Phototransduction Cascade In Photoreceptors Research Articles

Recombinant protein delivery enables modulation of the phototransduction cascade in mouse retina

Inherited retinal dystrophies are often associated with mutations in the genes involved in the phototransduction cascade in photoreceptors, a paradigmatic signaling pathway mediated by G protein-coupled receptors. Photoreceptor viability is strictly dependent on the levels of the second messengers cGMP and Ca2+. Here we explored the possibility of modulating the phototransduction cascade in mouse rods using direct or liposome-mediated administration of a recombinant protein crucial for regulating the interplay of the second messengers in photoreceptor outer segments. The effects of administration of the free and liposome-encapsulated human guanylate cyclase-activating protein 1 (GCAP1) were compared in biological systems of increasing complexity (in cyto, ex vivo, and in vivo). The analysis of protein biodistribution and the direct measurement of functional alteration in rod photoresponses show that the exogenous GCAP1 protein is fully incorporated into the mouse retina and photoreceptor outer segments. Furthermore, only in the presence of a point mutation associated with cone-rod dystrophy in humans p.(E111V), protein delivery induces a disease-like electrophysiological phenotype, consistent with constitutive activation of the retinal guanylate cyclase. Our study demonstrates that both direct and liposome-mediated protein delivery are powerful complementary tools for targeting signaling cascades in neuronal cells, which could be particularly important for the treatment of autosomal dominant genetic diseases.

Cyclic AMP as a Regulator of the Phototransduction Cascade

Until recently, cAMP has been believed to play an important role in the organization of circadian rhythms in the vertebrate eye but not to be directly involved in controlling the phototransduction cascade in photoreceptors. However, given that cAMP levels in photoreceptor cells vary significantly during the day and that cAMP metabolism in photoreceptor cells may itself be light-dependent, it is logical to suggest that cAMP-dependent protein phosphorylation may be a mechanism for tuning phototransduction processes in accordance with ongoing illumination. The current review addresses contemporary data on the structure of the circadian pacemaker in the retina, the involvement of intraphotoreceptor cAMP in its organization, and the known intracellular targets on which cAMP has or may have regulatory influences. Our recent data indicate that changes in cAMP levels have regulatory influences on the photoreceptor cascade. This action may significantly widen the range of adaptation of photoreceptors to illumination conditions, increasing its sensitivity during the dark part of the day and decreasing its sensitivity in bright light.

A transcription inhibitor, actinomycin D, partially inhibits the rhabdomeral renewal induced by a diacylglycerol lipase inhibitor, U-57908 in crab retinas

1. A previous study implies that diacylglycerols (DAGs) released by the phototransduction cascade in photoreceptors of a crab, Leptograpsus, activate protein kinase Cs (PKCs). PKCs were suggested to phosphorylate nuclear proteins that regulate the transcription of rhabdomeral membrane precursors. 2. We show that a transcription inhibitor, actinomycin D, reduces the rhabdomeral renewal induced by a diacylglycerol lipase inhibitor, U-57908, but does not eliminate it. 3. Within the particular experimental protocol, rhabdomeral maintenance and renewal by controls are inhibited by actinomycin D in an absence of U-57908. 4. We conclude that although rhabdomeral renewal induced by PKC activation involves the regulation of transcription, multiple pathways are implicated. 5. An alternative route for the degradation of DAGs by phosphorylation is eliminated because a specific inhibitor of DAG kinases, R-59022, exerts no effect. 6. U-57908 promotes photoreceptor shrinkage which is not compromised by actinomycin D. Shrinkage occurs independently of rhabdomeral renewal, suggesting a further consequence of PKC activation.