MPP3

MPP3 is a MAGUK-family scaffolding protein that organizes apical junctional complexes and multi-protein signaling assemblies at cell-cell contact sites in epithelia, retina, and developing cortex. At the retinal outer limiting membrane, MPP3 is recruited to the CRB1/MPP5 scaffold, maintains PALS1 (MPP5) levels in Müller glia, and its conditional loss causes late-onset retinal degeneration accelerated by light exposure (PMID:16519681, PMID:23893895). In cortical progenitors, MPP3 is required for adherens junction integrity, proper spindle orientation, and neuronal migration; conditional knockout produces ectopic mitoses and defective cortical stratification (PMID:23658188). MPP3 also bridges CADM1 to PI3K via a CADM1–MPP3–Dlg–p85 complex that activates Akt/Rac1 signaling and actin reorganization for epithelial cell spreading, and it modulates 5-HT2C receptor surface expression and desensitization through its PDZ domain (PMID:24503895, PMID:16914526).

1. 2003 Medium

   Identifying MPP3 as a direct PDZ-dependent binding partner of CADM1 (TSLC1) at cell-cell contacts established that this MAGUK scaffold operates in an intercellular adhesion cascade in epithelial cells.

   Evidence Reciprocal Co-IP and GST pulldown with PDZ-motif deletion controls in lung epithelial cells

   PMID:13679854

   Open questions at the time

   • No in vivo validation of the CADM1–MPP3 interaction
   • Downstream signaling consequences of the interaction were unknown
2. 2006 Medium

   Demonstrating that MPP3 localizes to the subapical region and photoreceptor synapse in retina, where it is recruited to the MPP5/CRB1 scaffold, placed MPP3 within the Crumbs polarity complex and suggested tissue-specific complex assembly with DLG1.

   Evidence Immunofluorescence in mouse and human retinae with endogenous Co-IP confirming MPP3–MPP5 and MPP3–DLG1 complexes

   PMID:16519681

   Open questions at the time

   • Loss-of-function consequences in the retina were not yet established
   • Stoichiometry and direct versus bridged interactions within the complex were unresolved
3. 2006 Medium

   Showing that MPP3 opposes PSD-95 by preventing 5-HT2C receptor desensitization and increasing its surface expression revealed a PDZ-dependent role for MPP3 in GPCR trafficking and signaling in neurons.

   Evidence Ca²⁺ response assays in HEK293 cells and primary cortical neurons with Tat-peptide PDZ inhibitor controls

   PMID:16914526

   Open questions at the time

   • Mechanism by which MPP3 stabilizes receptor surface expression (endocytic versus recycling) was not delineated
   • In vivo neuronal phenotype of MPP3 loss on serotonergic signaling was not tested
4. 2013 High

   Conditional knockout of Mpp3 in cortical progenitors proved that MPP3 is essential for maintaining the apical protein complex, adherens junctions, and correct spindle orientation, resolving whether it is required or redundant for cortical development.

   Evidence Cre-lox conditional KO in mouse cortex with immunofluorescence, BrdU/EdU cell cycle, and spindle orientation analysis

   PMID:23658188

   Open questions at the time

   • Which specific apical complex proteins are direct MPP3 interactors versus secondarily affected was not distinguished
   • Postnatal cortical circuit consequences were not assessed
5. 2013 High

   Conditional deletion of Mpp3 in retina demonstrated that MPP3 maintains PALS1 levels at the subapical region and that its loss causes light-accelerated retinal degeneration, with genetic epistasis showing synergy with Pals1 haploinsufficiency.

   Evidence Conditional KO mouse with ERG, EM ultrastructure, immunofluorescence, and Mpp3/Pals1 double-knockout epistasis

   PMID:23893895

   Open questions at the time

   • Whether MPP3 stabilizes PALS1 by direct protein-protein interaction or indirectly through complex integrity was not resolved
   • Contribution of individual retinal cell types (photoreceptors vs. Müller glia) was not fully separated
6. 2014 Medium

   Identification of the CADM1–MPP3–Dlg–p85 quaternary complex linked MPP3 scaffolding to PI3K/Akt/Rac1 activation and actin reorganization, providing a signaling mechanism downstream of trans-homophilic CADM1 adhesion.

   Evidence Co-IP of the multi-protein complex with functional inhibitor epistasis (LY294002, Akt and Rac1 inhibitors) in cell spreading assays

   PMID:24503895 PMID:25268382

   Open questions at the time

   • Direct binding interfaces between MPP3 and p85 or Dlg were not mapped
   • In vivo relevance of this signaling axis was not tested
7. 2015 Medium

   Quantitative FRAP analysis showed that MPP3 exists in two kinetically distinct pools — a long-lived CADM1-bound complex (τ ~16 min) and a rapidly exchanging free pool (τ ~40–60 s) — establishing the dynamic behavior of MPP3 at adhesion sites.

   Evidence FRAP and FLIP live-cell imaging with exponential curve-fitting in cultured cells

   PMID:25780926

   Open questions at the time

   • Whether the two pools have distinct functional roles was not determined
   • Dynamics were not measured in polarized epithelial or retinal contexts
8. 2025 Medium

   Demonstration that MPP3 expression in enteric neural crest cells is downregulated by Schwann cell-derived extracellular vesicles carrying MIR22HG, and that MPP3 knockdown blocks ENCC proliferation, migration, and distal colon colonization, extended MPP3's functional importance to enteric nervous system development.

   Evidence siRNA knockdown in ENCCs, extracellular vesicle treatment, m6A sequencing, and in vivo mouse colonization assay

   PMID:41286351

   Open questions at the time

   • Downstream effectors of MPP3 in ENCC migration were not identified
   • Whether known MPP3 complexes (CADM1–Dlg–PI3K) operate in this context is unknown
   • Single lab; awaits independent replication

• The structural basis of MPP3 scaffold assembly — how its SH3, GUK, and PDZ domains simultaneously engage distinct partners to form tissue-specific complexes — and the full range of physiological contexts requiring MPP3 remain to be determined.
• No crystal structure or cryo-EM model of MPP3 in complex with any partner
• No systematic interactome beyond candidate-based approaches
• Relationship between retinal, cortical, and enteric functions has not been integrated mechanistically