Behind sporadic ALS: A biophysical characterization of motor neurons in health and disease.

Abstract

Amyotrophic lateral sclerosis (ALS) is characterized by death of motor neurons due to accumulation of aggregates and perturbation of intracellular transport. The underlying causes of these changes remain yet mysterious. Although a fraction of cases are caused by inherited mutations, the majority are sporadic, and the risk increases with age. The cell interior is extremely crowded with organelles and macromolecular complexes. This intracellular molecular crowding crucially impacts molecular assembly and transport. We hypothesized that age-associated changes could perturb macromolecular crowding and participate in the pathogenesis of ALS. We generated and leveraged genetically encoded multimeric nanoparticles (GEMs) to quantify macromolecular crowding in human iPSC-derived motor neurons, for the first time. The diffusivity of tracer particles like GEMs is generally decreased by molecular crowding and increased by active cellular processes. GEMs’ diffusivity was different depending on the subcellular localization. We observed that GEMs’ diffusivity is sensitive to the compressive state of the cell overall, while is oriented and regulated by ATP-dependent processes only in the neurites (axons and dendrites). Mechanical and osmotic pressures can increase in the aged brain. Motor neurons from ALS patients did not show modified basal molecular crowding, but might be more sensitive to mechanical perturbations. We are now studying how molecular crowding perturbations impact aggregation and transport of TDP-43, a protein found in ALS aggregates, in human motor neurons. We are performing the first high-throughput characterization of the biophysical properties of motor neurons to decipher the relationship between intracellular biophysical changes and initiation of protein aggregation in ALS.