Expression of Galpha14 in sweet-transducing taste cells of the posterior tongue
- Marco Tizzano†1, 2,
- Gennady Dvoryanchikov†3,
- Jennell K Barrows1, 2,
- Soochong Kim4,
- Nirupa Chaudhari3, 5 and
- Thomas E Finger1Email author
© Tizzano et al; licensee BioMed Central Ltd. 2008
Received: 02 August 2008
Accepted: 13 November 2008
Published: 13 November 2008
"Type II"/Receptor cells express G protein-coupled receptors (GPCRs) for sweet, umami (T1Rs and mGluRs) or bitter (T2Rs), as well as the proteins for downstream signalling cascades. Transduction downstream of T1Rs and T2Rs relies on G-protein and PLCβ2-mediated release of stored Ca2+. Whereas Gαgus (gustducin) couples to the T2R (bitter) receptors, which Gα-subunit couples to the sweet (T1R2 + T1R3) receptor is presently not known. We utilized RT-PCR, immunocytochemistry and single-cell gene expression profiling to examine the expression of the Gαq family (q, 11, 14) in mouse taste buds.
By RT-PCR, Gα14 is expressed strongly and in a taste selective manner in posterior (vallate and foliate), but not anterior (fungiform and palate) taste fields. Gαq and Gα11, although detectable, are not expressed in a taste-selective fashion. Further, expression of Gα14 mRNA is limited to Type II/Receptor cells in taste buds. Immunocytochemistry on vallate papillae using a broad Gαq family antiserum reveals specific staining only in Type II taste cells (i.e. those expressing TrpM5 and PLCβ2). This staining persists in Gαq knockout mice and immunostaining with a Gα11-specific antiserum shows no immunoreactivity in taste buds. Taken together, these data show that Gα14 is the dominant Gαq family member detected. Immunoreactivity for Gα14 strongly correlates with expression of T1R3, the taste receptor subunit present in taste cells responsive to either umami or sweet. Single cell gene expression profiling confirms a tight correlation between the expression of Gα14 and both T1R2 and T1R3, the receptor combination that forms sweet taste receptors.
Gα14 is co-expressed with the sweet taste receptor in posterior tongue, although not in anterior tongue. Thus, sweet taste transduction may rely on different downstream transduction elements in posterior and anterior taste fields.
Taste buds, the end-organs for gustation, detect and respond to a variety of macronutrient and aversive compounds to generate taste perception. Compounds that evoke bitter taste bind to one or more G protein coupled receptors (GPCRs) of the T2R family of taste receptors [1–3]. Amino acids and compounds that elicit umami taste bind to a variety of GPCRs including metabotropic glutamate receptors, mGluR4 and mGluR1, and the heterodimeric taste receptor, T1R1+T1R3 [4–7]. Sugars and a variety of other sweeteners bind to the heterodimeric receptor, T1R2+T1R3[5, 8]. Most of these various taste GPCRs appear to all couple to a common transduction pathway that includes the heterotrimeric G protein subunits, Gβ3 and Gγ13, a phospholipase C, PLCβ2, and a transient receptor potential ion channel, TrpM5 [9–14].
In spite of the detailed exploration of the Gβγ-triggered signaling, much less is understood of which Gα subunits couple to various taste GPCRs, and which downstream signaling pathways they recruit. Taste buds are reported to express a number of different subunits including Gαgus (gustducin), Gαs, Gαi, Gαq, Gα14, Gα15 and two transducin isoforms, Gαt1 and Gαt2 [15–18]. Apart from Gαgus, it remains unknown which of these are expressed in the same cells as taste GPCRs and might be candidate signaling partners. In vitro, the sweet taste receptor, T1R2+T1R3, couples via Gαi to cAMP modulation. Although Gαi subunits are expressed in taste buds, it is unclear which receptors activate them in situ. Biochemical and physiological studies have suggested that bitter taste transduction includes the involvement of Gαgus, although the exact mechanism of such involvement remained unclear[13, 21, 22]. While Gαgus-knockout mice are quite insensitive to bitter tastants, they are also somewhat compromised in their detection of sweet tastants. Gustducin is co-expressed with the T1R2+T1R3 sweet receptor in the palate  and fungiform papillae but not in the posterior gustatory fields. Furthermore, the direct functional role of Gαgus in sweet detection has not been demonstrated. The Gαgus, Gαi and Gαs subunits alter cAMP levels when activated, while members of the Gαq family trigger release of stored Ca2+. The primary cellular response triggered by tastants appears to be a Ca2+ signal. Although much of this signal is produced via the action of Gβγ subunits, the contribution of the Gαq family has not been examined in taste buds.
