Copper homeostasis and aging in the fungal model system Podospora anserina: differential expression of PaCtr3 encoding a copper transporter

Impact of Mitochondrial Architecture, Function, Redox Homeostasis, and Quality Control on Organismic Aging: Lessons from a Fungal Model System

Significance: Mitochondria are eukaryotic organelles with various essential functions. They are both the source and the targets of reactive oxygen species (ROS). Different branches of a mitochondrial quality control system (mQCS), such as ROS balancing, degradation of damaged proteins, or whole mitochondria, can mitigate the adverse effects of ROS stress. However, the capacity of mQCS is limited. Overwhelming this capacity leads to dysfunctions and aging. Strategies to interfere into mitochondria-dependent human aging with the aim to increase the healthy period of life, the health span, rely on the precise knowledge of mitochondrial functions. Experimental models such as Podospora anserina, a filamentous fungus with a clear mitochondrial aging etiology, proved to be instrumental to reach this goal. Recent Advances: Investigations of the P. anserina mQCS revealed that it is constituted by a complex network of different branches. Moreover, mitochondrial architecture and lipid homeostasis emerged to affect aging. Critical Issues: The regulation of the mQCS is only incompletely understood. Details about the involved signaling molecules and interacting pathways remain to be elucidated. Moreover, most of the currently generated experimental data were generated in well-controlled experiments that do not reflect the constantly changing natural life conditions and bear the danger to miss relevant aspects leading to incorrect conclusions. Future Directions: In P. anserina, the precise impact of redox signaling as well as of molecular damaging for aging remains to be defined. Moreover, natural fluctuation of environmental conditions needs to be considered to generate a realistic picture of aging mechanisms as they developed during evolution.

The impact of biomembranes and their dynamics on organismic aging: insights from a fungal aging model

Biomembranes fulfill several essential functions. They delimitate cells and control the exchange of compounds between cells and the environment. They generate specialized cellular reaction spaces, house functional units such as the respiratory chain (RC), and are involved in content trafficking. Biomembranes are dynamic and able to adjust their properties to changing conditions and requirements. An example is the inner mitochondrial membrane (IMM), which houses the RC involved in the formation of adenosine triphosphate (ATP) and the superoxide anion as a reactive oxygen species (ROS). The IMM forms a characteristic ultrastructure that can adapt to changing physiological situations. In the fungal aging model Podospora anserina, characteristic age-related changes of the mitochondrial ultrastructure occur. More recently, the impact of membranes on aging was extended to membranes involved in autophagy, an important pathway involved in cellular quality control (QC). Moreover, the effect of oleic acid on the lifespan was linked to basic biochemical processes and the function of membranes, providing perspectives for the elucidation of the mechanistic effects of this nutritional component, which positively affects human health and aging.

To die or not to die - How mitochondrial processes affect lifespan of Podospora anserina

The filamentous ascomycete Podospora anserina is a well-established model system to study organismic aging. Its senescence syndrome has been investigated for more than fifty years and turned out to have a strong mitochondrial etiology. Several different mitochondrial pathways were demonstrated to affect aging and lifespan. Here, we present an update of the literature focusing on the cooperative interplay between different processes.

Superoxide Dismutases in Eukaryotic Microorganisms: Four Case Studies

Various components in the cell are responsible for maintaining physiological levels of reactive oxygen species (ROS). Several different enzymes exist that can convert or degrade ROS; among them are the superoxide dismutases (SODs). If left unchecked, ROS can cause damage that leads to pathology, can contribute to aging, and may, ultimately, cause death. SODs are responsible for converting superoxide anions to hydrogen peroxide by dismutation. Here we review the role of different SODs on the development and pathogenicity of various eukaryotic microorganisms relevant to human health. These include the fungal aging model, Podospora anserina; various members of the genus Aspergillus that can potentially cause aspergillosis; the agents of diseases such as Chagas and sleeping disease, Trypanosoma cruzi and Trypanosoma brucei, respectively; and, finally, pathogenic amoebae, such as Acanthamoeba spp. In these organisms, SODs fulfill essential and often regulatory functions that come into play during processes such as the development, host infection, propagation, and control of gene expression. We explore the contribution of SODs and their related factors in these microorganisms, which have an established role in health and disease.

A Network of Pathways Controlling Cellular Homeostasis Affects the Onset of Senescence in Podospora anserina

Research on Podospora anserina unraveled a network of molecular pathways affecting biological aging. In particular, a number of pathways active in the control of mitochondria were identified on different levels. A long-known key process active during aging of P. anserina is the age-related reorganization of the mitochondrial DNA (mtDNA). Mechanisms involved in the stabilization of the mtDNA lead to lifespan extension. Another critical issue is to balance mitochondrial levels of reactive oxygen species (ROS). This is important because ROS are essential signaling molecules, but at increased levels cause molecular damage. At a higher level of the network, mechanisms are active in the repair of damaged compounds. However, if damage passes critical limits, the corresponding pathways are overwhelmed and impaired molecules as well as those present in excess are degraded by specific enzymes or via different forms of autophagy. Subsequently, degraded units need to be replaced by novel functional ones. The corresponding processes are dependent on the availability of intact genetic information. Although a number of different pathways involved in the control of cellular homeostasis were uncovered in the past, certainly many more exist. In addition, the signaling pathways involved in the control and coordination of the underlying pathways are only initially understood. In some cases, like the induction of autophagy, ROS are active. Additionally, sensing and signaling the energetic status of the organism plays a key role. The precise mechanisms involved are elusive and remain to be elucidated.

Impaired F1Fo-ATP-Synthase Dimerization Leads to the Induction of Cyclophilin D-Mediated Autophagy-Dependent Cell Death and Accelerated Aging

Mitochondrial F1Fo-ATP-synthase dimers play a critical role in shaping and maintenance of mitochondrial ultrastructure. Previous studies have revealed that ablation of the F1Fo-ATP-synthase assembly factor PaATPE of the ascomycete Podospora anserina strongly affects cristae formation, increases hydrogen peroxide levels, impairs mitochondrial function and leads to premature cell death. In the present study, we investigated the underlying mechanistic basis. Compared to the wild type, we observed a slight increase in non-selective and a pronounced increase in mitophagy, the selective vacuolar degradation of mitochondria. This effect depends on the availability of functional cyclophilin D (PaCYPD), the regulator of the mitochondrial permeability transition pore (mPTP). Simultaneous deletion of PaAtpe and PaAtg1, encoding a key component of the autophagy machinery or of PaCypD, led to a reduction of mitophagy and a partial restoration of the wild-type specific lifespan. The same effect was observed in the PaAtpe deletion strain after inhibition of PaCYPD by its specific inhibitor, cyclosporin A. Overall, our data identify autophagy-dependent cell death (ADCD) as part of the cellular response to impaired F1Fo-ATP-synthase dimerization, and emphasize the crucial role of functional mitochondria in aging.

