MicroRNAs are single-stranded regulatory RNAs of 18-25 nucleotide length generated from endogenous transcripts that form local hairpin structures. The processing of microRNA transcripts involves the activities of two RNase III enzymes Drosha and Dicer. In this study we analyzed structural features of human microRNA precursors that make these transcripts Drosha and Dicer substrates. The structures of minimal functional primary precursors (pri-microRNAs) and secondary precursors (pre-microRNAs) were predicted. The frequency, nucleotide sequence content and the localization of various structure destabilizing motifs was analyzed. We identified numerous pri-microRNAs which structures strongly depart from the consensus structure and their processing is hard to explain by the existing model of the Microprocessor complex. We also found a biased distribution of symmetric and asymmetric motifs along the pre-microRNA hairpin stem and an overrepresentation of bulges on its 5′ arm (p < 0.000001), which may have considerable functional implications.

Regenerative medicine has become a new therapeutic approach in which stem cells or genetically reprogrammed cells are delivered to diseased areas in the body with the intention that such multipotent cells will differentiate into healthy tissue and exchange damaged tissue. The success of such cell-based therapeutic approaches depends on precise dosing and delivery of the cells to the desired site in the human body. To determine the accuracy and efficacy of the therapy, tracking of the engrafted cells in an intact living organism is crucial. There is a great need for sensitive, noninvasive imaging methods, which would allow clinicians to monitor viability, migration dynamics, differentiation towards specific cell type, regeneration potential and integration of transplanted cells with host tissues for an optimal time period. Various in vivo tracking methods are currently used including: MRI (Magnetic Resonance Imaging), PET (Positron Emission Tomography), SPECT (Single Photon Emission Computer Tomography), optical imaging (OI), photoacoustic imaging (PAI) and ultrasound (US). In order to carry out the detection with each of the aforementioned techniques, the cells must be labeled either exogenously (ex vivo) or endogenously (in vivo). For tracking the administrated cells, scientists usually manipulate cells outside the living organism by incorporating imaging contrast agents (CAs) or reporter genes. Strategies for stem cell labeling using CAs will be reviewed in the light of various imaging techniques.