Mechanotransduction in gastrointestinal smooth muscle cells: role of mechanosensitive ion channels

Abstract

Mechanosensation, the ability to properly sense mechanical stimuli and transduce them into physiologic responses, is an essential determinant of gastrointestinal (GI) function. Abnormalities in this process result in highly prevalent GI functional and motility disorders. In the GI tract, several cell types sense mechanical forces and transduce them into electrical signals, which elicit specific cellular responses. Some mechanosensitive cells like sensory neurons act as specialized mechanosensitive cells that detect forces and transduce signals into tissue-level physiological reactions. Nonspecialized mechanosensitive cells like smooth muscle cells (SMCs) adjust their function in response to forces. Mechanosensitive cells use various mechanoreceptors and mechanotransducers. Mechanoreceptors detect and convert force into electrical and biochemical signals, and mechanotransducers amplify and direct mechanoreceptor responses. Mechanoreceptors and mechanotransducers include ion channels, specialized cytoskeletal proteins, cell junction molecules, and G protein-coupled receptors. SMCs are particularly important due to their role as final effectors for motor function. Myogenic reflex—the ability of smooth muscle to contract in response to stretch rapidly—is a critical smooth muscle function. Such rapid mechanotransduction responses rely on mechano-gated and mechanosensitive ion channels, which alter their ion pores’ opening in response to force, allowing fast electrical and Ca²⁺ responses. Although GI SMCs express a variety of such ion channels, their identities remain unknown. Recent advancements in electrophysiological, genetic, in vivo imaging, and multi-omic technologies broaden our understanding of how SMC mechano-gated and mechanosensitive ion channels regulate GI functions. This review discusses GI SMC mechanosensitivity's current developments with a particular emphasis on mechano-gated and mechanosensitive ion channels.

INTRODUCTION

What Is Mechanotransduction?

Mechanotransduction is a process by which cells convert mechanical forces into electrical and chemical signals that result in specific cellular responses. Mechanotransduction is a two-step process. First, cells sense the mechanical input from their surroundings using mechanoreceptors. Second, cells convert the receptor signal into the appropriate cellular response using mechanotransducers. A cell may use a repertoire of mechanoreceptors: ion channels, transmembrane adhesion receptors, cell surface receptors, and sarcomeric proteins (1). Mechanotransduction is at the center of critical sensory processes, such as touch and hearing, and autonomic processes, such as blood pressure regulation, gastrointestinal (GI) motility, embryonic development, vascular tone, and muscle stretch (1–4). Mechanotransduction defects are associated with a range of genetic and acquired diseases, from muscular dystrophies, cardiomyopathies, and cancer, to GI functional and motility disorders (2, 5, 6).

Mechanotransduction in the GI Tract

Electromechanical organs like the GI tract, bladder, and heart generate tissue-specific mechanical forces sensed by mechanosensitive cells. In the GI tract, mechanotransduction is critical for normal function. Notable GI disorders that involve disrupted mechanotransduction include diverticulosis (7), ulcerative colitis (8), Crohn’s disease (8), chronic constipation (9), pseudo-obstruction, visceral hypersensitivity (10, 11), functional dyspepsia (12), irritable bowel syndrome, colon cancer (13), and even systemic conditions, like obesity (14). Since the GI tract is continually changing its volume and moving, all GI tract cells live in a mechanical world. We can classify the GI tract cells as specialized and nonspecialized mechanosensitive cells. Specialized mechanosensitive cells, like enterochromaffin cells, intrinsic primary afferent neurons, intestinofugal afferent neurons, and extrinsic sensory neurons, sense and convert mechanical forces into an appropriate physiological response. Meanwhile, nonspecialized mechanosensitive cells such as smooth muscle cells (SMCs), interstitial cells of Cajal (ICCs), and immune cells adjust their own function in response to mechanical stimuli (15–17).

