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Expression of Tas1 taste receptors in mammalian spermatozoa: functional role of Tas1r1 in regulating basal Ca²⁺ and cAMP concentrations in spermatozoa.

Meyer D, Voigt A, Widmayer P, Borth H, Huebner S, Breit A, Marschall S, de Angelis MH, Boehm U, Meyerhof W, Gudermann T, Boekhoff I - PLoS ONE (2012)

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Walther-Straub Institute of Pharmacology and Toxicology, Ludwig-Maximilians-University, Munich, Germany.

ABSTRACT

Background: 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.

Methodology/principal findings: 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 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.

Conclusions/significance: 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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Related in: MedlinePlus

Working model illustrating a possible functional role of tastereceptor signaling in taste cells and spermatozoa.[A] Model for the transduction cascade of theumami receptor in taste cells. On the left, a schematic drawing of theonion-like structure of a single taste bud formed by elongated tastecells is shown. The peripheral ends of the 50–100 taste cells inone taste bud terminate at the gustatory pore; taste information iscoded by afferent nerve fibers which innervate the taste buds and comeclose to type II receptor cells but only form conventional chemicalsynapses with the basolateral membrane of type III taste cells. In tastecells, the Tas1r1 and Tas1r3 receptors form a functional dimer which isable to recognize amino acids such as MSG. Upon ligand binding, theumami receptor activates a trimeric G Protein consisting ofα-gustducin [αGus] andβ3 and γ13[βγ]. The βγ subunitactivates phopholipase Cβ2[PLC] which cleaves phosphatidylinositol 4,5-bisphosphate [PIP2] to inositoltrisphoshate [IP3] anddiacylglycerol [DAG]. IP3 mediatesan increase in intracellular calcium by activation of calcium channelsin the endoplasmic reticulum [ER] andsubsequently an influx of calcium through ion channels in the plasmamembrane [TRPM5]. Simultaneously, releasedα-gustducin can activate phosphodiesterase, resulting in a decreaseof intracellular levels of cyclic adenosine monophosphate[cAMP]. A crosstalk between the twopathways exists through a cAMP regulated activation of protein kinas A[PKA] which inhibits PLC and theIP3-receptor in the ER. This mechanism may ensureadequate Ca2+ signaling to taste stimuli by keeping thetaste cell in a tonically suppressed state. The drawing was modifiedfrom Ref. [45] and [109].[B] Putative model of Tas1 taste receptorsignaling in spermatozoa. The schematic drawing in the left signifiesthe sperm's journey in the different sections of the female genitaltract [uterus, oviduct,ampulla] which sperm have to transit to reachthe egg in the ampullar region of the oviduct (dotted red line). Insperm cells, the Tas1r1 protein [Tas1r1] maydimerize with its taste partner Tas1r3 or with a yet not identifiedreceptor [R?]. G protein activation resultsin the release of a G protein α-subunit[Gα] which activatesphosphodiesterase [PDE], thus leading to thehydrolysis of cAMP. In this model, an activation of the receptor dimer[Tas1r1/R?] by chemosensory ligandswithin the different regions of the female genital tract (red rhoms) ora constitutively active receptor may ensure low cAMP levels, therebypreventing cAMP-triggered maturation processes of the sperm, likecapacitation, motility or acrosome reaction, before the sperm reachesthe egg in the ampullary part of the oviduct. If the simultaneouslyreleased Gβγ complex [βγ]indeed stimulates PLC in analogy to taste cells or alternativelyactivates potassium [K+] channels in sperm, iscurrently not clear. Constant cAMP hydrolysis can be overcome duringsperm maturation either by an decrease in taste receptor activationcontrolled by changes in the composition of chemical components in thedifferent fluids of the female genital tract or by an increase in[Ca2+]i, or high bicarbonateconcentration which would lead to an activation of the solubleadenylatecyclase [sAC] in spermatozoa. Forseek of simplicity, regulatory effects of PKA activation or EPACstimulation on calcium channels or the IP3 receptor areomitted in the model.
