Specificity of 14-3-3 isoform dimer interactions and phosphorylation

14-3-3 proteins: Regulators of cardiac excitation-contraction coupling and stress responses

14-3-3 proteins are highly conserved proteins that regulate numerous cellular processes mostly through phosphorylation-dependent protein-protein interactions. In the heart 14-3-3 proteins play critical roles in cardiac conduction pathways, excitation-contraction (EC) coupling, development and stress responses. This review summarizes the current understanding of cardiac 14-3-3 regulation and function, with particular emphasis on its role in ion channel regulation and β-adrenergic signalling. We discuss how 14-3-3 proteins act through three main mechanisms - masking, clamping, and scaffolding - to regulate target proteins, including Cx43, CaV1.2, NaV1.5, and various potassium channels. The seven mammalian 14-3-3 isoforms display distinct but overlapping functions, with tissue-specific expression patterns and isoform-specific regulation through phosphorylation and dimerization. Recent work has revealed 14-3-3's importance in cardiac development and stress responses, where it generally serves a cardioprotective role. However in some pathological contexts such as ischaemia-reperfusion injury, 14-3-3 can be detrimental. We highlight emerging themes in cardiac 14-3-3 biology, including its role in prolonging β-adrenergic signalling. Understanding the complex regulation of cardiac 14-3-3 and its numerous targets presents both opportunities and challenges for therapeutic development.

Dynamic interactions of dimeric hub proteins underlie their diverse functions and structures: A comparative analysis of 14-3-3 and LC8

Hub proteins interact with a host of client proteins and regulate multiple cellular functions. Dynamic hubs have a single binding interface for one client at a time resulting in competition among clients with the highest affinity. Dynamic dimeric hubs with two identical sites bind either two different client proteins or two chains of the same client to form homogenous complexes and could also form heterogeneous mixtures of interconverting complexes. Here, we review the interactions of the dimeric hubs 14-3-3 and LC8. 14-3-3 is a phosphoserine/threonine binding protein involved in structuring client proteins and regulating their phosphorylation. LC8 is involved in promoting the dimerization of client peptides and the rigidification of their disordered regions. Both 14-3-3 and LC8 are essential genes, with 14-3-3 playing a crucial role in apoptosis and cell cycle regulation, while LC8 is critical for the assembly of proteins involved in transport, DNA repair, and transcription. Interestingly, both protein dimers can dissociate by phosphorylation, which results in their interactome-wide changes. Their interactions are also regulated by the phosphorylation of their clients. Both form heterogeneous complexes with various functions including phase separation, signaling, and viral hijacking where they restrict the conformational heterogeneity of their dimeric clients that bind nucleic acids. This comparative analysis highlights the importance of dynamic protein-protein interactions in the diversity of functions of 14-3-3 and LC8 and how small differences in structures of interfaces explain why 14-3-3 is primarily involved in the regulation of phosphorylation states while LC8 is primarily involved in the regulation of assembly of large dynamic complexes.

Putative Biomarkers for Prognosis, Epithelial-to-Mesenchymal Transition, and Drug Response in Cell Lines Representing Oral Squamous Cell Carcinoma Progression

BACKGROUND/OBJECTIVES

Oral squamous cell carcinoma (OSCC) is the most common form of head and neck cancer and accounts for over 50,000 new cancer cases annually in the United States. The survival rates are markedly different for localized OSCC versus metastatic disease, for which the five-year survival rate is only 39%. Depending on its pathology and stage at diagnosis, the treatment may involve surgery, radiation, targeted therapy, or conventional chemotherapy. However, there is an unmet need for reliable biomarkers to predict the treatment response or link therapeutic efficacy to tumor progression. We sought to assemble a panel of OSCC tumor progression biomarkers that correlated with the epithelial-to-mesenchymal transition (EMT) and the response to cytotoxic drugs.

METHODS

We used four cell lines that represented the stepwise progression from normal oral mucosa to dysplastic, invasive, and metastatic OSCC lesions and performed a quantitative analysis via Western blot for putative markers. EMT phenotypes were assessed using wound healing migration assays. Live cell imaging was used to assess drug effectiveness over time.

RESULTS

The expression of stratifin, a tumor suppressor gene, is inversely correlated with both tumor progression steps and the expression of the EMT marker N-cadherin. Conversely, the E-cadherin and fibronectin expression was markedly decreased in the advanced-stage OSCC lines. In addition, metastatic Detroit 562 cells exhibited resistance to cell death following docetaxel treatment and showed clear migratory behavior.

CONCLUSIONS

We describe a molecular signature of advanced and drug-resistant OSCC tumors which encompasses multiple markers, warranting further investigation to establish their utility in predicting clinical outcomes and guiding the treatment options for patients afflicted with oral cancer.

YWHAG promotes colorectal cancer progression by regulating the CTTN-Wnt/β-catenin signaling axis

Colorectal cancer (CRC) ranks as the third most prevalent cancer type globally. Nevertheless, the fundamental mechanisms driving CRC progression remain ambiguous, and the prognosis for the majority of patients diagnosed at an advanced stage is dismal. YWHA/14-3-3 proteins serve as central nodes in several signaling pathways and are closely related to tumorigenesis and progression. However, their exact roles in CRC are still poorly elucidated. In this study, we revealed that YWHAG was the most significantly upregulated member of the YWHA/14-3-3 family in CRC tissues and was associated with a poor prognosis. Subsequent phenotypic experiments showed that YWHAG promoted the proliferation, migration, and invasion of CRC cells. Mechanistically, RNA-seq data showed that multiple signaling pathways, including Wnt and epithelial-mesenchymal transition, were potentially regulated by YWHAG. CTTN was identified as a YWHAG-associated protein, and mediated its tumor-promoting functions by activating the Wnt/β-catenin signaling in CRC cells. In summary, our data indicate that YWHAG facilitates the proliferation, migration, and invasion of CRC cells by modulating the CTTN-Wnt/β-catenin signaling pathway, which offers a novel perspective for the treatment of CRC.

In Silico Prediction and Biophysical Validation of Novel 14-3-3σ Homodimer Stabilizers

14-3-3σ plays an important role in controlling tumor metabolic reprogramming and cancer cell growth. However, its function is often compromised in many cancers due to its downregulation. Previous studies found that homodimerization of 14-3-3σ is critical for its activity. However, to date, it is not known if stabilization of 14-3-3σ homodimers can improve its activity or prevent its degradation. In our previous work, we have showed that GCP-Lys-OMe is a potential 14-3-3σ homodimer stabilizer. However, its stabilizing effect was not experimentally validated. Therefore, in this study, we have attempted to predict few potential peptides that can stabilize the dimeric form of 14-3-3σ using similar in silico techniques as described previously for GCP-Lys-OMe. Subsequent [1H]-CPMG NMR experiments confirmed the binding of the peptides (peptides 3, 5, 9, and 16) on 14-3-3σ, with peptide 3 showing the strongest binding. Competitive [1H]-CPMG assays further revealed that while peptide 3 does not compete with a 14-3-3σ binding peptide (ExoS) for the protein's amphipathic groove, it was found to improve ExoS binding on 14-3-3σ. When 14-3-3σ was subjected to dynamic light scattering experiments, the 14-3-3σ homodimer was found to undergo dissociation into monomers prior to aggregation. Intriguingly, the presence of peptide 3 increased 14-3-3σ stability against aggregation. Overall, our findings suggest that (1) docking accompanied by MD simulations can be used to identify potential homodimer stabilizing compounds of 14-3-3σ and (2) peptide 3 can slow down 14-3-3σ aggregation (presumably by preventing its dissociation into monomers), as well as improving the binding of 14-3-3σ to ExoS protein.

IgE receptor of mast cells signals mediator release and inflammation via adaptor protein 14-3-3ζ

BACKGROUND

Mast cells (MCs) are tissue-resident immune cells that mediate IgE-dependent allergic responses. Downstream of FcεRI, an intricate network of receptor-specific signaling pathways and adaptor proteins govern MC function. The 14-3-3 family of serine-threonine phosphorylation-dependent adapter proteins are known to organize intracellular signaling. However, the role of 14-3-3 in IgE-dependent activation remains poorly defined.

