Mutual independence of alkaline- and calcium-mediated signalling in Aspergillus fumigatus refutes the existence of a conserved druggable signalling nexus.

Functional coupling of calcium- and alkaline responsive signalling occurs in multiple fungi to afford efficient cation homeostasis. Host microenvironments exert alkaline stress and potentially toxic concentrations of Ca2+ , such that highly conserved regulators of both calcium- (Crz) and pH- (PacC/Rim101) responsive signalling are crucial for fungal pathogenicity. Drugs targeting calcineurin are potent antifungal agents but also perturb human immunity thereby negating their use as anti-infectives, abrogation of alkaline signalling has, therefore, been postulated as an adjunctive antifungal strategy. We examined the interdependency of pH- and calcium-mediated signalling in Aspergillus fumigatus and found that calcium chelation severely impedes hyphal growth indicating a critical requirement for this ion independently of ambient pH. Transcriptomic responses to alkaline pH or calcium excess exhibited minimal similarity. Mutants lacking calcineurin, or its client CrzA, displayed normal alkaline tolerance and nuclear translocation of CrzA was unaffected by ambient pH. Expression of a highly conserved, alkaline-regulated, sodium ATPase was tolerant of genetic or chemical perturbations of calcium-mediated signalling, but abolished in null mutants of the pH-responsive transcription factor PacC, and PacC proteolytic processing occurred normally during calcium excess. Taken together our data demonstrate that in A. fumigatus the regulatory hierarchy governing alkaline tolerance circumvents calcineurin signalling.

Developmental changes in trak-mediated mitochondrial transport in neurons.

Previous studies established that the kinesin adaptor proteins, TRAK1 and TRAK2, play an important role in mitochondrial transport in neurons. They link mitochondria to kinesin motor proteins via a TRAK acceptor protein in the mitochondrial outer membrane, the Rho GTPase, Miro. TRAKs also associate with enzyme, O-linked N-acetylglucosamine transferase (OGT), to form a quaternary, mitochondrial trafficking complex. A recent report suggested that TRAK1 preferentially controls mitochondrial transport in axons of hippocampal neurons whereas TRAK2 controls mitochondrial transport in dendrites. However, it is not clear whether the function of any of these proteins is exclusive to axons or dendrites and if their mechanisms of action are conserved between different neuronal populations and also, during maturation. Here, a comparative study was carried out into TRAK-mediated mitochondrial mobility in axons and dendrites of hippocampal and cortical neurons during maturation in vitro using a shRNA gene knockdown approach. It was found that in mature hippocampal and cortical neurons, TRAK1 predominantly mediates axonal mitochondrial transport whereas dendritic transport is mediated via TRAK2. In young, maturing neurons, TRAK1 and TRAK2 contribute similarly in mitochondrial transport in both axons and dendrites in both neuronal types. These findings demonstrate maturation regulation of mitochondrial transport which is conserved between at least two distinct neuronal subtypes.

Localization of the kinesin adaptor proteins trafficking kinesin proteins 1 and 2 in primary cultures of hippocampal pyramidal and cortical neurons.

Neuronal function requires regulated anterograde and retrograde trafficking of mitochondria along microtubules by using the molecular motors kinesin and dynein. Previous work has established that trafficking kinesin proteins (TRAKs),TRAK1 and TRAK2, are kinesin adaptor proteins that link mitochondria to kinesin motor proteins via an acceptor protein in the mitochondrial outer membrane, etc. the Rho GTPase Miro. Recent studies have shown that TRAK1 preferentially controls mitochondrial transport in axons of hippocampal neurons by virtue of its binding to both kinesin and dynein motor proteins, whereas TRAK2 controls mitochondrial transport in dendrites resulting from its binding to dynein. This study further investigates the subcellular localization of TRAK1 and TRAK2 in primary cultures of hippocampal and cortical neurons by using both commercial antibodies and anti-TRAK1 and anti-TRAK2 antibodies raised in our own laboratory (in-house). Whereas TRAK1 was prevalently localized in axons of hippocampal and cortical neurons, TRAK2 was more prevalent in dendrites of hippocampal neurons. In cortical neurons, TRAK2 was equally distributed between axons and dendrites. Some qualitative differences were observed between commercial and in-house-generated antibody immunostaining.

Modulation of inositol polyphosphate levels regulates neuronal differentiation.

