The differentiation of cardiac fibroblasts to myofibroblasts is one of the key events during cardiac remodeling, however, the molecular mechanism underlying this process is not well known. Calcium signaling plays an important role in the regulation of cardiac fibroblast function, but its role in the differentiation of fibroblasts is undefined. Recently four Transient Receptor Potential (TRP) channels TRPM7, TRPC3, TRPC6 and TRPV4 were shown to be crucial for the differentiation of cardiac fibroblasts to myofibroblasts. This addendum sums up the roles described for these four TRP channels in cardiac fibroblast differentiation, and discusses the possible molecular mechanisms underlying this process and its relevance for cardiac remodeling in disease.
cardiac fibroblast; differentiation; mechanical signaling; myofibroblast; integrin; ECM stiffness; TRPV4; Rho/ROCK
The phenotypic switch underlying the differentiation of cardiac fibroblasts into hypersecretory myofibroblasts is critical for cardiac remodeling following myocardial infarction. Myofibroblasts facilitate wound repair in the myocardium by secreting and organizing extracellular matrix (ECM) during the wound healing process. However, the molecular mechanisms involved in myofibroblast differentiation are not well known. TGF-β has been shown to promote differentiation and this, combined with the robust mechanical environment in the heart, lead us to hypothesize that the mechanotransduction and TGF-β signaling pathways play active roles in the differentiation of cardiac fibroblasts to myofibroblasts. Here, we show that the mechanosensitve ion channel TRPV4 is required for TGF-β1-induced differentiation of cardiac fibroblasts into myofibroblasts. We found that the TRPV4-specific antagonist AB159908 and siRNA knockdown of TRPV4 significantly inhibited TGFβ1-induced differentiation as measured by incorporation of α-SMA into stress fibers. Further, we found that TGF-β1-induced myofibroblast differentiation was dependent on ECM stiffness, a response that was attenuated by TRPV4 blockade. Finally, TGF-β1 treated fibroblasts exhibited enhanced TRPV4 expression and TRPV4-mediated calcium influx compared to untreated controls. Taken together these results suggest for the first time that the mechanosensitive ion channel, TRPV4, regulates cardiac fibroblast differentiation to myofibroblasts by integrating signals from TGF-β1 and mechanical factors.
calcium; cardiac remodeling; myocardial infarction; extracellular matrix; TRPV4
Cilia are mechanosensing organelles that communicate extracellular signals into intracellular responses. Altered functions of primary cilia play a key role in the development of various diseases including polycystic kidney disease. Here, we show that endothelial cells from the oak ridge polycystic kidney (Tg737orpk/orpk) mouse, with impaired cilia assembly, exhibit a reduction in the actin stress fibers and focal adhesions compared to wild type. In contrast, endothelial cells from polycystin-1 deficient mice (pkd1null/null), with impaired cilia function, display robust stress fibers and focal adhesion assembly. We found that the Tg737orpk/orpk cells exhibit impaired directional migration and endothelial cell monolayer permeability compared to the wild type and pkd1null/null cells. Finally, we found that the expression of heat shock protein 27 (hsp27) and the phosphorylation of FAK are down regulated in the Tg737orpk/orpk cells and overexpression of hsp27 restored both FAK phosphorylation and cell migration. Taken together, these results demonstrate that disruption of the primary cilia structure or function compromises the endothelium through the suppression of hsp27 dependent actin organization and focal adhesion formation, which may contribute to the vascular dysfunction in ciliopathies.
