Mixed lineage kinases (MLKs): a role in dendritic cells, inflammation and immunity?

Disrupting PIAS3-mediated SUMOylation of MLK3 ameliorates poststroke neuronal damage and deficits in cognitive and sensorimotor behaviors

Activated small ubiquitin-like modifiers (SUMOs) have been implicated in neuropathological processes following ischemic stroke. However, the target proteins of SUMOylation and their contribution to neuronal injury remain to be elucidated. MLK3 (mixed-lineage kinase 3), a member of the mitogen-activated protein kinase kinase kinase (MAPKKK) family, is a critical regulator of neuronal lesions following cerebral ischemia. Here, we found that SUMOylation of MLK3 increases in both global and focal ischemic rodent models and primary neuronal models of oxygen and glucose deprivation (OGD). SUMO1 conjugation at the Lys401 site of MLK3 promoted its activation, stimulated its downstream p38/c-Jun N-terminal kinase (JNK) cascades, and led to cell apoptosis. The interaction of MLK3 with PIAS3, a SUMO ligase, was elevated following ischemia and reperfusion. The PINIT domain of PIAS3 was involved in direct interactions with MLK3. Overexpression of the PINIT domain of PIAS3 disrupted the MLK3-PIAS3 interaction, inhibited SUMOylation of MLK3, suppressed downstream signaling, and reduced cell apoptosis and neurite damage. In rodent ischemic models, the overexpression of the PINIT domain reduced brain lesions and alleviated deficits in learning, memory, and sensorimotor functions. Our findings demonstrate that brain ischemia-induced MLK3 SUMOylation by PIAS3 is a potential target against poststroke neuronal lesions and behavioral impairments.

MLK3 localizes mainly to the cytoplasm and promotes oxidative stress injury via a positive feedback loop

Activation of mixed lineage kinase 3 (MLK3) by phosphorylation at Thr277/Ser281 stimulates downstream apoptotic pathways and ultimately leads to cell injury. MLK3 is reported to localize to both the cytoplasm and nucleus in human ovarian cancer cells and immortalized ovarian epithelial cells (T80 and T90 cells), and phosphorylation at Thr477 is required for the cytoplasmic retention of MLK3 in T80 cells. However, the subcellular distribution of MLK3 in other cell types has rarely been reported, and whether phosphorylation of MLK3 at Thr277/Ser281 affects its subcellular distribution is unknown. Here, our bioinformatics analysis predicted that MLK3 was mainly distributed in the cytoplasm and nucleus. In the human HEK293T embryonic kidney cell line and murine HT22 hippocampal neuronal cell line, endogenous MLK3 was more abundant in the cytoplasm and less abundant in the nucleus. In addition, overexpressed Myc-tagged MLK3 and EGFP-tagged MLK3 were also observed to localize mainly to the cytoplasm. MLK3 that was activated by phosphorylation at Thr277/Ser281 was mainly distributed in the cytoplasm, and phosphorylation deficient (T277A/S281A) and mimic (T277E/S281E) mutants both showed distributions similar to that of wild type (wt) MLK3, further proving that phosphorylation at Thr277/Ser281 was not involved in regulating MLK3 subcellular localization. In HEK293T cells, H2O2 stimulation accelerated MLK3 phosphorylation (activation), and this phosphorylation was reduced by the antioxidant N-acetylcysteine in a dose-dependent manner. Overexpressing wt MLK3 promoted the production of intracellular reactive oxygen species and increased cell apoptosis, both of which were enhanced by the phosphorylation-mimic (T277E/S281E) MLK3 variant but not by the phosphorylation-deficient (T277A/S281A) MLK3 variant. These findings provided additional evidence for the cytoplasmic and nuclear distribution of MLK3 in HEK293T cells or HT22 cells and revealed the pivotal role of MLK3 in the positive feedback loop of oxidative stress injury.

MLK3 Regulates Inflammatory Response via Activation of AP-1 Pathway in HEK293 and RAW264.7 Cells

