2016 Nobel Prize in Physiology or Medicine β Yoshinori Ohsumi, Unraveling the Cell's Self-Cleaning System
What You'll Learn in This Article
The 2016 Nobel Prize in Physiology or Medicine was awarded to Yoshinori Ohsumi of the Tokyo Institute of Technology for his discovery of the genes that map out the system by which cells engulf and break down their own components for recycling β autophagy. In the early 1990s, he screened for autophagy-deficient mutants in yeast and discovered the ATG gene family (autophagy-related genes). He later demonstrated that these genes are evolutionarily conserved even in humans. Autophagy is activated in response to starvation, oxidative stress, infection, and damaged organelles, breaking down old proteins and damaged organelles within the cell. The dysregulation of this system is linked to various conditions, including cancer, neurodegenerative diseases, autoimmune diseases, and type 2 diabetes, making autophagy one of the hottest topics in cell biology in the early 21st century.
A Different Perspective β Cells Survive by Eating Themselves
The common understanding that "cells create proteins and use them" is only half the story. Cells constantly break down old proteins and damaged organelles, recycling their components. It is only when this balance of breakdown and recycling is maintained that cells can remain healthy. Until the late 20th century, protein degradation in cells was primarily understood as the ubiquitin-proteasome system, a precise degradation process that tags and destroys a single misfolded protein in the proteasome. However, while this system is useful for handling individual proteins, it cannot handle the entire organelle, such as mitochondria or the endoplasmic reticulum.
Autophagy is the system responsible for this large-scale cleaning. The cell wraps its old components in a double membrane, creating an isolated area called an autophagosome, and then fuses this area with a lysosome to break it down. In CS terms, this is a system that creates an isolated sweep area and runs a garbage collection (GC) cycle. If the ubiquitin-proteasome system is a precise incineration based on reference counts, then autophagy is closer to a generation-based mark-and-sweep. It isolates damaged parts, sweeps them away, and reuses the resulting raw materials.
This system has been present since the early stages of evolution, but the gene components were identified after Ohsumi's discovery. The ATG gene family he identified is a list of components responsible for each stage of autophagy: initiation signal, double membrane formation, expansion, sealing, and lysosome fusion. Most of these components are evolutionarily conserved from yeast to humans, and the dysfunction of these components is linked to various human diseases.
The Zeitgeist β Candlelight Protests and Impeachment Politics
In 2016, South Korea transitioned into Park Geun-hye impeachment politics after October. The Choi Soon-sil scandal exploded in October with the release of a JTBC tablet PC report revealing the president's close associates' involvement in state affairs. A total of 17 million people participated in candlelight vigils across the country, including in Gwanghwamun Square. On December 9, the National Assembly passed the impeachment motion with 234 votes in favor, and the president's duties were suspended while awaiting a decision from the Constitutional Court. This decision was finalized on March 10 of the following year with the president's removal.
In July, the THAAD (Terminal High Altitude Area Defense) deployment decision was announced, leading to strong opposition from the Seongju region and economic retaliation from China. From August to October, the Samsung Galaxy Note 7 battery explosion incident occurred, leading to a global recall and discontinuation, causing significant damage to the company's reputation.
Globally, the 9-game match between Lee Sedol (9-dan) and AlphaGo was held from March 9 to 15, with AlphaGo winning 4-1. This was the first widely recognized event of artificial intelligence surpassing human intelligence in a cognitive game, and it marked the moment when deep learning became a popular topic. The June 23 Brexit referendum resulted in the UK's decision to leave the EU, marking a major turning point in world politics. In the November 8 US presidential election, Trump defeated Hillary Clinton, defying expectations and marking the beginning of a period of extreme change in world politics. The August Rio Olympics were held in South America for the first time, and the global spread of the Zika virus posed a new challenge in responding to emerging infectious diseases.
In the scientific community, this award was part of a trend of Japanese laureates winning consecutive Nobel Prizes. From the 2014 Physics Prize (Akasaki, Amano, Nakamura, for blue LEDs) to the 2015 Physiology or Medicine Prize (Omura, for ivermectin), to Ohsumi in 2016, Japanese scientists won Nobel Prizes in the natural sciences for three consecutive years, and Japan's basic science investment policy began to attract international attention. In his Nobel Prize acceptance speech, Ohsumi repeatedly emphasized that "basic science is necessary for humanity, even if it does not appear to have immediate practical applications," and this message resonated internationally as a reaffirmation of the value of pure basic research in the face of neoliberal pressures for university reform.
