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The Journal of Neuroscience, July 2, 2008, 28(27):6926-6937; doi:10.1523/JNEUROSCI.0800-08.2008

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Neurobiology of Disease
Autophagy Induction and Autophagosome Clearance in Neurons: Relationship to Autophagic Pathology in Alzheimer's Disease

Barry Boland,^1,2,3 Asok Kumar,^1,4 Sooyeon Lee,^1,4 Frances M. Platt,³ Jerzy Wegiel,⁶ W. Haung Yu,⁵ and Ralph A. Nixon^1,4

¹Center for Dementia Research, Nathan Kline Institute, Orangeburg, New York 10962, ²Laboratory for Neurodegenerative Research, School of Biomolecular and Biomedical Science, Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland, ³Department of Pharmacology, University of Oxford, Oxford OX13QT, United Kingdom, ⁴Departments of Psychiatry, Cell Biology, and Neuroscience, New York University, New York, New York 10016, ⁵Taub Institute and Department of Pathology, Columbia University, New York, New York 10032, and ⁶New York State Institute for Basic Research in Developmental Disabilities, Staten Island, New York 10314

Correspondence should be addressed to Dr. Barry Boland, Laboratory for Neurodegenerative Research, School of Biomolecular and Biomedical Science, Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland. Email: barry.boland{at}ucd.ie

Macroautophagy, a major pathway for organelle and protein turnover, has been implicated in the neurodegeneration of Alzheimer's disease (AD). The basis for the profuse accumulation of autophagic vacuoles (AVs) in affected neurons of the AD brain, however, is unknown. In this study, we show that constitutive macroautophagy in primary cortical neurons is highly efficient, because newly formed autophagosomes are rapidly cleared by fusion with lysosomes, accounting for their scarcity in the healthy brain. Even after macroautophagy is strongly induced by suppressing mTOR (mammalian target of rapamycin) kinase activity with rapamycin or nutrient deprivation, active cathepsin-positive autolysosomes rather than LC3-II-positive autophagosomes predominate, implying efficient autophagosome clearance in healthy neurons. In contrast, selectively impeding late steps in macroautophagy by inhibiting cathepsin-mediated proteolysis within autolysosomes with cysteine- and aspartyl-protease inhibitors caused a marked accumulation of electron-dense double-membrane-limited AVs, containing cathepsin D and incompletely degraded LC3-II in perikarya and neurites. Similar structures accumulated in large numbers when fusion of autophagosomes with lysosomes was slowed by disrupting their transport on microtubules with vinblastine. Finally, we find that the autophagic vacuoles accumulating after protease inhibition or prolonged vinblastine treatment strongly resembled AVs that collect in dystrophic neurites in the AD brain and in an AD mouse model. We conclude that macroautophagy is constitutively active and highly efficient in healthy neurons and that the autophagic pathology observed in AD most likely arises from impaired clearance of AVs rather than strong autophagy induction alone. Therapeutic modulation of autophagy in AD may, therefore, require targeting late steps in the autophagic pathway.

Key words: aging; Alzheimer; autophagy; culture; lysosome; neuron; neuron death; neuronal death; neuropathology; neuroprotection; neurotoxicity; storage

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Received Aug. 20, 2007; revised May 5, 2008; accepted May 15, 2008.

Correspondence should be addressed to Dr. Barry Boland, Laboratory for Neurodegenerative Research, School of Biomolecular and Biomedical Science, Conway Institute, University College Dublin, Belfield, Dublin 4, Ireland. Email: barry.boland{at}ucd.ie

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