Cyclin-Specific Docking Mechanisms Reveal the Complexity of M-CDK Function in the Cell Cycle.

Cyclin-dependent kinases (CDKs) coordinate hundreds of molecular events during the cell cycle. Multiple cyclins are involved, but the global role of cyclin-specific phosphorylation has remained unsolved. We uncovered a cyclin docking motif, LxF, that mediates binding of replication factor Cdc6 to mitotic cyclin. This interaction leads to phospho-adaptor Cks1-mediated inhibition of M-CDK to facilitate Cdc6 accumulation and sequestration in mitosis. The LxF motif and Cks1 also mediate the mutual inhibition between M-CDK and the tyrosine kinase Swe1. Additionally, the LxF motif is critical for targeting M-CDK to phosphorylate several mitotic regulators; for example, Spo12 is targeted via LxF to release the phosphatase Cdc14. The results complete the full set of G1, S, and M-CDK docking mechanisms and outline the unified role of cyclin specificity and CDK activity thresholds. Cooperation of cyclin and Cks1 docking creates a variety of CDK thresholds and switching orders, including combinations of last in, first out (LIFO) and first in, first out (FIFO) ordering.

LINE-1 expression in cancer correlates with p53 mutation, copy number alteration, and S phase checkpoint.

Retrotransposons are genomic DNA sequences that copy themselves to new genomic locations via RNA intermediates; LINE-1 is the only active and autonomous retrotransposon in the human genome. The mobility of LINE-1 is largely repressed in somatic tissues but is derepressed in many cancers, where LINE-1 retrotransposition is correlated with p53 mutation and copy number alteration (CNA). In cell lines, inducing LINE-1 expression can cause double-strand breaks (DSBs) and replication stress. Reanalyzing multiomic data from breast, ovarian, endometrial, and colon cancers, we confirmed correlations between LINE-1 expression, p53 mutation status, and CNA. We observed a consistent correlation between LINE-1 expression and the abundance of DNA replication complex components, indicating that LINE-1 may also induce replication stress in human tumors. In endometrial cancer, high-quality phosphoproteomic data allowed us to identify the DSB-induced ATM-MRN-SMC S phase checkpoint pathway as the primary DNA damage response (DDR) pathway associated with LINE-1 expression. Induction of LINE-1 expression in an in vitro model led to increased phosphorylation of MRN complex member RAD50, suggesting that LINE-1 directly activates this pathway.

Histone Variant H2A.Z.2 Mediates Proliferation and Drug Sensitivity of Malignant Melanoma.

Histone variants are emerging as key regulatory molecules in cancer. We report a unique role for the H2A.Z isoform H2A.Z.2 as a driver of malignant melanoma. H2A.Z.2 is highly expressed in metastatic melanoma, correlates with decreased patient survival, and is required for cellular proliferation. Our integrated genomic analyses reveal that H2A.Z.2 controls the transcriptional output of E2F target genes in melanoma cells. These genes are highly expressed and display a distinct signature of H2A.Z occupancy. We identify BRD2 as an H2A.Z-interacting protein, levels of which are also elevated in melanoma. We further demonstrate that H2A.Z.2-regulated genes are bound by BRD2 and E2F1 in an H2A.Z.2-dependent manner. Importantly, H2A.Z.2 deficiency sensitizes melanoma cells to chemotherapy and targeted therapies. Collectively, our findings implicate H2A.Z.2 as a mediator of cell proliferation and drug sensitivity in malignant melanoma, holding translational potential for novel therapeutic strategies.

Yeast Stn1 promotes MCM to circumvent Rad53 control of the S phase checkpoint.

