Mesenchymal stem cells stimulate intestinal stem cells to repair radiation-induced intestinal injury

The loss of stem cells residing in the base of the intestinal crypt has a key role in radiation-induced intestinal injury. In particular, Lgr5⁺ intestinal stem cells (ISCs) are indispensable for intestinal regeneration following exposure to radiation. Mesenchymal stem cells (MSCs) have previously been shown to improve intestinal epithelial repair in a mouse model of radiation injury, and, therefore, it was hypothesized that this protective effect is related to Lgr5⁺ ISCs. In this study, it was found that, following exposure to radiation, transplantation of MSCs improved the survival of the mice, ameliorated intestinal injury and increased the number of regenerating crypts. Furthermore, there was a significant increase in Lgr5⁺ ISCs and their daughter cells, including Ki67⁺ transient amplifying cells, Vil1⁺ enterocytes and lysozyme⁺ Paneth cells, in response to treatment with MSCs. Crypts isolated from mice treated with MSCs formed a higher number of and larger enteroids than those from the PBS group. MSC transplantation also reduced the number of apoptotic cells within the small intestine at 6 h post-radiation. Interestingly, Wnt3a and active β-catenin protein levels were increased in the small intestines of MSC-treated mice. In addition, intravenous delivery of recombinant mouse Wnt3a after radiation reduced damage in the small intestine and was radioprotective, although not to the same degree as MSC treatment. Our results show that MSCs support the growth of endogenous Lgr5⁺ ISCs, thus promoting repair of the small intestine following exposure to radiation. The molecular mechanism of action mediating this was found to be related to increased activation of the Wnt/β-catenin signaling pathway.The epithelium of the small intestine contains crypts and villi. Intestinal stem cells (ISCs) reside in the base of the crypts and are responsible for maintaining intestinal epithelial homeostasis and regeneration following injury.^{1, 2} Recent studies have identified two populations of stem cells in the small intestine of mice called Lgr5⁺ and Bmi1⁺ ISCs.^{3, 4, 5, 6, 7, 8, 9, 10, 11} Lgr5⁺ ISCs, also known as crypt base columnar cells (CBCs), are interspersed among the Paneth cells and are active rapidly cycling stem cells.¹² A single Lgr5⁺ ISC can grow to form ‘enteroids'' in vitro that develop into all the differentiated cell types found in the intestinal crypt.¹³ Conversely, Bmi1⁺ cells are a population of ISCs located at position +4 relative to the base of the crypt, and are quiescent, slowly cycling stem cells.¹⁴ The loss of ISCs has a critical role in radiation-induced intestinal injury (RIII).^{15, 16, 17, 18} Apoptosis of stem cells because of exposure to radiation prevents normal re-epithelialization of the intestines. Therefore, enhancing the survival of ISCs following radiation is a potential effective treatment for RIII.Mesenchymal stem cells (MSCs) possess significant potential as a therapeutic for tissue damage because of their ability to regulate inflammation, inhibit apoptosis, promote angiogenesis, and support the growth and differentiation of local stem and progenitor cells.^{19, 20} However, the mechanisms by which MSCs mediate these beneficial effects remain unclear, although it has been suggested that MSCs may actively secrete a broad range of bioactive molecules with immunomodulatory (PGE2, IDO, NO, HLA-G5, TSG-6, IL-6, IL-10 and IL-1RA), mitogenic (TGFα/β, HGF, IGF-1, bFGF and EGF), angiogenic (VEGF and TGF-β1) and/or anti-apoptotic (STC-1 and SFRP2) properties that function to modulate the regenerative environment at the site of injury.²¹ Upon re-establishment of the microenvironment following damage, the surviving endogenous stem and progenitor cells can then regenerate the injured tissue completely.Our previous study, as well as other published studies, has found that systemic administration of MSCs improves intestinal epithelial repair in an animal model of radiation injury.^{22, 23, 24, 25} Following MSC treatment, radiation-induced lesions in mice were significantly smaller than those in the control group. However, the mechanism behind this protective effect is not fully understood. Lgr5⁺ ISCs have been previously shown to be indispensable for radiation-induced intestinal regeneration.²⁶ Therefore, in this study, we tested whether the therapeutic effects of MSCs in response to RIII are related to the Lgr5⁺ population of resident ISCs.