Can gut bacteria make zombies?



L. rhamnosus: Not a zombie-
producing bacteria...yet.



If T. gondii weren't enough to worry about, now there is evidence that bacteria in our gut can influence brain function.  Lactobacillus and other probiotic organisms have long been speculated to have beneficial in vivo effects, and are most commonly known for improving the health of the digestive tract.  L. acidophilus is probably the most widely known, since it is used to make yogurt, but there are many other types of lactobacilli with alleged health benefits ranging from lowered cholesterol to improved mood.  Some of these benefits are speculative, and for many years any benefit (such as improved gut health) was thought to be due to local effects or secreted chemicals.  However, the impact of these bacterial colonies may be much more far-reaching.

A Proceeding of the National Academy of Sciences (PNAS) paper published by Heujtz et al. last January (abstract) showed that microbial colonization in the mouse gut led to the activation of signaling pathways involved with motor control and emotional response.  This was the latest in a growing body of literature that suggests gut bacteria could influence how we think and act.  Now, in the August edition of PNAS, Bravo and colleagues take this one step further (abstract).  They show that Lactobacillus rhamnosus can directly influence the expression of GABA receptors in the brain.  GABA is the primary neurotransmitter for regulating many physiological and psychological activities in humans.  For example, caffeine inhibits GABA and results in an overall increase in neurotransmitter activity.  In contrast, alcohol and sedatives tend to increase GABA activity, leading to reduced neurotransmitter activity. Bravo et al. showed that feeding mice L. rhamnosus reduced GABA expression in some areas of the brain, while increasing it in others.  The overall effect was to make the mice more calm.  Here in the Dark Lab, we would test this by subjecting the rats to endless episodes of Jersey Shore, Barney and Friends, and The Jonas Brothers, and then asking how long it takes before they fall into convulsions.  Bravo measured stress-induced hyperthermia (rise in core body temperature from stress) after a battery of different tests, including  forced swimming and mazes (less barbaric than listening to the Barney jingle, but presumably effective at producing stress) and showed that the L. rhamnosus-fed animals exhibited less stress during these activities.

The final point of the paper was what I found most interesting.  Some of the animals had the vagus nerve cut prior to the start of the experiment.  This nerve is a direct link between the gut and the brain and is responsible for transmitting signals about hunger and satiation.  In these animals,there was absolutely no effect from consuming L. rhamnosus.  No changes in GABA expression and no behavior changes versus the control animals.  This means that the probiotic bacteria that colonize in the gut might actually use this nerve to signal directly to the brain.  Although these bacteria appear to provide a health benefit, I can certainly imagine other strains that are more nefarious.  Yeah, I'm talking about zombies again.  This phenomenon is not all that different from what is seen in T. gondii or the zombie ants... microbial agents that manipulate the brain function of the host.  If a probiotic strain can mimic the effect of caffeine-overstimulation or, even worse, caffeine-deprivation, then that would be a whole new kind of scary.  Anyone who has seen me before that first cup of coffee knows what I'm talking about... it's bad enough to give a zombie nightmares!

Zombie rats are horny!

You may remember a very early post on zombies, where I talked about a parasite known as Toxoplasma gondii.  I just read a new article published in PLoS that is really mind-blowing.  It also should fall in the category of science known as what-kind-of-PhD-do-I-need-to-study-this-shit.  Very interesting, very strange, and very cool.  Here's the abstract to check out for yourself (abstract).  As you know, rats that are infected with T. gondii lose their fear of cats.  This is important for the parasite because part of its life-cycle occurs in the gut of the cat.  This report takes that one step further.  House et al. show that as part of this shift in rat behavior, the rats are actually attracted to cat urine.  Not attracted like "this smells like roses" but rather "yowsa, hot babes" (rats do say 'yowsa'... I've heard them).  Yes, the parasite changes the way the rat brain responds to the smell of cat urine.  Neurons in the ventromedial hypothalmus, dorsomedial part (mercifully abbreviated as VMHdm), which normally are active in response to fear, are silent when infected rats are exposed to urine.  In contrast, the area in the brain the DOES light up is the posterodorsal medial amygdala.  These are the same neurons that light up when rats watch porn (or are exposed to estrous females, but somehow that doesn't sound as sexy).  Their conclusion is that T. gondii makes rats think that if they follow the cat pee, they will get laid.  Although that may work in some dive bars, for these rats it ultimately leads to just a single dinner date.

