Glucan synthase complex of Aspergillus fumigatus

The RHO1-specific GTPase-activating protein LRG1 regulates polar tip growth in parallel to Ndr kinase signaling in Neurospora

Regulation of Rho GTPase signaling is critical for cell shape determination and polarity. Here, we investigated the role of LRG1, a novel member of the GTPase-activating proteins (GAPs) of Neurospora crassa. LRG1 is essential for apical tip extension and to restrict excessive branch formation in subapical regions of the hypha and is involved in determining the size of the hyphal compartments. LRG1 localizes to hyphal tips and sites of septation via its three LIM domains. The accumulation of LRG1 as an apical cap is dependent on a functional actin cytoskeleton and active growth, and is influenced by the opposing microtubule-dependent motor proteins dynein and kinesin-1. Genetic evidence and in vitro GTPase assays identify LRG1 as a RHO1-specific GAP affecting several output pathways of RHO1, based on hyposensitivity to the glucan inhibitor caspofungin, synthetic lethality with a hyperactive beta1,3-glucan synthase mutant, altered PKC/MAK1 pathway activities, and hypersensitivity to latrunculin A. The morphological defects of lrg-1 are highly reminiscent to the Ndr kinase/RAM pathway mutants cot-1 and pod-6, and genetic evidence suggests that RHO1/LRG1 function in parallel with COT1 in coordinating apical tip growth.

Pathogenesis of allergic bronchopulmonary aspergillosis in cystic fibrosis: current understanding and future directions

Allergic bronchopulmonary aspergillosis (ABPA) is an allergic disease characterized clinically by wheezing, pulmonary infiltrates, bronchiectasis, and fibrosis that affects patients with asthma and cystic fibrosis (CF). Although this disease has been characterized by a Th2 immune response to Aspergillus, the disease has some features such as central bronchiectasis which is not seen in other Th2 driven lung diseases such as atopic asthma. Here we will review the current pathophysiology of ABPA in CF and highlight new molecules that may affect immune responses against Aspergillus and ABPA disease pathogenesis.

Identification of phoslactomycin E as a metabolite inducing hyphal morphological abnormalities in Aspergillus fumigatus IFO 5840

In our survey for antifungal compounds, a fermentation broth of Streptomyces sp. HA81-2 was found to inhibit the in vitro growth of Aspergillus fumigatus IFO 5840 accompanied by hyphal morphological abnormalities. One of the isolated antibiotics was identified as phoslactomycin E based on LC-MS and NMR spectral data. In a preliminary assay using the membrane fractions of A. fumigatus, phoslactomycin E was found to inhibit the activity of 1,3-beta glucan synthase.

Chemosensitization prevents tolerance of Aspergillus fumigatus to antimycotic drugs

Tolerance of human pathogenic fungi to antifungal drugs is an emerging medical problem. We show how strains of the causative agent of human aspergillosis, Aspergillus fumigatus, tolerant to cell wall-interfering antimycotic drugs become susceptible through chemosensitization by natural compounds. Tolerance of the A. fumigatus mitogen-activated protein kinase (MAPK) mutant, sakADelta, to these drugs indicates the osmotic/oxidative stress MAPK pathway is involved in maintaining cell wall integrity. Using deletion mutants of the yeast, Saccharomyces cerevisiae, we first identified thymol and 2,3-dihydroxybenzaldehyde (2,3-D) as potent chemosensitizing agents that target the cell wall. We then used these chemosensitizing agents to act as synergists to commercial antifungal drugs against tolerant strains of A. fumigatus. Thymol was an especially potent chemosensitizing agent for amphotericin B, fluconazole or ketoconazole. The potential use of natural, safe chemosensitizing agents in antifungal chemotherapy of human mycoses as an alternative to combination therapy is discussed.

Stage-specific innate immune recognition of Aspergillus fumigatus and modulation by echinocandin drugs

The pulmonary innate immune system clears inhaled Aspergillus fumigatus conidia (spores) from terminal airways. Failure to control conidial germination in immune compromised hosts can result in hyphal tissue invasion and fatal disease. Insight into the molecular recognition of A. fumigatus by host leukocytes indicates that the innate immune system exploits obligate changes in fungal cell wall composition that occur at the first stage of germination, conidial swelling. Germinating spores activate at least two host signal transduction pathways. Surface exposure of fungal beta-(1,3) glucan, a polysaccharide constituent of the fungal cell wall, triggers dectin-1 signaling by host phagocytes. Spore germination leads to the induction of Toll-like receptor (TLR) signaling as well. This stage-specific recognition mechanism focuses host antifungal responses on cells with the potential for tissue invasion and may serve to limit potentially deleterious effects of inflammation in space and time. Fungal beta-(1,3) glucan not only activates host innate immune responses but also represents the target of echinocandin drugs. The activity of echinocandin drugs has largely been understood on the basis of pharmacologic growth inhibition of yeast and moulds, resulting in lysis of yeast cells and stunting of dysmorphic hyphae. The recognition that fungal beta-1,3 glucan activates dectin-1 signaling suggests that echinocandin drugs may exert immune modulatory effects by altering innate immune responses to drug-treated fungal cells, a view supported by recent data from studies on C. albicans, A. fumigatus, and non-Aspergillus moulds.

Impaired ribosome biogenesis disrupts the integration between morphogenesis and nuclear duplication during the germination of Aspergillus fumigatus

Aspergillus fumigatus is an important opportunistic fungal pathogen that is responsible for high mortality rates in the immunosuppressed population. CgrA, the A. fumigatus ortholog of a Saccharomyces cerevisiae nucleolar protein involved in ribosome biogenesis, contributes to the virulence of this fungus by supporting rapid growth at 37 degrees C. To determine how CgrA affects ribosome biogenesis in A. fumigatus, polysome profile and ribosomal subunit analyses were performed on both wild-type A. fumigatus and a DeltacgrA mutant. The loss of CgrA was associated with a reduction in the level of 80S monosomes as well as an imbalance in the 60S:40S subunit ratio and the appearance of half-mer ribosomes. The gene expression profile in the DeltacgrA mutant revealed increased abundance of a subset of translational machinery mRNAs relative to the wild type, suggesting a potential compensatory response to CgrA deficiency. Although DeltacgrA conidia germinated normally at 22 degrees C, they swelled excessively when incubated at 37 degrees C and accumulated abnormally high numbers of nuclei. This hypernucleated phenotype could be replicated pharmacologically by germinating wild-type conidia under conditions of reductive stress. These findings indicate that the germination process is particularly vulnerable to global disruption of protein synthesis and suggest that CgrA is involved in both ribosome biogenesis and polarized cell growth in A. fumigatus.

