Mek2 is dispensable for mouse growth and development

Mek1/2 gene dosage determines tissue response to oncogenic Ras signaling in the skin

Ras genes are commonly mutated in human cancers of the skin and other tissues. Oncogenic Ras signals through multiple effector pathways, including the Erk1/2 mitogen-activated protein kinase (MAPK), phosphatidylinositol-3 kinase (PI3K) and the Ral guanine nucleotide exchange factor (RalGEF) cascades. In epidermis, the activation of oncogenic Ras induces hyperplasia and inhibits differentiation, features characteristic of squamous cell carcinoma. The downstream effector pathways required for oncogenic Ras effects in epidermis, however, are undefined. In this study, we investigated the direct contribution of Mek1 and Mek2 MAPKKs to oncogenic Ras signaling. The response of murine epidermis to conditionally active oncogenic Ras was unimpaired by deletion of either Mek1 or Mek2 MAPKKs individually. In contrast, Ras effects were entirely abolished by combined deletion of all Mek1/2 alleles, whereas epidermis retaining only one allele of either Mek1 or Mek2 showed intermediate responsiveness. Thus, the effects of oncogenic Ras on proliferation and differentiation in skin show a gene dosage-dependent requirement for the Erk1/2 MAPK cascade at the level of Mek1/2 MAPKKs.

Noonan, Costello and cardio–facio–cutaneous syndromes: dysregulation of the Ras–MAPK pathway

A class of developmental disorders caused by dysregulation of the Ras-induced mitogen-activated protein kinase (MAPK) cascade (the Ras–MAPK pathway) has emerged. Three of these disorders – Noonan, Costello and cardio–facio–cutaneous syndromes – have overlapping phenotypic features characterised by distinctive facial dysmorphia, cardiac defects, musculoskeletal and cutaneous abnormalities, and neurocognitive delay. The germline mutations associated with these disorders are in genes that encode proteins of the Ras–MAPK pathway. In vitro studies have determined that the overwhelming majority of these mutations result in increased signal transduction down the pathway, but usually to a lesser degree than somatic mutations in the same genes that are associated with cancer. The Ras–MAPK pathway is essential in the regulation of the cell cycle, differentiation, growth and senescence, so it is not surprising that germline mutations that affect its function have profound effects on development. Here we review the clinical consequences of the known molecular lesions associated with Noonan syndrome, Costello syndrome and cardio–facio–cutaneous syndrome, and explore possible therapeutic modalities for treatment.

IGF signaling defects as causes of growth failure and IUGR

A substantial portion of children born small for gestational age (SGA) fail to catch up height, despite normal or even elevated insulin-like growth factor (IGF1) serum levels. In most cases, the etiology of the apparent IGF1 resistance is regarded as idiopathic. However, the recent identification of human IGF1 and IGF1 receptor (IGF1R) mutations, as well as information obtained from transgenic animals, points to a strong genetic component being of pivotal importance in the development of growth retardation. These findings direct attention to molecules downstream of the IGF1R, which have both growth-promoting and, to a lesser extent, metabolic functions. Therefore, defects in these molecules are likely to participate in the etiology of human SGA.

ERK1 and ERK2 MAPK are key regulators of distinct gene sets in zebrafish embryogenesis

Background

The MAPK signaling proteins are involved in many eukaryotic cellular processes and signaling networks. However, specific functions of most of these proteins in vertebrate development remain elusive because of potential redundancies. For instance, the upstream activation pathways for ERK1 and ERK2 are highly similar, and also many of their known downstream targets are common. In contrast, mice and zebrafish studies indicate distinct roles for both ERKs in cellular proliferation, oncogenic transformation and development. A major bottleneck for further studies is that relatively little is known of in vivo downstream signaling specific for these kinases.

Results

Microarray based gene expression profiling of ERK1 and ERK2 knockdown zebrafish embryos at various stages of early embryogenesis resulted in specific gene expression signature sets that showed pronounced differences in gene ontology analyses. In order to predict functions of these genes, zebrafish specific in silico signaling pathways involved in early embryogenesis were constructed using the GenMAPP program. The obtained transcriptome signatures were analyzed in the BMP, FGF, Nodal and Wnt pathways. Predicted downstream effects of ERK1 and ERK2 knockdown treatments on key pathways responsible for mesendoderm development were confirmed by whole mount in situ hybridization experiments.

