The Salmonella Deubiquitinase SseL Inhibits Selective Autophagy of Cytosolic Aggregates

Abstract

Cell stress and infection promote the formation of ubiquitinated aggregates in both non-immune and immune cells. These structures are recognised by the autophagy receptor p62/sequestosome 1 and are substrates for selective autophagy. The intracellular growth of Salmonella enterica occurs in a membranous compartment, the Salmonella-containing vacuole (SCV), and is dependent on effectors translocated to the host cytoplasm by the Salmonella pathogenicity island-2 (SPI-2) encoded type III secretion system (T3SS). Here, we show that bacterial replication is accompanied by the formation of ubiquitinated structures in infected cells. Analysis of bacterial strains carrying mutations in genes encoding SPI-2 T3SS effectors revealed that in epithelial cells, formation of these ubiquitinated structures is dependent on SPI-2 T3SS effector translocation, but is counteracted by the SPI-2 T3SS deubiquitinase SseL. In macrophages, both SPI-2 T3SS-dependent aggregates and aggresome-like induced structures (ALIS) are deubiquitinated by SseL. In the absence of SseL activity, ubiquitinated structures are recognized by the autophagy receptor p62, which recruits LC3 and targets them for autophagic degradation. We found that SseL activity lowers autophagic flux and favours intracellular Salmonella replication. Our data therefore show that there is a host selective autophagy response to intracellular Salmonella infection, which is counteracted by the deubiquitinase SseL.

Intracellular Salmonella induces aggrephagy of host endomembranes in persistent infections

“…The requirement of MT dynamics for formation of the LGP C aggregate provides in our opinion a strong evidence for differentiating this aggresome from the ALIS structures that are reported by Mesquita et al in epithelial cells infected with S. Typhimurium. 59 We also confirmed that incubation of fibroblasts with an extract of heat-killed bacteria (which contains lipopolysaccharide [LPS] and flagellin) does not trigger formation of the LGP C aggresome (Fig. S12).…”

“…This is in marked contrast to the formation of ALIS by S. Typhimurium in epithelial cells. 59 Live cell-imaging microscopy allowed us to uncover how the distinct compartments that interact with the autophagy machinery evolve in the S. Typhimurium-infected fibroblast. Besides the distinct structural features observed, we identified differences with epithelial cells regarding autophagy receptors, timing of the autophagy events and the pathogen functions involved.…”

“…57 Of interest, the simultaneous loss of SifA and other SPI2 effectors such as SseJ or SopD2, leads to increased stability of the phagosomal membrane. 58,59 Infection of fibroblasts with sifAD, sseJD and sopD2D single mutants showed that SifA is required for formation of the LGP C aggregate (Fig. 9A).…”

The ubiquitin system: a critical regulator of innate immunity and pathogen–host interactions

“…110 For example, the effector SseL is identified as a DUB and deubiquitinates the ubiquitin aggregates surrounding the SCV to help bacteria escape from autophagy. 111 Another DUB discovered in Sa. typhimurium is AvrA, which inhibits host inflammatory responses by deubiquitinating IκBα and β-catenin.…”

Structural and enzymatic characterization of a host-specificity determinant fromSalmonella

“…The catalytic activity of YopJ ultimately leads to the inhibition of innate and adaptive immunity responses and induction of cell death (Mukherjee et al, 2007;Paquette et al, 2012;Orth, 2002;Viboud & Bliska, 2005). In addition, S. Typhimurium produces a deubiquitinase, SseL, which functions by using a His-Asn-Cys triad to remove Lys63-linked ubiquitin chains from SCV-associated aggregates that are targeted for autophagic degradation (Mesquita et al, 2012;Rytkö nen et al, 2007). In this manner, SseL decreases the autophagic flux within the host, consequently contributing to down-modulation of NF-B-dependent cytokine production and macrophage-delayed cytotoxicity (Mesquita et al, 2012;Rytkö nen et al, 2007;Le Negrate et al, 2008).…”