Category Archives: Short Communication

Forcing virus to destroy itself

If virus can infect us, why can’t the intelligent humans design something that can trick the virus to self-destruct? This question regarding the ability of self-destruct was raised in one of the casual conversations I had last week. My immediate answer was that in the case of RNA viruses, some progeny viruses go through self-destruction due to the high mutation rate because of the absence of proofreading in the RNA Polymerase. But if the error is prevented, self-destruction will not happen, unless you change the cellular components (i.e. increasing the expression of antiviral mechanisms, or just reducing the expression of replication machinery that the virus needs). In sum, my answer was yes in some conditions, but no in the perfect scenario when virus can replicate without problems.

After researching for papers regarding the self-destructive virus, I came across an article about a researcher, Bently Fane, from University of Arizona, and also several publications from Dr. Fane on pubmed.com. In short, they basically engineered bacteria to keep producing the mutated virus’s coat protein. So why is it important for them to make mutated protein? The mutated protein serves as a decoy. Due to the high abundance of mutated and un-mutated coat proteins within the virus-infected bacteria, newly synthesized viruses incorporated both proteins by mistake. These viruses probably do not  have a proper structure to withstand the external environment (i.e. pH changes and high termperature). As a result, these viruses were destroyed due to instability. The bottom line is that viruses can be made to self-destruct by making decoy coat proteins. So now let’s step back a little. Theoretically, if we can make our infected cells to keep making mutated coat proteins, the progeny viruses will self-destruct because they incorporate some mutated coat proteins.

So, can this be a cure for viral infection? If we can somehow express mutated coat protein, can we evade infection? Sadly, the answer is no. Self-destruction can only be induced for a short period of time. The abundance of mutated coat proteins exerted pressure to select for a subgroup of viruses that can form a proper structure with mutated and un-mutated coat proteins. After a few generations, the fitter subgroup survives and becomes the dominant virus. Therefore, this surviving subgroup will continue without self-destructing (meaning we can never outrun viruses!).

References:

J Virol. 2011 Jul;85(13):6589-93. Epub 2011 Apr 13. From resistance to stimulation: the evolution of a virus in the presence of a dominant lethal inhibitory scaffolding protein. Cherwa JE Jr, Fane BA.

J Virol. 2009 Nov;83(22):11746-50. Epub 2009 Sep 2. Viral adaptation to an antiviral protein enhances the fitness level to above that of the uninhibited wild type.Cherwa JE Jr, Sanchez-Soria P, Wichman HA, Fane BA.

Self-destructiv virus – A tool to learn how viruses work. University Communications.

Why stem cells silence virus growth?

Retroviruses can integrate their viral genome into the human genomes. Embryonic stem cells are equipped with a mechanism to shut down the replication of retrovirus. But this mechanism is not commonly found. Thus, it is an interesting question to ask why our stem cells have this special silencing mechanism. Stem cells are transcriptionally active, as it gives rise to multiple lineages of cells. Therefore, it is important to silence the retrovirus transcription. Here, we will discuss the first article of the week, “Embryonic stem cells use ZFP809 to silence retroviral DNAs) by Daniel Wolf and Stephen P. Goff (do ic 10.1038/nature 07844).

Inside the Moloney MLV, there is a primer binding site to initiate replication of the MLV genome. This primer binding site binds to proline tRNA. Thus, using proline tRNA, we can detect the silencing factors that act at the primer binding site to stop replication inside stem cells. In the figure on the right, when Pro probe incubates with the stem cell extracts, lane 2 shows the Pro probe binds to a complex at around 45kDa. (The B2 is just a mutant version of Pro that they know is not going to bind to the silencing factor. Thus, it is a negative control). After mass spectroscopy and subsequent protein analysis on the complex, they figured that the complex composes of a factor called ZFP809. In the bottom figure, you see ZFP809 binds to the Pro probe. Then the author moved on to prove that the ZFP809 blocks the replication of Pro-dependent replication. In short, this paper is the first to find a silencing factor in stem cells to inhibit the replication of MLV.

A recurring theme: virus exploits the host’s machinery.

One of the most fascinating thing I found associated with all types of viruses is that they are capable of hijacking cellular machinery with only a few genes. In the case of dengue virus (DENV), it only codes for 7 nonstructural and 3 structural genes. And how these tiny viruses can take over host cells of about 20000-25000 genes is just fascinating. I attended a seminar, called “The ESCRT pathway in HIV budding and cell division” today down at GCIS. It was hosted by Dr. Wesley Sundquist from University of Utah (shown right). And here, I will talk about what the major concepts mentioned in the talk.

ImageAccording to wikipedia, “ESCRT (endosomal sorting complex required for transport) refers to a series of cytosolic protein complexes called ESCRT-0, ESCRT-I, ESCRT-II, and ESCRT-III.” There are 3 major roles of ESCRT machinery: (1) multivesicular bodies (MVB) formation, (2) virus budding, and (3) cytokinesis. The MVB formation is important in recycling receptors that are endocytosed into the cells. One of the prime examples is the EGF receptor that gets endocytosed after binding to the EGF molecules. After the EGFR is internalized in the lysosome, it is then localized in the MVB to traffick to the endosomes for ubiquitination, which is then expressed on the cell surface for another round of binding to EGFR. In the case of cytokinesis, ESCRT-III forms a multimer around the midbody between two daughter cells (in other words, it wrap around the junction between two daughter cells). Then, an ATPase interacts with ESCRT-III to facilitate the cleavage and separate the two daughter cells. (it is likely that this ATPase contribute a conformational change to the ESCRT-III proteins, which give rise to enough thermal energy to break apart the junction.

HIV exploits this pinching technique by ESCRT-III to bud off from the host cells. It was shown that the envelope protein, Gag, interacts with ALIX. ALIX then interacts with ESCRT-III, which then Vps4 ATPase comes in to facilitate the pinching. As a result, progeny HIV can then be released from the host cells to continue infect neighboring uninfected cells. In the case of ALIX-depleted (siRNA-ALIX treated) cells (see below), HIV is not able to bud well from the host cell, and there is some distinguishable difference in the envelope of the HIV between the wildtype HIV and ALIX binding site mutant HIV.

Taken together, HIV exploits the cellular cleavage mechanism used for cytokinesis to facilitate its budding from the host.

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Summary:

– Virus exploits cellular pathways to facilitate its replication cycle

– Cellular ESCRT pathway is important for HIV budding.

References:

Fujii et al. Freed, Functional role of Alix in HIV-1 replication, Virology, Volume 391, Issue 2, 1 September 2009, 284-292, J. Virol.

Wollert et al. The ESCRT machinery at a glance. July 1, 2009 J Cell Sci 122, 2163-2166.

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