Cellular source of hypothalamic macrophage accumulation in diet-induced obesity.

Obese mice on a high-fat diet (HFD) display signs of inflammation in the hypothalamic arcuate nucleus (ARC), a critical area for controlling systemic energy metabolism. This has been suggested as a key mechanism of obesity-associated hypothalamic dysfunction. We reported earlier that bone marrow-derived macrophages accumulate in the ARC to sustain hypothalamic inflammation upon chronic exposure to an HFD. However, the mechanism underlying hypothalamic macrophage accumulation has remained unclear.

We investigated whether circulating monocytes or myeloid precursors contribute to hypothalamic macrophage expansion during chronic HFD feeding. To trace circulating myeloid cells, we generated mice that express green fluorescent protein (GFP) in their lysozyme M-expressing myeloid cells (LysM^GFP mice). We conducted parabiosis and bone marrow transplantation experiments using these animals. Mice received an HFD for 12 or 30 weeks and were then sacrificed to analyze LysM^GFP cells in the hypothalamus. Hypothalamic vascular permeability in the HFD-fed obese mice was also tested by examining the extravascular leakage of Evans blue and fluorescence-labeled albumin. The timing of LysM^GFP cell entry to the hypothalamus during development was also evaluated.

Our parabiosis and bone marrow transplantation experiments revealed a significant infiltration of circulating LysM^GFP cells into the liver, skeletal muscle, choroid plexus, and leptomeninges but not in the hypothalamic ARC during chronic HFD feeding, despite increased hypothalamic vascular permeability. These results suggested that the recruitment of circulating monocytes is not a major mechanism for maintaining and expanding the hypothalamic macrophage population in diet-induced obesity. We demonstrated instead that LysM^GFP cells infiltrate the hypothalamus during its development. LysM^GFP cells appeared in the hypothalamic area from the late embryonic period. This cellular pool suddenly increased immediately after birth, peaked at the postnatal second week, and adopted an adult pattern of distribution after weaning.

Bone marrow-derived macrophages mostly populate the hypothalamus in early postnatal life and may maintain their pool without significant recruitment of circulating monocytes throughout life, even under conditions of chronic HFD feeding.