Ultrastructural insights into pathogen clearance by autophagy.

Area of Science:

• Cellular Biology
• Microbiology
• Immunology

Background:

• Autophagy is a cellular defense mechanism against intracellular bacterial pathogens like Salmonella.
• Upon escape from the Salmonella-containing vacuole (SCV), bacteria are ubiquitinated, but the precise localization of ubiquitin and its relation to autophagic membranes remain unclear.

Purpose of the Study:

• To investigate the spatial distribution of ubiquitin, the ruptured SCV membrane, and phagophores around cytosolic Salmonella.
• To elucidate the role of the SCV in the initiation of xenophagy.

Main Methods:

• Transmission electron microscopy (TEM) was employed to visualize the ultrastructural localization of ubiquitin, SCV remnants, and phagophores.
• Analysis of cytosolic Salmonella and their association with host cell membranes.

Main Results:

• Ubiquitin was found to be enriched on the fragmented SCV, rather than directly on the bacterial surface.
• Cytosolic bacteria lacking an intact SCV were less efficiently targeted by phagophores.
• Large autophagic vacuoles, associated with endoplasmic reticulum membranes, were observed engulfing multiple bacteria, with SCV remnants present within.

Conclusions:

• The fragmented Salmonella-containing vacuole plays a significant role in recruiting and forming autophagic phagophores during xenophagy.
• Mechanisms of xenophagy targeting damaged SCVs share parallels with mitophagy targeting damaged mitochondria.