Outcomes of hypothalamic oxytocin neuron-driven cardioprotection after acute myocardial infarction

Altered autonomic balance is a hallmark of numerous cardiovascular diseases, including myocardial infarction (MI). Although device-based vagal stimulation is cardioprotective during chronic disease, a non-invasive approach to selectively stimulate the cardiac parasympathetic system immediately after an infarction does not exist and is desperately needed. Cardiac vagal neurons (CVNs) in the brainstem receive powerful excitation from a population of neurons in the paraventricular nucleus (PVN) of the hypothalamus that co-release oxytocin (OXT) and glutamate to excite CVNs. We tested if chemogenetic activation of PVN-OXT neurons following MI would be cardioprotective. The PVN of neonatal rats was transfected with vectors to selectively express DREADDs within OXT neurons. At 6 weeks of age, an MI was induced and DREADDs were activated with clozapine-N-oxide. Seven days following MI, patch-clamp electrophysiology confirmed the augmented excitatory neurotransmission from PVN-OXT neurons to downstream nuclei critical for parasympathetic activity with treatment (43.7 ± 10 vs 86.9 ± 9 pA; MI vs. treatment), resulting in stark improvements in survival (85% vs. 95%; MI vs. treatment), inflammation, fibrosis assessed by trichrome blue staining, mitochondrial function assessed by Seahorse assays, and reduced incidence of arrhythmias (50% vs. 10% cumulative incidence of ventricular fibrillation; MI vs. treatment). Myocardial transcriptomic analysis provided molecular insight into potential cardioprotective mechanisms, which revealed the preservation of beneficial signaling pathways, including muscarinic receptor activation, in treated animals. These comprehensive results demonstrate that the PVN-OXT network could be a promising therapeutic target to quickly activate beneficial parasympathetic-mediated cellular pathways within the heart during the early stages of infarction.

Hypothalamic Oxytocin Neuron Activation Attenuates Intermittent Hypoxia-Induced Hypertension and Cardiac Dysfunction in an Animal Model of Sleep Apnea

BACKGROUND

Obstructive sleep apnea is a prevalent and poorly treated cardiovascular disease that leads to hypertension and autonomic imbalance. Recent studies that restore cardiac parasympathetic tone using selective activation of hypothalamic oxytocin neurons have shown beneficial cardiovascular outcomes in animal models of cardiovascular disease. This study aimed to determine if chemogenetic activation of hypothalamic oxytocin neurons in animals with existing obstructive sleep apnea-induced hypertension would reverse or blunt the progression of autonomic and cardiovascular dysfunction.

METHODS

Two groups of rats were exposed to chronic intermittent hypoxia (CIH), a model of obstructive sleep apnea, for 4 weeks to induce hypertension. During an additional 4 weeks of exposure to CIH, 1 group was treated with selective activation of hypothalamic oxytocin neurons while the other group was untreated.

RESULTS

Hypertensive animals exposed to CIH and treated with daily hypothalamic oxytocin neuron activation had lower blood pressure, faster heart rate recovery times after exercise, and improved indices of cardiac function compared with untreated hypertensive animals. Microarray analysis suggested that, compared with treated animals, untreated animals had gene expression profiles associated with cellular stress response activation, hypoxia-inducible factor stabilization, and myocardial extracellular matrix remodeling and fibrosis.

CONCLUSIONS

In animals already presenting with CIH-induced hypertension, chronic activation of hypothalamic oxytocin neurons blunted the progression of hypertension and conferred cardioprotection after an additional 4 weeks of CIH exposure. These results have significant clinical translation for the treatment of cardiovascular disease in patients with obstructive sleep apnea.

Activation of Oxytocin Neurons Improves Cardiac Function in a Pressure-Overload Model of Heart Failure

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Hypothalamic OXT neurons were chronically activated using a chemogenetic approach in an animal model of HF.

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Synaptic release of OXT onto parasympathetic autonomic targets was reduced in animals with HF but restored with daily treatment consisting of activation of OXT neurons.

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Long-term daily OXT neuron activation increased parasympathetic activity to the heart and reduced mortality, cardiac inflammation, and fibrosis and improved critical longitudinal in vivo indices of cardiac function.

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The benefits in cardiac function and autonomic balance in HF closely tracked the study-designed differences in initiation of OXT neuron activation in different groups.

