Could Gemfibrozil (Lopid) be used to slow down (or even treat) Alzheimer’s disease?

Is a treatment of Alzheimer’s disease at hand with a drug in clinical use for nearly 40 years? A paper in this week’s PNAS implies that it might (vol. 112 pp. 8445 – 8450 ’15 7 July ’15). First a lot more background than I usually provide, because some family members of the afflicted read everything they can get their hands on, and few of them have medical or biochemical training. The cognoscenti can skip past this to the text marked ***

One of the two pathologic hallmarks of Alzheimer’s disease is the senile plaque (the other is the neurofibrillary tangle). The major component of the plaque is a fragment of a protein called APP (Amyloid Precursor Protein). Normally it sits in the cellular membrane of nerve cells (neurons) with part sticking outside the cell and another part sticking inside. The protein as made by the cell contains anywhere from 563 to 770 amino acids linked together in a long chain. The fragment destined to make up the senile plaque (called the Abeta peptide) is much smaller (39 to 42 amino acids) and is found in the parts of APP embedded in the membrane and sticking outside the cell.

No protein lives forever in the cell, and APP is no exception. There are a variety of ways to chop it up, so its amino acids can be used for other things. One such chopper is called ADAM10 (aka Kuzbanian). ADAM10breaks down APP in such a way that Abeta isn’t formed. The paper essentially found that Gemfibrozil (commercial name Lopid) increases the amount of ADAM10 around. If you take a mouse genetically modified so that it will get senile plaques and decrease ADAM10 you get a lot more plaques.

The authors didn’t artificially increase the amount of ADAM10 to see if the animals got fewer plaques (that’s probably their next paper).

So there you have it. Should your loved one get Gemfibrozil? It’s a very long shot and the drug has significant side effects. For just how long a shot and the chain of inferences why this is so look at the text marked @@@@


How does Gemfibrozil increase the amount of ADAM10 around? It binds to a protein called PPARalpha which is a type of nuclear hormone receptor. PPARalpha binds to another protein called RXR, and together they turn on the transcription of a variety of genes, most of which are related to lipid metabolism. One of the genes turned on is ADAM10, which really has never been mentioned in the context of lipid metabolism. In any event Gemfibrozil binds to PPARalpha which binds more effectively to RAR which binds more effectively to the promoter of the ADAM10 gene which makes more ADAM10 which chops of APP in such fashion that Abeta isn’t made.

How in the world the authors got to PPARalpha from ADAM10 is unknown — but I’ve written the following to the lead author just before writing this post.

Dr. Pahan;

Great paper. People have been focused on ADAM10 for years. It isn’t clear to me how you were led to PPARgamma from reading your paper. I’m not sure how many people are still on Gemfibrozil. Probably most of them have some form of vascular disease, which increases the risk of dementia of all sorts (including Alzheimer’s). Nonetheless large HMOs have prescription data which can be mined to see if the incidence of Alzheimer’s is less on Gemfibrozil than those taking other lipid lowering agents, or the population at large. One such example (involving another class of drugs) is JAMA Intern Med. 2015;175(3):401-407, where the prescriptions of 3,434 individuals 65 years or older in Group Health, an integrated health care delivery system in Seattle, Washington. I thought the conclusions were totally unwarranted, but it shows what can be done with data already out there. Did you look at other fibrates (such as Atromid)?

Update: 22 July ’15

I received the following back from the author

Dear Dr.

Wonderful suggestion. However, here, we have focused on the basic science part because the NIH supports basic science discovery. It is very difficult to compete for NIH R01 grants using data mining approach.

It is PPARα, but not PPARγ, that is involved in the regulation of ADAM10. We searched ADAM10 gene promoter and found a site where PPAR can bind. Then using knockout cells and ChIP assay, we confirmed the participation of PPARα, the protein that controls fatty acid metabolism in the liver, suggesting that plaque formation is controlled by a lipid-lowering protein. Therefore, many colleagues are sending kudos for this publication.

Thank you.

Kalipada Pahan, Ph.D.

The Floyd A. Davis, M.D., Endowed Chair of Neurology


Departments of Neurological Sciences, Biochemistry and Pharmacology

So there you have it.  An idea worth pursuing according to Dr. Pahan, but one which he can’t (or won’t).  So, dear reader, take it upon yourself (if you can) to mine the data on people given Gemfibrozil to see if their risk of Alzheimer’s is lower.  I won’t stand in your way or compete with you as I’m a retired clinical neurologist with no academic affiliation. The data is certainly out there, just as it was for the JAMA Intern Med. 2015;175(3):401-407 study.  Bon voyage.


