Cell sorting after tracer injection to elucidate allocation of cellular FDG uptake in different cell types and different microglial activation states

Abstract

AbstractBackground2‐deoxy‐2‐[18F]fluoro‐d‐glucose positron‐emission‐tomography (FDG‐PET) is widely used to study cerebral glucose metabolism. Radiosorting revealed that the FDG‐PET signal is directly influenced by microglial glucose uptake in mouse models and we found similar associations in patients with neurodegenerative diseases. Now, we questioned the allocation of cellular FDG uptake in the rodent brain.MethodSingle cell [18F]FDG uptake as assessed by magnetic activated cell sorting (MACS) was multiplied with published total cell numbers in the mouse brain, i.e. 71e+6 neurons or 7% (≈ 7.6e+6) microglia cells to estimate the overall uptake per cell fraction. Estimated cellular microglial [18F]FDG uptake as assessed by MACS was cross‐validated with observed %‐FDG‐PET reductions after microglia depletion. Interstitial and vessel [18F]FDG was estimated by interstitial compartment volume and assumed equilibrium of non‐phosphorylated extra‐ and intracellular [14C]Deoxyglucose. Synaptic [18F]FDG uptake was estimated via an enriched synaptosome fraction relative to [18F]FDG uptake of whole‐brain homogenate. A microglia proliferation index of 1.3 was assumed for the whole brain in the Aβ mouse model.ResultMicroglia cells accounted for 8.6% of the overall brain [18F]FDG uptake in wild‐type mice and for 29.9% of the overall brain [18F]FDG uptake in PS2APP mice, confirmed by similar alterations after microglia depletion in PET. Synapses contributed to 13.2‐17.1% of the overall brain [18F]FDG uptake whereas interstitial (5.9‐7.7%) and vessel (2.6‐3.4%) [18F]FDG uptake was of minor magnitude. Neuronal and astrocyte cell bodies accounted for <10% of the overall brain [18F]FDG uptake, but a proportion of 42‐55% was not recovered and likely allocated in lost cell processes, oligodendrocytes and other cell types.ConclusionMicroglial FDG uptake alterations are the source of FDG‐PET changes in models of neurodegenerative diseases. Our data should increase the awareness to consider glia cells as key contributors of FDG uptake and the detailed proportions of these cellular contributions in healthy brains and under disease conditions deserve deeper investigation.