Subunit composition, molecular environment, and activation of native TRPC channels encoded by their interactomes.

In the mammalian brain TRPC channels, a family of Ca2+-permeable cation channels, are involved in a variety of processes from neuronal growth and synapse formation to transmitter release, synaptic transmission and plasticity. The molecular appearance and operation of native TRPC channels, however, remained poorly understood. Here, we used high-resolution proteomics to show that TRPC channels in the rodent brain are macro-molecular complexes of more than 1 MDa in size that result from the co-assembly of the tetrameric channel core with an ensemble of interacting proteins (interactome). The core(s) of TRPC1-, C4-, and C5-containing channels are mostly heteromers with defined stoichiometries for each subtype, whereas TRPC3, C6, and C7 preferentially form homomers. In addition, TRPC1/C4/C5 channels may co-assemble with the metabotropic glutamate receptor mGluR1, thus guaranteeing both specificity and reliability of channel activation via the phospholipase-Ca2+ pathway. Our results unveil the subunit composition of native TRPC channels and resolve the molecular details underlying their activation.

Alix is required for activity-dependent bulk endocytosis at brain synapses.

In chemical synapses undergoing high frequency stimulation, vesicle components can be retrieved from the plasma membrane via a clathrin-independent process called activity-dependent bulk endocytosis (ADBE). Alix (ALG-2-interacting protein X/PDCD6IP) is an adaptor protein binding to ESCRT and endophilin-A proteins which is required for clathrin-independent endocytosis in fibroblasts. Alix is expressed in neurons and concentrates at synapses during epileptic seizures. Here, we used cultured neurons to show that Alix is recruited to presynapses where it interacts with and concentrates endophilin-A during conditions triggering ADBE. Using Alix knockout (ko) neurons, we showed that this recruitment, which requires interaction with the calcium-binding protein ALG-2, is necessary for ADBE. We also found that presynaptic compartments of Alix ko hippocampi display subtle morphological defects compatible with flawed synaptic activity and plasticity detected electrophysiologically. Furthermore, mice lacking Alix in the forebrain undergo less seizures during kainate-induced status epilepticus and reduced propagation of the epileptiform activity. These results thus show that impairment of ADBE due to the lack of neuronal Alix leads to abnormal synaptic recovery during physiological or pathological repeated stimulations.

A longer isoform of Stim1 is a negative SOCE regulator but increases cAMP-modulated NFAT signaling.

Alternative splicing is a potent modifier of protein function. Stromal interaction molecule 1 (Stim1) is the essential activator of store-operated Ca2+ entry (SOCE) triggering activation of transcription factors. Here, we characterize Stim1A, a splice variant with an additional 31 amino acid domain inserted in frame within its cytosolic domain. Prominent expression of exon A is found in astrocytes, heart, kidney, and testes. Full-length Stim1A functions as a dominant-negative regulator of SOCE and ICRAC, facilitating sequence-specific fast calcium-dependent inactivation and destabilizing gating of Orai channels. Downregulation or absence of native Stim1A results in increased SOCE. Despite reducing SOCE, Stim1A leads to increased NFAT translocation. Differential proteomics revealed an interference of Stim1A with the cAMP-SOCE crosstalk by altered modulation of phosphodiesterase 8 (PDE8), resulting in reduced cAMP degradation and increased PIP5K activity, facilitating NFAT activation. Our study uncovers a hitherto unknown mechanism regulating NFAT activation and indicates that cell-type-specific splicing of Stim1 is a potent means to regulate the NFAT signalosome and cAMP-SOCE crosstalk.

A synaptic temperature sensor for body cooling.

Deep brain temperature detection by hypothalamic warm-sensitive neurons (WSNs) has been proposed to provide feedback information relevant for thermoregulation. WSNs increase their action potential firing rates upon warming, a property that has been presumed to rely on the composition of thermosensitive ion channels within WSNs. Here, we describe a synaptic mechanism that regulates temperature sensitivity of preoptic WSNs and body temperature. Experimentally induced warming of the mouse hypothalamic preoptic area in vivo triggers body cooling. TRPM2 ion channels facilitate this homeostatic response and, at the cellular level, enhance temperature responses of WSNs, thereby linking WSN function with thermoregulation for the first time. Rather than acting within WSNs, we-unexpectedly-find TRPM2 to temperature-dependently increase synaptic drive onto WSNs by disinhibition. Our data emphasize a network-based interoceptive paradigm that likely plays a key role in encoding body temperature and that may facilitate integration of diverse inputs into thermoregulatory pathways.

