The mitotic arrest deficient protein MAD2B interacts with the clathrin light chain A during mitosis

Backbone and ILV side-chain methyl NMR resonance assignments of human Rev7/Rev3-RBM1 and Rev7/Rev3-RBM2 complexes

Rev7 is a versatile HORMA (Hop1, Rev7, Mad2) family adaptor protein with multiple roles in mitotic regulation and DNA damage response, and an essential accessory subunit of the translesion synthesis (TLS) DNA polymerase Polζ employed in replication of damaged DNA. Within Polζ, the two copies of Rev7 interact with the two Rev7-bonding motifs (RBM1 and RBM2) of the catalytic subunit Rev3 by a mechanism characteristic of HORMA proteins whereby the "safety-belt" loop of Rev7 closes on the top of the ligand. Here we report the nearly complete backbone and Ile, Val, Leu side-chain methyl NMR resonance assignments of the 27 kDa human Rev7/Rev3-RBM1 and Rev7/Rev3-RBM2 complexes (BMRB deposition numbers 51651 and 51652) that will facilitate future NMR studies of Rev7 dynamics and interactions.

REV7 in Cancer Biology and Management

DNA repair and cell cycle regulation are potential biological fields to develop molecular targeting therapies for cancer. Human REV7 was originally discovered as a homologous molecule to yeast Rev7, which is involved in DNA damage response and mutagenesis, and as the second homolog of yeast Mad2, involved in the spindle assembly checkpoint. Although REV7 principally functions in the fields of DNA repair and cell cycle regulation, many binding partners of REV7 have been identified using comprehensive analyses in the past decade, and the significance of REV7 is expanding in various other biological fields, such as gene transcription, epigenetics, primordial germ cell survival, neurogenesis, intracellular signaling, and microbial infection. In addition, the clinical significance of REV7 has been demonstrated in studies using human cancer tissues, and investigations in cancer cell lines and animal models have revealed the greater impacts of REV7 in cancer biology, which makes it an attractive target molecule for cancer management. This review focuses on the functions of REV7 in human cancer and discusses the utility of REV7 for cancer management with a summary of the recent development of inhibitors targeting REV7.

Functions and Regulation of Meiotic HORMA-Domain Proteins

During meiosis, homologous chromosomes must recognize, pair, and recombine with one another to ensure the formation of inter-homologue crossover events, which, together with sister chromatid cohesion, promote correct chromosome orientation on the first meiotic spindle. Crossover formation requires the assembly of axial elements, proteinaceous structures that assemble along the length of each chromosome during early meiosis, as well as checkpoint mechanisms that control meiotic progression by monitoring pairing and recombination intermediates. A conserved family of proteins defined by the presence of a HORMA (HOp1, Rev7, MAd2) domain, referred to as HORMADs, associate with axial elements to control key events of meiotic prophase. The highly conserved HORMA domain comprises a flexible safety belt sequence, enabling it to adopt at least two of the following protein conformations: one closed, where the safety belt encircles a small peptide motif present within an interacting protein, causing its topological entrapment, and the other open, where the safety belt is reorganized and no interactor is trapped. Although functional studies in multiple organisms have revealed that HORMADs are crucial regulators of meiosis, the mechanisms by which HORMADs implement key meiotic events remain poorly understood. In this review, we summarize protein complexes formed by HORMADs, discuss their roles during meiosis in different organisms, draw comparisons to better characterize non-meiotic HORMADs (MAD2 and REV7), and highlight possible areas for future research.

Clathrin Light Chains: Not to Be Taken so Lightly

Clathrin is a cytosolic protein involved in the intracellular trafficking of a wide range of cargo. It is composed of three heavy chains and three light chains that together form a triskelion, the subunit that polymerizes to form a clathrin coated vesicle. In addition to its role in membrane trafficking, clathrin is also involved in various cellular and biological processes such as chromosomal segregation during mitosis and organelle biogenesis. Although the role of the heavy chains in regulating important physiological processes has been well documented, we still lack a complete understanding of how clathrin light chains regulate membrane traffic and cell signaling. This review highlights the importance and contributions of clathrin light chains in regulating clathrin assembly, vesicle formation, endocytosis of selective receptors and physiological and developmental processes.

