Category Archives: Chemistry (relatively pure)

Lactose intolerance and the proteins of the synaptic cleft

What does lactose intolerance have to do with the zillions of proteins happily infesting the synaptic cleft?  Only someone whose mind was warped into very abstract thinking by rooming with philosophy majors in college would see a connection.

The synaptic cleft is of immense theoretical interest to neuroscientists, drug chemists and pharmacologists, and of great practical interest to people affected by neurologic and psychiatric disease either in themselves or someone they know (e.g. just about everyone).

Almost exactly a year ago I wrote a post about a great paper on the proteins of the synaptic cleft by Thomas Sudhof.  You may read the post after the *****

Well Dr. Sudhof is back with another huge review of just how synapses are formed [ Neuron vol. 100 pp. 276 – 293 ’18 ], which covers very similar ground.

It is clear that he’s depressed by the state of the field.  Here are a few quotes

“I believe that we may need to pay more attention to technical details than customary because the pressures on investigators have increased the tendency to publish preliminary results, especially results obtained with new methods whose limitations are not yet clear.”

Translation: a lot of the stuff coming out is junk.

“Given the abundance of papers reporting non-validated protein interactions that cannot possibly be all correct, it seems that confidence in a possible protein-protein interaction requires either isolation of a stable complex or biophysical measurements of interactions using recombinant purified proteins.”

Translation:  Oy vey !

“Pre- or postsynaptic specializations are surprisingly easy to induce by diverse signals. This was first shown in pioneering studies demonstrating that polylysine beads induce formation of presynaptic nerve terminals in cultured neurons and in brain in vivo.” Obviously this means that you have to be very careful when you claim that a given protein or two causes a synapse to form, which researchers have not been.”

Translation not needed.

Then on to the meat of the review.  “An impressive number of candidate synaptic Cell Adhesion Molecules (CAMs) has been described (9 classes are given each with multiple members). For some of these CAMs, compelling data demonstrate their presence in synapses and suggest a functional role in synapses. Others, however, are less well documented. If one looks at the results in total, the overall impression is puzzlement: how do so many CAMs contribute to shaping a synapse?”

Then from 281 – 286 he goes into the various CAMs, showing the extent and variety of proteins found in the synaptic cleft.  Which ones are necessary and what are they doing?  Can they all be important.  There must be some redundancy as knockout of some doesn’t do much.

Here is where lactose tolerance/intolerance comes in to offer succor to the harried investigator.

Bluntly, they must be doing something, and something important,  or they wouldn’t be there.

People with lactose intolerance have nothing wrong with the gene which breaks down lactose.  Babies have no problem with breast milk.  The enzyme (lactase)  produced from the gene is quite normal in all of us.  However 10,000 years ago and earlier, cattle were not domesticated, so there was no dietary reason for a human weaned from the breast to make the enzyme.  Something turned off lactase production — from my reading, it’s not clear what.   The control region (lactase enhancer) for the lactase gene is 14,000 nucleotides upstream from the gene itself.  After domestication of cattle, so that people could digest milk their entire lives a mutation arose changing cytosine to thymine in the enhancer.  The farthest back the mutation has been found is 6.500 years. 3 other mutations are known, which keep the lactase gene expressed past weaning.  They arose independently.  All 4 spread in the population, because back then our ancestors were in a semi-starved state most of the time, and carriers had better nutrition.

How does this offer succor to Dr. Sudhof?  Simply this, here is a mechanism to turn off production of an enzyme our ancestors didn’t need past weaning.  Don’t you think this would be the case for all the proteins found in and around the synapse.  They must be doing something or they wouldn’t be there.  I realize that this is teleology writ large, but evolutionary adaptations make you think this way.

*****

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

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Thomas Gold lives !

