Telomerase Is Expressed in Tissue Progenitor Cells in Mouse and Human

One means of gaining insight into telomerase function is to assess how telomerase is expressed in mammalian tissues. This seemingly straightforward question has been complicated by the low abundance of the telomerase enzyme in cells and tissues. TERT protein and TERT mRNA are therefore difficult to detect, which explains why most studies have relied on the PCR-based TRAP assay for telomerase activity. Using the TRAP assay, telomerase is detected in many, and perhaps all, self-renewing tissues, but its expression may be restricted to certain cell compartments within tissues, particularly those enriched in progenitor cells (Forsyth et al. 2002). For example, in human hair follicles, telomerase is expressed most highly in the bulb region, the portion of the follicle that harbors actively dividing progenitor cells during anagen, the active growth phase of the hair follicle cycle (Ramirez et al. 1997). Telomerase was expressed at much lower levels in the bulge region, which contains quiescent multipotent stem cells. In human epidermis, telomerase is expressed

Steven E. Artandi

Department of Medicine, Cancer Biology Program, Stanford, California, United States. e-mail: [email protected]

K.L. Rudolph (ed.), Telomeres and Telomerase in Ageing, Disease, and Cancer. © 2008 Springer-Verlag Berlin Heidelberg in the basal layer, which contains dividing progenitor cells that differentiate to regenerate the skin. More differentiated layers of skin did not express telomerase (Harle-Bachor and Boukamp 1996). Similarly, dissection of gastrointestinal tissue showed that telomerase activity was highest in the crypts, which contain the rapidly proliferating progenitor cells that replenish the population of epithelial cells that migrate along the crypt-villus axis and are continuously shed into the lumen of the gut (Bachor et al. 1999, Tahara et al. 1999). Therefore, telomerase is expressed in normal self-renewing human epithelial tissues, particularly in compartments that contain proliferating progenitor cells.

In the hematopoietic system, telomerase expression patterns have been investigated extensively (Lansdorp 2005). In mouse, telomerase is expressed in purified hematopoietic stem cells (HSCs)(Morrison et al. 1996). Similarly, in humans, telomerase is detected in primitive HSCs, although these levels were found to be low when the stem cells were quiescent. Stimulation of HSCs in culture led to cell cycle entry and to a significant increase in the levels of telomerase activity (Chiu et al. 1996, Yui et al. 1998). These results could mean that in vivo telomerase is switched on as HSCs become activated to self-renew or to differentiate. One confounding set of observations that makes a simple model of telomerase regulation in HSC unlikely relates to the fact that telomeres shorten in HSC and in differentiated hematopoietic cells with advancing age (Vaziri et al. 1994, Rufer et al. 1999). Therefore, although telomerase may be activated as HSCs enter cycle, either the levels or the action of telomerase is insufficient to prevent the subsequent telomere shortening that accompanies cell division. A better understanding of telomere dynamics in hematopoiesis will require detailed analyses of telomerase regulation in more committed hematopoietic progenitors as well as in HSCs.

Telomerase activity is detected in mouse embryonic stem (ES) cells (Niida et al. 1998) and human ES cells (Thomson et al. 1998). Telomerase activity markedly diminishes with differentiation of mouse ES cells, consistent with the general paradigm that telomerase is shut off in differentiated cells. Through the use of a TERT promoter fragment driving green fluorescent protein (GFP) in mouse ES cells, silencing of the TERT promoter was shown to be the primary variable underlying the loss of telomerase activity with ES cell differentation (Armstrong et al. 2000). This phenomenon of TERT transcriptional silencing with terminal differentiation has been seen in many cell culture experiments, including cancer cells that retain the ability to differentiate under appropriate conditions (Greenberg et al. 1998). Under some circumstances, telomerase activity is reduced with cell cycle exit, for example, with serum starvation of NIH3T3 cells (Holt et al. 1996). This paradigm of telomerase silencing is recapitulated during embryogenesis in some tissues. For example, in brain, TERT mRNA and telomerase are well expressed at embryonic day 13 in mouse, but diminish during embryogenesis and the first few days of postnatal life. Similar patterns of telomerase expression were seen within the brain, in cerebral cortex, hippocampus, and brain stem (Klapper et al. 2001).

There are many commonalities between mouse and human in terms of patterns of telomerase expression. In both species, telomerase is not expressed in tissues comprised largely of terminally differentiated tissues, such as brain, muscle, and heart (Chadeneau et al. 1995, Prowse and Greider 1995, Artandi et al. 2002). As in human, telomerase is detected in gastrointestinal epithelium, anagen hair follicles (Sarin et al.

2005), and hematopoietic progenitor cells (Morrison et al. 1996) from mouse. Telomerase is detected in lymphocytes from both species and increases with cytokine stimulation of lymphocytes (Weng et al. 1996). There is some evidence that the increase in telomerase activity occurs at a posttranslational level. TERT protein was found to be cytoplasmic in resting human T cells, but with cytokine stimulation TERT translocated to the nucleus (Liu et al. 1999). Some notable differences between mouse and human include fibroblasts and liver. Mouse embryonic fibroblasts are tel-omerase positive, whereas human fibroblast lines typically derived from embryonic tissues or from foreskin are telomerase negative, at least after adaptation to culture. BJ fibroblasts were found to express low levels of telomerase transiently during S-phase, perhaps reflecting the fact that some human fibroblasts can express telomerase (Masutomi et al. 2003). Mouse liver is telomerase positive, whereas human liver is telomerase negative (Kim et al. 1994, Prowse and Greider 1995).

In many cases, it is unclear which cell populations express telomerase in a complex tissue comprising multiple different cell types. This question of which cells express telomerase would be greatly aided by in situ techniques that allow detection of telomerase components. These approaches have proven challenging because TERT mRNA and TERT protein are nonabundant. One study analyzed TERT mRNA by RNA in situ hybridization in human tissues and found TERT mRNA in the basal layer of the epidermis and the gastrointestinal crypts (Kolquist et al. 1998). Additional investigation of telomerase expression patterns by RNA in situ hybridization in human and mouse is clearly warranted. Detection of TERT protein has been similarly challenging. Our understanding of TERT regulation and the role that TERT plays in tissue homeostasis would be significantly advanced if it could reliably detected by immunohistochemistry or Western blot. In addition to the limitations associated with the low abundance of TERT, the specificity of certain anti-TERT antibodies has been problematic (Wu et al. 2006). Taken together, telomerase appears restricted in its expression pattern in mammalian tissues and is enriched in cycling tissue progenitor cells.

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