Category Archives: Neurology & Psychiatry

So much work, so little progress

Two years ago, I found going to a memorial service for a friend and classmate who died of Alzheimer’s curiously uplifting  (see the link at the end). The disease is far from ignored. A monster review in Neuron vol. 97 pp. 32 – 58 ’18 — http://www.cell.com/neuron/fulltext/S0896-6273(17)31081-4  contained references to over 400 research articles half of them published since January 2013.

Still I found it quite depressing.  Tons of work and tons findings, and yet no coherent path to the cause (or causes); something absolutely necessary for a rational treatment, unless we somehow stumble into a therapy.

In a way it’s like cancer.  The cancer genome atlas intensively studied the genome of various cancers, looking for ‘the’ or ‘the set of’ causative mutations.  They found way too much.  The average colon and breast cancer had an average of 93 mutated genes, of which 11 were thought to be cancer promoting.  Not only that, but the same 11 were not consistent from tumor to tumor.

So it is with this epic review.  Which of the myriad findings described are causative of the disease and which are responses of the nervous system to the ’cause’ (or causes).

In the review the authors posit that Alzheimer’s disease is due to failure of ‘homeostatic systems’ that maintain a ‘set point’ of neuronal firing.  Unfortunately what is measured to determine the set point isn’t known. This seems to be an example of redefining a question into an answer.  Clearly if you juice up neuronal firing rates by stimulation they come back down, or if you inhibit them, they come back up.  So you can operationally define set point without defining it mechanistically.   It must be due to some sort of feedback on whatever it is that is sensing ‘the set point’ , but what is it that is being sensed?

The following is from an earlier post but is quite relevant to homeostasis and set points.

The whole notion of control in the presence of feedback is far from clear cut.  Here’s the story of the first inklings of feedback in endocrinology.  I watched it happen.

Endocrinology was pretty simple in med school back in the 60s. All the target endocrine glands (ovary, adrenal, thyroid, etc.) were controlled by the pituitary; a gland about the size of a marble sitting an inch or so directly behind the bridge of your nose. The pituitary released a variety of hormones into the blood (one or more for each target gland) telling the target glands to secrete, and secrete they did. That’s why the pituitary was called the master gland back then.  The master gland ruled.

Things became a bit more complicated when it was found that a small (4 grams or so out of 1500) part of the brain called the hypothalamus sitting just above the pituitary was really in control, telling the pituitary what and when to secrete. Subsequently it was found that the hormones secreted by the target glands (thyroid, ovary, etc.) were getting into the hypothalamus and altering its effects on the pituitary. Estrogen is one example. Any notion of simple control vanished into an ambiguous miasma of setpoints, influences and equilibria. Goodbye linearity and simple notions of causation.

As soon as feedback (or simultaneous influence) enters the picture it becomes like the three body problem in physics, where 3 objects influence each other’s motion at the same time by the gravitational force. As John Gribbin (former science writer at Natureand now prolific author) said in his book ‘Deep Simplicity’, “It’s important to appreciate, though, that the lack of solutions to the three-body problem is not caused by our human deficiencies as mathematicians; it is built into the laws of mathematics.” The physics problem is actually much easier than endocrinology, because we know the exact strength and form of the gravitational force.

https://luysii.wordpress.com/2016/01/05/an-uplifting-way-to-start-the-new-year/

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Why drug development is hard #31: retroviruses at the synapse

What if I told you that a very important neuronal synaptic protein Arc (Arg3.1) is acting like like a virus, sending copies of itself (and its messenger RNA) across the synapse?  Would a team of shrinks, who’ve never examined me, tell you that I was crazy and unfit to blog?  Well there is very good evidence that exactly this occurs in one situation and probably many more [ Cell vol. 172 pp. 8 – 10, 262 – 274, 275 – 288 ’18] — http://www.cell.com/cell/fulltext/S0092-8674(17)31509-X.

Arc stands for Activity Regulated Cytoskeleton associated protein.  It’s messenger RNA (mRNA) is transcribed from the gene in response to neuronal activity.  More importantly, the mRNA for  Arc is rapidly distributed to active synapses through the cell body and dendrites, where it is translated into protein. It is locally and rapidly stimulated during the induction of long term depression and plays a critical role in removing a class of glutamic acid receptors (AMPA receptors) from the synapse.  To whet the interest of drug developers, Arc regulates the activity dependent cleavage of the Amyloid Precursor Protein (APP) and beta amyloid production by its interaction with presenilin

Several posts could easily be filled with what Arc does, but that’s not what is so amazing about these papers.  Parts of the Arc protein arose from one of the many transcriptionally dead retroviruses found in our genome.  Our species literally wouldn’t exist without other retroviral gifts.  For instance syncytin1 is a protein expressed a high levels in the placenta.  It is produced from the envelope gene of an endogenous retrovirus (HERV-W) which has undergon inactivating mutations in its other major genes (gag and pol).  Mutant mice in which the gene has been knocked out die in utero due to failure of placenta formation.

Part of the arc gene arose from the Gag gene (Group specific antigen gene) of a retrovirus.  Recall most viruses have proteins coating their genetic material when they’re on the move (e. g. a capsid).  In the case of retroviruses, the genetic material is RNA rather than DNA.  Well the gag elements of the Arc protein form a capsid containing the mRNA for Arc (just like a virus).  In some way or other the capsid containing mRNA gets outside the neuron at the nerve muscle junction and gets into muscle.  The evidence is good that this happens, but in a system somewhat removed from us — the fruitfly (Drosophila).  Fruitfly neuromuscular junctions lacking this mechanism are weaker.