Treating yeast cells with the replication inhibitor hydroxyurea activates the S phase checkpoint kinase Rad53, eliciting responses that block DNA replication origin firing, stabilize replication forks, and prevent premature extension of the mitotic spindle. We previously found overproduction of Stn1, a subunit of the telomere-binding Cdc13-Stn1-Ten1 complex, circumvents Rad53 checkpoint functions in hydroxyurea, inducing late origin firing and premature spindle extension even though Rad53 is activated normally. Here, we show Stn1 overproduction acts through remarkably similar pathways compared to loss of RAD53, converging on the MCM complex that initiates origin firing and forms the catalytic core of the replicative DNA helicase. First, mutations affecting Mcm2 and Mcm5 block the ability of Stn1 overproduction to disrupt the S phase checkpoint. Second, loss of function stn1 mutations compensate rad53 S phase checkpoint defects. Third Stn1 overproduction suppresses a mutation in Mcm7. Fourth, stn1 mutants accumulate single-stranded DNA at non-telomeric genome locations, imposing a requirement for post-replication DNA repair. We discuss these interactions in terms of a model in which Stn1 acts as an accessory replication factor that facilitates MCM activation at ORIs and potentially also maintains MCM activity at replication forks advancing through challenging templates.

Coronavirus Porcine Epidemic Diarrhea Virus Nucleocapsid Protein Interacts with p53 To Induce Cell Cycle Arrest in S-Phase and Promotes Viral Replication.

Subversion of the host cell cycle to facilitate viral replication is a common feature of coronavirus infections. Coronavirus nucleocapsid (N) protein can modulate the host cell cycle, but the mechanistic details remain largely unknown. Here, we investigated the effects of manipulation of porcine epidemic diarrhea virus (PEDV) N protein on the cell cycle and the influence on viral replication. Results indicated that PEDV N induced Vero E6 cell cycle arrest at S-phase, which promoted viral replication (P < 0.05). S-phase arrest was dependent on the N protein nuclear localization signal S71NWHFYYLGTGPHADLRYRT90 and the interaction between N protein and p53. In the nucleus, the binding of N protein to p53 maintained consistently high-level expression of p53, which activated the p53-DREAM pathway. The key domain of the N protein interacting with p53 was revealed to be S171RGNSQNRGNNQGRGASQNRGGNN194 (NS171-N194), in which G183RG185 are core residues. NS171-N194 and G183RG185 were essential for N-induced S-phase arrest. Moreover, small molecular drugs targeting the NS171-N194 domain of the PEDV N protein were screened through molecular docking. Hyperoside could antagonize N protein-induced S-phase arrest by interfering with interaction between N protein and p53 and inhibit viral replication (P < 0.05). The above-described experiments were also validated in porcine intestinal cells, and data were in line with results in Vero E6 cells. Therefore, these results reveal the PEDV N protein interacts with p53 to activate the p53-DREAM pathway, and subsequently induces S-phase arrest to create a favorable environment for virus replication. These findings provide new insight into the PEDV-host interaction and the design of novel antiviral strategies against PEDV. IMPORTANCE Many viruses subvert the host cell cycle to create a cellular environment that promotes viral growth. PEDV, an emerging and reemerging coronavirus, has led to substantial economic loss in the global swine industry. Our study is the first to demonstrate that PEDV N-induced cell cycle arrest during the S-phase promotes viral replication. We identified a novel mechanism of PEDV N-induced S-phase arrest, where the binding of PEDV N protein to p53 maintains consistently high levels of p53 expression in the nucleus to mediate S-phase arrest by activating the p53-DREAM pathway. Furthermore, a small molecular compound, hyperoside, targeted the PEDV N protein, interfering with the interaction between the N protein and p53 and, importantly, inhibited PEDV replication by antagonizing cell cycle arrest. This study reveals a new mechanism of PEDV-host interaction and also provides a novel antiviral strategy for PEDV. These data provide a foundation for further research into coronavirus-host interactions.

Exo1 phosphorylation inhibits exonuclease activity and prevents fork collapse in rad53 mutants independently of the 14-3-3 proteins.

