 results in an accelerated differentiation of retinal ganglion cells and cone photoreceptors during early and late embryonic development. We provide evidence that more retinal progenitor cells express the late progenitor marker Sox9 in Vav3-deficient mice than in wild-types. This premature differentiation is compensated during the postnatal period and late-born cell types such as bipolar cells and Müller glia display normal numbers. Taken together, our data imply that Vav3 is a regulator of retinal progenitor cell differentiation, thus highlighting a novel role for guanine nucleotide exchange factors in retinogenesis.

Rostral-caudal distribution of Emx1-lineage stem/transit amplifying cells and lineage progression in embryonic cortex depend on Hedgehog signaling

Lineage progression of neural precursors to an EGF-responsive state can be promoted by several extrinsic signals, including fibroblast growth factor 2 (FGF2) and Hedgehog (Hh). It has been suggested that EGF-responsive precursors in the embryonic cerebral cortex originate in the ventral telencephalon in an FGF-dependent manner and migrate dorsally. To determine whether cortical EGF-responsive cells originate locally from dorsal precursors, we marked these precursors using Emx1-cre and the cre reporter Z/EG and observed a local origin for EGF-responsive cells. We also found a rostral-caudal difference in the abundance of self-renewing, neurogenic Emx1-lineage precursors, with more present rostrally. Deleting the Hh receptor smoothened in Emx-1 lineage cells impaired their progression to an EGF-responsive state. Moreover, loss of smoothened increased the proportion of neurogenic, self-renewing Emx1-lineage cells in caudal regions of cortex, eliminating their asymmetric distribution. Our results support the idea that Hh signaling promotes lineage progression of stem/transit amplifying cells, particularly in caudal regions of the embryonic cortex, leading to rostral-caudal differences in the abundance of neurogenic, self-renewing precursors.

[Instability of cell phenotype and tumor initiating cells in gliomas]

Gliomas, the most frequent primitive CNS tumors, have been suggested to originate from astrocytes or from neural progenitors/stem cells. However, the precise identity of the cells at the origin of gliomas remains a matter of debate because no pre-neoplastic state has been yet identified. TGFα, an EGF family member, is frequently over-expressed in the early stages of glioma progression. We questioned whether prolonged TGFα exposure affects the stability of the normal mature astrocyte phenotype and, eventually, their propensity to cancerous transformation. Using mouse astrocyte cultures devoid of residual neural stem cells or progenitors, we demonstrate that several days of TGFα-treatment result in the functional conversion of a population of mature astrocytes into radial glial cells, a population of neural progenitors, without any accompanying sign of cancerous transformation. In contrast, when astrocytes de-differentiated with TGFα were submitted to oncogenic stress using gamma irradiation, they acquired cancerous properties, forming high-grade glioma-like tumors after brain grafting. Gamma irradiation was without effect on astrocytes which were not treated with TGFα. These results suggested that most gliomas should contain tumor cells with stem-like properties (TSCs). Our study of 55 pediatric brain tumors show that tumor cells with stem cell-like or progenitor-like properties can be isolated from a majority of gliomas. Survival analysis showed an association between isolation of TSCs with extended self-renewal capabilities and a patient's higher mortality rate.

The effect of epidermal growth factor in the injured brain after trauma in rats

Epidermal growth factor (EGF) is a known mitogen for neural stem and progenitor cells (NS/NPCs) in the central nervous system (CNS). In vitro, EGF maintains NS/NPCs in the proliferative state, whereas in the normal rodent brain it promotes their proliferation and migration in the subventricular zone (SVZ). Additionally, EGF administration can augment neuronal replacement in the ischemic-injured adult striatum. Recently we found that the SVZ and the hippocampus display an injury-induced proliferative response following traumatic brain injury (TBI) that is linked to increased EGF expression. As adult neurogenesis is associated with cognitive function, we hypothesized that post-TBI administration of EGF could affect neurogenesis and cognitive recovery. Adult rats were intraventricularly infused with EGF or vehicle for 7 days following TBI. 5-Bromo-2-deoxyuridine (BrdU) was administered to label proliferating cells and the animals were sacrificed at 1 or 4 weeks post-injury. Using immunohistochemistry and stereology, we found that at 1 week post-injury, compared to vehicle-infused animals EGF-infused animals had significantly more BrdU-positive cells in the SVZ and hippocampus concomitant with enhanced EGF receptor expression. At 4 weeks post-injury, the number of BrdU-positive cells in the hippocampus was similar in both groups, suggesting that EGF does not support long-term survival of newly generated cells. Furthermore, we found that the EGF-induced proliferative population differentiated preferentially toward astroglial phenotype. Nevertheless, animals treated with EGF showed significant improvement in cognitive function, which was accompanied by reduced hippocampal neuronal cell loss. Collectively, the data from this study demonstrate that EGF exerts a neuroprotective rather than neurogenic effect in protecting the brain from injury.

