Accumulation of c-src mRNA is developmentally regulated in embryonic neural retina

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.

Hormonal and non-hormonal regulation of glutamine synthetase in the developing neural retina

Two isoforms of the glucocorticoid receptor, with apparent molecular mass of 90 and 95 kDa, are expressed in embryonic chicken neural retina. The 95-kDa receptor represents a hyperphosphorylated form of the 90-kDa receptor. Activation of the glucocorticoid receptor by cortisol results in a dose-dependent increase in receptor phosphorylation, translocation of receptor molecules into the nucleus and a decline in the total amount of the receptor. Activation of the glucocorticoid receptor can also be observed in the developing retinal tissue in ovo. At late embryonic ages, when the systemic level of glucocorticoids increases, a substantial quantity of receptor molecules becomes translocated into the nucleus, the relative level of the 95-kDa isoform increases, and the total amount of receptor declines. Activation of the receptor molecules in ovo correlates directly with an increase in transcription of the glucocorticoid-inducible gene, glutamine synthetase. The close correlation between the increase in systemic glucocorticoids, activation of glucocorticoid receptor molecules and induction of glutamine synthetase gene transcription suggests that glucocorticoids are directly involved in the developmental control of glutamine synthetase expression. Long-term organ culturing of embryonic retinal tissue in the absence of hormone results in an increase in glutamine synthetase expression. This increase, which is only 5 to 10% of that observed in ovo, is not mediated by activated receptor molecules and represents a mechanism for non-hormonal regulation of glutamine synthetase.

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.

Neuronal pp60c-src(+) in the developing chick spinal cord as revealed with anti-hexapeptide antibody

Polyclonal antibody was raised in rabbits against a synthetic hexapeptide R-K-V-D-V-R corresponding to a unique amino acid sequence of the neuron-specific c-src gene product pp60c-src(+). The antibody was purified by affinity chromatography. A single band with an apparent molecular mass of 60 kDa was recognized when the supernatant of homogenates of brain and spinal cord from chick embryos and chicks was probed with the affinity purified anti-hexapeptide antibody after SDS-polyacrylamide gel electrophoresis followed by Western blotting. Specificity of the antibody was further characterized by autophosphorylation assay of immunoprecipitate in comparison with the monoclonal antibody 327. Immunocytochemical studies by light microscopy revealed that pp60c-src(+) was localized in flake-like aggregates in neuronal cell bodies of the spinal cord in 7-15-day-incubated chick embryos and newly hatched chicks. Developing spinal ganglia and muscle cells were also immunoreactive at early developmental stages. By electron microscopy, the reaction product was observed mainly in two regions. One region was at polysomes and along the membranes of the rough endoplasmic reticulum. The other region was along the neuronal plasma membrane--at subsurface cisterns and at synapses. At synapses, the postsynaptic density, presynaptic membrane and synaptic vesicle membranes were immunostained. Immunoreactivity at synapses were more frequently observed at earlier stages than at later stages of development. These findings suggest that pp60c-src(+) is actively produced in developing neurons and has some important roles in synaptogenesis. In mature synapses, pp60c-src(+) may be involved in the interaction of synaptic vesicles with the presynaptic membrane.

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.

Modulation of retinal differentiation by oncogenes: effect of the v-src gene on expression of choline acetyltransferase and glutamine synthetase

Expression of the protooncogene c-src in chick neural retina is developmentally regulated and associated with neural differentiation. In the present study, chick neural retina (NR) cell cultures from 7 day embryos were exposed to the exogenous src oncogene, the c-src counterpart, to establish the effect of expression of v-src on specific retinal cellular differentiation. NR cells from 7 day chick embryos were placed in monolayer or rotation culture and infected with Rous sarcoma virus (RSV) containing a single transforming gene. Other cultures were infected with a transforming defective mutant of RSV which still possesses mitogenic activity for NR cells. While control cultures showed typical neuronal and Muller cell morphologies at the light and electron microscopic level, NR cells infected with RSV exhibited dramatic morphological alterations in monolayer culture and cell aggregates. However, the mutant src gene induced mitosis without accompanying transforming properties. When aggregate cultures were treated with hydrocortisone to induce glutamine synthetase (GS) expression in Muller cells, control cultures showed the typical immunofluorescence pattern of GS staining, while RSV infected cultures showed no GS fluorescence. Cultures infected with mutant RSV showed some staining for GS. In contrast, choline acetyltransferase activity was shown to increase in both monolayer and aggregate cultures of retinal cells following v-src expression. These data indicate that the presence of excess v-src in differentiating cultures of NR inhibits the expression of some neural specific enzymes and enhances the presence of other specific proteins. Moreover, continually growing cultures of oncogene-altered retinal cells may be useful as models to study gene expression in development of the nervous system.

