Researchers rethink how 'beige' fat cells burn calories

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By IMPC

Published 5th July 2018

It has been known for decades that low temperatures can trigger specialized fat cells to burn energy to produce heat, but in a new study, UC San Francisco researchers have discovered a new heat-producing pathway in fat cells that works by burning excess blood glucose, suggesting a potential new approach to treating metabolic disorders such as obesity and type 2 diabetes.

The findings, published in Nature Medicine, represent a paradigm shift in scientists’ understanding of how mammals turn their fat reserves into heat, said senior investigator Shingo Kajimura, PhD, an associate professor of cell and tissue biology in UCSF’s School of Dentistry and a member of the UCSF Diabetes Center.

All mammals, including humans, use white fat cells to store energy, but can also have reserves of brown fat, which burns stored energy to produce heat. Human babies are born with brown fat as a natural defense against cold and hibernating animals such as bears build up large stores of brown fat for the same reason. However, adult humans do not usually have much brown fat.

In 2015, the Kajimura lab identified a new type of fat in adult humans — so-called “beige” fat — which exists in pockets within fat tissue and can convert white fat into brown fat in response to cold or other stresses. In follow-up studies, the lab showed that mice with more beige fat were protected from diabetes and diet-induced obesity — by burning calories to generate heat, the animals were more easily able to shed unhealthy excess white fat. However, the mechanism by which these cells turn energy into heat was a missing piece of the puzzle.

“Now we’ve found this new pathway by which beige fat cells make heat,” Kajimura said. “We’re very excited not only for the science, but also for its terrific therapeutic potential to treat obesity and type 2 diabetes.”

 

More than one way to make heat

When you are cold, your muscles initially shiver to produce heat, but with longer exposure to cold temperatures, your body needs other ways to keep you warm.

For many years, researchers interested in understanding how our bodies turn stored energy into heat have studied the function of a protein called uncoupling protein 1 (UCP1). This protein is present only in brown and beige fat cells, and works by redirecting energy flow in mitochondria, the power plants of our cells, so they produce heat rather than biologically available energy.

In many mammals, UCP1 is responsible for producing heat in response to environmental needs without muscle shivering: its activity shoots up with cold temperatures as well as after overfeeding. However, some species — such as pigs — don’t have a functional UCP1 protein, but are somehow still able to stay warm in cold environments. This led the Kajimura lab to wonder whether there might be another mechanism involved in beige and brown fat’s ability to generate heat.

In previous studies, the team deleted the UCP1 protein from mice with higher-than-normal levels of beige fat, and found the animals were still resistant to diabetes and diet-induced obesity, demonstrating that beige fat’s beneficial effects were completely independent of UCP1.

“This was conceptually very surprising to us and for the field, because UCP1 has been the only known thermogenic protein for over twenty years,” Kajimura said.

Now, study lead author Kenji Ikeda, MD, PhD, and colleagues have found a new mechanism that beige fat cells can use to generate heat when exposed to cold temperatures, which involves activating a pair of proteins called SERCA2b and ryanodine receptor 2 (RyR2).

Normally, these proteins are responsible for controlling the availability of calcium, a key ion within fat cells. When calcium levels are too high, SERCA2b can use some energy to pump the extra calcium into storage sites within the cell. Then, when calcium in the cell is low, RyR2 acts as a valve to release some of these stored calcium reserves. But in cold conditions, Kajimura’s team found, cells can activate both proteins at once. Like revving a car’s engine by hitting the gas and brake at the same time, the main consequence is to generate a lot of heat and burn a lot of fuel – in this case glucose.

In fact, the researchers found that this process can burn so much glucose that lowering SERCA2b activity in beige fat cells in mice impacted whole-body glucose usage, suggesting potential applications for the prevention of type 2 diabetes, which is thought to be triggered in part by long-term elevations in blood glucose levels.

“Now that we’ve found that beige fat burns glucose using SERCA2b, it explains many things,” Kajimura said. “This is why mice become diabetic when we reduce beige fat, but disrupting UCP1 does not cause diabetes, and this is why mice are protected from diabetes in the presence of more beige fat.”

An evolutionarily conserved mechanism

Experiments in isolated pig and human beige fat cells laboratory dishes showed that it isn’t just mice who can use SERCA2b to turn excess glucose into heat, the researchers found, though it isn’t yet clear whether this mechanism or UCB1-based thermogenesis dominate in human beige fat.

The discovery that beige fat has more than one mechanism to stave off the cold makes evolutionary sense, Kajimura explained: “Thermoregulation is so important to life that there must be multiple mechanisms — like in pigs that don’t have a functional UCP1.”

