Human P2Y11 Expression Level Affects Human P2X7 Receptor-Mediated Cell Death

Adenosine triphosphate (ATP) is known to induce cell death in T lymphocytes at high extracellular concentrations. CD4⁺ and CD8⁺ T lymphocytes have a differential response to ATP, which in mice is due to differences in the P2X7 receptor expression levels. By contrast, we observed that the difference in human CD4⁺ and CD8⁺ T lymphocyte response toward the synthetic ATP-analog BzATP is not explained by a difference in human P2X7 receptor expression. Rather, the BzATP-induced human P2X7 receptor response in naïve and immune-activated lymphocyte subtypes correlated with the expression of another ATP-binding receptor: the human P2Y₁₁ receptor. In a recombinant expression system, the coexpression of the human P2Y₁₁ receptor counteracted BzATP-induced human P2X7 receptor-driven lactate dehydrogenase release (a marker of cell death) and pore formation independent of calcium signaling. A mutated non-signaling human P2Y₁₁ receptor had a similar human P2X7 receptor-inhibitory effect on pore formation, thus demonstrating that the human P2X7 receptor interference was not caused by human P2Y₁₁ receptor signaling. In conclusion, we demonstrate an important species difference in the ATP-mediated cell death between mice and human cells and show that in human T lymphocytes, the expression of the human P2Y₁₁ receptor correlates with human P2X7 receptor-driven cell death following BzATP stimulation.

Transport of charged small molecules after electropermeabilization - drift and diffusion

Background

Applications of electric-field-induced permeabilization of cells range from cancer therapy to wastewater treatment. A unified understanding of the underlying mechanisms of membrane electropermeabilization, however, has not been achieved. Protocols are empirical, and models are descriptive rather than predictive, which hampers the optimization and expansion of electroporation-based technologies. A common feature of existing models is the assumption that the permeabilized membrane is passive, and that transport through it is entirely diffusive. To demonstrate the necessity to go beyond that assumption, we present here a quantitative analysis of the post-permeabilization transport of three small molecules commonly used in electroporation research — YO-PRO-1, propidium, and calcein — after exposure of cells to minimally perturbing, 6 ns electric pulses.

Results

Influx of YO-PRO-1 from the external medium into the cell exceeds that of propidium, consistent with many published studies. Both are much greater than the influx of calcein. In contrast, the normalized molar efflux of calcein from pre-loaded cells into the medium after electropermeabilization is roughly equivalent to the influx of YO-PRO-1 and propidium. These relative transport rates are correlated not with molecular size or cross-section, but rather with molecular charge polarity.

Conclusions

This comparison of the kinetics of molecular transport of three small, charged molecules across electropermeabilized cell membranes reveals a component of the mechanism of electroporation that is customarily taken into account only for the time during electric pulse delivery. The large differences between the influx rates of propidium and YO-PRO-1 (cations) and calcein (anion), and between the influx and efflux of calcein, suggest a significant role for the post-pulse transmembrane potential in the migration of ions and charged small molecules across permeabilized cell membranes, which has been largely neglected in models of electroporation.

Electronic supplementary material

The online version of this article (10.1186/s13628-018-0044-2) contains supplementary material, which is available to authorized users.

Membrane permeabilization of mammalian cells using bursts of high magnetic field pulses

BACKGROUND

Cell membrane permeabilization by pulsed electromagnetic fields (PEMF) is a novel contactless method which results in effects similar to conventional electroporation. The non-invasiveness of the methodology, independence from the biological object homogeneity and electrical conductance introduce high flexibility and potential applicability of the PEMF in biomedicine, food processing, and biotechnology. The inferior effectiveness of the PEMF permeabilization compared to standard electroporation and the lack of clear description of the induced transmembrane transport are currently of major concern.

METHODS

The PEMF permeabilization experiments have been performed using a 5.5 T, 1.2 J pulse generator with a multilayer inductor as an applicator. We investigated the feasibility to increase membrane permeability of Chinese Hamster Ovary (CHO) cells using short microsecond (15 µs) pulse bursts (100 or 200 pulses) at low frequency (1 Hz) and high dB/dt (>10⁶ T/s). The effectiveness of the treatment was evaluated by fluorescence microscopy and flow cytometry using two different fluorescent dyes: propidium iodide (PI) and YO-PRO®-1 (YP). The results were compared to conventional electroporation (single pulse, 1.2 kV/cm, 100 µs), i.e., positive control.

RESULTS

The proposed PEMF protocols (both for 100 and 200 pulses) resulted in increased number of permeable cells (70 ± 11% for PI and 67 ± 9% for YP). Both cell permeabilization assays also showed a significant (8 ± 2% for PI and 35 ± 14% for YP) increase in fluorescence intensity indicating membrane permeabilization. The survival was not affected.

DISCUSSION

The obtained results demonstrate the potential of PEMF as a contactless treatment for achieving reversible permeabilization of biological cells. Similar to electroporation, the PEMF permeabilization efficacy is influenced by pulse parameters in a dose-dependent manner.

