Drosophila AK-3, a homolog of human CKMT1B, is essential for spermiogenesis

The assembly and maintenance of axonemal microtubules during spermatogenesis is essential for production of functional sperm. This study shows that arginine kinase 3 (AK-3) (human ortholog CKMT1B) is necessary for Drosophila spermiogenesis and its biological function is conserved from flies to humans.

Spermatogenesis is a conserved process across animals, involving the proliferation and maintenance of germ stem cells, haploid spermatid production via meiosis and the generation of mature sperm with unique shapes. Our previous studies revealed that after ocnus (ocn) knockdown in germlines, the male flies Drosophila melanogaster were sterile, and the expression levels of many proteins were significantly changed. Among these proteins, CG4546 (arginine kinase 3, AK-3) was drastically downregulated, implying its crucial role in fly spermatogenesis. Here, we demonstrate that AK-3 was highly expressed in Drosophila testes. Knockdown of the AK-3 in testes leads to scattered nuclear bundles, loss of the central paired microtubule in the flagellar axoneme, disrupted individualization complexes, lack of mature sperm in seminal vesicles and thus resulting in male complete infertility. Notably, Drosophila AK-3 shares homology with human CKMT1B. Expression of human CKMT1B can rescue the defects in late spermatogenesis and male sterility caused by AK-3 knockdown in flies, indicating that this gene is evolutionarily and functionally conserved. These results suggest that AK-3 contributes to the regulation of spermiogenesis, especially the assembly and stabilizing of the axonemal microtubules during the sperm elongation. These data substantiate the importance of arginine kinase during spermatogenesis and its evolutionary conservation, and might provide fundamental information for studying the function of CKMT1B in male fertility in humans.