CD9, A MAJOR PLATELET CELL-SURFACE GLYCOPROTEIN, IS A ROCA ANTIGEN AND IS EXPRESSED IN THE NERVOUS-SYSTEM

We previously generated a monoclonal antibody (mAb), ROCA 1, which bin ds preferentially to rostral versus caudal sympathetic ganglia and int ercostal nerves. Two other mAbs, ROCA2 and B2C11, bind to the same str uctures but not in rostrocaudal gradients. All three mAbs recognize a 26 kDa cell surface protein. Amino acid sequence data obtained from th e affinity purified 26 kDa protein showed some homology with human CD9 , a tetraspan protein implicated in intercellular signaling in hematop oietic cells. Using the PCR, we obtained cDNA clones representing the entire rat CD9 coding sequence from sciatic nerve and sympathetic gang lia. ROCA1, ROCA2, and B2C11 each immunoprecipitate a 26 kDa protein f rom CHO cells stably transfected with one of the clones, demonstrating that the ROCA cell surface antigen is indeed rat CD9. We find that CD 9 mRNA is widely expressed, with particularly high levels present in a number of neural tissues. In situ hybridization demonstrates that per ipheral neurons and Schwann cells, as well as adrenal chromaffin cells express CD9 mRNA. Consistent with immunoblot analyses showing that, u nlike the ROCA1 epitope, the 26 kDa protein is not expressed in a rost rocaudal gradient, we find similar levels of rat CD9 mRNA in rostral a nd caudal intercostal nerves. In developing postnatal rat sciatic nerv e, CD9 mRNA levels are coordinately regulated with the expression of m yelin genes. These results provide another example of a cell surface p rotein expressed by both hematopoietic and neural cells, and suggest a role for CD9 in intercellular signaling in the nervous system.