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Mitochondrial dysfunction induces a signaling pathway, which culminates in changes in the expression of many nuclear genes. This retrograde response, as it is called, extends yeast replicative life span. It also results in a marked increase in the cellular content of extrachromsomal ribosomal DNA circles (ERCs), which can cause the demise of the cell. We have resolved the conundrum of how these two molecular mechanisms of yeast longevity operate in tandem. About 50% of the life-span extension elicited by the retrograde response involves processes other than those that counteract the deleterious effects of ERCs. Deletion of RTG2, a gene that plays a central role in relating the retrograde response signal to the nucleus, enhances the generation of ERCs in cells with (grande) or in cells without (petite) fully functional mitochondria, and it curtails the life span of each. In contrast, overexpression of RTG2 diminishes ERC formation in both grandes and petites. The excess Rtg2p did not augment the retrograde response, indicating that it was not engaged in retrograde signaling. FOB1, which is known to be required for ERC formation, and RTG2 were found to be in converging pathways for ERC production. RTG2 did not affect silencing of ribosomal DNA in either grandes or petites, which were similar to each other in the extent of silencing at this locus. Silencing of ribosomal DNA increased with replicative age in either the presence or the absence of Rtg2p, distinguishing silencing and ERC accumulation. Our results indicate that the suppression of ERC production by Rtg2p requires that it not be in the process of transducing the retrograde signal from the mitochondrion. Thus, RTG2 lies at the nexus of cellular metabolism and genome stability, coordinating two pathways that have opposite effects on yeast longevity.

Signal transducers and activators of transcription (Stats) play central roles in the conversion of extracellular signals, e.g., cytokines, hormones and growth factors, into tissue and cell type specific gene expression patterns. In normal cells, their signaling potential is strictly limited in extent and duration. The persistent activation of Stat3 or Stat5 is found in many human tumor cells and contributes to their growth and survival. Stat5 activation plays a pivotal role in nearly all hematological malignancies and occurs downstream of oncogenic kinases, e.g., Bcr-Abl in chronic myeloid leukemias (CML) and Jak2(V617F) in other myeloproliferative diseases (MPD). We defined the mechanisms through which Stat5 affects growth and survival of K562 cells, representative of Bcr-Abl positive CML, and HEL cells, representative for Jak2(V617F) positive acute erythroid leukemia. In our experiments we suppressed the protein expression levels of Stat5a and Stat5b through shRNA mediated downregulation and demonstrated the dependence of cell survival on the presence of Stat5. Alternatively, we interfered with the functional capacities of the Stat5 protein through the interaction with a Stat5 specific peptide ligand. This ligand is a Stat5 specific peptide aptamer construct which comprises a 12mer peptide integrated into a modified thioredoxin scaffold, S5-DBD-PA. The peptide sequence specifically recognizes the DNA binding domain (DBD) of Stat5. Complex formation of S5-DBD-PA with Stat5 causes a strong reduction of P-Stat5 in the nuclear fraction of Bcr-Abl-transformed K562 cells and a suppression of Stat5 target genes. Distinct Stat5 mediated survival mechanisms were detected in K562 and Jak2(V617F)-transformed HEL cells. Stat5 is activated in the nuclear and cytosolic compartments of K562 cells and the S5-DBD-PA inhibitor most likely affects the viability of Bcr-Abl+ K562 cells through the inhibition of canonical Stat5 induced target gene transcription. In HEL cells, Stat5 is predominantly present in the cytoplasm and the survival of the Jak2(V617F)+ HEL cells is impeded through the inhibition of the cytoplasmic functions of Stat5. [ABSTRACT FROM AUTHOR]

The signal transducer and activator of transcription Stat5 is transiently activated by growth factor and cytokine signals in normal cells, but its persistent activation has been observed in a wide range of human tumors. Aberrant Stat5 activity was initially observed in leukemias, but subsequently also found in carcinomas. We investigated the importance of Stat5 in human tumor cell lines. shRNA mediated downregulation of Stat5 revealed the dependence of prostate and breast cancer cells on the expression of this transcription factor. We extended these inhibition studies and derived a peptide aptamer (PA) ligand, which directly interacts with the DNA-binding domain of Stat5 in a yeast-two-hybrid screen. The Stat5 specific PA sequence is embedded in a thioredoxin (hTRX) scaffold protein. The resulting recombinant protein S5-DBD-PA was expressed in bacteria, purified and introduced into tumor cells by protein transduction. Alternatively, S5-DBD-PA was expressed in the tumor cells after infection with a S5-DBD-PA encoding gene transfer vector. Both strategies impaired the DNA-binding ability of Stat5, suppressed Stat5 dependent transactivation and caused its intracellular degradation. Our experiments describe a peptide based inhibitor of Stat5 protein activity which can serve as a lead for the development of a clinically useful compound for cancer treatment. [ABSTRACT FROM AUTHOR]

