A new study led by a team at the Scripps Research Institute has discovered a novel communication pathway between fat cells and the brain. The findings show that the brain doesn't just regulate fat burning by slowly responding to hormonal signals in the blood, but can send messages directly to adipose tissue and influence metabolic processes.
The traditional view on metabolism is that the body uses various signaling molecules such as hormones to regulate its energy production. Adipose tissue acts like the body's energy storage system, and when we need to use this supply for extra fuel during times of heavy physical activity or stress, certain signals trigger the sympathetic nervous system to start talking to these fat cells.
So far, it is thought that most of the nerves that extend into adipose tissue are related to this sympathetic nervous system pathway. However, understanding exactly what types of neurons are present in this tissue has been difficult to study. Therefore, in order to obtain the results of this study, the research team needed to develop completely new imaging methods.
In this study, two new methods were deployed: one, known as HYBRiD, makes adipose tissue transparent, giving researchers a unique window into neuronal pathways; the other, known as ROOT, allows researchers to study exactly how certain neurons located in adipose tissue communicate with other parts of the body.
The study was indeed made possible by the combination of these new methods," said Yu Wang, the first author of the paper on the study. When we first started this project, there were no readily available tools to answer these questions."
The biggest finding of the study was the discovery of sensory neurons that branch from the spine to the adipose tissue. These sensory neurons communicate directly with a part of the brain called the dorsal root ganglion.
"The discovery of these neurons shows for the first time that your brain is actively investigating your fat, not just passively receiving information about it," said co-senior author Li Ye, "The implications of this finding are far-reaching."
So what type of information is being sent from these sensory neurons to the brain? In response, the researchers blocked the communication of these sensory neurons and found an increase in metabolic activity in adipose tissue. When the communication of sensory neurons was silenced, the sympathetic nervous system kicked in and started converting white fat cells into brown fat. This mechanism enhanced the body's fat burning process.
The researchers speculate that these two opposing neural signals may work in concert as a way to maintain a metabolic balance. The sympathetic nervous system turns the fat-burning process on, while the sensory neuron pathway turns the process off.
"This tells us that the brain's instructions to adipose tissue are not one-size-fits-all. It's more subtle than that, and the two types of neurons act like a throttle and a brake for burning fat," Li said.
Fundamental discoveries often lead to a whole host of new questions that require further research, and this study is no exception. What is clear from this finding at this stage is that the newly discovered sensory neuron communication pathways are critical to maintaining healthy adipose tissue.
But from there, the problems quickly began to pile up. Such as how do these sensory neuron signals from adipose tissue interact mechanistically with sympathetic nervous system signals? Or more tellingly, what kind of information is actually going to and from the brain through these adipose tissue sensory neurons? Do different parts of the dorsal root ganglia direct different metabolic functions of adipose tissue? Most importantly, can these pathways be therapeutically modulated to treat obesity or metabolic disease?
