Kupffer cell-like syncytia replenish resident macrophage function in the fibrotic liver

Kupffer cells (KCs) are localized in liver sinusoids but extend pseudopods to parenchymal cells to maintain their identity and serve as the body's central bacterial filter. Liver cirrhosis drastically alters vascular architecture, but how KCs adapt is unclear. We used a mouse model of liver fibrosis and human tissue to examine immune adaptation. Fibrosis forced KCs to lose contact with parenchymal cells, down-regulating "KC identity," which rendered them incapable of clearing bacteria. Commensals stimulated the recruitment of monocytes through CD44 to a spatially distinct vascular compartment. There, recruited monocytes formed large aggregates of multinucleated cells (syncytia) that expressed phenotypical KC markers and displayed enhanced bacterial capture ability. Syncytia formed via CD36 and were observed in human cirrhosis as a possible antimicrobial defense that evolved with fibrosis.

The Calcium Score's power of zero in 82-Rubidium PET depending on age group and sex

Background

Coronary calcium score (CAC) is a well-validated method to detect coronary artery disease (CAD). A significant number of patients referred for functional ischaemia tests reveal a normal result. CAC could be a potential gatekeeper to reduce unnecessary testing.

Purpose

Study aims were to describe the frequency of zero calcium and its diagnostic power to exclude abnormal perfusion stratified by age and sex in a large cohort undergoing 82-Rubidium Positron Emission Tomography (PET).

Methods

All consecutive patients with suspected CAD who were referred for myocardial perfusion PET at our tertiary center between 2016 and 2021 were identified. Baseline and scan related data were extracted from the electronic database. Patients were included for this retrospective analysis if CAC and semi-quantitative analysis of perfusion study were available. Percentiles and test characteristics of zero calcium were calculated stratified by age and sex. Summed stress score (SSS) ≥4 on PET was considered abnormal.

Results

2640 patients were included. Mean age was 65±11 years; 54% were male. Angina and dyspnea were present in 39% and 60%, respectively. 21% (558/2640) of the scans were abnormal and median CAC was 62 [0–374]. 685 patients (26%) had no calcium. Stratified by age, the proportion of zero calcium was 89% (<40 years), 61% (40–49y), 40% (50–59y), 21% (60–69y), 14% (70–79y) and 6% (≥80y). CAC was higher in abnormal scans (median 561 vs. 27, p<0.001) and zero CAC was associated with a lower risk of abnormal PET (2.6% vs. 27.6%, p<0.001). The negative predictive value (NPV) of calcium zero to exclude abnormal PET was 97%, 96% and 98% for all patients, males and females, respectively. Test characteristics are displayed in Table 1. Percentiles of CAC according to scan result are depicted in Figure 1.

Conclusion

The proportion of zero calcium is frequent and declines with increasing age. The absence of coronary calcium is associated with less abnormal PET scans. Zero calcium to exclude an abnormal scan performed best in young patients (<50) with a NPV ≥98%. CAC could act as a gatekeeper. However, further studies including safety endpoints are needed.

The upper calcium score indicates the 90th percentile of patients with normal PET. The lower calcium score indicates the 5th percentile of patients with abnormal PET. Calcium scores within the white area are unlikely to be associated with abnormal PET (since this area includes only the 5% of abnormal PET scans). The dark grey area indicates patients with a high likelihood of positive PET. For better readability, different scales were used.

Funding Acknowledgement

Type of funding sources: None.

Using high-sensitivity cardiac troponin for the exclusion of inducible myocardial ischemia in patients without previously known coronary artery disease

Background

The rapid and safe exclusion of functionally relevant coronary artery disease (CAD) is a crucial, yet unmet clinical need. High-sensitivity cardiac troponin (hs-cTn) may be an attractive strategy, particularly in patients without previously known CAD.

Purpose

To derive and internally validate optimal rule-out cutoffs for an early and safe exclusion of functionally relevant CAD in symptomatic patients without previously known CAD.

Methods

In an ongoing single-center, prospective, cohort study, we enrolled consecutive patients without previously known CAD that were referred with symptoms possibly related to functionally relevant CAD. Cardiac troponin concentrations were measured at presentation using two high-sensitivity assays (Elecsys hs-cTnT and Architect hs-cTnI). Presence of functionally relevant CAD was adjudicated by 2 independent cardiologists, blinded to hs-cTn measurements, using MPI-SPECT/CT in all patients, as well as coronary angiography and fractional flow reserve measurements, whenever available. The primary diagnostic outcome was safety for early rule-out of functionally relevant CAD, quantified by sensitivity and the negative predictive value (NPV). The co-primary prognostic outcomes were cumulative incidences of cardiovascular death and all-cause death after 5 years. A NPV ≥90% and sensitivity ≥90% were predefined as acceptable performance criteria. The derived cutoffs were further evaluated in pre-specified subgroups. Internal validity was assessed with a bootstrapping procedure for a realistic estimate in similar future patients. Cumulative incidence curves stratified by the presence of functionally relevant CAD and hs-cTn concentrations below and above the derived cutoffs were constructed.

