Expression of the protein kinase D (PKD) family during mouse embryogenesis

Getting smart - Deciphering the neuronal functions of protein kinase D

Protein kinase D (PKD) is a family of serine/threonine kinases that play important roles in various signalling pathways in cells, including neuronal cells. In the nervous system, PKD has been shown to be involved in learning and memory formation by regulating neurotransmitter release, neurite outgrowth and dendrite development, synapse formation and synaptic plasticity. In addition, PKD has been implicated in pain perception or neuroprotection during oxidative stress. Dysregulation of PKD expression and activity has been linked to several neurological disorders, including autism and epilepsy. In this review, we summarize the current knowledge on the function of the PKD family members in neuronal cells, including the spatial regulation of their downstream signalling pathways. We will further discuss the potential role of PKD in the pathogenesis of neurological disorders.

Protein kinase D2 confers neuroprotection by promoting AKT and CREB activation in ischemic stroke

Ischemic stroke, constituting 80-90% of all strokes, is a leading cause of death and long-term disability in adults. There is an urgent need to discover new targets and therapies for this devastating condition. Protein kinase D (PKD), as a key target of diacylglycerol involved in ischemic responses, has not been well studied in ischemic stroke, particularly PKD2. In this study, we found that PKD2 expression and activity were significantly upregulated in the ipsilateral side of the brain after transient focal cerebral ischemia, which coincides with the upregulation of PKD2 in primary neurons in response to in vitro ischemia, implying a potential role of PKD2 in neuronal survival in ischemic stroke. Using kinase-dead PKD2 knock-in (PKD2-KI) mice, we examined whether loss of PKD2 activity affected stroke outcomes in mice subjected to 1 h of transient middle cerebral artery occlusion (tMCAO) and 24 h of reperfusion. Our data demonstrated that PKD2-KI mice exhibited larger infarction volumes and worsened neurological scores, indicative of increased brain injury, as compared to the wild-type (WT) mice, confirming a neuroprotective role of PKD2 in ischemia/reperfusion (I/R) injury. Mouse primary neurons obtained from PKD2-KI mice also exhibited increased cell death as compared to the WT neurons when subjected to in vitro ischemia. We have further identified AKT and CREB as two main signaling nodes through which PKD2 regulates neuronal survival during I/R injury. In summary, PKD2 confers neuroprotection in ischemic stroke by promoting AKT and CREB activation and targeted activation of PKD2 may benefit neuronal survival in ischemic stroke.

Protein kinase D promotes activity-dependent AMPA receptor endocytosis in hippocampal neurons

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) type glutamate receptors (AMPARs) mediate the majority of fast excitatory neurotransmission in the brain. The continuous trafficking of AMPARs into and out of synapses is a core feature of synaptic plasticity, which is considered as the cellular basis of learning and memory. The molecular mechanisms underlying the postsynaptic AMPAR trafficking, however, are still not fully understood. In this work, we demonstrate that the protein kinase D (PKD) family promotes basal and activity-induced AMPAR endocytosis in primary hippocampal neurons. Pharmacological inhibition of PKD increased synaptic levels of GluA1-containing AMPARs, slowed down their endocytic trafficking and increased neuronal network activity. By contrast, ectopic expression of constitutive active PKD decreased the synaptic level of AMPARs, while increasing their colocalization with early endosomes. Our results thus establish an important role for PKD in the regulation of postsynaptic AMPAR trafficking during synaptic plasticity.

A zebrafish forward genetic screen identifies an indispensable threonine residue in the kinase domain of PRKD2

Protein kinase D2 belongs to a family of evolutionarily conserved enzymes regulating several biological processes. In a forward genetic screen for zebrafish cardiovascular mutants, we identified a mutation in the prkd2 gene. Homozygous mutant embryos develop as wild type up to 36 h post-fertilization and initiate blood flow, but fail to maintain it, resulting in a complete outflow tract stenosis. We identified a mutation in the prkd2 gene that results in a T757A substitution at a conserved residue in the kinase domain activation loop (T714A in human PRKD2) that disrupts catalytic activity and drives this phenotype. Homozygous mutants survive without circulation for several days, allowing us to study the extreme phenotype of no intracardiac flow, in the background of a functional heart. We show dysregulation of atrioventricular and outflow tract markers in the mutants and higher sensitivity to the Calcineurin inhibitor, Cyclosporin A. Finally we identify TBX5 as a potential regulator of PRKD2. Our results implicate PRKD2 catalytic activity in outflow tract development in zebrafish.

