Canadian scientists from The Hospital for Sick Children in Toronto reported in the August 7th issue of Cell Stem Cell their findings on transcription factors that determine endodermal progentior cells. With a teratoma assay and growth factor-mediated differentiation, the investigators found that SOX7 and SOX17 are constitutive expressed in producing extraembryonic endoderm and endoderm progenitors. Extraembryonic endodermal cell expressed SOX7 and SOX17 cells were expressed in mesendodermal cells. The investigators noted that SOX17 mesendodermal progenitors could "undergo endoderm maturation in vitro in the absence of cytokine-mediated endoderm induction." The experimental results revealed that these endodermal progenitors could maintain their phenotype after many passages in culture.

In the August 14th online edition of Stem Cells, S. A. Ruiz and C. S. Chen from Johns Hopkins School of Medicine reported their in vitro study results on gradients of mechanical forces which drive differentiation of different tissue lineages in multicellular islands of human mesenchymal stem cells (MSCs). In these multicellular structures, the investigators found that in presence of soluble osteogenic and adipogenic growth factors, the cells in the edge differentiated into osteogenic precursors whereas cells in the center became adipocytes. With traction force maeasurements, the researchers found that regions of high stress resulted in osteogenesis and regions of low stress allowed the cells to differentiate into adipocytes. Manipulation of cytoskeleton tension could determine commitment to either adipogenesis or osteogenesis. The authors concluded that their experimental results "demonstrated a role for mechanical forces in linking multicellular organization to spatial differentials of cell differentiation, and represent an important guiding principle in tissue patterning that could be exploited in stem cell-based therapies."

In the August 1st issue of J. Clin. Investigation, K. E. Hoot et al. from Oregon Health Science Univ. reported their findings on the role of Smad2 during the formation and progression of skin cancer in mice. Smad2, -3, and -4 are mediators of the TGF-β signaling pathway and serve as tumor suppressors. The investigators found in mice with keratinocyte-specific deletion of Smad2 there were accelerated formation and malignant progression of chemically induced skin tumors in contrast to wild-type mice. The loss of Smad2 resulted in poorly differentiated human squamous cell carcinomas concomitant with the cells undergoing epithelial-mesenchymal transition (EMT) prior to spontaneous loss of Smad4. The epithelial cells in Smad2-/- mice also had a decreased expression in E-cahedrin while an upregulation int the transriptional repressor Snail. The experimental data revealed that knocking down Snail abrogated the EMT phase associated with Smad2 loss. It also demonstrated that Smad2 loss led to significant binding of Smad4 to the Snail promoter, while knocking down either Smad3 or Smad4 in keratinocytes abrograted Smad2 loss-associated Snail overexpression. The authors concluded that their data "suggest that enhanced Smad3/Smad4-mediated Snail transcription contributed to Smad2/loss-associated Emt during skin carcinogenesis."

In the July 10th issue of J. of Clinical Investigations, A. Bersenev et al. from Children's Hospital of Philadelphia reported their findings on identifying a inhibitory adaptor protein, Lnk, that modulates hematopoiesis through controlling homeostasis and self-renewal of hemapoietic stem cells (HSCs). The investigators found that Lnk serves as a negative regulator on the HSC homeostatic pathway involving thrombopoietin (TPO), its receptor Mpl, and activation of JAK2 pathway. The experimental results revealed that Lnk binds to JAK2 which represses the proliferative response in HSCs. In mice deficient in Lnk (Lnk-/-), there is a 10-fold increase in HSC frequency compared to wild type mice. However, a large fraction of the expanded HSCs were quiescent and resistant to 5-fluroouracil treatment. The quiescent HSCs were more effective in engrafting the bone marrow. The researchers also conducted experiments demonstrating that Lnk directly binds to phosphorylated tyrosine residues of JAK2 following TPO stimulation of Mpl. The authors concluded that their study reveals that Lnk is a "physiological negative regulator of JAK@ in stem cells and TPO/Mpl/JAK@/Lnk as a major regulatory pathway in controlling stem cell self-renewal and quiescence."

Medulloblastoma, an insidious brain cancer in children, is associated with mutation (90%) of the Sonic hedgehog (Shh) receptor, Patched. In the Feb. 16th post scientists from Duke University noted that aberrant Shh signaling from a mutation in the Shh receptor gave rise to hyperproliferative granule cell precursors (GCPs) of the cerebellum which can result in brain tumors in children. In the August 12th edition of Cancer Cell, Z-J Yang et al. from Duke Univ. Med. Ctr., published their findings on a study determining the cellular origin of medulloblastoma resulting from mutation in the Shh signaling pathway. The investigators found that activation of Shh in neuronal progenitors caused medulloblastoma by 3 months of age in mice. Surprisingly, activation of Shh signaling pathway in stem cells also caused brain tumors, once the stem cells were committed to the neuronal lineage. The study results revealed that mutations in the stem cells produced a more aggressive tumor than those initiated by progenitors, with all animals succumbing to the brain tumor by 3-4 weeks. The authors concluded that their studies "suggest that medulloblastoma can be initiated in progenitors or stem cells but that Shh-induced tumorigenesis is associated with neuronal lineage commitment."

