Of Brain and Toxoplasma

Posted by Kasra

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This  wonderful comic had stayed in my drafts folder for a very long time and I had decided finally not to post since Calamaties of Nature had stopped publishing comics. But then again, a recent PLOS ONE paper reminded me of it again.  Toxoplasma gondii is one of my favorite parasites. It is one of the most common parasites of humans and in majority of cases lays dormant throughout life, making it one of the most successful parasites in Nature. After infection (eating poorly cooked infected meat or contact with feces of an infected cat), T. gondii escapes the gut and migrates to the brain. At first glance, it does not seem to do much over there, at least nothing drastic. But studies including current work by Ingram et al. have shown that T. gondii infection can permanently alter animal behavior, permanently meaning the behavioral change stays even after infection has been cleared.  This recent study is another example demonstrating this ability of the parasite:

Ingram et al. checked how the behavior of an infected or uninfected mouse differ when exposed to predators, in this case Bobcat urine. As you can see in the figure below (Part A), they set up a field, one part of which was spotted with Bobcat urine or Rabbit urine (controlling for effect of just urine versus predator urine). Infected, uninfected and infected with a attenuated parasite (which could successfully clear the infection) mice were let in the area to see which spots they spend more or less time at, translated to which areas they avoid and which areas they do not. As you can see in sections Bi and Bii, the mouse partly avoided the section with rabbit urine (hence the importance of controls!) but this avoidance was way stronger when exposed to Bobcat urine. Surprisingly this behavior almost vanishes in infected mice, regardless of the strain, in other words, regardless of presence or absence of parasites in the brain.

From Ingram et al. PLOS ONE, 2013

From Ingram et al. PLOS ONE, 2013

There is a body of research on possible effects of T. gondii infection on human brain and behavior and correlations to increased suicide and schizophrenia have been suggested, although not confirmed. Still we all keep in mind that correlation does not mean causation.

Could this alteration of animal behavior also have an evolutionary explanation/impact on Toxoplasma’s life cycle? In other words, is there more to this phenomenon than just parasite infects brain, brain acts strange?

There is a difference between cats (big and small, domestic and wild) and the rest of the animals when it comes to Toxoplasma. In other animals, as I mentioned above, Toxoplasma escapes the gut after ingestion and moves  to muscle  and brain tissue and just stays there. So basically the infection is kind of a dead-end. It cannot be transferred to the next host until the current host dies and gets eaten up by another one. However,cats are the main hosts for Toxoplasma. That is where the parasite goes through the sexual stage of its life cycle. Also, Toxoplasma cysts are constantly shed from an infected cat through its feces. Not a dead-end infection. Now the loss of predator in Toxo-infected mice makes sense! It helps the parasite get back to its main host where it can complete the cycle! This is a trait that would have been highly favored by natural selection whenever it evolved. Because it would strongly increase the chance of the parasite being passed around to the next host.

At the end, what the comic made me think of was, could those parasites also have parasites that would alter their behavior in their own benefit? Is this is a classic example of the Extended Phenotype idea introduced by Richard Dawkins?

Reference:

Ingram WM, Goodrich LM, Robey EA, & Eisen MB (2013). Mice Infected with Low-Virulence Strains of Toxoplasma gondii Lose Their Innate Aversion to Cat Urine, Even after Extensive Parasite Clearance. PloS one, 8 (9) PMID: 24058668

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Helminths release anti-microbial peptide-like molecules that are immunomodulatory

Posted by Kasra

In this paper, the authors have studied peptides that are found secreted by helminths Schistosoma mansonai and Fasciola hepatica and closely resemble mammalian antimicrobial peptidescathelidins to be precise.

First, the backgrounds: S. mansonai and F. hepatica are both trematodes or flukes. Their cyst form can be ingested via contaminated food or water and in the gut they hatch and can migrate to the liver. Like many other parasites, they don’t kill but cause morbidity. They are categorized as neglected tropical diseases and their infections are treatable and obviously preventable.

Anti-microbial peptides are very diverse in sequence and functions among different organisms, but there are similarities in their secondary and tertiary structures. They are produced by many multicellular organisms and their activities can range from bacteriocidal to immune modulatory. Some peptides can have both functions simultaneously,

Thivierge et al. start their paper by an interesting notion: the similarity of the innate immune response to helminths with the immune response to wounds and tissue injury. They are both anti-inflammatory and pro-Th2. Skewing the immune response from Th1 to Th2 and thus leading to less pathology as well as parasite chronicity is a recurring theme in parasite immunology. Read a full review here.

