Cardiac expression of the microsomal triglyceride transport protein protects the heart function during ischemia

Aims: The microsomal triglyceride transport protein (MTTP) is critical for assembly and secretion of apolipoprotein B (apoB)-containing lipoproteins and is most abundant in the liver and intestine. Surprisingly, MTTP is also expressed in the heart. Here we tested the functional relevance of cardiac MTTP expression. Materials and methods: We combined clinical studies, advanced expression analysis of human heart biopsies and analyses in genetically modified mice lacking cardiac expression of the MTTP-A isoform of MTTP. Results: Our results indicate that lower cardiac MTTP expression in humans is associated with structural and perfusion abnormalities in patients with ischemic heart disease. MTTP-A deficiency in mice heart does not affect total MTTP expression, activity or lipid concentration in the heart. Despite this, MTTP-A deficient mice displayed impaired cardiac function after a myocardial infarction. Expression analysis of MTTP indicates that MTTP expression is linked to cardiac function and responses in the heart. Conclusions: Our results indicate that MTTP may play an important role for the heart function in conjunction to ischemic events

Glucosylceramide synthase deficiency in the heart compromises β1-adrenergic receptor trafficking

Aims: Cardiac injury and remodelling are associated with the rearrangement of cardiac lipids. Glycosphingolipids are membrane lipids that are important for cellular structure and function, and cardiac dysfunction is a characteristic of rare monogenic diseases with defects in glycosphingolipid synthesis and turnover. However, it is not known how cardiac glycosphingolipids regulate cellular processes in the heart. The aim of this study is to determine the role of cardiac glycosphingolipids in heart function.Methods and results: Using human myocardial biopsies, we showed that the glycosphingolipids glucosylceramide and lactosylceramide are present at very low levels in non-ischaemic human heart with normal function and are elevated during remodelling. Similar results were observed in mouse models of cardiac remodelling. We also generated mice with cardiomyocyte-specific deficiency in Ugcg, the gene encoding glucosylceramide synthase (hUgcg-/- mice). In 9- to 10-week-old hUgcg-/- mice, contractile capacity in response to dobutamine stress was reduced. Older hUgcg-/- mice developed severe heart failure and left ventricular dilatation even under baseline conditions and died prematurely. Using RNA-seq and cell culture models, we showed defective endolysosomal retrograde trafficking and autophagy in Ugcg-deficient cardiomyocytes. We also showed that responsiveness to β-adrenergic stimulation was reduced in cardiomyocytes from hUgcg-/- mice and that Ugcg knockdown suppressed the internalization and trafficking of β1-adrenergic receptors.Conclusions: Our findings suggest that cardiac glycosphingolipids are required to maintain β-adrenergic signalling and contractile capacity in cardiomyocytes and to preserve normal heart function.</p

Cardiac lipids and their role in the diseased heart

Lipids play an essential role within the heart as they are involved in energy storage, membrane stability and signaling. Changes in cardiac lipid composition and utilization may thus have profound effects on cardiac function. Importantly, the diseased heart is associated with intracellular metabolic abnormalities, including accumulation of lipids. In this thesis, I targeted cardiac lipid droplets (LDs) and membrane lipids using genetically modified mice and cultured cardiomyocytes to investigate how myocardial lipid content and composition affect cardiac function. In Paper I, we investigated the LD protein perilipin 2 (Plin2) and its role in myocardial lipid storage. Unexpectedly, Plin2 deficiency in mice result in increased triglyceride levels within the heart as a result of decreased lipophagy. Even though Plin2-/- mice had markedly enhanced lipid levels in the heart, they had normal heart function under baseline conditions and under mild stress. However, after an induced myocardial infarction, cardiac function reduced in Plin2-/- mice compared with Plin2+/+ mice. We have previously shown in both humans and mice that low levels of cardiac Plin5 are unfavorable for heart function. Therefore, in Paper II we tested the hypothesis that forced overexpression of cardiac Plin5 is beneficial for heart function. We found that Plin5 promotes exercise-like effects, inducing physiological hypertrophy with enhanced left ventricular mass, but with preserved heart function. Furthermore, calmodulin-dependent protein kinase II (CaMKII) and phospholamban activities were increased by Plin5 overexpression, indicating enhanced cardiac contractility and calcium handling. In Paper III, we found that the sphingolipid glucosylceramide (GlcCer) accumulates in the human heart following injury. We targeted cardiac Ugcg (the gene encoding GlcCer synthase) in mice (hUgcg–/– mice) and found that a significant decrease in GlcCer in cardiomyocytes results in Golgi dispersion and defective autophagy regulation, leading to compromised β-adrenergic signaling. hUgcg–/– mice developed dilated cardiomyopathy and died prematurely from heart failure. In conclusion, our studies show that dysfunctional cardiac lipid storage plays a role in heart function, both in the healthy and diseased heart. Thus, targeting cardiac lipid accumulation may be a future strategy to delay cardiovascular disease progression

