Vertebrate telomeres and telomere-interacting proteins

We have characterized telomere dynamics of normal and immortalized chicken cells in order to explore the possibility of using chicken cells as an alternate vertebrate model system to study telomere biology. We chose to study chicken cells because: (1) chicken primary cells have a limited life span in culture and exhibit low rates of spontaneous immortalization, and (2) chicken DT40 cells can be used to generate gene “knock-outs” in cell culture. After optimizing the conditions to isolate genomic DNA with minimal aggregation, we measured the length of telomeres from normal chicken cells using a novel non-denaturing Southern hybridization. We report that chicken telomeres are rather short (8–23kb). The telomerase expression profile in normal chicken tissues (three different donor ages) was also examined and we have found that telomerase is constitutively expressed in the germ line and in most somatic tissues examined. This expression profile mimics that observed in mice and differs from that of humans. The robust telomerase activity in embryonic tissues was rapidly down-regulated when a primary culture was established from this tissue and the culture senesced after undergoing 22–32 population doublings. The down-regulation of telomerase activity correlated well the with gradual loss of telomere length exhibited by the aging chicken embryonic fibroblasts. Therefore, like their human counterparts, telomere shortening acts as the signal that limits the proliferation of chicken cells in culture. The gene encoding chicken ortholog of human Tankyrase1, a protein with poly(ADP-ribose) polymerase activity localized to telomeres, has been isolated and found to be very well conserved (∼98% identical). The gene encoding another molecule called Tankyrase2, that has a similar modular architecture to Tankyrase1 and significant overall sequence identity, has also been isolated from chicken cells (∼85%). While Tankyrase1 displays a complex cellular localization, and is present at telomeres, the pericentriolar matrix and at nuclear pores, the cellular localization of Tankyrase2 is currently unknown. Based on sequence analysis, we hypothesize that Tankyrase1 and 2 may have both common and unique roles in cells. Their function(s) are currently being studied