PubMed: 18930136

Title
Autophagy and the ubiquitin-proteasome system: collaborators in neuroprotection.
Journal
Biochimica et biophysica acta
Volume
1782
Issue
None
Pages
691-9
Date
2008-12-01
Authors
Nedelsky NB | Taylor JP | Todd PK

Evidence 1c4c0fec2d

Recent evidence suggests that the autophagic turnover of amyloid beta precursor protein (APP) may underlie the generation of toxic amyloid-β species [61].

Evidence 18321cdca4

In addition, inducing autophagy in an TOR-independent manner using lithium [53] or trehalose [54–56] has been shown to accelerate clearance of disease proteins in vitro [56] and protect against neurodegeneration in mouse and Drosophila models of Huntington’s disease [53,54].

Evidence 11d23e013f

This role in recycling is complementary to that of the UPS, which degrades proteins to generate oligopeptides that are subsequently degraded into amino acids while replenishing the cell’s supply of free ubiquitin.

Evidence dbbe68fdea

Pharmacological upregulation of autophagy can be accomplished using the drug rapamycin, which works by inhibiting TOR (target of rapamycin), a pleiotropic molecule that negatively regulates autophagy, among other functions

Evidence 138eff1c46

Indeed, treatment with rapamycin ameliorates the degenerative phenotype in a Drosophila model of SBMA, as well as in Drosophila and mouse models of Huntington’s disease [48,50,52].

Evidence 27b89c4132

p62 localizes to a variety of ubiquitin-positive neuropathological inclusions including Lewy bodies in Parkinson’s disease, neurofibrillary tangles in tauopathies, polyglutamine-expanded huntingtin aggregates in Huntington’s disease, and aggregates of mutant SOD1 in familial amyotrophic lateral sclerosis [85–87].

Evidence 61c09c53a8

It has been inferred that aggresome formation in vitro is a cytoprotective response in cultured cells since their formation correlates inversely with cell death, whereas interventions that block aggresome formation enhance cytotoxicity and slow the rate of turnover of misfolded proteins [27,64,70–72]

Evidence a986a31482

In mammals, autophagosomes first fuse with endosomes and multivesicular bodies to form amphisomes, which subsequently fuse with lysosomes to create degradative vacuoles termed autolysosomes [17].

Evidence 78c1e4afb1

In Huntington’s disease, affected neurons show accumulation of huntingtin in cathepsin-D-positive vacuoles [24]. Cathepsin-D is a lysosomal protease enriched in neuronal tissues, suggesting that these are autolysosomes

Evidence f3659a0866

CLN3-related neurodegeneration appears to be a consequence of reduced autophagosome-lysosome fusion [42].

Evidence 4b89d90cf2

The initial step in autophagy involves expansion of a membranous structure called the “isolation membrane” or “phagophore” that engulfs a portion of the cell; the membrane eventually fuses to form a new double-membraned structure known as an autophagosome (Figure 1).

Evidence 6296796612

Once formed, new autophagosomes move through a stepwise maturation process that culminates with fusion to a lysosome permitting degradation of the lumenal contents.

Evidence 707aefb119

LC3-II staining is also used as a primary histological marker of autophagosomes.

Evidence 85537e9fef

This suggestion originates from the observed accumulation of autophagic vacuoles in neurons from affected brain regions in a number of neurodegenerative diseases, including Alzheimer’s disease, Parkinson’s disease, Creutzfeldt- Jakob disease, and many of the polyglutamine diseases [21–24].

Evidence 0465de834f

Alzheimer’s disease pathology features massive accumulation of autophagic vacuoles within large swellings along dystrophic and degenerating neurites in neocortical and hippocampal pyramidal neurons [21].

Evidence e4fc126420

The consequences of impaired lysosome function, for example, may be observed in cathepsin D knockout mice and Drosophila melanogaster cathepsin D mutants which show neurodegeneration and associated accumulation of autophagosomes and lysosomes [33–35].

Evidence c57c07fe66

Indeed, a mouse model of distal-SBMA that expresses mutant p150/dynactin is characterized by accumulation of ubiquitin-positive aggregates and autophagic vacuoles in affected neurons [49].

