SERCA2 phosphorylation at serine 663 is a key regulator of Ca2+ homeostasis in heart diseases

Despite advances in cardioprotection, new therapeutic strategies capable of preventing ischemia-reperfusion injury of patients are still needed. Here, we discover that sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA2) phosphorylation at serine 663 is a clinical and pathophysiological event of cardiac function. Indeed, the phosphorylation level of SERCA2 at serine 663 is increased in ischemic hearts of patients and mouse. Analyses on different human cell lines indicate that preventing serine 663 phosphorylation significantly increases SERCA2 activity and protects against cell death, by counteracting cytosolic and mitochondrial Ca2+ overload. By identifying the phosphorylation level of SERCA2 at serine 663 as an essential regulator of SERCA2 activity, Ca2+ homeostasis and infarct size, these data contribute to a more comprehensive understanding of the excitation/contraction coupling of cardiomyocytes and establish the pathophysiological role and the therapeutic potential of SERCA2 modulation in acute myocardial infarction, based on the hotspot phosphorylation level of SERCA2 at serine 663 residue.

Endoplasmic reticulum-mitochondria miscommunication is an early and causal trigger of hepatic insulin resistance and steatosis

BACKGROUND & AIMS

Hepatic insulin resistance in obesity and type 2 diabetes was recently associated with endoplasmic reticulum (ER)-mitochondria miscommunication. These contact sites (mitochondria-associated membranes: MAMs) are highly dynamic and involved in many functions; however, whether MAM dysfunction plays a causal role in hepatic insulin resistance and steatosis is not clear. Thus, we aimed to determine whether and how organelle miscommunication plays a role in the onset and progression of hepatic metabolic impairment.

METHODS

We analyzed hepatic ER-mitochondria interactions and calcium exchange in a time-dependent and reversible manner in mice with diet-induced obesity. Additionally, we used recombinant adenovirus to express a specific organelle spacer or linker in mouse livers, to determine the causal impact of MAM dysfunction on hepatic metabolic alterations.

RESULTS

Disruption of ER-mitochondria interactions and calcium exchange is an early event preceding hepatic insulin resistance and steatosis in mice with diet-induced obesity. Interestingly, an 8-week reversal diet concomitantly reversed hepatic organelle miscommunication and insulin resistance in obese mice. Mechanistically, disrupting structural and functional ER-mitochondria interactions through the hepatic overexpression of the organelle spacer FATE1 was sufficient to impair hepatic insulin action and glucose homeostasis. In addition, FATE1-mediated organelle miscommunication disrupted lipid-related mitochondrial oxidative metabolism and induced hepatic steatosis. Conversely, reinforcement of ER-mitochondria interactions through hepatic expression of a synthetic linker prevented diet-induced glucose intolerance after 4 weeks' overnutrition. Importantly, ER-mitochondria miscommunication was confirmed in the liver of obese patients with type 2 diabetes, and correlated with glycemia, HbA1c and HOMA-IR index.

CONCLUSIONS

ER-mitochondria miscommunication is an early causal trigger of hepatic insulin resistance and steatosis, and can be reversed by switching to a healthy diet. Thus, targeting MAMs could help to restore metabolic homeostasis.

LAY SUMMARY

The literature suggests that interactions between the endoplasmic reticulum and mitochondria could play a role in hepatic insulin resistance and steatosis during chronic obesity. In the present study, we reappraised the time-dependent regulation of hepatic endoplasmic reticulum-mitochondria interactions and calcium exchange, investigating reversibility and causality, in mice with diet-induced obesity. We also assessed the relevance of our findings to humans. We show that organelle miscommunication is an early causal trigger of hepatic insulin resistance and steatosis that can be improved by nutritional strategies.

Comparative proteomic analysis of cardiac mitochondria-associated reticulum membranes in type 2 diabetes

Funding Acknowledgements

Type of funding sources: Public grant(s) – National budget only. Main funding source(s): INSERM U1060

Introduction

The prevalence of type 2 diabetes (T2D) is increasing worldwide with cardiovascular complications being a leading cause of T2D-related morbi-mortality, notably diabetic cardiomyopathy (DCM). The involvement of the mitochondria-associated reticular membranes (MAM) in diabetic cardiomyopathy starts to be demonstrated, notably for Ca²+ regulation of cellular processes. However, while the MAM protein composition was studied in several organs, none was conducted on cardiac MAM.

