Telomeres as hotspots for innate immunity and inflammation

Abstract

Aging is marked by the gradual accumulation of deleterious changes that disrupt organ function, creating an altered physiological state that is permissive for the onset of prevalent human diseases. While the exact mechanisms governing aging remain a subject of ongoing research, there are several cellular and molecular hallmarks that contribute to this biological process. This review focuses on two factors, namely telomere dysfunction and inflammation, which have emerged as crucial contributors to the aging process. We aim to discuss the mechanistic connections between these two distinct hallmarks and provide compelling evidence highlighting the loss of telomere protection as a driver of pro-inflammatory states associated with aging. By reevaluating the interplay between telomeres, innate immunity, and inflammation, we present novel perspectives on the etiology of aging and its associated diseases.

Keywords: Aging; Cancer; Genome stability; Inflammation; Innate immunity; Telomeres.

Fig. 1.. DNA repair pathways at deprotected telomeres.

A) ssDNA activates the ATR kinase pathway. B) DSBs are recognized either by Ku heterodimer or MRN complex and activate the ATM kinase pathway. This triggers local phosphorylation of multiple ATM substrates in the surrounding chromatin, most notably the histone variant H2AX and the recruitment of additional DDR mediators, such as 53BP1 and MDC1, at the break site. These modifications give rise to TIF, macroscopic structures that mark sites of deprotected telomeres. This signaling switches from phosphorylation to ubiquitination through the recruitment of E3 ligases, including RNF8 and RNF168, resulting in the ubiquitination of several proteins, including H2A/H2AX. C) During c-NHEJ, broken ends are rapidly bound by Ku, followed by the recruitment and activation of the DNA-PKcs. Broken DNA ends are joined by DNA ligase IV. Depletion of TRF2 provokes undesired c-NHEJ activity at telomeres, potentially resulting in chromosomal end-to-end fusions. D) MRN/CtIP-mediated short-range resection disrupts the c-NHEJ pathway by displacing Ku from DNA ends and exposes short microhomologies of 2–20 bp (green boxes) required for alt-EJ. This repair process is contingent on PARP1 and Polθ and is finalized by DNA ligase III. E) Extensive resection is executed by EXO1 or combined action of BLM helicase and DNA2 nuclease. Generated 3’ ssDNA overhangs, rapidly coated with RPA heterotrimer, facilitate either inter- or intratelomeric strand invasion and recombination. This invasion results in break-induced telomere synthesis via a noncanonical Polδ-PCNA-RFC1 replisome. Formed D-loop can be resolved by the SMX endonuclease complex, causing crossover events without telomere elongation (telomeric sister chromatid exchange (T-SCE), indicated by yellow telomeres), or dissolved by the BTR complex, leading to telomere extension without crossover. Aberrant excision of T-loops formed by intratelomeric recombination via SLX4 or XRCC3 and NBS1 repair proteins can result in accumulation of ECTRs and rapid telomere shortening. Shelterin proteins and auxiliary factors, which inhibit specific stages of DNA repair mechanisms at deprotected telomeres, are highlighted with red boxes. (1) TPP1-POT1 complex represses ATR signaling by blocking RPA association with telomeric overhang. (2) TRF2 aids in forming the protective t-loop structure, shielding chromosome ends from DNA sensors. In addition, TRF2 inhibits Ku heterodimerization, thereby preventing c-NHEJ activation. (3) TRF2 through its iDDR domain impedes the recruitment of RNF168, limiting downstream signaling events. (4) Shelterin and the Ku heterodimer act in coordinated fashion to suppress alt-EJ. (5) components TRF2, RAP1, and POT1, in conjunction with Ku, impede sequence exchange between sister chromatid telomeres. (6) During the S phase, TRF2 recruits RTEL1 to unwind the t-loop, shielding it from cleavage by SLX4. Furthermore, TRF2 inhibits t-loop excision by suppressing NBS1–XRCC3 activity.

Fig. 2.. Telomere-associated molecular patterns.

Telomere-driven innate immunity involves the activation of interconnected DNA- and RNA-sensing pathways. A) Deprotected telomeres result in chromosome fusion and formation of dicentric chromosomes. Tension across the centromeres of a dicentric chromosome can lead to chromosomal bridge formation, which is often resolved via fragmentation. Broken chromosome fragments lacking centromere may be mis-segregated, resulting in formation of one or more MN. MN can also emerge from lagging chromosomes that detach from both centromeres and are left behind at anaphase. These MN, characterized by an unstable nuclear envelope provoke cGAS activation upon envelope rupture. DNA bridges may endure past anaphase forming a link between two newly assembled daughter nuclei. These NPBs share similar nuclear envelope defects with micronuclei, leading to cGAS-dependent innate immune response. B) ECTRs, products of telomere trimming and HDR activity, may exhibit linear or circular configurations. The latter includes t-circles (both strands), C-circles (entire C-rich circle with a G-rich primer), or G-circles (complete G-rich circle with a C-rich primer). When released into cytoplasm, ECTRs can activate cGAS pathway. C) TERRA is a long non-coding RNA transcribed by RNA Polymerase II from subtelomeric regions toward chromosome ends, using the C-rich strand as a template. The cytosolic TERRA molecules can adopt multiple structures, such as ssRNA, G-quadruplex configurations, or RNA:DNA hybrids excised from telomeric R-loops. All TERRA species have the potential to provoke an innate immune response. D) In the cytosol, telomere-derived dsDNA species are recognized and bound by cGAS, activating STING. On the other hand, ZBP1 interacts with cytosolic TERRA, triggering MAVS-dependent signaling. Both pathways induce the transcription of interferon genes and proinflammatory cytokines via the TBK1-IRF3 and IKK-NFκB signaling cascades.