Evidence c1d9fc9398

Indeed, primary lysosomal dysfunction in inherited congenital “lysosomal storage disorders” has long been recognized to cause severe neurodegenerative phenotypes characterized pathologically by accumulations of lysosomes and autophagic vacuoles [38]

Evidence 9f5cad46a5

For example, the neuronal ceroid lipofuscinoses (NCLs) are a heterogeneous group of inherited, neurodegenerative disorders with onset ranging from infancy to late adulthood that are caused by a variety of defects in lysosomal function.

Evidence 3763369575

Microtubule-based vesicular trafficking is essential for delivery of autophagosomes to lysosomes and subsequent fusion [46], and impaired dynein-mediated trafficking is associated with impaired autophagosome/ lysosome fusion and reduced protein turnover [47,48].

Evidence aac5be9801

The substrate-chaperone complex is then targeted to the lysosome by binding to lysosome-associated membrane protein 2A (LAMP-2A) which carries out receptor-mediated translocation of the substrate into the lysosome for degradation [11,13]

Evidence 2cc67725d7

HDAC6 activity was also reported to regulate chaperone expression in response to heat shock by deacetylating Hsp90 leading to release and activation of the transcription factor HSF-1 [79].

Evidence e328b63310

Recent advances have demonstrated that autophagy also serves a surprisingly diverse array of additional functions, including organelle clearance, antigen presentation, elimination of microbes, as well as regulation of development and cell death [9].

Evidence d35587bb84

Several lines of evidence suggest that there is an impairment of autophagy resulting from impaired autophagosome-lysosome fusion combined with decreasing efficiency of the lysosomal system [60].

Evidence c055cc7135

Considering the importance of protein catabolism in maintaining cell homeostasis, it is not surprising that dysregulation of protein turnover is associated with myriad disease states such as cancer and neurodegeneration [5].

Evidence fae4519148

Cellular stresses such as polyQ expression, proteasome impairment, oxidative stress, and increased misfolded protein burden activate transcription and translation of p62, suggesting that it functions broadly in stress situations [83,84]

Evidence 48545d4a0a

Microautophagy consists of direct engulfment of small volumes of cytosol by lysosomes [10], whereas chaperone-mediated autophagy (CMA) involves selective, receptor-mediated translocation of proteins into the lysosomal lumen [11].

Evidence 89bc46ebd7

CMA has also been found to contribute to the degradation of α-synuclein [29].

Evidence 421e0dcd8e

For example, α-synuclein is degraded at least in part by CMA [29].

Evidence bd4160d3ce

The neuronal protein α-synuclein, for example, can be degraded by the UPS, macroautophagy and chaperone-mediated autophagy [26,29].

Evidence d4a60ca241

UPS-mediated catabolism is also essential to mainten amino acid pools in acute starvation and contributes significantly to the degradation of defective proteins [1,2,7].

Evidence f7e67e13f4

Indeed, experimental evidence indicates that neurodegeneration is frequently associated with impaired UPS function, although whether this is a cause or consequence of neurodegeneration is a contested issue, as is reviewed elsewhere in this special issue

Evidence 9430fc4aa4

In the case of α-synuclein, for example, Webb et al. concluded that soluble forms of the disease protein are efficiently degraded by the UPS, while aggregated or oligomeric α-synuclein require autophagy for clearance [26].

Evidence be450d65df

In addition to aggresome formation, impairment of the UPS in vitro has been found to induce autophagy [63,64].

Evidence a48098b3d1

In cells that are exposed to stressors such as starvation or UPS inhibition, Alfy relocalizes from the nuclear envelope to filamentous cytoplasmic structures that are near autophagic membranes and ubiquitinated protein inclusions, as well as within autophagosomes [77].

Evidence 06b9d02500

Specifically, it is suggested that K63-linked polyubiquitin chains recruit p62 and HDAC6 providing a signal for autophagic degradation [92,93].

