Myt1 inhibition of Cyclin A/Cdk1 is essential for fusome integrity and premeiotic centriole engagement in Drosophila spermatocytes

A deficiency screen identifies genomic regions critical for sperm head-tail connection

The Sperm Neck provides a stable connection between the sperm head and tail, which is critical for fertility in species with flagellated sperm. Within the Sperm Neck, the Head-Tail Coupling Apparatus serves as the critical link between the nucleus (head) and the axoneme (tail) via the centriole. To identify regions of the Drosophila melanogaster genome that contain genetic elements that influence Head-Tail Coupling Apparatus formation, we undertook a 2 part screen using the Drosophila Deficiency kit. For this screen, we utilized a sensitized genetic background that overexpresses the pericentriolar material regulatory protein Pericentrin-Like Protein. We had previously shown that Pericentrin-Like Protein overexpression disrupts the head-tail connection in some spermatids, but not to a degree sufficient to reduce fertility. In the first step of the screen, we tested for deficiencies that in combination with Pericentrin-Like Protein overexpression causes a reduction in fertility. We ultimately identified 11 regions of the genome that resulted in an enhanced fertility defect when combined with Pericentrin-Like Protein overexpression. In the second step of the screen, we tested these deficiencies for their ability to enhance the head-tail connection defect caused by Pericentrin-Like Protein overexpression, finding 6 genomic regions. We then tested smaller deficiencies to narrow the region of the genome that contained these enhancers and examined the expression patterns of the genes within these deficiencies using publicly available datasets of Drosophila tissue RNAseq and Drosophila testes snRNAseq. In total, our analysis suggests that some deficiencies may contain single genes that influence Head-Tail Coupling Apparatus formation or fertility, while other deficiencies appear to be genomic regions rich in testis-expressed genes that might affect the Head-Tail Coupling Apparatus through complex, multigene interactions.

A deficiency screen identifies genomic regions critical for sperm head-tail connection

A stable connection between the sperm head and tail is critical for fertility in species with flagellated sperm. The head-tail coupling apparatus (HTCA) serves as the critical link between the nucleus (head) and the axoneme (tail) via the centriole. To identify regions of the Drosophila melanogaster genome that contain genetic elements that influence HTCA formation, we undertook a two part screen using the Drosophila deficiency (Df) kit. For this screen, we utilized a sensitized genetic background that overexpresses the pericentriolar material regulatory protein Pericentrin-Like Protein (PLP). We had previously shown that PLP overexpression (PLPOE) disrupts the head-tail connection in some spermatids, but not to a degree sufficient to reduce fertility. In the first step of the screen we tested for Dfs that in combination with PLPOE cause a reduction in fertility. We ultimately identified 11 regions of the genome that showed an enhanced fertility defect when combined with PLP overexpression. In the second step of the screen we tested these Dfs for their ability to enhance the HTCA defect caused by PLPOE, finding six. We then tested smaller Dfs to narrow the region of the genome that contained these enhancers. To further analyze the regions of the genome removed by these Dfs, we examined the expression patterns of the genes within these Dfs in publicly available datasets of RNAseq of Drosophila tissues and snRNAseq of Drosophila testes. In total, our analysis suggests that some of these Dfs may contain a single gene that might influence HTCA formation and / or fertility, while others appear to be regions of the genome especially rich in testis-expressed genes that might affect the HTCA because of complex, multi-gene interactions.

Cyclin B Export to the Cytoplasm via the Nup62 Subcomplex and Subsequent Rapid Nuclear Import Are Required for the Initiation of Drosophila Male Meiosis