This work shows long-term restoration of the hypothalamic oxytocin (OXT) network preserves OXT release, reduces mortality, cardiac inflammation, fibrosis, and improves autonomic tone and cardiac function in a model of heart failure. Intranasal administration of OXT in patients mimics the short-term changes seen in animals by increasing parasympathetic—and decreasing sympathetic—cardiac activity. This work provides the essential translational foundation to determine if approaches that mimic paraventricular nucleus (PVN) OXT neuron activation, such as safe, noninvasive, and well-tolerated intranasal administration of OXT, can be beneficial in patients with heart failure.

Abstract 373: Regulation of Hypoxia by Chromatin Reader Protein Kinase C Binding Protein 1 (PRKCBP1)

PRKCBP1 (also known as RACK7 and Zmynd8) is a polyvalent chromatin reader known to cooperatively bind acetylated and methylated nucleosomes. Recently it has been shown to regulate transcription and cancer progression by coordinating histone methylation modifications affecting enhancer and promoter regions of genes. The role of PRKCBP1 in the cardiac myocardium is unexplored. Hypoxia-inducible factor 1α (HIF-1α) upregulation and stabilization is a common feature of both cancer and myocardial ischemia, promoting cellular functions such as proliferation, glucose metabolism and angiogenesis. Here we investigated the mechanism by which PRKCBP1 modulates the cardiac hypoxic response.

We hypothesize that PRKCBP1 inhibits the HIF-1 response in the hypoxic heart by reducing enhancer activity of HIF-1 target genes and altering availability of HIF-1 binding sites.

We have found that in transgenic mice with a mutation that increases HIF-induced expression of PRKCBP, the effect of induced oxygen-stable HIF is markedly diminished. These mice did not exhibit the typical HIF-1 over-expression phenotype of dilated vessels, increased heart size and reduced ventricular function. Semi-quantitative rtPCR analysis of mouse neonatal cardiomyocytes transfected with CMV-driven expression plasmids for PRKCBP1 and oxygen-stable HIF-1α showed striking reduction of multiple HIF-1 target genes such as PDK1 (45% reduction relative to Ehbp1) compared to the HIF-1α plasmid alone. RNAi mediated knockdown of PRKCBP1 removed this negative regulation (65% increase). Analysis of human PRKCBP1 and HIF-1α ChIP-seq data indicate that PRKCBP1 binds to the enhancer of 78% of HIF-1 regulated genes. ATAC-seq data suggest that PRKCBP1 affects genome-wide chromatin accessibility, with loci-specific modifications at numerous HIF-1 target genes, such as EGLN3. These data suggest that PRKCBP1 may be acting both by modulating enhancer activity in cis - to HIF-1 target genes and by preventing HIF-1 binding to hypoxia response elements of target genes.

We have discovered a new regulator of HIF-1 action that modifies the hypoxic response, likely through chromatin remodeling. This new form of regulation may modify the pathophysiology of ischemia and provide new targets for therapy.

Abstract 307: Chemogenetic Activation of Paraventricular Oxytocin Neurons Reduces Cardiac Dysfunction During Heart Failure

There is a distinctive cardiac autonomic imbalance in HF consisting of increased sympathetic activity and decreased parasympathetic tone. Recent work has indicated that activation of paraventricular nucleus (PVN) oxytocin neurons increases co-release of oxytocin with glutamate to elevate cardiac vagal tone yet little is known regarding the long-term (16 week) benefit. We hypothesize that activation of oxytocin neurons in the PVN will blunt the progression of HF by reducing inflammation and fibrosis at 16 weeks post-HF initiation than in untreated counterparts. Rats underwent trans-ascending aortic constriction (TAC) to induce HF. In a subset of HF rats (TAC+OXT), oxytocin neurons were activated by excitatory designer receptors exclusively activated by designer drugs (DREADDs). Rats were implanted with ECG telemetry devices and underwent a treadmill protocol to measure peak effort capacity and heart rate recovery (HRR). Cardiac structural measurements were obtained using echocardiography. After 16 weeks, hearts were rapidly excised, Langendorff perfused to measure mechanical function, and prepared for histology and western blot analysis. Indices of cardiac function (stroke volume, ejection fraction, cardiac output, and fractional shortening) indicated preserved function in TAC+OXT compared to TAC. HRR after peak effort capacity values were lowest in TAC and were significantly greater in TAC+OXT (p≤0.05), indicating preservation of cardiac parasympathetic tone. LV developed pressure was significantly impaired in TAC compared to sham as well as significantly improved in TAC+OXT compared to TAC (p≤0.05). Contractility and relaxation were significantly impaired in TAC compared to sham (p≤0.05), while TAC+OXT animals had values not different from sham. TAC animals had collagen III levels 5X higher than TAC+OXT. Histological analysis indicated increased fibrosis in disease groups compared to sham. TAC+OXT had IL-1β levels significantly lower than TAC (p≤0.05). PVN oxytocin neuron activation slows the decline of cardiac function in 16 week post- TAC rats, likely by reinstating cardioprotective parasympathetic activity. This novel method has translational potential since intranasal oxytocin administration has been used in the clinic.