This article has an associated First Person interview with the first author of the paper.

Summary: We identified, through a zebrafish forward screen, an evolutionarily conserved residue in the catalytic domain of protein kinase D2 and its homologues.

Phos-tag SDS-PAGE resolves agonist- and isoform-specific activation patterns for PKD2 and PKD3 in cardiomyocytes and cardiac fibroblasts

Protein kinase D (PKD) consists of a family of three structurally related enzymes that are co-expressed in the heart and have important roles in many biological responses. PKD1 is activated by pro-hypertrophic stimuli and has been implicated in adverse cardiac remodeling. Efforts to define the cardiac actions of PKD2 and PKD3 have been less successful at least in part because conventional methods provide a general screen for PKD activation but are poorly suited to resolve activation patterns for PKD2 or PKD3. This study uses Phos-tag SDS-PAGE, a method that exaggerates phosphorylation-dependent mobility shifts, to overcome this technical limitation. Phos-tag SDS-PAGE resolves PKD1 as distinct molecular species (indicative of pools of enzyme with distinct phosphorylation profiles) in unstimulated cardiac fibroblasts and cardiomyocytes; as a result, attempts to track PKD1 mobility shifts that result from agonist activation were only moderately successful. In contrast, PKD2 and PKD3 are recovered from resting cardiac fibroblasts and cardiomyocytes as single molecular species; both enzymes display robust mobility shifts in Phos-tag SDS-PAGE in response to treatment with sphingosine-1-phosphate, thrombin, PDGF, or H₂O₂. Studies with GF109203X implicate protein kinase C activity in the stimulus-dependent pathways that activate PKD2/PKD3 in both cardiac fibroblasts and cardiomyocytes. Studies with C3 toxin identify a novel role for Rho in the sphingosine-1-phosphate and thrombin receptor-dependent pathways that lead to the phosphorylation of PKD2/3 and the downstream substrate CREB in cardiomyocytes. In conclusion, Phos-tag SDS-PAGE provides a general screen for stimulus-specific changes in PKD2 and PKD3 phosphorylation and exposes a novel role for these enzymes in specific stress-dependent pathways that influence cardiac remodeling.

Protein kinase D promotes plasticity-induced F-actin stabilization in dendritic spines and regulates memory formation

PKD regulates the stabilization of the F-actin network within dendritic spines upon chemically induced plasticity changes and is needed for proper hippocampal LTP and spatial memory formation.

Actin turnover in dendritic spines influences spine development, morphology, and plasticity, with functional consequences on learning and memory formation. In nonneuronal cells, protein kinase D (PKD) has an important role in stabilizing F-actin via multiple molecular pathways. Using in vitro models of neuronal plasticity, such as glycine-induced chemical long-term potentiation (LTP), known to evoke synaptic plasticity, or long-term depolarization block by KCl, leading to homeostatic morphological changes, we show that actin stabilization needed for the enlargement of dendritic spines is dependent on PKD activity. Consequently, impaired PKD functions attenuate activity-dependent changes in hippocampal dendritic spines, including LTP formation, cause morphological alterations in vivo, and have deleterious consequences on spatial memory formation. We thus provide compelling evidence that PKD controls synaptic plasticity and learning by regulating actin stability in dendritic spines.