Results

Among 2111 eligible patients, 498 (23.6%) had a final diagnosis of functionally relevant CAD. Median age was 68 years and 938 (44.4%) were female. For ruling out functionally relevant CAD, a hs-cTnT concentration <5 ng/L resulted in a sensitivity of 90.8% (95% CI: 87.9–93.0%) and a NPV of 90.2% (95% CI: 87.1–92.5), triaging 468 (22.2%) patients towards rule-out. Similarly, a hs-cTnI concentration <2 ng/L resulted in a sensitivity of 91.6% (95% CI: 88.8–93.7%) and a NPV of 90.0% (95% CI: 86.8–92.6), triaging 422 (20.0%) patients. Internal validation showed robustness of these findings. The diagnostic performance of the derived cutoffs did not significantly vary across the subgroups. Hs-cTn concentrations above the derived cutoffs were associated with a substantially higher cumulative event rate of cardiovascular death (hs-cTnT: 7.0% vs. 0.8%; hs-cTnI: 6.6% vs. 1.2%) and all-cause death (hs-cTnT: 14.3% vs. 2.4%; hs-cTnI: 13.1% vs. 4.4%) during 5-years follow-up (log rank p<0.001 for all).

Conclusion

In symptomatic patients without previously known CAD, very low hs-cTn concentrations may generally allow to safely and effectively exclude functionally relevant CAD.

Funding Acknowledgement

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): Swiss National Science FoundationSwiss Heart Foundation

P6436Soluble urokinase plasminogen activator receptor and functionally relevant coronary artery disease: a prospective cohort study

Background

The urokinase system is pivotal in the pathogenesis of atherosclerosis. Therefore, soluble urokinase plasminogen activator receptor (suPAR) concentrations may help in the detection of functionally relevant coronary artery disease (fCAD).

Purpose

To evaluate suPAR as diagnostic marker for fCAD.

Methods

Among consecutive patients with symptoms suggestive of fCAD, fCAD was adjudicated blinded to suPAR concentrations in two domains: first, diagnosis of fCAD according to myocardial perfusion single photon emission tomography (MPI-SPECT) and coronary angiography; second, fCAD according to cardiovascular death, non-fatal acute myocardial infarction (AMI) and all-cause death during 2-year follow-up.

Results

Among 968 patients, symptoms were adjudicated to be causally related to fCAD in 26% (255/968). SuPAR concentrations were higher in patients with fCAD as compared to those without (3.45 ng/mL versus 3.20 ng/mL, p=0.007), but overall had only low diagnostic accuracy (area under the curve [AUC]: 0.56, 95% CI 0.52–0.60) and were not independent predictors of fCAD after multivariable adjustment. Circulating suPAR concentrations were modestly correlated with high-sensitivity cardiac troponin (hs-cTn) T (Spearman's rho 0.393, p<0.001), NT-proBNP (Spearman's rho 0.327, p<0.001) and age (Spearman's rho 0.364, p<0.001), but only weakly correlated with the extent of coronary atherosclerosis as quantified by perfusion defects (Spearman's rho 0.123, p<0.001). Prognostically, suPAR concentrations had moderate-to-high accuracy in the prediction of cardiovascular death (AUC 0.72, 95% CI 0.62–0.81) and all-cause death (AUC 0.72, 95% CI 0.65–0.79) at 2-years, and remained a significant predictor for all-cause death after multivariable adjustment (p=0.001). SuPAR concentrations did not predict non-fatal AMI.

Conclusions

SuPAR is an independent predictor of death, but not helpful in the detection of fCAD.

