SNAREs: a double-edged sword for intravacuolar bacterial pathogens within host cells

Identification a novel syntaxin-like protein from silkworm Bombyx mori pathogen Nosema bombycis and characteristics its membrane fusion function

Pebrine is a serious disease of the silkworm, Bombyx mori, caused by the first identified microsporidium Nosema bombycis, which is an obligate parasitic single-celled eukaryote. The pathogen can spread both horizontally and vertically, severely affecting sericulture. SNARE proteins mainly mediate the transport of vesicles and membrane fusion, playing a key role in the biological processes. The microsporidium is known to have a well-developed membrane system, especially the polaroplast which occupies most of the volume of mature spores. In order to explore the function of microsporidian SNARE protein, the transcription and subcellular localization characteristics of a novel Syntaxin-like protein (NbSTX-like) from N. bombycis that had a conserved t-SNARE motif were analyzed. In the different development stages of N. bombycis, the NbSTX-like expressed in the nucleus of meronts, then transited to the cytoplasm in the sporonts, gradually gathered at the two ends of the sporoblasts, and finally concentrated at the polaroplast, posterior vacuole and plasma membrane region of mature spores. Interestingly, the rNbSTX-like protein could fuse liposomes to form large vesicular and tubular structures. The formation of sporoplasms was inhibited by the anti-NbSTX-like serum, implying that NbSTX-like protein participated in sporoplasm maturation. These findings laid a foundation for studying the function of SNARE proteins in microsporidia and provided new insights for the prevention and control of sericulture pathogens.

Self-Assembling Imidazolium Nanoaggregates Trigger a Unique Dynamin-Dependent Cell Death via Cytoplasmic Vacuolization and Mitochondrial Dysfunction in Human Lung Adenocarcinoma

The identification of alternative cell death pathways is key to developing therapies for apoptosis-resistant cancers. We investigated cell death induced by delocalized lipophilic cation (DLC) nanoaggregates in A549 lung carcinoma cells. These DLCs trigger a dynamin-dependent, nonapoptotic pathway involving cytoplasmic vesicle accumulation and mitochondrial dysfunction. Leveraging the mitochondria-targeting ability of lipophilic cations, we designed and synthesized fluorescent mitochondrion-toxic molecules with potent cytotoxicity against A549, MDA-MB-231, and MCF-7 cells. Dynamic light scattering revealed the nanoaggregate formation of the lead compound, L3, in the RPMI media. L3 inhibited metastasis and clonal expansion, induced vacuole formation post endocytosis, and impaired the mitochondrial function, disrupting ATP levels. This led to mitochondrial permeability transition pore (MPTP) opening and oxidative imbalance via glutathione perturbation. L3 demonstrated strong antitumor activity in vitro and in vivo, showing high potential for treating apoptosis-resistant cancers.

Trehalose dimycolate inhibits phagosome maturation and promotes intracellular M. tuberculosis growth via noncanonical SNARE interactions

Mycobacterial cell envelopes are rich in unusual lipids and glycans that play key roles during infection and vaccination. The most abundant envelope glycolipid is trehalose dimycolate (TDM). TDM compromises the host response to mycobacterial species via multiple mechanisms, including inhibition of phagosome maturation. The molecular mechanism by which TDM inhibits phagosome maturation has been elusive. We find that a clickable, photoaffinity TDM probe recapitulates key phenotypes of native TDM in macrophage host cells and binds several host Soluble N-ethylmaleimide-Sensitive Factor Attachment Proteins Receptor (SNARE) proteins, including Vesicle Transport through Interaction with t-SNAREs 1B (VTI1B), Syntaxin 8 (STX8), and Vesicle-Associated Membrane Protein 2 (VAMP2). VTI1B and STX8 normally promote endosome fusion by forming a complex with VAMP8. However, in the presence of Mycobacterium tuberculosis, VTI1B and STX8 complex with VAMP2, which in turn decreases VAMP8 binding. VAMP2 acts together with mycolate structure to inhibit phagosome maturation and promotes intracellular M. tuberculosis replication. Thus one mechanism by which TDM constrains the innate immune response to M. tuberculosis is via noncanonical SNARE complexation.

