Progeroid syndromes induced by mutations in lamin A or in its interactors – named progeroid laminopathies – are model systems for the dissection of the molecular pathways causing physiological and premature aging. A large amount of data, based mainly on the Hutchinson Gilford Progeria syndrome (HGPS), one of the best characterized progeroid laminopathy, has highlighted the role of lamins in multiple DNA activities, including replication, repair, chromatin organization and telomere function. On the other hand, the phenotypes generated by mutations affecting genes directly acting on DNA function, as mutations in the helicases WRN and BLM or in the polymerase polδ, share many of the traits of progeroid laminopathies. These evidences support the hypothesis of a concerted implication of DNA function and lamins in aging. We focus here on these aspects to contribute to the comprehension of the driving forces acting in progeroid syndromes and premature aging.

Chromosome ends are complex structures, which require a panel of factors for their elongation, replication, and protection. We describe here the mechanics of mammalian telomeres, dynamics and maintainance in relation to lamins. Multiple biochemical connections, including association of telomeres to the nuclear envelope and matrix, of telomeric proteins to lamins, and of lamin-associated proteins to chromosome ends, underline the interplay between lamins and telomeres. Paths toward senescence, such as defective telomere replication, altered heterochromatin organization, and impaired DNA repair, are common to lamins' and telomeres' dysfunction. The convergence of phenotypes can be interpreted through a model of dynamic, lamin-controlled functional platforms dedicated to the function of telomeres as fragile sites. The features of telomeropathies and laminopathies, and of animal models underline further overlapping aspects, including the alteration of stem cell compartments. We expect that future studies of basic biology and on aging will benefit from the analysis of this telomere-lamina interplay.

Telomeres are nucleoprotein complexes that protect the ends of linear chromosomes from incomplete replication, degradation and detection as DNA breaks. Mammalian telomeres are protected by shelterin, a multiprotein complex that binds the TTAGGG telomeric repeats and recruits a series of additional factors that are essential for telomere function. Although many shelterin-associated proteins have been so far identified, the inventory of shelterin-interacting factors required for telomere maintenance is still largely incomplete. Here, we characterize AKTIP/Ft1 (human AKTIP and mouse Ft1 are orthologous), a novel mammalian shelterin-bound factor identified on the basis of its homology with the Drosophila telomere protein Pendolino. AKTIP/Ft1 shares homology with the E2 variant ubiquitin-conjugating (UEV) enzymes and has been previously implicated in the control of apoptosis and in vesicle trafficking. RNAi-mediated depletion of AKTIP results in formation of telomere dysfunction foci (TIFs). Consistent with these results, AKTIP interacts with telomeric DNA and binds the shelterin components TRF1 and TRF2 both in vivo and in vitro. Analysis of AKTIP- depleted human primary fibroblasts showed that they are defective in PCNA recruiting and arrest in the S phase due to the activation of the intra S checkpoint. Accordingly, AKTIP physically interacts with PCNA and the RPA70 DNA replication factor. Ft1-depleted p53^-/- MEFs did not arrest in the S phase but displayed significant increases in multiple telomeric signals (MTS) and sister telomere associations (STAs), two hallmarks of defective telomere replication. In addition, we found an epistatic relation for MST formation between Ft1 and TRF1, which has been previously shown to be required for replication fork progression through telomeric DNA. Ch-IP experiments further suggested that in AKTIP-depleted cells undergoing the S phase, TRF1 is less tightly bound to telomeric DNA than in controls. Thus, our results collectively suggest that AKTIP/Ft1 works in concert with TRF1 to facilitate telomeric DNA replication. Chromosome ends are capped by specialized structures called telomeres, which protect chromosomes from deterioration, incomplete replication and end-to-end fusion. Defects in telomere structure and/or function may have a strong impact on human health, leading to premature aging and a variety of diseases including cancer. One of the most important tasks to understand and possibly prevent the consequences of telomere dysfunction is the identification and characterization of telomere-associated proteins. Here we show for the first time that human telomeric proteins can be identified on the basis of their homology with those that protect the telomeres of the fruit fly Drosophila melanogaster. Although flies and humans elongate their telomeres through different mechanisms, our studies suggested that a subset of Drosophila telomere-associated proteins have conserved human counterparts. Based on this hypothesis we identified and characterized a novel human telomeric protein called AKTIP. We show that AKTIP binds the components of the shelterin multiprotein complex, which caps and protects the human telomeres. AKTIP-depleted chromosomes exhibit an accumulation of DNA repair factors at their ends (telomere dysfunction foci), which are diagnostic of telomere damage. Loss of AKTIP results in a general impairment of DNA synthesis and in defective telomere replication. Collectively, our results indicate that AKTIP cooperates with the shelterin component TRF1 to ensure proper telomere replication.

Growing evidence shows that improper intake of vitamin B6 increases cancer risk and several studies indicate that diabetic patients have a higher risk of developing tumors. We previously demonstrated that in Drosophila the deficiency of Pyridoxal 5′ phosphate (PLP), the active form of vitamin B6, causes chromosome aberrations (CABs), one of cancer prerequisites, and increases hemolymph glucose content. Starting from these data we asked if it was possible to provide a link between the aforementioned studies. Thus, we tested the effect of low PLP levels on DNA integrity in diabetic cells. To this aim we generated two Drosophila models of type 2 diabetes, the first by impairing insulin signaling and the second by rearing flies in high sugar diet. We showed that glucose treatment induced CABs in diabetic individuals but not in controls. More interestingly, PLP deficiency caused high frequencies of CABs in both diabetic models demonstrating that hyperglycemia, combined to reduced PLP level, impairs DNA integrity. PLP-depleted diabetic cells accumulated Advanced Glycation End products (AGEs) that largely contribute to CABs as α-lipoic acid, an AGE inhibitor, rescued not only AGEs but also CABs. These data, extrapolated to humans, indicate that low PLP levels, impacting on DNA integrity, may be considered one of the possible links between diabetes and cancer.

AKTIP is a shelterin-interacting protein required for replication of telomeric DNA. Here, we show that AKTIP biochemically interacts with A- and B-type lamins and affects lamin A, but not lamin C or B, expression. In interphase cells, AKTIP localizes at the nuclear rim and in discrete regions of the nucleoplasm just like lamins. Double immunostaining revealed that AKTIP partially co-localizes with lamin B1 and lamin A/C in interphase cells, and that proper AKTIP localization requires functional lamin A. In mitotic cells, AKTIP is enriched at the spindle poles and at the midbody of late telophase cells similar to lamin B1. AKTIP-depleted cells show senescence-associated markers and recapitulate several aspects of the progeroid phenotype. Collectively, our results indicate that AKTIP is a new player in lamin-related processes, including those that govern nuclear architecture, telomere homeostasis and cellular senescence.

The nuclear envelope compartmentalizes chromatin in eukaryotic cells. The main nuclear envelope components are lamins that associate with a panoply of factors, including the LEM domain proteins. Th...