Sculpting organ innervation

The development of the neural crest–derived sensory, autonomic, and enteric nervous systems is an intricate process that directs the egress of crest cells from the dorsal neural tube, their migration and differentiation in distinct ganglia, and their ultimate projections to maturing peripheral organs. This feat requires the coordination of complex sets of target-derived cues that play instructive roles in neuronal differentiation, survival, and pathfinding in vivo, to ensure that neuron numbers precisely match target innervation requirements. Although neurotrophic factors, including the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3), as well as glial-derived neurotrophic factor (GDNF), are well-characterized target-derived factors that promote the survival and differentiation of peripheral neurons (1), little is known about the upstream mechanisms that regulate the temporal and spatial expression of these factors to dictate organ-specific patterns of axon guidance, aside from the regulation of NT-3 in mesenchyme by Wnt factors (2). In the heart, myocyte-derived NGF critically regulates sympathetic innervation (3), an event that occurs in late gestation. In this issue of the JCI, Ieda and colleagues (4) uncover a critical role for endothelin-1 (ET-1), a member of the endothelin family of peptides that have numerous roles in cardiovascular development, in regulating NGF expression in the heart. The endothelins are a family of 21–amino acid peptides originally characterized by their potent vasopressive effects. Enzymatically cleaved from a preproform by the sequential actions of endothelin-converting enzymes, ET-1 is the best-characterized member in the cardiovascular system and binds to two distinct seven-transmembrane G protein–coupled receptors, ETA and ETB (5). Prominent effects of endothelin signaling in cardiovascular development include the promotion of Purkinje fiber differentiation (6), craniofacial and aortic arch patterning via actions on cranial and cardiac neural crest (7), and cardiac outflow tract remodeling (8). In many regards, symptoms observed in mice deficient in ET-1, endothelin-converting enzyme-1, or ETA resemble those associated with human congenital cardiovascular malformations such as CATCH 22 syndrome, although only a few downstream targets of ET-1, including heart and neural crest derivatives (known as HANDs) and Goosecoid, have been characterized (8, 9). In the current study, Ieda and colleagues document that ET-1, but not several other growth factors known to act on embryonic cardiomyocytes, upregulates NGF mRNA and protein expression in myocyte cultures (4). This occurs in a Giβγ-dependent manner, consistent with the known expression of ETA by cardiac myocytes, and by established intracellular signaling pathways downstream of the ETA receptor. Using ET-1 gene–targeted mice, the authors demonstrate that ET-1 markedly induces NGF expression by cardiac myocytes between embryonic days 15.5 and 18.5, when stellate ganglion sympathetic neurons are extending their projections to the heart. In the absence of ET-1, fiber density is markedly diminished, cardiac norepinephrine levels fail to increase, and stellate ganglion neurons die. These results are consistent with recent studies of Glenbova and Ginty (10) using mice doubly deficient in both Bax, to impair neuronal apoptosis, and NGF. Their studies indicate that although NGF is not required for the formation of the stellate ganglion, it is required to promote extensive fiber growth to innervate coronary arteries and cardiac myocytes. Ieda and colleagues use genetic rescue experiments, crossing ET-1–null mice with transgenic mice overexpressing NGF in cardiac myocytes, to confirm that myocyte-derived NGF can restore sympathetic innervation despite the loss of ET-1 (4). Although the in vivo source of ET-1 in the late-gestational heart is not documented, these complementary approaches provide convincing evidence that endothelin receptor signaling in myocytes is required for optimal NGF induction and subsequent target innervation.