Skeletal phenotypes and molecular mechanisms in aging mice

Aging is an inevitable physiological process,often accompanied by age-related bone loss and subsequent bone-related diseases that pose serious health risks.Research on skeletal diseases caused by aging in humans is challenging due to lengthy study durations,difficulties in sampling,regional variability,and substantial investment.Consequently,mice are preferred for such studies due to their similar motor system structure and function to humans,ease of handling and care,low cost,and short generation time.In this review,we present a comprehensive overview of the characteristics,limitations,applicability,bone phenotypes,and treatment methods in naturally aging mice and prematurely aging mouse models(including SAMP6,POLG mutant,LMNA,SIRT6,ZMPSTE24,TFAM,ERCC1,WERNER,and KL/KL-deficient mice).We also summarize the molecular mechanisms of these aging mouse models,including cellular DNA damage response,senescence-related secretory phenotype,telomere shortening,oxidative stress,bone marrow mesenchymal stem cell(BMSC)abnormalities,and mitochondrial dysfunction.Overall,this review aims to enhance our understanding of the pathogenesis of aging-related bone diseases.

Star fruit extract and C-glycosylated flavonoid components have potential to prevent air pollutant-induced skin inflammation and premature aging

Air pollution adversely affects skin,leading to skin inflammation and premature skin aging.Plant derived antioxidant compounds have been considered to be promising in discovery of effective agents for the protection of skin from the damage by air pollutants.Our previous studies demonstrated that Averrhoa carambola fruit(known as star fruit)is rich in flavonoid C-glycosides with unique structures and potent antioxidant activity.Thus,the star fruit extract(SFE)and main flavonoid C-glycoside components,carambolasides I,J,and P(1-3),carambolaflavone B(4),and isovitexin 2″-O-α-l-rhamnoside(5),were investigated for the activity against air pollutant stress in human epidermis.As a result,SFE and compounds 1-5 exhibited significant inhibitory activity against protein carbonylation in oxidative-stressed stratum corneum with the best activity being shown by compound 3.SFE and compounds 2-5 were also active against engine exhaust-induced protein carbonylation in stratum corneum.When further evaluated,SFE and com-pound 3 significantly inhibited gene expression of the key inflammation mediators IL-1αand COX-2 in PM-stressed keratinocytes.The results indicated that SFE and the flavonoid C-glycosides are potentially effective against air pollutant-induced skin inflammation and premature aging.

The Concurrence of Hypercholesterolemia and Aging Promotes DNA Damage in Apolipoprotein E-Deficient Mice

Recent evidence shows that increased oxidative stress and aging contribute to DNA damage in various cardiovascular diseases such as lipid disorders and atherosclerosis. In the present study, we used the comet assay to evaluate the influ- ence of aging on DNA damage in whole blood cells from apolipoprotein E-deficient (apoE?/?) mice and compared the results to those found in cells from wild-type C57BL/6 (C57) mice. Using the alkaline comet assay and fluorescent ethidium bromide staining, DNA damage was analyzed in the peripheral whole blood (5 μL) cells that were isolated from either young (8-week-old) and elderly (72-week-old) apoE?/? mice or from age-matched C57 mice. The levels of total plasma cholesterol were approximately 6-fold higher in apoE?/? mice of both ages compared to C57 mice. Elderly apoE?/? mice showed significantly higher levels of DNA damage (19%) compared to elderly C57 mice (8%, p < 0.01) and young apoE?/? mice (10%, p < 0.01). The comet assay in whole blood cells is a suitable technique for the detection of DNA damage in the apoE?/? mouse;it is an easy, rapid, inexpensive and sensitive method. The novelty of this study is that DNA damage occurring in whole blood cells of this murine model requires the concurrence of aging and oxida- tive stress-related hypercholesterolemia.

