Site-specific phosphorylation of ZYG-1 regulates ZYG-1 stability and centrosome number

Spindle bipolarity is critical for genomic integrity. As centrosome number often dictates bipolarity, tight control of centrosome assembly is vital for faithful cell division. The master centrosome regulator ZYG-1/Plk4 plays a pivotal role in this process. In C. elegans, casein kinase II (CK2) negatively regulates centrosome duplication by controlling centrosome-associated ZYG-1 levels. Here, we investigated CK2 as a regulator of ZYG-1 and its impact on centrosome assembly. We show that CK2 phosphorylates ZYG-1 in vitro and physically interacts with ZYG-1 in vivo. Depleting CK2 or blocking ZYG-1 phosphorylation at CK2 target sites leads to centrosome amplification. Non-phosphorylatable ZYG-1 mutants exhibit elevated ZYG-1 levels, leading to increased ZYG-1 and downstream factors at centrosomes, thus driving centrosome amplification. Moreover, inhibiting the 26S proteasome prevents degradation of the phospho-mimetic ZYG-1. Our findings suggest that CK2-dependent phosphorylation of ZYG-1 controls ZYG-1 levels via proteasomal degradation to limit centrosome number.

Site-Specific Phosphorylation of ZYG-1 Regulates ZYG-1 Stability and Centrosome Number

Spindle bipolarity is critical for genomic integrity. Given that centrosome number often dictates mitotic bipolarity, tight control of centrosome assembly is vital for the fidelity of cell division. The kinase ZYG-1/Plk4 is a master centrosome factor that is integral for controlling centrosome number and is modulated by protein phosphorylation. While autophosphorylation of Plk4 has been extensively studied in other systems, the mechanism of ZYG-1 phosphorylation in C. elegans remains largely unexplored. In C. elegans, Casein Kinase II (CK2) negatively regulates centrosome duplication by controlling centrosome-associated ZYG-1 levels. In this study, we investigated ZYG-1 as a potential substrate of CK2 and the functional impact of ZYG-1 phosphorylation on centrosome assembly. First, we show that CK2 directly phosphorylates ZYG-1 in vitro and physically interacts with ZYG-1 in vivo. Intriguingly, depleting CK2 or blocking ZYG-1 phosphorylation at putative CK2 target sites leads to centrosome amplification. In the non-phosphorylatable (NP)-ZYG-1 mutant embryo, the overall levels of ZYG-1 are elevated, leading to an increase in centrosomal ZYG-1 and downstream factors, providing a possible mechanism of the NP-ZYG-1 mutation to drive centrosome amplification. Moreover, inhibiting the 26S proteasome blocks degradation of the phospho-mimetic (PM)-ZYG-1, while the NP-ZYG-1 mutant shows partial resistance to proteasomal degradation. Our findings suggest that site-specific phosphorylation of ZYG-1, partly mediated by CK2, controls ZYG-1 levels via proteasomal degradation, limiting centrosome number. We provide a mechanism linking CK2 kinase activity to centrosome duplication through direct phosphorylation of ZYG-1, which is critical for the integrity of centrosome number.

kin-3 genetically suppresses sur-6 in centrosome assembly during Caenorhabditis elegans embryogenesis

Protein phosphorylation plays a critical role in cell cycle progression. In Caenorhabditis elegans , Casein Kinase II (CK2) negatively regulates centrosome assembly, and Protein Phosphatase 2A (PP2A) ^{SUR-6 /B55} acts as a positive regulator of centrosome duplication, suggesting CK2 and PP2A ^{SUR-6 /B55} play opposing roles in centrosome assembly. Here, we examined the genetic interaction between kin-3 , encoding the catalytic subunit of CK2, and sur-6 , encoding the PP2Aregulatory subunit SUR-6 /B55, in Caenorhabditis elegans embryos. Our results show that kin-3 (RNAi) partially restores normal centrosome duplication and embryonic viability to hypomorphic sur-6 mutants, suggesting that kin-3 genetically suppresses sur-6 in centrosome assembly during Caenorhabditis elegans embryogenesis.

The C. elegans Casein Kinase II is associated with meiotic DNA in fertilized oocytes

By using CRISPR/Cas9 genome-editing, we have generated epitope-tagged KIN-3 and KIN-10 expressing strains at the endogenous C-terminal loci in Caenorhabditis elegans . We observed that both the catalytic (KIN-3::V5) and regulatory (KIN-10::2xMyc) subunits of the Casein Kinase II (CK2) holoenzyme complex are associated with meiotic DNA, enriched in the midvalent rings during meiotic divisions in fertilized C. elegans oocytes.