Conclusion

The gene ontology analyses showed that ERK1 and ERK2 target common and distinct gene sets, confirming the difference in knockdown phenotypes and diverse roles for these kinases during embryogenesis. For ERK1 we identified specific genes involved in dorsal-ventral patterning and subsequent embryonic cell migration. For ERK2 we identified genes involved in cell-migration, mesendoderm differentiation and patterning.

The specific function of ERK2 in the initiation, maintenance and patterning of mesoderm and endoderm formation was biologically confirmed.

The small molecule Mek1/2 inhibitor U0126 disrupts the chordamesoderm to notochord transition in zebrafish

Background

Key molecules involved in notochord differentiation and function have been identified through genetic analysis in zebrafish and mice, but MEK1 and 2 have so far not been implicated in this process due to early lethality (Mek1-/-) and functional redundancy (Mek2-/-) in the knockout animals.

Results

Here, we reveal a potential role for Mek1/2 during notochord development by using the small molecule Mek1/2 inhibitor U0126 which blocks phosphorylation of the Mek1/2 target gene Erk1/2 in vivo. Applying the inhibitor from early gastrulation until the 18-somite stage produces a specific and consistent phenotype with lack of dark pigmentation, shorter tail and an abnormal, undulated notochord. Using morphological analysis, in situ hybridization, immunhistochemistry, TUNEL staining and electron microscopy, we demonstrate that in treated embryos the chordamesoderm to notochord transition is disrupted and identify disorganization in the medial layer of the perinotochordal basement mebrane as the probable cause of the undulations and bulges in the notochord. We also examined and excluded FGF as the upstream signal during this process.

Conclusion

Using the small chemical U0126, we have established a novel link between MAPK-signaling and notochord differentiation. Our phenotypic analysis suggests a potential connection between the MAPK-pathway, the COPI-mediated intracellular transport and/or the copper-dependent posttranslational regulatory processes during notochord differentiation.

MAP kinase: it's been longer than fifteen minutes

The review highlights evidence for different functions in the cell cycle of the two MAP kinase kinases, MEK1 and MEK2, and the two MAP kinases, ERK1 and ERK2. Functional differences may explain why instances of cell cycle arrest can be MEK1 or MEK2 dependent.

Identification of extracellular signal-regulated kinase 1/2 and p38 MAPK as regulators of human sperm motility and acrosome reaction and as predictors of poor spermatozoan quality

Mature spermatozoa acquire progressive motility only after ejaculation. Their journey in the female reproductive tract also includes suppression of progressive motility, reactivation, capacitation, and hyperactivation of motility (whiplash), the mechanisms of which are obscure. MAPKs are key regulatory enzymes in cell signaling, participating in diverse cellular functions such as growth, differentiation, stress, and apoptosis. Here we report that ERK1/2 and p38 MAPK are primarily localized to the tail of mature human spermatozoa. Surprisingly, c-Jun N-terminal kinase 1/2, which is thought to be ubiquitously expressed, could not be detected in mature human spermatozoa. ERK1/2 stimulation is downstream to protein kinase C (PKC) activation, which is also present in the human sperm tail (PKCbetaI and PKCepsilon). ERK1/2 stimulates and p38 inhibits forward and hyperactivated motility, respectively. Both ERK1/2 and p38 MAPK are involved in the acrosome reaction. Using a proteomic approach, we identified ARHGAP6, a RhoGAP, as an ERK substrate in PMA-stimulated human spermatozoa. Inverse correlation was obtained between the relative expression level of ERK1 or the relative activation level of p38 and sperm motility, forward progression motility, sperm morphology, and viability. Therefore, increased expression of ERK1 and activated p38 can predict poor human sperm quality.

Biochemical characterization of novel germline BRAF and MEK mutations in cardio-facio-cutaneous syndrome

Cardio-facio-cutaneous syndrome (CFC) is a sporadic, complex developmental disorder involving characteristic craniofacial features, cardiac defects, ectodermal abnormalities, growth deficiency, hypotonia, and developmental delay. CFC is caused by alteration of activity through the mitogen-activated protein kinase (MAPK) pathway due to heterogeneous de novo germline mutations in B-Raf mutant proteins, MEK1 and MEK2. Approximately 75% of individuals with CFC have mutations in BRAF. In vitro functional studies demonstrate that many of these mutations confer increase activity upon the mutant protein as compared to the wildtype protein. However, as is seen cancer, some of the B-Raf mutant proteins are kinase impaired. Western blot analyses corroborate kinase assays as determined by mutant proteins phosphorylating downstream effectors MEK and ERK. Approximately 25% of individuals with CFC have mutations in either MEK1 or MEK2 that lead to increased MEK kinase activity as judged by increased phosphorylation of its downstream effector ERK. Unlike BRAF, no somatic mutations have ever been identified in MEK genes. The identification of novel germline BRAF and MEK mutations in CFC will help understand the pathophysiology of this syndrome. Furthermore, it will also provide insight to the normal function of B-Raf and MEK, and contribute to the knowledge of the role of the MAPK pathway in cancer. Since the MAPK pathway has been studied intensively in the context of cancer, numerous therapeutics that specifically target this pathway may merit investigation in this population of patients.

