Aged-vascular niche hinders osteogenesis of mesenchymal stem cells through paracrine repression of Wnt-axis.

Age-induced decline in osteogenic potential of bone marrow mesenchymal stem cells (BMSCs) potentiates osteoporosis and increases the risk for bone fractures. Despite epidemiology studies reporting concurrent development of vascular and bone diseases in the elderly, the underlying mechanisms for the vascular-bone cross-talk in aging are largely unknown. In this study, we show that accelerated endothelial aging deteriorates bone tissue through paracrine repression of Wnt-driven-axis in BMSCs. Here, we utilize physiologically aged mice in conjunction with our transgenic endothelial progeria mouse model (Hutchinson-Gilford progeria syndrome; HGPS) that displays hallmarks of an aged bone marrow vascular niche. We find bone defects associated with diminished BMSC osteogenic differentiation that implicate the existence of angiocrine factors with long-term inhibitory effects. microRNA-transcriptomics of HGPS patient plasma combined with aged-vascular niche analyses in progeria mice reveal abundant secretion of Wnt-repressive microRNA-31-5p. Moreover, we show that inhibition of microRNA-31-5p as well as selective Wnt-activator CHIR99021 boosts the osteogenic potential of BMSCs through de-repression and activation of the Wnt-signaling, respectively. Our results demonstrate that the vascular niche significantly contributes to osteogenesis defects in aging and pave the ground for microRNA-based therapies of bone loss in elderly.

Elongating RNA polymerase II and RNA:DNA hybrids hinder fork progression and gene expression at sites of head-on replication-transcription collisions.

Uncoordinated clashes between replication forks and transcription cause replication stress and genome instability, which are hallmarks of cancer and neurodegeneration. Here, we investigate the outcomes of head-on replication-transcription collisions, using as a model system budding yeast mutants for the helicase Sen1, the ortholog of human Senataxin. We found that RNA Polymerase II accumulates together with RNA:DNA hybrids at sites of head-on collisions. The replication fork and RNA Polymerase II are both arrested during the clash, leading to DNA damage and, in the long run, the inhibition of gene expression. The inactivation of RNA Polymerase II elongation factors, such as the HMG-like protein Spt2 and the DISF and PAF complexes, but not alterations in chromatin structure, allows replication fork progression through transcribed regions. Attenuation of RNA Polymerase II elongation rescues RNA:DNA hybrid accumulation and DNA damage sensitivity caused by the absence of Sen1, but not of RNase H proteins, suggesting that such enzymes counteract toxic RNA:DNA hybrids at different stages of the cell cycle with Sen1 mainly acting in replication. We suggest that the main obstacle to replication fork progression is the elongating RNA Polymerase II engaged in an R-loop, rather than RNA:DNA hybrids per se or hybrid-associated chromatin modifications.

The dark side of RNA:DNA hybrids.

RNA:DNA hybrids form when nascent transcripts anneal to the DNA template strand or any homologous DNA region. Co-transcriptional RNA:DNA hybrids, organized in R-loop structures together with the displaced non-transcribed strand, assist gene expression, DNA repair and other physiological cellular functions. A dark side of the matter is that RNA:DNA hybrids are also a cause of DNA damage and human diseases. In this review, we summarize recent advances in the understanding of the mechanisms by which the impairment of hybrid turnover promotes DNA damage and genome instability via the interference with DNA replication and DNA double-strand break repair. We also discuss how hybrids could contribute to cancer, neurodegeneration and susceptibility to viral infections, focusing on dysfunctions associated with the anti-R-loop helicase Senataxin.