Mechanisms of cortical microtubule organization in epidermal keratinocytes.

Microtubules in many differentiated cell types are reorganized from a radial, centrosome-bound array into a cell type-specific, non-centrosomal network. In epidermal keratinocytes, a subset of microtubules is organized from the cell cortex. These microtubules are anchored to desmosomes, with ninein serving as a linker protein. Details of this organization are poorly understood. We used immunofluorescence expansion microscopy to visualize directly the contact between cortical microtubules and desmosomes in murine skin tissue. Microtubule bound laterally to desmosomes, or with their ends at mixed polarity. Experiments including time-lapse microscopy of EB3-GFP, microtubule regrowth after depolymerization, and expression of ectopic ninein that was sequestered to the plasma membrane by a CAAX sequence motif, indicated that nucleation of microtubules doesn't occur at the cortex. Experimental severing of microtubules by spastin led to accumulation of microtubules next to ectopic, cortical ninein. Overall, our data suggest that microtubules accumulate by translocation from non-cortical sites towards sites of cortical ninein.