Abstract
Protein misfolding caused by inherited mutations leads to loss of protein function and potentially toxic ‘gain of function’, such as the dominant P23H rhodopsin mutation that causes retinitis pigmentosa (RP). Here, we tested whether the AMPK activator metformin could affect the P23H rhodopsin synthesis and folding. In cell models, metformin treatment improved P23H rhodopsin folding and traffic. In animal models of P23H RP, metformin treatment successfully enhanced P23H traffic to the rod outer segment, but this led to reduced photoreceptor function and increased photoreceptor cell death. The metformin-rescued P23H rhodopsin was still intrinsically unstable and led to increased structural instability of the rod outer segments. These data suggest that improving the traffic of misfolding rhodopsin mutants is unlikely to be a practical therapy, because of their intrinsic instability and long half-life in the outer segment, but also highlights the potential of altering translation through AMPK to improve protein function in other protein misfolding diseases.
Highlights
Inherited mutations in rhodopsin (RHO), the light sensitive protein of rod photoreceptor cells, are the most common cause of autosomal dominant retinitis pigmentosa
We tested the ability of the AMPK activators, metformin and 5-amino-1-b-D-ribofuranosyl-imidazole-4-carboxamide (AICAR), to partially reduce translation and investigate the hypothesis that mild inhibition of protein translation can improve the traffic of misfolded rod opsin
Following 18 h of treatment, non-permeabilised cells were stained with the Rho4D2 antibody, which has an epitope in the extracellular domain, to reveal the P23H-GFP that had trafficked to the cell surface (Fig. 1A)
Summary
Inherited mutations in rhodopsin (RHO), the light sensitive protein of rod photoreceptor cells, are the most common cause of autosomal dominant retinitis pigmentosa (adRP). An understanding of the molecular mechanisms underlying Class II rhodopsin mutants came from cell culture models which showed that misfolded rod opsin is retained in the endoplasmic reticulum (ER), as opposed to wild-type (WT) normal protein which translocates to the plasma membrane. These results suggested the existence of an ER quality control machinery that does not permit further traffic of misfolded rod opsin [4,5]. Mutant rod opsin can exert a dominant-negative effect on the processing of WT rod opsin [6,8] and can activate the unfolded protein response (UPR) [9]
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