Metabolic Adaptation of Paracoccidioides brasiliensis in Response to in vitro Copper Deprivation

Copper is an essential micronutrient for the performance of important biochemical processes such as respiration detoxification, and uptake of metals like iron. Studies have shown that copper deprivation is a strategy used by the host against pathogenic fungi such as Cryptoccocus neoformans and Candida albicans during growth and development of infections in the lungs and kidneys. Although there are some studies, little is known about the impact of copper deprivation in members of the Paracoccidioides genus. Therefore, using isobaric tag labeling (iTRAQ)-Based proteomic approach and LC-MS/MS, we analyzed the impact of in vitro copper deprivation in the metabolism of Paracoccidioides brasiliensis. One hundred and sixty-four (164) differentially abundant proteins were identified when yeast cells were deprived of copper, which affected cellular respiration and detoxification processes. Changes in cellular metabolism such as increased beta oxidation and cell wall remodeling were described.

The Rhizophagus irregularis Genome Encodes Two CTR Copper Transporters That Mediate Cu Import Into the Cytosol and a CTR-Like Protein Likely Involved in Copper Tolerance

Arbuscular mycorrhizal fungi increase fitness of their host plants under Cu deficient and toxic conditions. In this study, we have characterized two Cu transporters of the CTR family (RiCTR1 and RiCTR2) and a CTR-like protein (RiCTR3A) of Rhizophagus irregularis. Functional analyses in yeast revealed that RiCTR1 encodes a plasma membrane Cu transporter, RiCTR2 a vacuolar Cu transporter and RiCTR3A a plasma membrane protein involved in Cu tolerance. RiCTR1 was more highly expressed in the extraradical mycelia (ERM) and RiCTR2 in the intraradical mycelia (IRM). In the ERM, RiCTR1 expression was up-regulated by Cu deficiency and down-regulated by Cu toxicity. RiCTR2 expression increased only in the ERM grown under severe Cu-deficient conditions. These data suggest that RiCTR1 is involved in Cu uptake by the ERM and RiCTR2 in mobilization of vacuolar Cu stores. Cu deficiency decreased mycorrhizal colonization and arbuscule frequency, but increased RiCTR1 and RiCTR2 expression in the IRM, which suggest that the IRM has a high Cu demand. The two alternatively spliced products of RiCTR3, RiCTR3A and RiCTR3B, were more highly expressed in the ERM. Up-regulation of RiCTR3A by Cu toxicity and the yeast complementation assays suggest that RiCTR3A might function as a Cu receptor involved in Cu tolerance.

Transcriptomic analysis of basidiocarp development in Ustilago maydis (DC) Cda

Previously, we demonstrated that when Ustilago maydis (DC) Cda., a phytopathogenic basidiomycete and the causal agent of corn smut, is grown in the vicinity of maize embryogenic calli in a medium supplemented with the herbicide Dicamba, it developed gastroid-like basidiocarps. To elucidate the molecular mechanisms involved in the basidiocarp development by the fungus, we proceeded to analyze the transcriptome of the process, identifying a total of 2002 and 1064 differentially expressed genes at two developmental stages, young and mature basidiocarps, respectively. Function of these genes was analyzed with the use of different databases. MIPS analysis revealed that in the stage of young basidiocarp, among the ca. two thousand differentially expressed genes, there were some previously described for basidiocarp development in other fungal species. Additional elements that operated at this stage included, among others, genes encoding the transcription factors FOXO3, MIG3, PRO1, TEC1, copper and MFS transporters, and cytochromes P450. During mature basidiocarp development, important up-regulated genes included those encoding hydrophobins, laccases, and ferric reductase (FRE/NOX). The demonstration that a mapkk mutant was unable to form basidiocarps, indicated the importance of the MAPK signaling pathway in this developmental process.

Alternative oxidase: distribution, induction, properties, structure, regulation, and functions

The respiratory chain in the majority of organisms with aerobic type metabolism features the concomitant existence of the phosphorylating cytochrome pathway and the cyanide- and antimycin A-insensitive oxidative route comprising a so-called alternative oxidase (AOX) as a terminal oxidase. In this review, the history of AOX discovery is described. Considerable evidence is presented that AOX occurs widely in organisms at various levels of organization and is not confined to the plant kingdom. This enzyme has not been found only in Archaea, mammals, some yeasts and protists. Bioinformatics research revealed the sequences characteristic of AOX in representatives of various taxonomic groups. Based on multiple alignments of these sequences, a phylogenetic tree was constructed to infer their possible evolution. The ways of AOX activation, as well as regulatory interactions between AOX and the main respiratory chain are described. Data are summarized concerning the properties of AOX and the AOX-encoding genes whose expression is either constitutive or induced by various factors. Information is presented on the structure of AOX, its active center, and the ubiquinone-binding site. The principal functions of AOX are analyzed, including the cases of cell survival, optimization of respiratory metabolism, protection against excess of reactive oxygen species, and adaptation to variable nutrition sources and to biotic and abiotic stress factors. It is emphasized that different AOX functions complement each other in many instances and are not mutually exclusive. Examples are given to demonstrate that AOX is an important tool to overcome the adverse aftereffects of restricted activity of the main respiratory chain in cells and whole animals. This is the first comprehensive review on alternative oxidases of various organisms ranging from yeasts and protists to vascular plants.

Candida albicans adapts to host copper during infection by swapping metal cofactors for superoxide dismutase

Copper is both an essential nutrient and potentially toxic metal, and during infection the host can exploit Cu in the control of pathogen growth. Here we describe a clever adaptation to Cu taken by the human fungal pathogen Candida albicans. In laboratory cultures with abundant Cu, C. albicans expresses a Cu-requiring form of superoxide dismutase (Sod1) in the cytosol; but when Cu levels decline, cells switch to an alternative Mn-requiring Sod3. This toggling between Cu- and Mn-SODs is controlled by the Cu-sensing regulator Mac1 and ensures that C. albicans maintains constant SOD activity for cytosolic antioxidant protection despite fluctuating Cu. This response to Cu is initiated during C. albicans invasion of the host where the yeast is exposed to wide variations in Cu. In a murine model of disseminated candidiasis, serum Cu was seen to progressively rise over the course of infection, but this heightened Cu response was not mirrored in host tissue. The kidney that serves as the major site of fungal infection showed an initial rise in Cu, followed by a decline in the metal. C. albicans adjusted its cytosolic SODs accordingly and expressed Cu-Sod1 at early stages of infection, followed by induction of Mn-Sod3 and increases in expression of CTR1 for Cu uptake. Together, these studies demonstrate that fungal infection triggers marked fluctuations in host Cu and C. albicans readily adapts by modulating Cu uptake and by exchanging metal cofactors for antioxidant SODs.

