Absence of seasonal changes in FSHR gene expression in the cat cumulus–oocyte complex in vivo and in vitro

Abstract

Domestic cat oocytes are seasonally sensitive to FSH. Compared with those collected during the breeding season, oocytes from the nonbreeding (NB) season require more FSH during in vitro maturation to achieve comparable developmental competence. This study tested the hypothesis that this seasonal variation was due to altered expression of FSH receptors (FSHR) and/or FSH-induced genes. Relative expression levels of FSHR mRNA and FSH-enhanced gene estrogen receptor β (ESR2) were measured by qPCR in whole ovaries and immature cumulus-oocyte complexes (COCs) isolated from cat ovaries during the natural breeding vs NB seasons. Expression levels of FSH-induced genes prostaglandin-endoperoxide synthase 2 (PTGS2), early growth response protein-1 (EGR1), and epidermal growth factor receptor (EGFR) were examined in mature COCs from both seasons that were a) recovered in vivo or b) matured in vitro with conventional (1 μg/ml) or high (10 μg/ml) FSH concentrations. Overall, FSHR mRNA levels were lower in whole ovaries during the NB compared with breeding season but were similar in immature COCs, whereas ESR2 levels did not differ in either group between intervals. We observed changes in PTGS2, EGR1, and EGFR mRNA expression patterns across maturation in COCs within but not between the two seasons. The lack of seasonal differentiation in FSH-related genes was not consistent with the decreased developmental capacity of oocytes fertilized during the NB season. These findings reveal that the seasonal decrease in cat oocyte sensitivity to FSH occurs both in vivo and in vitro. Furthermore, this decline is unrelated to changes in expression of FSHR mRNA or mRNA of FSH-induced genes in COCs from antral follicles.