No abstract

Autophagy is a cytoplasmic degradation system, which is important for starvation adaptation and cellular quality control. Recent advances in understanding autophagy highlight its importance under physiological and pathological conditions. However, methods for monitoring autophagic activity are complicated and the results are sometimes misinterpreted. Here, we review the methods used to identify autophagic structures, and to measure autophagic flux in cultured cells and animals. We will also describe the existing autophagy reporter mice that are useful for autophagy studies and drug testing. Lastly, we will consider the attempts to monitor autophagy in samples derived from humans.

Upon mitochondrial depolarization, Parkin, a Parkinson disease-related E3 ubiquitin ligase, translocates from the cytosol to mitochondria and promotes their degradation by mitophagy, a selective type of autophagy. Here, we report that in addition to mitophagy, Parkin mediates proteasome-dependent degradation of outer membrane proteins such as Tom20, Tom40, Tom70, and Omp25 of depolarized mitochondria. By contrast, degradation of the inner membrane and matrix proteins largely depends on mitophagy. Furthermore, Parkin induces rupture of the outer membrane of depolarized mitochondria, which also depends on proteasomal activity. Upon induction of mitochondrial depolarization, proteasomes are recruited to mitochondria in the perinuclear region. Neither proteasome-dependent degradation of outer membrane proteins nor outer membrane rupture is required for mitophagy. These results suggest that Parkin regulates degradation of outer and inner mitochondrial membrane proteins differently through proteasome-and mitophagydependent pathways.Parkinson disease is a progressive neurodegenerative disease that is characterized by postural changes, resting tremor, muscle rigidity, and weakness (1, 2). These symptoms are mainly caused by loss of dopaminergic neurons in the substantia nigra, and mitochondrial dysfunction appears to be the primary pathogenic event. Indeed, many of the products of Parkinson disease-related genes such as ␣-synuclein/PARK1/4, PARKIN/ PARK2, PINK1/PARK6, DJ-I/PARK7, and OMI/HTRA2 are physically and functionally linked to mitochondria (3,4).Parkin is a RING domain-containing E3 ubiquitin ligase, and its mutation causes autosomal recessive juvenile Parkinson disease (5). Recent studies have revealed that Parkin is important for mitochondrial quality control through degradation of damaged mitochondria. Narendra et al. (6) first demonstrated that Parkin translocates from the cytosol to depolarized mitochondria and triggers elimination of these mitochondria by autophagy, which is known as mitophagy. Targeting of Parkin to mitochondria requires PTEN-induced putative kinase 1 (Pink1), 3 another Parkinson disease-associated gene product (7-14). Pink1 is an extremely unstable mitochondrial protein, but it is stabilized upon mitochondrial depolarization and subsequently recruits Parkin.Autophagy is a membrane-mediated intracellular degradation process. A portion of cytoplasm is first enclosed by the double-membraned autophagosome, and the autophagosome then fuses with a lysosome to degrade the enclosed materials. Although autophagy has been thought to be mainly non-selective, recent studies have revealed that the autophagosomal membrane can recognize some specific proteins and organelles. Parkin-mediated autophagy of damaged mitochondria is one of the best examples of selective autophagy. However, the precise role of Parkin in the induction of mitophagy has not been fully elucidated. To date, several mitochondrial proteins, voltage-dependent anion channel 1 (VDAC1) (8), mitofusin (a mitochondrial pro-fusion factor)...

(2015) The autophagy gene Wdr45/Wipi4 regulates learning and memory function and axonal homeostasis, Autophagy, 11:6, 881-890, DOI: 10.1080/15548627.2015 Abbreviations: ACTB, b-actin; AMC, aminomethylcoumarin; Atg, autophagy-related; BPAN, b-propeller protein-associated neurodegeneration; CALB, calbindin; CNS, central nervous system; DCN, deep cerebellar nuclei; Ei24, etoposide-induced gene 24; epg, ectopic P granule; fEPSP, field excitatory postsynaptic potential; GFAP, glial fibrillary acid protein; H&E, hematoxylin and eosin; KO, knockout; LC3, microtubule-associated protein 1 light chain 3; LTP, long-term potentiation; MBP, myelin basic protein; NBIA, neurodegeneration with brain iron accumulation; RBFOX3, RNA binding protein, fox-1 homolog (C. elegans) 3; rpm, rotations per min; SENDA, static encephalopathy of childhood with neurodegeneration in adulthood; SQSTM1, sequestosome-1; WDR5/WIPI4, WD repeat domain 45; WT, wild type.WDR45/WIPI4, encoding a WD40 repeat-containing PtdIns(3)P binding protein, is essential for the basal autophagy pathway. Mutations in WDR45 cause the neurodegenerative disease b-propeller protein-associated neurodegeneration (BPAN), a subtype of NBIA. We generated CNS-specific Wdr45 knockout mice, which exhibit poor motor coordination, greatly impaired learning and memory, and extensive axon swelling with numerous axon spheroids. Autophagic flux is defective and SQSTM1 (sequestosome-1)/p62 and ubiquitin-positive protein aggregates accumulate in neurons and swollen axons. Nes-Wdr45 fl/Y mice recapitulate some hallmarks of BPAN, including cognitive impairment and defective axonal homeostasis, providing a model for revealing the disease pathogenesis of BPAN and also for investigating the possible role of autophagy in axon maintenance.