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Activin Receptors, Type II: One of the two types of ACTIVIN RECEPTORS. They are membrane protein kinases belonging to the family of PROTEIN-SERINE-THREONINE KINASES. The major type II activin receptors are ActR-IIA and ActR-IIB.Activin Receptors: Receptors for ACTIVINS are membrane protein kinases belonging to the family of PROTEIN-SERINE-THREONINE KINASES, thus also named activin receptor-like kinases (ALK's). Activin receptors also bind TRANSFORMING GROWTH FACTOR BETA. As those transmembrane receptors of the TGF-beta superfamily (RECEPTORS, TRANSFORMING GROWTH FACTOR BETA), ALK's consist of two different but related protein kinases, Type I and Type II. Activins initiate cellular signal transduction by first binding to the type II receptors (ACTIVIN RECEPTORS, TYPE II ) which then recruit and phosphorylate the type I receptors (ACTIVIN RECEPTORS, TYPE I ) with subsequent activation of the type I kinase activity.Activins: Activins are produced in the pituitary, gonads, and other tissues. By acting locally, they stimulate pituitary FSH secretion and have diverse effects on cell differentiation and embryonic development. Activins are glycoproteins that are hetero- or homodimers of INHIBIN-BETA SUBUNITS.Activin Receptors, Type I: One of the two types of ACTIVIN RECEPTORS or activin receptor-like kinases (ALK'S). There are several type I activin receptors. The major active ones are ALK-2 (ActR-IA) and ALK-4 (ActR-IB).Myostatin: A growth differentiation factor that is a potent inhibitor of SKELETAL MUSCLE growth. It may play a role in the regulation of MYOGENESIS and in muscle maintenance during adulthood.Receptors, Transforming Growth Factor beta: Cell-surface proteins that bind transforming growth factor beta and trigger changes influencing the behavior of cells. Two types of transforming growth factor receptors have been recognized. They differ in affinity for different members of the transforming growth factor beta family and in cellular mechanisms of action.Receptors, Interleukin-1 Type II: An interleukin-1 receptor subtype that competes with the INTERLEUKIN-1 RECEPTOR TYPE I for binding to INTERLEUKIN-1ALPHA and INTERLEUKIN-1BETA. The interleukin-1 type II receptor appears to lack signal transduction capability. Therefore it may act as a "decoy" receptor that modulates the activity of its ligands. Both membrane-bound and soluble forms of the receptor have been identified.Inhibins: Glycoproteins that inhibit pituitary FOLLICLE STIMULATING HORMONE secretion. Inhibins are secreted by the Sertoli cells of the testes, the granulosa cells of the ovarian follicles, the placenta, and other tissues. Inhibins and ACTIVINS are modulators of FOLLICLE STIMULATING HORMONE secretions; both groups belong to the TGF-beta superfamily, as the TRANSFORMING GROWTH FACTOR BETA. Inhibins consist of a disulfide-linked heterodimer with a unique alpha linked to either a beta A or a beta B subunit to form inhibin A or inhibin B, respectivelyInhibin-beta Subunits: They are glycopeptides and subunits in INHIBINS and ACTIVINS. Inhibins and activins belong to the transforming growth factor beta superfamily.Follistatin: A broadly distributed protein that binds directly to ACTIVINS. It functions as an activin antagonist, inhibits FOLLICLE STIMULATING HORMONE secretion, regulates CELL DIFFERENTIATION, and plays an important role in embryogenesis. Follistatin is a single glycosylated polypeptide chain of approximately 37-kDa and is not a member of the inhibin family (INHIBINS). Follistatin also binds and neutralizes many members of the TRANSFORMING GROWTH FACTOR BETA family.Bone Morphogenetic Protein Receptors, Type II: A subtype of bone morphogenetic protein receptors with low affinity for BONE MORPHOGENETIC PROTEINS. They are constitutively active PROTEIN-SERINE-THREONINE KINASES that can interact with and phosphorylate TYPE I BONE MORPHOGENETIC PROTEIN RECEPTORS.Receptors, Growth Factor: Cell surface receptors that bind growth or trophic factors with high affinity, triggering intracellular responses which influence the