Role of laminins containing beta2 and gamma3 chains in regulating retinal neurogenesis and morphogenesis.

Abstract

Establishment and maintenance of cell polarity plays a critical role in the development, organization, and function of tissues. Disruptions of cell polarity and tissue organization lead to pathophysiology in many organ systems including epithelial gland cancers. A key element in establishing cell polarity and the maintenance of tissue organization is the attachment to laminin-rich basement membrane that underlies all epithelial derived tissues. Arising from the ectoderm, the CNS shares an epithelial lineage and indeed laminins containing the b2 and g3 chains have been isolated from both the brain and retina. In fact, laminin b2 chain is a major constituent of the basement membrane of the retina – the inner limiting membrane (ILM). In this study, we have specifically addressed questions to understand the contribution of laminins containing the b2 and g3 chains in retinal development. Specifically, we have investigated how laminins regulate proliferation and differentiation of the retinal progenitor cell (RPC), their involvement in the process of neuronal maturation and in regulation of structural and functional properties of Müller cells (MC), glial cells that are critical for retinal homeostasis. Therefore, experiments in this study were focused on three cell types that are adherent to the ILM – the RPCs, the MC (terminal progeny of RPC) and retinal ganglion cells (RGCs), the only output neuron of the retina. Deletion of laminin b2 and g3 chains results in a specific disruption of the ILM that leads to the loss of adhesion of the RPCs to the ILM, thereby disrupting the establishment of polarity in RPCs. The loss of polarity in the RPCs lead to changes in the rate of proliferation and affects photoreceptor production. Our data also suggests that altered proliferation due to laminin deletion is mediated by a decrease in Notch effectors. These studies provide novel insights into the role of laminin b2 and g3 chains in regulating proliferation of RPCs and additionally in suppressing the pool of photoreceptors in vivo. Further, loss of polarity in the MCs causes cytoskeleton changes and disruption in the spatial orientation of the water and ions channels, resulting in MC dysfunction leading to pathophysiological response in the retina. Finally, we analyzed the effects of laminin deletion on RGC development showed that RGC spacing, and dendritic elongations are disrupted along with more minor perturbations in intra-retinal axonal guidance. Moreover, our in vivo data that laminin in the ILM promote RGC survival. Taken together, these studies suggest a critical role of laminins in retinal development and the maintenance of retinal structural integrity. The insights gained from these studies will contribute to the development of rationally designed biomimetic niches that can be used in ocular tissue regeneration. Furthermore, the insights from the MC studies demonstrate the importance of MC adhesion to the ILM in maintenance of retinal architecture and will contribute to our understanding of retinal pathologies such as retinal detachment, proliferative vitreoretinopathy and ILM stripping. The insights gained from the role of laminin in RGC development may well be important in understanding CNS congenital dysgenesis as well as being relevant to CNS pathologies including glaucoma, neurodegeneration and the ischemia.