The cyclin-dependent kinase 1 (Cdk1)-cyclin B (CycB) complex plays critical roles in cell-cycle regulation. Before Drosophila male meiosis, CycB is exported from the nucleus to the cytoplasm via the nuclear porin 62kD (Nup62) subcomplex of the nuclear pore complex. When this export is inhibited, Cdk1 is not activated, and meiosis does not initiate. We investigated the mechanism that controls the cellular localization and activation of Cdk1. Cdk1-CycB continuously shuttled into and out of the nucleus before meiosis. Overexpression of CycB, but not that of CycB with nuclear localization signal sequences, rescued reduced cytoplasmic CycB and inhibition of meiosis in Nup62-silenced cells. Full-scale Cdk1 activation occurred in the nucleus shortly after its rapid nuclear entry. Cdk1-dependent centrosome separation did not occur in Nup62-silenced cells, whereas Cdk1 interacted with Cdk-activating kinase and Twine/Cdc25C in the nuclei of Nup62-silenced cells, suggesting the involvement of another suppression mechanism. Silencing of roughex rescued Cdk1 inhibition and initiated meiosis. Nuclear export of Cdk1 ensured its escape from inhibition by a cyclin-dependent kinase inhibitor. The complex re-entered the nucleus via importin β at the onset of meiosis. We propose a model regarding the dynamics and activation mechanism of Cdk1-CycB to initiate male meiosis.

The E3 ligase Poe promotes Pericentrin degradation

Centrosomes are essential parts of diverse cellular processes, and precise regulation of the levels of their constituent proteins is critical for their function. One such protein is Pericentrin (PCNT) in humans and Pericentrin-like protein (PLP) in Drosophila. Increased PCNT expression and its protein accumulation are linked to clinical conditions including cancer, mental disorders, and ciliopathies. However, the mechanisms by which PCNT levels are regulated remain underexplored. Our previous study demonstrated that PLP levels are sharply down-regulated during early spermatogenesis and this regulation is essential to spatially position PLP on the proximal end of centrioles. We hypothesized that the sharp drop in PLP protein was a result of rapid protein degradation during the male germ line premeiotic G2 phase. Here, we show that PLP is subject to ubiquitin-mediated degradation and identify multiple proteins that promote the reduction of PLP levels in spermatocytes, including the UBR box containing E3 ligase Poe (UBR4), which we show binds to PLP. Although protein sequences governing posttranslational regulation of PLP are not restricted to a single region of the protein, we identify a region that is required for Poe-mediated degradation. Experimentally stabilizing PLP, via internal PLP deletions or loss of Poe, leads to PLP accumulation in spermatocytes, its mispositioning along centrioles, and defects in centriole docking in spermatids.

Cyst stem cell lineage eIF5 non-autonomously prevents testicular germ cell tumor formation via eIF1A/eIF2γ-mediated pre-initiation complex

BACKGROUND

Stem cell niche maintains stem cell population identity and is essential for the homeostasis of self-renewal and differentiation in Drosophila testes. However, the mechanisms of CySC lineage signals-mediated soma-germline communications in response to external stimuli are unclear.

METHODS

Pre-initiation complex functions were evaluated by UAS-Gal4-mediated cell effects. RNA sequencing was conducted in NC and eIF5 siRNA-treated cells. Genetic interaction analysis was used to indicate the relationships between eIF5 and eIF1A/eIF2γ in Drosophila testes.

RESULTS

Here, we demonstrated that in CySCs, translation initiation factor eIF5 mediates cyst cell differentiation and the non-autonomously affected germ cell differentiation process. CySCs lacking eIF5 displayed unbalanced cell proliferation and apoptosis, forming testicular germ cell tumors (TGCTs) during spermatogenesis. eIF5 transcriptional regulation network analysis identified multiple metabolic processes and several key factors that might be involved in germ cell differentiation and TGCT formation. Importantly, knockdown of eIF1A and eIF2γ, key components of pre-initiation complex, mimicked the phenotype of knocking down eIF5 in the stem cell niche of Drosophila testes. Genetic interaction analysis indicated that eIF5 was sufficient to rescue the phenotype of tumorlike structures induced by down-regulating eIF1A or eIF2γ in CySCs.

CONCLUSIONS

These findings demonstrated that CySC lineage eIF5, together with eIF1A or eIF2γ, mediates soma-germline communications for the stem cell niche homeostasis in Drosophila testes, providing new insights for the prevention of TGCTs.

