Target cells promote the development and functional maturation of neurons derived from a sympathetic precursor cell line.

The role of target interactions in the development and functional maturation of peripheral neurons was investigated using an immortalized sympathetic precursor cell line. bMAH cells underwent neuronal differentiation in response to neurotrophic factors, but maintained an immature neuronal phenotype characterized by small cell bodies and continued cell division. Co-culture with cardiac myocytes, a target of sympathetic innervation, promoted the appearance of large-diameter postmitotic bMAH neurons. Analysis of bMAH maturation in the presence and absence of co-cultured myocytes indicated that myocyte-derived factors promoted the survival of maturing bMAH neurons prior to their acquisition of nerve growth factor dependence. Myocyte interactions also promoted the functional maturation of bMAH neurons, leading to an increase in the localization of synaptic vesicle proteins into neuritic varicosities and the acquisition of sympathetic-like intrinsic electrical properties. Like primary sympathetic neurons, mature bMAH neurons formed functional connections to cardiac myocytes as measured by evoked postsynaptic responses in connected myocytes. The effects of myocyte co-culture on developing bMAH neurons could be mimicked by myocyte conditioned medium, indicating that cardiac myocytes produce soluble factors that promote the appearance of mature neurons. These experiments indicate that targets of innervation play a role in directing the development and final maturation of peripheral neurons.

Nerve growth factor collaborates with myocyte-derived factors to promote development of presynaptic sites in cultured sympathetic neurons.

Nerve growth factor (NGF) acutely modulates synaptic transmission between sympathetic neurons and their cardiac myocyte targets. NGF also has developmental effects in establishing the level of synaptic transmission between sympathetic neurons and myocytes in culture, although little is known about the mechanisms by which NGF influences this synaptic connectivity. Here we report that NGF acts in conjunction with factors produced by cardiac myocytes to promote neuronal contact with the target and the extension of synaptic vesicle-containing growth cones. In conjunction with previously published results showing that NGF has long-term effects on synaptic transmission between sympathetic neurons and myocytes, this work suggests that NGF acts to promote sympathetic neurotransmission by increasing the number of sympathetic fibers establishing target contact. Further, we found that developmental changes in cardiac myocytes led to an increase in the density of synaptic vesicle-containing variocosities along sympathetic fibers, a process regulated by NGF. Thus, as myocytes mature they produce factors that promote the formation of sympathetic presynaptic structures. These results argue that multiple target interactions regulate the extent of synapse formation between sympathetic neurons and cardiac cells and suggest that NGF promotes presynaptic development by increasing neuronal contact with myocyte-derived cell surface or matrix-associated factors.