Heat exposure impairs porcine oocyte quality with suppressed actin expression in cumulus cells and disrupted F-actin formation in transzonal projections

Chao Yin,Bin He,Yang Yang,Ruqian Zhao,Jie Liu,Zhanglin Chang

doi:10.1186/s40104-020-00477-8

Abstract

BackgroundTranszonal projections (TZPs) constitute a structural basis for the communication between the oocyte and its surrounding cumulus cells (CCs), which play critical roles in promoting the oocyte maturation. Previously we found that heat stress (HS) causes loss of TZPs in porcine cumulus-oocyte complexes (COCs) with decreased density of filamentous actin (F-actin). However, the time-course responses of F-actin and its monomeric actins (β-actin and γ-actin) during the in vitro maturation of oocytes remain unclear.ResultsIn this study, excised porcine ovaries were exposed to HS at 41.5 °C for 1 h before COCs were isolated and matured in vitro for 44 h. HS significantly reduced oocyte quality, characterized by impaired cumulus expansion, delayed meiotic resumption and lower survival rate and polar body extrusion rate, as well as decreased expression of mitochondrial DNA-encoded genes and elevated mitochondrial reactive oxygen species concentration. Expression of β-actin and γ-actin in CCs increased gradually with oocytes maturation, which was significantly reduced in HS group, especially at 24 h and/or 44 h of in vitro maturation. By contrast, the number of TZPs and the fluorescence intensity of F-actin in zona pellucida decreased gradually during oocytes maturation, which were significantly reduced by HS at 24 h of in vitro maturation. Moreover, colocalization analyses revealed both β-actin and γ-actin contribute to the F-actin formation in porcine TZPs, and the colocalization of F-actin with GJ protein connexin 45 was significantly reduced in heat-exposed COCs.ConclusionsThe results indicate that the suppression of actin expressions in CCs, which may lead to the F-actin unstabilization in TZPs, will subsequently contribute to the compromised quality of oocytes under HS.

Highlights

Transzonal projections (TZPs) constitute a structural basis for the communication between the oocyte and its surrounding cumulus cells (CCs), which play critical roles in promoting the oocyte maturation
The meiotic progression of oocytes was delayed by heat stress (HS), with 55% of the oocytes lag behind at the stage from GV to GVBD at 24 h of in vitro maturation, and only 6% of the oocytes reached MII stage in HS group at 44 h compared to 77% in control group (Fig. 1b)
Results showed that the amount and the number of layers of CCs surrounding the control group oocytes increased gradually during the cumulus-oocyte complexes (COCs) in vitro maturation, whereas in HS group the COCs were not adequately expanded, characterized by remarkable decreases (P < 0.05) in cumulus expansions at 16 h, 24 h and 44 h of COCs in vitro maturation, as compared with control groups (Fig. 1c, d)

Summary

Introduction

Transzonal projections (TZPs) constitute a structural basis for the communication between the oocyte and its surrounding cumulus cells (CCs), which play critical roles in promoting the oocyte maturation. The time-course responses of F-actin and its monomeric actins (β-actin and γ-actin) during the in vitro maturation of oocytes remain unclear. Oocytes lose contacts with the external environment once they enter the antral follicle stages. Instead, their growing demands of materials are maintained by the surrounding cumulus cells (CCs), which form the cumulus-oocyte complexes (COCs) with oocytes. Transzonal projections (TZPs) are one class of intercellular structures linking CCs with oocytes. In pigs, the time-course responses of F-actin and its monomeric actins (β-actin and γ-actin) during the oocyte in vitro maturation remain unclear

Methods

Results

Discussion

Conclusion