Human cells lacking coilin and Cajal bodies are proficient in telomerase assembly,trafficking and telomere maintenance

The RNA component of human telomerase (hTR) localizes to Cajal bodies, and it has been proposed that Cajal bodies play a role in the assembly of telomerase holoenzyme and telomerase trafficking. Here, the role of Cajal bodies was examined in Human cells deficient of coilin (i.e. coilin-knockout (KO) cells), in which no Cajal bodies are detected. In coilin-KO cells, a normal level of telomerase activity is detected and interactions between core factors of holoenzyme are preserved, indicating that telomerase assembly occurs in the absence of Cajal bodies. Moreover, dispersed hTR aggregates and forms foci specifically during S and G2 phase in coilin-KO cells. Colocalization of these hTR foci with telomeres implies proper telomerase trafficking, independent of Cajal bodies. Therefore, telomerase adds similar numbers of TTAGGG repeats to telomeres in coilin-KO and controls cells. Overexpression of TPP1-OB-fold blocks cell cycle-dependent formation of hTR foci and inhibits telomere extension. These findings suggest that telomerase assembly, trafficking and extension occur with normal efficiency in Cajal bodies deficient human cells. Thus, Cajal bodies, as such, are not essential in these processes, although it remains possible that non-coilin components of Cajal bodies and/or telomere binding proteins (e.g. TPP1) do play roles in telomerase biogenesis and telomere homeostasis.     

Telomerase recruitment requires both TCAB1 and Cajal bodies independently

The ability of most cancer cells to grow indefinitely relies on the enzyme telomerase and its recruitment to telomeres. In human cells, recruitment depends on the Cajal body RNA chaperone TCAB1 binding to the RNA subunit of telomerase (hTR) and is also thought to rely on an N-terminal domain of the catalytic subunit, hTERT. We demonstrate that coilin, an essential structural component of Cajal bodies, is required for endogenous telomerase recruitment to telomeres but that overexpression of telomerase can compensate for Cajal body absence. In contrast, recruitment of telomerase was sensitive to levels of TCAB1, and this was not rescued by overexpression of telomerase. Thus, although Cajal bodies are important for recruitment, TCAB1 has an additional role in this process that is independent of these structures. TCAB1 itself localizes to telomeres in a telomerase-dependent but Cajal body-independent manner. We identify a point mutation in hTERT that largely abolishes recruitment yet does not affect association of telomerase with TCAB1, suggesting that this region mediates recruitment by an independent mechanism. Our results demonstrate that telomerase has multiple independent requirements for recruitment to telomeres and that the function of TCAB1 is to directly transport telomerase to telomeres.     

Human telomerase RNA accumulation in Cajal bodies facilitates telomerase recruitment to telomeres and telomere elongation

Telomerase is required for telomere maintenance and is responsible for the immortal phenotype of cancer cells. How telomerase is assembled and reaches telomeres in the context of nuclear architecture is not understood. Recently, the telomerase RNA subunit (hTR) was shown to accumulate in Cajal bodies (CBs), subnuclear structures implicated in ribonucleoprotein maturation. However, the functional relevance of this localization for telomerase was unknown. hTR localization to CBs requires a short sequence motif called the CAB box. Here, we reconstitute telomerase in human cells and determine the effects of CAB box mutations on telomere biology. We demonstrate that mutant hTR, which fails to accumulate in CBs, is fully capable of forming catalytically active telomerase in vivo but is strongly impaired in telomere extension. The functional deficiency is accompanied by a decreased association of telomerase with telomeres. Collectively, these data identify subnuclear localization as an important regulatory mechanism for telomere length homeostasis in human cells.     

A Cajal body-independent pathway for telomerase trafficking in mice

The intranuclear trafficking of human telomerase involves a dynamic interplay between multiple nuclear sites, most notably Cajal bodies and telomeres. Cajal bodies are proposed to serve as sites of telomerase maturation, storage, and assembly, as well as to function in the cell cycle-regulated delivery of telomerase to telomeres in human cells. Here, we find that telomerase RNA does not localize to Cajal bodies in mouse cells, and instead resides in separate nuclear foci throughout much of the cell cycle. However, as in humans, mouse telomerase RNA (mTR) localizes to subsets of telomeres specifically during S phase. The localization of mTR to telomeres in mouse cells does not require coilin-containing Cajal bodies, as mTR is found at telomeres at similar frequencies in cells from wild-type and coilin knockout mice. At the same time, we find that human TR localizes to Cajal bodies (as well as telomeres) in mouse cells, indicating that the distinct trafficking of mTR is attributable to an intrinsic property of the RNA (rather than a difference in the mouse cell environment such as the properties of mouse Cajal bodies). We also find that during S phase, mTR foci coalesce into short chains, with at least one of the conjoined mTR foci co-localizing with a telomere. These findings point to a novel, Cajal body-independent pathway for telomerase biogenesis and trafficking in mice.     

The snoRNA domain of vertebrate telomerase RNA functions to localize the RNA within the nucleus.

Telomerase RNA is an essential component of the ribonucleoprotein enzyme involved in telomere length maintenance, a process implicated in cellular senescence and cancer. Vertebrate telomerase RNAs contain a box H/ACA snoRNA motif that is not required for telomerase activity in vitro but is essential in vivo. Using the Xenopus oocyte system, we have found that the box H/ACA motif functions in the subcellular localization of telomerase RNA. We have characterized the transport and biogenesis of telomerase RNA by injecting labeled wild-type and variant RNAs into Xenopus oocytes and assaying nucleocytoplasmic distribution, intranuclear localization, modification, and protein binding. Although yeast telomerase RNA shares characteristics of spliceosomal snRNAs, we show that human telomerase RNA is not associated with Sm proteins or efficiently imported into the nucleus. In contrast, the transport properties of vertebrate telomerase RNA resemble those of snoRNAs; telomerase RNA is retained in the nucleus and targeted to nucleoli. Furthermore, both nuclear retention and nucleolar localization depend on the box H/ACA motif. Our findings suggest that the H/ACA motif confers functional localization of vertebrate telomerase RNAs to the nucleus, the compartment where telomeres are synthesized. We have also found that telomerase RNA localizes to Cajal bodies, intranuclear structures where it is thought that assembly of various cellular RNPs takes place. Our results identify the Cajal body as a potential site of telomerase RNP biogenesis.