Autophagy: A Nobel Prize Topic

Introduction

The word autophagy is derived from Greek words “auto” meaning self and “phagy” meaning eating. Autophagy is a normal physiological process in the body that deals with destruction of cells in the body.
It maintains homeostasis or normal functioning by protein degradation and turnover of the destroyed cell organelles for new cell formation.
During cellular stress the process of Autophagy is upscaled and increased. Cellular stress is caused when there is deprivation of nutrients and/or growth factors.
Thus Autophagy may provide an alternate source of intracellular building blocks and substrates that may generate energy to enable continuous cell survival.

History

Autophagy has been known for over 50 years but its fundamental importance in physiology and medicine was only recognized after Yoshinori Ohsumi's paradigm-shifting research in the 1990's. For his discoveries, he is awarded this year's Nobel Prize in physiology or medicine.

Autophagy and cell death

Autophagy also kills the cells under certain conditions. These are form of programmed cell death (PCD) and are called autophagic cell death. Programmed cell death is commonly termed apoptosis.
Autophagy is termed a nonapoptotic programmed cell death with different pathways and mediators from apoptosis.
Autophagy mainly maintains a balance between manufacture of cellular components and break down of damaged or unnecessary organelles and other cellular constituents.
There are some major degradative pathways that include proteasome that involves breaking down of most short-lived proteins.

Autophagy and stress

Autophagy enables cells to survive stress from the external environment like nutrient deprivation and also allows them to withstand internal stresses like accumulation of damaged organelles and pathogen or infective organism invasion.
Autophagy is seen in all eukaryotic systems including fungi, plants, slime mold, nematodes, fruit flies and insects, rodents (laboratory mice and rats), humans.

Mechanism Of Autophagy

Degradation – a central function in all living cells

in the mid 1950's scientists observed a new specialized cellular compartment, called an organelle, containing enzymes that digest proteins, carbohydrates and lipids. This specialized compartment is referred to as a "lysosome" and functions as a workstation for degradation of cellular constituents. The Belgian scientist Christian de Duve was awarded the Nobel Prize in Physiology or Medicine in 1974 for the discovery of the lysosome. New observations during the 1960's showed that large amounts of cellular content, and even whole organelles, could sometimes be found inside lysosomes. The cell therefore appeared to have a strategy for delivering large cargo to the lysosome. Further biochemical and microscopic analysis revealed a new type of vesicle transporting cellular cargo to the lysosome for degradation (Figure 1). Christian de Duve, the scientist behind the discovery of the lysosome, coined the term autophagy, "self-eating", to describe this process. The new vesicles were named autophagosomes.

Figure 1: Our cells have different specialized compartments. Lysosomes constitute one such compartment and contain enzymes for digestion of cellular contents. A new type of vesicle called autophagosome was observed within the cell. As the autophagosome forms, it engulfs cellular contents, such as damaged proteins and organelles. Finally, it fuses with the lysosome, where the contents are degraded into smaller constituents. This process provides the cell with nutrients and building blocks for renewal.

During the 1970's and 1980's researchers focused on elucidating another system used to degrade proteins, namely the "proteasome". Within this research field Aaron Ciechanover, Avram Hershko and Irwin Rose were awarded the 2004 Nobel Prize in Chemistry for "the discovery of ubiquitin-mediated protein degradation". The proteasome efficiently degrades proteins one-by-one, but this mechanism did not explain how the cell got rid of larger protein complexes and worn-out organelles. Could the process of autophagy be the answer and, if so, what were the mechanisms?

Autophagy – an essential mechanism in our cells

After the identification of the machinery for autophagy in yeast, a key question remained. Was there a corresponding mechanism to control this process in other organisms? Soon it became clear that virtually identical mechanisms operate in our own cells. The research tools required to investigate the importance of autophagy in humans were now available.

Thanks to Ohsumi and others following in his footsteps, we now know that autophagy controls important physiological functions where cellular components need to be degraded and recycled. Autophagy can rapidly provide fuel for energy and building blocks for renewal of cellular components, and is therefore essential for the cellular response to starvation and other types of stress. After infection, autophagy can eliminate invading intracellular bacteria and viruses. Autophagy contributes to embryo development and cell differentiation. Cells also use autophagy to eliminate damaged proteins and organelles, a quality control mechanism that is critical for counteracting the negative consequences of aging.

Disrupted autophagy has been linked to Parkinson's disease, type 2 diabetes and other disorders that appear in the elderly. Mutations in autophagy genes can cause genetic disease. Disturbances in the autophagic machinery have also been linked to cancer. Intense research is now ongoing to develop drugs that can target autophagy in various diseases.

Types of autophagy

There are several types of Autophagy. These are:-

microautophagy – in this process the cytosolic components are directly taken up by the lysosome itself through the lysosomal membrane.
macroautophagy – this involves delivery of cytoplasmic cargo to the lysosome through the intermediary of a double membrane-bound vesicle. This is called an autophagosome that fuses with the lysosome to form an autolysosome.
Chaperone-mediated autophagy – in this process the targeted proteins are translocated across the lysosomal membrane in a complex with chaperone proteins (such as Hsc-70).
micro- and macropexophagy
piecemeal microautophagy of the nucleus
cytoplasm-to-vacuole targeting (Cvt) pathway.

Difference Between Apoptosis, Necrosis and Autophagy

Apoptosis

The induction of apoptosis leads to characteristic cell changes and finally to death . These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation. Furthermore, changes affecting the membrane, nucleus, cytoplasm, and mitochondria occur. Apoptosis involves a complex cascade of reactions regulated by specific proteases called caspases and results in DNA degradation. Apoptotic processes have been researched in an extensive variety of diseases. Excessive apoptosis causes atrophy, whereas an insufficient amount results in uncontrolled cell proliferation, such as cancer.

Necroptosis
Necroptosis is indeed a programmed form of necrosis. As in all forms of necrotic processes, cells break open and leak their contents into the intercellular environment. In contrast to necrosis, leaking of the membrane during necroptosis is regulated by the cell. Necroptosis has been well described as a non-apoptotic “cellular suicide” process which represents a viral defense mechanism. Moreover, it plays a role in inflammatory diseases such as pancreatitis and Crohn’s disease.

Autophagy

Autophagy describes the fundamental catabolic mechanism during which cells degrade dysfunctional and unnecessary cellular components.This process is driven by the action of lysosomes and promotes survival during starvation periods, as the cellular energy level can thus be maintained. During autophagy autophagosomes are generated, organelles which are surrounded by double membranes and which contain the cellular components to be degraded. The autophagosome formation is induced by class 3 phosphoinositide-3-kinase, Atg 6 (autophagy-related gene 6) and ubiquitin or ubiquitin-like modifications of the target proteins. Subsequently, autophagosomes traffick through the cytosol of the cell and finally fuse with lysosomes to form an autolysome. After this fusion the cargo is degraded by lysosomal hydrolases. Excessive autophagy leads to cell death – a process which can be morphologically differentiated from apoptosis.

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