The peptides studied in this paper have similar secondary structure to alpha helical mammalian antimicrobial peptides (Cathelicidins) such as LL-37 and BMAP-28. A feature of these peptides is presence of amphi-pathic helices (hydrophobic on one side and hydrophilic on another side). This was also seen in the predicted secondary structures of peptides that were found to be secreted from S. mansonai and F. hepatica. 

Following this, the authors studied the peptides for a variety of anti-microbial and toxicity activities that are seen with mammalian peptides and found none to be present even at high doses (things such as pore formation, . However, what they did find was the peptides’ ability to modulate functions of immune cells. In this particular case they report inhibition of TNF secretion by macrophages and alteration of antibody secretion by B cells.

Similar secondary structure among mammalian and helminth peptides. (A) shows mammalian peptides with hydrophilic areas marked green and hydrophobic areas marked red. The dotted line and arrows in (B) show hydrophobic patches in the helmnith peptides. From Thivierge et al. 2013. PLoS Negl Trop Dis 7(7): e2307. doi:10.1371/journal.pntd.0002307

What the authors argue from their results is that the similar structure of these peptides to mammalian peptides and yet lack of toxicity allows them to effectively manipulate the immune response in their favor. These modulations could help in blunting of a strong Th1 response with lots of damage to the parasite as well as the host tissue and a milder response leading to parasite chronicity. Knocked-out parasites will better show the extent of importance of these peptides. Nontheless, longterm co-evolution of host and parasites has given rise to these peptides: they are nontoxic and modulatory at least in vitro.  This means plenty of potential in biotech and pharmaceutics!

Thivierge K, Cotton S, Schaefer DA, Riggs MW, To J, Lund ME, Robinson MW, Dalton JP, & Donnelly SM (2013). Cathelicidin-like Helminth Defence Molecules (HDMs): Absence of Cytotoxic, Anti-microbial and Anti-protozoan Activities Imply a Specific Adaptation to Immune Modulation. PLoS neglected tropical diseases, 7 (7) PMID: 23875042

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Bacteriophages may protect us against pathogens

Posted by Kasra

Given the extremely large amount of bacteria in our gastrointestinal track, it is not surprising to think that the gut would be also swarming with pathogens of bacteria, that is bacteriophages as well. In their recent work published in PNAS, Barr et al. take a look at what impact these particles could have on the population of bacteria in mucosal surfaces and what could it mean for us. Their work actually turns out very interesting results.

Mucosal surfaces are the body’s points of contact  with the outside. Being highly populated with bacteria, they can be suitable points of infection as well. That is why they are heavily guarded with various immune barriers and mechanisms, both innate and adaptive. Barr et al. point to a possible mechanism of protection against infection which not innate nor adaptive. They start by comparing the amounts of bacteria and bacteriophages in different mucosal and non-mucosal surfaces in various mucus producing animals. They interestingly observe that the bacteriophage to bacteria ratio in mucosal sites is way larger than those in adjacent non-mucosal sites (from average about 40fold to average about 3fold). They verify this in both invertebrates and vertebrates and thus suggest that this could be a phenomenon in all mucus-producing metazoans.

Next, they point to a previous recent study by Minot et al. who had found immunoglobulin (Ig)-like domains in the total analyzed genome of human gut viruses (or so called human gut virome).  These domains that usually act as in recognition and binding (as an antibody would do); they show that the bacteriophages actually bind to mucus through these proteins.  Barr et al. also show that presence of bacteriophages on a mucosal surface significantly reduces Escherichia coli invasion in vitro.

bacteriophage

Model for how presence of bacteriophage on the mucosal surface can help in protection against bacterial infection. From Barr et al. PNAS 2013 PMID: 23690590

This is an incredible system where the benefit of the bacteriophages and their hosts actually match. It is not clear at this point whether the animal body would have to do something other than producing mucus to keep the bacteriophages where they are or that it is just enjoying this protection more or less free of charge.

Barr JJ, Auro R, Furlan M, Whiteson KL, Erb ML, Pogliano J, Stotland A, Wolkowicz R, Cutting AS, Doran KS, Salamon P, Youle M, & Rohwer F (2013). Bacteriophage adhering to mucus provide a non-host-derived immunity. Proceedings of the National Academy of Sciences of the United States of America PMID: 23690590

Minot S, Grunberg S, Wu GD, Lewis JD, & Bushman FD (2012). Hypervariable loci in the human gut virome. Proceedings of the National Academy of Sciences of the United States of America, 109 (10), 3962-6 PMID: 22355105

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An intracellular receptor for antibodies

