Breakpoint Cluster Region Protein Research Articles

The Evaluation of Rac1 Signaling as a Potential Therapeutic Target of Alzheimer's Disease.

The Small GTPase Rac1 is critical for various fundamental cellular processes, including cognitive functions. The cyclical activation and inactivation of Rac1, mediated by Rac guanine nucleotide exchange factors (RacGEFs) and Rac GTPase-activating proteins (RacGAPs), respectively, are essential for activating intracellular signaling pathways and controlling cellular processes. We have recently shown that the Alzheimer's disease (AD) therapeutic drug donepezil activates the Rac1-PAK pathway in the nucleus accumbens (NAc) for enhanced aversive learning. Also, PAK activation itself in the NAc enhances aversive learning. As aversive learning allows short-term preliminary AD drug screening, here we tested whether sustained Rac1 activation by RacGAP inhibition can be used as an AD therapeutic strategy for improving AD-learning deficits based on aversive learning. We found that the RacGAP domain of breakpoint cluster region protein (Bcr) (Bcr-GAP) efficiently inhibited Rac1 activity in a membrane ruffling assay. We also found that, in striatal/accumbal primary neurons, Bcr knockdown by microRNA mimic-expressing adeno-associated virus (AAV-miRNA mimic) activated Rac1-PAK signaling, while Bcr-GAP-expressing AAV inactivated it. Furthermore, conditional knockdown of Bcr in the NAc of wild-type adult mice enhanced aversive learning, while Bcr-GAP expression in the NAc inhibited it. The findings indicate that Rac1 activation by RacGAP inhibition enhances aversive learning, implying the AD therapeutic potential of Rac1 signaling.

Biochemical and breakpoint cluster region-c-ABL oncogene 1 polymorphism study among Iraqi patients with chronic myeloid leukemia

BACKGROUND: Chronic myeloid leukemia (CML) has been well recognized as an exemplary instance of a malignant disease characterized by a distinctive molecular occurrence, namely the presence of the breakpoint cluster region (BCR)-c-ABL oncogene 1 (ABL1) oncogene. The Philadelphia chromosome gives rise to an anomalous fusion gene characterized by atypical kinase activity, resulting in the accumulation of reactive oxygen species and genetic instability that holds significance in the advancement of diseases. OBJECTIVE: The objective of this study was to investigate the detection rate of BCR-ABL1 polymorphism and BCR protein level in a group of Iraqi patients with CML. MATERIALS AND METHODS: This study has been carried out on 150 specimens, 120 patients subjected to CML included 20 patients diagnosed as newly diagnosis CML and 100 patients treated with CML. In addition to 30 apparently healthy persons as a control group (normal persons) from the National Center of Hematology/Mustansiryiah University/Baghdad, 65 out of 100 patients on imatinib while 35 nonimatinib (nilotinib and bosutinib). Fresh whole blood and serum were obtained from all patients and controls. We used total DNA genomic extraction extracted from ethylenediaminetetraacetic acid blood for genetic detection of Bcr/Abl Genes Polymorphism by sequencing technique in patients with CML and apparently control groups and used serum for biochemical tests include urea, lactate dehydrogenase (LDH), aspartate transaminase (AST), alanine transaminase (ALT), and creatinine using biochemicals methods (colorimetric and kinetic), respectively, as well as detection BCR protein level using sandwich enzyme-linked immunosorbent assays technique. RESULTS: According to age and sex, the patients’ groups were matching with the control group. Regarding the biochemical parameters (urea creatinine, ALT, AST, and LDH) serum level, there are no significant differences among new diagnosis CML, patients respond to treatments and failure group except in serum level of creatinine between new diagnosis CML group and failure group, there are significant differences (P = 0.01). The present results showed that DNA polymorphism distribution was according to C\C; G\C; A\T; and A\A were 32%, 26%, 18%, and 24%, respectively, in patients with CML and 28%; 20%;12%; and 40%, respectively, in the control group. There are significant statistical differences (P < 0.05) between different groups according to the genotyping of BCR\ABL, the results obtained from the sequenced 429 bp fragments, and the detailed positions of the observed variations are described in the NCBI reference sequences (rs766724113). The samples were submitted in NCBI, and the accession number of nucleotide sequences of BCR\ABL as new recording: LC 775148, LC 775149, and LC 775150, while regarding with BCR protein, there are significant differences in level between new diagnosis CML and CML on treatment and control groups, P < 0.001 for each comparison while there are no significant differences between treated group and control group (P = 0.729). CONCLUSION: The present results indicate that BCR-ABL1 polymorphism and BCR protein level in a group of Iraqi patients with CML may play a role in the tumor biology of the examined subset of CML and may contributed to their development.