Inflammation is a critically important barrier found in innate immunity. However, severe and sustained inflammatory conditions are regarded as causes of many different serious diseases, such as cancer, atherosclerosis, and diabetes. Although numerous studies have addressed how inflammatory responses proceed and what kinds of proteins and cells are involved, the exact mechanism and protein components regulating inflammatory reactions are not fully understood. In this paper, to determine the regulatory role of mixed lineage kinase 3 (MLK3), which functions as mitogen-activated protein kinase kinase kinase (MAP3K) in cancer cells in inflammatory response to macrophages, we employed an overexpression strategy with MLK3 in HEK293 cells and used its inhibitor URMC-099 in lipopolysaccharide (LPS)-treated RAW264.7 cells. It was found that overexpressed MLK3 increased the mRNA expression of inflammatory genes (COX-2, IL-6, and TNF-α) via the activation of AP-1, according to a luciferase assay carried out with AP-1-Luc. Overexpression of MLK3 also induced phosphorylation of MAPKK (MEK1/2, MKK3/6, and MKK4/7), MAPK (ERK, p38, and JNK), and AP-1 subunits (c-Jun, c-Fos, and FRA-1). Phosphorylation of MLK3 was also observed in RAW264.7 cells stimulated by LPS, Pam3CSK, and poly(I:C). Finally, inhibition of MLK3 by URMC-099 reduced the expression of COX-2 and CCL-12, phosphorylation of c-Jun, luciferase activity mediated by AP-1, and phosphorylation of MAPK in LPS-treated RAW264.7 cells. Taken together, our findings strongly suggest that MLK3 plays a central role in controlling AP-1-mediated inflammatory responses in macrophages and that this enzyme can serve as a target molecule for treating AP-1-mediated inflammatory diseases.

Advances in Immunosuppressive Agents Based on Signal Pathway

Immune abnormality involves in various diseases, such as infection, allergic diseases, autoimmune diseases, as well as transplantation. Several signal pathways have been demonstrated to play a central role in the immune response, including JAK/STAT, NF-κB, PI3K/AKT-mTOR, MAPK, and Keap1/Nrf2/ARE pathway, in which multiple targets have been used to develop immunosuppressive agents. In recent years, varieties of immunosuppressive agents have been approved for clinical use, such as the JAK inhibitor tofacitinib and the mTOR inhibitor everolimus, which have shown good therapeutic effects. Additionally, many immunosuppressive agents are still in clinical trials or preclinical studies. In this review, we classified the immunosuppressive agents according to the immunopharmacological mechanisms, and summarized the phase of immunosuppressive agents.

Cdc42/Rac Interactive Binding Containing Effector Proteins in Unicellular Protozoans With Reference to Human Host: Locks of the Rho Signaling

Small GTPases are the key to actin cytoskeleton signaling, which opens the lock of effector proteins to forward the signal downstream in several cellular pathways. Actin cytoskeleton assembly is associated with cell polarity, adhesion, movement and other functions in eukaryotic cells. Rho proteins, specifically Cdc42 and Rac, are the primary regulators of actin cytoskeleton dynamics in higher and lower eukaryotes. Effector proteins, present in an inactive state gets activated after binding to the GTP bound Cdc42/Rac to relay a signal downstream. Cdc42/Rac interactive binding (CRIB) motif is an essential conserved sequence found in effector proteins to interact with Cdc42 or Rac. A diverse range of Cdc42/Rac and their effector proteins have evolved from lower to higher eukaryotes. The present study has identified and further classified CRIB containing effector proteins in lower eukaryotes, focusing on parasitic protozoans causing neglected tropical diseases and taking human proteins as a reference point to the highest evolved organism in the evolutionary trait. Lower eukaryotes’ CRIB containing proteins fall into conventional effector molecules, PAKs (p21 activated kinase), Wiskoit-Aldrich Syndrome proteins family, and some have unique domain combinations unlike any known proteins. We also highlight the correlation between the effector protein isoforms and their selective specificity for Cdc42 or Rac proteins during evolution. Here, we report CRIB containing effector proteins; ten in Dictyostelium and Entamoeba, fourteen in Acanthamoeba, one in Trypanosoma and Giardia. CRIB containing effector proteins that have been studied so far in humans are potential candidates for drug targets in cancer, neurological disorders, and others. Conventional CRIB containing proteins from protozoan parasites remain largely elusive and our data provides their identification and classification for further in-depth functional validations. The tropical diseases caused by protozoan parasites lack combinatorial drug targets as effective paradigms. Targeting signaling mechanisms operative in these pathogens can provide greater molecules in combatting their infections.

Therapeutic potential of targeting mixed lineage kinases in cancer and inflammation

Dysregulation of intracellular signaling pathways is a key attribute of diseases associated with chronic inflammation, including cancer. Mitogen activated protein kinases have emerged as critical conduits of intracellular signal transmission, yet due to their ubiquitous roles in cellular processes, their direct inhibition may lead to undesired effects, thus limiting their usefulness as therapeutic targets. Mixed lineage kinases (MLKs) are mitogen-activated protein kinase kinase kinases (MAP3Ks) that interact with scaffolding proteins and function upstream of p38, JNK, ERK, and NF-kappaB to mediate diverse cellular signals. Studies involving gene silencing, genetically engineered mouse models, and small molecule inhibitors suggest that MLKs are critical in tumor progression as well as in inflammatory processes. Recent advances indicate that they may be useful targets in some types of cancer and in diseases driven by chronic inflammation including neurodegenerative diseases and metabolic diseases such as nonalcoholic steatohepatitis. This review describes existing MLK inhibitors, the roles of MLKs in various aspects of tumor progression and in the control of inflammatory processes, and the potential for therapeutic targeting of MLKs.