Tyrosine phosphorylated proteins accumulate in junctional regions of the developing chick neural retina

Antibodies specific for protein phosphotyrosyl residues were used to localize sites of action of tyrosine-specific protein kinases in developing chick neural retina by immunoperoxidase staining. Phosphotyrosine-modified proteins became prominent in growth cone- and process-rich regions of embryonic retina during neuronal differentiation. Maximal levels accumulated in the synaptic layers and limiting membranes of the adult retina, where numerous junctional complexes reside. Two major phosphotyrosine-modified proteins in adult retina (80, 42 kDal) increased markedly during maturation. In contrast, the synaptic layers of optic tectum and other brain regions exhibited low protein phosphotyrosine levels. These results suggest a specific role for protein tyrosine phosphorylation in the retina at sites of synapses and other intercellular junctions.

Evolution of the neuron-specific alternative splicing product of the c-src proto-oncogene

The observation of a slower migrating form of pp60c-src in neural tissue of chicken and mouse has recently been shown to be due to an alternative transcript form of the c-src gene (Martinez et al.: Science 237:411-415, 1987; Levy et al.: Mol Cell Biol 7:4142-4145, 1987). An insertion of 18 basepairs between exons 3 and 4, presumed to be due to alternative splicing of a mini-exon, gives rise to six amino acid residues not found in the non-neuronal (termed fibroblastic) form of pp60c-src. We have addressed the question of the evolutionary origin of the c-src neuronal insert and its functional significance regarding neural-specific expression of the c-src gene. To this end we have investigated whether the c-src gene of a lower vertebrate (the teleost fish Xiphophorus) gives rise to a neural-specific transcript in an analogous manner. We could show that the fish c-src gene does encode for a "fibroblastic" and a "neuronal" form of transcript and that the neuronal transcript does indeed arise by way of alternative splicing of a mini-exon. The mini-exon is also 18 basepairs long and we could demonstrate directly that this exon lies within the intron separating exons 3 and 4. For comparative purposes we have examined whether the fish c-yes gene, the member of the src gene family most closely related to c-src, also encodes a neural tissue-specific transcript. No evidence for a second transcript form in brain was obtained.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of pp60c-src tyrosine kinase activity during secretion in stimulated bovine adrenal chromaffin cells

High levels of the proto-oncogene product, pp60c-src, have been found in developing and adult neural tissues as well as in certain fully mature cells of the hematopoietic lineage, e.g., platelets and myelomonocytes. Adrenal medullary chromaffin cells exhibit characteristics of both types of cells, i.e., they are derived from the neural crest and carry out exocytosis in response to specific stimuli. Earlier studies have shown that pp60c-src localizes not only to the plasma membrane of chromaffin cells but also to the membranes of chromaffin granules, the secretory vesicles of these cells that store catecholamines and other secretory products. To investigate the possible involvement of pp60c-src in exocytosis, cultured bovine chromaffin cells were analyzed for changes in c-src tyrosine kinase activity in response to stimulation by several secretagogues. Results of in-vitro immune complex kinase assays showed that pp60c-src, derived from cells that had been stimulated for various lengths of time, exhibited decreased auto- and transphosphorylating activities as compared to pp60c-src immunoprecipitated from control cells. The greatest reduction in activity was observed 10 min post-stimulation, while normal levels were regained 2-6 hr after secretagogue treatment. Western immunoblot analysis of the immunoprecipitated pp60c-src revealed that approximately 50% less c-src protein was present in immune complexes prepared 10 min after stimulation as compared to those prepared from mock-stimulated controls, resulting in a specific autophosphorylating activity that was 42-47% of control and little or no reduction in the transphosphorylating specific activity. In experiments in which the rate of secretion of [3H]-norepinephrine from cells preloaded with this compound was compared to the rate of modulation of pp60c-src activity, 50% of the maximal reduction in pp60c-src activity occurred within 2-4 min while 50% maximal release of [3H]-norepinephrine occurred within 1-3 min. Taken together, these results suggest that pp60c-src may play some role (direct or indirect) in the exocytotic process.

