Biomedical subjects

Junjie Chen

Publications and source records attributed to Junjie Chen.

At least 19 recordsLinked to original sources

PubMed2026

Genome-wide identification and functional analysis of the BES1-like (VfBES1) gene family in Vernicia fordii reveals its role in floral development.

BACKGROUND: Vernicia fordii Hemsl (also known as Tung tree), an significant commercial oil-producing tree species, is a monoecious and diclinous species with male and female flowers on the same inflorescence; however, the molecular mechanisms governing its floral sex determination remain elusive, particularly the genetic basis underlying the skewed female-to-male flower ratio and the evolutionary dynamics of sex-related gene families, which severely restrict targeted breeding for yield enhancement. In the model plant Arabidopsis, the BRI1 EMS SUPPRESSOR 1 (BES1) transcription factor family plays a crucial role in Brassinosteroid (BR) signaling and reproductive development. However, its function remains largely unexplored in woody perennials. RESULTS: In this study, we introduce the genome-wide identification and functional characterization of the BES1-like (VfBES1) gene family in the Tung tree for the first time. Integrative multi-omics approaches reveal seven VfBES1 genes that are clustered into three phylogenetically distinct clades, each characterized by clade-specific motifs and structural simplicity. Segmental duplication events (VfBES1-1/VfBES1-5 and VfBES1-4/VfBES1-7) and promoter cis-element enrichment (hormone-responsive and abiotic stress-related motifs) highlight evolutionary innovation and functional diversification. Spatiotemporal expression profiling reveals VfBES1 genes' tissue- and stage-specific roles. VfBES1-1 predominantly expresses in female flowers and fruits, suggesting its possible roles in late-stage sex maintenance or ovule and fruit development. VfBES1-2 and VfBES1-6 exhibit male flower-specific and early floral developmental activation, respectively. Nuclear-localized VfBES1-6 displays co-expression with VfMYB35-1 gene, which is a regulator of male structure degeneration. CONCLUSIONS: Findings in this study shed light on the regulatory roles of VfBES1 genes in the floral development of the Tung tree, providing a reference for its precision breeding to enhance flowering synchrony and seed productivity. This study also provides a comparative framework for understanding the functional diversity of BES1-like genes in non-model woody plants.

Flowers↗

PubMed2007

Enhanced intra-switch region recombination during immunoglobulin class switch recombination in 53BP1-/- B cells.

Immunoglobulin class switch recombination (CSR) is initiated by activation-induced cytidine deaminase (AID), an enzyme that deaminates cytidine residues in single-stranded DNA. U:G mismatches created by AID are processed to produce lesions that recruit and activate DNA damage response proteins including Ataxia-telangiectasia mutated (ATM), histone H2AX, Nijmegen breakage syndrome 1 (Nbs1), and p53 binding protein 1 (53BP1). Among these proteins, absence of 53BP1 produces the most severe impairment of class switching. Here, we demonstrate that AID is targeted normally to switch region DNA and that intra-switch region recombination is enhanced in 53BP1-/- B cells. In addition, Smicro-Sgamma1 switch region junctions cloned from 53BP1-/- B cells show unusual insertions suggestive of failed class switching. Our data are consistent with a role for 53BP1 in stabilizing the synapsis of switch regions during CSR.

Animals↗

PubMed2006

Structural basis for the methylation state-specific recognition of histone H4-K20 by 53BP1 and Crb2 in DNA repair.

Histone lysine methylation has been linked to the recruitment of mammalian DNA repair factor 53BP1 and putative fission yeast homolog Crb2 to DNA double-strand breaks (DSBs), but how histone recognition is achieved has not been established. Here we demonstrate that this link occurs through direct binding of 53BP1 and Crb2 to histone H4. Using X-ray crystallography and nuclear magnetic resonance (NMR) spectroscopy, we show that, despite low amino acid sequence conservation, both 53BP1 and Crb2 contain tandem tudor domains that interact with histone H4 specifically dimethylated at Lys20 (H4-K20me2). The structure of 53BP1/H4-K20me2 complex uncovers a unique five-residue 53BP1 binding cage, remarkably conserved in the structure of Crb2, that best accommodates a dimethyllysine but excludes a trimethyllysine, thus explaining the methylation state-specific recognition of H4-K20. This study reveals an evolutionarily conserved molecular mechanism of targeting DNA repair proteins to DSBs by direct recognition of H4-K20me2.