The second generation mixed lineage kinase-3 (MLK3) inhibitor CLFB-1134 protects against neurotoxin-induced nigral dopaminergic neuron loss

Dopaminergic neurons express mixed lineage kinases which regulate the expression of cell death genes. In Parkinson's disease, cell death via apoptosis is prevalent, and previous work testing mixed lineage kinase inhibitors in animal models suggested the inhibitors had some neuroprotective potential. CLFB-1134 is a new, brain-penetrant inhibitor specific for MLK3, tested here in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) model of dopaminergic depletion and nigral neuron death in mice. After ensuring that treatment with CLFB-1134 did not alter conversion of MPTP to MPP+, we demonstrated CLFB-1134's inhibition of MLK3 and neuroprotective efficacy. Specifically we evaluated the integrity of the nigrostriatal dopamine system following MPTP by assessing protein expression, high performance liquid chromatography, and immunohistology with stereology. We found that CLFB-1134 achieves protection of striatal dopaminergic terminals and nigral cell bodies when dosed simultaneously or following MPTP treatment. By preventing phosphorylation of JNK and other downstream targets of MLK3, CLFB-1134 protects against the neurotoxin MPTP. Inhibition of MLK3 may be a valid target for future work investigating treatment of Parkinson's disease.

Broad Spectrum Mixed Lineage Kinase Type 3 Inhibition and HIV-1 Persistence in Macrophages

Mixed lineage kinases (MLKs) are a group of serine-threonine kinases that evolved in part to respond to endogenous and exogenous insults that result in oxidative stress and pro-inflammatory responses from innate immune cells. Human immunodeficiency virus type 1 (HIV-1) thrives in these conditions and is associated with the development of associated neurocognitive disorders (HAND). As part of a drug discovery program to identify new therapeutic strategies for HAND, we created a library of broad spectrum MLK inhibitors with drug-like properties. Serendipitously, the lead compound, URMC-099 has proved useful not only in reversing damage to synaptic architecture in models of HAND, but also serves to restore autophagy as a protective response when given in concert with nanoformulated antiretroviral therapy (nanoART) in persistently infected macrophages. These findings are reviewed in the context of MLK3 biology and cellular signaling pathways relevant to new HIV-1 therapies. Graphical abstract.

Breakthroughs in modern cancer therapy and elusive cardiotoxicity: Critical research-practice gaps, challenges, and insights

To date, five cancer treatment modalities have been defined. The three traditional modalities of cancer treatment are surgery, radiotherapy, and conventional chemotherapy, and the two modern modalities include molecularly targeted therapy (the fourth modality) and immunotherapy (the fifth modality). The cardiotoxicity associated with conventional chemotherapy and radiotherapy is well known. Similar adverse cardiac events are resurging with the fourth modality. Aside from the conventional and newer targeted agents, even the most newly developed, immune‐based therapeutic modalities of anticancer treatment (the fifth modality), e.g., immune checkpoint inhibitors and chimeric antigen receptor (CAR) T‐cell therapy, have unfortunately led to potentially lethal cardiotoxicity in patients. Cardiac complications represent unresolved and potentially life‐threatening conditions in cancer survivors, while effective clinical management remains quite challenging. As a consequence, morbidity and mortality related to cardiac complications now threaten to offset some favorable benefits of modern cancer treatments in cancer‐related survival, regardless of the oncologic prognosis. This review focuses on identifying critical research‐practice gaps, addressing real‐world challenges and pinpointing real‐time insights in general terms under the context of clinical cardiotoxicity induced by the fourth and fifth modalities of cancer treatment. The information ranges from basic science to clinical management in the field of cardio‐oncology and crosses the interface between oncology and onco‐pharmacology. The complexity of the ongoing clinical problem is addressed at different levels. A better understanding of these research‐practice gaps may advance research initiatives on the development of mechanism‐based diagnoses and treatments for the effective clinical management of cardiotoxicity.