Evidence 2d9ff93881

This view was initially challenged by the observation that monoubiquitination operates as a key signal in endocytosis, a process important for numerous cell functions including lysosomal biogenesis [62]

Evidence 1c3447856e

The process of autophagy is controlled by parallel activation cascades that involve ubiquitin-like (UBL) protein modification, strikingly similar to the activation cascade that regulates the UPS (Figure 2a).

Evidence 3e1d768cdc

Autophagy, by contrast, is primarily responsible for degrading long-lived proteins and maintaining amino acid pools in the setting of chronic starvation, although its contribution to the degradation of defective proteins may equal that of the UPS.

Evidence a73835af47

“Autophagy”, literally “self-eating”, describes a catabolic process in which cell constituents such as organelles and proteins are delivered to the lysosomal compartment for degradation.

Evidence 283940bc75

Autophagy is an evolutionarily conserved process whose primary task in lower organisms is the maintenance of metabolic homeostasis in the face of changing nutrient availability [8].

Evidence b12d4317bf

It has also been suggested that autophagy plays a role in the initiation or progression of some neurodegenerative diseases [20].

Evidence 1a224c9c9d

Insight into the role of autophagy in neurodegeneration has been provided by studies indicating that: 1) some neurodegenerative disease-related proteins are degraded by autophagy, 2) impairment of autophagy promotes neurodegeneration in animal models and several human neurodegenerative diseases, and 3) manipulation of autophagy modifies phenotypes in animal models of neurodegeneration.

Evidence 020ab1830b

That neurodegenerative disease-causing proteins are frequently degraded by autophagy was demonstrated by a series of in vitro studies which showed that pharmacological induction or inhibition of macroautophagy alters the rate of turnover of a number of disease-related proteins including polyglutamine-expanded proteins, polyalanine-expanded proteins, as well as wild type and mutant forms of α-synuclein [25,26]

Evidence fa31cfd80e

It is becoming increasingly evident that the autophagy-lysosomal system is essential to neuronal homeostasis, and may in some settings be neuroprotective

Evidence a44e362d88

Similarly, in transgenic mice expressing amyloid precursor protein, a mouse model of Alzheimer’s disease, genetic inhibition of autophagy by heterozygous depletion of beclin-1 results in enhancement of neurodegeneration [51].

Evidence 98d74948e9

For example, in a Drosophila model of X-linked spinobulbar muscular atrophy (SBMA), a polyglutamine disease, degeneration was strongly enhanced by genetic inhibition of autophagy [50].

Evidence 47671d96e6

It has now been established that clearance of misfolded proteins from aggresomes is mediated at least in part by autophagy, implicating this pathway as a compensatory mechanism for degrading misfolded proteins when the proteasome is impaired [27,64,71,73].

Evidence aad791a967

Similar induction of autophagy is observed in response to genetic impairment of the proteasome in Drosophila [50].

Evidence 81364b5897

These processes are distinguished from macroautophagy, in which an isolation membrane expands to engulf a portion of the cell, eventually fusing to form a new autophagic vacuole that subsequently fuses with a lysosome [12].

Evidence 0ef4f66fe3

There have been fewer efforts to manipulate UPS function for therapeutic benefit in neurodegenerative disease, but it was recently shown that use of a proteasome activator enhanced survival in an in vitro model of Huntington’s disease [58], suggesting that augmenting other routes of protein degradation may also provide neuroprotection.

Evidence 64d2ef12c0

While GABARAP and GATE-16 may also be conjugated to PE in experimental systems, at present MAP-LC3 (typically abbreviated LC3) is the only protein that is known to remain associated with the autophagosome in higher eukaryotes.

Evidence 88719ed42a

LC3-I becomes conjugated to PE to form “LC3-II” and thereby covalently associates with the phagophore

Evidence 649169a8a6

CMA is a process in which proteins harboring a pentapeptide motif related to the sequence KFERQ are specifically recognized by a cytosolic chaperone, the heat shock cognate protein of 70 kDa (hsc70).

Evidence 8bd800c242

These Atg12-Atg5 conjugates are further cross-linked to Atg16 to form a large (~350 kDa) multimeric complex, which has been thought to act as a structural support for membrane expansion [15].

