Calcium Signaling in the Ventricular Myocardium of the Goto-Kakizaki Type 2 Diabetic Rat

Mitochondrial glycerol 3-phosphate dehydrogenase deficiency exacerbates lipotoxic cardiomyopathy

Metabolic diseases such as obesity and diabetes induce lipotoxic cardiomyopathy, which is characterized by myocardial lipid accumulation, dysfunction, hypertrophy, fibrosis and mitochondrial dysfunction. Here, we identify that mitochondrial glycerol 3-phosphate dehydrogenase (mGPDH) is a pivotal regulator of cardiac fatty acid metabolism and function in the setting of lipotoxic cardiomyopathy. Cardiomyocyte-specific deletion of mGPDH promotes high-fat diet induced cardiac dysfunction, pathological hypertrophy, myocardial fibrosis, and lipid accumulation. Mechanically, mGPDH deficiency inhibits the expression of desuccinylase SIRT5, and in turn, the hypersuccinylates majority of enzymes in the fatty acid oxidation (FAO) cycle and promotes the degradation of these enzymes. Moreover, manipulating SIRT5 abolishes the effects of mGPDH ablation or overexpression on cardiac function. Finally, restoration of mGPDH improves lipid accumulation and cardiomyopathy in both diet-induced and genetic obese mouse models. Thus, our study indicates that targeting mGPDH could be a promising strategy for lipotoxic cardiomyopathy in the context of obesity and diabetes.

Effects of high glucose and insulin on the electrophysiological properties of cardiomyocytes derived from human-induced pluripotent stem cells

OBJECTIVES

The risk of arrhythmia increases in diabetic patients. However, the effects of hyperglycemia and insulin therapy on the electrophysiological properties of human cardiomyocytes remain unclear. This study is to explore the effects of high glucose and insulin on the electrophysiological properties and arrhythmias of cardiomyocytes derived from human-induced pluripotent stem cells (hiPSC-CMs).

METHODS

Immunofluorescent staining and flow cytometry were used to analyze the purity of hiPSC-CMs generated from human skin fibroblasts of a healthy donor. The hiPSC-CMs were divided into 3 group (treated with normal medium, high glucose and insulin for 4 days): a control group (NM group, containing 5 mmol/L glucose), a high glucose group (HG group, containing 15 mmol/L glucose), and a high glucose combined with insulin (HG+INS group, containing 15 mmol/L glucose+100 mg/L insulin). Electrophysiological changes of hiPSC-CMs were detected by microelectrode array (MEA) before or after treatment with glucose and insulin, including beating rate (BR), field potential duration (FPD) (similar to QT interval in ECG), FPDc (FPD corrected by BR), spike amplitude and conduction velocity (CV). Effects of sotalol on electrophysiological properties and arrhythmias of hiPSC-CMs were also evaluated.

RESULTS

The expression of cardiac-specific marker of cardiac troponin T was high in the hiPSC-CMs. The purity of hiPSC-CMs was 99.06%. Compared with the NM group, BR was increased by (9.14±0.8)% in the HG group (P<0.01). After treatment with high glucose, FPD was prolonged from (460.4±9.0) ms to (587.6±23.7) ms in the HG group, while it was prolonged from (462.5±14.5) ms to (512.6±17.6) ms in the NM group. Compared with the NM group, FPD of hiPSC-CMs was prolonged by (16.8±1.4)% in the HG group (P<0.01). The FPDc of hiPSC-CMs was prolonged from (389.1±13.7) ms to (478.3±31.5) ms in the HG group, and that was prolonged from (387.7±21.6) ms to (422.6±32.9) ms in the NM group. Compared with the NM group, the FPDc of hiPSC-CMs was prolonged by (13.9±1.3)% in HG group (P<0.01). The spike amplitude and CV remained unchanged between the HG group and the NM group (P>0.05). Ten µmol/L of sotalol can induce significant arrhythmias from all wells in the HG group. After treatment with insulin and high glucose, compared with the HG group, BR was increased by (8.3±0.5)% in the HG+INS group (P<0.05). The FPD was prolonged from (463.4±9.7) ms to (532.6±12.8) ms in the HG+INS group, while it was prolonged from (460.4±9.0) ms to (587.6±23.7) ms in the HG group. Compared with the HG group, the FPD of hiPSC-CMs was shortened by (12.7±1.9)% in the HG+INS group (P<0.01). The FPDc of hiPSC-CMs was prolonged from (387.4±4.1) ms to (422.4±10.0) ms in the HG+INS group, and that was prolonged from (384.8±4.0) ms to (476.3±11.5) ms in HG group. Compared with the HG group, the FPDc of hiPSC-CMs was shortened by (14.7±1.1)% in HG group (P<0.01). After the insulin treatment, the spike amplitude of hiPSC-CMs was increased from (3.12±0.46) mV to (4.35±0.64) mV in the HG+INS group, while it was enhanced from (3.06±0.35) mV to (3.33±0.41) mV in the HG group. The spike amplitude of hiPSC-CMs was increased by (30.8±3.7)% in the HG+INS group compared with that in the HG group (P<0.05). The CV in the HG+INS group was increased from (0.23±0.08) mm/ms to (0.32±0.08) mm/ms after insulin treatment, which was increased from (0.21±0.04) mm/ms to (0.30±0.07) mm/ms in the HG group, but there was no significant difference in CV between the HG+INS group and the HG group (P>0.05). The induction experiment showed that 10 μmol/L of sotalol could prolong the FPDc of hiPSC-CMs by (78.9±11.6)% in the HG+INS group, but no arrhythmia was induced in each well.