MEK1 and MEK2 regulate distinct functions by sorting ERK2 to different intracellular compartments

In this study, we provide novel insight into the mechanism of how ERK2 can be sorted to different intracellular compartments and thereby mediate different responses. MEK1-activated ERK2 accumulated in the nucleus and induced proliferation. Conversely, MEK2-activated ERK2 was retained in the cytoplasm and allowed survival. Localization was a determinant for ERK2 functions since MEK1 switched from providing proliferation to be a mediator of survival when ERK2 was routed to the cytoplasm by the attachment of a nuclear export site. MEK1-mediated ERK2 nuclear translocation and proliferation were shown to depend on phosphorylation of S298 and T292 sites in the MEK1 proline-rich domain. These sites are phosphorylated on cellular adhesion in MEK1 but not MEK2. Whereas p21-activated kinase phosphorylates S298 and thus enhances the MEK1-ERK2 association, ERK2 phosphorylates T292, leading to release of active ERK2 from MEK1. On the basis of these results, we propose that the requirement of adhesion for cells to proliferate in response to growth factors, in part, may be explained by the MEK1 S298/T292 control of ERK2 nuclear translocation. In addition, we suggest that ERK2 intracellular localization determines whether growth factors mediate proliferation or survival and that the sorting occurs in an adhesion-dependent manner.

Comparative analysis of Hoxa5 allelic series

Analysis of the Hoxa5(-/-) mutants has revealed the critical role of Hoxa5 in survival, specification of axial identity, and ontogeny of organs, including the respiratory tract. The presence of the selection cassette in the original Hoxa5(-/-) mutation may interfere with the interpretation of the phenotypes. To circumvent this aspect and to bypass the lethality of the Hoxa5 mutation, we have designed a conditional approach and generated Hoxa5 allelic variants. The conditional allele (Hoxa5(floxed)) behaves as a wild-type allele. In contrast, both the Hoxa5(Delta) and the Hoxa5(floxneo) alleles are characterized by the loss of the functional transcript and protein, the lethality due to lung defects and the skeletal homeotic transformations similar to those of the Hoxa5(-/-) mutants. Analysis of neighboring Hox gene expression patterns in the Hoxa5 mutants produced further confirmed that the Hoxa5 allelic variants are true null alleles.

Mek1/2 MAPK kinases are essential for Mammalian development, homeostasis, and Raf-induced hyperplasia

The p42/p44 mitogen-activated protein kinase (MAPK) cascade includes Ras, Raf, Mek, and Erk MAPK. To determine the effect of a full knockout at a single level of this signaling pathway in mammals, and to investigate functional redundancy between Mek1 and Mek2, we disrupted these genes in murine and human epidermis. Loss of either protein alone produced no phenotype, whereas combined Mek1/2 deletion in development or adulthood abolished Erk1/2 phosphorylation and led to hypoproliferation, apoptosis, skin barrier defects, and death. Conversely, a single copy of either allele was sufficient for normal development. Combined Mek1/2 loss also abolished Raf-induced hyperproliferation. Human tissue deficient in either Mek isoform was normal, whereas loss of both proteins led to hypoplasia, which was rescued by active Erk2 expression. These data indicate that Mek1/2 are functionally redundant in the epidermis, where they act as a linear relay in the MAPK pathway to mediate development and homeostasis.

ERK1 and ERK2 mitogen-activated protein kinases affect Ras-dependent cell signaling differentially

Background

The mitogen-activated protein (MAP) kinases p44^ERK1 and p42^ERK2 are crucial components of the regulatory machinery underlying normal and malignant cell proliferation. A currently accepted model maintains that ERK1 and ERK2 are regulated similarly and contribute to intracellular signaling by phosphorylating a largely common subset of substrates, both in the cytosol and in the nucleus.