SMC mechanotransduction is critical for normal GI tract development and subsequent function. During development, smooth muscle orients itself in response to mechanical forces, and this proper alignment of muscle layers is crucial for normal function (18). The role of SMC mechanotransduction extends and expands after development. It is critical for normal function since SMCs are the ultimate effectors—they generate resting tone, muscle contraction, and relaxation. As these functions require constant and rapid responses to mechanical stimuli, gut SMCs employ a wide array of membrane-associated molecules to sense and process the mechanical stimuli. These include surface receptors (ion channels, G protein-coupled receptors, and kinases), specialized intracellular cytoskeletal proteins, and extracellular cell-cell connections (1). Cells use these molecules to assimilate various mechanical stimuli like shear stress, stretch, pressure, torsion, and compression and convert them into short-term (i.e., changes in voltage and ion concentrations) and long-term (i.e., changes in gene expression) effects (5). The layers of the GI tract (mucosa, submucosa, muscularis, and serosa) have distinct mechanical properties and sense a range of passive and active mechanical forces, which ultimately result in efficient digestion, absorption, and propulsion of contents. Mechanical activity in the gut is wide ranging, including the peristaltic reflex, segmentation, propulsion, migratory motor activity, and structural rearrangements in response to function. In the muscle layer, the peristaltic, segmental, and ring contractions generate wall tension, whereas luminal content flow results in shear stress that drives the activation of various mechanosensitive components (19, 20).

Among the mechanically sensitive molecules, ion channels are indispensable for rapid mechanotransduction. Mechanically sensitive ion channels rapidly transduce force and quickly affect downstream mechanisms via electrical and calcium signaling. We may divide the mechanically sensitive ion channels into two categories. Mechano-gated ion channels are transmembrane proteins whose primary role is to respond to force (4, 21) and serve as classic mechanoreceptors. Mechanosensitive ion channels are gated by primary stimuli other than force, like chemical ligands and voltage, and force alters the channels' responses to their primary stimuli (22, 23). Mechano-gated ion channels sense forces, through either the membrane (force-from-lipid) (24) or structural proteins (force-from-tether) (25), by specialized mechanically sensitive gates that open (or close) in response to mechanical input (26). To be considered mechano-gated, an ion channel must meet specific criteria (4, 21): 1) It must be required for a response to mechanical stimuli and not for cellular development; 2) it must be expressed in a mechanosensitive cell, and the loss of the channel should abolish mechanical response; 3) alterations in the properties of the ion channel gene should alter the mechanical properties of the channel; and 4) it should retain the response to force when expressed in a heterologous system. The criteria for classifying the mechanosensitive ion channels do not currently exist. Eukaryotic mechano-gated and mechanosensitive channels' identities have been elusive, but recent advances in biophysics, bio-omics, and structural biology combined to substantially advance their identification (27–32). Indeed, the identification of mechano-gated ion channels is a critical step toward understanding the process of mechanotransduction. In this review, we highlight the importance of gut SMC mechanotransduction with a particular emphasis on SMC mechano-gated and mechanosensitive ion channels.

GI SMOOTH MUSCLE MECHANOSENSITIVITY

Myogenic Reflex as the Prototypical GI SMC Mechanotransduction

Coordinated contractility of GI SMCs is essential for normal GI motility. Synchronized interactions between myogenic, neurogenic, and hormonal mechanisms regulate GI smooth muscle tone and activity (33, 34). Neuronal and hormonal mechanisms adjust and synchronize motility patterns of the gut, whereas myogenic mechanisms may be fundamental processes since GI smooth muscle exhibits tone and contractility independent of neuronal input (33, 35).

In smooth muscle, the myogenic response, or reflex, refers to smooth muscle's ability to respond to mechanical forces independent from the nervous system. In a classic set of studies, Bayliss reported the contractile responses of arterial smooth muscle in response to increased transmural pressure (36). Although Edith Bulbring showed a myogenic response in GI smooth muscle (37), much of our mechanistic understanding of the myogenic response still derives from studies with vascular smooth muscle (38, 39). The smooth muscle syncytium consists of SMCs and interstitial cells (currently ICCs and PDGFR-α+ cells). ICCs exhibit spontaneous pacemaker activity in the stomach, small intestine, and colon, which is independent of hormonal and neuronal inputs. Pacemaker activity generates low-amplitude contractions that are further modulated by neuronal inputs (40). ICCs are mechanosensitive (16), and human ICCs express mechanosensitive ion channels such as Na_V1.5 (124), but their role in GI mechanosensitivity remains poorly understood.

