ISG15, a Small Molecule with Huge Implications: Regulation of Mitochondrial Homeostasis

Abstract

Viruses are responsible for the majority of infectious diseases, from the common cold to HIV/AIDS or hemorrhagic fevers, the latter with devastating effects on the human population. Accordingly, the development of efficient antiviral therapies is a major goal and a challenge for the scientific community, as we are still far from understanding the molecular mechanisms that operate after virus infection. Interferon-stimulated gene 15 (ISG15) plays an important antiviral role during viral infection. ISG15 catalyzes a ubiquitin-like post-translational modification termed ISGylation, involving the conjugation of ISG15 molecules to de novo synthesized viral or cellular proteins, which regulates their stability and function. Numerous biomedically relevant viruses are targets of ISG15, as well as proteins involved in antiviral immunity. Beyond their role as cellular powerhouses, mitochondria are multifunctional organelles that act as signaling hubs in antiviral responses. In this review, we give an overview of the biological consequences of ISGylation for virus infection and host defense. We also compare several published proteomic studies to identify and classify potential mitochondrial ISGylation targets. Finally, based on our recent observations, we discuss the essential functions of mitochondria in the antiviral response and examine the role of ISG15 in the regulation of mitochondrial processes, specifically OXPHOS and mitophagy.

Keywords: OXPHOS; interferon; mitochondria; mitophagy; ubiquitin-like modification.

Figure 1

Intracellular and extracellular activities of ISG15. Different stimuli trigger the expression of ISG15, which is produced as a precursor of 17 kDa with two ubiquitin-like domains linked by a hinge region (1). Intracellular ISG15 can be processed into its mature form and conjugated to de novo synthesized proteins in a process termed ISGylation. ISG15 processing exposes its carboxy-terminal LRLRGG motif, allowing its conjugation to lysine residues in target proteins to modulate their function. In addition, ISGylation is reversible due to the action of the protease USP18, which also regulates IFNAR-mediated signaling (2). ISG15 can remain unconjugated within the cell, regulating protein activity (3), or be secreted as a cytokine, acting as a chemotactic and stimulating factor for immune cells (4). Binding of ISG15 to LFA-1 integrin receptor on the surface of NK cells promotes the activation, production and release of IFN-γ IL-10 after IL-12 priming. Moreover, extracellular ISG15 is able to form dimers/multimers through cysteine residues, to modulate cytokine levels.

Figure 2

Predicted subcellular distribution of ISGylated proteins. Proteins identified as ISGylation targets in different proteomic studies were evaluated for their subcellular location. Percentage of the total ISGylated proteins located in each cellular organelle is shown.

Figure 3

Impact of ISG15 on mitochondrial activities. Mitochondria are targets of ISG15 and ISGylation in murine bone marrow-derived macrophages (BMDMs). ISGylated proteins can be found in all mitochondrial localizations, mainly in the mitochondrial intermembrane space (IMS) and inner mitochondrial membrane (IMM), where free ISG15 is also present. ISG15 and ISGylation are involved in the regulation of mitochondrial metabolism. Absence of ISG15 leads to alterations in OXPHOS, with lower oxygen consumption rates and ATP production levels, in addition to aberrant ETC supercomplexes assembly. Such disruption of OXPHOS mechanisms decreases ROS production, with repercussions for macrophage polarization. Mitophagy is also altered in cells lacking ISG15. Finally, ISG15-/- BMDM accumulate defective mitochondria and Parkin cannot be found in mitochondrial extracts, suggesting that ISG15 is important during the translocation of Parkin from the cytoplasm to mitochondria.