There are side effects, one of which is a severe muscle disease, and as a neurologist I saw someone so severely weakened by drugs of this class that they were on a respirator being too weak to breathe (they recovered). The use of Gemfibrozil rests on the assumption that the senile plaque and Abeta peptide are causative of Alzheimer’s. A huge amount of money has been spent and lost on drugs (antibodies mostly) trying to get rid of the plaques. None have helped clinically. It is possible that the plaque is the last gasp of a neuron dying of something else (e.g. a tombstone rather than a smoking gun). It is also possible that the plaque is actually a way the neuron was defending itself against what was trying to kill it (e.g. the plaque as a pile of spent bullets).

Why drug discovery is so hard (particularly in the brain): Reason #28: The brain processes its introns very differently

Useful drug discovery for neurologic and psychiatric disease is nearly at a standstill. It isn’t for want of trying by basic researchers and big and small pharma. A recent excellent review [ Neuron vol. 87 pp. 14 – 27 ’15 ] helps explain why. In short, the brain processes its protein coding genes rather differently.

This post assumes you know what introns, exons and alternate splicing are. For pretty much all the needed background see the following.


When splicing first came out I started making a list of proteins which were alternatively spliced. It is now safe to assume that any gene containing introns (95% of all protein coding genes [ Proc. Natl. Acad. Sci. vol. 112 pp. 17985 – 17990 ’08 ]) results in several protein products due to alternative splicing. The products produced vary from tissue to tissue, probably because most tissues express different splicing regulators.

Here are a few. A2BP1 (aka Rbfox1, aka FOX1) is a brain specific RNA splicing factor found only in postmitotic terminally differentiated neurons. It is deleted in 10% of glioblastomas. Another is nSR100 (neural Specific Related protein of 100 kiloDaltons) — see later.

To show how crucial alternative splicing is for the every existence of the brain, consider this. The neuronal splicing regulator PTBP2 is barely expressed in most tissues. It is upregulated in neurons. Both PTBP1 and PTBP2 are repressors of neural alternative splicing (but some genes are actually enhanced). In a given region of the brain either PTPB1 or PTBP2 is expressed (but not both). PTBP1 promotes skiping of a neural specific exon (exon #10) in PTBP2 transcripts. This exposes a premature termination codon in PTBP2 leading to nonsense mediated decay (NMD). PTPB1 is expressed in most nonNeural tissues and neural precursor cells, but is silenced in developing neurons by the microRNA miR-124. The mRNA for PTBP2 contains an alternative exon which triggers nonsense mediated decay (NMD) when skipped. Inclusion of the exon requires positive transacting factors such as nSR100 in neurons. Repression is mediated by PTBP1 in undifferentiation. microRNAs (which ones?) downregulate PTBP1 during neuronal differentiation, relieving the negative regulation of PTBP2. Depletion of PTBP1 in fibroblasts is enough for PTBP2 induction and neuronal transdifferentiation.

It gets more complicated still. PTBP1 inhibits splicing of introns at the 3′ end of some genes involved in presynaptic function. This results in nuclear retention and turnover via components of the nuclear RNA surveillance machinery. As PTBP1 is downregulated during neuronal differentiation, the target introns are spliced out and the mature mRNAs are found.

Now we get to microExons, something unknown until 2014. For more details see —
Briefly, microexons are defined as exons containing 50 nucleotides or less (the paper says 3 – 27 nucleotides). They have been overlooked, partially because their short length makes them computationally difficult to find. Also few bothered to look for them as they were thought to be unfavorable for splicing because they were too short to contain exonic splicing enhancers. They are so short that it was thought that the splicing machinery (which is huge) couldn’t physically assemble at both the 3′ and 5′ splice sites. So much for theory, they’re out there.

The inclusion in the final transcript of most identified neural microExons is regulated by a brain specific factor nSR100 (neural specific SR related protein of 100 kiloDaltons)/SRRM4 which binds to intronic enhancer UGC motifs close to the 3′ splice sites, resulting in their inclusion. They are ‘enhanced’ by tissue specific RBFox proteins. nSR100 is said to be reduced in Autism Spectrum Disorder (really? all? some?). nSR100 is strongly coexpressed in the developing human brain in a gene network module M2 which is enriched for rare de novo ASD assciated mutations.

MicroExons are enriched for lengths which are multiples of 3 nucleotides. Recall that every 3 nucleotides in mRNA codes for an amino acid. This implies strong selection pressure was used to preserve reading frames as 3n+1 and 3n+2 produce a frameshift. The microExons are enriched in charged amino acids. Most microExons show high inclusion at late stages of neuronal differentiation in genes associated with axon formation and synapse function. A neural specific microExon in Protrudin/Zfyve27 increases its interation with Vessicale Associated membrane protein associated Protein VAP) and to promote neurite outgrowth.

