Short talk by Raluca Niesner
Freie Universität Berlin;
Germany Rheumatology Research Center, Berlin (DRFZ) – A Leibniz Institute
MetaFLIMB – longitudinal in vivo NAD(P)H fluorescence lifetime imaging of the femoral marrow
It is well known that phenotypes and functions of immune cells, including myeloid cells, are tightly linked to their metabolic profile, while all three, phenotype, function and metabolic profile, are affected by the continuously changing tissue microenvironment. Focusing on bone regeneration after injury, we aim to understand changes in the metabolism of myeloid cells in bone marrow context, over time. However, therefore technologies providing such kind of information are missing.
We developed our longitudinal intravital imaging of the mouse femur, to enable longitudinal micro-endoscopic fluorescence lifetime imaging (FLIM) for metabolic profiling (“MetaFLIMB”). Using our previously developed reference system of enzyme-dependent fluorescence lifetimes derived from the ubiquitous metabolic co-enzymes NADH and NADPH (NAD(P)H), we can determine preferential enzymatic activities in vivo. We stratify enzymatic activities to identify dominant metabolic pathways voxel-wise pathways serving for energy production, i.e. (1) fatty acid β-oxidation (FAO, indicated by HADH activity), (2) anaerobic glycolysis-like pathways (indicated by LDH activity), (3) a group of pathways encompassing carbohydrate metabolism, including glycolysis (GAPDH activity) pentose-phosphate pathway (SDH and G6PDH activity), tricaboxylic acid cycle (IDH activity) and electron transport chain (Complex I activity), (4) oxidative mechanisms, including oxidative phosphorylation (Oxphos, indicated by PDH activity). Additionally, we distinguish pathways associated to cellular function and cellular state, i.e. oxidative burst (NADPH oxidase activity) and dormancy or death, indicated by low/no NAD(P)H-dependent enzymatic activity, reflected by an increase in unbound (free) NAD(P)H.
Using MetaFLIMB in osteotomized femurs of mice with red fluorescent myeloid cells (LysMCre/+R26LSL-tdRFP), we demonstrate that myeloid cells display highly heterogeneous metabolic profiles both spatially and temporally during bone regeneration. Our results go beyond the binary paradigm of myeloid cells using either glycolytic or oxidative signaling pathways (linked to pro- or anti-inflammatory functions, respectively) derived from in vitro experiment. We attribute the higher metabolic heterogeneity of myeloid cells in vivo compared to in vitro conditions to the dynamic metabolic microenvironment in the bone marrow. Under in vivo conditions, myeloid cells with various metabolic profiles, i.e. using other pathways for energy production than the anaerobic pathway associated with pro-inflammatory cells, performed the oxidative burst necessary for the process of phagocytosis. This demonstrates that a high metabolic flexibility of myeloid cells in vivo is related to their functional flexibility. It suggests that myeloid cells, which perform an oxidative burst and are responsible for clearing of debris at early stages after bone injury, and those responsible for vascular or bone remodeling during the later phases, use distinct pathways for energy production.
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