Hemodynamic Forces Regulate Cardiac Regeneration-Responsive Enhancer Activity during Ventricle Regeneration

Cardiac regenerative capacity varies widely among vertebrates. Zebrafish can robustly regenerate injured hearts and are excellent models to study the mechanisms of heart regeneration. Recent studies have shown that enhancers are able to respond to injury and regulate the regeneration process. However, the mechanisms to activate these regeneration-responsive enhancers (RREs) remain poorly understood. Here, we utilized transient and transgenic analysis combined with a larval zebrafish ventricle ablation model to explore the activation and regulation of a representative RRE. lepb-linked enhancer sequence (LEN) directed enhanced green fluorescent protein (EGFP) expression in response to larval ventricle regeneration and such activation was attenuated by hemodynamic force alteration and mechanosensation pathway modulation. Further analysis revealed that Notch signaling influenced the endocardial LEN activity as well as endogenous lepb expression. Altogether, our work has established zebrafish models for rapid characterization of cardiac RREs in vivo and provides novel insights on the regulation of LEN by hemodynamic forces and other signaling pathways during heart regeneration.

Hemodynamic Force as a Potential Regulator of Inflammation-Mediated Focal Growth of Saccular Aneurysms in a Rat Model

Past studies have elucidated the crucial role of macrophage-mediated inflammation in the growth of intracranial aneurysms (IAs), but the contributions of hemodynamics are unclear. Considering the size of the arteries, we induced de novo aneurysms at the bifurcations created by end-to-side anastomoses with the bilateral common carotid arteries in rats. Sequential morphological data of induced aneurysms were acquired by magnetic resonance angiography. Computational fluid dynamics analyses and macrophage imaging by ferumoxytol were performed. Using this model, we found that de novo saccular aneurysms with a median size of 3.2 mm were induced in 20/45 (44%) of animals. These aneurysms mimicked human IAs both in morphology and pathology. We detected the focal growth of induced aneurysms between the 10th and 17th day after the anastomosis. The regional maps of hemodynamic parameters demonstrated the area exposed to low wall shear stress (WSS) and high oscillatory shear index (OSI) colocalized with the regions of growth. WSS values were significantly lower in the growing regions than in ones without growth. Macrophage imaging showed colocalization of macrophage infiltration with the growing regions. This experimental model demonstrates the potential contribution of low WSS and high OSI to the macrophage-mediated growth of saccular aneurysms.

Intraventricular hemodynamic forces do not differentiate between healthy controls and heart failure patients with preserved ejection fraction

Background Hemodynamic force analysis has been proposed as a noninvasive marker of cardiac function. In a recent study, longitudinal (apical-to-basal) hemodynamic forces were derived from anatomical MRI images and found decreased in heart failure with preserved ejection fraction (HFpEF) patients compared to controls, indicating a potential use for prognostication and testing of therapeutic response. This issue has not been investigated using the reference method of measurement. Purpose To investigate whether intraventricular hemodynamic forces computed using gold-standard cardiac magnetic resonance flow maps can reproducibly differentiate between healthy controls and HFpEF patients. Methods 4D flow data were acquired in 59 subjects through cardiac magnetic resonance imaging using a 1.5T scanner (Siemens Healthcare, Erlangen, Germany). Hemodynamic forces within the LV were computed across the cardiac cycle using the Navier-Stokes equation to find the global pressure gradient, which was then integrated over the LV volume to produce the instantaneous hemodynamic force (unit: Newton) and subsequently normalized to ventricular volume, resulting in a force-volume index (N/l). Average longitudinal forces (root mean square, FRMS) were quantified over the entire cardiac cycle, with and without volume normalization. Results We studied 33 healthy subjects, 14 patients with HFpEF, 6 patients with HFmEF and 6 patients with HFrEF. Groups were similar with regards to sex, cardiac output, heart rate, systolic and diastolic blood pressure, and body surface area. Volume-normalized FRMS did not differ between controls and HFpEF (0.86±0.19 vs. 0.75±0.19 N/l, p=0.08) while lower values were found in HFmEF (0.60±0.19 N/l, p=0.004) and HFrEF (0.38±0.15 N/l, p<0.0001) compared to controls (Figure 1A). There was a significant positive correlation between EF and FRMS, both for the entire population (R2 = 0.54, Figure 1B) and for patients (R2 = 0.67, p<0.0001 for both). Importantly, non-normalized FRMS did not differ between controls (Figure 1C, 0.10±0.03 N) and HFpEF (0.09±0.03 N, p=0.25), HFmEF (0.11±0.02 N, p=0.18) or HFrEF (0.09±0.02 N, p=0.67). Moreover, no correlation was seen between non-normalized FRMS and EF (Figure 1D). Conclusions Hemodynamic forces computed from reference standard 4D flow CMR data do not differentiate between healthy controls and HFpEF patients regardless of whether volume normalization is used or not. Our findings do not support a role for hemodynamic forces in HFpEF assessment. Figure 1. (A) Volume-normalized hemodynamic forces over the entire cardiac cycle (lines: average values, shaded area: ±1SD for HFpEF), and (B) variation of volume-normalized average force, FRMS, with left ventricular ejection fraction (LVEF). (C), (D): When indexing to LV volume was not performed, the differences between groups was attenuated, and no correlation was seen between EF and FRMS. Funding Acknowledgement Type of funding source: Public grant(s) – National budget only. Main funding source(s): Swedish Heart and Lung Foundation, Region of Scania

