Increasing p16INK4a expression decreases forebrain progenitors and neurogenesis during ageing

Effects of mood stabilizers on adult dentate gyrus-derived neural precursor cells

Neurogenesis in the adult dentate gyrus (DG) is considered to be partly involved in the action of mood stabilizers. However, it remains unclear how mood stabilizers affect neural precursor cells in adult DG. We have established a culture system of adult rat DG-derived neural precursor cells (ADP) and have shown that lithium, a mood stabilizer, and dexamethasone, an agonist of glucocorticoid receptor, reciprocally regulate ADP proliferation. Neurogenesis constitutes not only proliferation of neural precursor cells but also apoptosis and differentiation. To develop further understanding of mood stabilizer effects on neural precursor cells in adult DG, we investigated and compared the effects of four common mood stabilizers-lithium, valproate, carbamazepine, and lamotrigine-on ADP proliferation, apoptosis, and differentiation. ADP proliferation, decreased by dexamethasone, was examined using Alamar Blue assay. Using TUNEL assay, ADP apoptosis induced by staurosporine was examined. The differentiated ADP induced by retinoic acid was characterized by immunostaining with anti-GFAP or anti-Tuj1 antibody. Lithium and valproate, but not carbamazepine and lamotrigine, recovered ADP proliferation decreased by dexamethasone. All four mood stabilizers decreased ADP apoptosis. Retinoic acid differentiated ADP into both neurons and astrocytes. Lithium and carbamazepine increased the ratio of neurons and decreased that of astrocytes. However, valproate and lamotrigine increased the ratio of astrocytes and decreased that of neurons. Therefore, these four stabilizers exhibited both common and differential effects on ADP proliferation, apoptosis, and differentiation.

Overexpression of the cell cycle inhibitor p16INK4a promotes a prothrombotic phenotype following vascular injury in mice

OBJECTIVE

Age-associated cellular senescence is thought to promote vascular dysfunction. p16(INK4a) is a cell cycle inhibitor that promotes senescence and is upregulated during normal aging. In this study, we examine the contribution of p16(INK4a) overexpression to venous thrombosis.

METHODS AND RESULTS

Mice overexpressing p16(INK4a) were studied with 4 different vascular injury models: (1) ferric chloride (FeCl(3)) and (2) Rose Bengal to induce saphenous vein thrombus formation; (3) FeCl(3) and vascular ligation to examine thrombus resolution; and (4) lipopolysaccharide administration to initiate inflammation-induced vascular dysfunction. p16(INK4a) transgenic mice had accelerated occlusion times (13.1 ± 0.4 minutes) compared with normal controls (19.7 ± 1.1 minutes) in the FeCl(3) model and 12.7 ± 2.0 and 18.6 ± 1.9 minutes, respectively in the Rose Bengal model. Moreover, overexpression of p16(INK4a) delayed thrombus resolution compared with normal controls. In response to lipopolysaccharide treatment, the p16(INK4a) transgenic mice showed enhanced thrombin generation in plasma-based calibrated automated thrombography assays. Finally, bone marrow transplantation studies suggested increased p16(INK4a) expression in hematopoietic cells contributes to thrombosis, demonstrating a role for p16(INK4a) expression in venous thrombosis.

CONCLUSIONS

Venous thrombosis is augmented by overexpression of the cellular senescence protein p16(INK4a).

Perturbed cellular response to brain injury during aging

Old age is associated with an enhanced susceptibility to stroke and poor recovery from brain injury, but the cellular processes underlying these phenomena are only partly understood. Therefore, studying the basic mechanisms underlying structural and functional recovery after brain injury in aged subjects is of considerable clinical interest. Behavioral and cytological analyses of rodents that have undergone experimental injury show that: (a) behaviorally, aged rodents are more severely impaired by ischemia than are young animals, and older rodents also show diminished functional recovery; (b) compared to young animals, aged animals develop a larger infarct area, as well as a necrotic zone characterized by a higher rate of cellular degeneration and a larger number of apoptotic cells; (c) both astrocytes and macrophages are activated strongly and early following stroke in aged rodents; (d) in older animals, the premature, intense cytoproliferative activity following brain injury leads to the precipitous formation of growth-inhibiting scar tissue, a phenomenon amplified by the persistent expression of neurotoxic factors; (e) though the timing is altered, the regenerative capability of the brain is largely preserved in rats, at least into early old age. Whether endogenous neurogenesis contributes to spontaneous recovery after stroke has not yet been established. If neurogenesis from endogenous neuronal stem cells is to be used therapeutically, an individual approach will be required to assess the possible extent of neurogenic response as well as the possibilities to alter this response for functional improvement or prevention of further loss of brain function.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

Wnt signaling and aging-related heart disorders

Aging is associated with various heart diseases, and this may be attributable, in part, to the prolonged exposure of the heart to cardiovascular risk factors. However, aging is also associated with heart disorders such as diastolic dysfunction that are not necessarily linked to the risk factors for cardiovascular diseases. Recent studies have demonstrated a mechanistic link between Wnt signaling and premature aging or aging-related phenotypes. As a part of the review series on Wnt signaling and the cardiovascular system, we discuss here the possible involvement of Wnt signaling in aging-associated heart diseases or heart disorders.

p38 MAPK and JNK antagonistically control senescence and cytoplasmic p16INK4A expression in doxorubicin-treated endothelial progenitor cells

Patients treated with low-dose anthracyclines often show late onset cardiotoxicity. Recent studies suggest that this form of cardiotoxicity is the result of a progenitor cell disease. In this study we demonstrate that Cord Blood Endothelial Progenitor Cells (EPCs) exposed to low, sub-apoptotic doses of doxorubicin show a senescence phenotype characterized by increased SA-b-gal activity, decreased TRF2 and chromosomal abnormalities, enlarged cell shape, and disarrangement of F-actin stress fibers accompanied by impaired migratory ability. P16 ^INK4A localizes in the cytoplasm of doxorubicin-induced senescent EPCs and not in the nucleus as is the case in EPCs rendered senescent by different stimuli. This localization together with the presence of an arrest in G2, and not at the G1 phase boundary, which is what usually occurs in response to the cell cycle regulatory activity of p16^INK4A, suggests that doxorubicin-induced p16 ^INK4A does not regulate the cell cycle, even though its increase is closely associated with senescence. The effects of doxorubicin are the result of the activation of MAPKs p38 and JNK which act antagonistically. JNK attenuates the senescence, p16 ^INK4A expression and cytoskeleton remodeling that are induced by activated p38. We also found that conditioned medium from doxorubicin-induced senescent cardiomyocytes does not attract untreated EPCs, unlike conditioned medium from apoptotic cardiomyocytes which has a strong chemoattractant capacity. In conclusion, this study provides a better understanding of the senescence of doxorubicin-treated EPCs, which may be helpful in preventing and treating late onset cardiotoxicity.

