Remove, Reuse, Recycle

OUTLOOK MEDICAL RESEARCH NILS-PETTER EKWALL NILS-PETTER

MOLECULAR BIOLOGY Remove, reuse, recycle Waste removal is not usually described as sexy, but the once-neglected field of autophagy — which plays a part in cancer and other diseases — is a hot topic in biomedical research.

BY MICHAEL EISENSTEIN Autophagy was once considered to be little Autophagy was discovered in the 1960s, more than a cellular recycling bin — a process based on microscopic observations of selec- hen Ana María Cuervo began by which cells break down unwanted biomol- tive degradation of cellular material within the researching her thesis in autophagy ecules into raw materials. But more recent lysosome (see ‘A history of autophagy’). Over — a cellular recycling mecha- research has revealed that autophagy is, in fact, time, scientists accumulated evidence that this Wnism — little did she know that two decades a nexus for the cellular stress response and a process helped cells to deal with nutrient-poor later she would be working in one of the most failure point for many diseases. In the past conditions, to eliminate excess proteins and dynamic fields of medical research. Randy ten years, researchers have made connections even to remove entire mitochondria — the Schekman, winner of last year’s Nobel Prize between autophagy and the immune response, cell’s metabolic power plants. However, most in Physiology or Medicine, even chose to talk cancer, neurodegeneration and ageing, says functions seemed to fall under the umbrella about autophagy in his opening address to the Daniel Klionsky of the University of Michigan of basic maintenance, and autophagy research 37 laureates and 600 young scientists at this in the United States. “The field just exploded.” remained a niche field. year’s meeting instead of cellular trafficking The turning point that showed autophagy — his prizewinning work. Cuervo is accus- A PROMOTION FROM HOUSEKEEPING was not simply cellular housekeeping came tomed to this rise in interest. “I did my thesis on There are different types of autophagy, but the in the mid-1990s, when a number of proteins autophagy in the early 1990s when autophagy best-understood pathway is known as ‘macro- (now known as Atg proteins) that collectively wasn’t cool,” says Cuervo, who is now co-direc- autophagy’ — a bulk mechanism for gather- mediate the formation and maturation of the tor of the Einstein Institute for Aging Research ing up and degrading proteins, organelles and phagophore were reported. Since then it has at the Albert Einstein College of Medicine in other cellular materials. The process begins become clear that the Atg machinery inter- New York City. “When I finished, everybody with the formation of a double-membrane sects with physiological processes underlying told me to change fields because autophagy was structure known as a phagophore, which elon- an array of disorders, but scientists are still a dead end,” she confesses. Studies have proved gates and engulfs nearby cellular components struggling to figure out the conditions that this prediction to be spectacularly wrong. (see ‘Eating up the cell’). autophagy prevents or promotes.

CANCER CONTROVERSY By contrast, some forms of Parkinson’s Autophagy seems to provide a crucial bulwark are associated with disruptions in a paral- EATING UP THE CELL against genetic and biochemical damage — for lel autophagy pathway called chaperone- Autophagy is part of a cell’s normal function, removing proteins, damaged organelles and other example, by eliminating damaged mitochon- mediated autophagy in which specific proteins unwanted material. Failure of the system is dria that would otherwise leak toxic molecules are delivered directly to the lysosome for deg- implicated in a number of conditions and ageing. into the cell. As such, it is perhaps unsurprising radation by means of a protein called LAMP2A that cancer was the first disease to be linked without involvement of the autophagosome. with autophagy. However, current evidence One of the proteins normally removed Lysosome suggests that autophagy can act as both an ena- by this process is α-synuclein, the plaque- Cell bler of and a protector against tumour growth, forming protein associated with Parkinson’s. Components Pre-phagophore creating some debate in the field. Mutant forms of the protein or an excessive (organelles structure and cytoplasm) In 1999, Beth Levine and her colleagues at production of it can gum up the system and to be recycled Columbia University, New York, showed that cause a gradual but steady decline in neuronal RELATED CONDITIONS a protein called beclin-1 suppresses tumour