The Inflammasome by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9781071621448
Publisher: Springer US

1 Introduction

The NLRP3 inflammasome , which has been linked to human inflammatory diseases, is activated by diverse stimuli [1]. It has been shown that almost all NLRP3 stimuli can increase the permeability of the cell membrane to K+, and the high-level extracellular K+ inhibits NLRP3 inflammasome activation [2], suggesting that efflux of intracellular K+ may orchestrate NLRP3 inflammasome assembly.

Such a scenario was built upon the assumption that high extracellular K+ abolishes its extracellular/intracellular gradient, stopping its efflux. Under physiological conditions, the main extracellular cation is Na+ while the main cytosolic cation is K+. As the plasma membrane is more permeable for K+, the tendency of K+ exodus outwards sets up a positive charge above the outer membrane, intracellular Cl− is thus drawn to be under the inner membrane, creating a −30 to −90 mV charge differential over the plasma membrane. This phenomenon, known as the membrane potential (MP), is essential for eukaryotic biology and affects many functions of the cell. Rise in extracellular K+, artificial or biological, disrupts the MP. While this may implicate the reduced efflux, the consequences of this disruption reaches far beyond K+ efflux [3].

In his discovery of inflammasome complexes [4], Jurg Tschopp noticed that increase in extracellular K+ blocked their activation. He attributed this effect to the result of K+ efflux blockage. Since then, this treatment has been used as the standard protocol to assert the rule of K+ efflux in inflammasome activation. However, as disruption of the MP has implications beyond this simple blockage. Other consequences that might modulate inflammasome activation have not been fully investigated.

The common protocols such as high extracellular K+ used in NLRP3 research disrupts the basic baseline of membrane potential, causing sustained depolarization [5]. Subsequent research has found that sustained hyperpolarization (CFTRinh-172 or Glyburide treatment) also blocked its activation. This is remarkable as hyperpolarization is diametrically opposite of depolarization induced by increased extracellular K+ [6]. This raises the issue whether membrane potential disruption is the true reason for inflammasome inhibition. We found that the activation of Ca2+-dependent calpain is a ubiquitous consequence of NLRP3 stimuli, and a relatively stable membrane potential is critically required for calpain activities, indicating the maintenance of membrane potential has great significance for NLRP3 inflammasome activation. Therefore, under physiological conditions, whether K+ efflux is truly central to inflammasome activation should be confirmed by via direct measurement.

ICP-OES is a multiple element quantitation technique based on the measurement of excited atoms and ions at the wavelengths characteristic for the specific elements being measured. The lower detection limit for ICP-OES can extend to parts per billion, sufficient for intracellular ion concentration detection. We have attempted to measure the intracellular K+ over time to elucidate how K+ flows during the activation of NLRP3 inflammasome by ICP-OES. The results showed that the ion concentration drops rapidly in a short time by nearly a half (see Fig. 1), seemingly in agreement with the K+ efflux proposal [6]. As we noted, the establishment of membrane potential is usually caused by the asymmetric distribution and the different permeability of ions across plasma membrane.

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