Ca²⁺ plays a crucial role in mechanotransduction through various mechanisms that involve changes in its cytosolic concentration. Ca²⁺ is essential for SMC function in the vascular system for maintaining arteriolar tone (41) and elicitation of myogenic constriction (38). The amplitudes and timing of Ca²⁺ signals are important signal transduction parameters. Rapid transient events or prolonged global responses are dependent on the intensity and timing of the stimulus (42). Mechano-gated ion channels are prime candidates for rapid Ca²⁺ entry in response to membrane force, and therefore, they are leading candidates to elicit the myogenic response. Various mechano-gated cation currents exist in vascular SMCs (43–45), carried by transient receptor potential (TRP) (39) and Piezo (46) channels. However, we know little about how GI SMCs contribute toward myogenic response, and even broadly, what molecular entities sense and mediate this response in gut SMCs.

Studies in toad stomach SMCs suggest that the opening of stretch-activated channels allows ions like Ca²⁺ to influx from the extracellular space, and subsequent membrane depolarization activates voltage-gated Ca²⁺ channels and Ca²⁺ release from internal stores that markedly raise intracellular Ca²⁺ (47). Small focal mechanical deformation of the plasma membrane of myocytes isolated from the rabbit distal colon's longitudinal muscle layer resulted in localized intracellular Ca²⁺ concentration transients that propagated throughout the cell (48). The mediators of mechanically induced Ca²⁺ influx may be L-type voltage-gated Ca²⁺ channels themselves (23), but since they are mechanosensitive, rather than mechano-gated, they may not be available to activate at SMC's resting potential. Further, in some experiments, Ca²⁺ channel blockers did not affect mechanically induced responses (38, 48), suggesting the involvement of other mechanically activated Ca²⁺ influx pathways and specifically mechano-gated cation channels. The cell membrane’s lipid composition and the interaction between the cytoskeleton and the extracellular membrane proteins may also influence the mechanosensitive ion channel activity (49–53). These studies demonstrate that SMCs exhibit diverse Ca²⁺ signaling patterns in response to mechanical stimuli, and mechano-gated ion channels take center stage in many rapid mechanosensing processes. In the subsequent sections, we discuss how different mechano-gated and mechanosensitive ion channels of gut SMCs might contribute toward gut myogenic response and mechanical sensitivity.

GI SMOOTH MUSCLE MECHANOSENSITIVE ION CHANNELS

In GI SMCs, the main candidates for rapid mechanotransduction required in myogenic response are mechanosensitive and mechano-gated ion channels (4, 52). Several types have been identified and characterized in GI SMCs, including but not limited to nonselective cation channels; voltage-gated Na⁺, Ca²⁺, and K⁺ channels; Ca²⁺-activated K⁺ channels; and two-pore K⁺ (K2P) channels (4, 38, 47) (Table 1).

Table 1.

Mechanosensitive and mechano-gated ion channels in GI SMCs

Mechano Channel	Current	Type	Characterized	Location in GI Smooth Muscle	Reference
Cav1.2	L-type Ca2+	Mechanosensitive	Yes	Human jejunum circular SMCs, human stomach SMCs	(23, 54)
Nav1.5	Na+	Mechanosensitive	Yes	Human jejunal circular SMCs	(55)
Ca2+-activated large conductance K+ channel (BKCa)	K+	Mechanosensitive	Yes	Rat colon SMCs	(56)
TREK-1	K+	Mechano-gated	Yes	Mouse small bowel and colon SMCs	(57–60)
TRPC4	Nonselective cation	Mechanosensitive	Yes	Mouse stomach, small bowel, and colon SMCs	(61–64)
TRPC6	Nonselective cation	Mechanosensitive	Yes	Mouse small bowel and colon SMCs	(61, 63,64)
TRPC7	Nonselective cation	Mechanosensitive	No, RNA expression	Mouse small bowel and colon SMCs	(63,64)

Ion Channels Whose Opening Depolarizes the Cell Membrane

Nonselective cation channels.

