ACS Chem Biol. 2016 Jun 17;11(6):1652-60. doi: 10.1021/acschembio.6b00103

Synchronous Bioimaging of Intracellular pH and Chloride Based on LSS Fluorescent Protein.

Paredes JM1,2, Idilli AI1, Mariotti L3, Losi G3, Arslanbaeva LR1,4, Sato SS5, Artoni P6, Szczurkowska J7, Cancedda L7, Ratto GM5,6, Carmignoto G3, Arosio D1,4.
  • 1Institute of Biophysics, CNR , Via alla Cascata 56/C, 38123 Trento, Italy.
  • 2Bruno Kessler Foundation , Via Sommarive 18, 38123 Trento, Italy.
  • 3Neuroscience Institute, CNR , Viale G. Colombo 3, 35121 Padova, Italy.
  • 4CIBIO, University of Trento , Via delle Regole 101, 38123 Trento, Italy.
  • 5Nanoscience Institute, CNR , Pisa, Italy.
  • 6Scuola Normale Superiore , Pisa, Italy.
  • 7Istituto Italiano di Tecnologia , Genoa, Italy.

 

Abstract

Ion homeostasis regulates critical physiological processes in the living cell. Intracellular chloride concentration not only contributes in setting the membrane potential of quiescent cells but it also plays a role in modulating the dynamic voltage changes during network activity. Dynamic chloride imaging demands new tools, allowing faster acquisition rates and correct accounting of concomitant pH changes. Joining a long-Stokes-shift red-fluorescent protein to a GFP variant with high sensitivity to pH and chloride, we obtained LSSmClopHensor, a genetically encoded fluorescent biosensor optimized for the simultaneous chloride and pH imaging and requiring only two excitation wavelengths (458 and 488 nm). LSSmClopHensor allowed us to monitor the dynamic changes of intracellular pH and chloride concentration during seizure like discharges in neocortical brain slices. Only cells with tightly controlled resting potential revealed a narrow distribution of chloride concentration peaking at about 5 and 8 mM, in neocortical neurons and SK-N-SH cells, respectively. We thus showed that LSSmClopHensor represents a new versatile tool for studying the dynamics of chloride and proton concentration in living systems.

PMID: 27031242;

 

SUPPLEMENTARY

The importance of chloride

Chloride ions (Cl) play important roles in different cellular functions and physiological processes, and its role is critical in several human diseases1, and neurodevelopmental disorders2. In particular, chloride homeostasis is increasingly investigated in neuroscience as it is involved in the synaptic signaling3, as well as in neuronal growth and migration4.

How to measure intracellular chloride

The use of non-invasive techniques in the determination of intracellular chloride brings multiple advantages, and in particular, the use of genetically encoded fluorescent biosensors is enabling us to image Clconcentration in cells with unprecedented resolution. Cl biosensors, however, are biased by a strong pH dependency, so that slight intracellular pH changes can lead to wrong determination of Clconcentration. This limitation can be overcome by measuring intracellular pH and Cl- concentration simultaneously.5 In this case, the biosensor is formed by a Cland pH sensitivity fluorescent protein (E2GFP) and a Cland pH insensitivity red protein (DsRed).

Our improvement in the chloride biosensor

Despite the increasing interest in Cl imaging, its widespread adoption is prevented by the need for a dedicated setups. To simplify these requirements and allow a wider adoption of quantitative Cl imaging with the correct analysis of concomitant pH changes, we developed a novel biosensor exploiting a novel red fluorescent protein with a large (>100 nm) Stokes shift (i.e. a large distance between excitation and emission maxima).6 Thus in our new biosensor, named LSSmClopHensor, a single excitation source is able to excite the green and red moiety simultaneously with a considerable simplification of the required setup.