Identification of high-affinity copper transporters in Aspergillus fumigatus

We investigated the copper metabolism of Aspergillus fumigatus, which has not been characterized well. We cloned the putative copper transporters ctrA2 and ctrC from A. fumigatus and investigated the functions of these transporters in copper metabolism. Four putative copper transporters were identified in the A. fumigatus genome; ctrA2 and ctrC complemented CTR1 functionally and localized to the plasma membrane in Saccharomyces cerevisiae. ctrA2 and ctrC single-deletion mutants and a double-deletion mutant of ctrA2 and ctrC were constructed in A. fumigatus. The ctrA2 and ctrC double-deletion mutant exhibited a growth defect on Aspergillus minimal medium (AMM) supplemented with bathocuproine disulfonic acid (BCS) and was sensitive to H2O2. Furthermore, the deletion of ctrA2 and ctrC reduced superoxide dismutase (SOD) activity, laccase activity, and intracellular copper contents. The activities of the ctrA2 and ctrC genes were up-regulated by BCS treatment. In addition, the deletion of ctrA2 up-regulated ctrC and vice versa. ctrA2 and ctrC were localized to the A. fumigatus plasma membrane. Although ctrA2 and ctrC failed to affect the mouse survival rate, these genes affected conidial killing activity. Taken together, these results indicate that ctrA2 and ctrC may function as membrane transporters and that the involvement of these genes in pathogenicity merits further investigation.

Overexpression of Pa_1_10620 encoding a mitochondrial Podospora anserina protein with homology to superoxide dismutases and ribosomal proteins leads to lifespan extension

In biological systems, reactive oxygen species (ROS) represent 'double edged swords': as signaling molecules they are essential for proper development, as reactive agents they cause molecular damage and adverse effects like degeneration and aging. A well-coordinated control of ROS is therefore of key importance. Superoxide dismutases (SODs) are enzymes active in the detoxification of superoxide. The number of isoforms of these proteins varies among species. Here we report the characterization of the putative protein encoded by Pa_1_10620 that has been previously annotated to code for a mitochondrial ribosomal protein but shares also sequence domains with SODs. We report that the gene is transcribed in P. anserina cultures of all ages and that the encoded protein localizes to mitochondria. In strains overexpressing Pa_1_10620 in a genetic background in which PaSod3, the mitochondrial MnSOD of P. anserina, is deleted, no SOD activity could be identified in isolated mitochondria. However, overexpression of the gene leads to lifespan extension suggesting a pro-survival function of the protein in P. anserina.

Effect of paraquat-induced oxidative stress on gene expression and aging of the filamentous ascomycete Podospora anserina

Aging of biological systems is influenced by various factors, conditions and processes. Among others, processes allowing organisms to deal with various types of stress are of key importance. In particular, oxidative stress as the result of the generation of reactive oxygen species (ROS) at the mitochondrial respiratory chain and the accumulation of ROS-induced molecular damage has been strongly linked to aging. Here we view the impact of ROS from a different angle: their role in the control of gene expression. We report a genome-wide transcriptome analysis of the fungal aging model Podospora anserina grown on medium containing paraquat (PQ). This treatment leads to an increased cellular generation and release of H₂O₂, a reduced growth rate, and a decrease in lifespan. The combined challenge by PQ and copper has a synergistic negative effect on growth and lifespan. The data from the transcriptome analysis of the wild type cultivated under PQ-stress and their comparison to those of a longitudinal aging study as well as of a copper-uptake longevity mutant of P. anserina revealed that PQ-stress leads to the up-regulation of transcripts coding for components involved in mitochondrial remodeling. PQ also affects the expression of copper-regulated genes suggesting an increase of cytoplasmic copper levels as it has been demonstrated earlier to occur during aging of P. anserina and during senescence of human fibroblasts. This effect may result from the induction of the mitochondrial permeability transition pore via PQ-induced ROS, leading to programmed cell death as part of an evolutionary conserved mechanism involved in biological aging and lifespan control.

Identification of autophagy as a longevity-assurance mechanism in the aging model Podospora anserina

The filamentous ascomycete Podospora anserina is a well-established aging model in which a variety of different pathways, including those involved in the control of respiration, ROS generation and scavenging, DNA maintenance, proteostasis, mitochondrial dynamics, and programmed cell death have previously been demonstrated to affect aging and life span. Here we address a potential role of autophagy. We provide data demonstrating high basal autophagy levels even in strains cultivated under noninduced conditions. By monitoring an N-terminal fusion of EGFP to the fungal LC3 homolog PaATG8 over the lifetime of the fungus on medium with and without nitrogen supplementation, respectively, we identified a significant increase of GFP puncta in older and in nitrogen-starved cultures suggesting an induction of autophagy during aging. This conclusion is supported by the demonstration of an age-related and autophagy-dependent degradation of a PaSOD1-GFP reporter protein. The deletion of Paatg1, which leads to the lack of the PaATG1 serine/threonine kinase active in early stages of autophagy induction, impairs ascospore germination and development and shortens life span. Under nitrogen-depleted conditions, life span of the wild type is increased almost 4-fold. In contrast, this effect is annihilated in the Paatg1 deletion strain, suggesting that the ability to induce autophagy is beneficial for this fungus. Collectively, our data identify autophagy as a longevity-assurance mechanism in P. anserina and as another surveillance pathway in the complex network of pathways affecting aging and development. These findings provide perspectives for the elucidation of the mechanisms involved in the regulation of individual pathways and their interactions.

