La FAM fatale: USP9X in development and disease

Abstract

Deubiquitylating enzymes (DUBs), act downstream of ubiquitylation. As such, these post-post-translational modifiers function as the final arbitrators of a protein substrate's ubiquitylation status, thus regulating its fate. In most instances, DUBs moderate the absolute level of a substrate, its locality or activity, rather than being an "all-or-none" phenomenon. Yet, disruption of this quantitative regulation can produce dramatic qualitative differences. The ubiquitin-specific protease 9X (USP9X/FAM) is a substrate-specific DUB, which displays an extraordinarily high level of sequence conservation from Drosophila to mammals. It is primarily the recent revelations of USP9X's pivotal role in human cancers, both as oncogene or tumour suppressor, in developmental disorders including intellectual disability, epilepsy, autism and developmental delay that has led to a subsequent re-examination of its molecular and cellular functions. Results from experimental animal models have implicated USP9X in neurodegeneration, including Parkinson's and Alzheimer's disease, as well as autoimmune diseases. In this review, we describe the current and accumulated knowledge on the molecular, cellular and developmental aspects of USP9X function within the context of the biological consequences during normal development and disease.

Fig. 1

Structural information of USP9X. Schematic of USP9X structure showing functional domains and nuclear localisation sequence (NLS) motifs. Below the schematic are the regions of USP9X known to facilitate binding to the listed interacting proteins. Above the schematic is a scale (in amino acids), the localisation of variants associated with ID¹⁸ and a histogram of variants found in cancer samples (cBioportal). Red indicates nonsense variants, green represents missense variants and purple indicates both

Fig. 2

Coordination of developmental signalling pathways by USP9X. Interactors of USP9X (hexagons) are key regulators, or signal transduction molecules, of signalling pathways important for many aspects of embryogenesis. In Notch signalling (orange), USP9X is required in the signal sending cells to antagonise the proteasomal degradation of both Mind bomb1 (MIB1) and Epsin1, which control key endocytic events of Notch ligands (Delta, Serrate and Lag2; DSL) required for signalling competence. In EGF signalling (dark blue), USP9X protects ITCH from proteasomal degradation, which then inhibits the delivery of the EGF receptor to the lysosome and hence promotes signalling output. USP9X interacts with multiple components of the mTOR signalling pathway (green) including mTOR, Raptor and Rictor. Although the direct consequence of these interactions is unknown, Usp9x negatively regulates the signalling output of the two mTOR signalling complexes, mTORC1 and 2. USP9X can promote canonical Wnt signalling (light blue) by protecting the central signal transduction component β-catenin (βcat) from proteasomal degradation, thus promoting its accumulation, which is associated with nuclear translocation. USP9X can also affect TGFβ signalling (purple) both positively and negatively. USP9X protects SMURF1 from proteasomal degradation, which enables it to directly downregulate TGFβ receptors from the cell surface. In contrast, USP9X promotes TGFβ signalling by reversing the effects of mono-ubiquitylation of the common Smad4, which inhibits its ability to form signal transducing complexes with receptor SMADs (R SMAD). Adding further complexity, USP9X substrates can themselves regulate multiple signalling pathways, for example ITCH also negatively regulates Notch and Wnt signalling (dark blue dotted lines), whilst SMURF1 also negatively regulates Wnt signalling (purple dotted lines)

Fig. 3

Overview of the contribution of USP9X to development and disease. USP9X (red) interacts with 35 known proteins (purple triangles), many of which are substrates. The interaction between USP9X and its binding partners is regulated by the sub-cellular localisations of both (green circle). The outcomes of USP9X interactions are known to control many cellular processes and behaviour (orange circle), which likely underlie the involvement of USP9X in development and disease processes. Each layer of USP9X involvement (i.e. the concentric circles representing substrates, localisation, cell behaviour and developmental and disease processes) can be rotated (arrowheads) so as to find combinations of relevant mechanisms. For example, the localisation of USP9X and AF6 at sites of cell–cell adhesion is required for the polarity of blastomeres and pre-implantation development, whilst the interaction of USP9X and MCL1 in the cytoplasm results in the activation of the anti-apoptotic pathways that can lead to tumour resistance against oncogenic therapies