Translational control by ketamine and its implications for comorbid cognitive deficits in depressive disorders.

Ketamine has shown antidepressant effects in patients with major depressive disorder (MDD) resistant to first-line treatments and approved for use in this patient population. Ketamine induces several forms of synaptic plasticity, which are proposed to underlie its antidepressant effects. However, the molecular mechanism of action directly responsible for ketamine's antidepressant effects remains under active investigation. It was recently demonstrated that the effectors of the mammalian target of rapamycin complex 1 (mTORC1) signalling pathway, namely, eukaryotic initiation factor 4E (eIF4E) binding proteins 1 and 2 (4E-BP1 and 4E-BP2), are central in mediating ketamine-induced synaptic plasticity and behavioural antidepressant-like effect. 4E-BPs are a family of messenger ribonucleic acid (mRNA) translation repressors inactivated by mTORC1. We observed that their expression in inhibitory interneurons mediates ketamine's effects in the forced swim and novelty suppressed feeding tests and the long-lasting inhibition of GABAergic neurotransmission in the hippocampus. In addition, another effector pathway that regulates translation elongation downstream of mTORC1, the eukaryotic elongation factor 2 kinase (eEF2K), has been implicated in ketamine's behavioural effects. We will discuss how ketamine's rapid antidepressant effect depends on the activation of neuronal mRNA translation through 4E-BP1/2 and eEF2K. Furthermore, given that these pathways also regulate cognitive functions, we will discuss the evidence of ketamine's effect on cognitive function in MDD. Overall, the data accrued from pre-clinical research have implicated the mRNA translation pathways in treating mood symptoms of MDD. However, it is yet unclear whether the pro-cognitive potential of subanesthetic ketamine in rodents also engages these pathways and whether such an effect is consistently observed in the treatment-resistant MDD population.

Antidepressant actions of ketamine engage cell-specific translation via eIF4E.

Effective pharmacotherapy for major depressive disorder remains a major challenge, as more than 30% of patients are resistant to the first line of treatment (selective serotonin reuptake inhibitors)1. Sub-anaesthetic doses of ketamine, a non-competitive N-methyl-D-aspartate receptor antagonist2,3, provide rapid and long-lasting antidepressant effects in these patients4-6, but the molecular mechanism of these effects remains unclear7,8. Ketamine has been proposed to exert its antidepressant effects through its metabolite (2R,6R)-hydroxynorketamine ((2R,6R)-HNK)9. The antidepressant effects of ketamine and (2R,6R)-HNK in rodents require activation of the mTORC1 kinase10,11. mTORC1 controls various neuronal functions12, particularly through cap-dependent initiation of mRNA translation via the phosphorylation and inactivation of eukaryotic initiation factor 4E-binding proteins (4E-BPs)13. Here we show that 4E-BP1 and 4E-BP2 are key effectors of the antidepressant activity of ketamine and (2R,6R)-HNK, and that ketamine-induced hippocampal synaptic plasticity depends on 4E-BP2 and, to a lesser extent, 4E-BP1. It has been hypothesized that ketamine activates mTORC1-4E-BP signalling in pyramidal excitatory cells of the cortex8,14. To test this hypothesis, we studied the behavioural response to ketamine and (2R,6R)-HNK in mice lacking 4E-BPs in either excitatory or inhibitory neurons. The antidepressant activity of the drugs is mediated by 4E-BP2 in excitatory neurons, and 4E-BP1 and 4E-BP2 in inhibitory neurons. Notably, genetic deletion of 4E-BP2 in inhibitory neurons induced a reduction in baseline immobility in the forced swim test, mimicking an antidepressant effect. Deletion of 4E-BP2 specifically in inhibitory neurons also prevented the ketamine-induced increase in hippocampal excitatory neurotransmission, and this effect concurred with the inability of ketamine to induce a long-lasting decrease in inhibitory neurotransmission. Overall, our data show that 4E-BPs are central to the antidepressant activity of ketamine.

Maternal Immune Activation and the Development of Dopaminergic Neurotransmission of the Offspring: Relevance for Schizophrenia and Other Psychoses.

