Chimeras and Insulin

When I first explained bioengineering to my husband, he asked me to make him a miniature winged bison. He’s asked for one for years, even though my work has nothing to do with making new breeds of bison. And, oh yeah, it’s impossible. But as my husband would say: that’s quitter’s talk. What good is a degree in bioengineering if it can’t be used to cook up cute little creatures?

A miniature winged bison is an example of a chimera, a mythical beast that’s a mix of two species. Like a griffin or a pegasus. In fact, wings are pretty common among chimera. Because how do you make a lion more bad-ass? Add wings.

Pure fantasy, right? Not entirely. Science has a way of making fantasy into reality. And the story of real chimeras begins with today’s gene of interest: insulin (INS).

Insulin is a hormone that regulates blood glucose levels. When blood glucose levels are too high (i.e. after a meal) insulin stimulates storage of glucose in the liver and muscle tissue so that it can be used later. We know it’s an incredibly important protein because it’s highly conserved across different species, meaning human insulin is nearly identical to that of cows, pigs, even fish [1]. In fact, pig insulin varies from human insulin by only one amino acid. In evolution, useful proteins hardly change, even while the organism evolves around it.

A type of venomous sea snail uses this similarity to hunt fish [2]. Insulin toxin in the snail venom mimics fish insulin. Once injected, the insulin toxin lowers blood glucose levels in the fish, making it sluggish (Snail. Slug-ish. See what I did there?).

This similarity also makes treating diabetes mellitus with insulin injection much easier. In the old days, insulin was harvested from animal sources [1]. Take the pancreas of a cow, a pig, or a fish, grind it up, extract the insulin, and inject. This worked ok, except for the odd allergic reaction caused by left over bits of animal pancreas in the injection.

What we needed was a source of easily-purified human insulin to replace animal-derived insulin. And the answer was bacteria! Well, bacteria and recombinant DNA technology.

In addition to their main store of DNA, bacteria have small loops of DNA called plasmids. Each plasmid contains the DNA for at least one gene. Scientists pasted the gene for human insulin (INS) into a plasmid and inserted it into bacteria [3]. Because DNA works the same way regardless of species, these bacteria read the gene as if it were bacterial DNA and produced human insulin.  They became tiny insulin producing factories. Human-bacteria hybrids. Chimeras!

The human insulin gene (yellow) is inserted into a bacterial plasmid.  This plasmid is inserted into bacteria to create a new strain that produces human insulin.  Insulin is then harvested and packaged for treatment of diabetes.

Today all insulin used in the US is made by recombinant DNA technology [4]. And this technique has been used to create other chimeras, or genetically modified organisms (GMO’s). Fish that glow in the dark from the fluorescent gene of a jellyfish [5]. Corn that produces an insecticide from a bacterial gene [6]. This technology gives us the power to mold the evolution to serve our needs and our fancies. Kind of a scary thought.

We’re still a long way off from ‘improving’ animals by adding wings, but the more we learn about genetic engineering, theoretically, the more complex Franken-creatures we can create. I guess that means I should get to work on that bison.

(teacup for scale)

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References

[1]  Insulin-www.idf.org
[2] “Deadly sea snail uses weaponised insulin to make its prey sluggish”. The Guardian. 19 January 2015.
[3] Watson, James D. (2007). Recombinant DNA: Genes and Genomes: A Short Course. San Francisco: W.H. Freeman. ISBN 0-7167-2866-4.
[4] “Why is US Insulin So Expensive?”. NPR. 1 April 2015.
[5] Glofish web page
[6] Mendelsohn, M.; Kough, J.; Vaituzis, Z.; Matthews, K. (2003). “Are Bt crops safe?”. Nature Biotechnology 21 (9): 1003–1009. PMID 12949561.

