Expression of v-src in embryonic neural retina alters cell adhesion, inhibits histogenesis, and prevents induction of glutamine synthetase

Protein-bound crotonaldehyde accumulates in the spinal cord of superoxide dismutase-1 mutation-associated familial amyotrophic lateral sclerosis and its transgenic mouse model

Growing evidence documents oxidative stress involvement in ALS. We previously demonstrated accumulation of a protein-bound form of the highly toxic lipid peroxidation product crotonaldehyde (CRA) in the spinal cord of sporadic ALS patients. In the present study, to the determine the role for CRA in the disease processes of superoxide dismutase-1 (SOD1) mutation-associated familial ALS (FALS), we performed immunohistochemical and semi-quantitative cell count analyses of protein-bound CRA (P-CRA) in the spinal cord of SOD1-mutated FALS and its transgenic mouse model. Immunohistochemical analysis revealed increased P-CRA immunoreactivity in the spinal cord of the FALS patients and the transgenic mice compared to their respective controls. In the FALS patients, P-CRA immunoreactivity was localized in almost all of the chromatolytic motor neurons, neurofilamentous conglomerates, spheroids, cordlike swollen axons, reactive astrocytes and microglia, and the surrounding neuropil in the affected areas represented by the anterior horns. In the transgenic mice, P-CRA immunoreactivity was localized in only a few ventral horn glia in the presymptomatic stage, in almost all of the vacuolated motor neurons and cordlike swollen axons and some of the ventral horn reactive astrocytes and microglia in the onset stage, and in many of the ventral horn reactive astrocytes and microglia in the advanced stage. Cell count analysis on mouse spinal cord sections disclosed a statistically significant increase in the density of P-CRA-immunoreactive glia in the ventral horns of the young to old G93A mice compared to the age-matched control mice. The present results indicate that enhanced CRA formation occurs in motor neurons and reactive glia in the spinal cord of SOD1-mutated FALS and its transgenic mouse model as well as sporadic ALS, sug- gesting implications for CRA in the pathomechanism common to these forms of ALS.

Cytoskeletal and cell contact control of the glucocorticoid pathway

The cytoskeleton is a dynamic network that undergoes restructuring during a variety of cellular events including cell contact formation, cell invasion and the mitotic phase of the cell cycle. Here, we review the contribution of the cytoskeletal network to the inductive activity of glucocorticoids by focusing on the hormonal control of glutamine synthetase in the chick neural retina. Depolymerization of the cytoskeleton in cells of the intact retinal tissue inhibits the hormonal induction of glutamine synthetase, but does not alter the cellular amount of the glucocorticoid-receptor protein or the ability of the receptor molecules to translocate into the nucleus. Inhibition of glutamine synthetase induction occurs via a mechanism that involves elevation of c-Jun protein accumulation and repression of glucocorticoid-receptor transcriptional activity. Unlike growth factors and other c-Jun inducing stimuli that control the transcription of the c-Jun gene, depolymerization of the cytoskeleton elevates c-Jun accumulation by upregulating the translation of the c-Jun transcript. We postulate that the cytoskeletal-dependent increase in c-Jun accumulation is involved in cell contact control of both cell proliferation and transcriptional activity of the glucocorticoid-receptor protein.

A morphogenetic wave of p27Kip1 transcription directs cell cycle exit during organ of Corti development

The molecular mechanisms coordinating cell cycle exit with cell differentiation and organogenesis are a crucial, yet poorly understood, aspect of normal development. The mammalian cyclin-dependent kinase inhibitor p27(Kip1) is required for the correct timing of cell cycle exit in developing tissues, and thus plays a crucial role in this process. Although studies of p27(Kip1) regulation have revealed important posttranscriptional mechanisms regulating p27(Kip1) abundance, little is known about how developmental patterns of p27(Kip1) expression, and thus cell cycle exit, are achieved. Here, we show that during inner ear development transcriptional regulation of p27(Kip1) is the primary determinant of a wave of cell cycle exit that dictates the number of postmitotic progenitors destined to give rise to the hair cells and supporting cells of the organ of Corti. Interestingly, transcriptional induction from the p27(Kip1) gene occurs normally in p27(Kip1)-null mice, indicating that developmental regulation of p27(Kip1) transcription is independent of the timing of cell cycle exit. In addition, cell-type-specific patterns of p27(Kip1) transcriptional regulation are observed in the mature organ of Corti and retina, suggesting that this mechanism is important in differential regulation of the postmitotic state. This report establishes a link between the spatial and temporal pattern of p27(Kip1) transcription and the control of cell number during sensory organ morphogenesis.

