Distinct cytoskeletal regulators of mechanical memory in cardiac fibroblasts and cardiomyocytes.

Recognizing that cells "feel" and respond to their mechanical environment, recent studies demonstrate that many cells exhibit a phenomenon of "mechanical memory" in which features induced by prior mechanical cues persist after the mechanical stimulus has ceased. While there is a general recognition that different cell types exhibit different responses to changes in extracellular matrix stiffening, the phenomenon of mechanical memory within myocardial cell types has received little attention to date. To probe the dynamics of mechanical memory in cardiac fibroblasts (CFs) and cardiomyocytes derived from human induced pluripotent stem cells (iPSC-CMs), we employed a magnetorheological elastomer (MRE) cell culture substrate with tunable and reversible stiffness spanning the range from normal to diseased myocardium. In CFs, using increased cell area and increases in α-smooth muscle actin as markers of cellular responses to matrix stiffening, we found that induction of mechanical memory required seven days of stiff priming. Both induction and maintenance of persistent CF activation were blocked with the F-actin inhibitor cytochalasin D, while inhibitors of microtubule detyrosination had no impact on CFs. In iPSC-CMs, mechanical memory was invoked after only 24 h of stiff priming. Moreover, mechanical memory induction and maintenance were microtubule-dependent in CMs with no dependence on F-actin. Overall, these results identify the distinct temporal dynamics of mechanical memory in CFs and iPSC-CMs with different cytoskeletal mediators responsible for inducing and maintaining the stiffness-activated phenotype. Due to its flexibility, this model is broadly applicable to future studies interrogating mechanotransduction and mechanical memory in the heart and might inform strategies for attenuating the impact of load-induced pathology and excess myocardial stiffness.

Transcriptomic analysis of a 3D blood-brain barrier model exposed to disturbed fluid flow.

Cerebral aneurysms are more likely to form at bifurcations in the vasculature, where disturbed fluid is prevalent due to flow separation at sufficiently high Reynolds numbers. While previous studies have demonstrated that altered shear stress exerted by disturbed flow disrupts endothelial tight junctions, less is known about how these flow regimes alter gene expression in endothelial cells lining the blood-brain barrier. Specifically, the effect of disturbed flow on expression of genes associated with cell-cell and cell-matrix interaction, which likely mediate aneurysm formation, remains unclear. RNA sequencing of immortalized cerebral endothelial cells isolated from the lumen of a 3D blood-brain barrier model reveals distinct transcriptional changes in vessels exposed to fully developed and disturbed flow profiles applied by both steady and physiological waveforms. Differential gene expression, validated by qRT-PCR and western blotting, reveals that lumican, a small leucine-rich proteoglycan, is the most significantly downregulated gene in endothelial cells exposed to steady, disturbed flow. Knocking down lumican expression reduces barrier function in the presence of steady, fully developed flow. Moreover, adding purified lumican into the hydrogel of the 3D blood-brain barrier model recovers barrier function in the region exposed to fully developed flow. Overall, these findings emphasize the importance of flow regimes exhibiting spatial and temporal heterogeneous shear stress profiles on cell-matrix interaction in endothelial cells lining the blood-brain barrier, while also identifying lumican as a contributor to the formation and maintenance of an intact barrier.

Implementation and characterization of a physiologically relevant flow waveform in a 3D microfluidic model of the blood-brain barrier.

Previous in vitro studies interrogating the endothelial response to physiologically relevant flow regimes require specialized pumps to deliver time-dependent waveforms that imitate in vivo blood flow. The aim of this study is to create a low-cost and broadly adaptable approach to mimic physiological flow, and then use this system to characterize the effect of flow separation on velocity and shear stress profiles in a three-dimensional (3D) topology. The flow apparatus incorporates a programmable linear actuator that superposes oscillations on a constant mean flow driven by a peristaltic pump to emulate flow in the carotid artery. The flow is perfused through a 3D in vitro model of the blood-brain barrier designed to induce separated flow. Experimental flow patterns measured by microparticle image velocimetry and modeled by computational fluid dynamics reveal periodic changes in the instantaneous shear stress along the channel wall. Moreover, the time-dependent flow causes periodic flow separation zones, resulting in variable reattachment points during the cycle. The effects of these complex flow regimes are assessed by evaluating the integrity of the in vitro blood-brain barrier model. Permeability assays and immunostaining for proteins associated with tight junctions reveal barrier breakdown in the region of disturbed flow. In conclusion, the flow system described here creates complex, physiologically relevant flow profiles that provide deeper insight into the fluid dynamics of separated flow and pave the way for future studies interrogating the cellular response to complex flow regimes.

