The trafficking of prosaposin (SGP‐1) and GM₂AP to the lysosomes of TM4 sertoli cells is mediated by sortilin and monomeric adaptor proteins

Abstract

Prosaposin (SGP-1) and GM2 activator protein (GM2AP) are soluble sphingolipid activator proteins (SAPs) that are targeted to the lysosomal compartment of Sertoli cells to aid hydrolases in the breakdown of glycosphingolipids. To reach the lysosome, most soluble proteins must interact with the mannose 6-phosphate receptor (MPR). To be sorted from the Golgi, the MPR must bind to the Golgi associated, gamma-adaptin homologous, ARF binding proteins (GGAs), a group of monomeric adaptor proteins responsible for the recruitment of clathrin. It is well established, however, that the lysosomes of I-cell disease (ICD) patients have near normal levels of several lysosomal proteins, including prosaposin and GM2AP. ICD results from a mutation in the phosphotransferase that adds mannose 6-phosphate to hydrolases. Thus, prosaposin and GM2AP can traffic to lysosomes in a MPR independent manner. Previous work has demonstrated that an interaction with sphingomyelin in the Golgi membrane is necessary for the targeting of prosaposin by an unknown receptor. Using a TM4 Sertoli cell line, we tested the hypothesis that prosaposin and GM2AP are targeted to the lysosomal compartment via the sortilin receptor, which has been recently shown to have a GGA binding motif. Interestingly, dominant-negative GGAs, unable to bind clathrin to shuttle from the Golgi, prevented the trafficking of prosaposin and GM2AP to lysosomes. A dominant negative construct of sortilin lacking the GGA binding domain retained prosaposin and GM2AP in the Golgi. In conclusion, our results showed that the trafficking of prosaposin and GM2AP to the lysosome is dependent on sortilin.

Plant Retromer, Localized to the Prevacuolar Compartment and Microvesicles inArabidopsis, May Interact with Vacuolar Sorting Receptors

“…This is true for the mannosyl 6-phosphate and sortilin receptors (Bonifacino and Traub, 2003;Hassan et al, 2004) in mammals, the Vps10p receptor in yeast (Dennes et al, 2002), and the BP-80-type vacuolar sorting receptors (VSRs) of plants (Happel et al, 2004). Another feature shared, at least, by the mammalian and yeast receptors is that they are recycled from the PVC to the TGN with the help of a cytosolic protein complex called the retromer (Seaman et al, 1998;Arighi et al, 2004;Seaman, 2004Seaman, , 2005.…”

WNK4 Enhances the Degradation of NCC through a Sortilin-Mediated Lysosomal Pathway

“…28,29 It is a homolog of the yeast vacuolar sorting receptor Vps10p and belongs to the type I Vps10p superfamily. 17 The human sortilin gene encodes 833 amino acids.…”

Matrix-coated transwell-cultured TM4 sertoli cell testosterone-regulated gene expression mimics in vivo expression

“…However, mechanisms downstream of testosterone stimulation by binding AR in Sertoli cells remain poorly defined (Sharpe 2005). While cultures of TM4 cells on plastic or glass slides provide valuable information (Nakhla et al 1989;Akerstrom and Walters 1992;Tokuda et al 1992;Catalano et al 2003;Hassan et al 2004;Kasumi et al 2006), these cultures maintain a non-Sertoli-like morphology without basal compartment uptake and polarization (Janecki and Steinberger 1986). Some data from TM4 cells grown on a plastic culture surface are relevant to Sertoli cells in vivo such as sorting and targeting of sphingolipid activator proteins to the lysosomal compartment that are mediated by sortilin (Hassan et al 2004).…”