Fluid flow-induced increase in inward Ba2+ current expressed in HEK293 cells transiently transfected with human neuronal L-type Ca2+ channels

Mechanical regulation of gene expression in cardiac myocytes and fibroblasts

The intact heart undergoes complex and multiscale remodelling processes in response to altered mechanical cues. Remodelling of the myocardium is regulated by a combination of myocyte and non-myocyte responses to mechanosensitive pathways, which can alter gene expression and therefore function in these cells. Cellular mechanotransduction and its downstream effects on gene expression are initially compensatory mechanisms during adaptations to the altered mechanical environment, but under prolonged and abnormal loading conditions, they can become maladaptive, leading to impaired function and cardiac pathologies. In this Review, we summarize mechanoregulated pathways in cardiac myocytes and fibroblasts that lead to altered gene expression and cell remodelling under physiological and pathophysiological conditions. Developments in systems modelling of the networks that regulate gene expression in response to mechanical stimuli should improve integrative understanding of their roles in vivo and help to discover new combinations of drugs and device therapies targeting mechanosignalling in heart disease.

L-type calcium channel modulates mechanosensitivity of the cardiomyocyte cell line H9c2

The application of mechanical stimuli to cells often induce increases in intracellular calcium, affecting the regulation of a variety of cell functions. Although the mechanism of mechanotransduction-induced calcium increases has not been fully resolved, the involvement of mechanosensitive ion channels in the plasma membrane and the endoplasmic reticulum has been reported. Here, we demonstrate that voltage-gated L-type calcium channels play a critical role in the mechanosensitive calcium response in H9c2 rat cardiomyocytes. The intracellular calcium level in H9c2 cells increased in a reproducible dose-dependent manner in response to uniaxial stretching. The stretch-activated calcium response (SICR) completely disappeared in calcium-free medium, whereas thapsigargin and cyclopiazonic acid, inhibitors of sarcoendoplasmic reticulum calcium ATPase, partially reduced the SICR. These findings suggest that both calcium influx across the cell membrane and calcium release from the sarcoendoplasmic reticulum are involved in the SICR. Nifedipine, diltiazem, and verapamil, inhibitors of L-type calcium channels, reduced the SICR in a dose-dependent manner. Furthermore, small interfering RNA against the L-type calcium channel α1c subunit diminished the SICR dramatically. Nifedipine also diminished the mechanosensitivity of Langendorff-perfused rat heart. These results suggest that the SICR in H9c2 cardiomyocytes involves the activation of L-type calcium channels and subsequent calcium release from the sarcoendoplasmic reticulum.

Protein phosphorylation maintains the normal function of cloned human Cav2.3 channels

Ca_v2.3 Ca²⁺ channels are subject to cytosolic regulation, which has been difficult to characterize in native cells. Neumaier et al. demonstrate the role of phosphorylation in the function of these channels and suggest a close relationship between voltage dependence and the phosphorylation state.

R-type currents mediated by native and recombinant Ca_v2.3 voltage-gated Ca²⁺ channels (VGCCs) exhibit facilitation (run-up) and subsequent decline (run-down) in whole-cell patch-clamp recordings. A better understanding of the two processes could provide insight into constitutive modulation of the channels in intact cells, but low expression levels and the need for pharmacological isolation have prevented investigations in native systems. Here, to circumvent these limitations, we use conventional and perforated-patch-clamp recordings in a recombinant expression system, which allows us to study the effects of cell dialysis in a reproducible manner. We show that the decline of currents carried by human Ca_v2.3+β₃ channel subunits during run-down is related to adenosine triphosphate (ATP) depletion, which reduces the number of functional channels and leads to a progressive shift of voltage-dependent gating to more negative potentials. Both effects can be counteracted by hydrolysable ATP, whose protective action is almost completely prevented by inhibition of serine/threonine but not tyrosine or lipid kinases. Protein kinase inhibition also mimics the effects of run-down in intact cells, reduces the peak current density, and hyperpolarizes the voltage dependence of gating. Together, our findings indicate that ATP promotes phosphorylation of either the channel or an associated protein, whereas dephosphorylation during cell dialysis results in run-down. These data also distinguish the effects of ATP on Ca_v2.3 channels from those on other VGCCs because neither direct nucleotide binding nor PIP₂ synthesis is required for protection from run-down. We conclude that protein phosphorylation is required for Ca_v2.3 channel function and could directly influence the normal features of current carried by these channels. Curiously, some of our findings also point to a role for leupeptin-sensitive proteases in run-up and possibly ATP protection from run-down. As such, the present study provides a reliable baseline for further studies on Ca_v2.3 channel regulation by protein kinases, phosphatases, and possibly proteases.

