Telomeres,senescence, and hematopoietic stem cells

The replicative lifespan of normal somatic cells is restricted by the erosion of telomeres, which are protective caps at the ends of linear chromosomes. The loss of telomeres induces antiproliferative signals that eventually lead to cellular senescence. The enzyme complex telomerase can maintain telomeres, but its expression is confined to highly proliferative cells such as stem cells and tumor cells. The immense regenerative capacity of the hematopoietic system is provided by a distinct type of adult stem cell: hematopoietic stem cells (HSCs). Although blood cells have to be produced continuously throughout life, the HSC pool seems not to be spared by aging processes. Indeed, limited expression of telomerase is not sufficient to prevent telomere shortening in these cells, which is thought ultimately to limit their proliferative capacity. In this review, we discuss the relevance of telomere maintenance for the hematopoietic stem cell compartment and consider potential functions of telomerase in this context. We also present possible clinical applications of telomere manipulation in HSCs and new insights affecting the aging of the hematopoietic stem cell pool and replicative exhaustion. This work was supported by European Community Grant LSHC-CT-2004-502943 (MOL CANCER MED).     

Differentiation rather than aging of muscle stem cells abolishes their telomerase activity

A general feature of stem cells is the ability to routinely proliferate to build, maintain, and repair organ systems. Accordingly, embryonic and germline, as well as some adult stem cells, produce the telomerase enzyme at various levels of expression. Our results show that, while muscle is a largely postmitotic tissue, the muscle stem cells (satellite cells) that maintain this biological system throughout adult life do indeed display robust telomerase activity. Conversely, primary myoblasts (the immediate progeny of satellite cells) quickly and dramatically downregulate telomerase activity. This work thus suggests that satellite cells, and early transient myoblasts, may be more promising therapeutic candidates for regenerative medicine than traditionally utilized myoblast cultures. Muscle atrophy accompanies human aging, and satellite cells endogenous to aged muscle can be triggered to regenerate old tissue by exogenous molecular cues. Therefore, we also examined whether these aged muscle stem cells would produce tissue that is “young” with respect to telomere maintenance. Interestingly, this work shows that the telomerase activity in muscle stem cells is largely retained into old age wintin inbred “long” telomere mice and in wild‐derived short telomere mouse strains, and that age‐specific telomere shortening is undetectable in the old differentiated muscle fibers of either strain. Summarily, this work establishes that young and old muscle stem cells, but not necessarily their progeny, myoblasts, are likely to produce tissue with normal telomere maintenance when used in molecular and regenerative medicine approaches for tissue repair. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009     

Telomere dynamics in hematopoietic stem cells

The hematopoietic system has an outstanding regenerative capacity which depends on a relatively small population of hematopoietic stem cells (HSC). In contrast to normal human cells, blood-forming stem cells, like most of their counterparts from other adult tissues, exhibit telomerase activity to a certain level. Nevertheless, this telomerase activity does not prevent telomere shortening in HSC, suggesting a restriction of their proliferative capacity. Here, we review recent studies on telomere dynamics in HSC of humans and mice. Furthermore, we discuss the impact of telomere manipulation in HSC for possible clinical applications and speculate on functions of telomerase beyond telomere lengthening.     

Telomere dysfunction induces environmental alterations limiting hematopoietic stem cell function and engraftment

Cell-intrinsic checkpoints limit the proliferative capacity of primary cells in response to telomere dysfunction. It is not known, however, whether telomere dysfunction contributes to cell-extrinsic alterations that impair stem cell function and organ homeostasis. Here we show that telomere dysfunction provokes defects of the hematopoietic environment that impair B lymphopoiesis but increase myeloid proliferation in aging telomerase knockout (Terc(-/-)) mice. Moreover, the dysfunctional environment limited the engraftment of transplanted wild-type hematopoietic stem cells (HSCs). Dysfunction of the hematopoietic environment was age dependent and correlated with progressive telomere shortening in bone marrow stromal cells. Telomere dysfunction impaired mesenchymal progenitor cell function, reduced the capacity of bone marrow stromal cells to maintain functional HSCs, and increased the expression of various cytokines, including granulocyte colony-stimulating factor (G-CSF), in the plasma of aging mice. Administration of G-CSF to wild-type mice mimicked some of the defects seen in aging Terc(-/-) mice, including impairment of B lymphopoiesis and HSC engraftment. Conversely, inhibition of G-CSF improved HSC engraftment in aged Terc(-/-) mice. Taken together, these results show that telomere dysfunction induces alterations of the environment that can have implications for organismal aging and cell transplantation therapies.     

