Roles of activin and its signal transduction mechanisms in reproductive tissues

Activins were identified initially as gonadal proteins having a stimulating effect on FSH production by the pituitary gland. Strong evidence has accum ulated that activins are important regulating factors for many reproductive processes. Activin may have paracrine or autocrine roles rather than solel y an endocrine action on FSH secretion. Activins together with their signal ling molecules must be shown to be produced locally in a particular tissue to provide support for their paracrine or autocrine action in that tissue. The discovery of the activin receptors, the intracellular signalling mediat ors (Smads) and some transcription co-factors involved in activin responses has helped to unravel the activin-transforming growth factor P signalling mechanism. However, few reports have clearly demonstrated the presence of a ll of the activin signalling molecules in reproductive tissues, despite the important roles of activin in these tissues. Several activin receptor type s and Smad molecules have been identified, indicating either a redundancy i n signalling molecules or different signalling pathways. At present, it is not clear which particular subset of these signalling molecules is importan t in reproductive processes. The aim of this review is to collate the infor mation available on activin actions, as well as on the signalling molecules , to understand how activins may transduce their paracrine or autocrine sig nals in reproductive tissues.