Doug Ethell has a clear and comparatively easily tested hypothesis on an important cause of Alzheimer's disease: that it results from the progressive failure of drainage of cerebrospinal fluid through one particularly crucial pathway in the skull. This traps ever greater levels of metabolic waste in the brain, such as amyloid-β, tau, and α-synuclein, and leads to the spectrum of well-known neurodegenerative diseases characterized by protein aggregates and resultant dysfunction and death of neurons.
Dave Gobel of the Methuselah Foundation backed the first work on this hypothesis a few years back, and the result is Leucadia Therapeutics, a company now well on the way to proving that restored drainage of cerebrospinal fluid can be a basis for treatment. Along the way, supporting evidence for the important of impaired cerebrospinal fluid flow in neurodegeneration has emerged from groups studying the glymphatic system in the brain. Given that the cost of this exercise is a tiny fraction of the funding put into the development of any one anti-amyloid immunotherapy, and it should impact all forms of metabolic waste in the brain, not just one, it seems like a good path forward.
I'd like to thank Aubrey de Grey for inviting me to this wonderful conference, because aging is still the biggest risk factor for Alzheimer's disease. There are 35 million cases in the world today, and another 200 million people are walking around today who will get this disease over the next 30 years. So solving this one is like curing cancer ten times over.
Let's start at the beginning. In 1901, a German psychiatrist working at the Frankfurt asylum had this patient Auguste Deter. Alois Alzheimer was her doctor. She was 51 years old and severely demented; confused, disoriented, paranoid. She couldn't make any new memories. He followed her for five years until she died, and then he took her brain to the Kraepelin lab in Munich, where they discovered the pathology that underlay her condition. He wrote up a paper for that, a small paper, "On a Peculiar Disease of the Cerebral Cortex." It wasn't until a year later that Kraepelin wrote his textbook and mentioned the case, and he referred to it as "Alzheimer's disease." That is how it got its name - he didn't name it after himself.
The most obvious pathological feature, on a gross level, of Alzheimer's disease is a several atrophy or wasting of the cerebral cortex. It is due to the death of billions of neurons. If you take a section of that and you cut it up you find the two pathological features that they identified in Kraepelin's lab. Firstly there are plaques, tiny waxy deposits of amyloid-β protein and some other things, but mostly amyloid-β. Secondly, neurofibrillary tangles. These are the insoluable remnants of the cytoskeletons of neurons that have died. They are a very good marker for where the cell death is happening in the brain, but neurofibrillary tangles are something that sick neurons make. So it doesn't really tell us why they are sick, just that there were sick neurons there.
Amyloid has been the center and a preoccupation in Alzheimer's disease research for 25 years. Unfortunately it has not gone so well. It has been an unbroken string of failed clinical trials, costing $20-30 billion in private and public funding - and the bloodletting continues to this day. In just February of this year, two companies dropped out of their Alzheimer's development program. Close to 1800 clinical trials have been conducted on something related to Alzheimer's disease. Do you know how many of those trials have slowed or stopped the progression of Alzheimer's disease? Zero. Not a single one. That is what I call a systematic error. So a few years ago I began to apply Occam's razor to this, looking for a more simplistic solution. Is there something basic that we are missing, something that can account for the age-dependent incidence of the disease? Something that accounts for the higher risk for people who have traumatic head injuries, and the near certainty that people with specific mutations will develop the disease at an early age, just like Auguste Deter.
I'm going to need to use a little neuroanatomy here, but I'll try to keep it simple. At the top of the slide here we have the right halt of the cerebral cortex, the right is the front and the left is the back. The image below that, since it is just the half, is what it looks like on the medial surface - on the inside. The last image is what it looks like from below. Here we have three columns of such images for the three stages of Alzheimer's disease, early, middle, and late. The coloration is neurofibrillary tangle staining. If you notice on the left hand side, early stage Alzheimer's, the small orange portion of neurofibrillary tangle staining is the specific area where the pathology starts. It is almost like a little campfire; it starts off with a spark that smoulders a bit and then it explodes - it goes to other regions of the brain.
