SUMMARY Soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNAREs) drive stagewise membrane fusion by zippering into membrane-bridging four-helix bundles. Yet the conformations underlying successive fusion stages and the coupling of folding energy to bilayer remodeling remain unclear. Using optical tweezers, we measured the intermediates, energetics, kinetics, and force dependence of individual synaptic SNARE complexes assembled in cis on single membranes and in trans between apposed membranes. Membrane-anchored cis-SNAREs assembled through N-terminal and cooperative C-terminal/linker-domain transitions, whereas their transmembrane domains showed little intrinsic dimerization. Syntaxin retained membrane-dependent helical continuity through its linker domain before zippering was complete. PIP₂ strengthened but slowed late zippering. In trans, membrane repulsion arrested single trans-SNARE complexes in a half-zippered state. Gβγ further clamped this intermediate and inhibited late zippering; Gα–GDP, but not Gα–GTPγS, relieved the clamp, revealing a nucleotide-dependent mechanism for GPCR-mediated inhibition of neurotransmitter release. Modeling suggests that cooperative late zippering, syntaxin linker helicity, and concerted action of multiple SNAREs focus folding energy released over a long distance onto short-range membrane apposition. Thus, mechanically gated SNARE zippering is regulated by membrane forces, lipids, and regulatory proteins.

Abstract Acute kidney injury (AKI) is a serious and common clinical syndrome that currently has no effective treatment. Emerging evidence links coagulation pathways to kidney injury, particularly through coagulation proteases. Protease-activated receptors (PARs) are a family of G-protein coupled receptors (GPCRs) that are activated by proteolytic cleavage of their N termini, exposing a tethered ligand that initiates receptor signaling. PARs have been shown to play a major role in inflammation, vascular regulation, and tissue injury. PARs play key roles in inflammation, vascular regulation, and tissue injury. Previous work from the Hamm laboratory demonstrated that PAR4 contributes to AKI progression, as PAR4 knockout mice were protected in both unilateral ureteral obstruction and ischemia-reperfusion–based models of kidney disease. In this study, we investigated the potential of a PAR4 antagonist, VU6073819, at mitigating AKI progression in an ischemia-reperfusion injury (IRI) mouse model. PAR4 antagonism not only alleviated kidney injury and inflammatory response, but it significantly improved the survival. These findings identify PAR4 as a promising therapeutic target for AKI.

Platelet activation may be a central mediator of a chain of events leading to microinfarcts, leakage of thrombin and fibrin through the blood-brain barrier and chronic neuroinflammation that typify Alzheimer’s Disease (AD). The platelet thrombin receptor Protease Activated Receptor 4 (PAR4) is responsible for platelet activation and amplification of thrombin generation. Thrombin cleaves fibrinogen to fibrin, and pathologic fibrin deposition in the cerebral microvasculature is itself a risk factor for Alzheimer’s disease. Fibrin interaction with vascular amyloid β (Aβ) leads to degradation-resistant blood clots. This process initiates inflammation both in the vessel and adjacent parenchyma. Inflammation in general has been reported to turn on expression of PAR4 in endothelial cells not typically expressing PAR4. 1) Data were acquired from the Religious Orders Study (ROS) and the Rush Memory and Aging Project (MAP) to study the correlation of the PAR4 gene expression or methylation with AD diagnosis and longitudinal cognitive decline. 2) 5xFAD amyloid model mice were crossed with PAR4KO mice to test for pathology and markers of inflammation. 1) PAR4 gene F2RL3 mRNA was elevated in AD cases and was associated with worse retrospective longitudinal cognitive performance. We also report a significant association of F2RL3 epigenetic demethylation with cognitive decline. 2) 5xFAD mice exhibit increased PAR4 protein expression on vascular endothelial cells. 5xFAD mice exhibit increased vascular fibrin deposits compared to WT mice, and this fibrin deposition is reduced in 5xFAD/PAR4KO compared to 5xFAD. In a human study the PAR4 gene F2RL3 mRNA expression is associated with multiple AD-relevant outcomes and its encoded product, PAR4, may play a role in disease pathogenesis. PAR4 is a platelet receptor that functions upstream of fibrin deposition in 5xFAD amyloid mice, leading to misexpression in the microvasculature. Fibrin deposits have been reported to be inflammatory in 5xFAD and future work will focus on teasing apart the contribution of PAR4 mediated fibrin deposits in inflammation and microglial activation.

