Summary: Aging is a multifactorial process where deterioration of body functions is driven by stochastic damage while counteracted by distinct genetically encoded repair systems. To better understand the genetic component of aging, many studies have addressed the gene and protein expression profiles of various aging model systems engaging different organisms from yeast to human. The recently identified small non‐coding miRNAs are potent post‐transcriptional regulators that can modify the expression of up to several hundred target genes per single miRNA, similar to transcription factors. Increasing evidence shows that miRNAs contribute to the regulation of most if not all important physiological processes, including aging. However, so far the contribution of miRNAs to age‐related and senescence‐related changes in gene expression remains elusive. To address this question, we have selected four replicative cell aging models including endothelial cells, replicated CD8⁺ T cells, renal proximal tubular epithelial cells, and skin fibroblasts. Further included were three organismal aging models including foreskin, mesenchymal stem cells, and CD8⁺ T cell populations from old and young donors. Using locked nucleic acid‐based miRNA microarrays, we identified four commonly regulated miRNAs, miR‐17 down‐regulated in all seven; miR‐19b and miR‐20a, down‐regulated in six models; and miR‐106a down‐regulated in five models. Decrease in these miRNAs correlated with increased transcript levels of some established target genes, especially the cdk inhibitor p21/CDKN1A. These results establish miRNAs as novel markers of cell aging in humans.

Damage to cells and tissues is one of the driving forces of aging and age-related diseases. Various repair systems are in place to counteract this functional decline. In particular, the property of adult stem cells to self-renew and differentiate is essential for tissue homeostasis and regeneration. However, their functionality declines with age (Rando, 2006). One organ that is notably affected by the reduced differentiation capacity of stem cells with age is the skeleton. Here, we found that circulating microvesicles impact on the osteogenic differentiation capacity of mesenchymal stem cells in a donor-age-dependent way. While searching for factors mediating the inhibitory effect of elderly derived microvesicles on osteogenesis, we identified miR-31 as a crucial component. We demonstrated that miR-31 is present at elevated levels in the plasma of elderly and of osteoporosis patients. As a potential source of its secretion, we identified senescent endothelial cells, which are known to increase during aging in vivo (Erusalimsky, 2009). Endothelial miR-31 is secreted within senescent cell-derived microvesicles and taken up by mesenchymal stem cells where it inhibits osteogenic differentiation by knocking down its target Frizzled-3. Therefore, we suggest that microvesicular miR-31 in the plasma of elderly might play a role in the pathogenesis of age-related impaired bone formation and that miR-31 might be a valuable plasma-based biomarker for aging and for a systemic environment that does not favor cell-based therapies whenever osteogenesis is a limiting factor.

Summary: Cellular senescence of normal human cells has by now far exceeded its initial role as a model system for aging research. Many reports show the accumulation of senescent cells in vivo, their effect on their microenvironment and its double‐edged role as tumour suppressor and promoter. Importantly, removal of senescent cells delays the onset of age‐associated diseases in mouse model systems. To characterize the role of miRNAs in cellular senescence of endothelial cells, we performed miRNA arrays from HUVECs of five different donors. Twelve miRNAs, comprising hsa‐miR‐23a, hsa‐miR‐23b, hsa‐miR‐24, hsa‐miR‐27a, hsa‐miR‐29a, hsa‐miR‐31, hsa‐miR‐100, hsa‐miR‐193a, hsa‐miR‐221, hsa‐miR‐222 and hsa‐let‐7i are consistently up‐regulated in replicatively senescent cells. Surprisingly, also miR‐21 was found up‐regulated by replicative and stress‐induced senescence, despite being described as oncogenic. Transfection of early passage endothelial cells with miR‐21 resulted in lower angiogenesis, and less cell proliferation mirrored by up‐regulation of p21^CIP1 and down‐regulation of CDK2. These two cell‐cycle regulators are indirectly regulated by miR‐21 via its validated direct targets NFIB (Nuclear factor 1 B‐type), a transcriptional inhibitor of p21^CIP1, and CDC25A, which regulates CDK2 activity by dephosphorylation. Knock‐down of either NFIB or CDC25A shows a phenocopy of over‐expressing miR‐21 in regard to cell‐cycle arrest. Finally, miR‐21 over‐epxression reduces the replicative lifespan, while stable knock‐down by sponges extends the replicative lifespan of endothelial cells. Therefore, we propose that miR‐21 is the first miRNA that upon its knock‐down extends the replicative lifespan of normal human cells.

Mutations in PHF8 are associated with X-linked mental retardation and cleft lip/cleft palate. PHF8 contains a plant homeodomain (PHD) in its N terminus and is a member of a family of JmjC domain-containing proteins. While PHDs can act as methyl lysine recognition motifs, JmjC domains can catalyze lysine demethylation. Here, we show that PHF8 is a histone demethylase that removes repressive histone H3 dimethyl lysine 9 marks. Our biochemical analysis revealed specific association of the PHF8 PHD with histone H3 trimethylated at lysine 4 (H3K4me3). Chromatin immunoprecipitation followed by high-throughput sequencing indicated that PHF8 is enriched at the transcription start sites of many active or poised genes, mirroring the presence of RNA polymerase II (RNAPII) and of H3K4me3-bearing nucleosomes. We show that PHF8 can act as a transcriptional coactivator and that its activation function largely depends on binding of the PHD to H3K4me3. Furthermore, we present evidence for direct interaction of PHF8 with the C-terminal domain of RNAPII. Importantly, a PHF8 disease mutant was defective in demethylation and in coactivation. This is the first demonstration of a chromatin-modifying enzyme that is globally recruited to promoters through its association with H3K4me3 and RNAPII.