C60 fullerene localization and membrane interactions in RAW 264.7 immortalized mouse macrophages

There continues to be a significant increase in the number and complexity of hydrophobic nanomaterials that are engineered for a variety of commercial purposes making human exposure a significant health concern. This study uses a combination of biophysical, biochemical and computational methods to probe potential mechanisms for uptake of C60 nanoparticles into various compartments of living immune cells. Cultures of RAW 264.7 immortalized murine macrophage were used as a canonical model of immune-competent cells that are likely to provide the first line of defense following inhalation. Modes of entry studied were endocytosis/pinocytosis and passive permeation of cellular membranes. The evidence suggests marginal uptake of C60 clusters is achieved through endocytosis/pinocytosis, and that passive diffusion into membranes provides a significant source of biologically-available nanomaterial. Computational modeling of both a single molecule and a small cluster of fullerenes predicts that low concentrations of fullerenes enter the membrane individually and produce limited perturbation; however, at higher concentrations the clusters in the membrane causes deformation of the membrane. These findings are bolstered by nuclear magnetic resonance (NMR) of model membranes that reveal deformation of the cell membrane upon exposure to high concentrations of fullerenes. The atomistic and NMR models fail to explain escape of the particle out of biological membranes, but are limited to idealized systems that do not completely recapitulate the complexity of cell membranes. The surprising contribution of passive modes of cellular entry provides new avenues for toxicological research that go beyond the pharmacological inhibition of bulk transport systems such as pinocytosis.

Probing the epigenetic regulation of HIF-1α transcription in developing tissue

HIF-1-mediated hypoxia signalling in embryonic tissue is probed by assessing the methylation status of the HIF-1α and EPO HRE promoters using bisulfite sequencing.

Dielectrophoretic assembly of insulinoma cells and fluorescent nanosensors into three-dimensional pseudo-islet constructs

Dielectrophoretic forces, generated by radio-frequency voltages applied to micromachined, transparent, indium tin oxide electrodes, have been used to condense suspensions of insulinoma cells (BETA-TC-6 and INS-1) into a 10 x 10 array of three-dimensional cell constructs. Some of these constructs, measuring approximately 150 microm in diameter, 120 microm in height and containing around 1000 cells, were of the same size and cell density as a typical islet of Langerhans. With the dielectrophoretic force maintained, these engineered cell constructs were able to withstand mechanical shock and fluid flow forces. Reproducibility of the process required knowledge of cellular dielectric properties, in terms of membrane capacitance and membrane conductance, which were obtained by electrorotation measurements. The ability to incorporate fluorescent nanosensors, as probes of cellular oxygen and pH levels, into these 'pseudo-islets' was also demonstrated. The footprint of the 10 x 10 array of cell constructs was compatible with that of a 1536 microtitre plate, and thus amenable to optical interrogation using automated plate reading equipment.

Electrokinetic measurements of membrane capacitance and conductance for pancreatic -cells

Membrane capacitance and membrane conductance values are reported for insulin secreting cells (primary -cells and INS-1 insulinoma cells), determined using the methods of dielectrophoresis and electrorotation. The membrane capacitance value of 12.57 (+/-1.46) mFm(-2), obtained for -cells, and the values from 9.96 (+/-1.89) mFm(-2) to 10.65 (+/-2.1) mFm(-2), obtained for INS-1 cells, fall within the range expected for mammalian cells. The electrorotation results for the INS-1 cells lead to a value of 36 (+/-22) Sm(-2) for the membrane conductance associated with ion channels, if values in the range 2-3 nS are assumed for the membrane surface conductance. This membrane conductance value falls within the range reported for INS cells obtained using the whole-cell patch-clamp technique. However, the total 'effective' membrane conductance value of 601 (+/-182) Sm(-2) obtained for the INS-1 cells by dielectrophoresis is significantly larger (by a factor of around three) than the values obtained by electrorotation. This could result from an increased membrane surface conductance, or increased passive conduction of ions through membrane pores, induced by the larger electric field stresses experienced by cells in the dielectrophoresis experiments.

