beta-Thymosin, a modulator of the actin cytoskeleton is increased in regenerating retinal ganglion cells

beta-Thymosins are actin monomer-binding polypeptides that are expressed in a neuronal growth-specific manner during embryonic development. Here, we s how that regenerating retinal ganglion cells and non-neuronal cells of the optic nerve transiently activate beta-thymosin transcription after optic ne rve lesion in th zebrafish. In retinal cell cultures, beta-thymosin is foun d at highest concentration in growth cones, branching points and varicositi es of neurite-extending retinal ganglion cells. These places often exhibit reduced phalloidin staining, indicating that beta-thymosin promotes the dis assembly of actin filaments. beta-Thymosin distribution within neurons in c ulture is distinct from actin, tubulin and th actin-severing protein gelsol in. Ectopic expression of beta-thymosin in a central nervous system (CNS) c atecholaminergic cell line leads to alterations in the shape of the cell bo dies and neurites. beta-Thymosin-positive cells spread more fully and exhib it an excessive degree of branching. We partially cloned two other actin-bi nding proteins, profilin and gelsolin, and analysed their expression patter ns. Profilin is constitutively expressed in virtually all cells. Gelsolin, like beta-thymosin, is selectively increased in regenerating retinal gangli on cells. During development, however, gelsolin mRNA is not detected in the nervous system. These findings indicate that distinct mechanisms control t he actin cytoskeleton in embryonic and regenerating neurons, and that beta- thymosin may be a major regulator of actin dynamics in th zebrafish CNS.