VEXAS
Vexas Syndrome: VEXAS ('Vacuoles, E1 Enzyme, X-linked, Autoinflammatory, Somatic syndrome') syndrome represents a new type of autoinflammatory syndrome. All patients with this condition have an acquired somatic mutation in UBA1, the gene encoding the first enzyme of the ubiquitin conjugation pathway. While somatic mutations are most commonly associated with malignancies, VEXAS syndrome is an unusual example of a somatic mutation causing a complex adult-onset inflammatory disease, and therefore represents the prototype for a new group of diseases. We are modelling this disease to identify genetic vulnerabilities
Genetic dissection of the role of ubiquitin and ubiquitin signaling pathways in regulating cellular homeostasis
Viruses recruit host E3 ubiquitin ligases to degrade key cellular immune effectors as exemplified by studies on cytomegalovirus (CMV) encoded US2/US11 genes which downregulate MHC molecules. Traditional efforts to identify the responsible host E3 ligases proved challenging, but our application of optical, phenotypic genetic screens e.g. CRISPR screens, have identified the host ubiquitin E3 ligases appropriated by viruses such as human CMV for receptor downregulation, including MHC-I molecules as well as the cellular ubiquitin E3 ligases used for homeostatic regulation of MHC-1, and the rate limiting enzyme of cholesterol synthesis, HMGCoA reductase.
The success of these genetic approaches emphasise the power and utility of genetic screening technologies to define rate-limiting enzymes of signalling pathways in human cells. E.g. our application of genetic screens identified RNF145 and Gp78 as cellular ubiquitin E3 ligases independently responsible for the post-translational regulation of HMGCoA reductase - the rate limiting enzyme of cholesterol synthesis.
Regulation of HMGCR is complex multistep process with built-in redundancy
•RNF145 and gp78 independently target HMGCR
•Sterols induce RNF145 to recognise HMGCR via Insigs
•If RNF145/gp78 unavailable: Hrd1 acts as a ‘backup’
•RNF145 is unique: An ER-resident sterol responsive E3 ligase
Polyubiquitination of HMGCR is RNF145, Gp78 & Ube2G2 dependent
Lipid homeostasis and membrane fluidity
Proteostatic adaptations to altered membrane composition and fluidity: ER-resident E3 ligase RNF145 responds to lipid challenge
How cells sense and respond to changes in membrane lipid composition
van den Boomen DJH, et al. A trimeric Rab7 GEF controls NPC1-dependent lysosomal cholesterol export. Nat Commun. 2020 Nov 3;11(1):5559
LDL receptor trafficking controls cholesterol homeostasis
References
Volkmar N, Gawden-Bone CM, Williamson JC, Nixon-Abell J, West JA, St George-Hyslop PH, Kaser A, Lehner PJ. Regulation of membrane fluidity by RNF145-triggered degradation of the lipid hydrolase ADIPOR2. EMBO J. 2022 Oct 4;41(19):e110777. doi: 10.15252/embj.2022110777. Epub 2022 Aug 22. PMID: 35993436; PMCID: PMC9531299.
van den Boomen DJH, Sienkiewicz A, Berlin I, Jongsma MLM, van Elsland DM, Luzio JP, Neefjes JJC, Lehner PJ. A trimeric Rab7 GEF controls NPC1-dependent lysosomal cholesterol export. Nat Commun. 2020 Nov 3;11(1):5559. doi: 10.1038/s41467-020-19032-0. PMID: 33144569; PMCID: PMC7642327.
van den Boomen DJH, Volkmar N, Lehner PJ. Ubiquitin-mediated regulation of sterol homeostasis. Curr Opin Cell Biol. 2020 Aug;65:103-111. doi: 10.1016/j.ceb.2020.04.010. Epub 2020 Jun 21. PMID: 32580085.
Menzies SA, Volkmar N, van den Boomen DJ, Timms RT, Dickson AS, Nathan JA,Lehner PJ. The sterol-responsive RNF145 E3 ubiquitin ligase mediates the degradation of HMG-CoA reductase together with gp78 and Hrd1. Elife. 2018 Dec 13;7:e40009. doi: 10.7554/eLife.40009. PMID: 30543180; PMCID: PMC6292692
Ongoing work in this area:
The regulation of membrane fluidity. Most recently we showed how the RNF145 ER-resident ubiquitin E3 ligase is an ER sensor of membrane fluidity. In response to altered lipid homeostasis, RNF145 regulates its substrate, ADIPOR2, an ER-resident lipid hydrolase to restore lipid homeostasis and prevent cytotoxicity. RNF145 together with ADIPOR2 therefore provide a fundamental mechanism by which mammalian cells rapidly sense and respond to membrane lipid imbalances. Together RNF145 and ADIPOR2 constitute in the ER. RNF145 senses changes in the membrane phospholipid environment and responds through regulation of ADIPOR2. Under saturated FA conditions, rapid RNF145 auto-ubiquitination stabilises ADIPOR2 enabling it to restore lipid homeostasis. In contrast, in unsaturated lipid membranes, RNF145 stabilisation promotes the lipid-sensitive degradation of ADIPOR2. RNF145 and ADIPOR2 are and maintain cell viability under conditions of increased lipid saturation and decreased membrane fluidity.