Acknowledgement/Funding

European Union, Swiss National Science Foundation, the Swiss Heart Foundation, the Cardiovascular Research Foundation Basel, the University of Basel,

Restricted role for methionine synthase reductase defined by subcellular localization

Methionine synthase reductase (MSR; gene name MTRR) is responsible for the reductive activation of methionine synthase. Cloning of the MTRR gene had revealed two major transcription start sites which, by alternative splicing, allows for two potential translation products of 698 and 725 amino acids. While the shorter protein was expected to target the cytosol where methionine synthase is located, the additional sequence in the longer protein was consistent with a role as a mitochondrial leader sequence. The possibility that MSR might target mitochondria was also suggested by the work of Leal et al. [N.A. Leal, H. Olteanu, R. Banerjee, T.A. Bobik, Human ATP:Cob(I)alamin adenosyltransferase and its interaction with methionine synthase reductase, J. Biol. Chem. 279 (2004) 47536-47542.] who showed that it can act as the reducing enzyme in combination with MMAB (ATP:Cob(I)alamin adenosyltransferase) to generate adenosylcobalamin from cob(II)alamin in vitro. Here we examined directly whether MSR protein is found in mitochondria. We show that, while two transcripts are produced by alternative splicing, the N-terminal segment of the putative mitochondrial form of MSR fused to GFP does not contain a sufficiently strong mitochondrial leader sequence to direct the fusion protein to the mitochondria of human fibroblasts. Further, antibodies to MSR protein localized MSR to the cytosol, but not to the mitochondria of human fibroblasts or the human hepatoma line Huh-1, as determined by Western blot analysis and immunofluorescence of cells in situ. These data confirm that MSR protein is restricted to the cytosol but, based on the Leal study, suggest that a similar protein may interact with MMAB to reduce the mitochondrial cobalamin substrate in the generation of adenosylcobalamin.

Structures of pulmonary surfactant films adsorbed to an air–liquid interface in vitro

Phospholipid films can be preserved in vitro when adsorbed to a solidifiable hypophase. Suspensions of natural surfactant, lipid extract surfactants, and artificial surfactants were added to a sodium alginate solution and filled into a captive bubble surfactometer (CBS). Surfactant film was formed by adsorption to the bubble of the CBS for functional tests. There were no discernible differences in adsorption, film compressibility or minimal surface tension on quasi-static or dynamic compression for films formed in the presence or absence of alginate in the subphase of the bubble. The hypophase-film complex was solidified by adding calcium ions to the suspension with the alginate. The preparations were stained with osmium tetroxide and uranyl acetate for transmission electron microscopy. The most noteworthy findings are: (1) Surfactants do adsorb to the surface of the bubble and form osmiophilic lining layers. Pure DPPC films could not be visualized. (2) A distinct structure of a particular surfactant film depends on the composition and the concentration of surfactant in the bulk phase, and on whether or not the films are compressed after their formation. The films appear heterogeneous, and frequent vesicular and multi-lamellar film segments are seen associated with the interfacial films. These features are seen already upon film formation by adsorption, but multi-lamellar segments are more frequent after film compression. (3) The rate of film formation, its compressibility, and the minimum surface tension achieved on film compression appear to be related to the film structure formed on adsorption, which in turn is related to the concentration of the surfactant suspension from which the film is formed. The osmiophilic surface associated surfactant material seen is likely important for the surface properties and the mechanical stability of the surfactant film at the air-fluid interface.

Fluorescence light microscopy of pulmonary surfactant at the air-water interface of an air bubble of adjustable size

The structural dynamics of pulmonary surfactant was studied by epifluorescence light microscopy at the air-water interface of a bubble as a model close to nature for an alveolus. Small unilamellar vesicles of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, a small amount of a fluorescent dipalmitoylphosphatidylcholine-analog, and surfactant-associated protein C were injected into the buffer solution. They aggregated to large clusters in the presence of Ca(2+) and adsorbed from these units to the interface. This gave rise to an interfacial film that eventually became fully condensed with dark, polygonal domains in a fluorescent matrix. When now the bubble size was increased or decreased, respectively, the film expanded or contracted. Upon expansion of the bubble, the dark areas became larger to the debit of the bright matrix and reversed upon contraction. We were able to observe single domains during the whole process. The film remained condensed, even when the interface was increased to twice its original size. From comparison with scanning force microscopy directly at the air-water interface, the fluorescent areas proved to be lipid bilayers associated with the (dark) monolayer. In the lung, such multilayer phase acts as a reservoir that guarantees a full molecular coverage of the alveolar interface during the breathing cycle and provides mechanical stability to the film.

Scanning force microscopy at the air-water interface of an air bubble coated with pulmonary surfactant

To study the structure-function relationship of pulmonary surfactant under conditions close to nature, molecular films of a model system consisting of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylglycerol, and surfactant-associated protein C were prepared at the air-water interface of air bubbles about the size of human alveoli (diameter of 100 microm). The high mechanical stability as well as the absence of substantial film flow, inherent to small air bubbles, allowed for scanning force microscopy (SFM) directly at the air-water interface. The SFM topographical structure was correlated to the local distribution of fluorescent-labeled dipalmitoylphosphatidylcholine, as revealed from fluorescence light microscopy of the same bubbles. Although SFM has proven before to be exceptionally well suited to probe the structure of molecular films of pulmonary surfactant, the films so far had to be transferred onto a solid support from the air-water interface of a film balance, where they had been formed. This made them prone to artifacts imposed by the transfer. Moreover, the supported monolayers disallowed the direct observation of the structural dynamics associated with expansion and compression of the films as upon breathing. The current findings are compared in this respect to our earlier findings from films, transferred onto a solid support.

