Removal of hypersignaling endosomes by simaphagy

Area of Science:

• Cell Biology
• Molecular Biology
• Cancer Research

Background:

• Activated transmembrane receptors signal after endocytosis.
• Sustained signaling from dysfunctional endosomes, due to impaired receptor internalization into intralumenal vesicles (ILVs), is linked to cancer.
• A cellular surveillance mechanism is needed to clear aberrant signaling endosomes.

Purpose of the Study:

• To investigate the cellular mechanism for detecting and clearing physically intact endosomes with accumulated signaling receptors.
• To understand the role of autophagy in terminating endosomal signaling.

Main Methods:

• Proximity biotinylation and proteomics on ESCRT-0 defective endosomes.
• Live cell microscopy to observe receptor ubiquitination, phosphorylation, and recruitment of autophagy proteins.
• Light and electron tomography to visualize endosome engulfment by phagophores.
• Inhibition of autophagy to assess its impact on signaling and cell migration.

Main Results:

• ESCRT-0 defective endosomes showed enrichment of ubiquitin-binding autophagy receptors NBR1 (NBR1 autophagy cargo receptor) and SQSTM1 (sequestosome 1).
• Loss of ESCRT-0 (HGS/HRS) or ESCRT-I (VPS37) subunits led to high levels of ubiquitinated and phosphorylated receptors, with recruitment of NBR1, SQSTM1, and autophagy initiation proteins.
• Endosomes with intact membranes but aberrant receptor downregulation were engulfed by phagophores, a process inhibited by blocking autophagy, leading to increased signaling and directed cell migration.

Conclusions:

• Dysfunctional endosomes are recognized and cleared by autophagy, termed simaphagy.
• Simaphagy acts as a failsafe mechanism to terminate aberrant receptor signaling.
• This process is crucial for preventing sustained signaling implicated in cancer progression.