Deletion of Sirt3 does not affect atherosclerosis but accelerates weight gain and impairs rapid metabolic adaptation in LDL receptor knockout mice: implications for cardiovascular risk factor development

Sirt3 is a mitochondrial NAD(+)-dependent deacetylase that governs mitochondrial metabolism and reactive oxygen species homeostasis. Sirt3 deficiency has been reported to accelerate the development of the metabolic syndrome. However, the role of Sirt3 in atherosclerosis remains enigmatic. We aimed to investigate whether Sirt3 deficiency affects atherosclerosis, plaque vulnerability, and metabolic homeostasis. Low-density lipoprotein receptor knockout (LDLR (-/-)) and LDLR/Sirt3 double-knockout (Sirt3 (-/-) LDLR (-/-)) mice were fed a high-cholesterol diet (1.25 % w/w) for 12 weeks. Atherosclerosis was assessed en face in thoraco-abdominal aortae and in cross sections of aortic roots. Sirt3 deletion led to hepatic mitochondrial protein hyperacetylation. Unexpectedly, though plasma malondialdehyde levels were elevated in Sirt3-deficient mice, Sirt3 deletion affected neither plaque burden nor features of plaque vulnerability (i.e., fibrous cap thickness and necrotic core diameter). Likewise, plaque macrophage and T cell infiltration as well as endothelial activation remained unaltered. Electron microscopy of aortic walls revealed no difference in mitochondrial microarchitecture between both groups. Interestingly, loss of Sirt3 was associated with accelerated weight gain and an impaired capacity to cope with rapid changes in nutrient supply as assessed by indirect calorimetry. Serum lipid levels and glucose tolerance were unaffected by Sirt3 deletion in LDLR (-/-) mice. Sirt3 deficiency does not affect atherosclerosis in LDLR (-/-) mice. However, Sirt3 controls systemic levels of oxidative stress, limits expedited weight gain, and allows rapid metabolic adaptation. Thus, Sirt3 may contribute to postponing cardiovascular risk factor development.

Genetic, metabolic, and molecular insights into the diverse outcomes of diet-induced obesity in mouse

Alexis Maximilien Bachmann

Overweight and obesity are increasingly common public health issues worldwide, leading to a wide range of diseases from metabolic syndrome to steatohepatitis and cardiovascular diseases. While the increase in the prevalence of obesity is partly attributable to changes in lifestyle (i.e. increased sedentarity and changes in eating behaviour), the metabolic and clinical impacts of these obesogenic conditions varies between sexes and genetic backgrounds. The conception of personalised treatments of obesity and its complications require a thorough understanding of the diversity of responses to environmental stimuli such as high-fat diet intake. In this thesis, by analysing nine genetically diverse mouse strains, we show that much like humans, mice respond to high-fat diet in a genetic- and sex-dependent manner. Physiological and molecular responses to high-fat diet are associated with the expression of genes involved in immunity and mitochondrial function. Finally, we find that mitochondrial function and mitochondrial supercomplex assembly may explain part of the diversity of physiological responses. By exploring the complex interactions between genetics and metabolic phenotypes via gene expression and molecular traits, we shed light on the importance of genetic background and sex in determining metabolic outcomes. In addition to providing the community with an extensive resource for optimizing future experiments, this work serves as an exemplary design for more generalizable translational studies.

EPFL2021

The role of the transcriptional regulator BCL6 in adipose tissue development and remodeling

