Endothelin receptor Aa regulates proliferation and differentiation of Erb-dependent pigment progenitors in zebrafish

Abstract

Skin pigment patterns are important, being under strong selection for multiple roles including camouflage and UV protection. Pigment cells underlying these patterns form from adult pigment stem cells (APSCs). In zebrafish, APSCs derive from embryonic neural crest cells, but sit dormant until activated to produce pigment cells during metamorphosis. The APSCs are set-aside in an ErbB signaling dependent manner, but the mechanism maintaining quiescence until metamorphosis remains unknown. Mutants for a pigment pattern gene, parade , exhibit ectopic pigment cells localised to the ventral trunk, but also supernumerary cells restricted to the Ventral Stripe. Contrary to expectations, these melanocytes and iridophores are discrete cells, but closely apposed. We show that parade encodes Endothelin receptor Aa, expressed in the blood vessels, most prominently in the medial blood vessels, consistent with the ventral trunk phenotype. We provide evidence that neuronal fates are not affected in parade mutants, arguing against transdifferentiation of sympathetic neurons to pigment cells. We show that inhibition of BMP signaling prevents specification of sympathetic neurons, indicating conservation of this molecular mechanism with chick and mouse. However, inhibition of sympathetic neuron differentiation does not enhance the parade phenotype. Instead, we pinpoint ventral trunk-restricted proliferation of neural crest cells as an early feature of the parade phenotype. Importantly, using a chemical genetic screen for rescue of the ectopic pigment cell phenotype of parade mutants (whilst leaving the embryonic pattern untouched), we identify ErbB inhibitors as a key hit. The time-window of sensitivity to these inhibitors mirrors precisely the window defined previously as crucial for the setting aside of APSCs in the embryo, strongly implicating adult pigment stem cells as the source of the ectopic pigment cells. We propose that a novel population of APSCs exists in association with medial blood vessels, and that their quiescence is dependent upon Endothelin-dependent factors expressed by the blood vessels.

Evolution of the potassium channel gene Kcnj13 underlies colour pattern diversification in Danio fish

“…In zebrafish, endothelin signalling is directly required in iridophores for their development and proliferation; iridophores indirectly promote and sustain melanophore development 32 . Several paralogs exist for endothelin receptors and ligands 74 , only one of each is specifically involved in patterning 7 , 29 , 73 , 75 . Indeed, it has been shown that in D. nigrofasciatus , due to cis-regulatory changes, the expression of the secreted ligand Endothelin 3b (Edn3b) is lower than in D. rerio 73 .…”

Thyroid hormone regulates distinct paths to maturation in pigment cell lineages

“…By sampling individual cell transcriptomes across NC-derived lineages, our study complements prior investigations of lineage relationships, morphogenetic behaviors, genetic requirements, and spatial and cell-type-specific gene expression profiles (Eom et al, 2015; Irion et al, 2016; Johnson et al, 1995; Kelsh et al, 2017; McMenamin et al, 2014; Parichy and Spiewak, 2015; Singh et al, 2016; Singh et al, 2014). Multipotent progenitors that give rise to adult melanophores, some xanthophores, and iridophores are established in the embryo and reside within peripheral nerves as development progresses (Budi et al, 2011; Budi et al, 2008; Camargo-Sosa et al, 2019; Dooley et al, 2013a; Singh et al, 2016). As the adult pattern forms, some of these cells migrate to the hypodermis where they differentiate and integrate into dark stripes or light interstripes.…”

GABA‐A receptor and mitochondrial TSPO signaling act in parallel to regulate melanocyte stem cell quiescence in larval zebrafish

“…For example, our finding that metformin, which itself had little effect on pigmentation, demonstrates opposing effects on melanocyte production when combined with GABA or TSPO pathway compounds suggests that melanocyte development must integrate both local and systemic signals during pigmentation. In addition to local and systemic cues, MSCs also appear to respond to spatial cues, as recent work indicates the ednraa RTK pathway regulates MSC quiescence specifically in the ventral trunk of larval zebrafish (Camargo‐Sosa et al, ). In the future, it will be interesting to determine how GABA and TSPO function integrate with ednraa activity to regulate MSC quiescence and activation.…”