Endoplasmic reticulum (ER) stress in cumulus-oocyte complexes impairs pentraxin-3 secretion, mitochondrial membrane potential (Δᴪm), and embryo development

Abstract

Fatty acids such as palmitic acid at high levels are known to induce endoplasmic reticulum (ER) stress and lipotoxicity in numerous cell types and thereby contribute to cellular dysfunctions in obesity. To understand the impact of high fatty acids on oocytes, ER stress and lipotoxicity were induced in mouse cumulus-oocyte complexes during in vitro maturation using the ER Ca2_ channel blocker thapsigargin or high physiological levels of palmitic acid; both of which significantly induced ER stress marker genes (Atf4, Atf6, Xbp1s, and Hspa5) and inositol-requiring protein-1_phosphorylation, demonstrating an ER stress response that was reversible with the ER stress inhibitor salubrinal. Assessment of pentraxin-3, an extracellular matrix protein essential for fertilization,by immunocytochemistry and Western blotting showed dramatically impaired secretion concurrent with ER stress. Mitochondrial activity in oocytes was assessed by 5,5',6,6'-tetrachloro- 1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide staining of inner mitochondrial membrane potential, and oocytes matured in thapsigargin or high-dose palmitic acid had significantly reduced mitochondrial activity, reduced in vitro fertilization rates, and were slower to develop to blastocysts. The deficiencies in protein secretion, mitochondrial activity, and oocyte developmental competence were each normalized by salubrinal, demonstrating that ER stress is a key mechanismmediating fatty acid-induced defects in oocyte developmental potential.