Cyclin-dependent kinase 1 expression is inhibited by p16(INK4a) at the post-transcriptional level through the microRNA pathway

The p16(INK4a) protein regulates cell cycle progression mainly by inhibiting the activity of G1-phase cyclin-dependent kinases (CDKs) 4 and 6, the subsequent retinoblastoma protein (pRb) phosphorylation and E2F transcription factor release. The p16(INK4a) protein can also repress the activity of other transcription factors, such as c-myc, nuclear factor-kappaB and c-Jun/AP1. Here, we report that, in two p16(-/-), pRb(WT) and p53(WT) cell lines (MCF7 and U87), p16(INK4a) overexpression induces a dramatic decrease in CDK1 protein expression. In response to p16(INK4a), the decreased rate of CDK1 protein synthesis, its unchanged protein half-life, unreduced CDK1 mRNA steady-state levels and mRNA half-life allow us to hypothesize that p16(INK4a) could regulate CDK1 expression at the post-transcriptional level. This CDK1 downregulation is mediated by the 3'-untranslated region (3'UTR) of CDK1 mRNA as shown by translational inhibition in luciferase assays and is associated with a modified expression balance of microRNAs (miRNAs) that potentially regulate CDK1, analyzed by TaqMan Human microRNA Array. The p16(INK4a)-induced expression of two miRNAs (miR-410 and miR-650 chosen as an example) in MCF7 cells is confirmed by individual reverse transcription-qPCR. Furthermore, we show the interaction of miR-410 or miR-650 with CDK1-3'UTR by luciferase assays. Endogenous CDK1 expression decreases upon both miRNA overexpression and increases with their simultaneous inhibition. The induction of miR-410, but not miR-650 could be related to the pRb/E2F pathway. These results demonstrate the post-transcriptional inhibition of CDK1 by p16(INK4a). We suggest that p16(INK4a) may regulate gene expression by modifying the functional equilibrium of transcription factors and consequently the expression balance of miRNAs.

mTOR: from growth signal integration to cancer, diabetes and ageing

In all eukaryotes, the target of rapamycin (TOR) signalling pathway couples energy and nutrient abundance to the execution of cell growth and division, owing to the ability of TOR protein kinase to simultaneously sense energy, nutrients and stress and, in metazoans, growth factors. Mammalian TOR complex 1 (mTORC1) and mTORC2 exert their actions by regulating other important kinases, such as S6 kinase (S6K) and Akt. In the past few years, a significant advance in our understanding of the regulation and functions of mTOR has revealed the crucial involvement of this signalling pathway in the onset and progression of diabetes, cancer and ageing.

A tale of tailless

Drosophila Tailless(Tll) and its vertebrate homologue Tlx are conserved orphan nuclear receptors specifically expressed in the eye and the forebrain. Tll and Tlx act primarily as transcriptional repressors through their interactions with transcriptional corepressors, Atrophin family proteins, and histone-tail/chromatin-modifying factors such as lysine-specific histone demethylase 1 and histone deacetylases. The functional importance of Tll and Tlx is made apparent by the recent discovery that they are expressed in neural stem cells (NSCs) and are required for self-renewal of these cells in both Drosophila and the mouse. This review provides a snapshot of current knowledge about Tll and Tlx and their transcriptional network, which maintains NSCs in developing and adult animals.

Human Umbilical Cord Blood Cells Have Trophic Effects on Young and Aging Hippocampal Neurons in Vitro

In experimental models of central nervous system (CNS) aging, injury and disease, administering human umbilical cord blood (HUCB) cells induce recovery, most likely by interacting with multiple cellular processes. The aim of this study was to examine whether the HUCB cells produce trophic factors that may enhance survival and maturation of hippocampal neurons in an in vitro test system. We co-cultured the mononuclear fraction of HUCB cells with hippocampal neurons isolated from either young (7-months of age) or aging (21 month of age) rat brain for 14, 21, 28, 35 and 42 days in vitro (DIV), respectively. Immunocytochemistry was then employed to identify neurons (MAP2(+)) and glial cells (GFAP(+)) as well as arborization of neurites. The average number of MAP2(+) hippocampal neurons cells in both young and aging neuronal-HUCB co-cultures was significantly higher than in the control cultures (hippocampal mono-cultures). These MAP2(+) neurons in co-culture were richly arborized, especially in 21 and 28 DIV co-cultures, and expressed functional enzymes (Synaptophysin, tyrosine hydryoxlase (TH)), gamma amino butyric acid receptor (GABAAr) and glutamate transporter (EAAC1). The majority of hippocampal neurons in both co-culture systems grew very well and survived for up to 42 DIV with an increment of immature neurons which were positive for Nestin and TuJ1. Using a multiplex protein array, a number of secreted proteins that could have trophic effects on the neurons were identified.

When neurogenesis encounters aging and disease

In this review, we consider the evidence that a reduction in neurogenesis underlies aging-related cognitive deficits and impairments in disorders such as Alzheimer's disease (AD). The molecular and cellular alterations associated with impaired neurogenesis in the aging brain are discussed. Dysfunction of presenilin-1, misprocessing of amyloid precursor protein and toxic effects of hyperphosphorylated tau and β-amyloid probably contribute to impaired neurogenesis in AD. Because factors such as exercise, environmental enrichment and dietary energy restriction enhance neurogenesis, and protect against age-related cognitive decline and AD, knowledge of the underlying neurogenic signaling pathways could lead to novel therapeutic strategies for preserving brain function. In addition, manipulation of endogenous neural stem cells and stem cell transplantation, as stand-alone or adjunct treatments, seems promising.

Transcription factor E4F1 is essential for epidermal stem cell maintenance and skin homeostasis

A growing body of evidence suggests that the multifunctional protein E4F1 is involved in signaling pathways that play essential roles during normal development and tumorigenesis. We generated E4F1 conditional knockout mice to address E4F1 functions in vivo in newborn and adult skin. E4F1 inactivation in the entire skin or in the basal compartment of the epidermis induces skin homeostasis defects, as evidenced by transient hyperplasia in the interfollicular epithelium and alteration of keratinocyte differentiation, followed by loss of cellularity in the epidermis and severe skin ulcerations. E4F1 depletion alters clonogenic activity of epidermal stem cells (ESCs) ex vivo and ends in exhaustion of the ESC pool in vivo, indicating that the lesions observed in the E4F1 mutant skin result, at least in part, from cell-autonomous alterations in ESC maintenance. The clonogenic potential of E4F1 KO ESCs is rescued by Bmi1 overexpression or by Ink4a/Arf or p53 depletion. Skin phenotype of E4F1 KO mice is also delayed in animals with Ink4a/Arf and E4F1 compound gene deficiencies. Our data identify a regulatory axis essential for ESC-dependent skin homeostasis implicating E4F1 and the Bmi1-Arf-p53 pathway.

