Interaction of ROMK2 channel with lipid kinases DGKE and AGK: Potential channel activation by localized anionic lipid synthesis

In this study, we utilized enzyme-catalyzed proximity labeling with the engineered promiscuous biotin ligase Turbo-ID to identify the proxisome of the ROMK2 channel. This channel resides in various cellular membrane compartments of the cell including the plasma membrane, endoplasmic reticulum and mitochondria. Within mitochondria, ROMK2 has been suggested as a pore-forming subunit of mitochondrial ATP-regulated potassium channel (mitoKATP). We found that ROMK2 proxisome in addition to previously known protein partners included two lipid kinases: acylglycerol kinase (AGK) and diacylglycerol kinase ε (DGKE), which are localized in mitochondria and the endoplasmic reticulum, respectively. Through co-immunoprecipitation, we confirmed that these two kinases are present in complexes with ROMK2 channels. Additionally, we found that the products of AGK and DGKE, lysophosphatidic acid (LPA) and phosphatidic acid (PA), stimulated the activity of ROMK2 channels in artificial lipid bilayers. Our molecular docking studies revealed the presence of acidic lipid binding sites in the ROMK2 channel, similar to those previously identified in Kir2 channels. Based on these findings, we propose a model wherein localized lipid synthesis, mediated by channel-bound lipid kinases, contributes to the regulation of ROMK2 activity within distinct intracellular compartments, such as mitochondria and the endoplasmic reticulum.

Electrochemical studies of the mitochondrial ROMK2 potassium channel activity reconstituted into the free-standing and tethered bilayer lipid membranes

The renal-outer-medullary‑potassium (ROMK2) channel modulates potassium transport in the kidney. It has been postulated that the ROMK2 is the pore-forming subunit of the mitochondrial ATP-sensitive potassium channel as a mediator of cardioprotection. In this study, cell-free synthesis of the ROMK2 was performed in presence of membrane scaffold protein (MSP1D1) nanodiscs. Activity measurements were achieved after channel reconstitution into the planar lipid bilayer and tethered bilayer lipid membranes. Both methods allowed for monitoring of channel function, verified with channel blocking and activation/re-activation experiments. The primary function of the mitochondrial potassium channels is to regulate the potential of the mitochondrial membrane, which allows them to play an important role in cytoprotection. This work focuses on obtaining the ROMK2 using a cell-free expression system, followed by the incorporation of the channel protein into the lipid bilayer and studying the influence of voltage changes and molecular modulators on channel activity. Channel activity was measured after its reconstitution into two models of lipid bilayers - BLM (Bilayer Lipid Membrane) and tBLM (Tethered Bilayer Lipid Membrane) deposited on a solid gold electrode. These two model membranes and electrochemical measurements made it possible to measure the flux of K⁺ ions in the presence of channel modulators.

Pharmacological Characterization of a Recombinant Mitochondrial ROMK2 Potassium Channel Expressed in Bacteria and Reconstituted in Planar Lipid Bilayers

In the inner mitochondrial membrane, several potassium channels that play a role in cell life and death have been identified. One of these channels is the ATP-regulated potassium channel (mitoK_ATP). The ROMK2 potassium channel is a potential molecular component of the mitoK_ATP channel. The current study aimed to investigate the pharmacological modulation of the activity of the ROMK2 potassium channel expressed in Escherichia coli bacteria. ROMK2 was solubilized in polymer nanodiscs and incorporated in planar lipid bilayers. The impact of known mitoK_ATP channel modulators on the activity of the ROMK2 was characterized. We found that the ROMK2 channel was activated by the mitoK_ATP channel opener diazoxide and blocked by mitoK_ATP inhibitors such as ATP/Mg²⁺, 5-hydroxydecanoic acid, and antidiabetic sulfonylurea glibenclamide. These results indicate that the ROMK2 potassium protein may be a pore-forming subunit of mitoK_ATP and that the impact of channel modulators is not related to the presence of accessory proteins.

The mitochondrial permeability transition pore: an evolving concept critical for cell life and death

In this review, we summarize current knowledge of perhaps one of the most intriguing phenomena in cell biology: the mitochondrial permeability transition pore (mPTP). This phenomenon, which was initially observed as a sudden loss of inner mitochondrial membrane impermeability caused by excessive calcium, has been studied for almost 50 years, and still no definitive answer has been provided regarding its mechanisms. From its initial consideration as an in vitro artifact to the current notion that the mPTP is a phenomenon with physiological and pathological implications, a long road has been travelled. We here summarize the role of mitochondria in cytosolic calcium control and the evolving concepts regarding the mitochondrial permeability transition (mPT) and the mPTP. We show how the evolving mPTP models and mechanisms, which involve many proposed mitochondrial protein components, have arisen from methodological advances and more complex biological models. We describe how scientific progress and methodological advances have allowed milestone discoveries on mPTP regulation and composition and its recognition as a valid target for drug development and a critical component of mitochondrial biology.