Differences in the Histopathology and Cytokine Expression Pattern between Chronological Aging and Photoaging of Hairless Mice Skin

Skin photoaging is a complex, multifactorial process resulting in functional and structural changes of the skin, and different phenotypes from chronological skin aging are well-recognized. Ultraviolet (UV)-irradiated hairless mice have been used as a skin photoaging animal model. However, differences in morphology and gene expression patterns between UV-induced and chronological skin changes in this mouse model have not been fully elucidated. Here we investigated differences in histopathology and cytokine expression between UV-irradiated and non-irradiated aged hairless mice to clarify the factor(s) that differentiate photoaging from chronological skin aging phenotypes. Eight-week-old HR-1 hairless mice were divided into UV-irradiated (UV-irradiated mice) and non-irradiated (control mice) groups. Irradiation was performed three times per week for 10 weeks. In addition, 30-week-old HR-1 hairless mice were reared until 70 weeks of age without UV irradiation (aged mice). Histopathologies revealed that the flattening of dermal-epidermal junctions and epidermal thickening were observed only in UV-irradiated mice. Decreases in fine elastic fibers just beneath the epidermis, the thickening of elastic fibers in the reticular dermis, and the accumulation of glycosaminoglycans were more prominent in UV-irradiated mice as compared to non-irradiated aged mice. Quantitative PCR analyses revealed that UV-irradiated mice showed an increase in the expression of IFN-γ. In contrast, aged mice exhibited proportional up-regulation of both pro-inflammatory and anti-inflammatory cytokines. The IFN-γ/IL-4 ratio, an indicator for the balance of pro-inflammatory and anti-inflammatory cytokines, was significantly higher in UV-irradiated mice as compared to control and non-irradiated aged mice. An elevated IFN-γ/IL-4 ratio was also observed in aged senescence-accelerated mouse-prone 1 (SAMP1) mice, a spontaneous skin photoaging model we recently reported. Thus, an imbalance between pro-inflammatory and anti-inflammatory cytokines might be a key factor to differentiate photoaged skin from chronologically-aged skin.

Paraquat induces selective dopaminergic nigrostriatal degeneration in aging C57BL/6 mice

Background Paraquat （PQ; 1,1 ＇-dimethyl-4,4＇-bipyridinium）, a widely used herbicide that is structurally similar to the known dopaminergic neurotoxicant MPTP （1-methyl-l, 2, 3,6-tetrahydropyridine ）, has been suggested as a potential etiologic factor for the development of Parkinson＇ s disease （PD）. Aging is an accepted risk factor for idiopathic Parkinson＇ s disease. The aim of this study was to test the hypothesis that paraquat could induce PD-like nigrostriatal dopaminergic degeneration in aging C57BL/6 mice. Methods Senile male C57BL/6 mice were intraperitoneally injected with either saline or PQ at 2-day intervals for a total of 10 doses. Locomotor activity and performance on the pole test were measured 7 days after the last injection and animals were sacrificed one day later. Level of dopamine （DA） and its metabolites levels in the striatum were measured by high-performance liquid chromatography with an electrochemical detector （ HPLCECD）, and numbers of tyrosine hydroxylase （TH） positive neurons were estimated using immunohistochemistry. Results Locomotor activities were significantly decreased and the behavioral performance on the pole test were significantly impaired in the PQ treated group. Level of DA and its metabolites levels in the striatum were declined by 8 days after the last injection. Immunohistochemical analyses showed that PQ was associated with a reduction in numbers of tyrosine hydroxylase positive neurons. Conclusions Long-term repeated exposes to PQ can selectively impair the nigrostriatal dopaminergic system of senile mice, suggesting that PQ could play an important role in the pathogenesis of Parkinson＇ s disease （PD）. Our results also validate a novel model of PD induced by exposure to a toxic environmental agent.

Maternal zinc deficiency impairs brain nestin expression in prenatal and postnatal mice

Effects of maternal dietary zinc deficiency on prenatal and postnatal brain development were investigated in ICR strain mice. From d 1 of pregnancy (E0) until postnatal d 20 (P20), maternal mice were fed experimental diets that contained 1 mg Zn/kg/day (severe zinc deficient, SZD), 5 mg Zn/kg/day (marginal zinc deficient, MZD), 30 mg Zn/kg/day (zinc adequately supplied, ZA) or 100 mg Zn/kg/day (zinc supplemented, ZS and pair-fed, PF). Brains of offspring from these dietary groups were examined at various developmental stages for expression of nestin, an intermediate filament protein found in neural stem cells and young neurons. Immunocytochemistry showed nestin expression in neural tube 10.5 d post citrus (dpc) as well as in the cerebral cortex and neural tube from 10.5 dpc to postnatal d 10 (P10). Nestin immunoreactivities in both brain and neural tube of those zinc-supplemented control groups (ZA, ZS, PF) were stronger than those in zinc-deficient groups (SZD and MZD). Western blot analysis confirmed that nestin levels in pooled brain extracts from each of the zinc-supplemented groups (ZA, ZS, PF) were much higher than those from the zinc-deficient groups (SZD and MZD) from 10.5 dpc to P10. Immunostaining and Western blots showed no detectable nestin in any of the experimental and control group brains after P20. These observations of an association between maternal zinc deficiency and decreased nestin protein levels in brains of offspring suggest that zinc deficiency suppresses development of neural stem cells, an effect which may lead to neuroanatomical and behavioral abnormalities in adults.

