Gradual meiosis-to-mitosis transition in the early mouse embryo

Maternal inheritance of functional centrioles in two parthenogenetic nematodes

Centrioles are the core constituent of centrosomes, microtubule-organizing centers involved in directing mitotic spindle assembly and chromosome segregation in animal cells. In sexually reproducing species, centrioles degenerate during oogenesis and female meiosis is usually acentrosomal. Centrioles are retained during male meiosis and, in most species, are reintroduced with the sperm during fertilization, restoring centriole numbers in embryos. In contrast, the presence, origin, and function of centrioles in parthenogenetic species is unknown. We found that centrioles are maternally inherited in two species of asexual parthenogenetic nematodes and identified two different strategies for maternal inheritance evolved in the two species. In Rhabditophanes diutinus, centrioles organize the poles of the meiotic spindle and are inherited by both the polar body and embryo. In Disploscapter pachys, the two pairs of centrioles remain close together and are inherited by the embryo only. Our results suggest that maternally-inherited centrioles organize the embryonic spindle poles and act as a symmetry-breaking cue to induce embryo polarization. Thus, in these parthenogenetic nematodes, centrioles are maternally-inherited and functionally replace their sperm-inherited counterparts in sexually reproducing species.

Developmental and tissue-specific roles of mammalian centrosomes

Centrosomes are dominant microtubule organizing centers in animal cells with a pair of centrioles at their core. They template cilia during interphase and help organize the mitotic spindle for a more efficient cell division. Here, we review the roles of centrosomes in the early developing mouse and during organ formation. Mammalian cells respond to centrosome loss-of-function by activating the mitotic surveillance pathway, a timing mechanism that, when a defined mitotic duration is exceeded, leads to p53-dependent cell death in the descendants. Mouse embryos without centrioles are highly susceptible to this pathway and undergo embryonic arrest at mid-gestation. The complete loss of the centriolar core results in earlier and more severe phenotypes than that of other centrosomal proteins. Finally, different developing tissues possess varying thresholds and mount graded responses to the loss of centrioles that go beyond the germ layer of origin.

Gradual centriole maturation associates with the mitotic surveillance pathway in mouse development

Centrosomes, composed of two centrioles and pericentriolar material, organize mitotic spindles during cell division and template cilia during interphase. The first few divisions during mouse development occur without centrioles, which form around embryonic day (E) 3. However, disruption of centriole biogenesis in Sas-4 null mice leads to embryonic arrest around E9. Centriole loss in Sas-4^-/- embryos causes prolonged mitosis and p53-dependent cell death. Studies in vitro discovered a similar USP28-, 53BP1-, and p53-dependent mitotic surveillance pathway that leads to cell cycle arrest. In this study, we show that an analogous pathway is conserved in vivo where 53BP1 and USP28 are upstream of p53 in Sas-4^-/- embryos. The data indicate that the pathway is established around E7 of development, four days after the centrioles appear. Our data suggest that the newly formed centrioles gradually mature to participate in mitosis and cilia formation around the beginning of gastrulation, coinciding with the activation of mitotic surveillance pathway upon centriole loss.

Building the right centriole for each cell type

Loncarek and Bettencourt-Dias review molecular mechanisms of centriole biogenesis amongst different organisms and cell types.

The centriole is a multifunctional structure that organizes centrosomes and cilia and is important for cell signaling, cell cycle progression, polarity, and motility. Defects in centriole number and structure are associated with human diseases including cancer and ciliopathies. Discovery of the centriole dates back to the 19th century. However, recent advances in genetic and biochemical tools, development of high-resolution microscopy, and identification of centriole components have accelerated our understanding of its assembly, function, evolution, and its role in human disease. The centriole is an evolutionarily conserved structure built from highly conserved proteins and is present in all branches of the eukaryotic tree of life. However, centriole number, size, and organization varies among different organisms and even cell types within a single organism, reflecting its cell type–specialized functions. In this review, we provide an overview of our current understanding of centriole biogenesis and how variations around the same theme generate alternatives for centriole formation and function.

Loss of function of KIF1B impairs oocyte meiotic maturation and early embryonic development in mice

Kinesin family member 1B (KIF1B) is an important microtubule-dependent monomeric motor in mammals, although little is known about its role in meiosis. We profiled KIF1B expression and localization during oocyte maturation and early embryonic development in mice, revealing a dynamic pattern throughout meiotic progression. Depletion or inhibition of KIF1B leads to abnormal polar body extrusion, disordered spindle dynamics, defects in chromosome congression, increased aneuploidy, and impaired embryonic development. Further, KIF1B depletion affects the distribution of mitochondria and abundance of ATP. Taken together, our study demonstrates that mouse KIF1B is important for spindle assembly, chromosome congression, and mitochondrial distribution during oocyte maturation and early embryonic development. Mol. Reprod. Dev. 83: 1027-1040, 2016 © 2016 Wiley Periodicals, Inc.

RBM14 prevents assembly of centriolar protein complexes and maintains mitotic spindle integrity

Formation of a new centriole adjacent to a pre-existing centriole occurs only once per cell cycle. Despite being crucial for genome integrity, the mechanisms controlling centriole biogenesis remain elusive. Here, we identify RBM14 as a novel suppressor of assembly of centriolar protein complexes. Depletion of RBM14 in human cells induces ectopic formation of centriolar protein complexes through function of the STIL/CPAP complex. Intriguingly, the formation of such structures seems not to require the cartwheel structure that normally acts as a scaffold for centriole formation, whereas they can retain pericentriolar material and microtubule nucleation activity. Moreover, we find that, upon RBM14 depletion, a part of the ectopic centriolar protein complexes in turn assemble into structures more akin to centrioles, presumably by incorporating HsSAS-6, a cartwheel component, and cause multipolar spindle formation. We further demonstrate that such structures assemble in the cytoplasm even in the presence of pre-existing centrioles. This study sheds light on the possibility that ectopic formation of aberrant structures related to centrioles may contribute to genome instability and tumorigenesis.