Downregulation of a unique photoreceptor protein correlates with improper outer segment assembly

A unique photoreceptor protein has been characterized. This protein, termed XAP-1 antigen, is expressed by photoreceptors exclusively under conditions in which the outer segment membranes are properly assembled. When the retinal pigment epithelium is adherent to the underlying neural retina, the XAP-1 antigen is localized to the plasma membrane that surrounds the inner and outer segments in the areas juxtaposed to the subretinal space. A similar labeling pattern is detected in retinal pigment epithelium-deprived retinas in which assembly of nascent outer segments is supported by lactose. In retinas that undergo degeneration subsequent to the removal of the retinal pigment epithelium, the expression of this protein is completely downregulated. Immunohistochemical analyses and subcellular fractionation along with Western blot analysis, indicate that the XAP-1 antigen is a membrane-associated soluble protein. Mass spectrometric analysis indicates that the XAP-1 antigen shares homology via 12 tryptic peptide masses with the gamma-crystallin (lens structural protein) subclasses, although it does not immunolocalize to the same ocular structures as reported for the gamma-crystallins. We propose that XAP-1 antigen is a unique protein that is expressed extensively by healthy photoreceptor cells; the expression of the XAP-1 antigen exclusively by photoreceptors with organized outer segments suggests that this protein may play a critical role in outer segment assembly.

Pigment epithelium-derived factor supports normal Müller cell development and glutamine synthetase expression after removal of the retinal pigment epithelium

In conditions in which the retinal pigment epithelium (RPE) is dystrophic, carries a genetic mutation, or is removed physically, Müller cells undergo degenerative changes that contribute to the retinal pathology. We previously demonstrated that pigment epithelium-derived factor (PEDF), a glycoprotein secreted by the RPE cells with neuroprotective and differentiation properties, protects against photoreceptor degeneration induced by RPE removal. The purpose of the present study was to analyze the putative gliosupportive activity of PEDF on Müller cells of RPE-deprived retinas and assess whether protection of Müller cells was correlated with improved photoreceptor outer segment assembly. Eyes were dissected from Xenopus laevis tadpoles, and the RPE was removed before culturing in medium containing purified PEDF, PEDF plus anti-PEDF, or medium alone. Control eyes matured with an adherent RPE or in medium containing PEDF plus nonimmune serum. Müller cell ultrastructure was examined. Glial fibrillary acidic protein (GFAP) and glutamine synthetase were localized immunocytochemically, and the corresponding protein levels were quantified. In control retinas, Müller cells were structurally intact and formed adherens junctions with neighboring photoreceptors. In addition, they did not express GFAP, whereas glutamine synthetase expression was high. RPE removal dramatically altered the ultrastructure and biosynthetic activity of Müller cells; Müller cells failed to form adherens junctions with photoreceptors and glutamine synthetase expression was suppressed. PEDF prevented the degenerative glial response; Müller cells were ultrastructurally normal and formed junctional complexes with photoreceptors. PEDF also preserved the expression of glutamine synthetase at near-normal levels. The morphogenetic effects of PEDF were blocked by the anti-PEDF antibody. Our study documents the glioprotective effects of PEDF and suggests that maintenance of the proper Müller cell ultrastructure and expression of glutamine synthetase may be necessary to support the proper assembly of photoreceptor outer segments.

