Mesodermal patterning defect in mice lacking the Ste20 NCK interacting kinase (NIK)

The mitogen-activated protein kinase kinase kinase kinase GCKR positively regulates canonical and noncanonical Wnt signaling in B lymphocytes

Wnt ligands bind receptors of the Frizzled (Fz) family to control cell fate, proliferation, and polarity. Canonical Wnt/Fz signaling stabilizes beta-catenin by inactivating GSK3beta, leading to the translocation of beta-catenin to the nucleus and the activation of Wnt target genes. Noncanonical Wnt/Fz signaling activates RhoA and Rac, and the latter triggers the activation of c-Jun N-terminal kinase (JNK). Here, we show that exposure of B-lymphocytes to Wnt3a-conditioned media activates JNK and raises cytosolic beta-catenin levels. Both the Rac guanine nucleotide exchange factor Asef and the mitogen-activated protein kinase kinase kinase kinase germinal center kinase-related enzyme (GCKR) are required for Wnt-mediated JNK activation in B cells. In addition, we show that GCKR positively affects the beta-catenin pathway in B cells. Reduction of GCKR expression inhibits Wnt3a-induced phosphorylation of GSK3beta at serine 9 and decreases the accumulation of cytosolic beta-catenin. Furthermore, Wnt signaling induces an interaction between GCKR and GSK3beta. Our findings demonstrate that GCKR facilitates both canonical and noncanonical Wnt signaling in B lymphocytes.

The Nck-interacting kinase phosphorylates ERM proteins for formation of lamellipodium by growth factors

The mammalian Ste20-like Nck-interacting kinase (NIK) and its orthologs Misshapen in Drosophila and Mig-15 in Caenorhabditis elegans have a conserved function in regulating cell morphology, although through poorly understood mechanisms. We report two previously unrecognized actions of NIK: regulation of lamellipodium formation by growth factors and phosphorylation of the ERM proteins ezrin, radixin, and moesin. ERM proteins regulate cell morphology and plasma membrane dynamics by reversibly anchoring actin filaments to integral plasma membrane proteins. In vitro assays show that NIK interacts directly with ERM proteins, binding their N termini and phosphorylating a conserved C-terminal threonine. In cells, NIK and phosphorylated ERM proteins localize at the distal margins of lamellipodia, and NIK activity is necessary for phosphorylation of ERM proteins induced by EGF and PDGF, but not by thrombin. Lamellipodium extension in response to growth factors is inhibited in cells expressing a kinase-inactive NIK, suppressed for NIK expression with siRNA oligonucleotides, or expressing ezrin T567A that cannot be phosphorylated. These data suggest that direct phosphorylation of ERM proteins by NIK constitutes a signaling mechanism controlling growth factor-induced membrane protrusion and cell morphology.

p38 and a p38-interacting protein are critical for downregulation of E-cadherin during mouse gastrulation

During vertebrate gastrulation, an epithelial to mesenchymal transition (EMT) is necessary for migration of mesoderm from the primitive streak. We demonstrate that p38 MAP kinase and a p38-interacting protein (p38IP) are critically required for downregulation of E-cadherin during gastrulation. In an ENU-mutagenesis screen we identified the droopy eye (drey) mutation, which affects splicing of p38IP. p38IP(drey) mutant embryos display incompletely penetrant defects in neural tube closure, eye development, and gastrulation. A stronger allele (p38IP(RRK)) exhibits gastrulation defects in which mesoderm migration is defective due to deficiency in E-cadherin protein downregulation in the primitive streak. We show that p38IP binds directly to p38 and is required for p38 activation in vivo. Moreover, both p38 and p38IP are required for E-cadherin downregulation during gastrulation. Finally, p38 regulates E-cadherin protein expression downstream from NCK-interacting kinase (NIK) and independently of the regulation of transcription by Fibroblast Growth Factor (Fgf) signaling and Snail.

