NRP/Optineurin Cooperates with TAX1BP1 to potentiate the activation of NF-kappaB by human T-lymphotropic virus type 1 tax protein

PCBP1 interacts with the HTLV-1 Tax oncoprotein to potentiate NF-κB activation

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma. The HTLV-1 Tax constitutively activates nuclear factor-κB (NF-κB) to promote the survival and transformation of HTLV-1-infected T cells. Despite extensive study of Tax, how Tax interacts with host factors to regulate NF-κB activation and HTLV-1-driven cell proliferation is not entirely clear. Here, we showed that overexpression of Poly (rC)-binding protein 1 (PCBP1) promoted Tax-mediated IκB kinase (IKK)-NF-κB signaling activation, whereas knockdown of PCBP1 attenuated Tax-dependent IKK-NF-κB activation. However, Tax activation of HTLV-1 long terminal repeat was unaffected by PCBP1. Furthermore, depletion of PCBP1 led to apoptosis and reduced proliferation of HTLV-1-transformed cells. Mechanistically, PCBP1 interacted and co-localized with Tax in the cytoplasm, and PCBP1 KH3 domain was indispensable for the interaction between PCBP1 and Tax. Moreover, PCBP1 facilitated the assembly of Tax/IKK complex. Collectively, our results demonstrated that PCBP1 may exert an essential effect in Tax/IKK complex combination and subsequent NF-κB activation, which provides a novel insight into the pathogenetic mechanisms of HTLV-1.

Ubiquitinome profiling reveals in vivo UBE2D3 targets and implicates UBE2D3 in protein quality control

Ubiquitination has crucial roles in many cellular processes and dysregulation of ubiquitin machinery enzymes can result in various forms of pathogenesis. Cells only have a limited set of ubiquitin-conjugating (E2) enzymes to support the ubiquitination of many cellular targets. As individual E2 enzymes have many different substrates and interactions between E2 enzymes and their substrates can be transient, it is challenging to define all in vivo substrates of an individual E2 and the cellular processes it affects. Particularly challenging in this respect is UBE2D3, an E2 enzyme with promiscuous activity in vitro but less defined roles in vivo. Here, we set out to identify in vivo targets of UBE2D3 by using SILAC-based and label-free quantitative ubiquitin diGly proteomics to study global proteome and ubiquitinome changes associated with UBE2D3 depletion. UBE2D3 depletion changed the global proteome, with the levels of proteins from metabolic pathways, in particular retinol metabolism, being the most affected. However, the impact of UBE2D3 depletion on the ubiquitinome was much more prominent. Interestingly, molecular pathways related to mRNA translation were the most affected. Indeed, we find that ubiquitination of the ribosomal proteins RPS10 and RPS20, critical for ribosome-associated protein quality control (RQC), is dependent on UBE2D3. We show by TULIP2 methodology that RPS10 and RPS20 are direct targets of UBE2D3 and demonstrate that UBE2D3's catalytic activity is required to ubiquitinate RPS10 in vivo. In addition, our data suggest that UBE2D3 acts at multiple levels in autophagic protein quality control (PQC). Collectively, our findings show that depletion of an E2 enzyme in combination with quantitative diGly-based ubiquitinome profiling is a powerful tool to identify new in vivo E2 substrates, as we have done here for UBE2D3. Our work provides an important resource for further studies on the in vivo functions of UBE2D3.

HTLV-1 Infection and Pathogenesis: New Insights from Cellular and Animal Models

Since the discovery of the human T-cell leukemia virus-1 (HTLV-1), cellular and animal models have provided invaluable contributions in the knowledge of viral infection, transmission and progression of HTLV-associated diseases. HTLV-1 is the causative agent of the aggressive adult T-cell leukemia/lymphoma and inflammatory diseases such as the HTLV-1 associated myelopathy/tropical spastic paraparesis (HAM/TSP). Cell models contribute to defining the role of HTLV proteins, as well as the mechanisms of cell-to-cell transmission of the virus. Otherwise, selected and engineered animal models are currently applied to recapitulate in vivo the HTLV-1 associated pathogenesis and to verify the effectiveness of viral therapy and host immune response. Here we review the current cell models for studying virus–host interaction, cellular restriction factors and cell pathway deregulation mediated by HTLV products. We recapitulate the most effective animal models applied to investigate the pathogenesis of HTLV-1-associated diseases such as transgenic and humanized mice, rabbit and monkey models. Finally, we summarize the studies on STLV and BLV, two closely related HTLV-1 viruses in animals. The most recent anticancer and HAM/TSP therapies are also discussed in view of the most reliable experimental models that may accelerate the translation from the experimental findings to effective therapies in infected patients.

Autophagy in Human T-Cell Leukemia Virus Type 1 (HTLV-1) Induced Leukemia

Viruses play an important role in the development of certain human cancers. They are estimated to contribute 16% to all human cancers. Human T-cell leukemia virus type 1 (HTLV-1) was the first human retrovirus to be discovered and is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), an aggressive T-cell malignancy with poor prognosis. HTLV-1 viral proteins interact with mechanisms and proteins present in host cells for their own benefit, evading the immune system and promoting the establishment of disease. Several viruses manipulate the autophagy pathway to achieve their infective goals, and HTLV-1 is not the exception. HTLV-1 Tax viral protein engages NF-κB and autophagy pathways prone favoring viral replication and T cell transformation. In this review we focus on describing the relationship of HTLV-1 with the autophagy machinery and its implication in the development of ATLL.

The E3/E4 ubiquitin conjugation factor UBE4B interacts with and ubiquitinates the HTLV-1 Tax oncoprotein to promote NF-κB activation

Human T-cell leukemia virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), and the neurological disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The HTLV-1 Tax protein persistently activates the NF-κB pathway to enhance the proliferation and survival of HTLV-1 infected T cells. Lysine 63 (K63)-linked polyubiquitination of Tax provides an important regulatory mechanism that promotes Tax-mediated interaction with the IKK complex and activation of NF-κB; however, the host proteins regulating Tax ubiquitination are largely unknown. To identify new Tax interacting proteins that may regulate its ubiquitination we conducted a yeast two-hybrid screen using Tax as bait. This screen yielded the E3/E4 ubiquitin conjugation factor UBE4B as a novel binding partner for Tax. Here, we confirmed the interaction between Tax and UBE4B in mammalian cells by co-immunoprecipitation assays and demonstrated colocalization by proximity ligation assay and confocal microscopy. Overexpression of UBE4B specifically enhanced Tax-induced NF-κB activation, whereas knockdown of UBE4B impaired Tax-induced NF-κB activation and the induction of NF-κB target genes in T cells and ATLL cell lines. Furthermore, depletion of UBE4B with shRNA resulted in apoptotic cell death and diminished the proliferation of ATLL cell lines. Finally, overexpression of UBE4B enhanced Tax polyubiquitination, and knockdown or CRISPR/Cas9-mediated knockout of UBE4B attenuated both K48- and K63-linked polyubiquitination of Tax. Collectively, these results implicate UBE4B in HTLV-1 Tax polyubiquitination and downstream NF-κB activation.

Infection with the retrovirus HTLV-1 leads to the development of either CD4+CD25+ leukemia/lymphoma (ATLL) or a demyelinating neuroinflammatory disease (HAM/TSP) in a subset of infected individuals. The HTLV-1 Tax protein is a regulatory protein which regulates viral gene expression and persistently activates cellular signaling pathways such as NF-κB to drive the clonal expansion and longevity of HTLV-1 infected CD4+ T cells. Polyubiquitination of Tax is a key mechanism of NF-κB activation by assembling and activating IκB kinase (IKK) signaling complexes; however, the host factors regulating Tax ubiquitination have remained elusive. Here, we have identified the E3/E4 ubiquitin conjugation factor UBE4B as a novel Tax binding protein that promotes both K48- and K63-linked polyubiquitination of Tax. Knockdown or knockout of UBE4B impairs Tax-induced NF-κB activation and triggers apoptosis of HTLV-1-transformed cells. Therefore, UBE4B is an integral host factor that supports HTLV-1 Tax polyubiquitination, NF-κB activation and cell survival.