GABAergic signaling provides the primary inhibitory mechanism of the brain, with defects linked to epilepsy, anxiety, depression, insomnia, schizophrenia and neurodegeneration. A key regulatory mechanism is autoinhibition of GABA release during repetitive activity via presynaptic Gi/o-coupled GABAB receptors, supporting synaptic tuning and memory formation, and limiting neurotransmitter spillover. Exogenous GABAB receptor agonists reduce presynaptic Ca2+ entry by inhibiting calcium channels. However, using transgenic mice expressing a mutant SNAP25 with diminished ability to bind Gβγ (SNAP25δ3), we show that suppression by GABAB autoreceptors requires intact Gβγ-SNARE interactions. Imaging of presynaptic Ca2+ transients in GABAergic axons showed no GABA-mediated autoreceptor suppression of Ca2+ entry during stimulus trains. In contrast, application of the exogenous GABAB receptor agonist baclofen profoundly inhibited Ca2+ entry, which could be partially reversed by exogenously elevating cAMP, indicating a complementary role of inhibition of adenylyl cyclase. Baclofen reduced spontaneous IPSC frequency and amplitude and both effects were diminished in SNAP25δ3 mice, consistent with inhibition at Ca2+ channels and SNARE complexes. Physiological GABA-mediated and exogenous GABAB receptor activation thus produce distinct outcomes on GABAergic neurotransmission, indicating that synthetic drug application to neurons does not faithfully recapitulate endogenous signaling pathways. We conclude that endogenous rapid GABAB autoreceptor signaling inhibits neurotransmitter release primarily by Gβγ-mediated inhibitions of SNARE mechanisms, whereas prolonged agonist application additionally suppresses Ca2+ influx via cAMP signaling. ### Competing Interest Statement The authors have declared no competing interest.

BackgroundPresynaptic Gi/o coupled GPCRs can act as negative feedback regulators of neurotransmitter release via Gβγ effector modulation through two mechanisms: decreased calcium influx and direct inhibition of membrane fusion by soluble N-ethylmaleimide—sensitive factor attachment protein (SNAP) receptor (SNARE). Previously, we discovered that truncation of the last three C-terminal amino acids of SNAP25 (SNAP25Δ3) prevents Gβγ-SNARE interaction, effectively removing the braking mechanism on neurotransmitter release. We have demonstrated enhanced metabolic protection in male SNAP25Δ3/Δ3 mice housed at room temperature (22°C), including increased adipose tissue beiging and glucose uptake and enhanced insulin sensitivity, rendering them resistant to diet-induced obesity (DIO). When male SNAP25Δ3/Δ3 mice were housed at thermoneutrality (30°C), all metabolic protection was abolished, suggesting sympathetic tone is important for the phenotypes.MethodsWe housed male and female mice at either standard room temperature (21°C) or at thermoneutrality (30°C) and fed them a high fat diet (HFD) for 8 weeks. Glucose tolerance tests were performed before and after the 8 weeks of HFD along with body composition analyses. Organs were then dissected for mass analysis as well as immunohistochemistry. Additionally, we ovariectomized female mice to investigate the role of sex hormones in our phenotypes. Finally, we housed mice in Sable Promethion chambers at various environmental temperatures to investigate the effect of environmental temperature on basal metabolic rates.ResultsWe found SNAP25Δ3/Δ3 female mice exhibited the same metabolic protection at RT (22°C) and displayed enhanced metabolic protection from DIO compared to standard chow just as males did. However, female SNAP25Δ3/Δ3 mice display persistent metabolic protection even when housed at thermoneutrality. In this study, we investigate the mechanisms behind this sex dependent persistent phenotype. Thermoneutral set point did not differ between sexes nor genotype, suggesting that metabolic protection is not due to a difference in hypothalamic temperature regulation. Metabolic protection in SNAP25Δ3/Δ3 persisted in ovariectomized mice despite increased weight gain compared to mice receiving sham operations.ConclusionThis study has identified that there is not a sex-dependent difference for thermoneutral set point in mice. Additionally, there is a sex hormone independent mechanism driving the persistent metabolic protection of female SNAP25Δ3/Δ3 mice housed in thermoneutrality.

