Excessive weight around our middle gives our brain’s resident immune cells heavy exposure to a signal that turns them against us, setting in motion a crescendo of inflammation that damages cognition, scientists say.
It’s known this visceral adiposity, characterized by an apple-shaped physique, is considered particularly bad for our bodies and brains.
But Medical College of Georgia scientists have shown for the first time one way visceral fat is bad for brains is by enabling easy, excessive access for the proinflammatory protein signal interleukin-1 beta, they report in The Journal of Clinical Investigation.
“We have moved beyond correlations saying there is a lot of visceral fat here, and there is cognitive decline here so they may be interacting with each other,” says Dr. Alexis M. Stranahan, neuroscientist in the MCG Department of Neuroscience and Regenerative Medicine at Augusta University.
“We have identified a specific signal that is generated in visceral fat, released into the blood that gets through the blood brain barrier and into the brain where it activates microglia and impairs cognition.”
The brain typically does not see much of this interleukin-1 beta, but Stranahan and her colleagues have found that visceral adiposity generates high, chronic levels of the signal that in turn over-activate the usually protective microglia, the resident immune cells in our brain.
A bit like a smoldering pot, this chronic inflammation from visceral fat prompts formation of inflammasome complexes that further amplify the immune response and inflammation.
The protein NLRP3 is a core component of the inflammasome complex in the fat, and it’s what promotes the production and release of interleukin-1 beta by fat cells, and stokes the inflammation fire.
It was known these reactions were causing problems in the body, and now the MCG scientists have evidence they are causing problems in the brain.
To explore brain effects, the scientists knocked NLRP3 out of mice and found the mice were protected against obesity-induced inflammation of the brain and the cognitive problems that can result.
They also transplanted visceral adipose tissue from obese mice and obese mice missing NLRP3 into lean mice recipients and found the transplant from the NLRP3 knockout mouse had essentially no effect.
But the transplant from the obese but genetically intact mice increased levels of interleukin-1 beta in the hippocampus, a center of learning and memory in the brain, and impaired cognition.
They looked further and found that just transplanting the visceral fat caused essentially the same impact as obesity resulting from a high-fat diet, including significantly increasing brain levels of interleukin-1 beta and activating microglia.
Mice missing interleukin-1 beta’s receptor on the microglia also were protected from these brain ravages.
Their findings enabled the scientists to start putting together the pieces that NLRP3 was working through interleukin-1 beta, which led them to also knock out the receptor for interleukin-1 beta on microglia and confirm that action in the brain.
Microglia typically function as watchdogs, constantly surveilling and roaming the brain, eliminating dead cells and other debris as well as a myriad of other tasks like forming and pruning connections between neurons. Microglia also have receptors for interleukin-1 beta, and the protein, whose many actions include promoting inflammation, easily passes through the protective blood brain barrier.
Microglia’s helpful – or harmful – actions likely result from signals they are exposed to, and another thing interleukin-1 beta appears to do is prompt microglia to wrap around synapses, possibly exerting damaging pressure and/or releasing substances that actually interfere with conversations between neurons, Stranahan says.
In the absence of disease, microglia also are known to embrace synapses but to release good things like brain-derived neurotrophic factor, which is like fertilizer for these invaluable connections.
Happy microglia also have long processes that enable them to reach out and do their many tasks; and inflammation retracts those processes. The scientists found much shorter processes and less complex microglia in mice on a high-fat diet, more changes that didn’t happen when NLRP3 was knocked out.
To measure cognitive ability, the scientists looked at mice’s ability to navigate a water maze after 12 weeks on a high- or low-fat diet.
They found it took the normal, or wild type, mice consuming the higher fat diet as well as the visceral transplant recipients with NLRP3 intact longer to negotiate the water maze. In fact, while they could reach a platform they could see, they had trouble finding one beneath the water’s surface that they had been taught to find.
Mice with the interleukin-1 receptor knocked out, could find it just fine, Stranahan says.
The high-fat diet, transplant mice also had weaker connections, or synapses, between neurons involved in learning and memory. Mice on a high-fat diet but missing NLRP3 were spared these changes, like mice on a low-fat diet.
Also, like many of us, mice tend to prefer new toys and those on a low-fat diet or with NLRP3 removed were better at recognizing novel objects to play with and their synapses were stronger. The high-fat diet transplant mice seemed not to remember so well which toy they’d already played with.
