CELL-DEATH PREVENTION, MITOGEN-ACTIVATED PROTEIN-KINASE STIMULATION, AND INCREASED SULFATIDE CONCENTRATIONS IN SCHWANN-CELLS AND OLIGODENDROCYTES BY PROSAPOSIN AND PROSAPTIDES

Prosaposin, the precursor of saposins A, B, C, and D, was recently ide ntified as a neurotrophic factor, Herein prosaposin was found to incre ase sulfatide concentrations in primary and transformed Schwann cells (iSC) and oligodendrocytes (differentiated CG4 cells), Of the four mat ure saposins, only saposin C was found to increase sulfatide concentra tions in these cell types, A similar result was obtained by using pept ides (prosaptides) encompassing tile neurotrophic sequence located in the saposin C domain, Dose-response curves demonstrated maximal enhanc ement by saposin C and prosaptides at low nanomolar concentrations (5- 10 nM). The increase in sulfatide concentration by a 14-mer prosaptide , TX14(A), in CG4 oligodendrocytes was about 3-fold greater than in pr imary Schwann cells. A mutant prosaptide with a single amino acid repl acement of Asn --> Asp was inactive. Prosaptides did not induce cell p roliferation of primary Schwann cells, iSC cells, or CG4 oligodendrocy tes but nanomolar concentrations of prosaptides prevented cell death o f iSC cells and CG4 oligodendrocytes. Immunoblot analysis demonstrated that phosphorylation of both mitogen-activated protein kinase p-42 an d p-44 isoforms were enhanced 3- to 5-fold after 5 min of treatment wi th prosaptides at concentrations of 1-5 nM. These findings suggest tha t prosaposin and prosaptides bind to a receptor that initiates signal transduction to promote meylin lipid synthesis and prolong cell surviv al in both Schwann cells and oligodendrocytes. Prosaposin may function as a myelinotrophic factor in vivo during development and repair of m yelinated nerves explaining the deficiency of myelin observed in prosa posin-deficient mice and humans.