Basic fibroblast growth factor: a potential inhibitor of glutamine synthetase expression in injured neural tissue

Basic fibroblast growth factor (bFGF) was recently shown to promote the survival of neural cells and tissues, raising hopes for its therapeutic potential in degenerative disorders of the CNS. Here we examine the effect of bFGF on the expression of glutamine synthetase, a key enzyme in the detoxification of the neurotransmitter glutamate. Expression of this enzyme is regulated by systemic glucocorticoids and, in chick neural retina tissue, is restricted to Müller glial cells. We report that exogenous supply of bFGF to retinal explants inhibits hormonal induction of glutamine synthetase expression. This inhibition appears to be mediated by the c-Jun protein which accumulated, in response to bFGF, exclusively in Müller glial cells. Ischemic conditions, which reportedly stimulate the release of endogenous bFGF, also led to an increase in c-Jun protein and a decline in glutamine synthetase expression. This decline could be competitively prevented by a soluble fibroblast growth factor receptor but not by a soluble epidermal growth factor receptor. The finding that endogenous release of bFGF or its exogenous supply down-regulates glutamine synthetase expression suggests that in addition to its reported neuroprotective effect, bFGF may exacerbate glutamate mediated neurotoxicity through direct down-regulation of glutamine synthetase.

From stem cells towards neural layers: a lesson from re-aggregated embryonic retinal cells

Cells from dissociated embryonic avian retinae have the capacity to re-aggregate in rotation culture and form cellular spheres reconstituting a complete arrangement of all retinal layers. This exquisite phenomenon is based upon in vitro proliferation of multipotent precursor stem cells and spatial organization of their differentiating descendants. The addition of soluble factors from cultured retinal pigmented epithelial (RPE) or radial glial cells is essential to revert inside-out spheres (rosetted retinal spheres) into correctly laminated outside-out spheres (stratified spheres). Such complete restoration of a laminated brain tissue by cell re-aggregation has been achieved only for the embryonic avian retina, but not the mammalian retina, nor for other brain parts. This review summarises the history of the re-aggregation approach, presents avian retinal re-aggregate models, and analyses roles of the RPE and Müller cells for successful retinal tissue regeneration. It is predicted that these results will become biomedically relevant, as stem cell biology will soon open ways to produce large amounts of human retinal precursors.

Targeted disruption of Müller cell metabolism induces photoreceptor dysmorphogenesis

Within the retina, the Müller cells and photoreceptors are in close physical proximity and are metabolically coupled. It is unknown, however, whether Müller cells affect photoreceptor differentiation and outer segment membrane assembly. The objective of this study was to determine whether targeted disruption of Müller cell metabolism would induce photoreceptor dysmorphogenesis. Intact isolated Xenopus laevis embryonic eyes were cultured in medium with or without Müller cell-specific inhibitors (i.e., alpha-aminoadipic acid and fluorocitrate). To assess Müller cell injury, the gross retinal morphology was examined along with immunocytochemical assessment of Müller cell-specific protein expression patterns. The steady-state levels of opsin were quantified to determine whether the Müller cell inhibitors negatively affected photoreceptor protein synthesis. Müller and photoreceptor cell ultrastructure was scrutinized and the organization of the outer segment membranes was graded. In control retinas, there was no swelling of Müller cell cytoplasm. Glial fibrillary acidic protein (GFAP) was undetectable, whereas glutamine synthetase was abundant. The steady-state level of opsin was high and photoreceptors elaborated properly folded outer segments. Exposure to both Müller cell-specific inhibitors induced swelling of Müller cell endfeet, cytoplasmic paling and alterations of Müller cell-specific protein expression patterns. The steady-state level of opsin in retinas exposed to alpha-aminoadipic acid was unchanged compared with control eyes, whereas, in eyes exposed to fluorocitrate, opsin levels were slightly reduced. The most significant finding was that targeted disruption of Müller cell metabolism adversely affected photoreceptor outer segment membrane assembly, causing dysmorphogenesis of nascent outer segments. These results suggest that the termination signal(s) necessary for proper outer segment folding were disrupted by targeted inhibition of Müller cells and support the hypothesis that Müller cells interact with photoreceptors through mechanisms that may regulate, at least in part, the assembly of photoreceptor outer segment membranes.

