A potential link between enteric glia and the pathophysiology of diet-induced obesity and related metabolic diseases

Enteric glia are a large population of peripheral neuroglia that accompany neurons in the enteric nervous system. These cells have diverse functions and engage in bidirectional communication with various cell types, including enteric neurons, immune cells, and possibly the gut microbiota.^{1, 2} Enteric glia play important roles in maintaining gastrointestinal (GI) homeostasis, and it is thought that alterations in their functions could be pivotal in the development of GI disorders. For instance, gains or losses in glial functions contribute to abnormal gut barrier function, inflammation, immune activation, and motor control. Understanding mechanisms by which enteric glia serve as “guardians” of the mucosal barrier has been an area of considerable interest; however, their involvement in mucosal barrier dysfunction is still debated.³

Inflammation caused by altered diet–gut microbiome–host interactions is considered an important driver of increased epithelial permeability in the development of obesity; yet the underlying mechanisms remain poorly understood.^{4, 5} A recent study by D'Antongiovanni et al.⁶ in Acta Physiologica Volume 240 addressed this issue by exploring potential contributions of enteric glia in gut barrier dysfunction driven by ingesting a Western (high-fat) diet. This study specifically focused on potential roles of inflammasome activation in glia as a potential contributor to diet-induced inflammation. The investigators approached this question using wild-type C57BL/6J and NLRP3-KO^−/− mice fed a 60-kcal high-fat diet (HFD) or standard diet for 8 weeks and studied mucosal integrity by histology, immunolabeling, and western blot. Potential reactive gliosis processes and inflammasome activation were assessed by immunolabeling for glial fibrillary acidic protein (GFAP) and co-labeling for inflammasome components.

The data show that mice consuming a HFD for 8 weeks increased body weight, altered colon mucus composition by decreasing acidic mucins, disrupted epithelial barrier integrity, increased GFAP-positive glial cells (gliosis), and triggered NLRP3 inflammasome activation. Surprisingly, HFD-NLRP3^−/− mice failed to gain weight on the HFD and did not exhibit signs of enteric gliosis or altered mucus composition and epithelial barrier integrity. Based on these results, the authors suggested that inflammasome activation is involved in causing obesity, impairing the mucosal barrier, and activating gliosis. To test this concept more directly, the investigators turned to in vitro coculture experiments with a rat-transformed cell line used to model enteric glia (CRL-2690) and a rat intestinal epithelial cell (IEC) line. Challenging cultures with a combination of lipopolysaccharide (LPS) and palmitate was then used to broadly test whether dietary saturated fatty acids and endotoxins disrupt the epithelial barrier and activate enteric gliosis and the NLRP3 inflammasome. In support, data showed that palmitate and LPS decreased tight junction protein expression and increased transepithelial permeability in vitro. Moreover, palmitate and LPS crossed the leaky epithelial barrier and activated the NLRP3/caspase-1 inflammasome in CRL-2690 cells. Subsequently, CRL-2690 cells produced IL-1β, which enhanced the increase of transepithelial permeability by augmenting the tight junction protein expression decrease. Collectively, the results of these experiments suggested that NLRP3 inflammasome activation in enteric glia, or other at least glial-like cells, contributes to the overall detrimental effects of inflammasome activation in multiple cell types in the gut mucosa. These effects involve promoting increased gut barrier permeability and inflammation in obesity.

The results of this study raise the interesting possibility that enteric glia and the NLRP3 inflammasome could contribute to the pathophysiology of mucosal barrier dysfunction associated with Western diets and other lifestyle-related disorders.⁷ However, many questions remain. For instance, why NLRP3 knockout mice failed to gain weight in this study is puzzling, given that multiple prior studies show robust weight gain in this line that is comparable to, or perhaps even more than, the wild-type background strains.^{8, 9} The lack of weight gain raises the question of whether the absence of changes to the glial was directly linked to NLRP3 or to a less or overall inflammatory stimulus. There are also major differences between the CRL-2690 cell line used for in vitro experiments and true enteric glia. These differences and the high variability among some data presented suggest that additional follow-up work is needed to increase confidence in the mechanisms proposed. Despite this, the results support the authors' view that the inflammasome can be targeted to mitigate the increases in transepithelial permeability and subsequent inflammation linked with causing obesity. This study also highlights a critical need for studies focused on elucidating the role of enteric glial cells in normal mucosal biology and the pathobiology of conditions characterized by increased transepithelial permeability. For example, work addressing potential crosstalk between glia, enterocytes, and specialized cells such as goblet, enteroendocrine, tuft, and Paneth cells would further our understanding of gut epithelial biology. In addition, identifying how specific dietary molecules, mainly pro-inflammatory fatty acids such as palmitic, stearic, and myristate acids and microbial derived endotoxins, metabolites, and toxins affect enteric glia, could provide insight into mechanisms that promote metabolic diseases. These studies are essential for advancing our understanding of the interplay between enteric glia, diet, and gut health, ultimately informing therapeutic strategies for GI disorders.

Onesmo B. Balemba: Writing – original draft; writing – review and editing; conceptualization. Brian D. Gulbransen: Writing – original draft; writing – review and editing; conceptualization.

Balemba's work was supported by National Institute of Health via NIDDK Diabetic Complication Consortium 5U24DK115255-04, and the University of Idaho. BDG receives support from grants R01DK103723 and R01DK120862 from the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK) of the National Institutes of Health (NIH). The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

The authors declare that they have no commercial or financial conflict of interest.

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