Microtubule and Actin Cytoskeletal Dynamics in Male Meiotic Cells of Drosophila melanogaster

Drosophila dividing spermatocytes offer a highly suitable cell system in which to investigate the coordinated reorganization of microtubule and actin cytoskeleton systems during cell division of animal cells. Like male germ cells of mammals, Drosophila spermatogonia and spermatocytes undergo cleavage furrow ingression during cytokinesis, but abscission does not take place. Thus, clusters of primary and secondary spermatocytes undergo meiotic divisions in synchrony, resulting in cysts of 32 secondary spermatocytes and then 64 spermatids connected by specialized structures called ring canals. The meiotic spindles in Drosophila males are substantially larger than the spindles of mammalian somatic cells and exhibit prominent central spindles and contractile rings during cytokinesis. These characteristics make male meiotic cells particularly amenable to immunofluorescence and live imaging analysis of the spindle microtubules and the actomyosin apparatus during meiotic divisions. Moreover, because the spindle assembly checkpoint is not robust in spermatocytes, Drosophila male meiosis allows investigating of whether gene products required for chromosome segregation play additional roles during cytokinesis. Here, we will review how the research studies on Drosophila male meiotic cells have contributed to our knowledge of the conserved molecular pathways that regulate spindle microtubules and cytokinesis with important implications for the comprehension of cancer and other diseases.

Identification of Survival-Associated Hub Genes in Pancreatic Adenocarcinoma Based on WGCNA

Pancreatic adenocarcinoma is one of the leading causes of cancer-related death worldwide. Since little clinical symptoms were shown in the early period of pancreatic adenocarcinoma, most patients were found to carry metastases when diagnosis. The lack of effective diagnosis biomarkers and therapeutic targets makes pancreatic adenocarcinoma difficult to screen and cure. The fundamental problem is we know very little about the regulatory mechanisms during carcinogenesis. Here, we employed weighted gene co-expression network analysis (WGCNA) to build gene interaction network using expression profile of pancreatic adenocarcinoma from The Cancer Genome Atlas (TCGA). STRING was used for the construction and visualization of biological networks. A total of 22 modules were detected in the network, among which yellow and pink modules showed the most significant associations with pancreatic adenocarcinoma. Dozens of new genes including PKMYT1, WDHD1, ASF1B, and RAD18 were identified. Further survival analysis yielded their valuable effects on the diagnosis and treatment of pancreatic adenocarcinoma. Our study pioneered network-based algorithm in the application of tumor etiology and discovered several promising regulators for pancreatic adenocarcinoma detection and therapy.

Cyclin-dependent kinases-based synthetic lethality: Evidence, concept, and strategy

Synthetic lethality is a proven effective antitumor strategy that has attracted great attention. Large-scale screening has revealed many synthetic lethal genetic phenotypes, and relevant small-molecule drugs have also been implemented in clinical practice. Increasing evidence suggests that CDKs, constituting a kinase family predominantly involved in cell cycle control, are synthetic lethal factors when combined with certain oncogenes, such as MYC, TP53, and RAS, which facilitate numerous antitumor treatment options based on CDK-related synthetic lethality. In this review, we focus on the synthetic lethal phenotype and mechanism related to CDKs and summarize the preclinical and clinical discoveries of CDK inhibitors to explore the prospect of CDK inhibitors as antitumor compounds for strategic synthesis lethality in the future.

