Z Gerontol Geriat 2013 · 46:629–634 DOI 10.1007/s00391-013-0531-7 Received: 4 June 2013 Revised: 19 June 2013 Accepted: 23 June 2013 Published online: 14 October 2013 © Springer-Verlag Berlin Heidelberg 2013 M. Hodzic 1 · Y. Naaldijk 2, 3 · A. Stolzing 2, 3 1 University of Berlin 2 Fraunhofer Institute for Cell Therapy and Immunology, Leipzig 3 Translational Research Centre for Regenerative Medicine (TRM), University of Leipzig Regulating aging in adult stem cells with microRNA Introduction MicroRNA biosynthesis MicroRNAs (miRNAs) originate from noncoding regions of the genome and from introns of coding regions [1]. MiR- NAs are transcribed by the RNA polymer- ases 2 and 3 [2]. Transcription of miRNA from noncoding regions yields longer primary transcripts called primary miR- NA (pri-miRNA). Pri-miRNA is cleaved in the nucleus by the Drosha/DGCR8 complex into a 70-nt long hairpin struc- ture, the precursor miRNA (pre-miRNA) (. Fig. 1). The pre-miRNA is exported from nucleus by the Ran-GTP and ex- portin 5 system through nuclear pores. In the cytosol, a complex of the RNase3-Dic- er and the transactivation response RNA- binding protein cleaves the hairpin struc- ture, resulting in a core RNA duplex of ap- proximately 22 nt. Only one strand of this duplex, the mature miRNA, is used fur- ther for loading onto the silencing com- plex consisting of Dicer/TRBP and the Argonaute 2 protein. This RNA-induced silencing complex (RISC) binds to mes- senger RNA transcripts and regulates gene expression by translational inhibition [3], deadenylation [4] and degradation [5]. This review concentrates on the in- volvement of miRNAs in the aging pro- cess of adult stem cells. More detailed de- scriptions of transcriptional regulation or the RNAi machinery are not given here. MSC aging and senescence Mesenchymal stem cells (MSC) are adult, multipotent stem cells, which can dif- ferentiate to osteoblasts, chondrocytes and adipocytes. Isolation of heterogen- ic MSC populations has been performed from various tissues such as bone marrow (BM), adipose tissue, muscle and umbili- cal cord blood (UCB), among others [6, 7]. The regenerative capacity of MSCs of- fers huge potential to the field of regen- erative medicine. New knowledge about aging and senescence processes after pro- longed cultivation will enable us to under- stand problems behind in vitro expansion of MSCs for clinical applications [8]. BM-MSC cultivation results in in- creased miRNA (miR-371, miR-369-5p, miR-29c, miR-499 and let-7f) due to pas- sage effects and not due to senescence [9]. Expression of miR-369-5p, miR-371 and miR-499 miRNAs reduces MSC prolifera- tive potential. MiR-369-5p further impairs adipogenic differentiation and down-reg- ulates de novo DNA methyltransferase DNMT3a, while miR-371 shows the op- posite effects [10]. The role of miR-499 in age-related up-regulation in MSCs is un- known. On the other hand, human UCB- MSCs (hUCB-MSC) cultured to reach replicative senescence showed a senes- cence-dependent decrease in histone deacetylases (HDACs), down-regulation of high mobility group AT-hook 2 protein (HMGA2), as well as increased expres- sion of p16 (. Fig. 2), p21 and p27 [11]. HDACs are involved in the transcription- al silencing of several miRNAs. Inhibition of HDACs induces changes in chromatin structure and increases the transcription of pre-let-7a1, pre-let-7d, pre-let-7f1, pre- miR-23a, pre-miR-26a and pre-miR-30a in hUCB-MSCs [11]. Interestingly, oxida- tive stress caused by H 2 O 2 induced senes- cence and decreased HMGA2 level, but lowered let-7a and miR-23a expression, suggesting an additional mechanism be- hind HMGA2 regulation. While p16 was increased in senescence caused by pro- longed passaging of hUCB-MSCs and HDAC inhibition, there was no change in p16 levels when cells were exposed to H 2 0 2 . However, p21 expression was in- duced by H 2 0 2 [11]. MiR-23a is involved in apoptosis and is a negative regulator of PGC-1α [12]. The peroxisome proliferator-activated recep- tor (PPAR) gamma co-activator 1 (PGC- 1α) is involved in mitochondrial biogen- esis and is induced by caloric restriction in mice [13, 14, 15]. Increased miR-23a ex- pression in senescent hUCB-MSCs could indicate impairment of mitochondrial biogenesis and function (. Fig. 2). A re- cent study demonstrated that the mito- chondrial deacetylase SIRT3 is a PGC-1α downstream target. SIRT3 mediates ROS protection and mitochondrial biogenesis [16]. These results further illuminate the positive association between SIRT3 pro- tein and human longevity [17]. A publication by Hackl et al. high- lights the importance of the involvement of miRNA in aging and senescence. The authors searched for differences and com- monalities between replicative and organ- ismal aging. In their model of MSC aging, five miRNAs were down-regulated, four of which belong to the polycistronic miR- 17~92 cluster that contains miR-19a, miR- 17, miR-19b, miR-20a [18, 19]. MiR-19 is one of the most important members of this cluster regarding oncogenic potential. By inhibiting the expression of its target, Beiträge zum Themenschwerpunkt 629 Zeitschrift für Gerontologie und Geriatrie 7 · 2013 |