Distinct functions of the dual leucine zipper kinase depending on its subcellular localization

The dual leucine zipper kinase DLK induces β-cell apoptosis by inhibiting the transcriptional activity conferred by the β-cell protective transcription factor cAMP response element binding protein CREB. This action might contribute to β-cell loss and ultimately diabetes. Within its kinase domain DLK shares high homology with the mixed lineage kinase (MLK) 3, which is activated by tumor necrosis factor (TNF) α and interleukin (IL)-1β, known prediabetic signals. In the present study, the regulation of DLK in β-cells by these cytokines was investigated. Both, TNFα and IL-1β induced the nuclear translocation of DLK. Mutations within a putative nuclear localization signal (NLS) prevented basal and cytokine-induced nuclear localization of DLK and binding to the importin receptor importin α, thereby demonstrating a functional NLS within DLK. DLK NLS mutants were catalytically active as they phosphorylated their down-stream kinase c-Jun N-terminal kinase to the same extent as DLK wild-type but did neither inhibit CREB-dependent gene transcription nor transcription conferred by the promoter of the anti-apoptotic protein BCL-xL. In addition, the β-cell apoptosis-inducing effect of DLK was severely diminished by mutation of its NLS. In a murine model of prediabetes, enhanced nuclear DLK was found. These data demonstrate that DLK exerts distinct functions, depending on its subcellular localization and thus provide a novel level of regulating DLK action. Furthermore, the prevention of the nuclear localization of DLK as induced by prediabetic signals with consecutive suppression of β-cell apoptosis might constitute a novel target in the therapy of diabetes mellitus.

Adjunctive and long-acting nanoformulated antiretroviral therapies for HIV-associated neurocognitive disorders

PURPOSE OF REVIEW

We are pleased to review current and future strategies being developed to modulate neuroinflammation while reducing residual viral burden in the central nervous system. This has been realized by targeted long-acting antiretroviral nano and adjunctive therapies being developed for HIV-infected people. Our ultimate goal is to eliminate virus from its central nervous system reservoirs and, in so doing, reverse the cognitive and motor dysfunctions.

RECENT FINDINGS

Herein, we highlight our laboratories' development of adjunctive and nanomedicine therapies for HIV-associated neurocognitive disorders. An emphasis is placed on drug-drug interactions that target both the viral life cycle and secretory proinflammatory neurotoxic factors and signaling pathways.

SUMMARY

Antiretroviral therapy has improved the quality and duration of life for people living with HIV-1. A significant long-term comorbid illness is HIV-associated neurocognitive disorders. Symptoms, although reduced in severity, are common. Disease occurs, in part, through continued low-level viral replication, inducing secondary glial neuroinflammatory activities. Our recent works and those of others have seen disease attenuated in animal models through the use of adjunctive and long-acting reservoir-targeted nanoformulated antiretroviral therapy. The translation of these inventions from animals to humans is the focus of this review.

MLK3 phophorylates AMPK independently of LKB1

Emerging evidence has shown that cellular energy metabolism is regulated by the AMPK and MLK3-JNK signaling pathways, but the functional link between them remains to be determined. The present study aimed to explore the crosstalk between MLK3 and AMPK. We found that both JNK and AMPK were phosphorylated at their activation sites by TNF-α, Anisomycin, H2O2 and sorbitol. Interestingly, sorbitol stimulated phosphorylation of AMPK at T172 in LKB1-deficient cells. Following the screening of more than 100 kinases, we identified that MLK3 induced phosphorylation of AMPK at T172. Our in vitro analysis further revealed that MLK3-mediated phosphorylation of AMPK at T172 was independent of AMP, but addition of AMP caused a mobility shift of AMPK, an indication of autophosphorylation, suggesting that AMP binding and phosphorylation of T172 leads to maximal activation of AMPK. GST-pull down assays showed a direct interaction between AMPKα1 subunit and MLK3. Altogether, our results indicate that MLK3 serves as a common upstream kinase of AMPK and JNK and functions as a direct upstream kinase for AMPK independent of LKB1.

MLK3 regulates fMLP-stimulated neutrophil motility

INTRODUCTION

Mixed lineage kinase 3 (MLK3) is part of the intracellular regulatory system that connects extracellular cytokine or mitogen signals received through G-protein coupled receptors to changes in gene expression. MLK3 activation stimulates motility of epithelial cells and epithelial-derived tumor cells, but its role in mediating the migration of other cell types remains unknown. Since neutrophils play a crucial role in innate immunity and contribute to the pathogenesis of several diseases, we therefore examined whether MLK3 might regulate the motility of mouse neutrophils responding to a chemotactic stimulus, the model bacterial chemoattractant fMLP.

METHODS

The expression of Mlk3 in mouse neutrophils was determined by immunocytochemistry and by RT-PCR. In vitro chemotaxis in a gradient of fMLP, fMLP-stimulated random motility, fMLP-stimulated F-actin formation were measured by direct microscopic observation using neutrophils pre-treated with a novel small molecule inhibitor of MLK3 (URMC099) or neutrophils obtained from Mlk3-/- mice. In vivo effects of MLK3 inhibition were measured by counting the fMLP-induced accumulation of neutrophils in the peritoneum following pre-treatment with URMC099 in wild-type C57Bl/6 or mutant Mlk3-/- mice.

