Iron regulatory pathways differentially expressed duringMadurella mycetomatisgrain development inGalleria mellonella

AbstractBackgroundMycetoma is a neglected, chronic granulomatous infection of the subcutaneous tissue, most often caused by the fungal pathogenMadurella mycetomatis. Characteristic of the infection is the formation of grains. However, knowledge of the function and formation of the grain is limited. To map the processes leading toM. mycetomatisgrain formation, we used aGalleria mellonellalarvae infection model and time-course transcriptomic profiling.MethodsGalleria mellonellalarvae were infected withM. mycetomatisgenome strain mm55. At 4h, 24h, 72h and 168h post-inoculation, RNA was extracted from larval content. Two types of sequencing libraries were prepared for time-course transcriptomic profiling and analysis.FindingsIn the infectedG. mellonella, 88.0% of the RNA sequence reads mapped toG. mellonella, while only 0.01% mapped toM. mycetomatis. Differential Gene Expression analysis revealed that 3,498G. mellonellaand 136M. mycetomatisgenes were differentially expressed during infection. Most of the enriched GO terms of both host and pathogen are linked to energy pathways, nucleobase metabolic process as well as cation and iron transport. Genes related to iron transport were highly expressed by bothG. mellonella (transferrin and ferritin) andM. mycetomatis(SidA, SidD and SidI). A protein-protein interaction network analysis ofD. melanogasterhomologous genes inM. mycetomatisrevealed the expression of the entire siderophore biosynthesis pathway throughout infection.InterpretationThe identification of the importance of iron acquisition during grain formation can be exploited as a potential novel diagnostic and therapeutic strategy for mycetoma.Research in contextEvidence before this studyMycetoma is a chronic, neglected tropical infectious disease, characterised by a large subcutaneous mass and the formation of black grains in the affected tissue. Treatment for mycetoma is disappointing as in 25-50% of the patients recurrences are noted and up to 15% of patients will have to undergo amputation. The main reason behind this poor treatment response is the formation of protective structures by the pathogen upon entering the human body. These structures are called grains and provide a strong barrier for antifungal agents. Although grains are the hallmark of mycetoma, it is currently not known how these grains are formed. To improve the current therapy, it is important to gain insights in grain formation.Added value of this studyWe unravel the processes leading to grain formation and development in an invertebrate model ofMadurella mycetomatisgrain. We were able to build a model of grain formation and demonstrated that iron sequestering plays an important role in this process. Our findings were an important milestone in understanding the pathogenesis of mycetoma which has been a mystery for decades.Implications of all the available evidenceThe findings, will provide leads for future drug development of mycetoma treatment and therefore, improve patients live and end the need for amputations.