Membrane stretch activates nonselective cation channels in the cells from the cardiovascular system—rat atrial myocytes (65), guinea pig arterial SMCs (44), and porcine arterial SMCs (43). Similarly, pressurized patches from single SMCs from the toad stomach contained nonselective cation currents (47). Hypotonic cell swelling also increased carbachol-evoked inward current in guinea pig ileal SMCs and gastric myocytes through potentiation of nonselective cation channels (66, 67). Although the majority of nonselective mechano-gated cation channels are primarily permeable to Na⁺ and K⁺, they also conduct Ca²⁺ and Mg²⁺ (43, 66, 67). Indeed, single stretch-activated cationic channels in SMCs conduct quantities of Ca²⁺ adequate for stimulating the myogenic response (68). The commonly used nonselective mechano-gated ion channel blocker gadolinium (Gd³⁺) (69) eliminated the myogenic reflex and prevented action potential firing in stretched tissue (70). These studies suggest that nonselective cation channels may be important mediators of the myogenic reflex. Indeed, a variety of nonselective cation channels is found in GI SMCs (16).

Transient receptor potential channels.

The mammalian transient receptor potential (TRP) channel family is large. It comprises 28 members and is subdivided into five classes: TRPC, TRPV, TRPA, TRPP, and TRPM (71). TRP channels are cation-permeable ion channels gated by various stimuli, including chemicals like capsaicin (e.g., TRPV1) (72), temperature (e.g., TRPA1) (73), and force (e.g., TRPC6) (71). TRP channels are sensitive to various forms of mechanical force, including fluid shear stress and membrane stretch, and are potential candidates for mediating stretch-activated cation currents and pressure-induced depolarization (71). Among different subgroups, TRPC1/6, TRPM4, TRPV2/4, and TRPP1 (PKD2) have been associated with mechanotransduction in vascular SMCs (39, 71, 74, 75). Ca²⁺ influx through TRPC channels controls a variety of biological functions, including regulation of SMC proliferation, endothelin-evoked arterial contraction, neuronal differentiation, and cardiac hypertrophy (76). TRP ion channels are expressed in the GI tract, where they act as molecular sensors and transducers regulating various functions (61, 64, 77, 78). Deletion of TRPC4 and TRPC6 in GI SMCs results in impaired smooth muscle contraction and intestinal motility in vivo (63). Similarly, TRPC4 has been demonstrated as an essential component of the nonselective cation channels in murine stomach (62).

TRP channels are involved in a variety of mechanosensory processes. However, it remains unclear whether some TRP channels are the primary receptors for mechanical force (mechano-gated channels), or they are mechanosensitive or they are involved in downstream mechanotransduction. For example, we discuss the involvement of TRPC1 and TRPC6 channels in mechanosensing by a range of cells in the vascular tree. However, direct mechano-gating of TRPC1 and TRPC6 remains in question. Some studies suggest that they are mechano-gated, whereas others show that they are not directly mechano-gated when expressed in a heterologous system (79) or liposomes (80). It could be that cell-specific cofactors are required. TRPC6 expressed in Caenorhabditis elegans neurons restored mechanosensitivity in a touch-insensitive mutant but still required diacylglycerol for activation (80).

Piezo channels.

Piezo mechano-gated ion channels have recently been established as major force sensors in mammalian cells (28). The two isoforms, Piezo1 and Piezo2, are essential components of many mechanosensory processes during development (46, 81), homeostasis (82), blood pressure regulation (83), and cell and tissue restructuring in response to endogenous forces (46, 81). Piezo1 and Piezo2 mechano-gated ion channels are expressed throughout the length of the GI tract (28). Piezo2 is generally restricted to specialized mechanosensory cells—like epithelial Merkel cells and gut epithelial enteroendocrine cells (84), and neurons (85, 86). Piezo1 is more broadly expressed (2), and although it is expressed in vascular SMCs (46), its expression and roles in GI SMCs remain unknown. Piezo1 biophysical properties, including nonselective cationic permeability, fast activation, and slow-to-intermediate inactivation (28), make it a strong candidate to contribute to physiological functions such as the myogenic reflex. Piezo channels can be inhibited by the broad mechanosensitive channel blockers Gd³⁺ and ruthenium red, as well as the more specific Piezo channel inhibitor GsMTx-4 (15, 28, 46, 87). Indeed, guinea pig gastric SMCs express mechanically sensitive nonselective cation currents inhibited by Gd³⁺ and with kinetic and conductance properties like Piezo1 channels (88). Based on these findings, studies have begun exploring Piezo channels in the gut (89). However, the precise distribution of Piezo channels and their function in GI SMCs remains unexplored.