Sonic Hedgehog and the Two-Faced Pig

For my first post, I’ve chosen one of my favorite genes: sonic hedgehog (abbreviated Shh or SHH).  I first learned about this gene in grad school.  One of my professors was interested in embryonic development, the study of how complex organisms develop from an amorphous ball of cells after fertilization.  This professor decorated the lab with ‘interesting’ biological specimens preserved in jars of formaldehyde, a la a ‘Cabinet of Curiosities’ from the 1800’s.  One of those specimens was Ditto, the two-faced pig.

But before I explain Ditto and his connection to sonic hedgehog, a quick note about the gene.  The name of this gene is actually a story in itself.  Shh is part of a set of genes discovered by a single lab.  They named each gene after a different variety of hedgehog (indian hedgehog, desert hedgehog, etc.).  But one of the genes, named after SEGA’s video game character, was later found to play a pivotal role in mammalian biology [1][2].  Since then, sonic hedgehog has be at the center of a controversy around the naming convention for genes.  At first, gene names may seem inconsequential, but when creatively-named genes are associated with a debilitating disorder, it might make a serious conversation with your doctor sound slightly insincere.  Other notorious examples include headcase, lunatic fringe, and smurf.

Nevertheless, the rules for naming a new gene remain akin to calling “dibs”.  So, for now, the wackiness of our genome is at the mercy of a few lucky postdocs and grad students.  What could go wrong?

Sonic hedgehog is one of the best examples of what’s called a morphogen: a protein that controls the patterning of tissues through a concentration gradient.  When a group of cells express a morphogen, they act as a point of reference, communicating to the rest of the embryo, for example, where the center is, or which end is heads or tails.

Control of hand patterning is a great example of Shh acting as a morphogen. As the hand develops, one area expresses high levels of Shh. This area marks where the pinky should develop.  Moving away from the pinky, Shh concentration drops directing development of the ring finger, middle finger, pointer, and finally, in area with the lowest concentration of Shh, the thumb [3].  So without sonic hedgehog, I’d guess we’d be all thumbs (Hahaha… why aren’t you laughing?).

The local concentration of sonic hedgehog determines where each finger will develop from the limb bud.

But let’s get back to that two-faced pig.  Sonic hedgehog controls patterning in the face, specifically the width of facial features.  As Shh expression increases, the width of the face increases.  But if expression of sonic hedgehog keeps increasing, something strange happens.  As the face widens, one mouth becomes two.  The nose splits into two.  A third eye appears.  And in extreme cases, organisms with overexpression of sonic hedgehog develop two complete faces, mirror images of one another.  Ditto had an overexpression of sonic hedgehog which resulted in the development of two snouts, and three eyes, although one of his snouts and his third eye weren’t functional [4][5].

So widening the face eventually results in the development of another face.  That means the pattern of the face is retained even when expression of Shh malfunctions.  This thought always leaves me in awe of the complexity of our genes.  It seems hard enough to create a face that’s recognizably human, but somehow our genes are so exact that the final product bears an uncanny resembles to its parents.

“He has your eye.”

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References

[1] Marigo V, Roberts DJ, Lee SM, Tsukurov O, Levi T, Gastier JM, Epstein DJ, Gilbert DJ, Copeland NG, Seidman CE (July 1995). “Cloning, expression, and chromosomal location of SHH and IHH: two human homologues of the Drosophila segment polarity gene hedgehog”. Genomics 28 (1): 44–51. PMID 7590746.
[2] “A Gene Named Sonic”. The New York Times. 1994-01-11.
[3] Harfe, Brian D.; Scherz, Paul J.; Nissim, Sahar; Tian, Hua; McMahon, Andrew P.; Tabin, Clifford J. (2004). “Evidence for an Expansion-Based Temporal Shh Gradient in Specifying Vertebrate Digit Identities”. Cell 118 (4): 517–528. PMID 15315763.
[4] Armand Marie Leroi (2005). Mutants: on the form, varieties and errors of the human body. New York, N.Y: Harper Perennial. ISBN 0-00-653164-4.
[5] “Talk about pigheaded… this one’s got two: Piglet fights for survival after being born with rare deformity”. Daily Mail. 2013-04-12.