Rod Phosphodiesterase-6 (PDE6) Catalytic Subunits Restore Cone Function in a Mouse Model Lacking Cone PDE6 Catalytic Subunit

Abstract

Rod and cone photoreceptor neurons utilize discrete PDE6 enzymes that are crucial for phototransduction. Rod PDE6 is composed of heterodimeric catalytic subunits (␣␤), while the catalytic core of cone PDE6 (␣) is a homodimer. It is not known if variations between PDE6 subunits preclude rod PDE6 catalytic subunits from coupling to the cone phototransduction pathway. To study this issue, we generated a cone-dominated mouse model lacking cone PDE6 (Nrl ؊/؊ cpfl1). In this animal model, using several independent experimental approaches, we demonstrated the expression of rod PDE6 (␣␤) and the absence of cone PDE6 (␣) catalytic subunits. The rod PDE6 enzyme expressed in cone cells is active and contributes to the hydrolysis of cGMP in response to light. In addition, rod PDE6 expressed in cone cells couples to the light signaling pathway to produce S-cone responses. However, S-cone responses and light-dependent cGMP hydrolysis were eliminated when the ␤-subunit of rod PDE6 was removed (Nrl ؊/؊ cpfl1 rd). We conclude that either rod or cone PDE6 can effectively couple to the cone phototransduction pathway to mediate visual signaling. Interestingly, we also found that functional PDE6 is required for trafficking of M-opsin to cone outer segments.Vertebrate visual perception is mediated by two types of photoreceptor cells, rods, and cones (1-3). Rods mediate vision in dim light and respond to a single photon. In contrast, cones mediate vision in bright light, are less sensitive than rods to light, respond faster, and adapt to light stimuli over several orders of magnitude (1). Although the transduction mechanism used by both rods and cones to detect and respond to light are similar, the protein components that mediate the visual signaling are distinct (3). The differences between rod and cone responses are likely due to levels and variations in the protein transduction machinery (2, 3). Phototransduction is initiated when photoisomerized rhodopsin or cone opsin activates transducin, which in turn activates light-activated rod phosphodiesterase-6 (PDE6), 2 the effector of the cascade (2, 3). PDE6 hydrolyzes cGMP resulting in closure of cation channels and subsequent hyperpolarization of photoreceptor cell membranes (4). No differences in light-dependent signaling between rods and cones are observed at the level of opsin activation (5). In contrast, cell-specific transducin subunits contributes to altered signaling properties between rods and cones (6). However, this finding is contradicted by another study showing that rod and cone transducin are able to substitute for each other efficiently (7).The last step in the phototransduction cascade, the activation of PDE6 by transducin subunits is different between rods and cones. Apart from transducin, rods and cones contain distinct PDE6 subunits (4). Rod PDE6 exists as a heteromer with two catalytic subunits (␣␤) and two inhibitory (␥) subunits. Rod PDE6 is the only member of a vast family of PDE proteins that functions as a catalytic heterodimer. Cone PDE6 is composed of two i...

Targeted Ablation of the Pde6h Gene in Mice Reveals Cross-species Differences in Cone and Rod Phototransduction Protein Isoform Inventory

“…ERG measurements of Nrl Ϫ/Ϫ / cpfl1 double mutants lacked, as expected, rod activity, but surprisingly showed S-cone-mediated responses under light-adapted conditions. Moreover, immunoblotting of Nrl Ϫ/Ϫ /cpfl1 retinae revealed the presence of cone PDE6H and rod PDE6G suggesting that the PDE6G protein is normally expressed in murine cones (70). In addition to the inhibitory PDE6 proteins, catalytic rod PDE6 subunits were detectable in cone photoreceptors of the all cone-like retina model.…”

A Mouse Model for Studying Cone Photoreceptor Pathologies

“…It has been recently demonstrated that cones in Nrl À/À ;Cpfl1 mice also express rod PDE6 catalytic subunits that can partially replace the nonfunctional cone PDE alpha subunit. 31 Potentially, this may also reduce toxic cGMP levels and support cone viability and function in R91W;Nrl À/À ;Cpfl1 mice. To completely ablate retinal function and potentially accelerate degeneration, we generated quadruple R91W;Nrl À/À ;Cpfl1;rd10 mice, which carry an additional mutation in the beta subunit of rod PDE6.…”

Rip3 knockdown rescues photoreceptor cell death in blind pde6c zebrafish

“…However, recently an elegant study in a mouse model lacking cone Pde6c showed that cones expressed the rod photoreceptor Pde6a/b subunit proteins and that these subunits were used instead for cone phototransduction. 32 Therefore, the expression of rod phototransduction genes in FACS Tg(TaC-P:EGFP) wild-type and mutant cones was examined. We found that wild-type cones expressed very low levels of pde6a and pde6b, whereas mutant cones had highly upregulated levels of these two genes at both 4 and 7 d.p.f.…”

“…For instance, in a genetically derived all-cone retina mouse model lacking cone Pde6c (Nrl À / À ;cpfl1), it was demonstrated that the cones instead expressed the rod Pde6 catalytic subunits conferring phototransduction capability to the cones. 32 Similarly, in the rd10 mouse (which lacks the rod Pde6b gene), the cone Pde6a 0 subunit was transgenically expressed in the mutant rd10 rods and this rescued the response to light, demonstrating that cone Pde6a 0 can functionally substitute for Pde6ab. 38 These studies suggest that the rod and cone Pde6 catalytic subunits can functionally substitute for one another and our work further implies that the local environment can determine rod and cone coupling.…”