Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron death. A hallmark of the disease is the appearance of protein aggregates in the affected motor neurons. We have found that p62, a protein implicated in protein aggregate formation, accumulated progressively in the G93A mouse spinal cord. The accumulation of p62 was in parallel to the increase of polyubiquitinated proteins and mutant SOD1 aggregates. Immunostaining studies showed that p62, ubiquitin, and mutant SOD1 co-localized in the protein aggregates in affected cells in G93A mouse spinal cord. The p62 protein selectively interacted with familial ALS mutants, but not WT SOD1. When p62 was co-expressed with SOD1 in NSC34 cells, it greatly enhanced the formation of aggregates of the ALS-linked SOD1 mutants, but not wild-type SOD1. Cell viability was measured in the presence and absence of overexpressed p62, and the results suggest that the large aggregates facilitated by p62 were not directly toxic to cells under the conditions in this study. Deletion of the ubiquitin-association (UBA) domain of p62 significantly decreased the p62-facilitated aggregate formation, but did not completely inhibit it. Further protein interaction experiments also showed that the truncated p62 with the UBA domain deletion remained capable of interacting with mutant SOD1. The findings of this study show that p62 plays a critical role in forming protein aggregates in familial ALS, likely by linking misfolded mutant SOD1 molecules and other cellular proteins together. Amyotrophic lateral sclerosis (ALS)3 is a progressive neurodegenerative disorder leading to the selective death of motor neurons (1, 2). The majority of cases are sporadic, but about 10% of all cases are familial (fALS). In ϳ20% of familial cases, a mutant allele of the copper-zinc superoxide dismutase (SOD1) enzyme has been identified (3-5). As of today, over 100 different mutations of SOD1 causing ALS have been reported, the majority of which are point mutations and act in a dominant fashion. Several ALS risk factors have been identified, but the etiology of the disease is largely unclear. A hallmark of the disease is the appearance of intracellular inclusions in degenerating motor neurons (6), both in the familial cases caused by mutations in SOD1 and in the more obscure sporadic cases (6 -11). The formation of such protein aggregates precedes neuronal death (12). The inclusions are typically SOD1 and ubiquitin immunoreactive in the mutant SOD1-mediated fALS cases (2, 7-9, 11-13). However, it is unclear how the SOD1 mutants form aggregates and whether other proteins play any role in the aggregation process or the aggregate-induced toxicity.p62 (also called sequestosome 1) was first identified as a phosphotyrosine-independent ligand for the Lck SH2 domain (14). It was later found to be a polyubiquitin-binding protein (15)(16)(17). The expression of p62 is up-regulated by several stress conditions, e.g. oxidative stress (18,19), proteasome inhibition (19 -21), or p...
, tetrameric red fluorescent protein; FL, full-length; GFP, green fluorescent protein; HA, hemagglutinin; HEK293, human embryonic kidney cells; KO, knockout; LC3, microtubule-associated protein 1 light chain 3B; LIR, LC3 interaction region; MEF, mouse embryonic fibroblast; PB1, Phox and Bem1; PBS, phosphate-buffered saline; SDS-PAGE, sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SMIR, SOD1 mutant interaction region; SOD, superoxide dismutase; UBA, ubiquitin association; WT, wild-type. AbstractThe p62/sequestosome 1 protein has been identified as a component of pathological protein inclusions in neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). P62 has also been implicated in autophagy, a process of mass degradation of intracellular proteins and organelles. Autophagy is a critical pathway for degrading misfolded and/or damaged proteins, including the copper-zinc superoxide dismutase (SOD1) mutants linked to familial ALS. We previously reported that p62 interacted with ALS mutants of SOD1 and that the ubiquitin-association domain of p62 was dispensable for the interaction. In this study, we identified two distinct regions of p62 that were essential to its binding to mutant SOD1: the N-terminal Phox and Bem1 (PB1) domain (residues 1-104) and a separate internal region (residues 178-224) termed here as SOD1 mutant interaction region (SMIR). The PB1 domain is required for appropriate oligomeric status of p62 and the SMIR is the actual region interacting with mutant SOD1. Within the SMIR, the conserved W184, H190 and positively charged R183, R186, K187, and K189 residues are critical to the p62-mutant SOD1 interaction as substitution of these residues with alanine resulted in significantly abolished binding. In addition, SMIR and the p62 sequence responsible for the interaction with LC3, a protein essential for autophagy activation, are independent of each other. In cells lacking p62, the existence of mutant SOD1 in acidic autolysosomes decreased, suggesting that p62 can function as an adaptor between mutant SOD1 and the autophagy machinery. This study provides a novel molecular mechanism by which mutant SOD1 can be recognized by p62 in an ubiquitin-independent fashion and targeted for the autophagy-lysosome degradation pathway. Keywords: autophagy, familial amyotrophic lateral sclerosis, mutant superoxide dismutase 1, sequestosome 1/p62.