Trehalose dimycolate inhibits phagosome maturation and promotes intracellular M. tuberculosis growth via noncanonical SNARE interaction

Mycobacterial cell envelopes are rich in unusual lipids and glycans that play key roles during infection and vaccination. The most abundant envelope glycolipid is trehalose dimycolate (TDM). TDM compromises the host response to mycobacterial species via multiple mechanisms, including inhibition of phagosome maturation. The molecular mechanism by which TDM inhibits phagosome maturation has been elusive. We find that a clickable, photoaffinity TDM probe recapitulates key phenotypes of native TDM in macrophage host cells and binds several host SNARE proteins, including VTI1B, STX8, and VAMP2. VTI1B and STX8 normally promote endosome fusion by forming a complex with VAMP8. However, in the presence of Mycobacterium tuberculosis , VTI1B and STX8 complex with VAMP2, which in turn decreases VAMP8 binding. VAMP2 acts together with mycolate structure to inhibit phagosome maturation and promotes intracellular M. tuberculosis replication. Thus one mechanism by which TDM constrains the innate immune response to M. tuberculosis is via non-canonical SNARE complexation.

Significance Statement

Glycolipids from the Mycobacterium tuberculosis cell envelope, particularly trehalose dimycolate (TDM), play major roles in subverting the immune response to this intracellular pathogen. How subversion occurs is often unclear because glycans and lipids are technically challenging to study in cells. We discovered that a TDM-mimicking chemical probe interacts with three host SNARE proteins, including two that regulate endosome fusion and one that does not. The presence of TDM or M. tuberculosis triggers abnormal binding of these SNAREs, which in turn inhibits the fusion of M. tuberculosis -containing phagosomes with lysosomes and promotes M. tuberculosis replication. Our work provides an unusual example of a bacterial pathogen restricting the immune response via glycolipid-SNARE interactions.

The Formation and Features of Massive Vacuole Induced by Nutrient Deficiency in Human Embryonic Kidney Cells

BACKGROUND

Cellular vacuolization is a commonly observed phenomenon under physiological and pathological conditions. However, the mechanisms underlying vacuole formation remain largely unresolved.

METHODS

LysoTracker Deep Red probes and Enhanced Green Fluorescent Protein-tagged light chain 3B (LC3B) plasmids were employed to differentiate the types of massive vacuoles. By confocal microscopy, lysosome-like massive vacuoles (LysoTracker Deep Red+), autophagosome-like massive vacuoles (LC3B-enhanced green fluorescent protein (EGFP+)), and autolysosome-like massive vacuoles (LC3B-EGFP+ LysoTracker Deep Red+) in starved HEK293T cells were observed.

RESULTS

In this study, we demonstrated that nutrient deficiency can induce the formation of massive vacuoles that appear highly electron-lucent in HEK293T cells. Additionally, these massive vacuoles, resulting from nutrient depletion, can originate from various organelles, including small vacuoles, autophagosomes, lysosomes, and autolysosomes. We found that massive vacuoles could form through two primary mechanisms: the accumulation of small vacuoles into larger vacuoles or the fusion of homogeneous or heterogeneous vacuoles. Further analysis revealed that the membranes of massive vacuoles, regardless of origin, were composed of a bilayer membrane structure. As the volume of the massive vacuoles increased, the cytoplasm and nucleus were displaced toward the periphery of the cells, leading to the formation of signet ring-like cells. Interestingly, we provided evidence that complete massive vacuoles or autophagosome-like massive vacuoles can be released and exist independently outside HEK293T cells.

CONCLUSIONS

Nutrient deprivation induces the formation of heterogeneous, massive vacuoles in human embryonic kidney cells, some of which contribute to the development of signet ring cells, while others lead to extracellular vacuole formation.