Prosaposin: A myelinotrophic protein that promotes expression of myelin constituents and is secreted after nerve injury

Recently, we demonstrated that prosaposin and prosaptides (peptides encompa ssing the neurotrophic sequence in prosaposin) prevent cell death and incre ase extracellular regulated kinase (ERK) phosphorylation and sulfatide cont ent in primary Schwann cells or oligodendrocytes (Hiraiwa et al., 1997a). H ere, we examine the effect of prosaptide on other myelin constituents, on S chwann cell morphology and proliferation, and characterize the time course of expression of prosaposin protein after sciatic nerve injury. After 24 h of treatment with 10 nM TX14(A), a 14-mer prosaptide, the specific activity of UDP-galactose:ceramide galactosyltransferase (GalT) in primary Schwann cells was increased by 150% over controls. Under the same conditions, the m aximum content of sulfatide increased 3-fold over controls after 48 h of tr eatment. Northern blot analysis, probed with oligonucleotide sequences from the GalT and PO cDNAs, revealed that the mRNA levels of GalT and PO protei n were elevated about 30 and 200%, respectively, over controls after 24 h o f treatment with TX14(A). Treatment of primary Schwann cells with TX14(A) a lso induced a morphological change at 10 nM; the peptide-treated cells had a bipolar (spindle-shaped) appearance after 48 h of treatment, compared to control cells which were irregular and multipolar. TX14(A) did not induce c ell proliferation, indicating that TX14(A), unlike IGF-I, is not mitogenic. After sciatic nerve transection, Western blot analysis demonstrated the pr esence of intact prosaposin in tubular fluid in a silicon chamber into whic h the proximal and distal nerve stumps were sutured. The concentration of p rosaposin in the fluid was maximum after 9 days post-surgery and returned t o normal after 28 days post-surgery. In uninjured and injured nerve, prosap osin immunolocalized to the smooth muscle of epineurial and endoneurial ves sels. These findings indicated that sciatic nerve secreted prosaposin after injury and that prosaposin is a naturally occurring injury-repair protein which acts to prevent degeneration and to promote regeneration of periphera l nerves. GLIA 26:353-360, 1999. (C) 1999 Wiley-Liss, Inc.