Innate Immune Nod1/RIP2 Signaling Is Essential for Cardiac Hypertrophy but Requires Mitochondrial Antiviral Signaling Protein for Signal Transductions and Energy Balance.

BACKGROUND: Cardiac hypertrophy is a key biological response to injurious stresses such as pressure overload and, when excessive, can lead to heart failure. Innate immune activation by danger signals, through intracellular pattern recognition receptors such as nucleotide-binding oligomerization domain 1 (Nod1) and its adaptor receptor-interacting protein 2 (RIP2), might play a major role in cardiac remodeling and progression to heart failure. We hypothesize that Nod1/RIP2 are major contributors to cardiac hypertrophy, but may not be sufficient to fully express the phenotype alone. METHODS: To elucidate the contribution of Nod1/RIP2 signaling to cardiac hypertrophy, we randomized Nod1-/-, RIP2-/-, or wild-type mice to transverse aortic constriction or sham operations. Cardiac hypertrophy, fibrosis, and cardiac function were examined in these mice. RESULTS: Nod1 and RIP2 proteins were upregulated in the heart after transverse aortic constriction, and this was paralleled by increased expression of mitochondrial proteins, including mitochondrial antiviral signaling protein (MAVS). Nod1-/- and RIP2-/- mice subjected to transverse aortic constriction exhibited better survival, improved cardiac function, and decreased cardiac hypertrophy. Downstream signal transduction pathways that regulate inflammation and fibrosis, including NF (nuclear factor) κB and MAPK (mitogen-activated protein kinase)-GATA4/p300, were reduced in both Nod1-/- and RIP2-/- mice after transverse aortic constriction compared with wild-type mice. Coimmunoprecipitation of extracted cardiac proteins and confocal immunofluorescence microscopy showed that Nod1/RIP2 interaction was robust and that this complex also included MAVS as an essential component. Suppression of MAVS expression attenuated the complex formation, NF κB signaling, and myocyte hypertrophy. Interrogation of mitochondrial function compared in the presence or ablation of MAVS revealed that MAVS serves to suppress mitochondrial energy output and mediate fission/fusion related dynamic changes. The latter is possibly linked to mitophagy during cardiomyocytes stress, which may provide an intriguing link between innate immune activation and mitochondrial energy balance under stress or injury conditions. CONCLUSIONS: We have identified that innate immune Nod1/RIP2 signaling is a major contributor to cardiac remodeling after stress. This process is critically joined by and regulated through the mitochondrial danger signal adapter MAVS. This novel complex coordinates remodeling, inflammatory response, and mitochondrial energy metabolism in stressed cardiomyocytes. Thus, Nod1/RIP2/MAVS signaling complex may represent an attractive new therapeutic approach toward heart failure.

Selective Imaging of Matrix Metalloproteinase-13 to Detect Extracellular Matrix Remodeling in Atherosclerotic Lesions.

PURPOSE: Overexpression and activation of matrix metalloproteinase-13 (MMP-13) within atheroma increases susceptibility to plaque rupture, a major cause of severe cardiovascular complications. In comparison to pan-MMP targeting [18F]BR-351, we evaluated the potential for [18F]FMBP, a selective PET radiotracer for MMP-13, to detect extracellular matrix (ECM) remodeling in vascular plaques possessing markers of inflammation. PROCEDURES: [18F]FMBP and [18F]BR-351 were initially assessed in vitro by incubation with en face aortae from 8 month-old atherogenic ApoE-/- mice. Ex vivo biodistributions, plasma metabolite analyses, and ex vivo autoradiography were analogously performed 30 min after intravenous radiotracer administration in age-matched C57Bl/6 and ApoE-/- mice under baseline or homologous blocking conditions. En face aortae were subsequently stained with Oil Red O (ORO), sectioned, and subject to immunofluorescence staining for Mac-2 and MMP-13. RESULTS: High-resolution autoradiographic image analysis demonstrated target specificity and regional concordance to lipid-rich lesions. Biodistribution studies revealed hepatobiliary excretion, low accumulation of radioactivity in non-excretory organs, and few differences between strains and conditions in non-target organs. Plasma metabolite analyses uncovered that [18F]FMBP exhibited excellent in vivo stability (≥74% intact) while [18F]BR-351 was extensively metabolized (≤37% intact). Ex vivo autoradiography and histology of en face aortae revealed that [18F]FMBP, relative to [18F]BR-351, exhibited 2.9-fold greater lesion uptake, substantial specific binding (68%), and improved sensitivity to atherosclerotic tissue (2.9-fold vs 2.1-fold). Immunofluorescent staining of aortic en face cross sections demonstrated elevated Mac-2 and MMP-13-positive areas within atherosclerotic lesions identified by [18F]FMBP ex vivo autoradiography. CONCLUSIONS: While both radiotracers successfully identified atherosclerotic plaques, [18F]FMBP showed superior specificity and sensitivity for lesions possessing features of destructive plaque remodeling. The detection of ECM remodeling by selective targeting of MMP-13 may enable characterization of high-risk atherosclerosis featuring elevated collagenase activity.