Off-target effects of epidermal growth factor receptor antagonists mediate retinal ganglion cell disinhibited axon growth

Inhibition of central nervous system axon growth is reportedly mediated in part by calcium-dependent phosphorylation of axonal epidermal growth factor receptor, with local administration of the epidermal growth factor receptor kinase inhibitors AG1478 and PD168393 to an optic nerve lesion site promoting adult retinal ganglion cell axon regeneration. Here, we show that epidermal growth factor receptor was neither constitutively expressed, nor activated in optic nerve axons in our non-regenerating and regenerating optic nerve injury models, a finding that is inconsistent with phosphorylated epidermal growth factor receptor-dependent intra-axonal signalling of central nervous system myelin-related axon growth inhibitory ligands. However, epidermal growth factor receptor was localized and activated within most glia in the retina and optic nerve post-injury, and thus an indirect glial-dependent mechanism for stimulated retinal ganglion cell axon growth by epidermal growth factor receptor inhibitors seemed plausible. Using primary retinal cultures with added central nervous system myelin extracts, we confirmed previous reports that AG1478/PD168393 blocks epidermal growth factor receptor activation and promotes disinhibited neurite outgrowth. Paradoxically, neurites did not grow in central nervous system myelin extract-containing cultures after short interfering ribonucleic acid-mediated knockdown of epidermal growth factor receptor. However, addition of AG1478 restored neurite outgrowth to short interfering ribonucleic acid-treated cultures, implying that epidermal growth factor receptor does not mediate AG1478-dependent effects. TrkA-/B-/C-Fc fusion proteins and the kinase blocker K252a abrogated the neuritogenic activity in these cultures, correlating with the presence of the neurotrophins brain derived neurotrophic factor, nerve growth factor and neurotrophin-3 in the supernatant and increased intracellular cyclic adenosine monophosphate activity. Neurotrophins released by AG1478 stimulated disinhibited retinal ganglion cell axon growth in central nervous system myelin-treated cultures by the induction of regulated intramembraneous proteolysis of p75(NTR) and Rho inactivation. Retinal astrocytes/Müller cells and retinal ganglion cells were the source of neurotrophins, with neurite outgrowth halved in the presence of glial inhibitors. We attribute AG1478-stimulated neuritogenesis to the induced release of neurotrophins together with raised cyclic adenosine monophosphate levels in treated cultures, leading to axon growth and disinhibition by neurotrophin-induced regulated intramembraneous proteolysis of p75(NTR). These off-target effects of epidermal growth factor receptor kinase inhibition suggest a novel therapeutic approach for designing treatments to promote central nervous system axon regeneration.

Current Prospects for Adult Stem Cell–Based Therapies in Ocular Repair and Regeneration

Recent advances in stem cell biology have led to the exploration of stem cell-based therapies to treat a wide range of human diseases. In the ophthalmic field, much hope has been placed on the potential use of these cells to restore sight, particularly in those conditions in which other established treatments have failed and in which visual function has been irreversibly damaged by disease or injury. At present, there are many limitations for the immediate use of embryonic stem cells to treat ocular disease, and as more evidence emerges that adult stem cells are present in the adult human eye, it is clear that these cells may have advantages to develop into feasible therapeutic treatments without the problems associated with embryonic research and immune rejection. Here we discuss the current prospects for the application of various adult ocular stem cells to human therapies for restoration of vision.

Development and neurogenic potential of Müller glial cells in the vertebrate retina

Considerable research on normal and diseased states within the retina has focused on neurons. Recent research on glia throughout the central nervous system, including within the retina where Müller glia are the main type of glia, has provided a more in depth view of glial functions in health and disease. Glial cells have been recognized as being vital for the maintenance of a healthy tissue environment, where they actively participate in neuronal activity. More recently, Müller glia have been recognized as being very similar to retinal progenitor cells, particularly when compared at the molecular level using comprehensive expression profiling techniques. The molecular similarities, as well as the developmental events that occur at the end of the genesis period of retinal cells, have led us to propose that Müller glia are a form of late stage retinal progenitor cells. These late stage progenitor cells acquire some specialized glial functions, but do not irreversibly leave the progenitor state. Indeed, Müller glia appear to be able to behave as a progenitor in that they have been shown to proliferate and produce neurons in several instances when an acute injury has been applied to the retina. Enhancement of this response is thus an exciting strategy for retinal repair.

The group E Sox genes Sox8 and Sox9 are regulated by Notch signaling and are required for Müller glial cell development in mouse retina

Although Müller glial cells play pivotal roles in the vertebrate retina, the regulation of their development is poorly understood. While Notch-Hes5 signaling has been shown to be involved in this developmental process, the presence of Müller glial cells in Hes5-deficient mice suggests the involvement of other molecules. We found that two group E Sox genes, Sox8 and Sox9, are expressed in proliferating progenitors and then exclusively in Müller glial cells in mouse retina. Knocking-down Sox8 and Sox9 by shRNA significantly reduced the population of Müller glial cells and relatively increased the proportion of rod photoreceptors, suggesting that the Sox genes play roles in the specification of Müller glial cells. Using an activated form of Notch and the gamma-secretase inhibitor DAPT, we also found that Notch signaling regulates the transcription of Sox8 and Sox9. This is the first evidence that group E Sox genes play important roles in the developing vertebrate retina.

Nf1 mutation expands an EGFR-dependent peripheral nerve progenitor that confers neurofibroma tumorigenic potential

Defining growth factor requirements for progenitors facilitates their characterization and amplification. We characterize a peripheral nervous system embryonic dorsal root ganglion progenitor population using in vitro clonal sphere-formation assays. Cells differentiate into glial cells, smooth muscle/fibroblast (SM/Fb)-like cells, and neurons. Genetic and pharmacologic tools revealed that sphere formation requires signaling from the EGFR tyrosine kinase. Nf1 loss of function amplifies this progenitor pool, which becomes hypersensitive to growth factors and confers tumorigenesis. DhhCre;Nf1(fl/fl) mouse neurofibromas contain a progenitor population with similar growth requirements, potential, and marker expression. In humans, NF1 mutation predisposes to benign neurofibromas, incurable peripheral nerve tumors. Prospective identification of human EGFR(+);P75(+) neurofibroma cells enriched EGF-dependent sphere-forming cells. Neurofibroma spheres contain glial-like progenitors that differentiate into neurons and SM/Fb-like cells in vitro and form benign neurofibroma-like lesions in nude mice. We suggest that expansion of an EGFR-expressing early glial progenitor contributes to neurofibroma formation.

