Ca2+-dependent phosphorylation of syntaxin-1A by the death-associated protein (DAP) kinase regulates its interaction with Munc18

Malic enzyme 2 maintains metabolic state and anti-tumor immunity of CD8+ T cells

The functional integrity of CD8+ T cells is closely linked to metabolic reprogramming; therefore, understanding the metabolic basis of CD8+ T cell activation and antitumor immunity could provide insights into tumor immunotherapy. Here, we report that ME2 is critical for mouse CD8+ T cell activation and immune response against malignancy. ME2 deficiency suppresses CD8+ T cell activation and anti-tumor immune response in vitro and in vivo. Mechanistically, ME2 depletion blocks the TCA cycle flux, leading to the accumulation of fumarate. Fumarate directly binds to DAPK1 and inhibits its activity by competing with ATP for binding. Notably, pharmacological inhibition of DAPK1 abolishes the anti-tumor function conferred by ME2 to CD8+ T cells. Collectively, these findings demonstrate a role for ME2 in the regulation of CD8+ T cell metabolism and effector functions as well as an unexpected function for fumarate as a metabolic signal in the inhibition of DAPK1.

Quantitative Proteomic and Phosphoproteomic Analyses Reveal a Role of Death-Associated Protein Kinase 1 in Regulating Hippocampal Synapse

Death-associated protein kinase 1 (DAPK1) is a stress-responsive calcium/calmodulin (CaM)-regulated serine/threonine protein kinase that is actively involved in stress-induced cell death. The dysregulation of DAPK1 has been established in various neurological disorders such as epilepsy, Alzheimer's disease (AD), and Parkinson's disease (PD). Recent research indicates a synaptic localization of DAPK1 in neurons, suggesting a potential role of DAPK1 in modulating synaptic structure and function. However, the key molecules and pathways underlying the influence of DAPK1 on synapses remain elusive. We utilized quantitative proteomic and phosphoproteomic analyses to compare the differences in protein expression and phosphorylation in hippocampal tissues of wild-type (WT) and DAPK1-knockout (KO) mice. Bioinformatic analysis of differentially expressed proteins and phosphoproteins revealed a preferential enrichment of proteins involved in regulating synaptic function, cytoskeletal structure, and neurotransmission. Gene set enrichment analysis (GESA) highlighted altered presynaptic functions including synaptic vesicle priming and glutamate secretion in KO mice. Besides, we observed that proteins with potential phosphorylation motifs of ERK and DAPK1 were overrepresented among the differential phosphoproteins and were highly enriched in neuronal function-related pathways. Furthermore, Western blot analysis validated differences in the expression of several proteins closely associated with presynaptic organization, dendrites and calcium transmembrane transport between KO and WT mice, further corroborating the potential involvement of DAPK1 in the regulation of synaptic functions. Overall, our data provide molecular evidence to elucidate the physiological links between DAPK1 and neuronal functions and help clarify the role of DAPK1 in the pathogenesis of neurodevelopmental and neurodegenerative diseases.

Death-associated protein kinase 1 as a therapeutic target for Alzheimer's disease

Alzheimer's disease (AD) is the most prevalent form of dementia in the elderly and represents a major clinical challenge in the ageing society. Neuropathological hallmarks of AD include neurofibrillary tangles composed of hyperphosphorylated tau, senile plaques derived from the deposition of amyloid-β (Aβ) peptides, brain atrophy induced by neuronal loss, and synaptic dysfunctions. Death-associated protein kinase 1 (DAPK1) is ubiquitously expressed in the central nervous system. Dysregulation of DAPK1 has been shown to contribute to various neurological diseases including AD, ischemic stroke and Parkinson's disease (PD). We have established an upstream effect of DAPK1 on Aβ and tau pathologies and neuronal apoptosis through kinase-mediated protein phosphorylation, supporting a causal role of DAPK1 in the pathophysiology of AD. In this review, we summarize current knowledge about how DAPK1 is involved in various AD pathological changes including tau hyperphosphorylation, Aβ deposition, neuronal cell death and synaptic degeneration. The underlying molecular mechanisms of DAPK1 dysregulation in AD are discussed. We also review the recent progress regarding the development of novel DAPK1 modulators and their potential applications in AD intervention. These findings substantiate DAPK1 as a novel therapeutic target for the development of multifunctional disease-modifying treatments for AD and other neurological disorders.

