Targeted deletion of Hand2 in cardiac neural crest-derived cells influences cardiac gene expression and outflow tract development.

The basic helix-loop-helix DNA binding protein Hand2 has critical functions in cardiac development both in neural crest-derived and mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest has allowed us to genetically dissect Hand2-dependent defects specifically in outflow tract and cardiac cushion independent of Hand2 functions in mesoderm-derived structures. Targeted deletion of Hand2 in the neural crest results in misalignment of the aortic arch arteries and outflow tract, contributing to development of double outlet right ventricle (DORV) and ventricular septal defects (VSD). These neural crest-derived developmental anomalies are associated with altered expression of Hand2-target genes we have identified by gene profiling. A number of Hand2 direct target genes have been identified using ChIP and ChIP-on-chip analyses. We have identified and validated a number of genes related to cell migration, proliferation/cell cycle and intracellular signaling whose expression is affected by Hand2 deletion in the neural crest and which are associated with development of VSD and DORV. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting expression of target genes associated with a number of functional interactions in neural crest-derived cells required for proper patterning of the outflow tract, generation of the appropriate number of neural crest-derived cells for elongation of the conotruncus and cardiac cushion organization. Our genetic model has made it possible to investigate the molecular genetics of neural crest contributions to outflow tract morphogenesis and cell differentiation.

Conditional deletion of Hand2 reveals critical functions in neurogenesis and cell type-specific gene expression for development of neural crest-derived noradrenergic sympathetic ganglion neurons.

Neural crest-derived structures that depend critically upon expression of the basic helix-loop-helix DNA binding protein Hand2 for normal development include craniofacial cartilage and bone, the outflow tract of the heart, cardiac cushion, and noradrenergic sympathetic ganglion neurons. Loss of Hand2 is embryonic lethal by E9.5, obviating a genetic analysis of its in-vivo function. We have overcome this difficulty by specific deletion of Hand2 in neural crest-derived cells by crossing our line of floxed Hand2 mice with Wnt1-Cre transgenic mice. Our analysis of Hand2 knock-out in neural crest-derived cells reveals effects on development in all neural crest-derived structures where Hand2 is expressed. In the autonomic nervous system, conditional disruption of Hand2 results in a significant and progressive loss of neurons as well as a significant loss of TH expression. Hand2 affects generation of the neural precursor pool of cells by affecting both the proliferative capacity of the progenitors as well as affecting expression of Phox2a and Gata3, DNA binding proteins important for the cell autonomous development of noradrenergic neurons. Our data suggest that Hand2 is a multifunctional DNA binding protein affecting differentiation and cell type-specific gene expression in neural crest-derived noradrenergic sympathetic ganglion neurons. Hand2 has a pivotal function in a non-linear cross-regulatory network of DNA binding proteins that affect cell autonomous control of differentiation and cell type-specific gene expression.

Expression of Hand2 is sufficient for neurogenesis and cell type-specific gene expression in the enteric nervous system.

The basic helix-loop-helix DNA binding protein Hand2 is expressed in neural crest-derived precursors of enteric neurons and has been shown to affect both neurogenesis and neurotransmitter specification of noradrenergic sympathetic ganglion neurons. In the current study, our goal was to determine whether Hand2 affects neurogenesis and/or expression of vasoactive intestinal polypeptide and choline acetyltransferase in developing enteric neurons. Gain-of-function of Hand2 in HNK-1(+) immmunoselected precursor cells resulted in increased neurogenesis. The number of neurons expressing vasoactive intestinal polypeptide increased in response to Hand2 overexpression although choline acetyltransferase was not affected. Targeted deletion of Hand2 in neural crest cells resulted in loss of all neurons expressing vasoactive intestinal polypeptide along the length of the gastrointestinal tract, patterning defects in the myenteric plexus of the stomach, and altered number and morphology of neurons expressing TH. Our data demonstrate that expression of Hand2 is sufficient and necessary for neurogenesis and expression of a subset of cell type-specific markers in the developing enteric nervous system.

The endocrine-disrupting compound, nonylphenol, inhibits neurotrophin-dependent neurite outgrowth.

Endocrine-disrupting compounds (EDCs) may interfere with neuronal development due to high levels of accumulation in biological tissue and potentially aberrant steroid signaling. Treatment of dissociated embryonic Xenopus spinal cord neurons with the EDC, nonylphenol (NP), did not alter cell survival or neurite outgrowth but inhibited neurotrophin-induced neurite outgrowth, effects that were recapitulated by treatment with comparable concentrations of 17 beta-estradiol (E2) and beta-estradiol 6-(O-carboxy-methyl)oxime: BSA (E2-BSA), but not a synthetic androgen. Effects of NP were not inhibited by the nuclear estrogen receptor antagonist, ICI 182,780, but were inhibited by the G protein antagonist, pertussis toxin. Nerve growth factor (NGF)-induced neurite outgrowth in Xenopus neurons was shown to require MAPK signaling. NP did not affect TrkA expression, MAPK signaling, or phosphatidylinositol 3' kinase-Akt-glycogen synthase kinase 3 beta (PI3K-Akt-GSK3 beta) signaling in Xenopus. The ability of NP to inhibit NGF-induced neurite outgrowth without altering survival was recapitulated in the rat pheochromocytoma (PC12) cell line. As with Xenopus neurons, the inhibitory actions of NP in PC12 cells were not antagonized by ICI 182,780 and did not involve alterations in signaling along either the MAPK or PI3K-Akt-GSK3 beta pathways. NP did significantly inhibit the ability of NGF to increase protein kinase A activity in this cell line. These data have important implications with respect to potentially deleterious effects of NP exposure during early neural development and highlight the fact that bioaccumulation of EDCs, such as NP, may elicit very disparate effects along divergent signaling pathways than those that arise from the actions of physiological levels of endogenous estrogens.