NPEPPS

NPEPPS (PSA/MP100) is a predominantly cytoplasmic, zinc-dependent M1-family metalloaminopeptidase that proteolytically clears neurotoxic substrates and thereby protects against neurodegeneration (PMID:16950154, PMID:11435692). It directly proteolyzes tau in vitro, and its activity is protective in vivo: PSA loss exacerbates and PSA gain suppresses tau-induced neurodegeneration in Drosophila, while PSA overexpression in TAU(P301L) mice lowers total and hyperphosphorylated tau, delays paralysis, and preserves motor neurons (PMID:16950154, PMID:21320871). PSA likewise regulates the abundance and clearance of SOD1, and its expression is reduced in motor neurons of SOD1G93A mice and sporadic ALS patients (PMID:21548977). Beyond its catalytic role, NPEPPS has a distinct non-enzymatic function as a structural inhibitory partner of the volume-regulated anion channel (VRAC): cryo-EM shows three PSA molecules clamping a single SWELL1 (LRRC8A) hexamer to couple adjacent leucine-rich-repeat domains, and PSA suppresses basal VRAC activation through physical binding that does not require aminopeptidase activity (PMID:41371222). Through this VRAC interaction NPEPPS controls intracellular cisplatin concentrations, driving cisplatin resistance such that its depletion or pharmacologic inhibition resensitizes resistant bladder cancer cells, xenografts, and patient-derived organoids, with the NPEPPS/VRAC expression ratio predicting cisplatin response across cancers (PMID:38535994, PMID:39671496). NPEPPS also stabilizes NRF2 by suppressing its ubiquitination in hepatocytes, restraining diet-induced hepatic lipid accumulation (PMID:34048566). Puromycin selectively inhibits NPEPPS by binding near the active-site entrance to block substrate access rather than being hydrolyzed (PMID:33045297).

1. 1994 Medium

   Established the basic biochemical identity of the protein, showing that brain-derived MP100 is a broad-specificity metalloprotease, the founding observation of its enzymatic character.

   Evidence Protein purification from human brain with in vitro chromogenic peptide assay

   PMID:8198608

   Open questions at the time

   • Physiological substrates not defined
   • Subcellular compartment not resolved
2. 1999 Medium

   Linked MP100 to PSA at the sequence level and tested an amyloidogenic role, resolving whether the enzyme acts as a beta-secretase by showing it cleaves free Abeta-spanning peptides but not intact beta-APP.

   Evidence cDNA cloning, recombinant peptide cleavage assays, co-IP and colocalization with beta-APP in rat brain and HEK/neuroblastoma cells; synaptic localization by immunoelectron microscopy and synaptosomal co-fractionation

   PMID:10037494 PMID:10434003

   Open questions at the time

   • No beta-secretase activity on native substrate
   • Functional significance of synaptic localization untested
3. 2001 Medium

   Defined NPEPPS as a cytoplasmic zinc-dependent exopeptidase and mapped the gene, providing genetic and compartmental grounding.

   Evidence FISH chromosomal mapping and coding-region sequencing

   PMID:11435692

   Open questions at the time

   • Functional consequence of the position-140 polymorphism unknown
4. 2006 High

   Identified tau as a direct proteolytic substrate and demonstrated a protective role against neurodegeneration, establishing NPEPPS as a modifier of tauopathy.

   Evidence In vitro proteolysis of tau plus Drosophila loss/gain-of-function neurodegeneration assays

   PMID:16950154

   Open questions at the time

   • Cleavage sites on tau not mapped
   • Regulation of PSA activity in neurons unknown
5. 2011 High

   Extended the neuroprotective tau model to mammals and added SOD1 as a substrate, broadening NPEPPS into a clearance factor for multiple neurodegeneration-associated proteins.

   Evidence TAU(P301L) transgenic mouse crosses with phospho-TAU quantification, cell KD/OE; in vitro SOD1 proteolysis with KD/OE and ALS tissue immunohistochemistry

   PMID:21320871 PMID:21548977

   Open questions at the time

   • Whether reduced PSA causes or follows ALS pathology unresolved
   • SOD1 evidence from a single lab
6. 2021 Medium

   Revealed a non-proteolytic regulatory function in metabolism, showing NPEPPS stabilizes NRF2 to restrain hepatic lipid accumulation.

   Evidence Hepatocyte and ob/ob mouse KD/OE with NRF2 ubiquitination and lipid assays

   PMID:34048566

   Open questions at the time

   • Mechanism by which NPEPPS suppresses NRF2 ubiquitination unknown
   • Whether catalytic activity is required untested
7. 2024 High

   Connected NPEPPS to chemoresistance, establishing that it controls intracellular cisplatin levels via VRAC interaction and serves as a predictive biomarker and therapeutic target.

   Evidence Whole-genome CRISPR screen, reciprocal KD/KO/OE with intracellular cisplatin measurement, xenografts, PDOs, and NPEPPS/VRAC expression-ratio analysis across cancer cohorts

   PMID:38535994 PMID:39671496

   Open questions at the time

   • Molecular basis of VRAC regulation not yet structural
   • Whether catalytic activity contributes to cisplatin handling unclear
8. 2025 High

   Provided the structural and mechanistic basis for NPEPPS as a non-enzymatic VRAC subunit, showing physical clamping of SWELL1 hexamers suppresses channel activity independent of aminopeptidase function.

   Evidence Cryo-EM of the PSA–SWELL1 complex, co-IP, electrophysiology with overexpression/deletion, catalytic-dead mutant, and cGAMP transport assays

   PMID:41371222

   Open questions at the time

   • How VRAC binding integrates with cytoplasmic proteolytic roles unknown
   • Physiological triggers governing PSA–VRAC stoichiometry undefined

• It remains unknown whether NPEPPS engages additional regulatory peptides such as angiotensin II and how its dual catalytic and structural-scaffold roles are partitioned in vivo.
• Angiotensin II interaction from a single thermal-shift assay, not functionally validated
• No integrated model reconciling proteolytic, NRF2-stabilizing, and VRAC-scaffold functions