health. “Chaperone-mediated autophagy A double membrane called activity in humans and promotes early forma- cannot remove the molecules at the normal the phagophore starts to grow tion of the phagophore1. The group also found rate, and the protein begins to accumulate,” inside the cell, encasing the components to be recycled. Herpes virus that several cellular pathways that drive tumour says Cuervo. The normal autophagic process Stalls growth inhibit autophagy, either by preventing can compensate to a certain extent. However, formation of activation of beclin-1 or by interfering with as Rubinsztein and others have observed, phagophore in other Atg proteins. Levine is waiting for proof α-synuclein can also exacerbate the condition. brain cells. before declaring that autophagy failure itself drives tumour growth, but she believes it makes CONSTRUCTIVE FEEDBACK Parkinson’s and The double membrane Huntington’s for a compelling hypothesis. “The general view The impact of autophagy goes beyond the encloses cell components, diseases is that autophagy plays a protective role against confines of an individual cell — this process forming a structure called Membrane fails the development of cancer,” she says. is also used to regulate metabolic function the autophagosome. to capture However, some scientists believe that throughout the entire body. Cuervo and her material. autophagy can also help advanced tumours to team recently found2 that the liver helps to thrive by allowing cancerous cells to cope with manage metabolism by using chaperone- Tuberculosis the stress associated with competing for lim- mediated autophagy to selectively destroy the Interferes with autophagosome ited nutrients and oxygen, not to mention the enzymes that convert sugar into energy. This maturation. toxicity caused by radiation or chemotherapy. is crucial, says Cuervo, because otherwise the Autophagy inhibitors could, therefore, render liver becomes a “selfish organ” that uses all Alzheimer’s A lysosome fuses with the disease established cancers more vulnerable to treat- the glucose for itself at the expense of other outer membrane of the ment, says oncologist Ravi Amaravadi at the tissues. Along with her colleague Rajat Singh, autophagosome, forming Autophagosome 3 the autolysosome. fails to fuse with University of Pennsylvania in Philadelphia. she has also found that nutrient-sensing lysosome. “The overarching theme is that autophagy is an functions mediated by autophagy help the adaptive stress response that protects the cancer brain to convey that it is time to eat by switch- Lysosome cell in advanced disease,” he says. ing on appetite signals and switching off those that indicate satiety. KEEPING A CLEAR MIND Elements of the autophagy machinery also But it is not only cancer that is linked to the act as a line of defence against viruses and bac- failure of autophagy — it also seems to play a teria by diverting would-be cell hijackers to Diabetes and key part in neurodegenerative disorders such the lysosome for destruction. Microbiologist obesity as Alzheimer’s, Parkinson’s and Huntington’s Vojo Deretic at the University of New Mexico in Enzymes from the lysosome Abnormal diseases. These conditions are characterized Albuquerque hypothesizes that the autophagy degrade the contents of regulation of the autolysosome. autophagy is a by the formation of dense protein aggregates, machinery may have served as a primordial feature of these which point to some sort of failure in cellular form of immunity in early evolutionary history, conditons. housekeeping, but disruptions vary consider- by helping the body to distinguish between ably between the conditions. molecular signatures that represent foreign For example, neurons in Alzheimer’s patients threats and those that are indicators of ‘self’ exhibit increased numbers of autophagosomes, and should be ignored. the membranes that enclose the cell compo- Many pathogens have evolved strategies that nents before they are broken down, yet they can can sabotage autophagy, which Deretic first no longer fuse effectively with the lysosome. encountered while attempting to understand The degraded molecules, or metabolites, Although the roots of Alzheimer’s pathology how Mycobacterium tuberculosis lives inside are released and are ready to be reused. remain unclear, with toxicity linked to accumu- immune cells. He found4 that the bacteria were lation of two proteins called tau and amyloid-β escaping destruction by selectively attacking a (Aβ), autophagic failure could provide a rea- molecule that would otherwise transport them With so many crucial processes seemingly sonable explanation