This Article outlines the optimized chemical synthesis and preliminary biochemical characterization of a new oligonucleotide analogue called thio­phosphor­amidate morpholinos (TMOs). Their rational design hinges upon integrating two well-studied pharmacophores, namely, phosphoro­thioates (pS) and morpholinos, to create morpholino–pS hybrid oligonucleotides. Our simple synthesis strategy enables the easy incorporation of morpholino–pS moieties and therapeutically relevant sugar modifications in tandem to create novel oligonucleotide (ON) analogues that are hitherto unexplored in the oligotherapeutics arena. Exclusively TMO-modified ONs demonstrate high stability toward 3′-exonuclease. Hybridization studies show that TMO chimeras consisting of alternating TMO and DNA–pS subunits exhibit higher binding affinity toward complementary RNA relative to the canonical DNA/RNA duplex (∼10 °C). Oligonucleotides that consist entirely of TMO linkages also show higher RNA binding affinity but do not recruit ribonuclease H1 (RNase H1). Chimeric TMO analogues demonstrate high gene silencing efficacy, comparable to that of a chimeric 2′-OMe–pS/pO control, during in vitro bioassay screens designed to evaluate their potential as microRNA inhibitors of hsa-miR-15b-5p in HeLa cells.

The spatial partitioning of the transcriptome in the cell is an important form of gene-expression regulation. Here, we address how intron retention influences the spatio-temporal dynamics of transcripts from two clinically relevant genes: TERT (Telomerase Reverse Transcriptase) pre-mRNA and TUG1 (Taurine-Upregulated Gene 1) lncRNA. Single molecule RNA FISH reveals that nuclear TERT transcripts uniformly and robustly retain specific introns. Our data suggest that the splicing of TERT retained introns occurs during mitosis. In contrast, TUG1 has a bimodal distribution of fully spliced cytoplasmic and intron-retained nuclear transcripts. We further test the functionality of intron-retention events using RNA-targeting thiomorpholino antisense oligonucleotides to block intron excision. We show that intron retention is the driving force for the nuclear compartmentalization of these RNAs. For both RNAs, altering this splicing-driven subcellular distribution has significant effects on cell viability. Together, these findings show that stable retention of specific introns can orchestrate spatial compartmentalization of these RNAs within the cell. This process reveals that modulating RNA localization via targeted intron retention can be utilized for RNA-based therapies.

Numerous global connections have been made between splicing and other layers of gene regulation, including the spatial partitioning of the transcriptome in the cell. Yet, there has been surprisingly little analysis of the spatio-temporal regulation of individual protein-coding and non-coding RNA molecules in single cells. Here we address how intron retention influences the spatio-temporal dynamics of transcripts from two clinically relevant genes: TERT (Telomerase Reverse Transcriptase) pre-mRNA and TUG1 (Taurine-Upregulated Gene 1) lncRNA. Single molecule RNA FISH revealed that nuclear TERT transcripts uniformly and robustly retain two specific introns whose splicing occurs during mitosis. In contrast, TUG1 has a bimodal distribution of fully spliced cytoplasmic and intron-retained nuclear transcripts. We further test the functionality of intron-retention events using RNA-targeting thiomorpholino antisense oligonucleotides to block intron excision. We show that intron retention is the driving force for the nuclear compartmentalization of these RNAs. For both RNAs, altering this splicing-driven subcellular distribution had significant effects on cell growth. Together, these findings show that stable retention of specific introns can orchestrate spatial compartmentalization of RNAs within the cell; this process reveals new targets for RNA-based therapies.

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