These results are also consistent with other findings that show an increased level of dopamine in infected rats.  Dopamine is, of course, the primary molecule in behavioral reward so it seems reasonable that this pathway would also be effective at shifting rat behaviors.  They cite a 2006 paper by Webster et al. (abstract) where it was shown that dopamine receptor antagonists prevent the attraction to cat urine.  Could this receptor be the first therapeutic target for treating zombies?  I'll have to propose that at the next New Target meeting. 

The ability of T. gondii to alter brain function and behavioral response is incredibly cool and a little bit scary.  Why scary?  Because it is estimated that at least one third of the human population has been exposed to the zombie-inducing T. gondii (and no, it's not just pop stars and politicians).  Even subtle changes in behavior on that scale can lead to massive changes in society.  Although it is unclear how well this study translates to humans, it does suggest that in the event of a zombie apocalypse, it's probably not a good idea to hide out in the girls' bathroom.

Highlights from the Protein Society Symposium

A week ago, I attended the 25th annual symposium of the Protein Society in Boston.  I've gone to this meeting three times and this was the best yet.  Very, very cool stuff.  I also got to see a lot of old friends from grad school, including my graduate advisor.  It was great to hear him talk fondly about the 'good ol' days' (ie, when I was his student) as I distinctly remember them being less fond and more frantic.  I think finishing a PhD thesis is the mental equivalent of giving birth to an elephant while running a marathon, but for several obvious reasons I will never be able to test the hypothesis.  I also met some cool new people.  I talked politics with a girl from Poland in a loud Irish pub and met another girl who is some kind of flute prodigy from a well-known coffee empire.  At the reception, I met a guy from, well, some European country who had done a postdoc in San Diego and I knew many of the trails he had hiked.  We tried watching the Red Sox game from the 50th floor of the Prudential building... great view but when the outfielders look like fleas on a green dog, it's really hard to see what is happening.  We kind of made it up as we went along and since the Polish chick didn't know the game, it was all good.  He still owes me pictures of Fenway.  The poster session was crazy, with two overlapping sessions and the very friendly (but bored) vendor who bribed me with chocolate every time I passed her booth.  I was also a poster judge this year, so I missed most of that session, but tracked the presenters down later to ask questions.  It's a little awkward at the coffee breaks, since everyone is staring at the nametags trying to find people they want to talk to.  I would try and catch a glimpse of their badge over the rim of my coffee and hope they were not offended when I simply walked away.  It's a very unusual hierarchy at conferences.  There is absolutely no guessing about where you stand in the pecking order.

Anyhow, I can't talk about the things I found most exciting because I was there for work, and work stuff has to stay off the radar.  However, let me briefly describe two (not work-related) things that were pretty cool.  One was a talk by Della David at UCSF on protein aggregation as a part of aging.  I don't know a lot about this field, but one of her early slides really caught my attention.  She was discussing the role of protein "aging" in inducing aggregation using C. elegans (a worm) as a model.  As the worm aged, she showed that the concentration of over 400 different proteins increased in the insoluble fraction.  In simpler terms, if you take all of the proteins out of the worm, many of them are soluble but some fraction are in an aggregated form, which is not soluble.  Although the total amount of protein seemed fairly constant with age, the proportion of aggregated protein increased and seemed to disrupt the natural process of homeostasis.  Then came the kicker... to show that this was an active process (that is, controlled by a cellular system) she used a C. elegans that had been engineered to have a specific mutation in the Daf-2 receptor.  These worms had twice the life span of a normal worm.  Whoa!  Sign me up for that mutation! Imagine living 160 years!  It turns out that the fraction of aggregates is independent of the lifespan, suggesting that the process is controlled.  Two things here... Daf2, which is part of the insulin/IGF-1 signaling pathway, can regulate lifespan (possibly related to the observation that mice that eat less live longer?) and that protein aggregation as a result of aging could also be controlled.  Here is a review on insulin/IGF-1 signaling in aging (abstract) and here is David's recent open-access paper covering some of this story (PLOS paper).  Listen folks, please hurry up with this important work... I'm not getting any younger.

