Somewhat to my surprise, I have recently found myself very interested in plant viruses. This started a few years ago when I ate a most delicious variety of hot pepper that apparently is infected with a virus that gives the peppers white stripes. I’ve never really given much thought to plants and plant viruses before but as I began to look into their biology it seems that plant viruses have some terrific tricks up their sleeves (if you will) to aid in their transmission.
Plants aren’t walking around coughing on each other, so most of them depend on insects to come and bite the infected plant and carry the virus to the next host. But a plant that is infected isn’t very attractive to insects, since unhealthy plants aren’t as likely to be a valuable source of food. But viruses are masters of host manipulation. A recent study looked at Cucumber mosaic virus, and its ability to attract aphids to infected leaves. It seems that aphids dont like to spend much time on infected leaves, and they dont have to. The virus sticks to the aphid mouthparts quickly and easily and the aphid can then bring it to the next plant. But the aphids still have to be attracted to the leaves, even if they dont stay for long. So how does the virus attract the aphid to the plant? Researchers set up a special chamber with a leaf from an infected and a leaf from an uninfected plant. The leaves were not visible, but could be smelled by the aphids through wire mesh. Aphids released into the chamber were more attracted to the uninfected leaf. Analysis of volitile organic compounds being released from leaves showed that both infected and uninfected leaves release compounds that aphids can smell but infected leaves release much more. So even though the meal may not be as good, the strong smell brings the aphids to the table.
All posts by David Esteban
Big Viruses for Small Hosts
Its a common misconception among my students that simpler hosts like bacteria or single celled eukaryotes would host simpler (ie smaller) viruses, but that is certainly not the case.
Another giant virus that infects a protist has been identified and sequenced. Like its close relative Mimivirus, this new virus called Cafeteria roenbergensis virus (CroV) has a very large genome and has many genes not typically found in viruses. Before the discovery of Mimivirus, viruses were not known to encode proteins involved in protein translation. That was a function on which viruses were totally dependent on the host. However, these giant viruses seem to have their fingers in protein translation too, showing us yet another strategy in manipulating host processes. There is also block of genes that appear to be derived from bacteria. The host species, C. roebergensis, eats bacteria, so it would be interesting to know if the bacterial genes were the result of a horizontal gene transfer event from a preferred host food.
Before CroV, all giant viruses identified infect amoebas. CroV infects C. roenbergensis, a marine protist. So what is it about protists that makes them good hosts for such big viruses? Why haven’t we found giant viruses infecting other eukaryotes?
Could the explanation lie in the still murky evolutionary origin of viruses? Another recent paper attempts to put some viruses (nucleocytoplasmic large DNA viruses, including giant viruses, poxes and herpesviruses) into the tree of life along with bacteria, archaea and eukaryotes. Using genes common to all, they showed that these viruses have a very ancient evolutionary origin, probably right around the time of the appearance of eukaryotes. Were the ancestral viruses more cell-like and over time progressively lost genes?
Should I Stay or Should I Go?
Some bacteriophages (viruses that infect bacteria) can undergo a special kind of replication cycle called lysogeny. Rather than making lots of new phages upon infection, these phages can pop their genome into the host chromosome. When the cell copies its DNA and divides, the integrated phage is copied too, so the daughter cells are infected. These integrated phages, or prophages turn out to be very important to us: cholera is caused by V. cholera bacteria with an integrated phage that expresses the toxin that causes diarrhea.
So what is the evolutionary advantage of the lysogenic/lytic switch for this phage? When would it “choose” one over the other? In my biol 105 class we discussed the evolutionary benefits of lysogeny. One of the most enjoyable parts of teaching is when students ask challenging questions that lead to fruitful discussion and deeper thought. Here is my perspective on this.
You can think of the lytic cycle as highly virulent (100% mortality) or lysogenic as non-virulent (no negative effect on the host while the phage is present as a prophage). Virulence is often related to transmission, such that viruses will evolve to have the optimal level of virulence to allow for efficient transmission. Since lysogenic phages can switch between high and low virulence, when would high virulence be favored for transmission and when would low virulence be favored?