Well that’s pretty far from us.  However one of the papers (275 – 288) showed that the Arc protein and its mRNA was found in extracellular vesicles released from mouse neurons cultured from their cerebral cortex.  Could viral-like particles be crossing the synapses in our brains (which are already pretty chockfull of stuff — see https://luysii.wordpress.com/2017/11/15/the-bouillabaisse-of-the-synaptic-cleft/).  It’s very early times (in fact the Cell issue came out 3 days ago) but people are sure to look.  There are at least 100 Gag derived genes in the human genome (Campillos, M., Doerks, T., Shah, P.K., and Bork, P. (2006). Computational characterization of multiple Gag-like human proteins. Trends Genet. 22, 585–589.).

Remarkable.  Remember CRISPR was hiding in plain sight for half a century.  We have a lot to learn.  No wonder drugs have unexpected side effects.

Are the inclusions found in neurologic disease attempts at defense rather then the cause?

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.

A fascinating recent paper [ Neuron vol. 97 pp. 3 – 4, 108 – 124 ’18 ] http://www.cell.com/neuron/pdf/S0896-6273(17)31089-9.pdf gives strong evidence that some inclusions can be defensive rather than toxic.  It contains the following;

“In these studies, we found that formation of large inclusions was correlated with protection from a-synuclein toxicity”

The paper is likely to be a landmark because it ties two neurologic diseases (Parkinsonism and Alzheimer’s) together by showing that they may due to toxicity produced by single mechanism — inhibition of mitochondrial function.

Basically, the paper says that overproduction of alpha synuclein (the major component of the Lewy body inclusion of Parkinsonism) and tau (the major component of the neurofibrillary tangle of Alzheimer’s disease) produce death and destruction by interfering with mitochondria.  The mechanism is mislocalization of a protein called Drp1 which is important in mitochondrial function (it’s required for mitochondrial fission).

Actin isn’t just found in muscle, but is part of the cytoskeleton of every cell.  Alpha-synuclein is held to alter actin dynamics by binding to another protein called spectrin (which also binds to actin).  The net effect is to mislocalize Drp1 so it doesn’t bind to mitochondria where it is needed.  It isn’t clear to me from reading the paper, just where the Drp1 actually goes.

In any event overexpressing spectrin causes the alpha-synuclein to bind to it forming inclusions and protecting the cells.

There is a similar mechanism proposed for tau, and co-expressing alpha synuclein with Tau significantly enhances the toxicity of both models of tau toxicity which implies that they work by a common mechanism.

Grains of salt are required because the organism used for the model is the humble fruitfly (Drosophila).

Not a great way to end 2017

Not a great way to end 2017

2017 ended with a rejection of the following letter to PNAS.

As a clinical neurologist with a long standing interest in muscular dystrophy(1), I was referred many patients who turned out to have chronic fatigue syndrome (CFS) . Medicine, then and now, has no effective treatment for CFS.

A paper (2) cited In an excellent review of cellular senescence (3) was able to correlate an intracellular marker of senescence (p16^INK4a) with the degree of fatigue experienced by patients undergoing chemotherapy for breast cancer. Chemotherapy induces cellular senescence, and the fatigue was thought to come from the various cytokines secreted by senescent cells (Senescence Associated Secretory Phenotype—SASP) It seems logical to me to test CFS patients for p16^INK4a (4).
I suggested this to the senior author; however, he was nominated as head of the National Cancer Institute just 9 days later. There the matter rested until the paper of Montoya et al. (5) appeared in July. I looked up the 74 individual elements of the SASP and found that 9 were among the 17 cytokines whose levels correlated with the degree of fatigue in CFS. However, this is not statistically significant as Montoya looked at 51 cytokines altogether.

In October, an article(6) on the possibility of killing senescent cells to prevent aging contained a statement that Judith Campisi’s group (which has done much of the work on SASP) had identified “hundreds of proteins involved in SASPs”. (These results have not yet been published.) It is certainly possible that many more of Montoya’s 17 cytokines are among them.

If this is the case, a rational therapy for CFS is immediately apparent; namely, the senolytics, a class of drugs which kills senescent cells. A few senolytics are currently available clinically and many more are under development as a way to attack the aging process (6).

If Montoya still has cells from the patients in the study, measuring p16^INK4a could prove or disprove the idea. However, any oncology service could do the test. If the idea proves correct, then there would be a way to treat the debilitating fatigue of both chemotherapy and CFS—not to mention the many more medical conditions in which severe fatigue is found.
Chemotherapy is a systemic process, producing senescent cells everywhere, which is why DeMaria (2) was able to use circulating blood cells to measure p16^INK4a. It is possible that the senescent cells producing SASP in CFS are confined to one tissue; in which case testing blood for p16^INK4a would fail. (That would be similar to pheochromocytoma cells, in which a few localized cells produce major systemic effects.)

Although senolytics might provide symptomatic treatment (something worthwhile having since medicine presently has nothing for the CFS patient), we’d still be in the dark about what initially caused the cells to become senescent. But this would be research well worth pursuing.

