Prosaptide activates the MAPK pathway by a G-protein-dependent mechanism essential for enhanced sulfatide synthesis by Schwann cells

Prosaposin, the precursor of saposins A, B, C, and D, was recently reported to be a neurotrophic factor in vivo and in vitro. The neurotrophic region of prosaposin has been localized to a 12-amino acid sequence within the saposin C domain and has been used to derive biologically active synthetic peptides (14-22 residues), called prosaptides. Treatment of primary Schwann cells and an immortalized Schwann cell line, iSC, with a 14-mer prosaptide, TX14(A) (10 nM), enhanced phosphorylation of mitogen-activated kinases ERK1 (p44 MAPK) and ERK2 (p42 MAPK) within 5 min, which was blocked by 4 h pretreatment with pertussis toxin. Furthermore, incubation of Schwann cells with the nonhydrolyzable GDP analog GDP-betaS inhibited TX14(A)-induced ERK phosphorylation. TX14(A) enhanced the sulfatide content of primary Schwann cells by 2.5-fold, which was inhibited by pretreatment with pertussis toxin or the synthetic MAP kinase kinase inhibitor PD098059. In addition, TX14(A) increased the tyrosine phosphorylation of all three isoforms of the adapter molecule, Shc, which coincided with the association of p60Src and PI(3)K. Inhibition of PI3(K) by wortmannin blocked TX14(A)-induced ERK phosphorylation. These data demonstrate that TX14(A) uses a pertussis toxin-sensitive G-protein pathway to activate ERKs, which is essential for enhanced sulfatide synthesis in Schwann cells.