Abstract
Retinal pigment epithelium (RPE) degeneration underpins diseases triggered by disparate genetic lesions, noxious insults, or both. The pleiotropic Ranbp2 controls the expression of intrinsic and extrinsic pathological stressors impinging on cellular viability. However, the physiological targets and mechanisms controlled by Ranbp2 in tissue homeostasis, such as RPE, are ill defined. We show that mice, RPE-cre::Ranbp2(-/-), with selective Ranbp2 ablation in RPE develop pigmentary changes, syncytia, hypoplasia, age-dependent centrifugal and non-apoptotic degeneration of the RPE, and secondary leakage of choriocapillaris. These manifestations are accompanied by the development of F-actin clouds, metalloproteinase-11 activation, deregulation of expression or subcellular localization of critical RPE proteins, atrophic cell extrusions into the subretinal space, and compensatory proliferation of peripheral RPE. To gain mechanistic insights into what Ranbp2 activities are vital to the RPE, we performed genetic complementation analyses of transgenic lines of bacterial artificial chromosomes of Ranbp2 harboring loss of function of selective Ranbp2 domains expressed in a Ranbp2(-/-) background. Among the transgenic lines produced, only Tg(RBD2/3*-HA)::RPE-cre::Ranbp2(-/-)-expressing mutations, which selectively impair binding of RBD2/3 (Ran-binding domains 2 and 3) of Ranbp2 to Ran-GTP, recapitulate RPE degeneration, as observed with RPE-cre::Ranbp2(-/-). By contrast, Tg(RBD2/3*-HA) expression rescues the degeneration of cone photoreceptors lacking Ranbp2. The RPE of RPE-cre::Ranbp2(-/-) and Tg(RBD2/3*-HA)::RPE-cre::Ranbp2(-/-) share proteostatic deregulation of Ran GTPase, serotransferrin, and γ-tubulin and suppression of light-evoked electrophysiological responses. These studies unravel selective roles of Ranbp2 and its RBD2 and RBD3 in RPE survival and functions. We posit that the control of Ran GTPase by Ranbp2 emerges as a novel therapeutic target in diseases promoting RPE degeneration.
Highlights
Ranbp2 and its Ran-GTP-binding domains’ roles in Retinal pigment epithelium (RPE) survival/function, a multidisease target, are elusive
The findings of this study establish a critical role of Ranbp2 and selective Ran-GTP binding domains of Ranbp2 in the function and survival of the RPE
2) The RBD1 and RBD4 of Ranbp2, and Ranbp1 cannot compensate physiologically in the RPE for the losses of function of RBD2 and RBD3 of Ranbp2, even though RBD1– 4 of Ranbp2 and Ranbp1 are structurally and biochemically equivalent [43, 50]; this finding indicates that the functions of RBD2 and RBD3 are spatially constrained within Ranbp2 and probably dependent on their neighboring domains
Summary
Ranbp and its Ran-GTP-binding domains’ roles in RPE survival/function, a multidisease target, are elusive. To identify targets and mechanisms controlling RPE viability that may be amenable to pharmacological manipulations and therapeutic interventions in disease and aging-related conditions promoting RPE degeneration, we assessed the physiological roles of Ranbp, its domains, and allied activities in RPE functions and survival. We uncovered that Ranbp is dispensable for RPE proliferation and that Ranbp ablation promotes the progressive, autonomous, and non-autonomous degeneration of mature RPE cells and secondary impairment of choriocapillaris with manifestations reminiscent of those observed in neovascular age-related macular degeneration These pathological features and other molecular disturbances are retained selectively by the RPE upon mutations impairing the binding of RBD2 and RBD3 of Ranbp to Ran-GTP but not loss of function of other domains of Ranbp
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