Hdac1 and Hdac2 act redundantly to control p63 and p53 functions in epidermal progenitor cells

Epidermal and hair follicle development from surface ectodermal progenitor cells requires coordinated changes in gene expression. Histone deacetylases alter gene expression programs through modification of chromatin and transcription factors. We find that deletion of ectodermal Hdac1 and Hdac2 results in dramatic failure of hair follicle specification and epidermal proliferation and stratification, phenocopying loss of the key ectodermal transcription factor p63. Although expression of p63 and its positively regulated basal cell targets is maintained in Hdac1/2-deficient ectoderm, targets of p63-mediated repression, including p21, 14-3-3σ, and p16/INK4a, are ectopically expressed, and HDACs bind and are active at their promoter regions in normal undifferentiated keratinocytes. Mutant embryos display increased levels of acetylated p53, which opposes p63 functions, and p53 is required for HDAC inhibitor-mediated p21 expression in keratinocytes. Our data identify critical requirements for HDAC1/2 in epidermal development and indicate that HDAC1/2 directly mediate repressive functions of p63 and suppress p53 activity.

Cellular senescence: putting the paradoxes in perspective

Cellular senescence arrests the proliferation of potential cancer cells, and so is a potent tumor suppressive mechanism, akin to apoptosis. Or is it? Why did cells evolve an anti-cancer mechanism that arrests, rather than kills, would-be tumor cells? Recent discoveries that senescent cells secrete growth factors, proteases and cytokines provide a shifting view--from senescence as a cell autonomous suppressor of tumorigenesis to senescence as a means to mobilize the systemic and local tissue milieu for repair. In some instances, this mobilization benefits the organism, but in others it can be detrimental. These discoveries provide potential mechanisms by which cellular senescence might contribute to the diverse, and seemingly incongruent, processes of tumor suppression, tumor promotion, tissue repair, and aging.

Cdk2: a key regulator of the senescence control function of Myc

Proto-oncogenes such as MYC and RAS promote normal cell growth but fuel tumor development when deregulated. However, over-activated Myc and Ras also trigger intrinsic tumor suppressor mechanisms leading to apoptosis and senescence, respectively. When expressed together MYC and RAS are sufficient for oncogenic transformation of primary rodent cells, but the basis for their cooperativity has remained unresolved. While Ras is known to suppress Myc-induced apoptosis, we recently discovered that Myc is able to repress Ras-induced senescence. Myc and Ras thereby together enable evasion of two main barriers of tumorigenesis. The ability of Myc to suppress senescence was dependent on phosphorylation of Myc at Ser-62 by cyclin-dependent kinase 2 (Cdk2), uncovering a new non-redundant role of this kinase. Further, utilizing Cdk2 as a cofactor, Myc directly controlled key genes involved in senescence. We speculate that this new role of Myc/Cdk2 in senescence has relevance for other Myc functions, such as regulation of stemness, self-renewal, immortalization and differentiation, which may have an impact on tissue regeneration. Importantly, selective pharmacological inhibition of Cdk2 forced Myc/Ras expressing cells into cellular senescence, highlighting this kinase as a potential therapeutic target for treatment of tumors driven by Myc or Ras.

Energy metabolism in adult neural stem cell fate

The adult mammalian brain contains a population of neural stem cells that can give rise to neurons, astrocytes, and oligodendrocytes and are thought to be involved in certain forms of memory, behavior, and brain injury repair. Neural stem cell properties, such as self-renewal and multipotency, are modulated by both cell-intrinsic and cell-extrinsic factors. Emerging evidence suggests that energy metabolism is an important regulator of neural stem cell function. Molecules and signaling pathways that sense and influence energy metabolism, including insulin/insulin-like growth factor I (IGF-1)-FoxO and insulin/IGF-1-mTOR signaling, AMP-activated protein kinase (AMPK), SIRT1, and hypoxia-inducible factors, are now implicated in neural stem cell biology. Furthermore, these signaling modules are likely to cooperate with other pathways involved in stem cell maintenance and differentiation. This review summarizes the current understanding of how cellular and systemic energy metabolism regulate neural stem cell fate. The known consequences of dietary restriction, exercise, aging, and pathologies with deregulated energy metabolism for neural stem cells and their differentiated progeny will also be discussed. A better understanding of how neural stem cells are influenced by changes in energy availability will help unravel the complex nature of neural stem cell biology in both the normal and diseased state.

At the stem of youth and health

Cellular senescence is a specialized form of growth arrest, confined to mitotic cells, induced by various stressful stimuli and characterized by a permanent growth arrest, resistance to apoptosis, an altered pattern of gene expression and the expression of some markers that are characteristic, although not exclusive, to the senescent state. Senescent cells profoundly modify neighboring and remote cells through the production of an altered secretome, eventually leading to inflammation, fibrosis and possibly growth of neoplastic cells. Mammalian aging has been defined as a reduction in the capacity to adequately maintain tissue homeostasis or to repair tissues after injury. Tissue homeostasis and regenerative capacity are nowadays considered to be related to the stem cell pool present in every tissue. For this reason, pathological and patho-physiological conditions characterized by altered tissue homeostasis and impaired regenerative capacity can be viewed as a consequence of the reduction in stem cell number and/or function. Last, cellular senescence is a double-edged sword, since it may inhibit the growth of transformed cells, preventing the occurrence of cancer, while it may facilitate growth of preneoplastic lesions in a paracrine fashion; therefore, interventions targeting this cell response to stress may have a profound impact on many age-related pathologies, ranging from cardiovascular disease to oncology. Aim of this review is to discuss both molecular mechanisms associated with stem cell senescence and interventions that may attenuate or reverse this process.

p16(INK4a) -mediated suppression of telomerase in normal and malignant human breast cells

The cyclin-dependent kinase inhibitor p16(INK4a) (CDKN2A) is an important tumor suppressor gene frequently inactivated in human tumors. p16 suppresses the development of cancer by triggering an irreversible arrest of cell proliferation termed cellular senescence. Here, we describe another anti-oncogenic function of p16 in addition to its ability to halt cell cycle progression. We show that transient expression of p16 stably represses the hTERT gene, encoding the catalytic subunit of telomerase, in both normal and malignant breast epithelial cells. Short-term p16 expression increases the amount of histone H3 trimethylated on lysine 27 (H3K27) bound to the hTERT promoter, resulting in transcriptional silencing, likely mediated by polycomb complexes. Our results indicate that transient p16 exposure may prevent malignant progression in dividing cells by irreversible repression of genes, such as hTERT, whose activity is necessary for extensive self-renewal.

