Structural Studies of the Apo and Ca^(2+)-Bound States of the Human BK (SLO1) Channel Gating Ring in Solution

The gating ring (GR) regulates the activity of large-conductance voltage- and Ca^(2+)-activated K^+ channels (BK) by interacting with intracellular signaling molecules. To understand the operation of this biological sensor under physiological conditions, we performed Small-Angle X-ray Scattering (SAXS) analysis, at beamline 4-2 at the Stanford Synchrotron Radiation laboratory. SAXS measurements of the purified GR were performed in the absence or in the presence of 35 μM free Ca^(2+), found to be a saturating concentration in previous work. The quality of the circularly-averaged scattering data was evaluated with Guinier analysis, while the ATSAS software suite was used to derive structural information. The radius of gyration (R_g) and maximum interparticle distance (D_(max)) of the apo GR were 48.65±1.372 Å and 185 Å, respectively. These values are comparable to data obtained from crystal structure of GR (3NAF), where the envelope R_g, calculated with CRYSOL, is 45.55 Å, and its diameter 155.6 Å. Ca^(2+)-bound GR shows a decrease in R_g to 42.77±1.058 Å and D_(max) to 160 Å, demonstrating the structural response of GR to Ca^(2+). Low-resolution structural models of the GR were generated from the experimental scattering pattern using DAMMIN. The Ca^(2+)-bound GR revealed notable changes in both flexible and assembly interfaces of the superstructure's constituent RCK1 (Regulator of Conductance for K^+) and RCK2 domains. Since the structural changes are resolved under physiologically-relevant conditions, we speculate that they represent the molecular transitions that initiate the Ca^(2+)-induced activation of human BK channels

Metal-driven Operation of the Human Large-conductance Voltage- and Ca^(2+)-dependent Potassium Channel (BK) Gating Ring Apparatus

Large-conductance voltage- and Ca^(2+)-dependent K^+ (BK, also known as MaxiK) channels are homo-tetrameric proteins with a broad expression pattern that potently regulate cellular excitability and Ca^(2+) homeostasis. Their activation results from the complex synergy between the transmembrane voltage sensors and a large (>300 kDa) C-terminal, cytoplasmic complex (the “gating ring”), which confers sensitivity to intracellular Ca^(2+) and other ligands. However, the molecular and biophysical operation of the gating ring remains unclear. We have used spectroscopic and particle-scale optical approaches to probe the metal-sensing properties of the human BK gating ring under physiologically relevant conditions. This functional molecular sensor undergoes Ca^(2+)- and Mg^(2+)-dependent conformational changes at physiologically relevant concentrations, detected by time-resolved and steady-state fluorescence spectroscopy. The lack of detectable Ba^(2+)-evoked structural changes defined the metal selectivity of the gating ring. Neutralization of a high-affinity Ca^(2+)-binding site (the “calcium bowl”) reduced the Ca^(2+) and abolished the Mg^(2+) dependence of structural rearrangements. In congruence with electrophysiological investigations, these findings provide biochemical evidence that the gating ring possesses an additional high-affinity Ca^(2+)-binding site and that Mg^(2+) can bind to the calcium bowl with less affinity than Ca^(2+). Dynamic light scattering analysis revealed a reversible Ca^(2+)-dependent decrease of the hydrodynamic radius of the gating ring, consistent with a more compact overall shape. These structural changes, resolved under physiologically relevant conditions, likely represent the molecular transitions that initiate the ligand-induced activation of the human BK channel

The RCK1 domain of the human BK_(Ca) channel transduces Ca^(2+) binding into structural rearrangements

Large-conductance voltage- and Ca^(2+)-activated K^+ (BK_(Ca)) channels play a fundamental role in cellular function by integrating information from their voltage and Ca2+ sensors to control membrane potential and Ca^(2+) homeostasis. The molecular mechanism of Ca^(2+)-dependent regulation of BKCa channels is unknown, but likely relies on the operation of two cytosolic domains, regulator of K^+ conductance (RCK)1 and RCK2. Using solution-based investigations, we demonstrate that the purified BK_(Ca) RCK1 domain adopts an α/β fold, binds Ca^(2+), and assembles into an octameric superstructure similar to prokaryotic RCK domains. Results from steady-state and time-resolved spectroscopy reveal Ca^(2+)-induced conformational changes in physiologically relevant [Ca^(2+)]. The neutralization of residues known to be involved in high-affinity Ca^(2+) sensing (D362 and D367) prevented Ca^(2+)-induced structural transitions in RCK1 but did not abolish Ca^(2+) binding. We provide evidence that the RCK1 domain is a high-affinity Ca^(2+) sensor that transduces Ca^(2+) binding into structural rearrangements, likely representing elementary steps in the Ca^(2+)-dependent activation of human BK_(Ca) channels

