Many cellular stresses activate senescence, a persistent hyporeplicative state characterized in part by expression of the p16INK4a cell cycle inhibitor. Senescent cell production occurs throughout life and plays beneficial roles in a variety of physiological and pathological processes including embryogenesis, wound healing, host immunity and tumor suppression. Meanwhile, the steady accumulation of senescent cells with age also has adverse consequences. These non-proliferating cells occupy key cellular niches and elaborate pro-inflammatory cytokines, contributing to aging-related diseases and morbidity. This model suggests that the abundance of senescent cells in vivo predicts ‘molecular’, as opposed to chronologic, age, and that senescent cell clearance may mitigate aging-associated pathology.

Mammalian ageing is associated with reduced regenerative capacity in tissues that contain stem cells 1,2 . It has been proposed that this is at least partially caused by the senescence of progenitors with age 3,4 ; however, it has not yet been tested whether genes associated with senescence functionally contribute to physiological declines in progenitor activity. Here we show that progenitor proliferation in the subventricular zone and neurogenesis in the olfactory bulb, as well as multipotent progenitor frequency and self-renewal potential, all decline with age in the mouse forebrain. These declines in progenitor frequency and function correlate with increased expression of p16 INK4a , which encodes a cyclin-dependent kinase inhibitor linked to senescence 5 . Ageing p16 INK4a -deficient mice showed a significantly smaller decline in subventricular zone proliferation, olfactory bulb neurogenesis, and the frequency and self-renewal potential of multipotent progenitors. p16 INK4a deficiency did not detectably affect progenitor function in the dentate gyrus or enteric nervous system, indicating regional differences in the response of neural progenitors to increased p16 INK4a expression during ageing. Declining subventricular zone progenitor function and olfactory bulb neurogenesis during ageing are thus caused partly by increasing p16 INK4a expression.Stem cells must persist throughout adult life in numerous tissues, including the central nervous system (CNS) 6 , in order to replace the mature cells that are lost to turnover, injury, or disease. However, the function of stem cells and other progenitors declines with age in diverse tissues including the haematopoietic system 7-9 , muscle 10,11 and brain 6,12,13 . Consistent with this, ageing tissues exhibit reduced repair capacity and an increased incidence of degenerative disease 1,4 . However, the mechanisms responsible for the age-related decline in the function of stem cells and other progenitors remain uncertain.

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Centrosome- and cilia-associated proteins play crucial roles in establishing polarity and regulating intracellular transport in post-mitotic cells. Using genetic mapping and positional candidate strategy, we have identified an in-frame deletion in a novel centrosomal protein CEP290 (also called NPHP6), leading to early-onset retinal degeneration in a newly identified mouse mutant, rd16. We demonstrate that CEP290 localizes primarily to centrosomes of dividing cells and to the connecting cilium of retinal photoreceptors. We show that, in the retina, CEP290 associates with several microtubule-based transport proteins including RPGR, which is mutated in approximately 15% of patients with retinitis pigmentosa. A truncated CEP290 protein (DeltaCEP290) is detected in the rd16 retina, but in considerably reduced amounts; however, the mutant protein exhibits stronger association with specific RPGR isoform(s). Immunogold labeling studies demonstrate the redistribution of RPGR and of phototransduction proteins in the photoreceptors of rd16 retina. Our findings suggest a critical function for CEP290 in ciliary transport and provide insights into the mechanism of early-onset photoreceptor degeneration.

Stem cells are proposed to segregate chromosomes asymmetrically during self-renewing divisions so that older ('immortal') DNA strands are retained in daughter stem cells whereas newly synthesized strands segregate to differentiating cells. Stem cells are also proposed to retain DNA labels, such as 5-bromo-2-deoxyuridine (BrdU), either because they segregate chromosomes asymmetrically or because they divide slowly. However, the purity of stem cells among BrdU-label-retaining cells has not been documented in any tissue, and the 'immortal strand hypothesis' has not been tested in a system with definitive stem cell markers. Here we tested these hypotheses in haematopoietic stem cells (HSCs), which can be highly purified using well characterized markers. We administered BrdU to newborn mice, mice treated with cyclophosphamide and granulocyte colony-stimulating factor, and normal adult mice for 4 to 10 days, followed by 70 days without BrdU. In each case, less than 6% of HSCs retained BrdU and less than 0.5% of all BrdU-retaining haematopoietic cells were HSCs, revealing that BrdU has poor specificity and poor sensitivity as an HSC marker. Sequential administration of 5-chloro-2-deoxyuridine and 5-iodo-2-deoxyuridine indicated that all HSCs segregate their chromosomes randomly. Division of individual HSCs in culture revealed no asymmetric segregation of the label. Thus, HSCs cannot be identified on the basis of BrdU-label retention and do not retain older DNA strands during division, indicating that these are not general properties of stem cells.

