Loss of CRB2 in Müller glial cells modifies a CRB1-associated retinitis pigmentosa phenotype into a Leber congenital amaurosis phenotype.

Variations in the human Crumbs homolog-1 (CRB1) gene lead to an array of retinal dystrophies including early onset of retinitis pigmentosa (RP) and Leber congenital amaurosis (LCA) in children. To investigate the physiological roles of CRB1 and CRB2 in retinal Müller glial cells (MGCs), we analysed mouse retinas lacking both proteins in MGC. The peripheral retina showed a faster progression of dystrophy than the central retina. The central retina showed retinal folds, disruptions at the outer limiting membrane, protrusion of photoreceptor nuclei into the inner and outer segment layers and ingression of photoreceptor nuclei into the photoreceptor synaptic layer. The peripheral retina showed a complete loss of the photoreceptor synapse layer, intermingling of photoreceptor nuclei within the inner nuclear layer and ectopic photoreceptor cells in the ganglion cell layer. Electroretinography showed severe attenuation of the scotopic a-wave at 1 month of age with responses below detection levels at 3 months of age. The double knockout mouse retinas mimicked a phenotype equivalent to a clinical LCA phenotype due to loss of CRB1. Localization of CRB1 and CRB2 in non-human primate (NHP) retinas was analyzed at the ultrastructural level. We found that NHP CRB1 and CRB2 proteins localized to the subapical region adjacent to adherens junctions at the outer limiting membrane in MGC and photoreceptors. Our data suggest that loss of CRB2 in MGC aggravates the CRB1-associated RP-like phenotype towards an LCA-like phenotype.

Crumbs 2 prevents cortical abnormalities in mouse dorsal telencephalon.

The formation of a functionally integrated nervous system is dependent on a highly organized sequence of events that includes timely division and differentiation of progenitors. Several apical polarity proteins have been shown to play crucial roles during neurogenesis, however, the role of Crumbs 2 (CRB2) in cortical development has not previously been reported. Here, we show that conditional ablation of Crb2 in the murine dorsal telencephalon leads to defects in the maintenance of the apical complex. Furthermore, within the mutant dorsal telencephalon there is premature expression of differentiation proteins. We examined the physiological function of Crb2 on wild type genetic background as well as on background lacking Crb1. Telencephalon lacking CRB2 resulted in reduced levels of PALS1 and CRB3 from the apical complex, an increased number of mitotic cells and expanded neuronal domain. These defects are transient and therefore only result in rather mild cortical abnormalities. We show that CRB2 is required for maintenance of the apical polarity complex during development of the cortex and regulation of cell division, and that loss of CRB2 results in cortical abnormalities.