ORCA/LRWD1 Regulates Homologous Recombination at ALT-Telomeres by Modulating Heterochromatin Organization

Chromatin's Influence on Pre-Replication Complex Assembly and Function

In all eukaryotes, the initiation of DNA replication requires a stepwise assembly of factors onto the origins of DNA replication. This is pioneered by the Origin Recognition Complex, which recruits Cdc6. Together, they bring Cdt1, which shepherds MCM2-7 to form the OCCM complex. Sequentially, a second Cdt1-bound hexamer of MCM2-7 is recruited by ORC-Cdc6 to form an MCM double hexamer, which forms a part of the pre-RC. Although the mechanism of ORC binding to DNA varies across eukaryotes, how ORC is recruited to replication origins in human cells remains an area of intense investigation. This review discusses how the chromatin environment influences pre-RC assembly, function, and, eventually, origin activity.

MicroRNA-320a enhances LRWD1 expression through the AGO2/FXR1-dependent pathway to affect cell behaviors and the oxidative stress response in human testicular embryonic carcinoma cells

BACKGROUND

Testicular cancer is fairly rare but can affect fertility in adult males. Leucine-rich repeats- and WD repeat domain-containing protein 1 (LRWD1) is a sperm-specific marker that mainly affects sperm motility in reproduction. Our previous study demonstrated the impact of LRWD1 on testicular cancer development; however, the underlying mechanisms remain unclear.

METHODS

In this study, various plasmids associated with LRWD1 and miR-320a manipulation were used to explore the roles and regulatory effects of these molecules in NT2D1 cellular processes. A Dual-Glo luciferin-luciferase system was used to investigate LRWD1 transcriptional activity, and qRT-PCR and western blotting were used to determine gene and protein expression.

RESULTS

The results suggested that miR-320a positively regulated LRWD1 and positively correlated with NT2D1 cell proliferation but negatively correlated with cell migration and invasion ability. In addition, the miRNA-ribonucleoprotein complex AGO2/FXR1 was shown to be essential in the mechanism by which miR-320a regulates LRWD1 mRNA expression. As miR-320a was required to regulate LRWD1 expression through the AGO2 and FXR1 complex, eEF2 and eLF4E were also found to be involved in miR-320a increasing LRWD1 expression. Furthermore, miR-320a and LRWD1 were responsive to oxidative stress, and NRF2 was affected by the presence of miR-320a in response to ROS stimulation.

CONCLUSIONS

This is the first study showing the role of miR-320a in upregulating the testicular cancer-specific regulator LRWD1 and the importance of the AGO2/FXR1 complex in miR-320a-mediated upregulation of LRWD1 during testicular cancer progression.

ORChestra coordinates the replication and repair music

Error-free genome duplication and accurate cell division are critical for cell survival. In all three domains of life, bacteria, archaea, and eukaryotes, initiator proteins bind replication origins in an ATP-dependent manner, play critical roles in replisome assembly, and coordinate cell-cycle regulation. We discuss how the eukaryotic initiator, Origin recognition complex (ORC), coordinates different events during the cell cycle. We propose that ORC is the maestro driving the orchestra to coordinately perform the musical pieces of replication, chromatin organization, and repair.

High-resolution ribosome profiling reveals translational selectivity for transcripts in bovine preimplantation embryo development

High-resolution ribosome fractionation and low-input ribosome profiling of bovine oocytes and preimplantation embryos has enabled us to define the translational landscapes of early embryo development at an unprecedented level. We analyzed the transcriptome and the polysome- and non-polysome-bound RNA profiles of bovine oocytes (germinal vesicle and metaphase II stages) and early embryos at the two-cell, eight-cell, morula and blastocyst stages, and revealed four modes of translational selectivity: (1) selective translation of non-abundant mRNAs; (2) active, but modest translation of a selection of highly expressed mRNAs; (3) translationally suppressed abundant to moderately abundant mRNAs; and (4) mRNAs associated specifically with monosomes. A strong translational selection of low-abundance transcripts involved in metabolic pathways and lysosomes was found throughout bovine embryonic development. Notably, genes involved in mitochondrial function were prioritized for translation. We found that translation largely reflected transcription in oocytes and two-cell embryos, but observed a marked shift in the translational control in eight-cell embryos that was associated with the main phase of embryonic genome activation. Subsequently, transcription and translation become more synchronized in morulae and blastocysts. Taken together, these data reveal a unique spatiotemporal translational regulation that accompanies bovine preimplantation development.

