Lactobacillus probiotic protects intestinal epithelium from radiation injury in a TLR-2/cyclo-oxygenase-2-dependent manner.

BACKGROUND: The small intestinal epithelium is highly sensitive to radiation and is a major site of injury during radiation therapy and environmental overexposure. OBJECTIVE: To examine probiotic bacteria as potential radioprotective agents in the intestine. METHODS: 8-week-old C57BL/6 wild-type or knockout mice were administered probiotic by gavage for 3 days before 12 Gy whole body radiation. The intestine was evaluated for cell-positional apoptosis (6 h) and crypt survival (84 h). RESULTS: Gavage of 5×107 Lactobacillus rhamnosus GG (LGG) improved crypt survival about twofold (p<0.01); the effect was observed when administered before, but not after, radiation. Conditioned medium (CM) from LGG improved crypt survival (1.95-fold, p<0.01), and both LGG and LGG-CM reduced epithelial apoptosis particularly at the crypt base (33% to 18%, p<0.01). LGG was detected in the distal ileal contents after the gavage cycle, but did not lead to a detectable shift in bacterial family composition. The reduction in epithelial apoptosis and improved crypt survival offered by LGG was lost in MyD88⁻/⁻, TLR-2⁻/⁻ and cyclo-oxygenase-2⁻/⁻ (COX-2) mice but not TLR-4⁻/⁻ mice. LGG administration did not lead to increased jejunal COX-2 mRNA or prostaglandin E2 levels or a change in number of COX-2-expressing cells. However, a location shift was observed in constitutively COX-2-expressing cells of the lamina propria from the villi to a position near the crypt base (villi to crypt ratio 80:20 for control and 62:38 for LGG; p<0.001). Co-staining revealed these COX-2-expressing small intestinal lamina propria cells to be mesenchymal stem cells. CONCLUSIONS: LGG or its CM reduce radiation-induced epithelial injury and improve crypt survival. A TLR-2/MyD88 signalling mechanism leading to repositioning of constitutive COX-2-expressing mesenchymal stem cells to the crypt base is invoked.

Growth factor regulation of prostaglandin-endoperoxide synthase 2 (Ptgs2) expression in colonic mesenchymal stem cells.

We previously found that a population of colonic stromal cells that constitutively express high levels of prostaglandin-endoperoxide synthase 2 (Ptgs2, also known as Cox-2) altered their location in the lamina propria in response to injury in a Myd88-dependent manner (Brown, S. L., Riehl, T. E., Walker, M. R., Geske, M. J., Doherty, J. M., Stenson, W. F., and Stappenbeck, T. S. (2007) J. Clin. Invest. 117, 258-269). At the time of this study, the identity of these cells and the mechanism by which they expressed high levels of Ptgs2 were unknown. Here we found that these colonic stromal cells were mesenchymal stem cells (MSCs). These colonic MSCs expressed high Ptgs2 levels not through interaction with bacterial products but instead as a consequence of mRNA stabilization downstream of Fgf9 (fibroblast growth factor 9), a growth factor that is constitutively expressed by the intestinal epithelium. This stabilization was mediated partially through a mechanism involving endogenous CUG-binding protein 2 (CUGbp2). These studies suggest that Fgf9 is an important factor in the regulation of Ptgs2 in colonic MSCs and may be a factor involved in its constitutive expression in vivo.

Myd88-dependent positioning of Ptgs2-expressing stromal cells maintains colonic epithelial proliferation during injury.

We identified cellular and molecular mechanisms within the stem cell niche that control the activity of colonic epithelial progenitors (ColEPs) during injury. Here, we show that while WT mice maintained ColEP proliferation in the rectum following injury with dextran sodium sulfate, similarly treated Myd88(-/-) (TLR signaling-deficient) and prostaglandin-endoperoxide synthase 2(-/-) (Ptgs2(-/-)) mice exhibited a profound inhibition of epithelial proliferation and cellular organization within rectal crypts. Exogenous addition of 16,16-dimethyl PGE(2) (dmPGE(2)) rescued the effects of this injury in both knockout mouse strains, indicating that Myd88 signaling is upstream of Ptgs2 and PGE(2). In WT and Myd88(-/-) mice, Ptgs2 was expressed in scattered mesenchymal cells. Surprisingly, Ptgs2 expression was not regulated by injury. Rather, in WT mice, the combination of injury and Myd88 signaling led to the repositioning of a subset of the Ptgs2-expressing stromal cells from the mesenchyme surrounding the middle and upper crypts to an area surrounding the crypt base adjacent to ColEPs. These findings demonstrate that Myd88 and prostaglandin signaling pathways interact to preserve epithelial proliferation during injury using what we believe to be a previously undescribed mechanism requiring proper cellular mobilization within the crypt niche.

Deciphering the 'black box' of the intestinal stem cell niche: taking direction from other systems.

PURPOSE OF REVIEW: Study of developmental signaling pathways suggests that the intestinal stem cell niche regulates the activity of the crypt-based epithelial progenitors during homeostasis and injury states. The cellular origin of these signals, however, remains poorly defined. Here, we examine the current state of knowledge regarding intestinal epithelial progenitor niches and highlight applicable lessons learned from other systems. RECENT FINDINGS: Cell-cell contact, regulatory factor delivery, stem cell polarity, and mesenchymal stem cells are considered. SUMMARY: Based on the findings in other niche systems as well as the overall complexity and unique organization of the intestinal progenitor niche, future studies will focus on defining peri-cryptal architecture, cellular sources of regulatory factors, and the dynamic nature of the niche during homeostasis and injury repair. These insights may lead to novel cell-based therapies for a variety of conditions that damage the mucosal lining of the gut.