Familial amyotrophic lateral sclerosis (ALS) has been linked to mutations in the copper/zinc superoxide dismutase (SOD1) gene. The mutant SOD1 protein exhibits a toxic gain‐of‐function that adversely affects the function of neurons. However, the mechanism by which mutant SOD1 initiates ALS is unclear. Lipid rafts are specialized microdomains of the plasma membrane that act as platforms for the organization and interaction of proteins involved in multiple functions, including vesicular trafficking, neurotransmitter signaling, and cytoskeletal rearrangements. In this article, we report a proteomic analysis using a widely used ALS mouse model to identify differences in spinal cord lipid raft proteomes between mice overexpressing wild‐type (WT) and G93A mutant SOD1. In total, 413 and 421 proteins were identified in the lipid rafts isolated from WT and G93A mice, respectively. Further quantitative analysis revealed a consortium of proteins with altered levels between the WT and G93A samples. Functional classification of the 67 altered proteins revealed that the three most affected subsets of proteins were involved in: vesicular transport, and neurotransmitter synthesis and release; cytoskeletal organization and linkage to the plasma membrane; and metabolism. Other protein changes were correlated with alterations in: microglia activation and inflammation; astrocyte and oligodendrocyte function; cell signaling; cellular stress response and apoptosis; and neuronal ion channels and neurotransmitter receptor functions. Changes of selected proteins were independently validated by immunoblotting and immunohistochemistry. The significance of the lipid raft protein changes in motor neuron function and degeneration in ALS is discussed, particularly for proteins involved in vesicular trafficking and neurotransmitter signaling, and the dynamics and regulation of the plasma membrane‐anchored cytoskeleton.
An important consequence of protein misfolding related to neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), is the formation of proteinaceous inclusions or aggregates within the central nervous system. We have previously shown that several familial ALS-linked copper-zinc superoxide dismutase (SOD1) mutants (A4V, G85R, and G93A) interact and co-localize with the dynein-dynactin complex in cultured cells and affected tissues of ALS mice. In this study, we report that the interaction between mutant SOD1 and the dynein motor plays a critical role in the formation of large inclusions containing mutant SOD1. Disruption of the motor by overexpression of the p50 subunit of dynactin in neuronal and non-neuronal cell cultures abolished the association between aggregation-prone SOD1 mutants and the dynein-dynactin complex. The p50 overexpression also prevented mutant SOD1 inclusion formation and improved the survival of cells expressing A4V SOD1. Furthermore, we observed that two ALS-linked SOD1 mutants, H46R and H48Q, which showed a lower propensity to interact with the dynein motor, also produced less aggregation and fewer large inclusions. Overall, these data suggest that formation of large inclusions depends upon association of the abnormal SOD1s with the dynein motor. Whether the misfolded SOD1s directly perturb axonal transport or impair other functional properties of the dynein motor, this interaction could propagate a toxic effect that ultimately causes motor neuron death in ALS. Amyotrophic lateral sclerosis (ALS)2 is a progressive and fatal neurodegenerative disease primarily affecting motor neurons in the spinal cord and brainstem. Approximately 10% of ALS cases are inherited, and of these ϳ20% are caused by mutations in the Cu,Zn-superoxide dismutase 1 (SOD1) gene (1, 2). To date, more than 100 mutations scattered throughout the SOD1 protein have been identified (3). Many SOD1 mutants retain nearly normal enzymatic activity, and SOD1 knock-out mice do not develop ALS (4, 5). Therefore, it is believed that mutant SOD1s acquire a toxic "gain-of-function" property.We recently identified dynein as a component of soluble SOD1-containing high molecular weight (HMW) complexes (6), which have been implicated in neuronal toxicity and may be precursors to SOD1 inclusions (7-9). Moreover, co-immunoprecipitation using tissue lysates from transgenic rodents expressing wild-type (WT), G93A, or G85R SOD1, showed that mutant SOD1 interacted with the dynein-dynactin complex to a much greater extent than did WT SOD1. The association was detected in the pre-symptomatic G93A mice (60 days), and the amount of mutant SOD1 interacting with the dynein complex increased over the disease progression (6).Dynein-mediated microtubule-dependent retrograde transport has been shown to be important for sequestration of misfolded proteins into large inclusions called aggresomes (10,11). We hypothesized that the interaction between the dynein motor and mutant SOD1 could play a role in the aggregation and formation of inc...
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