Changes In Mechanical Forces Research Articles

MicroRNA-19a mediates the suppressive effect of laminar flow on cyclin D1 expression in human umbilical vein endothelial cells

Endothelial cells (ECs) respond to changes in mechanical forces, leading to the modulation of signaling networks and cell function; an example is the inhibition of EC proliferation by steady laminar flow. MicroRNAs (miRs) are short noncoding 20-22 nucleotide RNAs that negatively regulate the expression of target genes at the posttranscriptional level. This study demonstrates that miRs are involved in the flow regulation of gene expression in ECs. With the use of microRNA chip array, we found that laminar shear stress (12 dyn/cm(2), 12 h) regulated the EC expression of many miRs, including miR-19a. We further showed that stable transfection of miR-19a significantly decreased the expression of a reporter gene controlled by a conserved 3'-untranslated region of the cyclinD1 gene and also the protein level of cyclin D1, leading to an arrest of cell cycle at G1/S transition. Laminar flow suppressed cyclin D1 protein level, and this suppressive effect was diminished when the endogenous miR-19a was inhibited. In conclusion, we demonstrated that miR-19a plays an important role in the flow regulation of cyclin D1 expression. These results revealed a mechanism by which mechanical forces modulate endothelial gene expression.

Flow increases superoxide production by NADPH oxidase via activation of Na-K-2Cl cotransport and mechanical stress in thick ascending limbs

Superoxide (O(2)(-)) regulates renal function and is implicated in hypertension. O(2)(-) production increases in response to increased ion delivery in thick ascending limbs (TALs) and macula densa and mechanical strain in other cell types. Tubular flow in the kidney acutely varies causing changes in ion delivery and mechanical stress. We hypothesized that increasing luminal flow stimulates O(2)(-) production by NADPH oxidase in TALs via activation of Na-K-2Cl cotransport. We measured intracellular O(2)(-) in isolated rat TALs using dihydroethidium in the presence and absence of luminal flow and inhibitors of NADPH oxidase, Na-K-2Cl cotransport, and Na/H exchange. In the absence of flow, the rate of O(2)(-) production was 5.8 +/- 1.4 AU/s. After flow was initiated, it increased to 29.7 +/- 4.3 AU/s (P < 0.001). O(2)(-) production was linearly related to flow. Tempol alone and apocynin alone blocked the flow-induced increase in O(2)(-) production (3.5 +/- 1.7 vs. 4.5 +/- 2.8 AU/s and 8.2 +/- 2.1 vs. 10.6 +/- 2.8 AU/s, respectively). Furosemide decreased flow-induced O(2)(-) production by 55% (37.3 +/- 5.2 to 16.8 +/- 2.8 AU/s; P < 0.002); however, dimethylamiloride had no effect. Finally, we examined whether changes in mechanical forces are involved in flow-induced O(2)(-) production by using a Na-free solution to perfuse TALs. In the absence of NaCl, luminal flow enhanced O(2)(-) production (1.5 +/- 0.5 to 13.5 +/- 1.1 AU/s; P < 0.001), approximately 50% less stimulation than when flow was increased in the presence of luminal NaCl. We conclude that flow stimulates O(2)(-) production in TALs via activation of NADPH oxidase and that NaCl absorption due to Na-K-2Cl cotransport and flow-associated mechanical factors contribute equally to this process.

Microbial responses to microgravity and other low-shear environments.

Microbial adaptation to environmental stimuli is essential for survival. While several of these stimuli have been studied in detail, recent studies have demonstrated an important role for a novel environmental parameter in which microgravity and the low fluid shear dynamics associated with microgravity globally regulate microbial gene expression, physiology, and pathogenesis. In addition to analyzing fundamental questions about microbial responses to spaceflight, these studies have demonstrated important applications for microbial responses to a ground-based, low-shear stress environment similar to that encountered during spaceflight. Moreover, the low-shear growth environment sensed by microbes during microgravity of spaceflight and during ground-based microgravity analogue culture is relevant to those encountered during their natural life cycles on Earth. While no mechanism has been clearly defined to explain how the mechanical force of fluid shear transmits intracellular signals to microbial cells at the molecular level, the fact that cross talk exists between microbial signal transduction systems holds intriguing possibilities that future studies might reveal common mechanotransduction themes between these systems and those used to sense and respond to low-shear stress and changes in gravitation forces. The study of microbial mechanotransduction may identify common conserved mechanisms used by cells to perceive changes in mechanical and/or physical forces, and it has the potential to provide valuable insight for understanding mechanosensing mechanisms in higher organisms. This review summarizes recent and future research trends aimed at understanding the dynamic effects of changes in the mechanical forces that occur in microgravity and other low-shear environments on a wide variety of important microbial parameters.