Prenatal infections have been linked to the development of schizophrenia (SCZ) and other neurodevelopmental disorders in the offspring, and work in animal models indicates that this is to occur through the maternal inflammatory response triggered by infection. Several studies in animal models demonstrated that acute inflammatory episodes are sufficient to trigger brain alterations in the adult offspring, especially in the mesolimbic dopamine (DA) system, involved in the pathophysiology of SCZ and other disorders involving psychosis. In the current review, we synthesize the literature on the clinical studies implicating prenatal infectious events in the development of SCZ. Then, we summarize evidence from animal models of maternal immune activation (MIA) and the behavioral and molecular alterations relevant for the function of the DAergic system. Furthermore, we discuss the evidence supporting the involvement of maternal cytokines, such as interleukin 6 (IL-6) and leptin (a hormone with effects on inflammation) in mediating the effects of MIA on the fetal brain, leading to the long-lasting effects on the offspring. In particular, IL-6 has been involved in mediating the effects of MIA animal models in the offspring through actions on the placenta, induction of IL-17a, or triggering the decrease in non-heme iron (hypoferremia). Maternal infection is very likely interacting with additional genetic and environmental risk factors in the development of SCZ; systematically investigating how these interactions produce specific phenotypes is the next step in understanding the etiology of complex psychiatric disorders.

Defining a Canonical Ligand-Binding Pocket in the Orphan Nuclear Receptor Nurr1.

Nuclear receptor-related 1 protein (Nurr1/NR4A2) is an orphan nuclear receptor (NR) that is considered to function without a canonical ligand-binding pocket (LBP). A crystal structure of the Nurr1 ligand-binding domain (LBD) revealed no physical space in the conserved region where other NRs with solvent accessible apo-protein LBPs bind synthetic and natural ligands. Using solution nuclear magnetic resonance spectroscopy, hydrogen/deuterium exchange mass spectrometry, and molecular dynamics simulations, we show that the putative canonical Nurr1 LBP is dynamic with high solvent accessibility, exchanges between two or more conformations on the microsecond-to-millisecond timescale, and can expand from the collapsed crystallized conformation to allow binding of unsaturated fatty acids. These findings should stimulate future studies to probe the ligandability and druggability of Nurr1 for both endogenous and synthetic ligands, which could lead to new therapeutics for Nurr1-related diseases, including Parkinson's disease and schizophrenia.

Translational control of depression-like behavior via phosphorylation of eukaryotic translation initiation factor 4E.

Translation of mRNA into protein has a fundamental role in neurodevelopment, plasticity, and memory formation; however, its contribution in the pathophysiology of depressive disorders is not fully understood. We investigated the involvement of MNK1/2 (MAPK-interacting serine/threonine-protein kinase 1 and 2) and their target, eIF4E (eukaryotic initiation factor 4E), in depression-like behavior in mice. Mice carrying a mutation in eIF4E for the MNK1/2 phosphorylation site (Ser209Ala, Eif4e ki/ki), the Mnk1/2 double knockout mice (Mnk1/2-/-), or mice treated with the MNK1/2 inhibitor, cercosporamide, displayed anxiety- and depression-like behaviors, impaired serotonin-induced excitatory synaptic activity in the prefrontal cortex, and diminished firing of the dorsal raphe neurons. In Eif4e ki/ki mice, brain IκBα, was decreased, while the NF-κB target, TNFα was elevated. TNFα inhibition in Eif4e ki/ki mice rescued, whereas TNFα administration to wild-type mice mimicked the depression-like behaviors and 5-HT synaptic deficits. We conclude that eIF4E phosphorylation modulates depression-like behavior through regulation of inflammatory responses.

Translational control of ERK signaling through miRNA/4EHP-directed silencing.

MicroRNAs (miRNAs) exert a broad influence over gene expression by directing effector activities that impinge on translation and stability of mRNAs. We recently discovered that the cap-binding protein 4EHP is a key component of the mammalian miRNA-Induced Silencing Complex (miRISC), which mediates gene silencing. However, little is known about the mRNA repertoire that is controlled by the 4EHP/miRNA mechanism or its biological importance. Here, using ribosome profiling, we identify a subset of mRNAs that are translationally controlled by 4EHP. We show that the Dusp6 mRNA, which encodes an ERK1/2 phosphatase, is translationally repressed by 4EHP and a specific miRNA, miR-145. This promotes ERK1/2 phosphorylation, resulting in augmented cell growth and reduced apoptosis. Our findings thus empirically define the integral role of translational repression in miRNA-induced gene silencing and reveal a critical function for this process in the control of the ERK signaling cascade in mammalian cells.