Remodelling of Ca2+ homeostasis is linked to enlarged endoplasmic reticulum in secretory cells.

The endoplasmic reticulum (ER) is extensively remodelled during the development of professional secretory cells to cope with high protein production. Since ER is the principal Ca2+ store in the cell, we characterised the Ca2+ homeostasis in NALM-6 and RPMI 8226 cells, which are commonly used as human pre-B and antibody secreting plasma cell models, respectively. Expression levels of Sec61 translocons and the corresponding Sec61-mediated Ca2+ leak from ER, Ca2+ storage capacity and store-operated Ca2+ entry were significantly enlarged in the secretory RPMI 8226 cell line. Using an immunoglobulin M heavy chain producing HeLa cell model, we found that the enlarged Ca2+ storage capacity and Ca2+ leak from ER are linked to ER expansion. Our data delineates a developmental remodelling of Ca2+ homeostasis in professional secretory cells in which a high Sec61-mediated Ca2+ leak and, thus, a high Ca2+ turnover in the ER is backed up by enhanced store-operated Ca2+ entry.

A short isoform of STIM1 confers frequency-dependent synaptic enhancement.

Store-operated Ca2+-entry (SOCE) regulates basal and receptor-triggered Ca2+ signaling with STIM proteins sensing the endoplasmic reticulum (ER) Ca2+ content and triggering Ca2+ entry by gating Orai channels. Although crucial for immune cells, STIM1's role in neuronal Ca2+ homeostasis is controversial. Here, we characterize a splice variant, STIM1B, which shows exclusive neuronal expression and protein content surpassing conventional STIM1 in cerebellum and of significant abundance in other brain regions. STIM1B expression results in a truncated protein with slower kinetics of ER-plasma membrane (PM) cluster formation and ICRAC, as well as reduced inactivation. In primary wild-type neurons, STIM1B is targeted by its spliced-in domain B to presynaptic sites where it converts classic synaptic depression into Ca2+- and Orai-dependent short-term synaptic enhancement (STE) at high-frequency stimulation (HFS). In conjunction with altered STIM1 splicing in human Alzheimer disease, our findings highlight STIM1 splicing as an important regulator of neuronal calcium homeostasis and of synaptic plasticity.

Synergistic actions of v-SNARE transmembrane domains and membrane-curvature modifying lipids in neurotransmitter release.

Vesicle fusion is mediated by assembly of SNARE proteins between opposing membranes. While previous work suggested an active role of SNARE transmembrane domains (TMDs) in promoting membrane merger (Dhara et al., 2016), the underlying mechanism remained elusive. Here, we show that naturally-occurring v-SNARE TMD variants differentially regulate fusion pore dynamics in mouse chromaffin cells, indicating TMD flexibility as a mechanistic determinant that facilitates transmitter release from differentially-sized vesicles. Membrane curvature-promoting phospholipids like lysophosphatidylcholine or oleic acid profoundly alter pore expansion and fully rescue the decelerated fusion kinetics of TMD-rigidifying VAMP2 mutants. Thus, v-SNARE TMDs and phospholipids cooperate in supporting membrane curvature at the fusion pore neck. Oppositely, slowing of pore kinetics by the SNARE-regulator complexin-2 withstands the curvature-driven speeding of fusion, indicating that pore evolution is tightly coupled to progressive SNARE complex formation. Collectively, TMD-mediated support of membrane curvature and SNARE force-generated membrane bending promote fusion pore formation and expansion.

Outcomes after thumb carpometacarpal joint stabilization with an abductor pollicis longus tendon strip for the treatment of chronic instability.

