Although chronic wounds are common and continue to be a major cause of morbidity and mortality, treatments for these conditions are lacking and often ineffective. A large body of evidence exists demonstrating the therapeutic potential of mesenchymal stem cells (MSCs) for repair and regeneration of damaged tissue, including acceleration of cutaneous wound healing. However, the exact mechanisms of wound healing mediated by MSCs are unclear. In this study, we examined the role of MSC exosomes in wound healing. We found that MSC exosomes ranged from 30 to 100-nm in diameter and internalization of MSC exosomes resulted in a dosedependent enhancement of proliferation and migration of fibroblasts derived from normal donors and chronic wound patients. Uptake of MSC exosomes by human umbilical vein endothelial cells also resulted in dosedependent increases of tube formation by endothelial cells. MSC exosomes were found to activate several signaling pathways important in wound healing (Akt, ERK, and STAT3) and induce the expression of a number of growth factors [hepatocyte growth factor (HGF), insulin-like growth factor-1 (IGF1), nerve growth factor (NGF), and stromal-derived growth factor-1 (SDF1)]. These findings represent a promising opportunity to gain insight into how MSCs may mediate wound healing.

Heart failure carries a poor prognosis with few treatment options. While myocardial stem cell therapeutic trials have traditionally relied on intracoronary infusion or intramyocardial injection routes, these cell delivery methods are invasive and can introduce harmful scar tissue, arrhythmia, calcification, or microinfarction in the heart. Given that patients with heart failure are at an increased surgical risk, the development of a noninvasive stem cell therapeutic approach is logistically appealing. Taking advantage of the trophic effects of bone marrow mesenchymal stem cells (MSCs) and using a hamster heart failure model, the present study demonstrates a novel noninvasive therapeutic regimen via the direct delivery of MSCs into the skeletal muscle bed. Intramuscularly injected MSCs and MSC-conditioned medium each significantly improved ventricular function 1 mo after MSC administration. MSCs at 4 million cells/animal increased fractional shortening by approximately 40%, enhanced capillary and myocyte nuclear density by approximately 30% and approximately 80%, attenuated apoptosis by approximately 60%, and reduced fibrosis by approximately 50%. Myocyte regeneration was evidenced by an approximately twofold increase in the expression of cell cycle markers (Ki67 and phosphohistone H(3)) and an approximately 13% reduction in mean myocyte diameter. Increased circulating levels of hepatocyte growth factor (HGF), leukemia inhibitory factor, and macrophage colony-stimulating factor were associated with the mobilization of c-Kit-positive, CD31-positive, and CD133-positive progenitor cells and a subsequent increase in myocardial c-Kit-positive cells. Trophic effects of MSCs further activated the expression of HGF, IGF-II, and VEGF in the myocardium. The work highlights a cardiac repair mechanism mediated by trophic cross-talks among the injected MSCs, bone marrow, and heart that can be explored for noninvasive stem cell therapy.

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We recently demonstrated a cardiac therapeutic regimen based on injection of bone marrow mesenchymal stem cells (MSCs) into the skeletal muscle. Although the injected MSCs were trapped in the local musculature, the extracardiac cell delivery approach repaired the failing hamster heart. This finding uncovers a tissue repair mechanism mediated by trophic factors derived from the injected MSCs and local musculature that can be explored for minimally invasive stem cell therapy. However, the trophic factors involved in cardiac repair and their actions remain largely undefined. We demonstrate here a role of MSC-derived IL-6-type cytokines in cardiac repair through engagement of the skeletal muscle JAK-STAT3 axis. The MSC IL-6-type cytokines activated JAK-STAT3 signaling in cultured C2C12 skeletal myocytes and caused increased expression of the STAT3 target genes hepatocyte growth factor (HGF) and VEGF, which was inhibited by glycoprotein 130 (gp130) blockade. These in vitro findings were corroborated by in vivo studies, showing that the MSC-injected hamstrings exhibited activated JAK-STAT3 signaling and increased growth factor/cytokine production. Elevated host tissue growth factor levels were also detected in quadriceps, liver, and brain, suggesting a possible global trophic effect. Paracrine actions of these host tissue-derived factors activated the endogenous cardiac repair mechanisms in the diseased heart mediated by Akt, ERK, and JAK-STAT3. Administration of the cell-permeable JAK-STAT inhibitor WP1066 abrogated MSC-mediated host tissue growth factor expression and functional improvement. The study illustrates that the host tissue trophic factor network can be activated by MSC-mediated JAK-STAT3 signaling for tissue repair.

Hsp90 functions in association with several cochaperones for folding of protein kinases and transcription factors, although the relative contribution of each to the overall reaction is unknown. We assayed the role of nine different cochaperones in the activation of Ste11, a Saccharomyces cerevisiae mitogen-activated protein kinase kinase kinase. Studies on signaling via this protein kinase pathway was measured by ␣-factor-stimulated induction of FIG1 or lacZ, and repression of HHF1. Several cochaperone mutants tested had reduced FIG1 induction or HHF1 repression, although to differing extents. The greatest defects were in cpr7⌬, sse1⌬, and ydj1⌬ mutants. Assays of Ste11 kinase activity revealed a pattern of defects in the cochaperone mutant strains that were similar to the gene expression studies. Overexpression of CDC37, a chaperone required for protein kinase folding, suppressed defects the sti1⌬ mutant back to wild-type levels. CDC37 overexpression also restored stable Hsp90 binding to the Ste11 protein kinase domain in the sti1⌬ mutant strain. These data suggest that Cdc37 and Sti1 have functional overlap in stabilizing Hsp90:client complexes. Finally, we show that Cns1 functions in MAP kinase signaling in association with Cpr7. INTRODUCTIONAmong several major classes of molecular chaperone that have been characterized, Hsp90 appears to function primarily in folding of signal transducing proteins such as transcription factors and protein kinases (Caplan, 1999;Prodromou and Pearl, 2003). Hsp90 does not act alone in this capacity, but in association with several cochaperones that may also have chaperone function as defined by their ability to prevent polypeptide aggregation in vitro.The current understanding of how Hsp90 cochaperones function derived from in vitro studies on progesterone receptors (PR) and glucocorticoid receptors (GR; Toft, 1997, 2003). The results of these studies showed that Hsp70 and Hsp40 first interact with the receptor ligandbinding domain. Hsp90 is recruited into this complex by the cochaperone called Hop, which interacts with both Hsp90 and Hsp70. Subsequently, Hsp70 and Hop are displaced by other cochaperones, such as one of many immunophilins and p23. Folding occurs in association with ATP hydrolysis by Hsp90, which is stimulated by another cochaperone called Aha1 (and its paralog Hch1; Panaretou et al., 2002;Lotz et al., 2003). It is unclear whether folding occurs while the receptor is in association with Hsp90 or after its release. Whether the cochaperones are required for this reaction is not clear, because purified Hsp70 and Hsp90 can together facilitate folding of GR in vitro, whereas cochaperones such as Hop stimulate the reaction (Morishima et al., 2000;Rajapandi et al., 2000). Furthermore, deletion of yeast Hop (STI1), or p23 (SBA1) does not result in a slow growth phenotype except under stress conditions (Nicolet and Craig, 1989;Chang et al., 1997;Bohen, 1998;Fang et al., 1998).Another question is whether the scheme described above for nuclear receptors reflects a set of ...