Expression of kin of irregular chiasm-like 3/mKirre in proprioceptive neurons of the dorsal root ganglia and its interaction with nephrin in muscle spindles.

Kin of irregular chiasm-like 3 (Kirrel3), a mammalian homolog of the kirre gene of Drosophila melanogaster, belongs to the immunoglobulin superfamily. Previously, we have reported that Kirrel3 is expressed in the developing and adult central nervous system. In the present study we investigated the expression of Kirrel3 in the mouse dorsal root ganglia (DRG) and their projection targets. In the adult DRGs, Kirrel3 mRNA was detected in 21.5 +/- 2.3% of total DRG neurons and the expression was mainly prevalent in the medium- and large-sized neurons. In addition, Kirrel3 mRNA predominantly colocalized with tyrosine kinase receptor (Trk) C-immunoreactivity. In the developing DRGs, Kirrel3 mRNA was first detected in a few cells at embryonic day (E) 11.5, gradually increased, and reached the adult level at E17.5. During the development, Kirrel3 was expressed in most TrkC-positive DRG neurons. The expression of Kirrel3 was observed in TrkC-positive nerve fibers around neurotrophin 3 (NT3)-positive intrafusal muscle fibers of muscle spindles at E17.5. However, Kirrel3 was not expressed in TrkC-positive nerve fibers projecting to the spinal cord throughout development. Furthermore, nephrin was expressed in the NT3-positive intrafusal muscle fibers and was in close apposition with Kirrel3-immunoreactivity. Coimmunoprecipitation assay revealed that nephrin interacted with Kirrel3 in the developing muscles. These results suggest that Kirrel3 might play a role in the axonal pathfinding, cell recognition, and synapse formation of DRG neurons on appropriate target cells, including the targeting of proprioceptive neurons on muscle spindles through the interaction with nephrin.