A heterologous in-cell assay for investigating intermicrovillar adhesion complex interactions and function

Solute transporting epithelial cells build arrays of microvilli on their apical surface to increase membrane scaffolding capacity and enhance function potential. In epithelial tissues such as the kidney and gut, microvilli are length-matched and assembled into tightly packed brush borders, which are organized by [~]50 nm thread-like links that form between the distal tips of adjacent protrusions. Composed of protocadherins CDHR2 and CDHR5, adhesion links are stabilized at the tips by a cytoplasmic tripartite module containing the scaffolds USH1C and ANKS4B, and the actin-based motor, MYO7B. As several questions about the formation and function of this intermicrovillar adhesion complex remain open, we devised a system that allows one to study individual binary interactions between specific complex components and MYO7B. Our approach employs a chimeric myosin consisting of the motor domain of MYO10 fused to the cargo-binding tail domain of MYO7B. When expressed in HeLa cells, which do not normally produce adhesion complex proteins, this motor exhibited robust trafficking to the tips of filopodia and was also able to transport individual components to these sites. Unexpectedly, the MYO10/MYO7B chimera was able to deliver CDHR2 and CDHR5 to distal tips in the absence USH1C or ANKS4B. Cells engineered to localize high levels of CDHR2 at filopodial tips acquired inter-filopodial adhesion and exhibited a striking dynamic length matching activity that aligned distal tips over time. These observations reveal a robust adhesion-dependent mechanism for matching the lengths of adjacent surface protrusions, and may offer insight on how epithelial cells minimize microvillar length variability.