Maybe the senile plaque really is a tombstone

“Thinking about pathologic changes in neurologic disease has been simplistic in the extreme.  Intially both senile plaques and neurofibrillary tangles were assumed to be causative for Alzheimer’s.  However there are 3 possible explanations for any microscopic change seen in any disease.  The first is that they are causative (the initial assumption).  The second is that they are a pile of spent bullets, which the neuron uses to defend itself against the real killer.  The third is they are tombstones, the final emanations of a dying cell.” I’ve thought this way for years, and the quote is from 2012: https://luysii.wordpress.com/2012/07/26/research-on-alzheimers-disease-the-bad-news-the-good-news/.

We now have some evidence that the senile plaque may be just a tombstone marking a dead neuron. Certainly attempts to remove the plaques haven’t helped patients despite billions spent in the attempt.

A recent paper [ Proc. Natl. Acad. Sci. vol. 117 pp. 28625–28631 ’20 –https://www.pnas.org/content/pnas/117/46/28625.full.pdf ] not only provides a new way to look at Alzheimer’s disease, but immediately suggests (to me at least) a way to test the idea. If the test proves correct, it will turn the focus of Alzheimer disease research on its ear.

Not to leave anyone behind, the senile plaque is largely made of a small fragment (the aBeta peptide 40 or 42 amino acids) from a much larger protein (the amyloid precursor protein [ APP ] — with well over 800 amino acids). Mutations in APP with the net effect of producing more aBeta are associated with familial Alzheimer’s disease, as are mutations in the enzymes chopping up APP to form Abeta (presenilin1, etc.).

The paper summarizes some evidence that the real culprit is neuronal uptake of the Abeta peptide either as a monomer, or a collection of monomers (an oligomer) or even the large aggregate of monomers seen under the microscope as the senile plaque.

The paper gives clear evidence that a 30 amino acid fragment of Abeta by itself without oligomerization can be taken up by neurons. Not only that but they found the protein on neuronal cell surface that Abeta binds to as well.

Ready to be shocked?

The protein taking Abeta into the neuron is the prion protein (PrPC) which can cause mad cow disease, wasting disease of elk and all sorts of horrible neurologic diseases among them Jakob Creutzfeldt disease.

Now to explain a bit of the jargon which follows. The amino acids making up our proteins come in two forms which are mirror images of each other. All our amino acids are of the L form, but the authors were able to synthesize the 42 amino acid Abeta peptide (Abeta42 below) using all L or all D amino acids.

It’s time to let the authors speak for themselves.

“In previous experiments we compared toxicity of L- and D-Aβ42. We found that, under conditions where L-Aβ42 reduced cell viability over 50%, D-Aβ42 was either nontoxic (PC12) or under 20% toxic . We later showed that L-Aβ is taken up approximately fivefold more efficiently than D-Aβ (28), suggesting that neuronal Aβ uptake and toxicity are linked.”

Well, if so, then the plaque is the tombstone of a neuron which took up too much Abeta.

Well how could you prove this? Any thoughts?

Cell models are nice, but animal models are probably better (although they’ve never resulted in useful therapy for stroke despite decades of trying).

Enter the 5xFAD mouse — it was engineered to have 3 mutations in APP known to cause Familial Alzheimer’s Disease + 2 more in Presenilin1 (which also cause FAD). The poor mouse starts getting Abeta deposition in its brain under two months of age (mice live about two years).

Now people aren’t really sure just what the prion protein (PrPC) actually does, and mice have been made without it (knockout mice). They are viable and fertile, and initially appear normal, but abnormalities appear as the mouse ages if you look hard enough. But still . . .

So what?

Now either knock out the PrPC gene in the 5xFAD mouse or mate the two different mouse strains.

The genes (APP, presenilin1 and PrPC) are on different chromosomes (#21, #14 and #20 respectively). So the first generation (F1) will have a normal counterpart of each of the 3 genes, along with a pathologic gene (e.g. they will be heterozygous for the 3 genes).

When members of F1 are bred with each other we’d expect some of them to have all mutant genes. If it were only 2 genes on two chromosomes, we’d expect 25% of he offspring (F2 generation) to have all abnormal genes. I’ll leave it for the mathematically inclined to figure out what the actual percentage of homozygous abnormal for all 3 would be).

What’s the point? Well, it’s easy to measure just what genes a mouse is carrying, so it’s time to look at mice with a full complement of 5xFAD genes and no PrPC.

If these mice don’t have any plaques in their brains, it’s game, set and match. Alzheimer research will shift from ways to remove the senile plaque, to ways to prevent it by inhibiting cellular uptake of the abeta peptide.

What could go wrong? Well, their could be other mechanisms and other proteins involved in getting Abeta into cells, but these could be attacked as well.

