X-linked mental retardation gene CUL4B targets ubiquitylation of H3K4 methyltransferase component WDR5 and regulates neuronal gene expression

The role of cullin4B in human cancers

Cullin 4B (CUL4B) is a scaffold of the Cullin4B-Ring E3 ligase complex (CRL4B) that plays an important role in proteolysis and is implicated in tumorigenesis. Aberrant expression of CUL4B has been reported in various types of human diseases. Recently, studies have shown that CUL4B was overexpressed in a multitude of solid neoplasms and affect the expression of several tumor suppressor genes. In this review, we aim to summarize the biological function of CUL4B in order to better understand its pathogenesis in human cancers.

A cullin 4B-RING E3 ligase complex fine-tunes pancreatic δ cell paracrine interactions

Somatostatin secreted by pancreatic δ cells mediates important paracrine interactions in Langerhans islets, including maintenance of glucose metabolism through the control of reciprocal insulin and glucagon secretion. Disruption of this circuit contributes to the development of diabetes. However, the precise mechanisms that control somatostatin secretion from islets remain elusive. Here, we found that a super-complex comprising the cullin 4B-RING E3 ligase (CRL4B) and polycomb repressive complex 2 (PRC2) epigenetically regulates somatostatin secretion in islets. Constitutive ablation of CUL4B, the core component of the CRL4B-PRC2 complex, in δ cells impaired glucose tolerance and decreased insulin secretion through enhanced somatostatin release. Moreover, mechanistic studies showed that the CRL4B-PRC2 complex, under the control of the δ cell-specific transcription factor hematopoietically expressed homeobox (HHEX), determines the levels of intracellular calcium and cAMP through histone posttranslational modifications, thereby altering expression of the Cav1.2 calcium channel and adenylyl cyclase 6 (AC6) and modulating somatostatin secretion. In response to high glucose levels or urocortin 3 (UCN3) stimulation, increased expression of cullin 4B (CUL4B) and the PRC2 subunit histone-lysine N-methyltransferase EZH2 and reciprocal decreases in Cav1.2 and AC6 expression were found to regulate somatostatin secretion. Our results reveal an epigenetic regulatory mechanism of δ cell paracrine interactions in which CRL4B-PRC2 complexes, Cav1.2, and AC6 expression fine-tune somatostatin secretion and facilitate glucose homeostasis in pancreatic islets.

Dysregulation of the miR-194-CUL4B negative feedback loop drives tumorigenesis in non-small-cell lung carcinoma

Cullin 4B (CUL4B), a scaffold protein that assembles CRL4B ubiquitin ligase complexes, is overexpressed in many types of cancers and represses many tumor suppressors through epigenetic mechanisms. However, the mechanisms by which CUL4B is upregulated remain to be elucidated. Here, we show that CUL4B is upregulated in non‐small‐cell lung carcinoma (NSCLC) tissues and is critically required for cell proliferation and migration in vitro and for xenograft tumor formation in vivo. We found that microRNA‐194 (miR‐194) and CUL4B protein were inversely correlated in cancer specimens and demonstrated that miR‐194 could downregulate CUL4B by directly targeting its 3′‐UTR. We also showed that CUL4B could be negatively regulated by p53 in a miR‐194‐dependent manner. miR‐194 was further shown to attenuate the malignant phenotype of lung cancer cells by downregulating CUL4B. Interestingly, CRL4B also epigenetically represses miR‐194 by catalyzing monoubiquitination at H2AK119 and by coordinating with PRC2 to promote trimethylation at H3K27 at the gene clusters encoding miR‐194. RBX1, another component in CRL4B complex, is also targeted by miR‐194 in NSCLC cells. Our results thus establish a double‐negative feedback loop between miR‐194 and CRL4B, dysregulation of which contributes to tumorigenesis. The function of miR‐194 as a negative regulator of CUL4B has therapeutic implications in lung cancer.

Guard the guardian: A CRL4 ligase stands watch over histone production

Histones are evolutionarily conserved proteins that together with DNA constitute eukaryotic chromatin in a defined stoichiometry. Core histones are dynamic scaffolding proteins that undergo a myriad of post-translational modifications, which selectively engage chromosome condensation, replication, transcription and DNA damage repair. Cullin4-RING ubiquitin E3 ligases are known to hold pivotal roles in a wide spectrum of chromatin biology ranging from chromatin remodeling and transcriptional repression, to sensing of cytotoxic DNA lesions. Our recent work uncovers an unexpected function of a CRL4 ligase upstream of these processes in promoting histone biogenesis. The CRL4^WDR23 ligase directly controls the activity of the stem-loop binding protein (SLBP), which orchestrates elemental steps of canonical histone transcript metabolism. We demonstrate that non-proteolytic ubiquitination of SLBP ensures sufficient histone reservoirs during DNA replication and is vital for genome integrity and cellular fitness.

Polycomb PRC2 complex mediates epigenetic silencing of a critical osteogenic master regulator in the hippocampus

During hippocampal neuron differentiation, the expression of critical inducers of non-neuronal cell lineages must be efficiently silenced. Runx2 transcription factor is the master regulator of mesenchymal cells responsible for intramembranous osteoblast differentiation and formation of the craniofacial bone tissue that surrounds and protects the central nervous system (CNS) in mammalian embryos. The molecular mechanisms that mediate silencing of the Runx2 gene and its downstream target osteogenic-related genes in neuronal cells have not been explored. Here, we assess the epigenetic mechanisms that mediate silencing of osteoblast-specific genes in CNS neurons. In particular, we address the contribution of histone epigenetic marks and histone modifiers on the silencing of the Runx2/p57 bone-related isoform in rat hippocampal tissues at embryonic to adult stages. Our results indicate enrichment of repressive chromatin histone marks and of the Polycomb PRC2 complex at the Runx2/p57 promoter region. Knockdown of PRC2 H3K27-methyltransferases Ezh2 and Ezh1, or forced expression of the Trithorax/COMPASS subunit Wdr5 activates Runx2/p57 mRNA expression in both immature and mature hippocampal cells. Together these results indicate that complementary epigenetic mechanisms progressively and efficiently silence critical osteoblastic genes during hippocampal neuron differentiation.

The antiobesity factor WDTC1 suppresses adipogenesis via the CRL4WDTC1 E3 ligase

WDTC1/Adp encodes an evolutionarily conserved suppressor of lipid accumulation. While reduced WDTC1 expression is associated with obesity in mice and humans, its cellular function is unknown. Here, we demonstrate that WDTC1 is a component of a DDB1-CUL4-ROC1 (CRL4) E3 ligase. Using 3T3-L1 cell culture model of adipogenesis, we show that disrupting the interaction between WDTC1 and DDB1 leads to a loss of adipogenic suppression by WDTC1, increased triglyceride accumulation and adipogenic gene expression. We show that the CRL4(WDTC) (1) complex promotes histone H2AK119 monoubiquitylation, thus suggesting a role for this complex in transcriptional repression during adipogenesis. Our results identify a biochemical role for WDTC1 and extend the functional range of the CRL4 complex to the suppression of fat accumulation.

Accelerated hepatocellular carcinoma development in CUL4B transgenic mice

Cullin 4B (CUL4B) is a component of the Cullin 4B-Ring E3 ligase (CRL4B) complex that functions in proteolysis and in epigenetic regulation. CUL4B possesses tumor-promoting properties and is markedly upregulated in many types of human cancers. To determine the role of CUL4B in liver tumorigenesis, we generated transgenic mice that expressed human CUL4B in livers and other tissues and evaluated the development of spontaneous and chemically-induced hepatocellular carcinomas. We observed that CUL4B transgenic mice spontaneously developed liver tumors at a high incidence at old ages and exhibited enhanced DEN-induced hepatocarcinogenesis. There was a high proliferation rate in the livers of CUL4B transgenic mice that was accompanied by increased levels of Cdk1, Cdk4 and cyclin D1 and decreased level of p16. The transgenic mice also exhibited increased compensatory proliferation after DEN-induced liver injury, which was accompanied by activation of Akt, Erk, p38 and NF-κB. We also found that Prdx3 was downregulated and that DEN induced a higher level of reactive oxygen species in the livers of transgenic mice. Together, our results demonstrate a critical role of CUL4B in hepatocarcinogenesis in mice.

