Zahra Mohri

Low shear stress induces M1 macrophage polarization in murine thin-cap atherosclerotic plaques.

Macrophages, a significant component of atherosclerotic plaques vulnerable to acute complications, can be pro-inflammatory (designated M1), regulatory (M2), lipid- (Mox) or Heme-induced (Mhem). We showed previously that low (LSS) and oscillatory (OSS) shear stress cause thin-cap fibroatheroma and stable smooth muscle cell-rich plaque formation respectively in ApoE-knockout (ApoE(-/-)) mice. Here we investigated whether different shear stress conditions relate to specific changes in macrophage polarization and plaque morphology by applying a shear stress-altering cast to the carotid arteries of high fat-fed ApoE(-/-) mice. The M1 markers iNOS and IRF5 were highly expressed in macrophage-rich areas of LSS lesions compared to OSS lesions 6weeks after cast placement, while the M2 marker Arginase-1, and Mox/Mhem markers HO-1 and CD163 were elevated in OSS lesions. Our data indicates shear stress could be an important determinant of macrophage polarization in atherosclerosis, with low shear promoting M1 programming.

High throughput en face mapping of arterial permeability using tile scanning confocal microscopy.

Elevated uptake of plasma macromolecules by the arterial wall has been implicated in the initiation of atherosclerosis. Here we describe a new method for mapping such uptake in laboratory animals. Albumin was labelled with a fluorescent dye and administered intravenously. After 10 min, the aorta was fixed in situ, excised and opened. En face confocal microscopy employing a computer-controlled stage was used to obtain contiguous tiles, each consisting of a stack of images of fluorescence emission at different depths in the wall. To obtain two-dimensional maps, intensities were summed in each column of voxels starting at the endothelial surface and extending 10 μm into the wall. Variation in the sensitivity of the system with time and in all three spatial directions was assessed and corrected using calibration standards and model specimens. In immature rabbits, uptake around aorto-intercostal branches was greatest in an arrowhead-shaped region around the downstream half of each ostium, and at its lateral margins. Uptake around branches in mature rabbits was more uniform; it was highest upstream of the ostium. Patches and streaks of high uptake were also seen at non-branch locations in the descending thoracic aorta. Transport was more uniform around branches in mice, except for small regions of high uptake at the ostial rim and at the leading edge of an intimal cushion upstream of the ostium, where lesions develop. The technique provides accurate quantification in three dimensions over large areas; it has high throughput, sensitivity and resolution and is suitable for widespread use.

Influence of shear stress magnitude and direction on atherosclerotic plaque composition

The precise flow characteristics that promote different atherosclerotic plaque types remain unclear. We previously developed a blood flow-modifying cuff for ApoE−/− mice that induces the development of advanced plaques with vulnerable and stable features upstream and downstream of the cuff, respectively. Herein, we sought to test the hypothesis that changes in flow magnitude promote formation of the upstream (vulnerable) plaque, whereas altered flow direction is important for development of the downstream (stable) plaque. We instrumented ApoE−/− mice (n = 7) with a cuff around the left carotid artery and imaged them with micro-CT (39.6 µm resolution) eight to nine weeks after cuff placement. Computational fluid dynamics was then performed to compute six metrics that describe different aspects of atherogenic flow in terms of wall shear stress magnitude and/or direction. In a subset of four imaged animals, we performed histology to confirm the presence of advanced plaques and measure plaque length in each segment. Relative to the control artery, the region upstream of the cuff exhibited changes in shear stress magnitude only (p < 0.05), whereas the region downstream of the cuff exhibited changes in shear stress magnitude and direction (p < 0.05). These data suggest that shear stress magnitude contributes to the formation of advanced plaques with a vulnerable phenotype, whereas variations in both magnitude and direction promote the formation of plaques with stable features.

Elevated Uptake of Plasma Macromolecules by Regions of Arterial Wall Predisposed to Plaque Instability in a Mouse Model

Atherosclerosis may be triggered by an elevated net transport of lipid-carrying macromolecules from plasma into the arterial wall. We hypothesised that whether lesions are of the thin-cap fibroatheroma (TCFA) type or are less fatty and more fibrous depends on the degree of elevation of transport, with greater uptake leading to the former. We further hypothesised that the degree of elevation can depend on haemodynamic wall shear stress characteristics and nitric oxide synthesis. Placing a tapered cuff around the carotid artery of apolipoprotein E -/- mice modifies patterns of shear stress and eNOS expression, and triggers lesion development at the upstream and downstream cuff margins; upstream but not downstream lesions resemble the TCFA. We measured wall uptake of a macromolecular tracer in the carotid artery of C57bl/6 mice after cuff placement. Uptake was elevated in the regions that develop lesions in hyperlipidaemic mice and was significantly more elevated where plaques of the TCFA type develop. Computational simulations and effects of reversing the cuff orientation indicated a role for solid as well as fluid mechanical stresses. Inhibiting NO synthesis abolished the difference in uptake between the upstream and downstream sites. The data support the hypothesis that excessively elevated wall uptake of plasma macromolecules initiates the development of the TCFA, suggest that such uptake can result from solid and fluid mechanical stresses, and are consistent with a role for NO synthesis. Modification of wall transport properties might form the basis of novel methods for reducing plaque rupture.

The emerging role of YAP/TAZ in mechanotransduction

Cells sense their physical surroundings through mechanotransduction—that is by translating mechanical forces and deformations into biochemical signals that can ultimately influence gene expression, cell shape and cell fate. Recently, YAP/TAZ—the main transcriptional effectors of the Hippo signalling pathway, a major growth regulatory pathway within metazoa—have been identified as key mechanotransducers acting by nuclear relays of mechanical stimuli (1).