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Expression of Tas1 Taste Receptors in Mammalian Spermatozoa: Functional Role of Tas1r1 in Regulating Basal Ca and cAMP Concentrations in Spermatozoa

Walther-Straub Institute of Pharmacology and Toxicology, Ludwig-Maximilians-University, Munich, Germany.
PLoS ONE (Impact Factor: 3.53). 02/2012; 7(2):e32354. DOI: 10.1371/journal.pone.0032354
Source: PubMed

ABSTRACT During their transit through the female genital tract, sperm have to recognize and discriminate numerous chemical compounds. However, our current knowledge of the molecular identity of appropriate chemosensory receptor proteins in sperm is still rudimentary. Considering that members of the Tas1r family of taste receptors are able to discriminate between a broad diversity of hydrophilic chemosensory substances, the expression of taste receptors in mammalian spermatozoa was examined.
The present manuscript documents that Tas1r1 and Tas1r3, which form the functional receptor for monosodium glutamate (umami) in taste buds on the tongue, are expressed in murine and human spermatozoa, where their localization is restricted to distinct segments of the flagellum and the acrosomal cap of the sperm head. Employing a Tas1r1-deficient mCherry reporter mouse strain, we found that Tas1r1 gene deletion resulted in spermatogenic abnormalities. In addition, a significant increase in spontaneous acrosomal reaction was observed in Tas1r1 null mutant sperm whereas acrosomal secretion triggered by isolated zona pellucida or the Ca²⁺ ionophore A23187 was not different from wild-type spermatozoa. Remarkably, cytosolic Ca²⁺ levels in freshly isolated Tas1r1-deficient sperm were significantly higher compared to wild-type cells. Moreover, a significantly higher basal cAMP concentration was detected in freshly isolated Tas1r1-deficient epididymal spermatozoa, whereas upon inhibition of phosphodiesterase or sperm capacitation, the amount of cAMP was not different between both genotypes.
Since Ca²⁺ and cAMP control fundamental processes during the sequential process of fertilization, we propose that the identified taste receptors and coupled signaling cascades keep sperm in a chronically quiescent state until they arrive in the vicinity of the egg - either by constitutive receptor activity and/or by tonic receptor activation by gradients of diverse chemical compounds in different compartments of the female reproductive tract.

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    • "Owing to the lack of anti-T1R2, the authors of the above study could not assess T1R2 expression in mature spermatozoa (Meyer et al., 2012). In one study, T1R1 knockout mice and wild-type controls showed similar reproductive characteristics, such as sperm concentration, testis histology, sperm morphology, sperm mobility and testosterone level (Meyer et al., 2012). In the tas1r1-null animals, treatment with zona pellucida induced significant acrosome reaction, indicating that tas1r1 deletion did not influence the ability of sperm to bind to the zona pellucida and activate coupled intracellular signalling cascades. "
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