The binding of neurotrophins to tropomyosin receptor kinase receptors initiates several signaling pathways, including the activation of phospholipase C-γ, which promotes the release of diacylglycerol and inositol 1,4,5-trisphosphate (IP(3)). In addition to recycling back to inositol, IP(3) serves as a precursor for the synthesis of higher phosphorylated inositols, such as inositol 1,3,4,5,6-pentakisphosphate (IP(5)) and inositol hexakisphosphate (IP(6)). Previous studies on the effect of neurotrophins on inositol signaling were limited to the analysis of IP(3) and its dephosphorylation products. Here we demonstrate that nerve growth factor (NGF) regulates the levels of IP(5) and IP(6) during PC12 differentiation. Furthermore, both NGF and brain-derived neurotrophic factor alter IP(5) and IP(6) intracellular ratio in differentiated PC12 cells and primary neurons. Neurotrophins specifically regulate the expression of IP(5)-2 kinase (IP(5)-2K), which phosphorylates IP(5) into IP(6). IP(5)-2K is rapidly induced after NGF treatment, but its transcriptional levels sharply decrease in fully differentiated PC12 cells. Reduction of IP(5)-2K protein levels by small interfering RNA has an effect on the early stages of PC12 cell differentiation, whereas fully differentiated cells are not affected. Conversely, perturbation of IP(5)-2K levels by overexpression suggests that both differentiated PC12 cells and sympathetic neurons require low levels of the enzyme for survival. Therefore maintaining appropriate intracellular levels of inositol polyphosphates is necessary for neuronal survival and differentiation.

Genetic bypass of Aspergillus nidulans crzA function in calcium homeostasis.

After dephosphorylation by the phosphatase calcineurin, the fungal transcription factor CrzA enters the nucleus and activates the transcription of genes responsible for calcium homeostasis and many other calcium-regulated activities. A lack of CrzA confers calcium-sensitivity to the filamentous fungus Aspergillus nidulans. To further understand calcium signaling in filamentous fungi and to identify genes that interact genetically with CrzA, we selected for mutations that were able to suppress crzAΔ calcium intolerance and identified three genes. Through genetic mapping, gene sequencing, and mutant rescue, we were able to identify these as cnaB (encoding the calcineurin regulatory subunit), folA (encoding an enzyme involved in folic acid biosynthesis, dihydroneopterin aldolase), and scrC (suppression of crzA(-), encoding a hypothetical protein). By using a calcium indicator, Fluo-3, we were able to determine that the wild-type and the suppressor strains were either able to regulate intracellular calcium levels or were able to take up and or store calcium correctly. The increased expression of calcium transporters, pmcA and/or pmcB, in suppressor mutants possibly enabled tolerance to high levels of calcium. Our results suggest that a cnaB suppressor mutation confers calcium tolerance to crzAΔ strains through restoration of calcium homeostasis. These results stress that in A. nidulans there are calcineurin-dependent and CrzA-independent pathways. In addition, it is possible that CrzA is able to contribute to the modulation of folic acid biosynthesis.

Preparation of quality inositol pyrophosphates.

Myo-inositol is present in nature either unmodified or in more complex phosphorylated derivates. Of the latest, the two most abundant in eukaryotic cells are inositol pentakisphosphate (IP(5;)) and inositol hexakisphosphate (phytic acid or IP(6;)). IP(5;) and IP(6;) are the precursors of inositol pyrophosphate molecules that contain one or more pyrophosphate bonds(1). Phosphorylation of IP(6;) generates diphoshoinositolpentakisphosphate (IP(7;) or PP-IP(5;)) and bisdiphoshoinositoltetrakisphosphate (IP(8;) or (PP)(2;)-IP(4;)). Inositol pyrophosphates have been isolated from all eukaryotic organisms so far studied. In addition, the two distinct classes of enzymes responsible for inositol pyrophosphate synthesis are highly conserved throughout evolution(2-4). The IP(6;) kinases (IP(6;)Ks) posses an enormous catalytic flexibility, converting IP(5;) and IP(6;) to PP-IP(4;) and IP(7;) respectively and subsequently, by using these products as substrates, promote the generation of more complex molecules(5,6). Recently, a second class of pyrophosphate generating enzymes was identified in the form of the yeast protein VIP(1;) (also referred as PP-IP(5;)K), which is able to convert IP(6;) to IP(7;) and IP(8;)(7,8). Inositol pyrophosphates regulate many disparate cellular processes such as insulin secretion(9), telomere length(10,11), chemotaxis(12), vesicular trafficking(13), phosphate homeostasis(14) and HIV-1 gag release(15). Two mechanisms of actions have been proposed for this class of molecules. They can affect cellular function by allosterically interacting with specific proteins like AKT(16). Alternatively, the pyrophosphate group can donate a phosphate to pre-phosphorylated proteins(17). The enormous potential of this research field is hampered by the absence of a commercial source of inositol pyrophosphates, which is preventing many scientists from studying these molecules and this new post-translational modification. The methods currently available to isolate inositol pyrophosphates require sophisticated chromatographic apparatus(18,19). These procedures use acidic conditions that might lead to inositol pyrophosphate degradation(20) and thus to poor recovery. Furthermore, the cumbersome post-column desalting procedures restrict their use to specialized laboratories. In this study we describe an undemanding method for the generation, isolation and purification of the products of the IP(6;)-kinase and PP-IP(5;)-kinases reactions. This method was possible by the ability of polyacrylamide gel electrophoresis (PAGE) to resolve highly phosphorylated inositol polyphosphates(20). Following IP(6;)K1 and PP-IP(5;)K enzymatic reactions using IP(6;) as the substrate, PAGE was used to separate the generated inositol pyrophosphates that were subsequently eluted in water.

