The phenoloxidases of the ascomycete Podospora anserina. XIII. Action and interaction of genes controlling the formation of laccase

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.

Recent advances in laccase activity assays: A crucial challenge for applications on complex substrates

Despite being one of the first enzymes discovered in 1883, the determination of laccase activity remains a scientific challenge, and a barrier to the full use of laccase as a biocatalyst. Indeed, laccase, an oxidase of the blue multi-copper oxidases family, has a wide range of substrates including substituted phenols, aromatic amines and lignin-related compounds. Its one-electron mechanism requires only oxygen and releases water as a reaction product. These characteristics make laccase a biocatalyst of interest in many fields of applications including pulp and paper industry, biorefineries, food, textile, and pharmaceutical industries. But to fully envisage the use of laccase at an industrial scale, its activity must be reliably quantifiable on complex substrates and in complex matrices. This review aims to describe current and emerging methods for laccase activity assays and place them in the context of a potential industrial use of the enzyme.

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.

Mac1-Dependent Copper Sensing Promotes Histoplasma Adaptation to the Phagosome during Adaptive Immunity

Intracellular pathogens residing within macrophage phagosomes are challenged with recognizing the phagosomal environment and appropriately responding to changing host defense strategies imposed in this organelle. One such phagocyte defense is the restriction of available copper as a form of nutritional immunity during the adaptive immune response to fungal pathogens. The intracellular fungal pathogen Histoplasma capsulatum adapts to this decreased copper through upregulation of the Ctr3 copper transporter. In this study, we show that Histoplasma recognizes the characteristic low-copper phagosomal environment of activated macrophages through the copper-dependent transcriptional regulator Mac1. Multiple cis-acting regulatory sequences in the CTR3 promoter control upregulation of CTR3 transcription under low-copper conditions, and the loss of Mac1 function prevents induction of Ctr3 under low-copper conditions. During adaptive immunity, this loss of copper sensing by Mac1 attenuates Histoplasma virulence more severely than loss of Ctr3 alone, indicating that Mac1 controls the expression of additional mechanisms important for pathogenesis. Transcriptional profiling of Histoplasma yeasts identified a small regulon of Mac1-dependent genes, with the most strongly regulated genes encoding proteins linked to copper, iron, and zinc homeostasis and defenses against reactive oxygen (iron-requiring catalase [CatB] and superoxide dismutase [Sod4]). Accordingly, the loss of Mac1 function increased sensitivity to copper and iron restriction and blocked low-copper-induced expression of extracellular catalase activity. Thus, Mac1-mediated sensing of low-copper signals to Histoplasma yeasts their residence within the activated macrophage phagosome, and in response, Histoplasma yeasts produce factors, including non-copper-dependent factors, to combat the enhanced defenses of activated macrophages.

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.

Phenotypic instability in fungi

Fungi are prone to phenotypic instability, that is, the vegetative phase of these organisms, be they yeasts or molds, undergoes frequent switching between two or more behaviors, often with different morphologies, but also sometime having different physiologies without any obvious morphological outcome. In the context of industrial utilization of fungi, this can have a negative impact on the maintenance of strains and/or on their productivity. Instabilities have been shown to result from various mechanisms, either genetic or epigenetic. This chapter will review different types of instabilities and discuss some lesser-known ones, mostly in filamentous fungi, while it will direct readers to additional literature in the case of well-known phenomena such as the amyloid prions or fungal senescence. It will present in depth the "white/opaque" switch of Candida albicans and the "crippled growth" degeneration of the model fungus Podospora anserina. These are two of the most thoroughly studied epigenetic phenotypic switches. I will also discuss the "sectors" presented by many filamentous ascomycetes, for which a prion-based model exists but is not demonstrated. Finally, I will also describe intriguing examples of phenotypic instability for which an explanation has yet to be provided.

