Late retinal progenitor cells show intrinsic limitations in the production of cell types and the kinetics of opsin synthesis.

The seven major cell classes of the vertebrate neural retina arise from a pool of multipotent progenitor cells. Several studies suggest a model of retinal development in which both the environment and the progenitor cells themselves change over time (). To test this model, we used a reaggregate culture system in which a labeled population of progenitor cells from the postnatal rat retina were cultured with an excess of embryonic retinal cells. The labeled cells were then assayed for their cell fate choices and their kinetics of rod differentiation, as measured by opsin synthesis. The kinetics of opsin synthesis remained unchanged, but fewer postnatal cells adopted the rod cell fate when cultured with embryonic cells. There was an increase in the percentage of bipolar cells produced by postnatal progenitor cells, indicating a possible respecification of fate. The increase in bipolar cells could occur even after progenitor cells had completed their terminal mitoses. These alterations in cell fates appeared to be caused at least in part by a secreted factor released by the embryonic cells that requires the LIFRbeta/gp130 complex for signaling. Finally, although surrounded by 20-fold more embryonic cells, the postnatal cells did not choose to adopt any fates normally produced only by embryonic cells.

Extrinsic and intrinsic factors control the genesis of amacrine and cone cells in the rat retina.

The seven major classes of cells of the vertebrate neural retina are generated from a pool of multipotent progenitor cells. Recent studies suggest a model of retinal development in which both the progenitor cells and the environment change over time (Cepko, C. L., Austin, C. P., Yang, X., Alexiades, M. and Ezzeddine, D. (1996). Proc. Natl. Acad. Sci. USA 93, 589-595). We have utilized a reaggregate culture system to test this model. A labeled population of progenitors from the embryonic rat retina were cultured with an excess of postnatal retinal cells and then assayed for their cell fate choices. We found that the postnatal environment had at least two signals that affected the embryonic cells' choice of fate; one signal inhibited the production of amacrine cells and a second affected the production of cone cells. No increase in cell types generated postnatally was observed. The source of the inhibitor of the amacrine cell fate appeared to be previously generated amacrine cells, suggesting that amacrine cell number is controlled by feedback inhibition. The progenitor cell lost its ability to be inhibited for production of an amacrine cell as it entered M phase of the cell cycle. We suggest that postmitotic cells influence progenitor cell fate decisions, but that they do so in a manner restricted by the intrinsic biases of progenitor cells.

Two phases of rod photoreceptor differentiation during rat retinal development.

We have conducted a comprehensive analysis of the relative timing of the terminal mitosis and the onset of rhodopsin expression in rod precursors in the rat retina in vivo. This analysis demonstrated that there are two distinct phases of rod development during retinal histogenesis. For the majority of rod precursors, those born on or after embryonic day 19 (E19), the onset of rhodopsin expression was strongly correlated temporally with cell cycle withdrawal. For these precursors, the lag between the terminal mitosis and rhodopsin expression was measured to be 5.5-6.5 d on average. By contrast, for rod precursors born before E19, the lag was measured to be significantly longer, averaging from 8.5 to 12.5 d. In addition, these early-born rod precursors seemed to initiate rhodopsin expression in a manner that was not correlated temporally with the terminal mitosis. In these cells, onset of rhodopsin expression appeared approximately synchronous with later-born cells, suggesting a synchronous recruitment to the rod cell fate induced by environmental signals. To examine this possibility, experiments in which the early-born precursors were exposed to a late environment were conducted, using a reaggregate culture system. In these experiments, the early-born precursors appeared remarkably uninfluenced by the late environment with respect to both rod determination and the kinetics of rhodopsin expression. These results support the idea that intrinsically distinct populations of rod precursors constitute the two phases of rod development and that the behavior exhibited by the early-born precursors is intrinsically programmed.

Schwannomin: new insights into this member of the band 4.1 superfamily.

Neurofibromatosis type 2 (NF2) is an autosomal dominant disease characterized by the development of central nervous system tumours. The NF2 gene was recently cloned and found to encode a protein, schwannomin (or merlin), with homology to the band 4.1 superfamily. This superfamily of proteins includes ezrin, moesin, radixin, and talin, as well as several protein tyrosine phosphatases. How does a cytoskeleton-associated protein act as a tumour suppressor? While this fundamental question remains unanswered, recent studies have begun to address key questions regarding the function of schwannomin. In this review, we examine what is known about the band 4.1 superfamily and how this information pertains to schwannomin. In addition, we summarize recent studies of schwannomin itself.

Vulnerability of oligodendroglia to glutamate: pharmacology, mechanisms, and prevention.

Periventricular white matter injury, the principal variety of brain injury of the human premature infant, involves differentiating oligodendroglia. Nothing is known of the biochemical mechanism of oligodendroglial death in this disorder. Because an early event in periventricular white matter injury is ischemia-induced axonal disruption and because such axonal destruction could lead to a marked increase in local concentrations of glutamate, we evaluated the vulnerability of differentiating oligodendroglia to glutamate in a culture model. Oligodendroglia were isolated from mixed-glial primary cultures by a selective detachment technique and grown in a primary culture under conditions that lead to differentiation. These oligodendroglia were found to be highly vulnerable to glutamate-induced cell death. The EC50 for glutamate for a 24 hr exposure was approximately 200 microM, comparable to the value reported for neurons in conventional cerebral cortical cultures. Astrocytes, in contrast, were shown to be resistant to as much as 5 mM glutamate. Study of glutamate receptor antagonists and glutamate transport substrates showed that the glutamate-induced oligodendroglial death was not related to a receptor mechanism, as operates in neurons, but rather was secondary to glutamate uptake by the oligodendroglia. Glutamate transport by high-affinity, sodium-dependent and by sodium-independent systems was shown. The central importance of glutamate uptake for the toxic effect of glutamate was shown by total prevention of the oligodendroglial toxicity by the simultaneous inhibition of glutamate uptake by the specific inhibitor D,L-threo-beta-hydroxyaspartate. Subsequent observations showed that the toxicity of glutamate was mediated by free radical attack, the consequence of glutathione depletion, apparently caused by the action of a glutamate-cystine exchange mechanism that results in cystine and thereby glutathione depletion. Thus, addition of cystine or cysteine totally prevented the glutamate toxicity to oligodendroglia. Second, glutamate exposure led to cystine efflux. Third, glutathione levels decreased markedly in cells exposed to glutamate, and this marked decrease preceded the loss of cell viability. Fourth, glutamate toxicity could be prevented totally by exposure to different free radical scavengers, vitamin E and idebenone. The data thus show that glutamate is highly toxic to oligodendroglia. Moreover, the findings raise the possibilities that such glutamate toxicity is operative in the oligodendroglial cell death associated with ischemic processes that disrupt axons, such as periventricular white matter injury of the premature infant, and that novel therapies directed against glutamate transport, glutathione depletion, and free radical attack might be beneficial in prevention of that injury.