The Collagen Receptor Discoidin Domain Receptor 1b Enhances Integrin β1-Mediated Cell Migration by Interacting With Talin and Promoting Rac1 Activation.

Integrins and discoidin domain receptors (DDRs) 1 and 2 promote cell adhesion and migration on both fibrillar and non fibrillar collagens. Collagen I contains DDR and integrin selective binding motifs; however, the relative contribution of these two receptors in regulating cell migration is unclear. DDR1 has five isoforms (DDR1a-e), with most cells expressing the DDR1a and DDR1b isoforms. We show that human embryonic kidney 293 cells expressing DDR1b migrate more than DDR1a expressing cells on DDR selective substrata as well as on collagen I in vitro. In addition, DDR1b expressing cells show increased lung colonization after tail vein injection in nude mice. DDR1a and DDR1b differ from each other by an extra 37 amino acids in the DDR1b cytoplasmic domain. Interestingly, these 37 amino acids contain an NPxY motif which is a central control module within the cytoplasmic domain of β integrins and acts by binding scaffold proteins, including talin. Using purified recombinant DDR1 cytoplasmic tail proteins, we show that DDR1b directly binds talin with higher affinity than DDR1a. In cells, DDR1b, but not DDR1a, colocalizes with talin and integrin β1 to focal adhesions and enhances integrin β1-mediated cell migration. Moreover, we show that DDR1b promotes cell migration by enhancing Rac1 activation. Mechanistically DDR1b interacts with the GTPase-activating protein (GAP) Breakpoint cluster region protein (BCR) thus reducing its GAP activity and enhancing Rac activation. Our study identifies DDR1b as a major driver of cell migration and talin and BCR as key players in the interplay between integrins and DDR1b in regulating cell migration.

DDR1 contributes to kidney inflammation and fibrosis by promoting the phosphorylation of BCR and STAT3.

Discoidin domain receptor 1 (DDR1), a receptor tyrosine kinase activated by collagen, contributes to chronic kidney disease. However, its role in acute kidney injury and subsequent development of kidney fibrosis is not clear. Thus, we performed a model of severe ischemia/reperfusion-induced acute kidney injury that progressed to kidney fibrosis in WT and Ddr1-null mice. We showed that Ddr1-null mice had reduced acute tubular injury, inflammation, and tubulointerstitial fibrosis with overall decreased renal monocyte chemoattractant protein (MCP-1) levels and STAT3 activation. We identified breakpoint cluster region (BCR) protein as a phosphorylated target of DDR1 that controls MCP-1 production in renal proximal tubule epithelial cells. DDR1-induced BCR phosphorylation or BCR downregulation increased MCP-1 secretion, suggesting that BCR negatively regulates the levels of MCP-1. Mechanistically, phosphorylation or downregulation of BCR increased β-catenin activity and in turn MCP-1 production. Finally, we showed that DDR1-mediated STAT3 activation was required to stimulate the secretion of TGF-β. Thus, DDR1 contributes to acute and chronic kidney injury by regulating BCR and STAT3 phosphorylation and in turn the production of MCP-1 and TGF-β. These findings identify DDR1 an attractive therapeutic target for ameliorating both proinflammatory and profibrotic signaling in kidney disease.

