Tag Archives: insulin

Life at the edge of foldability

Insulin has contains 51 amino acids, split into two chains held together by disulfide bonds. The two chains come from a single gene and a single mRNA. Clearly a lot of processing is required. There is an A chain containing 21 amino acids and a B chain containing 30.

Mutations of phenylalanine at position #24 on the B chain results in MODY (Maturity Onset Diabetes of the Young) in which not enough insulin is made. Every known vertebrate insulin contains phenylalanine at B#24.

A fascinating paper [ Proc. Natl. Acad. Sci. vol. 117 pp. 29618 – 29628 ’20 ] explains why.

The reason is that having phenylalanine at B#24 appears to be crucial in folding of the insulin into its final form. We have 20 amino acids, and changing phenylalanine at B#24 to any of the other 19 amino acids results in poor insulin production.

Well we can now make any protein this long by automated peptide synthesis. Which amino acid is closest in shape and structure to phenylalanine? Tyrosine clearly. So the authors made insulin with tyrosine at B#24 (outside the cell).

Guess what — insulin synthesized (outside the cells) B#24 tyrosine bound to the insulin receptor almost as well (20 fold less well), but in terms of biological activity there was no difference. The 3 dimensional structures of B#24 tyrosine and B#24 phenylalanine were nearly identical.

The problem was in the processing of the parent protein (proinsulin) with something other than B#24 phenylalanine to insulin, which involves breaking the chain and forming 3 disulfide bonds between 6 cysteines. So it isn’t structures which evolution is conserving by B#24 phenylalanine but the ability to be processed and folded correctly.

Time to let the authors speak for themselves “Our results suggest that sequences required for insulin’s bioactivity (similar in all vertebrates) are frozen at the edge of nonfoldability.”



An incredible paper which may turn diabetes treatment on its head

First off, why should a neurologist be interested in diabetes?  That’s for endocrinologists isn’t it?  Not really.  One of the most serious aspects of diabetes is its acceleration of vascular disease (atherosclerosis), with an increased risk of heart attack and stroke (which is where the neurologist comes in).  So you have a diabetic who has survived a stroke. You want to prevent the next one, and you know that they’re in a much higher risk group for another stroke (1) because they just had one (2) because they’re more likely to have another because of their diabetes.  Also vascular disease makes any neurologic problem worse, dementia, neuropathy you name it.

So I’ve always tried to stay current on diabetes.  Which brings me to Proc. Natl. Acad. Sci. vol. 109 pp. 14972 – 14976 ’12 (11 September issue).  You’d think that nearly a century after Banting and Best’s discovery of Insulin that there were no new wrinkles left.  Not so.

Now for a bit of anatomy and physiology.  In a very simplistic way, you can regard diabetes as not enough insulin.  Insulin lowers blood sugar and is needed after you eat.   It is made in the pancreas, an organ which secretes digestive enzymes into your gastrointestinal tract.  It also secretes insulin, but into the blood rather than the gut.  Insulin is made in relatively small collections of cells (1,000 – 10,0000) called islets dispersed through out the pancreas.  They are .1 – .2 milliMeters in diameter, and even though we have over 1,000,000 of them, they constitute 2% of the mass of the pancreas. 85% of the islet cells make insulin (the beta cells), another cell type (the alpha cells) make another protein (glucagon), also secreted into the blood, making it a hormone by definition.  Glucagon raises blood sugar, by causing the liver to make glucose and then pump it out.

Streptozotocin (a glucose derivative) made by soil bacteria, has the nasty property of selectively killing pancreatic beta cells, leaving everything else alone.  Naturally experimentalists love it and have used it to produce severe diabetes (fatal if untreated) in lab animals, and then try new treatments to see if they can come up with something better.

Amazingly, here’s one they didn’t try until now.  Given our present tools, you can pretty much knock out any gene you want in a mouse embryonic stem cell, implant it in another mouse, and (after a lot of failure), make a strain of mice lacking that particular gene (these are what’s called knockout mice).  What they knocked out was not glucagon (which has effects all over the body, including the brain), but the receptor for glucagon.  So even though there’s plenty of glucagon around, lacking the receptor cells don’t respond to it.  It’s like hearing a language you don’t know — the sound is there all right but you can’t internalize it and react to what’s being said.

It had been known for a while that giving streptozotocin to mice lacking the glucagon receptor doesn’t produce diabetes (or much else).  The finding as been treated with a good deal of skepticism, being blamed on other factors.  The authors of the present paper, found a way to put the glucagon receptor back into the liver (using a virus).  When they did this to a knockout mouse living happily despite being treated with strpetozotocin, their glucose shot up.  The virus didn’t hang around forever, glucagon receptor levels in the liver dropped and blood sugar dropped along with it.   Remember that a normal mouse dies of diabetic complications fairly quickly after receiving streptozotocin.

You couldn’t ask for much better proof, and a new way to treat diabetes may have been found.  Amazing.

As in all of medicine there are caveats.  The glucagon receptor is found in other organs, heart and brain among them, so blocking the action of glucagon this way  may have many other effects.