Eating Nothing is the Fountain of Youth???

I know this blog is about innate immunity, but since I work in an aging lab, it's something that mildly interests me. I ran across this paper where it was apparently found that by removing the food course of c. elegans, a round worm, they will actually have increased longevity.

I guess I should start by saying that c. elegans is a common model organism. It offers many of the same advantages that Drosophila do having a sequenced genome, being easy to manipulate and relatively short lived. From what I've read, since I actually know very little about c. elegans, they are cultured on agar plates, and are fed UV-killed bacterial culture. I should also mention that it is a well known phenomenon that putting animals on a calorically restricted diet extends lifespan. What is new here is that starving the animals completely also extends lifespan. I dont know about you, but I was skeptical.

Similar to the experiment in my last post, where exposing flies to bacteria extended lifespan only in a specific time window, starving worms extends lifespan optimally at certain ages. When worms are put on a normal diet, they live to a maximum age of about 42 days. However, when larvae are starved, the maximum lifespan increases a couple of days and when 2 day old adults are starved, the effect is maximised.

The next part of the paper was starving mutants to try and dissect the pathway through which lifespan is increased. The first mutant tested was a little off topic, but it was a long-lived mutant, and when it was starved, no increase in lifespan was seen. A series of other mutants were then tested, and it was found that the extension of lifespan during starvation is independent of the Insulin-IGF-signalling pathway, which is considered responsible for the effects of conventional caloric restriction.

Although the findings of this paper are pretty cool, I'm still a little skeptical as to how they were achieved. Their materials and methods are pretty skinny. I'd be interested to see if another lab can replicate these results.

Oral Infection of Drosophila by Bacteria

The long standing tradition in immunity research in Drosophila has been to inject a dose of bacteria into each individual fly of the experiment using a needle dipped in bacteria. While this method is relatively easy, it leaves much to be desired. Our researrch group has found that this method introduced a highly variable dose of bacteria into each fly. As a remedy to this scenario, a french research group has developed a protocol using an entomopathogenic Pseudomonas bacteria to infect flies orally.

Because of the mechanical and molecular defense mechanisms associated with the Drosophila gut, most pathogens cannot enter through simple ingestion. This fact led to the proliferation of the injection techniques in Drosophila immunity research. Recently however, three Erwinia carotovora bacterial species have been identified which are capable of infecting flies through ingestion. The eperiments were carried out on Drosophila larvae, where 200 larvae were fed an overnight culture of the bacteria. Probably the most useful part of this finding is that expression patterns in response to the Erwinia infection were similar to those seen in respones to infection with other gram negative bacteria. Moreover, when the amount of bacteria eaten was quantified, it was found that the deviation did not exceed that seen with standard injection protocols.

I know it probably only me, but this paper has me excited because it has the potential to make my life much much easier for my final year here.

The Trade-off Between Pathogen Resistence and Longevity

To date very little has been done to adress whether activation of the innate immune system has any effect on fitness or longevity. In a study by Libert et al., the innate immune response was consitutively activated and it was found that although there was a higher pathogen resistence in these flies they were short lived.

It was first found that the overexpression of a peptido-glycan recognition protein, PGRP, which is the first molecule in both the Imd and Toll pathways, was enough to consitutively turn on the immune response. This conferred a wide range of resistence to bacteria and fungi. It was also found, using mutants, that this resistence was dependent on the nuclear transcription factor Relish.

The next part of the experiment examined short term trade-offs to consitutive activation of the immune system in behavious such as climbing activity, fecundity and heat shock tolerance. None of these characteristics were affected in the short term. However, it was found that chronic activation, which is similar to the chronic imflammation state in elderly mammals, reduces lifespan. According to the authors, their data are consistent with the presence of a cost for persistent enhanced immunity, and more over indicate a link between immune system signaling and genes that influence longevity.

As I have found the more I read, much of the scientific discoveries, such as this one, are not surprising given even the littles amount of thought. However, it is impossible to move forward and get to the really cool stuff until basic research such as this is completed.