Mammalian taste buds are composed of up to 100 cells. Though most mature cells in taste buds have a generally fusiform shape, they can be distinguished into several distinct types based on their functional properties and the expression of diagnostic mRNA and protein markers. Based on ultrastructural and other criteria, cells in rodent taste buds are classified as "Type I", the glial-like or supporting cells, "Type II", the primary receptor cells, and "Type III", the cells that show specialized chemical synapses. Type II cells are characterized by the ubiquitous expression of PLCβ2 and TrpM5 [20, 27–29]. Subsets of Type II cells express either T2Rs or T1Rs, an observation that suggested the segregated detection of tastants of the sweet and bitter qualities. Subsequent functional studies have demonstrated the equivalence of cell types identified by expression patterns and cells with particular response profiles. For instance, cells that express NCAM and SNAP25 were shown to be those that display voltage-gated calcium channels[29, 30]. Cells that express TrpM5 or PLCβ2 are those that respond to bitter or sweet stimuli[29, 31]. Here, we have used RT-PCR, immunocytochemistry and single-cell gene expression profiling to examine the expression of the Gαq family (αq, α11, α14) in mouse taste buds and establish which of these are co-expressed with T1R2 and T1R3, subunits that constitute the sweet taste GPCR. The pattern of expression suggests that Gαgus is unlikely to be a signaling partner for T1R2+T1R3. Instead, the sweet receptor subunits consistently are co-expressed with Gα14.
Gαq family members are differentially expressed in different taste fields
First, we used end-point RT-PCR to evaluate the expression of Gαq family subunits in taste buds to assess which members, if any, of the Gαq family (Gαq, Gα11 and Gα14) are expressed in taste buds. We analyzed taste buds obtained from four different oral taste fields (vallate, foliate, fungiform and palate) as well as non-taste lingual and palatal epithelium.
To test this, we undertook quantitative RT-PCR experiments to compare expression levels of the Gαq family members in taste buds (Fig. 1B). The concentration of Gα14 mRNA in vallate taste buds was comparable to that of PLCβ2 mRNA. Neither mRNA was expressed in nontaste epithelium. In contrast, Gαq mRNA was found at similar concentration in CV taste buds and in nontaste epithelium while Gα11 was at much higher concentration in nontaste epithelium than in taste epithelium. Thus, neither Gα11 or Gαq are expressed in a taste-selective manner. Further, the mRNAs for Gαq and Gα11 are expressed at 14- and 80-fold lower concentrations respectively than Gα14 mRNA. These data support our interpretation from end-point RT-PCR, that the only Gα subunit of this family that is likely to have a taste-selective role is Gα14.
Immunoreactivity to Gαq-family in Gαgus-negative Type II cells
Gαq-family-immunoreactivity is not due to Gαq nor Gα11
To further test whether Gα11 is present in taste buds, we utilized an antiserum directed against the N-terminal region of Gα11 which shares no sequence similarity to the N-terminal region of either Gαq or Gα14. This Gα11-specific antiserum does not stain taste buds (Fig. 4C–C') although it does stain the cerebellar molecular layer (Fig. 4E) in which Gα11 is detectable by immunocytochemistry. These results essentially rule out Gα11 as the source of the taste bud immunoreactivity for the Gq/11/14 antibody. Since this Gq/11/14 antibody exhibits staining in Gαq knockout mice, we conclude that the staining is attributable to neither Gαq nor Gα11 leaving only Gα14 as the possible source of the immunoreactivity. These results are entirely consistent with our RT-PCR data (above) showing that Gα14 mRNA is the predominant Gαq family member isoform expressed in a taste-specific manner.
Gαq-family-immunoreactivity is in Type II Taste (Receptor) Cells
Taste buds (TBs) comprise at least three different types of mature cells, so we utilized type-specific markers to test whether Gαq-family expression correlates with a specific cell type. Type II (receptor) cells express the GPCR taste receptors (T1Rs and T2Rs in different cells), TrpM5 and PLCβ2 [20, 27–29]. Thus, we performed the first set of immunocytochemistry using tissues from TrpM5-GFP mice in which all GFP labeled taste cells express the TrpM5 protein. In vallate taste buds, PLCβ2 antibody stained over 92% (25 of 27 cells) of TrpM5-GFP-labeled cells, confirming the identification of Type II cells with these markers. Further, in vallate taste buds, immunoreactivity to PGP9.5 antibody, which stains mostly Type III cells ) was detectable only in a small percentage of TrpM5-GFP positive cells (~4%; 1 of 27 cells).