Cap-binding protein 4EHP effects translation silencing by microRNAs.

MicroRNAs (miRNAs) play critical roles in a broad variety of biological processes by inhibiting translation initiation and by destabilizing target mRNAs. The CCR4-NOT complex effects miRNA-mediated silencing, at least in part through interactions with 4E-T (eIF4E transporter) protein, but the precise mechanism is unknown. Here we show that the cap-binding eIF4E-homologous protein 4EHP is an integral component of the miRNA-mediated silencing machinery. We demonstrate that the cap-binding activity of 4EHP contributes to the translational silencing by miRNAs through the CCR4-NOT complex. Our results show that 4EHP competes with eIF4E for binding to 4E-T, and this interaction increases the affinity of 4EHP for the cap. We propose a model wherein the 4E-T/4EHP interaction engenders a closed-loop mRNA conformation that blocks translational initiation of miRNA targets.

Bound Waters Mediate Binding of Diverse Substrates to a Ubiquitin Ligase.

The N-end rule pathway controls the half-life of proteins based on their N-terminal residue. Positively charged type 1 N-degrons are recognized by a negatively charged pocket on the Zn finger named the UBR box. Here, we show that the UBR box is rigid, but bound water molecules in the pocket provide the structural plasticity required to bind different positively charged amino acids. Ultra-high-resolution crystal structures of arginine, histidine, and methylated arginine reveal that water molecules mediate the binding of N-degron peptides. Using a high-throughput binding assay and isothermal titration calorimetry, we demonstrate that the UBR box is able to bind methylated arginine and lysine peptides with high affinity and measure the preference for hydrophobic residues in the second position in the N-degron peptide. Finally, we show that the V122L mutation present in Johanson-Blizzard syndrome patients changes the specificity for the second position due to occlusion of the secondary pocket.

Control of embryonic stem cell self-renewal and differentiation via coordinated alternative splicing and translation of YY2.

Translational control of gene expression plays a key role during the early phases of embryonic development. Here we describe a transcriptional regulator of mouse embryonic stem cells (mESCs), Yin-yang 2 (YY2), that is controlled by the translation inhibitors, Eukaryotic initiation factor 4E-binding proteins (4E-BPs). YY2 plays a critical role in regulating mESC functions through control of key pluripotency factors, including Octamer-binding protein 4 (Oct4) and Estrogen-related receptor-ß (Esrrb). Importantly, overexpression of YY2 directs the differentiation of mESCs into cardiovascular lineages. We show that the splicing regulator Polypyrimidine tract-binding protein 1 (PTBP1) promotes the retention of an intron in the 5'-UTR of Yy2 mRNA that confers sensitivity to 4E-BP-mediated translational suppression. Thus, we conclude that YY2 is a major regulator of mESC self-renewal and lineage commitment and document a multilayer regulatory mechanism that controls its expression.

Identification of a Binding Site for Unsaturated Fatty Acids in the Orphan Nuclear Receptor Nurr1.

Nurr1/NR4A2 is an orphan nuclear receptor, and currently there are no known natural ligands that bind Nurr1. A recent metabolomics study identified unsaturated fatty acids, including arachidonic acid and docosahexaenoic acid (DHA), that interact with the ligand-binding domain (LBD) of a related orphan receptor, Nur77/NR4A1. However, the binding location and whether these ligands bind other NR4A receptors were not defined. Here, we show that unsaturated fatty acids also interact with the Nurr1 LBD, and solution NMR spectroscopy reveals the binding epitope of DHA at its putative ligand-binding pocket. Biochemical assays reveal that DHA-bound Nurr1 interacts with high affinity with a peptide derived from PIASγ, a protein that interacts with Nurr1 in cellular extracts, and DHA also affects cellular Nurr1 transactivation. This work is the first structural report of a natural ligand binding to a canonical NR4A ligand-binding pocket and indicates a natural ligand can bind and affect Nurr1 function.

Inhibition of Group I Metabotropic Glutamate Receptors Reverses Autistic-Like Phenotypes Caused by Deficiency of the Translation Repressor eIF4E Binding Protein 2.

