The current study substantially proves reno-protective effects of therapeutically administered EPCs that express Atg5 in a constitutive manner. Firstly, transfected cells improved persisting kidney dysfunction several weeks after ischemia and significantly reduced urinary protein excretion. Secondly, structural endpoints were modulated as well. While interstitial matrix deposition and mesenchymal transdifferentiation of endothelial cells, both occuring at week 6, were not halted, transfected cells completely prevented the loss of peritubular capillaries. The latter has been identified as hallmark of acute ischemia and as risk factor for chronic kidney disease [20]. Administration of transfected cells also increased endothelial p62 abundances, indicative for an increased autophagocytic flux [13, 18].
In the past, we attempted to activate the autophagocytic cascade in EPCs by pharmacological measures exclusively. In total, four substances were evaluated, none of these reliably improved protective effects of the cells in AKI [13, 14]. We assumed that the lack of specificity of such drug-based approaches most likely accounted for the findings reported in previous studies. Cell-based therapies of ischemic and inflammatory diseases have increasingly been investigated in recent years. Most studies have been performed in animals, however, particularly mesenchymal stem cells (MSCs) are currently being evaluated in human diseases including AKI [21, 22]. In parallel, the mechanisms of therapeutic cell actions within the (post)ischemic or inflammatory microenvironment have gained the interest of researchers worldwide. Regarding MSCs and EPCs, two mechanisms have been identified, the release of certain types of vesicles and paracrinic actions, mediated by humoral factors or the so-called secretome. For more informations regarding EPCs, we would like to refer to the literature [5, 6]. Very recent and yet unpublished data from our laboratory indicated that intact EPCs must home to the postischemic kidney in order to promote AKI protection. In fact, systemically injected microvesicles protected the kidney as well but these effects occurred excusively under very defined conditions. Previous studies revealed that AKI protection mediated by intact EPCs can be stimulated with several substances such as Angiopoietin-1 and − 2, and the hormone melatonin [4, 8, 9]. In our latest and yet unpublished series of experiments, such stimulatory effects were missing if microvesicles were injected alone. Since pharmacological strategies for AP activation failed so far and we generally strive to augment AKI-protective EPC effects in a persistent manner, genetic modification (herein: Atg5 transfection) became a promising option. To date, only few studies addressed the stimulation of autophagy in EPCs for therapeutic purposes. Zhou and colleagues [23] employed hypoxic preconditioning and found enhanced survival of the cells in a rat model of limb ischemia. Hu et al. [16] described reduced EPC migration and tube formation upon Atg5 inactivation, the authors applied a highly selective strategy (Atg5 gene silencing). Comparable approaches with other cell types such as mesenchymal stem cells (MSCs) have not been reported so far, although autophagy activation in MSCs has been shown to augment cell competence under different experimental conditions [24–26]. The mechanisms by which constitutive Atg5 activation enables the cells to protect the kidney are speculative in nature. Since AP is regarded as self-defensive strategy under certain conditions [11], one may hypothesize that Atg5 transduction increases the lifespan of injected EPCs or their stress-resistance within the post-ischemic microenvironment. However, our approach did not ensure a reliable labelling of EPCs, making any distinct conclusion about longer intra-renal engraftment impossible.
Although selective AP activation appears as promising option for enhancing EPC competence in cell-based AKI therapy, several problems remain. The two most important problems are related to the source of EPCs or other cell-types and the timing of cell administration. These aspects have recently been discussed [22].
In summary, we showed for the first time that constitutive Atg5 expression in therapeutically administered EPCs can substantially improve AKI protective effects of the cells. The potential consequences for the clinical management of the syndrome need to be defined.