Light modulates the melanophore response to a-MSH in Xenopus laevis : an analysis of the signal transduction crosstalk mechanisms involved.

Abstract

Melanin granule (melanosome) dispersion within Xenopus laevis melanophores is evoked either by light or a-MSH. We have previously demonstrated that the initial biochemical steps of light and a-MSH signaling are distinct, since the increase in cAMP observed in response to a-MSH was not seen after light exposure. cAMP concentrations in response to a-MSH were significantly lower in cells pre-exposed to light as compared to the levels in dark-adapted melanophores. Here we demonstrate the presence of an adenylyl cyclase (AC) in the Xenopus melanophore, similar to the mammalian type IX which is inhibited by Ca2+–calmodulin-activated phosphatase. This finding supports the hypothesis that the cyclase could be negatively modulated by a light-promoted Ca2+ increase. In fact, the activity of calcineurin PP2B phosphatase was increased by light, which could result in AC IX inhibition, thus decreasing the response to a-MSH. St-Ht31, a disrupting agent of protein kinase A (PKA)–anchoring kinase A protein (AKAP) complex totally blocked the melanosome dispersing response to a-MSH, but did not impair the photo-response in Xenopus melanophores. Sequence comparison of a melanophore AKAP partial clone with GenBank sequences showed that the anchoring protein was a gravin-like adaptor previously sequenced from Xenopus non-pigmentary tissues. Co-immunoprecipitation of Xenopus AKAP and the catalytic subunit of PKA demonstrated that PKA is associated with AKAP and it is released in the presence of a-MSH. We conclude that in X. laevis melanophores, AKAP12 (gravin-like) contains a site for binding the inactive PKA thus compartmentalizing PKA signaling and also possesses binding sites for PKC. Light diminishes a-MSH-induced increase of cAMP by increasing calcineurin (PP2B) activity, which in turn inhibits adenylyl cyclase type IX, and/or by activating PKC, which phosphorylates the gravin-like molecule, thus destabilizing its binding to the cell membrane.