Posted by: Kasra

We usually consider exiting the phagolysosome and entering the cell cytoplasm to be a immune evasion mechanism for pathogens. The pathogens inside the phagolysosome can be processed and presented via MHCII to the adaptive immune system, but once free of that compartment, the pathogen could potentially ‘hide’ from the immune system, well apparently not that much! Apart from the intracellular pattern recognition receptors (NLRs), researchers have found another receptor that responds to intracellular presence of antibodies. McEwan et al. showed that if antibody coated viruses or bacteria have entered the cytosol, presence of the Fc part of the antibody can be sensed by a protein called TRIM21. This could in turn result in an inflammatory and anti-viral response by activating NF-κB and AP-1 and production of cytokines. To me, this is an excellent example that shows how the host and the pathogens have evolved together for many years becoming more and more complex through an arms race.  A newly developed strategy by one party is followed by a counter strategy by the other party.

NatImm

From Geijtenbeek TB, & Gringhuis SI (2013). An inside job for antibodies: tagging pathogens for intracellular sensing. Nature immunology, 14 (4), 309-11 PMID: 23507635

McEwan WA, Tam JC, Watkinson RE, Bidgood SR, Mallery DL, & James LC (2013). Intracellular antibody-bound pathogens stimulate immune signaling via the Fc receptor TRIM21. Nature immunology, 14 (4), 327-36 PMID: 23455675

Geijtenbeek TB, & Gringhuis SI (2013). An inside job for antibodies: tagging pathogens for intracellular sensing. Nature immunology, 14 (4), 309-11 PMID: 23507635

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Did fungi help mammals dominate Earth?

Image

From SMBC-Comics

 

Posted by Kasra

 

The Cretaceous mass extinction is one of the most exciting topics in evolutionary biology. There are always discussions on what caused the mass extinction, what happened during the extinction, what happened to all the dinosaurs, why did the mammals and birds survive, and so on. A recent article in PLoS Pathogens by Arturo Casadevall brings forward an interesting hypothesis: Fungi might have given mammals an evolutionary advantage during this period of time.

Casadevall mentions that humans and other mammals are generally resistant to fungal infections and most pathogenic fungi are in fact only opportunistic pathogens. He suggests that this is due to the mammalian control of body temperature – higher than optimal growth temperature of fungi – and evolution of powerful adaptive immunity. On the other hand, he brings forward examples of amphibians, being ectothermic, and primitive mammals, having lower body temperature, are more susceptible to fungal infections.

He next states that the Cretaceous-Tertiary (K-T) boundary included a cooling period with plenty of dust in the atmosphere and lack of sufficient sunlight. This led to a fungal bloom on the Earth and possible growth of pathogenic fungi. The hypothesis states that this higher than normal presence of fungi selected against surviving ectothermic reptiles and in favour of endothermic mammals. Thus, fungi might have indirectly helped mammals and possibly warm-blooded birds by killing off their competitors for the limited food resources.

Like any other scientific hypothesis, this one also needs to be tested. Unfortunately fossil records cannot tell us much of how common fungal infections were at the time. However, one can first look more closely if warm-bloodedness or higher body temperature does indeed aid in protection from fungal infection, given similar immune systems. Comparing today’s amphibians, reptiles and mammals can be tricky as the host-pathogen interactions may greatly differ among the groups. This could lead to conclusions that are confounded by differences in pathogenicity of fungi or power of the host immune system. Pooling data together from larger numbers host-pathogen pairs can lead to more robust conclusions. Application of heat-resistant fungi can also be beneficial for performing more controlled experiments rather than comparing natural histories. Casadevall himself suggests that climate change can promote evolution of fungi that can better survive in elevated temperatures and be threats as emerging pathogens. In this case, we would be a step in advance knowing what to expect, should these new pathogens emerge.

Casadevall A (2012). Fungi and the rise of mammals. PLoS pathogens, 8 (8) PMID: 22916007

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Science answers WHY questions: programmed cell death in unicellular parasites

Posted by Kasra

Famous geneticist Josh Haldane once famously said “I would lay down my life for two brothers or eight cousins”.

Programmed cell death, otherwise known as apoptosis is a well justified procedure in multicellular organisms. All cells within a multicellular organism are originated from a single zygote and are genetically identical (except for some sets of immune cells that go through somatic mutation and genomic rearrangements, but that is different story). The purpose of the organism is to pass on the genetic material to the next generation and to maximize this genetic transfer. Therefore, if programmed death of some cells within the organism would help this purpose (i.e. increase the organism’s fitness), its evolution is logical. After all, often only a handful of cells (the gametes) get to pass their genetic material to the next generation and rest die eventually anyway.

