Cells continually experience stress and damage from exogenous and endogenous sources, and their responses range from complete recovery to cell death. Proliferating cells can initiate an additional response by adopting a state of permanent cell-cycle arrest that is termed cellular senescence. Understanding the causes and consequences of cellular senescence has provided novel insights into how cells react to stress, especially genotoxic stress, and how this cellular response can affect complex organismal processes such as the development of cancer and ageing.

Cellular senescence: when bad things happen to good cells

Added by telomerase, arrays of TTAGGG repeats specify the ends of human chromosomes. A complex formed by six telomere-specific proteins associates with this sequence and protects chromosome ends. By analogy to other chromosomal protein complexes such as condensin and cohesin, I will refer to this complex as shelterin. Three shelterin subunits, TRF1, TRF2, and POT1 directly recognize TTAGGG repeats. They are interconnected by three additional shelterin proteins, TIN2, TPP1, and Rap1, forming a complex that allows cells to distinguish telomeres from sites of DNA damage. Without the protective activity of shelterin, telomeres are no longer hidden from the DNA damage surveillance and chromosome ends are inappropriately processed by DNA repair pathways. How does shelterin avert these events? The current data argue that shelterin is not a static structural component of the telomere. Instead, shelterin is emerging as a protein complex with DNA remodeling activity that acts together with several associated DNA repair factors to change the structure of the telomeric DNA, thereby protecting chromosome ends. Six shelterin subunits: TRF1, TRF2, TIN2, Rap1, TPP1, and POT1.

/pdf/shelterin-the-protein-complex-that-shapes-and-safeguards-4nn8ijxqxb.pdf

Shelterin: the protein complex that shapes and safeguards human telomeres

Switching and Signaling at the Telomere

The genomes of prokaryotes and eukaryotic organelles are usually circular as are most plasmids and viral genomes. In contrast, the nuclear genomes of eukaryotes are organized on linear chromosomes, which require mechanisms to protect and replicate DNA ends. Eukaryotes navigate these problems with the advent of telomeres, protective nucleoprotein complexes at the ends of linear chromosomes, and telomerase, the enzyme that maintains the DNA in these structures. Mammalian telomeres contain a specific protein complex, shelterin, that functions to protect chromosome ends from all aspects of the DNA damage response and regulates telomere maintenance by telomerase. Recent experiments, discussed here, have revealed how shelterin represses the ATM and ATR kinase signaling pathways and hides chromosome ends from nonhomologous end joining and homology-directed repair.

http://delangelab.rockefeller.edu/pubs/108_Palm_AnnRevGen2008.pdf

How Shelterin Protects Mammalian Telomeres

https://www.cell.com/cell/pdf/S0092-8674(13)01602-4.pdf

Dynamic Imaging of Genomic Loci in Living Human Cells by an Optimized CRISPR/Cas System

https://delangelab.rockefeller.edu/pubs/50_Griffith_Cell_99.pdf

Mammalian Telomeres End in a Large Duplex Loop

Mammalian telomeres contain a duplex TTAGGG-repeat tract terminating in a 3′ single-stranded overhang. TRF2 protein has been implicated in remodeling telomeres into duplex lariats, termed t-loops, in vitro and t-loops have been isolated from cells in vivo. To examine the features of the telomeric DNA essential for TRF2-promoted looping, model templates containing a 500 bp double-stranded TTAGGG tract and ending in different single-stranded overhangs were constructed. As assayed by electron microscopy, looped molecules containing most of the telomeric tract are observed with TRF2 at the loop junction. A TTAGGG-3′ overhang of at least six nucleotides is required for loop formation. Termini with 5′ overhangs, blunt ends or 3′ termini with non-telomeric sequences at the junction are deficient in loop formation. Addition of non-telomeric sequences to the distal portion of a 3′ overhang beginning with TTAGGG repeats only modestly diminishes looping. TRF2 preferentially localizes to the junction between the duplex repeats and the single-stranded overhang. Based on these findings we suggest a model for the mechanism by which TRF2 remodels telomeres into t-loops.

/pdf/t-loop-assembly-in-vitro-involves-binding-of-trf2-near-the-3-livy1sgbws.pdf

T-loop assembly in vitro involves binding of TRF2 near the 3' telomeric overhang.

TRF1 is a key player in telomere length regulation. Because length control was proposed to depend on the architecture of telomeres, we studied how TRF1 binds telomeric TTAGGG repeat DNA and alters its conformation. Although the single Myb‐type helix–turn–helix motif of a TRF1 monomer can interact with telomeric DNA, TRF1 predominantly binds as a homodimer. Systematic Evolution of Ligands by Exponential enrichment (SELEX) with dimeric TRF1 revealed a bipartite telomeric recognition site with extreme spatial variability. Optimal sites have two copies of a 5′–YTAGGGTTR–3′ half‐site positioned without constraint on distance or orientation. Analysis of binding affinities and DNase I footprinting showed that both half‐sites are simultaneously contacted by the TRF1 dimer, and electron microscopy revealed looping of the intervening DNA. We propose that a flexible segment in TRF1 allows the two Myb domains of the homodimer to interact independently with variably positioned half‐sites. This unusual DNA binding mode is directly relevant to the proposed architectural role of TRF1.

/pdf/trf1-binds-a-bipartite-telomeric-site-with-extreme-spatial-vf0tzx1k4s.pdf

TRF1 binds a bipartite telomeric site with extreme spatial flexibility

p53 binds telomeric single strand overhangs and t-loop junctions in vitro.

Context.—Hereditary hemochromatosis is recognized as one of the most common autosomal recessive disorders, with a prevalence of 1 in 200 to 400 in the white population. Early detection and...

Rachel M. Stansel

Papers

Mammalian Telomeres End in a Large Duplex Loop

T-loop assembly in vitro involves binding of TRF2 near the 3' telomeric overhang.

TRF1 binds a bipartite telomeric site with extreme spatial flexibility

p53 binds telomeric single strand overhangs and t-loop junctions in vitro.

HFE Genotyping Using Multiplex Allele-Specific Polymerase Chain Reaction and Capillary Electrophoresis