5-Azacytidine-induced demethylation of DNA to senescent level does not block proliferation of human fibroblasts

The functional impact of nuclear reorganization in cellular senescence

Cellular senescence is the irreversible cell cycle arrest in response to DNA damage. Because senescent cells accumulate with age and contribute to chronic inflammation, they are promising therapeutic targets for healthspan extension. The senescent phenotype can vary depending on cell type and on the specific insults that induce senescence. This variability is also reflected in the extensive remodeling of the genome organization within the nucleus of senescent cells. Here, we give an overview of the nuclear changes that occur in different forms of senescence, including changes to chromatin state and composition and to the three-dimensional organization of the genome, as well as alterations to the nuclear envelope and to the accessibility of repetitive genomic regions. Many of these changes are shared across all forms of senescence, implicating nuclear organization as a fundamental driver of the senescent state and of how senescent cells interact with the surrounding tissue.

Peptidomimetics - An infinite reservoir of metal binding motifs in metabolically stable and biologically active molecules

The involvement of metal ions in interactions with therapeutic peptides is inevitable. They are one of the factors able to fine-tune the biological properties of antimicrobial peptides, a promising group of drugs with one large drawback - a problematic metabolic stability. Appropriately chosen, proteolytically stable peptidomimetics seem to be a reasonable solution of the problem, and the use of D-, β-, γ-amino acids, unnatural amino acids, azapeptides, peptoids, cyclopeptides and dehydropeptides is an infinite reservoir of metal binding motifs in metabolically stable, well-designed, biologically active molecules. Below, their specific structural features, metal-chelating abilities and antimicrobial potential are discussed.

Old cells, new tricks: chromatin structure in senescence

Cellular senescence is a stable form of cell cycle arrest with roles in many pathophysiological processes including development, tissue repair, cancer, and aging. Senescence does not represent a single entity but rather a heterogeneous phenotype that depends on the trigger and cell type of origin. Such heterogeneous features include alterations to chromatin structure and epigenetic states. New technologies are beginning to unravel the distinct mechanisms regulating chromatin structure during senescence. Here, we describe the multiple levels of chromatin organization associated with senescence: global and focal, linear, and higher order.

Suicidal function of DNA methylation in age-related genome disintegration

This article is dedicated to the 60th anniversary of 5-methylcytosine discovery in DNA. Cytosine methylation can affect genetic and epigenetic processes, works as a part of the genome-defense system and has mutagenic activity; however, the biological functions of this enzymatic modification are not well understood. This review will put forward the hypothesis that the host-defense role of DNA methylation in silencing and mutational destroying of retroviruses and other intragenomic parasites was extended during evolution to most host genes that have to be inactivated in differentiated somatic cells, where it acquired a new function in age-related self-destruction of the genome. The proposed model considers DNA methylation as the generator of 5mC>T transitions that induce 40-70% of all spontaneous somatic mutations of the multiple classes at CpG and CpNpG sites and flanking nucleotides in the p53, FIX, hprt, gpt human genes and some transgenes. The accumulation of 5mC-dependent mutations explains: global changes in the structure of the vertebrate genome throughout evolution; the loss of most 5mC from the DNA of various species over their lifespan and the Hayflick limit of normal cells; the polymorphism of methylation sites, including asymmetric mCpNpN sites; cyclical changes of methylation and demethylation in genes. The suicidal function of methylation may be a special genetic mechanism for increasing DNA damage and the programmed genome disintegration responsible for cell apoptosis and organism aging and death.

The heterochromatin loss model of aging

There are significant changes in gene expression that occur with cellular senescence and organismic aging. Genes residing in compacted heterochromatin domains are typically silenced due to an altered accessibility to transcription factors. Heterochromatin domains and gene silencing are set up in early development and were initially believed to be maintained for the remainder of the lifespan. Recent data suggest that there may be a net loss of heterochromatin with advancing age in both yeast and mice. The gradual loss of heterochromatin-induced gene silencing could explain the changes in gene expression that are closely linked with aging. A general model is proposed for heterochromatin loss as a major factor in generating alterations in gene expression with age. The heterochromatin loss model is supported by several lines of evidence and suggests that a fundamental genetic mechanism underlies most of the changes in gene expression observed with senescence.

