Expression of receptor tyrosine kinase RYK in developing rat central nervous system

Receptor tyrosine kinase RYK is a mammalian homologue of Drosophila Lio, which is involved in learning and memory and in axon guidance. We cloned a rat ryk gene and characterized its expression pattern in the central nervous system. Northern blot analysis of the whole brain revealed that the RYK mRNA was abundant during the period from 13 to 18 embryonic days (E13-18) and it decreased by E20. In the postnatal brain, the RYK signal was higher in postnatal one week (P1W) cerebrum and in P2W cerebellum than in later stages. In situ hybridization revealed that RYK was expressed throughout the central nervous system, mainly in the ventricular zone on E11 and E13. On E18 and E20, the remarkable level of RYK mRNA was detected in the ventricular zone as well as in the cortical plate of the forebrain. These two regions overlapped the immunoreactive areas of nestin and MAP2, a neural stem cell marker and a mature neural marker, respectively. Moreover, the double-labeling analysis showed that the same cells expressed both RYK and nestin in the ventricular zone. In the postnatal brain, RYK was predominantly expressed in neurons of various regions. These observations suggest that RYK plays a contributory role as a multifunctional molecule in the differentiation and maturation of neuronal cells in the central nervous system.