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Abstract:
The neural crest is a vertebrate specific, developmentally transient population of pluripotent stem cells capable of crossing germ layer boundaries and differentiating into a multitude of different tissues. In Zebrafish, one of these cell types, the pigment cells of the body, first appear and differentiate in early embryos contributing to a stereotypical larval pigmentation pattern composed of melanophores, xanthophores and iridophores. At juvenile stages the larval pigmentation pattern undergoes a rapid transformation with the appearance of large numbers of newly arriving pigment cells that develop into the adult striped pattern of zebrafish skin. These melanophores arise from an undifferentiated cell population. Although ontogenetically derived from the neural crest, their exact source in juvenile and adult fish has remained unclear. Here I present the source of the late appearing melanophores of the adult pattern to be derived from a stem cell population, set aside early in development at the future site of the dorsal root ganglia as part of the peripheral nervous system. The melanophore progenitors use the spinal nerves as migratory routes leading to the hypodermis at the periphery. Using mutants, morpholino and small molecule inhibition, I dissect the specific roles of ErbB and Kit signaling in the establishment of the melanophore stem cells and the role of dorsal root ganglia as their niche. By combining blastula transplantation and live imaging techniques I have identified melanophore progenitor cells during regeneration becoming active at the site of the dorsal root ganglia. I have followed their lineage until melanization, thereby confirming their specific fate. Furthermore, I show the requirement of these melanophore stem cells for the adult pigment cell population. Continuing along with melanophore maturation and differentiation, I have positionally cloned the zebrafish albino mutant and present mutations in slc45a2 encoding a solute carrier protein as causing the phenotype. In humans, mutations in SLC45A2 lead to oculocutaneous albinism type IV and single nucleotide polymorphisms (SNPs) at this locus are associated with skin color variation, but little is known about the function of SLC45A2 in melanization. I present evidence for a role of Slc45a2 in pH and ionic homeostasis inside melanosomes and show how variation of these conditions affects the key melanin producing enzyme, tyrosinase. I also show that mutations in sandy/tyrosinase lead to a destruction of melanosomes with toxic effects to neighboring tissues. Taken together these results provide insight into the large amount of variation in SLC45A2 across many animals as opposed to the relative low variation associated with TYROSINASE in humans.