Objective

We aim to characterize the cardiac MAM proteome by an exhaustive mass spectrometry-based proteomics together with the effect of early T2D using two well-established obesogenic diabetic mouse models (high-fat high-sucrose diet, HFHSD, and leptin-deficient ob/ob).

Methods

12 weeks old ob/ob mice versus 16 weeks HFHSD-fed mice were characterized at the subcellular level. MAM were enriched by subcellular fractionation, then protein compostion was assessed by mass spectrometry. Uniprot and Panther softwares were used to study the MAM proteomes. At the cellular level, mitochondrial calcium transfer was assessed using a mitochondrial Ca²+ sensor, while contractile function was evaluated by studying cardiomyocyte Ca²+ transients and echocardiography.

Results

Compartmental analysis of MAMs showed an enrichment of proteins from mitochondria, reticulum, cytoskeleton and plasma membrane. On the functional level, molecular and biological annotations revealed that cardiac MAM presents mainly enzymes and transporters, associated to 4 main biological processes: cellular and metabolic processes, localization and cellular component organization. Both the cellular response to stress and the lipid metabolism processes were found upregulated in both diabetic cardiac MAM. While Ca²+ transport was downregulated in the HFHSD MAM, this process was upregulated in the ob/ob MAM. However, no change in the histamine-induced reticulum-mitochondria Ca²+ transfer was observed in the ob/ob cardiomyocytes. Contrary to the HFHSD mice, the ob/ob mice did not exhibit any cardiac hypertrophy, by echocardiography or electrophysiology. Echocardiography further revealed no diastolic dysfunction in the ob/ob mice but a mild strain rate reduction with preserved ejection fraction.

Conclusion

Our data investigate the protein composition of the cardiac MAM and unravel the main alterations of the MAM proteome in ob/ob model versus our HFHSD results. Importantly, while being two models of early diabetic cardiomyopathy, the MAM proteome is altered differently, emphasizing on the complexity of diabetic animal models. Indeed, these ob/ob mice, recognized as a T2D and obese mouse model, do not recapitulate the main hallmarks of DCM, i.e. diastolic dysfunction and cardiac hypertrophy, contrary to the heart failure with preserved ejection fraction (HFpEF) observed in the HFHSD mice, yet the lipotoxicity in the ob/ob cardiomyocytes may contribute to the early systolic dysfunction.

Effect of Metformin on T2D-Induced MAM Ca2+ Uncoupling and Contractile Dysfunction in an Early Mouse Model of Diabetic HFpEF

Diabetic cardiomyopathy (DCM) is a leading complication in type 2 diabetes patients. Recently, we have shown that the reticulum-mitochondria Ca²⁺ uncoupling is an early and reversible trigger of the cardiac dysfunction in a diet-induced mouse model of DCM. Metformin is a first-line antidiabetic drug with recognized cardioprotective effect in myocardial infarction. Whether metformin could prevent the progression of DCM remains not well understood. We therefore investigated the effect of a chronic 6-week metformin treatment on the reticulum-mitochondria Ca²⁺ coupling and the cardiac function in our high-fat high-sucrose diet (HFHSD) mouse model of DCM. Although metformin rescued the glycemic regulation in the HFHSD mice, it did not preserve the reticulum-mitochondria Ca²⁺ coupling either structurally or functionally. Metformin also did not prevent the progression towards cardiac dysfunction, i.e., cardiac hypertrophy and strain dysfunction. In summary, despite its cardioprotective role, metformin is not sufficient to delay the progression to early DCM.

Adipose-Tissue-Derived Mesenchymal Stem Cells Mediate PD-L1 Overexpression in the White Adipose Tissue of Obese Individuals, Resulting in T Cell Dysfunction

The PD-L1/PD-1 immune checkpoint axis is the strongest T cell exhaustion inducer. As immune dysfunction occurs during obesity, we analyzed the impact of obesity on PD-L1/PD-1 expression in white adipose tissue (WAT) in mice and in human white adipocytes. We found that PD-L1 was overexpressed in WAT of diet-induced obese mice and was associated with increased expression of PD-1 in visceral but not subcutaneous WAT. Human in vitro cocultures with adipose-tissue-derived mesenchymal stem cells (ASC) and mononuclear cells demonstrated that the presence of ASC harvested from obese WAT (i) enhanced PD-L1 expression as compared with ASC from lean WAT, (ii) decreased Th1 cell cytokine secretion, and (iii) resulted in decreased cytolytic activity towards adipocytes. Moreover, (iv) the implication of PD-L1 in obese ASC-mediated T cell dysfunction was demonstrated through PD-L1 blockade. Finally, (v) conditioned media gathered from these cocultures enhanced PD-L1 expression in freshly differentiated adipocytes, depending on IFNγ. Altogether, our results suggest that PD-L1 is overexpressed in the WAT of obese individuals during IFNγ secretion, leading to T cell dysfunction and notably reduced cytolytic activity. Such a mechanism could shed light on why adipose-tissue-infiltrating viruses, such as SARS-CoV-2, can worsen disease in obese individuals.

