title: Light modulates the melanophore response to alpha-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 alpha-MSH. We have previously demonstrated that the initial biochemical steps of light and alpha-MSH signaling are distinct, since the increase in cAMP observed in response to alpha-MSH was not seen after light exposure. cAMP concentrations in response to alpha-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 Ca(2+)-calmodulin-activated phosphatase. This finding supports the hypothesis that the cyclase could be negatively modulated by a light-promoted Ca(2+) 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 alpha-MSH. St-Ht31, a disrupting agent of protein kinase A (PKA)-anchoring kinase A protein (AKAP) complex totally blocked the melanosome dispersing response to alpha-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 alpha-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 alpha-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.
authors.affiliation: Department of Biology, University of Virginia, Charlotesville, VA 22904-4328, USA.
authors.forename: Mauro César
authors.initials: MC
authors.lastname: Isoldi
authors.forename: Ignacio
authors.initials: I
authors.lastname: Provencio
authors.forename: Ana Maria de Lauro
authors.initials: AM
authors.lastname: Castrucci
mesh_terms: D054758:A Kinase Anchor Proteins / Q000235:genetics / Q000378:metabolism; D000262:Adenylyl Cyclases / Q000235:genetics / Q000378:metabolism; D000818:Animals; D015153:Blotting, Western; D002478:Cells, Cultured; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism; D047468:Immunoprecipitation; D008027:Light; D008547:Melanophores / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D011485:Protein Binding / Q000187:drug effects / Q000235:genetics; D015398:Signal Transduction / Q000187:drug effects / Q000528:radiation effects; D029867:Xenopus Proteins / Q000235:genetics / Q000378:metabolism*; D014982:Xenopus laevis / Q000378:metabolism*; D000521:alpha-MSH / Q000494:pharmacology*
chemical_list: D054758:A Kinase Anchor Proteins; D029867:Xenopus Proteins; D000521:alpha-MSH; D017868:Cyclic AMP-Dependent Protein Kinases; D000262:Adenylyl Cyclases