Abstract MP40: Breakpoint Cluster Region Protein Function As A Kinase Mediates The Atheroprotective Actions Of Hdl In Vascular Endothelium

Introduction: HDL has direct atheroprotective actions on endothelial cells (EC) that include the attenuation of vascular inflammation and promotion of endothelial monolayer integrity. These processes are mediated by scavenger receptor class B, type 1 (SR-BI) and its adaptor protein PDZK1. We have identified the breakpoint cluster region protein (BCR) as a PDZK1 interacting protein in EC. Best known as a gene in the BCR-ABL complex associated with the Philadelphia chromosome, BCR structure predicts both adaptor protein and kinase functions. Hypothesis: Via interaction with PDZK1 and kinase activity, BCR mediates the atheroprotective actions of HDL on EC. Methods: Newly-developed kinase assays that employ time-resolved Forster resonance energy transfer were used to detect kinase activity and identify kinase substrates. In vivo studies were done in ApoAI -/- versus ApoAI -/- ;BCR -/- mice administered control AAV or AAV-ApoAI. Leukocyte-EC interaction and carotid artery reendothelialization were evaluated by intravital microscopy and thermal injury, respectively. Atherosclerosis was invoked using AAV8-PCSK9. BCR expression was compared in human atherosclerotic and normal arteries in two independent publicly available patient cohorts. Results: In cultured EC, HDL activation of eNOS, blunting of EC-monocyte adhesion, and promotion of EC migration all required BCR, and reconstitution studies with BCR mutants revealed requirements for both BCR-PDZK1 interaction and kinase activity. In response to HDL, BCR directly phosphorylated Akt-Ser473 and it provoked Akt-Thr308 phosphorylation by PDK1. ApoAI restoration in vivo in ApoAI -/- mice prompted SR-BI-PDZK1-BCR protein complex formation and BCR phosphorylation of Akt-Ser473 in mouse aorta. It also blunted leukocyte-endothelial cell adhesion and enhanced carotid artery reendothelialization in BCR +/+ , but not in BCR -/- mice. Furthermore, the atheroprotective effect of ApoAI restoration was demonstrable in BCR +/+ , but not in BCR -/- mice. In both human cohorts BCR expression was decreased in atherosclerotic versus normal arteries. Conclusion: BCR function as a novel direct and indirect kinase of Akt mediates the atheroprotective actions of HDL in vascular endothelium.

Identification of the motifs of beta-turns and mutated amino acids studies on BCR (Breakpoint cluster region) protein using Insilico techniques

Recent investigations have rapidly added crucial new insights into the complex functions of the normal BCR gene and of the BCR-ABL chimaera. They are yielding potential therapeutic breakthroughs in the treatment of Philadelphia (Ph) chromosome-positive leukemias. The objective of the present in silico research investigation is to find out whether the functional part (beta-turns) is present in the mutated amino acids of BCR (Breakpoint cluster region) protein. Two significant steps are involved in this study. First, we performed protein sequence modeling of BCR using automated protein modeling servers and the 3D structure was visualized using molecular visualization software and tools. In the second step, the function domains and motifs regions of BCR gene-coded protein is predicted using “PDBsum generate” tool in order to show where exactly the beta-turns lie on the clinically-proven mutated amino acids of BCR protein. The results of our investigation can be used as potential drug binding sites in the field of drug docking studies. It can act as a potential therapeutic agent for Chronic Myeloid Leukemia (CML) type of Leukemia.

Discovery of Berberine that Targetedly Induces Autophagic Degradation of both BCR-ABL and BCR-ABL T315I through Recruiting LRSAM1 for Overcoming Imatinib Resistance.

Imatinib, the breakpoint cluster region protein (BCR)/Abelson murine leukemia viral oncogene homolog (ABL) inhibitor, is widely used to treat chronic myeloid leukemia (CML). However, imatinib resistance develops in many patients. Therefore, new drugs with improved therapeutic effects are urgently needed. Berberine (BBR) is a potent BCR-ABL inhibitor for imatinib-sensitive and -resistant CML. Protein structure analysis and virtual screening were used to identify BBR targets in CML. Molecular docking analysis, surface plasmon resonance imaging, nuclear magnetic resonance assays, and thermoshift assays were performed to confirm the BBR target. The change in BCR-ABL protein expression after BBR treatment was assessed by Western blotting. The effects of BBR were assessed in vitro in cell lines, in vivo in mice, and in human CML bone marrow cells as a potential strategy to overcome imatinib resistance. We discovered that BBR bound to the protein tyrosine kinase domain of BCR-ABL. BBR inhibited the activity of BCR-ABL and BCR-ABL with the T315I mutation, and it also degraded these proteins via the autophagic lysosome pathway by recruiting E3 ubiquitin-protein ligase LRSAM1. BBR inhibited the cell viability and colony formation of CML cells and prolonged survival in CML mouse models with imatinib sensitivity and resistance. The results show that BBR directly binds to and degrades BCR-ABL and BCR-ABL T315I via the autophagic lysosome pathway by recruiting LRSAM1. The use of BBR is a new strategy to improve the treatment of patients with CML with imatinib sensitivity or resistance.See related commentary by Elf, p. 3899.