Interestingly, Kajimura said, there is a human disease called malignant hyperthermia, in which mutations in the gene for ryanodine receptor 1 — the form of ryanodine receptor used in muscles — cause hyperthermia in the skeletal muscle, suggesting a related mechanism may be involved.

Kajimura and Ikeda are excited by the possibility of using drugs — or even nutritional supplements — to activate SERCA2b in beige fat in hopes of improving the body’s ability to process glucose for patients with type 2 diabetes and obesity, and also to help patients with diseases such as malignant hyperthermia that disrupt normal temperature regulation.

For example, it is well known that eating ginger warms up our bodies, Kajimura said, but the mechanism is poorly understood. Intriguingly, there is some evidence that gingerol, or ginger extract, activates SERCA proteins, suggesting interesting future opportunities for the scientists to test gingerol as a way to trigger beige fat to burn more calories.

Research article: UCP1-independent signaling involving SERCA2b-mediated calcium cycling regulates beige fat thermogenesis and systemic glucose homeostasis

Scientists from the University of Cambridge have identified a potential therapeutic target in the devastating genetic disease Hutchinson-Gilford Progeria Syndrome (HGPS), which is characterised by premature ageing. The research utilises IMPC resources through the use of mouse phenotypic data.

The paper is published in Nature Communications, with preclinical data showing that chemical inhibition or genetic deregulation of the enzyme N-acetyltransferase 10 (NAT10) leads to significant health and lifespan gains in a mouse model of HGPS.

HGPS is a rare condition: patients have an average life expectancy of around 15 years, suffering a variety of symptoms including short stature, low body weight, hair loss, skin thickening, problems with fat storage, osteoporosis, and cardiovascular disease, typically dying of a heart attack.

The disease arises from specific mutations in the gene for the protein Lamin A, which lead to production of a shorter, dysfunctional protein that accumulates in cells, specifically in the membranes surrounding the nucleus. This causes disorganisation of chromatin (the ‘packaging’ around DNA), deregulated transcription, accumulation of DNA damage and defective cell proliferation.

By screening candidate molecules for an effect on nuclear membranes in human HGPS patient-derived cells in vitro, the authors have previously identified a small molecule called remodelin as an effective ameliorative agent. They then identified which component of the cells was being affected by remodelin: an enzyme with a variety of cell functions, called NAT10.

Their aim in the new study was to take these findings into a mouse model with the same genetic defect as HGPS patients, to see whether inhibiting NAT10 – either chemically by administration of remodelin or genetically by engineering reduced production of NAT10 – could ameliorate the disease. The results show that these approaches indeed significantly improved the health of the diseased mice, increased their lifespan, and reduced the effects of the HGPS mutation across a variety of measures in body tissues and at the cellular level.

The research was led by Dr Gabriel Balmus from the Wellcome Trust/ Cancer Research UK Gurdon Institute and Dr Delphine Larrieu from the Cambridge Institute for Medical Research, University of Cambridge; and Dr David Adams from the Wellcome Sanger Institute.

 

Senior author Professor Steve Jackson commented: “We’re very excited by the possibility that drugs targeting NAT10 may, in future, be tested on people suffering from HGPS. I like to describe this approach as a ‘re-balancing towards the healthy state’.

“We first studied the cell biology to understand how the disease affects cells, and then used those findings to identify ways to re-balance the defect at the whole-organism level. Our findings in mice suggest a therapeutic approach to HGPS and other premature ageing diseases.”

 

Research article: https://www.nature.com/articles/s41467-018-03770-3.pdf

Nat10 gene on the IMPC website: http://www.mousephenotype.org/data/genes/MGI:2138939

Glucose is the energy that fuels cells, and the body likes to store glucose for later use. But too much glucose can contribute to obesity, and scientists have long wanted to understand what happens within a cell to tip the balance.

To solve this riddle, researchers at UT Southwestern’s Cecil H. and Ida Green Center for Reproductive Biology Sciences examined specialized compartments inside the cell to reveal the role of a molecule termed NAD+ in turning on genes that make fat cells.

The study utilised IMPC resources for the generation of Nmnat1 conditional knockout mice. Frozen Nmnat1tm1a(EUCOMM)Wtsi embryos on a C57BL/6N background were obtained and recovered at UT Southwestern’s Transgenic Core Facility. The research is reported in the journal Science.

NAD+ is found in every cell of the body and some scientists believe that boosting its production may be tied to better health and to the slowing down of the aging process.