Variants in CUL4B are associated with cerebral malformations

Variants in cullin 4B (CUL4B) are a known cause of syndromic X-linked intellectual disability. Here, we describe an additional 25 patients from 11 families with variants in CUL4B. We identified nine different novel variants in these families and confirmed the pathogenicity of all nontruncating variants. Neuroimaging data, available for 15 patients, showed the presence of cerebral malformations in ten patients. The cerebral anomalies comprised malformations of cortical development (MCD), ventriculomegaly, and diminished white matter volume. The phenotypic heterogeneity of the cerebral malformations might result from the involvement of CUL-4B in various cellular pathways essential for normal brain development. Accordingly, we show that CUL-4B interacts with WDR62, a protein in which variants were previously identified in patients with microcephaly and a wide range of MCD. This interaction might contribute to the development of cerebral malformations in patients with variants in CUL4B.

Distinct and overlapping functions of the cullin E3 ligase scaffolding proteins CUL4A and CUL4B

The cullin 4 subfamily of genes includes CUL4A and CUL4B, which share a mostly identical amino acid sequence aside from the elongated N-terminal region in CUL4B. Both act as scaffolding proteins for modular cullin RING ligase 4 (CRL4) complexes which promote the ubiquitination of a variety of substrates. CRL4 function is vital to cells as loss of both genes or their shared substrate adaptor protein DDB1 halts proliferation and eventually leads to cell death. Due to their high structural similarity, CUL4A and CUL4B share a substantial overlap in function. However, in some cases, differences in subcellular localization, spatiotemporal expression patterns and stress-inducibility preclude functional compensation. In this review, we highlight the most essential functions of the CUL4 genes in: DNA repair and replication, chromatin-remodeling, cell cycle regulation, embryogenesis, hematopoiesis and spermatogenesis. CUL4 genes are also clinically relevant as dysregulation can contribute to the onset of cancer and CRL4 complexes are often hijacked by certain viruses to promote viral replication and survival. Also, mutations in CUL4B have been implicated in a subset of patients suffering from syndromic X-linked intellectual disability (AKA mental retardation). Interestingly, the antitumor effects of immunomodulatory drugs are caused by their binding to the CRL4CRBN complex and re-directing the E3 ligase towards the Ikaros transcription factors IKZF1 and IKZF3. Because of their influence over key cellular functions and relevance to human disease, CRL4s are considered promising targets for therapeutic intervention.

S6 Kinase- and β-TrCP2-Dependent Degradation of p19Arf Is Required for Cell Proliferation

The kinase mTOR (mammalian target of rapamycin) promotes translation as well as cell survival and proliferation under nutrient-rich conditions. Whereas mTOR activates translation through ribosomal protein S6 kinase (S6K) and eukaryotic translation initiation factor 4E-binding protein (4E-BP), how it facilitates cell proliferation has remained unclear. We have now identified p19(Arf), an inhibitor of cell cycle progression, as a novel substrate of S6K that is targeted to promote cell proliferation. Serum stimulation induced activation of the mTOR-S6K axis and consequent phosphorylation of p19(Arf) at Ser(75). Phosphorylated p19(Arf) was then recognized by the F-box protein β-TrCP2 and degraded by the proteasome. Ablation of β-TrCP2 thus led to the arrest of cell proliferation as a result of the stabilization and accumulation of p19(Arf). The β-TrCP2 paralog β-TrCP1 had no effect on p19(Arf) stability, suggesting that phosphorylated p19(Arf) is a specific substrate of β-TrCP2. Mice deficient in β-TrCP2 manifested accumulation of p19(Arf) in the yolk sac and died in utero. Our results suggest that the mTOR pathway promotes cell proliferation via β-TrCP2-dependent p19(Arf) degradation under nutrient-rich conditions.

Akt Phosphorylates Wnt Coactivator and Chromatin Effector Pygo2 at Serine 48 to Antagonize Its Ubiquitin/Proteasome-mediated Degradation

Pygopus 2 (Pygo2/PYGO2) is an evolutionarily conserved coactivator and chromatin effector in the Wnt/β-catenin signaling pathway that regulates cell growth and differentiation in various normal and malignant tissues. Although PYGO2 is highly overexpressed in a number of human cancers, the molecular mechanism underlying its deregulation is largely unknown. Here we report that Pygo2 protein is degraded through the ubiquitin/proteasome pathway and is posttranslationally stabilized through phosphorylation by activated phosphatidylinositol 3-kinase/Akt signaling. Specifically, Pygo2 is stabilized upon inhibition of the proteasome, and its intracellular level is regulated by Cullin 4 (Cul4) and DNA damage-binding protein 1 (DDB1), components of the Cul4-DDB1 E3 ubiquitin ligase complex. Furthermore, Pygo2 is phosphorylated at multiple residues, and Akt-mediated phosphorylation at serine 48 leads to its decreased ubiquitylation and increased stability. Finally, we provide evidence that Akt and its upstream growth factors act in parallel with Wnt to stabilize Pygo2. Taken together, our findings highlight chromatin regulator Pygo2 as a common node downstream of oncogenic Wnt and Akt signaling pathways and underscore posttranslational modification, particularly phosphorylation and ubiquitylation, as a significant mode of regulation of Pygo2 protein expression.

A Non-Canonical Function of Gβ as a Subunit of E3 Ligase in Targeting GRK2 Ubiquitylation

G protein-coupled receptors (GPCRs) comprise the largest family of cell surface receptors, regulate a wide range of physiological processes, and are the major targets of pharmaceutical drugs. Canonical signaling from GPCRs is relayed to intracellular effector proteins by trimeric G proteins, composed of α, β, and γ subunits (Gαβγ). Here, we report that G protein β subunits (Gβ) bind to DDB1 and that Gβ2 targets GRK2 for ubiquitylation by the DDB1-CUL4A-ROC1 ubiquitin ligase. Activation of GPCR results in PKA-mediated phosphorylation of DDB1 at Ser645 and its dissociation from Gβ2, leading to increase of GRK2 protein. Deletion of Cul4a results in cardiac hypertrophy in male mice that can be partially rescued by the deletion of one Grk2 allele. These results reveal a non-canonical function of the Gβ protein as a ubiquitin ligase component and a mechanism of feedback regulation of GPCR signaling.

Lack of CUL4B leads to increased abundance of GFAP-positive cells that is mediated by PTGDS in mouse brain

Astrocytes are the most abundant cell type in the mammalian brain and are important for the functions of the central nervous system. Glial fibrillary acidic protein (GFAP) is regarded as a hallmark of mature astrocytes, though some GFPA-positive cells may act as neural stem cells. Missense heterozygous mutations in GFAP cause Alexander disease that manifests leukodystrophy and intellectual disability. Here, we show that CUL4B, a scaffold protein that assembles E3 ubiquitin ligase, represses the expression of GFAP in neural progenitor cells (NPCs) during brain development. Lack of Cul4b in NPCs in cultures led to increased generation of astrocytes, marked by GFAP and S100β. The GFAP+ cells were also found to be more abundant in the brains of nervous system-specific Cul4b knockout mice in vivo. Moreover, we demonstrated that the increased generation of GFAP+ cells from Cul4b-null NPCs was mediated by an upregulation of prostaglandin D2 synthase PTGDS. We showed that the increased GFAP expression can be attenuated by pharmacological inhibition of the PTGDS enzymatic activity or by shRNA-mediated knockdown of Ptgds. Importantly, exogenously added PTGDS could promote the generation of GFAP+ cells from wild-type NPCs. We further observed that Ptgds is targeted and repressed by the CUL4B/PRC2 complex. Together, our results demonstrate CUL4B as a negative regulator of GFAP expression during neural development.

