Today’s post will cover a ground-breaking study out of Sweden that examined the interactions between gut flora and the blood-brain barrier (BBB). (1)
This definition of the blood-brain barrier from Wikipedia sums up its structure and function quite nicely:
“The blood–brain barrier (BBB) is a highly selective permeability barrier that separates the circulating blood from the brain extracellular fluid (BECF) in the central nervous system (CNS). The blood–brain barrier is formed by capillary endothelial cells, which are connected by tight junctions with an extremely high electrical resistivity of at least 0.1 Ω⋅m. The blood–brain barrier allows the passage of water, some gases, and lipid soluble molecules by passive diffusion, as well as the selective transport of molecules such as glucose and amino acids that are crucial to neural function. On the other hand, the blood–brain barrier may prevent the entry of lipophilic, potential neurotoxins by way of an active transport mechanism mediated by P-glycoprotein.”
Preventing neurotoxins from reaching the brain is a necessary condition for preserving normal brain function. Failure to do so will inevitably lead to brain damage.
But how exactly does something like intestinal gut flora affect this barrier?
Using a germ-free mouse model, these researchers were able to demonstrate that the absence of gut-flora is significantly associated with an increase in the permeability of the brain to substances that would normally be prevented from entering this organ.
In a first series of experiments, both pathogen-free (yet colonized by gut flora) pregnant mice and germ-free pregnant mice were injected with infrared-labeled immunoglobulin antibodies to see whether they would be detected in the brains of their developing babies.
In the pathogen-free conventional mice, the antibody was shown to initially diffuse through the embryonic brain, but quickly became confined to blood vessels over a short time. By contrast, in the fetal brains of germ-free mothers, the diffusion of this antibody in the brain persisted.
After assessing levels of blood-brain barrier tight-junction proteins in the two groups of developing embryos, no difference was noted between expression of claudin-5 or ZO-1. The same could not be said of occludin. This protein was significantly lower in the blood-brain barrier of embryos carried by pregnant mice devoid of gut flora, suggesting that the persistent diffusion of infrared-labeled antibodies in their brain was directly related to increased permeability of the fetal brain barrier.
In another series of experiments, but this time in adult mice, the blood-brain barrier was again found to be much leakier in adult mice that were devoid of gut microbes.
Using PET scan imaging to trace the flow of raclopride across this barrier, the researchers found an increased leakage into the brain of germ-free mice when compared to conventional pathogen-free mice, although this effect was noted only during the first four minutes after injection.
Using Evans blue dye tracer in a process known as fluorescence microscopy, the dye was only detected in the blood vessels of conventional mice, which is as it should be. However, in germ-free mice not only was the dye detected in their blood vessels, it had also entered the brain again demonstrating compromised blood-brain barrier function.
In yet another experiment, injecting monoclonal antibody R4A into adult germ-free mice was associated with abnormal neurons in the hippocampus. This contrasted with conventional mice who experienced no cellular damage when administered this substance.
The hippocampus is vital for the formation of both short-term and long-term memory. In Alzheimer’s disease, this is one of the first regions of the brain to become dysfunctional.
In the absence of gut flora, germ-free adult mice were found to express significantly lower levels of two tight-junction blood-brain barrier proteins: occludin and claudin-5. No difference was noted in the expression of ZO-1. Increased permeability was noted in the frontal cortex, hippocampus and the striatum.
Utilizing transmission electron microscopy to access the shape of these tight-junctions in germ-free rodents, these scientists found them to be “diffuse, disorganized structures compared with those in the pathogen-free [conventional] group.”
On the left we see a healthy tight junction from the brain of a mouse colonized by beneficial gut flora. On the right (highlighted by the two white arrows) we see the disorganized appearance of a similar tight-junction from a germ-free mouse.
It should be kept in mind that there was no difference noted between male and female adult mice when it came to the morphology of these junctions. In other words, the increase in brain permeability to systemic substances was independent of sex.
In another series of experiments, colonizing germ-free mice with gut flora from pathogen-free controls led to decreases in permeability as evidenced by less dye crossing over into the brain from the blood stream. Analyzing tight junction proteins in these newly colonized mice showed a significant increase in occludin in both the frontal cortex and hippocampus, and an increased expression of claudin-5 in the hippocampus and striatum:
Here we see slides that traced Evans blue dye (shown in red) and DAPI nuclear staining (shown in blue) in three regions of the brain: the frontal cortex, hippocampus and striatum.
The vertical column on the left shows very low levels of staining in the brain of pathogen-free (PF) conventional mice. However, the middle column shows extensive staining indicative of a very leaky blood-brain barrier in the germ-free (GF) mice. The last column shows what happens after these germ-free mice were colonized with beneficial gut flora. Note the significant decrease in staining.
So what is it about beneficial gut flora that leads to the strengthening of the blood-brain barrier? Answer: Short-chain saturated fatty acids, the same fatty acids produced by gut flora when they ferment prebiotics.
These fatty acids—propionate, acetate and particularly butyrate —have been shown to have favorable effects on the integrity of the gut wall. For the first time ever, these researchers discovered they also have the same positive effects on the blood-brain barrier.
Colonizing germ-free mice with Clostridium tyrobutyricum, a beneficial strain of Clostridium that produces butyrate when fermenting prebiotics, resulted in a more robust blood-brain barrier. Likewise, colonizing another group of germ-free mice with Bacteroides thetaiotaomicron, a gut microbe that produces mainly acetate and propionate when also fermenting soluble fiber, led to a reduction in blood-brain permeability. Finally, administrating sodium butyrate to germ-free mice for three days also reduced leakiness.
This animal study proves that these short-chain fatty acids have quite wide-ranging effects, including regulating what does or doesn’t enter the brain. I’ve already written how important these fatty acids are for the regulation of blood pressure and the treatment of type 2 diabetes, so none of this should surprise any of you.
Increased intestinal permeability, itself symptomatic of low short-chain fatty acid production resulting from either reduced beneficial gut flora populations or lack of these soluble fibers in the diet, increases inflammation and cortisol generation, and skewers tryptophan metabolism. Is it any wonder then that more and more studies are finding a robust association between gut dysbiosis and deranged mental functioning?
I have yet to speak with anyone experiencing gut dysbiosis who doesn’t also complain of mental distress. Whether that distress is depression or anxiety or insomnia or memory lapses or brain fog or mood swings or whatever, this study demonstrates that these issues are intimately related to the state of a person’s gut flora.
Given the reality of the gut-brain axis, it’s time for modern medicine to accept the premise that a leaky gut also equates to a leaky blood-brain barrier. And once again, I remind all my readers that the slogan at the top of this blog speaks to a truth that is ignored at your own peril.