Out, in and back again: PtdIns(4,5)P(2) regulates cadherin trafficking in epithelial morphogenesis

Abstract

The morphogenesis of epithelial cells in the tissue microenvironment depends on the regulation of the forces and structures that keep cells in contact with their neighbours. The formation of cell-cell contacts is integral to the establishment and maintenance of epithelial morphogenesis. In epithelial tissues, the misregulation of the signalling pathways that control epithelial polarization induces migratory and invasive cellular phenotypes. Many cellular processes influence cadherin targeting and function, including exocytosis, endocytosis and recycling. However, the localized generation of the lipid messenger PtdIns(4,5)P(2) is emerging as a fundamental signal controlling all of these processes. The PtdIns(4,5)P(2)-generating enzymes, PIPKs (phosphatidylinositol phosphate kinases) are therefore integral to these pathways. By the spatial and temporal targeting of PIPKs via the actions of its functional protein associates, PtdIns(4,5)P(2) is generated at discrete cellular locales to provide the cadherin-trafficking machinery with its required lipid messenger. In the present review, we discuss the involvement of PtdIns(4,5)P(2) and the PIPKs in the regulation of the E-cadherin (epithelial cadherin) exocytic and endocytic machinery, the modulation of actin structures at sites of adhesion, and the direction of cellular pathways which determine the fate of E-cadherin and cell-cell junctions. Recent work is also described that has defined phosphoinositide-mediated E-cadherin regulatory pathways by the use of organismal models.

Figure 1. Specific phosphoinositide messengers regulate protein activities

Polyphosphoinositides can be generated from PtdIns3P (PI3P), PtdIns4P (PI4P) and PtdIns5P (PI5P) by enzymes of the PIPK family. Importantly, each of these polyphosphoinositides possesses a specific group of protein effectors within the cell, and the binding of these lipids to their protein partners acts to modulate protein activity. PI3,4,5P₃, PtdIns(3,4,5)P₃; PI3,5P₂, PtdIns(3,5)P₂; PI3,4P₂, PtdIns(3,4)P₂; PI4,5P₂, PtdIns(4,5)P₂.

Figure 2. PtdIns(4,5) P₂ generation is spatially and temporally regulated

In order to act as a succinct signalling molecule, PtdIns(4,5)P₂ (PIP₂) is generated only where and when it is needed for function. PIPKI activity is modulated downstream of signalling cues such as G-proteins, tyrosine and serine phosphorylation, or other post-translational modifications. PIPKIs are targeted to discrete subcellular domains by the binding of proteins that can act as targeting factors. At these sites, PtdIns(4,5)P₂ is generated for use by its effectors. In addition, many proteins which physically associate with PIPKIs are also PtdIns(4,5)P₂ effectors. PIP, PtdInsP. An animated version of this Figure can be found at http://www.BiochemJ.org/bj/418/0247/bj4180247add.htm.

Figure 3. AP1B/PIPKIγ dependent E-cadherin exocytosis

PIPKIγ 661 acts as both a scaffolding and signalling molecule by linking E-cadherin (ECD) to AP1B, allowing for the trafficking of E-cadherin from the recycling endosome to the basolateral plasma membrane. At the plasma membrane, the exocyst complex assembles in a PtdIns(4,5)P₂-dependent manner to mediate fusion of the exocytic vesicle with the plasma membrane. Further regulation of PIPKIs and the exocyst at the site of exocytosis occur via the actions of the small G-proteins Ral, Cdc42 and Rho.

Figure 4. Tyrosine-compared with dileucine-facilited endocytosis of E-cadherin

The facilitation of E-cadherin internalization via two individual clathrin-dependent pathways suggests that the entry into one system compared with the other must be regulated via specific cellular signals. (A) AP2-mediated E-cadherin (ECD) endocytosis via the tyrosine sorting motif (Yxxφ) and a PIPKIγ-scaffold may be the pathway in which E-cadherin could recycle back to the plasma membrane after internalization. This pathway is subject to regulation by Arf6, as Arf6 can associate with AP2 and PIPKIγ and targets both proteins to the plasma membrane. As this pathway does not require dissociation of p120-catenin for endocytosis, E-cadherin would probably remain in a stabilized complex at the recycling endosome and be quickly returned to the cell surface, rather than being sent to the lysosome. In this model, the tyrosine-sorting-motif-mediated pathway could be a mechanism for constitutive E-cadherin endocytosis, as there is no net loss in E-cadherin. (B) In contrast, stimulated dissociation of p120-catenin from E-cadherin (ECD) would open up the dileucine (LL) motif for recognition by an as-yet undefined AP complex, and the destabilized E-cadherin complex would then be endocytosed in an SNX9- and dynamin (Dyn)-dependent manner. Generation of PtdIns(4,5)P₂ by a PIPKI would likely be required to activate the AP complex and to drive dynamin and SNX9 function at the site of vesicle fission. In this system, the fate of endocytosed E-cadherin is less clear. However, the identification of the specific clathrin adaptor(s) which mediate dileucine-based E-cadherin internalization will shed light on this pathway.