The diverse role of optineurin in pathogenesis of disease

Optineurin is a widely expressed protein that possesses multiple functions. Growing evidence suggests that mutation or dysregulation of optineurin can cause several neurodegenerative diseases, including amyotrophic lateral sclerosis, primary open-angle glaucoma, and Huntington's disease, as well as inflammatory digestive disorders such as Crohn's disease. Optineurin engages in vesicular trafficking, receptor regulation, immune reactions, autophagy, and distinct signaling pathways including nuclear factor kappa beta, by which optineurin contributes to cellular death and related diseases, indicating its potential as a therapeutic target. In this review, we discuss the major functions and signaling pathways of optineurin. Furthermore, we illustrate the influence of optineurin mutation or dysregulation to region-specific pathogenesis as well as potential applications of optineurin in therapeutic strategies.

Mechanisms of Oncogenesis by HTLV-1 Tax

The human T-cell lymphotropic virus type 1 (HTLV-1) is the etiological agent of adult T-cell leukemia/lymphoma (ATLL), a neoplasm of CD4+CD25+ T cells that occurs in 2-5% of infected individuals after decades of asymptomatic latent infection. Multiple HTLV-1-encoded regulatory proteins, including Tax and HTLV-1 basic leucine zipper factor (HBZ), play key roles in viral persistence and latency. The HTLV-1 Tax oncoprotein interacts with a plethora of host cellular proteins to regulate viral gene expression and also promote the aberrant activation of signaling pathways such as NF-κB to drive clonal proliferation and survival of T cells bearing the HTLV-1 provirus. Tax undergoes various post-translational modifications such as phosphorylation and ubiquitination that regulate its function and subcellular localization. Tax shuttles in different subcellular compartments for the activation of anti-apoptotic genes and deregulates the cell cycle with the induction of DNA damage for the accumulation of genomic instability that can result in cellular immortalization and malignant transformation. However, Tax is highly immunogenic and therefore HTLV-1 has evolved numerous strategies to tightly regulate Tax expression while maintaining the pool of anti-apoptotic genes through HBZ. In this review, we summarize the key findings on the oncogenic mechanisms used by Tax that set the stage for the development of ATLL, and the strategies used by HTLV-1 to tightly regulate Tax expression for immune evasion and viral persistence.

E3 Ligase ITCH Interacts with the Z Matrix Protein of Lassa and Mopeia Viruses and Is Required for the Release of Infectious Particles

Lassa virus (LASV) and Mopeia virus (MOPV) are two closely related, rodent-born mammarenaviruses. LASV is the causative agent of Lassa fever, a deadly hemorrhagic fever endemic in West Africa, whereas MOPV is non-pathogenic in humans. The Z matrix protein of arenaviruses is essential to virus assembly and budding by recruiting host factors, a mechanism that remains partially defined. To better characterize the interactions involved, a yeast two-hybrid screen was conducted using the Z proteins from LASV and MOPV as a bait. The cellular proteins ITCH and WWP1, two members of the Nedd4 family of HECT E3 ubiquitin ligases, were found to bind the Z proteins of LASV, MOPV and other arenaviruses. The PPxY late-domain motif of the Z proteins is required for the interaction with ITCH, although the E3 ubiquitin-ligase activity of ITCH is not involved in Z ubiquitination. The silencing of ITCH was shown to affect the replication of the old-world mammarenaviruses LASV, MOPV, Lymphocytic choriomeningitis virus (LCMV) and to a lesser extent Lujo virus (LUJV). More precisely, ITCH was involved in the egress of virus-like particles and the release of infectious progeny viruses. Thus, ITCH constitutes a novel interactor of LASV and MOPV Z proteins that is involved in virus assembly and release.

NF-κB and MicroRNA Deregulation Mediated by HTLV-1 Tax and HBZ

The risk of developing adult T-cell leukemia/lymphoma (ATLL) in individuals infected with human T-cell lymphotropic virus 1 (HTLV-1) is about 3-5%. The mechanisms by which the virus triggers this aggressive cancer are still an area of intensive investigation. The viral protein Tax-1, together with additional regulatory proteins, in particular HTLV-1 basic leucine zipper factor (HBZ), are recognized as relevant viral factors required for both viral replication and transformation of infected cells. Tax-1 deregulates several cellular pathways affecting the cell cycle, survival, and proliferation. The effects of Tax-1 on the NF-κB pathway have been thoroughly studied. Recent studies also revealed the impact of Tax-1 and HBZ on microRNA expression. In this review, we summarize the recent progress in understanding the contribution of HTLV-1 Tax- and HBZ-mediated deregulation of NF-κB and the microRNA regulatory network to HTLV-1 pathogenesis.

SQSTM-1/p62 potentiates HTLV-1 Tax-mediated NF-κB activation through its ubiquitin binding function

The NF-κB pathway is constitutively activated in adult T cell leukemia, an aggressive malignancy caused by Human T Leukemia Virus type 1 (HTLV-1). The viral oncoprotein Tax triggers this constitutive activation by interacting with the ubiquitin-rich IKK complex. We previously demonstrated that Optineurin and TAX1BP1, two members of the ubiquitin-binding, Sequestosome-1 (SQSTM-1/p62)-like selective autophagy receptor family, are involved in Tax-mediated NF-κB signaling. Here, using a proximity-dependent biotinylation approach (BioID), we identify p62 as a new candidate partner of Tax and confirm the interaction in infected T cells. We then demonstrate that p62 knock-out in MEF cells as well as p62 knock-down in HEK293T cells significantly reduces Tax-mediated NF-κB activity. We further show that although p62 knock-down does not alter NF-κB activation in Jurkat T cells nor in infected T cells, p62 does potentiate Tax-mediated NF-κB activity upon over-expression in Jurkat T cells. We next show that p62 associates with the Tax/IKK signalosome in cells, and identify the 170–206 domain of p62 as sufficient for the direct, ubiquitin-independent interaction with Tax. However, we observe that this domain is dispensable for modulating Tax activity in cells, and functional analysis of p62 mutants indicates that p62 could potentiate Tax activity in cells by facilitating the association of ubiquitin chains with the Tax/IKK signalosome. Altogether, our results identify p62 as a new ubiquitin-dependent modulator of Tax activity on NF-κB, further highlighting the importance of ubiquitin in the signaling activity of the viral Tax oncoprotein.

SQSTM-1/p62 potentiates HTLV-1 Tax-mediated NF-κB activation through its ubiquitin binding function

The NF-κB pathway is constitutively activated in adult T cell leukemia, an aggressive malignancy caused by Human T Leukemia Virus type 1 (HTLV-1). The viral oncoprotein Tax triggers this constitutive activation by interacting with the ubiquitin-rich IKK complex. We previously demonstrated that Optineurin and TAX1BP1, two members of the ubiquitin-binding, Sequestosome-1 (SQSTM-1/p62)-like selective autophagy receptor family, are involved in Tax-mediated NF-κB signaling. Here, using a proximity-dependent biotinylation approach (BioID), we identify p62 as a new candidate partner of Tax and confirm the interaction in infected T cells. We then demonstrate that p62 knock-out in MEF cells as well as p62 knock-down in HEK293T cells significantly reduces Tax-mediated NF-κB activity. We further show that although p62 knock-down does not alter NF-κB activation in Jurkat T cells nor in infected T cells, p62 does potentiate Tax-mediated NF-κB activity upon over-expression in Jurkat T cells. We next show that p62 associates with the Tax/IKK signalosome in cells, and identify the 170–206 domain of p62 as sufficient for the direct, ubiquitin-independent interaction with Tax. However, we observe that this domain is dispensable for modulating Tax activity in cells, and functional analysis of p62 mutants indicates that p62 could potentiate Tax activity in cells by facilitating the association of ubiquitin chains with the Tax/IKK signalosome. Altogether, our results identify p62 as a new ubiquitin-dependent modulator of Tax activity on NF-κB, further highlighting the importance of ubiquitin in the signaling activity of the viral Tax oncoprotein.

Comparative virology of HTLV-1 and HTLV-2

Human T cell leukemia virus type 1 (HTLV-1) was the first discovered human retrovirus and the etiologic agent of adult T-cell leukemia and HTLV-1-associated myelopathy/tropical spastic paraparesis. Shortly after the discovery of HTLV-1, human T-cell leukemia virus type 2 (HTLV-2) was isolated from a patient with hairy cell leukemia. Despite possession of similar structural features to HTLV-1, HTLV-2 has not been definitively associated with lymphoproliferative disease. Since their discovery, studies have been performed with the goal of highlighting the differences between HTLV-1 and HTLV-2. A better understanding of these differences will shed light on the specific pathogenic mechanisms of HTLV-1 and lead to novel therapeutic targets. This review will compare and contrast the two oldest human retroviruses with regards to epidemiology, genomic structure, gene products, and pathobiology.

