Characterization of the constitutive pig ovary heat shock chaperone machinery and its response to acute thermal stress or to seasonal variations

Reduced oocyte competence causes the lower fertility reported in domestic sows during the warm months of the year. Somatic cells express heat shock proteins (HSPs) to protect themselves from damage caused by thermal stress. HSPs are classified as molecular chaperones and control the correct folding of newly synthetized or damaged proteins. Aim of the present work was to perform a comprehensive survey of the different components of the heat shock chaperone machinery in the pig ovary, which included HSP40, HSP70, HSP90 and HSP110 families as well as heat shock factors (HSF) 1 and 2. Pig ovarian follicles constitutively expressed different members of these families therefore we examined their ability to respond to a heat stress. In order to take into account the role of the complex follicular architecture, whole pig ovaries were exposed to 41.5°C for 1 h. This significantly disrupted oocyte maturation and determined the up regulation of HSP70, HSP40, HSPH1, HSPA4, HSPA4L, HSF1, and HFS2 genes whereas HSP90A, HSP90B as well as stress unrelated genes were not altered. Unexpectedly HSPs and HSFs expression changed only in the oocytes but not in the cumulus cells. COCs isolated from ovaries collected in summer and in winter showed the same pattern. We conclude that the HSP chaperone machinery is constitutively fully operational in the pig ovary. However following a thermal stimulus or seasonal variations, cumulus cells HS-related gene expression remains unchanged and only oocytes activate a response, suggesting why this is insufficient to preserve their competence both in vitro and in vivo.