Cardiac Characteristics of Transgenic Mice Overexpressing Refsum Disease Gene-Associated Protein within the Heart

PEX3 promotes regenerative repair after myocardial injury in mice through facilitating plasma membrane localization of ITGB3

The peroxisome is a versatile organelle that performs diverse metabolic functions. PEX3, a critical regulator of the peroxisome, participates in various biological processes associated with the peroxisome. Whether PEX3 is involved in peroxisome-related redox homeostasis and myocardial regenerative repair remains elusive. We investigate that cardiomyocyte-specific PEX3 knockout (Pex3-KO) results in an imbalance of redox homeostasis and disrupts the endogenous proliferation/development at different times and spatial locations. Using Pex3-KO mice and myocardium-targeted intervention approaches, the effects of PEX3 on myocardial regenerative repair during both physiological and pathological stages are explored. Mechanistically, lipid metabolomics reveals that PEX3 promotes myocardial regenerative repair by affecting plasmalogen metabolism. Further, we find that PEX3-regulated plasmalogen activates the AKT/GSK3β signaling pathway via the plasma membrane localization of ITGB3. Our study indicates that PEX3 may represent a novel therapeutic target for myocardial regenerative repair following injury.

Disruption of mitochondrial bioenergetics and calcium homeostasis by phytanic acid in the heart: Potential relevance for the cardiomyopathy in Refsum disease

Refsum disease is an inherited peroxisomal disorder caused by severe deficiency of phytanoyl-CoA hydroxylase activity. Affected patients develop severe cardiomyopathy of poorly known pathogenesis that may lead to a fatal outcome. Since phytanic acid (Phyt) concentrations are highly increased in tissues of individuals with this disease, it is conceivable that this branched-chain fatty acid is cardiotoxic. The present study investigated whether Phyt (10-30 μM) could disturb important mitochondrial functions in rat heart mitochondria. We also determined the influence of Phyt (50-100 μM) on cell viability (MTT reduction) in cardiac cells (H9C2). Phyt markedly increased mitochondrial state 4 (resting) and decreased state 3 (ADP-stimulated) and uncoupled (CCCP-stimulated) respirations, besides reducing the respiratory control ratio, ATP synthesis and the activities of the respiratory chain complexes I-III, II, and II-III. This fatty acid also reduced mitochondrial membrane potential and induced swelling in mitochondria supplemented by exogenous Ca²⁺, which were prevented by cyclosporin A alone or combined with ADP, suggesting the involvement of the mitochondrial permeability transition (MPT) pore opening. Mitochondrial NAD(P)H content and Ca²⁺ retention capacity were also decreased by Phyt in the presence of Ca²⁺. Finally, Phyt significantly reduced cellular viability (MTT reduction) in cultured cardiomyocytes. The present data indicate that Phyt, at concentrations found in the plasma of patients with Refsum disease, disrupts by multiple mechanisms mitochondrial bioenergetics and Ca²⁺ homeostasis, which could presumably be involved in the cardiomyopathy of this disease.

SIRT3 improved peroxisomes-mitochondria interplay and prevented cardiac hypertrophy via preserving PEX5 expression

The present study identified a novel mechanism underlying the protective effect of Sirtuin 3 (SIRT3) against pathological cardiac hypertrophy, beyond its well-accepted role as a deacetylase in mitochondria. SIRT3 modulates the peroxisomes-mitochondria interplay by preserving the expression of peroxisomal biogenesis factor 5 (PEX5), thereby improving mitochondrial function. Downregulation of PEX5 was observed in the hearts of Sirt3^-/- mice and angiotensin II-induced cardiac hypertrophic mice, as well as in cardiomyocytes with SIRT3 silencing. PEX5 knockdown abolished the protective effect of SIRT3 against cardiomyocyte hypertrophy, whereas PEX5 overexpression alleviated the hypertrophic response induced by SIRT3 inhibition. PEX5 was involved in the regulation of SIRT3 in mitochondrial homeostasis, including mitochondrial membrane potential, mitochondrial dynamic balance, mitochondrial morphology and ultrastructure, as well as ATP production. In addition, SIRT3 alleviated peroxisomal abnormalities in hypertrophic cardiomyocytes via PEX5, as implied by improvement of peroxisomal biogenesis and ultrastructure, as well as increase of peroxisomal catalase and repression of oxidative stress. Finally, the role of PEX5 as a key regulator of the peroxisomes-mitochondria interplay was confirmed, since peroxisomal defects caused by PEX5 deficiency led to mitochondrial impairment. Taken together, these observations indicate that SIRT3 could maintain mitochondrial homeostasis by preserving the peroxisomes-mitochondria interplay via PEX5. Our findings provide a new understanding of the role of SIRT3 in mitochondrial regulation via interorganelle communication in cardiomyocytes.

