Targeting central inflammation to treat obesity and obesity related disorders
Creators: name: De Luca, Simone
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 320903
subject: Central nervous system
classificationCode: 320999
subject: Neurosciences not elsewhere classified

description: Microglia perform a variety of functions both during development and throughout life, including regulating neuronal survival and death, synaptogenesis, angiogenesis and responding to brain injury. They are also involved in the acute and long-term responses to satiety signalling that become disrupted in overweight and obesity. Obesity is becoming a worldwide epidemic in today’s society. The prevalence of overweight and obese children is particularly concerning as the rate of childhood obesity is increasing. Obesity leads to a higher risk of developing short and long-term obesity-associated disorders, such as type 2 diabetes, cardiovascular diseases, cognitive dysfunction, as well as increased susceptibility to infections. Obesity, in both children and adults, is associated with chronic low-grade central inflammation with an increase in microglial number and activity. However, it is not completely known how this activated microglial profile impacts childhood obesity and the effects on learning and memory.<br><br>Here we used an established animal model of childhood obesity in rats to investigate microglia’s function in establishing and maintaining obesity-associated comorbidities. This model induces litter manipulations so that pups are suckled in small litters (of four rats) to allow neonatal overfeeding or control litters (of 12 pups). This neonatal overfeeding model allows a reduced competition for consumption of milk therefore inducing an accelerated growth and weight gain that persists at least into young adulthood. We have previously identified that rats suckled in small litters during development have a pro-inflammatory profile with an increase in microglial number and density in the hypothalamus. Using this model, we examined whether this pro-inflammatory profile seen in the neonatally overfed adult rats would extend into extra-hypothalamic regions, such as the hippocampus, and impair learning and memory. We found that neonatal overfeeding induces microgliosis in the hippocampus during development and this persists into adulthood. To assess cognitive function in the neonatally overfed rats, we examined working memory in the novel object recognition task (NOR) and spatial memory in the delayed win-shift radial arm maze (RAM). Hippocampal microgliosis in our neonatally overfed rats was associated with poor performance in both the NOR and RAM tasks relative to controls. Interestingly, in control-fed rats, the RAM learning task reduced cell proliferation in the dentate gyrus and neuronal numbers in the Cornu Ammonis (CA)3 as well as suppressed the microglial profile in the hippocampus and retrosplenial cortex. The neonatally overfed rats showed an impaired sensitivity to learning with a less effective microglial suppression and no neuronal or cell proliferation effects. Thus, neonatal over nutrition impairs learning and memory with a role for microglia.<br><br>Subsequently, we investigated whether this hippocampal microglial profile induced by early life over nutrition would cause a hyper-activated response to a lipopolysaccharide (LPS) stimulus and whether this inflammation could be prevented by early life minocycline, a tetracycline antibiotic known to inhibit microglial activity. We found that neonatally overfed adult rats were hyper-responsive to an immune challenge, LPS, inducing a dramatic increase in the number of hippocampal microglial cells compared to control-fed rats. We also demonstrated that repeated neonatal injections, regardless whether with minocycline or with saline, markedly suppressed microglial number and density throughout the hippocampus, completely abolishing differences between the neonatally overfed and control-fed groups in responses to LPS. Collectively, this suggests that the neonatal nutritional environment can have long-term effects, including effects on the hippocampal immune responses in adulthood, but also that exposure to a repeated injection protocol during development, irrespective of treatment, has prominent long-term impact on the microglial profile.<br><br>The initial studies performed in this project outlined that microglia play an important role in cognition, showing that a reduction in microglial number and density in the hippocampus and retrosplenial cortex is necessary to learn a spatial task. Therefore, we generated a conditional microglial ablation model in rats (Cx3cr1- Dtr (diphtheria toxin receptor) model) to investigate whether specific depletion of microglia would alter learning and memory. Under basal conditions, insertion of Dtr on the promoter for the Cx3cr1 gene did not affect the microglial or monocyte profile. We also established that exposure to diphtheria toxin (DT) induced microglial depletion within 48 hrs of DT administration and microglial repopulation after 7 days. We identified that depletion of microglia did not alter cognitive function in a Y maze or NOR task. However, microglial repopulation transiently enhanced working memory in the NOR and novel place recognition tasks and this was associated with alterations in both immature and mature neuronal numbers in the hippocampus.<br><br>Using this microglial ablation model, we have also revealed a novel role for microglia in acutely regulating feeding and metabolism. We demonstrated that microglial depletion induced acute weight loss and reductions in food intake. Further investigations into how depletion of microglia induced anorexia revealed that these Cx3cr1-Dtr rats did not display a sickness response and that despite the acute and reversible anorexia and weight loss, these rats had enhanced orexigenic signalling after elimination of microglia.<br><br>Findings from this thesis indicate that microglia have additional roles outside of the historically-described functions. This includes acute and long-term responses to overfeeding, as well as acute regulation of cognition and satiety signalling. The findings provide further insight into how microglia can affect cognition and responses to an immune challenge during childhood obesity and into how they can acutely influence satiety signalling.<br>