Constitutive EGFR signaling in oligodendrocyte progenitors leads to diffuse hyperplasia in postnatal white matter

Gliogenesis requires the careful orchestration of migration, differentiation, and proliferation of progenitors. Signaling through the epidermal growth factor receptor (EGFR) has been implicated in regulating these processes in a variety of cell types, including neural progenitors. By retroviral infection, we constitutively expressed an EGFR-GFP fusion protein in white matter glial progenitors at postnatal day 3 of the rat forebrain in vivo and analyzed the development of these cells over the subsequent 15 weeks. EGFR-GFP+ cells remained proliferative and migratory, gradually populating the brains ipsilateral and contralateral to the side of viral infection, but never differentiated into mature glia. The accumulation of these cells doubled the total cell density in white matter and led to a 10-fold increase in the abundance of glial progenitors, giving rise to a progenitor "hyperplasia." The marker profile of infected cells, NG2+, olig2+, PDGFR-alpha+, nestin+, GFAP-, and CC1-, indicated a close resemblance to oligodendrocyte progenitors. Positive immunostaining for phosphorylated EGFR colocalized with punctate accumulation of EGFR-GFP, indicating that a subset of receptors was engaged in active signaling. Furthermore, EGF was required to observe phospho-tyrosine EGFR immunostaining of glial progenitors in culture. These observations suggest that constitutive EGFR expression can inhibit glial differentiation, but requires ligand as well.

Revisiting nestin expression in retinal progenitor cells in vitro and after transplantation in vivo

The purpose of this study is to characterize the co-expression of nestin--a neuroectodermal stem cell and a reactive glial marker-with various mature retinal cell markers in retinal progenitor cells (RPCs) expanded in vitro, followed either by in vitro induction or subretinal transplantation. Rat RPCs derived from embryonic day (E) 17 rat retina were expanded in serum free defined culture, and induced to differentiate by all-trans retinoic acid (RA). Following induction, cells were stained for nestin in combination with retinal neuronal and glial markers. Cultured cells were collected for quantitative RT-PCR gene expression analysis prior to and after induction. In a second series, passage 2 RPCs were transplanted into the subretinal space of S334ter-3 retinal degeneration rats at postnatal day 28. After 1-4 weeks, sections through the transplant were double immunostained for nestin and various retinal specific neuronal markers. The cultured RPCs treated with RA exhibited nestin co-expression with various retinal specific markers, including protein kinase C alpha (PKC), neurofilament 200 (NF200), cellular retinaldehyde binding protein (CRALBP), and rhodopsin. Following RA induction, quantitative RT-PCR analysis demonstrated downregulation of nestin, PAX-6, thy1.1, and PKCalpha, and upregulation of rhodopsin, glial fibrillary acidic protein (GFAP), and CrX. No nestin coexpression was observed with any of the retinal specific neuronal markers in RPC transplants in vivo except for some nestin-immunoreactivity overlapping with GFAP positive cells in the host retina. The role of nestin as a unique neural stem/progenitor cell marker should be reconsidered. Nestin expression during RPC maturation appears to be different in vitro versus in vivo.

VEGF activates divergent intracellular signaling components to regulate retinal progenitor cell proliferation and neuronal differentiation

During vertebrate neurogenesis, multiple extracellular signals influence progenitor cell fate choices. The process by which uncommitted progenitor cells interpret and integrate signals is not well understood. We demonstrate here that in the avascular chicken retina, vascular endothelial growth factor (VEGF) secreted by postmitotic neurons acts through the FLK1 receptor present on progenitor cells to influence cell proliferation and commitment. Augmenting VEGF signals increases progenitor cell proliferation and decreases retinal ganglion cell genesis. Conversely, absorbing endogenous VEGF ligand or disrupting FLK1 activity attenuates cell proliferation and enhances retinal ganglion cell production. In addition, we provide evidence that VEGF signals transmitted by the FLK1 receptor activate divergent intracellular signaling components, which regulate different responses of progenitor cells. VEGF-induced proliferation is influenced by the MEK-ERK pathway, as well as by the basic helix-loop-helix factor HES1. By contrast, VEGF-dependent ganglion cell suppression does not require MEK-ERK activation, but instead relies on VEGF-stimulated HES1 activity, which is independent of NOTCH signaling. Moreover, elevated HES1 expression promotes progenitor cell proliferation and prevents overproduction of retinal ganglion cells owing to the loss of VEGF or sonic hedgehog (SHH), another signal that suppresses ganglion cell development. Based on previous and current findings, we propose that HES1 serves as a convergent signaling node within early retinal progenitor cells to integrate various cell-extrinsic cues, including VEGF and SHH, in order to control cell proliferation and neuronal specification.