Teresa Didonna

The adipose tissue is increasingly recognized for the important role it plays in various diseases and pathological conditions, such as obesity, diabetes, cardiovascular diseases, osteoporosis, cancer, and aging. Despite its long-held reputation for being a simple fat storage tissue, we now know the adipose tissue is a singular organ playing a crucial role in whole-body metabolism and energy homeostasis. Understanding the development and functioning of adipose tissue and, in particular, of its structural and functional fat depots, is crucial to understanding human health and disease, and can pave the way for personalized therapeutic and preventive approaches. Transcription factors are key regulators of adipose tissue biogenesis, development, and responses to changes in environmental stimuli such as feeding, fasting, and exercise. The transcription repressor BCL6 - well known in immunology and cancer - has been recently linked to adipogenesis and metabolism. In the context of this thesis, in order to better understand the role of BCL6 in adipose tissue development and remodeling, we sought to further investigate the physiological function of BCL6 in white and brown adipose tissue. We developed and characterized a fat-specific conditional knockout mouse model for Bcl6 (Bcl6 FKO), and a Pdgfra-Cre driver was used to obtain in vivo ablation of Bcl6 in adipogenic precursors and stem cells (ASPCs). We discovered that ablation of Bcl6 in adipocyte precursors has a major impact on white fat mass and its distribution in the bodies of mice. Moreover, being fed a standard and a high-fat diet has a prominent effect on visceral adipose tissue, the mass of which was significantly reduced compared to that of control mice. Additionally, Bcl6 FKO mice did not demonstrate sensitivity to high-fat diet-induced mass gain and related comorbidities, such as hepatic steatosis and glucose-homeostasis alterations. The increased number of adipocytes observed in the gonadal adipose tissue occurred without increasing overall adiposity and body weight compared to control mice under a high-caloric diet regime. In conclusion, Bcl6 FKO mice represent a lean mouse model with a healthy balance between subcutaneous and visceral white adipose tissue depots. BCL6 could be an interesting adipose-specific target for therapeutic interventions in the context of obesity and related metabolic dysfunctions.

EPFL2019

Thermal microsensors for in vitro and in vivo monitoring of chemical and biological processes

Rima Padovani

Metabolism is a highly coordinated biochemical cellular activity indispensable for sustaining life, and is often related to mitochondrial functioning. Innovative technological approaches allowing the study of the thermal signature of chemical and metabolic processes are presented in this thesis. In particular, two solutions are proposed: (i) a nanocalorimetric platform enabling measurements of biochemical heat production in vitro, and (ii) a method for localized in vivo temperature measurements in mice using implantable miniaturized sensors. First, the design and development of the nanocalorimetric platform is presented. In particular, this platform is built around a commercial thermopile-based sensor chip, on which an open-well reservoir holding a sample with a volume of a few tens of microliter is directly positioned. Both components are embedded into an isothermal housing providing an excellent temperature stability (± 1 mK), which is a prerequisite for accurate heat power measurements. Furthermore, the platform is characterized by a fast thermalization time to the target temperature (less than 30 minutes) and a fast sensing response time (a few seconds). Numerical simulations, which allowed the optimization of the platform design, are also reported. Electrical calibration of the platform was carried out using resistive heaters positioned either on the sensor chip or in the sample reservoir. A heat power limit of detection of 70 nW was estimated. Then, two reactions for the validation of the nanocalorimetric platform were investigated: (i) the mixing of 1-propanol in water, and (ii) the oxidation of glucose catalyzed by glucose oxidase. The results showed good agreement with literature, confirming that our versatile platform may be applied to many thermochemical studies, including thermodynamic analysis and kinetic reaction analysis. Furthermore, localized in vivo temperature measurements in mice by means of implantable miniaturized sensors are presented. The aim was to monitor mouse metabolism during cold exposure, and to record possible temperature differences between the body temperature measured in the abdomen and the temperature of the brown adipose tissue (BAT) situated in the interscapular area. This approach is of biological interest as it may help unravelling the question whether biochemical activation of BAT is associated with local increase in metabolic heat production. Specifically, the preparation and calibration of miniaturized thermistor sensors are reported and the surgical procedure is also described. Temperature measurements, carried out on mice during cold exposure (6 °C) for a maximum duration of 6 hours, are presented and analyzed. Control measurements with a conventional probe confirmed the good performance of the implanted sensors. Moreover, analysis of the experimental results allowed the identification of two different mouse phenotypes, distinguishable in terms of their metabolic resistance to cold exposure. This difference was investigated from the thermal point of view by computational simulations. A simple physical model of the mouse body allowed to reproduce the global evolution of hypothermia and to explain qualitatively the temperature difference between abdomen and BAT locations. While with this approach, we have demonstrated the importance and feasibility of localized temperature measurements on mice, further optimization of this technique is foreseen to better identify local metabolism variations.