If the experiment shows what it might, this would be the best Thanksgiving present I could imagine.

So go to it. I’m an 80+ year old retired neurologist with no academic affiliation. I’d love to see someone try it.

Kuru continues to inform

Neurologists of my generation were fascinated with Kuru, a disease of the (formerly) obscure Fore tribe of New Guinea. Who would have thought they would tell us a good deal about protein structure and dynamics?

It is a fascinating story including a Nobelist pedophile (Carleton Gajdusek) https://en.wikipedia.org/wiki/Daniel_Carleton_Gajdusek and another (future) Nobelist who I probably ate lunch with when we were both medical students in the same Medical Fraternity but don’t remember –https://en.wikipedia.org/wiki/Stanley_B._Prusiner

Kuru is a horrible neurodegeneration starting with incoordination, followed by dementia and death in a vegetative state in 4 months to 2 years. For the cognoscenti — the pathology is neuronal loss, astrocytosis, microglial proliferation, loss of myelinated fibers and the kuru plaque.

It is estimated that it killed 3,000 members of the 30,000 member tribe. The mode of transmission turned out to be ritual cannibalism (flesh of the dead was eaten by the living before burial). Once that stopped the disease disappeared.

It is a prion disease, e.g. a disease due to a protein (called PrP) we all have but in an abnormal conformation (called PrpSc). Like Vonnegut’s Ice-9 (https://en.wikipedia.org/wiki/Ice-nine) PrPSc causes normal PrP to assume its conformation, causing it to aggregate and form an insoluble mess. We still don’t know the structure of PrPSc (because it’s an insoluble mess). Even now, “the detailed structure of PrPSc remains unresolved” but ‘it seems to be’ very similar to amyloid [ Nature vol. 512 pp. 32 – 34 ’14]. Not only that, but we don’t know what PrP actually does, and mice with no PrP at all are normal [ Nature vol. 365 p. 386 ’93 ]. For much more on prions please see https://luysii.wordpress.com/2014/03/30/a-primer-on-prions/

Prusiner’s idea that prion diseases were due to a protein, with no DNA or RNA involved met with incredible resistance for several reasons. This was the era of DNA makes RNA makes protein, and Prisoner was asking us to believe that a protein could essentially reproduce without any DNA or RNA. This was also the era in which X-ray crystallography was showing us ‘the’ structure of proteins, and it was hard to accept that there could be more than one.

There are several other prion diseases of humans (all horrible) — mad cow disease, Jakob Creutzfeldt disease, Familial fatal insomnia, etc. etc. and others in animals. All involve the same protein PrP.

One can take brain homogenates for an infected animal, inoculate it into a normal animal and watch progressive formation of PrPSc insoluble aggregates and neurodegeneration. A huge research effort has gone into purifying these homogenates so the possibility of any DNA or RNA causing the problem is very low. There still is one hold out — Laura Manuelidis who would have been a classmate had I gone to Yale Med instead of Penn. n

Enter [ Nature vol. 522 pp. 423 – 424, 478 – 481 ’15 ] which continued to study the genetic makeup of the Fore tribe. In an excellent example of natural selection in action, a new variant of PrP appeared in the tribe. At amino acid #127, valine is substituted for glycine (G127V is how this sort of thing is notated). Don’t be confused if you’re somewhat conversant with the literature — we all have a polymorphism at amino acid #129 of the protein, which can be either methionine or valine. It is thought that people with one methionine and one valine on each gene at 129 were somewhat protected against prion disease (presumably it affects the binding between identical prion proteins required for conformational change to PrPSc.

What’s the big deal? Well, this work shows that mice with one copy of V127 are protected against kuru prions. The really impressive point is that the mice are also protected against variant Creutzfedlt disease prions. Mice with two copies of V127 are completely protected against all forms of human prion disease . So something about V/V at #127 prevents the conformation change to PrPSc. We don’t know what it is as the normal structure of the variant hasn’t been determined as yet.

This is quite exciting, and work is certain to go on to find short peptide sequences mimicking the conformation around #127 to see if they’ll also work against prion diseases.

This won’t be a huge advance for the population at large, as prion diseases, as classically known, are quite rare. Creutzfeldt disease hits 1 person out of a million each year.

There are far bigger fish to fry however. There is some evidence that the neurofibrillary tangles (tau protein) of Alzheimer’s disease and the Lewy bodies (alpha-Synuclein) of Parkinsonism, spread cell to cell by a ‘prionlike’ mechanism [ Nature vol.485 pp. 651 – 655 ’12, Neuron vol. 73 pp. 1204 – 1215 ’12 ]. Could this sort of thing be blocked by a small amino acid change in one of them (or better a small drug like peptide?).

Stay tuned.