Inhibition of the calcineurin-like protein phosphatase activity in Limulus ventral eye photoreceptor cells alters the characteristics of the spontaneous quantal bumps and the light-mediated inward currents, and enhances arrestin phosphorylation

Abstract

Changes in intracellular calcium are involved in phototransduction processes in both vertebrate and invertebrate photoreceptors. During this phototransduction process in the Limulus ventral eye, there is a biochemical change in the protein phosphatase, calcineurin, such that it becomes capable of activation by calcium and calmodulin. Here we show that the calcium/calmodulin-dependent calcineurin-like activity in light-adapted ventral eye was completely inhibited by the CaN autoinhibitory peptide, CaN A457-482 and the Merck analog of the membrane-permeable, immunosuppressant drug, FK 506, L-683, 590, but not an inactive analogue, L-685, 818. Whole-cell, voltage-clamp recordings of spontaneous quantal bump activity present in dark-adapted photoreceptors injected with either CaN A457-482 (500 microM) or superfused with L-683, 590 (20 microM) or L-685, 818 revealed that both CaN A457-482 and L-683, 590, but not L-685, 818, caused rapid decreases in quantal bump amplitude, rise time and fall time, resulting in smaller, sharper bumps. This was correlated with enhanced phosphorylation of arrestin in light-adapted ventral eye photoreceptors exposed to L-683, 590 or less reliably okadaic acid. Both CaN A457-482 and L-683, 590 markedly affected the light-stimulated inward currents recorded from light-adapted ventral photoreceptors, causing a "terracing" of the inward current, and an intensity-dependent delay in the time required to reach peak amplitude. Consequently, inhibition of calcineurin markedly affects two major rhodopsin-dependent electrophysiological processes, and implicates CaN as an integral component in the phototransduction cascade.

Immunocytochemical localization of opsin, visual arrestin, myosin III, and calmodulin in Limulus lateral eye retinular cells and ventral photoreceptors

“…In Limulus , myosin III is a major target for modulation by the circadian clock and by light (Edwards and Battelle, 1987; Edwards et al, 1989, 1990; Battelle et al, 1998). Calmodulin, together with calcium, has been implicated in regulating the light response by several different mechanisms (Richard et al, 1997; Scott et al, 1997; Kass et al, 1998). The immunocytochemical localization of these proteins provides an important underpinning for understanding the photoresponse in Limulus , a classical model for visual system studies.…”

Light modulates the melanophore response to α-MSH in Xenopus laevis: An analysis of the signal transduction crosstalk mechanisms involved

“…Calcineurin is a Ser-Thr phosphatase PP2B activated by Ca 2+ /calmodulin, and extremely conserved evolutionarily from fungi to humans (Hemenway and Heitman, 1999). Furthermore, inhibition of the phosphatase PP2B (calcineurin) decreases light-induced inward currents in Limulus eye, enhancing arrestin phosphorylation (Kass et al, 1998). These facts led us to determine calcineurin activity in Xenopus melanophores.…”