Showing papers on "Mechanotransduction published in 1997"

Tensegrity: the architectural basis of cellular mechanotransduction

Abstract: Physical forces of gravity, hemodynamic stresses, and movement play a critical role in tissue development. Yet, little is known about how cells convert these mechanical signals into a chemical response. This review attempts to place the potential molecular mediators of mechanotransduction (e.g. stretch-sensitive ion channels, signaling molecules, cytoskeleton, integrins) within the context of the structural complexity of living cells. The model presented relies on recent experimental findings, which suggests that cells use tensegrity architecture for their organization. Tensegrity predicts that cells are hard-wired to respond immediately to mechanical stresses transmitted over cell surface receptors that physically couple the cytoskeleton to extracellular matrix (e.g. integrins) or to other cells (cadherins, selectins, CAMs). Many signal transducing molecules that are activated by cell binding to growth factors and extracellular matrix associate with cytoskeletal scaffolds within focal adhesion complexes. Mechanical signals, therefore, may be integrated with other environmental signals and transduced into a biochemical response through force-dependent changes in scaffold geometry or molecular mechanics. Tensegrity also provides a mechanism to focus mechanical energy on molecular transducers and to orchestrate and tune the cellular response.

Mechanotransduction in bone: osteoblasts are more responsive to fluid forces than mechanical strain.

Abstract: Mechanical force applied to bone produces two localized mechanical signals on the cell: deformation of the extracellular matrix (substrate strain) and extracellular fluid flow. To study the effects of these stimuli on osteoblasts, MC3T3-E1 cells were grown on type I collagen-coated plastic plates and subjected to four-point bending. This technique produces uniform levels of physiological strain and fluid forces on the cells. Each of these parameters can be varied independently. Osteopontin (OPN) mRNA expression was used to assess the anabolic response of MC3T3-E1 cells. When fluid forces were low, neither strain magnitude nor strain rate was correlated with OPN expression. However, higher-magnitude fluid forces significantly increased OPN message levels independently of the strain magnitude or rate. These data indicate that fluid forces, and not mechanical stretch, influence OPN expression in osteoblasts and suggest that fluid forces induced by extracellular fluid flow within the bone matrix may play an important role in bone formation in response to mechanical loading.

Spatial relationships in early signaling events of flow-mediated endothelial mechanotransduction

Abstract: Blood flow interactions with the vascular endothelium represent a specialized example of mechanical regulation of cell function that has important physiological and pathological cardiovascular consequences. The endothelial monolayer in vivo acts as a signal transduction interface for forces associated with flowing blood (hemodynamic forces) in the acute regulation of artery tone and chronic structural remodeling of arteries, including the pathology of atherosclerosis. Mechanisms related to spatial relationships at the cell surfaces and throughout the cell that influence flow-mediated endothelial mechanotransduction are discussed. In particular, flow-mediated ion channel activation and cytoskeletal dynamics are considered in relation to topographic analyses of the luminal and abluminal surfaces of living endothelial cells.

Molecular modeling of mechanotransduction in the nematode caenorhabditis elegans

Abstract: ▪ Abstract Genetic and molecular studies of touch avoidance in the nematode Caenorhabditis elegans have resulted in a molecular model for a mechanotransducing complex. mec-4 and mec-10 encode proteins hypothesized to be subunits of a mechanically gated ion channel that are related to subunits of the vertebrate amiloride-sensitive epithelial Na+ channel. Products of mec-5, a novel collagen, and mec-9, a protein that includes multiple Kunitz-type protease inhibitor repeats and EGF repeats, may interact with the channel in the extracellular matrix. Inside the cell, specialized 15-protofilament microtubules composed of mec-12 α-tubulin and mec-7 β-tubulin may be linked to the mechanosensitive channel by stomatin-homologous MEC-2. MEC-4 and MEC-10 are members of a large family of C. elegans proteins, the degenerins. Two other degenerins, UNC-8 and DEL-1, are candidate components of a stretch-sensitive channel in motor neurons. Implications for advancing understanding of mechanotransduction in other systems are...