growth, differentiation, or survival of cells.Transforming Growth Factor beta: A factor synthesized in a wide variety of tissues. It acts synergistically with TGF-alpha in inducing phenotypic transformation and can also act as a negative autocrine growth factor. TGF-beta has a potential role in embryonal development, cellular differentiation, hormone secretion, and immune function. TGF-beta is found mostly as homodimer forms of separate gene products TGF-beta1, TGF-beta2 or TGF-beta3. Heterodimers composed of TGF-beta1 and 2 (TGF-beta1.2) or of TGF-beta2 and 3 (TGF-beta2.3) have been isolated. The TGF-beta proteins are synthesized as precursor proteins.Protein-Serine-Threonine Kinases: A group of enzymes that catalyzes the phosphorylation of serine or threonine residues in proteins, with ATP or other nucleotides as phosphate donors.Smad2 Protein: A receptor-regulated smad protein that undergoes PHOSPHORYLATION by ACTIVIN RECEPTORS, TYPE I. It regulates TRANSFORMING GROWTH FACTOR BETA and ACTIVIN signaling.Signal Transduction: The intracellular transfer of information (biological activation/inhibition) through a signal pathway. In each signal transduction system, an activation/inhibition signal from a biologically active molecule (hormone, neurotransmitter) is mediated via the coupling of a receptor/enzyme to a second messenger system or to an ion channel. Signal transduction plays an important role in activating cellular functions, cell differentiation, and cell proliferation. Examples of signal transduction systems are the GAMMA-AMINOBUTYRIC ACID-postsynaptic receptor-calcium ion channel system, the receptor-mediated T-cell activation pathway, and the receptor-mediated activation of phospholipases. Those coupled to membrane depolarization or intracellular release of calcium include the receptor-mediated activation of cytotoxic functions in granulocytes and the synaptic potentiation of protein kinase activation. Some signal transduction pathways may be part of larger signal transduction pathways; for example, protein kinase activation is part of the platelet activation signal pathway.Smad Proteins: A family of proteins that are involved in the translocation of signals from TGF-BETA RECEPTORS; BONE MORPHOGENETIC PROTEIN RECEPTORS; and other surface receptors to the CELL NUCLEUS. They were originally identified as a class of proteins that are related to the mothers against decapentaplegic protein, Drosophila and sma proteins from CAENORHABDITIS ELEGANS.Bone Morphogenetic Proteins: Bone-growth regulatory factors that are members of the transforming growth factor-beta superfamily of proteins. They are synthesized as large precursor molecules which are cleaved by proteolytic enzymes. The active form can consist of a dimer of two identical proteins or a heterodimer of two related bone morphogenetic proteins.Receptors, Interleukin-1: Cell surface receptors that are specific for INTERLEUKIN-1. Included under this heading are signaling receptors, non-signaling receptors and accessory proteins required for receptor signaling. Signaling from interleukin-1 receptors occurs via interaction with SIGNAL TRANSDUCING ADAPTOR PROTEINS such as MYELOID DIFFERENTIATION FACTOR 88.Receptors, Tumor Necrosis Factor, Type II: A tumor necrosis factor receptor subtype that is expressed primarily in IMMUNE SYSTEM cells. It has specificity for membrane-bound form of TUMOR NECROSIS FACTORS and mediates intracellular-signaling through TNF RECEPTOR ASSOCIATED FACTORS.RNA, Messenger: RNA sequences that serve as templates for protein synthesis. Bacterial mRNAs are generally primary transcripts in that they do not require post-transcriptional processing. Eukaryotic mRNA is synthesized in the nucleus and must be exported to the cytoplasm for translation. Most eukaryotic mRNAs have a sequence of polyadenylic acid at the 3' end, referred to as the poly(A) tail. The function of this tail is not known for certain, but it may play a role in the export of mature mRNA from the nucleus as well as in helping stabilize some mRNA molecules by retarding their degradation in the cytoplasm.