Multifaceted modes of γ-tubulin complex recruitment and microtubule nucleation at mitotic centrosomes

Anti-Cancer Potential of a new Derivative of Caffeic Acid Phenethyl Ester targeting the Centrosome

Anaplastic Large Cell Lymphoma (ALCL) is an aggressive T-cell lymphoma affecting children and young adults. About 30% of patients develop therapy resistance therefore new precision medicine drugs are highly warranted. Multiple rounds of structure-activity optimization of Caffeic Acid Phenethyl Ester have resulted in CM14. CM14 causes upregulation of genes involved in oxidative stress response and downregulation of DNA replication genes leading to G2/M arrest and subsequent apoptosis induction. In accordance with this, an unbiased proteomics approach, confocal microscopy and molecular modeling showed that TUBGCP2, member of the centrosomal γ-TuRC complex, is a direct interaction partner of CM14. CM14 overcomes ALK inhibitor resistance in ALCL and is also active in T-cell Acute Lymphoblastic Leukemia and Acute Myeloid Leukemia. Interestingly, CM14 also induced cell death in docetaxel-resistant prostate cancer cells thus suggesting an unexpected role in solid cancers. Thus, we synthesized and thoroughly characterized a novel TUBGCP2 targeting drug that is active in ALCL but has also potential for other malignancies.

Centrosomal and acentrosomal microtubule nucleation during neuronal development

Neurons rely on the microtubule cytoskeleton to create and maintain their sophisticated cellular architectures. Advances in cryogenic electron microscopy, expansion microscopy, live imaging, and gene editing have enabled novel insights into mechanisms of centrosomal and acentrosomal microtubule nucleation, the key process generating new microtubules. This has paved the way for the functional dissection of distinct microtubule networks that regulate various processes during neuronal development, including neuronal delamination, polarization, migration, maturation, and synapse function. We review recent progress in understanding the molecular concepts of microtubule nucleation, how these concepts underlie neurodevelopmental processes, and pinpoint the open questions. Since microtubules play a pivotal role in axon regeneration within the adult central nervous system, understanding the processes of microtubule nucleation could inform strategies to enhance the regenerative capabilities of neurons in the future.

Proteasome inhibition induces microtubule-dependent changes in nuclear morphology

Cancers and neurodegenerative disorders are associated with both disrupted proteostasis and altered nuclear morphology. Determining if changes in nuclear morphology contribute to pathology requires an understanding of the underlying mechanisms, which are difficult to elucidate in cells where pleiotropic effects of altering proteostasis might indirectly influence nuclear morphology. To investigate direct effects, we studied nuclei assembled in Xenopus egg extract where potentially confounding effects of transcription, translation, cell cycle progression, and actin dynamics are absent. We report that proteasome inhibition causes acute microtubule-dependent changes in nuclear morphology and stability and altered microtubule dynamics and organization. Proteomic analysis of proteasome-inhibited extracts identified an increased abundance of microtubule nucleator TubGCP6, and TubGCP6 depletion partially rescued nuclear morphology. Key results were confirmed in HeLa cells. We propose that accumulation of TubGCP6 leads to altered microtubule dynamics proximal to the nucleus, producing forces that deform the nucleus and impact nuclear morphology and integrity.

Heat application in live cell imaging

Thermal heating of biological samples allows to reversibly manipulate cellular processes with high temporal and spatial resolution. Manifold heating techniques in combination with live-cell imaging were developed, commonly tailored to customized applications. They include Peltier elements and microfluidics for homogenous sample heating as well as infrared lasers and radiation absorption by nanostructures for spot heating. A prerequisite of all techniques is that the induced temperature changes are measured precisely which can be the main challenge considering subcellular structures or multicellular organisms as target regions. This article discusses heating and temperature sensing techniques for live-cell imaging, leading to future applications in cell biology.

Amplified centrosomes-more than just a threat

Centrosomes are major organizing components of the tubulin-based cytoskeleton. In recent years, we have gained extensive knowledge about their structure, biogenesis, and function from single cells, cell-cell interactions to tissue homeostasis, including their role in human diseases. Centrosome abnormalities are linked to, among others primary microcephaly, birth defects, ciliopathies, and tumorigenesis. Centrosome amplification, a state where two or more centrosomes are present in the G1 phase of the cell cycle, correlates in cancer with karyotype alterations, clinical aggressiveness, and lymph node metastasis. However, amplified centrosomes also appear in healthy tissues and, independent of their established role, in multi-ciliation. One example is the liver where hepatocytes carry amplified centrosomes owing to whole-genome duplication events during organogenesis. More recently, amplified centrosomes have been found in neuronal progenitors and several cell types of hematopoietic origin in which they enhance cellular effector functions. These findings suggest that extra centrosomes do not necessarily pose a risk for genome integrity and are harnessed for physiological processes. Here, we compare established and emerging 'non-canonical functions' of amplified centrosomes in cancerous and somatic cells and discuss their role in cellular physiology.

