Transforming growth factor-beta-3 is mitogenic for rat retinal progenitor cells in vitro

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.

Pleiotropic functions of pituitary adenylyl cyclase-activating polypeptide on retinal ontogenesis: involvement of KLF4 in the control of progenitor cell proliferation

We showed previously that the neuropeptide pituitary adenylyl cyclase-activating polypeptide (PACAP) negatively regulates proliferation of postnatal rat retinal progenitor cells through the downregulation of cyclin D1 in a cAMP/protein kinase A dependent manner. In the present study, we describe by microarray analysis several putative PACAP targets regulated by different transcription factor families. One of these families is the Sp/Klf family of transcriptional factors capable of regulating cyclin D1, and among members, we demonstrate by immunocytochemistry that KLF4 is expressed throughout rat retinal development by retinal progenitor cells and in most differentiated cell types. Using retinal explants preparations, PACAP treatment can transiently increase Klf4 mRNA levels; from electrophoretic mobility shift assays, PACAP is also able to increase the nuclear KLF4 content. From these results, we suggest that KLF4 may be involved in the anti-proliferative effects of PACAP as one mechanism regulating progenitor cell transition from proliferation to differentiation throughout retinal development.

The role of cell cycle in retinal development: cyclin-dependent kinase inhibitors co-ordinate cell-cycle inhibition, cell-fate determination and differentiation in the developing retina

The mature retina is formed through multi-step developmental processes, including eye field specification, optic vesicle evagination, and cell-fate determination. Co-ordination of these developmental events with cell-proliferative activity is essential to achieve formation of proper retinal structure and function. In particular, the molecular and cellular dynamics of the final cell cycle significantly influence the identity that a cell acquires, since cell fate is largely determined at the final cell cycle for the production of postmitotic cells. This review summarizes our current understanding of the cellular mechanisms that underlie the co-ordination of cell-cycle and cell-fate determination, and also describes a molecular role of cyclin-dependent kinase inhibitors (CDKIs) as co-ordinators of cell-cycle arrest, cell-fate determination and differentiation.

Pituitary adenylyl cyclase-activating polypeptide controls the proliferation of retinal progenitor cells through downregulation of cyclin D1

During retinal development, cell proliferation and exit from the cell cycle must be precisely regulated to ensure the generation of the appropriate numbers and proportions of the various retinal cell types. Previously, we showed that pituitary adenylyl cyclase-activating polypeptide (PACAP) exerts a neuroprotective effect in the developing retina of rats, through the cAMP-cAMP-dependent protein kinase (protein kinase A) (PKA) pathway. Here, we show that PACAP also regulates the proliferation of retinal progenitor cells. PACAP, PACAP-specific receptor (PAC1), and the receptors activated by both PACAP and vasoactive intestinal peptide (VIP), VPAC1 and VPAC2, are expressed during embryonic and postnatal development of the rat retina. Treatment of retinal explants with PACAP38 reduced the incorporation of [(3)H]thymidine as well as the number of 5-bromo-2'-deoxyuridine-positive and cyclin D1-positive cells. Pharmacological experiments indicated that PACAP triggers this antiproliferative effect through the activation of both PAC1 and VPACs, and the cAMP-PKA pathway. In addition, PACAP receptor activation decreased both cyclin D1 mRNA and protein content. Altogether, the data support the hypothesis that PACAP is a cell-extrinsic regulator with multiple roles during retinal development, including the regulation of proliferation in a subpopulation of retinal progenitor cells.

AMP-activated protein kinase is involved in neural stem cell growth suppression and cell cycle arrest by 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside and glucose deprivation by down-regulating phospho-retinoblastoma protein and cyclin D

The fate of neural stem cells (NSCs), including their proliferation, differentiation, survival, and death, is regulated by multiple intrinsic signals and the extrinsic environment. We had previously reported that 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR) directly induces astroglial differentiation of NSCs by activation of the Janus kinase (JAK)/Signal transducer and activator of transcription 3 (STAT3) pathway independently of AMP-activated protein kinase (AMPK). Here, we reported the observation that AICAR inhibited NSC proliferation and its underlying mechanism. Analysis of caspase activity and cell cycle showed that AICAR induced G1/G0 cell cycle arrest in NSCs, associated with decreased levels of poly(ADP-ribose) polymerase, phospho-retinoblastoma protein (Rb), and cyclin D but did not cause apoptosis. Iodotubericidin and Compound C, inhibitors of adenosine kinase and AMPK, respectively, or overexpression of a dominant-negative mutant of AMPK, but not JAK inhibitor, were able to reverse the anti-proliferative effect of AICAR. Glucose deprivation also activated the AMPK pathway, induced G0/G1 arrest, and suppressed the proliferation of NSCs, an effect associated with decreased levels of phospho-Rb and cyclin D protein. Furthermore, Compound C and overexpression of dominant-negative AMPK in C17.2 NSCs could block the glucose deprivation-mediated down-regulation of cyclin D and partially reverse the suppression of proliferation. These results suggest that AICAR and glucose deprivation might induce G1/G0 cell cycle arrest and suppress proliferation of NSCs via phospho-Rb and cyclin D down-regulation. AMPK, but not JAK/STAT3, activation is key for this inhibitory effect and may play an important role in the responses of NSCs to metabolic stresses such as glucose deprivation.