Single nucleotide substitutions effectively block Cas9 and allow for scarless genome editing in Caenorhabditis elegans

In Caenorhabditis elegans, germline injection of Cas9 complexes is reliably used to achieve genome editing through homology-directed repair of Cas9-generated DNA breaks. To prevent Cas9 from targeting repaired DNA, additional blocking mutations are often incorporated into homologous repair templates. Cas9 can be blocked either by mutating the PAM sequence that is essential for Cas9 activity or by mutating the guide sequence that targets Cas9 to a specific genomic location. However, it is unclear how many nucleotides within the guide sequence should be mutated, since Cas9 can recognize "off-target" sequences that are imperfectly paired to its guide. In this study, we examined whether single-nucleotide substitutions within the guide sequence are sufficient to block Cas9 and allow for efficient genome editing. We show that a single mismatch within the guide sequence effectively blocks Cas9 and allows for recovery of edited animals. Surprisingly, we found that a low rate of edited animals can be recovered without introducing any blocking mutations, suggesting a temporal block to Cas9 activity in C. elegans. Furthermore, we show that the maternal genome of hermaphrodite animals is preferentially edited over the paternal genome. We demonstrate that maternally provided haplotypes can be selected using balancer chromosomes and propose a method of mutant isolation that greatly reduces screening efforts postinjection. Collectively, our findings expand the repertoire of genome editing strategies in C. elegans and demonstrate that extraneous blocking mutations are not required to recover edited animals when the desired mutation is located within the guide sequence.

APC/CFZR-1 regulates centrosomal ZYG-1 to limit centrosome number

Aberrant centrosome numbers are associated with human cancers. The levels of centrosome regulators positively correlate with centrosome number. Thus, tight control of centrosome protein levels is critical. In Caenorhabditis elegans, the anaphase-promoting complex/cyclosome and its co-activator FZR-1 (APC/CFZR-1), a ubiquitin ligase, negatively regulates centrosome assembly through SAS-5 degradation. In this study, we report the C. elegans ZYG-1 (Plk4 in humans) as a potential substrate of APC/CFZR-1. Inhibiting APC/CFZR-1 or mutating a ZYG-1 destruction (D)-box leads to elevated ZYG-1 levels at centrosomes, restoring bipolar spindles and embryonic viability to zyg-1 mutants, suggesting that APC/CFZR-1 influences centrosomal ZYG-1 via the D-box motif. We also show the Slimb/βTrCP-binding (SB) motif is critical for ZYG-1 degradation, substantiating a conserved mechanism by which ZYG-1/Plk4 stability is regulated by the SKP1-CUL1-F-box (Slimb/βTrCP)-protein complex (SCFSlimb/βTrCP)-dependent proteolysis via the conserved SB motif in C. elegans. Furthermore, we show that co-mutating ZYG-1 SB and D-box motifs stabilizes ZYG-1 in an additive manner, suggesting that the APC/CFZR-1 and SCFSlimb/βTrCP ubiquitin ligases function cooperatively for timely ZYG-1 destruction in C. elegans embryos where ZYG-1 activity remains at threshold level to ensure normal centrosome number.

microRNA strand selection: Unwinding the rules

microRNAs (miRNAs) play a central role in the regulation of gene expression by targeting specific mRNAs for degradation or translational repression. Each miRNA is post‐transcriptionally processed into a duplex comprising two strands. One of the two miRNA strands is selectively loaded into an Argonaute protein to form the miRNA‐Induced Silencing Complex (miRISC) in a process referred to as miRNA strand selection. The other strand is ejected from the complex and is subject to degradation. The target gene specificity of miRISC is determined by sequence complementarity between the Argonaute‐loaded miRNA strand and target mRNA. Each strand of the miRNA duplex has the capacity to be loaded into miRISC and possesses a unique seed sequence. Therefore, miRNA strand selection plays a defining role in dictating the specificity of miRISC toward its targets and provides a mechanism to alter gene expression in a switch‐like fashion. Aberrant strand selection can lead to altered gene regulation by miRISC and is observed in several human diseases including cancer. Previous and emerging data shape the rules governing miRNA strand selection and shed light on how these rules can be circumvented in various physiological and pathological contexts.

