Imaging in osteogenesis imperfecta: Where we are and where we are going

Osteogenesis imperfecta (OI) is a rare phenotypically and genetically heterogeneous group of inherited skeletal dysplasias. The hallmark features of OI include bone fragility and susceptibility to fractures, bone deformity, and diminished growth, along with a plethora of associated secondary features (both skeletal and extraskeletal). The diagnosis of OI is currently made on clinical grounds and may be confirmed by genetic testing. However, imaging remains pivotal in the evaluation of this disease. The aim of this article is to review the current role played by the various radiologic techniques in the diagnosis and monitoring of OI in the postnatal setting as well as to discuss recent advances and future perspectives in OI imaging. Conventional Radiography and Dual-energy X-ray Absorptiometry (DXA) are currently the two most used imaging modalities in OI. The cardinal radiographic features of OI include generalized osteopenia/osteoporosis, bone deformities, and fractures. DXA is currently the most available technique to assess Bone Mineral Density (BMD), specifically areal BMD (aBMD). However, DXA has important limitations and cannot fully characterize bone fragility in OI based on aBMD. Novel DXA-derived parameters, such as Trabecular Bone Score (TBS), may provide further insight into skeletal changes induced by OI, but evidence is still limited. Techniques like Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) can be useful as problem-solvers or in specific settings, including the evaluation of cranio-cervical abnormalities. Recent evidence supports the use of High-Resolution peripheral Quantitative Computed Tomography (HR-pQCT) as a promising tool to improve the characterization of bone fragility in OI. However, HR-pQCT remains a primarily research technique at present. Quantitative Computed Tomography (QCT) is an alternative to DXA for the determination of BMD at central sites, with distinct advantages but considerably higher radiation exposure. Quantitative Ultrasound (QUS) is a portable, inexpensive, and radiation-free modality that may complement DXA evaluation, providing information on bone quality. However, evidence of usefulness of QUS in OI is poor. Radiofrequency Echographic Multi Spectrometry (REMS) is an emerging non-ionizing imaging method that holds promise for the diagnosis of low BMD and for the prediction of fracture risk, but so far only one published study has investigated its role in OI. To conclude, several different radiologic techniques have proven to be effective in the diagnosis and monitoring of OI, each with their own specificities and peculiarities. Clinicians should be aware of the strategic role of the various modalities in the different phases of the patient care process. In this scenario, the development of international guidelines including recommendations on the role of imaging in the diagnosis and monitoring of OI, accompanied by continuous active research in the field, could significantly improve the standardization of patient care.

A high affinity switch for cAMP in the HCN pacemaker channels

Binding of cAMP to Hyperpolarization activated cyclic nucleotide gated (HCN) channels facilitates pore opening. It is unclear why the isolated cyclic nucleotide binding domain (CNBD) displays in vitro lower affinity for cAMP than the full-length channel in patch experiments. Here we show that HCN are endowed with an affinity switch for cAMP. Alpha helices D and E, downstream of the cyclic nucleotide binding domain (CNBD), bind to and stabilize the holo CNBD in a high affinity state. These helices increase by 30-fold cAMP efficacy and affinity measured in patch clamp and ITC, respectively. We further show that helices D and E regulate affinity by interacting with helix C of the CNBD, similarly to the regulatory protein TRIP8b. Our results uncover an intramolecular mechanism whereby changes in binding affinity, rather than changes in cAMP concentration, can modulate HCN channels, adding another layer to the complex regulation of their activity.

Combination of hydrophobicity and codon usage bias determines sorting of model K+ channel protein to either mitochondria or endoplasmic reticulum

When the K+ channel-like protein Kesv from Ectocarpus siliculosus virus 1 is heterologously expressed in mammalian cells, it is sorted to the mitochondria. This targeting can be redirected to the endoplasmic reticulum (ER) by altering the codon usage in distinct regions of the gene or by inserting a triplet of hydrophobic amino acids (AAs) into the protein's C-terminal transmembrane domain (ct-TMD). Systematic variations in the flavor of the inserted AAs and/or its codon usage show that a positive charge in the inserted AA triplet alone serves as strong signal for mitochondria sorting. In cases of neutral AA triplets, mitochondria sorting are favored by a combination of hydrophilic AAs and rarely used codons; sorting to the ER exhibits the inverse dependency. This propensity for ER sorting is particularly high when a common codon follows a rarer one in the AA triplet; mitochondria sorting in contrast is supported by codon uniformity. Since parameters like positive charge, hydrophobic AAs, and common codons are known to facilitate elongation of nascent proteins in the ribosome the data suggest a mechanism in which local changes in elongation velocity and co-translational folding in the ct-TMD influence intracellular protein sorting.

Alkali metal cations modulate the geometry of different binding sites in HCN4 selectivity filter for permeation or block

Hyperpolarization-activated cyclic-nucleotide gated (HCN) channels are important for timing biological processes like heartbeat and neuronal firing. Their weak cation selectivity is determined by a filter domain with only two binding sites for K+ and one for Na+. The latter acts as a weak blocker, which is released in combination with a dynamic widening of the filter by K+ ions, giving rise to a mixed K+/Na+ current. Here, we apply molecular dynamics simulations to systematically investigate the interactions of five alkali metal cations with the filter of the open HCN4 pore. Simulations recapitulate experimental data like a low Li+ permeability, considerable Rb+ conductance, a block by Cs+ as well as a punch through of Cs+ ions at high negative voltages. Differential binding of the cation species in specific filter sites is associated with structural adaptations of filter residues. This gives rise to ion coordination by a cation-characteristic number of oxygen atoms from the filter backbone and solvent. This ion/protein interplay prevents Li+, but not Na+, from entry into and further passage through the filter. The site equivalent to S3 in K+ channels emerges as a preferential binding and presumably blocking site for Cs+. Collectively, the data suggest that the weak cation selectivity of HCN channels and their block by Cs+ are determined by restrained cation-generated rearrangements of flexible filter residues.