The other talk I liked was by Ken Dill (a long-time favorite of mine and also from UCSF).  I'm used to him talking about transfer free energies and lattice models for proteins but this time he was talking about the stability of the proteome.  He (and others) have shown pretty convincingly that on a macro scale, protein stability is roughly dependent on the length of the protein.  (Seems simple but it has taken decades to model it in a way that makes physical sense).  Armed with this model, he determined the stability of the entire proteome and found that it is only marginally stable.  Over 500 proteins have stabilities less than 3 kcal/mol, which means they are barely folded and functional.  The implication of this result is that even slight increases in temperature can cause many of these proteins to unfold.  The resulting denaturation catastrophe overwhelms the cell and causes cell death.  This is the most plausible explanation yet for why slight increases in temperatures cause such problems (even for humans, an increase in body temp of 7-8 degrees can be fatal).  I asked him about the proteome of thermophilic bacteria and whether it might explain their ability to survive extreme temperatures and he said he is working on that now.  I'm guessing that might explain some of the adaptability, although the detailed mechanism is still a mystery.  For you DIYbio people out there, this model provides a pretty simple way to do this type of analysis yourself. The simplicity of the model, and the fact that minor ensemble changes can be magnified into major improvements for the organism tell me that life might be lurking everywhere there is an energy gradient (I'm looking at you, Titan).  On the flip side, it shows how sensitive life can be to slight changes in the environment. Here are the links to the articles (proteome stability and Dill's model)

  Dill also started off with a joke: "There are three kinds of mathematicians... those that can count, and those that can't."  Nothing like a geek joke to start off a talk... but hopefully his material will be better in San Diego next year.

GFP Beagles: Disease model or designer pet?

A recent paper in the journal genesis describes the production of transgenic beagles that glow when exposed to UV light (abstract).  The eGFP transgene was introduced into a beagle embryo using a similar (but much improved) technique that was used to clone Dolly the sheep.  Making a puppy that glows is not really new, as it has been demonstrated by the same group in 2009 (abstract) and others (in dogs, as well as other small mammals) but there are two cool things about the recent work.  First of all, they put the transgene under a promotor.  This means that the puppy does not glow green until the gene gets switched on, in this case by the drug doxycycline.  Feeding the dog low levels of doxycycline induced the expression of the GFP protein (green glow) and the effect could be turned off simply by removing the drug from the dog's diet.  This is pretty easy to do in small animals (like rats and mice) but pretty complicated in large animals. 

  In a separate article, also in genesis, they report the transmission of the transgene to offspring (abstract).  This was also interesting, as the GFP-containing females were totally fertile and had normal pregnancies and births.  The dads were wild type beagles so roughly 50% of the offspring carried the transgene.  This is consistent with stable germline transmission.  This result suggests that we are really not far off from having designer dogs.  The initial cloning will be hard (efficiency is still in the 1-5% range) but if the transgene is stable, simple breeding techniques should be enough to make zillions of glowing puppies!  Although this is a crude example, one can envision a vastly different world where hair color, facial and body features, and perhaps even personality traits could be engineered.  Genetic defects and disease determinants could be engineered out.  One could build the perfect dog.  You did realize I was still talking about dogs... right?