[ Proc. Natl. Acad. Sci. vol. 112 pp. 3445 – 3450 ’15 ] Deep mRNA sequencing of mouse cerebral cortex expanded the list of alternative splicing events TENfold and showed that 72% of multiexon genes express multiple splice variants. Among the newly discovered alternatively spliced exon are 1,104 exons involved in nonsense mediated decay (NMD). THey are enriched in RNA binding proteins including splicing factors. Another set of alternatively spliced NMD exons is found in genes coding for chromatin regulators. Conservation of NMD exons is found in lower vertebrates, but those involving chromatin regulators are found later into the mammalian lineage. So the transcriptome in the brain is even more complicated.

A bit more about the actual effects on protein structure of alternate splicing. The sites chosen for this aren’t random. Cell and tissue differentially regulated alternative splicing events are significantly UNDERrepresented in functionally defined folded domains in proteins, they are enriched in regions of protein disorder that typically are surface accessible and embed short linear interaction motifs (with other proteins and ligands). Among a set of analyzed neural specific exons enriched in disordered regions, 1/3 promoted or disrupted interactions with partner proteins. So regulated exon splicing might specify tissue and cell type specific protein interaction networks. They regard their inclusion/exclusion as protein surface microsurgery.

How much can a little microexon do to protein function? Here’s an example of a 6 nucleotide microexon (two amino acids). Insertion of the microExon in the nuclear adaptor protein Apbb1 enhances its interaction with Kat5/Tip60 a histone deacetylase. The microExon adds Arginine and Glutamic acid to a phosphotyrosine binding domain (PTB domain) which binds Kat4. This enhances binding.

Had enough? The complexity is staggering and I haven’t even talked about recursive splicing — that’s for another post, but here’s a reference if you can’t wait — [ Nature vol. 521 pp. 300 – 301, 371 – 375, 376 – 379 ’15 ]. Pity the drug chemist figuring out which alternatively spliced form of a brain protein to attack (particularly if it hasn’t been studied for microExons).

I’m not making this up

Docs hate res ipsa loquitur — a favorite of malpractice lawyers — e.g. the thing speaks for itself — If accepted this means that negligence or malpractice requires no proof. So here I am, using it as a rhetorical device in an important malpractice case.

Below are the 7 priority areas set out by Katherine Archuleta, the late unlamented director of the Office of Personnel management. It’s a marvelously politically correct document, with all the appropriate buzzwords to warm the heart (and numb the mind). Unfortunately security wasn’t one of the seven.

OPM has set out seven priority areas:

Honoring the Workforce: OPM will be the champion of the Federal workforce. Through such programs as the OPM Innovation Lab and the Learning Center, we will provide career training and skill development for Federal employees. OPM will serve as the thought leader in research and data-driven human resource management and policy decision-making.

Build a More Diverse and Engaged Workforce: OPM, which by Executive Order is the lead agency on increasing diversity and inclusion in the Federal workforce, will recruit qualified individuals to serve and expand access to the job pipeline from entry and mid-level positions to leadership posts. OPM will provide leadership in helping agencies create work environments where a diverse Federal workforce is fully engaged and energized.

World Class Customer Service: OPM will respond to the interests of its many and diverse customers throughout the lifecycle of an employee. Whether it’s a recent graduate seeking to start a Federal career, a current employee looking for a training opportunity, or a retiree, OPM will provide timely, accurate and responsive service.

IT Improvement: Under the leadership of a new Chief Information Officer and Chief Technology Officer, OPM will implement its IT Strategic Plan to streamline and update IT systems to better serve Federal employees from resume through retirement.

Background Investigations: In partnership with the Office of the Director of National Intelligence, OPM will implement the revised Federal Investigative Standards and will lead efforts to strengthen the background investigations program across government as we maintain the highest standards of quality and timeliness.

Retirement: OPM is closing in on our goal to process 90 percent of cases within 60 days. The agency will continue to update our systems as we continue to transition to a paperless process.

Health Care: OPM will fully implement the Multi-State Plan provision of the Affordable Care Act, provide coverage to Tribal employees and continue providing high quality health insurance benefits to the Federal workforce.

Here’s a link to the original

Res ipsa loquitur

Why drug discovery is so hard: Reason #27 Moonlighting effects.