Thomas Gold was a scientific jack of all trades being involved in physics, cosmology and geochemistry, the latter of interest to us here.  He thought petroleum and other hydrocarbons were actually produced by micro-organisms below the surface of the earth, providing us with a replenishable supply (how ecological !)  Here’s part of a Wiki article about him —  Hydrocarbons are not biology reworked by geology (as the traditional view would hold), but rather geology reworked by biology.– https://en.wikipedia.org/wiki/Thomas_Gold.

Why bring him up now?  Because [ Proc. Natl. Acad. Sci. vol. 115 pp. 10702 – 10707 ’18 ] (http://www.pnas.org/content/115/42/10702) showed that 600 meters below the surface (where light and molecular oxygen never go) Cyanobacteria were found.  They use the electrons stripped from hydrogen (which is said to be produced in the subsurface by several (unspecified) abiotic mechanisms) as an energy source.  The electrons have to go somewhere, and they postulate that the electron acceptors are iron or manganese oxides. Wherever microorganisms have been found in deep continental settings hydrogen concentration decreases.

Basically life acts as the middleman, taking an energy cut from the flow of electrons from reductant to oxidant.

Seriously, life may have actually arisen in such situations.

Triplets and TADs

Neurologists have long been interested in triplet diseases — https://en.wikipedia.org/wiki/Trinucleotide_repeat_disorder.  The triplet is made of a string of 3 nucleotides.  Example —  cytosine adenosine guanosine or CAG — which accounts for a lot of them.  We have lots of places in our genome where such repeats normally occur, with the triplets repeated up to 42 times.  However in diseases like Huntington’s chorea the repeats get to be as many as 250 CAGs in a row.  You normally are quite fine as long as you have under 36 of them, and no one has fewer than 6 at this particular location.

Subsequently, expansions of 4, 5, and 6 nucleotide repeats have also been shown to cause disease, bring the total of repeat expansion diseases to over 40.  Why more than half of them should affect the nervous system entirely or for the most part is a mystery.  Needless to say there are plenty of theories.

This leads to three questions (1) there are repeats all over the genome, why do only 40 or so of them expand (2) since we all have repeats in front of the genes where they cause disease why don’t we all have the diseases (3) why do the number of repeats expand with each succeeding generation — the phenomenon is called anticipation.  I saw one such example where a father brought his son to my muscular dystrophy clinic.  The boy had moderately severe myotonic dystrophy.  When I shook the father’s hand, it was clear that he had mild myotonia, which had in no way impaired his life (he was a successful banker).

A recent paper in Cell may help answer the first question and has a hint about the second [ Cell vol. 175 pp. 38 – 40, 224 – 238 ’18 ].  21 of 27 disease associated short tandem repeats (daSTRs) localize to something called a topologically associated domain (TAD) or subdomain (subTAD) boundary. These are defined as contiguous intervals in the genome in which every pair has an elevated interaction frequency compared to loci out side the domain.  TADs and subTADs are measured using chromosome conformation capture assays (acronyms for them include 3C, CCC, 4C, 5C, Hi-C).

Briefly they are performed as follows.  Intact nuclei are isolate from live cel cultures.  These are subjected to paraformaldehye crosslinking to fix segment of genome in close physical proximity. The crosslinked genomic DNA is digested with a restriction endonuclease, and the products expanded by PCR using primers in all possible combinations.  Then having a complete genome sequence in hand, you see what regions of the genome got close enough together to show up in the assay.

This may help explain question one, and the paper gives some speculation about question two — we don’t all have these diseases, because unlike the unfortunates with them, we don’t have problems in our genes for DNA replication, repair and recombination.  There is some evidence for this;  studies in model organisms with these mutations do have short tandem repeat instability.

Unfortunately the paper doesn’t discuss anticipation, because no clinicians appear to be among the authors, even though they’re from Penn which 50+ years ago was very strong in clinical neurology.

None of this work discusses the fascinating questions of how the expanded repeats cause disease, or why so many of them affect the nervous system.