INTRODUCTION: Instabilities of the thumb carpometacarpal (CMC) joint, caused by idiopathic ligamentous hyperlaxity, trauma or other conditions may lead to pain, functional impairment and eventually osteoarthritis. Several techniques have been described to enhance stability of the CMC 1. The aim of this study was to evaluate postoperative outcomes after CMC 1 joint stabilization using a soft-tissue procedure in patients with chronic instability. MATERIALS AND METHODS: This study was designed as a retrospective study with a single follow-up visit after a minimum of 1 year postoperatively. All patients who underwent stabilization of the CMC 1 with an abductor pollicis longus (APL) tendon strip for chronic, habitual instability were re-assessed using clinical examination, dedicated outcome scores [Visual Analogue Scale (VAS); The Disability of the Arm, Shoulder and Hand (DASH) score; Nelson score; Kapandji opposition score], grip and pinch strength measurements, and radiographic examination. RESULTS: 12 patients (15 operated thumbs) with a mean age at surgery of 23.2 (± 9.3) years were included after a mean follow-up period of 3.5 (± 1.3) years. The postoperative outcomes indicated excellent results, with a mean DASH score of 13.3 (± 11.3), VAS 1.1 at rest (and 2.8 during stress) and Nelson score of 87.7 (± 11.3). Postoperative grip, pinch strength and passive stability were not significantly different between operated and non-operated sides (p = 0.852; p = 0.923 and p = 0.428, respectively). We observed one case of recurrent instability besides no other complications. However, patients with trapezium hypoplasia (5 of 12) were more prone to signs of radiographic instability during stress testing. CONCLUSIONS: Thumb carpometacarpal stabilization with an APL tendon strip yielded excellent clinical outcomes and low morbidity in the mid-term. However, long-term follow-up is needed to assess specifically whether patients with trapezium hypoplasia may be more prone to clinical symptom recurrence than those with normal anatomy. LEVEL OF EVIDENCE: Level IV.

TRPC channels regulate Ca2+-signaling and short-term plasticity of fast glutamatergic synapses.

Transient receptor potential (TRP) proteins form Ca2+-permeable, nonselective cation channels, but their role in neuronal Ca2+ homeostasis is elusive. In the present paper, we show that TRPC channels potently regulate synaptic plasticity by changing the presynaptic Ca2+-homeostasis of hippocampal neurons. Specifically, loss of TRPC1/C4/C5 channels decreases basal-evoked secretion, reduces the pool size of readily releasable vesicles, and accelerates synaptic depression during high-frequency stimulation (HFS). In contrast, primary TRPC5 channel-expressing neurons, identified by a novel TRPC5-τ-green fluorescent protein (τGFP) knockin mouse line, show strong short-term enhancement (STE) of synaptic signaling during HFS, indicating a key role of TRPC5 in short-term plasticity. Lentiviral expression of either TRPC1 or TRPC5 turns classic synaptic depression of wild-type neurons into STE, demonstrating that TRPCs are instrumental in regulating synaptic plasticity. Presynaptic Ca2+ imaging shows that TRPC activity strongly boosts synaptic Ca2+ dynamics, showing that TRPC channels provide an additional presynaptic Ca2+ entry pathway, which efficiently regulates synaptic strength and plasticity.

Large scale, unbiased analysis of elementary calcium signaling events in cardiac myocytes.

The identification of spatiotemporally restricted Ca2+ signals, Ca2+ sparks, was instrumental for our understanding of cardiac Ca2+ homeostasis. High-speed 2D confocal imaging enables acquisition of such Ca2+ sparks with high-content information but their full appreciation is constrained by the lack of unbiased and easy-to-use analysis tools. We developed a software toolset for unbiased and automatic Ca2+ spark analysis for huge data sets of subcellular Ca2+ signals. iSpark was developed to be scanner and detector independent. In myocytes from hearts subjected to various degrees of hypertrophy we acquired >5.000.000 Ca2+ sparks from 14 mice. The iSpark-enabled analysis of this large Ca2+ spark data set showed that the highly organized distribution of Ca2+ sparks present in healthy cells disarrayed concomitant with the development of aberrant transverse tubules and disease severity. Thus, iSpark represents a versatile and universal tool for analyzing local Ca2+ signaling in healthy as well as diseased, aberrant local Ca2+ signal transduction. The results from the unbiased analysis of large data sets provide a deeper insight into possible mechanisms contributing to the onset and progression of cardiac diseases such as hypertrophy.

