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Introduction
Obesity is a growing epidemic that causes many serious health related complications. While the causes of obesity are complex, there is conclusive evidence that overconsumption coupled with a sedentary lifestyle is the primary cause of this medical condition. Dietary consumption is controlled by appetite which is in turn regulated by multiple neuronal systems, including the taste system. However, the relationship between taste and obesity has not been well defined. Growing evidence suggests that taste perception in the brain is altered in obese animals and humans, however no studies have determined if there are altered taste responses in the peripheral taste receptor cells, which is the initiation site for the detection and perception of taste stimuli.Methodology/Principal Findings
In this study, we used C57Bl/6 mice which readily become obese when placed on a high fat diet. After ten weeks on the high fat diet, we used calcium imaging to measure how taste-evoked calcium signals were affected in the obese mice. We found that significantly fewer taste receptor cells were responsive to some appetitive taste stimuli while the numbers of taste cells that were sensitive to aversive taste stimuli did not change. Properties of the taste-evoked calcium signals were also significantly altered in the obese mice. Behavioral analyses found that mice on the high fat diet had reduced ability to detect some taste stimuli compared to their littermate controls.Conclusions/Significance
Our findings demonstrate that diet-induced obesity significantly influences peripheral taste receptor cell signals which likely leads to changes in the central taste system and may cause altered taste perception. 相似文献3.
Mercè Izquierdo-Serra Dirk Trauner Artur Llobet Pau Gorostiza 《Biochimica et Biophysica Acta (BBA)/General Subjects》2013
Background
Neurons signal to each other and to non-neuronal cells as those in muscle or glands, by means of the secretion of neurotransmitters at chemical synapses. In order to dissect the molecular mechanisms of neurotransmission, new methods for directly and reversibly triggering neurosecretion at the presynaptic terminal are necessary. Here we exploit the calcium permeability of the light-gated channel LiGluR in order to reversibly manipulate cytosolic calcium concentration, thus controlling calcium-regulated exocytosis.Methods
Bovine chromaffin cells expressing LiGluR were stimulated with light. Exocytic events were detected by amperometry or by whole-cell patch-clamp to quantify membrane capacitance and calcium influx.Results
Amperometry reveals that optical stimulation consistently triggers exocytosis in chromaffin cells. Secretion of catecholamines can be adjusted between zero and several Hz by changing the wavelength of illumination. Differences in secretion efficacy are found between the activation of LiGluR and native voltage-gated calcium channels (VGCCs). Our results show that the distance between sites of calcium influx and vesicles ready to be released is longer when calcium influx is triggered by LiGluR instead of native VGCCs.Conclusion
LiGluR activation directly and reversibly increases the intracellular calcium concentration. Light-gated calcium influx allows for the first time to control calcium-regulated exocytosis without the need of applying depolarizing solutions or voltage clamping in chromaffin cells.General significance
LiGluR is a useful tool to study the secretory mechanisms and their spatiotemporal patterns in neurotransmission, and opens a window to study other calcium-dependent processes such as muscular contraction or cell migration. 相似文献4.
Yu-Kyong Shin Bronwen Martin Wook Kim Caitlin M. White Sunggoan Ji Yuxiang Sun Roy G. Smith Jean Sévigny Matthias H. Tsch?p Stuart Maudsley Josephine M. Egan 《PloS one》2010,5(9)
Background
The gustatory system plays a critical role in determining food preferences, food intake and energy balance. The exact mechanisms that fine tune taste sensitivity are currently poorly defined, but it is clear that numerous factors such as efferent input and specific signal transduction cascades are involved.Methodology/Principal Findings
Using immunohistochemical analyses, we show that ghrelin, a hormone classically considered to be an appetite-regulating hormone, is present within the taste buds of the tongue. Prepro-ghrelin, prohormone convertase 1/3 (PC 1/3), ghrelin, its cognate receptor (GHSR), and ghrelin-O-acyltransferase (GOAT , the enzyme that activates ghrelin) are expressed in Type I, II, III and IV taste cells of mouse taste buds. In addition, ghrelin and GHSR co-localize in the same taste cells, suggesting that ghrelin works in an autocrine manner in taste cells. To determine a role for ghrelin in modifying taste perception, we performed taste behavioral tests using GHSR null mice. GHSR null mice exhibited significantly reduced taste responsivity to sour (citric acid) and salty (sodium chloride) tastants.Conclusions/Significance
These findings suggest that ghrelin plays a local modulatory role in determining taste bud signaling and function and could be a novel mechanism for the modulation of salty and sour taste responsivity. 相似文献5.
Background
NMDA receptors are traditionally viewed as being located postsynaptically, at both synaptic and extrasynaptic locations. However, both anatomical and physiological studies have indicated the presence of NMDA receptors located presynaptically. Physiological studies of presynaptic NMDA receptors on neocortical GABAergic terminals and their possible role in synaptic plasticity are lacking.Methodology/Principal Findings
We report here that presynaptic NMDA receptors are present on GABAergic terminals in developing (postnatal day (PND) 12-15) but not older (PND21-25) rat frontal cortex. Using MK-801 in the recording pipette to block postsynaptic NMDA receptors, evoked and miniature IPSCs were recorded in layer II/III pyramidal cells in the presence of AMPA/KA receptor antagonists. Bath application of NMDA or NMDA receptor antagonists produced increases and decreases in mIPSC frequency, respectively. Physiologically patterned stimulation (10 bursts of 10 stimuli at 25 Hz delivered at 1.25 Hz) induced potentiation at inhibitory synapses in PND12-15 animals. This consisted of an initial rapid, large increase in IPSC amplitude followed by a significant but smaller persistent increase. Similar changes were not observed in PND21-25 animals. When 20 mM BAPTA was included in the recording pipette, potentiation was still observed in the PND12-15 group indicating that postsynaptic increases in calcium were not required. Potentiation was not observed when patterned stimulation was given in the presence of D-APV or the NR2B subunit antagonist Ro25-6981.Conclusions/Significance
The present results indicate that presynaptic NMDA receptors modulate GABA release onto neocortical pyramidal cells. Presynaptic NR2B subunit containing NMDA receptors are also involved in potentiation at developing GABAergic synapses in rat frontal cortex. Modulation of inhibitory GABAergic synapses by presynaptic NMDA receptors may be important for proper functioning of local cortical networks during development. 相似文献6.
Horio N Yoshida R Yasumatsu K Yanagawa Y Ishimaru Y Matsunami H Ninomiya Y 《PloS one》2011,6(5):e20007
Background
The polycystic kidney disease-like ion channel PKD2L1 and its associated partner PKD1L3 are potential candidates for sour taste receptors. PKD2L1 is expressed in type III taste cells that respond to sour stimuli and genetic elimination of cells expressing PKD2L1 substantially reduces chorda tympani nerve responses to sour taste stimuli. However, the contribution of PKD2L1 and PKD1L3 to sour taste responses remains unclear.Methodology/Principal Findings
We made mice lacking PKD2L1 and/or PKD1L3 gene and investigated whole nerve responses to taste stimuli in the chorda tympani or the glossopharyngeal nerve and taste responses in type III taste cells. In mice lacking PKD2L1 gene, chorda tympani nerve responses to sour, but not sweet, salty, bitter, and umami tastants were reduced by 25–45% compared with those in wild type mice. In contrast, chorda tympani nerve responses in PKD1L3 knock-out mice and glossopharyngeal nerve responses in single- and double-knock-out mice were similar to those in wild type mice. Sour taste responses of type III fungiform taste cells (GAD67-expressing taste cells) were also reduced by 25–45% by elimination of PKD2L1.Conclusions/Significance
These findings suggest that PKD2L1 partly contributes to sour taste responses in mice and that receptors other than PKDs would be involved in sour detection. 相似文献7.
