Topographical depth reveals contact guidance mechanism distinct from focal adhesion confinement

Cellular response to the topography of their environment, known as contact guidance, is a crucial aspect to many biological processes yet remains poorly understood. A prevailing model to describe cellular contact guidance involves the lateral confinement of focal adhesions (FA) by topography as an underlying mechanism governing how cells can respond to topographical cues. However, it is not clear how this model is consistent with the well-documented depth-dependent contact guidance responses in the literature. To investigate this model, we fabricated a set of contact guidance chips with lateral dimensions capable of confining focal adhesions and relaxing that confinement at various depths. We find at the shallowest depth of 330 nm, the model of focal adhesion confinement is consistent with our observations. However, the cellular response at depths of 725 and 1000 nm is inadequately explained by this model. Instead, we observe a distinct reorganization of F-actin at greater depths in which topographically induced cell membrane deformation alters the structure of the cytoskeleton. These results are consistent with an alternative curvature-hypothesis to explain cellular response to topographical cues. Together, these results indicate a confluence of two molecular mechanisms operating at increased induced membrane curvature that govern how cells sense and respond to topography.

Interfacing Live Cells with Surfaces: A Concurrent Control Technique for Quantifying Surface Ligand Activity

Surface ligand activity is a key design parameter for successfully interfacing surfaces with cells─whether in the context of in vitro investigations for understanding cellular signaling pathways or more applied applications in drug delivery and medical implants. Unlike other crucial surface parameters, such as stiffness and roughness, surface ligand activity is typically based on a set of assumptions rather than directly measured, giving rise to interpretations of cell adhesion that can vary with the assumptions made. To fill this void, we have developed a concurrent control technique for directly characterizing in vitro ligand surface activity. Pairs of gold-coated glass chips were biofunctionalized with RGD ligand in a parallel workflow: one chip for in vitro applications and the other for surface plasmon resonance (SPR)-based RGD activity characterization. Recombinant α_Vβ₃ integrins were injected over the SPR chip surface as mimics of the cellular-membrane-bound receptors and the resulting binding kinetics parameterized to quantify surface ligand activity. These activity measurements were correlated with cell morphological features, measured by interfacing MDA-MB-231 cells with the in vitro chip surfaces on the live cell microscope. We demonstrate how the interpretation of a cell phenotype based on direct activity measurements can vary markedly from interpretations based on assumed activity. The SPR concurrent control approach has multiple advantages due to the fact that SPR is a standardized technique and has the sensitivity to measure ligand activity across the most relevant range of extracellular surface densities, while the in vitro chip design can be used with all commonly used light microscopy modalities (e.g., phase contrast, DIC, and fluorescence) so that a wide range of phenotypic and molecular markers can be correlated to the ligand surface activity.

Abstract MP114: Cardiotropic Adenovirus Increases Arrhythmia Susceptibility During Acute Infection

Myocarditis is responsible for up to 42% of sudden cardiac death in young adults yet mechanisms underlying virally-induced arrhythmia remain elusive. Adenovirus is a leading etiological agent of myocarditis but due to species-specificity, in vivo models are limited. Altered cardiac gap junction function underlies sudden cardiac death and given that gap junctions also propagate antiviral immune responses, we hypothesized adenovirus would target intercellular coupling, leading to arrhythmogenesis. Employing a recently described strain of mouse adenovirus, MAdV-3, we investigated cardiotropism, arrhythmia susceptibility, and molecular mechanisms of a virally-induced arrhythmogenic substrate. Adult C57BL/6 mice were inoculated with MAdV-3 and infection allowed to progress for one week. Viral genomes were detected by qPCR to measure tissue tropism together with whole animal histopathology. Echo- and electrocardiography were used to measure cardiac function and electrical activity. Viral effects on transcription and junctional protein/ion channel expression and localization were quantified using RT-qPCR, confocal microscopy, and western blotting. Finally, ex vivo optical mapping was employed to test conduction defects and susceptibility to arrhythmia. We find MAdV-3 viral genomes specifically enriched in heart tissue, confirming cardiotropism. Immune cell infiltration was not detected in the heart and no cardiomyopathy was apparent by echocardiography. RR interval widening, however, does occur in MAdV-3 infected mice with reductions in cardiac ion channel and gap junction mRNA transcripts. Optical mapping experiments show that conduction velocity is altered in acutely infected hearts and norepinephrine challenge uncovered electrical impairment presenting as atrioventricular dissociation with increased premature ventricular contraction burden. Our data demonstrate that MAdV-3 is cardiotropic and acute infection result in changes in cardiac electrical activity prior to development of cardiomyopathy or a detectable immune response. MAdV-3 infection is a novel model for adenoviral myocarditis, providing insights into viral subversion of cardiac intercellular coupling and mechanisms of arrhythmogenesis.