Telomere length, stem cells and aging

Telomere shortening occurs concomitant with organismal aging, and it is accelerated in the context of human diseases associated with mutations in telomerase, such as some cases of dyskeratosis congenita, idiopathic pulmonary fibrosis and aplastic anemia. People with these diseases, as well as Terc-deficient mice, show decreased lifespan coincidental with a premature loss of tissue renewal, which suggests that telomerase is rate-limiting for tissue homeostasis and organismal survival. These findings have gained special relevance as they suggest that telomerase activity and telomere length can directly affect the ability of stem cells to regenerate tissues. If this is true, stem cell dysfunction provoked by telomere shortening may be one of the mechanisms responsible for organismal aging in both humans and mice. Here, we will review the current evidence linking telomere shortening to aging and stem cell dysfunction.     

Historical claims and current interpretations of replicative aging

Replicative aging is the process by which most normal human cells "count" the number of times they have divided, eventually undergoing a growth arrest termed cellular senescence. This process is dependent on the shortening of telomeres, repeated sequences at the ends of the chromosomes. The loss of telomeric sequences with each cell division eventually induces a growth arrest that has a similar phenotype to that of cells stressed by inadequate culture or other conditions. Experiments over the past several years have identified species in which replicative aging does not occur and many examples in which a failure to proliferate has been misinterpreted as replicative senescence. Insights from these studies now permit a reevaluation of much of the seemingly contradictory data concerning replicative aging. There are good theoretical reasons for believing a limited proliferative capacity contributes to declining tissue homeostasis with increasing age. Although the presence of telomere shortening provides strong circumstantial evidence that replicative aging is occurring in vivo, thus far there is only very limited direct evidence for actual physiological effects of replicative aging.     

The role of telomere shortening in somatic stem cells and tissue aging: lessons from telomerase model systems

The analysis of model systems has broadened our understanding of telomere-related aging processes. Telomerase-deficient mouse models have demonstrated that telomere dysfunction impairs tissue renewal capacity and shortens lifespan. Telomere shortening limits cell proliferation by activating checkpoints that induce replicative senescence or apoptosis. These checkpoints protect against an accumulation of genomically instable cells and cancer initiation. However, the induction of these checkpoints can also limit organ homeostasis, regeneration, and survival during aging and in the context of diseases. The decline in tissue regeneration in response to telomere shortening has been related to impairments in stem cell function. Telomere dysfunction impairs stem cell function by activation of cell-intrinsic checkpoints and by the induction of alterations in the micro- and macro-environment of stem cells. In this review, we discuss the current knowledge about the impact of telomere shortening on disease stages induced by replicative cell aging as indicated by studies on telomerase model systems.     

Proliferative senescence in hematopoietic stem cells during ex-vivo expansion

The good outcome of hematopoietic stem cell (HSC) transplantation is hampered by low doses of CD34+ cell infusion. Transplanted HSCs undergo a replicative stress that causes accelerated senescence due to rapid telomere shortening. The expansion of human cord blood HSCs is instrumental in obtaining a large number of "good quality" cells, in terms of telomere length and telomerase activity compared to adult HSCs.     

Multiple facets of TPP1 in telomere maintenance

Telomeres are nucleoprotein complexes that cap the ends of all linear chromosomes and function to prevent aberrant repair and end-to-end chromosome fusions. In somatic cells, telomere shortening is a natural part of the aging process as it occurs with each round of cell division. In germ and stem cells, however, the enzyme telomerase synthesizes telomere DNA to counter-balance telomere shortening and help maintain cellular proliferation. Of the primary telomere end-binding proteins, TPP1 has recently emerged as a primary contributor in protecting telomere DNA and in recruiting telomerase to the telomere ends. In this review, we summarize the current knowledge regarding the role of TPP1 in telomere maintenance.