Ivonne Torres-Atencio Erola Ainsua-Enrich Fernando de Mora César Picado Margarita Martín 《PloS one》2014,9(10)
Background
Mast cells play a critical role in allergic and inflammatory diseases, including exercise-induced bronchoconstriction (EIB) in asthma. The mechanism underlying EIB is probably related to increased airway fluid osmolarity that activates mast cells to the release inflammatory mediators. These mediators then act on bronchial smooth muscle to cause bronchoconstriction. In parallel, protective substances such as prostaglandin E2 (PGE2) are probably also released and could explain the refractory period observed in patients with EIB.Objective
This study aimed to evaluate the protective effect of PGE2 on osmotically activated mast cells, as a model of exercise-induced bronchoconstriction.Methods
We used LAD2, HMC-1, CD34-positive, and human lung mast cell lines. Cells underwent a mannitol challenge, and the effects of PGE2 and prostanoid receptor (EP) antagonists for EP1–4 were assayed on the activated mast cells. Beta-hexosaminidase release, protein phosphorylation, and calcium mobilization were assessed.Results
Mannitol both induced mast cell degranulation and activated phosphatidyl inositide 3-kinase and mitogen-activated protein kinase (MAPK) pathways, thereby causing de novo eicosanoid and cytokine synthesis. The addition of PGE2 significantly reduced mannitol-induced degranulation through EP2 and EP4 receptors, as measured by beta-hexosaminidase release, and consequently calcium influx. Extracellular-signal-regulated kinase 1/2, c-Jun N-terminal kinase, and p38 phosphorylation were diminished when compared with mannitol activation alone.Conclusions
Our data show a protective role for the PGE2 receptors EP2 and EP4 following osmotic changes, through the reduction of human mast cell activity caused by calcium influx impairment and MAP kinase inhibition. 相似文献8.
Yukio Akaneya Kazuhiro Sohya Akihiko Kitamura Fumitaka Kimura Chris Washburn Renping Zhou Ipe Ninan Tadaharu Tsumoto Edward B. Ziff 《PloS one》2010,5(8)
Background
Synaptogenesis is a fundamental step in neuronal development. For spiny glutamatergic synapses in hippocampus and cortex, synaptogenesis involves adhesion of pre and postsynaptic membranes, delivery and anchorage of pre and postsynaptic structures including scaffolds such as PSD-95 and NMDA and AMPA receptors, which are glutamate-gated ion channels, as well as the morphological maturation of spines. Although electrical activity-dependent mechanisms are established regulators of these processes, the mechanisms that function during early development, prior to the onset of electrical activity, are unclear. The Eph receptors and ephrins provide cell contact-dependent pathways that regulate axonal and dendritic development. Members of the ephrin-A family are glycosyl-phosphatidylinositol-anchored to the cell surface and activate EphA receptors, which are receptor tyrosine kinases.Methodology/Principal Findings
Here we show that ephrin-A5 interaction with the EphA5 receptor following neuron-neuron contact during early development of hippocampus induces a complex program of synaptogenic events, including expression of functional synaptic NMDA receptor-PSD-95 complexes plus morphological spine maturation and the emergence of electrical activity. The program depends upon voltage-sensitive calcium channel Ca2+ fluxes that activate PKA, CaMKII and PI3 kinase, leading to CREB phosphorylation and a synaptogenic program of gene expression. AMPA receptor subunits, their scaffolds and electrical activity are not induced. Strikingly, in contrast to wild type, stimulation of hippocampal slices from P6 EphA5 receptor functional knockout mice yielded no NMDA receptor currents.Conclusions/Significance
These studies suggest that ephrin-A5 and EphA5 signals play a necessary, activity-independent role in the initiation of the early phases of synaptogenesis. The coordinated expression of the NMDAR and PSD-95 induced by eprhin-A5 interaction with EphA5 receptors may be the developmental switch that induces expression of AMPAR and their interacting proteins and the transition to activity-dependent synaptic regulation. 相似文献9.
Ettorre M Lorenzetto E Laperchia C Baiguera C Branca C Benarese M Spano P Pizzi M Buffelli M 《PloS one》2012,7(2):e31451
Background
The functioning of the nervous system depends upon the specificity of its synaptic contacts. The mechanisms triggering the expression of the appropriate receptors on postsynaptic membrane and the role of the presynaptic partner in the differentiation of postsynaptic structures are little known.Methods and Findings
To address these questions we cocultured murine primary muscle cells with several glutamatergic neurons, either cortical, cerebellar or hippocampal. Immunofluorescence and electrophysiology analyses revealed that functional excitatory synaptic contacts were formed between glutamatergic neurons and muscle cells. Moreover, immunoprecipitation and immunofluorescence experiments showed that typical anchoring proteins of central excitatory synapses coimmunoprecipitate and colocalize with rapsyn, the acetylcholine receptor anchoring protein at the neuromuscular junction.Conclusions
These results support an important role of the presynaptic partner in the induction and differentiation of the postsynaptic structures. 相似文献10.
Mei-pian Chen Z. Ioav Cabantchik Shing Chan Godfrey Chi-fung Chan Yiu-fai Cheung 《PloS one》2014,9(11)
Background
Iron overload cardiomyopathy that prevails in some forms of hemosiderosis is caused by excessive deposition of iron into the heart tissue and ensuing damage caused by a raise in labile cell iron. The underlying mechanisms of iron uptake into cardiomyocytes in iron overload condition are still under investigation. Both L-type calcium channels (LTCC) and T-type calcium channels (TTCC) have been proposed to be the main portals of non-transferrinic iron into heart cells, but controversies remain. Here, we investigated the roles of LTCC and TTCC as mediators of cardiac iron overload and cellular damage by using specific Calcium channel blockers as potential suppressors of labile Fe(II) and Fe(III) ingress in cultured cardiomyocytes and ensuing apoptosis.Methods
Fe(II) and Fe(III) uptake was assessed by exposing HL-1 cardiomyocytes to iron sources and quantitative real-time fluorescence imaging of cytosolic labile iron with the fluorescent iron sensor calcein while iron-induced apoptosis was quantitatively measured by flow cytometry analysis with Annexin V. The role of calcium channels as routes of iron uptake was assessed by cell pretreatment with specific blockers of LTCC and TTCC.Results
Iron entered HL-1 cardiomyocytes in a time- and dose-dependent manner and induced cardiac apoptosis via mitochondria-mediated caspase-3 dependent pathways. Blockade of LTCC but not of TTCC demonstrably inhibited the uptake of ferric but not of ferrous iron. However, neither channel blocker conferred cardiomyocytes with protection from iron-induced apoptosis.Conclusion
Our study implicates LTCC as major mediators of Fe(III) uptake into cardiomyocytes exposed to ferric salts but not necessarily as contributors to ensuing apoptosis. Thus, to the extent that apoptosis can be considered a biological indicator of damage, the etiopathology of cardiosiderotic damage that accompanies some forms of hemosiderosis would seem to be unrelated to LTCC or TTCC, but rather to other routes of iron ingress present in heart cells. 相似文献11.
Background
Outer hair cells are the specialized sensory cells that empower the mammalian hearing organ, the cochlea, with its remarkable sensitivity and frequency selectivity. Sound-evoked receptor potentials in outer hair cells are shaped by both voltage-gated K+ channels that control the membrane potential and also ligand-gated K+ channels involved in the cholinergic efferent modulation of the membrane potential. The objectives of this study were to investigate the tonotopic contribution of BK channels to voltage- and ligand-gated currents in mature outer hair cells from the rat cochlea.Methodology/Principal
Findings In this work we used patch clamp electrophysiology and immunofluorescence in tonotopically defined segments of the rat cochlea to determine the contribution of BK channels to voltage- and ligand-gated currents in outer hair cells. Although voltage and ligand-gated currents have been investigated previously in hair cells from the rat cochlea, little is known about their tonotopic distribution or potential contribution to efferent inhibition. We found that apical (low frequency) outer hair cells had no BK channel immunoreactivity and little or no BK current. In marked contrast, basal (high frequency) outer hair cells had abundant BK channel immunoreactivity and BK currents contributed significantly to both voltage-gated and ACh-evoked K+ currents.Conclusions/Significance
Our findings suggest that basal (high frequency) outer hair cells may employ an alternative mechanism of efferent inhibition mediated by BK channels instead of SK2 channels. Thus, efferent synapses may use different mechanisms of action both developmentally and tonotopically to support high frequency audition. High frequency audition has required various functional specializations of the mammalian cochlea, and as shown in our work, may include the utilization of BK channels at efferent synapses. This mechanism of efferent inhibition may be related to the unique acetylcholine receptors that have evolved in mammalian hair cells compared to those of other vertebrates. 相似文献12.
