Abstract:
Autophagy is an intracellular pathway that has a degradative function that helps the cell to
keep up with cytoplasmic homeostasis. Ohsumi worked on autophagy in yeast. His research
found that mutations in autophagy genes can cause sickness. Many organisms depend on
autophagy to withstand starvations. Ohsumi's examination from the last part of the 1980s and
mid 1990s through today has shown autophagy has a job in assurance against dementia and
Parkinson's. Yoshinori Ohsumi discovered the mechanism underlying autophagy. He further
explained how the cellular components are degraded and recycled. The idea that fasting
initiates autophagy, which hinders the maturing cycle and decidedly prevents cells to renew,
was laid down by his research.
This report examines Ohsumi's Nobel prize-winning work in setting, prior to clarifying the
clinical pertinence of autophagy.
Keywords: Autophagy, Nobel Prize, Ohsumi, Yoshinori, medicine or physiology
About the Nobel Laureate: Yoshinori Ohsumi
Yoshinori Ohsumi was awarded the 2016 Nobel Prize in Physiology or Medicine “for his
discoveries of mechanisms for autophagy” by The Nobel Assembly at Karolinska Institutet.
● Place of Birth: Fukuoka, Japan
● Date of Birth: 9 February, 1945
● Education: University of Tokyo (B.Sc) (D.Sc), Rockefeller University (Post-Doc)
● Specialization in: Cell Biology, Autophagy.
Introduction
To understand the process let us first take a look at what we mean by autophagy. Autophagy
is a Greek word where auto- means “self” and, phagien means “to eat”. Hence, autophagy
means “self-devouring”. Autophagy is a mechanism used to sustain cellular homeostasis by
redirecting material present in cytoplasm to lysosome for degradation by use of enzymes.
Ohsumi worked on Saccharomyces cerevisiae and found out that when they are starved they
show a mechanism similar to that of mammalian cells where the cell contents are sent to
vacuoles. Upon using an electron microscope he found out that these are double membranous
structures named as auto phagosomes which are precursors of autophagic bodies in yeasts.
The intermediate structures formed from the fusion of autophagosomes and vacuoles which
are only visible in yeasts and not in mammalian cells.
Ohsumi’s Experiment
The focal point of Ohsumi’s research was protein degradation in vacuole (equivalent to
lysosomes in mammalian cells). He selected yeast as the model organism for his research due
to the ease of identification of genes used in biochemical pathways in cells. In order to prove
that autophagy was taking place in yeast Ohsumi devised that if he disrupts the process of
degradation in vacuole and meanwhile autophagy is active then if he viewed vacuole under
microscope he would see a lot of accumulated auto phagosomes. In order for him to execute
this plan he took mutated yeasts which lacked vacuole degradation enzymes and cultured
these cells and then starved these cells in order to induce autophagy. He found out after
examining through microscope that a lot of vesicles were accumulating in the vacuoles.
These vesicles were identified to me autophagosomes. This not only helped him to prove
existence of autophagy in yeast cells but also helped him to devise a method for identification
of key genes involved in autophagy.
I. Discovery of Autophagy genes
Using the yeast strains in which autophagosomes accumulated in vacuoles due to starvation,
Ohsumi thought of a method to discover genes involved in autophagy. The thought process
was that if genes involved in autophagy were inactive the accumulation of autophagosomes
in vacuoles would not have happened in the first place. So in order to test this hypothesis
Ohsumi exposed yeast cells to certain chemicals that caused random mutations in yeast cells.
After this he caused autophagy in these cells by inducing starvation in them. This hypothesis
yielded wonders for his research as he was able to identify many genes and proteins involved
in autophagy using this method.
Image courtesy : https://www.nobelprize.org/. (n.d.). [Figure 2: In yeast (left panel) a large compartment called the vacuole corresponds to the lysosome in mammalian cells. Ohsumi generated yeast lacking vacuolar degradation enzymes. When these yeast cells were starved, autophagosomes rapidly accumulated in the vacuole (middle panel). His experiment demonstrated that autophagy exists in yeast. As a next step, Ohsumi studied thousands of yeast mutants (right panel) and identified 15 genes that are essential for autophagy.]. https://www.nobelprize.org/uploads/2018/06/med-press-2-en.jpg
Molecular mechanics of Autophagy
Ohsumi and group used protease deficient yeast to to detect 15 autophagy-defective mutant
yeast strains that were unsuccessful to form autophagic bodies. This led them to detect 15
new autophagy genes aka ATG genes. First such gene to be characterized by them was 11-13
ATG1. Autophagy requires the kinase function of Atg1P protein in mutant yeasts. It was seen
that phosphorylation of ATG1p was controlled by accessibility to nutrients. Ohsumi was able
to show that autophagy in yeasts can be induced by the inhibiting effects of rapamycin on
TOR and that it’s signalling is upstream to Atg protein. This clearly stated a similar
autophagic mechanism between yeasts and mammalian cells which was later used in many
researches involving autophagy.
