Tag Archives: Microglia

Will flickering light treat Alzheimer’s disease ?

Big pharma has spent zillions trying to rid the brain of senile plaques, to no avail. A recent paper shows that light flickering at 40 cycles/second (40 Hertz) can do it — this is not a misprint [ Nature vol. 540 pp. 207 – 208, 230 – 235 ’16 ]. As most know the main component of the senile plaque of Alzheimer’s disease is a fragment (called the aBeta peptide) of the amyloid precursor protein (APP).

The most interesting part of the paper showed that just an hour or so of light flickering at 40 Hertz temporarily reduced the amount of Abeta peptide in visual cortex of aged mice. Nothing invasive about that.

Should we try this in people? How harmful could it be? Unfortunately the visual cortex is relatively unaffected in Alzheimer’s disease — the disease starts deep inside the head in the medial temporal lobe, particularly the hippocampus — the link shows just how deep it is -https://en.wikipedia.org/wiki/Hippocampus#/media/File:MRI_Location_Hippocampus_up..png

You might be able to do this through the squamous portion of the temporal bone which is just in front of and above the ear. It’s very thin, and ultrasound probes placed here can ‘see’ blood flowing in arteries in this region. Another way to do it might be a light source placed in the mouth.

The technical aspects of the paper are fascinating and will be described later.

First, what could go wrong?

The work shows that the flickering light activates the scavenger cells of the brain (microglia) and then eat the extracellular plaques. However that may not be a good thing as microglia could attack normal cells. In particular they are important in the remodeling of the dendritic tree (notably dendritic spines) that occurs during experience and learning.

Second, why wouldn’t it work? So much has been spent on trying to remove abeta, that serious doubt exists as to whether excessive extracellular Abeta causes Alzheimer’s and even if it does, would removing it be helpful.

Now for some fascinating detail on the paper (for the cognoscenti)

They used a mouse model of Alzheimer’s disease (the 5XFAD mouse). This poor creature has 3 different mutations associated with Alzheimer’s disease in the amyloid precursor protein (APP) — these are the Swedish (K670B), Florida (I716V) and London (V717I). If that wasn’t enough there are two Alzheimer associated mutations in one of the enzymes that processes the APP into Abeta (M146L, L286V) — using the single letter amino acid code –http://www.biochem.ucl.ac.uk/bsm/dbbrowser/c32/aacode.html.hy1. Then the whole mess is put under control of a promoter particularly active in mice (the Thy1 promoter). This results in high expression of the two mutant proteins.

So the poor mice get lots of senile plaques (particularly in the hippocampus) at an early age.

The first experiment was even more complicated, as a way was found to put channelrhodopsin into a set of hippocampal interneurons (this is optogenetics and hardly simple). Exposing the channel to light causes it to open the membrane to depolarize and the neuron to fire. Then fiberoptics were used to stimulate these neurons at 40 Hertz and the effects on the plaques were noted. Clearly a lot of work and the authors (and grad students) deserve our thanks.

Light at 8 Hertz did nothing to the plaques. I couldn’t find what other stimulation frequencies were used (assuming they were tried).

It would be wonderful if something so simple could help these people.

For other ideas about Alzheimer’s using physics rather than chemistry please see — https://luysii.wordpress.com/2014/11/30/could-alzheimers-disease-be-a-problem-in-physics-rather-than-chemistry/


The most interesting thing to an evolutionist is not that APOE4 increases the risk of Alzheimer’s disease

Neurologists were immensely excited by the discovery 25 years ago that the APOE4 variant of APOlipoprotein E increases the risk of Late Onset Alzheimer’s Disease (LOAD). 24,000 papers later (Google Scholar) we still don’t know how it does it. Should all this work have been done ? Of course ! !  Once we know the mechanism(s) by which APOE4 increases Alzheimer’s risk we’ll have new ideas to help us attack.

The APOE gene has 3 variants (alleles) APOE2, 3 and 4. The protein is average sized (299 amino acids). The 3 alleles differ at two positions (amino acids #112 and #158) where either cysteine or arginine can be found. The frequency of APOE4 is 14% in the adult white population, that of E3 is 78% and that of E2 is 8%.

Fascinating as this all is, it’s not what’s interesting from an evolutionary point of view.

[ Proc. Natl. Acad. Sci. vol. 113 pp. 17 – 18, 74 – 79 ’16 ] Postmenpausal longevity in females is not limited to humans. Humans, orcas and pilot whales are the only vertebrate species known to have prolonged postreproductive lifespans. Our fertility ends at about the same age that fertility ends in other female hominids (the great apes). However, apes rarely live into their 40s (even in captivity).

Unlike APOE4, APOE2 and APOE3 protect against late onset Alzheimer’s.

The fascinating point is that APOE2 and APOE3 aren’t found in the great apes. They are a human invention. Now LOAD occurs well past reproduction, so there should be no reason in terms of reproductive success for them to arise and be more common in human populations than the original APOE4.

Even more interesting is some work on another protein CD33, found on immune cells and glia in the brain [ Neuron vol. 78 pp. 575 – 577, 631 – 643 ’13 ] A minor allele (21% frequency in human populations) of CD33 (SNP rs 3865444) protects against Alzheimer’s. The allele is associated with reductions in CD33 expression in microglia, and also with reduction in levels of insoluble Abeta42 in (Alzheimer’s) brain. The numbers of CD33+ microglia correlate with insoluble Abeta42 levels and amyloid plaque burden. So decreasing (or inhibiting) CD33 function might help Alzheimer patients.

Again the protective allele is only found in man. The great apes don’t have it just the major (nonprotective) allele.

Again, there is no way that having the allele directly improves your reproductive success. By the time it is protecting you, you’re infertile.

What in the world is going on? Why did alleles protective against Alzheimer’s arise in two very different proteins in the course of human evolution?

“There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.” — Mark Twain.

The reason these alleles probably arose gets us in to an ancient battle in evolutionary theory — what is the actual unit of selection? It may be the group rather than the individual. Face it, human infants and children are helpless for longer than other primates, and need others to care for them, for at least 5 years. Who better than grandma and grandpa? So the fact that with granny around more children survive to reproduce constitutes group selection (I think).

As Theodosius Dobzhansky said “Nothing in Biology Makes Sense Except in the Light of Evolution”