Potential Therapeutic Role of HDAC Inhibitors in FUS-ALS

Abstract

Mutations in the FUS gene cause amyotrophic lateral sclerosis (ALS-FUS). However, the exact pathogenic mechanism of mutant fused in sarcoma (FUS) protein is not completely understood. FUS is an RNA binding protein (RBP) localized predominantly in the nucleus, but ALS-linked FUS mutations can affect its nuclear localization signal impairing its import into the nucleus. This mislocalization to the cytoplasm facilitates FUS aggregation in cytoplasmic inclusions. Therapies targeting post translational modifications are rising as new treatments for ALS, in particular acetylation which could have a role in the dynamics of RBPs. Research using histone deacetylase (HDAC) inhibitors in FUS-ALS models showed that HDACs can influence cytoplasmic FUS localization. Inhibition of HDACs could promote acetylation of the FUS RNA binding domain (RRM) and altering its RNA interactions resulting in FUS maintenance in the nucleus. In addition, acetylation of FUS RRMs might also favor or disfavor its incorporation into pathological inclusions. In this review, we summarize and discuss the evidence for the potential role of HDACs in the context of FUS-ALS and we propose a new hypothesis based on this overview.

Keywords: ALS; FUS; HDAC inhibitors; LLPS; RNA; RRM; acetylation; mislocalization.

Figure 1

Protein domains in FUS. The FUS folded domains are: RRM, ZnF (Zinc Finger), NLS (Nuclear location signal). In addition, NES (nuclear export signal) is a putative domain that may be located in the RRM domain. The FUS intrinsically disordered regions (indicated by a red triangle) are: Low complexity domain (LCD), Glycine-rich domains, and RGGs. The number of residues that covers each domain is indicated.

Figure 2

Summary of the acetylation sites in FUS and its potential effects on FUS-ALS pathology [based on the results obtained in the study of Arenas et al. (2020)]. Acetylation of the FUS RRM (K315/316) by both acetylation mimetics and HDAC pan-inhibition (using a DACi cocktail) decreased FUS ability to bind to RNA. Prevention of RNA binding reduces cytoplasmic inclusions and FUS colocalization with stress granules. Thus, FUS acetylation on RRM sites has a positive effect on FUS-ALS pathology. On the other hand, FUS NLS region (K510 site) proved to be acetylated by acetylation mimetics, CBP or pan-HDAC inhibition. Acetylation of the K510 residue results in the loss of affinity with Transportin-1 leading to increased FUS mislocalization. Thus, FUS acetylation on the NLS site has a negative effect on FUS-ALS pathology.

Figure 3

Influence of RNA binding on FUS toxicity compared to the effect of FUS acetylation of RRM residues on FUS toxicity. (A) Influence of RNA binding on FUS LLPS. Binding of FUS to high RNA concentrations prevents it from undergoing LLPS. (B) Influence of RRM acetylation on FUS LLPS. RRM acetylation prevents RNA-RRM interactions, increasing FUS LLPS. (C) Influence of RNA binding on FUS recruitment to SGs. RNA binding increases FUS recruitment to SG. (D) Influence of RRM acetylation on FUS recruitment to SGs. Acetylation of RRM domains prevents FUS from binding to RNA, decreasing FUS recruitment to SGs. (E) Influence of RRM acetylation on FUS droplets and stress granules formation. RRM acetylation promotes droplet formation that can further turn into stress granules.

Figure 4

Potential molecular mechanism of HDACs to drive FUS mislocalization and related ALS features. (A) Motor neurons with NLS-mutated FUS. Mutations in NLS domain impairs FUS binding to Transportin-1 resulting in FUS cytoplasm sequestration. Lower concentrations of nuclear FUS not activates FUS autoregulation resulting in an overproduction of mutant FUS producing further increase on FUS cytoplasmic levels. Nuclear HATs acetylate FUS RRM preventing its binding to RNA and hence FUS egress from the nucleus. Nuclear HDACs (like HDAC3) increases FUS mislocalization by deacetylating RRM. A higher ratio of nuclear HDACs/HATs has been associated with ALS, hence driving FUS mislocalization through HDACs. Moreover, the reduction of HDAC1-FUS interactions caused by FUS mislocalization impairs DNA repair resulting in DNA damage. In the cytoplasm, HATs acetylation of the RRM can promote LLPS of FUS. HDAC6 participates in the proteasomal clearance of FUS aggregates. Abnormal cytoplasmic expression of HDAC6 impairs axonal transport and, also, generates DNA damage. (B) Motor neurons with NLS-mutated FUS treated by HDAC inhibitors. In absence of nuclear HDACs, nuclear FUS is only subject of the HATs effect, thereby acetylation of FUS RRM keeps FUS within the nucleus. Therefore, maintaining FUS in the nucleus reactivates the autoregulatory mechanism of FUS reducing the production of mutant FUS. Prevention of FUS mislocalization also avoids FUS recruitment into stress granules and stress impairments associated to ALS. Moreover, HDAC6 inhibition ameliorates axonal damage from α-tubulin deacetylation and DNA damage caused by HDAC6.