Hand2 elevates cardiomyocyte production during zebrafish heart development and regeneration.

Embryonic heart formation requires the production of an appropriate number of cardiomyocytes; likewise, cardiac regeneration following injury relies upon the recovery of lost cardiomyocytes. The basic helix-loop-helix (bHLH) transcription factor Hand2 has been implicated in promoting cardiomyocyte formation. It is unclear, however, whether Hand2 plays an instructive or permissive role during this process. Here, we find that overexpression of hand2 in the early zebrafish embryo is able to enhance cardiomyocyte production, resulting in an enlarged heart with a striking increase in the size of the outflow tract. Our evidence indicates that these increases are dependent on the interactions of Hand2 in multimeric complexes and are independent of direct DNA binding by Hand2. Proliferation assays reveal that hand2 can impact cardiomyocyte production by promoting division of late-differentiating cardiac progenitors within the second heart field. Additionally, our data suggest that hand2 can influence cardiomyocyte production by altering the patterning of the anterior lateral plate mesoderm, potentially favoring formation of the first heart field at the expense of hematopoietic and vascular lineages. The potency of hand2 during embryonic cardiogenesis suggested that hand2 could also impact cardiac regeneration in adult zebrafish; indeed, we find that overexpression of hand2 can augment the regenerative proliferation of cardiomyocytes in response to injury. Together, our studies demonstrate that hand2 can drive cardiomyocyte production in multiple contexts and through multiple mechanisms. These results contribute to our understanding of the potential origins of congenital heart disease and inform future strategies in regenerative medicine.

Hand2 inhibits kidney specification while promoting vein formation within the posterior mesoderm.

Proper organogenesis depends upon defining the precise dimensions of organ progenitor territories. Kidney progenitors originate within the intermediate mesoderm (IM), but the pathways that set the boundaries of the IM are poorly understood. Here, we show that the bHLH transcription factor Hand2 limits the size of the embryonic kidney by restricting IM dimensions. The IM is expanded in zebrafish hand2 mutants and is diminished when hand2 is overexpressed. Within the posterior mesoderm, hand2 is expressed laterally adjacent to the IM. Venous progenitors arise between these two territories, and hand2 promotes venous development while inhibiting IM formation at this interface. Furthermore, hand2 and the co-expressed zinc-finger transcription factor osr1 have functionally antagonistic influences on kidney development. Together, our data suggest that hand2 functions in opposition to osr1 to balance the formation of kidney and vein progenitors by regulating cell fate decisions at the lateral boundary of the IM.

Identification of ZBP-89 as a novel GATA-1-associated transcription factor involved in megakaryocytic and erythroid development.

A complete understanding of the transcriptional regulation of developmental lineages requires that all relevant factors be identified. Here, we have taken a proteomic approach to identify novel proteins associated with GATA-1, a lineage-restricted zinc finger transcription factor required for terminal erythroid and megakaryocytic maturation. We identify the Krüppel-type zinc finger transcription factor ZBP-89 as being a component of multiprotein complexes involving GATA-1 and its essential cofactor Friend of GATA-1 (FOG-1). Using chromatin immunoprecipitation assays, we show that GATA-1 and ZBP-89 cooccupy cis-regulatory elements of certain erythroid and megakaryocyte-specific genes, including an enhancer of the GATA-1 gene itself. Loss-of-function studies in zebrafish and mice demonstrate an in vivo requirement for ZBP-89 in megakaryopoiesis and definitive erythropoiesis but not primitive erythropoiesis, phenocopying aspects of FOG-1- and GATA-1-deficient animals. These findings identify ZBP-89 as being a novel transcription factor involved in erythroid and megakaryocytic development and suggest that it serves a cooperative function with GATA-1 and/or FOG-1 in a developmental stage-specific manner.