AP-3 complex subunit delta gene, ap3d1, regulates melanogenesis and melanophore survival via autophagy in zebrafish (Danio rerio).

Zebrafish are an emerging model organism to study the syndromic albinism disorder, Hermansky-Pudlak syndrome (HPS), due to visible pigment development at 24 hours postfertilization, and conserved melanogenesis mechanisms. We describe crasher, a novel HPS type 10 (HPS10) zebrafish model, with a mutation in AP-3 complex subunit delta gene, ap3d1. Exon 14 of ap3d1 is overexpressed in crasher mutants, while the expression of ap3d1 as a whole is reduced. ap3d1 knockout in *AB zebrafish recapitulates the mutant crasher phenotype. We show ap3d1 loss-of-function mutations cause significant expression changes in the melanogenesis genes, dopachrome tautomerase (dct) and tyrosinase-related protein 1b (tyrp1b), but not tyrosinase (tyr). Last, Generally Applicable Gene-set Enrichment (GAGE) analysis suggests autophagy pathway genes are upregulated together in crasher. Treatment with autophagy-inhibitor, bafilomycin A1, significantly decreases melanophore number in crasher, suggesting ap3d1 promotes melanophore survival by limiting excessive autophagy. crasher is a valuable model to explore the regulation of melanogenesis gene expression and pigmentation disease.

Maintenance of melanophore morphology and survival is cathepsin and vps11 dependent in zebrafish.

Here, we characterize a Danio rerio zebrafish pigment cell mutant (melanophore integrity mutant), which displays a defect in maintenance of melanophore and iridophore number. Mapping and candidate gene analysis links the melanophore integrity mutant mutation to the vacuolar protein sorting 11 (vps11(w66)) gene. Quantification of vps11(w66) chromatophores during larval stages suggests a decrease in number as compared to wildtype siblings. TUNEL analysis and treatment with the caspase inhibitor, zVAD-fmk, indicate that vps11(w66) chromatophore death is caspase independent. Western blot analysis of PARP-1 cleavage patterns in mutant lysates suggests that increases in pH dependent cathepsin activity is involved in the premature chromatophore death observed in vps11(w66) mutants. Consistently, treatment with ALLM and Bafilomycin A1 (cathepsin/calpain and vacuolar-type H+-ATPase inhibitors, respectively), restore normal melanophore morphology and number in vps11(w66) mutants. Last, LC3B western blot analysis indicates an increase in autophagosome marker, LC3B II in vps11(w66) mutants as compared to wildtype control, but not in ALLM or Bafilomycin A1 treated mutants. Taken together, these data suggest that vps11 promotes normal melanophore morphology and survival by inhibiting cathepsin release and/or activity.