Protein Kinase C Activators Inhibit the Visual Cascade inLimulusVentral Photoreceptors at an Early Stage

Abstract

The phosphoinositide cascade mediates visual transduction in invertebrate photoreceptors. Phospholipase C (PLC) catalyzes the hydrolysis of phosphatidylinositol bisphosphate, producing inositol trisphosphate (InsP 3 ) and diacylglycerol (DAG). Protein kinase C (PKC) is a major target of DAG in many cell types. We have used PKC activators to investigate the function of the kinase in the phototransduction cascade in Limulus polyphemus ventral photoreceptors. Extracellular application of (Ϫ)-indolactam V (0.03-30 M) or phorbol-12,13-dibutyrate (10 M) reversibly reduced the sensitivity of the electrical response of the photoreceptors to light by up to 1000-fold. The inert stereoisomer (ϩ)-indolactam V and 4␣-phorbol had no effect. The effect of (Ϫ)-indolactam V was antagonized by the PKC inhibitors bisindolylmaleimide I and Gö 6976. Coapplication of bisindolylmaleimide V, used as a negative control compound for PKC inhibition, did not reduce the effectiveness of (Ϫ)-indolactam V. These findings are consistent with (Ϫ)-indolactam V activating PKC and desensitizing the light response. Furthermore, our pharmacological results indicate that PKC activation does not appear to play a role in light adaptation. We localized the position of the target of PKC in the visual cascade. We chemically excited the cascade at various stages to determine the kinase's target. PKC activation by (Ϫ)-indolactam V decreased the light-induced elevation of intracellular calcium but had no effect on the photoreceptor's excitatory response to intracellular injection of InsP 3 . However, the PKC activator greatly reduced the excitation caused by GTP-␥-S injection. We propose that PKC inhibits the visual transduction cascade at the G-protein and/or PLC stage.

Protein kinase C subtypes and retinal ischemic preconditioning

“…Although functionally both are involved in IPC, they operate via different cell signaling pathways. PKCδ inhibits the visual transduction cascade at the G-protein and/or PLC stage (Dabdoub and Payne, 1999). PKCε has not been previously shown in photoreceptors, and we cannot define involvement in visual transduction from this study.…”

Protein kinase C‐induced disorganization and endocytosis of photosensitive membrane in Limulus ventral photoreceptors

“…Our previous pharmacological studies of PKC activation via (-)-ILV and phorbol esters established that a putative Limulus PKC is capable of desensitizing the phototransduction cascade in ventral nerve photoreceptors (Dabdoub and Payne, 1999). Desensitization caused by PKC activation is upstream from the inositol triphosphate (InsP 3 )-induced Ca 2ϩ release from the endoplasmic reticulum; that is, the desensitization is at the microvillar level of the visual cascade.…”

“…To determine whether PKC-mediated desensitization of the photoresponse in Limulus ventral nerve photoreceptors (Dabdoub and Payne, 1999) is accompanied by ultrastructural modification of the receptor's phototransducing apparatus (rhabdom), we examined ventral nerve photoreceptors incubated in darkness with the PKC activator (-)-ILV. (-)-ILV is an indole alkaloid that mimics the endogenous PKC activator diacylglycerol (DAG; Heikkilä and Akerman, 1989).…”

“…0.03 and 25 M (Dabdoub and Payne, 1999). We wished to determine whether ultrastructural changes are graded with (-)-ILV concentration over the same range.…”

Calcium imaging demonstrates colocalization of calcium influx and extrusion in fly photoreceptors

“…Accordingly, the dissociation constant of the buffer (K d,B ) was taken to be 20 M, the value reported for OG5N (29,30). As only the steady-state distribution of Ca 2ϩ was investigated here, This paper was submitted directly (Track II) to the PNAS office.…”