Heat Shock Proteins (HSPs) are highly conserved molecular chaperones, which increase during cytoprotective responses against cellular stress. Their main functions include protein folding and maintenance of cell viability; and some HSPs have been implicated in regulation of normal ovarian function and embryonic viability.
To understand how ovarian cells may respond to various stressors, we examined the expression and localisation of several key HSPs (namely, HSPA1A/B, HSPA5, HSP60, HSP90AA1, HSP90AB1, and HSP90B1) as well as transcription factor HSF1 during oocyte maturation and ovulation in normal mouse ovarian follicles. Subsequent experiments will determine whether ovarian HSP expression is altered during metabolic stress, namely obesity, and its impact on ovarian function.
Ovaries were obtained from CBAF1 mice treated with PMSG to stimulate folliculogenesis, followed by ovulatory hCG for 0h, 8h, 11.5h or 13h. Paraffin sections were immuno-stained using antibodies against specific HSPs and peroxidase detection methods. Separately, cumulus-oocyte complexes and denuded oocytes were stained by immunofluorescence and visualised using confocal microscopy.
Every HSP was detected in granulosa cells as well as in the ovarian theca and showed dynamic regulation of expression in response to the LH surge. For instance, HSP90AB1 localised to granulosa and cumulus cells, with peak expression at the time of ovulation. Interestingly, the remaining studied HSPs showed dramatic changes in localization within the oocyte. Prior to the LH surge HSF1, HSPA5 and HSP60 localized within the Germinal Vesicle. Following the LH surge, HSPA1A/B and HSP90AA1 localized to the spindle; while HSP60 and HSP90B1 surrounded the meiotic spindle complex. In ovulated oocytes, only HSPA1A/B and HSP90B1 remained highly expressed respectively in the cytoplasm and the plasma membrane.
We show that the LH surge regulates HSF1 and HSPs expression in granulosa cells and triggers intracellular re-localization of these proteins in oocytes; likely reflective of their importance in cytoprotection and oocyte competence.