Preventing excess replication origin activation to ensure genome stability

Cells activate distinctive regulatory pathways that prevent excessive initiation of DNA replication to achieve timely and accurate genome duplication. Excess DNA synthesis is constrained by protein-DNA interactions that inhibit initiation at dormant origins. In parallel, specific modifications of pre-replication complexes prohibit post-replicative origin relicensing. Replication stress ensues when the controls that prevent excess replication are missing in cancer cells, which often harbor extrachromosomal DNA that can be further amplified by recombination-mediated processes to generate chromosomal translocations. The genomic instability that accompanies excess replication origin activation can provide a promising target for therapeutic intervention. Here we review molecular pathways that modulate replication origin dormancy, prevent excess origin activation, and detect, encapsulate, and eliminate persistent excess DNA.

The Essential Role of Stathmin in Myoblast C2C12 for Vertical Vibration-Induced Myotube Formation

Vertical vibration (VV) is a type of whole body vibration, which induces muscle contraction through vibration to improve muscle strength and bone density. However, the mechanism of VV on muscle cell myotube formation is still unclear. In the current study, we aim to clarify the mechanism involved in VV's stimulation of myotube formation. In order to identify the molecules regulated by VV, we performed proteomics analysis including 2D electrophoresis combined with MALDI-TOF/TOF Mass. Stathmin was identified as a high potential molecule responding to VV stimulation, and we found that under VV stimulation, the expression of stathmin gene and protein increased in a time-dependent manner. In addition, we also confirmed that the increase of stathmin stimulated by VV is mediated through the PI3K/Akt pathway. Furthermore, stathmin siRNA significantly down-regulated the expression of myogenic regulatory factor (MRF) MyoD, decorin, and type I collagen (Col-I), and down-regulated the cellular process regulators such as FGF7, TGFBr1 and PAK3. Taken together, our results confirm that under the stimulation of VV, PI3K/Akt and stathmin would be activated, as well as the up-regulation of MRFs, such as FGF7, TGFBr1 and PAK3 to initiate myogenesis. It also showed that the response of MRF to VV stimulation was significantly related to stathmin expression, which also confirmed the importance of stathmin in the entire myotube formation process. This study may provide evidence of stathmin as a biological indicator of VV to increase muscle strength.

Replication initiation: Implications in genome integrity

In every cell cycle, billions of nucleotides need to be duplicated within hours, with extraordinary precision and accuracy. The molecular mechanism by which cells regulate the replication event is very complicated, and the entire process begins way before the onset of S phase. During the G1 phase of the cell cycle, cells prepare by assembling essential replication factors to establish the pre-replicative complex at origins, sites that dictate where replication would initiate during S phase. During S phase, the replication process is tightly coupled with the DNA repair system to ensure the fidelity of replication. Defects in replication and any error must be recognized by DNA damage response and checkpoint signaling pathways in order to halt the cell cycle before cells are allowed to divide. The coordination of these processes throughout the cell cycle is therefore critical to achieve genomic integrity and prevent diseases. In this review, we focus on the current understanding of how the replication initiation events are regulated to achieve genome stability.

The Telomeric Protein TRF2 Regulates Replication Origin Activity within Pericentromeric Heterochromatin

Heterochromatic regions render the replication process particularly difficult due to the high level of chromatin compaction and the presence of repeated DNA sequences. In humans, replication through pericentromeric heterochromatin requires the binding of a complex formed by the telomeric factor TRF2 and the helicase RTEL1 in order to relieve topological barriers blocking fork progression. Since TRF2 is known to bind the Origin Replication Complex (ORC), we hypothesized that this factor could also play a role at the replication origins (ORI) of these heterochromatin regions. By performing DNA combing analysis, we found that the ORI density is higher within pericentromeric satellite DNA repeats than within bulk genomic DNA and decreased upon TRF2 downregulation. Moreover, we showed that TRF2 and ORC2 interact in pericentromeric DNA, providing a mechanism by which TRF2 is involved in ORI activity. Altogether, our findings reveal an essential role for TRF2 in pericentromeric heterochromatin replication by regulating both replication initiation and elongation.

The E3 ligase RFWD3 stabilizes ORC in a p53-dependent manner

RFWD3 is an E3 ubiquitin ligase that plays important roles in DNA damage response and DNA replication. We have previously demonstrated that the stabilization of RFWD3 by PCNA at the replication fork enables ubiquitination of the single-stranded binding protein, RPA and its subsequent degradation for replication progression. Here, we report that RFWD3 associates with the Origin Recognition Complex (ORC) and ORC-Associated (ORCA/LRWD1), components of the pre-replicative complex required for the initiation of DNA replication. Overexpression of ORC/ORCA leads to the stabilization of RFWD3. Interestingly, RFWD3 seems to stabilize ORC/ORCA in cells expressing wild type p53, as the depletion of RFWD3 reduces the levels of ORC/ORCA. Further, the catalytic activity of RFWD3 is required for the stabilization of ORC. Our results indicate that the RFWD3 promotes the stability of ORC, enabling efficient pre-RC assembly.