Centrosome loss or amplification does not dramatically perturb global gene expression in Drosophila

Centrosome Loss Triggers a Transcriptional Program To Counter Apoptosis-Induced Oxidative Stress

Centrosomes play a critical role in mitotic spindle assembly through their role in microtubule nucleation and bipolar spindle assembly. Loss of centrosomes can impair the ability of some cells to properly conduct mitotic division, leading to chromosomal instability, cell stress, and aneuploidy. Multiple aspects of the cellular response to mitotic error associated with centrosome loss appear to involve activation of JNK signaling. To further characterize the transcriptional effects of centrosome loss, we compared gene expression profiles of wild-type and acentrosomal cells from Drosophila wing imaginal discs. We found elevation of expression of JNK target genes, which we verified at the protein level. Consistent with this, the upregulated gene set showed significant enrichment for the AP-1 consensus DNA-binding sequence. We also found significant elevation in expression of genes regulating redox balance. Based on those findings, we examined oxidative stress after centrosome loss, revealing that acentrosomal wing cells have significant increases in reactive oxygen species (ROS). We then performed a candidate genetic screen and found that one of the genes upregulated in acentrosomal cells, glucose-6-phosphate dehydrogenase, plays an important role in buffering acentrosomal cells against increased ROS and helps protect those cells from cell death. Our data and other recent studies have revealed a complex network of signaling pathways, transcriptional programs, and cellular processes that epithelial cells use to respond to stressors, like mitotic errors, to help limit cell damage and maintain normal tissue development.

Drosophila PLP assembles pericentriolar clouds that promote centriole stability, cohesion and MT nucleation

Pericentrin is a conserved centrosomal protein whose dysfunction has been linked to several human diseases. It has been implicated in many aspects of centrosome and cilia function, but its precise role is unclear. Here, we examine Drosophila Pericentrin-like-protein (PLP) function in vivo in tissues that form both centrosomes and cilia. Plp mutant centrioles exhibit four major defects: (1) They are short and have subtle structural abnormalities; (2) They disengage prematurely, and so overduplicate; (3) They organise fewer cytoplasmic MTs during interphase; (4) When forming cilia, they fail to establish and/or maintain a proper connection to the plasma membrane—although, surprisingly, they can still form an axoneme-like structure that can recruit transition zone (TZ) proteins. We show that PLP helps assemble “pericentriolar clouds” of electron-dense material that emanate from the central cartwheel spokes and spread outward to surround the mother centriole. We propose that the partial loss of these structures may largely explain the complex centriole, centrosome and cilium defects we observe in Plp mutant cells.

Centrioles are complex, microtubule (MT) based structures that organise two important cell organelles, the centrosome and the cilium. The centrosome is a major MT organising centre in many cell types, while the cilium functions as a cellular “antenna” responsible for regulating several cellular signalling pathways. Pericentrin is conserved centriole-binding protein that plays an important part in centrosome and cilium function, and mutations in the Pericentrin gene are linked to several human diseases. Here we use the fruit-fly Drosophila melanogaster to investigate how Pericentrin-Like-Protein (the fly homolog of Pericentrin) contributes to centriole, centrosome and cilium function. We find that Plp mutant fly centrioles have subtle structural defects, organize less microtubules, and do not properly migrate to the cell membrane to form cilia. We also observe that PLP helps assemble “pericentriolar clouds”—dense structures that emanate from the centriole, and appear to interact with microtubules, as well as connect existing centrioles to newly formed ones. In mutant flies these structures are significantly reduced in size. We propose that the defects in these PLP structures can explain most, if not all, the complex defects observed in Plp mutants.

Mitotic spindle assembly in animal cells: a fine balancing act

The mitotic spindle has a crucial role in ensuring the accurate segregation of chromosomes into the two daughter cells during cell division, which is paramount for maintaining genome integrity. It is a self-organized and dynamic macromolecular structure that is constructed from microtubules, microtubule-associated proteins and motor proteins. Thirty years of research have led to the identification of centrosome-, chromatin- and microtubule-mediated microtubule nucleation pathways that each contribute to mitotic spindle assembly. Far from being redundant pathways, data are now emerging regarding how they function together to ensure the timely completion of mitosis. We are also beginning to comprehend the multiple mechanisms by which cells regulate spindle scaling. Together, this research has increased our understanding of how cells coordinate hundreds of proteins to assemble the dynamic, precise and robust structure that is the mitotic spindle.

Centrosome function and assembly in animal cells

It has become clear that the role of centrosomes extends well beyond that of important microtubule organizers. There is increasing evidence that they also function as coordination centres in eukaryotic cells, at which specific cytoplasmic proteins interact at high concentrations and important cell decisions are made. Accordingly, hundreds of proteins are concentrated at centrosomes, including cell cycle regulators, checkpoint proteins and signalling molecules. Nevertheless, several observations have raised the question of whether centrosomes are essential for many cell processes. Recent findings have shed light on the functions of centrosomes in animal cells and on the molecular mechanisms of centrosome assembly, in particular during mitosis. These advances should ultimately allow the in vitro reconstitution of functional centrosomes from their component proteins to unlock the secrets of these enigmatic organelles.