for either pathway. “At to the lysosome. Likewise, Levine has observed5 converging on a single cellular pathway, the late stages of disease you get what looks like an that the herpes virus thwarts autophagy to sur- expectation is that failures in autophagy have autophagy blockade that might compromise vive within neurons. “It has a protein that binds far-reaching consequences throughout the the whole process,” says neuroscientist David to the beclin-1 protein and blocks its function,” body. The evidence now strongly suggests that Rubinsztein at the University of Cambridge, she says. “This is not necessary for viral replica- ageing is associated with a decline in autophagy, UK. “That’s going to affect not only tau and Aβ tion in vitro but is essential for replication in and some researchers are intrigued by the strik- clearance but also removal of damaged mito- neurons, and this meant that viral evasion of ing overlap among conditions that are associ- chondria and other processes.” autophagy was necessary for disease.” ated with both ageing and autophagy, such as

16 OCTOBER 2014 | VOL 514 | NATURE | S3 © 2014 Macmillan Publishers Limited. All rights reserved OUTLOOK MEDICAL RESEARCH diabetes, cancer and neurodegenerative dis- is very important because it’s randomized, so ease. Cuervo and her colleagues have found BACK IN TIME if there’s a signal we’ll know that it’s due to the evidence that chaperone-mediated autophagy hydroxychloroquine,” he says. might be an important factor in healthy human A history of autophagy From a therapeutic perspective, hitting the ageing. For instance, her team learned that the wrong target could have dire consequences. “If receptor in this pathway (LAMP2A) normally The story of autophagy begins with you’re having problems with autophagosome decreases with age, and therefore reduces the Belgian cell biologist Christian de Duve, clearance, as has been shown with Alzheimer’s, cell’s ability to degrade proteins, which Cuervo who shared the Nobel prize in 1974 then a drug that promotes autophagosome believes could increase the risk of metabolic for his exploration of the structural formation will just create more vesicles that diseases. This initiates a vicious circle, whereby and functional organization of the cell. aren’t going anywhere and make a bad traffic failure to control enzymes that break down De Duve discovered the lysosome, an jam worse,” says Cuervo. Furthermore, some sugar and fat leads to their steady accumulation acidic membrane within the cell that is viruses actually make use of the autophagy in the body which, in turn, further suppresses loaded with enzymes that can digest machinery to assist in replication, so the same autophagy. The inability of the cell to main- biomolecules. In the 1960s, scientists drug that thwarts, say, herpes might encourage tain its internal environment could be linked learned that proteins and structures poliovirus proliferation and release. to other ageing-associated disorders, too. “It’s called organelles within cells were being Additionally, many of the drugs being tested like my mother used to say: in a clean house, scooped up from the cytoplasm and affect autophagy either incidentally or in con- everything works better,” says Cuervo. delivered to the lysosome for destruction junction with other cellular pathways, making Conversely, other tricks to boost longevity and recycling. De Duve coined the term it harder to determine whether autophagy is the seem to demand healthy autophagic function. ‘autophagy’ to describe such cellular culprit or the cure for a given condition. For example, caloric restriction — in which self-cannibalization. subjects greatly reduce their food consump- Early studies linked autophagy to BACK TO BASICS tion without crossing the line into malnutri- the body’s ability to sense nutrients, Deretic has obtained promising early data from tion — has been strongly linked with increased suggesting that the process enables cells a compound that may help to contain the pro- lifespan in many animal models. These same to obtain raw materials during starvation. liferation of HIV by means of autophagy, but physiological conditions also stimulate Once this model was established, interest wants to get a better insight into how the mole- autophagy, offering tantalizing evidence that in the subject waned. Things changed cule works before getting too excited. “We have these two processes — autophagy and the in the mid-1990s when researchers to be very careful about how we interpret the longevity gains associated with restricted began to untangle the mechanisms that data and what we expect to see before we