Caveolin in the retina: regulation of cellular survival signalling in glaucoma
Glaucoma, one of the most common retinal disorders, is the leading cause of irreversible loss of vision around the world. The main physiological hallmark of this disease is a progressive loss of retinal ganglion cells (RGCs) and optic nerve alterations with the major risk factor of enhanced intraocular pressure (IOP). Unfortunately, still, some patients demonstrate disease progression despite IOP lowering management which is currently the primary treatment for glaucoma. Therefore, unravelling the IOP-independent mechanisms remains rudimentary. Recently, genome-wide association studies have linked variations in Caveolin1/2 (CAV-1/2) gene loci as a risk factor in glaucoma. However, the biochemical role of Cav-1 and the potential mechanisms underlying this genetic association remain to be explored. Cav-1 is the prominent signature protein of caveolae membrane invaginations and is involved in a variety of signalling pathways. Absence of Cav-1 has previously been demonstrated to impair retinal function, and our investigations established that it is implicated in regulation of brain-derived neurotrophic factor (BDNF)/TrkB signalling in the RGCs through mediating the function of a non-membrane phosphatase Shp2. BDNF and its receptor, TrkB have been extensively shown to play critical roles in maintaining the health of RGCs and protect them from apoptosis. This study aimed to investigate the function of Cav-1 in the inner retina specifically with respect to its interaction with Shp2 phosphatase under both healthy and experimental glaucoma conditions.
In the first part of this study I demonstrated that in the normal physiological situation, the absence of Cav-1 was associated with impaired inner retinal function in Cav-1 KO retinas compare to WT retina suggesting that this adaptor protein have critical functions in the maintenance of inner retinal integrity under normal conditions. Cav-1 ablation however, linked to a partially functional and structural protection of the inner retina under chronically elevated IOP conditions. I further investigated the biochemical consequence of this protection which illustrated higher phosphorylation of both Shp2 and Cav-1 in WT retinas which was in accordance with a decline in TrkB receptor activity. This result elucidated the in vivo interaction between two molecules associated with negative regulation of TrkB receptor under chronic but not acute elevated IOP.
In the second section, I extended my study to further examine the in vivo interactions of Shp2 with Cav-1 by genetically downregulating Shp2 expression using AAV gene therapy under two models of experimental glaucoma conditions. The results showed that AAV-mediated Shp2 suppression could impart partial protection into the retina against damage caused by chronic exposure to increased IOP, which was only evident in WT retinas. This result also suggested that protective effects of Shp2 ablation under experimental glaucoma conditions were dependent on Cav-1, suggesting in vivo interactions between the two proteins. However, the beneficial effect of Shp2 downregulation on either retinal function or BDNF/TrkB signalling seen in chronic models was not observed in acutely increased IOP, and this was regardless of Cav-1 status, probably due to other mechanisms of damage in this model. In the last section, ultimately using an in silico study, we investigated the impact of known Cav isoforms on the structure of these proteins. The results revealed that while non-synonymous single nucleotide polymorphism (nsSNP) in Cav-3 gene may have a damaging effect on the protein stability, the known nsSNPs did not have a negative effect on the Cav-1 and Cav-2 protein structures. Together, these results suggest a novel role of Cav-1 in mediating Shp2 functions in the retina; particularly RGCs. Loss of Cav-1 exerts a partial protective effect on the inner retina under experimental glaucoma conditions which is in association with its interaction with Shp2 phosphatase. Future studies will help establish the pathophysiological cross-talk between these two proteins.