Novel high-throughput assay to assess cellular manganese levels in a striatal cell line model of Huntington's disease confirms a deficit in manganese accumulation

In spite of the essentiality of manganese (Mn) as a trace element necessary for a variety of physiological processes, Mn in excess accumulates in the brain and has been associated with dysfunction and degeneration of the basal ganglia. Despite the high sensitivity, limited chemical interference, and multi-elemental advantages of traditional methods for measuring Mn levels, they lack the feasibility to assess Mn transport dynamics in a high-throughput manner. Our lab has previously reported decreased net Mn accumulation in a mutant striatal cell line model of Huntington's disease (STHdhQ111/Q111) relative to wild-type following Mn exposure. To evaluate Mn transport dynamics in these striatal cell lines, we have developed a high-throughput fluorescence-quenching extraction assay (Cellular Fura-2 Manganese Extraction Assay – CFMEA). CFMEA utilizes changes in fura-2 fluorescence upon excitation at 360 nm (Ca2+ isosbestic point) and emission at 535 nm, as an indirect measurement of total cellular Mn content. Here, we report the establishment, development, and application of CFMEA. Specifically, we evaluate critical extraction and assay conditions (e.g. extraction buffer, temperature, and fura-2 concentration) required for efficient extraction and quantitative detection of cellular Mn from cultured cells. Mn concentrations can be derived from quenching of fura-2 fluorescence with standard curves based on saturation one-site specific binding kinetics. Importantly, we show that extracted calcium and magnesium concentrations below 10 μM have negligible influence on measurements of Mn by fura-2. CFMEA is able to accurately measure extracted Mn levels from cultured striatal cells over a range of at least 0.1–10 μM. We have used two independent Mn supplementation approaches to validate the quantitative accuracy of CFMEA over a 0–200 μM cellular Mn-exposure range. Finally, we have utilized CFMEA to experimentally confirm a deficit in net Mn accumulation in the mutant HD striatal cell line versus wild-type cells. To conclude, we have developed and applied a novel assay to assess Mn transport dynamics in cultured striatal cell lines. CFMEA provides a rapid means of evaluating Mn transport kinetics in cellular toxicity and disease models.