The RNA-binding motif protein 14 regulates telomere integrity at the interface of TERRA and telomeric R-loops.

Telomeric repeat-containing RNA (TERRA) and its formation of RNA:DNA hybrids (or TERRA R-loops), influence telomere maintenance, particularly in human cancer cells that use homologous recombination-mediated alternative lengthening of telomeres. Here, we report that the RNA-binding motif protein 14 (RBM14) is associated with telomeres in human cancer cells. RBM14 negatively regulates TERRA expression. It also binds to TERRA and inhibits it from forming TERRA R-loops at telomeres. RBM14 depletion has several effects, including elevated TERRA levels, telomeric R-loops, telomere dysfunction-induced DNA damage foci formation, particularly in the presence of DNA replication stress, pRPA32 accumulation at telomeres and telomere signal-free ends. Thus, RBM14 protects telomere integrity via modulating TERRA levels and its R-loop formation at telomeres.

Boosting NAD ameliorates hematopoietic impairment linked to short telomeres in vivo.

Short telomeres are a defining feature of telomere biology disorders (TBDs), including dyskeratosis congenita (DC), for which there is no effective general cure. Patients with TBDs often experience bone marrow failure. NAD, an essential metabolic coenzyme, is decreased in models of DC. Herein, using telomerase reverse transcriptase null (Tert-/-) mice with critically short telomeres, we investigated the effect of NAD supplementation with the NAD precursor, nicotinamide riboside (NR), on features of health span disrupted by telomere impairment. Our results revealed that NR ameliorated body weight loss in Tert-/- mice and improved telomere integrity and telomere dysfunction-induced systemic inflammation. NR supplementation also mitigated myeloid skewing of Tert-/- hematopoietic stem cells. Furthermore, NR alleviated villous atrophy and inflammation in the small intestine of Tert-/- transplant recipient mice. Altogether, our findings support NAD intervention as a potential therapeutic strategy to enhance aspects of health span compromised by telomere attrition.

Aberrant expression and localization of the RAP1 shelterin protein contribute to age-related phenotypes.

Short telomeres induce a DNA damage response (DDR) that evokes apoptosis and senescence in human cells. An extant question is the contribution of telomere dysfunction-induced DDR to the phenotypes observed in aging and telomere biology disorders. One candidate is RAP1, a telomere-associated protein that also controls transcription at extratelomeric regions. To distinguish these roles, we generated a knockin mouse carrying a mutated Rap1, which was incapable of binding telomeres and did not result in eroded telomeres or a DDR. Primary Rap1 knockin embryonic fibroblasts showed decreased RAP1 expression and re-localization away from telomeres, with an increased cytosolic distribution akin to that observed in human fibroblasts undergoing telomere erosion. Rap1 knockin mice were viable, but exhibited transcriptomic alterations, proinflammatory cytokine/chemokine signaling, reduced lifespan, and decreased healthspan with increased body weight/fasting blood glucose levels, spontaneous tumor incidence, and behavioral deficits. Taken together, our data present mechanisms distinct from telomere-induced DDR that underlie age-related phenotypes.

A dual function for canonical Wnt/ß-catenin signaling in the developing mammalian cochlea.

The canonical Wnt/ß-catenin signaling pathway is known to play crucial roles in organogenesis by regulating both proliferation and differentiation. In the inner ear, this pathway has been shown to regulate the size of the otic placode from which the cochlea will arise; however, direct activity of canonical Wnt signaling as well as its function during cochlear mechanosensory hair cell development had yet to be identified. Using TCF/Lef:H2B-GFP reporter mice and transfection of an independent TCF/Lef reporter construct, we describe the pattern of canonical Wnt activity in the developing mouse cochlea. We show that prior to terminal mitosis, canonical Wnt activity is high in early prosensory cells from which hair cells and support cells will differentiate, and activity becomes reduced as development progresses. Using an in vitro model we demonstrate that Wnt/ß-catenin signaling regulates both proliferation and hair cell differentiation within the developing cochlear duct. Inhibition of Wnt/ß-catenin signaling blocks proliferation during early mitotic phases of development and inhibits hair cell formation in the differentiating organ of Corti. Conversely, activation increases the number of hair cells that differentiate and induces proliferation in prosensory cells, causing an expansion of the Sox2-positive prosensory domain. We further demonstrate that the induced proliferation of Sox2-positive cells may be mediated by the cell cycle regulator cyclin D1. Lastly, we provide evidence that the mitotic Sox2-positive cells are competent to differentiate into hair cells. Combined, our data suggest that Wnt/ß-catenin signaling has a dual function in cochlear development, regulating both proliferation and hair cell differentiation.

Regulation of cell fate and patterning in the developing mammalian cochlea.

PURPOSE OF REVIEW: A significant proportion of hearing loss and deafness is caused by defects in the structure or function of cells within the organ of Corti. Identification of the molecular factors that regulate the development of this structure should provide valuable insights regarding inner ear formation and the signaling pathways that underlie congenital auditory deficits. In addition, targeted modulation of these same factors could be developed as therapies for hair cell regeneration. RECENT FINDINGS: Results from experiments using transgenic and mutant mice, as well as in-vitro techniques, have identified genes and signaling pathways that are required to either specify unique auditory cell types, such as hair cells or supporting cells, or to generate the highly ordered cellular pattern that is characteristic for the organ of Corti. In particular, the hedgehog and fibroblast growth factor signaling pathways modulate the formation of the progenitor cells that will give rise to the organ of Corti. SRY-box containing gene 2, a transcription factor that is required for the formation of the cochlear progenitor cell population, has paradoxically been shown to also act as an inhibitor of hair cell development. Finally, the motor protein myosin II regulates extension of the organ of Corti and the alignment of hair cells and supporting cells into ordered rows. SUMMARY: A better understanding of the signaling pathways that direct different aspects of cochlear development, such as specific of cell fates or cellular patterning, offers the potential to identify new pathways or molecules that could be targeted for therapeutic interventions.