CONCLUSIONS

High glucose can induce FPD/FPDc of hiPSC-CMs prolongation and increase the risk of arrhythmia induced by drugs. Insulin can reduce the FPD/FPDc prolongation and the risk of induced arrhythmia by high glucose.These results are important to understand the electrophysiological changes of the myocardium in diabetic patients and the impact of insulin therapy on its electrophysiology. Further study on the mechanism may provide new ideas and methods for the treatment of acquired and even inherited long QT syndrome.

The Goto Kakizaki rat: Impact of age upon changes in cardiac and renal structure, function

BACKGROUND

Patients with diabetes are at a high risk for developing cardiac dysfunction in the absence of coronary artery disease or hypertension, a condition known as diabetic cardiomyopathy. Contributing to heart failure is the presence of diabetic kidney disease. The Goto-Kakizaki (GK) rat is a non-obese, non-hypertensive model of type 2 diabetes that, like humans, shares a susceptibility locus on chromosome 10. Herein, we perform a detailed analysis of cardio-renal remodeling and response to renin angiotensin system blockade in GK rats to ascertain the validity of this model for further insights into disease pathogenesis.

METHODS

Study 1: Male GK rats along with age matched Wistar control animals underwent longitudinal assessment of cardiac and renal function for 32 weeks (total age 48 weeks). Animals underwent regular echocardiography every 4 weeks and at sacrifice, early (~24 weeks) and late (~48 weeks) timepoints, along with pressure volume loop analysis. Histological and molecular characteristics were determined using standard techniques. Study 2: the effect of renin angiotensin system (RAS) blockade upon cardiac and renal function was assessed in GK rats. Finally, proteomic studies were conducted in vivo and in vitro to identify novel pathways involved in remodeling responses.

RESULTS

GK rats developed hyperglycaemia by 12 weeks of age (p<0.01 c/w Wistar controls). Echocardiographic assessment of cardiac function demonstrated preserved systolic function by 48 weeks of age. Invasive studies demonstrated left ventricular hypertrophy, pulmonary congestion and impaired diastolic function. Renal function was preserved with evidence of hyperfiltration. Cardiac histological analysis demonstrated myocyte hypertrophy (p<0.05) with evidence of significant interstitial fibrosis (p<0.05). RT qPCR demonstrated activation of the fetal gene program, consistent with cellular hypertrophy. RAS blockade resulted in a reduction blood pressure(P<0.05) cardiac interstitial fibrosis (p<0.05) and activation of fetal gene program. No significant change on either systolic or diastolic function was observed, along with minimal impact upon renal structure or function. Proteomic studies demonstrated significant changes in proteins involved in oxidative phosp4horylation, mitochondrial dysfunction, beta-oxidation, and PI3K/Akt signalling (all p<0.05). Further, similar changes were observed in both LV samples from GK rats and H9C2 cells incubated in high glucose media.

CONCLUSION

By 48 weeks of age, the diabetic GK rat demonstrates evidence of preserved systolic function and impaired relaxation, along with cardiac hypertrophy, in the presence of hyperfiltration and elevated protein excretion. These findings suggest the GK rat demonstrates some, but not all features of diabetes induced "cardiorenal" syndrome. This has implications for the use of this model to assess preclinical strategies to treat cardiorenal disease.

