Mechanisms of mutant PDE6 proteins underlying retinal diseases

Abstract

Mutations in PDE6 genes encoding the effector enzymes in rods and cones underlie severe retinal diseases including retinitis pigmentosa (RP), autosomal dominant congenital stationary night blindness (adCSNB), and achromatopsia (ACHM). Here we examined a spectrum of pathogenic missense mutations in PDE6 using the system based on co-expression of cone PDE6C with its specialized chaperone AIPL1 and the regulatory Pγ subunit as a potent co-chaperone. We uncovered two mechanisms of PDE6C mutations underlying ACHM: (a) folding defects leading to expression of catalytically inactive proteins and (b) markedly diminished ability of Pγ to co-chaperone mutant PDE6C proteins thereby dramatically reducing the levels of functional enzyme. The mechanism of the Rambusch adCSNB associated with the H258N substitution in PDE6B was probed through the analysis of the model mutant PDE6C-H262N. We identified two interrelated deficits of PDE6C-H262N: disruption of the inhibitory interaction of Pγ with mutant PDE6C that markedly reduced the ability of Pγ to augment the enzyme folding. Thus, we conclude that the Rambusch adCSNB is triggered by low levels of the constitutively active PDE6. Finally, we examined PDE6C-L858V, which models PDE6B-L854V, an RP-linked mutation that alters the protein isoprenyl modification. This analysis suggests that the type of prenyl modifications does not impact the folding of PDE6, but it modulates the enzyme affinity for its trafficking partner PDE6D. Hence, the pathogenicity of PDE6B-L854V likely arises from its trafficking deficiency. Taken together, our results demonstrate the effectiveness of the PDE6C expression system to evaluate pathogenicity and elucidate the mechanisms of PDE6 mutations in retinal diseases.

Structural analysis of allosteric regulation of the regulatory GAF domains of cone photoreceptor phosphodiesterase-6 (PDE6) by cGMP and the inhibitory γ-subunit

“…8, gray spheres). For example, several mutations have been identified in the vicinity of the GAFa β1/β2 loop, including Arg102Trp 45,46 , Arg104Trp 44,47 , and Glu109Val 46 that are likely to disrupt the function of this element of the allosteric communication network. The disease-causing mutation Leu298His 46 is located in the GAFb β1/β2 loop ( Fig.…”

AIPL1: A specialized chaperone for the phototransduction effector

“…Reciprocally, the system based on co-expression of PDE6C, AIPL1, and Pγ is well-suited to evaluate the pathogenicity and mechanisms of PDE6 mutations underlying retina diseases [33]. Mutations in the PDE6A and PDE6B genes cause autosomal recessive retinitis pigmentosa (RP), a progressive degeneration of rods leading to blindness [20, 21].…”

“…Mutations in PDE6C lead to a loss of cone function and cause autosomal recessive achromatopsia [34–36]. The first study of the achromatopsia–linked mutations in the context of the wild-type PDE6C template uncovered two general mechanisms triggering this retina disorder [33]. First, for a group of PDE6C mutations, AIPL1 was unable to fold mutant PDE6 proteins leading to complete catalytic inactivity.…”

“…First, for a group of PDE6C mutations, AIPL1 was unable to fold mutant PDE6 proteins leading to complete catalytic inactivity. For the second group of PDE6C mutations, the key deficiency was the failure of Pγ to serve as a co-chaperone with AIPL1, which led to dramatic reductions in the levels of functional enzyme [33]. Future studies are expected to reveal the mechanisms of rod PDE6 in retina disease.…”

RNA-Seq reveals differential expression profiles and functional annotation of genes involved in retinal degeneration in Pde6c mutant Danio rerio

“…Due to the high cooperativity of cGMP binding to the channel [8], a small increase in cGMP levels will have a profound effect on the number of open channels and the cations (Na + , Ca 2+ ) that pass through them. Humans harboring loss-offunction PDE6b mutations develop RP, progressing to total blindness as a function of age [9,10]. This mutation has been predicted to cause a frameshift in the coding sequence and result either in a truncated pde6c or degradation of pde6c mRNA through nonsense-mediated decay; it ultimately affects both cone and rod photoreceptors [7].…”