Relative motion of transmembrane segments S0 and S4 during voltage sensor activation in the human BKCa channel

Large-conductance voltage- and Ca2+-activated K+ (BKCa) channel α subunits possess a unique transmembrane helix referred to as S0 at their N terminus, which is absent in other members of the voltage-gated channel superfamily. Recently, S0 was found to pack close to transmembrane segments S3 and S4, which are important components of the BKCa voltage-sensing apparatus. To assess the role of S0 in voltage sensitivity, we optically tracked protein conformational rearrangements from its extracellular flank by site-specific labeling with an environment-sensitive fluorophore, tetramethylrhodamine maleimide (TMRM). The structural transitions resolved from the S0 region exhibited voltage dependence similar to that of charge-bearing transmembrane domains S2 and S4. The molecular determinant of the fluorescence changes was identified in W203 at the extracellular tip of S4: at hyperpolarized potential, W203 quenches the fluorescence of TMRM labeling positions at the N-terminal flank of S0. We provide evidence that upon depolarization, W203 (in S4) moves away from the extracellular region of S0, lifting its quenching effect on TMRM fluorescence. We suggest that S0 acts as a pivot component against which the voltage-sensitive S4 moves upon depolarization to facilitate channel activation

Osteosynthesis-associated infection of the lower limbs by multidrug-resistant and extensively drug-resistant Gram-negative bacteria: a multicentre cohort study

Purpose: The purpose of this study was the clinical and therapeutic assessment of lower-limb osteosynthesis-associated infection (OAI) by multidrug-resistant (MDR) and extensively drug-resistant (XDR) Gram-negative bacteria (GNB), which have been poorly studied to date. Methods: A prospective multicentre observational study was conducted on behalf of ESGIAI (the European Society of Clinical Microbiology and Infectious Diseases (ESCMID) Study Group on Implant-Associated Infections). Factors associated with remission of the infection were evaluated by multivariate and Cox regression analysis for a 24-month follow-up period. Results: Patients (n=57) had a history of trauma (87.7 %), tumour resection (7 %) and other bone lesions (5.3 %). Pathogens included Escherichia coli (n=16), Pseudomonas aeruginosa (n=14; XDR 50 %), Klebsiella spp. (n=7), Enterobacter spp. (n=9), Acinetobacter spp. (n=5), Proteus mirabilis (n=3), Serratia marcescens (n=2) and Stenotrophomonas maltophilia (n=1). The prevalence of ESBL (extended-spectrum β-lactamase), fluoroquinolone and carbapenem resistance were 71.9 %, 59.6 % and 17.5 % respectively. Most patients (n=37; 64.9 %) were treated with a combination including carbapenems (n=32) and colistin (n=11) for a mean of 63.3 d. Implant retention with debridement occurred in early OAI (66.7 %), whereas the infected device was removed in late OAI (70.4 %) (p=0.008). OAI remission was achieved in 29 cases (50.9 %). The type of surgery, antimicrobial resistance and duration of treatment did not significantly influence the outcome. Independent predictors of the failure to eradicate OAI were age &gt;60 years (hazard ratio, HR, of 3.875; 95 % confidence interval, CI95 %, of 1.540–9.752; p=0.004) and multiple surgeries for OAI (HR of 2.822; CI95 % of 1.144–6.963; p=0.024). Conclusions: Only half of the MDR/XDR GNB OAI cases treated by antimicrobials and surgery had a successful outcome. Advanced age and multiple surgeries hampered the eradication of OAI. Optimal therapeutic options remain a challenge.</p

Management of the ataxias : towards best clinical practice

This document aims to provide recommendations for healthcare professionals on the diagnosis and management of people with progressive ataxia. The progressive ataxias are rare neurological conditions, and are often poorly understood by healthcare professionals. Diagnosis has generally been a long process because of the rarity and complexity of the different ataxias1. In addition, many healthcare professionals are unsure how best to manage the conditions and there is sometimes a feeling that little can be done for these patients1,2 Although there are no disease-modifying treatments for the majority of the progressive ataxias, there are many aspects of the conditions that are treatable and it is thus important that this is recognised by the relevant healthcare professionals. The diagnosis and management of the few treatable causes is also of paramount importance. All this highlights the importance of producing these guidelines: in order to increase awareness and understanding of these conditions, and lead to their improved diagnosis and management. With new developments in genetic technologies and the discovery of more genes, diagnosis is improving and has great scope to continue to do so. In addition, research is advancing and many human trials to test medications are taking place, making us more optimistic that disease-modifying treatments will be found for the progressive ataxias