Quality Control

Symposium Preview: "Quality Control"Protein folding is a problem that was born complex and shows few signs of getting simpler. Ever since Cyrus Levinthal posed his eponymous paradox in 1969 that an unfolded polypeptide chain offered an astronomical number of theoretical folding states, researchers have wrestled with the problem of how cells actually coax most proteins into proper shape. This daunting process is not always successful, and the resulting misfolded proteins must be identified and destroyed promptly by cells to maintain protein homeostasis (often termed proteostasis). The various pathways of protein quality control and cellular stress responses that maintain proteostasis are intimately connected to human disease including cancer and neurodegenerative disorders, according to Ramanujan Hegde of the Laboratory of Molecular Biology (LMB) in Cambridge, England. Hegde will be one of three featured speakers at the ASCB 2016 Annual Meeting symposium on Quality Control, Monday, December 5, in San Francisco. The other speakers are Anne Bertolotti, who is also at the LMB in Cambridge, and Laurie Glimcher, who just became President and CEO of the Dana-Farber Cancer Institute in Boston.

 

Hegde says that each of the three symposium speakers pursues a different question in protein quality control and stress responses, but their common theme is balance. All three are exploring the mechanisms that maintain proteostasis during health, and how these mechanisms go awry in various diseases.

 

Previewing the symposium, Hegde explains that the research of his LMB colleague, Bertolotti, is addressing the gradual failure of protein quality control in cells of the aging brain. “For a very long time, it’s been known that many neurodegenerative diseases such as Alzheimer’s or Parkinson’s are associated with accumulation and aggregates of misfolded proteins… and the brain’s ability to deal with these aberrant proteins seems to fail over time.” By studying how cells respond to protein misfolding stress, Bertolotti has discovered a new class of small molecules that temporarily limits protein synthesis selectively during stress, thereby giving the stressed cell a chance to recover its balance.

 

A major insight from Bertolotti’s studies is how precisely cells seem to balance protein synthesis with protein quality control capacity by using a multi-pronged system of feedback mechanisms to fine-tune both halves of this equation. Deviations from this homeostatic state, especially over long periods of time, are now thought by many in the field to play a decisive role in various neurodegenerative diseases. With her small molecule interventions that can subtly shift this balance, Bertolotti has opened exciting new therapeutic possibilities, says Hegde.

 

The quality control links to cancer come from the pathways cells use to respond to different types of stress.  Recent research by Glimcher in tumor cells and their surrounding environment points to a particular reliance of one such stress response pathway for tumor growth, says Hegde. This realization means that inhibiting key stress pathways might provide new therapeutic opportunities for certain types of cancer.

 

Glimcher is a physician-scientist who discovered a conserved transcription factor, XBP1, that proved to be a critical link in the pathway that communicates protein misfolding stress in the endoplasmic reticulum (ER) to the nucleus. This unfolded protein response (UPR) pathway, originally described in yeast, is now seen playing roles in many areas of human physiology. In recent years, Glimcher has uncovered an unexpected reliance of cancer cells on XBP1, perhaps due to the stressful hypoxic environment in which they grow. Beyond this cell intrinsic role of XBP1, Glimcher has discovered that the unique tumor microenvironment also affects nearby immune cells. Dendritic cells in the tumor have elevated XBP1 activity, and this indirectly affects their ability to stimulate the immune system against the nascent tumor. The dual dependence on XBP1 both within and outside the tumor cell has led Glimcher to propose XBP1 as a novel therapeutic strategy.

 

Hegde says that he approaches protein quality control from a different perspective than Bertolotti or Glimcher. They are looking at protein quality control and stress responses on a macro level, while the Hegde lab takes a micro approach. “We want to know all the different pathways in the cell that are utilized to detect when a protein is made incorrectly.” Hegde explains that there is no one-size-fits-all mechanism. “If a ribosome doesn’t complete protein synthesis, or if a protein is synthesized but it doesn’t get taken to the right organelle, or if it gets to the right organelle but it doesn’t get folded in the right way—each of these steps is monitored by different quality control pathways.”

 

Finding how they are monitored and what exactly is being recognized to judge protein quality is the challenge, Hegde says. Cells synthesize millions of proteins a minute. “How do cells find the rare few who don’t get folded properly or taken to the right place? What are the cues that the cell recognizes that say this is not made correctly.”

 

To get a closer look, Hegde is using a combination of classical biochemical methods and new technologies, such as cryo-electron microscopy (cryo-EM). His lab is among the few that exploits the capacity of cellular extracts to carry out protein synthesis and quality control. Using such cell-free systems, Hegde is defining the factors that recognize mis-localized or misfolded proteins, some of which participate in certain neurodegenerative diseases. Yet quality control is intrinsically a dynamic process, making it challenging to isolate intermediates in sufficient quantities for detailed analysis. Technological advances in cryo-EM are offering a way around this problem by allowing structural information to be extracted from extremely small amounts of material from a heterogeneous sample. This has allowed Hegde and other researchers to begin direct observation of quality control factors recognizing their targets.

 

With so many proteins, so many pathways, and so many chaperones, it makes finding needles in haystacks seem like child’s play. Yet the protein quality control haystack is slowly giving up its secrets, Hegde believes, and at ASCB 2016, attendees can expect many more to come to light.

About the Author:


John Fleischman was the ASCB Senior Science Writer from 2000 to 2016. Best unpaid perk of the job? Working with new grad students and Nobel Prize winners.