Regulation of histone H3K4 methylation in brain development and disease

The growing list of mutations implicated in monogenic disorders of the developing brain includes at least seven genes (ARX, CUL4B, KDM5A, KDM5C, KMT2A, KMT2C, KMT2D) with loss-of-function mutations affecting proper regulation of histone H3 lysine 4 methylation, a chromatin mark which on a genome-wide scale is broadly associated with active gene expression, with its mono-, di- and trimethylated forms differentially enriched at promoter and enhancer and other regulatory sequences. In addition to these rare genetic syndromes, dysregulated H3K4 methylation could also play a role in the pathophysiology of some cases diagnosed with autism or schizophrenia, two conditions which on a genome-wide scale are associated with H3K4 methylation changes at hundreds of loci in a subject-specific manner. Importantly, the reported alterations for some of the diseased brain specimens included a widespread broadening of H3K4 methylation profiles at gene promoters, a process that could be regulated by the UpSET(KMT2E/MLL5)-histone deacetylase complex. Furthermore, preclinical studies identified maternal immune activation, parental care and monoaminergic drugs as environmental determinants for brain-specific H3K4 methylation. These novel insights into the epigenetic risk architectures of neurodevelopmental disease will be highly relevant for efforts aimed at improved prevention and treatment of autism and psychosis spectrum disorders.

DNA damage-induced activation of CUL4B targets HUWE1 for proteasomal degradation

The E3 ubiquitin ligase HUWE1/Mule/ARF-BP1 plays an important role in integrating/coordinating diverse cellular processes such as DNA damage repair and apoptosis. A previous study has shown that HUWE1 is required for the early step of DNA damage-induced apoptosis, by targeting MCL-1 for proteasomal degradation. However, HUWE1 is subsequently inactivated, promoting cell survival and the subsequent DNA damage repair process. The mechanism underlying its regulation during this process remains largely undefined. Here, we show that the Cullin4B-RING E3 ligase (CRL4B) is required for proteasomal degradation of HUWE1 in response to DNA damage. CUL4B is activated in a NEDD8-dependent manner, and ubiquitinates HUWE1 in vitro and in vivo. The depletion of CUL4B stabilizes HUWE1, which in turn accelerates the degradation of MCL-1, leading to increased induction of apoptosis. Accordingly, cells deficient in CUL4B showed increased sensitivity to DNA damage reagents. More importantly, upon CUL4B depletion, these phenotypes can be rescued through simultaneous depletion of HUWE1, consistent with the role of CUL4B in regulating HUWE1. Collectively, these results identify CRL4B as an essential E3 ligase in targeting the proteasomal degradation of HUWE1 in response to DNA damage, and provide a potential strategy for cancer therapy by targeting HUWE1 and the CUL4B E3 ligase.

NEDDylation is essential for Kaposi's sarcoma-associated herpesvirus latency and lytic reactivation and represents a novel anti-KSHV target

Kaposi’s sarcoma-associated herpesvirus (KSHV) is the causative agent of Kaposi's sarcoma (KS) and primary effusion lymphoma (PEL), which are aggressive malignancies associated with immunocompromised patients. For many non-viral malignancies, therapeutically targeting the ubiquitin proteasome system (UPS) has been successful. Likewise, laboratory studies have demonstrated that inhibition of the UPS might provide a promising avenue for the treatment of KSHV-associated diseases. The largest class of E3 ubiquitin ligases are the cullin-RING ligases (CRLs) that are activated by an additional ubiquitin-like protein, NEDD8. We show that pharmacological inhibition of NEDDylation (using the small molecule inhibitor MLN4924) is cytotoxic to PEL cells by inhibiting NF-κB. We also show that CRL4B is a novel regulator of latency as its inhibition reactivated lytic gene expression. Furthermore, we uncovered a requirement for NEDDylation during the reactivation of the KSHV lytic cycle. Intriguingly, inhibition prevented viral DNA replication but not lytic cycle-associated gene expression, highlighting a novel mechanism that uncouples these two features of KSHV biology. Mechanistically, we show that MLN4924 treatment precluded the recruitment of the viral pre-replication complex to the origin of lytic DNA replication (OriLyt). These new findings have revealed novel mechanisms that regulate KSHV latency and reactivation. Moreover, they demonstrate that inhibition of NEDDylation represents a novel approach for the treatment of KSHV-associated malignancies.

Kaposi’s sarcoma-associated herpesvirus (KSHV) causes Kaposi’s sarcoma (KS) and primary effusion lymphoma (PEL), often fatal malignancies afflicting HIV-infected patients. Previous research has shown that blockade of the ubiquitin proteasome system (UPS, a normal quality control pathway that degrades cellular proteins) is able to kill KSHV-infected lymphoma cells. A large component of the UPS is made up by the protein family known as the cullin-RING ubiquitin ligases (CRLs), which are activated by NEDD8 (a process known as NEDDylation). Recently, an inhibitor of NEDDylation (MLN4924) was developed and is currently in clinical trials as an anti-cancer drug. As NEDDylation has not been investigated for many viruses, we used this to compound examine its importance in KSHV biology. Firstly we show that NEDDylation is essential for the viability of KSHV-infected lymphoma cells, and MLN4924 treatment killed these cells by blocking NF-κB activity (required for KSHV latency gene expression and KSHV-associated cancer). Furthermore, we show that NEDDylation is required for KSHV to replicate its genome, a critical step in the production of new virus particles. Therefore, this research has identified a novel molecular mechanism that governs KSHV replication. Furthermore, it demonstrates that NEDDylation is a viable target for the treatment of KSHV-associated malignancies.

WD repeat-containing protein 5 (WDR5) localizes to the midbody and regulates abscission

Cytokinesis partitions the cytoplasm of a parent cell into two daughter cells and is essential for the completion of cell division. The final step of cytokinesis in animal cells is abscission, which is a process leading to the physical separation of two daughter cells. Abscission requires membrane traffic and microtubule disassembly at a specific midbody region called the secondary ingression. Here, we report that WD repeat-containing protein 5 (WDR5), a core subunit of COMPASS/MLL family histone H3 lysine 4 methyltransferase (H3K4MT) complexes, resides at the midbody and associates with a subset of midbody regulatory proteins, including PRC1 and CYK4/MKLP1. Knockdown of WDR5 impairs abscission and increases the incidence of multinucleated cells. Further investigation revealed that the abscission delay is primarily due to slower formation of secondary ingressions in WDR5 knockdown cells. Consistent with these defects, midbody microtubules in WDR5 knockdown cells also display enhanced resistance to depolymerization by nocodazole. Recruitment of WDR5 to the midbody dark zone appears to require integrity of the WDR5 central arginine-binding cavity, as mutations that disrupt histone H3 and MLL1 binding to this pocket also abolish the midbody localization of WDR5. Taken together, these data suggest that WDR5 is specifically targeted to the midbody in the absence of chromatin and that it promotes abscission, perhaps by facilitating midbody microtubule disassembly.