Epidermal Growth Factor Is a Neuronal Differentiation Factor for Retinal Stem Cells In Vitro

Stem cells are a tool for in vitro elucidation of the putative role of factors on cell fate. Herein we analyze the role of epidermal growth factor (EGF) on progeny derived from retinal stem cells (RSCs). We isolated cells from neuroretinas of neonate mice. All the proliferating cells harbored the radial glia marker RC2, expressed transcription factors usually found in radial glia (Mash1, Pax6), and met the criteria of stem cells: high capacity of expansion, maintenance of an undifferentiated state, and multipotency demonstrated by clonal analysis. We analyzed the differentiation 7 days after transfer of the cells in different culture media. In absence of serum, EGF led to the expression of the neuronal marker beta-tubulin-III and acquisition of neuronal morphology in 15% of the cells. Analysis of cell proliferation by bromodeoxyuridine incorporation revealed that EGF mainly induced the formation of neurons without stimulating cell cycle progression. Moreover, a pulse of 2-hour EGF stimulation was sufficient to induce neuronal differentiation. Some neurons were committed to the retinal ganglion cell (RGC) phenotype, as revealed by the expression of retinal ganglion markers (Ath5, Brn3b, and melanopsin) and in a few cases to other retinal phenotypes (photoreceptors [PRs] and bipolar cells). We confirmed that the late RSCs were not restricted over time and that they conserved their multipotency by generating retinal phenotypes that usually appear at early (RGC) or late (PRs) developmental stages. Our results show that EGF is not only a factor controlling glial development, as previously shown, but also a potent differentiation factor for retinal neurons, at least in vitro.

glial cells missing and gcm2 cell autonomously regulate both glial and neuronal development in the visual system of Drosophila

The transcription factors Glial cells missing (Gcm) and Gcm2 are known to play a crucial role in promoting glial-cell differentiation during Drosophila embryogenesis. Our findings reveal a central function for gcm genes in regulating neuronal development in the postembryonic visual system. We demonstrate that Gcm and Gcm2 are expressed in both glial and neuronal precursors within the optic lobe. Removal of gcm and gcm2 function shows that the two genes act redundantly and are required for the formation of a subset of glial cells. They also cell-autonomously control the differentiation and proliferation of specific neurons. We show that the transcriptional regulator Dachshund acts downstream of gcm genes and is required to make lamina precursor cells and lamina neurons competent for neuronal differentiation through regulation of epidermal growth factor receptor levels. Our findings further suggest that gcm genes regulate neurogenesis through collaboration with the Hedgehog-signaling pathway.

Epidermal growth factor receptor expression regulates proliferation in the postnatal rat retina

Epidermal growth factor (EGF) is known to promote proliferation of both retinal progenitors and Muller glia in vitro, but several questions remain concerning an in vivo role for this factor. In this study, we investigated whether the EGF receptor (EGFR) is necessary for the maintenance of normal levels of progenitor and Muller glial proliferation in vivo. Here, we show that (1) mice with homozygous deletion of the Egfr gene have reduced proliferation in late stages of retinal histogenesis, (2) EGF is mitogenic for Müller glia in vivo during the first two postnatal weeks in the rodent retina, (3) the effectiveness of EGF as a Müller glial mitogen declines in parallel with the decline in EGFR expression as the retina matures, and (4) following damage to the retina from continuous light exposure, EGFR expression is up-regulated in Müller glia to levels close to those in the neonatal retina, resulting in a renewed mitotic response to EGF. Together with previous results from other studies, these data indicate that the downregulation of a growth factor receptor is one mechanism by which glial cells maintain mitotic quiescence in the mature nervous system.

Cytokine-induced activation of signal transducer and activator of transcription in photoreceptor precursors regulates rod differentiation in the developing mouse retina

Ciliary neurotrophic factor (CNTF) exhibits multiple biological effects during vertebrate retinogenesis, including regulation of photoreceptor cell differentiation. In the early postnatal mouse retina, CNTF induces rapid and transient phosphorylation of signal transducer and activator of transcription (STAT) 1 and STAT3 and the extracellular signal-regulated kinase (ERK). Although both proliferating progenitor cells and postmitotic neurons respond directly to cytokine signals, CNTF elicits distinct phosphorylation patterns of STAT3 and ERK. CNTF stimulation induces low levels of STAT3 phosphorylation in progenitors and differentiated neurons but a robust STAT3 activation among postmitotic photoreceptor precursors expressing the cone-rod homeobox gene Crx and newly differentiated rod photoreceptors. In contrast, CNTF causes preferential phosphorylation of ERK in progenitor cells and photoreceptor precursors. Inhibition of the cytokine receptor gp130 using neutralizing antibodies reveals that gp130 is required for both CNTF-induced STAT3 and ERK phosphorylation. Perturbation of STAT signaling by a STAT inhibitor peptide or a dominant-negative STAT3 mutant causes enhanced production of rod photoreceptors in the absence of exogenous cytokines, whereas inhibiting ERK activation by a MEK (mitogen-activated protein kinase kinase)-specific inhibitor has no effect on rod photoreceptor differentiation in vitro. Furthermore, disrupting the function of epidermal growth factor (EGF) receptors, which modulate rod development in vivo, indicates that the EGF family of ligands does not mediate the inhibitory effect of cytokine on rod differentiation. These results demonstrate that cytokine signal transduction is dynamic and heterogeneous in the developing retina, and that endogenous ligand-induced STAT activation in retinal progenitor and/or photoreceptor precursor cells plays an important role in regulating photoreceptor development.

Developmental profile of ErbB receptors in murine central nervous system: Implications for functional interactions

The ErbB family, ErbB1 (also known as the epidermal growth factor receptor EGFR), ErbB2, ErbB3, and ErbB4 comprise a group of receptor tyrosine kinases that interact with ligands from the epidermal growth factor (EGF) superfamily, subsequently dimerize, catalytically activate each other by cross-phosphorylation, and then stimulate various signaling pathways. To gain a better understanding of in vivo functions of ErbB receptors in the central nervous system, the current study examined their mRNA expression throughout development in the mouse brain via in situ hybridization. EGFR, ErbB2, and ErbB4 exhibited distinct but sometimes overlapping distributions in multiple cell types within germinal zones, cortex, striatum, and hippocampus in prenatal and postnatal development. In addition, a subpopulation of cells positive for ErbB4 mRNA in postnatal cortex and striatum coexpressed mRNA for either EGFR or GAD67, a marker for gamma-aminobutyric acid (GABA)ergic interneurons, suggesting that both ErbB4 and EGFR are coexpressed in GABAergic interneurons. In contrast, ErbB3 mRNA was not detected within the brain during development and only appeared in white matter tracts in adulthood. Together, these findings suggest that ErbB receptors might mediate multiple functions in central nervous system development, some of which may be initiated by EGFR/ErbB4 heterodimers in vivo.

