The most common presentation of male infertility are men that produce low numbers of spermatozoa, that are poor in motility and morphology (also known as oligoasthenoteratozoospermia, or OAT). From an aetiology perspective, there is evidence to suggest that testicular hyperthermia is a leading cause of OAT. However, it is unknown why the process of spermatogenesis must remain 3-4oC below core body temperature.
To understand this further, the testes of mice were placed in a water bath and exposed to either control or elevated temperatures. A phospho-proteomic analysis of testicular cells that were exposed to these conditions showed changes in the phosphorylation level of Serine Rich Splicing Factor 10 (SRSF10). SRSFs are known to play a major role in the regulation of mRNA alternative splicing, particularly controlling the inclusion of exons during RNA maturation. To understand the kinetics of phosphorylation further, isolated round spermatid cells were placed at different temperatures. We found SRSF10 was highly phosphorylated when round cells were incubated below 35oC suggesting kinase involvement. However, above 35oC, SRSF10 was dephosphorylated suggesting phosphatase involvement.
Using siRNA, we have begun to identify the family of thermo-sensitive kinases that directly regulate the phosphorylation of SRSF10. It appears that CLK-like kinase 2 (CDK2) phosphorylates SRSF10 in a temperature dependent manner. Work is now continuing to identify the phosphatase(s) that are responsible for the removal of phosphorylation.
The finding that SRSF10 is regulated by temperature, suggests testicular hyperthermia results in aberrant alternative splicing, leading to both a loss of meiotic cells and poor sperm quality. The data suggests that a key reason for lower testicular temperature is the regulation of alternate mRNA transcripts.