Glucose Regulated Protein 78 Phosphorylation in Sperm Undergoes Dynamic Changes during Maturation

PLoS One. 2015 Nov 30;10(11):e0141858. doi: 10.1371/journal.pone.0141858. eCollection 2015.

Abstract

GRP78, a resident endoplasmic reticulum (ER) chaperone involved in protein transport, folding and assembly, has been reported in sperm. It is shown to be localized in the neck region of human sperm. We have previously reported GRP78 to be less phosphorylated in asthenozoosperm.The present study aimed to determine whether sperm GRP78 undergoes phosphorylation changes during epididymal maturation and whether there are any differences in GRP78 phosphoforms in asthenozoosperm vis-à-vis normozoosperm. Testicular- and cauda epididymal- sperm from adult male Holtzman rats, and semen ejaculates collected from normal and asthenozoospermic individuals were investigated. DIGE carried out to determine phosphorylation of GRP78 in asthenozoosperm and normal sperm reveals a shift in the location of GRP78 of asthenozoosperm towards the alkaline pH, indicative of reduced GRP78 phosphorylation. Immunoprecipitation studies using antibodies specific to GRP78, serine-, threonine-, and tyrosine phosphorylation and Pan phospho antibody demonstrates GRP78 to be phosphorylated at all three residues in rat spermatozoa. Phosphatase assays using Calf intestinal alkaline phosphatase and Lambda protein phosphatase followed by nanofluidic proteomic immunoassay (NIA) show that in rat, GP4.96, GP4.94 and GP4.85 are the three phosphoforms in mature (caudal) sperm as against two phosphoforms GP4.96and GP4.94in immature (testicular) sperm. In mature human sperm GP5.04, GP4.96, and GP4.94were the 3 phosphoforms observed. GP4.94[P = 0.014]andGP5.04 [P = 0.02] are significantly reduced in asthenozoosperm. Ours is the first report indicating GRP78 in sperm to be phosphorylated at serine, threonine and tyrosine residues contrary to published literature reporting GRP78 not to be tyrosine phosphorylated. We report the presence of GRP78 phosphoforms in rat- and human- sperm and our data suggest that GRP78 phosphorylation in sperm undergoes spatial reorganization during epididymal maturation. Significant differences observed in 2 out of 3 phosphoforms in asthenozoosperm suggest that GRP78 phosphorylation may have functional relevance in sperm with consequent clinical implications.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Asthenozoospermia / metabolism
  • Case-Control Studies
  • Endoplasmic Reticulum Chaperone BiP
  • Epididymis / growth & development
  • Epididymis / metabolism
  • Heat-Shock Proteins / metabolism*
  • Humans
  • Male
  • Phosphorylation
  • Protein Processing, Post-Translational*
  • Rats, Sprague-Dawley
  • Spermatogenesis
  • Spermatozoa / metabolism
  • Testis / metabolism
  • Testis / pathology

Substances

  • Endoplasmic Reticulum Chaperone BiP
  • HSPA5 protein, human
  • Heat-Shock Proteins

Grants and funding

This work (RA/181/09-2014) was carried out with the financial support from the Indian Council of Medical Research and Department of Biotechnology (DBT), India and their support is gratefully acknowledged. The authors also thank the University Grants Commission (UGC) for the ‘Maulana Azad Fellowship’ provided to V. L. and DBT, India for Junior Research Fellowship (JRF) and Senior Research Fellowship (SRF) to P. R.