Animal Models
The study was approved by the institutional Animal Care and Use Committee and conformed to guidelines in the PHS Guide for the Care and Use of Laboratory Animals. Mice with homozygous genetic deletion of both the low-density lipoprotein receptor and the Apobec-1 mRNA editing peptide for Apolipoprotein-B (LDLR-/-Apobec1-/-) were studied and were fed a standard laboratory diet (PicoLab 5L0D, LabDiet, Northlake, TX). These mice develop reproducible age-dependent atherosclerotic plaque at lesion-prone sites of the aorta on standard diets.[15] Mice entered into the study at 20–25 weeks of age and mice of either sex were used based on the sex independent nature of disease development in this strain [16]. For all procedures, mice were anesthetized with inhaled isoflurane (1.0–2.0%) and kept euthermic. A jugular vein was cannulated for intravenous access when required for CEUS molecular imaging.
Myocardial Infarction
A closed-chest model of MI was used in order to minimize the confounding effects of thoracotomy-related inflammation on early post-MI molecular imaging studies [5]. At five to seven days prior to MI, mice were anesthetized, intubated, and placed on positive pressure mechanical ventilation. A left lateral thoracotomy was performed through which an 8 − 0 nylon suture was passed under the left anterior descending (LAD) coronary artery but was not tied. The free ends of the suture were exteriorized through the chest wall and left subcutaneously on layered closure. After 5–7 days, mice were anesthetized, and tension was placed on the exteriorized sutures for forty minutes, during which ST-segment elevation on electrocardiographic monitoring and wall motion abnormalities on high-frequency transthoracic two-dimensional echocardiography (Vevo 2100, Visualsonic Inc., Toronto, Canada) were used to confirm ischemia in the LAD territory. Non-ischemic sham-treated control animals had sutures placed without tightening (n = 25) and were studied at the same time intervals as animals undergoing MI. Naïve animals without surgical intervention were also studied for the molecular imaging component of the study (n = 14).
Post-MI Interventions
Mice were randomized to receive either no therapy or one of three therapies intended to reduce thromboinflammation. Recombinant human ADAMTS13 (rADAMTS13) was generated from Tet-On HEK293 cells transfected with the pNBioSec vector [17], and administered at a continuous rate of 2 µg/day for three weeks by osmotic minipump (model 1004, Alzet, Cupertino, CA) placed at the time of MI (n = 19). NAC (Sigma Aldrich, St. Louis, MO) was given orally for three weeks (600 mg/kg/day) by supplementing the drinking water at a concentration calibrated to intake (n = 19). FXI was inhibited by intraperitoneal administration of a monoclonal antibody (clone 14E11) (1 mg/kg) immediately after MI and on post-MI days 3, 9, and 15 (n = 4) [18]. Because of the differences in administration routes and vehicles for these therapies, non-treated mice undergoing MI did not receive any sham therapies.
Aortic Molecular Imaging
CEUS molecular imaging of the proximal thoracic aorta, a remote atheroprone site, was performed at either day 3 or 21 after MI or sham procedure using microbubble probes targeted to either the A1-domain of VWF, GPIbα as an indicator of platelet adhesion, or to the extracellular domains of P-selectin or vascular cell adhesion molecule (VCAM)-1. These probes were constructed from biotinylated lipid-shelled decafluorobutane microbubbles that were prepared by sonication of gas-saturated aqueous lipid suspension of distearoylphosphatidylcholine (2 mg/mL), polyoxyethylene-40-stearate (1 mg/mL), and distearoylphosphatidylethanolamine-PEG (2000) biotin (0.1 mg/mL; Avanti Polar Lipids, Alabaster, AL). Biotinylated ligands were conjugated to the surface as described previously using a streptavidin bridge [15]. Ligands used for targeting were: dimeric recombinant murine VWF A1 domain (mature VWF amino acids 445 to 716) for targeting platelet GPIbα [5, 19]; a cell-derived biotinylated polypeptide representing the N-terminal 300 amino acids of GPIbα for targeting endothelial VWF;[19] and monoclonal antibodies against the extracellular domain of either P-selectin (RB40.34, BD Biosciences, San Jose, California), or VCAM-1 (clone 429, BD Biosciences). Control microbubbles were prepared with isotype control antibody (R3-34, BD Biosciences). Microbubble concentrations and size distributions were measured by electrozone sensing (Multisizer III, Beckman Coulter). CEUS was performed with a linear-array transducer and power-modulation pulse-inversion imaging (Sequoia, Siemens Medical Systems, Mountainview, California). Imaging was performed at 7 MHz, a dynamic range of 55 dB, and a mechanical index of 1.0. Gain was set at a level that eliminated pre-contrast background speckle and was kept constant. The ascending aorta was imaged in long-axis gated to end-diastole. Images were acquired 8 min after intravenous injection of 1×106 targeted or control microbubbles performed in random order. Animals were randomized to receive three of the five microbubble agents according to safety-based limits on injected volumes. Signal intensity was measured using established protocols that differentiate signal from adherent versus circulating microbubbles [5, 15]. Regions-of-interest were standardized encompassing the aorta from the sinuses to the proximal arch.
