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Unveiling the life cycle of a microvillus

By Colbie Chinowsky

The surface of the intestinal tract is the sole site of nutrient absorption—a life-sustaining process—and disturbances to this tissue have the potential for deadly consequences. The small intestine has evolved a variety of structures that maximize the surface area available for nutrient uptake, including microvilli, fingerlike projections that protrude from the cell. The collection of microvilli on the surface of intestinal cells forms a larger structure known as the brush border, which resembles the bristles on a toothbrush.

In a recent Current Biology paper, graduate student Isabella Gaeta, from the lab of Professor of Cell and Developmental Biology Matt Tyska, reveals how microvilli are born and begin to grow. The assembly of new microvilli is critical for the function of the intestine, but until now little was known about how cells build these structures.

Fluorescence microscopy image of an epithelial cell. The cell is a dark spot in the center with a wide and blurry yellow halo around it. You can see short, stick-like projections mostly clustered in groups of 2 or 3 (these are the microvilli). The microvilli are colored in different hues (depending on the depth of field), going from red through the rest of the colors of the rainbow.
A polarized kidney epithelial cell is shown in this image. The microvilli are color coded by depth, and the actin binding protein EPS8 is shown in white. Image by Isabella Gaeta.

The intestine is different from most other tissues because of its high rate of cell turnover—old cells constantly die off and are replaced by new, robust cells. Each new cell, however, must also build brand new microvilli. This process is driven by the end-to-end assembly of a cellular protein known as actin, and the resulting actin “filaments” are in turn bundled together to form a strong support for the cell membrane. Using live-cell microscopy, Gaeta showed the various proteins that come to the cell surface to “birth” a microvillus and revealed that EPS8, a protein known to help form actin structures, is required for microvilli to continue to grow.

Gaeta also showed that a “mother microvillus” is capable of birthing a “daughter” by creating a new branch off an existing projection that in turn splits into an independent structure. This exciting new finding illuminates a novel pathway for microvilli formation, and additional studies along these lines will lead to a better understanding of the birth and growth of microvilli. This work will also allow researchers to understand how the complex structures of the intestine may be disrupted in diseases characterized by loss of microvilli, such as enteric (intestinal) infections with pathogenic Escherichia coli.