Amino Acid Sequence↗

PubMed2006

The tandem BRCT domain of 53BP1 is not required for its repair function.

53BP1 plays an important role in cellular response to DNA damage. It is thought to be the mammalian homologue of budding yeast Rad9 and/or fission yeast Crb2. Rad9/Crb2 are bona fide checkpoint proteins whose activation requires their corresponding C-terminal tandem BRCT (BRCA1 C-terminal) motifs, which mediate their oligomerization and phosphorylation at multiple sites following DNA damage. Here we show that the function of human 53BP1 similarly depends on its oligomerization and phosphorylation at multiple sites but in a BRCT domain-independent manner. Moreover, unlike its proposed yeast counterparts, human 53BP1 only has limited checkpoint functions but rather acts as an adaptor in the repair of DNA double strand breaks. This difference in function may reflect the higher complexity of the DNA damage response network in metazoa including the evolution of other BRCT domain-containing proteins that may have functions redundant or overlapping with those of 53BP1.

Cell Line↗

PubMed2006

CDK2-dependent phosphorylation of FOXO1 as an apoptotic response to DNA damage.

The function of cyclin-dependent kinase 2 (CDK2) is often abolished after DNA damage. The inhibition of CDK2 plays a central role in DNA damage-induced cell cycle arrest and DNA repair. However, whether CDK2 also influences the survival of cells under genotoxic stress is unknown. Forkhead box O (FOXO) transcription factors are emerging as key regulators of cell survival. CDK2 specifically phosphorylated FOXO1 at serine-249 (Ser249) in vitro and in vivo. Phosphorylation of Ser249 resulted in cytoplasmic localization and inhibition of FOXO1. This phosphorylation was abrogated upon DNA damage through the cell cycle checkpoint pathway that is dependent on the protein kinases Chk1 and Chk2. Moreover, silencing of FOXO1 by small interfering RNA diminished DNA damage-induced death in both p53-deficient and p53-proficient cells. This effect was reversed by restored expression of FOXO1 in a manner depending on phosphorylation of Ser249. Functional interaction between CDK2 and FOXO1 provides a mechanism that regulates apoptotic cell death after DNA strand breakage.

Animals↗

PubMed2006

The role of Vpr in HIV-1 disease progression is independent of its G2 arrest induction function.

Vpr (viral protein R) is a vital HIV-1 accessory protein with multiple functions in the viral life cycle, including nuclear import of preintegration complex, induction of apoptosis and G(2) cell cycle arrest. The cell cycle perturbation activity of Vpr requires activation of the ATR (Ataxia-Telangiectasia and Rad3-related) pathway and the integrity of Vpr C-terminal motif that is crucial for chromatin binding. Recent studies also demonstrated Vpr as one of the viral factors that influence HIV disease progression, as mutations in Vpr were overrepresented in some cohorts of long-term nonprogressors (LTNP). The LTNP-associated mutations of Vpr are frequently observed in the C-terminal domain. This raises the question whether the LTNP phenotype of Vpr is the result of the loss its ability to induce G(2) arrest. Here we report that the LTNP-associated mutants of Vpr function normally in the induction of G(2) arrest. No defects in ATR activation and direct binding to chromatin are observed. These mutants also show similar levels of apoptosis induction as wild-type Vpr. These data differentiate the LTNP-associated mutations of Vpr with those defective in inducing G(2) arrest. We propose that the G(2) arrest function of Vpr is separated from the LTNP phenotype, and the role of Vpr in HIV disease progression may involve other functions of Vpr.

Apoptosis↗

PubMed2006

Repeated phosphopeptide motifs in human Claspin are phosphorylated by Chk1 and mediate Claspin function.

