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Linking the VPS35 and EIF4G1 Pathways in Parkinson’s Disease

2015; Cell Press; Volume: 85; Issue: 1 Linguagem: Inglês

10.1016/j.neuron.2014.12.045

1097-4199

Owen A. Ross, Casey Cook, Leonard Petrucelli,

Neurological diseases and metabolism

Elucidating the underlying pathogenic pathways in Parkinson's disease will be critical for targeted drug development. In this issue of Neuron, Dhungel et al., 2015Dhungel N. Eleutri S. Li L. Kramer N. Chartron J. Spencer B. Kosberg K. Fields J. Klodjan S. Adame A. et al.Neuron. 2015; 85 (this issue): 76-87Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar utilize a yeast model to establish a link between VPS35 and EIF4G1 in α-synuclein-related neurodegeneration. Elucidating the underlying pathogenic pathways in Parkinson's disease will be critical for targeted drug development. In this issue of Neuron, Dhungel et al., 2015Dhungel N. Eleutri S. Li L. Kramer N. Chartron J. Spencer B. Kosberg K. Fields J. Klodjan S. Adame A. et al.Neuron. 2015; 85 (this issue): 76-87Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar utilize a yeast model to establish a link between VPS35 and EIF4G1 in α-synuclein-related neurodegeneration. Parkinson's disease (PD) is the most common age-related neurodegenerative movement disorder. It is defined by the asymmetric onset of motor symptoms, including bradykinesia, rigidity, resting tremor, and postural instability (Lees et al., 2009Lees A.J. Hardy J. Revesz T. Lancet. 2009; 373: 2055-2066Abstract Full Text Full Text PDF PubMed Scopus (1607) Google Scholar). Given the prominence of motor symptoms, PD is typically considered a movement disorder, though a number of nonmotor features (including sleep, gastrointestinal, and cognitive disturbances) are also common and often predate the motor dysfunction (Lees et al., 2009Lees A.J. Hardy J. Revesz T. Lancet. 2009; 373: 2055-2066Abstract Full Text Full Text PDF PubMed Scopus (1607) Google Scholar). In addition, a positive response to dopamine replacement therapy supports a clinical diagnosis of probable PD. A definitive determination of PD requires the presence of α-synuclein-positive Lewy-related pathology (Dickson, 2012Dickson D.W. Cold Spring Harb Perspect Med. 2012; 2Crossref PubMed Scopus (473) Google Scholar). Although predominantly a sporadic disorder in nature, significant disease insights have been gleaned from gene discovery studies on rare multi-incident families. To date, mutations in six key genes have been shown to cause what we consider familial forms of PD (SNCA, LRRK2, and VPS35 in dominantly inherited patterns and PARKIN, PINK1, and DJ1 in recessively inherited patterns) (Trinh and Farrer, 2013Trinh J. Farrer M. Nat Rev Neurol. 2013; 9: 445-454Crossref PubMed Scopus (344) Google Scholar). These genes cause disease in multiple unrelated families, and pathogenicity for specific mutations is well established. A number of other genes have been implicated in single or small families with inherited PD (e.g., EIF4G1 and DNAJC13) or other forms of parkinsonism (e.g., ATP13A2 and PLA2G6) (Trinh and Farrer, 2013Trinh J. Farrer M. Nat Rev Neurol. 2013; 9: 445-454Crossref PubMed Scopus (344) Google Scholar). Furthermore, large population-based genome-wide association studies have nominated 24 independent genomic loci that influence the individual susceptibility to PD, including both SNCA and LRRK2 (Nalls et al., 2014Nalls M.A. Pankratz N. Lill C.M. Do C.B. Hernandez D.G. Saad M. DeStefano A.L. Kara E. Bras J. Sharma M. et al.International Parkinson's Disease Genomics Consortium (IPDGC)Parkinson's Study Group (PSG) Parkinson's Research: The Organized GENetics Initiative (PROGENI)23andMeGenePDNeuroGenetics Research Consortium (NGRC)Hussman Institute of Human Genomics (HIHG)Ashkenazi Jewish Dataset InvestigatorCohorts for Health and Aging Research in Genetic Epidemiology (CHARGE)North American Brain Expression Consortium (NABEC)United Kingdom Brain Expression Consortium (UKBEC)Greek Parkinson's Disease ConsortiumAlzheimer Genetic Analysis GroupNat. Genet. 