Brown adipose tissue macrophages control tissue innervation and homeostatic energy expenditure

Tissue macrophages provide immune defense and contribute to establishment and maintenance of tissue homeostasis. Here we used constitutive and inducible mutagenesis to delete the nuclear transcription regulator methyl-CpG binding protein 2 (Mecp2) in defined tissue macrophages. Animals lacking the Rett syndrome-associated gene in macrophages did not show signs of neurodevelopmental disorder, but displayed spontaneous obesity, which could be linked to impaired brown adipose tissue (BAT) function. Specifically, mutagenesis of a BAT-resident CX(3)CR1(+) macrophage subpopulation compromised homeostatic, though not acute cold-induced thermogenesis. Mechanistically, BAT malfunction of pre-obese mice harboring mutant macrophages was associated with decreased sympathetic innervation and local norepinephrine titers, resulting in reduced adipocyte expression of thermogenic factors. Mutant macrophages over-expressed PlexinA4, which might contribute to the phenotype by repulsion of Sema6A-expressing sympathetic axons. Collectively, we report a previously unappreciated homeostatic role of macrophages in the control of tissue innervation, disruption of which in BAT results in metabolic imbalance.