Cell Senescence and Its Implications for Nephrology

In 1985, Kaysen and Meyers (1) stated that “the mechanisms and the full biochemical and physiologic consequences of renal senescence(definition) remain to be fully elucidated.” The present discussion highlights recent advances in cell biology that have implications for renal senescence. The study of cellular senescence is an emerging field of research with implications for aging, cancer, and chronic disease. It is the study of the limitations on the survival and function of somatic cells. Cells, tissues, organs, and organisms deteriorate over time and deteriorate more rapidly with stress. This deterioration may contribute to normal aging, chronic diseases, the performance of transplanted cells and tissues, repair of injury, and cancer. The cellular and molecular events in this deterioration may present opportunities for understanding and modifying these processes. This article highlights recent developments and their potential relevance in nephrology. This discussion reflects in part a symposium held at the American Society of Nephrology meeting in November 1999. The issue of senescence in renal transplants has been covered elsewhere (2). Given that detailed reviews of renal senescence are available (3,4,5,6,7), we concentrate on basic developments in studies of somatic cell senescence in vitro and then examine their in vivo significance. Definitions We use the term age to mean the time elapsed since birth. The term renal senescence reflects the structural and functional phenotype associated with aged kidneys. Cellular senescence and replicative senescence refer to an in vitro phenotype of cultured somatic cells that have reached their finite limit for replication, a state that may or may not exist for similar cells in vivo. In vitro studies of aged cells, i.e., derived from an old donor, have to be distinguished from senescent cells, which have developed the in vitro senescence phenotype. There is a difference between replicative senescence and terminal differentiation. Replicative senescence is arrived at by cell division. Terminal differentiation is a programmed phenotype, which responds to environmental clues. Whereas terminal differentiation may be a beneficial phenotype, the senescence phenotype in contrast may be a mixed blessing or even detrimental. Molecular Events in Replicative SenescenceIn Vitro The molecular basis of the in vitro cellular senescence phenotype probably differs between cell types and between species, e.g., humans versus mice (8). Hayflick and Moorhead (9) recognized that cultured human somatic cells in vitro displayed a limitation in their number of cycles. This number of cycles was called their Hayflick number. It was lower in cells from older donors and was unaffected by pausing. Thus, human somatic cells have a mechanism for counting the number of times that they have divided, a “mitotic clock.” They stop irreversibly when this cycle number is reached, and they manifest the state of replicative senescence. Telomere(definition) Shortening In human cells, shortening of telomeres is critical to replicative senescence. Telomeres are DNA repeats (TTAGGG) at the ends of chromosomes that shorten in dividing normal cells. Telomeres prevent chromosome ends from being confused with DNA breaks and probably have other functions in tethering and sorting chromosomes. The ends of telomeres must be replicated by the enzyme telomerase, a ribonucleoprotein expressed in germline and in immortal cell populations that maintains telomere length constant. In 1973, Olovnikov (10) proposed the telomere theory: namely, that somatic cells were limited because they cannot fully replicate their telomeres (Figure 1). The Hayflick limit was validated by the demonstration that human fibroblasts in culture lack telomerase, shorten their telomeres with each cycle, and develop replicative senescence when telomere length becomes critical. The critical experiment was the demonstration that transfection of telomerase into cultured human cells extends their life span and replication remarkably (11), thus bypassing the Hayflick limit.Figure 1.: Telomere Hypothesis: Telomerase is active in germline cells, maintaining long stable telomeres, but is repressed in most normal somatic cells, resulting in telomere loss in dividing cells. At M1, the Hayflick limit, there is a presumed critical telomere loss in one or perhaps a few chromosomes signaling irreversible cell cycle arrest. This corresponds to the phenotype of replicative senescence. Transformation events may allow somatic cells to bypass M1 without activating telomerase. When chromosomes become critically short on a large number of telomeres, cells are genomically unstable and enter crisis (M2). Rare clones that activate telomerase escape M2, stabilize their genome, and acquire indefinite growth capacity.Telomeric DNA diminishes by approximately 100 bp in dividing normal somatic cells at each cell doubling. The loss of telomeres can trigger the response to DNA breaks, which results in an organized cellular state, the senescence phenotype (M1 in Figure 1). Cells that are driven to continue dividing by abnormal stimuli develop massive genomic instability or crisis (M2). Germline cells and immortal cell populations like most cancer cell lines possess mechanisms, which are either telomerase activation or an alternative mechanism, to preserve their telomere length indefinitely despite cell division, thus protecting their genome. The state of replicative senescence in human skin fibroblasts includes cessation in replication, altered patterns of gene expression, and resistance to apoptosis. Senescent fibroblasts remain viable for many months, with ongoing RNA and protein synthesis. However, senescent cells cannot be stimulated to enter the S phase of the cell cycle by any combination of growth factors or physiologic mitogens. Senescent human fibroblasts show an enlarged and flat morphology and accumulate lipofuscin pigment and senescence associated β galactosidase (SA-β-GAL)