But it starts here, and this is called the medial temporal gyrus. It has the hippocampus in there, and big parts of the olfactory system. So there is something peculiar about this area. No other animal gets this disease, and in humans it starts in this one specific area. This is actually a different part of the cerebral cortex. In the early Alzheimer's images, the unstained yellow area is the neocortex, a six-layered neocortex. But the stained areas with neurofibrillary tangles are allocortex, three layer and five layer cortex. They are a more ancient part of the brain. What is it about this part of the brain that seeds Alzheimer's pathology, seeds the deposition of amyloid-β in the interstitial spaces of this region of the brain?
I think it comes back to the evolutionary origin. Here is an image of an alligator brain. The blue area is where the olfactory system is. It used to be a third of the forebrain - very important. If we look at other animals, we can see that in most mammals it is very large, because it is important for smell, which is important for survival, breeding, and evolutionary fitness. But in humans, notice how small the olfactory system is in the brain. It is just a tiny thing, and sits just below the prefrontal cortex, where executive function and abstract reasoning happen. So it is almost as it there was a turf war for space, and the olfactory system lost. So we think very well, but we can't smell very well. This is my dog Rex; when I take him for a walk, he smells as if it is an 80" plasma ultra-high-definition TV, and I smell like it is a little 1950s black and white with the fuzzy lines and stuff - a dog is 700-800 times better than the average human in sense of smell.
Now, the olfactory system is where Alzheimer's disease starts. In the image, these are the olfactory bulbs here, the little bulbs. So what is it about this area that seeds Alzheimer's pathology? I think it might have to do with the clearance of interstitial spaces. Here is an allegory. Think of a forest; there is a little stream in the forest, and leaves fall down from the trees. If they fall on the ground, they sit there and they rot. If they fall into the stream it carries them away. Later in the summer, the stream starts to dry up, and the leaves keep falling in. Eventually it hits a point at which the water can't carry them away, and then they form mats - or plaques. So I think it has to do with the clearance of the tissue.
In most of the body this is done in the following way: capillaries have these tiny holes, or fenestrations, or some kind of gap between the cells. Blood plasma comes out of these holes, and enters the interstitial compartment, where it becomes interstitial fluid. It passes slowly through there, picks up soluable macromolecules, maybe pieces of apoptotic cells, and other debris, and carries it along to the lymphatic vessels. They take it up, lymph nodes eventually taking it back to the bloodstream. Of course any cancer cells will also take this route, and that his how they so frequently wind up in the lymph nodes.
But in the brain, this system doesn't exist. The blood-brain barrier prevents any formation of fenestrations or gaps between the endothelial cells. The brain does it a different way. It has these large fluid-filled cavities called ventricles, and those have choroid plexuses that produce cerebrospinal fluid (CSF) - about half a liter per day per person. It percolates through the tissue, through the cortex, through all the spaces, and along the blood vessels, and it works its way up to the surface, to the subarachnoid spaces. From there some is resorbed, but some of it passes all the way down to the spinal cord, where you can get a lumbar puncture or a spinal tap to sample the CSF.
Now remember that I said that the olfactory system used to be a very big, very important part of the forebrain. CSF comes into the hippocampus, and works its way up to the surface at the medial temporal gyrus. But then it goes along the olfactory route towards the olfactory bulb, not towards the spinal cord. In this diagram, here is the medial temporary gyrus; fluid comes in, goes towards the surface, goes towards the front, and there is a rudimentary cone here - the lateral olfactory stria. This is a loose fiber bundle, and the CSF passes through little channels in there, along past the basal forebrain, and to the olfactory bulb.
