In vivo mitotic spindle scaling can be modulated by changing the levels of a single protein: the microtubule polymerase XMAP215

Abstract

In many organisms, early embryonic development is characterized by a series of reductive cell divisions that result in rapid increases in cell number and concomitant decreases in cell size. Intracellular organelles, such as the nucleus and mitotic spindle, also become progressively smaller during this developmental window, but the molecular and mechanistic underpinnings of these scaling relationships are not fully understood. For the mitotic spindle, changes in cytoplasmic volume are sufficient to account for size scaling during early development in certain organisms. This observation is consistent with models that evoke a limiting component, whereby the smaller absolute number of spindle components in smaller cells limits spindle size. Here we investigate the role of a candidate factor for developmental spindle scaling, the microtubule polymerase XMAP215. Microinjection of additional XMAP215 protein into Xenopus laevis embryos was sufficient to induce the assembly of larger spindles during developmental stages 6.5, 7, and 8, whereas addition of a polymerase-incompetent XMAP215 mutant resulted in a downward shift in the in vivo spindle scaling curve. In sum, these results indicate that even small cells are able to produce larger spindles if microtubule growth rates are increased and suggest that structural components are not limiting.

Cytoplasmic volume and limiting nucleoplasmin scale nuclear size duringXenopus laevisdevelopment

“…Limiting component models have been described for mitotic spindle length scaling (Good et al, 2013;Hazel et al, 2013;Milunovic-Jevtic et al, 2018;Reber et al, 2013) and centrosome size regulation (Decker et al, 2011), however less studied is whether nuclear size might be regulated by limiting components (Goehring and Hyman, 2012;Marshall, 2015;Reber and Goehring, 2015). Microinjection of nuclei into Xenopus oocytes resulted in nuclear growth with clustered nuclei growing less (Gurdon, 1976), similar to what has been observed in multinucleate fission yeast cells (Neumann and Nurse, 2007).…”

XMAP215 promotes microtubule–F-actin interactions to regulate growth cone microtubules during axon guidance in Xenopus laevis

“…Given that MT advancement into the growth cone periphery depends upon MT extension along F-actin bundles, we wondered whether XMAP215 might be specifically involved in the regulation of MT-F-actin interactions within the growth cone. XMAP215 family members have received significant attention as critical regulators of MT polymerization and nucleation (Ayaz et al, 2012;Brouhard et al, 2008;Flor-Parra et al, 2018;Milunovic-Jevtićet al, 2018;Thawani et al, 2018;Widlund et al, 2011;Zanic et al, 2013), but there are no previous studies that examine whether XMAP215 can bind directly to F-actin or mediate MT-F-actin interactions in any system.…”

Spindle scaling is governed by cell boundary regulation of microtubule nucleation

“…However, variability in spindle size across species for a given cell size could still be explained by differences in the overall microtubule dynamics (7,16,17,22). We propose that these differences in microtubule dynamics‚ and possibly nucleation, reflect interspecies differences in relative protein amounts within a cell (7,43,44).…”

“…Current models propose that spindles scale by changing the length of microtubules via a limiting cellular component that regulates microtubule polymerization dynamics (10,11,17,21,22). Consistent with this proposal, microtubule growth velocity correlates with spindle size in early C. elegans and sea urchin embryos (17), and increased activity of the microtubule polymerase XMAP215 is sufficient to change spindle length in vitro and in vivo (16,22). These studies on spindle size have been limited to measurements of microtubule dynamics in relatively small spindles (spindle length L < 20 µm) or in biochemically perturbed large spindles in vitro (L ~ 50 µm).…”

“…One essential structure that needs to scale to maintain the robust segregation of chromosomes during early embryogenesis is the mitotic spindle. Spindles can scale an order of magnitude and fall into what appears to be a universal scaling relationship with cell size (4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20).…”