Mechanism of how the universal module XMAP215 γ-TuRC nucleates microtubules

It has become increasingly evident in recent years that nucleation of microtubules from a diverse set of MTOCs requires both the γ-tubulin ring complex (γ-TuRC) and the microtubule polymerase XMAP215. Despite their essentiality, little is known about how these nucleation factors interact and work together to generate microtubules. Using biochemical domain analysis of XMAP215 and structural approaches, we find that a sixth TOG domain in XMAP215 binds γ-TuRC via γ-tubulin as part of a broader interaction involving the C-terminal region. Moreover, TOG6 is required for XMAP215 to promote nucleation from γ-TuRC to its full extent. Interestingly, we find that XMAP215 also depends strongly on TOG5 for microtubule lattice binding and nucleation. Accordingly, we report a cryo-EM structure of TOG5 bound to the microtubule lattice that reveals promotion of lateral interactions between tubulin dimers. Finally, we find that while XMAP215 constructs' effects on nucleation are generally proportional to their effects on polymerization, formation of a direct complex with γ-TuRC allows cooperative nucleation activity. Thus, we propose that XMAP215's C-terminal TOGs 5 and 6 play key roles in promoting nucleation by promoting formation of longitudinal and lateral bonds in γ-TuRC templated nascent microtubules at cellular MTOCs.

γ-TuRCs and the augmin complex are required for the development of highly branched dendritic arbors in Drosophila

Microtubules are nucleated by γ-tubulin ring complexes (γ-TuRCs) and are essential for neuronal development. Nevertheless, γ-TuRC depletion has been reported to perturb only higher-order branching in elaborated Drosophila larval class IV dendritic arborization (da) neurons. This relatively mild phenotype has been attributed to defects in microtubule nucleation from Golgi outposts, yet most Golgi outposts lack associated γ-TuRCs. By analyzing dendritic arbor regrowth in pupae, we show that γ-TuRCs are also required for the growth and branching of primary and secondary dendrites, as well as for higher-order branching. Moreover, we identify the augmin complex (hereafter augmin), which recruits γ-TuRCs to the sides of pre-existing microtubules, as being required predominantly for higher-order branching. Augmin strongly promotes the anterograde growth of microtubules in terminal dendrites and thus terminal dendrite stability. Consistent with a specific role in higher-order branching, we find that augmin is expressed less strongly and is largely dispensable in larval class I da neurons, which exhibit few higher-order dendrites. Thus, γ-TuRCs are essential for various aspects of complex dendritic arbor development, and they appear to function in higher-order branching via the augmin pathway, which promotes the elaboration of dendritic arbors to help define neuronal morphology.

Multivalent coiled-coil interactions enable full-scale centrosome assembly and strength

The outermost layer of centrosomes, called pericentriolar material (PCM), organizes microtubules for mitotic spindle assembly. The molecular interactions that enable PCM to assemble and resist external forces are poorly understood. Here, we use crosslinking mass spectrometry (XL-MS) to analyze PLK-1-potentiated multimerization of SPD-5, the main PCM scaffold protein in C. elegans. In the unassembled state, SPD-5 exhibits numerous intramolecular crosslinks that are eliminated after phosphorylation by PLK-1. Thus, phosphorylation induces a structural opening of SPD-5 that primes it for assembly. Multimerization of SPD-5 is driven by interactions between multiple dispersed coiled-coil domains. Structural analyses of a phosphorylated region (PReM) in SPD-5 revealed a helical hairpin that dimerizes to form a tetrameric coiled-coil. Mutations within this structure and other interacting regions cause PCM assembly defects that are partly rescued by eliminating microtubule-mediated forces, revealing that PCM assembly and strength are interdependent. We propose that PCM size and strength emerge from specific, multivalent coiled-coil interactions between SPD-5 proteins.

Regulation of centrosome size by the cell-cycle oscillator in Drosophila embryos

Mitotic centrosomes assemble when centrioles recruit large amounts of pericentriolar material (PCM) around themselves. In early C. elegans embryos, mitotic centrosome size appears to be set by the limiting amount of a key component. In Drosophila syncytial embryos, thousands of mitotic centrosomes are assembled as the embryo proceeds through 13 rounds of rapid nuclear division, driven by a core cell cycle oscillator. These divisions slow during nuclear cycles 11-13, and we find that centrosomes respond by reciprocally decreasing their growth rate, but increasing their growth period-so that they grow to a relatively consistent size at each cycle. At the start of each cycle, moderate CCO activity initially promotes centrosome growth, in part by stimulating Polo/PLK1 recruitment to centrosomes. Later in each cycle, high CCO activity inhibits centrosome growth by suppressing the centrosomal recruitment and/or maintenance of centrosome proteins. Thus, in fly embryos, mitotic centrosome size appears to be regulated predominantly by the core cell cycle oscillator, rather than by the depletion of a limiting component.