Serine-129 phosphorylation of α-synuclein is an activity-dependent trigger for physiologic protein-protein interactions and synaptic function

Phosphorylation of α-synuclein at the serine-129 site (α-syn Ser129P) is an established pathologic hallmark of synucleinopathies and a therapeutic target. In physiologic states, only a fraction of α-syn is phosphorylated at this site, and most studies have focused on the pathologic roles of this post-translational modification. We found that unlike wild-type (WT) α-syn, which is widely expressed throughout the brain, the overall pattern of α-syn Ser129P is restricted, suggesting intrinsic regulation. Surprisingly, preventing Ser129P blocked activity-dependent synaptic attenuation by α-syn-thought to reflect its normal function. Exploring mechanisms, we found that neuronal activity augments Ser129P, which is a trigger for protein-protein interactions that are necessary for mediating α-syn function at the synapse. AlphaFold2-driven modeling and membrane-binding simulations suggest a scenario where Ser129P induces conformational changes that facilitate interactions with binding partners. Our experiments offer a new conceptual platform for investigating the role of Ser129 in synucleinopathies, with implications for drug development.

Ablation of Death-Associated Protein Kinase 1 Changes the Transcriptomic Profile and Alters Neural-Related Pathways in the Brain

Death-associated protein kinase 1 (DAPK1), a Ca2+/calmodulin-dependent serine/threonine kinase, mediates various neuronal functions, including cell death. Abnormal upregulation of DAPK1 is observed in human patients with neurological diseases, such as Alzheimer’s disease (AD) and epilepsy. Ablation of DAPK1 expression and suppression of DAPK1 activity attenuates neuropathology and behavior impairments. However, whether DAPK1 regulates gene expression in the brain, and whether its gene profile is implicated in neuronal disorders, remains elusive. To reveal the function and pathogenic role of DAPK1 in neurological diseases in the brain, differential transcriptional profiling was performed in the brains of DAPK1 knockout (DAPK1-KO) mice compared with those of wild-type (WT) mice by RNA sequencing. We showed significantly altered genes in the cerebral cortex, hippocampus, brain stem, and cerebellum of both male and female DAPK1-KO mice compared to those in WT mice, respectively. The genes are implicated in multiple neural-related pathways, including: AD, Parkinson’s disease (PD), Huntington’s disease (HD), neurodegeneration, glutamatergic synapse, and GABAergic synapse pathways. Moreover, our findings imply that the potassium voltage-gated channel subfamily A member 1 (Kcna1) may be involved in the modulation of DAPK1 in epilepsy. Our study provides insight into the pathological role of DAPK1 in the regulatory networks in the brain and new therapeutic strategies for the treatment of neurological diseases.

Regulation of DAPK1 by Natural Products: An Important Target in Treatment of Stroke

Stroke is a sudden neurological disorder that occurs due to impaired blood flow to an area of the brain. Stroke can be caused by the blockage or rupture of a blood vessel in the brain, called ischemic stroke and hemorrhagic stroke, respectively. Stroke is more common in men than women. Atrial fibrillation, hypertension, kidney disease, high cholesterol and lipids, genetic predisposition, inactivity, poor nutrition, diabetes mellitus, family history and smoking are factors that increase the risk of stroke. Restoring blood flow by repositioning blocked arteries using thrombolytic agents or endovascular therapy are the most effective treatments for stroke. However, restoring circulation after thrombolysis can cause fatal edema or intracranial hemorrhage, and worsen brain damage in a process known as ischemia-reperfusion injury. Therefore, there is a pressing need to find and develop more effective treatments for stroke. In the past, the first choice of treatment was based on natural compounds. Natural compounds are able to reduce the symptoms and reduce various diseases including stroke that attract the attention of the pharmaceutical industry. Nowadays, as a result of the numerous studies carried out in the field of herbal medicine, many useful and valuable effects of plants have been identified. The death-associated protein kinase (DAPK) family is one of the vital families of serine/threonine kinases involved in the regulation of some biological functions in human cells. DAPK1 is the most studied kinase within the DAPKs family as it is involved in neuronal and recovery processes. Dysregulation of DAPK1 in the brain is involved in the developing neurological diseases such as stroke. Natural products can function in a variety of ways, including reducing cerebral edema, reducing brain endothelial cell death, and inhibiting TNFα and interleukin-1β (IL-1β) through regulating the DAPK1 signal against stroke. Due to the role of DAPK1 in neurological disorders, the aim of this article was to investigate the role of DAPK1 in stroke and its modulation by natural compounds.