Macrophage Jak2 deficiency accelerates atherosclerosis through defects in cholesterol efflux.

Atherosclerosis is a chronic inflammatory condition in which macrophages play a major role. Janus kinase 2 (JAK2) is a pivotal molecule in inflammatory and metabolic signaling, and Jak2V617F activating mutation has recently been implicated with enhancing clonal hematopoiesis and atherosclerosis. To determine the essential in vivo role of macrophage (M)-Jak2 in atherosclerosis, we generate atherosclerosis-prone ApoE-null mice deficient in M-Jak2. Contrary to our expectation, these mice exhibit increased plaque burden with no differences in macrophage proliferation, recruitment or bone marrow clonal expansion. Notably, M-Jak2-deficient bone marrow derived macrophages show a significant defect in cholesterol efflux. Pharmacologic JAK2 inhibition with ruxolitinib also leads to defects in cholesterol efflux and accelerates atherosclerosis. Liver X receptor agonist abolishes the efflux defect and attenuates the accelerated atherosclerosis that occurs with M-Jak2 deficiency. Macrophages of individuals with the Jak2V617F mutation show increased efflux which is normalized when treated with a JAK2 inhibitor. Together, M-Jak2-deficiency leads to accelerated atherosclerosis primarily through defects in cholesterol efflux from macrophages.

Cardiac Sympathetic Positron Emission Tomography Imaging with Meta-[18F]Fluorobenzylguanidine is Sensitive to Uptake-1 in Rats.

Dysfunction of the cardiac sympathetic nervous system contributes to the development of cardiovascular diseases including ischemia, heart failure, and arrhythmias. Molecular imaging probes such as meta-[123I]iodobenzylguanidine have demonstrated the utility of assessing neuronal integrity by targeting norepinephrine transporter (NET, uptake-1). However, current radiotracers can report only on innervation due to suboptimal kinetics and lack sensitivity to NET in rodents, precluding mechanistic studies in these species. The objective of this work was to characterize myocardial sympathetic neuronal uptake mechanisms and kinetics of the positron emission tomography (PET) radiotracer meta-[18F]fluorobenzylguanidine ([18F]mFBG) in rats. Automated synthesis using spirocyclic iodonium(III) ylide radiofluorination produces [18F]mFBG in 24 ± 1% isolated radiochemical yield and 30-95 GBq/µmol molar activity. PET imaging in healthy rats delineated the left ventricle, with monoexponential washout kinetics (kmono = 0.027 ± 0.0026 min-1, Amono = 3.08 ± 0.33 SUV). Ex vivo biodistribution studies revealed tracer retention in the myocardium, while pharmacological treatment with selective NET inhibitor desipramine, nonselective neuronal and extraneuronal uptake-2 inhibitor phenoxybenzamine, and neuronal ablation with neurotoxin 6-hydroxydopamine reduced myocardial retention by 33, 76, and 36%, respectively. Clearance of [18F]mFBG from the myocardium was unaffected by treatment with uptake-1 and uptake-2 inhibitors following peak myocardial activity. These results suggest that myocardial distribution of [18F]mFBG in rats is dependent on both NET and extraneuronal transporters and that limited reuptake to the myocardium occurs. [18F]mFBG may therefore prove useful for imaging intraneuronal dysfunction in small animals.

The Future of Cardiac Molecular Imaging.

Molecular imaging with positron emission tomography (PET) and single-photon emission computed tomography (SPECT) serves numerous applications in clinical cardiology and research. Similar to other medical imaging technologies, this area has undergone and continues to experience rapid changes resulting from technological and medical advances. These have immediate impacts on diagnosis, treatment planning, and patient care, as well as supplying innovative tools for fundamental and translational research. A broad shift toward hybrid PET systems and incorporation of advanced computational tools has been accompanied by mechanism-specific, targeted radiopharmaceuticals that seek to address long-standing limitations in cardiac imaging. While this review addresses some of the still-emerging clinical uses of established radiopharmaceuticals, it too highlights newer imaging probes, applications, and imaging techniques and instrumentation on the horizon. We highlight molecular imaging advances in inflammatory and infiltrative myocardial conditions, heart metabolism, vascular and valvular diseases, neurohormonal dysregulation, and transformational technical advances such as the rise of artificial intelligence and theranostic approaches to cardiovascular disease.