Results

Here, we show that ablation of ERK1 in mouse embryo fibroblasts and NIH 3T3 cells by gene targeting and RNA interference results in an enhancement of ERK2-dependent signaling and in a significant growth advantage. By contrast, knockdown of ERK2 almost completely abolishes normal and Ras-dependent cell proliferation. Ectopic expression of ERK1 but not of ERK2 in NIH 3T3 cells inhibits oncogenic Ras-mediated proliferation and colony formation. These phenotypes are independent of the kinase activity of ERK1, as expression of a catalytically inactive form of ERK1 is equally effective. Finally, ectopic expression of ERK1 but not ERK2 is sufficient to attenuate Ras-dependent tumor formation in nude mice.

Conclusion

These results reveal an unexpected interplay between ERK1 and ERK2 in transducing Ras-dependent cell signaling and proliferation. Whereas ERK2 seems to have a positive role in controlling normal and Ras-dependent cell proliferation, ERK1 probably affects the overall signaling output of the cell by antagonizing ERK2 activity.

Raf kinase signaling functions in sensory neuron differentiation and axon growth in vivo

To define the role of the Raf serine/threonine kinases in nervous system development, we conditionally targeted B-Raf and C-Raf, two of the three known mammalian Raf homologs, using a mouse line expressing Cre recombinase driven by a nestin promoter. Targeting of B-Raf, but not C-Raf, markedly attenuated baseline phosphorylation of Erk in neural tissues and led to growth retardation. Conditional elimination of B-Raf in dorsal root ganglion (DRG) neurons did not interfere with survival, but instead caused marked reduction in expression of the glial cell line-derived neurotrophic factor receptor Ret at postnatal stages, associated with a profound reduction in levels of transcription factor CBF-beta. Elimination of both alleles of Braf, which encodes B-Raf, and one allele of Raf1, which encodes C-Raf, affected DRG neuron maturation as well as proprioceptive axon projection toward the ventral horn in the spinal cord. Finally, conditional elimination of all Braf and Raf1 alleles strongly reduced neurotrophin-dependent axon growth in vitro as well as cutaneous axon terminal arborization in vivo. We conclude that Raf function is crucial for several aspects of DRG neuron development, including differentiation and axon growth.

MKK signaling and vascularization

In 1998, George Vande Woude's lab discovered that anthrax lethal factor (LF), the principal virulence component of anthrax toxin, was a zinc-metalloprotease that cleaved and inactivated mitogen-activated protein kinase kinases (MKK). It was perhaps not surprising, given the known roles of MKK1 and 2 in cell proliferation, that LF was subsequently found to dramatically inhibit tumor growth in vivo. What was not anticipated, however, was that the tumors treated with LF would have a substantially reduced vascular content. This intriguing result was one of the first indications that MKK signaling plays an important role in promoting tumor vascularization in vivo. In the following short review, we will compare in vitro and in vivo evidence that supports the hypothesis that MKK signaling pathways are essential for vascularization.

MEK1 activation by PAK: a novel mechanism

Extracellular signal-Regulated Kinase (ERK) controls a variety of cellular processes, including cell proliferation and cell motility. While oncogenic mutations in Ras and B-Raf result in deregulated ERK activity and proliferation and migration in some tumor cells, other tumors exhibit elevated ERK signaling in the absence of these mutations. Here we provide evidence that PAK can directly activate MEK1 by a mechanism distinct from conventional Ras/Raf mediated activation. We find that PAK phosphorylation of MEK1 serine 298 stimulates MEK1 autophosphorylation on the activation loop, and activation of MEK1 activity towards ERK in in vitro reconstitution experiments. Serines 218 and/or 222 in the MEK1 activation loop are required for PAK-stimulated MEK1 activity towards ERK. MEK2, which is a poor target for PAK phosphorylation in cells, is not activated in this manner. Tissue culture experiments verify that this mechanism is used in suspended fibroblasts expressing mutationally activated PAK1. We speculate that aberrant signaling through PAK may directly induce anchorage-independent MEK1 activation in tumor cells lacking oncogenic Ras or Raf mutations, and that this mechanism may contribute to localized MEK signaling in focal contacts and adhesions during cell adhesion or migration.