Ether lipids, sphingolipids and toxic 1-deoxyceramides as hallmarks for lean and obese type 2 diabetic patients

AIM

The worldwide increase in obesity and type 2 diabetes (T2D) represents a major health challenge. Chronically altered lipids induced by obesity further promote the development of T2D, and the accumulation of toxic lipid metabolites in serum and peripheral organs may contribute to the diabetic phenotype.

METHODS

To better understand the complex metabolic pattern of lean and obese T2D and non-T2D individuals, we analysed the lipid profile of human serum, skeletal muscle and visceral adipose tissue of two cohorts by systematic mass spectrometry-based lipid analysis.

RESULTS

Lipid homeostasis was strongly altered in a disease- and tissue-specific manner, allowing us to define T2D signatures associated with obesity from those that were obesity independent. Lipid changes encompassed lyso-, diacyl- and ether-phospholipids. Moreover, strong changes in sphingolipids included cytotoxic 1-deoxyceramide accumulation in a disease-specific manner in serum and visceral adipose tissue. The high amounts of non-canonical 1-deoxyceramide present in human adipose tissue most likely come from cell-autonomous synthesis because 1-deoxyceramide production increased upon differentiation to adipocytes in mouse cell culture experiments.

CONCLUSION

Taken together, the observed lipidome changes in obesity and T2D will facilitate the identification of T2D patient subgroups and represent an important step towards personalized medicine in diabetes.

Reduced reticulum-mitochondria Ca2+ transfer is an early and reversible trigger of mitochondrial dysfunctions in diabetic cardiomyopathy

Type 2 diabetic cardiomyopathy features Ca²⁺ signaling abnormalities, notably an altered mitochondrial Ca²⁺ handling. We here aimed to study if it might be due to a dysregulation of either the whole Ca²⁺ homeostasis, the reticulum-mitochondrial Ca²⁺ coupling, and/or the mitochondrial Ca²⁺ entry through the uniporter. Following a 16-week high-fat high-sucrose diet (HFHSD), mice developed cardiac insulin resistance, fibrosis, hypertrophy, lipid accumulation, and diastolic dysfunction when compared to standard diet. Ultrastructural and proteomic analyses of cardiac reticulum-mitochondria interface revealed tighter interactions not compatible with Ca²⁺ transport in HFHSD cardiomyocytes. Intramyocardial adenoviral injections of Ca²⁺ sensors were performed to measure Ca²⁺ fluxes in freshly isolated adult cardiomyocytes and to analyze the direct effects of in vivo type 2 diabetes on cardiomyocyte function. HFHSD resulted in a decreased IP3R-VDAC interaction and a reduced IP3-stimulated Ca²⁺ transfer to mitochondria, with no changes in reticular Ca²⁺ level, cytosolic Ca²⁺ transients, and mitochondrial Ca²⁺ uniporter function. Disruption of organelle Ca²⁺ exchange was associated with decreased mitochondrial bioenergetics and reduced cell contraction, which was rescued by an adenovirus-mediated expression of a reticulum-mitochondria linker. An 8-week diet reversal was able to restore cardiac insulin signaling, Ca²⁺ transfer, and cardiac function in HFHSD mice. Therefore, our study demonstrates that the reticulum-mitochondria Ca²⁺ miscoupling may play an early and reversible role in the development of diabetic cardiomyopathy by disrupting primarily the mitochondrial bioenergetics. A diet reversal, by counteracting the MAM-induced mitochondrial Ca²⁺ dysfunction, might contribute to restore normal cardiac function and prevent the exacerbation of diabetic cardiomyopathy.

