Role of glycogen synthase kinase 3 beta as a negative regulator of dorsoventral axis formation in Xenopus embryos.

The dorsoventral axis is established early in Xenopus development and may involve signaling by Wnts, a family of Wnt1-protooncogene-related proteins. The protein kinase shaggy functions in the wingless/Wnt signaling pathway, which operates during Drosophila development. To assess the role of a closely related kinase, glycogen synthase kinase 3 beta (GSK-3 beta), in vertebrate embryogenesis, we cloned a cDNA encoding a Xenopus homolog of GSK-3 beta (XGSK-3 beta). XGSK-3 beta-specific transcripts were detected by Northern analysis in Xenopus eggs and early embryos. Microinjection of the mRNA encoding a catalytically inactive form of rat GSK-3 beta into a ventrovegetal blastomere of eight-cell embryos caused ectopic formation of a secondary body axis containing a complete set of dorsal and anterior structures. Furthermore, in isolated ectodermal explants, the mutant GSK-3 beta mRNA activated the expression of neural tissue markers. Wild-type XGSK-3 beta mRNA suppressed the dorsalizing effects of both the mutated GSK-3 beta and Xenopus dishevelled, a proposed upstream signaling component of the same pathway. These results strongly suggest that XGSK-3 beta functions to inhibit dorsoventral axis formation in the embryo and provide evidence for conservation of the Wnt signaling pathway in Drosophila and vertebrates.

Limbic epilepsy in transgenic mice carrying a Ca2+/calmodulin-dependent kinase II alpha-subunit mutation.

Multifunctional Ca2+/calmodulin-dependent protein kinase II (CaMK) phosphorylates proteins pivotally involved in diverse neuronal processes and thereby coordinates cellular responses to external stimuli that regulate intracellular Ca2+ [Hanson, P. I. & Schulman, H. (1992) Annu. Rev. Biochem. 61, 559-664]. Despite extensive study, the impact of this enzyme on control of the excitability of neuron populations in the mammalian nervous system in situ is unknown. To address this question, we studied transgenic mice carrying a null mutation (-/-) for the alpha subunit of CaMK. In contrast to wild-type littermates, null mutants exhibit profound hyperexcitability, evident in epileptic seizures involving limbic structures including the hippocampus. No evidence of increased excitability was detected in mice carrying null mutations of the gamma isoform of protein kinase C, underscoring the specificity of the effect of CaMK. CaMK plays a powerful and previously underappreciated role in control of neuronal excitability in the mammalian nervous system. These insights have important implications for analyses of mechanisms of epilepsy and, perhaps, learning and memory.

Calcium/Calmodulin-Dependent Protein Kinase II Serves as a Biochemical Integrator of Calcium Signals for the Induction of Synaptic Plasticity

Repetitive Ca2+ transients in dendritic spines induce various forms of synaptic plasticity by transmitting information encoded in their frequency and amplitude. CaMKII plays a critical role in decoding these Ca2+ signals to initiate long-lasting synaptic plasticity. However, the properties of CaMKII that mediate Ca2+ decoding in spines remain elusive. Here, I measured CaMKII activity in spines using fast-framing two-photon fluorescence lifetime imaging. Following each repetitive Ca2+ elevations, CaMKII activity increased in a stepwise manner. This signal integration, at the time scale of seconds, critically depended on Thr286 phosphorylation. In the absence of Thr286 phosphorylation, only by increasing the frequency of repetitive Ca2+ elevations could high peak CaMKII activity or plasticity be induced. In addition, I measured the association between CaMKII and Ca2+/CaM during spine plasticity induction. Unlike CaMKII activity, association of Ca2+/CaM to CaMKII plateaued at the first Ca2+ elevation event. This result indicated that integration of Ca2+ signals was initiated by the binding of Ca2+/CaM and amplified by the subsequent increases in Thr286-phosphorylated form of CaMKII. Together, these findings demonstrate that CaMKII functions as a leaky integrator of repetitive Ca2+ signals during the induction of synaptic plasticity, and that Thr286 phosphorylation is critical for defining the frequencies of such integration.

