PKCζ Mediates Breakdown of Outer Blood-Retinal Barriers in Diabetic Retinopathy

Aims/hypothesis: Diabetic macular edema represents the main cause of visual loss in diabetic retinopathy. Besides inner blood retinal barrier breakdown, the role of the outer blood retinal barrier breakdown has been poorly analyzed. We characterized the structural and molecular alterations of the outer blood retinal barrier during the time course of diabetes, focusing on PKC zeta a critical protein for tight junction assembly, known to be overactivated by hyperglycemia. Methods: Studies were conducted on a type2 diabetes Goto-Kakizaki rat model. PKC zeta level and subcellular localization were assessed by immunoblotting and immunohistochemistry. Cell death was detected by TUNEL assays. PKC zeta level on specific layers was assessed by laser microdissection followed by Western blotting. The functional role of PKC zeta was then evaluated in vivo, using intraocular administration of its specific inhibitor. Results: PKC zeta was localized in tight junction protein complexes of the retinal pigment epithelium and in photoreceptors inner segments. Strikingly, in outer segment PKC zeta staining was restricted to cone photoreceptors. Short-term hyperglycemia induced activation and delocalization of PKC zeta from both retinal pigment epithelium junctions and cone outer segment. Outer blood retinal barrier disruption and photoreceptor cone degeneration characterized long-term hyperglycemia. In vivo, reduction of PKC zeta overactivation using a specific inhibitor, restored its tight-junction localization and not only improved the outer blood retinal barrier, but also reduced photoreceptor cell-death. Conclusions: In the retina, hyperglycemia induced overactivation of PKC zeta is associated with outer blood retinal barrier breakdown and photoreceptor degeneration. In vivo, short-term inhibition of PKC zeta restores the outer barrier structure and reduces photoreceptor cell death, identifying PKC zeta as a potential target for early and underestimated diabetes-induced retinal pathology.