Yu-Qing Zhou

GLP-1R Agonist Liraglutide Activates Cytoprotective Pathways and Improves Outcomes After Experimental Myocardial Infarction in Mice

OBJECTIVE Glucagon-like peptide-1 receptor (GLP-1R) agonists are used to treat type 2 diabetes, and transient GLP-1 administration improved cardiac function in humans after acute myocardial infarction (MI) and percutaneous revascularization. However, the consequences of GLP-1R activation before ischemic myocardial injury remain unclear. RESEARCH DESIGN AND METHODS We assessed the pathophysiology and outcome of coronary artery occlusion in normal and diabetic mice pretreated with the GLP-1R agonist liraglutide. RESULTS Male C57BL/6 mice were treated twice daily for 7 days with liraglutide or saline followed by induction of MI. Survival was significantly higher in liraglutide-treated mice. Liraglutide reduced cardiac rupture (12 of 60 versus 46 of 60; P = 0.0001) and infarct size (21 ± 2% versus 29 ± 3%, P = 0.02) and improved cardiac output (12.4 ± 0.6 versus 9.7 ± 0.6 ml/min; P = 0.002). Liraglutide also modulated the expression and activity of cardioprotective genes in the mouse heart, including Akt, GSK3β, PPARβ-δ, Nrf-2, and HO-1. The effects of liraglutide on survival were independent of weight loss. Moreover, liraglutide conferred cardioprotection and survival advantages over metformin, despite equivalent glycemic control, in diabetic mice with experimental MI. The cardioprotective effects of liraglutide remained detectable 4 days after cessation of therapy and may be partly direct, because liraglutide increased cyclic AMP formation and reduced the extent of caspase-3 activation in cardiomyocytes in a GLP-1R–dependent manner in vitro. CONCLUSIONS These findings demonstrate that GLP-1R activation engages prosurvival pathways in the normal and diabetic mouse heart, leading to improved outcomes and enhanced survival after MI in vivo.

Genetic Deletion or Pharmacological Inhibition of Dipeptidyl Peptidase-4 Improves Cardiovascular Outcomes After Myocardial Infarction in Mice

OBJECTIVE Glucagon-like peptide-1 (7-36)amide (GLP-1) is cleaved by dipeptidyl peptidase-4 (DPP-4) to GLP-1 (9-36)amide. We examined whether chemical inhibition or genetic elimination of DPP-4 activity affects cardiovascular function in normoglycemic and diabetic mice after experimental myocardial infarction. RESEARCH DESIGN AND METHODS Cardiac structure and function was assessed by hemodynamic monitoring and echocardiography in DPP-4 knockout (Dpp4−/−) mice versus wild-type (Dpp4+/+) littermate controls and after left anterior descending (LAD) coronary artery ligation–induced myocardial infarction (MI). Effects of sustained DPP-4 inhibition with sitagliptin versus treatment with metformin were ascertained after experimental MI in a high-fat diet–streptozotocin model of murine diabetes. Functional recovery from ischemia-reperfusion (I/R) injury was measured in isolated hearts from Dpp4−/− versus Dpp4+/+ littermates and from normoglycemic wild-type (WT) mice treated with sitagliptin or metformin. Cardioprotective signaling in the murine heart was examined by RT-PCR and Western blot analyses. RESULTS Dpp4−/− mice exhibited normal indexes of cardiac structure and function. Survival post-MI was modestly improved in normoglycemic Dpp4−/− mice. Increased cardiac expression of phosphorylated AKT (pAKT), pGSK3β, and atrial natriuretic peptide (ANP) was detected in the nonischemic Dpp4−/− heart, and HO-1, ANP, and pGSK3β proteins were induced in nonischemic hearts from diabetic mice treated with sitagliptin or metformin. Sitagliptin and metformin treatment of wild-type diabetic mice reduced mortality after myocardial infarction. Sitagliptin improved functional recovery after I/R injury ex vivo in WT mice with similar protection from I/R injury also manifest in hearts from Dpp4−/− versus Dpp4+/+ mice. CONCLUSIONS Genetic disruption or chemical inhibition of DPP-4 does not impair cardiovascular function in the normoglycemic or diabetic mouse heart.

Chromatin remodelling complex dosage modulates transcription factor function in heart development

Dominant mutations in cardiac transcription factor genes cause human inherited congenital heart defects (CHDs); however, their molecular basis is not understood. Interactions between transcription factors and the Brg1/Brm-associated factor (BAF) chromatin remodelling complex suggest potential mechanisms; however, the role of BAF complexes in cardiogenesis is not known. In this study, we show that dosage of Brg1 is critical for mouse and zebrafish cardiogenesis. Disrupting the balance between Brg1 and disease-causing cardiac transcription factors, including Tbx5, Tbx20 and Nkx2–5, causes severe cardiac anomalies, revealing an essential allelic balance between Brg1 and these cardiac transcription factor genes. This suggests that the relative levels of transcription factors and BAF complexes are important for heart development, which is supported by reduced occupancy of Brg1 at cardiac gene promoters in Tbx5 haploinsufficient hearts. Our results reveal complex dosage-sensitive interdependence between transcription factors and BAF complexes, providing a potential mechanism underlying transcription factor haploinsufficiency, with implications for multigenic inheritance of CHDs.