Molecular mechanism of activation-triggered subunit exchange in Ca(2+)/calmodulin-dependent protein kinase II.

Activation triggers the exchange of subunits in Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), an oligomeric enzyme that is critical for learning, memory, and cardiac function. The mechanism by which subunit exchange occurs remains elusive. We show that the human CaMKII holoenzyme exists in dodecameric and tetradecameric forms, and that the calmodulin (CaM)-binding element of CaMKII can bind to the hub of the holoenzyme and destabilize it to release dimers. The structures of CaMKII from two distantly diverged organisms suggest that the CaM-binding element of activated CaMKII acts as a wedge by docking at intersubunit interfaces in the hub. This converts the hub into a spiral form that can release or gain CaMKII dimers. Our data reveal a three-way competition for the CaM-binding element, whereby phosphorylation biases it towards the hub interface, away from the kinase domain and calmodulin, thus unlocking the ability of activated CaMKII holoenzymes to exchange dimers with unactivated ones.

Molecular mechanism of activation-triggered subunit exchange in Ca(2+)/calmodulin-dependent protein kinase II.

Activation triggers the exchange of subunits in Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), an oligomeric enzyme that is critical for learning, memory, and cardiac function. The mechanism by which subunit exchange occurs remains elusive. We show that the human CaMKII holoenzyme exists in dodecameric and tetradecameric forms, and that the calmodulin (CaM)-binding element of CaMKII can bind to the hub of the holoenzyme and destabilize it to release dimers. The structures of CaMKII from two distantly diverged organisms suggest that the CaM-binding element of activated CaMKII acts as a wedge by docking at intersubunit interfaces in the hub. This converts the hub into a spiral form that can release or gain CaMKII dimers. Our data reveal a three-way competition for the CaM-binding element, whereby phosphorylation biases it towards the hub interface, away from the kinase domain and calmodulin, thus unlocking the ability of activated CaMKII holoenzymes to exchange dimers with unactivated ones.

Isolation and expression analysis of multiple isoforms of putative farnesoic acid O-methyltransferase in several crustacean species

Farnesoic acid O-methyltransferase (FaMeT) is the enzyme responsible for the conversion of farnesoic acid (FA) to methyl farnesoate (MF) in the final step of MF synthesis. Multiple isoforms of putative FaMeT were isolated from six crustacean species belonging to the families Portunidae, Penaeidae, Scyllaridae and Parastacidae. The portunid crabs Portunus pelagicus and Scylla serrata code for three forms: short, intermediate and long. Two isoforms (short and long) were isolated from the penaeid prawns Penaeus monodon and Fenneropenaeus merguiensis. Two isoforms were also identified in the scyllarid Thenus orientalis and parastacid Cherax quadricarinatus. Putative FaMeT sequences were also amplified from the genomic DNA of P. pelagicus and compared to the putative FaMeT transcripts expressed. Each putative FaMeT cDNA isoform was represented in the genomic DNA, indicative of a multi-gene family. Various tissues from P. pelagicus were individually screened for putative FaMeT expression using PCR and fragment analysis. Each tissue type expressed all three isoforms of putative FaMeT irrespective of sex or moult stage. Protein domain analysis revealed the presence of a deduced casein kinase II phosphorylation site present only in the long isoform of putative FaMeT.

