Kinesin-12 influences axonal growth during zebrafish neural development

Kinesin-12 (also called KIF15) is a microtubule-based motor protein best known for its role in cell division. We previously reported that kinesin-12 is robustly expressed in developing terminally post-mitotic neurons, with levels diminishing as neurons reach maturity. We found that axons of cultured rodent neurons grow faster and longer if kinesin-12 is experimentally depleted, leading us to conclude that kinesin-12 plays a role in modulating axonal growth. Here we used zebrafish to explore whether these results apply to an in vivo system and whether they apply across different kinds of vertebrates. In whole mount in situ hybridization, kinesin-12 mRNA was detectable at 2-cell and 1K-cell stages. At 5.3 and 8 hours post-fertilization (hpf), hybridization signal for kinesin-12 mRNA was observed in the ectoderm. From 14 to 36 hpf, the signal had expanded to the central nervous system. At 60 hpf, the hybridization signal was concentrated in the brain. After 5 days post-fertilization, kinesin-12 expression was reduced. Kinesin-12 knockdown resulted in notably longer fast-growing axons with fewer branches by injection of a splice-blocking morpholino into Tg(huC:egfp) or Tg(hb9:gfp) zebrafish embryos. Kinesin-12 overexpression resulted in shorter axons than controls. These results are consistent with our previous observations on rodents using primary cultures for the experimental manipulations, and suggest a key role of kinesin-12 as a modulator of axonal development.