The Kavanaugh Ford confrontation will be to this decade what the Patty Hearst kidnapping was to a previous one  — https://en.wikipedia.org/wiki/Patty_Hearst.  Since I suffered 4 episodes of physical (not sexual) abuse as a kid, and dealt with this extensively as a neurologist, I’m trying to decide whether to write about it.  Emotions are high and there are a lot of nuts out there on the net. There is even a reasonable possibility that both Ford and Kavanaugh are right and not lying.

The chemical ingenuity of the AIDs virus

Pop quiz:  You are a virus with under 10,000 nucleotides in your genome.  To make the capsid enclosing your genome, you need to make 250 hexamers of a particular protein.  How do you do it?

 

Give up?

 

You grab a cellular metabolite with a mass under 1,000 Daltons to bind the 6 monomers together.  The metabolite occurs at fairly substantial concentrations (for a metabolite) of 10 – 40 microMolar.

What is the metabolite?

Give up?

 

It has nearly perfect 6 fold symmetry.

 

Still give up?

[ Nature vol. 560 pp. 509 – 512 ’18 ]  https://www.nature.com/articles/s41586-018-0396-4 says that it’s inositol hexakisphosphate (IP6)  — nomenclature explained at the end. http://www.refinebiochem.com/pages/InositolHexaphosphate.html

Although IP6 looks like a sugar (with 6 CHOH groups forming a 6 membered ring), it is not a typical one because it is not an acetal (no oxygen in the ring).  All 6 hydroxyls of IP6 are phosphorylated.  They bind to two lysines on a short (21 amino acids) alpha helix found in the protein (Gag which has 500 amino acids).  That’s how IP6 binds the 6 Gag proteins together. The paper has great pictures.

It is likely that IP6 is use by other cellular proteins to form hexamers (but the paper doesn’t discuss this).

IP6 is quite symmetric, and 5 of the 6 phosphorylated hydroxyls can be equatorial, so this is likely the energetically favored conformation, given the bulk (and mass) of the phosphate group.

I think that the AIDS virus definitely has more chemical smarts than we do.  Humility is definitely in order.

Nomenclature note:  We’re all used to ATP (Adenosine TriPhosphate) and ADP (Adenosine DiPhosphate) — here all 3 or 2 phosphates form a chain.  Each of the 6 hydroxyls of inositol can be singly phosphorylated, leading to inositol bis, tris, tetrakis, pentakis, hexakis phosphates.  Phosphate chains can form on them as well, so IP7 and IP8 are known (heptakis?, Octakis??)

When the dissociation constant doesn’t tell you what you want to know

Drug chemists spend a lot of time getting their drugs to bind tightly to their chosen target.  Kd’s (dissociation constants) are measured with care –https://en.wikipedia.org/wiki/Dissociation_constant.  But Kd’s are only  a marker for the biologic effects that are the real reason for the drug.  That’s why it was shocking to find that Kd’s don’t seem to matter in a very important and very well studied system.

It’s not the small molecule ligand protein receptor most drug chemists deal with, it’s the goings on at the immunologic synapse between antigen presenting cell and T lymphocyte (a much larger ligand target interface — 1,000 – 2,000 Angstroms^2 — than the usual site of drug/protein binding).   A peptide fragment lies down in a groove on the Major Histocompatibility Complex (pMHC) where it is presented to the T lymphoCyte Receptor (TCR) — another protein complex.  The hope is that an immune response to the parent protein of the peptide fragment will occur.

 

However, the Kd’s (affinities)of strong (e.g. producing an immune response) peptide agonist ligands and those producing not much (e.g. weak) are similar and at times overlapping.  High affinity yet nonStimulatory interactions occur with high frequency in the human T cell repertoire [ Cell vol. 174 pp. 672 – 687 ’18 ].  The authors  determined the structure of both weak and strong ligands bound to the TCR.  One particular TCR had virtually the same structure when bound to strong and weak agonist ligands. When studied in two dimensional membranes, the dwell time of ligand with receptor didn’t distinguish strong from weak antigens (surprising).