Evidence 6f3b42e1bb

As mentioned above, dramatic illustration of the interrelatedness of the UPS and autophagy was provided by characterizations of mice with conditional knockout of the essential autophagy genes Atg5 or Atg7 in the central nervous system, which resulted in neurodegeneration with accumulation of ubiquitin-positive pathology [36,37].

Evidence 2be2b8594e

More recent work has demonstrated that the Atg12-Atg5 conjugate can function as an E3-like enzyme in the second arm of the Atg conjugation cascade to promote lipidation of Atg8 [16]

Evidence 3f84f29595

In a second arm of the Atg conjugation system, Atg4 cleaves the UBL protein Atg8 to promote interaction with Atg7.

Evidence 78602b43ef

Similar results have recently been observed in vitro using the proteasome inhibitor lactacystin, as pre-treatment with rapamycin attenuates lactacystin-induced apoptosis and reduces lactacystin-induced ubiquitinated protein aggregation [74].

Evidence 13f58393d3

In the first arm of the Atg conjugation system, phagophore membrane elongation is triggered through the sequential action of an E1-like protein (Atg7) and an E2-like protein (Atg10) leading to an isopeptide linkage between the C-terminal glycine the UBL protein Atg12 and a lysine residue of Atg5 (Figure 2b).

Evidence c47363192f

Atg8 is then conjugated with the phospholipid phosphotidylethanolamine (PE) by the concerted action of the E2-like Atg3 and the E3-like Atg12-Atg5 conjugate

Evidence a66c26d161

Diseasecausing mutations in ATP13A2 result in protein retention in the endoplasmic reticulum and enhanced proteasomal degradation, suggesting that neurodegeneration could be caused by overwhelming the UPS and/or loss of function in lysosomal protein degradation [39].

Evidence 394f7ba338

Mutations in CLN3, a transmembrane protein that localizes to the late endosomal/lysosomal membrane, cause a form of NCL

Evidence 6e124e16e5

Although the mechanism whereby autophagy and UPS function are coordinated is little understood, several regulators have emerged as important players in mediating this crosstalk, including histone deacetylase 6 (HDAC6) [50,64,75], p62/sequestosome 1 (p62) [76], and the FYVE-domain containing protein Alfy [77]; notably, these proteins have all been found to regulate or be essential for aggresome formation

Evidence 7ffaa01b11

Recent models propose that p62 and HDAC6 function analogously to facilitate autophagic degradation of proteins that display specific polyubiquitin topology.

Evidence 84aa75dc0b

HDAC6 activity appears to be important for trafficking ubiquitinated proteins and lysosomes in vitro and this has led to the suggestion that HDAC6 coordinates delivery of substrates to autophagic machinery [64,70,78].

Evidence b2c3b76b86

Rab7 participates in trafficking autophagosomes and fusion with lysosomes and disease-causing mutations are predicted to impair this process [43–45].

Evidence e80e43e50d

Mutations in α-synuclein that are causative of familial Parkinson’s disease are poorly transferred to the lysosomal lumen and accumulate on the lysosomal surface, resulting in blockade of receptor-mediated translocation.

Evidence 10f9f3c42c

Thus, it has been suggested that p62 provides a key link between autophagy and the UPS by facilitating autophagic degradation of ubiquitinated proteins.

Evidence 22534a00ac

This model is consistent with an older study showing that inactivation of the ubiquitin-activating enzyme E1 leads to a defect in autolysosomal degradation and to an absence of ubiquitin-positive proteins within lysosomes [68].

Evidence 4b01de82e5

Mutations in blue cheese, the Drosophila homology of human Alfy, lead to reduced longevity and the accumulation of ubiquitinated neural aggregates, suggesting that its role in autophagic degradation may be involved in the clearance of ubiquitin aggregates [77,94]

Evidence 25ed961fad

Many neurodegenerative diseases are characterized by accumulation of misfolded protein deposits in affected brain regions, suggesting a failure in the cell’s degradative capacity [19].

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