Characterization of a novel quiescence responsive element downregulated by v-Src in the promoter of the neuroretina specific QR1 gene

The neuroretina is a functional unit of the central nervous system which arises through successive steps of division, growth arrest and differentiation of neuroectodermal precursors. Postmitotic quail neuroretina (QNR) cells are conditionally induced to divide upon infection with temperature sensitive mutants of Rous sarcoma virus (RSV), since QNR cell division can be arrested by either inactivating p60v-Src at the nonpermissive temperature (41 degrees C) or by serum deprivation at 37 degrees C. We are studying the transcriptional control of QR1, a neuroretina specific gene, whose expression is down-regulated in proliferating cells at 37 degrees C and is fully restored when these cells are made quiescent. We previously showed that this quiescence specific upregulation implicates a promoter region named A box, which binds Maf transcription factors. We report the identification of the C box, a second promoter sequence that activates QR1 transcription in non dividing cells. This sequence is able to form two DNA-protein complexes, one of which (C4) is predominantly detected in growth arrested NR cells. We identified the DNA binding site for C4 and described mutations that abolish both C4 binding and promoter activity in quiescent cells. Moreover, we show that a multimerized C box is able to stimulate a heterologous promoter in non dividing cells and constitutes, therefore, a novel quiescence responsive enhancer. Finally, we report that QR1 transcriptional response to cell quiescence requires cooperation between the C box and A box.

Closer look at lactose-mediated support of retinal morphogenesis

We have previously shown in intact isolated eye rudiments from Xenopus laevis that lactose, but not mannose, permits the formation of organized photoreceptor outer segments in the absence of the retinal pigment epithelium (RPE). The purpose of this study was to determine, using electron microscopic analysis, the key ultrastructural differences between healthy retinas, lactose-protected retinas, and retinas that developed aberrantly to reveal which subcellular structures were exclusively present in healthy retinas. Filamentous actin was also localized in retinas to determine its distribution under the various conditions. In healthy retinas, calycal processes extending approximately three-fourths of the length of the outer segment surrounded highly organized photoreceptor outer segments. Adherens junctions were localized between adjacent photoreceptors and Müller cells at the outer limiting membrane. In addition, Müller cells possessed apical processes that extended for a short distance beyond the adherens junctions. These fine cytoarchitectural details were missing in retinas that completed differentiation in the absence of the RPE; both calycal and apical processes were no longer present and adherens junctions were sparsely intermittent. Müller cells appeared atrophic. Similarly, mannose promoted none of the fine cytoarchitectural details of the retina. Lactose, however, supported the formation of the proper subcellular cytoarchitecture of both photoreceptor and Müller cells. These results suggest that these subcellular structures may be fundamental for the proper assembly and stability of organized outer segments and are necessary to allow for normal cytogenesis of the outer retina.

Wild-type and mutant forms of v-src differentially alter neuronal migration and differentiation in vivo

The effects of three different forms of v-src on brain cell development were determined in vivo. Recombinant retroviral vectors encoding the marker lacZ (control) and either wild-type v-src or SH2 or SH3 domain-deleted forms of v-src (deltaSH2 or deltaSH3, respectively) were used to infect neuronal progenitor cells in the embryonic chicken midbrain (optic tectum; OT). Embryos were injected in the OT with retroviral concentrates on embryonic day (E) 3 and sacrificed at E6, E9, and later in development. Patterns of cell proliferation, migration, and differentiation of lacZ-marked clonal cell progeny were then analyzed. Relative to lacZ-only controls, cell clone size at E6 was significantly increased for v-src-, unchanged for deltaSH2-, and smaller for deltaSH3-injected embryos. At E9, deltaSH2 cell clones were significantly larger than controls, suggesting increased survival from normal programmed cell death. Radial neuronal migration was impaired for v-src and deltaSH3 clones, whereas tangential neuronal migration was enhanced along fiber tracts in v-src and deltaSH2 clones. Moreover, radial glial cell development and differentiation was hindered in v-src and deltaSH3 clones. These experiments demonstrate that ectopic v-src signaling alters proliferation, migration, survival, and differentiation of developing brain cells and suggest that src signaling pathways are involved in these developmental processes. Furthermore, certain effects of v-src on brain cells require specific src homology domains.