Exacerbated mRNA translation during brain development has been linked to autism spectrum disorders (ASDs). Deletion of the eukaryotic initiation factor 4E (eIF4E)-binding protein 2 gene (Eif4ebp2), encoding the suppressor of mRNA translation initiation 4E-BP2, leads to an imbalance in excitatory-to-inhibitory neurotransmission and ASD-like behaviors. Inhibition of group I metabotropic glutamate receptors (mGluRs) mGluR1 and mGluR5 reverses the autistic phenotypes in several ASD mouse models. Importantly, these receptors control synaptic physiology via activation of mRNA translation. We investigated the potential reversal of autistic-like phenotypes in Eif4ebp2(-/-) mice by using antagonists of mGluR1 (JNJ16259685) or mGluR5 (fenobam). Augmented hippocampal mGluR-induced long-term depression (LTD; or chemically induced mGluR-LTD) in Eif4ebp2(-/-) mice was rescued by mGluR1 or mGluR5 antagonists. While rescue by mGluR5 inhibition occurs through the blockade of a protein synthesis-dependent component of LTD, normalization by mGluR1 antagonists requires the activation of protein synthesis. Synaptically induced LTD was deficient in Eif4ebp2(-/-) mice, and this deficit was not rescued by group I mGluR antagonists. Furthermore, a single dose of mGluR1 (0.3 mg/kg) or mGluR5 (3 mg/kg) antagonists in vivo reversed the deficits in social interaction and repetitive behaviors (marble burying) in Eif4ebp2(-/-) mice. Our results demonstrate that Eif4ebp2(-/-) mice serve as a relevant model to test potential therapies for ASD symptoms. In addition, we provide substantive evidence that the inhibition of mGluR1/mGluR5 is an effective treatment for physiological and behavioral alterations caused by exacerbated mRNA translation initiation. SIGNIFICANCE STATEMENT: Exacerbated mRNA translation during brain development is associated with several autism spectrum disorders (ASDs). We recently demonstrated that the deletion of a negative regulator of mRNA translation initiation, the eukaryotic initiation factor 4E-binding protein 2, leads to ASD-like behaviors and increased excitatory synaptic activity. Here we demonstrated that autistic behavioral and electrophysiological phenotypes can be treated in adult mice with antagonists of group I metabotropic glutamate receptors (mGluRs), which have been previously used in other ASD models (i.e., fragile X syndrome). These findings support the use of group I mGluR antagonists as a potential therapy that extends to autism models involving exacerbated mRNA translation initiation.

Structural mechanism for signal transduction in RXR nuclear receptor heterodimers.

A subset of nuclear receptors (NRs) function as obligate heterodimers with retinoid X receptor (RXR), allowing integration of ligand-dependent signals across the dimer interface via an unknown structural mechanism. Using nuclear magnetic resonance (NMR) spectroscopy, x-ray crystallography and hydrogen/deuterium exchange (HDX) mass spectrometry, here we show an allosteric mechanism through which RXR co-operates with a permissive dimer partner, peroxisome proliferator-activated receptor (PPAR)-γ, while rendered generally unresponsive by a non-permissive dimer partner, thyroid hormone (TR) receptor. Amino acid residues that mediate this allosteric mechanism comprise an evolutionarily conserved network discovered by statistical coupling analysis (SCA). This SCA network acts as a signalling rheostat to integrate signals between dimer partners, ligands and coregulator-binding sites, thereby affecting signal transmission in RXR heterodimers. These findings define rules guiding how NRs integrate two ligand-dependent signalling pathways into RXR heterodimer-specific responses.

The MLLE domain of the ubiquitin ligase UBR5 binds to its catalytic domain to regulate substrate binding.

E3 ubiquitin ligases catalyze the transfer of ubiquitin from an E2-conjugating enzyme to a substrate. UBR5, homologous to the E6AP C terminus (HECT)-type E3 ligase, mediates the ubiquitination of proteins involved in translation regulation, DNA damage response, and gluconeogenesis. In addition, UBR5 functions in a ligase-independent manner by prompting protein/protein interactions without ubiquitination of the binding partner. Despite recent functional studies, the mechanisms involved in substrate recognition and selective ubiquitination of its binding partners remain elusive. The C terminus of UBR5 harbors the HECT catalytic domain and an adjacent MLLE domain. MLLE domains mediate protein/protein interactions through the binding of a conserved peptide motif, termed PAM2. Here, we characterize the binding properties of the UBR5 MLLE domain to PAM2 peptides from Paip1 and GW182. The crystal structure with a Paip1 PAM2 peptide reveals the network of hydrophobic and ionic interactions that drive binding. In addition, we identify a novel interaction of the MLLE domain with the adjacent HECT domain mediated by a PAM2-like sequence. Our results confirm the role of the MLLE domain of UBR5 in substrate recruitment and suggest a potential role in regulating UBR5 ligase activity.

