Mesenchymal and neuronal cell expression of the p75 neurotrophin receptor gene occur by different mechanisms.

The p75 neurotrophin receptor serves as a receptor for all known neurotrophins, including NGF, BDNF, NT-3, and NT-4/5. The expression pattern of p75 is known to be widespread and extends outside the nervous system, suggesting that neurotrophins may have functions beyond neuronal cell survival and differentiation. To investigate the expression of p75, a human minigene was constructed that contained the full-length receptor cDNA directed by its own promoter and 4 kb of 5' upstream sequence. This minigene was used to generate transgenic mice and was found to be expressed in selective neuronal populations such as the cerebellar external granular layer. Expression of the transgene was not observed in sensory or sympathetic ganglia or in the basal forebrain. Transection of the sciatic nerve revealed that the human transgene was appropriately upregulated after nerve injury, indicating that sequences responsible for proper induction during nerve regeneration reside in the minigene construct. In contrast, the p75 transgene was found to be extensively expressed in mesenchymal cells during development, closely mimicking the in vivo pattern of expression. The detection of p75 receptors in the mesenchyme implies that neurotrophins are likely to exert effects during morphogenesis of mesodermal tissues and that separate signals are likely to direct neuronal versus nonneuronal expression of the p75 gene.