Well, we all know what heat shock proteins (Hsps) do — they bind to proteins which have lost their shape due to heat (or other stressors), cuddle them hydrolyze ATP and nurse them back to health. But what  if some of them do other things? The phenomenon is called moonlighting.

The case of Hsp70 is instructive. Some background first. The Hsp70 chaperone transiently associates with its substrates in a manner controlled by its ATPase cycle. ATP binding to the amino terminal nucleotide binding domain (NBD) induces a conformational change in the carboxy terminal substrate binding domain (SBD) which results in low affinity for substrate. Hydrolysis of ATP converts the Hsp70 to the ADP state, which binds substrates with higher affinity. Exchange of ADP for ATP releases substrate completing the cycle. The hydrolysis of ATP is stimulated by J-domain containing cochaperones. These are the nucleotide exchange factors.  Back and forth Hsp70 and the damaged protein go through the cycle until the protein is nursed back to normal or, failing this, is destroyed.

The Hsp70 family acts early in protein synthesis by binding to a small stretch of hydrophobic amino acids on a protein’s surface. Aided by a set of smaller Hsp40 proteins (also known as J proteins), a hsp70 monomer binds to its target protein and then hydrolyzes ATP to ADP, undergoing a conformational change that causes the hsp70 to clamp down very tightly on the target. After the hsp40 dissociates (see below), the dissociation of the hsp70 protein is induced by the rapid rebinding of ATP after ADP release. Repeated cycles of hsp protein binding and release help the target protein to refold.

Enter [ Proc. Natl. Acad. Sci. vol. 112 pp. E3327 – E3336 ’15 ] This work shows Hsp70 is methylated on arginine #469 by Coactivator Associated aRginine Methyltransferase 1/Protein aRginine MethylTransferase 4 (CARM1/PRMT4) and demethylated by JuMonJi Domain containing 6 (JMJD6) — hideous acronyms shortening even more hideous names. Methylated Hsp70 then functions in transcription as a ‘regulator’ of Retinoid Acid Receptor beta 2 (RARbeta2) transcriptional acitivty. R468Mmethylated Hsp70 mediates the interaction between Hsp70 and TFIIH (Transcription Factor IIH).

The regulation of gene transcription is an entirely novel and unsuspected function for a heat shock protein. A classic example of moonlighting.

Drug chemists and pharmacologists are always concerned about off-target effects. For an interesting example please see  Off-target effects occur when their drug hits something else in the cell producing an unexpected (and usually untoward) effect.

If you are unaware that your target of choice is doing a little something else on the side (e.g. moonlighting) you can get an off target effect even when you hit your desired target. It’s a tough business. How many more moonlighters are out there that we don’t know about?

Hsp70 is a good example. Here are two more — no background provided, so you’re on your own — except to point out that glucocorticoids are a widely used class of drug.

[ Proc. Natl. Acad. Sci. vol. 112 pp. E1540 – 1549 ’15 ] Amazingly, the glucocorticoid receptor (GR)plays a role in mRNA degradation by acting as an RNA binding protein. When loaded onto the 5′ UnTranslated Region (5′ UTR) of a target mRNA, the GR recruits UPF1 through Proline-rich Nuclear Receptor Coregulatory protein 2 (PNRC2) in a ligand (of itself?) dependent manner to cuase rapid mRNA degradation. They call this GMD (Glurocorticoid receptor Mediated Decay). Along with Staufen Mediated mRNA Decay (SMD) and Nonsense Mediated mRNA Decay (NMD), they share UPF1 (Upstream Frameshift 1) and PNRC2.

[ Science vol. 323 pp. 723 – 724, 793 – 797 ’09 ] Stat3 proteins represent the canonical mediators of signals elicited by cytokines binding to type I cytokine receptors. However, GRIM19 (Gene associated with Retinoid Interferon Mortality 19), a mitochondrial protein, interacts with Stat3 and inhibits its transcriptional activity (where?). This work shows that Stat3 associates with GRIM19 containing complexes I and II (components of the electron transport chain) in mouse liver and muscle mitochondria. Levels of Stat3 in mitochondria are 10% of cytosolic levels.

Cells lacking Stat3 show decreased activity of mitochondrial complexes I and II. Effects on complex I and II don’t require Stat3’s DNA binding domain, the dimerization motif, or the tyrosine phosphorylation site controlling Stat3 nuclear localization and transcriptional activity — so this is a ‘moonlighting’ role for State3 having nothing to do with gene transcription. The serine phosporylation site on Stat3 is important. So Stat3 is required to maintain normal mitochondrial function.