Clathrin: the molecular shape shifter

Clathrin is best known for its contribution to clathrin-mediated endocytosis yet it also participates to a diverse range of cellular functions. Key to this is clathrin's ability to assemble into polyhedral lattices that include curved football or basket shapes, flat lattices or even tubular structures. In this review, we discuss clathrin structure and coated vesicle formation, how clathrin is utilised within different cellular processes including synaptic vesicle recycling, hormone desensitisation, spermiogenesis, cell migration and mitosis, and how clathrin's remarkable 'shapeshifting' ability to form diverse lattice structures might contribute to its multiple cellular functions.

REV7: Jack of many trades

The HORMA domain protein REV7, also known as MAD2L2, interacts with a variety of proteins and thereby contributes to the establishment of different complexes. With doing so, REV7 impacts a diverse range of cellular processes and gained increasing interest as more of its activities became uncovered. REV7 has important roles in translesion synthesis and mitotic progression, and acts as a central component in the recently discovered shieldin complex that operates in DNA double-strand break repair. Here we discuss the roles of REV7 in its various complexes, focusing on its activity in genome integrity maintenance. Moreover, we will describe current insights on REV7 structural features that allow it to be such a versatile protein.

Genome-wide DNA copy number analysis and targeted transcriptional analysis of canine histiocytic malignancies identifies diagnostic signatures and highlights disruption of spindle assembly complex

Canine histiocytic malignancies (HM) are rare across the general dog population, but overrepresented in certain breeds, such as Bernese mountain dog and flat-coated retriever. Accurate diagnosis relies on immunohistochemical staining to rule out histologically similar cancers with different prognoses and treatment strategies (e.g., lymphoma and hemangiosarcoma). HM are generally treatment refractory with overall survival of less than 6 months. A lack of understanding regarding the mechanisms of disease development and progression hinders development of novel therapeutics. While the study of human tumors can benefit veterinary medicine, the rarity of the suggested orthologous disease (dendritic cell sarcoma) precludes this. This study aims to improve the understanding of underlying disease mechanisms using genome-wide DNA copy number and gene expression analysis of spontaneous HM across several dog breeds. Extensive DNA copy number disruption was evident, with losses of segments of chromosomes 16 and 31 detected in 93% and 72% of tumors, respectively. Droplet digital PCR (ddPCR) evaluation of these regions in numerous cancer specimens effectively discriminated HM from other common round cell tumors, including lymphoma and hemangiosarcoma, resulting in a novel, rapid diagnostic aid for veterinary medicine. Transcriptional analysis demonstrated disruption of the spindle assembly complex, which is linked to genomic instability and reduced therapeutic impact in humans. A key signature detected was up-regulation of Matrix Metalloproteinase 9 (MMP9), supported by an immunohistochemistry-based assessment of MMP9 protein levels. Since MMP9 has been linked with rapid metastasis and tumor aggression in humans, the data in this study offer a possible mechanism of aggression in HM.

Dynamic feature of mitotic arrest deficient 2-like protein 2 (MAD2L2) and structural basis for its interaction with chromosome alignment-maintaining phosphoprotein (CAMP)