Intellectual disability-associated dBRWD3 regulates gene expression through inhibition of HIRA/YEM-mediated chromatin deposition of histone H3.3

Many causal mutations of intellectual disability have been found in genes involved in epigenetic regulations. Replication-independent deposition of the histone H3.3 variant by the HIRA complex is a prominent nucleosome replacement mechanism affecting gene transcription, especially in postmitotic neurons. However, how HIRA-mediated H3.3 deposition is regulated in these cells remains unclear. Here, we report that dBRWD3, the Drosophila ortholog of the intellectual disability gene BRWD3, regulates gene expression through H3.3, HIRA, and its associated chaperone Yemanuclein (YEM), the fly ortholog of mammalian Ubinuclein1. In dBRWD3 mutants, increased H3.3 levels disrupt gene expression, dendritic morphogenesis, and sensory organ differentiation. Inactivation of yem or H3.3 remarkably suppresses the global transcriptome changes and various developmental defects caused by dBRWD3 mutations. Our work thus establishes a previously unknown negative regulation of H3.3 and advances our understanding of BRWD3-dependent intellectual disability.

CUL4B activates Wnt/β-catenin signalling in hepatocellular carcinoma by repressing Wnt antagonists

Activation of Wnt/β-catenin signalling is frequently observed in many types of cancer including hepatocellular carcinoma (HCC). We recently reported that cullin 4B (CUL4B), a scaffold protein that assembles CRL4B ubiquitin ligase complexes, is overexpressed in many types of solid tumours and contributes to epigenetic silencing of tumour suppressors. In this study, we characterized the function of CUL4B in HCC and investigated whether CUL4B is involved in the regulation of Wnt/β-catenin signalling. CUL4B and β-catenin were frequently up-regulated and positively correlated in HCC tissues. CUL4B activated Wnt/β-catenin signalling by protecting β-catenin from GSK3-mediated degradation, achieved through CUL4B-mediated epigenetic silencing of Wnt pathway antagonists. Knockdown of CUL4B resulted in the up-regulation of Wnt signal antagonists such as DKK1 and PPP2R2B. Simultaneous knockdown of PPP2R2B partially reversed the down-regulation of β-catenin signalling caused by CUL4B depletion. Furthermore, CRL4B promoted the recruitment and/or retention of PRC2 at the promoters of Wnt antagonists and CUL4B knockdown decreased the retention of PRC2 components as well as H3K27me3. Knockdown of CUL4B reduced the proliferation, colony formation, and invasiveness of HCC cells in vitro and inhibited tumour growth in vivo, and these effects were attenuated by introduction of exogenous β-catenin or simultaneous knockdown of PPP2R2B. Conversely, ectopic expression of CUL4B enhanced the proliferation and invasiveness of HCC cells. We conclude that CUL4B can up-regulate Wnt/β-catenin signalling in human HCC through transcriptionally repressing Wnt antagonists and thus contributes to the malignancy of HCC.

CRL4(VprBP) E3 ligase promotes monoubiquitylation and chromatin binding of TET dioxygenases

DNA methylation at the C-5 position of cytosine (5mC) regulates gene expression and plays pivotal roles in various biological processes. The TET dioxygenases catalyze iterative oxidation of 5mC, leading to eventual demethylation. Inactivation of TET enzymes causes multistage developmental defects, impaired cell reprogramming, and hematopoietic malignancies. However, little is known about how TET activity is regulated. Here we show that all three TET proteins bind to VprBP and are monoubiquitylated by the VprBP-DDB1-CUL4-ROC1 E3 ubiquitin ligase (CRL4(VprBP)) on a highly conserved lysine residue. Deletion of VprBP in oocytes abrogated paternal DNA hydroxymethylation in zygotes. VprBP-mediated monoubiquitylation promotes TET binding to chromatin. Multiple recurrent TET2-inactivating mutations derived from leukemia target either the monoubiquitylation site (K1299) or residues essential for VprBP binding. Cumulatively, our data demonstrate that CRL4(VprBP) is a critical regulator of TET dioxygenases during development and in tumor suppression.

CUL4A-DDB1-Rbx1 E3 ligase controls the quality of the PTS2 receptor Pex7p

Pex7p is the cytosolic receptor for peroxisomal matrix proteins harbouring PTS2 (peroxisome-targeting signal type-2). Mutations in the PEX7 gene cause RCDP (rhizomelic chondrodysplasia punctata) type 1, a distinct PTS2-import-defective phenotype of peroxisome biogenesis disorders. The mechanisms by which the protein level and quality of Pex7p are controlled remain largely unknown. In the present study we show that dysfunctional Pex7p, including mutants from RCDP patients, is degraded by a ubiquitin-dependent proteasomal pathway involving the CRL4A (Cullin4A-RING ubiquitin ligase) complex. Furthermore, we demonstrate that the degradation of dysfunctional Pex7p is essential for maintaining normal PTS2 import, thereby suggesting that CRL4A functions as an E3 ligase in the quality control of Pex7p. Our results define a mechanism underlying Pex7p homoeostasis and highlight its importance for regulating PTS2 import. These findings may lead to a new approach to Pex7p-based therapies for the treatment of peroxisome biogenesis disorders such as RCDP.

CRL4B interacts with and coordinates the SIN3A-HDAC complex to repress CDKN1A and drive cell cycle progression

CUL4B, a scaffold protein that assembles the CRL4B ubiquitin ligase complex, participates in the regulation of a broad spectrum of biological processes. Here, we demonstrate a crucial role of CUL4B in driving cell cycle progression. We show that loss of CUL4B results in a significant reduction in cell proliferation and causes G1 cell cycle arrest, accompanied by the upregulation of the cyclin-dependent kinase (CDK) inhibitors (CKIs) p21 and p57 (encoded by CDKN1A and CDKN1C, respectively). Strikingly, CUL4B was found to negatively regulate the function of p21 through transcriptional repression, but not through proteolysis. Furthermore, we demonstrate that CRL4B and SIN3A-HDAC complexes interact with each other and co-occupy the CDKN1A and CDKN1C promoters. Lack of CUL4B led to a decreased retention of SIN3A-HDAC components and increased levels of acetylated H3 and H4. Interestingly, the ubiquitylation function of CRL4B is not required for the stable retention of SIN3A-HDAC on the promoters of target genes. Thus, in addition to directly contributing to epigenetic silencing by catalyzing H2AK119 monoubiquitylation, CRL4B also facilitates the deacetylation function of SIN3A-HDAC. Our findings reveal a coordinated action between CRL4B and SIN3A-HDAC complexes in transcriptional repression.

CRL4A(CRBN) E3 ubiquitin ligase restricts BK channel activity and prevents epileptogenesis

Ion channels regulate membrane excitation, and mutations of ion channels often cause serious neurological disorders including epilepsy. Compared with extensive analyses of channel protein structure and function, much less is known about the fine tuning of channel activity by post-translational modification. Here we report that the large conductance, Ca(2+)- and voltage-activated K(+) (BK) channels are targeted by the E3 ubiquitin ligase CRL4A(CRBN) for polyubiquitination and retained in the endoplasmic reticulum (ER). Inactivation of CRL4A(CRBN) releases deubiquitinated BK channels from the ER to the plasma membrane, leading to markedly enhanced channel activity. Mice with CRL4A(CRBN) mutation in the brain or treated with a CRL4A(CRBN) inhibitor are very sensitive to seizure induction, which can be attenuated by blocking BK channels. Finally, the mutant mice develop spontaneous epilepsy when aged. Therefore, ubiquitination of BK channels before their cell surface expression is an important step to prevent systemic neuronal excitability and epileptogenesis.

Genome instability in blood cells of a BRCA1+ breast cancer family

Background

BRCA1 plays an essential role in maintaining genome stability. Inherited BRCA1 germline mutation (BRCA1+) is a determined genetic predisposition leading to high risk of breast cancer. While BRCA1+ induces breast cancer by causing genome instability, most of the knowledge is known about somatic genome instability in breast cancer cells but not germline genome instability.