Dysfunction of Optineurin in Amyotrophic Lateral Sclerosis and Glaucoma

Neurodegenerative disorders, including amyotrophic lateral sclerosis (ALS), frontotemporal dementia, and glaucoma, affect millions of people worldwide. ALS is caused by the loss of motor neurons in the spinal cord, brainstem, and brain, and genetic mutations are responsible for 10% of all ALS cases. Glaucoma is characterized by the loss of retinal ganglion cells and is the most common cause of irreversible blindness. Interestingly, mutations in OPTN, encoding optineurin, are associated with both ALS and glaucoma. Optineurin is a highly abundant protein involved in a wide range of cellular processes, including the inflammatory response, autophagy, Golgi maintenance, and vesicular transport. In this review, we summarize the role of optineurin in cellular mechanisms implicated in neurodegenerative disorders, including neuroinflammation, autophagy, and vesicular trafficking, focusing in particular on the consequences of expression of mutations associated with ALS and glaucoma. This review, therefore showcases the impact of optineurin dysfunction in ALS and glaucoma.

Molecular effect of an OPTN common variant associated to Paget's disease of bone

Paget’s disease of bone (PDB) is a chronic bone disorder and although genetic factors appear to play an important role in its pathogenesis, to date PDB causing mutations were identified only in the Sequestosome 1 (SQSTM1) gene at the PDB3 locus. PDB6 locus, also previously linked to PDB, contains several candidate genes for metabolic bone diseases. We focused our analysis in the most significantly associated variant with PDB, within the Optineurin (OPTN) gene, i.e. the common variant rs1561570. Although it was previously shown to be strongly associated with PDB in several populations, its contribution to PDB pathogenesis remains unclear. In this study we have shown that rs1561570 may contribute to PDB since its T allele results in the loss of a methylation site in patients’ DNA, leading to higher levels of OPTN gene expression and a corresponding increase in protein levels in patients’ osteoclasts. This increase in OPTN expression leads to higher levels of NF-κB translocation into the nucleus and increasing expression of its target genes, which may contribute to the overactivity of osteoclasts observed in PDB. We also reported a tendency for a more severe clinical phenotype in the presence of a haplotype containing the rs1561570 T allele, which appear to be re-enforced with the presence of the SQSTM1/P392L mutation. In conclusion, our work provides novel insight towards understanding the functional effects of this variant, located in OPTN intron 7, and its implication in the contribution to PDB pathogenesis.

Optineurin: A Coordinator of Membrane-Associated Cargo Trafficking and Autophagy

Optineurin is a multifunctional adaptor protein intimately involved in various vesicular trafficking pathways. Through interactions with an array of proteins, such as myosin VI, huntingtin, Rab8, and Tank-binding kinase 1, as well as via its oligomerisation, optineurin has the ability to act as an adaptor, scaffold, or signal regulator to coordinate many cellular processes associated with the trafficking of membrane-delivered cargo. Due to its diverse interactions and its distinct functions, optineurin is an essential component in a number of homeostatic pathways, such as protein trafficking and organelle maintenance. Through the binding of polyubiquitinated cargoes via its ubiquitin-binding domain, optineurin also serves as a selective autophagic receptor for the removal of a wide range of substrates. Alternatively, it can act in an ubiquitin-independent manner to mediate the clearance of protein aggregates. Regarding its disease associations, mutations in the optineurin gene are associated with glaucoma and have more recently been found to correlate with Paget’s disease of bone and amyotrophic lateral sclerosis (ALS). Indeed, ALS-associated mutations in optineurin result in defects in neuronal vesicular localisation, autophagosome–lysosome fusion, and secretory pathway function. More recent molecular and functional analysis has shown that it also plays a role in mitophagy, thus linking it to a number of other neurodegenerative conditions, such as Parkinson’s. Here, we review the role of optineurin in intracellular membrane trafficking, with a focus on autophagy, and describe how upstream signalling cascades are critical to its regulation. Current data and contradicting reports would suggest that optineurin is an important and selective autophagy receptor under specific conditions, whereby interplay, synergy, and functional redundancy with other receptors occurs. We will also discuss how dysfunction in optineurin-mediated pathways may lead to perturbation of critical cellular processes, which can drive the pathologies of number of diseases. Therefore, further understanding of optineurin function, its target specificity, and its mechanism of action will be critical in fully delineating its role in human disease.

NF-κB signaling mechanisms in HTLV-1-induced adult T-cell leukemia/lymphoma

The human T-cell leukemia virus type 1 (HTLV-1) is a complex deltaretrovirus linked to adult T-cell leukemia/lymphoma (ATLL), a fatal CD4 +  malignancy in 3-5% of infected individuals. The HTLV-1 Tax regulatory protein plays indispensable roles in regulating viral gene expression and activating cellular signaling pathways that drive the proliferation and clonal expansion of T cells bearing HTLV-1 proviral integrations. Tax is a potent activator of NF-κB, a key signaling pathway that is essential for the survival and proliferation of HTLV-1-infected T cells. However, constitutive NF-κB activation by Tax also triggers a senescence response, suggesting the possibility that only T cells capable of overcoming NF-κB-induced senescence can selectively undergo clonal expansion after HTLV-1 infection. Tax expression is often silenced in the majority of ATLL due to genetic alterations in the tax gene or DNA hypermethylation of the 5'-LTR. Despite the loss of Tax, NF-κB activation remains persistently activated in ATLL due to somatic mutations in genes in the T/B-cell receptor (T/BCR) and NF-κB signaling pathways. In this review, we focus on the key events driving Tax-dependent and -independent mechanisms of NF-κB activation during the multistep process leading to ATLL.

NF-κB in Hematological Malignancies

NF-κB (Nuclear Factor Κ-light-chain-enhancer of activated B cells) transcription factors are critical regulators of immunity, stress response, apoptosis, and differentiation. Molecular defects promoting the constitutive activation of canonical and non-canonical NF-κB signaling pathways contribute to many diseases, including cancer, diabetes, chronic inflammation, and autoimmunity. In the present review, we focus our attention on the mechanisms of NF-κB deregulation in hematological malignancies. Key positive regulators of NF-κB signaling can act as oncogenes that are often prone to chromosomal translocation, amplifications, or activating mutations. Negative regulators of NF-κB have tumor suppressor functions, and are frequently inactivated either by genomic deletions or point mutations. NF-κB activation in tumoral cells is also driven by the microenvironment or chronic signaling that does not rely on genetic alterations.

Distinct Contributions of Autophagy Receptors in Measles Virus Replication

Autophagy is a potent cell autonomous defense mechanism that engages the lysosomal pathway to fight intracellular pathogens. Several autophagy receptors can recognize invading pathogens in order to target them towards autophagy for their degradation after the fusion of pathogen-containing autophagosomes with lysosomes. However, numerous intracellular pathogens can avoid or exploit autophagy, among which is measles virus (MeV). This virus induces a complete autophagy flux, which is required to improve viral replication. We therefore asked how measles virus interferes with autophagy receptors during the course of infection. We report that in addition to NDP52/CALCOCO₂ and OPTINEURIN/OPTN, another autophagy receptor, namely T6BP/TAXIBP1, also regulates the maturation of autophagosomes by promoting their fusion with lysosomes, independently of any infection. Surprisingly, only two of these receptors, NDP52 and T6BP, impacted measles virus replication, although independently, and possibly through physical interaction with MeV proteins. Thus, our results suggest that a restricted set of autophagosomes is selectively exploited by measles virus to replicate in the course of infection.

