Photoreceptor Cell Death and Proliferation in the STK38L Mutant Dog Retina Precedes Development of Hybrid Rod/S-Cone Photoreceptors

Abstract

Purpose: : Early retinal degeneration (erd) is an autosomal recessive, early onset canine retinal degeneration characterized by aberrant functional and structural development of rod inner and outer segments, and rod and cone synapses. Abnormal development is followed by degeneration of rods and cones. The disease results from a mutation in the STK 38L gene that causes exon 4 to be skipped. We now report that photoreceptor (PR) cell death and proliferation occur at the same time period, and cell proliferation is followed by formation of hybrid rod/S-cone photoreceptors.

Methods: : Paraformaldehyde fixed retinas from affected and controls dogs (ages 4-14 wks), were embedded in OCT. Cell death was assessed by TUNEL, and cell division by PCNA and KI-67 labeling. Rod and cone-specific markers were examined by immunocytochemistry; qRT-PCR and western blotting was used to assess expression of photoreceptor specific genes and transcription factors. Outer segment renewal was evaluated by autoradiography and opsin biosynthesis.

Results: : PR cell death and proliferation occur concurrently, and demonstrate a bimodal pattern that peaks at 8 and 12 wks. The newly generated rods express rod opsin and S-cone opsin (hybrid photoreceptors). Rod outer segment renewal is diffuse rather than through addition of discrete discs. Compared to control retina, qRT-PCR shows ~10 fold increase in S- and L/M-cone opsin mRNA expression which is confirmed by western analysis.

Conclusions: : The erd-mutant retina appears to be unique in showing a burst of PR proliferation that parallels the time window for cell death. The newly generated PRs are hybrid rod/S-cones. The results suggest that STK38L may be involved in photoreceptor cell specification.

Keywords: photoreceptors • degenerations/dystrophies • retinal development