Anyone intrigued by the idea should feel free to go ahead and test it. I am a retired neurologist with no academic affiliation, lacking the means to test it.
References

1 Robinson, L (1979) Split genes and musclar dystrophy. Muscle Nerve 2: 458 – 464

2. He S, Sharpless N (2017) Senescence in Health and Disease. Cell 170: 1000 – 1011

3. Demaria M, et al. (2014) Cellular senescence promotes adverse effects of chemotherapy and cancer relapse. Cancer Discov. 7: 165 – 176

4. https://luysii.wordpress.com/2017/09/04/is-the-era-of-precision-medicine-for-chronic-fatigue-syndrome-at-hand/

5. Montoya JG, et al., (2017) Cytokine signature associated with disease severity in chronic fatigue syndrome patients, Proc Natl Acad Sci USA 114: E7150-E7158

6. Scudellari M, (2017) To stay young, kill zombie cells Nature 551: 448 – 450

The flying Wallendas of the synapse

Is anything similar to the flying Wallendas ( https://en.wikipedia.org/wiki/The_Flying_Wallendas) going on in the synapse? The first electron micrographs of the synaptic cleft back in the day showed a clear space about 400 Angstroms (40 nanoMeters) thick.  Well we now know that there are tons of proteins occupying this space — a copy of a previous post

The bouillabaisse of the synaptic cleft

appears after the **** at the end of this post.  It shows just how many proteins occupy that clear space. Could a presynaptic protein directly bond to a postsynaptic protein across the cleft (perhaps with the help of a third or fourth Wallenda protein between the two?  A nice review [ Neuron vol. 96 pp. 680 – 696 ’17b ] http://www.cell.com/neuron/fulltext/S0896-6273(17)30935-2 sets out what is known.

We know that neurexins (presynaptic) bind to neuroligins (postsynaptic) across the cleft.  This is the best studied pair, and most of the earlier post discusses what is known about them.

Figure 1c p. 682 is particularly fascinating as it shows that there are many more molecules which shake hands across the cleft.  Even more interesting is the fact that just where they are relative to the center/periphery of  the synapses isn’t shown for the neurexin/neuroligin pair and the LAR/Strk pair (e.g. one of the best studied pairs) because apparently this isn’t known.   The ephrins/ephrin pair and the syncam pair are in the center, while N-cadherin is shown at the edge.

One of the crucial elements of the post-synaptic membrane, the AMPAR receptor for glutamic protrudes its amino terminal domain 1/3 of the way across the cleft (assuming it is 40 nanoMeters thick).

Postsynaptic receptors are said to be clustered in nanoDomains 80 – 100 nanoMeters in diameter, Similarly, presynaptic RIM nanoClusters are the same size and are said to be aligned with postSynaptic nanoClusters of PSD95 as measured by 3D-STORM, the current most cutting edge technique we have for visualizing these things [ Nature vol. 536 pp. 210 – 214 ’17 ].

So, all in all, the paper is fascinating and shows how much more there is to know.

Unfortunately the paper contains one statement which raises my chemical hackles;  “A consistent prediction across models is that the glutamate concentration profile reaches a very high peak (over 1 milliMolar), but only for a brief time period (100 microSeconds) and over a small distance (100 nanoMeters).” Glutamate is the major excitatory neurotransmitter in brain and is what binds to AMPAR.

Models are lovely, but how many molecules of glutamic acid are they talking about?  It’s easy (but tedious) to figure this out.

We know the volume they are talking about: a cylinder 100 nanoMeters in diameter and 40 nanoMeters tall (the width of the synaptic cleft).   So it contains pi * 100 * 40 = 12,566 cubic nanometers –round this down to 10^4 cubic nanoMeters. A liter is a cube .1 meters (10 centimeters) on a side. So 10 centimeters is 10^8 nanoMeters, meaning that a liter contains (10^8)^3 = 10^24 cubic nanoMeters.

A 1 molar solution of anything contains 6 * 10^23 molecules per liter (Avogadro’s number), so a 1 milliMolar solution (of glutamate in this case) contains 6 * 10^20 molecules/liter or  6 * 10^-4 molecules per cubic nanoMeter. Multiply this by the volume of the cylinder and you get a grand total of 6 molecules of glutamic acid in the cylinder.

If I’ve done the calculations correctly (and I think I have), “a very high peak (over 1 milliMolar)” is basically scientific garbage, the concept of concentration being stretched far beyond its range of meaningful applicability.

I’d love to stand corrected if my calculations are incorrect. Just make a comment.

Addendum 12 Dec — well my calculation is wrong. Here’s the dialog

APAJ — “We know the volume they are talking about: a cylinder 100 nanoMeters in diameter and 40 nanoMeters tall (the width of the synaptic cleft). So it contains pi * 100 * 40 = 12,566 cubic nanometers –round this down to 10^4 cubic nanoMeters.”
Just one err0r in the maths: the volume is r^2*pi*h so it’s closer to 3^5 cubic nm. This leads to ~188 glumate molecules following your further calculations. A more significant number, but I agree concentrations should not be used in these kind of volumes.

APAJ — Thanks — you’re correct and I’m embarrassed — pi * diameter is circumference not volume. so its pi * 50^2 * 40 = 314,259 cubic microns == 25 x more than 12,566 bringing the number of glutamic acids up to 150 (when 12,566 is rounded down to 10^4).

The criticism still stands. Concentration is meaningless in such small volumes.

 

*****

The bouillabaisse of the synaptic cleft

The synaptic cleft is so small ( under 400 Angstroms — 40 nanoMeters ) that it can’t be seen with the light microscope ( the smallest wavelength of visible light 3,900 Angstroms — 390 nanoMeters).  This led to a bruising battle between Cajal and Golgi a just over a century ago over whether the brain was actually made of cells.  Even though Golgi’s work led to the delineation of single neurons he thought the brain was a continuous network.  They both won the Nobel in 1906.