In general the Kds  pMHC/TCR  are quite low — not in the nanoMolar range beloved by drug chemists (and found in antigen/antibody binding), but 1000 times weaker in the micromolar range.  So [ Proc. Natl. Acad. Sci. vol. 115 pp. E7369 – E7378 ’18 ] cleverly added an extra few amino acids which they call molecular velcro, to boost the affinity x 10 (actually this decreases Kd tenfold).

One rationale for the weak binding is that it facilitates scanning by the TCR of  the pMHC  repertoire allowing the TCR to choose the best.  So they added the velcro, expecting the repertoire to be less diverse (since the binding was tighter).  It was just the same. Again the Kd didn’t seem to matter.

https://en.wikipedia.org/wiki/Catch_bond

Even more interesting, the first paper noted that productive TCR/pMHC bonds had catch bonds — e.g. bonds which get stronger the more you pull on them. The authors were actually able to measure the phenomenon. Catch bonds been shown to exist in a variety of systems (white cells sticking to blood vessel lining, bacterial adhesion), but their actual mechanism is still under debate.  The great thing about this paper (p. 682) is molecular dynamics simulation showed the conformational changes which occurred during catch bond formation in one case..   They even have videos.  Impressive.

This sort of thing is totally foreign to all solution chemistry, as there is no way to pull on a bond in solution.  Optical tweezers allow you to pull and stretch molecules (if you can attach them to large styrofoam balls).

A creation myth

Sigmund Freud may have been wrong about penis envy, but most lower forms of scientific life (chemists, biologists) do have physics envy — myself included.  Most graduate chemists have taken a quantum mechanics course, if only to see where atomic and molecular orbitals come from.  Anyone doing physical chemistry has likely studied statistical mechanics. I was fortunate enough to audit one such course given by E. Bright Wilson (of Pauling and Wilson).

Although we no longer study physics per se, most of us read books about physics.  Two excellent such books have come out in the past year.  One is “What is Real?” — https://www.basicbooks.com/titles/adam-becker/what-is-real/9780465096053/, the other is “Lost in Math” by Sabine Hossenfelder whose blog on physics is always worth reading, both for herself and the heavies who comment on what she writes — http://backreaction.blogspot.com

Both books deserve a long discursive review here. But that’s for another time.  Briefly, Hossenfelder thinks that physics for the past 30 years has become so fascinated with elegant mathematical descriptions of nature, that theories are judged by their mathematical elegance and beauty, rather than agreement with experiment.  She acknowledges that the experiments are both difficult and expensive, and notes that it took a century for one such prediction (gravitational waves) to be confirmed.

The mathematics of physics can certainly be seductive, and even a lowly chemist such as myself has been bowled over by it.  Here is how it hit me

Budding chemists start out by learning that electrons like to be in filled shells. The first shell has 2 elements, the next 2 + 6 elements etc. etc. It allows the neophyte to make some sense of the periodic table (as long as they deal with low atomic numbers — why the 4s electrons are of lower energy than the 3d electons still seems quite ad hoc to me). Later on we were told that this is because of quantum numbers n, l, m and s. Then we learn that atomic orbitals have shapes, in some wierd way determined by the quantum numbers, etc. etc.

Recursion relations are no stranger to the differential equations course, where you learn to (tediously) find them for a polynomial series solution for the differential equation at hand. I never really understood them, but I could use them (like far too much math that I took back in college).

So it wasn’t a shock when the QM instructor back in 1961 got to them in the course of solving the Schrodinger equation for the hydrogen atom (with it’s radially symmetric potential). First the equation had to be expressed in spherical coordinates (r, theta and phi) which made the Laplacian look rather fierce. Then the equation was split into 3 variables, each involving one of r, theta or phi. The easiest to solve was the one involving phi which involved only a complex exponential. But periodic nature of the solution made the magnetic quantum number fall out. Pretty good, but nothing earthshaking.