Cytotoxic agents, alone or in combination, are being used in the treatment of colorectal cancer. Despite progress in the therapeutic regimes, this common malignancy is still the cause of considerable morbidity and mortality, and further improvements are required. Cancer cells often exhibit intrinsic resistance against chemotherapeutic agents or they develop resistance over the time of treatment. Several mechanisms have been made responsible, e.g., drugs may fail to reach tumor cells or drugs may fail to elicit cytotoxicity. The molecular characterization of drug resistance in cancer cells may lead to strategies to overcome it and enhance the sensitivity to chemotherapy. Irinotecan is one of the main treatments of colorectal cancer; it is converted into its active metabolite SN38 and acts as a topoisomerase I inhibitor. Inhibition of this enzyme prevents DNA relegation following uncoiling. Irinotecan has been used as a chemotherapeutic agent either as a single agent or in combination with 5-fluorouracil and targeted therapies directed against the epidermal growth factor receptor, such as cetuximab. The transcription factor signal transducer and activator of transcription 3 (Stat3) is a member of the signal transducer and activator of transcription protein family. Its persistent activation is found in tumor cells and has been associated with drug and radiation resistance. The treatment of colorectal cancer cells with irinotecan leads to senescence or apoptosis following DNA double-strand break induction. This process is impaired by the activation of Stat3. We have derived a Stat3 specific peptide aptamer [recombinant Stat3 inhibitory peptide aptamer (rS3-PA)] that recognizes the dimerization domain of Stat3 and effectively inhibits its function. The delivery of rS3-PA into colon cancer cells and the resulting inhibition of Stat3 strongly enhanced the cytotoxic action of SN38. These data show that the targeted inhibition of Stat3 decreases drug resistance and enhances SN38-mediated cell death. The combination of these agents has a potent antitumor effect and could become beneficial for the treatment of patients with colorectal cancer. [ABSTRACT FROM AUTHOR]

Signal transducer and activator of transcription 3 (Stat3) assumes central functions in the regulation of apoptosis, proliferation, angiogenesis, and immune responses in normal cells. It also plays crucial roles in inflammatory and malignant diseases and in the cellular communication in the tissue microenvironment. Signaling interactions among normal endothelial cells, immune cells, and tumor cells, mediated by the release of cytokines, chemokines, and growth factors, often result in the activation of Stat3 and promotion of cancer cell proliferation, invasion, angiogenesis, and immune evasion. Stat3 also causes the differentiation and activation of T helper 17 (Th17) cells, which is involved, e.g., in psoriasis, an inflammatory autoimmune disease of the skin. Here, we describe molecular characteristics of a mouse model triggered by the treatment of mouse skin with the immune modulator imiquimod. The application of this compound causes the local release of proinflammatory cytokines and symptoms that resemble human psoriasis. We show that this process is accompanied by strong Stat3 activation. We also investigated the effects of a membrane-permeable, peptide-based Stat3 inhibitor, recombinant Stat3-specific peptide aptamer (rS3-PA). This molecule specifically interacts with Stat3 and prevents its transactivation potential in cultured cells. rS3-PA is able to penetrate the skin, enter cells, and reduce the level of activated Stat3. The topical applications of rS3-PA to the skin could thus possibly become useful in the treatment of inflammatory skin diseases and skin cancer. [ABSTRACT FROM AUTHOR]

