Mechanisms of mechanotransduction by specialized low-threshold mechanoreceptors in the guinea pig rectum

The guinea pig rectum, but not the colon, is innervated by a specialized class of distension-sensitive mechanoreceptors that have transduction sites corresponding to rectal intraganglionic laminar endings (rIGLEs). Rectal mechanoreceptors recorded in vitro had low threshold to circumferential stretch, adapted slowly, and could respond within 2 ms to mechanical stimulation by a piezo-electric probe. Antagonists to ionotropic N-methyl-D-aspartate (NMDA; CGS 19755, memantine) and non-NMDA (6,7-dinitroquinoxaline-2,3-dione) glutamate receptors did not affect mechanotransduction. In normal Krebs solution, the P2X purinoreceptor agonist alpha,beta-methylene ATP reduced mechanoreceptor firing evoked by distension but simultaneously relaxed circular smooth muscle and inhibited stretch-induced contractions. Neither ATP nor alpha,beta-methylene ATP affected mechanotransduction when transduction sites were directly compressed with von Frey hairs. The P2 purinoreceptor antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid did not affect stretch-induced firing but reduced the inhibitory effect of alpha,beta-methylene ATP on stretch-induced firing. Under isometric conditions, blocking synaptic transmission in Ca2+-free solution reduced stretch-evoked firing but not when basal tension was restored to control levels. Under isotonic condition, Ca2+-free solution did not significantly affect load-evoked firing. The blockers of mechanogated and/or transient receptor potential channels, benzamil, Gd3+, SKF 96365, and ruthenium red inhibited stretch-induced firing but, in parallel, significantly reduced stretch-induced contractions. Benzamil and SKF 96365 were able to inhibit mechanotransduction when transduction sites were compressed with von Frey hairs. The results show that mechanotransduction is rapid but does not depend on fast exocytotic release of mediators. It is likely that stretch-activated ion channels on rIGLEs are involved in direct, physical mechanotransduction by rectal low-threshold mechanoreceptors.     

Integrins in mechanotransduction

Mechanical forces are crucial to the regulation of cell and tissue morphology and function. At the cellular level, forces influence cytoskeletal organization, gene expression, proliferation, and survival. Integrin-mediated adhesions are intrinsically mechanosensitive and a large body of data implicates integrins in sensing mechanical forces. We review the relationship between integrins and mechanical forces, the role of integrins in cellular responses to stretch and fluid flow, and propose that some of these events are mechanistically related.     

Interactions of mechanotransduction pathways

Integrins may serve as mechanosensors in endothelial cells (ECs): shear stress causes integrin-Shc association, assembly of the signaling complex and then leads to JNK activation. Flow also mediates selective and cell-specific alterations in vascular cell G-protein expression that correlate with changes in cell-signalling, G-protein functionality and modulate Ca2+ concentration. In this study, we explored the cross-talks between EC membrane mechanosensors, such as integrins, ion channels, and G-proteins in shear stress-induced signal transduction by their specific inhibition. Confluent monolayer of bovine aortic endothelial cells (BAECs) were incubated with or without specific inhibitors prior to shearing experiments. Our results showed an attenuation of integrin-Shc association under shear stress with RGD, and with PTX, but not with BAPTA/AM. The inhibitions of shear-activated JNK are similar for RGD and PTX. However, unlike for integrin association, the chelation of calcium reduced JNK activation. These results provide several lines of evidence of the interactions between different mechanosensors in ECs. First, integrin-Shc association required cell attachment and G-protein activity, but not intracellular calcium. Second, shear-induced JNK activation is regulated by multiple mechano-sensing mechanisms such as integrin, G-protein and calcium concentration.     

Computational modeling of extracellular mechanotransduction

Mechanotransduction may occur through numerous mechanisms, including potentially through autocrine signaling in a dynamically changing extracellular space. We developed a computational model to analyze how alterations in the geometry of an epithelial lateral intercellular space (LIS) affect the concentrations of constitutively shed ligands inside and below the LIS. The model employs the finite element method to solve for the concentration of ligands based on the governing ligand diffusion-convection equations inside and outside of the LIS, and assumes idealized parallel plate geometry and an impermeable tight junction at the apical surface. Using the model, we examined the temporal relationship between geometric changes and ligand concentration, and the dependence of this relationship on system characteristics such as ligand diffusivity, shedding rate, and rate of deformation. Our results reveal how the kinetics of mechanical deformation can be translated into varying rates of ligand accumulation, a potentially important mechanism for cellular discrimination of varying rate-mechanical processes. Furthermore, our results demonstrate that rapid changes in LIS geometry can transiently increase ligand concentrations in underlying media or tissues, suggesting a mechanism for communication of mechanical state between epithelial and subepithelial cells. These results underscore both the plausibility and complexity of the proposed extracellular mechanotransduction mechanism.     