Cyclin E and Cdk1 regulate the termination of germline transit-amplification process in Drosophila testis

An extension of the G1 is correlated with stem cell differentiation. The role of cell cycle regulation during the subsequent transit amplification (TA) divisions is, however, unclear. Here, we report for the first time that in the Drosophila male germline lineage, the transit amplification divisions accelerate after the second TA division. The cell cycle phases, marked by Cyclin E and Cyclin B, are progressively altered during the TA. Antagonistic functions of the bag-of-marbles and the Transforming-Growth-Factor-β signaling regulate the cell division rates after the second TA division and the extent of the Cyclin E phase during the fourth TA division. Furthermore, loss of Cyclin E during the fourth TA cycle retards the cell division and induces premature meiosis in some cases. A similar reduction of Cdk1 activity during this stage arrests the penultimate division and subsequent differentiation, whereas enhancement of the Cdk1 activity prolongs the TA by one extra round. Altogether, the results suggest that modification of the cell cycle structure and the rates of cell division after the second TA division determine the extent of amplification. Also, the regulation of the Cyclin E and CDK1 functions during the penultimate TA division determines the induction of meiosis and subsequent differentiation.

PKMYT1 is associated with prostate cancer malignancy and may serve as a therapeutic target

Prostate cancer (PCa) is the third most common malignancy worldwide. Novel and effective therapeutic targets are needed for PCa. The purpose of this study was to discover novel therapeutic targets for PCa by performing advanced analysis on PCa RNA sequencing (RNAseq) data from The Cancer Genome Atlas (TCGA). Weighted correlation-network analysis (WGCNA) was performed on the RNAseq data of tumor samples, and the module most relevant to the Gleason score was identified. Combining differential gene-expression analysis and survival analysis, we narrowed down potential therapeutic target genes and found that PKMYT1 might be one. Subsequently, functional studies (i.e., cell-proliferation assays, cell cycle analysis, and colony-formation assays) demonstrated that knockdown of PKMYT1 significantly inhibited the growth of PCa cells. Further investigation illustrated that PKMYT1 promoted the growth of PCa cells through targeting CCNB1 and CCNE1 expression. In addition, fostamatinib, an inhibitor of PKMYT1, effectively inhibited the proliferation of PCa cells. Taken together, our results suggest that PKMYT1 is a gene associated with malignancy of PCa and is a novel therapeutic target.

Sperm Head-Tail Linkage Requires Restriction of Pericentriolar Material to the Proximal Centriole End

The centriole, or basal body, is the center of attachment between the sperm head and tail. While the distal end of the centriole templates the cilia, the proximal end associates with the nucleus. Using Drosophila, we identify a centriole-centric mechanism that ensures proper proximal end docking to the nucleus. This mechanism relies on the restriction of pericentrin-like protein (PLP) and the pericentriolar material (PCM) to the proximal end of the centriole. PLP is restricted proximally by limiting its mRNA and protein to the earliest stages of centriole elongation. Ectopic positioning of PLP to more distal portions of the centriole is sufficient to redistribute PCM and microtubules along the entire centriole length. This results in erroneous, lateral centriole docking to the nucleus, leading to spermatid decapitation as a result of a failure to form a stable head-tail linkage.

Phosphorylation of keratin 18 serine 52 regulates mother-daughter centriole engagement and microtubule nucleation by cell cycle-dependent accumulation at the centriole

Serine-52 (Ser52) is the major physiologic site of keratin 18 (K18) phosphorylation. Here, we report that serine-52 phosphorylated K18 (phospho-Ser52 K18) accumulated on centrosomes in a cell cycle-dependent manner. Moreover, we found that phospho-Ser52 K18 was located at the proximal end of the mother centriole. Transfection with the K18 Ser52 → Ala (K18 S52A) mutant prevented centriole localization of phospho-Ser52 K18 and resulted in separation of the mother-daughter centrioles. Inhibition of microtubule polymerization led to the disappearance of aggregated phospho-Ser52 K18 on the centrosome; removal of inhibitors resulted in reaccumulation of phospho-Ser52 K18 in microtubule-organizing centers. Transfection with a K18 S52A mutant inhibited microtubule nucleation. These results reveal a cell cycle-dependent change in centrosome localization of phospho-Ser52 k18 and strongly suggest that the phosphorylation status of Ser52 K18 of mother centrioles plays a critical role in maintaining a tight engagement between mother and daughter centrioles and also contributes to microtubule nucleation.