Solubilization, purification, and functional reconstitution of human ROMK potassium channel in copolymer styrene-maleic acid (SMA) nanodiscs

Expression, purification, and functional reconstitution of mammalian ion channels are often challenging. Heterologous expression of mammalian channels in bacteria can be advantageous due to unrelated protein environment and the lack of risk of copurification of endogenous proteins, e.g., accessory channel subunits that can influence the channel activity. Also, direct recording of channel activity could be challenging due to their intracellular localization like in the case of mitochondrial channels. The activity of purified channels can be characterized at the single-molecule level by electrophysiological techniques, such as planar lipid bilayers (PLB). In this work, we describe a simple approach to accomplish PLB recording of the activity of single renal outer medullary potassium channels ROMK expressed in E. coli. We focused on the ROMK2 isoform that is present at low levels in the mitochondria and can be responsible for mitoK_ATP activity. We screened for the best construct to express the codon-optimized ROMK proteins with a 6xHis tag for protein purification. The strategy involved the use of optimal styrene-maleic acid (SMA) copolymer, which forms so-called polymer nanodiscs, to solubilize and purify ROMK-containing SMA lipid particles (SMALPs), which were amenable for fusion with PLB. Reconstituted ROMK channels exhibited ion selectivity, rectification, and pharmacological properties, which are in agreement with previous work on ROMK channels.

Patch-Clamp Recording of the Activity of Ion Channels in the Inner Mitochondrial Membrane

Mitochondria are intracellular organelles, which play a crucial role in the generation of ATP. Mitochondria are surrounded by a double membrane, consisting of a smooth outer membrane (OMM) and a markedly folded inner mitochondrial membrane (IMM). Mitochondrion that has been stripped of its outer membrane, leaving the inner membrane intact is called mitoplast. There is a number of different transport proteins located in the inner mitochondrial membrane including ion channels that mediate fluxes of potassium, calcium, and chloride ions. These channels regulate the mitochondrial membrane potential, respiration, and production of reactive oxygen species. The stability of mitoplasts offers the possibility of measuring the activity of ion channels from IMM using the patch-clamp technique. Electrophysiological measurements of currents through ion channels in the IMM permit discovery of unique properties of these channels with the aim of new specific pharmacological therapies. In this chapter, we describe the isolation of mitochondria, preparation of mitoplast for patch-clamp recordings and single-mitoplast PCR experiments, which can be helpful in mastering the technique of recording the activity of mitochondrial ion channels.

Heme is required for carbon monoxide activation of mitochondrial BKCa channel

Carbon monoxide (CO) is an endogenously synthesized gaseous mediator and is involved in the regulation of numerous physiological processes. Mitochondria, in which hemoproteins are abundant, are among the targets for CO action. Large-conductance calcium-activated (mitoBK_Ca) channels in the inner mitochondrial membrane share multiple biophysical similarities with the BK_Ca channels of the plasma membrane and could be a potential target for CO. To test this hypothesis, the activity of the mitoBK_Ca channels in human astrocytoma U-87 MG cell mitochondria was assessed with the patch-clamp technique. The effects of CO-releasing molecules (CORMs), such as CORM-2, CORM-401, and CORM-A1, were compared to the application of a CO-saturated solution to the mitoBK_Ca channels in membrane patches. The applied CORMs showed pleiotropic effects including channel inhibition, while the CO-containing solution did not significantly modulate channel activity. Interestingly, CO applied to the mitoBK_Ca channels, which were inhibited by exogenously added heme, stimulated the channel. To summarize, our findings indicate a requirement of heme binding to the mitoBK_Ca channel for channel modulation by CO and suggest that CORMs might have complex unspecific effects on mitoBK_Ca channels.

The monomers, oligomers, and fibrils of amyloid-β inhibit the activity of mitoBKCa channels by a membrane-mediated mechanism

A causative agent of Alzheimer's disease (AD) is a short amphipathic peptide called amyloid beta (Aβ). Aβ monomers undergo structural changes leading to their oligomerization or fibrillization. The monomers as well as all aggregated forms of Aβ, i.e., oligomers, and fibrils, can bind to biological membranes, thereby modulating membrane mechanical properties. It is also known that some isoforms of the large-conductance calcium-activated potassium (BK_Ca) channel, including the mitochondrial BK_Ca (mitoBK_Ca) channel, respond to mechanical changes in the membrane. Here, using the patch-clamp technique, we investigated the impact of full-length Aβ (Aβ_1-42) and its fragment, Aβ_25-35, on the activity of mitoBK_Ca channels. We found that all forms of Aβ inhibited the activity of the mitoBK_Ca channel in a concentration-dependent manner. Since monomers, oligomers, and fibrils of Aβ exhibit different molecular characteristics and structures, we hypothesized that the inhibition was not due to direct peptide-protein interactions but rather to membrane-binding of the Aβ peptides. Our findings supported this hypothesis by showing that Aβ peptides block mitoBK_Ca channels irrespective of the side of the membrane to which they are applied. In addition, we found that the enantiomeric peptide, D-Aβ_1-42, demonstrated similar inhibitory activity towards mitoBK_Ca channels. As a result, we proposed a general model in which all Aβ forms i.e., monomers, oligomers, and amyloid fibrils, contribute to the progression of AD by exerting a modulatory effect on mechanosensitive membrane components.

Mitochondrial potassium channels - an overview

Mitochondria play a fundamental role in ATP synthesis within the majority of mammalian cells. Potassium channels present in the inner mitochondrial membrane are fine regulators of mitochondrial function, based on inner membrane K+ permeability. These channels are regulated by a plethora of factors and conditions in a way similar to plasma membrane potassium channels. Regulators of mitochondrial potassium channels include the membrane potential, calcium ions, free fatty acids and ATP levels within the cells. Recently, it was shown that these channels are regulated by the respiratory chain, stretching of the membrane and phosphorylation. The essential interest that has driven studies of mitochondrial potassium channels for nearly 25 years is their role in cytoprotection and in cell death. Mitochondrial potassium channels have been described in neurons, astrocytoma, cardiac and skeletal muscles, fibroblasts, keratinocytes and endothelial cells. In this overview, we summarize the current knowledge of mitochondrial potassium channels. This summary will be done with a special focus on studies performed over the last 20 years in the Laboratory of Intracellular Ion Channels at the Nencki Institute. These include studies on the electrophysiological and pharmacological properties of mitochondrial potassium channels and on their regulation by endogenous intracellular substances. Additionally, the regulation of mitochondrial potassium channels by the respiratory chain and by stretching of the inner mitochondrial membrane will be reviewed. Properties of mitochondrial potassium channels in various organisms will also be summarized.

