ATXN3: a multifunctional protein involved in the polyglutamine disease spinocerebellar ataxia type 3

Abstract

ATXN3 is a ubiquitin hydrolase (or deubiquitinase, DUB), product of the ATXN3 gene, ubiquitously expressed in various cell types including peripheral and neuronal tissues and involved in several cellular pathways. Importantly, the expansion of the CAG trinucleotides within the ATXN3 gene leads to an expanded polyglutamine domain in the encoded protein, which has been associated with the onset of the spinocerebellar ataxia type 3, also known as Machado-Joseph disease, the most common dominantly inherited ataxia worldwide. ATXN3 has therefore been under intensive investigation for decades. In this review, we summarize the main functions of ATXN3 in proteostasis, DNA repair and transcriptional regulation, as well as the emerging role in regulating chromatin structure. The mentioned molecular functions of ATXN3 are also reviewed in the context of the pathological expanded form of ATXN3.

Keywords: ATXN3; DNA damage response; chromatin organization; neurodegeneration; polyglutamine neurodegenerative disorders; proteostasis; spinocerebellar ataxia type-3; ubiquitin hydrolase; ubiquitin proteasome system.

Figure 1.

Domain organization and molecular structure of ATXN3. (A) Schematic representation of the most common isoform of ATXN3 (UniProt P54252-2). ATXN3 is composed of an N-terminal catalytic domain (Josephin domain, JD), followed by a C-terminal tail containing three UIMs and a polyglutamine sequence of variable length (PolyQ). ATXN3 holds a nuclear localization signal (NLS) located upstream of the PolyQ region and two nuclear-export signals (NES) within the JD. The VBM sequence around the PolyQ track is responsible for the binding to VCP/p97. (B) Cartoon (upper panel) and space-filling (lower panel) models of the Josephin domain crystal structure (PDB ID: 1YZB). Cysteine 14 (Cys14) is labelled in purple. (C) Cartoon predicted model of ATXN3 (AlphaFold ID: P54252-F1). Functional domains are indicated as in (A).

Figure 2.

Roles of ATXN3 in proteostasis. (A) ATXN3 is required for efficient ERAD, assisting p97 in the extraction and unfolding of proteins for subsequent delivery to the proteasome. (B) Interaction of Bcl-2 with BECN1 inhibits autophagy initiation, Parkin monoubiquitinates Bcl-2, leading to an increase on Bcl-2 levels. ATXN3 counteracts Parkin E3 activity, by interacting with it, and hampering the auto-ubiquitination of Parkin. Also, ATXN3 removes ubiquitin chains from BECN1, leading to its stabilization and stimulation of autophagy. ATXN3 interacts with LC3C and GABARAP in early stages of autophagy, promoting autophagy. ATXN3 also regulates the activity of the E3 ligase Parkin, important for the ubiquitination of autophagy substrates and the autophagy inhibitor Bcl-2.

Figure 3.

Involvement of ATXN3 in genome integrity maintenance. (A) The association between ATXN3 and PNKP promotes the 3′ phosphatase activity of PNKP, removing the 3′-P group, and consequent SB repair. (B) Under unperturbed conditions and upon DNA damage, ATXN3 interacts with Chk1 and antagonizes polyubiquitination and degradation of Chk1, facilitating DNA damage checkpoints and repair. Under prolonged replicative stress, phosphorylation of Chk1 results in dissociation of Chk1 from ATXN3. (C) During the early phase of DSB response, ATXN3 antagonizes RNF4-induced polyubiquitination and subsequent chromatin-release of MDC1. By prolonging the residence time of MDC1 at chromatin, ATXN3 ensures that the DDR is accurately activated. The p97 ATXN3 complex promotes RNF8 extraction from damage sites to avoid RNF8 overaccumulation and inhibition of NHEJ, ensuring proper DSBs repair. (D) p53 ubiquitination is mainly governed by the E3 ligase Mdm2 and leads to proteasomal degradation of p53. Among other DUBs, ATXN3 stabilizes p53 by direct deubiquitination of the protein.

Figure 4.

Normal and pathogenic roles of ATXN3 in the DDR. ATXN3-WT regulates the kinetics of early and late events in the DDR process. As a DUB, ATXN3 stabilizes MDC1, Chk1 and p53 through the removal of polyubiquitin chains, and promotes PNKP phosphatase activity, stimulating DNA repair and cell survival. On the other hand, ATXN3-PolyQ inhibits PNKP-dependent DNA repair and prolongs p53-signaling through enhanced p53-binding and stabilization. This leads to increased activation of pro-apoptotic pathways and cell vulnerability to DNA damage.