Work Package 3

Signaling centres in embryonic pattern formation and organogenesis

Introduction
The embryonic development of body structures requires orchestration of cell-cell communication by signaling centres. Any disturbance of this orchestration can lead to dysmorphogenesis and cancer. Similar constellations of molecular players are implicated in functioning of different embryonic signaling centers, and much can be learned from comparison of the molecular and cellular mechanisms involved. We propose to unite our efforts to elucidate essential gene hierarchies and cell-cell interactions involved in different but mechanistically related signaling centres. The functions of the primitive streak, one of the earliest signaling centers, include control of posterior elongation of the early embryo from a stem cell population persisting until midgestation. Its activity depends on the integrity of the Wnt and Fgf pathways, and requires activities of families of transcription factors such as the T-box and Hox/Cdx homeodomain proteins. Coordination of Wnt-dependent development of the limbs requires a largely overlapping set of regulators. The Hedgehog family of proteins plays a central role in several signaling centres, involved in development of limbs, external genitalia, craniofacial structures, the peripheral nervous system, and also in gonadal development and gametogenesis. In mammals, three Hedgehog proteins exist: Sonic hedgehog (Shh), Indian hedgehog (Ihh), and Desert hedgehog (Dhh). Early polarisation of the limb bud results in a posterior localisation of expression of Shh and hence the polarising region. In parallet to the pathway responsible for Shh activation, a restrictive cascade including Gli3 and Alx4 prevent anterior activation of Shh signaling. Dhh, and probably also Ihh, are critical factors in development of the gonadal architecture and maintenance of the germ cell lineage. Dhh is further known to be expressed in Schwann cells of the peripheral nervous system, where it plays an important role in development of the protective nerve sheath.

Objective
In this project, we aim to identify interactions within and between gene expression cascades in signaling centres controlling the above-mentioned embryonic processes. We anticipate that identified components and mechanisms will play key regulatory roles in embryonic pattern formation and organogenesis, and that our studies will point to additional roles in other aspects of health and disease.

Approach
We propose to investigate the genetic mechanisms driven by signaling in the different morphogenetic centres, by using gain and loss of function mutant mice (existing and novel models). Expression of different mutant alleles and combinations thereof will be analysed in specific signaling centres, using strategies such as gene knock-in and conditional knockout. Changes in the expression of target genes will be monitored. We also will use appropriate cellular and tissue explant systems to characterize cell behaviour and cell-cell interactions in the different signaling centers. These investigations should generate essential novel information on signaling processes driving normal development, and will designate genes and molecules that are causally involved in pathogenic events and are candidate targets in treatment of dysmorphogenesis and cancer.

Links with other projects: In the context of the present proposal, direct links are to WP1, 2, 8 and 9.

Keywords: signaling centres, gene cascades, parallel/interactive pathways, dysmorphogenesis, cancer

Participants: Grootegoed, Deschamps, Meijer, Meijlink