The biochemistry of AD and the implementation of the Amyloid hypothesis

What is Alzheimer’s and its history

Alzheimer’s is a disease that leads to the death of nerve cells and tissue loss throughout the brain. This over time causes the brain to dramatically shrink and the cell death causes memory failure, personality changes and problems carrying out what we consider to be everyday activities.

The disease was first discovered in 1906, by Dr Alois Alzheimer who conducted a post-mortem examination on a woman who had recently died of an unusual mental illness (she displayed symptoms such as memory loss, language problems and unpredictable behaviour)1. The changes in the brain tissue of this women led to a further in-depth examination by Dr Alzheimer. He noticed that she had many abnormal clumps (which we now know as Amyloid plaques) and entangled neurofibrillary bundles.

These two findings are still considered to be the main features of Alzheimer’s disease. The other feature is the connections between the neurons and the damage to their dendrites and axon terminals due to plaque forming upon them, disrupting their chemical passage of signals.

These conclusions led to a new outlook on why the older generation had a frequency of memory loss, and difficulty just simply breathing and speaking by the end of their life. Not all were affected but many had symptoms, and it had always alluded neurologists as to why this occurs. This was a huge breakthrough in the neuroscience industry, however, not the huge change that will be the main feature of this essay.

The breakthrough that I am most concerned with, is the breakthrough discovered by Kim et al. But before discussing this breakthrough, we need to make sense of the theory that these academics built upon.

The Amyloid Cascade theory and its use in treatment breakthroughs

The main theory that has always been tested is the Amyloid cascade theory. This hypothesis states that the excessive abnormal clumps that Dr Alois Alzheimer had discovered, may be an accumulation of a peptide known as amyloid-beta. This accumulation sets off a series of events that results in the death of neurons and brain cells, progressively leading to AD.

The first question to propose, is what is this Amyloid-beta (A-B) peptide? It is believed that it is a by-product of the proteolytic breakdown of the amyloid precursor protein (APP). It is not quite clear what the exact function of APP is, but it is thought to heavily influence damage to the activity of brain cells, due to the findings of a mass of their by-product (A-B) in the brain tissue in post-mortem examinations of AD patients.

Amyloid beta is also not a hugely understood protein. What is known of them, is that these A-B monomers are soluble across membranes and are largely alpha helical in membranes. The link to AD is through the fact that under high concentrations, they are able to undergo dramatic conformational change to form a beta-sheet rich tertiary structure that aggregates to form amyloid fibrils. These fibrils deposit outside the neurons and form senile or neurotic plaques in dense formations.

Genetically engineered mice were used in a test (by Kim et al) that identified A-B causes the destruction of synapses before they clump into plaques that lead to nerve cell death. The researchers identified a new protein, called PirB in mice and LilrB2 in humans, which chemically attracts amyloid-beta clusters as if it were a phagocyte being attracted to a pathogen. This A-B cluster is set off and cascades into a state of overactive biochemical activity that results in the destruction of the synapses.

To add even more influence on this theory: It was found that the mice without PirB were resistant to memory breakdown and synapse loss that is associated with AD. The similar protein in humans, LilrB2, could therefore be the reason for AD and the attraction of A-B clusters, and so my own hypothesis is, what if we could genetically identify the presence of this protein LilrB2 with in the human body and more importantly the brain? And if we are able to identify this, could this perhaps be genetically modified to code DNA against the production of this protein or an enzyme bind to the LilrB2 to reduce the radicals influencing the proteolysis of APP to A-B.

The amyloid theory also encapsulates the belief that proteins, and especially APP can be broken down to a negatively charged protein however it has never been clear how this could be impactful in the progressive destruction of the brain by AD.

From what I’ve personally researched and considered about synaptic transmission, I believe that these AB monomers are able to attach and surround these neurons due to the Ca+ concentrations that are expressed by the neurons. The AB monomers are likely to be negatively charged proteins that are then “attracted” to these monomers and are able to attach and deposit themselves onto the membrane of these neurons. This disrupts the function of the synapse due to the charges fluctuating and their inability to be able to convert Ca+ ions so are unable to connect transmissions through synapses. By doing this it basically induces unrequested apoptosis as the cell is unable to carry out its function.

Even further, it could be considered that the “complex” that the A-B forms with the LilrB2 induces a negative charge due to the imbalance of hydrogen bonding to the pH of the liquid in the brain, as it is much more alkaline than the rest of the body, results in its negative charge. As the complex means that it is a potential radical due to unstable bonds, the pH of the cerebrospinal fluid could cause it to donate OH- ions to stabilise the radical meaning that it is now in the perfect condition within the brain. As it is a protein, it will likely want to attach to a surface membrane, with the cortex of the brain being the volunteer of that.

If the LilrB2 and A-B complex attaches to the cortex, it may mean that the LilrB2 detaches from the A-B leaving it still in its radical state, it will react with a possible attached protein on the membrane surface of the cortex and so break it off. Slowly it will carry on doing this, what with radical influencing A-B to carry on cascading into further radicals, as says the theory. If this is the case, though extremely hard to test with our current technology, could we possibly find a cure that can either form a complex with the LilrB2 or APP or A-B? Or could we use treatment to induce the brain into a water pH level or acidic state that could still keep or processes running but stop the conformational change in shape of A-B/APP?