A highly virulent pathogen runs the risk of wiping out its host population. If the cells are growing actively in an environment like the gut, and the virus is replicating to high levels, it could spread to the entire host population eventually killing every cell. The virus would then depend on either more V. cholera entering the gut, or getting out of the gut and spreading to a new human host infected with cholera. Alternatively, cholera can also grow in the environment, so the phage could infect a cell in the environment. However there is some obvious risk there, that of finding the next host cholera cell either in a gut or the environment. Its a big world out there for a tiny phage and a tiny bacterial cell to meet each other. A less risky approach might be to limit virulence and allow prophage infected cells to survive. The cholera cells will be returned to the environment, where they can replicate or to a new human host where it can also replicate. Either way, the phage is guaranteed to find a host, because its already in it.
Now if the prophage finds itself in cells that are no longer growing, there may be an advantage to getting out and finding “happy” hosts. Cells that are not growing could be at greater risk of cell damage and death, perhaps they are not acquiring the nutrients and energy necessary to grow or repair cellular damage. If the cell dies, the phage will not be able to replicate. So the phage would enter the lytic cycle and release progeny. The risk of not finding a new host would presumably be lower than the risk of staying within a dying host. High virulence therefore is advantageous for transmission in this situation.
Does anyone have a different perspective?
Of Aliens and Arsenic
Or How to Screw up an Interesting Discovery with Mediocrity and Media Hype
NASA annonced an “astrobiology discovery”….Aliens! No, sorry, bacteria…in California! WOW! OK, this is no ordinary bacteria as it can grow in astonishing concentrations of arsenic, a highly toxic compound. It’s nothing new that bacteria can grow in all kinds of extreme environments, but this is growth in a lot more arsenic than we’ve seen before. The claim however is that this bacterium actually replaces phosphorus with arsenic. Phosphorus is used as part of the structure of DNA, as well as proteins and many important small metabolites, most notably ATP, used as an energy source. Arsenic is much like phosphorus but arsenic containing compounds are quite unstable in water (which is why arsenic is toxic). This would be novel in cellular life as we know it; DNA containing arsenic! I was initially very interested in this discovery…and then I read the paper.
When you make a claim as big as replacing one of the most import elements in a cell, it seems to me your data better be rock solid. The paper is quite underwhelming. Although it seems there might be arsenic associated with the DNA it certainly doesn’t provide good evidence that it’s a part of its structure. In fact, others have argued more convincingly that the trace amount of phosphorus contaminating the preparation is sufficient for the cells to use to make everything they would need to.
It seems to me this is a case of the researchers jumping ahead of themselves and interpreting something from the data that is just not there, a failure of the peer review system, and an overly enthusiastic media running with a thrilling but incorrect story about alternative branches of life and our need to re-write biology textbooks. Whats sad is that there actually is something interesting here that has been missed and would be a more reasonable interpretation of the data. This bacteria can grow at extreme concentrations of arsenic, in the near absence of phosphorus, but can still somehow scavenge those few available atoms of phosporus and use them, where probably all other organisms would have long since died of arsenic poisoning. How does it do that? This is still a fascinating bacterium…it just wont redefine life.
FInally, why am I writing about bacteria on a virology blog? I learned some time ago never to underestimate viruses. After being surprised about the existence of viruses infecting organisms in places like bubbling geothermal acid pools, I realized that where there are cells, there are viruses. I wonder if this arsenic loving bacteria is host to some interesting viruses? I bet Mono Lake, where this discovery was made, is full of interesting viruses.