A mechanism for exocytotic arrest by the Complexin C-terminus.

ComplexinII (CpxII) inhibits non-synchronized vesicle fusion, but the underlying mechanisms have remained unclear. Here, we provide evidence that the far C-terminal domain (CTD) of CpxII interferes with SNARE assembly, thereby arresting tonic exocytosis. Acute infusion of a CTD-derived peptide into mouse chromaffin cells enhances synchronous release by diminishing premature vesicle fusion like full-length CpxII, indicating a direct, inhibitory function of the CTD that sets the magnitude of the primed vesicle pool. We describe a high degree of structural similarity between the CpxII CTD and the SNAP25-SN1 domain (C-terminal half) and show that the CTD peptide lowers the rate of SDS-resistant SNARE complex formation in vitro. Moreover, corresponding CpxII:SNAP25 chimeras do restore complexin's function and even 'superclamp' tonic secretion. Collectively, these results support a so far unrecognized clamping mechanism wherein the CpxII C-terminus hinders spontaneous SNARE complex assembly, enabling the build-up of a release-ready pool of vesicles for synchronized Ca2+-triggered exocytosis.

Circulating bile acids in healthy adults respond differently to a dietary pattern characterized by whole grains, legumes and fruits and vegetables compared to a diet high in refined grains and added sugars: A randomized, controlled, crossover feeding study.

OBJECTIVE: The effects of diets high in refined grains on biliary and colonic bile acids have been investigated extensively. However, the effects of diets high in whole versus refined grains on circulating bile acids, which can influence glucose homeostasis and inflammation through activation of farnesoid X receptor (FXR) and G protein-coupled bile acid receptor 1 (TGR5), have not been studied. MATERIALS AND METHODS: We conducted a secondary analysis from a randomized controlled crossover feeding trial (NCT00622661) in 80 healthy adults (40 women/40 men, age 18-45 years) from the greater Seattle Area, half of which were normal weight (BMI 18.5-25.0 kg/m2) and half overweight to obese (BMI 28.0-39.9 kg/m2). Participants consumed two four-week controlled diets in randomized order: 1) a whole grain diet (WG diet), designed to be low in glycemic load (GL), high in whole grains, legumes, and fruits and vegetables, and 2) a refined grain diet (RG diet), designed to be high GL, high in refined grains and added sugars, separated by a four-week washout period. Quantitative targeted analysis of 55 bile acid species in fasting plasma was performed using liquid chromatography tandem mass spectrometry. Concentrations of glucose, insulin, and CRP were measured in fasting serum. Linear mixed models were used to test the effects of diet on bile acid concentrations, and determine the association between plasma bile acid concentrations and HOMA-IR and CRP. Benjamini-Hochberg false discovery rate (FDR) < 0.05 was used to control for multiple testing. RESULTS: A total of 29 plasma bile acids were reliably detected and retained for analysis. Taurolithocholic acid (TLCA), taurocholic acid (TCA) and glycocholic acid (GCA) were statistically significantly higher after the WG compared to the RG diet (FDR < 0.05). There were no significant differences by BMI or sex. When evaluating the association of bile acids and HOMA-IR, GCA, taurochenodeoxycholic acid, ursodeoxycholic acid (UDCA), 5ß­cholanic acid­3ß,12α­diol, 5­cholanic acid­3ß­ol, and glycodeoxycholic acid (GDCA) were statistically significantly positively associated with HOMA-IR individually, and as a group, total, 12α­hydroxylated, primary and secondary bile acids were also significant (FDR < 0.05). When stratifying by BMI, chenodeoxycholic acid (CDCA), cholic acid (CA), UDCA, 5ß-cholanic acid-3ß, deoxycholic acid, and total, 12α-hydroxylated, primary and secondary bile acid groups were significantly positively associated with HOMA-IR among overweight to obese individuals (FDR < 0.05). When stratifying by sex, GCA, CDCA, TCA, CA, UDCA, GDCA, glycolithocholic acid (GLCA), total, primary, 12α­hydroxylated, and glycine-conjugated bile acids were significantly associated with HOMA-IR among women, and CDCA, GDCA, and GLCA were significantly associated among men (FDR < 0.05). There were no significant associations between bile acids and CRP. CONCLUSIONS: Diets with comparable macronutrient and energy composition, but differing in carbohydrate source, affected fasting plasma bile acids differently. Specifically, a diet characterized by whole grains, legumes, and fruits and vegetables compared to a diet high in refined grains and added sugars led to modest increases in concentrations of TLCA, TCA and GCA, ligands for FXR and TGR5, which may have beneficial effects on glucose homeostasis.