Claspin is a checkpoint protein involved in ATR (ataxia telangiectasia mutated- and Rad3-related)-dependent Chk1 activation in Xenopus and human cells. In Xenopus, Claspin interacts with Chk1 after DNA damage through a region containing two highly conserved repeats, which becomes phosphorylated during the checkpoint response. Because this region is also conserved in human Claspin, we investigated the regulation and function of these potential phosphorylation sites in human Claspin. We found that Claspin is phosphorylated in vivo at Thr-916 in response to replication stress and UV damage. Mutation of these phosphorylation sites on Claspin inhibited Claspin-Chk1 interaction in vivo, impaired Chk1 activation, and induced premature chromatin condensation in cells, indicating a defect in replication checkpoint. In addition, we found that Thr-916 on Claspin is phosphorylated by Chk1, suggesting that Chk1 regulates Claspin during checkpoint response. These results together indicate that phosphorylation of Claspin repeats in human Claspin is important for Claspin function and the regulation of Claspin-Chk1 interaction in human cells.

Adaptor Proteins, Signal Transducing↗

PubMed2006

BRCA1 ubiquitinates its phosphorylation-dependent binding partner CtIP.

BRCA1 (Breast Cancer Susceptibility Gene 1) possesses an N-terminal Ring domain and tandem C-terminal BRCT motifs. While the Ring domain has E3 ubiquitin ligase activity, the BRCA1 BRCT domains specifically recognize phospho-serine motifs. Here, we demonstrate that BRCA1 Ring domain catalyzes CtIP ubiquitination in a manner that depends on a phosphorylation-mediated interaction between CtIP and BRCA1 BRCT domains. The BRCA1-dependent ubiquitination of CtIP does not target CtIP for degradation. Instead, ubiquitinated CtIP associates with chromatin following DNA damage and participates in G2/M checkpoint control. Thus, we propose that BRCA1 can regulate the functions of its substrates through nonproteasomal pathways that do not involve substrate degradation.

BRCA1 Protein↗

PubMed2006

Cellular senescence and DNA repair.

Aging is a complex process that results in functional decline and mortality of the organisms. On the cellular lever, cellular senescence has been used as a model for aging. Therefore, understanding cellular senescence has important health implications. Initial observations suggest that cellular senescence is the result of telomere shortening. Recent findings suggest that cellular senescence could be triggered by DNA damage. In fact, both telomere shortening and DNA-damage-induced cellular senescence share a common mechanism, the DNA damage response pathway. This review will discuss the link between DNA repair defects and cellular senescence.

Aging↗

PubMed2006

MR tagging demonstrates quantitative differences in regional ventricular wall motion in mice, rats, and men.

Rats and genetically manipulated mouse models have played an important role in the exploration of molecular causes of cardiovascular diseases. However, it has not been fully investigated whether mice or rats and humans manifest similar patterns of ventricular wall motion. Although similarities in anatomy and myofiber architecture suggest that fundamental patterns of ventricular wall motion may be similar, the considerable differences in heart size, heart rate, and sarcomeric protein isoforms may yield quantitative differences in ventricular wall mechanics. To further our understanding of the basic mechanisms of myofiber contractile performance, we quantified regional and global indexes of ventricular wall motion in mice, rats, and men using magnetic resonance (MR) imaging. Both regular cine and tagged MR images at apical, midventricular, and basal levels were acquired from six male volunteers, six Fischer 344 rats, and seven C57BL/6 mice. Morphological parameters and ejection fraction were computed directly from cine images. Myocardial twist (rotation angle), torsion (net twist per unit length), circumferential strain, and normalized radial shortening were calculated by homogeneous strain analysis from tagged images. Our data show that ventricular twist was conserved among the three species, leading to a significantly smaller torsion, measured as net twist per unit length, in men. However, both circumferential strain and normalized radial shortening were the largest in male subjects. Although other parameters, such as circumferential-longitudinal shear strain, need to be evaluated, and the causes of these differences in contractile mechanics remain to be elucidated, the preservation of twist appears fundamental to cardiac function and should be considered in studies that extrapolate data from animals to humans.

Adolescent↗

PubMed2006

MDC1 maintains genomic stability by participating in the amplification of ATM-dependent DNA damage signals.

