InteractiveFly: GeneBrief

Transmembrane channel-like: Biological Overview | References

Gene name - Transmembrane channel-like

Synonyms -

Cytological map position - 67C2-67C2

Function - mechanosensitive ion channel

Keywords - larval proprioception - defensive response to mechanical stimuli - Tmc cells undergo restructuring engendering an enhanced response to touching in females following mating - critical for sensing subtle differences in substrate stiffness during ovoposition - required for sensing two key textural features of food-hardness and viscosity - presumably activated by membrane curvature in dendrites that are exposed to strain

Symbol - Tmc

FlyBase ID: FBgn0267796

Genetic map position - chr3L:9,597,764-9,614,001

NCBI classification - TMC domain

Cellular location - surface transmembrane

NCBI links: EntrezGene, Nucleotide, Protein

Tmc orthologs: Biolitmine

Drosophila Transmembrane channel-like (Tmc) is a protein that functions in larval proprioception. The closely related TMC1 protein is required for mammalian hearing and is a pore-forming subunit of the hair cell mechanotransduction channel. In hair cells, TMC1 is gated by small deflections of microvilli that produce tension on extracellular tip-links that connect adjacent villi. How Tmc might be gated in larval proprioceptors, which are neurons having a morphology that is completely distinct from hair cells, is unknown. This study has used high-speed confocal microscopy both to measure displacements of proprioceptive sensory dendrites during larval movement and to optically measure neural activity of the moving proprioceptors. Unexpectedly, the pattern of dendrite deformation for distinct neurons was unique and differed depending on the direction of locomotion: ddaE neuron dendrites were strongly curved by forward locomotion, while the dendrites of ddaD were more strongly deformed by backward locomotion. Furthermore, GCaMP6f calcium signals recorded in the proprioceptive neurons during locomotion indicated tuning to the direction of movement. ddaE showed strong activation during forward locomotion, while ddaD showed responses that were strongest during backward locomotion. Peripheral proprioceptive neurons in animals mutant for Tmc showed a near-complete loss of movement related calcium signals. As the strength of the responses of wild-type animals was correlated with dendrite curvature, it is proposed that Tmc channels may be activated by membrane curvature in dendrites that are exposed to strain. These findings begin to explain how distinct cellular systems rely on a common molecular pathway for mechanosensory responses (He, 2019).

For stimuli in motion, sensory systems must encode the direction of movement. This is perhaps best studied in the visual system, where neurons in the vertebrate and invertebrate retina are activated by moving edges in a visual scene. In the retina, specific neurons are tuned to be activated by stimuli moving in a preferred direction but are inhibited by stimuli with non-preferred motion. More poorly understood is how mechanosensory systems might encode the direction of movement. Nevertheless, direction selectivity has been observed in several mechanosensory systems. In the best-understood example, the hair cells of the inner ear show a preferred mechanosensory response when the actin-rich bundles of stereocilia are displaced toward the microvilli on the taller side of the bundle. Another example is found in the neurons that innervate the mechanosensory bristles of adult Drosophila. These neurons are activated by forces that displace the bristle toward the body, but not by displacements away from the body. Similarly, texture sensing in the adult fly proboscis involves a directional deflection of taste bristles that depends on Drosophila Tmc (Zhang, 2016). Low-threshold mechanoreceptors with lanceolate endings that innervate hair follicles in the mouse respond preferentially to deflection of hairs in the caudal to rostral direction (He, 2019).

This study has also discovered another example of preferred directional mechanosensory responses in identified non-ciliated sensory neurons of the Drosophila larva. The ddaE neuron shows preferential responses to forward locomotion, while the ddaD neuron responds preferentially to backward locomotion. Interestingly, the molecular basis of these mechanosensory responses depends on the Drosophila Tmc gene, which encodes a putative ion channel gene that is homologous to a pore forming subunit of the mechanotransduction channel of mammalian hearing (TMC1) (He, 2019).

In the hair cell, direction selectivity is an emergent property of the actin-rich bundle of stereovilli. The villi possess extracellular tip-links that transmit tugging forces to the mechanosensory channels localized near the tips of the actin bundles. The tip-link tension that is needed for mechanosensory channel gating is generated when the bundle is deflected toward the tallest side, but not when deflected toward the shortest side. A dimeric TMC1 protein complex comprises an ion channel that may be activated by the tugging forces of the tip-link. It is remarkable that Drosophila proprioceptive neurons, which bear no apparent structural resemblance to the inner ear hair cell, rely on a homologous gene (Tmc) for mechanosensory responses that are direction sensitive. These observations raise interesting questions for future study. How can Tmc family channel members function for mechanosensation in such structurally distinct cells as class I neurons and hair cells? Do class I neurons possess extracellular or intracellular links that are involved in activating the Tmc channels? If not, it may be that membrane curvature or tension alone is an important feature for the activation Tmc channels. The latter idea is consistent with proposed models for activation of mechanosensory transduction channels via the forces imposed on them by the plasma membrane (He, 2019).

An additional question that comes from these studies underlies finding the mechanism that generates the preferred direction responses of the class I neurons. Several potential possibilities are envisioned that are not mutually exclusive. The first possibility is that the direction preference is entirely explained by the magnitude of dendrite curvature that occurs in the different neurons during forward and backward movement. Estimates of dendrite curvature were found to be higher in ddaE relative to ddaD during forward locomotion and higher in ddaD than in ddaE during backward locomotion. Thus, in the current experiments, the degree of curvature was correlated with the strength of the calcium signals that was observed in the different neurons during movement. Although the total curvature, and the peak GCaMP signals, were higher for the cells in the preferred direction, these findings may not provide a complete explanation for the direction-selective responses. For instance, evidence for possible differences in adaptation mechanisms is found in the sustained recordings on the tracking microscope, which revealed a higher baseline calcium level in neurons that were responding to prolonged bouts of movement in the preferred direction (He, 2019).

A second possibility would invoke a circuit mechanism that involves inhibition. The current results have shown that the dendrite deformations observed in ddaE and ddaD occur at distinct phases of the segmental contraction cycle. During forward locomotion, ddaE dendrites deform earlier than those of ddaD, and the dendrites of ddaD deform earlier during reverse locomotion. Thus, the more strongly activated neuron is the first to experience deformation, and it is possible that inhibition of the less strongly activated cell occurs during the delay. This model has similarities to the mechanisms that allow starburst amacrine cells to shape responses of direction-selective ganglion cells of the vertebrate retina (He, 2019).

A third possibility is that dendrite deformations that progress in a distal to proximal direction are more strongly activating than those that progress in proximo-distal direction. Ionic currents that progress from distal to proximal might summate at a spike initiation zone reflected by calcium signals at the cell soma. In contrast, proximal-to-distal dendrite deformations would show reduced summation since the currents would progress in a direction that is moving away from the cell body. This model predicts passive dendrites in class I neurons that lack strongly voltage-gated currents. Fourth, as with other mechanosensory systems the cellular transduction machinery of the class I neurons may be constructed with an inherent asymmetry that causes it to be more sensitive to the forces that are generated in the preferred direction of movement. This model is appealing due to the involvement of the Tmc family of ion channels in the mechanically driven responses of both the class I neurons and hair cells of the inner ear. Thus, the cellular ultrastructure of the Tmc-dependent transduction machinery of class I neurons will be a fascinating subject for future study (He, 2019).

Finally, the results indicate that the responses of the class I neurons are consistent with the previously proposed mission-accomplished model, but this study adds into this model the feature of direction selectivity. The highest responses of the neurons coincide with the phase of the segmental contraction cycle in which the muscles of the segment are most fully contracted (i.e., mission accomplished). The timing of this peak class I response may facilitate the progression of the wave of neural activity in the larval ganglion to initiate contraction of the next segment, and the signals may also help to terminate the contraction of the preceding segment and within the contracting segment of the traveling wave. It is noteworthy that neurons of the larval ganglion have been identified that show specific activity during bouts of forward locomotion and backward locomotion, respectively. In addition, the larva has a suite of neurons beyond ddaE and ddaD that are thought to participate in proprioception. These neurons include the chordotonal neurons, the bipolar dendritic neurons, and possibly the dmd1 neuron. The activities of many of these neurons (such as the bipolar dendritic neurons, dmd1, and the class I cells ddaD, ddaE, and vpda) have been recently investigated using SCAPE microscopy of moving larvae (see the accompanying paper by Vaadia (2019), and the results indicate that each cell shows a relatively unique response that is timed to various phases of the forward locomotion contraction cycle (as was also seen with ddaE and ddaD). As the larval connectome mapping proceeds, it will be interesting to determine how sensory input from each of these neurons impacts CNS circuits that are specifically engaged during forward and backward locomotion, respectively (He, 2019).

