Cell-cell communication is essential for maintaining cell coordination and tissue level functions. Communication is mediated by receptors which facilitate signal prorogation across the cell membrane. The biochemical steps of receptor activation are generally well studied; however, the contributions of spatial, dynamic, and mechanical cues are less well understood. Juxtacrine signaling receptors, receptors activated by ligands on adjacent cells, are particularly dependent on the spatial organization of receptor, ligand, and other pathway components in order to potentiate signaling. Understanding, how ligands and receptors are presented at the membrane, interact across cells, and ultimately integrate cues to induce signaling is essential for understanding cellular communication. Here we present our work on unraveling the function of two essential juxtacrine signaling and adhesion receptors, Notch and Cadherin.
Notch is a type I transmembrane receptor that plays a critical role in development, relaying signals from neighboring cells in order to affect numerous cell fate decisions. Signal transduction by the Notch receptor is activated by interaction with ligand presented as a transmembrane protein on adjacent cell surfaces. This interaction potentiates the receptor for proteolysis and activates downstream signaling. Notch’s activation has long been theorized to be dependent on mechanical force. Using a newly developed nanoprobe system, magnetoplasmonic plasmonic nanoparticles (MPNs), we specifically test for mechanical activation of Notch and rule out clustering and allosteric interactions as required for Notch activation. Force is also known to play a role in Cadherin function and given the versatility of the MPN system we were able to test for force dependent recruitment of the actin cytoskeleton and downstream adapter proteins.
Furthermore, using single particle methods and ensemble imaging we have investigated the regulation of Notch activation at the cell surface. We found that Notch exhibits a distinct spatial distribution and is subject to diffusional barriers at the cell surface, specifically cellular adhesions, including Cadherin junctions and focal adhesions. This work implicates adhesions as regulators of Notch presentation and activation at the cell membrane, a new paradigm for Notch signal regulation.