Young DAPK1 knockout mice have altered presynaptic function

The death-associated protein kinase 1 (DAPK1) has recently been shown to have a physiological function in long-term depression (LTD) of glutamatergic synapses: acute inhibition of DAPK1 blocked the LTD that is normally seen at the hippocampal CA1 synapse in young mice, and a pharmacogenetic combination approach showed that this specifically required DAPK1-mediated suppression of postsynaptic Ca²⁺/calmodulin-dependent protein kinase II binding to the NMDA-type glutamate receptor (NMDAR) subunit GluN2B during LTD stimuli. Surprisingly, we found here that genetic deletion of DAPK1 (in DAPK1^-/- mice) did not reduce LTD. Paired pulse facilitation experiments indicated a presynaptic compensation mechanism: in contrast to wild-type mice, LTD stimuli in DAPK1^-/- mice decreased presynaptic release probability. Basal synaptic strength was normal in young DAPK1^-/- mice, but basal glutamate release probability was reduced, an effect that normalized with maturation.NEW & NOTEWORTHY Young death-associated protein kinase 1 (DAPK1) knockout mice have reduced basal glutamate release probability, an effect that normalized with maturation. This provided a compensatory mechanism that may have prevented a reduction of long-term depression in the young DAPK1 knockout mice.

Heteromeric Solute Carriers: Function, Structure, Pathology and Pharmacology

Solute carriers form one of three major superfamilies of membrane transporters in humans, and include uniporters, exchangers and symporters. Following several decades of molecular characterisation, multiple solute carriers that form obligatory heteromers with unrelated subunits are emerging as a distinctive principle of membrane transporter assembly. Here we comprehensively review experimentally established heteromeric solute carriers: SLC3-SLC7 amino acid exchangers, SLC16 monocarboxylate/H⁺ symporters and basigin/embigin, SLC4A1 (AE1) and glycophorin A exchanger, SLC51 heteromer Ost α-Ost β uniporter, and SLC6 heteromeric symporters. The review covers the history of the heteromer discovery, transporter physiology, structure, disease associations and pharmacology - all with a focus on the heteromeric assembly. The cellular locations, requirements for complex formation, and the functional role of dimerization are extensively detailed, including analysis of the first complete heteromer structures, the SLC7-SLC3 family transporters LAT1-4F2hc, b^0,+AT-rBAT and the SLC6 family heteromer B⁰AT1-ACE2. We present a systematic analysis of the structural and functional aspects of heteromeric solute carriers and conclude with common principles of their functional roles and structural architecture.

Protein Phosphorylation in Depolarized Synaptosomes: Dissecting Primary Effects of Calcium from Synaptic Vesicle Cycling

Synaptic transmission is mediated by the regulated exocytosis of synaptic vesicles. When the presynaptic membrane is depolarized by an incoming action potential, voltage-gated calcium channels open, resulting in the influx of calcium ions that triggers the fusion of synaptic vesicles (SVs) with the plasma membrane. SVs are recycled by endocytosis. Phosphorylation of synaptic proteins plays a major role in these processes, and several studies have shown that the synaptic phosphoproteome changes rapidly in response to depolarization. However, it is unclear which of these changes are directly linked to SV cycling and which might regulate other presynaptic functions that are also controlled by calcium-dependent kinases and phosphatases. To address this question, we analyzed changes in the phosphoproteome using rat synaptosomes in which exocytosis was blocked with botulinum neurotoxins (BoNTs) while depolarization-induced calcium influx remained unchanged. BoNT-treatment significantly alters the response of the synaptic phoshoproteome to depolarization and results in reduced phosphorylation levels when compared with stimulation of synaptosomes by depolarization with KCl alone. We dissect the primary Ca2+-dependent phosphorylation from SV-cycling-dependent phosphorylation and confirm an effect of such SV-cycling-dependent phosphorylation events on syntaxin-1a-T21/T23, synaptobrevin-S75, and cannabinoid receptor-1-S314/T322 on exo- and endocytosis in cultured hippocampal neurons.

Phosphoproteomics reveals that the hVPS34 regulated SGK3 kinase specifically phosphorylates endosomal proteins including Syntaxin-7, Syntaxin-12, RFIP4 and WDR44