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Taste buds are aggregates of 50–100 polarized neuroepithelial cells that detect nutrients and other compounds. Combined analyses of gene expression and cellular function reveal an elegant cellular organization within the taste bud. This review discusses the functional classes of taste cells, their cell biology, and current thinking on how taste information is transmitted to the brain.
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Taste: our most intrepid sense
Sampling the environment through our sense of taste.
Taste is the sensory modality that guides organisms to identify and consume nutrients while avoiding toxins and indigestible materials. For humans, this means recognizing and distinguishing sweet, umami, sour, salty, and bitter—the so-called “basic” tastes (Fig. 1). There are likely additional qualities such as fatty, metallic, and others that might also be considered basic tastes. Each of these is believed to represent different nutritional or physiological requirements or pose potential dietary hazards. Thus, sweet-tasting foods signal the presence of carbohydrates that serve as an energy source. Salty taste governs intake of Na+ and other salts, essential for maintaining the body’s water balance and blood circulation. We generally surmise that umami, the taste of l-glutamate and a few other l-amino acids, reflects a food’s protein content. These stable amino acids and nucleotide monophosphates are naturally produced by hydrolysis during aging or curing. Bitter taste is innately aversive and is thought to guard against consuming poisons, many of which taste bitter to humans. Sour taste signals the presence of dietary acids. Because sour taste is generally aversive, we avoid ingesting excess acids and overloading the mechanisms that maintain acid–base balance for the body. Moreover, spoiled foods often are acidic and are thus avoided. Nonetheless, people learn to tolerate and even seek out certain bitter- and sour-tasting compounds such as caffeine and citric acid (e.g., in sweet-tart citrus fruits), overcoming innate taste responses. Variations of taste preference may arise from genetic differences in taste receptors and may have important consequences for food selection, nutrition, and health (Drayna, 2005; Kim and Drayna, 2005; Dotson et al., 2008; Shigemura et al., 2009).Open in a separate windowFigure 1.Taste qualities, the taste receptors that detect them, and examples of natural stimuli. Five recognized taste qualities—sweet, sour, bitter, salty, and umami—are detected by taste buds. Bitter taste is thought to protect against ingesting poisons, many of which taste bitter. Sweet taste signals sugars and carbohydrates. Umami taste is elicited by l-amino acids and nucleotides. Salty taste is generated mainly by Na+ and sour taste potently by organic acids. Evidence is mounting that fat may also be detected by taste buds via dedicated receptors. The names of taste receptors and cartoons depicting their transmembrane topology are shown outside the perimeter. Bitter is transduced by G protein–coupled receptors similar to Class I GPCRs (with short extracellular N termini). In contrast, sweet and umami are detected by dimers of Class III GPCRs (with long N termini that form a globular extracellular ligand-binding domain). One of the receptors for Na+ salts is a cation channel composed of three subunits, each with two transmembrane domains. Membrane receptors for sour and fat are as yet uncertain.An important, if unrecognized aspect of taste is that it serves functions in addition to guiding dietary selection. Stimulating taste buds initiates physiological reflexes that prepare the gut for absorption (releasing digestive enzymes, initiating peristalsis, increasing mesenteric flow) and other organs for metabolic adjustments (insulin release, sympathetic activation of brown adipose tissue, increased heart rate; Giduck et al., 1987; Mattes, 1997). Collectively, these reflexes that are triggered by the sensory (sight, smell, taste) recognition of food are termed cephalic phase responses.| Glossary | |
| Afferent | Neuron or nerve fiber that carries signals from peripheral sensory receptors to the central nervous system. |
| Autocrine | Referring to the action of a transmitter or hormone onto the same cell from which it was secreted. |
| Ecto-ATPase | An enzyme that degrades extracellular ATP; associated with the extracellular face of the plasma membrane of some taste bud cells. |
| GPCR | G protein–coupled receptor; integral plasma membrane proteins with 7 transmembrane domains; detect and signal neurotransmitters, hormones, sensory and other stimuli. |
| Gustation | The sense of taste; beginning with excitation of cells in taste buds and leading to perception of taste qualities (sweet, bitter, etc.). |
| Gustducin | Heterotrimeric G protein that includes a taste-selective Gα subunit, α-gustducin. |
| Pannexin | A family of ion channels (Panx1, 2, 3) related to the gap junction–forming connexin proteins; pannexins may only form hemichannels and transfer molecules from cytoplasm to extracellular space. |
| Paracrine | Referring to the action of a transmitter or hormone onto cells adjacent to or near the cell from which it was secreted. |
| Sensory code | The pattern of action potentials in sensory nerves that denotes the quality, intensity, duration, etc., of a sensory stimulus. |
| Somatosensory | The sense of pain, temperature, touch, pressure, texture (and other tactile stimuli). |
| T1Rs | A family of taste GPCRs (T1R1, R2, R3) that detect sweet or umami tastants; they function as heterodimers, e.g., T1R2 plus T1R3. |
| T2Rs | A family of taste GPCRs that detect bitter tastants; there are 20–40 members in different species. |
| Tastants | Compounds that elicit taste. |
| Taste GPCR | Families of GPCRs that are expressed in taste bud cells and bind sweet, bitter, or umami tastants. |
| Umami taste | A Japanese term (“good taste”), used for the taste of certain amino acids (especially glutamate), nucleotides (esp. IMP, GMP). Roughly translates as “savory”. |
Diverse sensory inputs tickle our taste buds.
