The Nonsense-mediated mRNA decay (NMD) pathway selectively degrades mRNAs harboring premature termination codons (PTCs) but also regulates the abundance of a large number of cellular RNAs. The central role of NMD in the control of gene expression requires the existence of buffering mechanisms that tightly regulate the magnitude of this pathway. Here, we will focus on the mechanism of NMD with an emphasis on the role of RNA helicases in the transition from NMD complexes that recognize a PTC to those that promote mRNA decay. We will also review recent strategies aimed at uncovering novel trans-acting factors and their functional role in the NMD pathway. Finally, we will describe recent progress in the study of the physiological role of the NMD response.

The physical ends of chromosomes, known as telomeres, protect chromosome ends from nucleolytic degradation and DNA repair activities. Conventional DNA replication enzymes lack the ability to fully replicate telomere ends. In addition, nucleolytic activities contribute to telomere erosion. Short telomeres trigger DNA damage checkpoints, which mediate cellular senescence. Telomere length homeostasis requires telomerase, a cellular reverse transcriptase, which uses an internal RNA moiety as a template for the synthesis of telomere repeats. Telomerase elongates the 3′ ends of chromosomes, whereas the complementary strand is filled in by conventional DNA polymerases. In humans, telomerase is ubiquitously expressed only during the first weeks of embryogenesis, and is subsequently downregulated in most cell types. Correct telomere length setting is crucial for long-term survival. The telomere length reserve must be sufficient to avoid premature cellular senescence and the acceleration of age-related disease. On the other side, telomere shortening suppresses tumor formation through limiting the replicative potential of cells. In recent years, novel insight into the regulation of telomerase at chromosome ends has increased our understanding on how telomere length homeostasis in telomerasepositive cells is achieved. Factors that recruit telomerase to telomeres in a cell cycle-dependent manner have been identified in Saccharomyces cerevisiae. In humans, telomerase assembles with telomeres during S phase of the cell cycle. Presumably through mediating formation of alternative telomere structures, telomere-binding proteins regulate telomerase activity in cis to favor preferential elongation of the shortest telomeres. Phosphoinositide 3-kinase related kinases are also required for telomerase activation at chromosome ends, at least in budding and fission yeast. In vivo analysis of telomere elongation kinetics shows that telomerase does not act on every telomere in each cell cycle but that it exhibits an increasing preference for telomeres as their lengths decline. This suggests a model in which telomeres switch between extendible and nonextendible states in a length-dependent manner. In this review we expand this model to incorporate the finding that telomerase levels also limit telomere length and we propose a second switch between a non-telomerase-associated "extendible" and a telomerase-associated "extending" state.

Telomeres can fold into t-loops that may result from the invasion of the 3' overhang into duplex DNA. Their formation is facilitated in vitro by the telomeric protein TRF2, but very little is known regarding the mechanisms involved. Here we reveal that TRF2 generates positive supercoiling and condenses DNA. Using a variety of TRF2 mutants, we demonstrate a strong correlation between this topological activity and the ability to stimulate strand invasion. We also report that these properties require the combination of the TRF-homology (TRFH) domain of TRF2 with either its N- or C-terminal DNA-binding domains. We propose that TRF2 complexes, by constraining DNA around themselves in a right-handed conformation, can induce untwisting of the neighboring DNA, thereby favoring strand invasion. Implications of this topological model in t-loop formation and telomere homeostasis are discussed.