Telomeres--what's new at the end?

Mitofusin 1 and optic atrophy 1 shift metabolism to mitochondrial respiration during aging

Replicative and chronological lifespan are two different modes of cellular aging. Chronological lifespan is defined as the duration during which quiescent normal cells retain their capacity to re‐enter the proliferative cycle. This study investigated whether changes in metabolism occur during aging of quiescent normal human fibroblasts (NHFs) and the mechanisms that regulate these changes. Bioenergetics measurements were taken in quiescent NHFs from younger (newborn, 3‐day, 5‐month, and 1‐year) and older (58‐, 61‐, 63‐, 68‐, and 70‐year) healthy donors as well as NHFs from the same individual at different ages (29, 36, and 46 years). Results show significant changes in cellular metabolism during aging of quiescent NHFs: Old NHFs exhibit a significant decrease in glycolytic flux and lactate levels, and increase in oxygen consumption rate (OCR) and ATP levels compared to young NHFs. Results from the Seahorse XF Cell Mito Stress Test show that old NHFs with a lower Bioenergetic Health Index (BHI) are more prone to oxidative stress compared to young NHFs with a higher BHI. The increase in OCR in old NHFs is associated with a shift in mitochondrial dynamics more toward fusion. Genetic knockdown of mitofusin 1 (MFN1) and optic atrophy 1 (OPA1) in old NHFs decreased OCR and shifted metabolism more toward glycolysis. Downregulation of MFN1 and OPA1 also suppressed the radiation‐induced increase in doubling time of NHFs. In summary, results show that a metabolic shift from glycolysis in young to mitochondrial respiration in old NHFs occurs during chronological lifespan, and MFN1 and OPA1 regulate this process.

MnSOD activity regulates hydroxytyrosol-induced extension of chronological lifespan

Chronological lifespan (CLS) is defined as the duration of quiescence in which normal cells retain the capacity to reenter the proliferative cycle. This study investigates whether hydroxytyrosol (HT), a naturally occurring polyphenol found in olives, extends CLS in normal human fibroblasts (NHFs). Quiescent NHFs cultured for a long duration (30-60 days) lose their capacity to repopulate. Approximately 60% of these cells exit the cell cycle permanently; a significant increase in the doubling time of the cell population was observed. CLS was extended in quiescent NHFs that were cultured in the presence of HT for 30-60 days. HT-induced extension of CLS was associated with an approximately 3-fold increase in manganese superoxide dismutase (MnSOD) activity while there was no change in copper-zinc superoxide dismutase, catalase, or glutathione peroxidase protein levels. Quiescent NHFs overexpressing a dominant-negative mutant form of MnSOD failed to extend CLS. HT suppressed age-associated increase in mitochondrial ROS levels. Results from spectroscopy assays indicate that HT in the presence of peroxidases can undergo catechol-semiquinone-quinone redox cycling generating superoxide, which in a cellular context can activate the antioxidant system, e.g., MnSOD expression. These results demonstrate that HT extends CLS by increasing MnSOD activity and decreasing age-associated mitochondrial reactive oxygen species accumulation.

Bladder tumor markers: from hematuria to molecular diagnostics--where do we stand?

Bladder cancer is a common malignancy in the USA. Currently, the detection of initial tumors and recurrent disease is based on evaluation of voided urinary specimens, often followed by cystoscopy. With the high rate of recurrence, cystoscopies are regularly repeated with the aim of halting progression of the disease. For patients, this process is fraught with anxiety, pain and high cost. As a result, intense work is being done in the field of bladder tumor markers with the goal of identifying bladder cancer earlier, both in the initial diagnosis and in recurrences of known tumor. The possibility of identifying a marker that could noninvasively differentiate benign and malignant causes of hematuria, and identify recurrences prior to their pathologic progression is the objective of this area of research. Currently, a large number of tumor markers exist, each scrutinized in both the laboratory and in clinical trials. Here we present many of the most widely used and tested markers. Background details are provided as to the mechanism of detection of malignant cells, the results of recent trials and future directions of study. Some novel modalities for tumor detection are also presented. The next few years will no doubt bring newer markers and lead to the elimination of others. Studies continue to refine the role of these markers in clinical practice, but their ultimate efficacy will need to be borne out in large-scale clinical trials in a multitude of settings.