Mechanotransduction in Endothelial Cells: Temporal Signaling Events in Response to Shear Stress

Abstract: Fluid shear stress is one of the most important mechanical forces acting upon vascular endothelium, because of its location at the interface between the bloodstream and vascular wall. Recent evidence

Signal transduction of mechanical stimuli is dependent on microfilament integrity : Identification of osteopontin as a mechanically induced gene in osteoblasts

Abstract: Mechanical perturbation has been shown to modulate a wide variety of changes in second message signals and patterns of gene expression in osteoblasts. Embryonic chick osteoblasts were subjected to a dynamic spatially uniform biaxial strain (1.3% applied strain) at 0.25 Hz for a single 2-h period, and osteopontin (OPN), an Arg-Gly-Asp (RGD)-containing protein, was shown to be a mechanoresponsive gene. Expression of opn mRNA reached a maximal 4-fold increase 9 h after the end of the mechanical perturbation that was not inhibited by cycloheximide, thus demonstrating that mechanoinduction of opn expression is a primary response through the activation of pre-existing transcriptional factors. The signal transduction pathways, which mediated the increased expression of opn in response to mechanical stimuli, were shown to be dependent on the activation of a tyrosine kinase(s) and protein kinase A (PKA) or a PKA-like kinase. Selective inhibition of protein kinase C (PKC) had no effect on the mechanoinduction of osteopontin even though opn has been demonstrated to be an early response gene to phorbol 12-myristate 13-acetate (PMA) stimulation. Mechanotransduction was dependent on microfilament integrity since cytochalasin-D blocked the up-regulation of the opn expression; however, microfilament disruption had no effect on the PMA induction of the gene. The microtubule component of the cytoskeleton was not related to the mechanism of signal transduction involved in controlling opn expression in response to mechanical stimulation since colchicine did not block opn expression. Mechanical stimulus was shown to activate focal adhesion kinase (FAK), which specifically became associated with the cytoskeleton after mechanical perturbation, and its association with the cytoskeleton was dependent on tyrosine kinase activity. In conclusion, these results demonstrate that the signal transduction pathway for mechanical activation of opn is uniquely dependent on the structural integrity of the microfilament component of the cytoskeleton. In contrast, the PKC pathway, which also activates this gene in osteoblasts, acts independently of the cytoskeleton in the transduction of its activity.

The molecules of mechanosensation

Abstract: ▪ Abstract Mechanosensation, the transduction of mechanical forces into a cellular electrochemical signal, enables living organisms to detect touch; vibrations, such as sound; accelerations, including gravity; body movements; and changes in cellular volume and shape. Ion channels directly activated by mechanical tension are thought to mediate mechanosensation in many systems. Only one channel has been cloned that is unequivocably mechanically gated: the MscL channel in bacteria. Genetic screens for touch-insensitive nematodes or flies promise to identify the proteins that constitute a mechanosensory apparatus in eukaryotes. In Caenorhabditis elegans, the mec genes thus identified encode molecules for a candidate structure, which includes a “degenerin” channel tethered to specialized extracellular and intracellular structural proteins. In hair cells of the inner ear, evidence suggests that an extracellular tip link pulls on a channel, which attached intracellularly to actin via a tension-regulating myosin ...

Effects of Fluid Shear Stress on Gene Regulation of Vascular Cells

Abstract: Hemodynamic forces such as fluid shear stress play an active role in many physiological and pathophysiological processes of the cardiovascular system. Shear stress resulting from blood flow and transmural plasma flux alters the function of vascular cell (primarily endothelial cells), leading to both rapid and slower adaptive tissue responses. Transmission of the shear stress signal throughout the vascular cell involves a complex interplay between cytoskeletal and biochemical elements and results in changes in structure, metabolism, and gene expression. Herein we review current knowledge on flow-induced mechanotransduction in the vascular endothelial cell and the molecular mechanisms believed responsible for shear-induced endothelial and smooth muscle cell gene regulation with an emphasis on signal transduction.

