Post-Translational Modifications Modulate Proteinopathies of TDP-43, FUS and hnRNP-A/B in Amyotrophic Lateral Sclerosis

Abstract

It has been shown that protein low-sequence complexity domains (LCDs) induce liquid-liquid phase separation (LLPS), which is responsible for the formation of membrane-less organelles including P-granules, stress granules and Cajal bodies. Proteins harbouring LCDs are widely represented among RNA binding proteins often mutated in ALS. Indeed, LCDs predispose proteins to a prion-like behaviour due to their tendency to form amyloid-like structures typical of proteinopathies. Protein post-translational modifications (PTMs) can influence phase transition through two main events: i) destabilizing or augmenting multivalent interactions between phase-separating macromolecules; ii) recruiting or excluding other proteins and/or nucleic acids into/from the condensate. In this manuscript we summarize the existing evidence describing how PTM can modulate LLPS thus favouring or counteracting proteinopathies at the base of neurodegeneration in ALS.

Keywords: RNA binding proteins; amyotrophic lateral sclerosis; low-complexity domain; post-translational modifications; protein aggregations.

FIGURE 1

The domain structure of hnRNP-A1, TDP-43 and FUS. The RNA-binding proteins hnRNP-A1, TDP-43 and FUS share structure similarities. Particularly, they harbor Prion-like domain, RNA-recognition motif (RRM) and nuclear localization signal (NLS). hnRNP-A1 and FUS are both characterized by the presence of Arg-Gly-Gly-rich (RGG) domains while TDP-43 has a Gly-rich (G-rich) domain and hnRNP-A1 display a M9 motif. Moreover, both FUS and TDP-43 present nuclear export signal (NES) but only FUS has a zinc-finger (ZnF) domain and a Gln-Gly-Ser-Tyr-rich (QGSY-rich) domain.

FIGURE 2

Assembly of condensates and PTMs. Panel A. LCD-containing proteins (including TDP-43, FUS and hnRNPA1) under certain conditions assume a misfolded state in which several intra and inter-molecular interaction can be established. Different PTMs tend to stimulate or dampen the formation of insoluble condensate in which LCD-containing proteins are sequestered in a toxic β-sheet structure. However, PTM has protein- and residue-specific impact on protein aggregate formation. Panel B. hnRNPA1, FUS and TDP-43 can be target of several PTMs catalyzed by specific enzymes. hnRNP-A1 and FUS can be PARylated by PARPs and PARP1, respectively. FUS protein can be methylated by PRMT1 acetylated by CBP/p300 and NatA. Moreover, the Prion like domain of FUS is phosphorylated at multiple sites by the two kinases ATM and DNA-PK. TDP-43 is actively acetylated by CBP and phosphorylated by Casein Kinase. Finally, UBE2E and Parkin catalyze the ubiquitination of TDP-43.