OBJECTIVE

We sought to determine whether 14-3-3 proteins are required for IgE-dependent MC activation and whether 14-3-3 is a viable target for the treatment of MC-mediated inflammatory diseases.

METHODS

Genetic manipulation of 14-3-3ζ expression in human and mouse MCs was performed and IgE-dependent mediator release assessed. Pharmacologic inhibitors of 14-3-3 and 14-3-3ζ knockout mice were used to assess 14-3-3ζ function in a MC-dependent in vivo passive cutaneous anaphylaxis (PCA) model of allergic inflammation. Expression and function of 14-3-3ζ were assessed in human nasal polyp tissue MCs.

RESULTS

IgE-dependent mediator release from human MCs was decreased by 14-3-3ζ knockdown and increased by 14-3-3ζ overexpression. Deletion of the 14-3-3ζ gene decreased IgE-dependent activation of mouse MCs in vitro and PCA responses in vivo. Furthermore, the 14-3-3 inhibitor, RB-11, which impairs dimerization of 14-3-3, inhibited cultured MC and polyp tissue MC activation and signaling downstream of the FcεRI receptor and dose-dependently attenuated PCA responses.

CONCLUSION

IgE/FcεRI-mediated MC activation is positively regulated by 14-3-3ζ. We identify a critical role for this p-Ser/Thr-binding protein in the regulation of MC FcεRI signaling and IgE-dependent immune responses and show that this pathway may be amenable to pharmacologic targeting.

In Silico Studies on GCP-Lys-OMe as a Potential 14-3-3σ Homodimer Stabilizer

14-3-3 sigma is a vital negative cell cycle regulator. Its expression is consistently downregulated in many types of cancer through gene promoter hypermethylation or proteasomal degradation. 14-3-3 sigma needs to form a homodimer to be functional, while dimers are less prone to degradation than monomers. This suggests that a homodimer stabilizer may increase the tumor suppressive activities of 14-3-3 sigma. However, no known homodimer stabilizer of 14-3-3 sigma has been reported to date. Therefore, this study attempts to test the potential capability of GCP-Lys-OMe (previously reported to bind at the dimer interface of 14-3-3 zeta isoform), to bind and stabilize the 14-3-3 sigma homodimer. In silico docking of GCP-Lys-OMe on 14-3-3 sigma showed more favorable interaction energy (−9.63 kcal/mole) to the dimer interface than 14-3-3 zeta (−7.73 kcal/mole). Subsequent 100 ns molecular dynamics simulation of the GCP-Lys-OMe/14-3-3 sigma complex revealed a highly stable interaction with an average root-mean-square deviation of 0.39 nm (protein backbone) and 0.77 nm (ligand atoms). More contacts between residues at the homodimer interface and a smaller coverage of conformational space of protein atoms were detected for the bound form than for the apo form. These results suggest that GCP-Lys-OMe is a potential homodimer stabilizer of 14-3-3 sigma.

Structural insights into the functional roles of 14-3-3 proteins

Signal transduction cascades efficiently transmit chemical and/or physical signals from the extracellular environment to intracellular compartments, thereby eliciting an appropriate cellular response. Most often, these signaling processes are mediated by specific protein-protein interactions involving hundreds of different receptors, enzymes, transcription factors, and signaling, adaptor and scaffolding proteins. Among them, 14-3-3 proteins are a family of highly conserved scaffolding molecules expressed in all eukaryotes, where they modulate the function of other proteins, primarily in a phosphorylation-dependent manner. Through these binding interactions, 14-3-3 proteins participate in key cellular processes, such as cell-cycle control, apoptosis, signal transduction, energy metabolism, and protein trafficking. To date, several hundreds of 14-3-3 binding partners have been identified, including protein kinases, phosphatases, receptors and transcription factors, which have been implicated in the onset of various diseases. As such, 14-3-3 proteins are promising targets for pharmaceutical interventions. However, despite intensive research into their protein-protein interactions, our understanding of the molecular mechanisms whereby 14-3-3 proteins regulate the functions of their binding partners remains insufficient. This review article provides an overview of the current state of the art of the molecular mechanisms whereby 14-3-3 proteins regulate their binding partners, focusing on recent structural studies of 14-3-3 protein complexes.

Similarity of the non-amyloid-β component and C-terminal tail of monomeric and tetrameric alpha-synuclein with 14-3-3 sigma

Alpha-synuclein (αSyn) is often described as a predominantly disordered protein that has a propensity to self-assemble into toxic oligomers that are found in patients with Parkinson's and Alzheimer's diseases. αSyn's chaperone behavior and tetrameric structure are proposed to be protective against toxic oligomerization. In this paper, we extended the previously proposed similarity between αSyn and 14-3-3 proteins to the α-helical tetrameric species of αSyn in detail. 14-3-3 proteins are a family of well-folded proteins with seven human isoforms, and function in signal transduction and as molecular chaperones. We investigated protein homology, using sequence alignment, amyloid, and disorder prediction, as well as three-dimensional visualization and protein-interaction networks. Our results show sequence homology and structural similarity between the aggregation-prone non-amyloid-β component (NAC) residues Val-52 to Gly-111 in αSyn and 14-3-3 sigma residues Leu-12 to Gly-78. We identified an additional region of sequence homology in the C-terminal region of αSyn (residues Ser-129 to Asp-135) and a C-terminal loop of 14-3-3 between helix αH and αI (residues Ser-209 to Asp-215). This data indicates αSyn shares conserved domain architecture with small heat shock proteins. We show predicted regions of high amyloidogenic propensity and intrinsic structural disorder in αSyn coincide with amyloidogenic and disordered predictions for 14-3-3 proteins. The homology in the NAC region aligns with residues involved in dimer- and tetramerization of the non-amyloidogenic 14-3-3 proteins. Because 14-3-3 proteins are generally not prone to misfolding, our results lend further support to the hypothesis that the NAC region is critical to the assembly of αSyn into the non-toxic tetrameric state.

The protein YWHAE (14-3-3 epsilon) in spermatozoa is essential for male fertility

BACKGROUND

Spermatogenesis is a complex biological process highlighted by synthesis and activation of proteins that regulate meiosis and cellular differentiation occur during spermatogenesis. 14-3-3 proteins are adaptor proteins that play critical roles in kinase signaling, especially for regulation of cell cycle and apoptosis in eukaryotic cells. There are seven isoforms of the 14-3-3 family proteins encoded by seven genes (β, ε, γ, η, θ/τ, ζ and σ). 14-3-3 isoforms have been shown to have many interacting partners in several tissues including testis.

OBJECTIVE

While it is known that 14-3-3 proteins are expressed in the functions of testis and spermatozoon, the role for each of the seven isoforms is not known. In this study, we investigated the roles of 14-3-3η and 14-3-3ε isoforms in spermatogenesis.

MATERIALS AND METHODS

To study the in vivo function of 14-3-3η and 14-3-3ε in spermatogenesis, we generated testis-specific and global knockout mice for each of 14-3-3η and 14-3-3ε isoforms (CKO and GKO, respectively). Computer-assisted semen analysis was used to assess sperm motility, while immunohistochemical studies were conducted to check spermatogenesis.

RESULTS

Although both 14-3-3η and 14-3-3ε isoforms were present in mouse testis, only the expression of 14-3-3ε, but not 14-3-3η, was detected in spermatozoa. Mice lacking 14-3-3η were normal and fertile while 14-3-3ε CKO and GKO males showed infertility. Low sperm count with higher abnormal spermatozoa was seen in 14-3-3ε CKO mice. The motility of 14-3-3ε knockout spermatozoa was lower than that of the control. A reduction in the phosphorylation of both glycogen synthase kinase 3 and PP1γ2 was also seen in spermatozoa from 14-3-3ε CKO mice, suggesting a specific role of 14-3-3ε in spermatogenesis, sperm motility, and fertility.

DISCUSSION AND CONCLUSION

This is the first demonstration that of the seven 14-3-3 isoforms, 14-3-3ε is essential for normal sperm function and male fertility.