Taste is commonly confused with flavor, the combined sensory experience of olfaction and gustation. Gustatory signals originate in sensory end organs in the oral cavity—taste buds—and are triggered by water-soluble compounds that contact the apical tips of the epithelial cells of taste buds. In contrast, olfactory signals are generated by neurons in a specialized patch of nasal epithelium and are triggered by volatile compounds. Although the peripheral sensory organs for taste and smell are quite distinct, their signals are integrated in the orbitofrontal and other areas of the cerebral cortex to generate flavors and mediate food recognition (Rolls and Baylis, 1994; Small and Prescott, 2005).Taste is also commonly confused with somatosensory sensations such as the cool of menthol or the heat of chili peppers. Strictly speaking, gustation is the sensory modality generated when chemicals activate oral taste buds and transmit signals to a specific region of the brainstem (the rostral solitary nucleus). Capsaicin (the active compound in chilies) and menthol principally stimulate ion channels in somatosensory nerve fibers (Caterina et al., 1997; McKemy et al., 2002). Capsaicin and related compounds may stimulate important interactions between somatosensory trigeminal (cranial nerve V) nerve fibers in the tongue and taste buds, and thus modulate taste (Wang et al., 1995; Whitehead et al., 1999). Additional somatosensory modalities such as texture and visual cues such as color also significantly influence the “taste” of foods (Small and Prescott, 2005).Fatty taste lies at an intersection of somatosensory and gustatory perception. For many years, the recognition of dietary fat was considered primarily a function of its texture, and thus of somatosensory origin. Free fatty acids are potent gustatory stimuli (Gilbertson, 1998; Gilbertson et al., 2005; Laugerette et al., 2005). They are abundant in the human diet and, in some species, may be produced when salivary lipases rapidly hydrolyze ingested triglycerides in the oral cavity (Kawai and Fushiki, 2003). Specific membrane receptors essential for detecting fatty acids are present on taste bud cells (Laugerette et al., 2005; Sclafani et al., 2007; Wellendorph et al., 2009).Thus, fatty taste may also come to be recognized as another basic taste quality (Mattes, 2009).In this review, we address only the molecular recognition and cellular processing that occurs in oral taste buds and that is conveyed in gustatory afferent nerves. Many of the proteins that underlie transduction for sweet, bitter, and umami tastes are also expressed in sensory cells lining the stomach and intestine. Chemosensory cells in the gut detect amino acids, peptides, sugars, and bitter compounds and respond by locally releasing peptides (e.g., GLP-1). These cells may also stimulate the vagus nerve that sends signals from the gut to the brain (Rozengurt and Sternini, 2007; Kokrashvili et al., 2009b). Yet, it is unlikely that this information contributes to the conscious perception or discrimination of sweet, sour, salty, etc., tastes. These “taste-like” chemosensory cells, although interesting and likely important, are not discussed further.The structure of taste buds and other matters of taste
Taste buds are clusters of up to 100 polarized neuroepithelial cells that form compact, columnar pseudostratified “islands” embedded in the surrounding stratified epithelium of the oral cavity (Fig. 2 A). In humans, there are ∼5,000 taste buds in the oral cavity, situated on the superior surface of the tongue, on the palate, and on the epiglottis (Miller, 1995). Taste buds across the oral cavity serve similar functions. Although there are subtle regional differences in sensitivity to different compounds over the lingual surface, the oft-quoted concept of a “tongue map” defining distinct zones for sweet, bitter, salty, and sour has largely been discredited (Lindemann, 1999).Open in a separate windowFigure 2.Cell types and synapses in the taste bud. (A) Electron micrograph of a rabbit taste bud showing cells with dark or light cytoplasm, and nerve profiles (arrows). Asterisks mark Type II (receptor) cells. Reprinted with permission from J. Comp. Neurol. (Royer and Kinnamon, 1991). (B) A taste bud from a transgenic mouse expressing GFP only in receptor (Type II) cells. Presynaptic cells are immunostained (red) for aromatic amino acid decarboxylase (a neurotransmitter-synthesizing enzyme that is a marker for these cells), and are distinct from receptor cells, identified by GFP (green). Reprinted with permission from J. Neurosci. (C) Taste buds immunostained for NTPDase2 (an ectonucleotidase associated with the plasma membrane of Type I cells) reveal the thin lamellae (red) of Type I cells. These cytoplasmic extensions wrap around other cells in the taste bud. GFP (green) indicates receptor cells as in B. Bar, 10 µm. Image courtesy of M.S. Sinclair and N. Chaudhari. (D) High magnification electron micrograph of a synapse between a presynaptic taste cell and a nerve terminal (N) in a hamster taste bud. The nucleus (Nu) of the presynaptic cell is at the top, and neurotransmitter vesicles cluster near the synapse(s). The nerve profile includes mitochondria (m) and electron-dense postsynaptic densities. mt, microtubule. Image courtesy of J.C. Kinnamon.The elongate cells of taste buds are mature differentiated cells. Their apical tips directly contact the external environment in the oral cavity and thus experience wide fluctuations of tonicity and osmolarity, and the presence of potentially harmful compounds. Hence, taste bud cells, similar to olfactory neurons, comprise a continuously renewing population, quite unlike the sensory receptors for vision and hearing: photoreceptors and hair cells. It is now clear that adult taste buds are derived from local epithelium. At least some precursor cells are common between taste buds and the stratified nonsensory epithelium surrounding them (Stone et al., 1995; Okubo et al., 2009).Tight junctions connecting the apical tips of cells were noted in electron micrographs of taste buds from several species (Murray, 1973, 1993). Typical tight junction components such as claudins and ZO-1 are detected at the apical junctions (Michlig et al., 2007). Taste buds, like most epithelia, impede the permeation of water and many solutes through their intercellular spaces. Nevertheless, paracellular pathways through taste buds have been demonstrated for certain ionic and nonpolar compounds (Ye et al., 1991). Indeed, permeation of Na+ into the interstitial spaces within taste buds may contribute to the detection of salty taste (Simon, 1992; Rehnberg et al., 1993).Considering the strongly polarized shapes of taste cells, relatively few proteins have been shown to be partitioned into the apical membrane. Examples include aquaporin-5 (Watson et al., 2007) and a K channel, ROMK (Dvoryanchikov et al., 2009).Electron micrographs of taste buds reveal cells of varying electron densities that were interpreted as reflecting a continuum of stages of differentiation or maturation. However, precise morphometric analyses (i.e., electron density of cytoplasm, shape of nucleus, length and thickness of microvilli, and the presence of specialized chemical synapses) demonstrated that cells in taste buds were of discrete types (Murray, 1993; Pumplin et al., 1997; Yee et al., 2001). Ultrastructural features served as the basis for a reclassification of taste cells. Taste buds were described as containing cells imaginatively termed Types I, II, and III, and Basal, a nonpolarized, presumably undifferentiated cell, sometimes termed Type IV. What was missing was a convincing argument that these morphotypes represented distinct functional classes.Subsequently, investigators have probed taste buds with antibodies at both light and electron microscopic levels, thus associating a few protein markers with the ultrastructurally defined cell types. These markers included α-gustducin (a taste-selective Gα subunit involved in taste transduction) in Type II cells and SNAP25 (a core component of SNARE complexes that regulate exocytosis of synaptic vesicles) in Type III cells (Yang et al., 2000; Yee et al., 2001; Clapp et al., 2004). Immunostaining in pairwise combinations then expanded the numbers of taste-specific proteins that could be assigned exclusively to cells of Type I, II, or III. Fig. 2 B demonstrates the clear distinction between cell Types II and III, with few if any cells exhibiting an intermediate pattern of gene expression. Similarly, cell Types I and II are separate populations (Fig. 2 C). Type III cells are the only cells that exhibit well-differentiated synapses (Fig. 2 D). An important advance has been with the generation of transgenic mice with GFP expressed from promoters selectively active in Type II or III cells. This has allowed a precise integration between functional properties, morphological features, and gene expression patterns of the cell types within taste buds. For instance, by combining patch-clamp and immunostaining on tissues from such mice, Medler et al. (2003) showed that voltage-gated Ca2+ currents, de rigeur components of synapses, are limited to the Type III cells. In contrast, Ca2+ imaging in combination with transgenic markers demonstrated that Type II cells respond to sweet, bitter, or umami taste stimuli while lacking voltage-gated Ca channels (Clapp et al., 2006; DeFazio et al., 2006).Type I cells.
Type I cells are the most abundant cells in taste buds, with extended cytoplasmic lamellae that engulf other cells (Fig. 2 C). Type I cells express GLAST, a transporter for glutamate, indicating that they may be involved in glutamate uptake (Lawton et al., 2000). Type I cells also express NTPDase2, a plasma membrane–bound nucleotidase that hydrolyzes extracellular ATP (Bartel et al., 2006). ATP serves as a neurotransmitter in taste buds (Finger et al., 2005) and glutamate also is a candidate neurotransmitter. Thus, Type I cells appear to be involved in terminating synaptic transmission and restricting the spread of transmitters, a role performed in the central nervous system by glial cells.Type I cells also express ROMK, a K channel that may be involved in K+ homeostasis within the taste bud (Dvoryanchikov et al., 2009). During prolonged trains of action potentials elicited by intense taste stimulation, Type I cells may serve to eliminate K+ (see blue cell in Fig. 3) that would accumulate in the limited interstitial spaces of the taste bud and lead to diminished excitability of Type II and III cells. This is another stereotypic glial function. Patch-clamp studies have suggested that some taste cells, presumably Type I cells, possesses electrophysiological properties, such as inexcitability and high resting k+ conductance, also characteristic of glia (Bigiani, 2001). Thus, Type I cells appear overall to function as glia in taste buds. A caveat is that not all Type I cells necessarily participate in each of the glial roles described above.Open in a separate windowFigure 3.The three major classes of taste cells. This classification incorporates ultrastructural features, patterns of gene expression, and the functions of each of Types I, II (receptor), and III (presynaptic) taste cells. Type I cells (blue) degrade or absorb neurotransmitters. They also may clear extracellular K+ that accumulates after action potentials (shown as bursts) in receptor (yellow) and presynaptic (green) cells. K+ may be extruded through an apical K channel such as ROMK. Salty taste may be transduced by some Type I cells, but this remains uncertain. Sweet, bitter, and umami taste compounds activate receptor cells, inducing them to release ATP through pannexin1 (Panx1) hemichannels. The extracellular ATP excites ATP receptors (P2X, P2Y) on sensory nerve fibers and on taste cells. Presynaptic cells, in turn, release serotonin (5-HT), which inhibits receptor cells. Sour stimuli (and carbonation, not depicted) directly activate presynaptic cells. Only presynaptic cells form ultrastructurally identifiable synapses with nerves. Tables below the cells list some of the proteins that are expressed in a cell type–selective manner.Lastly, Type I cells may exhibit ionic currents implicated in salt taste transduction (Vandenbeuch et al., 2008). Despite their being the most abundant cell type in taste buds, the least is known about Type I cells.Type II (receptor) cells.
