SUMOylation and Viral Infections of the Brain

Abstract

The small ubiquitin-like modifier (SUMO) system regulates numerous biological processes, including protein localization, stability and/or activity, transcription, and DNA repair. SUMO also plays critical roles in innate immunity and antiviral defense by mediating interferon (IFN) synthesis and signaling, as well as the expression and function of IFN-stimulated gene products. Viruses including human immunodeficiency virus-1, Zika virus, herpesviruses, and coronaviruses have evolved to exploit the host SUMOylation system to counteract the antiviral activities of SUMO proteins and to modify their own proteins for viral persistence and pathogenesis. Understanding the exploitation of SUMO is necessary for the development of effective antiviral therapies. This review summarizes the interplay between viruses and the host SUMOylation system, with a special emphasis on viruses with neuro-invasive properties that have pathogenic consequences on the central nervous system.

Keywords: HIV; SUMOylation; ZIKA; brain; coronavirus; cytomegalovirus; microglia; neuroinflammation; post-translational modifications.

Figure 1

SUMOylation: the Small Ubiquitin-related Modifier (SUMO) pathway. SUMO proteins are processed by a SUMO-specific protease at the C-terminal tail to expose a diglycine (-GG) motif, resulting in a mature SUMO peptide. SUMO is subsequently activated in an ATP-dependent reaction, creating an intermediate thioester bond between the active site of SUMO and the heterodimeric E1-activating enzyme (SAE1/SAE2). Following activation, SUMO is transferred from the E1 enzyme to Ubc9, and finally attached to a target lysine in the protein substrate, which is usually located within the consensus site. This final step is mediated by E3 ligase enzymes that function in substrate recognition and specificity. SUMOylation is reversible (deconjugation), and the same SUMO proteases involved in SUMO maturation catalyze the removal of SUMO from the target substrate. Cross-talk exists between SUMOylation and ubiquitylation through SUMO-targeted ubiquitin ligases (STUbLs). STUbLs are enzymes that catalyze the addition of ubiquitin to proteins that have been previously SUMOylated with SUMO chains. STUbL activity results in target proteins that are modified by both SUMO and ubiquitin, which can be targeted to the proteasome for degradation. AMP: adenosine monophosphate; PPi: pyrophosphate; Ub: ubiquitin. Adapted with permission from [26] 2021, Springer Nature. Figure was created with Biorender.

Figure 2

CTIP2 promotes the establishment of HIV-1 latency in microglia. CTIP2 participates in the establishment of HIV-1 latency in microglia by recruiting a chromatin modifying complex (or viral latency complex) to the viral promoter (HIV-1 LTR). This complex consists of: Sp1, which anchors CTIP2 to the viral promoter and acts as a scaffold for the recruitment of chromatin modifying proteins; HDAC1 and HDAC2, which are responsible for deacetylation of Nuc-1, one of the nucleosomes positioned immediately downstream of the transcriptional start site of the HIV-1 LTR; and the histone methyltransferase Suv39H1, which contributes to HIV-1 silencing through methylation of Nuc-1. CTIP2 also recruits the demethylase, LSD1, and the SUMO E3 ligase, TRIM28, which, in association with CTIP2, contributes to HIV-1 gene silencing. Several of the CTIP2-associated proteins in the viral latency complex interact with the host SUMOylation system. Accordingly, determining the role of the SUMOylation in the establishment and/or persistence of HIV-1 latency in microglia could aid in the design of new pharmacological agents that target HIV-1 viral reservoirs. Sp1: specificity protein 1; COUP-TF: chicken ovalbumin upstream promoter transcription factor; CTIP2: COUP-TF interacting protein 2; HDAC1/2: histone deacetylase 1/2; TRIM28: tripartite motif containing 28; SUMO: small ubiquitin-related modifier. Figure was created with Biorender.