The serum- and glucocorticoid-regulated kinase (SGK) isoforms contribute resistance to cancer therapies targeting the PI3K pathway. SGKs are homologous to Akt and these kinases display overlapping specificity and phosphorylate several substrates at the same residues, such as TSC2 to promote tumor growth by switching on the mTORC1 pathway. The SGK3 isoform is up-regulated in breast cancer cells treated with PI3K or Akt inhibitors and recruited and activated at endosomes, through its phox homology domain binding to PtdIns(3)P. We undertook genetic and pharmacological phosphoproteomic screens to uncover novel SGK3 substrates. We identified 40 potential novel SGK3 substrates, including four endosomal proteins STX7 (Ser126) and STX12 (Ser139), RFIP4 (Ser527) and WDR44 (Ser346) that were efficiently phosphorylated in vitro by SGK3 at the sites identified in vivo, but poorly by Akt. We demonstrate that these substrates are inefficiently phosphorylated by Akt as they possess an n + 1 residue from the phosphorylation site that is unfavorable for Akt phosphorylation. Phos-tag analysis revealed that stimulation of HEK293 cells with IGF1 to activate SGK3, promoted phosphorylation of a significant fraction of endogenous STX7 and STX12, in a manner that was blocked by knock-out of SGK3 or treatment with a pan SGK inhibitor (14H). SGK3 phosphorylation of STX12 enhanced interaction with the VAMP4/VTI1A/STX6 containing the SNARE complex and promoted plasma membrane localization. Our data reveal novel substrates for SGK3 and suggest a mechanism by which STX7 and STX12 SNARE complexes are regulated by SGK3. They reveal new biomarkers for monitoring SGK3 pathway activity.

Presynaptic Caytaxin prevents apoptosis via deactivating DAPK1 in the acute phase of cerebral ischemic stroke

Death-associated protein kinase 1 (DAPK1) is a key protein that mediates neuronal death in ischemic stroke. Although the substrates of DAPK1 and molecular signal in stroke have been gradually discovered, the modulation of DAPK1 itself is still unclear. Here we first reveal that Caytaxin, a brain-specific member of BCL2/adenovirus E1B -interacting protein (BNIP-2), increases and interacts with DAPK1 as early as 2 h after middle cerebral artery occlusion (MCAO) in the penumbra area of mouse brain. Furthermore, Caytaxin binds to DAPK1 at the presynaptic site and inhibits DAPK1 catalytic activity. Silencing Caytaxin by Caytaxin shRNA (Sh-Caytaxin) enhances DAPK1 activity, deteriorates neuronal apoptosis and brain injuries both in vivo and in vitro. Thus, elevating presynaptic Caytaxin could prevent neuronal apoptosis by inhibiting DAPK1 activation in the acute stage of ischemic stroke. Caytaxin may physiologically protect neuronal cells and represent a potential prevention and therapeutic target in the early phase of cerebral ischemic stroke.

The selective disruption of presynaptic JNK2/STX1a interaction reduces NMDA receptor-dependent glutamate release

The neuronal loss caused by excessive glutamate release, or ‘excitotoxicity’, leads to several pathological conditions, including cerebral ischemia, epilepsy, and neurodegenerative diseases. Over-stimulation of presynaptic N-methyl-D-aspartate (NMDA) receptors is known to trigger and support glutamate spillover, while postsynaptic NMDA receptors are responsible for the subsequent apoptotic cascade. Almost all molecules developed so far are unable to selectively block presynaptic or postsynaptic NMDA receptors, therefore a deeper knowledge about intracellular NMDA pathways is required to design more specific inhibitors. Our previous work showed that presynaptic c-Jun N-terminal kinase 2 (JNK2) specifically regulates NMDA-evoked glutamate release and here we demonstrate that an interaction between Syntaxin-1a and JNK2 is fundamental to this mechanism. Based on this evidence, a new cell permeable peptide (CPP), “JGRi1”, has been developed to disrupt the JNK2/STX1a interaction to indirectly, but specifically, inhibit presynaptic NMDA receptor signaling. JGRi1 reduces the NMDA-evoked release of glutamate both in in-vitro and ex-vivo experiments while also being able to widely diffuse throughout brain tissue via intraperitoneal administration. In conclusion, the JNK2/STX1 interaction is involved in presynaptic NMDA-evoked glutamate release and the novel CPP, JGRi1, acts as a pharmacological tool that promotes neuroprotection.

Diverse exocytic pathways for mast cell mediators

Mast cells play pivotal roles in innate and adaptive immunities but are also culprits in allergy, autoimmunity, and cardiovascular diseases. Mast cells respond to environmental changes by initiating regulated exocytosis/secretion of various biologically active compounds called mediators (e.g. proteases, amines, and cytokines). Many of these mediators are stored in granules/lysosomes and rely on intricate degranulation processes for release. Mast cell stabilizers (e.g. sodium cromoglicate), which prevent such degranulation processes, have therefore been clinically employed to treat asthma and allergic rhinitis. However, it has become increasingly clear that different mast cell diseases often involve multiple mediators that rely on overlapping but distinct mechanisms for release. This review illustrates existing evidence that highlights the diverse exocytic pathways in mast cells. We also discuss strategies to delineate these pathways so as to identify unique molecular components which could serve as new drug targets for more effective and specific treatments against mast cell-related diseases.