Semifast forward to the mid 60s when I was in medical school.  We finally had the electron microscope, so we could see synapses. They showed up as a small CLEAR spaces (e.g. electrons passed through it easily leaving it white) between neurons.  Neurotransmitters were being discovered at the same time and the synapse was to be the analogy to vacuum tubes, which could pass electricity in just one direction (yes, the transistor although invented hadn’t been used to make anything resembling a computer — the Intel 4004 wasn’t until the 70s).  Of course now we know that information flows back and forth across the synapse, with endocannabinoids (e. g. natural marihuana) being the major retrograde neurotransmitter.

Since there didn’t seem to be anything in the synaptic cleft, neurotransmitters were thought to freely diffuse across it to being to receptors on the other (postsynaptic) side e.g. a free fly zone.

Fast forward to the present to a marvelous (and grueling to read because of the complexity of the subject not the way it’s written) review of just what is in the synaptic cleft [ Cell vol. 171 pp. 745 – 769 ’17 ] http://www.cell.com/cell/fulltext/S0092-8674(17)31246-1 (It is likely behind a paywall).  There are over 120 references, and rather than being just a catalogue, the single author Thomas Sudhof extensively discusseswhich experimental work is to be believed (not that Sudhof  is saying the work is fraudulent, but that it can’t be used to extrapolate to the living human brain).  The review is a staggering piece of work for one individual.

The stuff in the synaptic cleft is so diverse, and so intimately involved with itself and the membranes on either side what what is needed for comprehension is not a chemist but a sociologist.  Probably most of the molecules to be discussed are present in such small numbers that the law of mass action doesn’t apply, nor do binding constants which rely on large numbers of ligands and receptors. Not only that, but the binding constants haven’t been been determined for many of the players.

Now for some anatomic detail and numbers.  It is remarkably hard to find just how far laterally the synaptic cleft extends.  Molecular Biology of the Cell ed. 5 p. 1149 has a fairly typical picture with a size marker and it looks to be about 2 microns (20,000 Angstroms, 2,000 nanoMeters) — that’s 314,159,265 square Angstroms (3.14 square microns).  So let’s assume each protein takes up a square 50 Angstroms on a side (2,500 square Angstroms).  That’s room for 125,600 proteins on each side assuming extremely dense packing.  However the density of acetyl choline receptors at the neuromuscular junction is 8,700/square micron, a packing also thought to be extremely dense which would give only 26,100 such proteins in a similarly distributed CNS synapse. So the numbers are at least in the right ball park (meaning they’re within an order of magnitude e.g. within a power of 10) of being correct.

What’s the point?

When you see how many different proteins and different varieties of the same protein reside in the cleft, the numbers for  each individual element is likely to be small, meaning that you can’t use statistical mechanics but must use sociology instead.

The review focuses on the neurExins (I capitalize the E  to help me remember that they are prEsynaptic).  Why?  Because they are the best studied of all the players.  What a piece of work they are.  Humans have 3 genes for them. One of the 3 contains 1,477 amino acids, spread over 1,112,187 basepairs (1.1 megaBases) along with 74 exons.  This means that just over 1/10 of a percent of the gene is actually coding for for the amino acids making it up.  I think it takes energy for RNA polymerase II to stitch the ribonucleotides into the 1.1 megabase pre-mRNA, but I couldn’t (quickly) find out how much per ribonucleotide.  It seems quite wasteful of energy, unless there is some other function to the process which we haven’t figured out yet.

Most of the molecule resides in the synaptic cleft.  There are 6 LNS domains with 3 interspersed EGFlike repeats, a cysteine loop domain, a transmembrane region and a cytoplasmic sequence of 55 amino acids. There are 6 sites for alternative splicing, and because there are two promoters for each of the 3 genes, there is a shorter form (beta neurexin) with less extracellular stuff than the long form (alpha-neurexin).  When all is said and done there are over 1,000 possible variants of the 3 genes.

Unlike olfactory neurons which only express one or two of the nearly 1,000 olfactory receptors, neurons express mutiple isoforms of each, increasing the complexity.

The LNS regions of the neurexins are like immunoglobulins and fill at 60 x 60 x 60 Angstrom box.  Since the synaptic cleft is at most 400 Angstroms long, the alpha -neurexins (if extended) reach all the way across.

Here the neurexins bind to the neuroligins which are always postsynaptic — sorry no mnemonic.  They are simpler in structure, but they are the product of 4 genes, and only about 40 isoforms (due to alternative splicing) are possible. Neuroligns 1, 3 and 4 are found at excitatory synapses, neuroligin 2 is found at inhibitory synapses.  The intracleft part of the neuroligins resembles an important enzyme (acetylcholinesterase) but which is catalytically inactive.  This is where the neurexins.

This is complex enough, but Sudhof notes that the neurexins are hubs interacting with multiple classes of post-synaptic molecules, in addition to the neuroligins — dystroglycan, GABA[A] receptors, calsystenins, latrophilins (of which there are 4).   There are at least 50 post-synaptic cell adhesion molecules — “Few are well understood, although many are described.”

The neurexins have 3 major sites where other things bind, and all sites may be occupied at once.  Just to give you a taste of he complexity involved (before I go on to  larger issues).

The second LNS domain (LNS2)is found only in the alpha-neurexins, and binds to neuroexophilin (of which there are 4) and dystroglycan .

The 6th LNS domain (LNS6) binds to neuroligins, LRRTMs, GABA[A] receptors, cerebellins and latrophilins (of which there are 4)_

The juxtamembrane sequence of the neurexins binds to CA10, CA11 and C1ql.

The cerebellins (of which there are 4) bind to all the neurexins (of a particular splice variety) and interestingly to some postsynaptic glutamic acid receptors.  So there is a direct chain across the synapse from neurexin to cerebellin to ion channel (GLuD1, GLuD2).