Mitotic arrest deficient 2-like protein 2 (MAD2L2), also termed MAD2B or REV7, is involved in multiple cellular functions including translesion DNA synthesis (TLS), signal transduction, transcription, and mitotic events. MAD2L2 interacts with chromosome alignment-maintaining phosphoprotein (CAMP), a kinetochore-microtubule attachment protein in mitotic cells, presumably through a novel "WK" motif in CAMP. Structures of MAD2L2 in complex with binding regions of the TLS proteins REV3 and REV1 have revealed that MAD2L2 has two faces for protein-protein interactions that are regulated by its C-terminal region; however, the mechanisms underlying the MAD2L2-CAMP interaction and the mitotic role of MAD2L2 remain unknown. Here we have determined the structures of human MAD2L2 in complex with a CAMP fragment in two crystal forms. The overall structure of the MAD2L2-CAMP complex in both crystal forms was essentially similar to that of the MAD2L2-REV3 complex. However, the residue interactions between MAD2L2 and CAMP were strikingly different from those in the MAD2L2-REV3 complex. Furthermore, structure-based interaction analyses revealed an unprecedented mechanism involving CAMP's WK motif. Surprisingly, in one of the crystal forms, the MAD2L2-CAMP complex formed a dimeric structure in which the C-terminal region of MAD2L2 was swapped and adopted an immature structure. The structure provides direct evidence for the dynamic nature of MAD2L2 structure, which in turn may have implications for the protein-protein interaction mechanism and the multiple functions of this protein. This work is the first structural study of MAD2L2 aside from its role in TLS and might pave the way to clarify MAD2L2's function in mitosis.

Rev7, the regulatory subunit of Polζ, undergoes UV-induced and Cul4-dependent degradation

In eukaryotic cells, Rev7 interacts with Rev3 and functions as a regulatory subunit of Polζ, a translesion DNA synthesis (TLS) polymerase. In addition to its role in TLS, mammalian Rev7, also known as Mad2B/Mad2L2, participates in multiple cellular activities including cell cycle progression and double-strand break repair through its interaction with several proteins. Here we show that in mammalian cells, Rev7 undergoes ubiquitin/proteasome-mediated degradation upon UV irradiation in a time-dependent manner. We identified the Rev7 N-terminal destruction box as the degron and Cul4A/B as putative E3 ligases in this process. We also show that the nucleotide excision repair (NER) pathway protein HR23B physically interacts and colocalizes with Rev7 in the nuclear foci after UV irradiation and protects Rev7 from accelerated degradation. Furthermore, a similar Rev7 degradation profile was observed in cells treated with the UV-mimetic agent 4-nitroquinoline 1-oxide but not with cisplatin or camptothecin, suggesting a role of the NER pathway protein(s) in UV-induced Rev7 degradation. These data and the observation that cells deficient in Rev7 are sensitized to UV irradiation while excessive Rev7 protects cells from UV-induced DNA damage provide a new insight into the potential interplay between TLS and NER.

Rev7/Mad2B plays a critical role in the assembly of a functional mitotic spindle

The spindle assembly checkpoint (SAC) acts as a guardian against cellular threats that may lead to chromosomal missegregation and aneuploidy. Mad2, an anaphase-promoting complex/cyclosome-Cdc20 (APC/C(Cdc20)) inhibitor, has an additional homolog in mammals known as Mad2B, Mad2L2 or Rev7. Apart from its role in Polζ-mediated translesion DNA synthesis and double-strand break repair, Rev7 is also believed to inhibit APC/C by negatively regulating Cdh1. Here we report yet another function of Rev7 in cultured human cells. Rev7, as predicted earlier, is involved in the formation of a functional spindle and maintenance of chromosome segregation. In the absence of Rev7, cells tend to arrest in G2/M-phase and display increased monoastral and abnormal spindles with misaligned chromosomes. Furthermore, Rev7-depleted cells show Mad2 localization at the kinetochores of metaphase cells, an indicator of activated SAC, coupled with increased levels of Cyclin B1, an APC(Cdc20) substrate. Surprisingly unlike Mad2, depletion of Rev7 in several cultured human cell lines did not compromise SAC activity. Our data therefore suggest that besides its role in APC/C(Cdh1) inhibition, Rev7 is also required for mitotic spindle organization and faithful chromosome segregation most probably through its physical interaction with RAN.

