The hyperprolificacy phenotype of Booroola ewes is due to the presence of the FecB(B) allele at the FecB locus, recently identified as a single amino acid substitution (Q249R) in the bone morphogenetic protein (BMP) type-IB receptor (BMPR1B), and is associated with a more precocious differentiation of ovarian granulosa cells (GCs). To evaluate the consequences of the Booroola mutation on BMPR1B functions, the action of ligands of the transforming growth factor-beta (TGFbeta)/BMP family that act through (growth and differentiation factor-5, BMP-4) or independently of (activin A, TGFbeta-1) BMPR1B were studied on primary cultures of GCs from homozygous FecB(+) and FecB(B) ewes. All the tested TGFbeta/BMP family ligands inhibited progesterone secretion by FecB(+) GCs. Those inhibitory effects were lower for GCs from preovulatory (5-7 mm diameter) than from small antral follicles (1-3 mm diameter). The presence of the Booroola mutation was associated with a 3- to 4-fold (P<0.001) decreased responsiveness of GCs from FecB(B) compared with FecB(+) small follicles to the action of BMPR1B ligands. In contrast, TGFbeta-1 and activin A had similar inhibitory effects on progesterone secretion by GCs from FecB(+) and FecB(B) small follicles. No difference between genotypes was observed with GCs from preovulatory follicles. In transfection experiments with HEK-293 cells, co-expression of FecB(+) BMPR1B and BMPR2 resulted in a 2.6-fold (P<0.01) induction of the activity of a BMP-specific luciferase reporter construct by BMP-4. Interestingly, no response to BMP-4 was observed when cells were transfected with the FecB(B) form of the BMPR1B receptor. Overall, these data strongly suggest that the Q249R mutation is associated with a specific alteration of BMPR1B signaling in hyperprolific Booroola ewes.

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