SIRTain regulators of premature senescence and accelerated aging

The sirtuin proteins constitute class III histone deacetylases （HDACs）. These evolutionarily conserved NAD＋-dependent enzymes form an important component in a variety of cellular and biological processes with highly divergent as well as convergent roles in maintaining metabolic homeostasis, safeguarding genomic integrity, regulating cancer metabolism and also inflammatory responses. Amongst the seven known mammalian sirtuin proteins, SIRT1 has gained much attention due to its widely acknowledged roles in pro- moting longevity and ameliorating age-associated pathologies. The contributions of other sirtuins in the field of aging are also gradually emerging. Here, we summarize some of the recent discoveries in sirtuins biology which clearly implicate the functions of sirtuin proteins in the regulation of premature cellular senescence and accelerated aging. The roles of sirtuins in various cellular processes have been extrapolated to draw inter-linkage with anti-aging mechanisms. Also, the latest findings on sirtuins which might have potential effects in the process of aging have been reviewed.

Synaptic aging disrupts synaptic morphology and function in cerebellar Purkinje cells

Synapses are key structures in neural networks,and are involved in learning and memory in the central nervous system.Investigating synaptogenesis and synaptic aging is important in understanding neural development and neural degeneration in diseases such as Alzheimer disease and Parkinson’s disease.Our previous study found that synaptogenesis and synaptic maturation were harmonized with brain development and maturation.However,synaptic damage and loss in the aging cerebellum are not well understood.This study was designed to investigate the occurrence of synaptic aging in the cerebellum by observing the ultrastructural changes of dendritic spines and synapses in cerebellar Purkinje cells of aging mice.Immunocytochemistry,Di I diolistic assays,and transmission electron microscopy were used to visualize the morphological characteristics of synaptic buttons,dendritic spines and synapses of Purkinje cells in mice at various ages.With synaptic aging in the cerebellum,dendritic spines and synaptic buttons were lost,and the synaptic ultrastructure was altered,including a reduction in the number of synaptic vesicles and mitochondria in presynaptic termini and smaller thin specialized zones in pre-and post-synaptic membranes.These findings confirm that synaptic morphology and function is disrupted in aging synapses,which may be an important pathological cause of neurodegenerative diseases.

Tibolone modulates neuronal plasticity through regulating Tau, GSK3β/Akt/PI3K pathway and CDK5 p35/p25 complexes in the hippocampus of aged male mice

Aging is a key risk factor for cognitive decline and age-related neurodegenerative disorders. Also, an age-related decrease in sex steroid hormones may have a negative impact on the formation of neurofibrillary tangles （NFTs）; these hormones can regulate Tau phosphorylation and the principal kinase GSK3β involved in this process. Hormone replacement therapy decreases NFTs, but it increases the risk of some types of cancer. However, other synthetic hormones such as tibolone （TIB） have been used for hormone replacement therapy. The aim of this work was to evaluate the long-term effects of TIB （0.01 mg/kg and 1mg/kg, intragastrically for 12 weeks） on the content of total and hyperphosphorylated Tau （PHF-1） proteins and the regulation of GSK3β/Akt/PI3K pathway and CDK5/p35/p25 complexes in the hippocampus of aged male mice. We observed that the content of PHF-1 decreased with TIB administration. In contrast, no changes were observed in the active form of GSK3β or PI3K. TIB decreased the expression of the total and phosphorylated form of Akt while increased that of p110 and p85. The content of CDK5 was differentially modified with TIB： it was increased at low doses and decreased at high doses. When we analyzed the content of CDK5 activators, an increase was found on p35; however, the content of p25 decreased with administration of low dose of TIB. Our results suggest a possible mechanism of action of TIB in the hippocampus of aged male mice. Through the regulation of Tau and GSK3β/Akt/PI3K pathway, and CDK5/p35/p25 complexes, TIB may modulate neuronal plasticity and regulate learning and memory processes.

Can outbred mice be used as a mouse model of mild cognitive impairment?

