Calcium-induced alterations in mitochondrial morphology quantified in situ with optical scatter imaging

Nanocluster model of photothermal assay: application for high-sensitive monitoring of nicotine-induced changes in metabolism, apoptosis, and necrosis at a cellular level

This study evaluates the capability of a photothermal (PT) assay to monitor the impact of nicotine on pancreatic cancer cells (AR42J). The specific PT response is closely proportional to nicotine concentrations at concentration range 1 nM to 100 microM, while at high concentrations of nicotine ranging from 1 mM to 50 mM, PT response shows dramatic decrease. According to the theoretical model, the mechanism of the PT assay is associated with metabolic and apoptotic-related shrinking of local cellular absorbing nanoscale zones caused by increased local absorption at low nicotine doses, while high doses of nicotine lead to apoptotic release of absorbing component (cytochrome c) into the intracellular space, and necrotic swelling of organelles, thereby causing a decrease in local absorption. This model is verified with conventional imaging and with Annexin-V Propidium iodide kits. The PT assay, in addition to its high sensitivity (3 orders of magnitude better than conventional assay), shows the potential to distinguish between various functional states of cells that are associated with changes in metabolism, early and late stages of apoptosis, and necrosis. Comparison of PT responses of pancreatic tumor cells AR42J with isolated primary pancreatic acinar cells and HepG2 cells shows a universal nature of PT assay.

Light scattering from intact cells reports oxidative-stress-induced mitochondrial swelling

Angularly resolved light scattering measurements were performed on suspensions of EMT6 cells and on mitochondria isolated from rabbit liver. Mie theory analysis of the scattering from intact cells indicated that mitochondrial-sized organelles dominated scattering in the range 5-90 degrees . This interpretation was supported by the analysis of scattering from isolated mitochondria. Intact cells were subjected to oxidative stress by photodynamic insult. After 3 h of incubation in the heme precursor aminolevulinic acid hexylester, EMT6 cells accumulated abundant protoporphyrin IX, an endogenous photosensitizer formed in mitochondria. Irradiation of aminolevulinic acid/protoporphyrin IX-sensitized cells with 10 J cm(-2) of 514 nm light led to pronounced changes in angularly resolved light scattering consistent with mitochondrial swelling. Electron microscopy of similarly treated EMT6 cell monolayers showed significant changes in mitochondrial morphology, which included distension of the outer unit membrane and bloating of the internal mitochondrial compartment. Informed by these electron microscopy results, we implemented a coated sphere model to interpret the scattering from intact cells subjected to oxidative stress. The coated sphere interpretation was compatible with the scattering measurements from these cells, whereas simpler Mie theory models based on homogenous swelling were dramatically unsuccessful. Thus, in this system, angularly resolved light scattering reports oxidative-stress-induced changes in mitochondrial morphology.

Photothermal detection of nicotine-induced apoptotic effects in pancreatic cancer cells

The objective of this study was to evaluate the capability of a photothermal (PT) assay in determining the effects of graded doses of nicotine in a pancreatic acinar cell line (AR42J). The cellular response to nicotine was detected through the monitoring of PT signals from light-absorbing endogenous cellular structures that have been used as natural indicators for nicotine's action. It was demonstrated that introducing nicotine to cultured acinar cells in vitro leads to changes in cellular absorbing structures, thereby altering the microstructure of PT cell images and the temporal shape of PT signals. The results showed that the dependence of specific PT parameters was almost proportional to nicotine concentrations ranging from 1 nM to 100 microM, with the saturation maximum at and around 100 microM - 1 mM; thereafter, PT signals decreased rapidly to control levels and even lower, in the range of 1 - 50 mM. Conventional fluorescent tests (Annexin V--Propidium Iodide) performed in parallel showed no effect with nicotine at a concentration <1 microM (three orders of magnitude greater than the sensitivity threshold of the PT assay). With an increase in nicotine concentration from 1 mM to 50 mM, rapidly growing apoptotic and necrotic cells were detected. Thus, the PT assay demonstrated the capability for high-sensitivity detection of nicotine's impact, which may be related to a change in cell metabolism, apoptosis, or necrosis, depending on nicotine concentration.

