MCOLN2

MCOLN2 encodes TRPML2, an inwardly rectifying, nonselective cation channel permeable to Ca2+ that operates in the endolysosomal system of immune cells to control cargo recycling and innate immune signaling (PMID:19940139, PMID:26432893, PMID:30479274). The channel is constitutively active at the plasma membrane and conducts Ca2+, with constitutive activity sufficient to cause Ca2+-overload cell death, and a varitint-waddler (Ala→Pro) substitution in TM5 produces a gain-of-function channel (PMID:19940139); gating is tuned by extracytosolic pH, low extracellular sodium, and selective small-molecule agonists shared with TRPML3 (PMID:19940139, PMID:22753890). Structural work defines a homotetramer whose large extracytosolic/lumenal domain binds Ca2+ at an acidic pre-pore loop in a pH-dependent manner, and full-length cryo-EM captures the inactive apo state and the ion-conducting pathway (PMID:31178222, PMID:34915027). TRPML2 also behaves as a hypotonicity/mechanosensitive channel that requires an intact phosphoinositide-binding pocket (abolished by L314R), and this mechanosensitivity is functionally needed for fast endolysosomal recycling (PMID:33177082). In macrophages the endogenous channel localizes to early/recycling endosomes, and its activation—genetically or by the selective agonist ML2-SA1—drives CCL2 chemokine secretion and macrophage migration, with TRPML2-knockout mice showing reduced CCL2 and impaired macrophage recruitment to LPS or bacteria (PMID:26432893, PMID:30479274). Beyond Ca2+, TRPML2 conducts Mg2+ out of endolysosomes to restrict intracellular Salmonella Typhi replication through magnesium deprivation (PMID:37228749). TRPML2 forms homo- and heteromultimers with TRPML1 and TRPML3 and dictates lysosomal targeting of TRPML3 (PMID:16606612), and MCOLN2 transcription is driven by PAX5 acting at a proximal promoter element (PMID:25445271). In cancer contexts TRPML2 activates IL-1β/NF-κB signaling in prostate cancer and the VEGFA/Notch2 axis in glioblastoma (PMID:34548638, PMID:35054871).

1. 2006 High

   Established that TRPML2 assembles into homo- and heteromultimers and acts as a dominant determinant of subcellular targeting within the TRPML family, framing it as a lysosomal channel that organizes its paralogs.

   Evidence Reciprocal co-immunoprecipitation and coexpression microscopy with localization mutants across TRPML1/2/3 combinations

   PMID:16606612

   Open questions at the time

   • Did not establish channel activity or physiological function
   • Localization assessed under overexpression, not endogenous conditions
2. 2009 High

   Defined TRPML2 biophysically as a constitutively active, inwardly rectifying, nonselective Ca2+-permeable channel regulated by pH, answering whether MCOLN2 encodes a functional ion channel.

   Evidence Whole-cell patch-clamp electrophysiology with site-directed mutagenesis and viability assays in heterologous mammalian cells

   PMID:19940139

   Open questions at the time

   • Recordings at the plasma membrane rather than the native endolysosomal compartment
   • Endogenous activators and physiological gating context not defined
3. 2009 Medium

   Linked TRPML2 expression to TRPML1 activity, raising the possibility of cross-regulation within the channel family at the transcriptional level.

   Evidence RNAi knockdown with overexpression rescue and pharmacological TRPML1 activation, monitored by qRT-PCR

   PMID:19763610

   Open questions at the time

   • Transcriptional mechanism linking TRPML1 activity to MCOLN2 expression unresolved
   • Single-lab, cell-type-restricted observation
4. 2012 Medium

   Identified low extracellular sodium and TRPML3-class small molecules as activators, indicating shared gating determinants and providing pharmacological tools.

   Evidence Whole-cell patch-clamp with a small-molecule screen and mutagenesis of extracellular loop residues

   PMID:22753890

   Open questions at the time

   • Sodium-block residue mapping done on TRPML3, not directly on TRPML2
   • Physiological relevance of sodium sensitivity not established
5. 2014 Medium

   Identified PAX5 as the transcriptional activator of MCOLN2, explaining its restricted/immune-cell expression pattern at the promoter level.

   Evidence Luciferase reporter assays, PAX5 overexpression and RNAi, and promoter site-directed mutagenesis

   PMID:25445271

   Open questions at the time

   • Direct PAX5 occupancy at the endogenous promoter (e.g. ChIP) not shown
   • Whether PAX5 fully accounts for tissue specificity unaddressed
6. 2015 High

   Placed TRPML2 in recycling endosomes of macrophages and tied it to innate immunity via CCL2 production and macrophage recruitment, defining its first in vivo function.

   Evidence Endogenous immunofluorescence, TRPML2-knockout mice, chemokine measurements, and in vivo macrophage recruitment assays

   PMID:26432893

   Open questions at the time

   • Mechanistic link from channel activity to CCL2 secretion not yet resolved
   • Ion-flux dependence of the phenotype not directly tested
7. 2018 High

   Connected TRPML2 channel activity directly to CCL2 secretion and migration using an isoform-selective agonist, demonstrating that activation accelerates early/recycling endosomal trafficking.

