Biogenesis of Lysosomes
Lysosomal Sphingolipid Activator Proteins (SAPs)
Targeting Mechanisms of lysosomal Proteins
Studies on Reproduction
Sphingolipid activator proteins (SAPs) are five non-enzymatic cofactors required for the lysosomal degradation of glycosphingolipids. Four of them are the saposins A, B, C and D, which derive from the precursor protein, prosaposin. Prosaposin mutations are responsible for the variant forms of the lysosomal storage disorders, Gaucher’s disease and metachromatic leukodystrophy. The fifth activator, the GM2 Activator Protein (GM2AP), is the product of a distinct gene and is an essential cofactor for hexosaminidase A in the degradation of GM2 to GM3 ganglioside. Defects in GM2AP are the underlying causes for AB Variant of GM2 Gangliosidosis characterized by lysosomal accumulation of undegraded GM2 in neurons, fibroblasts and in several tissues of affected patients. Therefore, the study of SAPs is relevant to human health. In 1996 we discovered that prosaposin was sorted in the Golgi apparatus and transported to the lysosomes in a mannose-6-phosphate independent manner. Later we identified the internal amino acid sequence of prosaposin involved in this process and such a study suggested the existence of a lysosomal alternative receptor.
Recently, we have identified sortilin to be this novel lysosomal alternative receptor. We have demonstrated that sortilin is involved in the alternative sorting of the SAPs, prosaposin and GM2AP. We and other investigators showed that sortilin has a cytoplasmic GGA (Golgi-localized, γ-ear-containing, ARF-binding protein) binding motif similar to the M6P-R. GGA acts as adaptor proteins bridging the receptor and clathrin, a required step for the targeting of sorted proteins to lysosomes. With the identification of this novel receptor our laboratory has taken the lead in the study of lysosomal targeting of soluble proteins and our goal is to unfold the important clinical implications of this discovery. Although this alternative receptor was initially implicated in the sorting and trafficking of sphingolipid activator proteins, we have recently found that sortilin is also involved in the targeting of several lysosomal hydrolases including cathepsin H, cathepsin D and acid sphingomyelinase. We have cloned the sortilin gene and analyzed its structure and inactivated this gene by homologous recombination.
During the past 5 years, in collaboration with Dr. Scott Argraves (Medical University of South Carolina), we have pioneered functional studies on the role of the LDL receptor-like related protein-2 (LRP-2/megalin) in the male reproductive system. We demonstrated for the first time that apoliprotein J is the major ligand of megalin in the epididymis and that both proteins may play a role in sperm maturation and sperm capacitation. In addition, we have identified in the sperm plasma membrane several proteins involved in cholesterol efflux, a process that is essential for fertilization.
Carlos R. Morales, Professor
Jibin Zeng, Research Assistant
Maryssa Canuel, Postdoctoral Fellow
Yuan Libin, Ph.D. Student
Xiaoyan Ni, Research Assistant
Medical Histology (Units 2 and 3)
Introduction to Dynamic Histology (ANAT-261)
Research Course in Cell Biology (ANAT-432)