Uncovering the Complexity of a Simple Retrovirus: A Study of Glycosylated Gag and Flow Virometry

• Main Author: Renner, Tyler
• Other Authors: Langlois, Marc-André
• Format: Others
• Language: en
• Published: Université d'Ottawa / University of Ottawa 2020
• Subjects: Retrovirus; MLV; HIV; Flow Virometry; Glycosylated Gag; APOBEC3; A3G; mA3; SERINC
• Online Access: http://hdl.handle.net/10393/40062; http://dx.doi.org/10.20381/ruor-24301

Murine leukemia virus (MLV), classified as a gammaretrovirus, has been studied extensively to enhance our understanding of the biology and replication of retroviral infection. Typically referred to as a simple retrovirus, its usefulness as a model is highlighted owing to its minimal genome. The genetic material for MLV was thought to only code the basic and essential defining features of a retrovirus. Through the understanding developed from the use of simple retroviruses, the clinical and research communities were immeasurably more prepared to combat the more complex and decidedly infamous human immunodeficiency virus (HIV). Interestingly, a scenario of convergent evolution has directed MLV to encode an accessory protein, termed Glycosylated Gag (gGag), that shares functionality reminiscent of several HIV proteins. Herein, I present a dissection of a novel function of this enigmatic protein, paired with an improved understanding of the biology of MLV that was revealed by the development of small particle flow cytometry performed on viruses, also known as flow virometry. Initially, we elucidated that gGag is responsible for the resistance of MLV towards the restriction factor murine APOBEC3 (mA3). I showed that even endogenous mA3 from primary cells exhibited an enhanced enzymatic activity towards MLV with mutant gGag proteins which have lost glycosylation sites. In our following study, I illustrated that these mutants displayed a reduced viral core stability, the severity of which was correlated directly with susceptibility to mA3. These results are in line with the hypothesis that viral core stability and APOBEC3-susceptibility are directly linked. Furthermore, I showed for the first time that unprocessed gGag was associated with viral particles released from producer cells in the orientation of a type I membrane protein, with the structural regions directed within the viral core. This may be the direct evidence of how gGag improves capsid stability, a mechanism which is still unresolved. On the flip side, gGag as a type II membrane protein was observed exclusively on virus-like particles devoid of detectable envelope glycoprotein (Env). This marks a potential new function for gGag in the context of infection. Given the ubiquitous necessity of an optimized core stability for any virus, combined with the overlapping function of gGag with HIV accessory proteins, continuation of this work represents an as of yet clinically unexplored avenue for the development of HIV therapeutics. At the same time, in order to characterize individual viral particles, I played an instrumental role in developing the technique of flow virometry within our core facility. I illustrated that the Env of MLV does not significantly accumulate on extracellular vesicles (EVs) and acts as an effective marker for viral particles. With this evidence in hand, the enumeration of MLV virions was made possible. By correlating this information with an absolute viral genome determination, I was able to estimate the packaging efficiency for MLV in a quantitative manner. This information suggests that roughly 80-85% of MLV particles are missing their essential genetic information. These findings may implicate the disease progression of MLV infection may be enhanced by the use of defective-interfering particles, a theory that has been suggested for HIV. This work highlighted the fact that flow virometry is uniquely capable to discriminate viral particles from other cell-derived membraned vesicles in a highly sensitive manner. Overall, my work has unveiled new complexities of a simple retrovirus, while laying the groundwork towards both diagnostics and therapeutics for the ongoing battle with HIV.