HCN channels sense temperature and determine heart rate responses to heat

Heart rate increases with heat, [1-3] constituting a fundamental physiological relationship in vertebrates. Each normal heartbeat is initiated by an action potential generated in a sinoatrial nodal pacemaker cell. Pacemaker cells are enriched with hyperpolarization activated cyclic nucleotide-gated ion channels (HCN) that deliver cell membrane depolarizing inward current that triggers action potentials. HCN channel current increases due to cAMP binding, a mechanism coupling adrenergic tone to physiological 'fight or flight' heart rate acceleration. However, the mechanism(s) for heart rate response to thermal energy is unknown. We used thermodynamical and homology computational modeling, site-directed mutagenesis and mouse models to identify a concise motif on the S4-S5 linker of the cardiac pacemaker HCN4 channels (M407/Y409) that determines HCN4 current (If) and cardiac pacemaker cell responses to heat. This motif is required for heat sensing in cardiac pacemaker cells and in isolated hearts. In contrast, the cyclic nucleotide binding domain is not required for heat induced HCN4 current increases. However, a loss of function M407/Y409 motif mutation prevented normal heat and cAMP responses, suggesting that heat sensing machinery is essential for operating the cAMP allosteric pathway and is central to HCN4 modulation. The M407/Y409 motif is conserved across all HCN family members suggesting that HCN channels participate broadly in coupling heat to changes in cell membrane excitability.

Tailoring baker's yeast Saccharomyces cerevisiae for functional testing of channelrhodopsin

Channelrhodopsin 2 (ChR2) and its variants are the most frequent tools for remote manipulation of electrical properties in cells via light. Ongoing attempts try to enlarge their functional spectrum with respect to ion selectivity, light sensitivity and protein trafficking by mutations, protein engineering and environmental mining of ChR2 variants. A shortcoming in the required functional testing of large numbers of ChR2 variants is the lack of an easy screening system. Baker's yeast, which was successfully employed for testing ion channels from eukaryotes has not yet been used for screening of ChR2s, because they neither produce the retinal chromophore nor its precursor carotenoids. We found that addition of retinal to the external medium was not sufficient for detecting robust ChR activity in yeast in simple growth assays. This obstacle was overcome by metabolic engineering of a yeast strain, which constitutively produces retinal. In proof of concept experiments we functionally express different ChR variants in these cells and monitor their blue light induced activity in simple growth assays. We find that light activation of ChR augments an influx of Na+ with a consequent inhibition of cell growth. In a K+ uptake deficient yeast strain, growth can be rescued in selective medium by the blue light induced K+ conductance of ChR. This yeast strain can now be used as chassis for screening of new functional ChR variants and mutant libraries in simple yeast growth assays under defined selective conditions.

Seizures, behavioral deficits and adverse drug responses in two new genetic mouse models of HCN1 epileptic encephalopathy

De novo mutations in voltage- and ligand-gated channels have been associated with an increasing number of cases of developmental and epileptic encephalopathies, which often fail to respond to classic antiseizure medications. Here, we examine two knock-in mouse models replicating de novo sequence variations in the HCN1 voltage-gated channel gene, p.G391D and p.M153I (Hcn1^G380D/+ and Hcn1^M142I/+ in mouse), associated with severe drug-resistant neonatal- and childhood-onset epilepsy, respectively. Heterozygous mice from both lines displayed spontaneous generalized tonic-clonic seizures. Animals replicating the p.G391D variant had an overall more severe phenotype, with pronounced alterations in the levels and distribution of HCN1 protein, including disrupted targeting to the axon terminals of basket cell interneurons. In line with clinical reports from patients with pathogenic HCN1 sequence variations, administration of the antiepileptic Na⁺ channel antagonists lamotrigine and phenytoin resulted in the paradoxical induction of seizures in both mouse lines, consistent with an effect to further impair inhibitory neuron function. We also show that these variants can render HCN1 channels unresponsive to classic antagonists, indicating the need to screen mutated channels to identify novel compounds with diverse mechanism of action. Our results underscore the necessity of tailoring effective therapies for specific channel gene variants, and how strongly validated animal models may provide an invaluable tool towards reaching this objective.

X-ray irradiation triggers immune response in human T-lymphocytes via store-operated Ca2+ entry and NFAT activation

Radiation therapy efficiently eliminates cancer cells and reduces tumor growth. To understand collateral agonistic and antagonistic effects of this treatment on the immune system, we examined the impact of x-ray irradiation on human T cells. We find that, in a major population of leukemic Jurkat T cells and peripheral blood mononuclear cells, clinically relevant radiation doses trigger delayed oscillations of the cytosolic Ca²⁺ concentration. They are generated by store-operated Ca²⁺ entry (SOCE) following x-ray–induced clustering of Orai1 and STIM1 and formation of a Ca²⁺ release–activated Ca²⁺ (CRAC) channel. A consequence of the x-ray–triggered Ca²⁺ signaling cascade is translocation of the transcription factor nuclear factor of activated T cells (NFAT) from the cytosol into the nucleus, where it elicits the expression of genes required for immune activation. The data imply activation of blood immune cells by ionizing irradiation, with consequences for toxicity and therapeutic effects of radiation therapy.