Issues with this hypothesis

However, the issue with this hypothesis is that with any protein there is always a mutation and these AB proteins are highly prone to mutating. As a result, even if we do secure drugs that could possibly bind to the AB proteins to reduce or cancel their effect, the cost effectiveness due to the proportion of people that may have a mutated version of the AB protein would not be sufficient enough to consider the drug.

The aim that everyone has when curing dementia is “drugs that prevent or remove the amyloid should slow the onset of dementia”3. But, no one has been able to cure or even find a slow for AD using this proposed theory, with 90% of drug trials failing and reaping no results.

Maybe the issue is that we don’t consider the effect of a protein. The issue with this hypothesis in my view is that with any protein there is always a mutation and these AB proteins are highly prone to mutating. As a result, even if we do secure drugs that could possibly bind to the AB proteins to reduce or cancel their effect, the cost effectiveness due to the proportion of people that may have a mutated version of the AB protein would not be sufficient enough to consider the drug.

The A-B protein is highly conformational, with it theoretically easily changing shape just at pH 8, and not all of these A-B proteins will have the same style of conformational change. The aim of all the drug trials are, “removing amyloid once people have established dementia”3 will suitably stop AD. However, we don’t consider the fact that our drugs will have one tertiary structure that fits one specific form of the A-B protein, the A-B that was been changed AFTER AD has occurred.

Perhaps the idea should be, what could we do to bind to LilrB2 or even APP to stop A-B from even forming. The basis we are all looking at is a theory that takes place during the progressive AD destruction. Maybe a better idea would be what could we do to prevent A-B full stop.

My idea for a possible cure basing my research on the Amyloid hypothesis breakthrough by Kim

As I discussed before, A-B has to be released into spaces between the cells, with the nerve cells the gap is the synapse. As you can see in the diagram on the right, there are a number of enzymes involved in the process of the formation of A-B. So the idea that Kim had is based on the formation of Oligomers by ONE protein but does not take into account the enzymes that are used.

Every drug trial that I have researched such as the use of solanezumab or bapineuzumab focuses on one protein: the A-B protein. But proteins could possibly have a different tertiary structure when they form their oligomers or their plaques and so a drug that targets their bonding to break the bonding down would not be able to battle the large number of different bonding that is formed. Yes most of the boding is di-sulfide and hydrogen bonding but we are basing our drugs on a POTENTIAL theory. The cascade theory could be completely wrong but the facts that we do know are that APP breaks down to form the product of A-B and that the enzymes labelled in the above diagram are indeed included in the APP break down. So if we know these two, why not work on possibly creating a drug that can be used as a competitive or even non-competitive synthetic protein to bond with the B-secretase, stopping the proteolysis of APP and so stopping the aggregation of the A-B.

More research definitely needs to be conducted on the beginning stages of AD but this is so hard to predict. The issue with the research that has currently been conducted is that we are basing our hypothesises and ideas on post-mortem examinations and MRI scans of people who already have AD. So the drugs that we are producing are targeted at late stage AD and dementia. If we could predict a pre-disposition of the enzyme B-sucrase, LilrB2 or a mass amount of APP we could predict the likelihood of someone potentially having dementia in the future and prescribe them the medication to bind with any of these proteins to prevent the aggregation and accumulation of A-B.

The issue with our lack of knowledge with the brain

The entire issue is the lack of knowledge of the brain. We understand which parts of the brain are mainly affected by the A-B due to our knowledge of the lobes of the brain.4 The parietal, temporal and frontal lobes are the most likely to be damaged, with brain scans indicating this as well as the changes in personality and morality (frontal lobe), our ability in language and writing (parietal lobe) and most severely memory (temporal).

We can see from this imaging that the lack of red area shows a lack of activity in the middle core of the brain, inferring this with the image on the right, we can see mainly the frontal parietal and temporal lobes are affected the most but we don’t know why. We haven’t been able to test on healthy brains (pre mortem examinations) sufficiently enough to be able to make accurate suggestions about if each lobe has different structures that influence it’s normal function and its potential to be affected by diseases such as AD. This is what I’d like to research on more and this is the next step in the breakthrough in neurotics that I believe is desperately needed.

Included Bibliography in Vancouver format

1. What Happens to the Brain in Alzheimer's Disease? [Internet]. National Institute on Aging. 2019 [cited 10 July 2019]. Available from: https://www.nia.nih.gov/health/what-happens-brain-alzheimers-disease

2. The Amyloid Cascade Hypothesis of Alzheimer's Disease [Internet]. Verywell Health. 2019 [cited 5 July 2019]. Available from: https://www.verywellhealth.com/amyloid-cascade-hypothesis-of-alzheimers-disease-98791

3. Zhang S. Is the Leading Theory about Alzheimer's Wrong? [Internet]. The Atlantic. 2019 [cited 6 July 2019]. Available from: https://www.theatlantic.com/health/archive/2017/02/alzheimers-amyloid-hypothesis/517185/

4. Synthetic peptide can inhibit toxicity, aggregation of protein in Alzheimer's disease [Internet]. Medicalxpress.com. 2019 [cited 4 July 2019]. Available from: https://medicalxpress.com/news/2019-04-synthetic-peptide-inhibit-toxicity-aggregation.html

By Chandini :)