A time frame permissive for Protein Kinase D2 activity to direct angiogenesis in mouse embryonic stem cells

The protein kinase D isoenzymes PKD1/2/3 are prominent downstream targets of PKCs (Protein Kinase Cs) and phospholipase D in various biological systems. Recently, we identified PKD isoforms as novel mediators of tumour cell-endothelial cell communication, tumour cell motility and metastasis. Although PKD isoforms have been implicated in physiological/tumour angiogenesis, a role of PKDs during embryonic development, vasculogenesis and angiogenesis still remains elusive. We investigated the role of PKDs in germ layer segregation and subsequent vasculogenesis and angiogenesis using mouse embryonic stem cells (ESCs). We show that mouse ESCs predominantly express PKD2 followed by PKD3 while PKD1 displays negligible levels. Furthermore, we demonstrate that PKD2 is specifically phosphorylated/activated at the time of germ layer segregation. Time-restricted PKD2-activation limits mesendoderm formation and subsequent cardiovasculogenesis during early differentiation while leading to branching angiogenesis during late differentiation. In line, PKD2 loss-of-function analyses showed induction of mesendodermal differentiation in expense of the neuroectodermal germ layer. Our in vivo findings demonstrate that embryoid bodies transplanted on chicken chorioallantoic membrane induced an angiogenic response indicating that timed overexpression of PKD2 from day 4 onwards leads to augmented angiogenesis in differentiating ESCs. Taken together, our results describe novel and time-dependent facets of PKD2 during early cell fate determination.

Circadian Clocks and the Interaction between Stress Axis and Adipose Function

Many physiological processes and most endocrine functions show fluctuations over the course of the day. These so-called circadian rhythms are governed by an endogenous network of cellular clocks and serve as an adaptation to daily and, thus, predictable changes in the organism's environment. Circadian clocks have been described in several tissues of the stress axis and in adipose cells where they regulate the rhythmic and stimulated release of stress hormones, such as glucocorticoids, and various adipokine factors. Recent work suggests that both adipose and stress axis clock systems reciprocally influence each other and adrenal-adipose rhythms may be key players in the development and therapy of metabolic disorders. In this review, we summarize our current understanding of adrenal and adipose tissue rhythms and clocks and how they might interact to regulate energy homoeostasis and stress responses under physiological conditions. Potential chronotherapeutic strategies for the treatment of metabolic and stress disorders are discussed.

A 2.0 Mb microdeletion in proximal chromosome 14q12, involving regulatory elements of FOXG1, with the coding region of FOXG1 being unaffected, results in severe developmental delay, microcephaly, and hypoplasia of the corpus callosum

We identified 2.0 Mb of a novel deletion on chromosome 14q12, involving 8 genes and putative regulatory elements of FOXG1 by array CGH in a patient with severe growth and psychomotor retardation, hypotonia, microcephaly, dysmorphic face, and hypoplasia of the corpus callosum. Case of a submicroscopic 14q12 deletion, involving regulatory elements of FOXG1, with the coding region of FOXG1 being unaffected, is extremely rare. Using fibroblast cell line established from the patient, we showed that the expression level of FOXG1 in our patient was decreased. Our finding provides additional evidence that not only over-dosage of FOXG1 as previously mentioned, under-dosage of FOXG1 is also associated with phenotype, overlapping between congenital variant of Rett syndrome with FOXG1 mutations and 14q12 microdeletion, not including the coding region of FOXG1. Though the gene dosage of FOXG1 appears to be critical for the normal development of brain, the complex mechanism of its regulation of gene expression remains to be elucidated.

Physiological functions of protein kinase D in vivo

The cellular functions of the serine/threonine protein kinase D (PKD) have been extensively studied within the last decade and distinct roles such as fission of vesicles at the Golgi compartment, coordination of cell migration and invasion, and regulation of gene transcription have been correlated with this kinase family. Here, we highlight the current state of in vivo studies on PKD function with a focus on animal models and discuss the molecular basis of the observed phenotypic characteristics associated with this kinase family.

Discovery of diverse small molecule chemotypes with cell-based PKD1 inhibitory activity