Adenovirus targets transcriptional and posttranslational mechanisms to limit gap junction function

Adenoviruses are responsible for a spectrum of pathogenesis including viral myocarditis. The gap junction protein connexin43 (Cx43, gene name GJA1) facilitates rapid propagation of action potentials necessary for each heartbeat. Gap junctions also propagate innate and adaptive antiviral immune responses, but how viruses may target these structures is not understood. Given this immunological role of Cx43, we hypothesized that gap junctions would be targeted during adenovirus type 5 (Ad5) infection. We find reduced Cx43 protein levels due to decreased GJA1 mRNA transcripts dependent upon β-catenin transcriptional activity during Ad5 infection, with early viral protein E4orf1 sufficient to induce β-catenin phosphorylation. Loss of gap junction function occurs prior to reduced Cx43 protein levels with Ad5 infection rapidly inducing Cx43 phosphorylation events consistent with altered gap junction conductance. Direct Cx43 interaction with ZO-1 plays a critical role in gap junction regulation. We find loss of Cx43/ZO-1 complexing during Ad5 infection by co-immunoprecipitation and complementary studies in human induced pluripotent stem cell derived-cardiomyocytes reveal Cx43 gap junction remodeling by reduced ZO-1 complexing. These findings reveal specific targeting of gap junction function by Ad5 leading to loss of intercellular communication which would contribute to dangerous pathological states including arrhythmias in infected hearts.

Abstract 505: Adenovirus Targets Connexin43 Gap Junction Expression and Function During Infection

Introduction: Myocarditis leads to pathologic remodeling of the myocardium and arrhythmia. A leading etiological agent of viral myocarditis is human Adenovirus type 5 (Ad5). Individuals affected are at a higher risk of arrhythmogenesis which can lead to sudden cardiac death. Gap junctions, comprised of connexin proteins, provide direct intercellular communication between cardiomyocytes to facilitate electrical coupling. Connexin43 (Cx43) is the predominant connexin expressed in the working myocardium and is tightly regulated by post-translational modifications. Gap junctions are also known to propagate the innate antiviral immune response but the impact of Ad5 infection on Cx43 expression and function is essentially unexplored.

Hypothesis: During Ad5 infection Cx43 gap junctions are directly targeted to limit intercellular communication thus precipitating an arrhythmogenic substrate.

Methods: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), keratinocytes (HaCaT), and lentivirus-transduced HaCaTs stably overexpressing Cx43 were infected with Ad5. Biochemical fractionation and pulse chase assays were implemented to determine gap junction and Cx43 stability. Cx43 protein levels and phosphorylation status were determined by western blotting and immunoprecipitation experiments. Confocal immunofluorescence microscopy was employed determine localization and functional of Cx43 during Ad5 infection.

Results: Cx43 expression is targeted early in infection with remaining protein localized at the cell-cell boarder during infection. Phosphorylation of Cx43-Ser368 was detected during early Ad5 infection implicating reduced gap junction conductance. In contrast phosphorylation of Cx43-Ser262 is reduced during Ad5 infection with a concomitant increase in protein half-life despite losses in functional gap junction intercellular communication.

Conclusion: Cx43 gap junction structures are stabilized during Ad5 infection but directly targeted by specific phosphorylation events to effect channel closure. Insight provided into mechanisms by which Ad5 manipulates Cx43 regulation will provide novel therapeutic avenues to target gap junction function in a broad spectrum of heart disease states.

Modulating cardiac conduction during metabolic ischemia with perfusate sodium and calcium in guinea pig hearts