Peroxisomal Stress Response and Inter-Organelle Communication in Cellular Homeostasis and Aging

Peroxisomes are key regulators of cellular and metabolic homeostasis. These organelles play important roles in redox metabolism, the oxidation of very-long-chain fatty acids (VLCFAs), and the biosynthesis of ether phospholipids. Given the essential role of peroxisomes in cellular homeostasis, peroxisomal dysfunction has been linked to various pathological conditions, tissue functional decline, and aging. In the past few decades, a variety of cellular signaling and metabolic changes have been reported to be associated with defective peroxisomes, suggesting that many cellular processes and functions depend on peroxisomes. Peroxisomes communicate with other subcellular organelles, such as the nucleus, mitochondria, endoplasmic reticulum (ER), and lysosomes. These inter-organelle communications are highly linked to the key mechanisms by which cells surveil defective peroxisomes and mount adaptive responses to protect them from damages. In this review, we highlight the major cellular changes that accompany peroxisomal dysfunction and peroxisomal inter-organelle communication through membrane contact sites, metabolic signaling, and retrograde signaling. We also discuss the age-related decline of peroxisomal protein import and its role in animal aging and age-related diseases. Unlike other organelle stress response pathways, such as the unfolded protein response (UPR) in the ER and mitochondria, the cellular signaling pathways that mediate stress responses to malfunctioning peroxisomes have not been systematically studied and investigated. Here, we coin these signaling pathways as "peroxisomal stress response pathways". Understanding peroxisomal stress response pathways and how peroxisomes communicate with other organelles are important and emerging areas of peroxisome research.

PEX5 prevents cardiomyocyte hypertrophy via suppressing the redox-sensitive signaling pathways MAPKs and STAT3

Peroxisomal biogenesis factor 5 (PEX5) is a member of peroxisome biogenesis protein family which serves as a shuttle receptor for the import of peroxisome matrix protein. The function of PEX5 on cardiomyocyte hypertrophy remained to be elucidated. Our study demonstrated that the protein expression level of PEX5 was declined in primary neonatal rat cardiomyocytes treated with phenylephrine (PE) and hearts from cardiac hypertrophic rats induced by abdominal aortic constriction (AAC). Overexpression of PEX5 alleviated cardiomyocyte hypertrophy induced by PE, while silencing of PEX5 exacerbated cardiomyocyte hypertrophy. PEX5 improved redox imbalance by decreasing cellular reactive oxygen species level and preserving peroxisomal catalase. Moreover, PEX5 knockdown aggravated PE-induced activation of redox-sensitive signaling pathways, including mitogen-activated protein kinase (MAPK) pathway and signal transducer and activator of transcription 3 (STAT3); whereas PEX5 overexpression suppressed activation of MAPK and STAT3. But PEX5 did not affect PE-induced phosphorylation of mammalian target of rapamycin (mTOR). In conclusion, the present study suggests that PEX5 protects cardiomyocyte against hypertrophy via regulating redox homeostasis and inhibiting redox-sensitive signaling pathways MAPK and STAT3.

Peroxisomes in cardiomyocytes and the peroxisome / peroxisome proliferator-activated receptor-loop

It is well established that the heart is strongly dependent on fatty acid metabolism. In cardiomyocytes there are two distinct sites for the β-oxidisation of fatty acids: the mitochondrion and the peroxisome. Although the metabolism of these two organelles is believed to be tightly coupled, the nature of this relationship has not been fully investigated. Recent research has established the significant contribution of mitochondrial function to cardiac ATP production under normal and pathological conditions. In contrast, limited information is available on peroxisomal function in the heart. This is despite these organelles harbouring metabolic pathways that are potentially cardioprotective, and findings that patients with peroxisomal diseases, such as adult Refsum’s disease, can develop heart failure. In this article, we provide a comprehensive overview on the current knowledge of peroxisomes and the regulation of lipid metabolism by PPARs in cardiomyocytes. We also present new experimental evidence on the differential expression of peroxisome-related genes in the heart chambers and demonstrate that even a mild peroxisomal biogenesis defect (Pex11α-/- ) can induce profound alterations in the cardiomyocyte’s peroxisomal compartment and related gene expression, including the concomitant deregulation of specific PPARs. The possible impact of peroxisomal dysfunction in the heart is discussed and a model for the modulation of myocardial metabolism via a peroxisome/PPAR-loop is proposed.

