Microtubules help seal layers of epidermal cells by strengthening intercellular junctions, Sumigray et al. reveal.
Microtubules perform a crucial job in mitotic cells—helping to separate chromosomes—but researchers haven’t discerned all of their roles in differentiated cells. One mystery, for instance, is why microtubules accumulate at the cortex in epidermal cells. Sumigray et al. devised a system to test the functions of these microtubules in vitro. The researchers dosed layers of epidermal keratinocytes with taxol, which stabilized microtubules at the cell cortex, mimicking their organization in vivo.
Sumigray et al. found that cortical microtubules reinforce sheets of keratinocytes but not by directly providing structural support. Instead, the fibers fortified adherens junctions that link adjacent cells, changing the composition and dynamics of these cell–cell junctions. More of the adherens junction component β-catenin gathered at the cortex, for instance. The microtubules also drew myosin II to the cortex, enabling the motor protein to apply tension and ensure that the junctions couple to the underlying actin cytoskeleton.
Adherens junctions fasten neighboring cells together, but tight junctions, another type of intercellular connection, provide a barrier that is impermeable to small molecules. The researchers found that, by modifying adherens junctions, cortical microtubules enable the tight junctions between epidermal cells to create a tighter seal. They tested this ability by injecting newborn mice with biotin, a compound that normally can’t diffuse through the epidermis because of tight junctions. But biotin penetrated deeper into the skin in mice treated with nocodazole to break down their microtubules. An unanswered question, the researchers say, is how the microtubules attract myosin II to the cortex.