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Betaglycan (BG) is a transmembrane co-receptor of the transforming growth factor-β (TGF-β) family of signaling ligands. It is essential for embryonic development, tissue homeostasis and fertility in adults. It functions by enabling binding of the three TGF-β isoforms to their signaling receptors and is additionally required for inhibin A (InhA) activity. Despite its requirement for the functions of TGF-βs and InhA in vivo, structural information explaining BG ligand selectivity and its mechanism of action is lacking. Here, we determine the structure of TGF-β bound both to BG and the signaling receptors, TGFBR1 and TGFBR2. We identify key regions responsible for ligand engagement, which has revealed binding interfaces that differ from those described for the closely related co-receptor of the TGF-β family, endoglin, thus demonstrating remarkable evolutionary adaptation to enable ligand selectivity. Finally, we provide a structural explanation for the hand-off mechanism underlying TGF-β signal potentiation. Betaglycan is a co-receptor for selective TGF-β family ligands. Here, the authors solve its structure in complex with TGF-β and the signaling receptors, which explains its ligand selectivity and reveals its mechanism in potentiating TGF-β signaling.

Gonadotropin-releasing hormone (GnRH) is the primary neuropeptide controlling reproduction in vertebrates. GnRH stimulates follicle-stimulating hormone (FSH) and luteinizing hormone (LH) synthesis via a G-protein-coupled receptor, GnRHR, in the pituitary gland. In mammals, GnRHR lacks a C-terminal cytosolic tail (Ctail) and does not exhibit homologous desensitization. This might be an evolutionary adaptation that enables LH surge generation and ovulation. To test this idea, we fused the chicken GnRHR Ctail to the endogenous murine GnRHR in a transgenic model. The LH surge was blunted, but not blocked in these mice. In contrast, they showed reductions in FSH production, ovarian follicle development, and fertility. Addition of the Ctail altered the nature of agonist-induced calcium signaling required for normal FSH production. The loss of the GnRHR Ctail during mammalian evolution is unlikely to have conferred a selective advantage by enabling the LH surge. The adaptive significance of this specialization remains to be determined.

Disclosure: Y. Lin: None. E. Brule: None. M. Cowan: None. U. Boehm: None. D.J. Bernard: None. Loss of inhibin negative feedback in the pituitary leads to enhanced ovulation but pregnancy failure in mice Follicle-stimulating hormone (FSH) is an essential regulator of gonadal function, particularly in females. Inhibins are TGFβ family ligands made in the gonads that suppress FSH synthesis. Inhibins require a co-receptor, betaglycan or TGFBR3L, to mediate their actions in pituitary gonadotrope cells. Female mice with a gonadotrope-specific knockout of betaglycan or global deletion of Tgfbr3l exhibit increased ovarian follicle development, numbers of ovulated eggs, and litter sizes compared to controls. Females with both co-receptors knocked out (hereafter dKO) show dramatic increases in FSH levels, numbers of antral follicles and corpora lutea, serum progesterone levels, and ovarian aromatase expression. They ovulate 3-4 times as many eggs in natural cycles compared to control females. dKO females can become pregnant and show an increased number of implanted embryos at 7.5 days post coitum (7.5 dpc). Nevertheless, dKO females do not give birth to live offspring. By 10.5 dpc, the weight of the fetoplacental unit is decreased in dKO females, and many embryos display morphological abnormalities. By 14.5 dpc, most embryos in dKO females are either dead or resorbing. Wild-type surrogates give birth to live young following transplantation of embryos from control or dKO females. Conversely, control but not dKO females carry wild-type embryos to term. These data suggest that the maternal environment in dKO mice cannot support full-term pregnancies. Based on the hormonal profile of the cycling females, we suspected a deleterious effect of elevated estrogens. Consistent with this idea, treatment with the aromatase inhibitor anastrozole increased fetal survival in some dKO mice, suggesting a role for estrogens in embryo death. We are currently more thoroughly characterizing the maternal hormonal environment throughout gestation and investigating placental structure/function to gain more insight into the precise nature of the pregnancy failure. Our results will show how loss of inhibin action in the pituitary impede embryo survival and whether pregnancy loss is linked to increases in FSH. Presentation: Friday, June 16, 2023

Betaglycan (BG) is a transmembrane co-receptor of the transforming growth factor-β (TGF-β) family of signaling ligands. It is essential for embryonic development and tissue homeostasis and fertility in adults. It functions by enabling binding of the three TGF-β isoforms to their signaling receptors and is additionally required for inhibin A (InhA) activity. Despite its requirement for the functions of TGF-βs and InhA in vivo, structural information explaining BG ligand selectivity and its mechanism of action is lacking. Here, we determine the structure of TGF-β bound both to BG and the signaling receptors, TGFBR1 and TGFBR2. We identify key regions responsible for ligand engagement, which has revealed novel binding interfaces that differ from those described for the closely related co-receptor of the TGF-β family, endoglin, thus demonstrating remarkable evolutionary adaptation to enable ligand selectivity. Finally, we provide a structural explanation for the hand-off mechanism underlying TGF-β signal potentiation.

Myostatin is a paracrine myokine that regulates muscle mass in a variety of species, including humans. Here, we report a functional role for myostatin as an endocrine hormone directly promoting pituitary follicle-stimulating hormone (FSH) synthesis and thereby ovarian function. Previously, this FSH-stimulating role was attributed to other members of the transforming growth factor β family, the activins. The results both challenge activin’s eponymous role in FSH synthesis and establish an endocrine axis between skeletal muscle and the pituitary gland. The data also suggest that efforts to antagonize myostatin to treat muscle wasting disorders may have unintended consequences on fertility. Hormone synthesis and reproduction depend on crosstalk between skeletal muscle and the pituitary gland.

Gonadotropin-releasing hormone (GnRH) is the primary neuropeptide controlling reproduction in vertebrates. GnRH stimulates follicle-stimulating hormone (FSH) and luteinizing hormone (LH) synthesis via a G protein-coupled receptor, GnRHR, in the pituitary gland. In mammals, GnRHR lacks a C-terminal cytosolic tail (Ctail) and does not exhibit homologous desensitization. This might be an evolutionary adaptation that enables LH surge generation and ovulation. To test this idea, we fused the chicken GnRHR Ctail to the endogenous murine GnRHR in a transgenic model. The LH surge was blunted, but not blocked in these mice. In contrast, they showed reductions in FSH production, ovarian follicle development, and fertility. Addition of the Ctail altered the nature of agonist-induced calcium signaling required for normal FSH production. The loss of the GnRHR Ctail during mammalian evolution is unlikely to have conferred a selective advantage by enabling the LH surge. The adaptive significance of this specialization remains to be determined. Competing Interest Statement The authors have declared no competing interest.