Weak Cation Selectivity in HCN Channels Results From K+-Mediated Release of Na+ From Selectivity Filter Binding Sites

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate the pacemaker current which plays an important role in the timing of various biological processes like the heart beat. We used umbrella sampling to explore the potential of mean force for the conduction of potassium and sodium through the open HCN4 pore. Our data explain distinct functional features like low unitary conductance and weak selectivity as a result of high energetic barriers inside the selectivity filter of this channel. They exceed the 3-5 kJ/mol threshold which is presumed as maximal barrier for diffusion-limited conductance. Furthermore, simulations provide a thermodynamic explanation for the weak cation selectivity of HCN channels that contain only two ion binding sites in the selectivity filter (SF). We find that sodium ions bind more strongly to the SF than potassium and are easier released by binding of potassium than of another sodium. Hence ion transport and selectivity in HCN channels is not determined by the same mechanism as in potassium-selective channels; it rather relies on sodium as a weak blocker that can only be released by potassium.

Bioelectrical Impedance Vector Analysis (BIVA) for the monitoring of body composition in pregnancy

BACKGROUND/OBJECTIVES

During pregnancy, body composition alterations can be considered as markers of complications and in this context, a non-invasive and low-cost method such as Bioelectrical Impedance Vector Analysis (BIVA), can be employed to monitor such changes. This study aimed at identifying body compartments trend during physiological pregnancy.

SUBJECTS/METHODS

Classic and specific BIVA variables have been measured in a sample of 37 pregnant women approximately every 4 weeks of gestation and once postpartum. Researchers used both longitudinal and cross-sectional approach. The first case included data of women from the 11th to the 15th week along with data from the 28th to the 32nd week of gestation. The cross-sectional approach regarded two more specific moments (11th-12th weeks and 30th-31st weeks) and data within two months postpartum RESULTS: The longitudinal approach showed a significant decrease in classic BIVA variables (R/H, Xc/H, Z/H p < 0.001) and a shortening of the vector, pointing out that TBW and hydration increased significantly. Specific vector length increased significantly, indicating a physiological gain of FM% (p < 0.01). The cross-sectional approach showed lower values of R/H, Xc/H, Z/H between 12th-13th and 30th-31st weeks (p < 0.01), while in the postpartum period values tended to those registered at the beginning of pregnancy. No changes have been found for the phase angle in both approaches, indicating that ECW/ICW ratio remained constant CONCLUSIONS: Among physiological pregnancies, bioelectric values showed a coherent trend and these results represent a first contribution to support routine exams, leading to an early detection of anomalous values potentially correlated to pathologies.

Distinct classes of potassium channels fused to GPCRs as electrical signaling biosensors

Ligand-gated ion channels (LGICs) are natural biosensors generating electrical signals in response to the binding of specific ligands. Creating de novo LGICs for biosensing applications is technically challenging. We have previously designed modified LGICs by linking G protein-coupled receptors (GPCRs) to the Kir6.2 channel. In this article, we extrapolate these design concepts to other channels with different structures and oligomeric states, namely a tetrameric viral Kcv channel and the dimeric mouse TREK-1 channel. After precise engineering of the linker regions, the two ion channels were successfully regulated by a GPCR fused to their N-terminal domain. Two-electrode voltage-clamp recordings showed that Kcv and mTREK-1 fusions were inhibited and activated by GPCR agonists, respectively, and antagonists abolished both effects. Thus, dissimilar ion channels can be allosterically regulated through their N-terminal domains, suggesting that this is a generalizable approach for ion channel engineering.

Do the functional properties of HCN1 mutants correlate with the clinical features in epileptic patients?

The altered function of the Hyperpolarization-activated Cyclic-Nucleotide-gated (HCN) ion channels plays an important role in the pathogenesis of epilepsy in humans. In particular, HCN1 missense mutations have been recently identified in patients with different epileptic phenotypes, varying from mild to severe. Their electrophysiological characterization shows that mutated channels can act both with loss-of-function and gain-of-function mechanisms of action, without an evident correlation with the phenotype. In search for a correlation between clinical features and biophysical properties of the mutations, in this work we considered sixteen HCN1 mutations, found in eighteen Early Infantile Epileptic Encephalopathy (EIEE) patients. Statistical analysis did not establish any significant correlation between the clinical parameters and the current properties of the mutant channels. The lack of significance of our results could depend on the small number of mutations analyzed, epilepsy-associated with certainty. With the progressive increase of Next Generation Sequencing in patients with early-onset epilepsy, it is expected that the number of patients with HCN1 mutations will grow steadily. Functional characterization of epilepsy-associated HCN1 mutations remains a fundamental tool for a better understanding of the pathogenetic mechanisms leading to the disease in humans.

Gating movements and ion permeation in HCN4 pacemaker channels

The HCN1-4 channel family is responsible for the hyperpolarization-activated cation current If/Ih that controls automaticity in cardiac and neuronal pacemaker cells. We present cryoelectron microscopy (cryo-EM) structures of HCN4 in the presence or absence of bound cAMP, displaying the pore domain in closed and open conformations. Analysis of cAMP-bound and -unbound structures sheds light on how ligand-induced transitions in the channel cytosolic portion mediate the effect of cAMP on channel gating and highlights the regulatory role of a Mg2+ coordination site formed between the C-linker and the S4-S5 linker. Comparison of open/closed pore states shows that the cytosolic gate opens through concerted movements of the S5 and S6 transmembrane helices. Furthermore, in combination with molecular dynamics analyses, the open pore structures provide insights into the mechanisms of K+/Na+ permeation. Our results contribute mechanistic understanding on HCN channel gating, cyclic nucleotide-dependent modulation, and ion permeation.