Happy fourth of July

Two encounters in the past 2 days brought home just how fortunate we are to live in the USA, along with the realization that only the immigrants truly appreciate this country. One was with a Greek friend who is a professor of engineering at a local university. His wife is on her way to Greece acting as a money launderer (well not really) bringing US dollars to her family over there. We get his brains for free and probably those of his 15 year old daughter who grew up here.

The other was with our tile man Sergey, a Russian immigrant of 17 years. He describes how his grandfather was sent by Stalin to Siberia, surviving for 14 years. Why? He wasn’t at all active politically, but was a devout Christian. That was all it took. If you find this difficult to accept — look at the following post — Some of the most brilliant mathematicians of the Soviet Union were persecuted for the same reason.

Not convinced? Over 25 years ago, the local paper where we were living at the time had an interview with a Ukrainian woman newly arrived in the states. She was asked what it was she liked best about this country. She said it was being able to have people over to her house for prayer without having the draw the curtains.

Yes, we complain a lot about how things could be better, probably a genetic heritage, as only those who were unsatisfied with their condition where they were had the gumption to get up and come here.

However, as great as we are, tonight’s fireworks pale in comparison to those of the Chinese New Year (they invented fireworks after all). Earlier this year my wife and I viewed a 30 minute display from 3 ships firing away in Hong Kong harbor. Some of them even spelled out Chinese characters according to our daughter in law. If you like loud, go to a Buddist temple for their new year celebrations.

Kuru continues to inform

Neurologists of my generation were fascinated with Kuru, a disease of the (formerly) obscure Fore tribe of New Guinea. Who would have thought they would tell us a good deal about protein structure and dynamics?

It is a fascinating story including a Nobelist pedophile (Carleton Gajdusek) and another (future) Nobelist who I probably ate lunch with when we were both medical students in the same Medical Fraternity but don’t remember –

Kuru is a horrible neurodegeneration starting with incoordination, followed by dementia and death in a vegetative state in 4 months to 2 years. For the cognoscenti — the pathology is neuronal loss, astrocytosis, microglial proliferation, loss of myelinated fibers and the kuru plaque.

It is estimated that it killed 3,000 members of the 30,000 member tribe. The mode of transmission turned out to be ritual cannibalism (flesh of the dead was eaten by the living before burial). Once that stopped the disease disappeared.

It is a prion disease, e.g. a disease due to a protein (called PrP) we all have but in an abnormal conformation (called PrpSc). Like Vonnegut’s Ice-9 ( PrPSc causes normal PrP to assume its conformation, causing it to aggregate and form an insoluble mess. We still don’t know the structure of PrPSc (because it’s an insoluble mess). Even now, “the detailed structure of PrPSc remains unresolved” but ‘it seems to be’ very similar to amyloid [ Nature vol. 512 pp. 32 – 34 ’14]. Not only that, but we don’t know what PrP actually does, and mice with no PrP at all are normal [ Nature vol. 365 p. 386 ’93 ]. For much more on prions please see

Prusiner’s idea that prion diseases were due to a protein, with no DNA or RNA involved met with incredible resistance for several reasons. This was the era of DNA makes RNA makes protein, and Prisoner was asking us to believe that a protein could essentially reproduce without any DNA or RNA. This was also the era in which X-ray crystallography was showing us ‘the’ structure of proteins, and it was hard to accept that there could be more than one.

There are several other prion diseases of humans (all horrible) — mad cow disease, Jakob Creutzfeldt disease, Familial fatal insomnia, etc. etc. and others in animals. All involve the same protein PrP.

One can take brain homogenates for an infected animal, inoculate it into a normal animal and watch progressive formation of PrPSc insoluble aggregates and neurodegeneration. A huge research effort has gone into purifying these homogenates so the possibility of any DNA or RNA causing the problem is very low. There still is one hold out — Laura Manuelidis who would have been a classmate had I gone to Yale Med instead of Penn. n

Enter [ Nature vol. 522 pp. 423 – 424, 478 – 481 ’15 ] which continued to study the genetic makeup of the Fore tribe. In an excellent example of natural selection in action, a new variant of PrP appeared in the tribe. At amino acid #127, valine is substituted for glycine (G127V is how this sort of thing is notated). Don’t be confused if you’re somewhat conversant with the literature — we all have a polymorphism at amino acid #129 of the protein, which can be either methionine or valine. It is thought that people with one methionine and one valine on each gene at 129 were somewhat protected against prion disease (presumably it affects the binding between identical prion proteins required for conformational change to PrPSc.