14-3-3σ and Its Modulators in Cancer

14-3-3σ is an acidic homodimer protein with more than one hundred different protein partners associated with oncogenic signaling and cell cycle regulation. This review aims to highlight the crucial role of 14-3-3σ in controlling tumor growth and apoptosis and provide a detailed discussion on the structure-activity relationship and binding interactions of the most recent 14-3-3σ protein-protein interaction (PPI) modulators reported to date, which has not been reviewed previously. This includes the new fusicoccanes stabilizers (FC-NAc, DP-005), fragment stabilizers (TCF521-123, TCF521-129, AZ-003, AZ-008), phosphate-based inhibitors (IMP, PLP), peptide inhibitors (2a-d), as well as inhibitors from natural sources (85531185, 95911592). Additionally, this review will also include the discussions of the recent efforts by a different group of researchers for understanding the binding mechanisms of existing 14-3-3σ PPI modulators. The strategies and state-of-the-art techniques applied by various group of researchers in the discovery of a different chemical class of 14-3-3σ modulators for cancer are also briefly discussed in this review, which can be used as a guide in the development of new 14-3-3σ modulators in the near future.

sORF-Encoded MicroPeptides: New players in inflammation, metabolism, and precision medicine

Significant technological advances have enabled the discovery and identification of a new class of molecules, micropeptides or small ORF encoded peptides (SEPs) within non-coding RNAs (ncRNAs). As ncRNAs are well known to be transcriptionally silent, the discovery of SEPs implies that many ncRNAs are misannotated or play both coding and non-coding functions. SEPs have reportedly diverse regulatory roles in embryogenesis, myogenesis, inflammation, diseases, and cancer. SEPs appearing in different subcellular compartments show distinct functions. In this review, we summarized the functions of SEPs that have been characterized thus far. As SEPs are amenable to therapeutic development as biologics, understanding their underlying functions will provide novel targets for the treatment of inflammatory or metabolic disorders.

Emerging roles of 14-3-3γ in the brain disorder

14-3-3 proteins are mostly expressed in the brain and are closely involved in numerous brain functions and various brain disorders. Among the isotypes of the 14-3-3 proteins, 14-3-3γ is mainly expressed in neurons and is highly produced during brain development, which could indicate that it has a significance in neural development. Furthermore, the distinctive levels of temporally and locally regulated 14-3-3γ expression in various brain disorders suggest that it could play a substantial role in brain plasticity of the diseased states. In this review, we introduce the various brain disorders reported to be involved with 14-3-3γ, and summarize the changes of 14-3-3γ expression in each brain disease. We also discuss the potential of 14-3-3γ for treatment and the importance of research on specific 14-3-3 isotypes for an effective therapeutic approach.

14-3-3 σ: A potential biomolecule for cancer therapy

As more studies have focused on the function of 14-3-3 proteins, their role in tumor progression has gradually improved. In the 14-3-3 protein family, 14-3-3σ is the protein that is most associated with tumor occurrence and development. In some malignancies, 14-3-3σ acts as a tumor suppressor via p53 and tumor suppressor genes. In most tumors, 14-3-3σ overexpression increases resistance to chemotherapy and radiotherapy and mediates the G2-M checkpoint after DNA damage. Although 14-3-3σ overexpression has been closely associated with poorer prognosis in pancreatic, gastric and colorectal cancer, its role in gallbladder and nasopharyngeal cancer remains less clear. As such, the function of 14-3-3σ in specific cancer types needs to be further clarified. It has been hypothesized that a role may be related to its molecular chaperone function combined with various protein ligands. In this review, we examine the role of 14-3-3σ in tumor development and drug resistance. We discuss the potential of targeting 14-3-3σ regulators in cancer therapy and treatment.

Ex vivo Quantitative Proteomic Analysis of Serotonin Transporter Interactome: Network Impact of the SERT Ala56 Coding Variant

Altered serotonin (5-HT) signaling is associated with multiple brain disorders, including major depressive disorder (MDD), obsessive-compulsive disorder (OCD), and autism spectrum disorder (ASD). The presynaptic, high-affinity 5-HT transporter (SERT) tightly regulates 5-HT clearance after release from serotonergic neurons in the brain and enteric nervous systems, among other sites. Accumulating evidence suggests that SERT is dynamically regulated in distinct activity states as a result of environmental and intracellular stimuli, with regulation perturbed by disease-associated coding variants. Our lab identified a rare, hypermorphic SERT coding substitution, Gly56Ala, in subjects with ASD, finding that the Ala56 variant stabilizes a high-affinity outward-facing conformation (SERT^∗) that leads to elevated 5-HT uptake in vitro and in vivo. Hyperactive SERT Ala56 appears to preclude further activity enhancements by p38α mitogen-activated protein kinase (MAPK) and can be normalized by pharmacological p38α MAPK inhibition, consistent with SERT Ala56 mimicking, constitutively, a high-activity conformation entered into transiently by p38α MAPK activation. We hypothesize that changes in SERT-interacting proteins (SIPs) support the shift of SERT into the SERT^∗ state which may be captured by comparing the composition of SERT Ala56 protein complexes with those of wildtype (WT) SERT, defining specific interactions through comparisons of protein complexes recovered using preparations from SERT^–/– (knockout; KO) mice. Using quantitative proteomic-based approaches, we identify a total of 459 SIPs, that demonstrate both SERT specificity and sensitivity to the Gly56Ala substitution, with a striking bias being a loss of SIP interactions with SERT Ala56 compared to WT SERT. Among this group are previously validated SIPs, such as flotillin-1 (FLOT1) and protein phosphatase 2A (PP2A), whose functions are believed to contribute to SERT microdomain localization and regulation. Interestingly, our studies nominate a number of novel SIPs implicated in ASD, including fragile X mental retardation 1 protein (FMR1) and SH3 and multiple ankyrin repeat domains protein 3 (SHANK3), of potential relevance to long-standing evidence of serotonergic contributions to ASD. Further investigation of these SIPs, and the broader networks they engage, may afford a greater understanding of ASD as well as other brain and peripheral disorders associated with perturbed 5-HT signaling.

Cofilin participates in regulating alpha-epithelial sodium channel by interaction with 14-3-3 isoforms

Renal epithelial sodium channel (ENaC) plays a crucial role in maintaining homeostasis and sodium absorption. While insulin participates in controlling sodium transport across the renal epithelium, the underlying molecular mechanism remain unclear. In this study, we found that insulin increased the expression and function of alpha-epithelial sodium channel (α-ENaC) as well as phosphorylation of cofilin, a family of actin-binding proteins which disassembles actin filaments, in mouse cortical collecting duct (mpkCCD_c14) cells. The wild-type (WT) cofilin and its constitutively phosphorylated form (S3D), but not its constitutively non-phosphorylable form (S3A), contributed to the elevated expression on α-ENaC. Overexpression of 14-3-3ε, β, or γ increased the expression of α-ENaC and cofilin phosphorylation, which was blunted by knockdown of 14-3-3ε, β, or γ. Moreover, it was found that insulin increased the interaction between cofilin and 14-3-3 isoforms, which indicated relevance of 14-3-3 isoforms with cofilin. Furthermore, LIMK1/SSH1 pathway was involved in regulation of cofilin and α-ENaC expression by insulin. The results from this work indicate that cofilin participates in the regulation of α-ENaC by interaction with 14-3-3 isoforms.

YWHA (14-3-3) protein isoforms and their interactions with CDC25B phosphatase in mouse oogenesis and oocyte maturation

Background

Immature mammalian oocytes are held arrested at prophase I of meiosis by an inhibitory phosphorylation of cyclin-dependent kinase 1 (CDK1). Release from this meiotic arrest and germinal vesicle breakdown is dependent on dephosphorylation of CDK1 by the protein, cell cycle division 25B (CDC25B). Evidence suggests that phosphorylated CDC25B is bound to YWHA (14-3-3) proteins in the cytoplasm of immature oocytes and is thus maintained in an inactive form. The importance of YWHA in meiosis demands additional studies.