Recursion relations made their appearance with the solution of the radial and the theta equations. So it was plug and chug time with series solutions and recursion relations so things wouldn’t blow up (or as Dr. Gouterman, the instructor, put it: the electron has to be somewhere, so the wavefunction must be zero at infinity). MEGO (My Eyes Glazed Over) until all of a sudden there were the main quantum number (n) and the azimuthal quantum number (l) coming directly out of the recursion relations.

When I first realized what was going on, it really hit me. I can still see the room and the people in it (just as people can remember exactly where they were and what they were doing when they heard about 9/11 or (for the oldsters among you) when Kennedy was shot — I was cutting a physiology class in med school). The realization that what I had considered mathematical diddle, in some way was giving us the quantum numbers and the periodic table, and the shape of orbitals, was a glimpse of incredible and unseen power. For me it was like seeing the face of God.

But what interested me the most about “Lost in Math” was Hossenfelder’s discussion of the different physical laws appearing at different physical scales (e.g. effective laws), emergent properties and reductionism (pp. 44 –> ).  Although things at larger scales (atoms) can be understood in terms of the physics of smaller scales (protons, neutrons, electrons), the details of elementary particle interactions (quarks, gluons, leptons etc.) don’t matter much to the chemist.  The orbits of planets don’t depend on planetary structure, etc. etc.  She notes that reduction of events at one scale to those at a smaller one is not an optional philosophical position to hold, it’s just the way nature is as revealed by experiment.  She notes that you could ‘in principle, derive the theory for large scales from the theory for small scales’ (although I’ve never seen it done) and then she moves on

But the different structures and different laws at different scales is what has always fascinated me about the world in which we exist.  Do we have a model for a world structured this way?

Of course we do.  It’s the computer.

 

Neurologists have always been interested in computers, and computer people have always been interested in the brain — von Neumann wrote “The Computer and the Brain” shortly before his death in 1958.

Back in med school in the 60s people were just figuring out how neurons talked to each other where they met at the synapse.  It was with a certain degree of excitement that we found that information appeared to flow just one way across the synapse (from the PREsynaptic neuron to the POST synaptic neuron).  E.g. just like the vacuum tubes of the earliest computers.  Current (and information) could flow just one way.

The microprocessors based on transistors that a normal person could play with came out in the 70s.  I was naturally interested, as having taken QM I thought I could understand how transistors work.  I knew about energy gaps in atomic spectra, but how in the world a crystal with zillions of atoms and electrons floating around could produce one seemed like a mystery to me, and still does.  It’s an example of ’emergence’ about which more later.

But forgetting all that, it’s fairly easy to see how electrons could flow from a semiconductor with an abundance of them (due to doping) to a semiconductor with a deficit — and have a hard time flowing back.  Again a one way valve, just like our concept of the synapses.

Now of course, we know information can flow the other way in the synapse from POST synaptic to PREsynaptic neuron, some of the main carriers of which are the endogenous marihuana-like substances in your brain — anandamide etc. etc.  — the endocannabinoids.

In 1968 my wife learned how to do assembly language coding with punch cards ones and zeros, the whole bit.  Why?  Because I was scheduled for two years of active duty as an Army doc, a time in which we had half a million men in Vietnam.  She was preparing to be a widow with 2 infants, as the Army sent me a form asking for my preferences in assignment, a form so out of date, that it offered the option of taking my family with me to Vietnam if I’d extend my tour over there to 4 years.  So I sat around drinking Scotch and reading Faulkner waiting to go in.

So when computers became something the general populace could have, I tried to build a mental one using and or and not logical gates and 1s and 0s for high and low voltages. Since I could see how to build the three using transistors (reductionism), I just went one plane higher.  Note, although the gates can be easily reduced to transistors, and transistors to p and n type semiconductors, there is nothing in the laws of semiconductor physics that implies putting them together to form logic gates.  So the higher plane of logic gates is essentially an act of creation.  They do not necessarily arise from transistors.

What I was really interested in was hooking the gates together to form an ALU (arithmetic and logic unit).  I eventually did it, but doing so showed me the necessity of other components of the chip (the clock and in particular the microcode which lies below assembly language instructions).