A neural circuit encoding mating states tunes defensive behavior in Drosophila

Social context can dampen or amplify the perception of touch, and touch in turn conveys nuanced social information. However, the neural mechanism behind social regulation of mechanosensation is largely elusive. This study reports that fruit flies exhibit a strong defensive response to mechanical stimuli to their wings. In contrast, virgin female flies being courted by a male show a compromised defensive response to the stimuli, but following mating the response is enhanced. This state-dependent switch is mediated by a functional reconfiguration of a neural circuit labelled with the Tmc-L gene in the ventral nerve cord. The circuit receives excitatory inputs from peripheral mechanoreceptors and coordinates the defensive response. While male cues suppress it via a doublesex (dsx) neuronal pathway, mating sensitizes it by stimulating a group of uterine neurons and consequently activating a leucokinin-dependent pathway (see Lkr). Such a modulation is crucial for the balance between defense against body contacts and sexual receptivity (Liu, 2020).

Touch can be pleasant or aversive. Animals' responses to external stimuli, including mechanical force, are tuned by internal states and social contexts. The perception of touch is affected with social context and in turn conveys nuanced social information. This versatility of mechanosensory behaviors is essential for animals to adapt to different environments. These modulations can occur at both the peripheral and central nervous systems. However, the neural mechanism behind social regulation of mechanosensation is largely elusive (Liu, 2020).

In animals with intra-species interaction, response to body touch is actively regulated with social contexts. For example, activation of mechanosensory neurons on the fore-leg induced by tapping can trigger collective behavior in Drosophila. During courtship, a major form of social interaction between flies, the role of touch sensation is poorly understood. Unlike visual, auditory and chemical cues known to play important roles during sexual behaviors of flies, the tactile communication between a pair of flies during courtship is often overlooked. One reason is that body touch can occur to most parts of the body surface and the females' response is highly diverse, and unlike other sensory modes, there are multiple regions in the brain and the ventral nerve cord that receive mechanosensory inputs. Moreover, mechanical stimuli on the body surface are usually alert signals and flies tend to provoke escape or defensive response. This gives rise to a profound question: how do female flies adjust their responsive state to body touch according to the context of sexual activity? In female mice, different groups of neurons in the ventromedial hypothalamus seem to play different roles in regulating sexual receptivity and defensive behaviors, implicating that defensive response is affected with mating activities. However, the dissection of neural circuits that mediate the interplay between mating states and sensory inputs has not been achieved (Liu, 2020).

In recent years, progress has been made to uncover the genetic and neuronal basis of sexual behaviors, although the studies on males far outnumber those on females. Female flies have well-established sexually dimorphic behaviors that are under rigorous control. For instance, the doublesex (dsx) circuits in the brain are activated with male pheromone and courtship song to slow down the female's locomotion. Meanwhile, an increase of receptivity causes a reduced level of kicking and fencing against male's tap or lick. In contrast, mating can trigger a switch of both behaviors and internal states in female, including reduced receptivity and changes in diet preference. They also become more aggressive when competing for food resources. Whether mated female flies become more responsive to or alert against body contact stimuli compared to pre-mated females is unknown (Liu, 2020).

To answer these questions, this study combined genetic and behavioral approaches to investigate the versatility of female flies' defensive response to body contacts. A somatosensory center in the ventral nerve cord was identified that transmits somatosensory input to motor actions. The functional plasticity of this neural structure is responsible for the mating-induced switch in defensive response. This study reveals the neural basis for the sexual activity inducing modulation of touch sensitivity (Liu, 2020).

Female flies show a profound behavioral switch after mating. For example, they become hypersensitive to many sensory stimuli after mating. This paper showns that the behavioral responses to tactile stimuli are also exaggerated. The sensitivity to a mechanical touch on the body in female flies was tuned by different mating states. Upon touch on the wing margins, the mechanical information was relayed to Central Tmc-L neurons (CTNs) to trigger the defensive response. The activity of CTNs was inhibited by a dsx neural circuit when male courtship cues were present; After copulation, a group of Gr32a neurons on the uterus were activated, culminating in the enhancement of defensive response, which presumably resulted from an elevated responsiveness of CTNs by the action of LK. The switch between pre-mating and post-mating states in the defensive response is fast and reversible and of decisive importance for female receptivity (Liu, 2020).

Previous studies demonstrate that the post-mating response (PMR) is mainly mediated by male ejaculate. Suppression of re-mating, as a major PMR effect, emerges in two phases: the fast phase commences immediately after mating, whereas the slow phase takes hours after mating to appear and can last for as long as two weeks. A recent study identified a sensory (likely mechanosensory) pathway that directly encodes mating experience to suppress re-mating immediately after mating. This study has unraveled an additional pathway that originates from the uterus, yet is independent of SP signaling, to suppress female receptivity to mate, in which the uterine neurons (UNs) was involved in enhancing the defensive response and consequently reducing receptivity after mating. Gr32a is known to form, together with other Gr molecules, a gustatory receptor complex responsive to a wide array of bitter compounds including some male cuticular hydrocarbons that inhibits male courtship to conspecific males or females from other species. While it's highly plausible that UNs function as an internal sensor to detect male ejaculate, another intriguing possibility is that, during copulation, a male fly transmits to his mate certain cuticular hydrocarbons, which activate Gr32a in UNs, leading to a post-mating change in the defensive response in the recipient female (Liu, 2020).

Why do females need this alternative pathway from the uterus to the CTNs to regulate their PMR? Some potential functions of this pathway are envisioned: 1) As sensory neurons directly innervating the uterus, the UNs are well positioned to sense the ingredients in the seminal fluid. Besides SP, the seminal fluid contains a cocktail of proteins and other molecules, many of which take actions on unidentified sites. It is tantalizing to hypothesize that Gr32a and UNs are involved in the detection of some of the molecules. 2) UNs may act as a compliment of the LASN pathway to mediate the short-term post-mating response (PMR). This notion is supported by the fact that females with their LASN silenced still had substantial experience index around 4 h after mating and ablating the Crz neurons can't block the transfer of all the components of seminal fluid. 3) As a downstream of UNs, ABLK neurons were found to regulate water homeostasis and food intake, implicating that the activation of UNs may be linked with changes of other physiological states that can last longer that hours. Although in this study focus was placed on the defensive behavior, it is conceivable that UNs' activation may impact other post-mating behaviors such as feeding, egg-laying or aggression (Liu, 2020).

A cohort of neuropeptides mediate the PMR. Leucokinin was initially found critical for body water balance as it is a neurohormone to increase Malpighian tubule fluid secretion and hindgut motility. It also plays a role in the regulation of meal size. This study may provide a link between the water/nutrition balance system and the mechanism for female receptivity via a common signaling channel mediated by LK and LKR. Although there are only a few of clusters of LK neurons in the fly, LKR is expressed broadly in the fly central nervous system and other tissues. Notably, among the 20 LK neurons in the VNC, at least 4-6 abdominal LK (ABLK) neurons also express DH44, a neuropeptide with established roles in female reproductive behaviors. It is an open question whether there is any functional heterogeneity among the ABLK neurons. Intriguingly, the LK receptor LKR is homologous to vertebrate tachykinin receptors. In rodents, tachykinins and all neurokinin receptors are present in the uterus and their abundance is regulated during pregnancy. It thus seems plausible that the LK pathway plays an evolutionarily conserved role in the reproductive behaviors of female animals (Liu, 2020).