Mechanosensitivity of mitochondrial large-conductance calcium-activated potassium channels

Potassium channels have been discovered in the inner mitochondrial membrane of various cells. These channels can regulate the mitochondrial membrane potential, the matrix volume, respiration and reactive species generation. Therefore, it is believed that their activation is cytoprotective in various tissues. In our study, the single-channel activity of a large-conductance calcium-activated potassium channel (mitoBK_Ca) was measured by the patch-clamp technique on mitoplasts derived from mitochondria isolated from human glioma U-87 MG cells. Here, we show for the first time that mechanical stimulation of mitoBK_Ca channels results in an increased probability of channel opening. However, the mechanosensitivity of mitoBK_Ca channels was variable with some channels exhibiting no mechanosensitivity. We detected the expression of mechanosensitive BK_Ca-STREX exon in U-87 MG cells and hypotesize, based on previous studies demonstrating the presence of multiple BK_Ca splice variants that variable mechanosensitivity of mitoBK_Ca could be the result of the presence of diverse BK_Ca isoforms in mitochondria of U-87 MG cells. Our findings indicate the possible involvement of the mitoBK_Ca channel in mitochondria activities in which changes in membrane tension and shape play a crucial role, such as fusion/fission and cristae remodeling.

cGMP-Elevating Compounds and Ischemic Conditioning Provide Cardioprotection Against Ischemia and Reperfusion Injury via Cardiomyocyte-Specific BK Channels

BACKGROUND

The nitric oxide-sensitive guanylyl cyclase/cGMP-dependent protein kinase type I signaling pathway can afford protection against the ischemia/reperfusion injury that occurs during myocardial infarction. Reportedly, voltage and Ca²⁺-activated K⁺ channels of the BK type are stimulated by cGMP/cGMP-dependent protein kinase type I, and recent ex vivo studies implicated that increased BK activity favors the survival of the myocardium at ischemia/reperfusion. It remains unclear, however, whether the molecular events downstream of cGMP involve BK channels present in cardiomyocytes or in other cardiac cell types.

METHODS

Gene-targeted mice with a cardiomyocyte- or smooth muscle cell-specific deletion of the BK (CMBK or SMBK knockouts) were subjected to the open-chest model of myocardial infarction. Infarct sizes of the conditional mutants were compared with litter-matched controls, global BK knockout, and wild-type mice. Cardiac damage was assessed after mechanical conditioning or pharmacological stimulation of the cGMP pathway and by using direct modulators of BK. Long-term outcome was studied with respect to heart functions and cardiac fibrosis in a chronic myocardial infarction model.

RESULTS

Global BK knockouts and CMBK knockouts, in contrast with SMBK knockouts, exhibited significantly larger infarct sizes compared with their respective controls. Ablation of CMBK resulted in higher serum levels of cardiac troponin I and elevated amounts of reactive oxygen species, lower phosphorylated extracellular receptor kinase and phosphorylated AKT levels and an increase in myocardial apoptosis. Moreover, CMBK was required to allow beneficial effects of both nitric oxide-sensitive guanylyl cyclase activation and inhibition of the cGMP-degrading phosphodiesterase-5, ischemic preconditioning, and postconditioning regimens. To this end, after 4 weeks of reperfusion, fibrotic tissue increased and myocardial strain echocardiography was significantly compromised in CMBK-deficient mice.

CONCLUSIONS

Lack of CMBK channels renders the heart more susceptible to ischemia/reperfusion injury, whereas the pathological events elicited by ischemia/reperfusion do not involve BK in vascular smooth muscle cells. BK seems to permit the protective effects triggered by cinaciguat, riociguat, and different phosphodiesterase-5 inhibitors and beneficial actions of ischemic preconditioning and ischemic postconditioning by a mechanism stemming primarily from cardiomyocytes. This study establishes mitochondrial CMBK channels as a promising target for limiting acute cardiac damage and adverse long-term events that occur after myocardial infarction.

What do we not know about mitochondrial potassium channels?

In this review, we summarize our knowledge about mitochondrial potassium channels, with a special focus on unanswered questions in this field. The following potassium channels have been well described in the inner mitochondrial membrane: ATP-regulated potassium channel, Ca(2+)-activated potassium channel, the voltage-gated Kv1.3 potassium channel, and the two-pore domain TASK-3 potassium channel. The primary functional roles of these channels include regulation of mitochondrial respiration and the alteration of membrane potential. Additionally, they modulate the mitochondrial matrix volume and the synthesis of reactive oxygen species by mitochondria. Mitochondrial potassium channels are believed to contribute to cytoprotection and cell death. In this paper, we discuss fundamental issues concerning mitochondrial potassium channels: their molecular identity, channel pharmacology and functional properties. Attention will be given to the current problems present in our understanding of the nature of mitochondrial potassium channels. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

[What we don't know about mitochondrial potassium channels?]