Rho1 has distinct functions in morphogenesis, cell wall biosynthesis and virulence of Fusarium oxysporum

Rho-type GTPases regulate polarized growth in yeast by reorganization of the actin cytoskeleton and through signalling pathways that control the expression of cell wall biosynthetic genes. We report the cloning and functional analysis of rho1 from Fusarium oxysporum, a soilborne fungal pathogen causing vascular wilt on plants and opportunistic infections in humans. F. oxysporum strains carrying either a Deltarho1 loss-of-function mutation or a rho1(G14V) gain-of-function allele were viable, but displayed a severely restricted colony phenotype which was partially relieved by the osmotic stabilizer sorbitol, indicating structural alterations in the cell wall. Consistent with this hypothesis, Deltarho1 strains showed increased resistance to cell wall-degrading enzymes and staining with Calcofluor white, as well as changes in chitin and glucan synthase gene expression and enzymatic activity. Re-introduction of a functional rho1 allele into the Deltarho1 mutant fully restored the wild-type phenotype. The Deltarho1 strain had dramatically reduced virulence on tomato plants, but was as virulent as the wild type on immunodepressed mice. Thus, Rho1 plays a key role during fungal infection of plants, but not of mammalian hosts.

The pharmacology and clinical use of caspofungin

Caspofungin was the first echinocandin to be licensed for the treatment of invasive fungal infections. Caspofungin has in vitro and in vivo activity against Candida spp. and Aspergillus spp., which constitute the majority of medically important opportunistic fungal pathogens. Caspofungin inhibits the synthesis of the 1,3-beta-glucan, with resultant osmotic instability and lysis. The pharmacology of caspofungin is relatively complex. Trafficking of drug into tissues is an important determinant of the shape of the concentration-time relationship. Caspofungin has demonstrated efficacy in experimental models of invasive candidiasis and aspergillosis, which reflect its activity in the treatment of oropharyngeal, esophageal and disseminated candidiasis, as well as salvage therapy for patients with invasive aspergillosis.

The echinocandin micafungin: a review of the pharmacology, spectrum of activity, clinical efficacy and safety

Micafungin is a relatively broad-spectrum antifungal agent available for clinical use in the US and Japan. By inhibiting the production of beta-1,3-glucan, an essential fungal cell wall component, micafungin has reduced toxicity to mammalian cells while maintaining potent antifungal activity against many pathogenic fungi including polyene- and azole-resistant isolates. Indeed, micafungin has been shown to be efficacious in the treatment of infections caused by Candida and Aspergillus species in clinical trials without the associated toxicities of amphotericin B formulations and drug interactions that occur with the azoles. In this review, the pharmacology, spectrum of activity, clinical efficacy and safety profile of micafungin are discussed.

Testing the efficacy of RNA interference constructs in Aspergillus fumigatus

We recently developed a silencing vector in Aspergillus fumigatus which carries a hygromycin resistance marker and a transcriptional unit for hairpin RNA expression under the control of the inducible glucoamylase promoter (pGla) (Mouyna et al. in FEMS Microbiol Lett 237:317-324, 2004). We showed previously that this vector can be used for the RNA interference application of two genes ALB1 and FKS1 of which reduced mRNA levels occurred for both, with phenotypic consequences resembling disruptions of genes involved in melanin (ALB1) and beta(1-3)glucan biosynthesis (FKS1). We reported here the silencing of KRE6 and CRH1, two other genes putatively involved in cell wall biosynthesis using a similar construction under the control of the constitutive promoter glyceraldehyde-3-phosphate dehydrogenase (pgpdA). Silencing of the expression of these two genes was obtained. Further analysis of the transformants showed however that (1) a 100% loss of expression was never achieved for all genes tested (2) the vector used for RNAi is lost or modified over successive transfers resulting in an inhibition of the silencing. These disadvantages of RNAi indicate that classical gene disruption by gene replacement remains the most efficient method for a molecular analysis of gene function in A. fumigatus.

The echinocandins: comparison of their pharmacokinetics, pharmacodynamics and clinical applications

Caspofungin, micafungin and anidulafungin are three drugs of the echinocandin class of antifungals available for intravenous treatment of invasive candidiasis and aspergillosis. They exhibit high in vitro and in vivo activities against Candida spp. and Aspergillus spp. In various clinical studies investigating candidemia and invasive candidiasis, Candida esophagitis, and fever in neutropenia, the clinical efficacy of the echinocandin tested was similar to that of established antifungals. Antifungal activity against strains no longer susceptible to conventional antifungal agents, such as fluconazole and amphotericin B suggests that echinocandins can be used as salvage therapy in life-threatening fungal infections. There is no cross-resistance to other antifungals. Excellent safety and tolerability of treatment with caspofungin has been documented over a total of 4.3 million patient days. Echinocandins are poor substrates of the cytochrome P450 enzyme family and can be safely co-administered with most drugs without the need for dosage adaptation. No dose reduction is required in renal impairment. A reduction in the daily maintenance dose has been recommended for caspofungin, but not for micafungin and anidulafungin in patients presenting with mild to moderate hepatic failure.

FsFKS1, the 1,3-beta-glucan synthase from the caspofungin-resistant fungus Fusarium solani

The cell wall, a mesh of carbohydrates and proteins, shapes and protects the fungal cell. The enzyme responsible for the synthesis of one of the main components of the fungal wall, 1,3-beta-glucan synthase, is targeted by the antifungal caspofungin acetate (CFA). Clinical isolates of Candida albicans and Aspergillus fumigatus are much more sensitive to CFA than clinical isolates of Fusarium species. To better understand CFA resistance in Fusarium species, we cloned and sequenced FsFKS1, which encodes the Fusarium solani f. sp. pisi beta(1,3)-D-glucan synthase, used RNA interference to reduce its expression and complemented deletion of the essential fks gene of the CFA-sensitive fungus A. fumigatus with FsFKS1. Reduction of the FsFKS1 message in F. solani f. sp. pisi reduced spore viability and caused lysis of spores and hyphae, consistent with cell wall defects. Compensating for the loss of A. fumigatus fks1 with FsFKS1 caused only a modest increase in the tolerance of A. fumigatus for CFA. Our results suggest that FsFKS1 is required for the proper construction of F. solani cell walls and that the resistance of F. solani to CFA is at best only partially due to resistance of the FsFKS1 enzyme to this antifungal agent.

