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Vesicle-Based Sensors for Extracellular Potassium Detection

Abstract

Introduction

The design of sensors that can detect biological ions in situ remains challenging. While many fluorescent indicators exist that can provide a fast, easy readout, they are often nonspecific, particularly to ions with similar charge states. To address this issue, we developed a vesicle-based sensor that harnesses membrane channels to gate access of potassium (K+) ions to an encapsulated fluorescent indicator.

Methods

We assembled phospholipid vesicles that incorporated valinomycin, a K+ specific membrane transporter, and that encapsulated benzofuran isophthalate (PBFI), a K+ sensitive dye that nonspecifically fluoresces in the presence of other ions, like sodium (Na+). The specificity, kinetics, and reversibility of encapsulated PBFI fluorescence was determined in a plate reader and fluorimeter. The sensors were then added to E. coli bacterial cultures to evaluate K+ levels in media as a function of cell density.

Results

Vesicle sensors significantly improved specificity of K+ detection in the presence of a competing monovalent ion, sodium (Na+), and a divalent cation, calcium (Ca2+), relative to controls where the dye was free in solution. The sensor was able to report both increases and decreases in K+ concentration. Finally, we observed our vesicle sensors could detect changes in K+ concentration in bacterial cultures.

Conclusion

Our data present a new platform for extracellular ion detection that harnesses ion-specific membrane transporters to improve the specificity of ion detection. By changing the membrane transporter and encapsulated sensor, our approach should be broadly useful for designing biological sensors that detect an array of biological analytes in traditionally hard-to-monitor environments.

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Acknowledgments

This work was supported by the Cornew Innovation Award from the Chemistry of Life Processes Institute at Northwestern University (to NPK) and the National Science Foundation (CBET-1844219 and CBET-1844336 to NPK). MAB was supported by the National Defense Science and Engineering Graduate Fellowship through the Department of Defense. AMD and NRC were assisted by a grant from the Undergraduate Research Grant Program, which is administered by Northwestern University’ s Office of Undergraduate Research, and by a grant from Northwestern’s Biomedical Engineering Department.

Conflict of interest

Margrethe Boyd, Anna Davis, Nora Chambers, Peter Tran, Arthur Prindle, and Neha P. Kamat declare that they have no conflicts of interest.

Ethical Approval

No animal or human studies were performed in this work.

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Correspondence to Neha P. Kamat.

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Boyd, M.A., Davis, A.M., Chambers, N.R. et al. Vesicle-Based Sensors for Extracellular Potassium Detection. Cel. Mol. Bioeng. 14, 459–469 (2021). https://doi.org/10.1007/s12195-021-00688-7

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  • DOI: https://doi.org/10.1007/s12195-021-00688-7

Keywords

  • Membrane
  • Biosensing
  • Liposome
  • Ionophore
  • Fluorescence