primary cilia; hsp27; cardiovascular; endothelial
Idiopathic pulmonary fibrosis (IPF) is a fatal fibrotic lung disorder with no effective medical treatments available. The generation of myofibroblasts, which are critical for fibrogenesis, requires both a mechanical signal and activated TGF-β; however, it is not clear how fibroblasts sense and transmit the mechanical signal(s) that promote differentiation into myofibroblasts. As transient receptor potential vanilloid 4 (TRPV4) channels are activated in response to changes in plasma membrane stretch/matrix stiffness, we investigated whether TRPV4 contributes to generation of myofibroblasts and/or experimental lung fibrosis. We determined that TRPV4 activity is upregulated in lung fibroblasts derived from patients with IPF. Moreover, TRPV4-deficient mice were protected from fibrosis. Furthermore, genetic ablation or pharmacological inhibition of TRPV4 function abrogated myofibroblast differentiation, which was restored by TRPV4 reintroduction. TRPV4 channel activity was elevated when cells were plated on matrices of increasing stiffness or on fibrotic lung tissue, and matrix stiffness–dependent myofibroblast differentiation was reduced in response to TRVP4 inhibition. TRPV4 activity modulated TGF-β1–dependent actions in a SMAD-independent manner, enhanced actomyosin remodeling, and increased nuclear translocation of the α-SMA transcription coactivator (MRTF-A). Together, these data indicate that TRPV4 activity mediates pulmonary fibrogenesis and suggest that manipulation of TRPV4 channel activity has potential as a therapeutic approach for fibrotic diseases.
We previously reported that type VI collagen deposition increases in the infarcted myocardium in vivo. To date, a specific role for this non-fibrillar collagen has not been explored in the setting of myocardial infarction (MI).
To determine whether deletion of type VI collagen in an in vivo model of post-MI wound healing would alter cardiac function and remodeling in the days to weeks after injury.
Methods and Results
Wild type (WT) and Col6a1-/- mice were subjected to MI followed by serial echocardiographic and histological assessments. At 8 weeks post-MI, infarct size was significantly reduced, ejection fraction was significantly preserved (43.9 ± 3.3% vs. 29.1 ± 4.3% for WT) and left ventricular (LV) chamber dilation was attenuated in the Col6a1-/- MI group (25.8 ± 7.9% increase vs. 62.6 ± 16.5% for WT). The improvement in cardiac remodeling was evident as early as 10 days post-MI in the Col6a1-/- mice. Myocyte apoptosis within the infarcted zones was initially greater in the Col6a1-/- group 3 days post-MI but by day 14 this was significantly reduced. Collagen deposition was also reduced in the infarcted and remote areas of the Col6a1-/- hearts. The reductions in chronic myocyte apoptosis and fibrosis are critical events leading to improved long-term remodeling and functional outcomes.
These unexpected results demonstrate for the first time that deletion of type VI collagen in this knockout model plays a critical protective role following MI by limiting infarct size, chronic apoptosis, aberrant remodeling and fibrosis leading to preservation of cardiac function.
post-MI remodeling; cardioprotection; myofibroblast; non-fibrillar collagen; cell-matrix interactions
Cysteinyl leukotrienes (cys-LTs), LTC4, LTD4, LTE4 are potent inflammatory lipid mediators that act through two distinct G-protein-coupled receptors, CysLT1R and CysLT2R. Although cys-LTs are shown to induce vascular leakage and atherosclerosis, the molecular mechanism by which cys-LTs modulate endothelial function is not known. Here, we show that cys-LTs (LTC4 and LTD4) induce robust calcium influx in human umbilical vein endothelial cells (HUVECs) through CysLT2R, but not CysLT1R. Further, cys-LT treatment induced endothelial cell (EC) contraction leading to monolayer disruption via CysLT2R/Rho kinase dependent pathway. Furthermore, stimulation with cys-LTs potentiated TNFα-induced VCAM-1 expression and leukocyte recruitment to ECs through CysLT2R. In contrast, we found that both LTC4 and LTD4 stimulated EC proliferation through CysLT1R. Taken together, these results suggest that cys-LTs induce endothelial inflammation and proliferation via CysLT2R/Rho kinase and CysLT1R/Erk dependent pathways, respectively, which play critical role in the etiology of cardiovascular diseases such as atherosclerosis and myocardial infarction.
