Effect of age and apoptosis on the mouse homologue of the huWRN gene

Ageing–apoptosis relation in murine spleen

Relatively few studies have been published with regard to modification of apoptosis in normal tissues as a function of ageing. The majority of these studies demonstrated an increase in programmed cell death (PCD) with age. However, opposite results, namely loss of apoptotic control with age, have also been reported. In the present study, we examined proliferation and apoptotic cell death in spleens of C57/BL mice of different ages. A tendency towards decrease in cell proliferative capacity was seen with age. By contrast, apoptosis was increased in spleens from aged animals. Moreover, the proliferative cell/apoptotic cell ratio decreased in function of age. Ladder type DNA degradation was much more pronounced in DNA derived from splenocytes of old mice. These results were supported by a decrease of Bcl-2 and an increase in Fas receptor expression as well as by increased activation of caspases 8, 3 and 9 in splenocytes from aged animals. In addition, cell surface molecular markers recognizable by macrophages in apoptotic cells, namely decreased sialic acid concomitant with increased unmasking of galactose residues, were more pronounced on splenocytes from old mice than on those from young animals. In addition to the experimental evidence which supports a role of apoptotic cell death in ageing, a series of theoretical reasoning, which could also favor this possibility, are discussed.

Biochemical characterization of an exonuclease from Arabidopsis thaliana reveals similarities to the DNA exonuclease of the human Werner syndrome protein

The human Werner syndrome protein (hWRN-p) possessing DNA helicase and exonuclease activities is essential for genome stability. Plants have no homologue of this bifunctional protein, but surprisingly the Arabidopsis genome contains a small open reading frame (ORF) (AtWRNexo) with homology to the exonuclease domain of hWRN-p. Expression of this ORF in Escherichia coli revealed an exonuclease activity for AtWRN-exo-p with similarities but also some significant differences to hWRN-p. The protein digests recessed strands of DNA duplexes in the 3' --> 5' direction but hardly single-stranded DNA or blunt-ended duplexes. In contrast to the Werner exonuclease, AtWRNexo-p is also able to digest 3'-protruding strands. DNA with recessed 3'-PO4 and 3'-OH termini is degraded to a similar extent. AtWRNexo-p hydrolyzes the 3'-recessed strand termini of duplexes containing mismatched bases. AtWRNexo-p needs the divalent cation Mg2+ for activity, which can be replaced by Mn2+. Apurinic sites, cholesterol adducts, and oxidative DNA damage (such as 8-oxoadenine and 8-oxoguanine) inhibit or block the enzyme. Other DNA modifications, including uracil, hypoxanthine and ethenoadenine, did not inhibit AtWRNexo-p. A mutation of a conserved residue within the exonuclease domain (E135A) completely abolished the exonucleolytic activity. Our results indicate that a type of WRN-like exonuclease activity seems to be a common feature of the DNA metabolism of animals and plants.

Caspases, apoptosis and aging

Caspases are a group of cysteine dependent aspartate-specific proteases. Originally found as the homolog of Ced-3 in C. elegans, 14 caspases have now been identified in mammals to date. Caspases play important roles in both the intrinsic and extrinsic apoptotic pathways and interact with the non-caspase apoptotic pathways. A number of recent published observations have suggested a strong association between apoptosis, age-related diseases and aging. Findings from our group and others reveal a strong correlation between alterations in caspase activity and aging. In this view point, we summarize current knowledge of the connection between caspases and aging observed in a variety of model systems from cultured cells in vitro to the in vivo models of rodents and humans.

RecQ helicases: suppressors of tumorigenesis and premature aging

The RecQ helicases represent a subfamily of DNA helicases that are highly conserved in evolution. Loss of RecQ helicase function leads to a breakdown in the maintenance of genome integrity, in particular hyper-recombination. Germ-line defects in three of the five known human RecQ helicases give rise to defined genetic disorders associated with cancer predisposition and/or premature aging. These are Bloom's syndrome, Werner's syndrome and Rothmund-Thomson syndrome, which are caused by defects in the genes BLM, WRN and RECQ4 respectively. Here we review the properties of RecQ helicases in organisms from bacteria to humans, with an emphasis on the biochemical functions of these enzymes and the range of protein partners that they operate with. We will discuss models in which RecQ helicases are required to protect against replication fork demise, either through prevention of fork breakdown or restoration of productive DNA synthesis.