Covalent conjugation of reduced graphene oxide with oligos for current–volt signal determination on leukemia

Oncogene-mutated breakpoint cluster region protein gene (BCR/ABL) has been found in chronic myelogenous leukemia (CML) patients. In particular, b3a2 is a common mutation type with CML, tested in this research as deoxyribonucleic acid sensor to identify the specific b3a2 sequence by a suitable capture probe. The aminated capture probe was attached on interdigitated electrode surface through the chemical linker, glycidyloxypropyltrimethoxysilane. Covalently linked reduced graphene oxide (rGO) with target sequence was identified by the capture probe. The prepared rGO from vacuum-assisted low-temperature exfoliated graphite was characterized by field emission scanning electron microscope, field emission transmission electron microscope, energy-dispersive X-ray spectroscopy and fourier-transform infrared spectroscopy. The limit of detection with duplex formation was attained between 1 and 10 aM, and the saturation point was noticed from 10 fM. The sensitivity was calculated to be 1 aM. Further, control experiments with single-, triple- and complete mismatch-sequences instead of target complementary failed to yield the changes in current signal indicating the selective identification of targeted b3a2 sequence. This method of detection helps in identifying the minimal leukemia cells residual with CML patient.

Design and Synthesis of a Photoisomerizable Second Generation Kinase Inhibitor

BCR/ABL is a chimeric oncogene made up of breakpoint cluster region protein (BCR) from chromosome 22 and Abelson murine leukemia (ABL) from chromosome 9. This genetic recombination results in a shortened chromosome 22, which is also known as the Philadelphia Chromosome. The BCR/ABL fusion protein produced as a result of this translocation mutation causes the resulting tyrosine kinase enzyme to become constitutively active. Given the critical regulatory role of ABL in cell division, its constitutive activation results in uncontrollable cell division, which in this case, most often leads to chronic myeloid leukemia (CML). BCR/ABL has also been associated with acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML).The first‐generation treatments for diseases caused by this mutation were approved for use in 2001 and were kinase inhibitors. These inhibitors effectively shut off the dysfunctional kinase and shut down uncontrolled cell division. Over time, cancer cells can become resistant to drugs and many CML patients begin to experience resistance to first‐generation therapies. These resistances are caused by mutations in the target kinase, which change the drug binding site and/or protein conformation. Although the development of second‐generation therapeutics has addressed many of these issues, one common problem remains off‐target interactions of these drugs, which result in undesirable side effects. One way to address both of those issues is create drugs that can be switched on/off when and where necessary. Light can be used to change the shape of molecules. Since the shape of molecules is important for their binding to a biomolecule, changing shape can change bioactivity. The purpose of this research is to test the photo‐induced bioactivity of azologues of a second‐generation BCR/ABL inhibitor. Azologues are light activated versions of known biologically active molecules, whose photoisomerizable properties are imparted by the presence of an azo‐stilbene group. Compounds that can be selectively spatially and temporally activated by light could circumvent these off‐target interactions. Additionally, photoisomerizable molecules can be useful tools for investigating biological systems. Given the spatial and temporal control that photoisomerization allow, they may be utilized as powerful chemical tools for the study of time and location dependent cellular processes. The design and synthesis of the target molecule will be outlined as well as the unique synthetic challenges associated with this class of biomolecule.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Collision induced unfolding and dissociation differentiates ATP-competitive from allosteric protein tyrosine kinase inhibitors