Human platelets express protease-activated receptor (PAR) 1 and 4 on the platelet membrane. PAR1 activation is crucial for hemostasis, while PAR4 activation produces a prolonged pro-inflammatory platelet response. Once activated, the PAR receptors cause platelet activation and aggregation. The PARs are activated by proteolytic cleavage of their N-terminus, revealing a new N-terminus called the tethered ligand. This tethered ligand folds back and activates the receptor. PAR1 and PAR4 differ in their tethered ligand sequence as well as their signaling cascades; however, they are both activated by various serine proteases, specifically thrombin.Much is known about PAR4 activation via thrombin activation; however, little is known about activation by other proteases. Thrombin cleavage of PAR4 and subsequent activation leads to G proteins, Gαq and Gα12/13, and β-arrestin signaling. The objective of this study is to characterize trypsin activation of PAR4 on human platelets.Platelets were isolated from the whole blood of healthy human volunteers by centrifugation. Platelets were utilized in flow cytometry assays, calcium mobilization assays, and western blot experiments to characterize trypsin activation of human platelets.Here, we demonstrate that trypsin activates human platelets solely through PAR4. Trypsin activation of PAR4 signaling through both Gαq and Gα12/13 is as effective as α-thrombin activation of PAR4.Thrombin may be the main protease to activate PAR4 under typical conditions; however, under pathological conditions such as acute pancreatitis (AP), trypsin activation of PAR4 may be a main driver of platelet activation and disease progression.

The Hamm laboratory recently published a cohort of PAR4 antagonists that were effective against the tethered ligand activation of PAR4. These compounds were generated from an ultralarge virtual screen using a homology model of PAR4. Upon further investigation, it appears the protease-activated receptor antagonists highlighted in this work have some thrombin liability. The Hamm laboratory further characterized the activity of these compounds using various methods, including a fluorescent thrombin activity assay, a chromogenic thrombin activity assay, and flow cytometry assays. We conclude that they do indeed antagonize PAR4, but thrombin is an additional target.

Retinal rod photoreceptors generate reproducible quantal responses, enabling them to "count" single photons. Interestingly, in mammalian rods, one photoisomerization in several hundred elicits an aberrant response that is larger than normal and persists for a variable period lasting up to tens of seconds. Although rare, aberrant responses influence signaling because many rods converge onto downstream neurons and because "normal" and aberrant single-photon responses temporally summate in steady light. Bicarbonate increases the normal photon response and the maximal response of rods, but its effect on the aberrant responses is not known. To find out, we used a fully space-resolved, biophysical model of visual transduction in the murine rod and corroborated the results with ex vivo electroretinogram (ERG) recordings. In our simulations, the increased circulating current with bicarbonate raised [Ca2+]in, which suppressed the shutoff of photoexcited rhodopsin during the normal single-photon response but not during the aberrant response. Consequently, the normal single-photon response was enlarged to a greater extent than the aberrant response. In ERG recordings, aberrant responses gave rise to a long-lived tail in the bright flash response, which was used to assess how they were affected by bicarbonate. Indeed, the increase in aberrant response amplitude with bicarbonate was less than that for the normal photon response, consistent with modeling. In simulations, the accumulation of aberrant responses produced a slow, secondary rise in the step response to subsaturating intensities, but in the ERG, the secondary rise seemed to disappear with bicarbonate. By boosting normal photon responses and the maximal response, bicarbonate elicited a droop in the step response due to light adaptation (not included in the model) that was more prominent and appeared at lower intensities. Because bicarbonate also reduced the relative contribution of the aberrant response component, the droop merged with and obscured the secondary rise.

