10th November 2019
The Blood Brain Barrier
Here I am going backwards in time and space to try and explain, from a different angle, a fundamental problem with the LDL/cholesterol hypothesis. In doing so I hope to again make clear why I am certain the entire process of cardiovascular disease (CVD) requires a complete re-think.
In a medical school long, long ago, on a planet far, far, away, I was part of a small group teaching session on cardiology… Aberdeen 1980, actually. I have mentioned this event before, a critical moment in my life. The tutor was Dr Elspeth Smith, who was researching heart disease at the time. Research that, to my chagrin, I knew little about until several years later, when I began more detailed research into cardiovascular disease.
At one point in the tutorial, Dr Smith stated that LDL (low density lipoprotein) cannot get past, or through, the endothelium. At that time, I hadn’t much of a clue what LDL was, and very little idea about the endothelium. However, something about the intensity of her comment created an itch, one that I have spent very nearly forty years scratching.
I say this because, if LDL cannot get past the endothelium, then the widely accepted, and supposedly primary causal mechanism of heart disease, must be wrong!
Just to remind you that the central mechanism underpinning the ‘cholesterol hypothesis’ has always been that LDL leaks out of the blood past, or through, the endothelium, and into the arterial wall behind.
This then stimulates a whole series of downstream processes whereby you end up with thickenings in the artery wall narrowing the artery – known as atherosclerotic plaques. The higher the LDL level, the faster the leakage? I put a question mark there, because I don’t think I have ever seen this stated explicitly – I suppose it is implied as self-evident.
Clearly, however, if LDL cannot pass through the endothelium – the single layer of cells that lines all artery walls – then the ‘cholesterol hypothesis’ is a busted flush. Which is sort of interesting in a ‘hold the front page’ sort of fashion. ‘LDL hypothesis completely wrong – shock horror.’ Dr Elspeth Smith explains that LDL cannot get through the endothelium. Experts around the world, agree, and look for other explanations for heart disease.
This is a headline that I must have missed.
At this point you may be thinking, how do we get from LDL, and the endothelium, to the Blood Brain Barrier (BBB), which is the title of this blog. Well, are you sitting comfortably? Then I shall begin, at the end, with the blood brain barrier itself.
All doctors are taught there is a barrier between the bloodstream and the brain called the Blood Brain Barrier (BBB). Very few know what it actually consists of, other than that there is a barrier, of some kind, that prevents various things from entering the brain at will.
A barrier between the blood and the brain is critical because the brain is a highly delicate organ, which copes very badly with noxious substances, such as bacterial toxins. Which makes the BBB essential for life. However, it can become a problem if you have, for example, a brain tumour and the doctors want to give you chemotherapy. Because most anti-cancer drugs cannot get past the BBB. Some drugs can, most can’t.
However, a certain number of things must enter our brains, or we would almost instantly die. Glucose, for example. If our brain cannot get enough glucose, we go into a coma, then die. We also need amino acids (the building block of proteins), some fats and vitamins and suchlike.
Clearly, therefore, the BBB needs to be a selective barrier. It must block some things, but allow entry – and exit – of those substances that are required for brain function. Ethanol from malt whisky, for example, distilled from glucose. Well it is essential for my life anyway – in moderation obviously … obviously.
At this point you may be thinking, we are getting further and further away from LDL and heart disease, but please bear with me on this, because it does all come together at the end.
Next question, what is the BBB? The answer is that it is comprised of endothelial cells that are tightly bound to each other, and have a strong support structure underneath called the basement membrane. This membrane keeps the endothelial cells wrapped even more closely, further protecting them from any possible disruption.
This ‘tight’ binding of endothelial cells means that anything that wants to get into the brain must first pass through an endothelial cell. As you may imagine, this is an extraordinarily complicated and tightly controlled process. Substances in the blood cannot flow down a concentration gradient and straight through an endothelial cell.
LDL, for example, can only enter a cell, if there is an LDL receptor on the cell wall. The LDL links onto the receptor and then LDL and the receptor (the ‘LDL receptor complex’) is dragged inside the cell in a process known as ‘endocytosis.’
It does not matter what the concentration of LDL in the blood is – without a receptor, LDL is unable to gain entry to a cell. If it cannot get in, it cannot pass through. This is why the concentration of LDL becomes very high in Familial Hypercholesterolaemia (FH).
In this condition there is a lack of LDL receptors on all cells in the body, so LDL cannot easily enter cells, therefore the concentration in the blood rises very high.