The next level up, is what my wife was doing — sending assembly language instructions of 1’s and 0’s to the computer, and watching how gates were opened and shut, registers filled and emptied, transforming the 1’s and 0’s in the process.  Again note that there is nothing necessary in the way the gates are hooked together to make them do anything.  The program is at yet another higher level.

Above that are the higher level programs, Basic, C and on up.  Above that hooking computers together to form networks and then the internet with TCP/IP  etc.

While they all can be reduced, there is nothing inherent in the things that they are reduced to which implies their existence.  Their existence was essentially created by humanity’s collective mind.

Could something be going on in the levels of the world seen in physics.  Here’s what Nobel laureate Robert Laughlin (he of the fractional quantum Hall effect) has to say about it — http://www.pnas.org/content/97/1/28.  Note that this was written before people began taking quantum computers seriously.

“However, it is obvious glancing through this list that the Theory of Everything is not even remotely a theory of every thing (2). We know this equation is correct because it has been solved accurately for small numbers of particles (isolated atoms and small molecules) and found to agree in minute detail with experiment (35). However, it cannot be solved accurately when the number of particles exceeds about 10. No computer existing, or that will ever exist, can break this barrier because it is a catastrophe of dimension. If the amount of computer memory required to represent the quantum wavefunction of one particle is Nthen the amount required to represent the wavefunction of k particles is Nk. It is possible to perform approximate calculations for larger systems, and it is through such calculations that we have learned why atoms have the size they do, why chemical bonds have the length and strength they do, why solid matter has the elastic properties it does, why some things are transparent while others reflect or absorb light (6). With a little more experimental input for guidance it is even possible to predict atomic conformations of small molecules, simple chemical reaction rates, structural phase transitions, ferromagnetism, and sometimes even superconducting transition temperatures (7). But the schemes for approximating are not first-principles deductions but are rather art keyed to experiment, and thus tend to be the least reliable precisely when reliability is most needed, i.e., when experimental information is scarce, the physical behavior has no precedent, and the key questions have not yet been identified. There are many notorious failures of alleged ab initio computation methods, including the phase diagram of liquid 3He and the entire phenomenonology of high-temperature superconductors (810). Predicting protein functionality or the behavior of the human brain from these equations is patently absurd. So the triumph of the reductionism of the Greeks is a pyrrhic victory: We have succeeded in reducing all of ordinary physical behavior to a simple, correct Theory of Everything only to discover that it has revealed exactly nothing about many things of great importance.”

So reductionism doesn’t explain the laws we have at various levels.  They are regularities to be sure, and they describe what is happening, but a description is NOT an explanation, in the same way that Newton’s gravitational law predicts zillions of observations about the real world.     But even  Newton famously said Hypotheses non fingo (Latin for “I feign no hypotheses”) when discussing the action at a distance which his theory of gravity entailed. Actually he thought the idea was crazy. “That Gravity should be innate, inherent and essential to Matter, so that one body may act upon another at a distance thro’ a Vacuum, without the Mediation of any thing else, by and through which their Action and Force may be conveyed from one to another, is to me so great an Absurdity that I believe no Man who has in philosophical Matters a competent Faculty of thinking can ever fall into it”

So are the various physical laws things that are imposed from without, by God only knows what?  The computer with its various levels of phenomena certainly was consciously constructed.

Is what I’ve just written a creation myth or is there something to it?

Will acyclovir be a treatment for Alzheimer’s ?

When I was a first year medical student my aunt died of probable acute herpes simplex encephalitis at Columbia University Hospital in New York City.  That was 55 years ago and her daughters (teenagers at the time) still bear the scars.  Later, as a neurologist I treated it, and after 1977, when acyclovir, which effectively treats herpes encephalitis came out, I would always wonder if acyclovir would have saved her.

