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Transmembrane protein 63: Biological Overview | References

An animal's decision to accept or reject a prospective food is based only, in part, on its chemical composition. Palatability is also greatly influenced by textural features including smoothness versus grittiness, which is influenced by particle sizes. This study demonstrates that Drosophila melanogaster is endowed with the ability to discriminate particle sizes in food and uses this information to decide whether a food is appealing. The decision depends on a mechanically activated channel, OSCA/TMEM63, which is conserved from plants to humans. tmem63 is expressed in a multidendritic neuron (md-L) in the fly tongue (proboscis). Loss of tmem63 impairs the activation of md-L by mechanical stimuli and the ability to choose food based on particle size. These findings reveal the first role for this evolutionarily conserved, mechanically activated TMEM63 channel in an animal and provide an explanation of how flies can sense and behaviorally respond to the texture of food provided by particles (Li, 2021).

Food selection is among the most critical and ancient behaviors exhibited by animals. Multiple classes of chemosensory receptors have been defined in flies, mice, and other animals that contribute to the discrimination of nutritious from noxious foods. In humans and other mammals, the textural features of food also have a major impact on food appeal. The texture of food is influenced by its hardness and viscosity, as well as the size of food particles. These features comprise the mouthfeel of food and illustrate the importance of somatosensation for assessing palatability. Nevertheless, the receptors that detect food texture are unknown in mammals. Recently, several groups have begun to exploit the fruit fly, Drosophila melanogaster, to unravel the receptors and neurons that allow animals to evaluate palatability based on physical features of food including hardness viscosity and temperature (Li, 2021).

For humans, one of the key physical features of food that impacts food appeal is particle size. For example, ice crystals of 10-20 μm cause ice cream to be perceived as very smooth, whereas larger crystals of 50 μm confer a perception of grittiness Particle sizes in chocolate also influence the evaluation of desirability. In experiments with human volunteers, the addition of garnet, polyethylene, or mica particles of different sizes to flavored syrups impacts food appeal (Li, 2021).

The influence of particle size on food appeal might be conserved throughout the animal kingdom. Drosophila feed on many types of fruit and are especially attracted to ripened and decaying fruit. The size of starch granules in mango and kiwifruit range from ~10-15 μm. In addition, decaying fruit is laden with yeast, which is also a food source for flies. The size of budding yeast is ~8-10 μm. Thus, flies might prefer foods with particle sizes that are typically found in fruits and budding yeast. However, the underlying molecular and cellular basis through which the size of particles in food alters acceptance or rejection remains unexplored (Li, 2021).

This study established Drosophila as an animal model for revealing the mechanism through which particle size influences food attraction. Flies prefer sucrose-containing food with particles of a particular size. This ability to discriminate between foods based on particle size depends on TMEM63, a member of a recently discovered family of mechanically activated channels that are conserved from plants (OSCA) to animals including humans (TMEM63). OSCA/TMEM63 represents the only protein family shown to be mechanically gated ion channels in both plants and animals. However, their roles in animals are unknown. This study found that the role of TMEM63 in sensing particles in food is dependent on its expression in a pair of mechanically activated multidendritic neurons (md-L) in the labellum, which is the fly's major taste organ. Although TMEM63 is required for sensing small particles in food, which exert subtle mechanical forces, it is dispensable for detecting other textural features of food such as hardness and viscosity, which cause stronger mechanical interactions with the labellum. Another mechanosensitive channel, TMC, is also expressed in md-L neurons. However, in contrast to TMEM63, the TMC protein functions in detecting the hardness and viscosity of food (Zhang, 2016) but is dispensable for particle sensation. These results reveal the first molecular and cellular underpinning for sensing particle sizes in food and a requirement for a TMEM63 channel in an animal (Li, 2021).