Electrophysiological responses of human bone cells to mechanical stimulation: evidence for specific integrin function in mechanotransduction.

Abstract: Bone cells respond to mechanical stimuli, but the transduction mechanisms responsible are not fully understood. Integrins, a family of heterodimeric transmembrane glycoproteins, which link components of the extracellular matrix with the actin cytoskeleton, have been implicated as mechanoreceptors. We have assessed the roles of integrins in the transduction of cyclical mechanical stimuli to human bone cells (HBCs), which results in changes in membrane potential. HBC showed membrane depolarization following 0.104 Hz mechanical stimulation and membrane hyperpolarization following stimulation at 0.33 Hz. The membrane depolarization response involved tetrodotoxin-sensitive sodium channels and could be inhibited by antibodies against alpha V, beta 1, and beta 5 integrins. In contrast, the hyperpolarization response was inhibited by gadolinium and antibodies to the integrin-associated protein (CD47), alpha 5 and beta 1 integrin. Both responses could be abrogated by ARg-Gly-Asp (RGD)-containing peptides, inhibition of tyrosine kinase activity, and disruption of the cytoskeleton. These results demonstrate differential electrophysiological responses of HBC to different frequencies of mechanical strain. Furthermore, they suggest that integrins act as HBC mechanoreceptors with distinct signaling pathways being activated by different frequencies of mechanical stimuli.

Integrins, tensegrity, and mechanotransduction.

Abstract: Physical forces, such as those due to gravity, play an important role in tissue development and remodeling. Yet, little is known about how individual cells sense mechanical signals or how they transduce them into a chemical response. Rather than listing the numerous signal pathways that have been found to be sensitive to mechanical stimulation, we need to place potential molecular signaling mechanisms within the context of the entire cell. The model presented is based on the concept that cells use tensegrity architecture to organize their cytoskeleton and stabilize their form. Studies with stick and string tensegrity cell models predict that living cells are hard-wired to respond immediately to external mechanical stresses. This hard-wiring exists in the form of discrete cytoskeletal filament networks that mechanically couple specific cell surface receptors, such as integrins, to nuclear matrix scaffolds and to potential transducing molecules that physically associate with the cytoskeleton. If these signaling molecules do function in a "solid-state", then mechanical stresses may be transduced into biochemical responses through force-dependent changes in cytoskeletal geometry or through local alterations in thermodynamic or kinetic parameters. Changes in cytoskeletal tension (prestress) also may play a role in signal amplification and adaptation. Recent experimental results are described which provide direct support for the tensegrity theory.

The low-affinity neurotrophin receptor p75 regulates the function but not the selective survival of specific subpopulations of sensory neurons.

Abstract: Mice with a targeted deletion of the low-affinity neurotrophin receptor p75 (p75−/−) exhibit a 50% loss of large- and small-diameter sensory neurons in the dorsal root ganglion. Using neurophysiological recording techniques, we now show that p75 is not required for the survival of specific, functionally defined subpopulations of sensory neurons. Rather, p75−/− mice exhibit losses of neurons that subserve nociceptive as well as non-nociceptive functions. The receptive properties of large myelinated afferent fibers were normal in p75−/− mice. However, the receptive properties of subpopulations of afferent fibers with thin myelinated or unmyelinated axons were strikingly impaired in mice lacking p75. Furthermore, the presence of p75 is required for normal mechanotransduction in C fibers and D-hair receptors and normal heat sensitivity in A-fiber nociceptors.