Deficits in spatial learning and memory are some of the earliest symptoms in mild cognitive impairment （MCI）. However, there are few valid MCI animal models available to evaluate putative therapeutic strategies. The aim of this study was to obtain a natural animal model of MCI. Outbred Kunming （aged 5 and 12.5 months） and ICR （7 and 12 months） mice were utilized in the present study. Morris water maze and radial six-arm water maze （RAWM） were simultaneously used to evaluate impaired spatial learning and memory in middle-aged mice （approximately 12 months of age）. Compared with younger mice in the respective groups, the middle-aged mice suffered visible impairment of spatial memory in the Morris water maze and RAWM, and mild spatial learning deficiency occurred in the RAWM study alone. Thus outbred Kunming and ICR mice could be utilized as a natural animal model for MCI, in particular for memory impairment studies.

Interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints during aging

The interfacial microstructure evolution of 12Cr1MoV/TP347H dissimilar steel welded joints with a nickel-based filler metal during aging was studied in detail to elucidate the mechanism of premature failures of this kind of joints.The results showed that not only a band of granular Cr_(23)C_(6)carbides were formed along the fusion boundary in the ferritic steel during aging,but also a large number of granular or plate-like Cr_(23)C_(6)carbides,which have a cube-cube orientation relationship with the matrix,were also precipitated on the weld metal side of the fu-sion boundary,making this zone be etched more easily than the other zone and become a dark etched band.Stacking faults were found in some Cr_(23)C_(6)carbides.In the as-welded state,deformation twins were observed in the weld metal with a fully austenitic structure.The peak micro-hardness was shifted from the ferritic steel side to the weld metal side of the fusion boundary after aging and the peak value increased signific-antly.Based on the experimental results,a mechanism of premature failures of the joints was proposed.

Age-related Changes in Familial Hypertrophic Cardiomyopathy Phenotype in Transgenic Mice and Humans

β-myosin heavy chain mutations are the most frequently identified basis for hypertrophic cardiomyopathy （HCM）. A transgenic mouse model （αMHC403） has been extensively used to study various mechanistic aspects of HCM. There is general skepticism whether mouse and human disease features are similar. Herein we compare morphologie and functional characteristics, and disease evolu- tion, in a transgenic mouse and a single family with a MHC mutation. Ten male αWHC403 transgenic mice （at -5 weeks, -12 weeks, and -24 weeks） and 10 HCM patients from the same family with a β-myosin heavy chain mutation were enrolled. Morphometric, conventional echocardiographic, tissue Doppler and strain analytic characteristics of transgenic mice and HCM patients were assessed. Ten male transgenic mice （αMHC403） were examined at ages -5 weeks, -12 weeks, and -24 Weeks. In the transgenic mice, aging was associated with a significant increase in septal （0.59±0.06 vs. 0.64±-0.05 vs. 0.69±0.11 mm, P〈0.01） and anterior wall thickness （0.58±0.1 vs. 0.62±0.07 vs. 0.80-1-0.16 mm, P〈0.001）, which was coincident with a significant decrease in circumferential strain （-22%=1=4% vs. -20%-4-3% vs. -19%-4-3%, P=0.03）, global longitudinal strain （-19%-4-3% vs. -17%-4-2% vs. -16%±3%, P=0.001） and E/A ratio （1.9±0.3 vs. 1.7-4-0.3 vs. 1.4-4-0.3, P=0.01）. The HCM patients were classified into 1st generation （n=6; mean age 534-6 years）, and 2nd generation （n=4; mean age 32＋8 years）. Septal thickness （2.2±0.9 vs. 1.4±0.1 cm, P〈0.05）, left atrial （LA） volume （62±16 vs. 41±5 mL, P=0.03）, E/A ratio （0.77±0.21 vs. 1.1±0.1, P=0.01）, E/e＇ ratio （25±10 vs. 12±2, P=0.03）, global left ventricular （LV） strain （-14%±3% vs. -20%±3%, P=0.01） and global LV early diastolic strain rate （0.76±0.17 s1 vs. 1.3±0.2 s-1, P=0.01） were significantly worse in the older generation. In β-myosin heavy chain muta- tions, transgenic mice and humans have similar progression in morphologic and functional abnormali- ties. The αMHC4±3 transgenic mouse model closely recapitulates human disease.