Protein kinase C binding protein 1 inhibits hypoxia-inducible factor-1 in the heart

AIMS

Hypoxia-inducible factor-1 alpha (HIF-1α) is a key transcription factor responsible for the induction of genes that facilitate adaptation to hypoxia. To study HIF-1 signalling in the heart, we developed a mouse model in which an oxygen-stable form of HIF-1α can be inducibly expressed in cardiac myocytes, under the regulation of tetracycline.

METHODS AND RESULTS

Remarkably, expression of the transgene in mice generated two distinct phenotypes. One was the expected expression of HIF-regulated transcripts and associated changes in cardiac angiogenesis and contractility. The other was an unresponsive phenotype with much less expression of typical HIF-response genes and substantial expression of a zinc-finger protein, Protein Kinase C Binding Protein 1 (PRKCBP1). We have demonstrated that this second phenotype is due to an insertion of a fragment of DNA upstream of the PRKCBP1 gene that contains two additional canonical HIF binding sites and leads to substantial HIF binding, assessed by chromatin immunoprecipitation, and transcriptional activation. This insertion is found only in the FVB strain of mice that contributed the αMHC-tet binding protein transgene to these biallelic mice. In HEK293 cells transfected with oxygen-stable HIF-1α and PRKCBP1, we demonstrated inhibition of HIF-1 activity by a luciferase reporter assay. Using mouse primary cells and cell lines, we show that transfection with oxygen-stable HIF-1α and PRKCBP1 reduced expression of direct HIF-1 gene targets and that knockdown of PRKCBP1 removes that negative inhibition. Consistent with previous reports suggesting that PRKCBP1 modulates the chromatin landscape, we found that HL-1 cells transfected with oxygen-stable HIF-1α and PRKCBP1 have reduced global 5-methyl cytosine compared to HIF-1 alone.

CONCLUSION

We show genetic, transcriptional, biochemical, and physiological evidence that PRKCBP1 inhibits HIF activity. Identification of a new oxygen-dependent and previously unsuspected regulator of HIF may provide a target for new therapeutic approaches to ischaemic heart disease.

HIF1 mediates a switch in pyruvate kinase isoforms after myocardial infarction

Alternative splicing of RNA is an underexplored area of transcriptional response. We expect that early changes in alternatively spliced genes may be important for responses to cardiac injury. Hypoxia inducible factor 1 (HIF1) is a key transcription factor that rapidly responds to loss of oxygen through alteration of metabolism and angiogenesis. The goal of this study was to investigate the transcriptional response after myocardial infarction (MI) and to identify novel, hypoxia-driven changes, including alternative splicing. After ligation of the left anterior descending artery in mice, we observed an abrupt loss of cardiac contractility and upregulation of hypoxic signaling. We then performed RNA sequencing on ischemic heart tissue 1 and 3 days after infarct to assess early transcriptional changes and identified 89 transcripts with altered splicing. Of particular interest was the switch in Pkm isoform expression (pyruvate kinase, muscle). The usually predominant Pkm1 isoform was less abundant in ischemic hearts, while Pkm2 and associated splicing factors (hnRNPA1, hnRNPA2B1, Ptbp1) rapidly increased. Despite increased Pkm2 expression, total pyruvate kinase activity remained reduced in ischemic myocardial tissue. We also demonstrated HIF1 binding to PKM by chromatin immunoprecipitation, indicating a direct role for HIF1 in mediating this isoform switch. Our study provides a new, detailed characterization of the early transcriptome after MI. From this analysis, we identified an HIF1-mediated alternative splicing event in the PKM gene. Pkm1 and Pkm2 play distinct roles in glycolytic metabolism and the upregulation of Pkm2 is likely to have important consequences for ATP synthesis in infarcted cardiac muscle.

Separating fast and slow exchange transfer and magnetization transfer using off-resonance variable-delay multiple-pulse (VDMP) MRI

PURPOSE

To develop a method that can separate and quantify the fast (>1 kHz) and slow exchange transfer and magnetization transfer components in Z-spectra.