What’s the big deal? Well, this work shows that mice with one copy of V127 are protected against kuru prions. The really impressive point is that the mice are also protected against variant Creutzfedlt disease prions. Mice with two copies of V127 are completely protected against all forms of human prion disease . So something about V/V at #127 prevents the conformation change to PrPSc. We don’t know what it is as the normal structure of the variant hasn’t been determined as yet.

This is quite exciting, and work is certain to go on to find short peptide sequences mimicking the conformation around #127 to see if they’ll also work against prion diseases.

This won’t be a huge advance for the population at large, as prion diseases, as classically known, are quite rare. Creutzfeldt disease hits 1 person out of a million each year.

There are far bigger fish to fry however. There is some evidence that the neurofibrillary tangles (tau protein) of Alzheimer’s disease and the Lewy bodies (alpha-Synuclein) of Parkinsonism, spread cell to cell by a ‘prionlike’ mechanism [ Nature vol.485 pp. 651 – 655 ’12, Neuron vol. 73 pp. 1204 – 1215 ’12 ]. Could this sort of thing be blocked by a small amino acid change in one of them (or better a small drug like peptide?).

Stay tuned.

Outside information on the Greek financial crisis

Definitely off topic, but I wrote the following to a an international banker friend of 50+ years experience about the Greek financial crisis

“But of more immediate import, what sayeth the banker about shutting the banks in Greece for a week. In this article people were taking Euros out, but if the banks don’t have any and the Europeans won’t give more, what happens then. Gotterdammerung?”

I got the following back

Possibly. Martial law? It happened here in the 30s – the “bank holiday(s)”. Having lived thru 2 bank runs, I can tell you it is a time when rationality is absent. The presumption is the Greek authorities have been planning for this. Greeks are used to violent demonstrations,. I would expect some bank buildings being burned. For a few days the country can function. More and I can not see anything but Grexit. I see a gigantic game of chicken. The Germans have been blinking up until now but the internal political cost to Merkel may be too great. Germany has MUCH to lose if Greece leaves the EU.

Are you as smart as the (inanimate) blind watchmaker

Here’s a problem the cell has solved. Can you? Figure out a way to send a protein to two different membranes in the cell (the membrane encoding it { aka the plasma membrane }, and the endoplasmic reticulum) in the proportions you wish.

The proteins must have exactly the same sequence and content of amino acids, ruling out alternative splicing of exons in the mRNA (if this is Greek to you have a look at the following post — and the others collected under —

The following article tells you how the cell does it. Recall that not all of the messenger RNA (mRNA) is translated into protein. The ribosome latches on to the 5′ end of the mRNA,  subsequently moving toward the 3′ end until it finds the initiator codon (AUG which codes for methionine). This means that there is a 5′ untranslated region (5′ UTR). It then continues moving 3′ ward stitching amino acids together.  Similarly after the ribosome reaches the last codon (one of 3 stop codons) there is a 3′ untranslated region (3′ UTR) of the mRNA. The 3′ UTR isn’t left alone but is cleaved and a polyAdenine tail added to it. The 3′ UTR is where most microRNAs bind controlling mRNA stability (hence the amount of protein produced from a given mRNA).

The trick used by the cell is described in [ Nature vol. 522 pp. 363 – 367 ’15 ]. The 3’UTR is alternatively processed producing a variety of short and long 3’UTRs. One such protein where this happens is CD47 — which is found on the surface of most cells where it stops the cell from being eaten by scavenger cells such as macrophages. The long 3′ UTR of CD47 allows efficient cell surface expression, while the short 3′ UTR localizes it to the endoplasmic reticulum.

How could this possibly work? Once the protein is translated by the ribosome, it leaves the ribosome and the mRNA doesn’t it? Not quite.

They say that the long 3′ UTR of CD47 acts as a scaffold to recruit a protein complex which contains HuR (aka ELAVL1), an RNA binding protein and SET to the site of translation. The allows interaction of SET with the newly translated cytoplasmic domains of CD47, resulting in subsequent translocation of CD47 to the plasma membrane via activated RAC1.

The short 3′ UTR of CD47 doesn’t have the sequence binding HuR and SET, so CD47 doesn’t get to the plasma membrane, rather to the endoplasmic reticulum.

The mechanism may be quite general as HuR binds to thousands of mRNAs. The paper gives two more examples of proteins where this happens.

It’s also worth noting that all this exquisite control, does NOT involve covalent bond formation and breakage (e.g. not what we consider classic chemical reactions). Instead it’s the dance of one large molecular object binding to another in other ways. The classic chemist isn’t smiling. The physical chemist is.

The twists and turns of topoisomerase (pun intended)

It is very sad that my late friend Nick Cozzarelli isn’t around to enjoy the latest exploits of the enzyme class he did so much great work on — the topoisomerases. For a social note about him see the end of the post.

