Understanding Sperm Function required for Fertilisation Using Phenotypic Screening

Infertility is a global health problem, estimated to affect 1 in 7 couples (Barratt et al., 2017). It has significant impacts on those affected, and its treatment places substantial financial strain both on infertile couples and health services. Whilst male factor is a common cause of infertility, standard semen analysis (sperm morphology and motility) is assessed as normal in 30% of all cases. In the absence of female factors this is termed ‘unexplained infertility’. However, diagnostic semen analysis is a quantitative rather than qualitative evaluation, and does not specifically assess sperm function required for fertilisation. Unfortunately there is currently a lack of diagnostic tests within the clinical setting to assess sperm dysfunction, yet it is responsible for poor treatment success rates and failed fertilisation, which causes significant patient distress as well as financial loss for couples and health services.

The reality is that absence of diagnostic tests for sperm dysfunction is because of very limited understanding of the molecular events in human sperm that are necessary for fertilisation. We, and others, have progressed the study of plasma membrane ion channels in human sperm by applying methods for direct recording of ion channel currents and demonstrated that they have an essential role in sperm function (Lishko et al., 2011; Mansell et al., 2014). By applying this approach to surplus aliquots from patient samples used for treatment, we have shown that up to 10% of apparently normal semen samples carry abnormalities of the progesterone/prostaglandin E1 (P4/PGE1)-sensitive Ca2+ channel CatSper and the sperm K+ channel (KSper), such that fertilisation at IVF is very limited or fails completely (Brown et al., 2016; Williams et al., 2015). Importantly, in only one case have we been able to identify a genetic defect in a channel-coding region (Brown et al., 2018) As such, our data clearly indicate that sperm dysfunction largely occurs independently of known genetic factors. Phenotypic screening of patient sperm is therefore fundamental to identify these cases, as well as to increase our understanding of the contribution of the sperm to fertilisation events.

Human sperm motility is complex and involves a number of functionally different behaviours that may be elicited in response to cues experienced in female tract (Brown et al., 2017). Intracellular calcium, membrane potential and pH are critical to these processes, and ion channel dysfunction (ICD) is thus likely to be a significant contributing cause of unexplained infertility. This PhD project proposes to use phenotypic screening strategies to increase understanding of human sperm function required for fertilisation, including intracellular calcium responses, acrosome reaction and phospholipase C zeta protein expression. The project will be an unrivalled opportunity for a student to learn a raft of advance biological techniques in a research facility closely affiliated to NHS Tayside Assisted Conception Unit.

References
Barratt CLR, De Jonge CJ, Sharpe RM. ‘Man Up’: The importance and strategy for placing male reproductive health centre stage in the political and research agenda. Hum Reprod 2018;33(4):541-545

Lishko PV, Botchkina IL, Kirichok Y. Progesterone activates the principal Ca2+ channel of human sperm. Nature 2011;471(7338):387-391.

Mansell SA, Publicover SJ, Barratt CL, Wilson SM. Patch clamp studies of human sperm under physiological ionic conditions reveal three functionally and pharmacologically distinct cation channels. Mol Hum Reprod 2014 May;20(5):392-408.

Brown SG, Publicover SJ, Mansell SA, Lishko PV, Williams HL, Ramalingam M, Wilson SM, Barratt CL, Sutton KA, Da Silva SM. Depolarization of sperm membrane potential is a common feature of men with subfertility and is associated with low fertilization rate at IVF. Hum Reprod 2016;31(6):1147-1157.

Williams HL, Mansell S, Alasmari W, Brown SG, Wilson SM, Sutton KA, Miller MR, Lishko PV, Barratt CL, Publicover SJ, Martins da Silva S. Specific loss of CatSper function is sufficient to compromise fertilizing capacity of human spermatozoa. Hum Reprod 2015 30(12):2737-2746.

Brown SG, Miller MR, Lishko PV, Lester DH, Publicover SJ, Barratt CLR, Martins Da Silva SJ Homozygous in-frame deletion in CATSPERE in a man producing spermatozoa with loss of CatSper function and compromised fertilizing capacity. Hum Reprod 2018 33(10):1812–1816

Brown SG, Costello S, Kelly MC, Ramalingam M, Drew E, Publicover SJ, Barratt CLR, Da Silva SM. Complex CatSper-dependent and independent [Ca2+]i signalling in human spermatozoa induced by follicular fluid. Hum Reprod 2017;32(10):1995-2006.

This project will be based at the University of Dundee’s School of Medicine, with a proposed start date of 1 October 2019. Please contact Dr Sarah Martins da Silva with any enquiries ([Email Address Removed])

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To apply please send a cover letter, curriculum vitae and two references to: [Email Address Removed]