FUS and TDP-43 Phases in Health and Disease

Abstract

The distinct prion-like domains (PrLDs) of FUS and TDP-43, modulate phase transitions that result in condensates with a range of material states. These assemblies are implicated in both health and disease. In this review, we examine how sequence, structure, post-translational modifications, and RNA can affect the self-assembly of these RNA-binding proteins (RBPs). We discuss how our emerging understanding of FUS and TDP-43 liquid-liquid phase separation (LLPS) and aggregation, could be leveraged to design new therapies for neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), frontotemporal dementia (FTD), and limbic-predominant age-related TDP-43 encephalopathy (LATE).

Keywords: aggregation; neurodegenerative diseases; phase separation; prion-like domains: RNA-binding proteins; stress granules.

Figure 1.. Model Phase Transitions.

(A) A phase diagram describing homotypic phase separation. Crossing the phase boundary leads to a transition from a well-mixed solution (position 1) to a biphasic solution, with light and dense phases (position 2). Further increasing the concentration (position 3) increases the volume fraction of the dense phase, without altering the light or dense phase concentrations. (B) High ratios of RNA:RNA binding proteins (RBPs), including FUS and TDP-43, can prevent phase separation and aggregation [21,32]. Conversely, lower ratios of RNA to RBPs can lead multiple copies of a protein bound to a single RNA, creating a high local protein concentration and facilitating intermolecular interactions between RBPs, thus nucleating a phase transition [20,139].

Figure 2.. Interaction Interfaces Regulating FUS LLPS.

Residues 1–239 of FUS harbor the prion-like domain (PrLD), which is followed by a glycine-rich region extending to residue 267, a RNA-recognition motif (RRM), two arginine-glycine rich regions (RGGs), a zinc finger (ZnF) domain, and a PY-nuclear localization signal (PY-NLS) [13]. Low complexity aromatic-rich kinked segments (LARKS) in the PrLD are capable of forming homotypic cross-β interactions [–54]. Arginine residues in RGG domains and tyrosine residues in the PrLD can form intramolecular (orange arrows) and intermolecular interactions [13,46]. RRM, RGG, and ZnF domains, further mediate interactions between FUS and RNA molecules (black arrows) [13]. The RGG domains also interact with poly(ADP Ribose) (PAR) (red arrows) [55]. The PY-NLS interacts with importins which regulate FUS condensation (purple arrow) [39,48,57,58]. Collectively, these interactions govern FUS assembly via competition between inter- and intramolecular interactions tuned by post-translational modifications and scaffolds like RNA and PAR.

Figure 3.. Interaction Interfaces Regulating TDP-43 Liquid–Liquid Phase Separation (LLPS).

TDP-43 consists of an N-terminal domain (NTD) that can form homotypic interactions (orange arrow) [18,76], and which contains a nuclear localization signal (NLS) harboring two poly(ADP Ribose) (PAR)-binding motifs (red arrow) [13,22]. The NLS also engages importins, which can regulate TDP-43 condensation (purple arrow) [39]. The NTD is followed by two RNA-recognition motif (RRM) domains and a prion-like domain (PrLD). The two RRM domains facilitate the interactions between TDP-43 and RNA molecules (black arrows), which are important for regulation of TDP-43 LLPS [32]. The PrLD itself contains various subdomains capable of forming intermolecular interactions with other copies of TDP-43 (top). These subdomains include a region with α-helical propensity, a Q/N-rich region, numerous Zippers, and low complexity aromatic-rich kinked segments (LARKS) [78,80,83].