Genome-wide association study revealed genomic regions related to white/red earlobe color trait in the Rhode Island Red chickens

Background

Earlobe color is a naturally and artificially selected trait in chicken. As a head furnishing trait, it has been selected as a breed characteristic. Research has demonstrated that white/red earlobe color was related to at least three loci and sex-linked. However, there has been little work to date to identify the specific genomic regions and genes response to earlobe color in Rhode Island Red chickens. Currently, it is possible to identify the genomic regions responsible for white/red earlobe in Rhode Island Red chicken to eliminate this gap in knowledge by using genome-wide association (GWA) analysis.

Results

In the present study, genome-wide association (GWA) analysis was conducted to explore the candidate genomic regions response to chicken earlobe color phenotype. Hens with red dominant and white dominant earlobe was used for case-control analysis by Illumina 600 K SNP arrays. The GWA results showed that 2.38 Mb genomic region (50.13 to 52.51 Mb) with 282 SNPs on chromosome Z were significantly correlated to earlobe color, including sixteen known genes and seven anonymous genes. The sixteen genes were PAM, SLCO4C1, ST8SIA4, FAM174A, CHD1, RGMB, RIOK2, LIX1, LNPEP, SHB, RNF38, TRIM14, NANS, CLTA, GNE, and CPLX1.

Conclusions

The study has revealed the white/red earlobe trait is polygenic and sex-linked in Rhode Island Red chickens. In the genome significant ~2.38 Mb region, twenty-three genes were found and some of them could play critical roles in the formation of white/red earlobe color, especially gene SLCO4C1. Taken together, the candidate genes findings herein can help elucidate the genomic architecture of response to white/red earlobe and provide a new insight on mechanisms underlying earlobe color in Rhode Island Red chickens and other breeds.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0422-1) contains supplementary material, which is available to authorized users.

MAD2B-mediated SnoN downregulation is implicated in fibroblast activation and tubulointerstitial fibrosis

MAD2B, an anaphase-promoting complex/cyclosome (APC/C) inhibitor and a small subunit of DNA polymerase ζ, is indispensible for mitotic checkpoint control and DNA repair. Previously, we established that MAD2B is expressed in glomerular and tubulointerstitial compartments and participates in high glucose-induced podocyte injury. However, its role in other renal diseases remains elusive. In the present study, we aim to illustrate the potential role of MAD2B in the pathogenesis of renal fibrosis. By immunofluorescence and Western blotting, we found MAD2B expression is obviously increased in tubulointerstitial fibrosis (TIF) patients and unilateral ureteral obstruction (UUO) mice. It is widely accepted that resident fibroblasts are the major source of collagen-producing myofibroblasts during TIF. Therefore, we evaluated the level of MAD2B in fibroblasts (NRK-49F) exposed to transforming growth factor (TGF)-β1 by immunoblotting and revealed that MAD2B is upregulated in a time-dependent manner. Intriguingly, SnoN, a transcriptional repressor of the TGF-β1/Smad signaling pathway, is decreased in TGF-β1-treated fibroblasts as well as the kidney cortex from TIF patients and UUO mice. Either in vitro or in vivo, local genetic depletion of MAD2B by lentiviral transfection could preserve SnoN abundance and suppress Smad3 phosphorylation, which finally dampens fibroblast activation, ECM accumulation, and alleviates the severity of TIF. However, the ubiquitin ligase APC/C is not involved in the MAD2B-mediated SnoN decline, although this process is ubiquitination dependent. In conclusion, our observation proposes that besides cell cycle management, MAD2B has a profibrotic role during fibroblast activation and TIF by suppressing SnoN expression. Targeting the MAD2B-SnoN pathway is a promising intervention for TIF.

The multifaceted roles of the HORMA domain in cellular signaling

The HORMA domain is a multifunctional protein–protein interaction module found in diverse eukaryotic signaling pathways including the spindle assembly checkpoint, numerous DNA recombination/repair pathways, and the initiation of autophagy. In all of these pathways, HORMA domain proteins occupy key signaling junctures and function through the controlled assembly and disassembly of signaling complexes using a stereotypical “safety belt” peptide interaction mechanism. A recent explosion of structural and functional work has shed new light on these proteins, illustrating how strikingly similar structural mechanisms give rise to radically different functional outcomes in each family of HORMA domain proteins.