Phosphorylation status of the phosphoprotein P of rinderpest virus modulates transcription and replication of the genome

The phosphoprotein P of paramyxoviruses is known to play more than one role in genome transcription and replication. Phosphorylation of P at the NH2 terminus by cellular casein kinase II has been shown to be necessary for transcription of the genome in some of the viruses, while it is dispensable for replication. The phosphorylation null mutant of rinderpest virus P protein, in which three serine residues have been mutated, has been shown earlier to be non-functional in an in vivo minigenome replication/transcription system. In this work, we have shown that the phosphorylation of P protein is essential for transcription, whereas the null mutant is active in replication of the genome in vivo. The null mutant P acts as a transdominant repressor of transcriptional activity of wild-type P and as an activator of replication carried out by wild-type P protein. These results suggest the phosphorylation status of P may act as a replication switch during virus replication. We also show that the phosphorylation null mutant P is capable of interacting with L and N proteins and is able to form a tripartite complex of L-(N-P) when expressed in insect cells, similar to wild-type P protein.

Bioengineering of AAV2 Capsid at Specific Serine, Threonine, or Lysine Residues Improves Its Transduction Efficiency in Vitro and in Vivo

We hypothesized that the AAV2 vector is targeted for destruction in the cytoplasm by the host cellular kinase/ubiquitination/proteasomal machinery and that modification of their targets on AAV2 capsid may improve its transduction efficiency. In vitro analysis with pharmacological inhibitors of cellular serine/threonine kinases (protein kinase A, protein kinase C, casein kinase II) showed an increase (20-90%) on AAV2-mediated gene expression. The three-dimensional structure of AAV2 capsid was then analyzed to predict the sites of ubiquitination and phosphorylation. Three phosphodegrons, which are the phosphorylation sites recognized as degradation signals by ubiquitin ligases, were identified. Mutation targets comprising eight serine (S) or seven threonine (T) or nine lysine (K) residues were selected in and around phosphodegrons on the basis of their solvent accessibility, overlap with the receptor binding regions, overlap with interaction interfaces of capsid proteins, and their evolutionary conservation across AAV serotypes. AAV2-EGFP vectors with the wild-type (WT) capsid or mutant capsids (15 S/T -> alanine A] or 9 K -> arginine R] single mutant or 2 double K -> R mutants) were then evaluated in vitro. The transduction efficiencies of 11 S/T -> A and 7 K -> R vectors were significantly higher (similar to 63-90%) than the AAV2-WT vectors (similar to 30-40%). Further, hepatic gene transfer of these mutant vectors in vivo resulted in higher vector copy numbers (up to 4.9-fold) and transgene expression (up to 14-fold) than observed from the AAV2-WT vector. One of the mutant vectors, S489A, generated similar to 8-fold fewer antibodies that could be cross-neutralized by AAV2-WT. This study thus demonstrates the feasibility of the use of these novel AAV2 capsid mutant vectors in hepatic gene therapy.

cDNA cloning and overexpression of acidic ribosomal phosphoprotein P1 gene (RPLP1) from the Giant Panda

RPLP1 is one of acidic ribosomal phosphoproteins encoded by RPLP1 gene, which plays an important role in the elongation step of protein synthesis. The cDNA of RPLP1 was cloned successfully for the first time from the Giant Panda (Ailuropoda melanoleuca) using RT-PCR technology, which was also sequenced, analyzed preliminarily and expressed in E. coli. The cDNA fragment cloned is 449bp in size, containing an open reading frame of 344bp encoding 114 amino acids. Alignment analysis indicated that the nucleotide sequence and the deduced amino acid sequence are highly conserved to other five species studied, including Homo sapiens, Mus musculus, Rattus norvegicus, Bos Taurus and Sus scrofa. The homologies for nucleotide sequences of Giant Panda PPLP1 to that of these species are 92.4%, 89.8%, 89.0%, 91.3% and 87.5%, while the homologies for amino acid sequences are 96.5%, 94.7%, 95.6%, 96.5% and 88.6%. Topology prediction showed there are three Casein kinase II phosphorylation sites and two N-myristoylation sites in the RPLP1 protein of the Giant Panda (Ailuropoda melanoleuca). The RPLP1 gene was overexpressed in E. coli and the result indicated that RPLP1 fusion with the N-terminally His-tagged form gave rise to the accumulation of an expected 18kDa polypeptide, which was in accordance with the predicted protein and could also be used to purify the protein and study its function.