There is far more to the review. But here is something I didn’t see there.  People have talked about proton wires — sites on proteins that allow protons to jump from one site to another, and move much faster than they would if they had to bump into everything in solution.  Remember that molecules are moving quite rapidly — water is moving at 590 meters a second at room temperature. Since the synaptic cleft is 40 nanoMeters (40 x 10^-9 meters, it should take only 40 * 10^-9 meters/ 590 meters/second   60 trillionths of a second (60 picoSeconds) to cross, assuming the synapse is a free fly zone — but it isn’t as the review exhaustively shows.

It it possible that the various neurotransmitters at the synapse (glutamic acid, gamma amino butyric acid, etc) bind to the various proteins crossing the cleft to get their target in the postsynaptic membrane (e.g. neurotransmitter wires).  I didn’t see any mention of neurotransmitter binding to  the various proteins in the review.  This may actually be an original idea.

I’d like to put more numbers on many of these things, but they are devilishly hard to find.  Both the neuroligins and neurexins are said to have stalks pushing them out from the membrane, but I can’t find how many amino acids they contain.  It can’t find how much energy it takes to copy the 1.1 megabase neurexin gene in to mRNA (or even how much energy it takes to add one ribonucleotide to an existing mRNA chain).

Another point– proteins have a finite lifetime.  How are they replenished?  We know that there is some synaptic protein synthesis — does the cell body send packages of mRNAs to the synapse to be translated there.  There are at least 50 different proteins mentioned in the review, and don’t forget the thousands of possible isoforms, each of which requires a separate mRNA.

Old Chinese saying — the mountains are high and the emperor is far away. Protein synthesis at the synaptic cleft is probably local.  How what gets made and when is an entirely different problem.

A large part of the review concerns mutations in all these proteins associated with neurologic disease (particularly autism).  This whole area has a long and checkered history.  A high degree of cynicism is needed before believing that any of these mutations are causative.  As a neurologist dealing with epilepsy I saw the whole idea of ion channel mutations causing epilepsy crash and burn — here’s a link — https://luysii.wordpress.com/2011/07/17/we’ve-found-the-mutation-causing-your-disease-not-so-fast-says-this-paper/

Once again, hats off to Dr. Sudhof for what must have been a tremendous amount of work

The bouillabaisse of the synaptic cleft

The synaptic cleft is so small ( under 400 Angstroms — 40 nanoMeters ) that it can’t be seen with the light microscope ( the smallest wavelength of visible light 3,900 Angstroms — 390 nanoMeters).  This led to a bruising battle between Cajal and Golgi a just over a century ago over whether the brain was actually made of cells.  Even though Golgi’s work led to the delineation of single neurons he thought the brain was a continuous network.  They both won the Nobel in 1906.

Semifast forward to the mid 60s when I was in medical school.  We finally had the electron microscope, so we could see synapses. They showed up as a small CLEAR spaces (e.g. electrons passed through it easily leaving it white) between neurons.  Neurotransmitters were being discovered at the same time and the synapse was to be the analogy to vacuum tubes, which could pass electricity in just one direction (yes, the transistor although invented hadn’t been used to make anything resembling a computer — the Intel 4004 wasn’t until the 70s).  Of course now we know that information flows back and forth across the synapse, with endocannabinoids (e. g. natural marihuana) being the major retrograde neurotransmitter.

Since there didn’t seem to be anything in the synaptic cleft, neurotransmitters were thought to freely diffuse across it to being to receptors on the other (postsynaptic) side e.g. a free fly zone.

Fast forward to the present to a marvelous (and grueling to read because of the complexity of the subject not the way it’s written) review of just what is in the synaptic cleft [ Cell vol. 171 pp. 745 – 769 ’17 ] http://www.cell.com/cell/fulltext/S0092-8674(17)31246-1 (It is likely behind a paywall).  There are over 120 references, and rather than being just a catalogue, the single author Thomas Sudhof extensively discusseswhich experimental work is to be believed (not that Sudhof  is saying the work is fraudulent, but that it can’t be used to extrapolate to the living human brain).  The review is a staggering piece of work for one individual.

The stuff in the synaptic cleft is so diverse, and so intimately involved with itself and the membranes on either side what what is needed for comprehension is not a chemist but a sociologist.  Probably most of the molecules to be discussed are present in such small numbers that the law of mass action doesn’t apply, nor do binding constants which rely on large numbers of ligands and receptors. Not only that, but the binding constants haven’t been been determined for many of the players.

Now for some anatomic detail and numbers.  It is remarkably hard to find just how far laterally the synaptic cleft extends.  Molecular Biology of the Cell ed. 5 p. 1149 has a fairly typical picture with a size marker and it looks to be about 2 microns (20,000 Angstroms, 2,000 nanoMeters) — that’s 314,159,265 square Angstroms (3.14 square microns).  So let’s assume each protein takes up a square 50 Angstroms on a side (2,500 square Angstroms).  That’s room for 125,600 proteins on each side assuming extremely dense packing.  However the density of acetyl choline receptors at the neuromuscular junction is 8,700/square micron, a packing also thought to be extremely dense which would give only 26,100 such proteins in a similarly distributed CNS synapse. So the numbers are at least in the right ball park (meaning they’re within an order of magnitude e.g. within a power of 10) of being correct.

What’s the point?

When you see how many different proteins and different varieties of the same protein reside in the cleft, the numbers for  each individual element is likely to be small, meaning that you can’t use statistical mechanics but must use sociology instead.

