Distinct localization of two serine-threonine kinase receptors for activin and TGF-beta in the rat brain and down-regulation of type I activin receptor during peripheral nerve regeneration

The localizations of serine-threonine kinase receptor mRNA for the novel type I TGF-beta and/or activin receptor named B1 (rat), ALK-4 (mouse) or ActR-IB (human) were demonstrated by in situ hybridization. As the putative ligand for this receptor in the brain has not yet been clearly determined, we compared its localization to type II activin receptor (ActR-II) which is the counterpart of the type I activin receptor. B1 mRNA was widely observed in neuronal cells throughout the brain, and especially strong positive signals were found in the cerebral cortex, olfactory tubercle, and hippocampus. The localization of B1 mRNA coincided well with that of ActR-II. This strongly suggests that B1 (ALK-4/ActR-IB) could be the type I activin receptor, as type I and type II activin receptor were supposed to form a receptor complex. In addition, we examined the localization of type II TGF-beta receptor (T beta RII) mRNA which is an essential counterpart of the type I TGF-beta receptors for TGF-beta signaling. T beta RII mRNA was expressed mainly in non-neuronal cells such as choroid plexus. In addition, T beta RII mRNA expression was also found in a minor population of neuronal cells. T beta RII mRNA-positive neurons were observed in the reticular thalamus, laterodorsal tegmental nucleus, pedunculopontine tegmental nucleus and the ventral tegmental nucleus. The localization of T beta RII was markedly different from that of activin receptors in the rat brain. Since TGF-Ps and activins are known as growth factors and/or survival factors, we examined changes in levels of B1 and T beta RII mRNA expression during peripheral nerve regeneration. Expression of B1 mRNA in the axotomized hypoglossal motoneurons was substantially decreased from day 3 after axotomy and this decrease was significant until postoperative day 28, whereas no T beta RII signal was observed in hypoglossal nucleus prior or after axotomy. This transient down-regulation of B1 mRNA expression suggests that activin signaling is somehow suppressed during peripheral nerve regeneration.