50 ways to build a spindle: the complexity of microtubule generation during mitosis

Abstract

The accurate segregation of duplicated chromosomes, essential for the development and viability of a eukaryotic organism, requires the formation of a robust microtubule (MT)-based spindle apparatus. Entry into mitosis or meiosis precipitates a cascade of signalling events which result in the activation of pathways responsible for a dramatic reorganisation of the MT cytoskeleton: through changes in the properties of MT-associated proteins, local concentrations of free tubulin dimer and through enhanced MT nucleation. The latter is generally thought to be driven by localisation and activation of γ-tubulin-containing complexes (γ-TuSC and γ-TuRC) at specific subcellular locations. For example, upon entering mitosis, animal cells concentrate γ-tubulin at centrosomes to tenfold the normal level during interphase, resulting in an aster-driven search and capture of chromosomes and bipolar mitotic spindle formation. Thus, in these cells, centrosomes have traditionally been perceived as the primary microtubule organising centre during spindle formation. However, studies in meiotic cells, plants and cell-free extracts have revealed the existence of complementary mechanisms of spindle formation, mitotic chromatin, kinetochores and nucleation from existing MTs or the cytoplasm can all contribute to a bipolar spindle apparatus. Here, we outline the individual known mechanisms responsible for spindle formation and formulate ideas regarding the relationship between them in assembling a functional spindle apparatus.

Aurora kinase inhibitors reveal mechanisms of HURP in nucleation of centrosomal and kinetochore microtubules

“…Although centrosome-derived microtubules are radial initially, they begin growing with directional bias so that the density of microtubules between centrosomes and the mitotic chromosomes is greater than between centrosomes and the cell cortex (42). These observations suggest a lack of equality in the extension of microtubules to the cortex or to chromosomes.…”

“…These observations suggest a lack of equality in the extension of microtubules to the cortex or to chromosomes. It remains unclear what elements, including microtubule-associated proteins or motor proteins, cause this directional activity (42). HURP is one element that participates in this asymmetry through its association with microtubules that grow toward the mitotic chromosomes rather than the cortex (Fig.…”

Characterization of the Arabidopsis Augmin Complex Uncovers Its Critical Function in the Assembly of the Acentrosomal Spindle and Phragmoplast Microtubule Arrays

“…This raises a possibility that the augmin complex may contain more than one MAP. Structurally, Msd1/ Dgt9 and HAUS2/Cep27 can be paired with each other (Duncan and Wakefield, 2011). Because of its sequence similarity to HAUS2, it would be interesting to test whether AUG2 also interacts with MTs directly.…”

“…Multiple mechanisms are known to contribute to the assembly of the spindle apparatus in animal cells (Duncan and Wakefield, 2011). Whereas some are dependent on the g-tubulin complex, others rely on proteins like the Ran GTPase and MAP/ motors.…”

Using Photobleaching to Measure Spindle Microtubule Dynamics in Primary Cultures of Dividing Drosophila Meiotic Spermatocytes

“…However, recent reports from Drosophila indicate that spindle assembly pathways vary between the two dividing cell types. 34,[46][47][48] Therefore, it remains to be determined whether the plus and minus end effectors are shared or whether spermatocytes use alternative depolymerization mechanisms. Elucidating the molecules involved is an exciting future challenge that will require the application of the photobleaching assay to cells manipulated by genetic or pharmacological methods.…”