Methods

Using the exome-sequencing method, we analyzed the genomes of blood cells in a typical BRCA1+ breast cancer family with an exon 13-duplicated founder mutation, including six breast cancer-affected and two breast cancer unaffected members.

Results

We identified 23 deleterious mutations in the breast cancer-affected family members, which are absent in the unaffected members. Multiple mutations damaged functionally important and breast cancer-related genes, including transcriptional factor BPTF and FOXP1, ubiquitin ligase CUL4B, phosphorylase kinase PHKG2, and nuclear receptor activator SRA1. Analysis of the mutations between the mothers and daughters shows that most mutations were germline mutation inherited from the ancestor(s) while only a few were somatic mutation generated de novo.

Conclusion

Our study indicates that BRCA1+ can cause genome instability with both germline and somatic mutations in non-breast cells.

Cullin4A and cullin4B are interchangeable for HIV Vpr and Vpx action through the CRL4 ubiquitin ligase complex

Human immunodeficiency virus (HIV) seizes control of cellular cullin-RING E3 ubiquitin ligases (CRLs) to promote viral replication. HIV-1 Vpr and HIV-2/simian immunodeficiency virus (SIV) Vpr and Vpx engage the cullin4 (CUL4)-containing ubiquitin ligase complex (CRL4) to cause polyubiquitination and proteasomal degradation of host proteins, including ones that block infection. HIV-1 Vpr engages CRL4 to trigger the degradation of uracil-N-glycosylase 2 (UNG2). Both HIV-1 Vpr and HIV-2/SIV Vpr tap CRL4 to initiate G2 cell cycle arrest. HIV-2/SIV Vpx secures CRL4 to degrade the antiviral protein SAMHD1. CRL4 includes either cullin4A (CUL4A) or cullin4B (CUL4B) among its components. Whether Vpr or Vpx relies on CUL4A, CUL4B, or both to act through CRL4 is not known. Reported structural, phenotypic, and intracellular distribution differences between the two CUL4 types led us to hypothesize that Vpr and Vpx employ these in a function-specific manner. Here we determined CUL4 requirements for HIV-1 and HIV-2/SIV Vpr-mediated G2 cell cycle arrest, HIV-1 Vpr-mediated UNG2 degradation, and HIV-2 Vpx-mediated SAMHD1 degradation. Surprisingly, CUL4A and CUL4B are exchangeable for CRL4-dependent Vpr and Vpx action, except in primary macrophages, where Vpx relies on both CUL4A and CUL4B for maximal SAMHD1 depletion. This work highlights the need to consider both CUL4 types for Vpr and Vpx functions and also shows that the intracellular distribution of CUL4A and CUL4B can vary by cell type.

IMPORTANCE

The work presented here shows for the first time that HIV Vpr and Vpx do not rely exclusively on CUL4A to cause ubiquitination through the CRL4 ubiquitin ligase complex. Furthermore, our finding that intracellular CUL4 and SAMHD1 distributions can vary with cell type provides the basis for reconciling previous disparate findings regarding the site of SAMHD1 depletion. Finally, our observations with primary immune cells provide insight into the cell biology of CUL4A and CUL4B that will help differentiate the functions of these similar proteins.

Intellectual disability and autism spectrum disorders: causal genes and molecular mechanisms

Intellectual disability (ID) and autism spectrum disorder (ASD) are the most common developmental disorders present in humans. Combined, they affect between 3 and 5% of the population. Additionally, they can be found together in the same individual thereby complicating treatment. The causative factors (genes, epigenetic and environmental) are quite varied and likely interact so as to further complicate the assessment of an individual patient. Nonetheless, much valuable information has been gained by identifying candidate genes for ID or ASD. Understanding the etiology of either ID or ASD is of utmost importance for families. It allows a determination of the risk of recurrence, the possibility of other comorbidity medical problems, the molecular and cellular nature of the pathobiology and hopefully potential therapeutic approaches.

Essential role of lncRNA binding for WDR5 maintenance of active chromatin and embryonic stem cell pluripotency

The WDR5 subunit of the MLL complex enforces active chromatin and can bind RNA; the relationship between these two activities is unclear. Here we identify a RNA binding pocket on WDR5, and discover a WDR5 mutant (F266A) that selectively abrogates RNA binding without affecting MLL complex assembly or catalytic activity. Complementation in ESCs shows that WDR5 F266A mutant is unable to accumulate on chromatin, and is defective in gene activation, maintenance of histone H3 lysine 4 trimethylation, and ESC self renewal. We identify a family of ESC messenger and lncRNAs that interact with wild type WDR5 but not F266A mutant, including several lncRNAs known to be important for ESC gene expression. These results suggest that specific RNAs are integral inputs into the WDR5-MLL complex for maintenance of the active chromatin state and embryonic stem cell fates. DOI: http://dx.doi.org/10.7554/eLife.02046.001.

A Cul4 E3 ubiquitin ligase regulates histone hand-off during nucleosome assembly

Nucleosome assembly following DNA replication and gene transcription is important to maintain genome stability and epigenetic information. Newly synthesized histones H3-H4 first bind histone chaperone Asf1 and are then transferred to other chaperones for nucleosome assembly. However, it is unknown how H3-H4 is transferred from the Asf1-H3-H4 complex to other chaperones because Asf1 binds H3-H4 with high affinity. Here, we show that yeast Rtt101(Mms1) E3 ubiquitin ligase preferentially binds and ubiquitylates new histone H3 acetylated at lysine 56. Inactivation of Rtt101 or mutating H3 lysine residues ubiquitylated by the Rtt101(Mms1) ligase impairs nucleosome assembly and promotes Asf1-H3 interactions. Similar phenotypes occur in human cells in which the ortholog of Rtt101(Mms1), Cul4A(DDB1), is depleted. These results indicate that the transfer of H3-H4 from the Asf1-H3-H4 complex to other histone chaperones is regulated by a conserved E3 ligase and provide evidence for crosstalk between histone acetylation and ubiquitylation in nucleosome assembly.

E3 ubiquitin ligase Cullin4B mediated polyubiquitination of p53 for its degradation

Controlled protein ubiquitination through E3 ubiquitin ligases and degradation via 26S proteasome machinery is required for orderly progression through cell cycle, chromatin remodeling, DNA repair, and development. Each cullin-dependent ubiquitin ligase (E3) complex can recruit various substrates for their degradation. Cullin 4A (CUL4A) and Cullin 4B (CUL4B) are members of cullin family proteins that mediate ubiquitin dependent proteolysis. Though, these two cul4 genes are functionally redundant, Cullin 4B is not a substitute for all the Cullin 4A functions. Published report has shown that CUL4A interacts with p53 and induces its decay. Although, CUL4A has been known to control several cellular processes, little is known about CUL4B functions. Therefore, in this study, we analyzed the role of CUL4B on p53 polyubiquitination. Our stable cell line and transient transfection studies show that CUL4B indeed interacts with p53 and induces its polyubiquitination. Importantly, both CUL4A and CUL4B overexpressing cells show almost equal levels of p53 polyubiquitination. Moreover, we observed an increased level of polyubiquitination on p53 in CUL4B overexpressing stable cell line upon treatment with siRNA specific for CUL4A indicating that CUL4B plays a vital role in p53 stability. In addition, we have observed the differential expression of CUL4B in various eukaryotic cell lines and mouse tissues suggesting the important role of CUL4B in various tissues. Together, these observations establish an important negative regulatory role of CUL4B on p53 stability.