New Vaccine Protects Against Ebola Virus
Contributed by Guest Blogger: S. Goldberg ’14
Ebola Virus (EBOV) is a fairly new infection of the Filoviridae family, that causes Ebola Hemorrhagic Fever, or EHF. This infection can often be severe or fatal in humans and primates. Because it would be difficult to create a vaccine against all four different virus species of Ebola, scientists came up with a plan to develop a vaccine protective against a single species of the infection. An experiment was designed to test to see if a “prime-boost” strategy would work, with a vaccine protecting against one Ebola Virus species and protecting a different species at the same time. In the process of doing this experiment, it was found that the vaccine developed to protect primates against the two most lethal Ebola Virus species also protected against the newer Ebola virus species that was founded in 2007. The prime-boost vaccination provides immunity against newly emerging EBOV species and shows cross-protection against EBOV infection. In this strategy, the “prime” is a DNA vaccine that has a small amount of genetic material with surface proteins of the Zaire Ebola virus species and the Sudan Ebola virus, the two most lethal species of EBOV. The “boost” is made of a weak cold virus that delivers the Zaire EBOV surface protein. The experiment, conducted and overseen by the National Institute of Allergy and Infectious Diseases and the US Army Medical, gathered eight 3 to 5 year old cynomolgus monkeys as their test subjects, to see if such a vaccine actually protected against the two older Ebola virus species and the newer strain. Each of the monkeys were given the immunization and then transferred to a laboratory where they were exposed to the EBOV infection. The monkeys stayed their for the duration of the experiment. Using a blood analyzer, liver enzyme levels were examined on “days 0, 3, 6, 10, 14, 21 and 32”. During this test, samples of T-cell intracellular cytokines were taken. CD8+ T-cells were stained with antibodies, against intracellular cytokines. This technique allows for the frequency of antigen-specific T-cells to be determined. The production of cytokines plays an important role in the immune response of the body. After isolating the RNA of each subject, each of the monkeys given a vaccine that contained Zaire EBOV and Sudan EBOV glycoprotein (GP). After the GP was exposed within each of the bodies, the subjects developed “robust antigen-specific…immune responses against the GP from [Zaire EBOV] as well as cellular immunity against lethal…[Bundibugyo EBOV]”. After concluding this experiment, scientists have learned that current vaccines that can bring about T-cell immunity will have a greater possibility of “protecting against other [new] pathogenic EBOV species”.
If we have the ability to protect against new and emerging EBOV species, does that mean that, in the long-run, mutation of the virus will stop? Will existence of any EBOV species disappear? If it disappears, could it reappear? Could similar experiments be done to discover if a other currently used vaccines, aside those that display T-cell immune responses, are protecting against other unknown or new viral strains?
New Treatment Found for Chronic Hepatitis B Virus
Contributed by Guest Blogger: S. Bhutani ’14
Chronic Hepatitis B virus (HBV) is a common infectious disease. HBV infects hepatocytes, which are liver cells, thereby impairing liver function and leading to disease such as liver cancer. In chronic HBV the responses from the CD8 cytotoxic T-cell and CD4 helper T-cell are not substantial enough because high amounts of HBV impair T-cell immunity. Thus, ideal anti-viral treatments would need to reduce the amount of virus. In the past decade, the introduction of nucleoside and nucleotide analogs, compounds that look similar to nucleotides and deceive the virus into thinking it is being replicated, has improved treatment for HBV. However, the prominent nucleoside analog, lamivudine, that inhibits HBV replication, is no longer as effective because the virus has developed mutations that resist the drug. One new nucleotide analog, called adefovir dipivoxil, has been tested on chronic HBV patients in China.
In China 22 patients were tested along with 20 healthy controls. The patients were treated with adefovir dipivoxil once daily for 10 weeks. The presence of cytokines producing certain T-helper cells was measured, as was the amount of HBV DNA using biochemical markers. Before adefovir dipivoxil, the cytokines producing helper and killer T-cells were lower in the patients than the healthy individuals. There were two patients with HBV mutations, one which was lamivudine resistant and one which was adefovir dipivoxil resistant. Adefovir dipivoxil treatment resulted in an increase of Th1 and Th2 cytokines that produce CD4 T-cells in all patients except for the one with the adefovir dipivoxil resistant mutation, proving that this new drug can combat lamivudine resistant HBV mutations. The treated patients’ levels of cytokines peaked and then dropped and stayed at the cytokine levels of the healthy individuals. There was an inverse correlation between the amount of HBV DNA and cytokine levels; after treatment as cytokine levels increased, HBV DNA decreased.
