Occurrence of prosaposin as a neuronal surface membrane component.

Prosaposin is a precursor of four saposins that are required for the lysosomal hydrolysis of sphingolipids by specific hydrolases. Besides its precursor role, prosaposin also exists as a secreted protein. The present investigation reveals that prosaposin also exists as an integral component of the surface membranes of neuronal cells. Subcellular fractionation studies demonstrate that the membrane-bound prosaposin occurs specifically in plasma membranes of NS20Y rat neuroblastoma cells. An immunohistochemical study of the neuroblastoma cells using rat prosaposin-specific antibodies also showed that a portion of prosaposin is located on the surface of neurites as well as on cell bodies. Similar histochemical studies with antibodies that specifically recognized human prosaposin revealed the presence of prosaposin in dendrites, axons, and cell bodies of subcortical and spinal cord neurons in both human adult brain and in fetal brain (24-wk gestation). These findings suggest an important role of prosaposin in neuronal development.