Figure 3

Inflammatory pathology in COVID-19 brains. (A) Section of the hypoglossal nucleus shows several motor neurons and a microglial nodule (arrow). (B) An adjacent section immunolabeled for CD68 (brown) shows clustered microglia within the nodule. Inset: Microglia in close apposition to a hypoglossal neuron (CD68⁺). (E) The locus coeruleus contains a microglial nodule with a degenerating neuron in the center, identified by its residual neuromelanin (arrow). (F,G) Neurons of the dorsal motor nucleus of the vagus surrounded by CD68⁺ microglia. (H,I) Microglial nodules in the dentate nucleus (arrows in (H)), neuron in the middle of a nodule (arrow in (I)), CD68. Scale bar in (A,B) = 200 µm; (E) = 10 µm; (F,G) = 50 µm; (H) = 100 µm; (I) = 50 µm. Adapted with permission from [61] 2021, Oxford University Press. Panels C, D, J, K from the original publication are not shown.

Figure 4

ZIKV NS5 forms unique nuclear speckles that disrupt PML/SUMO-1 NBs. (A) hBMECs grown on microslides were infected with ZIKV-PRV (MOI, 2) and fixed 12, 18, and 24 hpi; immunolabeled for ZIKV NS5 and SUMO-1 or PML; and visualized by confocal microscopy. (B) Mock-infected or ZIKV-infected hBMECs were immunolabeled for PML and SUMO-1 24 hpi and visualized by confocal microscopy. (C) ZIKV-infected hBMECs were immunolabeled for ZIKV NS5 and fibrillarin at 24 hpi and visualized by confocal microscopy. Experiments were conducted in triplicate, repeated at least three times, and representative data are presented. Bars = 5 μm. Adapted with permission from [76]. 2020, American Society for Microbiology. PML: promyelocytic leukemia; ZIKV-PRV: PRVABC59 ZIKV strain; hBMEC: human brain microvascular endothelial cells; MOI: multiplicity of infection; hpi: hours post-infection.

Figure 5

Molecular docking model of the binding between the putative non-structural 5 (NS5) SUMO-interacting motifs of flaviviruses and the SUMO1 protein. (A) Top panel: multiple sequence alignment of the amino acid sequences of the putative NS5 protein SUMO-interacting motifs (SIM) of Zika virus (ZIKV), dengue virus (DENV) (serotype 3), Japanese encephalitis virus (JEV), West Nile virus (WNV), and yellow fever virus (YFV). Bottom panel: Stick representation of the structural similarities among the five flaviviruses’ putative NS5 SIM peptides. (B) Schematic representation of the binding between the SUMO1 protein and the five flaviviruses’ putative NS5 SIM peptides. The SUMO1 protein is shown in tan and the NS5 SIM peptides are shown in different colors. (C) Ribbon representation showing the interacting amino acid residues of the putative ZIKV NS5 SIM peptide and the active sites of the SUMO1 protein. The putative ZIKV NS5 SIM peptide and SUMO1 protein are displayed in magenta and blue, respectively. The interacting residues are shown as sticks with hydrogen bonds represented by yellow dashed lines. Adapted with permission from [74]. 2019, MDPI.

Figure 6

Interactions between human cytomegalovirus (HCMV) and SUMO. HCMV entry to the central nervous system (CNS) is secondary to peripheral organ infection. Passage across the blood-brain barrier is thought to be mediated by monocytes. Upon crossing the BBB, HCMV infects resident cells and has been shown to interact with the host SUMOylation system. Immediate–early protein-1 (IE1) was the first viral protein identified as a SUMO interactor and has been shown to inhibit the SUMOylation of promyelocytic leukemia (PML) bodies, which suppresses innate immune responses. Similarly, the HCMV latency-associated protein, LUNA, functions as a de-SUMOylase to promote PML de-SUMOylation. The HCMV pp71 protein promotes the SUMOylation of its cellular substrate, Daxx, though the functional consequence of this interaction is unknown. pp71 has also been shown to mediate Daxx degradation through a ubiquitin-independent pathway. Figure was created with Biorender.