Localization of Brachyury (T) in embryonic and extraembryonic tissues during mouse gastrulation

T-box gene family members have important roles during murine embryogenesis, gastrulation, and organogenesis. Although relatively little is known about how T-box genes are regulated, published gene expression studies have revealed dynamic and specific patterns in both embryonic and extraembryonic tissues of the mouse conceptus. Mutant alleles of the T-box gene Brachyury (T) have identified roles in formation of mesoderm and its derivatives, such as somites and the allantois. However, given the cell autonomous nature of T gene activity and conflicting results of gene expression studies, it has been difficult to attribute a primary function to T in normal allantoic development. We report localization of T protein by sectional immunohistochemistry in both embryonic and extraembryonic tissues during mouse gastrulation, emphasizing T localization within the allantois. T was detected in all previously reported sites within the conceptus, including the primitive streak and its derivatives, nascent embryonic mesoderm, the node and notochord, as well as notochord-associated endoderm and posterior neurectoderm. In addition, we have clarified T within the allantois, where it was first detected in the proximal midline of the late allantoic bud (approximately 7.5 days postcoitum, dpc) and persisted within an expanded midline domain until 6-somite pairs (s; approximately 8.5 dpc). Lastly, we have discovered several novel T sites, including the developing heart, visceral endoderm, extraembryonic ectoderm, and its derivative, chorionic ectoderm. Together, these data provide a unified picture of T in the mammalian conceptus, and demonstrate T's presence in unrelated cell types and tissues in highly dynamic spatiotemporal patterns in both embryonic and extraembryonic tissues.

A small interfering RNA screen for modulators of tumor cell motility identifies MAP4K4 as a promigratory kinase

Cell motility is a complex biological process, involved in development, inflammation, homeostasis, and pathological processes such as the invasion and metastatic spread of cancer. Here, we describe a genomic screen designed to identify inhibitors of cell migration. A library of 10,996 small interfering RNAs (targeting 5,234 human genes) was screened for their ability to block the migration of a highly motile ovarian carcinoma cell line, SKOV-3, by using a 384-well wound-healing assay coupled with automated microscopy and wound quantification. Two or more small interfering RNAs against four genes, CDK7, DYRK1B, MAP4K4 (NIK/HGK) (MAP4K4, mitogen-activated protein 4 kinase 4), and SCCA-1 (SerpinB3), potently blocked the migration of SKOV-3 cells, concordant with reduced transcript levels. Further studies of the promigratory role of MAP4K4 showed that the knockdown of this transcript inhibited the migration of multiple carcinoma cell lines, indicating a broad role in cell motility and potently suppressed the invasion of SKOV-3 cells in vitro. The effect of MAP4K4 on cellular migration was found to be mediated through c-Jun N-terminal kinase, independent of AP1 activation and downstream transcription. Accordingly, small molecule inhibition of c-Jun N-terminal kinase suppressed SKOV-3 cell migration, underscoring the potential therapeutic utility of mitogen-activated protein kinase pathway inhibition in cancer progression.

Identification of Direct Serum-response Factor Gene Targets during Me2SO-induced P19 Cardiac Cell Differentiation

Serum-response factor (SRF) is an obligatory transcription factor, required for the formation of vertebrate mesoderm leading to the origin of the cardiovascular system. Protein A-TEV-tagged chromatin immunoprecipitation technology was used to collect direct SRF-bound gene targets from pluripotent P19 cells, induced by Me2SO treatment into an enriched cardiac cell population. From 242 sequenced DNA fragments, we identified 188 genomic DNA fragments as potential direct SRF targets that contain CArG boxes and CArG-like boxes. Of the 92 contiguous genes that were identified, a subgroup of 43 SRF targets was then further validated by co-transfection assays with SRF. Expression patterns of representative candidate genes were compared with the LacZ reporter expression activity of the endogenous SRF gene. According to the Unigene data base, 84% of the SRF target candidates were expressed, at least, in the heart. In SRF null embryonic stem cells, 81% of these SRF target candidates were greatly affected by the absence of SRF. Among these SRF-regulated genes, Raf1, Map4k4, and Bicc1 have essential roles in mesoderm formation. The 12 regulated SRF target genes, Mapk10 (JNK3), Txnl2, Azi2, Tera, Sema3a, Lrp4, Actc1, Myl3, Hspg2, Pgm2, Hif3a, and Asb5, have been implicated in cardiovascular formation, and the Ski and Hes6 genes have roles in muscle differentiation. SRF target genes related to cell mitosis and cycle, E2f5, Npm1, Cenpb, Rbbp6, and Scyl1, expressed in the heart tissue were differentially regulated in SRF null ES cells.