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pone-0032354-g012: Working model illustrating a possible functional role of tastereceptor signaling in taste cells and spermatozoa.[A] Model for the transduction cascade of theumami receptor in taste cells. On the left, a schematic drawing of theonion-like structure of a single taste bud formed by elongated tastecells is shown. The peripheral ends of the 50–100 taste cells inone taste bud terminate at the gustatory pore; taste information iscoded by afferent nerve fibers which innervate the taste buds and comeclose to type II receptor cells but only form conventional chemicalsynapses with the basolateral membrane of type III taste cells. In tastecells, the Tas1r1 and Tas1r3 receptors form a functional dimer which isable to recognize amino acids such as MSG. Upon ligand binding, theumami receptor activates a trimeric G Protein consisting ofα-gustducin [αGus] andβ3 and γ13[βγ]. The βγ subunitactivates phopholipase Cβ2[PLC] which cleaves phosphatidylinositol 4,5-bisphosphate [PIP2] to inositoltrisphoshate [IP3] anddiacylglycerol [DAG]. IP3 mediatesan increase in intracellular calcium by activation of calcium channelsin the endoplasmic reticulum [ER] andsubsequently an influx of calcium through ion channels in the plasmamembrane [TRPM5]. Simultaneously, releasedα-gustducin can activate phosphodiesterase, resulting in a decreaseof intracellular levels of cyclic adenosine monophosphate[cAMP]. A crosstalk between the twopathways exists through a cAMP regulated activation of protein kinas A[PKA] which inhibits PLC and theIP3-receptor in the ER. This mechanism may ensureadequate Ca2+ signaling to taste stimuli by keeping thetaste cell in a tonically suppressed state. The drawing was modifiedfrom Ref. [45] and [109].[B] Putative model of Tas1 taste receptorsignaling in spermatozoa. The schematic drawing in the left signifiesthe sperm's journey in the different sections of the female genitaltract [uterus, oviduct,ampulla] which sperm have to transit to reachthe egg in the ampullar region of the oviduct (dotted red line). Insperm cells, the Tas1r1 protein [Tas1r1] maydimerize with its taste partner Tas1r3 or with a yet not identifiedreceptor [R?]. G protein activation resultsin the release of a G protein α-subunit[Gα] which activatesphosphodiesterase [PDE], thus leading to thehydrolysis of cAMP. In this model, an activation of the receptor dimer[Tas1r1/R?] by chemosensory ligandswithin the different regions of the female genital tract (red rhoms) ora constitutively active receptor may ensure low cAMP levels, therebypreventing cAMP-triggered maturation processes of the sperm, likecapacitation, motility or acrosome reaction, before the sperm reachesthe egg in the ampullary part of the oviduct. If the simultaneouslyreleased Gβγ complex [βγ]indeed stimulates PLC in analogy to taste cells or alternativelyactivates potassium [K+] channels in sperm, iscurrently not clear. Constant cAMP hydrolysis can be overcome duringsperm maturation either by an decrease in taste receptor activationcontrolled by changes in the composition of chemical components in thedifferent fluids of the female genital tract or by an increase in[Ca2+]i, or high bicarbonateconcentration which would lead to an activation of the solubleadenylatecyclase [sAC] in spermatozoa. Forseek of simplicity, regulatory effects of PKA activation or EPACstimulation on calcium channels or the IP3 receptor areomitted in the model.