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.

Retinal neurons regulate proliferation of postnatal progenitors and Müller glia in the rat retina via TGF beta signaling

The number of proliferating cells in the rodent retina declines dramatically after birth. To determine if extrinsic factors in the retinal micro-environment are responsible for this decline in proliferation, we established cultures of retinal progenitors or Muller glia, and added dissociated retinal neurons from older retinas. The older cells inhibited proliferation of progenitor cells and Muller glia. When these experiments were performed in the presence of TGF(beta)RII-Fc fusion protein, an inhibitor of TGF(beta) signaling, proliferation was restored. This suggests a retina-derived TGF(beta) signal is responsible for the developmental decline in retinal proliferation. TGFbeta receptors I and II are expressed in the retina and are located in nestin-positive progenitors early in development and glast-positive Muller glia later in development. RT-PCR and immunofluorescence data show TGF(beta)2 is the most highly expressed TGF(beta)ligand in the postnatal retina, and it is expressed by inner retinal neurons. Addition of either TGF(beta)1 or TGF(beta)2 to postnatal day 4 retinas significantly inhibited progenitor proliferation, while treatment of explanted postnatal day 6 retinas with TGF(beta) signaling inhibitors resulted in increased proliferation. Last, we tested the effects of TGF(beta) in vivo by injections of TGF(beta) signaling inhibitors: when TGF(beta) signaling is inhibited at postnatal day 5.5, proliferation is increased in the central retina; and when co-injected with EGF at postnatal day 10, TGF(beta)inhibitors stimulate Muller glial proliferation. In sum, these results show that retinal neurons produce a cytostatic TGF(beta) signal that maintains mitotic quiescence in the postnatal rat retina.

Comparison of the Proliferation and Differentiation Ability between Adult Rat Retinal Stem Cells and Cerebral Cortex-Derived Neural Stem Cells

Recent studies have demonstrated that retinal stem cells (RSCs) and stem cells of the central nervous system both exhibited the abilities of self-renewal, proliferation and differentiation into multilineage. In the present study, we compared the proliferation and differentiation abilities between RSCs and cerebral corticex-derived neural stem cells (CNSCs) of adult rats. Stem cells isolated from pigmented ciliary margins of eyes and cerebral cortical tissues of adult rats were cultured in 96-well plates that contained serum-free medium with epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). In contrast to RSCs, which stopped proliferating after the 8th week, the total cell count of neurospheres in CNSCs increased twofold at the 5th week and more than fourfold at the 10th week after in vitro culture. In contrast, RSCs stopped proliferating after 8 weeks of culture. After adding 2% fetal calf serum and withdrawing EGF and bFGF from the culture medium, the percentages of nestin-positive cells(20.6 +/- 2.7%), microtubule-associated-protein-2-positive neurons (33.2 +/- 3.9%) and glial-fibrillary-acidic-protein-positive glial cells(51.3 +/- 6.2%) in the differentiated CNSCs were significantly higher than those in the differentiated RSCs (10.2 +/- 1.9, 22.3 +/- 1.3 and 44.6 +/- 5.1%, respectively; p < 0.05). We also found that the combination of transforming growth factor beta type III with retinoic acid played an important role in the induction of CNSCs to differentiate into opsin-positive cells. Our data demonstrated that CNSCs displayed a higher ability of proliferation and retinal lineage. This report also offers an alternative protocol of cell reproduction for producing retinal cells.

Antibody microarray for correlating cell phenotype with surface marker

To correlate cell surface markers with the cell phenotype, an antibody microarray prepared by covalently immobilizing antibodies onto a cellulose membrane and subsequent immunocytochemical staining were employed. The direct binding assay of a lymphoblastic leukemia cell line on the microarray showed that the immobilized antibody served to capture cells expressing the specific antigen. The density of bound cells increased linearly with an increasing content of antigen-expressing cells in suspension. The method was further applied to the analysis of surface antigens expressed on neural stem cells. A binding assay was performed with neural cells obtained from the neurosphere culture of the rat fetal striatum on a microarray spotted with eight kinds of antibodies and four different proteins, followed by immunocytochemical staining of cells bound to the microarray using antibodies to the intracellular markers of immature (nestin and vimentin) and mature (beta-tubulin III and glial fibrillary acidic protein) neural cells. As a result, the phenotype of bound cells could be correlated to surface antigen expression, which illustrated the potential of the solid-phase cytometry developed here for the identification of surface markers.

Neural regeneration in the chick retina

In warm-blooded vertebrates, possibilities for retinal regeneration have recently become reality with the discovery of neural stem cells in the mature eye. A number of different cellular sources of neural stem cells have been identified. These sources include stem cells at the retinal margin, pigmented cells in the ciliary body and iris, non-pigmented cells in the ciliary body and Müller glia within the retina. This review focuses on recent reports of neural stem cells and regeneration in the postnatal chicken retina. In the chicken eye sources of neurogensis and regeneration include: (1) retinal stem cells at the peripheral edge of the retina; (2) Müller glia in central regions of the retina; (3) non-pigmented epithelial cells in the posterior portion of the ciliary body; and (4) possibly pigmented cells in the pars plana of the ciliary body. This review discusses the similarities between the retinal progenitor cells in the postnatal eye and those found in the embryo. In addition, I discuss combinations of growth factors, (insulin, IGF-I, EGF and FGF2) that are capable of stimulating the proliferation and production of neurons from neural progenitors, non-neural epithelial cells, and postmitotic support cells in the avian eye. In summary, the mechanisms that regulate the proliferation and differentiation of cells with neurogenic potential are beginning to be understood and the postnatal chicken eye has proven to be a useful model system to study retinal regeneration.