The Drosophila Citrate Lyase Is Required for Cell Division during Spermatogenesis

The Drosophila melanogasterDmATPCL gene encodes for the human ATP Citrate Lyase (ACL) ortholog, a metabolic enzyme that from citrate generates glucose-derived Acetyl-CoA, which fuels central biochemical reactions such as the synthesis of fatty acids, cholesterol and acetylcholine, and the acetylation of proteins and histones. We had previously reported that, although loss of Drosophila ATPCL reduced levels of Acetyl-CoA, unlike its human counterpart, it does not affect global histone acetylation and gene expression, suggesting that its role in histone acetylation is either partially redundant in Drosophila or compensated by alternative pathways. Here, we describe that depletion of DmATPCL affects spindle organization, cytokinesis, and fusome assembly during male meiosis, revealing an unanticipated role for DmATPCL during spermatogenesis. We also show that DmATPCL mutant meiotic phenotype is in part caused by a reduction of fatty acids, but not of triglycerides or cholesterol, indicating that DmATPCL-derived Acetyl-CoA is predominantly devoted to the biosynthesis of fatty acids during spermatogenesis. Collectively, our results unveil for the first time an involvement for DmATPCL in the regulation of meiotic cell division, which is likely conserved in human cells.

Drosophila Doublefault protein coordinates multiple events during male meiosis by controlling mRNA translation

During the extended prophase of Drosophila gametogenesis, spermatocytes undergo robust gene transcription and store many transcripts in the cytoplasm in a repressed state, until translational activation of select mRNAs in later steps of spermatogenesis. Here, we characterize the Drosophila Doublefault (Dbf) protein as a C2H2 zinc-finger protein, primarily expressed in testes, that is required for normal meiotic division and spermiogenesis. Loss of Dbf causes premature centriole disengagement and affects spindle structure, chromosome segregation and cytokinesis. We show that Dbf interacts with the RNA-binding protein Syncrip/hnRNPQ, a key regulator of localized translation in Drosophila We propose that the pleiotropic effects of dbf loss-of-function mutants are associated with the requirement of dbf function for translation of specific transcripts in spermatocytes. In agreement with this hypothesis, Dbf protein binds cyclin B mRNA and is essential for translation of cyclin B in mature spermatocytes.

The Rove Beetle Creophilus maxillosus as a Model System to Study Asymmetric Division, Oocyte Specification, and the Germ-Somatic Cell Signaling

Creophilus maxillosus (Staphylinidae, Coleoptera, Polyphaga) has a meroistic-telotrophic ovary composed of tropharium, which contains trophocytes (nurse cells) and vitellarium, which contains growing oocytes. The trophocytes are connected to the oocytes by cytoplasmic nutritive cords, which deliver nutrients to the oocytes. The formation/differentiation of the oocytes and trophocytes takes place in the pupal ovary within linear chains of sibling cells. Each chain is composed of a single oocyte connected to a linear chain of sister trophocytes. The nuclei of the oocytes contain an extrachromosomal DNA body (extra DNA body) consisting of amplified ribosomal DNA (rDNA). During oogenesis, the prospective oocyte, located at the base (posterior) of each chain, is the only cell within the chain that amplifies rDNA and retains permanent contact with the somatic pre-follicular cells. The oogonial divisions leading to the formation of the oocyte/trophocytes chain are asymmetric, and during consecutive divisions, the rDNA body always segregates basally (posteriorly) to the prospective oocyte abutted on the somatic cells. However, the segregation of rDNA is imperfect, and within each oocyte/trophocytes chain, there is a gradient of rDNA: the prospective oocyte has the highest amount of rDNA and the trophocyte that is most distant (most anterior) from the oocyte has no or the lowest amount of rDNA. In addition, the divisions within each chain are parasynchronous, with the pro-oocyte being the most mitotically advanced cell in the chain. These observations indicate the presence of a signaling gradient emanating from the somatic cells and/or oocyte; this gradient diminishes in strength with the increasing distance from its source, i.e., the oocyte/somatic cells. Because of this phenomenon, C. maxillosus is the perfect model in which to study the germ-somatic cell interactions and signaling. This chapter describes the methods for the collection and laboratory culture of C. maxillosus and the analysis of divisions and signaling in the C. maxillosus ovary.

