| Structural highlights
Disease
[AVR2B_HUMAN] Defects in ACVR2B are the cause of visceral heterotaxy autosomal type 4 (HTX4) [MIM:613751]. A form of visceral heterotaxy, a complex disorder due to disruption of the normal left-right asymmetry of the thoracoabdominal organs. It results in an abnormal arrangement of visceral organs, and a wide variety of congenital defects. Clinical features of visceral heterotaxy type 4 include dextrocardia, right aortic arch and a right-sided spleen, anomalies of the inferior and the superior vena cava, atrial ventricular canal defect with dextro-transposed great arteries, pulmonary stenosis, polysplenia and midline liver.[1]
Function
[AVR2B_HUMAN] Transmembrane serine/threonine kinase activin type-2 receptor forming an activin receptor complex with activin type-1 serine/threonine kinase receptors (ACVR1, ACVR1B or ACVR1c). Transduces the activin signal from the cell surface to the cytoplasm and is thus regulating many physiological and pathological processes including neuronal differentiation and neuronal survival, hair follicle development and cycling, FSH production by the pituitary gland, wound healing, extracellular matrix production, immunosuppression and carcinogenesis. Activin is also thought to have a paracrine or autocrine role in follicular development in the ovary. Within the receptor complex, the type-2 receptors act as a primary activin receptors (binds activin-A/INHBA, activin-B/INHBB as well as inhibin-A/INHA-INHBA). The type-1 receptors like ACVR1B act as downstream transducers of activin signals. Activin binds to type-2 receptor at the plasma membrane and activates its serine-threonine kinase. The activated receptor type-2 then phosphorylates and activates the type-1 receptor. Once activated, the type-1 receptor binds and phosphorylates the SMAD proteins SMAD2 and SMAD3, on serine residues of the C-terminal tail. Soon after their association with the activin receptor and subsequent phosphorylation, SMAD2 and SMAD3 are released into the cytoplasm where they interact with the common partner SMAD4. This SMAD complex translocates into the nucleus where it mediates activin-induced transcription. Inhibitory SMAD7, which is recruited to ACVR1B through FKBP1A, can prevent the association of SMAD2 and SMAD3 with the activin receptor complex, thereby blocking the activin signal. Activin signal transduction is also antagonized by the binding to the receptor of inhibin-B via the IGSF1 inhibin coreceptor.[2]
Publication Abstract from PubMed
The TGF-beta family ligands myostatin, GDF11, and activins are negative regulators of skeletal muscle mass, which have been reported to primarily signal via the ActRIIB receptor on skeletal muscle and thereby induce muscle wasting described as cachexia. Use of a soluble ActRIIB-Fc "trap," to block myostatin pathway signaling in normal or cachectic mice leads to hypertrophy or prevention of muscle loss, perhaps suggesting that the ActRIIB receptor is primarily responsible for muscle growth regulation. Genetic evidence demonstrates however that both ActRIIB- and ActRIIA-deficient mice display a hypertrophic phenotype. Here, we describe the mode of action of bimagrumab (BYM338), as a human dual-specific anti-ActRIIA/ActRIIB antibody, at the molecular and cellular levels. As shown by X-ray analysis, bimagrumab binds to both ActRIIA and ActRIIB ligand binding domains in a competitive manner at the critical myostatin/activin binding site, hence preventing signal transduction through either ActRII. Myostatin and the activins are capable of binding to both ActRIIA and ActRIIB, with different affinities. However, blockade of either single receptor through the use of specific anti-ActRIIA or anti-ActRIIB antibodies achieves only a partial signaling blockade upon myostatin or activin A stimulation, and this leads to only a small increase in muscle mass. Complete neutralization and maximal anabolic response are achieved only by simultaneous blockade of both receptors. These findings demonstrate the importance of ActRIIA in addition to ActRIIB in mediating myostatin and activin signaling and highlight the need for blocking both receptors to achieve a strong functional benefit.
Blockade of activin type II receptors with a dual anti-ActRIIA/IIB antibody is critical to promote maximal skeletal muscle hypertrophy.,Morvan F, Rondeau JM, Zou C, Minetti G, Scheufler C, Scharenberg M, Jacobi C, Brebbia P, Ritter V, Toussaint G, Koelbing C, Leber X, Schilb A, Witte F, Lehmann S, Koch E, Geisse S, Glass DJ, Lach-Trifilieff E Proc Natl Acad Sci U S A. 2017 Nov 6. pii: 201707925. doi:, 10.1073/pnas.1707925114. PMID:29109273[3]
From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine.
References
- ↑ Kosaki R, Gebbia M, Kosaki K, Lewin M, Bowers P, Towbin JA, Casey B. Left-right axis malformations associated with mutations in ACVR2B, the gene for human activin receptor type IIB. Am J Med Genet. 1999 Jan 1;82(1):70-6. PMID:9916847
- ↑ Attisano L, Wrana JL, Montalvo E, Massague J. Activation of signalling by the activin receptor complex. Mol Cell Biol. 1996 Mar;16(3):1066-73. PMID:8622651
- ↑ Morvan F, Rondeau JM, Zou C, Minetti G, Scheufler C, Scharenberg M, Jacobi C, Brebbia P, Ritter V, Toussaint G, Koelbing C, Leber X, Schilb A, Witte F, Lehmann S, Koch E, Geisse S, Glass DJ, Lach-Trifilieff E. Blockade of activin type II receptors with a dual anti-ActRIIA/IIB antibody is critical to promote maximal skeletal muscle hypertrophy. Proc Natl Acad Sci U S A. 2017 Nov 6. pii: 201707925. doi:, 10.1073/pnas.1707925114. PMID:29109273 doi:https://dx.doi.org/10.1073/pnas.1707925114
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