PhD Discussion Session: Pilar Rodríguez and Montserrat López

Long-ranged force patterns and waves during the formation and maintenance of repulsive epithelial barriers

For an organism to develop and maintain homeostasis, cell types with distinct functions must often be separated by physical boundaries. A prevalent mechanism for the formation and maintenance of such boundaries is the repulsive interaction between Eph receptor tyrosine kinases and their ligands Ephrins. Upon contact, cells expressing Eph and Ephrin trigger diverse local mechanisms that prevent intercellular adhesion, including receptor endocytosis, extracellular cadherin cleavage, and local contractility. Here we show that, besides these local subcellular mechanisms, Eph/Ephrin boundary formation involves cooperative physical forces generated by cells located many rows behind the boundary. Contact between two epithelial monolayers, one expressing Eph and one expressing Ephrin, results in the buildup of two supracellular acto-myosin cables that line epithelial edges at both sides of the boundary. Besides these cables, both monolayers exhibit long-lived periodic patterns of traction forces that expand several cell rows and tend to pull the monolayer away from the boundary, thereby contributing to sustain tissue segregation. The formation of these patterns is paralleled by the generation of soliton-like deformation waves that propagate away from the boundary. Finally, we show that periodic traction patterns and mechanical waves are observed not only during Eph/Ephrin repulsion but also during formation of diverse types of barriers. Our findings thus unveil a global physical mechanism that sustains tissue separation.

Nanoscale Conductance mapping of redox proteins

Electron Transfer (ET) plays essential roles in crucial biological processes such as cell respiration and photosynthesis. It takes place between redox proteins and in protein complexes that display an outstanding efficiency and environmental adaptability. Although the fundamental aspects of ET processes are well understood, more experimental methods are needed to determine electronic pathways in these redox protein structures. Understanding how ET works is important not only for fundamental reasons, but also for the potential technological applications of these redox-active nanoscale systems.
Electrochemical Scanning Tunneling Microscopy (ECSTM) is an excellent tool to study redox molecules including proteins. It offers single molecule resolution and allows working in nearly physiological conditions, with full electrochemical control. Beyond imaging, ECSTM allows performing current-voltage and current-distance tunneling spectroscopy. We adapted the current-voltage spectroscopy mode of ECSTM to obtain simultaneous topographic and differential conductance images under potentiostatic control. After validation of the method we applied it to the study of the redox protein Azurin immobilized on to a Au <111> surface, a model system to study biological ET processes.

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