The Role of Host Factors in the Control of Gut Microbiota and Inflammation in Diet-Induced Obesity and During Aging

Abstract

Disruptions of microbiota and inflammatory homeostasis in the gut have been implicated in the metabolic dysregulation of diet-induced obesity (DIO) and aging. However, the involvement of host factors in these disturbances remains unclear. Lipocalin 2 (Lcn2) has previously been identified as an adipocytokine and characterized as an important regulator of diet-induced obesity and inflammation. Lcn2, as an anti-microbial peptide is also expressed in intestine, and the upregulation of intestinal Lcn2 has been previously reported in inflammatory bowel disease to protect against colitis-associated intestinal inflammation and to regulate gut microbiota composition. In chapter 2 and 3, we investigated the role and mechanism of Lcn2 in the regulation of gut microbiota composition and microbial metabolism in diet-induced obesity. We also investigated the role of Lcn2 in the control of age-related reshaping of gut microbiota and brain health via a gut-brain axis during aging. We demonstrate that intestinal Lcn2 expression and secretion into the gut lumen is regulated by high fat diet (HFD) consumption and aging in a time-dependent manner. Lcn2 plays a protective role in HFD-induced and age-associated gut microbiota dysbiosis. Our results suggest that Lcn2 regulates gut microbiota composition and intestinal inflammation through an independent mechanism, and Lcn2 likely regulates the growth of siderophore-producing bacteria directly. Lcn2 deficiency alters the production of microbial metabolites, particularly short-chain fatty acids (SCFAs) during DIO and aging, thereby contributing to the development of obesity and age-related disease. In chapter 2, our findings suggest that the induction of Lcn2 expression and secretion at the early stage of HFD consumption is a host defensive mechanism against HFD-induced disruption of gut commensal and inflammatory homeostasis, whereas long-term HFD challenge disrupts the protective role of Lcn2 by disturbing the secretion of Lcn2 into the gut lumen. Moreover, we have identified 45 bacteria with altered abundances in Lcn2 knockout (LKO) mice during DIO. Through the correlation analysis between body weight and bacteria/metabolites, we highlight that Lcn2 controls the development of obesity through regulating the growth of the microbes (Dubosiella and SCFA-producing bacteria) and the production of microbial metabolites (e.g. SCFAs). In chapter 3, we found that Lcn2 deficiency impairs cognitive function and alters behavioral outcomes in old (18-month-old) mice. Sixteen bacteria at the family level were identified to be differentially abundant in LKO mice. Of those 16 bacteria, health-promoting bacteria (e.g. SCFA-producing bacteria Muribaculaceae) have significantly less prevalent in old LKO mice. Consistently, Old LKO mice display lower levels of fecal butyrate compared to old wild-type (WT) mice. Our data suggest a potential mechanism involving gut microbiota and its associated metabolites for the role of Lcn2 in the control of age-related neurodegenerative diseases. Chronic Low-grade inflammation in multiple tissues and organs is commonly accompanied with aging and its-related diseases, such as obesity, type 2 diabetes, neurodegenerative diseases, and cancers. Macrophages play a critical role in local tissue inflammation, which contributes to metabolic complications. Macrophage polarization is tightly associated with its metabolic reprograming and immune dysfunction. In chapter 4, we explored intracellular molecules/pathways that connect these alterations in inflammatory macrophages. We found that the expression of guanylate binding protein 1 (Gbp1), an intracellular anti-microbial protein is significantly decreased in white adipose tissue of HFD-fed and aged mice. Downregulation of Gbp1 expression results in macrophage polarization towards a pro-inflammatory phenotype. Gbp1 knockdown (Kd) macrophages have impaired mitochondrial function and decreased mitophagy activity. More interestingly, Gbp1 Kd macrophages experience senescence as evidenced by increased activation of AMPK-p53 pathway and positive staining of β-galactosidase. Collectively, these observations highlight important roles of Gbp1 in improving mitochondrial dysfunction, preserving immune function, and attenuating senescence of macrophages during inflammatory stress and aging.