Global view of transcriptome in the brains of aged NR2B transgenic mice

NR2B subunits are involved in regulating aging, in particular, age-related learning and memory deficits. We examined 19-month-old NR2B transgenic mice and their littermate controls. First, we detected expression of the NR2B subunit gene, Grin2b, in the neocortex of transgenic mice using real-time PCR. Next, we used microarrays to examine differences in neocortical gene expression. Pathway and signal-net analyses identified multiple pathways altered in the transgenic mice, in-cluding the P53, Jak-STAT, Wnt, and Notch pathways, as well as regulation of the actin cytoskeleton and neuroactive ligand-receptor interactions. Further signal-net analysis highlighted the P53 and insulin-like growth factor pathways as key regulatory pathways. Our results provide new insight into understanding the molecular mechanisms of NR2B regulated age-related memory storage, normal organismal aging and age-related disease.

The Murine (C3H/He) Epidermal la^+ Dendritic Cells (la^+ DECs), Thy-1^+ Dendritic Cells (Thy-1^+DECs) and Aging

Identification and enumeration of both dendritic Ia^+ epi-dermal cells (Ia^+DECs) and dendritic Thy-1^+ epidermalcells (Thy-1^+DECa) from various parts of the body wereexamined by using epidermal sheets of C3H/He inbred miceof different age groups and indirect immunofluorescent tech-nique. A significant decline of both Ia^+DECs and Thy-1^+DECs in the mice of the aged group was demonstrated anddifferent densities and different distribution patterns betweenIa^+DECs and Thy-1^+DECs were obserged. These findingsmay imply that the decline of both Ia^+DECs and Thy-1^+DECs in the aged group may reflect the alterations of im-mune response in aging.

Effect of SOD1 Overexpression on the 20S Proteasome during Aging

Metabolism of oxygen derivatives has been shown to be altered in Down syndrome (DS) due to the overexpression of the Cu/Zn superoxide dismutase gene ( SOD-1) on chromosome 21. Transgenic mice for the human SOD1 gene (h SOD1) exhibit some features of the syndrome. Oxidation of proteins and oxidative stress are involved in normal and pathological aging. The proteasome is an adaptative system to eliminate the modified proteins which can be deleterious. As SOD1 overexpression has been shown to be either deleterious or protective according to tissues and paradigms, we have measured in function of age the 20S proteasome activities in neural tissues (cerebral hemisphere, cerebellum and cortex) and in the thymus and the heart from control and transgenic mice. Indeed, although SOD1 overexpression is very deleterious in thymus and heart, it has little effect in cerebral hemisphere and cortex depending on the proteolytic activity measured. Conversely in the cerebellum the three proteolytic activities decrease dramatically in transgenic old mice while it was not modified in control mice during aging. The results of this study suggest that some phenotypes of DS present in thymus, heart and neural tissues of h SOD1 transgenic mice might be partially due to the modulation of the 20S proteasome expression during aging.

Effect of aging on stem cells

Pluripotent stem cells have the remarkable self-renewal ability and are capable of differentiating into multiple diverse cells. There is increasing evidence that the aging process can have adverse effects on stem cells. As stem cells age, their renewal ability deteriorates and their ability to differentiate into the various cell types is altered. Accordingly, it is suggested aging-induced deterioration of stem cell functions may play a key role in the pathophysiology of the various aging-associated disorders. Understanding the role of the aging process in deterioration of stem cell function is crucial, not only in understanding the pathophysiology of agingassociated disorders, but also in future development of novel effective stem cell-based therapies to treat agingassociated diseases. This review article first focuses on the basis of the various aging disease-related stem cell dysfunction. It then addresses the several concepts on the potential mechanism that causes aging-related stem cell dysfunction. It also briefly discusses the current potential therapies under development for aging-associated stem cell defects.

Effects of chitooligosaccharide-zinc on the ovarian function of mice with premature ovarian failure via the SESN2/NRF2 signaling pathway

