Background:
Obesity is a multifactorial disease process with contributions from the microbiome as well as other factors. Hormone products from enteroendocrine cells (EECs) have emerged as therapeutic strategies in the management of obesity, yet the mechanisms by which colonic EECs might alter microbial and host responses that result in obesity remain obscure. We sought to examine the role colonic EECs have on obesity development.

Methods:
We utilized the Cre-lox system to generate constitutive (EECΔCol) and inducible (iEECΔCol) mouse models of colonic EEC loss through deletion of a conditional Neurog3 allele. We performed experiments to examine the effects of colonic EEC loss under high fat diet and regular chow, genotype-segregation, and after antibiotic exposure. Analyses included 16s sequencing, metatranscriptomics, stool and serum metabolomics, MRI, and hormone analysis. We also performed colonic motility studies. We performed oral, rectal, and intraperitoneal (IP) glutamate administration experiments in conjunction with hypothalamic cFos staining. Finally, stool transfer studies were conducted on germ-free (GF) animals.

Results:
Constitutive and inducible loss of colonic EECs resulted in hyperphagia and weight gain that solely occurred upon genotype segregation. The increased weight is secondary to increased fat content and is enhanced by a high fat diet. Co-housing animals with wild-type (WT) littermates or exposure to antibiotics abrogated the phenotype. Genotype segregated mutant animals are dysbiotic even prior to the onset of obesity and exhibit colonic dysmotility. Stool transfer from mutant animals into GF animals resulted in transmission of the phenotype. Through stool metabolomics, we identified an increase in fecal L-glutamic acid that correlated with the hyperphagia obesity phenotype. Administration of oral or rectal glutamate, but not IP glutamate, into wild type animals resulted in hyperphagia and obesity. Fecal glutamate level is diet-independent and metatrancriptomics identified enhanced pathways of bacterial glutamate production.

Conclusion:
Here we define a novel role for colonic EECs in regulating host metabolic processes through shaping the colonic microbiome. Central to this observation is the finding that bacterially derived glutamic acid plays a role in this interaction and is a potential target for therapeutic development options for obesity.

Jeff McDonald

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