In many ways, our brain and our digestive tract are deeply connected, and recent studies have even suggested that the bacteria living in our gut can influence some neurological diseases. To help researchers better understand how this gut-brain axis communicates, Massachusetts Institute of Technology (MIT) researchers have developed an “organs-on-a-chip” system that replicates interactions between the brain, liver, and colon.
Using their system, the researchers were able to model the influence that gut microbiome organisms have on both healthy brain tissue and tissue samples derived from patients with Parkinson’s disease. They found that short-chain fatty acids (SCFAs), which are produced by gut bacteria and are transported to the brain, can have very different effects on healthy and diseased brain cells. “While short-chain fatty acids are largely beneficial to human health, we observed that under certain conditions they can further exacerbate certain brain pathologies, such as protein misfolding and neuronal death, related to Parkinson’s disease (PD),” said Martin Trapecar, PhD, an MIT postdoc and the lead author of the study.
Trapecar, together with colleagues including senior researchers Linda Griffith, PhD, the School of Engineering professor of teaching innovation and a professor of biological engineering and mechanical engineering, and Rudolf Jaenisch, PhD, an MIT professor of biology and a member of MIT’s Whitehead Institute for Medical Research, reported on their developments in Science Advances, in a paper titled, “Human physiomimetic model integrating microphysiological systems of the gut, liver, and brain for studies of neurodegenerative diseases.”
The gut-brain axis operates as a two-way communication system that integrates the central nervous system (CNS) with endocrine, metabolic, and immune signalling pathways, the authors wrote. This connection between the gut and the brain can even be tangible. Feeling nervous, for example, may lead to physical pain in the stomach, while hunger signals from the gut make us feel irritable. But attempting to model complex interactions between the brain and digestive tract and the gut microbiome in animals such as mice is difficult, because mouse physiology is very different to that of humans.
As a critical part of this system, the gut microbiome and its metabolic products, including short-chain fatty acids, directly and indirectly affect the broader gut-immune-liver-brain axis. In fact, the investigators continued, “Accumulating data implicate dysregulation of the gut-brain axis in a variety of pathologies from inflammatory bowel disease to neurodegenerative diseases (NDs).” However, they noted, “Multifactorial NDs remain one of the biggest medical challenges of our time, because both environmental and genetic factors are intertwined, obscuring causality. While most of our current knowledge about PD comes from valuable animal experimentation and human clinical data, the overwhelming disease complexity on a whole organismal level is a roadblock to progress in its own right.”
Griffith’s lab has for several years been developing microphysiological systems (MPS), constructed as small devices that can be used to grow engineered tissue models of different organs, connected by microfluidic channels. In some cases, these models can offer more accurate information on human disease than animal models can, Griffith said. “MPSs are in vitro models that, under perfusion, mimic facets of physiological organ behavior,” the authors further explained. “The goal of physiomimetic models is to define the essential elements of complex disease states involving multiple organ systems and capture these in the simplest possible MPS experimental configuration that will reveal useful insights.”
Griffith and Trapecar last year published data describing use of a microphysiological system to model interactions between the liver and the colon. In that study, they found that short-chain fatty acids produced by gut microbes can under certain conditions worsen autoimmune inflammation associated with ulcerative colitis. SCFAs, which include butyrate, propionate, and acetate, can also have beneficial effects on tissues, including increased immune tolerance, and they account for about 10% of the energy that we get from food.
For their newly reported study, the MIT team decided to add the brain and circulating immune cells to their multi-organ system, given that the brain has many interactions with the digestive tract, which can occur via the enteric nervous system or through the circulation of immune cells, nutrients, and hormones between organs. Several years ago, Caltech researchers had discovered a connection between SCFAs and Parkinson’s disease (PD) in mice. Their research had shown that SCFAs, which are produced by bacteria as they consume undigested fiber in the gut, sped up the progression of PD disease, while mice raised in a germ-free environment were slower to develop the disease. “A potentially important signaling link between the gut microbiome and the brain in the context of PD involves SCFA, the MIT team noted. “A previous study with gnotobiotic mice implicated the presence of SCFA to faster progression toward PD in a mouse model of the disease.”
Griffith and Trapecar decided to further explore these findings, using their microphysiological model. For their work, they teamed up with Jaenisch’s lab at the Whitehead Institute. Jaenisch had previously developed a way to transform fibroblast cells from Parkinson’s patients into pluripotent stem cells, which can then be induced to differentiate into different types of brain cells—neurons, astrocytes, and microglia.
More than 80% of Parkinson’s cases cannot be linked to a specific gene mutation, but the rest do have a genetic cause. The cells that the MIT researchers used for their Parkinson’s model carry a mutation (A53T) that causes accumulation of the alpha synuclein protein, which damages neurons and causes inflammation in brain cells. Jaenisch’s lab has also generated brain cells that have this mutation corrected, but are otherwise genetically identical and from the same patient as the diseased cells.
Griffith and Trapecar first studied these two sets of brain cells in microphysiological systems that were not connected to any other tissues, and found that the Parkinson’s disease cells showed more inflammation than the healthy, corrected cells. The PD cells also had impairments in their ability to metabolize lipids and cholesterol.
The researchers then connected the brain cells to tissue models of the colon and liver, using channels that allow immune cells and nutrients, including SCFAs, to flow between them. They found that for healthy brain cells, exposure to SCFAs was beneficial, and helped them to mature. However, when brain cells derived from Parkinson’s patients were exposed to SCFAs, the beneficial effects disappeared. Instead, the cells experienced higher levels of protein misfolding and cell death.
These effects were seen even when immune cells were removed from the system, leading the researchers to hypothesize that the effects are mediated by changes to lipid metabolism. “It seems that short-chain fatty acids can be linked to neurodegenerative diseases by affecting lipid metabolism rather than directly affecting a certain immune cell population,” Trapecar said. “Now the goal for us is to try to understand this.”
The findings offer support for the notion that human tissue models could provide insights that animal models cannot, Griffith said. The researchers plan to model other types of neurological diseases that may be influenced by the gut microbiome.
“We have created a human gut-liver-cerebral physiomimetic system that incorporates cells of both the innate and adaptive immune system,” the authors stated. “Using this approach and advanced genetic tools, we were able to observe increased maturation of hiPSC-derived neurons, astrocytes, and microglia on the transcriptomic level. Our conceptual and experimental model of continuous and prolonged interactive immune-metabolic cross-talk between organ systems represents a significant advance in modeling the human gut-brain axis in the context of NDs in vitro.”
Griffith further noted, “We should be really pushing development of these, because it is important to start bringing more human features into our models. We have been able to start getting insights into the human condition that are hard to get from mice.” The team is now working on a new version of the model that will include micro blood vessels connecting different tissue types, which will allow researchers to study how blood flow between tissues influences them.