Deletion of Protein Phosphatase 2A Accelerates Retinal Degeneration in GRK1-and Arr1-Deficient Mice

PURPOSE. Light detection in retinal rod photoreceptors is initiated by activation of the visual pigment rhodopsin. A critical, yet often-overlooked, step enabling efficient perception of light is rhodopsin dephosphorylation mediated by protein phosphatase 2A (PP2A). PP2A deficiency has been reported to impair rhodopsin regeneration after phosphorylation by G protein receptor kinase 1 (GRK1) and binding of arrestin (Arr1), thereby delaying rod dark adaptation. However, its effects on the viability of photoreceptors in the absence of GRK1 and Arr1 remain unclear. Here, we investigated the effects of PP2A deficiency in the absence of GRK1 or Arr1, both of which have been implicated in Oguchi disease, a form of night blindness. METHODS. Rod-specific mice lacking the predominant catalytic C alpha-subunit of PP2A were crossed with the Grk1(-/-) or Arr1(-/- )strains to obtain double knockout lines. Rod photoreceptor viability was analyzed in histological cross-sections of the retina stained with hematoxylin and eosin, and rod function was evaluated by ex vivo electroretinography. RESULTS. PP2A deficiency alone did not impair photoreceptor viability up to 12 months of age. Retinal degeneration was more pronounced in rods lacking GRK1 compared to rods lacking Arr1, and degeneration was accelerated in both Grk1(-/-) or Arr1(-/- )strains where PP2A was also deleted. In Arr1(-/-) mice, rod maximal photoresponse amplitudes were reduced by 80% at 3 months, and this diminution was enhanced further with concomitant PP2A deficiency. CONCLUSIONS. These results suggest that although PP2A is not required for the survival of rods, its deletion accelerates the degeneration induced by the absence of either GRK1 or Arr1.