Reorganization of the Endosomal System in Salmonella-Infected Cells: The Ultrastructure of Salmonella-Induced Tubular Compartments

Abstract

During the intracellular life of Salmonella enterica, a unique membrane-bound compartment termed Salmonella-containing vacuole, or SCV, is formed. By means of translocated effector proteins, intracellular Salmonella also induce the formation of extensive, highly dynamic membrane tubules termed Salmonella-induced filaments or SIF. Here we report the first detailed ultrastructural analyses of the SCV and SIF by electron microscopy (EM), EM tomography and live cell correlative light and electron microscopy (CLEM). We found that a subset of SIF is composed of double membranes that enclose portions of host cell cytosol and cytoskeletal filaments within its inner lumen. Despite some morphological similarities, we found that the formation of SIF double membranes is independent from autophagy and requires the function of the effector proteins SseF and SseG. The lumen of SIF network is accessible to various types of endocytosed material and our CLEM analysis of double membrane SIF demonstrated that fluid phase markers accumulate only between the inner and outer membrane of these structures, a space continual with endosomal lumen. Our work reveals how manipulation of the endosomal membrane system by an intracellular pathogen results in a unique tubular membrane compartmentalization of the host cell, generating a shielded niche permissive for intracellular proliferation of Salmonella.

Proteomes of Host Cell Membranes Modified by Intracellular Activities of Salmonella enterica

“…LAMP1 is a prominent membrane-integral component of SIFs, and the fusion with GFP enabled us to continuously monitor their development. LAMP1-GFP transfection of HeLa cells had no influence on the infection process of Salmonella as previous experiments demonstrated (15,33). As expected, the tagged effector protein SseF was present in SIFs and co-localized with LAMP1-GFP (Fig.…”

Intracellular Salmonella induces aggrephagy of host endomembranes in persistent infections

“…2C), a feature reported for the SIF formed in epithelial cells infected with S. Typhimurium. 40 The LGP C aggregate is therefore compatible with membranous SIF-like tubules packed like a "crumpled" paper ball. Altogether, these data suggest that, despite their different morphologies, the LGP C aggregates in fibroblasts and the SIF in infected epithelial cells have a common origin.…”

The metabolic pathways utilized by Salmonella Typhimurium during infection of host cells

“…Fusion of vesicles or endosomes containing transporters directly with the SCV would therefore result in an orientation in the SCV membrane, which would deplete the SCV of nutrients by transport out of the SCV into the cytosol. Hints as to how Salmonella may have solved this nontrivial topological problem may be found in recent ultrastructural studies of Sifs and other intracellular Salmonella-induced tubule structures (Krieger et al, 2014;Liss et al, 2017). Double membranes were observed within Sifs which contained host cell cytosol and cytoskeletal filaments, i.e., a 'vacuole within a vacuole', in which the inner, membrane-bound vacuole contained engulfed cytosolic material, including entrapped vesicles.…”