Development of resting membrane potentials in differentiating murine neuroblastoma cells (N1E-115) evaluated by flow cytometry

Fluorescence Imaging of Cell Membrane Potential: From Relative Changes to Absolute Values

Membrane potential is a fundamental property of biological cells. Changes in membrane potential characterize a vast number of vital biological processes, such as the activity of neurons and cardiomyocytes, tumorogenesis, cell-cycle progression, etc. A common strategy to record membrane potential changes that occur in the process of interest is to utilize organic dyes or genetically-encoded voltage indicators with voltage-dependent fluorescence. Sensors are introduced into target cells, and alterations of fluorescence intensity are recorded with optical methods. Techniques that allow recording relative changes of membrane potential and do not take into account fluorescence alterations due to factors other than membrane voltage are already widely used in modern biological and biomedical studies. Such techniques have been reviewed previously in many works. However, in order to investigate a number of processes, especially long-term processes, the measured signal must be corrected to exclude the contribution from voltage-independent factors or even absolute values of cell membrane potential have to be evaluated. Techniques that enable such measurements are the subject of this review.

Interfacing SH-SY5Y human neuroblastoma cells with SU-8 microstructures

Microwell structures were fabricated using SU-8 photoresist for engineering a quasi-three-dimensional (quasi-3D) microenvironment for cultured neuronal cells. SH-SY5Y human neuroblastoma cells were successfully integrated into microwells of a nominal diameter of 100 microm, with or without 10-microm wide microchannels connecting neighboring microwells, in an aspect ratio (ratio of structure depth over width) of approximately 1. With the help of polyethylene glycol stamping and laminin coating, a neuronal-like network was achieved by integrating populations of SH-SY5Y cells with a microwell network pattern. Resting membrane potential establishment was evaluated with confocal microscopy and the potentiometric fluorescent dye tetramethylrhodamine methyl ester. It was found that the intra/extracellular fluorescent intensity ratio (R) was 2.4+/-1.4 [n (number of cells measured)=112] for SH-SY5Y cells on flat SU-8 substrates on day 5 into differentiation, which was not significantly different from the ratio on day 13 into differentiation, 2.0+/-1.8 (n=104) (P>0.05). For cells in the microwell network structures, R was 4.8+/-4.7 (n=51) and 3.9+/-3.2 (n=62) on days 5 and 13 into differentiation, respectively (P>0.5). Cells within the network structures had higher R ratios than on flat substrates, for either day 5 or 13 into differentiation (P<0.01). These results demonstrated that the well network structures, or topographically patterned substrates, were more suitable formats for promoting SH-SY5Y cell resting membrane potential establishment than flat substrates, suggesting the potential to control cellular function through substrate topography engineering.

Determination of resting membrane potential of individual neuroblastoma cells (IMR-32) using a potentiometric dye (TMRM) and confocal microscopy

The potentiometric dye, Tetramethylrhodamine methyl ester (TMRM) has been extensively used with fluorometry or optical microscopy to evaluate the electric potential across plasma or mitochondrial membranes. We present here a TMRM confocal microscopy-based potential measurement technique. Corrections are introduced to minimize nonspecific dye binding and insensitivity to low background levels. We have used this technique to compare the resting membrane potential of proliferating and differentiated human neuroblastoma cells (IMR-32).

Characterization of 3-D collagen hydrogels for functional cell-based biosensing

To address the growing demand for functional cell-based assay technologies with accelerated drug discovery applications, we have proposed the use of human neuroblastoma cells (IMR-32) immobilized in three-dimensional (3-D) collagen hydrogel matrices. The gel protects weakly adherent cells from fluid mechanical forces while providing a more physiologically relevant 3-D environment. Hydrogels made up of collagen, between 0.5 and 1.0mg/ml, exhibited mechanical stability adequate to withstand fluid mechanical forces (<0.11 mN) typical of automated commercial fluid transfer equipment. Collagen-entrapped cells visualized with the aid of confocal microscopy and a potentiometric-sensitive dye, TMRM, exhibited round morphology in comparison to flat morphology typical of cells in two-dimensional (2-D) monolayer cultures. Morphological differentiation characterized by neurite extension and cell aggregation was observed for both 2-D and 3-D cultures. Differentiated IMR-32 cells failed to develop a resting membrane potential typical of excitable cells. Free intracellular calcium was monitored with Calcium Green-1. Depolarization-induced Ca 2+influx was only observed with differentiated 3-D cells unlike 2-D cells, where calcium flux was observed in both differentiated and undifferentiated cells. Taken together, the results revealed that collagen hydrogels (0.5 mg/ml collagen) were suitable structural supports for weakly adherent cells. However, for voltage-dependent calcium channel function applications, further investigations are needed to explain the difference between 2-D monolayer and 3-D collagen-entrapped cells.

Biochemical and electrophysiological differentiation profile of a human neuroblastoma (IMR-32) cell line

A human neuroblastoma cell line (IMR-32), when differentiated, mimics large projections of the human cerebral cortex and under certain tissue culture conditions, forms intracellular fibrillary material, commonly observed in brains of patients affected with Alzheimer's disease. Our purpose is to use differentiated IMR-32 cells as an in vitro system for magnetic field exposure studies. We have previously studied in vitro differentiation of murine neuroblastoma (N1E-115) cells with respect to resting membrane potential development. The purpose of this study was to extend our investigation to IMR-32 cells. Electrophysiological (resting membrane potential, V(m)) and biochemical (neuron-specific enolase activity [NSE]) measurements were taken every 2 d for a period of 16 d. A voltage-sensitive oxonol dye together with flow cytometry was used to measure relative changes in V(m). To rule out any effect due to mechanical cell detachment, V(m) was indirectly measured by using a slow potentiometric dye (tetramethylrhodamine methyl ester) together with confocal digital imaging microscopy. Neuron-specific enolase activity was measured by following the production of phosphoenolpyruvate from 2-phospho-d-glycerate at 240 nm. Our results indicate that in IMR-32, in vitro differentiation as characterized by an increase in NSE activity is not accompanied by resting membrane potential development. This finding suggests that pathways for morphological-biochemical and electrophysiological differentiations in IMR-32 cells are independent of one another.