Investigation of a Lifestyle Intervention in Women at High Risk of Breast Cancer.

One fourth of breast cancer can be attributed to sedentary lifestyles and being overweight or obese. This pilot study was conducted to explore whether a 6-month lifestyle intervention affected body composition and obesity-related biomarkers among women at high risk of breast cancer. Overweight/obese women at high risk of breast cancer were randomized to the control group or to the intervention. The intervention was an individually tailored, cognitive-behavioral therapy program that assists women in identifying strategies to improve their nutrition and physical activity habits with the goal of reduced adiposity. We compared changes in body composition and plasma biomarkers from baseline to 6 months. Body weight, adiposity, leptin, insulin resistance, and C-reactive protein were significantly reduced in the intervention group versus controls. No significant differences were observed in adiponectin, insulin, glucose, or interleukin-6. Our findings suggest that this intervention improves the metabolic and inflammatory profiles of overweight/obese women at risk of breast cancer.

Astrocytes control synaptic strength by two distinct v-SNARE-dependent release pathways.

Communication between glia cells and neurons is crucial for brain functions, but the molecular mechanisms and functional consequences of gliotransmission remain enigmatic. Here we report that astrocytes express synaptobrevin II and cellubrevin as functionally non-overlapping vesicular SNARE proteins on glutamatergic vesicles and neuropeptide Y-containing large dense-core vesicles, respectively. Using individual null-mutants for Vamp2 (synaptobrevin II) and Vamp3 (cellubrevin), as well as the corresponding compound null-mutant for genes encoding both v-SNARE proteins, we delineate previously unrecognized individual v-SNARE dependencies of astrocytic release processes and their functional impact on neuronal signaling. Specifically, we show that astroglial cellubrevin-dependent neuropeptide Y secretion diminishes synaptic signaling, while synaptobrevin II-dependent glutamate release from astrocytes enhances synaptic signaling. Our experiments thereby uncover the molecular mechanisms of two distinct v-SNARE-dependent astrocytic release pathways that oppositely control synaptic strength at presynaptic sites, elucidating new avenues of communication between astrocytes and neurons.

Heteromeric channels formed by TRPC1, TRPC4 and TRPC5 define hippocampal synaptic transmission and working memory.

Canonical transient receptor potential (TRPC) channels influence various neuronal functions. Using quantitative high-resolution mass spectrometry, we demonstrate that TRPC1, TRPC4, and TRPC5 assemble into heteromultimers with each other, but not with other TRP family members in the mouse brain and hippocampus. In hippocampal neurons from Trpc1/Trpc4/Trpc5-triple-knockout (Trpc1/4/5-/-) mice, lacking any TRPC1-, TRPC4-, or TRPC5-containing channels, action potential-triggered excitatory postsynaptic currents (EPSCs) were significantly reduced, whereas frequency, amplitude, and kinetics of quantal miniature EPSC signaling remained unchanged. Likewise, evoked postsynaptic responses in hippocampal slice recordings and transient potentiation after tetanic stimulation were decreased. In vivo, Trpc1/4/5-/- mice displayed impaired cross-frequency coupling in hippocampal networks and deficits in spatial working memory, while spatial reference memory was unaltered. Trpc1/4/5-/- animals also exhibited deficiencies in adapting to a new challenge in a relearning task. Our results indicate the contribution of heteromultimeric channels from TRPC1, TRPC4, and TRPC5 subunits to the regulation of mechanisms underlying spatial working memory and flexible relearning by facilitating proper synaptic transmission in hippocampal neurons.