The S phase checkpoint is crucial to maintain genome stability under conditions that threaten DNA replication. One of its critical functions is to prevent Exo1-dependent fork degradation, and Exo1 is phosphorylated in response to different genotoxic agents. Exo1 seemed to be regulated by several post-translational modifications in the presence of replicative stress, but the specific contribution of checkpoint-dependent phosphorylation to Exo1 control and fork stability is not clear. We show here that Exo1 phosphorylation is Dun1-independent and Rad53-dependent in response to DNA damage or dNTP depletion, and in both situations Exo1 is similarly phosphorylated at multiple sites. To investigate the correlation between Exo1 phosphorylation and fork stability, we have generated phospho-mimic exo1 alleles that rescue fork collapse in rad53 mutants as efficiently as exo1-nuclease dead mutants or the absence of Exo1, arguing that Rad53-dependent phosphorylation is the mayor requirement to preserve fork stability. We have also shown that this rescue is Bmh1-2 independent, arguing that the 14-3-3 proteins are dispensable for fork stabilization, at least when Exo1 is downregulated. Importantly, our results indicated that phosphorylation specifically inhibits the 5' to 3'exo-nuclease activity, suggesting that this activity of Exo1 and not the flap-endonuclease, is the enzymatic activity responsible of the collapse of stalled replication forks in checkpoint mutants.

A novel system of bacterial cell division arrest implicated in horizontal transmission of an integrative and conjugative element.

Integrative and conjugative elements (ICEs) are widespread mobile DNA elements in the prokaryotic world. ICEs are usually retained within the bacterial chromosome, but can be excised and transferred from a donor to a new recipient cell, even of another species. Horizontal transmission of ICEclc, a prevalent ICE in proteobacteria, only occurs from developed specialized transfer competent (tc) cells in the donor population. tc cells become entirely dedicated to the ICE transmission at the cost of cell proliferation. The cell growth impairment is mediated by two ICEclc located genes, parA and shi, but the mechanistic and dynamic details of this process are unknown. To better understand the function of ParA and Shi, we followed their intracellular behavior from fluorescent protein fusions, and studied host cell division at single-cell level. Superresolution imaging revealed that ParA-mCherry colocalized with the host nucleoid while Shi-GFP was enriched at the membrane during the growth impairment. Despite being enriched at different cellular locations, the two proteins showed in vivo interactions, and mutations in the Walker A motif of ParA dislocalized both ParA and Shi. In addition, ParA mutations in the ATPase motif abolished the growth arrest on the host cell. Time-lapse microscopy revealed that ParA and Shi initially delay cell division, suggesting an extension of the S phase of cells, but eventually completely inhibit cell elongation. The parA-shi locus is highly conserved in other ICEclc-related elements, and expressing ParA-Shi from ICEclc in other proteobacterial species caused similar growth arrest, suggesting that the system functions similarly across hosts. The results of our study provide mechanistic insight into the novel and unique system on ICEs and help to understand such epistatic interaction between ICE genes and host physiology that entails efficient horizontal gene transfer.

Ddc2ATRIP promotes Mec1ATR activation at RPA-ssDNA tracts.

The DNA damage checkpoint response is controlled by the phosphatidylinositol 3-kinase-related kinases (PIKK), including ataxia telangiectasia-mutated (ATM) and ATM and Rad3-related (ATR). ATR forms a complex with its partner ATRIP. In budding yeast, ATR and ATRIP correspond to Mec1 and Ddc2, respectively. ATRIP/Ddc2 interacts with replication protein A-bound single-stranded DNA (RPA-ssDNA) and recruits ATR/Mec1 to sites of DNA damage. Mec1 is stimulated by the canonical activators including Ddc1, Dpb11 and Dna2. We have characterized the ddc2-S4 mutation and shown that Ddc2 not only recruits Mec1 to sites of DNA damage but also stimulates Mec1 kinase activity. However, the underlying mechanism of Ddc2-dependent Mec1 activation remains to be elucidated. Here we show that Ddc2 promotes Mec1 activation independently of Ddc1/Dpb11/Dna2 function in vivo and through ssDNA recognition in vitro. The ddc2-S4 mutation diminishes damage-induced phosphorylation of the checkpoint mediators, Rad9 and Mrc1. Rad9 controls checkpoint throughout the cell-cycle whereas Mrc1 is specifically required for the S-phase checkpoint. Notably, S-phase checkpoint signaling is more defective in ddc2-S4 mutants than in cells where the Mec1 activators (Ddc1/Dpb11 and Dna2) are dysfunctional. To understand a role of Ddc2 in Mec1 activation, we reconstituted an in vitro assay using purified Mec1-Ddc2 complex, RPA and ssDNA. Whereas ssDNA stimulates kinase activity of the Mec1-Ddc2 complex, RPA does not. However, RPA can promote ssDNA-dependent Mec1 activation. Neither ssDNA nor RPA-ssDNA efficiently stimulates the Mec1-Ddc2 complex containing Ddc2-S4 mutant. Together, our data support a model in which Ddc2 promotes Mec1 activation at RPA-ssDNA tracts.