We tend to be quite glib about just what goes on inside a nucleus when DNA is opened up and transcribed into mRNA by RNA polymerase II (Pol II). We think of DNA has a linear sequence of 4 different elements (which it is) and stop there. But DNA is a double helix, and the two strands of the helix wind around each other every 10 elements (nucleotides), meaning that within the confines of our nuclei this happens 320,000,000 times.

I’ve written a series of six posts on what we would see if our nuclei were enlarged  by a factor of 100,000 (which is the amount of compaction our DNA must undergo to fit inside the 10 micron (10 millionths of a meter) in diameter nucleus (since if fully extended our DNA would be 1 meter long. So if you compacted the distance from New York to Seattle (2840 miles or 14,995,200 feet) down by this factor you’d get a sphere 150 feet in diameter or half the length of a football (US) field. Now imagine blowing up the diameter and length of the DNA helix by 100,000 and you’d get something looking like a 2,840 mil long strand of linguini which twists on itself  320,000,000 times. The two strands are 3/8th of an inch thick. They twist around each other every 9/16ths of an inch.

For the gory details start at and follow the links.

Well, we know that for DNA to be copied into mRNA it must be untwisted, the strands separated so RNA polymerase II (Pol II) can get to it.  Pol II is enormous — a mass of 500 kiloDaltons and 7 times thicker at 140 Angstroms than the DNA helix of 20 Angstrom thickness.

Consider the fos gene (which we’ll be talking about later). It contains 380 amino acids (meaning that the gene contains at least 1140 nucleotides ). The actual gene is longer because of introns (3,461 nucleotides), which means that the gene contains 346 complete turns of the double helix, all of which must be unwound to transcribe it into mRNA.

So it’s time for an experiment. Get about 3 feet of cord roughly 3/8 of an inch thick. Tie the ends together, loop one end around a hook in your closet, put a pencil in the other end and rotate it about 100 times (or until you get tired). Keeping everything the same, have a friend put another pencil between the two strands in the middle, separating them. Now pull on the strands to make the separation wider and move the middle pencil toward one end. In the direction of motion the stands will coil even tighter (supercoiling) and behind they’ll unwind.

This should make it harder for Pol II to do its work (or for enzymes which copy DNA to more DNA). This is where the various topoisomerase come in. They cut DNA allowing supercoils to unwind. They remain attached to the DNA they cut so that the DNA can be put back together. There are basically two classes of topoisomerase — Type I topoisomerase cuts one strand, leaving the other intact, type II cuts both.

Who would have thought that type II topoisomerase would be involved in the day to day function of our brain.

Neurons are extended things, with information flowing from dendrites on one side of the cell body to much longer axons on the other. The flow involves depolarization of the cell body as impulses travel toward the axon. We know that certain genes are turned on by this activity (e.g. the DNA coding for the protein is transcribed into mRNA which is translated into protein by the ribosome). They are called activity dependent genes.

This is where [ Cell vol. 1496 – 1498, 1592 – 1605 ’15 ] comes in. Prior to neuronal activity, when activity dependent genes are expressed at low levels, the genes still show the hallmarks of highly expressed genes (e.g. binding by transcription factors and RNA polymerase II, Histone H3 trimethylation of lysine #4 {H3K4Me3 } at promoters).

This work shows that such genes are highly negatively supercoiled (see above) preventing RNA polymerase II (Pol II) from extending into the gene body. On depolarization of the cell body in some way Topoisomerase IIB is activated, leading to double strand breaks (dsbs) within promoters allowing the DNA to unwind and Pol II to productively elongate through gene bodies.

There is evidence that neuronal stimulation leads to dsbs ( Nature NeuroScience vol. 16 pp. 613 – 621 ’13 ) throughout the transcription of immediate early genes (e.g. genes turned on by neural activity). The evidence is that there is phosphorylation of serine #139 on histone variant H2AX (gammaH2AX) which is a chromatin mark deposited on adjacent histones by the DNA damage response pathway immediately after DSBs are found.

Etoposide (a topoisomerase inhibitor) traps the enzyme in a state where it remains bound to the DNA of the dsb. On etoposide Rx, there is an increase in activity dependent genes (Fos, FosB, Npas4). Inhibition of topiosomerase IIB (the most prevalent topoisomerase in neurons) by RNA interference (RNAi) leads to blunted activity dependent induction of these genes. This implies that DNA cutting by topoisomerase IIB is required for gene activation in response to neuronal activity.  Other evidence is that knocking down topoisomerase  using RNA interference (RNAi) stops activity dependent gene transcription.