We are characterizing peptides which are able to interact with functional domains of oncoproteins and thus inhibit their activity. The yeast two-hybrid system was used to derive a peptide sequence which specifically interacts with the dimerization domain of the transcription factor Stat3. The activated form of Stat3 is required for the survival of many transformed cells and Stat3 inhibition can cause tumor cell death. The genetic selection of specific peptide sequences from random peptide libraries requires the integration into a scaffold protein and the expression in yeast cells. The scaffold protein, a variant of the human thioredoxin protein, has previously been optimized and also allows for effective bacterial expression of the recombinant protein and the cellular uptake of the purified, recombinant protein. We investigated the contributions of the scaffold protein to the inhibitory properties of rS3-PA. For this purpose we compared rS3-PA in which the ligand peptide is embedded within the thioredoxin scaffold protein with a minimal Stat3-interacting peptide sequence. sS3-P45 is a synthetic peptide of 45 amino acids in length and consists only of the Stat3-binding sequence of 20 amino acids, a protein transduction domain (PTD) and a Flag-tag. Both, the recombinant rS3-PA of 19.3 kDa and the synthetic sS3-P45 of 5.1 kDa, were taken up into the cytoplasm of cells by the PTD-mediated transduction process, inhibited Stat3 target gene expression and caused the death of Stat3-dependent tumor cells. Stat3-independent normal cells were unaffected. rS3-PA effectively inhibited Stat3 function at 2 μM, however, sS3-P45 was required at a concentration of 100 μM to exert the same effects. The more potent action of rS3-PA is most probably due to a conformational stabilization of the Stat3-interacting peptide in the context of the scaffold protein. [ABSTRACT FROM AUTHOR]

Background: Signal transducer and activator of transcription 3 and 5 (Stat3 and Stat5) play important roles in cell differentiation, proliferation, apoptosis and inflammation. They are transiently activated by ligand-receptor interactions in normal cells but are often found to be constitutively active in cancer cells. Analysis of their activation pattern is therefore important for the description of developmental processes and the understanding of cellular transformation. Materials and methods: To visualize Stat3 and Stat5 transactivation activity in different cell types, we designed novel reporter constructs. These constructs comprise Stat3 or Stat5 specific promoter elements and reporter genes encoding bgalactosidase or fluorescent proteins. These constructs were integrated into lentiviral gene transfer vectors facilitating efficient transduction of most cell types. Results: The lentiviral reporter constructs were used to infect different cell types and their inducibility by activated Stat3 or Stat5 was measured. The Stat3-mCherry reporter was active in transduced tumor cells, which exhibit high levels of phosphorylated Stat3 and it was inducible in HepG2 liver cells by interleukin-6 treatment. The Stat5-LacZ reporter was active in cultured cells upon hormone induction of Stat5 and in primary mammary epithelial cells transplanted into cleared fat pads of mice during late pregnancy. Conclusion: These novel reporter constructs are valuable tools to investigate and to distinguish between Stat3 and Stat5 activity in primary cells and cancer cells. They will also be useful in the discovery of drugs targeting Stat3 or Stat5. They can also be employed to generate transgenic mice and track Stat activity during development. [ABSTRACT FROM AUTHOR]

The completion of the human genome sequence and the development of new techniques, which allow the visualisation of comprehensive gene expression patterns, has led to the identification of a large number of gene products differentially expressed in tumours and corresponding normal tissues. The task at hand is the sorting of these genes into correlative and causative ones. Correlative genes are merely changed as a consequence of transformation and have no decisive effects upon transformation. In contrast, causative genes play a direct role in the process of cellular transformation and the maintenance of the transformed state, which can be exploited for therapeutic purposes. Oncogenes and tumour suppressor genes are prime targets for the development of new inhibitors and gene therapeutic strategies. However, many target oncogene products do not exhibit enzymatic activity that can be inhibited by conventional small molecular weight compounds. They exert their functions through regulated protein-protein or protein-DNA interactions and might require other compounds for efficient interference with such functions. Peptides are emerging as a novel class of drugs for cancer therapy, which could fulfil these tasks. Peptide therapy aims at the specific inhibition of inappropriately activated oncogenes. This review will focus on the selection procedures, which can be employed to identify useful peptides for the treatment of cancer. Before peptide-based therapeutics can become useful, it will be necessary to increase their stability by modifications or the use of scaffolds. Additionally, various delivery methods including liposomes and particularly the use of protein transduction domains (PTDs) have to be explored. These strategies will yield highly specific and more effective peptides and improve the potential of peptide-based anti-cancer therapeutics. Copyright © 2005 European Peptide Society and John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Lifespan extension of Podospora anserina mutant grisea is caused by a loss-of-function mutation in the nuclear gene Grisea. This gene encodes the copper regulated transcription factor GRISEA recently shown to be involved in the expression of PaSod2 encoding the mitochondrial manganese superoxide dismutase. Here we report the identification and characterization of a second target gene. This gene, PaCtr3, encodes a functional homologue of the Saccharomyces cerevisiae high affinity copper permease yCTR3. PaCtr3 is not expressed in the grisea mutant confirming the assumption that the extension of lifespan is primarily caused by cellular copper limitation and a switch from a cytochrome oxidase (COX)-dependent to and alternative oxidase (AOX)-dependent respiration. Transcript levels of PaCtr3 and PaSod2 respond to copper, iron, manganese and zinc. Transcription of PaCtr3 was found to be down-regulated during senescence of wild-type cultures suggesting that the intracellular copper concentration is raised in old cultures. A two hybrid analysis suggested that GRISEA acts as a homodimer. In accordance, an inverted repeat was identified as a putative binding sequence in the promoter region of PaCtr3 and of PaSod2. Finally, the expression of PaCtr3 in transformants of the grisea mutant led to lifespan shortening. This effect correlates with the activity of the copper-dependent COX demonstrating a strong link between copper-uptake, respiration and lifespan. [Copyright &y& Elsevier]

