Effects of Lys to Glu mutations in GsMTx4 on membrane binding, peptide orientation, and self-association propensity, as analyzed by molecular dynamics simulations

Two types of peptides derived from the neurotoxin GsMTx4 inhibit a mechanosensitive potassium channel by modifying the mechano-gate

Atrial fibrillation is the most common sustained cardiac arrhythmia in humans. Current atrial fibrillation antiarrhythmic drugs have limited efficacy and carry the risk of ventricular proarrhythmia. GsMTx4, a mechanosensitive channel–selective inhibitor, has been shown to suppress arrhythmias through the inhibition of stretch-activated channels (SACs) in the heart. The cost of synthesizing this peptide is a major obstacle to clinical use. Here, we studied two types of short peptides derived from GsMTx4 for their effects on a stretch-activated big potassium channel (SAKcaC) from the heart. Type I, a 17-residue peptide (referred to as Pept 01), showed comparable efficacy, whereas type II (i.e., Pept 02), a 10-residue peptide, exerted even more potent inhibitory efficacy on SAKcaC compared with GsMTx4. We identified through mutagenesis important sequences required for peptide functions. In addition, molecular dynamics simulations revealed common structural features with a hydrophobic head followed by a positively charged protrusion that may be involved in peptide channel–lipid interactions. Furthermore, we suggest that these short peptides may inhibit SAKcaC through a specific modification to the mechanogate, as the inhibitory effects for both types of peptides were mostly abolished when tested with a mechano-insensitive channel variant (STREX-del) and a nonmechanosensitive big potassium (mouse Slo1) channel. These findings may offer an opportunity for the development of a new class of drugs in the treatment of cardiac arrhythmia generated by excitatory SACs in the heart.

Exploring the Molecular Level Interaction of Human Serum Albumin with Calcium Oxalate Monohydrate Crystals

BACKGROUND

Human serum albumin (HSA) is one of the most abundant proteins in the blood plasma, urine as well as in the organic matrix of renal calculi. Macromolecules present in the urine modulate kidney stone formation either by stimulating or inhibiting crystallization process.

OBJECTIVE

In the present study, effect of HSA protein on the growth of calcium oxalate monohydrate crystal (COM) was investigated.

METHODS

Crystal growth assay was used to measure oxalate depletion in the crystal seeded solution in the presence of HSA. HSA concentrations exhibiting effect on crystal growth were selected for FTIR and XRD analysis. In silico docking was performed on seven different binding sites of HSA.

RESULTS

Albumin is playing dual role in growth of calcium oxalate crystallization. FTIR and XRD studies further revealed HSA exerted strain over crystal thus affecting its structure by interacting with amino acids of its pocket 1. Docking results indicate that out of 7 binding pocket in protein, calcium oxalate interacts with Arg-186 and Lys-190 amino acids of pocket 1.

CONCLUSION

Our study confirms the role of HSA in calcium oxalate crystallization where acidic amino acids arginine and lysine are binding with COM crystals, revealing molecular interaction of macromolecule and crystal in urolithiasis.

Manipulation of a spider peptide toxin alters its affinity for lipid bilayers and potency and selectivity for voltage-gated sodium channel subtype 1.7

Huwentoxin-IV (HwTx-IV) is a gating modifier peptide toxin from spiders that has weak affinity for the lipid bilayer. As some gating modifier toxins have affinity for model lipid bilayers, a tripartite relationship among gating modifier toxins, voltage-gated ion channels, and the lipid membrane surrounding the channels has been proposed. We previously designed an HwTx-IV analogue (gHwTx-IV) with reduced negative charge and increased hydrophobic surface profile, which displays increased lipid bilayer affinity and in vitro activity at the voltage-gated sodium channel subtype 1.7 (NaV1.7), a channel targeted in pain management. Here, we show that replacements of the positively-charged residues that contribute to the activity of the peptide can improve gHwTx-IV's potency and selectivity for NaV1.7. Using HwTx-IV, gHwTx-IV, [R26A]gHwTx-IV, [K27A]gHwTx-IV, and [R29A]gHwTx-IV variants, we examined their potency and selectivity at human NaV1.7 and their affinity for the lipid bilayer. [R26A]gHwTx-IV consistently displayed the most improved potency and selectivity for NaV1.7, examined alongside off-target NaVs, compared with HwTx-IV and gHwTx-IV. The lipid affinity of each of the three novel analogues was weaker than that of gHwTx-IV, but stronger than that of HwTx-IV, suggesting a possible relationship between in vitro potency at NaV1.7 and affinity for lipid bilayers. In a murine NaV1.7 engagement model, [R26A]gHwTx-IV exhibited an efficacy comparable with that of native HwTx-IV. In summary, this study reports the development of an HwTx-IV analogue with improved in vitro selectivity for the pain target NaV1.7 and with an in vivo efficacy similar to that of native HwTx-IV.

