Microglial Inflammatory Signaling Orchestrates the Hypothalamic Immune Response to Dietary Excess and Mediates Obesity Susceptibility

Abstract

Dietary excess triggers accumulation of pro-inflammatory microglia in the mediobasal hypothalamus (MBH), but the components of this microgliosis and its metabolic consequences remain uncertain. Here, we show that microglial inflammatory signaling determines the immunologic response of the MBH to dietary excess and regulates hypothalamic control of energy homeostasis in mice. Either pharmacologically depleting microglia or selectively restraining microglial NF-κB-dependent signaling sharply reduced microgliosis, an effect that includes prevention of MBH entry by bone-marrow-derived myeloid cells, and greatly limited diet-induced hyperphagia and weight gain. Conversely, forcing microglial activation through cell-specific deletion of the negative NF-κB regulator A20 induced spontaneous MBH microgliosis and cellular infiltration, reduced energy expenditure, and increased both food intake and weight gain even in absence of a dietary challenge. Thus, microglial inflammatory activation, stimulated by dietary excess, orchestrates a multicellular hypothalamic response that mediates obesity susceptibility, providing a mechanistic rationale for non-neuronal approaches to treat metabolic diseases.

Keywords: energy balance; gliosis; hypothalamus; infiltration; inflammation; microglia; myeloid cell; obesity.

Figure 1. Microglial Depletion Reduces Body Weight and Food Intake in Mice Fed a HFD

(A) Representative MBH sections showing Iba1⁺ microglial depletion by feeding mice either a CD or a HFD containing PLX5622 (1.2 g/kg) versus a nutrient-matched control diet. (B) Quantification of (A) (n = 6–8, two-tailed t test, **p < 0.01 versus control). (C) Reduced body weight in PLX5622-treated mice, specifically when fed a HFD (n = 12, two-way repeated-measures [RM] ANOVA, *p < 0.05 or **p < 0.01 HFD versus HFD+ PLX5622). (D) Unchanged lean mass and reduced fat mass in PLX5622-treated mice fed a 4-week HFD (n = 12, two-tailed t test, *p < 0.05 versus HFD). (E) Equivalent food intake in PLX5622-treated and control mice fed a low-fat CD. (F) Reduced food intake in PLX5622-treated mice fed a HFD (cumulative intake normalized to intial body weight, n = 12, two-way RM-ANOVA, *p < 0.05 or **p < 0.01 versus HFD). Values are mean ± SEM (3V, third ventricle; scale bar, 20 μm). See also Figure S1.

Figure 2. Deleting Microglial IKKβ Reduces Body Weight and Food Intake in Mice Fed a HFD

(A and B) Genomic PCR for truncated IKKβ exon 3 (A) and qPCR for Ikbkb mRNA (B) from sorted CD11b⁺/CD45^low microglia 4 weeks after tamoxifen-induced recombination (n = 5–7, two-tailed t test; ***p < 0.001). (C) Tnfa (TNF) mRNA induction in extracted microglia treated with LPS ex vivo showing reduced response in IKKβ^MGKO microglia (n = 4, two-tailed t test; *p < 0.05 versus non-LPS control). (D) Reduced body weight in IKKβ^MGKO mice fed a HFD (n = 12–16; two-way RM-ANOVA, *p < 0.05, **p < 0.01 KO HFD versus Ctl HFD). (E) Equivalent food intake in IKKβ^MGKO and control mice fed a CD (n = 5–6). (F) Reduced food intake in IKKβ^MGKO mice fed a HFD (cumulative intake normalized to intial body weight, n = 6–7; two-way RM-ANOVA; **p < 0.01, ***p < 0.001). (G) Equivalent lean body mass in IKKβ^MGKO and control mice fed a CD and HFD (n = 10–12). (H) Reduced total body fat in IKKβ^MGKO mice fed a HFD (n = 10–12/group; two-way ANOVA; *p < 0.05 KO HFD versus Ctl HFD). (I) Equivalent energy expenditure in IKKβ^MGKO and control mice (n = 8). (J) Reduced number and size of Iba1⁺ microglia in the hypothalami of IKKβ^MGKO mice fed a HFD. Values are mean ± SEM (scale bar, 20 μm). (K) Quantification of (J). Values are mean ± SEM. *p < 0.05 versus control. See also Figures S2 and S7.

Figure 3. Diet-Responsive Microglia Are Spatially Distinct within the Hypothalamus

(A) MBH sections showing a distinct spatial distribution of Iba1⁺, P2Y12⁺, and Tmem119⁺ cells in the median eminence (ME), arcuate nucleus (ARC), and ventromedial hypothalamus (VMH). (B) Quantification of (A). (C) Representative hypothalamic sections stained for P2Y12 or Tmem119 showing lack of co-localization with GFP (arrowheads) in CX3CR1-GFP mice following a 4-week HFD. (D) Increased hypothalamic CX3CR1⁺/P2Y12⁻ and Cx3cr1⁺/Tmem119⁻ cell numbers in response to HFD. (E) Representative sections showing that a 4-week HFD induces MBH infiltration by CD169⁺ monocytes acquiring a microglia-like morphology on arrival in the niche (inset). (F) Quantification of HFD-induced CD169⁺ MBH infiltration. Values are mean ± SEM (n = 8, *p < 0.05 versus CD; 3V, third ventricle; scale bar, 20 μm).