Identification of the first Oomycete annexin as a (1→3)-β-d-glucan synthase activator

(1-->3)-beta-D-Glucans are major components of the cell walls of Oomycetes and as such they play an essential role in the morphogenesis and growth of these microorganisms. Despite the biological importance of (1-->3)-beta-D-glucans, their mechanisms of biosynthesis are poorly understood. Previous studies on (1-->3)-beta-D-glucan synthases from Saprolegnia monoica have shown that three protein bands of an apparent molecular weight of 34, 48 and 50 kDa co-purify with enzyme activity. However, none of the corresponding proteins have been identified. Here we have identified, purified, sequenced and characterized a protein from the 34 kDa band and clearly shown that it has all the biochemical properties of proteins from the annexin family. In addition, we have unequivocally demonstrated that the purified protein is an activator of (1-->3)-beta-D-glucan synthase. This represents a new type of function for proteins belonging to the annexin family. Two other proteins from the 48 and 50 kDa bands were identified as ATP synthase subunits, which most likely arise from contaminations by mitochondria during membrane preparation. The results, which are discussed in relation with the possible regulation mechanisms of (1-->3)-beta-D-glucan synthases, represent a first step towards a better understanding of cell wall polysaccharide biosynthesis in Oomycetes.

Cell wall assembly in Saccharomyces cerevisiae

An extracellular matrix composed of a layered meshwork of beta-glucans, chitin, and mannoproteins encapsulates cells of the yeast Saccharomyces cerevisiae. This organelle determines cellular morphology and plays a critical role in maintaining cell integrity during cell growth and division, under stress conditions, upon cell fusion in mating, and in the durable ascospore cell wall. Here we assess recent progress in understanding the molecular biology and biochemistry of cell wall synthesis and its remodeling in S. cerevisiae. We then review the regulatory dynamics of cell wall assembly, an area where functional genomics offers new insights into the integration of cell wall growth and morphogenesis with a polarized secretory system that is under cell cycle and cell type program controls.

Dectin-1 and TLRs permit macrophages to distinguish between different Aspergillus fumigatus cellular states

Aspergillus fumigatus is a common cause of invasive and allergic pulmonary disease. Resting conidia of the filamentous fungus are constantly inhaled, but cause infection only after initiating hyphal growth. In this study, we have explored whether macrophages can distinguish between resting spores and the maturing, potentially invasive form of the fungus. Although macrophages bind and ingest A. fumigatus resting conidia efficiently, there is little inflammatory response; NF-kappabeta is not activated, inflammatory cytokines are not induced, and reactive oxygen species are not produced. However, maturing A. fumigatus conidia and germ tubes stimulate NF-kappabeta, secretion of proinflammatory cytokines and production of reactive oxygen by human monocyte-derived macrophages and murine macrophages from multiple anatomical sites. These responses are in part mediated by dectin-1, which binds cell wall beta-glucan that is not present on the surface of dormant conidia, but is present after cellular swelling and loss of the hydrophobic proteinaceous cell wall. Dectin-1 binding to germ tubes augments, but is not required for, TLR2-mediated inflammatory cytokine secretion. Dectin-1 recognition of germ tubes also stimulates TNF-alpha production in the absence of both TLR2 and MyD88 signaling. These data demonstrate one mechanism by which the pulmonary inflammatory response is tailored toward metabolically active cells, thereby avoiding unnecessary tissue damage with frequent inhalation of ubiquitous spores.

Differential expression of chitin synthase (CHS) and glucan synthase (FKS) genes correlates with the formation of a modified, thinner cell wall in in vivo-produced Beauveria bassiana cells

During infection (in vivo), the entomopathogenic fungus Beauveria bassiana produces yeast-like cells that are surrounded by modified cell walls. These modifications have been related to the fungus ability to limit recognition by the host defense system. The composition of the in vivo cell wall was analyzed using a combination of cytochemical and molecular techniques. The in vivo cell walls still contained both chitin and 1,3-beta-glucan, but they were significantly thinner than in vitro cell walls (50-60 nm versus 100-160 nm, respectively). The difference in cell wall thickness was correlated with transcriptional regulation of cell wall-related genes: quantitative RT-PCR reactions demonstrated that B. bassiana chitin synthase (CHS) and glucan synthase (FKS) genes are down regulated in vivo. These analyses indicate that in vivo-triggered phenotypic modifications, including cell wall adjustments, are controlled by molecular mechanisms that include regulation of gene expression at the transcriptional level.

Current state of three-dimensional characterisation of antifungal targets and its use for molecular modelling in drug design

The alarming rise in life-threatening systemic fungal infections due to the emergence of drug-resistant fungal strains had produced an increased demand for new antimycotics, especially those targeting novel antifungal structures. Drug discovery has developed from screening natural products and chemical synthesis to a modern approach, namely structure-based drug design. Whilst many antifungal agents currently in use were discovered more than 30 years ago, characterisation of various drug targets has only been achieved recently, contributing immensely to understanding the structure-activity relationships of antifungals and their targets. Three-dimensional characterisation has become a well established tool for modern antifungal drug research and should play an important role in investigations for new antifungal agents.

Deletion of GEL2 encoding for a beta(1-3)glucanosyltransferase affects morphogenesis and virulence in Aspergillus fumigatus

The first fungal glycosylphosphatidylinositol anchored beta(1-3)glucanosyltranferase (Gel1p) has been described in Aspergillus fumigatus and its encoding gene GEL1 identified. Glycosylphosphatidylinositol-anchored glucanosyltransferases play an active role in the biosynthesis of the fungal cell wall. We characterize here GEL2, a homologue of GEL1. Both homologues share common characteristics: (i) GEL1 and GEL2 are constitutively expressed during over a range of growth conditions; (ii) Gel2p is also a putative GPI-anchored protein and shares the same beta(1-3)glucanosyltransferase activity as Gel1p and (iii) GEL2, like GEL1, is able to complement the Deltagas1 deletion in Saccharomyces cerevisiae confirming that Gelp and Gasp have the same enzymatic activity. However, disruption of GEL1 did not result in a phenotype whereas a Deltagel2 mutant and the double mutant Deltagel1Deltagel2 exhibit slower growth, abnormal conidiogenesis, and an altered cell wall composition. In addition, the Deltagel2 and the Deltagel1Deltagel2 mutant have reduced virulence in a murine model of invasive aspergillosis. These data suggest for the first time that beta(1-3)glucanosyltransferase activity is required for both morphogenesis and virulence in A. fumigatus.