TGA2 dual activity: N-terminus regulation of reversible DNA binding influenced by NPRI and CK2

TGA2 is a dual-function Systemic Acquired Resistance (SAR) transcription factor involved in the activation and repression of pathogenesis-related (PR) genes. Recent studies have shown that TGA2 is able to switch from a basal repressor to activator, likely, through regulatory control from its N-terminus. The N-terminus has also been shown to affect DNA binding of the TGA2 bZIP domain when phosphorylated by Casein Kinase II (CK2). The mechanisms involved for directing a switch from basal repressor to activator, and the role of kinase activity, have not previously been looked at in detail. This study provides evidence for the involvement of a CK2-like kinase in the switch of TGA2 activity from repressor to activator, by regulating the DNA-binding activity of TGA2 by phosphorylating residues in the N terminus of the protein.

Untersuchungen zur Regulation der Basenfehlpaarungsreparatur und zur Zytostatikaresistenz in Säugerzellen

In der vorliegenden Arbeit konnte gezeigt werden, daß eine Behandlung von Säugerzellen mit O6-Methylguanin generierenden Substanzen zu einer Zunahme der GT-Bindungsaktivität und zu einer Translokation der MMR-Proteine MSH2, MSH6 und PMS2 aus dem Cytoplasma in den Zellkern führt. Versuche mit MSH6-defizienten DLD1-Zellen und Coimmunpräzipitationsversuche zeigten, daß der MutSalpha-Komplex (MSH2+MSH6) bereits im Cytoplasma gebildet wird und daß die Translokation von MSH2 nur im Komplex mit MSH6 erfolgt. Durch die Untersuchung von Zellinien mit unterschiedlichem MGMT-Status konnte gezeigt werden, daß der DNA-Alkylierungsschaden O6-Methylguanin (O6-MeG) in Form des O6-MeGC oder O6-MeGT-Basenpaars das initiale Signal für die nukleäre Translokation von MutSalpha darstellt. Untersuchungen zur post-translationalen Modifikation der MMR-Proteine zeigten, daß MSH2 und MSH6 sowohl in vivo als auch in vitro durch die Proteinkinase C (PKC) und die Casein Kinase II (CKII) phosphoryliert werden. Es konnte gezeigt werden, daß die Phosphorylierung von MutSalpha die Effektivität der Bindung an Basenfehlpaarungen beeinflußt, da unphosphoryliertes MutSalpha in vitro nicht effektiv an GT-Fehlpaarungen binden kann. Bei der Untersuchung der Resistenzentwicklung von Melanomzellen gegenüber Zytostatika konnte gezeigt werden daß die Resistenz gegenüber Fotemustin auf einer Erhöhung der Menge des Reparaturproteins O6-Methylguanin-DNA-Methyltransferase (MGMT) beruht. Die Reaktivierung des MGMT-Gens beruht seinerseits auf einer CpG-Hypermethylierung im kodierenden Bereichs des Gens.