In vivo functions of mitogen-activated protein kinases: conclusions from knock-in and knock-out mice

Multicellular organisms achieve intercellular communication by means of signalling molecules whose effect on the target cell is mediated by signal transduction pathways. Such pathways relay, amplify and integrate signals to elicit appropriate biological responses. Protein kinases form crucial intermediate components of numerous signalling pathways. One group of protein kinases, the mitogen-activated protein kinases (MAP kinases) are kinases involved in signalling pathways that respond primarily to mitogens and stress stimuli. In vitro studies revealed that the MAP kinases are implicated in several cellular processes, including cell division, differentiation, cell survival/apoptosis, gene expression, motility and metabolism. As such, dysfunction of specific MAP kinases is associated with diseases such as cancer and immunological disorders. However, the genuine in vivo functions of many MAP kinases remain elusive. Genetically modified mouse models deficient in a specific MAP kinase or expressing a constitutive active or a dominant negative variant of a particular MAP kinase offer valuable tools for elucidating the biological role of these protein kinases. In this review, we focus on the current status of MAP kinase knock-in and knock-out mouse models and their phenotypes. Moreover, examples of the application of MAP kinase transgenic mice for validating therapeutic properties of specific MAP kinase inhibitors, and for investigating the role of MAP kinase in pathogen-host interactions will be discussed.

The MEK/ERK cascade: from signaling specificity to diverse functions

The ERK signaling cascade is a central MAPK pathway that plays a role in the regulation of various cellular processes such as proliferation, differentiation, development, learning, survival and, under some conditions, also apoptosis. The ability of this cascade to regulate so many distinct, and even opposing, cellular processes, raises the question of signaling specificity determination by this cascade. Here we describe mechanisms that cooperate to direct MEK-ERK signals to their appropriate downstream destinations. These include duration and strength of the signals, interaction with specific scaffolds, changes in subcellular localization, crosstalk with other signaling pathways, and presence of multiple components with distinct functions in each tier of the cascade. Since many of the mechanisms do not function properly in cancer cells, understanding them may shed light not only on the regulation of normal cell proliferation, but also on mechanisms of oncogenic transformation.

MAPK signalling: ERK5 versus ERK1/2

Extracellular-signal-regulated kinase 5 (ERK5) is a member of the mitogen-activated protein kinase (MAPK) family and, similar to ERK1/2, has the Thr-Glu-Tyr (TEY) activation motif. Both ERK5 and ERK1/2 are activated by growth factors and have an important role in the regulation of cell proliferation and cell differentiation. Moreover, both the ERK5 and the ERK1/2 pathways are sensitive to PD98059 and U0126, which are two well-known inhibitors of the ERK pathway. Despite these similarities, recent studies have revealed distinctive features of the ERK5 pathway: ERK5 has a key role in cardiovascular development and neural differentiation; ERK5 nuclear translocation is controlled by its own nuclear localizing and nuclear export activities; and the carboxy-terminal half of ERK5, which follows its kinase catalytic domain, has a unique function.

Requirement for Map2k1 (Mek1) in extra-embryonic ectoderm during placentogenesis

Map2k1(-/-) embryos die at mid-gestation from abnormal development and hypovascularization of the placenta. We now show that this phenotype is associated with a decreased labyrinth cell proliferation and an augmented cell apoptosis. Although the activation of MAP2K1 and MAP2K2 is widespread in the labyrinthine region, MAPK1 and MAPK3 activation is restricted to the cells lining the maternal sinuses, suggesting an important role for the ERK/MAPK cascade in these cells. In Map2k1(-/-) placenta, ERK/MAPK cascade activation is perturbed. Abnormal localization of the syncytiotrophoblasts is also observed in Map2k1(-/-) placenta, even though this cell lineage is specified at the correct time during placentogenesis. The placental phenotype can be rescued in tetraploid experiments. In addition, Map2k1-specific deletion in the embryo leads to normal embryo development and to the birth of viable Map2k1(-/-) mice. Altogether, these data enlighten the essential role of Map2k1 in extra-embryonic ectoderm during placentogenesis. In the embryo, the Map2k1 gene function appears dispensable.

Negative regulation of the alpha interferon-induced antiviral response by the Ras/Raf/MEK pathway

Interferon (IFN) is one of the molecules released by virus-infected cells, resulting in the establishment of an antiviral state within infected and neighboring cells. IFN-induced antiviral response may be subject to modulation by the cellular signaling environment of host cells which impact the effectiveness of viral replication. Here, we show that cells with an activated Ras/Raf/MEK signaling cascade allow propagation of viruses in the presence of IFN. Ras-transformed (RasV12) and vector control NIH 3T3 cells were infected with vesicular stomatitis virus (VSV) or an IFN-sensitive vaccinia virus (delE3L) in the presence of alpha interferon. While IFN protected vector control cells from infection by both viruses, RasV12 cells were susceptible to viral infection regardless of the presence of IFN. IFN sensitivity was restored in RasV12 cells upon RNA interference (RNAi) knockdown of Ras. We further investigated which elements downstream of Ras are responsible for counteracting IFN-induced antiviral responses. A Ras effector domain mutant that can only stimulate the Raf kinase family of effectors was able to suppress the IFN response and allow VSV replication. IFN-induced antiviral mechanisms were also restored in RasV12 cells by treatment with a MEK inhibitor (U0126 or PD98059). Moreover, by using RNAi to MEK1 and MEK2, we determined that MEK2, rather than MEK1, is responsible for suppression of the IFN response. In conclusion, our results suggest that activation of the Ras/Raf/MEK pathway downregulates IFN-induced antiviral response.

