Molecular basis of selective antagonism of the P2X1 receptor for ATP by NF449 and suramin: contribution of basic amino acids in the cysteine-rich loop.

BACKGROUND AND PURPOSE: The cysteine-rich head region, which is adjacent to the proposed ATP-binding pocket in the extracellular ligand-binding loop of P2X receptors for ATP, is absent in the antagonist-insensitive Dictyostelium receptors. In this study we have determined the contribution of the head region to the antagonist action of NF449 and suramin at the human P2X1 receptor. EXPERIMENTAL APPROACH: Chimeras and point mutations in the cysteine-rich head region were made between human P2X1 and P2X2 receptors. Mutant receptors were expressed in Xenopus oocytes and P2X receptor currents characterized using two-electrode voltage clamp. KEY RESULTS: The chimera replacing the region between the third and fourth conserved cysteine residues of the P2X1 receptor with the corresponding part of P2X2 reduced NF449 sensitivity a thousand fold from an IC(50) of ∼1 nM at the P2X1 receptor to that of the P2X2 receptor (IC(50) ∼1 µM). A similar decrease in sensitivity resulted from mutation of four positively charged P2X1 receptor residues in this region that are absent from the P2X2 receptor. These chimeras and mutations were also involved in determining sensitivity to the antagonist suramin. Reciprocal chimeras and mutations in the P2X2 receptor produced modest increases in antagonist sensitivity. CONCLUSIONS AND IMPLICATIONS: These data indicate that a cluster of positively charged residues at the base of the cysteine-rich head region can account for the highly selective antagonism of the P2X1 receptor by the suramin derivative NF449.

Death-associated protein kinase (DAPK1) in cerebral cortex of late-onset Alzheimer's disease patients and aged controls.

AIMS: Here our objective was to detect the pro-apoptotic serine/threonine kinase death-associated protein kinase (DAPK1) in aged human cerebral cortex and to test the hypothesis that DAPK1 abundance is associated with late-onset Alzheimer's disease (AD). METHODS: Using Western analysis and immunohistochemistry we evaluated post mortem frontal cerebral cortex from patients with severe AD (mean age 76 years, range 66-91, n = 11, all male), and from control cases without serious central nervous system illness (mean age 77 years, range 61-95, n = 12, all male). We also examined brains of Tg2576 transgenic mice (males, aged 16-21 months), a model for chronic amyloid-induced brain injury. RESULTS: Immunohistochemical labelling showed DAPK1 expression in cortical neurones of human cortex and axonal tracts within subcortical white matter, both in AD and in control brains. Western analysis confirmed DAPK1 expression in all samples, although expression was very low in some control cases. DAPK1 abundance in the AD group was not significantly different from that in controls (P = 0.07, Mann-Whitney test). In brains of Tg2576 mice DAPK1 abundance was very similar to that in wild-type littermates (P = 0.96, Mann-Whitney test). CONCLUSION: We found that DAPK1 was expressed in neurones of aged human frontal cortex, both in AD and in control cases.

Telomere shortening accompanies increased cell cycle activity during serial transplantation of hematopoietic stem cells.

Reactivation of telomerase and maintenance of telomere length can lead to the prevention of replicative senescence in some human somatic cells grown in vitro. To investigate whether telomere shortening might also play a role in the limitation of hematopoietic stem cell (HSC) division capacity in vivo, we analyzed telomere length during serial transplantation of murine HSCs. Southern blot analysis of telomere length in donor bone marrow cells revealed extensive shortening ( approximately 7 kb) after just two rounds of HSC transplantation. The number of cycling HSCs increased after transplantation and remained elevated for at least 4 mo, while the frequency of HSCs in the bone marrow was completely regenerated by 2 mo after transplantation. Direct analysis of telomeres in HSCs by fluorescent in situ hybridization during serial transplantation also revealed a reduction in telomere size. Together, these data show that telomeres shorten during division of HSCs in vivo, and are consistent with the hypothesis that telomere shortening may limit the replicative capacity of HSCs.

Expression of mouse telomerase reverse transcriptase during development, differentiation and proliferation.

We have identified the mouse telomerase reverse transcriptase component (mTERT) and demonstrate both substantial sequence homology to the human ortholog (hTERT), and the presence of reverse transcriptase and telomerase specific motifs. Furthermore, we show functional interchangeability with hTERT in in vitro telomerase reconstitution experiments, as mTERT produces strong telomerase activity in combination with the human telomerase RNA component hTR. The mouse TERT is widely expressed at low levels in adult tissues, with greatest abundance during embryogenesis and in adult thymus and intestine. The mTERT component mRNA levels were regulated during both differentiation and proliferation, while mTR levels remained constant throughout both processes. Comparison of mTERT and mTR levels to telomerase activity indicates that mTERT expression is more tightly linked to the regulation of telomerase activity during these processes than is mTR. In contrast to the situation in human cell cultures, mTERT transcript levels are present at readily detectable levels in primary cultured cells and are not upregulated following crisis. The widespread expression of mTERT in primary cells and mouse tissues could explain the increased frequency of spontaneous immortalization of mouse cells in culture and tumorigenesis in vivo.

