Journal Article

Serum albumin and HCO<sub>3</sub><sup>−</sup> regulate separate pools of ATP in human spermatozoa

T.H. Hereng, K.B.P. Elgstøen, L. Eide, K.R. Rosendal and B.S. Skålhegg

in Human Reproduction

Published on behalf of European Society of Human Reproduction and Embryology

Volume 29, issue 5, pages 918-930
Published in print May 2014 | ISSN: 0268-1161
Published online February 2014 | e-ISSN: 1460-2350 | DOI: https://dx.doi.org/10.1093/humrep/deu028
Serum albumin and HCO3− regulate separate pools of ATP in human spermatozoa

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STUDY QUESTION

Do the known capacitating agents HCO3 and serum albumin regulate the generation of ATP required for sperm motility and capacitation?

SUMMARY ANSWER

Serum albumin and HCO3 seem to regulate two separate pools of ATP by different mechanisms in human spermatozoa.

WHAT IS KNOWN ALREADY

Sperm capacitation is a maturation process that naturally occurs in the female reproductive tract preparing the sperm cell for fertilization. It is a highly energy-depending process as it involves hyperactive motility and substantial levels of protein phosphorylation.

STUDY DESIGN, SIZE, DURATION

Human sperm cells from four (motility experiments) and three (all other experiments) healthy donors were used. Untreated cells were compared with cells treated with HCO3 and serum albumin for up to 4 h.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Changes in glycolysis and mitochondrial respiration rates upon treatment with serum albumin and HCO3 were analysed by metabolic tracing of 13C-labelled substrates and respirometry studies, respectively. Levels of hyperactive spermatozoa and ATP content were measured during 4 h of incubation under capacitating conditions.

MAIN RESULTS AND THE ROLE OF CHANCE

We found that HCO3 significantly (P < 0.05) increased glycolytic flux by >3-folds via a cAMP/PKA sensitive pathway. This was accompanied by an increase in hyperactive motility. In contrast, serum albumin significantly increased endogenous ATP levels by 50% without stimulating hyperactive motility or glycolysis, indicating that this pool of ATP is separately located from the HCO3-induced ATP. The increase in ATP induced by albumin could be mimicked by treatment with the cholesterol acceptors 2-hydroxypropyl- and methyl-β-cyclodextrin and counteracted by co-incubation with cholesterol sulphate to the level of the non-treated control (P < 0.05), pointing to cholesterol extraction from the sperm cell membrane as the main mechanism. However, the concentration of cyclodextrins needed to directly detect cholesterol extraction from the sperm cells was not compatible with maintenance of sperm viability. The increase in ATP seemed not to be dependent on the sperm-specific Ca2+ channel CatSper. Finally, we demonstrated that neither HCO3 nor serum albumin stimulated mitochondrial respiration rates. However, serum albumin increased the respiratory capacity of mitochondria by >50%, an effect that was counteracted by HCO3.

LIMITATIONS, REASONS FOR CAUTION

Great variation in motility and capacitation is observed between sperm cells from different species. Hence, caution should be taken when extrapolating the findings in this work on human spermatozoa to sperm from other species.

WIDER IMPLICATIONS OF THE FINDINGS

It is already established that an efficient energy-generation is required to support sperm motility and capacitation. However, the mechanisms explaining how ATP production is regulated in spermatozoa are not fully understood. Our findings indicate that HCO3 stimulates hyperactive motility by increasing glycolytic flux and ATP production in a cAMP/PKA sensitive fashion. On the other hand, serum albumin seems to increase ATP concentration at a different location and by a mechanism different from glycolysis that involves extraction of cholesterol from the sperm cell membrane. These new insights into sperm metabolism may pave the way for both the development of new and improved male contraceptives and optimized assisted reproduction techniques.

STUDY FUNDING

The work was funded by Spermatech AS, The University of Oslo and the Research Council of Norway.

COMPETING INTEREST(S)

T.H.H. and K.R.R. are employees at Spermatech. B.S.S is a shareholder in Spermatech.

Keywords: human serum albumin; HCO3−; sperm metabolism; protein kinase A; capacitation

Journal Article.  8477 words.  Illustrated.

Subjects: Reproductive Medicine

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