Echocardiography
High-frequency (40 MHz) transthoracic echocardiography (Vevo 2100, Visualsonics, Toronto, Canada) was performed within 10 minutes of reperfusion, and at post-MI days 3 and 21. Parasternal long-axis and mid-ventricular short-axis views were obtained to measure left ventricular (LV) dimensions and volumes at end-systole and end-diastole, LV ejection fraction (LVEF), and limited-view wall motion score index. Stroke volume was calculated by the product of the LV outflow tract cross-sectional area and time-velocity integral on angle-corrected pulsed-wave Doppler. Cardiac output was calculated as the product of stroke volume and heart rate. Global longitudinal strain (GLS) and circumferential strain (GCS) were calculated using speckle-tracking echocardiography from mid-ventricular parasternal short-axis view and a modified apical view, and were quantified as the average of a six-segment model.
Histology
Histology of the aortic root and the mid-ascending aorta for LDLR-/-Apobec1-/- mice was performed 21 days after MI or sham procedure. Blood was removed from the arterial system by infusion of isothermic buffered saline after which the aorta was perfusion-fixed. Trans-axial sections of the aortic root at the level of the sinuses were stained with Masson’s trichrome to assess the plaque area within the internal elastic lamina, and collagen content. Immunofluorescence histology was performed with anti-mouse primary mAb against Mac-2 (M3/38, Invitrogen, Waltham, Massachusetts, USA) for monocytes/macrophages, against CD41 (ab181582, Abcam, Cambridge, United Kingdom) for platelets, and against matrix metalloproteinase-9 (MMP-9) (PA5-13199, ThermoFisher). Staining for evidence of vascular cell signaling through platelet-derived TGFβ1 was performed with mAb against β-catenin (51067-2-AP, Proteintech, Rosemount, IL) and phosphorylated SMAD2 (pSMAD2) (H.205.4, Invitrogen). Secondary staining was performed with species-appropriate secondary polyclonal antibodies labeled with ALEXA fluorochromes (Fluor-488, Fluor-568, or Fluor-647) and observations were made by confocal fluorescent microscopy (TCS SP5, Leica Microsystems, Buffalo Grove, IL). Spatial extent of plaque size, collagen content, or fluorescent staining area were quantified using Image-J.
Arterial NF-κB
For assessment of treatment-related changes in arterial NF-κB, the proximal descending thoracic aorta was obtained immediately after euthanasia, homogenized in cell extraction solution, and incubated at 4° C for 20 minutes. After centrifugation at 4°C for 20 minutes, the supernatant was stored at − 80° C. Murine NF-κB transcription factor complex p65 subunit was quantified by enzyme-linked immunosorbent assay (SimpleStep NF-κB-p65, pS536, Abcam). Data were normalized to tissue weight.
Statistical Analysis
Data were analyzed statistically using Prism version 9.0 (GraphPad La Jolla, CA). Continuous variables that were normally distributed are displayed as mean ± SD unless stated. Tests for normal versus non-normal distribution were made using a Shapiro-Wilk test. A Student t test was performed for comparisons of normally distributed data. For non-normally distributed data, Mann-Whitney U test was used as appropriate according to experimental conditions and data were displayed using box-whisker plots depicting median (bar), interquartile range (box), and range (whiskers). For multiple comparisons, one-way analysis of variance was performed for normally distributed data and a Kruskal-Wallis test was performed for non-normally distributed data. Post-hoc individual comparisons were performed using Dunn’s test for multiple comparisons. Differences were considered significant at P < 0.05.