Left atrial morpho-functional changes in hypertrophic cardiomyopathy and Fabry disease: a CMR-feature tracking study

Funding Acknowledgements

Type of funding sources: None.

Background

Left ventricular (LV) diastolic dysfunction (DD) is a hallmark of hypertrophic cardiomyopathy (HCM) and its phenocopies, such as Fabry disease (FD). Together with left atrial (LA) size, LA function is emerging as a sensitive marker of the adaptive changes to backward transmission of LV cardiac filling pressure, thus implementing DD assessment. Additionally, both HCM and FD are characterized by a primitive atrial myopathy, but LA morpho-functional changes in HCM and FD have never been directly compared. More recently, LA strain by Cardiovascular Magnetic Resonance Feature Tracking (CMR-FT) has been demonstrated to be a feasible and reproducible tool to explore LA function.

Purpose

To compare LA morpho-functional changes in HCM and FD and to explore their correlation with tissue alterations.

Methods

15 HCM and 15 sex-, age- and LV mass index-matched FD patients underwent CMR (Magnetom Aera 1.5T, Siemens) and Doppler Echocardiography for LV diastolic function assessment (E/e’ and DD grading from 0 to 3). LA phasic function was evaluated by CMR-FT strain (Qstrain Medis). The software output included passive (εe, conduit function), active (εa, booster pump function) and total strain (εs, reservoir function), along with LA volumes and ejection fraction (EF). Late gadolinium enhancement (LGE) was quantified as a percentage of LV mass using the standard deviations (SDs) method (≥ 5 SDs). Interstitial fibrosis was assessed by extracellular volume (ECV) quantification in remote myocardium. All patients were in sinus rhythm.

Results

In the HCM group, the proportion of patients with DD grade 2-3 was only slightly higher than in FD (p 0.26). Accordingly, no significant difference was found in E/e’ value (p 0.78). Compared to FD, HCM patients showed more severe LA morpho-functional changes, including larger LA end-systolic volume (ESV) (113 ± 35 vs 84 ± 23 ml), lower LA EF (37 ± 7 vs 44 ± 9 %) and a greater reduction of εs (-20 ± 5 vs -25 ± 6 %) and εa (-10 ± 4 vs -15 ± 4 %) (all p &lt; 0.05). LV size and function and the burden of fibrosis (LGE quantification and ECV) were comparable between the two groups. Interestingly, in HCM population, unlike in FD, LA morpho-functional measurements significantly correlated with tissue characterization parameters (LA ESV with LGE, r 0.56, p 0.03; εs and εa with ECV, r -0.51, p 0.05 and r -0.59, p 0.02, respectively).

Conclusions

LA morpho-functional alterations are much more severe in HCM compared to FD with similar degree of LV hypertrophy. A more severe atrial myopathy or different mechanisms of atrial damage in the two cardiomyopathies may explain these findings. LA CMR-FT analysis may represent a sensitive tool to discriminate between HCM and FD, although larger studies are needed to confirm this finding and the possible correlation with the occurrence of atrial arrhythmias and thromboembolic risk.

Left atrial strain analysis in hypertensive heart disease and hypertrophic cardiomyopathy by cardiovascular magnetic resonance feature tracking

Funding Acknowledgements

Type of funding sources: None.

Background

Increasing evidence suggests that left atrial (LA) deformation is a sensitive marker of diastolic dysfunction in hypertrophic phenotypes. However, there is little data about the impact of hypertension on LA function; furthermore, LA deformation in hypertensive heart disease (HHD) and hypertrophic cardiomyopathy (HCM) has not been compared yet. 

Purpose

The aim of this study is to compare atrial dimensions and function, evaluated by cardiovascular magnetic resonance feature tracking (CMR-FT) in patients with HHD, HCM and healthy subjects (HS).

Methods

67 patients (20 HHD, 27 HCM, 20 HS) underwent CMR and were included in the study. Patients were matched for age, sex and BSA; HHD and HCM were also comparable for LV mass index and ejection fraction (EF). CMR-FT atrial strain analysis was performed using Qstrain, Medis software to obtain i) LA conduit function, ii) LA booster pump function), iii) LA reservoir function, iv) LA volumes and EF. Tissue Doppler echocardiography was used to assess diastolic function, including E/e’. LA stiffness was calculated as the ratio between E/e’ and LA reservoir.

Both focal and interstitial myocardial fibrosis were assessed with LGE and extracellular volume (ECV) quantification. 

Results

HHD and HCM showed impaired LA reservoir, conduit function and higher LA volumes vs HS (reservoir: 28 ± 11% and 28 ± 13% vs 41 ± 17%; conduit: 13 ± 7% and 13 ± 7% vs 22 ± 11%; LAESV: 76 ± 21 and 87 ± 22 vs 57 ± 19 ml respectively; all p ≤ 0.03).

HHD and HCM were comparable for bi-ventricular morpho-functional parameters and ECV. HHD showed lower E/e’ values (8 ± 2 vs 16 ± 7, p = 0.002) and LA stiffness (0.23 ± 0.3 vs 0.74 ± 0.6, p 0.03), LA dimensions (LA area 13 ± 3 vs 16 ± 3 cm2/m2, p = 0.02 , LAESVi 41 ± 12 vs 48 ± 11 ml/m2, p = 0.05) and LGE extent (1 ± 2% vs 5 ± 5%, p = 0.001) as compared to HCM. Interestingly, HHD revealed a comparable reduced LA reservoir and conduit function (all p = 0.9) vs HCM. 

