Pathways That Synthesize Phosphatidylethanolamine Impact Candida albicans Hyphal Length and Cell Wall Composition through Transcriptional and Posttranscriptional Mechanisms

Candida albicans is a leading cause of systemic bloodstream infections, and synthesis of the phospholipid phosphatidylethanolamine (PE) is required for virulence. The psd1 Delta/Delta psd2 Delta/Delta mutant, which cannot synthesize PE by the cytidine diphosphate diacylglycerol (CDP-DAG) pathway, is avirulent in the mouse model of systemic candidiasis. Similarly, an ept1 Delta/Delta, mutant, which cannot produce PE by the Kennedy pathway, exhibits decreased kidney fungal burden in systemically infected mice. Conversely, overexpression of EPT1 results in a hypervirulent phenotype in this model. Thus, mutations that increase PE synthesis increase virulence, and mutations that decrease PE synthesis decrease virulence. However, the mechanism by which virulence is regulated by PE synthesis is only partially understood. RNA sequencing was performed on strains with deficient or excessive PE biosynthesis to elucidate the mechanism. Decreased PE synthesis from loss of EPT1 or PSD1 and PSD2 leads to downregulation of genes that impact mitochondrial function. Losses of PSD1 and PSD2, but not EPT1, cause significant increases in transcription of glycosylation genes, which may reflect the substantial cell wall defects in the psd1 Delta/Delta psd2 Delta/Delta mutant. These accumulated defects could contribute to the decreased virulence observed for mutants with deficient PE synthesis. In contrast to mutants with decreased PE synthesis, there were no transcriptional differences between the EPT1 overexpression strain and the wild type, indicating that the hypervirulent phenotype is a consequence of posttranscriptional changes. It was found that overexpression of EPT1 causes increased chitin content and increased hyphal length. These phenotypes may help to explain the previously observed hypervirulence in the EPT1 overexpressor.