There is little ambiguity in how Type II cells function within taste buds. Embedded in the plasma membrane of these cells are receptors that bind sweet, bitter, or umami compounds. These taste receptors are G protein–coupled receptors with seven transmembrane domains. Signaling events downstream of these receptors are well documented and are discussed under “Transduction” below (for review see Margolskee, 2002; Breslin and Huang, 2006; Simon et al., 2006). In addition, Type II cells express voltage-gated Na and K channels essential for producing action potentials, and hemichannel subunits, key players in taste-evoked secretion of ATP (yellow cell in Fig. 3). Any given Type II cell expresses taste GPCRs specific for only one taste quality, such as sweet or bitter, but not both (Nelson et al., 2001). Correspondingly, a given receptor cell responds only to stimulation with ligands that activate those receptors. In brief, Type II cells are “tuned” to sweet, bitter, or umami taste (Tomchik et al., 2007). In recognition of their role as the primary detectors of these classes of tastants, Type II cells were renamed “receptor” cells (DeFazio et al., 2006). Type II cells do not appear to be directly stimulated by sour or salty stimuli.Curiously, receptor cells do not form ultrastructurally identifiable synapses. Instead, nerve fibers, presumably gustatory afferents, are closely apposed to these cells (Murray, 1973, 1993; Yang et al., 2000; Yee et al., 2001; Clapp et al., 2004). Signals transmitted from receptor cells to sensory afferents or other cells within the taste bud must do so by unconventional mechanisms, i.e., without the involvement of synaptic vesicles, as will be described below.Type III (presynaptic) cells.
The consensus is that Type III cells (green cell in Fig. 3) express proteins associated with synapses and that they form synaptic junctions with nerve terminals (Murray et al., 1969; Murray, 1973, 1993; Yang et al., 2000; Yee et al., 2001). These cells express a number of neuronal-like genes including NCAM, a cell surface adhesion molecule, enzymes for the synthesis of at least two neurotransmitters, and voltage-gated Ca channels typically associated with neurotransmitter release (DeFazio et al., 2006; Dvoryanchikov et al., 2007). Type III cells, expressing synaptic proteins and showing depolarization-dependent Ca2+ transients typical of synapses, have been labeled “presynaptic” cells (DeFazio et al., 2006). Like receptor cells, presynaptic cells also are excitable and express a complement of voltage-gated Na and K channels to support action potentials (Medler et al., 2003; Gao et al., 2009; Vandenbeuch and Kinnamon, 2009a,b). The origin of nerve fibers that synapse with Type III cells, and whether they represent taste afferents, is not known. In addition to these neuronal properties, presynaptic cells also respond directly to sour taste stimuli and carbonated solutions and are presumably the cells responsible for signaling these sensations (Huang et al., 2006; Tomchik et al., 2007; Huang et al., 2008b; Chandrashekar et al., 2009).A key feature of presynaptic cells is that they receive input from and integrate signals generated by receptor cells (see below). Hence, in the intact taste bud, unlike receptor cells, presynaptic cells are not tuned to specific taste qualities but instead respond broadly to sweet, salty, sour, bitter, and umami compounds (Tomchik et al., 2007). Although presynaptic cells share many neuron-like properties, it is clear that they are not a homogeneous population (Tomchik et al., 2007; Roberts et al., 2009).Basal cells.
This category describes spherical or ovoid cells that do not extend processes into the taste pore and are likely to be undifferentiated or immature taste cells (Farbman, 1965). It is not clear whether all basal cells within taste buds represent a common undifferentiated class of cells. Unambiguous markers for these cells have not been identified, and the exact significance of basal cells as a cell population remains to be elucidated.Nerve fibers.
Taste buds are innervated by sensory neurons whose cell bodies are located in clusters nestled against the brain (the geniculate, petrosal, and nodose cranial ganglia). In the adult, each taste bud is innervated by 3–14 sensory ganglion neurons, depending on the species (mouse, rat, hamster) and oral region (tongue, palate; Krimm and Hill, 1998; Whitehead et al., 1999). Gustatory nerve fibers comingle with a rich plexus of other nerve fibers under the taste epithelium. In the absence of clear markers to distinguish them, one cannot discern which of these fibers carry taste information as opposed to pain, tactile, or thermal signals. Taste axons branch and penetrate the basal lamina to enter taste buds. Although some fibers terminate in synaptic structures on Type III cells, others course intimately among taste cells without forming specialized synapses (Farbman, 1965; Murray et al., 1969; Murray, 1973).As will be explained next, the concerted action of Type I, Type II (receptor), and Type III (presynaptic) cells underlies taste reception. There are synaptic interactions, both feed-forward and feedback, between these cells when taste stimuli activate the taste bud.Beyond the tasty morsel: the underlying molecular mechanisms for nutrient detection
Transduction of gustatory stimuli in receptor (Type II) cells.