2014; 46: 989-993Crossref PubMed Scopus (1266) Google Scholar). However, despite the numerous genetic insights into disease etiology, current understanding of the basic mechanisms and pathways that contribute to the pathogenesis of PD remains limited. In this issue of Neuron, Dhungel et al., 2015Dhungel N. Eleutri S. Li L. Kramer N. Chartron J. Spencer B. Kosberg K. Fields J. Klodjan S. Adame A. et al.Neuron. 2015; 85 (this issue): 76-87Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar employ a yeast-based model system to provide functional insight into the human PD-related genes VPS35 and EIF4G1, which are known to function in regulating vacuolar transport (VPS35) and transcription/translation mechanisms (EIF4G1). For both genes, the mutations identified in the original families are missense, resulting in amino acid substitutions (VPS35 p.D620N and EIF4G1 p.R1205H) that could result in gain, loss, or toxic gain of function mechanisms. Therefore, in order to determine the consequence of these mutations, a greater understanding of the normal functions and molecular pathways that involve VPS35 and EIF4G1 is imperative. One way to gain insight into a gene's function is to identify interacting genes. The authors begin by performing two independent genome-wide screens in yeast for genes that interact with the yeast homologs of EIF4G1 (called TIF4631) and VPS35. Surprisingly, while the investigators identify a key list of genes with synthetic lethal effects from these two independent analyses, a handful of genes that were synthetic lethal with both yeast Vps35 and Tif4631 (EIF4G1) deletions were also discovered. Given the known functions of VPS35 and EIF4G1, synthetic lethal genes involved in vacuolar transport (VPS35) and transcription/translation mechanisms (EIF4G1) were predicted, while overlapping pathways unexpectedly included protein targeting and endosomal transport. Despite uncovering common pathways that include both VPS35 and EIF4G1, simultaneous deletion of VPS35 and EIF4G1 did not cause synthetic lethality, prompting the authors to evaluate alternate modes of genetic interactions. Intriguingly, overexpression of EIF4G1 (or yeast homolog tif4631) in yeast lacking VPS35 was highly toxic, while restoring wild-type, but not mutant (p.D620N), VPS35 could rescue this phenotype. Given that VPS35 is a known component of the retromer complex (which regulates recycling, sorting, and trafficking between the endosomal and Golgi network), the authors predicted that EIF4G1 toxicity under conditions of VPS35 deficiency may be due to loss of retromer function. In support of this hypothesis, deficiency of other retromer components (e.g., VPS26) similarly enhanced EIF4G1-related toxicity. The authors then showed that the toxicity was due to proteotoxic stress, as concomitant EIF4G1 overexpression and VPS35 deficiency resulted in focal accumulation of Hsp104-GFP (a reporter of protein misfolding/aggregation), as well as an increase in endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). These findings suggest that a functional retromer complex plays an essential role in combatting and ameliorating the abnormal accumulation and misfolding of proteins instigated by an upregulation in EIF4G1 expression. In addition, overexpression of the yeast sortilin homolog Vps10 (a transmembrane protein that regulates neuronal viability and neurotrophic signaling) was also protective in the EIF4G1 upregulation-VPS35 deletion system, which could indicate that VPS35 alleviates EIF4G1 toxicity through VPS10. As members of the human VPS10 family (including sortilin, SorL1, and SorCS1/2/3) have also been implicated in neurodegeneration, these observations may pinpoint a general process by which protein accumulation/aggregation disorders could be targeted therapeutically. Given the central role of α-synuclein in PD pathophysiology, building connections between the other PD-associated genes should eventually lead back to the universal pathognomonic Lewy-related pathology. Gitler and colleagues had previously used a similar approach to investigate toxicity related to α-synuclein overexpression in yeast (Gitler et al., 2009Gitler A.D. Chesi A. Geddie M.L. Strathearn K.E. Hamamichi S. Hill K.J. Caldwell K.A. Caldwell G.A. Cooper A.A. Rochet J.C. Lindquist S. Nat. Genet. 2009; 41: 308-315Crossref PubMed Scopus (448) Google Scholar, Yeger-Lotem et al., 2009Yeger-Lotem E. Riva L. Su L.J. Gitler A.D. Cashikar A.G. King O.D. Auluck P.K. Geddie M.L. Valastyan J.S. Karger D.R. et al.Nat. Genet. 