Clinical meaning
Candida albicans pathogenicity is driven by a coordinated arsenal of virulence factors at the molecular level. Secreted aspartyl proteases (SAPs 1-10) are a family of extracellular enzymes that degrade host mucosal proteins including E-cadherin, secretory IgA, complement components, and extracellular matrix proteins. SAPs 1-3 are predominantly expressed during mucosal infection and facilitate tissue invasion by disrupting epithelial integrity. SAP expression is regulated by environmental pH and nutrient availability, with acidic conditions favoring SAP2 production.
Agglutinin-like sequence (ALS) adhesins, particularly ALS3, mediate initial attachment to host epithelial cells and are essential for biofilm initiation. ALS3 functions as an invasin that triggers receptor-mediated endocytosis by binding to E-cadherin and N-cadherin on host cells, enabling Candida to actively penetrate epithelial barriers without requiring hyphal mechanical force alone.
The yeast-to-hypha morphogenetic switch is governed by multiple signaling pathways including the cAMP-PKA pathway, MAP kinase cascades, and transcription factors (Efg1, Cph1, Rim101). Environmental cues such as temperature (37°C), serum, CO₂ levels, neutral pH, and N-acetylglucosamine trigger this transition. The hyphal form expresses hypha-specific genes including HWP1 (hyphal wall protein 1), which serves as a substrate for host transglutaminases, covalently bonding the fungus to epithelial cells.
Azole resistance in Candida involves multiple mechanisms: (1) upregulation of ERG11 encoding the target enzyme lanosterol 14α-demethylase, (2) point mutations in ERG11 reducing drug binding affinity, (3) overexpression of efflux pumps (CDR1/CDR2 belonging to the ABC transporter family and MDR1 of the major facilitator superfamily), and (4) biofilm-mediated resistance through extracellular matrix sequestration of drug molecules. Echinocandin resistance develops through mutations in FKS1 and FKS2 genes encoding the catalytic subunit of β-1,3-glucan synthase, the drug target. These hot-spot mutations (particularly S645 in FKS1) reduce enzyme sensitivity to echinocandins by 50-3000 fold.