Transcriptomic uniqueness and commonality of the ion channels and transporters in the four heart chambers

Myocardium transcriptomes of left and right atria and ventricles from four adult male C57Bl/6j mice were profiled with Agilent microarrays to identify the differences responsible for the distinct functional roles of the four heart chambers. Female mice were not investigated owing to their transcriptome dependence on the estrous cycle phase. Out of the quantified 16,886 unigenes, 15.76% on the left side and 16.5% on the right side exhibited differential expression between the atrium and the ventricle, while 5.8% of genes were differently expressed between the two atria and only 1.2% between the two ventricles. The study revealed also chamber differences in gene expression control and coordination. We analyzed ion channels and transporters, and genes within the cardiac muscle contraction, oxidative phosphorylation, glycolysis/gluconeogenesis, calcium and adrenergic signaling pathways. Interestingly, while expression of Ank2 oscillates in phase with all 27 quantified binding partners in the left ventricle, the percentage of in-phase oscillating partners of Ank2 is 15% and 37% in the left and right atria and 74% in the right ventricle. The analysis indicated high interventricular synchrony of the ion channels expressions and the substantially lower synchrony between the two atria and between the atrium and the ventricle from the same side.

Mechanisms of diabetic cardiomyopathy and potential therapeutic strategies: preclinical and clinical evidence

The pathogenesis and clinical features of diabetic cardiomyopathy have been well-studied in the past decade, but effective approaches to prevent and treat this disease are limited. Diabetic cardiomyopathy occurs as a result of the dysregulated glucose and lipid metabolism associated with diabetes mellitus, which leads to increased oxidative stress and the activation of multiple inflammatory pathways that mediate cellular and extracellular injury, pathological cardiac remodelling, and diastolic and systolic dysfunction. Preclinical studies in animal models of diabetes have identified multiple intracellular pathways involved in the pathogenesis of diabetic cardiomyopathy and potential cardioprotective strategies to prevent and treat the disease, including antifibrotic agents, anti-inflammatory agents and antioxidants. Some of these interventions have been tested in clinical trials and have shown favourable initial results. In this Review, we discuss the mechanisms underlying the development of diabetic cardiomyopathy and heart failure in type 1 and type 2 diabetes mellitus, and we summarize the evidence from preclinical and clinical studies that might provide guidance for the development of targeted strategies. We also highlight some of the novel pharmacological therapeutic strategies for the treatment and prevention of diabetic cardiomyopathy.

Bioenergetic functions in subpopulations of heart mitochondria are preserved in a non-obese type 2 diabetes rat model (Goto-Kakizaki)

A distinct bioenergetic impairment of heart mitochondrial subpopulations in diabetic cardiomyopathy is associated with obesity; however, many type 2 diabetic (T2DM) patients with high-risk for cardiovascular disease are not obese. In the absence of obesity, it is unclear whether bioenergetic function in the subpopulations of mitochondria is affected in heart with T2DM. To address this issue, a rat model of non-obese T2DM was used to study heart mitochondrial energy metabolism, measuring bioenergetics and enzyme activities of the electron transport chain (ETC). Oxidative phosphorylation in the presence of substrates for ETC and ETC activities in both populations of heart mitochondria in T2DM rats were unchanged. Despite the preservation of mitochondrial function, aconitase activity in T2DM heart was reduced, suggesting oxidative stress in mitochondria. Our study indicate that metabolic function of heart mitochondria is unchanged in the face of oxidative stress and point to a critical role of obesity in T2DM cardiomyopathy.

Retinal Deficits Precede Cognitive and Motor Deficits in a Rat Model of Type II Diabetes

Purpose

To investigate the temporal appearance of retinal, cognitive, and motor deficits in Goto-Kakizaki (GK) rats, a spontaneously occurring, polygenic model of type II diabetes. GK rats develop impaired insulin secretion at 2 weeks and fasting hyperglycemia at 4 weeks.

Methods

In male and female GK rats and Wistar controls, glucose tolerance test (hyperglycemia) and electroretinogram (ERG, retinal function) were performed at 4 and 8 weeks of age. Spectral domain-optical coherence tomography (retinal structure) was assessed at 6 weeks. Spatial alternation (cognitive function) and number of entries (exploratory behavior) were assessed via Y-maze at 4, 5, 6, 7, and 8 weeks. Rotarod (motor function) was performed at 4, 6, and 8 weeks.

Results

By 4 weeks, the GK rats exhibited significant glucose intolerance (P < 0.001) and retinal deficits, including delays in ERG implicit times (flicker, P < 0.01; oscillatory potentials, P < 0.001). In addition, the GK rats showed greater ERG amplitudes (P < 0.001) and thinner retinas (P < 0.001). At 7 weeks, the GK rats showed deficits in cognitive function (P < 0.001) and exploratory behavior (P < 0.01). However, no motor function deficits were observed by 8 weeks. Interestingly, the male GK rats showed greater hyperglycemia (P < 0.05), but the female rats showed greater ERG delays (P < 0.001).

Conclusions

In GK rats, retinal function deficits developed prior to cognitive or motor deficits. Future studies will investigate common mechanistic links, long-term functional and vascular changes, and whether early retinal deficits can predict cognitive dysfunction or late-stage retinal disease.