A show of Hands: Novel and conserved expression patterns of teleost hand paralogs during craniofacial, heart, fin, peripheral nervous system and gut development

BACKGROUND

Hand genes are required for the development of the vertebrate jaw, heart, peripheral nervous system, limb, gut, placenta, and decidua. Two Hand paralogues, Hand1 and Hand2, are present in most vertebrates, where they mediate different functions yet overlap in expression. In ray-finned fishes, Hand gene expression and function is only known for the zebrafish, which represents the rare condition of having a single Hand gene, hand2. Here we describe the developmental expression of hand1 and hand2 in the cichlid Copadichromis azureus.

RESULTS

hand1 and hand2 are expressed in the cichlid heart, paired fins, pharyngeal arches, peripheral nervous system, gut, and lateral plate mesoderm with different degrees of overlap.

CONCLUSIONS

Hand gene expression in the gut, peripheral nervous system, and pharyngeal arches may have already been fixed in the lobe- and ray-finned fish common ancestor. In other embryonic regions, such as paired appendages, hand2 expression was fixed, while hand1 expression diverged in lobe- and ray-finned fish lineages. In the lateral plate mesoderm and arch associated catecholaminergic cells, hand1 and hand2 swapped expression between divergent lineages. Distinct expression of cichlid hand1 and hand2 in the epicardium and myocardium of the developing heart may represent the ancestral pattern for bony fishes.

Cardiac Na+-Ca2+ exchanger 1 (ncx1h) is critical for the ventricular cardiomyocyte formation via regulating the expression levels of gata4 and hand2 in zebrafish

Ca²⁺ signaling is critical for heart development; however, the precise roles and regulatory pathways of Ca²⁺ transport proteins in cardiogenesis remain largely unknown. Sodium-calcium exchanger 1 (Ncx1) is responsible for Ca²⁺ efflux in cardiomyocytes. It is involved in cardiogenesis, while the mechanism is unclear. Here, using the forward genetic screening in zebrafish, we identified a novel mutation at a highly-conserved leucine residue in ncx1 gene (mutant^LDD353/ncx1h^L154P) that led to smaller hearts with reduced heart rate and weak contraction. Mechanistically, the number of ventricular but not atrial cardiomyocytes was reduced in ncx1h^L154P zebrafish. These defects were mimicked by knockdown or knockout of ncx1h. Moreover, ncx1h^L154P had cytosolic and mitochondrial Ca²⁺ overloading and Ca²⁺ transient suppression in cardiomyocytes. Furthermore, ncx1h^L154P and ncx1h morphants downregulated cardiac transcription factors hand2 and gata4 in the cardiac regions, while overexpression of hand2 and gata4 partially rescued cardiac defects including the number of ventricular myocytes. These findings demonstrate an essential role of the novel 154th leucine residue in the maintenance of Ncx1 function in zebrafish, and reveal previous unrecognized critical roles of the 154th leucine residue and Ncx1 in the formation of ventricular cardiomyocytes by at least partially regulating the expression levels of gata4 and hand2.

barx1 represses joints and promotes cartilage in the craniofacial skeleton

The evolution of joints, which afford skeletal mobility, was instrumental in vertebrate success. Here, we explore the molecular genetics and cell biology that govern jaw joint development. Genetic manipulation experiments in zebrafish demonstrate that functional loss, or gain, of the homeobox-containing gene barx1 produces gain, or loss, of joints, respectively. Ectopic joints in barx1 mutant animals are present in every pharyngeal segment, and are associated with disrupted attachment of bone, muscles and teeth. We find that ectopic joints develop at the expense of cartilage. Time-lapse experiments suggest that barx1 controls the skeletal precursor cell choice between differentiating into cartilage versus joint cells. We discovered that barx1 functions in this choice, in part, by regulating the transcription factor hand2. We further show that hand2 feeds back to negatively regulate barx1 expression. We consider the possibility that changes in barx1 function in early vertebrates were among the key innovations fostering the evolution of skeletal joints.