This said, occurrence programmed cell death in unicellular organisms brings up an evolutionary dilemma. Scientists have found markers of apoptosis in unicellular organisms as wide as Plasmodium, Trypanosoma, Giardia and Saccharomyceses. Unicellular organisms within an ecosystem are usually competing with one another for resources, the same way animals and plants do in a larger scale. Therefore, evolution of a trait as strong and costly as altruistic death is worth a closer look, both mechanistically and rationally.

Reece et al. have discussed this matter in a recent review article published in PLoS Pathogens. Mainly focusing on Plasmodium, this review addresses hard questions such as why would altruism evolve in unicellular organisms? What could be the benefits of it? What are the factors that control its occurrence?

Discussing the ideas and hypotheses presented in the review here would be spoilers for those who are eager to read it. In that case you may stop right now and download the open access article right here. If not, you can continue reading as I bring up a few highlights of the paper that I found most interesting.

Hamilton’s rule in evolutionary biology states that cooperation can evolve under these circumstances:

rb – c > 0

Where <r> is relatedness, (for instance the ratio of relatedness between siblings is 0.5 because they share 50% of their genome), <b> is the benefit the receiver gets, and <c> is the cost of the giver. So if benefit x relatedness is larger than cost, then cooperation or sacrifice can be actually be worth it. It is with reference to this rule that Josh Haldane was willing to give away his life for two brothers or eight cousins.

Within multicellular organisms, r = 1 because all cells have been deriven from a single clone. Therefore, wherever, b > c, cell death can evolve. But in unicellular organisms, the story is different. For instance, the population of Plasmodium or Leishmania parasites in the mosquito can be genetically very close or very diverse. Now the evolutionary theory would predict that if occurrence of apoptosis is for altruism and cooperation, it is favourable if the parasites within the population are genetically close to one another. This is a theory that can be tested in a lab: both genetic diversity of different parasite populations and occurence of apoptosis within those populations are measurable with today’s techniques.

Going further from the evolutionary strategy behind evolution of programmed cell death in unicellular organisms, we should also think about what could be possibly the benefits of the receivers that would favor death of the others. One of the theories is that overgrowth of parasites can result in limitation of resources and/or harm to the vector (in this case the Phlebotomine mosquito). If the vector gets overwhelmed by the parasites, it cannot transfer them to the next stage in the life cycle. Thus as Reece et al. suggest, death can be density dependent to control the parasite popultion, another hypothesis which is also readily testable.

And finally, we get to (in my opinion) the difficult part, which is to discover the mechanism underneath these strategies. How can parasites detect relatedness or density? It is possible that sophisticated strategies and mechanisms simply do not exist in certain populations because infecting populations have always been clonal and measurement of relatedness has not been needed. But it cannot always be case. Reece et al. point out to some studies which show evidence of detection of genetic diversity by parasites and existence of mechanisms similar to bacterial quorum sensing.

There is still a lot more to learn and be amazed with.

Reece SE, Pollitt LC, Colegrave N, & Gardner A (2011). The meaning of death: evolution and ecology of apoptosis in protozoan parasites. PLoS pathogens, 7 (12) PMID: 22174671

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Host-pathogen coevolution in a test tube: C. elegans running with the red queen

Posted by Kasra

 

“Well, in our country,” said Alice, still panting a little, “you’d generally get to somewhere else — if you run very fast for a long time, as we’ve been doing.”

“A slow sort of country!” said the Queen. “Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!”

***

The Red queen hypothesis was first proposed to explain why the fitness of some species stays the same, although they are constantly evolving. Arms race between a predator and prey, or a host and a pathogen are classic examples. Constant evolution occurs when the species is in a constantly changing environment. For instance, the host is infected by more virulent species of the pathogen each generation; therefore it is under constant selection for having a better and better immune system.  Similar selection pressure holds for the pathogen that is facing stronger and stronger immune systems, so it is under selection for more virulence. In such situations, the host and the pathogen coevolve, but their fitness doesn’t change, ‘it is as the Red queen says it takes all the running you can do to keep in the same place’.

There is another important implementation of the Red Queen hypothesis and that is in explaining sexual reproduction. In species that reproduce sexually, males charge an extra cost. They contribute little or nothing to the offspring, compared to females or hermaphrodites. They consume resources that could be otherwise consumed by offspring or females that could actually produce offspring. Theoretically, an asexually reproducing population of females could grow faster and produce more offspring than a sexually reproducing one. But it doesn’t just end there. Sexual reproduction creates a new selective force called sexual selection. This selective pressure is exerted on one sex that is being chosen by the other. It can result in selection of very bizzare traits that might actually reduce the fitness of the individual (think of the Peacock’s mganificant tail). Then why are higher eukaroytes reproducing sexually? What are the males for?