Plasma metabolite abundances are associated with urinary enterolactone excretion in healthy participants on controlled diets.

Enterolignans, products of gut bacterial metabolism of plant lignans, have been associated with reduced risk of chronic diseases, but their association with other plasma metabolites is unknown. We examined plasma metabolite profiles according to urinary enterolignan excretion in a cross-sectional analysis using data from a randomized crossover, controlled feeding study. Eighty healthy adult males and females completed two 28-day feeding periods differing by glycemic load, refined carbohydrate, and fiber content. Lignan intake was calculated from food records using a polyphenol database. Targeted metabolomics was performed by LC-MS on plasma from fasting blood samples collected at the end of each feeding period. Enterolactone (ENL) and enterodiol, were measured in 24 h urine samples collected on the penultimate day of each study period using GC-MS. Linear mixed models were used to test the association between enterolignan excretion and metabolite abundances. Pathway analyses were conducted using the Global Test. Benjamini-Hochberg false discovery rate (FDR) was used to control for multiple testing. Of the metabolites assayed, 121 were detected in all samples. ENL excretion was associated positively with plasma hippuric acid and melatonin, and inversely with epinephrine, creatine, glycochenodeoxycholate, and glyceraldehyde (P < 0.05). Hippuric acid only satisfied the FDR of q < 0.1. END excretion was associated with myristic acid and glycine (q < 0.5). Two of 57 pathways tested were associated significantly with ENL, ubiquinone and terpenoid-quinone biosynthesis, and inositol phosphate metabolism. These results suggest a potential role for ENL or ENL-metabolizing gut bacteria in regulating plasma metabolites.

Large dense-core vesicle exocytosis from mouse dorsal root ganglion neurons is regulated by neuropeptide Y.

Peptidergic dorsal root ganglion (DRG) neurons transmit sensory and nociceptive information from the periphery to the central nervous system. Their synaptic activity is profoundly affected by neuromodulatory peptides stored and released from large dense-core vesicles (LDCVs). However, the mechanism of peptide secretion from DRG neurons is poorly understood. Using total internal reflection fluorescence microscopy (TIRFM), we visualized individual LDCVs loaded with fluorescent neuropeptide Y (NPY) and analyzed their stimulation-dependent release. We tested several protocols and found an overall low stimulation-secretion coupling that increased after raising intracellular Ca2+ concentration by applying a weak pre-stimulus. Interestingly, the stimulation protocol also influenced the mechanism of LDCV fusion. Depolarization of DRG neurons with a solution containing 60mM KCl triggered full fusion, kiss-and-run, and kiss-and-stay exocytosis with equal frequency. In contrast, field electrode stimulation primarily induced full fusion exocytosis. Finally, our results indicate that NPY can promote LDCV secretion. These results shed new light on the mechanism of NPY action during modulation of DRG neuron activity, an important pathway in the treatment of chronic pain.

Dietary biomarker evaluation in a controlled feeding study in women from the Women's Health Initiative cohort.

