Actomyosin-mediated statolith positioning in gravisensing plant cells studied in microgravity

The positioning and gravity-induced sedimentation of statoliths is crucial for gravisensing in most higher and lower plants. In positively gravitropic rhizoids and, for the first time, in negatively gravitropic protonemata of characean green algae, statolith positioning by actomyosin forces was investigated in microgravity (<10(-4) g) during parabolic flights of rockets (TEXUS/MAXUS) and during the Space-Shuttle flight STS 65. In both cell types, the natural position of statoliths is the result of actomyosin forces which compensate the statoliths' weight in this position. When this balance of forces was disturbed in microgravity or on the fast-rotating clinostat (FRC), a basipetal displacement of the statoliths was observed in rhizoids. After several hours in microgravity, the statoliths were loosely arranged over an area whose apical border was in the same range as in 1 g, whereas the basal border had increased its distance from the tip. In protonemata, the actomyosin forces act net-acropetally. Thus, statoliths were transported towards the tip when protonemata were exposed to microgravity or rotated on the FRC. In preinverted protonemata, statoliths were transported away from the tip to a dynamically stable resting position. Experiments in microgravity and on the FRC gave similar results and allowed us to distinguish between active and passive forces acting on statoliths. The results indicate that actomyosin forces act differently on statoliths in the different regions of both cell types in order to keep the statoliths in a position where they function as susceptors and initiate gravitropic reorientation, even in cells that had never experienced gravity during their growth and development.     

The influence of the HGMF on mass-charge transfer in gravisensing cells

The present work is focused on the influence of the high-gradient-magnetic field (HGMF) on spatial distribution of ion fluxes along the roots (a), cytoplasmic streaming (b), and the processes of plant cell growth connected with intracellular mass and charge transfer (c).     

Actomyosin-mediated statolith positioning and sedimentation in gravisensing plant cells studied in microgravity

Graviresponding and tip-growing characean rhizoids and protonemata possess a highly efficient actin-based system to control and correct the position of their statoliths, a prerequisite for gravisensing. Acropetally and basipetally acting actomyosin forces and gravity are the components of the statolith positioning system that also directs sedimenting statoliths to cell-type specific ,oraviperception sites at the plasma membrane where the graviresponse is initiated. These results encourage to propose that similar cytoskeleton-mediated mechanisms for gravity sensing may exist in higher plant statocytes.     

Mechanotransduction in Caenorhabditis elegans

One of the looming mysteries in signal transduction today is the question of how mechanical signals, such as pressure or mechanical force delivered to a cell, are interpreted to direct biological responses. All living organisms, and probably all cells, have the ability to sense and respond to mechanical stimuli. At the single-cell level, mechanical signaling underlies cell-volume control and specialized responses such as the prevention of poly-spermy in fertilization. At the level of the whole organism, mechanotransduction underlies processes as diverse as stretch-activated reflexes in vascular epithelium and smooth muscle; gravitaxis and turgor control in plants; tissue development and morphogenesis; and the senses of touch, hearing, and balance. Intense genetic, molecular, and elecrophysiological studies in organisms ranging from nematodes to mammals have highlighted members of the recently discovered DEG/ENaC family of ion channels as strong candidates for the elusive metazoan mechanotransducer. Here, we discuss the evidence that links DEG/ENaC ion channels to mechanotransduction and review the function of Caenorhabiditis elegans members of this family called degenerins and their role in mediating mechanosensitive behaviors in the worm.     

Mechanotransduction of stretch-induced prostanoid release by fetal lung epithelial cells

