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

Reticulons 1 and 3 are essential for axonal growth and synaptic maintenance associated with intellectual development

Reticulon (RTN) proteins are a family of proteins biochemically identified for shaping tubular endoplasmic reticulum, a subcellular structure important for vesicular transport and cell-to-cell communication. In our recent study of mice with knockout of both reticulon 1 (Rtn1) and Rtn3, we discovered that Rtn1-/-;Rtn3-/- (brief as R1R3dKO) mice exhibited neonatal lethality, despite the fact that mice deficient in either RTN1 or RTN3 alone exhibit no discernible phenotypes. This has been the first case to find early lethality in animals with deletion of partial members of RTN proteins. The complete penetrance for neonatal lethality can be attributed to multiple defects including the impaired neuromuscular junction found in the diaphragm. We also observed significantly impaired axonal growth in a regional-specific manner, detected by immunohistochemical staining with antibodies to neurofilament light chain and neurofilament medium chain. Ultrastructural examination by electron microscopy revealed a significant reduction in synaptic active zone length in the hippocampus. Mechanistic exploration by unbiased proteomic assays revealed reduction of proteins such as FMR1, Staufen2, Cyfip1, Cullin-4B and PDE2a, which are known components in the fragile X mental retardation pathway. Together, our results reveal that RTN1 and RTN3 are required to orchestrate neurofilament organization and intact synaptic structure of the central nervous system.

Calcifying pseudoneoplasms of the neuraxis (CAPNON). A case report

Calcifying pseudoneoplasms of the neuraxis (CAPNON) are rare, slow-growing, benign lesions occurring throughout the neuroaxis that are frequently misdiagnosed and overlooked by clinicians. Here, we report a case of a 56-year-old woman who presented with a history of recurrent headache for the previous six years. Magnetic resonance imaging (MRI) revealed a 2.3-cm-sized solid mass in the right frontal lobe that was surrounded by marked edematous areas. The lesion demonstrated dense calcification and avid enhancement. The lesion was initially diagnosed as oligodendroglioma, and then found to be CAPNON based on histopathology of a surgically resected tissue. Genetic analysis revealed a nonsense mutation in the CUL4B gene. The patient's condition appeared to reflect a reactive, rather than neoplastic, process. Clinicians should be prepared to detect such pseudotumors histopathologically in order to avoid unnecessary differential tests of neoplastic or infectious diseases, as well as potentially harmful therapies.

Cul4b Promotes Progression of Malignant Cutaneous Melanoma Patients by Regulating CDKN2A

Although several molecular targeted therapy and immunotherapy have been developed, cutaneous melanoma prognosis is still not satisfying. Cul4b promotes the progression of several malignant tumors by regulating cell proliferation. However, its prognostic role in malignant cutaneous melanoma has not been evaluated. In this study, immunohistochemistry was performed to assess the expression of Cul4b in a consecutive patient cohort. The prognostic role of Cul4b was estimated with univariate and multivariate analysis. Cul4b was knocked down in melanoma cell line to evaluate its role in promoting cell proliferation. The results revealed that Cul4b was highly expressed in some of the cutaneous malignant melanoma patients and high expression of Cul4b was associated with poor melanoma-specific overall survival and poor disease-free survival. Cul4b expression was associated with Breslow categories, Clark level, and Ki67 expression. Univariate and multivariate analysis revealed that Cul4b is an independent prognosis risk factor of cutaneous melanoma. Downregulation of Cul4b inhibited the proliferation ability of melanoma cells and downregulated the expression of CDKN2A. These results suggest that Cul4b plays an essential role in cutaneous melanoma progression and may serve as a promising treatment target.

CUL4B negatively regulates Toll-like receptor-triggered proinflammatory responses by repressing Pten transcription

Toll-like receptors (TLRs) play critical roles in innate immunity and inflammation. The molecular mechanisms by which TLR signaling is fine-tuned remain to be completely elucidated. Cullin 4B (CUL4B), which assembles the CUL4B-RING E3 ligase complex (CRL4B), has been shown to regulate diverse developmental and physiological processes by catalyzing monoubiquitination for histone modification or polyubiquitination for proteasomal degradation. Here, we identified the role of CUL4B as an intrinsic negative regulator of the TLR-triggered inflammatory response. Deletion of CUL4B in macrophages increased the production of proinflammatory cytokines and decreased anti-inflammatory cytokine IL-10 production in response to pathogens that activate TLR3, TLR4, or TLR2. Myeloid cell-specific Cul4b knockout mice were more susceptible to septic shock when challenged with lipopolysaccharide, polyinosinic-polycytidylic acid or Salmonella typhimurium infection. We further demonstrated that enhanced TLR-induced inflammatory responses in the absence of CUL4B were mediated by increased GSK3β activity. Suppression of GSK3β activity efficiently blocked the TLR-triggered increase in proinflammatory cytokine production and attenuated TLR-triggered death in Cul4b mutant mice. Mechanistically, CUL4B was found to negatively regulate TLR-triggered signaling by epigenetically repressing the transcription of Pten, thus maintaining the anti-inflammatory PI3K-AKT-GSK3β pathway. The upregulation of PTEN caused by CUL4B deletion led to uncontrolled GSK3β activity and excessive inflammatory immune responses. Thus, our findings indicate that CUL4B functions to restrict TLR-triggered inflammatory responses through regulating the AKT-GSK3β pathway.

