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Aims A key mechanism responsible for atrial fibrillation is multi-wavelet reentry (MWR). We have previously demonstrated that ablation in regions of increased circuit density reduces the duration of, and decreases the inducibility of MWR. In this study, we demonstrate a method for identifying local circuit density using electrogram frequency and validated its effectiveness for map-guided ablation in a computer model of MWR. Methods and results We simulated MWR in tissues with variation of action potential duration and intercellular resistance. Electrograms were calculated using various electrode sizes and configurations. We measured and compared the number of circuits to the tissue activation frequency and electrogram frequency using three recording configurations [unipolar, contact bipolar, orthogonal closed unipolar (OCU)] and two frequency measurements (dominant frequency, centroid frequency). We then used the highest resolution electrogram frequency map (OCU centroid frequency) to guide the placement of lesions to high frequency regions. Map-guided ablation was compared with no ablation and random/blind ablation lesions of equal length. Electrogram frequency correlated with tissue frequency and circuit density as a function of electrode spatial resolution. Map-guided ablation resulted in a significant reduction in MWR duration (142 ± 174 vs. 41 ± 63 s). Conclusion Electrogram frequency correlates with circuit density in MWR provided electrodes have high spatial resolution. Map-guided ablation is superior to no ablation and to blind/random ablation.

During the epithelial to mesenchymal transition (EMT), polarized cells in the epithelium can undergo a transformation characterized by the loss of cell-cell junctions and increased migratory activity into nonpolarized invasive cells. These cells adopt a mesenchymal shape and migrate into the basal lamina. Such transitions have been observed in developmental processes and have been linked to cancer cell metastasis. Most experimental studies on EMT search for molecular markers indicating an epithelial or mesenchymal conformation, focussing on afferent signaling pathways received by cells undergoing this transformation; however, these approaches are unable to track mechanical changes in the cell and the possible role this plays in EMT. In order to address this gap in our understanding, we have used a quantitative approach to study population level effects of single cell changes typically occurring during EMT. We have developed a computational model making use of the advantages of both single cell migratory models and agent-based cell population models to study the effect of cellular molecular processes in EMT. The disruption of a cell sheet representing the epithelium over a dense extracellular matrix (ECM) is simulated using interaction forces between different cells and between cells and discrete fibers representing the ECM. In our study, two different parameters were varied: protrusion force magnitude and E-cadherin (cell-cell junction) concentration. The cell population was tracked for 3 days and the number of cells that leave the layer, the depth of invasion, and the percentage of initial number of cells that remain in the layer (a measure of epithelium disruption) were monitored. Our studies suggest that having a high protrusion force or a reduction in cell-cell attachments is enough to cause EMT. Our results also demonstrate that the morphological progression in membrane disruption has an effect on the number of cells becoming invasive, with epithelial layers broken into clusters hindering the further exodus of cells. The results reveal the quantitative interplay between two key parameters involved in EMT and suggest potential avenues for further exploration of a systems level understanding of EMT.

Integrins are transmembrane proteins that allow cells to bind to their external environment. They are the primary regulators of cell-matrix interactions, with direct roles in cell motility and signaling, which in turn regulate numerous physiological processes. Under common experimental conditions, integrins tend to cluster for sturdy and effective binding to extra-cellular matrix molecules. These clusters often evolve into focal adhesions, which regulate downstream signaling. However, integrin clusters are more pronounced and have longer lifetimes in two-dimensional assays than in more realistic three-dimensional environments. While a number of models and theoretical approaches have focused on integrin binding and diffusion, the reasons for the differences between two- and three-dimensional clustering have remained elusive. In this study, we model an individual cluster attached to a two-dimensional collagen film and attached to collagen fibers of various sizes in three-dimensional matrices. We then discuss how our results explain differences in size and lifetime, and how they hint at reasons for other differences between the two environments. Further, we make predictions regarding the stability of clusters based on different overall intracellular conditions. Our results show good agreement with experiments and provide a quantitative basis for understanding how matrix dimensionality and structure regulate integrin behavior in environments that mimic in vivo conditions.

Autoantibodies and the immunoreceptors to which they bind can contribute to the pathogenesis of autoimmune diseases such as rheumatoid arthritis (RA). Spleen Tyrosine Kinase (Syk) is a non-receptor tyrosine kinase with a central role in immunoreceptor (FcR) signaling and immune cell functionality. Syk kinase inhibitors have activity in antibody-dependent immune cell activation assays, in preclinical models of arthritis, and have progressed into clinical trials for RA and other autoimmune diseases. Here we describe the characterization of a novel triazolopyridine-based Syk kinase inhibitor, CC-509. This compound is a potent inhibitor of purified Syk enzyme, FcR-dependent and FcR-independent signaling in primary immune cells, and basophil activation in human whole blood. CC-509 is moderately selective across the kinome and against other non-kinase enzymes or receptors. Importantly, CC-509 was optimized away from and has modest activity against cellular KDR and Jak2, kinases that when inhibited in a preclinical and clinical setting may promote hypertension and neutropenia, respectively. In addition, CC-509 is orally bioavailable and displays dose-dependent efficacy in two rodent models of immune-inflammatory disease. In passive cutaneous anaphylaxis (PCA), CC-509 significantly inhibited skin edema. Moreover, CC-509 significantly reduced paw swelling and the tissue levels of pro-inflammatory cytokines RANTES and MIP-1α in the collagen-induced arthritis (CIA) model. In summary, CC-509 is a potent, moderately selective, and efficacious inhibitor of Syk that has a differentiated profile when compared to other Syk compounds that have progressed into the clinic for RA.

