Ubiquitin-Mediated Regulation of Autophagy During Viral Infection

Purpose of Review

Virus infections skew the host autophagic response to meet their replication and transmission demands by tapping into the critical host regulatory mechanisms that control the autophagic flux. This review is a compendium of previous reports highlighting the mechanisms that viruses adapt to hijack the host ubiquitination machinery to repurpose autophagy for their sustenance.

Recent Findings

Emerging evidence suggests a critical role of host ubiquitin machinery in the manifestation of the antiviral or proviral functions of autophagy. Lately, more emphasis has been laid to identify specific host E3 ubiquitin ligases, their targets (viral or host), and characterizing corresponding ubiquitin linkages by biochemical or genome-wide genetic screening approaches.

Summary

Here, we highlight how viruses ingeniously engage and subvert the host ubiquitin-autophagy system to promote virus replication and antagonize intracellular innate immune responses.

Chandipura virus induces cell death in cancer cell lines of human origin and promotes tumor regression in vivo

Chandipura virus (CHPV) is an emerging human pathogen of great clinical significance. In this study, we have investigated the susceptibility pattern of both normal and cancer cell lines of human origin to wild-type (wt) CHPV in order to explore the possibility of developing CHPV as an oncolytic vector (OV). Marked cytopathic effect along with enhanced virus output was observed in cancer cell lines (HeLa, A549, U-138, PC-3, and HepG2) in comparison to normal human adult dermal fibroblast (HADF) cells. At an MOI of 0.1, cancer cell lines were differentially susceptible to CHPV, with cells like HeLa and U-138 having pronounced cell death, while the PC-3 were comparatively resistant. All cell lines used in the study except U-138 restricted CHPV infection to varying degrees with IFN-β pre-treatment and supplementation of interferon (IFN) could neither activate the IFN signaling pathway in U-138 cells. Finally, U-138 tumor xenografts established in non-obese diabetic severe combined immunodeficiency (NOD/SCID) mice showed significant delay in tumor growth in the CHPV-challenged animals. Thus, targeted cytopathic effect in cancer cells at a very low dose with restricted replication in normal cells offers a rationale to exploit CHPV as an oncolytic vector in the future.

Functional Dissection of the Dominant Role of CD55 in Protecting Vesicular Stomatitis Virus against Complement-Mediated Neutralization

The human complement system is an important part of the innate immune system. Its effector pathways largely mediate virus neutralization. Vesicular stomatitis virus (VSV) activates the classical pathway of the complement, leading to virus neutralization by lysis. Two host-derived membrane-associated regulators of complement activation (RCA), CD55 and CD46, which are incorporated into the VSV envelope during egress, confer protection by delaying/resisting complement-mediated neutralization. We showed previously that CD55 is more effective than CD46 in the inhibition of neutralization. In this study, we identified that, at the protein level, VSV infection resulted in the down-regulation of CD46 but not CD55. The mRNA of both the RCAs was significantly down-regulated by VSV, but it was delayed in the case of CD55. The immunoblot analysis of the levels of RCAs in the progeny virion harvested at three specific time intervals, points to an equal ratio of its distribution relative to viral proteins. Besides reconfirming the dominant role of CD55 over CD46 in shielding VSV from complement, our results also highlight the importance of the subtle modulation in the expression pattern of RCAs in a system naturally expressing them.

Ultrafast changes in lattice symmetry probed by coherent phonons

The electronic and structural properties of a material are strongly determined by its symmetry. Changing the symmetry via a photoinduced phase transition offers new ways to manipulate material properties on ultrafast timescales. However, to identify when and how fast these phase transitions occur, methods that can probe the symmetry change in the time domain are required. Here we show that a time-dependent change in the coherent phonon spectrum can probe a change in symmetry of the lattice potential, thus providing an all-optical probe of structural transitions. We examine the photoinduced structural phase transition in VO(2) and show that, above the phase transition threshold, photoexcitation completely changes the lattice potential on an ultrafast timescale. The loss of the equilibrium-phase phonon modes occurs promptly, indicating a non-thermal pathway for the photoinduced phase transition, where a strong perturbation to the lattice potential changes its symmetry before ionic rearrangement has occurred.