Knockout of Ku86 accelerates cellular senescence induced by high NaCl

Heat shock induces apoptosis in human embryonic stem cells but a premature senescence phenotype in their differentiated progeny

Embryonic stem cells (ESC) are able to self-renew and to differentiate into any cell type. To escape error transmission to future cell progeny, ESC require robust mechanisms to ensure genomic stability. It was stated that stress defense of mouse and human ESC against oxidative stress and irradiation is superior compared with differentiated cells. Here, we investigated heat shock response of human ESC (hESC) and their differentiated progeny. Fibroblast-like cells were generated by spontaneous hESC differentiation via embryoid bodies. Like normal human diploid fibroblasts, these cells have a finite lifespan in culture, undergo replicative senescence and die. We found that sublethal heat shock affected survival of both cell types, but in hESC it induced apoptosis, whereas in differentiated cells it produced cell cycle arrest and premature senescence phenotype. Heat shock survived hESC and differentiated cells restored the properties of initial cells. Heated hESC progeny exhibited pluripotent markers and the capacity to differentiate into the cells of three germ layers. Fibroblast-like cells resisted heat shock, proliferated for a limited number of passages and entered replicative senescence as unheated parental cells. Taken together, these results show for the first time that both hESC and their differentiated derivatives are sensitive to heat shock, but the mechanisms of their stress response are different: hESC undergo apoptosis, whereas differentiated cells under the same conditions exhibit stress-induced premature senescence (SIPS) phenotype. Both cell types that survived sublethal heat shock sustain parental cell properties.

Increased insensible water loss contributes to aging related dehydration

Dehydration with aging is attributed to decreased urine concentrating ability and thirst. We further investigated by comparing urine concentration and water balance in 3, 18 and 27 month old mice, consuming equal amounts of water. During water restriction, 3 month old mice concentrate their urine sufficiently to maintain water balance (stable weight). 18 month old mice concentrate their urine as well, but still lose weight (negative water balance). 27 month old mice do not concentrate their urine as well and lose even more weight than the 18 month old mice, indicating a larger negative water balance. Negative water balance in older mice is accompanied by increased vasopressin excretion, providing further evidence of dehydration. All 3 groups maintain water balance while consuming only the water in gel food containing 56% water. However, both older groups excrete a smaller volume of urine of higher osmolality, indicating greater extra urinary water loss. Since their feces also contain less water, the excess water lost by the older mice apparently is through other routes, presumably insensible loss through the respiratory tract and skin. The greater insensible water loss occurs at an earlier age (18 months) than decreased urine concentrating ability (27 months). We propose that insensible water loss through skin and respiration increases with age, making a major contribution to aging related dehydration.

Ku80 facilitates chromatin binding of the telomere binding protein, TRF2

The Ku70/80 heterodimer is central to non-homologous end joining repair of DNA double-strand breaks and the Ku80 gene appears to be essential for human but not rodent cell survival. The Ku70/80 heterodimer is located at telomeres but its precise function in telomere maintenance is not known. In order to examine the role of Ku80 beyond DNA repair in more detail, we have taken a knockdown approach using a human fibroblast strain. Following targeted Ku80 knockdown, telomere defects are observed and the steady state levels of the TRF2 protein are reduced. Inhibitor studies indicate that this loss of TRF2 is mediated by the proteasome and degradation of TRF2 following Ku depletion appears to involve a decrease in chromatin binding of TRF2, suggesting that the Ku heterodimer enhances TRF2 chromatin association and that non-chromatin bound TRF2 is targeted to the proteasome.