The fly VNC has attracted less attention as a site for neural integration for controlling behavior, despite that its critical role in generating motor output is well-documented. This study unequivocally demonstrated that the mating state-dependent, and LK-mediated changes in defensive response rely on neural plasticity occurring exclusively within the VNC. The CTNs neural circuit characterized in this study provides an entry point to delineate the neural mechanism whereby ongoing behavior is fine-tuned at every moment of actions by sensory-guided neural plasticity. Interestingly, the sheet-holder shaped arborization of CTNs in the accessary mesothoracic ganglion is very similar to the structure in the same VNC region reported to be the neural substrate that balances locomotion and feeding (Mann, 2013). In that study, the arborizations were visualized with E564-Gal4, which labels a group of brain-descending neurons involved in gustatory information processing. These descending neurons were also reported to be downstream of sensory afferents from multiple appendages. An attractive scenario is that the sheet-holder shaped arborization serves as the integrative module that coordinates feeding, locomotion and defensive response in a sexual activity-dependent manner with and/or without the brain involvement. Further study is needed to elucidate whether CTNs and E564 neurons connect to each other, and whether the regulatory dsx and LK inputs impinge on this specific neural structure (Liu, 2020).

The neural circuits in the fly brain encoding mating rejection are poorly understood. The copulation rate or latency are commonly measured as a readout of receptivity. However, in view of the rich repertoire of the motor programs for rejection that includes kicking, fencing, wing flicking and ovipositor extrusion, it is conceivable that a multilayered neural network distributing the brain and VNC operates to organize rejection behaviors. One of the key neural elements controlling receptivity is the Dsx-positive pC1 cluster in the female brain, the male counterpart of which includes the P1 cluster, the neural center that makes the decision to court. Although this study has established a synaptic connection between a class of dsx neurons and CTNs, it remains to be examined whether pC1 has any role in controlling CTNs and if it does, which descending neurons deliver the pC1 output to CTNs (Liu, 2020).

Parallel mechanosensory pathways direct oviposition decision-making in Drosophila

Female Drosophila choose their sites for oviposition with deliberation. Female flies employ sensitive chemosensory systems to evaluate chemical cues for egg-laying substrates, but how they determine the physical quality of an oviposition patch remains largely unexplored. This study reports that flies evaluate the stiffness of the substrate surface using sensory structures on their appendages. The TRPV family channel Nanchung is required for the detection of all stiffness ranges tested, whereas two other proteins, Inactive and DmPiezo, interact with Nanchung to sense certain spectral ranges of substrate stiffness differences. Furthermore, Tmc is critical for sensing subtle differences in substrate stiffness. The Tmc channel is expressed in distinct patterns on the labellum and legs and the mechanosensory inputs coordinate to direct the final decision making for egg laying. This study thus reveals the machinery for deliberate egg-laying decision making in fruit flies to ensure optimal survival for their offspring (Zhang, 2020).

This study revealed an unexpected complexity of stiffness assessment when female flies select their egg-laying site. Multiple peripheral appendages and mechanosensory channels are employed to determine the stiffness difference between adjacent egg-laying substrates, and the parallel information from different mechanosensory pathways is integrated to make the final decision for softer substrate. At the moderate stiffness range (0.25%-0.5%), a group of nan+ mechanosensory neurons in the leg tarsal bristles are activated. Similarly, a lower stiffness difference (0.25%-0.4%) activates a group of nan+/Dmpiezo+ tarsal bristle mechanosensory neurons. The detection of subtle stiffness differences is small, as 0.05% agarose relies on sd-L and md-L neurons. Activation of each pathway imparts an inhibitory tone on egg laying and thus guides the flies to softer substrate. Although it remains to be tested whether nan+/Dmpiezo+ tarsal mechanosensory neurons can be activated by moderate stiffness or sd-L/md-L neurons can be activated by moderate and mild stiffness, behavioral data argue that there is functional redundancy among the sensory pathways (Zhang, 2020).

Together with previous findings that flies choose egg-laying sites based on internal and external cues, this study demonstrates that the decision-making process for egg-laying sites in female Drosophila is a highly deliberative process that employs multiple sensory modalities and multiple sensory structures within each modality. This deliberateness is essential because choosing the best egg-laying site is the most critical parental behavior among female flies to maximize their offspring's survival. Female flies in the wild certainly face a more difficult task in making such decisions for a far more complicated environment than is available in a lab experiment. Further investigation will be needed to understand how flies make decisions when evaluating complex or conflicting cues from multiple sensory pathways (Zhang, 2020).

This study has revealed the exquisite ability of female flies to discriminate a texture difference as small as 0.05% in agarose. To do this, flies employ both external sensory structures and proprioceptive sensors to assess the stiffness of the surface. Upon touching the substrate with the legs, tarsal bristles are the first structures to be deformed, leading to the activation of mechanosensory neurons underneath the bristles. In the later probing step, as the proboscis pushes against the substrate, the cuticle of the distal labellum starts to be compressed against the substrate. With innervation to most of the labellum bristles, the Tmc+ md-L neurons are well positioned to detect this information. Proboscis extension will also cause a change of the angle between the labellum and haustrum, and consequently activates the proprioceptive Iav+ sd-L neurons. Loss of either md-L or sd-L neurons on the labellum results in a complete disability to identify a subtle stiffness difference, suggesting that the two structures cooperate functionally to detect weak mechanical stimuli. It remains to be explored how these two sensors coordinate to represent stiffness values in the brain to make the final, accurate selection of softer substrate (Zhang, 2020).

Under the experimental conditions used in this study, the labellum and legs are the predominant appendages that detect substrate stiffness during egg laying. Nevertheless, the role of the ovipositor structure that executes the oviposition maneuver cannot be overlooked. This notion is supported by a previous study, but the exact neurons or genes remain elusive due to the structural complexity of the ovipositor. Moreover, a female fly pushes her lower abdomen against the substrate in order to insert the eggs into the substrate, and this abdominal bending action may require proprioceptive feedback to represent her body position and strength, although this notion requires further experimental evidence. Although it is possible to build a cumulative picture of mechanosensory regulation of decision making, a comprehensive understanding cannot be achieved before the roles of ovipositor and abdominal proprioception are elucidated (Zhang, 2020).

So far, a bona fide center in the fly brain for the integration of mechanosensory inputs has not been established. Unlike visual or olfactory pathways, each of which are encoded and represented by discrete brain regions, mechanosensory inputs appear sparsely distributed throughout the brain and neural transduction from the peripheral to the central nervous system (CNS) seems to be largely parallel. In the egg-laying neuronal circuit, the labellum mechanosensory neurons for detecting subtle stiffness differences project extensive arborizations over the SEZ, a brain region critical for gustatory perception. By contrast, leg bristle neurons that sense greater stiffness send their axons to the ventral nerve cord (VNC) and the projections are then relayed to the higher brain regions including the SEZ, ventrolateral protocerebrum (VLP), superior lateral protocerebrum (SLP), and others. This segregation complicates the identification of brain circuitry that integrates parallel mechanosensory inputs from different appendages to direct egg-laying decision making. Previous studies have raised working models for this interaction, most of which are supported by the fact that mechanosensory and gustatory pathways antagonize or facilitate each other in the local SEZ circuits. Based on the results that leg mechanosensory neurons project to multiple brain regions, however, it would seem more likely that integration may also occur at higher brain areas outside the SEZ (Zhang, 2020).

Furthermore, mechanosensory and gustatory information unambiguously influence one another during decision making for egg laying or feeding. Wu (2019) found that Tmc neurons were required for the loss of softness preference when sugar was provided. This study more symmetrically deciphered the mechanosensory pathways involved in the stiffness detection. Both studies agree that the tarsus and labellum are essential for the flies to choose egg-laying substrates of the optimal stiffness. Wu focused on the discrimination between 0.5% and 1.5% agarose whereas this study focuses on substrates from 0.25% to 0.5% agarose. A major difference in the two experimental setups for these two studies is that the stiffness difference ranges in this study were smaller (0.25%-0.5%), which allowed uncovering of additional mechanosensory mechanisms underlying egg-laying site choice. Nevertheless, the two studies are mutually complementary in deciphering how female flies recognize and integrate substrate texture and chemical cues into final decision making for egg-deposition sites (Zhang, 2020).