RESULTS

The expression of Mlk3 mRNA and protein was observed in neutrophils purified from wild-type C57Bl/6 mice but not in neutrophils from mutant Mlk3-/- mice. Chemotaxis by wild-type neutrophils induced by a gradient of fMLP was reduced by pre-treatment with URMC099. Neutrophils from C57Bl/6 mice pretreated with URMC099 and neutrophils from Mlk3-/- mice moved far less upon fMLP-stimulation and did not form F-actin as readily as untreated neutrophils from C57Bl/6 controls. In vivo recruitment of neutrophils into the peritoneum by fMLP was significantly reduced in wild-type mice treated with URMC099, as well as in untreated Mlk3-/- mice-thereby confirming the role of MLK3 in neutrophil migration.

CONCLUSIONS

Mlk3 mRNA is expressed in murine neutrophils. Genetic or pharmacologic inhibition of MLK3 blocks fMLP-mediated motility of neutrophils both in vitro and in vivo, suggesting that MLK3 may be a therapeutic target in human diseases characterized by exuberant neutrophil migration.

Discovery, synthesis, and characterization of an orally bioavailable, brain penetrant inhibitor of mixed lineage kinase 3

Inhibition of mixed lineage kinase 3 (MLK3) is a potential strategy for treatment of Parkinson's disease and HIV-1 associated neurocognitive disorders (HAND), requiring an inhibitor that can achieve significant brain concentration levels. We report here URMC-099 (1) an orally bioavailable (F = 41%), potent (IC50 = 14 nM) MLK3 inhibitor with excellent brain exposure in mouse PK models and minimal interference with key human CYP450 enzymes or hERG channels. The compound inhibits LPS-induced TNFα release in microglial cells, HIV-1 Tat-induced release of cytokines in human monocytes and up-regulation of phospho-JNK in Tat-injected brains of mice. Compound 1 likely functions in HAND preclinical models by inhibiting multiple kinase pathways, including MLK3 and LRRK2 (IC50 = 11 nM). We compare the kinase specificity and BBB penetration of 1 with CEP-1347 (2). Compound 1 is well tolerated, with excellent in vivo activity in HAND models, and is under investigation for further development.

MLK3 is a direct target of biochanin A, which plays a role in solar UV-induced COX-2 expression in human keratinocytes

Solar UV (sUV) is an important environmental carcinogen. Recent studies have shown that sUV is associated with numerous human skin disorders, such as wrinkle formation and inflammation. In this study, we found that the isoflavone, biochanin A, inhibited the expression of sUV-induced COX-2, which is a well-characterized sUV-induced enzyme, in both human HaCaT keratinocytes and JB6 P+ mouse skin epidermal cells. Several studies have demonstrated the beneficial effects of biochanin A. However, its direct molecular target is unknown. We found that biochanin A inhibited sUV-induced phosphorylation of MKK4/JNK/c-Jun and MKK3/6/p38/MSK1. Mixed-lineage kinase 3 (MLK3) is an upstream kinase of MKK4 and MKK3/6. Thus, we evaluated the effect of biochanin A on MLK3. We found that sUV-induced MLK3 phosphorylation was not affected, whereas MLK3 kinase activity was significantly suppressed by biochanin A. Furthermore, direct binding of biochanin A in the MLK3 ATP-binding pocket was detected using pull-down assays. Computer modeling supported our observation that MLK3 is a novel target of biochanin A. These results suggest that biochanin A exerts chemopreventive effects by suppressing sUV-induced COX-2 expression mediated through MLK3 inhibition.

HBx activates FasL and mediates HepG2 cell apoptosis through MLK3-MKK7-JNKs signal module

AIM

To investigate the possible mechanism by which hepatitis B virus X protein (HBx) mediates apoptosis of HepG2 cells.

METHODS

HBx expression vector pcDNA3.1-X was transfected into HepG2 cells to establish an HBx high-expression cellular model as pcDNA3.1-X transfected group. The pcDNA3.1-X and pSilencer3.1-shHBX (HBx antagonist) were cotransfected into HepG2 cells to establish an HBx low-expression model as RNAi group. Untransfected HepG2 cells and HepG2 cells transfected with negative control plasmid were used as controls. Apoptosis rate, the expression of Fas/FasL signaling pathway-related proteins and the phosphorylation levels of MLK3, MKK7 and JNKs, which are upstream molecules of death receptor pathways and belong to the family of mitogen-activated protein kinases (MAPKs), were measured in each group.

