Expression and putative functions of KIFC1 for nuclear reshaping and midpiece formation during spermiogenesis of Phascolosoma esculenta.

Kinesin-14 KIFC1 plays an important role in vesicular transport, microtubule organization, and spermiogenesis. In this study, we first investigated the microtubule distribution and expression pattern of KIFC1 during spermiogenesis of P. esculenta. Microtubules are abundant during spermiogenesis of P. esculenta and may be related to the generation and maintenance of pseudopodia-like cytoplasmic protrusions and nuclear reshaping. The Pe-KIFC1 protein is conserved with a motor domain where microtubule and ATP binding sites are predicted, a coiled-coil domain and a divergent tail domain. The Pe-kifc1 gene was extensively expressed and showed the highest expression in coelomic fluid where spermiogenesis occurs. We further observed the expression of kifc1 mRNA and protein and found that Pe-KIFC1 protein primarily co-localized with microtubules during spermiogenesis, indicating that KIFC1 might play several roles during this process via its cargo transport and/or microtubule organization function. In addition, co-localization of mitochondria and KIFC1 was also detected during spermiogenesis, which were located in the midpiece in mature sperm, suggesting that mitochondria might be a cargo of Pe-KIFC1 that participates in the intracellular distribution of mitochondria and formation of the midpiece. Based on our detailed observations of the dynamic distribution of microtubules, KIFC1, and mitochondria during spermiogenesis and the conserved function of KIFC1 in cargo transport and microtubule organization, functional models of Pe-KIFC1 during spermiogenesis are proposed, including the participation of KIFC1 in nuclear reshaping and midpiece formation.