Chemical Inducers of Autophagy That Enhance the Clearance of Mutant Proteins in Neurodegenerative Diseases
2010; Elsevier BV; Volume: 285; Issue: 15 Linguagem: Inglês
10.1074/jbc.r109.072181
ISSN1083-351X
AutoresMaurizio Renna, María Jiménez-Sánchez, Sovan Sarkar, David C. Rubinsztein,
Tópico(s)Genetic Neurodegenerative Diseases
ResumoMany of the neurodegenerative diseases that afflict people are caused by intracytoplasmic aggregate-prone proteins. These include Parkinson disease, tauopathies, and polyglutamine expansion diseases such as Huntington disease. In Mendelian forms of these diseases, the mutations generally confer toxic novel functions on the relevant proteins. Thus, one potential strategy for dealing with these mutant proteins is to enhance their degradation. This can be achieved by up-regulating macroautophagy, which we will henceforth call autophagy. In this minireview, we will consider the reasons why autophagy up-regulation may be a powerful strategy for these diseases. In addition, we will consider some of the drugs and associated signaling pathways that can be used to induce autophagy with these therapeutic aims in mind. Many of the neurodegenerative diseases that afflict people are caused by intracytoplasmic aggregate-prone proteins. These include Parkinson disease, tauopathies, and polyglutamine expansion diseases such as Huntington disease. In Mendelian forms of these diseases, the mutations generally confer toxic novel functions on the relevant proteins. Thus, one potential strategy for dealing with these mutant proteins is to enhance their degradation. This can be achieved by up-regulating macroautophagy, which we will henceforth call autophagy. In this minireview, we will consider the reasons why autophagy up-regulation may be a powerful strategy for these diseases. In addition, we will consider some of the drugs and associated signaling pathways that can be used to induce autophagy with these therapeutic aims in mind. Intracellular protein misfolding and aggregation are features of many late-onset neurodegenerative diseases called proteinopathies. These include Alzheimer disease, Parkinson disease, tauopathies, and polyQ 3The abbreviations used are: polyQpolyglutamineHDHuntington diseaseSCAspinocerebellar ataxiaGFPgreen fluorescent proteinALSamyotrophic lateral sclerosismTORmammalian target of rapamycinIP3inositol 1,4,5-trisphosphateIMPaseinositol monophosphataseIP3RIP3 receptorSMERsmall molecule enhancer of rapamycinPLCϵphospholipase CϵERendoplasmic reticulumGSK-3βglycogen synthase kinase-3β. expansion diseases such as HD and various SCAs such as SCA3 (1Rubinsztein D.C. Nature. 2006; 443: 780-786Crossref PubMed Scopus (1332) Google Scholar, 2Ross C.A. Poirier M.A. Nat. Med. 2004; 10: S10-S17Crossref PubMed Scopus (2474) Google Scholar). Currently, there are no effective strategies to slow or prevent the neurodegeneration resulting from these diseases in humans. polyglutamine Huntington disease spinocerebellar ataxia green fluorescent protein amyotrophic lateral sclerosis mammalian target of rapamycin inositol 1,4,5-trisphosphate inositol monophosphatase IP3 receptor small molecule enhancer of rapamycin phospholipase Cϵ endoplasmic reticulum glycogen synthase kinase-3β. All known polyQ mutant proteins form intracellular aggregates (inclusions) with amyloid-like structures in susceptible neurons (3Zoghbi H.Y. Orr H.T. Annu. Rev. Neurosci. 