Disturbed flow disrupts the blood-brain barrier in a 3D bifurcation model.

The effect of disturbed flow profiles on the endothelium have been studied extensively in systemic vasculature, but less is known about the response of the blood-brain barrier (BBB) to these flow regimes. Here we investigate the effect of disturbed flow on the integrity of the BBB using a three-dimensional, perfusable bifurcation model consisting of a co-culture of endothelial cells with mural and glial cells. Experimental flow patterns predicted by computational fluid dynamics mimic in vivo flow regimes, specifically the presence of a recirculation zone immediately downstream of the bifurcation. Dextran permeability assays and immunostaining with markers for tight junctions show that barrier disruption is significantly greater in areas of disturbed flow compared to fully developed regions downstream of the bifurcation. Probing crosstalk between cell types suggests that disturbed flow causes barrier breakdown independent of endothelial-mural and endothelial-glial interaction. Overall, disturbed flow-induced disruption of the blood-brain barrier suggests that flow-mediated mechanisms may contribute to vascular pathologies in the central nervous system.

Hyaluronan Disrupts Cardiomyocyte Organization within 3D Fibrin-Based Hydrogels.

The extracellular matrix in vivo contains variable but often large amounts of glycosaminoglycans that influence cell and tissue function. Hyaluronan (HA) is an abundant glycosaminoglycan within the extracellular matrix of the myocardium during early development and in the aftermath of a myocardial infarction. Its flexible anionic structure has a strong influence on mechanical response and interstitial fluid flow within the matrix. Additionally, HA has a direct, biochemical effect on cells through an array of cell-surface receptors, including CD44, RHAMM/CD168, and other surface-exposed structures. Recent studies have shown that HA modulates the response of cardiomyocytes and other cell types to two-dimensional substrates of varying elastic moduli. This study investigates the force response to HA of cardiomyocytes and cardiac fibroblasts within three-dimensional matrices of variable composition and mechanical properties in vitro. HA significantly decreased the force exerted by the cell-matrix constructs in a tensiometer testing platform and within microfabricated tissue gauges. However, its effect was no different from that of alginate, an anionic polysaccharide with the same charge density but no specific transmembrane receptors. Therefore, these results establish that HA exerts a generic physical-chemical effect within three-dimensional hydrogels that must be accounted for when interrogating cell-matrix interactions.

Impact of Design Changes in Gastrostomy Tube (G-tube) Devices for Patients Who Rely on Home-Based Blenderized Diets for Enteral Nutrition.

OBJECTIVE: Blenderized diets are gaining increasing popularity among enteral tube users. Connectors in gastrostomy tubes (G-tubes) are undergoing standardization to reduce misconnections. These standardized G-tubes are referred to as ENFit G-tubes. This study was performed to quantify the in vitro performance of existing (legacy) G-tubes and compare them with ENFit G-tubes for blenderized diets. METHOD: Patient blenderized diet recipes and practices were obtained through patient advocacy groups. Different blenders and blending times were studied. Five legacy G-tube brands and three corresponding ENFit brands, sized between 14 Fr and 24 Fr, were studied under gravity and push modes of feeding. RESULTS: Considering both thin and thick blenderized gravity mode diets, an average increase in feeding time from 20 minutes to 32 ± 18 minutes in transitioning from legacy to ENFit was observed with standard G-tubes, compared to 22 ± 3.5 minutes for low profiles. For push-mode diets, a 60-second push with standard ENFit G-tubes was easier compared to standard legacy G-tubes (61% ± 21% as much force), but faster 5-second pushes required considerably more effort for ENFit standard G-tubes (167% ± 96%). Low-profile ENFit G-tubes required slightly less effort compared to low-profile legacies for both 60-second and 5-second pushes (72% ± 22% and 90% ± 19%, respectively). Clogging was common in both legacy and ENFit devices, particularly under gravity mode. CONCLUSIONS: For a push mode of feeding, patients will largely be unimpacted after the transition to ENFit. For a gravity mode of feeding, some ENFit users may need higher-powered blenders and should expect increased feeding times.