Treatment of painful osteoporotic vertebral fractures with percutaneous vertebroplasty or kyphoplasty.

Vertebral fracture is the most common complication of osteoporosis. It results in significant mortality and morbidity, including prolonged and intractable pain in a minority of patients. Vertebroplasty and kyphoplasty, procedures that involve percutaneous injection of bone cement into a collapsed vertebra, have recently been introduced for treatment of osteoporotic patients who have prolonged pain (several weeks or longer) following vertebral fracture. To determine the details of the procedures and to gather information on their safety and efficacy, we performed a MEDLINE search using the terms 'vertebroplasty' and 'kyphoplasty.' We reviewed reports of these procedures in patients with osteoporosis. We supplemented the articles found with other papers known to the authors and with presentations at national meetings. Randomized trials of vertebroplasty and kyphoplasty have not been reported. Case reports suggest that these procedures are associated with pain relief in 67% to 100% of cases. Short-term complications, mainly the result of extravasation of cement, include increased pain and damage from heat or pressure to the spinal cord or nerve roots. Proper patient selection and good technique should minimize complications, but rarely, decompressive surgery is needed. Long-term benefits have not yet been shown, but potentially include prevention of recurrent pain at the treated level(s) with both procedures, and, with kyphoplasty, reversal of height loss and spinal deformity, an improved level of function, and avoidance of chronic pain and restriction of internal organs. Possible long-term complications, again not fully evaluated, include local acceleration of bone resorption caused by the treatment itself or by foreign-body reaction at the cement-bone interface, and increased risk of fracture in treated or adjacent vertebrae through changes in mechanical forces. Controlled trials are needed to determine both short-term and long-term safety and efficacy of vertebroplasty and kyphoplasty. Both procedures may be useful for osteoporotic patients who have prolonged pain following acute vertebral fracture. Until there is conclusive evidence for efficacy and long-term safety, these procedures should be done only in carefully selected patients, only by experienced operators with appropriate high-quality imaging equipment, and ideally at centers that are participating in controlled trials.

Calcium-dependent activation of phospholipase C by mechanical distension in renin-expressing As4.1 cells.

One of the major physiological regulators for the production and release of renin from the kidney is blood pressure. The juxtaglomerular (JG) cells, located primarily at the afferent arterioles leading to the glomerulus, are thought to be the baroreceptor of the kidney and adjust their ability to secrete renin in an inverse relationship to changes in pressure (mechanical force). The characteristics of JG cells that allow them to sense and respond to changes in mechanical force at the cellular level are not clear. By use of a renin-expressing clonal cell line (As4.1) as a model for JG cells, it was the purpose of this paper to identify cellular pathways that are activated by mechanical distension. Fura 2-labeled As4.1 cells were mechanically probed to observe changes of intracellular calcium concentration ([Ca(2+)](i)). Mechanical distension of As4.1 cells resulted in an influx of Ca(2+) to the cytosol, mediated by stretch-activated ion channels and dependent on the presence of extracellular Ca(2+). Furthermore, cyclic mechanical distension elevated total inositol phosphates (IP) in As4.1 cells. This response was also dependent on the presence of extracellular Ca(2+), and the addition of U-73122, a phospholipase C (PLC) antagonist, significantly attenuated the increase of IP. Taken together, these findings demonstrate the calcium-dependent activation of PLC and the subsequent increase of IP and [Ca(2+)](i) to be a potentially important pathway for the modality of pressure sensing by renin-expressing cells in response to mechanical stimulation.