Transient receptor potential vanilloid 2-mediated shear-stress responses in C2C12 myoblasts are regulated by serum and extracellular matrix

The developmental sensitivity of skeletal muscle to mechanical forces is unparalleled in other tissues. Calcium entry via reputedly mechanosensitive transient receptor potential (TRP) channel classes has been shown to play an essential role in both the early proliferative stage and subsequent differentiation of skeletal muscle myoblasts, particularly TRP canonical (TRPC) 1 and TRP vanilloid (TRPV) 2. Here we show that C2C12 murine myoblasts respond to fluid flow-induced shear stress with increments in cytosolic calcium that are largely initiated by the mechanosensitive opening of TRPV2 channels. Response to fluid flow was augmented by growth in low extracellular serum concentration (5 vs. 20% fetal bovine serum) by greater than 9-fold and at 18 h in culture, coincident with the greatest TRPV2 channel expression under identical conditions (P < 0.02). Fluid flow responses were also enhanced by substrate functionalization with laminin, rather than with fibronectin, agreeing with previous findings that the gating of TRPV2 is facilitated by laminin. Fluid flow-induced calcium increments were blocked by ruthenium red (27%) and SKF-96365 (38%), whereas they were unaltered by 2-aminoethoxydiphenyl borate, further corroborating that TRPV2 channels play a predominant role in fluid flow mechanosensitivity over that of TRPC1 and TRP melastatin (TRPM) 7.

Mechanical regulation of native and the recombinant calcium channel

L-type calcium channels are modulated by a host of mechanisms that include voltage, calcium ions (Ca(2+) dependent inactivation and facilitation), cytosolic proteins (CAM, CAMKII, PKA, PKC, etc.), and oxygen radicals. Here we describe yet another Ca(2+) channel regulatory mechanism that is induced by pressure-flow (PF) forces of ∼25dyn/cm(2) producing 35-60% inhibition of channel current. Only brief periods (300ms) of such PF pulses were required to suppress reversibly the current. Recombinant Ca(2+) channels (α1c77/β2a/α2δ and α1c77/β1/α2δ), expressed in HEK293 cells, were similarly suppressed by PF pulses. To examine whether Ca(2+) released by PF pulses triggered from different sub-cellular compartments (SR, ER, mitochondria) underlies the inhibitory effect of PF on the channel current, pharmacological agents and ionic substitutions were employed to probe this possibility. No significant difference in effectiveness of PF pulses to suppress ICa or IBa (used to inhibit CICR) was found between control cells and those exposed to U73122 and 2-APB (PLC and IP3R pathway modulators), thapsigargin and BAPTA (SERCA2a modulator), dinitrophenol, FCCP and Ru360 (mitochondrial inhibitors), l-NAME (NOS inhibitor signaling), cAMP and Pertussis toxin (Gi protein modulator). We concluded that the rapid and reversible modulation of the Ca(2+) channel by PF pulses is independent of intracellular release of Ca(2+) and Ca(2+) dependent inactivation of the channel and may represent direct mechanical regulatory effect on the channel protein in addition to previously reported Ca(2+)-release or entry dependent mechanism.

Application of single-cell microfluorimetry to neurotoxicology assays

Intracellular signaling events play fundamental roles in regulating physiological function. In neurons, these include inducing growth and differentiation, secretion, gene expression, and controlling processes associated with learning and memory. All of these processes have in common the vital dependence on changes in intracellular Ca²(+) [Ca²(+)](i). Numerous toxicants, including metals, polychlorinated biphenyls, and biological neurotoxins, can disrupt [Ca²(+)](i). Understanding how toxicants disrupt Ca²(+)-dependent neuronal signaling, and thus induce neuronal death or dysfunction, requires the ability to monitor [Ca²(+)](i) at the level of individual cells. A series of fluorophores that can report on changes in [Ca²(+)](i) has been pivotal in this process. This section describes how to use these fluorophores to study effects of neurotoxicants on two types of processes: changes in [Ca²(+)](i) in individual cells and changes in mitochondrial membrane potential. Similar techniques using distinct fluorophores can be applied to other physiological processes.