Hand factors as regulators of cardiac morphogenesis and implications for congenital heart defects

Almost 15 years of careful study have established the related basic Helix-Loop-Helix (bHLH) transcription factors Hand1 and Hand2 as critical for heart development across evolution. Hand factors make broad contributions, revealed through animal models, to the development of multiple cellular lineages that ultimately contribute to the heart. They perform critical roles in ventricular cardiomyocyte growth, differentiation, morphogenesis, and conduction. They are also important for the proper development of the cardiac outflow tract, epicardium, and endocardium. Molecularly, they function both through DNA binding and through protein-protein interactions, which are regulated transcriptionally, posttranscriptionally by microRNAs, and posttranslationally through phosphoregulation. Although direct Hand factor transcriptional targets are progressively being identified, confirmed direct targets of Hand factor transcriptional activity in the heart are limited. Identification of these targets will be critical to model the mechanisms by which Hand factor bHLH interactions affect developmental pathways. Improved understanding of Hand factor-mediated transcriptional cascades will be necessary to determine how Hand factor dysregulation translates to human disease phenotypes. This review summarizes the insight that animal models have provided into the regulation and function of these factors during heart development, in addition to the recent findings that suggest roles for HAND1 and HAND2 in human congenital heart disease.

Endoderm-derived Sonic hedgehog and mesoderm Hand2 expression are required for enteric nervous system development in zebrafish

The zebrafish enteric nervous system (ENS), like those of all other vertebrate species, is principally derived from the vagal neural crest cells (NCC). The developmental controls that govern the migration, proliferation and patterning of the ENS precursors are not well understood. We have investigated the roles of endoderm and Sonic hedgehog (SHH) in the development of the ENS. We show that endoderm is required for the migration of ENS NCC from the vagal region to the anterior end of the intestine. We show that the expression of shh and its receptor ptc-1 correlate with the development of the ENS and demonstrate that hedgehog (HH) signaling is required in two phases, a pre-enteric and an enteric phase, for normal ENS development. We show that HH signaling regulates the proliferation of vagal NCC and ENS precursors in vivo. We also show the zebrafish hand2 is required for the normal development of the intestinal smooth muscle and the ENS. Furthermore we show that endoderm and HH signaling, but not hand2, regulate gdnf expression in the intestine, highlighting a central role of endoderm and SHH in patterning the intestine and the ENS.

Transcripts encoding HAND genes are differentially expressed and regulated by BMP4 and GDNF in developing avian gut

Growth and transcription factors provide important developmental cues to neural crest-derived precursors of enteric neurons. The basic helix-loop-helix transcription factors, HAND2 and HAND1, are expressed in the gastrointestinal tract, but neither the growth factors that induce their expression nor the cell types that express them in the gut are known. We show that transcripts encoding HAND2 are expressed in all segments of the developing gut while those encoding HAND1 are confined to the small intestine and colon. Using in situ hybridization combined with immunostaining using cell type-specific antigens, we demonstrate that transcripts encoding HAND2 are expressed in neurons of both the myenteric and submucosal ganglia. Transcripts encoding HAND1 are expressed by cells in the epithelial lining of the small intestine and colon. The differential localization of HAND2 and HAND1 is reflected in nonoverlapping patterns of regulation by gut-derived factors. The expression of transcripts encoding HAND2 is increased in neural crest-derived cells when cocultured with E4 gut, suggesting a gut-derived factor regulates expression of HAND genes. Exposure of gut-derived neural crest-derived cells to BMP4 significantly increased the expression of HAND2 in all gut segments. In the esophagus and gizzard, where HAND1 is not normally expressed, treatment with BMP4 induced the expression of transcripts encoding HAND1 in nonneural crest-derived cells. GDNF failed to induce consistent expression of transcripts encoding HAND2 in neural crest cells but did support a modest increase in HAND2 expression in gut-derived crest cells obtained from the esophagus and colon. GDNF had no detectable effect on the expression of transcripts encoding HAND1. These results suggest; 1) that HAND2 has a function in the development of enteric neurons, and 2) that BMP and GDNF differentially regulate HAND2 and HAND1 gene expression in the developing gastrointestinal tract.