Id expression during mouse development: a role in morphogenesis.

We have characterized the spatial and temporal pattern of Id transcription during mouse embryogenesis. The Id gene encodes a helix-loop-helix (HLH) protein which can heterodimerize with the ubiquitously expressed HLH protein products of the E2A gene, and prevent them from binding DNA either alone or as a heterodimer with tissue specific HLH transcription factors such as the muscle determination gene, MyoD1 (Benezra et al., 1990: Cell 61:49-59). Since Id has been shown to be down-regulated during induced differentiation in several cell lines, it has been postulated that Id plays a general inhibitory role in cell differentiation (Benezra et al., 1990). In situ analysis of Id mRNA expression in the mouse embryo was performed in order to determine whether the pattern of Id expression is consistent with this postulate. A detailed study throughout the entirety of mouse postimplantation development reveals that Id is expressed upon gastrulation at very high levels in almost all regions of the mouse embryo and expression declines as embryogenesis proceeds. In skeletal muscle, in which the inhibitory action of Id has been established in tissue culture models (Benezra et al., 1990), Id and the HLH myogenic factors are expressed in a mutually exclusive manner suggesting that myogenic precursors do not express both types of HLH gene products. In addition, Id colocalizes both spatially and temporally with Hox-7.1, a murine homeobox gene which is associated with regions of high cell proliferation and positional fate assignment.