Telomeres, telomerase, and aging: origin of the theory

In 1971 I published a theory in which I first formulated the DNA end replication problem and explained how it could be solved. The solution to this problem also provided an explanation for the Hayflick Limit, which underpins the discovery of in vitro and in vivo cell senescence. I proposed that the length of telomeric DNA, located at the ends of chromosomes consists of repeated sequences, which play a buffer role and should diminish in dividing normal somatic cells at each cell doubling. I also proposed that the loss of sequences containing important information that could occur after buffer loss could cause the onset of cellular senescence. I also suggested that for germline cells and for the cells of vegetatively propagated organisms and immortal cell populations like most cancer cell lines, an enzyme might be activated that would prevent the diminution of DNA termini at each cell division, thus protecting the information containing part of the genome. In the last few years, most of my suggestions have been authenticated by laboratory evidence. the DNA sequences that shorten in dividing normal cells are telomeres and the enzyme that maintains telomere length constant in immortal cell populations is telomerase.

Identification and characterization of a putative telomere end-binding protein from Tetrahymena thermophila

Telomeric DNA of Tetrahymena thermophila consists of a long stretch of (TTGGGG)n double-stranded repeats with a single-stranded (TTGGGG)2 3' overhang at the end of the chromosome. We have identified and characterized a protein that specifically binds to a synthetic telomeric substrate consisting of duplex DNA and the 3' telomeric repeat overhang. This protein is called TEP (telomere end-binding protein). A change from G to A in the third position of the TTGGGG overhang repeat converts the substrate to a human telomere analog and reduces the binding affinity approximately threefold. Changing two G's to C's in the TTGGGG repeats totally abolishes binding. However, permutation of the Tetrahymena repeat sequence has only a minor effect on binding. A duplex structure adjacent to the 3' overhang is required for binding, although the duplex need not contain telomeric repeats. TEP does not bind to G-quartet DNA, which is formed by many G-rich sequences. TEP has a greatly reduced affinity for RNA substrates. The copy number of TEP is at least 2 x 10(4) per cell, and it is present under different conditions of cell growth and development, although its level varies. UV cross-linking experiments show that TEP has an apparent molecular mass of approximately 65 kDa. Unlike other telomere end-binding proteins, TEP is sensitive to high salt concentrations.

TLC1: template RNA component of Saccharomyces cerevisiae telomerase

Telomeres, the natural ends of linear eukaryotic chromosomes, are essential for chromosome stability. Because of the nature of DNA replication, telomeres require a specialized mechanism to ensure their complete duplication. Telomeres are also capable of silencing the transcription of genes that are located near them. In order to identify genes in the budding yeast Saccharomyces cerevisiae that are important for telomere function, a screen was conducted for genes that, when expressed in high amounts, would suppress telomeric silencing. This screen lead to the identification of the gene TLC1 (telomerase component 1). TLC1 encodes the template RNA of telomerase, a ribonucleoprotein required for telomere replication in a variety of organisms. The discovery of TLC1 confirms the existence of telomerase in S. cerevisiae and may facilitate both the analysis of this enzyme and an understanding of telomere structure and function.

Mammalian telomere dynamics: healing, fragmentation shortening and stabilization

Telomeres are essential for stable chromosome maintenance. The simple G-rich sequence motif d(TTAGGG)n is all that is required in cis for telomere function in mammalian cells, as in other eukaryotes. Using this fact, telomeres have been used to specifically fragment mammalian chromosomes to dissect their structure and function. Telomere length maintenance is altered in cancer cells. Trans-acting factors, such as telomerase and telomere-binding proteins, may determine telomere function in both normal and cancer cells. Current experiments are aimed at understanding the role of telomerase and telomere-binding proteins in cellular senescence and immortalization.