This article is categorized under:

RNA Processing > Processing of Small RNAs

Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs

Regulatory RNAs/RNAi/Riboswitches > Biogenesis of Effector Small RNAs

The Zebrafish curly fry Is Required for Proper Centrosome and Mitotic Spindle Assembly

The zebrafish curly fry (cfy) mutation leads to a dramatic increase in mitotic index and cell death starting during neural tube formation. The mutant phenotype is cell autonomous and does not result from defects in apical/basal polarity within the neuroepithelium. The increase in mitotic index could be due to increased proliferation or cell cycle arrest in mitosis. cfy embryos were analyzed to examine these two possibilities. By labeling embryos with a pulse of BrdU and anti-phospho-histone 3 and examining the DNA content by fluorescence activated cell sorting, we show that cfy mutants exhibit no increase in proliferation, but a significant increase in the number of cells arrested in mitosis. Furthermore, time-lapse microscopy in vivo confirmed that a great majority of dividing cells arrest during mitosis and that these mitotically arrested cells die in cfy embryos. Finally, immunostaining and confocal microscopy in cfy mutant embryos revealed that mitotic cells in mutants contain aberrant centrosomes and often exhibit monopolar spindles, thereby leading to mitotic cell cycle arrest. Our results suggest that the cfy gene is required for proper centrosome assembly and mitotic spindle formation, therefore critical for normal cell division.

Casein kinase II is required for proper cell division and acts as a negative regulator of centrosome duplication in Caenorhabditis elegans embryos

Centrosomes are the primary microtubule-organizing centers that orchestrate microtubule dynamics during the cell cycle. The correct number of centrosomes is pivotal for establishing bipolar mitotic spindles that ensure accurate segregation of chromosomes. Thus, centrioles must duplicate once per cell cycle, one daughter per mother centriole, the process of which requires highly coordinated actions among core factors and modulators. Protein phosphorylation is shown to regulate the stability, localization and activity of centrosome proteins. Here, we report the function of Casein kinase II (CK2) in early Caenorhabditis elegans embryos. The catalytic subunit (KIN-3/CK2α) of CK2 localizes to nuclei, centrosomes and midbodies. Inactivating CK2 leads to cell division defects, including chromosome missegregation, cytokinesis failure and aberrant centrosome behavior. Furthermore, depletion or inhibiting kinase activity of CK2 results in elevated ZYG-1 levels at centrosomes, restoring centrosome duplication and embryonic viability to zyg-1 mutants. Our data suggest that CK2 functions in cell division and negatively regulates centrosome duplication in a kinase-dependent manner.

Summary: The conserved protein kinase CK2 negatively regulates centrosome assembly and is required for proper cell cycle progression and cytokinesis in early C. elegans embryos.

ATX-2, the C. elegans Ortholog of Human Ataxin-2, Regulates Centrosome Size and Microtubule Dynamics

Centrosomes are critical sites for orchestrating microtubule dynamics, and exhibit dynamic changes in size during the cell cycle. As cells progress to mitosis, centrosomes recruit more microtubules (MT) to form mitotic bipolar spindles that ensure proper chromosome segregation. We report a new role for ATX-2, a C. elegans ortholog of Human Ataxin-2, in regulating centrosome size and MT dynamics. ATX-2, an RNA-binding protein, forms a complex with SZY-20 in an RNA-independent fashion. Depleting ATX-2 results in embryonic lethality and cytokinesis failure, and restores centrosome duplication to zyg-1 mutants. In this pathway, SZY-20 promotes ATX-2 abundance, which inversely correlates with centrosome size. Centrosomes depleted of ATX-2 exhibit elevated levels of centrosome factors (ZYG-1, SPD-5, γ-Tubulin), increasing MT nucleating activity but impeding MT growth. We show that ATX-2 influences MT behavior through γ-Tubulin at the centrosome. Our data suggest that RNA-binding proteins play an active role in controlling MT dynamics and provide insight into the control of proper centrosome size and MT dynamics.

The microtubule (MT) cytoskeleton undergoes dynamic rearrangements during the cell cycle. As the primary microtubule-organizing center, centrosomes orchestrate MT dynamics and play a key role in establishing bipolar spindles in mitosis. Errors in centrosome assembly lead to missegregation of genomic content and aneuploidy. Thus, stringent regulation of centrosome assembly is of vital importance for the fidelity of cell division and survival. Using the nematode Caenorhabditis elegans (C. elegans) as a model, we study the role of the RNA-binding protein, ATX-2, a C. elegans homolog of Human Ataxin-2 in early cell division. A number of RNAs and RNA-binding proteins are shown to be associated with centrosomes and MTs, and influence the assembly of mitotic spindles. In C. elegans, the RNA-binding role of SZY-20 is implicated in regulating centrosome size. We show that ATX-2 functions together with SZY-20 in centrosome size and MT behavior. SZY-20 promotes ATX-2 protein levels, and the amount of ATX-2 influences centrosome size and subsequent MT dynamics. Our work provides evidence that RNA-binding proteins have an active role in controlling MT dynamics.