ScienceWeek

CELL BIOLOGY: ON AUTOPHAGY

The following points are made by T. Shintani and D.J. Klionsky (Science 2004 306:990):

1) Cellular homeostasis requires a constant balance between biosynthetic and catabolic processes. Eukaryotic cells primarily use two distinct mechanisms for large-scale degradation, the proteasome and autophagy; but only autophagy has the capacity to degrade entire organelles. The three types of autophagy are macroautophagy, microautophagy, and chaperone-mediated autophagy (1).

2) Macroautophagy (hereafter called autophagy) plays an important physiological role in human health. In autophagy, a double- or multi-membrane-bound structure, called the autophagosome or autophagic vacuole, is formed de novo to sequester cytoplasm. Then, the vacuole membrane fuses with the lysosome to deliver the contents into the organelle lumen, where the contents are degraded and the resulting macromolecules recycled.

3) Autophagy occurs at basal levels in most tissues and contributes to the routine turnover of cytoplasmic components. However, autophagy can be induced by a change of environmental conditions such as nutrient depletion. In addition to turnover of cellular components, autophagy is involved in development, differentiation, and tissue remodeling in various organisms (2). Autophagy is also implicated in certain human diseases. Paradoxically, autophagy can serve to protect cells but may also contribute to cell damage.

4) Autophagy is involved in programmed cell death (PCD). Type I PCD, apoptosis, is characterized by condensation of cytoplasm and chromatin, DNA fragmentation, and cell fragmentation into apoptotic bodies, followed by removal and degradation of the dying cells by phagocytosis. Type II PCD (autophagic) is characterized by the accumulation of autophagic vesicles (autophagosomes and autophagolysosomes) and is often observed when massive cell elimination is demanded or when phagocytes do not have easy access to the dying cells. One feature that distinguishes apoptosis from autophagic cell death is the source of the lysosomal enzymes used for most of the dying-cell degradation. Apoptotic cells use phagocytic cell lysosomes for this process, whereas cells with autophagic morphology use the endogenous lysosomal machinery of dying cells. It has been unclear whether autophagy directly executes cell death or is the secondary effect of apoptosis. A recent study, however, suggests that autophagy might cause cell death (3). Caspase inhibitor-induced autophagic cell death is severely affected by RNA interference (RNAi) with ATG7 and beclin 1 expression, two genes whose products are essential for autophagy (3).

5) In summary: Autophagy, the process by which cells recycle cytoplasm and dispose of excess or defective organelles, has entered the research spotlight largely owing to the discovery of the protein components that drive this process. Identifying the autophagy genes in yeast and finding orthologs in other organisms reveals the conservation of the mechanism of autophagy in eukaryotes and allows the use of molecular genetics and molecular biology in different model systems to study this process. By mostly morphological studies, autophagy has been linked to disease processes. Whether autophagy protects from or causes disease is unclear.(4,5)

References (abridged):

1. D. J. Klionsky, Ed., Autophagy (Landes Bioscience, Georgetown, TX, 2004), pp. 1-303

2. B. Levine, D. J. Klionsky, Dev. Cell 6, 463 (2004)

3. L. Yu et al., Science 304, 1500 (2004)

4. T. Raveh, G. Droguett, M. S. Horwitz, R. A. DePinho, A. Kimchi, Nature Cell Biol. 3, 1 (2001)

5. B. Inbal, S. Bialik, I. Sabanay, G. Shani, A. Kimchi, J. Cell Biol. 157, 455 (2002)

Science http://www.sciencemag.org

--------------------------------

Related Material:

CELL BIOLOGY: ON AUTOPHAGY IN BIOLOGICAL CELLS

The following points are made by Daniel J. Klionsky (Nature 2004 431:31):

1) When you are fasting or are otherwise deprived of food, your body begins to break down stored nutrients to keep essential processes and organs (such as your brain) supplied with fuel. Similarly, when a cell is deprived of nutrients it will degrade some of its own constituents to stay alive. It does this by the process of autophagy -- literally, "self-eating".

2) Autophagy is marked by the formation of autophagosomes --large vesicles, bounded by a double membrane, that sequester cytosol and organelles such as mitochondria. Fusion of an autophagosome with a lysosome releases the inner vesicle into the lysosome, where enzymes break the vesicle down. Degradation and recycling of the vesicle's constituents enable the cell to continue to carry out essential processes.

3) Because autophagy has the capacity to degrade entire organelles, it could be harmful if it occurred randomly. Autophagy must therefore be tightly regulated. The question is: how is this regulation accomplished? How does a cell or organism sense its environment and trigger an appropriate signal to induce or suppress autophagy? The process occurs in organisms ranging from yeast to worms to humans(3).

