Myomegalin is necessary for the formation of centrosomal and Golgi-derived microtubules.

The generation of cellular microtubules is initiated at specific sites such as the centrosome and the Golgi apparatus that contain nucleation complexes rich in γ-tubulin. The microtubule growing plus-ends are stabilized by plus-end tracking proteins (+TIPs), mainly EB1 and associated proteins. Myomegalin was identified as a centrosome/Golgi protein associated with cyclic nucleotide phosphodiesterase. We show here that Myomegalin exists as several isoforms. We characterize two of them. One isoform, CM-MMG, harbors a conserved domain (CM1), recently described as a nucleation activator, and is related to a family of γ-tubulin binding proteins, which includes Drosophila centrosomin. It localizes at the centrosome and at the cis-Golgi in an AKAP450-dependent manner. It recruits γ-tubulin nucleating complexes and promotes microtubule nucleation. The second isoform, EB-MMG, is devoid of CM1 domain and has a unique N-terminus with potential EB1-binding sites. It localizes at the cis-Golgi and can localize to microtubule plus-ends. EB-MMG binds EB1 and affects its loading on microtubules and microtubule growth. Depletion of Myomegalin by small interfering RNA delays microtubule growth from the centrosome and Golgi apparatus, and decreases directional migration of RPE1 cells. In conclusion, the Myomegalin gene encodes different isoforms that regulate microtubules. At least two of these have different roles, demonstrating a previously unknown mechanism to control microtubules in vertebrate cells.

Casein kinase I delta controls centrosome positioning during T cell activation.

Although termed central body, the centrosome is located off-center in many polarized cells. T cell receptor (TCR) engagement by antigens induces a polarity switch in T cells. This leads to the recruitment of the centrosome to the immunological synapse (IS), a specialized cell-cell junction. Despite much recent progress, how TCR signaling triggers centrosome repositioning remains poorly understood. In this paper, we uncover a critical requirement for the centrosomal casein kinase I delta (CKIδ) in centrosome translocation to the IS. CKIδ binds and phosphorylates the microtubule plus-end-binding protein EB1. Moreover, a putative EB1-binding motif at the C terminus of CKIδ is required for centrosome translocation to the IS. We find that depletion of CKIδ in T lymphocytes and inhibition of CKI in epithelial cells reduce microtubule growth. Therefore, we propose that CKIδ-EB1 complexes contribute to the increase in microtubule growth speeds observed in polarized T cells, a mechanism that might serve to generate long-stable microtubules necessary for centrosome translocation.

Centrosome function in cancer: guilty or innocent?

The regulation of centrosome number and function underlies bipolar mitotic spindle formation and genetic integrity. Cancer cells both in culture and in situ exhibit a wide range of centrosome abnormalities. Here, we briefly review advances in our understanding of the pathways that govern normal centrosome function and outline the potential causes and consequences of their deregulation in disease. There is ample observational but little experimental evidence to support the conventional model that centrosome dysfunction causes genomic instability and, as a result, cancer. This model has been challenged by recent studies that have uncovered evidence of a direct link between centrosome function in asymmetric cell division and tumourigenesis. Thus, it is timely to discuss the provocative idea that, in certain tissues, abnormal centrosomes drive malignant transformation not by generating genomic instability but by deregulating asymmetric cell division.

Knock-in and knock-out: the use of reverse genetics in somatic cells to dissect mitotic pathways.

Reverse genetic methods, such as homologous gene targeting, have greatly contributed to our understanding of molecular pathways in mitosis, especially in yeast. The chicken B-lymphocyte line, DT40, represents a unique example among vertebrate somatic cells where homologous gene targeting occurs at very high frequency. DT40 cells therefore provide a useful and accessible somatic genetic system for wide-ranging biochemical and cell biological assays. In this chapter, we describe the main principles of homologous gene targeting, the concept of targeting construct design and the detailed experimental protocol of how to achieve successful knockouts. We also mention methods for conditional disruption of essential genes and conclude with specific procedures for the study of mitosis in DT40 cells.

The Syk tyrosine kinase localizes to the centrosomes and negatively affects mitotic progression.

We showed previously that the spleen tyrosine kinase Syk is expressed by mammary epithelial cells and that it suppresses malignant growth of breast cancer cells. The exact molecular mechanism of its tumor-suppressive activity remains, however, to be identified. Here, we show that Syk colocalizes and copurifies with the centrosomal component gamma-tubulin and exhibits a catalytic activity within the centrosomes. Moreover, its centrosomal localization depends on its intact kinase activity. Centrosomal Syk expression is persistent in interphase but promptly drops during mitosis, obviously resulting from its ubiquitinylation and proteasomal degradation. Conversely, unrestrained exogenous expression of a fluorescently tagged Discosoma sp. red fluorescent protein (DsRed)-Syk chimera engenders abnormal cell division and cell death. Transient DsRed-Syk overexpression triggers an abrupt cell death lacking hallmarks of classic apoptosis but reminiscent of mitotic catastrophe. Surviving stable DsRed-Syk-transfected cells exhibit multipolar mitotic spindles and contain multiple abnormally sized nuclei and supernumerary centrosomes, revealing anomalous cell division. Taken together, these results show that Syk is a novel centrosomal kinase that negatively affects cell division. Its expression is strictly controlled in a spatiotemporal manner, and centrosomal Syk levels need to decline to allow customary progression of mitosis.