Phytanic acid disturbs mitochondrial homeostasis in heart of young rats: a possible pathomechanism of cardiomyopathy in Refsum disease

Phytanic acid (Phyt) accumulates in tissues and biological fluids of patients affected by Refsum disease. Although cardiomyopathy is an important clinical manifestation of this disorder, the mechanisms of heart damage are poorly known. In the present study, we investigated the in vitro effects of Phyt on important parameters of oxidative stress in heart of young rats. Phyt significantly increased thiobarbituric acid-reactive substances levels (P < 0.001) and carbonyl formation (P < 0.01), indicating that this fatty acid induces lipid and protein oxidative damage, respectively. In contrast, Phyt did not alter sulfhydryl oxidation. Phyt also decreased glutathione (GSH) concentrations (P < 0.05), an important non-enzymatic antioxidant defense. Moreover, Phyt increased 2',7'-dichlorofluorescin oxidation (DCFH) (P < 0.01), reflecting increased reactive species generation. We also found that the induced lipid and protein oxidative damage, as well as the decreased GSH levels and increased DCFH oxidation provoked by this fatty acid were prevented or attenuated by the reactive oxygen species scavengers melatonin, trolox, and GSH, but not by the nitric oxide inhibitor N: (ω)-nitro-L: -arginine methyl ester, suggesting that reactive oxygen species were involved in these effects. Next, we verified that Phyt strongly inhibited NADH-cytochrome c oxidoreductase (complex I-III) activity (P < 0.001) in heart supernatants, and decreased membrane potential and the NAD(P)H pool in heart mitochondria, indicating that Phyt acts as a metabolic inhibitor and as an uncoupler of the electron transport chain. Therefore, it can be presumed that disturbance of cellular energy and redox homeostasis induced by Phyt may possibly contribute to the cardiomyopathy found in patients affected by Refsum disease.

The promoter of brain-specific angiogenesis inhibitor 1-associated protein 4 drives developmentally targeted transgene expression mainly in adult cerebral cortex and hippocampus

Restricting transgene expression to specific cell types and maintaining long-term expression are major goals for gene therapy. Previously, we cloned brain-specific angiogenesis inhibitor 1-associated protein 4 (BAI1-AP4), a novel brain-specific protein that interacts with BAI1, and found that it was developmentally upregulated in the adult brain. In this report, we isolated 5 kb of the 5' upstream sequence of the mouse BAI1-AP4 gene and analyzed its promoter activity. Functional analyses demonstrated that an Sp1 site was the enhancer, and the region containing the transcription initiation site and an AP2-binding site was the basal promoter. We examined the ability of the BAI1-AP4 promoter to drive adult brain-specific expression by using it to drive lacZ expression in transgenic (TG) mice. Northern blot analyses showed a unique pattern of beta-galactosidase expression in TG brain, peaking at 1 month after birth, like endogenous BAI1-AP4. Histological analyses demonstrated the same localization and developmental expression of beta-galactosidase and BAI1-AP4 in most neurons of the cerebral cortex and hippocampus. Our data indicate that TG mice carrying the BAI1-AP4 promoter could be a valuable model system for region-specific brain diseases.

Changes underlying arrhythmia in the transgenic heart overexpressing Refsum disease gene-associated protein

Previously, we identified a novel neuron-specific protein (PAHX-AP1) that binds to Refsum disease gene product (PAHX), and we developed transgenic (TG) mice that overexpress heart-targeted PAHX-AP1. These mice have atrial tachycardia and increased susceptibility to aconitine-induced arrhythmia. This study was undertaken to elucidate the possible changes in ion channels underlying the susceptibility to arrhythmia in these mice. RT-PCR analyses revealed that the cardiac expression of adrenergic beta(1)-receptor (ADRB1) was markedly lower, whereas voltage-gated potassium channel expression (Kv2.1) was higher in PAHX-AP1 TG mice compared with non-TG mice. However, the expression of voltage-sensitive sodium and calcium channels, and muscarinic receptor was not significantly different. Propranolol pretreatment, a non-specific beta-adrenoceptor antagonist, blocked aconitine-induced arrhythmia in non-TG mice, but not in PAHX-AP1 TG mice. Our results indicate that, in the PAHX-AP1 TG heart, the modulation of voltage-gated potassium channel and ADRB1 expression seem to be important in the electrophysiological changes associated with altered ion channel functions, but ADRB1 is not involved in the greater susceptibility to aconitine-induced arrhythmia.