The big advantage of sexual reproduction is generation of genetic diversity. Without shuffling of genes through sexual reproduction, it would take a lot longer (or never) for a new advantageous mutation to be fixed in the population. In addition, new genotypes are created because different array of alleles are put together with each round of shuffling. Without this shuffling, these potentially advantageous allele combinations might never occure. Yeasts switch from asexual reproduction to sexual reproduction when they are under stress or in an unhospitable environment. In other words, although they are well fit to live in that environment, they hope that by shuffling their genes, some of the offspring would have better chances. Obviously if they continue reproducing asexually, their offspring would be identical to them and doomed for extinction.

However, this sexual-asexual switch does not exist in animals and plants (there are always exceptions in biology). This would make you infer that they are in a constantly changing environment that requires constant evolution or it would lead to extinction. In other words, it is like running with the red queen to keep fitness at the same place. But what is it that is constantly changing and imposing this strong selective pressure for such a high costly method of reproduction? It was proposed that presence of constantly evolving pathogens and parasites puts their hosts under selective pressure for adaptation. Sexual reproduction offers the selective advantage of genetic diversity and possibility of more resistance in the offspring. Same argument stands for selfing versus outcrossing strategies. Outcrossing is more costly. It requires actively seeking (dating!) another outcrossing partner. Also, outcrossing populations may have mixed subpopulations of hermaphrodites and males. In this case again, the males are an extra cost, while selfing limits the genetic diversity of the offspring.

It is difficult to create controlled evolutionary systems in a lab environment, thus evolutionary hypotheses are difficult to test. This is what makes the paper by Morran et al. published in Science in July 2011 very exciting. They tested the validity of the Red Queen hypothesis in the coevolution of hosts and pathogens.

The question was whether outcrossing offers selective advantage over selfing in the presence of a coevolving pathogen. They used Caenohabditis elegans and the deadly bacteria Serratia marcescenes as the host-pathogen couple. Three different strains of C. elegans were used:  strictly selfing, strictly outcrossing and wildtype (a mixture of both). ‘Coevolution’ of C. elegans and S. marcescenes was done by sequentially infecting surviving C. elegans worms with bacteria that were isolated from dead worms in the same culture. This was in comparison with ‘evolution’ of C. elegans where it was being reinfected with the ancestral strain that both cultures had started with. In the coevolution model, the bacteria were also supposedly under selection for more infectivity since the surviving worms were more resistant to infection. This experiment was carried over for 30 generations.

What would the red queen hypothesis predict for these conditions? Infection with deadly bacteria is supposed to induce more outcrossing as opposed to selfing in order to increase the chances of resistance in the offspring. However, once resistance has been acheieved, we would expect the worms to revert back to selfing as they are no longer under selective pressure. However, if resistance never occurs, outcrossing rates are expected to remain high compared to control. And this is what Morran et al. observed with the wildtype C. elegans worms. While evolving C. elegans worms that were exposed to a nonevolving strain of S. mersescenes peaked in outcrossing rate within 8 generations, by generation 30 the outcrossing rate was down back to control levels. However, the outcrossing coevolving worms showed continuously increasing outcrossing rates plateauing after 16 generations.

This clearly supports the red queen hypothesis in explaining the underlying reason for outcrossing and presence of non-offspring-bearing males in a population. However, the authors did not stop here and further backed up their argument:

They showed that the coevolving bacteria indeed have higher infectivity in strictly selfing, or non-coevolving C. elegans. Furthermore, the strictly selfing worms did not adapt to infection. Higher mortality rates were observed after multiple generations and also the coevolving bacteria were shown to be more infectious compared to the ancestral strain; this was not the case with the strictly outcrossing or wildtype strains.

So all together, this is an elegantly performed evolution experiment that highlights the importance of outcrossing in adaptation to pathogens. As it is beautifully put in Science2.0: The Red Queen is not dead. Long live the Red Queen.

Genes are not the only things that get better with sex. Matt Ridley, the author of The Red Queen , discusses how technology advances in the human society when ideas have sex in a TED talk.

Oh, and happy 2012!

Morran LT, Schmidt OG, Gelarden IA, Parrish RC 2nd, & Lively CM (2011). Running with the Red Queen: host-parasite coevolution selects for biparental sex. Science (New York, N.Y.), 333 (6039), 216-8 PMID: 21737739

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