Further supporting this idea, the authors induced dsbs at promoters of activity dependent genes (Fos, fosB, Npas4) using the CRISPR system. A significant increase in transcription was found when the Fos promoter was targeted.

I frankly find this incredible. Double strand breaks are considered bad things for good reason and the cell mounts huge redundant machines to repair them, yet apparently neurons, the longest lived cells in our bodies are doing this day in and day out. The work is so fantastic that it needs to be replicated.

Social Note: Nick Cozzarelli is one of the reasons Princeton was such a great institution back in the 50s (and hopefully still is). Nick’s father was an immigrant shoemaker living in Jersey City, N. J. Princeton recognized his talent, took him in, allowing him to work his way through on scholarship, waiting tables in commons, etc. etc. He obtained a PhD in biochemistry from Harvard and later became a prof at Berkeley, where he edited the Proceedings of the National Academy of Sciences USA for 10 years. He passed away far too soon of Burkitt’s lymphoma in his late 60s. We were friends as undergraduates and in grad school.

I can only wonder what Nick would say about the latest twists of the topoisomerase story

The uses of disorder

There was a lot of shock and awe about a report showing how seemingly minor changes in an aliphatic group on benzene led to markedly different conformations in its protein target (lysozyme from bacteriophage T4)

Our noses are being rubbed in just how floppy proteins are, in contrast to the first glimpses of protein structure obtained by Xray crystallography. Back then we knew so little about proteins, that seeing all the atoms laid out in alpha helices and beta sheets was incredibly compelling. We talked about the structure of a protein rather than a structure. Even back then, with hemoglobin (one of the first solved proteins) it was obvious that proteins had to have more than one structure. The porphyrin ring in heme that oxygen binds to is buried deep in hemoglobin, and the initial structure had to move in some way to allow oxygen to find its way in (because the initial structure showed no obvious channel for oxygen). So hemoglobin had to breathe.

We now know that many proteins have intrinsically disordered segments. Amazingly, the most recent estimate I could find in my notes (or in Wikipedia) is this — It is estimated that over 30% of eukaryotic proteins have stretches of over 30 amino acids that are intrinsically disordered [ J. Mol. Biol. vol. 337 pp. 635 – 645 ’04 ]. Does anyone out there know of more recent data?

We’re a lot smarter now — here’s a comment on Derek’s post — “I have always thought crystal structures of proteins/enzymes are more a guide than actually useful. You are crystallizing a protein first-proteins don’t pack like that in vivo. Then you are settling on the conformation that freezes out- is this the lowest energy form? Then you are ignoring hte fact that these are highly dynamic structures that are constantly moving, sliding, shaking, adjusting. Then if you put a ligand in there you get the lowest energy form-which is what it would look like after reaction and before ligand dissociation- this is quite different from what it can look like at other stages of the reaction.”

Here is an interesting example of the uses of protein disorder going on right now in just about every neuron in your body. Most neurons have long processes, far too long for diffusion to move a needed protein to their ends. For that purpose we have microtubules (aka neurotubules in neurons) stretching the length of the processes, onto which two types of motors attach (dyneins which moves things to negative end of the microtubule and kinesins which move things to the positive end).

The microtubule is built from a heterodimer of two proteins (alpha and beta tubulin). Each contains about 450 amino acids and forms a globule 40 Angstroms (4 nanoMeters) in diameter. The heterodimers pack end to end to form a protofilament. 13 protofilaments line up side by side to form the microtubule, a hollow structure about 250 Angstroms in diameter. In cells microtubules are 1 to 10 microns long, but in nerve process they can be ‘up to’ 100 microns in length. Even at 1 micron (1,000 nanoMeters) that’s 13 * 250 heterodimers in a microtubule.

Any protein structure this important has a lot of modifications imposed on it to alter structure and function. Examples include phosphorylation and the addition of glutamic acid chains (polyglutamylation). The carboxy terminal tails of alpha and beta tubulin are flexible and stick out from the tubulin rod (which is why they aren’t seen on Xray crystallography). The carboxy terminal tail is the site of post-translational glutamylation. The enzyme polyglutamylating the carboxy terminal tail of beta tubular is TTLL7 (you don’t want to know what the acronym stands for). It binds to the alpha/beta tubular heterodimer by an intrinsically disordered region of its own (becoming structured in the process), then it binds to the intrinsically disordered carboxyl terminal tails, structuring them and modifying them. It’s basically a mating dance. There is a precedent for this — see

So disordered regions of proteins although structureless are far from functionless


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