Importance of a Laccase Gene (Lcc1) in the Development of Ganoderma tsugae

In this study, a novel laccase gene (Lcc1) from Ganoderma tsugae was isolated and its functions were characterized in detail. The results showed that Lcc1 has the highest expression activity during mycelium development and fruit body maturation based on the analysis of Lcc1 RNA transcripts at different developmental stages of G. tsugae. To investigate the exact contribution of Lcc1 to mycelium and fruit body development in G. tsugae, Lcc1 transgenic strains were constructed by targeted gene replacement and over-expression approaches. The results showed that the lignin degradation rate in Lcc1 deletion mutant was much lower than the degradation efficiency of the wild-type (WT), over-expression and rescue strains. The lignin degradation activity of G. tsugae is dependent on Lcc1 and the deletion of Lcc1 exerted detrimental influences on the development of mycelium branch. Furthermore, the study uncovered that Lcc1 deletion mutants generated much shorter pale grey fruit bodies, suggesting that Lcc1 contributes directly to pigmentation and stipe elongation during fruit body development in G. tsugae. The information obtained in this study provides a novel and mechanistic insight into the specific role of Lcc1 during growth and development of G. tsugae.

Transformation to senescence with plasmid like DNA in the ascomycete Podospora anserina

In the ascomycete Podospora anserina senescence through strain aging is under nucleo-cytoplasmic control and inducible in juvenile mycelia by an 'infective principle' transferred after cytoplasmic contact via anastomoses. A specific DNA called plasmid-like (pl) DNA, present exclusively in aging mycelia, was found to be identical with this 'infective principle', since it was possible to transform juvenile protoplasts to senescence by using purified p1DNA. Therefore a specific function may be attributed to this ccc DNA. Its direct involvement in a genetically programed senescence is confirmed and its development as a vector for transfer of genetic information in eukaryotes can be undertaken.

Determination of oxidoreductase activity using a high-throughput microplate respiratory measurement

High-throughput multiparallel activity profiling for oxygen consuming cell layers has been recently developed for extracellular flux analysis. This technology has great potential for determining the enzymatic activity of oxidoreductases (i.e., laccase) both in vivo and in vitro, which is usually measured using photometrical tests monitoring the colored oxidation products. Improvements in terms of sample throughput, comparability, and gain of information (i.e., stoichiometry, electron transfer rate) can be achieved by means of a multiwell plate-based fluorimetric oxygen sensor. In the present study, various laccases have been applied to develop protocols that allow the multiparallel measurement of O(2)-consumption by enzymatic reactions. The developed and validated method enables the comparative quantitation of laccase characteristics (i.e., profiles of activity at various pH values) and minimizes the time it usually takes to collect respiratory data of oxygen-consuming enzymes. Furthermore, the possibility to assess differences between single and multisubstrate kinetics of laccases has been demonstrated.

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.

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.

Importance of laccase in vegetative growth of pleurotus Florida

Mycelial culture of Pleurotus florida produced highest extracellular laccase in optimum growth medium. At least two laccases (L(inf1) and L(inf2)) were shown to be present in the culture filtrate. Low-laccase-yielding mutants with impaired L(inf2) activity had poor mycelial growth and could not form fruit body, whereas the revertants from the same mutants were similar to the parent in mycelial growth and fruit body formation.

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.

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.

A potential impact of DNA repair on ageing and lifespan in the ageing model organism Podospora anserina: decrease in mitochondrial DNA repair activity during ageing

The free radical theory of ageing states that ROS play a key role in age-related decrease in mitochondrial function via the damage of mitochondrial DNA (mtDNA), proteins and lipids. In the sexually reproducing ascomycete Podospora anserina ageing is, as in other eukaryotes, associated with mtDNA instability and mitochondrial dysfunction. Part of the mtDNA instabilities may arise due to accumulation of ROS induced mtDNA lesions, which, as previously suggested for mammals, may be caused by an age-related decrease in base excision repair (BER). Alignments of known BER protein sequences with the P. anserina genome revealed high homology. We report for the first time the presence of BER activities in P. anserina mitochondrial extracts. DNA glycosylase activities decrease with age, suggesting that the increased mtDNA instability with age may be caused by decreased ability to repair mtDNA damage and hence contribute to ageing and lifespan control in this ageing model. Additionally, we find low DNA glycosylase activities in the long-lived mutants grisea and DeltaPaCox17::ble, which are characterized by low mitochondrial ROS generation. Overall, our data identify a potential role of mtDNA repair in controlling ageing and life span in P. anserina, a mechanism possibly regulated in response to ROS levels.

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.

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.

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.