Inducible turnover of optineurin regulates T cell activation

Optineurin (Optn) is an adaptor protein with homology to NF-κB essential modulator (NEMO), the regulatory subunit of the IκB kinase (IKK) complex. Dysregulation of Optn has been linked to neurodegenerative, autoimmune and bone diseases. Optn shares a high degree of homology with NEMO, but is not part of the same high-molecular weight complex containing IKKα and IKKβ. Despite its homology with NEMO and the fact that it has been the subject of extensive study in several cell types, there are no published studies addressing the role of Optn during T cell activation. Here we demonstrate that ectopic expression of Optn down-regulates TCR-induced NF-κB activation and TNF-α production, in a manner dependent on ubiquitin-binding. Conversely, knock-down of Optn enhances NF-κB activation and the production of TNF-α. Consistent with a negative regulatory role for this protein, we observed transient loss of Optn after TCR stimulation in both cell lines and in primary murine T cells. The acute loss of Optn appears to be due to both protein degradation and exocytosis, the latter via activation-induced exosomes. This study therefore provides novel information regarding the role of Optn during TCR activation, suggesting the possible importance of Optn during inflammation and/or autoimmune diseases.

Human T-Cell Leukemia Virus Type 1 Infection and Adult T-Cell Leukemia

Human T-cell leukemia virus type 1 (HTLV-1) is the first retrovirus discovered to cause adult T-cell leukemia (ATL), a highly aggressive blood cancer. HTLV-1 research in the past 35 years has been most revealing in the mechanisms of viral oncogenesis. HTLV-1 establishes a lifelong persistent infection in CD4⁺ T lymphocytes. The infection outcome is governed by host immunity. ATL develops in 2-5% of infected individuals 30-50 years after initial exposure. HTLV-1 encodes two oncoproteins Tax and HBZ, which are required for initiation of cellular transformation and maintenance of cell proliferation, respectively. HTLV-1 oncogenesis is driven by a clonal selection and expansion process during which both host and viral factors cooperate to impair genome stability, immune surveillance, and other mechanisms of tumor suppression. A better understanding of HTLV-1 biology and leukemogenesis will reveal new strategies and modalities for ATL prevention and treatment.

Comparative interactomics analysis of different ALS-associated proteins identifies converging molecular pathways

Amyotrophic lateral sclerosis (ALS) is a devastating neurological disease with no effective treatment available. An increasing number of genetic causes of ALS are being identified, but how these genetic defects lead to motor neuron degeneration and to which extent they affect common cellular pathways remains incompletely understood. To address these questions, we performed an interactomic analysis to identify binding partners of wild-type (WT) and ALS-associated mutant versions of ATXN2, C9orf72, FUS, OPTN, TDP-43 and UBQLN2 in neuronal cells. This analysis identified several known but also many novel binding partners of these proteins. Interactomes of WT and mutant ALS proteins were very similar except for OPTN and UBQLN2, in which mutations caused loss or gain of protein interactions. Several of the identified interactomes showed a high degree of overlap: shared binding partners of ATXN2, FUS and TDP-43 had roles in RNA metabolism; OPTN- and UBQLN2-interacting proteins were related to protein degradation and protein transport, and C9orf72 interactors function in mitochondria. To confirm that this overlap is important for ALS pathogenesis, we studied fragile X mental retardation protein (FMRP), one of the common interactors of ATXN2, FUS and TDP-43, in more detail in in vitro and in vivo model systems for FUS ALS. FMRP localized to mutant FUS-containing aggregates in spinal motor neurons and bound endogenous FUS in a direct and RNA-sensitive manner. Furthermore, defects in synaptic FMRP mRNA target expression, neuromuscular junction integrity, and motor behavior caused by mutant FUS in zebrafish embryos, could be rescued by exogenous FMRP expression. Together, these results show that interactomics analysis can provide crucial insight into ALS disease mechanisms and they link FMRP to motor neuron dysfunction caused by FUS mutations.

Cellular Functions of Optineurin in Health and Disease

Optineurin (OPTN) was initially identified as a regulator of NF-κB and interferon signaling, but attracted most attention because of its association with various human disorders such as glaucoma, Paget disease of bone, and amyotrophic lateral sclerosis. Importantly, OPTN has recently been identified as an autophagy receptor important for the autophagic removal of pathogens, damaged mitochondria, and protein aggregates. This activity is most likely compromised in patients carrying OPTN mutations, and contributes to the observed phenotypes. In this review we summarize recent studies describing the molecular mechanisms by which OPTN controls immunity and autophagy, and discuss these findings in the context of several diseases that have been associated with OPTN (mal)function.

Regulation of TBK1 activity by Optineurin contributes to cell cycle-dependent expression of the interferon pathway

The innate immune system has evolved to detect and neutralize viral invasions. Triggering of this defense mechanism relies on the production and secretion of soluble factors that stimulate intracellular antiviral defense mechanisms. The Tank Binding Kinase 1 (TBK1) is a serine/threonine kinase in the innate immune signaling pathways including the antiviral response and the host defense against cytosolic infection by bacteries. Given the critical roles of TBK1, important regulatory mechanisms are required to regulate its activity. Among these, Optineurin (Optn) was shown to negatively regulate the interferon response, in addition to its important role in membrane trafficking, protein secretion, autophagy and cell division. As Optn does not carry any enzymatic activity, its functions depend on its precise subcellular localization and its interaction with other proteins, especially with components of the innate immune pathway. This review highlights advances in our understanding of Optn mechanisms of action with focus on the relationships between Optn and TBK1 and their implication in host defense against pathogens. Specifically, how the antiviral immune system is controlled during the cell cycle by the Optn/TBK1 axis and the physiological consequences of this regulatory mechanism are described. This review may serve to a better understanding of the relationships between the different functions of Optn, including those related to immune responses and its associated pathologies such as primary open-angle glaucoma, amyotrophic lateral sclerosis and Paget's disease of bone.

The Major Histocompatibility Complex Class II Transactivator CIITA Inhibits the Persistent Activation of NF-κB by the Human T Cell Lymphotropic Virus Type 1 Tax-1 Oncoprotein

Human T cell lymphotropic virus type 1 (HTLV-1) Tax-1, a key protein in HTLV-1-induced T cell transformation, deregulates diverse cell signaling pathways. Among them, the NF-κB pathway is constitutively activated by Tax-1, which binds to NF-κB proteins and activates the IκB kinase (IKK). Upon phosphorylation-dependent IκB degradation, NF-κB migrates into the nucleus, mediating Tax-1-stimulated gene expression. We show that the transcriptional regulator of major histocompatibility complex class II genes CIITA (class II transactivator), endogenously or ectopically expressed in different cells, inhibits the activation of the canonical NF-κB pathway by Tax-1 and map the region that mediates this effect. CIITA affects the subcellular localization of Tax-1, which is mostly retained in the cytoplasm, and this correlates with impaired migration of RelA into the nucleus. Cytoplasmic and nuclear mutant forms of CIITA reveal that CIITA exploits different strategies to suppress Tax-1-mediated NF-κB activation in both subcellular compartments. CIITA interacts with Tax-1 without preventing Tax-1 binding to both IKKγ and RelA. Nevertheless, CIITA affects Tax-1-induced IKK activity, causing retention of the inactive p50/RelA/IκB complex in the cytoplasm. Nuclear CIITA associates with Tax-1/RelA in nuclear bodies, blocking Tax-1-dependent activation of NF-κB-responsive genes. Thus, CIITA inhibits cytoplasmic and nuclear steps of Tax-1-mediated NF-κB activation. These results, together with our previous finding that CIITA acts as a restriction factor inhibiting Tax-1-promoted HTLV-1 gene expression and replication, indicate that CIITA is a versatile molecule that might also counteract Tax-1 transforming activity. Unveiling the molecular basis of CIITA-mediated inhibition of Tax-1 functions may be important in defining new strategies to control HTLV-1 spreading and oncogenic potential.

IMPORTANCE

HTLV-1 is the causative agent of human adult T cell leukemia-lymphoma (ATLL). The viral transactivator Tax-1 plays a central role in the onset of ATLL, mostly by deregulating the NF-κB pathway. We demonstrate that CIITA, a key regulator of adaptive immunity, suppresses Tax-1-dependent activation of NF-κB by acting at several levels: it retains most of Tax-1 and RelA in the cytoplasm and inhibits their residual functional activity in the nucleus. Importantly, this inhibition occurs in cells that are targets of HTLV-1 infection. These findings are of interest in the field of virology because they expand the current knowledge of the functional relationship between viral products and cellular interactors and provide the basis for a better understanding of the molecular countermeasures adopted by the host cell to antagonize HTLV-1 spreading and transforming properties. Within this framework, our results may contribute to the establishment of novel strategies against HTLV-1 infection and virus-dependent oncogenic transformation.