Distribution of the surfactant-associated protein C within a lung surfactant model film investigated by near-field optical microscopy

Lung surfactant films at the air/water interface exhibit the particularity that surfactant molecules are expelled from the surface monolayer into a surface associated multilamellar phase during compression. They are able to re-enter the surface film during the following expansion. The underlying mechanism for this behavior is not fully understood yet. However, an important role is ascribed to the surfactant-associated protein C (SP-C). Here, we studied a model lung surfactant, consisting of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and SP-C, by means of scanning near-field optical microscopy (SNOM). Attaching a fluorescent dye to the protein allowed the localization of its lateral distribution at various surface pressures with high resolution. At an early stage of compression, the film appears demixed into a pure lipid phase and a protein-enriched phase. Within the latter phase, protein aggregations are revealed. They show a uniform density, having three times the fluorescence intensity of their surroundings. Across the phase boundary between the lipid phase and the protein-rich phase, there is a protein density gradient rather than an abrupt border. When the film is highly compressed, we observe the formation of multilamellar structures that are fluorescent. They are often surrounded by a slightly fluorescent monolayer. The fluorescence of the multilayer stacks (i. e., the protein content per unit area) is proportional to the height of the stacks.

A scanning force- and fluorescence light microscopy study of the structure and function of a model pulmonary surfactant

The structure of an artificial pulmonary surfactant was studied by scanning force- and fluorescence light microscopy (SFM, and FLM, respectively). The surfactant--a mixture of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG) and recombinant surfactant-associated protein C (SP-C)--was prepared at the air-water interface of a Langmuir film balance and imaged by FLM under various states of compression. In order to visualize their topography by SFM, the films were transferred onto a solid mica support by the Langmuir-Blodgett (LB) technique. We found that a region of high film compressibility of the spread monolayer close to its equilibrium surface pressure (pi = 50 mN/m) was due to the exclusion of layered protrusions with each layer 5.5 to 6.5 nm thick. They remained associated with the monolayer and readily reinserted upon expansion of the film. Comparison with the FLM showed that the protrusions contained the protein in high concentration. The more the film was compressed, the larger was the number of layers on top of each other. The protrusions arose from regions of the monolayer with a distinct microstructure that may have been responsible for their formation. The molecular architecture of the microstructure remains to be elucidated, although some of it can be inferred from spectroscopic data in combination with the SFM topographical images. We illustrate our current understanding of the film structure with a molecular model.

Adsorption of biological molecules to a solid support for scanning probe microscopy

Scanning probe microscopes are now established tools to study the surface structure of biological macromolecules under physiological conditions. Sample preparation methods for this microscopy all have the objective to attach the specimen firmly to a support. Here we analyse the commonly used method of adsorbing biological specimens to freshly cleaved mica. This is facilitated by adjusting the electrolyte concentration and the pH of the buffer solution. Native macromolecular systems absorbed to mica in this way can be reproducibly imaged at submolecular resolution.

The structure of a model pulmonary surfactant as revealed by scanning force microscopy

The structures formed by a pulmonary surfactant model system of dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylglycerol (DPPG), and recombinant surfactant-associated protein C (SP-C) were studied using scanning force microscopy (SFM) on Langmuir-Blodgett films. The films appeared to be phase separated, in agreement with earlier investigations by fluorescence light microscopy. There were smooth polygonal patches of mostly lipid, surrounded by a corrugated rim rich in SP-C. When the films were compressed beyond the equilibrium surface pressure, the protein-rich phase mediated the formation of layered protrusions. The height of these multilamellar structures embodied equidistant steps slightly higher than a DPPC double layer in the gel phase. At the air-water interface too, a high compressibility at low surface tension was indicative of the exclusion of matter. The exclusion process proved to be fully reversible. The present study demonstrates that some of the matter of the model pulmonary surfactant can move in and out of the active monolayer. The SFM images revealed a lipid-protein complex that was responsible for the reversible exclusion of double-layer structures. This mechanism may be important in the natural system too, to keep the surface tension of the alveolar air/water interface constantly low over the range of area encountered upon breathing.

