Identification of a hypaxial somite enhancer element regulating Pax3 expression in migrating myoblasts and characterization of hypaxial muscle Cre transgenic mice.

Pax3 encodes a transcription factor that functions in the embryonic central nervous system, neural crest, and somitic mesoderm. Prior studies suggest that distinct regulatory elements regulate temporal and spatial expression of Pax3 in neural crest and mesoderm. Here, we describe a discrete enhancer element, conserved between mouse and human genomes, that directs Pax3 expression in the ventral-lateral lip of interlimb somites. These regions give rise to hypaxial musculature including limb, ventral body wall, diaphragm, and tongue muscles. Transgenic mice harboring the hypaxial muscle enhancer driving lacZ expression initiate beta-galactosidase expression at E10.0, significantly later than endogenous Pax3 expression in presomitic and segmented mesoderm. Initiation of transgene expression is not dependent on Pax3 itself, since expression is detectable in homozygous Splotch embryos. Transgenic mice expressing Cre recombinase in hypaxial myoblasts were generated and characterized. These results suggest that Pax3 is differentially regulated within the somite in both spatial and temporal domains. Hypaxial muscle Cre mice will allow for specific manipulation of gene expression in this subset of developing skeletal muscle.

Cre-mediated excision of Fgf8 in the Tbx1 expression domain reveals a critical role for Fgf8 in cardiovascular development in the mouse.

Tbx1 has been implicated as a candidate gene responsible for defective pharyngeal arch remodeling in DiGeorge/Velocardiofacial syndrome. Tbx1(+/-) mice mimic aspects of the DiGeorge phenotype with variable penetrance, and null mice display severe pharyngeal hypoplasia. Here, we identify enhancer elements in the Tbx1 gene that are conserved through evolution and mediate tissue-specific expression. We describe the generation of transgenic mice that utilize these enhancer elements to direct Cre recombinase expression in endogenous Tbx1 expression domains. We use these Tbx1-Cre mice to fate map Tbx1-expressing precursors and identify broad regions of mesoderm, including early cardiac mesoderm, which are derived from Tbx1-expressing cells. We test the hypothesis that fibroblast growth factor 8 (Fgf8) functions downstream of Tbx1 by performing tissue-specific inactivation of Fgf8 using Tbx1-Cre mice. Resulting newborn mice display DiGeorge-like congenital cardiovascular defects that involve the outflow tract of the heart. Vascular smooth muscle differentiation in the great vessels is disrupted. This data is consistent with a model in which Tbx1 induces Fgf8 expression in the pharyngeal endoderm, which is subsequently required for normal cardiovascular morphogenesis and smooth muscle differentiation in the aorta and pulmonary artery.