Mild replication stress causes chromosome mis-segregation via premature centriole disengagement

Replication stress, a hallmark of cancerous and pre-cancerous lesions, is linked to structural chromosomal aberrations. Recent studies demonstrated that it could also lead to numerical chromosomal instability (CIN). The mechanism, however, remains elusive. Here, we show that inducing replication stress in non-cancerous cells stabilizes spindle microtubules and favours premature centriole disengagement, causing transient multipolar spindles that lead to lagging chromosomes and micronuclei. Premature centriole disengagement depends on the G2 activity of the Cdk, Plk1 and ATR kinases, implying a DNA-damage induced deregulation of the centrosome cycle. Premature centriole disengagement also occurs spontaneously in some CIN+ cancer cell lines and can be suppressed by attenuating replication stress. Finally, we show that replication stress potentiates the effect of the chemotherapeutic agent taxol, by increasing the incidence of multipolar cell divisions. We postulate that replication stress in cancer cells induces numerical CIN via transient multipolar spindles caused by premature centriole disengagement.

Chromosome instability can be caused by replication stress, although the mechanism is unclear. Here, the authors show that inducing mild replication stress in cancerous and non-cancerous cell lines leads to centriole disengagement and the subsequent formation of lagging chromosomes and micronuclei.

Coordinative control of G2/M phase of the cell cycle by non-coding RNAs in hepatocellular carcinoma

Objective

To investigate the interaction of non-coding RNAs (ncRNAs) in hepatocellular carcinoma.

Methods

We compared the ncRNAs and mRNAs expression profiles of hepatocellular carcinoma and adjacent tissue by microarray and RT-PCR. The relationship between different ncRNAs and mRNA was analyzed using bioinformatics tools. A regulatory model of ncRNAs in hepatocellular carcinoma cells was developed.

Results

A total of 1,704 differentially expressed lncRNAs, 57 miRNAs, and 2,093 mRNAs were identified by microarray analyses. There is a co-expression relationship between two ncRNAs (miRNA-125b-2-3p and lncRNA P26302). Bioinformatics analysis demonstrated cyclin-dependent kinases 1 and CyclinA2 as potential targets of miR-125b-2-3p and Polo-like kinase 1 as potential target of lncRNAP26302. All three gene are important components in the G2/M phase of cell cycle. Subsequently real-time polymerase chain reaction (PCR) studies confirmed these microarray results.

Conclusion

MiR-125b-2-3p and lncRNAP26302 may affect the G2/M phase of the cell cycle through the regulation of their respective target genes. This study shows a role of ncRNAs in pathogenesis of hepatocellular carcinoma at molecular level, providing a basis for the future investigation aiming at early diagnosis and novel treatment of hepatocellular carcinoma.

Subcellular Specialization and Organelle Behavior in Germ Cells

Gametes, eggs and sperm, are the highly specialized cell types on which the development of new life solely depends. Although all cells share essential organelles, such as the ER (endoplasmic reticulum), Golgi, mitochondria, and centrosomes, germ cells display unique regulation and behavior of organelles during gametogenesis. These germ cell-specific functions of organelles serve critical roles in successful gamete production. In this chapter, I will review the behaviors and roles of organelles during germ cell differentiation.

Stay Connected: A Germ Cell Strategy

Germ cells develop as a cyst of interconnected sibling cells in a broad range of organisms in both sexes. A well-established function of intercellular connectivity is to transport cytoplasmic materials from 'nurse' cells to oocytes, a critical process for developing functional oocytes in ovaries of many species. However, there are situations where connectivity exists without a nursing mechanism, and the biological meaning of such connectivity remains obscure. In this review, we summarize current knowledge on the formation of intercellular connectivity, and discuss its meaning by visiting multiple examples of germ cell connectivity observed in evolutionarily distant species.