Chronic myeloid leukemia is caused by a chimeric oncoprotein comprised of the breakpoint cluster region protein and the Abelson protein tyrosine kinase, and is often treated using protein tyrosine kinase inhibitors that target the ATP-binding domain of the kinase. A more recently-discovered class of inhibitors target the myristoylation site in the C-terminal region of the kinase domain, and offer allosteric control over kinase activity. The discovery process surrounding such inhibitors, however, is exceptionally challenging as allosteric sites are frequently not apparent within X-ray structures and their role in kinase function is not always obvious. As such, there is currently a general lack of assays capable of rapidly identifying allosteric kinase inhibitors. In this report, we describe an assay for detecting allosteric ligand binding in the Abelson protein kinase domain using gas-phase protein unfolding and dissociation. Our data strongly differentiate ATP-competitive and myristate pocket-binding ligands through analyses of both gas-phase unfolding patterns and dissociative charge stripping observed in kinase-inhibitor complex ions produced by electrospray ionization. Furthermore, we discuss and quantify these results, as well as project the utility of this technique for future efforts in high-throughput kinase inhibitor discovery.

Sestd1 Encodes a Developmentally Dynamic Synapse Protein That Complexes With BCR Rac1-GAP to Regulate Forebrain Dendrite, Spine and Synapse Formation.

SEC14 and Spectrin domain-1 (Sestd1) is a synapse protein that exhibits a striking shift from the presynaptic to postsynaptic space as neurons mature postnatally in the mouse hippocampus. Hippocampal pyramidal neurons from mice with global genetic deletion of Sestd1 have reduced dendrite arbors, spines, and excitatory synapses. Electrophysiologically this correlates with cell-autonomous reductions in both AMPA- and NMDA-excitatory postsynaptic currents in individual hippocampal neurons from which Sestd1 has been deleted in vivo. These neurodevelopmental and functional deficits are associated with increased activation of the Rho family GTPases Rac1 and RhoA. Co-immunoprecipitation and mass spectrometry reveal that the Breakpoint Cluster Region protein, a Rho GTPase activating protein (GAP), forms complexes with Sestd1 in brain tissue. This complements earlier findings that Sestd1 can also partner with other Rho family GAPs and guanine nucleotide exchange factors. Our findings demonstrate that Sestd1 is a developmentally dynamic synaptic regulator of Rho GTPases that contributes to dendrite and excitatory synapse formation within differentiating pyramidal neurons of the forebrain.

Prevalence of Abelson murine leukemia viral oncogene homolog-breakpoint cluster region fusions and correlation with peripheral blood parameters in chronic myelogenous leukemia patients in Lorestan Province, Iran.

Context:Chronic myelogenous leukemia (CML) is a chronic malignancy of myeloid linage associated with a significant increase in granulocytes in bone marrow and peripheral blood. CML diagnosis is based on detection of Philadelphia chromosome and “Abelson murine leukemia viral oncogene homolog” (ABL)-“breakpoint cluster region protein” fusions (ABL-BCR fusions).Aims:In this study, patients with CML morphology were studied according to ABL-BCR fusions and the relationship between the fusions and peripheral blood cell changes was examined.Materials and Methods:All patients suspected to chronic myeloproliferative disorders in Lorestan Province visiting subspecialist hematology clinics who were confirmed by oncologist were studied over a period of 5 years. After completing basic data questionnaire, blood samples were obtained with informed consent from the patients. Blood cell count and morphology were investigated and RNA was extracted from blood samples. cDNA was synthesized from RNA and ABL-BCR fusions including b3a2 and b2a2 (protein 210 kd or p210), e1a2 (protein 190 kdor p190), and e19a2 (protein 230 kdor p230) were studied by multiplex reverse transcription polymerase chain reaction method. Coexistence of e1a2 and b2a2 (p210/p190) fusions was also studied. The prevalence of mutations and their correlation with the blood parameters were statistically analyzed.Results:Of 58 patients positive for ABL-BCR fusion, 18 (30.5%) had b2a2 fusion, 37 (62.71%) had b3a2 fusion and three (3.08%) had e1a2 fusion. Coexistence of e1a2 and b2a2 (p210/p190) was not observed. There was no significant correlation between ABL-BCR fusions and white blood cell count, platelet count, and hemoglobin concentration.Conclusions:The ABL-BCR fusions in Lorestan Province were similar to other studies in Iran, and b3a2 fusion had the highest prevalence in the studied patients studied.