In HHD patients LA reservoir function was correlated with E/e’ (r -0.8, p = 0.02), but not in HCM. Conversely, LA reservoir function was correlated with LV mass index in HCM (r -0.5, p &lt; 0.01). 

Conclusions

HHD patients showed a similar and significant impairment of LA function, with lower LA dimensions and E/e’ compared to HCM with similar LV mass index and preserved function.

CMR-FT atrial strain analysis could represent a useful tool for HHD management, able to detect diastolic dysfunction (and/or atrial dysfunction) earlier than traditional markers. Further studies are needed to explore the relationship of LA deformation to heart failure symptoms and atrial fibrillation occurrence and potential changes related to response to therapy.

The mutation L69P in the PAS domain of the hERG potassium channel results in LQTS by trafficking deficiency

The congenital long QT syndrome (LQTS) is a cardiac disorder characterized by a prolonged QT interval on the electrocardiogram and an increased susceptibility to ventricular arrhythmias and sudden cardiac death. A frequent cause for LQTS is mutations in the KCNH2 gene (also known as the human ether-a-go-go-related gene or hERG), which reduce or modulate the potassium current I_Kr and hence alter cardiac repolarization. In a patient with a clinically diagnosed LQTS, we identified the mutation L69P in the N-terminal PAS (Per-Arnt-Sim) domain of hERG. Functional expression in HEK293 cells shows that a homotetrameric hERG channel reconstituted with only mutant subunits exhibits a drastically reduced surface expression of the channel protein thus leading to a diminished hERG current. Unlike many other mutations in the hERG-PAS domain the negative impact of the L69P substitution cannot be rescued by facilitated protein folding at a lower incubation temperature. Further, co-expression of wt and mutant monomers does not restore either wt like surface expression or the full hERG current. These results indicate L69P is a dominant negative mutation, with deficits which most likely occurs at the level of protein folding and subsequently inhibits trafficking to the plasma membrane. The functional deficits of the mutant channel support the clinical diagnosis of a LQTS.

Codon Bias Can Determine Sorting of a Potassium Channel Protein

Due to the redundancy of the genetic code most amino acids are encoded by multiple synonymous codons. It has been proposed that a biased frequency of synonymous codons can affect the function of proteins by modulating distinct steps in transcription, translation and folding. Here, we use two similar prototype K⁺ channels as model systems to examine whether codon choice has an impact on protein sorting. By monitoring transient expression of GFP-tagged channels in mammalian cells, we find that one of the two channels is sorted in a codon and cell cycle-dependent manner either to mitochondria or the secretory pathway. The data establish that a gene with either rare or frequent codons serves, together with a cell-state-dependent decoding mechanism, as a secondary code for sorting intracellular membrane proteins.

Detection of ligand binding to purified HCN channels using fluorescence-based size exclusion chromatography

Biochemical measurements of ligand binding to eukaryotic membrane proteins are challenging because they can require large amounts of purified protein. For this reason, ligand binding is preferentially evaluated on soluble domains rather than on the full length proteins. In this chapter, we describe the use of fluorescence size exclusion chromatography-based thermostability (FSEC-TS) as an assay to monitor ligand binding to the full length mammalian ion channel HCN4. FSEC-TS monitors the effect of the ligand on the thermal denaturation curve of the protein by following the fluorescence of a fused GFP protein. Changes in the melting temperature (Tm) provide a quantitative value for measuring ligand-protein interaction. As a proof of concept, we describe here the protocol for monitoring the binding of the second messenger cAMP and of the known HCN drug Ivabradine to the purified GFP-HCN4 channel. cTMP, a known non-binder of HCN channels, is used as a control. Due to the small amount of protein required, the assay represents a high-throughput screening system for evaluating binding of small molecules to full length proteins.

Distinct lipid bilayer compositions have general and protein-specific effects on K+ channel function

It has become increasingly apparent that the lipid composition of cell membranes affects the function of transmembrane proteins such as ion channels. Here, we leverage the structural and functional diversity of small viral K⁺ channels to systematically examine the impact of bilayer composition on the pore module of single K⁺ channels. In vitro–synthesized channels were reconstituted into phosphatidylcholine bilayers ± cholesterol or anionic phospholipids (aPLs). Single-channel recordings revealed that a saturating concentration of 30% cholesterol had only minor and protein-specific effects on unitary conductance and gating. This indicates that channels have effective strategies for avoiding structural impacts of hydrophobic mismatches between proteins and the surrounding bilayer. In all seven channels tested, aPLs augmented the unitary conductance, suggesting that this is a general effect of negatively charged phospholipids on channel function. For one channel, we determined an effective half-maximal concentration of 15% phosphatidylserine, a value within the physiological range of aPL concentrations. The different sensitivity of two channel proteins to aPLs could be explained by the presence/absence of cationic amino acids at the interface between the lipid headgroups and the transmembrane domains. aPLs also affected gating in some channels, indicating that conductance and gating are uncoupled phenomena and that the impact of aPLs on gating is protein specific. In two channels, the latter can be explained by the altered orientation of the pore-lining transmembrane helix that prevents flipping of a phenylalanine side chain into the ion permeation pathway for long channel closings. Experiments with asymmetrical bilayers showed that this effect is leaflet specific and most effective in the inner leaflet, in which aPLs are normally present in plasma membranes. The data underscore a general positive effect of aPLs on the conductance of K⁺ channels and a potential interaction of their negative headgroup with cationic amino acids in their vicinity.

