N 1-Methylpseudouridine substitution enhances the performance of synthetic mRNA switches in cells.

Synthetic messenger RNA (mRNA) tools often use pseudouridine and 5-methyl cytidine as substitutions for uridine and cytidine to avoid the immune response and cytotoxicity induced by introducing mRNA into cells. However, the influence of base modifications on the functionality of the RNA tools is poorly understood. Here we show that synthetic mRNA switches containing N1-methylpseudouridine (m1Ψ) as a substitution of uridine substantially out-performed all other modified bases studied, exhibiting enhanced microRNA and protein sensitivity, better cell-type separation ability, and comparably low immune stimulation. We found that the observed phenomena stem from the high protein expression from m1Ψ containing mRNA and efficient translational repression in the presence of target microRNAs or proteins. In addition, synthetic gene circuits with m1Ψ significantly improve performance in cells. These findings indicate that synthetic mRNAs with m1Ψ modification have enormous potentials in the research and application of biofunctional RNA tools.

The amyloid precursor protein binds to ß-catenin and modulates its cellular distribution.

Accumulating evidence has shown that the processing of the amyloid precursor protein (APP) and the formation of amyloid-ß are associated with the canonical Wnt/ ß-catenin signalling pathway. It was recently published that the drosophila homologue of APP is a conserved modulator of Wnt PCP signalling, suggesting a potential regulation of this pathway by APP. The aim of this study was to investigate the potential interaction of APP with the canonical Wnt pathway. APP overexpression in N2a cells led to alterations in the subcellular distribution of ß-catenin by physically binding to it, preventing its translocation to the nucleus and precluding the transcription of Wnt target genes. In addition, studies in APP transgenic mice and human Alzheimer's disease (AD) brain tissue showed the cellular co-localization of APP and ß-catenin and binding of both proteins, suggesting the formation physical complexes of APP and ß-catenin, yet not present in healthy controls. Furthermore, a reduction in the levels of nuclear ß-catenin was detected in AD brains compared to controls as well as a decrease in the expression of the inactive phosphorylated Glycogen synthase kinase 3 (GSK3) isoform. Therefore, these findings indicate a reciprocal regulation of Wnt/ ß-catenin signalling pathway and APP processing involving a physical interaction between APP and ß-catenin.

Cell-type-specific genome editing with a microRNA-responsive CRISPR-Cas9 switch.

The CRISPR-Cas9 system is a powerful genome-editing tool useful in a variety of biotechnology and biomedical applications. Here we developed a synthetic RNA-based, microRNA (miRNA)-responsive CRISPR-Cas9 system (miR-Cas9 switch) in which the genome editing activity of Cas9 can be modulated through endogenous miRNA signatures in mammalian cells. We created miR-Cas9 switches by using a miRNA-complementary sequence in the 5΄-UTR of mRNA encoding Streptococcus pyogenes Cas9. The miR-21-Cas9 or miR-302-Cas9 switches selectively and efficiently responded to miR-21-5p in HeLa cells or miR-302a-5p in human induced pluripotent stem cells, and post-transcriptionally attenuated the Cas9 activity only in the target cells. Moreover, the miR-Cas9 switches could differentially control the genome editing by sensing endogenous miRNA activities within a heterogeneous cell population. Our miR-Cas9 switch system provides a promising framework for cell-type selective genome editing and cell engineering based on intracellular miRNA information.

Efficient, Selective Removal of Human Pluripotent Stem Cells via Ecto-Alkaline Phosphatase-Mediated Aggregation of Synthetic Peptides.

The incomplete differentiation of human induced pluripotent stem cells (iPSCs) poses a serious safety risk owing to their potential tumorigenicity, hindering their clinical application. Here, we explored the potential of phospho-D-peptides as novel iPSC-eliminating agents. Alkaline phosphatases overexpressed on iPSCs dephosphorylate phospho-D-peptides into hydrophobic peptides that aggregate and induce cell death. We isolated a peptide candidate, D-3, that selectively and rapidly induced toxicity in iPSCs within 1 hr but had little influence on various non-iPSCs, including primary hepatocytes and iPSC-derived cardiomyocytes. Two hours of D-3 treatment efficiently eliminated iPSCs from both single cultures and co-cultures spiked with increasing ratios of iPSCs. In addition, D-3 prevented residual iPSC-induced teratoma formation in a mouse tumorigenicity assay. These results suggest the enormous potential of D-3 as a low-cost and effective anti-iPSC agent for both laboratory use and for the safe clinical application of iPSC-derived cells in regenerative medicine.

MicroRNA-302 switch to identify and eliminate undifferentiated human pluripotent stem cells.

The efficiency of pluripotent stem cell differentiation is highly variable, often resulting in heterogeneous populations that contain undifferentiated cells. Here we developed a sensitive, target-specific, and general method for removing undesired cells before transplantation. MicroRNA-302a-5p (miR-302a) is highly and specifically expressed in human pluripotent stem cells and gradually decreases to basal levels during differentiation. We synthesized a new RNA tool, miR-switch, as a live-cell reporter mRNA for miR-302a activity that can specifically detect human induced pluripotent stem cells (hiPSCs) down to a spiked level of 0.05% of hiPSCs in a heterogeneous population and can prevent teratoma formation in an in vivo tumorigenicity assay. Automated and selective hiPSC-elimination was achieved by controlling puromycin resistance using the miR-302a switch. Our system uniquely provides sensitive detection of pluripotent stem cells and partially differentiated cells. In addition to its ability to eliminate undifferentiated cells, miR-302a switch also holds great potential in investigating the dynamics of differentiation and/or reprograming of live-cells based on intracellular information.