Amino acid biosynthetic routes as drug targets for pulmonary fungal pathogens: what is known and why do we need to know more?

Abstract

Amongst 1.5 million fatal mycoses of humans occurring annually [1], the vast majority involve the human lung as the primary site of pathogenesis, and are derived from organisms which occupy environmental niches. On entry into the respiratory system pathogenic fungi must draw upon metabolic versatility for survival and proliferation as the mammalian lung is a nutritionally limiting environment. The nutritional stresses encountered have exposed vulnerabilities which have long been viewed as potential antifungal targets, since humans lack several of the metabolic pathways which fungi rely upon for pathogenic growth. However the ability of saprophytic fungi to proteolytically liberate amino acids from exogenous protein sources, and the differential availabilities of amino acids in diverse host niches have undermined confidence in amino acid metabolism as a target for selectively toxic antifungal therapies. Recent studies have reopened this debate by revealing a number of anabolic amino acid pathways in pathogenic fungi as being essential for viability per se. This review examines new knowledge on fungal amino acid metabolism in fungal pathogens of the human lung with a view to highlighting important new advances and gaps in understanding. TITLE Role of amino acid metabolism in fungal pathogenicityThroughout the kingdoms of life, amino acids are important building blocks of proteins and critical sources of macroelements such as nitrogen and sulphur which, if not acquired via intrinsic anabolic metabolism, must be sourced from extrinsic sources. Although fungi can biosynthesise all 20 of the proteinogenic amino acids the uptake of exogenous amino acids is significantly energetically preferable [2]. Accordingly, saprophytic fungal genomes harbour a multitude of protease and peptidase, and amino acid transporter encoding genes to ensure the liberation and uptake of amino acids from exogenous proteinaceous substrates.Amongst the hierarchy of preferred nitrogen sources the nitrogen-rich amino acids glutamine and arginine are dominant, but other amino acids can also be utilised when starvation threatens. Several wide domain regulatory mechanisms operate to ensure that starvation for nitrogen, carbon or amino acids is met with an appropriate metabolic response. In response to intracellular glutamine levels AreA-type GATA factors interact with other transcription factors specific for the biosynthesis of alternative nitrogen sources eg proline [3,4], while amino acid starvation increases translation of the transcriptional activator CpcA to generate a broad-acting cellular response governing amino acid biosynthesis and uptake [5]. Cellular levels of cysteine and methionine reflect intracellular sulphur content and modulate sulphur uptake via sulphur metabolite repression involving the positive-acting

Role of Amino Acid Metabolism in the Virulence of Human Pathogenic Fungi

“…In contrast to what has been reported for C. neoformans and A. fumigatus, only a few reports exist concerning amino acid biosynthetic pathways essential for virulence in C. albicans [24,108]. For example, the disruption of the threonine and branched-chain amino acid biosynthetic pathways result in attenuated virulence in intravenous infection model in mice [110,111].…”

“…This is particularly the case for the biosynthesis of 10 amino acids which are at least semi-essential for humans: Ile, Leu, Lys, Met, Phe, Thr, Trp, Val, Arg, and His. The potential of essential amino acid biosynthetic pathways as antifungal targets has been summarized previously [24,108]. Another option would be targeting or exploiting fungal amino acid transport mechanisms.…”

Antifungal Resistance, Metabolic Routes as Drug Targets, and New Antifungal Agents: An Overview about Endemic Dimorphic Fungi

“…Evidence shows that efficient responses to amino acid starvation and requirements are important for fungal pathogenicity; the absence of CpcA, a transcriptional activator in amino acid starvation, and AreA, a nitrogen metabolic repressor activated when preferable nitrogen sources are not available, impairs A. fumigatus virulence in a murine model of pulmonary aspergillosis [57, 58]. Growing knowledge suggests that biosynthetic pathways have purposes beyond nutritional requirements; additional functions may exists that could be even more important for pathogenicity [59]. During fungal infection, the microorganism encounters specific niches with variable amounts of amino acids/nitrogen sources.…”

“…Based on our current knowledge, amino acid biosynthetic routes are suitable targets for antifungal development, because they are essential for fungal pathogens and have enzymes not found in humans. However, some aspects must be considered in targeting amino acid pathways: auxotrophy for certain amino acids can be restored by uptake of exogenous molecules from proteolytic products of fungal proteases and the niche-specific requirement varies dramatically [59]. Thus, the generation of conditional expressing mutants appears to be the best strategy to analyze in vivo amino acid acquisition, when null mutants are not obtainable.…”

Arginine Auxotrophy Affects Siderophore Biosynthesis and Attenuates Virulence of Aspergillus fumigatus

“…Fungi are eukaryotes and consequently share many of their metabolic pathways with humans, hindering the development of novel fungal-specific drugs. To identify potential targets for antifungal drug development, mutant strains deficient in amino acid biosynthetic pathways not present in higher eukaryotes have been characterized in several fungal pathogens [2]. In A. fumigatus, enzymes participating in the biosynthesis of the amino acids tyrosine, phenylalanine, tryptophan, lysine, histidine, cysteine, methionine, and leucine were validated as potential antifungal targets due to virulence Starting with glutamate, ornithine is synthesized in the mitochondrion in five enzymatic steps, and either directly transported to the cytoplasm via the ornithine transporter AmcA or converted into citrulline before being transported to the cytoplasm.…”