Prionoids in amyotrophic lateral sclerosis

Abstract

Amyotrophic lateral sclerosis (ALS) is the third most frequent neurodegenerative disease after Alzheimer's and Parkinson's disease. ALS is characterized by the selective and progressive loss of motoneurons in the spinal cord, brainstem and cerebral cortex. Clinical manifestations typically occur in midlife and start with focal muscle weakness, followed by the rapid and progressive wasting of muscles and subsequent paralysis. As with other neurodegenerative diseases, the condition typically begins at an initial point and then spreads along neuroanatomical tracts. This feature of disease progression suggests the spreading of prion-like proteins called prionoids in the affected tissues, which is similar to the spread of prion observed in Creutzfeldt-Jakob disease. Intensive research over the last decade has proposed the ALS-causing gene products Cu/Zn superoxide dismutase 1, TAR DNA-binding protein of 43 kDa, and fused in sarcoma as very plausible prionoids contributing to the spread of the pathology. In this review, we will discuss the molecular and cellular mechanisms leading to the propagation of these prionoids in ALS.

Keywords: amyotrophic lateral sclerosis; physiopathology; prion-like; protein aggregation; therapy.

Graphical Abstract

Figure 1

Cellular events involved in the generation and spread of prionoids. Seeding/nucleation: intracellular accumulation of cytoplasmic misfolded proteins induces prionoid seeding and nucleation in non-amyloid or amyloid-like structures termed ‘aggregates’. Oligomers are presumably intermediate structural protein species in the nucleation process. Propagation: mechanisms of cellular extrusion of misfolded proteins, comprising intercellular bridges like tubular nanotubes and unconventional secretion of proteins in a soluble state or contained in various secreted vesicles (lysosomes, endosomes, exosomes, extracellular vesicles, exophers). Cellular uptake: uptake of prionoids by macropinocytosis. This process seems to be regulated by several receptors that recognize extracellular protein aggregates.

Figure 2

Unconventional secretion pathways of SOD1, TDP-43 and FUS. The cytoplasmic cargos SOD1, FUS and TDP-43, are represented by red, orange and purple circles, respectively. SOD1 is released either by a specific secretory pathway for cytoplasmic ubiquitinated misfolded proteins called MAPS or by an autophagic-like secretory pathway involving a specialized organelle called CUPS. SOD1 mutants are also secreted via exocytosis from motoneurons. The MAPS pathway seems also to promote wildtype TDP-43 secretion. FUS has, for instance, only been detected in extracellular vesicles from LC3⁺ autophagosomes. MAPS involves the targeting of cargos in Rab9⁺ (green circles) late endosomes allowing protein secretion in a vesicles-free form. This process depends on the binding of HSPA8 (marine blue triangle) and the deubiquitinase USP19 (brown stick) to polyubiquitinated (black circle) proteins. The CUPS-dependent pathway involves the Golgi reassembly-stacking protein (GRASP55) that appears to shuttle between the Golgi and CUPS.

Figure 3

Pathological aggregations of SOD1, FUS and TDP-43. Schematic view of pathways leading to the aggregation of SOD1 and the two RBPs, FUS and TDP-43. SOD1 aggregation results in the accumulation of misfolded oligomeric or monomeric species during the folding process that can be due to intrinsically disordered structure (fALS mutations) and defects of the protein quality control and folding machineries. Alternatively, the properly folded SOD1 dimer might be also destabilized due to deleterious physicochemical perturbations (e.g. reactive oxygen species). FUS and TDP-43 aggregation might be the result of an imbalance in their oscillations between disordered monomers and highly ordered oligomeric structures. These structural properties give rise to labile phase transitions from liquid to liquid (liquid droplets) or to liquid to solid (hydrogel). ALS-causing mutations, cellular dysregulation (RNA metabolism, trafficking defects…) favour aberrant and irreversible phase transitions (liquid/solid, solid/solid) producing fibrils (seeding). NTD (N-terminal domain), MBD (metal binding domain), RRM (RNA recognition motif), ZnF (zing finger domain) and NLS (nuclear-localization sequence).

Figure 4

Experimental paradigms demonstrating the prion-like properties of SOD1 and TDP-43 in mice. This schema is a compilation of several published results obtained by the administration of misfolded SOD1 and TDP-43 aggregates in different mouse models. The nature of protein extracts (crude homogenates, purified proteins, recombinant proteins), the route of delivery and the genetic background of injected mice are indicated. For the observed physiopathological outcome, three criteria are mentioned: the levels of protein inclusions/aggregates in the CNS, the motor symptoms evaluated by behaviour tests and the lifespan of injected mice.