Oocytes in sheep homozygous for a mutation in bone morphogenetic protein receptor 1B express lower mRNA levels of bone morphogenetic protein 15 but not growth differentiation factor 9

Abstract

The aim of this study was to test the hypothesis that the high ovulation rate in ewes (BB) homozygous for a mutation in the bone morphogenetic protein receptor type 1B (BMPR1B) gene is linked to lower BMP15 and/or GDF9 mRNA in oocytes compared with those in wild-type (CC) ewes. Cumulus cell-oocyte complexes (COC) and granulosa cells (GC) were recovered from R1 mm diameter follicles of BB and CC ewes during a prostaglandin-induced follicular phase. Expression levels of GDF9 and BMP15 were measured by multiplex qPCR from individual COC. The gonadotropin-induced cAMP responses of the GC from each non-atretic follicle were measured following treatment with FSH or human chorionic gonadotropin. In a separate validation experiment, GDF9 and BMP15 expression was present only in oocytes and not in cumulus cells. There was no effect of follicular diameter on oocyte-derived GDF9 or BMP15 mRNA levels. The mean expression levels of BMP15, but not GDF9, were significantly lower in all non-atretic follicles, including the subsets containing either FSH-or LH-responsive GC in BB, compared with CC, ewes. No genotype effects were noted for FSH-induced cAMP production by GC either with respect to dose of, or number of follicles responding to, FSH. However, ovaries from BB ewes contained significantly more follicles responsive to LH, with respect to cAMP production in GC. We propose that these findings are consistent with the hypothesis that the higher ovulation rate in BB sheep is due, at least in part, to lower oocyte-derived BMP15 mRNA levels together with the earlier onset of LH-responsiveness in GC.

Using sheep lines with mutations in single genes to better understand ovarian function

“…A possible mechanism for these interactions was identified when it was shown that BMP15 mRNA levels were decreased in animals carrying the Woodlands gene (Feary et al 2007) or homozygous for the Booroola gene (Crawford et al 2011) compared with their wild-type contemporaries. However, BMP15 mutations have also been observed in flocks carrying other putative major genes influencing ovulation rate wherein the effect is additive rather than synergistic.…”

“…The follicle therefore develops characteristics of an ovulatory follicle at a smaller size. This can be demonstrated through the increased proportions of smaller diameter follicles in Booroola and Inverdale animals with granulosa cells capable of responding to LH in vitro , Crawford et al 2011. Originally, granulosa cells from Booroola and Inverdale animals were thought to have an increased responsiveness to FSH, but it has been shown recently that this is not the case.…”

“…However, it is important to note that the effects of the Booroola mutation on follicle numbers are inconsistent (Smith et al 1994, Ruoss et al 2009). In addition, the acquisition of LH responsiveness in granulosa cells in a greater proportion of the pool of small antral follicles may allow more follicles to undergo preovulatory maturation , Crawford et al 2011.…”

Oocyte-Somatic Cell Interactions and Ovulation Rate: Effects on Oocyte Quality and Embryo Yield

“…54,57 The latter finding is due to significant interactive effects of follicular diameter and genotype where the GC populations were often lower in follicles 1.0 mm in I+ ewes than in ++ ewes. 53 Moreover, from detailed analyses of individual follicles, the higher ovulation rate in I+ ewes was due to a greater proportion of follicles acquiring LH-responsive GC at smaller diameters, compared to that in ++ animals. 57 Thus, the ovulation rate in I+ ewes occurs because follicles of different diameters respond to LH and ovulate with a total number of GC, and subsequent CL cells, that are not different from that in ++ animals.…”

“…52 It's also noteworthy that the ALK6-mutant sheep express lower levels of BMP15 mRNA. 53 Evidence that a partial reduction in BMP15 or GDF9 bioactivity is directly responsible for the significant increases in ovulation rate was demonstrated by immuno-neutralization experiments 12 : the increase was due to significantly more animals having two or more ovulations. In a large follow-up study involving more than 400 ewes, BMP15 immuno-neutralization significantly increased pregnancyrates without affecting embryo or fetal survival.…”