Dynamic expression pattern of leucine-rich repeat neuronal protein 4 in the mouse dorsal root ganglia during development.

A member of leucine-rich repeat neuronal protein (Lrrn) family, Lrrn4, is a type I transmembrane protein and functions as a cell adhesion molecule. In our previous report, Lrrn4 is expressed in a subset of small-sized dorsal root ganglion (DRG) neurons of the adult mice. In the present study, we investigated the expression pattern of Lrrn4 in the developing DRGs. The expression of Lrrn4 was first observed in 7% of total DRG neurons at embryonic day (E) 13.5, gradually increasing to 44% at E17.5, reached the maximum level between E17.5 and postnatal day (P) 7, decreased drastically after P7, and became the adult level by P14. Interestingly, the expression of Lrrn4 was mainly observed in TrkC-positive neurons at E13.5, and the predominant expression was shifted from TrkC-positive neurons to TrkA-positive neurons between E15.5 and E17.5. As the central afferents of TrkC-positive and TrkA-positive neurons begin to penetrate into the spinal cord to form synapse with secondary neurons at E13.5 and E15.5, respectively, the time course of Lrrn4 expression may suggest the contribution of Lrrn4 to synaptic formation. In addition, some cell adhesion molecules containing leucine-rich repeat are identified as synaptic adhesion molecules, suggesting that the spatiotemporal expression pattern of Lrrn4 contributes to the development of synaptic function in the DRG neurons.