As stated above, sweet, umami, and bitter compounds each activate different taste GPCRs that are expressed in discrete sets of receptor cells. For instance, receptor cells that express members of the T2R family of GPCRs sense bitter compounds (Chandrashekar et al., 2000). In different mammals, 20–35 separate genes encode members of the T2R family. These taste receptors exhibit heterogeneous molecular receptive ranges: some are narrowly tuned to 2–4 bitter-tasting compounds, whereas others are promiscuously activated by numerous ligands (Meyerhof et al., 2010). On the basis of in situ hybridizations with mixed probes on rodent taste buds, the T2Rs were reported either to be expressed as overlapping subsets of mRNAs (Matsunami et al., 2000) or coexpressed in a single population of taste cells (Adler et al., 2000). More recently, detailed analyses on human taste buds confirm that different bitter-responsive taste cells express subsets of 4–11 of the T2Rs in partially overlapping fashion (Behrens et al., 2007). This observation is important insofar as it provides a molecular basis for discriminating between different bitter compounds. Bitter-sensing taste cells are known to functionally discriminate among bitter compounds (Caicedo and Roper, 2001). This pattern of T2R expression, along with polymorphisms across the gene family, is thought to allow humans and animals to detect the enormous range of potentially toxic bitter compounds found in nature (Drayna, 2005).Receptor cells expressing the heterodimer T1R2+T1R3 respond to sugars, synthetic sweeteners, and sweet-tasting proteins such as monellin and brazzein (Nelson et al., 2001; Jiang et al., 2004; Xu et al., 2004). Although the persistence of sensitivity to some sugars in mice lacking T1R3 suggests that additional receptors for sweet may exist (Damak et al., 2003), candidate receptors have yet to be identified.A third class of receptor cells expresses the heterodimeric GPCR, T1R1+T1R3, which responds to umami stimuli, particularly the combination of l-glutamate and GMP/IMP, compounds that accumulate in many foods after hydrolysis of proteins and NTPs (Li et al., 2002; Nelson et al., 2002). Nevertheless, robust physiological responses and behavioral preference for umami tastants persist in mice in which T1R3 is knocked out, suggesting that additional taste receptors may contribute to umami detection (Damak et al., 2003; Maruyama et al., 2006; Yasumatsu et al., 2009). Functional responses to various umami tastants occur in distinct subsets of cells within taste buds (Maruyama et al., 2006) and neural responses show similarly heterogeneous patterns (Yoshida et al., 2009b), observations that further suggest that umami taste is complex, and likely mediated through multiple types of taste receptors. In summary, although the T1R1+T1R3 dimer clearly acts as an umami receptor, additional GPCRs may play complementary roles. Candidates for additional umami receptors include a taste-specific variant or other isoforms of G protein–coupled glutamate receptors expressed in taste buds (Chaudhari et al., 2000; Li et al., 2002; Nelson et al., 2002; San Gabriel et al., 2009).The T1Rs are dimeric Class III GPCRs, with large N-terminal extracellular domains (Max et al., 2001). This domain forms a Venus Flytrap structure as in other family members. T1Rs also possess a multitude of additional ligand-binding sites on the exterior faces of the flytrap, in the linker, and perhaps even in the plane of the membrane (Cui et al., 2006; Temussi, 2009). In contrast, T2Rs resemble Class I GPCRs with binding sites in the transmembrane helices, in keeping with the nonpolar nature of many bitter ligands (Floriano et al., 2006).When they bind taste molecules, taste GPCRs activate heterotrimeric GTP-binding proteins (Fig. 4 A). For example, the bitter receptors (T2Rs) are coexpressed with and activate the taste-selective Gα subunit, α-gustducin, and the closely related α-transducin (Ruiz-Avila et al., 1995). Taste receptors that include T1R3 may couple to Gα14 and other Gα subunits (Tizzano et al., 2008). Despite this apparent selectivity of taste GPCRs for Gα subunits, the principal pathway for taste transduction appears to be via Gβγ, including Gγ13 and Gβ1 or Gβ3 (Huang et al., 1999). Upon ligand binding, the Gβγ subunits are freed from the taste GPCR and interact functionally with a phospholipase, PLCβ2, an unusual isoform that is activated by Gβγ rather than the more common Gαq family subunits (Rössler et al., 1998). Knocking out PLCβ2 severely diminishes, but does not eliminate taste sensitivity (Zhang et al., 2003; Dotson et al., 2005). PLCβ2 stimulates the synthesis of IP3, which opens IP3R3 ion channels on the endoplasmic reticulum, releasing Ca2+ into the cytosol of receptor cells (Simon et al., 2006; Roper, 2007). The elevated intracellular Ca2+ appears to have two targets in the plasma membrane: a taste-selective cation channel, TRPM5, and a gap junction hemichannel, both found in receptor cells (Pérez et al., 2002; Huang et al., 2007). The Ca2+-dependent opening of TRPM5 produces a depolarizing generator potential in receptor cells (Liu and Liman, 2003). If sufficiently large, generator potentials evoke action potentials in receptor cells. The two signals elicited by tastants: strong depolarization and increased cytoplasmic Ca2+, are integrated by gap junction hemichannels. The outcome of this convergence is that the taste bud transmitter, ATP, and possibly other molecules, are secreted through the hemichannel pores into the extracellular space surrounding the activated receptor cell (Fig. 3, yellow cell; and Fig. 4 A; Huang et al., 2007; Romanov et al., 2007; Huang and Roper, 2010).Open in a separate windowFigure 4.Mechanisms by which five taste qualities are transduced in taste cells. (A) In receptor (Type II) cells, sweet, bitter, and umami ligands bind taste GPCRs, and activate a phosphoinositide pathway that elevates cytoplasmic Ca2+ and depolarizes the membrane via a cation channel, TrpM5. The combined action of elevated Ca2+ and membrane depolarization opens the large pores of gap junction hemichannels, likely composed of Panx1, resulting in ATP release. Shown here is a dimer of T1R taste GPCRs (sweet, umami). T2R taste GPCRs (bitter) do not have extensive extracellular domains and it is not known whether T2Rs form multimers. (B) In presynaptic (Type III) cells, organic acids (HAc) permeate through the plasma membrane and acidify the cytoplasm where they dissociate to acidify the cytosol. Intracellular H+ is believed to block a proton-sensitive K channel (as yet unidentified) and depolarize the membrane. Voltage-gated Ca channels would then elevate cytoplasmic Ca2+ to trigger exocytosis of synaptic vesicles (not depicted). (C) The salty taste of Na+ is detected by direct permeation of Na+ ions through membrane ion channels, including ENaC, to depolarize the membrane. The cell type underlying salty taste has not been definitively identified.Although most researchers agree that ATP release occurs through a plasma membrane hemichannel, whether these channels are formed of pannexin (Panx) or connexin (Cx) subunits is not fully resolved. Panx1 is robustly expressed in receptor cells, whereas several Cx subunits are expressed at more modest levels (Huang et al., 2007; Romanov et al., 2007). Although there may be gap junctions presumably formed of connexins between cells in mammalian taste buds (Yoshii, 2005), such junctions would not be expected to secrete ATP into extracellular spaces. A principal argument for Cx hemichannels in taste cells was based on the blocking action of certain isoform-selective mimetic peptides. However, the specificity of such peptides has recently been called into question (Wang et al., 2007). Finally, Panx1 hemichannels are gated open by elevated cytoplasmic Ca2+ and/or membrane depolarization (Locovei et al., 2006). ATP release from taste cells similarly is mediated by both Ca2+ and voltage (Huang and Roper, 2010). In contrast, Cx hemichannels usually open only in the absence of extracellular Ca2+ and typically are blocked by elevated cytoplasmic Ca2+. Further, Panx1-selective antagonists block taste-evoked ATP secretion (Huang et al., 2007; Dando and Roper, 2009). Thus, the weight of the evidence strongly favors ATP release through Panx1 hemichannels in receptor cells. Nevertheless, the ideal test to resolve this question, namely testing ATP release from taste cells from Panx1 or Cx knockout mice, has yet to be reported.Presynaptic (Type III) cells also detect some taste stimuli.
Presynaptic cells exhibit very different taste sensitivity and transduction mechanisms when compared with receptor cells. Sour taste stimuli (acids) excite presynaptic cells (Tomchik et al., 2007). The membrane receptor or ion channel that transduces acid stimuli remains as yet unidentified. Non-selective cation channels formed by PKD2L1 and PKD1L3 were proposed as candidate sour taste receptors (Huang et al., 2006; Ishimaru et al., 2006; LopezJimenez et al., 2006). Yet, this channel is sensitive to extracellular pH rather than a drop in cytoplasmic pH, which is known to be the proximate stimulus for sour taste (Fig. 4 B; Lyall et al., 2001; Huang et al., 2008b). Further, mice lacking PKD1L3 remain capable of detecting acid taste stimuli (Nelson et al., 2010). More likely candidate acid receptors in Type III cells are plasma membrane channels that are modulated by cytoplasmic acidification, such as certain K channels (Lin et al., 2004; Richter et al., 2004). Presynaptic cells also detect carbonation, partly through the action of carbonic anhydrase that produces protons and thus acidifies the environment (Graber and Kelleher, 1988; Simons et al., 1999; Chandrashekar et al., 2009). The complete transduction pathways for carbonation and sour taste have not been completely described.Salt detection and transduction.