Mechanical ventilation is the primary supportive treatment for infants and adults suffering from severe respiratory failure. Adverse mechanical ventilation (overdistension of the lung) triggers a proinflammatory response. Along with cytokines, inflammatory mediators such as bioactive lipids are involved in the regulation of the inflammatory response. The arachidonic acid pathway is a key source of bioactive lipid mediators, including prostanoids. Although ventilation has been shown to influence the production of prostanoids in the lung, the mechanotransduction pathways are unknown. Herein, we established that cyclic stretch of fetal lung epithelial cells, but not fibroblasts, can evoke an extremely sensitive, rapid alteration in eicosanoid metabolism through a cyclooxygenase (COX)-2 dependent mechanism. Cyclic stretch significantly increased PGI(2), PGF(2alpha), PGD(2), PGE(2), and thromboxane B(2) levels in the media of epithelial cells, but did not alter leukotriene B(4) or 12-hydroxyeicosatetraenoic acid levels. Inhibition of COX-2, but not COX-1, attenuated the cyclic stretch-induced PG increase in the media, suggesting that cyclic stretch primarily affected PG synthesis. Substrate (free arachidonic acid) availability for PG generation was increased because of a cyclic stretch-induced activation of cytosolic phospholipase A(2) (cPLA(2)) via an influx of extracellular calcium and phosphorylation by mitogen-activated protein kinase, p44/42MAPK. The data are compatible with cPLA(2) and COX-2 being intimately involved in regulating the injury response to adverse mechanical ventilation.     

Mechanotransduction in spider slit sensilla

Mechanoreception is a vital constituent of several sensory modalities and a wide range of internal regulatory processes, but fundamental mechanisms for neural detection of mechanical stimuli have been difficult to characterize because of the morphological properties of most mechanoreceptors and the nature of the stimulus itself. An invertebrate preparation, the VS-3 lyriform slit sense organ of the spider, Cupiennius salei, has proved useful because it possesses large mechanosensory neurons, whose cell bodies are close to the sites of sensory transduction, and accessible to intracellular recording during mechanotransduction. This has made it possible to observe and experiment with all the major stages of mechanosensation. Here, we describe several important findings from this preparation, including the estimated number, conductance and ionic selectivity of the ion channels responsible for mechanotransduction, the major voltage-activated ion channels responsible for action potential encoding and control of the dynamic properties of the neurons, the location of action potential initiation following mechanical stimulation, and the efferent control of mechanoreception. While many details of mechanosensation remain to be discovered, the VS-3 system continues to offer important opportunities to advance our understanding of this crucial physiological process.     

Mechanotransduction in embryonic vascular development

A plethora of biochemical signals provides spatial and temporal cues that carefully orchestrate the complex process of vertebrate embryonic development. The embryonic vasculature develops not only in the context of these biochemical cues, but also in the context of the biomechanical forces imparted by blood flow. In the mature vasculature, different blood flow regimes induce distinct genetic programs, and significant progress has been made toward understanding how these forces are perceived by endothelial cells and transduced into biochemical signals. However, it cannot be assumed that paradigms that govern the mature vasculature are pertinent to the developing embryonic vasculature. The embryonic vasculature can respond to the mechanical forces of blood flow, and these responses are critical in vascular remodeling, certain aspects of sprouting angiogenesis, and maintenance of arterial–venous identity. Here, we review data regarding mechanistic aspects of endothelial cell mechanotransduction, with a focus on the response to shear stress, and elaborate upon the multifarious effects of shear stress on the embryonic vasculature. In addition, we discuss emerging predictive vascular growth models and highlight the prospect of combining signaling pathway information with computational modeling. We assert that correlation of precise measurements of hemodynamic parameters with effects on endothelial cell gene expression and cell behavior is required for fully understanding how blood flow-induced loading governs normal vascular development and shapes congenital cardiovascular abnormalities.     

Mechanotransduction pathways in skeletal muscle hypertrophy

In the last decade, molecular biology has contributed to define some of the cellular events that trigger skeletal muscle hypertrophy. Recent evidence shows that insulin like growth factor 1/phosphatidyl inositol 3-kinase/protein kinase B (IGF-1/PI3K/Akt) signaling is not the main pathway towards load-induced skeletal muscle hypertrophy. During load-induced skeletal muscle hypertrophy process, activation of mTORC1 does not require classical growth factor signaling. One potential mechanism that would activate mTORC1 is increased synthesis of phosphatidic acid (PA). Despite the huge progress in this field, it is still early to affirm which molecular event induces hypertrophy in response to mechanical overload. Until now, it seems that mTORC1 is the key regulator of load-induced skeletal muscle hypertrophy. On the other hand, how mTORC1 is activated by PA is unclear, and therefore these mechanisms have to be determined in the following years. The understanding of these molecular events may result in promising therapies for the treatment of muscle-wasting diseases. For now, the best approach is a good regime of resistance exercise training. The objective of this point-of-view paper is to highlight mechanotransduction events, with focus on the mechanisms of mTORC1 and PA activation, and the role of IGF-1 on hypertrophy process.     