The neuropeptide GsMTx4 inhibits a mechanosensitive BK channel through the voltage-dependent modification specific to mechano-gating

The cardiac mechanosensitive BK (Slo1) channels are gated by Ca²⁺, voltage, and membrane stretch. The neuropeptide GsMTx4 is a selective inhibitor of mechanosensitive (MS) channels. It has been reported to suppress stretch-induced cardiac fibrillation in the heart, but the mechanism underlying the specificity and even the targeting channel(s) in the heart remain elusive. Here, we report that GsMTx4 inhibits a stretch-activated BK channel (SAKcaC) in the heart through a modulation specific to mechano-gating. We show that membrane stretching increases while GsMTx4 decreases the open probability (P _o) of SAKcaC. These effects were mostly abolished by the deletion of the STREX axis-regulated (STREX) exon located between RCK1 and RCK2 domains in BK channels. Single-channel kinetics analysis revealed that membrane stretch activates SAKcaC by prolonging the open-time duration (τ_O) and shortening the closed-time constant (τ_C). In contrast, GsMTx4 reversed the effects of membrane stretch, suggesting that GsMTx4 inhibits SAKcaC activity by interfering with mechano-gating of the channel. Moreover, GsMTx4 exerted stronger efficacy on SAKcaC under membrane-hyperpolarized/resting conditions. Molecular dynamics simulation study revealed that GsMTx4 appeared to have the ability to penetrate deeply within the bilayer, thus generating strong membrane deformation under the hyperpolarizing/resting conditions. Immunostaining results indicate that BK variants containing STREX are also expressed in mouse ventricular cardiomyocytes. Our results provide common mechanisms of peptide actions on MS channels and may give clues to therapeutic suppression of cardiac arrhythmias caused by excitatory currents through MS channels under hyper-mechanical stress in the heart.

Enantiomeric Aβ peptides inhibit the fluid shear stress response of PIEZO1

Traumatic brain injury (TBI) elevates Abeta (Aβ) peptides in the brain and cerebral spinal fluid. Aβ peptides are amphipathic molecules that can modulate membrane mechanics. Because the mechanosensitive cation channel PIEZO1 is gated by membrane tension and curvature, it prompted us to test the effects of Aβ on PIEZO1. Using precision fluid shear stress as a stimulus, we found that Aβ monomers inhibit PIEZO1 at femtomolar to picomolar concentrations. The Aβ oligomers proved much less potent. The effect of Aβs on Piezo gating did not involve peptide-protein interactions since the D and L enantiomers had similar effects. Incubating a fluorescent derivative of Aβ and a fluorescently tagged PIEZO1, we showed that Aβ can colocalize with PIEZO1, suggesting that they both had an affinity for particular regions of the bilayer. To better understand the PIEZO1 inhibitory effects of Aβ, we examined their effect on wound healing. We observed that over-expression of PIEZO1 in HEK293 cells increased cell migration velocity ~10-fold, and both enantiomeric Aβ peptides and GsMTx4 independently inhibited migration, demonstrating involvement of PIEZO1 in cell motility. As part of the motility study we examined the correlation of PIEZO1 function with tension in the cytoskeleton using a genetically encoded fluorescent stress probe. Aβ peptides increased resting stress in F-actin, and is correlated with Aβ block of PIEZO1-mediated Ca2+ influx. Aβ inhibition of PIEZO1 in the absence of stereospecific peptide-protein interactions shows that Aβ peptides modulate both cell membrane and cytoskeletal mechanics to control PIEZO1-triggered Ca2+ influx.

Investigating the interaction of Grammostola rosea venom peptides and model lipid bilayers with solid-state NMR and electron microscopy techniques

Spider venom contains a number of small peptides that can control the gating properties of a wide range of ion channels with high affinity and specificity. These ion channels are responsible for coordination and control of many bodily functions such as transducing signals into sensory functions, smooth muscle contractions as well as serving as sensors in volume regulation. Hence, these peptides have been the topic of many research efforts in hopes that they can be used as biomedical therapeutics. Several peptides are known to control the gating properties of ion channels by involving the lipid membrane. GsMTx4, originally isolated from the Chilean Rose tarantula (Grammostola rosea), is known to selectively inhibit mechanosensitive ion channels by partitioning into the lipid bilayer. To further understand this indirect gating mechanism, we investigated the interactions between native GsAF2, VsTx1 and a synthetic form of GsMTx4 with model DMPC lipid bilayers using ³¹P solid-state NMR, ¹³C CP-MAS NMR, NS-TEM and cryo-TEM. The results reveal that these inhibitor cystine knot peptides perforate the DMPC lipid vesicles similarly with some subtle differences and ultimately create small spherical vesicles and anisotropic cylindrical and discoidal vesicles at concentrations near 1.0-1.5 mol% peptide. The anisotropic components align with their long axes along the NMR static B₀ magnetic field, a property that should be useful in future NMR structural investigations of these systems. These findings move us forward in our understanding of how these peptides bind and interact with the lipid bilayer.