Taste buds detect Na salts by directly permeating Na+ through apical ion channels and depolarizing taste cells. An ion channel that has long been thought to mediate this action is the amiloride-sensitive epithelial Na channel, ENaC (Fig. 4 C; Heck et al., 1984; Lin et al., 1999; Lindemann, 2001). This notion was recently confirmed by knocking out a critical ENaC subunit in taste buds, which impaired salt taste detection (Chandrashekar et al., 2010). This study did not assign salt sensitivity to any of the established taste cell types, but patch-clamp studies suggested that Na+-detecting cells are Type I cells (Vandenbeuch et al., 2008). Pharmacological and other evidence suggests that salt transduction in human and animal models also occurs via additional membrane receptors or ion channels. Although a modified TrpV1 channel has been proposed as a candidate Na+ taste transducer, knockout mice show a minimal phenotype with respect to salt detection (Ruiz et al., 2006; Treesukosol et al., 2007).Information processing and cell-to-cell signaling in taste buds: teasing apart our taste response
Transmitters and information flow.
Receptor and presynaptic cells each release different neurotransmitters (Huang et al., 2007). To date, receptor cells are known to release only ATP, via pannexin channels as described above. Presynaptic cells on the other hand, secrete serotonin (5-HT) and norepinephrine (NE). In some instances presynaptic cells co-release both these amines (Dvoryanchikov et al., 2007; Huang et al., 2008a). Secretion of these biogenic amines appears to be via conventional Ca2+-dependent exocytosis. Clusters of monoaminergic vesicles are present at synapses in electron micrographs of mouse presynaptic cells (Takeda and Kitao, 1980).Gustatory stimuli initiate a sequence of chemical signals that are passed between cells in the taste bud. When sweet, bitter, or umami tastants excite taste buds, ATP secreted from receptor cells stimulates gustatory afferent nerve fibers. At the same time, ATP also excites adjacent presynaptic cells and stimulates them to release 5-HT and/or NE. ATP secreted during taste stimulation has a third target, namely the receptor cells, themselves. ATP, acting as an autocrine transmitter, exerts positive feedback onto receptor cells, increasing its own secretion and presumably counteracting its degradation by ecto-ATPase (Huang et al., 2009; Fig. 3).The 5-HT released by presynaptic cells also may have multiple targets. One effect of 5-HT is to inhibit receptor cells. That is, 5-HT exerts a negative feedback onto receptor cells. The opposing effects of positive (purinergic autocrine) and negative (serotonergic paracrine) feedback in the taste bud during gustatory activation combine to shape the signals transmitted from taste buds to the hindbrain. However, details of how these feedback pathways are balanced to shape the eventual sensory output awaits experimentation and many questions remain. One might speculate that 5-HT mediates “lateral inhibition,” suppressing the output of adjacent receptor (e.g., bitter) cells when a particular (e.g., sweet) receptor cell is stimulated. Alternatively, the negative feedback loop may participate in sensory adaptation by decreasing the afferent signal over time.Other sites of action for 5-HT (and NE) possibly include the nerve fibers that form synapses with presynaptic taste cells. Quite possibly, there are parallel purinergic and serotonergic outputs from taste buds and parallel information pathways leading into the hindbrain. At present, this is only a speculation (Roper, 2009).In summary (see Fig. 3), receptor cells detect and discriminate sweet, bitter, or umami tastants, generate Ca2+ signals, and release ATP transmitter onto afferent nerves. The ATP from different receptor cells converges onto and produces secondary excitation of presynaptic cells, thereby integrating signals representing all three taste qualities (Tomchik et al., 2007). It is not clear that the secondary responses of presynaptic cells to sweet, bitter, and umami stimuli are necessary for identifying or discriminating these taste qualities. Primary signals in presynaptic cells are only generated by sour tastants, and this is the only quality that is lost when presynaptic cells are ablated (Huang et al., 2006).Cracking the taste code.
Taste afferent nerve fibers transmit information from taste buds to the brain. How the activation of receptor and presynaptic cells during gustatory stimulation translates into a neural code that specifies different taste qualities (sweet, bitter, etc.) remains unclear. Two opposing solutions to this logic problem are much discussed. On the one hand, dedicated nerve fibers (“labeled lines”) could transmit each quality, e.g., “bitter” cells, “bitter” fibers, and “bitter” neurons at each successive relay in the brain. On the other hand, a combinatorial system would have qualities encoded by patterns of activity across several fibers. In the latter case, any given fiber could transmit information for more than a single quality. A third, less-discussed option is a temporal code in which quality would be denoted by a timing pattern of action potentials such as occurs in auditory fibers.The question of coding has been addressed through genetic manipulations and physiological and behavioral assays. Electrophysiological recordings from single afferent fibers or their parent sensory ganglion cells indicated that some neurons respond strongly to a single taste quality (usually sweet), but also have weak responses to other tastes. In contrast, other afferent neurons are excited by multiple tastes, i.e., are broadly responsive (Hellekant et al., 1997; Frank et al., 2008; Breza et al., 2010). Thus, afferent taste neurons show response profiles similar to both narrowly tuned taste bud receptor cells and to broadly tuned presynaptic cells. The pattern of afferent neuron activity mirrors the heterogeneity of taste bud cellular responses (Gilbertson et al., 2001; Caicedo et al., 2002; Tomchik et al., 2007; Yoshida et al., 2009a; Breza et al., 2010) and suggests that neural activity encoding taste does not follow a simple dedicated labeled-line logic. That is, “bitter-specific,” “sour-specific,” etc., afferent sensory fibers and subsequent neurons in the network—obligatory components of labeled-line coding—have never been reported.An argument for labeled-line coding has been made based on the results of replacing a modified opioid receptor for the bitter or sweet receptors in taste cells (Zhao et al., 2003; Mueller et al., 2005). Mice engineered with this foreign receptor in “sweet” receptor cells strongly preferred and copiously drank solutions of a synthetic ligand for the modified receptor, as if the compound tasted sweet. For normal mice, the ligand was tasteless. Conversely, when the opioid receptor was targeted to “bitter” receptor cells, the same ligand was strongly aversive. Although this was presented as firm proof of labeled-line coding, the logic bears reexamining. Take for example a computer keyboard. Striking the “A” key activates a combination of electronic signals that results in the illumination of a combination of pixels to produce the first letter of the alphabet on screen. If the plastic key (the “receptor”) on the keyboard were changed, striking the replacement key would still produce the letter “A” on screen. The experiment does not inform one about the electronic coding that is out of sight between the two visible events, and does not imply that labeled wires link the base of the key to particular pixels. Chemosensory researchers agree that labeled taste cells exist. Labeled lines remain controversial.In summary, sweet, bitter, and umami cells all secrete the same neurotransmitter, ATP, onto afferent fibers. Discrete synapses are lacking that might couple receptor cells with sensory afferent fibers to transmit single taste qualities. Although some taste cells and sensory afferent neurons are tightly tuned, others are responsive to multiple taste qualities. Thus, it remains an open question exactly how information gathered by well-differentiated receptor cells in taste buds is “coded” for the eventual perception of distinct taste qualities.Future directions in taste research
Taste research, although making tremendous strides in recent years, has exposed major gaps in our understanding. Among the open questions is the molecular identification of additional taste receptors. Known taste receptors do not account for all ligands and sensory characteristics for sweet and umami tastes. It is likely that there are additional, undiscovered sweet and umami receptors (Chaudhari et al., 2009). There is also the question of transduction mechanisms for some of the less-studied qualities such as sour, fatty, metallic, and astringent. Solving these may require combining molecular and population genetic analyses on human or mouse populations along with more conventional expression studies.Another area of intense investigation is how gustatory signals are encoded by the nervous system. The principles of sensory coding from the retina to visual cortex were elucidated decades ago. We have a sound understanding of tonotopic and computational maps for the auditory system. Lateral inhibition and somatosensory receptive fields are well defined. Comparable insights into taste are lacking and we still do not understand how the brain distinguishes sweet, sour, salty, and so forth. If taste does not follow a simple labeled-line code, how are gustatory signals transmitted and deciphered? Ongoing studies include the possibility that taste is encoded in the time domain, i.e., by the frequency and pattern of action potentials in hindbrain and cortical neurons (Di Lorenzo et al., 2009; Miller and Katz, 2010). Other laboratories are exploring higher-order cortical processing via functional magnetic resonance imaging to address the interaction between taste detection, preference, and appetite regulation (Rolls, 2006; Small et al., 2007; Accolla and Carleton, 2008).A critical chasm in our understanding of taste is how gustatory mechanisms are linked to mood, appetite, obesity, and satiety. The obvious link is that taste guides and to a large extent determines food selection, with salty, sweet, and fat tastes being the main actors. A fascinating link between appetite and moods is that serotonin-enhancing drugs, commonly used for treating mood disorders and depression, were shown to influence taste thresholds (Heath et al., 2006). Whether the mechanism of this action depends on the inhibitory action of 5-HT in taste buds remains to be determined, but the findings are intriguing (Kawai et al., 2000).Cracks in the hard nut of appetite regulation are exposing a new dimension of taste—the impact of appetite-regulating hormones on peripheral gustatory sensory organs. A number of neuropeptide hormones activate hypothalamic and hindbrain circuits that regulate appetite. We are now learning that several of these same peptide hormones, including leptin, glucagon-like peptide, and oxytocin, modulate chemosensory transduction at the level of the taste bud. Circulating leptin, acting directly on taste receptor cells, reduces sweet responses measured in taste buds, in afferent nerves, and by behavioral tests (Kawai et al., 2000; Nakamura et al., 2008). Circulating oxytocin, another anorectic peptide, also acts on taste buds (Sinclair et al., 2010). Blood-delivered satiety peptides may be ideal candidates for integrating sensory and motivational drivers of appetite. Additional satiety peptides, including glucagon-like peptide-1, are synthesized within taste buds and act on taste cells or nerves (Shin et al., 2008). This research might provide avenues into therapeutic approaches for obesity, and at a minimum further help explain the seemingly insatiable human drive to consume calories.Finally, another new direction for taste research is the presence of taste receptors and their downstream intracellular effectors in sensory cells of the gut (Rozengurt and Sternini, 2007; Kokrashvili et al., 2009a). The existence of these “taste” mechanisms in the gut is perhaps not surprising, given the importance of sensing the chemical nature of luminal contents at all points along the GI tract. However, the findings have generated new excitement in understanding how the gut participates in detecting and controlling appetite in general, and digestive processes in particular. 相似文献14.