Mck1 defines a key S-phase checkpoint effector in response to various degrees of replication threats.

The S-phase checkpoint plays an essential role in regulation of the ribonucleotide reductase (RNR) activity to maintain the dNTP pools. How eukaryotic cells respond appropriately to different levels of replication threats remains elusive. Here, we have identified that a conserved GSK-3 kinase Mck1 cooperates with Dun1 in regulating this process. Deleting MCK1 sensitizes dun1Δ to hydroxyurea (HU) reminiscent of mec1Δ or rad53Δ. While Mck1 is downstream of Rad53, it does not participate in the post-translational regulation of RNR as Dun1 does. Mck1 phosphorylates and releases the Crt1 repressor from the promoters of DNA damage-inducible genes as RNR2-4 and HUG1. Hug1, an Rnr2 inhibitor normally silenced, is induced as a counterweight to excessive RNR. When cells suffer a more severe threat, Mck1 inhibits HUG1 transcription. Consistently, only a combined deletion of HUG1 and CRT1, confers a dramatic boost of dNTP levels and the survival of mck1Δdun1Δ or mec1Δ cells assaulted by a lethal dose of HU. These findings reveal the division-of-labor between Mck1 and Dun1 at the S-phase checkpoint pathway to fine-tune dNTP homeostasis.

The Nup84 complex coordinates the DNA damage response to warrant genome integrity.

DNA lesions interfere with cellular processes such as transcription and replication and need to be adequately resolved to warrant genome integrity. Beyond their primary role in molecule transport, nuclear pore complexes (NPCs) function in other processes such as transcription, nuclear organization and DNA double strand break (DSB) repair. Here we found that the removal of UV-induced DNA lesions by nucleotide excision repair (NER) is compromised in the absence of the Nup84 nuclear pore component. Importantly, nup84Δ cells show an exacerbated sensitivity to UV in early S phase and delayed replication fork progression, suggesting that unrepaired spontaneous DNA lesions persist during S phase. In addition, nup84Δ cells are defective in the repair of replication-born DSBs by sister chromatid recombination (SCR) and rely on post-replicative repair functions for normal proliferation, indicating dysfunctions in the cellular pathways that enable replication on damaged DNA templates. Altogether, our data reveal a central role of the NPC in the DNA damage response to facilitate replication progression through damaged DNA templates by promoting efficient NER and SCR and preventing chromosomal rearrangements.

O6-methylguanine-induced transcriptional mutagenesis reduces p53 tumor-suppressor function.

Altered protein function due to mutagenesis plays an important role in disease development. This is perhaps most evident in tumorigenesis and the associated loss or gain of function of tumor-suppressor genes and oncogenes. The extent to which lesion-induced transcriptional mutagenesis (TM) influences protein function and its contribution to the development of disease is not well understood. In this study, the impact of O6-methylguanine on the transcription fidelity of p53 and the subsequent effects on the protein's function as a regulator of cell death and cell-cycle arrest were examined in human cells. Levels of TM were determined by RNA-sequencing. In cells with active DNA repair, misincorporation of uridine opposite the lesion occurred in 0.14% of the transcripts and increased to 14.7% when repair by alkylguanine-DNA alkyltransferase was compromised. Expression of the dominant-negative p53 R248W mutant due to TM significantly reduced the transactivation of several established p53 target genes that mediate the tumor-suppressor function, including CDKN1A (p21) and BBC3 (PUMA). This resulted in deregulated signaling through the retinoblastoma protein and loss of G1/S cell-cycle checkpoint function. In addition, we observed impaired activation of apoptosis coupled to the reduction of the tumor-suppressor functions of p53. Taking these findings together, this work provides evidence that TM can induce phenotypic changes in mammalian cells that have important implications for the role of TM in tumorigenesis.