2000; 23: 217-247Crossref PubMed Scopus (1102) Google Scholar). HD, the most prevalent of the nine polyQ expansion diseases, is caused by an abnormally expanded CAG trinucleotide repeat tract in the IT15 gene (>35 repeats). These repeats are translated into an elongated polyQ tract close to the N-terminal end of the huntingtin protein. Huntingtin is mainly cytosolic, but a small proportion is nuclear (4Imarisio S. Carmichael J. Korolchuk V. Chen C.W. Saiki S. Rose C. Krishna G. Davies J.E. Ttofi E. Underwood B.R. Rubinsztein D.C. Biochem. J. 2008; 412: 191-209Crossref PubMed Scopus (327) Google Scholar). In HD, intranuclear inclusions are seen in the rarer juvenile-onset cases, but extranuclear inclusions predominate in the more typical adult-onset cases. The causal role for inclusions in these diseases is debated because some have reported dissociations between cell death and inclusion formation (4Imarisio S. Carmichael J. Korolchuk V. Chen C.W. Saiki S. Rose C. Krishna G. Davies J.E. Ttofi E. Underwood B.R. Rubinsztein D.C. Biochem. J. 2008; 412: 191-209Crossref PubMed Scopus (327) Google Scholar, 5Arrasate M. Mitra S. Schweitzer E.S. Segal M.R. Finkbeiner S. Nature. 2004; 431: 805-810Crossref PubMed Scopus (1614) Google Scholar). Strong genetic and transgenic data argue that the primary consequence of the polyQ expansion mutations is to confer toxic gain of function on the mutant proteins (1Rubinsztein D.C. Nature. 2006; 443: 780-786Crossref PubMed Scopus (1332) Google Scholar, 2Ross C.A. Poirier M.A. Nat. Med. 2004; 10: S10-S17Crossref PubMed Scopus (2474) Google Scholar, 4Imarisio S. Carmichael J. Korolchuk V. Chen C.W. Saiki S. Rose C. Krishna G. Davies J.E. Ttofi E. Underwood B.R. Rubinsztein D.C. Biochem. J. 2008; 412: 191-209Crossref PubMed Scopus (327) Google Scholar, 6Rubinsztein D.C. Trends Genet. 2002; 18: 202-209Abstract Full Text Full Text PDF PubMed Scopus (252) Google Scholar). Indeed, a gain-of-function mechanism appears to underlie most of the Mendelian disorders caused by aggregate-prone proteins, including tauopathies and other polyQ expansion disorders. This does not exclude that the gain-of-function toxicity in diseases like HD may be modulated to some degree by loss-of-function effects, although transgenic data suggest that such putative effects are likely to be small (7Van Raamsdonk J.M. Pearson J. Rogers D.A. Bissada N. Vogl A.W. Hayden M.R. Leavitt B.R. Hum. Mol. Genet. 2005; 14: 1379-1392Crossref PubMed Scopus (127) Google Scholar). Because the mutations causing many proteinopathies (e.g. polyQ diseases and tauopathies) confer novel toxic functions on the specific proteins and because disease severity frequently correlates with expression levels, it is important to understand the factors regulating the synthesis and clearance of these aggregate-prone proteins. Our data suggest that accelerating the removal of toxic huntingtin fragments may be a tractable therapeutic strategy for HD (Fig. 1). We showed that the ubiquitin-proteasome and autophagy-lysosome pathways are the major routes for mutant huntingtin fragment clearance (8Ravikumar B. Duden R. Rubinsztein D.C. Hum. Mol. Genet. 2002; 11: 1107-1117Crossref PubMed Scopus (938) Google Scholar). Although the narrow proteasome barrel precludes entry of oligomers/aggregates of mutant huntingtin (or other aggregate-prone intracellular proteins), such substrates can be degraded efficiently by macroautophagy (which we will call autophagy). Autophagy involves the formation of double-membrane isolation structures called phagophores, which expand and engulf portions of the cytoplasm, forming double-membrane vesicles called autophagosomes (Fig. 1) (9Ravikumar B. Futter M. Jahreiss L. Korolchuk V.I. Lichtenberg M. Luo S. Massey D.C. Menzies F.M. Narayanan U. Renna M. Jimenez-Sanchez M. Sarkar S. Underwood B. Winslow A. Rubinsztein D.C. J. Cell Sci. 2009; 122: 1707-1711Crossref PubMed Scopus (149) Google Scholar, 10Mizushima N. Levine B. Cuervo A.M. Klionsky D.J. Nature. 