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  • Studies have shown that the cultivation of BMSCs under


    Studies have shown that the cultivation of BMSCs under hypoxic conditions, treating with cytokines (such as HGF, IGF-1 and IL-6) and viral-mediated CXCR-4 gene delivery have been shown to re-establish CXCR-4 expression on the surface of stem niclosamide (Liu et al., 2010, Liu et al., 2014a, Ziaei et al., 2014). Over-expression of CXCR-4 on the surface of sub-cultured BMSCs was shown to promote stem cell migration and increase their therapeutic potential for restoring normal renal function after AKI (Liu et al. 2013). The key limitations of all above methods include the following: the in vitro culture condition is different from endogenous inflammation micro-environment in vivo and viral transduction may alter the cell biology properties of experimental BMSCs and induce long-term safety issues (Liu et al., 2010, Won et al., 2014). Non-viral vectors could serve as a safe alternative for targeted gene delivery in BMSCs for clinical settings, but they have relatively low transfection efficiency (Hsu et al. 2011). Currently, ultrasound-targeted micro-bubble destruction (UTMD) has become a promising gene transfection alternative because of its high efficiency, safety and simplicity. The interaction between ultrasound irradiation and micro-bubbles could lead to micro-streaming, cavitation and jetting, which are responsible for increasing gene delivery efficiency (Chen et al., 2013, Escoffre et al., 2013b, Geis et al., 2012). Our group has previously proved that combining UTMD and non-viral vectors could enhance gene transfection efficiency and maintain the multi-directional differentiation and reproductive activity of the transfected BMSCs (Li et al. 2013). However, no studies have reported whether this method is feasible for enhancing BMSCs' homing ability to kidney tissues in AKI. Based on previous findings, we hypothesize that combing UTMD and polyethylenimine (PEI) would enhance CXCR-4 gene transfection efficiency in BMSCs, which may augment the site-targeted engraftment of BMSCs to AKI kidney tissues in rats. To test this hypothesis, the cooperation of UTMD and PEI was used to induce CXCR-4 expression in sub-cultured BMSCs in vitro. Then, the experimental BMSCs were injected to rats' tail veins to assess their homing ability to targeted AKI kidney tissues. The long-term goal of this study is to provide an effective alternative for improving BMSC therapy in vivo.
    Materials and Methods
    Discussion Numerous studies have reported that BMSCs can migrate to AKI kidney tissues and repair injured tissues by transdifferentiating into local cell types and paracrine abilities (Jia et al., 2012, Qi and Wu, 2013). However, the repair efficiency is limited mainly because only a small proportion of intravenously injected stem cells can migrate specifically to the damaged renal tissues, leaving the majority of transplanted BMSCs in other blood-rich organs (Baer and Geiger, 2010, Feng et al., 2013). This insufficient homing efficiency has made it an unviable therapy. Similar to that of inflammatory cells homing to inflamed tissues, BMSCs' homing process involves several steps, including chemotactic response by chemokine-chemokine receptor interactions, attachment to vascular endothelial cells, and transendothelial migration into the parenchyma (Deak et al., 2010, Karp and Teol, 2009). It requires the right combination of signaling molecules from the injured tissue and the corresponding receptors on BMSCs to home and target specific tissues. Recently, it has been shown that inflammatory cytokines (IL-1b, TNF-alpha, etc.) and some adhesion molecules (P-selecin, VCAM-1, etc.) are essential components involved in the migration of stem cells into the inflamed sites (Chamberlain et al., 2008, Li Li, 2011, Lotfinegad et al., 2014). Among these factors, SDF-1 and its unique receptor, CXCR-4, are considered to be the most important chemokines in BMSC homing and survival in organ-specific tissues (Herberg et al., 2013, Lau and Wang, 2011). Studies have demonstrated that expression of SDF-1 is predominantly promoted in AKI kidney tissues (Liu et al. 2013). Our study has also proved that SDF-1 protein expression in kidney tissues was significantly increased after AKI model establishment and reached the maximal level by day 7. However, the surface expression of CXCR-4 in culture-expanded BMSCs was reduced during in vitro expansion, which may lead to low efficiency of injected BMSCs homing toward the SDF-1 gradient in the injured tissues (Ahmadbeigi et al. 2010). It was observed in our study that fewer than 1% of BMSCs at passage 3 expressed CXCR-4 receptors on the cell surface. Therefore, over-expression of CXCR-4 would be an effective strategy to improve the directional migratory capacity of sub-cultured BMSCs.