Rhinovirus-Induced Shutoff of Cellular Responses

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The RNA picornaviruses have evolved wonderfully robust and effective mechanisms to express their proteins and override host defenses. Novel internal ribosomal entry sites (IRESs) allow these genomes to bypass normal translational requirements for 5' cap structures and efficiently lure the ribosomes into viral instead of cellular pathways. The captured ribosomes pass down a single, long open reading frame (ORF), creating polyproteins that are in reality, tandem linkages of all structural and enzymatic units necessary for rapid and virulent infection. Individual protein fragments are liberated co-translationally and posttranslationally in an elaborate proteolytic cascade that is a defining feature of this family. The molecular biology of picornaviruses, and in particular that of poliovirus and the closely related human rhinovirus, is perhaps one of the best understood and most thoroughly accessible experimental systems in all of biology. Basic mechanisms of IRES translation, protein processing or genome replication have been under investigation for decades and there are superb molecular and recombinant tools available for the study of every aspect of the virus lifecycle. The focus of this project is the way in which the human rhinoviruses can seditiously subvert a cell's innate immunity traps, crippling the capacity of that cell to trigger an alarm or altering the nature of the alarm itself. The molecular battleground in that first infected cell, pits two key viral proteases, 2Apro and 3Cpro, enzymes strategically honed by evolution for this particular purpose, against cellular defenses that may vary significantly according to cell-type, cell-cycle, host genetics, immune history and current medical predisposition. We propose that substantially different disease phenotypes can be triggered at their core, by the ability or inability of these proteases to shutoff of crucial cellular transcription and translational processes. Every subsequent immune and disease response is consequent to this outcome. At the ultimate molecular level, the activities of these proteases instigate the cascade of events that set off or prevent an episode of disease. The experiments in this project will examine the cleavage processes and molecular consequences by which rhinovirus infection inactivates cytoplasmic-nuclear transport, thus crippling the ability of the cell to carry out mRNA transcription, or cap-dependent translation. They will determine whether an unconditional viral-induced shutoff of nuclear transport is necessary to allow viral replication and cell lysis. Or, under conditions when these processes are only partially effective, whether a host-induced up-regulation of innate triggers then quickly escalates into a full-blown immune response, that may or may not be inappropriate for the extent and severity of actual infection. The project will also examine host shutoff activities are ubiquitous and similar in multiple human cell types, or are modulated by the genetics and/or innate immunity status of the cultures and the genotype of virus.