2008; 451: 1069-1075Crossref PubMed Scopus (5180) Google Scholar). Autophagosomes are formed randomly in the cytoplasm and are then trafficked along microtubules in a dynein-dependent fashion toward the microtubule-organizing center, where they fuse with lysosomes, forming autolysosomes, after which their contents are degraded (11Jahreiss L. Menzies F.M. Rubinsztein D.C. Traffic. 2008; 9: 574-587Crossref PubMed Scopus (334) Google Scholar, 12Ravikumar B. Acevedo-Arozena A. Imarisio S. Berger Z. Vacher C. O'Kane C.J. Brown S.D. Rubinsztein D.C. Nat. Genet. 2005; 37: 771-776Crossref PubMed Scopus (373) Google Scholar). The only known mammalian protein that specifically associates with the autophagosome membrane (as opposed to other vesicles) is MAP1 LC3 (microtubule-associated protein 1 light chain 3), which is post-translationally modified into cytosolic LC3-I, which conjugates with phosphatidylethanolamine upon autophagy induction to form autophagosome-associated LC3-II (13Kabeya Y. Mizushima N. Ueno T. Yamamoto A. Kirisako T. Noda T. Kominami E. Ohsumi Y. Yoshimori T. EMBO J. 2000; 19: 5720-5728Crossref PubMed Scopus (5467) Google Scholar). Recent studies have shown that constitutive autophagy may play a pivotal role in the clearance of normally occurring cellular misfolded proteins, as loss of basal autophagy by conditional knock-out of key autophagy genes, such as Atg5 and Atg7, in mouse brains resulted in a neurodegenerative phenotype and the formation of protein aggregates (14Hara T. Nakamura K. Matsui M. Yamamoto A. Nakahara Y. Suzuki-Migishima R. Yokoyama M. Mishima K. Saito I. Okano H. Mizushima N. Nature. 2006; 441: 885-889Crossref PubMed Scopus (3130) Google Scholar, 15Komatsu M. Waguri S. Chiba T. Murata S. Iwata J. Tanida I. Ueno T. Koike M. Uchiyama Y. Kominami E. Tanaka K. Nature. 2006; 441: 880-884Crossref PubMed Scopus (2850) Google Scholar). We have shown that mutant huntingtin fragments, expanded polyalanines tagged with GFP, and mutant forms of α-synuclein (associated with forms of Parkinson disease) are highly dependent on autophagy for their clearance in cell models (8Ravikumar B. Duden R. Rubinsztein D.C. Hum. Mol. Genet. 2002; 11: 1107-1117Crossref PubMed Scopus (938) Google Scholar, 16Webb J.L. Ravikumar B. Atkins J. Skepper J.N. Rubinsztein D.C. J. Biol. Chem. 2003; 278: 25009-25013Abstract Full Text Full Text PDF PubMed Scopus (1154) Google Scholar). The clearance of these mutant proteins is delayed by autophagy inhibitors like 3-methyladenine and bafilomycin A1 or by knockdown of autophagy genes, whereas autophagy induction with rapamycin enhances their clearance (8Ravikumar B. Duden R. Rubinsztein D.C. Hum. Mol. Genet. 2002; 11: 1107-1117Crossref PubMed Scopus (938) Google Scholar, 17Shibata M. Lu T. Furuya T. Degterev A. Mizushima N. Yoshimori T. MacDonald M. Yankner B. Yuan J. J. Biol. Chem. 2006; 281: 14474-14485Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, 18Iwata A. Christianson J.C. Bucci M. Ellerby L.M. Nukina N. Forno L.S. Kopito R.R. Proc. Natl. Acad. Sci. U.S.A. 2005; 102: 13135-13140Crossref PubMed Scopus (273) Google Scholar). Subsequently, other aggregate-prone proteins, such as Tau causing frontotemporal dementias, mutant ataxin-3 associated with SCA3, mutant SOD1 (superoxide dismutase 1) associated with ALS, and mutant prion proteins causing prion diseases, have been shown to be autophagy substrates (19Fornai F. Longone P. Cafaro L. Kastsiuchenka O. Ferrucci M. Manca M.L. Lazzeri G. Spalloni A. Bellio N. Lenzi P. Modugno N. Siciliano G. Isidoro C. Murri L. Ruggieri S. Paparelli A. Proc. Natl. Acad. Sci. U.S.A. 2008; 105: 2052-2057Crossref PubMed Scopus (482) Google Scholar, 20Berger Z. Ravikumar B. Menzies F.M. Oroz L.G. Underwood B.R. Pangalos M.N. Schmitt I. Wullner U. Evert B.O. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2006; 15: 433-442Crossref PubMed Scopus (562) Google Scholar, 21Aguib Y. Heiseke A. Gilch S. Riemer C. Baier M. Schätzl H.M. Ertmer A. Autophagy. 2009; 5: 361-369Crossref PubMed Scopus (187) Google Scholar, 22Heiseke A. Aguib Y. Riemer C. Baier M. Schätzl H.M. J. Neurochem. 2009; 109: 25-34Crossref PubMed Scopus (168) Google Scholar). Although most of the wild-type counterparts of these mutant proteins are poor autophagy substrates, wild-type α-synuclein has been shown to be degraded by chaperone-mediated autophagy, a distinct lysosome pathway (23Cuervo A.M. Stefanis L. Fredenburg R. Lansbury P.T. Sulzer D. Science. 2004; 305: 1292-1295Crossref PubMed Scopus (1572) Google Scholar). Thus, up-regulating autophagy may be beneficial for the treatment of neurodegenerative diseases, and identification of autophagy enhancers could provide potential therapeutic candidates (Table 1) (24Sarkar S. Ravikumar B. Floto R.A. Rubinsztein D.C. Cell Death Differ. 2009; 16: 46-56Crossref PubMed Scopus (435) Google Scholar, 25Sarkar S. Ravikumar B. Rubinsztein D.C. Methods Enzymol. 2009; 453: 83-110Crossref PubMed Scopus (81) Google Scholar).TABLE 1List of autophagy enhancers for neurodegenerative diseases and their mode of actionAutophagy enhancersMode of actionRefs.Rapamycin, CCI-779, Glc, Glc-6-P, Torin1, perhexiline, niclosamide, rottlerinInhibit mTORC18Ravikumar B. Duden R. Rubinsztein D.C. Hum. Mol. Genet. 2002; 11: 1107-1117Crossref PubMed Scopus (938) Google Scholar, 20Berger Z. Ravikumar B. Menzies F.M. Oroz L.G. Underwood B.R. Pangalos M.N. Schmitt I. Wullner U. Evert B.O. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2006; 15: 433-442Crossref PubMed Scopus (562) Google Scholar, 30Thoreen C.C. Kang S.A. Chang J.W. Liu Q. Zhang J. Gao Y. Reichling L.J. Sim T. Sabatini D.M. Gray N.S. J. Biol. Chem. 2009; 284: 8023-8032Abstract Full Text Full Text PDF PubMed Scopus (1405) Google Scholar, 31Balgi A.D. Fonseca B.D. Donohue E. Tsang T.C. Lajoie P. Proud C.G. Nabi I.R. Roberge M. PloS One. 2009; 4: e7124Crossref PubMed Scopus (289) Google Scholar, 37Ravikumar B. Vacher C. Berger Z. Davies J.E. Luo S. Oroz L.G. Scaravilli F. Easton D.F. Duden R. O'Kane C.J. Rubinsztein D.C. Nat. Genet. 2004; 36: 585-595Crossref PubMed Scopus (1990) Google Scholar, 39Ravikumar B. Stewart A. Kita H. Kato K. Duden R. Rubinsztein D.C. Hum. Mol. Genet. 2003; 12: 985-994Crossref PubMed Scopus (108) Google ScholarLithium, L-690,330Inhibit IMPase and reduce inositol and IP3 levels; mTOR-independent41Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. Cook L.J. Rubinsztein D.C. J. Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (815) Google Scholar, 47Sarkar S. Krishna G. Imarisio S. Saiki S. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2008; 17: 170-178Crossref PubMed Scopus (299) Google ScholarCarbamazepine, sodium valproateReduce inositol and IP3 levels; mTOR-independent41Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. Cook L.J. Rubinsztein D.C. J. Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (815) Google Scholar, 48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google ScholarVerapamil, loperamide, amiodarone, nimodipine, nitrendipine, niguldipine, pimozideCa2+ channel blockers; reduce intracytosolic Ca2+ levels; mTOR-independent48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google Scholar, 54Zhang L. Yu J. Pan H. Hu P. Hao Y. Cai W. Zhu H. Yu A.D. Xie X. Ma D. Yuan J. Proc. Natl. Acad. Sci. U.S.A. 2007; 104: 19023-19028Crossref PubMed Scopus (399) Google ScholarCalpastatin, calpeptinCalpain inhibitors; mTOR-independent48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google ScholarClonidine, rilmenidineImidazoline-1 receptor agonists; reduce cAMP levels; mTOR-independent48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google Scholar2′,5′-DideoxyadenosineAdenylyl cyclase inhibitor; reduces cAMP levels; mTOR-independent48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google ScholarNF449Gαs inhibitor; mTOR-independent48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google ScholarMinoxidilK+ATP channel opener; mTOR-independent48Williams A. Sarkar S. Cuddon P. Ttofi E.K. Saiki S. Siddiqi F.H. Jahreiss L. Fleming A. Pask D. Goldsmith P. O'Kane C.J. Floto R.A. Rubinsztein D.C. Nat. Chem. Biol. 2008; 4: 295-305Crossref PubMed Scopus (665) Google ScholarPenitrem AInhibits high conductance Ca2+-activated K+ channel; mTOR-independent54Zhang L. Yu J. Pan H. Hu P. Hao Y. Cai W. Zhu H. Yu A.D. Xie X. Ma D. Yuan J. Proc. Natl. Acad. Sci. U.S.A. 2007; 104: 19023-19028Crossref PubMed Scopus (399) Google ScholarFluspirilene, trifluoperazineDopamine antagonists; mTOR-independent54Zhang L. Yu J. Pan H. Hu P. Hao Y. Cai W. Zhu H. Yu A.D. Xie X. Ma D. Yuan J. Proc. Natl. Acad. Sci. U.S.A. 2007; 104: 19023-19028Crossref PubMed Scopus (399) Google ScholarTrehaloseUnknown; mTOR-independent60Sarkar S. Davies J.E. Huang Z. Tunnacliffe A. Rubinsztein D.C. J. Biol. Chem. 2007; 282: 5641-5652Abstract Full Text Full Text PDF PubMed Scopus (905) Google ScholarSMER10, SMER18, SMER28, SMER analogsUnknown; mTOR-independent50Sarkar S. Perlstein E.O. Imarisio S. Pineau S. Cordenier A. Maglathlin R.L. Webster J.A. Lewis T.A. O'Kane C.J. Schreiber S.L. Rubinsztein D.C. Nat. Chem. Biol. 2007; 3: 331-338Crossref PubMed Scopus (508) Google Scholar Open table in a new tab At the time we did the first of these studies, the only drug that was known to induce autophagy that was in clinical use (for other indications) was rapamycin. Rapamycin is a highly specific inhibitor of mTOR. The mTOR pathway, which is essential for controlling cell growth, protein synthesis, ribosome biogenesis, nutrient metabolism, and autophagy, involves two functional complexes called mTORC1 and mTORC2 (26Guertin D.A. Sabatini D.M. Sci. Signal. 2009; 2: pe24Crossref PubMed Scopus (485) Google Scholar). In mammalian cells, rapamycin forms a complex with the immunophilin FKBP12 (FK506-binding protein of 12 kDa), which binds to mTORC1 and inhibits its activity (Fig. 2) (27Kim D.H. Sarbassov D.D. Ali S.M. King J.E. Latek R.R. Erdjument-Bromage H. Tempst P. Sabatini D.M. Cell. 2002; 110: 163-175Abstract Full Text Full Text PDF PubMed Scopus (2371) Google Scholar). However, recent studies have shown that prolonged treatment with rapamycin can inhibit mTORC2 activity in certain mammalian cell types (28Sarbassov D.D. Ali S.M. Sengupta S. Sheen J.H. Hsu P.P. Bagley A.F. Markhard A.L. Sabatini D.M. Mol. Cell. 2006; 22: 159-168Abstract Full Text Full Text PDF PubMed Scopus (2192) Google Scholar, 29Zeng Z. Sarbassov D.D. Samudio I.J. Yee K.W. Munsell M.F. Jackson C.E. Giles F.J. Sabatini D.M. Andreeff M. Konopleva M. Blood. 