CNNM2

CNNM2 is a basolateral membrane protein of the renal distal convoluted tubule that governs cellular and systemic Mg2+ homeostasis (PMID:21397062, PMID:33859252). In heterologous systems it mediates Mg2+-sensitive transmembrane cation currents and saturable Mg2+ flux (PMID:15899945, PMID:21397062), and in mammalian cells it promotes cellular Mg2+ uptake at least in part through regulation of the Mg2+-permeable channel TRPM7, while also supporting Mg2+ efflux (PMID:24699222, PMID:35846113). The protein adopts an extracellular N-terminus/intracellular C-terminus topology, is N-glycosylated at Asn-112 to stabilize its plasma-membrane expression, and forms homo- and heterodimers between its isoforms (PMID:22399287). Its intracellular CBS-domain (Bateman) module binds AMP/ADP/ATP — with Mg2+ facilitating ATP binding — and switches from a twisted to a flat disc-like conformation upon nucleotide binding, a transition that transmits regulatory signals toward the transmembrane region; the pathogenic T568I mutation locks this module in the nucleotide-bound flat state and disrupts Mg2+ transport regulation (PMID:25184538). The same Bateman module mediates a heterotetrameric interaction with the oncogenic phosphatase PRL-1, anchored by CBS2 residue Asp-558 (PMID:27899452). CNNM2 is essential for embryonic development and contributes to renal and intestinal Mg2+ handling, as homozygous knockout is largely perinatally lethal and produces hypomagnesemia with increased fecal Mg2+ and Ca2+ excretion (PMID:33859252). Independent of its transport role, CNNM2 acts directly in neurons, where reduced expression alters dendritic spine morphogenesis and impairs cognition and sensorimotor gating (PMID:37715107). Loss-of-function patient mutations recurrently reduce plasma-membrane expression, cause cytoplasmic mislocalization, and abolish the protein's ability to normalize intracellular Mg2+ (PMID:24699222, PMID:33600043, PMID:40612795).

1. 2005 Medium

   Established that CNNM2 itself can mediate Mg2+ movement, defining it as a candidate Mg2+ transport entity with broad divalent-cation selectivity.

   Evidence Two-electrode voltage-clamp of mouse ACDP2/CNNM2 in Xenopus oocytes with substrate characterization

   PMID:15899945

   Open questions at the time

   • Single heterologous system, single lab
   • Did not establish whether currents reflect direct transport or channel regulation
   • No in vivo or renal context
2. 2010 Medium

   Showed that only CNNM2 isoform 1 is a functional Mg2+-transporting entity, distinguishing functional from non-functional splice variants.

   Evidence Bacterial complementation in Salmonella MM281 and Mg2+ influx measurement

   PMID:20519162

   Open questions at the time

   • Bacterial surrogate may not reflect mammalian function
   • Did not resolve direct transport vs. regulatory role
3. 2011 High

   Localized CNNM2 to the basolateral membrane of distal renal tubule and linked patient mutations to diminished Mg2+-sensitive currents, anchoring it as a physiological renal Mg2+ handler.

   Evidence Immunolocalization plus electrophysiology of wild-type vs. mutant CNNM2 in kidney epithelial cells across two families

   PMID:21397062

   Open questions at the time

   • Mechanism by which Mg2+ sensitivity is conferred not defined
   • Did not establish whether CNNM2 is a transporter or regulator
4. 2012 High

   Defined the membrane topology, N-glycosylation at Asn-112 required for plasma-membrane stability, and isoform heterodimerization, providing the structural framework for the protein's surface expression.

   Evidence Epitope-insertion topology mapping, glycosylation-site mutagenesis, and co-immunoprecipitation

   PMID:22399287

   Open questions at the time

   • Functional consequence of heterodimerization on transport not established
   • Did not address the intracellular regulatory domain function
5. 2014 High

   Placed CNNM2 in a Mg2+ uptake pathway via TRPM7 and demonstrated organismal requirement, showing patient mutations are loss-of-function for cellular Mg2+ uptake and neurodevelopment.