MiCroKiTS 4.0: a database of midbody, centrosome, kinetochore, telomere and spindle

We reported an updated database of MiCroKiTS 4.0 (http://microkit.biocuckoo.org) for proteins temporally and spatially localized in distinct subcellular positions including midbody, centrosome, kinetochore, telomere and mitotic spindle during cell division/mitosis. The database was updated from our previously developed database of MiCroKit 3.0, which contained 1489 proteins mostly forming super-complexes at midbody, centrosome and kinetochore from seven eukaryotes. Since the telomere and spindle apparatus are critical for cell division, the proteins localized at the two positions were also integrated. From the scientific literature, we curated 1872 experimentally identified proteins which at least locate in one of the five positions from eight species. Then the ortholog detection was performed to identify potential MiCroKiTS proteins from 144 eukaryotic organisms, which contains 66, 45 and 33 species of animals, fungi and plants, respectively. In total, 87 983 unique proteins with corresponding localization information were integrated into the database. The primary references of experimentally identified localizations were provided and the fluorescence microscope figures for the localizations of human proteins were shown. The orthologous relations between predicted and experimental localizations were also present. Taken together, we anticipate the database can serve as a useful resource for further analyzing the molecular mechanisms during cell division.

Molecular genetics and cellular features of TFE3 and TFEB fusion kidney cancers

Despite nearly two decades passing since the discovery of gene fusions involving TFE3 or TFEB in sporadic renal cell carcinoma (RCC), the molecular mechanisms underlying the renal-specific tumorigenesis of these genes remain largely unclear. The recently published findings of The Cancer Genome Atlas Network reported that five of the 416 surveyed clear cell RCC tumours (1.2%) harboured SFPQ-TFE3 fusions, providing further evidence for the importance of gene fusions. A total of five TFE3 gene fusions (PRCC-TFE3, ASPSCR1-TFE3, SFPQ-TFE3, NONO-TFE3, and CLTC-TFE3) and one TFEB gene fusion (MALAT1-TFEB) have been identified in RCC tumours and characterized at the mRNA transcript level. A multitude of molecular pathways well-described in carcinogenesis are regulated in part by TFE3 or TFEB proteins, including activation of TGFβ and ETS transcription factors, E-cadherin expression, CD40L-dependent lymphocyte activation, mTORC1 signalling, insulin-dependent metabolism regulation, folliculin signalling, and retinoblastoma-dependent cell cycle arrest. Determining which pathways are most important to RCC oncogenesis will be critical in discovering the most promising therapeutic targets for this disease.

Sequestration of CDH1 by MAD2L2 prevents premature APC/C activation prior to anaphase onset

MAD2L2 is rapidly degraded by APC/C^CDC20 at the onset of anaphase, allowing release of sequestered CDH1 to activate the dephosphorylated APC/C.

The switch from activation of the anaphase-promoting complex/cyclosome (APC/C) by CDC20 to CDH1 during anaphase is crucial for accurate mitosis. APC/C^CDC20 ubiquitinates a limited set of substrates for subsequent degradation, including Cyclin B1 and Securin, whereas APC/C^CDH1 has a broader specificity. This switch depends on dephosphorylation of CDH1 and the APC/C, and on the degradation of CDC20. Here we show, in human cells, that the APC/C inhibitor MAD2L2 also contributes to ensuring the sequential activation of the APC/C by CDC20 and CDH1. In prometaphase, MAD2L2 sequestered free CDH1 away from the APC/C. At the onset of anaphase, MAD2L2 was rapidly degraded by APC/C^CDC20, releasing CDH1 to activate the dephosphorylated APC/C. Loss of MAD2L2 led to premature association of CDH1 with the APC/C, early destruction of APC/C^CDH1 substrates, and accelerated mitosis with frequent mitotic aberrations. Thus, MAD2L2 helps to ensure a robustly bistable switch between APC/C^CDC20 and APC/C^CDH1 during the metaphase-to-anaphase transition, thereby contributing to mitotic fidelity.

