Proteostasis and movement disorders: Parkinson's disease and amyotrophic lateral sclerosis

Abstract

Parkinson's disease (PD) is a movement disorder that afflicts over one million in the U.S.; amyotrophic lateral sclerosis (ALS or Lou Gehrig's disease) is less prevalent but also has a high incidence. The two disorders sometimes present together, making a comparative study of interest. Both ALS and PD are neurodegenerative diseases, and are characterized by the presence of intraneuronal inclusions; however, different classes of neurons are affected and the primary protein in the inclusions differs between the diseases, and in some cases is different in distinct forms of the same disease. These observations might suggest that the more general approach of proteostasis pathway alteration would be a powerful one in treating these disorders. Examining results from human genetics and studies in model organisms, as well as from biochemical and biophysical characterization of the proteins involved in both diseases, we find that most instances of PD can be considered as arising from the misfolding, and self-association to a toxic species, of the small neuronal protein α-synuclein, and that proteostasis strategies are likely to be of value for this disorder. For ALS, the situation is much more complex and less clear-cut; the available data are most consistent with a view that ALS may actually be a family of disorders, presenting similarly but arising from distinct and nonoverlapping causes, including mislocalization of some properly folded proteins and derangement of RNA quality control pathways. Applying proteostasis approaches to this disease may require rethinking or broadening the concept of what proteostasis means.

Figure 1.

Schematic of the protein sequence of human α-synuclein, showing various domains. Human α-synuclein is a small protein whose amino acid sequence is often thought of as comprising three domains: an amino-terminal region that is the site of the three known point mutations that cause autosomal dominant, familial PD (yellow); the so-called nonamyloid component (black), which, paradoxically, is required for the formation of α-synuclein amyloid fibrils; and the acidic carboxy-terminal domain (red), which is thought to be disordered in all forms of α-synuclein. Cleavage of approximately the last 20 residues of this region produces a truncated protein that aggregates much more readily than the full-length molecule, suggesting that at least one function of the disordered third of the sequence is to hinder aggregation. Although α-synuclein is widely believed to be a natively unfolded protein except when bound to membranes (where the first 95 residues are believed to become helical), analysis of the sequence by methods designed to detect intrinsically unfolded proteins predicts that only the last 40 residues of α-synuclein have no propensity to fold (J Sussman, pers. comm.). Interestingly, the first ∼100 residues of the sequence contain a mixture of both helix-favoring and β-sheet-forming residues, which may explain the ability of this protein to adopt the cross-β structure of an amyloid when perturbed.

Figure 2.

Parkin proteostasis. The solubility and activity of parkin can be influenced by inherited mutations within the coding sequence of the protein but also the tissue environment and prevalence of cell stressors. Changes in the cytoplasmic concentrations of dopamine, mitochondrial stress and both reactive oxygen species (ROS), and reactive nitrogen species (RNS) can influence the function of parkin and be risk factors in idiopathic PD via modulation of wild-type parkin function.

Figure 3.

A pathway for Parkinson’s. A possibly fictitious attempt to fit the known environmental and genetic factors associated with sporadic and familial PD into a common pathological pathway involving α-synuclein proteostasis. Many parts of this diagram are hypothetical, and it is predicated on the assumption that there is a nonpathological, folded form of synuclein that must be unfolded before aggregation can commence. Further, it is assumed that Lewy body formation is off the pathway of toxicity, but this is far from certain. The direct connection between organelle dysfunction/damage and cell death is purely speculative. The pathway is intended to provide a framework for thinking about therapeutic strategies based on proteostasis, but it should be taken with more than just a pinch of salt.

Figure 4.

Probing for misfolded WT-SOD1 in SALS patient tissues with conformation specific antibodies. The epitopes for a series of six anti-SOD1 conformation-specific antibodies designed to detect misfolded SOD1 are mapped onto the structure of the WT-SOD1 dimer (Protein database, accession #2C9V) (Strange et al. 2006), where each β-strand is labeled (I–VIII), copper atoms are colored brown, and zinc atoms cyan. The left view of SOD1 has undergone a 90° rotation about the x-axis to generate the right view. Forsberg et al. generated three polyclonal antibodies that epitope mapped to either SOD1 sequences 4–20, 57–72, or 131–153 (highlighted in blue, except for residues 143–151 that are highlighted in green); all three antibodies detected aggregated WT-SOD1 in SALS spinal cord tissues (Forsberg et al. 2010). The monoclonal C4F6 antibody that detected misfolded, soluble WT-SOD1 in SALS spinal cord tissues was epitope mapped to exon 4 (highlighted in red) (Bosco and Landers 2010). Anti-SOD1 antibodies that failed to detect misfolded SOD1 in SALS tissues include the USOD antibody, which epitope mapped to residues 42–48 (highlighted in yellow) (Kerman et al. 2010), and the SEDI antibody, which epitope mapped to 143–151 (highlighted in green) (Liu et al. 2009). Note that the SOD1 sequence recognized by SEDI (143–151; green) is encoded within the epitope of the Forsberg et al. antibody (131–153; blue and green) that detected WT-SOD1 aggregates.

Figure 5.

Domain structures of TDP-43 and FUS/TLS. TDP-43 and FUS have somewhat similar domain structures and each contain multiple, functional domains (see color coded key). More than 35 ALS-linked mutations have been identified in TDP-43, the vast majority of which are located within the Gly-rich domain (denoted by **) (Lagier-Tourenne et al. 2010). A similar number (∼30) of ALS-linked mutations have been reported in FUS; however, most FUS mutations are within the nuclear localization signal (NLS; denoted by **) with additional mutations in the Gly-rich region (denoted by *) (Lagier-Tourenne et al. 2010).

Figure 6.

FUS incorporates into stress granules. (A) GFP-FUS WT is predominately localized to the nucleus of stably-transfected HEK-239 cells. Both a point mutation (R521G) within the nuclear localization signal (NLS; Fig. 5) and a truncation mutation (R495X) that eliminates the NLS increase the cytoplamsic expression level of GFP-FUS according to the following trend R495X > R521G > WT (Bosco et al. 2010b). DAPI (blue) is used to stain the nuclei. (B) On exposure to sodium arsenite, which induces oxidative stress, mutant-FUS proteins become incorporated into cytoplasmic foci called stress granules, which are detected with the stress granule marker anti-G3BP (red) (Bosco et al. 2010b). The extent to which FUS incorporates into stress granules correlates with the cytoplamsic levels of the FUS protein (R495X > R521G > WT).