The drug is simplicity itself.  It’s just guanosine (https://en.wikipedia.org/wiki/Guanosine) with two of the carbons of the ribose missing.  Herpesviruses have an enzyme which forms the triphosphate incorporating it into its DNA killing the virus.  Well, actually we have the same enzyme, but the virus’s enzyme is 3,000,000 times more efficient than ours, so acyclovir is relatively nontoxic to us.  People with compromised renal function shouldn’t take it.

What does this have to do with Alzheimer’s disease?  The senile plaque of Alzheimers is mostly the aBeta peptide (39 – 43 amino acids) from the amyloid precursor protein (APP).  This has been known for years, and my notes on various papers about over the years contain 150,000 characters or so.

Even so, there’s a lot we don’t understand about APP and the abeta peptide — e.g. what are they doing for us?  You can knockout the APP gene in mice and they appear normal and fertile.  The paper cited below notes that APP has been present in various species for the past 400,000,000 years of evolutionary time remaining pretty much unchanged throughout, so it is probably doing something useful

A recent paper in Neuron (vol. 99 pp. 56 – 63 ’18) noted that aBeta is actually an antimicrobial peptide.  When exposed to herpes simplex it binds to glycoproteins on its surface and then  oligomerizes forming amyloid (just like in the senile plaque) trapping the virus.  Abeta will protect mice against herpes simplex 1 (HSV1) encephalitis.  Even more important — infection of the mice with HSV1 induced abeta production in their brains.

People have been claiming infections as the cause of just about every neurodegeneration since I’ve been a neurologist, and papers have been written about HSV1 and Alzheimer’s.

Which brings me to the second paper (ibid. pp. 64 – 82) that looked for the viral RNAs and DNAs in over 900 or so brains, some with and some without Alzheimer’s.  They didn’t find HSV but they found two other herpes viruses known to infect man (HHV6, HHV7 — which cause roseola infantum).  Humans are subject to infection with 8 different herpes virus (Epstein Barr — mononucleosis, H. Zoster — chickenpox etc. etc.).   Just about everyone of us has herpes virus in latent form in the trigeminal ganglion — which gets sensory information from our faces.

So could some sort of indolent herpesvirus infection be triggering abeta peptide production as a defense with the senile plaque as a byproduct?  That being the case, given the minimal benefits of any therapy we have for Alzheimer’s disease so far, why not try acyclovir (Zovirax) on Alzheimer’s.

I find it remarkable that neither paper mentioned this possibility, or even discussed any of the antivirals active against herpesviruses.

How many more metabolites like this are out there?

3′ deoxy 3′ 4′ didehydro cytidine triphosphate — doesn’t roll’ tripgingly on the tongue’ does it? (Hamlet Act 3, scene 2, 1–4).  Can you draw the structure?  It is the product of another euphoniously named enzyme — Viperin.  Abbreviated ddhCTP it is just cytidine triphosphate with a double bond between carbons 3 and 4 of the sugar.

Viperin is an enzyme induced by interferon which inhibits the replication of a variety of viruses. [ Nature vol. 558 pp. 610 – 614 ’18 ] describes a  beautiful sequence  of reactions for ddhCTP’s formation using S-Adenosyl Methionine (SAM).  ddhCTP acts as a chain terminator for the RNA dependent RNA polymerases of multiple members of Flaviviruses (including Zika).

However the paper totally blows it for not making the point that ddhCTP is extremely close to a drug (which has been used against AIDS for years — Zalcitabine (Hivid) — http://www.molbase.com/en/name-Zalcitibine.html which is just ddC.  ddhCTP is almost the same as ddC — except that there is no triphosphate on the 5′ hydroxyl (which enzymes in the body add), and instead of a double bond between carbons 3 and 4 of the sugar, both carbons are fully reduced (CH2 and CH2).  So ddhCTP is Nature’s own Zalcitabine.

It is worth reflecting on just how many other metabolites are out there acting as ‘natural’ drugs that we just haven’t found yet.