Cysteinyl leukotrienes (cys-LTs) are a group of lipid mediators that are potent bronchoconstrictors, powerful inducers of vascular leakage and potentiators of airway hyperresponsiveness. Cys-LTs play an essential role in asthma and are synthesized as well as activated in mast cells (MCs). Cys-LTs relay their effects mainly through two known GPCRs, CysLT1R and CysLT2R. Although protein kinase C (PKC) isoforms are implicated in the regulation of CysLT1R function, neither the role of PKCs in cys-LT-dependent MC inflammatory signaling nor the involvement of specific isoforms in MC function are known. Here, we show that PKC inhibition augmented LTD4 and LTE4-induced calcium influx through CysLT1R in MCs. In contrast, inhibition of PKCs suppressed c-fos expression as well MIP1β generation by cys-LTs. Interestingly, cys-LTs activated both PKCα and PKCε isoforms in MC. However, knockdown of PKCα augmented cys-LT mediated calcium flux, while knockdown of PKCε attenuated cys-LT induced c-fos expression and MIP1β generation. Taken together, these results demonstrate for the first time that cys-LT signaling downstream of CysLT1R in MCs is differentially regulated by two distinct PKCs which modulate inflammatory signals that have significant pathobiologic implications in allergic reactions and asthma pathology.
Physical interactions between cells and the extracellular matrix (ECM) guide directional migration by spatially controlling where cells form focal adhesions (FAs), which in turn regulate the extension of motile processes. Here we show that physical control of directional migration requires the FA scaffold protein paxillin. Using single-cell sized ECM islands to constrain cell shape, we found that fibroblasts cultured on square islands preferentially activated Rac and extended lamellipodia from corner, rather than side regions after 30 min stimulation with PDGF, but that cells lacking paxillin failed to restrict Rac activity to corners and formed small lamellipodia along their entire peripheries. This spatial preference was preceded by non-spatially constrained formation of both dorsal and lateral membrane ruffles from 5–10 min. Expression of paxillin N-terminal (paxN) or C-terminal (paxC) truncation mutants produced opposite, but complementary, effects on lamellipodia formation. Surprisingly, pax−/− and paxN cells also formed more circular dorsal ruffles (CDRs) than pax+ cells, while paxC cells formed fewer CDRs and extended larger lamellipodia even in the absence of PDGF. In a two-dimensional (2D) wound assay, pax−/− cells migrated at similar speeds to controls but lost directional persistence. Directional motility was rescued by expressing full-length paxillin or the N-terminus alone, but paxN cells migrated more slowly. In contrast, pax−/− and paxN cells exhibited increased migration in a three-dimensional (3D) invasion assay, with paxN cells invading Matrigel even in the absence of PDGF. These studies indicate that paxillin integrates physical and chemical motility signals by spatially constraining where cells will form motile processes, and thereby regulates directional migration both in 2D and 3D. These findings also suggest that CDRs may correspond to invasive protrusions that drive cell migration through 3D extracellular matrices.
Integrins are ubiquitous transmembrane mechanoreceptors that elicit changes in intracellular biochemistry in response to mechanical force application, but these alterations generally proceed over seconds to minutes. Stress-sensitive ion channels represent another class of mechanoreceptors that are activated much more rapidly (within msec), and recent findings suggest that calcium influx through Transient Receptor Potential Vanilloid-4 (TRPV4) channels expressed in the plasma membrane of bovine capillary endothelial cells is required for mechanical strain-induced changes in focal adhesion assembly, cell orientation and directional migration. However, whether mechanically stretching a cell’s extracellular matrix (ECM) adhesions might directly activate cell surface ion channels remains unknown. Here we show that forces applied to β1 integrins result in ultra-rapid (within 4 msec) activation of calcium influx through TRPV4 channels. The TRPV4 channels were specifically activated by mechanical strain in the cytoskeletal backbone of the focal adhesion, and not by deformation of the lipid bilayer or submembranous cortical cytoskeleton alone. This early-immediate calcium signaling response required the distal region of the β1 integrin cytoplasmic tail that contains a binding site for the integrin-associated transmembrane CD98 protein, and external force application to CD98 within focal adhesions activated the same ultra-rapid calcium signaling response. Local direct strain-dependent activation of TRPV4 channels mediated by force transfer from integrins and CD98 may therefore enable compartmentalization of calcium signaling within focal adhesions that is critical for mechanical control of many cell behaviors that underlie cell and tissue development.