Effect of age on proliferation-regulating factors in human adult neurogenic regions

Neurogenesis, the birth of new neurons, continues throughout adulthood in the human subventricular zone (SVZ) and hippocampus. It is not known how levels of putative proliferation-regulating factors change with age in human adult neurogenic areas. The current project employed ELISAs to investigate changes in levels of putative proliferation-regulating factors in the healthy human SVZ and dentate gyrus throughout the adult lifespan (18-104 years). Levels of brain-derived neurotrophic factor, basic fibroblast growth factor and interleukin (IL)-1β were significantly higher in the hippocampus than in the SVZ and levels of glial-derived neurotrophic factor and transforming growth factor-α were significantly higher in the SVZ (p < 0.005), suggesting that factors with predominant influences on neurogenesis differ between the two human adult neurogenic areas. Hippocampal levels of transforming growth factor-β1 strongly increased with age (n = 9, p < 0.01), whereas hippocampal and SVZ levels of brain-derived neurotrophic factor, epidermal growth factor, basic fibroblast growth factor, glial-derived neurotrophic factor, heparin-binding epidermal growth factor, insulin-like growth factor-1, IL-1β, IL-6 and transforming growth factor-α did not change significantly with age in the SVZ or hippocampus. These findings suggest regulation of the adult neurogenic environment in the human brain may differ over time from that in other species.

Advances in understanding tissue regenerative capacity and mechanisms in animals

Questions about how and why tissue regeneration occurs have captured the attention of countless biologists, biomedical engineers and clinicians. Regenerative capacity differs greatly across organs and organisms, and a range of model systems that use different regenerative strategies and that offer different technical advantages have been studied to understand regeneration. Making use of this range of systems and approaches, recent advances have allowed progress to be made in understanding several key issues that are common to natural regenerative events. These issues include: the determination of regenerative capacity; the importance of stem cells, dedifferentiation and transdifferentiation; how regenerative signals are initiated and targeted; and the mechanisms that control regenerative proliferation and patterning.

Notch activity levels control the balance between quiescence and recruitment of adult neural stem cells

The limited generation of neurons during adulthood is controlled by a balance between quiescence and recruitment of neural stem cells (NSCs). We use here the germinal zone of the zebrafish adult telencephalon to examine how the frequency of NSC divisions is regulated. We show, using several in vivo techniques, that progenitors transit back and forth between the quiescent and dividing state, according to varying levels of Notch activity: Notch induction drives progenitors into quiescence, whereas blocking Notch massively reinitiates NSC division and subsequent commitment toward becoming neurons. Notch activation appears predominantly triggered by newly recruited progenitors onto their neighbors, suggesting an involvement of Notch in a self-limiting mechanism, once neurogenesis is started. These results identify for the first time a lateral inhibition-like mechanism in the context of adult neurogenesis and suggest that the equilibrium between quiescence and neurogenesis in the adult brain is controlled by fluctuations of Notch activity, thereby regulating the amount of adult-born neurons.

Brain tumor stem cells: the cancer stem cell hypothesis writ large

Brain tumors, which are typically very heterogeneous at the cellular level, appear to have a stem cell foundation. Recently, investigations from multiple groups have found that human as well as experimental mouse brain tumors contain subpopulations of cells that functionally behave as tumor stem cells, driving tumor growth and generating tumor cell progeny that form the tumor bulk, but which then lose tumorigenic ability. In human glioblastomas, these tumor stem cells express neural precursor markers and are capable of differentiating into tumor cells that express more mature neural lineage markers. In addition, modeling brain tumors in mice suggests that neural precursor cells more readily give rise to full blown tumors, narrowing potential cells of origin to those rarer brain cells that have a proliferative potential. Applying stem cell concepts and methodologies is giving fresh insight into brain tumor biology, cell of origin and mechanisms of growth, and is offering new opportunities for development of more effective treatments. The field of brain tumor stem cells remains very young and there is much to be learned before these new insights are translated into new patient treatments.

c-MYC overexpression with loss of Ink4a/Arf transforms bone marrow stromal cells into osteosarcoma accompanied by loss of adipogenesis

The development of cancer is due to the growth and proliferation of transformed normal cells. Recent evidence suggests that the nature of oncogenic stress and the state of the cell of origin critically affect both tumorigenic activity and tumor histological type. However, this mechanistic relationship in mesenchymal tumors is currently largely unexplored. To clarify these issues, we established a mouse osteosarcoma (OS) model through overexpression of c-MYC in bone marrow stromal cells (BMSCs) derived from Ink4a/Arf (-/-) mice. Single-cell cloning revealed that c-MYC-expressing BMSCs are composed of two distinctly different clones: highly tumorigenic cells, similar to bipotent-committed osteochondral progenitor cells, and low-tumorigenic tripotent cells, similar to mesenchymal stem cells (MSCs). It is noteworthy that both bipotent and tripotent cells were capable of generating histologically similar, lethal OS, suggesting that both committed progenitor cells and MSCs can become OS cells of origin. Shifting mesenchymal differentiation by depleting PPARγ in tripotent MSC-like cells and overexpressing PPARγ in bipotent cells affected cell proliferation and tumorigenic activity. Our findings indicate that differentiation potential has a key role in OS tumorigenic activity, and that the suppression of adipogenic ability is a critical factor for the development of OS.

Cross-species genomics matches driver mutations and cell compartments to model ependymoma

Understanding the biology that underlies histologically similar but molecularly distinct subgroups of cancer has proven difficult because their defining genetic alterations are often numerous, and the cellular origins of most cancers remain unknown. We sought to decipher this heterogeneity by integrating matched genetic alterations and candidate cells of origin to generate accurate disease models. First, we identified subgroups of human ependymoma, a form of neural tumour that arises throughout the central nervous system (CNS). Subgroup-specific alterations included amplifications and homozygous deletions of genes not yet implicated in ependymoma. To select cellular compartments most likely to give rise to subgroups of ependymoma, we matched the transcriptomes of human tumours to those of mouse neural stem cells (NSCs), isolated from different regions of the CNS at different developmental stages, with an intact or deleted Ink4a/Arf locus (that encodes Cdkn2a and b). The transcriptome of human supratentorial ependymomas with amplified EPHB2 and deleted INK4A/ARF matched only that of embryonic cerebral Ink4a/Arf(-/-) NSCs. Notably, activation of Ephb2 signalling in these, but not other, NSCs generated the first mouse model of ependymoma, which is highly penetrant and accurately models the histology and transcriptome of one subgroup of human supratentorial tumour. Further, comparative analysis of matched mouse and human tumours revealed selective deregulation in the expression and copy number of genes that control synaptogenesis, pinpointing disruption of this pathway as a critical event in the production of this ependymoma subgroup. Our data demonstrate the power of cross-species genomics to meticulously match subgroup-specific driver mutations with cellular compartments to model and interrogate cancer subgroups.