Corpus callosum and visual cortex of mice with deletion of the NMDA-NR1 receptor: I. Accelerated development of callosal projection neurons

Many pharmacological experiments show that the ionotropic receptor NMDA has both neurotrophic and neuroexcitotoxic effects. The neurotrophic function is manifested in many ways including acceleration of neuronal development, enhancement of neuronal migration, neuroprotection, blockage of apoptosis, prevention of aging and prematurity, as well as effects on synaptic plasticity and synaptogenesis. On the other hand, the neuroexcitotoxic function is manifested in its role in neurological and psychiatric diseases such as epilepsy, Parkinson's disease and schizophrenia. The present study explores the consequences of complete and partial absence of NMDA-NR1 receptors throughout development. Using DiI tracing in vitro, the development of corpus callosum projection neurons in transgenic mice with deletion of the NMDA-NR1 receptor was observed in visual cortex. Compared to littermate controls, the histogenesis and neuronal development of corpus callosum cells of origin was found to be accelerated in NR1-/- mice. That is, the corpus callosum projection neurons in NR1 knockout mice developed earlier and faster than in littermate heterozygous and wild-type mice. However, the corpus callosum projection neurons in NR1 heterozygous mice developed earlier and faster than in littermate wild-type mice. This suggests that NMDA-NR1 receptors are involved in sequencing and/or temporal regulation of neuronal development, and that there is a gene-dose effect. Studies from other laboratories suggest that the observed phenomenon of prematurity or accelerated development is a direct effect of altered expression of genes found in mice with deletion of the NMDA-NR1 receptor.

Helicase activity is only partially required for Schizosaccharomyces pombe Rqh1p function

The RecQ-related family of DNA helicases is required for the maintenance of genomic stability in organisms ranging from bacteria to humans. In humans, mutation of three RecQ-related helicases, BLM, WRN and RecQL4, cause the cancer-prone and premature ageing diseases of Bloom syndrome, Werner's syndrome and Rothmund-Thompson syndrome, respectively. In the fission yeast Schizosaccharomyces pombe, disruption of the rqh1(+) gene, which encodes the single Sz. pombe RecQ-related helicase, causes cells to display reduced viability and elevated levels of chromosome loss. After S-phase arrest or DNA damage, cells lacking rqh1(+) function display elevated levels of homologous recombination and defective chromosome segregation. Here we show that, like other RecQ family members, the Rqh1p protein displays 3' to 5' DNA helicase activity. Interestingly, however, unlike other RecQ family members, the helicase activity of Rqh1p is only partially required for its function in recovery from S-phase arrest or DNA damage. We also report that high cellular levels of Rqh1p result in lethal chromosome segregation defects, while more moderate levels of Rqh1p cause significantly elevated rates of chromosome loss. This suggests that careful regulation of RecQ-like protein levels in eukaryotic cells is vital for maintaining genome stability.

The Werner syndrome. A model for the study of human aging

Human aging is a complex process that leads to the gradual deterioration of body functions with time. Various models to approach the study of aging have been launched over the years such as the genetic analysis of life span in the yeast S. cerevisiae, the worm C. elegans, the fruitfly, and mouse, among others. In human models, there have been extensive efforts using replicative senescence, the study of centenerians, comparisons of young versus old at the organismal, cellular, and molecular levels, and the study of premature aging syndromes to understand the mechanisms leading to aging. One good model for studying human aging is a rare autosomal recessive disorder known as the Werner syndrome (WS), which is characterized by accelerated aging in vivo and in vitro. A genetic defect implicated in WS was mapped to the WRN locus. Mutations in this gene are believed to be associated, early in adulthood, with clinical symptoms normally found in old individuals. WRN functions as a DNA helicase, and recent evidence, summarized in this review, suggests specific biochemical roles for this multifaceted protein. The interaction of WRN protein with RPA (replication protein A) and p53 will undoubtedly direct efforts to further dissect the genetic pathway(s) in which WRN protein functions in DNA metabolism and will help to unravel its contribution to the human aging process.