Pax6 induces ectopic eyes in a vertebrate

We report here that misexpression of the transcription factor Pax6 in the vertebrate Xenopus laevis leads to the formation of differentiated ectopic eyes. Multiple molecular markers indicated the presence of mature lens fiber cells, ganglion cells, Muller cells, photoreceptors and retinal pigment epithelial cells in a spatial arrangement similar to that of endogenous eyes. Lineage tracing experiments showed that lens, retina and retinal pigment epithelium arose as a consequence of the cell-autonomous function of Pax6. These experiments also reveal that the cell autonomous activity of misexpressed Pax6 causes the ectopic expression of a number of genes including Rx, Otx2, Six3 and endogenous Pax6, each of which has been implicated in eye development. The formation of ectopic and endogenous eyes could be suppressed by coexpression of a dominant-negative form of Pax6. These data show that in vertebrates, as in the invertebrate Drosophila melanogaster, Pax6 is both necessary and sufficient to trigger the cascade of events required for eye formation.

Glucocorticoid control of glial gene expression

The glucocorticoid signaling pathway is responsive to a considerable number of internal and external signals and can therefore establish diverse patterns of gene expression. A glial-specific pattern, for example, is shown by the glucocorticoid-inducible gene glutamine synthetase. The enzyme is expressed at a particularly high level in glial cells, where it catalyzes the recycling of the neurotransmitter glutamate, and at a low level in most other cells, for housekeeping duties. Glial specificity of glutamine synthetase induction is achieved by the use of positive and negative regulatory elements, a glucocorticoid response element and a neural restrictive silencer element. Though not glial specific by themselves, these elements may establish a glial-specific pattern of expression through their mutual activity and their combined effect. The inductive activity of glucocorticoids is markedly repressed by the c-Jun protein, which is expressed at relatively high levels in proliferating glial cells. The signaling pathway of c-Jun is activated by the disruption of glia-neuron cell contacts, by transformation with v-src, and in proliferating retinal cells of early embryonic ages. The c-Jun protein inhibits the transcriptional activity of the glucocorticoid receptor and thus represses glutamine synthetase expression. This repressive mechanism might also affect the ability of glial cells to cope with glutamate neurotoxicity in injured tissues.

Regulation of the spatiotemporal pattern of expression of the glutamine synthetase gene

Glutamine synthetase, the enzyme that catalyzes the ATP-dependent conversion of glutamate and ammonia into glutamine, is expressed in a tissue-specific and developmentally controlled manner. The first part of this review focuses on its spatiotemporal pattern of expression, the factors that regulate its levels under (patho)physiological conditions, and its role in glutamine, glutamate, and ammonia metabolism in mammals. Glutamine synthetase protein stability is more than 10-fold reduced by its product glutamine and by covalent modifications. During late fetal development, translational efficiency increases more than 10-fold. Glutamine synthetase mRNA stability is negatively affected by cAMP, whereas glucocorticoids, growth hormone, insulin (all positive), and cAMP (negative) regulate its rate of transcription. The signal transduction pathways by which these factors may regulate the expression of glutamine synthetase are briefly discussed. The second part of the review focuses on the evolution, structure, and transcriptional regulation of the glutamine synthetase gene in rat and chicken. Two enhancers (at -6.5 and -2.5 kb) were identified in the upstream region and two enhancers (between +156 and +857 bp) in the first intron of the rat glutamine synthetase gene. In addition, sequence analysis suggests a regulatory role for regions in the 3' untranslated region of the gene. The immediate-upstream region of the chicken glutamine synthetase gene is responsible for its cell-specific expression, whereas the glucocorticoid-induced developmental appearance in the neural retina is governed by its far-upstream region.

Overexpression of c-src and n-src in the developing Xenopus retina differentially impairs axonogenesis

To compare the roles of the nonreceptor tyrosine kinase c-src and its neuronal splice form n-src in developing neurons, Xenopus retinal precursors were transfected in vivo with c-src, n-src, or constitutively active mutants. Axonogenesis of retinal ganglion cells was markedly impaired by the expression of constitutively active c-src and only mildly affected by the expression of constitutively active n-src. This differential phenotype could not be accounted for by raised levels of intracellular tyrosine phosphorylation alone because the average anti-phosphotyrosine staining intensity of retinal neurons expressing mutant n-src was almost twofold greater than that of neurons expressing mutant c-src. The expression of either constitutively active isoform inhibited photoreceptor differentiation by 72% but did not influence other cell fates. These results suggest that c-src and n-src have both overlapping and distinct activities in differentiating retinal neurons.