Hemodynamic Effects of Concentric and Eccentric Outflow Graft of LVADs on the Aortic Valve

Background: Aortic valve disease is a common complication of left ventricular assist device (LVAD) support. Optimizing the outflow graft anastomotic type of LVADs might be an alternative that can reduce this complication. However, the effect of this type of LVAD on the biomechanical states of the aortic valve remains unclear.Methods: In this study, a finite element-smoothed particle hydrodynamics-coupled model was established. Two kinds of anastomotic types (concentric and eccentric graft cases) were designed.Results: The anastomotic type could significantly affect the biomechanical states of the aortic valve. During the opening phase, the motion, deformation, and biomechanical states of the leaflet in both cases were similar to each other. The axial hemodynamic force (AHF) imposed on the leaflet in the eccentric graft case (0.9 N) was slightly larger than that in the concentric graft case (0.3 N). During the closing phase, the rapid closing time of the leaflet in the eccentric graft case (40 ms) was longer than that in the concentric graft case (15 ms). In addition, the peak value of the AHF in the concentric graft case was much larger (13 N) than that in the eccentric graft case (4.5 N). The oscillation of the AHF was observed only in the concentric graft case.Conclusions: The eccentric graft could lead to better biomechanical and hemodynamic states of the aortic valve than the concentric graft.

Similar Biophysical Abnormalities in Glomeruli and Podocytes from Two Distinct Models

Background FSGS is a pattern of podocyte injury that leads to loss of glomerular function. Podocytes support other podocytes and glomerular capillary structure, oppose hemodynamic forces, form the slit diaphragm, and have mechanical properties that permit these functions. However, the biophysical characteristics of glomeruli and podocytes in disease remain unclear.Methods Using microindentation, atomic force microscopy, immunofluorescence microscopy, quantitative RT-PCR, and a three-dimensional collagen gel contraction assay, we studied the biophysical and structural properties of glomeruli and podocytes in chronic (Tg26 mice [HIV protein expression]) and acute (protamine administration [cytoskeletal rearrangement]) models of podocyte injury.Results Compared with wild-type glomeruli, Tg26 glomeruli became progressively more deformable with disease progression, despite increased collagen content. Tg26 podocytes had disordered cytoskeletons, markedly abnormal focal adhesions, and weaker adhesion; they failed to respond to mechanical signals and exerted minimal traction force in three-dimensional collagen gels. Protamine treatment had similar but milder effects on glomeruli and podocytes.Conclusions Reduced structural integrity of Tg26 podocytes causes increased deformability of glomerular capillaries and limits the ability of capillaries to counter hemodynamic force, possibly leading to further podocyte injury. Loss of normal podocyte mechanical integrity could injure neighboring podocytes due to the absence of normal biophysical signals required for podocyte maintenance. The severe defects in podocyte mechanical behavior in the Tg26 model may explain why Tg26 glomeruli soften progressively, despite increased collagen deposition, and may be the basis for the rapid course of glomerular diseases associated with severe podocyte injury. In milder injury (protamine), similar processes occur but over a longer time.

Force and torque on spherical particles in micro-channel flows using computational fluid dynamics

To delineate the influence of hemodynamic force on cell adhesion processes, model in vitro fluidic assays that mimic physiological conditions are commonly employed. Herein, we offer a framework for solution of the three-dimensional Navier–Stokes equations using computational fluid dynamics (CFD) to estimate the forces resulting from fluid flow near a plane acting on a sphere that is either stationary or in free flow, and we compare these results to a widely used theoretical model that assumes Stokes flow with a constant shear rate. We find that while the full three-dimensional solutions using a parabolic velocity profile in CFD simulations yield similar translational velocities to those predicted by the theoretical method, the CFD approach results in approximately 50% larger rotational velocities over the wall shear stress range of 0.1–5.0 dynes cm −2 . This leads to an approximately 25% difference in force and torque calculations between the two methods. When compared with experimental measurements of translational and rotational velocities of microspheres or cells perfused in microfluidic channels, the CFD simulations yield significantly less error. We propose that CFD modelling can provide better estimations of hemodynamic force levels acting on perfused microspheres and cells in flow fields through microfluidic devices used for cell adhesion dynamics analysis.