An unexpected inhibition of antiviral signaling by virus-encoded tumor suppressor p53 in pancreatic cancer cells

Virus-encoded tumor suppressor p53 transgene expression has been successfully used in vesicular stomatitis virus (VSV) and other oncolytic viruses (OVs) to enhance their anticancer activities. However, p53 is also known to inhibit virus replication via enhanced type I interferon (IFN) antiviral responses. To examine whether p53 transgenes enhance antiviral signaling in human pancreatic ductal adenocarcinoma (PDAC) cells, we engineered novel VSV recombinants encoding human p53 or the previously described chimeric p53-CC, which contains the coiled-coil (CC) domain from breakpoint cluster region (BCR) protein and evades the dominant-negative activities of endogenously expressed mutant p53. Contrary to an expected enhancement of antiviral signaling by p53, our global analysis of gene expression in PDAC cells showed that both p53 and p53-CC dramatically inhibited type I IFN responses. Our data suggest that this occurs through p53-mediated inhibition of the NF-κB pathway. Importantly, VSV-encoded p53 or p53-CC did not inhibit antiviral signaling in non-malignant human pancreatic ductal cells, which retained their resistance to all tested VSV recombinants. To the best of our knowledge, this is the first report of p53-mediated inhibition of antiviral signaling, and it suggests that OV-encoded p53 can simultaneously produce anticancer activities while assisting, rather than inhibiting, virus replication in cancer cells.

Huntingtin-associated protein 1 interacts with breakpoint cluster region protein to regulate neuronal differentiation.

Alterations in microtubule-dependent trafficking and certain signaling pathways in neuronal cells represent critical pathogenesis in neurodegenerative diseases. Huntingtin (Htt)-associated protein-1 (Hap1) is a brain-enriched protein and plays a key role in the trafficking of neuronal surviving and differentiating cargos. Lack of Hap1 reduces signaling through tropomyosin-related kinases including extracellular signal regulated kinase (ERK), resulting in inhibition of neurite outgrowth, hypothalamic dysfunction and postnatal lethality in mice. To examine how Hap1 is involved in microtubule-dependent trafficking and neuronal differentiation, we performed a proteomic analysis using taxol-precipitated microtubules from Hap1-null and wild-type mouse brains. Breakpoint cluster region protein (Bcr), a Rho GTPase regulator, was identified as a Hap1-interacting partner. Bcr was co-immunoprecipitated with Hap1 from transfected neuro-2a cells and co-localized with Hap1A isoform more in the differentiated than in the nondifferentiated cells. The Bcr downstream effectors, namely ERK and p38, were significantly less activated in Hap1-null than in wild-type mouse hypothalamus. In conclusion, Hap1 interacts with Bcr on microtubules to regulate neuronal differentiation.

Kaposi's sarcoma-associated herpesvirus microRNAs repress breakpoint cluster region protein expression, enhance Rac1 activity, and increase in vitro angiogenesis.

MicroRNAs (miRNAs) are small, ∼ 22-nucleotide-long RNAs that regulate gene expression posttranscriptionally. Kaposi's sarcoma-associated herpesvirus (KSHV) encodes 12 pre-miRNAs during latency, and the functional significance of these microRNAs during KSHV infection and their cellular targets have been emerging recently. Using a previously reported microarray profiling analysis, we identified breakpoint cluster region mRNA (Bcr) as a cellular target of the KSHV miRNA miR-K12-6-5p (miR-K6-5). Bcr protein levels were repressed in human umbilical vein endothelial cells (HUVECs) upon transfection with miR-K6-5 and during KSHV infection. Luciferase assays wherein the Bcr 3' untranslated region (UTR) was cloned downstream of a luciferase reporter showed repression in the presence of miR-K6-5, and mutation of one of the two predicted miR-K6-5 binding sites relieved this repression. Furthermore, inhibition or deletion of miR-K6-5 in KSHV-infected cells showed increased Bcr protein levels. Together, these results show that Bcr is a direct target of the KSHV miRNA miR-K6-5. To understand the functional significance of Bcr knockdown in the context of KSHV-associated disease, we hypothesized that the knockdown of Bcr, a negative regulator of Rac1, might enhance Rac1-mediated angiogenesis. We found that HUVECs transfected with miR-K6-5 had increased Rac1-GTP levels and tube formation compared to HUVECs transfected with control miRNAs. Knockdown of Bcr in latently KSHV-infected BCBL-1 cells increased the levels of viral RTA, suggesting that Bcr repression by KSHV might aid lytic reactivation. Together, our results reveal a new function for both KSHV miRNAs and Bcr in KSHV infection and suggest that KSHV miRNAs, in part, promote angiogenesis and lytic reactivation. Kaposi's sarcoma (KS)-associated herpesvirus (KSHV) infection is linked to multiple human cancers and lymphomas. KSHV encodes small nucleic acids (microRNAs) that can repress the expression of specific human genes, the biological functions of which are still emerging. This report uses a variety of approaches to show that a KSHV microRNA represses the expression of the human gene called breakpoint cluster region (Bcr). Repression of Bcr correlated with the activation of a protein previously shown to cause KS-like lesions in mice (Rac1), an increase in KS-associated phenotypes (tube formation in endothelial cells and vascular endothelial growth factor [VEGF] synthesis), and modification of the life cycle of the virus (lytic replication). Our results suggest that KSHV microRNAs suppress host proteins and contribute to KS-associated pathogenesis.