Die Rolle von LRP1 in der Funktion des NMDA-Rezeptors

Ein funktionelles Zusammenspiel von LRP1, einem Mitglied der LDL-Rezeptorfamilie, mit dem NMDA-Rezeptor, einem Glutamat Rezeptor, wurde durch die Interaktion beider Proteine sowie eine tPa-vermittelte, LRP1-abhängige Signalübertragung durch den NMDA-Rezeptor belegt. Darüber hinaus zeigen Mäuse mit einem konditionellen neuronalen knock-out des Lrp1 Gens Verhaltensänderungen, die mit einer beeinträchtigten Signalübertragung durch NMDA-Rezeptoren assoziiert werden könnten. Die genaue Rolle von LRP1 in der NMDA-Rezeptor-Funktion bleibt allerdings noch unklar. In der vorliegenden Arbeit wurde die Rolle von LRP1 bei der Expression der NR2B-Untereinheit des NMDA-Rezeptors an der Zelloberfläche primärer kortikaler Neurone untersucht. Zu diesem Zweck wurde die knock-in Mauslinie LRP1ΔNPxY2, die sich durch eine Alanin Substitution im NPxY2 Motiv des LRP1 auszeichnet, eingesetzt. rnEs konnte gezeigt werden, dass diese knock-in Mutation in einer erhöhten Expression von LRP1 und der NMDA-Rezeptoruntereinheiten NR1 und NR2B an der Zelloberfläche primärer kortikaler Neurone resultiert. Der Effekt konnte durch eine reduzierte Endozytoserate von LRP1 und der NR1-und NR2B-Untereinheiten in primären LRP1ΔNPxY2 Neuronen erklärt werden. Darüber hinaus wurde ein verändertes Phosphorylierungsmuster der Internalisierungssignale der NR2B-Rezeptoruntereinheit Serin S1480 und Tyrosin Y1472 an der Zelloberfläche primärer LRP1ΔNPxY2 Neurone detektiert. Die verantwortlichen Kinasen Fyn und Kasein-Kinase II sind allerdings in LRP1ΔNPxY2 Neuronen im Vergleich zu den Wildtyp-Kontrollen nicht abweichend reguliert. In den Co-Immunopräzipitationsexperimenten wurde gezeigt, dass die Bindung von LRP1 mit NR2B durch die Phosphorylierung reguliert wird und dieser Regulationsmechanismus in LRP1ΔNPxY2 Neuronen beeinträchtigt ist. Dies resultiert in einer stärkeren Bindung von NR2B-Rezeptoruntereinheit an LRP1. Aufgrund reduzierter Internalisierungsraten von LRP1 in LRP1ΔNPxY2 Neuronen führt dieser Umstand zu einer Akkumulation beider Rezeptorproteine an der Zelloberfläche. Schließlich wurden die NMDA-Rezeptor-assoziierten Verhaltensänderungen wie die Hyperaktivität und die Defizite im direkten und umgekehrten räumlichen Lernvermögen in den LRP1ΔNPxY2 Tieren nachgewiesen. Zusammengefasst, demonstrieren diese Ergebnisse, dass LRP1 eine kritische Rolle in der Regulierung der NR2B-Expression an der Zelloberfläche spielt.

Multisite phosphorylation of ornithine decarboxylase in transformed macrophages

Ornithine decarboxylase (ODC), the initial inducible enzyme in the polyamine biosynthetic pathway, exists in the transformed macrophage RAW264 cell line as a phosphoprotein following cell stimulation. The hypothesis that ODC is phosphorylated at multiple sites in stimulated RAW264 cells was investigated. ODC isolated from tetradecanoyl-phorbol-13-acetate (TPA)-stimulated cells metabolically radiolabeled in the presence of $\sp{32}$P$\sb{\rm i}$ was subjected to cyanogen bromide (CNBr) cleavage followed by phosphopeptide mapping and two dimensional phosphoamino acid analysis. These phosphorylation studies demonstrated six in situ phosphorylated CNBr-generated fragments having apparent molecular weights of 17, 14.3, 8, 6.5, 4, and 2.7 kDa and also revealed that ODC is phosphorylated in RAW264 cells on at least 5 serine and 2 threonine residues.^ In addition, the in vivo specific activity and phosphorylation pattern of ODC in response to various kinase cascade stimulants was studied. A differential response in ODC specific activity and a variation in the relative distribution of $\sp{32}$P-labeling of serine and threonine residues on the ODC molecule was noted in response to fetal bovine serum, cAMP and isobutylmethylxanthine, lipopolysaccharide, or TPA.^ Based on information derived from consensus sequence motifs, three protein kinases responsible for the phosphorylation of ODC in vitro were identified. Purified ODC was phosphorylated in vitro by casein kinase II (CK II), extracellular signal-regulated kinase 1 (ERK1), and its activator, extracellular signal-regulated kinase kinase (MEK). CK II phosphorylated ODC on serine residues contained on three CNBr-generated peptides with apparent molecular weights of 14.3, 6.5, and 2.7 kDa. Both ERK1 and MEK phosphorylated ODC on serine and threonine residues on a CNBr-generated peptide fragment with an apparent molecular weight of 6.5 kDa. The in vitro radiolabeled peptides corresponded in molecular mass with some of the CNBr fragments of ODC phosphorylated in situ in stimulated RAW264 cells.^ This study concludes that ODC is phosphorylated in the transformed macrophage RAW264 cell line at multiple sites in response to various kinase cascade stimulants. These stimulants also led to a differential response in specific activity and phosphorylation pattern of ODC in RAW264 cells. Three protein kinases have been identified which phosphorylate ODC in vitro on peptides and amino acid residues which correspond with those phosphorylated in situ. ^

