Abstract
In zebrafish (Danio rerio), iridophores are specified from neural crest cells and represent a tractable system for examining mechanisms of cell fate and differentiation. Using this system, we have investigated the role of cAMP protein kinase A (PKA) signaling in pigment cell differentiation. Activation of PKA with the adenylyl cyclase activator forskolin reduces the number of differentiated iridophores in wildtype larvae, with insignificant changes to melanophore number. Inhibition of PKA with H89 significantly increases iridophore number, supporting a specific role for PKA during iridophore development. To determine the effects of altering PKA activity on iridophore and melanophore gene expression, we examined expression of iridophore marker pnp4a, melanophore marker mitfa, and the mitfa repressor foxd3. Consistent with our cell counts, forskolin significantly decreased pnp4a expression as detected by in situ hybridization and quantification of pnp4a+ cells. Forskolin had the opposite effect on mitfa and foxd3 gene activity, increasing the area of expression. As mitfa/nacre mutants have extra iridophores as compared to wildtype larvae, we examined the function of mitfa during PKA-sensitive iridophore development. Forskolin treatment of mitfa/nacre mutants did significantly reduce the number of iridophores but to a lesser extent than that observed in treated wildtype larvae. Taken together, our data suggests that PKA inhibits iridophore development in a subset of iridophore precursors, potentially via a foxd3-independent pathway.
Highlights
Chromatophores, or pigment/light-reflecting cells, develop from neural crest cells, a population of cells that arise near the dorsal aspect of the developing vertebrate neural tube
As mitfa/nacre mutants have extra iridophores as compared to wildtype larvae, we examined the function of mitfa during protein kinase A (PKA)-sensitive iridophore development
Forskolin treatment of mitfa/nacre mutants did significantly reduce the number of iridophores but to a lesser extent than that observed in treated wildtype larvae
Summary
Chromatophores, or pigment/light-reflecting cells, develop from neural crest cells, a population of cells that arise near the dorsal aspect of the developing vertebrate neural tube. PKA signaling regulates transcriptional programming important for neural crest cell fate choice in quail cell cultures, resulting in the induction of the melanocyte differentiation gene Mitf while simultaneously inducing expression of the RE-1 silencing transcription factor, a neuron-specific transcriptional repressor. These results were duplicated in zebrafish models where involvement of the Wnt signaling responsive transcription factor, CtBP2, was required for increased melanophore differentiation [18], suggesting a cross-talk between Wnt and PKA signaling. Our data suggests that PKA signaling inhibits iridophore differentiation while promoting differentiation of melanophores in zebrafish larvae
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