Structural close-related aromatic compounds have different effects on laccase activity and on lcc gene expression in the ligninolytic fungus Trametes sp. I-62

Nine phenolic compounds (p-coumaric acid, ferulic acid, guaiacol, syringol, p-methoxyphenol, pyrocatechol, phloroglucinol, 3,5-dihydroxybenzoic acid, and syringaldazine) were tested for their ability to increase laccase production in the ligninolytic basidiomycete Trametes sp. I-62. All these compounds resulted in increases in laccase activity, with the highest levels being detected in the presence of p-coumaric acid (273-fold) and guaiacol (73-fold). The three laccase isozyme genes in this fungus lcc1, lcc2, and lcc3 are differentially expressed in the presence of some of these aromatics with total lcc transcript levels differing markedly depending on the aromatic compound tested. Guaiacol (the best inducer of lcc gene transcription) and p-coumaric acid selectively induced expression of lcc1 and lcc2; ferulic acid induced lcc3 expression, while 3,5-dihydroxybenzoic acid had no marked effect on laccase gene transcription. The results demonstrate that close-related aromatic compounds appear to have different effects on both laccase activity levels and lcc gene expression in this basidiomycete.

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.

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

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.

Genes, mitochondria and aging in filamentous fungi

Fungi are eukaryotic microorganisms studied in various areas of general and applied biology. A few species were among the first systems in which specific aspects of aging were addressed experimentally. Various factors, both environmental and genetic, were found to affect lifespan and aging. Mitochondrial pathways play a paramount role. Since mitochondria are semiautonomous organelles and depend on both nuclear as well as mitochondrial genes, mitochondrial-nuclear interactions are of major relevance. As a main generator of reactive oxygen species (ROS), mitochondria are prone to molecular damage. However, cells can cope with the negative effects of ROS utilizing different scavenging systems and, once defects became manifested, by repair of damaged molecules. Both, lowering ROS generation and increasing mitochondrial "caretaker" systems bear great potential to interfere with natural aging processes.

Aging in fungi: role of mitochondria in Podospora anserina

In experimental gerontology, there is a long tradition in the use of both unicellular and filamentous species of fungi. In the last three decades, biochemical, genetic and molecular approaches have proved very fruitful in elucidating different aspects of ageing. It was shown that various genes and molecular pathways are involved in life span control. The oxygenic energy metabolism plays a central role. During mitochondrial energy transduction, reactive oxygen species (ROS) are generated as by-products. These molecules are able to damage all cellular compounds leading to cellular dysfunctions. Within certain limits, however, cells are able to cope with ROS-related problems. First, ROS scavengers can be induced which are effective in lowering the molecular burden of ROS on cellular functions. Second, if damage occurs, specific repair mechanisms and the general turnover of affected molecules can maintain cellular functions. Finally, if damage of essential components is too severe, cells may induce specific pathways to compensate for the corresponding impairments. A coordinated interaction between different cellular compartments is involved in these processes. In this review I shall concentrate on the ageing in the filamentous ascomycete Podospora anserina. It is clear that both environmental as well as genetic traits are involved in the control of life span and that mitochondrial-nuclear interactions play a paramount role.

Mitochondrial functions and aging

In the filamentous ascomycete Podospora anserina mitochondria play a major role in lifespan control. Since the function of these organelles depends on a large number of individual components it is no surprise that a complex network of interacting branches of individual molecular pathways is involved in this process. Recently, the nuclear encoded transcription factor GRISEA was found to significantly affect mitochondrial functions. GRISEA is involved in the control of cellular copper homeostasis. Most importantly, the high affinity uptake of copper from the environment is controlled by this transcription factor. Once copper has entered the cell, it becomes distributed to different compartments and different target molecules. This process depends on a group of proteins, termed copper chaperones. PaCOX17, a homologue of the yeast copper chaperone yCOX17, appears to be involved in copper delivery to mitochondria. Most importantly, the metal is crucial for the assembly and the function of complex IV of the respiratory chain. However, although P. anserina is an obligate aerobe and therefore depends on mitochondrial energy transduction, impairments in the copper delivery pathway are not lethal. This is due to the induction of a molecular back-up system able to compensate for deficiencies in complex IV. The system utilizes an alternative oxidase (PaAOX) which uses iron instead of copper as a cofactor. The alternative respiratory pathway is characterized by a decreased ATP generation but, most significantly, also a decrease in the production of reactive oxygen species. Consequently, molecular damage is reduced which contributes to an increased lifespan of this type of mutant. In addition, modifications in the availability of cellular copper have other relevant consequences. Most significantly, the characteristic age-related rearrangements occurring in the mitochondrial DNA of wild-type strains of P. anserina were found to be dependent on the availability of copper.