CRL4B promotes tumorigenesis by coordinating with SUV39H1/HP1/DNMT3A in DNA methylation-based epigenetic silencing

Cullin 4B (CUL4B) is a component of the Cullin4B-Ring E3 ligase complex (CRL4B) that functions in proteolysis and is implicated in tumorigenesis. Here, we report that CRL4B is associated with histone methyltransferase SUV39H1, heterochromatin protein 1 (HP1) and DNA methyltransferases 3A (DNMT3A). We showed that CRL4B, through catalyzing H2AK119 monoubiquitination, facilitates H3K9 tri-methylation and DNA methylation, two key epigenetic modifications involved in DNA methylation-based gene silencing. Depletion of CUL4B resulted in loss of not only H2AK119 monoubiquitination but also H3K9 trimethylation and DNA methylation, leading to derepression of a collection of genes, including the tumor suppressor IGFBP3. We demonstrated that CUL4B promotes cell proliferation and invasion, which are consistent with a tumorigenic phenotype, at least partially by repressing IGFBP3. We found that the expression of CUL4B is markedly upregulated in samples of human cervical carcinoma and is negatively correlated with the expression of IGFBP3. Our experiments unveiled a coordinated action between histone ubiquitination/methylation and DNA methylation in transcription repression, providing a mechanism for CUL4B in tumorigenesis.

Cullin-RING Ligases as attractive anti-cancer targets

The ubiquitin-proteasome system (UPS) promotes the timely degradation of short-lived proteins with key regulatory roles in a vast array of biological processes, such as cell cycle progression, oncogenesis and genome integrity. Thus, abnormal regulation of UPS disrupts the protein homeostasis and causes many human diseases, particularly cancer. Indeed, the FDA approval of bortezomib, the first class of general proteasome inhibitor, for the treatment of multiple myeloma, demonstrated that the UPS can be an attractive anti-cancer target. However, normal cell toxicity associated with bortezomib, resulting from global inhibition of protein degradation, promotes the focus of drug discovery efforts on targeting enzymes upstream of the proteasome for better specificity. E3 ubiquitin ligases, particularly those known to be activated in human cancer, become an attractive choice. Cullin-RING Ligases (CRLs) with multiple components are the largest family of E3 ubiquitin ligases and are responsible for ubiquitination of ~20% of cellular proteins degraded through UPS. Activity of CRLs is dynamically regulated and requires the RING component and cullin neddylation. In this review, we will introduce the UPS and CRL E3s and discuss the biological processes regulated by each of eight CRLs through substrate degradation. We will further discuss how cullin neddylation controls CRL activity, and how CRLs are being validated as the attractive cancer targets by abrogating the RING component through genetic means and by inhibiting cullin neddylation via MLN4924, a small molecule indirect inhibitor of CRLs, currently in several Phase I clinical trials. Finally, we will discuss current efforts and future perspectives on the development of additional inhibitors of CRLs by targeting E2 and/or E3 of cullin neddylation and CRL-mediated ubiquitination as potential anti-cancer agents.

CRL4A-FBXW5-mediated degradation of DLC1 Rho GTPase-activating protein tumor suppressor promotes non-small cell lung cancer cell growth

DLC1 encodes a RhoA GTPase-activating protein and tumor suppressor lost in cancer by genomic deletion or epigenetic silencing and loss of DLC1 gene transcription. We unexpectedly identified non-small cell lung cancer (NSCLC) cell lines and tumor tissue that expressed DLC1 mRNA yet lacked DLC1 protein expression. We determined that DLC1 was ubiquitinated and degraded by cullin 4A-RING ubiquitin ligase (CRL4A) complex interaction with DDB1 and the FBXW5 substrate receptor. siRNA-mediated suppression of cullin 4A, DDB1, or FBXW5 expression restored DLC1 protein expression in NSCLC cell lines. FBXW5 suppression-induced DLC1 reexpression was associated with a reduction in the levels of activated RhoA-GTP and in RhoA effector signaling. Finally, FBXW5 suppression caused a DLC1-dependent decrease in NSCLC anchorage-dependent and -independent proliferation. In summary, we identify a posttranslational mechanism for loss of DLC1 and a linkage between CRL4A-FBXW5-associated oncogenesis and regulation of RhoA signaling.

Osteogenic genes related to the canonic WNT pathway are down-regulated in ameloblastoma

OBJECTIVE

The aim of this study was to determine the expression of essential osteogenic genes related to the canonic WNT pathway, i.e., WDR5, sFRP-2, and their downstream genes, in ameloblastoma and to clarify their biologic impact on this neoplasm.

STUDY DESIGN

Forty-six paraffin-embedded ameloblastoma samples and ameloblastic (AM-1) and preosteoblastic (KUSA/A1) cell lines were used. Immunohistochemistry, Western blot, reverse-transcription polymerase chain reaction, and alkaline phosphatase (ALP) activity assay were performed.

RESULTS

WDR5, essential for osteoblast differentiation and canonic WNT pathway activation, was negative in most ameloblastoma cases and weakly expressed in AM-1 cells. Conversely, sFRP-2s was overexpressed. RUNX2 and C-MYC, downstream inductions of canonic WNT pathway activation, demonstrated weak mRNA expressions in ameloblastoma, suggesting WNT pathway impairment and WDR5 functional inactivity. Recombinant WDR5 weakly induced ALP activity of KUSA/A1 cells cultured in AM-1 conditioned medium.

CONCLUSIONS

These findings suggest that WNT-related bone-forming genes are down-regulated in ameloblastoma. Concurrent sFRP-2 overexpression suggests that both bone-forming and bone-inhibiting genes equally contributed to reduced bone formation in this neoplasm.

CCDC22 deficiency in humans blunts activation of proinflammatory NF-κB signaling

NF-κB is a master regulator of inflammation and has been implicated in the pathogenesis of immune disorders and cancer. Its regulation involves a variety of steps, including the controlled degradation of inhibitory IκB proteins. In addition, the inactivation of DNA-bound NF-κB is essential for its regulation. This step requires a factor known as copper metabolism Murr1 domain-containing 1 (COMMD1), the prototype member of a conserved gene family. While COMMD proteins have been linked to the ubiquitination pathway, little else is known about other family members. Here we demonstrate that all COMMD proteins bind to CCDC22, a factor recently implicated in X-linked intellectual disability (XLID). We showed that an XLID-associated CCDC22 mutation decreased CCDC22 protein expression and impaired its binding to COMMD proteins. Moreover, some affected individuals displayed ectodermal dysplasia, a congenital condition that can result from developmental NF-κB blockade. Indeed, patient-derived cells demonstrated impaired NF-κB activation due to decreased IκB ubiquitination and degradation. In addition, we found that COMMD8 acted in conjunction with CCDC22 to direct the degradation of IκB proteins. Taken together, our results indicate that CCDC22 participates in NF-κB activation and that its deficiency leads to decreased IκB turnover in humans, highlighting an important regulatory component of this pathway.

Ramshackle (Brwd3) promotes light-induced ubiquitylation of Drosophila Cryptochrome by DDB1-CUL4-ROC1 E3 ligase complex

Cryptochrome (CRY) is the primary circadian photoreceptor in Drosophila. It resets the circadian clock by promoting light-induced degradation of the clock proteins Timeless and Period, as well as its own proteolysis. The E3 ligases that ubiquitylate Timeless and Period before degradation are known and it is known that Drosophila (d) CRY is degraded by the ubiquitin-proteasome system as well. To identify the E3 ligase for dCRY we screened candidates in S2 cells by RNAi. Knockdown of each of the 25 putative F-box proteins identified by bioinformatics did not attenuate the light-induced degradation of dCRY. However, knockdown of a WD40 protein, Bromodomain and WD repeat domain containing 3 (Brwd3) (CG31132/Ramshackle) caused strong attenuation of dCRY degradation following light exposure. We found that BRWD3 functions as a Damage-specific DNA binding protein 1 (DDB1)- and CULLIN (CUL)4-associated factor in a Cullin4-RING Finger E3 Ligase (CRL4) that mediates light-dependent binding of dCRY to CUL4-ROC1-DDB1-BRWD3, inducing ubiquitylation of dCRY and its light-induced degradation. Thus, this study identifies a light-activated E3 ligase complex essential for light-mediated CRY degradation in Drosophila cells.

