Regulation of Genes That Modulate the Response to Injury and Repair

Studies looking at changes in gene expression in recipients following MSC administration point to the role of MSCs in “reprogramming” the inflammatory response network [97]. MSC administration results in a coordinated modulation of the host transcriptional response characterized by an overall downregulation of inflammation and inflammation-related genes and a shift toward upregulation of genes involved in reducing inflammation, phagocytosis, and bacterial killing. MSC administration results in transcriptional reprogramming of multiple genes involved in the host response to injury and repair. As well, MSC administration alters the expression of genes responsible for the pro-inflammatory M1 macrophage phenotype together with an increase in antiinflammatory M2 macrophage determinants. Our own group employed a genome-wide expression microarray approach to perform an “unsupervised” network analysis in major organs commonly affected by sepsis following MSC administration. In addition to the expected changes in pathways associated with inflammation and tissue repair, we found that treatment with MSCs returned transcriptional pathways responsible for maintaining mitochondrial function and consequently cellular bioenergetics to control levels. This finding is consistent with the possibility of mitochondrial transfer between MSCs and cells of the host organs as proposed by Islam et al. [31]. Moreover, reduction in innate immune pro-inflammatory transcription responses was also observed concurrently with upregulation of genes tasked with maintaining microvasculature integrity [98]. This data indicates that the paradigm that a single MSC-derived paracrine mediator is responsible for the global pleotropic effect of MSCs on transcriptional and network reprogramming is absurd. A much more plausible explanation is that MSC-conferred protection from acute injury involves a range of complementary activities, resulting in mitigation of the innate and acquired immune and inflammatory responses while also affecting complex networks of host cell-cell, cell-matrix, metabolism and bioenergy substrate utilization, and functional pathways. The dramatic effect of stem cells on gene transcription suggests MSCs may generate a coordinated genetic response by regulating gene expression at a higher gene organizational level that depends on complex network of transcription factors and histone modifiers, in concert with specific transcriptional co-activators and co-repressors that activate or repress MSCs [99, 100].

Regulation of gene expression at the epigenetic level occurs via modifications of chromatin architecture by facilitating the opening of DNA (euchromatin) to permit transcription or the condensing of DNA (heterochromatin) to repress transcription [101]. Accordingly, the architecture of chromatin is essential for the regulation of various chromatin-based cellular processes and is dynamically modulated through the orchestration of multiple mechanisms including histone modification, DNA methylation, chromatin remodeling, and noncoding RNA. At present, a large number of studies are focusing on identifying extrinsic regulators and their intrinsic target transcription factors that regulate MSC properties and functions. So far we know that a variety of histone-modifying enzymes are involved in the dynamic regulation of MSC trilineage differentiation [100]. In addition, epigenetic changes seem to play a critical role in the ability of MSCs to cross oligolineage boundaries and differentiate into different cell types, such as neurons, cardiomyocytes, hepato- cytes, and endothelial cells, under appropriate culture conditions, indicating that these cells are extremely plastic. Importantly, various chemical genetic approaches, specific combination of small cell-permeable biological active compounds that are involved in the regulation of chromatin structure and function, and interfere with specific cell signaling pathways, may be used for reprogramming MSCs [102]. For example, histone modifiers alter the differentiation of MSCs into neuronal [103,

104] or cardiomyocyte [102, 105] cells. Moreover, the epigenetic-modifying drug such as BIX-01294 (a histone G9a methyltransferase inhibitor) can enhance endothelial differentiation of adipose tissue-derived MSCs through upregulation of several endothelial markers and factors associated with blood vessel formation [106]. The relative biological significance of the role of epigenetic regulation to the therapeutic potential of MSCs comes from the diabetes and the senescence literature. MSCs are responsive to metabolic cues including circulating glucose levels. Longterm exposure of undifferentiated human adipose tissue stromal cells to high glucose upregulates a subset of inflammation response genes by altering their promoter histone methylation patterns in a manner consistent with transcriptional derepression [107]. Genetic alteration studies demonstrate that DNA methyltransferases are responsible for maintaining methylation, and their loss results in significant myeloid skewing and self-renewal defects (reviewed in detail in [108]). Evidence that MSCs can be primed to express epigenetic “marks” provides an argument for priming cells in an effort to engineer MSCs that can impart specific biological properties to recipients [109, 110].