In the current work the authors present the most interesting, yet not fully understood issues regarding origin, function and pharmacology of the mitochondrial potassium channels. There are eight potassium channels known to contribute to the potassium permeability of the inner mitochondrial membrane: ATP-regulated channel, calcium-regulated channels of large, intermediate and small conductance, voltage-regulated Kv1.3 and Kv7.4 channels, two-pore-domain TASK-3 channel and SLO2 channel. The primary function of the mitochondrial potassium channels is regulation of the mitochondrial membrane potential. Additionally, mitochondrial potassium channels alter cellular respiration, regulation of the mitochondrial volume and ROS synthesis. However, mechanisms underlying these processes are not fully understood yet. In this work, the authors not only present available knowledge about this topic, but also put certain hypotheses that may set the direction for the future research on these proteins.

Cytoplasmic Domain of MscS Interacts with Cell Division Protein FtsZ: A Possible Non-Channel Function of the Mechanosensitive Channel in Escherichia Coli

Bacterial mechano-sensitive (MS) channels reside in the inner membrane and are considered to act as emergency valves whose role is to lower cell turgor when bacteria enter hypo-osmotic environments. However, there is emerging evidence that members of the Mechano-sensitive channel Small (MscS) family play additional roles in bacterial and plant cell physiology. MscS has a large cytoplasmic C-terminal region that changes its shape upon activation and inactivation of the channel. Our pull-down and co-sedimentation assays show that this domain interacts with FtsZ, a bacterial tubulin-like protein. We identify point mutations in the MscS C-terminal domain that reduce binding to FtsZ and show that bacteria expressing these mutants are compromised in growth on sublethal concentrations of β-lactam antibiotics. Our results suggest that interaction between MscS and FtsZ could occur upon inactivation and/or opening of the channel and could be important for the bacterial cell response against sustained stress upon stationary phase and in the presence of β-lactam antibiotics.

The CSC proteins FAP61 and FAP251 build the basal substructures of radial spoke 3 in cilia

Motile cilia have nine doublet microtubules, with hundreds of associated proteins that repeat in modules. Each module contains three radial spokes, which differ in their architecture, protein composition, and function. The conserved proteins FAP61 and FAP251 are crucial for the assembly and stable docking of RS3 and cilia motility.

Dynein motors and regulatory complexes repeat every 96 nm along the length of motile cilia. Each repeat contains three radial spokes, RS1, RS2, and RS3, which transduct signals between the central microtubules and dynein arms. Each radial spoke has a distinct structure, but little is known about the mechanisms of assembly and function of the individual radial spokes. In Chlamydomonas, calmodulin and spoke-associated complex (CSC) is composed of FAP61, FAP91, and FAP251 and has been linked to the base of RS2 and RS3. We show that in Tetrahymena, loss of either FAP61 or FAP251 reduces cell swimming and affects the ciliary waveform and that RS3 is either missing or incomplete, whereas RS1 and RS2 are unaffected. Specifically, FAP251-null cilia lack an arch-like density at the RS3 base, whereas FAP61-null cilia lack an adjacent portion of the RS3 stem region. This suggests that the CSC proteins are crucial for stable and functional assembly of RS3 and that RS3 and the CSC are important for ciliary motility.

Coexisting psoriasis,psoriatic arthritis, gout and Dupuytren's contracture

The authors describe the case of a 60-year-old Caucasian man with psoriasis who subsequently developed psoriatic arthritis, gout and Dupuytren's contracture. This is the first report documenting the simultaneous occurrence of psoriasis, psoriatic arthritis, gout and Dupuytren's contracture in a single patient. The etiopathology underlying the concurrent presence of these diseases is discussed.

Visualization of cholesterol deposits in lysosomes of Niemann-Pick type C fibroblasts using recombinant perfringolysin O

BACKGROUND

Niemann-Pick disease type C (NPC) is caused by defects in cholesterol efflux from lysosomes due to mutations of genes coding for NPC1 and NPC2 proteins. As a result, massive accumulation of unesterified cholesterol in late endosomes/lysosomes is observed. At the level of the organism these cholesterol metabolism disorders are manifested by progressive neurodegeneration and hepatosplenomegaly. Until now filipin staining of cholesterol deposits in cells has been widely used for NPC diagnostics. In this report we present an alternative method for cholesterol visualization and estimation using a cholesterol-binding bacterial toxin, perfringolysin O.

METHODS

To detect cholesterol deposits, a recombinant probe, perfringolysin O fused with glutathione S-transferase (GST-PFO) was prepared. GST-PFO followed by labeled antibodies or streptavidin was applied for immunofluorescence and immunoelectron microscopy to analyze cholesterol distribution in cells derived from NPC patients. The identity of GST-PFO-positive structures was revealed by a quantitative analysis of their colocalization with several organelle markers. Cellular ELISA using GST-PFO was developed to estimate the level of unesterified cholesterol in NPC cells.

RESULTS

GST-PFO recognized cholesterol with high sensitivity and selectivity, as demonstrated by a protein/lipid overlay assay and surface plasmon resonance analysis. When applied to stain NPC cells, GST-PFO decorated abundant deposits of cholesterol in intracellular vesicles that colocalized with filipin-positive structures. These cholesterol deposits were resistant to 0.05%-0.2% Triton X-100 used for cells permeabilization in the staining procedure. GST-PFO-stained organelles were identified as late endosomes/lysosomes based on their colocalization with LAMP-1 and lysobisphosphatidic acid. On the other hand, GST-PFO did not colocalize with markers of the Golgi apparatus, endoplasmic reticulum, peroxisomes or with actin filaments. Only negligible GST-PFO staining was seen in fibroblasts of healthy individuals. When applied to cellular ELISA, GST-PFO followed by anti-GST-peroxidase allowed a semiquantitative analysis of cholesterol level in cells of NPC patients. Binding of GST-PFO to NPC cells was nearly abolished after extraction of cholesterol with methyl-β-cyclodextrin.