Genes and molecules involved in Aspergillus fumigatus virulence

Aspergillus fumigatus causes a wide range of diseases that include mycotoxicosis, allergic reactions and systemic diseases (invasive aspergillosis) with high mortality rates. Pathogenicity depends on immune status of patients and fungal strain. There is no unique essential virulence factor for development of this fungus in the patient and its virulence appears to be under polygenetic control. The group of molecules and genes associated with the virulence of this fungus includes many cell wall components, such as beta-(1-3)-glucan, galactomannan, galactomannanproteins (Afmp1 and Afmp2), and the chitin synthetases (Chs; chsE and chsG), as well as others. Some genes and molecules have been implicated in evasion from the immune response, such as the rodlets layer (rodA/hyp1 gene) and the conidial melanin-DHN (pksP/alb1 gene). The detoxifying systems for Reactive Oxygen Species (ROS) by catalases (Cat1p and Cat2p) and superoxide dismutases (MnSOD and Cu, ZnSOD), had also been pointed out as essential for virulence. In addition, this fungus produces toxins (14 kDa diffusible substance from conidia, fumigaclavin C, aurasperon C, gliotoxin, helvolic acid, fumagilin, Asp-hemolysin, and ribotoxin Asp fI/mitogilin F/restrictocin), allergens (Asp f1 to Asp f23), and enzymatic proteins as alkaline serin proteases (Alp and Alp2), metalloproteases (Mep), aspartic proteases (Pep and Pep2), dipeptidyl-peptidases (DppIV and DppV), phospholipase C and phospholipase B (Plb1 and Plb2). These toxic substances and enzymes seems to be additive and/or synergistic, decreasing the survival rates of the infected animals due to their direct action on cells or supporting microbial invasion during infection. Adaptation ability to different trophic situations is an essential attribute of most pathogens. To maintain its virulence attributes A. fumigatus requires iron obtaining by hydroxamate type siderophores (ornitin monooxigenase/SidA), phosphorous obtaining (fos1, fos2, and fos3), signal transductional falls that regulate morphogenesis and/or usage of nutrients as nitrogen (rasA, rasB, rhbA), mitogen activated kinases (sakA codified MAP-kinase), AMPc-Pka signal transductional route, as well as others. In addition, they seem to be essential in this field the amino acid biosynthesis (cpcA and homoaconitase/lysF), the activation and expression of some genes at 37 degrees C (Hsp1/Asp f12, cgrA), some molecules and genes that maintain cellular viability (smcA, Prp8, anexins), etc. Conversely, knowledge about relationship between pathogen and immune response of the host has been improved, opening new research possibilities. The involvement of non-professional cells (endothelial, and tracheal and alveolar epithelial cells) and professional cells (natural killer or NK, and dendritic cells) in infection has been also observed. Pathogen Associated Molecular Patterns (PAMP) and Patterns Recognizing Receptors (PRR; as Toll like receptors TLR-2 and TLR-4) could influence inflammatory response and dominant cytokine profile, and consequently Th response to infec tion. Superficial components of fungus and host cell surface receptors driving these phenomena are still unknown, although some molecules already associated with its virulence could also be involved. Sequencing of A. fumigatus genome and study of gene expression during their infective process by using DNA microarray and biochips, promises to improve the knowledge of virulence of this fungus.

Coccidioides posadasii contains a single 1,3-beta-glucan synthase gene that appears to be essential for growth

1,3-beta-Glucan synthase is responsible for the synthesis of beta-glucan, an essential cell wall structural component in most fungi. We sought to determine whether Coccidioides posadasii possesses genes homologous to known fungal FKS genes that encode the catalytic subunit of 1,3-beta-glucan synthase. A single gene, designated FKS1, was identified, and examination of its predicted protein product showed a high degree of conservation with Fks proteins from other filamentous fungi. FKS1 is expressed at similar levels in mycelia and early spherulating cultures, and expression decreases as the spherules mature. We used Agrobacterium-mediated transformation to create strains that harbor DeltaFKS1::hygB, a null allele of FKS1, and hypothesize that Fks1p function is essential, due to our inability to purify this allele away from a complementing wild-type FKS1 allele in a heterokaryotic strain. The heterokaryon appears normal with respect to growth rate and arthroconidium production; however, microscopic examination of strains with DeltaFKS1::hygB alleles revealed abnormal swelling of hyphal elements.

Two alpha(1-3) glucan synthases with different functions in Aspergillus fumigatus

Alpha(1-3) glucan is a main component of the Aspergillus fumigatus cell wall. In spite of its importance, synthesis of this amorphous polymer has not been investigated to date. Two genes in A. fumigatus, AGS1 and AGS2, are highly homologous to the AGS genes of Schizosaccharomyces pombe, which encode putative alpha(1-3) glucan synthases. The predicted Ags proteins of A. fumigatus have an estimated molecular mass of 270 kDa. AGS1 and AGS2 were disrupted in A. fumigatus. Both Deltaags mutants have similar altered hyphal morphologies and reduced conidiation levels. Only Deltaags1 presented a reduction in the alpha(1-3) glucan content of the cell wall. These results showed that Ags1p and Ags2p were functionally different. The cellular localization of the two proteins was in agreement with their different functions: Ags1p was localized at the periphery of the cell in connection with the cell wall, whereas Ags2p was intracellularly located. An original experimental model of invasive aspergillosis based on mixed infection and quantitative PCR was developed to analyze the virulence of A. fumigatus mutant and wild-type strains. Using this model, it was shown that the cell wall and morphogenesis defects of Deltaags1 and Deltaags2 were not associated with a reduction in virulence in either mutant. This result showed that a 50% reduction in the content of the cell wall alpha(1-3) glucan does not play a significant role in A. fumigatus pathogenicity.