CUL4B promotes replication licensing by up-regulating the CDK2-CDC6 cascade

CUL4B up-regulates CDK2 by repressing miR-372 and miR-373, leading to increased phosphorylation and stabilization of CDC6, thus promoting replication licensing.

Cullin-RING ubiquitin ligases (CRLs) participate in the regulation of diverse cellular processes including cell cycle progression. Mutations in the X-linked CUL4B, a member of the cullin family, cause mental retardation and other developmental abnormalities in humans. Cells that are deficient in CUL4B are severely selected against in vivo in heterozygotes. Here we report a role of CUL4B in the regulation of replication licensing. Strikingly, CDC6, the licensing factor in replication, was positively regulated by CUL4B and contributed to the loading of MCM2 to chromatin. The positive regulation of CDC6 by CUL4B depends on CDK2, which phosphorylates CDC6, protecting it from APC^CDH1-mediated degradation. Thus, aside being required for cell cycle reentry from quiescence, CDK2 also contributes to pre-replication complex assembly in G1 phase of cycling cells. Interestingly, the up-regulation of CDK2 by CUL4B is achieved via the repression of miR-372 and miR-373, which target CDK2. Our findings thus establish a CUL4B–CDK2–CDC6 cascade in the regulation of DNA replication licensing.

X-linked intellectual disability gene CUL4B targets Jab1/CSN5 for degradation and regulates bone morphogenetic protein signaling

Cullin 4B (CUL4B) is a scaffold protein involved in the assembly of cullin-RING ubiquitin ligase (E3) complexes. Contemporary reports have identified multiple mutations of CUL4B gene as being causally associated with X-linked intellectual disability (XLID). Identifying the specific protein substrates will help to better understand the physiological functions of CUL4B. The current study identified Jun activation domain-binding protein (Jab1/CSN5) in the COP9 signalosome (CSN) complex as a novel proteolytic target for the CUL4B ubiquitin ligase complex. The impaired degradation of Jab1 was observed in cells after RNAi-mediated CUL4B depletion. Integrity of DDB1-CUL4B-ROC1 was further demonstrated to be indispensable for the degradation of Jab1. In addition, the degradation of Jab1 is independent of CUL4A, a cullin family member closely related to CUL4B. In vitro and in vivo ubiquitination assays revealed that CUL4B promoted the polyubiquitination of Jab1. Interestingly, CUL4B-silenced cells were shown to exhibit abnormal upregulation of bone morphogenetic protein (BMP) signaling. Furthermore, in vivo studies of embryonic fibroblasts in Cul4b-deficient mice demonstrated Jab1 accumulation and increased activation of the BMP signaling pathway. Together, the current findings demonstrate the CUL4B E3 ubiquitin ligase plays a key role in targeting Jab1 for degradation, potentially revealing a previously undocumented mechanism for regulation of the BMP signaling pathway involved with the CUL4B-based E3 complex. This observation may provide novel insights into the molecular mechanisms underlying CUL4B-associated XLID pathogenesis.

An epigenetic framework for neurodevelopmental disorders: from pathogenesis to potential therapy

Neurodevelopmental disorders (NDDs) are characterized by aberrant and delayed early-life development of the brain, leading to deficits in language, cognition, motor behaviour and other functional domains, often accompanied by somatic symptoms. Environmental factors like perinatal infection, malnutrition and trauma can increase the risk of the heterogeneous, multifactorial and polygenic disorders, autism and schizophrenia. Conversely, discrete genetic anomalies are involved in Down, Rett and Fragile X syndromes, tuberous sclerosis and neurofibromatosis, the less familiar Phelan-McDermid, Sotos, Kleefstra, Coffin-Lowry and "ATRX" syndromes, and the disorders of imprinting, Angelman and Prader-Willi syndromes. NDDs have been termed "synaptopathies" in reference to structural and functional disturbance of synaptic plasticity, several involve abnormal Ras-Kinase signalling ("rasopathies"), and many are characterized by disrupted cerebral connectivity and an imbalance between excitatory and inhibitory transmission. However, at a different level of integration, NDDs are accompanied by aberrant "epigenetic" regulation of processes critical for normal and orderly development of the brain. Epigenetics refers to potentially-heritable (by mitosis and/or meiosis) mechanisms controlling gene expression without changes in DNA sequence. In certain NDDs, prototypical epigenetic processes of DNA methylation and covalent histone marking are impacted. Conversely, others involve anomalies in chromatin-modelling, mRNA splicing/editing, mRNA translation, ribosome biogenesis and/or the regulatory actions of small nucleolar RNAs and micro-RNAs. Since epigenetic mechanisms are modifiable, this raises the hope of novel therapy, though questions remain concerning efficacy and safety. The above issues are critically surveyed in this review, which advocates a broad-based epigenetic framework for understanding and ultimately treating a diverse assemblage of NDDs ("epigenopathies") lying at the interface of genetic, developmental and environmental processes. This article is part of the Special Issue entitled 'Neurodevelopmental Disorders'.

CRL4B catalyzes H2AK119 monoubiquitination and coordinates with PRC2 to promote tumorigenesis

We reported that Cullin4B-Ring E3 ligase complex (CRL4B) is physically associated with Polycomb-repressive complex 2 (PRC2). We showed that CRL4B possesses an intrinsic transcription repressive activity by promoting H2AK119 monoubiquitination. Ablation of Cul4b or depletion of CUL4B, the main component of CRL4B, resulted in loss of not only H2AK119 monoubiquitination but also H3K27 trimethylation, leading to derepression of target genes that are critically involved in cell growth and migration. We demonstrated that CUL4B promotes cell proliferation, invasion, and tumorigenesis in vitro and in vivo and found that its expression is markedly upregulated in various human cancers. Our data indicate that CUL4B promotes tumorigenesis, supporting the pursuit of CUL4B as a target for cancer therapy.

Cul4B regulates neural progenitor cell growth

Background

Cullin ubiquitin ligases are activated via the covalent modification of Cullins by the small ubiquitin-like protein nedd8 in a process called neddylation. Genetic mutations of cullin-4b (cul4b) cause a prevalent type of X-linked intellectual disability (XLID) in males, but the physiological function of Cul4B in neuronal cells remains unclear.

Results

There are three major isoforms of Cul4B (1, 2, and 3) in human and rodent tissues. By examining the endogenous Cul4B isoforms in the brain, this study demonstrates that Cul4B-1 and Cul4B-2 isoforms are unneddylated and more abundant in the brain whereas the lesser species Cul4B-3 that misses the N-terminus present in the other two isoforms is neddylated. The data suggest that the N-terminus of Cul4B inhibits neddylation in the larger isoforms. Immunostaining of human NT-2 cells also shows that most Cul4B is unneddylated, especially when it is localized in the process in G0-synchronized cells. This study demonstrates that Cul4B accumulates during mitosis and downregulation of Cul4B arrests NPCs and NT-2 cells in the G2/M phase of the cell cycle. In both human and rodent brain tissues, Cul4B-positive cells accumulate β-catenin in the dentate subgranular zone and the subventricular zone. These Cul4B-positive cells also co-express the MPM-2 mitotic epitope, suggesting that Cul4B is also necessary for mitosis progression in vivo.