Potential for neural regeneration after neurotoxic injury in the adult mammalian retina

It has long been believed that the retina of mature mammals is incapable of regeneration. In this study, using the N-methyl-D-aspartate neurotoxicity model of adult rat retina, we observed that some Müller glial cells were stimulated to proliferate in response to a toxic injury and produce bipolar cells and rod photoreceptors. Although these newly produced neurons were limited in number, retinoic acid treatment promoted the number of regenerated bipolar cells. Moreover, misexpression of basic helix-loop-helix and homeobox genes promoted the induction of amacrine, horizontal, and rod photoreceptor specific phenotypes. These findings demonstrated that retinal neurons regenerated even in adult mammalian retina after toxic injury. Furthermore, we could partially control the fate of the regenerated neurons with extrinsic factors or intrinsic genes. The Müller glial cells constitute a potential source for the regeneration of adult mammalian retina and can be a target for drug delivery and gene therapy in retinal degenerative diseases.

Stem cells and retinal repair

Retinal stem cells (RSCs) are multipotent central nervous system (CNS) precursors that give rise to the retina during the course of development. RSCs are present in the embryonic eyecup of all vertebrate species and remain active in lower vertebrates throughout life. Mammals, however, exhibit little RSC activity in adulthood and thus little capacity for retinal growth or regeneration. Because CNS precursors can now be isolated from immature and mature mammals and expanded ex vivo, it is possible to study these cells in culture as well as following transplantation to the diseased retina. Such experiments have revealed a wealth of unanticipated findings, both in terms of the instructive cues present in the mature mammalian retina as well as the ability of grafted CNS precursors to respond to them. This review examines current knowledge regarding RSCs, together with other CNS precursors, from the perspective of investigators who wish to isolate, propagate, genetically modify, and transplant these cells as a regenerative strategy with application to retinal disease.

The neurogenic competence of progenitors from the postnatal rat retina in vitro

The mammalian retina develops from stem or progenitor cells that are of neuroectodermal origin and derive from bilateral invaginations of the neuroepithelium, the optic vesicles. Shortly after birth, around 12 days postnatal in rats, the retina is fully developed in its cellular parts. Even though different cell types in the adult might be potential sources for retinal stem cells or progenitor cells, the retina is a non-neurogenic region and the diseased retina is devoid of any spontaneous regeneration. In an attempt to link late developmental processes to the adult situation, we analyzed the presence and the neurogenic potential of retinal progenitors during the postnatal period and compared it to adult ciliary body (CB) derived retinal progenitors and subventricular zone (SVZ) derived neural stem cells. Retinal progenitor properties were identified by the capacity to proliferate and by the expression of the progenitor markers Nestin, Flk-1, Chx10, Pax6 and the radial glia marker BLBP. The neurogenic potential was assayed by the expression of the neuronal markers doublecortin, betaIII Tubulin, Map2 and NSE, the glial makers A2B5, NG2, GalC and GFAP, and by incorporation of BrdU. The number of Flk-1 positive cells and concomitantly the number of newly born betaIII Tubulin-positive cells decreased within the first postnatal week in retinal progenitor cultures and no newly generated betaIII Tubulin, but GFAP positive cells were detected thereafter. In contrast to neural stem cells derived from the adult SVZ, postnatal and adult CB derived progenitors had a lower and a restricted proliferation potential and did not generate oligodendrocytes. The work demonstrates, however, that the existence of retinal progenitor cells is not restricted to embryonic development. In the sensory retina the differentiation potential of late retinal progenitors becomes restricted to the glial lineage, whereas neurogenic progenitor cells are still present in the CB. In addition, major differences in growth and differentiation potential of adult neural stem cells and postnatal and adult retinal progenitors are presented.

Growth factors induce neurogenesis in the ciliary body

The ciliary body of the eye is a nonneural tissue that is derived from the anterior rim of the optic cup, an extension of the neural tube. This tissue normally does not contain neurons and functions to produce components of the aqueous humor. We found that intraocular injections of insulin, EGF, or FGF2 stimulate NPE cells to proliferate and differentiate into neurons. These growth factors had region-specific effects along the radial axis of the ciliary body, with insulin and EGF stimulating proliferation of NPE cells close to the retina, while FGF2 stimulated the proliferation of NPE cells further toward the lens. Similar region-specific effects were observed for accumulations of neurons in the NPE in response to injections of different growth factors. The neurons derived from NPE cells express neurofilament, beta3 tubulin, RA4, calretinin, Islet1, or Hu, and a few produced long axonal projections, several millimeters in length that extend across the ciliary body. Our results suggest that the ciliary body has the capacity to generate retinal neurons, but normally neurogenesis is actively inhibited.