The multiple activities of CtBP/BARS proteins: the Golgi view

The C terminal-binding protein (CtBP) family functions in the nucleus as co-repressors of transcription and has a crucial role in differentiation, apoptosis, oncogenesis and development. Recently, the products of the CtBP1 gene have been implicated in important cytoplasmic functions, including membrane fission in intracellular trafficking, the partitioning of the Golgi complex during mitosis and the organization of ribbon synapses. This has led to a redefinition of the CtBPs as multifunctional proteins. Shuttling of CtBPs between the nucleus and the cytoplasm can be finely regulated by post-translational modifications. In addition, the structural homology with the dehydrogenase family of proteins and the ability of CtBPs to bind NAD(+) and acyl-CoAs have offered clues to the molecular mechanisms that enable these proteins to have different functions. Here, we discuss the cytoplasmic roles of the CtBPs and the possible mechanisms that enable them to switch between cell compartments and multiple functions.

Germline mutations in genes within the MAPK pathway cause cardio-facio-cutaneous syndrome

Cardio-facio-cutaneous (CFC) syndrome is a sporadic developmental disorder involving characteristic craniofacial features, cardiac defects, ectodermal abnormalities, and developmental delay. We demonstrate that heterogeneous de novo missense mutations in three genes within the mitogen-activated protein kinase (MAPK) pathway cause CFC syndrome. The majority of cases (18 out of 23) are caused by mutations in BRAF, a gene frequently mutated in cancer. Of the 11 mutations identified, two result in amino acid substitutions that occur in tumors, but most are unique and suggest previously unknown mechanisms of B-Raf activation. Furthermore, three of five individuals without BRAF mutations had missense mutations in either MEK1 or MEK2, downstream effectors of B-Raf. Our findings highlight the involvement of the MAPK pathway in human development and will provide a molecular diagnosis of CFC syndrome.

Mitogen-activated protein kinases, adherens junction dynamics, and spermatogenesis: a review of recent data

Mitogen-activated protein kinases (MAPKs) are important regulators of many cellular processes. In mammalian testes, these kinases are involved in controlling cell division, differentiation, survival and death, and are therefore critical to spermatogenesis. Recent studies have also illustrated their involvement in junction restructuring in the seminiferous epithelium, especially at the ectoplasmic specialization (ES), a testis-specific adherens junction (AJ) type. ES contributes to the adhesion between Sertoli cells at the blood-testis barrier, as well as between Sertoli and developing spermatids (step 9 and beyond) at the adluminal compartment. MAPKs regulate AJ dynamics in the testis via their effects on the turnover of junction-associated protein complexes, the production of proteases and protease inhibitors, and the cytoskeleton structure. In this review, roles of the three major MAPK members, namely extracellular signal-regulated kinase (ERK), c-Jun N-terminal kinase (JNK), and p38 MAPK, in ES dynamics are critically discussed. An integrated model of how these three MAPKs regulate adhesion function in the seminiferous epithelium is also presented. This model will serve as the framework for future investigation in the field.

IQGAP1 is a scaffold for mitogen-activated protein kinase signaling

IQGAP1 modulates many cellular functions such as cell-cell adhesion, transcription, cytoskeletal architecture, and selected signaling pathways. We previously documented that IQGAP1 binds extracellular signal-regulated kinase (ERK) 2 and regulates growth factor-stimulated ERK activity. Here we show that MEK, the molecule immediately upstream of ERK in the Ras/mitogen-activated protein (MAP) kinase signaling cascade, also interacts directly with IQGAP1. Both MEK1 and MEK2 bound IQGAP1 in vitro and coimmunoprecipitated with IQGAP1. The addition of ERK2 enhanced by fourfold the in vitro interaction of MEK2 with IQGAP1 without altering binding of MEK1. Similarly, ERK1 promoted MEK binding to IQGAP1, but either MEK protein altered the association between IQGAP1 and ERK. Epidermal growth factor (EGF) differentially regulated binding, enhancing MEK1 interaction while reducing MEK2 binding to IQGAP1. In addition, both knockdown and overexpression of IQGAP1 reduced EGF-stimulated activation of MEK and ERK. Analyses with selective IQGAP1 mutant constructs indicated that MEK binding is crucial for IQGAP1 to modulate EGF activation of ERK. Our data strongly suggest that IQGAP1 functions as a molecular scaffold in the Ras/MAP kinase pathway.