Expression of Genes Encoding Multi-Transmembrane Proteins in Specific Primate Taste Cell Populations
Bryan D. Moyer Peter Hevezi Na Gao Min Lu Dalia Kalabat Hortensia Soto Fernando Echeverri Bianca Laita Shaoyang Anthony Yeh Mark Zoller Albert Zlotnik 《PloS one》2009,4(12)
Background
Using fungiform (FG) and circumvallate (CV) taste buds isolated by laser capture microdissection and analyzed using gene arrays, we previously constructed a comprehensive database of gene expression in primates, which revealed over 2,300 taste bud-associated genes. Bioinformatics analyses identified hundreds of genes predicted to encode multi-transmembrane domain proteins with no previous association with taste function. A first step in elucidating the roles these gene products play in gustation is to identify the specific taste cell types in which they are expressed.Methodology/Principal Findings
Using double label in situ hybridization analyses, we identified seven new genes expressed in specific taste cell types, including sweet, bitter, and umami cells (TRPM5-positive), sour cells (PKD2L1-positive), as well as other taste cell populations. Transmembrane protein 44 (TMEM44), a protein with seven predicted transmembrane domains with no homology to GPCRs, is expressed in a TRPM5-negative and PKD2L1-negative population that is enriched in the bottom portion of taste buds and may represent developmentally immature taste cells. Calcium homeostasis modulator 1 (CALHM1), a component of a novel calcium channel, along with family members CALHM2 and CALHM3; multiple C2 domains; transmembrane 1 (MCTP1), a calcium-binding transmembrane protein; and anoctamin 7 (ANO7), a member of the recently identified calcium-gated chloride channel family, are all expressed in TRPM5 cells. These proteins may modulate and effect calcium signalling stemming from sweet, bitter, and umami receptor activation. Synaptic vesicle glycoprotein 2B (SV2B), a regulator of synaptic vesicle exocytosis, is expressed in PKD2L1 cells, suggesting that this taste cell population transmits tastant information to gustatory afferent nerve fibers via exocytic neurotransmitter release.Conclusions/Significance
Identification of genes encoding multi-transmembrane domain proteins expressed in primate taste buds provides new insights into the processes of taste cell development, signal transduction, and information coding. Discrete taste cell populations exhibit highly specific gene expression patterns, supporting a model whereby each mature taste receptor cell is responsible for sensing, transmitting, and coding a specific taste quality. 相似文献15.
Nurdan ?zkucur Thomas K. Monsees Srikanth Perike Hoa Quynh Do Richard H. W. Funk 《PloS one》2009,4(7)
Background
Investigation of the mechanisms of guided cell migration can contribute to our understanding of many crucial biological processes, such as development and regeneration. Endogenous and exogenous direct current electric fields (dcEF) are known to induce directional cell migration, however the initial cellular responses to electrical stimulation are poorly understood. Ion fluxes, besides regulating intracellular homeostasis, have been implicated in many biological events, including regeneration. Therefore understanding intracellular ion kinetics during EF-directed cell migration can provide useful information for development and regeneration.Methodology/Principal Findings
We analyzed the initial events during migration of two osteogenic cell types, rat calvarial and human SaOS-2 cells, exposed to strong (10–15 V/cm) and weak (≤5 V/cm) dcEFs. Cell elongation and perpendicular orientation to the EF vector occurred in a time- and voltage-dependent manner. Calvarial osteoblasts migrated to the cathode as they formed new filopodia or lamellipodia and reorganized their cytoskeleton on the cathodal side. SaOS-2 cells showed similar responses except towards the anode. Strong dcEFs triggered a rapid increase in intracellular calcium levels, whereas a steady state level of intracellular calcium was observed in weaker fields. Interestingly, we found that dcEF-induced intracellular calcium elevation was initiated with a local rise on opposite sides in calvarial and SaOS-2 cells, which may explain their preferred directionality. In calcium-free conditions, dcEFs induced neither intracellular calcium elevation nor directed migration, indicating an important role for calcium ions. Blocking studies using cadmium chloride revealed that voltage-gated calcium channels (VGCCs) are involved in dcEF-induced intracellular calcium elevation.Conclusion/Significance
Taken together, these data form a time scale of the morphological and physiological rearrangements underlying EF-guided migration of osteoblast-like cell types and reveal a requirement for calcium in these reactions. We show for the first time here that dcEFs trigger different patterns of intracellular calcium elevation and positional shifting in osteogenic cell types that migrate in opposite directions. 相似文献16.
17.
18.
Background
Caffeine stimulates calcium-induced calcium release (CICR) in many cell types. In neurons, caffeine stimulates CICR presynaptically and thus modulates neurotransmitter release.Methodology/Principal Findings
Using the whole-cell patch-clamp technique we found that caffeine (20 mM) reversibly increased the frequency and decreased the amplitude of miniature excitatory postsynaptic currents (mEPSCs) in neocortical neurons. The increase in mEPSC frequency is consistent with a presynaptic mechanism. Caffeine also reduced exogenously applied glutamate-activated currents, confirming a separate postsynaptic action. This inhibition developed in tens of milliseconds, consistent with block of channel currents. Caffeine (20 mM) did not reduce currents activated by exogenous NMDA, indicating that caffeine block is specific to non-NMDA type glutamate receptors.Conclusions/Significance
Caffeine-induced inhibition of mEPSC amplitude occurs through postsynaptic block of non-NMDA type ionotropic glutamate receptors. Caffeine thus has both pre and postsynaptic sites of action at excitatory synapses. 相似文献19.
20.
Melissa A Kovach Kathleen A Stringer Rachel Bunting Xiaoying Wu Lani San Mateo Michael W Newstead Robert Paine III Theodore J Standiford 《Respiratory research》2015,16(1)