The fungal pathogen Candida is a common organism that resides harmlessly in the human body, but under certain conditions, it can cause infections ranging from superficial candidiasis to life-threatening systemic disease. Candida albicans is the species most frequently associated with these infections, especially in individuals with compromised immune systems. Understanding how this fungus transitions from a harmless resident to an aggressive invader is central to developing effective treatments. The flow of genetic information (DNA to RNA to protein) reveals that RNA is more than just a simple genetic intermediary. In Candida species, RNA acts as a sophisticated regulator, controlling the precise timing and amount of proteins produced, which dictates the fungus’s ability to survive, adapt, and cause disease within the host environment.
The Classes of RNA in Fungal Pathogenesis
Fungal cells, like human cells, utilize different types of RNA molecules, which can be broadly categorized based on their function. Messenger RNA (mRNA) carries the genetic blueprint from DNA to the ribosomes, where it is translated into a protein. The stability and localization of Candida mRNA are tightly controlled by various RNA-binding proteins, determining when and where a protein is ultimately made. This post-transcriptional control allows the fungus to rapidly change its behavior in response to environmental cues, such as temperature or nutrient availability inside the host.
Beyond coding for proteins, non-coding RNAs (ncRNAs) play regulatory roles in the fungal cell. These molecules, including small (sncRNAs) and long (lncRNAs) non-coding RNAs, do not translate into proteins but directly influence gene expression. LncRNAs are particularly intriguing because they exhibit high sequence divergence even between closely related Candida species, making them potential targets for highly specific therapies. Researchers have identified hundreds of lncRNA fragments that are specifically expressed during different stages of the Candida infection process, suggesting they act as switches in pathogenesis.
RNA Regulation of Virulence and Biofilm Formation
A virulence mechanism for Candida albicans is morphological switching, the ability to change shape, which is regulated by RNA activity. When colonizing a host, Candida typically exists as a budding yeast, but to invade tissue, it must transition into a filamentous form called hyphae. This transition is necessary for tissue penetration and evading host immune cells.
Specific RNA-binding proteins (RBPs) govern this morphological change by controlling the fate of mRNAs that encode for hyphal-associated genes. RBPs can localize certain mRNAs to the growing tip of the hypha, ensuring proteins needed for polarized growth are synthesized where they are needed for tissue invasion. Furthermore, RNA decay pathways, involving enzymes that break down mRNA, are also implicated; mutants lacking certain mRNA decay components display defects in filamentous growth.
RNA also coordinates the formation of biofilms, which are complex communities of fungal cells encased in a protective matrix that adhere to host surfaces or medical devices. Biofilm formation is essential for persistent infection and resistance to host defenses and antifungal drugs. The genes producing adhesins, proteins that enable the fungus to stick to surfaces, are subject to post-transcriptional control mediated by RNA. By regulating these genes, Candida can coordinate the transition from individual cells to a robust, multicellular structure that shields the fungus from external threats.
RNA and Antifungal Drug Resistance
Antifungal drug resistance in Candida species presents a challenge in clinical settings, and RNA regulation is involved in this resistance mechanism. A common strategy is to overexpress efflux pumps, membrane proteins that actively pump the drug out of the cell before it reaches its target. The two main types of efflux pumps are ATP-binding cassette (ABC) transporters (e.g., Cdr1) and Major Facilitator Superfamily (MFS) transporters (e.g., Mdr1).
Elevated expression of CDR1 and MDR1 genes often results from gain-of-function mutations in specific transcription factors, reflected in the abundance of their mRNA targets. For example, the transcription factor Tac1 governs the expression of CDR1 and CDR2, while Mrr1 controls MDR1 expression. Mutations that render these transcription factors constitutively active lead to high levels of CDR1 and MDR1 mRNA, resulting in continuous drug efflux and resistance.
Resistance to azole drugs, which target the Erg11 enzyme involved in ergosterol synthesis, is also linked to RNA control. Upon exposure to azoles, Candida species can increase the transcription rate of the ERG11 gene, leading to higher levels of ERG11 mRNA and subsequently more target enzyme. This mechanism, often mediated by transcription factors like Upc2, allows the fungus to compensate for the drug’s inhibitory effect by producing more target protein.
Therapeutic Strategies Based on RNA Targeting
Understanding RNA’s role in Candida pathogenesis is opening new avenues for developing treatments that circumvent traditional drug resistance. One promising approach is RNA interference (RNAi), a natural cellular process that can be harnessed to silence specific fungal genes. By introducing double-stranded RNA molecules matching a target mRNA, researchers can trigger the degradation of that mRNA, preventing the production of the corresponding protein. This strategy can be used to target genes responsible for virulence factors or drug resistance, such as CDR1 or ERG11.
A challenge for RNA-based therapies is delivering the RNA molecule into the fungal cell, which is protected by a cell wall composed of chitin, \(\beta\)-glucans, and mannoproteins. Researchers are exploring methods to overcome this barrier, including the use of lipid nanoparticles (LNPs) or cationic polymers, like chitosan, to encapsulate the RNA and facilitate its uptake. These delivery systems protect the RNA from degradation and enhance its ability to penetrate the fungal cell.
Fungal RNA signatures are also being investigated as rapid diagnostic biomarkers. Traditional blood cultures for candidemia are slow, delaying the start of appropriate antifungal treatment and increasing patient mortality. Highly sensitive molecular techniques, such as reverse transcription-polymerase chain reaction (RT-PCR) and novel rRNA hybridization assays, are being developed to detect fungal RNA directly in clinical samples. Targeting ribosomal RNA (rRNA) provides a strong signal for identification, allowing for rapid and accurate detection of Candida species directly from blood cultures, improving the timely diagnosis and management of invasive candidiasis.