What is NAD+?

NAD+ stands for nicotinamide adenine dinucleotide. It’s a molecule found inside cells in the body that helps transfer energy between molecules.

Why is it important?

NAD+ is believed to play important roles in longevity, aging, and diseases ranging from neurodegenerative disorders to cancer.

UT Southwestern biologists examined individual compartments inside cells that house NAD+ molecules to determine how they control genes that are essential to the fat-storing process – knowledge that could help in a wide range of ailments, including metabolic disorders, neurodegenerative diseases, inflammation and aging, and cancer.

“This compartmentalization ends up having profound effects on gene expression in the nucleus, as well as metabolism in the cytoplasm,” (the jellylike substance outside the cell’s nucleus), said Dr. W. Lee Kraus, Director of the Green Center and senior author on the research. “We found that these processes play key roles in fat cell differentiation and in cancer cells.”

“The previous thinking in the field was that NAD+ was evenly distributed throughout cells and moved freely between different subcellular compartments,” said Dr. Kraus, Professor of Obstetrics and Gynecology and Pharmacology. “We showed that NAD+ is actually compartmentalized – there are separate nuclear and cytoplasmic pools of NAD+ whose levels change under certain cellular conditions.”

Accounting for the levels of NAD+ biosynthesis separately rather than in their totality helped increase the understanding of the biology involved, said first author Dr. Keun Ryu, a postdoctoral researcher in Obstetrics and Gynecology.

“Our study provides a new understanding of NAD+ biology,” he said.

Research article: http://science.sciencemag.org/content/sci/360/6389/eaan5780.full.pdf

Scientists have identified a network of genes that could play an important role in the development of metabolic diseases such as diabetes. A research team from the Helmholtz Center Munich and the International Mouse Phenotyping Consortium (IMPC) led the work that is published in ‘Nature Communications‘.

The development of metabolic diseases like diabetes is a complex process. As well as lifestyle and environmental factors, many different genes are responsible for the pathogenesis of both type 1 and type 2 diabetes. These genes encode information on how to assemble individual proteins that function in glucose metabolism.

Many genes that play an important role in the development of human diseases are still unknown. It is only by deciphering the causal genetic links that we can understand diseases, develop therapeutic interventions, or even prevent an outbreak. Thus, the new diabetes genes discovered in this study could be used, for example, as biomarkers for individual risk prediction, early diagnosis of the disease, or personalised approaches for treatment.

Twenty-three new candidate genes for diabetes in humans

As part of the IMPC, knockout mice – each lacking a specific gene – were examined for metabolic dysfunction. Using this method, researchers are trying to establish whether the missing gene is involved in important metabolic processes and can be linked to human diseases.

“Our analysis of the phenotyping data has identified a total of 974 genes whose loss has strong effects on glucose and lipid metabolism,” said Martin Hrabě de Angelis, who led the study and is the Chair of Experimental Genetics at the Technical University of Munich. “For more than a third of the genes no connection to metabolism was known previously.”

In addition, the researchers that teamed up with first author Dr. Jan Rozman, report that the functions of 51 of the discovered metabolic genes in the mouse were hitherto completely unknown. When compared with genome data collected in humans, they found that 23 of these also appear to play a role in human diabetes. “They are new candidate genes, and mice that lack these genes may be important models to investigate impaired glucose metabolism and diabetes,” explains Rozman, who coordinates the metabolic phenotyping at the German Mouse Clinic as part of the IMPC. One of these genes is C4orf22, which appears to be involved in insulin action in participants of the diabetes study “Tübingen Family Study (TÜF)”.

Interestingly, according to the bioinformatician and co-author Dr. Thomas Werner, these genes were also similar in their structure – many had common genetic elements. The scientists therefore assume that these genes belong to a network. In the future, they want to investigate these new regulatory structures and to explore to what extent they allow the prediction of gene functions of so-far unknown genes.

Publication
Rozman et al. Nature Communications 2018

Further links 
https://www.medizin.uni-tuebingen.de/en/Research/Collaborative+Research/…

To see the data, click here

This study in Nature Genetics reveals hundreds of new insights into gene function and human disease. The paper describes the analysis of 3,328 genes by the IMPC, representing approximately 15% of the mouse genome. 360 new disease models were identified. Moreover, the team identified new candidate genes for diseases with unknown molecular mechanisms. More than half of the genes analysed have never been investigated in a mouse before, and, for 1,092 genes no molecular function or biological process were previously known.

(more…)

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By IMPC

Published 20th March 2018

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