Optineurin deficiency in mice is associated with increased sensitivity to Salmonella but does not affect proinflammatory NF-κB signaling

Optineurin (OPTN) is an evolutionary conserved and ubiquitously expressed ubiquitin-binding protein that has been implicated in glaucoma, Paget bone disease, amyotrophic lateral sclerosis, and other neurodegenerative diseases. From in vitro studies, OPTN was shown to suppress TNF-induced NF-κB signaling and virus-induced IRF signaling, and was identified as an autophagy receptor required for the clearance of cytosolic Salmonella upon infection. To assess the in vivo functions of OPTN in inflammation and infection, we generated OPTN-deficient mice. OPTN knockout mice are born with normal Mendelian distribution and develop normally without any signs of spontaneous organ abnormality or inflammation. However, no differences in NF-κB activation could be observed in OPTN knockout mice or fibroblasts derived from these mice upon TNF or LPS treatment. Primary bone marrow-derived macrophages from OPTN-deficient mice had slightly impaired IRF signaling and reduced IFN type I production in response to LPS or poly(I,C). Finally, OPTN-deficient mice were more susceptible to infection with Salmonella, confirming in vivo the importance of OPTN in bacterial clearance.

Tax secretion from peripheral blood mononuclear cells and Tax detection in plasma of patients with human T-lymphotropic virus-type 1-associated myelopathy/tropical spastic paraparesis and asymptomatic carriers

Human T-lymphotropic virus-type 1 (HTLV-1) is the etiologic agent of the neurologic disease HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). Tax viral protein plays a critical role in viral pathogenesis. Previous studies suggested that extracellular Tax might involve cytokine-like extracellular effects. We evaluated Tax secretion in 18 h-ex vivo peripheral blood mononuclear cells (PBMCs) cultures from 15 HAM/TSP patients and 15 asymptomatic carriers. Futhermore, Tax plasma level was evaluated from other 12 HAM/TSP patients and 10 asymptomatic carriers. Proviral load and mRNA encoding Tax were quantified by PCR and real-time RT-PCR, respectively. Intracellular Tax in CD4(+)CD25(+) cells occurred in 100% and 86.7% of HAM/TSP patients and asymptomatic carriers, respectively. Percentage of CD4(+)CD25(+) Tax+, proviral load and mRNA encoding Tax were significantly higher in HAM/TSP patients. Western blot analyses showed higher secretion levels of ubiquitinated Tax in HAM/TSP patients than in asymptomatic carriers. In HTLV-1-infected subjects, Western blot of plasma Tax showed higher levels in HAM/TSP patients than in asymptomatic carriers, whereas no Tax was found in non-infected subjects. Immunoprecipitated plasma Tax resolved on SDS-PAGE gave two major bands of 57 and 48 kDa allowing identification of Tax and Ubiquitin peptides by mass spectrometry. Relative percentage of either CD4(+)CD25(+) Tax+ cells, or Tax protein released from PBMCs, or plasma Tax, correlates neither with tax mRNA nor with proviral load. This fact could be explained by a complex regulation of Tax expression. Tax secreted from PBMCs or present in plasma could potentially become a biomarker to distinguish between HAM/TSP patients and asymptomatic carriers.

A Glaucoma-Associated Variant of Optineurin, M98K, Activates Tbk1 to Enhance Autophagosome Formation and Retinal Cell Death Dependent on Ser177 Phosphorylation of Optineurin

Certain missense mutations in optineurin/OPTN and amplification of TBK1 are associated with normal tension glaucoma. A glaucoma-associated variant of OPTN, M98K, induces autophagic degradation of transferrin receptor (TFRC) and death in retinal cells. Here, we have explored the role of Tbk1 in M98K-OPTN-induced autophagy and cell death, and the effect of Tbk1 overexpression in retinal cells. Cell death induced by M98K-OPTN was dependent on Tbk1 as seen by the effect of Tbk1 knockdown and blocking of Tbk1 activity by a chemical inhibitor. Inhibition of Tbk1 also restores M98K-OPTN-induced transferrin receptor degradation. M98K-OPTN-induced autophagosome formation, autophagy and cell death were dependent on its phosphorylation at S177 by Tbk1. Knockdown of OPTN reduced starvation-induced autophagosome formation. M98K-OPTN expressing cells showed higher levels of Tbk1 activation and enhanced phosphorylation at Ser177 compared to WT-OPTN expressing cells. M98K-OPTN-induced activation of Tbk1 and its ability to be phosphorylated better by Tbk1 was dependent on ubiquitin binding. Phosphorylated M98K-OPTN localized specifically to autophagosomes and endogenous Tbk1 showed increased localization to autophagosomes in M98K-OPTN expressing cells. Overexpression of Tbk1 induced cell death and caspase-3 activation that were dependent on its catalytic activity. Tbk1-induced cell death possibly involves autophagy, as shown by the effect of Atg5 knockdown, and requirement of autophagic function of OPTN. Our results show that phosphorylation of Ser177 plays a crucial role in M98K-OPTN-induced autophagosome formation, autophagy flux and retinal cell death. In addition, we provide evidence for cross talk between two glaucoma associated proteins and their inter-dependence to mediate autophagy-dependent cell death.

HTLV-1 Tax Stimulates Ubiquitin E3 Ligase, Ring Finger Protein 8, to Assemble Lysine 63-Linked Polyubiquitin Chains for TAK1 and IKK Activation

Human T lymphotropic virus type 1 (HTLV-1) trans-activator/oncoprotein, Tax, impacts a multitude of cellular processes, including I-κB kinase (IKK)/NF-κB signaling, DNA damage repair, and mitosis. These activities of Tax have been implicated in the development of adult T-cell leukemia (ATL) in HTLV-1-infected individuals, but the underlying mechanisms remain obscure. IKK and its upstream kinase, TGFβ-activated kinase 1 (TAK1), contain ubiquitin-binding subunits, NEMO and TAB2/3 respectively, which interact with K63-linked polyubiquitin (K63-pUb) chains. Recruitment to K63-pUb allows cross auto-phosphorylation and activation of TAK1 to occur, followed by TAK1-catalyzed IKK phosphorylation and activation. Using cytosolic extracts of HeLa and Jurkat T cells supplemented with purified proteins we have identified ubiquitin E3 ligase, ring finger protein 8 (RNF8), and E2 conjugating enzymes, Ubc13:Uev1A and Ubc13:Uev2, to be the cellular factors utilized by Tax for TAK1 and IKK activation. In vitro, the combination of Tax and RNF8 greatly stimulated TAK1, IKK, IκBα and JNK phosphorylation. In vivo, RNF8 over-expression augmented while RNF8 ablation drastically reduced canonical NF-κB activation by Tax. Activation of the non-canonical NF-κB pathway by Tax, however, is unaffected by the loss of RNF8. Using purified components, we further demonstrated biochemically that Tax greatly stimulated RNF8 and Ubc13:Uev1A/Uev2 to assemble long K63-pUb chains. Finally, co-transfection of Tax with increasing amounts of RNF8 greatly induced K63-pUb assembly in a dose-dependent manner. Thus, Tax targets RNF8 and Ubc13:Uev1A/Uev2 to promote the assembly of K63-pUb chains, which signal the activation of TAK1 and multiple downstream kinases including IKK and JNK. Because of the roles RNF8 and K63-pUb chains play in DNA damage repair and cytokinesis, this mechanism may also explain the genomic instability of HTLV-1-transformed T cells and ATL cells.

Activation of the NF-κB family of transcription factors by the HTLV-1 oncoprotein, Tax, is causally linked to adult T cell leukemia (ATL) development in HTLV-1-infected individuals, but the underlying mechanisms are not fully understood. NF-κB activation requires the phosphorylation of its inhibitor, IκBα, by IκB kinase (IKK), which marks IκBα for degradation. In this study, we demonstrate that Tax inappropriately activates a ubiquitin E3 ligase, RNF8, and ubiquitin E2 conjugating enzymes, Ubc13:Uev1A/Uev2, to assemble long lysine 63-linked polyubiquitin (K63-pUb) chains, which function as signaling platforms for polyubiquitin-binding TGFβ-activated kinase 1 (TAK1) and IKK to congregate and become activated. Because TAK1 mediates the activation of multiple downstream signaling pathways, the mechanism described here can explain the complex effect of Tax on cell signaling. The major functions of RNF8 are to signal cellular DNA damage repair (DDR) and cell division by assembling K63-pUb chains at the site of DNA damage and cell cleavage. As such, the inappropriate activation of RNF8 and the over-abundance of K63-pUb chains in Tax-expressing cells may explain how Tax causes DNA damage and cell division defect.