Then what happens to the fluid? Well, here is an image of the interior of a skull, looking downwards. The front is at the top, the back is to the bottom. There are two little depressions or pockets, olfactory fossa, in the front of the skull. The olfactory bulbs sit in there, and the CSF drains through. In this cross-section figure, we can see the olfactory bulbs on top, a small gap, and below the cribriform plate and then nasal mucosa. Here are the olfactory nerves, sending information to the olfactory bulb about the odors they perceive. But there are gaps in there, and the metabolite-laden CSF makes its way through into the nasal mucosa, where there are plenty of lymphatic vessels. For anyone who has ever had a nasal vaccine - the presence of so many lymphatic vessels in the nasal mucosa is why this delivery method is used.
The cribriform plate is a natural choke point for the clearance of this fluid from the brain region where the disease starts. Here is an image of a 26 year old skull, and a CT scan of the same. You can see that there is some thick bone in the cribriform plate here, but not very much. Here is a CT scan of the same region of an 80 year old skull, and you can see that there is a lot more bone deposition in the cribriform plate. This structure at the top of the image is a piece of bone that sticks up, and I'll show you more of it in a minute. It is called the crista galli and notice how much bigger it is in the older person. We have continuing bone deposition in this area. One of the effects of that is that it closes off the holes, the apertures, reducing the ability to clear the CSF.
Here is an image of a CT scan of the skull, and this is seen from the front. The cribriform plate is on the bottom here, and this vertical structure is the crista galli. Notice it goes down the middle, right here, all the way and connects up with the bone in the middle of your nose, the nasal septum. The second image shows a cribriform plate from the interior of the skull. You can see the apertures there. What we've been doing at Leucadia is high resolution micro-CT imaging of about 70 human cribiform plates. These are taken from control subjects at different ages, and from Alzheimer's patients.
The image here is of a 26-year-old control cribriform plate. I actually have a 3-D print with me, as we do a lot of that as well. You can see the holes, plenty of apertures. There is a little thick bone, but lots of apertures - Swiss cheese, practically. If we look at 70-year-old control, we can see that there is a bit of a bony veil that has developed, and there are not so many apertures in here. There is a little bit of bone deposition. If we go a little further, and look at an Alzheimer's disease cribriform plate, notice the big bony veil convering pretty much all of the apertures at the very back of the olfactory fossa.
See also a big bony area in the middle, because this cribriform plate is uneven. The crista galli is in the middle: if you break your nose, it is going to deflect the crista galli, and when it heals there will be extra bone around the injury. So in this case we can tell that it has been deviated due to past injury, and thus there is thicker bone on that side. So it seems that if you injure the cribriform plate, it is going to affect this system. This next image is the cribriform plate for a different Alzheimer's patient. You can see that the channels in the middle are different, but the big bony veil is similar. So we are talking about how much capacity there is to drain the CSF, and that seems to be different in Alzheimer's patients.
We are carrying out these high resolution micro-CT scans on cadavers, but we want to start doing them on humans, because we want to be able to relate what we see in the high resolution samples with what we will see in a lower resolution clinical sample. We do that, we'll get our scans, and the hard-working interns at the company are going through and segmenting these files: I have to go through and say that is bone and that is other tissues. We produce these 3-D models of the cribriform plate, like the one I just showed you, and if we take just the one aperture and expand it like this diagram here, we can see it is made up of dots, and each dot has an (x,y,z) coordinate. We get a big file that contains thousands upon thousands of these coordinates, and that's how we're matching up the samples, using computers.
If we do this, what we should be able to accomplish is, theoretically, when we are all finished with our database, is look at someone's scan and determine the theoretical CSF flow capacity. If we can do that, then we can use CT scans to say, well this person obvious has a lot of occlusion here, or this person is fine. This will come in very handy for mild cognitive impairment, which is the pre-dementia state. Only about 10% of those patients progress on to Alzheimer's disease. The big question has always been which ones? If we can do these scans and say this one and that one and so on, it is going to be a big help. It could also be part of an annual wellness assessment for seniors. Maybe they get a scan every two years or five years, and we can actually watch the progression of ossification, and calculate how long it is going to be before they hit the critical threshold.