Identification and functional characterization of a novel human misshapen/Nck interacting kinase-related kinase, hMINK beta

Misshapen/NIKs-related kinase (MINK) is a member of the germinal center family of kinases that are homologous to the yeast sterile 20 (Ste20) kinases and regulate a wide variety of cellular processes, including cell morphology, cytoskeletal rearrangement, and survival. Here, we present the cloning and functional characterization of a novel human Misshapen/NIKs-related kinase beta (hMINK beta) that encodes a polypeptide of 1312 amino acids. hMINK beta is ubiquitously expressed in most tissues with at least five alternatively spliced isoforms. Similar to Nck interacting kinase (NIK) and Traf2 and Nck-interacting kinase (TNIK), hMINK beta moderately activates c-Jun N-terminal kinase (JNK) and associates with Nck via the intermediate domain in the yeast two-hybrid system and in a glutathione S-transferase (GST) pull-down assay. Interestingly, overexpression of the kinase domain deleted and kinase-inactive mutants of hMINK beta in human fibrosarcoma HT1080 cells enhanced cell spreading, actin stress fiber formation, and adhesion to extracellular matrix, as well as decreased cell motility and cell invasion. Furthermore, these mutants also promoted cell-cell adhesion in human breast carcinoma MCF7 cells, evidenced with cell growth in clusters and increased membrane localization of beta-catenin, a multifunctional protein involved in E-cadherin-mediated cell adhesion. Finally, hMINK beta protein was found to colocalize with the Golgi apparatus, implicating that hMINK beta might exert its functions, at least in part, through the modulation of intracellular protein transport. Taken together, these results suggest that hMINK beta plays an important role in cytoskeleton reorganization, cell adhesion, and cell motility.

The murine Nck SH2/SH3 adaptors are important for the development of mesoderm-derived embryonic structures and for regulating the cellular actin network

Mammalian Nck1 and Nck2 are closely related adaptor proteins that possess three SH3 domains, followed by an SH2 domain, and are implicated in coupling phosphotyrosine signals to polypeptides that regulate the actin cytoskeleton. However, the in vivo functions of Nck1 and Nck2 have not been defined. We have mutated the murine Nck1 and Nck2 genes and incorporated beta-galactosidase reporters into the mutant loci. In mouse embryos, the two Nck genes have broad and overlapping expression patterns. They are functionally redundant in the sense that mice deficient for either Nck1 or Nck2 are viable, whereas inactivation of both Nck1 and Nck2 results in profound defects in mesoderm-derived notochord and embryonic lethality at embryonic day 9.5. Fibroblast cell lines derived from Nck1(-/-) Nck2(-/-) embryos have defects in cell motility and in the organization of the lamellipodial actin network. These data suggest that the Nck SH2/SH3 adaptors have important functions in the development of mesodermal structures during embryogenesis, potentially linked to a role in cell movement and cytoskeletal organization.

The STE20 kinase HGK is broadly expressed in human tumor cells and can modulate cellular transformation, invasion, and adhesion

HGK (hepatocyte progenitor kinase-like/germinal center kinase-like kinase) is a member of the human STE20/mitogen-activated protein kinase kinase kinase kinase family of serine/threonine kinases and is the ortholog of mouse NIK (Nck-interacting kinase). We have cloned a novel splice variant of HGK from a human tumor line and have further identified a complex family of HGK splice variants. We showed HGK to be highly expressed in most tumor cell lines relative to normal tissue. An active role for this kinase in transformation was suggested by an inhibition of H-Ras(V12)-induced focus formation by expression of inactive, dominant-negative mutants of HGK in both fibroblast and epithelial cell lines. Expression of an inactive mutant of HGK also inhibited the anchorage-independent growth of cells yet had no effect on proliferation in monolayer culture. Expression of HGK mutants modulated integrin receptor expression and had a striking effect on hepatocyte growth factor-stimulated epithelial cell invasion. Together, these results suggest an important role for HGK in cell transformation and invasiveness.

