The influence of neuronal-glial interactions on glia-specific gene expression in embryonic retina

Ectopic expression of L1CAM ectodomain alters differentiation and motility, but not proliferation, of human neural progenitor cells

Adult human neural progenitor and stem cells have been implicated as a potential source of brain cancer causing cells, but specific events that might cause cells to progress towards a transformed phenotype remain unclear. The L1CAM (L1) cell adhesion/recognition molecule is expressed abnormally by human glioma cancer cells and is released as a large extracellular ectodomain fragment, which stimulates cell motility and proliferation. This study investigates the effects of ectopic overexpression of the L1 long ectodomain (L1LE; ˜180 kDa) on the motility, proliferation, and differentiation of human neural progenitor cells (HNPs). L1LE was ectopically expressed in HNPs using a lentiviral vector. Surprisingly, overexpression of L1LE resulted in reduced HNP motility in vitro, in stark contrast to the effects on glioma and other cancer cell types. L1LE overexpression resulted in a variable degree of maintenance of HNP proliferation in media without added growth factors but did not increase proliferation. In monolayer culture, HNPs expressed a variety of differentiation markers. L1LE overexpression resulted in loss of glutamine synthetase (GS) and β3-tubulin expression in normal HNP media, and reduced vimentin and increased GS expression in the absence of added growth factors. When co-cultured with chick embryonic brain cell aggregates, HNPs show increased differentiation potential. Some HNPs expressed p-neurofilaments and oligodendrocytic O4, indicating differentiation beyond that in monolayer culture. Most HNP-L1LE cells lost their vimentin and GFAP (glial fibrillary acidic protein) staining, and many cells were positive for astrocytic GS. However, these cells rarely were positive for neuronal markers β3-tubulin or p-neurofilaments, and few HNP oligodendrocyte progenitors were found. These results suggest that unlike for glioma cells, L1LE does not increase HNP cell motility, but rather decreases motility and influences the differentiation of normal brain progenitor cells. Therefore, the effect of L1LE on increasing motility and proliferation appears to be limited to already transformed cells.

Challenges in the study of neuronal differentiation: a view from the embryonic eye

Progress in the study of the molecular mechanisms that regulate neuronal differentiation has been quite impressive in recent years, and promises to continue to an equally fast pace. This should not lead us into a sense of complacency, however, because there are still significant barriers that cannot be overcome by simply conducting the same type of experiments that we have been performing thus far. This article will describe some of these challenges, while highlighting the conceptual and methodological breakthroughs that will be necessary to overcome them.

A model of retinal cell differentiation in the chick embryo

This article presents an overview of retinal cell differentiation in the chick embryo, in the context of a hypothetical model based on information generated during the last several years. The model proposes that: (1) most (if not all) proliferating neuroepithelial cells have the potential to give rise to a progeny comprising two or more different cell types; (2) the time at which cells undergo their terminal mitosis does not determine their differentiated fate; (3) many postmitotic precursor cells remain plastic (i.e., uncommitted) for some time after terminal mitosis, during which they encounter position-dependent signals as they migrate toward their definitive laminar position within the retina; (4) as a consequence of these inductive stimuli, precursor cells that migrate to different retinal layers express different transcriptional regulators; (5) morphologically undifferentiated precursor cells are committed to cell type-specific, complex patterns of differentiation, which they can express even when isolated from their normal microenvironment, and (6) even after precursor cells become committed to a specific identity, additional inductive signals are necessary for the cells to complete the development of a fully mature phenotype. The article presents a summary of the supportive evidence, as well as a critical evaluation of the model, and concludes with an overview of unanswered questions regarding retinal cell differentiation and a brief evaluation of the prospects for further progress in this field.

A morphological and immunohistochemical study of human retinal pigment epithelial cells, retinal glia, and fibroblasts grown on Gelfoam matrix in an organ culture system. A comparison of structural and nonstructural proteins and their application to cell type identification

Retinal pigment epithelial (RPE) cells, retinal glia, and fibroblasts undergo marked phenotypical change when outside their usual microenvironment, as occurs in epiretinal membrane formation. To explore their phenotypic potential without the influence of other cell types, each was cultured on Gelfoam matrix and assessed immunohistochemically and ultrastructurally. All cell types demonstrated vimentin and a universal beta-tubulin epitope, TU27. RPE and retinal glial cells were positive for cytokeratin, Leu 7, and neuron-specific (gamma gamma) enolase, as were glia and fibroblasts for S-100 protein and RPE cells and fibroblasts for glutamine synthetase. RPE cells alone showed positivity for class III beta-tubulin and retinal S-antigen (monolayer cultures only); occasional retinal glia, which immunohistochemical findings suggest are Müller cell derived, demonstrated GFA protein. Therefore, class III beta-tubulin may be useful in distinguishing RPE cells from retinal glia and fibroblasts, and Leu-7 may help to identify RPE cells and fibroblasts; these cell types are difficult to distinguish in clinical material using more traditional morphological criteria.

Effects on the fetal rat eye of maternal benomyl exposure and protein malnutrition

Benomyl, a benzimidazole fungicide, produced ocular and craniocerebral malformations in fetal rats when administered to the dams by gavage in a dose of 62.4 mg/kg of maternal body weight/day on days 7-21 of gestation. Ocular anomalies included retinal dysplasia, cataracts, microphthalmia, and anophthalmia. These anomalies occurred in 43.3% of fetuses exposed to benomyl and a normal protein diet but increased to 62.5% when benomyl administration was combined with a protein deficient (8% casein) diet. Microscopic examination of the malformed eyes revealed that the most common abnormality, retinal dysplasia, consisted of rosettes of retinal cells and retinal infolding. The majority of rosettes had a single layer and a limiting membrane. Rosettes with two or three layers were also observed, particularly in fetuses exposed both to protein deficiency and benomyl. Although anophthalmia was identified macroscopically in five fetuses, only a single instance of true anophthalmia was found microscopically. These data support the results of previous investigators that benomyl induces ocular malformation and that protein deficiency enhances the teratogenic effects of benomyl. The disorderly proliferation of retinal cells and rosette formation resembled the periventricular cell masses that accumulate in brains exposed to benomyl and certain other teratogenic agents. The anti-tubulin action of benomyl is known to impair microtubule formation and it may produce brain and ocular malformations by disruption of neuronal proliferation and migration.