Visceral fat – body fat that is stored in the abdominal cavity in close proximity to important internal organs such as the liver, pancreas, and intestines – plays a central role in the interrelationship between obesity and systemic inflammation. Excess visceral fat, often referred to as central or abdominal obesity, is a strong predictor of age-related cognitive decline. A new study in mice demonstrates that having excess visceral fat may impair cognition by activating the NLRP3 inflammasome and promoting the release of interleukin-1 beta (IL-1β).
Inflammasomes are large, intracellular complexes that detect and respond to internal and external threats. Activation of inflammasomes has been implicated in a host of inflammatory disorders. Cryopyrin, also known as NLRP3, is a protein that drives the formation and activation of the NLRP3 inflammasome.
Interleukin-1 beta is a proinflammatory protein present in many cells. NLRP3 inflammasome-driven release of IL-1β activates microglia, the brain’s resident immune cells. Microglia serve an essential role in maintaining brain microenvironment homeostasis. Acute activation of microglia modulates inflammation and neurotoxicity, but chronic activation promotes brain inflammation and harm.
The authors of the study first determined that mice lacking the gene for NLRP3 did not experience visceral fat-induced brain inflammation and cognitive decline. They also determined that when visceral fat from normal, obese mice was transplanted into these mice, they exhibited higher levels of IL-1β in their hippocampus, an area of the brain associated with memory (in particular, the consolidation of short-term memories to long-term memories), learning, and spatial navigation.
To understand the effects of IL-1β on brain function, the authors of the study fed the mice a high- or low-fat diet for 12 weeks and then assessed the animals' capacity to navigate a water maze. The mice that ate the higher-fat diet experienced greater difficulties negotiating the water maze, compared to those that ate the lower-fat diet. Examination of the animals' brains revealed that the mice that ate the high-fat diet (as well as those that received the fat transplants) had weaker synapses between the neurons involved in learning and memory.
These findings suggest that chronic inflammation driven by excess visceral fat may contribute to cognitive decline by promoting the release of IL-1β and increasing inflammation. Inflammation drives other aspects of brain dysfunction, including those associated with depression. Watch this clip in which Dr. Charles Raison discusses how a pro-inflammatory environment can contribute to the risk of depression.