Taking Centrioles to the Elimination Round

Coping with centriole loss: pericentriolar material maintenance after centriole degeneration

Shortly after the onset of ciliogenesis in Caenorhabditis elegans sensory neurons, the centrioles/basal bodies undergo degeneration. The fate of the pericentriolar material (PCM) that was associated with those centrioles has, however, remained unknown. Two recent studies by the Dammermann and the Feldman groups now show that not only does the PCM persist at the ciliary base, it also continues to assemble in the absence of canonical centrioles. Importantly, these neuronal centrosomes retain the ability to function as the cell’s main microtubule-organizing center and support ciliary function.

Notch signaling induces either apoptosis or cell fate change in multiciliated cells during mucociliary tissue remodeling

Multiciliated cells (MCCs) are extremely highly differentiated, presenting >100 cilia and basal bodies. Therefore, MCC fate is thought to be terminal and irreversible. We analyzed how MCCs are removed from the airway-like mucociliary Xenopus epidermis during developmental tissue remodeling. We found that a subset of MCCs undergoes lateral line-induced apoptosis, but that the majority coordinately trans-differentiate into goblet secretory cells. Both processes are dependent on Notch signaling, while the cellular response to Notch is modulated by Jak/STAT, thyroid hormone, and mTOR signaling. At the cellular level, trans-differentiation is executed through the loss of ciliary gene expression, including foxj1 and pcm1, altered proteostasis, cilia retraction, basal body elimination, as well as the initiation of mucus production and secretion. Our work describes two modes for MCC loss during vertebrate development, the signaling regulation of these processes, and demonstrates that even cells with extreme differentiation features can undergo direct fate conversion.

Animal Female Meiosis: The Challenges of Eliminating Centrosomes

Sexual reproduction requires the generation of gametes, which are highly specialised for fertilisation. Female reproductive cells, oocytes, grow up to large sizes when they accumulate energy stocks and store proteins as well as mRNAs to enable rapid cell divisions after fertilisation. At the same time, metazoan oocytes eliminate their centrosomes, i.e., major microtubule-organizing centres (MTOCs), during or right after the long growth phases. Centrosome elimination poses two key questions: first, how can the centrosome be re-established after fertilisation? In general, metazoan oocytes exploit sperm components, i.e., the basal body of the sperm flagellum, as a platform to reinitiate centrosome production. Second, how do most metazoan oocytes manage to build up meiotic spindles without centrosomes? Oocytes have evolved mechanisms to assemble bipolar spindles solely around their chromosomes without the guidance of pre-formed MTOCs. Female animal meiosis involves microtubule nucleation and organisation into bipolar microtubule arrays in regulated self-assembly under the control of the Ran system and nuclear transport receptors. This review summarises our current understanding of the molecular mechanism underlying self-assembly of meiotic spindles, its spatio-temporal regulation, and the key players governing this process in animal oocytes.

Loss and Rebirth of the Animal Microtubule Organizing Center: How Maternal Expression of Centrosomal Proteins Cooperates with the Sperm Centriole in Zygotic Centrosome Reformation

Centrosomes are the main microtubule organizing centers in animal cells. In particular during embryogenesis, they ensure faithful spindle formation and proper cell divisions. As metazoan centrosomes are eliminated during oogenesis, they have to be reassembled upon fertilization. Most metazoans use the sperm centrioles as templates for new centrosome biogenesis while the egg's cytoplasm re-prepares all components for on-going centrosome duplication in rapidly dividing embryonic cells. We discuss our knowledge and the experimental challenges to analyze zygotic centrosome reformation, which requires genetic experiments to enable scrutinizing respective male and female contributions. Male and female knockout animals and mRNA injection to mimic maternal expression of centrosomal proteins could point a way to the systematic molecular dissection of the process. The most recent data suggest that timely expression of centrosome components in oocytes is the key to zygotic centrosome reformation that uses male sperm as coordinators for de novo centrosome production.