Expression and activation of STAT proteins during mouse retina development

Cytokines and growth factors play important roles in mammalian ocular development and maintenance. Recent studies have indicated that some of these ligands can activate signal transducer and activator of transcription factors (STATs) and modulate gene transcription. The purpose of this study was to investigate the expression and activation of STAT proteins in the developing mouse retina. Anti-STAT and anti-phosphorylated STAT antibodies were used to detect the expression and activation of STATs in embryonic and postnatal neuronal retina, ciliary margin, and retinal pigment epithelium (RPE). In situ hybridization and Western blot were also employed. In embryonic stages, all STAT proteins were expressed in the neuronal retina in distinct cell populations at different embryonic stages. For example, Stat3 expression and activation gradually increased in the inner neuroblast layer and ciliary margin during development. In adult retina, Stat3 was detected in the inner nuclear layer and ganglion cells layers. Stat1 was strongly expressed in both outer and inner plexiform layers. Stat5a was clearly expressed in the outer/inner nuclear layer, the ganglion cell layer, and the inner plexiform layer. Strong expression of Stat3, Stat5a, and Stat6 was observed in the RPE. Activated Stat3 and Stat5a were found in the neural retina and the RPE. Distinct STAT proteins were present in different cell populations in neuronal retina and RPE suggesting multiple functions of STATs in mammalian eye development. Studies of STAT signal pathways in the eye may contribute to the understanding of molecular mechanisms in control of ocular development and pathogenesis.

Genetic rescue of cell number in a mouse model of microphthalmia: interactions between Chx10 and G1-phase cell cycle regulators

Insufficient cell number is a primary cause of failed retinal development in the Chx10 mutant mouse. To determine if Chx10 regulates cell number by antagonizing p27(Kip1) activity, we generated Chx10, p27(Kip1) double null mice. The severe hypocellular defect in Chx10 single null mice is alleviated in the double null, and while Chx10-null retinas lack lamination, double null retinas have near normal lamination. Bipolar cells are absent in the double null retina, a defect that is attributable to a requirement for Chx10 that is independent of p27(Kip1). We find that p27(Kip1) is abnormally present in progenitors of Chx10-null retinas, and that its ectopic localization is responsible for a significant amount of the proliferation defect in this microphthalmia model system. mRNA and protein expression patterns in these mice and in cyclin D1-null mice suggest that Chx10 influences p27(Kip1) at a post-transcriptional level, through a mechanism that is largely dependent on cyclin D1. This is the first report of rescue of retinal proliferation in a microphthalmia model by deletion of a cell cycle regulatory gene.

Differentiation of human neural stem cells into retinal cells

We have previously reported that transplanted human neural stem cells (HNSCs) display extensive migration and positional incorporation into the aged rat brain, which is associated with an improvement in cognitive function. In the current study, to investigate whether HNSCs are capable of differentiating into retinal cells, we treated HNSCs with human transforming growth factor-beta3 (TGF-beta3) under a serum-free differentiation condition. After 5 days of differentiation in vitro we detected opsin-immunopositive cells in the culture treated with TGF-beta3. We also transplanted TGF-beta3-treated HNSCs into the rat vitreous cavity. The donor cells migrated and differentiated into opsin-positive cells in the host retinal cell layer. Here we show for the first time that TGF-beta3-treated HNSCs differentiate into retinal cells.

Insulin and fibroblast growth factor 2 activate a neurogenic program in Müller glia of the chicken retina

We have reported previously that neurotoxic damage to the chicken retina causes Müller glia to dedifferentiate, proliferate, express transcription factors common to retinal progenitors, and generate new neurons and glia, whereas the majority of newly produced cells remain undifferentiated (Fischer and Reh, 2001). Because damaged retinal cells have been shown to produce increased levels of insulin-related factors and FGFs, in the current study we tested whether intraocular injections of growth factors stimulate Müller glia to proliferate and produce new neurons. We injected growth factors and bromodeoxyuridine into the vitreous chamber of the eyes of chickens and assayed for changes in glial phenotype and proliferation within the retina. Although insulin or FGF2 alone had no effect, the combination of insulin and FGF2 caused Müller glia to coexpress transcription factors common to retinal progenitors (Pax6 and Chx10) and initiated a wave of proliferation in Müller cells that began at the retinal margin and spread into peripheral regions of the retina. Most of the newly formed cells remain undifferentiated, expressing Pax6 and Chx10, whereas some differentiate into Müller glia, and a few differentiate into neurons that express the neuronal markers Hu or calretinin. There was no evidence of retinal damage in eyes treated with insulin and FGF2. We conclude that the combination of insulin and FGF2 stimulated Müller glia to dedifferentiate, proliferate, and generate new neurons. These findings imply that exogenous growth factors might be used to stimulate endogenous glial cells to regenerate neurons in the CNS.

Neuronal differentiation of adult rat hippocampus-derived neural stem cells transplanted into embryonic rat explanted retinas with retinoic acid pretreatment

The purpose of this study was to evaluate the effects of the retinal environment and retinoic acid (RA) pretreatment on the differentiation of transplanted adult rat hippocampus-derived neural stem cells (AHSCs). AHSCs were transplanted into embryonic (E18) or neonatal (P6) rat retinal explants and the mixture was cultured for 2 weeks. Other AHSCs were stimulated by 0.5 microM all-trans RA for 6 days before transplantation. Immunofluorescent double staining showed that a larger number of AHSCs became beta-tubulin III-positive neurons in the E18 than in P6 retinas. In addition, many AHSCs became MAP2ab-positive and MAP5-positive neurons following RA pretreatment and transplantation. Only a few AHSCs became HPC-1-, calbindin-, PKC- or rhodopsin-positive cells under these conditions. We conclude that the microenvironment supplied by embryonic retinas is conductive to neuronal differentiation in general. RA stimulation before transplantation was also effective in stimulating differentiation.

