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New Studies Suggest microRNA Pathway May Influence Stem Cell Maintenance, Formation

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The growing body of evidence suggesting that microRNAs play a role in cellular functioning grew some more this month with the publication of two papers showing that the non-coding RNAs influence certain types of stem cells.
 
In one paper, published this week in Current Biology, researchers from the Stowers Institute for Medical Research present data indicating that Dicer-1, one of two Dicers required for mature miRNA generation in Drosophila, is also vital in maintaining germline and somatic stem cells in the Drosophila ovary.
 
“The findings were interesting to us because they demonstrated that the microRNA pathway is essential for controlling self-renewal or maintenance of two types of stem cells," Zhigang Jin, one of the study’s co-authors, said in a statement. "In the future, the small RNAs responsible for stem cell regulation could potentially be used to control stem cell functions in vivo and stem cell expansion in vitro."
 
According to the Current Biology paper, Jin and Xie aimed to determine whether a lack of Dcr-1 would impact the maintenance of stem cells in the Drosophila ovary by creating mutants lacking the enzyme.
 
“Our results show that [the mutant germline stem cells] cannot be maintained and are lost rapidly from the niche without discernable features of cell death, indicating that Dcr-1 controls GSC self-renewal but not survival,” the researchers wrote.
 
However, a gene known as “bag of marbles”, which encodes a key differentiating factor in the Drosophila germline, is not up-regulated in the mutant stem cells, “and its removal does not slow down … mutant [germline stem cell] loss,” they added. This suggests that Dcr-1 controls stem cell self-renewal by repressing a differentiation pathway independent of the gene.
 
Additional experiments showed that Dcr-1 is also essential for the maintenance of somatic stem cells in the Drosophila ovary.
 
In light of these findings, “we further propose that miRNAs processed by Dcr-1 are essential for controlling self-renewal” of germline and somatic stem cells, an hypothesis supported by earlier work showing that the Dcr-1 partner Loquacious is also required for germline stem cell maintenance, the researchers wrote.
 
“Without one or more miRNAs generated by Dcr-1, ovarian [germline and somatic stem cells] undergo premature differentiation that leads to the depletion of these stem cells in their corresponding niches,” they added.
 
"We are in the process of identifying the microRNAs that are important for stem cell self-renewal," Ting Xie, the paper’s other co-author, noted. "Understanding the mechanisms controlling stem cell self-renewal will be crucial to our developing the ability to expand stem cell populations for performing tissue repair."
 
Hematopoietry in Motion
 
The second paper — published earlier this month in the Proceedings of the National Academy of Sciences by researchers from Ohio State University, Johns Hopkins University, and the Fred Hutchinson Cancer Research Center — shows that a specific microRNA controls the formation of CD34+ hematopoietic stem-progenitor cells.
 
“To better understand the role of miRNAs in normal human hematopoiesis, we determined the miRNA expression profile of human hematopoietic stem-progenitor cells,” the researchers wrote. “We then bioinformatically combined these miRNA expression data, mRNA expression data obtained for human CD34+ cells that we had determined in a previous study, and the predicted mRNA targets of all known miRNAs.”
 

“The findings were interesting to us because they demonstrated that the microRNA pathway is essential for controlling self-renewal or maintenance of two types of stem cells. In the future, the small RNAs responsible for stem cell regulation could potentially be used to control stem cell functions in vivo and stem cell expansion in vitro." in vivo in vitro

By combining these data sets, the researchers noted, they were able to examine in silico the interactions of hematopoietic stem-progenitor cell-expressed miRNAs and mRNAs and predict which miRNAs might control hepatopoietic differentiation.
 
“We found that 33 miRNAs are expressed in CD34+ [hematopoietic stem-progenitor cells] obtained from both bone marrow and mobilized blood peripheral blood stem cell harvests, and we demonstrate that translation of several mRNAs associated with hematopoietic differentiation is controlled by these” miRNAs, according to the PNAS paper’s authors.
 
Each of the miRNAs targets multiple hematopoietic differentiation-associated mRNAs, “suggesting that a relatively small subset of [the miRNAs] negatively regulates hematopoiesis,” they added. “Taken together, these observations led us to propose a model in which many of the protein-coding genes that specify hematopoietic differentiation are expressed at an early time point by undifferentiated” hematopoietic stem-progenitor cells, but are held in check by miRNAs until differentiation occurs.
 
The researchers also functionally validated one of the miRNAs expressed in CD34+ cells, miR-155, as a key regulator of myelopoiesis and erythropoiesis. “As predicted, miRNA-155 transduction greatly reduced by myeloid and erythroid colony formation of normal human” hematopoietic stem-progenitor cells, they wrote.
 
“We hope that this study will serve to guide, and will be greatly refined by, the work of other researchers examining the role of miRNAs in hematopoiesis,” the researchers concluded. “Specifically, we look forward to results of experiments testing our model in which many of the genetic components associated with hematopoietic differentiation are expressed at an early time point by undifferentiated [hematopoietic stem-progenitor cells] but are controlled by … [miRNA-mediated] repression of protein translation until differentiation is initiated.”

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