The mid-piece of the sperm tail is a major site of energy production for sperm motility and defects in its formation are strongly associated with human male infertility. Despite, this the processes by which mitochondria are recruited from the cytoplasm, and then assembled into the characteristic, helical mitochondrial sheath of the mid-piece, remain virtually unexplored. Herein, through the characterisation of a Vdac2 knockout mouse model, we reveal novel insights into this mysterious process.
VDAC2 is a voltage-gated porin in the outer mitochondrial membrane. It has well-established roles in apoptosis and cellular metabolism, and decreased VDAC2 levels have been previously correlated with human male infertility. Consistent with this, here, we show that VDAC2 is essential for male fertility and that spermatogenesis is uniquely sensitive to even partial VDAC2 reduction. Indeed, mice heterozygous for our Vdac2 deletion allele (Vdac2+/-) are male sterile. Unexpectedly however, our analyses suggest this phenotype is not due the disruption of VDAC2 apoptotic function. Instead, Vdac2+/- male sterility is due to an inability to produce functionally motile sperm. Both the percentage of motile sperm and the percentage of progressively motile sperm were minimal in Vdac2+/- males (reduced by 62% and 93%, respectively). Intriguingly, these motility defects are structural in origin. Most notably trafficking and incorporation of mitochondria into the sperm flagella mid-piece was severely compromised. Consistent with these data, we also show that VDAC2 is highly testis enriched, and while it is present in all ages of the postnatal mouse testis, it is particularly upregulated after the onset of spermiogenesis. Collectively, our data unequivocally establishes VDAC2 as an essential component of the spermiogenesis machinery and supports a model wherein VDAC2 facilitates mitochondrial loading into the mid-piece. Further, given that the VDAC2 has previously been established to bind the microtubule motor protein, KLC3, these data raise the distinct possibility that VDAC2 functions as an ‘adaptor’ between motor proteins and mitochondria during mid-piece formation.