ATM-dependent telomere loss in aging human diploid fibroblasts and DNA damage lead to the post-translational activation of p53 protein involving poly(ADP-ribose) polymerase.

Telomere loss has been proposed as a mechanism for counting cell divisions during aging in normal somatic cells. How such a mitotic clock initiates the intracellular signalling events that culminate in G1 cell cycle arrest and senescence to restrict the lifespan of normal human cells is not known. We investigated the possibility that critically short telomere length activates a DNA damage response pathway involving p53 and p21(WAF1) in aging cells. We show that the DNA binding and transcriptional activity of p53 protein increases with cell age in the absence of any marked increase in the level of p53 protein, and that p21(WAF1) promoter activity in senescent cells is dependent on both p53 and the transcriptional co-activator p300. Moreover, we detected increased specific activity of p53 protein in AT fibroblasts, which exhibit accelerated telomere loss and undergo premature senescence, compared with normal fibroblasts. We investigated the possibility that poly(ADP-ribose) polymerase is involved in the post-translational activation of p53 protein in aging cells. We show that p53 protein can associate with PARP and inhibition of PARP activity leads to abrogation of p21 and mdm2 expression in response to DNA damage. Moreover, inhibition of PARP activity leads to extension of cellular lifespan. In contrast, hyperoxia, an activator of PARP, is associated with accelerated telomere loss, activation of p53 and premature senescence. We propose that p53 is post-translationally activated not only in response to DNA damage but also in response to the critical shortening of telomeres that occurs during cellular aging.

Models of initiation of replicative senescence by loss of telomeric DNA.

Situated at the ends of all eukaryotic chromosomes are telomeres, genetic elements that are essential for genomic stability. It has recently been established that telomere length shortens during replicative aging of normal human somatic cells. Although the cause of replicative senescence of somatic cells is still debated, we believe that telomere shortening plays a causal role in this process. In support of this hypothesis, mutant strains of yeast and ciliates that are incapable of maintaining telomere length during cell division eventually acquire a senescent-like phenotype wherein the cells become sickly, stop growing and die. Also, replicative capacity of cultured human skin fibroblast strains shows a strong positive correlation with telomere length. Several theories explaining how telomere shortening could lead to the induction of replicative senescence are now presented. We favor a model in which replicative senescence is caused by the shortening of telomeres below a length that is critical for the maintenance of proper telomere structure and function.

Telomere shortening is associated with cell division in vitro and in vivo.

In humans, the amount of terminal (TTAGGG)n, telomeric DNA decreases during aging of various somatic cell types in vitro and in vivo. While the factors accounting for telomere shortening have not been thoroughly established, the inability of the DNA replication machinery to completely copy chromosomal termini (the "end replication problem") and the absence in somatic cells of telomerase, the enzyme that synthesizes telomeric DNA de novo, is a likely mechanism. One prediction of this hypothesis is that telomere shortening should be dependent on cell division. Thus we analyzed telomere length in actively dividing and quiescent cells in vitro and in vivo. In circular outgrowths of cultured human diploid fibroblasts (HDF), cells at the outer periphery had a significantly lower mean terminal restriction fragment (TRF) length (P = 0.011) and telomeric signal intensity (P = 0.024) than cells at the center. Also, the rate of telomere shortening over time for HDFs held quiescent was not statistically significant (m = -12 bp/day, P = 0.16) while that for serially passaged cells was significant (m = -34 bp/day, P = 0.017). To examine the rate of telomere shortening for quiescent cells in vivo, we measured mean TRF length in brain tissue from adult donors ranging in age from 32-75 years. No significant decrease was observed as a function of donor age (P = 0.087), in contrast to the shortening of telomere length that occurs during in vivo aging of mitotically active cells (P = 0.0001). These observations show that telomere shortening is largely, if not entirely, dependent on cell division and support the end replication problem as a mechanism for this process and the use of telomere length as a biomarker for replicative capacity.

The RNA component of human telomerase.

Eukaryotic chromosomes are capped with repetitive telomere sequences that protect the ends from damage and rearrangements. Telomere repeats are synthesized by telomerase, a ribonucleic acid (RNA)-protein complex. Here, the cloning of the RNA component of human telomerase, termed hTR, is described. The template region of hTR encompasses 11 nucleotides (5'-CUAACCCUAAC) complementary to the human telomere sequence (TTAGGG)n. Germline tissues and tumor cell lines expressed more hTR than normal somatic cells and tissues, which have no detectable telomerase activity. Human cell lines that expressed hTR mutated in the template region generated the predicted mutant telomerase activity. HeLa cells transfected with an antisense hTR lost telomeric DNA and began to die after 23 to 26 doublings. Thus, human telomerase is a critical enzyme for the long-term proliferation of immortal tumor cells.

Evidence for a critical telomere length in senescent human fibroblasts.

