Phase separation and pathologic transitions of RNP condensates in neurons: implications for amyotrophic lateral sclerosis, frontotemporal dementia and other neurodegenerative disorders

Abstract

Liquid-liquid phase separation results in the formation of dynamic biomolecular condensates, also known as membrane-less organelles, that allow for the assembly of functional compartments and higher order structures within cells. Multivalent, reversible interactions between RNA-binding proteins (RBPs), including FUS, TDP-43, and hnRNPA1, and/or RNA (e.g., RBP-RBP, RBP-RNA, RNA-RNA), result in the formation of ribonucleoprotein (RNP) condensates, which are critical for RNA processing, mRNA transport, stability, stress granule assembly, and translation. Stress granules, neuronal transport granules, and processing bodies are examples of cytoplasmic RNP condensates, while the nucleolus and Cajal bodies are representative nuclear RNP condensates. In neurons, RNP condensates promote long-range mRNA transport and local translation in the dendrites and axon, and are essential for spatiotemporal regulation of gene expression, axonal integrity and synaptic function. Mutations of RBPs and/or pathologic mislocalization and aggregation of RBPs are hallmarks of several neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and Alzheimer's disease. ALS/FTD-linked mutations of RBPs alter the strength and reversibility of multivalent interactions with other RBPs and RNAs, resulting in aberrant phase transitions. These aberrant RNP condensates have detrimental functional consequences on mRNA stability, localization, and translation, and ultimately lead to compromised axonal integrity and synaptic function in disease. Pathogenic protein aggregation is dependent on various factors, and aberrant dynamically arrested RNP condensates may serve as an initial nucleation step for pathologic aggregate formation. Recent studies have focused on identifying mechanisms by which neurons resolve phase transitioned condensates to prevent the formation of pathogenic inclusions/aggregates. The present review focuses on the phase separation of neurodegenerative disease-linked RBPs, physiological functions of RNP condensates, and the pathologic role of aberrant phase transitions in neurodegenerative disease, particularly ALS/FTD. We also examine cellular mechanisms that contribute to the resolution of aberrant condensates in neurons, and potential therapeutic approaches to resolve aberrantly phase transitioned condensates at a molecular level.

Keywords: RNA-binding proteins; aggregation; biomolecular condensates; neurodegenerative disease; phase separation.

Figure 1

A schematic representation depicting a healthy neuron (left) and an ALS/FTD-affected neuron (right) along with the major differences in RBPs and RNP condensates under physiologic and pathologic conditions. 1. Physiological RBPs are found predominantly in the nucleus whereas RBPs are mislocalized to cytoplasm in disease. 2. Resistance to proteasomal degradation: Aggregates composed of RBPs in affected neurons show increased resistance to degradation by the cellular proteasome system. 3. Defects in mRNA/protein transport: There is impaired stability and transport of mRNA. 4. Defects in local translation: Affected neurons show deficiencies in local translation. The bottom of the figure represents several therapeutic options to target neurodegenerative diseases aimed to resolve aberrant RNP condensates and prevent the aggregation of RBPs in the affected neurons.

Figure 2

Schematic highlighting RNA-binding proteins (RBPs) and intrinsically disordered proteins (IDPs), domain structure and disease-linked mutations: TAR DNA-binding protein 43 (TDP-43 - Uniprot Q13148), Fused in sarcoma (FUS - Uniprot P35637), Heterogeneous nuclear ribonucleoprotein A1 (hnRNPA1 Uniprot P09651), T-cell intracellular antigen 1 (TIA1 - Uniprot P31483), Fragile X mental retardation protein (FMRP – Uniprot Q06787), α-Synuclein (Alpha-synuclein – Uniprot P37840), and Microtubule-associated protein tau (Tau - Uniprot P10636-5). The major domains for each protein and disease-linked mutations retrieved from Uniprot, are depicted. RNA recognition motif (RRM); Low complexity domain (LCD); Zinc finger (ZnF); Tyrosine- and glycine-rich region (YG-box); Frameshift (FS); Loss of function (LOF); Nuclear localization signal (NLS); hnRNP K protein homology (KH); arginine-glycine–glycine box (RGG); Non-Amyloid-beta component (NAC) region, Microtubule binding region (MTBR).