   Evidence Stable Mg2+ isotope uptake in HEK293 cells and zebrafish morpholino knockdown with WT/mutant rescue

   PMID:24699222

   Open questions at the time

   • Whether CNNM2 directly transports Mg2+ or only regulates TRPM7 unresolved
   • Molecular basis of TRPM7 regulation not defined
6. 2014 High

   Revealed nucleotide-gated conformational regulation of the CBS/Bateman module and explained a pathogenic mutation as a conformational lock, establishing the intracellular regulatory mechanism.

   Evidence X-ray crystallography of apo and nucleotide-bound Bateman module and structural comparison with the T568I mutant

   PMID:25184538

   Open questions at the time

   • How the conformational change is transmitted to the transmembrane region remains structurally unmapped
   • Full-length structure unavailable
7. 2016 High

   Defined the structural basis of the CNNM2–PRL-1 interaction, linking CNNM2 to oncogenic phosphatase signaling through a heterotetrameric interface.

   Evidence Crystal structure of PRL-1/CNNM2 Bateman module complex with Asp-558 mutagenesis

   PMID:27899452

   Open questions at the time

   • Functional consequence of PRL-1 binding on CNNM2 transport not resolved
   • In vivo relevance of the interaction not tested here
8. 2016 Medium

   Challenged the direct-transporter model by failing to detect Mg2+ transport by either human isoform and observing broad subcellular distribution and higher-order complexes.

   Evidence Electrogenic and electroneutral Mg2+ transport assays, fractionation, and complex analysis in HEK293 cells

   PMID:27068403

   Open questions at the time

   • Negative result from a single lab, contradicts earlier transport findings
   • Interactors not identified
   • Reason for discrepancy with prior assays unresolved
9. 2021 High

   Established the organismal physiology of CNNM2 loss, showing perinatal lethality and disturbed renal and intestinal Mg2+/Ca2+ handling.

   Evidence CRISPR/Cas9 Cnnm2 knockout mice with serum/fecal electrolyte and microCT analysis

   PMID:33859252

   Open questions at the time

   • Cause of perinatal lethality not defined
   • Tissue-specific contributions not dissected
10. 2021 Medium

    Demonstrated a transport-independent neuronal role for CNNM2 in dendritic spine morphogenesis and cognition.

    Evidence In vivo mPFC knockdown with behavioral assays, primary neuron knockdown morphology, and proteomics

    PMID:37715107

    Open questions at the time

    • Molecular mechanism in neurons not defined
    • Relationship to Mg2+ transport unclear
    • Single lab
11. 2021 Medium

    Consolidated the loss-of-function mechanism for patient variants as reduced plasma-membrane expression and failure to raise cellular Mg2+ uptake.

    Evidence 25Mg2+ uptake assays and plasma-membrane immunofluorescence in HEK293 cells across multiple variants

    PMID:33600043

    Open questions at the time

    • Does not distinguish trafficking defect from intrinsic transport defect
    • Single lab
12. 2022 Medium

    Provided direct evidence for a Mg2+ efflux role using a relevant DCT cell type, linking a CBS-domain mutation to impaired ATP-Mg2+ binding and intracellular Mg2+ accumulation.

    Evidence MgGreen efflux assay in mDCT cells with WT vs. R480L CNNM2 and molecular dynamics simulation

    PMID:35846113

    Open questions at the time

    • Efflux mechanism supported partly by computation
    • Direct vs. regulatory efflux not distinguished
    • Single lab
13. 2025 Medium

    Showed that some pathogenic variants act by mislocalization and cytoplasmic clumping despite elevated expression, causing intracellular Mg2+ accumulation.

    Evidence qPCR, Western blot, immunofluorescence, and flow cytometry in cells expressing E298del and P360R variants

    PMID:40612795

    Open questions at the time

    • No direct transport reconstitution
    • Mechanism linking mislocalization to Mg2+ accumulation not defined

• Whether CNNM2 is itself a Mg2+ transporter or primarily a regulator of channels such as TRPM7 — and how nucleotide-driven CBS-domain conformational change is mechanically coupled to transmembrane Mg2+ flux — remains unresolved.
• No full-length structure
• Conflicting transport assay outcomes across labs
• Direct vs. regulatory transport mechanism undefined