Preservation of epithelial proliferation is crucial for proper maintenance of the colonic epithelial barrier. ColEPs from mice that lack either Ptgs2
generate an inappropriate hypoproliferative state when exposed to DSS (18
). In this article, we establish that Ptgs2 and PGE2
are key downstream targets of Myd88 signaling during DSS-mediated injury that can maintain epithelial proliferation in the face of this injury stimulus. An unanticipated finding was that the mechanistic link was not through the modulation of either Ptgs2 expression or PGE2
production by Myd88/TLR activity, as we initially hypothesized. Instead, Myd88 signaling controlled the localization of a population of PSCs, a major source of Ptgs2 in the mouse colon (Figure ). During injury, PSCs altered their localization to become more enriched within the crypt base–associated mesenchyme. In addition, the PSCs in this region were more closely associated with the crypt base epithelium, where proliferative ColEPs reside. These 2 PSC redistributions depended on Myd88 signaling and were necessary to maintain epithelial proliferation in the response to injury.
Model of the key cells that maintain ColEP proliferation and crypt morphology during DSS-mediated injury.
Recent studies have begun to dissect the cellular components and molecular interactions of cellular niches and their associated stem and progenitor cells in a variety of mammalian systems such as bone marrow and skin as well as in simpler model organisms such the gonads of Drosophila melanogaster
and Caenorhabditis elegans
(for reviews, see refs. 13
). All of these systems emphasize the close contact of the niche with the target stem cell in order to effect proper communication. This study along, with our previous study (18
), emphasizes that during specific perturbations, the colonic niche is not a fixed vessel. One mechanism of injury adjustment is the recruitment of additional cellular constituents. We previously found that activated macrophages, likely recruited from blood monocytes, are one key cell type that performs this function in the intestine (18
). PSCs appear to be recruited from a much more local cellular pool that is not expanded during injury and are a second cell type that is mobilized to the crypt niche to become juxtaposed to ColEPs during injury.
Several lines of evidence suggest that epithelial proliferation is preserved in WT mice treated with DSS by the altered distribution of PSCs and their generation of PGE2
. First, in the absence of TLR signaling, neither PSC repositioning nor preservation of proliferation during injury occurs. Second, in the absence of Ptgs2, there is no preservation of proliferation. Third, in both Myd88–/–
mice, epithelial proliferation is maintained by the administration of a stable analog of PGE2
. The key event in preserving epithelial proliferation appears to be the achievement of high levels of PGE2
in the pericryptal niche. PGE2
has a short half-life and acts over short distances (34
). Therefore, PSCs positioned closer to crypt epithelial cells should increase the local concentration of PGE2
. In WT mice, PGE2
levels in the pericryptal region are increased by moving the PGE2
-producing cells closer; we were able to achieve the same effect in Myd88–/–
mice by administering a stable analog of PGE2
. The mechanism of the redistribution of the PSCs is unknown, and we hypothesize that this occurs by cell migration in response to either a chemotactic factor or basement membrane modification. It is possible that this same factor could recruit activated macrophages as well.
Total rectal PGE2
content is similar in all the conditions tested, including in DSS-treated Ptgs2–/–
mice, suggesting that the majority of the rectal PGE2
is synthesized through Ptgs1. These data are consistent with previous studies demonstrating the presence of large numbers of Ptgs1-expressing cells and small numbers of Ptgs2-expressing cells in the gastrointestinal tract (22
) and that the intestinal PGE2
levels are decreased in Ptgs1–/–
). It is important to note that the rectal PGE2
level represents the integration of numerous PGE2
microenvironments within this tissue. Current methods do not allow measurement of PGE2
levels in specific microenvironments. It is reasonable to assume that although Ptgs2 does not contribute extensively to total rectal PGE2
levels, it does contribute significantly to PGE2
synthesis in the immediate neighborhood of PSCs.
In these studies, epithelial proliferation was maintained during DSS-mediated injury by repositioning PSCs without any increase in Ptgs2 expression or PGE2
production. Ex vivo, TLR ligands given in pharmacologic quantities can induce Ptgs2 expression in cells that do not constitutively express Ptgs2, such as macrophages and myofibroblasts (36
). Increased Ptgs2 expression is commonly associated with markedly increased PGE2
production. Here, in response to the modest level of injury seen in DSS colitis, TLR signaling induces the repositioning of PSCs without (a) inducing higher levels of Ptgs2 expression in those cells, (b) inducing Ptgs2 expression in cells that do not normally express Ptgs2, or (c) increasing overall PGE2
production in the colon. The importance of the positioning of PSCs to locally provide PGE2
to ColEPs during injury is supported by a recently published study that shows PGE2
affects epithelial proliferation by enhancing Wnt signaling in colonic tumor cells (25
). Importantly, loss of Wnt signaling in a normal mouse creates a colonic phenotype that is similar to that of DSS-treated Myd88–/–
). Thus, PSCs may be mobile elements that can support critical growth factor pathways during certain types of damage.
These findings may have applications for injury in other organs outside of the gut. Ptgs2 and PGE2
play positive roles in wound response and healing in bone marrow injured by chemotherapeutic drugs and in bone injured by fractures (38
). Myd88 signaling has recently been shown to be required for resistance of lung epithelial cells to bleomycin injury (40
). Our studies suggest that the linkage of Myd88 and Ptgs2 signaling may occur in these other tissues. If so, then the role of the positioning of PSCs with regard to tissue stem cells would be of interest. The advantage of the colonic injury system as a model for uncovering and quantifying cellular redistribution is the well-defined microanatomy and organization of the epithelium and its associated mesenchyme.
In conclusion, this model supports the concept that the niche established by pericryptal cells is important in regulating epithelial cell proliferation in the colon. It also supports the suggestion that PGE2
promotes epithelial proliferation and epithelial homeostasis in the face of injury. One aspect of the model that we believe to be novel is the demonstration that a population of PSCs is repositioned in response to injury and promotes epithelial homeostasis through PGE2
production. Thus, the pericryptal niche is even more plastic than previously thought, and the cellular composition of the niche is modulated in response to injury. The second aspect of this model that we believe to be novel is the demonstration that the repositioning of the PSCs is Myd88 dependent. This observation provides a mechanism for the more severe DSS-induced colitis seen in Myd88–/–
). A future goal will be to better define the PSC cell type and its properties in the intestine and extraintestinal sites. The challenge will be to isolate these mesenchymal cells without inducing Ptgs2 in additional cell populations.