Lysosomen, Autophagy (2023)

You usually reach for food when you're hungry, but have you ever wondered what happens inside your cells when food isn't available? Astonishing,eukaryoticCells have evolved a way to withstand food over long periods of time by digesting their own components. When starvation conditions persist, cells digest some of their own cytoplasmic components to recycle metabolites needed for the synthesis of essential molecules. For example, cells can digest long-lived proteins to release amino acids. How did this self-feeding process evolve? How is it controlled by the cell? Today, autophagy research is a growing field of increasing importance as understanding the basic mechanisms of autophagy is key to understanding how cells maintain themselves.

cellular activity

metabolismis the set of chemical reactions occurring in cells (and consequently in living organisms) involved in cell growth,Reproduction, and maintenance. Metabolism is a balance of two antagonistic processes: anabolism and catabolism. Anabolism synthesizes molecules and builds structures. On the other end of the spectrum, catabolism breaks down molecules and structures. Autophagy (a Greek word meaning "self-eating") is a catabolic process in eukaryotic cells that delivers cytoplasmic components and organelles to lysosomes for digestion. Lysosomes are specialized organelles that break down macromolecules, allowing the cell to recycle materials.

The discovery of the lysosomes

In 1949, Christian de Duve, then President of the Laboratory of Physiological Chemistry at the University of Louvain, Belgium, studied how toInsulinacted on liver cells. He wanted to determine the location of onedifficult(Kind ofproteininvolved in chemical reactions) called glucose-6-phosphatase in cells. He and his group knew that this enzyme plays a key role in regulating blood sugar levels. They obtained cell extracts by mixing rat liver fragments in distilled water and centrifuging the mixture at high speed. They observed high phosphatase activity in the extracts. However, when they tried to purify the enzyme from cell extracts, they ran into an unexpected problem - they could precipitate the enzyme, but they couldn't redissolve it.

Instead of using cell extracts, they opted for a gentler technique, using differential centrifugation to fractionate cells. This technique separates different cell components based on their size and density. The researchers ruptured the mouse liver cells and then fractionated the samples in sucrose medium by centrifugation. They were able to detect enzyme activity in the so-called microsomal fraction of the cell. Then coincidence entered the picture.

The scientists used an enzyme called acid phosphatase as a control for their experiments. To their surprise, the acid phosphatase activity after differential centrifugation was only 10% of what was expected.enzymaticActivity (i.e. the activity they obtained in their previous experiments with cell extracts). One day, a scientist happened to purify some cell fractions and left them in the refrigerator. When they returned five days later to measure the enzyme activity of the fractions, they found the enzyme activity values ​​they were looking for! To ensure there was no error, they repeated the experiment several times. The results were the same every time: when the enzyme activity was measured with fresh samples, the activity was only 10% of the activity obtained after storing the samples in the refrigerator for 5 days. How could you explain these results?

They hypothesized that a membrane-like barrier restricts the enzyme's accessibility to its substrate. Leaving the samples for a few days gave the enzymes time to diffuse. They described the membranous barrier as a "sac-like structure surrounded by a membrane and containing acid phosphatase". In 1955, additional hydrolases (enzymes that break chemical bonds) were discovered in these sac-like structures, suggesting that they are a new type of organelle with a lytic structure.function(Bainton 1981). De Duve named these new organelles "lysosomes" to reflect their lytic nature.

That same year, Alex Novikoff of the University of Vermont visited Duve's lab. Novikoff, a skilled microscopist, was able to obtain the first electron micrographs of the new organelle from samples of partially purified lysosomes. Using an acid phosphatase staining procedure, de Duve and Novikoff confirmed its localization in the lysosome using light and electron microscopy studies (Essner & Novikoff 1961).

We now know that lysosomes contain hydrolases capable of digesting all types of macromolecules. Christian de Duve was recognized for his role in discovering the lysosome when he received the 1974 Nobel Prize in Physiology or Medicine. The discovery of lysosomes raised many new questions. The most critical question was: What was the physiological function of this "pocket" of enzymes?