RESULTS

Compared with HepG2 cell group and RNAi group, apoptosis rate, the expression of Fas and FasL proteins, and the activation of MLK3, MKK7 and JNKs were increased in the pcDNA3.1-X transfected group. The activation of JNKs and expression of FasL protein were inhibited in the pcDNA3.1-X transfected group when treated with a known JNK inhibitor, SP600125. When authors treated pcDNA3.1-X transfected group with K252a, a known MLK3 inhibitor, the activation of MLK3, MKK7 and JNKs as well as expression of FasL protein was inhibited. Furthermore, cell apoptosis rate was also significantly declined in the presence of K252a in the pcDNA3.1-X transfected group.

CONCLUSION

HBx can induce HepG2 cell apoptosis via a novel active MLK3-MKK7-JNKs signaling module to upregulate FasL protein expression.

Saturated fatty acids induce c-Src clustering within membrane subdomains, leading to JNK activation

Saturated fatty acids (FA) exert adverse health effects and are more likely to cause insulin resistance and type 2 diabetes than unsaturated FA, some of which exert protective and beneficial effects. Saturated FA, but not unsaturated FA, activate Jun N-terminal kinase (JNK), which has been linked to obesity and insulin resistance in mice and humans. However, it is unknown how saturated and unsaturated FA are discriminated. We now demonstrate that saturated FA activate JNK and inhibit insulin signaling through c-Src activation. FA alter the membrane distribution of c-Src, causing it to partition into intracellular membrane subdomains, where it likely becomes activated. Conversely, unsaturated FA with known beneficial effects on glucose metabolism prevent c-Src membrane partitioning and activation, which are dependent on its myristoylation, and block JNK activation. Consumption of a diabetogenic high-fat diet causes the partitioning and activation of c-Src within detergent insoluble membrane subdomains of murine adipocytes.

Cloning and Initial Functional Characterization of Mlk4α and Mlk4β

We have cloned a novel human mixed-lineage kinase gene, MLK4. Two alternatively spliced forms, MLK4α (580 aa) and MLK4β (1036 aa), have been identified and mapped to chromosomal band 1q42. MLK4 shows high amino acid homology to the kinase catalytic domain of MLK3 (72%), MLK1 (71%) and MLK2 (69%). Strong expression of MLK4 was detected in the human pancreas and kidneys. pCMV-MLK4β c-myc-tagged protein (human) was expressed in the cytoplasm and nucleus of transiently transfected COS-1 cells, while pCMV-MLK4α c-myc-tagged protein (human) was expressed in cytoplasm only. Both MLK4 isoforms reduced the colony formation ability of MCF7 cells by 85%-95% and almost totally suppressed cell proliferation in the CyQUANT cell proliferation assay. Human pCMV-MLK4β transgenic mice expressed the MLK4β in all tissues examined but no phenotypic abnormalities were observed. Thus, in this work, we present the cloning and sequencing of MLK4α and MLK4β for the first time; the data obtained suggest that MLK4 may function as a MAP kinase.

The MAP kinase signaling cascades: a system of hundreds of components regulates a diverse array of physiological functions

Sequential activation of kinases within the mitogen-activated protein (MAP) kinase (MAPK) cascades is a common, and evolutionary-conserved mechanism of signal transduction. Four MAPK cascades have been identified in the last 20 years and those are usually named according to the MAPK components that are the central building blocks of each of the cascades. These are the extracellular signal-regulated kinase 1/2 (ERK1/2), c-Jun N-Terminal kinase (JNK), p38, and ERK5 cascades. Each of these cascades consists of a core module of three tiers of protein kinases termed MAPK, MAPKK, and MAP3K, and often two additional tiers, the upstream MAP4K and the downstream MAPKAPK, which can complete five tiers of each cascade in certain cell lines or stimulations. The transmission of the signal via each cascade is mediated by sequential phosphorylation and activation of the components in the sequential tiers. These cascades cooperate in transmitting various extracellular signals and thus control a large number of distinct and even opposing cellular processes such as proliferation, differentiation, survival, development, stress response, and apoptosis. One way by which the specificity of each cascade is regulated is through the existence of several distinct components in each tier of the different cascades. About 70 genes, which are each translated to several alternatively spliced isoforms, encode the entire MAPK system, and allow the wide array of cascade's functions. These components, their regulation, as well as their involvement together with other mechanisms in the determination of signaling specificity by the MAPK cascade is described in this review. Mis-regulation of the MAPKs signals usually leads to diseases such as cancer and diabetes; therefore, studying the mechanisms of specificity-determination may lead to better understanding of these signaling-related diseases.

