Similarly to Mstn, Act-A signals by binding with high affinity to activin receptor type llA (ActRllA) or less so to activin receptor type IIB followed by the recruitment of the ActRI (ALK4, ALK7, or ALK2)
[33] (
Figure 1). Subsequently, the canonical SMAD pathway is activated. However, Act-A can induce MAPKs, which affect cell migration and differentiation
[34][35], and controls the Wnt signaling pathway involved in developmental and injury processes
[36]. Act-A action is regulated by several molecules. At the extracellular level, FSTL binds to Act-A with high affinity and prevents the engagement of type II receptors
[37] and inhibin, binding to betaglycan, a type III TGF-B receptor, and then to ActRII, and competes with Act-A for the receptor site
[38]. At the intracellular level, BAMBI (BMP and activin membrane-bound inhibitor), a transmembrane pseudoreceptor structurally similar to type I receptors, inhibits activin signaling
[39] due to lacking the intracellular kinase domain. However, Cripto reduces Act-A efficacy, inhibiting the ability of the activin/ActRII complex to recruit the type I receptor, thereby inhibiting the activin downstream signaling pathway
[9]. Act-A was first described as gonadal protein stimulating FSH secretion by the pituitary gland
[40]. Then, it is expressed in the embryonal ovary, uterus, and testis, and in glands such as the breast and prostate. Interestingly, Act-A is also present in the human placenta, amnion, and chorion, and its levels are elevated in pregnant women
[41]. However, apart from exerting diverse biological functions in the reproductive tract, Act-A and its receptors have been fully characterized in virtually all body systems
[42]. Act-A is strongly expressed in different compartments of the central nervous system, where it seems to exert neuroprotective effects
[43]. In addition, Act-A is present in both developing and adult heart, kidney, lung, and gastrointestinal tract
[44]. In the heart, Act-A (and ActRs) may influence cardiomyocyte differentiation and remodeling after different kinds of injuries
[45]. In the kidney, Act-A exerts a profibrotic effect, both during organ development and following acute and chronic damage
[46][47]. Moreover, Act-A operates in concert with Mstn in negatively regulating muscle growth and may play a significant role in bone remodeling
[48]. In bone, Act-A is secreted by osteoblasts and during bone matrix resorption by osteoclasts
[49]. Different animal models showed that Act-A induces osteoblastogenesis, osteoclastogenesis, chondrogenesis, and collagen synthesis
[50]. Coherently, inhibition of Act-A signaling obtained by administration of soluble ActRIIA, or the use of a ligand trap, was effective in preventing muscle wasting in different mouse models of experimental CKD and promoted osteogenesis and increased bone mass in healthy mice and primates
[51][52]. Finally, Act-A modulates innate and adaptive immune mechanisms and mediates inflammatory responses
[53]. Most immune cell types, including macrophages and T and B lymphocytes, can produce and respond to Act-A, and in inflammatory conditions, high levels of interleukin (IL)-1β and tumor necrosis factor-alpha (TNF-α) can promote, through NF-kB, Act-A expression and secretion, boosting the inflammatory process
[54]. Furthermore, Act-A is proapoptotic in several cells, such as hepatocytes
[55], renal proximal tubular cells
[56], B cells
[57], chronic myeloid leukaemia cells
[58], and cardiac myocytes
[59]. Finally, recently it has been proved that activin-A is involved in the pathogenesis of vascular calcification and CKD-related mineral bone disorders.
[60]