Copper-modulated gene expression and senescence in the filamentous fungus Podospora anserina

We have previously shown that the control of cellular copper homeostasis by the copper-modulated transcription factor GRISEA has an important impact on the phenotype and lifespan of Podospora anserina. Here we demonstrate that copper depletion leads to the induction of an alternative respiratory pathway and to an increase in lifespan. This response compensates mitochondrial dysfunctions via the expression of PaAox, a nuclear gene coding for an alternative oxidase. It resembles the retrograde response in Saccharomyces cerevisiae. In P. anserina, this pathway appears to be induced by specific impairments of the copper-dependent cytochrome c oxidase. It is not induced as the result of a general decline of mitochondrial functions during senescence. We cloned and characterized PaAox. Transcript levels are decreased when cellular copper, superoxide, and hydrogen peroxide levels are raised. Copper also controls transcript levels of PaSod2, the gene encoding the mitochondrial manganese superoxide dismutase (PaSOD2). PaSod2 is a target of transcription factor GRISEA. During the senescence of wild-type strain s, the activity of PaSOD2 decreases, whereas the activity of the cytoplasmic copper/zinc superoxide dismutase (PaSOD1) increases. Collectively, the data explain the postponed senescence of mutant grisea as a defined consequence of copper depletion, ultimately leading to a reduction of oxidative stress. Moreover, they suggest that during senescence of the wild-type strain, copper is released from mitochondria. The involved mechanism is unknown. However, it is striking that the permeability of mitochondrial membranes in animal systems changes during apoptosis and that mitochondrial proteins with an important impact on this type of cellular death are released.

ami1, an orthologue of the Aspergillus nidulans apsA gene, is involved in nuclear migration events throughout the life cycle of Podospora anserina

The Podospora anserina ami1-1 mutant was identified as a male-sterile strain. Microconidia (which act as male gametes) form, but are anucleate. Paraphysae from the perithecium beaks are also anucleate when ami1-1 is used as the female partner in a cross. Furthermore, in crosses heterozygous for ami1-1, some crozier cells are uninucleate rather than binucleate. In addition to these nuclear migration defects, which occur at the transition between syncytial and cellular states, ami1-1 causes abnormal distribution of the nuclei in both mycelial filaments and asci. Finally, an ami1-1 strain bearing information for both mating types is unable to self-fertilize. The ami1 gene is an orthologue of the Aspergillus nidulans apsA gene, which controls nuclear positioning in filaments and during conidiogenesis (at the syncytial/cellular transition). The ApsA and AMI1 proteins display 42% identity and share structural features. The apsA gene complements some ami1-1 defects: it increases the percentage of nucleate microconidia and restores self-fertility in an ami1-1 mat+ (mat-) strain. The latter effect is puzzling, since in apsA null mutants sexual reproduction is quite normal. The functional differences between the two genes are discussed with respect to their possible history in these two fungi, which are very distant in terms of evolution.

Laccase activity tests and laccase inhibitors

Sulfhydryl organic compounds described as laccase inhibitors: dithiothreitol, thioglycolic acid, cysteine, diethyldithiocarbamic acid, and sodium azide were tested for their activity toward laccase of Trametes versicolor in different test systems utilising 2, 2'-azino-bis-(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and 2, 6-dimethoxyphenol as enzyme substrates. Only sodium azide acted as a true laccase inhibitor and showed no significant interference with the enzyme tests. All other substances did not significantly inhibit the laccase activity and the previously reported inhibitory effects result from the reductions of the reaction products such as ABTS radical cation and diquinone or subsequent non-enzymatic interactions during substrate oxidation. The latter apparently forms a complex with unreacted ABTS displaying varied spectral characteristics and resulting in an underestimation of enzyme activity.

Mitochondrial-nuclear interactions and lifespan control in fungi

In fungi, mitochondrial-nuclear interactions are part of a complex molecular network involved in the control of aging processes. The generation of reactive oxygen species at the mitochondrial respiratory chain plays a major role in this network. Mitochondrial DNA instabilities, which are under the control of nuclear genes, affect the generation of reactive oxygen species and modulate the rate of aging. As mitochondria become dysfunctional, they transduce signals to the nucleus and induce the expression of a set of nuclear genes, a process termed retrograde regulation. Molecular data are emerging which suggest that retrograde regulation is involved in lifespan control.