Results

Messenger RNA for multiple isoforms of the YWHA protein family was detected in mouse oocytes and eggs. All seven mammalian YWHA isoforms previously reported to be expressed in mouse oocytes, were found to interact with CDC25B as evidenced by in situ proximity ligation assays. Interaction of YWHAH with CDC25B was indicated by Förster Resonance Energy Transfer (FRET) microscopy. Intracytoplasmic microinjection of oocytes with R18, a known, synthetic, non-isoform-specific, YWHA-blocking peptide promoted germinal vesicle breakdown. This suggests that inhibiting the interactions between YWHA proteins and their binding partners releases the oocyte from meiotic arrest. Microinjection of isoform-specific, translation-blocking morpholino oligonucleotides to knockdown or downregulate YWHA protein synthesis in oocytes suggested a role for a specific YWHA isoform in maintaining the meiotic arrest. More definitively however, and in contrast to the knockdown experiments, oocyte-specific and global deletion of two isoforms of YWHA, YWHAH (14-3-3 eta) or YWHAE (14-3-3 epsilon) indicated that the complete absence of either or both isoforms does not alter oocyte development and release from the meiotic prophase I arrest.

Conclusions

Multiple isoforms of the YWHA protein are expressed in mouse oocytes and eggs and interact with the cell cycle protein CDC25B, but YWHAH and YWHAE isoforms are not essential for normal mouse oocyte maturation, fertilization and early embryonic development.

Differential effects of 14-3-3 dimers on Tau phosphorylation, stability and toxicity in vivo

Neurodegenerative dementias collectively known as Tauopathies involve aberrant phosphorylation and aggregation of the neuronal protein Tau. The largely neuronal 14-3-3 proteins are also elevated in the central nervous system (CNS) and cerebrospinal fluid of Tauopathy patients, suggesting functional linkage. We use the simplicity and genetic facility of the Drosophila system to investigate in vivo whether 14-3-3s are causal or synergistic with Tau accumulation in precipitating pathogenesis. Proteomic, biochemical and genetic evidence demonstrate that both Drosophila 14-3-3 proteins interact with human wild-type and mutant Tau on multiple sites irrespective of their phosphorylation state. 14-3-3 dimers regulate steady-state phosphorylation of both wild-type and the R406W mutant Tau, but they are not essential for toxicity of either variant. Moreover, 14-3-3 elevation itself is not pathogenic, but recruitment of dimers on accumulating wild-type Tau increases its steady-state levels ostensibly by occluding access to proteases in a phosphorylation-dependent manner. In contrast, the R406W mutant, which lacks a putative 14-3-3 binding site, responds differentially to elevation of each 14-3-3 isoform. Although excess 14-3-3ζ stabilizes the mutant protein, elevated D14-3-3ɛ has a destabilizing effect probably because of altered 14-3-3 dimer composition. Our collective data demonstrate the complexity of 14-3-3/Tau interactions in vivo and suggest that 14-3-3 attenuation is not appropriate ameliorative treatment of Tauopathies. Finally, we suggest that 'bystander' 14-3-3s are recruited by accumulating Tau with the consequences depending on the composition of available dimers within particular neurons and the Tau variant.

The dynamic and stress-adaptive signaling hub of 14-3-3: emerging mechanisms of regulation and context-dependent protein-protein interactions

14-3-3 proteins are a family of structurally similar phospho-binding proteins that regulate essentially every major cellular function. Decades of research on 14-3-3s have revealed a remarkable network of interacting proteins that demonstrate how 14-3-3s integrate and control multiple signaling pathways. In particular, these interactions place 14-3-3 at the center of the signaling hub that governs critical processes in cancer, including apoptosis, cell cycle progression, autophagy, glucose metabolism, and cell motility. Historically, the majority of 14-3-3 interactions have been identified and studied under nutrient-replete cell culture conditions, which has revealed important nutrient driven interactions. However, this underestimates the reach of 14-3-3s. Indeed, the loss of nutrients, growth factors, or changes in other environmental conditions (e.g., genotoxic stress) will not only lead to the loss of homeostatic 14-3-3 interactions, but also trigger new interactions, many of which are likely stress adaptive. This dynamic nature of the 14-3-3 interactome is beginning to come into focus as advancements in mass spectrometry are helping to probe deeper and identify context-dependent 14-3-3 interactions-providing a window into adaptive phosphorylation-driven cellular mechanisms that orchestrate the tumor cell's response to a variety of environmental conditions including hypoxia and chemotherapy. In this review, we discuss emerging 14-3-3 regulatory mechanisms with a focus on post-translational regulation of 14-3-3 and dynamic protein-protein interactions that illustrate 14-3-3's role as a stress-adaptive signaling hub in cancer.

Function of Translationally Controlled Tumor Protein in Organ Growth: Lessons from Drosophila Studies

Regulation of cell growth and proliferation is crucial for development and function of organs in all animals. Genetic defects in growth control can lead to developmental disorders and cancers. Translationally controlled tumor protein (TCTP) is a family of evolutionarily conserved proteins implicated in cancer. Recent studies have revealed multiple roles of TCTP in diverse cellular events, but TCTP functions in vivo are poorly understood in vertebrate systems. We have used Drosophila melanogaster, the fruit fly, as a model organism for genetic dissection of Tctp function. Our studies have shown that Tctp is essential for organ development by regulating growth signaling. Furthermore, it is required for genome stability by promoting DNA repair and chromatin remodeling in the nucleus. Thus, Tctp acts as a multifaceted cytosolic and nuclear factor for regulating organ growth and genome stability. In this chapter, we describe an overview of our findings on Tctp functions in Drosophila and discuss their implications in cancer.

Protein kinase B (AKT) regulates SYK activity and shuttling through 14-3-3 and importin 7

The Protein kinase B (AKT) regulates a plethora of intracellular signaling proteins to fine-tune signaling of multiple pathways. Here, we found that following B-cell receptor (BCR)-induced tyrosine phosphorylation of the cytoplasmic tyrosine kinase SYK and the adaptor BLNK, the AKT/PKB enzyme strongly induced BLNK (>100-fold) and SYK (>100-fold) serine/threonine phosphorylation (pS/pT). Increased phosphorylation promoted 14-3-3 binding to BLNK (37-fold) and SYK (2.5-fold) in a pS/pT-concentration dependent manner. We also demonstrated that the AKT inhibitor MK2206 reduced pS/pT of both BLNK (3-fold) and SYK (2.5-fold). Notably, the AKT phosphatase, PHLPP2 maintained the activating phosphorylation of BLNK at Y84 and increased protein stability (8.5-fold). In addition, 14-3-3 was required for the regulation SYK's interaction with BLNK and attenuated SYK binding to Importin 7 (5-fold), thereby perturbing shuttling to the nucleus. Moreover, 14-3-3 proteins also sustained tyrosine phosphorylation of SYK and BLNK. Furthermore, substitution of S295 or S297 for alanine abrogated SYK's binding to Importin 7. SYK with S295A or S297A replacements showed intense pY525/526 phosphorylation, and BLNK pY84 phosphorylation correlated with the SYK pY525/526 phosphorylation level. Conversely, the corresponding mutations to aspartic acid in SYK reduced pY525/526 phosphorylation. Collectively, these and previous results suggest that AKT and 14-3-3 proteins down-regulate the activity of several BCR-associated components, including BTK, BLNK and SYK and also inhibit SYK's interaction with Importin 7.

ROS1 fusions in cancer: a review

The ROS1 gene belongs to the sevenless subfamily of tyrosine kinase insulin receptor genes. A literature review identified a ROS1 fusion in 2.54% of the patients with lung adenocarcinoma and even higher frequencies in spitzoid neoplasms and inflammatory myofibroblastic tumors. At present, 26 genes were found to fuse with ROS1, some of them already known to fuse with RET and ALK. All the fusion proteins retain the ROS1 kinase domain, but rarely its transmembrane domain. Most of the partners have dimerization domains that are retained in the fusion, presumably leading to constitutive ROS1 tyrosine kinase activation. Some partners have transmembrane domains that are retained or not in the chimeric proteins. Therefore, different ROS1 fusions have distinct subcellular localization, suggesting that they may activate different substrates in vivo.