Optineurin regulates the interferon response in a cell cycle-dependent manner

Viral invasion into a host is initially recognized by the innate immune system, mainly through activation of the intracellular cytosolic signaling pathway and coordinated activation of interferon regulatory factor 3 (IRF3) and nuclear factor kappa B (NF-κB) transcription factors that promote type I interferon gene induction. The TANK-binding Kinase 1 (TBK1) phosphorylates and activates IRF3. Here, we show that Optineurin (Optn) dampens the antiviral innate immune response by targeting the deubiquitinating enzyme CYLD to TBK1 in order to inhibit its enzymatic activity. Importantly, we found that this regulatory mechanism is abolished at the G2/M phase as a consequence of the nuclear translocation of CYLD and Optn. As a result, we observed, at this cell division stage, an increased activity and phosphorylation of TBK1 that lead to its relocalization to mitochondria and to enhanced interferon production, suggesting that this process, which relies on Optn function, might be of major importance to mount a preventive antiviral response during mitosis.

The innate immune system has evolved to detect and neutralize viral invasion. Triggering of this defense mechanism relies on the production and secretion of soluble factors that stimulate an intracellular antiviral defense mechanism. The protein Optineurin was shown to negatively regulate this process. Importantly, we discovered the mechanism by which Optineurin inhibits antiviral activity and showed that this regulation is prevented during a critical step of cell division leading to enhancement of the cellular defense mechanism. This paper shows that the antiviral immune system is controlled during the cell cycle and that Optineurin-mediated induction of this system might serve to protect cells from infection during cell division.

Human T-cell leukemia virus type 1 (HTLV-1) tax requires CADM1/TSLC1 for inactivation of the NF-κB inhibitor A20 and constitutive NF-κB signaling

Persistent activation of NF-κB by the Human T-cell leukemia virus type 1 (HTLV-1) oncoprotein, Tax, is vital for the development and pathogenesis of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). K63-linked polyubiquitinated Tax activates the IKK complex in the plasma membrane-associated lipid raft microdomain. Tax also interacts with TAX1BP1 to inactivate the NF-κB negative regulatory ubiquitin-editing A20 enzyme complex. However, the molecular mechanisms of Tax-mediated IKK activation and A20 protein complex inactivation are poorly understood. Here, we demonstrated that membrane associated CADM1 (Cell adhesion molecule1) recruits Ubc13 to Tax, causing K63-linked polyubiquitination of Tax, and IKK complex activation in the membrane lipid raft. The c-terminal cytoplasmic tail containing PDZ binding motif of CADM1 is critical for Tax to maintain persistent NF-κB activation. Finally, Tax failed to inactivate the NF-κB negative regulator ubiquitin-editing enzyme A20 complex, and activate the IKK complex in the lipid raft in absence of CADM1. Our results thus indicate that CADM1 functions as a critical scaffold molecule for Tax and Ubc13 to form a cellular complex with NEMO, TAX1BP1 and NRP, to activate the IKK complex in the plasma membrane-associated lipid rafts, to inactivate NF-κB negative regulators, and maintain persistent NF-κB activation in HTLV-1 infected cells.

HTLV-1 infection leads to the development of Adult T-cell Leukemia (ATL) or HTLV-1 associated myelopathy/ tropical spastic paraparesis (HAM/TSP). One of the major causes responsible for the development of HTLV-1 associated diseases is chronic inflammation directed by NF-kappaB (NF-κB). NF-κB activation in response to a wide variety of signals is transient and tightly controlled by ubiquitin-editing enzyme A20. One of the mechanisms of persistent NF-κB activation in HTLV-1 infected cells is inactivation of NF-κB negative regulators; however, the precise mechanism is unknown. Here, we focused on host tumor suppressor Cell adhesion molecule 1 (CADM1) that is robustly upregulated in HTLV-1 infected cells. The expression of CADM1 is frequently silenced in several cancers; however, it is critical for HTLV-1 associated ATL tumor cell survival. We characterized the role of CADM1 in persistent NF-κB activation in HTLV-1 infected cells. We found that CADM1 is required for the HTLV-1 oncoprotein, Tax, to form a cellular complex with Ubc13, TAX1BP1, NRP and NEMO in the membrane lipid rafts micorodomain. We further demonstrated that Tax requires CADM1 to inactivate NF-κB negative regulator and maintain persistent NF-κB activation. Our study reveals a novel mechanism of chronic NF-κB activation by CADM1 in HTLV-1 infected cells.

ADAR1 enhances HTLV-1 and HTLV-2 replication through inhibition of PKR activity

Background

The role of innate immunity in general and of type I interferon (IFN-I) in particular in HTLV-1 pathogenesis is still a matter of debate. ADAR1-p150 is an Interferon Stimulated Gene (ISG) induced by IFN-I that can edit viral RNAs. We therefore investigated whether it could play the role of an anti-HTLV factor.

Results

We demonstrate here that ADAR1 is also expressed in the absence of IFN stimulation in activated primary T-lymphocytes that are the natural target of this virus and in HTLV-1 or HTLV-2 chronically infected T-cells. ADAR1 expression is also increased in primary lymphocytes obtained from HTLV-1 infected individuals. We show that ADAR1 enhances HTLV-1 and HTLV-2 infection in T-lymphocytes and that this proviral effect is independent from its editing activity. ADAR1 expression suppresses IFN-α inhibitory effect on HTLV-1 and HTLV-2 and acts through the repression of PKR phosphorylation.

Discussion

This study demonstrates that two interferon stimulated genes, i.e. PKR and ADAR1 have opposite effects on HTLV replication in vivo. The balanced expression of those proteins could determine the fate of the viral cycle in the course of infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12977-014-0093-9) contains supplementary material, which is available to authorized users.

Regulation of HTLV-1 tax stability, cellular trafficking and NF-κB activation by the ubiquitin-proteasome pathway

Human T-cell leukemia virus type 1 (HTLV-1) is a complex retrovirus that infects CD4+ T cells and causes adult T-cell leukemia/lymphoma (ATLL) in 3%–5% of infected individuals after a long latent period. HTLV-1 Tax is a trans-activating protein that regulates viral gene expression and also modulates cellular signaling pathways to enhance T-cell proliferation and cell survival. The Tax oncoprotein promotes T-cell transformation, in part via constitutive activation of the NF-κB transcription factor; however, the underlying mechanisms remain unknown. Ubiquitination is a type of post-translational modification that occurs in a three-step enzymatic cascade mediated by E1, E2 and E3 enzymes and regulates protein stability as well as signal transduction, protein trafficking and the DNA damage response. Emerging studies indicate that Tax hijacks the ubiquitin machinery to activate ubiquitin-dependent kinases and downstream NF-κB signaling. Tax interacts with the E2 conjugating enzyme Ubc13 and is conjugated on C-terminal lysine residues with lysine 63-linked polyubiquitin chains. Tax K63-linked polyubiquitination may serve as a platform for signaling complexes since this modification is critical for interactions with NEMO and IKK. In addition to NF-κB signaling, mono- and polyubiquitination of Tax also regulate its subcellular trafficking and stability. Here, we review recent advances in the diverse roles of ubiquitin in Tax function and how Tax usurps the ubiquitin-proteasome pathway to promote oncogenesis.