Mitochondrial DNA rearrangements of Podospora anserina are under the control of the nuclear gene grisea

Podospora anserina is a filamentous fungus with a limited life span. Life span is controlled by nuclear and extranuclear genetic traits. Herein we report the nature of four alterations in the nuclear gene grisea that lead to an altered morphology, a defect in the formation of female gametangia, and an increased life span. Three sequence changes are located in the 5' upstream region of the grisea ORF. One mutation is a G --> A transition at the 5' splice site of the single intron of the gene, leading to a RNA splicing defect. This loss-of-function affects the amplification of the first intron of the mitochondrial cytochrome c oxidase subunit I gene (COI) and the specific mitochondrial DNA rearrangements that occur during senescence of wild-type strains. Our results indicate that the nuclear gene grisea is part of a molecular machinery involved in the control of mitochondrial DNA reorganizations. These DNA instabilities accelerate but are not a prerequisite for the aging of P. anserina cultures.

Isolation and characterization of a laccase gene from Podospora anserina

The genome of the filamentous ascomycete Podospora anserina contains at least four non-adjacent regions that are homologous to the laccase gene of Neurospora crassa. One of these regions contains a gene (lac2) encoding a protein that displays 62% identity with the N. crassa laccase. In shaken cultures, lac2 mRNA is present at low basal levels throughout the growth phase but increases at least 20-fold at the beginning of the autolytic phase and decreases again thereafter. Addition of aromatic xenobiotics (guaiacol, hydroquinone, benzoquinone) to the medium during the growth phase results in a rapid, drastic and temporary increase in the abundance of lac2 mRNA. The promoter region of lac2 contains two sequences which display complete homology with the eukaryotic Xenobiotic Responsive Element and two sequences homologous to the eukaryotic Antioxidant Responsive Element. The identity and function of the laccase encoded by lac2 are discussed.

GRISEA, a putative copper-activated transcription factor from Podospora anserina involved in differentiation and senescence

Podospora anserina is a filamentous fungus with a limited lifespan. Lifespan is controlled by both environmental and genetic factors. Using a combination of genetic and molecular approaches we have cloned one of these factors, gerontogene grisea. The cloned wild-type copy of grisea complements the altered morphological characteristics (e.g., colony and ascospore color), the defect in gametangia development, and the increased lifespan of the pleiotropic mutant grisea. A molecular analysis revealed that grisea is a discontinuous gene with a single intron. The deduced amino acid sequence shows significant homology to MAC1, ACE1 and AMT1, indicating that GRISEA, like the proteins from Saccharomyces cerevisiae (MAC1 and ACE1) and Candida glabrata (AMT1), codes for a copper-activated transcription factor. This conclusion is consistent with the pleiotropic nature of the grisea phenotype. We suggest that the gerontoprotein GRISEA is one component of a transcription apparatus involved in the genetic control of morphogenesis and aging.

Reversion of a long-living, undifferentiated mutant of Podospora anserina by copper

The Podospora anserina nuclear mutant grisea displays an undifferentiated growth phenotype (diminished production of aerial hyphae), is female sterile (lack of perithecia), has a prolonged life-span compared to the wild-type strain, and lacks detectable phenoloxidase (laccase and tyrosinase) activity. Reversion of all of these characteristics to those of the wild-type phenotype was accomplished by supplementing the growth medium with extra amounts of copper salts. These results indicate that the primary defect of the grisea strain is in its copper uptake and/or distribution in the cells.

The role of mitochondrial DNA rearrangements in aging and human diseases

Instabilities and point mutations of the high molecular weight mitochondrial DNA (mtDNA) were shown to be correlated with various degenerative processes in both lower eukaryotes as well as in mammals. In filamentous fungi, circular and linear plasmids were demonstrated to be involved in mtDNA rearrangements and in the genetic control of senescence. In addition, in these eukaryotic microorganisms, which have proved to be ideal model systems in experimental gerontology, a number of nuclear genes were identified controlling the stability of the mitochondrial genome. Although the mitochondrial genome of mammals, including humans, appears to be quite stable in comparison to other species, mtDNA instabilities of the type described in fungi were observed in mitochondria of patients with different mitochondrial degenerative disorders (CPEO, KSS, Pearson syndrome, LHON, MERRF, MELAS). It was later demonstrated that such mtDNA rearrangements appear to accumulate progressively during aging in human subjects. These data suggest that instabilities of the mitochondrial genome may play an important role in the control of life span not only in lower eukaryotes, but also in humans.

