The regulatory and modulatory roles of TRP family channels in malignant tumors and relevant therapeutic strategies.

Transient receptor potential (TRP) channels are one primary type of calcium (Ca2+) permeable channels, and those relevant transmembrane and intracellular TRP channels were previously thought to be mainly associated with the regulation of cardiovascular and neuronal systems. Nowadays, however, accumulating evidence shows that those TRP channels are also responsible for tumorigenesis and progression, inducing tumor invasion and metastasis. However, the overall underlying mechanisms and possible signaling transduction pathways that TRP channels in malignant tumors might still remain elusive. Therefore, in this review, we focus on the linkage between TRP channels and the significant characteristics of tumors such as multi-drug resistance (MDR), metastasis, apoptosis, proliferation, immune surveillance evasion, and the alterations of relevant tumor micro-environment. Moreover, we also have discussed the expression of relevant TRP channels in various forms of cancer and the relevant inhibitors' efficacy. The chemo-sensitivity of the anti-cancer drugs of various acting mechanisms and the potential clinical applications are also presented. Furthermore, it would be enlightening to provide possible novel therapeutic approaches to counteract malignant tumors regarding the intervention of calcium channels of this type.

The push-to-open mechanism of the tethered mechanosensitive ion channel NompC.

NompC is a mechanosensitive ion channel responsible for the sensation of touch and balance in Drosophila melanogaster. Based on a resolved cryo-EM structure, we performed all-atom molecular dynamics simulations and electrophysiological experiments to study the atomistic details of NompC gating. Our results showed that NompC could be opened by compression of the intracellular ankyrin repeat domain but not by a stretch, and a number of hydrogen bonds along the force convey pathway are important for the mechanosensitivity. Under intracellular compression, the bundled ankyrin repeat region acts like a spring with a spring constant of ~13 pN nm-1 by transferring forces at a rate of ~1.8 nm ps-1. The linker helix region acts as a bridge between the ankyrin repeats and the transient receptor potential (TRP) domain, which passes on the pushing force to the TRP domain to undergo a clockwise rotation, resulting in the opening of the channel. This could be the universal gating mechanism of similar tethered mechanosensitive TRP channels, which enable cells to feel compression and shrinkage.

Mechanotransduction Ion Channels in Hearing and Touch.

The ability of living organisms to detect mechanical force originates from mechanotransduction ion channels, which convert membrane tension into electrical or chemical signals that are transmitted to the brain. A variety of studies on touch and sound perception in both vertebrates and invertebrates have broadened our understanding of mechanotransduction and identified promising candidates for mechanotransduction ion channels. Here, we discussed the physiological properties of mechanotransduction ion channels in hearing and touch, the identification of their molecular entities, and recent structural studies providing insights to their gating mechanisms in force sensing. We present an updated review of the evidence supporting several candidates, including NOMPC, Brv1, and TMC channels, as mechanotransduction ion channels and highlight their qualifications satisfying the specific criteria proposed for a mechanotransducer.

Mechanosensitive Ion Channels: Structural Features Relevant to Mechanotransduction Mechanisms.

Activation of mechanosensitive ion channels underlies a variety of fundamental physiological processes that require sensation of mechanical force. Different mechanosensitive channels adapt distinctive structures and mechanotransduction mechanisms to fit their biological roles. How mechanosensitive channels work, especially in animals, has been extensively studied in the past decade. Here we review key findings in the functional and structural characterizations of these channels and highlight the structural features relevant to the mechanotransduction mechanism of each specific channel.

Ultrastructural organization of NompC in the mechanoreceptive organelle of Drosophila campaniform mechanoreceptors.

Mechanoreceptive organelles (MOs) are specialized subcellular entities in mechanoreceptors that transform extracellular mechanical stimuli into intracellular signals. Their ultrastructures are key to understanding the molecular nature and mechanics of mechanotransduction. Campaniform sensilla detect cuticular strain caused by muscular activities or external stimuli in Drosophila Each campaniform sensillum has an MO located at the distal tip of its dendrite. Here we analyzed the molecular architecture of the MOs in fly campaniform mechanoreceptors using electron microscopic tomography. We focused on the ultrastructural organization of NompC (a force-sensitive channel) that is linked to the array of microtubules in these MOs via membrane-microtubule connectors (MMCs). We found that NompC channels are arranged in a regular pattern, with their number increasing from the distal to the proximal end of the MO. Double-length MMCs in nompC29+29ARs confirm the ankyrin-repeat domain of NompC (NompC-AR) as a structural component of MMCs. The unexpected finding of regularly spaced NompC-independent linkers in nompC3 suggests that MMCs may contain non-NompC components. Localized laser ablation experiments on mechanoreceptor arrays in halteres suggest that MMCs bear tension, providing a possible mechanism for why the MMCs are longer when NompC-AR is duplicated or absent in mutants. Finally, mechanical modeling shows that upon cuticular deformation, sensillar architecture imposes a rotational activating force, with the proximal end of the MO, where more NOMPC channels are located, being subject to larger forces than the distal end. Our analysis reveals an ultrastructural pattern of NompC that is structurally and mechanically optimized for the sensory functions of campaniform mechanoreceptors.