The transcription factor BcLTF1 regulates virulence and light responses in the necrotrophic plant pathogen Botrytis cinerea

Botrytis cinerea is the causal agent of gray mold diseases in a range of dicotyledonous plant species. The fungus can reproduce asexually by forming macroconidia for dispersal and sclerotia for survival; the latter also participate in sexual reproduction by bearing the apothecia after fertilization by microconidia. Light induces the differentiation of conidia and apothecia, while sclerotia are exclusively formed in the absence of light. The relevance of light for virulence of the fungus is not obvious, but infections are observed under natural illumination as well as in constant darkness. By a random mutagenesis approach, we identified a novel virulence-related gene encoding a GATA transcription factor (BcLTF1 for light-responsive TF1) with characterized homologues in Aspergillus nidulans (NsdD) and Neurospora crassa (SUB-1). By deletion and over-expression of bcltf1, we confirmed the predicted role of the transcription factor in virulence, and discovered furthermore its functions in regulation of light-dependent differentiation, the equilibrium between production and scavenging of reactive oxygen species (ROS), and secondary metabolism. Microarray analyses revealed 293 light-responsive genes, and that the expression levels of the majority of these genes (66%) are modulated by BcLTF1. In addition, the deletion of bcltf1 affects the expression of 1,539 genes irrespective of the light conditions, including the overexpression of known and so far uncharacterized secondary metabolism-related genes. Increased expression of genes encoding alternative respiration enzymes, such as the alternative oxidase (AOX), suggest a mitochondrial dysfunction in the absence of bcltf1. The hypersensitivity of Δbctlf1 mutants to exogenously applied oxidative stress - even in the absence of light - and the restoration of virulence and growth rates in continuous light by antioxidants, indicate that BcLTF1 is required to cope with oxidative stress that is caused either by exposure to light or arising during host infection.

Both fungal pathogens and their host plants respond to light, which represents an important environmental cue. Unlike plants using light for energy generation, filamentous fungi use light, or its absence, as a general signal for orientation (night/day, underground/on the surface). Therefore, dependent on the ecological niche of the fungus, light may control the development of reproductive structures (photomorphogenesis), the dispersal of propagules (phototropism of reproductive structures) and the circadian rhythm. As in other organisms, fungi have to protect themselves against the detrimental effects of light, i.e. the damage to macromolecules by emerging singlet oxygen. Adaptive responses are the accumulation of pigments, especially in the reproductive and survival structures such as spores, sclerotia and fruiting bodies. Light is sensed by fungal photoreceptors leading to quick responses on the transcriptional level, and is furthermore considered to result in the accumulation of reactive oxygen species (ROS). In this study, we provide evidence that an unbalanced ROS homoeostasis (generation outweighs detoxification) caused by the deletion of the light-responsive transcription factor BcLTF1 impairs the ability of the necrotrophic pathogen Botrytis cinerea to grow in the presence of additional oxidative stress arising during illumination or during infection of the host.

A genome-wide longitudinal transcriptome analysis of the aging model Podospora anserina

Aging of biological systems is controlled by various processes which have a potential impact on gene expression. Here we report a genome-wide transcriptome analysis of the fungal aging model Podospora anserina. Total RNA of three individuals of defined age were pooled and analyzed by SuperSAGE (serial analysis of gene expression). A bioinformatics analysis identified different molecular pathways to be affected during aging. While the abundance of transcripts linked to ribosomes and to the proteasome quality control system were found to decrease during aging, those associated with autophagy increase, suggesting that autophagy may act as a compensatory quality control pathway. Transcript profiles associated with the energy metabolism including mitochondrial functions were identified to fluctuate during aging. Comparison of wild-type transcripts, which are continuously down-regulated during aging, with those down-regulated in the long-lived, copper-uptake mutant grisea, validated the relevance of age-related changes in cellular copper metabolism. Overall, we (i) present a unique age-related data set of a longitudinal study of the experimental aging model P. anserina which represents a reference resource for future investigations in a variety of organisms, (ii) suggest autophagy to be a key quality control pathway that becomes active once other pathways fail, and (iii) present testable predictions for subsequent experimental investigations.

A differential genome-wide transcriptome analysis: impact of cellular copper on complex biological processes like aging and development

The regulation of cellular copper homeostasis is crucial in biology. Impairments lead to severe dysfunctions and are known to affect aging and development. Previously, a loss-of-function mutation in the gene encoding the copper-sensing and copper-regulated transcription factor GRISEA of the filamentous fungus Podospora anserina was reported to lead to cellular copper depletion and a pleiotropic phenotype with hypopigmentation of the mycelium and the ascospores, affected fertility and increased lifespan by approximately 60% when compared to the wild type. This phenotype is linked to a switch from a copper-dependent standard to an alternative respiration leading to both a reduced generation of reactive oxygen species (ROS) and of adenosine triphosphate (ATP). We performed a genome-wide comparative transcriptome analysis of a wild-type strain and the copper-depleted grisea mutant. We unambiguously assigned 9,700 sequences of the transcriptome in both strains to the more than 10,600 predicted and annotated open reading frames of the P. anserina genome indicating 90% coverage of the transcriptome. 4,752 of the transcripts differed significantly in abundance with 1,156 transcripts differing at least 3-fold. Selected genes were investigated by qRT-PCR analyses. Apart from this general characterization we analyzed the data with special emphasis on molecular pathways related to the grisea mutation taking advantage of the available complete genomic sequence of P. anserina. This analysis verified but also corrected conclusions from earlier data obtained by single gene analysis, identified new candidates of factors as part of the cellular copper homeostasis system including target genes of transcription factor GRISEA, and provides a rich reference source of quantitative data for further in detail investigations. Overall, the present study demonstrates the importance of systems biology approaches also in cases were mutations in single genes are analyzed to explain the underlying mechanisms controlling complex biological processes like aging and development.

Copper ability to induce premature senescence in human fibroblasts

Human diploid fibroblasts (HDFs) exposed to subcytotoxic concentrations of oxidative or stressful agents, such as hydrogen peroxide, tert-butylhydroperoxide, or ethanol, undergo stress-induced premature senescence (SIPS). This condition is characterized by the appearance of replicative senescence biomarkers such as irreversible growth arrest, increase in senescence-associated β-galactosidase (SA β-gal) activity, altered cell morphology, and overexpression of several senescence-associated genes. Copper is an essential trace element known to accumulate with ageing and to be involved in the pathogenesis of some age-related disorders. Past studies using either yeast or human cellular models of ageing provided evidence in favor of the role of intracellular copper as a longevity modulator. In the present study, copper ability to cause the appearance of senescent features in HDFs was assessed. WI-38 fibroblasts exposed to a subcytotoxic concentration of copper sulfate presented inhibition of cell proliferation, cell enlargement, increased SA β-gal activity, and mRNA overexpression of several senescence-associated genes such as p21, apolipoprotein J (ApoJ), fibronectin, transforming growth factor β-1 (TGF β1), insulin growth factor binding protein 3, and heme oxygenase 1. Western blotting results confirmed enhanced intracellular p21, ApoJ, and TGF β1 in copper-treated cells. Thus, similar to other SIPS-inducing agents, HDF exposure to subcytotoxic concentration of copper results in premature senescence. Further studies will unravel molecular mechanisms and the biological meaning of copper-associated senescence and lead to a better understanding of copper-related disorder establishment and progression.

