Emerging mechanisms of molecular pathology in ALS

Abstract

Amyotrophic lateral sclerosis (ALS) is a devastating degenerative disease characterized by progressive loss of motor neurons in the motor cortex, brainstem, and spinal cord. Although defined as a motor disorder, ALS can arise concurrently with frontotemporal lobal dementia (FTLD). ALS begins focally but disseminates to cause paralysis and death. About 10% of ALS cases are caused by gene mutations, and more than 40 ALS-associated genes have been identified. While important questions about the biology of this disease remain unanswered, investigations of ALS genes have delineated pathogenic roles for (a) perturbations in protein stability and degradation, (b) altered homeostasis of critical RNA- and DNA-binding proteins, (c) impaired cytoskeleton function, and (d) non-neuronal cells as modifiers of the ALS phenotype. The rapidity of progress in ALS genetics and the subsequent acquisition of insights into the molecular biology of these genes provide grounds for optimism that meaningful therapies for ALS are attainable.

Figure 3. Disruption of neuroaxonal structure and axonal vesicle trafficking in ALS.

(i) Mutations in profilin-1 disrupt multiple functions of this protein, including its role in facilitating polymerization of monomeric G-actin to its filamentous F-actin form. (ii) Reduced expression of EphA4 correlates with increased survival in sALS, hypothetically through reducing this protein’s normal activity in signaling axon repulsion and arresting axonal growth. Reduction in levels of EphA4 or its activity is believed to enhance the capacity of a distal motor terminal to extend, remodel, and potentially reinnervate the target muscle. (iii) Disruption in endocytosis and the transport of vesicles from the golgi apparatus and ER are a likely consequence of ALS-associated mutations in VAPB. (iv) Mutations have been detected in the p150 subunit of dynactin, which is required for tethering cargos to the dynein retrograde transport complex. Some studies have suggested that altered expression of KIFAP1 enhances ALS risk by impairing function of the anterograde transport motor complex.

Figure 2. Pathogenic mechanisms associated with hexanucleotide repeat-expanded C9orf72 and various DNA/RNA-binding proteins.

(A) The mutant C9orf72 gene associated with fALS contains an intronic G₄C₂ motif often expanded to several hundred (and even several thousand) repeats. This GC-rich domain is transcribed in both the sense and antisense directions, producing mRNA prone to forming large intranuclear foci that are believed to sequester some RNA-binding proteins. The sense or antisense transcripts undergo noncanonical RAN translation in all six possible reading frames, generating five dipeptide-repeat peptides (GA, GR, GP, PR, PA), which form inclusions that are associated with the protein p62. (B) Under normal conditions, the DNA/RNA-binding proteins mutated in fALS, most notably TDP-43 and FUS, are typically located within the nucleus, where they serve multiple functions. These proteins are also able to translocate to the cytoplasm, where they may localize to stress granules under some adverse cellular conditions. When these proteins are defective (e.g., bearing ALS-related mutations) the normal range of interactions with DNA and RNA are disrupted; this can lead to marked changes in transcription, splicing, and translation. Furthermore, the presence of the low-complexity, prion-like domains is thought to facilitate oligomer self-assembly under conditions of cellular stress, thereby promoting prion-like toxicity and intercellular spread.

Figure 1. Structure and functional domains of DNA/RNA-binding proteins associated with fALS.

Several functional domains are common to the ALS-associated DNA/RNA-binding proteins, including TDP-43, hnRNPA1, FUS, and TAF15. These common domains include a nuclear localization motif, RNA-repeat binding domains, and glycine-rich domains of low structural complexity that possess prion-like activity. ALS-related mutations are annotated in their corresponding residues. Structurally ordered, disordered and prion-like domains are noted beneath each peptide. Sources of sequence data: Uniprot (domain prediction; http://www.uniprot.org), FoldIndex (ordered/disordered domain prediction; http://bip.weizmann.ac.il/fldbin/findex). Prion domain mapping data provided by refs. , , and . Adapted with permission from Elsevier (139).