Protein kinase D (PKD) is a novel family of serine/threonine kinases regulated by diacylglycerol, which is involved in multiple cellular processes and various pathological conditions. The limited number of cell-active, selective inhibitors has historically restricted biochemical and pharmacological studies of PKD. We now markedly expand the PKD1 inhibitory chemotype inventory with eleven additional novel small molecule PKD1 inhibitors derived from our high throughput screening campaigns. The in vitro IC(50)s for these eleven compounds ranged in potency from 0.4 to 6.1 µM with all of the evaluated compounds being competitive with ATP. Three of the inhibitors (CID 1893668, (1Z)-1-(3-ethyl-5-methoxy-1,3-benzothiazol-2-ylidene)propan-2-one; CID 2011756, 5-(3-chlorophenyl)-N-[4-(morpholin-4-ylmethyl)phenyl]furan-2-carboxamide; CID 5389142, (6Z)-6-[4-(3-aminopropylamino)-6-methyl-1H-pyrimidin-2-ylidene]cyclohexa-2,4-dien-1-one) inhibited phorbol ester-induced endogenous PKD1 activation in LNCaP prostate cancer cells in a concentration-dependent manner. The specificity of these compounds for PKD1 inhibitory activity was supported by kinase assay counter screens as well as by bioinformatics searches. Moreover, computational analyses of these novel cell-active PKD1 inhibitors indicated that they were structurally distinct from the previously described cell-active PKD1 inhibitors while computational docking of the new cell-active compounds in a highly conserved ATP-binding cleft suggests opportunities for structural modification. In summary, we have discovered novel PKD1 inhibitors with in vitro and cell-based inhibitory activity, thus successfully expanding the structural diversity of small molecule inhibitors available for this important pharmacological target.

Protein kinase D2 controls cardiac valve formation in zebrafish by regulating histone deacetylase 5 activity

BACKGROUND

The molecular mechanisms that guide heart valve formation are not well understood. However, elucidation of the genetic basis of congenital heart disease is one of the prerequisites for the development of tissue-engineered heart valves.

METHODS AND RESULTS

We isolated here a mutation in zebrafish, bungee (bng(jh177)), which selectively perturbs valve formation in the embryonic heart by abrogating endocardial Notch signaling in cardiac cushions. We found by positional cloning that the bng phenotype is caused by a missense mutation (Y849N) in zebrafish protein kinase D2 (pkd2). The bng mutation selectively impairs PKD2 kinase activity and hence Histone deacetylase 5 phosphorylation, nuclear export, and inactivation. As a result, the expression of Histone deacetylase 5 target genes Krüppel-like factor 2a and 4a, transcription factors known to be pivotal for heart valve formation and to act upstream of Notch signaling, is severely downregulated in bungee (bng) mutant embryos. Accordingly, the expression of Notch target genes, such as Hey1, Hey2, and HeyL, is severely decreased in bng mutant embryos. Remarkably, downregulation of Histone deacetylase 5 activity in homozygous bng mutant embryos can rescue the mutant phenotype and reconstitutes notch1b expression in atrioventricular endocardial cells.

CONCLUSIONS

We demonstrate for the first time that proper heart valve formation critically depends on Protein kinase D2-Histone deacetylase 5-Krüppel-like factor signaling.

Protein kinase D2 is an essential regulator of murine myoblast differentiation

Muscle differentiation is a highly conserved process that occurs through the activation of quiescent satellite cells whose progeny proliferate, differentiate, and fuse to generate new myofibers. A defined pattern of myogenic transcription factors is orchestrated during this process and is regulated via distinct signaling cascades involving various intracellular signaling pathways, including members of the protein kinase C (PKC) family. The protein kinase D (PKD) isoenzymes PKD1, -2, and -3, are prominent downstream targets of PKCs and phospholipase D in various biological systems including mouse and could hence play a role in muscle differentiation. In the present study, we used a mouse myoblast cell line (C2C12) as an in vitro model to investigate the role of PKDs, in particular PKD2, in muscle stem cell differentiation. We show that C2C12 cells express all PKD isoforms with PKD2 being highly expressed. Furthermore, we demonstrate that PKD2 is specifically phosphorylated/activated during the initiation of mouse myoblast differentiation. Selective inhibition of PKCs or PKDs by pharmacological inhibitors blocked myotube formation. Depletion of PKD2 by shRNAs resulted in a marked inhibition of myoblast cell fusion. PKD2-depleted cells exhibit impaired regulation of muscle development-associated genes while the proliferative capacity remains unaltered. Vice versa forced expression of PKD2 increases myoblast differentiation. These findings were confirmed in primary mouse satellite cells where myotube fusion was also decreased upon inhibition of PKDs. Active PKD2 induced transcriptional activation of myocyte enhancer factor 2D and repression of Pax3 transcriptional activity. In conclusion, we identify PKDs, in particular PKD2, as a major mediator of muscle cell differentiation in vitro and thereby as a potential novel target for the modulation of muscle regeneration.