2007; 109: 3509-3512Crossref PubMed Scopus (305) Google Scholar). Recently, a selective ATP-competitive small molecule mTOR inhibitor called Torin1 has been found to induce autophagy to a much greater extent than rapamycin (30Thoreen C.C. Kang S.A. Chang J.W. Liu Q. Zhang J. Gao Y. Reichling L.J. Sim T. Sabatini D.M. Gray N.S. J. Biol. Chem. 2009; 284: 8023-8032Abstract Full Text Full Text PDF PubMed Scopus (1405) Google Scholar). Recently, Roberge and co-workers (31Balgi A.D. Fonseca B.D. Donohue E. Tsang T.C. Lajoie P. Proud C.G. Nabi I.R. Roberge M. PloS One. 2009; 4: e7124Crossref PubMed Scopus (289) Google Scholar) reported a study in which they screened a library of 3500 chemicals with an automated cell-based assay to detect increases in autophagosome numbers. The screen identified four compounds (perhexiline, niclosamide, amiodarone, and rottlerin) that stimulated autophagy by inhibiting mTORC1 (but not mTORC2) signaling (Fig. 2). Rottlerin inhibited mTORC1 signaling via TSC2 (tuberous sclerosis complex 2), whereas the other drugs inhibited mTORC1 signaling in a TSC2-independent manner (31Balgi A.D. Fonseca B.D. Donohue E. Tsang T.C. Lajoie P. Proud C.G. Nabi I.R. Roberge M. PloS One. 2009; 4: e7124Crossref PubMed Scopus (289) Google Scholar). Interestingly, three of the identified compounds (amiodarone, perhexiline, and niclosamide) are drugs already approved for other therapeutic indications, thereby reinforcing the rationale for targeting mTORC1 activity in diseases in which positive modulation of autophagy may be beneficial. Recent studies have identified some of the molecular components in mammalian autophagy downstream of mTORC1. Rapamycin appears to regulate mammalian autophagy by inhibiting the mTOR-mediated phosphorylation of Atg13 and ULK1, which are involved in autophagosome formation. This leads to dephosphorylation-dependent activation of ULK1 (and ULK2) and ULK1-mediated phosphorylation of Atg13, FIP200, and ULK1 itself, which triggers autophagy (Fig. 2). Thus, the ULK1-Atg13-FIP200 complex appears to integrate the autophagy signals downstream of mTORC1 (32Ganley I.G. Lam D.H. Wang J. Ding X. Chen S. Jiang X. J. Biol. Chem. 2009; 284: 12297-12305Abstract Full Text Full Text PDF PubMed Scopus (1075) Google Scholar, 33Hosokawa N. Hara T. Kaizuka T. Kishi C. Takamura A. Miura Y. Iemura S. Natsume T. Takehana K. Yamada N. Guan J.L. Oshiro N. Mizushima N. Mol. Biol. Cell. 2009; 20: 1981-1991Crossref PubMed Scopus (1493) Google Scholar, 34Jung C.H. Jun C.B. Ro S.H. Kim Y.M. Otto N.M. Cao J. Kundu M. Kim D.H. Mol. Biol. Cell. 2009; 20: 1992-2003Crossref PubMed Scopus (1502) Google Scholar). However, it is not yet clear how phosphorylation of these proteins regulates their activities. Subsequent studies have provided robust support for our assertions using genetic and chemical approaches and suggest that autophagy is important for clearance of mutant huntingtin fragments, at least as large as the first one-third of the protein as well as full-length mutant huntingtin, and that wild-type forms are far less dependent on autophagy for their clearance compared with the mutant forms (17Shibata M. Lu T. Furuya T. Degterev A. Mizushima N. Yoshimori T. MacDonald M. Yankner B. Yuan J. J. Biol. Chem. 2006; 281: 14474-14485Abstract Full Text Full Text PDF PubMed Scopus (381) Google Scholar, 18Iwata A. Christianson J.C. Bucci M. Ellerby L.M. Nukina N. Forno L.S. Kopito R.R. Proc. Natl. Acad. Sci. U.S.A. 2005; 102: 13135-13140Crossref PubMed Scopus (273) Google Scholar, 24Sarkar S. Ravikumar B. Floto R.A. Rubinsztein D.C. Cell Death Differ. 2009; 16: 46-56Crossref PubMed Scopus (435) Google Scholar, 35Qin Z.H. Wang Y. Kegel K.B. Kazantsev A. Apostol B.L. Thompson L.M. Yoder J. Aronin N. DiFiglia M. Hum. Mol. Genet. 2003; 12: 3231-3244Crossref PubMed Scopus (234) Google Scholar, 36Rubinsztein D.C. Gestwicki J.E. Murphy L.O. Klionsky D.J. Nat. Rev. Drug Discov. 