Cells build a new trash-sorting station inside stressed mitochondria — in lab models

Edited by Samael A. Tejada, Founder & Editor-in-Chief

Mitochondria are bound by a double membrane and cannot easily call for help when their own proteins misfold. Yu, Jiang, Chen and colleagues at Nankai University asked how proteostatic stress inside mitochondria is relayed to the rest of the cell — and found a structure the field had not described: SIMPA, a stress-induced aggregate that forms inside mitochondria and is handed off to other organelles for disposal.

What is SIMPA, and what builds it?

Under proteostatic stress — hydrogen peroxide, heat shock and respiratory-chain inhibitors in human U-2 OS and HeLa cells — damaged proteins aggregated inside mitochondria. The aggregation depended on GRP75, a mitochondrial chaperone: knocking GRP75 down sharply reduced SIMPA formation, while its substrate-binding and ATPase domains and its mitochondrial localization were all required for the aggregates to form and later clear.

How does the cell take the trash out?

Clearance ran through mitochondria-associated membranes (MAM) — the contact sites where mitochondria touch the endoplasmic reticulum. GRP75 interacted with the MAM proteins VDAC1 and ITPR3 during SIMPA turnover, and knocking either one down slowed clearance. The endoplasmic reticulum's own stress pathways were part of the machinery too: phosphorylated IRE1α gathered at SIMPA sites, XBP1 knockdown left residual aggregates behind, and ER-associated degradation markers rose during recovery.

Does this happen in a living animal — and can it help?

Yes, at the preclinical level. SIMPA appeared in mouse liver tissue across three injury models — alcohol, carbon tetrachloride and acetaminophen (APAP) — and mice engineered to overexpress Grp75 in the liver showed less necrosis, lower liver-enzyme markers and better outcomes after APAP injury. That makes SIMPA a candidate quality-control pathway worth targeting, not yet a therapy: everything shown is cells and mice, and no human data exist.

Where the evidence stands

• Established: Mitochondria maintain protein quality through chaperones, proteases and mitophagy, and ER–mitochondria contact sites coordinate stress signals between organelles.
• Preliminary: In human cell models and mouse liver-injury models (Nature, Oct 21, 2026), stress-induced mitochondrial protein aggregates (SIMPA) formed in a GRP75-dependent manner, cleared via MAM contacts (VDAC1/ITPR3) and ER stress pathways, and promoting the pathway via Grp75 overexpression ameliorated liver-injury phenotypes in mice.
• Absent: Human data of any kind, and any drug or intervention shown to act through SIMPA in people.