Hemotin, a Regulator of Phagocytosis Encoded by a Small ORF and Conserved across Metazoans

Translation of hundreds of small ORFs (smORFs) of less than 100 amino acids has recently been revealed in vertebrates and Drosophila. Some of these peptides have essential and conserved cellular functions. In Drosophila, we have predicted a particular smORF class encoding ~80 aa hydrophobic peptides, which may function in membranes and cell organelles. Here, we characterise hemotin, a gene encoding an 88aa transmembrane smORF peptide localised to early endosomes in Drosophila macrophages. hemotin regulates endosomal maturation during phagocytosis by repressing the cooperation of 14-3-3ζ with specific phosphatidylinositol (PI) enzymes. hemotin mutants accumulate undigested phagocytic material inside enlarged endo-lysosomes and as a result, hemotin mutants have reduced ability to fight bacteria, and hence, have severely reduced life span and resistance to infections. We identify Stannin, a peptide involved in organometallic toxicity, as the Hemotin functional homologue in vertebrates, showing that this novel regulator of phagocytic processing is widely conserved, emphasizing the significance of smORF peptides in cell biology and disease.

Klotho Regulates 14-3-3ζ Monomerization and Binding to the ASK1 Signaling Complex in Response to Oxidative Stress

The reactive oxygen species (ROS)-sensitive apoptosis signal-regulating kinase 1 (ASK1) signaling complex is a key regulator of p38 MAPK activity, a major modulator of stress-associated with aging disorders. We recently reported that the ratio of free ASK1 to the complex-bound ASK1 is significantly decreased in Klotho-responsive manner and that Klotho-deficient tissues have elevated levels of free ASK1 which coincides with increased oxidative stress. Here, we tested the hypothesis that: 1) covalent interactions exist among three identified proteins constituting the ASK1 signaling complex; 2) in normal unstressed cells the ASK1, 14-3-3ζ and thioredoxin (Trx) proteins simultaneously engage in a tripartite complex formation; 3) Klotho's stabilizing effect on the complex relied solely on 14-3-3ζ expression and its apparent phosphorylation and dimerization changes. To verify the hypothesis, we performed 14-3-3ζ siRNA knock-down experiments in conjunction with cell-based assays to measure ASK1-client protein interactions in the presence and absence of Klotho, and with or without an oxidant such as rotenone. Our results show that Klotho activity induces posttranslational modifications in the complex targeting 14-3-3ζ monomer/dimer changes to effectively protect against ASK1 oxidation and dissociation. This is the first observation implicating all three proteins constituting the ASK1 signaling complex in close proximity.

Protein-protein interactions as drug targets

Modulation of protein-protein interactions (PPIs) is becoming increasingly important in drug discovery and chemical biology. While a few years ago this 'target class' was deemed to be largely undruggable an impressing number of publications and success stories now show that targeting PPIs with small, drug-like molecules indeed is a feasible approach. Here, we summarize the current state of small-molecule inhibition and stabilization of PPIs and review the active molecules from a structural and medicinal chemistry angle, especially focusing on the key examples of iNOS, LFA-1 and 14-3-3.

The role of the 14-3-3 protein family in health, disease, and drug development

14-3-3 proteins regulate intracellular signaling pathways, such as signal transduction, protein trafficking, cell cycle, and apoptosis. In addition to the ubiquitous roles of 14-3-3 isoforms, unique tissue-specific functions are also described for each isoform. Owing to their role in regulating cell cycle, protein trafficking, and steroidogenesis, 14-3-3 proteins are prevalent in human diseases, such as cancer, neurodegeneration, and reproductive disorders, and, therefore, serve as valuable drug targets. In this review, we summarize the role of 14-3-3 proteins in normal and disease states, with a focus on 14-3-3γ and ɛ. We also discuss drug compounds targeting 14-3-3 proteins and their potential therapeutic uses.

Integrate Omics Data and Molecular Dynamics Simulations toward Better Understanding of Human 14-3-3 Interactomes and Better Drugs for Cancer Therapy

The 14-3-3 protein family is among the most extensively studied, yet still largely mysterious protein families in mammals to date. As they are well recognized for their roles in apoptosis, cell cycle regulation, and proliferation in healthy cells, aberrant 14-3-3 expression has unsurprisingly emerged as instrumental in the development of many cancers and in prognosis. Interestingly, while the seven known 14-3-3 isoforms in humans have many similar functions across cell types, evidence of isoform-specific functions and localization has been observed in both healthy and diseased cells. The strikingly high similarity among 14-3-3 isoforms has made it difficult to delineate isoform-specific functions and for isoform-specific targeting. Here, we review our knowledge of 14-3-3 interactome(s) generated by high-throughput techniques, bioinformatics, structural genomics and chemical genomics and point out that integrating the information with molecular dynamics (MD) simulations may bring us new opportunity to the design of isoform-specific inhibitors, which can not only be used as powerful research tools for delineating distinct interactomes of individual 14-3-3 isoforms, but also can serve as potential new anti-cancer drugs that selectively target aberrant 14-3-3 isoform.

FoxO1 inhibits transcription and membrane trafficking of epithelial Na+ channel

The epithelial Na(+) channel (ENaC), regulated by insulin, is of fundamental importance in the control of Na(+) reabsorption in the distal nephron. The potential role of Forkhead box O1 (FoxO1), downstream of insulin signaling, in the regulation of ENaC remains to be investigated. Here, we found that the overexpression of a constitutively active form of FoxO1 (ADA-FoxO1) suppressed the mRNA level of the ENaC α subunit (α-ENaC; also known as SCCN1A) and the apical density of ENaC in mouse cortical collecting duct (mCCD) cells. Conversely, knockdown of FoxO1 increased the apical membrane levels of α-ENaC and Na(+) transport under basal conditions. Insulin elevated α-ENaC expression and induced FoxO1 phosphorylation; however, the increase in α-ENaC and phosphorylated FoxO1 expression observed with insulin treatment was blunted ∼ 60% in cells expressing ADA-FoxO1. Moreover, insulin induced the interaction between phosphorylated FoxO1 and 14-3-3ε, indicating that FoxO1 phosphorylation promotes ENaC membrane trafficking by binding to 14-3-3ε. FoxO1 also suppressed activity of the α-ENaC promoter, and the putative FoxO1 target site is located in the -500 to -200 nt region of the α-ENaC promoter. These findings indicate that FoxO1 is a key negative regulatory factor in the insulin-dependent control of ENaC expression and forward trafficking in mCCD epithelia.

14-3-3 proteins in plant-pathogen interactions

14-3-3 proteins define a eukaryotic-specific protein family with a general role in signal transduction. Primarily, 14-3-3 proteins act as phosphosensors, binding phosphorylated client proteins and modulating their functions. Since phosphorylation regulates a plethora of different physiological responses in plants, 14-3-3 proteins play roles in multiple signaling pathways, including those controlling metabolism, hormone signaling, cell division, and responses to abiotic and biotic stimuli. Increasing evidence supports a prominent role of 14-3-3 proteins in regulating plant immunity against pathogens at various levels. In this review, potential links between 14-3-3 function and the regulation of plant-pathogen interactions are discussed, with a special focus on the regulation of 14-3-3 proteins in response to pathogen perception, interactions between 14-3-3 proteins and defense-related proteins, and 14-3-3 proteins as targets of pathogen effectors.

Pharmacological regulation of the cholesterol transport machinery in steroidogenic cells of the testis

Reduced serum testosterone (T), or hypogonadism, is estimated to affect about 5 million American men, including both aging and young men. Low serum T has been linked to mood changes, worsening cognition, fatigue, depression, decreased lean body mass and bone mineral density, increased visceral fat, metabolic syndrome, decreased libido, and sexual dysfunction. Administering exogenous T, known as T-replacement therapy (TRT), reverses many of the symptoms of low T levels. However, this treatment can result in luteinizing hormone suppression which, in turn, can lead to reduced sperm numbers and infertility, making TRT inappropriate for men who wish to father children. Additionally, TRT may result in supraphysiologic T levels, skin irritation, and T transfer to others upon contact; and there may be increased risk of prostate cancer and cardiovascular disease, particularly in aging men. Therefore, the development of alternate therapies for treating hypogonadism would be highly desirable. To do so requires greater understanding of the series of steps leading to T formation and how they are regulated, and the identification of key steps that are amenable to pharmacological modulation so as to induce T production. We review herein our current understanding of mechanisms underlying the pharmacological induction of T formation in hypogonadal testis.