Scientists from the University of Toronto, R. A. Rose et al., reported in the August 7th online edition of Stem Cells the results of their study on mouse bone marrow-derived mesenchymal cells which had been co-cultured with rat embryonic cardiomyocytes (rCMs). The study was conducted in order to determine whether the mesenchymal stromal cells (MSCs) were able to differentiate into functional cardiomyocytes. The investigators used transgenic female mice expressing GFP under control of the a-myosin heavy chain promoter. After 5 days in culture with male rCMs, 6.3% of the MSCs became GFP+ as well as expressed the cardiomyocyte markers troponi I and a-actinin. Additionally, the expressed the cardiac-specific genes atrial natriuretic factor, Nkx2.5 and a-cardiac actin. However, the researchers found that the GFP+ MSCs did not generate the action potential or ionic currents typical of cardiomyocytes. Further analysis of the MSC phenotype revealed the cells expressed the following markers: CD45-, CD34+, CD73+,CD105+, CD90+, CD44+, SDF1+, CD134L, type IV collagen, vimentin+, troponinT+, troponinI+, a-actinin+, and connexin43+. The authors concluded that the MSC population they isolated "displayed plasticity towards the cardiomyocyte lineage while retaining mesenchymal stromal cell properties, including a non-excitable electrophysiological phenotype." (It is interesting to note that co-culturing mouse MSCs with rat rCMs may have impacted the study results, particularly, in the functional analysis of the differentiated cardiomyocytes.)

In the August 7th issue of Cell Stem Cell, University of Connecticut's R-H Xu et al. published the results of their study on the signaling pathways for self-renewal and differentiation in human embryonic stem cells (ESCs). Both TGFβ and FGF signaling maintain pluriplotency in ESCs, whereas BMP signaling induces differentiation in ESCs. With a defined medium, the investigators found that both TGFβ and FGF synergistically inhibited BMP signaling, while maintained sustained expression of Nanog, Oct4, and Sox2. The experimental results revealed that both TGFb and BMP signaling produced SMADs which could bind Nanog's proximal promoter. However, Nanog promoter activity is enhanced by TGFβ-mediated SMAD signaling and reduced by BMP-mediated SMADs. The authors concluded that their study results "suggest that direct binding of TGFβ/Activin-responsive SMADs to the Nanog promoter plays an essential role in sustaining human ESC self-renewal."

August 8, 2008---
Category: Signaling & Pathways
In a study investigating the control of microRNA gene expression and transcriptional regulators of embryonic stem cells, scientists from the Whitehead Institute, A. Marson et al., mapped a transcriptional regulatory circuitry of embryonic stem cells (ESCs) involving master transcription factors which modulate self-renewal and pluripotency of the ESCs. In the August 8th issue of Cell, the investigators reported their experimental results using high-resolution ChIP-seq data, to identify miRNA promoters as well as quanitative sequencing of short transcripts. The researchers found that four master transcription factors (Oct4, Sox2, Nanog, and Tcf3) in both human and mouse ESCs interacted with two key sets of microRNA genes. One set of microRNA genes are upregulated in ESCs while a second set of genes are silenced by Polycomb proteins which co-occupy the promoter regions. The experimental data also revealed that the second set of repressed microRNA genes are involved expressed in specific tissues during differentiation of the ESCs. The authors concluded that their data revealed how "key ESC transcription factors promote the ESC microRNA expression program and integrate microRNAs into the regulatory circuitry controlling ESC identity."

Scientists from Harvard University, A. Kobayashi et al., reported their study results on the identifying nephron progenitor cells which express Six2. In the August 7th issue of Cell Stem Cell, the investigators found that the cells expressing Six2 represented multipotent nephron progenitor cells from the cap mesenchyme during nephrogenesis. Some of the Six2-expressing progenitors were able to self-renew and they contributed to muliple domains of the nephron. Cells of the cap mesenchyme which lack Six2 activity were found to form ectopic nephron tubules which was dependent upon Wnt9b signaling. The authors concluded that their "observations suggested that Six2 activity cell-autonomously regulates a multipotent nephron progenitor population."

Belgium scientists from the Université Libre de Bruxelles, A. Bondue et al., reported in the July 3rd issue of Cell Stem Cell the results of their study specification of multipotent cardiovascular progenitor cells with mouse embryonic stem cells (ESCs) during early embryonic development. The investigators used genome-wide transcriptional analysis to identify a transcription factor, Mesp1, which simultaneously activates and represses a discrete set of genes that regulate differentiation of the ESCs into cardiovascular progenitor cells. Mesp1 binds to the DNA sequences located in the promoter region of genes that result in upregulation of the core cardiac transcriptional machinery. Additionally, Mesp1 represses the expression of key genes that regulates mesodermal and endodermal fates during embryogenesis. The authors concluded that their experimental data suggest that "Mesp1 acts as a key regulatory switch during cardiovascular specification, residing at the top of the hierarchy fo the gene network responsible for cardiovascular cell-fate determination."