Distinct reduction of relative microglial glucose uptake compared to astrocytes and neurons upon isolation from brain environment

Matthias Brendel^1*Sebastian T Kunte¹Johannes Gnörich¹Philipp Beumers¹Laura Bartos¹Stephan Wagner¹Karin Wind-Mark¹Adrien Holzgreve¹Dennis Pötter¹Rudolf A Werner¹Sibylle Ziegler¹Nathalie Lisa Albert¹Alessio Colombo²Sabina Tahirovic²

• ¹Ludwig Maximilian University of Munich, Munich, Germany
• ²German Center for Neurodegenerative Diseases, Helmholtz Association of German Research Centers (HZ), Bonn, North Rhine-Westphalia, Germany

Microglial energy metabolism has gained high attention in neurodegenerative diseases. In vitro methods provide important insights, but it remains unclear if metabolism of brain highly motile microglia is preserved outside their regular environment. Thus, we directly compared microglial in vivo and in vitro glucose uptake in mice. Microglia and astrocytes were isolated from brain using immunomagnetic cell sorting upon [ 18 F]FDG injection in living mice, followed by gamma and cell counting (scRadiotracing). Enriched cell fractions were incubated with excessive [ 18 F]FDG (50,000-fold) in vitro, washed and measured equivalently. For all fractions, radioactivity per cell was normalized to injected or incubated radioactivity and ratios of microglial uptake were calculated relative to astrocytes and the microglia/astrocyte-negative fraction. The experiment was repeated using glucose-free buffer and validated by in vitro incubation without prior in vivo [ 18 F]FDG injection, to exclude influences of fasting and glucose injection. scRadiotracing results were compared against cell culture [ 18 F]FDG incubation. Microglial in vivo glucose uptake was higher when compared to astrocytes (50.4-fold, p<0.0001) and non-microglia/non-astrocyte cells (10.6-fold, p<0.0001). Microglia still comprised highest glucose uptake in vitro, but with a distinct reduction of the microglia-to-astrocyte (5.7-fold, p<0.0015) and microglia-to-microglia/astrocyte-negative ratios (1.7-fold, p<0.0001). Fasting and in vitro incubation validated the results. Cell culture indicated low microglial uptake compared against neurons (1:100) or astrocytes (1:10). Relative to astrocytes and other cells, microglia show a distinct reduction of glucose uptake in vitro when compared against in vivo uptake. Our results emphasize that in vitro experiments should be interpreted cautiously, when studying microglial energy metabolism.