Role of the Non-Canonical RNAi Pathway in the Antifungal Resistance and Virulence of Mucorales

Abstract

Mucorales are the causal agents for the lethal disease known as mucormycosis. Mortality rates of mucormycosis can reach up to 90%, due to the mucoralean antifungal drug resistance and the lack of effective therapies. A concerning urgency among the medical and scientific community claims to find targets for the development of new treatments. Here, we reviewed different studies describing the role and machinery of a novel non-canonical RNAi pathway (NCRIP) only conserved in Mucorales. Its non-canonical features are the independence of Dicer and Argonaute proteins. Conversely, NCRIP relies on RNA-dependent RNA Polymerases (RdRP) and an atypical ribonuclease III (RNase III). NCRIP regulates the expression of mRNAs by degrading them in a specific manner. Its mechanism binds dsRNA but only cuts ssRNA. NCRIP exhibits a diversity of functional roles. It represses the epimutational pathway and the lack of NCRIP increases the generation of drug resistant strains. NCRIP also regulates the control of retrotransposons expression, playing an essential role in genome stability. Finally, NCRIP regulates the response during phagocytosis, affecting the multifactorial process of virulence. These critical NCRIP roles in virulence and antifungal drug resistance, along with its exclusive presence in Mucorales, mark this pathway as a promising target to fight against mucormycosis.

Comprehensive Review on the Virulence Factors and Therapeutic Strategies with the Aid of Artificial Intelligence against Mucormycosis

“…68 Mutations in rdrp1 and rdrp3 genes also influence epimutant formation. 158 Biofilm Formation (Fungal Cell Wall and Cell Membrane). Studies on M. circinelloides highlight the role of cell wall components and surface proteins in pathogenicity since avirulent strain had a lower tolerance to heat and cell wall stress.…”

“…QIP functions as an exonuclease essential for transgene-induced RNAi pathway in M. lusitanicus , akin to its role in other fungi. RnhA, resembling Sad-3 helicase, contributes to RNAi-induced heterochromatin formation. , …”

Mucorales and Mucormycosis: Recent Insights and Future Prospects

“…The n on- c anonical R NA i p athway (NCRIP) is another particularity described only in Mucorales ; therefore, it is another interesting target for developing new antifungal drugs. The NCRIP is RdRP-dependent and Dicer-independent, relying on the RNase III activity in the protein R3B2 [ 70 , 72 ]. The functional role of NCRIP has been related to RNA degradation, being postulated as an evolutionary link between the ancient bacterial RNA degradation mechanisms and the modern eukaryotic RNAi canonical mechanisms [ 69 , 70 ].…”

Genetic Manipulation in Mucorales and New Developments to Study Mucormycosis

“…Before discovering RNAi in M. lusitanicus , the implementation of genomic libraries and an accidentally silenced dark-yellow transformant led to the identification of a gene involved in the process of carotenogenesis, the negative regulator crgA [ 49 ]. This discovery led to the further complete dissection of the silencing mechanism in M. lusitanicus [ 26 , 53 , 54 , 55 , 56 ]. The development of a hairpin-RNA expression plasmid permitted the determination of the contribution of each component of the canonical RNAi pathway of M. lusitanicus.…”

“…Later, the study of this endogenous regulatory mechanism led to the discovery of two novel silencing phenomena, the generation of epimutants [ 22 ] and the non-canonical RNAi pathway (NCRIP) [ 35 ]. Both RNAi pathways immediately attracted the attention of researchers due to their role in the pathogenic potential of M. lusitanicus [ 29 , 55 , 71 , 72 ]. After all, the ability of the fungus to generate transient resistance against antifungal drugs [ 22 , 73 , 74 ], and the identification of NCRIP, a dicer -independent silencing mechanism that negatively regulates the generation of epimutants [ 35 , 68 , 75 ], supposed important implications for antifungal therapies.…”