Mentions: Our results indicate that the onset of Tas1r3 expression resembles that of Tas1r1during spermatogenesis (Fig.3), and that their sub-cellular localization in mature spermatozoa iscomparable (Fig. 4; Fig. S2).However, we currently cannot definitively affirm that the observedco-localization indeed results in a physical interaction of the tongue-specificdimerization partners to constitute a functional receptor complex in male germcells. Given that sperm provide a unique response spectrum which obviously doesnot include [Ca2+]i responses to MSG (Fig. 9) one might suggest thattaste receptors in sperm form functional hetero-dimers or even largerhetero-oligomers [92] which are different from the ones in taste buds ofthe tongue and which may also show some compensatory effect upon Tas1r1deletion, thus providing an explanation for the inconspicuous reproductivephenotype of Tas1r1 (−/−) animals. Sincehetero-dimerization between distinct GPCRs was described to be responsible forthe generation of pharmacologically defined receptors with a unique mode ofactivation (e. g. agonist affinity, efficacy, signaling properties, positive ornegative allosteric modulation) [90], [93], one may speculate that such a“new” receptor entity which had also been proposed for gastricendocrine cells [94] might specifically recognize endogenous reproductiveagonists. This assumption seems even more attractive considering that theTas1r1/Tas1r3 dimer mediating umami taste sensation on the tongue is potentiatedby purine nucleotides, like inosine monophosphate (IMP) [95], a “finetuning” mechanism, which for taste receptors in spermatozoa may berealized by reproductive specific allosteric modulators. In this context, it isinteresting to mention that glutamate concentrations in the female genital tractare high in the uterus und decline constantly on the way to the egg in theampullary region of the follicular tube (s. Model, Fig. 12B), whereas all other amino acids showtheir highest concentration in the oviductal region [3], thus indicating thatdistinct gradients of potential taste receptor ligands indeed exist within thedifferent compartments of the female genital tract. Thus, sperm may senseincrements of such chemical compounds on their way to the mature egg in theampullary part of the fallopian tube. However, at present we cannot definitivelydecide whether MSG can induce cAMP signals in Tas1R1 mutant sperm due toelevated basal cAMP levels in uncapacitated Tas1r1 sperm (s. Fig. 11C/D and Fig. S4).Together with the observation that MSG did not elicit an increase in[Ca2+]i (Fig. 9), and that MSG was ineffective ininducing acrosome reaction in spermatozoa (Fig. S3),it still remains debatable whether glutamate is indeed an active ligand of theTas1r1 in spermatozoa.


Expression of Tas1 taste receptors in mammalian spermatozoa: functional role of Tas1r1 in regulating basal Ca²⁺ and cAMP concentrations in spermatozoa.

Meyer D, Voigt A, Widmayer P, Borth H, Huebner S, Breit A, Marschall S, de Angelis MH, Boehm U, Meyerhof W, Gudermann T, Boekhoff I - PLoS ONE (2012)