Abstract: Stat3 was initially recognized as a transcription factor and mediates the nuclear action of many different cytokines and growth factors. In addition to its roles in normal cell function, the inappropriate activation of Stat3 in tumor cells has attracted the attention of tumor biologists and has led to the consideration of Stat3 as a drug target. The induction of Stat3 activity under physiological circumstances is transient and many different levels of activation and deactivation have been defined. In addition to kinases and phosphatases, the SOCS proteins and the PIAS proteins have been recognized as negatively regulating components, which fine-tune the extent and the duration of Stat3 function. Its nuclear cytoplasmic shuttling is exquisitely regulated and adds to the complexity of Stat3 action. Newly discovered associations with cytoplasmic molecules suggest functions outside the conventional transcriptional regulation context. High molecular weight transcription complexes suggest that Stat3 might assume roles in transcriptional induction as well as in transcriptional suppression. The aberrant activation in tumor cells and the central function of Stat3 in the communication between cells of the immune system and tumor cells are of great interest for translational research projects and innovative drug development. [Copyright &y& Elsevier]

Investigates the mechanisms of age asymmetry between mother and daughter cells in Saccharomyces cerevisiae. Association of mitochondrial dysfunction with aging; Reproduction of yeast S. cerevisiae by asymmetric cell division; Importance of mitochondrial segregation in the maintenance of age asymmetry.

Signal transducers and activators of transcription (Stats) play central roles in the conversion of extracellular signals, e.g., cytokines, hormones and growth factors, into tissue and cell type specific gene expression patterns. In normal cells, their signaling potential is strictly limited in extent and duration. The persistent activation of Stat3 or Stat5 is found in many human tumor cells and contributes to their growth and survival. Stat5 activation plays a pivotal role in nearly all hematological malignancies and occurs downstream of oncogenic kinases, e.g., Bcr-Abl in chronic myeloid leukemias (CML) and Jak2(V617F) in other myeloproliferative diseases (MPD). We defined the mechanisms through which Stat5 affects growth and survival of K562 cells, representative of Bcr-Abl positive CML, and HEL cells, representative for Jak2(V617F) positive acute erythroid leukemia. In our experiments we suppressed the protein expression levels of Stat5a and Stat5b through shRNA mediated downregulation and demonstrated the dependence of cell survival on the presence of Stat5. Alternatively, we interfered with the functional capacities of the Stat5 protein through the interaction with a Stat5 specific peptide ligand. This ligand is a Stat5 specific peptide aptamer construct which comprises a 12mer peptide integrated into a modified thioredoxin scaffold, S5-DBD-PA. The peptide sequence specifically recognizes the DNA binding domain (DBD) of Stat5. Complex formation of S5-DBD-PA with Stat5 causes a strong reduction of P-Stat5 in the nuclear fraction of Bcr-Abl-transformed K562 cells and a suppression of Stat5 target genes. Distinct Stat5 mediated survival mechanisms were detected in K562 and Jak2(V617F)-transformed HEL cells. Stat5 is activated in the nuclear and cytosolic compartments of K562 cells and the S5-DBD-PA inhibitor most likely affects the viability of Bcr-Abl+ K562 cells through the inhibition of canonical Stat5 induced target gene transcription. In HEL cells, Stat5 is predominantly present in the cytoplasm and the survival of the Jak2(V617F)+ HEL cells is impeded through the inhibition of the cytoplasmic functions of Stat5.