Mechanical stresses that are preferentially transmitted across the cell surface via transmembrane integrin receptors activate gene transcription by triggering production of intracellular chemical second messengers, such as cAMP. Here we show that the sensitivity of the cAMP signaling pathway to mechanical stresses transferred across β1 integrins is mediated by force-dependent activation of the heterotrimeric G protein subunit Gαs within focal adhesions at the site of stress application. Gαs is recruited to focal adhesions that form within minutes following clustering of β1 integrins induced by cell binding to magnetic microbeads coated with activating integrin ligands, and β1 integrin and Gαs co-precipitate when analyzed biochemically. Stress application to activated β1 integrins using magnetic twisting cytometry increases Gαs recruitment and activates these large G proteins within focal adhesions, as measured by binding of biotinylated azido-anilido-GTP, whereas application of similar stresses to inactivated, integrins or control histocompatibility antigens has little effect. This response is relevant physiologically as application of mechanical strain to cells bound to flexible extracellular matrix-coated substrates induce translocation of phospho-CREB to the nucleus, which can be attenuated by inhibiting Gαs activity, either using the inhibitor melittin or suppressing its expression using siRNA. Although integrins are not typical G protein-coupled receptors, these results show that integrins focus mechanical stresses locally on heterotrimeric G proteins within focal adhesions at the site of force application, and transduce mechanical stimuli into an intracellular cAMP signaling response by activating Gαs at these membrane signaling sites.
mechanotransduction; G protein; integrin; focal adhesion; shear stress; mechanical strain; magnetic
Cyclic mechanical strain produced by pulsatile blood flow regulates the orientation of endothelial cells lining blood vessels, and influences critical processes such as angiogenesis. Mechanical stimulation of stretch-activated calcium channels is known to mediate this reorientation response, however, the molecular basis remains unknown. Here we show that cyclically stretching capillary endothelial cells adherent to flexible extracellular matrix substrates activates mechanosensitive TRPV4 ion channels that, in turn, stimulate phosphatidyl inositol-3-kinase-dependent activation and binding of additional ·1 integrin receptors, which promotes cytoskeletal remodeling and cell reorientation. Inhibition of integrin activation using blocking antibodies and knockdown of TRPV4 channels using specific siRNA suppress strain-induced capillary cell reorientation. Thus, mechanical forces that physically deform extracellular matrix may guide capillary cell reorientation through a strain-dependent ‘integrin to integrin’ signaling mechanism mediated by force-induced activation of mechanically-gated TRPV4 ion channels on the cell surface.
mechanical strain; integrin; TRPV4; endothelial cell; reorientation; cytoskeleton
The formation of focal adhesions governs cell shape and function; however, there are few measurements of the binding kinetics of focal adhesion proteins in living cells. Here, we used the fluorescence recovery after photobleaching (FRAP) technique, combined with mathematical modeling and scaling analysis to quantify dissociation kinetics of focal adhesion proteins in capillary endothelial cells. Novel experimental protocols based on mathematical analysis were developed to discern the rate-limiting step during FRAP. Values for the dissociation rate constant kOFF ranged over an order of magnitude from 0.009 ± 0.001/s for talin to 0.102 ± 0.010/s for FAK, indicating that talin is bound more strongly than other proteins in focal adhesions. Comparisons with in vitro measurements reveal that multiple focal adhesion proteins form a network of bonds, rather than binding in a pair-wise manner in these anchoring structures in living cells.