Morphogenesis Molecular Genetics (URA CNRS 2578)

Dr. Benoit ROBERT

We are studying the mechanisms that govern the ontogeny of organs and the elaboration of shape in vertebrates. We focus on the role of Msx genes in these processes. Msx are non-clustered homeobox genes that encode transcription factors and form small multigene families in vertebrates. In the mouse embryo, Msx1 and Msx2 are expressed at many sites of inductive interactions between ectoderm and mesoderm, 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). Msx gene expression has been proposed to be associated with maintenance of cellular plasticity. We are concentrating on two major developmental models in which Msx genes are involved.

Function of Msx genes in limb morphogenesis

In vertebrates, morphogenesis first involves patterning of embryonic fields, i.e. their partition into different territories characterized by specific expression programs. This leads to local changes in cell physiology that promote concerted proliferation, oriented cell division, apoptosis, migration, differentiation, all functions that contribute to growth, the motor for morphogenesis. In the limb bud, Msx1 and Msx2 are expressed both in the apical ectoderm and underlying mesenchyme, according to dynamic expression patterns. Mutation of both Msx genes results in a complex limb phenotype that ranges from oligo- to polydactyly, but always leads to the loss of structures with an anterior identity (pollex/radius or hallux/Tibia). On the contrary, a hypomorphic combination of mutant alleles systematically results in anterior outgrowth and polydactyly. We are analyzing the changes in cell physiology associated with this outgrowth, and the differences in transcriptome between normal and mutant limbs to pinpoint the genes that may underlie changes in cell behaviour.

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).

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 investigate the role of Msx1 and Msx2 in these processes in the embryonic spinal cord. Using mouse strains constructed in the laboratory that express lacZ or Gfp reporters under the control of Msx1 or Msx2 regulatory sequences, respectively, we have analyzed the dynamic expression pattern of these two transcription factors during neurogenesis. Inactivation of both genes results in mispatterning of the dorsal aspect of the embryonic spinal cord and neuron specification defects. We are working towards the identification of Msx gene 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 in the mammalian central nervous system.