MDC1 functions in checkpoint activation and DNA repair following DNA damage. To address the physiological role of MDC1, we disrupted the MDC1 gene in mice. MDC1-/- mice recapitulated many phenotypes of H2AX-/- mice, including growth retardation, male infertility, immune defects, chromosome instability, DNA repair defects, and radiation sensitivity. At the molecular level, H2AX, MDC1, and ATM form a positive feedback loop, with MDC1 directly mediating the interaction between H2AX and ATM. MDC1 binds phosphorylated H2AX through its BRCT domain and ATM through its FHA domain. Through these interactions, MDC1 accumulates activated ATM flanking the sites of DNA damage, facilitating further ATM-dependent phosphorylation of H2AX and the amplification of DNA damage signals. In the absence of MDC1, many downstream ATM signaling events are defective. These results suggest that MDC1, as a signal amplifier of the ATM pathway, is vital in controlling proper DNA damage response and maintaining genomic stability.

Adaptor Proteins, Signal Transducing↗

PubMed2006

H2AX prevents DNA breaks from progressing to chromosome breaks and translocations.

Histone H2AX promotes DNA double-strand break (DSB) repair and immunoglobulin heavy chain (IgH) class switch recombination (CSR) in B-lymphocytes. CSR requires activation-induced cytidine deaminase (AID) and involves joining of DSB intermediates by end joining. We find that AID-dependent IgH locus chromosome breaks occur at high frequency in primary H2AX-deficient B cells activated for CSR and that a substantial proportion of these breaks participate in chromosomal translocations. Moreover, activated B cells deficient for ATM, 53BP1, or MDC1, which interact with H2AX during the DSB response, show similarly increased IgH locus breaks and translocations. Thus, our findings implicate a general role for these factors in promoting end joining and thereby preventing DSBs from progressing into chromosomal breaks and translocations. As cellular p53 status does not markedly influence the frequency of such events, our results also have implications for how p53 and the DSB response machinery cooperate to suppress generation of lymphomas with oncogenic translocations.

Animals↗

PubMed2006

Role of genomic instability and p53 in AID-induced c-myc-Igh translocations.

Chromosomal translocations involving the immunoglobulin switch region are a hallmark feature of B-cell malignancies. However, little is known about the molecular mechanism by which primary B cells acquire or guard against these lesions. Here we find that translocations between c-myc and the IgH locus (Igh) are induced in primary B cells within hours of expression of the catalytically active form of activation-induced cytidine deaminase (AID), an enzyme that deaminates cytosine to produce uracil in DNA. Translocation also requires uracil DNA glycosylase (UNG), which removes uracil from DNA to create abasic sites that are then processed to double-strand breaks. The pathway that mediates aberrant joining of c-myc and Igh differs from intrachromosomal repair during immunoglobulin class switch recombination in that it does not require histone H2AX, p53 binding protein 1 (53BP1) or the non-homologous end-joining protein Ku80. In addition, translocations are inhibited by the tumour suppressors ATM, Nbs1, p19 (Arf) and p53, which is consistent with activation of DNA damage- and oncogenic stress-induced checkpoints during physiological class switching. Finally, we demonstrate that accumulation of AID-dependent, IgH-associated chromosomal lesions is not sufficient to enhance c-myc-Igh translocations. Our findings reveal a pathway for surveillance and protection against AID-dependent DNA damage, leading to chromosomal translocations.

Animals↗

PubMed2006

Characterization of CHEK2 mutations in prostate cancer.

The checkpoint kinase 2 (CHEK2, also known as CHK2) is a tumor suppressor that participates in the DNA damage-signaling pathway. It is phosphorylated and activated following DNA damage, resulting in cell cycle arrest and apoptosis. Previously, we reported germline CHEK2 mutations in patients with prostate cancer. In this study, we have identified two novel somatic CHEK2 mutations, c.349A > G (p.R117G) and c.967A > C (p.E321K), in prostate tumor specimens and investigated the functions of these mutants in vivo. We have shown that most of the germline CHEK2 mutations and one somatic mutation (p.R117G) within FHA domain have modestly reduced CHEK2 kinase activity in comparison with wild-type CHEK2 while the other somatic mutation (p.E321K) within the kinase domain of CHEK2 totally abolished CHEK2 kinase activity. Given that several clinical CHEK2 mutations reside in the Forkhead-associated (FHA) domain, we further generated a series of missense mutations within this domain and demonstrated the requirement of an intact FHA domain for the full activation of CHEK2. Taken together, these results provide evidence that both germline and somatic CHEK2 mutations identified in prostate cancer may contribute to the development of prostate cancer through the reduction of CHEK2 activation in response to DNA damage and/or oncogenic stress.