Identification of an evolutionarily conserved family of inorganic polyphosphate endopolyphosphatases.

Inorganic polyphosphate (poly-P) consists of just a chain of phosphate groups linked by high energy bonds. It is found in every organism and is implicated in a wide variety of cellular processes (e.g. phosphate storage, blood coagulation, and pathogenicity). Its metabolism has been studied mainly in bacteria while remaining largely uncharacterized in eukaryotes. It has recently been suggested that poly-P metabolism is connected to that of highly phosphorylated inositol species (inositol pyrophosphates). Inositol pyrophosphates are molecules in which phosphate groups outnumber carbon atoms. Like poly-P they contain high energy bonds and play important roles in cell signaling. Here, we show that budding yeast mutants unable to produce inositol pyrophosphates have undetectable levels of poly-P. Our results suggest a prominent metabolic parallel between these two highly phosphorylated molecules. More importantly, we demonstrate that DDP1, encoding diadenosine and diphosphoinositol phosphohydrolase, possesses a robust poly-P endopolyphosphohydrolase activity. In addition, we prove that this is an evolutionarily conserved feature because mammalian Nudix hydrolase family members, the three Ddp1 homologues in human cells (DIPP1, DIPP2, and DIPP3), are also capable of degrading poly-P.

Sub-telomere directed gene expression during initiation of invasive aspergillosis.

Aspergillus fumigatus is a common mould whose spores are a component of the normal airborne flora. Immune dysfunction permits developmental growth of inhaled spores in the human lung causing aspergillosis, a significant threat to human health in the form of allergic, and life-threatening invasive infections. The success of A. fumigatus as a pathogen is unique among close phylogenetic relatives and is poorly characterised at the molecular level. Recent genome sequencing of several Aspergillus species provides an exceptional opportunity to analyse fungal virulence attributes within a genomic and evolutionary context. To identify genes preferentially expressed during adaptation to the mammalian host niche, we generated multiple gene expression profiles from minute samplings of A. fumigatus germlings during initiation of murine infection. They reveal a highly co-ordinated A. fumigatus gene expression programme, governing metabolic and physiological adaptation, which allows the organism to prosper within the mammalian niche. As functions of phylogenetic conservation and genetic locus, 28% and 30%, respectively, of the A. fumigatus subtelomeric and lineage-specific gene repertoires are induced relative to laboratory culture, and physically clustered genes including loci directing pseurotin, gliotoxin and siderophore biosyntheses are a prominent feature. Locationally biased A. fumigatus gene expression is not prompted by in vitro iron limitation, acid, alkaline, anaerobic or oxidative stress. However, subtelomeric gene expression is favoured following ex vivo neutrophil exposure and in comparative analyses of richly and poorly nourished laboratory cultured germlings. We found remarkable concordance between the A. fumigatus host-adaptation transcriptome and those resulting from in vitro iron depletion, alkaline shift, nitrogen starvation and loss of the methyltransferase LaeA. This first transcriptional snapshot of a fungal genome during initiation of mammalian infection provides the global perspective required to direct much-needed diagnostic and therapeutic strategies and reveals genome organisation and subtelomeric diversity as potential driving forces in the evolution of pathogenicity in the genus Aspergillus.

Functional characterization of the Aspergillus fumigatusPHO80 homologue.