T1R & G-protein co-localization in Taste Buds
Single Cell RT-PCR
Gq/11/14 + Gαgus
Gα14 + Gαgus
T1R3 + T1R2
(includes T1R1 & TT1R2)
Gαq-family immunoreactivity in T1R3-expressing cells
Single-cell RT-PCR shows co-expression of Gα14 with T1R2/T1R3
Using the complementary techniques of immunocytochemistry and gene expression profiling via single-cell RT-PCR, we have explored the pattern of expression of the Gαq family of heterotrimeric G protein subunits. We show that while Gαq and Gα11 are expressed at low levels in taste buds, Gα14 is expressed in a highly taste bud-selective manner. This observation is consistent with many previous reports of the ubiquitous distribution of Gαq and Gα11, while Gα14 is restricted to a few highly specialized cell types. Further, we show that Gα14 is limited to Type II taste cells, i.e. those that express taste GPCRs.
Mammalian taste buds use specialized taste GPCRs (including the T1R and T2R families) to detect bitter, umami and sweet tastants. Both families of taste receptors activate the downstream signalling elements, PLCβ2 and IP3R3[9, 12, 20, 27, 39]. The T2R receptors couple to Gαgus[22, 40] but the Gα subunits activated in situ by the T1R receptors are not yet defined. Whereas Gαgus is substantially co-expressed with T1R receptors in the anterior taste fields, such is not the case for posterior taste fields[24, 25] where only a fraction of T1R3-expressing taste cells co-express Gαgus. Hence, Gαgus is unlikely to be the Gα subunit associated with the T1R receptors in the posterior tongue. We report here the obligatory co-expression of Gα14 with the sweet receptor T1R2+T1R3 in foliate and vallate taste fields. Accordingly, we suggest that sweet transduction in posterior, but not anterior gustatory fields involves Gα14. Whether Gα14 couples directly to the sweet taste receptor or to other GPCRs intimately associated with sweet detection remains to be determined.
The different taste receptors are not homogeneous across the taste fields of the tongue but show regional differences[25, 41]. The T1R1+T1R3 umami receptor is much more prevalent in the anterior taste fields (fungiform and palate) than in the posterior lingual fields (foliate and vallate). In posterior lingual taste fields, T1R3 more commonly partners with T1R2 forming a sweet receptor. Our single cell RT-PCR data show that over 70% (10/14) of the T1R3-expressing cells also express T1R2. With in situ hybridization, two groups reported previously that 92% and 98% of the T1R3 expressing cells of the vallate papilla co-express T1R2. The lower percentage of co-localization we report may reflect differences in technique, given the higher sensitivity of single cell RT-PCR over in situ hybridization. Most of the T1R3-positive, non T1R2-expressing cells in our profiling displayed low levels of T1R3 (e.g. cells 8, 10, 11 of Fig. 6) and may not have been detected with in situ hybridization.
The T1R2+T1R3 heterodimer can functionally couple to a variety of Gαi/o subunits in heterologous systems[19, 43]. Since sweeteners activate adenylyl cyclase in vallate taste buds, Margolskee has postulated that the T1R2+T1R3 receptor may couple to Gαs. Indeed, Kusakabe et al showed the expression of Gαs, Gαi2 and Gαi3 in vallate taste buds, but not the tight association with sweet receptors. In contrast, our data show clearly that Gα14, a member of the Gαq family is consistently co-expressed with T1R2+T1R3 in posterior taste buds. Because sweet taste receptors are co-expressed with PLCβ2, and mice in which PLCβ2 is knocked out show a severe deficit of sweet signaling, it is widely accepted that sweet transduction occurs via Gβγ-mediated activation of PLCβ2. Yet PLCβ2 also can be activated robustly by members of the Gαq family. Our study is the first demonstration of a consistent association of Gα14 (a Gαq family member) with taste receptors that activate PLCβ2.