Molecular basis of environmentally induced birth defects

Exposure of the developing conceptus to selected environmental agents can lead to deleterious and often times lethal birth defects. These malformations result in serious emotional and financial consequences to families and societies worldwide. As we continue to progress technologically, we face challenges from the introduction of new pharmacological agents and chemical compounds into the environment. This results in a concomitant need to more fully understand the relationship between in utero exposure to environmental teratogens and the risk of congenital malformations. The goal of this review is to provide a current perspective of the major concepts related to the molecular basis of environmentally induced birth defects. Starting with a discussion of commonly occurring birth defects, we consider important fundamental facets of embryonic development, teratology, and gene-environment interactions. The review then summarizes our current understanding of the molecular mechanisms involved in selected birth defects following exposure to pharmacological compounds, including thalidomide, retinoids, and valproic acid. Understanding these signaling pathways may lead to the development of safer pharmaceutical compounds and a reduction in the number of infants born with preventable birth defects.

Transforming growth factor beta in experimentally detached retina and periretinal membranes

This study was undertaken to determine whether experimental retinal detachment produces changes in retinal localization of three isoforms of transforming growth factor beta (TGF-beta) and the type II receptor for this protein. Neural retinas of young adult cats were detached from the pigment epithelium. Survival times varied from 3 to 28 days to study the temporal course of TGF-beta localization during retinal degeneration. ELISA assay for TGF-beta1 and -beta2 was performed on samples of fluid from the vitreous chamber to determine whether active or inactive TGF-beta was present. Confocal microscopy was used to localize TGF-beta1, -beta2 and -beta3 and the type II TGF-beta receptor at the various detachment durations. Following experimental retinal detachment the levels of TGF-beta2 increased in the vitreous chamber but no changes in TGF-beta1 were detected. Levels were increased 3 days post-detachment and continued throughout the 28 day period studied. The most prominent changes in immunolocalization occurred in the TGF-beta1 and -beta2 isoforms. Increased immunolabeling was seen in Müller cells and ganglion cell bodies. Hypertrophied Müller cell processes formed periretinal membranes that were heavily labeled by the TGF-beta2 antibody. Some increased immunostaining for TGF-beta3 was observed in the ganglion cell bodies. Labeling for the TGF-beta type II receptor was seen in Müller cells, ganglion cells and the inner and outer plexiform layers in both normal and detached retinas. Changes in localization of the receptor after detachment paralleled the changes seen in TGF-beta protein localization. These results demonstrate that retinal detachment induces the synthesis and secretion of TGF-beta2. Growth factor and receptor immunolabeling were increased in Müller cells suggesting that this isoform is involved in the retinal gliotic response and may contribute to the development of proliferative vitreoretinopathy.

Trk C signaling is required for retinal progenitor cell proliferation

Although neurotrophin actions in the survival of specific retinal cell types have been identified, the biological functions for neurotrophin-3 (NT-3) in early retinal development remain unclear. Having localized NT-3 and trk C expression at early developmental stages when retinal neuroepithelial progenitor cells predominate, we sought to modulate NT-3 signaling in these cells by overexpressing a truncated isoform of the NT-3 receptor, trk C. We have demonstrated that this non-catalytic receptor can inhibit NT-3 signaling when coexpressed with the full-length kinase-active trk C receptor. Using a replication-deficient retrovirus to ectopically express the truncated trk C receptor to limited numbers of progenitor cells in ovo, we examined the effects of disrupted trk C signaling on the proliferation or differentiation of retinal cells. Clones expressing truncated trk C exhibited a 70% reduction in clone size, compared with clones infected with a control virus, indicating that inhibition of trk C signaling decreased the clonal expansion of cells derived from a single retinal progenitor cell. Additionally, impaired NT-3 signaling resulted in a reduction of all retinal cell types, suggesting that NT-3 targets retinal precursor cells rather than differentiated cell types. BrdU labeling studies performed at E6 indicate that this reduction in cell number occurs through a decrease in cell proliferation. These studies suggest that NT-3 is an important mitogen early in retinal development and serves to establish the size of the progenitor pool from which all future differentiated cells arise.

