Point of
VIEW
Autophagy and Neurodegeneration Congcong He and Daniel J. Klionsky*
Life Sciences Institute, and Departments of Molecular, Cellular and Developmental Biology and Biological Chemistry, University of Michigan, Ann Arbor, Michigan 48109
R
emoval of unwanted proteins is crucial for the maintenance of cellular homeostasis, especially in postmitotic cells, such as neurons, since the accumulation of abnormal proteins will not be diluted by cell division. Indeed, misfolded protein aggregation characterizes most neurodegenerative diseases (1). Eukaryotic cells primarily utilize two mechanisms for protein clearance, the ubiquitin– proteasome pathway and the autophagy– lysosome pathway. Compared with the proteasome, which recognizes and degrades protein substrates conjugated to a polyubiquitin chain, autophagy is a less selective clearance mechanism (Figure 1). Autophagy is induced under conditions of physiological stress, such as starvation. Autophagy also occurs at a basal level in normal conditions. Yet, little is known about the physiological importance of constitutive autophagy in nondividing cells. Two recent papers published in Nature by Hara et al. (2) and Komatsu et al (3). proposed a neuroprotective role of basal autophagy, based on their striking findings that mice with autophagy defects specifically in neurons formed intracellular inclusions in the brain and developed neurodegenerative symptoms, even without the expression of any disease-causing, aggregate-prone proteins. The autophagic process requires the concerted coordination of proteins encoded by Atg (autophagy-related) genes (4). The ubiquitous knockout of Atg5 or Atg7, two essential genes for autophagy, causes early postnatal lethality in mice. Accordingly, Hara et al. (2) and Komatsu et al. (3) used a
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Cre-loxP system to disrupt Atg5 and Atg7, respectively, in the mouse central nervous system (CNS) by expressing Cre recombinase driven by a CNS-specific promoter. This approach allowed the authors to investigate the role of autophagy in the CNS in adult animals and study long-term neurodegenerative progression. The neuron-specific, autophagy-deficient mice (referred to as Atg5−/− and Atg7−/−) were born normally but developed progressive behavioral defects resembling those of aging-related neurodegeneration in humans and mouse models, for example, limb-clasping, tremor, and ataxic walking pattern. The behavioral phenotypes were accompanied by mass death of Purkinje cells and the presence of ubiquitin-positive inclusion bodies in certain regions of the brain including the cerebral cortex, cerebellum, and hypothalamus, which is thought to be the pathological hallmark of neurodegeneration. In addition, the autophagy-deficient mice had a dramatically decreased survival rate; most animals died within 28 weeks. In Atg7−/− mice, polyubiquitinated proteins accumulated and aggregated into inclusions despite apparently normal proteasome function, which suggests that basal autophagy may be essential in clearing some misfolded proteins that are beyond the degradative capability of the proteasome. Thus, basal autophagy may be of more biological importance than was previously recognized. If defects in autophagy alone are sufficient to elicit protein aggregation and degeneration in normal neurons
A B S T R A C T Autophagy mediates the bulk degradation of cytosolic proteins and organelles. Recent studies using neuronspecific knockout mouse models demonstrate that autophagy deficiency leads to protein aggregation and neurodegeneration, even in the absence of disease-related aggregate-prone proteins.
*To whom correspondence should be addressed. E-mail:
[email protected].
Published online May 19, 2006 10.1021/cb600182h CCC: $33.50 © 2006 by American Chemical Society
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Figure 1. Schematic representation of autophagy in a mammalian cell. A portion of cytosol destined for degradation is sequestered in a double-membrane vesicle termed an autophagosome, by elongation of a small pre-autophagosome membrane structure that derives from an unknown origin. The autophagosome then fuses with the lysosome to form an autolysosome. The inner vesicle is degraded by resident hydrolases together with its cargoes.
(Figure 2, panel a), it is interesting to consider cells that are burdened with aggregate-prone proteins related to human diseases, for example, ␣-synuclein and expanded polyglutamine proteins. In a line of inducible transgenic mice, neurodegeneration could be promisingly reversed when Figure 2. Hypothetical models of clearance mechanisms of normal proteins and diseaserelated aggregate-prone proteins. a) In normal neurons, misfolded proteins are ubiquitinated and actively degraded by proteasomes. The basal level of autophagy also constitutively degrades excess or misfolded proteins, maintaining a favorable environment for various cellular functions. In the neurons of Atg5ⴚ/ⴚ or Atg7ⴚ/ⴚ mice where autophagy is blocked, an elevated level of misfolded proteins accumulates in the cell and forms inclusions, leading to neurodegeneration and cell death. b) In patients or transgenic animals with neurodegenerative disorders, aggregate-prone proteins, for example, expanded polyglutamine proteins, express and adopt aberrant structures that easily aggregate into inclusions that are highly resistant to proteases. Autophagy may be a primary mechanism by which cells clear diffuse monomeric or oligomeric forms of aggregateprone proteins, although a role for the proteasome in clearance cannot be ruled out.
