Windows to cell function and dysfunction: Signatures written in the boundary layers

Abstract

The medium surrounding cells either in culture or in tissues contains a chemical mix varying with cell state. As solutes move in and out of the cytoplasmic compartment they set up characteristic signatures in the cellular boundary layers. These layers are complex physical and chemical environments whose profiles both reflect cell physiology and provide conduits for intercellular messaging. Here we review some of the most relevant characteristics of the extracellular/intercellular space. Our initial focus is primarily with cultured cells but we extend our consideration to the far more complex environment of tissues and discuss how chemical signatures in the boundary layer can or may affect cell function. Critical to the entire essay are the methods used, or being developed, to monitor chemical profiles in the boundary layers. We review recent developments in ultramicro electrochemical sensors and tailored optical reporters suitable for the task 20 in hand. IntroductionThe advent of electron tomography with 3 dimensional image reconstruction has revealed a fundamental beauty and complexity behind the structure of the living cell. Notable contributions have come from several imaging laboratories but one of the most compelling can be found in Marsh et al.(1) From such reconstructions the complexity within a cell is quite staggering and in the 4 th dimension of time this entire structure is in motion. The question that becomes extremely interesting is how molecular and ionic signals are regulated and move within these matrices, where chemical diffusion will be significantly altered. (2) However, complexity, both structural and functional, does not stop at the plasma membrane but extends outwards 30 into the extracellular/intercellular space (EICS: Fig.1). The chemical dynamics within this space are hypothesized to play key roles in cancer, spreading depression, epilepsy and sleep disorders. (3)(4)(5)(6) The purpose of this essay is to consider what we can learn from this space and the methods for following the dynamics of chemical movement within the diffusive boundary layers that comprise an integral part of a cell. We will start by noting the physical and chemical features involved.

Review and Hypothesis: A Potential Common Link Between Glial Cells, Calcium Changes, Modulation of Synaptic Transmission, Spreading Depression, Migraine, and Epilepsy—H+

“…External membrane-associated voltage gradients are too small to be detected by the H + -selective electrode, a very different situation from operating ion-selective electrodes in an intracellular environment. The possible but unlikely impact of surface charge influencing measurements is discussed in detail in Smith et al (2010). An example of the actual size and time-dependence of electrical drift associated with ion-selective electrodes is shown in Figures 2C,D (note that this signal was generated by creating a steep artificial gradient from a source pipette far larger than would be expected when recording responses from a single cell, which generate signals in the microvolt range).…”

“…Electrodes are typically translated in an approximate square wave between the poles with an interval of 0.3 Hz, with data collected at each position for around 1 s. The electrode must be moved quickly enough so that the contribution from electrical drift is roughly the same at both locations, but not so quickly as to significantly stir the solution and thus potentially disrupt the differential H + gradient at the heart of the measurement. With these operating parameters and dimensions, and taking into account the diffusion constant of H + , mixing is not seen as a problem (Messerli et al, 2006;Smith et al, 2010).…”

“…Finally, self-referencing H + -selective electrodes report an ion flux-that is, an alteration in H + -activity as it changes over a specified period of time and distance, typically given in units of µmol cm −2 s −1 . As noted above, an exact determination of the flux value is dependent on distance, response times, buffering capacity, the source strength, and in vivo, on the intracellular volume and tortuosity (Syková and Nicholson, 2008;Smith et al, 2010). Given these considerations, we have opted to present results simply as the difference in the voltage reported between the two poles of translation, i.e., ∆ µV.…”