4) Drosophila larvae have a storage organ called the fat body, which has analogous functions to those of the liver and adipose (fat) tissue in vertebrates: it is a source of large amounts of potential energy. If an animal has just eaten, of course, there is no need to tap into these energy stores. In response to food, the hormone insulin is produced; this binds a receptor on the surface of cells in the fat body and throughout the organism, and triggers a signalling cascade. An important part of this cascade is an enzyme termed class I phosphatidylinositol-3-OH kinase (PI(3)K). This adds a phosphate group to a particular position on the lipid phosphatidylinositol, which is part of the cell membrane. Various proteins in turn bind the phosphorylated lipid and become activated, thus transmitting the external signal (in this case, insulin) into the cell. A central player in this pathway is the enzyme Tor, which is involved in many regulatory events connected with energy metabolism, and which suppresses autophagy(4).

5) So this pathway provides a mechanism by which cells can block self-eating if the organism has just fed. On the other hand, the pathway also provides a means of inducing autophagy when the organism is starved: a lack of food (particularly of the sugar glucose) leads to a lack of insulin, leaving the pathway inactive and enabling the cell to tap into its storage reserves. But how does the cell regulate the degree of autophagy so that it does not get out of hand? An explanation for this additional level of control comes from Scott et al(1). The activity of the enzyme p70 S6 kinase has been shown to increase when Tor is turned on --that is, when nutrients are abundant(5). This led to the proposal that p70 S6 kinase is itself a negative regulator of autophagy. However, Scott et al(1). now show that this enzyme must be active for autophagy to be maximally induced.(2)

References (abridged):

1. Scott, R. C., Schuldiner, O. & Neufeld, T. P. Dev. Cell 7, 167-178 (2004)

2. Rusten, T. E. et al. Dev. Cell 7, 179-192 (2004)

3. Reggiori, F. & Klionsky, D. J. Eukaryot. Cell 1, 11-21 (2002)

4. Jacinto, E. & Hall, M. N. Nature Rev. Mol. Cell Biol. 4, 117-126 (2003) (Erratum: 4, 249 (2003))

5. Chung, J., Kuo, C. J., Crabtree, G. R. & Blenis, J. Cell 69, 1227-1236 (1992)

Nature http://www.nature.com/nature

--------------------------------

Related Material:

ON PROGRAMMED CELL DEATH

The following points are made by Scott F. Gilbert (citation below):

1) "To be, or not to be: that is the question." While we all are poised at life-or-death decisions, this existential dichotomy is exceptionally stark for embryonic cells. Programmed cell death, called apoptosis, is a normal part of development. In the nematode worm C. elegans, in which we can count the number of cells, exactly 131 cells die according to the normal developmental pattern. All the cells of this nematode are "programmed" to die unless they are actively told not to undergo apoptosis. In humans, as many as 10^(11) cells die in each adult each day and are replaced by other cells. (Indeed, the mass of cells we lose each year through normal cell death is close to our entire body weight!) Within the uterus, during our fetal development, we were constantly making and destroying cells, and we generated about three times as many neurons as we eventually ended up with when we were born.

2) Apoptosis is necessary not only for the proper spacing and orientation of neurons, but also for generating the middle ear space, the vaginal opening, and the spaces between our fingers and toes. Apoptosis prunes away unneeded structures, controls the number of cells in particular tissues, and sculpts complex organs.

3) Different tissues use different signals for apoptosis. One of the signals often used in vertebrates is bone morphogenetic protein 4 (BMP4). Some tissues, such as connective tissue, respond to BMP4 by differentiating into bone. Others, such as the frog gastrula ectoderm, respond to BMP4 by differentiating into skin. Still others, such as neural crest cells and tooth primordia, respond by degrading their DNA and dying. In the developing tooth, for instance, numerous growth and differentiation factors are secreted by the enamel knot. After the cusp has grown, the enamel knot synthesizes BMP4 and shuts itself down by apoptosis.

4) In other tissues, the cells are "programmed" to die, and they will remain alive only if some growth or differentiation factor is present to "rescue" them. This happens during the development of mammalian red blood cells. The red blood cell precursors in the mouse liver need the hormone erythropoietin in order to survive. If they do not receive it, they undergo apoptosis. The erythropoietin receptor works through the JAK-STAT pathway, activating the Stat5 transcription factor. In this way, the amount of erythropoietin present can determine how many red blood cells enter the circulation.

Adapted from: Scott F. Gilbert: Developmental Biology. 6th Edition. Sinauer 2000, p.165.

ScienceWeek http://scienceweek.com