Thus, this study suggests that adefovir dipivoxil inhibits HBV DNA polymerase thereby reducing the amount HBV and restoring T-cell immunity. However, it should be noted that even after the recent development of the drug, HBV mutations were still found. Thus will there ever be development of a treatment for which there no resistant mutations have already evolved?
Using HIV to treat HCV
Contributed by Guest Blogger: R. Trenchard ’14
Traditionally, the go-to method used to treat patients with hepatitis C virus has been to administer a combination of antiviral agents called pegylated interferon-a (PEG-IFN) and ribavirin. These agents combat the virus particles that cause HCV, and regulate the immune system. Clinical studies of this type of treatment however, show that the mixture of these two agents achieves the desired sustained virological response (SVR) for only 36%-46% of HCV patients. This sad statistic has led to recent research that has proposed a more advanced treatment method that includes specifically targeted antiviral therapy for HCV (STAT-C). This treatment would work like HIV therapy and involve the inclusion of protease and polymerase inhibitors to the standard HCV antiviral concoction. It is expected to improve treatment outcomes, because every stage of the HCV life cycle could be a target for STAT-C agents.
The STAT-C agents mentioned above have worked in trials because they target enzymes that are essential for viral replication. Currently they have been used in addition to PEG-IFN and ribavirin, but once there are a sufficient amount of STAT-C agents licensed they can be used alone as a combination therapy used to act at distinct stages of viral replication and to create a barrier to resistance. Like HIV therapy however, these drug combinations increase the risk for drug-drug interactions (DDI) in patients. Some of these drug reactions include anaemia, haematological adverse events, and mitochondrial toxicity. Aside from this some patients for whom the treatment works for can suffer from side effects ranging from flu-like symptoms to anemia and cardiovascular problems.
This evokes a number of questions. With such dangerous risks involved, is the treatment worthwhile? What characteristics of HIV and HCV allow them both to be treated with protease and polymerase inhibitors? Can this type of treatment be used on other viruses as well?
Ekybion: Inhibitor of influenza or immune-response?
Contributed by Guest Blogger: H. Cushing ’14
Ekybion is a drug complex that was developed to treat inflammation in the respiratory tract caused by infectious agents. A series of experiments were implemented to test if Ekybion was capable of inhibiting the growth of influenza A/PR/8/34 H1N1 strain in vivo (mice).
The first experiment was performed in vitro. MDCK cells were treated with Ekybion 1 hour pre-infection, 1, 2, 4, or 12 hours post-infection. Samples of the different cultures were taken 24 and 48 hours after infection and the number of influenza viruses were counted using the hemagglutination assay method. The results showed that the treatment of MDCK cells (whether pre/post infection) significantly reduced viral growth for at least 48 hours and that pre-infection treatment was most efficient.
The first experiment done in vivo was testing the possible toxicity of Ekybion. They treated one of two groups of mice three times a day with Ekybion. The toxicity was determined by the mice’s weight and normalcy of lung tissue. The test was done with several different concentrations of Ekybion. The results showed that no toxicity was observed in the treated mice until 50x concentration level was reached – indicating that no toxicity would result from Ekybion use at 1x (concentration intended for medical use).
Ekybion’s inhibition of the virus in infected mice was tested at different treatment times and concentrations. Mice were weighted daily for 16 days and 2-5 mice were euthanized from each group on the second day post-infection to determine the amount of virus in lung tissue. The results showed no significant differences in weight loss and that the treatment with 1x concentration for 2 minutes was the most effective with a 46% survival rate (compared to 0% survival in the control). The lung tissues collected from the euthanized mice were used to determine cytokine levels at the site of infection. The results showed that mice treated with Ekybion had a lower cytokine quantity. This determined that Ekybion could suppress immune response as well as reduce viral growth. The question resulting from the experiments is whether or not the anti-viral benefits outweigh the immunosuppression effect of the drug.