METHODS

Z-spectra were recorded as a function of mixing time using a train of selective pulses providing variable-delay multipulse build-up curves. Fast and slow transfer components in the Z-spectra were separated and quantified on a voxel-by-voxel basis by fitting the mixing time-dependent CEST signal using a 3-pool model.

RESULTS

Phantom studies of glutamate solution, bovine serum albumin solution, and hair conditioner showed the capability of the proposed method to separate fast and slow transfer components. In vivo mouse brain studies showed a strong contrast between white matter and gray matter in the slow-transferring map, corresponding to an asymmetric component of the conventional semisolid magnetization transfer contrast. In addition, a fast-transferring proton map was found that was homogeneous across the brain and attributed to the total contributions of the fast-exchanging protons from proteins, metabolites, and a symmetric magnetization transfer contrast component.

CONCLUSIONS

This new method provides a simple way to extract fast and slow transfer components from the Z-spectrum, leading to novel MRI contrasts, and providing insight into the different magnetization transfer contrast contributions.

Abstract 120: HIF1 Alpha Can be Inhibited in the Heart by Protein Kinase C Binding Protein 1

Hypoxia-inducible factor 1 alpha (HIF-1α) is a key transcription factor responsible for the induction of genes that facilitate adaptation to hypoxia. The HIF-1 heterodimer (HIF-1α and HIF-1ß) recognizes a consensus element designated the hypoxia response element (HRE) within the enhancer regions of target genes that are implicated in many different cellular functions that ameliorate hypoxic stress. To study HIF-1 signaling in the heart, our lab developed a mouse model in which an oxygen-stable form of HIF-1α can be inducibly expressed in cardiac myocytes with a tet-off system . Remarkably, expression of the transgene generated two distinct phenotypes. One was the expected expression of HIF-regulated transcripts and associated HIF–related changes in cardiac angiogenesis and contractility. The other was a “resistant phenotype” with much less expression of typical HIF-response genes and substantial expression of a zinc-finger protein, a receptor for protein kinase C called PRKCBP1, also known as ZMYND8. We have demonstrated that this second phenotype is due to an insertion of a fragment of DNA upstream of the PRKCBP1 gene that contains two additional HREs and leads to substantial HIF binding, assessed by ChIP, and transcriptional activation. This inserted allele is found only in the FVB strain of mice that contributed the αMHC-tet binding protein allele to the transgenic strain. In HEK293 cells transfected with oxygen-stable HIF1α and PRKCBP1 we demonstrated inhibition of HIF-1 activity by a luciferase reporter assay. In the MCF-7 breast cancer cell line we have shown that increased expression of PRKCBP1 produces inhibition of HIF degradation in normoxia. This and co-immunoprecipitation of HIF with PRKCBP1 suggests a direct interaction of the two proteins that acts to inhibit HIF action. We have genetic, transcriptional, biochemical, and physiological evidence that PRKCBP1 inhibits HIF activity through direct interaction in a homeostatic mechanism mediated by transcriptional control.

Abstract W P236: Role of Leukocyte Cathepsin G in an Atherothrombotic Stroke Model

The majority of ischemic strokes in humans are caused by atheroemboli that originate from extracranial vessels, activate platelets and produce thrombi that occlude intracranial vessels. Most of the current stroke models do not simulate the mechanisms of endogenous thrombus interaction with blood elements, the vascular wall and its extracellular matrix, and thus do not translate well for the study of atherothrombotic mechanisms. To overcome these shortcomings, we developed an atherothrombotic stroke model. We used this model to determine the pathophysiologic role of leukocyte cathepsin G, a serine protease with pro-thrombotic properties, in stroke and to determine its suitability as a molecular target for therapy. We hypothesized that deficiency of cathepsin G will reduce intravascular thrombus formation, decrease infarct volume and improve neurobehavioral deficit in a mouse atherothrombotic stroke model.

Male C57Bl/6 (WT) and cathepsin G knockout mice (CG-/-) were used. A PE8 catheter was advanced into the ICA near the MCA origin and a reduction in cortical laser-Doppler flow was induced after a series of intra-arterial injections of collagen, a platelet activator and component of ruptured atherosclerotic plaque. Mice were sacrificed 3h post-collagen injection for histologic examination, or 48h post-collagen for infarct volume quantification and neurobehavioral assessment.