5-Azacytidine and BDNF enhance the maturation of neurons derived from EGF-generated neural stem cells

EGF-generated neural stem cells can form astrocytes, neurons, and oligodendrocytes upon differentiation; however, the proportion of cells that actually form neurons is very small. In the present study, we have studied the effect that 5-azacytidine (5AzaC), a demethylation agent, and brain-derived growth factor (BDNF) have on the differentiation and maturation of neurons originating from EGF-generated neural stem cells. Stem cells were maintained under a variety of culture conditions using combinations of 5AzaC and BDNF either alone or together. More neurons, as determined by the number of beta-tubulin III-immunoreactive somata, were present in cultures maintained in BDNF medium (a nearly fourfold increase compared to control cultures). 5AzaC did not significantly affect neuronal number, regardless of the presence of BDNF. In addition to neuronal number, the effect of 5AzaC and BDNF on the distribution of the microtubule proteins MAP2 and Tau was analyzed. In most cultures, MAP2 and Tau were colocalized throughout the neuron. In contrast, neurons cotreated with 5AzaC and BDNF contained neurons that began to exhibit cytoskeletal segregation of MAP2 into the somatodendritic compartments. Tau remained dispersed within the somata and the axon. This effect was not produced when 5AzaC or BDNF was used individually. These results demonstrate that 5AzaC and BDNF cooperate to produce more mature neurons from EGF-generated neural stem cells then either molecule can alone.

Replicative senescence: considerations relating to the stability of heterochromatin domains

Replicative senescence of human diploid fibroblasts (HDF) cultured in vitro is characterized by a progressive and irreversible loss of responsiveness to mitogenic stimulation by serum. While some constraints have been placed on the nature of HDF senescence, its underlying molecular mechanism(s) remain obscure. Here, the possibility is considered that defects in cell cycle-coupled reassembly of repressive chromatin domains may contribute to HDF senescence. Features of this model are discussed in relation to established models of HDF senescence based on telomere shortening and loss of DNA methylation.

Life span prediction from the rate of age-related DNA demethylation in normal and cancer cell lines

A method has been proposed for the Hayflick Limit prediction by the analysis of the 5-methylcytosine content in DNA at earlier and later cell passages. The following facts were used as the basis of the method: (i) the rate of m5C loss from DNA remains approximately constant during cell divisions and it does not depend on the cell donor age; (ii) this rate is inversely proportional to the Hayflick Limit as well as to the life span of cell donor species; (iii) the period corresponded to loss of all m5C residues from the genome coincides with or somewhat exceeds the Hayflick Limit of normal cells. The prognosis of the Hayflick Limit has usually been found in good agreement with the experimental evidences for various human, hamster, and mouse cell lines. The method proposed may be used for early detection of precrisis and cancer cells. The age-related m5C loss may result from accumulation of the m5C-->T+C transitions occurring with DNA methylation in every cell division.

Changes of the methylation pattern of the c-myc gene during in vitro aging of IMR90 human embryonic fibroblasts

DNA modification by cytosine methylation has received considerable interest in the context of mammalian cell differentiation but is discussed controversially with respect to cellular aging. As the expression of c-myc affects strongly cellular aging and terminal differentiation, we have analysed the sequence-specific methylation pattern of the c-myc gene during proliferative aging in vitro of human embryonic fibroblasts. In this study, both, 5-methylcytidine sensitive restriction enzymes as well as genomic sequencing were used. The overall methylation pattern was found essentially stable during proliferative aging. However, specific hypermethylation of exon II during aging was observed. Furthermore, one specific cytidine located in the consensus sequence of the DNA binding factor PEBP2 was found completely methylated during most of the course of proliferative aging of the cells but became demethylated as the cells reached the end of their proliferative life span. Our results indicate the importance of establishing the sequence-specific changes of the methylation pattern of the genome during in vitro aging.