Identification of a proliferating marginal zone of retinal progenitors in postnatal chickens

In warm-blooded vertebrates it is generally accepted that after early stages of development new neurons are not added to the retina. Contrary to this belief, we show here that hatched chickens have a zone of proliferating cells at the peripheral margin of the retina, similar to that of fish and amphibians. We found that cells at the peripheral edge of the retina incorporated the thymidine analog BrdU and expressed the cell cycle regulator proliferating cell nuclear antigen (PCNA). Furthermore, cells in the ciliary epithelium and retinal margin coexpressed the homeodomain transcription factors Pax6 and Chx-10, similar to multipotent progenitors of embryonic retina. Expression of PCNA, Pax6, and Chx-10 in cells at the retinal margin was maintained in adult birds. Double-labeling studies showed that BrdU-labeled cells that were integrated into the retina expressed proteins found only in differentiated neurons. Increased rates of ocular growth, induced by visual deprivation, resulted in increased numbers of BrdU-labeled cells at the retinal margin. Unlike the progenitors in the retinal marginal zone of fish and amphibians, the progenitors of the chick retina do not increase their rate of proliferation in response to acute damage. Furthermore, insulin, insulin-like growth factor-I, and epidermal growth factor increased proliferation of progenitors at the retinal margin, while basic fibroblast growth factor had no effect. These results indicate that the avian retina has a marginal growth zone containing proliferating cells that share similarities with multipotent embryonic retinal progenitors and the retinal stem cells of cold-blooded vertebrates.

Function for Hedgehog genes in zebrafish retinal development

The hedgehog (hh) genes encode secreted signaling proteins that have important developmental functions in vertebrates and invertebrates. In Drosophila, expression of hh coordinates retinal development by propagating a wave of photoreceptor differentiation across the eye primordium. Here we report that two vertebrate hh genes, sonic hedgehog (shh) and tiggy-winkle hedgehog (twhh), may perform similar functions in the developing zebrafish. Both shh and twhh are expressed in the embryonic zebrafish retinal pigmented epithelium (RPE), initially in a discrete ventral patch which then expands outward in advance of an expanding wave of photoreceptor recruitment in the subjacent neural retina. A gene encoding a receptor for the hedgehog protein, ptc-2, is expressed by retinal neuroepithelial cells. Injection of a cocktail of antisense (alphashh/alphatwhh) oligonucleotides reduces expression of both hh genes in the RPE and slows or arrests the progression of rod and cone photoreceptor differentiation. Zebrafish strains known to have mutations in Hh signaling pathway genes similarly exhibit retardation of photoreceptor differentiation. We propose that hedgehog genes may play a role in propagating photoreceptor differentiation across the developing eye of the zebrafish.

Characterization of a chicken retinoid X receptor-gamma gene promoter and identification of sequences that direct expression in retinal cells

Development of the cellular complexity of the vertebrate neural retina relies on an intricate interplay between extracellular signals and intracellular factors. In particular, transcription factors play a key role in determining the competence of cells to respond to extracellular signals. We have previously shown that, in the developing chick neural retina, expression of the retinoid X receptor-gamma (RXR-gamma2) nuclear receptor gene is restricted to photoreceptors. To characterize the mechanisms that regulate expression of this gene in the neural retina, we isolated a chicken RXR-gamma genomic clone containing the RXR-gamma2 promoter and mapped the transcription initiation site by means of ribonuclease protection. We analysed promoter activity by transient transfection of luciferase reporter gene constructs into cultured cells isolated from embryonic-chick neural retina or facial mesenchyme, which does not normally express detectable RXR-gamma2 transcripts. The DNA fragment lying between nucleotides -657 and +37 with respect to the transcription initiation site had basal promoter activity in both cell types. The fragment lying between nucleotides -1198 and -991 directed 10-20-fold higher levels of luciferase activity in neural retina cells, but only basal levels in facial mesenchyme cells. This 208 bp fragment also enhanced the activity of the simian-virus-40 promoter, when placed upstream in either orientation. Electrophoretic-mobility-shift assays using this 208 bp fragment demonstrated the formation of four neural retina-specific protein-DNA complexes. These results indicate that regulation of RXR-gamma2 transcription in the developing chick neural retina involves the binding of one or more neural retina-specific protein factors to an enhancer element located approx. 1 kbp upstream of the transcription initiation site.

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.

Valproic acid-induced alterations in growth and neurotrophic factor gene expression in murine embryos [corrected]

Although the teratogenicity of valproic acid (VPA) has been well established, the mechanism(s) by which this anticonvulsant drug induces malformations remains controversial. Using the combined molecular techniques of in situ-transcription (IST) and antisense RNA (aRNA) amplification we analyzed VPA-induced alterations in the gene expression for 10 genes within the neural tubes of embryos from two murine strains that have been shown to differ in their susceptibility to VPA-induce neural tube defects (NTD). Pregnant dams from both SWV (susceptible) and LM/Bc (resistant) strains were either treated with saline (control) or VPA (600 mg/kg) on gestational day (GD) 8:12 (day:hour). Neural tubes were isolated from control or VPA exposed embryos at three gestational time points, which represented the beginning (GD 8:18), middle (GD 9:00), and end (GD 9:12) of neural tube closure (NTC) in both of these murine strains. Using univariant statistics we demonstrated that in LM/Bc embryos with NTDs, the expression of bdnf, ngf, and trk, ngf-R were significantly elevated at all three time points, and the cytokine, cntf was significantly decreased at GD 9:00. In contrast, the major gene alterations observed in SWV embryos were a significant increase in tfgalpha and tgfbeta1-3 at GD 9:00. In an effort to better define the more intricate interactions between VPA exposure and the expression of these genes, we analyzed our data using Principal Component Analysis. The results from this analysis demonstrated that embryos from these two stains behaved differently, not only in response to a VPA exposure, but also under control conditions, which may explain the multifactorial nature of NTDs in these mice.