BCL-xL-dependent light scattering by apoptotic cells

We measured the intensity ratio of wide-to-narrow angle scatter, optical scatter image ratio (OSIR), in single cells during apoptosis and after overexpression of the mitochondria-bound antiapoptotic protein BCL-xL. OSIR is sensitive to particle size/shape for objects with wavelength-scale dimensions, and was used as a morphometric measure of cellular response. Three cell variants were treated with staurosporine (STS): nontransfected parental CSM14.1, CSM14.1 stably expressing yellow fluorescent protein (YFP) with diffuse YFP fluorescence, and apoptosis-resistant CSM14.1 stably expressing the fusion protein construct YFP-BCL-xL with YFP fluorescence localized on the mitochondria. After treatment with 1 or 2 muM STS, the measured OSIR decreased monotonically by approximately 25% in the nontransfected and YFP variants, and reached a steady-state value 40-60 min after STS treatment. The decrease in OSIR at the onset of apoptosis preceded phosphatidyl serine exposure by 5 h. In the YFP-BCL-xL cell variant, the initial OSIR was already approximately 24% lower than the initial OSIR in YFP and nontransfected cells, and only decreased by <10% after STS treatment. Alterations in light scattering by cells overexpressing BCL-xL even before apoptosis induction raise interesting questions as to the role of BCL-xL in conferring apoptosis resistance by preconditioning the cells and possibly altering mitochondrial morphology.

Anoxia induces matrix shrinkage accompanied by an increase in light scattering in isolated brain mitochondria

It is important to monitor mitochondrial conditions, and light scattering (LS) measurements have been applied to the detection of morphological changes in mitochondria in vivo. Little is known about the morphological and LS responses of brain mitochondria to oxygen withdrawal, a critical factor in cell death. We have therefore investigated the morphological and LS responses of isolated brain mitochondria to anoxia. Anoxia induced an increase in LS, reflecting mitochondrial matrix shrinkage. This response was reversible, but was reduced by adding digitonin, which disrupted the outer membrane selectively. This suggested that integrity of the outer membrane was necessary for the matrix response. We further examined the effects of Mg2+ and ATP on the responses because both exist in cells and modulate the changes in matrix volume. Although Mg2+ and ATP reduced the rates of increase and decrease in LS, respectively, the magnitudes of the increases in LS caused by anoxia stayed at over 80% of the control level (no Mg2+) in the presence of Mg2+ and ATP. This suggested that the increase in LS occurred in cells containing Mg2+ and ATP during anoxia. In contrast, that caused by inhibitors of the electron transport chain was reduced to below 30% of the control level in the presence of Mg2+. The present in vitro study provides a basis for interpretation of LS signals from mitochondria in brain research during oxygen withdrawal.

Detectability of contrast agents for confocal reflectance imaging of skin and microcirculation

Confocal reflectance microscopy of skin and other tissues in vivo is currently limited to imaging at the cellular, nuclear and general architectural levels due to the lack of microstructure-specific contrast. Morphologic and functional imaging at specific organelle and microstructure levels may require the use of exogenous contrast agents in small (nontoxic) concentrations, from which weakly backscattered light must be detected in real time. We report an analysis based on Mie theory to predict detectability, in terms of signal-to-background and signal-to-noise ratios, of reflectance contrast agents within skin and microcirculation. The analysis was experimentally verified by detectability of (a) intravenously injected polystyrene microspheres that enhance the contrast of dermal microcirculation in Sprague-Dawley rats, and (b) acetic acid-induced compaction of chromatin that enhances nuclear morphology in normal and cancerous human skin. Such analyses and experiments provide a quantitative basis for developing the opto-biochemical properties and use of contrast agents and for designing confocal instrumentation to enable real-time detectability in vivo.