Comparison of newly generated doublecortin-immunoreactive neuronal progenitors in the main olfactory bulb among variously aged gerbils

In the present study, we investigated age-related differences in neuronal progenitors in the gerbil main olfactory bulb (MOB) using doublecortin (DCX), a marker for neuronal progenitors which differentiate into neurons in the brain. No difference in the number of neuronal nuclei (NeuN)-immunoreactive neurons was found in the MOB at variously aged gerbils. At postnatal month (PM) 1, DCX immunoreaction was detected in all layers of the MOB except for the olfactory nerve layer. At this time point, DCX-immunoreactive cells (neuronal progenitors) were very abundant; however, they did not have fully developed-processes. From PM 3, the number of DCX-immunoreactive neuronal progenitors was decreased with age. At PM 6, DCX-immunoreactive cells showed very well-developed processes. In western blot analysis, DCX protein level in the MOB was highest at PM 1. Thereafter, levels of DCX protein were decreased with age. In the subventricular zone of the lateral ventricle, the number of Ki-67-immunoractive cells (proliferating cells) was also significantly decreased with age. In addition, increases of α-synuclein-immunoreactive structures were observed in the MOB with age. These results suggest that decrease in DCX-immunoreactive neuronal progenitors and its protein levels in the MOB with age may be associated with reduction of cell proliferation in the SVZ and with an increase in α-synuclein in the MOB.

Caloric restriction: from soup to nuts

Caloric restriction (CR), reduced protein, methionine, or tryptophan diets; and reduced insulin and/or IGFI intracellular signaling can extend mean and/or maximum lifespan and delay deleterious age-related physiological changes in animals. Mice and flies can shift readily between the control and CR physiological states, even at older ages. Many health benefits are induced by even brief periods of CR in flies, rodents, monkeys, and humans. In humans and nonhuman primates, CR produces most of the physiologic, hematologic, hormonal, and biochemical changes it produces in other animals. In primates, CR provides protection from type 2 diabetes, cardiovascular and cerebral vascular diseases, immunological decline, malignancy, hepatotoxicity, liver fibrosis and failure, sarcopenia, inflammation, and DNA damage. It also enhances muscle mitochondrial biogenesis, affords neuroprotection; and extends mean and maximum lifespan. CR rapidly induces antineoplastic effects in mice. Most claims of lifespan extension in rodents by drugs or nutrients are confounded by CR effects. Transcription factors and co-activators involved in the regulation of mitochondrial biogenesis and energy metabolism, including SirT1, PGC-1alpha, AMPK and TOR may be involved in the lifespan effects of CR. Paradoxically, low body weight in middle aged and elderly humans is associated with increased mortality. Thus, enhancement of human longevity may require pharmaceutical interventions.

Deletion analysis of BMI1 oncoprotein identifies its negative regulatory domain

BACKGROUND

The polycomb group (PcG) protein BMI1 is an important regulator of development. Additionally, aberrant expression of BMI1 has been linked to cancer stem cell phenotype and oncogenesis. In particular, its overexpression has been found in several human malignancies including breast cancer. Despite its established role in stem cell maintenance, cancer and development, at present not much is known about the functional domains of BMI1 oncoprotein. In the present study, we carried out a deletion analysis of BMI1 to identify its negative regulatory domain.

RESULTS

We report that deletion of the C-terminal domain of BMI1, which is rich in proline-serine (PS) residues and previously described as PEST-like domain, increased the stability of BMI1, and promoted its pro-oncogenic activities in human mammary epithelial cells (HMECs). Specifically, overexpression of a PS region deleted mutant of BMI1 increased proliferation of HMECs and promoted an epithelial-mesenchymal transition (EMT) phenotype in the HMECs. Furthermore, when compared to the wild type BMI1, exogenous expression of the mutant BMI1 led to a significant downregulation of p16INK4a and an efficient bypass of cellular senescence in human diploid fibroblasts.

CONCLUSIONS

In summary, our data suggest that the PS domain of BMI1 is involved in its stability and that it negatively regulates function of BMI1 oncoprotein. Our results also suggest that the PS domain of BMI1 could be targeted for the treatment of proliferative disorders such as cancer and aging.

Presenilin 1 mutants impair the self-renewal and differentiation of adult murine subventricular zone-neuronal progenitors via cell-autonomous mechanisms involving notch signaling

The vast majority of pedigrees with familial Alzheimer's disease (FAD) are caused by inheritance of mutations in the PSEN1 1 gene. While genetic ablation studies have revealed a role for presenilin 1 (PS1) in embryonic neurogenesis, little information has emerged regarding the potential effects of FAD-linked PS1 variants on proliferation, self-renewal and differentiation, key events that control cell fate commitment of adult brain neural progenitors (NPCs). We used adult brain subventricular zone (SVZ)-derived NPC cultures transduced with recombinant lentivirus as a means to investigate the effects of various PS1 mutants on self-renewal and differentiation properties. We now show that viral expression of several PS1 mutants in NPCs leads to impaired self-renewal and altered differentiation toward neuronal lineage, in vitro. In line with these observations, diminished constitutive proliferation and steady-state SVZ progenitor pool size was observed in vivo in transgenic mice expressing the PS1DeltaE9 variant. Moreover, NPC cultures established from the SVZ of adult mice expressing PS1DeltaE9 exhibit reduced self-renewal capacity and premature exit toward neuronal fates. To these findings, we show that both the levels of endogenous Notch/CBF-1-transcriptional activity and transcripts encoding Notch target genes are diminished in SVZ NPCs expressing PS1DeltaE9. The deficits in self-renewal and multipotency are restored by expression of Notch1-ICD or a downstream target of the Notch pathway, Hes1. Hence, we argue that a partial reduction in PS-dependent gamma-secretase processing of the Notch, at least in part, accounts for the impairments observed in SVZ NPCs expressing the FAD-linked PS1DeltaE9 variant.