Protein kinase D isoforms are activated in an agonist-specific manner in cardiomyocytes

Protein kinase D (PKD) exists as a family of structurally related enzymes that are activated through similar phosphorylation-dependent mechanisms involving protein kinase C (PKC). While individual PKD isoforms could in theory mediate distinct biological functions, previous studies identify a high level of functional redundancy for PKD1 and PKD2 in various cellular contexts. This study shows that PKD1 and PKD2 are activated in a stimulus-specific manner in neonatal cardiomyocytes. The α(1)-adrenergic receptor agonist norepinephrine selectively activates PKD1, thrombin and PDGF selectively activate PKD2, and endothelin-1 and PMA activate both PKD1 and PKD2. PKC activity is implicated in the α(1)-adrenergic receptor pathway that activates PKD1 and the thrombin- and PDGF-dependent pathways that activate PKD2. Endothelin-1 activates PKD via both rapid PKC-dependent and more sustained PKC-independent mechanisms. The functional consequences of PKD activation were assessed by tracking phosphorylation of CREB and cardiac troponin I (cTnI), two physiologically relevant PKD substrates in cardiomyocytes. We show that overexpression of an activated PKD1-S744E/S748E transgene increases CREB-Ser(133) and cTnI-Ser(23)/Ser(24) phosphorylation, but agonist-dependent pathways that activate native PKD1 or PKD2 selectively increase CREB-Ser(133) phosphorylation; there is no associated increase in cTnI-Ser(23)/Ser(24) phosphorylation. Gene silencing studies provide unanticipated evidence that PKD1 down-regulation leads to a compensatory increase in PKD2 activity and that down-regulation of PKD1 (alone or in combination with PKD2) leads to an increase in CREB-Ser(133) phosphorylation. Collectively, these studies identify distinct roles for native PKD1 and PKD2 enzymes in stress-dependent pathways that influence cardiac remodeling and the progression of heart failure.

Unique functions for protein kinase D1 and protein kinase D2 in mammalian cells

Mammalian PKD (protein kinase D) isoforms have been implicated in the regulation of diverse biological processes in response to diacylglycerol and PKC (protein kinase C) signalling. To compare the functions of PKD1 and PKD2 in vivo, we generated mice deficient in either PKD1 or PKD2 enzymatic activity, via homozygous expression of PKD1^S744A/S748A or PKD2^S707A/S711A ‘knockin’ alleles. We also examined PKD2-deficient mice generated using ‘gene-trap’ technology. We demonstrate that, unlike PKD1, PKD2 catalytic activity is dispensable for normal embryogenesis. We also show that PKD2 is the major PKD isoform expressed in lymphoid tissues, but that PKD2 catalytic activity is not essential for the development of mature peripheral T- and B-lymphocytes. PKD2 catalytic activity is, however, required for efficient antigen receptor-induced cytokine production in T-lymphocytes and for optimal T-cell-dependent antibody responses in vivo. Our results reveal a key in vivo role for PKD2 in regulating the function of mature peripheral lymphocytes during adaptive immune responses. They also confirm the functional importance of PKC-mediated serine phosphorylation of the PKD catalytic domain for PKD activation and downstream signalling and reveal that different PKD family members have unique and non-redundant roles in vivo.

Protein kinase D controls the integrity of Golgi apparatus and the maintenance of dendritic arborization in hippocampal neurons

