Biology subjects

del Alamo, J. C.

Publications and source records attributed to del Alamo, J. C..

25 records · Page 2Linked to original sources

bioRxiv2021

Non-Newtonian Patient-specific Simulations of Left Atrial Hemodynamics

The lack of mechanically effective contraction of the left atrium (LA) during atrial fibrillation (AF) disturbs blood flow, increasing the risk of thrombosis and ischemic stroke. Thrombosis is most likely in the left atrial appendage (LAA), a small narrow sac where blood is prone to stagnate. Slow flow promotes the formation of erythrocyte aggregates in the LAA, also known as rouleaux, causing viscosity gradients that are usually disregarded in patient-specific simulations. To evaluate these non-Newtonian effects, we built atrial models derived from 4D computed tomography scans of patients and carried out computational fluid dynamics simulations using the Carreau-Yasuda constitutive relation. We examined six patients, three of whom had AF and LAA thrombosis or a history of transient ischemic attacks (TIAs). We modeled the effects of hematocrit and rouleaux formation kinetics by varying the parameterization of the Carreau-Yasuda relation and modulating non-Newtonian viscosity changes based on residence time. Comparing non-Newtonian and Newtonian simulations indicates that slow flow in the LAA increases blood viscosity, altering secondary swirling flows and intensifying blood stasis. While some of these effects can be subtle when examined using instantaneous metrics like shear rate or kinetic energy, they are manifested in the blood residence time, which accumulates over multiple heartbeats. Our data also reveal that LAA blood stasis worsens when hematocrit increases, offering a potential new mechanism for the clinically reported correlation between hematocrit and stroke incidence. In summary, we submit that hematocrit-dependent non-Newtonian blood rheology should be considered in calculating patient-specific blood stasis indices by computational fluid dynamics.

bioengineering↗

bioRxiv2020

A balance between matrix deformation and the coordination of turning events governs directed neutrophil migration in 3-D matrices

Three-dimensional (3-D) neutrophil migration is essential for immune surveillance and inflammatory responses. During 3-D migration, especially through extravascular spaces, neutrophils rely on frontal protrusions and rear contractions to squeeze and maneuver through extracellular matrices containing narrow pores. However, the role of matrix density and the cells ability to probe and remodel matrix pores during 3-D chemotaxis are far from being understood. We investigated these processes by tracking the trajectories of over 20,000 neutrophils in a 3-D migration device containing collagen matrices of varying concentrations and analyzing the shape of these trajectories at multiple scales. Additionally, we quantified the transient 3-D matrix deformations caused by the migrating cells. The mean pore size of our reconstituted collagen matrices decreased when the collagen concentration ([col]) was increased. In low-[col] matrices, neutrophils exerted large transient deformations and migrated in relatively straight trajectories. In contrast, they were not able to appreciably deform high- [col] matrices and adapted to this inability by turning more often to circumvent these narrow matrix pores. While this adaptation resulted in slower migration, the cells were able to balance the more frequent turning with the long-range directional bias necessary for chemotaxis. Based on our statistical analysis of cell trajectories, we postulate that neutrophils achieve this balance by using matrix obstacles as pivoting points to steer their motion towards the chemoattractant. Inhibiting myosin-II contractility or Arp2/3-mediated pseudopod protrusions not only compromised the cells ability to deform the matrix, but also made them switch to increased turning in more restrictive matrices when compared to untreated control cells. Both myosin-II contractility and Arp2/3-mediated branched polymerization of actin played a role in fast migration, but Arp2/3 was also crucial for neutrophils when coordinating the orientations of successive turns to prevent veering away from the chemotactic path. These results may contribute to an improved understanding of the mechanisms employed by migrating neutrophils in confined 3-D environments, as well as the molecular and environmental regulators for maintaining persistent motion.

biophysics↗

bioRxiv2020

Demonstration of Patient-Specific Simulations To Assess Left Atrial Appendage Thrombogenesis Risk

Atrial fibrillation (AF) alters left atrial (LA) hemodynamics, which can lead to thrombosis in the left atrial appendage (LAA), systemic embolism and stroke. A personalized risk-stratification of AF patients for stroke would permit improved balancing of preventive anticoagulation therapies against bleeding risk. We investigated how LA anatomy and function impact LA and LAA hemodynamics, and explored whether patient-specific analysis by computational fluid dynamics (CFD) can predict the risk of LAA thrombosis. We analyzed 4D-CT acquisitions of LA wall motion with an in-house immersed-boundary CFD solver. We considered six patients with diverse atrial function, three without a LAA thrombus (LAAT/TIA-neg), and three with either a LAA thrombus (removed digitally before running the simulations) or a history of transient ischemic attacks (LAAT/TIA-pos). We found that blood inside the left atrial appendage of LAAT/TIA-pos patients had marked alterations in residence time and kinetic energy when compared with LAAT/TIA-neg patients. In addition, we showed how the LA conduit, reservoir and booster functions distinctly affect LA and LAA hemodynamics. While the flow dynamics of fixed-wall and moving-wall simulations differ significantly, fixed-wall simulations risk-stratified our small cohort for LAA thrombosis only slightly worse than moving-wall simulations.