Chitooligosaccharide-zinc(COS·Zn)is a powerful anti-oxidant and anti-aging scavenger,whose anti-oxidative ability immensely exceeds vitamin C.Therefore,this study was aimed to investigate the protective effects of COS·Zn against premature ovarian failure(POF)and potential mechanisms.Female KM adult mice were divided into the following groups:a treatment group(150 mg·kg^(−1)·d^(−1) COS·Zn),a treatment group(300 mg·kg^(−1)·d^(−1) COS·Zn),a prevention group,two control groups and two CY/BUS groups.COS·Zn(150,300 mg·kg^(−1)·d^(−1))and COS·Zn(300 mg·kg^(−1)·d^(−1))were therapeutically and preventatively administered to POF mice in the treatment and prevention studies,respectively.All the groups were administered for 21 days.Fewer primary and secondary follicles were observed in the COS·Zn-treated groups(including the treatment and prevention groups)than those of the control groups.Meanwhile,the ovarian index and the levels of FSH and LH notably increased in the treatment and prevention groups compared with those in the CY/BUS group.The levels of MVH,OCT4 and PCNA in the treatment group(300·kg^(−1)·d^(−1) COS·Zn)and MVH in the prevention group remarkably increased compared with those in the CY/BUS groups.Meanwhile,the levels of P53 and P16 protein were down-regulated in the treatment and prevention groups compared with those in the CY/BUS groups.Additionally,the amounts of Sestrin2(SESN2)and SOD2 protein were obviously higher in the treatment group(150 mg·kg^(−1)·d^(−1) COS·Zn)than those in the CY/BUS groups.Similarly,the amounts of NRF2 and SESN2 protein were up-regulated in the prevention group.Besides,an increased GSH level was observed in the two treatment groups,compared with that in the CY/BUS groups,and the same trend was also present in the prevention group.Taken together,COS·Zn improves the ovarian and follicular development through regulating the SESN2/NRF2 signaling pathway.These results suggest the role of COS·Zn as a novel agent for the treatment and prevention of POF.

Prologue: Juvenility Inferences of Parental Advance Aging

Couples are escalating delay in childbearing to the late 35 s (female), the 40 s (males) and afar. The surmising of this collective and societal transformation on youth constitution and salubriousness has just at present been a spotlight of research. There are distinguished intensified perinatal risks related with expanding maternal age, notwithstanding the way that fatherly age seems to have a presumably predominant adverse impingement on youth well-being. Although the preeminent rate of poor gravidness consequence may contrast from individual to singular point of view, the impingement of postponing childbearing from a general wellbeing perspective cannot be swelled and should be in the pattern of general wellbeing plan for the coming years. Recognizing the part of components and black box, characteristically the maturing of the gametes, and how this change effects on preparation, blastulation lastly the posterity, is an essential and consequent advance as we attempt to help patients outline sound families.

Extracellular vesicles from human urine-derived stem cells delay aging through the transfer of PLAU and TIMP1

Aging increases the risks of various diseases and the vulnerability to death.Cellular senescence is a hallmark of aging that contributes greatly to aging and aging-related diseases.This study demonstrates that extracellular vesicles from human urine-derived stem cells(USC-EVs)efficiently inhibit cellular senescence in vitro and in vivo.The intravenous injection of USC-EVs improves cognitive function,increases physical fitness and bone quality,and alleviates aging-related structural changes in different organs of senescence-accelerated mice and natural aging mice.The anti-aging effects of USC-EVs are not obviously affected by the USC donors’ages,genders,or health status.Proteomic analysis reveals that USC-EVs are enriched with plasminogen activator urokinase(PLAU)and tissue inhibitor of metalloproteinases 1(TIMP1).These two proteins contribute importantly to the anti-senescent effects of USC-EVs associated with the inhibition of matrix metalloproteinases,cyclin-dependent kinase inhibitor 2A(P16INK4a),and cyclin-dependent kinase inhibitor 1A(P21cip1).These findings suggest a great potential of autologous USC-EVs as a promising anti-aging agent by transferring PLAU and TIMP1 proteins.

Cochlear mitochondrial DNA3867bp deletion in aged mice

OBJECTIVES: To study the status of cochlear mitochondrial DNA (mtDNA) and to determine the location of mtDNA deletion in aged mice. METHODS: We detected cochlear mtDNA in 2, 7 - 10 and 17 - 19 month old mice by nested polymerase chain reaction (PCR) and DNA sequencing. RESULTS: mtDNA3867bp deletions were found in the cochleae of aged mice. The deletion occurred within nt9103-nt12970 and were flanked by 15 base pair direct repeats. Comparing the incidence of mtDNA3867bp deletions, 17 - 19 month old mice (7/8) were significantly higher than 7 - 10 month old mice (4/16). The deletion was not observed in 2 month old mice (0/7). The ratio of deleted mtDNA/total mtDNA in 17 - 19 month old mice was higher than in 7 - 10 month old mice (P