Cellular and molecular characterization of early and late retinal stem cells/progenitors: Differential regulation of proliferation and context dependent role of Notch signaling

Retinal stem cells/progenitors that define the evolutionarily conserved early and late stages of retinal histogenesis are known to have distinct competence to give rise to stage-specific retinal cell types. However, the information regarding their innate proliferative behavior and phenotypic potential in terms of generating neurons and glia is lacking. Here we demonstrate that, like their counterparts in other central nervous system (CNS) regions during early and late stages of embryonic development, the early and late retinal stem cells/progenitors display different proliferative response to fibroblast growth factor 2 (FGF2) and epidermal growth factor (EGF) and bias towards generating neurons or glia. Although the former predominantly generate neurons, the latter are partial towards giving rise to glia. Transcription profiling identified classes of genes that are differentially expressed in early and late retinal stem cells/progenitors in proliferating conditions and suggested that the distinct proliferative response to FGF2 and EGF is likely due to differential expression of FGF receptor 1 (FGFR1) and EGF receptor (EGFR). However, the proliferative maintenance of retinal stem cells/progenitors is likely to include other signaling pathways such as those mediated by insulin-like growth factors (IGFs) and stem cell factor (SCF). Transcription profiling of early and late retinal stem cells/progenitors in proliferating and differentiating conditions suggested a context dependent role for Notch signaling, which may constitute one of the mechanisms underlying the stage-dependent phenotypic potential of retinal stem cells/progenitors.

Activation of epidermal growth factor receptors directs astrocytes to organize in a network surrounding axons in the developing rat optic nerve

In postnatal developing optic nerves, astrocytes organize their processes in a cribriform network to group axons into bundles. In neonatal rat optic nerves in vivo, the active form of EGFR tyrosine kinase is abundantly present when the organization of astrocytes and axons is most actively occurring. Blocking activity of EGFR tyrosine kinase during the development of rat optic nerves in vivo inhibits astrocytes from extending fine processes to surround axons. In vitro, postnatal optic nerve astrocytes, stimulated by EGF, organize into cribriform structures which look remarkably like the in vivo structure of astrocytes in the optic nerve. In addition, when astrocytes are co-cultured with neonatal rat retinal explants in the presence of EGF, astrocytes that are adjacent to the retinal explants, re-organize to an astrocyte-free zone into which neurites grow out from the retinal tissue. We hypothesize that in the developing optic nerve, EGFR activity directs the formation of a histo-architectural structure of astrocytes which surrounds axons and provides a permissive environment for axon development.

Alternative culture conditions for isolation and expansion of retinal progenitor cells

PURPOSE

To investigate different in vitro model systems for retinal progenitor cell (RPC) isolation and expansion.

METHODS

RPCs were isolated from embryonic day (E) 17 Long Evans rat retinas. Three different culture media: (1) modified serum free defined media (2) serum-containing media and (3) embryonic stem cell (ES)-conditioned media were used for RPC isolation and long term expansion. Expression of various cellular markers and cell morphologies were compared among the three culture systems at different passages by immunostaining and confocal microscopy.

RESULTS

All three culture systems could maintain RPCs as nestin-positive cells (78-87%) after long-term in vitro expansion. However, RPCs appeared to proliferate faster in the serum-free culture system. The ES-conditioned media provided the best RPC survival. Cells appeared smaller at early passages compared with later passages. This morphology change occurred at P9-P10 in the serum-free medium, and at P5-P6 in the other two culture systems.

CONCLUSIONS

The serum-free medium may be superior for preventing RPC differentiation during expansion.

Antagonistic relationship between Dpp and EGFR signaling in Drosophila head patterning

The Drosophila eye field that gives rise to the visual system and dorsal head epidermis forms an unpaired anlage located in the dorsal head ectoderm. The eye field expresses and requires both Dpp and EGFR signaling for its development. As shown in previous studies, EGFR is required for cell maintenance in the developing visual system. Dpp initially switches on the early eye genes so and eya in the eye field. Consecutively, high levels of Dpp in the dorsal midline inhibit these genes and promote development of head epidermis. We show that Dpp negatively regulates EGFR signaling, thereby increasing the amount of cell death in the dorsal midline. By this mechanism, Dpp controls the formation of a bilateral visual system and indirectly modulates cell death, which is essential for normal head morphogenesis. Loss of either Dpp or its downstream target, Zen, abolishes head epidermis fate and leads to the misexpression of dp-ERK in the dorsal midline. The resulting morphological phenotype consists of cyclopia, reduction of cell death, and failure of head involution. Ectopic expression of activated EGFR inhibits the Dpp target race and thereby causes cyclopia and defective head involution. We discuss possible mechanisms of Dpp and EGFR interaction in the embryo.