Gross examination of excised brains 3h post-collagen revealed thrombi within the main ICA in 71% (5/7) of WT mice and 0% of CG-/- mice (p=0.005), branches of the ICA in 0 WT mice and 29% (2/7) of CG-/- mice (p=0.13), in the main MCA in 43% (3/7) of WT mice and 0 CG-/- mice (p=0.05), and distal branches of the MCA in 100% (7/7) of WT and CG-/- mice. Immunofluorescence microscopy confirmed that platelets and fibrin were major components of these thrombi. Cerebral infarct in WT mice was 33% ± 9% of hemispheric volume and 19% ± 1% in CG-/- mice (p=0.02). Neurobehavioral deficit 48h post-collagen was reduced in CG-/- mice compared to WT.

Using a novel atherothrombotic stroke model, these data reveal an important role of cathepsin G in cerebrovascular thrombus formation and a novel mechanism linking inflammatory cells to ischemic brain injury- a link commonly observed in human clinical studies.

A novel atherothrombotic model of ischemic stroke induced by injection of collagen into the cerebral vasculature

BACKGROUND

Most ischemic strokes in humans are caused by ruptured arterial atheroma, which activate platelets and produce thrombi that occlude cerebral vessels.

METHODS

To simulate these events, we threaded a catheter through the internal carotid artery toward the middle cerebral artery (MCA) orifice and injected collagen directly into the cerebral circulation of male C57Bl/6 mice and Wistar rats.

RESULTS

Laser-Doppler flowmetry demonstrated reductions in cerebral blood flow (CBF) of ∼80% in mice and ∼60% in rats. CBF spontaneously increased but remained depressed after catheter withdrawal. Magnetic resonance imaging showed that ipsilateral CBF was reduced at 3h after collagen injection and markedly improved at 48 h. Micro-computed tomography revealed reduced blood vessel density in the ipsilateral MCA territory at 3 h. Gross examination of excised brains revealed thrombi within ipsilateral cerebral arteries at 3 h, but not 24 h, after collagen injection. Immunofluorescence microscopy confirmed that platelets and fibrinogen/fibrin were major components of these thrombi at both macrovascular and microvascular levels. Cerebral infarcts comprising ∼30% of hemispheric volume and neurobehavioral deficits were observed 48 h after ischemic injury in both mice and rats.

COMPARISON WITH EXISTING METHODS

Collagen injection caused brain injury that was similar in magnitude and variability to mechanical MCA occlusion or injection of a pre-formed clot; however, alterations in CBF and the mechanism of vascular occlusion were more consistent with clinical ischemic stroke.

CONCLUSION

This novel rodent model of ischemic stroke has pathophysiologic characteristics consistent with clinical atherothrombotic stroke, is technically feasible, and creates reproducible brain injury.

Acetaminophen attenuates doxorubicin-induced cardiac fibrosis via osteopontin and GATA4 regulation: reduction of oxidant levels

It is well documented in animal and human studies that therapy with the anti-cancer drug doxorubicin (DOX) induces fibrosis, cardiac dysfunction, and cell death. The most widely accepted mechanism of cardiac injury is through production of reactive oxygen species (ROS), which cause mitochondrial damage, sarcomere structural alterations, and altered gene expression in myocytes and fibroblasts. Here we investigated the effects of acetaminophen (APAP, N-acetyl-para-aminophenol) on DOX-induced cardiac injury and fibrosis in the presence or absence of osteopontin (OPN). H9c2 rat heart-derived embryonic myoblasts were exposed to increasing concentrations of DOX ± APAP; cell viability, oxidative stress, and OPN transcript levels were analyzed. We found a dose-dependent decrease in cell viability and a corresponding increase in intracellular oxidants at the tested concentrations of DOX. These effects were attenuated in the presence of APAP. RT-PCR analysis revealed a small increase in OPN transcript levels in response to DOX, which was suppressed by APAP. When male 10-12-week-old mice (OPN(+/+) or OPN(-/-)) were given weekly injections of DOX ± APAP for 4 weeks there was substantial cardiac fibrosis in OPN(+/+) and, to a lesser extent, in OPN(-/-) mice. In both groups, APAP decreased fibrosis to near baseline levels. Activity of the pro-survival GATA4 transcription factor was diminished by DOX in both mouse genotypes, but retained baseline activity in the presence of APAP. These effects were mediated, in part, by the ability of APAP, acting as an anti-inflammatory agent, to decrease intracellular ROS levels, consequently diminishing the injury-induced increase in OPN levels.