Personal Narrative β 40 Years of Autophagy Research in a Yeast Lab
Yoshinori Ohsumi (1945β) was born in Fukuoka City, Japan. He received his bachelor's and doctoral degrees from the University of Tokyo and spent three years as a postdoctoral researcher at Rockefeller University in the United States (where Gerald Edelman, the 1972 Nobel laureate, was his supervisor) before returning to Japan. He established his own laboratory at the University of Tokyo and later moved to the National Institute of Basic Biology and the Tokyo Institute of Technology, dedicating his life to autophagy research.
Ohsumi became fascinated by autophagy in 1988, when he started his own laboratory at the University of Tokyo. The phenomenon of autophagy itself had already been observed in the 1960s by Christian de Duve's team (1974 Nobel laureate, for the discovery of lysosomes) using electron microscopy, but the genes responsible for this process had remained unsolved for 30 years.
Ohsumi's approach was straightforward. He used Saccharomyces cerevisiae, the yeast that Hartwell used to discover the cell cycle in 2001, and created a system to screen for mutants in which autophagy fails. The problem was that autophagy was not very active under normal conditions, making it difficult to observe. Ohsumi circumvented this by using starvation conditions β nutrient-deprived media, where autophagy is dramatically activated. He then selected for mutants that failed in autophagy under these conditions, allowing him to identify the essential genes for autophagy.
Another key observation method was the use of autophagosome accumulation. In yeast with mutations in proteases (protein-degrading enzymes), autophagosomes accumulate inside the cell because they cannot be broken down in the lysosome. This accumulation became large enough to be observed by optical microscopy, and Ohsumi was able to quantify autophagy activity by measuring this accumulation.
By combining these two tools, Ohsumi reported in 1993 the first 15 yeast gene families involved in autophagy failure. He named these genes APG (autophagy) genes, which were later internationally standardized as ATG (autophagy-related) genes. In the following 20 years, Ohsumi's laboratory and other collaborating laboratories elucidated the function of each of these genes. How is the double membrane of autophagosomes formed? Which components are responsible for which stages? What are the human counterparts?
The decisive link to humans came in the early 2000s. It was confirmed that the yeast ATG genes are evolutionarily conserved in Drosophila, mice, and humans, and that these genes in human cells are responsible for autophagy in the same way. After this, autophagy research expanded dramatically, and Ohsumi's more than 40 years of dedicated basic research became a key tool for understanding human diseases. At the time of the Nobel Prize, Ohsumi was 71 years old and still actively conducting research, and he said in his acceptance speech that "there is still much more to be discovered about autophagy," which resonated internationally.
Key Achievements β The Cell's GC System in CS Terms
The basic autophagy pipeline can be summarized as follows:
- Trigger: When the mTOR complex, which monitors the cell's nutritional status, is inhibited, autophagy begins. If amino acids, glucose, and growth factors are abundant, mTOR is active and inhibits autophagy. However, when these are depleted or oxidative stress is high, mTOR is inhibited and autophagy is activated.
- Initiation: The ULK1/2 kinase complex starts the formation of autophagosomes at a specific point in the cytoplasm. This point is usually near the endoplasmic reticulum.
- Nucleation: The PI3K complex, centered on Beclin-1, creates the initial seed for the new double membrane.
- Elongation: The LC3 protein binds to the new double membrane, expanding the membrane. The lipidization of LC3 is the key indicator of autophagosome maturation and is now the standard marker for measuring autophagy activity.
- Closure: The double membrane completely closes, forming a complete autophagosome that isolates some of the cytoplasm (old proteins and organelles).
- Fusion and degradation: The autophagosome fuses with the lysosome, and the proteases, lipases, and nucleases in the lysosome break down the isolated contents. Amino acids, fatty acids, and nucleotides are released into the cytoplasm and reused.
The essence of this system is an architecture that creates an isolated sweep area and runs a large-scale GC. Unlike the ubiquitin-proteasome system, which manages individual proteins based on reference counts, autophagy sweeps a specific area of the cytoplasm. This is surprisingly similar to a memory pressure-responsive mark-and-sweep GC. mTOR is a heuristic that monitors available resources, and when resource depletion is detected, the GC cycle begins, and the raw materials from the swept area are reused.