BACKGROUND: Controlled human feeding studies are necessary for robust nutritional biomarker development and validation. Previous feeding studies have typically evaluated single nutrients and tested relatively few diets. OBJECTIVES: The objectives were 1) to simultaneously associate dietary intake with a range of potential nutritional biomarkers in postmenopausal women by using a controlled feeding study whereby each participant was provided a diet similar to her usual diet and 2) to evaluate serum concentrations of select nutrients as potential biomarkers with the use of established urinary recovery biomarkers of energy and protein as benchmarks for evaluation. DESIGN: Postmenopausal women from the Women's Health Initiative (n = 153) were provided with a 2-wk controlled diet in which each individual's menu approximated her habitual food intake as estimated from her 4-d food record and adjusted for estimated energy requirements. Serum biomarkers, including carotenoids, tocopherols, folate, vitamin B-12, and phospholipid fatty acids, were collected at the beginning and end of the feeding period. Doubly labeled water and urinary nitrogen biomarkers were used to derive estimates of energy and protein consumption, respectively. RESULTS: Linear regression of (ln-transformed) consumed nutrients on (ln-transformed) potential biomarkers and participant characteristics led to the following regression (R2) values for serum concentration biomarkers: folate, 0.49; vitamin B-12, 0.51; α-carotene, 0.53; ß-carotene, 0.39; lutein + zeaxanthin, 0.46; lycopene, 0.32; and α-tocopherol, 0.47. R2 values for percentage of energy from polyunsaturated fatty acids and urinary recovery biomarkers of energy and protein intakes were 0.27, 0.53, and 0.43, respectively. Phospholipid saturated fatty acids and monounsaturated fatty acids and serum γ-tocopherol were weakly associated with intake (R2 < 0.25). CONCLUSIONS: Serum concentration biomarkers of several vitamins and carotenoids performed similarly to established energy and protein urinary recovery biomarkers in representing nutrient intake variation in a feeding study, and thus are likely suitable for application in this population of postmenopausal women. Further work is needed to identify objective measures of categories of fatty acid intake. This trial was registered at clinicaltrials.gov as NCT00000611.

v-SNARE transmembrane domains function as catalysts for vesicle fusion.

Vesicle fusion is mediated by an assembly of SNARE proteins between opposing membranes, but it is unknown whether transmembrane domains (TMDs) of SNARE proteins serve mechanistic functions that go beyond passive anchoring of the force-generating SNAREpin to the fusing membranes. Here, we show that conformational flexibility of synaptobrevin-2 TMD is essential for efficient Ca(2+)-triggered exocytosis and actively promotes membrane fusion as well as fusion pore expansion. Specifically, the introduction of helix-stabilizing leucine residues within the TMD region spanning the vesicle's outer leaflet strongly impairs exocytosis and decelerates fusion pore dilation. In contrast, increasing the number of helix-destabilizing, ß-branched valine or isoleucine residues within the TMD restores normal secretion but accelerates fusion pore expansion beyond the rate found for the wildtype protein. These observations provide evidence that the synaptobrevin-2 TMD catalyzes the fusion process by its structural flexibility, actively setting the pace of fusion pore expansion.

A low-fat high-carbohydrate diet reduces plasma total adiponectin concentrations compared to a moderate-fat diet with no impact on biomarkers of systemic inflammation in a randomized controlled feeding study.

PURPOSE: We compared the effects of a eucaloric moderate-fat diet (18% protein, 36% fat, and 46% carbohydrate), a eucaloric low-fat high-carbohydrate diet (18% protein, 18% fat, and 64% carbohydrate), and a low-calorie (33% reduced) low-fat high-carbohydrate diet on biomarkers of systemic inflammation. METHODS: We randomly assigned 102 participants (age 21-76 years and BMI 19.2-35.5 kg/m(2)) to the three different diets for 6 weeks in a parallel design intervention trial. All foods were provided. Ninety-three participants completed all study procedures; 92 were included in the analyses. Endpoints included plasma C-reactive protein (CRP), interleukin-6 (IL-6), soluble tumor necrosis factor receptors I and II (sTNFRI and II), and adiponectin. RESULTS: In the unadjusted primary analyses, none of the endpoints were differentially affected by the dietary interventions despite the significantly greater reductions in body weight and fat mass in participants consuming the low-calorie low-fat diet compared to the eucaloric diets (p < 0.001). When including weight change in the model in secondary analysis, adiponectin tended to be increased with weight loss (time × weight change interaction, p = 0.051). Adjusted for weight change, adiponectin was reduced in the groups consuming the low-fat diets relative to the moderate-fat diet (p = 0.008). No effect of the intervention diets or weight loss on CRP, IL-6, or sTNFRI and II was seen in these secondary analyses. CONCLUSIONS: In relatively healthy adults, moderate weight loss had minimal effects on systemic inflammation, and raised plasma adiponectin only modestly. A lower dietary fat and higher carbohydrate content had little impact on measures of systemic inflammation, but reduced adiponectin concentrations compared to a moderate-fat diet. The latter may be of concern given the consistent and strong inverse association of plasma adiponectin with many chronic diseases.