The expression of Gαgus in association with the sweet receptor in the palate and fungiform papilla is likely to account for the profound decrease of intake of sucrose and sweeteners in long-term taste behaviours in Gαgus knockout mice[36, 48, 49]. On the basis of these previous results along with our current findings, Gαgus knockout would be predicted to affect only the palatal and fungiform taste fields where Gαgus is co-expressed with the sweet receptors. Transection of the gustatory nerves innervating only anterior taste fields results in near-total elimination of sweet taste preference in some behavioural assays despite the presence of an intact posterior lingual taste system. Thus the loss of sweet-driven taste behaviours reported in Gαgus knockouts is likely attributable to loss of function in the sweet-detecting taste buds of the anterior taste fields. Indeed more rigorous behavioural analysis of gustducin null mice reveals substantial residual sweet-guided taste behavior. Electrophysiological analysis of taste nerves of Gαgus-null mice reveals a profound loss of sweet-evoked activity in anterior tongue but only partial loss in posterior. This is consistent with our finding that relatively few sweet receptive taste cells also express Gαgus. The degree of functional loss as measured by the magnitude of the glossopharyngeal nerve response in Gαgus null mice, is, however, greater than would be predicted on the basis of the co-expression patterns. Perhaps Gαgus-expressing sweet-receptive cells disproportionately activate gustatory nerve fibers, or act synergistically with other cells within a taste bud to effect activation of the nerve fibers. Indeed, Roper suggests the likelihood of intrabud intercellular communication playing an essential role in transmission of taste information.
The sweet-receptive taste cells of the posterior tongue express Gα14 while those in anterior taste buds express Gαgus. These findings show that even within a single sensory system, the same receptor may couple with different G-protein alpha subunits in different functional parts of the system. Our results also account for the residual behavioural and neural activity to sweet tastants in Gαgus-null animals.
All experiments were performed under protocols approved by the Animal Care and Use Committees of the CU Denver School of Medicine and the University of Miami School of Medicine. We used three strains of adult transgenic mice in which GFP is expressed from the promoters of T1R3, TRPM5, PLCβ2, or GAD1[31, 34, 53]. In taste buds from each of these strains, previous studies have shown that all cells expressing the relevant endogenous proteins also express GFP [31, 33, 34]. Gαq-deficient mice and littermate controls were obtained from Satya Kunapuli (Temple University, Philadelphia, PA)  with permission from Stefan Offermanns (University of Heidelberg, Heidelberg, Germany). This Gαq-null line was generated by replacement of sequence coding for amino acids 246–297 of Gαq by the neo gene and is described in detail in Cho et al. The genotype of Gq-null mice was determined by PCR and was confirmed by platelet aggregation assay.
We used a Gq-family antibody that is labelled and sold as anti-Gαq/11 (Santa Cruz Biotechnology; sc# 392) although the manufacturer indicates that it likely will react with Gα14. The antigenic peptide, VFAAVKDTILQLNLKEYNLV, is located near the C-terminus, is 100% identical between Gαq and Gα11 and is 90% identical/100% similar in Gα14 (all sequences from mouse). In contrast, Gα15 is only 45% identical/75% similar in this region. Thus, the antibody likely reacts with Gαq, Gα11 and Gα14, but not Gα15. To test the specificity of this broad Gαq family antibody, we also used an affinity-purified Gα11 specific antiserum (Santa Cruz Biotechnology; sc# 394) directed against an N-terminal peptide (aa 13–29 from mouse sequence) that lacks any sequence similarity to either Gαq or Gα14. The cerebellum was used as a positive control tissue for these antisera since Purkinje cells express both Gαq and Gα11 but not Gα14 [35, 56].
Mice were perfusion-fixed in 4% paraformaldehyde (PFA) in 0.1 M phosphate buffer (PB). Taste tissues (vallate, foliate and fungiform papillae and soft palate) were post-fixed (4°C, 60 min); 14 μm cryosections were washed with phosphate-buffered saline (PBS), blocked with 1% normal goat serum, and incubated with rabbit anti-Gαq-family antibody (1:500); at 4°C overnight. The secondary antiserum used was Rhodamine RedTM-X-conjugated AffiniPure™ Fab Fragment goat anti-Rabbit IgG (H+L) (1:100; Jackson ImmunoResearch Lab.; 111-297-003).
To examine co-expression of the Gαq family with other proteins in taste buds, and to avoid cross reactivity of multiple rabbit primary antibodies with a common secondary antibody, we used the Zenon Rabbit IgG Labeling Kit (Invitrogen, Z25308). For this, each primary antibody other than Gαq/11/14 was pre-conjugated to Alexa Fluor 647 so that it did not require a secondary antibody for visualization. After the binding of the first primary and secondary antibodies was complete, the primary antibody-Zenon647 complex was applied to the slides for 80 min at RT in the dark, washed in PB with 0.2% Triton X-100 and postfixed in PFA/PB. Slides were then coverslipped with Fluormount-G. Omission of primary antibodies (detected with Rhodamine Red-X or Zenon) resulted in no apparent fluorescent signal. The primary antibodies used as Zenon complexes were rabbit anti-PLCβ2 (1:200; Santa Cruz Biotechnology; sc# 206); rabbit anti-Gαgus (1:200: Santa Cruz Biotechnology; sc# 395) and rabbit anti-PGP9.5 (Ubiquitin C-terminal hydrolase-L1; 1:200; AbD Serotec; 7863-0504).