We previously demonstrated that altering extracellular sodium (Na_o) and calcium (Ca_o) can modulate a form of electrical communication between cardiomyocytes termed "ephaptic coupling" (EpC), especially during loss of gap junction coupling. We hypothesized that altering Na_o and Ca_o modulates conduction velocity (CV) and arrhythmic burden during ischemia. Electrophysiology was quantified by optically mapping Langendorff-perfused guinea pig ventricles with modified Na_o (147 or 155 mM) and Ca_o (1.25 or 2.0 mM) during 30 min of simulated metabolic ischemia (pH 6.5, anoxia, aglycemia). Gap junction-adjacent perinexal width ( W_P), a candidate cardiac ephapse, and connexin (Cx)43 protein expression and Cx43 phosphorylation at S368 were quantified by transmission electron microscopy and Western immunoblot analysis, respectively. Metabolic ischemia slowed CV in hearts perfused with 147 mM Na_o and 2.0 mM Ca_o; however, theoretically increasing EpC with 155 mM Na_o was arrhythmogenic, and CV could not be measured. Reducing Ca_o to 1.25 mM expanded W_P, as expected during ischemia, consistent with reduced EpC, but attenuated CV slowing while delaying arrhythmia onset. These results were further supported by osmotically reducing W_P with albumin, which exacerbated CV slowing and increased early arrhythmias during ischemia, whereas mannitol expanded W_P, permitted conduction, and delayed the onset of arrhythmias. Cx43 expression patterns during the various interventions insufficiently correlated with observed CV changes and arrhythmic burden. In conclusion, decreasing perfusate calcium during metabolic ischemia enhances perinexal expansion, attenuates conduction slowing, and delays arrhythmias. Thus, perinexal expansion may be cardioprotective during metabolic ischemia. NEW & NOTEWORTHY This study demonstrates, for the first time, that modulating perfusate ion composition can alter cardiac electrophysiology during simulated metabolic ischemia.

Altered translation initiation of Gja1 limits gap junction formation during epithelial-mesenchymal transition

Epithelial-mesenchymal transition (EMT) is activated during development, wound healing, and pathologies including fibrosis and cancer metastasis. Hallmarks of EMT are remodeling of intercellular junctions and adhesion proteins, including gap junctions. The GJA1 mRNA transcript encoding the gap junction protein connexin43 (Cx43) has been demonstrated to undergo internal translation initiation, yielding truncated isoforms that modulate gap junctions. The PI3K/Akt/mTOR pathway is central to translation regulation and is activated during EMT, leading us to hypothesize that altered translation initiation would contribute to gap junction loss. Using TGF-β-induced EMT as a model, we find reductions in Cx43 gap junctions despite increased transcription and stabilization of Cx43 protein. Biochemical experiments reveal suppression of the internally translated Cx43 isoform, GJA1-20k in a Smad3 and ERK-dependent manner. Ectopic expression of GJA1-20k does not halt EMT, but is sufficient to rescue gap junction formation. GJA1-20k localizes to the Golgi apparatus, and using superresolution localization microscopy we find retention of GJA1-43k at the Golgi in mesenchymal cells lacking GJA1-20k. NativePAGE demonstrates that levels of GJA1-20k regulate GJA1-43k hexamer oligomerization, a limiting step in Cx43 trafficking. These findings reveal alterations in translation initiation as an unexplored mechanism by which the cell regulates Cx43 gap junction formation during EMT.

TNFα Modulates Cardiac Conduction by Altering Electrical Coupling between Myocytes

Background: Tumor Necrosis Factor α (TNFα) upregulation during acute inflammatory response has been associated with numerous cardiac effects including modulating Connexin43 and vascular permeability. This may in turn alter cardiac gap junctional (GJ) coupling and extracellular volume (ephaptic coupling) respectively. We hypothesized that acute exposure to pathophysiological TNFα levels can modulate conduction velocity (CV) in the heart by altering electrical coupling: GJ and ephaptic.

Methods and Results: Hearts were optically mapped to determine CV from control, TNFα and TNFα + high calcium (2.5 vs. 1.25 mM) treated guinea pig hearts over 90 mins. Transmission electron microscopy was performed to measure changes in intercellular separation in the gap junction-adjacent extracellular nanodomain—perinexus (W_P). Cx43 expression and phosphorylation were determined by Western blotting and Cx43 distribution by confocal immunofluorescence. At 90 mins, longitudinal and transverse CV (CV_L and CV_T, respectively) increased with control Tyrode perfusion but TNFα slowed CV_T alone relative to control and anisotropy of conduction increased, but not significantly. TNFα increased W_P relative to control at 90 mins, without significantly changing GJ coupling. Increasing extracellular calcium after 30 mins of just TNFα exposure increased CV_T within 15 mins. TNFα + high calcium also restored CV_T at 90 mins and reduced W_P to control values. Interestingly, TNFα + high calcium also improved GJ coupling at 90 mins, which along with reduced W_P may have contributed to increasing CV.

Conclusions: Elevating extracellular calcium during acute TNFα exposure reduces perinexal expansion, increases ephaptic, and GJ coupling, improves CV and may be a novel method for preventing inflammation induced CV slowing.