Expression of pp60c-src in adult and developing rat central nervous system

Brains of adult and fetal (E13-E19) rats were assayed by region for the presence of the proto-oncogene product pp60c-src. pp60c-src was abundant in the adult brain with the highest levels found in the cerebral hemispheres and localized to the cortical cellular layers. In the embryonic nervous system the levels of pp60c-src activity were much higher throughout the brain than those observed in the adult. The expression of pp60c-src was developmentally regulated, but demonstrated a regionally distinct pattern of expression. In the cortex src activity steadily rose during gestation, while in the basal forebrain and midbrain maximal activity was observed at E17 which then declined to adult levels. The data demonstrate that pp60c-src is differentially expressed in regions of the brain, both during development and in the adult.

Expression of proto-oncogenes in neural tissues

Several proto-oncogenes have been shown to be expressed in tissues of neural origin. In most cases, their expression is developmentally regulated and they encode proteins similar in their sequence to a variety of known proteins involved in transferring information from the cell surface to the nucleus. Some of the proto-oncogenes, including src and yes, are expressed preferentially in neural tissues and one of them, src+, is expressed there exclusively. Many of neurally expressed proto-oncogenes, including src, yes, ras and myc, are also found in organs containing epithelial cells involved in ion transport. It is possible that proteins encoded by these proto-oncogenes are themselves involved in some aspects of ion transport. Among defined categories of neurons expressing proto-oncogenes, cerebellar Purkinje cells are most frequently mentioned. They express at least 3 proto-oncogenes, src, yes, myc, as well as protein kinase C. Purkinje cells make an attractive model for functional studies of these proteins. Although an integrated picture-illuminating cooperative action of proto-oncogenes in neural or other tissues is missing, it is hoped that discovery of new classes of proto-oncogenes, and functional interactions among them, may help us to understand not only oncogenesis but also biology of the nervous system.

Control of myogenic differentiation by cellular oncogenes

The establishment of a differentiated phenotype in skeletal muscle cells requires withdrawal from the cell cycle and termination of DNA synthesis. Myogenesis can be inhibited by serum components, purified mitogens, and transforming growth factors, but the intracellular signaling pathways utilized by these molecules are unknown. Recent studies have confirmed a role for proteins encoded by cellular proto-oncogenes in transduction of growth factor effects that lead to cell proliferation. To test the contrasting hypothesis that cellular oncogenes might also regulate tissue-specific gene expression in developing muscle cells, myoblasts have been modified by incorporation of the cognate viral oncogenes, the corresponding normal or oncogenic cellular homologs, and chimeric oncogenes, whose expression can be induced reversibly. Regulation of the endogenous cellular oncogenes also has been examined in detail. Down-regulation of c-myc is not obligatory for myogenesis; rather, inhibitory effects of myc on muscle differentiation are contingent on sustained proliferation. In contrast, activated src and ras genes block myocyte differentiation directly, through a mechanism that is independent of DNA synthesis and is rapidly reversible, resembling the effects of inhibitory growth factors. The coordinate regulation of diverse tissue-specific gene products including muscle creatine kinase, nicotinic acetylcholine receptors, sarcomeric proteins, and voltage-gated ion channels, raises the hypothesis that inhibitors such as transforming growth factor-beta and ras proteins might exert their effects through a transacting transcriptional signal shared by multiple muscle-specific genes.

A molecular view of vertebrate retinal development

Immunological probes have begun to identify molecules that delineate cell layers and cell types during the formation of the retina and other parts of the optic cup. Within the developing retina, cell-type-specific monoclonal antibodies have been used to show that differentiation occurs before cells reach their final laminar position. Cell surface molecules have been found expressed in position-dependent gradients across the retina. These molecules may convey positional information to the retinal cells and their topographic connections. One such molecule is a modified carbohydrate group on a ganglioside, suggesting that such groups may play a role in neural development. A variety of molecules that are expressed by rod photoreceptors at defined stages of their differentiation have been characterized. These molecules have been used to show the development of subcellular compartments within rods. In vitro studies have suggested that photoreceptor molecules expressed at different times are under different forms of regulation. Some of these cell-specific molecules have been shown to be under transcriptional control and thus defined cell interactions seem to be linked to changes in gene expression during retinal development.