Cyclin Y binds and activates CDK4 to promote the G1/S phase transition in hepatocellular carcinoma cells via Rb signaling.

Inactivation of Rb is a major event in the development of hepatocellular carcinoma (HCC). The activity of CDK4, determined by T172 phosphorylation, correlates with the onset of RB phosphorylation and G1/S cell cycle transition. However, the regulation of CDK4 activation and of the Rb pathway in HCC remain unclear. Here, we report that cyclin Y, a novel member of the cyclin family, is a potential regulator of the Rb pathway. We demonstrate that the Cyclin Y protein was overexpressed in human HCC tissues and that it was associated with poor patient prognosis. Cyclin Y could regulate the G1/S phase transition in human HCC cell lines. We found that CDK4 can bind to Cyclin Y in vitro. Furthermore, the accumulation of Cyclin Y could activate CDK4 through T172 phosphorylation of CDK4, inactivate Rb with increasing Rb phosphorylation, and enable the expression of E2F target genes such as CDK2 and Cyclin A. Thus, our findings suggest that Cyclin Y plays a role in the G1/S phase transition of HCC cells via Cyclin Y/CDK4/Rb signaling and that Cyclin Y could be used as a potential prognostic biomarker in HCC.

Safe and Efficacious Diphtheria Toxin-Based Treatment for Melanoma: Combination of a Light-On Gene-Expression System and Nanotechnology.

The controversy surrounding the use of diphtheria toxin (DT) as a therapeutic agent against tumor cells arises mainly from its unexpected harmfulness to healthy tissues. We encoded the cytotoxic fragment A of DT (DTA) as an objective gene in the Light-On gene-expression system to construct plasmids pGAVPO (pG) and pU5-DTA (pDTA). Meanwhile, a cRGD-modified ternary complex comprising plasmids, chitosan, and liposome (pG&pDTA@cRGD-CL) was prepared as a nanocarrier to ensure transfection efficiency. Benefiting from spatiotemporal control of this light-switchable transgene system and the superior tumor targeting of the carrier, toxins were designed to be expressed selectively in illuminated lesions. In vitro studies suggested that pG&pDTA@cRGD-CL exerted arrest of the S phase in B16F10 cells upon blue light irradiation and, ultimately, induced the apoptosis and necrosis of tumor cells. Such DTA-based treatment exerted enhanced antitumor activity in mice bearing B16F10 xenografts and displayed prolonged survival time with minimal side effects. Hence, we described novel DTA-based therapy combined with nanotechnology and the Light-On gene-expression system: such treatment could be a promising strategy against melanoma.

miR-144-3p inhibits cell proliferation of colorectal cancer cells by targeting BCL6 via inhibition of Wnt/ß-catenin signaling.

BACKGROUND: It has been shown that miR-144-3p regulates cell proliferation, apoptosis, migration and invasion in various cancers. However, the function and expression of miR-144-3p in colorectal cancer (CRC) remained obscure. METHODS: Immunohistochemical (IHC) staining was performed to investigate the protein expression of BCL6 in CRC tissues. The effect of BCL6 and miR-144-3p on CRC cells was explored through methylthiazolyl tetrazolium (MTT) assay, colony formation and cell cycle assays. Luciferase reporter assays, reverse transcription quantitative polymerase chain reaction (RT-qPCR) and Western blot assay were carried out to determine that BCL6 is directly regulated by miR-144-3p. RESULTS: Our results showed that miR-144-3p is down-regulated in CRC and correlates with the tumor progression of CRC patients. miR-144-3p inhibits cell proliferation and delays G1/S phase transition of CRC cells. Moreover, we found that BCL6 is a new target of miR-144-3p. Furthermore, BCL6 is a mediator of miR-144-3p repression of cell proliferation and cell cycle arrest in CRC cells. miR-144-3p repression of Wnt/ß-catenin signaling is mediated by BCL6 in CRC cells. CONCLUSIONS: Overall, the effect of the miR-144-3p/BCL6 axis on regulating CRC carcinogenesis was demonstrated, and miR-144-3p was identified as a potential prognostic and therapeutic target in colorectal cancer.