The review focuses on the neurExins (I capitalize the E  to help me remember that they are prEsynaptic).  Why?  Because they are the best studied of all the players.  What a piece of work they are.  Humans have 3 genes for them. One of the 3 contains 1,477 amino acids, spread over 1,112,187 basepairs (1.1 megaBases) along with 74 exons.  This means that just over 1/10 of a percent of the gene is actually coding for for the amino acids making it up.  I think it takes energy for RNA polymerase II to stitch the ribonucleotides into the 1.1 megabase pre-mRNA, but I couldn’t (quickly) find out how much per ribonucleotide.  It seems quite wasteful of energy, unless there is some other function to the process which we haven’t figured out yet.

Most of the molecule resides in the synaptic cleft.  There are 6 LNS domains with 3 interspersed EGFlike repeats, a cysteine loop domain, a transmembrane region and a cytoplasmic sequence of 55 amino acids. There are 6 sites for alternative splicing, and because there are two promoters for each of the 3 genes, there is a shorter form (beta neurexin) with less extracellular stuff than the long form (alpha-neurexin).  When all is said and done there are over 1,000 possible variants of the 3 genes.

Unlike olfactory neurons which only express one or two of the nearly 1,000 olfactory receptors, neurons express mutiple isoforms of each, increasing the complexity.

The LNS regions of the neurexins are like immunoglobulins and fill at 60 x 60 x 60 Angstrom box.  Since the synaptic cleft is at most 400 Angstroms long, the alpha -neurexins (if extended) reach all the way across.

Here the neurexins bind to the neuroligins which are always postsynaptic — sorry no mnemonic.  They are simpler in structure, but they are the product of 4 genes, and only about 40 isoforms (due to alternative splicing) are possible. Neuroligns 1, 3 and 4 are found at excitatory synapses, neuroligin 2 is found at inhibitory synapses.  The intracleft part of the neuroligins resembles an important enzyme (acetylcholinesterase) but which is catalytically inactive.  This is where the neurexins.

This is complex enough, but Sudhof notes that the neurexins are hubs interacting with multiple classes of post-synaptic molecules, in addition to the neuroligins — dystroglycan, GABA[A] receptors, calsystenins, latrophilins (of which there are 4).   There are at least 50 post-synaptic cell adhesion molecules — “Few are well understood, although many are described.”

The neurexins have 3 major sites where other things bind, and all sites may be occupied at once.  Just to give you a taste of he complexity involved (before I go on to  larger issues).

The second LNS domain (LNS2)is found only in the alpha-neurexins, and binds to neuroexophilin (of which there are 4) and dystroglycan .

The 6th LNS domain (LNS6) binds to neuroligins, LRRTMs, GABA[A] receptors, cerebellins and latrophilins (of which there are 4)_

The juxtamembrane sequence of the neurexins binds to CA10, CA11 and C1ql.

The cerebellins (of which there are 4) bind to all the neurexins (of a particular splice variety) and interestingly to some postsynaptic glutamic acid receptors.  So there is a direct chain across the synapse from neurexin to cerebellin to ion channel (GLuD1, GLuD2).

There is far more to the review. But here is something I didn’t see there.  People have talked about proton wires — sites on proteins that allow protons to jump from one site to another, and move much faster than they would if they had to bump into everything in solution.  Remember that molecules are moving quite rapidly — water is moving at 590 meters a second at room temperature. Since the synaptic cleft is 40 nanoMeters (40 x 10^-9 meters, it should take only 40 * 10^-9 meters/ 590 meters/second   60 trillionths of a second (60 picoSeconds) to cross, assuming the synapse is a free fly zone — but it isn’t as the review exhaustively shows.

It it possible that the various neurotransmitters at the synapse (glutamic acid, gamma amino butyric acid, etc) bind to the various proteins crossing the cleft to get their target in the postsynaptic membrane (e.g. neurotransmitter wires).  I didn’t see any mention of neurotransmitter binding to  the various proteins in the review.  This may actually be an original idea.

I’d like to put more numbers on many of these things, but they are devilishly hard to find.  Both the neuroligins and neurexins are said to have stalks pushing them out from the membrane, but I can’t find how many amino acids they contain.  It can’t find how much energy it takes to copy the 1.1 megabase neurexin gene in to mRNA (or even how much energy it takes to add one ribonucleotide to an existing mRNA chain).

Another point– proteins have a finite lifetime.  How are they replenished?  We know that there is some synaptic protein synthesis — does the cell body send packages of mRNAs to the synapse to be translated there.  There are at least 50 different proteins mentioned in the review, and don’t forget the thousands of possible isoforms, each of which requires a separate mRNA.

Old Chinese saying — the mountains are high and the emperor is far away. Protein synthesis at the synaptic cleft is probably local.  How what gets made and when is an entirely different problem.

A large part of the review concerns mutations in all these proteins associated with neurologic disease (particularly autism).  This whole area has a long and checkered history.  A high degree of cynicism is needed before believing that any of these mutations are causative.  As a neurologist dealing with epilepsy I saw the whole idea of ion channel mutations causing epilepsy crash and burn — here’s a link — https://luysii.wordpress.com/2011/07/17/we’ve-found-the-mutation-causing-your-disease-not-so-fast-says-this-paper/

Once again, hats off to Dr. Sudhof for what must have been a tremendous amount of work

We don’t understand amyloid very well

I must admit I was feeling pretty snarky about our understanding of amyloid and Alzheimer’s after the structure of Abeta42 was published.  In particular the structure explained why the alanine 42–> threonine 42 mutation was protective against Alzheimer’s disease while the alanine 42 –> valine 42 mutation increases the risk.  That’s all explained in the last post — https://luysii.wordpress.com/2017/10/12/abeta42-at-last/ — but a copy will appear at the end.