Signaling components, which confer an "addiction" phenotype on cancer cells, represent promising drug targets. The transcription factor signal transducers and activators of transcription 3 (STAT3) is constitutively activated in many different types of tumor cells and its activity is indispensible in a large fraction. We found that the expression of the endogenous inhibitor of STAT3, protein inhibitor of activated STAT3 (PIAS3), positively correlates with STAT3 activation in normal cells. This suggests that PIAS3 controls the extent and the duration of STAT3 activity in normal cells and thus prevents its oncogenic function. In cancer cells, however, the expression of PIAS3 is posttranscriptionally suppressed, possibly enhancing the oncogenic effects of activated STAT3. We delimited the interacting domains of STAT3 and PIAS3 and identified a short fragment of the COOH-terminal acidic region of PIAS3, which binds strongly to the coiled-coil domain of STAT3. This PIAS3 fragment was used to derive the recombinant STAT3-specific inhibitor rPP-C8. The addition of a protein transduction domain allowed the efficient internalization of rPP-C8 into cancer cells. This resulted in the suppression of STAT3 target gene expression, in the inhibition of migration and proliferation, and in the induction of apoptosis at low concentrations [half maximal effective concentration (EC(50)), <3 micromol/L]. rPP-C8 did not affect normal fibroblasts and represents an interesting lead for the development of novel cancer drugs targeting the coiled-coil domain of STAT3., ((c) 2010 AACR.)

Peptide aptamers are molecules that bind to protein targets and are able to interfere with their functions. In the past, important achievements have been made using such peptide aptamers in different approaches and for various purposes. Peptide aptamers are comprised of a variable peptide region of 8 to 20 amino acids in length, which is displayed by a scaffold protein. An overview of the numerous scaffold proteins that have been investigated for their suitability to present peptide aptamers will be given. To identify peptide aptamers efficiently and specifically binding to a predetermined target, two eukaryotic systems have been used in multiple studies: a modified version of the Gal4 yeast-two-hybrid system and the optimized LexA interaction trap system. The two yeast systems are compared and the design of high-complexity peptide aptamer libraries for these systems is described. Although the yeast-two-hybrid system is based on intracellular interactions mammalian screens, performed in cell culture experiments, are sometimes preferred or required. We will give an overview of the mammalian selection systems available, which are based on the expression of peptide aptamers in retroviral or lentiviral vectors. We will show that the isolation and use of peptide aptamers as inhibitors of individual signaling components represents a new challenge for drug development.

Signal transduction events often involve the assembly of protein complexes dependent on modular interactions. The inappropriate assembly of modular components plays a role in oncogenic transformation and can be exploited for therapeutic purposes. Selected peptides embedded in the context of a scaffold protein can serve as competitive inhibitors of intracellular protein functions in cancer cells. Therapeutic application depends on binding specificities and affinities, as well as on the production and purification characteristics of the peptide aptamers and their delivery into cells. We carried out experiments to improve the properties of the scaffold. We found that the commonly used bacterial thioredoxin scaffold is suboptimal for therapeutic purposes because it aggregates during purification and is most likely immunogenic in humans. We compared the properties of peptide aptamers embedded in three alternative scaffold structures: a coiled-coil stem-loop structure, a dimerization domain, and human thioredoxin (hTrx). We found that only the hTrx molecule can be efficiently produced in bacteria and purified with high yield. We removed five internal cysteines of hTrx to circumvent aggregation during purification, which is a prerequisite for efficient transduction. Insertion of our previously characterized peptide aptamers [e.g., specifically binding signal transducer and activator of transcription 3 (Stat3)] into the modified hTrx scaffold retained their target binding properties. Addition of a protein transduction domain, consisting of nine arginines, results in a fusion protein, which is taken up by cultured cells. We show that treatment of glioblastoma cells, expressing constitutively activated Stat3, with the purified peptide aptamers strongly inhibits Stat3 signaling, causing cell growth arrest and inducing apoptosis.