Protein kinase D (PKD) is known to participate in various cellular functions, including secretory vesicle fission from the Golgi and plasma membrane-directed transport. Here, we report on expression and function of PKD in hippocampal neurons. Expression of an enhanced green fluorescent protein (EGFP)-tagged PKD activity reporter in mouse embryonal hippocampal neurons revealed high endogenous PKD activity at the Golgi complex and in the dendrites, whereas PKD activity was excluded from the axon in parallel with axonal maturation. Expression of fluorescently tagged wild-type PKD1 and constitutively active PKD1(S738/742E) (caPKD1) in neurons revealed that both proteins were slightly enriched at the trans-Golgi network (TGN) and did not interfere with its thread-like morphology. By contrast, expression of dominant-negative kinase inactive PKD1(K612W) (kdPKD1) led to the disruption of the neuronal Golgi complex, with kdPKD1 strongly localized to the TGN fragments. Similar findings were obtained from transgenic mice with inducible, neuron-specific expression of kdPKD1-EGFP. As a prominent consequence of kdPKD1 expression, the dendritic tree of transfected neurons was reduced, whereas caPKD1 increased dendritic arborization. Our results thus provide direct evidence that PKD activity is selectively involved in the maintenance of dendritic arborization and Golgi structure of hippocampal neurons.

PKD3 is the predominant protein kinase D isoform in mouse exocrine pancreas and promotes hormone-induced amylase secretion

The protein kinase D (PKD) family of serine/threonine kinases, which can be activated by gastrointestinal hormones, consists of three distinct isoforms that modulate a variety of cellular processes including intracellular protein transport as well as constitutive and regulated secretion. Although isoform-specific functions have been identified in a variety of cell lines, the expression and function of PKD isoforms in normal, differentiated secretory tissues is unknown. Here, we demonstrate that PKD isoforms are differentially expressed in the exocrine and endocrine cells of the pancreas. Specifically, PKD3 is the predominant isoform expressed in exocrine cells of the mouse and human pancreas, whereas PKD1 and PKD2 are more abundantly expressed in the pancreatic islets. Within isolated mouse pancreatic acinar cells, PKD3 undergoes rapid membrane translocation, trans-activating phosphorylation, and kinase activation after gastrointestinal hormone or cholinergic stimulation. PKD phosphorylation in pancreatic acinar cells occurs viaaCa2+-independent, diacylglycerol- and protein kinase C-dependent mechanism. PKD phosphorylation can also be induced by physiologic concentrations of secretagogues and by in vivo stimulation of the pancreas. Furthermore, activation of PKD3 potentiates MEK/ERK/RSK (RSK, ribosomal S6 kinase) signaling and significantly enhances cholecystokinin-mediated pancreatic amylase secretion. These findings reveal a novel distinction between the exocrine and endocrine cells of the pancreas and further identify PKD3 as a signaling molecule that promotes hormone-stimulated amylase secretion.

Expression patterns of protein kinase D 3 during mouse development

Background

The PKD family of serine/threonine kinases comprises a single member in Drosophila (dPKD), two isoforms in C. elegans (DKF-1 and 2) and three members, PKD1, PKD2 and PKD3 in mammals. PKD1 and PKD2 have been the focus of most studies up to date, which implicate these enzymes in very diverse cellular functions, including Golgi organization and plasma membrane directed transport, immune responses, apoptosis and cell proliferation. Concerning PKD3, a role in the formation of vesicular transport carriers at the trans-Golgi network (TGN) and in basal glucose transport has been inferred from in vitro studies. So far, however, the physiological functions of the kinase during development remain unknown.

Results

We have examined the expression pattern of PKD3 during the development of mouse embryos by immunohistochemistry. Using a PKD3 specific antibody we demonstrate that the kinase is differentially expressed during organogenesis. In the developing heart a strong PKD3 expression is constantly detected from E10 to E16.5. From E12.5 on PKD3 is increasingly expressed in neuronal as well as in the supporting connective tissue and in skeletal muscles.

Conclusion

The data presented support an important role for PKD3 during development of these tissues.

Requirement of protein kinase D1 for pathological cardiac remodeling

The adult heart responds to biomechanical stress and neurohormonal signaling by hypertrophic growth, accompanied by fibrosis, diminished pump function, and activation of a fetal gene program. Class II histone deacetylases (HDACs) suppress stress-dependent remodeling of the heart via their association with the MEF2 transcription factor, an activator of heart disease. Protein kinase D (PKD) is a stress-responsive kinase that phosphorylates class II HDACs, resulting in their dissociation from MEF2 with consequent activation of MEF2 target genes. To test whether PKD1 is required for pathological cardiac remodeling in vivo, we generated mice with a conditional PKD1-null allele. Mice with cardiac-specific deletion of PKD1 were viable and showed diminished hypertrophy, fibrosis, and fetal gene activation as well as improved cardiac function in response to pressure overload or chronic adrenergic and angiotensin II signaling. We conclude that PKD1 functions as a key transducer of stress stimuli involved in pathological cardiac remodeling in vivo.