GsMTx4-D provides protection to the D2.mdx mouse

Duchenne muscular dystrophy is a life-limiting muscle disease that has no current effective therapy. Despite mounting evidence that dysregulation of mechanosensitive ion channels is a significant contributor to dystrophy pathogenesis, effective pharmacologic strategies targeting these channels are lacking. GsMTx4, and its enantiomer GsMTx4-D, are peptide inhibitors of mechanosensitive channels with identical activity. In previous studies, acute in vitro application of GsMTx4 to dystrophic murine muscle effectively reduced the excess MSC dependent calcium influx linked to contraction-induced muscle damage. Here we sought to determine if in vivo treatment with GsMTx4-D proffered benefit in the D2.mdx mouse. GsMTx4-D showed a 1-week half-life when administered by subcutaneous injection over four weeks. Informed by these results, D2.mdx mice were then treated by a subcutaneous injection regimen of GsMTx4-D for six weeks followed by determination of muscle mass, muscle susceptibility to eccentric contraction injury and multiple histological indicators of disease progression. The mice showed a reduction in the loss of muscle mass and a decrease in susceptibility to contraction induced injury. These protective effects were realized without reduction in fibrosis, supporting a model where GsMTx4-D acts directly on muscle cells. We propose GsMTx4-D represents a promising new therapy to slow disease progression and may complement other therapies such as anti-inflammatory agents and gene-replacement strategies.

Piezo channels and GsMTx4: Two milestones in our understanding of excitatory mechanosensitive channels and their role in pathology

Discovery of Piezo channels and the reporting of their sensitivity to the inhibitor GsMTx4 were important milestones in the study of non-selective cationic mechanosensitive channels (MSCs) in normal physiology and pathogenesis. GsMTx4 had been used for years to investigate the functional role of cationic MSCs, especially in muscle tissue, but with little understanding of its target or inhibitory mechanism. The sensitivity of Piezo channels to bilayer stress and its robust mechanosensitivity when expressed in heterologous systems were keys to determining GsMTx4's mechanism of action. However, questions remain regarding Piezo's role in muscle function due to the non-selective nature of GsMTx4 inhibition toward membrane mechanoenzymes and the implication of MCS channel types by genetic knockdown. Evidence supporting Piezo like activity, at least in the developmental stages of muscle, is presented. While the MSC targets of GsMTx4 in muscle pathology are unclear, its muscle protective effects are clearly demonstrated in two recent in situ studies on normal cardiomyocytes and dystrophic skeletal muscle. The muscle protective function may be due to the combined effect of GsMTx4's inhibitory action on cationic MSCs like Piezo and TRP, and its potentiation of repolarizing K⁺ selective MSCs like K2P and SAKCa. Paradoxically, the potent in vitro action of GsMTx4 on many physiological functions seems to conflict with its lack of in situ side-effects on normal animal physiology. Future investigations into cytoskeletal control of sarcolemma mechanics and the suspected inclusion of MSCs in membrane micro/nano sized domains with distinct mechanical properties will aide our understanding of this dichotomy.

Gating modifier toxin interactions with ion channels and lipid bilayers: Is the trimolecular complex real?

Spider peptide toxins have attracted attention because of their ability to target voltage-gated ion channels, which are involved in several pathologies including chronic pain and some cardiovascular conditions. A class of these peptides acts by modulating the gating mechanism of voltage-gated ion channels and are thus called gating modifier toxins (GMTs). In addition to their interactions with voltage-gated ion channels, some GMTs have affinity for lipid bilayers. This review discusses the potential importance of the cell membrane on the mode of action of GMTs. We propose that peptide-membrane interactions can anchor GMTs at the cell surface, thereby increasing GMT concentration in the vicinity of the channel binding site. We also propose that modulating peptide-membrane interactions might be useful for increasing the therapeutic potential of spider toxins. Furthermore, we explore the advantages and limitations of the methodologies currently used to examine peptide-membrane interactions. Although GMT-lipid membrane binding does not appear to be a requirement for the activity of all GMTs, it is an important feature, and future studies with GMTs should consider the trimolecular peptide-lipid membrane-channel complex. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

