Expression of avian Groucho-related genes (Grgs) during embryonic development

Groucho-related genes (Grgs) encode transcriptional co-repressors of Lef/Tcf and Hes proteins, which are mediators of Wnt and Notch signalling, respectively. Thus, they are important players in the developmental processes controlled by Wnt and Notch signalling, including lateral inhibition, segmentation and dorso-ventral patterning. We have cloned the avian homologues of Grg genes and examined their expression pattern by whole-mount in situ hybridisation between Hamburger-Hamilton (HH) stages 3 and 24. At HH stage 3, Grg gene expression is detected in the primitive streak and Hensen's node. Later, Grg genes are expressed at high levels in the developing head fold and by HH stage 11, throughout the anterior CNS and in the ventricular zone of the neural tube. In addition, Grg2, Grg4 and Grg5 are expressed in the notochord. In the paraxial mesoderm, Grg genes are activated as soon as somites form. As somites mature, Grg1 and Grg5/AES are expressed predominantly in the medial myotome and dermomyotome, whereas Grg2, Grg3 and Grg4 are expressed throughout the myotome. In HH stage 20 limbs, Grg1, Grg3 and Grg4 transcripts are more abundant in the posterior limb bud, whereas Grg2 and Grg5/AES are expressed throughout. By HH stage 24, Grg1, Grg2 and Grg3 become localized to the dorsal and ventral limb muscle masses, whereas Grg4 and Grg5/AES occupy a more central and ventro-proximal domain, respectively. Overall, our expression data are consistent with a role for Grg genes in Lef/Tcf and Wnt signalling during somitogenesis and with a role in Hes and Notch signalling in neurogenesis.