Src Homology 2 Domain Containing Protein 5 (SH2D5) Binds the Breakpoint Cluster Region Protein, BCR, and Regulates Levels of Rac1-GTP

SH2D5 is a mammalian-specific, uncharacterized adaptor-like protein that contains an N-terminal phosphotyrosine-binding domain and a C-terminal Src homology 2 (SH2) domain. We show that SH2D5 is highly enriched in adult mouse brain, particularly in Purkinjie cells in the cerebellum and the cornu ammonis of the hippocampus. Despite harboring two potential phosphotyrosine (Tyr(P)) recognition domains, SH2D5 binds minimally to Tyr(P) ligands, consistent with the absence of a conserved Tyr(P)-binding arginine residue in the SH2 domain. Immunoprecipitation coupled to mass spectrometry (IP-MS) from cultured cells revealed a prominent association of SH2D5 with breakpoint cluster region protein, a RacGAP that is also highly expressed in brain. This interaction occurred between the phosphotyrosine-binding domain of SH2D5 and an NxxF motif located within the N-terminal region of the breakpoint cluster region. siRNA-mediated depletion of SH2D5 in a neuroblastoma cell line, B35, induced a cell rounding phenotype correlated with low levels of activated Rac1-GTP, suggesting that SH2D5 affects Rac1-GTP levels. Taken together, our data provide the first characterization of the SH2D5 signaling protein.

Single-cell analysis of K562 cells: An imatinib-resistant subpopulation is adherent and has upregulated expression of BCR-ABL mRNA and protein

In chronic myeloid leukemia (CML) cells from different stages of maturation may have differential expression of BCR-ABL at both messenger RNA (mRNA) and protein level. However, the significance of such differential expression to clinical disease behavior is unknown. Using the CML-derived, BCR-ABL expressing cell line, K562, distinct plastic-adherent (K562/Adh) and nonadherent (K562/NonAdh) subpopulations were established and then analyzed both as single cells and as bulk cell populations. BCR-ABL mRNA was upregulated in K562/Adh compared with K562/NonAdh cells in both single cell and bulk population analyses (p < 0.0001). Similarly, phosphorylation of BCR protein was upregulated in K562/Adh, compared with K562/NonAdh cells (63.42% vs. 23.1%; p = 0.007), and these two K562 subpopulations were found to express significantly different microRNA species. Furthermore, treatment with the BCR-ABL tyrosine kinase inhibitor, imatinib, reduced cell viability more rapidly in K562/NonAdh compared with K562/Adh cells (p < 0.005) both at single and bulk cell levels. This discovery of an adherent subpopulation of K562 cells with increased BCR-ABL mRNA, increased phosphorylated BCR protein expression, differential microRNA expression, and increased imatinib resistance suggests that a similar subpopulation of cells can also mediate clinical resistance to imatinib during treatment of patients with CML.

