Pax6–dependent cell adhesion molecules assume important functions in retinal morphogenesis.

Abstract

Abstract: : Purpose: To identify Pax6 target genes coding for cell adhesion molecules and to assess their contribution to Pax6–controlled laminar organization of the retina in Xenopus. Methods: The mRNA levels of the putative Pax6 target genes XmN–cadherin, a maternal cadherin related to mammalian cadherin–4, XN–cadherin, homologous to cadherin–2, and N–CAM were measured by real–time PCR in control and Pax6–deficient embryos, in which Pax6 expression was blocked by antisense morpholino molecules. The Pax6 target genes XmN– and XN–cadherin as well as N–CAM were individually inhibited by specific antisense morpholinos. Phenotypes were analyzed by external, morphological and immunocytochemical examination. Results: In Pax6–deficient embryos, transcription of the adhesion molecules tested was strongly reduced at neurula stage and did not reach normal levels throughout ocular morphogenesis. Direct inhibition of XmN– or XN–cadherin by antisense morpholino caused phenotypes that closely resembled Pax6–inhibited embryos with ocular deficiencies such as reduced eye size and photoreceptor rosettes around clusters of pigment epithelial cells displaced into the neural retinal layers. Inner retinal layers were often split into fragments or duplicated, forming an additional structure external to the photoreceptor layer. Inhibition of N–CAM did not cause a significant reduction in eye size but also led to severe laminar disruption and rosette formation. Conclusions: The expression of at least three genes coding for different cell adhesion molecules in the retina depends on Pax6. These cell adhesion systems, involving XmN–, XN–cadherin, or N–CAM assume crucial functions in establishing correct lamination of retinal layers.

Keywords: cell adhesions/cell junctions • retinal development • gene/expression