Targeting LUNX inhibits non-small cell lung cancer growth and metastasis

There remains a great need for effective therapies for lung cancer, the majority of which are non-small cell lung cancers (NSCLC). Here, we report the identification of a novel candidate therapeutic target, LUNX, as a molecule overexpressed in primary NSCLC and lymph node metastases that is associated with reduced postoperative survival. Functional studies demonstrated that LUNX overexpression promoted lung cancer cell migration and proliferation by interactions with the chaperone protein 14-3-3. Conversely, LUNX silencing disrupted primary tumor growth, local invasion, and metastatic colonization. The finding that LUNX was expressed on cell membranes prompted us to generate and characterize LUNX antibodies as a candidate therapeutic. Anti-LUNX could downregulate LUNX and reduce lung cancer cell proliferation and migration in vitro. Administered in vivo to mice bearing lung cancer xenografts, anti-LUNX could slow tumor growth and metastasis and improve mouse survival. Together, our work provides a preclinical proof of concept for LUNX as a novel candidate target for immunotherapy in lung cancer.

Protein modifications regulate the role of 14-3-3γ adaptor protein in cAMP-induced steroidogenesis in MA-10 Leydig cells

The 14-3-3 protein family comprises adaptors and scaffolds that regulate intracellular signaling pathways. The 14-3-3γ isoform is a negative regulator of steroidogenesis that is hormonally induced and transiently functions at the initiation of steroidogenesis by delaying maximal steroidogenesis in MA-10 mouse tumor Leydig cells. Treatment of MA-10 cells with the cAMP analog 8-bromo-cAMP (8-Br-cAMP), which stimulates steroidogenesis, triggers the interaction of 14-3-3γ with the steroidogenic acute regulatory protein (STAR) in the cytosol, limiting STAR activity to basal levels. Over time, this interaction ceases, allowing for a 2-fold induction in STAR activity and maximal increase in the rate of steroid formation. The 14-3-3γ/STAR pattern of interaction was found to be opposite that of the 14-3-3γ homodimerization pattern. Phosphorylation and acetylation of 14-3-3γ showed similar patterns to homodimerization and STAR binding, respectively. 14-3-3γ Ser(58) phosphorylation and 14-3-3γ Lys(49) acetylation were blocked using trans-activator of HIV transcription factor 1 peptides coupled to 14-3-3γ sequences containing Ser(58) or Lys(49). Blocking either one of these modifications further induced 8-Br-cAMP-induced steroidogenesis while reducing lipid storage, suggesting that the stored cholesterol is used for steroid formation. Taken together, these results indicate that Ser(58) phosphorylation and Lys(49) acetylation of 14-3-3γ occur in a coordinated time-dependent manner to regulate 14-3-3γ homodimerization. 14-3-3γ Ser(58) phosphorylation is required for STAR interactions under control conditions, and 14-3-3γ Lys(49) acetylation is important for the cAMP-dependent induction of these interactions.

The application of an emerging technique for protein-protein interaction interface mapping: the combination of photo-initiated cross-linking protein nanoprobes with mass spectrometry

Protein–protein interaction was investigated using a protein nanoprobe capable of photo-initiated cross-linking in combination with high-resolution and tandem mass spectrometry. This emerging experimental approach introduces photo-analogs of amino acids within a protein sequence during its recombinant expression, preserves native protein structure and is suitable for mapping the contact between two proteins. The contact surface regions involved in the well-characterized interaction between two molecules of human 14-3-3ζ regulatory protein were used as a model. The employed photo-initiated cross-linking techniques extend the number of residues shown to be within interaction distance in the contact surface of the 14-3-3ζ dimer (Gln8–Met78). The results of this study are in agreement with our previously published data from molecular dynamic calculations based on high-resolution chemical cross-linking data and Hydrogen/Deuterium exchange mass spectrometry. The observed contact is also in accord with the 14-3-3ζ X-ray crystal structure (PDB 3dhr). The results of the present work are relevant to the structural biology of transient interaction in the 14-3-3ζ protein, and demonstrate the ability of the chosen methodology (the combination of photo-initiated cross-linking protein nanoprobes and mass spectrometry analysis) to map the protein-protein interface or regions with a flexible structure.

Genetic and physical interaction of Ssp1 CaMKK and Rad24 14-3-3 during low pH and osmotic stress in fission yeast

The Ssp1 calmodulin kinase kinase (CaMKK) is necessary for stress-induced re-organization of the actin cytoskeleton and initiation of growth at the new cell end following division in Schizosaccharomyces pombe. In addition, it regulates AMP-activated kinase and functions in low glucose tolerance. ssp1⁻ cells undergo mitotic delay at elevated temperatures and G₂ arrest in the presence of additional stressors. Following hyperosmotic stress, Ssp1-GFP forms transient foci which accumulate at the cell membrane and form a band around the cell circumference, but not co-localizing with actin patches. Hyperosmolarity-induced localization to the cell membrane occurs concomitantly with a reduction of its interaction with the 14-3-3 protein Rad24, but not Rad25 which remains bound to Ssp1. The loss of rad24 in ssp1⁻ cells reduces the severity of hyperosmotic stress response and relieves mitotic delay. Conversely, overexpression of rad24 exacerbates stress response and concomitant cell elongation. rad24⁻ does not impair stress-induced localization of Ssp1 to the cell membrane, however this response is almost completely absent in cells overexpressing rad24.

A NudE/14-3-3 pathway coordinates dynein and the kinesin Khc73 to position the mitotic spindle

Mitotic spindle position is controlled by interactions of cortical molecular motors with astral microtubules. In animal cells, Partner of Inscuteable (Pins) acts at the cortex to coordinate the activity of Dynein and Kinesin-73 (Khc73; KIF13B in mammals) to orient the spindle. Though the two motors move in opposite directions, their synergistic activity is required for robust Pins-mediated spindle orientation. Here, we identify a physical connection between Dynein and Khc73 that mediates cooperative spindle positioning. Khc73's motor and MBS domains link Pins to microtubule plus ends, while its stalk domain is necessary for Dynein activation and precise positioning of the spindle. A motif in the stalk domain binds, in a phospho-dependent manner, 14-3-3ζ, which dimerizes with 14-3-3ε. The 14-3-3ζ/ε heterodimer binds the Dynein adaptor NudE to complete the Dynein connection. The Khc73 stalk/14-3-3/NudE pathway defines a physical connection that coordinates the activities of multiple motor proteins to precisely position the spindle.

Switching responses: spatial and temporal regulators of axon guidance

The ability of the axonal growth cone to switch between attraction and repulsion in response to guidance cues in the extracellular environment during nervous system development is fundamental to the precise wiring of complex neural circuits. Regulation of cell-surface receptors by means of transcriptional control, local translation, trafficking and proteolytic processing are powerful mechanisms to regulate the response of the growth cone. Important work has also revealed how intracellular signalling pathways, including calcium and cyclic nucleotide signalling, can alter the directional response elicited by a particular cue. Here, we describe how these multiple regulatory mechanisms influence growth cone turning behaviour. We focus on recent evidence that suggests a significant role for 14-3-3 adaptor proteins in modifying growth cone turning behaviour and mediating directional polarity switches during development. Characterizing how 14-3-3 s regulate growth cone signalling will provide invaluable insight into nervous system development and may facilitate the identification of novel targets for promoting nerve regeneration following injury.

14-3-3s are potential biomarkers for HIV-related neurodegeneration

Over the last decade, it has become evident that 14-3-3 proteins are essential for primary cell functions. These proteins are abundant throughout the body, including the central nervous system and interact with other proteins in both cell cycle and apoptotic pathways. Examination of cerebral spinal fluid in humans suggests that 14-3-3s including 14-3-3ε (YWHAE) are up-regulated in several neurological diseases, and loss or duplication of the YWHAE gene leads to Miller-Dieker syndrome. The goal of this review is to examine the utility of 14-3-3s as a marker of human immune deficiency virus (HIV)-dependent neurodegeneration and also as a tool to track disease progression. To that end, we describe mechanisms implicating 14-3-3s in neurological diseases and summarize evidence of its interactions with HIV accessory and co-receptor proteins.