Iroquois homeobox gene 3 establishes fast conduction in the cardiac His–Purkinje network

Rapid electrical conduction in the His–Purkinje system tightly controls spatiotemporal activation of the ventricles. Although recent work has shed much light on the regulation of early specification and morphogenesis of the His–Purkinje system, less is known about how transcriptional regulation establishes impulse conduction properties of the constituent cells. Here we show that Iroquois homeobox gene 3 (Irx3) is critical for efficient conduction in this specialized tissue by antithetically regulating two gap junction–forming connexins (Cxs). Loss of Irx3 resulted in disruption of the rapid coordinated spread of ventricular excitation, reduced levels of Cx40, and ectopic Cx43 expression in the proximal bundle branches. Irx3 directly represses Cx43 transcription and indirectly activates Cx40 transcription. Our results reveal a critical role for Irx3 in the precise regulation of intercellular gap junction coupling and impulse propagation in the heart.

Ultrasound-guided left-ventricular catheterization: a novel method of whole mouse perfusion for microimaging

We describe a novel technique to perform whole-body perfusion fixation in mice with specific relevance to micro-imaging. With the guidance of high-frequency ultrasound imaging, we were able to perfuse fixative and contrast agents via a catheter inserted into the left ventricle, and therefore preserved the integrity of the chest and abdominal cavity. In this preliminary study, our success rate over 15 animals was 73%. We demonstrate applications of this technique for magnetic resonance imaging and micro-CT, but we expect that this method can be generally applied to whole-body perfusions of other small animals in which the intact body is necessary.

Endothelial Nitric Oxide Synthase Gene Expression During Murine Embryogenesis Commencement of Expression in the Embryo Occurs With the Establishment of a Unidirectional Circulatory System

To elucidate the role of endothelial NO synthase (eNOS)-derived NO during mammalian embryogenesis, we assessed the expression of the eNOS gene during development. Using transgenic eNOS promoter/reporter mice (with &bgr;-galactosidase and green fluorescent protein reporters), in situ cRNA hybridization, and immunohistochemistry to assess transcription, steady-state mRNA levels, and protein expression, respectively, we noted that eNOS expression in the developing cardiovascular system was highly restricted to endothelial cells of medium- and large-sized arteries and the endocardium. The onset of transcription of the native eNOS gene and reporters coincided with the establishment of robust, unidirectional blood flow at embryonic day 9.5, as assessed by Doppler ultrasound biomicroscopy. Interestingly, reporter transgene expression and native eNOS mRNA were also observed in discrete regions of the developing skeletal musculature and the apical ectodermal ridge of developing limbs, suggesting a role for eNOS-derived NO in limb development. In vitro studies of promoter/reporter constructs indicated that similar eNOS promoter regions operate in both embryonic skeletal muscle and vascular endothelial cells. In summary, transcriptional activity of the eNOS gene in the murine circulatory system occurred following the establishment of embryonic blood flow. Thus, the eNOS gene is a late-onset gene in endothelial ontogeny.

Tbx5-dependent pathway regulating diastolic function in congenital heart disease

At the end of every heartbeat, cardiac myocytes must relax to allow filling of the heart. Impaired relaxation is a significant factor in heart failure, but all pathways regulating the cardiac relaxation apparatus are not known. Haploinsufficiency of the T-box transcription factor Tbx5 in mouse and man causes congenital heart defects (CHDs) as part of Holt–Oram syndrome (HOS). Here, we show that haploinsufficiency of Tbx5 in mouse results in cell-autonomous defects in ventricular relaxation. Tbx5 dosage modulates expression of the sarco(endo)plasmic reticulum Ca2+-ATPase isoform 2a encoded by Atp2a2 and Tbx5 haploinsufficiency in ventricular myocytes results in impaired Ca2+ uptake dynamics and Ca2+ transient prolongation. We also demonstrate that Tbx5 can activate the Atp2a2 promoter. Furthermore, we find that patients with HOS have significant diastolic filling abnormalities. These results reveal a direct genetic pathway that regulates cardiac diastolic function, implying that patients with structural CHDs may have clinically important underlying anomalies in heart function that merit treatment.

