USP40

USP40 is a deubiquitinating enzyme that stabilizes diverse substrates by removing K48-linked polyubiquitin chains, thereby preventing their proteasomal degradation, with established roles in endothelial barrier integrity, podocyte homeostasis, and hepatocellular carcinoma progression. In endothelial cells, USP40 deubiquitinates and inactivates HSP90β, suppressing RhoA/MLC signaling and NF-κB–dependent ICAM1 expression; global and endothelial-specific USP40 knockout in mice exacerbates experimental lung injury, while lentiviral USP40 gene transfer or the small-molecule activator indole-3-acetic acid (IAA) is protective (PMID:38307937, PMID:38761166). In podocytes, USP40 co-localizes with nestin and stabilizes HINT1, thereby sustaining p53 expression; USP40 loss in zebrafish disrupts glomerular permeability (PMID:28148530, PMID:35605301). In hepatocellular carcinoma, USP40 deubiquitinates YAP at K252/K315 and stabilizes Claudin1, promoting tumor cell proliferation, migration, and stemness through a YAP–USP40 positive-feedback loop (PMID:38537774, PMID:38308285).

1. 2017 Medium

   Whether USP40 had any physiological role in vivo was unknown; demonstration that USP40 binds nestin in glomerular cells and that its loss disrupts glomerular permeability in zebrafish established USP40 as a regulator of kidney filtration barrier integrity.

   Evidence Immunoprecipitation, siRNA knockdown in podocytes, and zebrafish morpholino knockdown with permeability assays

   PMID:28148530

   Open questions at the time

   • Deubiquitination activity toward nestin was not directly demonstrated
   • Mechanism linking USP40 loss to nestin reduction (direct deubiquitination vs. indirect stabilization) was not resolved
   • Mammalian in vivo kidney phenotype not yet tested
2. 2019 Medium

   Whether USP40 had catalytic substrates was unresolved; identification of CFLARL (c-FLIPL) as a USP40 substrate, with GMEB1 acting as a bridging factor, established USP40 as a K48-directed deubiquitinase that regulates DISC formation and apoptotic signaling in NSCLC cells.

   Evidence Co-immunoprecipitation, GST pull-down, shRNA knockdown, and flow cytometry in NSCLC cell lines

   PMID:31046799

   Open questions at the time

   • Single-lab study; independent replication absent
   • Catalytic-dead mutant control not reported
   • In vivo relevance for apoptosis regulation not tested
3. 2022 Medium

   How USP40 supports podocyte homeostasis mechanistically was unclear; showing that USP40 deubiquitinates HINT1, preventing its degradation and sustaining p53 expression, linked USP40 catalytic activity to a defined tumor-suppressor signaling axis in podocytes.

   Evidence Co-immunoprecipitation, gene knockdown in cultured podocytes, USP40 knockout mouse, and FSGS disease model

   PMID:35605301

   Open questions at the time

   • Direct ubiquitin-chain type specificity for HINT1 not defined
   • Whether HINT1 stabilization is the sole mechanism underlying the glomerular phenotype is unknown
   • Rescue experiment restoring USP40 in KO podocytes not reported
4. 2024 High

   Establishing the endothelial-intrinsic function of USP40 and its substrate in vascular barrier regulation, unbiased proteomics identified HSP90β as a direct USP40 substrate whose deubiquitination suppresses RhoA/MLC/NF-κB signaling; endothelial-specific knockout exacerbated lung injury while gene transfer rescued it, and the small molecule IAA was shown to activate USP40-dependent HSP90β deubiquitination.

   Evidence Unbiased proteomics, Co-IP, ubiquitination assays, global and EC-specific KO mice, lentiviral rescue, TEER assays, IAA pharmacological activation in KO mice

   PMID:38307937 PMID:38761166

   Open questions at the time

   • Structural basis of IAA-mediated USP40 activation is unknown
   • Whether HSP90β deubiquitination is activating or inactivating in other cell types is not resolved
   • USP40 catalytic-domain crystal structure not available
5. 2024 High

   Whether USP40 contributes to oncogenesis was untested; identification of YAP (at K252/K315) and Claudin1 as USP40 substrates, and discovery of a YAP→USP40 transcriptional positive-feedback loop, established USP40 as a driver of HCC cell proliferation, migration, and stemness.

   Evidence Co-IP, ubiquitination assays, site-directed mutagenesis, RNA-seq, knockdown/overexpression, in vivo xenograft, and mechanical stiffness models in HCC cells

   PMID:38308285 PMID:38537774

   Open questions at the time

   • Whether YAP and Claudin1 stabilization are independent or epistatic is not resolved
   • Relevance to patient HCC survival or in vivo genetic models (conditional KO) not established
   • Selectivity of USP40 for YAP versus other Hippo pathway components not tested

• No structural or biochemical reconstitution data exist for USP40; the catalytic mechanism, substrate selectivity determinants, and regulation of USP40 enzymatic activity remain undefined at the molecular level.
• No crystal or cryo-EM structure of USP40
• In vitro reconstitution of deubiquitinase activity with purified components not reported
• Relative specificity across its multiple identified substrates has not been compared under standardized conditions