Optimized transfection strategy for expression and electrophysiological recording of recombinant voltage-gated ion channels in HEK-293T cells

The in vitro expression and electrophysiological recording of recombinant voltage-gated ion channels in cultured human embryonic kidney cells (HEK-293T) is a ubiquitous research strategy. HEK-293T cells must be plated onto glass coverslips at low enough density so that they are not in contact with each other in order to allow for electrophysiological recording without confounding effects due to contact with adjacent cells. Transfected channels must also express with high efficiency at the plasma membrane for whole-cell patch clamp recording of detectable currents above noise levels. Heterologous ion channels often require long incubation periods at 28°C after transfection in order to achieve adequate membrane expression, but there are increasing losses of cell-coverslip adhesion and membrane stability at this temperature. To circumvent this problem, we developed an optimized strategy to transfect and plate HEK-293T cells. This method requires that cells be transfected at a relatively high confluency, and incubated at 28°C for varying incubation periods post-transfection to allow for adequate ion channel protein expression. Transfected cells are then plated onto glass coverslips and incubated at 37°C for several hours, which allows for rigid cell attachment to the coverslips and membrane restabilization. Cells can be recorded shortly after plating, or can be transferred to 28°C for further incubation. We find that the initial incubation at 28°C, after transfection but before plating, is key for the efficient expression of heterologous ion channels that normally do not express well at the plasma membrane. Positively transfected, cultured cells are identified by co-expressed eGFP or eGFP expressed from a bicistronic vector (e.g. pIRES2-EGFP) containing the recombinant ion channel cDNA just upstream of an internal ribosome entry site and an eGFP coding sequence. Whole-cell patch clamp recording requires specialized equipment, plus the crafting of polished recording electrodes and L-shaped ground electrodes from borosilicate glass. Drug delivery to study the pharmacology of ion channels can be achieved by directly micropipetting drugs into the recording dish, or by using microperfusion or gravity flow systems that produce uninterrupted streams of drug solution over recorded cells.

Amiloride-sensitive channels are a major contributor to mechanotransduction in mammalian muscle spindles

We investigated whether channels of the epithelial sodium/amiloride-sensitive degenerin (ENaC/DEG) family are a major contributor to mechanosensory transduction in primary mechanosensory afferents, using adult rat muscle spindles as a model system. Stretch-evoked afferent discharge was reduced in a dose-dependent manner by amiloride and three analogues - benzamil, 5-(N-ethyl-N-isopropyl) amiloride (EIPA) and hexamethyleneamiloride (HMA), reaching > or = 85% inhibition at 1 mm. Moreover, firing was slightly but significantly increased by ENaC delta subunit agonists (icilin and capsazepine). HMA's profile of effects was distinct from that of the other drugs. Amiloride, benzamil and EIPA significantly decreased firing (P < 0.01 each) at 1 microm, while 10 microm HMA was required for highly significant inhibition (P < 0.0001). Conversely, amiloride, benzamil and EIPA rarely blocked firing entirely at 1 mm, whereas 1 mm HMA blocked 12 of 16 preparations. This pharmacology suggests low-affinity ENaCs are the important spindle mechanotransducer. In agreement with this, immunoreactivity to ENaC alpha, beta and gamma subunits was detected both by Western blot and immunocytochemistry. Immunofluorescence intensity ratios for ENaC alpha, beta or gamma relative to the vesicle marker synaptophysin in the same spindle all significantly exceeded controls (P < 0.001). Ratios for the related brain sodium channel ASIC2 (BNaC1alpha) were also highly significantly greater (P < 0.005). Analysis of confocal images showed strong colocalisation within the terminal of ENaC/ASIC2 subunits and synaptophysin. This study implicates ENaC and ASIC2 in mammalian mechanotransduction. Moreover, within the terminals they colocalise with synaptophysin, a marker for the synaptic-like vesicles which regulate afferent excitability in these mechanosensitive endings.

