TMEM168

TMEM168 is a nuclear membrane-localized transmembrane protein that acts as a scaffold controlling the stability of the cardiac sodium channel Nav1.5 (SCN5A) and that modulates neurotransmission and tumor cell proliferation in other contexts (PMID:32175648, PMID:29211814, PMID:30940290). In cardiomyocytes it localizes to the nuclear membrane, and a heterozygous R539Q mutation reduces Nav1.5 protein and Na+ current by increasing Nedd4-2 binding to Nav1.5 and promoting its ubiquitination and degradation (PMID:32175648). Mechanistically, the R539Q mutant binds αB-crystallin with higher affinity than wild-type protein and sequesters it to the perinuclear region, diminishing αB-crystallin's ability to compete with Nedd4-2 for Nav1.5, so that proteasome-dependent Nav1.5 degradation increases and cell-surface channel expression falls (PMID:34086898). In the nucleus accumbens, TMEM168 binds the matrix protein osteopontin and raises its levels, and its overexpression lowers basal and stimulated GABA release and blunts methamphetamine-induced dopamine elevation (PMID:29026117, PMID:29211814). In glioblastoma cells, TMEM168 supports proliferation upstream of the Wnt/β-catenin pathway, since its knockdown lowers β-catenin, C-myc, cyclin D1 and survivin, arrests cells in G0/G1 and triggers apoptosis, effects rescued by the Wnt activator LiCl (PMID:30940290). Beyond these contexts, no unifying molecular activity for TMEM168 has been characterized in the available corpus.

1. 2017 Medium

   Establishing a physical binding partner gave TMEM168 its first molecular handle and tied it to extracellular matrix signaling and dopaminergic function in the brain.

   Evidence Co-immunoprecipitation in COS-7 cells and AAV overexpression in mouse nucleus accumbens with biochemical and in vivo microdialysis readouts

   PMID:29026117

   Open questions at the time

   • Direct binding interface and stoichiometry with osteopontin undefined
   • Mechanism linking osteopontin binding to dopamine modulation unresolved
   • Gain-of-function only; loss-of-function phenotype untested
2. 2017 Medium

   Linking TMEM168 to GABA release extended its neuromodulatory role beyond dopamine to inhibitory neurotransmission.

   Evidence AAV-mediated overexpression in mouse nucleus accumbens with microdialysis measurement of extracellular GABA

   PMID:29211814

   Open questions at the time

   • Molecular mechanism by which TMEM168 suppresses GABA release unknown
   • Whether the effect is cell-autonomous to GABAergic neurons untested
3. 2019 Medium

   A loss-of-function screen in tumor cells placed TMEM168 upstream of a defined proliferative signaling axis, distinguishing a cancer-relevant role.

   Evidence siRNA knockdown in U87/U373 GBM cells with cell cycle, apoptosis, Wnt-component western blots, and LiCl epistasis rescue

   PMID:30940290

   Open questions at the time

   • Direct biochemical link between TMEM168 and Wnt/β-catenin components not identified
   • Whether effect generalizes beyond GBM lines untested
4. 2020 High

   Connecting TMEM168 to Nav1.5 turnover via Nedd4-2 defined a concrete cardiac mechanism and a disease-linked mutation, the strongest mechanistic anchor in the timeline.

   Evidence Immunofluorescence localization, whole-cell patch clamp, co-IP, ubiquitination assay, and Tmem168 R539Q knock-in mouse cardiomyocyte electrophysiology

   PMID:32175648

   Open questions at the time

   • Whether wild-type TMEM168 directly contacts Nav1.5 or Nedd4-2 not shown
   • Structural basis of nuclear membrane localization unresolved
5. 2021 Medium

   Identifying αB-crystallin sequestration explained how the R539Q mutation shifts the balance toward Nedd4-2-driven Nav1.5 degradation, completing the cardiac mechanistic model.

   Evidence Co-IP of WT/mutant TMEM168 with αB-crystallin, αB-crystallin knockdown, localization imaging, and MG-132 proteasome rescue in HL-1 cardiomyocytes

   PMID:34086898

   Open questions at the time

   • Quantitative affinity difference between WT and mutant not measured
   • Single lab; reciprocal validation of competition model pending

• It remains unknown whether TMEM168's roles in cardiac channel stability, NAc neurotransmission, and GBM proliferation share a common molecular activity or reflect context-specific scaffolding functions.
• No unifying biochemical activity or domain function defined across tissues
• No structural model of the transmembrane protein