Differential HIF and NOS Responses to Acute Anemia: Defining Organ Specific Hemoglobin Thresholds for Tissue Hypoxia

Tissue hypoxia likely contributes to anemia-induced organ injury and mortality. Severe anemia activates hypoxia-inducible factor (HIF) signaling by hypoxic- and neuronal nitric oxide (NO) synthase- (nNOS) dependent mechanisms. However, organ-specific hemoglobin (Hb) thresholds for increased HIF expression have not been defined. To assess organ-specific Hb thresholds for tissue hypoxia, HIF-α (oxygen-dependent degradation domain, ODD) luciferase mice were hemodiluted to mild, moderate, or severe anemia corresponding to Hb levels of 90, 70, and 50 g/l, respectively. HIF luciferase reporter activity, HIF protein, and HIF-dependent RNA levels were assessed. In the brain, HIF-1α was paradoxically decreased at mild anemia, returned to baseline at moderate anemia, and then increased at severe anemia. Brain HIF-2α remained unchanged at all Hb levels. Both kidney HIF-1α and HIF-2α increased earlier (Hb ∼70-90 g/l) in response to anemia. Liver also exhibited an early HIF-α response. Carotid blood flow was increased early (Hb ∼70, g/l), but renal blood flow remained relatively constant, only increased at Hb of 50 g/l. Anemia increased nNOS (brain and kidney) and endothelia NOS (eNOS) (kidney) levels. Whereas anemia-induced increases in brain HIFα were nNOS-dependent, our current data demonstrate that increased renal HIFα was nNOS independent. HIF-dependent RNA levels increased linearly (∼10-fold) in the brain. However, renal HIF-RNA responses (MCT4, EPO) increased exponentially (∼100-fold). Plasma EPO levels increased near Hb threshold of 90 g/l, suggesting that the EPO response is sensitive. Collectively, these observations suggest that each organ expresses a different threshold for cellular HIF/NOS hypoxia responses. This knowledge may help define the mechanism(s) by which the brain and kidney maintain oxygen homeostasis during anemia.

Aortic Regurgitation Dramatically Alters the Distribution of Atherosclerotic Lesions and Enhances Atherogenesis in Mice

Objective—Hemodynamics plays a critical role in atherogenesis, but the association between flow pattern and preferential localization of lesion is not fully understood. We developed a mouse model of aortic valve regurgitation (AR) to change the aortic flow pattern and observed the effects on plaque formation. Methods and Results—High-frequency Doppler ultrasound imaging of 10 untreated C57BL/6J mice and 6 sham-treated low-density lipoprotein receptor-deficient (Ldlr−/−) mice revealed consistent antegrade blood flow throughout the aorta and oscillatory flow only along the lesser curvature of the aortic arch. Catheter-induced AR in 7 Ldlr−/− mice produced various degrees of diastolic retrograde flow throughout the aorta. After the mice were fed a cholesterol-enriched diet for 6 weeks, the burden of atherosclerotic lesions was increased 6-fold, with the naturally plaque-resistant descending aorta becoming susceptible. The AR severity correlated positively with the lesion burden in the descending thoracic and abdominal aorta but negatively with the lesions in the ascending aorta and aortic arch. Conclusion—This AR model is valuable for elucidating the relationship between hemodynamics and predisposition of the artery wall to atherosclerosis, because of the significant alterations in local flow patterns and the conversion of large regions in the descending aorta from lesion resistant to lesion prone.

Acoustic and Kinetic Behaviour of Definity in Mice Exposed to High Frequency Ultrasound

Microbubble contrast agents have shown clinical potential for characterising blood flow using 1 to 10 MHz ultrasound; however, scaling their use for similar applications in the mouse with high frequency ultrasound (20 to 60 MHz) has not been addressed. The goal was to determine the utility of microbubbles for mouse imaging with 30 MHz ultrasound by investigating their attenuation and backscatter characteristics as a function of concentration in vitro and dose response in vivo. The agent was exposed to a 30 MHz, 20% bandwidth pulse with a peak negative pressure of 244 kPa. In vitro results showed that the attenuation and backscatter increased linearly for concentrations between 2.8 x 10(6) and 28 x 10(6) bubbles per mL of deionized water. In vivo experiments where performed in the jugular vein of CD-1 mice and time intensity curves were acquired for doses between 10 and 100 microL kg(-1). These doses corresponded to the range of concentrations used in vitro. In vivo results showed that the peak enhancement of the agent increased linearly for doses between 10 and 60 microL kg(-1), the duration of enhancement varied between 200 to 300 s and the integrated enhancement (area under the curve) increased linearly up to 100 microL kg(-1). A maximum enhancement of 13 dB over the blood pool was observed for a dose of 100 microL kg(-1). The intra- and inter-mouse variabilities were 10% to 40% and indicate that further optimisations are required. These results suggest that quantitative contrast flow studies in the mouse using high frequency ultrasound are possible for doses between 10 and 60 microL kg(-1).