Filter-exchange spectroscopy is sensitive to gradual cell membrane degradation.

ABSTRACT

Transmembrane water permeability changes occur after initialization of necrosis and are a mechanism for early detection of cell death. Filter-exchange spectroscopy (FEXSY) is sensitive to transmembrane water permeability and enables its quantification by magnetic resonance via the apparent exchange rate (AXR). In this study, we investigate AXR changes during necrotic cell death. FEXSY measurements of yeast cells in different necrotic stages were performed and compared with established fluorescence cell death markers and pulsed gradient spin echo measurements. Furthermore, the influence of T2 relaxation on AXR was examined in a two-compartment system. The AXR of yeast cells increased slightly after incubation with 20% isopropanol, whereas it peaked sharply after incubation with 25% isopropanol. At this point, almost all the yeast cells were vital but showed compromised membranes. After incubation with 30% isopropanol, AXR measurements showed high variability, at a point corresponding to a majority of the yeast cells being in late-stage necrosis with disrupted cell membranes. Simulations revealed that, for FEXSY measurements in a two-compartment system, a long filter echo time (TEf), compared with the T2 of the slow-diffusing compartment, filters out a fraction of the slow-diffusing compartment signal and leads to overestimation of apparent diffusion coefficient (ADC) and underestimation of AXR. Our results demonstrate that AXR is sensitive to gradual permeabilization of the cell membrane of living cells in different permeabilization stages without exogenous contrast agents. AXR measurements were sensitive to permeability changes induced by relatively low concentrations of isopropanol, at levels for which no measurable effect was detectable by ADC measurements. TEf may act as a signal filter that affects the estimated AXR value of a system consisting of a variety of local diffusivities and a range of T2 that includes T2 values shorter or comparable with the TEf.