Properties, Regulation, and in Vivo Functions of a Novel Protein Kinase D

Protein kinase D (PKD) isoforms are protein kinase C effectors in signaling cascades controlled by diacylglycerol (DAG). All PKDs are regulated by DAG/phorbol 12-myristate 13-acetate-binding C1 domains and an activation loop (A-loop). To understand how PKD isoforms diversify DAG signaling networks, it is essential to determine redundant and novel properties of their regulatory domains, characterize factors controlling PKD gene expression, and discover their in vivo physiological roles. Studies on a novel PKD, Caenorhabditis elegans DKF-2 (D kinase family-2), addressed these topics. The C1b domain mediates phorbol 12-myristate 13-acetate-induced translocation and activation of DKF-2. However, when DAG is elevated, C1a and C1b contribute equally to targeting/activation of DKF-2. DKF-2 C1 domains do not inhibit catalytic activity; they mediate delivery of DKF-2 to a membrane where protein kinase C phosphorylates Ser(925) and Ser(929) in the A-loop. This potently stimulates DKF-2 catalytic activity. Phosphorylation of Ser(925) alone switches on 70% of maximal kinase activity. Persistent phosphorylation of Ser(929) tags DKF-2 for proteasomal degradation; Ser(P)(925) plays a minor role in DKF-2 degradation. GATA enhancer sequences govern DKF-2 expression in intestine in vivo. Adult life span increases 40% in animals lacking DKF-2. In thermally stressed wild type animals, the DAF-16 transcription factor is segregated from the nuclei of adult intestinal cells. In contrast, DAF-16 enters adult intestinal nuclei of DKF-2-deficient, thermally stressed animals, where it can trigger gene transcription that protects against various insults. The results suggest a mechanism for increased longevity and show that a PKD links DAG signals to regulation of stress responses and life span.

Cloning and expression analysis of a protein kinase C gene, PKCmu, and its regulation of the promoter region in zebrafish

The cDNA and genomic DNA of zebrafish (Danio rerio) protein kinase Cmu (PKCmu), with its promoter region, were obtained. The 508-amino acid zebrafish PKCmu has 86.17% similarity to human PKCmu. Real-time reverse-transcription polymerase chain reaction analysis with starvation and hormonal treatment found significant differences between the control group and the experimental group after 14 days of starvation. After injecting insulin-like growth factor II (IGF-II), growth hormone (GH), insulin, or human chorionic gonadotropin, significant differences were observed between the control and experimental groups 24 h after treatment. After injecting the gonadotropin-releasing hormone or luteotropin-releasing hormone, significant differences were seen between the control and experimental groups 15 h after treatment. These results suggest that in vivo PKCmu expression is regulated by the insulin family or by the GH, but other sex hormones produced a significant expression level more quickly than the insulin family and GH. The zebrafish PKCmu gene is located on zebrafish chromosome 17 and consists of 16 exons. A 2.6 kilobase pair on the 5' flanking region displayed maximal promoter activity in the zebrafish liver (ZFL) cell line after treatment with IGF-I, IGF-II, and GH. However, a 1.6 kilobase pair on the 5' flanking region displayed maximal promoter activity in the HeLa cell line after treatment with IGF-I, IGF-II, and GH. Finally, PKCmu may have important nuclear effects on cell growth and may involve nuclear localization. By transiently transfecting ZFL cells with various zebrafish PKCmu segments, we identified a nuclear localization signal: the amino acid sequence between amino acids 206 and 209 was able to predominantly direct enhanced green fluorescence protein (EGFP) into the nucleus, whereas a deletion of this motif abrogated the nuclear localization property.

Protein kinase D regulates several aspects of development in Drosophila melanogaster

Background

Protein Kinase D (PKD) is an effector of diacylglycerol-regulated signaling pathways. Three isoforms are known in mammals that have been linked to diverse cellular functions including regulation of cell proliferation, differentiation, motility and secretory transport from the trans-Golgi network to the plasma membrane. In Drosophila, there is a single PKD orthologue, whose broad expression implicates a more general role in development.