The NompC channel regulates Nilaparvata lugens proprioception and gentle-touch response.

NompC channel is a member of the transient receptor potential (TRP) ion channel superfamily. It can regulate gentle-touch, locomotion, hearing and food texture detection in Drosophila. We cloned the NompC gene of Nilaparvata lugens (NlNompC). The full length NlNompC possessed similar structure as DmNompC, which belongs to TRPN subfamily. The expression pattern analysis of different developmental stages and body parts showed that the transcription of NlNompC was more abundant in adult stage and in the abdomen. Injection of double-stranded RNA (dsRNA) of NlNompC in the third-instar nymphs successfully knocked down the target gene with 75% suppression. At nine days after injection, the survival rate of dsRNA injected nymphs was as low as 9.84%. Behavioral observation revealed that the locomotion of the dsRNA injected nymphs was defective with much less movement compared to the negative control. Feeding and honeydew excretion of the dsRNA injected insects also decreased significantly. These results suggested that NlNompC is a classical mechanotransduction channel that plays important roles in proprioception and locomotion, and is essential for the survival of N. lugens. The results also contribute to the understanding of how TRP channels regulate proprioception.

Brv1 Is Required for Drosophila Larvae to Sense Gentle Touch.

How we sense touch is fundamental for many physiological processes. However, the underlying mechanism and molecular identity for touch sensation are largely unknown. Here, we report on defective gentle-touch behavioral responses in brv1 loss-of-function Drosophila larvae. RNAi and Ca2+ imaging confirmed the involvement of Brv1 in sensing touch and demonstrated that Brv1 mediates the mechanotransduction of class III dendritic arborization neurons. Electrophysiological recordings further revealed that the expression of Brv1 protein in HEK293T cells gives rise to stretch-activated cation channels. Purified Brv1 protein reconstituted into liposomes were found to sense stretch stimuli. In addition, co-expression studies suggested that Brv1 amplifies the response of mechanosensitive ion channel NOMPC (no mechanoreceptor potential C) to touch stimuli. Altogether, these findings demonstrate a molecular entity that mediates the gentle-touch response in Drosophila larvae, providing insights into the molecular mechanisms of touch sensation.

Proprioceptive Opsin Functions in Drosophila Larval Locomotion.

Animals rely on mechanosensory feedback from proprioceptors to control locomotory body movements. Unexpectedly, we found that this movement control requires visual opsins. Disrupting the Drosophila opsins NINAE or Rh6 impaired larval locomotion and body contractions, independently of light and vision. Opsins were detected in chordotonal proprioceptors along the larval body, localizing to their ciliated dendrites. Loss of opsins impaired mechanically evoked proprioceptor spiking and cilium ultrastructure. Without NINAE or Rh6, NOMPC mechanotransduction channels leaked from proprioceptor cilia and ciliary Inactive (Iav) channels partly disappeared. Locomotion is shown to require opsins in proprioceptors, and the receptors are found to express the opsin gene Rh7, in addition to ninaE and Rh6. Besides implicating opsins in movement control, this documents roles of non-ciliary, rhabdomeric opsins in cilium organization, providing a model for a key transition in opsin evolution and suggesting that structural roles of rhabdomeric opsins preceded their use for light detection.

NOMPC-dependent mechanotransduction shapes the dendrite of proprioceptive neurons.

Animal locomotion depends on proprioceptive feedback which is generated by mechanosensory neurons. We recently demonstrated that the evolutionarily conserved stumble (stum) gene is essential for mechanical transduction in a group of proprioceptive neurons in Drosophila leg joints. A specialized dendritic ending of the stum-expressing neurons is stretched by changes in joint position, suggesting that the dendritic site is specifically tuned for joint proprioception. Here, we show that the stum-expressing neurons express the mechanosensory channel NOMPC. In nompC mutants responses to joint position were abolished, indicating that NOMPC is the mechanosensitive channel in stum-expressing neurons. The NOMPC protein had a similar subcellular distribution as STUM, being located specifically at the dendritic site that is stretched by joint motions, thus validating that this site is a specialized sensory compartment. In the absence of NOMPC the sensory portion of the dendrite was misshapen, generating membrane protrusions. Thus, we have shown that mechanical responsiveness at a specialized dendritic site is essential for determination of the fine dendritic structure. The abnormal morphology at the sensory compartment of non-active neurons suggests that the dendrite samples for a responsive anchoring site and stabilizes the structure that elicits the effective mechanotransduction.