Dynamic expression pattern of Neuronal leucine-rich repeat 4 in mouse sensory neurons during development

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In this study, we examined the expression of NLRR-4 in the adult and developing dorsal root ganglia (DRGs). Northern blotting analysis revealed that the expression of NLRR-4 mRNA was detected between postnatal day (P) 0 and adult DRGs, in particular, the strong expression was observed at P0 and P7. In situ hybridization histochemistry showed that NLRR-4 mRNA was exclusively detected in the small-sized DRG neurons of the adult mice. Likewise, LacZ staining in NLRR-4 heterozygote mice revealed that LacZ-positive neurons were observed in 8% of the total DRG neurons, and this frequency was similar to the results of in situ hybridization histochemistry. Approximately 42% of NLRR-4-positive neurons contained receptor tyrosine kinase (Trk) A immunoreactivity, and 58% expressed Ret immunoreactivity in the adult mice. In the developing DRGs, NLRR-4 expression was first detected in 7% of total DRG neurons at embryonic day (E) 13.5, gradually increased, and reached the maximum level to 44% at E17.5. The percentage of LacZ-positive neurons in total neurons changed little between E17.5 and P7, and drastically reduced after P7. As a results of characterization for NLRR4-expressing neurons during development, NLRR4-positive neurons mainly expressed TrkC at E13.5. Between E17.5 and P0, NLRR-4-positive neurons contained predominantly TrkA-immunoreactivities. TrkC-positive neurons are earlier maturated compared with TrkA-positive neurons such as axonal innervation into the spinal cord, synaptic formation, and synaptic maturation. These results suggested that NLRR-4 might play a role in the synapse formation of DRG neurons contacting to the secondary neurons in the spinal cord.