Conclusions

This study provides first evidence that unneddylated Cul4B isoforms exist in the brain and are necessary for mitosis progression in NPCs. The data suggest that unneddylated Cul4B isoforms specifically inhibits β-catenin degradation during mitosis. Furthermore, unneddylated Cul4B may play a role in addition to cell cycle since it is exclusively localized to the processes in starved NT-2 cells. Further analyses of the different isoforms of Cul4B will help understand the cognitive deficits in Cul4B-linked XLID and give insights into drug and biomarker discoveries.

Rescue of the genetically engineered Cul4b mutant mouse as a potential model for human X-linked mental retardation

Mutation in CUL4B, which encodes a scaffold protein of the E3 ubiquitin ligase complex, has been found in patients with X-linked mental retardation (XLMR). However, early deletion of Cul4b in mice causes prenatal lethality, which has frustrated attempts to characterize the phenotypes in vivo. In this report, we successfully rescued Cul4b mutant mice by crossing female mice in which exons 4-5 of Cul4b were flanked by loxP sequences with Sox2-Cre male mice. In Cul4b-deficient (Cul4b(Δ)/Y) mice, no CUL4B protein was detected in any of the major organs, including the brain. In the hippocampus, the levels of CUL4A, CUL4B substrates (TOP1, β-catenin, cyclin E and WDR5) and neuronal markers (MAP2, tau-1, GAP-43, PSD95 and syn-1) were not sensitive to Cul4b deletion, whereas the number of parvalbumin (PV)-positive GABAergic interneurons was decreased in Cul4b(Δ)/Y mice, especially in the dentate gyrus (DG). Some dendritic features, including the complexity, diameter and spine density in the CA1 and DG hippocampal neurons, were also affected by Cul4b deletion. Together, the decrease in the number of PV-positive neurons and altered dendritic properties in Cul4b(Δ)/Y mice imply a reduction in inhibitory regulation and dendritic integration in the hippocampal neural circuit, which lead to increased epileptic susceptibility and spatial learning deficits. Our results identify Cul4b(Δ)/Y mice as a potential model for the non-syndromic model of XLMR that replicates the CUL4B-associated MR and is valuable for the development of a therapeutic strategy for treating MR.

CUL4B ubiquitin ligase in mouse development: a model for human X-linked mental retardation syndrome?

CUL4B, a member of the cullin-RING ubiquitin ligase family, is frequently mutated in X-linked mental retardation (XLMR) patients. The study by Liu et al. showed that Cul4b plays an essential developmental role in the extra-embryonic tissues, while it is dispensable in the embryo proper during mouse embryogenesis. Viable Cul4b-null mice provide the first animal model to study neuronal and behavioral deficiencies seen in human CUL4B XLMR patients.

Lack of Cul4b, an E3 ubiquitin ligase component, leads to embryonic lethality and abnormal placental development

Cullin-RING ligases (CRLs) complexes participate in the regulation of diverse cellular processes, including cell cycle progression, transcription, signal transduction and development. Serving as the scaffold protein, cullins are crucial for the assembly of ligase complexes, which recognize and target various substrates for proteosomal degradation. Mutations in human CUL4B, one of the eight members in cullin family, are one of the major causes of X-linked mental retardation. We here report the generation and characterization of Cul4b knockout mice, in which exons 3 to 5 were deleted. In contrast to the survival to adulthood of human hemizygous males with CUL4B null mutation, Cul4b null mouse embryos show severe developmental arrest and usually die before embryonic day 9.5 (E9.5). Accumulation of cyclin E, a CRL (CUL4B) substrate, was observed in Cul4b null embryos. Cul4b heterozygotes were recovered at a reduced ratio and exhibited a severe developmental delay. The placentas in Cul4b heterozygotes were disorganized and were impaired in vascularization, which may contribute to the developmental delay. As in human CUL4B heterozygotes, Cul4b null cells were selected against in Cul4b heterozygotes, leading to various degrees of skewed X-inactivation in different tissues. Together, our results showed that CUL4B is indispensable for embryonic development in the mouse.

Essential role of the CUL4B ubiquitin ligase in extra-embryonic tissue development during mouse embryogenesis

Mutations of the CUL4B ubiquitin ligase gene are causally linked to syndromic X-linked mental retardation (XLMR). However, the pathogenic role of CUL4B mutations in neuronal and developmental defects is not understood. We have generated mice with targeted disruption of Cul4b, and observed embryonic lethality with pronounced growth inhibition and increased apoptosis in extra-embryonic tissues. Cul4b, but not its paralog Cul4a, is expressed at high levels in extra-embryonic tissues post implantation. Silencing of CUL4B expression in an extra-embryonic cell line resulted in the robust accumulation of the CUL4 substrate p21(Cip1/WAF) and G2/M cell cycle arrest, which could be partially rescued by silencing of p21(Cip1/WAF). Epiblast-specific deletion of Cul4b prevented embryonic lethality and gave rise to viable Cul4b null mice. Therefore, while dispensable in the embryo proper, Cul4b performs an essential developmental role in the extra-embryonic tissues. Our study offers a strategy to generate viable Cul4b-deficient mice to model the potential neuronal and behavioral deficiencies of human CUL4B XLMR patients.

Pathogenic Role of the CRL4 Ubiquitin Ligase in Human Disease

The cullin 4-RING ubiquitin ligase (CRL4) family employs multiple DDB1–CUL4 associated factors substrate receptors to direct the degradation of proteins involved in a wide spectrum of cellular functions. Aberrant expression of the cullin 4A (CUL4A) gene is found in many tumor types, while mutations of the cullin 4B (CUL4B) gene are causally associated with human X-linked mental retardation. This focused review will summarize our current knowledge of the two CUL4 family members in the pathogenesis of human malignancy and neuronal disease, and discuss their potential as new targets for cancer prevention and therapeutic intervention.

Cullin 4B is recruited to tristetraprolin-containing messenger ribonucleoproteins and regulates TNF-α mRNA polysome loading

TNF-α is a central mediator of inflammation and critical for host response to infection and injury. TNF-α biosynthesis is controlled by transcriptional and posttranscriptional mechanisms allowing for rapid, transient production. Tristetraprolin (TTP) is an AU-rich element binding protein that regulates the stability of the TNF-α mRNA. Using a screen to identify TTP-interacting proteins, we identified Cullin 4B (Cul4B), a scaffolding component of the Cullin ring finger ligase family of ubiquitin E3 ligases. Short hairpin RNA knockdown of Cul4B results in a significant reduction in TNF-α protein and mRNA in LPS-stimulated mouse macrophage RAW264.7 cells as well as a reduction in TTP protein. TNF-α message t(1/2) was reduced from 69 to 33 min in LPS-stimulated cells. TNF-3' untranslated region luciferase assays utilizing wild-type and mutant TTP-AA (S52A, S178A) indicate that TTP function is enhanced in Cul4B short hairpin RNA cells. Importantly, the fold induction of TNF-α mRNA polysome loading in response to LPS stimulation is reduced by Cul4B knockdown. Cul4B is present on the polysomes and colocalizes with TTP to exosomes and processing bodies, which are sites of mRNA decay. We conclude that Cul4B licenses the TTP-containing TNF-α messenger ribonucleoprotein for loading onto polysomes, and reduction of Cul4B expression shunts the messenger ribonucleoproteins into the degradative pathway.

CUL4B: trash talking at chromatin

In this issue, Nakagawa and Xiong (2011) reveal a mechanism targeting WDR5 for proteolysis dependent on the X-linked mental retardation gene, CUL4B. This provides a link between the stability of a chromatin factor and gene expression implicated in neurological pathogenesis.