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Signals from target tissues play critical roles in the functional differentiation of neuronal cells, and in their subsequent adaptations to peripheral changes in the adult. Sensory neurons in the dorsal root ganglia (DRG) provide an excellent model system for the study of signals that regulate the development of neuronal diversity. DRG have been well characterized and contain both neurons that convey information from muscles about limb position, as well as other neurons that provide sensations from skin about pain information. Sensory neurons involved in pain sensation can be distinguished physiologically and antigenically, and one hallmark characteristic is that these neurons contain neuropeptides important for their functions. The transforming growth factor (TGF) beta family member activin A has recently been implicated in neural development and response to injury. During sensory neuron development, peripheral target tissues containing activin or activin itself can regulate pain neuropeptide expression. Long after development has ceased, skin target tissues retain the capacity to signal neurons about changes or injury, to functionally refine synapses. This review focuses on the role of activin as a target-derived differentiative factor in neural development that has additional roles in response to cutaneous injuries in the adult.

Successful healing of skin wounds requires sensory innervation and the release of vasoactive neuropeptides that dilate blood vessels and deliver serum proteins to the wound, and that cause pain that protects from further injury. Activin has been proposed as a target-derived regulator of sensory neuropeptides during development, but its role in the mature nervous system is unknown. While adult skin contains a low level of activin, protein levels in skin adjacent to a wound increase rapidly after an excision. Neurons containing the neuropeptide calcitonin gene-related peptide (CGRP) increased in sensory ganglia that projected to the wounded skin, but not in ganglia that projected to unwounded skin, suggesting that neurons respond to a local skin signal. Indeed, many adult sensory neurons respond with increased CGRP expression to the application of activin in vitro and utilize a smad-mediated signal transduction pathway in this response. A second skin-derived factor nerve growth factor (NGF) also increased in wounded skin and increased CGRP in cultured adult dorsal root ganglia (DRG) neurons but with lower efficacy. Together, these data support the hypothesis that activin made by skin cells regulates changes in sensory neuropeptides following skin injury, thereby promoting vasodilation and wound healing.

Target skin cells induce the neuropeptide calcitonin gene-related peptide (CGRP) in naïve embryonic dorsal root ganglion (DRG) neurons in vitro, but the molecular basis of that induction is not known. Recombinant activin or bone morphogenetic proteins (BMPs) dramatically increase the number of sensory neurons with CGRP and substance P in vitro (X. Ai et al., 1999, Mol. Cell. Neurosci. 14, 506-518). These experiments were designed to test if activin or BMPs accounted for the CGRP-inductive activity by skin cells. To identify factors from skin that induce CGRP, we developed a bioassay in which embryonic DRG neurons isolated before peripheral target contact in vivo are challenged in vitro with specific factors. Conditioned medium from an embryonic rat skin cell line induced neuronal CGRP expression, and induction was blocked by follistatin, implicating transforming growth factor family members. Immunoblot analysis revealed that the skin cell line medium contained several activin and bone morphogenetic protein moieties. Antibody specific to activin neutralized most of the CGRP-inductive activity in skin conditioned medium. These data indicate that the CGRP-inductive action of skin cells involves activin and establish activin as a candidate regulator of this sensory neuropeptide phenotype during development., (Copyright 2000 Academic Press.)

Sympathetic neurons, chromaffin cells, and small, intensely fluorescent (SIF) cells are thought to derive from a common sympathoadrenal precursor cell. Sympathoadrenal precursor cells and adrenal chromaffin cells have been shown to react with an antibody, SA-1; we now report that, like sympathoadrenal precursors, SIF cells during their proliferative phase transiently possess this epitope. Precursors and SIF cells were identified in double-label studies of the superior cervical ganglion (SCG) using SA-1 and an antibody that identifies a noradrenergic trait, tyrosine hydroxylase (TH). At E16 when the earliest SIF precursors were detected and at birth, while postmitotic principal neurons had lost SA-1 reactivity, many SIF cells expressed both TH and SA-1. As development proceeded, the proportion of SIF cells expressing only TH increased. In addition, some small SIF-like cells possessed only SA-1 reactivity at birth. Some SIF cells at P7 possessed SA-1, but it was absent at P10 and in the adult. Bromodeoxyuridine (BrdU) was used to identify proliferating SIF cells, and SA-1+ expression was correlated with the period of SIF cell proliferation. At late embryogenesis, the proportion of SA-1+ SIF cells that possessed BrdU was relatively large (17% at E20), and decreased as SIF cell division ceased (6% at P1). Our results indicate that SA-1 is present on SIF cells when these cells are capable of cell division. In addition, mature SIF cells lack SA-1 and are therefore antigenically distinct from the sympathoadrenal precursor. These data suggest that the expression of SA-1 is correlated with the ability of sympathoadrenal cells to proliferate, in the SCG early during embryogenesis, chromaffin cells both during embryogenesis and in the adult, and SIF cells during their transient period of division in the SCG.

The phylogeny of adhesion among cells derived from neural tissue has been examined using a combination of functional and immunological analyses. The presence of the neural cell adhesion molecule (NCAM) was evaluated with respect to NCAM-specific antigenic determinants attached to a polypeptide chain with appropriate electrophoretic properties. By these criteria, NCAM-like molecules were detected in all embryonic and adult vertebrates tested, and an adult mollusc, but not in an adult insect, crustacean, or nematode. The functional assays measured adhesiveness by simple aggregation of neural membrane vesicles, as well as by NCAM-specific binding between membranes from different species. The presence of the NCAM antigen in vertebrate membranes correlated with binding activity in both the NCAM-specific and general adhesion assays, implying that the adhesiveness of these membranes largely reflects NCAM-mediated binding. The results also indicate that NCAM function has been conserved during the evolution of vertebrates, and supports the possibility that mechanisms of nerve-nerve, nerve-muscle, and nerve-glial interaction, which have been demonstrated previously to involve NCAM, may be similar for many chordates. Whereas NCAM was not detected in adult fly and worm, these species did express NCAM-like antigens transiently during early development. These results are consistent with the hypothesis that NCAM is required during several periods of development, and that the functions of this molecule in nematodes and insects may be distinct from or a subset of those that occur in vertebrates. The expanded role of the molecule represented by its expression during later stages of vertebrate development may thus have been an important contribution to the evolution of chordates.