High error rate on production line triggers slowdown
Living cells function like miniature factories, producing more than 25,000 distinct proteins that must fold into precise three-dimensional shapes to work correctly. When cellular stresses overwhelm these production systems, proteins can be synthesized incorrectly and remain unfolded or misfolded, which impairs function and can trigger broader cellular problems.
Researchers at Duke University, working with collaborators in Singapore, have discovered that cells detect the accumulation of misfolded proteins and respond by reorganizing where protein synthesis occurs. The study, published Sept. 11, 2014 in Cell, reveals a dynamic relocalization of mRNA and ribosomes between the endoplasmic reticulum (ER) and the cytosol during stress, offering new insight into how cells manage protein misfolding and the unfolded protein response (UPR).
For decades, cell biologists have understood the broad architecture of protein production: DNA in the nucleus is transcribed into messenger RNA (mRNA), which exits the nucleus and is translated by ribosomes. Many ribosomes attach to the rough endoplasmic reticulum, an accordion-shaped membrane system that coordinates synthesis and folding of secreted and membrane proteins. Other ribosomes remain free in the cytosol, translating proteins that function in the cytoplasm.
When cells undergo stress—such as heat shock, nutrient deprivation, or chemical insults—protein folding can fail. The unfolded protein response (UPR) is a well-characterized alarm that reduces overall protein synthesis and activates pathways to refold or degrade misfolded proteins. The Duke-led team asked whether the UPR is accompanied by additional, spatially organized changes to the protein production machinery.
Using cultured cells exposed to thapsigargin, a compound known to induce ER stress, the researchers separated ribosome-associated mRNAs into two pools: those bound to ER-attached ribosomes and those associated with free cytosolic ribosomes. They found that, under stress, many mRNAs normally translated at the ER are rapidly released and shifted to the cytosolic ribosome pool. After stress is resolved, those mRNAs return to the ER, resuming their previous localization and synthesis patterns.
“We have identified an entirely new mechanism for how the cell responds to stress,” said Christopher V. Nicchitta, Ph.D., professor of cell biology at Duke University School of Medicine. “Essentially, the cell remodels the organization of its protein production machinery in order to compartmentalize the tasks at hand.” According to Nicchitta, this relocalization acts like a holding pattern: rather than only slowing production or boosting clearance of damaged proteins, the cell temporarily shifts mRNA away from the ER to limit production of proteins that are most likely to misfold under stress.
Importantly, the relocalization selectively affected the subset of mRNAs encoding secreted proteins and membrane proteins—such as hormones or growth factor receptors—whose misfolding can most strongly trigger ER stress and the UPR. Cytosolic translation of other classes of proteins was less affected. The selective nature of this response suggests the cell can target the production of high-risk proteins during acute stress while preserving synthesis of others.
The findings have implications for understanding diseases in which misfolded proteins accumulate, including Alzheimer’s disease, amyotrophic lateral sclerosis (ALS), Huntington’s disease, Parkinson’s disease, and type 2 diabetes. In these neurodegenerative and metabolic disorders, chronic protein-misfolding stress overwhelms cellular quality-control systems. Revealing how cells temporarily reorganize their translation machinery provides new context for how protective responses succeed or fail in disease.
Nicchitta and colleagues are now investigating the molecular factors that determine which mRNAs are released from the ER and which remain, and how manipulating those factors alters the stress response. The team has identified at least one promising candidate factor and is testing how its modulation affects mRNA localization and cell survival under stress.
The research was supported by the National Institutes of Health (GM101533) and the Singapore Ministry of Health (Duke/Duke-NUS Research Collaboration Award).
Contact: Karl Bates – Duke University
Source: Duke University press release
Image Source: University of Edinburgh, via the Wellcome Trust (adapted from the Duke University press release)
Original Research: Abstract for “The Unfolded Protein Response Triggers Selective mRNA Release from the Endoplasmic Reticulum” by David W. Reid, Qiang Chen, Angeline S.-L. Tay, Shirish Shenolikar, and Christopher V. Nicchitta in Cell. Published online September 11, 2014, doi:10.1016/j.cell.2014.08.012