Mouthfeel results from the physical features of food including hardness, viscosity, and particle size. The human gustatory experience is influenced by particle size, because it impacts on whether a food is smooth or gritty. However, the mechanisms are unknown. This study established that flies are also able to discriminate between foods based on particle size. Their favorite size was 9.2 μm, which falls within the size range of the budding yeast in their diet and the starch granules in many of the fruit that they consume. The width of their pharynx is ~15 μm. Thus, there might be a selection for a preference for particles <15 μm. However, the width of the pharynx may not define the upper size limit of particles. Although flies do not chew food, they can process food extra-orally by expelling enzymes from the proboscis (Li, 2021).

The sensation of particle size in food is a type of somatosensation, which is proposed to depend on the detection of particles physically interacting with gustatory sensilla. Deflection of sensilla then activates a mechanically activated channel in md-L neurons that is critical for particle sensation. In support of this model, particles in food cause small deflections of gustatory sensilla. When the cuticle was depressed with a probe, thereby resulting in small angle deflections similar to those produced by particles, the md-L neurons displayed increased GCaMP6f responses and action potentials (Li, 2021).

The mechanically activated channel is TMEM63, because loss of this channel impairs food discrimination based on particle size. TMEM63 is expressed in md-L neurons, and mutation of tmem63 disrupts activation of md-L neurons by deflection of taste sensilla. Another mechanically activated channel, TMC, is also required in md-L neurons but for different aspects of texture sensation: hardness and viscosity. The overall structure and dimeric composition of OSCA/TMEM63 channels is similar to the structure and dimeric architecture of TMCs. Thus, this common structure may be well suited for sensing small physical differences in food texture (Li, 2021).

Despite the structural similarities and co-expression of TMEM63 and TMC in md-L neurons, it is suggested that it is unlikely that TMEM63 heterodimerizes with TMC. Expression of Drosophila TMEM63 in vitro is sufficient to produce a mechanically activated channel. Moreover, the phenotypes of tmem63 and tmc mutants are distinct. Although loss of tmem63 disrupts particle size sensation, the mutants can still discriminate between sugary foods with differences in hardness and viscosity. Conversely, mutation of tmc impairs the ability to detect the hardness and viscosity of sucrose-containing food but does not alter the behavioral preference for sucrose with particles (Li, 2021).

The observation that flies can discern different textural features of food, such as hardness from particle size, raises questions concerning the coding mechanism. This issue is particularly provocative given that the md-L neuron functions in the detection of these distinct textural features. It is suggested that one potential explanation is that md-L, as well as the MSNs that are associated with each taste hair, are all required for the detection of food hardness and viscosity, whereas only the md-L neuron functions in particle size discrimination. Indeed, hMSNs also contribute to food hardness detection (Li, 2021).

Alternatively, but not mutually exclusive, is that TMEM63 and TMC function in sensing different levels of deflection of gustatory hairs. The responses of md-L neurons to smaller deflections of a single sensillum are eliminated by the tmem63 mutation but not impacted by the tmc mutation. Conversely, the responses to larger deflections are impaired by the tmc mutation but not the tmem63 mutation. This suggests that TMC is required for larger mechanical stimulation caused by hard and viscous food, whereas TMEM63 is critical for more subtle mechanical stimulation that mimics the effects of particles in food. Indeed, TMEM63 could even provide md-L neurons with sensitivity to the smallest particles tested (1 μm), because 1 μm particles in food elicit small deflections of sensilla that are intermediate between those produced by the steps 1 and 2 cuticle depressions that induce action potentials in control, but not tmem63 mutants (Li, 2021).

It is also plausible that there are differences in the spatial and temporal deflections due to hardness/viscosity versus particles. Particles would cause only some adjacent sensilla in any group to be transiently deflected at any given time as they are contacted by the particles. In contrast, hardness/viscosity would result in neighboring sensilla (e.g., L-type) to be simultaneously deflected. In addition, TMEM63 may be more rapidly inactivated than TMC, enabling TMEM63 to be better suited to sense transient deflections from particles. However, a direct comparison between TMEM63 and TMC is not possible because most TMCs, including fly TMC, have been refractory to biophysical analyses in vitro (Li, 2021).