The function of the lysosomes

One of the definitive clues to the function of lysosomes comes from the work of Werner Strauss and his group. Strauss wanted to understand how extracellular molecules enter the cell, a process known as endocytosis. He marked proteins and followed them on their way through the cell. He observed that the lysosomes described by de Duve contained fragments of the tagged proteins and concluded that the proteins were degraded in the lysosome (Straus 1954). In another set of experiments, ZanvilCohn fed macrophages (a type ofcellin the immune system) radioactively markedbacteriaHe observed that all types of radioactively labeled bacterial molecules (lipids, amino acids and carbohydrates) accumulated in lysosomes (Cohn 1963). Cohn concluded that lysosomes acted as the cell's digestive system, "eating" both compounds entering the cell from the outside and compounds inside the cell. Therefore, lysosomes are comparable to recycling facilities responsible for disposal of waste and reuse of components.

Autophagy and Lysosomes

Over the next few years, researchers examined different cell types with electron microscopes and discovered a variety of vesicles. Some of the vesicles contained entrapped cytoplasmic material. What did these vesicles do? Marilyn Farquhar and her collaborators at the University of California, San Francisco first suggested that these specific vesicles were pre-lysosomes (Smith & Farquhar 1966).

Forms prelysosomesonce againNOZytoplasmamade up of a cup-shaped membrane called an aphagophore. The rims of the phagophore expand as they become spherical until they close, enclosing the convoluted pieces of cytoplasm with whatever is inside, giving rise to a double-membrane vesicle. Farquhar observed these closed vesicles known as autophagosomes. Autophagosomes take up damaged molecules or organelles and carry this cargo to the lysosomes. Observing autophagosomes, deDuve realized that cells could break down their own components, and he called the process "autophagy" (Figure 1).

Autophagy: a process of self-digestion

For many years, scientists could only study autophagy by examining cells with electron microscopes. Using this tool, they found that after forming autophagosomes, they fuse with the lysosomal membrane to form a structure known as an autolysosome (Figure 1). The cargo is then broken down or recycled, depending on the stimulus that initiated the autophagy process.

Lysosomen, Autophagy (1)

Figure 1: The formation of phagolysosomes.

During autophagy, sequestration begins with the formation of a phagophore, which expands into a double-membrane autophagosome while enclosing part of the cytoplasm. The autophagosome can fuse with an endosome (the product of endocytosis), which is a form of heterophagy (heterophagy occurs when the cell internalizes and breaks down material that originates outside the cell. In contrast, autophagy occurs when the cell part consumed by itself). . The product of the endosome-autophagosome fusion is called an amphysoma. The complete autophagosome or amphysome fuses with a lysosome that supplies acid hydrolases. Enzymes in the resulting compartment, an autolysosome, rupture the inner membrane of the autophagosome and break down the cargo. The resulting macromolecules are released and recycled in the cytosol.

© 2007Nature publishing groupKlionsky, D. J. Autophagy: From phenomenology to molecular understanding in less than a decade. Nature Reviews Molecular Cell Biology 8, 931-937 (2007) doi:10.1038/nrm2245. All rights reserved.

figure detail

In 1992, Yoshinori Ohsumi and his colleagues at the University of Tokyo discovered that autophagy also occurs in humansYeast. Using a light microscope, they found that a few hours after depriving the yeast of nutrients, the vacuole (which works like our lysosomes) filled with vesicles containing bits of cytoplasm. These vesicles originate in the cytoplasm and then fuse with the lysosome, just like in animal and plant cells. Yeast than to be able to useModelopened the doors to the study of the molecular biology of the autophagic machinery (and to the identification of the major proteins involved in this process) (Takeshigeand other. 1992).

Real-time autophagy

Figure 2: Autophagosome formation

© 2007 Noboru Mizushima, Department of Biochemistry and Molecular Biology, University of Tokyo. All rights reserved.

In 2004, fifty years after Novikoff and de Duve observed lysosomes under the electron microscope, Noboru Mizushima and colleagues from Tokyo Medical and Dental University tracked the formation of autophagosomes by taking pictures every five minutes. For their experiments, they used a fluorescently labeled protein that localizes specifically to phagophores and observed the formation of autophagosomes in real time (Klionsky 2007) (see video, Figure 2).

A riddle that remained unanswered for many years was: From which cell membrane do predators arise? In 2010, Jennifer Lippincott-Schwartz and her colleagues used fluorescently labeled proteins to study the origin of phagophores. They observed that the outer membrane of themitochondriawas the main source of the membrane, with some contribution fromEndoplasmatisches Retikulum(a network of membranes in the cytoplasm of the cell) (McEwan & Dikic 2010).