Myosin phosphatase and RhoA-activated kinase modulate neurotransmitter release by regulating SNAP-25 of SNARE complex

Reversible phosphorylation of neuronal proteins plays an important role in the regulation of neurotransmitter release. Myosin phosphatase holoenzyme (MP) consists of a protein phosphatase-1 (PP1) catalytic subunit (PP1c) and a regulatory subunit, termed myosin phosphatase targeting subunit (MYPT1). The primary function of MP is to regulate the phosphorylation level of contractile proteins; however, recent studies have shown that MP is localized to neurons, and is also involved in the mediation of neuronal processes. Our goal was to investigate the effect of RhoA-activated kinase (ROK) and MP on the phosphorylation of one potential neuronal substrate, the synaptosomal-associated protein of 25 kDa (SNAP-25). SNAP-25 is a member of the SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex, along with synaptobrevin and syntaxin, and the primary role of SNAP25 is to mediate vesicle fusion. We showed that MYPT1 interacts with SNAP-25, as revealed by immunoprecipitation and surface plasmon resonance based binding studies. Mass spectrometry analysis and in vitro phosphorylation/dephosphorylation assays demonstrated that ROK phosphorylates, while MP dephosphorylates, SNAP-25 at Thr138. Silencing MYPT1 in B50 neuroblastoma cells increased phosphorylation of SNAP-25 at Thr138. Inhibition of PP1 with tautomycetin increased, whereas inhibition of ROK by H1152, decreased the phosphorylation of SNAP-25 at Thr138 in B50 cells, in cortical synaptosomes, and in brain slices. In response to the transduction of the MP inhibitor, kinase-enhanced PP1 inhibitor (KEPI), into synaptosomes, an increase in phosphorylation of SNAP-25 and a decrease in the extent of neurotransmitter release were detected. The interaction between SNAP-25 and syntaxin increased with decreasing phosphorylation of SNAP-25 at Thr138, upon inhibition of ROK. Our data suggest that ROK/MP play a crucial role in vesicle trafficking, fusion, and neurotransmitter release by oppositely regulating the phosphorylation of SNAP-25 at Thr138.

DAPK1 Signaling Pathways in Stroke: from Mechanisms to Therapies

Death-associated protein kinase 1 (DAPK1), a Ca²⁺/calmodulin (CaM)-dependent serine/threonine protein kinase, plays important roles in diverse apoptosis pathways not only in tumor suppression but also in neuronal cell death. The requirement of DAPK1 catalytic activity for its proposed cell functions and the elevation of catalytic activity of DAPK1 in injured neurons in models of neurological diseases, such as ischemia and epilepsy, validate that DAPK1 can be taken as a potential therapeutic target in these diseases. Recent studies show that DAPK1-NR2B, DAPK1-DANGER, DAPK1-p53, and DAPK1-Tau are currently known pathways in stroke-induced cell death, and blocking these cascades in an acute treatment effectively reduces neuronal loss. In this review, we focus on the role of DAPK1 in neuronal cell death after stroke. We hope to provide exhaustive summaries of relevant studies on DAPK1 signals involved in stroke damage. Therefore, disrupting DAPK1-relevant cell death pathway could be considered as a promising therapeutic approach in stroke.

Death Associated Protein Kinase 1 (DAPK1): A Regulator of Apoptosis and Autophagy

Death-Associated Protein Kinase 1 (DAPK1) belongs to a family of five serine/threonine (Ser/Thr) kinases that possess tumor suppressive function and also mediate a wide range of cellular processes, including apoptosis and autophagy. The loss and gain-of–function of DAPK1 is associated with various cancer and neurodegenerative diseases respectively. In recent years, mechanistic studies have broadened our knowledge of the molecular mechanisms involved in DAPK1-mediated autophagy/apoptosis. In the present review, we have discussed the structural information and various cellular functions of DAPK1 in a comprehensive manner.