Sterile or Feral? Preventing Dengue Fever via Mosquito Population Control
Contributed by Guest Blogger: E. Doyle ’14
In order to disseminate successfully, viruses, being immobile, must adapt and evolve to utilize their surroundings to effectively propagate. One group of viruses that’s done this particularly well are the Flaviviruses, which are transmitted to humans via bites by infected mosquitoes. Though beneficial for the spread of the virus, Flaviviruses such as yellow fever, dengue fever, and West Nile virus cause a significant amount of serious and painful illnesses and even death in human beings. Of particular concern are the dengue fever outbreaks that have, according to the CDC, recently been common in many parts of the world. Since there is no cure for this virus, prevention is the only way to stop the spread. Scientists are currently looking to do this by controlling the mosquito population.
In a recent study, scientists considered two known methods of mosquito population control (sterilization of male mosquitoes, and genetic alteration of male mosquitoes that would cause them to be genetically programmed to die, as well as any offspring they produced) and mathematically projected how these methods would be most effectively utilized. The variable in this experiment was the frequency of the release of the mosquitoes altered by these two techniques. Would fewer mosquitoes be produced if these altered males were released frequently in small bursts, or less frequently in larger numbers? The projected effectiveness of the different timelines was shown by the calculated number of mosquitoes present in the environment afterwards, keeping in mind the as well as the mating competitive ability of mosquitoes that have been altered to control their reproductive success. If the altered males are able to mate as successfully as the wild type males are and lower the population of mosquitoes below a certain level, the virus will no longer successfully transmit.
Though genetically altered or sterilized male mosquitoes may often lose out when it comes to reproductive success, as has been shown in other real-life experiments, the numbers showed that the release of these mosquitoes into the environment still works when they are released very frequently rather than at lower frequencies. It should be kept in mind, however, that releasing these mosquitoes more frequently also results in higher costs. Also, since the results of these experiments were merely projected using mathematical analysis, it begs the question of whether the anticipated results of frequent mosquito release would be as successful in real life as they are on the page.
Using Viruses to Battle Breast Cancer
Contributed by Guest Blogger: J. Warren ’14
Cancer is the second leading cause of death in the world today. This fact has made cancer research one of the leading studies in medicine, and any advancement in the field of cancer treatment is quite impactful. One of the more modern (and promising) approaches to treating cancer is through the use of viruses. An oncylitic virus is one that has been modified to infect cancer tissue in the body while not causing disease to the host. This should be possible due to the heightened susceptibility to viral infection of tumor cells, which have defects in certain antiviral response. The appeal of oncylitic viral infection is its ability to spread through the host, allowing it to attack any and all cancerous cells.
One highly attractive oncylitic virus is a mutant of vesicular stomatitis virus (VSV). A recent study investigated the effects of a mutant strain of VSV (rM51R-M) on breast cancer in both rats and humans. Though the virus was able to infect and kill the breast cancer cells without causing disease in the host mice, and could effectively destroy the tumors in vitro, unfortunately the research found that in vivo the viral infection is not sufficient for curing a host of breast cancer, and that VSV does not infect tumorigenic cells any more than normal cells.
Researchers grew several strains of human mammory epithelial cells with varying oncogenicity (chance to turn cancerous) and exposed them to varying multitudes of infection by rM51R-M. They observed that there was no significant difference in viral proliferation between the strains. They also looked at the effects of rM51R-M in mice with breast cancer, tracking the growth rate of the tumors according to the amount of infection.
Though the findings don’t amount to an effective way of defeating breast cancer, it does add valuable knowledge to the field. Future experiments can easily expand on these methods: using this data as a control, researchers could test other types of cancer, other oncylitic viruses, or perhaps the effects of compounding additional treatments. Perhaps this will also drive scientists to create completely new, engineered viruses that are able to infect only the tumor cells present and, some day, be able to completely eliminate multiple types of cancer.