Characterization ofAspergillus fumigatusmutants with reduced susceptibility to caspofungin

Caspofungin acetate (CAS) is a member of a new class of clinically-approved echinocandin drugs to treat invasive aspergillosis. CAS inhibits the activity of beta-1,3-D-glucan synthase (GS), thus damaging the fungal cell wall. Although no clinical resistance of Aspergillus to CAS has been reported as yet, the development of in vitro reduced susceptibility is presumed to be inevitable. By contrast, echinocandin resistance in laboratory strains of Candida albicans and Saccharomyces cerevisiae has been well documented. To study the potential for clinical resistance in Aspergillus, two classes of Aspergillus fumigatus mutant strains were isolated that exhibited reduced susceptibility to CAS. In the first class, a site-directed mutation within the target gene (AfFKS1, encoding the putative catalytic subunit of GS) was introduced and shown to confer low-level (16-fold) reduced susceptibility. A second class of spontaneous mutants were sensitive to low levels of drug but displayed nearly normal growth above 0.5 microg/ml, suggesting induction of an unknown resistance mechanism. At higher levels of drug (> or = 16 microg/ml), the mutants displayed partially restored sensitivity. Preliminary studies indicate that neither target site mutations, nor changes in target gene expression are present in these strains, as has been documented for several yeasts. Instead, preliminary results indicate that the molecular mechanism(s) underlying reduced susceptibility of CAS in the A. fumigatus strains is novel, possibly due to remodeling of the cell wall components.

Retinal Function Assessed by ERG Before and After Induction of Ocular Aspergillosis and Treatment by the Anti-fungal, Micafungin, in Rabbits

This study was conducted to evaluate the effectiveness of a new antifungal drug, micafungin, and standard antifungal drugs against endophthalmitis induced in a rabbit by intravitreal injection of Aspergillus fumigatus, an important fungal pathogen. Effectiveness was evaluated by the preservation of b-wave amplitude at 72 h after injection of the fungus relative to the b-wave amplitude at baseline before any intravitreal injections. A 0.06 ml inoculum of 10(6) conidia of A. fumigatus was injected into the vitreous of the right eye of all rabbits; and, 12 h later, a 0.06 ml solution containing one of 3 antifungal drugs or saline was injected into the vitreous of both eyes. All three antifungal drugs produced significant b-wave preservation at 72 h in infected eyes compared to that in infected eyes receiving saline injections. There was no statistically significant difference between the effects of micafungin and amphotericin B in the right eyes with fungal endophthalmitis, and both produced significantly more preservation of b-wave amplitude than voriconazole. Amphotericin B, but neither micafungin nor voriconazole produced significant reduction of the b-wave amplitude in the left eyes.

Specific molecular features in the organization and biosynthesis of the cell wall ofAspergillus fumigatus

The cell wall of Aspergillus fumigatus is composed of a branched beta1,3 glucan covalently bound to chitin, beta1,3, beta1,4 glucans, and galactomannan, that is embedded in an amorphous cement composed of alpha1,3 glucan, galactomannan and polygalactosamin. The mycelial cell wall of A. fumigatus is very different from the yeast Saccharomyces cerevisiae cell wall, and in particular lacks beta1,6 glucans and proteins covalently bound to cell wall polysaccharides. The differences in cell wall composition between the mould A. fumigatus and the yeast S. cerevisiae are also reflected at the genomic level where unique features have been identified in A. fumigatus. A single gene codes for the glucan synthase catalytic subumit; this finding has lead to the development of a RNAi methodology for the disruption of essential genes in A. fumigatus. In contrast to the glucan synthase, multiple genes have been found in the chitin synthase and the alpha glucan synthase families; in spite of homologous sequences, each gene in each family have very different function. Similarly homologous mannosyltransferase genes are found in yeast and moulds but they lead to the synthesis of very different N-mannan structures. This chemo-genomic comparative analysis has also suggested that GPI-anchored proteins do not have a role of linker in the three dimensional organization of the fungal cell wall.

Piperazine Propanol Derivative as a Novel Antifungal Targeting 1,3-.BETA.-D-Glucan Synthase

1,3-beta-D-Glucan synthase, which synthesizes a main component of fungal cell wall, is one of the promising targets for antifungal agents. In order to identify novel chemical classes of 1,3-beta-D-glucan synthase inhibitors, we screened a chemical library monitoring inhibition of the Candida albicans 1,3-beta-D-glucan synthase activity. The piperazine propanol derivative GSI578 [(2,6-difluoro-phenyl)-carbamic acid 3-(4-benzothiazol-2-yl-piperazine-1-yl)-propyl ester] was identified as a potent inhibitor against 1,3-beta-D-glucan synthase with an IC50 value of 0.16 microM. GSI578 exhibited in vitro antifungal activity against pathogenic fungi including C. albicans and Aspergillus fumigatus. Temperature-sensitive mutations of the FKS1 gene in the Deltafks2 background of Saccharomyces cerevisiae, where FKS1 and FKS2 encode putative catalytic subunits of 1,3-beta-D-glucan synthase, altered sensitivity to GSI578. This suggests that the antifungal activity of the piperazine propanol derivative has an effect on 1,3-beta-D-glucan synthase inhibition. Results of our initial evaluation suggest that the piperazine propanol derivative is a novel chemical structure of the class of antifungals which inhibit fungal cell growth by inhibiting fungal 1,3-beta-D-glucan synthase.

Expression of agsA, one of five 1,3-alpha-D-glucan synthase-encoding genes in Aspergillus niger, is induced in response to cell wall stress

1,3-alpha-D-Glucan is an important component of the cell wall of filamentous fungi. We have identified a family of five 1,3-alpha-D-glucan synthase-encoding genes in Aspergillus niger. The agsA gene was sequenced and the predicted protein sequence indicated that the overall domain structure of 1,3-alpha-D-glucan synthases is conserved in fungi. Using RT-PCR and Northern blot analysis, we found that expression of the agsA gene and to a lesser extent also of agsE were induced in the presence of the cell wall stress-inducing compounds such as Calcofluor White (CFW), SDS, and caspofungin. Loss of agsA function did not result in an apparent phenotype under normal growth conditions but rendered the cells more sensitive to CFW. The induction of 1,3-alpha-D-glucan synthase-encoding genes in response to cell wall stress was not limited to A. niger, but was also observed in Penicillium chrysogenum. We propose that this response to cell wall stress commonly occurs in filamentous fungi.

