John Parker, PhD
Baker Institute for Animal Health
Dr. Parker joined the faculty at Cornell in May 2003. His laboratory primarily studies the cellular pathobiology of mammalian reovirus infections. The Reoviridae include viruses with genomes comprising 10-12 segments of double-stranded RNA (dsRNA) enclosed within double or triple-shelled particles, and they include significant human and animal pathogens such as rotaviruses and bluetongue virus. The mammalian orthoreoviruses are used as models to study the molecular genetics of viral replication and pathogenesis, and are particularly tractable agents for the study of viral pathobiology in vitro. In addition, reoviruses are showing considerable promise as viral oncotherapeutic agents.
The major focus of the work in this laboratory is in defining the mechanism(s) by which viral and host factors are recruited to viral factories (VFs) – the cytoplasmic sites of reovirus replication and assembly – and how those factors interact to promote the efficient assembly of new viral particles. VFs form in the perinuclear cytoplasm of infected cells. During his post-doctoral studies Dr. Parker identified the roles of two viral proteins that are responsible for the formation of VFs: The viral nonstructural protein μNS forms the matrix of the VFs and will form factory-like inclusions (VFLIs) when expressed alone in cells; and the minor structural protein μ2, which is a microtubule-associated protein that interacts with the amino-terminal 41 amino acids of μNS, and links the VFs to the microtubule cytoskeleton. When μ2 and μNS are coexpressed filamentous VFLIs, which are morphologically indistinguishable from VFs, form in transfected cells. The nonstructural ssRNA binding protein μNS, viral cores, and the viral core surface proteins λ1, λ2, and μ2 are recruited to μNS VFLIs, and the cellular chaperones Hsp40, Hsc70, and Hsp70 are all strongly concentrated within both VFs and NS VFLIs. More recently the Parker laboratory has shown that the protein matrix of the VFs is highly dynamic and can allow vesicular structures to pass through. They are therefore now investigating the molecular basis for the dynamic nature of the factory matrix. It appears likely that the matrix of VFs plays a critical organizing function during viral replication and assembly – acting to organize and coordinate the viral and cellular factors needed to couple replication with assembly. Using tandem affinity purification the lab has identified the molecular chaperones Hsc70 and Hsp70 as directly interacting with the μNS protein. Current work is focused on dissecting the functional role of this interaction.
An additional focus of the Parker lab is on reovirus-induced apoptosis. This work focuses on the role of the viral outer capsid protein μ1 in apoptosis induction during the viral replicative cycle. These studies have defined a 30 amino acid region at the C-terminus of μ1 as necessary and sufficient for apoptosis induction. The capacity of μ1 to induce apoptosis correlates with its capacity to associate with mitochondria, endoplasmic reticulum, and lipid droplets. In newer work these findings have been extended to show that the viral 3 protein counteracts the proapoptotic effect ofμ1 by coassembling with free μ1 to form heterohexameric capsid subunits. In addition, the Parker lab has shown that apoptosis in infected cells appears to be regulated by the ubiquitin-proteasomal system.
In other studies, the Parker laboratory is examining the molecular and cellular controls of pathogenesis of caliciviruses by examining the differences in virulence between strains of caliciviruses that infect cats. Feline caliciviruses (FCV) are related to similar caliciviruses that infect dogs, mink, sea lions, rabbits, and humans. The human caliciviruses, Norwalk disease virus and the Sapporo-like viruses, are the most common causes of acute adult-onset viral gastroenteritis. These viruses are highly contagious, persist in the environment, are antigenically variable, and as a consequence are difficult to control. For these reasons the NIH has identified caliciviruses as potential bioterrorism agents; however, because the human caliciviruses cannot as yet be propagated in tissue culture cell, FCV has emerged as a model agent for studying the caliciviruses generally as it is easily grown in tissue culture and shares many of the biological properties of human caliciviruses.
The Parker laboratory has isolated and characterized several hypervirulent strains of FCV that cause high morbidity and mortality in infected cats, and compared those strains to others that cause mild or inapparent disease. They have identified several in vitro correlates of virulence and are using molecular genetics to identify the genetic determinants of virulence. At present the determinants of virulence for caliciviruses are unknown. Identifying the genetic determinants of FCV virulence will help in identifying other potentially virulent strains and in the design of better vaccines. In newer work they have confirmed that feline junctional adhesion molecule A is a bona fide receptor for FCV and have purified the FCV capsid and the ectodomain of the JAM-A receptor for structural and functional studies in collaborations they have established with Drs. B. V. Prasad at Baylor College of Medicine and T. Dermody at Vanderbilt University.