Rebuilding synaptic architecture in HIV-1 associated neurocognitive disease: a therapeutic strategy based on modulation of mixed lineage kinase

Work from our laboratories has validated mixed lineage kinase type 3 (MLK3) as an enzyme pathologically activated in the CNS by human immunodeficiency virus 1 (HIV-1) neurotoxins. In this review, we discuss MLK3 activation in the context of the neuropathogenesis of HIV-1 associated neurocognitive deficits (HAND). We use findings from the literature to substantiate the neuropathologic relevance of MLK3 to neurodegenerative disease, with an emphasis on Parkinson's disease that shares a number of important phenotypic and neuropathologic characteristics with HAND. We discuss signal transduction pathways downstream from MLK3 activation, with an emphasis on their involvement in microglia and neurons in preclinical models of HAND. Finally, we make a case for pharmacologic intervention targeted at inhibition of MLK3 as a strategy to reverse HAND, in light of the fact that combination antiretroviral therapy, despite successfully managing systemic infection of HIV-1, has been largely unsuccessful in eradicating HAND.

Mixed Lineage Kinase 3 deficiency delays viral clearance in the lung and is associated with diminished influenza-induced cytopathic effect in infected cells

Influenza virus leads to acute respiratory disease resulting in seasonal epidemics and periodic pandemics. Little is known about the signaling events that regulate host defense to influenza. One particular pathway, the c-Jun amino-terminal kinase (JNK) cascade is activated following influenza infection and blocking JNK leads to enhanced viral replication. We hypothesize that Mixed Lineage Kinase 3 (MLK3), an upstream regulator of JNK, is involved in the host response to influenza. To test this, wild-type and MLK3-/- mice were infected with pathogenic strain of influenza A virus, A/PR/8/34 (PR8). Although, cellular and humoral immune responses were similar between wild-type and MLK3-/- hosts, the viral load in the lungs was comparatively higher in MLK3-/- mice at day 8 post-infection. Consistent with this, MLK3-/- murine lung fibroblast and epithelial cells had prolonged survival and increased virion production following infection compared to wild-type. These findings support a role for MLK3 in viral production during influenza infection.

Mixed lineage kinase 3 gene mutations in mismatch repair deficient gastrointestinal tumours

Mixed lineage kinase 3 (MLK3) is a serine/threonine kinase, regulating MAPkinase signalling, in which cancer-associated mutations have never been reported. In this study, 174 primary gastrointestinal cancers (48 hereditary and 126 sporadic forms) and 7 colorectal cancer cell lines were screened for MLK3 mutations. MLK3 mutations were significantly associated with MSI phenotype in primary tumours (P = 0.0005), occurring in 21% of the MSI carcinomas. Most MLK3 somatic mutations identified were of the missense type (62.5%) and more than 80% of them affected evolutionarily conserved residues. A predictive 3D model points to the functional relevance of MLK3 missense mutations, which cluster in the kinase domain. Further, the model shows that most of the altered residues in the kinase domain probably affect MLK3 scaffold properties, instead of its kinase activity. MLK3 missense mutations showed transforming capacity in vitro and cells expressing the mutant gene were able to develop locally invasive tumours, when subcutaneously injected in nude mice. Interestingly, in primary tumours, MLK3 mutations occurred in KRAS and/or BRAF wild-type carcinomas, although not being mutually exclusive genetic events. In conclusion, we have demonstrated for the first time the presence of MLK3 mutations in cancer and its association to mismatch repair deficiency. Further, we demonstrated that MLK3 missense mutations found in MSI gastrointestinal carcinomas are functionally relevant.

Genetically engineered mouse models in drug discovery research

Genetically modified mouse models have been proven to be a powerful tool in drug discovery. The ability to genetically modify the mouse genome by removing or replacing a specific gene has enhanced our ability to identify and validate target genes of interest. In addition, many human diseases can be mimicked in the mouse and signaling pathways have been shown to be conserved. In spite of these advantages the technology has limitations. In transgenic animals there may be significant heterogeneity among different founders. In knock-out animals the predicted phenotypes are not always readily observed and occasionally a completely novel and unexpected phenotype emerges. To address the latter and ensure that a deep knowledge of the target of interest is obtained, we have developed a comprehensive phenotyping program which has identified novel phenotypes as well as any potential safety concerns which may be associated with a particular target. Finally we continue to explore innovative technologies as they become available such as RNAi for temporal and spatial gene knock-down and humanized models that may better simulate human disease states.