A significant question in the field asks how multiple mechanotransduction channels function in overlapping or parallel pathways to coordinate behavioral responses, as more than one channel type is typically expressed in the same type of mechanosensory neurons. This study found that the mechanosensory channels Nanchung and DmPiezo are required for the discrimination of a mild stiffness difference. However, how the combination of these two channels drives the function of the same neurons remains elusive. Two possibilities are suggested: first, multiple mechanosensitive channels co-express and function in the same neurons in a parallel manner. For example, DmPiezo and PPK function in larval class VI da neurons to mediate mechanical nociceptive response. Another case comes from larval class I da neurons, in which both NompC and Tmc are required for proprioceptive feedback to control larval locomotion. In this scenario, Nanchung and DmPiezo channels may function in parallel signaling pathways required for normal preference to 0.25% over 0.4%. When either pathway is disrupted, females would show a decreased ability to distinguish stiffness differences. Second, the two channels may function in series in the same pathway, with one acting as a sensor and the other as an amplifier. For instance, in fly Cho organ neurons, three TRP channels, Nanchung, NompC, and Inactive, are all required for sound transduction. Nanchung is expressed in most mechanosensory neurons for hearing and proprioception. It is plausible that Nanchung maintains basal neuronal activity and DmPiezo functions as a specific receptor for mechanical force. The current data support this view, as a nanGal4 mutant lost nearly all spike firing whereas DmpiezoKO still maintained a reduced firing activity. Behaviorally, the nanGal4 mutant showed much more severe defects in selecting softer substrate than DmpiezoKO in the mild range. The data also implicate other mechanosensors such as NompC as working in concert with Nanchung in bristle mechanosensory neurons (Zhang, 2020).

Sweet neurons inhibit texture discrimination by signaling TMC-expressing mechanosensitive neurons in Drosophila

Integration of stimuli of different modalities is an important but incompletely understood process during decision making. This study shows that Drosophila are capable of integrating mechanosensory and chemosensory information of choice options when deciding where to deposit their eggs. Specifically, females switch from preferring the softer option for egg-laying when both options are sugar free to being indifferent between them when both contain sucrose. Such sucrose-induced indifference between options of different hardness requires functional sweet neurons, and, curiously, the Transmembrane Channel-like (TMC)-expressing mechanosensitive neurons that have been previously shown to promote discrimination of substrate hardness during feeding. Further, axons of sweet neurons directly contact axons of TMC-expressing neurons in the brain and stimulation of sweet neurons increases Ca(2+) influx into axons of TMC-expressing neurons. These results uncover one mechanism by which Drosophila integrate taste and tactile information when deciding where to deposit their eggs and reveal that TMC-expressing neurons play opposing roles in hardness discrimination in two different decisions (Wu, 2019).

This work showed that activation of sweet neurons by sucrose can promote Drosophila females to become indifferent between two substrates of different hardness during egg-laying, and that such sucrose-induced indifference required input from the TMC-expressing mechanosensitive neurons on the labellum. Specifically, Drosophila females were shown to generally preferred the softer substrate for egg-laying in a two-choice assay when both options were sugar free, but their preference for the softer substrate reduced significantly when both options contained 100 mM sucrose. Such sugar-induced indifference between substrates of different hardness depended on functional molecular sugar receptors and sweet neurons as well as, interestingly, functional TMC channel and TMC-expressing mechanosensitive neurons. Further, anatomical-labeling and Ca2+-imaging results showed that axons of sweet neurons directly contacted those of TMC-expressing neurons in the brain and that depolarizing the sweet neurons increased Ca2+ influx into axon termini of TMC neurons. Thus, such axon-axon contacts provide an anatomical basis for sweet neurons to directly modulate the output of TMC neurons in the brain. Together, these findings suggest that, during egg-laying site selection, activation of sweet neurons can act to inhibit discrimination of substrates of different hardness by enhancing the output of TMC neurons directly. The results thus demonstrate a novel means by which Drosophila integrate specific chemosensory and mechanosensory properties of two competing substrates when evaluating them during a simple decision-making task. However, it is worth pointing out that the mechanism described in this study may not be the only path by which sweet neurons can act to modify discrimination of substrate hardness during egg-laying site selection. First, input from tarsi and antennae played a role, too. While no tmc transcripts were detected on them, it is unclear whether tmc-expressing neurons on these structures (that were missed by the tmc-GAL4) have the same interaction with sweet neurons as the ones on the labellum. Second, while the function of tmc-GAL4-expressing neurons was required for sucrose to dampen hardness discrimination, it was not possible to ascertain that direct artificial activation of these neurons was sufficient to do so in the absence of sucrose as such activation severely reduced females' egg-laying rate. Thus, one important next task is to identify the relevant mechanosensitive input from tarsi and antennae and assess how information they relay might be modulated by activation of sweet neurons during egg-laying site selection (Wu, 2019).

A second point that is worth discussing is whether the conclusions are compatible with findings from previous reports. While the results suggest that sweet neurons can act to potentiate the output of TMC neurons via axon-axon interaction, two recent studies have shown that activation of mechanosensitive neurons can inhibit the output of sweet neurons. Specifically, Zhang (2016) has shown that activation of TMC neurons can inhibit PER, a motor response triggered by activation of sweet neurons. Further, Jeong (2016) has shown that Nanchung-expressing neurons can inhibit PER and that axons of Nanchung-expressing neurons form inhibitory synapses with axons of sweet neurons. It is proposed that the current conclusions are not incompatible with these earlier reports. First, it is conceivable that axons of mechanosensitive neurons and sweet neurons can have two distinct types of interactions: presynaptic inhibition from mechanosensitive neurons to sweet neurons as well as presynaptic facilitation from sweet neurons to TMC neurons. Second, while 100 mM sucrose may facilitate TMC neurons less when flies were sampling 1.5% agarose than on 0.5% agarose (taking into account that sweet neurons should be suppressed more on 1.5% agarose than on 0.5% agarose), this should reduce the difference in perceived hardness of 0.5% and 1.5% agarose substrates, thus not inconsistent with what was seen. Moreover, it is unclear whether 0.5% and 1.5% agarose exerted very different levels of suppression on output of sweet neurons in this task. For example, Jeong (2016) showed that 0.2% vs. 2% agarose had significantly different impacts on feeding preference for 0.5 mM vs. 1 mM sucrose, however, the concentration of sucrose used in this study was 100 mM. For these reasons, the idea is favored that the conclusions expand the view of the relationship between sweet neurons and mechanosensitive neurons provided by the previous studies (Wu, 2019).

Another point worth discussing after comparing this work with previous reports is that flies appeared to use two different sensory mechanisms to discriminate substrates of different hardness during feeding and egg-laying, even though they generally preferred the softer substrate in both tasks. Previous studies have shown that flies rely on TMC, Nan, and NompC channels and two specific groups of labellum sensory neurons that express these channels to discriminate substrates of different hardness during feeding (Jeong, 2016; Zhang, 2016; Sanchez-Alcaniz, 2017). In contrast, the current results showed that neither these channels nor these neurons were essential for flies to discriminate substrates of different hardness during egg-laying. More curiously, the results suggest input from mechanosensitive neurons on the labellum (as well as possibly ones on antennae and tarsi) can act to inhibit discrimination of substrates of different hardness during egg-laying. This conclusion is supported in part by the observations that animals without intact labellum or functional TMC-expressing neurons on the labellum showed enhanced discrimination in the presence of sucrose during egg-laying. In contrast, tmc mutants did not discriminate substrates of different hardness well for feeding when given the exact same choices. The striking difference in the requirement of labellum and TMC on substrate hardness discrimination during feeding and egg-laying raises the question of what are the identities of the specific sensory neurons that promote discrimination of substrate hardness during egg-laying. The totality of the current results are consistent with a very tentative model that Drosophila likely use some as-yet-unidentified mechanosensitive neurons on their ovipositor to sense and discriminate substrates of different hardness. This tentative model is based on the following reasons. First, ovipositor is known to possess mechanosensitive neurons; second, flies have been shown to actively probe the substrates with their ovipositor prior to depositing each egg; third and most important, animals that lacked the a significant portion of virtually all other appendages (e.g., labellum, tarsi, wings) but had intact ovipositor were still capable of discriminating substrates of different hardness. Thus, another important next task is to identify the mechanosensitive neurons on the ovipositor -- or possibly on other body parts -- that are critical for discriminating substrate hardness during egg-laying and the central targets of these neurons. Identities of these neurons will provide a much-needed molecular and anatomical basis to start elucidating how texture discrimination and substrate selection during egg-laying site selection is enabled and modulated (Wu, 2019).

