Axotomy of vertebrate neurons leads to the transient upregulation of GAP-43 and alpha-tubulin. In adult zebrafish retina, mRNA levels of both genes were increased in retinal ganglion cells after optic nerve lesion following a similar time course. At 5 days after crush, the mRNA level of GAP-43 was increased nearly 20 times, whereas a 6-fold increase was observed for alpha-tubulin. Subsequently, upon target reinnervation, mRNA levels of both genes were downregulated and were 2-fold higher than normal at 25 days after crush. Stretching the optic nerve that results in diffuse axonal lesions led to the expression of both genes in identical subsets of retinal ganglion cells. When regeneration was prevented by removing a piece of the optic nerve, mRNA levels remained elevated. Disruption of axonal transport by colchicine and vinblastine led to the induction of both genes in normal retina. Blocking electrical activity with tetrodotoxin had no effect. This indicates that retrogradely transported signals induced by target contact regulate GAP-43 and alpha-tubulin transcription. Furthermore, the joint regulation of GAP-43 and alpha-tubulin mRNA levels after different kinds of lesion suggests that a common pathway underlies the regulation of neuronal GAP-43 and alpha-tubulin gene expression. In contrast, distinct mechanisms may control the extent and maintenance of increased mRNA levels of these genes.

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Beta-thymosins are actin monomer-binding polypeptides that are expressed in a neuronal growth-specific manner during embryonic development. Here, we show that regenerating retinal ganglion cells and non-neuronal cells of the optic nerve transiently activate beta-thymosin transcription after optic nerve lesion in the zebrafish. In retinal cell cultures, beta-thymosin is found at highest concentration in growth cones, branching points and varicosities of neurite-extending retinal ganglion cells. These places often exhibit reduced phalloidin staining, indicating that beta-thymosin promotes the disassembly of actin filaments. Beta-thymosin distribution within neurons in culture is distinct from actin, tubulin and the actin-severing protein gelsolin. Ectopic expression of beta-thymosin in a central nervous system (CNS) catecholaminergic cell line leads to alterations in the shape of the cell bodies and neurites. Beta-thymosin-positive cells spread more fully and exhibit an excessive degree of branching. We partially cloned two other actin-binding proteins, profilin and gelsolin, and analysed their expression patterns. Profilin is constitutively expressed in virtually all cells. Gelsolin, like beta-thymosin, is selectively increased in regenerating retinal ganglion cells. During development, however, gelsolin mRNA is not detected in the nervous system. These findings indicate that distinct mechanisms control the actin cytoskeleton in embryonic and regenerating neurons, and that beta-thymosin may be a major regulator of actin dynamics in the zebrafish CNS.

beta-Thymosins are polypeptides that bind monomeric actin and thereby function as actin buffers in many cells. We show that during zebrafish development, β-thymosin expression is tightly correlated with neuronal growth and differentiation. It is transiently expressed in a subset of axon-extending neurons, essentially primary neurons that extend long axons, glia and muscle. Non-neuronal expression in the brain is restricted to a subset of glia surrounding newly forming axonal tracts. Skeletal muscle cells in somites, jaw and fin express beta-thymosin during differentiation, coinciding with the time of innervation. Injection of beta-thymosin antisense RNA into zebrafish embryos results in brain defects and impairment of the development of beta-thymosin-associated axon tracts. Furthermore, irregularities in somite formation can be seen in a subset of embryos. Compared to wild-type, antisense-injected embryos show slightly weaker and more diffuse engrailed staining at the midbrain-hindbrain boundary and a strong reduction of Isl-1 labeling in Rohon Beard and trigeminal neurons. The decreased expression is not based on a loss of neurons indicating that beta-thymosin may be involved in the maintenance of the expression of molecules necessary for neuronal differentiation. Taken together, our results strongly indicate that beta-thymosin is an important regulator of development.