Working model illustrating a possible functional role of tastereceptor signaling in taste cells and spermatozoa.[A] Model for the transduction cascade of theumami receptor in taste cells. On the left, a schematic drawing of theonion-like structure of a single taste bud formed by elongated tastecells is shown. The peripheral ends of the 50–100 taste cells inone taste bud terminate at the gustatory pore; taste information iscoded by afferent nerve fibers which innervate the taste buds and comeclose to type II receptor cells but only form conventional chemicalsynapses with the basolateral membrane of type III taste cells. In tastecells, the Tas1r1 and Tas1r3 receptors form a functional dimer which isable to recognize amino acids such as MSG. Upon ligand binding, theumami receptor activates a trimeric G Protein consisting ofα-gustducin [αGus] andβ3 and γ13[βγ]. The βγ subunitactivates phopholipase Cβ2[PLC] which cleaves phosphatidylinositol 4,5-bisphosphate [PIP2] to inositoltrisphoshate [IP3] anddiacylglycerol [DAG]. IP3 mediatesan increase in intracellular calcium by activation of calcium channelsin the endoplasmic reticulum [ER] andsubsequently an influx of calcium through ion channels in the plasmamembrane [TRPM5]. Simultaneously, releasedα-gustducin can activate phosphodiesterase, resulting in a decreaseof intracellular levels of cyclic adenosine monophosphate[cAMP]. A crosstalk between the twopathways exists through a cAMP regulated activation of protein kinas A[PKA] which inhibits PLC and theIP3-receptor in the ER. This mechanism may ensureadequate Ca2+ signaling to taste stimuli by keeping thetaste cell in a tonically suppressed state. The drawing was modifiedfrom Ref. [45] and [109].[B] Putative model of Tas1 taste receptorsignaling in spermatozoa. The schematic drawing in the left signifiesthe sperm's journey in the different sections of the female genitaltract [uterus, oviduct,ampulla] which sperm have to transit to reachthe egg in the ampullar region of the oviduct (dotted red line). Insperm cells, the Tas1r1 protein [Tas1r1] maydimerize with its taste partner Tas1r3 or with a yet not identifiedreceptor [R?]. G protein activation resultsin the release of a G protein α-subunit[Gα] which activatesphosphodiesterase [PDE], thus leading to thehydrolysis of cAMP. In this model, an activation of the receptor dimer[Tas1r1/R?] by chemosensory ligandswithin the different regions of the female genital tract (red rhoms) ora constitutively active receptor may ensure low cAMP levels, therebypreventing cAMP-triggered maturation processes of the sperm, likecapacitation, motility or acrosome reaction, before the sperm reachesthe egg in the ampullary part of the oviduct. If the simultaneouslyreleased Gβγ complex [βγ]indeed stimulates PLC in analogy to taste cells or alternativelyactivates potassium [K+] channels in sperm, iscurrently not clear. Constant cAMP hydrolysis can be overcome duringsperm maturation either by an decrease in taste receptor activationcontrolled by changes in the composition of chemical components in thedifferent fluids of the female genital tract or by an increase in[Ca2+]i, or high bicarbonateconcentration which would lead to an activation of the solubleadenylatecyclase [sAC] in spermatozoa. Forseek of simplicity, regulatory effects of PKA activation or EPACstimulation on calcium channels or the IP3 receptor areomitted in the model.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3303551&req=5