Fluid pressure modulates L-type Ca2+ channel via enhancement of Ca2+-induced Ca2+ release in rat ventricular myocytes

This study examines whether fluid pressure (FP) modulates the L-type Ca2+ channel in cardiomyocytes and investigates the underlying cellular mechanism(s) involved. A flow of pressurized (∼16 dyn/cm2) fluid, identical to that bathing the myocytes, was applied onto single rat ventricular myocytes using a microperfusion method. The Ca2+ current ( ICa) and cytosolic Ca2+ signals were measured using a whole cell patch-clamp and confocal imaging, respectively. It was found that the FP reversibly suppressed ICa (by 25%) without altering the current-voltage relationships, and it accelerated the inactivation of ICa. The level of ICa suppression by FP depended on the level and duration of pressure. The Ba2+ current through the Ca2+ channel was only slightly decreased by the FP (5%), suggesting an indirect inhibition of the Ca2+ channel during FP stimulation. The cytosolic Ca2+ transients and the basal Ca2+ in field-stimulated ventricular myocytes were significantly increased by the FP. The effects of the FP on the ICa and on the Ca2+ transient were resistant to the stretch-activated channel inhibitors, GsMTx-4 and streptomycin. Dialysis of myocytes with high concentrations of BAPTA, the Ca2+ buffer, eliminated the FP-induced acceleration of ICa inactivation and reduced the inhibitory effect of the FP on ICa by ≈80%. Ryanodine and thapsigargin, abolishing sarcoplasmic reticulum Ca2+ release, eliminated the accelerating effect of FP on the ICa inactivation, and they reduced the inhibitory effect of FP on the ICa. These results suggest that the fluid pressure indirectly suppresses the Ca2+ channel by enhancing the Ca2+-induced intracellular Ca2+ release in rat ventricular myocytes.

The sodium-calcium exchanger is a mechanosensitive transporter

This report describes the influence of fluid flow and osmotically induced volume changes on Na(+)-Ca(2+) exchange (NCX) activity in transfected CHO cells. Exchange activity was measured as Na(+)-dependent Ca(2+) or Ba(2+) fluxes using the fluorescent probe fura-2. When exchange activity was initiated by superfusing Ba(2+)-containing solutions over the cells for a 20 s interval, a high rate of Ba(2+) uptake was observed while the solution was being applied but the rate of Ba(2+) uptake declined > 10-fold when the solution flow ceased. Ba(2+) efflux in exchange for extracellular Na(+) or Ca(2+) (Ba(2+)-Ca(2+) exchange) was similarly biphasic. During NCX-mediated Ca(2+) uptake, a rapid increase in cytosolic [Ca(2+)] to a peak value occurred, followed by a decline in [Ca(2+)](i) to a lower steady-state value after solution flow ceased. When NCX activity was initiated by an alternate procedure that minimized the duration of solution flow, the rapid phase of Ba(2+) influx was greatly reduced in magnitude and Ca(2+) uptake became nearly monophasic. Solution superfusion did not produce any obvious changes in cell shape or volume. NCX-mediated Ba(2+) and Ca(2+) influx were also sensitive to osmotically induced changes in cell volume. NCX activity was stimulated in hypotonic media and inhibited in hypertonic media; the osmotically induced changes in activity occurred within seconds and were rapidly reversible. We conclude that NCX activity is modulated by both solution flow and osmotically induced volume changes.