Crystallographic and modelling studies suggest that the SKICH domains from different protein families share a common Ig-like fold but harbour substantial structural variations

TAX1BP1 is a pleiotropic multi-domain protein involved in many important biological processes such as signal transduction, cell growth and apoptosis, transcriptional coactivation, membrane trafficking, neurotransmission and autophagy. The N-terminus of TAX1BP1 contains a SKICH domain implicated in autophagy. SKICH domains are also present in four other proteins including NDP52, CALCOCO1, SKIP and PIPP. The SKICH domains of SKIP and PIPP mediate plasma membrane localisation. The functions of the SKICH domains of NDP52 and CALCOCO1 are not known. Here we report the crystal structure of the TAX1BP1 SKICH domain, which has an Ig-like fold similar to the NDP52 SKICH domain. Extensive pairwise and clustered aromatic π-stacking interactions are present in the TAX1BP1 SKICH domain. The aromatic residues mediating these interactions can be classified into four groups with varying degrees of conservation among different protein families. The interactions mediated by highly conserved residues are found in the interior and one outward face of the Ig-like β-barrel, representing common structural features of the SKICH domains. Three TAX1BP1-specific pairwise interactions locate in the loop regions, each augmented by a proline-aromatic interaction. The three proline-aromatic clusters are linked together by more generic hydrophobic interactions, forming a unique hydrophobic surface at one end of the TAX1BP1 SKICH domain. The structures and homologous models of SKICH domains from different proteins reveal substantial differences in electrostatic surface properties of the domains. Together with existing biochemical data, results from the structural study suggest that an intact SKICH domain is required for the autophagy function of TAX1BP1.

The intricate interplay between RNA viruses and NF-κB

RNA viruses have rapidly evolving genomes which often allow cross-species transmission and frequently generate new virus variants with altered pathogenic properties. Therefore infections by RNA viruses are a major threat to human health. The infected host cell detects trace amounts of viral RNA and the last years have revealed common principles in the biochemical mechanisms leading to signal amplification that is required for mounting of a powerful antiviral response. Components of the RNA sensing and signaling machinery such as RIG-I-like proteins, MAVS and the inflammasome inducibly form large oligomers or even fibers that exhibit hallmarks of prions. Following a nucleation event triggered by detection of viral RNA, these energetically favorable and irreversible polymerization events trigger signaling cascades leading to the induction of antiviral and inflammatory responses, mediated by interferon and NF-κB pathways. Viruses have evolved sophisticated strategies to manipulate these host cell signaling pathways in order to ensure their replication. We will discuss at the examples of influenza and HTLV-1 viruses how a fascinating diversity of biochemical mechanisms is employed by viral proteins to control the NF-κB pathway at all levels.

Expression, purification and crystallization of the SKICH domain of human TAX1BP1

TAX1BP1 is a highly conserved, pleiotropic protein that plays many essential functions in human cells, including negative regulation of inflammatory and antimicrobial responses mediated by NF-κB and IRF3 signaling, inhibition of apoptosis, transcriptional coactivation and autophagy etc. TAX1BP1 contains a SKICH domain at the N-terminus, three coiled-coil domains in the middle and two ubiquitin-binding zinc-finger motifs at the C-terminus. The SKICH domain and the linker sequence between the SKICH domain and the coiled-coil region mediate interaction with ubiquitin-like proteins of the LC3/GABARAP family, which are autophagosome markers. For structure determination of the SKICH domain of TAX1BP1, a protein construct (amino acids 15-148) corresponding to the SKICH domain plus the linker region was expressed, purified and crystallized. A native diffraction data set has been collected to 1.9 Å resolution. A molecular-replacement solution has been found by using the structure of the SKICH domain of NDP52, a paralog of TAX1BP1.

E50K-OPTN-induced retinal cell death involves the Rab GTPase-activating protein, TBC1D17 mediated block in autophagy

The protein optineurin coded by OPTN gene is involved in several functions including regulation of endocytic trafficking, autophagy and signal transduction. Certain missense mutations in the gene OPTN cause normal tension glaucoma. A glaucoma-causing mutant of optineurin, E50K, induces death selectively in retinal cells. This mutant induces defective endocytic recycling of transferrin receptor by causing inactivation of Rab8 mediated by the GTPase-activating protein, TBC1D17. Here, we have explored the mechanism of E50K-induced cell death. E50K-OPTN-induced cell death was inhibited by co-expression of a catalytically inactive mutant of TBC1D17 and also by shRNA mediated knockdown of TBC1D17. Endogenous TBC1D17 colocalized with E50K-OPTN in vesicular structures. Co-expression of transferrin receptor partially protected against E50K-induced cell death. Overexpression of the E50K-OPTN but not WT-OPTN inhibited autophagy flux. Treatment of cells with rapamycin, an inducer of autophagy, reduced E50K-OPTN-induced cell death. An LC3-binding-defective mutant of E50K-OPTN showed reduced cell death, further suggesting the involvement of autophagy. TBC1D17 localized to autophagosomes and inhibited autophagy flux dependent on its catalytic activity. Knockdown of TBC1D17 rescued cells from E50K-mediated inhibition of autophagy flux. Overall, our results suggest that E50K mutant induced death of retinal cells involves impaired autophagy as well as impaired transferrin receptor function. TBC1D17, a GTPase-activating protein for Rab GTPases, plays a crucial role in E50K-induced impaired autophagy and cell death.

Highlights on distinctive structural and functional properties of HTLV Tax proteins

Human T cell leukemia viruses (HTLVs) are complex human retroviruses of the Deltaretrovirus genus. Four types have been identified thus far, with HTLV-1 and HTLV-2 much more prevalent than HTLV-3 or HTLV-4. HTLV-1 and HTLV-2 possess strictly related genomic structures, but differ significantly in pathogenicity, as HTLV-1 is the causative agent of adult T cell leukemia and of HTLV-associated myelopathy/tropical spastic paraparesis, whereas HTLV-2 is not associated with neoplasia. HTLVs code for a protein named Tax that is responsible for enhancing viral expression and drives cell transformation. Much effort has been invested to dissect the impact of Tax on signal transduction pathways and to identify functional differences between the HTLV Tax proteins that may explain the distinct oncogenic potential of HTLV-1 and HTLV-2. This review summarizes our current knowledge of Tax-1 and Tax-2 with emphasis on their structure, role in activation of the NF-κB (nuclear factor kappa-B) pathway, and interactions with host factors.

M98K-OPTN induces transferrin receptor degradation and RAB12-mediated autophagic death in retinal ganglion cells

Mutations in the autophagy receptor OPTN/optineurin are associated with the pathogenesis of glaucoma and amyotrophic lateral sclerosis, but the underlying molecular basis is poorly understood. The OPTN variant, M98K has been described as a risk factor for normal tension glaucoma in some ethnic groups. Here, we examined the consequence of the M98K mutation in affecting cellular functions of OPTN. Overexpression of M98K-OPTN induced death of retinal ganglion cells (RGC-5 cell line), but not of other neuronal and non-neuronal cells. Enhanced levels of the autophagy marker, LC3-II, a post-translationally modified form of LC3, in M98K-OPTN-expressing cells and the inability of an LC3-binding-defective M98K variant of OPTN to induce cell death, suggested that autophagy contributes to cell death. Knockdown of Atg5 reduced M98K-induced death of RGC-5 cells, further supporting the involvement of autophagy. Overexpression of M98K-OPTN enhanced autophagosome formation and potentiated the delivery of transferrin receptor to autophagosomes for degradation resulting in reduced cellular transferrin receptor levels. Coexpression of transferrin receptor or supplementation of media with an iron donor reduced M98K-induced cell death. OPTN complexes with RAB12, a GTPase involved in vesicle trafficking, and M98K variant shows enhanced colocalization with RAB12. Knockdown of Rab12 increased transferrin receptor level and reduced M98K-induced cell death. RAB12 is present in autophagosomes and knockdown of Rab12 resulted in reduced formation of autolysosomes during starvation-induced autophagy, implicating a role for RAB12 in autophagy. These results also show that transferrin receptor degradation and autophagy play a crucial role in RGC-5 cell death induced by M98K variant of OPTN.

HTLV tax: a fascinating multifunctional co-regulator of viral and cellular pathways

Human T-cell lymphotropic virus type 1 (HTLV-1) has been identified as the causative agent of adult T-cell leukemia (ATL) and HTLV-1-associated myelopathy/tropical spastic paraparesis (HAM/TSP). The virus infects between 15 and 20 million people worldwide of which approximately 2-5% develop ATL. The past 35 years of research have yielded significant insight into the pathogenesis of HTLV-1, including the molecular characterization of Tax, the viral transactivator, and oncoprotein. In spite of these efforts, the mechanisms of oncogenesis of this pleiotropic protein remain to be fully elucidated. In this review, we illustrate the multiple oncogenic roles of Tax by summarizing a recent body of literature that refines our understanding of cellular transformation. A focused range of topics are discussed in this review including Tax-mediated regulation of the viral promoter and other cellular pathways, particularly the connection of the NF-κB pathway to both post-translational modifications (PTMs) of Tax and subcellular localization. Specifically, recent research on polyubiquitination of Tax as it relates to the activation of the IkappaB kinase (IKK) complex is highlighted. Regulation of the cell cycle and DNA damage responses due to Tax are also discussed, including Tax interaction with minichromosome maintenance proteins and the role of Tax in chromatin remodeling. The recent identification of HTLV-3 has amplified the importance of the characterization of emerging viral pathogens. The challenge of the molecular determination of pathogenicity and malignant disease of this virus lies in the comparison of the viral transactivators of HTLV-1, -2, and -3 in terms of transformation and immortalization. Consequently, differences between the three proteins are currently being studied to determine what factors are required for the differences in tumorogenesis.

