Iron regulatory pathways differentially expressed during Madurella mycetomatis grain development in Galleria mellonella

Background Mycetoma is a neglected, chronic granulomatous infection of the subcutaneous tissue, most often caused by the fungal pathogen Madurella 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 to M. mycetomatis grain formation, we used a Galleria mellonella larvae infection model and time-course transcriptomic profiling. Methods Galleria mellonella larvae were infected with M. mycetomatis genome 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. Findings In the infected G. mellonella, 88.0% of the RNA sequence reads mapped to G. mellonella, while only 0.01% mapped to M. mycetomatis. Differential Gene Expression analysis revealed that 3,498 G. mellonella and 136 M. mycetomatis genes 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 both G. mellonella (transferrin and ferritin) and M. mycetomatis (SidA, SidD and SidI). A protein-protein interaction network analysis of D. melanogaster homologous genes in M. mycetomatis revealed the expression of the entire siderophore biosynthesis pathway throughout infection. Interpretation The 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 context Evidence before this study Mycetoma 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 study We unravel the processes leading to grain formation and development in an invertebrate model of Madurella mycetomatis grain. 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 evidence The findings, will provide leads for future drug development of mycetoma treatment and therefore, improve patients live and end the need for amputations.