Calcyclin gene expression is increased by mechanical strain in fibroblasts and lung.

Mechanical tension extending throughout the structural elements of the lung is a potential stimulus for cell proliferation and gene expression. Pulmonary fibroblasts located in the interstitial space of the capillary wall throughout the lung parenchyma and within the large vessels and airways are uniquely situated to sense changes in mechanical force. Therefore, we used the polymerase chain reaction-based method of differential display analysis to screen for altered gene expression in fetal human lung fibroblasts exposed to increased cyclic stretch. IMR-90 cells were seeded at 3 x 10(4) cells/cm(2) on laminin-coated plates. Cells were subsequently exposed to mechanical strain on a Flexercell apparatus, resulting in a maximal elongation of 20% at a rate of 60 cycles/min over a period of 48 h. A complementary DNA corresponding to the cell cycle-regulated gene calcyclin was identified in mechanically strained fibroblasts. Increased calcyclin messenger RNA levels were confirmed by Northern blot analysis. Further, calcyclin gene expression was upregulated in isolated-perfused rat lungs exposed to increased mechanical strain by ventilation at high states of lung inflation for 4 h. These data suggest that calcyclin gene expression plays a role in the response of pulmonary fibroblasts to increased mechanical tension and may alter the regulation of the fibroblast cell cycle.

Force effects on biochemical kinetics.

Force is an important component in the proper functioning of tissues and cells. In processes ranging from the contraction of muscles to the alignment of chromosomes at the metaphase plate, forces must be adjusted to the proper levels by cells. At the molecular level, it is clear that the motor molecules and other enzymes must respond to changes in mechanical forces by altering enzymatic function. Recent technical advances, primarily the atomic force microscope and laser tweezers, enable us to measure forces at the single molecule level to test how force is transduced into a change in enzyme activity. A priori, four basic mechanisms of coupling enzyme rate and force are considered. The mechanisms extend from the cellular to the molecular level. For example, polymer assembly rates and cytoskeletal matrix concentration are potentially modified by force in ways that feed back on critical enzyme rates. In studies of the known mechanosensitive enzymes, myosin and other motors, the bacterial flagellar rotor, and the F0F1 ATPase, the molecular mechanisms used to transduce force changes into activity changes have not been clearly defined, although it is reasonable to speculate about the nature of these mechanisms from the atomic structures and nanometer measurements of movement.

The assessment of mechanical and neuromuscular response strategies during landing

The purpose of this study was to assess relative contributions of mechanical and neuromuscular mechanisms to control of landing. Proposed mechanical and neuromuscular response strategies were evaluated relative to lower extremity impact force attenuation. Four subjects performed three conditions of 25 landings from a 60-cm height on each of two days. Additional masses were attached at each ankle for the second and removed for the third condition. Vertical ground reaction force and integrated electromyographic (IEMG) data were used to assess responses. Fourteen of 32 total single subject comparisons were significant in a mechanically predicted direction, indicating the presence of both mechanical and neuromuscular response strategies among subjects. Responses in the mechanical direction for rearfoot impact were consistent with a model predicting mechanical force changes. The presence of mechanical responses for forefoot impact not completely accounted for by the model suggested that the definition of a neuromuscular response not be limited to responses functioning in a protective manner. Multiple regression analyses relating added mass and IEMG to impact force magnitude demonstrated the complex nature of all responses. Vastus medialis IEMG was the most common independent variable included in regression models, emphasizing the importance of knee musculature in landing control.

