The peripheral sensory nervous system in the vertebrate head has a dual origin from cranial ectodermal placodes and neural crest cells. Cranial placodes arise from regions of thickened ectoderm in the embryonic head that invaginate and/or delaminate to give rise to portions of the cranial ganglia as well as sensory structures like the ear, lens, and nose. While peripheral ganglia of the trunk are exclusively neural crest-derived, those arising at cranial levels have a dual origin from both neural crest and placodes. Despite their importance to peripheral sensory innervation of the head, comparatively little is known about the early development of the placodes and how they become specified to adopt different lineages. This proposal aims to further characterize the cellular and molecular events underlying placode development, focusing on the olfactory placode domain, and how these cells obtain their prospective fates rather than becoming neural crest. Appropriate acquisition of placode fate is critical for proper craniofacial development. 1) Single cell lineage analysis of olfactory/lens primordium. We have shown that precursors to both lens and olfactory structures arise from a common territory next to the anterior neural plate and that they segregate over time by directional movements. Single cell lineage analysis will be performed to resolve whether there is a common lens/olfactory precursor or whether cells already know their fates within this common domain. Surprisingly, preliminary data indicate that these lineages may be segregated even within the preplacodal domain. 2) Environmental control of cell fate in the anterior neural folds: olfactory versus neural crest fate. The anterior neural fold is the only region of the neural tube that does not form neural crest, but rather forms olfactory placode. We will test whether olfactory placode can form neural crest when grafted caudally and if more caudal neural folds can form olfactory placode if grafted rostrally. 3) Epigenetic control of cell fate within anterior neural folds: the role of PHD12, a histone deacetylase complex member, in olfactory versus neural crest fate. PHD12 is selectively expressed in anterior neural folds and its knock-down causes anterior expansion of neural crest specifier genes in this domain. We will test whether PHD12 expression in the neural folds alters after heterotopic grafting and whether its gain- or loss-of-function modulates the olfactory/neural crest fate switch. 4) Transcriptional control of cell fate in anterior neural folds: the role of Pea3 transcription factor in olfactory versus neural crest fate. The transcription factor Pea3, which is a known downstream effector of FGF signaling, is expressed in olfactory and otic placodes at early stages. Preliminary data suggest that knock-down of Pea3 in the olfactory territory up-regulates the neural crest marker, Sox10. We will test whether this and other factors may act as switch points between placodal versus neural crest fate.
The peripheral nervous system of vertebrates arises from two sources: ectodermal placodes and neural crest cells and the neural crest is a multipotent stem-cell-like population that migrates extensively and gives rise to an amazingly diverse set of derivatives. Similarly, placodes migrate or invaginate, and give rise to sensory neurons of ganglia that innervate the face. Because placodes and neural crest-derived cells are involved in a variety of birth defects (e.g. cleft palate, cardiac septal defects) and cancers such as neurofibromatosis, melanoma, neuroblastoma, our results on the normal mechanisms of placode and neural crest development provide important clues regarding the mistakes that may lead to abnormal development or loss of the differentiated state.
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