Frontline defenders: goblet cell mediators dictate host-microbe interactions in the intestinal tract during health and disease

Abstract

Goblet cells (GCs) are the predominant secretory epithelial cells lining the luminal surface of the mammalian gastrointestinal (GI) tract. Best known for their apical release of mucin 2 (Muc2), which is critical for the formation of the intestinal mucus barrier, GCs have often been overlooked for their active contributions to intestinal protection and host defense. In part, this oversight reflects the limited tools available to study their function but also because GCs have long been viewed as relatively passive players in promoting intestinal homeostasis and host defense. In light of recent studies, this perspective has shifted, as current evidence suggests that Muc2 as well as other GC mediators are actively released into the lumen to defend the host when the GI tract is challenged by noxious stimuli. The ability of GCs to sense and respond to danger signals, such as bacterial pathogens, has recently been linked to inflammasome signaling, potentially intrinsic to the GCs themselves. Moreover, further work suggests that GCs release Muc2, as well as other mediators, to modulate the composition of the gut microbiome, leading to both the expansion as well as the depletion of specific gut microbes. This review will focus on the mechanisms by which GCs actively defend the host from noxious stimuli, as well as describe advanced technologies and new approaches by which their responses can be addressed. Taken together, we will highlight current insights into this understudied, yet critical, aspect of intestinal mucosal protection and its role in promoting gut defense and homeostasis.

Keywords: goblet cell; gut infections; inflammatory bowel disease; microbes; mucus.

Fig. 1.

The role of intestinal goblet cells (GCs) and their mediators in health and disease. Each panel represents several crypts within a region of colonic mucosa. Light green shows the outer mucus layer, whereas dark green shows the inner mucus layer, and light blue represents the lamina propria, as labeled in A. Relative expression/release of mediators by GCs is represented by arrows of different sizes (large arrows, heavy or increased secretion; smallest arrows, minimal or decreased secretion; medium arrows, baseline secretion state). A: bacterial colonization after birth triggers an increase in GC numbers and differentiation (rose-colored cells). This leads to dramatically increased production/secretion of Muc2 (green arrows), as well as Relm-β (purple arrows), into the gut lumen, whereas Tff3 (black arrows) also increases but to a lesser degree. B: under healthy (homeostatic) conditions, there is limited Relm-β secretion (purple arrows), whereas Tff3 is constantly secreted (black arrows), and Muc2 secretion (green arrows) is heavy and equivalent to the loss of mucus through defecation and its degradation by commensal bacteria (bacteria within white circle). C: during many enteric infections, GCs release significant amounts of Muc2 (green arrows), whereas Relm-β (purple arrows) as well Tff3 (black arrows) production and release also increase. An infection-induced increase in Muc2 secretion promotes the flushing of pathogens, as well as commensals from the gut (brown arrow). Relm-β secretion into the submucosa promotes immune cell (CD4⁺ T cell) recruitment that drives a variety of host defense mechanisms, including increased epithelial cell proliferation. D: in IBD and experimental colitis, host-driven changes in GC function lead to GC depletion, a reduction in Muc2 secretion (green arrows), and a reduction in overall mucus-layer thickness (light and dark green layers). Whereas Tff3 expression may undergo only modest changes in expression (black arrows), Relm-β secretion (purple arrows) is increased, which leads to the induction, production, and release of antimicrobial peptides and lectins (RegIIIβ and RegIIIγ; blue dots) by epithelial cells into the gut lumen. This increased antimicrobial activity leads to intestinal microbial dysbiosis, including loss of beneficial microbes, and the overgrowth of bacterial pathobionts associated with IBD.

Fig. 2.

Citrobacter rodentium localizes within the mucus layer and on the apical surface of the intestinal epithelium. Immunofluorescent staining of colonic tissue from a C57BL/6 mouse, 6 days post-C. rodentium infection. Antibodies were used against the translocated intimin receptor (Tir; red) to identify C. rodentium, mucus (Muc2; green), and DNA [4′,6-diamidino-2-phenylindole (DAPI); blue]. During infection, C. rodentium is found within the intestinal mucus layer (white arrowheads), as well as infecting the apical surface of the intestinal epithelium (white arrows). L, lumen; C, crypts, tissue surface, dotted, white lines. Original scale bars, 50 μm.

Fig. 3.

Expression of Tff3 and Relm-β in colonic goblet cells (GCs) during Citrobacter rodentium infection. Immunofluorescent staining of colonic tissue from a C57BL/6 mouse, 6 days post-C. rodentium infection. A, left: presence of GCs stained by using Santa Cruz anti-Muc2 (H300) antibody (green), targeting the mature form of Muc2. Middle: the same GCs on a serial cut section expressing both the Tff3 (red) and precursor (immature) form of Muc2, stained by using a Santa Cruz anti-Muc2 (P18) antibody (green; white arrows). Host cell nuclei are stained with DAPI (blue). Right: the higher magnification inset confirms that GCs express both Tff3 and Muc2 (white arrow). Colonic crypts (dashed white lines), GCs expressing Tff3 (white arrows), and GCs expressing Relm-β from B (arrowheads). Original scale bars, 50 μm. B, left: presence of GCs stained by using Santa Cruz anti-Muc2 (H300) antibody (green), targeting the mature form of Muc2. Middle: the same GCs on a serial cut section expressing both Relm-β (red) and the precursor form of Muc2, stained by using a Santa Cruz anti-Muc2 (P18) antibody (green; white arrowheads). Host cell nuclei are stained with DAPI (blue). Right: the higher magnification inset confirms that GCs express both Relm-β and Muc2 (white arrowhead). Colonic crypts (dashed white lines), GCs expressing Tff3 from A (white arrows), and GCs expressing Relm-β (arrowheads). Original scale bars, 50 μm. C: a cartoon outlining the possibility that intestinal GCs produce secretory granules containing Muc2, Relm-β, and Tff3 that are intermixed throughout the theca or an alternative possibility: that the different granules are segregated within the body of the theca.

Fig. 4.

Enteroids provide a useful model to study microbe interactions with goblet cells. Immunostaining of mouse colonic enteroids for E-cadherin (red), Muc2 (green), and DNA (DAPI; blue) under (A) baseline conditions or (B) following enteroid treatment with the Notch inhibitor dibenzazepine (DBZ) to increase goblet cell numbers. Muc2-producing goblet cells are seen under baseline conditions but are dramatically increased in number following DBZ treatment (white arrows). C: contrast-phase micrograph showing a mouse intestine enteroid microinjected with Salmonella enterica serovar Typhimurium. D: immunostaining of a mouse intestine enteroid showing the presence of luminal S. Typhimurium (green) in contact with epithelial cells stained with β-actin (red), whereas epithelial cell nuclei are stained with DAPI (blue). Original scale bars, 50 μm.