Characterization of an N-terminal Nav1.5 channel variant - a potential risk factor for arrhythmias and sudden death?

Background

Alterations in the SCN5A gene encoding the cardiac sodium channel Na_v1.5 have been linked to a number of arrhythmia syndromes and diseases including long-QT syndrome (LQTS), Brugada syndrome (BrS) and dilative cardiomyopathy (DCM), which may predispose to fatal arrhythmias and sudden death. We identified the heterozygous variant c.316A > G, p.(Ser106Gly) in a 35-year-old patient with survived cardiac arrest. In the present study, we aimed to investigate the functional impact of the variant to clarify the medical relevance.

Methods

Mutant as well as wild type GFP tagged Na_v1.5 channels were expressed in HEK293 cells. We performed functional characterization experiments using patch-clamp technique.

Results

Electrophysiological measurements indicated, that the detected missense variant alters Nav1.5 channel functionality leading to a gain-of-function effect. Cells expressing S106G channels show an increase in Na_v1.5 current over the entire voltage window.

Conclusion

The results support the assumption that the detected sequence aberration alters Na_v1.5 channel function and may predispose to cardiac arrhythmias and sudden cardiac death.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12881-020-01170-3.

cyclic AMP Regulation and Its Command in the Pacemaker Channel HCN4

Direct regulation of the pacemaker “funny” current (I_f) by cyclic AMP (cAMP) underlies heart rate modulation by the autonomic nervous system. At the molecular level, cAMP activates hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that drive I_f in sinoatrial node (SAN) myocytes. Even though HCN channel genes were identified more than 20 years ago, the understanding of how cAMP regulates their gating is still fragmented. Here we summarize present understanding on how the cAMP signal is transmitted from the cytosolic to the transmembrane (TM) domain in HCN4. We further discuss how detailed structural knowledge prompted the development of pharmacological/genetic tools for the control of cAMP regulation in these channels.

A small viral potassium ion channel with an inherent inward rectification

Some algal viruses have coding sequences for proteins with structural and functional characteristics of pore modules of complex K⁺ channels. Here we exploit the structural diversity among these channel orthologs to discover new basic principles of structure/function correlates in K⁺ channels. The analysis of three similar K⁺ channels with ≤ 86 amino acids (AA) shows that one channel (Kmpv₁) generates an ohmic conductance in HEK293 cells while the other two (Kmpv_SP1, Kmpv_PL1) exhibit typical features of canonical Kir channels. Like Kir channels, the rectification of the viral channels is a function of the K⁺ driving force. Reconstitution of Kmpv_SP1 and Kmpv_PL1 in planar lipid bilayers showed rapid channel fluctuations only at voltages negative of the K⁺ reversal voltage. This rectification was maintained in KCl buffer with 1 mM EDTA, which excludes blocking cations as the source of rectification. This means that rectification of the viral channels must be an inherent property of the channel. The structural basis for rectification was investigated by a chimera between rectifying and non-rectifying channels as well as point mutations making the rectifier similar to the ohmic conducting channel. The results of these experiments exclude the pore with pore helix and selectivity filter as playing a role in rectification. The insensitivity of the rectifier to point mutations suggests that tertiary or quaternary structural interactions between the transmembrane domains are responsible for this type of gating.

Structural basis for ion selectivity in TMEM175 K+ channels

The TMEM175 family constitutes recently discovered K+channels that are important for autophagosome turnover and lysosomal pH regulation and are associated with the early onset of Parkinson Disease. TMEM175 channels lack a P-loop selectivity filter, a hallmark of all known K+ channels, raising the question how selectivity is achieved. Here, we report the X-ray structure of a closed bacterial TMEM175 channel in complex with a nanobody fusion-protein disclosing bound K+ ions. Our analysis revealed that a highly conserved layer of threonine residues in the pore conveys a basal K+ selectivity. An additional layer comprising two serines in human TMEM175 increases selectivity further and renders this channel sensitive to 4-aminopyridine and Zn2+. Our findings suggest that large hydrophobic side chains occlude the pore, forming a physical gate, and that channel opening by iris-like motions simultaneously relocates the gate and exposes the otherwise concealed selectivity filter to the pore lumen.

The HCN domain couples voltage gating and cAMP response in hyperpolarization-activated cyclic nucleotide-gated channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels control spontaneous electrical activity in heart and brain. Binding of cAMP to the cyclic nucleotide-binding domain (CNBD) facilitates channel opening by relieving a tonic inhibition exerted by the CNBD. Despite high resolution structures of the HCN1 channel in the cAMP bound and unbound states, the structural mechanism coupling ligand binding to channel gating is unknown. Here we show that the recently identified helical HCN-domain (HCND) mechanically couples the CNBD and channel voltage sensing domain (VSD), possibly acting as a sliding crank that converts the planar rotational movement of the CNBD into a rotational upward displacement of the VSD. This mode of operation and its impact on channel gating are confirmed by computational and experimental data showing that disruption of critical contacts between the three domains affects cAMP- and voltage-dependent gating in three HCN isoforms.