Mechanotransduction at cadherin-mediated adhesions

Cell-to-cell junctions are crucial mechanical and signaling hubs that connect cells within tissues and probe the mechanics of the surrounding environment. Although the capacity of cell-to-extracellular-matrix (ECM) adhesions to sense matrix mechanics and proportionally modify cell functions is well established, cell-cell adhesions only recently emerged as a new class of force sensors. This finding exposes new pathways through which force can instruct cell functions. This review highlights recent findings, which demonstrate that protein complexes associated with classical cadherins, the principal architectural proteins at cell-cell junctions in all soft tissues, are mechanosensors. We further discuss the current understanding of the rudiments of a cadherin-based mechanosensing and transduction pathway, which is distinct from the force sensing machinery of cell-ECM adhesions.     

Mechanotransduction in Colonic Smooth Muscle Cells

We evaluated mechanisms which mediate alterations in intracellular biochemical events in response to transient mechanical stimulation of colonic smooth muscle cells. Cultured myocytes from the circular muscle layer of the rabbit distal colon responded to brief focal mechanical deformation of the plasma membrane with a transient increase in intracellular calcium concentration ([Ca²⁺] _i ) with peak of 422.7 ± 43.8 nm above an average resting [Ca²⁺] _i of 104.8 ± 10.9 nm (n= 57) followed by both rapid and prolonged recovery phases. The peak [Ca²⁺] _i increase was reduced by 50% in the absence of extracellular Ca²⁺, while the prolonged [Ca²⁺] _i recovery was either abolished or reduced to ≤15% of control values. In contrast, no significant effect of gadolinium chloride (100 μm) or lanthanum chloride (25 μm) on either peak transient or prolonged [Ca²⁺] _i recovery was observed. Pretreatment of cells with thapsigargin (1 μm) resulted in a 25% reduction of the mechanically induced peak [Ca²⁺] _i response, while the phospholipase C inhibitor U-73122 had no effect on the [Ca²⁺] _i transient peak. [Ca²⁺] _i transients were abolished when cells previously treated with thapsigargin were mechanically stimulated in Ca²⁺-free solution, or when Ca²⁺ stores were depleted by thapsigargin in Ca²⁺-free solution. Pretreatment with the microfilament disrupting drug cytochalasin D (10 μm) or microinjection of myocytes with an intracellular saline resulted in complete inhibition of the transient. The effect of cytochalasin D was reversible and did not prevent the [Ca²⁺] _i increases in response to thapsigargin. These results suggest a communication, which may be mediated by direct mechanical link via actin filaments, between the plasma membrane and an internal Ca²⁺ store. Received: 24 March 1997/Revised: 21 July 1997     

Mechanotransduction activates RhoA in the neighbors of apoptotic epithelial cells to engage apical extrusion

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Mechanotransduction via TRPV4 regulates inflammation and differentiation in fetal mouse distal lung epithelial cells

Background

Mechanical ventilation plays a central role in the injury of premature lungs. However, the mechanisms by which mechanical signals trigger an inflammatory cascade to promote lung injury are not well-characterized. Transient receptor potential vanilloid 4 (TRPV4), a calcium-permeable mechanoreceptor channel has been shown to be a major determinant of ventilator-induced acute lung injury in adult models. However, the role of these channels as modulators of inflammation in immature lungs is unknown. In this study, we tested the hypothesis that TRPV4 channels are important mechanotransducers in fetal lung injury.

Methods

Expression of TRPV4 in the mouse fetal lung was investigated by immunohistochemistry, Western blot and qRT-PCR. Isolated fetal epithelial cells were exposed to mechanical stimulation using the Flexcell Strain Unit and inflammation and differentiation were analyzed by ELISA and SP-C mRNA, respectively.