Putting the squeeze on mechanotransduction

Both mechanical and chemical stimuli guide tissue function. In a recent paper, Tschumperlin et al. proposed that pressure acting on airway epithelium elicits mechanotransduction not by directly altering biochemical signaling but by regulating extracellular fluid volume to modulate ligand-receptor interactions.     

Hair-cell mechanotransduction and cochlear amplification

In the inner ear, sensory hair cells not only detect but also amplify the softest sounds, allowing us to hear over an extraordinarily wide intensity range. This amplification is frequency specific, giving rise to exquisite frequency discrimination. Hair cells detect sounds with their mechanotransduction apparatus, which is only now being dissected molecularly. Signal detection is not the only role of this molecular network; amplification of low-amplitude signals by hair bundles seems to be universal in hair cells. "Fast adaptation," the rapid closure of transduction channels following a mechanical stimulus, appears to be intimately involved in bundle-based amplification.     

Cell adhesion receptors in mechanotransduction

Integrins and cadherins are tri-functional: they bind ligands on other cells or in the extracellular matrix, connect to the cytoskeleton inside the cell, and regulate intracellular signaling pathways. These adhesion receptors therefore transmit mechanical stresses and are well positioned to mediate mechanotransduction. Studies of cultured cells have shown that both integrin- and cadherin-mediated adhesion are intrinsically mechanosensitive. Strengthening of adhesions in response to mechanical stimulation may be a central mechanism for mechanotransduction. Studies of developing organisms suggest that these mechanisms contribute to tissue level responses to tension and compression, thereby linking morphogenetic movements to cell fate decisions.     

Nuclear mechanotransduction in stem cells

In development and in homeostatic maintenance of tissues, stem cells and progenitor cells are constantly subjected to forces. These forces can lead to significant changes in gene expression and function of stem cells, mediating self-renewal, lineage specification, and even loss of function. One of the ways that has been proposed to mediate these functional changes in stem cells is nuclear mechanotransduction — the process by which forces are converted to signals in the nucleus. The purpose of this review is to discuss the means by which mechanical signals are transduced into the nucleus, through the linker of nucleoskeleton and cytoskeleton (LINC) complex and other nuclear envelope transmembrane (NET) proteins, which connect the cytoskeleton to the nucleus. We discuss how LINC/NETs confers tissue-specific mechanosensitivity to cells and further elucidate how LINC/NETs acts as a control center for nuclear mechanical signals, regulating both gene expression and chromatin organization. Throughout, we primarily focus on stem cell–specific examples, notwithstanding that this is a nascent field. We conclude by highlighting open questions and pointing the way to enhanced research efforts to understand the role nuclear mechanotransduction plays in cell fate choice.     

Integrins and extracellular matrix in mechanotransduction

Integrins bind extracellular matrix fibrils and associate with intracellular actin filaments through a variety of cytoskeletal linker proteins to mechanically connect intracellular and extracellular structures. Each component of the linkage from the cytoskeleton through the integrin-mediated adhesions to the extracellular matrix therefore transmits forces that may derive from both intracellular, myosin-generated contractile forces and forces from outside the cell. These forces activate a wide range of signaling pathways and genetic programs to control cell survival, fate, and behavior. Additionally, cells sense the physical properties of their surrounding environment through forces exerted on integrin-mediated adhesions. This article first summarizes current knowledge about regulation of cell function by mechanical forces acting through integrin-mediated adhesions and then discusses models for mechanotransduction and sensing of environmental forces.     

Decoding mechanical cues by molecular mechanotransduction

Cells are exposed to a variety of mechanical cues, including forces from their local environment and physical properties of the tissue. These mechanical cues regulate a vast number of cellular processes, relying on a repertoire of mechanosensors that transduce forces into biochemical pathways through mechanotransduction. Forces can act on different parts of the cell, carry information regarding magnitude and direction, and have distinct temporal profiles. Thus, the specific cellular response to mechanical forces is dependent on the ability of cells to sense and transduce these physical parameters. In this review, we will highlight recent findings that provide insights into the mechanisms by which different mechanosensors decode mechanical cues and how their coordinated response determines the cellular outcomes.     

Integrin diversity brings specificity in mechanotransduction

Cells sense and respond to the biochemical and physical properties of the extracellular matrix (ECM) through adhesive structures that bridge the cell cytoskeleton and the surrounding environment. Integrin‐mediated adhesions interact with specific ECM proteins and sense the rigidity of the substrate to trigger signalling pathways that, in turn, regulate cellular processes such as adhesion, motility, proliferation and differentiation. This process, called mechanotransduction, influenced by the involvement of different integrin subtypes and their high ECM–ligand binding specificity, contributes to the cell‐type‐specific mechanical responses. In this review, we describe how the expression of particular integrin subtypes affects cellular adaptation to substrate rigidity. We then explain the role of integrins and associated proteins in mechanotransduction, focusing on their specificity in mechanosensing and force transmission.