Neurotransmitter release is a complex process involving tightly controlled co-factors and protein-protein interactions. G-protein coupled receptors negatively regulate exocytosis via the interaction of G-protein βγ (Gβγ) heterodimers with the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) complex. The neuronal ternary SNARE complex comprises synaptosomal-associated protein-25 (SNAP25), syntaxin-1A, and synaptobrevin-2. The regions of the SNARE complex that are important for interactions with Gβγ have been extensively characterized, but the critical sites on Gβγ are not well understood. Furthermore, the molecular basis for the specificity of different Gβ and Gγ isoforms for SNARE proteins remains elusive. Thus, we holistically probed the entire family of human Gβ and Gγ isoforms for regions critical for the target-SNARE (tSNARE) interaction using a peptide screening approach. Gβ and γ peptides with high affinities for tSNARE were then subjected to alanine scanning mutagenesis to identify the interaction sites. We found that the N-terminal coiled-coil domain of Gβγ as well as the β-propeller domain of Gβ are hotspots for SNARE interactions. Additionally, we found that the N-terminal Gγ2 peptide is a potent inhibitor of interactions between full-length Gβ1γ2 and SNAP25. We discovered that Gβ1γ2 preferentially interacts with ternary SNARE in the pre-fusion, partially zipped conformation, likely due to increased exposure of the C-terminus of SNAP25. Our combined results suggest that specific Gβγ heterodimers bind to ternary SNARE in the docked and primed state via critical residues of the β-propeller and N-terminal coil-coil domains. We propose that Gβγ binding disrupts zippering up of the SNARE complex and thereby vesicle fusion.

Platelet activation of protease-activated receptor 4 (PAR4) and thrombin are at the top of a chain of events leading to fibrin deposition, microinfarcts, blood-brain barrier disruption, and inflammation. We evaluated mRNA expression of the PAR4 gene F2RL3 in human brain and global cognitive performance in participants with and without cognitive impairment or dementia. Data were acquired from the Religious Orders Study (ROS) and the Rush Memory and Aging Project (MAP).F2RL3 mRNA was elevated in AD cases and was associated with worse retrospective longitudinal cognitive performance. Moreover, F2RL3 expression interacted with clinical AD diagnosis on longitudinal cognition whereas this relationship was attenuated in individuals without cognitive impairment. Additionally, when adjusting for the effects of AD neuropathology, F2RL3 expression remained a significant predictor of cognitive decline. F2RL3 expression correlated positively with transcript levels of proinflammatory markers including TNFα, IL-1β, NFκB, and fibrinogen α/β/γ. Together, these results reveal that F2RL3 mRNA expression is associated with multiple AD-relevant outcomes and its encoded product, PAR4, may play a role in disease pathogenesis.

Here, we demonstrate a structure-based small molecule virtual screening and lead optimization pipeline using a homology model of a difficult-to-drug G-protein-coupled receptor (GPCR) target. Protease-activated receptor 4 (PAR4) is activated by thrombin cleavage, revealing a tethered ligand that activates the receptor, making PAR4 a challenging target. A virtual screen of a make-on-demand chemical library yielded a one-hit compound. From the single-hit compound, we developed a novel series of PAR4 antagonists. Subsequent lead optimization via simultaneous virtual library searches and structure-based rational design efforts led to potent antagonists of thrombin-induced activation. Interestingly, this series of antagonists was active against PAR4 activation by the native protease thrombin cleavage but not the synthetic PAR4 agonist peptide AYPGKF.