There is already potential protection out there from brain effects, Stranahan says, noting biologics in use in humans for problems like rheumatoid arthritis and Crohn’s disease, that target interleukin-1 beta.
“Obesity-induced inflammation occurs over years and so does inflammation in some of these chronic inflammatory diseases,” Stranahan says.
There is also emerging evidence that bariatric surgery, which sometimes includes removing visceral fat, can improve attention, mood and executive function.
There are many hypotheses about why visceral fat is so inflamed, including its proximity to the gut microbiota, a centerpiece of our immune response, which is programmed to attack invaders.
Increased rates of cognitive decline have been linked to obesity in humans, including shrinkage of key brain areas like the hippocampus, although there also have been contradicting reports about the overall health impact of obesity, the scientists report.
The contradiction in impact may relate to where the fat is found, says Stranahan, whose next goals include studying the apparent protective effects of fat deposited under the skin, called subcutaneous fat, whose benefits may include allowing you to store energy away from the highly inflammatory abdominal area.
Waist to hip ratio is a better indicator of visceral adiposity than the standard body mass index, or BMI, that divides weight by height.
Adipose tissue is host to various immune cells and it is well established that during obesity, the amount of inflammatory macrophages increase in adipose tissue1,2. Visceral adipose tissue (VAT), surrounding the inner organs, has been shown to be more inflammatory active than subcutaneous adipose tissue (SAT), as increased amounts of visceral/abdominal fat associates with high levels of circulating inflammatory markers3–6, and a high number of pro-inflammatory cells in their adipose tissue1,2,7.
Moreover, due to its anatomical location, VAT directly supplies the liver with venous blood via the portal vein, rendering a prominent role in directing whole body metabolism.
Thus, it is possible that visceral fat is the source of circulating low grade inflammation, which might be important for the development of life-style related chronic diseases8–10.
Interestingly, in human and rodent studies, ageing is associated with an increase in the amount of visceral adipose tissue and/or level of inflammation11–15.
It is, however, unclear to what extent these age-related changes are a result of ageing per se or rather the result of changes in life-style with e.g. reduced levels of physical activity without a corresponding reduction in caloric intake.
A human cross sectional study reported that whereas ageing is associated with increased inflammation, life-long endurance training resulted in lower circulating levels of inflammatory markers in both young and elderly individuals16.
Although endurance exercise has been demonstrated to counteract pathological changes in visceral adipose tissue by reducing the amount of total and visceral adipose tissue under conditions of excess fat17,18, the effects of exercise training on VAT during ageing under lean conditions currently remains elusive.
Indeed, it has been shown that acute endurance exercise can result in an upregulation of the brown fat marker UCP-1 in mice, suggesting a shift of the visceral fat towards a more metabolic active profile (WAT “browning”), but whether this pertains to long term endurance training remains unknown19.
Indicators of WAT browning, besides UCP-1 upregulation, include increase in mitochondrial enzymes and involvement of macrophages20–22.
Finally, the mode of exercise might be of importance as some human studies show that endurance but not strength training can reduce the amount of adipose tissue and thereby inflammation23.
In the current study, we wanted to investigate the inflammatory status and tissue integrity of VAT in an exercise-training model of lean adult and old mice. The model consisted of adult and old mice that underwent either voluntary RT or voluntary ET treadmill running, followed by examination of visceral (epididymal) fat both with regards to size/structure, markers of inflammation, oxidative capacity, browning, and fibrosis. We hypothesized, that ageing would be related to increased visceral fat mass, larger adipocytes, higher inflammatory activity and lower oxidative capacity, and that regular physical training of different types would counteract these age-related changes and “rejuvenate” visceral adipose cells.
Discussion
We here describe an age-dependent phenotype of visceral adipose tissue of exercising mice and report an accumulation of alternatively activated (CD206+) M2 macrophages in old mice in response to endurance training.
This phenotype was accompanied by less visceral fat, smaller adipocytes, as well as higher Ucp-1 and IL-10 mRNA expression while Tgf-β1 mRNA expression was lower compared to the younger counterparts.
When interpreting our data in the light of the literature, it is important to bear in mind that we utilized a model of very old (23 months) mice, which we compared to adult mice. This could explain why our results conflicted with previous reports on increased visceral fat in old mice12,13,37, as a bimodal pattern with decreased visceral fat has been observed38–40.
In accordance with our study, Donato and colleagues found that 30 months old (ancient) mice exhibited less visceral fat and smaller adipocytes compared to adult mice (6 months)38. However, in sharp contrast to our findings, that study reported a decrease in Ucp-1 and an increase in fibrosis, while we demonstrate the opposite phenotype.