Biological networks are typically comprised of many parts whose interactions are governed by nonlinear dynamics. This potentially imbues them with the ability to support multiple attractors, and therefore to exhibit correspondingly distinct patterns of behavior. In particular, multiple attractors have been demonstrated for the electrical activity of the diseased heart in situations where cardioversion is able to convert a reentrant arrhythmia to a stable normal rhythm. Healthy hearts, however, are typically resilient to abnormal rhythms. This raises the question as to how a healthy cardiac cell network must be altered so that it can support multiple distinct behaviors. Here we demonstrate how anatomic defects can give rise to multi-stability in the heart as a function of the electrophysiological properties of the cardiac tissue and the timing of activation of ectopic foci. This leads to a form of hysteretic behavior, which we call dynamic entrapment, whereby the heart can become trapped in aberrant attractor as a result of a transient change in tissue properties. We show that this can lead to a highly inconsistent relationship between clinical symptoms and underlying pathophysiology, which raises the possibility that dynamic entrapment may underlie other forms of chronic idiopathic illness.

The goal of this study was to determine quantitative relationships between electrophysiologic parameters and the propensity of cardiac tissue to undergo atrial fibrillation. We used a computational model to simulate episodes of fibrillation, which we then characterized in terms of both their duration and the population dynamics of the electrical waves which drove them. Monte Carlo sampling revealed that episode durations followed an exponential decay distribution and wave population sizes followed a normal distribution. Half-lives of reentrant episodes increased exponentially with either increasing tissue area to boundary length ratio (A/BL) or decreasing action potential duration (APD), resistance (R) or capacitance (C). We found that the qualitative form of fibrillatory activity (e.g., multi-wavelet reentry (MWR) vs. rotors) was dependent on the ratio of resistance and capacitance to APD; MWR was reliably produced below a ratio of 0.18. We found that a composite of these electrophysiologic parameters, which we term the fibrillogenicity index (Fb = A/(BL*APD*R*C)), reliably predicted the duration of MWR episodes (r2 = 0.93). Given that some of the quantities comprising Fb are amenable to manipulation (via either pharmacologic treatment or catheter ablation), these findings provide a theoretical basis for the development of titrated therapies of atrial fibrillation.

Background— A key mechanism responsible for atrial fibrillation is multiwavelet re-entry (MWR). We have previously demonstrated improved efficiency of ablation when lesions were placed in regions of high circuit-density. In this study, we undertook a quantitative assessment of the relative effect of ablation on the probability of MWR termination and the inducibility of MWR, as a function of lesion length and circuit-density overlap. Methods and Results— We used a computational model to simulate MWR in tissues with (and without) localized regions of decreased action potential duration and increased intercellular resistance. We measured baseline circuit-density and distribution. We then assessed the effect of various ablation lesion sets on the inducibility and duration of MWR as a function of ablation lesion length and overlap with circuit-density. Higher circuit-density reproducibly localized to regions of shorter wavelength. Ablation lines with high circuit-density overlap showed maximum decreases in duration of MWR at lengths equal to the distance from the tissue boundary to the far side of the high circuit-density region (high-overlap, −43.5% [confidence interval, −22.0% to −65.1%] versus low-overlap, −4.4% [confidence interval, 7.3% to −16.0%]). Further ablation (beyond the length required to cross the high circuit-density region) provided minimal further reductions in duration and increased inducibility. Conclusions— Ablation at sites of high circuit-density most efficiently decreased re-entrant duration while minimally increasing inducibility. Ablation lines delivered at sites of low circuit-density minimally decreased duration yet increased inducibility of MWR.

Four species of Helicobacter--H. muridarum, "H. rappini," H. hepaticus, and H. bilis--have been identified in the gastrointestinal tract of rodents. The association of Helicobacter species with chronic gastrointestinal diseases in mice has raised concern about their impact on research results. In this study, different methods for detection of Helicobacter species in the mouse intestinal tract were compared: polymerase chain reaction (PCR) amplification of 16S rRNA gene sequences, bacterial culture, electron microscopy, and histologic examination (Steiner stain). The PCR method was more sensitive in detecting murine Helicobacter species than was culture, electron microscopy, or histologic examination. Of the cecal specimens identified as Helicobacter species-positive by PCR, approximately 60% were identified as positive by each of the other methods. An 87.5% concordance was obtained by PCR screening of DNA from fecal and cecal specimens. Differentiation among murine Helicobacter species by colony morphologic or histologic features was not possible. Scanning electron microscopy and histologic examination indicated greater numbers of helical microorganisms, specifically H. hepaticus, in the cecum than in the colon. These results indicate that the PCR assays used can be performed on feces as a noninvasive means for rapidly screening large numbers of colony mice for murine Helicobacter species.

A systematic approach to the management of patients with pancreatic abscess is presented. The method includes an aggressive diagnostic approach relying on radiographic and ultrasound examinations, adequate transperitoneal drainage of the abscess, coupled with gastrostomy and feeding jejunostomy. The importance of nutritional support using an elemental diet administered through a jejunostomy is emphasized.