2009; 41: 316-323Crossref PubMed Scopus (234) Google Scholar). In the present study, the authors show that loss of EIF4G1 does not impact α-synuclein toxicity, yet EIF4G1 overexpression (toxic in the VPS35 deletion model) suppressed α-synuclein-related toxicity in their earlier work. Conversely, Dhungel et al., 2015Dhungel N. Eleutri S. Li L. Kramer N. Chartron J. Spencer B. Kosberg K. Fields J. Klodjan S. Adame A. et al.Neuron. 2015; 85 (this issue): 76-87Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar demonstrated that VPS35 deletion enhanced α-synuclein-related toxicity and increased inclusion formation. In an established transgenic α-synuclein murine model, either knockdown or expression of mutant VPS35 increased neuronal loss, while overexpression of wild-type VPS35 was neuroprotective and lowered the accumulation of α-synuclein. Overexpression of wild-type VPS35 also appeared to lower or block the uptake of extracellular recombinant α-synuclein fibrils in a seeding model. The hypothesis that α-synuclein pathology can propagate or spread to neighboring cells in PD has recently gained considerable interest (Guo and Lee, 2014Guo J.L. Lee V.M. Nat. Med. 2014; 20: 130-138Crossref PubMed Scopus (432) Google Scholar), and the current study may implicate a role for the retromer complex in this process. The results of this study provide the field with important mechanistic insights into the possible pathogenic processes associated with PD due to mutations in VPS35 or EIF4G1. Dysfunction of the noted pathways of protein translation, transport, and recycling/clearance is believed to be fundamental in protein aggregation disorders in general. Deficiency of the retromer complex has also been implicated in Alzheimer's disease, so why this specific VPS35 mutant (p.D620N) results in a typical PD phenotype remains to be resolved. In addition, the results highlight a list of genes that exacerbate toxicity under conditions of either VPS35 or EIF4G1 loss of function, providing additional insight into individual and shared pathways linked to these two PD-associated genes. In future studies, it will be interesting to determine whether the pathways linked to VPS35 and EIF4G1 are also associated with other PD genes. As the most frequent and clinically relevant genetic mutations and risk factors in PD are in the LRRK2 gene (Ross et al., 2011Ross O.A. Soto-Ortolaza A.I. Heckman M.G. Aasly J.O. Abahuni N. Annesi G. Bacon J.A. Bardien S. Bozi M. Brice A. et al.Genetic Epidemiology Of Parkinson's Disease (GEO-PD) ConsortiumLancet Neurol. 2011; 10: 898-908Abstract Full Text Full Text PDF PubMed Scopus (241) Google Scholar), the fact that either VPS35 or VPS26 overexpression can rescue locomotor and lifespan deficits in a mutant LRRK2 Drosophila model (Linhart et al., 2014Linhart R. Wong S.A. Cao J. Tran M. Huynh A. Ardrey C. Park J.M. Hsu C. Taha S. Peterson R. et al.Mol. Neurodegener. 2014; 9: 23Crossref PubMed Scopus (72) Google Scholar) may implicate the involvement of a common pathogenic pathway(s) in PD. The complexity and systemic nature of PD means it is likely that dysfunction can occur directly at the specific protein level or at a higher level of cellular processes. The current lack of progress in developing therapeutics to either slow/halt the progression or even prevent clinical onset has been disappointing, though the mere identification of the primary gene targets may not be sufficient to accomplish this endeavor. In the future, it will likely be critical to characterize the dysfunctional pathway(s), which may also identify upstream or downstream targets that could be more amenable to intervention, as well as provide insight to facilitate biomarker development. Considering that the original pathogenic substitution (p.R1205H) is being detected in an increasing number of apparently healthy subjects, the pathogenicity of EIF4G1 mutations in PD is still under investigation. The work of Dhungel et al., 2015Dhungel N. Eleutri S. Li L. Kramer N. Chartron J. Spencer B. Kosberg K. Fields J. Klodjan S. Adame A. et al.Neuron. 