Figure 4. Peripheral Myeloid Cells Recruited to the MBH Participate in Diet-Induced Microgliosis

(A) Scheme to distinguish between resident microglia and infiltrating cells using CX3CR1^CreER/+: Rosa26-lox-stop-lox-tdTomato mice. 4 weeks after tamoxifen-induced Cre-LoxP recombination, microglia are tdTomato⁺ while circulating monocytes are tdTomato⁻. (B) Representative Iba1⁺/tdTomato⁻ cells, indicating their blood-borne origin (arrowheads), at the edges of the MBH in mice generated as in (A). (C) Increased MBH parenchymal infiltration by Iba1⁺/tdTomato⁻ cells in response to HFD. (D) Bone marrow transplantation strategy to reconstitute irradiated mice (with head shielding) with ubiquitin-GFP marrow (BMT^GFP). (E) Representative hypothalamic sections from BMT^GFP mice fed a CD or HFD for 4 weeks and stained for markers of bone-marrow-derived infiltrating cells (GFP), resident microglia (Tmem119), and pan-myeloid identity (CD68), showing marked diet-induced myeloid cell infiltration (GFP⁺/CD68⁺/Tmem119⁻) and an increased number of atypical myeloid cells (CD68⁺/GFP⁻/Tmem119⁻) in the MBH (see insets). (F) Data from singly and multiply stained MBH sections in (E), quantifying increased absolute numbers of total CD68⁺ cells, infiltrating myeloid cells and atypical myeloid cells, and a decreased number of resident microglia (Tmem119⁺) in response to a HFD ([+], positive staining by indicated antibody; [−], negative staining by indicated antibody; [blank], unstained for indicated antibody). (G) Quantification of the relative myeloid (CD68⁺) composition of the MBH from sections in (E), showing a diet-induced emergence of infiltrating cells (GFP⁺/Tmem119⁻) cells, a decrease in the contribution of resident yolk sac-derive microglia (GFP⁻/Tmem119⁺), and an increase in the contribution of atypical microglia/hypothalamic macrophages (CD68⁺ cells negative for the other two markers). Values are mean ± SEM (n = 6–8, *p < 0.05 versus CD; 3V, third ventricle; scale bar, 20 μm). See also Figure S3.

Figure 5. Depleting Microglia or Restraining Their Activation Potential Abolishes HFD-Induced Myeloid Cell Infiltration into the MBH

(A) Representative sections showing that PLX5622-induced depletion of microglia (Iba1⁺) abolishes bone-marrow-derived (GFP⁺) cell recruitment into the MBH of HFD-fed mice. (B) Similar impact on HFD-induced CD169⁺ cell infiltration into the MBH of IKKβ^MGKO mice. (C) Quantification of (A). (D) Quantification of (B). Values are mean ± SEM (n = 6–8, *p < 0.05; 3V, third ventricle; scale bar, 20 μm). See also Figure S4.

Figure 6. Deleting Microglial Tnfaip3 Increases Inflammatory Activation and Hypo-thalamic Microgliosis in Response to HFD

(A) Reduced Tnfaip3 (A20) mRNA levels in the A20^MGKO microglia. *p < 0.05 versus control. (B) Increased TNF and IL-6 secretion by microglia isolated from Cx3cr1^CreER/+: A20^F/F (A20^MGKO) mice, treated with 4-hydroxytamoxifen (5 μM, 48 hr), and then treated with LPS (100 ng/mL, 16 hr) (n = 5, *p < 0.05 versus control). (C) Increased number and size of Iba1⁺ microglia in the MBH of A20^MGKO mice fed a HFD for 4 weeks. (D) Quantification of (C) (n = 6, *p < 0.05 versus control). (E) Increased infiltration of CD169⁺ cells showing co-localization with the marker Iba1 (arrowheads) into the MBH of A20^MGKO fed a HFD for 4 weeks. (F) Quantification of (E). (G) Co-localization of CD169 with the microglial marker Tmem119 (arrowheads) in the MBH of A20^MGKO mice fed a HFD for 4 weeks. (H) Quantification of (G). Values are mean ± SEM (n = 6–8, *p < 0.05 versus control; 3V, third ventricle; scale bar, 20 μm). See also Figure S5.

Figure 7. Deleting Microglial A20 Increases Body Weight and Food Intake and Reduces Energy Expenditure

(A) Strategy to restrict A20 deletion to microglia by reconstituting irradiated and head-shielded CX3CR1^CreER/+: A20^F/F mice with WT bone marrow and then treating the resulting mice with tamoxifen following their recovery (A20^MGKO-BMT). (B) Spontaneous weight gain following tamoxifen treatment in A20^MGKO-BMT mice fed a standard low-fat CD (n = 6, two-way RM-ANOVA, *p < 0.05 versus control). (C) Increased food intake in CD-fed A20^MGKO-BMT mice singly housed in metabolic cages (cumulative intake normalized to intial body weight, n = 6, two-way RM-ANOVA, *p < 0.05, **p < 0.01 versus control). (D and E) Similar respiratory exchange ratio (RER) values (D) and reduced energy expenditure (E) in A20^MGKO-BMT versus control mice (n = 6, *p < 0.05 versus control). (F) Representative MBH sections showing reduced pSTAT3 staining in the MBH of A20^MGKO-BMT mice 45 min after i.p. leptin injection (3 mg/kg). Values are mean ± SEM (scale bar, 20 μm). (G) Quantification of (F). Values are mean ± SEM. *p < 0.05 versus control. See also Figures S6 and S7.