Telomerase expression and telomere length in immortal leukocyte lines fromchannel catfish

Normal channel catfish leukocytes readily undergo spontaneous in vitro immo rtalization yielding functionally active diploid cell lines. Since telomera se activation appears to be a critical step in the establishment of immorta l mammalian cells, studies were undertaken to determine if and when telomer ase expression occurs during the in vitro immortalization process of channe l catfish leukocytes. To this end, freshly isolated peripheral blood leukoc ytes (PBL) from normal fish were shown to exhibit low to undetectable level s of telomerase activity and within four days after culture initiation show ed dramatic increases in telomerase activity which typically remained high for at least four weeks. This activity then declined, concomitant with decr eases in cellular proliferation and increases in cell death. Cells which es caped this culture "crisis" re-expressed high levels of telomerase activity indefinitely. Although telomerase activity was expressed early in the immo rtalization process, clonal cell lines derived from these cultures had rela tively short telomeres. These results suggest that telomerase expression in catfish leukocytes is activation-induced, and its expression does not nece ssarily stabilize telomere length until a critically, albeit ill-defined, s hort length is reached. (C) 2000 Elsevier Science Ltd. All rights reserved.