Proof, if proof were needed, that LDL cannot ‘escape’ from the bloodstream and find refuge in the artery wall, or any other tissue. No matter what the concentration in the blood. Osmosis and/or diffusion of LDL through any cell is biologically impossible.
Therefore, getting back to the BBB, for substances to move through the BBB they first must be ‘endocytosed’ and then need to be shuttled through the cell via an active transportation system. This is known scientifically as ‘transcytosis.’ Literally, transport through the ‘cytoplasm’ where cytoplasm is the name for the jelly-like substance that fills up cells.
Once the substance has been transcytosed through the cell it must be ejected out through the cell membrane on the opposite side. Another complex process known as exocytosis.
Not everything needs a receptor to enter a cell. However, nothing gets in without the cell controlling the entry and exit. Even individual ions (charged atoms), tiny as they are, must pass through ‘gates’, or channels, to get into a cell. Calcium, sodium, potassium etc.
Their passage is carefully monitored and controlled. If this were not the case, you would instantly die. If a cell loses control of its internal environment it is either dying – or dead. See under, hyponatremic encephalopathy (for those with a scientific bent).
It has been argued that LDL molecules do not need to pass through endothelial cells, they can simply slip through the ‘cracks’ between endothelial cells. I have seen this argument used as to why ‘small dense LDL’ can cause CVD because, in this form, it is small enough to get through the cracks between the endothelial cells.
The problem with this hypothesis is that, first, small dense LDL is almost exactly the same size as normal LDL. Second, and most important, there are no cracks, or gaps, between the endothelial cells in our arteries – or the BBB. Endothelial cells in large arteries are bound together very tightly indeed. They are linked together by zips, buttons, and super-glue, then welded to create what is called a ‘tight junction’.
You can Google ‘tight junctions’ under ‘images’ to see how complex they are. There are over twenty protein bonds, sealing any gap shut. Tight junctions are so tight that they, too, can prevent the entry and exit of single ions. So, there is absolutely no way through for LDL here, either. For a quick size comparison, if a human were the size of an ion, an LDL molecule would proportionately be around the size of a super tanker.
In short, the idea that LDL can simply leak through the endothelium and into the artery wall behind requires that several key mechanisms – required for life – do not exist.
As I hope you can now see, Elspeth Smith was quite correct. LDL cannot pass through the endothelium. At least it cannot pass through a healthy endothelium, and nor can anything else either. Unless, unless the endothelium enables its passage.
Complication number one – yes, there is always a complication.
As blood vessels get smaller and smaller, like the branches on a tree, the endothelium changes dramatically. As arteries shrink down to became arterioles, then capillaries, the endothelium is no longer an impenetrable barrier.
In these very small blood vessels, the endothelium develops holes (fenestrations). Gaps also appear between individual endothelial cells, and the supporting basement membrane becomes loose. What you have is more like a sieve than a castle wall.
This ‘sieve like’ quality allows substances to move in and out of arterioles and capillaries, almost at will. Which, of course, makes perfect sense. There is little point in blood arriving at, say, the kidneys, where various waste products are removed, if it was all stuck behind an impenetrable endothelial barrier. The blood would flow into your kidneys, then flow out again, unchanged. This is not a good recipe for life.
So, yes, in the very small blood vessels, the endothelium allows the free passage of substances. But absolutely not in the larger blood vessels. If the blood simply leaked out as it passed through larger blood vessels, it would never reach the smaller blood vessels, nor get back to the heart again. It would be like having a garden hose that was full of holes along its entire length. Nothing would reach the sprinkler head.
Anyway, bringing some strands together here, the endothelium lining the large arteries has the same impenetrable structure as the endothelium lining the BBB, and therefore represents a barrier. Substances just cannot leak through this – and that includes LDL.
But can things be transcytosed through? More specifically, can LDL transcytose through it? Because, if it can, this would be a possible mechanism in support of the cholesterol hypothesis. Although, once again, you have to ask the question, why would the body have a system for actively transporting LDL through the endothelium and into the artery wall underneath. What would be the purpose of such a system? To cause atherosclerosis?
The mystery of this question is further deepened by the fact that the larger blood vessels in the body are supplied with nutrients via their own, small, blood vessels known as vasa vasorum. Literally, blood vessels of the blood vessels. These are arterioles, and capillaries, that penetrate/permeate the vessel wall.
These very small blood vessels, lying within the artery wall, have fenestrations (holes) and loose junctions that allow the free movement of LDL – in an out of artery walls. So, any LDL in the bloodstream can quite easily enter the artery – and exit the artery (and the veins) – without having to cross any barrier at all.
