Spindle Scaling Is Governed by Cell Boundary Regulation of Microtubule Nucleation

Abstract

Highlights d Microtubule nucleation scales spindles in zebrafish embryos and Xenopus egg extracts d Microtubule nucleation is regulated by the surface of the cell d Microtubule polymerization only contributes to scaling below a critical cell size d Microtubule number and length are regulated using distinct geometric cues

Volumetric morphometry reveals mitotic spindle width as the best predictor of spindle scaling

“…More recent studies, however, show that the nature of spindle scaling and size control is more complex. In different cells, spindle length spans over an order of magnitude and various molecular scaling mechanisms are likely to contribute to different degrees to cover the entire length regime (Rieckhoff et al, 2020). For example, the length of mitotic spindles increases with cell length in small cells, but in very large cells spindle length approaches an upper limit (Wuhr et al, 2008;Lacroix et al, 2018;Rieckhoff et al, 2020).…”

“…In different cells, spindle length spans over an order of magnitude and various molecular scaling mechanisms are likely to contribute to different degrees to cover the entire length regime (Rieckhoff et al, 2020). For example, the length of mitotic spindles increases with cell length in small cells, but in very large cells spindle length approaches an upper limit (Wuhr et al, 2008;Lacroix et al, 2018;Rieckhoff et al, 2020). More precisely, spindle length scales linearly with cytoplasmic volume (Hazel et al, 2013;Good et al, 2013).…”

“…While most experimental studies still measure spindle length and cell diameter or collapse them into area information, theoretical arguments regularly use volumetric data (Good et al, 2013;Reber et al, 2013;Rieckhoff et al, 2020). However, most tools available for analysing spindle size and geometry only allow for 2D analysis (Crowder et al, 2015;Grenfell et al, 2016).…”

“…In many systems, cell or cytoplasmic volume is a major determinant of spindle size (Good et al, 2013;Hazel et al, 2013;Farhadifar et al, 2015;Wang et al, 2016;Lacroix et al, 2018;Rieckhoff et al, 2020). So, to reliably measure cell volume, we took advantage of the fact that mitotic cells expressing fluorescently-tagged tubulin show, next to the prominent spindle signal, distinctive fluorescence of soluble tubulin throughout the cytoplasm (Figure 5A).…”

“…We live-imaged and analysed HeLa Kyoto, Ptk2 and mESCs expressing fluorescently-tagged tubulin. With an average cell volume of ≈ 6100 µm 3 (HeLa Kyoto, Figure 5C), 7450 µm 3 (Ptk2) and 2850 µm 3 (mESCs), all cell types are expected to fall into the linear scaling regime (cell volume, Vc < 10 6 µm 3 (Rieckhoff et al, 2020), cell diameter, dc < 140 µm, (Crowder et al, 2015)).…”