Lessons from the genome sequence of Neurospora crassa: tracing the path from genomic blueprint to multicellular organism

We present an analysis of over 1,100 of the approximately 10,000 predicted proteins encoded by the genome sequence of the filamentous fungus Neurospora crassa. Seven major areas of Neurospora genomics and biology are covered. First, the basic features of the genome, including the automated assembly, gene calls, and global gene analyses are summarized. The second section covers components of the centromere and kinetochore complexes, chromatin assembly and modification, and transcription and translation initiation factors. The third area discusses genome defense mechanisms, including repeat induced point mutation, quelling and meiotic silencing, and DNA repair and recombination. In the fourth section, topics relevant to metabolism and transport include extracellular digestion; membrane transporters; aspects of carbon, sulfur, nitrogen, and lipid metabolism; the mitochondrion and energy metabolism; the proteasome; and protein glycosylation, secretion, and endocytosis. Environmental sensing is the focus of the fifth section with a treatment of two-component systems; GTP-binding proteins; mitogen-activated protein, p21-activated, and germinal center kinases; calcium signaling; protein phosphatases; photobiology; circadian rhythms; and heat shock and stress responses. The sixth area of analysis is growth and development; it encompasses cell wall synthesis, proteins important for hyphal polarity, cytoskeletal components, the cyclin/cyclin-dependent kinase machinery, macroconidiation, meiosis, and the sexual cycle. The seventh section covers topics relevant to animal and plant pathogenesis and human disease. The results demonstrate that a large proportion of Neurospora genes do not have homologues in the yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe. The group of unshared genes includes potential new targets for antifungals as well as loci implicated in human and plant physiology and disease.

Invasive aspergillosis in patients with hematologic malignancies

Invasive aspergillosis is an increasingly common and often fatal opportunistic fungal infection in patients with hematologic malignancies. Prolonged and profound neutropenia remains a key risk factor for the development of invasive aspergillosis. However, qualitative deficiencies in host immune responses resulting from prolonged corticosteroid therapy, graft-versus-host disease, and cytomegalovirus infection are important risk factors for the recurrence and progression of Aspergillus infections after bone marrow recovery. Early diagnosis of invasive aspergillosis remains a challenge, and few tools are available for monitoring its course once the diagnosis is established. Even with the recent introduction of new antifungal therapies, mortality in patients with invasive aspergillosis remains high, and uniformly effective prophylaxis or preemptive therapeutic strategies are lacking. Strategies such as combination antifungal therapy and immunotherapy often are used as first-line treatment approaches in patients with documented invasive aspergillosis despite a paucity of clinical trial data. Recent advances in our understanding of the epidemiology, pathogenesis, and treatment of invasive aspergillosis in patients with hematologic malignancies are reviewed. The problems and controversies associated with defining optimal treatment strategies for invasive aspergillosis in this heavily immunocompromised population are highlighted.

Aspergillus nidulans RhoA is involved in polar growth, branching, and cell wall synthesis

Growth of the filamentous fungus Aspergillus nidulans begins when the conidium breaks dormancy and grows isotropically. Eventually a germ tube emerges and the axis of growth remains fixed in the primary hypha while new growth axes are established basally to form secondary germ tubes and lateral branches. Rho1 is a Rho family GTPase that has been shown to be involved in polarity establishment and cell wall deposition in Saccharomyces cerevisiae. A gene predicted to encode a Rho1 homolog was cloned from A. nidulans and named rhoA. Strains carrying ectopic copies of the constitutively active rhoA(G14V) allele or the dominant rhoA(E40I) allele were created and characterized. The constitutively active rhoA(G14V) strain grew slowly relative to wild type and showed an abnormal clustered pattern of branch emergence. The rhoA(G14V) strain also labeled intensely with calcofluor, showed elevated levels of cell wall N-acetylglucosamine and had unusually thick cell walls. The dominant rhoA(E40I)strain was accelerated in the emergence of secondary and tertiary germ tubes, and lateral branches relative to wild type and showed lysis with prolonged incubation. The rhoA(E40I) strain also was hypersensitive to the cell wall disrupting agents calcofluor and caspofungin acetate and showed an increase in cell wall N-acetylglucosamine levels. Our results suggest that rhoA plays a role in polarity, proper branching pattern, and cell wall deposition.

Echinocandin antifungal drugs

The echinocandins are large lipopeptide molecules that are inhibitors of beta-(1,3)-glucan synthesis, an action that damages fungal cell walls. In vitro and in vivo, the echinocandins are rapidly fungicidal against most Candida spp and fungistatic against Aspergillus spp. They are not active at clinically relevant concentrations against Zygomycetes, Cryptococcus neoformans, or Fusarium spp. No drug target is present in mammalian cells. The first of the class to be licensed was caspofungin, for refractory invasive aspergillosis (about 40% response rate) and the second was micafungin. Adverse events are generally mild, including (for caspofungin) local phlebitis, fever, abnormal liver function tests, and mild haemolysis. Poor absorption after oral administration limits use to the intravenous route. Dosing is once daily and drug interactions are few. The echinocandins are widely distributed in the body, and are metabolised by the liver. Results of studies of caspofungin in candidaemia and invasive candidiasis suggest equivalent efficacy to amphotericin B, with substantially fewer toxic effects. Absence of antagonism in combination with other antifungal drugs suggests that combination antifungal therapy could become a general feature of the echinocandins, particularly for invasive aspergillosis.

An ordered collection of expressed sequences from Cryphonectria parasitica and evidence of genomic microsynteny with Neurospora crassa and Magnaporthe grisea

Cryphonectria parasitica, the causative agent of chestnut blight, has proven to be a tractable experimental system for studying fungal pathogenesis. Moreover, the development of infectious cDNA clones of C. parasitica hypoviruses, capable of attenuating fungal virulence, has provided the opportunity to examine molecular aspects of fungal plant pathogenesis in the context of biological control. In order to establish a genomic base for future studies of C. parasitica, the authors have analysed a collection of expressed sequences. A mixed cDNA library was prepared from RNA isolated from wild-type (virus-free) and hypovirus-infected C. parasitica strains. Plasmid DNA was recovered from individual transformants and sequenced from the 5' end of the insert. Contig analysis of the collected sequences revealed that they represented approximately 2200 individual ORFs. An assessment of functional diversity present in this collection was achieved by using the BLAST software utilities and the NCBI protein database. Candidate genes were identified with significant potential relevance to C. parasitica growth, development, pathogenesis and vegetative incompatibility. Additional investigations of a 12.9 kbp genomic region revealed microsynteny between C. parasitica and both Neurospora crassa and Magnaporthe grisea, two closely related fungi. These data represent the largest collection of sequence information currently available for C. parasitica and are now forming the basis of further studies using microarray analyses to determine global changes in transcription that occur in response to hypovirus infection.