Glucose Uptake Measurement and Response to Insulin Stimulation in In Vitro Cultured Human Primary Myotubes

Skeletal muscle is the largest glucose deposit in mammals and largely contributes to glucose homeostasis. Assessment of insulin sensitivity of muscle cells is of major relevance for all studies dedicated to exploring muscle glucose metabolism and characterizing metabolic alterations. In muscle cells, glucose transporter type 4 (GLUT4) proteins translocate to the plasma membrane in response to insulin, thus allowing massive entry of glucose into the cell. The ability of muscle cells to respond to insulin by increasing the rate of glucose uptake is one of the standard readouts to quantify muscle cell sensitivity to insulin. Human primary myotubes are a suitable in vitro model, as the cells maintain many features of the donor phenotype, including insulin sensitivity. This in vitro model is also suitable for the test of any compounds that could impact insulin responsiveness. Measurements of the glucose uptake rate in differentiated myotubes reflect insulin sensitivity. In this method, human primary muscle cells are cultured in vitro to obtain differentiated myotubes, and glucose uptake rates with and without insulin stimulation are measured. We provide a detailed protocol to quantify passive and active glucose transport rates using radiolabeled [³H] 2-deoxy-D-Glucose ([³H]2dG). Calculation methods are provided to quantify active basal and insulin-stimulated rates, as well as stimulation fold.

Human skeletal myotubes display a cell-autonomous circadian clock implicated in basal myokine secretion

OBJECTIVE

Circadian clocks are functional in all light-sensitive organisms, allowing an adaptation to the external world in anticipation of daily environmental changes. In view of the potential role of the skeletal muscle clock in the regulation of glucose metabolism, we aimed to characterize circadian rhythms in primary human skeletal myotubes and investigate their roles in myokine secretion.

METHODS

We established a system for long-term bioluminescence recording in differentiated human myotubes, employing lentivector gene delivery of the Bmal1-luciferase and Per2-luciferase core clock reporters. Furthermore, we disrupted the circadian clock in skeletal muscle cells by transfecting siRNA targeting CLOCK. Next, we assessed the basal secretion of a large panel of myokines in a circadian manner in the presence or absence of a functional clock.

RESULTS

Bioluminescence reporter assays revealed that human skeletal myotubes, synchronized in vitro, exhibit a self-sustained circadian rhythm, which was further confirmed by endogenous core clock transcript expression. Moreover, we demonstrate that the basal secretion of IL-6, IL-8 and MCP-1 by synchronized skeletal myotubes has a circadian profile. Importantly, the secretion of IL-6 and several additional myokines was strongly downregulated upon siClock-mediated clock disruption.

CONCLUSIONS

Our study provides for the first time evidence that primary human skeletal myotubes possess a high-amplitude cell-autonomous circadian clock, which could be attenuated. Furthermore, this oscillator plays an important role in the regulation of basal myokine secretion by skeletal myotubes.

Adipose Tissue-Derived Stem Cells From Obese Subjects Contribute to Inflammation and Reduced Insulin Response in Adipocytes Through Differential Regulation of the Th1/Th17 Balance and Monocyte Activation

Obesity, through low-grade inflammation, can drive insulin resistance and type 2 diabetes. While infiltration of adipose tissue (AT) with mononuclear cells (MNCs) is well established in obesity, the functional consequences of these interactions are less understood. Herein, we cocultured human adipose-derived stem cells (ASCs) from obese individuals with MNCs and analyzed their reciprocal behavior. Presence of ASCs 1) enhanced interleukin (IL)-17A secretion by Th17 cells, 2) inhibited γ-interferon and tumor necrosis factor α secretion by Th1 cells, and 3) increased monocyte-mediated IL-1β secretion. IL-17A secretion also occurred in stromal vascular fractions issued from obese but not lean individuals. Th17 polarization mostly depended on physical contacts between ASCs and MNCs-with a contribution of intracellular adhesion molecule-1-and occurred through activation of the inflammasome and phosphatidylinositol 3-kinase pathways. ASCs favored STAT3 over STAT5 transcription factor binding on STAT binding sites within the IL-17A/F gene locus. Finally, conditioned media from activated ASC-MNC cocultures inhibited adipocyte differentiation mRNA markers and impaired insulin-mediated Akt phosphorylation and lipolysis inhibition. In conclusion, we report that obese- but not lean-derived ASCs induce Th17 promotion and monocyte activation. This proinflammatory environment, in turn, inhibits adipogenesis and adipocyte insulin response. The demonstration of an ASC-Th17-monocyte cell axis reveals a novel proinflammatory process taking place in AT during obesity and defines novel putative therapeutic targets.