88 siRNA SOP SVZ TBI QA QA/QC VEGF Introduction Small interfering ribonucleic acid Standard operating procedure Subventricular zone Traumatic brain Injury Quinolinic acid Quality assurance/quality control Vascular endothelial growth factor Human mesenchymal stem cells are known to secrete a variety of cytokines and growth factors that have both paracrine and autocrine activities for damaged tissues, including the brain (reviewed in [1]). The mechanism of action of adult MSC obtained from bone marrow or adipose tissue is based on the innate functions of these stem cells: the injected cells home to the injured area, in particular to hypoxic, apoptotic, or inflamed areas, and release trophic factors that hasten endogenous repair. These secreted bioactive products can suppress local inflammation, enhance angiogenesis, reduce levels of free radicals, inhibit fibrosis and apoptosis, and stimulate recruitment, retention, proliferation, and differentiation of tissue-residing stem cells (reviewed in [2]). These paracrine effects are distinct from the classical model of direct differentiation of stem cells into the tissue to be regenerated. MSC are ideally suited for cellular therapy due to their ease of isolation, manipulation, and safety. They can be expanded from normal qualified human donors in large quantities and can be infused without tissue matching, since they shield themselves from the immune system [3]. The ability to be transplanted without tissue matching has allowed large multicenter trials to be conducted with direct comparison of the same batches of MSC across hundreds of patients, without adverse events or rejection reactions [4, 5]. Due to the promise of MSC in cellular therapies, a variety of studies have focused initially not only on their characterization but also on their utility in treatment of several diseases in animal models. MSC contributed significantly to the recovery of tissues in models of myocardial infarction [6], stroke [7, 8], meniscus injury [9], and limb ischemia [10]. However, the number of engrafted MSC was consistently low in the damaged tissue, suggesting that their efficacy relies upon actions other than direct differentiation. Kinnaird et al. [11] demonstrated that MSC-conditioned media stimulated endothelial cell prolifer- ation and migration in vitro, and the injection of MSC- conditioned media into mice that had undergone hind limb ischemia was sufficient to mediate regeneration of the blood flow in the injured limb. Similar results have been shown with a cardiac infarction model [12], and the secretion of multiple angiogenic cytokines from MSC has been demonstrated. Mol Neurobiol (2012) 45:87–98 Hepatocyte growth factor (HGF), fibroblast growth factor-2 (FGF-2), insulin-like growth factor-1 (IGF-1), and vascular endothelial growth factor (VEGF) have all been detected in MSC-conditioned medium. The increasing body of evidence points toward the theory that a complex set of trophic factors secreted by MSC significantly contributes to injury repair in vivo, through stimulating angiogenesis, reducing oxidative stress, and decreasing apoptosis. MSC have been found to produce improvements in disease models even though a limited number of the cells could be demonstrated to be stably engrafted. A mystery that remains in the MSC field is that, while MSC persist long-term in the tissues of non-injured or chronically damaged mice, in cases of acute injury or inflammation, MSC respond to the injury robustly, but only transiently and do not become an enduring part of the repaired tissue or vasculature to any significant degree. We have studied this disparity in immune deficient mice that cannot reject the human cells, so the disappearance of MSC is indepen- dent of an immune rejection. One month post-infusion MSC are often virtually undetectable at the area of acute tissue damage [13–15]. The same phenomenon has been observed in large animal models. In contrast, when labeled MSC, cultured in the same way, are infused intravenously into immune deficient mice that have low- level systemic damage from irradiation, a chronic disease, or no damage at all, the cells migrate through all tissues and persist in a relatively evenly dispersed and long- lasting manner [16, 17]. We have recovered human MSC from numerous organs of the mice at time points from 1 to 18 months post-transplantation in those cases [16–20], whereas in the acute injury setting, they are only transiently recovered. In our previous studies, hypoxic preconditioning was found to enhance MSC efficacy in a severely hypoxic tissue ischemia model, perhaps by priming MSC to respond to the low oxygen environment [14]. We are currently using the hypoxic preconditioning method to prepare MSC for injection into the striatum in murine models of HD. Further studies must be done to better understand the duration of the residence of infused MSC in tissues, including the brain, and the mechanisms that recruit and retain them in chronic vs. acute tissue damage environments. This knowledge will allow the most effective use of MSC- based cellular therapies. MSC in Neural Repair—Overview There is currently much interest in the use of MSC to treat neurodegenerative diseases. In particular degenerative dis- eases like Huntington’s disease that are fatal and difficult to treat, MSC may prove therapeutic by providing neuro-