Results

We have employed tissue specific overexpression of various PKD variants as well as tissue specific RNAi, in order to investigate the function of the PKD gene in Drosophila. Apart from a wild type (WT), a kinase dead (kd) and constitutively active (SE) Drosophila PKD variant, we also analyzed two human isoforms hPKD2 and hPKD3 for their capacity to substitute PKD activity in the fly. Overexpression of either WT or kd-PKD variants affected primarily wing vein development. However, overexpression of SE-PKD and PKD RNAi was deleterious. We observed tissue loss, wing defects and degeneration of the retina. The latter phenotype conforms to a role of PKD in the regulation of cytoskeletal dynamics. Strongest phenotypes were larval to pupal lethality. RNAi induced phenotypes could be rescued by a concurrent overexpression of Drosophila wild type PKD or either human isoform hPKD2 and hPKD3.

Conclusion

Our data confirm the hypothesis that Drosophila PKD is a multifunctional kinase involved in diverse processes such as regulation of the cytoskeleton, cell proliferation and death as well as differentiation of various fly tissues.

Protein kinase D2 potentiates MEK/ERK/RSK signaling, c-Fos accumulation and DNA synthesis induced by bombesin in Swiss 3T3 cells

Protein kinase D (PKD) plays an important role in mediating cellular DNA synthesis in response to G protein-coupled receptor (GPCR) agonists but the function of other isoforms of the PKD family has been much less explored. Here, we examined whether PKD2 overexpression in Swiss 3T3 cells facilitates DNA synthesis and the activation of the extracellular regulated protein kinase (ERK) pathway in response to the mitogenic GPCR agonist bombesin. We show that PKD2 overexpression markedly potentiated the ability of this agonist to induce DNA synthesis. Addition of bombesin to Swiss 3T3 cells overexpressing PKD2 also induced a striking increase in the duration of MEK/ERK/RSK activation as compared with cultures of control cells. In contrast, neither DNA synthesis nor the duration of ERK activation in response to epidermal growth factor, which acts via protein kinase C/PKD2-independent pathways, was increased. Furthermore, bombesin promoted a striking accumulation of c-Fos protein in cells overexpressing PKD2. Our study demonstrates that PKD2, like PKD, facilitates mitogenesis and supports the hypothesis that an increase in the duration of the ERK signaling leading to accumulation of immediate gene products is one of the mechanisms by which isoforms of the PKD family enhance re-initiation of DNA synthesis by Gq-coupled receptor activation.

Protein kinase D2 mediates lysophosphatidic acid-induced interleukin 8 production in nontransformed human colonic epithelial cells through NF-kappaB

The signaling pathways mediating lysophosphatidic acid (LPA)-stimulated PKD(2) activation and the potential contribution of PKD(2) in regulating LPA-induced interleukin 8 (IL-8) secretion in nontransformed, human colonic epithelial NCM460 cells were examined. Treatment of serum-deprived NCM460 cells with LPA led to a rapid and striking activation of PKD(2), as measured by in vitro kinase assay and phosphorylation at the activation loop (Ser706/710) and autophosphorylation site (Ser876). PKD(2) activation induced by LPA was abrogated by preincubation with selective PKC inhibitors GF-I and Ro-31-8220 in a dose-dependent manner. These inhibitors did not have any direct inhibitory effect on PKD(2) activity. LPA induced a striking increase in IL-8 production and stimulated NF-kappaB activation, as measured by NF-kappaB-DNA binding, NF-kappaB-driven luciferase reporter activity, and IkappaBalpha phosphorylation. PKD(2) gene silencing utilizing small interfering RNAs targeting distinct PKD(2) sequences dramatically reduced LPA-stimulated NF-kappaB promoter activity and IL-8 production. PKD(2) activation is a novel early event in the biological action of LPA and mediates LPA-stimulated IL-8 secretion in NCM460 cells through a NF-kappaB-dependent pathway. Our results demonstrate, for the first time, the involvement of a member of the PKD family in the production of IL-8, a potent proinflammatory chemokine, by epithelial cells.