Lastly, what is the potential advantage in allowing sugar detection to inhibit discrimination of egg-laying substrates of different hardness? Strong selectiveness likely costs effort and delays emergence of progenies. Thus, when deciding between two competing substrates that do not differ significantly in values, it might be more advantageous for flies to deposit their eggs on both. In the experiments carried out in this study, difference in values between the plain 0.5% agarose and the plain 1.5% agarose maybe relatively small because while flies preferred the 0.5% agarose over the 1.5% agarose in the two-choice assay, they laid comparable numbers of eggs on them when each was presented in single-choice assays. Thus, the presence of high concentration of sucrose in both substrates may further reduce their differences in values, thereby largely eliminating flies' soft preference. (However, it is worth noting that the idea is favored that adding sucrose to the 0.5% and 1.5% agarose substrates may equalize their values by dampening them, at least in the context of regular assays performed in this study. This is because in regular assays, adding sucrose to an agarose substrate reduces as opposed to increases its value: while flies readily accepted the sucrose-containing substrate for egg-laying, they consistently preferred the plain one when given a choice between a plain one and a sucrose-containing one to choose from. Finally, from an evolutionary point of view, it is proposed that allowing sweet neurons to directly enhance the output of mechanosensitive neurons that can inhibit hardness discrimination during egg-laying may provide a neural substrate for different species to adopt different texture selectivity. For example, in contrast to Drosophila melanogaster, the fruit pest Drosophila suzukii is more receptive to lay eggs on harder substrates and attack both ripe (harder) and rotten (softer) fruits. It may be interesting to test whether modifications of the structure and function of sweet and TMC neurons, and/or the connection between them, contribute to Drosophila suzukii's acceptance of harder substrate during egg-laying (Wu, 2019).

Transmembrane channel-like (tmc) gene regulates Drosophila larval locomotion

Drosophila larval locomotion, which entails rhythmic body contractions, is controlled by sensory feedback from proprioceptors. The molecular mechanisms mediating this feedback are little understood. By using genetic knock-in and immunostaining, this study found that the Drosophila melanogaster transmembrane channel-like (tmc) gene is expressed in the larval class I and class II dendritic arborization (da) neurons and bipolar dendrite (bd) neurons, both of which are known to provide sensory feedback for larval locomotion. Larvae with knockdown or loss of tmc function displayed reduced crawling speeds, increased head cast frequencies, and enhanced backward locomotion. Expressing Drosophila TMC or mammalian TMC1 and/or TMC2 in the tmc-positive neurons rescued these mutant phenotypes. Bending of the larval body activated the tmc-positive neurons, and in tmc mutants this bending response was impaired. This implicates TMC's roles in Drosophila proprioception and the sensory control of larval locomotion. It also provides evidence for a functional conservation between Drosophila and mammalian TMCs (Guo, 2016).

Proprioception is vital for animals to control their locomotion behavior, although the underlying mechanisms remain to be worked out in Drosophila and other animals. This study reports that the tmc gene contributes to proprioception and sensory feedback for normal forward crawling behavior in Drosophila larvae. tmc is expressed in Drosophila larval sensory neurons. Behavioral and calcium imaging studies indicate that Drosophila TMC plays an important role in proprioception and regulation of crawling behavior. Moreover, behavioral defects due to loss of tmc function in Drosophila were rescued by expressing mammalian TMC proteins, indicative of an evolutionarily conserved function (Guo, 2016).

Several types of body-wall sensory neurons appear to play a role in the larval locomotion regulation. Silencing chordotonal cho) neurons results in increased frequency and duration of turning and reduced duration of linear locomotion, a phenotype similar to that caused by tmc mutation, suggesting that the Cho neurons and the tmc-expressing neurons might converge to the same motor output pathway. Interestingly, blocking class IV da neurons produces an opposite phenotype: fewer turns. Given that the central projection of class IV da neurons in the VNC is distinct from that of class I da neurons and bd neurons, it will be interesting to see how they regulate the same behavior in opposing manners (Guo, 2016).

Different neurons might use different mechanosensitive ion channels in coordinating proprioceptive cues, similar to what has been found in the touch-sensitive neurons. The TRPN channel NOMPC functions in class III da neurons to mediate gentle touch sensation whereas the DEG/ENaC ion channels PPK and PPK26, the TRP channel Painless, and Piezo function in class IV da neurons to mediate mechanical nociception. As to proprioception, chordotonal organs, class I and class IV da neurons and bd neurons may all contribute to proprioception to regulate larval locomotion behavior. It is reported that NOMPC is expressed in class I da neurons and bd neurons, and mutations of NOMPC cause prolonged stride duration and reduced crawling speed of mutant larvae. In contrast, the DEG/ENaC ion channels PPK and PPK26 function in class IV da neurons to modulate the extent of linear locomotion; reduction of these channel functions leads to decreased turning frequency and enhanced directional crawling (Guo, 2016).

Drosophila TMC protein exhibits sequence conservation with TMC family members in other species in the putative transmembrane domains, although it is much larger than its mouse or human homologs. It is of interest to determine whether the Drosophila TMC functions encompass a combination of functions of its mammalian homologs (Guo, 2016).

Among eight tmc genes in human and mice, tmc1 and tmc2 are found to be required for sound transduction in the hair cells of the inner ear. However, these genes are very broadly expressed, so it is possible that they might also function in other tissues. In light of the finding that Drosophila TMC functions in sensory neurons to regulate locomotion and mouse TMC1 or TMC2 functionally rescue the fly mutant phenotype, it will be interesting to test whether TMC1 and TMC2 have similar functions in addition to their involvement in hearing. This work indicates that the Drosophila tmc gene participates in proprioception. Whether mammalian tmc genes, including tmc1 and tmc2, participate in proprioception is an interesting open question (Guo, 2016).

In contrast to tmc1 and tmc2 in mammals and the Drosophila tmc gene, the tmc-1 gene of Caenorhabditis elegans was reported to contribute to high sodium sensation in ASH polymodal avoidance neurons, in which TMC-1 ion channels could be activated by high concentrations of extracellular sodium salts and permeate cations. It will be of interest to explore the potential roles of tmc genes in various species in mechanosensation or osmosensation (Guo, 2016).

How mammalian TMC1 and TMC2 function in sound transduction is still not fully understood, and whether they are the pore-forming channel subunits is under debate. It remains to be shown whether TMC1 and TMC2 can yield channel activities in heterologous expression systems, and they likely require other proteins for their function in mechanotransduction. Attempts were made to ectopically express the Drosophila tmc gene product in a variety of heterologous systems. However, no obvious mechanosensitive currents could be detected when these cells are exposed to mechanical stimuli. One possibility is that the Drosophila TMC protein fails to be trafficked to plasma membrane in the expression system that was used. Alternatively, additional components are required to form a mechanosensitive complex as gating of certain mechanogated ion channels such as NOMPC might require interactions of ion channels with extracellular matrix and/or intracellular cytoskeleton. Analyses of Drosophila tmc gene functions in larval locomotion regulation in this study, and in other future behavioral studies, may provide an opportunity to search for additional components that are necessary for the function of TMC proteins (Guo, 2016).

The basis of food texture sensation in Drosophila

Food texture has enormous effects on food preferences. However, the mechanosensory cells and key molecules responsible for sensing the physical properties of food are unknown. This study shows that akin to mammals, the fruit fly, Drosophila melanogaster, prefers food with a specific hardness or viscosity. This food texture discrimination depends upon a previously unknown multidendritic (md-L) neuron, which extends elaborate dendritic arbors innervating the bases of taste hairs. The md-L neurons exhibit directional selectivity in response to mechanical stimuli. Moreover, these neurons orchestrate different feeding behaviors depending on the magnitude of the stimulus. It was demonstrated that the single Drosophila transmembrane channel-like (TMC) protein is expressed in md-L neurons, where it is required for sensing two key textural features of food-hardness and viscosity. The study proposes that md-L neurons are long sought after mechanoreceptor cells through which food mechanics are perceived and encoded by a taste organ, and that this sensation depends on TMC (Zhang, 2016).