GsMTx4: Mechanism of Inhibiting Mechanosensitive Ion Channels

GsMTx4 is a spider venom peptide that inhibits cationic mechanosensitive channels (MSCs). It has six lysine residues that have been proposed to affect membrane binding. We synthesized six analogs with single lysine-to-glutamate substitutions and tested them against Piezo1 channels in outside-out patches and independently measured lipid binding. Four analogs had ∼20% lower efficacy than the wild-type (WT) peptide. The equilibrium constants calculated from the rates of inhibition and washout did not correlate with the changes in inhibition. The lipid association strength of the WT GsMTx4 and the analogs was determined by tryptophan autofluorescence quenching and isothermal calorimetry with membrane vesicles and showed no significant differences in binding energy. Tryptophan fluorescence-quenching assays showed that both WT and analog peptides bound superficially near the lipid-water interface, although analogs penetrated deeper. Peptide-lipid association, as a function of lipid surface pressure, was investigated in Langmuir monolayers. The peptides occupied a large fraction of the expanded monolayer area, but that fraction was reduced by peptide expulsion as the pressure approached the monolayer-bilayer equivalence pressure. Analogs with compromised efficacy had pressure-area isotherms with steeper slopes in this region, suggesting tighter peptide association. The pressure-dependent redistribution of peptide between "deep" and "shallow" binding modes was supported by molecular dynamics (MD) simulations of the peptide-monolayer system under different area constraints. These data suggest a model placing GsMTx4 at the membrane surface, where it is stabilized by the lysines, and occupying a small fraction of the surface area in unstressed membranes. When applied tension reduces lateral pressure in the lipids, the peptides penetrate deeper acting as "area reservoirs" leading to partial relaxation of the outer monolayer, thereby reducing the effective magnitude of stimulus acting on the MSC gate.

Adaptation Independent Modulation of Auditory Hair Cell Mechanotransduction Channel Open Probability Implicates a Role for the Lipid Bilayer

The auditory system is able to detect movement down to atomic dimensions. This sensitivity comes in part from mechanisms associated with gating of hair cell mechanoelectric transduction (MET) channels. MET channels, located at the tops of stereocilia, are poised to detect tension induced by hair bundle deflection. Hair bundle deflection generates a force by pulling on tip-link proteins connecting adjacent stereocilia. The resting open probability (P(open)) of MET channels determines the linearity and sensitivity to mechanical stimulation. Classically, P(open) is regulated by a calcium-sensitive adaptation mechanism in which lowering extracellular calcium or depolarization increases P(open). Recent data demonstrated that the fast component of adaptation is independent of both calcium and voltage, thus requiring an alternative explanation for the sensitivity of P(open) to calcium and voltage. Using rat auditory hair cells, we characterize a mechanism, separate from fast adaptation, whereby divalent ions interacting with the local lipid environment modulate resting P(open). The specificity of this effect for different divalent ions suggests binding sites that are not an EF-hand or calmodulin model. GsMTx4, a lipid-mediated modifier of cationic stretch-activated channels, eliminated the voltage and divalent sensitivity with minimal effects on adaptation. We hypothesize that the dual mechanisms (lipid modulation and adaptation) extend the dynamic range of the system while maintaining adaptation kinetics at their maximal rates.

GsMTx4-D is a cardioprotectant against myocardial infarction during ischemia and reperfusion

GsMTx4 is a selective inhibitor of cationic mechanosensitive ion channels (MSCs) and has helped establish the role of MSCs in cardiac physiology. Inhomogeneous local mechanical stresses due to hypercontracture and swelling during ischemic reperfusion injury (IRI) likely induce elevated MSC activity that can contribute to cation imbalance. The aim of this study was to determine if the D enantiomer of GsMTx4 can act as a cardioprotectant in a mouse IRI model. Ischemia and reperfusion involved ligating a coronary artery followed by release of the ligature. GsMTx4-D was tested by either acute intravenous injection during the ischemic event or by two day pretreatment by intraperitoneal injection, both methods achieving similar results. Based on pharmacokinetic studies, GsMTx4-D dosage was set to achieve expected plasma concentrations between 50 and 5000nM and heart tissue concentrations between 1 and 200nM by intravenous injection. Relative to vehicle injected animals, GsMTx4-D reduced infarct area by ~40% for acute and pretreated animals for both 20 and 45min ischemic challenges. Many indicators of cardiac output were indistinguishable from sham-treated control hearts after GsMTx4-D treatment showing improvement at both 4 and 48h post ischemia, and premature ventricular beats immediately following reperfusion were also significantly reduced. To determine if GsMTx4-D cardioprotection could act directly at the level of cardiomyocytes, we tested its effects in vitro on indicators of IRI damage like cation influx and activation of inflammatory kinases in isolated myocytes cultured under hypoxic conditions. Hypoxia challenged cardiomyocytes treated with 10μM GsMTx4-D showed improved contractility and near normal contraction-related Ca(2+) influx. GsMTx4-D inhibited indicators of ischemic damage such as the apoptotic signaling system JNK/c-Jun, but also inhibited the energy response signaling system Akt kinase. We conclude that GsMTx4-D is a potent cardioprotectant in vivo that may act directly on cardiomyocytes and potentially be useful in multidrug strategies to treat IRI.