Als3 Is a Candida albicans Invasin That Binds to Cadherins and Induces Endocytosis by Host Cells

Quynh T Phan,Yue Fu,Ashraf S Ibrahim,Carter L Myers,William H Welch,Michael R Yeaman,Donald C Sheppard,Scott G Filler,John E Edwards,Joseph Heitman

doi:10.1371/journal.pbio.0050064

Abstract

Candida albicans is the most common cause of hematogenously disseminated and oropharyngeal candidiasis. Both of these diseases are characterized by fungal invasion of host cells. Previously, we have found that C. albicans hyphae invade endothelial cells and oral epithelial cells in vitro by inducing their own endocytosis. Therefore, we set out to identify the fungal surface protein and host cell receptors that mediate this process. We found that the C. albicans Als3 is required for the organism to be endocytosed by human umbilical vein endothelial cells and two different human oral epithelial lines. Affinity purification experiments with wild-type and an als3Δ/als3Δ mutant strain of C. albicans demonstrated that Als3 was required for C. albicans to bind to multiple host cell surface proteins, including N-cadherin on endothelial cells and E-cadherin on oral epithelial cells. Furthermore, latex beads coated with the recombinant N-terminal portion of Als3 were endocytosed by Chinese hamster ovary cells expressing human N-cadherin or E-cadherin, whereas control beads coated with bovine serum albumin were not. Molecular modeling of the interactions of the N-terminal region of Als3 with the ectodomains of N-cadherin and E-cadherin indicated that the binding parameters of Als3 to either cadherin are similar to those of cadherin–cadherin binding. Therefore, Als3 is a fungal invasin that mimics host cell cadherins and induces endocytosis by binding to N-cadherin on endothelial cells and E-cadherin on oral epithelial cells. These results uncover the first known fungal invasin and provide evidence that C. albicans Als3 is a molecular mimic of human cadherins.

Highlights

The fungus Candida albicans causes both hematogenously disseminated and oropharyngeal disease
We discovered that Als3, a protein expressed on the surface of C. albicans, is required for this invasion process
Cadherins on the surface of human cells normally bind other cadherins for adhesion and signaling; we found that Als3 binds to cadherins on endothelial cells and oral epithelial cells, and this binding induces these host cells to take up the fungus

Summary

Introduction

The fungus Candida albicans causes both hematogenously disseminated and oropharyngeal disease. The invasion of either endothelial or oral epithelial cells by C. albicans is necessary for the organism to damage these cell types in vitro [4,5]. Host cell invasion and damage are likely critical virulence attributes of C. albicans because mutants with defects in these processes in vitro are highly likely to have attenuated virulence in experimental animal models of hematogenously disseminated or oropharyngeal candidiasis [5,6,7,8]. We have found that C. albicans invades both endothelial cells and oral epithelial cells in vitro by inducing its own endocytosis [4,5,7]. Endothelial cell endocytosis of C. albicans is induced when the organism binds to N-cadherin and other endothelial cell surface proteins. Endothelial cell endocytosis of C. albicans is dependent on extracellular calcium and is governed at least in part by the tyrosine phosphorylation of endothelial cell proteins [9,11]

Results

Discussion

Conclusion