Characterization of transcription initiation, translation initiation, and poly(A) addition sites in the gene-sized macronuclear DNA molecules of Euplotes

The DNA in the transcriptionally active macronucleus of the hypotrichous ciliate Euplotes crassus exists as short, linear molecules with each molecule encoding a single genetic function. Previous work has indicated that coding regions occupy the majority of macronuclear DNA molecules. In the present study we have defined the transcription initiation sites and poly(A) addition sites for a number of different macronuclear genes in Euplotes crassus. Our results indicate that mature transcripts represent all but approximately 100-200 bases of the non-telomeric sequences in macronuclear DNA molecules. We have also examined the sequences in the vicinity of transcription start sites, poly(A) addition sites, and translation initiation sites for Euplotes species genes in an attempt to define the cis-acting elements that control these processes. Our results indicate that some of the common sequence elements known to control these processes in higher eukaryotes are likely not utilized by Euplotes genes. The data do indicate the presence of other conserved sequences both preceding and at the site of poly(A) addition, as well as at the site of translation initiation. These conserved sequences may serve an analogous role in these organisms. Finally, we have found that most macronuclear DNA molecules have transcription initiation sites within 30 bp of the telomere, suggesting that the telomere may play a role in promoting transcription.

Centromeric and telomeric repeats are stable in nonagenarians as revealed by the double hybridization fluorescent in situ technique

We report a simple method for simultaneous identification of centromeric and telomeric repeat sequences of human chromosomes. Employing this technique, we investigated the stability of centromeres and telomeres in individuals over 90 years of age and compared them with younger controls (< 40 years). Our findings suggest that centromeric and telomeric repeats remain apparently stable in nonagenarians. These findings are enigmatic because it has been suggested that centromeres are lost in older individuals. Furthermore, telomeric shortening has been observed in aged lymphocytes and cellular senescence. However, stability of telomeric repeats noted in nonagenarians may be masked by loss followed by compensation by a process called telomeric elongation.

Functional analysis of the promoters of the human CaMIII calmodulin gene and of the intronless gene coding for a calmodulin-like protein

More than 1 kb of the 5'-flanking DNA of the human CaMIII calmodulin gene and of the calmodulin-like protein (CLP) gene have been sequenced. Notable features are the absence of a TATA-box and the presence of AGGGA elements in both upstream regions, the presence of several sequences with homology to known regulatory elements (cAMP-, retinoic acid- and interferon-responsive elements) in the CLP gene, and a high G + C-content and several putative Sp1-factor-binding sites in the CaMIII gene. 1 kb of the CaMIII upstream region was driving high-level growth hormone (hGH) reporter gene expression in human teratoma and monkey COS cells. Promoter activity dropped to about 30% and 10% when only 252 bp and 114 bp, respectively, of the CaMIII sequence were present. In contrast to the CaMIII promoter, the CLP gene upstream region was driving hGH expression only in human teratoma, but not in monkey COS cells. Addition of retinoic acid to the transfected cells had minimal effects on both promoters, leading to a 10-30% decrease of activity. The results show that the human CaMIII gene contains a strong and ubiquitously active promoter, whereas the promoter of the intronless CLP gene appears to be regulated in a cell-specific manner.

Localization of the intronless gene coding for calmodulin-like protein CLP to human chromosome 10p13-ter

The functional intronless gene coding for a calmodulin-like protein (CLP) has been localized to human chromosome 10p13-ter. Chromosomal assignment was performed by Southern blot analysis of DNA from human-rodent somatic cell hybrids and amplification of a CLP gene-specific 1090-bp DNA fragment by the polymerase chain reaction (PCR) on DNA from human-hamster cell hybrids. Chromosomal sublocalization was carried out by in situ hybridization of human chromosome metaphase spreads. The CLP gene is the first member of the human calmodulin/calmodulin-like gene family to be chromosomally sublocalized. Its presence near the telomeric end of the short arm of chromosome 10 may be of significance with respect to its highly (epithelial) cell-type restricted expression in vivo and strong downregulation upon malignant transformation. The generation of a human CLP gene-specific sequence tag site specified by the two primers used for PCR should prove useful for future linkage studies.