LIN7C

LIN7C (MALS-3/Veli3) is a PDZ/L27-domain scaffold protein that organizes membrane polarity complexes at epithelial junctions and neuronal synapses (PMID:17923534). It is the only shared core component of both the Crumbs apical tight junction complex and the Discs Large basolateral polarity complex in mammalian kidney, where it mediates stable assembly of both; loss of LIN7C disrupts both complexes and produces hypomorphic, cystic, fibrotic kidneys (PMID:17923534). LIN7C does not localize autonomously but is recruited to specific membrane domains by L27-domain heterodimerization with MAGUK partners: MPP4 positions it at presynaptic photoreceptor terminals (PMID:16520334, PMID:15558731), the Crumbs members Crb2b and MPP5/Pals1 drive its apical localization in retinal epithelium (PMID:24143272), and CASK targets it to basolateral membranes in intestinal epithelium (PMID:19726564). Through this scaffolding role it controls apical localization of PATJ and PALS1 in cortical neural progenitors, where its loss slows cell cycle progression and causes premature neuronal differentiation (PMID:18403412). LIN7C also facilitates Golgi-to-lateral-membrane trafficking of the GPCR BLT2, with its depletion trapping BLT2 in the Golgi and impairing epithelial barrier function (PMID:33481310). In cancer and retinal pigment epithelial cells LIN7C acts through the β-catenin pathway to suppress epithelial-mesenchymal transition, invasion, and metastasis (PMID:17942893, PMID:35761801).

1. 2005 Medium

   Established that LIN7C physically engages MAGUK scaffolds via L27-domain heterodimerization, defining its mode of membrane recruitment.

   Evidence In vitro L27 heterodimerization assay with MPP4 plus immunofluorescence colocalization in mouse retina

   PMID:15558731

   Open questions at the time

   • Did not establish whether the interaction recruits LIN7C in vivo
   • Distinct MPP4 vs MPP5 junctional contexts not mechanistically resolved
2. 2006 Medium

   Demonstrated that MPP4 recruits LIN7C to the synaptic membrane in vivo, showing LIN7C localization is partner-dependent rather than autonomous.

   Evidence Mpp4 knockout mouse with immunofluorescence and western blot showing mislocalization without protein loss

   PMID:16520334

   Open questions at the time

   • Functional consequence of LIN7C mislocalization for synaptic transmission not addressed
   • Single tissue (photoreceptor OPL)
3. 2007 High

   Identified LIN7C as the shared scaffold linking the Crumbs and Discs Large polarity complexes and required for their assembly, defining its core organizing role in epithelial polarity.

   Evidence MALS-3 knockout mice with reciprocal mass spectrometry proteomics, immunofluorescence, and renal histology

   PMID:17923534

   Open questions at the time

   • Structural basis for simultaneous engagement of both complexes not resolved
   • Whether assembly defect is direct or secondary to mislocalization unclear
4. 2007 Medium

   Connected LIN7C to suppression of invasion and metastasis through the β-catenin pathway, extending its role beyond polarity scaffolding.

   Evidence Inducible LIN7C overexpression in OSCC cells with in vivo metastasis assay and β-catenin western blot

   PMID:17942893

   Open questions at the time

   • Mechanistic link between scaffold function and β-catenin regulation undefined
   • Gain-of-function only; loss-of-function not tested
5. 2008 Medium

   Showed LIN7C is required for apical localization of PATJ/PALS1 in neural progenitors and links polarity to cell cycle and differentiation timing.

   Evidence MALS triple knockout mice with immunofluorescence, BrdU labeling, and cell cycle analysis in developing cortex

   PMID:18403412

   Open questions at the time

   • Redundancy among MALS paralogs requires triple knockout, obscuring LIN7C-specific contribution
   • Direct vs indirect coupling of polarity to cell cycle not resolved
6. 2009 Medium

   Defined tissue-specific and partner-specific recruitment: CASK targets LIN7C basolaterally in intestine but not stomach, and LIN7C/CASK localization is mutually independent of DLG1.

   Evidence Intestine-specific CASK knockout and mosaic mouse models with immunofluorescence

   PMID:19726564

   Open questions at the time

   • Identity of CASK-independent recruiting factor in stomach unknown
   • Functional consequence of basolateral mislocalization not characterized
7. 2013 Medium

   Demonstrated LIN7C is a dynamic Crumbs complex member whose apical recruitment depends on Crb2b and MPP5 as polarity matures.

   Evidence Live RFP-Lin7c imaging with crb/mpp5 genetic epistasis in zebrafish retinal epithelium

   PMID:24143272

   Open questions at the time

   • Trigger for cytosolic-to-apical transition not identified
   • Whether dynamics are conserved in mammalian epithelia not shown
8. 2021 Medium

   Revealed a trafficking function: LIN7C mediates Golgi-to-lateral-membrane transport of the GPCR BLT2 and supports epithelial barrier integrity.

   Evidence LIN7C siRNA knockdown with chimeric/deletion receptor analysis, APEX2 proximity proteomics, and immunofluorescence

   PMID:33481310

   Open questions at the time

   • Whether LIN7C traffics other cargoes beyond BLT2 unknown
   • Direct vs scaffold-mediated role in transport not distinguished
9. 2022 Medium

   Placed LIN7C under miR-124 control and confirmed its suppression of EMT/migration through epithelial marker regulation.

   Evidence Luciferase reporter validation of miR-124 targeting, LIN7C overexpression, transwell migration, EMT marker western blot in retinal pigment epithelial cells

   PMID:35761801

   Open questions at the time

   • Mechanistic link between LIN7C scaffolding and EMT marker changes not defined
   • Single cell context
10. 2014 Low

    Proposed HTR2C as an upstream negative regulator of the LIN7C/β-catenin axis, pharmacologically targetable to suppress metastasis.

    Evidence Mirtazapine treatment in vitro and in vivo metastasis assay with western blot and pathway analysis

    PMID:24961284

    Open questions at the time

    • Pathway inference is indirect; direct HTR2C–LIN7C regulatory link not demonstrated
    • Drug effects may act through targets other than LIN7C
    • Not independently confirmed

• How LIN7C structurally coordinates simultaneous engagement of distinct MAGUK partners and polarity complexes, and how its scaffold function mechanistically connects to β-catenin/EMT control, remains unresolved.
• No structural model of multi-partner assembly
• Direct molecular link between scaffolding and β-catenin pathway undefined
• Cargo repertoire for trafficking function unknown