Remember entropy – take III

Pop quiz.  How would you make an enzyme in a cold dwelling organism (0 Centrigrade) as catalytically competent as its brothers living in us at 37 C?

We know that reactions go faster the hotter it is, because there is more kinetic energy of the reactants to play with.  So how do you make an enzyme move more when it’s cold and there is less kinetic energy to play with.

Well for most cold tolerance enzymes (psychrophilic enzymes — a great scrabble word), evolution mutates surface amino acids to glycine.  Why glycine?  Well it’s lighter, and there is no side chain to get in the way  when the backbone moves.  The mutations aren’t in the active site but far away.   This means more wiggling of the backbone — which means more entropy of the backbone.

The following papers [ Nature vol. 558 pp. 195 – 196, 324 – 218 ’81 ] studied adenylate kinase, an enzyme found in most eukaryotes  which catalyzes

ATP + AMP < — > 2 ADP.

They studied the enzyme from E. Coli which happily lives within us at 37 C, and mutated a few surface valines and isoleucines to glycine, lowered the temperature and found the enzyme works as well (the catalytic rate of the mutated enzyme at 0 C is the same as the rate of the unmutated enzyme at 37).

Chemists have been studying transition state theory since the days of Eyring, and reaction rates are inversely proportional the the amount of free energy (not enthalpy) to raise the enzyme to the transition state.

F = H – TS (Free energy = enthalpy – Temperature * Entropy).

So to increase speed decrease the enthalpy of activation (deltaH) or increase the amount of entropy.

It is possible to separately measure enthalpy and entropies of activation, and the authors did just that (figure 4 p. 326) and showed that the enthalpy of activation of the mutated enzyme (glycine added) was the same as the unmutated enzyme, but that the free energy of activation of the mutated enzyme was less because of an increase in entropy (due to unfolding of different parts of the enzyme).

Determining these two parameters takes an enormous amount of work (see the story from grad school at the end). You have to determine rate constants at various temperatures, plot the rate constant divided by temperature and then measure the slope of the line you get to obtain the enthalpy of activation.   Activation entropy is determined by the intercepts of the straight line (which hopefully IS straight) with the X axis.  Determining the various data points is incredibly tedious and uninteresting.

So enzymes  of cold tolerant organisms are using entropy to make their enzymes work.

Grad school story — back in the day, studies of organic reaction mechanisms were very involved with kinetic measurements (that’s where Sn1 and Sn2 actually come from).  I saw the following happen several times, and resolved never get sucked in to having to actually do kinetic measurements.  Some hapless wretch would present his kinetic data to a seminar, only to have Frank Westheimer think of something else and suggest another 6 months of kinetic measurements, so back he went to the lab for yet more drudgery.

 

 

Molecular biology’s oxymoron

Dear reader.  What does a gene do?  It codes for something.  What does a nonCoding Gene do?  It also codes for something, just RNA instead of protein. It’s molecular biology’s very own oxymoron, a throwback to the heroic protein-centric early days of molecular biology. The term has been enshrined by usage for so long that it’s impossible to get rid of.  Nonetheless, the the latest work found even more nonCoding genes than genes actually coding for  protein.

An amusing article from Nature (vol. 558 pp. 354 – 355 ’18) has the current state of play.   The latest estimate is from GTex which sequenced 900 billion RNAs found in various human tissues, matched them to the sequence(s) of the human genome and used computer algorithms to determine which  of them were the product of genes coding for proteins and genes coding for something else.

The report from GTex  (Genotype Tissue expression Project) found 21,306 protein-coding genes and 21,856 non-coding genes — amazingly there are more nonCoding genes than protein coding ones.  This  is many more genes than found in the two most widely used human gene databases. The GENCODE gene set, maintained by the EBI, includes 19,901 protein-coding genes and 15,779 non-coding genes. RefSeq, a database run by the US National Center for Biotechnology Information (NCBI), lists 20,203 protein-coding genes and 17,871 non-coding genes.

Stay tuned.  The fat lady hasn’t sung.