Scanning tunneling microscopy of biological macromolecular structures coated with a conducting film

We have studied the capability of scanning tunneling microscopy (STM) to reveal the three-dimensional structure of biological macromolecular structures that have been rendered conductive by metal-coating. The sample preparation used has been derived from a well established method in transmission electron microscopy (TEM). It includes adsorption, freezing and dehydration by vacuum-sublimation, followed by metal-shadowing of the specimen. As an alternative to adsorption and coating, fluid biomaterials can be replaced by conductive freeze-fracture replica. We give an introduction into the sample preparation of metal-coated specimens and discuss how each step can affect the structural preservation and thereby the quality of the data. Some aspects of the data acquisition and the quantitative evaluation of STM data are shown. Possible contributions of STM in the biological macromolecular research are pointed out.

Platinum/iridium/carbon: a high-resolution shadowing material for TEM, STM and SEM of biological macromolecular structures

Thin Pt/Ir/C coating films (1.5 nm) show a fine granularity and provide a high structural resolution in the transmission electron microscope (TEM) when applied to freeze-dried biological macromolecules. They keep their structure when exposed to atmospheric conditions, without the need of an additional stabilizing carbon layer, in contrast to conventional high-resolution shadowing materials such as Ta/W and Pt/C. However, the correct ratio of the components has turned out to be crucial. When evaporating Pt/Ir/C from the source electrode in an electron-beam-heated evaporator, the ratio of the three elements changes progressively, and, consequently, the properties of such films depend strongly on the mass that has been pre-evaporated. In this paper we present a quantitative analysis of the composition of Pt/Ir/C films by wavelength-dispersive X-ray analysis (WDX) undertaken in association with TEM experiments. We applied Pt/Ir/C shadowing to two regular biological test specimens, the phage T4 type III polyhead and the HPI-layer of Deinococcus radiodurans. It turns out that Pt/Ir/C films containing at least 25% C are three-dimensionally stable on the freeze-dried macromolecular samples. By the dramatically improved resolution power of the latest scanning electron microscopes (SEM) and the invention of the scanning tunnelling microscope (STM), two new surface-sensitive tools for the investigation of biological macromolecular structures became available. The Pt/Ir/C coating has proved to be well suited for STM and SEM imaging of freeze-dried biological structures because of its good electrical conductivity and its direct three-dimensional stability. We compare STM, SEM and TEM images of freeze-dried and Pt/Ir/C-coated polyheads.

The Sendai virus nucleocapsid exists in at least four different helical states

Sendai virus nucleocapsids have been observed by electron microscopy to coexist in three different helical pitch conformations, 5.3, 6.8, and 37.5 nm. The 5.3- and 6.8-nm conformations are present both in uranyl acetate negatively stained preparations and in tantalum-tungsten metal-shadowed preparations, whereas the 37.5-nm conformation, which has not been previously reported, is present only in the shadowed preparations. The 5.3-nm pitch conformation appears to be a mixture of two discrete structural states, with a small difference in the twist of the structure between the two. We have used image reconstruction techniques on an averaged data set from eight negatively stained nucleocapsids to produce a three-dimensional reconstruction at 2.4-nm resolution of the structure in one of the 5.3-nm pitch states. There are 13.07 nucleocapsid protein (NP) subunits in each turn of the helix in this state. The helical repeat is 79.5 nm, containing 196 subunits in 15 turns of the left-handed 5.3-nm helix. The arrangement of subunits produces a 5.0-nm-diameter hollow core which forms an internal helical groove. The RNA accounts for about 3% of the mass of the nucleocapsid, and so its location is not conspicuous in the reconstruction. Because of the RNA remains associated with the NP subunits during mRNA transcription and genome replication, structural transitions in the nucleocapsid may determine the accessibility of the genome to polymerases. Alternatively, the large hollow core and internal helical groove we have reconstructed may allow access to the RNA even in the tightly coiled 5.3-nm pitch conformation.

Scanning tunneling microscopy of uncoated recA-DNA complexes

Uncoated recA-DNA complexes were imaged with the scanning tunneling microscope (STM). The images, which reveal the right-handed helical structure of the complexes with subunits clearly resolved, are comparable in quality to STM images of metal-coated specimens. Possible conduction mechanisms that allow STM imaging of biological macromolecules are discussed.

Scanning tunneling microscopy of recA-DNA complexes coated with a conducting film

A link between scanning tunneling microscopy (STM) and conventional transmission electron microscopy has been established for biological material by applying STM on freeze-dried recA-DNA complexes coated with a conducting film. The topography of the complexes observed by means of STM revealed a right-handed single helix composed of about six recA monomers per helical turn.