Image Courtesy:
https://www.nobelprize.org/
(n.d.). [Figure 2: In yeast (left panel) a large compartment called the vacuole corresponds to the lysosome in mammalian cells. Ohsumi generated yeast lacking vacuolar degradation enzymes. When these yeast cells were starved, autophagosomes rapidly accumulated in the vacuole (middle panel). His experiment demonstrated that autophagy exists in yeast. As a next step, Ohsumi studied thousands of yeast mutants (right panel) and identified 15 genes that are essential for autophagy.].
II. Conjugation systems
After the discovery of ATG genes, the next focus was to identify how these Atg proteins
interact. We already know that ubiquitin protein is used to tag substrates, but what was found
out is that Atg12p was seen to covalently bind with Atg5P in a similar manner to ubiquitin,
despite there being no homology between the two. When Ohsumi studied wild type yeast he
found that most Atg5P/ Atg12P existed as a conjugate. But when he studies the mutant strains
i.e. ATG7 and ATG10 he didn’t see any such conjugate. In order to understand how the
conjugate is formed he induced predetermined mutations in yeast.
Accordingly he deduced the following process for conjugation
Step 1: Atg7p is used to activate the carboxyl-terminal glycine of Atg12p.
Step 2: Atg10p receives Atg12p and further transfers it to Atg5p.
Atg7p capacities at the substrate sequestration step of autophagosome development was
shown by Klionsky's lab. These examinations lead to the distinguishing proof of the primary
mammalian ATG genes; human ATG12 and ATG5, which form by means of responses analogous to those in yeast and are communicated across the full scope of human tissues. Ohsumi and his team identified the 3rd member of the conjugate i.e. Atg16p when they performed secondary screening for Atg12p binding partners. This trimer conjugate complex plays an essential role in the initiation of the autophagy process by forming Atg5p-Atg12p/Atg16p multimers.
Figure 2: Ubiquitin-like conjugation systems in yeast autophagy. (a) The first conjugation reaction, the carboxyl-terminal glycine of Atg12p is activated by Atg7p. Atg12p is then transferred to Atg10p and finally onto Atg5p. The final Atg5p-Atg12p/Atg16p complex is formed when Atg16p binds Atg5p. (b) In the second conjugation reaction, Atg4p first cleaves nascent Atg8p to reveal a carboxyl-terminal glycine. Atg8p is then activated by Atg7p, transferred to Atg3p and finally onto the membrane component phosphatidylethanolamine (PE). Homologous proteins undergo equivalent conjugation reactions in mammalian autophagy.]. (n.d.). Yoshinori Ohsumi’s Nobel Prize for Mechanisms of Autophagy: From Basic Yeast Biology to Potential. Image Courtesy : Research Gate
Two years after this another ubiquitin like conjugation system was discovered where Atg8p
acted as the ubiquitin-like protein. Ohsumi recognised Atg8p as a marker of autophagic
assemblies in yeast22 and this elucidates how the hydrophilic protein Atg8p is capable of
associating with autophagic membranes.
Following are the steps to this association:
Step 1: Atg8p is cleaved by Atg4p to expose a carboxyl-terminal glycine, and this marks the
initiation of the process.
Step 2: Atg7p activates Atg8p and is then transferred to Atg3p.
Step 3: Atg8p lastly binds to phosphatidylethanolamine on the membrane.
Later on Yoshinori and team identified LC3 which is microtubule-associated protein 1 light
chain 3 which is the mammalian homologue of Atg8p.
LC3 has two forms
1. Non-lipidated LC3-I in the cytosol.
2. Phosphatidylethanolamine-conjugated LC3-II on autophagic membranes.
Conclusion
The research of Ohsumi in yeast gave scientists a tool with which to take apart the apparatus
of autophagy. This has worked with a more noteworthy comprehension of autophagy in
human wellbeing and illness, particularly in the fields of carcinoma, immunology, and
neurodegeneration. The expectation is that autophagy will turn into a perpetually manageable
restorative objective, with autophagy modulators moving into standard clinical practice. As
the investigation into autophagy has extended, it has become evident that it isn't just a
reaction to starvation. It likewise adds to a scope of physiological capacities, for example,
repressing disease cells and maturing, taking out microorganisms, and cleaning the inner
parts of cells. Nonetheless, there is still a lot we don't understand about the system of
autophagy and this calls for a genuine investigation.
Apurv Mehta
Department of Biochemistry and Biotechnology
St. Xavier’s College, Ahmedabad
References
1. Frake, Becca & Rubinsztein, David. (2016). Yoshinori Ohsumi’s Nobel Prize for
mechanisms of autophagy: From basic yeast biology to therapeutic potential. Journal of the
Royal College of Physicians of Edinburgh. 46. 228-233. 10.4997/JRCPE.2016.403.
2. Kotifani, A. (2020, June 02). Fasting for Health and Longevity: Nobel Prize Winning
Research on Cell Aging. Retrieved May 10, 2021, from
https://www.bluezones.com/2018/10/fasting-for-health-and-longevity-nobel-prize-winning-
research-on-cell-aging/#:~:text=Japanese cell biologist Yoshinori Ohsumi,positive impact on cell renewal.
3. The Nobel Prize in Physiology or Medicine 2016. (n.d.). Retrieved from
https://www.nobelprize.org/prizes/medicine/2016/press-release/