Certainly, the present study has limitations. For one, variation in rest before sacrifice ranged from 3–5 hours thus introducing possible spatial variation in the conducted gene-expression measurements. Hence, our measurements do not reflect acute work/training response, but rather a habitual adaptation.
Further, we are aware that CD206 is not a marker exclusively reserved for alternatively activated macrophages41, but is a generally accepted M2 marker. Importantly, a previous study applying the M2 macrophage markers CD163 and Mrc1, support our findings of an increase in M2 macrophages in mice with ageing, although in this study, mice were only aged for 30 weeks42.
Furthermore, the fact that no circulating blood samples were available, limits the ability to conclude regarding the coupling between local adipose tissue changes and alterations in circulating levels of inflammatory markers.
Nevertheless, we here describe an anti-inflammatory phenotype of visceral adipose tissue in old mice, whereas ageing (and obesity) -induced changes in adipose tissue is originally presumed to be based upon an increasingly inflammatory, and not anti-inflammatory, skewing43,44.
Therefore, our data represent an important contribution to the literature, indicating that (pronounced) aging per se, does not generate a pro-inflammatory phenotype or visceral fat accumulation in mice. Interestingly, a cross sectional study on human ageing found that from around the 8th to the 9th decade, a reduction in waist circumference (surrogate marker for abdominal obesity) was observed45, supporting the notion of decreased visceral fat with pronounced ageing.
However, whether the immunological phenotype of visceral adipose tissue in very old humans is indeed dominated by anti-inflammatory processes as our data would suggest, remains unanswered.
In our study, the visceral adipose tissue of the old mice seemed either more lipolytic or had lost lipid storage capacity. This was observed at rest and was accentuated following exercise training as epidydimal fat mass was reduced in combination with smaller adipocytes (Fig. 1A–C). The interaction between adipose tissue macrophages and lipolysis has been previously discussed27, and it has been shown that local lipid fluxes is a potent mediator of macrophage recruitment to adipose tissue26.
Indeed, exercise is a powerful mediator of lipolysis, and it has been shown in vivo in humans that the lipolytic activity is higher in abdominal depot (represented by both visceral and subcutaneous adipose tissue) than in the gluteal depot (consisting of subcutaneous adipose tissue)46.
This is in line with our finding in which ET decreased visceral adipocyte size in both age groups, a phenomenon already touched upon by other researchers47,48. Moreover, our finding that only ET seemed to reduce visceral fat mass is consistent with one of the few available meta-analysis on the subject23.
Interestingly, in concordance with our observations, exercise has previously been established to generate an anti-inflammatory response, by increasing the expression and release of anti-inflammatory mediators such as IL-10, arginase-1 and IL-6 (acute release without TNF-α) from human leukocytes and skeletal muscles49,50. It has also been suggested that exercise might confer a shift from M1 to M2 macrophage phenotype in adipose tissue51,52.
As endurance training promote browning of white adipose tissue in mice20,35, it is interesting to note that pre-adipocytes obtained from epididymal fat tissue have the ability to acquire a brown-like phenotype regulated by PPAR-γ, PGC1-α and norepinephrine, which are all known to be involved in response to exercise53.
Interestingly, some studies advocate that M2 macrophages, through norepinephrine release, can increase UCP-1 expression and mediate browning of white adipose tissue21,54, which was later questioned by other groups, discarding this idea55. Here, we report a modestly elevated expression of UCP-1 (Fig. 4B) in old mice, while the mechanism or direct link to the higher amount of M2 macrophages could not be determined within the scope of the current study.
Importantly, our results support an increased oxidative phenotype of VAT generated by exercise, as PGC1-α expression was increased (Fig. 4A). This is consistent with existing literature where PGC1-α is seen upregulated in both muscle and adipose tissue following an intense exercise protocol56, supporting the concept that exercise training can convey a beneficial metabolically effect on visceral adipose tissue57.
In conclusion, our study emphasizes the dynamics of adipose tissue and describe the visceral adipose tissue of lean old mice as an anti-inflammatory and highly lipolytic tissue with endurance exercise further enhancing these characteristics.
More information: De-Huang Guo et al. Visceral adipose NLRP3 impairs cognition in obesity via IL1R1 on Cx3cr1+ cells, Journal of Clinical Investigation (2020). DOI: 10.1172/JCI126078