bioengineering↗

bioRxiv2019

Regional dynamics of fractal dimension of the LV endocardium from cine CT images

We present a method to leverage the high fidelity of CT to quantify regional left ventricular function using topography variation of the endocardium as a surrogate measure of strain. 4DCT images of 10 normal and 10 abnormal subjects, acquired with standard clinical protocols, were used. The topography of the endocardium was characterized by its regional values of fractal dimension (FD), computed using a box-counting algorithm developed in-house. The average FD in each of the 16 American Heart Association segments was calculated for each subject as a function of time over the cardiac cycle. The normal subjects showed a peak systolic percentage change in FD of 5.9% {+/-} 2% in all free-wall segments, while the abnormal cohort experienced a change of 2% {+/-} 1.2% (p < 0.00001). Septal segments, being smooth, did not undergo large changes in FD. Additionally, a principal component analysis was performed on the temporal profiles of FD to highlight the possibility for unsupervised classification of normal and abnormal function. The method developed is free from manual contouring and does not require any feature tracking or registration algorithms. The FD values in the free wall segments correlated well with radial strain and with endocardial regional shortening measurements.

bioengineering↗

bioRxiv2019

Symmetry breaking transition towards directional locomotion in Physarum microplasmodia

True slime mold Physarum polycephalum has been widely used as a model organism to study flow-driven amoeboid locomotion as well as the dynamics of its complex mechanochemical self-oscillations. The aim of this work is to quantify the mechanical aspects of symmetry breaking and its transition into directional flow-driven amoeboid locomotion in small (<[~] 200 {micro}m) fragments of Physarum polycephalum. To this end, we combined measurements of traction stresses, fragment morphology, and ectoplasmic microrheology with experimental manipulations of cell-substrate adhesion, cortical strength and microplasmodium size. These measurements show that initiation of locomotion is accompanied by the symmetry breaking of traction stresses and the polarization of ectoplasmic mechanical properties, with the rear part of the microplasmodium becoming significantly stiffer after the onset of locomotion. Our experimental data suggests that the initiation of locomotion in Physarum could be analogous to an interfacial instability process and that microplasmodial size is a critical parameter governing the instability. Specifically, our results indicate that the instability driving the onset of locomotion is strengthened by substrate adhesiveness and weakened by cortical stiffness. Furthermore, the Fourier spectral analysis of morphology revealed lobe number n = 2 as the consistent dominant mode number across various experimental manipulations, suggesting that the instability mechanism driving the onset of Physarum locomotion is robust with respect to changes in environmental conditions and microplasmodial properties.

biophysics↗

bioRxiv2019

Three-dimensional Monolayer Stress Microscopy

Many biological processes involve the collective generation and transmission of mechanical stresses across cell monolayers. In these processes, the monolayer undergoes lateral deformation and bending due to the tangential and normal components of the cell-generated stresses. Monolayer Stress Microscopy (MSM) methods have been developed to measure the intracellular stress distribution in cell monolayers. However, current methods assume plane monolayer geometry and neglect the contribution of bending to the intracellular stresses.\n\nThis work introduces a three-dimensional (3D) MSM method that calculates monolayer stress from measurements of the 3D traction stresses exerted by the cells on a flexible substrate. The calculation is carried out by imposing equilibrium of forces and moments in the monolayer, subject to external loads given by the 3D traction stresses. The equilibrium equations are solved numerically, and the algorithm is validated for synthetic loads with known analytical solutions.\n\nWe present 3D-MSM measurements of monolayer stress in micropatterned islands of endothelial cells of different sizes and shapes. These data indicate that intracellular stresses caused by lateral deformation emerge collectively over long distances; they increase with the distance from the island edge until they reach a constant value that is independent of island size. On the other hand, bending-induced intracellular stresses are more concentrated spatially and remain confined to within 1-2 cell lengths of bending sites. The magnitude of these bending stresses is highest at the edges of the cell islands, where they can exceed the intracellular stresses caused by lateral deformations. Our data from non-patterned monolayers suggests that biomechanical perturbations far away from monolayer edges also cause significant localized alterations in bending tension. The localized effect of bending-induced stresses may be important in processes like cellular extravasation, which are accompanied by significant normal deflections of a cell monolayer (i.e. the endothelium), and which require localized changes in monolayer permeability.

biophysics↗

bioRxiv2019

Quantifying the mechanics of locomotion of the schistosome pathogen with respect to changes in its physical environment

Schistosomiasis is a chronic and morbid disease of poverty affecting approximately 200 million people worldwide. Mature schistosome flatworms wander in the hosts hepatic portal and mesenteric venous system where they encounter a range of blood flow conditions and geometrical confinement. However, the mechanisms that support schistosome locomotion and underlie the pathogens adaptation to its physical environment are largely unknown. By combining microfabrication and traction force microscopy, we developed various in vitro assays to quantify the mechanics of locomotion of adult male S. mansoni in different physiologically relevant conditions. We show that in unconfined settings, the parasite undergoes two-anchor marching mediated by the coordinated action of its oral and ventral suckers. This mode of locomotion is maintained when the worm faces an external flow, to which it responds by adjusting the strength of its suckers. In geometrically confined conditions, S. mansoni switches to a different crawling modality by generating retrograde peristaltic waves along its body, a mechanism shared with terrestrial and marine worms. But while the surface of most worms has backward-pointing bristles that rectify peristaltic waves and facilitate forward locomotion, S mansoni has isotropically oriented tubercles. This requires tight coordination between muscle contraction and substrate friction but confers S. mansoni the ability to reverse its direction of locomotion without turning its body, which is likely advantageous to maneuver in narrow bore vessels. We show that the parasite can also coordinate the action of its suckers with its peristaltic body contractions to increase crawling speed. Throughout this study, we report on a number of biomechanical parameters to quantify the motility of adult schistosomes (e.g. sucker grabbing strength, rate of detachment under flow, peristaltic wave properties and traction stresses). The new series of in vitro assays make it possible to quantify key phenotypical aspects of S. mansoni motility that could guide the discovery of new drugs to treat schistosomiasis.

biophysics↗