2007; 6: 304-312Crossref PubMed Scopus (890) Google Scholar). We showed that rapamycin attenuated mutant huntingtin fragment toxicity in cells and in transgenic Drosophila and mouse models of HD (37Ravikumar B. Vacher C. Berger Z. Davies J.E. Luo S. Oroz L.G. Scaravilli F. Easton D.F. Duden R. O'Kane C.J. Rubinsztein D.C. Nat. Genet. 2004; 36: 585-595Crossref PubMed Scopus (1990) Google Scholar). The protective effects of rapamycin were blocked in flies expressing mutant polyalanines or expanded polyQ when the expression of different autophagy genes was reduced (20Berger Z. Ravikumar B. Menzies F.M. Oroz L.G. Underwood B.R. Pangalos M.N. Schmitt I. Wullner U. Evert B.O. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2006; 15: 433-442Crossref PubMed Scopus (562) Google Scholar, 38Pandey U.B. Nie Z. Batlevi Y. McCray B.A. Ritson G.P. Nedelsky N.B. Schwartz S.L. DiProspero N.A. Knight M.A. Schuldiner O. Padmanabhan R. Hild M. Berry D.L. Garza D. Hubbert C.C. Yao T.P. Baehrecke E.H. Taylor J.P. Nature. 2007; 447: 859-863Crossref PubMed Scopus (999) Google Scholar), suggesting that the major benefits of this drug are autophagy-dependent and not mediated by alternative mechanisms such as impaired translation (at least in these in vivo settings). Our data in cell and fly models show that rapamycin-mediated autophagy up-regulation may be valuable for many other intracellular proteinopathies, including SCA3, and both mutant and wild-type Tau (20Berger Z. Ravikumar B. Menzies F.M. Oroz L.G. Underwood B.R. Pangalos M.N. Schmitt I. Wullner U. Evert B.O. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2006; 15: 433-442Crossref PubMed Scopus (562) Google Scholar). Tau was of particular interest, as it is mutated in certain frontotemporal dementias, and wild-type Tau is the major component of the neurofibrillary tangles that are believed to contribute to pathology in sporadic Alzheimer disease (20Berger Z. Ravikumar B. Menzies F.M. Oroz L.G. Underwood B.R. Pangalos M.N. Schmitt I. Wullner U. Evert B.O. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2006; 15: 433-442Crossref PubMed Scopus (562) Google Scholar). Furthermore, elevated intracellular glucose or glucose 6-phosphate also induces autophagy by inhibiting mTOR (Fig. 2) (39Ravikumar B. Stewart A. Kita H. Kato K. Duden R. Rubinsztein D.C. Hum. Mol. Genet. 2003; 12: 985-994Crossref PubMed Scopus (108) Google Scholar). An additional benefit of autophagy up-regulation in these diseases is that it appears to protect cells against apoptotic insults (40Ravikumar B. Berger Z. Vacher C. O'Kane C.J. Rubinsztein D.C. Hum. Mol. Genet. 2006; 15: 1209-1216Crossref PubMed Scopus (353) Google Scholar). Thus, enhancing autophagy may have two beneficial effects in the context of neurodegenerative diseases. First, it enhances removal of the toxic aggregate-prone protein, and second, it protects cells from apoptosis. The autosomal-dominant proteinopathies that are potentially amenable to autophagy up-regulation present an important opportunity for delaying the onset of disease. Most patients will have a positive family history, and thus, it is possible to identify most cases at risk of developing disease with a simple genetic test (4Imarisio S. Carmichael J. Korolchuk V. Chen C.W. Saiki S. Rose C. Krishna G. Davies J.E. Ttofi E. Underwood B.R. Rubinsztein D.C. Biochem. J. 2008; 412: 191-209Crossref PubMed Scopus (327) Google Scholar). Ideally, one would like to start treatment at the earliest possible age in such individuals to aim to delay the onset of disease. For instance, in HD, one would aim to delay onset from a median age of 40 until after normal life expectancy and thus effectively prevent the disease. One issue that remains unresolved is whether long-term autophagy up-regulation may have deleterious effects. It is important to point out that our mouse studies involved rapamycin administration regimes that were pulsatile (37Ravikumar B. Vacher C. Berger Z. Davies J.E. Luo S. Oroz L.G. Scaravilli F. Easton D.F. Duden R. O'Kane C.J. Rubinsztein D.C. Nat. Genet. 