Cell Line↗

PubMed2006

Signaling networks controlled by the MRN complex and MDC1 during early DNA damage responses.

Cells activate complex signaling networks in response to DNA damage. Several proteins and protein complexes are involved in sensing DNA lesions and initiating the DNA damage response networks. The subsequent DNA damage responses, including the initiation of DNA repair pathways, the activation of cell cycle checkpoint controls and the induction of apoptosis, help maintain genomic stability in mammalian systems. Failure to establish the appropriate DNA damage signaling networks results in genomic instability, which is a known causal factor in tumorigenesis. This review will discuss recent progress in the understanding of the mechanisms by which mammalian cells sense DNA lesions and transduce DNA damage signals during early DNA damage responses.

Adaptor Proteins, Signal Transducing↗

PubMed2006

Targeted PARACEST nanoparticle contrast agent for the detection of fibrin.

A lipid-encapsulated perfluorocarbon nanoparticle molecular imaging contrast agent that utilizes a paramagnetic chemical exchange saturation transfer (PARACEST) chelate is presented. PARACEST agents are ideally suited for molecular imaging applications because one can switch the contrast on and off at will simply by adjusting the pulse sequence parameters. This obviates the need for pre- and postinjection images to define contrast agent binding. Spectroscopy (4.7T) of PARACEST nanoparticles revealed a bound water peak at 52 ppm, in agreement with results from the water-soluble chelate. Imaging of control nanoparticles showed no appreciable contrast, while PARACEST nanoparticles produced >10% signal enhancement. PARACEST nanoparticles were targeted to clots via antifibrin antibodies and produced a contrast-to-noise ratio (CNR) of 10 at the clot surface.

Contrast Media↗

PubMed2005

The mitotic checkpoint in cancer and aging: what have mice taught us?

The spindle assembly checkpoint is a cellular surveillance mechanism that functions to ensure faithful chromosome segregation during mitosis. Failure of this checkpoint can result in aneuploidy, a state of having abnormal numbers of chromosomes. Most human cancers consist of aneuploid cells, but it is unclear if the aneuploidy is a cause or a consequence of tumorigenesis. Over recent years, mouse models for spindle assembly checkpoint failure have been generated to investigate the biological relevance of the different spindle assembly checkpoint genes and the pathologies associated with chromosome number instability. Most of these models exhibit susceptibility to carcinogenesis. Moreover, one model has led to the identification of the spindle checkpoint protein BubR1 as a regulator of the normal aging process.

Aging↗

PubMed2005

Gemcitabine-induced activation of checkpoint signaling pathways that affect tumor cell survival.

Two signaling pathways are activated by antineoplastic therapies that damage DNA and stall replication. In one pathway, double-strand breaks activate ataxia-telangiectasia mutated kinase (ATM) and checkpoint kinase 2 (Chk2), two protein kinases that regulate apoptosis, cell-cycle arrest, and DNA repair. In the second pathway, other types of DNA lesions and replication stress activate the Rad9-Hus1-Rad1 complex and the protein kinases ataxia-telangiectasia mutated and Rad3-related kinase (ATR) and checkpoint kinase 1 (Chk1), leading to changes that block cell-cycle progression, stabilize stalled replication forks, and influence DNA repair. Gemcitabine and cytarabine are two highly active chemotherapeutic agents that disrupt DNA replication. Here, we examine the roles these pathways play in tumor cell survival after treatment with these agents. Cells lacking Rad9, Chk1, or ATR were more sensitive to gemcitabine and cytarabine, consistent with the fact that these agents stall replication forks, and this sensitization was independent of p53 status. Interestingly, ATM depletion sensitized cells to gemcitabine and ionizing radiation but not cytarabine. Together, these results demonstrate that 1) gemcitabine triggers both checkpoint signaling pathways, 2) both pathways contribute to cell survival after gemcitabine-induced replication stress, and 3) although gemcitabine and cytarabine both stall replication forks, ATM plays differential roles in cell survival after treatment with these agents.

Ataxia Telangiectasia Mutated Proteins↗