Alternative oxidase dependent respiration leads to an increased mitochondrial content in two long-lived mutants of the aging model Podospora anserina

The retrograde response constitutes an important signalling pathway from mitochondria to the nucleus which induces several genes to allow compensation of mitochondrial impairments. In the filamentous ascomycete Podospora anserina, an example for such a response is the induction of a nuclear-encoded and iron-dependent alternative oxidase (AOX) occurring when cytochrome-c oxidase (COX) dependent respiration is affected. Several long-lived mutants are known which predominantly or exclusively respire via AOX. Here we show that two AOX-utilising mutants, grisea and PaCox17::ble, are able to compensate partially for lowered OXPHOS efficiency resulting from AOX-dependent respiration by increasing mitochondrial content. At the physiological level this is demonstrated by an elevated oxygen consumption and increased heat production. However, in the two mutants, ATP levels do not reach WT levels. Interestingly, mutant PaCox17::ble is characterized by a highly increased release of the reactive oxygen species (ROS) hydrogen peroxide. Both grisea and PaCox17::ble contain elevated levels of mitochondrial proteins involved in quality control, i. e. LON protease and the molecular chaperone HSP60. Taken together, our work demonstrates that AOX-dependent respiration in two mutants of the ageing model P. anserina is linked to a novel mechanism involved in the retrograde response pathway, mitochondrial biogenesis, which might also play an important role for cellular maintenance in other organisms.

Calorie restriction up-regulates iron and copper transport genes in Saccharomyces cerevisiae

Calorie restriction (CR) is a non genetic intervention, known to confer longevity benefits across the various phyla from unicellular yeast to mammals. CR also invokes homeostatic responses similar to stress, however the sequence of molecular events leading to longevity is still illusive. In this study, we analysed the whole genome gene expression profile in response to CR, mutations mimicking CR, heat shock and H(2)O(2) from a gene ontology perspective. Our analysis revealed that mitochondrion is a common hub in the gene expression programme under these conditions and the electron transport chain (ETC) is a major player. Consequently the genes involved in the metal ion transport were also significantly up-regulated. We confirmed the results of the in silico analysis using quantitative real time PCR which showed up-regulation of genes involved in respiration and transport of iron and copper. The promoter activity of one of the representative genes, FET3, was also found to be higher upon calorie restriction. Altogether, our results indicate that upon calorie restriction the levels of iron and copper fall in cells, which elicits a transcriptional response up-regulating the genes involved in their uptake to maintain cellular homeostasis.

Mitochondrial pathways governing stress resistance, life, and death in the fungal aging model Podospora anserina

Work from more than 50 years of research has unraveled a number of molecular pathways that are involved in controlling aging of the fungal model system Podospora anserina. Early research revealed that wild-type strain aging is linked to gross reorganization of the mitochondrial DNA. Later it was shown that aging of P. anserina does also take place, although at a slower pace, when the wild-type specific mitochondrial DNA rearrangements do not occur. Now it is clear that a network of different pathways is involved in the control of aging. Branches of these pathways appear to be connected and constitute a hierarchical system of responses. Although cross talk between the individual pathways seems to be fundamental in the coordination of the overall system, the precise underlying interactions remain to be unraveled. Such a systematic approach aims at a holistic understanding of the process of biological aging, the ultimate goal of modern systems biology.

A copper-transporting ATPase BcCCC2 is necessary for pathogenicity of Botrytis cinerea

Copper is an essential trace element that serves as a cofactor for numerous enzymes. In eukaryotes, copper-transporting ATPases deliver copper to various copper-containing proteins in the trans-golgi network. This study identified a copper-transporting ATPase gene BcCcc2 in a fungus pathogenic to plants, Botrytis cinerea. We investigated the biological roles of BcCCC2 by generating null mutants for BcCcc2. Melanization, conidiation and the formation of sclerotia were severely affected in DeltaBcCcc2 mutants. Moreover, a pathogenicity assay using tomato leaves and carnation petals revealed the mutants to be nonpathogenic. Further analysis indicated that they formed fewer appressoria and infection cushions than the wild-type. These structures were aberrant in morphology and in many cases had a significantly reduced ability to penetrate the plant epidermis. An assay also indicated that DeltaBcCcc2 mutants were defective in infection through wounds. BcCCC2 is necessary not only for penetrating a host but also for fungal growth within plant tissues. Our results also imply that B. cinerea requires copper-containing proteins for infection that are inactive in the absence of the copper-transporting ATPase BcCCC2.

Characterization of Ctr family genes and the elucidation of their role in the life cycle of Neurospora crassa

Transcriptional analysis using qRT-PCR of 62 metal ion transporters during conidial germination of Neurospora crassa showed a significant up regulation of a hypothetical copper transporter gene, tcu-1, that belongs to the Ctr family. Herein we characterised the Ctr family genes (tcu-1, tcu-2 and tcu-3) and deciphered their role in various developmental phases of the N. crassa life cycle. Cross complementation assays in copper uptake mutant of Saccharomyces cerevisiae revealed that tcu-1, tcu-2 and tcu-3 are functional homologs of S. cerevisiae copper transporters. Expression studies of Ctr family members in various developmental phases of N. crassa showed differential expression pattern for high-affinity copper transporter, TCU1. Functional analysis of their gene knockout mutants showed that tcu-1 is essential for saprophytic conidial germination, vegetative growth and perithecia development under copper limited conditions while conidiation remained unaffected.

Molecular basis of and interference into degenerative processes in fungi: potential relevance for improving biotechnological performance of microorganisms

Biological systems, from simple microorganisms to humans, are characterized by time-dependent degenerative processes which lead to reduced fitness, disabilities, severe diseases, and, finally, death. These processes are under genetic control but also influenced by environmental conditions and by stochastic processes. Studying the mechanistic basis of degenerative processes in the filamentous ascomycete Podospora anserina and in other systems demonstrated that mitochondria play a key role in the expression of degenerative phenotypes and unraveled a number of underlying molecular pathways. Reactive oxygen species (ROS) which are mainly, but not exclusively, formed at the mitochondrial respiratory chain are crucial players in this network. While being essential for signaling processes and development, ROS are, at the same time, a potential danger because they lead to molecular damage and degeneration. Fortunately, a number of interacting pathways including ROS scavenging, DNA and protein repair, protein degradation, and mitochondrial fission and fusion are involved in keeping cellular damage low. If these pathways are overwhelmed by extensive damage, programmed cell death is induced. The current knowledge of this hierarchical system of mitochondrial quality control, although still incomplete, appears now to be ready for the development of strategies effective in interventions into those pathways leading to degeneration and loss of performance also in microorganisms used in biotechnology. Very promising interdisciplinary interactions and collaborations involving academic and industrial research teams can be envisioned to arise which bear a great potential, in particular, when system biology approaches are used to understand relevant networks of pathways in a holistic way.

