Doctoral Dissertations

Date of Award

8-1997

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Life Sciences

Major Professor

Gerald L. Vaughan

Committee Members

Roland Bagby, John Dunlap, Jeff MacCabe

Abstract

Skin of the lizard, Anolis carolinensis, changes color from green to brown when stimulated with visible light or melanocyte-stimulating hormone (MSH). MSH and light-mediated responses involve melanosome dispersion within dermal melanophores. Color change by these agents was studied reflectometrically in intact skin, and melanophore responses were visually assessed in cultured cells. The MSH response, but not the dermal photic response (DPR) , utilizes adenosine 3',5' cyclic AMP (cAMP). cAMP levels increase in skins darkened with MSH, but not light or calcium ionophore a23187. cAMP-elevating agents also caused dispersion in cultured melanophores. There is no intracellular calcium requirement since the MSH response of skin samples or cultured melanophores remains intact in the presence of lanthanum, and cultured melanophores loaded with the intracellular calcium chelator, BAPTA-AM, respond normally. The action spectrum of DPR reveals the presence of a blue-light sensitive photopigment (&ligma;=425 nm) . Western blotting and immunolocalization suggests the presence of a cone-like opsin protein present on the melanophore plasmalemma and possibly on the melanosome. Calcium is apparently released from intracellular stores during DPR. X-ray microanalysis suggests that this store may be the melanosome. Studies with 8-Bromo-cGMP, which inhibits DPR but not MSH response, suggests that cGMP may inhibit calcium release. The mechanism of DPR may have similarities to the transduction mechanism of the retinal rod cell which shares a similar embryological origin. The MSH and DPR responses converge on a common microfilament-based dispersion system. A 57 kd protein (P57) is phosphorylated by light or MSH. Phosphorylation is inhibited by 8-Bromo-cGMP in the assay buffer. Since the protein phosphatase inhibitor, okadaic acid, triggers pigment dispersion in cultured cells, P57 is hypothesized to be the molecular switch which controls pigment movement. Phosphorylation of P57 may be the point at which the two pathways converge. Finally, we report MSH-sensitivity of the iridophore. Cultured iridophores shift transmittance from red to blue when treated with MSH, cAMP-elevating agents, or colchicine, but not light. Using spectral measurements from xanthophores and iridophores, theoretical skin reflectance spectra were calculated to examine the contribution of iridophore activity to skin color and color change.

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