BAF45D knockdown decreases cell viability, inhibits colony formation, induces cell apoptosis and S-phase arrest in human pancreatic cancer cells.

Pancreatic cancer, an extremely aggressive malignancy, is resistant to chemo- or radiotherapy. The rapid progression of pancreatic cancer without distinctive clinical sign makes early diagnosing and/or treating very difficult. BAF45D, a member of the d4 domain family, is involved in oncogenic processes. However, the role of BAF45D in pancreatic tumorigenesis is largely unclear. Our goal is to examine BAF45D protein expression after lentivirus-mediated Baf45d RNAi and explore the effects of BAF45D knockdown on cell proliferation, cell apoptosis, and cell cycle of human pancreatic cancer cells. Here our results showed that Baf45d RNAi downregulated BAF45D protein levels and decreased cell viability, increased cell apoptosis, and decreased colony formation in BxPC-3 cells. Moreover, BAF45D knockdown induced S-phase arrest in BxPC-3 cells. Our results here suggest that BAF45D may play a crucial role in tumorigenic properties of human pancreatic cancer cells.

Three new cases of ataxia-telangiectasia-like disorder: No impairment of the ATM pathway, but S-phase checkpoint defect.

Ataxia-telangiectasia-like disorder (ATLD) is a rare genomic instability syndrome caused by biallelic variants of MRE11 (meiotic recombination 11) characterized by progressive cerebellar ataxia and typical karyotype abnormalities. These symptoms are common to those of ataxia-telangiectasia, which is consistent with the key role of MRE11 in ataxia-telangiectasia mutated (ATM) activation after DNA double-strand breaks. Three unrelated French patients were referred with ataxia. Only one had typical karyotype abnormalities. Unreported biallelic MRE11 variants were found in these three cases. Interestingly, one variant (c.424G>A) was present in two cases and haplotype analysis strongly suggested a French founder variant. Variants c.544G>A and c.314+4_314+7del lead to splice defects. The level of MRE11 in lymphoblastoid cell lines was consistently and dramatically reduced. Functional consequences were evaluated on activation of the ATM pathway via phosphorylation of ATM targets (KAP1 and CHK2), but no consistent defect was observed. However, an S-phase checkpoint activation defect after camptothecin was observed in these patients with ATLD. In conclusion, we report the first three French ATLD patients and a French founder variant, and propose an S-phase checkpoint activation study to evaluate the pathogenicity of MRE11 variants.

microRNA-374 inhibits proliferation and promotes apoptosis of mouse melanoma cells by inactivating the Wnt signalling pathway through its effect on tyrosinase.