CONCLUSIONS

Our data indicate that a recombinant protein GST-PFO can be used to detect cholesterol accumulated in NPC cells by immunofluorescence and cellular ELISA. GST-PFO can be a convenient and reliable probe for revealing cholesterol deposits in cells and can be useful in diagnostics of NPC disease.

Genetic screen for potassium leaky small mechanosensitive channels (MscS) in Escherichia coli: recognition of cytoplasmic β domain as a new gating element

Mechanosensitive membrane channels in bacteria respond to the mechanical stretching of the membrane. They will open when bacteria are subjected to rapid osmotic down shock. MscS is a bacterial mechanosensitive channel of small conductance. It is a heptameric membrane protein whose transmembrane part, including the gate and its kinetics, has been well characterized. MscS has a large cytoplasmic domain of a cage-like shape that changes its conformation upon gating, but its involvement in gating is not understood. We screened MscS for mutations that cause potassium leak in Escherichia coli strains deficient in potassium transport systems. We did a phenotypic analysis of single and multiple mutants and recorded the single channel activities of some of them. After these analyses, we attributed the effects of a number of mutations to particular functional states of the channel. Our screen revealed that MscS leaks potassium in a desensitized and in an inactivated state. It also appeared that the lower part of TM3 (transmembrane, pore-forming helix) and the cytoplasmic β domain are tightly packed in the inactivated state but are dissociated in the open state. We attribute the TM3-β interaction to stabilization of the inactivated state in MscS and to the control of tight closure of its membrane pore.

Elliptical head hemiarthroplasty in the treatment of femoral neck and head fractures--preliminary report

BACKGROUND

The aim of his study is to evaluate efficacy of elliptical head hemiarthroplasty in treatment of femoral neck and head fractures and related complications in young patients under 55 years of age.

MATERIAL AND METHODS

Uncemented elliptical head hemiarthroplasty was performed in 14 patients aged 28-55, mean age of 46,7 years, out of whom 12 diagnosed with a femoral neck fracture or related complications and 2 with a femoral head fracture. Mean follow-up period was 4,5 years, range from 10 months to 6 years. The Harris Hip Score was applied for evaluation of functional results. The radiographic analysis was based on standard anteroposterior and axial radiographic projections. Acetabular cartilage erosion was assessed according to the Wetherell et al radiographic method of measurement.

RESULTS

The functional results ranged from 58 to 97 points, mean 85,28 points (according to the HHS). The radiographs revealed no signs of stem loosening. No cartilage erosion or migration of the prosthesis were observed in the study.

CONCLUSION

An elliptical shape of the prosthetic head reflects the anatomy of the hip reducing the risk of acetabular cartilage erosion and migration of the prosthesis. We conclude that modular elliptical head hemiarthroplasty can be a useful method of management of femoral neck or head fractures and related complications in younger patients.

Assessment of functional abilities of patients with contraindications to operative treatment of femoral neck fractures

BACKGROUND

Major obstacles to operative treatment of elderly patients with femoral neck fractures frequently encountered in clinical practice include poor overall health, lack of consent to have an operation, and general medical or anaesthesiological contraindications. In such cases, conservative treatment is the only possibility. This aims at maximally improving the patient's functional status, an approach best termed "movement-oriented patient management".

MATERIAL AND METHODS

The study population consisted of 51 patients (31 F, 20 M) with femoral neck fractures in whom surgery was contraindicated. The patients took part in a movement-oriented management programme. The study consisted in evaluating movement abilities, self-care and locomotion of the patients using the Harris and Lazansky scales. Patients were evaluated twice: at discharge from hospital and at three months after discharge.

RESULTS

In patients assessed using the Lazansky score, Evaluation 1 revealed 88% poor results and 12% satisfactory results, compared to 63% poor results and 37% satisfactory results at evaluation 2. Harris scores did not differ between Evaluations 1 and 2, with 100% of the patients achieving the lowest scores.

CONCLUSIONS

1) The study population of patients with femoral neck fractures in whom surgery was contraindicated did not demonstrate a definite improvement or deterioration of their functional status following "movement-oriented management"; 2) "movement-oriented management" cannot be regarded as having any effect on change in functional status, which should therefore be improved using a more beneficial method of treatment.

Evaluation of nonunions of the tibia diaphysis - own experience

BACKGROUND

Delayed union is not an expected outcome of treatment of a bone fracture. The cause of the delayed union must be established before treatment of the complication can be undertaken. The method of treatment depends mainly on the specific nature of delayed bone union. The study evaluates the causes of delayed bone union during treatment of tibial shaft fractures.

MATERIAL AND METHODS

Seventy-two patients with delayed union following a fracture of the tibial diaphysis were treated in the Department of Orthopaedics and Musculoskeletal Traumatology of Wrocław Medical University between 1995 and 2005. All patients had received previous treatment of the fractures. Depending on the morphology of the delayed union, the treatment was with Ilizarov's compression-distraction method or autologous osteoplasty with intramedullary, internal or external fixation. The choice of a method was based on an analysis of the features of the non-union while fracture morphology, presence of other concomitant sequelae of the original injury, fixation method and possible surgical errors, post-operative patient management, co-morbidity and patient compliance were evaluated as risk factors of delayed bone union. The risk factors were subdivided into treatment-dependent (external) and treatment-independent (internal). Treatment outcomes were evaluated on the basis of an orthopaedic examination and radiographic evidence.