The combination of hydrogen/deuterium exchange or chemical cross-linking techniques with mass spectrometry: mapping of human 14-3-3ζ homodimer interface

Hydrogen/deuterium (H/D) exchange or chemical cross-linking by soluble carbodiimide (EDC) was employed in combination with high-resolution mass spectrometry (MS) to extend our knowledge about contact surface regions involved in the well-characterized model of interaction between two molecules of human 14-3-3ζ regulatory protein. The H/D exchange experiment provided low resolution mapping of interaction in the homodimeric 14-3-3ζ complex. A lower level of deuteration, suggesting structural protection, of two sequential segments has been demonstrated for dimeric 14-3-3ζ wild type relative to the monomeric mutant 14-3-3ζ S58D. The N-terminal sequence (the first 27 residues) from one subunit interacts with region αC'and αD'-helices (residues 45-98) of the other molecule across the dimer interface. To identify interacting amino acid residues within the studied complex, a chemical cross-linking reaction was carried out to produce the covalent homodimer, which was detected by SDS-PAGE. The MS analysis (following tryptic in-gel digestion) employing both high resolution and tandem mass spectrometry revealed cross-linked amino acid residues. Two alternative salt bridges between Glu81 and either Lys9 or the N-terminal amino group have been found to participate in transient interactions of the 14-3-3ζ isotype homodimerization. The data obtained, which have never previously been reported, were used to modify the published 14-3-3 crystal structure using molecular modeling. Based on our findings, utilization of this combination of experimental approaches, which preserve protein native structures, is suitable for mapping the contact between two proteins and also allows for the description of transient interactions or of regions with flexible structure in the studied protein complexes.

Transcriptome sequencing revealed significant alteration of cortical promoter usage and splicing in schizophrenia

Background

While hybridization based analysis of the cortical transcriptome has provided important insight into the neuropathology of schizophrenia, it represents a restricted view of disease-associated gene activity based on predetermined probes. By contrast, sequencing technology can provide un-biased analysis of transcription at nucleotide resolution. Here we use this approach to investigate schizophrenia-associated cortical gene expression.

Methodology/Principal Findings

The data was generated from 76 bp reads of RNA-Seq, aligned to the reference genome and assembled into transcripts for quantification of exons, splice variants and alternative promoters in postmortem superior temporal gyrus (STG/BA22) from 9 male subjects with schizophrenia and 9 matched non-psychiatric controls. Differentially expressed genes were then subjected to further sequence and functional group analysis. The output, amounting to more than 38 Gb of sequence, revealed significant alteration of gene expression including many previously shown to be associated with schizophrenia. Gene ontology enrichment analysis followed by functional map construction identified three functional clusters highly relevant to schizophrenia including neurotransmission related functions, synaptic vesicle trafficking, and neural development. Significantly, more than 2000 genes displayed schizophrenia-associated alternative promoter usage and more than 1000 genes showed differential splicing (FDR<0.05). Both types of transcriptional isoforms were exemplified by reads aligned to the neurodevelopmentally significant doublecortin-like kinase 1 (DCLK1) gene.

Conclusions

This study provided the first deep and un-biased analysis of schizophrenia-associated transcriptional diversity within the STG, and revealed variants with important implications for the complex pathophysiology of schizophrenia.

Hormone-induced 14-3-3γ adaptor protein regulates steroidogenic acute regulatory protein activity and steroid biosynthesis in MA-10 Leydig cells

Cholesterol is the sole precursor of steroid hormones in the body. The import of cholesterol to the inner mitochondrial membrane, the rate-limiting step in steroid biosynthesis, relies on the formation of a protein complex that assembles at the outer mitochondrial membrane called the transduceosome. The transduceosome contains several mitochondrial and cytosolic components, including the steroidogenic acute regulatory protein (STAR). Human chorionic gonadotropin (hCG) induces de novo synthesis of STAR, a process shown to parallel maximal steroid production. In the hCG-dependent steroidogenic MA-10 mouse Leydig cell line, the 14-3-3γ protein was identified in native mitochondrial complexes by mass spectrometry and immunoblotting, and its levels increased in response to hCG treatment. The 14-3-3 proteins bind and regulate the activity of many proteins, acting via target protein activation, modification and localization. In MA-10 cells, cAMP induces 14-3-3γ expression parallel to STAR expression. Silencing of 14-3-3γ expression potentiates hormone-induced steroidogenesis. Binding motifs of 14-3-3γ were identified in components of the transduceosome, including STAR. Immunoprecipitation studies demonstrate a hormone-dependent interaction between 14-3-3γ and STAR that coincides with reduced 14-3-3γ homodimerization. The binding site of 14-3-3γ on STAR was identified to be Ser-194 in the STAR-related sterol binding lipid transfer (START) domain, the site phosphorylated in response to hCG. Taken together, these results demonstrate that 14-3-3γ negatively regulates steroidogenesis by binding to Ser-194 of STAR, thus keeping STAR in an unfolded state, unable to induce maximal steroidogenesis. Over time 14-3-3γ homodimerizes and dissociates from STAR, allowing this protein to induce maximal mitochondrial steroid formation.

Phosphorylation-dependent 14-3-3 protein interactions regulate CFTR biogenesis

cAMP/PKA stimulation elicited posttranslational increases in CFTR expression and the interaction of specific 14-3-3 proteins with phosphorylated sites within the R region. This improved the efficiency of nascent CFTR biogenesis and reduced its interaction with the COPI retrograde retrieval mechanism, making more CFTR available for anion secretion.

Cystic fibrosis transmembrane conductance regulator (CFTR) is a cAMP/protein kinase A (PKA)–regulated chloride channel whose phosphorylation controls anion secretion across epithelial cell apical membranes. We examined the hypothesis that cAMP/PKA stimulation regulates CFTR biogenesis posttranslationally, based on predicted 14-3-3 binding motifs within CFTR and forskolin-induced CFTR expression. The 14-3-3β, γ, and ε isoforms were expressed in airway cells and interacted with CFTR in coimmunoprecipitation assays. Forskolin stimulation (15 min) increased 14-3-3β and ε binding to immature and mature CFTR (bands B and C), and 14-3-3 overexpression increased CFTR bands B and C and cell surface band C. In pulse-chase experiments, 14-3-3β increased the synthesis of immature CFTR, reduced its degradation rate, and increased conversion of immature to mature CFTR. Conversely, 14-3-3β knockdown decreased CFTR B and C bands (70 and 55%) and elicited parallel reductions in cell surface CFTR and forskolin-stimulated anion efflux. In vitro, 14-3-3β interacted with the CFTR regulatory region, and by nuclear magnetic resonance analysis, this interaction occurred at known PKA phosphorylated sites. In coimmunoprecipitation assays, forskolin stimulated the CFTR/14-3-3β interaction while reducing CFTR's interaction with coat protein complex 1 (COP1). Thus 14-3-3 binding to phosphorylated CFTR augments its biogenesis by reducing retrograde retrieval of CFTR to the endoplasmic reticulum. This mechanism permits cAMP/PKA stimulation to make more CFTR available for anion secretion.

14-3-3 checkpoint regulatory proteins interact specifically with DNA repair protein human exonuclease 1 (hEXO1) via a semi-conserved motif

Human exonuclease 1 (hEXO1) acts directly in diverse DNA processing events, including replication, mismatch repair (MMR), and double strand break repair (DSBR), and it was also recently described to function as damage sensor and apoptosis inducer following DNA damage. In contrast, 14-3-3 proteins are regulatory phosphorserine/threonine binding proteins involved in the control of diverse cellular events, including cell cycle checkpoint and apoptosis signaling. hEXO1 is regulated by post-translation Ser/Thr phosphorylation in a yet not fully clarified manner, but evidently three phosphorylation sites are specifically induced by replication inhibition leading to protein ubiquitination and degradation. We demonstrate direct and robust interaction between hEXO1 and six of the seven 14-3-3 isoforms in vitro, suggestive of a novel protein interaction network between DNA repair and cell cycle control. Binding experiments reveal weak affinity of the more selective isoform 14-3-3σ but both 14-3-3 isoforms η and σ significantly stimulate hEXO1 activity, indicating that these regulatory proteins exert a common regulation mode on hEXO1. Results demonstrate that binding involves the phosphorable amino acid S746 in hEXO1 and most likely a second unidentified binding motif. 14-3-3 associations do not appear to directly influence hEXO1 in vitro nuclease activity or in vitro DNA replication initiation. Moreover, specific phosphorylation variants, including hEXO1 S746A, are efficiently imported to the nucleus; to associate with PCNA in distinct replication foci and respond to DNA double strand breaks (DSBs), indicating that 14-3-3 binding does not involve regulating the subcellular distribution of hEXO1. Altogether, these results suggest that association may be related to regulation of hEXO1 availability during the DNA damage response to plausibly prevent extensive DNA resection at the damage site, as supported by recent studies.

