Analysis of Development Edited by Prof Benjamin H Willier, Prof Paul A Weiss and Prof Viktor Hamburger Pp xii + 735 (Philadelphia and London: W B Saunders Company, 1955) 105s

Mechanisms of Development

Morpholino oligos: making sense of antisense?

Melanocytes are melanin-producing cells found in skin, hair follicles, eyes, inner ear, bones, heart and brain of humans. They arise from pluripotent neural crest cells and differentiate in response to a complex network of interacting regulatory pathways. Melanins are pigment molecules that are endogenously synthesized by melanocytes. The light absorption of melanin in skin and hair leads to photoreceptor shielding, thermoregulation, photoprotection, camouflage and display coloring. Melanins are also powerful cation chelators and may act as free radical sinks. Melanin formation is a product of complex biochemical events that starts from amino acid tyrosine and its metabolite, dopa. The types and amounts of melanin produced by melanocytes are determined genetically and are influenced by a variety of extrinsic and intrinsic factors such as hormonal changes, inflammation, age and exposure to UV light. These stimuli affect the different pathways in melanogenesis. In this review we will discuss the regulatory mechanisms involved in melanogenesis and explain how intrinsic and extrinsic factors regulate melanin production. We will also explain the regulatory roles of different proteins involved in melanogenesis.

https://www.mdpi.com/1422-0067/17/7/1144/pdf

Signaling Pathways in Melanogenesis

Contact Inhibition of Locomotion was discovered by Abercrombie more than 50 years ago to describe the behaviour of fibroblast cells confronting each other in vitro, where they retract their protrusions and change direction upon contact1,2. Its failure was suggested to contribute to malignant invasion3-6. However, the molecular basis of Contact Inhibition of Locomotion and whether it also occurs in vivo are still unknown. Here we show that neural crest cells, a highly migratory and multipotent embryonic cell population, whose behaviour has been likened to malignant invasion6-8, exhibit Contact Inhibition of Locomotion both in vivo and in vitro, and that this accounts for their directional migration. When two migrating neural crest cells meet, they stop, collapse their protrusions and change direction. In contrast, when a neural crest cell meets another cell type, it fails to display Contact Inhibition of Locomotion; instead, it invades the other tissue, like metastatic cancer cells3,5,9. We show that inhibition of non-canonical Wnt signalling abolishes both Contact Inhibition of Locomotion and the directionality of neural crest migration. Wnt signalling members localise at the site of cell contact, leading to activation of RhoA in this region. These results provide the first example of Contact Inhibition of Locomotion in vivo, present an explanation for coherent directional migration of group of cells and establish a novel role for non-canonical Wnt signalling.

/pdf/contact-inhibition-of-locomotion-in-vivo-controls-neural-4s1rkhww4q.pdf

Contact inhibition of locomotion in vivo controls neural crest directional migration

The enteric nervous system (ENS) has been explored by developmental neurobiologists and medical researchers for decades. Whereas developmental biologists have been unravelling the molecular mechanisms underlying the migration, proliferation and differentiation of the neural crest derivatives that give rise to the ENS, human geneticists have been uncovering the genetic basis for diseases of the ENS, notably Hirschsprung's disease. Here we discuss the exciting recent advances, including novel transgenic and genetic tools, a broadening range of model organisms, and the pursuit of ENS stem cells as a therapeutic tool, that are bringing these fields closer together.

Enteric nervous system development and Hirschsprung's disease: advances in genetic and stem cell studies.