RESULTS

In the vast majority of patients, delayed union of the tibial diaphysis was due to external factors.

CONCLUSIONS

Delayed union of the tibia can be avoided through early and simultaneous treatment of all sequelae of the original trauma. Patient management must be based on state-of-the-art medical knowledge.

[Rationalizing blood management in orthopedics and traumatology]

In recent years there has been growing interest in blood conservation and avoidance of transfusion in patients undergoing orthopedic surgery. The benefits of blood transfusion must be considered and evaluated in terms of risk factors relating to the adverse effects of transfusion. A number of strategies are available to reduce the need for blood transfusion. These strategies are maximally effective if combined to span the pre-operative, intra-operative and post-operative periods. Surgical, anesthetic and pharmacological techniques can reduce blood loss during operation and the use of allogenic blood. This article presents current opinions, on the base of contemporary literature, regarding risks of transfusion and several simple techniques that will reduce the need for transfusion in orthopedic procedures.

Surface changes of the mechanosensitive channel MscS upon its activation, inactivation, and closing

MscS is a bacterial mechanosensitive channel that shows voltage dependence. The crystal structure of MscS revealed that the channel is a homoheptamer with a large chamber on the intracellular site. Our previous experiments indicated that the cytoplasmic chamber of the channel is not a rigid structure and changes its conformation upon the channel activation. In this study, we have applied various sized cosolvents that are excluded from protein surfaces. It is well known that such cosolvents induce compaction of proteins and prevent thermal fluctuations. It is also known that they shift channel equilibrium to the state of lower volume. We have found that large cosolvents that cannot enter the channel interior accelerate channel inactivation when applied from the cytoplasmic side, but they slow down inactivation when applied from the extracellular side. We have also found that small cosolvents that can enter the channel cytoplasmic chamber prevent the channel from opening, unlike the large ones. These data support our idea that the channel cytoplasmic chamber shrinks upon inactivation but also give new clues about conformational changes of the channel upon transitions between its functional states.

Repair of oxidative damage in mitochondrial DNA of Saccharomyces cerevisiae: involvement of the MSH1-dependent pathway

Mitochondrial DNA (mtDNA) is located close to the respiratory chain, a major source of reactive oxygen species (ROS). This proximity makes mtDNA more vulnerable than nuclear DNA to damage by ROS. Therefore, the efficient repair of oxidative lesions in mtDNA is essential for maintaining the stability of the mitochondrial genome. A series of genetic and biochemical studies has indicated that eukaryotic cells, including the model organism Saccharomyces cerevisiae, use several alternative strategies to prevent mutagenesis induced by endogenous oxidative damage to nuclear DNA. However, apart from base excision repair (BER), no other pathways involved in the repair of oxidative damage in mtDNA have been identified. In this study, we have examined mitochondrial mutagenesis in S. cerevisiae cells which lack the activity of the Ogg1 glycosylase, an enzyme playing a crucial role in the removal of 8-oxoG, the most abundant oxidative lesion of DNA. We show that the overall frequency of the mitochondrial oligomycin-resistant (Olir) mutants is increased in the ogg1 strain by about one order of magnitude compared to that of the wild-type strain. Noteworthy, in the mitochondrial oli1 gene, G:C to T:A transversions are generated approximately 50-fold more frequently in the ogg1 mutant relative to the wild-type strain. We also demonstrate that the increased frequency of Olir mutants in the ogg1 strain is markedly reduced by the presence of plasmids encoding Msh1p, a homologue of the bacterial mismatch protein MutS, which specifically functions in mitochondria. This suppression of the mitochondrial mutator phenotype of the ogg1 strain seems to be specific, since overexpression of the mutant allele msh1-R813W failed to exert this effect. Finally, we also show that the increased frequency of Olir mutants arising in an msh1/MSH1 heterozygote grown in glucose-containing medium is further enhanced if the cells are cultivated in glycerol-containing medium, i.e. under conditions when the respiratory chain is fully active. Taken together, these results strongly suggest that MSH1-dependent repair represents a significant back-up to mtBER in the repair of oxidative damage in mtDNA.

Assessment of Ilizarov's method in intraarticular calcaneal fractures

Background. Hindfoot dysfunction after intrarticular fracture of calcaneus are evokated by disturbances of muscle balance caused by change of bone shape and disturbances of talo-calcanear join function caused by secondary arthrosis of the joint. Goal of fracture treatment is restoring joint surfaces and the shape of calcaneus. It allow restoring function of foot and a correct gait. Material and methods. In the years 1997-2002 in Department of Orthopedic and Traumatology of Wrocław Medical University 45 patients with intrarticular fractures of calcaneus were treated. Closed reduction as well as external stabilisation with the use of Illizarov fixator was applied in 20 patients as method of choice, in accordance to our criteria. Observation period of the treated group extended from 6 months to 6 years. Results. To investigation answered 25 patients (21 men and 4 woman). In this group 11 patients with 14 fractures of calcaneal bone were treated with Ilizarov method. Restoration of anatomical conditions and secondary arthrosis were evaluated on the basis of x-ray examination and functional result of treatment simultaneously according to Olerud, Molander and Rowe evaluation form. Conclusions. In our analysis application of closed reduction and external fixation with Ilizarov device is a method of choice in cases when selected criteria are fulfilled.

