Luminescent Dual Sensors Reveal Extracellular pH-Gradients and Hypoxia on Chronic Wounds That Disrupt Epidermal Repair

Wound repair is a quiescent mechanism to restore barriers in multicellular organisms upon injury. In chronic wounds, however, this program prematurely stalls. It is known that patterns of extracellular signals within the wound fluid are crucial to healing. Extracellular pH (pH(e)) is precisely regulated and potentially important in signaling within wounds due to its diverse cellular effects. Additionally, sufficient oxygenation is a prerequisite for cell proliferation and protein synthesis during tissue repair. It was, however, impossible to study these parameters in vivo due to the lack of imaging tools. Here, we present luminescent biocompatible sensor foils for dual imaging of pH(e) and oxygenation in vivo. To visualize pH(e) and oxygen, we used time-domain dual lifetime referencing (tdDLR) and luminescence lifetime imaging (LLI), respectively. With these dual sensors, we discovered centripetally increasing pH(e)-gradients on human chronic wound surfaces. In a therapeutic approach, we identify pH(e)-gradients as pivotal governors of cell proliferation and migration, and show that these pH(e)-gradients disrupt epidermal barrier repair, thus wound closure. Parallel oxygen imaging also revealed marked hypoxia, albeit with no correlating oxygen partial pressure (pO(2))-gradient. This highlights the distinct role of pH(e)-gradients in perturbed healing. We also found that pH(e)-gradients on chronic wounds of humans are predominantly generated via centrifugally increasing pH(e)-regulatory Na(+)/H(+)-exchanger-1 (NHE1)-expression. We show that the modification of pH(e) on chronic wound surfaces poses a promising strategy to improve healing. The study has broad implications for cell science where spatial pH(e)-variations play key roles, e.g. in tumor growth. Furthermore, the novel dual sensors presented herein can be used to visualize pH(e) and oxygenation in various biomedical fields.