Bilateral hypoglossal nerve stimulation for treatment of adult obstructive sleep apnoea.

BACKGROUND AND AIM: Hypoglossal nerve stimulation (HNS) decreases obstructive sleep apnoea (OSA) severity via genioglossus muscle activation and decreased upper airway collapsibility. This study assessed the safety and effectiveness at 6 months post-implantation of a novel device delivering bilateral HNS via a small implanted electrode activated by a unit worn externally, to treat OSA: the Genio™ system. METHODS: This prospective, open-label, non-randomised, single-arm treatment study was conducted at eight centres in three countries (Australia, France and the UK). Primary outcomes were incidence of device-related serious adverse events and change in the apnoea-hypopnoea index (AHI). The secondary outcome was the change in the 4% oxygen desaturation index (ODI). Additional outcomes included measures of sleepiness, quality of life, snoring and device use. This trial was registered with ClinicalTrials.gov, number NCT03048604. RESULTS: 22 out of 27 implanted participants (63% male, aged 55.9±12.0 years, body mass index (BMI) 27.4±3.0 kg·m-2) completed the protocol. At 6 months BMI was unchanged (p=0.85); AHI decreased from 23.7±12.2 to 12.9±10.1 events·h-1, a mean change of 10.8 events·h-1 (p<0.001); and ODI decreased from 19.1±11.2 to 9.8±6.9 events·h-1, a mean change of 9.3 events·h-1 (p<0.001). Daytime sleepiness (Epworth Sleepiness Scale; p=0.01) and sleep-related quality of life (Functional Outcomes of Sleep Questionnaire-10; p=0.02) both improved significantly. The number of bed partners reporting loud, very intense snoring, or leaving the bedroom due to participant snoring decreased from 96% to 35%. 91% of participants reported device use >5 days per week, and 77% reported use for >5 h per night. No device-related serious adverse events occurred during the 6-month post-implantation period. CONCLUSIONS: Bilateral HNS using the Genio™ system reduces OSA severity and improves quality of life without device-related complications. The results are comparable with previously published HNS systems despite minimal implanted components and a simple stimulation algorithm.

A new role for Escherichia coli DsbC protein in protection against oxidative stress.

We report a new function for Escherichia coli DsbC, a protein best known for disulfide bond isomerization in the periplasm. We found that DsbC regulates the redox state of the single cysteine of the L-arabinose-binding protein AraF. This cysteine, which can be oxidized to a sulfenic acid, mediates the formation of a disulfide-linked homodimer under oxidative stress conditions, preventing L-arabinose binding. DsbC, unlike the homologous protein DsbG, reduces the intermolecular disulfide, restoring AraF binding properties. Thus, our results reveal a new link between oxidative protein folding and the defense mechanisms against oxidative stress.

Folding mechanisms of periplasmic proteins.

More than one fifth of the proteins encoded by the genome of Escherichia coli are destined to the bacterial cell envelope. Over the past 20years, the mechanisms by which envelope proteins reach their three-dimensional structure have been intensively studied, leading to the discovery of an intricate network of periplasmic folding helpers whose members have distinct but complementary roles. For instance, the correct assembly of ß-barrel proteins containing disulfide bonds depends both on chaperones like SurA and Skp for transport across the periplasm and on protein folding catalysts like DsbA and DsbC for disulfide bond formation. In this review, we provide an overview of the current knowledge about the complex network of protein folding helpers present in the periplasm of E. coli and highlight the questions that remain unsolved. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Overexpression of the rhodanese PspE, a single cysteine-containing protein, restores disulphide bond formation to an Escherichia coli strain lacking DsbA.