Genetic variants at chromosomes 1q31-32 and 10q26 are strongly associated with susceptibility to age-related macular degeneration (AMD), a common blinding disease of the elderly. We demonstrate, by evaluating 45 tag SNPs spanning HTRA1, PLEKHA1, and predicted gene LOC387715/ARMS2, that rs10490924 SNP alone, or a variant in strong linkage disequilibrium, can explain the bulk of association between the 10q26 chromosomal region and AMD. A previously suggested causal SNP, rs11200638, and other examined SNPs in the region are only indirectly associated with the disease. Contrary to previous reports, we show that rs11200638 SNP has no significant impact on HTRA1 promoter activity in three different cell lines, and HTRA1 mRNA expression exhibits no significant change between control and AMD retinas. However, SNP rs10490924 shows the strongest association with AMD (P ‫؍‬ 5.3 ؋ 10 ؊30 ), revealing an estimated relative risk of 2.66 for GT heterozygotes and 7.05 for TT homozygotes. The rs10490924 SNP results in nonsynonymous A69S alteration in the predicted protein LOC387715/ARMS2, which has a highly conserved ortholog in chimpanzee, but not in other vertebrate sequences. We demonstrate that LOC387715/ARMS2 mRNA is detected in the human retina and various cell lines and encodes a 12-kDa protein, which localizes to the mitochondrial outer membrane when expressed in mammalian cells. We propose that rs10490924 represents a major susceptibility variant for AMD at 10q26. A likely biological mechanism is that the A69S change in the LOC387715/ARMS2 protein affects its presumptive function in mitochondria.aging ͉ genetic association ͉ mitochondria ͉ neurodegeneration ͉ retinal disease

Nephronophthisis (NPHP) is the most frequent genetic cause of chronic renal failure in children. Identification of four genes mutated in NPHP subtypes 1-4 (refs. 4-9) has linked the pathogenesis of NPHP to ciliary functions. Ten percent of affected individuals have retinitis pigmentosa, constituting the renal-retinal Senior-Loken syndrome (SLSN). Here we identify, by positional cloning, mutations in an evolutionarily conserved gene, IQCB1 (also called NPHP5), as the most frequent cause of SLSN. IQCB1 encodes an IQ-domain protein, nephrocystin-5. All individuals with IQCB1 mutations have retinitis pigmentosa. Hence, we examined the interaction of nephrocystin-5 with RPGR (retinitis pigmentosa GTPase regulator), which is expressed in photoreceptor cilia and associated with 10-20% of retinitis pigmentosa. We show that nephrocystin-5, RPGR and calmodulin can be coimmunoprecipitated from retinal extracts, and that these proteins localize to connecting cilia of photoreceptors and to primary cilia of renal epithelial cells. Our studies emphasize the central role of ciliary dysfunction in the pathogenesis of SLSN.

It is unclear whether neurogenesis occurs in the adult mammalian enteric nervous system (ENS). Neural crestderived cells capable of forming multilineage colonies in culture, and neurons and glia upon transplantation into chick embryos, persist throughout adult life in the mammalian ENS. In this study we sought to determine the physiological function of these cells. We discovered that these cells could be identified based on CD49b expression and that they had characteristics of enteric glia, including p75, GFAP, S100B, and SOX10 expression. To test whether new neurons or glia arise in the adult gut under physiological conditions, we marked dividing progenitors with a thymidine analog in rodents under steady-state conditions, or during aging, pregnancy, dietary changes, hyperglycemia, or exercise. We also tested gut injuries including inflammation, irradiation, benzalkonium chloride treatment, partial gut stenosis, and glial ablation. We readily observed neurogenesis in a neurogenic region of the central nervous system, but not reproducibly in the adult ENS. Lineage tracing of glial cells with GFAP-Cre and GFAP-CreERT2 also detected little or no adult ENS neurogenesis. Neurogenesis in the adult gut is therefore very limited under the conditions we studied. In contrast, ENS gliogenesis was readily observed under steady-state conditions and after injury. Adult enteric glia thus have the potential to form neurons and glia in culture but are fated to form mainly glia under physiological conditions and after the injuries we studied.