Analysis of protein phosphorylation in nerve terminal reveals extensive changes in active zone proteins upon exocytosis

Neurotransmitter release is mediated by the fast, calcium-triggered fusion of synaptic vesicles with the presynaptic plasma membrane, followed by endocytosis and recycling of the membrane of synaptic vesicles. While many of the proteins governing these processes are known, their regulation is only beginning to be understood. Here we have applied quantitative phosphoproteomics to identify changes in phosphorylation status of presynaptic proteins in resting and stimulated nerve terminals isolated from the brains of Wistar rats. Using rigorous quantification, we identified 252 phosphosites that are either up- or downregulated upon triggering calcium-dependent exocytosis. Particularly pronounced were regulated changes of phosphosites within protein constituents of the presynaptic active zone, including bassoon, piccolo, and RIM1. Additionally, we have mapped kinases and phosphatases that are activated upon stimulation. Overall, our study provides a snapshot of phosphorylation changes associated with presynaptic activity and provides a foundation for further functional analysis of key phosphosites involved in presynaptic plasticity.

DOI:http://dx.doi.org/10.7554/eLife.14530.001

The human nervous system contains more than a hundred billion neurons that are connected with each other via junctions called synapses. When an electrical impulse travelling along a neuron arrives at a synapse, it triggers bubble-like packages called synaptic vesicles within the neuron to merge with the neuron’s surface membrane. The contents of these vesicles – chemical messengers called neurotransmitters – are then released into the synapse and carry the signal to the next neuron.

Complex molecular machines made from many different proteins control the release of neurotransmitters. Quite a few of these proteins are regulated by the addition of phosphate groups at specific sites. However, not all of the proteins involved in the release of neurotransmitters have been studied in detail and it is largely unclear how most of them are regulated.

Now, Kohansal-Nodehi et al. have used techniques involving mass spectrometry to find out which proteins have phosphate groups added or removed in neurons that are releasing neurotransmitters. The experiments used pinched-off synapses isolated from rat brains. These structures, referred to as “synaptosomes”, lend themselves to this kind of study because they can be induced to continuously release neurotransmitters for several minutes. Kohansal-Nodehi et al. identified over 250 specific sites on proteins in the synaptosomes where phosphate groups are attached, including many on the key proteins known to operate in neurotransmitter release. Moreover, some proteins were modified at multiple sites, especially the proteins that form a scaffold to capture synaptic vesicles close to the membrane and prepare them for release. The data also revealed important clues about the enzymes that either attach or remove the phosphate groups.

Together, these findings provide new insights into the regulatory networks that control many proteins at the same time. The next challenge is to sort out which of these modifications change the interactions between the proteins that control neurotransmitter release, and to understand how these changes influence the trafficking of synaptic vesicles.

DOI:http://dx.doi.org/10.7554/eLife.14530.002

Drak Is Required for Actomyosin Organization During Drosophila Cellularization

The generation of force by actomyosin contraction is critical for a variety of cellular and developmental processes. Nonmuscle myosin II is the motor that drives actomyosin contraction, and its activity is largely regulated by phosphorylation of the myosin regulatory light chain. During the formation of the Drosophila cellular blastoderm, actomyosin contraction drives constriction of microfilament rings, modified cytokinesis rings. Here, we find that Drak is necessary for most of the phosphorylation of the myosin regulatory light chain during cellularization. We show that Drak is required for organization of myosin II within the microfilament rings. Proper actomyosin contraction of the microfilament rings during cellularization also requires Drak activity. Constitutive activation of myosin regulatory light chain bypasses the requirement for Drak, suggesting that actomyosin organization and contraction are mediated through Drak's regulation of myosin activity. Drak is also involved in the maintenance of furrow canal structure and lateral plasma membrane integrity during cellularization. Together, our observations suggest that Drak is the primary regulator of actomyosin dynamics during cellularization.

Death-associated protein kinase: A molecule with functional antagonistic duality and a potential role in inflammatory bowel disease (Review)

The cytoskeleton-associated serine/threonine kinase death-associated protein kinase (DAPK) has been described as a cancer gene chameleon with functional antagonistic duality in a cell type and context specific manner. The broad range of interaction partners and substrates link DAPK to inflammatory processes especially in the gut. Herein we summarize our knowledge on the role of DAPK in different cell types that play a role under inflammatory conditions in the gut. Besides some promising experimental data suggesting DAPK as an interesting drug target in inflammatory bowel disease there are many open questions regarding direct evidence for a role of DAPK in intestinal inflammation.