The bone is kind of only part of the story, because the other part is the soft tissue. This image is the highest resolution MRI ever done of a human cribriform plate. What we see here is the soft tissue is blue and the bone is a firey color. You can see where the apertures are, but that's not quite good enough, because there are so many apertures, a couple of dozen apertures, and some of them have blood vessels, some of them have nerves, some of them probably have the channels for CSF, but we need to know which are which. So we came up with another method: we figured out a way to enhance the staining, so that we could use CT to resolve bone and nerve and other tissues in the micro-CT images. Then we go in and digitize it. In this digitized image you can see the bone, the nerve, and these blue things here are the fluid-filled conduits where the CSF is actually flowing through. We can use this with our database to figure out how much CSF flow capacity there is.
Here is another digitized image showing the same, you can see the nerves in orange and the flow channels in blue. We can remove the bone from the image to show just the structure within. Now this yellow structure that is something else; it is a little interesting and nobody else have ever seen that before. These are actually channels within the bone, CSF conduits that are connected to the soft tissue drainage channels. It is almost like an equilibriation of the CSF flow mechanism. These CSF conduits in the bone, we can see them in the control cases, but looking at the Alzheimer's patients they are almost completely gone. We are losing these conduits and we're also getting ossification. So there is really something going on here with the CSF flow.
The proof of concept we are doing for this is we are blocking the cribriform plate in ferrets. It turns out that mice and rats, for a number of reasons, are probably the worst model you can pick for Alzheimer's disease. It is ironic, because the NIH has been very draconian about "you must use mice." In this picture you can see our team of human neurosurgeons, who brag now that they are the greatest ferret neurosurgeons in the world. The ferret pictured here has had surgery a few months before; the surgeons go in through the top of the skull, go into the nasal cavity, make a little window, peel the tissue off the cribriform plate, and put bone cement on there to block it up. We want to see after six months do we get plaques and tangles, hopefully so and that would be great. But in the meantime we're doing behavior studies. Pictured here is the maze that they run through. We are assessing them every two weeks, and when we finish this study we'll put it all together.
It is one thing to tell someone that we have done a scan and you are going to get Alzheimer's disease in five years or seven years, or you have a very high risk because of your cribriform plate. It is another thing to be able to do something about it. This slide is an image from Greek mythology, the water nymph Arethusa, and she attracted the attention of a river god, and she couldn't get away from him. Artemis saved her by turning her into a hidden underground stream. That is what we want to do; we want to make a hidden under-the-tissue channel to drain the fluid from this area. We call it Arethusta.
Here is a simple diagram. We go up through the nose, the cribriform plate is up there at the very top of the nasal cavity, and we put in the device. This is a very old version of the device. We have much newer, better ones, but it is in the patent, which is publicly available. What we feel we'll do is put the shunt in maybe in people who have mild cognitive impairment. That should actually reverse their mild cognitive impairment (MCI), because the MCI is happening because the amyloid-β has become oligomeric, which is toxic to synapses. So the synapses aren't working as well. This is even before the formation of plaques. If we restore the flow, we should be able to reduce the level of oligomeric amyloid-β and hopefully prevent the disease from progressing to Alzheimer's disease. That is our plan.
We think it is minimally invasive, although we do have neurosurgeons do it because we are puncturing into a part of the brain, and we have to be very careful about that. We think it should be pretty straightforward, and the trial, since we're targeting MCI, isn't going to be the 10 or 20 year trial for an Alzheimer's study. It will only be a couple of years, because we should see the effects in a matter of weeks to months. Then hopefully five years later they don't progress to Alzheimer's disease.
So in summary, I talked about the cribriform plate as the final outlet for CSF in the region where the disease starts. It is a natural chokepoint. We saw age-dependent changes in the cribriform plate, and the Alzheimer's patients show more occlusion. Then there are the flow channels, the fluidics. Looking at these flow channels is just a lot of work because these files are enormous, and it takes weeks and weeks to go through one and identify all of the channels and all of the nerves. But we are making good progress. Then what we really want to do is use all of our information to make a diagnostic algorithm that will work with an appropriate CT scan from anyone, and figure out where they are in the spectrum - and then treat MCI.