The Role of Prostaglandins in Bone Formation

Prostaglandins of the E series have been shown to be effective inducers of bone formation in vivo. In this study, the effects of PGE2 were evaluated in vivo using subcutaneous administration (3 mg/kg/d for 25 days) to ovariectomized rats or local application in the marrow cavity of tibiae of rats using biodegradable implants (0.13, 1.4 and 32 microg released over 8 days). Systemic treatment of rats with PGE2 stimulated cancellous bone formation in the metaphysis of the proximal tibiae as well as endocortical bone formation and de novo trabecular bone formation in the marrow cavity. Local delivery of PGE2 increased cancellous bone volume in the secondary spongiosa and cortical thickness (at 32 microg). Comparisons of prostanoid effects in vitro, in a bone-derived cell line, showed that PGF2alpha was a better stimulator of DNA synthesis than PGE2. PGF2alpha increased the steady state levels of IGF-I receptor mRNA while PGE2 increased IGF-I expression. Although the mechanism of bone formation by PGE2 is not known at this time, it is clear that PGE2 has powerful local anabolic effects on bone formation in vivo possibly by mediating responses to signals such as changes in mechanical force.

Cellular Tensegrity: Exploring How Mechanical Changes in the Cytoskeleton Regulate Cell Growth, Migration, and Tissue Pattern during Morphogenesis

Publisher Summary This chapter focuses on the role of the intracellular cytoskeleton (CSK) in cell shape determination and tissue morphogenesis. The role of mechanical changes in the CSK during embryological development is reviewed. The chapter focuses on the mechanism by which mechanical forces are transmitted across the cell surface and through the CSK, as well as how they regulate cell shape. An analysis of the biomechanical basis of cell shape control addresses two central questions: (1) how do changes in mechanical forces alter CSK organization, and (2) how do changes in CSK structure regulate cell growth and function. The results from recent studies showing that the CSK can respond directly to mechanical stress are also reviewed. The particular type of mechanical response that living cells exhibit is consistent with a theory of CSK architecture that is based on tensional integrity and is known as “tensegrity”. Inherent to the tensegrity model is an efficient mechanism for integrating changes in structure and function at the tissue, cell, nuclear, and molecular levels. The chapter explores the possibility that CSK tensegrity may also provide a mechanical basis for cell locomotion as well as a structural mechanism for coupling mechanical and chemical signaling pathways inside the cell.

Effects and regulation of prostaglandins in bone: Possible implications for the pathogenesis of osteoporosis

Prostaglandins (PG's) are complex regulators of bone metabolism which have biphasic effects on both bone formation and resorption. PG's, particularly PGE2, are produced abundantly by bone cells of the osteoblast lineage, and their production is highly regulated. Of particular importance for a potential role in the pathogenesis of osteoporosis are two recent sets of observations. The first is that sex hormones can regulate PG production in bone. This may be relevant because hormone deficiency is so important in the pathogenesis of osteoporosis. The second is that mechanical forces can alter PG production by bone cells and that some of the responses to changes in mechanical force in vitro and in organ culture may be mediated by PG.

Mechanoreception by the Endothelium: Mediators and Mechanisms of Pressure- and Flow-Induced Vascular Responses

Mechanoreception, a widely distributed sensory modality, has been shown to be present in certain blood vessels. Changes in physical forces, like sudden increase of transmural pressure or flow velocity (shear stress), trigger changes in blood vessel diameter; the former reduces it while the latter increases vessel caliber. These changes in diameter, which are the result of contraction and relaxation of vascular smooth muscle in the blood vessel media, can serve the purpose of physiological regulation of blood flow (autoregulation) and protection of the intima against damages from high shear forces. The precise location of mechanosensor(s) and the mechanism of mechanoreception and signal transduction are poorly understood. Accumulating evidence suggests that the endothelium may be a site of mechanoreception and that changes in the synthesis/release of endothelium-derived relaxing (EDRF, EDHF, PGI2) and contracting factors (EDCF) result in altered vascular smooth muscle tone and vessel caliber. Increased shear stress stimulates the release of EDRF and PGI2 probably via activation of a K+ channel (inward rectifier) in endothelial cell membrane. Endothelium-dependent vascular contraction evoked by increased transmural pressure may be the result of (1) reduced release of EDRF (canine carotid artery) and (2) stimulation of the release of a still unidentified EDCF(s) (feline cerebral artery). Thus the endothelium can serve as pressure and flow sensor and is capable of transducing changes in mechanical forces into changes of vascular smooth muscle tone by modulating the release of endothelium-derived vasoactive factors. The physiological importance of the mechanoreception by endothelial cells in the intact circulation remains to be determined.