Activin A promotes progenitor differentiation into photoreceptors in rodent retina

Activins are TGF beta-like proteins that were first discovered for their actions on the reproductive system, but have subsequently been shown to play a role in a variety of developmental processes. Previous studies have demonstrated that activins and their receptors are present in the developing retina, as well as other regions of the embryonic nervous system. We used both in vitro and in vivo approaches to test for functions of activin during retinal development. We found that activin A treatment of embryonic day 18 rat retinal cultures causes the progenitor cells in the cultures to exit the cell cycle and differentiate into rod photoreceptors. This effect is dose-dependent and the promotion of rod photoreceptor differentiation is specific, since the other primary retinal neurons generated in these cultures, the C1+ amacrine cells, are not affected by activin A treatment. Mice with homozygous deletion of the activin betaA gene show a specific decrease in the number of rod photoreceptors compared to wild-type or heterozygous littermates. These data demonstrate that activin A is an important regulator of photoreceptor differentiation in the developing retina.

OP-1 enhances dendritic growth from cerebral cortical neurons in vitro

Osteogenic protein-1 (OP-1), a member of the transforming growth factor-beta (TGF-beta) superfamily, has been demonstrated to stimulate dendrite growth from sympathetic neurons in culture. However, it is not known whether OP-1 affects dendrite growth from central nervous system neurons. Therefore we quantified axon and primary, secondary, and total dendritic growth from embryonic mouse cortical neurons (E 18) grown in vitro in a chemically defined medium. Morphology and double immunolabeling (MAP2, NF-H) were used to identify cortical dendrites and axons after 3 days in vitro. Cell morphology, neuron survival, and axon length were similar under all experimental conditions. The number of primary dendrites also was similar; however, the length of primary dendrites and the length and number of secondary dendrites were significantly increased by the addition of OP-1 to the culture medium. This increase in dendrite growth was dose-dependent; maximal dendritic growth was observed after the addition of 30-100 ng/ml of OP-1 to the culture medium. Specific support of dendrite growth was not observed when neurons were exposed to other members of the TGF-beta superfamily. These findings demonstrate that OP-1 selectively increases dendrite growth from cerebral cortical neurons in vitro.

Analysis of chicken Wnt-13 expression demonstrates coincidence with cell division in the developing eye and is consistent with a role in induction

We used a degenerate polymerase chain reaction (PCR) strategy to search for Wnt RNA in developing ocular tissues. We isolated a Macaca monkey Wnt-13 PCR fragment, orthologous to the human and murine Wnt-13 and Xenopus Wnt-2b, and a chick Wnt13 cDNA. Wnt-13 is a member of the Wnt-1 class of transforming Wnt molecules. In situ RNA hybridization revealed a dynamic Wnt-13 expression pattern in numerous developing tissues. Within the eye, Wnt-13 is expressed in the proliferative epithelium of the lens and both pigmented and non-pigmented layers of the ciliary margin. In vitro BrdU incorporation studies coupled with in situ hybridization showed that cWnt-13 expression domains in the lens were coincident with cell division. In addition to the eye, cWnt-13 was expressed in head ectoderm, prospective forelimb mesenchyme, lung bud, pharyngeal arches, the brain, as well as the otic vesicle. Our data are consistent with previous observations linking transforming Wnts with cell division and implicate a cascade of events involving cWnt-13 first in dorsoventral patterning and later in cell proliferation regulation associated with lens development. Dev Dyn 1999;215:215-224.

Involvement of P2 purinoceptors in the regulation of DNA synthesis in the neural retina of chick embryo

The activation of P2 purinoceptors induces Ca2+ mobilization in the early embryonic chick neural retina. This purinergic Ca2+ response declines parallel with the decrease in mitotic activity during retinal development. To investigate the role of P2 purinoceptors in the regulation of retinal cell proliferation, we studied the effects of the P2 purinoceptor antagonists suramin and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS), and of the agonist ATP on DNA synthesis in retinal organ cultures from embryonic day 3 (E3) chick. Suramin inhibited [3H]-thymidine incorporation in a dose-dependent manner (IC50: approximately 70 microM). PPADS also reduced [3H]-thymidine incorporation with maximum inhibition of 46% at 100 microM. Exogenous ATP enhanced [3H]-thymidine incorporation in a dose-dependent manner to maximally 200% of control (EC50: approximately 70 microM). In dissociated retinal cultures from E7 chick, both antagonists showed similar inhibitory effects on [3H]-thymidine incorporation without affecting cell viability. In line with these observations, the presence of extracellular ATP was demonstrated both in vitro and in vivo. In the medium of E3 retinal organ cultures, the concentration of ATP increased 25-fold within 1 h of incubation and this concentration was kept for at least 24 h. In the chick amniotic fluid, the ATP concentration was nearly 3 microM at E3 and declined to 0.15 microM at E7. The results indicate that P2 purinoceptors activated by autocrine or paracrine release of ATP are involved in the regulation of DNA synthesis in the neural retina at early embryonic stages.

Molecular regulators involved in vertebrate eye development

Development of the eye can be subdivided into three phases. The first phase is the formation of the major structures of the eye by the processes of induction and regional specification. The second is the maturation of these structures to form the functional eye, and the third phase is the formation of neuronal connections between retina and the optic tectum. These processes are tightly regulated by signalling cascades that direct axonal outgrowth, cellular proliferation and differentiation. Some members of these signalling cascades have been identified in recent studies. These include secreted factors which transmit signals extracellularly, and receptors and transcription factors which are members of intracellular signalling pathways that respond to extracellular signals. This review summarizes the recent research that has implicated these factors in playing a role in eye development on the basis of functional or expression criteria.