Mixed lineage kinase phosphorylates transcription factor E47 and inhibits TrkB expression to link neuronal death and survival pathways

E47 is a basic helix-loop-helix transcription factor involved in neuronal differentiation and survival. We had previously shown that the basic helix-loop-helix protein E47 binds to E-box sequences within the promoter of the TrkB gene and activates its transcription. Proper expression of the TrkB receptor plays a key role in development and function of the vertebrate nervous system, and altered levels of TrkB have been associated with important human diseases. Here we show that E47 interacts with MLK2, a mixed lineage kinase (MLK) involved in JNK-mediated activation of programmed cell death. MLK2 enhances phosphorylation of the AD2 activation domain of E47 in vivo in a JNK-independent manner and phosphorylates in vitro defined serine and threonine residues within a loop-helix structure of AD2 that also contains a putative MLK docking site. Although these residues are essential for MLK2-mediated inactivation of E47, inhibition of MLKs by CEP11004 causes up-regulation of TrkB at a transcriptional level in cerebellar granule neurons and differentiating neuroblastoma cells. These findings allow us to propose a novel mechanism by which MLK regulates TrkB expression through phosphorylation of an activation domain of E47. This molecular link would explain why MLK inhibitors not only prevent activation of cell death processes but also enhance cell survival signaling as a key aspect of their neuroprotective potential.

ZAK negatively regulates RhoGDIbeta-induced Rac1-mediated hypertrophic growth and cell migration

RhoGDIbeta, a Rho GDP dissociation inhibitor, induced hypertrophic growth and cell migration in a cultured cardiomyoblast cell line, H9c2. We demonstrated that RhoGDIbeta plays a previously undefined role in regulating Rac1 expression through transcription to induce hypertrophic growth and cell migration and that these functions are blocked by the expression of a dominant-negative form of Rac1. We also demonstrated that knockdown of RhoGDIbeta expression by RNA interference blocked RhoGDIbeta-induced Rac1 expression and cell migration. We demonstrated that the co-expression of ZAK and RhoGDIbeta in cells resulted in an inhibition in the activity of ZAK to induce ANF expression. Knockdown of ZAK expression in ZAK-RhoGDIbeta-expressing cells by ZAK-specific RNA interference restored the activities of RhoGDIbeta.

CEP-1347 reduces mutant huntingtin-associated neurotoxicity and restores BDNF levels in R6/2 mice

Huntington's disease (HD) is a devastating neurodegenerative disorder caused by an expanded polyglutamine repeat within the protein Huntingtin (Htt). We previously reported that mutant Htt expression activates the ERK1/2 and JNK pathways [Apostol, B.L., Illes, K., Pallos, J., Bodai, L., Wu, J., Strand, A., Schweitzer, E.S., Olson, J.M., Kazantsev, A., Marsh, J.L., Thompson, L.M., 2006. Mutant huntingtin alters MAPK signaling pathways in PC12 and striatal cells: ERK1/2 protects against mutant huntingtin-associated toxicity. Hum. Mol. Genet. 15, 273-285]. Chemical and genetic modulation of these pathways promotes cell survival and death, respectively. Here we test the ability of two closely related compounds, CEP-11004 and CEP-1347, which inhibit Mixed Lineage Kinases (MLKs) and are neuroprotective, to suppress mutant Htt-mediated pathogenesis in multiple model systems. CEP-11004/CEP-1347 treatment significantly decreased toxicity in mutant Htt-expressing cells that evoke a strong JNK response. However, suppression of cellular dysfunction in cell lines that exhibit only mild Htt-associated toxicity and little JNK activation was associated with activation of ERK1/2. These compounds also reduced neurotoxicity in immortalized striatal neurons from mutant knock-in mice and Drosophila expressing a mutant Htt fragment. Finally, CEP-1347 improved motor performance in R6/2 mice and restored expression of BDNF, a critical neurotrophic factor that is reduced in HD. These studies suggest a novel therapeutic approach for a currently untreatable neurodegenerative disease, HD, via CEP-1347 up-regulation of BDNF.

Metabolic stress signaling mediated by mixed-lineage kinases

Saturated free fatty acid (FFA) is a major source of metabolic stress that activates the c-Jun NH(2)-terminal kinase (JNK). This FFA-stimulated JNK pathway is relevant to hallmarks of metabolic syndrome, including insulin resistance. Here we used gene ablation studies in mice to demonstrate a central role for mixed-lineage protein kinases (MLK) in this signaling pathway. Saturated FFA causes protein kinase C (PKC)-dependent activation of MLK3 that subsequently causes increased JNK activity by a mechanism that requires the MAP kinase kinases MKK4 and MKK7. Loss of PKC, MLK3, MKK4, or MKK7 expression prevents FFA-stimulated JNK activation. Together, these data establish a signaling pathway that mediates effects of metabolic stress on insulin resistance.