Figure 1.

Activin A signaling pathway. A simplified illustration of the canonical and non-canonical signaling pathways of activin A. All the pathways are initiated by the formation of the activin A-heteropolymer receptor complex. The canonical pathway is Smad-dependent, while the non-canonical pathway can be mediated by ERK1/2, p38, Wnt, or PI3K. Endogenous ligand follistatin and activin family member inhibin can both bind to activin A directly to avoid the formation of activin A-heteropolymer receptors complex. Inhibin can also compete for the type II receptor. Antibodies targeting ALK4 are able to suppress downstream pathways as well. Created with BioRender.com.