Cell type-specific development of rodent central nervous system progenitor cells in culture

OBJECT

The aim of the study was to assess the pluripotential central nervous system (CNS) progenitor cells that give rise to the many differentiated neuronal and glial cell types of the adult mammalian brain and the role of peptide growth factors such as the epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). The action of these factors is crucial to the survival and ultimate differentiation of these CNS progenitor cells. However, the precise role of EGF and bFGF in the time course of cellular development, the acquisition of cell type specificity, and possible differential mitogenic behavior has not been clearly defined.

METHODS

The authors defined the time course of CNS progenitor cell development in cultured embryonic rodent cells by using immunocytochemical analysis to identify the expression of pluripotential (nestin)-, neuron (microtubule-associated protein-2 [MAP-2])-, and glia (glial fibrillary acidic protein [GFAP])-specific proteins in response to treatment with EGF and bFGF alone or in combination. The bromodeoxyuridine (BUdR) labeling index for each treatment group was used to define the mitogenic effects of each growth factor. In this investigation, the authors observed that progenitor cells develop in a stereotypical fashion when exposed to bFGF or EGF. Marked staining for nestin was evident soon after plating. This declined over time as staining for MAP-2 and GFAP increased. When treated with EGF alone, cells maintained their nestin immunoreactivity longer than those treated with bFGF alone or in combination with EGF. Treatment with bFGF alone promoted a significant increase in MAP-2 and, to a much lesser extent, GFAP reactivity. This was observed concomitant with the decline in nestin staining. The BUdR labeling index was similar among the different treatment groups and declined similarly over time in all treatment groups.

CONCLUSIONS

The effects of EGF and/or bFGF on the expression of development- and lineage-specific markers likely reflect the specific effects of these factors on developmental processes. These data indicate that bFGF exerts a preferential effect on neuronal development and, to a lesser extent, glial development, which is not explained by selective mitogenicity. The persistence of nestin staining seen in the cells treated with EGF alone indicates that EGF may function as a stem cell survival factor. This study provides evidence that CNS cell type-specific development can be altered by the manipulation of peptide growth factors that act as differentiation agents.

Cellular diversification in the vertebrate retina

The discovery of heterogeneous populations of retinal precursors with sequentially modified fates may help solve the conundrum of conserved histogenesis in the absence of determination either by birthdate or lineage. Combined with a wealth of new data on the exogenous and endogenous factors that influence cellular fate in the retina, models of how complexity is generated are beginning to emerge.

Sonic hedgehog promotes rod photoreceptor differentiation in mammalian retinal cells in vitro

The hedgehog gene family encodes secreted proteins important in many developmental patterning events in both vertebrates and invertebrates. In the Drosophila eye disk, hedgehog controls the progression of photoreceptor differentiation in the morphogenetic furrow. To investigate whether hedgehog proteins are also involved in the development of the vertebrate retina at stages of photoreceptor differentiation, we analyzed expression of the three known vertebrate hedgehog genes. We found that Sonic hedgehog and Desert hedgehog are expressed in the developing retina, albeit at very low levels, whereas Indian hedgehog (Ihh) is expressed in the developing and mature retinal pigmented epithelium, beginning at embryonic day 13. To determine whether hedgehog proteins have activities on developing retinal cells, we used an in vitro system in which much of retinal histogenesis is recapitulated. N-terminal recombinant Sonic Hedgehog protein (SHH-N) was added to rat retinal cultures for 3-12 d, and the numbers of retinal cells of various phenotypes were analyzed by immunohistochemistry. We found that SHH-N caused a transient increase in the number of retinal progenitor cells, and a 2- to 10-fold increase in the number of photoreceptors differentiating in the cultures when analyzed with three different photoreceptor-specific antigens. In contrast, the numbers of retinal ganglion cells and amacrine cells were similar to those in control cultures. These results show that Hedgehog proteins can regulate mitogenesis and photoreceptor differentiation in the vertebrate retina, and Ihh is a candidate factor from the pigmented epithelium to promote retinal progenitor proliferation and photoreceptor differentiation.

Effects of transforming growth factors on dopaminergic neurons in culture

It was recently reported that TGF-beta 1 either had no significant effect on or increased the survival of dopaminergic neurons in culture. TGF-beta 2 and TGF-beta 3 were reported to cause increased survival or to greatly inhibit survival. The transforming growth factors are a highly pleiotropic group of compounds, and the above results suggest that their actions may be critically dependent on the conditions of the assay. We have therefore tested these compounds under optimal conditions of culture, in a medium containing a low (2.5%) concentration of fetal bovine serum. TGF-beta 2 and 3 inhibited neuronal (MAP2-pos) survival only at the highest concentration (10 ng/ml) tested, while inhibition of survival of dopaminergic neurons was observed at 1.0 and 10 ng/ml. These results therefore suggest that the inhibitory action of TGF-beta 2 and 3 on the survival of dopaminergic neurons in culture, under the experimental conditions outlined, may be relatively specific.