Death Associated Protein Kinase (DAPK) -mediated neurodegenerative mechanisms in nematode excitotoxicity

Background

Excitotoxicity (the toxic overstimulation of neurons by the excitatory transmitter Glutamate) is a central process in widespread neurodegenerative conditions such as brain ischemia and chronic neurological diseases. Many mechanisms have been suggested to mediate excitotoxicity, but their significance across diverse excitotoxic scenarios remains unclear. Death Associated Protein Kinase (DAPK), a critical molecular switch that controls a range of key signaling and cell death pathways, has been suggested to have an important role in excitotoxicity. However, the molecular mechanism by which DAPK exerts its effect is controversial. A few distinct mechanisms have been suggested by single (sometimes contradicting) studies, and a larger array of potential mechanisms is implicated by the extensive interactome of DAPK.

Results

Here we analyze a well-characterized model of excitotoxicity in the nematode C. elegans to show that DAPK is an important mediator of excitotoxic neurodegeneration across a large evolutionary distance. We further show that some proposed mechanisms of DAPK’s action (modulation of synaptic strength, involvement of the DANGER-related protein MAB-21, and autophagy) do not have a major role in nematode excitotoxicity. In contrast, Pin1/PINN-1 (a DAPK interaction-partner and a peptidyl-prolyl isomerase involved in chronic neurodegenerative conditions) suppresses neurodegeneration in our excitotoxicity model.

Conclusions

Our studies highlight the prominence of DAPK and Pin1/PINN-1 as conserved mediators of cell death processes in diverse scenarios of neurodegeneration.

DAPK and cytoskeleton-associated functions

Death-associated protein kinase (DAPK) undergoes activation in response to various death stimuli, and they have been associated with an increase in DAPK catalytic activity. One of the most prominent features of DAPK-induced cell death is the effect on the cytoskeleton, including loss of matrix attachment, and membrane blebbing. One known cytoskeletal-associated substrate of DAPK is the myosin-II light chain, phosphorylated by DAPK on Ser(19), thus stabilizing actin stress fibres. Moreover, paxillin, a component of focal adhesions, was found to be localized in close proximity to the tips of the DAPK-positive filaments, indicating that stress fibres containing DAPK extend to focal contacts. Forced expression of DAPK in multiple cell types results in morphological changes such as cell rounding, membrane blebbing, shrinking and detachment. During directed migration, DAPK functions as a potent inhibitor of cell polarization, as evidenced by its perturbation of the formation of static protrusion at the leading edge. Furthermore, DAPK inhibits random migration by suppressing directional persistence. One of the studies considered DAPK as an anoikis inducer. Others showed that DAP-kinase inhibits the activities of cell surface integrins by converting them into an inactive conformation. Biochemical experiments have established the DAPK binding to Syntaxin1 and its subsequent phosphorylation at Ser(188) in a Ca(2+) dependent manner. This phosphorylation event has been shown to decrease the binding of Syntaxin to MUNC18-1, a protein critically involved in synaptic vesicle docking. Here, we have investigated the structural interactions that modulate DAPK phosphorylation with Syntaxin and its functional role in binding to the MUNC18-1 to regulate vesicle docking. This review will summarize our current knowledge of the role of DAPK on cytoskeleton reorganization and report the mechanisms that regulate these changes.

DAP-kinase and autophagy

DAP-kinase (DAPK) is a Ca(2+)-calmodulin regulated kinase with various, diverse cellular activities, including regulation of apoptosis and caspase-independent death programs, cytoskeletal dynamics, and immune functions. Recently, DAPK has also been shown to be a critical regulator of autophagy, a catabolic process whereby the cell consumes cytoplasmic contents and organelles within specialized vesicles, called autophagosomes. Here we present the latest findings demonstrating how DAPK modulates autophagy. DAPK positively contributes to the induction stage of autophagosome nucleation by modulating the Vps34 class III phosphatidyl inositol 3-kinase complex by two independent mechanisms. The first involves a kinase cascade in which DAPK phosphorylates protein kinase D, which then phosphorylates and activates Vps34. In the second mechanism, DAPK directly phosphorylates Beclin 1, a necessary component of the Vps34 complex, thereby releasing it from its inhibitor, Bcl-2. In addition to these established pathways, we will discuss additional connections between DAPK and autophagy and potential mechanisms that still remain to be fully validated. These include myosin-dependent trafficking of Atg9-containing vesicles to the sites of autophagosome formation, membrane fusion events that contribute to expansion of the autophagosome membrane and maturation through the endocytic pathway, and trafficking to the lysosome on microtubules. Finally, we discuss how DAPK's participation in the autophagic process may be related to its function as a tumor suppressor protein, and its role in neurodegenerative diseases.

