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Background 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. 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. Results In 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. Conclusions The 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.

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The rapid and efficient clearance of apoptotic germ cells (GCs) by Sertoli cells (SCs) is important for spermatogenesis. High mitochondrial activity in phagocytes is critical for continued clearance of apoptotic cells. However, the underlying molecular mechanism is poorly understood. Glycogen synthase kinase-3α (GSK3α) is a protein kinase that participates in the regulation of mitochondrial activity. Immunohistochemistry evidenced the predominant presence of the Ser21 phosphorylation GSK3α (inactivation) signal in SCs. Heat shock-induced apoptosis of GCs and dephosphorylation of GSK3α in SCs is a perfect model to investigate the role of GSK3α in phagocytic action. The number of apoptotic GCs was significantly lower in GSK3α inhibitor pre-treated mice with HS compared to normal control. In vitro phagocytosis assays shown that the phagocytic activity in GSK3α activated SCs was downregulated, while GSK3α inhibitor supplementation restored this process. Moreover, GSK3α activation participates in the alteration of the mitochondrial ultrastructure and activity. In particular, GSK3α activation inhibits mitochondrial fission via phosphorylation of dynamin related protein 1 at Ser637. Changes of mitochondrial activity resulted in the accumulation of lipid droplets and the alteration of metabolism pattern in SCs. In summary, our results demonstrate that inactivation of GSK3α is required for mitochondria-mediated apoptotic GCs phagocytosis in SCs.