Dissecting the function and assembly of acentriolar microtubule organizing centers in Drosophila cells in vivo

Acentriolar microtubule organizing centers (aMTOCs) are formed during meiosis and mitosis in several cell types, but their function and assembly mechanism is unclear. Importantly, aMTOCs can be overactive in cancer cells, enhancing multipolar spindle formation, merotelic kinetochore attachment and aneuploidy. Here we show that aMTOCs can form in acentriolar Drosophila somatic cells in vivo via an assembly pathway that depends on Asl, Cnn and, to a lesser extent, Spd-2--the same proteins that appear to drive mitotic centrosome assembly in flies. This finding enabled us to ablate aMTOC formation in acentriolar cells, and so perform a detailed genetic analysis of the contribution of aMTOCs to acentriolar mitotic spindle formation. Here we show that although aMTOCs can nucleate microtubules, they do not detectably increase the efficiency of acentriolar spindle assembly in somatic fly cells. We find that they are required, however, for robust microtubule array assembly in cells without centrioles that also lack microtubule nucleation from around the chromatin. Importantly, aMTOCs are also essential for dynein-dependent acentriolar spindle pole focusing and for robust cell proliferation in the absence of centrioles and HSET/Ncd (a kinesin essential for acentriolar spindle pole focusing in many systems). We propose an updated model for acentriolar spindle pole coalescence by the molecular motors Ncd/HSET and dynein in conjunction with aMTOCs.

Functional genomic analysis of the periodic transcriptome in the developing Drosophila wing

The eukaryotic cell cycle, driven by both transcriptional and posttranslational mechanisms, is the central molecular oscillator underlying tissue growth throughout animals. Although genome-wide studies have investigated cell-cycle-associated transcription in unicellular systems, global patterns of periodic transcription in multicellular tissues remain largely unexplored. Here we define the cell-cycle-associated transcriptome of the developing Drosophila wing epithelium and compare it with that of cultured Drosophila S2 cells, revealing a core set of periodic genes and a surprising degree of context specificity in periodic transcription. We further employ RNAi-mediated phenotypic profiling to define functional requirements for more than 300 periodic genes, with a focus on those required for cell proliferation in vivo. Finally, we investigate uncharacterized genes required for interkinetic nuclear migration. Combined, these findings provide a global perspective on cell-cycle control in vivo, and they highlight a critical need to understand the context-specific regulation of cell proliferation.

CP110 exhibits novel regulatory activities during centriole assembly in Drosophila

Although loss of CP110 is tolerated in Drosophila, CP110 is important for limiting centriole length, limiting centriolar microtubule length, and suppressing centriole overduplication when duplication proteins are overexpressed.

CP110 is a conserved centriole protein implicated in the regulation of cell division, centriole duplication, and centriole length and in the suppression of ciliogenesis. Surprisingly, we report that mutant flies lacking CP110 (CP110Δ) were viable and fertile and had no obvious defects in cell division, centriole duplication, or cilia formation. We show that CP110 has at least three functions in flies. First, it subtly influences centriole length by counteracting the centriole-elongating activity of several centriole duplication proteins. Specifically, we report that centrioles are ∼10% longer than normal in CP110Δ mutants and ∼20% shorter when CP110 is overexpressed. Second, CP110 ensures that the centriolar microtubules do not extend beyond the distal end of the centriole, as some centriolar microtubules can be more than 50 times longer than the centriole in the absence of CP110. Finally, and unexpectedly, CP110 suppresses centriole overduplication induced by the overexpression of centriole duplication proteins. These studies identify novel and surprising functions for CP110 in vivo in flies.

Genes involved in centrosome-independent mitotic spindle assembly in Drosophila S2 cells

Animal mitotic spindle assembly relies on centrosome-dependent and centrosome-independent mechanisms, but their relative contributions remain unknown. Here, we investigated the molecular basis of the centrosome-independent spindle assembly pathway by performing a whole-genome RNAi screen in Drosophila S2 cells lacking functional centrosomes. This screen identified 197 genes involved in acentrosomal spindle assembly, eight of which had no previously described mitotic phenotypes and produced defective and/or short spindles. All 197 genes also produced RNAi phenotypes when centrosomes were present, indicating that none were entirely selective for the acentrosomal pathway. However, a subset of genes produced a selective defect in pole focusing when centrosomes were absent, suggesting that centrosomes compensate for this shape defect. Another subset of genes was specifically associated with the formation of multipolar spindles only when centrosomes were present. We further show that the chromosomal passenger complex orchestrates multiple centrosome-independent processes required for mitotic spindle assembly/maintenance. On the other hand, despite the formation of a chromosome-enriched RanGTP gradient, S2 cells depleted of RCC1, the guanine-nucleotide exchange factor for Ran on chromosomes, established functional bipolar spindles. Finally, we show that cells without functional centrosomes have a delay in chromosome congression and anaphase onset, which can be explained by the lack of polar ejection forces. Overall, these findings establish the constitutive nature of a centrosome-independent spindle assembly program and how this program is adapted to the presence/absence of centrosomes in animal somatic cells.