In that post I breathlessly hoped for the structure of aBeta40 which is known to be less toxic to neurons.  Well it’s here and it shows how little we understand about what does and what doesn’t form amyloid.  The structure appears in a paper about the amyloid formed by another protein (FUS) to be described later — Cell 171, 615–627, October 19, 2017 — figure 7 p. 624.

Now all Abeta40 lacks are the last 2 amino acids of Abeta42 — isoleucine at 41 and alanine at 42.  So solve the Schrodinger equation for it, and stack it up so it forms amyloid, or use your favorite molecular dynamics or other modeling tool.  Take a guess what it looks like.

Abeta42 is a dimer, a beta40 is a trimer, even though the first 40 amino acids of both are identical.

It gets worse. FUS (FUsed in Sarcoma) is a 526 amino acid protein which binds to RNA and is mostly found in the nucleus.  Neurologists are interested in it because over 50 mutations in have been found in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).   FUS contains a low complexity domain (LCD) of 214 amino acids, 80% of which are one of 4 amino acids (glycine, serine, glutamine and tyrosine).  At high protein concentrations this domain of FUS forms long unbundled fibrils with the characteristic crossBeta structure of amyloid.  Only 57/214 of the LCD amino acids are part of the structured core of the amyloid — the rest are disordered.

Even worse the amino acids forming the amyloid core (#39 -#95) are NOT predicted by a variety of computational methods predicting amyloid formation (Agrescan, FISH, FOLDamyloid, Metamyl, PASTA 2.0).  The percentages of gly, ser, gln and tyr in the core forming region are pretty much the same as in the whole protein.  The core forming region has no repeats longer than 4 amino acids.

The same figure 7 has the structure of the amyloid formed by alpha-synuclein, which accumulates in the Lewy bodies of Parkinson’s disease.  It just has one peptide per layer of amyloid.

When you really understand something you can predict things, not just describe them as they are revealed.

 

Abeta42 at last

It’s easy to see why cryoEM got the latest chemistry Nobel.  It is telling us so much.  Particularly fascinating to me as a retired neurologist is the structure of the Abeta42 fibril reported in last Friday’s Science (vol. 358 pp. 116 – 119 ’17).

Caveats first.  The materials were prepared using an aqueous solution at low pH containing an organic cosolvent — so how physiologic could the structure actually be?  It probably is physiologic as the neurotoxicity of the fibrils to neurons in culture was the same as fibrils grown at neutral pH.  This still isn’t the same as fibrils grown in the messy concentrated chemical soup known as the cytoplasm.  Tending to confirm their findings is the fact that NMR and Xray diffraction on the crystals produced the same result.

The fibrils were unbranched and microns long (implying at least 2,000 layers of the beta sheets to be described).  The beta sheets stack in parallel and in register giving the classic crossBeta sheet structure.  They were made of two protofilaments winding around each other.  Each protofilament contains all 42 amino acids of Abeta42 and all of them form a completely flat beta sheet structure.

Feast your eyes on figure 2 p. 117.  In addition to showing the two beta sheets of the two protofilaments, it shows how they bind to each other.  Aspartic acid #1 of one sheet binds to lysine #28 of the other.  Otherwise the interface is quite hydrophobic.  Alanine2 of one sheet binds to alanine42 of the other, valine39 of one sheet binds to valine 39 of the other.  Most importantly isoLeucine 41 of one sheet binds to glycine38 of the other.

This is important since the difference between the less toxic Abeta40 and the toxic Abeta 42 are two hydrophobic amino acids Isoleucine 41 and Alanine 42.  This makes for a tighter, longer, more hydrophobic interface between the protofilaments stabilizing them.

That’s just a guess.  I can’t wait for work on Abeta40 to be reported at this resolution.

A few other points.  The beta sheet of each protomer is quite planar, but the planes of the two protomers are tilted by 10 degrees accounting for the helicity of the fibril. The fibril is a rhombus whose longest edge is about 70 Angstroms.

Even better the structure explains a mutation which is protective against Alzheimer’s.  This remains the strongest evidence (to me at least) that Abeta peptides are significantly involved in Alzheimer’s disease, therapeutic failures based on this idea notwithstanding.  The mutation is a change of alanine2 to threonine which can’t possibly snuggle up hydrophobically to isoleucine nearly as well as alanine did. This should significantly weaken the link between the two protofilaments and make fibril formation more difficult.

The Abeta structure of the paper also explains another mutation. This one increases the risk of Alzheimer’s disease (like many others which have been discovered).  It involves the same amino acid (alanine2) but this time it is changed to the morehydrophobic valine, probably resulting in a stronger hydrophobic interaction with isoLeucine41 (assuming that valine’s greater bulk doesn’t get in the way sterically).

Wonderful stuff to think and speculate about, now that we actually have some solid data to chew on.

Abeta raises its head again

Billions have been spent (and lost) by big Pharma on attempts to decrease Abeta peptide in the brain as a therapy for Alzheimer’s. Yet the theory that Abeta has something to do with Alzheimer’s won’t die because it is so compelling.

Here’s another example [Neuron vol. 96 pp. 355 – 372 ’17 ] Neurons in hippocampal slices stop forming new synapses when exposed to Abeta.  We think that synapse formation and elimination is going on all the time in our brains — it certainly is in mice.  For details see an excellent review [ Neuron vol. 96 pp. 43 – 55 ’17 ].  This is thought to be important in learning, something lost in Alzheimer’s as well as old memories. Two Alzheimer mouse models have shown defects in new synaptic spine formation.