Escherichia coli uses the DsbA/DsbB system for introducing disulphide bonds into proteins in the cell envelope. Deleting either dsbA or dsbB or both reduces disulphide bond formation but does not entirely eliminate it. Whether such background disulphide bond forming activity is enzyme-catalysed is not known. To identify possible cellular factors that might contribute to the background activity, we studied the effects of overexpressing endogenous proteins on disulphide bond formation in the periplasm. We find that overexpressing PspE, a periplasmic rhodanese, partially restores substantial disulphide bond formation to a dsbA strain. This activity depends on DsbC, the bacterial disulphide bond isomerase, but not on DsbB. We show that overexpressed PspE is oxidized to the sulphenic acid form and reacts with substrate proteins to form mixed disulphide adducts. DsbC either prevents the formation of these mixed disulphides or resolves these adducts subsequently. In the process, DsbC itself gets oxidized and proceeds to catalyse disulphide bond formation. Although this PspE/DsbC system is not responsible for the background disulphide bond forming activity, we suggest that it might be utilized in other organisms lacking the DsbA/DsbB system.

Dissecting the Escherichia coli periplasmic chaperone network using differential proteomics.

ß-Barrel proteins, or outer membrane proteins (OMPs), perform many essential functions in Gram-negative bacteria, but questions remain about the mechanism by which they are assembled into the outer membrane (OM). In Escherichia coli, ß-barrels are escorted across the periplasm by chaperones, most notably SurA and Skp. However, the contributions of these two chaperones to the assembly of the OM proteome remained unclear. We used differential proteomics to determine how the elimination of Skp and SurA affects the assembly of many OMPs. We have shown that removal of Skp has no impact on the levels of the 63 identified OM proteins. However, depletion of SurA in the skp strain has a marked impact on the OM proteome, diminishing the levels of almost all ß-barrel proteins. Our results are consistent with a model in which SurA plays a primary chaperone role in E. coli. Furthermore, they suggest that while no OMPs prefer the Skp chaperone pathway in wild-type cells, most can use Skp efficiently when SurA is absent. Our data, which provide a unique glimpse into the protein content of the nonviable surA skp mutant, clarify the roles of the periplasmic chaperones in E. coli.

Crystal structure of the outer membrane protein RcsF, a new substrate for the periplasmic protein-disulfide isomerase DsbC.

The bacterial Rcs phosphorelay is a stress-induced defense mechanism that controls the expression of numerous genes, including those for capsular polysaccharides, motility, and virulence factors. It is a complex multicomponent system that includes the histidine kinase (RcsC) and the response regulator (RcsB) and also auxiliary proteins such as RcsF. RcsF is an outer membrane lipoprotein that transmits signals from the cell surface to RcsC. The physiological signals that activate RcsF and how RcsF interacts with RcsC remain unknown. Here, we report the three-dimensional structure of RcsF. The fold of the protein is characterized by the presence of a central 4-stranded ß sheet, which is conserved in several other proteins, including the copper-binding domain of the amyloid precursor protein. RcsF, which contains four conserved cysteine residues, presents two nonconsecutive disulfides between Cys(74) and Cys(118) and between Cys(109) and Cys(124), respectively. These two disulfides are not functionally equivalent; the Cys(109)-Cys(124) disulfide is particularly important for the assembly of an active RcsF. Moreover, we show that formation of the nonconsecutive disulfides of RcsF depends on the periplasmic disulfide isomerase DsbC. We trapped RcsF in a mixed disulfide complex with DsbC, and we show that deletion of dsbC prevents the activation of the Rcs phosphorelay by signals that function through RcsF. The three-dimensional structure of RcsF provides the structural basis to understand how this protein triggers the Rcs signaling cascade.

The protein-disulfide isomerase DsbC cooperates with SurA and DsbA in the assembly of the essential ß-barrel protein LptD.

