Tissue-specific binding of radiolabeled activin A by activin receptors and follistatin in postimplantation rat and mouse embryos.

Activin affects the growth and differentiation of many cultured cell types, including rat anterior pituitary cells and gonadal and neuronal cell lines. Endogenous activins regulate mesoderm induction, body axis formation, and organogenisis in the developing embryo. The messenger RNAs encoding inhibin/activin subunits, follistatin (an activin-binding protein), and activin type II receptors (ActRII and IIB) are expressed in various cell types and tissues of the embryonic rat and mouse. Follistatin-deficient mice have numerous embryonic defects, including shiny taut skin, allowing relatively easy identification by the later stages of embryogenesis. ActRII-deficient mice, on the other hand, show limited developmental defects, with some (22%) embryonic day 18.5 (E18.5) ActRII-deficient embryos showing various skeletal and facial abnormalities. The present study was undertaken to identify the target tissues for biologically active activin A and assess the significance of its association with ActRII and follistatin in developing rat and mouse embryos. Fresh-frozen, slide-mounted, rat (E13 to E19) and mouse (E18.5) embryo sections were incubated with 125I-labeled recombinant human activin A. Nonspecific binding was evaluated by competition with an excess of cold activin A. As determined by image analysis, the highest levels of activin A binding were observed throughout the brain, spinal cord, and trigeminal and spinal ganglia at all ages. Lower levels of binding were found in the dermis of the skin starting on E15. Follistatin-deficient mice demonstrated similar patterns and levels of activin A binding in the neural tissues compared to wild-type controls, but binding was absent in the skin. In ActRII-deficient mice, activin A binding was completely absent in neural tissues, but was similar to wild-type control levels in the dermal layer of the skin. The data indicate that activin A binds to specific tissues of mouse and rat embryos and that binding is dependent upon the presence of ActRII in the central and peripheral nervous system and on follistatin in the skin.