Physiological role of autophagy

How much and which parts of the cell can be ingested without cell death occurring? The scientists hypothesized that the level of autophagy and stress specificity must be tightly controlled to ensure cellular health. For example, when there is an abundance of nutrients, autophagy levels should be low, but autophagy should increase during starvation.

It is important for unicellular organisms to maintain a pool of metabolites such as amino acids. Therefore, starvation-induced autophagy was probably first selected in unicellular organisms and then maintained in multicellular organisms. In mammals, autophagy is triggered not only by starvation, but also by physiological stimuli such as growth factors and hormones, and by invading pathogens. In general, autophagy is used to engulf non-specific components, but it can also selectively degrade damaged organelles, pathogen inclusions, orattackingBakterien (Nakatogawa)and other. 2009). Therefore, autophagy likely evolved in response to cellular starvation and later likely served as a primitive immune defense.

The process of autophagy is constantly taking place, whether the cell is starving or not, but at a basal level. Under normal conditions, autophagy removes damaged proteins and organelles to prevent cell damage. However, under stress (e.g. starvation, lack of growth factors or lack of oxygen), phagosome assembly increases. Under these conditions, intracellular molecules are digested to provide the nutrients that the cell needs.

A few years ago, scientists made a connection between autophagy andillness. Beth Levine and colleagues from Columbia University College of Physicians and Surgeons have shown that tumors develop after one of the two copies of the cell is deletedBeclin1Gen.Beclin1is a mammalian homologue of yeastAtg6Gen,what is needed for autophagy. In fact, 40-75% of sporadic human breast and ovarian cancers are missing a copyBeclin1. When Levine and his colleagues reinforced the phraseBeclin1they observed more autophagy in human carcinoma cells, and when these cells were injected into a mouse model, they were less able to develop tumors (Liangand other. 1999). In another series of studies, Eileen White and her colleagues at the University of Medicine and Dentistry in New Jersey found that autophagy protects against DNA damage. When they inhibited autophagy, they observed more chromosomal abnormalities typically associated with tumorigenesis (Mathewand other. 2007).


These discoveries have prompted many scientists around the world to study the various physiological functions of autophagy. There are currently more than 2,500 publications on autophagy, and several recent findings link this cellular process to immune and metabolic diseases. There are still many fundamental questions about the mechanisms that control autophagy that need to be addressed. Many scientists believe that studying autophagy regulation is crucial to understanding its biological role and to developing alternative therapies for diseases associated with autophagy dysregulation.

References and recommended reading

Bainton, D. F. The discovery of lysosomes.J CellBiol91,66-76 (1981).

Cohn, Z.A.The fate of bacteria in scavenger cells. I. Degradation of isotopically labeled bacteria by polymorphonuclear leukocytes and macrophages.J Exp. Med 117,27–42 (1963)

Essner, E. & Novikoff, A.B.Localization of acid phosphatase activity in hepatic lysosomes by electron microscopy.J Biophys BiochemCytol 9,773–784 (1961).

Klionsky, D. J. Autophagy: From phenomenology to molecular understanding in less than a decade.Nat. Rev. Mol. Cell Biol 8,931–937 (2007) doi:10.1038/nrm2245.

Liang, X.H.and other. Induction of autophagy and inhibition of tumorigenesis by beclin 1.Nature 402,672–676 (1999) doi: 10.1038/45257.

Matthäus, R.and other.Autophagy suppresses tumor progression by limiting chromosomal instability.Gene Dev 21,1367-1381 (2007).

McEwan, D.G. and Dikic I. Not all autophagy membranes are created equal.cell141,564-566 (2010).

Nakatogawa, H.and other. Dynamics and diversity in autophagy mechanisms: lessons from yeast.NatRev Mol Cell Biol 10,458–467 (2009) doi: 10.1038/nrm2708.

Smith, R.E. & Farquhar M.G.Lysosomal role in regulation of the secretory process in anterior pituitary cells.J CellBiol31,319–347 (1966).

Strauss, WIsolation and biochemical properties of mouse kidney cell droplets.JBiochem 207,745-755 (1954).

Takeshige, K.and other.Yeast autophagy demonstrated with proteinase-deficient mutants and conditions for their induction.JCB 119, 301–311 (1992).

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