CUL4B regulates autophagy via JNK signaling in diffuse large B-cell lymphoma

Aberrant expression of CUL4B was identified in various types of solid cancers. Cumulative evidences support the oncogenic role of CUL4B in cancers, including regulation of cell proliferation and signal transduction. However, its clinical value and potential pathogenic mechanism in diffuse large B-cell lymphoma (DLBCL) have not been described previously. Therefore, we hypothesize that overexpressed CUL4B may contribute to the pathogenesis of DLBCL. The aim of this study is to assess the expression and the biological function of CUL4B in DLBCL progression. In our study, CUL4B overexpression was observed in DLBCL tissues, and its upregulation was closely associated with poor prognosis in patients. Furthermore, the functional roles of CUL4B was detected both in vitro and in vivo. We demonstrated that silencing CUL4B could not only induce cell proliferation inhibition, cell cycle arrest, and motility attenuation of DLBCL cells in vitro, but also decrease tumor growth in DLBCL xenografts mice. In addition, we identified that CUL4B may act as a potent inductor of JNK phosphorylation in regulation of autophagy. Our findings demonstrated a significant role of CUL4B in the development and progression of DLBCL. CUL4B may act as a useful biomarker and a novel therapeutic target in DLBCL.

CUL4B promotes metastasis and proliferation in pancreatic cancer cells by inducing epithelial-mesenchymal transition via the Wnt/β-catenin signaling pathway

This study determines whether cullin 4B (CUL4B) promotes pancreatic cancer (PC) metastasis by inducing epithelial-mesenchymal transition (EMT) via the Wnt/β-catenin signaling pathway. A total of 64 PC patients were enrolled in this study. Human PC cell lines were distributed into blank, negative control, shCUL4B, PLOC, PLOC-CUL4B, and PLOC-CUL4B + siRNA-β-catenin groups. The expressions of CUL4B, Wnt/β-catenin signaling pathway-related proteins, and EMT-related proteins were determined using RT-qPCR and Western blotting. The positive expressions of CUL4B and β-catenin protein in tissues were detected by immunohistochemistry. MTT assay and flow cytometry was performed for cell proliferation and cell cycle, scratch test, and transwell assay for cell migration and invasion ability. CUL4B and β-catenin were expressed at a higher level in PC tissues than in paracancerous tissues though paracancerous tissues had higher expressions of CUL4B and β-catenin than normal tissues. The PLOC-CUL4B group showed increased CUL4B, Wnt, β-catenin, LEF-1, c-Jun, Cyclin D1, N-cadherin, Vimentin, Snail, and ZEB1 expression; decreased E-cadherin expression; accelerated cell proliferation; increased S-phase cell percentages; increased cell migration ability; more liver metastases; and enlarged tumor than the PLOC and PLOC-CUL4B + siRNA-β-catenin groups. The shCUL4B group showed decreased CUL4B, Wnt, β-catenin, LEF-1, c-Jun, Cyclin D1, N-cadherin, Vimentin, Snail, and ZEB1 expression; increased E-cadherin expression; decelerated cell proliferation; decreased S-phase cell percentages; reduced cell migration ability; less liver metastases; and decreased tumor weight than the blank and negative control groups. We demonstrate that CUL4B promotes PC metastasis by inducing EMT via the Wnt/β-catenin signaling pathway. Therefore, CUL4B might be the clinical target for treating PC.