Killing activity of micafungin against Aspergillus fumigatus hyphae assessed by specific fluorescent staining for cell viability

We studied the anti-Aspergillus activity of micafungin by using two fluorescent dyes to detect cell viability. Micafungin induced flattened hyphae, caused by the bursting of cells, which had lost their viability. Micafungin has killing activity against actively growing hyphae, even though it is not fungicidal against the whole burden of Aspergillus fumigatus.

Caspofungin

Caspofungin, the first inhibitor of fungal beta-1,3 glucan synthesis to receive approval by the United States Food and Drug Administration, is effective for the treatment of mucosal and invasive candidiasis and invasive aspergillosis. It is also active in vitro and in animal models against a number of other filamentous and dimorphic endemic fungi and in animal models of Pneumocystis carinii infection. In vitro studies and some animal studies almost always indicate an absence of antagonism when caspofungin is combined with azole or polyene antifungal agents. Caspofungin has an excellent safety profile. Caspofungin may prove to be useful in empirical therapy for suspected invasive fungal infections. Additional clinical trial data that expand our knowledge of the usefulness of caspofungin for these and other mycoses, including its administration in combination with other antifungal agents, is anticipated. Caspofungin is an important addition to the antifungal pharmacopoeia.

Caspofungin: first approved agent in a new class of antifungals

Caspofungin (Cancidas, Merck & Co. Inc.) is the first echinocandin antifungal agent to gain FDA-approval for use in the US. It has excellent clinical activity against Candida spp. and Aspergillus spp. but lacks significant activity against Cryptococcus neoformans. Caspofungin may have some activity against dimorphic fungi such as Histoplasma capsulatum and Coccidioides immitis, but no clinical data is available for treatment of these infections. Caspofungin has demonstrated poor activity against most filamentous fungi in vitro. Several clinical trials have demonstrated its efficacy in the treatment of oropharyngeal, oesophageal and invasive candidiasis, as well as invasive aspergillosis. As a result of caspofungin's unique mechanism of action, and the high morbidity and mortality of invasive fungal infections, there is considerable interest in using this new antifungal agent as part of a combination antifungal therapy. In vitro studies and small case series indicate that caspofungin does not appear to be antagonistic when combined with other antifungals, such as itraconazole, voriconazole or amphotericin B against Aspergillus spp. Caspofungin exerts concentration-dependent killing effects in many different in vitro and animal models of disseminated fungal infection. The usual daily dose is 50 mg/day i.v. following a 70 mg i.v. loading dose. However, higher caspofungin doses have been safely administered and up to 70 mg/day can be administered for patients who fail to respond to lower doses. Caspofungin has an excellent safety profile with reduced toxicities, compared to other licensed antifungal agents. Fever, thrombophlebitis, headache and liver enzyme elevations were the most common drug-related side effects reported in clinical trials so far. Additional data are needed to document its safety in long-term use, and with higher doses in patients with invasive fungal infections. Caspofungin is a promising agent as first-line therapy for invasive candidiasis, and as salvage therapy for invasive aspergillosis. However, more clinical data are needed to define its role as primary therapy for invasive aspergillosis, and its role in combination antifungal therapy.

The antifungal echinocandin caspofungin acetate kills growing cells of Aspergillus fumigatus in vitro

Caspofungin acetate is an antifungal antibiotic that inhibits synthesis of 1,3-beta-D-glucan, an essential component of the fungal cell wall. While caspofungin causes cell death in yeasts and dimorphic fungi such as Candida albicans, its effect on Aspergillus fumigatus is less well understood. We used the fluorescent dyes 5,(6)-carboxyfluorescein diacetate (CFDA) and bis-(1,3-dibutylbarbituric acid) trimethine oxonol (DiBAC), which stain live and dead cells, respectively, to further characterize the antifungal activity of caspofungin. For comparison, compounds whose mode of action was either fungistatic (fluconazole, itraconazole) or fungicidal (amphotericin B) were also evaluated. A correlation between caspofungin-induced loss of viability, decreased CFDA staining, and increased DiBAC staining was established first with C. albicans. For A. fumigatus, caspofungin caused similar dye-staining changes, which were quantified by fluorimetric analysis of stained hyphae grown in a medium that promoted dispersed growth. The minimum concentration of caspofungin required to produce these changes also decreased the level of growth-dependent reduction of the indicator dye Alamar Blue. We observed a differential effect of caspofungin as a function of cell position: 88% of apical cells and 61% of subapical branching cells failed to stain with the viable dye CFDA, but only 24% of subapical cells were unstained. Complementary results were seen with germlings from DiBAC-stained, caspofungin-treated cultures. Extended incubation of A. fumigatus with a single dose of caspofungin affected the same proportion of apical and subapical branching cells for up to 72 h. The dye-staining patterns illustrate that the cells at the active centers for new cell wall synthesis within A. fumigatus hyphae are killed when they are exposed to caspofungin.

Dynamics of cell wall structure in Saccharomyces cerevisiae

The cell wall of Saccharomyces cerevisiae is an elastic structure that provides osmotic and physical protection and determines the shape of the cell. The inner layer of the wall is largely responsible for the mechanical strength of the wall and also provides the attachment sites for the proteins that form the outer layer of the wall. Here we find among others the sexual agglutinins and the flocculins. The outer protein layer also limits the permeability of the cell wall, thus shielding the plasma membrane from attack by foreign enzymes and membrane-perturbing compounds. The main features of the molecular organization of the yeast cell wall are now known. Importantly, the molecular composition and organization of the cell wall may vary considerably. For example, the incorporation of many cell wall proteins is temporally and spatially controlled and depends strongly on environmental conditions. Similarly, the formation of specific cell wall protein-polysaccharide complexes is strongly affected by external conditions. This points to a tight regulation of cell wall construction. Indeed, all five mitogen-activated protein kinase pathways in bakers' yeast affect the cell wall, and additional cell wall-related signaling routes have been identified. Finally, some potential targets for new antifungal compounds related to cell wall construction are discussed.