Age-related cellular copper dynamics in the fungal ageing model Podospora anserina and in ageing human fibroblasts

In previous investigations an impact of cellular copper homeostasis on ageing of the ascomycete Podospora anserina has been demonstrated. Here we provide new data indicating that mitochondria play a major role in this process. Determination of copper in the cytosolic fraction using total reflection X-ray fluorescence spectroscopy analysis and eGfp reporter gene studies indicate an age-related increase of cytosolic copper levels. We show that components of the mitochondrial matrix (i.e. eGFP targeted to mitochondria) become released from the organelle during ageing. Decreasing the accessibility of mitochondrial copper in P. anserina via targeting a copper metallothionein to the mitochondrial matrix was found to result in a switch from a copper-dependent cytochrome-c oxidase to a copper-independent alternative oxidase type of respiration and results in lifespan extension. In addition, we demonstrate that increased copper concentrations in the culture medium lead to the appearance of senescence biomarkers in human diploid fibroblasts (HDFs). Significantly, expression of copper-regulated genes is induced during in vitro ageing in medium devoid of excess copper suggesting that cytosolic copper levels also increase during senescence of HDFs. These data suggest that the identified molecular pathway of age-dependent copper dynamics may not be restricted to P. anserina but may be conserved from lower eukaryotes to humans.

Regulation of Metallothionein Gene Expression

Organisms from bacteria to humans use elaborate systems to regulate levels of bioavailable zinc, copper, and other essential metals. An excess of them, or even traces of non-essential metals such as cadmium and mercury, can be highly toxic. Metallothioneins (MTs), short, cysteine-rich proteins, play pivotal roles in metal homeostasis and detoxification. With their sulfhydryl groups they avidly bind toxic metals and also play a role in cellular redox balance and radical scavenging. The intracellular concentration of MTs is adjusted to cellular demand primarily via regulated transcription. Especially upon heavy metal load, metallothionein gene transcription is strongly induced. From insects to mammals, the major regulator of MT transcription is MTF-1 (metal-responsive transcription factor 1), a zinc finger protein that binds to specific DNA sequence motifs (MREs) in the promoters of MT genes and other metal-regulated genes. This chapter provides an overview of our current knowledge on the expression and regulation of MT genes in higher eukaryotes, with some reference also to fungi which apparently have independently evolved their own regulatory systems.

Over-expression of an S-adenosylmethionine-dependent methyltransferase leads to an extended lifespan of Podospora anserina without impairments in vital functions

PaMTH1, a putative methyltransferase previously described to increase in abundance in total protein extracts during aging of Podospora anserina is demonstrated to accumulate in the mitochondrial cell fraction of senescent cultures. The protein is localized in the mitochondrial matrix and displays a methyltransferase activity utilizing flavonoids as substrates. Constitutive over-expression of PaMth1 in P. anserina results in a reduced carbonylation of proteins and an extended lifespan without impairing vital functions suggesting a protecting role of PaMTH1 against oxidative stress.

Podospora anserina: a model organism to study mechanisms of healthy ageing

The filamentous ascomycete Podospora anserina has been extensively studied as an experimental ageing model for more than 50 years. As a result, a huge body of data has been accumulated and various molecular pathways have been identified as part of a molecular network involved in the control of ageing and life span. The aim of this review is to summarize data on P. anserina ageing, including aspects like respiration, cellular copper homeostasis, mitochondrial DNA (mtDNA) stability/instability, mitochondrial dynamics, apoptosis, translation efficiency and pathways directed against oxidative stress. It becomes clear that manipulation of several of these pathways bears the potential to extend the healthy period of time, the health span, within the life time of the fungus. Here we put special attention on recent work aimed to identify and characterize this type of long-lived P. anserina mutants. The study of the molecular pathways which are modified in these mutants can be expected to provide important clues for the elucidation of the mechanistic basis of this type of 'healthy ageing' at the organism level.

Functional characterization of CgCTR2, a putative vacuole copper transporter that is involved in germination and pathogenicity in Colletotrichum gloeosporioides

Copper is a cofactor and transition metal involved in redox reactions that are essential in all eukaryotes. Here, we report that a vacuolar copper transporter that is highly expressed in resting spores is involved in germination and pathogenicity in the plant pathogen Colletotrichum gloeosporioides. A screen of C. gloeosporioides transformants obtained by means of a promoterless green fluorescent protein (GFP) construct led to the identification of transformant N159 in which GFP signal was observed in spores. The transforming vector was inserted 70 bp upstream of a putative gene with homology to the Saccharomyces cerevisiae vacuolar copper transporter gene CTR2. The C. gloeosporioides CTR2 (CgCTR2) gene fully complemented growth defects of yeast ctr2Delta mutants, and a CgCTR2-cyan fluorescent protein (CFP) fusion protein accumulated in vacuole membranes, confirming the function of the protein as a vacuolar copper transporter. Expression analysis indicated that CgCTR2 transcript is abundant in resting conidia and during germination in rich medium and downregulated during "pathogenic" germination and the early stages of plant infection. CgCTR2 overexpression and silencing mutants were generated and characterized. The Cgctr2 mutants had markedly reduced Cu superoxide dismutase (SOD) activity, suggesting that CgCTR2 is important in providing copper to copper-dependent cytosolic activities. The Cgctr2-silenced mutants had increased sensitivity to H2O2 and reduced germination rates. The mutants were also less virulent to plants, but they did not display any defects in appressorium formation and penetration efficiency. An external copper supply compensated for the hypersensitivity to H2O2 but not for the germination and pathogenicity defects of the mutants. Similarly, overexpression of CgCTR2 enhanced resistance to H2O2 but had no effect on germination or pathogenicity. Our results show that copper is necessary for optimal germination and pathogenicity and that CgCTR2 is involved in regulating cellular copper balance during these processes.

Pichia pastoris "just in time" alternative respiration

Alternative oxidases (Aox or Aod) are present in the mitochondria of plants, fungi and many types of yeast. These enzymes transfer electrons from the ubiquinol pool directly to oxygen without contributing to the proton transfer across the mitochondrial membrane. Alternative oxidases are involved in stress responses, programmed cell death and maintenance of the cellular redox balance. The alternative oxidase gene of the methylotrophic yeast Pichia pastoris was isolated and cloned to study its regulation and the effects of deregulation of the alternative respiration by overexpression or disruption of the gene. Both disruption and overexpression had negative effects on the biomass yield; however, the growth rate and substrate uptake rate of the strain overexpressing the alternative oxidase were slightly increased. These effects were even more pronounced when higher glucose concentrations were used. The occurrence of free intracellular radicals and cell death phenomena was investigated using dihydrorhodamine 123 and the TUNEL test. The results suggest a major contribution of the alternative oxidase to P. pastoris cell viability. The negative effects of deregulated alternative respiration clearly indicated the importance of precise regulation of the alternative oxidase in this yeast.