Monitoring Cl− Movement in Single Cells Exposed to Hypotonic Solution

Self-referencing ion--selective electrodes (ISEs), made with Chloride Ionophore I-Cocktail A (Fluka), were positioned 1-3 microm from human embryonic kidney cells (tsA201a) and used to record chloride flux during a sustained hyposmotic challenge. The ISE response was close to Nernstian when comparing potentials (VN) measured in 100 and 10 mM NaCl (deltaVN = 57 +/- 2 mV), but was slightly greater than ideal when comparing 1 and 10 mM NaCl (deltaVN = 70 +/- 3 mV). The response was also linear in the presence of 1 mM glutamate, gluconate, or acetate, 10 microM tamoxifen, or 0.1, 1, or 10 mM HEPES at pH 7.0. The ISE was approximately 3 orders of magnitude more selective for Cl- over glutamate or gluconate but less than 2 orders of magnitude move selective for Clover bicarbonate, acetate, citrate or thiosulfate. As a result this ISE is best described as an anion sensor. The ISE was 'poisoned' by 50 microM 5-nitro-2-(3phenylpropyl-amino)-benzoic acid (NPPB), but not by tamoxifen. An outward anion efflux was recorded from cells challenged with hypotonic (250 +/- 5 mOsm) solution. The increase in efflux peaked 7-8 min before decreasing, consistent with regulatory volume decreases observed in separate experiments using a similar osmotic protocol. This anion efflux was blocked by 10 microM tamoxifen. These results establish the feasibility of using the modulation of electrochemical, anion-selective, electrodes to monitor anions and, in this case, chloride movement during volume regulatory events. The approach provides a real-time measure of anion movement during regulated volume decrease at the single-cell level.

Mitochondrial metabolism reveals a functional architecture in intact islets of Langerhans from normal and diabetic Psammomys obesus

The cells within the intact islet of Langerhans function as a metabolic syncytium, secreting insulin in a coordinated and oscillatory manner in response to external fuel. With increased glucose, the oscillatory amplitude is enhanced, leading to the hypothesis that cells within the islet are secreting with greater synchronization. Consequently, non-insulin-dependent diabetes mellitus (NIDDM; type 2 diabetes)-induced irregularities in insulin secretion oscillations may be attributed to decreased intercellular coordination. The purpose of the present study was to determine whether the degree of metabolic coordination within the intact islet was enhanced by increased glucose and compromised by NIDDM. Experiments were performed with isolated islets from normal and diabetic Psammomys obesus. Using confocal microscopy and the mitochondrial potentiometric dye rhodamine 123, we measured mitochondrial membrane potential oscillations in individual cells within intact islets. When mitochondrial membrane potential was averaged from all the cells in a single islet, the resultant waveform demonstrated clear sinusoidal oscillations. Cells within islets were heterogeneous in terms of cellular synchronicity (similarity in phase and period), sinusoidal regularity, and frequency of oscillation. Cells within normal islets oscillated with greater synchronicity compared with cells within diabetic islets. The range of oscillatory frequencies was unchanged by glucose or diabetes. Cells within diabetic (but not normal) islets increased oscillatory regularity in response to glucose. These data support the hypothesis that glucose enhances metabolic coupling in normal islets and that the dampening of oscillatory insulin secretion in NIDDM may result from disrupted metabolic coupling.

REGENERATING ADULT COCKROACH DORSAL UNPAIRED MEDIAN NEURONES IN VITRO RETAIN THEIR IN VIVO MEMBRANE CHARACTERISTICS

The ability of differentiated neurones to recover from disease or injury depends upon both intrinsic and extrinsic factors. Whereas most mammalian neurones have a limited capacity for regeneration, regulated, in part, by physical and chemical cues in the brain microenvironment (Bray et al. 1987; Caroni and Schwab, 1988, 1989), invertebrates, and in particular insects, exhibit a far greater capacity for repair of central neurones and circuits (Treherne et al. 1988). Studies of the cues that regulate the regenerative process are made easier by the use of individual, identified neurones, cultured under controlled conditions. Invertebrates are particularly useful in this regard; neurones from mature nervous systems of both annelids and molluscs have been grown successfully in culture and their growth can be influenced by changes in the culture conditions (Acklin and Nicholls, 1990; Dagan and Levitan, 1981; Ready and Nicholls, 1979; Syed et al. 1990). Routine and long-term culture of identified neurones from the insect central nervous system (CNS) has proved more elusive, preventing the use of neurones from these well-studied systems. Recently, however, cultures of cockroach (Howes et al. 1991), locust (Kirchoff and Bicker, 1992) and moth (Hayashi and Levine, 1992) adult neurones have been described.