Characterization of essential genes by parasexual genetics in the human fungal pathogen Aspergillus fumigatus: impact of genomic rearrangements associated with electroporation of DNA

We have evaluated the usefulness of parasexual genetics in the identification of genes essential for the growth of the human fungal pathogen Aspergillus fumigatus. First, essentiality of the A. fumigatus AfFKS1 gene, encoding the catalytic subunit of the beta-(1,3)-glucan synthase complex, was assessed by inactivating one allele of AfFKS1 in a diploid strain of A. fumigatus obtained using adequate selectable markers in spore color and nitrate utilization pathways and by performing haploidization under conditions that select for the occurrence of the disrupted allele. Haploid progeny could not be obtained, demonstrating that AfFKS1 and, hence, beta-(1,3)-glucan synthesis are essential in A. fumigatus. Second, random heterozygous insertional mutants were generated by electroporation of diploid conidia with a heterologous plasmid. A total of 4.5% of the transformants failed to produce haploid progeny on selective medium. Genomic analysis of these heterozygous diploids led in particular to the identification of an essential A. fumigatus gene encoding an SMC-like protein resembling one in Schizosacccharomyces pombe involved in chromosome condensation and cohesion. However, significant plasmid and genomic DNA rearrangements were observed at many of the identified genomic loci where plasmid integration had occurred, thus suggesting that the use of electroporation to build libraries of A. fumigatus insertional mutants has relatively limited value and cannot be used in an exhaustive search of essential genes.

Fungal beta(1,3)-D-glucan synthesis

The polysaccharide beta(1,3)-D-glucan is a component of the cell wall of many fungi. Synthesis of the linear polymer is catalysed by UDP-glucose beta(1,3)-D-glucan beta(3)-D-glucosyltransferase. Because this enzyme has a key role in fungal cell-wall synthesis, and because many organisms that are responsible for human mycoses, including Candida albicans, Aspergillus fumigatus and Cryptococcus neoformans, produce walls that are rich in beta(1,3)-glucan, it has been and remains the focus of intensive study. From early characterization of the enzymatic activity in Saccharomyces cerevisiae, advances have been made in purification of the enzyme, identification of essential subunits and description of regulatory circuitry that controls expression and localization of different components of the multisubunit enzyme complex. Progress in each of these areas has been enhanced dramatically by the availability of specific inhibitors of the enzymatic reaction that produces beta(1,3)-glucan. These natural product inhibitors have utility both as tools to dissect the biology of beta(1,3)-glucan synthase and as sources for development of semisynthetic derivatives with clinical utility in treatment of human fungal disease. This review will focus on the biochemistry, genetics and regulation of the enzyme.

Protection of killer antiidiotypic antibodies against early invasive aspergillosis in a murine model of allogeneic T-cell-depleted bone marrow transplantation

Antiidiotypic monoclonal antibodies (MAbs) representing the internal image of a yeast killer toxin (KT) have therapeutic potential against several fungal infections. The efficacy of KT MAbs against Aspergillus fumigatus was investigated in a mouse model of T-cell-depleted allogeneic bone marrow transplantation (BMT) with invasive pulmonary aspergillosis. Mice were highly susceptible to infection at 3 days post-BMT, when profound neutropenia was observed both in the periphery and in the lungs. Treatment with KT MAbs protected the mice from infection, as judged by the long-term survival and decreased pathology associated with inhibition of fungal growth and hyphal development in the lungs. In vitro, similar to polymorphonuclear neutrophils, KT MAbs significantly inhibited the hyphal development and metabolic activity of germinated Aspergillus conidia. These results indicate that mimicking the action of neutrophils could be a strategy through which KT MAbs exert therapeutic efficacy in A. fumigatus infections.

Quantitative PCR assay to measure Aspergillus fumigatus burden in a murine model of disseminated aspergillosis: demonstration of efficacy of caspofungin acetate

Caspofungin acetate (MK-0991) is an antifungal antibiotic that inhibits the synthesis of 1,3-beta-D-glucan, an essential component of the cell wall of several pathogenic fungi. Caspofungin acetate was recently approved for the treatment of invasive aspergillosis in patients who are refractory to or intolerant of other therapies. The activity of 1,3-beta-D-glucan synthesis inhibitors against Aspergillus fumigatus has been evaluated in animal models of pulmonary or disseminated disease by using prolongation of survival or reduction in tissue CFU as assay endpoints. Because these methods suffer from limited sensitivity or poor correlation with fungal growth, we have developed a quantitative PCR-based (qPCR) (TaqMan) assay to monitor disease progression and measure drug efficacy. A. fumigatus added to naïve, uninfected kidneys as either ungerminated conidia or small germlings yielded a linear qPCR response over at least 4 orders of magnitude. In a murine model of disseminated aspergillosis, a burden of A. fumigatus was detected in each of five different organs at 4 days postinfection by the qPCR assay, and the mean fungal load in these organs was 1.2 to 3.5 log(10) units greater than mean values determined by CFU measurement. When used to monitor disease progression in infected mice, the qPCR assay detected an increase of nearly 4 log(10) conidial equivalents/g of kidney between days 1 and 4 following infection, with a peak fungal burden that coincided with the onset of significant mortality. Traditional CFU methodology detected only a marginal increase in fungal load in the same tissues. In contrast, when mice were infected with Candida albicans, which does not form true mycelia in tissues, quantitation of kidney burden by both qPCR and CFU assays was strongly correlated as the infection progressed. Finally, treatment of mice with induced disseminated aspergillosis with either caspofungin or amphotericin B reduced the A. fumigatus burden in infected kidneys to the limit of detection for the qPCR assay. Because of its much larger dynamic range, the qPCR assay is superior to traditional CFU determination for monitoring the progression of disseminated aspergillosis and evaluating the activity of antifungal antibiotics against A. fumigatus.

Membrane and cell wall targets in Aspergillus fumigatus

Antifungal drugs directed against the human opportunistic fungal pathogen Aspergillus fumigatus are limited in number and ergosterol-targeted: the polyenes bind to the membrane ergosterol and the azoles block the ergosterol biosynthesis pathway. The efficacy of the drugs currently available for clinical use (amphotericin B and itraconazole) is limited and the frequent occurrence of therapeutic failures in the treatment of invasive aspergillosis emphasizes the need for the development of new agents. Cell wall biosynthetic pathways have been recognized for a long time as essential and unique specific drug targets. Recent studies of the chemical organization of the cell wall of A. fumigatus together with comparative analysis of yeast cell wall data have shown that beta 1-3 glucan branching and chitin-beta 1-3 glucan binding are essential exocellular enzymatic steps in cell wall biosynthesis. The enzymes involved in the biosynthesis and remodeling of cell wall polysaccharides especially in A. fumigatus are reviewed.