Emerging role of the MORF/MRG gene family in various biological processes, including aging

Cellular senescence is the dominant phenotype over immortality. In our studies to identify senescence-related genes, we cloned Morf4, which induced senescence in a subset of tumor cells. Morf4 is a member of a family of seven genes, and Morf-related genes (Mrg) on chromosomes 15 (Mrg15) and X (MrgX) are also expressed. In contrast to MORF4, MRG15 and MRGX are positive regulators of cell division. All three proteins interact with histone deacetylases and acetyltransferases, suggesting that they function in regulation of chromatin dynamics. Mrg15 knockout mice are embryonic lethal, and mouse embryonic fibroblasts derived from Mrg15 null embryos proliferate poorly, enter senescence rapidly, and have impaired DNA repair compared to the wild type. Mrg15 null embryonic neural stem and progenitor cells also have a decreased capacity for proliferation and differentiation. Further studies are needed to determine the function of this gene family in various biological processes, including neural stem and progenitor cell aging.

Ionizing radiation-induced long-term expression of senescence markers in mice is independent of p53 and immune status

Exposure to IR has been shown to induce the formation of senescence markers, a phenotype that coincides with lifelong delayed repair and regeneration of irradiated tissues. We hypothesized that IR-induced senescence markers could persist long-term in vivo, possibly contributing to the permanent reduction in tissue functionality. Here, we show that mouse tissues exposed to a sublethal dose of IR display persistent (up to 45 weeks, the maximum time analyzed) DNA damage foci and increased p16(INK4a) expression, two hallmarks of cellular senescence and aging. BrdU-labeling experiments revealed that IR-induced damaged cells are preferentially eliminated, at least partially, in a tissue-dependent manner. Unexpectedly, the accumulation of damaged cells was found to occur independent from the DNA damage response modulator p53, and from an intact immune system, as their levels were similar in wild-type and Rag2(-/-) gammaC(-/-) mice, the latter being deficient in T, B, and NK cells. Together, our results provide compelling evidence that exposure to IR induces long-term expression of senescence markers in vivo, an effect that may contribute to the reduced tissue functionality observed in cancer survivors.

Esophageal cancer-related gene 4 is a secreted inducer of cell senescence expressed by aged CNS precursor cells

Mammalian aging is thought to be partially caused by the diminished capacity of stem/precursor cells to undergo self-renewing divisions. Although many cell-cycle regulators are involved in this process, it is unknown to what extent cell senescence, first identified as irreversible growth arrest in vitro, contributes to the aging process. Here, using a serum-induced mouse oligodendrocyte precursor cell (mOPC) senescence model, we identified esophageal cancer-related gene 4 (Ecrg4) as a senescence inducer with implications for the senescence-like state of postmitotic cells in the aging brain. Although mOPCs could proliferate indefinitely when cultured using the appropriate medium (OPC medium), they became senescent in the presence of serum and maintained their senescent phenotype even when the serum was subsequently replaced by OPC medium. We show that Ecrg4 was up-regulated in the senescent OPCs, its overexpression in OPCs induced senescence by accelerating the proteasome-dependent degradation of cyclins D1 and D3, and that its knockdown by a specific short hairpin RNA prevented these phenotypes. We also show that senescent OPCs secreted Ecrg4 and that recombinant Ecrg4 induced OPC senescence in culture. Moreover, increased Ecrg4 expression was observed in the OPCs and neural precursor cells in the aged mouse brain; this was accompanied by the expression of senescence-associated beta-galactosidase activity, indicating the cells' entrance into senescence. These results suggest that Ecrg4 is a factor linking neural-cell senescence and aging.

The nuclear receptor tailless induces long-term neural stem cell expansion and brain tumor initiation

Malignant gliomas are the most common primary brain tumors, and are associated with frequent resistance to therapy as well as poor prognosis. Here we demonstrate that the nuclear receptor tailless (Tlx), which in the adult is expressed exclusively in astrocyte-like B cells of the subventricular zone, acts as a key regulator of neural stem cell (NSC) expansion and brain tumor initiation from NSCs. Overexpression of Tlx antagonizes age-dependent exhaustion of NSCs in mice and leads to migration of stem/progenitor cells from their natural niche. The increase of NSCs persists with age, and leads to efficient production of newborn neurons in aged brain tissues. These cells initiate the development of glioma-like lesions and gliomas. Glioma development is accelerated upon loss of the tumor suppressor p53. Tlx-induced NSC expansion and gliomagenesis are associated with increased angiogenesis, which allows for the migration and maintenance of brain tumor stem cells in the perivascular niche. We also demonstrate that Tlx transcripts are overexpressed in human primary glioblastomas in which Tlx expression is restricted to a subpopulation of nestin-positive perivascular tumor cells. Our study clearly demonstrates how NSCs contribute to brain tumorgenesis driven by a stem cell-specific transcription factor, thus providing novel insights into the histogenesis and molecular pathogenesis of primary brain tumors.

Linking functional decline of telomeres, mitochondria and stem cells during ageing

The study of human genetic disorders and mutant mouse models has provided evidence that genome maintenance mechanisms, DNA damage signalling and metabolic regulation cooperate to drive the ageing process. In particular, age-associated telomere damage, diminution of telomere 'capping' function and associated p53 activation have emerged as prime instigators of a functional decline of tissue stem cells and of mitochondrial dysfunction that adversely affect renewal and bioenergetic support in diverse tissues. Constructing a model of how telomeres, stem cells and mitochondria interact with key molecules governing genome integrity, 'stemness' and metabolism provides a framework for how diverse factors contribute to ageing and age-related disorders.

The senescence-associated secretory phenotype: the dark side of tumor suppression

Cellular senescence is a tumor-suppressive mechanism that permanently arrests cells at risk for malignant transformation. However, accumulating evidence shows that senescent cells can have deleterious effects on the tissue microenvironment. The most significant of these effects is the acquisition of a senescence-associated secretory phenotype (SASP) that turns senescent fibroblasts into proinflammatory cells that have the ability to promote tumor progression.

FoxOs cooperatively regulate diverse pathways governing neural stem cell homeostasis

The PI3K-AKT-FoxO pathway is integral to lifespan regulation in lower organisms and essential for the stability of long-lived cells in mammals. Here, we report the impact of combined FoxO1, 3, and 4 deficiencies on mammalian brain physiology with a particular emphasis on the study of the neural stem/progenitor cell (NSC) pool. We show that the FoxO family plays a prominent role in NSC proliferation and renewal. FoxO-deficient mice show initial increased brain size and proliferation of neural progenitor cells during early postnatal life, followed by precocious significant decline in the NSC pool and accompanying neurogenesis in adult brains. Mechanistically, integrated transcriptomic, promoter, and functional analyses of FoxO-deficient NSC cultures identified direct gene targets with known links to the regulation of human brain size and the control of cellular proliferation, differentiation, and oxidative defense. Thus, the FoxO family coordinately regulates diverse genes and pathways to govern key aspects of NSC homeostasis in the mammalian brain.