Cation-selective channels.

Voltage-gated ion channels regulate membrane potential and endow GI smooth muscle with electrical excitability, which is required for contractility. GI SMCs express voltage-gated Ca²⁺, Na⁺, and K⁺ channels, and some of these channels are mechanosensitive.

Voltage-gated Ca²⁺ channels.

Ca²⁺ influx into SMCs is essential for intestinal smooth muscle contractility. The main pathway for Ca²⁺ entry into SMCs is through the dihydropyridine-sensitive L-type Ca²⁺ channels. Calcium channels identified in human jejunum circular SMCs (CACNA1C-encoded Ca_V1.2) can be activated by an increase in positive pressure or external shear forces (23, 53) or by osmotic stress (54). In gastric SMCs, although f-actin cytoskeleton disruption diminished L-type currents in isotonic solutions, cytoskeletal disruption did not alter L-type currents in response to osmotic stress (54). Similarly, in rat myocytes cell swelling and application of negative or positive pressure increased the dihydropyridine-sensitive inward current. Interestingly, bilayer composition rather than intact cytoskeleton modifies L-type channel mechanosensitivity, leading to altered activity and Ca²⁺ entry in human jejunum circular layer myocytes (90).

The external muscle layer SMCs also express a low voltage-activated, Ca²⁺-permeable ionic conductance in mouse (91, 92) and human colon (93), as well as ICCs from dog and mouse colon, and mouse small intestine (94). Early studies in colonic myocytes characterized the currents as T-type (92), and later work identified T-type Ca²⁺ channel (Cacna1h-encoded Ca_V3.2) in mouse jejunum ICC and SMCs (91). Ca_V3.2 can be blocked by the gut antispasmodic otilonium bromide, and T-type blocker mibefradil can modulate human colonic smooth muscle contraction (95). Though T-type Ca²⁺ channels are involved in mechanotransduction (96), their mechanosensitivity and roles in GI SMCs remain unclear.

Voltage-gated Na⁺ channels.

SMCs are thought of as Ca²⁺-centric systems with minimal contribution from Na⁺ channels. However, this dogma has been successfully challenged, especially for human SMCs. SCN5A-encoded Na_V1.5 is expressed in cardiomyocytes (97). Interestingly, there is species dependence, as mouse GI SMCs do not express Na_V1.5 (55, 98, 99). Indeed, in human GI SMCs, the influx of Na⁺ through Na_V1.5 quickly depolarizes the membrane potential, which was found to be important for the regulation of the electrical slow waves in GI smooth muscle (100). Na_V1.5 is also mechanosensitive (22, 101, 102). Mechanical forces have several important effects on Na_V1.5, including their gating, kinetics, and inactivation (22, 53). In human jejunum circular SMCs, Na_V1.5 mechanosensitivity is diminished by f-actin disruption (53), and the mechanosensitive response of heterologously expressed Na_V1.5 is regulated by the cytoskeletal proteins, syntrophin γ2 and telethonin (49, 50). The bilayer also has important effects on Na_V1.5 mechanosensitivity. Lipid-permeable anesthetics, such as lidocaine and ranolazine, known to alter GI sensorimotor function, diminish Na_V1.5 mechanosensitivity (9, 22, 103). Perhaps most importantly, human SCN5A mutations are associated with multiple clinical phenotypes, including conduction disorders in the heart and irritable bowel syndrome (IBS) in the GI tract (104, 105). IBS-associated SCN5A mutations frequently impact voltage gating (106) and/or mechanosensitivity (101, 102).

Ion Channels Whose Opening Hyperpolarizes the Cell Membrane

Several types of potassium channels exist in GI SMCs, including Ca²⁺-activated, voltage-gated, and two-pore K⁺ (K2P) channels. Variable K⁺ channel expression in SMCs contributes to the wide range of resting potentials and electrical patterns in GI smooth muscle (107). Except for inward-rectifying potassium channels, K⁺ channel opening results in K⁺ efflux, membrane hyperpolarization, and a consequent decrease in cell excitability (2). Therefore, although K⁺ channels are unlikely contributors to activation of the myogenic reflex, they may contribute as mechanical brakes, as recently shown in neurons (108) and vascular SMCs, the latter of which were activated during myogenic vasoconstriction to suppress response (109). They also contribute to active relaxation precipitated by organ filling, as in the proximal colon (58).

