Activin receptors are single-pass transmembrane serine/threonine kinases in the TGF-beta receptor superfamily. Signalling requires two: a constitutively active type II receptor (ActRIIA/ACVR2A or ActRIIB/ACVR2B) binds the ligand dimer first, then recruits and trans-phosphorylates a type I receptor (ALK4/ACVR1B, ALK7, or ALK5 for some ligands). The activated type I kinase phosphorylates SMAD2 and SMAD3, which complex with SMAD4 and move to the nucleus.
Why the receptor, not the ligand, is often the target
ActRIIB is the shared entry point for activin A and B, myostatin, GDF-11 and several BMPs. That promiscuity explains both the appeal and the difficulty of receptor-level intervention: soluble ActRIIB-Fc decoys such as ACE-031 sequester the whole ligand set at once. Rodent studies reported large muscle-mass increases, and a human trial programme in Duchenne muscular dystrophy was halted in 2013 after vascular findings including epistaxis and telangiectasia, attributed to blockade of BMP9/10 signalling as well as myostatin.
Inhibin subunits act as endogenous competitive antagonists at the same receptors, and follistatin intercepts ligands before they reach them.
Related terms and material
recombinant · preclinical · in vivo. Reference material: ACE-031, GDF-8 / myostatin propeptide, IGF & muscle peptides.