Even better the authors found what Abeta is binding to — a well known brain protein — Nogo receptor 1 (Ngr1).  When it was knocked down in the slice (by bolistic short hairpin RNA infererence — shRNAi), spines started reforming.

So the work may explain some of the problems in Alzheimer’s disease but it says nothing about the neuronal loss which is also found.

Also, there is something fishy about the results.  The Abeta preparation used in the experiment was mostly oligomers of about 100 monomers (with a molecular mass of 500 kiloDaltons).  Monomers had no effect.  It is much easier to conceptualize a monomer binding to a receptor than an oligomer.  However, oligomer binding would tend to cluster receptors, something important in immune responses.

The strongest evidence for Abeta in my opinion is the fact that certain mutations PROTECT against Alzheimer’s — and given the structure just worked out we have a plausible explanation of just how this works — for details see — https://luysii.wordpress.com/2017/10/12/abeta42-at-last/

 

Abeta42 at last

It’s easy to see why cryoEM got the latest chemistry Nobel.  It is telling us so much.  Particularly fascinating to me as a retired neurologist is the structure of the Abeta42 fibril reported in last Friday’s Science (vol. 358 pp. 116 – 119 ’17).  

Caveats first.  The materials were prepared using an aqueous solution at low pH containing an organic cosolvent — so how physiologic could the structure actually be?  It probably is physiologic as the neurotoxicity of the fibrils to neurons in culture was the same as fibrils grown at neutral pH.  This still isn’t the same as fibrils grown in the messy concentrated chemical soup known as the cytoplasm.  Tending to confirm their findings is the fact that NMR and Xray diffraction on the crystals produced the same result.

The fibrils were unbranched and microns long (implying at least 2,000 layers of the beta sheets to be described).  The beta sheets stack in parallel and in register giving the classic crossBeta sheet structure.  They were made of two protofilaments winding around each other.  Each protofilament contains all 42 amino acids of Abeta42 and all of them form a completely flat beta sheet structure.

Feast your eyes on figure 2 p. 117.  In addition to showing the two beta sheets of the two protofilaments, it shows how they bind to each other.  Aspartic acid #1 of one sheet binds to lysine #28 of the other.  Otherwise the interface is quite hydrophobic.  Alanine2 of one sheet binds to alanine42 of the other, valine39 of one sheet binds to valine 39 of the other.  Most importantly isoLeucine 41 of one sheet binds to glycine38 of the other.

This is important since the difference between the less toxic Abeta40 and the toxic Abeta 42 are two hydrophobic amino acids Isoleucine 41 and Alanine 42.  This makes for a tighter, longer, more hydrophobic interface between the protofilaments stabilizing them.

That’s just a guess.  I can’t wait for work on Abeta40 to be reported at this resolution.

A few other points.  The beta sheet of each protomer is quite planar, but the planes of the two protomers are tilted by 10 degrees accounting for the helicity of the fibril. The fibril is a rhombus whose longest edge is about 70 Angstroms.

Even better the structure explains a mutation which is protective against Alzheimer’s.  This remains the strongest evidence (to me at least) that Abeta peptides are significantly involved in Alzheimer’s disease, therapeutic failures based on this idea notwithstanding.  The mutation is a change of alanine2 to threonine which can’t possibly snuggle up hydrophobically to isoleucine nearly as well as alanine did. This should significantly weaken the link between the two protofilaments and make fibril formation more difficult.

The Abeta structure of the paper also explains another mutation. This one increases the risk of Alzheimer’s disease (like many others which have been discovered).  It involves the same amino acid (alanine2) but this time it is changed to the more hydrophobic valine, probably resulting in a stronger hydrophobic interaction with isoLeucine41 (assuming that valine’s greater bulk doesn’t get in the way sterically).

Wonderful stuff to think and speculate about, now that we actually have some solid data to chew on.

The worst name for a drug I’ve ever heard of

It is simply impossible for me to think of a worse name for a drug which might help people with Down syndrome than ALGERNON.   The authors can be excused as they’re all from Japan, but the editor of the paper Fred Gage should have known about ‘Flowers for Algernon’– https://en.wikipedia.org/wiki/Flowers_for_Algernon.  Briefly, it’s a story about a drug which tripled the intelligence of Algernon a laboratory mouse which was then given to a retarded individual (Charlie Gordon) whose intelligence similarly tripled, only to decline like Algernon’s.  It was originally a short story, then a book, then a play etc. etc.

The drug is potentially quite exciting — ALGERNON is an acronym forALtered GenERatioN Of Neurons).  It increases the number of neurons form by mice with a model of trisomy 21.  The brain is bigger, and the animals do better on tests.  It is thought to work by inhibiting an enzyme (DYRK1A) which adds phosphate to serine, threonine and tyrosine, making it a dual specificity kinase.  It phosphorylates a variety of proteins known to have significant effects on brain development (tau, cyclin D1, caspase9, Notch, gli1, etc). The net effect of DYRK1A inhibition is to increase neural stem cell proliferation during fetal life.

Chemists will be interested in just how simple the structure of ALGERNON is — it’s an all aromatic compound made of a pyridine linked to a fused 6:5 ring system in which the 5 membered ring contains 2 nitrogens.  That’s it.  No alcohols, methyls, ethyls, ..  amines, amides, ethers etc., etc.

The authors blue-sky a bit at the end.  They note that mice show neural proliferation during adult life (we do as well, but to a much lesser extent).  It might be useful to improve function in living Down syndrome individuals, and just about any other neurological problem in which neural proliferation would be beneficial.  It might also be offered to women carrying a Down fetus who object to abortion on moral grounds.  Exciting stuff, but for god’s sake change the name.