Investigating the role of Yes-associated protein (YAP) in neural crest development

The neural crest (NC) is a multipotent embryonic cell type derived from the ectoderm during neurulation giving rise to a variety of cell lineages such as neurons, glia and pigment cells. Most genes associated with the correct initiation, differentiation and migration of the neural crest have been found through reverse genetics. Similarities between neural crest development and some features of cancer progression are remarkable. For instance, it has been suggested that some cancer types recapitulate NC processes in an unregulated manner such as epithelial-mesenchymal transition or active cell migration throughout the body to form distant metastases. However, to date very little is known about initiators and drivers that direct neural crest cell migration to specific target sites. The Medaka mutant hirame represents an interesting melanocyte specific migration defect on the yolk sac caused by a loss of functional Yes-associated protein (YAP). Medaka hirame mutants were initially studied for their profound changes in body morphology. Genomic mapping identified the causal mutation as a nonsense point mutation within the first WW domain in the Yes-associated protein 1 (YAP1), causing translation of a dysfunctional YAP protein. YAP is a downstream transcriptional co-activator of the recently discovered and evolutionarily conserved Hippo pathway. Alterations within Hippo signalling are linked to cell survival, proliferation and abnormal tissue overgrowth. We demonstrate that hirame melanocyte precursors (melanoblasts) are initially present in normal abundance, but show an early migration defect with a lack of melanoblasts on the yolk sac, and corresponding accumulation in the lateral parts of the body. Subsequently, we observe an overall decline in differentiated melanocyte numbers during late stage embryogenesis. We designed an overexpression cassette linking enhanced GFP to either wild type or a mutated activated version of YAP and present evidence that it can efficiently rescue the melanocyte defect after injection of mRNA into one-cell stage embryos. Furthermore, analysis of the yolk sac anatomy via transmission electron microscopy indicates that a fraction of yolk membrane cells undergo apoptosis and we propose that this may contribute to the establishment of altered environmental cues leading to abnormal melanoblast migration onto the yolk sac. Injection of yap mRNA directly into the yolk sac however, failed to rescue melanoblast patterning. To advance our study, we isolated and characterised a 3.6 kb Medaka dopachrome tautomerase (Dct) promoter fragment, and used it to drive expression of enhanced green fluorescent protein (eGFP) in vivo. We generated germline transgenics with this construct that showed lineage-specific expression of eGFP within early migrating melanoblasts, a phenotype that is maintained in differentiated melanocytes throughout embryogenesis. In addition, using this promoter we overexpressed our egfp-yap fusion cassette and established transgenic lines to assess the cell autonomy of YAP within the melanocyte lineage. However, no fluorescent signal could be detected in the latter transgenics, necessitating future experimentation to properly characterise these lines. Finally, we analysed a range of neural crest markers to examine the extent of the neural crest defects in hirame mutants. In addition to the melanocyte phenotype, we identified a dramatic reduction in xanthophore numbers, although early leucophore development appears unaffected. We also observed a decreased number of dorsal root ganglia in the peripheral nervous system as well as smaller and partly ectopic cranial neural crest ganglia populations within the epibranchial arches. The characterisation of a novel Medaka melanocyte specific promoter as well as additional novel NC markers will be widely applicable and useful to the wider Medaka research community as a tool for the study of neural crest related mechanisms during development.