QRI, a retina-specific gene, encodes an extracellular matrix protein exclusively expressed during neural retina differentiation

Neural retina development results from growth arrest of neuroectodermal precursors and differentiation of postmitotic cells. The QRI gene is specifically expressed in Müller retinal glial cells. Its expression coincides with the stage of withdrawal from the cell cycle and establishment of differentiation and is repressed upon induction of retinal cell proliferation by the v-src gene product. In this report, we show that the QR1 gene encodes several glycosylated proteins that are secreted and can either associate with the extracellular matrix or remain diffusible in the medium. By using pulse-chase experiments, the 100-103 kDa forms seem to appear first and are specifically incorporated into the extracellular matrix, whereas the 108 and 60 kDa polypeptides appear later and are detected as soluble forms in the culture medium. We also report that expression of the QR1 gene is developmentally regulated in the chicken. Its mRNA is first detectable at embryonic day 10, reaches a maximal level at embryonic day 15 and is no longer detected at embryonic day 18. Immunolocalization of the QR1 protein in chicken retina sections during development shows that expression of the protein parallels the differentiation pattern of post-miotic cells (in particular Müller cells and rods), corresponding to the two differentiation gradients in the retina: from the ganglion cell layer to the inner nuclear layer and outer nuclear layer, and from the optic nerve to the iris. At embryonic day 10, expression of the QR1 protein(s) is restricted to the optic nerve region and the inner nuclear layer, colocalizing with Müller cell bodies. As development proceeds, QR1 protein localization spreads towards the iris and towards the outer nuclear layer, following Müller cell elongations towards the photoreceptors. Between embryonic days 16 and 18, the QR1 protein is no longer detectable in the optic nerve region and is concentrated around the basal segment of the photoreceptors in the peripheral retina. Our results suggest a role for the QR1 gene product in the process of growth arrest and establishment of photoreceptor differentiation.

Molecular basis for differential expression of glutamine synthetase in retina glia and neurons

Glutamine synthetase (GS) is a differentiation marker of retina glial cell. It is expressed in the chicken neural retina at a particularly high level, is inducible by glucocorticoids and is always confined to Müller glia. This study investigated the molecular basis for tissue and cell-type specific expression of the GS gene. A high level of GS expression in the retina was found to coincide with the accumulation of a relatively high level of GS mRNA in this tissue. The gliatoxic agent alpha-aminoadipic acid, which can selectively destroy glia cells, was used to demonstrate that restriction of GS induction to Müller glia is controlled at a transcriptional level. Cortisol could induce accumulation of GS mRNA and transcription of the GS gene in Müller glia but not in retina neurons. Glia and neurons were also found to differ in their ability to express the glucocorticoid inducible CAT construct, p delta G46TCO, which is controlled by a 'simple GRE' promoter. When introduced into cells of retina tissue, this construct was cortisol-inducible in glia whereas in neurons it was only slightly inducible or not at all. Introduction of a glucocorticoid receptor expression vector into the cells facilitated induction of the CAT construct in neurons. Analysis by immunoblotting revealed that expression of the glucocorticoid receptor protein is predominantly restricted to Müller glia. These results suggest that differential levels of glucocorticoid receptor expression in glia and neurons might be the basis for cell-type specific induction of GS.

Molecular control of glutamine synthetase expression in the developing retina tissue

Glutamine synthetase is a differentiation marker of the neural retina, whose expression is restricted to Müller glia cells, is inducible by glucocorticoids and is dependent on tissue development. The retina tissue acquires the competence to express GS in response to glucocorticoids with development, although the level of hormone binding activity in the cells does not alter with age. Using CAT constructs that are controlled by "simple GRE" promoters we demonstrated that glucocorticoid receptor transcription activity in retina cells increases with development. The increase in receptor activity correlates directly with the increase in inducibility of the glutamine synthetase gene and inversely with the rate of retina cell proliferation. At early developmental ages, when retina cells are still proliferating, the glucocorticoid receptor is transcriptionally inactive and glutamine synthetase expression cannot be induced. Receptor activity increases progressively with development and by day 12, when cell proliferation ceases, competence for glutamine synthetase induction is high. This competence for glutamine synthetase induction can be repressed by overexpressing the oncogene v-src, which stimulates retina cell proliferation. We discuss possible mechanisms for developmental-dependent modulation of glucocorticoid receptor transcriptional activity.