Plin2-deficiency reduces lipophagy and results in increased lipid accumulation in the heart

Myocardial dysfunction is commonly associated with accumulation of cardiac lipid droplets (LDs). Perilipin 2 (Plin2) is a LD protein that is involved in LD formation, stability and trafficking events within the cell. Even though Plin2 is highly expressed in the heart, little is known about its role in myocardial lipid storage. A recent report shows that cardiac overexpression of Plin2 result in massive myocardial steatosis suggesting that Plin2 stabilizes LDs. In this study, we hypothesized that deficiency in Plin2 would result in reduced myocardial lipid storage. In contrast to our hypothesis, we found increased accumulation of triglycerides in hearts, and specifically in cardiomyocytes, from Plin2−/− mice. Although Plin2−/− mice had markedly enhanced lipid levels in the heart, they had normal heart function under baseline conditions and under mild stress. However, after an induced myocardial infarction, stroke volume and cardiac output were reduced in Plin2−/− mice compared with Plin2+/+ mice. We further demonstrated that the increased triglyceride accumulation in Plin2-deficient hearts was caused by altered lipophagy. Together, our data show that Plin2 is important for proper hydrolysis of LDs

Glucosylceramide synthase deficiency in the heart compromises β1-adrenergic receptor trafficking

AIMS : Cardiac injury and remodelling are associated with the rearrangement of cardiac lipids. Glycosphingolipids are membrane lipids that are important for cellular structure and function, and cardiac dysfunction is a characteristic of rare monogenic diseases with defects in glycosphingolipid synthesis and turnover. However, it is not known how cardiac glycosphingolipids regulate cellular processes in the heart. The aim of this study is to determine the role of cardiac glycosphingolipids in heart function. METHODS AND RESULTS : Using human myocardial biopsies, we showed that the glycosphingolipids glucosylceramide and lactosylceramide are present at very low levels in non-ischaemic human heart with normal function and are elevated during remodelling. Similar results were observed in mouse models of cardiac remodelling. We also generated mice with cardiomyocyte-specific deficiency in Ugcg, the gene encoding glucosylceramide synthase (hUgcg(–/–) mice). In 9- to 10-week-old hUgcg(–/–) mice, contractile capacity in response to dobutamine stress was reduced. Older hUgcg(–/–) mice developed severe heart failure and left ventricular dilatation even under baseline conditions and died prematurely. Using RNA-seq and cell culture models, we showed defective endolysosomal retrograde trafficking and autophagy in Ugcg-deficient cardiomyocytes. We also showed that responsiveness to β-adrenergic stimulation was reduced in cardiomyocytes from hUgcg(–/–) mice and that Ugcg knockdown suppressed the internalization and trafficking of β1-adrenergic receptors. CONCLUSIONS : Our findings suggest that cardiac glycosphingolipids are required to maintain β-adrenergic signalling and contractile capacity in cardiomyocytes and to preserve normal heart function

Perilipin 5 is protective in the ischemic heart

Background: Myocardial ischemia is associated with alterations in cardiac metabolism, resulting in decreased fatty acid oxidation and increased lipid accumulation. Here we investigate how myocardial lipid content and dynamics affect the function of the ischemic heart, and focus on the role of the lipid droplet protein perilipin 5 (Plin5) in the pathophysiology of myocardial ischemia. Methods and results: We generated Plin5(-/-) mice and found that Plin5 deficiency dramatically reduced the triglyceride content in the heart. Under normal conditions, Plin5(-/-) mice maintained a close to normal heart function by decreasing fatty acid uptake and increasing glucose uptake, thus preserving the energy balance. However, during stress or myocardial ischemia, Plin5 deficiency resulted in myocardial reduced substrate availability, severely reduced heart function and increased mortality. Importantly, analysis of a human cohort with suspected coronary artery disease showed that a common noncoding polymorphism, rs884164, decreases the cardiac expression of PLIN5 and is associated with reduced heart function following myocardial ischemia, indicating a role for Plin5 in cardiac dysfunction. Conclusion: Our findings indicate that Plin5 deficiency alters cardiac lipid metabolism and associates with reduced survival following myocardial ischemia, suggesting that Plin5 plays a beneficial role in the heart following ischemia. (C) 2016 The Authors. Published by Elsevier Ireland Ltd