Copper homeostasis in Drosophila by complex interplay of import, storage and behavioral avoidance

Copper is an essential but potentially toxic trace element. In Drosophila, the metal-responsive transcription factor (MTF-1) plays a dual role in copper homeostasis: at limiting copper concentrations, it induces the Ctr1B copper importer gene, whereas at high copper concentrations, it mainly induces the metallothionein genes. Here we find that, despite the downregulation of the Ctr1B gene at high copper concentrations, the protein persists on the plasma membrane of intestinal cells for many hours and thereby fills the intracellular copper stores. Drosophila may risk excessive copper accumulation for the potential benefit of overcoming a period of copper scarcity. Indeed, we find that copper-enriched flies donate a vital supply to their offspring, allowing the following generation to thrive on low-copper food. We also describe two additional modes of copper handling: behavioral avoidance of food containing high (>or=0.5 mM) copper levels, as well as the ability of DmATP7, the Drosophila homolog of Wilson/Menkes disease copper exporters, to counteract copper toxicity. Regulated import, storage, export, and avoidance of high-copper food establish an adequate copper homeostasis under variable environmental conditions.

Copper supplementation increases yeast life span under conditions requiring respiratory metabolism

To further exploit yeast as a model for cellular aging we have modified the replicative life span assay to force respiration, by replacing glucose with the non-fermentable carbon source glycerol. The growth rates of several different strains varied greatly, with doubling times ranging from 2.7 to 7 h. Life spans of all strains were lower on media containing glycerol than on media containing glucose. However, supplementation of glycerol-containing media with copper resulted in increases in life span of between 17 and 72%; life spans equivalent to or beyond those obtained on glucose media. Addition of copper to glucose medium had no effect on life span. Microarray analysis showed that genes responsible for high affinity import of copper display reduced expression upon addition of copper, while most genes showed no change in expression. No differences in growth rate, oxygen uptake, or the levels of subunit II of the copper-containing cytochrome c oxidase were found between cultures of yeast grown with or without copper supplementation. Copper supplementation greatly extended the life span of sod1 and sod2 strains, suggesting that addition of copper may reduce the generation of superoxide. Forcing yeast to respire places an emphasis on mitochondrial function and may aid in the identification of factors involved in aging in other respiratory-dependent organisms.

Impact of ROS on ageing of two fungal model systems: Saccharomyces cerevisiae and Podospora anserina

To provide a foundation for the development of effective interventions to counteract various age-related diseases in humans, ageing processes have been extensively studied in various model organisms and systems. However, the mechanisms underlying ageing are still not unravelled in detail in any system including rather simple organisms. In this article, we review some of the molecular mechanisms that were found to affect ageing in two fungal models, the unicellular ascomycete Saccharomyces cerevisiae and the filamentous ascomycete Podospora anserina. A selection of issues like retrograde response, genomic instability, caloric restriction, mtDNA reorganisation and apoptosis is presented and discussed with special emphasis on the role reactive oxygen species (ROS) play in these diverse molecular pathways.

Reducing mitochondrial fission results in increased life span and fitness of two fungal ageing models

Ageing of biological systems is accompanied by alterations in mitochondrial morphology, including a transformation from networks and filaments to punctuate units. The significance of these alterations with regard to ageing is not known. Here, we demonstrate that the dynamin-related protein 1 (Dnm1p), a mitochondrial fission protein conserved from yeast to humans, affects ageing in the two model systems we studied, Podospora anserina and Saccharomyces cerevisiae. Deletion of the Dnm1 gene delays the transformation of filamentous to punctuate mitochondria and retards ageing without impairing fitness and fertility typically observed in long-lived mutants. Our data further suggest that reduced mitochondrial fission extends life span by increasing cellular resistance to the induction of apoptosis and links mitochondrial dynamics, apoptosis and life-span control.

Identification of a copper-inducible promoter for use in ectopic expression in the fungal pathogen Histoplasma capsulatum

Despite the existence of a number of genetic tools to study the fungal pathogen Histoplasma capsulatum, strategies for conditional gene expression have not been developed. We used microarray analysis to identify genes that are transcriptionally induced or repressed by the addition of copper sulfate (CuSO(4)) to H. capsulatum yeast cultures. One of these genes, CRP1, encodes a putative copper efflux pump that is significantly induced in the presence of CuSO(4). The upstream regulatory region of CRP1 was sufficient to drive copper-regulated expression of two reporter genes, lacZ and the gene encoding green fluorescent protein. Microarray experiments were performed to determine a copper concentration that triggers accumulation of the CRP1 transcript without significant perturbation of global gene expression. These studies show that the CRP1 upstream regulatory region can be used for ectopic expression of heterologous genes in H. capsulatum. Furthermore, they demonstrate the strategic use of microarrays to identify conditional promoters that confer induction in the absence of large-scale shifts in gene expression.

OXPHOS Supercomplexes: respiration and life-span control in the aging model Podospora anserina

Recent biochemical evidence has indicated the existence of respiratory supercomplexes as well as ATP synthase oligomers in the inner mitochondrial membrane of different eukaryotes. We have studied the organization of the respiratory chain of a wild-type strain and of two long-lived mutants of the filamentous fungus Podospora anserina. This aging model is able to respire by either the standard or the alternative pathway. In the latter, electrons are directly transferred from ubiquinol to the alternative oxidase (AOX) and thus bypass complexes III and IV. We showed that the two pathways are composed of distinct respiratory supercomplexes. These data are of significance for the understanding of both respiratory pathways as well as of life-span control and aging.

Identification and functional characterisation of ctr1, a Pleurotus ostreatus gene coding for a copper transporter

Copper homeostasis is crucial for the maintenance of life. In lignin-degrading fungi, copper is essential for the phenol oxidase enzymes that provide this activity. In this paper we report the characterization of a gene (ctr1) coding for a copper transporter in the white rot fungus Pleurotus ostreatus. The gene was identified in a cDNA library constructed from 4-day-old vegetative mycelium grown in liquid culture. The results presented here demonstrate that: (1) ctr1 functionally complements the respiratory deficiency of a yeast mutant defective in copper transport, supporting the idea that the Ctr1 protein is itself a copper transporter; (2) transcription of ctr1 is detectable in P. ostreatus at all developmental stages and in all tissues (with the exception of lamellae), and is negatively regulated by the presence of copper in the culture medium; (3) ctr1 is a single-copy gene that maps to P. ostreatus linkage group III; and (4) the regulatory sequence elements found in the promoter of ctr1 are similar to those found in other copper-related genes described in other systems. These results provide the first description of a copper transporter in this white rot fungus and should be useful for further studies on copper metabolism in higher basidiomycetes.