Making and repairing the mammalian brain--signaling toward neurogenesis and gliogenesis

Neural stem cells (NSCs) are subscribed extraordinary potential in repair of the damaged nervous system. However, the molecular identity of NSCs has not been established. Most NSC cultures contain large numbers of multipotent, bipotent, and lineage restricted neural progenitors, the majority of which appear to lose neurogenic potential after expansion. This review first discusses the neurogenic to gliogenic switch that is characteristic of progenitor development in vivo and in NSC cultures, and then the cell intrinsic and extrinsic mechanisms regulating the sequential differentiation of neurons and glia. Finally, we discuss potential methods for maintaining the neurogenic potential of NSC cultures after expansion.

EGF amplifies the replacement of parvalbumin-expressing striatal interneurons after ischemia

EGF promotes proliferation and migration of stem/progenitor cells in the normal adult brain. The effect of epidermal growth factor on neurogenesis in ischemic brain is unknown, however. Here we show that intraventricular administration of EGF and albumin augments 100-fold neuronal replacement in the injured adult mouse striatum after cerebral ischemia. Newly born immature neurons migrate into the ischemic lesion and differentiate into mature parvalbumin-expressing neurons, replacing more than 20% of the interneurons lost by 13 weeks after ischemia and representing 2% of the total BrdU-labeled cells. These data suggest that administration of EGF and albumin could be used to manipulate endogenous neurogenesis in the injured brain and to promote brain self-repair.

Xrx1 controls proliferation and multipotency of retinal progenitors

We investigated the function of Xrx1 during Xenopus retinogenesis. Xrx1 overexpression lengthens mitotic activity and ectopically activates the expression of markers of undifferentiated progenitors in the developing retina. We assayed Xrx1 ability to support proliferation with a cell-autonomous mechanism by in vivo lipofection of single retinal progenitors. Xrx1 overexpression increases clonal proliferation while Xrx1 functional inactivation exerts the opposite effect. We also compared the effects of Xrx1 with those of the cyclin-dependent kinase cdk2, a strong mitotic promoter. Despite the similar increase in clonal proliferation displayed by both factors, Xrx1 and cdk2 act differently on retinal cell fate determination. cdk2/cyclinA2 lipofected retinas show a decrease in early-born cell types as ganglion cells and cones and an increase in late-born types such as bipolar neurons. On the contrary, Xrx1 lipofected retinas show no changes in the proportions of the different cell types, thus suggesting a role in supporting multipotency of retinal progenitors.

The role of tangential dispersion in retinal mosaic formation

Individual types of retinal nerve cell are spaced across the retina in an orderly manner, ensuring a uniform sampling of the visual field. This regularity in cellular spacing has been commonly attributed to fate determination mechanisms operating around the time of cell birth, an hypothesis presuming that the position of a nerve cell is fixed within the plane of the retina from the time of its determination. At odds with this view, recent results from X-inactivation mosaic mice indicate that certain classes of retinal nerve cell, those known to form orderly mosaics in the adult retina, disperse tangentially during development. Furthermore, studies defining the spatial characteristics of developing and mature retinal mosaics suggest that cell-cell interactions around the time of morphological differentiation lead to mutual repulsion. Modelling studies in turn show that nothing more than a simple minimal spacing rule between neighboring cells of the same type is sufficient for the creation of the global patterning observed in biological retinal mosaics. For some cell types, the size of this "exclusion zone" surrounding individual cells is shown to be an intrinsic characteristic of each cell type, invariant across the retina, and accounting for the variation in mosaic regularity across changes in cell density. These results show how short-distance movements driven by intercellular interactions at the local level may mediate the emergence of the global patterning characteristic of retinal mosaics during development.

Generating neuronal diversity in the retina: one for nearly all

Visual perception of our environment essentially depends on the correct assembly of seven principal cell types into the functional architecture of the neuroretina. During retinogenesis these cell types derive from a common population of multipotent retinal progenitor cells (RPCs) residing in the inner layer of the optic cup. In contrast to other well studied regions of the developing CNS, retinal cell diversification is apparently not achieved by spatial prepatterning into distinct progenitor domains, but rather by the sequential production of cell types in a defined histogenetic order. Several lines of evidence suggest that this observation reflects substantial intrinsic changes in the retinogenic potential of RPCs. Recent advances, however, point at the existence of a common molecular framework underlying the retinogenic potential of RPCs throughout retinal neurogenesis.

Pax6 lights-up the way for eye development

Recent reports have exposed the temporal and spatial functions of the transcription factor Pax6 in the developing vertebrate eye. Pax6 is demonstrated to play essential roles in successive steps triggering lens differentiation while in the retina it functions to maintain multipotency and proliferation of retinal progenitor cells. These findings, together with the identification of Pax6 protein partners and downstream targets, pave the way for future work aimed to understand the molecular mechanism of eye development.

Cellular competence plays a role in photoreceptor differentiation in the developing Xenopus retina

Factors in the environment appear to be responsible for inducing many of the cell fates in the retina, including, for example, photoreceptors. Further, there is a conserved order of histogenesis in the vertebrate retina, suggesting that a temporal mechanism interacts in the control of cellular determination. The temporal mechanism involved could result from different inducing signals being released at different times. Alternatively, the inducing signals might be present at many stages, but an autonomous clock could regulate the competence of cells to respond to them. To differentiate between these mechanisms, cells from young embryonic retinas were dissociated and grown together with those from older embryos, and the timing of photoreceptor determination assayed. Young cells appeared uninfluenced by older cells, expressing photoreceptor markers on the same time schedule as when cultured alone. A similar result was obtained when the heterochronic mixing was done in vivo by grafting a small plug of optic vesicle from younger embryos into older hosts. Even the graft cells at the immediate margin of the transplant failed to express photoreceptor markers earlier than normal, despite their being in contact with older, strongly expressing host cells. We conclude that retinal progenitors intrinsically acquire the ability to respond to photoreceptor-inducing cues by a mechanism that runs on a cell autonomous schedule, and that the conserved order of histogenesis is based in part on this competence clock.