Ca²⁺-activated K⁺ channels.

GI SMCs express several Ca²⁺-activated K⁺ channels, which contribute to GI smooth muscle excitability (107). Mechanosensitive Ca²⁺-activated large conductance K⁺ channels (BK_Ca) produce large outward currents in GI SMCs, decreasing excitability (56). Compared to other GI segments, BK_Ca expression and current densities are highest in the colon (56). BK_Ca in rat colonic SMCs was not only sensitive to stretch, but BK_Ca blocker charybdotoxin attenuated the relaxation response of colonic smooth muscle strips to stretch (56).

Two-pore K⁺ channels.

Among the two-pore K⁺ (K2P) channel family, TREK-1 is specific to smooth muscle tissue in both mouse ileum and colon, whereas TREK-2 and TRAAK channels were found in enteric neurons but not in smooth muscle (58–60). In organ bath experiments, K2P channel activators induced the relaxation of ileum and colon tissues precontracted by KCl and carbachol and reduced the amplitude of spontaneous contractions (59). These channels contribute to membrane hyperpolarization and relaxation in the GI tract regions where relaxation is required upon mechanical stimulation, such as ascending colon that serves an important storage function (60). The K2P channel TREK-1 is mechano-gated (57). Interestingly, both dietary (51) and membrane lipids affect the mechanical sensitivity of several ion channels, and this is strongly established for TREK-1 (110). Studies have also demonstrated the presence of TREK-1 in rat bladder myocytes where the channel is activated by arachidonic acid and may have an important role in the regulation of bladder smooth muscle tone during urine storage (111).

Voltage-gated K⁺ channels.

Vascular SMCs express variety of voltage-gated K⁺ (K_V) channel isoforms, which play important roles in regulating their contraction (3, 107), and several studies showed that K_V channels are mechanosensitive (112, 113). In vascular SMCs, multiple K_V channels contribute to resting membrane potential and exhibit negative feedback regulation of myogenic tone (3, 114, 115). GI SMCs also express several types of K_V channels that contribute to the resting membrane potential in these cells (107, 116, 117). K_V channel mechanosensitivity may serve as a “mechanical brake” to electrical excitation (108), but the roles of K_V channel in SMC mechanosensitivity remain poorly understood.

To summarize, several types of mechano-gated and mechanosensitive ion channels are expressed in GI SMCs, and they regulate muscle contraction primarily through increasing the intracellular Ca²⁺ concentration. In many instances, these channels coordinate mechanotransduction. For example, TRPC1 and TRPC6 channels cooperate with TRPV4 in neurons to mediate mechanical hyperalgesia and primary afferent nociceptor sensitization (118). Similarly, TRPC3 and TRPC6 are co-expressed in sensory neuron cell lines, which are required for the normal function of cells involved in touch and hearing (119). Furthermore, activation of TRPV1 channels with capsaicin, in either dorsal root ganglion neurons or a heterologous expression system, inhibited Piezo1 and Piezo2 by depleting phosphatidylinositol 4,5-bisphosphate [PI(4,5)P]₂ and its precursor PI(4)P from the plasma membrane through Ca²⁺-induced phospholipase Cδ (PLCδ) activation (120). ENaC, TRPM4, and TRPC6 also play important roles in the pressure-induced myogenic response, and ENaC and TRPM4 interact in rat posterior cerebral arteries (121). Piezo1 and Piezo2 channels are rarely co-expressed in the same cell, but when they are as in baroreceptors (122) and chondrocytes (123), they collaborate to produce a physiologic response.

CONCLUSIONS AND FUTURE DIRECTIONS

Coordinated myogenic and neurogenic mechanisms regulate smooth muscle function and hence gut motility. Smooth muscle contraction in response to mechanical force occurs after blocking neuronal activity, a phenomenon called myogenic reflex. However, the mechanistic understanding of the myogenic reflex and other mechanosensing behavior of SMCs is derived mostly from the vascular system, where these concepts are deeply explored, whereas these mechanisms in GI SMCs remain mostly undetermined.