De novo PHIP-predicted deleterious variants are associated with developmental delay, intellectual disability, obesity, and dysmorphic features

Using whole-exome sequencing, we have identified novel de novo heterozygous pleckstrin homology domain-interacting protein (PHIP) variants that are predicted to be deleterious, including a frameshift deletion, in two unrelated patients with common clinical features of developmental delay, intellectual disability, anxiety, hypotonia, poor balance, obesity, and dysmorphic features. A nonsense mutation in PHIP has previously been associated with similar clinical features. Patients with microdeletions of 6q14.1, including PHIP, have a similar phenotype of developmental delay, intellectual disability, hypotonia, and obesity, suggesting that the phenotype of our patients is a result of loss-of-function mutations. PHIP produces multiple protein products, such as PHIP1 (also known as DCAF14), PHIP, and NDRP. PHIP1 is one of the multiple substrate receptors of the proteolytic CUL4-DDB1 ubiquitin ligase complex. CUL4B deficiency has been associated with intellectual disability, central obesity, muscle wasting, and dysmorphic features. The overlapping phenotype associated with CUL4B deficiency suggests that PHIP mutations cause disease through disruption of the ubiquitin ligase pathway.

HUWE1 plays important role in mouse preimplantation embryo development and the dysregulation is associated with poor embryo development in humans

HUWE1 is a HECT domain containing ubiquitin ligase implicated in neurogenesis, spermatogenesis and cancer development. The purpose of the current study is to investigate the role of HUWE1 in early embryo development. Here we demonstrate that Huwe1 is expressed in both nucleus and cytoplasm of preimplantation mouse embryos as well as gametes. Hypoxia (5% O₂) treatment could significantly increase Huwe1 expression during mouse embryo development process. HUWE1 knockdown inhibited normal embryonic development and reduced blastocyst formation, and increased apoptotic cell numbers were observed in the embryos of HUWE1 knockdown group. Human embryo staining result showed that reduced HUWE1 staining was observed in the poor-quality embryos. Furthermore, Western blot result showed that significantly reduced expression of HUWE1 was observed in the villi of miscarriage embryos compared with the normal control, indicating that reduced expression of HUWE1 is related to poor embryo development. Oxidative reagent, H₂O₂ inhibited HUWE1 expression in human sperm, indicating that HUWE1 expression in sperm is regulated by oxidative stress. In conclusion, these results suggest that HUWE1 protein could contribute to preimplantation embryo development and dysregulated expression of HUWE1 could be related to poor embryo development and miscarriage in IVF clinic.

The Role of the COP9 Signalosome and Neddylation in DNA Damage Signaling and Repair

The maintenance of genomic integrity is an important process in organisms as failure to sense and repair damaged DNA can result in a variety of diseases. Eukaryotic cells have developed complex DNA repair response (DDR) mechanisms to accurately sense and repair damaged DNA. Post-translational modifications by ubiquitin and ubiquitin-like proteins, such as SUMO and NEDD8, have roles in coordinating the progression of DDR. Proteins in the neddylation pathway have also been linked to regulating DDR. Of interest is the COP9 signalosome (CSN), a multi-subunit metalloprotease present in eukaryotes that removes NEDD8 from cullins and regulates the activity of cullin-RING ubiquitin ligases (CRLs). This in turn regulates the stability and turnover of a host of CRL-targeted proteins, some of which have established roles in DDR. This review will summarize the current knowledge on the role of the CSN and neddylation in DNA repair.

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.

Genetically engineered mouse models for functional studies of SKP1-CUL1-F-box-protein (SCF) E3 ubiquitin ligases

The SCF (SKP1 (S-phase-kinase-associated protein 1), Cullin-1, F-box protein) E3 ubiquitin ligases, the founding member of Cullin-RING ligases (CRLs), are the largest family of E3 ubiquitin ligases in mammals. Each individual SCF E3 ligase consists of one adaptor protein SKP1, one scaffold protein cullin-1 (the first family member of the eight cullins), one F-box protein out of 69 family members, and one out of two RING (Really Interesting New Gene) family proteins RBX1/ROC1 or RBX2/ROC2/SAG/RNF7. Various combinations of these four components construct a large number of SCF E3s that promote the degradation of many key regulatory proteins in cell-context, temporally, and spatially dependent manners, thus controlling precisely numerous important cellular processes, including cell cycle progression, apoptosis, gene transcription, signal transduction, DNA replication, maintenance of genome integrity, and tumorigenesis. To understand how the SCF E3 ligases regulate these cellular processes and embryonic development under in vivo physiological conditions, a number of mouse models with transgenic (Tg) expression or targeted deletion of components of SCF have been established and characterized. In this review, we will provide a brief introduction to the ubiquitin-proteasome system (UPS) and the SCF E3 ubiquitin ligases, followed by a comprehensive overview on the existing Tg and knockout (KO) mouse models of the SCF E3s, and discuss the role of each component in mouse embryogenesis, cell proliferation, apoptosis, carcinogenesis, as well as other pathogenic processes associated with human diseases. We will end with a brief discussion on the future directions of this research area and the potential applications of the knowledge gained to more effective therapeutic interventions of human diseases.