the expression of the harmful protein is shut off (5). This observation implies a protective role of protein clearance. Increasing evidence indicates autophagy is actively involved in this process (6, 7). For example, chemical inhibition of autophagy at the autophagosome formation or autophago-
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Misfolded protein Proteasome
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some–lysosome fusion stages is associated with enhanced protein aggregation and death in cell models (8). Conversely, rapamycin, a chemical inducer of autophagy, reduces aggregate formation and improves behavioral tasks in transgenic animal models (9, 10). Recent studies suggest that the cytosolic early protein species (monomer or oligomer) in the aggregation process, rather than the inclusion itself, are the main source of toxicity (1, 11). In Atg5−/− neurons and hepatocytes, inclusions formed after diffuse-ubiquitinated proteins had accumulated to a high level. This observation supports the idea that inclusions may be a consequence rather than the cause of pathogenesis. The other intriguing hypothesis that stems from this finding is that autophagy may protect against neurode-
HE AND KLIONSKY
Inclusion
Autophagy
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Point of
VIEW generation by reducing the level of potentially toxic diffuse protein species rather than the inclusions per se (Figure 2, panel b). In fact, the size of inclusions usually exceeds that of typical mammalian autophagosomes. There is some controversy regarding the ability of the proteasome to degrade these amyloidogenic proteins (12–14). In some diseases, the proteasomal function is significantly impaired in the presence of aggregate-prone proteins (15, 16). In addition, it has been found that proteasomes are unable to cleave within expanded polyglutamine sequences (17), which may result in steric hindrance and inhibition of the proteasome. Despite recent progress, there are many open questions to be answered with regard to misfolded protein clearance and neurodegeneration. For example, it is not clear if, or how, the proteasome and autophagy coordinate to accomplish the task of protein quality control. In addition, we do not know exactly the forms of aggregate-prone proteins (diffuse or inclusion) degraded through autophagy, or the molecular components and mechanism of autophagic degradation of these proteins. Since it is suggested that toxicity has been initiated before aggregates are formed (18), it will be helpful to have additional data that identify the factors/pathways that regulate the clearance of abnormal proteins before or during early stages of the aggregation process, as these may serve as potential therapeutic targets in neurodegenerative disorders. The studies by Hara et al. (2) and Komatsu et al. (3) provide a direct causative connection between impaired autophagy and neurodegeneration in the absence of genetic
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mutations that predispose an individual to disease and, thus, unveil a critical role of basal autophagic clearance. Neurons may be especially sensitive to proteolytic stress due to their quiescent state and highly specific function. The neuron-specific, autophagy-deficient mouse models will be useful in further investigating the role of autophagy in neurological diseases, specifically, in protection against misfolded protein accumulation and aggregation. Acknowledgement: This article is dedicated to the memory of Cecile M. Pickart. REFERENCES 1. Ross, C. A., and Poirier, M. A. (2004) Protein aggregation and neurodegenerative disease. Nat. Med. 10 (Suppl.), S10–S17. 2. Hara, T., Nakamura, K., Matsui, M., Yamamoto, A., Nakahara, Y., Suzuki-Migishima, R., Yokoyama, M., Mishima, K., Saito, I., Okano, H., and Mizushima, N. (2006) Suppression of basal autophagy in neural cells causes neurodegenerative disease in mice. Nature, published online April 19, http://dx.doi.org/10.1038/nature04724. 3. Komatsu, M., Waguri, S., Chiba, T., Murata, S., Iwata, J. I., Tanida, I., Ueno, T., Koike, M., Uchiyama, Y., Kominami, E., and Tanaka, K. (2006) Loss of autophagy in the central nervous system causes neurodegeneration in mice. Nature, published online April 19, http://dx.doi.org/10.1038/nature04723. 4. Levine, B., and Klionsky, D. J. (2004) Development by self-digestion: molecular mechanisms and biological functions of autophagy. Dev. Cell 6, 463–477. 5. Yamamoto, A., Lucas, J. J., and Hen, R. (2000) Reversal of neuropathology and motor dysfunction in a conditional model of Huntington’s disease. Cell 101, 57–66. 6. Bjorkoy, G., Lamark, T., Brech, A., Outzen, H., Perander, M., Overvatn, A., Stenmark, H., and Johansen, T. (2005) p62/SQSTM1 forms protein aggregates degraded by autophagy and has a protective effect on huntingtin-induced cell death. J. Cell Biol. 171, 603–614. 7. Qin, Z. H., Wang, Y., Kegel, K. B., Kazantsev, A., Apostol, B. L., Thompson, L. M., Yoder, J., Aronin, N., and DiFiglia, M. (2003) Autophagy regulates the processing of amino terminal huntingtin fragments. Hum. Mol. Genet. 12, 3231–3244.
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