Differentiation of reptilian neural crest cells in vitro

Melanoblast development coincides with the late emerging cells from the dorsal neural tube in turtle Trachemys scripta

Ectothermal reptiles have internal pigmentation, which is not seen in endothermal birds and mammals. Here we show that the development of the dorsal neural tube-derived melanoblasts in turtle Trachemys scripta is regulated by similar mechanisms as in other amniotes, but significantly later in development, during the second phase of turtle trunk neural crest emigration. The development of melanoblasts coincided with a morphological change in the dorsal neural tube between stages mature G15 and G16. The melanoblasts delaminated and gathered in the carapacial staging area above the neural tube at G16, and differentiated into pigment-forming melanocytes during in vitro culture. The Mitf-positive melanoblasts were not restricted to the dorsolateral pathway as in birds and mammals but were also present medially through the somites similarly to ectothermal anamniotes. This matched a lack of environmental barrier dorsal and lateral to neural tube and the somites that is normally formed by PNA-binding proteins that block entry to medial pathways. PNA-binding proteins may also participate in the patterning of the carapacial pigmentation as both the migratory neural crest cells and pigment localized only to PNA-free areas.

Transcriptional and signaling regulation in neural crest stem cell-derived melanocyte development: do all roads lead to Mitf?

Human neurocristopathies include a number of syndromes, tumors, and dysmorphologies of neural crest (NC) stem cell derivatives. In recent years, many white spotting genes have been associated with hypopigmentary disorders and deafness in neurocristopathies resulting from NC stem cell-derived melanocyte deficiency during development. These include PAX3, SOX10, MITF, SNAI2, EDNRB, EDN3, KIT, and KITL. Recent studies have revealed surprising new insights into a central role of MITF in the complex network of interacting genes in melanocyte development. In this perspective, we provide an overview of some of the current findings and explore complex functional roles of these genes during NC stem cell-derived melanocyte development.

Signaling and transcriptional regulation in the neural crest-derived melanocyte lineage: interactions between KIT and MITF

Genetic and cell culture analyses have shown that the development of melanocytes from neural crest-derived precursor cells critically depends on the tyrosine kinase receptor KIT and the basic-helix-loop-helix-leucine zipper transcription factor MITF. KIT and MITF show complex interactions in that MITF is needed for the maintenance of Kit expression in melanoblasts and KIT signaling modulates MITF activity and stability in melanocyte cell lines. Using primary neural crest cell cultures from embryos homozygous for a Kit null allele marked by an inserted LacZ gene (Kit(W-LacZ)), we show that the onset of Mitf expression in melanoblasts does not require KIT. In fact, provided that the melanocyte growth factor endothelin-3 is present, a small number of MITF/beta-Gal-positive cells can be maintained for at least 2 weeks in Kit(W-LacZ)/Kit(W-LacZ) cultures. These cells express several pigment cell-specific genes that are thought or have been shown to be activated by MITF, including dautochrome tautomerase, pMel 17/Silver and tyrosinase-related protein-1, but lack expression of the MITF target gene tyrosinase, which encodes the rate-limiting enzyme in melanin synthesis. Consequently, the cells remain unpigmented. Addition of cholera toxin, which elevates cAMP levels and mimics part of the KIT signaling pathway, increases the number of MITF-positive cells in Kit(W-LacZ)/Kit(W-LacZ) cultures, leads to tyrosinase expression, and induces the differentiation of melanoblasts into mature, pigmented melanocytes. Even when added on day 5–6 of culture, cholera toxin still rescues tyrosinase expression and differentiation. The results thus demonstrate that the presence of MITF is not sufficient for tyrosinase expression in melanoblasts and that KIT signaling influences gene expression during melanocyte development in a gene-selective manner.

Effects of local tissue environment on the differentiation of neural crest cells in turtle, with special reference to understanding the spatial distribution of pigment cells

The spatial distribution of neural crest-derived pigment cells in turtles differs markedly from those found in chickens and mice. One hypothesis to explain such differences in the spatial distribution of pigment cells is that local tissue factors interact with neural crest cells, thereby determining their differentiation into pigment-synthesizing cells. It is reported here that local tissue factors in the soft-shell turtle (Trionyx sinensis japonicus) play a critical role in the development of melanophores from neural crest cells during embryogenesis. Undifferentiated neural crest cells derived from trunk neural tubes were co-cultured in vitro with homochronous somites, or with heterochronous dermis, lung or liver for 14 days. Melanophore differentiation from neural crest cells was significantly promoted when co-cultured with cells from lung, somites or dermis, but not when co-cultured with liver cells. These results suggest that local tissue factors stimulate the differentiation of pluripotent neural crest derivatives toward pigment cells. It is proposed that specific environmental cues play an important role in the spatial distribution of pigment cells in a variety of vertebrate species.

Turtle lung cells produce a melanization-stimulating activity that promotes melanocytic differentiation of avian neural crest cells

We found previously that neural crest cells in turtle embryos migrated into the lung buds and melanocytes were located in the lungs. The finding suggested to us that the lungs provide a stimulatory factor(s) to the differentiation of neural crest cells into melanocytes. We have established lung cell lines to facilitate analysis of the interactions of neural crest cells with the environment in melanocyte development. One cell line, TLC-2, was found to produce a putative melanization-stimulating activity (MSA), which promoted the melanocyte differentiation in vitro of avian neural crest cells. The TLC-2-derived MSA was different from that of basic fibroblast growth factor (bFGF), alpha-melanocyte stimulating hormone (alpha-MSH), and steel factor (SLF). Its molecular weight was estimated to be within the range of 150 kD. Our findings suggest that MSA may be a novel factor exercising a positive control over melanocyte differentiation.