Neuron specific expression of a membrane protein, HPC-1: tissue distribution, and cellular and subcellular localization of immunoreactivity and mRNA.

The monoclonal antibody HPC-1 recognizes a protein antigen in the hippocampus, and its specific reactivity to the plasma membrane of the amacrine cell somas and the inner plexiform layer in rat retina has been reported. Sequencing the cDNA indicated in our previous study that the HPC-1 antigen was a membrane protein. By means of immunoblotting, an antiserum against the fusion protein of Escherichia coli beta-galactosidase and the HPC-1 antigen detected several proteins of about 35 kDa in the nervous tissues including retina, cerebral cortex, hippocampus, cerebellum and spinal cord, but no signal was obtained in the non-neuronal tissues. Immunofluorescent histochemistry of the various rat tissues revealed that the HPC-1 antigen was confined to the nervous system, including the matrices of the cerebral cortex and hippocampus, the molecular layer, membranes of granular cell somas and glomeruli in the cerebellum and gray matter of spinal cord. However, little staining was seen in the white matter of the central nervous tissues. Thus, the HPC-1 antigen was accumulated in the synapse-rich regions of neuronal cells. In situ hybridization revealed that the HPC-1 mRNA was present in most, if not all, neurons in the central and peripheral nervous systems except for the retina. In the retina, mRNA signals were detected in amacrine and ganglion cells in which HPC-1 immunoreactivity was absent in their soma, suggesting polarized localization of the HPC-1 mRNA on the ganglion cell axon terminal.