The Histo-CLEM Workflow for tissues of model organisms

The cellular mechanism(s) linking macrophages to norepinephrine (NE)-mediated regulation of thermogenesis has been a topic of debate. Here, we identify sympathetic neuron-associated macrophages (SAMs) as a population of cells that mediate clearance of NE via expression of Slc6a2, an NE transporter, and monoamine oxidase A (MAOa), a degradation enzyme. Optogenetic activation of the SNS upregulates NE uptake by SAMs and shifts the SAM profile to a more pro-inflammatory state. NE uptake by SAMs is prevented by genetic deletion of Slc6a2 or inhibition of the transporter. We also observed increased SAM content in the SNS of two obesity mouse models. Genetic ablation of Slc6a2 in SAMs increases brown adipose tissue (BAT) content, causes browning of white fat, increases thermogenesis, and leads to significant and sustained weight loss of obese mice. We further show that this pathway is conserved, as human sympathetic ganglia also contain SAMs expressing the analogous molecular machinery for NE clearance, thus constituting a potential target for obesity treatment.

Microscopy has gone hand in hand with the study of living systems since van Leeuwenhoek observed living microorganisms and cells in 1674 using his light microscope. A spectrum of dyes and probes now enable the localization of molecules of interest within living cells by fluorescence microscopy. With electron microscopy (EM), cellular ultrastructure has been revealed. Bridging these two modalities, correlated light microscopy and EM (CLEM) opens new avenues. Studies of protein dynamics with fluorescent proteins (FPs), which leave the investigator 'in the dark' concerning cellular context, can be followed by EM examination. Rare events can be preselected at the light microscopy level before EM analysis. Ongoing development-including of dedicated probes, integrated microscopes, large-scale and three-dimensional EM and super-resolution fluorescence microscopy-now paves the way for broad CLEM implementation in biology.