Effects of fluid flow on voltage-dependent calcium channels in rat vascular myocytes: fluid flow as a shear stress and a source of artifacts during patch-clamp studies

We examined the effects of fluid flow on L-type voltage-dependent Ca(2+) channel (VDCC(L)) currents in rat vascular myocytes using the nystatin perforated patch-clamp technique. The effect of fluid flow on the liquid (bathing solution)-metal (Ag/AgCl ground electrode) junction potential was also studied. With a fluid flow of 0-10 ml/min, changes in the junction potential of up to 5 mV were observed in proportion to the flow rate. Accordingly, fluid flow shifted the current-voltage (I-V) relationship of the recorded VDCC(L) currents in a positive direction. In addition to these shifts, fluid flow also increased the peak VDCC(L) current, suggesting some modulatory role for fluid flow in VDCC(L) currents. The use of a 3-M KCl agar-bridge between the ground electrode and bathing solution abnegated the potential shifts, and fluid flow increased the VDCC(L) currents in a voltage-independent manner. These results suggest that the bathing fluid flow can be both a source of erroneous voltage shift between liquid and metal junctions in the patch-clamp configuration and an important shear stress for the cells. The facilitation of VDCC(L) currents by fluid flow in vascular myocytes may contribute to the myogenic contraction of blood vessels. The mechanism by which fluid flow causes the voltage shift is vigorously discussed.

Death don't have no mercy and neither does calcium: Arabidopsis CYCLIC NUCLEOTIDE GATED CHANNEL2 and innate immunity

Plant innate immune response to pathogen infection includes an elegant signaling pathway leading to reactive oxygen species generation and resulting hypersensitive response (HR); localized programmed cell death in tissue surrounding the initial infection site limits pathogen spread. A veritable symphony of cytosolic signaling molecules (including Ca(2+), nitric oxide [NO], cyclic nucleotides, and calmodulin) have been suggested as early components of HR signaling. However, specific interactions among these cytosolic secondary messengers and their roles in the signal cascade are still unclear. Here, we report some aspects of how plants translate perception of a pathogen into a signal cascade leading to an innate immune response. We show that Arabidopsis thaliana CYCLIC NUCLEOTIDE GATED CHANNEL2 (CNGC2/DND1) conducts Ca(2+) into cells and provide a model linking this Ca(2+) current to downstream NO production. NO is a critical signaling molecule invoking plant innate immune response to pathogens. Plants without functional CNGC2 lack this cell membrane Ca(2+) current and do not display HR; providing the mutant with NO complements this phenotype. The bacterial pathogen-associated molecular pattern elicitor lipopolysaccharide activates a CNGC Ca(2+) current, which may be linked to NO generation due to buildup of cytosolic Ca(2+)/calmodulin.

The marine secondary metabolites 2,4-dibromophenol and 2,4,6-tribromophenol differentially modulate voltage dependent ion currents in neuroendocrine (PC12) cells

2,4-Dibromophenol (2,4-DBP) and 2,4,6-tribromophenol (2,4,6-TBP) are marine secondary metabolites, with 2,4,6-tribromophenol playing an important role as industrially produced flame retardant and pesticide. Both substances disturb cellular calcium signals in neuroendocrine cells as previously shown by Hassenklöver et al. (2006) [Hassenklöver, T., Predehl, S., Pilli, J., Ledwolorz, J., Assmann, M., Bickmeyer, U., 2006. Bromophenols, both present in marine organisms and in industrial flame retardants, disturb cellular Ca(2+) signaling in neuroendocrine cells (PC12). Aquat. Toxicol. 76, 37-45]. We investigated calcium channel currents in detail and outward membrane currents as potential cellular targets of both bromophenols. In this electrophysiological approach, 2,4-DBP reduced voltage dependent calcium channel currents with a half-maximal concentration of 45+/-32 microM (S.D.) and a Hill coefficient of 0.87+/-0.49 (S.D.). 2,4,6-TBP reduced calcium channel currents with a half-maximal concentration of 28+/-19 microM (S.D.) and a Hill coefficient of 0.79+/-0.31 (S.D.). The major contribution to calcium channel currents was mediated by L-type (67%) and N-type channels (30%) in PC12 cells; both bromophenols modulated both current types. Whole cell outward currents, mainly carried by potassium ions, were reduced by 2,4-DBP with a half-maximal concentration of 41+/-9 microM (S.D.) showing a Hill coefficient of 1.71+/-0.31 (S.D.). 2,4,6-TBP showed a weak reduction of outward currents at high concentrations of 300 microM. 2,4,6-TBP selectively decreased calcium entry via calcium channels as revealed in whole cell patch clamp experiments, whereas 2,4-DBP reduced both in- and outward currents.