Morphogenesis Molecular Genetics (URA CNRS 2578)
We are studying the mechanisms that govern the functional ontogeny of organs and the elaboration of shape. In vertebrates, morphogenesis initially involves patterning of embryonic fields, i.e. their partition into different territories characterized by specific expression programs. This lead to the local activation of signalling centres that will locally change cell physiology to promote concerted proliferation, apoptosis, migration, differentiation, all functions that contribute to growth, the motor for morphogenesis. We focus on the role of Msx genes in these processes. Msx are non-clustered homeobox genes that form small multigene families in vertebrates. In the mouse, Msx1 and Msx2 are expressed at many sites of inductive interactions before and during organogenesis, such as the apical region of the limb bud, branchial arches, hair follicles, tooth buds, mammary gland primordia, genital bud, etc... They are also expressed in the dorsalmost aspect of the neural tube (roof plate). Noticeably, Msx1 and Msx2 are preferentially expressed in undifferentiated cells, and expression is shut down before cells differentiate, which may suggest a role in maintenance of cellular plasticity. We are concentrating on three major developmental models in which Msx genes are involved:
Function of Msx genes in limb bud formation
In the limb bud, Msx1 and Msx2 are expressed both in the apical ectoderm and underlying mesenchyme, according to dynamic expression patterns. Compound mutation of both Msx genes results in a complex limb phenotype that ranges from oligo- to polydactyly. The common feature between all these phenotypes is the loss of structures with an anterior identity (thumb/radius or Big toe/Tibia). We have shown that Msx act downstream of the Shh/Gli3 pathway that controls antero-posterior limb patterning. As Msx are well-known targets of BMP signalling, this may explain how BMP and Shh signalling are linked. We are studying the gene interactions in which Msx genes are engaged to fulfil their functions. A new inducible Msx2 mutant allele we produced permits us to analyse separately the role of Msx genes into ectoderm and mesoderm limb components, and to decipher the complex network in which Msx are involved during limb development.
Forelimbs of a wild type (central picture) and two Msx1 Msx2 null mutants (upper and lower picture). The mutant displays abnormal anterior structures. The radius (R) is missing (*) and the digit 1 (d1) in either missing (upper specimen) or duplicated (lower specimen).
Role of Msx genes on vascularization
Blood vessels are composed of two interacting cells types. Endothelial cells constitute the internal layer. They are covered by a layer of murals cells, namely vascular smooth muscle cells (VSMCs) in arteries and veins, and pericytes in capillaries. Mural cells play a critical role in vessel maturation and stabilization, which has particular relevance to tumour biology, wound healing and retinal angiogenesis. Taking advantage of MsxlacZ alleles we produced, we have observed expression of the Msx genes in the murine vascular network at embryonic and adult stages. Msx2nLacZ is mainly expressed in a restricted subpopulation of VSMCs of peripheral arteries and veins (brachial, femoral and caudal), and in a few cells of the aorta. Mxs1-nlacZ is expressed to a lesser extent by smooth muscle of peripheral arteries, but is highly expressed in capillaries where it labels pericytes. These results suggest that Msx gene activity is essential for mural cell formation and remodelling. Our current objective is to understand the role of Msx expression in blood vessel formation and physiopathology, using conditional alleles of Msx1 and Msx2 to direct gene inactivation in specific cells (endothelium, smooth muscle, retinal pericytes…). We expect to better define the involvement of these homeogenes in the development and maturation of blood vessels, during development and in disease.
Blood vessel from the choroid: endothelial cells in green (ib4 antibody) and mural cells in red (Sma antibody).
Msx genes, neural tube patterning and neuronal cell type specification
Neural tube patterning and subsequent neuronal cell type specification constitute keys events during neurogenesis. We initiated a project to investigate the role of Msx genes in the embryonic spinal cord. Using mouse strains constructed in the lab that express lacZ or Gfp reporters under the control of Msx1 or Msx2 regulatory sequences, we have studied the dynamic expression pattern of these two transcription factors during neurogenesis. We are currently analysing the consequences of the genetic inactivation of Msx1 and Msx2 on embryonic spinal cord patterning. Because of their early dorsal expression, we specifically investigate the potential role of these genes as a relay in the BMP signalling cascade, a key pathway in dorsal neural tube patterning. We are further working towards the identification of Msx genes targets, to dissect the transcriptional network that leads from undifferentiated neural progenitors to mature neurons. This project should provide new insights into the mechanisms of neuronal cell specification as well as reprogramming of neuronal progenitors during adult neurogenesis in the central nervous system.