Effects of active MEK1 expression in vivo

Cell transformation is often a result of constitutive activation of genes in signaling pathways that regulate cell proliferation and differentiation. Indeed, the Ras/Raf/MEK/ERK mitogen-activated protein kinase (MAPK) signaling pathway is constitutively activated in a large number of cancers. The extent to which a single-gene mutation can alter cell fate, however, remains questionable. In vitro studies have addressed this issue, but organs are comprised of multiple cell types, and in vitro models often poorly approximate these interactions. In response to these limitations, cell-type specific mouse models have been generated as a means to examine the effect of altering a single element of the MAPK pathway in vivo. This review summarizes data from transgenic murine and human tissue models expressing constitutive active forms of MEK1.

Mek1 alters epidermal growth and differentiation

The highly homologous kinases, Mek1 and Mek2, act downstream of Ras and Raf to activate extracellular signal-regulated kinase (ERK) mitogen-activated protein kinases. In epidermis, Ras and Raf promote hyperplasia; however, they act on multiple Mek-independent effectors, and the extent to which Meks can mediate these effects is unknown. To address this, we expressed inducible Meks in transgenic murine and human epidermis. Both Mek1 and Mek2 triggered ERK phosphorylation. Only Mek1, however, recapitulated Ras/Raf effects in increasing proliferation and integrin expression while suppressing differentiation, which are impacts characteristic of epidermal neoplasia. Furthermore, a kinase-dead Mek1 mutant incapable of phosphorylating ERK proteins retained ability to mediate Mek1-driven epidermal proliferation. Mek1 is thus sufficient to promote the proliferative epithelial phenotype in a manner independent of intact kinase function.

Mitogen-Activated Protein Kinase Dynamics During the Meiotic G2/MI Transition of Mouse Spermatocytes1

Cellular and genetic approaches were used to investigate the requirements for activation during spermatogenesis of the extracellular signal-regulated protein kinases (ERKs), more commonly known as the mitogen-activated protein kinases (MAPKs). The MAPKS and their activating kinases, the MEKs, are expressed in specific developmental patterns. The MAPKs and MEK2 are expressed in all premeiotic germ cells and spermatocytes, while MEK1 is not expressed abundantly in pachytene spermatocytes. Phosphorylated (active) variants of these kinases are diminished in pachytene spermatocytes. Treatment of pachytene spermatocytes with okadaic acid (OA), to induce transition from meiotic prophase to metaphase I (G2/MI), resulted in phosphorylation and enzymatic activation of ERK1/2. However, U0126, an inhibitor of the ERK-activating kinases, MEK1/2, did not inhibit OA-induced MAPK activation or chromosome condensation. Analysis of spermatocytes lacking MOS, a mitogen-activated protein kinase kinase kinase responsible for MEK and MAPK activation, revealed that MOS is not required for OA-induced activation of the MAPKs. OA-induced MAPK activation was inhibited by butyrolactone I, an inhibitor of cyclin-dependent kinases 1 and 2 (CDK1, CDK2); thus, these kinases may regulate MAPK activity. Additionally, spermatocytes lacking CDC25C condensed bivalent chromosomes and activated both MPF and MAPKs in response to OA treatment; therefore, there is a CDC25C-independent pathway for MPF and MAPK activation. These studies reveal that spermatocytes do not require either MOS or CDC25C for onset of the meiotic division phase or for activation of MPF and the MAPKs, thus implicating a novel pathway for activation of the ERK1/2 MAPKs in spermatocytes.

MEK1 and MEK2, different regulators of the G1/S transition

The ERK cascade is activated by hormones, cytokines, and growth factors that result in either proliferation or growth arrest depending on the duration and intensity of the ERK activation. Here we provide evidence that the MEK1/ERK module preferentially provides proliferative signals, whereas the MEK2/ERK module induces growth arrest at the G1/S boundary. Depletion of either MEK subtype by RNA interference generated a unique phenotype. The MEK1 knock down led to p21cip1 induction and to the appearance of cells with a senescence-like phenotype. Permanent ablation of MEK1 resulted in reduced colony formation potential, indicating the importance of MEK1 for long term proliferation and survival. MEK2 deficiency, in contrast, was accompanied by a massive induction of cyclin D expression and, thus, CDK4/6 activation followed by nucleophosmin hyperphosphorylation and centrosome over-amplification. Our results suggest that the two MEK subtypes have distinct ways to contribute to a regulated ERK activity and cell cycle progression.

