REGULATION OF CELLULAR AND SYSTEM FUNCTION BY ACTIVIN

Activin is an important molecule that regulates hormonogenesis, cellul ar homeostasis (divide or die pathways), and differentiation programs (developmentally and in adult cells). The cellular mechanisms that int egrate an activin signal into a physiological response include a binar y receptor complex and tandem serine threonine kinases, intracellular signal mediators, and nuclear transcription factors. Activin antagonis ts (inhibins) and bioneutralizing binding proteins (follistatins) act as gating molecules to ensure accurate delivery of activin signals to cellular machinery. Correct execution of an activin cue intracellularl y permits actions as fundamental as embryonic mesoderm development, ne uronal survival, hematopoietic function, and reproductive cyclicity. A bsent or incorrect activin signaling results in phenotypes as catastro phic as embryonic lethality, tumor formation, and infertility. The gen eral ways in which a cell senses and responds to an activin signal wil l be reviewed in the first part of this paper. The role of this ligand in reproductive function will also be examined as a specific example of activin activity. (C) 1998 Elsevier Science Inc.