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. 2018 Jan 22;44(2):139-163.
doi: 10.1016/j.devcel.2017.12.020.

A Futile Battle? Protein Quality Control and the Stress of Aging

Free PMC article

A Futile Battle? Protein Quality Control and the Stress of Aging

Ryo Higuchi-Sanabria et al. Dev Cell. .
Free PMC article


There exists a phenomenon in aging research whereby early life stress can have positive impacts on longevity. The mechanisms underlying these observations suggest a robust, long-lasting induction of cellular defense mechanisms. These include the various unfolded protein responses of the endoplasmic reticulum (ER), cytosol, and mitochondria. Indeed, ectopic induction of these pathways, in the absence of stress, is sufficient to increase lifespan in organisms as diverse as yeast, worms, and flies. Here, we provide an overview of the protein quality control mechanisms that operate in the cytosol, mitochondria, and ER and discuss how they affect cellular health and viability during stress and aging.

Keywords: ER UPR; aging; heat-shock response; mitochondrial UPR; stress response.


Fig. 1
Fig. 1. Role of HSF1 in the heat shock response
In normal, steady-state conditions, HSF1 exists as an inactive monomer bound by HSP chaperones, including Hsp70 and Hsp90 (A). Under conditions of stress, HSF1 is released from Hsp70 and Hsp90, allowing it to trimerize, become phosphorylated, and gain DNA binding activity (B). In the nucleus, HSF1 trimers localize to conserved pentameric sequence motifs (nGAAn), called HSEs, to promote the expression of HSR genes that are necessary for protein repair and cell survival under heat stress (C).
Fig. 2
Fig. 2. Mitochondrial quality control is maintained by a series of protective mechanisms
The mitochondria has several dedicated quality control mechanisms, which repair and preserve the organelle both in the presence and absence of stress. UPRMT is regulated by ATFS-1, which is imported into the mitochondria and subsequently degraded by Lon protease under steady-state conditions (A). When the mitochondria is damaged and membrane potential is insufficient to drive import of ATFS-1 into the matrix, ATFS-1 shuttles to the nucleus to act as a transcription factor to promote expression of mitochondrial chaperones and other genes important in repairing damaged mitochondrial proteins and other organelle components (B). Damaged proteins in the mitochondria can also be selectively removed through mitochondria-associated degradation (C). Here, damaged proteins are presented to the outer surface of the mitochondria, ubiquitinated by substrate-specific E3 ubiquitin ligases, then pulled out of the mitochondria by the concerted efforts of Vms1p/Doa1p and Cdc48p. Once outside of the mitochondria, damaged proteins can be degraded by the cytosolic proteasome. On a larger scale, regions of damaged mitochondria can be teased apart from the rest of the mitochondria via fission (D). This damaged mitochondrial unit is subsequently targeted for degradation by the lysosome in a process known as mitophagy (E). Less critical damage, such as minor mutations in mtDNA or damage to specific mitochondrial proteins, can be complemented by fusion to mitochondria harboring fully functional mitochondrial components (F). Here, minor damage is demarcated with an asterisk (*). A mitochondrion harboring a damaged protein B*, but a fully functional protein A, can complement and itself be complemented via fusion to another mitochondrion harboring a fully functional protein B, but a damaged protein A*.
Fig. 3
Fig. 3. Multi-faceted quality control of the endoplasmic reticulum
Like the mitochondria, the ER also has a collection of unique quality control mechanisms to maintain ER function. There are three branches of UPRER, each consisting of a transmembrane protein with a luminal facing sensor for unfolded proteins, which signals damage to the nucleus through a unique transcription factor. Here, the IRE1-XBP1 arm of UPRER serves as a general representation of UPRER (A). When IRE-1 senses ER stress (e.g. presence of misfolded proteins), it produces homodimers, is phosphorylated, and promotes splicing of XBP1u mRNA localized at the ER. The resulting splice variant of XBP1 is capable of producing functional protein, XBP1s, which can shuttle to the nucleus, acting as a transcription factor to upregulate UPRER genes. Some proteins of downstream UPRER genes, such as chaperones, will shuttle back to the ER to repair damaged components. Damaged proteins in the ER can also be marked by ubiquitination, removed from the ER by the protein VCP, and targeted for degradation by the cytosolic proteasome in a process known as ERAD (B). Larger regions of ER damage can be resolved by sequestering the damage to a specific region of the ER. This damaged unit can then be specifically targeted for degradation upon binding of ER-phagy receptors (C). After this region of the ER buds into a vesicle, it fuses with a lysosome to destroy and recycle its contents.

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