The nucleus accumbens (NAc) guides reward-related motivated behavior implicated in pathological behavioral states, including addiction and depression. These behaviors depend on the precise neuromodulatory actions of Gi/o-coupled G-protein-coupled receptors (GPCRs) at glutamatergic synapses onto medium spiny projection neurons (MSNs). Previous work has shown that discrete classes of Gi/o-coupled GPCR mobilize Gβγ to inhibit vesicular neurotransmitter release via t-SNARE protein, SNAP25. However, it remains unknown which Gαi/o systems in the NAc utilize Gβγ-SNARE signaling to dampen glutamatergic transmission. Utilizing patch-clamp electrophysiology and pharmacology in a transgenic mouse line with a C-terminal three-residue deletion of SNAP25 (SNAP25Δ3) weaking the Gβγ-SNARE interaction, we surveyed a broad cohort of Gi/o-coupled GPCRs with robust inhibitory actions at glutamatergic synapses in the NAc. We find that basal presynaptic glutamate release probability is reduced in SNAP25Δ3 mice. While κ opioid, CB1, adenosine A1, group II metabotropic glutamate receptors, and histamine H3 receptors inhibit glutamatergic transmission onto MSNs independent of SNAP25, we report that SNAP25 contributes significantly to the actions of GABAB, 5-HT1B/D, and μ opioid receptors. These findings demonstrate that presynaptic Gi/o-coupled GPCRs recruit heterogenous effector mechanisms at glutamatergic synapses in the NAc, with a subset requiring SNA25-dependent Gβγ signaling.

The effect of U46619 and CPI211 on migration and invasion of 4T1 and MDA-MB-231 tumor cells through Matrigel-coated transwells was assessed. Tumor cells were seeded in the upper chambers of a Matrigel-coated transwell insert in serum-free media. Cell migration towards 1% serum in the lower chamber was assessed by crystal violet staining of the lower side of the transwell filter. Digital images of stained transwell filters were used to count the number of migrated cells. Representative images are shown (A). Each data point shown (B) represents the number of cells migrating to the lower side of the filter. Midlines are the average {plus minus} S.D., N = 4 (4T1) and 5 (MDA-MB-231), each assessed in triplicate.

Chemical structure of CPI211 is shown in Panel A. Western analysis of 293T cells expressing lentiviral empty vector, TPr-WT, or TPr-T399A s shown in Panel B. Antibodies used are shown to the left of each panel.

Negative regulation of exocytosis from secretory cells is accomplished through inhibitory signals from Gi/o GPCRs by Gβγ subunit inhibition of 2 mechanisms: decreased calcium entry and direct interaction of Gβγ with soluble N-ethylmaleimide–sensitive factor attachment protein (SNAP) receptor (SNARE) plasma membrane fusion machinery. Previously, we disabled the second mechanism with a SNAP25 truncation (SNAP25Δ3) that decreased Gβγ affinity for the SNARE complex, leaving exocytotic fusion and modulation of calcium entry intact and removing GPCR-Gβγ inhibition of SNARE-mediated exocytosis. Here, we report substantial metabolic benefit in mice carrying this mutation. Snap25Δ3/Δ3 mice exhibited enhanced insulin sensitivity and beiging of white fat. Metabolic protection was amplified in Snap25Δ3/Δ3 mice challenged with a high-fat diet. Glucose homeostasis, whole-body insulin action, and insulin-mediated glucose uptake into white adipose tissue were improved along with resistance to diet-induced obesity. Metabolic protection in Snap25Δ3/Δ3 mice occurred without compromising the physiological response to fasting or cold. All metabolic phenotypes were reversed at thermoneutrality, suggesting that basal autonomic activity was required. Direct electrode stimulation of sympathetic neuron exocytosis from Snap25Δ3/Δ3 inguinal adipose depots resulted in enhanced and prolonged norepinephrine release. Thus, the Gβγ-SNARE interaction represents a cellular mechanism that deserves further exploration as an additional avenue for combating metabolic disease.