FoxO3 regulates neural stem cell homeostasis

In the nervous system, neural stem cells (NSCs) are necessary for the generation of new neurons and for cognitive function. Here we show that FoxO3, a member of a transcription factor family known to extend lifespan in invertebrates, regulates the NSC pool. We find that adult FoxO3(-/-) mice have fewer NSCs in vivo than wild-type counterparts. NSCs isolated from adult FoxO3(-/-) mice have decreased self-renewal and an impaired ability to generate different neural lineages. Identification of the FoxO3-dependent gene expression profile in NSCs suggests that FoxO3 regulates the NSC pool by inducing a program of genes that preserves quiescence, prevents premature differentiation, and controls oxygen metabolism. The ability of FoxO3 to prevent the premature depletion of NSCs might have important implications for counteracting brain aging in long-lived species.

Bmi1 is a MYCN target gene that regulates tumorigenesis through repression of KIF1Bbeta and TSLC1 in neuroblastoma

Recent advances in neuroblastoma (NB) research addressed that epigenetic alterations such as hypermethylation of promoter sequences, with consequent silencing of tumor-suppressor genes, can have significant roles in the tumorigenesis of NB. However, the exact role of epigenetic alterations, except for DNA hypermethylation, remains to be elucidated in NB research. In this paper, we clarified the direct binding of MYCN to Bmi1 promoter and upregulation of Bmi1 transcription by MYCN. Mutation introduction into an MYCN binding site in the Bmi1 promoter suggests that MYCN has more important roles in the transcription of Bmi1 than E2F-related Bmi1 regulation. A correlation between MYCN and polycomb protein Bmi1 expression was observed in primary NB tumors. Expression of Bmi1 resulted in the acceleration of proliferation and colony formation in NB cells. Bmi1-related inhibition of NB cell differentiation was confirmed by neurite extension assay and analysis of differentiation marker molecules. Intriguingly, the above-mentioned Bmi1-related regulation of the NB cell phenotype seems not to be mediated only by p14ARF/p16INK4a in NB cells. Expression profiling analysis using a tumor-specific cDNA microarray addressed the Bmi1-dependent repression of KIF1Bbeta and TSLC1, which have important roles in predicting the prognosis of NB. Chromatin immunoprecipitation assay showed that KIF1Bbeta and TSLC1 are direct targets of Bmi1 in NB cells. These findings suggest that MYCN induces Bmi1 expression, resulting in the repression of tumor suppressors through Polycomb group gene-mediated epigenetic chromosome modification. NB cell proliferation and differentiation seem to be partially dependent on the MYCN/Bmi1/tumor-suppressor pathways.

Neural stem cell systems: physiological players or in vitro entities?

Neural stem cells (NSCs) can be experimentally derived or induced from different sources, and the NSC systems generated so far are promising tools for basic research and biomedical applications. However, no direct and thorough comparison of their biological and molecular properties or of their physiological relevance and possible relationship to endogenous NSCs has yet been carried out. Here we review the available information on different NSC systems and compare their properties. A better understanding of these systems will be crucial to control NSC fate and functional integration following transplantation and to make NSCs suitable for regenerative efforts following injury or disease.

Loss of ATM impairs proliferation of neural stem cells through oxidative stress-mediated p38 MAPK signaling

Ataxia-telangiectasia (A-T) is a genetic disorder caused by a mutation of the Atm gene, which controls DNA repair, cell cycling, and redox homeostasis. Even though oxidative stress has been implicated in the neurological anomalies in A-T, the effects of ATM loss on neural stem cell (NSC) survival has remained elusive. In this study, we investigated the effects of oxidative stress on NSC proliferation in an animal model for A-T neurodegeneration. We found that cultured subventricular zone neurosphere cells from Atm(-/-) mice show impaired proliferation, as well as intrinsic elevation of reactive oxygen species (ROS) levels, compared with those from Atm(+/+) mice. We also show that increasing the levels of ROS by H(2)O(2) treatment significantly reduces Atm(+/+) neurosphere formation and proliferation. In Atm(-/-) neurosphere cells, the Akt and Erk1/2 pathways are disrupted, together with enhanced activity of the p38 mitogen-activated protein kinase (MAPK). Treatment of these cells with the antioxidant N-acetyl-L-cysteine (NAC) or with a p38 MAPK inhibitor restores normal proliferation and reduced expression of p21(cip1) and p27(kip1) in the Atm(-/-) NSCs. These observations indicate that ATM plays a crucial role in NSC proliferation, by activating Akt and Erk1/2 pathways and by suppressing ROS-p38 MAPK signaling. Together, our results suggest that p38 MAPK signaling acts as a negative regulator of NSC proliferation in response to oxidative stress. These findings suggest a potential mechanism for neuronal cell loss as a result of oxidative stress in NSCs in progressive neurodegenerative diseases such as A-T.

Histone deacetylase controls adult stem cell aging by balancing the expression of polycomb genes and jumonji domain containing 3

Aging is linked to loss of the self-renewal capacity of adult stem cells. Here, we observed that human multipotent stem cells (MSCs) underwent cellular senescence in vitro. Decreased expression of histone deacetylases (HDACs), followed by downregulation of polycomb group genes (PcGs), such as BMI1, EZH2 and SUZ12, and by upregulation of jumonji domain containing 3 (JMJD3), was observed in senescent MSCs. Similarly, HDAC inhibitors induced cellular senescence through downregulation of PcGs and upregulation of JMJD3. Regulation of PcGs was associated with HDAC inhibitor-induced hypophosphorylation of RB, which causes RB to bind to and decrease the transcriptional activity of E2F. JMJD3 expression regulation was dependant on histone acetylation status at its promoter regions. A histone acetyltransferase (HAT) inhibitor prevented replicative senescence of MSCs. These results suggest that HDAC activity might be important for MSC self-renewal by balancing PcGs and JMJD3 expression, which govern cellular senescence by p16^INK4A regulation.

Mechanisms of stem cell self-renewal

Self-renewal is the process by which stem cells divide to make more stem cells, perpetuating the stem cell pool throughout life. Self-renewal is division with maintenance of the undifferentiated state. This requires cell cycle control and often maintenance of multipotency or pluripotency, depending on the stem cell. Self-renewal programs involve networks that balance proto-oncogenes (promoting self-renewal), gate-keeping tumor suppressors (limiting self-renewal), and care-taking tumor suppressors (maintaining genomic integrity). These cell-intrinsic mechanisms are regulated by cell-extrinsic signals from the niche, the microenvironment that maintains stem cells and regulates their function in tissues. In response to changing tissue demands, stem cells undergo changes in cell cycle status and developmental potential over time, requiring different self-renewal programs at different stages of life. Reduced stem cell function and tissue regenerative capacity during aging are caused by changes in self-renewal programs that augment tumor suppression. Cancer arises from mutations that inappropriately activate self-renewal programs.