Results

TRPV4 is developmentally regulated in the fetal mouse lung; it is expressed in the lung epithelium and increases with advanced gestation. In contrast, in isolated epithelial cells, TRPV4 expression is maximal at E17-E18 of gestation. Mechanical stretch increases TRPV4 in isolated fetal epithelial cells only during the canalicular stage of lung development. Using the TRPV4 agonist GSK1016790A, the antagonist HC-067047, and the cytokine IL-6 as a marker of inflammation, we observed that TRPV4 regulates release of IL-6 via p38 and ERK pathways. Interestingly, stretch-induced differentiation of fetal epithelial cells was also modulated by TRPV4.

Conclusion

These studies demonstrate that TRPV4 may play an important role in the transduction of mechanical signals in the fetal lung epithelium by modulating not only inflammation but also the differentiation of fetal epithelial cells.     

Mechanotransduction in Stretch-Induced Hypertrophy of Cardiac Myocytes

Abstract

Mechanical loading of cardiac muscles causes rapid activation of a number of immediate-early (IE) genes and hypertrophy. However, little is known as to how muscle cells sense mechanical load and regulate gene expression. We examined roles of several putative mechanotransducers in stretch-induced hypertrophy of cardiac myocytes grown on a deformable silicone sheet. Using the patch-clamp technique, we found a single class of stretch-activated cation channels which was completely and reversibly blocked by gadolinium. The inhibition of this channel by gadolinium did not affect either stretch-induced expression of the IE genes or hypertrophy. Neither disruption of microtubules with colchicine nor that of actin microfilaments by cytochalasin D prevented the stretch-induced IE gene expression. Arresting contractile activity by tetrodotoxin did not affect the stretch-induced IE gene expression or hypertrophy. These results suggest that stretch-activated cation channels, microtubules, microfilaments, and contractile activity are not the mechanotransducers. Preliminary results suggest that cell stretch may cause a release of a growth factor(s), which in turn initiates a cascade of hypertrophic response of cardiac myocytes.     

Mechanotransduction in leukocyte activation: a review

We review recent evidence which suggests that leukocytes in the circulation and in the tissue may readily respond to physiological levels of fluid shear stress in the range between about 1 and 10 dyn/cm 2, a range that is below the level to achieve a significant passive, viscoelastic response. The response of activated neutrophilic leukocytes to fluid shear consists of a rapid retraction of lamellipodia with membrane detachment from integrin binding sites. In contrast, a subgroup of non-activated neutrophils may project pseudopods after exposure to fluid shear stress. The evidence suggests that G-protein coupled receptor downregulation by fluid shear with concomitant downregulation of Rac-related small GTPases and depolymerization of F-actin serves to retract the lamellipodia in conjunction with proteolytic cleavage of beta 2 integrin to facilitate membrane detachment. Furthermore, there exists a mechanism to up- and down-regulate the fluid shear-response, which involves nitric oxide and the second messenger cyclic guanosine monophosphate (cGMP). Many physiological activities of circulating leukocytes are under the influence of fluid shear stress, including transendothelial migration of lymphocytes. We describe a disease model with chronic hypertension that suffers from an attenuated fluid shear-response with far reaching implications for microvascular blood flow.     

Mechanotransduction in bone cells proceeds via activation of COX-2, but not COX-1

Cyclooxygenase (COX) is the key enzyme in the production of prostaglandins, which are essential for the response of bone to mechanical loading. We determined which COX-isoform, COX-1 or COX-2, determines loading-induced prostaglandin production in primary bone cells in vitro. Mouse and human bone cells reacted to 1 h of pulsating fluid flow (PFF, 0.6+/-0.3 Pa at 5 Hz) with an increased prostaglandin E(2) production, which continued 24 h after cessation of PFF. Inhibition of COX-2 activity with NS-398 abolished the stimulating effect of PFF both at 1 h and at 24 h post-incubation, while inhibition of COX-1 by SC-560 affected neither the early nor the late response to flow. PFF rapidly stimulated COX-2 mRNA expression at 1 h but did not affect COX-1 mRNA expression. COX-2 mRNA expression was still significantly enhanced 24 h after cessation of PFF. We conclude that COX-2 is the mechanosensitive form of COX that determines the response of bone tissue to mechanical loading.