The doublecortin-family kinase ZYG-8DCLK1 regulates microtubule dynamics and motor-driven forces to promote the stability of C. elegans acentrosomal spindles.

Although centrosomes help organize spindles in most cell types, oocytes of most species lack these structures. During acentrosomal spindle assembly in C. elegans oocytes, microtubule minus ends are sorted outwards away from the chromosomes where they form poles, but then these outward forces must be balanced to form a stable bipolar structure. Simultaneously, microtubule dynamics must be precisely controlled to maintain spindle length and organization. How forces and dynamics are tuned to create a stable bipolar structure is poorly understood. Here, we have gained insight into this question through studies of ZYG-8, a conserved doublecortin-family kinase; the mammalian homolog of this microtubule-associated protein is upregulated in many cancers and has been implicated in cell division, but the mechanisms by which it functions are poorly understood. We found that ZYG-8 depletion from oocytes resulted in overelongated spindles with pole and midspindle defects. Importantly, experiments with monopolar spindles revealed that ZYG-8 depletion led to excess outward forces within the spindle and suggested a potential role for this protein in regulating the force-generating motor BMK-1/kinesin-5. Further, we found that ZYG-8 is also required for proper microtubule dynamics within the oocyte spindle and that kinase activity is required for its function during both meiosis and mitosis. Altogether, our findings reveal new roles for ZYG-8 in oocytes and provide insights into how acentrosomal spindles are stabilized to promote faithful meiosis. Author summary: When a cell divides, a football-shaped spindle made up of dynamic microtubule polymers functions to partition chromosomes. Usually, structures called centrosomes organize microtubules at the two ends of the spindle. However, in female reproductive cells ("oocytes") of many organisms, the spindle is formed without the help of centrosomes. In this study, we used the model organism C. elegans to understand how this type of "acentrosomal" spindle forms and maintains its shape. Specifically, we investigated a protein called ZYG-8 and we found that when this protein was removed from oocytes, the spindles became overelongated and their typical football shape was disrupted. Follow-up studies revealed that ZYG-8 plays two important functions that promote acentrosomal spindle assembly and stability in oocytes: 1) ZYG-8 helps control the dynamic properties of the microtubule polymers so that they can be properly arranged into the spindle structure and 2) ZYG-8 tunes forces within the spindle, to help the structure maintain it shape. Taken together, our findings have increased our understanding of how spindles form in the absence of centrosomes, shedding light on how female reproductive cells divide. [ABSTRACT FROM AUTHOR]