La-related Protein 1 (LARP1) Represses Terminal Oligopyrimidine (TOP) mRNA Translation Downstream of mTOR Complex 1 (mTORC1).

The mammalian target of rapamycin complex 1 (mTORC1) is a critical regulator of protein synthesis. The best studied targets of mTORC1 in translation are the eukaryotic initiation factor-binding protein 1 (4E-BP1) and ribosomal protein S6 kinase 1 (S6K1). In this study, we identify the La-related protein 1 (LARP1) as a key novel target of mTORC1 with a fundamental role in terminal oligopyrimidine (TOP) mRNA translation. Recent genome-wide studies indicate that TOP and TOP-like mRNAs compose a large portion of the mTORC1 translatome, but the mechanism by which mTORC1 controls TOP mRNA translation is incompletely understood. Here, we report that LARP1 functions as a key repressor of TOP mRNA translation downstream of mTORC1. Our data show the following: (i) LARP1 associates with mTORC1 via RAPTOR; (ii) LARP1 interacts with TOP mRNAs in an mTORC1-dependent manner; (iii) LARP1 binds the 5'TOP motif to repress TOP mRNA translation; and (iv) LARP1 competes with the eukaryotic initiation factor (eIF) 4G for TOP mRNA binding. Importantly, from a drug resistance standpoint, our data also show that reducing LARP1 protein levels by RNA interference attenuates the inhibitory effect of rapamycin, Torin1, and amino acid deprivation on TOP mRNA translation. Collectively, our findings demonstrate that LARP1 functions as an important repressor of TOP mRNA translation downstream of mTORC1.

Structure of REV-ERBß ligand-binding domain bound to a porphyrin antagonist.

REV-ERBα and REV-ERBß are members of the nuclear receptor (NR) superfamily of ligand-regulated transcription factors that play important roles in the regulation of circadian physiology, metabolism, and immune function. Although the REV-ERBs were originally characterized as orphan receptors, recent studies have demonstrated that they function as receptors for heme. Here, we demonstrate that cobalt protoporphyrin IX (CoPP) and zinc protoporphyrin IX (ZnPP) are ligands that bind directly to the REV-ERBs. However, instead of mimicking the agonist action of heme, CoPP and ZnPP function as antagonists of REV-ERB function. This was unexpected because the only distinction between these ligands is the metal ion that is coordinated. To understand the structural basis by which REV-ERBß can differentiate between a porphyrin agonist and antagonist, we characterized the interaction between REV-ERBß with heme, CoPP, and ZnPP using biochemical and structural approaches, including x-ray crystallography and NMR. The crystal structure of CoPP-bound REV-ERBß indicates only minor conformational changes induced by CoPP compared with heme, including the porphyrin ring of CoPP, which adopts a planar conformation as opposed to the puckered conformation observed in the heme-bound REV-ERBß crystal structure. Thus, subtle changes in the porphyrin metal center and ring conformation may influence the agonist versus antagonist action of porphyrins and when considered with other studies suggest that gas binding to the iron metal center heme may drive alterations in REV-ERB activity.

Structure of the HECT C-lobe of the UBR5 E3 ubiquitin ligase.

UBR5 ubiquitin ligase (also known as EDD, Rat100 or hHYD) is a member of the E3 protein family of HECT (homologous to E6-AP C-terminus) ligases as it contains a C-terminal HECT domain. In ubiquitination cascades involving E3s of the HECT class, ubiquitin is transferred from an associated E2 ubiquitin-conjugating enzyme to the acceptor cysteine of the HECT domain, which consists of structurally distinct N- and C-lobes connected by a flexible linker. Here, the high-resolution crystal structure of the C-lobe of the HECT domain of human UBR5 is presented. The structure reveals important features that are unique compared with other HECT domains. In particular, a distinct four-residue insert in the second helix elongates this helix, resulting in a strikingly different orientation of the preceding loop. This protruding loop is likely to contribute to specificity towards the E2 ubiquitin-conjugating enzyme UBCH4, which is an important functional partner of UBR5. Ubiquitination assays showed that the C-lobe of UBR5 is able to form a thioester-linked E3-ubiquitin complex, although it does not physically interact with UBCH4 in NMR experiments. This study contributes to a better understanding of UBR5 ubiquitination activity.