Ionic selectivity of mechanically activated channels in spider mechanoreceptor neurons

Abstract: The lyriform slit-sense organ on the patella of the spider, Cupiennius salei, consists of seven or eight slits, with each slit innervated by a pair of mechanically sensitive neurons. Mechanotransduction is believed to occur at the tips of the dendrites, which are surrounded by a Na+-rich receptor lymph. We studied the ionic basis of sensory transduction in these neurons by voltage-clamp measurement of the receptor current, replacement of extracellular cations, and application of specific blocking agents. The relationship between mechanically activated current and membrane potential could be approximated by the Goldman-Hodgkin-Katz current equation, with an asymptotic inward conductance of approximately 4.6 nS, indicating that 50-230 channels of 20-80 pS each would suffice to produce the receptor current. Amiloride and gadolinium, which are known to block mechanically activated ion channels, also blocked the receptor current. Ionic replacement showed that the channels are not permeable to choline or Rb+, but are partly permeable to Li+. The receptor current was inward at all membrane potentials (-200 to +200 mV) and never reversed, indicating high selectivity for Na+ over K+. This situation contrasts strongly with insect mechanoreceptors, vertebrate hair cells, and mechanically activated ion channels in nonsensory cells, most of which are either unselective for monovalent cations or selective for K+.

Mechanotransduction and intracellular signaling mechanisms of stretch-induced remodeling in endothelial cells.

Abstract: We investigated the signaling mechanism of stretch-induced cell remodeling in human umbilical vein endothelial cells (HUVECs). Freshly dissociated HUVECs were cultured on an elastic silicon membrane and subjected to uniaxial cyclic stretch (20% in length, 1 Hz). The cells started to change their morphology as early as 15 min after stretch onset, and most cells eventually aligned perpendicularly to the stretch axis within 1 h. This remodeling was dependent on the increase in intracellular calcium concentration ([Ca2+]i) via a Ca(2+)-permeable stretch-activated (SA) channel. During the process of remodeling, extensive rearrangement of stress fibers and focal adhesions was observed, which may be close to the final step in the intracellular signaling cascade. This event was [Ca2+]i-dependent, suggesting the existence of a Ca(2+)-dependent intermediate cascade that links [Ca2+]i to the rearrangement of cytoskeletons and focal adhesions. We found that some proteins, including pp125FAK (focal adhesion kinase) and paxillin, were tyrosine phosphorylated during cyclic stretch in a Ca(2+)-dependent manner. Inhibition of this tyrosine phosphorylation prohibited the stretch-dependent rearrangement of cytoskeletons and focal adhesions as well as the remodeling. Finally the tyrosine kinase src, which could phosphorylate pp125FAK, was found to be activated in a [Ca2+]i-dependent way during stretch. All of the above molecular events were consistently Ca(2+)-dependent, which led us to propose the signaling cascade: SA channel activation-->[Ca2+]i increase-->src activation-->protein tyrosine phosphorylation-->rearrangement of cytoskeletons and focal adhesions-->cell remodeling.

Mechanotransduction in Bone Cells: Induction of Nitric Oxide and Prostaglandin Synthesis by Fluid Shear Stress, but Not by Mechanical Strain

Abstract: The skeleton acts as a support for the rest of the body, and it has long been known that bone tissue adapts to its local mechanical environment: mechanical loading leads to an increase in the bone mass and density, whereas disuse results in a net loss of bone. It has been found that bones respond primarily to dynamic rather that static loads1. Mechanical loading of bone cells is associated with increased prostaglandin E2 (PGE2) production2, and inhibition of prostaglandin synthesis suppresses the osteogenic response3. Inhibition of nitric oxide synthase (NOS) also abolishes the loading response, indicating that nitric oxide (NO) production has an essential early role in bone remodeling, too4.

Mechanical Forces: Their Effects on Cells and Tissues

Abstract: 1. Introduction.- 2. Endothelial Cells.- 3. Bone Cells.- 4. Chondrocytes.- 5. Muscle Cells.- 6. Mechanotransduction.- 7. Production Systems.- 8. Overview.

Molecules that mediate touch transduction in the nematode Caenorhabditis elegans.

