Lecture neurodegenerative disorders 6: treatment for neurodegenerative disorders (therapy 2) Flashcards
Question already discussed in lecture about AD, but just for repetition:
Describe how amyloid beta plaques are synthesized (i.e. describe APP processing).
Amyloid Precursor Protein (APP) is normally cut by α-secretase, where non-amyloidogenic amyloid protein is produced as a consequence.
But if APP is cleaved by β- en γ-secretase, amyloidogenic amyloid is produced that is either 40 or 42 amino acids long.
Why is it more interesting to target γ-secretase instead of β-secretase?
Because β-secretase is responsible for the initial cleavage of APP. It cuts APP at its first amino acid. γ-secretase is responsible for the final cleavage of APP and determines whether Aβ is 40 or 42 amino acids long. This also determines the toxicity of Aβ, since Aβ(1-42) is more prone to aggregation.
What type of drugs are γ-secretase inhibitors and why is this important?
They’re small molecule inhibitors, these inhibitors are able to pass the blood-brain-barrier.
Semagestat is a γ-secretase inhibitor, but failed in clinical trial phase III. Why?
It failed due to peripheral side effects, the fact that Aβ CSF levels were not affected and that the cognition of patients worsened during the trial.
They did realize γ-secretase inhibitors could be used for the treatment of cancers. Why?
γ-secretase inhibitors had side effects, since it targets Notch signaling.
What was thought to be the reason behind the failure of semagestat in clinical trial?
The drug administration was not correct. The drug was administered daily and this resulted in concentration peaks of semagetat in the CSF.
In the picture you see that each time the drug runs low in concentration, the generation of Aβ increases. So a more moderate and continuous inhibition would be preferred.
So what became clear rather quickly after the failure of semagestat in clinical trial?
That there were critical knowledge gaps in γ-secretase pharmacology. The trial was started too soon and they needed to know more before trying again.
Describe β-secretase characteristics.
β-secretase (1) is also called BACE(1). It is an aspartic protease, where part of the protein is localized in the membrane (transmembrane).
The interest was then taken in BACE inhibiton. Was BACE actually a good target?
No, as you can see in the picture BACE has more substrates that it can cleave then only APP. The pictures also shows that some substrates are better cleaved by BACE then others, where you can see that APP is not a good substrate for BACE. Thus pharmacologically, APP is not a good substrate for BACE.
Besides this, BACE knockouts showed severe side effect and companies that produces BACE inhibitors also had severe side effects like liver toxicity.
Is there still possibility to use BACE, even though BACE inhibition showed severe side effects?
Yes, but not complete inhibiton of BACE. A reduced expression of BACE could be a possibility, where the dosing would be most important to take into account.
Why is it too simplistic too state that Aβ(1-40) and Aβ(1-42) are the amyloidogenic amyloids that are prone to aggregation and cause AD?
Because Aβ as multiple molecular appearances, as can be seen in the picture.
What can happen during APP processing or after production of Aβ?
Aβ can be modified by an enzyme called glutaminyl cyclase (QC). It modifies the N-terminal glutamate to a cyclic molecule called pyroglutamate (pE), which changes the properties of the end of Aβ.
What are characteristics of (pE)Aβ peptides?
- Abundantly present in AD brain
- Accelerated aggregation (more hydrophobic due to loss of charge)
- More neurotoxic
- Proteolytic resistance
So what could we do to prevent formation of (pE)Aβ peptides?
Inhibit glutaminyl cyclase (QC)
PBD150 is a developed drug that inhibits glutaminyl cyclase (QC). What is seen when this drug is administered in mice in regard to the plaques?
PBD150 reduces plaque formation (i.e. plaque load), but plaques that are already present are not targeted by this inhibitor.
What is another possibility besides administration of PBD150?
Developing a vaccine against pyroglutamate (pE3), this is shown to reduce plaques.
It is thought that antibodies are able to reduce the amount of Aβ according to two hypothesis. Name these.
- Antibodies cross the blood-brain-barrier and bind to Aβ.
- Peripheral sink hypothesis: Aβ crosses the blood-brain-barrier and binds to antibodies in the bloodstream. Since there is a equilibrium between Aβ in the brain and in the bloodstream: if you reduce the concentration of Aβ in the bloodstream, you titrate the Aβ from the brain to the bloodstream.
Is active immunization of an APP model possible?
Yes, they performed this kind of immune therapy in animals. In the picture, left is the brain of a mouse with plaques and right is the brain of a mouse that got active immune therapy.
A memory task is used to test the effectiveness of mice treated with antibodies against Aβ. This memory task makes use of the curiosity of the mouse, where first object A is shown and after a while object B is also shown. If they’re cognitively capable, mice will always go to object B out of curiosity. This can be measured with an index that indicates the time exploring the object. What is seen here?
An index of >50% means that the memory is capable of recognition. If a mouse has plaques, the memory is severely reduced. Transgenic APP mice with plaques that are treated with antibodies, show a repaired memory → very promising.
What is seen in regard to synaptophysin reactivity upon Aβ vaccination (either passive or active)?
The reactivity of synaptophysin increases drastically. There’s massive rescue of the phenotype.
What happened when Aβ vaccination went into clinical trial?
Even though synapthophysin reactivity went up, which seemed like a good thing: the clinical trial was halted when people developed severe side effects. Two people even died due to a too high immune response.
As you can see in the picture, synapthophysin reactivity is increased (brighter green in lower row). But if you compare the two rows, there’s no difference in the pictures.
What is shown in autopsies of patient with Aβ vaccination?
The black dot in the graph are patients with Aβ vaccination. You can see that in several brain areas of these patients, the Aβ load had decreased compared to other patients.