The contribution of the thrombin receptor protease-activated receptor 4 (PAR4) to acute kidney injury (AKI) and chronic kidney disease (CKD) is not well understood. Here we report that PAR4 expression is upregulated after kidney injury and PAR4 knockout (KO) mice are protected against fibrosis following kidney injury in two mouse models. First, PAR4 KO mice are protected against unilateral ureter obstruction. Second, PAR4 KO mice are protected against an AKI-CKD model of ischemia-reperfusion followed by contralateral nephrectomy.

Flow chart depicting the decision making steps towards selecting TBXA2R as a potential therapeutic target for drug repurposing of CPI211.

Retinal rods evolved to be able to detect single photons. Despite their exquisite sensitivity, rods operate over many log units of light intensity. Several processes inside photoreceptor cells make this incredible light adaptation possible. Here, we added to our previously developed, fully space resolved biophysical model of rod phototransduction, some of the mechanisms that play significant roles in shaping the rod response under high illumination levels: the function of RGS9 in shutting off G protein transducin, and calcium dependences of the phosphorylation rates of activated rhodopsin, of the binding of cGMP to the light-regulated ion channel, and of two membrane guanylate cyclase activities. A well stirred version of this model captured the responses to bright, saturating flashes in WT and mutant mouse rods and was used to explain “Pepperberg plots,” that graph the time during which the response is saturated against the natural logarithm of flash strength for bright flashes. At the lower end of the range, saturation time increases linearly with the natural logarithm of flash strength. The slope of the relation (τD) is dictated by the time constant of the rate-limiting (slowest) step in the shutoff of the phototransduction cascade, which is the hydrolysis of GTP by transducin. We characterized mathematically the X-intercept (Φo) which is the number of photoisomerizations that just saturates the rod response. It has been observed that for flash strengths exceeding a few thousand photoisomerizations, the curves depart from linearity. Modeling showed that the “upward bend” for very bright flash intensities could be explained by the dynamics of RGS9 complex and further predicted that there would be a plateau at flash strengths giving rise to more than ~107 photoisomerizations due to activation of all available PDE. The model accurately described alterations in saturation behavior of mutant murine rods resulting from transgenic perturbations of the cascade targeting membrane guanylate cyclase activity, and expression levels of GRK, RGS9, and PDE. Experimental results from rods expressing a mutant light-regulated channel purported to lack calmodulin regulation deviated from model predictions, suggesting that there were other factors at play.

Supplemental Table S1. Kaplan-Meier analysis of clinical cancer expression datasets (RNA-Seq) to assess the relationship between disease free patient survival and TBXA2R expression levels, as determined using Kmplot.com software, using the software-generated auto-select best cutoff for TBXA2R expression. N = the number of tumors with correlating clinical patient data. HR = hazard ratio. P value calculated using Log-Rank test.

While the ability of G protein βγ subunits (Gβγ) to bind to and functionally inhibit the neuronal SNARE proteins Stx1A, SNAP25, and synaptobrevin in the presence of the calcium sensor synaptotagmin I is well documented, these three SNARE proteins, which form the core SNARE complex for synchronous evoked release in neurons, are but a subset of the larger family of SNARE proteins, which participate in many other exocytic processes within the cell and in other populations of secretory cells throughout the body, from which the release of neurotransmitters, hormones, and other factors is regulated by G i/o -coupled GPCRs. The ability of Gβγ to regulate these processes is unknown. To investigate the feasibility of this mechanism to inhibit SNARE function more broadly, we utilized a series of biochemical assays of binding and function with four Qa-SNAREs (Stx1A, Stx2, Stx3, and Stx4) and four Qb,c-SNAREs (SNAP25, SNAP23, SNAP29, and SNAP47) in tandem with the R-SNARE synaptobrevin, synaptotagmin I, and Gβγ. Gβγ was found to bind to multiple Qa-SNARE isoforms as well as SNAP23, and inhibit the lipid mixing function of these SNAREs, as well as SNAP29. Together, this data suggests a more broad role for the Gβγ-SNARE pathway in the regulation of exocytosis beyond cells that express Stx1A or SNAP25.