Telomeres, the G/C-rich DNA sequences capping the ends of all eukaryotic chromosomes, have been shown to shorten during replicative aging of normal cells in vitro and in vivo. Moreover, variation in the initial length of terminal restriction fragments (TRF) accounts for much of the variation in replicative capacity of fibroblast cultures from different donors. Since replicative capacity also varies significantly between clones in a mass culture of fibroblasts from a single donor, we wished to further test the hypothesis that the shortening of telomeres to a critical or threshold length acts as a signal for cell senescence. Thus, we measured TRF length and total telomeric signal intensity for 35 clonal fibroblast populations at early passage and at senescence. Replicative capacity was found to be directly proportional to mean TRF length (m = 7.2 population doublings/kbp, r = 0.65, P = 0.0004) and total signal intensity (m = 25.0 population doublings/unit, r = 0.63, P < 0.003) at early passage. More importantly, the variability in both mean TRF length and signal intensity (F = 2.0 and 2.9; P = 0.02 and 0.03, respectively) at senescence was markedly less than that at early passage. Although initial telomere length cannot account for all of the interclonal variability in replicative capacity, our observations support the existence of a critical telomere length in senescing cells and a causal role of telomere shortening in cell senescence.

Evidence for a mitotic clock in human hematopoietic stem cells: loss of telomeric DNA with age.

The proliferative life-span of the stem cells that sustain hematopoiesis throughout life is not known. It has been proposed that the sequential loss of telomeric DNA from the ends of human chromosomes with each somatic cell division eventually reaches a critical point that triggers cellular senescence. We now show that candidate human stem cells with a CD34+CD38lo phenotype that were purified from adult bone marrow have shorter telomeres than cells from fetal liver or umbilical cord blood. We also found that cells produced in cytokine-supplemented cultures of purified precursor cells show a proliferation-associated loss of telomeric DNA. These findings strongly suggest that the proliferative potential of most, if not all, hematopoietic stem cells is limited and decreases with age, a concept that has widespread implications for models of normal and abnormal hematopoiesis as well as gene therapy.

Telomere length predicts replicative capacity of human fibroblasts.

When human fibroblasts from different donors are grown in vitro, only a small fraction of the variation in their finite replicative capacity is explained by the chronological age of the donor. Because we had previously shown that telomeres, the terminal guanine-rich sequences of chromosomes, shorten throughout the life-span of cultured cells, we wished to determine whether variation in initial telomere length would account for the unexplained variation in replicative capacity. Analysis of cells from 31 donors (aged 0-93 yr) indicated relatively weak correlations between proliferative ability and donor age (m = -0.2 doubling per yr; r = -0.42; P = 0.02) and between telomeric DNA and donor age (m = -15 base pairs per yr; r = -0.43; P = 0.02). However, there was a striking correlation, valid over the entire age range of the donors, between replicative capacity and initial telomere length (m = 10 doublings per kilobase pair; r = 0.76; P = 0.004), indicating that cell strains with shorter telomeres underwent significantly fewer doublings than those with longer telomeres. These observations suggest that telomere length is a biomarker of somatic cell aging in humans and are consistent with a causal role for telomere loss in this process. We also found that fibroblasts from Hutchinson-Gilford progeria donors had short telomeres, consistent with their reduced division potential in vitro. In contrast, telomeres from sperm DNA did not decrease with age of the donor, suggesting that a mechanism for maintaining telomere length, such as telomerase expression, may be active in germ-line tissue.

Telomere end-replication problem and cell aging.

Since DNA polymerase requires a labile primer to initiate unidirectional 5'-3' synthesis, some bases at the 3' end of each template strand are not copied unless special mechanisms bypass this "end-replication" problem. Immortal eukaryotic cells, including transformed human cells, apparently use telomerase, an enzyme that elongates telomeres, to overcome incomplete end-replication. However, telomerase has not been detected in normal somatic cells, and these cells lose telomeres with age. Therefore, to better understand the consequences of incomplete replication, we modeled this process for a population of dividing cells. The analysis suggests four things. First, if single-stranded overhangs generated by incomplete replication are not degraded, then mean telomere length decreases by 0.25 of a deletion event per generation. If overhangs are degraded, the rate doubles. Data showing a decrease of about 50 base-pairs per generation in fibroblasts suggest that a full deletion event is 100 to 200 base-pairs. Second, if cells senesce after 80 doublings in vitro, mean telomere length decreases about 4000 base-pairs, but one or more telomeres in each cell will lose significantly more telomeric DNA. A checkpoint for regulation of cell growth may be signalled at that point. Third, variation in telomere length predicted by the model is consistent with the abrupt decline in dividing cells at senescence. Finally, variation in length of terminal restriction fragments is not fully explained by incomplete replication, suggesting significant interchromosomal variation in the length of telomeric or subtelomeric repeats. This analysis, together with assumptions allowing dominance of telomerase inactivation, suggests that telomere loss could explain cell cycle exit in human fibroblasts.