GI SMCs express a wide range of mechanosensitive ion channels. Like vascular SMCs, mechanosensitive ion channels present in GI SMCs may contribute to the myogenic response, but their identities and roles remain unclear. There are important, but not insurmountable, barriers to the progress in this area. Perhaps the biggest hurdle is the identification of mechano-gated channels. As we described above, an ion channel needs to fulfill certain criteria to be considered mechano-gated (4, 21) or mechanosensitive. Several ion channels initially considered as candidate mechano-gated channels failed to fulfill some of these criteria. A classic example is the TRP channel family. Many TRP channels were initially considered as mechano-gated (124, 125). However, recent studies demonstrated that many of these TRP channels are not mechano-gated (79). Instead, they either are involved in downstream mechanotransduction processes or require additional cellular components like the cytoskeleton to function as mechanoreceptors (124, 126–128). To define the identities and roles of mechano-gated channels in mechanosensitive cells like GI SMCs, we must clearly define the mechanically modulated channels as mechano-gated or mechanosensitive, and we need to pay close attention to these channels' subcellular localization and mechanical nano-environment. These data will be required to determine how GI SMCs integrate different mechanical inputs; whether force is distributed between the lipid bilayer, cytoskeletal proteins, and cell surface mechano-gated channels; whether different gating mechanisms are involved for a specific mechanical input; and how different types of mechanosensitive ion channels (depolarizing and hyperpolarizing) are regulated. Recent developments in function-transcription approaches, single-cell RNA sequencing, and spatial transcriptomics technologies are giving new opportunities in understanding the biochemical processes at the cellular level. Single-cell RNA sequencing of vascular SMCs has yielded knowledge about their heterogeneity (129), ability to phenotypically modulate in response to various stimuli (130, 131), and developmental pathways (132, 133). Further, single-cell RNA sequencing following whole cell electrophysiology (Patch-seq) shed light on functional diversity in excitable cell types (134–137). Although single-cell RNA sequence analysis gives valuable information regarding cellular heterogeneity, spatial transcriptomics renders spatially defined characteristics of these cells. Combination of single-cell sequence and spatial analysis simultaneously provides cellular heterogeneity and their spatial distribution and association with each other in a tissue (138). Applying these techniques in GI SMCs will help to understand distinct smooth muscle subtypes in different regions of the GI tract, their functional diversity, and how they integrate with other cell types to exhibit physiologic functions, like the classic myogenic reflex riddle.

The progress in our understanding of the GI SMC myogenic response is further hampered by species, sex, age, and regional differences in their mechanotransduction mechanisms. GI SMCs are not a homogenous population, and they have varying morphological (139), electrical, and mechanical properties (40). Further, myogenic properties vary in different regions of the GI tract and between circular and longitudinal muscle layers, and these systems are likely optimized to perform the requisite functions. Apparent differences in myogenic responses also exist between species and with age. Notable example abounds. Na⁺ currents carried by Na_V1.5 were identified in human and dog SMCs, but not in mouse pig or guinea pig SMCs (99), and myogenic peristaltic contractions can be stimulated by muscarinic agonists in isolated rabbit and rat colon, but not in guinea pig or mouse colon (140). Age is a critical factor as well. For example, in colonic smooth muscle tissue of baboons, carbachol-induced contractile responses were reduced with age (141), and changes in gut anatomy and function with age are deeply established (142).

Smooth muscles are capable of autoregulation and actively contract in response to stretch. Ion channels modulated by mechanical forces likely contribute to the initiation and transduction of the myogenic response. Studies in the vascular SMCs and other cell types have identified several potential ion channels involved in mechanotransduction. However, knowledge regarding GI SMCs mechanotransduction is still limited. Further research on mechano-gated and mechanosensitive ion channel complexes of GI SMCs, along with their molecular mechanisms, distribution, and coregulation/colocalization with other cellular components in the GI tract, will advance our understanding of smooth muscle tissue and help to develop novel treatment strategies for GI motility disorders.

GRANTS

The authors are supported by National Institutes of Health Grants DK106456, DK123549, AT010875, and DK052766.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the authors.

AUTHOR CONTRIBUTIONS

A.B. conceived and designed research; V.J. and P.R.S. drafted manuscript; V.J., P.R.S., G.F., and A.B. edited and revised manuscript; V.J., P.R.S., G.F., and A.B. approved final version of manuscript.