Dynamics and remodeling of the enterocyte brush border during bacterial infection: Implications for intestinal host defense

Abstract

A dense array of parallel actin-based protrusions called microvilli extend from the apical surface of intestinal epithelial cells (IECs), collectively called the brush border. In addition to serving as the sole site of nutrient absorption, this domain also contains host defense proteins. As the brush border is located at the interface between intestinal contents and tissue it must prevent translocation of pathogens and toxins. However, little is known about its role in host defense. Using a combination of cell biological and microscopy techniques, we asked how the brush border responds to microbial infection. Microvilli release vesicles laden with the host defense enzyme intestinal alkaline phosphatase (IAP) into the intestinal lumen. Here, we find that IAP on these lumenal vesicles (LVs) is biochemically active, able to detoxify a bacterial toxin and limit inflammation. Independent of IAP activity, LVs inhibit attachment of enteropathogenic E. coli (EPEC) to IECs. LVs also limit growth of bacteria, while the presence of microbes stimulates increased LV shedding. Thus, LVs represent a multi-faceted form of host intestinal defense. When EPEC do reach the host cell surface, the brush border is dramatically remodeled, resulting in microvillar effacement and formation of actin-rich pedestals beneath the bacteria. Though many molecular aspects of attachment have been characterized, contributions of the brush border to the attachment process have not been investigated. We find that while brush border integrity is critical for limiting bacterial attachment, EPEC can utilize this domain to recruit actin bundles to sites of attachment. Using live cell microscopy, we show that EPEC stimulates flow of the brush border across the cell surface to sites of attachment, as well as directed elongation of microvilli towards bacterial cells. Microvillar actin also appears in nascent pedestals, and pedestal formation is inhibited in cells overexpressing an actin bundling protein. This work suggests a novel mechanism wherein EPEC-stimulated pedestal formation does not occur exclusively by de novo formation of a branched actin network, but also progresses through repurposing of existing microvillar parallel actin bundles.