TBK1 kinase addiction in lung cancer cells is mediated via autophagy of Tax1bp1/Ndp52 and non-canonical NF-κB signalling

K-Ras dependent non-small cell lung cancer (NSCLC) cells are 'addicted' to basal autophagy that reprograms cellular metabolism in a lysosomal-sensitive manner. Here we demonstrate that the xenophagy-associated kinase TBK1 drives basal autophagy, consistent with its known requirement in K-Ras-dependent NSCLC proliferation. Furthermore, basal autophagy in this context is characterised by sequestration of the xenophagy cargo receptor Ndp52 and its paralogue Tax1bp1, which we demonstrate here to be a bona fide cargo receptor. Autophagy of these cargo receptors promotes non-canonical NF-κB signalling. We propose that this TBK1-dependent mechanism for NF-κB signalling contributes to autophagy addiction in K-Ras driven NSCLC.

Low nuclear body formation and tax SUMOylation do not prevent NF-kappaB promoter activation

Background

The Tax protein encoded by Human T-lymphotropic virus type 1 (HTLV-1) is a powerful activator of the NF-κB pathway, a property critical for HTLV-1-induced immortalization of CD4⁺ T lymphocytes. Tax permanently stimulates this pathway at a cytoplasmic level by activating the IκB kinase (IKK) complex and at a nuclear level by enhancing the binding of the NF-κB factor RelA to its cognate promoters and by forming nuclear bodies, believed to represent transcriptionally active structures. In previous studies, we reported that Tax ubiquitination and SUMOylation play a critical role in Tax localization and NF-κB activation. Indeed, analysis of lysine Tax mutants fused or not to ubiquitin or SUMO led us to propose a two-step model in which Tax ubiquitination first intervenes to activate IKK while Tax SUMOylation is subsequently required for promoter activation within Tax nuclear bodies. However, recent studies showing that ubiquitin or SUMO can modulate Tax activities in either the nucleus or the cytoplasm and that SUMOylated Tax can serve as substrate for ubiquitination suggested that Tax ubiquitination and SUMOylation may mediate redundant rather than successive functions.

Results

In this study, we analyzed the properties of a new Tax mutant that is properly ubiquitinated, but defective for both nuclear body formation and SUMOylation. We report that reducing Tax SUMOylation and nuclear body formation do not alter the ability of Tax to activate IKK, induce RelA nuclear translocation, and trigger gene expression from a NF-κB promoter. Importantly, potent NF-κB promoter activation by Tax despite low SUMOylation and nuclear body formation is also observed in T cells, including CD4⁺ primary T lymphocytes. Moreover, we show that Tax nuclear bodies are hardly observed in HTLV-1-infected T cells. Finally, we provide direct evidence that the degree of NF-κB activation by Tax correlates with the level of Tax ubiquitination, but not SUMOylation.

Conclusions

These data reveal that the formation of Tax nuclear bodies, previously associated to transcriptional activities in Tax-transfected cells, is dispensable for NF-κB promoter activation, notably in CD4⁺ T cells. They also provide the first evidence that Tax SUMOylation is not a key determinant for Tax-induced NF-κB activation.

Molecular hallmarks of adult T cell leukemia

The molecular hallmarks of adult T cell leukemia (ATL) comprise outstanding deregulations of signaling pathways that control the cell cycle, resistance to apoptosis, and proliferation of leukemic cells, all of which have been identified by early excellent studies. Nevertheless, we are now confronted the therapeutic difficulties of ATL that is a most aggressive T cell leukemia/lymphoma. Using next-generation strategies, emerging molecular characteristics such as specific surface markers and an additional catalog of signals affecting the fate of leukemic cells have been added to the molecular hallmarks that constitute an organizing principle for rationalizing the complexities of ATL. Although human T cell leukemia virus type 1 is undoubtedly involved in ATL leukemogenesis, most leukemic cells do not express the viral protein Tax. Instead, cellular gene expression changes dominate homeostasis disorders of infected cells and characteristics of ATL. In this review, we summarize the state of the art of ATL molecular pathology, which supports the biological properties of leukemic cells. In addition, we discuss the recent discovery of two molecular hallmarks of potential generality; an abnormal microRNA pattern and epigenetic reprogramming, which strongly involve the imbalance of the molecular network of lymphocytes. Global analyses of ATL have revealed the functional impact of crosstalk between multifunctional pathways. Clinical and biological studies on signaling inhibitory agents have also revealed novel oncogenic drivers that can be targeted in future. ATL cells, by deregulation of such pathways and their interconnections, may become masters of their own destinies. Recognizing and understanding of the widespread molecular applicability of these concepts will increasingly affect the development of novel strategies for treating ATL.

Human T-lymphotropic virus proteins and post-translational modification pathways

Cell life from the cell cycle to the signaling transduction and response to stimuli is finely tuned by protein post-translational modifications (PTMs). PTMs alter the conformation, the stability, the localization, and hence the pattern of interactions of the targeted protein. Cell pathways involve the activation of enzymes, like kinases, ligases and transferases, that, once activated, act on many proteins simultaneously, altering the state of the cell and triggering the processes they are involved in. Viruses enter a balanced system and hijack the cell, exploiting the potential of PTMs either to activate viral encoded proteins or to alter cellular pathways, with the ultimate consequence to perpetuate through their replication. Human T-lymphotropic virus type 1 (HTLV-1) is known to be highly oncogenic and associates with adult T-cell leukemia/lymphoma, HTLV-1-associated myelopathy/tropical spastic paraparesis and other inflammatory pathological conditions. HTLV-1 protein activity is controlled by PTMs and, in turn, viral activity is associated with the modulation of cellular pathways based on PTMs. More knowledge is acquired about the PTMs involved in the activation of its proteins, like Tax, Rex, p12, p13, p30, HTLV-I basic leucine zipper factor and Gag. However, more has to be understood at the biochemical level in order to counteract the associated fatal outcomes. This review will focus on known PTMs that directly modify HTLV-1 components and on enzymes whose activity is modulated by viral proteins.

Toward an integrative view of Optineurin functions

This review highlights recent advances in our understanding of the mechanisms of Optineurin (Optn) action and its implication in diseases. Optn has emerged as a key player regulating various physiological processes, including membrane trafficking, protein secretion, cell division and host defense against pathogens. Furthermore, there is growing evidence for an association of Optn mutations with human diseases such as primary open-angle glaucoma, amyotrophic lateral sclerosis and Paget's disease of bone. Optn functions depend on its precise subcellular localization and its interaction with other proteins. Here, we review the mechanisms that allow Optn to ensure a timely and spatially coordinated integration of different physiological processes and discuss how their deregulation may lead to different pathologies.

The multifaceted oncoprotein Tax: subcellular localization, posttranslational modifications, and NF-κB activation

The human T-cell lymphotropic virus type-I (HTLV-I) is the etiologic agent of adult T-cell leukemia/lymphoma (ATL) and of tropical spastic paraparesis/HTLV-I-associated myelopathy. Constitutive NF-κB activation by the viral oncoprotein Tax plays a crucial role in the induction and maintenance of cellular proliferation, transformation, and inhibition of apoptosis. In an attempt to provide a general view of the molecular mechanisms of constitutive Tax-induced NF-κB activation, we summarize in this review the recent body of literature that supports a major role for Tax posttranslational modifications, chiefly ubiquitination, and SUMOylation, in the NF-κB activity of Tax. These modifications indeed participate in the control of Tax subcellular localization and modulate its protein-protein interaction potential. Tax posttranslational modifications, which highlight the ability of HTLV-I to optimize its limited viral genome size, might represent an attractive target for the design of new therapies for ATL.