What do cells do when they are “hungry”? Eukaryotic cells cope with starving conditions by eating their own components, a process called autophagy.
Normally, when you are hungry you look for something to eat, but have you ever wondered what happens inside your cells when no food is available? As incredible as it sounds, eukaryotic cells have evolved a way to resist eating for long periods of time by digesting their own components. When starving conditions are prolonged, cells digest part of their own cytoplasmic components to recycle metabolites needed to synthesize essential molecules. For example, cells can digest long-lived proteins to release amino acids. How did this process of self-eating evolve? How is it controlled by the cell? Today, research on autophagy is a growing field with increasing prominence because understanding the basic mechanisms of autophagy is key to understanding how cells sustain themselves.
Metabolism is the set of chemical reactions that occur in cells (and consequently, in living organisms) that are 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 side of the spectrum, catabolism breaks down molecules and structures. Autophagy (a Greek word that means "self-eating") is a catabolic process in eukaryotic cells that delivers cytoplasmic components and organelles to the lysosomes for digestion. Lysosomes are specialized organelles that break up macromolecules, allowing the cell to reuse the materials.
In 1949, Christian de Duve, then chairman of the Laboratory of Physiological Chemistry at the University of Louvain in Belgium, was studying how insulin acted on liver cells. He wanted to determine the location of an enzyme (a type of protein involved in chemical reactions) called glucose-6-phosphatase inside the cells. He and his group knew that this enzyme played a key role in regulating blood sugar levels. They obtained cellular extracts by blending rat liver fragments in distilled water and centrifuging the mixture at high speeds. They observed high phosphatase activity in the extracts. However, when they tried to purify the enzyme from cellular extracts, they had an unexpected problem-they could precipitate the enzyme, but they could not redissolve it.
Instead of using cellular extracts, they decided to use a more gentle technique that fractionated the cells with differential centrifugation. This technique separates different components of cells based on their sizes and densities. The researchers ruptured the rat liver cells and then fractionated the samples in a sucrose medium using centrifugation. They succeeded in detecting the enzyme's activity in what was known as the microsomal fraction of the cell. Then serendipity entered the picture.
The scientists were using 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 the expected enzymatic activity (i.e., the activity they obtained in their previous experiments using cellular extracts). One day, by chance, a scientist purified some cell fractions and then left them in the fridge. Five days later, after returning to measure the enzymatic activity of the fractions, they observed the enzymatic activity levels they were looking for! To ensure there was no mistake, they repeated the experiment a number of times. Each time, the results were the same: if they measured the enzymatic activity using fresh samples, then the activity was only 10% of the activity obtained when they let the samples rest for five days in the fridge. How could they explain these results?
They hypothesized that a membrane-like barrier limited the accessibility of the enzyme to its substrate. Letting the samples rest for a few days gave the enzymes time to diffuse. They described the membrane-like barrier as a "saclike structure surrounded by a membrane and containing acid phosphatase." By 1955, additional hydrolases (enzymes that break chemical bonds) were discovered in these saclike structures, suggesting that they were a new type of organelle with a lytic function (Bainton 1981). De Duve named these new organelles "lysosomes" to reflect their lytic nature.
That same year, Alex Novikoff from the University of Vermont visited de Duve´s laboratory. An experienced microscopist, Novikoff was able to obtain the first electron micrographs of the new organelle from samples of partially purified lysosomes. Using a staining method for acid phosphatase, de Duve and Novikoff confirmed its location in the lysosome using light and electron microscopic studies (Essner & Novikoff 1961).
Nowadays, we know that lysosomes contain hydrolases that are capable of digesting all kinds of macromolecules. Christian de Duve was recognized for his role in the discovery of lysosomes when he was awarded the Nobel Prize in Physiology or Medicine in 1974. The discovery of lysosomes led to many new questions. The most critical question was: what was the physiological function of this "bag" of enzymes?
