We now understand what amyloid actually is

Lately we have received an embarrassment of riches about amyloid and the diseases it causes.  I’ll start with the latest — the structure of TDP amyloid.

I must say it is a pleasure to get back to chemistry and away from the pandemic, however briefly.  So relax and prepare to enjoy some great chemistry and protein structure.

TDP43 (you don’t to know what the acronym stands for) is a protein which binds to RNA (among other things).  It also forms aggregates, and some 50 mutations are known producing FrontoTemporal  Dementia (FTD) and/or Amyotrophic Lateral Dementia (ALS).  I saw a case as a resident (before things were worked out) and knew something was screwy because while ALS is a horrible disease, patients are clear to the end (witness Stephen Hawking) and my patient was clearly dementing.

Mutations in TDP43 occur in 5% of familial ALS.  More to the point cytoplasmic aggregates of TDP43 occur in 95% of sporadic cases of ALS (no mutations), so neurologists have been fascinated with TDP43 for years.

Back before we knew much about the structure of amyloid, it was characterized by the dyes that would bind to it (Congo Red, thioflavin etc.) and birefringence (see below).  None of this is true for the aggregates of TDP43.

Well we now know what the structure of amyloid is.  You simply can’t do better than  Cell vol. 184 pp. 4857 – 4873 ’21 — but it might be behind a paywall.

So here’s the skinny about what amyloid actually is —

 

It is a significantly long polypeptide chain  flattening  out into a 4.8 Angstrom thick sheet, essentially living in 2 dimensions.  Thousands of sheets then pile on top of each other forming amyloid.  So amyloid is not a particular protein, but a type of conformation a protein can assume (like the alpha helices, beta pleated sheets etc. etc. ).

The structure also explained why planar molecules like Congo Red bind to amyloid (it slips between the sheets).   Or at least that’s what I thought.

 

Enter Nature vol. 601 pp. 29 – 30, 139 – 143 ’22 showing that some 79 amino acids of the 414 amino acids of TDP43 flatten out into single sheet in the aggregates, with the sheets piling on top of each other.  If that isn’t amyloid, what is?

 

Where are the beta strands producing birefringence if this is amyloid.  In fact where is the birefringence? (see below). The paper says that there are 10 beta strands in the 79 amino acids, but they are short with only two of them containing more than 3 amino acids (I guess they can see beta strands by measuring backbone angles a la Ramachandran plots).  The high number of glycine mediated turns prevents beta sheets from stacking next to each other precluding the crossBeta  structure (and birefringence).

 

Why doesn’t Congo Red bind?  My idea about how it binds to other amyloids (slipping between the sheets) clearly is incorrect.

 

There are all sorts of fascinating points about the amyloid of TDP43.  The filaments derived from patients are stable to heating to 65 C.   The structure of the TDP43 fibrils derived from patients with FTD/ALS are quite different in structure from synthetic filaments made from parts of TDP43, so possibly a lot of work will have to be done again.

 

Here is some more detail on amyloid structure:

 

So start with NH – CO – CHR.  NH  CO and C in the structure all lie in the same plane (the H and the side chain of the amino acid < R >  project out of the plane).

Here’s a bit of elaboration for those of you whose organic chemistry is a distant memory.  The carbon in the carbonyl bond (CO) has 3 bonding orbitals in one plane 120 degrees apart, with the 4th orbital perpendicular to the plane — this is called sp2 hybridization.  The nitrogen can also be hybridized to sp2.  This lets the pair of electrons above the plane roam around moving toward the carbon.  Why is this good?  Because any time you let electrons roam around you increase their entropy (S) and anything increasing entropy lowers their free energy (F)which is given by the formula F = H – TS where H is enthalpy (a measure of bond strength, and T is the absolute temperature in Kelvin.

 

So N and CO are in one plane, and so are the bonds from  N and C to the adacent atoms (C in both cases).

 

You can fit the plane atoms into a  rectangle 4.8 Angstroms high.  Well that’s one 2 dimensional rectangle, but the peptide bond between NH and CO in adjacent rectangles allows you to tack NH – CO – C s together while keeping them in a 3 dimensional parallelopiped 4.8 Angstroms high

 

Notice that in the rectangle the NH and CO bonds are projecting toward the top and bottom of the rectangle, which means that in each plane  NH – CO – CHR s, the NH and CO are pointing out of the 2 dimensional plane (and in opposite directions to boot). This is unlike protein structure in which the backbone NHs and COs hydrogen bond to each other.  There is nothing in this structure for them to bond to

 

What they do is hydrogen bond to another 3 dimensional parallelopiped (call it a sheet, but keep in mind that this is NOT the beta sheet you know about from the 3 dimensional structures of proteins we’ve had for years).

So thousands of sheets stacked together form the amyloid fibril.

 

Where does the 9 Angstrom reflection of cross beta (and birefringence) come from?  Consider the  [ NH – CHR – CO ]  backbone as it lies in the 4.8  thick plane (Having studied proteins structure since entering med school in ’62, I never thought such a thing would even be possible ! ).  It curves around like a snake lying flat.  Where are the side chains?  They are in the 4.8 thick plane, separating parts of the meandering backbone from each other — by an average of 9 Angstroms.

Here is an excellent picture of the Alzheimer culprit — the aBeta42 peptide as it forms the amyloid of the senile plaque

https://pubmed.ncbi.nlm.nih.gov/27355699/#&gid=article-figures&pid=figure-7-uid-6.

You can see the meandering backbone and the side chains keeping the backbone apart.

Then Nature [ vol. 598,  pp. 359 – 363 ’21] blows the field wide open, finding 19 different conformations of tau in clinically distinct diseases. Each clinical disease appears to be associated with a distinct polymorphism.  This is also true for the polymorphisms of alpha-synuclein, with distinct conformations being seen in each of Parkinsonism, multiple system atrophy and Lewy body dementia.

In none of the above diseases is there a mutation (change in amino acid sequence) in the protein.

Henry J. Heinz claimed to have 57 varieties of pickles in 1896, but Cell [ vol. 184 pp. 4857 – 4873 ’21  ] Page 4862 claims that 24 amyloid polymorphs of alpha-synuclein have been found and structurally characterized.  Recall that alpha-synuclein amyloid is the principal component of the Lewy body of Parkinsonism  and Lewy Body disese

How did they get the 24 different conformations?  They incubated the protein under different conditions (e.g. different salt concentrations, different alpha-synuclein concentrations, different salts).

Why is this incredibly good news? 

Because it moves us past amyloid itself, to the conditions which cause amyloid to form.  Certainly, removing amyloid or attacking it hasn’t resulted in any clinical benefit for the Alzheimer patient despite billions being spent by Big Pharma to do so.

We will start to study the ‘root causes’ of amyloid formation.   The amino acid sequence of each protein is identical despite the different conformations of the chain in the amyloid. Clearly the causes must be different for each of the different polymorphs of the protein.  This just has to be true.