Melanoma is one of the most malignant skin tumours with constantly increasing incidence worldwide. Previous studies have demonstrated that microRNA-374 (miR-374) is a novel biomarker for cancer therapy. Therefore, this study explores whether miR-374 targeting tyrosinase (TYR) affects melanoma and its underlying mechanism. We constructed subcutaneous melanoma models to carry out the following experiments. The cells were transfected with a series of miR-374 mimics, miR-374 inhibitors or siRNA against TYR. Dual luciferase reporter gene assay was used for the verification of the targeting relationship between miR-374 and TYR. Reverse transcription quantitative polymerase chain reaction and western blot analysis were conducted to determine the expression of miR-374, TYR, ß-catenin, B-cell leukaemia 2 (Bcl-2), Bcl-2 associated X protein (Bax), Low-density lipoprotein receptor-related protein 6 (LRP6), Leucine-rich repeat G protein-coupled receptor 5 (LGR5) and CyclinD1. Cell proliferation, migration, invasion, cell cycle distribution and apoptosis were evaluated using cell counting kit-8 assay, scratch test, transwell assay and flow cytometry respectively. TYR was proved as a putative target of miR-374 as the evidenced by the result. It was observed that up-regulated miR-374 or down-regulated TYR increased expression of Bax and decreased expressions of TYR, ß-catenin, LRP6, Bcl-2, CyclinD1 and LGR5, along with diminished cell proliferation, migration, invasion and enhanced apoptosis. Meanwhile, cells with miR-374 inhibitors showed an opposite trend. These findings indicated that up-regulated miR-374 could inhibit the expression of TYR to suppress cell proliferation, migration, invasion and promote cell apoptosis in melanoma cells by inhibiting the Wnt signalling pathway.

DDR Inc., one business, two associates.

Eukaryotic cells activate cell cycle checkpoints in response to DNA damage. In Saccharomyces cerevisiae, the DNA damage response is achieved by the activation of the sensor kinases Mec1 and Tel1 and transmitted to the effector kinase Rad53. Rad9 and Mrc1 are thought to differentially mediate the activation of Rad53 depending on the cell cycle phase. Rad9 can respond to DNA lesions throughout the cell cycle, whereas Mrc1 responds to replication impediments in S phase. It was not clear if Rad9 and Mrc1 were triggering the same response to DNA damage occurring in S phase. By carefully studying the kinetics of activation of Rad53 by different types of replication stresses, we recently showed that Rad9 and Mrc1 cooperate in time and space to trigger a unique response to DNA damage in S phase. This primarily includes the control of both DNA replication initiation and elongation. After showing that Rad9 plays a preponderant role during S phase, the data presented here provocatively suggest that Mrc1 could also mediate the activation of Rad53 outside of S phase.

Retinoid signaling controls spermatogonial differentiation by regulating expression of replication-dependent core histone genes.

Retinoic acid (RA) signaling is crucial for spermatogonial differentiation, which is a key step for spermatogenesis. We explored the mechanisms underlying spermatogonial differentiation by targeting expression of a dominant-negative mutant of retinoic acid receptor α (RARα) specifically to the germ cells of transgenic mice to subvert the activity of endogenous receptors. Here we show that: (1) inhibition of retinoid signaling in germ cells completely blocked spermatogonial differentiation identical to vitamin A-deficient (VAD) mice; (2) the blockage of spermatogonial differentiation by impaired retinoid signaling resulted from an arrest of entry of the undifferentiated spermatogonia into S phase; and (3) retinoid signaling regulated spermatogonial differentiation through controlling expression of its direct target genes, including replication-dependent core histone genes. Taken together, our results demonstrate that the action of retinoid signaling on spermatogonial differentiation in vivo is direct through the spermatogonia itself, and provide the first evidence that this is mediated by regulation of expression of replication-dependent core histone genes.

Cap-independent translation ensures mTOR expression and function upon protein synthesis inhibition.

The mechanistic/mammalian target of rapamycin (mTOR) is a conserved serine/threonine kinase that integrates cellular signals from the nutrient and energy status to act, namely, on the protein synthesis machinery. While major advances have emerged regarding the regulators and effects of the mTOR signaling pathway, little is known about the regulation of mTOR gene expression. Here, we show that the human mTOR transcript can be translated in a cap-independent manner, and that its 5' untranslated region (UTR) is a highly folded RNA scaffold capable of binding directly to the 40S ribosomal subunit. We further demonstrate that mTOR is able to bypass the cap requirement for translation both in normal and hypoxic conditions. Moreover, our data reveal that the cap-independent translation of mTOR is necessary for its ability to induce cell-cycle progression into S phase. These results suggest a novel regulatory mechanism for mTOR gene expression that integrates the global protein synthesis changes induced by translational inhibitory conditions.