Mesenchymal stem cells cultured under hypoxia escape from senescence via down-regulation of p16 and extracellular signal regulated kinase

Hypoxia has been considered to affect the properties of tissue stem cells including mesenchymal stem cells (MSCs). Effects of long periods of exposure to hypoxia on human MSCs, however, have not been clearly demonstrated. MSCs cultured under normoxic conditions (20% pO(2)) ceased to proliferate after 15-25 population doublings, while MSCs cultured under hypoxic conditions (1% pO(2)) retained the ability to proliferate with an additional 8-20 population doublings. Most of the MSCs cultured under normoxic conditions were in a senescent state after 100days, while few senescent cells were found in the hypoxic culture, which was associated with a down-regulation of p16 gene expression. MSCs cultured for 100days under hypoxic conditions were superior to those cultured under normoxic conditions in the ability to differentiate into the chondro- and adipogenic, but not osteogenic, lineage. Among the molecules related to mitogen-activated protein kinase (MAPK) signaling pathways, extracellular signal regulated kinase (ERK) was significantly down-regulated by hypoxia, which helped to inhibit the up-regulation of p16 gene expression. Therefore, the hypoxic culture retained MSCs in an undifferentiated and senescence-free state through the down-regulation of p16 and ERK.

Healing and hurting: molecular mechanisms, functions, and pathologies of cellular senescence

Cellular senescence is a proliferation arrest that is typically irreversible and caused by various cellular stresses, including excess rounds of cell division and cancer-causing genetic alterations. Senescence actively contributes to a tissue-level response to tissue wounding and incipient cancer, healing the tissue and suppressing tumor formation. However, in the long term, the same senescence program may hurt the tissue, thereby contributing to tissue aging. Tumor suppression, wound healing, and aging are each associated with inflammation, and here it is proposed that cellular senescence contributes to a "nonimmune cell" component of the tissue inflammatory response.

Early-life nutrition influences thymic growth in male mice that may be related to the regulation of longevity

Nutrition and growth rate during early life can influence later health and lifespan. We have demonstrated previously that low birthweight, resulting from maternal protein restriction during pregnancy followed by catch-up growth in rodents, was associated with shortened lifespan, whereas protein restriction and slow growth during lactation increased lifespan. The underlying mechanisms by which these differences arise are unknown. In the present study, we report that maternal protein restriction in mice influences thymic growth in early adult life. Offspring of dams fed a low-protein diet during lactation (PLP offspring) had significant thymic growth from 21 days to 12 weeks of age, whereas this was not observed in control mice or offspring of dams fed a low-protein diet during pregnancy (recuperated offspring). PCNA (proliferating-cell nuclear antigen) and SIRT1 (silent information regulator 1) protein levels at 21 days of age were significantly higher in the thymus from both PLP mice (P<0.001 and P<0.05 respectively) and recuperated mice (P<0.001 and P<0.01 respectively) compared with controls. At 12 weeks, PLP mice maintained a higher SIRT1 level, whereas PCNA was decreased in the thymus from recuperated offspring. This suggests that mitotic activity was initially enhanced in the thymus from both PLP and recuperated offspring, but remained sustained into adulthood only in PLP mice. The differential mitotic activity in the thymus from PLP and recuperated mice appeared to be influenced by changes in sex hormone concentrations and the expression of p53, p16, the androgen receptor, IL-7 (interleukin-7) and the IL-7 receptor. In conclusion, differential thymic growth may contribute to the regulation of longevity by maternal diet.

Aged mouse ovaries possess rare premeiotic germ cells that can generate oocytes following transplantation into a young host environment

Of all the major organ systems in the body, the ovaries of females are the first to exhibit impaired function with advancing age. Until recently, traditional thinking was that female mammals are provided with a non-renewable pool of oocyte-containing follicles at birth that are depleted during postnatal life to exhaustion, driving ovarian failure. However, a growing body of evidence, including the isolation of germline stem cells (GSC) from adult mouse ovaries that produce developmentally-competent oocytes, has challenged this belief. In addition, rare germline stem-like cells capable of generating oocytes in vitro that undergo parthenogenesis to form blastocyst-like structures have recently been identified in postmenopausal human ovaries. Here we show that the germline-specific meiosis-commitment genes,Stimulated by retinoic acid gene 8 (Stra8) and Deleted in azoospermia-like (Dazl), are highly expressed in aged mouse ovaries. However, histological and marker analyses fail to demonstrate the presence of oocytes, supporting that Stra8 and Dazl are expressed in premeiotic germ cells that do not undergo further differentiation. Through the use of aged germline-specific GFP-expressing transgenic mice, we further show that these germ cells can generate GFP-positive oocytes that co-express the primordial oocyte marker NOBOX and form follicles when grafted into young adult wild-type female hosts. Thus, aged mouse ovaries possess a rare population of premeiotic germ cells that retain the capacity to form oocytes if exposed to a young host environment.

Repair of senescent myocardium by mesenchymal stem cells is dependent on the age of donor mice

Myocardial infarction is one of the leading causes of mortality in aged people. Whether age of donors of mesenchymal stem cells (MSCs) affects its ability to repair the senescent heart tissue is unknown. In the present study, MSCs from young (2 months) and aged (18 months) green fluorescent protein expressing C57BL/6 mice were characterized with p16(INK4a) and β-gal associated senescence. Myocardial infarction was produced in 18-month-old wild-type C57BL/6 mice transplanted with MSCs from young and aged animals in the border of the infarct region. Expression of p16(INK4a) in MSCs from aged animals was significantly higher (21.5%± 1.2, P < 0.05) as compared to those from young animals (9.2%± 2.8). A decline in the tube-forming ability on Matrigel was also observed in aged MSCs as well as down-regulation of insulin-like growth factor-1, fibroblast growth factor (FGF-2), vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF) compared to young cells. Mice transplanted with young MSCs exhibited significant improvement in their left ventricle (LV) systolic and diastolic function as demonstrated by dp/dt(max) , dp/dt(min) , P(max) . Reduction in the LV fibrotic area was concomitant with neovascularization as demonstrated by CD31 and smooth muscle actin (SMA) expression. Real-time RT-PCR analysis for VEGF, stromal cell derived factor (SDF-1α) and GATA binding factor 4 (GATA-4) genes further confirmed the effect of age on MSC differentiation towards cardiac lineages and enhanced angiogenesis. These studies lead to the conclusion that repair potential of MSCs is dependent on the age of donors and the repair of senescent infarcted myocardium requires young healthy MSCs.