Ion channel-mediated uptake of cationic vital dyes into live cells: a potential source of error when assessing cell viability

Ionic "vital dyes" are commonly used to assess cell viability based on the idea that their permeation is contingent on a loss of membrane integrity. However, the possibility that dye entry is conducted into live cells by endogenous membrane transporters must be recognized and controlled for. Several cation-selective plasma membrane-localized ion channels, including the adenosine 5'-triphosphate (ATP)-gated P2X receptors, have been reported to conduct entry of the DNA-binding fluorescence dye, YO-PRO-1, into live cells. Extracellular ATP often becomes elevated as a result of release from dying cells, and so it is possible that activation of P2X channels on neighboring live cells could lead to exaggerated estimation of cytotoxicity. Here, we screened a number of fluorescent vital dyes for ion channel-mediated uptake in HEK293 cells expressing recombinant P2X2, P2X7, or TRPV1 channels. Our data shows that activation of all three channels caused substantial uptake and nuclear accumulation of YO-PRO-1, 4',6-diamidino-2-phenylindole (DAPI), and Hoechst 33258 into transfected cells and did so well within the time period usually used for incubation of cells with vital dyes. In contrast, channel activation in the presence of propidium iodide and SYTOX Green caused no measurable uptake and accumulation during a 20-min exposure, suggesting that these dyes are not likely to exhibit measurable uptake through these particular ion channels during a conventional cell viability assay. Caution is encouraged when choosing and employing cationic dyes for the purpose of cell viability assessment, particularly when there is a likelihood of cells expressing ion channels permeable to large ions.

Selection of nonapoptotic sperm by magnetic-activated cell sorting in Senegalese sole (Solea senegalensis)

Senegalese sole (Solea senegalensis) is a promising species in aquaculture. However, owing to decreased sperm quality in F1 generations and the absence of courtship in those individuals born in captivity, artificial fertilization is being used to generate new progenies. The objective of this study was to implement a sperm selection method for nonapoptotic sperm subpopulation recovery before sperm cryopreservation. In particular, magnetic-activated cell sorting is used to eliminate apoptotic spermatozoa. This study represents the proof-of-concept for magnetic-activated cell sorting applicability in teleost species relevant in aquaculture. Apoptotic cell population was studied by flow cytometry using YO-PRO-1 and a caspase detection kit. Also, reactive oxygen species were measured in sperm samples. Our data demonstrated that caspase detection is more specific than YO-PRO-1 in the identification of apoptotic cells in S senegalensis seminal samples. The results showed that the percentage of apoptotic cells (caspase positive) was significantly higher (P = 0.04) in seminal samples from F1 than that from wild individuals. Magnetic-activated cell sorting removed a significant number of apoptotic cells from the samples (54% and 75% in wild and F1 individuals, respectively), decreasing the level of cells positive for reactive oxygen species (P = 0.17). In conclusion, this technique reduces the percentage of nonfunctional spermatozoa in a seminal sample before cryopreservation. This novel technique can be applied directly in the aquaculture industry.

Two open states of P2X receptor channels

The occupancy of the orthosteric ligand binding sites of P2X receptor (P2XR) channels causes the rapid opening of a small cation-permeable pore, followed by a gradual dilation that renders the pore permeable to large organic cations. Electrophysiologically, this phenomenon was shown using whole-cell current recording on P2X2R-, P2X2/X5R-, P2X4R- and P2X7R-expressing cells that were bathed in N-methyl-D-glucamine (NMDG⁺)-containing buffers in the presence and/or absence of small monovalent and divalent cations. The pore dilation of P2X4R and P2X7R caused a secondary current growth, whereas that of P2X2R showed a sustained kinetic coupling of dilation and desensitization, leading to receptor channel closure. The pore size of the P2X7R open and dilated states was estimated to be approximately 0.85 nm and greater than 1 nm, respectively. The P2XR pore dilation was also observed in intact cells by measurement of fluorescent dye uptake/release, application of polyethylene glycols of different sizes, and atomic force microscopy. However, pore dilation was not observed at the single channel level. Structural data describing the dilated state are not available, and the relevance of orthosteric and allosteric ligand interactions to pore dilation was not studied.

Chlorobenzenes, lindane and dieldrin induce apoptotic alterations in human peripheral blood lymphocytes (in vitro study)

In this study, we have assessed apoptotic effect of 1,2,4-trichlorobenzene, hexachlorobenzene, lindane and dieldrin on human peripheral blood lymphocytes. We observed an increase in ROS formation and a decrease in mitochondrial transmembrane potential in the cells incubated with low concentrations of all compounds studied, in particular lindane and dieldrin. ROS formation and changes in mitochondrial transmembrane potential may have influenced caspase-3 activation, a crucial enzyme in the apoptotic process. Moreover, chlorobenzenes, and in particular lindane and dieldrin changed cells' membrane permeability and induced phosphatidylserine translocation, which confirmed that they are capable of inducing apoptosis in human lymphocytes. Apoptotic changes in human lymphocytes provoked by biologically relevant concentrations of these substances suggest that they may disturb function of immunological system especially among people occupationally exposed to their action.

Autophagy-dependent ATP release from dying cells via lysosomal exocytosis

ATP not only represents a key molecule for the intracellular storage and utilization of energy, but also plays a central role in the extracellular milieu, as it both recruits and activates immune cells. The secretion of ATP in the course of immunogenic cell death (ICD), a peculiar type of apoptosis that elicits adaptive immune responses, critically relies on the molecular machinery for autophagy. We have recently demonstrated that autophagy allows for the ICD-associated secretion of ATP as it contributes to the maintenance of lysosomal ATP stores. In addition, we found that ATP release in this setting is mediated by a molecular mechanism that involves lysosomal-associated membrane protein 1 (LAMP1)- and pannexin 1 (PANX1)-dependent lysosomal exocytosis.