Waardenburg-Shah syndrome combines the reduced enteric nervous system characteristic of Hirschsprung's disease with reduced pigment cell number, although the cell biological basis of the disease is unclear. We have analysed a zebrafish Waardenburg-Shah syndrome model. We show that the colourless gene encodes a sox10 homologue, identify sox10 lesions in mutant alleles and rescue the mutant phenotype by ectopic sox10 expression. Using iontophoretic labelling of neural crest cells, we demonstrate that colourless mutant neural crest cells form ectomesenchymal fates. By contrast, neural crest cells which in wild types form non-ectomesenchymal fates generally fail to migrate and do not overtly differentiate. These cells die by apoptosis between 35 and 45 hours post fertilisation. We provide evidence that melanophore defects in colourless mutants can be largely explained by disruption of nacre/mitf expression. We propose that all defects of affected crest derivatives are consistent with a primary role for colourless/sox10 in specification of non-ectomesenchymal crest derivatives. This suggests a novel mechanism for the aetiology of Waardenburg-Shah syndrome in which affected neural crest derivatives fail to be generated from the neural crest.

Zebrafish colourless encodes sox10 and specifies non-ectomesenchymal neural crest fates

sox10 is necessary for development of neural and pigment cell derivatives of the neural crest (NC). However, whereas a direct role for Sox10 activity has been established in pigment and glial lineages, this is more controversial in NC-derived sensory neurons of the dorsal root ganglia (DRGs). We proposed that sox10 functioned in specification of sensory neurons, whereas others suggested that sensory neuronal defects were merely secondary to absence of glia. Here we provide evidence that in zebrafish, early DRG sensory neuron survival is independent of differentiated glia. Critically, we demonstrate that Sox10 is expressed transiently in the sensory neuron lineage, and specifies sensory neuron precursors by regulating the proneural gene neurogenin1. Consistent with this, we have isolated a novel sox10 mutant that lacks glia and yet displays a neurogenic DRG phenotype. In conjunction with previous findings, these data establish the generality of our model of Sox10 function in NC fate specification.

A direct role for Sox10 in specification of neural crest-derived sensory neurons

Expression of zebrafish fkd6 in neural crest-derived glia

SUMMARY In humans, mutations in the SOX10 gene are a cause of the auditory-pigmentary disorder Waardenburg syndrome type IV (WS4) and related variants. SOX10 encodes an Sry-related HMG box protein essential for the development of the neural crest; deafness in WS4 and other Waardenburg syndromes is usually attributed to loss of neural-crest-derived melanocytes in the stria vascularis of the cochlea. However, SOX10 is strongly expressed in the developing otic vesicle and so direct roles for SOX10 in the otic epithelium might also be important. Here, we examine the otic phenotype of zebrafish sox10 mutants, a model for WS4. As a cochlea is not present in the fish ear, the severe otic phenotype in these mutants cannot be attributed to effects on this tissue. In zebrafish sox10 mutants, we see abnormalities in all otic placodal derivatives. Gene expression studies indicate deregulated expression of several otic genes, including fgf8, in sox10 mutants. Using a combination of mutant and morphant data, we show that the three sox genes belonging to group E ( sox9a, sox9b and sox10 ) provide a link between otic induction pathways and subsequent otic patterning: they act redundantly to maintain sox10 expression throughout otic tissue and to restrict fgf8 expression to anterior macula regions. Single-cell labelling experiments indicate a small and transient neural crest contribution to the zebrafish ear during normal development, but this is unlikely to account for the strong defects seen in the sox10 mutant. We discuss the implication that the deafness in WS4 patients with SOX10 mutations might reflect a haploinsufficiency for SOX10 in the otic epithelium, resulting in patterning and functional abnormalities in the inner ear.

/pdf/a-zebrafish-model-for-waardenburg-syndrome-type-iv-reveals-5zh2css41i.pdf

A zebrafish model for Waardenburg syndrome type IV reveals diverse roles for Sox10 in the otic vesicle

https://onlinelibrary.wiley.com/doi/pdf/10.1002/gene.1062

Kirsten A. Dutton

Papers

Zebrafish colourless encodes sox10 and specifies non-ectomesenchymal neural crest fates

A direct role for Sox10 in specification of neural crest-derived sensory neurons

Expression of zebrafish fkd6 in neural crest-derived glia

A zebrafish model for Waardenburg syndrome type IV reveals diverse roles for Sox10 in the otic vesicle

A morpholino phenocopy of the colourless mutant.