Physical and molecular determinants for the allometric control of spindle size – CytoScale

During metazoan development, embryonic cells decrease in size by up to two orders of magnitude (from 1.2 mm to 12 µm in Xenopus laevis) as a consequence of multiple rounds of cell division without growth of the early embryo. Using Xenopus, Caenorhabditis elegans or mouse embryos as model organisms, it has been shown that mitotic structures, including mitotic spindle length, centrosome size, and nuclear size, all scale with cell size. The scaling relationship at the base of this adaptation to cell volume changes is usually not perfectly proportional (i.e. not isometric). Instead, a subproportional relationship, termed allometric adaptation is often observed.

By measuring spindle size, microtubule growth rate and blastomere volumes in C. elegans embryos at various stages using a combination of live cell imaging, 2-photon microscopy and 3-D reconstructions, we found that spindle size and microtubule growth rate scale logarithmically with cell volume across divisions. The relationship between these three parameters is consistent with an allometric adaptation. However, the molecular mechanism that controls it and the functional consequences of its deregulation are not fully understood. While a spindle cannot obviously be larger than the blastomere that contains it, we suspect that the allometric adaptation of spindle size to cell volume could play a more specific function in regulating spindle positioning essential for proper cell division and/or chromosome segregation (the primary function of mitotic spindles) accuracy of which is crucial for genetic integrity of dividing cells. Another unanswered fundamental question is whether a similar spindle scaling process exists outside of early embryonic tissues. To address this question, we identified a somatic tissue in C. elegans that undergoes several rounds of cell division in a fixed volume. Daughter cells thus become smaller after each round of division and their spindle length and microtubule growth rate decrease accordingly. These results demonstrate that allometric adaptation of spindle size to cell volume is not specific to the early embryonic context. Altogether, our preliminary results suggest that, as proposed by previous in vitro and theoretical models, microtubule growth rate and microtubule dynamics modulation in general are major regulators of spindle size that could account for the allometric adaptation of spindle length to cell volume variations in different embryonic and somatic contexts.

By combining genetic manipulations of C. elegans, live cell imaging, in vitro biochemical assays and in silico modeling, our project aims at understanding the regulation and function of the allometric control of spindle size during cell division in embryonic and somatic tissues.