The Rac-GAP Bcr is a novel regulator of the Par complex that controls cell polarity

Cell polarization is essential for many biological processes, including directed cell migration, and loss of polarity contributes to pathological conditions such as cancer. The Par complex (Par3, Par6, and PKCζ) controls cell polarity in part by recruiting the Rac-specific guanine nucleotide exchange factor T-lymphoma invasion and metastasis 1 (Tiam1) to specialized cellular sites, where Tiam1 promotes local Rac1 activation and cytoskeletal remodeling. However, the mechanisms that restrict Par-Tiam1 complex activity to the leading edge to maintain cell polarity during migration remain unclear. We identify the Rac-specific GTPase-activating protein (GAP) breakpoint cluster region protein (Bcr) as a novel regulator of the Par-Tiam1 complex. We show that Bcr interacts with members of the Par complex and inhibits both Rac1 and PKCζ signaling. Loss of Bcr results in faster, more random migration and striking polarity defects in astrocytes. These polarity defects are rescued by reducing PKCζ activity or by expressing full-length Bcr, but not an N-terminal deletion mutant or the homologous Rac-GAP, Abr, both of which fail to associate with the Par complex. These results demonstrate that Bcr is an integral member of the Par-Tiam1 complex that controls polarized cell migration by locally restricting both Rac1 and PKCζ function.

Proteomics Strategy to Identify Substrates of LNX, a PDZ Domain-containing E3 Ubiquitin Ligase

Ubiquitin ligases (E3s) confer specificity to ubiquitination by recognizing target substrates. However, the substrates of most E3s have not been extensively discovered, and new methods are needed to efficiently and comprehensively identify these substrates. Mostly, E3s specifically recognize substrates via their protein interaction domains. We developed a novel integrated strategy to identify substrates of E3s containing protein interaction domains on a proteomic scale. The binding properties of the protein interaction domains were characterized by screening a random peptide library using a yeast two-hybrid system. Artificial degrons, consisting of a preferential ubiquitination sequence and particular interaction domain-binding motifs, were tested as potential substrates by in vitro ubiquitination assays. Using this strategy, not only substrates but also nonsubstrate regulators can be discovered. The detailed substrate recognition mechanisms, which are useful for drug discovery, can also be characterized. We used the Ligand of Numb protein X (LNX) family of E3s, a group of PDZ domain-containing RING-type E3 ubiquitin ligases, to demonstrate the feasibility of this strategy. Many potential substrates of LNX E3s were identified. Eight of the nine selected candidates were ubiquitinated in vitro, and two novel endogenous substrates, PDZ-binding kinase (PBK) and breakpoint cluster region protein (BCR), were confirmed in vivo. We further revealed that the LNX1-mediated ubiquitination and degradation of PBK inhibited cell proliferation and enhanced sensitivity to doxorubicin-induced apoptosis. The substrate recognition mechanism of LNX E3s was also characterized; this process involves the recognition of substrates via their specific PDZ domains by binding to the C-termini of the target proteins. This strategy can potentially be extended to a variety of E3s that contain protein interaction domain(s), thereby serving as a powerful tool for the comprehensive identification of their substrates on a proteomic scale.

Regulation of Synaptic Rac1 Activity, Long-Term Potentiation Maintenance, and Learning and Memory by BCR and ABR Rac GTPase-Activating Proteins

Rho family small GTPases are important regulators of neuronal development. Defective Rho regulation causes nervous system dysfunctions including mental retardation and Alzheimer's disease. Rac1, a member of the Rho family, regulates dendritic spines and excitatory synapses, but relatively little is known about how synaptic Rac1 is negatively regulated. Breakpoint cluster region (BCR) is a Rac GTPase-activating protein known to form a fusion protein with the c-Abl tyrosine kinase in Philadelphia chromosome-positive chronic myelogenous leukemia. Despite the fact that BCR mRNAs are abundantly expressed in the brain, the neural functions of BCR protein have remained obscure. We report here that BCR and its close relative active BCR-related (ABR) localize at excitatory synapses and directly interact with PSD-95, an abundant postsynaptic scaffolding protein. Mice deficient for BCR or ABR show enhanced basal Rac1 activity but only a small increase in spine density. Importantly, mice lacking BCR or ABR exhibit a marked decrease in the maintenance, but not induction, of long-term potentiation, and show impaired spatial and object recognition memory. These results suggest that BCR and ABR have novel roles in the regulation of synaptic Rac1 signaling, synaptic plasticity, and learning and memory, and that excessive Rac1 activity negatively affects synaptic and cognitive functions.