HCN1 mutation spectrum: from neonatal epileptic encephalopathy to benign generalized epilepsy and beyond

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels control neuronal excitability and their dysfunction has been linked to epileptogenesis but few individuals with neurological disorders related to variants altering HCN channels have been reported so far. In 2014, we described five individuals with epileptic encephalopathy due to de novo HCN1 variants. To delineate HCN1-related disorders and investigate genotype-phenotype correlations further, we assembled a cohort of 33 unpublished patients with novel pathogenic or likely pathogenic variants: 19 probands carrying 14 different de novo mutations and four families with dominantly inherited variants segregating with epilepsy in 14 individuals, but not penetrant in six additional individuals. Sporadic patients had epilepsy with median onset at age 7 months and in 36% the first seizure occurred during a febrile illness. Overall, considering familial and sporadic patients, the predominant phenotypes were mild, including genetic generalized epilepsies and genetic epilepsy with febrile seizures plus (GEFS+) spectrum. About 20% manifested neonatal/infantile onset otherwise unclassified epileptic encephalopathy. The study also included eight patients with variants of unknown significance: one adopted patient had two HCN1 variants, four probands had intellectual disability without seizures, and three individuals had missense variants inherited from an asymptomatic parent. Of the 18 novel pathogenic missense variants identified, 12 were associated with severe phenotypes and clustered within or close to transmembrane domains, while variants segregating with milder phenotypes were located outside transmembrane domains, in the intracellular N- and C-terminal parts of the channel. Five recurrent variants were associated with similar phenotypes. Using whole-cell patch-clamp, we showed that the impact of 12 selected variants ranged from complete loss-of-function to significant shifts in activation kinetics and/or voltage dependence. Functional analysis of three different substitutions altering Gly391 revealed that these variants had different consequences on channel biophysical properties. The Gly391Asp variant, associated with the most severe, neonatal phenotype, also had the most severe impact on channel function. Molecular dynamics simulation on channel structure showed that homotetramers were not conducting ions because the permeation path was blocked by cation(s) strongly complexed to the Asp residue, whereas heterotetramers showed an instantaneous current component possibly linked to deformation of the channel pore. In conclusion, our results considerably expand the clinical spectrum related to HCN1 variants to include common generalized epilepsy phenotypes and further illustrate how HCN1 has a pivotal function in brain development and control of neuronal excitability.

Biofilm formation - what we can learn from recent developments

Although biofilms have been observed early in the history of microbial research, their impact has only recently been fully recognized. Biofilm infections, which contribute to up to 80% of human microbial infections, are associated with common human disorders, such as diabetes mellitus and poor dental hygiene, but also with medical implants. The associated chronic infections such as wound infections, dental caries and periodontitis significantly enhance morbidity, affect quality of life and can aid development of follow-up diseases such as cancer. Biofilm infections remain challenging to treat and antibiotic monotherapy is often insufficient, although some rediscovered traditional compounds have shown surprising efficiency. Innovative anti-biofilm strategies include application of anti-biofilm small molecules, intrinsic or external stimulation of production of reactive molecules, utilization of materials with antimicrobial properties and dispersion of biofilms by digestion of the extracellular matrix, also in combination with physical biofilm breakdown. Although basic principles of biofilm formation have been deciphered, the molecular understanding of the formation and structural organization of various types of biofilms has just begun to emerge. Basic studies of biofilm physiology have also resulted in an unexpected discovery of cyclic dinucleotide second messengers that are involved in interkingdom crosstalk via specific mammalian receptors. These findings even open up new venues for exploring novel anti-biofilm strategies.

Mechanical transduction of cytoplasmic-to-transmembrane-domain movements in a hyperpolarization-activated cyclic nucleotide-gated cation channel

Hyperpolarization-activated cyclic nucleotide–gated cation (HCN) channels play a critical role in the control of pacemaking in the heart and repetitive firing in neurons. In HCN channels, the intracellular cyclic nucleotide–binding domain (CNBD) is connected to the transmembrane portion of the channel (TMPC) through a helical domain, the C-linker. Although this domain is critical for mechanical signal transduction, the conformational dynamics in the C-linker that transmit the nucleotide-binding signal to the HCN channel pore are unknown. Here, we use linear response theory to analyze conformational changes in the C-linker of the human HCN1 protein, which couple cAMP binding in the CNBD with gating in the TMPC. By applying a force to the tip of the so-called “elbow” of the C-linker, the coarse-grained calculations recapitulate the same conformational changes triggered by cAMP binding in experimental studies. Furthermore, in our simulations, a displacement of the C-linker parallel to the membrane plane (i.e. horizontally) induced a rotational movement resulting in a distinct tilting of the transmembrane helices. This movement, in turn, increased the distance between the voltage-sensing S4 domain and the surrounding transmembrane domains and led to a widening of the intracellular channel gate. In conclusion, our computational approach, combined with experimental data, thus provides a more detailed understanding of how cAMP binding is mechanically coupled over long distances to promote voltage-dependent opening of HCN channels.

A synthetic peptide that prevents cAMP regulation in mammalian hyperpolarization-activated cyclic nucleotide-gated (HCN) channels

Binding of TRIP8b to the cyclic nucleotide binding domain (CNBD) of mammalian hyperpolarization-activated cyclic nucleotide-gated (HCN) channels prevents their regulation by cAMP. Since TRIP8b is expressed exclusively in the brain, we envisage that it can be used for orthogonal control of HCN channels beyond the central nervous system. To this end, we have identified by rational design a 40-aa long peptide (TRIP8b_nano) that recapitulates affinity and gating effects of TRIP8b in HCN isoforms (hHCN1, mHCN2, rbHCN4) and in the cardiac current I_f in rabbit and mouse sinoatrial node cardiomyocytes. Guided by an NMR-derived structural model that identifies the key molecular interactions between TRIP8b_nano and the HCN CNBD, we further designed a cell-penetrating peptide (TAT-TRIP8b_nano) which successfully prevented β-adrenergic activation of mouse I_f leaving the stimulation of the L-type calcium current (I_CaL) unaffected. TRIP8b_nano represents a novel approach to selectively control HCN activation, which yields the promise of a more targeted pharmacology compared to pore blockers.