Fruit Fly Astronauts?

So it turns out that not just us academic types are using Drosophila these days. The shuttle Discovery astronauts, who just lifted off today, have taken with them populations of fuit flies to study the effects of weightlessness on gene expression. Studies have shown that immune systems of astronauts don't defend against germs as ferociously as they do on Earth. If you get sick in space, it might be harder to get well again. Also, astronauts bones weaken during long voyages, and without lots of exercise, their muscles atrophy. All of these side effects of space travel are rooted in gene expression.

The flies will be kept in special breeding chambers made of clear plastic, where their courtships behaviours, locomotion and flight ability can me monitored. As many as nine generations will be bred during the mission, with as many as 120 flies per generation. From each generation, a candidate sample will be frozen such that mRNA levels can be tested once the team returns to Earth, allowing the team to monitor gene expression. Perhaps most interesting, some of the flies will be submitted to centrifugation to simulate different gravitational environments, such as that of the moon or Mars, to see if prolonged exposure to these environments will affect gene expression.

For further reading, check out the article here.

Does the Immune System Age?

It is fairly well known that increased activation of the innate immune system is a common feature of aging animals. This paper by some friends of ours looks to determine whether the increase in the activation of the immune system in flies is a functional one, or just a by product of aging.

This experiment is actually very simple, luckily for us, a short one! This paper found that while in a non-infected state, the antimicrobial peptides (the end product of immune system activation) older flies express more than younger flies, which was expected. The interesting part is when flies of varying age were infected, it as found that older flies, while having a higher basal level of immune activation, could not induce the immune system to as high an extent as young flies in response to infection. Moreover, it took older flies longer to turn off their immune response than it did for young flies. The authors hypothesize that this decreased capacity to induce the immune system in older flies leads to persistent infection, which could be why we see the immune system always turned on a little bit in old flies.

Well, believe it or not, that's all there is to the paper. I told you it was short!

Viral Immune Response in Drosophila

A common question until late 2005 was whether viruses infect insects, and if they do, how do the insects defend against the viruses. This question was answered by a French research group, where it was found that the Jak-stat pathway (on the right hand most side of the diagram from the post titles "Drosophila Immunity) had conserved functions in the Drosophila anti-viral response.

The virus used was the Drosophila C virus, DCV, which is an RNA virus in the same family as polio and foot-and-mouth disease. Depending on the dose, injected DCV can kill a fly within 4-10 days. Interstingly, ingestion of the virus does not make the flies sick. As such, the natural of DCV infection of Drosophila is unkown. Upon injection with DCV, approximately 140 genes are upregulated by at least two fold. Ingestion of DCV leads to the upregulation of only 10 genes. Of the 140 genes induced, only one third are common to those induced during bacterial or fungal infection, indicating that the Jak-Stat pathway may defend against viruses independently of the Imd or Toll pathways. In mammals, Jak-Stat pathways are responsible for T cell activation in aquired immunity as well as viral defense. While Drosophila only have one Jak-Stat pathway, it is unsurprising that it shares homology to mammalian Jak-Stats or that it is involved in viral immunity.

That Jak-Stat pathway, however, is not solely responsible for the Drosophila viral response. Not all of the 140 genes induced upon injection with DCV are Jak-Stat dependent. Moreover, activation of genes in response to another virus, the Flock House virus, are also independent of the Jak-Stat pathway. What is puzzling is that mutant flies with a lethal allele of Hopscotch, which leads to the constitutive activation of the Jak-Stat pathway, there was no activation of the DCV induced genes, but other targets of the pathway, not induced by DCV were activated. The aurthors hypothesize that Jak-Stat pathway does not act alone, but rather has to cooperate with another pathway(s) to recognize and defend against DCV.

This, while very interesting, goes to show that even though the Drosophila genome has been sequenced, there is still much we dont know in terms of the existence and function of genes.

