Energy homeostasis

Summary

In biology, energy homeostasis, or the homeostatic control of energy balance, is a biological process that involves the coordinated homeostatic regulation of food intake (energy inflow) and energy expenditure (energy outflow). The human brain, particularly the hypothalamus, plays a central role in regulating energy homeostasis and generating the sense of hunger by integrating a number of biochemical signals that transmit information about energy balance. Fifty percent of the energy from glucose metabolism is immediately converted to heat. Energy homeostasis is an important aspect of bioenergetics. Definition In the US, biological energy is expressed using the energy unit Calorie with a capital C (i.e. a kilocalorie), which equals the energy needed to increase the temperature of 1 kilogram of water by 1 °C (about 4.18 kJ). Energy balance, through biosynthetic reactions, can b

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https://en.wikipedia.org/wiki/Energy_homeostasis

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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

Biochemical Membrane Lipidomics during Drosophila Development

Xue Li Guan, Ernst Hafen, Françoise Gisou van der Goot Grunberg

Lipids play critical roles in energy homeostasis, membrane structure, and signaling. Using liquid chromatography and mass spectrometry, we provide a comprehensive semiquantification of lipids during the life cycle of Drosophila melanogaster (230 glycerophospholipids, 210 sphingolipids, 6 sterols and sterol esters, and 60 glycerolipids) and obtain biological insights through this biochemical resource. First, we find a high and constant triacylglycerol-to-membrane lipid ratio during pupal stage, which is nonobvious in the absence of nutrient uptake and tissue remodeling. Second, sphingolipids undergo specific changes in headgroup (glycosylation) and tail configurations (unsaturation and hydroxylation on sphingoid base and fatty acyls, respectively), which correlate with gene expression of known (GIcT/CG6437; FA2H/ CG30502) and putative (Cyt-b5-r/CG13279) enzymes. Third, we identify a gender bias in phosphoethanolamine-ceramides as a lead for future investigation into sexual maturation. Finally, we partially characterize ghiberti, required for male meiotic cytokinesis, as a homolog of mammalian serine palmitoyltransferase.

Elsevier2013

Mitochondrial diseases are rare and severe conditions with debilitating symptoms. Biochemical defects in mitochondria however are common. The difference between these two frequencies is suspected to lie in the capability of the cells to adapt to the homeostatic disbalance. Cells activate retrograde signalling pathways in response to mitochondrial dysfunction, activating cellular transcriptional programs to correct for the disbalance. The full extent of the retrograde signals that are induced by mitochondrial dysfunctions still remains unknown. Studying these retrograde signalling pathways can lead to insights how to make the difference between restoring the homeostasis and a detrimental disease. Investigating the transcriptional responses to artificially induced mitochondrial dysfunction has proven to be lucrative in unveiling novel responses to mitochondrial defects. In this work we studied the transcriptional responses to three chemical models of mitochondrial respiratory chain dysfunction. In line with recently published work, we found a commonly upregulated stress response in all models. Interestingly, shared between all the models was a downregulation of genes enriched in cholesterol biosynthesis pathways. Upon closer examination this encompassed, almost without exception, all transcripts of the mevalonate pathway. This coordinated downregulation of transcripts had functional consequences on the output of the pathway, lowering the abundance of cholesterol precursor metabolites. Other metabolites in more upstream regions of the pathway were changed differently depending on the complex affected, suggesting other regulatory influences involved. Upon closer mechanistic investigation to the cause of the transcriptional suppression we found the main transcription factor responsible for mevalonate pathway gene transcription, the Sterol Response Element Binding Protein 2 (SREBP2), to be inhibited during complex I impairment. This was most evident in sterol depleted conditions where SREBP2 did not activate to normal levels. A decreased activation of SREBP2 is signature for an elevated concentration of ER-cholesterol. Indeed, other ER-bound cholesterol sensitive proteins confirmed this, showing a similar decreased abundance following complex I impairment in both normal and low lipid conditions. To investigate the source of this cholesterol we measured various cholesterol levels. No changes in the levels of total cholesterol (free cholesterol plus esterified cholesterol) were detected. Also, the fraction of free cholesterol was not significantly affected by complex I inhibition. Using the fluorescent cholesterol stain filipin, we were able to measure a modest but reproducible increase in intracellular free cholesterol. This provided novel mechanistic insight into the inhibition of ER sterol sensors and mevalonate pathway activity by mitochondrial dysfunction. Several well-known retrograde signalling pathways AMPK, ISR and mTOR did not explain the changes observed. The exact mechanism through which mitochondria incite an increase in intracellular free cholesterol remains unknown. Taken together this work can provide a mechanistic link between mitochondrial function and intracellular cholesterol homeostasis and could possibly link mitochondrial function to circulating lipoprotein homeostasis. Further work should focus on elucidating the possible mechanism, where differences in complex functions could provide a hint in the right direction.

EPFL2022

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

Teresa Didonna

EPFL2019