Temperature regulation of the cryptococcal phenoloxidase

Melanin formation at 37 degrees C has been proposed as a virulence factor in Cryptococcus neoformans. However, whereas catecholamine uptake is maintained at this temperature, phenoloxidase, which catalyses the oxidation of catecholamine to melanin, is severely decreased in most wild type strains cultivated at 37 degrees C.

The onset of senescence is affected by DNA rearrangements of a discontinuous mitochondrial gene in Podospora anserina

Mapping and transcription studies have revealed that in Podospora anserina the causative agent of senescence, a mitochondrial plasmid (plDNA), is identical with intron 1 of the discontinuous gene for cytochrome-c-oxidase subunit 1 (COI), which is 2 kpb from the discontinuous gene for cytochrome b (Cytb). A mitochondrial mutant (ex1) devoid of the COI, but not of the Cytb gene provides longevity. A molecular model for the onset of senescence is presented.

Chromosomal and extrachromosomal control of senescence in the ascomycete Podospora anserina

In Podospora anserina senescence leading to cellular death occurs regularly after prolonged vegetative propagation. However, the life span of this ascomycete may be extended by various means: 1. Mutations in a least 8 morphogenetic genes belonging to 4 linkage groups postpone drastically or even prevent in certain pairwise combinations (e.g. i viv) the onset of senescence. 2. Inhibitors of mt DNA and of mitochondrial protein synthesis show a life prolonging effect when added in low concentrations to the growth medium. 3. A similar effect was found when mycelia were fed exclusively on non repressive carbon sources. Whereas the anti-aging effect of specific mutated genes is rather permanent, the life prolonging action of the inhibitors and carbon sources is restricted and temporary. These substances have no long lasting effect, since after their removal from the medium aging proceeds. Physiological experiments have further shown the existence of three phases in the life span of Podospora anserina. During the juvenile phase aging is prevented by all of these compounds; during the presenescent phase aging is prevented by inhibitors of mt DNA only, and during the senescent phase aging is irreversible. Senescence may be induced in juvenile protoplasts by DNA extracted from senescent mycelia. This, together with the well known fact that senescence is extrachromosomically inherited, points to extrachromosomal DNA as the causative agent of senescence. This kind of DNA may be connected with or perhaps located in the mitochondria. Collectively, the data are consistent in showing that the syndrome of senescence in Podospora anserina is controlled by a chromosomal-extrachromosomal interaction. In this system, extrachromosomal DNA, perhaps a mt DNA, is identical with the infectious principle initiating the decay of the cell, and nuclear genes supervise its expression.

The phenoloxidases of the ascomycete Podospora anserina. Structural differences between laccases of high and low molecular weight

In order to investigate the extent of the relationship between the three copper-containing glycoproteins, laccases I, II and III (Mr70000, 80000 and 390000 respectively) of Podospora anserina, the following experiments were carried out on laccases II and III: (a) determination of amino acid composition; (b) determination of N-terminal and C-terminal amino acid; (c) determination of sugar composition; (d) dissociation studies on native and denatured laccases and also after removal of copper from the enzymes; (e) digestion of the carbohydrate moieties with the aid of glycosylhydrolases. A comparison between the results of these experiments and data previously obtained with laccase I allows the following conclusions to be drawn. 1. Laccases II and III are not identical. 2. Neither of these low molecular weight laccases are as complete molecules subunits of the oligomeric laccase I. 3. The possibility of partial identity of amino acid sequences of laccases I and III can not be excluded. 4. Laccase II possibly consists of subunits of Mr37000 whereas laccase III does not. 5. Digestion of 50% of the carbohydrate content leads to complete loss of serological specificity (serological reaction and cross reaction). This finding is discussed with regard to the possible role of the carbohydrate moiety as antigenic determinants and thus as the reason for the immunological relationship. As a consequence, at least three independent structural genes for laccases must be assumed.