The Nck family of adapter proteins: regulators of actin cytoskeleton

SH2/SH3 domain-containing adapter proteins, such as the Nck family, play a major role in regulating tyrosine kinase signalling. They serve to recruit proline-rich effector molecules to tyrosine-phosphorylated kinases or their substrates. Initially, it was not clear why cells from nematodes to vertebrates contain redundant and closely related SH2/SH3 adapters, such as Grb2, Crk and Nck. Recent evidence suggests that their biological roles are clearly different, whereas, for example, Grb2 connects activated receptor tyrosine kinases to Sos and Ras, leading to cell proliferation. The proteins of Nck family are implicated in organisation of actin cytoskeleton, cell movement or axon guidance in flies. In this review, the author attempts to summarise signalling pathways in which Nck plays a critical role.

The essential role of RI alpha in the maintenance of regulated PKA activity

Cloning of the individual regulatory (R) and catalytic (C) subunits of the cAMP-dependent protein kinase (PKA) and expression of these subunits in cell culture have provided mechanistic answers about the rules for PKA holoenzyme assembly. One of the central findings of these studies is the essential role of the RI alpha regulatory subunit in maintaining the catalytic subunit under cAMP control. The role of RI alpha as the key compensatory regulatory subunit in this enzyme family was confirmed by gene knockouts of the three other regulatory subunits in mice. In each case, RI alpha has demonstrated the capacity for significant compensatory regulation of PKA activity in tissues where the other regulatory subunits are expressed, including brain, brown and white adipose tissue, skeletal muscle, and sperm. The essential requirement of the RI alpha regulatory subunit in maintaining cAMP control of PKA activity was further corroborated by the knockout of RI alpha in mice, which results in early embryonic lethality due to failed cardiac morphogenesis. Closer examination of RI alpha knockout embryos at even earlier stages of development revealed profound deficits in the morphogenesis of the mesodermal embryonic germ layer, which gives rise to essential structures including the embryonic heart tube. Failure of the mesodermal germ layer in RI alpha knockout embryos can be rescued by crossing RI alpha knockout mice to C alpha knockout mice, supporting the conclusion that inappropriately regulated PKA catalytic subunit activity is responsible for the phenotype. Isolation of primary embryonic fibroblasts from RI alpha knockout embryos reveals profound alterations in the actin-based cytoskeleton, which may account for the failure in mesoderm morphogenesis at gastrulation.

A conserved interaction between beta1 integrin/PAT-3 and Nck-interacting kinase/MIG-15 that mediates commissural axon navigation in C. elegans

BACKGROUND

Integrins are heterodimeric (alphabeta) transmembrane receptors for extracellular matrix (ECM) ligands. Through interactions with molecular partners at cell junctions, they provide a connection between the ECM and the cytoskeleton and regulate many aspects of cell behavior. A number of integrin-associated molecules have been identified; however, in many cases, their function and role in the animal remain to be clarified.

RESULTS

We have identified the Nck-interacting kinase (NIK), a member of the STE20/germinal center kinase (GCK) family, as a partner for the beta1A integrin cytoplasmic domain. We find that NIK is expressed in the nervous system and other tissues in mouse embryos and colocalizes with actin and beta1 integrin in cellular protrusions in transfected cells. To demonstrate the functional significance of this interaction, we used Caenorhabditis elegans, since it has only one beta (PAT-3) integrin chain, two alpha (INA-1 and PAT-2) integrin chains, and a well-conserved NIK ortholog (MIG-15). Using three methods, we show that reducing mig-15 activity results in premature branching of commissures. A significant aggravation of this defect is observed when mig-15 activity is compromised in a weak ina-1 background. Neuronal-specific RNA interference against mig-15 or pat-3 leads to similar axonal defects, thus showing that both mig-15 and pat-3 act cell autonomously in neurons. Finally, we show a genetic interaction between mig-15, ina-1, and genes that encode Rac GTPases.

CONCLUSIONS

Using several models, we provide the first evidence that the kinase NIK and integrins interact in vitro and in vivo. This interaction is required for proper axonal navigation in C. elegans.