How use LSSmClopHensor

LSSClopHensor works as follows: (Figure 1)

  • By using alternatively two wavelength of excitation (488 and 458 nm); we record the emission of the two different moieties of the biosensor, recording fluorescence from three channels:

o   Green: Emission of E2GFP (ex 488 nm)

o   Cyan: Emission of E2GFP (ex 458 nm)

o   Red: Emission of LSSmKate2 (ex 458 nm)

  • pH map are calculated from the ratio between Green (Cland pH sensitivity) and Cyan channels (Cl sensitivity and pH insensitivity). Being both channels affected in the same way by Cl, their ratio is determined only by the pH value. (Figure 2A) A simple calibration curve is used to derive pH values from the ratio.
  • Once known the pH, we can determinate the Clconcentration (corrected by the influence of the pH value) using the ratio of the Cyan (Clsensitivity and pH insensitivity) and Red channels (Cland pH insensitivity). (Figure 2B). A great advantage of LSSClopHensor is the precise determination of Clconcentration even in the presence of concomitant pH changes. A previous calibration correlates the ratio with the Clconcentration value.

 

In case of steady pH during measurement intervals, the use of LSSmClopHemsor is further simplified, because we only need a single wavelength of excitation (458 nm) to determine the changes in Cl concentration.

Cland pH maps

We extended the use of LSSmClopHensor to monitor dynamic changes of Cl and H+ concentration in neocortical neuron networks in ex vivo preparations. We inducted an epileptic activity to neurons and record in real time the changes in the intensity in every channel. The epileptic activity produces an alteration in the fluxes of H+ and Cl at the plasma membrane, causing an intracellular acidification and Cl accumulation in the imaged neurons (Figure 3).

 

The importance of this study

Here, we developed a new ratiometric fluorescent biosensor that requires a single excitation to optimally excite both green and red FP and exploit the ratio between their measured emissions is the used to calculate maps ofintracellular pH and Clconcentration in time-lapse experiments.

Thus, we expect LSSmClopHensor to have wide use in imaging studies, in ex vivo and potentially also in in vivo, investigating the role of Cl and H+ in the cellular physiology and physiopathology.

 

Bibliography

  1. Planells-Cases, R., and Jentsch, T. J. (2009) Chloride channelopathies. Biochim. Biophys. Acta, Mol. Basis Dis. 1792, 173−189.
  2. Deidda, G., Bozarth, I. F., and Cancedda, L. (2014) Modulation of GABAergic transmission in development and neurodevelopmental disorders: Investigating physiology and pathology to gain therapeutic perspectives. Front. Cell. Neurosci. 8, DOI: 10.3389/fncel.2014.00119.
  3. Payne, J. A., Rivera, C., Voipio, J., and Kaila, K. (2003) Cation−chloride co-transporters in neuronal communication, development and trauma. Trends Neurosci. 26, 199−206.
  4. Schwab, A., Fabian, A., Hanley, P. J., and Stock, C. (2012) Role of Ion Channels and Transporters in Cell Migration. Physiol. Rev. 92, 1865−1913.
  5. Arosio, D., Ricci, F., Marchetti, L., Gualdani, R., Albertazzi, L., and Beltram, F. (2010) Simultaneous intracellular chloride and pH measurements using a GFP-based sensor. Nat. Methods 7, 516−U44.
  6. Piatkevich, K. D., Malashkevich, V. N., Almo, S. C., and Verkhusha, V. V. (2010) Engineering ESPT Pathways Based on Structural Analysis of LSSmKate Red Fluorescent Proteins with Large Stokes Shift. J. Am. Chem. Soc. 132, 10762−10770.

 

 

Acknowledgments:

We gratefully acknowledge the financial support of Telethon Italy (Grant GGP10138), RESTATE programme cofunded by the European Union under the FP7 COFUND Marie Curie Action – Grant agreement no. 267224 to J.M.P., MIUR (PRIN2009XPTWM2) and University of Trento (Progetti di Ricerca 2014).

 

Contact:

Daniele Arosio, Ph.D.

Institute of Biophysics

Via alla Cascata 56/C, 38123

CNR

Trento (Italy)

daniele.arosio@cnr.it

 

José M. Paredes, Ph.D.

Department of Physical Chemistry

Faculty of Pharmacy

18017 Campus of Cartuja

University of Granada (Spain)

jmparedes@ugr.es

 

 

 

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