The heart LIM protein gene (Hlp), expressed in the developing and adult heart, defines a new tissue-specific LIM-only protein family

In a subtraction designed to identify transcripts accompanying mesodermal lineage specification in mouse ES differentiation cultures, we identified a gene encoding a two LIM-domain protein which we named heart LIM protein (Hlp). Hlp is most closely related to thymus LIM protein, and these two genes comprise a new gene family related to the cysteine-rich protein (CRP) gene family. In the embryo, Hlp expression is primarily restricted to the developing heart. In situ hybridization showed expression at E7.75 in the paired heart-forming primordia prior to linear heart-tube formation. At E8.5, strong expression is detected in the heart, with equal expression in both heart chambers. Hlp expression is detected in both myocardium and endocardium, and in vascular endothelium. Later in fetal development low levels of Hlp expression are detected outside the heart, including dorsal root ganglia and the spinal cord. In the adult, Hlp is expressed at highest levels in the heart, and at lower levels in the brain, skeletal muscle and aorta. Hlp expression is unchanged in hypertrophic hearts induced by aortic constriction. These data suggest a role for the two LIM-domain gene Hlp in the very earliest stages of heart differentiation and development.