Roles of neurotrophin-3 during early development of the peripheral nervous system.

The neurotrophins are structurally related proteins regulating cell numbers in the developing vertebrate nervous system. They are necessary survival factors preventing the death of specific neuronal populations. Previous experiments have indicated that the administration of nerve growth factor or of brain-derived neurotrophic factor during the formation of sensory ganglia and of target innervation increases the number of neurons by preventing normally occurring neuronal death. These results support the view that during development, neuronal numbers are adjusted to the size of the target tissue by the release of limiting amounts of neurotrophins. However, increasing the levels of neurotrophin-3 during the formation of sensory ganglia results in a marked decrease in neuronal numbers, possibly as a consequence of premature cessation of sensory neuroblast proliferation. In sympathetic ganglia, the application of neurotrophin-3 during the formation of the sympathetic chain causes cell numbers to increase, a result also observed following the application of nerve growth factor. It thus appears that neurotrophin-3 and nerve growth factor can regulate cell numbers well before the period of target-derived control, and that neurotrophin-3 affects neuronal numbers in sensory and sympathetic ganglia in opposite ways.

Early expression of the nerve growth factor receptor ctrkA in chick sympathetic and sensory ganglia.

Avian sympathetic and sensory ganglia are useful models to study the biological effects of nerve growth factor (NGF) in vitro as well as in vivo. In order to examine the expression pattern of the NGF tyrosine kinase receptor during embryogenesis, we cloned a full-length cDNA encoding chick trkA (ctrkA). Compared with human trkA, the sequence identity is 46% in the extracellular domain and is unevenly distributed between the subdomains. Between embryonic Days 6.5 and 16 (E6.5 and E16), a single 3-kb ctrkA transcript is detected in sympathetic and sensory ganglia, and in situ hybridization experiments reveal the presence of ctrkA mRNA in both ganglia from E4.5 onward. The detection of ctrkA in the primary sympathetic chain is unexpected in view of previous experiments with cultured sympathetic neurons indicating a lack of a survival response to NGF at early developmental stages. However, it fits with the observation that, in vivo, the administration of NGF markedly affects cell numbers substantially before the period of target control of neuronal survival in the sympathetic chain.

Epithelial-mesenchymal conversion of dermatome progenitors requires neural tube-derived signals: characterization of the role of Neurotrophin-3.

Development of the somite-derived dermatome involves conversion of the epithelial dermatome progenitors into mesenchymal cells of the dermis. In chick embryos, neural tube-derived signals are required for this conversion, as the interposition of a membrane between neural tube and somites results in a failure of the dermatome to lose its epithelial arrangement. However, dermis formation can be completely rescued by coating the membranes with Neurotrophin-3, but not with the related molecule Nerve growth factor. Neurotrophin-3 was also found to be necessary for dermatome dissociation using in vitro explants or partially dissociated dermomyotomes. The functional relevance of these observations was investigated by neutralizing endogenous Neurotrophin-3 using a specific blocking antibody. Antibody-treated embryos revealed the presence of tightly aggregated cells between myotome and ectoderm instead of the loose dermal mesenchyme observed in embryos treated with control antibodies. As previous studies have demonstrated the presence of Neurotrophin-3 in the neural tube, these results suggest that it may be a necessary neural tube-derived signal required for early stages of dermis formation.