The ErbB2 receptor tyrosine kinase is overexpressed in approximately 30% of breast tumor cases and its overexpression correlates with an unfavorable prognosis. A major contributor for this course of the disease is the insensitivity of these tumors toward chemotherapy. Monoclonal antibodies, inhibiting the ligand-induced activation of the receptor and tyrosine kinase inhibitors acting on the intrinsic enzymatic activity of the intracellular domain, have been developed as targeted drugs. Both have been shown to be beneficial for breast cancer patients. We targeted a third aspect of receptor function: its association with intracellular signaling components. For this purpose, we selected peptide aptamers, which specifically interact with defined domains of the intracellular part of the receptor. The peptide aptamers were selected from a random peptide library using a yeast two-hybrid system with the intracellular tyrosine kinase domain of ErbB2 as a bait construct. The peptide aptamer AII-7 interacts with high specificity with the ErbB2 receptor in vitro and in vivo. The aptamers colocalized with the intracellular domain of ErbB2 within cells. We investigated the functional consequences of the aptamer interaction with the ErbB2 receptor within tumor cells. The aptamer sequences were either expressed intracellularly or introduced into the cells as recombinant aptamer proteins. The phosphorylation of p42/44 mitogen-activated protein kinase was nearly unaffected and the activation of signal transducers and activators of transcription-3 was only modestly reduced. In contrast, they strongly inhibited the induction of AKT kinase in MCF7 breast cancer cells treated with heregulin, whereas AKT activation downstream of insulin-like growth factor I or epidermal growth factor receptor was not or only slightly affected. High AKT activity is responsible for the enhanced resistance of ErbB2-overexpressing cancer cells toward chemotherapeutic agents. Peptide aptamer interference with AKT activation resulted in the restoration of regular sensitivity of breast cancer cells toward Taxol.

During the past two decades, our understanding of oncogenesis has advanced considerably and many new signalling pathways have been identified. Differences in signalling events that distinguish normal cells from tumour cells provide new targets for the development of anticancer agents. Peptide aptamers are small peptide sequences that have been selected to recognise a predetermined target protein domain and are potentially able to interfere with its function. They represent useful molecules for manipulating protein function in vivo. The isolation and use of specific peptide aptamers as inhibitors of individual signalling components, essential in cancer development and progression, provides a new challenge for drug development. Although peptides make up only a small fraction of current therapeutics, their potential is being enhanced by new developments affecting their modification, stability, delivery and their successful application in preclinical settings. This review summarises the methods that can be used for the isolation and delivery of peptide aptamers, as well as the important achievements that have been made using such peptide aptamers in different systems. The applicability of peptide aptamers as novel cancer therapeutics will be discussed.

The yeast Saccharomyces cerevisiae reproduces by asymmetric cell division, or budding. In each cell division, the daughter cell is usually smaller and younger than the mother cell, as defined by the number of divisions it can potentially complete before it dies. Although individual yeast cells have a limited life span, this age asymmetry between mother and daughter ensures that the yeast strain remains immortal. To understand the mechanisms underlying age asymmetry, we have isolated temperature-sensitive mutants that have limited growth capacity. One of these clonal-senescence mutants was in ATP2, the gene encoding the beta-subunit of mitochondrial F(1), F(0)-ATPase. A point mutation in this gene caused a valine-to-isoleucine substitution at the ninetieth amino acid of the mature polypeptide. This mutation did not affect the growth rate on a nonfermentable carbon source. Life-span determinations following temperature shift-down showed that the clonal-senescence phenotype results from a loss of age asymmetry at 36 degrees, such that daughters are born old. It was characterized by a loss of mitochondrial membrane potential followed by the lack of proper segregation of active mitochondria to daughter cells. This was associated with a change in mitochondrial morphology and distribution in the mother cell and ultimately resulted in the generation of cells totally lacking mitochondria. The results indicate that segregation of active mitochondria to daughter cells is important for maintenance of age asymmetry and raise the possibility that mitochondrial dysfunction may be a normal cause of aging. The finding that dysfunctional mitochondria accumulated in yeasts as they aged and the propensity for old mother cells to produce daughters depleted of active mitochondria lend support to this notion. We propose, more generally, that age asymmetry depends on partition of active and undamaged cellular components to the progeny and that this "filter" breaks down with age.