Cellular quiescence is a dormant but reversible cellular state in which cell-cycle entry and proliferation are prevented. Recent studies both in vivo and in vitro demonstrate that quiescence is actively maintained through synergistic interactions between intrinsic and extrinsic signals. Subtypes of adult mammalian stem cells can be maintained in this poised, quiescent state, and subsequently reactivated upon tissue injury to restore homeostasis. However, quiescence can become deregulated in pathological settings. In this review, we discuss the recent advances uncovering intracellular signaling pathways, transcriptional changes, and extracellular cues within the stem cell niche that control induction and exit from quiescence in tissue stem cells. We discuss the implications of quiescence as well as the pharmacological and genetic approaches that are being explored to either induce or prevent quiescence as a therapeutic strategy.

The primary cilium is able to maintain a specific protein composition, which is critical for its function as a signaling organelle. Here we introduce a system to synchronize biosynthetic trafficking of ciliary proteins that is based on conditional aggregation domains (CADs). This approach enables to create a wave of ciliary proteins that are transported together, which opens novel avenues for visualizing and studying ciliary import mechanisms. By using somatostatin receptor 3 (SSTR3) as model protein we studied intracellular transport and ciliary import with high temporal and spatial resolution in epithelial Madin-Darby canine kidney (MDCK) cells. This yielded the interesting discovery that SSTR3, besides being transported to the primary cilium, is also targeted to the basolateral plasma membrane. In addition, we found a similar behavior for another ciliary protein, nephrocystin-3 (NPHP3), thus suggesting a potential correlation between ciliary and basolateral trafficking. Furthermore, our CAD-based system allowed assembling a large dataset in which apical and basolateral surface SSTR3 signals could be compared to ciliary SSTR3 signals on a single cell level. This enabled to generate novel complementary evidence for the previously proposed lateral import mechanism of SSTR3 into the cilium along the plasma membrane.

Designer effectors based on the DNA binding domain (DBD) of Xanthomonas transcription activator-like effectors (TALEs) are powerful sequence-specific tools with an excellent reputation for their specificity in editing the genome, transcriptome, and more recently the epigenome in multiple cellular systems. However, the repetitive structure of the TALE arrays composing the DBD impedes their generation as gene synthesis product and prevents the delivery of TALE-based genes using lentiviral vectors (LVs), a widely used system for human gene therapy. To overcome these limitations, we aimed at chimerizing the DNA sequence encoding for the TALE-DBDs by introducing sufficient diversity to facilitate both their gene synthesis and enable their lentiviral delivery. To this end, we replaced three out of 17 Xanthomonas TALE repeats with TALE-like units from the bacterium Burkholderia rhizoxinica. This was combined with extensive codon variation and specific amino acid substitutions throughout the DBD in order to maximize intra- and inter-repeat sequence variability. We demonstrate that chimerized TALEs can be easily generated using conventional Golden Gate cloning strategy or gene synthesis. Moreover, chimerization enabled the delivery of TALE-based designer nucleases, transcriptome and epigenome editors using lentiviral vectors. When delivered as plasmid DNA, chimerized TALEs targeting the CCR5 and CXCR4 loci showed comparable activities in human cells. However, lentiviral delivery of TALE-based transcriptional activators was only successful in the chimerized form. Similarly, delivery of a chimerized CXCR4-specific epigenome editor resulted in rapid silencing of endogenous CXCR4 expression. In conclusion, extensive codon variation and chimerization of TALE-based DBDs enables both the simplified generation and the lentiviral delivery of designer TALEs, and therefore facilitates the clinical application of these tools to precisely edit the genome, transcriptome and epigenome.