SMAD6 inhibits granulosa cell proliferation and follicle growth rate in carrier and noncarrier heifers of the Trio allele

Cattle are generally considered a monovular species; however, recently a bovine high fecundity allele, termed the Trio allele, was discovered. Carriers of Trio have an elevated ovulation rate (3 to 5) while half-sibling noncarriers are monovular. Carriers of the Trio allele have overexpression in granulosa cells of SMAD6, an inhibitor of oocyte-derived regulators of granulosa cell proliferation and differentiation. In experiment 1, follicle size was tracked for each follicle during a follicular wave. Follicle growth rate was greater before vs after follicle deviation in both carriers and noncarriers. Additionally, follicle growth rate was consistently less in carriers vs noncarriers. In experiment 2, we collected granulosa cells from follicles before and after deviation for evaluation of granulosa cell gene expression. Granulosa cell proliferation was less in carriers vs noncarriers and after vs before follicle deviation (decreased expression of cell cycle genes CCNB1 and CCNA2). The decreased granulosa cell proliferation in noncarriers after deviation was associated with increased SMAD6 expression. Similarly, in experiment 3 decreased expression of SMAD6 in granulosa cells of noncarriers cultured in vitro for 60 hours was associated with increased expression of cell cycle genes. This suggests that SMAD6 may not just be inhibiting follicle growth rate in carriers of Trio but may also play a role in the decreased follicle growth after deviation in noncarriers. The hypotheses were supported that (1) follicle growth and granulosa cell proliferation decreases after deviation in both carriers and noncarriers and that (2) granulosa cell proliferation is reduced in carriers compared to noncarriers.