The impact of selected biological factors on bone regeneration in distraction osteogenesis

Background. Comparatively few reports have analyzed the biological factors influencing the course of distraction osteogenesis, such as the etiology and magnitude of shortening or the patient's age at surgery. The aim of our study was to assess the impact of selected biological factors on the course of bone regeneration in distraction osteogenesis of the lower leg. Material and method. We examined 126 patients, on whom 138 lower leg lengthening operations were performed. Congenital shortening was the most numerous cause (54.9%), followed by postinfectious (16.6%), posttraumatic (14.3%), neurogenic (7.9%), and achondroplastic (6.3%). The course of distraction osteogenesis was analyzed on the basis of duration of treatment, expressed by the lengthening index. Results. The highest lengthening index values were found in neurogenic patients, somewhat lower in congenital. The lowest lengthening index values were observed in postinfectious and posttraumatic shortenings, and in the group of bi-level lengthenings in achondroplastic patients. The lengthening index decreased along with increasing elongation. Lower lengthening index values were found in patients younger than 20 years, regardless of the method and number of osteotomy levels. Conclusions. There is clinical evidence for the impact of etiology, age, and magnitude of shortening on the course of distraction osteogenesis. The shortest treatment duration was found in shortening of acquired etiology and achondroplasia, and in patients older than 20 years. The values of the lengthening index were the lowest in lengthenings over 6 cm, whereas the greatest values were observed in lengthenings less than 4 cm.

Enhanced expression of the DNA damage-inducible gene DIN7 results in increased mutagenesis of mitochondrial DNA in Saccharomyces cerevisiae

We reported previously that the product of DIN7, a DNA damage-inducible gene of Saccharomyces cerevisiae, belongs to the XPG family of proteins, which are involved in DNA repair and replication. This family includes the S. cerevisiae protein Rad2p and its human homolog XPGC, Rad27p and its mammalian homolog FEN-1, and Exonuclease I (Exo I). Interestingly, Din7p is the only member of the XPG family which specifically functions in mitochondria. We reported previously that overexpression of DIN7 results in a mitochondrial mutator phenotype. In the present study we wished to test the hypothesis that this phenotype is dependent on the nuclease activity of Din7p. For this purpose, we constructed two alleles, din7-D78A and din7-D173A, which encode proteins in which highly conserved aspartates important for the nuclease activity of the XPG proteins have been replaced by alanines. Here, we report that overexpression of the mutant alleles, in contrast to DIN7, fails to increase the frequency of mitochondrial petite mutants or erythromycin-resistant (Er) mutants. Also, overproduction of din7-D78Ap does not result in destabilization of poly GT tracts in mitochondrial DNA (mtDNA), the phenotype observed in cells that overexpress Din7p. We also show that petite mutants induced by enhanced synthesis of wild-type Din7p exhibit gross rearrangements of mtDNA, and that this correlates with enhanced recombination within the mitochondrial cyt b gene. These results suggest that the stability of the mitochondrial genome of S. cerevisiae is modulated by the level of the nuclease Din7p.

C termini of the Escherichia coli mechanosensitive ion channel (MscS) move apart upon the channel opening

Heptameric YggB is a mechanosensitive ion channel (MscS) from the inner membrane of Escherichia coli. We demonstrate, using the patch clamp technique, that cross-linking of the YggB C termini led to irreversible inhibition of the channel activities. Application of Ni(2+) to the YggB-His(6) channels with the hexahistidine tags added to the ends of their C termini also resulted in a marked but reversible decrease of activities. Western blot revealed that YggB-His(6) oligomers are more stable in the presence of Ni(2+), providing evidence that Ni(2+) is coordinated between C termini from different subunits of the channel. Intersubunit coordination of Ni(2+) affecting channel activities occurred in the channel closed conformation and not in the open state. This may suggest that the C termini move apart upon channel opening and are involved in the channel activation. We propose that the as yet undefined C-terminal region may form a cytoplasmic gate of the channel. The results are discussed and interpreted based on the recently released quaternary structure of the channel.

[Molecular basis of ion selectivity based on crystalline structure of bacterial channels]

The principles of ionic selectivity of the two crystallised bacterial ion channels are described. These channels are: the potassium channel KcsA, whose amino-acid sequence is homologous to the eukaryotic voltage--dependent potassium channels and the chloride channel EcClC that is a prokaryotic member of the ClC family of chloride channels.

IN CONCLUSION

although the overall molecular architecture of KcsA is different from that of EcClC, the selectivity filters in both cases show similarities. They both utilise helix dipoles organised within the channel molecule in such a fashion to produce electrostatically favourable environment for anions (in the case of EcClC) or cations (in the case of KcsA).

