Cell Biology


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0 Q&A 788 Views Oct 20, 2023

For the analysis of cellular architecture during mitosis, nanometer resolution is needed to visualize the organization of microtubules in spindles. Here, we present a detailed protocol that can be used to produce 3D reconstructions of whole mitotic spindles in cells grown in culture. For this, we attach mammalian cells enriched in mitotic stages to sapphire discs. Our protocol further involves cryo-immobilization by high-pressure freezing, freeze-substitution, and resin embedding. We then use fluorescence light microscopy to stage select mitotic cells in the resin-embedded samples. This is followed by large-scale electron tomography to reconstruct the selected and staged mitotic spindles in 3D. The generated and stitched electron tomograms are then used to semi-automatically segment the microtubules for subsequent quantitative analysis of spindle organization. Thus, by providing a detailed correlative light and electron microscopy (CLEM) approach, we give cell biologists a toolset to streamline the 3D visualization and analysis of spindle microtubules (http://kiewisz.shinyapps.io/asga). In addition, we refer to a recently launched platform that allows for an interactive display of the 3D-reconstructed mitotic spindles (https://cfci.shinyapps.io/ASGA_3DViewer/).

Key features

• High-throughput screening of mitotic cells by correlative light and electron microscopy (CLEM).

• Serial-section electron tomography of selected cells.

• Visualization of mitotic spindles in 3D and quantitative analysis of microtubule organization.

Graphical overview

0 Q&A 430 Views Jul 20, 2023

Microtubule structure is commonly investigated using single-particle analysis (SPA) or subtomogram averaging (STA), whose main objectives are to gather high-resolution information on the αβ-tubulin heterodimer and on its interactions with neighboring molecules within the microtubule lattice. The maps derived from SPA approaches usually delineate a continuous organization of the αβ-tubulin heterodimer that alternate regularly head-to-tail along protofilaments, and that share homotypic lateral interactions between monomers (α-α, β-β), except at one unique region called the seam, made of heterotypic ones (α-β, β-α). However, this textbook description of the microtubule lattice has been challenged over the years by several studies that revealed the presence of multi-seams in microtubules assembled in vitro from purified tubulin. To analyze in deeper detail their intrinsic structural heterogeneity, we have developed a segmented subtomogram averaging (SSTA) strategy on microtubules decorated with kinesin motor-domains that bind every αβ-tubulin heterodimer. Individual protofilaments and microtubule centers are modeled, and sub-volumes are extracted at every kinesin motor domain position to obtain full subtomogram averages of the microtubules. The model is divided into shorter segments, and subtomogram averages of each segment are calculated using the main parameters of the full-length microtubule settings as a template. This approach reveals changes in the number and location of seams within individual microtubules assembled in vitro from purified tubulin and in Xenopus egg cytoplasmic extracts.

Key features

• This protocol builds upon the method developed by J.M. Heumann to perform subtomogram averages of microtubules and extends it to divide them into shorter segments.

• Microtubules are decorated with kinesin motor-domains to determine the underlying organization of its constituent αβ-tubulin heterodimers.

• The SSTA approach allows analysis of the structural heterogeneity of individual microtubules and reveals multi-seams and changes in their number and location within their shaft.

Graphical overview

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