All images were collected with an Olympus Fluoview confocal laser scanning microscope (LSCM) FV300 (Olympus Corporation). For each image, the channels were collected sequentially with single wavelength excitation and then merged to produce the composite image using the software Fluoview v5.0. This avoids the problem of bleed-through of images resulting from side-band excitation of the fluorochromes. Brightness and contrast were adjusted in Adobe Photoshop 7.0. Images for Fig. 3 are displayed as negatives to permit visualization of faint fluorescent signal. This was accomplished by using the "Invert" command in Photoshop on the Red channel of the confocal image.
For quantification of immunocytochemical data, we counted immunoreactive cells from 3 sections through the vallate papilla from 2 different animals. An immunoreactive profile was included if it had an elongate morphology extending at least half the height of the taste bud and included an obvious nucleus. Cell fragments not including a nuclear profile were not included in the sample.
Adult PLCβ2-GFP and GAD-GFP mice were killed by CO2 asphyxiation followed by decapitation, the tongue and palate were removed and a protease mixture consisting of 1 mg/ml collagenase, type A, 2.5 mg/ml dispase (both from Roche Products, Indianapolis, IN) and 1 mg/ml trypsin inhibitor (Sigma, St. Louis, MO) in Tyrode buffer was injected. Tyrode buffer consisted of, in mM: 139 NaCl, 5 KCl, 2 CaCl2, 1 MgCl2, 10 Hepes, 10 glucose, 10 Na pyruvate, and 5 NaHCO3; pH 7.2, 318–323 mOsm. Epithelium was peeled from underlying tissue after 20 min and incubated in Ca/Mg-free Tyrode's solution for 14 min. For Ca-Mg-free Tyrode buffer, CaCl2 and MgCl2 were replaced with 2 mM each EGTA and BAPTA.
Primers for RT-PCR
Protein & Gene
Forward Primer (5'→3')
Reverse Primer (5'→3')
Vallate taste buds were collected from PLCβ2-GFP mice (as above), then cells were dissociated by gentle trituration. GFP-labeled single cells were collected each into 60 μl of lysis buffer containing 200 ng of poly-Inosinic acid as a carrier, processed for RNA purification and cDNA synthesis. For preliminary analyses of cell type specific expression (Fig. 2), we collected individual GFP-labeled vallate taste cells, pooled them in lysis buffer and isolated RNA and linear amplified it as previously described . The aRNA (amplified RNA) was subjected to reverse transcription and 0.1% of the cDNA was used as template for RT-PCR (40 cycles). For analyses of single cells, mRNA from each cell was reverse transcribed and then each 20 μl single cell cDNA was divided as follows: 5% each for β-actin and TrpM5, 10% each for Gαgus and Gα14, 20% each for T1R2 and T1R3 and 30% for Gαq. PCR was performed in 20 μl for 40 cycles.
Quantitative RT-PCR was carried out as previously described using the same primers as for end point PCR, and SYBR Green PCR mix (Bio-Rad) in a Bio-Rad iQ iCycler. Three independent samples of CV taste buds and of adjacent non-taste epithelium were purified and analyzed in parallel. The concentration of each mRNA was compared to a standard curve generated from a sequence-validated template and calculated using MyiQ software (Bio-Rad). All mRNA concentrations were normalized to β-actin mRNA which was run in parallel.
The authors thank Robert Margolskee (Mt. Siani Medical Ctr.) and Sami Damak (Nestle Research Ctr.) for making the TrpM5-GFP and T1R3-GFP animals available for study. Similarly, the authors thank Satya Kunapuli (Temple Univ.) and Stefan Offermanns (Univ. Heidelberg) for facilitating and permitting use of the Gαq-KO mice.
This work was supported by NIH grants to T.E.F. (P30DC004657; DC007495), N.C. (DC006021; DC006308).
Aspects of this work were presented at the International Symposium on Olfaction & Taste, San Francisco, CA, July 2008 and published in abstract form in Chem Senses 2008, 38(4) S144. Since the initial submission and acceptance of this manuscript a related paper was published: Shindo Y, Miura H, Carninci P, Kawai J, Hayashizaki Y, Ninomiya Y, Hino A, Kanda T, Kusakabe Y, G alpha14 is a candidate mediator of sweet/umami signal transduction in the posterior region of the mouse tongue: Biochem Biophys Res Commun. 2008, Nov 21; 376(3): 504–8. Epub 2008 Sep 16.
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