Food preferences are affected greatly by the qualities of food, including nutrient value, texture, and the taste valence of sweet, bitter, salty, and sour qualities. During the last 15 years, many of the gustatory receptor proteins that participate in the discrimination of the chemical composition of food have been defined. In sharp contrast, the basis through which food texture is detected is enigmatic, despite the universal appreciation that the physical properties of food greatly influence decisions to consume a prospective offering. There are specific tactile features associated with liquid or solid food. Viscosity and creaminess are typical textural features of liquid food, whereas hardness, crispiness, and softness are the main physical characteristics of solid food. Similar to food tastes, food texture provides important information concerning food quality, including freshness and wholesomeness. For instance, people prefer freshly baked bread with relatively soft texture, and tend to reject older bread with a harder texture, even though the chemical composition has not changed significantly over the course of a couple of days. Furthermore, while exploring the food landscape, an animal must make assessments of food hardness and viscosity in order to exert the appropriate force to chew or ingest. Insufficient chewing force results in poor food processing, while excessive force can cause injury to the tongue or teeth (Zhang, 2016).

Food texture in mammals is predominantly detected through poorly understood mechanisms in taste organs. In rodents and humans, a subset of trigeminal nerves such as the lingual nerve provides somatosensitive afferents to the tongue. Due to the intrinsic mechanical properties of food, mastication produces compression and shearing forces, which in turn activate mechanosensory neuronsin taste organs. However, the molecular identities of mechanosensory neurons and signaling proteins that enable animals to detect food texture are unknown. To address the fundamental issue concerning the cellular and molecular mechanisms that function in the sensation of food texture, this study turned to the fruit fly, Drosophila melanogaster, as an animal model. In flies, food quality is evaluated largely through external sensory hairs (sensilla), which decorate the fly tongue (the labellum) and several other body parts. These sensilla, which house several sensory neurons, allow the chemical composition of foods, such as sugars and bitter compounds, to be detected prior to entering the mouthparts (Zhang, 2016).

This study found that Drosophila can discriminate between foods on the basis of hardness and viscosity. A previously unknown type of mechanosensory neuron was identified in the fly tongue that is dedicated to detecting food mechanics. These multidendritic neurons in the labellum (md-L) extend their projections into the bases of most of the external sensilla and are activated by deflections induced by hard and viscous food. The ability of md-L neurons to sense food mechanics is virtually lost due to elimination of the only Drosophila member of the transmembrane channel-like (TMC) family. Mice and humans each encode eight TMC proteins, and mutations in the founding member of this family, TMC1, cause deafness in mammals. This study found that tmc is broadly tuned to detect both soft and hard food textures. Remarkably, optogenetic stimulation of the md-L neurons with different light intensities yields opposing behavioral outcomes-weak light promotes feeding, while strong light represses feeding. It is concluded that md-L neurons and TMC are critical cellular and molecular components that enable external sensory bristles on the fly tongue to communicate textural features to the brain, and do so through a pre-ingestive mechanism (Zhang, 2016).

This study demonstrates that the attraction of wild-type flies to the same concentration of sucrose is altered by the viscosity or hardness of the food. If the sucrose-containing substrate is too sticky, soft, or hard, the appeal of the food declines. These observations establish the Drosophila taste system as a model to explore the cellular and molecular underpinnings that allow an animal to sense food texture. Moreover, similar to the chemosensory evaluation of food by external sensilla decorating the labellum, the textural assessment of foods is pre-ingestive in flies (Zhang, 2016).

This study has identified md-L, a previously undefined neuron in each of the two bilateral symmetrical labella, which extend a complex array of dendrites to the bases of many sensilla. Several observations demonstrate that md-L neurons play an indispensable role in food texture sensation. First, selective abolition of neurotransmission from md-L caused significant impairments in food texture discrimination. Second, laser ablation of md-L resulted in severe defects in perceiving the viscosity or hardness of foods. Third, low or moderate artificial activation of md-L neurons was sufficient to trigger proboscis extension. Thus, the loss-of-function and gain-of-function analyses of md-L neurons has lead to a conclusion that md-L neurons are key mechanoreceptor cells controlling sensation of food mechanics (Zhang, 2016).

Unexpectedly, while low-intensity optogenetic stimulation of md-L provoked proboscis extension, high-intensity light induced contraction of the proboscis. Thus, md-L neurons are tuned to different levels of mechanical stimuli that give rise to drastically different feeding behaviors. it is proposed that weak or moderate light mimics the response to softer foods that simulates feeding, while strong light induces a higher level of activity that mimics hard foods and discourages feeding. When a fly is offered sucrose in combination with optogenetic stimulation of md-L neurons with strong light, this caused the animal to reject the otherwise appetitive food. It is proposed that this rejection occurred because the animal perceived the texture of the sucrose as too hard. Thus, it is suggested that texture sensation is mediated by md-L neurons through an intensity-dependent rather than a labeled-line mechanism. While md-L are required, it is not excluded that other neurons in the labella contribute to food texture sensation. Ultrastructural studies of taste sensilla led to the proposal that a neuron positioned at the base of each taste sensillum is a mechanosensory neuron. However, it currently remains unclear as to whether these neurons contribute to some aspect of food texture detection (Zhang, 2016).

In Drosophila, most taste sensilla point toward the ventral direction. The md-L neuron produced much stronger neuronal activity in response to forces applied to taste hairs that were deflected dorsally than those deflected in other directions. Thus, taste sensilla are most sensitive to force applied opposite to the direction in which they point. Notably, this direction-dependent feature of taste sensilla is reminiscent of the directional sensitivity of hair in mammals, suggesting that it is a widely used neural coding strategy for sensation in the animal kingdom (Zhang, 2016).

The directional sensitivity of taste sensilla differs from the macrochaete bristles in the thorax, since these latter bristles are most sensitive to force applied in the same direction in which they point. The profound differences inmforce-directional sensitivity reflect the functional divergence between these two types of mechanosensory bristles. The direction-tuning feature of md-L neurons might be an evolutionary adaptation to help fruit flies sample food. While exploring the food landscape, a fruit fly normally extends its proboscis in the ventral direction. As a consequence, the forces arising from the food will bend taste sensilla in the opposite dorsal direction (Zhang, 2016).

Thus, it is suggested that md-L neurons evolved to become most sensitive to forces emanating from the dorsal direction It is concluded that Drosophila TMC is required for detecting food hardness. TMC is expressed and required in md-L neurons. Furthermore, loss of tmc greatly reduced the ability to behaviorally discriminate the preferred softness (1% agarose) or smoothness (sucrose solution only) from harder or stickier food options, respectively. However, the responses to tastants, such as sucrose, salt, or caffeine, were unaffected in tmc1, indicating that TMC was specifically required for sensing food texture rather than the chemical composition of food (Zhang, 2016).

An important question concerns the mechanism through which TMC enables md-L neurons to sense food hardness. It is proposed that deflection of gustatory sensilla by food hardness imposes mechanical force on these neurons. The harder the food, the greater the stimulation of md-L neurons, which sense force through the dendrites innervating the bases of many sensilla. Given the expression of TMC in dendrites, an appealing possibility is that TMC is a key component of a mechanically activated channel that endows the fly tongue with the ability to sense food hardness. A TMC protein (TMC-1) is expressed in worms and is proposed to be required for salt sensation (Chatzigeorgiou, 2013). Furthermore, TMC-1 plays a critical role in alkali sensation in vivo (Wang, 2016). As such, it appears that the worm TMC-1 controls multiple aspects of chemosensation. Mammalian TMC1 and TMC2 are required for hearing and expressed in the inner ear (Kawashima, 2011; Pan, 2013). Currently, it is not known if mammalian TMCs are subunits of a channel, or whether they are mechanically activated, since problems with cell-surface expression of these proteins in heterologous expression systems have precluded biophysical characterizations. It is possible that TMCs may depend on additional subunits for trafficking or to form functional ion channels. Drosophila TMC may also be one subunit of a mechanically activated channel, and it is proposed that this feature might allow md-L neurons to be stimulated in response to bending of taste sensilla by hard foods (Zhang, 2016).

In conclusion, this study has elucidated a cellular mechanism through which food mechanics influence the taste preference of an animal. The md-L neurons define a novel class of mechanosensory neurons that enable flies to detect food hardness and viscosity. A future question concerns the mapping of the brain region where mechanical and chemosensory pathways converge to dictate gustatory decisions. An appealing possibility is that md-L and GRN axons coordinately signal to a pair of command interneurons (Fdg neurons) that have extensive arborizations in the SEZ and control feeding behavior. Finally, the results demonstrate that TMC is essential for food texture sensation. These results raise the possibility that homologs of fly TMC may be dedicated to the gustatory discrimination of texture in many other animals, including mammals (Zhang, 2016).

