Promotion of the development of enteric neurons and glia by neuropoietic cytokines: Interactions with neurotrophin-3

Growth factor synergism and antagonism in early neural crest development

This review article focuses on data that reveal the importance of synergistic and antagonistic effects in growth factor action during the early phases of neural crest development. Growth factors act in concert in different cell lineages and in several aspects of neural crest cell development, including survival, proliferation, and differentiation. Stem cell factor (SCF) is a survival factor for the neural crest stem cell. Its action is neutralized by neurotrophins, such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) through apoptotic cell death. In contrast, SCF alone does not support the survival of melanogenic cells (pigment cell precursors). They require the additional presence of a neurotrophin (NGF, BDNF, or NT-3). Fibroblast growth factor-2 (FGF-2) is an important promoter of proliferation in neuronal progenitor cells. In neural crest cells, fibroblast growth factor treatment alone does not lead to cell expansion but also requires the presence of a neurotrophin. The proliferative stimulus of the fibroblast growth factor - neurotrophin combination is antagonized by transforming growth factor beta-1 (TGFbeta-1). Moreover, TGFbeta-1 promotes the concomitant expression of neuronal markers from two cell lineages, sympathetic neurons and primary sensory neurons, indicating that it acts on a pluripotent neuronal progenitor cell. Moreover, the combination of FGF-2 and NT3, but not other neurotrophins, promotes expression or activation of one of the earliest markers expressed by presumptive sympathetic neuroblasts, the norepinephrine transporter. Taken together, these data emphasize the importance of the concerted action of growth factors in neural crest development at different levels and in several cell lineages. The underlying mechanisms involve growth-factor-induced dependence of the cells on other factors and susceptibility to growth-factor-mediated apoptosis.Key words: neural crest, melanocyte, stem cell factor, neurotrophin-3, transforming growth factor-beta1, apoptosis, norepinephrine transporter.

V. Genes, lineages, and tissue interactions in the development of the enteric nervous system

The enteric nervous system is derived from the vagal, rostral-truncal, and sacral levels of the neural crest. Because the crest-derived population that colonizes the bowel contains multipotent cells, terminal differentiation occurs in the gut and is influenced by both the enteric microenvironment and the responsivity of multiple lineages of precursors. Enteric growth factor-receptor combinations, which promote the development of enteric neurons and/or glia in most of the gastrointestinal (GI) tract, include glial cell line-derived neurotrophic factor-GFRalpha-1-Ret, NT-3-TrkC, a still-to-be-identified neuropoietic cytokine-ciliary neurotrophic factor receptor-alpha, serotonin (5-HT)-5-HT2B, and LBP110, a 110-kDa laminin-1 binding protein. A qualitatively different effect is shown by the peptide-receptor combination ET-3-ETB, which inhibits neuronal differentiation and appears to prevent the premature differentiation of enteric neurons before colonization of the GI tract has been completed (resulting in aganglionosis of the terminal colon).