Nucleation and dissolution mechanism underlying amyotrophic lateral sclerosis/frontotemporal lobar dementia-linked fused in sarcoma condensates

Single-cell RNA-sequencing reveals early mitochondrial dysfunction unique to motor neurons shared across FUS- and TARDBP-ALS

Mutations in FUS and TARDBP cause amyotrophic lateral sclerosis (ALS), but the precise mechanisms of selective motor neuron degeneration remain unresolved. To address if pathomechanisms are shared across mutations and related to either gain- or loss-of-function, we performed single-cell RNA sequencing across isogenic induced pluripotent stem cell-derived neuron types, harbouring FUS P525L, FUS R495X, TARDBP M337V mutations or FUS knockout. Transcriptional changes were far more pronounced in motor neurons than interneurons. About 20% of uniquely dysregulated motor neuron transcripts were shared across FUS mutations, half from gain-of-function. Most indicated mitochondrial impairments, with attenuated pathways shared with mutant TARDBP M337V as well as C9orf72-ALS patient motor neurons. Mitochondrial motility was impaired in ALS motor axons, even with nuclear localized FUS mutants, demonstrating shared toxic gain-of-function mechanisms across FUS- and TARDBP-ALS, uncoupled from protein mislocalization. These early mitochondrial dysfunctions unique to motor neurons may affect survival and represent therapeutic targets in ALS.

The roles of FUS-RNA binding domain and low complexity domain in RNA-dependent phase separation

Fused in sarcoma (FUS) is an archetypal phase separating protein asymmetrically divided into a low complexity domain (LCD) and an RNA binding domain (RBD). Here, we explore how the two domains contribute to RNA-dependent phase separation, RNA recognition, and multivalent complex formation. We find that RBD drives RNA-dependent phase separation but forms large and irregularly shaped droplets that are rescued by LCD in trans. Electrophoretic mobility shift assay (EMSA) and single-molecule fluorescence assays reveal that, while both LCD and RBD bind RNA, RBD drives RNA engagement and multivalent complex formation. While RBD alone exhibits delayed RNA recognition and a less dynamic RNP complex compared to full-length FUS, LCD in trans rescues full-length FUS activity. Likewise, cell-based data show RBD forms nucleolar condensates while LCD in trans rescues the diffuse nucleoplasm localization of full-length FUS. Our results point to a regulatory role of LCD in tuning the RNP interaction and buffering phase separation.

Protocol for single-molecule pull-down of fluorescently tagged oligomers from cell lysates

Mutations in intrinsically disordered proteins drive the irreversible formation of pathological aggregates, a hallmark of neurodegenerative diseases. Here, we present a protocol to pull down fluorescently tagged proteins to characterize their basal oligomeric states. We describe steps for transfection and cell lysis, single-molecule slide preparation and pull-down, and oligomer dissolution. This protocol enables visualization of protein oligomers with single-molecule resolution. In addition, differences in oligomerization may provide insight on condensation or aggregation propensity in differing mutated or cell stress conditions. For complete details on the use and execution of this protocol, please refer to Djaja et al.1.

Advanced imaging techniques for studying protein phase separation in living cells and at single-molecule level

Protein-protein and protein-RNA interactions are essential for cell function and survival. These interactions facilitate the formation of ribonucleoprotein complexes and biomolecular condensates via phase separation. Such assembly is involved in transcription, splicing, translation and stress response. When dysregulated, proteins and RNA can undergo irreversible aggregation which can be cytotoxic and pathogenic. Despite technical advances in investigating biomolecular condensates, achieving the necessary spatiotemporal resolution to deduce the parameters that govern their assembly and behavior has been challenging. Many laboratories have applied advanced microscopy methods for imaging condensates. For example, single molecule imaging methods have enabled the detection of RNA-protein interaction, protein-protein interaction, protein conformational dynamics, and diffusional motion of molecules that report on the intrinsic molecular interactions underlying liquid-liquid phase separation. This review will outline advances in both microscopy and spectroscopy techniques which allow single molecule detection and imaging, and how these techniques can be used to probe unique aspects of biomolecular condensates.