The Role of Fat Signalling in Craniofacial Development

Abstract

Fat/Dachsous signalling regulates planar cell polarity (PCP), which is the polarisation of tissue structures perpendicular to the apical-basal axis, and interacts with the Hippo pathway to suppress overgrowth of tissues in Drosophila. Recent studies in vertebrates have implicated Fat4 and Dchs1, the receptor-ligand pair and the vertebrate homologues of Drosophila Ft and Ds, respectively, in regulating PCP in the kidney and cochlea. However, the role of Fat signalling is largely undetermined in vertebrate development.
To determine the role of Fat signalling in craniofacial development, a basic characterisation was carried out using histology, OPT scanning, skeletal preparations, immunohistochemistry and in situ hybridisations. This revealed that loss of Fat4 and/or Dchs1 results in arrested growth of salivary glands, disruption of hair polarity of the utricle, delayed differentiation of osteoblasts of the cranial bones and arrested lateral tangential migration of the Facial Branchiomotor neurons (FBNs).
In Drosophila and vertebrates, PCP is controlled by two pathways; the Frizzled-PCP and the Fat-PCP pathway. During development, FBNs undergo tangential caudal and lateral tangential migrations within the plane of the neuroepithelium and are a model system to study PCP. Previous studies have shown a critical role for Fz-PCP during caudal migration. The role of Fat-PCP signalling during FBN migration was analysed by using mouse mutants for Fat4 and Dchs1 and by expression analysis.
Loss of Fat4/Dchs1 results in an arrest of lateral migration of the FBNs and a loss of polarity as revealed by cell shape and Golgi orientation analysis with no effect on FBN specification. Fat4 and Dchs1 are expressed as complementary gradients in the hindbrain. Generation of chimeric tissue revealed that the gradient of Dchs1 is necessary for polarised FBN migration suggesting that the role and mechanisms of Fat-PCP signalling are conserved between vertebrates and Drosophila. The Islet-1cre and Hoxa3cre conditional knockouts revealed that there is a requirement for Dchs1 both cell autonomously within the FBNs as well as non-cell autonomously in the neuroepithelium whereas Fat4 is largely required non-cell autonomously.
Analysis of double Fat4-/-Vangl2Lp/Lp mutants revealed that Fz-PCP exclusively regulates caudal FBN migration whilst Fat-PCP is necessary for the lateral polarised migration of the FBNs even in the absence of Fz-PCP. The two pathways work on orthogonal axes to regulate FBN migration. This study establishes that Fat signalling is largely required during craniofacial morphogenesis, provides further evidence of Fat signalling in regulating PCP in vertebrates and the first evidence that gradients of Fat4/Dchs1 may establish PCP in vertebrates.

Date of Award	2013
Original language	English
Awarding Institution	King's College London
Supervisor	Philippa Francis-West (Supervisor), Sarah Guthrie (Supervisor) & Andrea Streit (Supervisor)