Fluidic Force Microscopy Demonstrates That Homophilic Adhesion by Candida albicans Als Proteins Is Mediated by Amyloid Bonds between Cells

Abstract

The fungal pathogen Candida albicans frequently forms drug-resistant biofilms in hospital settings and in chronic disease patients. Cell adhesion and biofilm formation involve a family of cell surface Als (agglutinin-like sequence) proteins. It is now well documented that amyloid-like clusters of laterally arranged Als proteins activate cell-cell adhesion under mechanical stress, but whether amyloid-like bonds form between aggregating cells is not known. To address this issue, we measure the forces driving Als5-mediated intercellular adhesion using an innovative fluidic force microscopy platform. Strong cell-cell adhesion is dependent on expression of amyloidforming Als5 at high cell surface density and is inhibited by a short antiamyloid peptide. Furthermore, there is greatly attenuated binding between cells expressing amyloid-forming Als5 and cells with a nonamyloid form of Als5. Thus, homophilic bonding between Als5 proteins on adhering cells is the major mode of fungal aggregation, rather than protein-ligand interactions. These results point to a model whereby amyloid-like β-sheet interactions play a dual role in cellcell adhesion, that is, in formation of adhesin nanoclusters (cis-interactions) and in homophilic bonding between amyloid sequences on opposing cells (trans-interactions). Because potential amyloid-forming sequences are found in many microbial adhesins, we speculate that this novel mechanism of amyloid-based homophilic adhesion might be widespread and could represent an interesting target for treating biofilm-associated infections.

An Amyloid Core Sequence in the Major Candida albicans Adhesin Als1p Mediates Cell-Cell Adhesion

“…S1 in the supplemental material). These findings are consistent with the idea that amyloid-like β-aggregation both in cis on the surface of a cell and in trans between cells is the basis for strong cell-cell bonding in both adhesins (42).…”

“…A novel statistical tool quantifies the clustering, and it shows frequencies far higher than expected from random associations. This result was similar to observations with Als5p, in which amyloid-like β-aggregation is necessary for clustering through interactions of the identical sequence on many molecules of the protein (28, 35, 38, 42).…”

“…2) (30). For cells expressing Als1p as well as cells expressing Als5p, cell-to-cell binding is dependent on V326: the V326N variant shows highly reduced binding probability and bond strength (Table 1) (42). In fact, in Als1p-expressing cells, the probability of adhesion dropped 14-fold from 97% between cells expressing Als1p WT to 7% for cells expressing Als1p V326N , and the cell-cell bond strength was reduced threefold, despite similarity in the cell surface concentration of the two forms (Fig.…”

Binding Strength of Gram-Positive Bacterial Adhesins

“…Interestingly, SMFS studies detected the massive surface exposure of Candida albicans Als adhesins after treatment with the antifungal caspofungin ( El-Kirat-Chatel et al, 2013 ). Homophilic interactions between Als adhesins play an important role in cellular aggregation ( Dehullu et al, 2019 ), highlighting the importance of studying the effects of antimicrobials on adhesion.…”

What makes bacterial pathogens so sticky?

“…In the baker's yeast Saccharomyces cerevisiae , flocculin (Flo)‐mediated co‐adhesion involved well‐known weak lectin‐binding forces, but also unanticipated strong unfolding forces reflecting the force‐induced extension of the hydrophobic tandem repeats of Flo proteins (El‐Kirat‐Chatel et al, ). In the pathogen Candida albicans , a novel form of homophilic cell–cell adhesion was identified, that is, β‐sheet bonding of ~500 pN between amyloid sequences in Als (agglutinin‐like sequence) proteins on opposing cells (Dehullu et al, ). Strong adhesion required Als expression on both interacting cells at high surface density, and were strongly inhibited by a short anti‐amyloid peptide.…”