The finding that tmem63 is expressed in many GRNs raises future questions as to the roles of TMEM63 in GRNs. This study found that the GRNs do not function in particle sensation, and mutation of tmem63 does not cause defects in sensation of sucrose, caffeine, a carboxylic acid, or NaCl. Finally, this and a previous study on TMC7 raise questions as to whether TMEM63 and TMC function in food texture sensation in mammals (Li, 2021).

Humidity response in Drosophila olfactory sensory neurons requires the mechanosensitive channel TMEM63

Birds, reptiles and insects have the ability to discriminate humidity levels that influence their survival and geographic distribution. Insects are particularly susceptible to humidity changes due to high surface area to volume ratios, but it remains unclear how humidity sensors transduce humidity signals. This study identified Or42b-expressing olfactory sensory neurons, which are required for moisture attraction in Drosophila. The sensilla housing Or42b neurons show cuticular deformations upon moist air stimuli, indicating a conversion of humidity into mechanical force. Accordingly, this study found Or42b neurons directly respond to humidity changes and rely on the mechanosensitive ion channel TMEM63 to mediate humidity sensing (hygrosensation). Expressing human TMEM63B in Tmem63 mutant flies rescued their defective phenotype in moisture attraction, demonstrating functional conservation. Thus, these results reveal a role of Tmem63 in hygrosensation and support the strategy to detect humidity by transforming it into a mechanical stimulus, which is unique in sensory transduction (Li, 2022).

In existing concepts explaining the mechanism of hygrosensory transduction, humidity receptors are either mechanosensitive or thermosensitive molecules. Previous studies have revealed the essential roles of IR40a, IR68a, IR93a and IR25a in hygrosensation, but due to the difficulty of function analysis of theses molecules in heterologous expression system, how they respond to humidity still need further investigation. By neuron ablation methods and mutant analysis, this study revealed a mechanosensory pathway that specifically contributes to 70% relative humidity (RH)-induced attraction. The data provide structural and molecular evidence supporting the concept that humidity changes can be transformed into mechanical cues which evoke hygrosensory inputs via mechanosensitive molecules. Moisture changes the cuticular curvature of ab1 sensilla, which may alter the membrane tension of associated sensory cilia expressing the mechanosensitive channel TMEM63. Previous in vitro studies have shown that TMEM63/OSCAs are sensitive to both osmotic stress and negative pressure (Zhang, 2018; Murthy, 2018), the present study shows that Drosophila TMEM63 confers stretch-activated currents when transfected into S2 cells. Thus DmTMEM63 might be a potential primary molecular receptor that can sense the membrane deformation resulting from humidity changes, although it could not be ruled out that TMEM63 could mediate humidity perception by sensing osmotic pressure changes. Intriguingly, this study reveals an evolutionary strategy to detect the obscure humidity or water vapor by converting it into a defined mechanical deformation, which is unique in the sensory system (Li, 2022).

Moreover, distinct Drosophila species show diverse humidity preferences and therefore have different geographical distributions, for example, experiments showed that rainforest flies prefer a high level of humidity while desert-dwelling flies prefer 20% RH. Apart from insects, other poikilotherms also display similar humidity-related geographical distributions. The current findings may provide a clue to study how distinct animal species evolve to select their native habitats (Li, 2022).

The data show that TMEM63 mediates hygrosensation in Or42b neurons, a neuron group for food odor detection in Drosophila, suggesting an early integration of hygrosensory and olfactory inputs. This is not surprising since insects rely on multisensory integration for locating vital resources. Disease-transmitting mosquitoes also exploit multimodal cues, such as CO2, odors, body heat and moisture, for finding and selecting potential hosts. Molecular receptors for chemicals and temperature have been unraveled in mosquitoes, but the receptors that incorporate humidity into multimodal sensory inputs have remained elusive. It will be interesting to test if Tmem63 functions as a humidity sensor in mosquitoes and guide host attraction by detecting the elevated humidity levels at the proximity of the host, which might provide insights into the molecular underpinnings of host approach behavior in disease vectors (Li, 2022).

