The molecular chaperone Cysteine String Protein is required to stabilize trans-SNARE complexes during human sperm acrosomal exocytosis

Abstract

Membrane fusion in sperm cells is crucial for acrosomal exocytosis and must be preserved to assure fertilizing capacity. Evolutionarily conserved protein machinery regulates acrosomal exocytosis. Molecular chaperones play a vital role in spermatogenesis and post-testicular maturation. Cysteine String Protein (CSP) is a member of the Hsp40 co-chaperones, and for more than 20 years, most research published focused on CSP´s role in synapsis. However, the participation of molecular chaperones in acrosomal exocytosis is poorly understood. Using western blot and indirect immunofluorescence, we showed that CSP is present in human sperm, is predominantly bound to membranes, and is palmitoylated. Moreover, using electron microscopy and functional assays, we reported that sequestration of CSP avoided the assembly of trans-complexes and inhibited exocytosis. In summary, our data demonstrated that CSP is necessary and mediates the trans-SNARE complex assembly between the outer acrosomal and plasma membranes, thereby regulating human sperm acrosomal exocytosis. Understanding CSP´s role is critical in identifying new biomarkers and generating new rational-based approaches to treating male infertility.