Cellular heterogeneity of mesenchymal stem cells (MSCs) impedes their use in regenerative medicine. The objective of this research is to identify potential biomarkers for the enrichment of progenitors from heterogeneous MSC cultures. To this end, the present study examines variation in expression of neuron-glial antigen 2 (NG2) and melanoma cell adhesion molecule (CD146) on the surface of MSCs derived from human bone marrow in response to culture conditions and among cell populations. Multipotent cells isolated from heterogeneous MSC cultures exhibit a greater than three-fold increase in surface expression for NG2 and greater than two-fold increase for CD146 as compared with parental and lineage-committed MSCs. For both antigens, surface expression is downregulated by greater than or equal to six-fold when MSCs become confluent. During serial passage, maximum surface expression of NG2 and CD146 is associated with minimum doubling time. Upregulation of NG2 and CD146 during loss of adipogenic potential at early passage suggests some limits to their utility as potency markers. A potential relationship between proliferation and antigen expression was explored by sorting heterogeneous MSCs into rapidly and slowly dividing groups. Fluorescence-activated cell sorting revealed that rapidly dividing MSCs display lower scatter and 50% higher NG2 surface expression than slowly dividing cells, but CD146 expression is comparable in both groups. Heterogeneous MSCs were sorted based on scatter properties and surface expression of NG2 and CD146 into high (HI) and low (LO) groups. Sc LO NG2 HI and Sc LO NG2 HI CD146 HI MSCs have the highest proliferative potential of the sorted groups, with colony-forming efficiencies that are 1.5-2.2 times the value for the parental controls. The Sc LO gate enriches for rapidly dividing cells. Addition of the NG2 HI gate increases cell survival to 1.5 times the parental control. Further addition of the CD146 HI gate does not significantly improve cell division or survival. The combination of low scatter and high NG2 surface expression is a promising selection criterion to enrich a proliferative phenotype from heterogeneous MSCs during ex vivo expansion, with potentially numerous applications.

Histone deacetylases (HDACs) catalyze deacetylation of histones that results in altered transcriptional activity. Inhibitors of HDACs have been shown to induce transcriptional changes that contribute positively to reprogramming somatic cells either by nuclear transfer or inducing a pluripotent state. However, the exact molecular mechanisms whereby HDAC inhibitors function and the specificity of the HDAC isoforms in cell reprogramming are not yet fully understood. Herein, we report the ability of individual isoform-specific HDACs to modulate endogenous expression of pluripotency-associated genes in bovine somatic cells. This in vitro study showed that a transient selective depletion of HDACs resulted in elevated mRNA levels of Oct-4, Sox2, and Nanog. In particular, we found that inhibition of specific HDAC isoforms using small interfering (si) RNA significantly increased expression of Nanog, a key factor required for totipotency induced by somatic cell nuclear transfer and for maintaining pluripotency in embryonic and induced pluripotent stem cells. Our study suggests that this gene might be the most susceptible to HDAC activity inhibition. Moreover, a regulatory role of the class III HDAC, SIRT3, on an Oct4-Sox2-Nanog transcriptional network was revealed. We observed the upregulation of pluripotency-related genes by depletion of SIRT3. SIRT3 is localized to mitochondria and is associated with energy metabolism processes, suggesting metabolic changes may be linked to reprogramming in bovine fibroblasts. In conclusion, we show that targeting selective HDACs can potentially be useful to enhance reprogramming and that sirtuins may play a pivotal role in somatic cell reprogramming by upregulating an Oct4-Sox2-Nanog transcriptional network. Dedifferentiating donor somatic cells by upregulating developmentally important genes through specific knockdown of epigenetic targets, in particular HDACs, may provide a path to improving livestock cloning and the in vitro production of pluripotent cells.

Human somatic cells can be directly reprogrammed to induced pluripotent stem (iPS) cells by forced expression of the transcription factors Oct4, Sox2, and either Klf4 and cMyc or Nanog and Lin28, using virus-based systems. However, low reprogramming efficiency and the potential for deleterious virus-induced genomic modification limit the clinical potential of this technology. Recent reports indicate, however, that the generation of iPS cells can be enhanced by the addition of synthetic small molecules, including epigenetic modulators. In this report, we demonstrate that the epigenetic modifiers Valproic Acid (VPA) and 5-azacytidine activate the reciprocal transcriptional regulation of endogenous pluripotency transcription factor genes in human dermal fibroblasts and that VPA alone can directly activate endogenous Oct4 in the absence of transgenes. Moreover, using human adipose cells, we demonstrate that histone deacetylase inhibition, prior to reprogramming factor transfection, increases embryonic stem (ES) cell-like colony formation ~2 - 3 fold. In addition, DNA methyltransferase (DNMT) inhibition during human ES cell culture promotes maturation of reprogrammed somatic cells, increasing the yield ~4 fold. These data provide proof of principle that reprogramming efficiency can be improved by inhibiting specific repressive epigenetic regulatory components at the levels of ES cell-like colony formation and maturation. In addition, these studies raise the interesting possibility that a more efficient small molecule-based reprogramming system may provide a superior alternative to current virus-based approaches

The identification of a single, early marker for full developmental potential of induced pluripotent stem (iPS) cells has proven elusive. Recently, however, activation of the imprinted gene cluster, Dlk1-Dio3 has emerged as a viable candidate in the mouse. To explore the relationship between Dlk1-Dio3 expression and developmental potential more fully, we used murine ear mesenchymal stem cells (mEMSC) for iPS cell induction. Mouse EMSC are easily obtained and share functional characteristics with embryonic stem (ES) cells and therefore, may be a reliable non-embryonic source for iPS cell production. We report that mEMSC express high levels of Gtl2, a maternally expressed gene within the Dlk1-Dio3 imprinted cluster. Moreover, mEMSC produce Gtl2 expressing (Gtl2 on ) iPSC clones that share functional characteristics with ES cell clones. The production of Gtl2 on iPS cell clones from mEMSC provides a new model with which to investigate the regulation of Dlk1-Dio3 cluster activity during direct cell reprogramming.

Small molecules will need to be identified and/or developed that target protein classes limiting reprogramming efficiency. A specific class of proteins includes epigenetic regulators that silence, or minimize expression, of pluripotency genes in differentiated cells. To better understand the role of specific epigenetic modulators in reprogramming, we have used shRNA delivered by lentivirus to assess the significance of individual epi-proteins in reprogramming pluripotent gene expression.