Functions of Tmc orthologs in other species

TMC1 is an essential component of a leak channel that modulates tonotopy and excitability of auditory hair cells in mice

Hearing sensation relies on the mechano-electrical transducer (MET) channel of cochlear hair cells, in which transmembrane channel-like 1 (TMC1) and transmembrane channel-like 2 (TMC2) have been proposed to be the pore-forming subunits in mammals. TMCs were also found to regulate biological processes other than MET in invertebrates, ranging from sensations to motor function. However, whether TMCs have a non-MET role remains elusive in mammals. This study reports that in mouse hair cells, TMC1, but not TMC2, provides a background leak conductance, with properties distinct from those of the MET channels. By cysteine substitutions in TMC1, four amino acids were characterized that are required for the leak conductance. The leak conductance is graded in a frequency-dependent manner along the length of the cochlea and is indispensable for action potential firing. Taken together, these results show that TMC1 confers a background leak conductance in cochlear hair cells, which may be critical for the acquisition of sound-frequency and -intensity (Liu, 2019).

A Tmc1 mutation reduces calcium permeability and expression of mechanoelectrical transduction channels in cochlear hair cells

Mechanoelectrical transducer (MET) currents were recorded from cochlear hair cells in mice with mutations of transmembrane channel-like protein TMC1 to study the effects on MET channel properties. A Tmc1 mouse was characterized with a single-amino-acid mutation (D569N), homologous to a dominant human deafness mutation. Measurements were made in both Tmc2 wild-type and Tmc2 knockout mice. By 30 d, Tmc1 pD569N heterozygote mice were profoundly deaf, and there was substantial loss of outer hair cells (OHCs). MET current in OHCs of Tmc1 pD569N mutants developed over the first neonatal week to attain a maximum amplitude one-third the size of that in Tmc1 wild-type mice, similar at apex and base, and lacking the tonotopic size gradient seen in wild type. The MET-channel Ca(2+) permeability was reduced 3-fold in Tmc1 pD569N homozygotes, intermediate deficits being seen in heterozygotes. Reduced Ca(2+) permeability resembled that of the Tmc1 pM412K Beethoven mutant, a previously studied semidominant mouse mutation. The MET channel unitary conductance, assayed by single-channel recordings and by measurements of current noise, was unaffected in mutant apical OHCs. In contrast to the Tmc1 M412K mutant, there was reduced expression of the TMC1 D569N channel at the transduction site assessed by immunolabeling, despite the persistence of tip links. The reduction in MET channel Ca(2+) permeability seen in both mutants may be the proximate cause of hair-cell apoptosis, but changes in bundle shape and protein expression in Tmc1 D569N suggest another role for TMC1 apart from forming the channel (Beurg, 2019).

Variable number of TMC1-dependent mechanotransducer channels underlie tonotopic conductance gradients in the cochlea

Functional mechanoelectrical transduction (MET) channels of cochlear hair cells require the presence of transmembrane channel-like protein isoforms TMC1 or TMC2. TMCs are required for normal stereociliary bundle development and distinctively influence channel properties. TMC1-dependent channels have larger single-channel conductance and in outer hair cells (OHCs) support a tonotopic apex-to-base conductance gradient. Each MET channel complex exhibits multiple conductance states in ~50 pS increments, basal MET channels having more large-conductance levels. Using mice expressing fluorescently tagged TMCs, a three-fold increase was seen in number of TMC1 molecules per stereocilium tip from cochlear apex to base, mirroring the channel conductance gradient in OHCs. Single-molecule photobleaching indicates the number of TMC1 molecules per MET complex changes from ~8 at the apex to ~20 at base. The results suggest there are varying numbers of channels per MET complex, each requiring multiple TMC1 molecules, and together operating in a coordinated or cooperative manner (Beurg, 2018).

Tmc2 expression partially restores auditory function in a mouse model of DFNB7/B11 deafness caused by loss of Tmc1 function

Mouse Tmc1 and Tmc2 are required for sensory transduction in cochlear and vestibular hair cells. Homozygous Tmc1(Δ/Δ) mice are deaf, Tmc2(Δ/Δ) mice have normal hearing, and double homozygous Tmc1(/); Tmc2(/) mice have deafness and profound vestibular dysfunction. These phenotypes are consistent with their different spatiotemporal expression patterns. Tmc1 expression is persistent in cochlear and vestibular hair cells, whereas Tmc2 expression is transient in cochlear hair cells but persistent in vestibular hair cells. On the basis of these findings, it is hypothesized that persistent Tmc2 expression in mature cochlear hair cells could restore auditory function in Tmc1(Δ/Δ) mice. To express Tmc2 in mature cochlear hair cells, a transgenic mouse line, Tg[PTmc1::Tmc2], was generated in which Tmc2 cDNA is expressed under the control of the Tmc1 promoter. The Tg[PTmc1::Tmc2] transgene slightly but significantly restored hearing in young Tmc1(Δ/Δ) mice, though hearing thresholds were elevated with age. The elevation of hearing thresholds was associated with deterioration of sensory transduction in inner hair cells and loss of outer hair cell function. Although sensory transduction was retained in outer hair cells, their stereocilia eventually degenerated. These results indicate distinct roles and requirements for Tmc1 and Tmc2 in mature cochlear hair cells (Nakanishi, 2018).

Structural relationship between the putative hair cell mechanotransduction channel TMC1 and TMEM16 proteins

The hair cell mechanotransduction (MET) channel complex is essential for hearing, yet it's molecular identity and structure remain elusive. The transmembrane channel-like 1 (TMC1) protein localizes to the site of the MET channel, interacts with the tip-link responsible for mechanical gating, and genetic alterations in TMC1 alter MET channel properties and cause deafness, supporting the hypothesis that TMC1 forms the MET channel. This study generated a model of TMC1 based on X-ray and cryo-EM structures of TMEM16 proteins, revealing the presence of a large cavity near the protein-lipid interface that also harbors the Beethoven mutation, suggesting that it could function as a permeation pathway. This study also found that hair cells are permeable to 3 kDa dextrans, and that dextran permeation requires TMC1/2 proteins and functional MET channels, supporting the presence of a large permeation pathway and the hypothesis that TMC1 is a pore forming subunit of the MET channel complex (Ballesteros, 2018).

TMC-1 mediates alkaline sensation in C. elegans through nociceptive neurons

Noxious pH triggers pungent taste and nocifensive behavior. While the mechanisms underlying acidic pH sensation have been extensively characterized, little is known about how animals sense alkaline pH in the environment. TMC genes encode a family of evolutionarily conserved membrane proteins whose functions are largely unknown. This study characterize C. elegans TMC-1, which was suggested to form a Na(+)-sensitive channel mediating salt chemosensation. Interestingly, TMC-1 was found to be required for worms to avoid noxious alkaline environment. Alkaline pH evokes an inward current in nociceptive neurons, which is primarily mediated by TMC-1 and to a lesser extent by the TRP channel OSM-9. However, unlike OSM-9, which is sensitive to both acidic and alkaline pH, TMC-1 is only required for alkali-activated current, revealing a specificity for alkaline sensation. Ectopic expression of TMC-1 confers alkaline sensitivity to alkali-insensitive cells. These results identify an unexpected role for TMCs in alkaline sensation and nociception (Vaadia, 2019).

The effects of Tmc1 Beethoven mutation on mechanotransducer channel function in cochlear hair cells

Sound stimuli are converted into electrical signals via gating of mechano-electrical transducer (MT) channels in the hair cell stereociliary bundle. The molecular composition of the MT channel is still not fully established, although transmembrane channel-like protein isoform 1 (TMC1) may be one component. This study found that in outer hair cells of Beethoven mice containing a M412K point mutation in TMC1, MT channels had a similar unitary conductance to that of wild-type channels but a reduced selectivity for Ca(2+). The Ca(2+)-dependent adaptation that adjusts the operating range of the channel was also impaired in Beethoven mutants, with reduced shifts in the relationship between MT current and hair bundle displacement for adapting steps or after lowering extracellular Ca(2+); these effects may be attributed to the channel's reduced Ca(2+) permeability. Moreover, the density of stereociliary CaATPase pumps for Ca(2+) extrusion was decreased in the mutant. The results suggest that a major component of channel adaptation is regulated by changes in intracellular Ca(2+). Consistent with this idea, the adaptive shift in the current-displacement relationship when hair bundles were bathed in endolymph-like Ca(2+) saline was usually abolished by raising the intracellular Ca(2+) concentration (Beurg, 2015).