Impact of a disruption of a pathway delivering copper to mitochondria on Podospora anserina metabolism and life span

A global depletion of cellular copper as the result of a deficiency in high-affinity copper uptake was previously shown to affect the phenotype and life span of the filamentous fungus Podospora anserina. We report here the construction of a strain in which the delivery of copper to complex IV of the mitochondrial respiratory chain is affected. This strain, PaCox17::ble, is a PaCox17-null mutant that does not synthesize the molecular chaperone targeting copper to cytochrome c oxidase subunit II. PaCox17::ble is characterized by a decreased growth rate, a reduction in aerial hyphae formation, reduced female fertility, and a dramatic increase in life span. The mutant respires via a cyanide-resistant alternative pathway, displays superoxide dismutase (SOD) activity profiles significantly differing from those of the wild-type strain and is characterized by a stabilization of the mitochondrial DNA. Collectively, the presented data define individual components of a molecular network effective in life span modulation and copper as an element with a dual effect. As a cofactor of complex IV of the respiratory chain, it is indirectly involved in the generation of reactive oxygen species (ROS) and thereby plays a life span-limiting role. In contrast, Cu/Zn SOD as a ROS-scavenging enzyme lowers molecular damage and thus positively affects life span. Such considerations explain the reported differences in life span of independent mutants and spread more light on the delicate tuning of the molecular network influencing biological ageing.

From aging to virulence: forging connections through the study of copper homeostasis in eukaryotic microorganisms

Recent years have witnessed an explosion in the breadth of investigations on transition metal homeostasis and the subsequent depth of our understanding of metals in biology. Many genes and proteins that serve in the uptake, distribution, sensing and detoxification of one such transition metal, copper, have been identified. Through genetic and biochemical studies, the molecular details of copper uptake are being elucidated, and evidence suggests a largely conserved mechanism for copper acquisition and distribution from yeast to humans. Investigations of the mitochondrial copper pathway reveal the complexity surrounding copper delivery to cytochrome oxidase and highlight additional roles for some of the participants in copper homeostasis, such as a copper chaperone that influences the subcellular distribution of its target for copper incorporation. Furthermore, our understanding of the structure and function of copper transporters, chaperones and cupro-proteins, coupled with the emergence of additional model systems, is providing surprising examples of the integration of copper homeostasis with other physiological and pathophysiological processes and states, such as cancer, aging and virulence.

Supramolecular Organization of Cytochrome c Oxidase- and Alternative Oxidase-dependent Respiratory Chains in the Filamentous Fungus Podospora anserina

To elucidate the molecular basis of the link between respiration and longevity, we have studied the organization of the respiratory chain of a wild-type strain and of two long-lived mutants of the filamentous fungus Podospora anserina. This established aging model is able to respire by either the standard or the alternative pathway. In the latter pathway, electrons are directly transferred from ubiquinol to the alternative oxidase and thus bypass complexes III and IV. We show that the cytochrome c oxidase pathway is organized according to the mammalian "respirasome" model (Schägger, H., and Pfeiffer, K. (2000) EMBO J. 19, 1777-1783). In contrast, the alternative pathway is composed of distinct supercomplexes of complexes I and III (i.e. I(2) and I(2)III(2)), which have not been described so far. Enzymatic analysis reveals distinct functional properties of complexes I and III belonging to either cytochrome c oxidase- or alternative oxidase-dependent pathways. By a gentle colorless-native PAGE, almost all of the ATP synthases from mitochondria respiring by either pathway were preserved in the dimeric state. Our data are of significance for the understanding of both respiratory pathways as well as lifespan control and aging.

Role of alternative oxidase gene in pathogenesis of Cryptococcus neoformans

We identified a homologue of the alternative oxidase gene in a screen to identify genes that are preferentially transcribed in response to a shift to 37 degrees C in the human-pathogenic yeast Cryptococcus neoformans. Alternative oxidases are nucleus-encoded mitochondrial proteins that have two putative roles: they can function in parallel with the classic cytochrome oxidative pathway to produce ATP, and they may counter oxidative stress within the mitochondria. The C. neoformans alternative oxidase gene (AOX1) was found to exist as a single copy in the genome, and it encodes a putative protein of 401 amino acids. An aox1 mutant strain was created using targeted gene disruption, and the mutant strain was reconstituted to wild type using a full-length AOX1. Compared to both the wild-type and reconstituted strains, the aox1 mutant strain was not temperature sensitive but did have significant impairment of both respiration and growth when treated with inhibitors of the classic cytochrome oxidative pathway. The aox1 mutant strain was also found to be more sensitive to the oxidative stressor tert-butyl hydroperoxide. The aox1 mutant strain was significantly less virulent than both the wild type and the reconstituted strain in the murine inhalational model, and it also had significantly impaired growth within a macrophage-like cell line. These data demonstrate that the alternative oxidase of C. neoformans can make a significant contribution to metabolism, has a role in the yeast's defense against exogenous oxidative stress, and contributes to the virulence composite of this organism, possibly by improving survival within phagocytic cells.

Knockout of 'metal-responsive transcription factor' MTF-1 in Drosophila by homologous recombination reveals its central role in heavy metal homeostasis

'Metal-responsive transcription factor-1' (MTF-1), a zinc finger protein, is conserved from mammals to insects. In the mouse, it activates metallothionein genes and other target genes in response to several cell stress conditions, notably heavy metal load. The knockout of MTF-1 in the mouse has an embryonic lethal phenotype accompanied by liver degeneration. Here we describe the targeted disruption of the MTF-1 gene in Drosophila by homologous recombination. Unlike the situation in the mouse, knockout of MTF-1 in Drosophila is not lethal. Flies survive well under laboratory conditions but are sensitive to elevated concentrations of copper, cadmium and zinc. Basal and metal-induced expression of Drosophila metallothionein genes MtnA (Mtn) and MtnB (Mto), and of two new metallothionein genes described here, MtnC and MtnD, is abolished in MTF-1 mutants. Unexpectedly, MTF-1 mutant larvae are sensitive not only to copper load but also to copper depletion. In MTF-1 mutants, copper depletion prevents metamorphosis and dramatically extends larval development/lifespan from normally 4-5 days to as many as 32 days, possibly reflecting the effects of impaired oxygen metabolism. These findings expand the roles of MTF-1 in the control of heavy metal homeostasis.