Calcineurin Is Required for Pseudohyphal Growth, Virulence, and Drug Resistance in <em>Candida lusitaniae</em>

<div><p><em>Candida lusitaniae</em> is an emerging fungal pathogen that infects immunocompromised patients including HIV/AIDS, cancer, and neonatal pediatric patients. Though less prevalent than other <em>Candida</em> species, <em>C. lusitaniae</em> is unique in its ability to develop resistance to amphotericin B. We investigated the role of the calcium-activated protein phosphatase calcineurin in several virulence attributes of <em>C. lusitaniae</em> including pseudohyphal growth, serum survival, and growth at 37°C. We found that calcineurin and Crz1, a <em>C. albicans</em> Crz1 homolog acting as a downstream target of calcineurin, are required for <em>C. lusitaniae</em> pseudohyphal growth, a process for which the underlying mechanism remains largely unknown in <em>C. lusitaniae</em> but hyphal growth is fundamental to <em>C. albicans</em> virulence. We demonstrate that calcineurin is required for cell wall integrity, ER stress response, optimal growth in serum, virulence in a murine systemic infection model, and antifungal drug tolerance in <em>C. lusitaniae</em>. To further examine the potential of targeting the calcineurin signaling cascade for antifungal drug development, we examined the activity of a calcineurin inhibitor FK506 in combination with caspofungin against echinocandin resistant <em>C. lusitaniae</em> clinical isolates. Broth microdilution and drug disk diffusion assays demonstrate that FK506 has synergistic fungicidal activity with caspofungin against echinocandin resistant isolates. Our findings reveal that pseudohyphal growth is controlled by the calcineurin signaling cascade, and highlight the potential use of calcineurin inhibitors and caspofungin for emerging drug-resistant <em>C. lusitaniae</em> infections.</p>