Clathrin heavy chain 1 is required for spindle assembly and chromosome congression in mouse oocytes

Clathrin heavy chain 1 (CLTC) has been considered a “moonlighting protein” which acts in membrane trafficking during interphase and in stabilizing spindle fibers during mitosis. However, its roles in meiosis, especially in mammalian oocyte maturation, remain unclear. This study investigated CLTC expression and function in spindle formation and chromosome congression during mouse oocyte meiotic maturation. Our results showed that the expression level of CLTC increased after germinal vesicle breakdown (GVBD) and peaked in the M phase. Immunostaining results showed CLTC distribution throughout the cytoplasm in a cell cycle-dependent manner. Appearance and disappearance of CLTC along with β-tubulin (TUBB) could be observed during spindle dynamic changes. To explore the relationship between CLTC and microtubule dynamics, oocytes at metaphase were treated with taxol or nocodazole. CLTC colocalized with TUBB at the enlarged spindle and with cytoplasmic asters after taxol treatment; it disassembled and distributed into the cytoplasm along with TUBB after nocodazole treatment. Disruption of CLTC function using stealth siRNA caused a decreased first polar body extrusion rate and extensive spindle formation and chromosome congression defects. Taken together, these results show that CLTC plays an important role in spindle assembly and chromosome congression through a microtubule correlation mechanism during mouse oocyte maturation.

A critical function of Mad2l2 in primordial germ cell development of mice

The development of primordial germ cells (PGCs) involves several waves of epigenetic reprogramming. A major step is following specification and involves the transition from the stably suppressive histone modification H3K9me2 to the more flexible, still repressive H3K27me3, while PGCs are arrested in G2 phase of their cycle. The significance and underlying molecular mechanism of this transition were so far unknown. Here, we generated mutant mice for the Mad2l2 (Mad2B, Rev7) gene product, and found that they are infertile in both males and females. We demonstrated that Mad2l2 is essential for PGC, but not somatic development. PGCs were specified normally in Mad2l2^−/− embryos, but became eliminated by apoptosis during the subsequent phase of epigenetic reprogramming. A majority of knockout PGCs failed to arrest in the G2 phase, and did not switch from a H3K9me2 to a H3K27me3 configuration. By the analysis of transfected fibroblasts we found that the interaction of Mad2l2 with the histone methyltransferases G9a and GLP lead to a downregulation of H3K9me2. The inhibitory binding of Mad2l2 to Cyclin dependent kinase 1 (Cdk1) could arrest the cell cycle in the G2 phase, and also allowed another histone methyltransferase, Ezh2, to upregulate H3K27me3. Together, these results demonstrate the potential of Mad2l2 in the regulation of both cell cycle and the epigenetic status. The function of Mad2l2 is essential in PGCs, and thus of high relevance for fertility.

Primordial germ cells (PGCs) are the origin of sperm and oocytes, and are responsible for transferring genetic information to the next generation faithfully. PGCs are first specified from pluripotent epiblast cells early in embryonic development. Second, they reprogram their epigenetic signature by changing histone modifications. This developmental event is specific to germ cells but not somatic cells. Although many players in the specification of PGCs are identified, only little is known about the genes essential for the regulation of the second phase. Here, we report that the Mad2l2 gene product plays an important role in the epigenetic reprogramming of PGCs. In wild type PGCs the cell cycle is arrested, and the methylation of histone 3 on residue K9 is replaced by methylation on K27. Our findings indicate that Mad2l2 is involved in this coordination of cell cycle and epigenetic reprogramming. The elucidation of this mechanism would help to identify the genetic basis of infertility.

Genomic imbalances in patients with a clinical presentation in the spectrum of Cornelia de Lange syndrome

Background

Cornelia de Lange syndrome (CdLS) is a rare autosomal-dominant disorder characterised by facial dysmorphism, growth and psychomotor developmental delay and skeletal defects. To date, causative mutations in the NIPBL (cohesin regulator) and SMC1A (cohesin structural subunit) genes account for > 50% and 6% of cases, respectively.