Sensory transduction in auditory and vestibular hair cells requires expression of transmembrane channel-like (Tmc) 1 and 2 genes, but the function of these genes is unknown. To investigate the hypothesis that TMC1 and TMC2 proteins are components of the mechanosensitive ion channels that convert mechanical information into electrical signals, whole-cell and single-channel currents were recorded from mouse hair cells that expressed Tmc1, Tmc2, or mutant Tmc1. Cells that expressed Tmc2 had high calcium permeability and large single-channel currents, while cells with mutant Tmc1 had reduced calcium permeability and reduced single-channel currents. Cells that expressed Tmc1 and Tmc2 had a broad range of single-channel currents, suggesting multiple heteromeric assemblies of TMC subunits. The data demonstrate TMC1 and TMC2 are components of hair cell transduction channels and contribute to permeation properties. Gradients in TMC channel composition may also contribute to variation in sensory transduction along the tonotopic axis of the mammalian cochlea (Pan, 2013).

tmc-1 encodes a sodium-sensitive channel required for salt chemosensation in C. elegans

Transmembrane channel-like (TMC) genes encode a broadly conserved family of multipass integral membrane proteins in animals. Human TMC1 and TMC2 genes are linked to human deafness and required for hair-cell mechanotransduction; however, the molecular functions of these and other TMC proteins have not been determined. This study shows that the Caenorhabditis elegans tmc-1 gene encodes a sodium sensor that functions specifically in salt taste chemosensation. tmc-1 is expressed in the ASH polymodal avoidance neurons, where it is required for salt-evoked neuronal activity and behavioural avoidance of high concentrations of NaCl. However, tmc-1 has no effect on responses to other stimuli sensed by the ASH neurons including high osmolarity and chemical repellents, indicating a specific role in salt sensation. When expressed in mammalian cell culture, C. elegans TMC-1 generates a predominantly cationic conductance activated by high extracellular sodium but not by other cations or uncharged small molecules. Thus, TMC-1 is both necessary for salt sensation in vivo and sufficient to generate a sodium-sensitive channel in vitro, identifying it as a probable ionotropic sensory receptor (Chatzigeorgiou, 2013).


Search PubMed for articles about Drosophila Tmc

Ballesteros, A., Fenollar-Ferrer, C. and Swartz, K. J. (2018). Structural relationship between the putative hair cell mechanotransduction channel TMC1 and TMEM16 proteins. Elife 7. PubMed ID: 30063209

Beurg, M., Goldring, A. C. and Fettiplace, R. (2015). The effects of Tmc1 Beethoven mutation on mechanotransducer channel function in cochlear hair cells. J Gen Physiol 146(3): 233-243. PubMed ID: 26324676

Beurg, M., Cui, R., Goldring, A. C., Ebrahim, S., Fettiplace, R. and Kachar, B. (2018). Variable number of TMC1-dependent mechanotransducer channels underlie tonotopic conductance gradients in the cochlea. Nat Commun 9(1): 2185. PubMed ID: 29872055

Beurg, M., Barlow, A., Furness, D. N. and Fettiplace, R. (2019). A Tmc1 mutation reduces calcium permeability and expression of mechanoelectrical transduction channels in cochlear hair cells. Proc Natl Acad Sci U S A 116(41): 20743-20749. PubMed ID: 31548403

Chatzigeorgiou, M., Bang, S., Hwang, S. W. and Schafer, W. R. (2013). tmc-1 encodes a sodium-sensitive channel required for salt chemosensation in C. elegans. Nature 494(7435): 95-99. PubMed ID: 23364694

Guo, Y., Wang, Y., Zhang, W., Meltzer, S., Zanini, D., Yu, Y., Li, J., Cheng, T., Guo, Z., Wang, Q., Jacobs, J. S., Sharma, Y., Eberl, D. F., Gopfert, M. C., Jan, L. Y., Jan, Y. N. and Wang, Z. (2016). Transmembrane channel-like (tmc) gene regulates Drosophila larval locomotion. Proc Natl Acad Sci U S A [Epub ahead of print]. PubMed ID: 27298354

He, L., Gulyanon, S., Mihovilovic Skanata, M., Karagyozov, D., Heckscher, E. S., Krieg, M., Tsechpenakis, G., Gershow, M. and Tracey, W. D., Jr. (2019). Direction selectivity in Drosophila proprioceptors requires the mechanosensory channel Tmc. Curr Biol 29(6): 945-956 e943. PubMed ID: 30853433

Jeong, Y. T., Oh, S. M., Shim, J., Seo, J. T., Kwon, J. Y. and Moon, S. J. (2016). Mechanosensory neurons control sweet sensing in Drosophila. Nat Commun 7: 12872. PubMed ID: 27641708

Kawashima, Y., Geleoc, G. S., Kurima, K., Labay, V., Lelli, A., Asai, Y., Makishima, T., Wu, D. K., Della Santina, C. C., Holt, J. R. and Griffith, A. J. (2011). Mechanotransduction in mouse inner ear hair cells requires transmembrane channel-like genes. J Clin Invest 121(12): 4796-4809. PubMed ID: 22105175

Liu, C., Zhang, B., Zhang, L., Yang, T., Zhang, Z., Gao, Z. and Zhang, W. (2020). A neural circuit encoding mating states tunes defensive behavior in Drosophila. Nat Commun 11(1): 3962. PubMed ID: 32770059

Liu, S., Wang, S., Zou, L., Li, J., Song, C., Chen, J., Hu, Q., Liu, L., Huang, P. and Xiong, W. (2019). TMC1 is an essential component of a leak channel that modulates tonotopy and excitability of auditory hair cells in mice. Elife 8. PubMed ID: 31661074

Mann, K., Gordon, M. D. and Scott, K. (2013). A pair of interneurons influences the choice between feeding and locomotion in Drosophila. Neuron 79(4): 754-765. PubMed ID: 23972600

Nakanishi, H., Kurima, K., Pan, B., Wangemann, P., Fitzgerald, T. S., Geleoc, G. S., Holt, J. R. and Griffith, A. J. (2018). Tmc2 expression partially restores auditory function in a mouse model of DFNB7/B11 deafness caused by loss of Tmc1 function. Sci Rep 8(1): 12125. PubMed ID: 30108230

Pan, B., Geleoc, G. S., Asai, Y., Horwitz, G. C., Kurima, K., Ishikawa, K., Kawashima, Y., Griffith, A. J. and Holt, J. R. (2013). TMC1 and TMC2 are components of the mechanotransduction channel in hair cells of the mammalian inner ear. Neuron 79(3): 504-515. PubMed ID: 23871232

Sanchez-Alcaniz, J. A., Zappia, G., Marion-Poll, F. and Benton, R. (2017). A mechanosensory receptor required for food texture detection in Drosophila. Nat Commun 8: 14192. PubMed ID: 28128210

Vaadia, R. D., Li, W., Voleti, V., Singhania, A., Hillman, E. M. C. and Grueber, W. B. (2019). Characterization of proprioceptive system dynamics in behaving Drosophila larvae using high-speed volumetric microscopy. Curr Biol 29(6): 935-944. PubMed ID: 30853438

Wang, X., Li, G., Liu, J., Liu, J. and Xu, X. Z. (2016). TMC-1 mediates alkaline sensation in C. elegans through nociceptive neurons. Neuron 91(1): 146-154. PubMed ID: 27321925

Wu, S. F., Ja, Y. L., Zhang, Y. J. and Yang, C. H. (2019). Sweet neurons inhibit texture discrimination by signaling TMC-expressing mechanosensitive neurons in Drosophila. Elife 8. PubMed ID: 31184585

Zhang, L., Yu, J., Guo, X., Wei, J., Liu, T. and Zhang, W. (2020). Parallel mechanosensory pathways direct oviposition decision-making in Drosophila. Curr Biol. PubMed ID: 32649914

Zhang, Y. V., Aikin, T. J., Li, Z. and Montell, C. (2016). The basis of food texture sensation in Drosophila. Neuron 91(4): 863-877. PubMed ID: 27478019

Biological Overview

date revised: 10 January 2021

Home page: The Interactive Fly © 2011 Thomas Brody, Ph.D.