New twists in the unfolded protein response

Alterations in inter-organelle crosstalk and Ca2+ signaling through mitochondria during proteotoxic stresses

BACKGROUND

Biogenesis and function of mitochondria is profoundly dependent on cytosolic translation of mitochondrial pre-proteins and its subsequent translocation and folding inside the organelle. Continuous exposure of non-native precursor proteins, exposure to damaging by-products of oxidative phosphorylation, load of mis-targeted or misfolded proteins from neighbouring compartments and unremitting demand of communication between mitochondrial and nuclear genomes, continuously pose proteotoxic threats to the organelle. Our knowledge of cellular mechanisms to cope up with such impending threat of proteotoxicity to mitochondria, is currently evolving. In recent years, several unique response and survival pathways have been discovered shedding light on cellular strategies to cope with stressed and dysfunctional mitochondria. As mitochondria compulsorily communicate with nucleus, cytosol and endoplasmic reticulum (ER) for its own biogenesis and function and in turn maintain critical cellular processes for survival, any impairment in communication by stressed or dysfunctional mitochondria may end up with fatal consequences.

DISCUSSION AND IMPLICATION

In this review, we have discussed about possible sources of mitochondrial proteotoxicity and the recent developments regarding cellular strategies to counter such stress to overcome dysfunctions of the organelle. Mitochondrial communication with neighbouring subcellular compartments like ER and cytosol during proteotoxic stress have been explored. In the context of mitochondrial proteotoxicity, alterations of crucial inter-organelle connections like ER-mitochondria contact sites and its implication on mitochondrial signaling activity like Ca²⁺ signaling have been dissected. Furthermore, an overview of pathological conditions, mainly neurodegenerative disorders that are known to be associated with mitochondrial proteotoxicity and Ca²⁺ dysregulation has been presented.

Endoplasmic reticulum quality control by garbage disposal

Various types of intracellular and extracellular stresses disturb homeostasis in the endoplasmic reticulum (ER) and, thus, trigger the ER stress response. Unavoidable and/or prolonged ER stress causes cell toxicity and occasionally cell death. The malfunction or death of irreplaceable cells leads to conformational diseases, including diabetes mellitus, ischemic diseases, metabolic diseases, and neurodegenerative diseases. In the past several decades, many studies have revealed the molecular mechanisms of the ER quality control system. Cells resolve ER stress by promptly and accurately reducing the amount of malfolded proteins. Recent reports have revealed that cells possess several types of ER-related disposal systems, including mRNA decay, proteasomal degradation, and autophagy. The removal of dispensable RNAs, proteins, and organelle parts may enable the effective maintenance of a functional ER. Here, we provide a comprehensive understanding of the ER quality control system by focusing on ER-related garbage disposal systems.

The UPR branch IRE1-bZIP60 in plants plays an essential role in viral infection and is complementary to the only UPR pathway in yeast

The unfolded protein response (UPR) signaling network encompasses two pathways in plants, one mediated by inositol-requiring protein-1 (IRE1)-bZIP60 mRNA and the other by site-1/site-2 proteases (S1P/S2P)-bZIP17/bZIP28. As the major sensor of UPR in eukaryotes, IRE1, in response to endoplasmic reticulum (ER) stress, catalyzes the unconventional splicing of HAC1 in yeast, bZIP60 in plants and XBP1 in metazoans. Recent studies suggest that IRE1p and HAC1 mRNA, the only UPR pathway found in yeast, evolves as a cognate system responsible for the robust UPR induction. However, the functional connectivity of IRE1 and its splicing target in multicellular eukaryotes as well as the degree of conservation of IRE1 downstream signaling effectors across eukaryotes remains to be established. Here, we report that IRE1 and its substrate bZIP60 function as a strictly cognate enzyme-substrate pair to control viral pathogenesis in plants. Moreover, we show that the S1P/S2P-bZIP17/bZIP28 pathway, the other known branch of UPR in plants, does not play a detectable role in virus infection, demonstrating the distinct function of the IRE1-bZIP60 pathway in plants. Furthermore, we provide evidence that bZIP60 and HAC1, products of the enzyme-substrate duet, rather than IRE1, are functionally replaceable to cope with ER stress in yeast. Taken together, we conclude that the downstream signaling of the IRE1-mediated splicing is evolutionarily conserved in yeast and plants, and that the IRE1-bZIP60 UPR pathway not only confers overlapping functions with the other UPR branch in fundamental biology but also may exert a unique role in certain biological processes such as virus-plant interactions.