2004; 36: 585-595Crossref PubMed Scopus (1990) Google Scholar), and thus, it is very unlikely that autophagy was induced all the time; rather, autophagy would have been induced between periods of normal autophagy. Rapamycin is a drug designed for long-term use, and although it has some side effects in patients and mice due to mTOR inhibition, these do not appear to be mediated by autophagy. Although one may argue that the side effect profile of rapamycin is outweighed by its potential benefits in many of these devastating diseases, it would be desirable to identify compounds that are better tolerated because one may need to treat patients who are at risk for developing these diseases for decades. Hence, we have tried to identify drugs that act independently of mTOR (Table 1). The first hint that there may be mTOR-independent pathways controlling autophagy was the discovery that intracellular IP3 levels negatively regulate autophagy (41Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. Cook L.J. Rubinsztein D.C. J. Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (815) Google Scholar). We have shown that autophagy can be induced by lowering intracellular inositol or IP3 levels independently of mTOR. Lithium and other mood-stabilizing agents used for treatment of bipolar disorder, such as carbamazepine and sodium valproate, enhances the clearance of autophagy substrates by reducing intracellular inositol levels (Fig. 3) (41Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. Cook L.J. Rubinsztein D.C. J. Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (815) Google Scholar, 42Williams R.S. Cheng L. Mudge A.W. Harwood A.J. Nature. 2002; 417: 292-295Crossref PubMed Scopus (549) Google Scholar). The ability of lithium to induce autophagy is due to inhibition of IMPase, which prevents inositol recycling, leading to depletion of cellular inositol and inhibition of the phosphoinositol cycle (41Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. Cook L.J. Rubinsztein D.C. J. Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (815) Google Scholar, 43Berridge M.J. Nature. 1993; 361: 315-325Crossref PubMed Scopus (6174) Google Scholar). Accordingly, the specific IMPase inhibitor L-690,330 mimics the effects of lithium on the clearance of autophagy substrates. Sodium valproate induces autophagy by inhibiting inositol synthesis and decreasing IP3 levels (41Sarkar S. Floto R.A. Berger Z. Imarisio S. Cordenier A. Pasco M. Cook L.J. Rubinsztein D.C. J. Cell Biol. 2005; 170: 1101-1111Crossref PubMed Scopus (815) Google Scholar, 44Shaltiel G. Shamir A. Shapiro J. Ding D. Dalton E. Bialer M. Harwood A.J. Belmaker R.H. Greenberg M.L. Agam G. Biol. Psychiatry. 2004; 56: 868-874Abstract Full Text Full Text PDF PubMed Scopus (99) Google Scholar). Consistent with a role of IP3 in autophagy, pharmacological inhibition of the IP3R by xestospongin B also induces autophagy (45Criollo A. Maiuri M.C. Tasdemir E. Vitale I. Fiebig A.A. Andrews D. Molgó J. Díaz J. Lavandero S. Harper F. Pierron G. di Stefano D. Rizzuto R. Szabadkai G. Kroemer G. Cell Death Differ. 2007; 14: 1029-1039Crossref PubMed Scopus (269) Google Scholar). It was further shown that xestospongin B induces autophagy by disrupting the IP3R-beclin 1 complex, which can also be modulated
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