The assembly of the ß-barrel proteins present in the outer membrane (OM) of Gram-negative bacteria is poorly characterized. After translocation across the inner membrane, unfolded ß-barrel proteins are escorted across the periplasm by chaperones that reside within this compartment. Two partially redundant chaperones, SurA and Skp, are considered to transport the bulk mass of ß-barrel proteins. We found that the periplasmic disulfide isomerase DsbC cooperates with SurA and the thiol oxidase DsbA in the folding of the essential ß-barrel protein LptD. LptD inserts lipopolysaccharides in the OM. It is also the only ß-barrel protein with more than two cysteine residues. We found that surAdsbC mutants, but not skpdsbC mutants, exhibit a synthetic phenotype. They have a decreased OM integrity, which is due to the lack of the isomerase activity of DsbC. We also isolated DsbC in a mixed disulfide complex with LptD. As such, LptD is identified as the first substrate of DsbC that is localized in the OM. Thus, electrons flowing from the cytoplasmic thioredoxin system maintain the integrity of the OM by assisting the folding of one of the most important ß-barrel proteins.

Disulfide bond formation in the bacterial periplasm: major achievements and challenges ahead.

SIGNIFICANCE: The discovery of the oxidoreductase disulfide bond protein A (DsbA) in 1991 opened the way to the unraveling of the pathways of disulfide bond formation in the periplasm of Escherichia coli and other Gram-negative bacteria. Correct oxidative protein folding in the E. coli envelope depends on both the DsbA/DsbB pathway, which catalyzes disulfide bond formation, and the DsbC/DsbD pathway, which catalyzes disulfide bond isomerization. RECENT ADVANCES: Recent data have revealed an unsuspected link between the oxidative protein-folding pathways and the defense mechanisms against oxidative stress. Moreover, bacterial disulfide-bond-forming systems that differ from those at play in E. coli have been discovered. CRITICAL ISSUES: In this review, we discuss fundamental questions that remain unsolved, such as what is the mechanism employed by DsbD to catalyze the transfer of reducing equivalents across the membrane and how do the oxidative protein-folding catalysts DsbA and DsbC cooperate with the periplasmic chaperones in the folding of secreted proteins. FUTURE DIRECTIONS: Understanding the mechanism of DsbD will require solving the structure of the membranous domain of this protein. Another challenge of the coming years will be to put the knowledge of the disulfide formation machineries into the global cellular context to unravel the interplay between protein-folding catalysts and chaperones. Also, a thorough characterization of the disulfide bond formation machineries at work in pathogenic bacteria is necessary to design antimicrobial drugs targeting the folding pathway of virulence factors stabilized by disulfide bonds.

Protein disulfide bond formation in the periplasm: determination of the in vivo redox state of cysteine residues.

Many proteins secreted to the bacterial cell envelope contain cysteine residues that are involved in disulfide bonds. These disulfides either play a structural role, increasing protein stability, or reversibly form in the catalytic site of periplasmic oxidoreductases. Monitoring the in vivo redox state of cysteine residues, i.e., determining whether those cysteines are oxidized to a disulfide bond or not, is therefore required to fully characterize the function and structural properties of numerous periplasmic proteins. Here, we describe a reliable and rapid method based on trapping reduced cysteine residues with 4'-acetamido-4'-maleimidylstilbene-2,2'-disulfonic acid (AMS), a maleimide compound. We use the Escherichia coli DsbA protein to illustrate the method, which can be applied to all envelope proteins.

A periplasmic reducing system protects single cysteine residues from oxidation.

The thiol group of the amino acid cysteine can be modified to regulate protein activity. The Escherichia coli periplasm is an oxidizing environment in which most cysteine residues are involved in disulfide bonds. However, many periplasmic proteins contain single cysteine residues, which are vulnerable to oxidation to sulfenic acids and then irreversibly modified to sulfinic and sulfonic acids. We discovered that DsbG and DsbC, two thioredoxin-related proteins, control the global sulfenic acid content of the periplasm and protect single cysteine residues from oxidation. DsbG interacts with the YbiS protein and, along with DsbC, regulates oxidation of its catalytic cysteine residue. Thus, a potentially widespread mechanism controls sulfenic acid modification in the cellular environment.