A dominant mitochondrial mutator phenotype of Saccharomyces cerevisiae conferred by msh1 alleles altered in the sequence encoding the ATP-binding domain

In order to improve our understanding of the role of the yeast MSH1 gene in error avoidance in mitochondrial DNA, two msh1 alleles were constructed, which encode proteins with amino acid substitutions in an ATP-binding domain that is highly conserved among MutS homologs. Here, we report that moderate overexpression of the msh1-R813W or msh1-G776D allele, in strains which also carry the wild-type MSH1 allele, slightly increases the frequency of mutations conferring resistance to erythromycin (E(r)) and elevates the frequency of alterations within a polyGT tract present in mitochondrial DNA (mtDNA). This result indicates that the mutant alleles confer a dominant mitochondrial mutator phenotype and strongly suggests that the ATP-binding domain plays a crucial role in the in vivo function of Msh1p. Interestingly, we have found that overexpression of wild-type MSH1 has opposite effects on the stability of polyGT vs. polyAT tracts present in mtDNA; excess of Msh1p slightly increases the stability of polyGT tracts, whereas the stability of polyAT tracts is dramatically decreased. We show that although overexpression of msh1-R813W or msh1-G776D also results in a marked overall increase in the frequency of alterations in polyAT tracts, the spectrum of alterations differs from that found in cells overexpressing MSH1; large deletions predominate in the latter case, while 2-bp deletions are generated in cells that overproduce the mutant msh1p. This result strongly suggests that the mutations in the ATP binding domain change the specificity of the protein with respect to the recognition of potentially mutagenic structures in mtDNA.

Bacterial ion channels and their eukaryotic homologues

Due to the relative ease of obtaining their crystal structures, bacterial ion channels provide a unique opportunity to analyse structure and function of their eukaryotic homologues. This review describes prokaryotic channels whose structures have been determined. These channels are KcsA, a bacterial homologue of eukaryotic potassium channels, MscL, a bacterial mechanosensitive ion channel and ClC0, a prokaryotic homologue of the eukaryotic ClC family of anion-selective channels. General features of their structure and function are described with a special emphasis on the advantages that these channels offer for understanding the properties of their eukaryotic homologues. We present amino-acid sequences of eukaryotic proteins related in their primary sequences to bacterial mechanosensitive channels. The usefulness of bacterial mechanosensitive channels for the studies on general principles of mechanosensation is discussed.

[Bacterial ion canals]

After an overview of potassium and chloride ion channels found in bacterial inner membrane this review focuses on mechanosensitive ion channels from the inner membrane of Escherichia coli. Mechanosensitive channels, MscL and MscS, have major roles in managing the transition from high to low environments. Biochemical and genetic studies of MscL, combined with structural information derived from X-ray crystalography, have brought the knowledge of how a mechanosensitive channel senses membrane tension. Physiological studies on MscL and on MscS have demonstrated how the mechanosensitive channels contribute to the bacterial membrane response upon hypo-osmotic stress.

The product of the DNA damage-inducible gene of Saccharomyces cerevisiae, DIN7, specifically functions in mitochondria

We reported previously that the product of the DNA damage-inducible gene of Saccharomyces cerevisiae, DIN7, belongs to a family of proteins that are involved in DNA repair and replication. The family includes S. cerevisiae proteins Rad2p and its human homolog XPGC, Rad27p and its mammalian homolog FEN-1, and Exonuclease I (Exo I). Here, we report that Din7p specifically affects metabolism of mitochondrial DNA (mtDNA). We have found that dun1 strains, defective in the transcriptional activation of the DNA damage-inducible genes RNR1, RNR2, and RNR3, exhibit an increased frequency in the formation of the mitochondrial petite (rho(-)) mutants. This high frequency of petites arising in the dun1 strains is significantly reduced by the din7::URA3 allele. On the other hand, overproduction of Din7p from the DIN7 gene placed under control of the GAL1 promoter dramatically increases the frequency of petite formation and the frequency of mitochondrial mutations conferring resistance to erythromycin (E(r)). The frequencies of chromosomal mutations conferring resistance to canavanine (Can(r)) or adenine prototrophy (Ade(+)) are not affected by enhanced synthesis of Din7p. Experiments using Din7p fused to the green fluorescent protein (GFP) and cell fractionation experiments indicate that the protein is located in mitochondria. A possible mechanism that may be responsible for the decreased stability of the mitochondrial genome in S. cerevisiae cells with elevated levels of Din7p is discussed.

Glutathione (GSH) reduces the open probability of mechanosensitive channels in Escherichia coli protoplasts

The effects of glutathione (reduced GSH, and oxidized GSSG) on mechanosensitive (MS) ion channels from Escherichia coli protoplasts were investigated using excised, inside-out membrane patches. Our studies demonstrate here that 5 mM GSH irreversibly reduces the activities of the 560-pS MS channel by approximately 70-75% whereas 5 mM GSSG did not alter the MS channel currents. In addition, millimolar concentrations of dithiothreitol had similar although weaker effects to GSH. The physiological concentration of GSH in E. coli cytoplasm ranges from 3.5 mM to 6.6 mM, which may indicate that under normal conditions these MS channels open less due to membrane stretch.

Voltage-independent adaptation of mechanosensitive channels in Escherichia coli protoplasts

Mechanosensitive (MS) ion channels, with 560 pS conductance, opened transiently by rapid application of suction pulses to patches of E. coli protoplast membrane. The adaptation phase of the response was voltage-independent. Application of strong suction pulses, which were sufficient to cause saturation of the MS current, did not abolish the adaptation. Multiple-pulse experimental protocols revealed that once MS channels had fully adapted, they could be reactivated by a second suction pulse of similar amplitude, providing the time between pulses was long enough and suction had been released between pulses. Limited proteolysis (0.2 mg/ml pronase applied to the cytoplasmic side of the membrane patch) reduced the number of open channels without affecting the adaptation. Exposing patches to higher levels of pronase (1 mg/ml) removed responsiveness of the channel to suction and abolished adaptation consistent with disruption of the tension transmission mechanism responsible for activating the MS channel. Based on these data we discuss a mechanism for mechanosensitivity mediated by a cytoplasmic domain of the MS channel molecule or associated protein.