Structural basis of substrate recognition and specificity in the N-end rule pathway.

The N-end rule links the half-life of a protein to the identity of its N-terminal residue. Destabilizing N-terminal residues are recognized by E3 ubiquitin ligases, termed N-recognins. A conserved structural domain called the UBR box is responsible for their specificity. Here we report the crystal structures of the UBR boxes of the human N-recognins UBR1 and UBR2, alone and in complex with an N-end rule peptide, Arg-Ile-Phe-Ser. These structures show that the UBR box adopts a previously undescribed fold stabilized through the binding of three zinc ions to form a binding pocket for type 1 N-degrons. NMR experiments reveal a preference for N-terminal arginine. Peptide binding is abrogated by N-terminal acetylation of the peptide or loss of the positive charge of the N-terminal residue. These results rationalize and refine the empirical rules for the classification of type 1 N-degrons. We also confirm that a missense mutation in UBR1 that is responsible for Johanson-Blizzard syndrome leads to UBR box unfolding and loss of function.

Structural and functional study of YER067W, a new protein involved in yeast metabolism control and drug resistance.

The genome of Saccharomyces cerevisiae is arguably the best studied eukaryotic genome, and yet, it contains approximately 1000 genes that are still relatively uncharacterized. As the majority of these ORFs have no homologs with characterized sequence or protein structure, traditional sequence-based approaches cannot be applied to deduce their biological function. Here, we characterize YER067W, a conserved gene of unknown function that is strongly induced in response to many stress conditions and repressed in drug resistant yeast strains. Gene expression patterns of YER067W and its paralog YIL057C suggest an involvement in energy metabolism. We show that yeast lacking YER067W display altered levels of reserve carbohydrates and a growth deficiency in media that requires aerobic metabolism. Impaired mitochondrial function and overall reduction of ergosterol content in the YER067W deleted strain explained the observed 2- and 4-fold increase in resistance to the drugs fluconazole and amphotericin B, respectively. Cell fractionation and immunofluorescence microscopy revealed that Yer067w is associated with cellular membranes despite the absence of a transmembrane domain in the protein. Finally, the 1.7 A resolution crystal structure of Yer067w shows an alpha-beta fold with low similarity to known structures and a putative functional site.YER067W's involvement with aerobic energetic metabolism suggests the assignment of the gene name RGI1, standing for respiratory growth induced 1. Altogether, the results shed light on a previously uncharacterized protein family and provide basis for further studies of its apparent role in energy metabolism control and drug resistance.

Apoptotic regulation by MCL-1 through heterodimerization.

Myeloid cell leukemia 1 (MCL-1), an anti-apoptotic BCL-2 family member active in the preservation of mitochondrial integrity during apoptosis, has fundamental roles in development and hematopoiesis and is dysregulated in human cancers. It bears a unique, intrinsically unstructured, N-terminal sequence, which leads to its instability in cells and hinders protein production and structural characterization. Here, we present collective data from NMR spectroscopy and titration calorimetry to reveal the selectivity of MCL-1 in binding BCL-2 homology 3 (BH3) ligands of interest for mammalian biology. The N-terminal sequence weakens the BH3 interactions but does not affect selectivity. Its removal by calpain-mediated limited proteolysis results in a stable BCL-2-like core domain of MCL-1 (cMCL-1). This core is necessary and sufficient for BH3 ligand binding. Significantly, we also characterized the in vitro protein-protein interaction between cMCL-1 and activated BID by size exclusion chromatography and NMR titrations. This interaction occurs in a very slow manner in solution but is otherwise similar to the interaction between cMCL-1 and BID-BH3 peptides. We also present the solution structure of complex cMCL-1xhBID-BH3, which completes the family portrait of MCL-1 complexes and may facilitate drug discovery against human tumors.