The TMEM63 proteins have been found to be evolutionarily conserved from Drosophila to mammals (Murthy, 2018; Hou, 2014; Yuan, 2014), but research on their physiological function is still in its infancy. This study showed that human TMEM63B can substitute for Drosophila TMEM63 to mediate moisture attraction behavior, an evolutionary solution to satisfy the internal needs for water. Further studies are necessary to determine whether mammalian Tmem63 genes function in behaviors related to osmotic regulation or water homeostasis (Li, 2022).

Search PubMed for articles about Drosophila Tmem63

Hou, C., Tian, W., Kleist, T., He, K., Garcia, V., Bai, F., Hao, Y., Luan, S. and Li, L. (2014). DUF221 proteins are a family of osmosensitive calcium-permeable cation channels conserved across eukaryotes. Cell Res 24(5): 632-635. PubMed ID: 24503647

Jojoa-Cruz, S., Saotome, K., Murthy, S. E., Tsui, C. C. A., Sansom, M. S., Patapoutian, A. and Ward, A. B. (2018). Cryo-EM structure of the mechanically activated ion channel OSCA1.2. Elife 7. PubMed ID: 30382939

Li, Q. and Montell, C. (2021). Mechanism for food texture preference based on grittiness. Curr Biol. PubMed ID: 33657409

Li, S., Li, B., Gao, L., Wang, J. and Yan, Z. (2022). Humidity response in Drosophila olfactory sensory neurons requires the mechanosensitive channel TMEM63. Nat Commun 13(1): 3814. PubMed ID: 35780140

Marques, M. C., Albuquerque, I. S., Vaz, S. H. and Bernardes, G. J. L. (2019). Overexpression of Osmosensitive Ca(2+)-Permeable Channel TMEM63B Promotes Migration in HEK293T Cells. Biochemistry 58(26): 2861-2866. PubMed ID: 31243992

Murthy, S. E., Dubin, A. E., Whitwam, T., Jojoa-Cruz, S., Cahalan, S. M., Mousavi, S. A. R., Ward, A. B. and Patapoutian, A. (2018). OSCA/TMEM63 are an Evolutionarily Conserved Family of Mechanically Activated Ion Channels. Elife 7. PubMed ID: 30382938

Yuan, F., Yang, H., Xue, Y., Kong, D., Ye, R., Li, C., Zhang, J., Theprungsirikul, L., Shrift, T., Krichilsky, B., Johnson, D. M., Swift, G. B., He, Y., Siedow, J. N. and Pei, Z. M. (2014). OSCA1 mediates osmotic-stress-evoked Ca2+ increases vital for osmosensing in Arabidopsis. Nature 514(7522): 367-371. PubMed ID: 25162526

Yue, X., Sheng, Y., Kang, L. and Xiao, R. (2019). Distinct functions of TMC channels: a comparative overview. Cell Mol Life Sci 76(21): 4221-4232. PubMed ID: 31584127

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

Zhang, M., Wang, D., Kang, Y., Wu, J. X., Yao, F., Pan, C., Yan, Z., Song, C. and Chen, L. (2018). Structure of the mechanosensitive OSCA channels. Nat Struct Mol Biol 25(9): 850-858. PubMed ID: 30190597

Zhao, X., Yan, X., Liu, Y., Zhang, P. and Ni, X. (2016). Co-expression of mouse TMEM63A, TMEM63B and TMEM63C confers hyperosmolarity activated ion currents in HEK293 cells. Cell Biochem Funct 34(4): 238-241. PubMed ID: 27045885

date revised: 26 July 2022

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