Ionizing Radiation Induces Morphological Changes and Immunological Modulation of Jurkat Cells

Impairment or stimulation of the immune system by ionizing radiation (IR) impacts on immune surveillance of tumor cells and non-malignant cells and can either foster therapy response or side effects/toxicities of radiation therapy. For a better understanding of the mechanisms by which IR modulates T-cell activation and alters functional properties of these immune cells, we exposed human immortalized Jurkat cells and peripheral blood lymphocytes (PBL) to X-ray doses between 0.1 and 5 Gy. This resulted in cellular responses, which are typically observed also in naïve T-lymphocytes in response of T-cell receptor immune stimulation or mitogens. These responses include oscillations of cytosolic Ca²⁺, an upregulation of CD25 surface expression, interleukin-2 and interferon-γ synthesis, elevated expression of Ca²⁺ sensitive K⁺ channels and an increase in cell diameter. The latter was sensitive to inhibition by the immunosuppressant cyclosporine A, Ca²⁺ buffer BAPTA-AM, and the CDK1-inhibitor RO3306, indicating the involvement of Ca²⁺-dependent immune activation and radiation-induced cell cycle arrest. Furthermore, on a functional level, Jurkat and PBL cell adhesion to endothelial cells was increased upon radiation exposure and was highly dependent on an upregulation of integrin beta-1 expression and clustering. In conclusion, we here report that IR impacts on immune activation and functional properties of T-lymphocytes that may have implications in both toxic effects and treatment response to combined radiation and immune therapy in cancer patients.

Reconstitution and functional characterization of ion channels from nanodiscs in lipid bilayers

Recent studies have shown that membrane proteins can be efficiently synthesized in vitro before spontaneously inserting into soluble nanoscale lipid bilayers called nanodiscs (NDs). In this paper, we present experimental details that allow a combination of in vitro translation of ion channels into commercially available NDs followed by their direct reconstitution from these nanobilayers into standard bilayer setups for electrophysiological characterization. We present data showing that two model K+ channels, Kcv and KcsA, as well as a recently discovered dual-topology F- channel, Fluc, can be reliably reconstituted from different types of NDs into bilayers without contamination from the in vitro translation cocktail. The functional properties of Kcv and KcsA were characterized electrophysiologically and exhibited sensitivity to the lipid composition of the target DPhPC bilayer, suggesting that the channel proteins were fully exposed to the target membrane and were no longer surrounded by the lipid/protein scaffold. The single-channel properties of the three tested channels are compatible with studies from recordings of the same proteins in other expression systems. Altogether, the data show that synthesis of ion channels into NDs and their subsequent reconstitution into conventional bilayers provide a fast and reliable method for functional analysis of ion channels.

Comparison among Three Different Biocoatings for Orthopaedic Prostheses. An Experimental Animal Study

One-hundred-eighty cylindrical monocortical titanium implants, 4mm diameter and 12mm long, with three different coatings: fluorohydroxyapatite (group A), hydroxyapatite (group B), and titanium oxide (group C), all applied by vacuum plasma spray were bilaterally, randomly implanted into the femurs and tibiae of twelve adult mongrel sheep. The sheep were divided into four groups (1, 2, 3 and 4) numbering three sheep each. Sheep of groups 1, 2, 3 and 4 were euthanized at two weeks, one month, three and nine months after implantation, respectively. Biomechanical and histomorphological analysis were performed. Extraction torque increased over time in all groups until the nine months period. At all the studied periods, the bone-implant contact was higher in Groups A and B compared to Group C. However, only at nine months did this difference reach statistical significance (p<0.005 comparing Groups A and B to C). The results of this study show that all the three coatings could be recommended for clinical applications.

Complement Activation in Dialysis: Effects on Cytokines, Lymphocyte Activation and β2 Microglobulin

Anaphylatoxins generated by complement activation by filter membranes are present in plasma during hemodialysis (HD). In the presence of endotoxins which may contaminate the dialysate, they can trigger monocytes to produce interleukin-1 (IL-1) and tumor necrosis factor (TNF), with detrimental effects for the patients. We have investigated whether or not the use of complement activating (cuprophan) and non- (or less-) activating membranes (polysulfone, polymethylmethacrylate or polyacrylonitrile) per se influences cytokine levels in HD patients. Our results indicate that if a sterile bicarbonate solution is used as dialysate, there are no significant increases in IL-1, TNF, interleukin-2 (IL-2) and soluble IL-2 receptors (sIL-2r) throughout HD, even with cuprophan membranes. Moreover even a prolonged use of this membrane (three months) did not change pre-dialysis levels of cytokines and receptors. Use of complement activating membranes also does not influence β2 microglobulin levels.

Yeast-Based Screening System for the Selection of Functional Light-Driven K+ Channels

Ion channels control the electrical properties of cells by opening and closing (gating) in response to a wide palette of environmental and physiological stimuli. Endowing ion channels with the possibility to be gated by remotely applied stimuli, such as light, provides a tool for in vivo control of cellular functions in behaving animals. We have engineered a synthetic light-gated potassium (K⁺) channel by connecting an exogenous plant photoreceptor LOV2 domain to the K⁺ channel pore Kcv. Here, we describe the experimental strategy that we have used to evolve the properties of the channel toward full control of light on pore gating. Our method combines rational and random mutagenesis of the channel followed by a yeast-based screening system for light-activated K⁺ conductance.