2015; 85 (this issue): 76-87Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar provides an innovative new approach to identify potential genetic factors, as well as disease-modifying candidates in PD. Importantly, the authors do show similar effects across different model systems, from yeast to worm to mouse. A similar approach identified ataxin 2 as a strong modifier of TDP-43 toxicity in yeast, and the gene was subsequently established as a major risk factor for ALS in humans (Elden et al., 2010Elden A.C. Kim H.J. Hart M.P. Chen-Plotkin A.S. Johnson B.S. Fang X. Armakola M. Geser F. Greene R. Lu M.M. et al.Nature. 2010; 466: 1069-1075Crossref PubMed Scopus (876) Google Scholar). Given the lack of large families available with Mendelian forms of PD without a known genetic cause, it is becoming increasingly important to generate novel approaches to identify candidate genes for PD. Next-generation sequencing methods have now revolutionized the way we view gene discovery studies. Identification of the pathogenic VPS35 mutation via exome sequencing on multi-incident families was the first example of this approach in PD (Vilariño-Güell et al., 2011Vilariño-Güell C. Wider C. Ross O.A. Dachsel J.C. Kachergus J.M. Lincoln S.J. Soto-Ortolaza A.I. Cobb S.A. Wilhoite G.J. Bacon J.A. et al.Am. J. Hum. Genet. 2011; 89: 162-167Abstract Full Text Full Text PDF PubMed Scopus (622) Google Scholar, Zimprich et al., 2011Zimprich A. Benet-Pagès A. Struhal W. Graf E. Eck S.H. Offman M.N. Haubenberger D. Spielberger S. Schulte E.C. Lichtner P. et al.Am. J. Hum. Genet. 2011; 89: 168-175Abstract Full Text Full Text PDF PubMed Scopus (623) Google Scholar). Despite this early success, the advances in next-generation sequencing technology have not led to the avalanche of PD-linked genes as hoped, with only a small number of additional rare variants being identified and proposed to be pathogenic (e.g., DNAJC13) (Vilariño-Güell et al., 2014Vilariño-Güell C. Rajput A. Milnerwood A.J. Shah B. Szu-Tu C. Trinh J. Yu I. Encarnacion M. Munsie L.N. Tapia L. et al.Hum. Mol. Genet. 2014; 23: 1794-1801Crossref PubMed Scopus (215) Google Scholar). However, as next-generation sequencing becomes more commonplace in genetics, we will likely witness an exponential growth in the abundance of rare or unique genetic variants of unknown clinical significance. Gene burden testing (i.e., the total number of rare mutations in a series of patients compared to controls) is a population-based genetic approach to confirm the role of candidate genes in disease. Although gene burden tests may pinpoint a few key genes that harbor rare disease-relevant variants, determining pathogenicity of the individual variants may not be possible from a genetics perspective. It will be crucial to develop functional readouts into which each rare/unique variant can be placed and functional pathogenicity determined. Thus, the results of the present study provide a good example of how this could work; the demonstration that either VPS35 knockdown or mutant VPS35 influences α-synuclein accumulation or fibril uptake provides a platform for the testing of other rare VPS35 mutants detected in patients. In summary, the present work of Dhungel et al., 2015Dhungel N. Eleutri S. Li L. Kramer N. Chartron J. Spencer B. Kosberg K. Fields J. Klodjan S. Adame A. et al.Neuron. 2015; 85 (this issue): 76-87Abstract Full Text Full Text PDF PubMed Scopus (127) Google Scholar provides a foundation for future genetic and functional studies involving EIF4G1, VPS35, and α-synuclein in PD. Further genetic studies may help to determine whether EIF4G1 mutations are pathogenic and clarify the disease mechanisms associated with specific mutations. However, it is likely that unbiased screens, such as those presented herein, and hypothesis-driven functional studies will be necessary to resolve the pathways involved and drive the generation of novel strategies for therapeutic intervention. Parkinson's Disease Genes VPS35 and EIF4G1 Interact Genetically and Converge on α-SynucleinDhungel et al.NeuronDecember 18, 2014In BriefParkinson's disease is associated with diverse genetic and environmental susceptibilities. Dhungel et al. use a combination of approaches and model systems, from yeast to worms to transgenic mice, to discover functional interactions between Parkinson's disease genes, EIF4G1, VPS35, and α-synuclein. Full-Text PDF Open Archive