Characterisation of microRNAs in Human Stem Cells

In collaboration with David Baulcombe and Attila Molnar we have generated microRNA libraries for human embryonic stem cells (hESCs) before and after differentiation along the neuronal lineage and also from human mesenchymal stem cells (hMSCs). Both cell types are of medical importance and understanding how their proliferation and differentiation is regulated by microRNAs is also of scientific interest. The hMSC library was sequenced by 454 technology and the two subsequent hESC libraries by Solexa sequencing. Approximately a quarter of all currently known microRNAs were identified between the libraries, in addition to 3 novel microRNAs and 25 annotated piRNAs. For the hESC libraries, we verified the presence of embryonic specific microRNAs (miR-302 family) and neuronal specific microRNAs (miR-9/miR-9*), and demonstrated that expression of these miRNAs is regulated at the transcriptional level. Additionally, promoter assessments of miR-9 transcription revealed that multiple upstream regions may be important in neuronal specific expression. Almost half of all known human microRNAs are located within the introns of host genes. We used microarrays to analyse host gene expression and found that there was little correlation with microRNA expression, indicating that many microRNAs are not regulated at the transcriptional level by their host promoter. Furthermore, the expression of microRNAs from the same cluster, and also from the same hairpin precursor, did not always correlate when compared between the stem cell libraries. Taken together, this data indicates that microRNAs are regulated at a variety of levels both pre- and post-transcriptionally. Many microRNA isomers were also detected that differed in expression between human cell types, and upon differentiation of the hMSCs through the osteoblastic lineage. Interestingly, microRNAs and some of their isomers showed different affinities for Argonaute proteins in pulldown assays. We also profiled mRNAs that were immunoprecipitated with Argonaute in order to identify miRNA target