Xenopus egg extract model – with fluorescent tubulin

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Xenopus egg extract model – with fluorescent tubulin

Xenopus egg extract model – with fluorescent tubulin

Why are we interested in studying microtubule-rich cellular structures?

The centrosome is a membraneless organelle that operates as the microtubule (MT) organizing center, which is critical for the spatial organization of the cell^{(reviewed in 1-3)}. Hundreds of proteins are found within the pericentriolar material that are important for many cellular processes³, including regulating the organization and nucleation of MTs.

MTs from centrosomes play an essential role during mitosis through the formation of key structures, including the mitotic spindle, which is important for proper chromosome segregation. Asters are also key MT structures that radiate outward from the centrosome toward the cell membrane and participate in positioning the spindle apparatus.

Dysfunctional centrosomes lead to impaired cellular functions that contribute to several human diseases^{2, 3}.Thus, a proper understanding of the essential components of the centrosome and how they impact MT structure and function is vital.

Several approaches are routinely used to study the centrosome, including fluorescence microscopy, proteomics, and in vitro systems like cell extracts. For example, key studies utilizing Xenopus laevis egg extracts lacking centrosomes revealed that other factors can also control spindle formation and bipolarity^{4, 5}. Since then, pathways such as the mitotic chromatin, augmin complex, acentriolar MTOC pathway have been shown to drive spindle formation through various mechanisms^{(reviewed in 3)}.

Below, we take a closer look at the Xenopus egg extract model to study MT structures and recent enhancements to this approach.

Figure Legend: Schematic depicting the steps involved in isolating the cytoplasmic fraction from Xenopus egg extracts. Fluorescently-labeled tubulin is then added to the mixture to label microtubules. This cell-free mixture is further processed for specific applications and used in a range of biological studies.

What is the Xenopus egg extract model, and how is it used to study MT structures?

The Xenopus egg extract model is a cell-free system that has been used for several decades to decipher how MTs behave, as it effectively recapitulates the structure, composition, and function of the cytoplasmic machinery. Established protocols are available for the production of high-quality in vitro spindle and aster structures from these extracts⁶.

Advantages of this model include – high yield, depletion of target proteins via immunodepletion, simple labeling of MTs through the addition of fluorescently-labeled tubulin (Cat. # TL670M or TL590M), and compatibility with advanced imaging techniques. Recently, the Xenopus egg extract model has been used in several studies on MTs in aster and spindle formation^7-9.

Understanding the nanomolar details of spindle morphology is essential, but traditional microscopy methods have limited resolution. Recently, Guilloux et al. developed an optimized expansion microscopy approach that is compatible with Xenopus egg extracts labeled with rhodamine tubulin (Cat. #TL590M)⁸. This approach delivered unprecedented clarity on microtubule arrangements and provides a detailed protocol for high-resolution imaging of spindle structures⁸.

The Xenopus egg extract model is also beneficial when trying to characterize regulatory proteins controlling MT formation and function. For example, a recent study by the Cheng Lab utilized this model system to determine that the motor MKLP2 and Aurora kinase B is critical for centrosome-independent aster formation⁷.

In a third example, the Xenopus egg extract model was used to characterize the effects of TPX2 depletion in spindle assembly⁹. The group was studying the effects of targeting the intrinsically disordered protein, TPX2, using a proteolysis antibody.

These studies highlight the utility of the Xenopus egg extract model to examine spindle structure with advanced microscopy approaches, as well as identify and characterize MT-regulatory proteins.

References

1. Hannaford, M.R. and N.M. Rusan, Positioning centrioles and centrosomes. J Cell Biol, 2024. 223(4).

2. Chavali, P.L., M. Putz, and F. Gergely, Small organelle, big responsibility: the role of centrosomes in development and disease. Philos Trans R Soc Lond B Biol Sci, 2014. 369(1650).

3. Conduit, P.T., A. Wainman, and J.W. Raff, Centrosome function and assembly in animal cells. Nat Rev Mol Cell Biol, 2015. 16(10): p. 611-24.

4. Heald, R., et al., Self-organization of microtubules into bipolar spindles around artificial chromosomes in Xenopus egg extracts. Nature, 1996. 382(6590): p. 420-5.

5. Hyman, A.A. and E. Karsenti, Morphogenetic properties of microtubules and mitotic spindle assembly. Cell, 1996. 84(3): p. 401-10.

6. Field, C.M. and T.J. Mitchison, Assembly of Spindles and Asters in Xenopus Egg Extracts. Cold Spring Harb Protoc, 2018. 2018(6): p. pdb prot099796.

7. Jin, L., M. Liu, and X. Cheng, An acentrosomal aster with atypical microtubule polarity recruits cytokinesis signals to its center in Xenopus egg extracts. J Cell Sci, 2025. 138(18).

8. Guilloux, G., et al., Optimized expansion microscopy reveals species-specific spindle microtubule organization in Xenopus egg extracts. Mol Biol Cell, 2025. 36(6): p. ar73.

9. Sun, X., et al., Degradation of intrinsically disordered proteins using proteolysis targeting nanobody conjugate. bioRxiv, 2024: p. 2024.11.27.622182.