The β subunit of CKII negatively regulates Xenopus oocyte maturation

CKII (formerly known as casein kinase II) is a ubiquitously expressed enzyme that plays an important role in regulating cell growth and differentiation. The β subunit of CKII (CKIIβ) is not catalytic but forms heterotetramers with the catalytic subunit α to generate an α2β2 holoenzyme. In Xenopus oocytes, CKIIβ also associates with another serine/threonine kinase, Mos. As a key regulator of meiosis, Mos is necessary and sufficient to initiate oocyte maturation. We have previously shown that the binding of CKIIβ to Mos represses Mos-mediated mitogen-activated protein kinase (MAPK) activation and that the ectopic expression of CKIIβ inhibits progesterone-induced Xenopus oocyte maturation. We have now used an antisense oligonucleotide technique to reduce the endogenous CKIIβ protein level in Xenopus oocytes, and we find that oocytes with a reduced content of CKIIβ are more sensitive to low doses of progesterone and show accelerated MAPK activation and germinal vesicle breakdown. Furthermore, ectopic expression of a Mos-binding fragment of CKIIβ suppressed the effect of antisense oligonucleotide. These results suggest that the endogenous CKIIβ normally sets a threshold level for Mos protein, which must be exceeded for Mos to activate the MAPK signaling pathway and induce oocyte maturation.

Sequences within the Cytoplasmic Domain of Gp180/Carboxypeptidase D Mediate Localization to the Trans-Golgi Network

Gp180, a duck protein that was proposed to be a cell surface receptor for duck hepatitis B virus, is the homolog of metallocarboxypeptidase D, a mammalian protein thought to function in the trans-Golgi network (TGN) in the processing of proteins that transit the secretory pathway. Both gp180 and mammalian metallocarboxypeptidase D are type I integral membrane proteins that contain a 58-residue cytosolic C-terminal tail that is highly conserved between duck and rat. To investigate the regions of the gp180 tail involved with TGN retention and intracellular trafficking, gp180 and various deletion and point mutations were expressed in the AtT-20 mouse pituitary corticotroph cell line. Full length gp180 is enriched in the TGN and also cycles to the cell surface. Truncation of the C-terminal 56 residues of the cytosolic tail eliminates the enrichment in the TGN and the retrieval from the cell surface. Truncation of 12–43 residues of the tail reduced retention in the TGN and greatly accelerated the turnover of the protein. In contrast, deletion of the C-terminal 45 residues, which truncates a potential YxxL-like sequence (FxxL), reduced the protein turnover and caused accumulation of the protein on the cell surface. A point mutation of the FxxL sequence to AxxL slowed internalization, showing that this element is important for retrieval from the cell surface. Mutation of a pair of casein kinase II sites within an acidic cluster showed that they are also important for trafficking. The present study demonstrates that multiple sequence elements within the cytoplasmic tail of gp180 participate in TGN localization.