Selective autophagy is an extension of this basic system.
- Mitophagy: Selectively removes damaged mitochondria via autophagy. Specific proteins on the surface of mitochondria (PINK1 and Parkin) detect damage and send a targeting signal.
- Reticulophagy: Selectively removes damaged areas of the endoplasmic reticulum.
- Xenophagy: Targets and removes bacteria that have invaded the cell via autophagy.
- Aggrephagy: Targets and removes aggregates of misfolded proteins.
These selective autophagies are selective sweep GC. Targets with a specific targeting signal (e.g., PINK1 marking mitochondria) are swept.
It is also important to note the limitations of this analogy. Autophagy is not just a GC but also a recycling system. Software GC simply recovers memory and returns it to the free list, but autophagy breaks down the contents and reuses the resulting raw materials (amino acids, fatty acids, and nucleotides) to synthesize new proteins and organelles. In particular, under starvation conditions, cells prioritize the use of amino acids obtained through autophagy to synthesize essential proteins, which is a crucial survival strategy.
Why It Matters: Neurodegeneration, Cancer, Aging, and Metabolism
First, it became a key axis in understanding neurodegenerative diseases. The common features of Parkinson's disease, Alzheimer's disease, and Huntington's disease are the accumulation of misfolded protein aggregates (Ξ±-synuclein Lewy bodies in Parkinson's, Ξ²-amyloid and tau in Alzheimer's, and polyglutamine aggregates in Huntington's). If autophagy functions normally, these aggregates are removed, but if there is an autophagy defect, they accumulate. The fact that PINK1 and Parkin were identified as causative genes for hereditary Parkinson's disease was a crucial clinical basis for this axis, and autophagy-promoting drugs have become a major direction for the development of neuroprotective therapies.
Second, it reveals the dual nature of cancer research. Autophagy plays a complex role in cancer. In the early stages of tumor formation, autophagy acts to suppress cancer by clearing damaged cells. However, in established tumors, autophagy is used as a self-survival strategy. This is because tumor cells, which are stressed by anticancer treatment, use autophagy to secure metabolic resources and continue to survive. Because of this duality, whether activating or inhibiting autophagy is beneficial for anticancer treatment depends on the type of tumor and the stage of treatment, requiring a targeted approach. Hydroxychloroquine, an autophagy inhibitor, is being used in several anticancer clinical trials.
Third, it is a new target for aging research. It has been observed in several species that autophagy activity decreases with age, and the view has taken hold that this decrease is a common root cause of various age-related pathologies (protein aggregation, mitochondrial dysfunction, and inflammation). It has been observed that intermittent fasting, calorie restriction, and exercise activate autophagy, and this is one of the molecular bases for the anti-aging effects of these interventions. Rapamycin, an mTOR inhibitor and autophagy activator, has repeatedly shown increased lifespan in mice, and clinical trials are underway in humans to test its effects on age-related markers.
Fourth, it is related to understanding type 2 diabetes and metabolism. If autophagy is deficient in pancreatic Ξ² cells, insulin synthesis errors accumulate, damaging the Ξ² cells, which is one of the axes of type 2 diabetes progression. It has also been observed that autophagy defects are a cause of pathology in fatty liver and cardiovascular diseases.
Fifth, it is a new point of contact with infectious diseases. Xenophagy, the process of removing intracellular bacteria by autophagy, is an important part of innate immunity. Several genetic risk factors for Crohn's disease (a chronic inflammatory bowel disease), such as ATG16L1 and IRGM, are autophagy genes. It is now believed that defects in autophagy against intestinal bacteria are one of the factors in the development of Crohn's disease.
Sixth, it reconfirms the value of basic science. Osmi's screening of autophagy genes in yeast was in the early 1990s, long before the connection to human diseases was confirmed. If this research had been started under a research support policy that demanded only immediate practical application, it would never have received funding. Osmi's repeated advocacy for the value of basic science in his Nobel lecture served as an impetus for universities around the world in the early 21st century to reconsider the balance between basic research and applied research, and it has been cited in scientific policy debates in many countries.
Osmi's 40-year experiment, which began on a yeast plate, has become the root of a major trend that now extends to the development of new drugs for Parkinson's disease, the molecular understanding of intermittent fasting, and clinical trials of mTOR inhibitors for aging. Autophagy is the quiet cleaning cycle that cells perform every moment, and we now have the list of its genes in our hands.
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