Phosphate is an ion that is essential for fungal growth. The systems for inorganic phosphate (P(i)) acquisition in eukaryotic cells (PHO) have been characterized as a low-affinity (that assures a supply of P(i) at normal or high external P(i) concentrations) and a high-affinity (activated in response to P(i) starvation). Here, as an initial step to understand the PHO pathway in Aspergillus fumigatus, we characterized the PHO80 homologue, PhoB(PHO80). We show that the DeltaphoB(PHO80) mutant has a polar growth defect (i.e., a delayed germ tube emergence) and, by phenotypic and phosphate uptake analyses, establish a link between PhoB(PHO80), calcineurin and calcium metabolism. Microarray hybridizations carried out with RNA obtained from wild-type and DeltaphoB(PHO80) mutant cells identify Afu4g03610 (phoD(PHO84)), Afu7g06350 (phoE(PHO89)), Afu4g06020 (phoC(PHO81)), and Afu2g09040 (vacuolar transporter Vtc4) as more expressed both in the DeltaphoB(PHO80) mutant background and under phosphate-limiting conditions of 0.1mM P(i). Epifluorescence microscopy revealed accumulation of poly-phosphate in DeltaphoB(PHO80) vacuoles, which was independent of extracellular phosphate concentration. Surprisingly, a phoD(PHO84) deletion mutant is indistinguishable phenotypically from the corresponding wild-type strain. mRNA analyses suggest that protein kinase A absence supports the expression of PHO genes in A. fumigatus. Furthermore, DeltaphoB(PHO80) and DeltaphoD(PHO84) mutant are fully virulent in a murine low dose model for invasive aspergillosis.

Functional characterization of the Aspergillus fumigatus CRZ1 homologue, CrzA.

The protein phosphatase calcineurin is an important mediator connecting calcium-dependent signalling to various cellular responses in multiple organisms. In fungi calcineurin acts largely through regulating Crz1p-like transcription factors. Here we characterize an Aspergillus fumigatus CRZ1 homologue, CrzA and demonstrate its mediation of cellular tolerance to increased concentrations of calcium and manganese. In addition to acute sensitivity to these ions, and decreased conidiation, the crzA null mutant suffers altered expression of calcium transporter mRNAs under high concentrations of calcium, and loss of virulence when compared with the corresponding complemented and wild-type strains. We use multiple expression analyses to probe the transcriptional basis of A. fumigatus calcium tolerance identifying several genes having calA and/or crzA dependent mRNA accumulation patterns. We also demonstrate that contrary to previous findings, the gene encoding the Aspergillus nidulans calcineurin subunit homologue, cnaA, is not essential and that the cnaA deletion mutant shares the morphological phenotypes observed in the corresponding A. fumigatus mutant, DeltacalA. Exploiting the A. nidulans model system, we have linked calcineurin activity with asexual developmental induction, finding that CrzA supports appropriate developmental induction in a calcineurin and brlA-dependent manner in both species.

Distinct roles for intra- and extracellular siderophores during Aspergillus fumigatus infection.

Siderophore biosynthesis by the highly lethal mould Aspergillus fumigatus is essential for virulence, but non-existent in humans, presenting a rare opportunity to strategize therapeutically against this pathogen. We have previously demonstrated that A. fumigatus excretes fusarinine C and triacetylfusarinine C to capture extracellular iron, and uses ferricrocin for hyphal iron storage. Here, we delineate pathways of intra- and extracellular siderophore biosynthesis and show that A. fumigatus synthesizes a developmentally regulated fourth siderophore, termed hydroxyferricrocin, employed for conidial iron storage. By inactivation of the nonribosomal peptide synthetase SidC, we demonstrate that the intracellular siderophores are required for germ tube formation, asexual sporulation, resistance to oxidative stress, catalase A activity, and virulence. Restoration of the conidial hydroxyferricrocin content partially rescues the virulence of the apathogenic siderophore null mutant Delta sidA, demonstrating an important role for the conidial siderophore during initiation of infection. Abrogation of extracellular siderophore biosynthesis following inactivation of the acyl transferase SidF or the nonribosomal peptide synthetase SidD leads to complete dependence upon reductive iron assimilation for growth under iron-limiting conditions, partial sensitivity to oxidative stress, and significantly reduced virulence, despite normal germ tube formation. Our findings reveal distinct cellular and disease-related roles for intra- and extracellular siderophores during mammalian Aspergillus infection.