Neural nicotinic acetylcholine receptor and GAP-43 gene expression and regulation in the developing mammalian retina.

Abstract

During development of the retina multipotential progenitor cells commit to a particular fate and differentiate into mature neurons. One of the earliest differentiations to take place in the developing retina is the formation of retinal ganglion cells (RGCs). The molecular mechanisms by which RGCs differentiate are unknown. One approach to this problem is to identify genes which are expressed during RGC differentiation and characterize their mechanisms of induction. We have taken this approach using genes encoding nicotinic acetylcholine receptors (nAChR) and the growth associated protein, GAP-43. In situ hybridization assays were used to identify the temporal and spatial patterns of nAChR and GAP-43 gene expression in the developing retina. Both genes were induced around embryonic day 14 (E14) in recently born RGCs located at the vitreal surface. This corresponds to a time when RGC axons were growing towards their brain targets. Based on these results, we hypothesized that either the environment the RGC axons traversed or their brain targets were initiating a signal that resulted in nAChR and GAP-43 gene expression. This hypothesis was tested by either transplanting E12 retinas to different brain environments or by explanting them into organ culture. The E12 retina was chosen since it represents a developmental time prior to RGC birth and therefore does not express nAChR or GAP-43 genes. These studies showed that both transplanted and explanted retinae induce nAChR and GAP-43 genes on schedule. Therefore, neither the environment through which the RGC axon grows, nor its target is required for induction of nAChR or GAP-43 genes. We investigated nAChR and GAP-43 gene expression in the ocular retardation mutant mouse, $or\sp{J}$. This mutant arrests retinal development around E14, a time when progenitor cells are differentiating into RGCs. Therefore, we investigated whether these early differentiating RGCs expressed their nAChR and GAP-43 genes. In situ hybridization experiments showed GAP-43 but not nAChR gene induction in the RGCs. This result indicates that although these genes are temporally induced in RGCs, their mechanism of induction is different. The $or\sp{J}$ mouse model should be useful to facilitate the characterization of these mechanisms.