Abstract: Despite the widespread importance of mechanotransduction in biology, remarkably little is known about the nature of the molecules that mediate mechanical signaling. Mechanosensation in the nematode Caenorhabditis elegans is mediated by six mechanosensory neurons called touch receptor cells. Genetic analysis has resulted in the identification of over 400 mutations that disrupt the function of the touch receptors. Molecular characterization of the genes revealed has identified subunits of a candidate mechanosensory ion channel, tubulins expressed specifically in the touch receptors, and extracellular matrix proteins needed for mechanotransduction. mec-4 and mec-10 encode members of a C. elegans gene family related to the vertebrate epithelial Na+ channel that are hypothesized to encode subunits of a mechanosensory channel. mec-6 may encode another channel subunit. Inside the cell, ty-tubulin MEC12, li-tubulin MEC-7 and a candidate linker protein MEC-2 may interact with the mechanotransducing channel to deliver gating tension. In the extracelluar matrix, collagen MEC-5 and MEC-9 and MEC-1 may interact with extracellular channel domains. A molecular model for mechanotransduction is discussed.

The functional matrix hypothesis revisited. of an osseous connected cellular network 2. The role

Abstract: Intercellular gap junctions permiz bone cells to intercellularly transmit, and subsequently process, periosteal functional matrix information, after its initial intraceilular mechanotransduction. In addition, gap junctions, as electrical synapses, underlie the organization of bone tissue as a connected cellular network, and the fact that all bone adaptation processes are multicellular. The structural and operational characteristics of such biologic networks are outlined and their specific bone cell attributes described. Specifically, bone is "tuned" to the precise frequencies of skeletal muscle activity. The inclusion of the concepts and databases that are related to the intracellular and intercellular bone cell mechanisms and processes of mechanotransduction and the organization of bone as a biologic connected cellular network permit revision of the functional matrix hypothesis, which offers an explanatory chain, extending from the epigenetic event of muscle contraction hierarchically downward to the regulation of the bone cell genome. (Am J Orthod Dentofac Orthop 1997;112:221-6.) The first article in this series considered the implications for the functional matrix hypothesis (FMH) of the ability of bone cells to carry out intracellular mechanosensation and transduction and intercellular communication. In this article, we will consider the implications for the FMH of the inclusion of connectionist network theory.

Methods for the Organogenesis of Skeletal Muscle in Tissue Culture

Abstract: Skeletal muscle structure is regulated by many factors, including nutrition, hormones, electrical activity, and tension. The muscle cells are subjected to both passive and active mechanical forces at all stages of development and these forces play important but poorly understood roles in regulating muscle organogenesis and growth. For example, during embryogenesis, the rapidly growing skeleton places large passive mechanical forces on the attached muscle tissue. These forces not only help to organize the proliferating mononucleated myoblasts into the oriented, multinucleated myofibers of a functional muscle but also tightly couple the growth rate of muscle to that of bone. Postnatally, the actively contracting, innervated muscle fibers are subjected to different patterns of active and passive tensions which regulate longitudinal and cross sectional myofiber growth. These mechanically-induced organogenic processes have been difficult to study under normal tissue culture conditions, resulting in the development of numerous methods and specialized equipment to simulate the in vivo mechanical environment.These techniques have led to the "engineering" of bioartificial muscles (organoids) which display many of the characteristics of in vivo muscle including parallel arrays of postmitotic fibers organized into fascicle-like structures with tendon-like ends. They are contractile, express adult isoforms of contractile proteins, perform directed work, and can be maintained in culture for long periods. The in vivo-like characteristics and durability of these muscle organoids make them useful for long term in vitro studies on mechanotransduction mechanisms and on muscle atrophy induced by decreased tension. In this report, we described a simple method for generating muscle organoids from either primary embrionic avain or neonatal rodent myoblasts.

Modulation of mechanotransduction in aortic baroreceptor neurons

Abstract: BaFomeptor Activity Baroleceptor Activity The arterial baroreceptors (BR) exert a major regulatory influence on the autonomic f-2 6.w nervous system. The sensitivity of BR is modulated by paracrine factors which may be released from the endothelium during stretch of the carotid sinus or the aortic arch. Prostacyclin (PGI2) increases, whereas nitric oxide decreases, BR activity in the isolated 0 25 75 125 175