Which raises a further conundrum for the cholesterol hypothesis. Why would LDL that enters the artery wall, via the vasa vasorum, cause no problems. Whilst the LDL – that is claimed to enter the artery wall by forcing itself past the endothelium – creates atherosclerotic plaques. Same artery wall, same LDL.
Which then raises another question. Why do atherosclerotic plaques only form in artery walls? Why not everywhere else, within every other organ and tissue in the body. If LDL, once it leaves the bloodstream, is so destructive, acting as the focus for plaque development, why doesn’t it cause plaques within the liver, or the kidneys, or the muscles, or the gut. Only, it seems, in artery walls. [Please don’t say xanthelasma, until you have thought very carefully about it]
But to get back on track. I wanted to see if I could answer a final question. We know that LDL cannot simply flow through endothelial cells, nor can it squeeze between non-existent gaps. But can it be actively transported through? Because, if it cannot, then this is the final nail in the cholesterol hypothesis. There is, literally, no way past. Elspeth Smith was quite correct.
[Other than via the vasa vasorum, obviously. However, veins have more vasa vasorum than arteries, so this if this is the route for LDL to enter blood vessel walls, then veins should have more atherosclerosis than arteries, which they do not. In fact, veins never develop atherosclerosis – ever.]
So, deep breath.
I wanted to know if LDL could get past the BBB. If not, then it could not get past the endothelium in the larger arteries either, end game. This is the sort of question to which you would think there should be a straightforward, yes or no answer. It took me a long time to find a definitive answer.
I did know that the brain manufactures its own cholesterol, because cholesterol is absolutely vital for brain function. Neurones are wrapped in myelin/cholesterol sheaths. New synapses have a very high proportion of cholesterol in them, and animal work has shown that – without cholesterol – new synapses cannot be made.
‘Brain cholesterol accounts for a large proportion of the body’s total cholesterol, existing in two pools: the plasma membranes of neurons and glial cells and the myelin membranes Cholesterol has been recently shown to be important for synaptic transmission, and a link between cholesterol metabolism defects and neurodegenerative disorders is now recognized.’ 1
This ‘brain’ cholesterol is synthesized in support cells, known as glial cells. These cells surround neurones and nourish neurones, and the cholesterol is transported about in its own lipoprotein called Apo E.
So, on initial analysis, it did seem unlikely that the brain would have developed its own, specific, cholesterol manufacturing capability if it could simply absorb it from the bloodstream. As it turns out, and as I eventually discovered, the brain cannot absorb LDL from the bloodstream.
‘… the blood brain barrier (BBB) prevents the uptake of lipoprotein-bound cholesterol from the circulation.’ 1
In quick summary here, the brain requires cholesterol, yet the BBB prevents it from entering the brain. Which means that an intact endothelium can completely block the passage of LDL Which means that LDL cannot get past, or through, the endothelium. Elspeth Smith was quite right.
But what did she think caused CVD, or atherosclerosis, or atherosclerotic plaques – or whatever term is currently in vogue? Well, this is one thing that she wrote about the matter:
‘After many years of neglect, the role of thrombosis in myocardial infarction is being reassessed. It is increasingly clear that all aspects of the haemostatic (blood clotting) system are involved: not only in the acute occlusive event, but also in all stages of atherosclerotic plaque development from the initiation of atherogenesis to the expansion and growth of large plaques.’ 2
She believed, as I have been trying to outline for a few years now, that to start atherosclerosis, you need to damage the endothelium, then a blood clots forms, then we are on the pathway to the final occlusive thrombosis (a blood clot that completely blocks an artery).
She believed, as I believe, that LDL has no part to play in this process. It does not, because it cannot. If you believe in the ‘thrombogenic’ hypothesis, you cannot believe in the LDL hypothesis, and vice-versa.
Which hypothesis is correct? Well, it is certainly true that the LDL hypothesis is currently in the ascendancy. But science is not, thankfully, a popularity contest like Strictly Come Dancing. Science is built on facts. Well, it is eventually. The fact is that the LDL hypothesis requires a central fact to be true but which can be proven to be wrong.
Elspeth Smith proved it to be wrong over fifty years ago, but no-one was listening. She was a hero, and I intend to do all that I can to ensure that it is her name, not that of Ancel Keys, that will echo through the history of CVD research. Because she was a true scientist. Whereas he….
Next time, why LDL also cannot damage the endothelium.