Hot New Field

Innate immunity has become a very hot topic in the Drosophila research community, especially in regards to aging research. Recently, it was found that exposing young fruit flies to bacteria actually extends lifespan, and if flies are kept in axenic conditions to for their entire lifespan, they actually live shorter. The paper can be found here.

Flies were initially raised in axenic conditions were short lived, however, lifespan can be returned to normal or extended depending on when they are exposed to bacteria. Non-axenically rasied embryos transferred to sterile food at various stages of adult life indicate that the first two days of adult life are hugely important in extending mean lifespan. Similarly, axenically rasied embryos transferred to non-sterile food before two days old show the greatest increase in mean lifespan.

To test the effect of genetic makeup in the flies, similar experiments were conducted on an array of long lived mutants. A spectrum of results was seen, from no lifespan extension, to large lifespan extension, indicating that it is possible to dissect the response using mutants.

When these results are discussed in light of similar findings in other model organisms, it is suggested by the authors that a likely reason for the need for bacteria early in life is to aide in gut developement, and the establishment of an internal gut flora.

Drosophila Immunity

Ever wonder how your body prevents your cuts from getting infected or how it fights off that cold virus? Well, the answer, at least in part, is the innate immune system. Your innate immune system is responsible for detecting intruders and fighting them off, regardless of what kind of intruder it is. One of the things that differentiates us from insects is that we have another layer of immunity called adaptive immunity, which remembers previous intruders and caters a response that is specific to each one. Because insects lack adaptive immunity, a layer of complication is removed and it is much easier to study their immunity. Luckily for us, innate immunity in insects such as Drosophila and innate immunity in humans is almost identical, which is why we can use Drosophila as a model organism.

Drosophila immunity conisists of two protein cascade pathways, which both culminate in the expression of antimicrobial peptides. One of the pathways, called the Imd pathway, is reponsible for fighting off gram negative bacterial infection. The other pathway, the Toll pathway, is responsible for fighting off gram positve bacteria and fungi. Below is an image of the Drosophila immune pathways taken from the Deveal et al paper I have linked in my first post. The diagram beautifully illustrates the complexity and elegance of the immune pathways. To this point, even though much of the pathways has been deduced, their complete functions remain to be seen. This field is just coming into its own.

What is innate immunity?

So what is innate immunity I bet you're asking? Well, it turns out that's a great quesiton. Innate immunity is the branch of the immune system that helps us fights off foreign invaders without having to have previously seen the pathogens. It is non-specific. The other side of the coin is the adaptive immune system which remember pathogens that have previously infected us, and can mount a very quick response specific to the pathogen. I study the innate branch of immunity and how it is intertwined into the aging process. To do so, I use the model organism fruit fly Drosophila melanogaster. And just in case you're curious, I use fruit flies because they dont have an adaptive immune system, so it avoids the complicating interplay between the two systems.

Innate immunity was implicated in aging in 2001/2002 where several aging studies found that many immune related genes are upregulated with age (DeVeal et al. 2004). From this stemmed my project. I am infecting fruit flies with a dose of bacteria and monitoring their survival as well as how long it takes them to clear the infection. We hope to find out whether or not immune system functionality increases with age as the genes are upregulated.

Innately Immune, an Introduction to the Forgotten Sibling

When we think of immunity and all that is asosciated with it, we often think of acquired or adaptive immunity and neglect its lesser known cousin, innate immunity. By writing this blog, I not only wish to spread the awesomeness that is innate immunity, but also to keep myself and readers abreast of current developments in the field. Because of my area of research, I will focus mainly on innate immunity's role in aging.

I am in my second year of my Master's in Biology in Laurent Seroude's Aging lab at Queen's University in Kingston, Ontario, Canada. I started working for Laurent in 2004 during my fourth year project, thinking I'd be there for the year, then be on my merry way after I finished my undergrad. Little did I know, I'd be here for another 2 years. At the moment, we're working on submitting a manuscript to Aging Cell, so things are a little hectic as one might imagine.

In my next post, I'll introduce the innate immune system in more depth, with a focus on the immune system of my model organism, Drosophila Melanogaster.