CNNM4

CNNM4 is a plasma-membrane Mg²⁺ efflux transporter that couples intracellular magnesium homeostasis to diverse signaling outputs including AMPK/mTOR energy metabolism, Ca²⁺ influx, and EGFR activation in epithelia, sperm, adipocytes, and hepatocytes (PMID:25347473, PMID:27006114, PMID:30670776, PMID:39517124). Its CBS-pair domain binds ATP and is the site of pathogenic missense variants that reduce protein and mRNA stability and impair Mg²⁺ extrusion without mislocalizing the transporter (PMID:39580587). Basolateral sorting in polarized epithelia depends on three dileucine motifs recognized by AP-1A and AP-1B clathrin adaptor complexes, and the oncogenic phosphatase PRL directly binds CNNM4 to inhibit Mg²⁺ efflux, thereby promoting tumor-associated metabolic reprogramming (PMID:25449265, PMID:25347473). Loss-of-function mutations in CNNM4 cause Jalili syndrome, a recessive disorder of cone-rod dystrophy and amelogenesis imperfecta, reflecting the transporter's essential roles in retinal photoreceptors and ameloblasts (PMID:19200525, PMID:19200527).

1. 2005 Medium

   Before CNNM4's transport function was known, a yeast two-hybrid screen identified the metal chaperone COX11 as a physical interactor and showed that co-expression with CNNM4 sensitized cells to copper, manganese, and cobalt, providing the first evidence that CNNM4 participates in metal ion homeostasis.

   Evidence Yeast two-hybrid screen plus metal toxicity assay in HEK293 cells

   PMID:15840172

   Open questions at the time

   • No direct metal transport assay was performed
   • COX11 interaction not validated by reciprocal Co-IP
   • Relevance to Mg²⁺ versus other divalent cations unclear
2. 2009 High

   Positional cloning in multiple consanguineous families established CNNM4 as the causative gene for Jalili syndrome (cone-rod dystrophy with amelogenesis imperfecta), linking a putative metal transporter to both retinal and dental mineralization biology.

   Evidence Homozygosity mapping and candidate-gene sequencing in seven independent families by two groups, with expression confirmed in neural retina and ameloblasts

   PMID:19200525 PMID:19200527

   Open questions at the time

   • The mechanism by which CNNM4 loss causes photoreceptor degeneration was not resolved
   • Which metal ion CNNM4 transports was still unknown
3. 2011 Medium

   Crystallization of the CBS-pair regulatory domain provided the first structural information on CNNM4, establishing the feasibility of atomic-resolution studies of its regulatory module.

   Evidence Protein purification, crystallization, and preliminary X-ray diffraction at 3.6 Å

   PMID:21393841

   Open questions at the time

   • Resolution too low for detailed atomic model
   • No ligand-bound structure obtained
   • Functional consequences of CBS domain conformation not tested
4. 2014 High

   Two concurrent studies resolved the core molecular function and trafficking mechanism of CNNM4: it mediates Mg²⁺ efflux at the plasma membrane, the PRL phosphatase binds and inhibits this efflux to activate AMPK/mTOR signaling in colorectal tumors, and basolateral sorting in polarized epithelia requires three dileucine motifs recognized by AP-1A/AP-1B adaptors.

   Evidence Co-IP of PRL–CNNM4, cellular Mg²⁺ flux assays, Cnnm4-KO mice on Apc-mutant background with rapamycin rescue; dileucine mutagenesis with μ1A/μ1B siRNA in MDCK cells

   PMID:25347473 PMID:25449265

   Open questions at the time

   • Direct structural basis of PRL–CNNM4 inhibition was not determined
   • Whether AP-1A and AP-1B are fully redundant or context-dependent remained unresolved
   • Whether Mg²⁺ transport is electrogenic or coupled to a counter-ion was not established
5. 2016 High

   CNNM4 was shown to be required for Ca²⁺ influx during sperm capacitation, revealing that its Mg²⁺ efflux activity indirectly regulates Ca²⁺ signaling; Cnnm4-deficient sperm phenocopied CatSper-null sperm, and forced Ca²⁺ entry rescued tyrosine phosphorylation.

