THE HAND1 LINEAGE REVEALS DISTINCT ROLES FOR HAND FACTORS DURING CARDIOVASCULAR DEVELOPMENT

Abstract

The basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 play critical roles in the development of multiple organ systems during embryogenesis. The dynamic expression patterns of these two factors within developing tissues obfuscates their respective unique and redundant organogenic functions. To define cell lineages potentially dependent upon Hand gene expression, we generated a mutant allele in which the coding region of Hand1 is replaced by Cre recombinase. Subsequent Cre-mediated activation of β-galactosidase or eYFP reporter alleles enabled lineage trace analyses that clearly define the fate of Hand1-expressing cells. Comparisons between Hand1 expression and Hand1-lineage greatly refine our understanding of its dynamic spatio-temporal expression domains and raise the possibility of novel Hand1 functions in structures not thought to be Hand1-dependent. To genetically examine functional overlap between Hand1 and Hand2, we conditionally deleted Hand2 from Hand1-expressing cells. Hand2 conditional knockout mice die midgestation and exhibit cardiovascular and limb defects. Moreover, Hand2 lineage-restricted deletion from the proepicardial organ results in defective epicardialization and failure to form coronary arteries. Together, these novel data demonstrate a hierarchal relationship whereby transient Hand1 expression within the septum transversum defines epicardial precursors that depend upon subsequent Hand2 function.