   Evidence Endolysosomal patch-clamp of endogenous channel, ML2-SA1 agonist, chemokine ELISA, migration and fluorescence trafficking assays

   PMID:30479274

   Open questions at the time

   • Cargo identity and the recycling step coupling Ca2+ flux to CCL2 export not defined
   • Whether Ca2+ versus other ions drives the effect not separated
8. 2019 High

   Provided a structural basis for pH-dependent regulation by resolving the extracytosolic/lumenal domain and its acidic Ca2+-binding pre-pore loop.

   Evidence X-ray crystallography at two pH values with ITC, SAXS, and native mass spectrometry

   PMID:31178222

   Open questions at the time

   • Isolated lumenal domain, not the full-length channel in membrane
   • Gating transitions coupled to Ca2+/pH binding not visualized
9. 2020 High

   Identified TRPML2 as a hypotonicity/mechanosensitive channel whose phosphoinositide-binding pocket is required for fast endolysosomal recycling, linking a gating stimulus to a cellular output.

   Evidence Endolysosomal patch-clamp, L314R mutagenesis, hypotonic stimulation, and recycling assays in immune cells

   PMID:33177082

   Open questions at the time

   • How membrane tension is sensed structurally remains undefined
   • Physiological source of the hypotonic/tension stimulus in vivo unclear
10. 2021 High

    Resolved the full-length homotetrameric channel in lipid nanodiscs, defining the apo conformation and ion-conducting pathway and enabling family-wide structural comparison.

    Evidence Cryo-EM of full-length mouse TRPML2 reconstituted in lipid nanodiscs

    PMID:34915027

    Open questions at the time

    • Captured only the inactive apo state; activated/agonist-bound states not solved
    • Mechanosensory and PIP2-engaged conformations not visualized
11. 2023 High

    Demonstrated that TRPML2 conducts Mg2+ out of endolysosomes to restrict intracellular Salmonella replication, extending its ion repertoire and role to nutritional immunity.

    Evidence cGWAS, endolysosomal patch-clamp measurement of Mg2+ currents, bacterial transcriptomics, and magnesium availability manipulation

    PMID:37228749

    Open questions at the time

    • Relationship between Mg2+ conduction and the established Ca2+/recycling functions unresolved
    • Selectivity mechanism for Mg2+ not structurally defined
12. 2021 Medium

    Implicated TRPML2 in tumor growth via IL-1β production and NF-κB activation, extending its signaling role beyond normal immunity.

    Evidence MCOLN2 knockdown/overexpression, cytokine array/ELISA, Ca2+ release, NF-κB luciferase reporter, and xenograft model

    PMID:34548638

    Open questions at the time

    • Direct mechanistic chain from channel flux to NF-κB not dissected
    • Single-lab, single-cancer-type evidence
13. 2022 Medium

    Tied TRPML2 to VEGFA/Notch2 angiogenic signaling and EMT regulation in glioblastoma, and showed functional interdependence with TRPML1.

    Evidence siRNA single/double knockdown with epistasis, overexpression, ML2-SA1, confocal colocalization, ELISA, and invasion assays

    PMID:35054871 PMID:35887088

    Open questions at the time

    • Direction of channel-to-pathway causality not fully established
    • Mutual stabilization mechanism of TRPML1/2 protein levels unknown
14. 2026 Medium

    Defined a sphingomyelin–TRPML2 axis controlling lysosomal Ca2+ release and membrane integrity, relevant to anti-CD20 antibody-induced lysosomal stress.

    Evidence Colocalization imaging, genetic TRPML2 manipulation, sphingomyelinase rescue, Ca2+ release and LMP assays

    PMID:41634107

    Open questions at the time

    • Whether sphingomyelin acts directly on the channel or via membrane environment unresolved
    • Single-study, antibody-specific context
15. 2026 Medium

    Identified scorpion venom peptides as TRPML2 agonists whose potency correlates with antiviral activity against Zika virus, expanding the agonist toolkit and an antiviral link.

    Evidence Co-IP/LC-MS/MS binding screen, molecular docking, calcium imaging, and antiviral replication assays

    PMID:41745776

    Open questions at the time

    • Mechanism connecting TRPML2 Ca2+ flux to ZIKV restriction not established
    • Direct binding site and selectivity over other TRPMLs not defined

• How distinct gating stimuli (pH, mechanical tension, PIP2, sodium, agonists) and distinct conducted ions (Ca2+ versus Mg2+) are integrated to produce specific cargo-recycling and immune outputs remains unresolved.
• No activated-state structure linking gating stimuli to pore opening
• Cargo/effector machinery coupling channel flux to CCL2 secretion not identified
• Functional separation of Ca2+- versus Mg2+-dependent roles not achieved