   Evidence Germline and germ-cell-specific Cnnm4 KO mice with Ca²⁺ imaging, motility analysis, and forced Ca²⁺ entry rescue

   PMID:27006114

   Open questions at the time

   • Molecular mechanism linking Mg²⁺ efflux to CatSper-dependent Ca²⁺ influx was not identified
   • Whether CNNM4 loss contributes to human male infertility was not tested
6. 2018 Medium

   A physical interaction between CNNM4 and IQCB1 (a Leber congenital amaurosis gene product) was identified, and a Jalili-associated truncation (p.R605X) enhanced this interaction and increased apoptosis, suggesting a functional bridge between the two retinal-disease pathways.

   Evidence Co-IP of CNNM4 and IQCB1, apoptosis assay with truncated construct

   PMID:29322253

   Open questions at the time

   • Single Co-IP without reciprocal validation
   • Physiological relevance of enhanced interaction to retinal degeneration not demonstrated in vivo
   • IQCB1 interaction not confirmed by an independent group
7. 2019 High

   In colonic epithelium, CNNM4 deficiency was shown to suppress TRPV1-mediated Ca²⁺ influx and constitutively activate EGFR signaling, directly linking CNNM4-dependent Mg²⁺ homeostasis to proliferative control; gefitinib rescue proved EGFR as the downstream effector.

   Evidence Cnnm4-KO mice, organoid Ca²⁺ imaging, EGFR phosphorylation assay, gefitinib pharmacological rescue

   PMID:30670776

   Open questions at the time

   • How altered intracellular Mg²⁺ suppresses TRPV1 activity at the channel level is unknown
   • Whether this EGFR axis operates in non-colonic epithelia was not tested
8. 2024 High

   Multiple studies expanded CNNM4's physiological scope: pathogenic CBS-domain missense variants were shown to reduce protein/mRNA stability (not localization) and impair Mg²⁺ extrusion, cold-induced CNNM4 expression in adipocytes established a paracrine Mg²⁺ signal that activates mTORC2 in macrophages, and hepatic CNNM4 silencing restored PCMT1 activity by improving mitochondrial SAM metabolism.

   Evidence Mg²⁺ efflux assay and stability measurements for CBS-domain variants; ADRB3-PKA-CREB pathway dissection with CNNM4 gain/loss-of-function in adipocytes and mTORC2 activity in macrophages; GalNAc-siRNA Cnnm4 silencing in alcohol-fed mice with PCMT1/SAM assays

   PMID:39517124 PMID:39580587 PMID:39641635

   Open questions at the time

   • Whether CNNM4 is the sole Mg²⁺ efflux pathway in adipocytes is unclear
   • The DFG-motif Mg²⁺ sensor model on mTOR lacks structural validation
   • Hepatic CNNM4 silencing effects on Mg²⁺-dependent enzymes beyond PCMT1 are unexplored
9. 2025 Medium

   Post-transcriptional regulation of CNNM4 by miRNAs was demonstrated through direct binding to its 3′UTR in hepatocytes, showing that cellular Mg²⁺ levels can be tuned at the mRNA level.

   Evidence High-throughput miRNA–3′UTR binding assay with cellular Mg²⁺ concentration measurement in hepatocytes

   PMID:40862638

   Open questions at the time

   • Specific miRNAs that are physiologically dominant in regulating CNNM4 in vivo are not identified
   • Whether miRNA regulation is tissue-specific was not addressed
10. 2026 Medium

    CNNM4 silencing in cholangiocarcinoma was shown to attenuate ferroptosis via a NUPR1-dependent pathway; iron chelation reversed the effect while HO-1 inhibition did not, establishing a Mg²⁺-linked ferroptosis axis distinct from canonical heme oxygenase pathways.

    Evidence siRNA/shRNA KD in CCA cell lines, GalNAc-siRNA in transposon-based mouse CCA model, proteomics, deferiprone rescue

    PMID:40764063

    Open questions at the time

    • Mechanism connecting Mg²⁺ efflux to NUPR1 expression is not resolved
    • Single-lab finding awaiting independent replication
    • Whether this ferroptosis link generalizes beyond cholangiocarcinoma is unknown

• Key open questions include the transport mechanism (electrogenic vs. coupled), a high-resolution full-length structure, the molecular basis of PRL-mediated inhibition, and whether CNNM4-dependent Mg²⁺ efflux directly or indirectly regulates CatSper and TRPV1 channel gating.
• No full-length CNNM4 structure exists
• Ion coupling stoichiometry and electrogenicity unresolved
• PRL inhibition mechanism at atomic detail unknown