Diet-Induced Obesity Elicits Macrophage Infiltration and Reduction in Spine Density in the Hypothalami of Male but Not Female Mice

Abstract

Increasing prevalence in obesity has become a significant public concern. C57BL/6J mice are prone to diet-induced obesity (DIO) when fed high-fat diet (HFD), and develop chronic inflammation and metabolic syndrome, making them a good model to analyze mechanisms whereby obesity elicits pathologies. DIO mice demonstrated profound sex differences in response to HFD with respect to inflammation and hypothalamic function. First, we determined that males are prone to DIO, while females are resistant. Ovariectomized females, on the other hand, are susceptible to DIO, implying protection by ovarian hormones. Males, but not females, exhibit changes in hypothalamic neuropeptide expression. Surprisingly, ovariectomized females remain resistant to neuroendocrine changes, showing that ovarian hormones are not necessary for protection. Second, obese mice exhibit sex differences in DIO-induced inflammation. Microglial activation and peripheral macrophage infiltration is seen in the hypothalami of males, while females are protected from the increase in inflammatory cytokines and do not exhibit microglia morphology changes nor monocyte-derived macrophage infiltration, regardless of the presence of ovarian hormones. Strikingly, the anti-inflammatory cytokine IL-10 is increased in the hypothalami of females but not males. Third, this study posits a potential mechanism of obesity-induced impairment of hypothalamic function whereby obese males exhibit reduced levels of synaptic proteins in the hypothalamus and fewer spines in GnRH neurons, located in the areas exhibiting macrophage infiltration. Our studies suggest that inflammation-induced synaptic remodeling is potentially responsible for hypothalamic impairment that may contribute to diminished levels of gonadotropin hormones, testosterone, and sperm numbers, which we observe and corresponds to the observations in obese humans. Taken together, our data implicate neuro-immune mechanisms underlying sex-specific differences in obesity-induced impairment of the hypothalamic function with potential consequences for reproduction and fertility.

Leukemia Inhibitory Factor Represses GnRH Gene Expression via cFOS during Inflammation in Male Mice

“…Obesity is considered a state of chronic inflammation as opposed to acute inflammation elicited by LPS injection [24, 25, 81]. Thus, we exposed male C57BL/6J mice to HFD or to CTR diet for 12 weeks.…”

“…Centrally administered cytokines also provoked reduced LH and GnRH levels, but the mechanism whereby these cytokines mediate their effects is unknown [16, 17, 21-23]. More recently, our group determined that low-grade, chronic inflammation caused by high-fat diet (HFD)-induced obesity may also directly affect GnRH neurons, resulting in reduced levels of LH in circulation and diminished GnRH mRNA levels in the hypothalamus, specifically in male mice [24]. We, and others, have reported that diet-induced obese mice and people exhibited increased levels of proinflammatory cytokines, TNF-α, IL-1β, and IL-6, in circulation [24, 25] and in the hypothalamus, at the mRNA level and protein level [24, 26].…”

“…More recently, our group determined that low-grade, chronic inflammation caused by high-fat diet (HFD)-induced obesity may also directly affect GnRH neurons, resulting in reduced levels of LH in circulation and diminished GnRH mRNA levels in the hypothalamus, specifically in male mice [24]. We, and others, have reported that diet-induced obese mice and people exhibited increased levels of proinflammatory cytokines, TNF-α, IL-1β, and IL-6, in circulation [24, 25] and in the hypothalamus, at the mRNA level and protein level [24, 26]. Additionally, we also identified that leukemia inhibitory factor (LIF), a member of the IL-6 family, is increased in the circulation and locally produced in male mice hypothalami in obesity [24].…”

“…We, and others, have reported that diet-induced obese mice and people exhibited increased levels of proinflammatory cytokines, TNF-α, IL-1β, and IL-6, in circulation [24, 25] and in the hypothalamus, at the mRNA level and protein level [24, 26]. Additionally, we also identified that leukemia inhibitory factor (LIF), a member of the IL-6 family, is increased in the circulation and locally produced in male mice hypothalami in obesity [24]. Interestingly, LIF is increased in a sex-specific manner only in males that exhibit reduction in GnRH mRNA and gonadotropin hormones, but not in females that lack changes in GnRH or gonadotropin hormones.…”

“…IL-6, a prototypical member of the family, on the other hand, is increased in both sexes. Our previous study postulated that impairment of GnRH neurons stems from reduction in spine density and consequently the connectivity of the GnRH network [24]. However, GnRH neurons express several cytokine receptors [27] and inflammation-induced cytokines may directly regulate intracellular signaling pathways in the GnRH neurons.…”