Identification of two distinct types of multipotent neural precursors that appear sequentially during CNS development

Epidermal growth factor (EGF) and fibroblast growth factor (FGF)-2 control neural stem cell proliferation in vitro and the formation of neurospheres. Neurospheres contain precursors that respond to both EGF and FGF-2 (E/F cells). E/F cells appear to originate from cells that initially respond to FGF-2 only but undergo a transition in growth factor responsiveness during in vitro culturing. It is unclear whether a similar change in growth factor responsiveness of multipotent precursors takes place in vivo and how this may affect neural precursor properties. Here I provide evidence that FGF-2-responsive precursors and E/F cells appear sequentially during CNS development. This transition from the early precursors (FGF-2-responsive cells) to the late precursors (E/F cells) takes place between E14 and E18. The two types of precursors are morphologically and antigenically distinct. E/F cells are very large and show strong nestin immunoreactivity. Thus the putative neurosphere-forming E/F cells are present in vivo and their generation is developmentally programmed. Their unique morphology may provide a basis for their isolation.

Cell cycle and cell fate in the nervous system

Recently, a number of molecules originally thought to have a primary role in cell determination have been shown to affect the cell cycle at specific check points, while other molecules discovered for their roles in the cell cycle progression are known to affect the determination and differentiation of neurons. These discoveries have led to a more detailed investigation of the complex molecular machinery that co-ordinates proliferation and differentiation.

Becoming glial in the neural retina

During development of the vertebrate neural retina, multipotent stem cells give rise to retinal neurons as well as to Müller cells, the principal glial population in the retina. Recent studies have shed light upon the extracellular and intracellular signaling pathways that regulate Müller glial cell genesis. Emerging evidence demonstrates that activation of the Notch signaling pathway can play a role in regulating Müller cell development as well as gliogenesis in other parts of the central nervous system. Cyclin dependent kinase (CDK) inhibitors of the Cip/Kip subfamily are cell cycle regulators that can regulate progenitor proliferation during retinal development, but also regulate the proliferation of Müller glia when they become activated in response to stress or injury. Surprisingly this class of proteins can also promote the development of Müller glia. In this review we discuss the role of both Notch and the CDK inhibitors in regulating Müller cell development.

In vivo induction of massive proliferation, directed migration, and differentiation of neural cells in the adult mammalian brain

The development of an in vivo procedure for the induction of massive proliferation, directed migration, and neurodifferentiation (PMD) in the damaged adult central nervous system would hold promise for the treatment of human neurodegenerative disorders such as Parkinson's disease. We investigated the in vivo induction of PMD in the forebrain of the adult rat by using a combination of 6-hydroxydopamine lesion of the substantia nigra dopaminergic neurons and infusions of transforming growth factor alpha (TGFalpha) into forebrain structures. Only in animals with both lesion and infusion of TGFalpha was there a rapid proliferation of forebrain stem cells followed by a timed migration of a ridge of neuronal and glial progenitors directed toward the region of the TGFalpha infusion site. Subsequently, increasing numbers of differentiated neurons were observed in the striatum. In behavioral experiments, there was a significant reduction of apomorphine-induced rotations in animals receiving the TGFalpha infusions. These results show that the brain contains stem cells capable of PMD in response to an exogenously administered growth factor. This finding has significant implications with respect to the development of treatments for both acute neural trauma and neurodegenerative diseases.

BMP and FGF regulate the development of EGF-responsive neural progenitor cells

Temporal changes in progenitor cell responses to extrinsic signals play an important role in development, but little is known about the mechanisms that determine how these changes occur. In the rodent CNS, expression of epidermal growth factor receptors (EGFRs) increases during embryonic development, conferring mitotic responsiveness to EGF among multipotent stem cells. Here we show that cell-cell signaling controls this change. Whereas EGF-responsive stem cells develop on schedule in explant and aggregate cultures of embryonic cortex, co-culture with younger cortical cells delays their development. Exogenous BMP4 mimics the effect of younger cells, reversibly inhibiting changes in EGFR expression and responsiveness. Moreover, blocking endogenous BMP receptors in progenitors with a virus transducing dnBMPR1B accelerates changes in EGFR signaling. This involves a non-cell-autonomous mechanism, suggesting that BMP negatively regulates signal(s) that promote the development of EGF-responsive stem cells. FGF2 is a good candidate for such a signal, as we find that it antagonizes the inhibitory effects of younger cortical cells and exogenous BMP4. These findings suggest that a balance between antagonistic extrinsic signals regulates temporal changes in an intrinsic property of neural progenitor cells.

p27(Kip1) regulates cell cycle withdrawal of late multipotent progenitor cells in the mammalian retina

The cyclin-dependent kinase inhibitor protein, p27(Kip1), is necessary for the timing of cell cycle withdrawal that precedes terminal differentiation in oligodendrocytes of the optic nerve. Although p27(Kip1) is widely expressed in the developing central nervous system, it is not known whether this protein has a similar role in neuronal differentiation. To address this issue, we have examined the expression and function of p27(Kip1) in the developing retina, a well-characterized part of the central nervous system. p27(Kip1) is expressed in a pattern coincident with the onset of differentiation of most retinal cell types. In vitro analyses show that p27(Kip1) accumulation in retinal cells correlates with cell cycle withdrawal and differentiation, and when overexpressed, p27(Kip1) inhibits proliferation of the progenitor cells. Furthermore, the histogenesis of photoreceptors and Müller glia is extended in the retina of p27(Kip1)-deficient mice. Finally, we examined the adult retinal dysplasia in p27(Kip1)-deficient mice with cell-type-specific markers. Contrary to previous suggestions that the dysplasia is caused by excess production of photoreceptors, we suggest that the dysplasia is due to the displacement of reactive Müller glia into the layer of photoreceptor outer segments. These results demonstrate that p27(Kip1) is part of the molecular mechanism that controls the decision of multipotent central nervous system progenitors to withdraw from the cell cycle. Second, postmitotic Müller glia have a novel and intrinsic requirement for p27(Kip1) in maintaining their differentiated state.

