A Chinese study has discovered that very long-chain fatty acids (VLCFAs) mediate septin organization in pathogenic fungi and that VLCFA biosynthesis represents a new target for controlling pathogenic fungi, which could lead to the development of new fungicides for use in plants and humans.

New fungal disease targets are urgently required due to the rapidly increasing global problem of fungicidal resistance, which poses a severe threat to food security and human health, the authors reported in the September 21, 2020, edition of Nature Microbiology.

Like antimicrobial resistance, fungicidal resistance is primarily driven by the widespread use/misuse of fungicides.

"Widespread antifungal use imposes selection pressures on fungal pathogens, allowing accumulation of mutations promoting resistance against antifungals," said study leader Xuewei Chen, director of the State Key Laboratory of Exploration and Utilization of Crop Gene Resources in Southwest China at Sichuan Agricultural University in Chengdu.

"This will affect human health, with nosocomial fungal resistance having been observed against many antifungals, such as amphotericin B, which is commonly used to treat a wide range of invasive mycoses in humans," said Chen.

"For example, resistance to amphotericin B is becoming increasingly common and the greater use of different classes of antifungals will further drive amphotericin B resistance in an increasing number of fungal species," he said, noting the need for new targets.

New targets

New antifungal targets may include appressoria, specialized cells helping fungi to infect plants or animals that are involved in controlling cell-shape, which is necessary for many important processes, including fungal pathogenicity.

Cell-shape changes mainly occur at cell membranes, but few membrane-associated components have been shown to play roles in cell-shape control, notably septin conserved guanosine triphosphate (GTP)-binding proteins.

Septins

Septins regulate membrane remodeling-associated processes, including exocytosis and neuronal spine morphogenesis, with mutations in septin-encoding genes leading to human diseases, including cancer and neurodegeneration.

"Most septin mutations occur in tumors of the large intestine, skin, endometrium and stomach," Chen told BioWorld Science.

Septin mutations are also associated with neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's disease, and frontotemporal lobar degeneration.

Septins are organized into higher-order structures that associate with plasma membranes, where they function as a lateral diffusion barrier.

On membranes, septins associate with lipids, notably phosphatidylinositol phosphates (PIPs), but it is unclear how PIPs mediate septin recruitment to specific membrane regions to control cell shape, as does how the composition of PIPs affects septin organization, which may involve VLCFAs.

VLCFAs

VLCFAs are known to be important for cell-shape regulation and are required, for example, for skin-barrier formation and myelin maintenance.

Furthermore, VLCFAs possess unique molecular properties allowing them to facilitate generation and stabilization of the curved membrane domains needed in highly specialized cell formation.

Understanding the interactions between septins and VLCFAs might allow further insights into septin-mediated processes in fungal pathogenesis, thereby providing clues to how such processes might be impaired for disease control.

In fungi, septins are required for generating cellular polar out-growths that allow pathogens to use appressoria to facilitate host entry, so impairing these structures might provide a potential means for disease prevention.

Nevertheless, despite this potential, relatively few anti-penetrant drugs have been developed to control fungal diseases of either plants or animals.

In their new Nature Microbiology study, Chen and his team reported having identified novel compounds that act specifically to prevent fungal infection.

Notably, they presented genetic and pharmaceutical evidence that organization of septin GTPases, which are essential for appressorium-mediated infection in the rice blast fungus Magnaporthe oryzae, require VLCFAs to mediate septin organization at membrane interfaces.

"Genetic disruption of the ELO1 gene required for VLCFA biosynthesis inhibited VLCFA production, impairing septin ring formation within the appressorium of M. oryzae," noted Chen.

Moreover, "pharmaceutical application of the VLCFA inhibitor molecules metazachlor, cafenstrole or diallate inhibited VLCFAs biosynthesis, impairing septin ring formation," he said.

"Both approaches were effective in preventing host penetration by M. oryzae; disruption of ELO1 gene or VLCFA inhibitors at a concentration of 500 mcM were both shown to reduce the pathogenicity of M. oryzae by over 50%."

VLCFAs were also shown to promote septin recruitment to curved plasma membranes, while VLCFA depletion prevented septin assembly and host penetration by M. oryzae.

Moreover, VLCFA biosynthesis inhibitors not only prevented rice blast disease, but also showed effective, broad-spectrum activity against a wide range of fungal pathogens of maize, wheat and locusts, without affecting their respective hosts.

"These antifungal observations suggest that VLCFA biosynthesis might be a useful target for controlling fungal disease in various species, including humans," said Chen.

"Given that animal fungal pathogens also have the VLCFA biosynthesis pathway, in future it will be interesting to test antifungal activity in human pathogens," he said.

However, before such testing, "I think two criteria need to be fulfilled, one of which is to determine the working concentration of VLCFA biosynthesis inhibitors that are effective against human fungal growth," said Chen.

"The other is that VLCFA biosynthesis inhibitors must show relatively specificity towards fungal pathogens and shown to be safe in human cell lines and animal models at effective antipathogenic concentrations."

In future, "we will continue to conduct research including both conceptual discovery and practical applications in this area," he said.

"Conceptually, we would like to determine how VLCFA biosynthesis is regulated at both the transcriptional and post-transcriptional levels for coupling with the PIP biosynthesis and proper septin distribution.

"Practically, we will investigate the effects of inhibitors on other fungal pathogens in different species, including humans. We are also interested in identifying more molecules that are effective in impairing septin-related fungal infection by screening chemical libraries." (He, M. et al. Nat Microbiol 2020, Advanced publication).

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