pone-0032354-g012: Working model illustrating a possible functional role of tastereceptor signaling in taste cells and spermatozoa.[A] Model for the transduction cascade of theumami receptor in taste cells. On the left, a schematic drawing of theonion-like structure of a single taste bud formed by elongated tastecells is shown. The peripheral ends of the 50–100 taste cells inone taste bud terminate at the gustatory pore; taste information iscoded by afferent nerve fibers which innervate the taste buds and comeclose to type II receptor cells but only form conventional chemicalsynapses with the basolateral membrane of type III taste cells. In tastecells, the Tas1r1 and Tas1r3 receptors form a functional dimer which isable to recognize amino acids such as MSG. Upon ligand binding, theumami receptor activates a trimeric G Protein consisting ofα-gustducin [αGus] andβ3 and γ13[βγ]. The βγ subunitactivates phopholipase Cβ2[PLC] which cleaves phosphatidylinositol 4,5-bisphosphate [PIP2] to inositoltrisphoshate [IP3] anddiacylglycerol [DAG]. IP3 mediatesan increase in intracellular calcium by activation of calcium channelsin the endoplasmic reticulum [ER] andsubsequently an influx of calcium through ion channels in the plasmamembrane [TRPM5]. Simultaneously, releasedα-gustducin can activate phosphodiesterase, resulting in a decreaseof intracellular levels of cyclic adenosine monophosphate[cAMP]. A crosstalk between the twopathways exists through a cAMP regulated activation of protein kinas A[PKA] which inhibits PLC and theIP3-receptor in the ER. This mechanism may ensureadequate Ca2+ signaling to taste stimuli by keeping thetaste cell in a tonically suppressed state. The drawing was modifiedfrom Ref. [45] and [109].[B] Putative model of Tas1 taste receptorsignaling in spermatozoa. The schematic drawing in the left signifiesthe sperm's journey in the different sections of the female genitaltract [uterus, oviduct,ampulla] which sperm have to transit to reachthe egg in the ampullar region of the oviduct (dotted red line). Insperm cells, the Tas1r1 protein [Tas1r1] maydimerize with its taste partner Tas1r3 or with a yet not identifiedreceptor [R?]. G protein activation resultsin the release of a G protein α-subunit[Gα] which activatesphosphodiesterase [PDE], thus leading to thehydrolysis of cAMP. In this model, an activation of the receptor dimer[Tas1r1/R?] by chemosensory ligandswithin the different regions of the female genital tract (red rhoms) ora constitutively active receptor may ensure low cAMP levels, therebypreventing cAMP-triggered maturation processes of the sperm, likecapacitation, motility or acrosome reaction, before the sperm reachesthe egg in the ampullary part of the oviduct. If the simultaneouslyreleased Gβγ complex [βγ]indeed stimulates PLC in analogy to taste cells or alternativelyactivates potassium [K+] channels in sperm, iscurrently not clear. Constant cAMP hydrolysis can be overcome duringsperm maturation either by an decrease in taste receptor activationcontrolled by changes in the composition of chemical components in thedifferent fluids of the female genital tract or by an increase in[Ca2+]i, or high bicarbonateconcentration which would lead to an activation of the solubleadenylatecyclase [sAC] in spermatozoa. Forseek of simplicity, regulatory effects of PKA activation or EPACstimulation on calcium channels or the IP3 receptor areomitted in the model.
Mentions: Our results indicate that the onset of Tas1r3 expression resembles that of Tas1r1during spermatogenesis (Fig.3), and that their sub-cellular localization in mature spermatozoa iscomparable (Fig. 4; Fig. S2).However, we currently cannot definitively affirm that the observedco-localization indeed results in a physical interaction of the tongue-specificdimerization partners to constitute a functional receptor complex in male germcells. Given that sperm provide a unique response spectrum which obviously doesnot include [Ca2+]i responses to MSG (Fig. 9) one might suggest thattaste receptors in sperm form functional hetero-dimers or even largerhetero-oligomers [92] which are different from the ones in taste buds ofthe tongue and which may also show some compensatory effect upon Tas1r1deletion, thus providing an explanation for the inconspicuous reproductivephenotype of Tas1r1 (−/−) animals. Sincehetero-dimerization between distinct GPCRs was described to be responsible forthe generation of pharmacologically defined receptors with a unique mode ofactivation (e. g. agonist affinity, efficacy, signaling properties, positive ornegative allosteric modulation) [90], [93], one may speculate that such a“new” receptor entity which had also been proposed for gastricendocrine cells [94] might specifically recognize endogenous reproductiveagonists. This assumption seems even more attractive considering that theTas1r1/Tas1r3 dimer mediating umami taste sensation on the tongue is potentiatedby purine nucleotides, like inosine monophosphate (IMP) [95], a “finetuning” mechanism, which for taste receptors in spermatozoa may berealized by reproductive specific allosteric modulators. In this context, it isinteresting to mention that glutamate concentrations in the female genital tractare high in the uterus und decline constantly on the way to the egg in theampullary region of the follicular tube (s. Model, Fig. 12B), whereas all other amino acids showtheir highest concentration in the oviductal region [3], thus indicating thatdistinct gradients of potential taste receptor ligands indeed exist within thedifferent compartments of the female genital tract. Thus, sperm may senseincrements of such chemical compounds on their way to the mature egg in theampullary part of the fallopian tube. However, at present we cannot definitivelydecide whether MSG can induce cAMP signals in Tas1R1 mutant sperm due toelevated basal cAMP levels in uncapacitated Tas1r1 sperm (s. Fig. 11C/D and Fig. S4).Together with the observation that MSG did not elicit an increase in[Ca2+]i (Fig. 9), and that MSG was ineffective ininducing acrosome reaction in spermatozoa (Fig. S3),it still remains debatable whether glutamate is indeed an active ligand of theTas1r1 in spermatozoa.

Bottom Line: 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.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.

View Article: PubMed Central - PubMed

Affiliation: Walther-Straub Institute of Pharmacology and Toxicology, Ludwig-Maximilians-University, Munich, Germany.

ABSTRACT

Background: 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.

Methodology/principal findings: 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 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.

Conclusions/significance: 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.

Show MeSH
Related in: MedlinePlus