Tiam1 is recruited to β1-integrin complexes by 14-3-3ζ where it mediates integrin-induced Rac1 activation and motility

14-3-3 is an adaptor protein that localizes to the leading edge of spreading cells, returning to the cytoplasm as spreading ceases. Previously, we showed that integrin-induced Rac1 activation and spreading were inhibited by sequestration of 14-3-3ζ and restored by its overexpression. Here, we determined whether 14-3-3 mediates integrin signaling by localizing a guanine nucleotide exchange factor (GEF) to Rac1-activating integrin complexes. We showed that GST-14-3-3ζ recruited the Rac1-GEF, Tiam1, from cell lysates through Tiam1 residues 1-182 (N(1-182) Tiam1). The physiological relevance of this interaction was examined in serum-starved Hela cells plated on fibronectin. Both Tiam1 and N(1-182) Tiam1 were recruited to 14-3-3-containing β1-integrin complexes, as shown by co-localization and co-immunoprecipitation. Integrin-induced Rac1 activation was inhibited when Tiam1 was depleted with siRNA or by overexpression of catalytically inactive N(1-182) Tiam1, which was incorporated into 14-3-3/β1-integrin complexes and inhibited spreading in a manner that was overcome by constitutively active Rac1. Integrin-induced Rac1 activation, spreading, and migration were also inhibited by overexpression of 14-3-3ζ S58D, which was unable to recruit Tiam1 from lysates, co-immunoprecipitate with Tiam1, or mediate its incorporation into β1-integrin complexes. Taken together, these findings suggest a previously unrecognized mechanism of integrin-induced Rac1 activation in which 14-3-3 dimers localize Tiam1 to integrin complexes, where it mediates integrin-dependent Rac1 activation, thus initiating motility-inducing pathways. Moreover, since Tiam1 is recruited to other sites of localized Rac1 activation through its PH-CC-EX domain, the present findings show that a mechanism involving its N-terminal 182 residues is utilized to recruit Tiam1 to motility-inducing integrin complexes.

14-3-3 proteins as potential therapeutic targets

The 14-3-3 family of phosphoserine/phosphothreonine-binding proteins dynamically regulates the activity of client proteins in various signaling pathways that control diverse physiological and pathological processes. In response to environmental cues, 14-3-3 proteins orchestrate the highly regulated flow of signals through complex networks of molecular interactions to achieve well-controlled physiological outputs, such as cell proliferation or differentiation. Accumulating evidence now supports the concept that either an abnormal state of 14-3-3 protein expression, or dysregulation of 14-3-3/client protein interactions, contributes to the development of a large number of human diseases. In particular, clinical investigations in the field of oncology have demonstrated a correlation between upregulated 14-3-3 levels and poor survival of cancer patients. These studies highlight the rapid emergence of 14-3-3 proteins as a novel class of molecular target for potential therapeutic intervention. The current status of 14-3-3 modulator discovery is discussed.

Critical residue that promotes protein dimerization: a story of partially exposed Phe25 in 14-3-3σ

Many proteins exist and function as oligomers. While hydrophobic interactions have been recognized as the major driving force for oligomerization, detailed molecular mechanisms for the assembly are unknown. Here, we used 14-3-3σ as a model protein and investigated the role of hydrophobic residues at the dimeric interface using MD simulations and coimmunoprecipitations. We found that a half-exposed and half-buried residue in the interface, Phe(25), plays a more important role in promoting homodimerization than the hydrophobic core residues by organizing both favorable hydrophobic and hydrophilic interactions. Phe(25) is critical in packing and stabilizing hydrophobic core residues. We conclude that the structural stability of hydrophobic cores is critical for a stable homodimer complex and this stable property can be bestowed by residues outside of hydrophobic core. The important organizing activity of Phe(25) for homodimerization of 14-3-3σ originates from its unique physical location, rigidity, size, and hydrophobicity. Thus, hydrophobic residues that are not deeply buried at the oligomeric interface may play important but different roles from the buried core residues and they may promote oligomerization by organizing co-operativity of core and other residues for favorable hydrophobic and electrostatic interactions.

14-3-3 proteins in neurodegeneration

Among the first reported functions of 14-3-3 proteins was the regulation of tyrosine hydroxylase (TH) activity suggesting a possible involvement of 14-3-3 proteins in Parkinson's disease. Since then the relevance of 14-3-3 proteins in the pathogenesis of chronic as well as acute neurodegenerative diseases, including Alzheimer's disease, polyglutamine diseases, amyotrophic lateral sclerosis and stroke has been recognized. The reported function of 14-3-3 proteins in this context are as diverse as the mechanism involved in neurodegeneration, reaching from basal cellular processes like apoptosis, over involvement in features common to many neurodegenerative diseases, like protein stabilization and aggregation, to very specific processes responsible for the selective vulnerability of cellular populations in single neurodegenerative diseases. Here, we review what is currently known of the function of 14-3-3 proteins in nervous tissue focussing on the properties of 14-3-3 proteins important in neurodegenerative disease pathogenesis.

Modulation of neoplastic gene regulatory pathways by the RNA-binding factor AUF1

The mRNA-binding protein AUF1 regulates the expression of many key players in cancer including proto-oncogenes, regulators of apoptosis and the cell cycle, and pro-inflammatory cytokines, principally by directing the decay kinetics of their encoded mRNAs. Most studies support an mRNA-destabilizing role for AUF1, although other findings suggest additional functions for this factor. In this review, we explore how changes in AUF1 isoform distribution, subcellular localization, and post-translational protein modifications can influence the metabolism of targeted mRNAs. However, several lines of evidence also support a role for AUF1 in the initiation and/or development of cancer. Many AUF1-targeted transcripts encode products that control pro- and anti-oncogenic processes. Also, overexpression of AUF1 enhances tumorigenesis in murine models, and AUF1 levels are enhanced in some tumors. Finally, signaling cascades that modulate AUF1 function are deregulated in some cancerous tissues. Together, these features suggest that AUF1 may play a prominent role in regulating the expression of many genes that can contribute to tumorigenic phenotypes, and that this post-transcriptional regulatory control point may be subverted by diverse mechanisms in neoplasia.

14-3-3ζ as a prognostic marker and therapeutic target for cancer

IMPORTANCE OF THE FIELD

The ubiquitously expressed 14-3-3ζ protein is involved in numerous important cellular pathways involved in cancer. Recent research suggests 14-3-3ζ may play a central role regulating multiple pathways responsible for cancer initiation and progression. This review will provide an overview of 14-3-3 proteins and address the role of 14-3-3ζ overexpression in cancer.

AREAS COVERED IN THIS REVIEW

The review covers the basic role of 14-3-3 in regulation of multiple pathways with a focus on 14-3-3ζ as a clinically relevant biomarker for cancer recurrence.

WHAT THE READER WILL GAIN

14-3-3ζ overexpression has been found in multiple cancers; however, the clinical implications were unclear. Recently, 14-3-3ζ has been identified as a biomarker for poor prognosis and chemoresistance in multiple tumor types, indicating a potential clinical application for using 14-3-3ζ in selecting treatment options and predicting cancer patients' outcome.

TAKE HOME MESSAGE

14-3-3ζ is a potential prognostic marker of cancer recurrence and predictive marker for therapeutic resistance. The overexpression of 14-3-3ζ in multiple cancers suggests that it may be a common target to intervene tumor progression; therefore, more efforts are needed for the development of 14-3-3 inhibitors.