Ischemic reperfusion injury is one of the main causes of AKI and more attention has been focused on stem cell therapy for ameliorating this injury. There has been mounting evidence for the existence of stem cells in the adult kidney, including the glomerulus , interstitium [21, 24–26], tubules [8, 27], and papilla . In this paper we demonstrated protective roles of MRPC, MRPC/EPO and MRPC/suramin after injection in I/R AKI C57BL/6 mice. MRPC, spindle-shaped with a large nucleus, were purified from the kidneys of adult C57BL/6-gfp mice (see Additional file 1 and Additional file 2: Figure S2). They exhibited features of renal progenitor cells with expression of renal progenitor markers Oct-4 and Pax-2, Wnt-4 and WT-1, which are expressed in the renal progenitors of metanephric mesenchyme during embryonic development . MRPC possessed the mesenchymal markers vimentin and α-SMA but not the epithelial marker E-cadherin. Furthermore, there was no expression of hematogenous or endothelial progenitor cell markers in MRPC, such as CD45 or CD34, which negated the possibility that MRPC originated from extrarenal tissues. In addition, MRPC were multipotent for their differentiation into osteoblast and adipocyte lineages in vitro and in vivo (see Additional file 1 and Additional file 3: Figure S3). Moreover, we studied the roles of MRPC alone and in combination with EPO or suramin in the I/R AKI mice model. In agreement with previous studies that showed that MKPC accelerate renal regeneration and prolong survival after ischemic injury [15, 21], these findings identify a suitable cell population, MRPC, for possible use in future studies of cell therapy for AKI. Here, we found that the effect of MRPC/EPO or MRPC/suramin was considerably stronger than MRPC alone very early (day 2) after injection. However, MRPC alone played a sustaining renal regeneration role in I/R AKI C57BL/6 mice. The reasons for this difference still remain to be clarified. A possible explanation is MRPC/EPO or MRPC/suramin formed more CD34+ and E-cadherin+ cells with fast incorporation into renal tubules and capillaries than MRPC alone, consistent with differentiation mechanisms that some MKPC formed vessels with red blood cells inside (CD34+ cells) and some incorporated into renal tubules (E-cadherin+ cells) .
However, MRPC alone played a sustaining renal regeneration role in I/R AKI C57BL/6 mice. The reasons for this still remain to be clarified. It is interesting that whether MRPC homed to the injured region. Our results showed that, seven days after ischemic injury and MRPC injection, GFP fluorescence was detected in some tubules of the kidney by immunofluorescence. One possible explanation may be based on the damaged vascular system in I/R AKI C57BL/6 mice. Acute ischemic injury of the kidney induced hypoxia in the injured region and, therefore, upregulated the expression of SDF-1 which attracted CXCR4+ cells (MRPC) to mobilize to the injured region . As the renal protection effect of MRPC was fast and immediate, there may be many mechanisms involved in the recovery process. Reduction of the inflammatory response was considered as a possible mechanism in the treatment of AKI. It was found that MRPC reduced the post-ischemic inflammatory response and obviously decreased macrophage infiltration, especially when combined with EPO or suramin (see Additional file 1 and Additional file 4: Figure S4).
How MRPC combine with EPO or suramin in the treatment of AKI is still not fully understood. As we know, EPO, a glycoprotein hormone, can stimulate the formation and differentiation of erythroid precursor cells in the bone marrow. However, further studies have been done on the undiscovered roles of EPO on other cell types that express EPO receptors [31–33]. Recent studies have shown that there are EPO receptors on the surfaces of tubular epithelial cells [31, 34]. Furthermore, EPO plays an important role in these cells to protect kidneys against acute injury in animal studies [31–33, 35]. Mechanisms involved in this protection appear to be associated with anti-apoptotic, anti-oxidative and anti-inflammatory properties as well as with the proangiogenic potential of EPO . It was reported that rhEPO treatment significantly attenuated the upregulation of transforming growth factor 1 (TGF-1) and α-SMA and the downregulation of E-cadherin in the obstructed kidney in a mouse model . Further, EPO treatment can increase the expression of CD34  after adriamycin-induced kidney injury. Moreover, E-cadherin is highly positively regulated by EPO in a PI3K-dependent manner in CD34+ progenitor cells . These findings may explain the greater improvement in renal histology and function in the mice treated with MRPC/EPO than in those treated with MRPC alone very early after injection. Suramin, a common drug in the treatment of trypanosomiasis, has recently been found to be useful in accelerating kidney recovery after AKI although the exact mechanism is still incompletely known. Recently, it was reported that the death of renal epithelial cells could directly cause necrosis of renal fibroblasts by releasing ATP immediately into the interstitium of the kidney as a death factor and the P2X7 receptor as a crucial mediator . Since peritubular fibroblasts in the kidney are the major EPO-producing cells, inhibition of P2X7 may promote renal structural and functional recovery after AKI. Since suramin is a general P2 inhibitor, it can inhibit the P2X7 receptor to prevent the death of renal fibroblasts and then raise the EPO level during the AKI process. Thus, suramin may protect against kidney injury by increasing EPO production. There is a close intrinsic correlation between EPO and suramin. However, it is still unclear how MRPC combine with EPO or suramin in the treatment of AKI and advanced research work needs to be done.
Recently, some studies have proven that the therapeutic efficiency of MSC in AKI and many other diseases may be improved by combination with a molecular treatment. La Manna et al.  showed that hyaluronan monoesters with butyric acid (HB) act as a preconditioning agent increasing angiogenesis and vascular regeneration efficiency of FMhMSCs. Mias et al.  found that pretreatment with melatonin could increase the survival, paracrine activity and efficiency of MSCs. Similarly, the protective effects of EPO compounds and MSC combinations are supported by a study which evaluated the effect of this combination on a rat model of ischemia . Although these data are from MSC, it is still reasonable to speculate that the efficiency of MRPC may also be enhanced by combination with molecular treatment. Our data show that MRPC treatment was an efficient approach for recovery from injury. There was no teratoma formed in the kidney six weeks after MRPC injection (see Additional file 1), and there are currently no reports about tumor genesis originating from MRPC. Moreover, our data show that combined MRPC/EPO and MRPC/suramin treatment was a more efficient approach for recovery from injury than MRPC alone very early (day 2) after injection and that MRPC alone played a sustaining renal repair role in I/R AKI C57BL/6 mice. Even though this potentiated effect might be related to the addition of independent beneficial effects of the treatment agents, combination of stem cell-based therapy with pharmacy therapy might offer a novel therapeutic approach for the treatment of I/R-induced AKI in humans.