RT Journal Article
SR Electronic
T1 Autophagy Induction and Autophagosome Clearance in Neurons: Relationship to Autophagic Pathology in Alzheimer's Disease
JF The Journal of Neuroscience
JO J. Neurosci.
FD Society for Neuroscience
SP 6926
OP 6937
DO 10.1523/JNEUROSCI.0800-08.2008
VO 28
IS 27
A1 Barry Boland
A1 Asok Kumar
A1 Sooyeon Lee
A1 Frances M. Platt
A1 Jerzy Wegiel
A1 W. Haung Yu
A1 Ralph A. Nixon
YR 2008
UL http://www.jneurosci.org/content/28/27/6926.abstract
AB 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.