The Cardiac Dysfunction Caused by Metabolic Alterations in Alzheimer's Disease

Abstract

A progressive defect in the energy generation pathway is implicated in multiple aging-related diseases, including cardiovascular conditions and Alzheimer's Disease (AD). However, evidence of the pathogenesis of cardiac dysfunction in AD and the associations between the two organ diseases need further elucidation. This study aims to characterize cellular defects resulting in decreased cardiac function in AD-model. 5XFAD mice, a strain expressing five mutations in human APP and PS1 that shows robust Aβ production with visible plaques at 2 months and were used in this study as a model of AD. 5XFAD mice and wild-type (WT) counterparts were subjected to echocardiography at 2-, 4-, and 6-month, and 5XFAD had a significant reduction in cardiac fractional shortening and ejection fraction compared to WT. Additionally, 5XFAD mice had decreased observed electrical signals demonstrated as decreased R, P, T wave amplitudes. In isolated cardiomyocytes, 5XFAD mice showed decreased fraction shortening, rate of shortening, as well as the degree of transient calcium influx. To reveal the mechanism by which AD leads to cardiac systolic dysfunction, the immunoblotting analysis showed increased activation of AMP-activated protein kinase (AMPK) in 5XFAD left ventricular and brain tissue, indicating altered energy metabolism. Mito Stress Assays examining mitochondrial function revealed decreased basal and maximal oxygen consumption rate, as well as defective pyruvate dehydrogenase activity in the 5XFAD heart and brain. Cellular inflammation was provoked in the 5XFAD heart and brain marked by the increase of reactive oxygen species accumulation and upregulation of inflammatory mediator activities. Finally, AD pathological phenotype with increased deposition of Aβ and defective cognitive function was observed in 6-month 5XFAD mice. In addition, elevated fibrosis was observed in the 6-month 5XFAD heart. The results implicated that AD led to defective mitochondrial function, and increased inflammation which caused the decrease in contractility of the heart.

Keywords: Alzheimer's Disease; cardiac dysfunction; glucose metabolic alterations; metabolic regulation; mitochondrial deficits.

Figure 1

Cardiac systolic dysfunction in 5XFAD compared to WT mice chronologically. (A) Echocardiography showed that 5XFAD mice developed progressive cardiac systolic dysfunction over time with reduced left ventricular function as shown by ejection fraction (EF) and fractional shortening (FS). Upper: Representative images of M-mode echocardiography. Lower: Quantification of echocardiography measurements for EF and FS. Biological replicates N = 9 for each group. P-value was determined by two-way ANOVA with Tukey's post hoc test. (B) Electrocardiography (ECG) showed that 5XFAD mice developed a decreased electrical signal over time with a reduced P wave, QRT complex, and the T wave. Upper: Representative images of ECG parameters. Lower: Quantification of ECG measurements. Biological replicates N = 8 for each group. P-value was determined by two-way ANOVA with Tukey's post hoc test.

Figure 2

Cardiomyocytes of 5XFAD mice exhibited impaired extend and rate of contraction associated with decreased calcium influx, (A) The contractile properties of isolated cardiomyocytes from 5XFAD (6 months) and WT (6 months) hearts. Biological replicates N = 47–68 for each group, these cardiomyocytes were isolated from a total of six mice. P-value determined by two-tailed students t-test. (B) The transient calcium signal response of the isolated cardiomyocytes from 5XFAD (6 months) and WT (6 months) hearts. Biological replicates N = 24–40 for each group, these cardiomyocytes were isolated from a total of six mice. P-value determined by two-tailed students t-test. (C) Immunoblotting showed the phosphorylation of AMPK at Threonine 172 in left ventricles from 5XFAD (6 months) and WT (6 months) hearts. Biological replicates N = 6 for each group. P-value determined by two-tailed students t-test. (D) Immunoblotting showed the phosphorylation of AMPK at Threonine 172 in the hippocampus and cortex from 5XFAD (6 months) and WT (6 months). Biological replicates N = 6 for each group. P-value determined by two-tailed students t-test.

Figure 3

Impairs mitochondrial oxidative phosphorylation in the 5XFAD heart, hippocampus, and cortex. (A) Mitochondrial stress assay examined the mitochondrial oxidative phosphorylation (OXPHOS) complexes activity in the heart of 5XFAD (6 months) and WT (6 months) mice demonstrated by measuring the oxygen consumption rate (OCR). N = 6 for each group. For each mouse, we collected 9–10 wells using isolated cardiomyocytes for OCR measurement and averaged the results to obtain a single value per mouse. P-value determined by two-tailed students t-test. (B) Mitochondrial stress assay examined the mitochondrial oxidative phosphorylation (OXPHOS) complexes activity in the hippocampus of 5XFAD (6 months) and WT (6 months) mice demonstrated by measuring the oxygen consumption rate (OCR). N = 6 for each group. For each mouse, we collected 1-2 tissue samples for OCR measurement and averaged the results to obtain a single value per mouse. P-value determined by twotailed students t-test. (C) Mitochondrial stress assay examined the mitochondrial oxidative phosphorylation (OXPHOS) complexes activity in the cortex of 5XFAD (6 months) and WT (6 months) mice demonstrated by measuring the oxygen consumption rate (OCR). N = 6 for each group. For each mouse, we collected 1-2 tissue samples for OCR measurement and averaged the results to obtain a single value per mouse. P-value determined by two-tailed students t-test.

Figure 4

Excessive oxidative stress and provoked cellular proinflammatory signaling in 5XFAD heart and brain. (A) MitoSox staining showed increased superoxide accumulation in the heart of 5XFAD (6 months) mice vs. WT (6 months) mice. Biological replicates N = 3 for each group. For each mouse, we collected 3–4 tissue sections for superoxide accumulation measurement and averaged the results to obtain a single value per mouse. P-value determined by two-tailed students t-test. (B) MitoSox staining showed increased superoxide accumulation in the hippocampus and cortex of 5XFAD (6 months) mice vs. WT (6 months) mice. Biological replicates N = 3 for each group. For each mouse, we collected 3–4 tissue sections for superoxide accumulation measurement and averaged the results to obtain a single value per mouse. P-value determined by two-tailed students t-test.

Figure 5

Aβ burden and cognitive function assessment in AD mice. (A) Representative images and quantification of Aβ stained with 6E10 antibody in the hippocampus and cortex of 5XFAD (6 months) mice and WT (6 months) mice. Biological replicates N = 8 for each group. P-value determined by two-tailed students t-test. (B) Radial arm water maze test showed the latencies to hidden platform and the errors happen in arms in WT (6 months) and 5XFAD (6 months) mice. Biological replicates N = 8 for each group. P-value determined by two-tailed students t-test. (C) Myocardial histology analysis showed elevated fibrosis in 5XFAD (6 months) mice heart compared to the WT (6 months) group. Biological replicates N = 3 for each group. For each mouse, we collected 4 tissue sections for measurement and averaged the results to obtain a single value per mouse. P-value determined by two-tailed students t-test. Black arrows highlighted the inflammatory infiltration and fibrosis formation in 5XFAD (6 months) heart H&E staining and Trichrome staining, respectively.