Regulation of FGF soluble receptor type 1 (SR1) expression and distribution in developing, degenerating, and FGF2-treated retina

The spatial and temporal patterns of expression and content of the fibroblast growth factor (FGF) soluble receptor SR1, a specific inhibitor of FGF, were investigated during embryonic and postnatal development of the retina in Fisher rats. As early as at embryonic day 18 (E18), SR1 mRNA and protein were detected in the retina. SR1 protein was strongly associated with the differentiating ganglion cells and its distribution paralleled the radial pattern of retinal development, from center to periphery. From E18 to postnatal day 5, the levels of both SR1 mRNA and SR1 protein remained constant. Thereafter, they decreased rapidly, by a factor of 5 in the adult retina. SR1 was labeled in the inner nuclear layer, but never in the photoreceptor nuclei. In the neural retina of RCS dystrophic rats, the levels SR1 mRNA and SR1 protein were 2 to 3 times higher than those in the normal congenic controls, before and during photoreceptor degeneration. These results provide the first evidence that a natural FGF inhibitor is regulated during retina development and degeneration and suggest that changes in SR1 content may be involved in the regulation of FGF activities in retina. This was confirmed in vivo in RCS rats, in which delayed photoreceptor apoptosis by intravitreal injection of FGF2 was accompanied by a downregulation of SR1 expression. Dev Dyn 2000;217:24-36.

p27Xic1, a Cdk inhibitor, promotes the determination of glial cells in Xenopus retina

p27Xic1, a member of the Cip/Kip family of Cdk inhibitors, besides its known function of inhibiting cell division, induces Müller glia from retinoblasts. This novel gliogenic function of p27Xic1 is mediated by part of the N-terminal domain near but distinct from the region that inhibits cyclin-dependent kinases. Cotransfections with dominant-negative and constitutively active Delta and Notch constructs indicate that the gliogenic effects of p27Xic1 work within the context of an active Notch pathway. The gradual increase of p27Xic1 in the developing retina thus not only limits the number of retinal cells but also increasingly favors the fate of the last cell type to be born in the retina, the Müller glia.

Neurotrophins and other growth factors in the generation of retinal neurons

The generation of neurons in the vertebrate retina, as in other areas of the developing nervous system, largely depends on extracellular signals. Of the known signaling molecules, neurotrophins play decisive, defined, and distinct roles. The three neurotrophins identified in the chick, namely, neurotrophin-3 (NT-3), brain-derived neurotrophic factor (BDNF), and nerve growth factor (NGF), are expressed in either the pigment epithelium (NT-3 and BDNF) or in the neural retina (NGF) at the onset of neuron birth. In addition, trkC and trkB, receptors for NT-3 and BDNF, respectively, together with p75, the low-affinity neurotrophin receptor, are expressed in the retina at the same developmental period. The role of these three neurotrophins in the differentiation of neurons in the chick retina has been elucidated by a combination of in vitro and in vivo experiments. Thus, NT-3 promotes the conversion of neuroepithelial cells into neurons, whereas BDNF and NGF control the programmed cell death (apoptosis) that affects early postmitotic neuroblasts. BDNF, acting via its trkB receptor, is a survival factor for these cells, whereas NGF, binding to p75 receptor, acts as a killing factor, thereby controlling the provisional number of newly generated neurons.

Expression of receptor tyrosine phosphatases during development of the retinotectal projection of the chick

Receptor tyrosine kinases and receptor protein tyrosine phosphatases (RPTPs) appear to coordinate many aspects of neural development, including axon growth and guidance. Here, we focus on the possible roles of RPTPs in the developing avian retinotectal system. Using both in situ hybridization analysis and immunohistochemistry, we show for the first time that five RPTP genes--CRYPalpha, CRYP-2, PTPmu, PTPgamma, and PTPalpha--have different but overlapping expression patterns throughout the retina and the tectum. PTPalpha is restricted to Muller glia cells and radial glia of the tectum, indicating a possible function in controlling neuronal migration. PTPgamma expression is restricted to amacrine neurons. CRYPalpha and CRYP-2 mRNAs in contrast are expressed throughout the retinal ganglion cell layer from where axons grow out to their tectal targets. PTPmu is expressed in a subset of these ganglion cells. CRYPalpha, CRYP-2, and PTPmu proteins are also localized in growth cones of retinal ganglion cell axons and are present in defined laminae of the tectum. Thus, the spatial and temporal expression of three distinct RPTP subtypes--CRYPalpha, CRYP-2, and PTPmu--are consistent with the possibility of their involvement in axon growth and guidance of the retinotectal projection.

Neural progenitors and stem cells: mechanisms of progenitor heterogeneity

Heterogeneity among progenitor cells in the vertebrate nervous system has been documented with increasing frequency over the past few years. It has become clear that differences in progenitor cells help to determine when and how they respond to environmental signals. More recent studies have begun to elucidate the molecular basis of the differences in progenitor cell subpopulations that control their developmental potential and responsiveness to environmental signals.