Intercellular junctions and cell polarity

All epithelial cells are interconnected through three major types of junctions – Tight Junctions (TJs), Adherens Junctions (AJs), and Desmosomes (DSMs). In simple epithelia, these junctions are positioned along the apical end of the cell lateral membrane, where they form the Apical Junctional Complex (AJC). AJs and DSMs, but not TJs, are also present along the basolateral plasma membrane, where they are much smaller in size. This asymmetry of junction organization is a distinctive architectural feature of simple epithelia. Such asymmetry is controlled by a set of conserved polarity proteins. The main question that we set out to address in this proposal is: how are two polarity proteins, basolateral protein Scrib and apical protein Par3, interconnected with proteins regulating the positioning of the cell-cell junctions in AJC. Our recently published results suggested how Scrib maintains “basolateral identity” of simple epithelial cell membrane, the mechanism that has been elusive for many years. We found that this function of Scrib is based on regulation of two enzymes, phosphatase PP1 (PP1) and Rho/Rac Guanine Nucleotide Exchange Factor, GEF-H1. Our results suggest that Scrib holds PP1 in inactive state and releases it in an active form through interactions with other polarity proteins, Llgl, Dlg, and Band 4.1. We also show that a protein that links the polarity apparatus with DSM assembly is an ARM protein, plakophilin 3 (Pkp3). We provide evidence that this protein is stored around TJs in a “dormant” pool that is established under control of the apical polarity protein Par3. The active form of Pkp3, released from this pool, drives formation of DSMs using the mechanism that is radically distinct from the assembly mechanism of the evolutionary related junctions, AJs. Furthermore, we present some evidence suggesting that a similar Pkp-dependent mechanism of junction assembly is involved in formation of a previously unrecognized subtype of AJs, which we call here Pkp4-dependent AJs. The advances made in previous years allow us to come up with this proposal, which includes the investigation of: (i) how Scrib regulates PP1 and what are the effectors of this pathway; (ii) how Pkp3 regulates adhesion activity of desmosomal cadherins and how its DSM assembly activity is regulated by the polarity system; and (iii) how Pkp4 controls adhesion activity of classic cadherins in assembly of specific subset of AJs. This work will pave the way toward the development of reagents modulating the adhesive and barrier properties of epithelial tissues.