Methods

We recruited 50 patients with a CdLS clinical diagnosis or with features that overlap with CdLS, who were negative for mutations at NIPBL and SMC1A at molecular screening. Chromosomal rearrangements accounting for the clinical diagnosis were screened for using array Comparative Genomic Hybridisation (aCGH).

Results

Four patients were shown to carry imbalances considered to be candidates for having pathogenic roles in their clinical phenotypes: patient 1 had a 4.2 Mb de novo deletion at chromosome 20q11.2-q12; patient 2 had a 4.8 Mb deletion at chromosome 1p36.23-36.22; patient 3 carried an unbalanced translocation, t(7;17), with a 14 Mb duplication of chromosome 17q24.2-25.3 and a 769 Kb deletion at chromosome 7p22.3; patient 4 had an 880 Kb duplication of chromosome 19p13.3, for which his mother, who had a mild phenotype, was also shown to be a mosaic.

Conclusions

Notwithstanding the variability in size and gene content of the rearrangements comprising the four different imbalances, they all map to regions containing genes encoding factors involved in cell cycle progression or genome stability. These functional similarities, also exhibited by the known CdLS genes, may explain the phenotypic overlap between the patients included in this study and CdLS. Our findings point to the complexity of the clinical diagnosis of CdLS and confirm the existence of phenocopies, caused by imbalances affecting multiple genomic regions, comprising 8% of patients included in this study, who did not have mutations at NIPBL and SMC1A. Our results suggests that analysis by aCGH should be recommended for CdLS spectrum cases with an unexplained clinical phenotype and included in the flow chart for diagnosis of cases with a clinical evaluation in the CdLS spectrum.

Rev3, the catalytic subunit of Polζ, is required for maintaining fragile site stability in human cells

It has been long speculated that mammalian Rev3 plays an important, yet unknown role(s) during mammalian development, as deletion of Rev3 causes embryonic lethality in mice, whereas no other translesion DNA synthesis polymerases studied to date are required for mouse embryo development. Here, we report that both subunits of Polζ (Rev3 and Rev7) show an unexpected increase in expression during G₂/M phase, but they localize independently in mitotic cells. Experimental depletion of Rev3 results in a significant increase in anaphase bridges, chromosomal breaks/gaps and common fragile site (CFS) expression, whereas Rev7 depletion primarily causes lagging chromosome defect with no sign of CFS expression. The genomic instability induced by Rev3 depletion seems to be related to replication stress, as it is further enhanced on aphidicolin treatment and results in increased metaphase-specific Fanconi anemia complementation group D type 2 (FANCD2) foci formation, as well as FANCD2-positive anaphase bridges. Indeed, a long-term depletion of Rev3 in cultured human cells results in massive genomic instability and severe cell cycle arrest. The aforementioned observations collectively support a notion that Rev3 is required for the efficient replication of CFSs during G₂/M phase, and that the resulting fragile site instability in Rev3 knockout mice may trigger cell death during embryonic development.

Novel functions of endocytic player clathrin in mitosis

Clathrin has been widely recognized as a pivotal player in endocytosis, in which several adaptors and accessory proteins are involved. Recent studies suggested that clathrin is also essential for cell division. Here this review mainly focuses on the clathrin-dependent mechanisms involved in spindle assembly and chromosome alignment. In mitosis, clathrin forms a complex with phosphorylated TACC3 to ensure spindle stability and proper chromosome alignment. The clathrin-regulated mechanism in mitosis requires the crosstalk among clathrin, spindle assembly factors (SAFs), Ran-GTP and mitotic kinases. Meanwhile, a coordinated mechanism is required for role transitions of clathrin during endocytosis and mitosis. Taken together, the findings of the multiple functions of clathrin besides endocytosis have expanded our understanding of the basic cellular activities.