Mitogen-activated protein kinases in apoptosis regulation

Cells are continuously exposed to a variety of environmental stresses and have to decide 'to be or not to be' depending on the types and strength of stress. Among the many signaling pathways that respond to stress, mitogen-activated protein kinase (MAPK) family members are crucial for the maintenance of cells. Three subfamilies of MAPKs have been identified: extracellular signal-regulated kinases (ERKs), c-Jun N-terminal kinases (JNKs), and p38-MAPKs. It has been originally shown that ERKs are important for cell survival, whereas JNKs and p38-MAPKs were deemed stress responsive and thus involved in apoptosis. However, the regulation of apoptosis by MAPKs is more complex than initially thought and often controversial. In this review, we discuss MAPKs in apoptosis regulation with attention to mouse genetic models and critically point out the multiple roles of MAPKs.

Reduced fertility in male mice deficient in the zinc metallopeptidase NL1

Members of the M13 family of zinc metalloendopeptidases have been shown to play critical roles in the metabolism of various neuropeptides and peptide hormones, and they have been identified as important therapeutic targets. Recently, a mouse NL1 protein, a novel member of the family, was identified and shown to be expressed mainly in the testis as a secreted protein. To define its physiological role(s), we used a gene targeting strategy to disrupt the endogenous murine Nl1 gene by homologous recombination and generate Nl1 mutant mice. The Nl1(-/-) mice were viable and developed normally, suggesting that zygotic expression of Nl1 is not required for development. However, Nl1(-/-) males produced smaller litters than their wild-type siblings, indicating specific male fertility problems. Reduced fertility may be explained by two impaired processes, decreased egg fertilization and perturbed early development of fertilized eggs. These two phenotypes did not result from gross anatomical modifications of the testis or from impaired spermatogenesis. Basic sperm parameters were also normal. Thus, our findings suggest that one of the roles of NL1 in mice is related to sperm function and that NL1 modulates the processes of fertilization and early embryonic development in vivo.

The MAP kinase pathway is required for entry into mitosis and cell survival

In this communication, we examined the role of the MAP kinase pathway in the G2/M phase of the cell cycle. Activation of the Plk1 and MAP kinase pathways was initially evaluated in FT210 cells, which arrest at G2 phase at the restrictive temperature (39 degrees C), due to a mutation in the cdc2 gene. Previous studies had shown that these cells enter mitosis at the nonpermissive temperature upon incubation with okadaic acid, a protein phosphatase 1 and 2A inhibitor. We show that treatment of FT210 cells at 39 degrees C with okadaic acid activated Plk1, as shown by hyperphosphorylation and elevated protein kinase activity, and also induced activation of the MAP kinase pathway. The specific Mek inhibitor PD98059 antagonized the okadaic acid-induced activation of both Plk1 and MAP kinases. This suggests that activation of the MAP kinase pathway may contribute to the okadaic acid-induced activation of Plk1 in FT210 cells at 39 degrees C. We also found that PD98059 strongly attenuated progression of HeLa cells through mitosis, and active Mek colocalizes with Plk1 at mitotic structures. To study the potential function of the MAP kinase pathway during mitosis, RNAi was used to specifically deplete five members of this pathway (Raf1, Mek1/2, Erk1/2). Each of these five protein kinases is required for cell proliferation and survival, and depletion of any of these proteins eventually leads to apoptosis. Treatment with Mek inhibitors also inhibited cell proliferation and caused apoptosis. A dramatic increase of Plk1 activities and a moderate increase of Cdc2 activities in Raf1-depleted cells indicate that Raf1-depleted cells arrest in the late G2 or M phase. Mek1 and Erk1 depletion also caused cell cycle arrest at G2, suggesting that these enzymes are required for the G2/M transition, whereas the loss of Mek2 or Erk2 caused arrest at G1.

Regulation of signal transduction by integrins

The integrin family of cell membrane receptors plays an important role in signal transduction cascades. Ligation of integrins by extracellular matrix proteins can lead to direct activation of Rho-family GTPases and MAP kinase pathways. However, perhaps the most significant signaling function of integrins is to modulate signal transduction events initiated by receptor tyrosine kinases and G protein-coupled receptors. This probably plays a role in coordinating information about cell shape and position with information about the availability of soluble growth factors.