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Cell-cell junctions are essential for tissue homeostasis. Prominent among these junctions are adherens junctions and tight junctions. Adherens junctions mediate adhesion between adjacent cells while tight junctions are responsible for establishing apical-basolateral polarity and limiting paracellular permeability. Loss or disruption of either adherens junctions or tight junctions leads to a myriad of disease states, thus these junctions need to be tightly regulated to prevent dysfunction. A unique property of tight junctions is their dependence on adherens junctions for proper assembly and maintenance. Loss or disruption of adherens junction leads to abnormal tight junctions. Understanding the mechanisms that mediate tight junction coupling to adherens junctions is important for treating diseases that arise from disrupted cell-cell junctions.
Currently, two controversial models exist for how tight junctions are coupled to adherens junctions. In the first model, the adherens junction protein α-catenin is critical for tight junction assembly. The second model suggests that a second adherens junction protein, nectin is critical for tight junction assembly through binding the tight junction protein ZO-1, and disruption of tight junction assembly is independent of E-cadherin. α-catenin also binds ZO-1, but the consequences of this interaction are unknown. I hypothesized that α-catenin binding to ZO-1 plays a critical role in coupling tight junctions to adherens junctions. To test this, I mapped the ZO-1 binding site on α-catenin and engineered a point mutant of α-catenin that failed to bind ZO-1. Expression of this point mutant in epithelial cells showed that ZO-1 binding to α-catenin is essential for tight junction assembly and maintenance, while adherens junctions were unaffected. These findings established a role for ZO-1 binding to α-catenin in coupling tight junctions to adherens junctions during junction assembly, as well as at steady-state conditions.
After discovering the importance of ZO-1 binding to α-catenin in coupling tight junctions to adherens junctions, I wanted to study whether this interaction is critical in a physiological setting. Tight junctions and adherens junctions are both strengthened in response to mechanical force; however the mechanisms responsible for tight junction strengthening were unknown. Using the system I previously developed, I show that ZO-1 binding to α-catenin is essential for increased tight junction integrity in response to mechanical force, coupling changes in tight junctions to increased stability of adherens junctions. Together, these findings identify a novel interaction that is critical for coupling tight junctions to adherens junctions under several conditions, and provide mechanistic insight into the cellular response to mechanical force.