even caloric intake — may be linked. Research from drive autophagy. From studies in simple start the experiment,” he says. “Is it an inducer Levine’s group has also shown that exercise organisms, such as yeast, scientists or an inhibitor, and is it driving the whole pro- can stimulate autophagy, and she speculates built genetic and functional maps of the cess or just half of it? A lot of screening data stop that our well-fed and sedentary contemporary machinery used in autophagy. It became short of answering these questions.” lifestyles may suppress our capacity to maintain clear that autophagy was conserved These questions become even harder the high level of autophagy that helped to keep throughout evolution and served a more to answer in the clinical setting, where our ancestors healthy. crucial purpose than just providing researchers often rely on proxy indicators to emergency rations to cells. M.E. glean static snapshots of a highly dynamic HUNGRY FOR NEW THERAPEUTICS process. Klionsky has worked with many of The potential link between increased the field’s top researchers to devise best prac- autophagy and better health could be good different cancer treatments with hydroxy- tices for studying autophagy, but it can still be news from a therapeutic perspective. The chloroquine, an antimalarial drug that also fiendishly difficult to determine how a given Levine group has developed a promising impairs lysosomal degradation. Although the experimental manipulation is altering the pro- molecule that can stimulate autophagy, pro- results were ambiguous in terms of efficacy, the cess — especially when one is targeting cells tecting mice against otherwise-lethal viral safety profile seems favourable. Amaravadi also deep within the brain or liver. infections and blocking the accumulation of reported evidence of stalled autophagy in blood For this reason, some of the most important proteins associated with neurodegenerative cells and tumour tissue from patients treated near-term studies in autophagy will be basic disease in cultured cells. with the highest doses of hydroxychloroquine, research efforts that monitor the nuts and Rubinsztein’s team has obtained promising suggesting that this drug or a related compound bolts of the process. “We need to understand preliminary results in mice with an autophagy- might be able to thwart a mechanism by which how autophagosomes are built, what regulates stimulating drug called rilmenidine, which has cancer eludes destruction. the way they form and what regulates their already been approved for treating high blood Given the ambiguous role of autophagy itinerary within the cell and fusion with lys- pressure in the United States and Europe. The in helping or hindering cancer, experts osomes,” says Rubinsztein. “Having that toolkit drug is being tested in an ongoing clinical trial have expressed concern that the genetic expanded will give us more potential insights for safety in patients with Huntington’s disease, heterogeneity found within a typical tumour into links with different types of disease.” ■ and Rubinsztein hopes to move towards effi- could make cancer too challenging a target for cacy trials in patients with early stage neuro- such a broad therapeutic approach. “I’m not Michael Eisenstein is a freelance journalist degenerative disease — an area where many optimistic that this pro-survival function of based in Philadelphia, Pennsylvania. clinical researchers see the greatest promise in autophagy is going to be a good therapeutic in autophagy-targeting therapeutics. all or even most cancers,” says Levine. At least 1. Liang, X. H. et al. Nature 402, 672–676 (1999). Several pharmaceutical companies, includ- one study6 suggests that inhibiting autophagy 2. Schneider, J. L., Suh, Y. & Cuervo, A. M. Cell Metab. 20, 417–432 (2014). ing Novartis, Pfizer and Millennium, are test- might instead provoke more aggressive tumour 3. Rubinsztein, D. C. EMBO Rep. 13, 173–174 (2012). 7 ing the waters, primarily focusing on inhibiting growth, although another study has contested 4. Vergne, I. et al. Proc. Natl Acad. Sci. USA 102, autophagy to make cancers more susceptible those findings. For now, this remains a topic of 4033–4038 (2005). 5. Orvedahl, A. et al. Cell Host Microbe 1, 23–35 to chemotherapy treatment. In August 2014, considerable debate, and Amaravadi hopes to (2007). Amaravadi and his colleagues published half gain deeper insights in an upcoming phase II 6. Rosenfeldt, M. T. et al. Nature 504, 296–300 (2013). a dozen phase I trials in which they paired trial in patients with pancreatic cancer. “This 7. Yang, A. et al. Cancer Discov. 4, 905–913 (2014).