Tag Archives: PrPc

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.

The prion battles continue with a historical note at the end

Now that we know proteins don’t have just one shape, and that 30% of them have unstructured parts, it’s hard to remember just how radical that Prusiner’s proposal that a particular conformation (PrPSc) of the normal prion protein (PrPC) caused other prion proteins to adopt it and cause disease was back in the 80s. Actually Kurt Vonnegut got there first with Ice-9 in “Cat’s Cradle ” in 1963. If you’ve never read it, do so, you’ll like it.

There was huge resistance to Prusiner’s idea, but eventually it became accepted except by Laura Manuelidis (about which more later). People kept saying the true infectious agent was a contaminant in the preparations Prusiner used to infect mice and that the prion protein (called PrPC) was irrelevant.

The convincing argument that Prusiner was right (for me at least) was PMCA (Protein Misfolding Cyclic Amplification) in which you start with a seed of PrPSc (the misfolded form of the normal prion protein PrPC), incubate it with a 10,000 fold excess of normal PrPC, which is converted by the evil PrPSC to more of itself. Then you sonicate what you’ve got breaking it into small fragments, and continue the process with another 10,000 fold excess of normal PrPC. Repeat this 20 times. This should certainly dilute out any infectious agent along for the ride (no living tissue is involved at any point). You still get PrPSc at the end. For details see Cell vol. 121 pp. 195 – 206 ’05.

Now comes [ Proc. Natl. Acad. Sci. vol. 117 pp. 23815 – 23822 ’20 ] which claims to be able to separate the infectivity of prions from their toxicity. Highly purified mouse prions show no signs of toxicity (neurite fragmentation, dendritic spine density changes) in cell culture, but are still infectious producing disease when injected into another mouse brain.

Even worse treatment of brain homogenates from prion infected mice with sodium laroylsarcosine destroys the toxicity to cultured neurons without reducing infectivity to the next mouse.

So if this paper can be replicated it implies that the prion protein triggers some reaction in the intact brain which then kills the animals.

Manuelidis thought in 2011 that the prion protein is a reaction to infection, and that we haven’t found the culprit. I think the PCMA pretty much destroyed that idea.

So basically we’re almost back to square one with what causes prion disease. Just to keep you thinking. Consider this. We can knock out the prion protein gene in mice. Guess what? The mice are normal. However, injection of the abnormal prion protein (PrPSc) into their brains (which is what researchers do to transmit the disease) doesn’t cause any disease.

Historical notes: I could have gone to Yale Med when Manuelidis was there, but chose Penn instead. According to Science vol. 332 pp. 1024 – 1027 ’11 she was one of 6 women in the class, and married her professor (Manuelidis) aged 48 when she was 24 graduating in 1967. In today’s rather Victorian standards of consent, power differential between teacher and student, that would probably have gotten both of them bounced out.

So I went to Penn Med. graduating in ’66. Prusiner graduated in ’68. He and I were in the same medical fraternity (Nu Sigma Nu). Don’t think animal house, medical fraternities were a place to get some decent food, play a piano shaped object and very occasionally party. It’s very likely that we shared a meal, but I have no recollection.

Graduating along with me was another Nobel Laureate to be — Mike Brown, he of the statins. Obviously a smart guy, but he didn’t seem outrageously smarter than the rest of us.