Microtubule stiffening by doublecortin-domain protein ZYG-8 contributes to spindle orientation during C. elegans zygote division

In Caenorhabditis elegans, mutations in zyg-8DCLK1, the sole Doublecortin-family member, led to incorrect mitotic spindle positioning, as seen by immunofluorescence. Doublecortin proteins bind to microtubules and are thought to stabilise or rigidify microtubules. Upon targeting zyg-8, we observed altered spindle-pole oscillations and perturbations in microtubule cortical-contact behaviour. They were not accounted for solely by the previously reported changes in microtubule dynamics. Instead, the phenotypes were consistent with ZYG-8 increasing microtubule rigidity. ZYG-8 depletion or mutation led to more frequent microtubule bending, and higher curvature and tortuosity. Microtubule softening in zyg-8 mutants likely reduced the efficiency of centring forces, causing exaggerated spindle oscillations and subsequent mispositioning during late anaphase. We propose that sufficient microtubule rigidity is essential for generating effective cortical pushing forces, which act as centring mechanisms to ensure accurate spindle orientation in late mitosis. These findings may have implications for cancer therapy, since the expression of DCLK1 and Tau, both known regulators of microtubule rigidity, is often deregulated in human cancers.