The DAP-kinase interactome

DAP-kinase (DAPK) is a Ca(2+)/calmodulin regulated Ser/Thr kinase that activates a diverse range of cellular activities. It is subject to multiple layers of regulation involving both intramolecular signaling, and interactions with additional proteins, including other kinases and phosphatases. Its protein stability is modulated by at least three distinct ubiquitin-dependent systems. Like many kinases, DAPK participates in several signaling cascades, by phosphorylating additional kinases such as ZIP-kinase and protein kinase D (PKD), or Pin1, a phospho-directed peptidyl-prolyl isomerase that regulates the function of many phosphorylated proteins. Other substrate targets have more direct cellular effects; for example, phosphorylation of the myosin II regulatory chain and tropomyosin mediate some of DAPK's cytoskeletal functions, including membrane blebbing during cell death and cell motility. DAPK induces distinct death pathways of apoptosis, autophagy and programmed necrosis. Among the substrates implicated in these processes, phosphorylation of PKD, Beclin 1, and the NMDA receptor has been reported. Interestingly, not all cellular effects are mediated by DAPK's catalytic activity. For example, by virtue of protein-protein interactions alone, DAPK activates pyruvate kinase isoform M2, the microtubule affinity regulating kinases and inflammasome protein NLRP3, to promote glycolysis, influence microtubule dynamics, and enhance interleukin-1β production, respectively. In addition, a number of other substrates and interacting proteins have been identified, the physiological significance of which has not yet been established. All of these substrates, effectors and regulators together comprise the DAPK interactome. By presenting the components of the interactome network, this review will clarify both the mechanisms by which DAPK function is regulated, and by which it mediates its various cellular effects.

FUS-regulated region- and cell-type-specific transcriptome is associated with cell selectivity in ALS/FTLD

FUS is genetically and pathologically linked to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). To clarify the RNA metabolism cascade regulated by FUS in ALS/FTLD, we compared the FUS-regulated transcriptome profiles in different lineages of primary cells from the central nervous system. The profiles of FUS-mediated gene expression and alternative splicing in motor neurons were similar to those of cortical neurons, but not to those in cerebellar neurons despite the similarity of innate transcriptome signature. The gene expression profiles in glial cells were similar to those in motor and cortical neurons. We identified certain neurological diseases-associated genes, including Mapt, Stx1a, and Scn8a, among the profiles of gene expression and alternative splicing events regulated by FUS. Thus, FUS-regulated transcriptome profiles in each cell-type may determine cellular fate in association with FUS-mediated ALS/FTLD, and identified RNA targets for FUS could be therapeutic targets for ALS/FTLD.

Death associated protein kinases: molecular structure and brain injury

Perinatal brain damage underlies an important share of motor and neurodevelopmental disabilities, such as cerebral palsy, cognitive impairment, visual dysfunction and epilepsy. Clinical, epidemiological, and experimental studies have revealed that factors such as inflammation, excitotoxicity and oxidative stress contribute considerably to both white and grey matter injury in the immature brain. A member of the death associated protein kinase (DAPk) family, DAPk1, has been implicated in cerebral ischemic damage, whereby DAPk1 potentiates NMDA receptor-mediated excitotoxicity through interaction with the NR2BR subunit. DAPk1 also mediate a range of activities from autophagy, membrane blebbing and DNA fragmentation ultimately leading to cell death. DAPk mRNA levels are particularly highly expressed in the developing brain and thus, we hypothesize that DAPk1 may play a role in perinatal brain injury. In addition to reviewing current knowledge, we present new aspects of the molecular structure of DAPk domains, and relate these findings to interacting partners of DAPk1, DAPk-regulation in NMDA-induced cerebral injury and novel approaches to blocking the injurious effects of DAPk1.

The ubiquitin ligase Mindbomb 1 coordinates gastrointestinal secretory cell maturation

After cell fate specification, differentiating cells must amplify the specific subcellular features required for their specialized function. How cells regulate such subcellular scaling is a fundamental unanswered question. Here, we show that the E3 ubiquitin ligase Mindbomb 1 (MIB1) is required for the apical secretory apparatus established by gastric zymogenic cells as they differentiate from their progenitors. When Mib1 was deleted, death-associated protein kinase-1 (DAPK1) was rerouted to the cell base, microtubule-associated protein 1B (MAP1B) was dephosphorylated, and the apical vesicles that normally support mature secretory granules were dispersed. Consequently, secretory granules did not mature. The transcription factor MIST1 bound the first intron of Mib1 and regulated its expression. We further showed that loss of MIB1 and dismantling of the apical secretory apparatus was the earliest quantifiable aberration in zymogenic cells undergoing transition to a precancerous metaplastic state in mouse and human stomach. Our results reveal a mechanistic pathway by which cells can scale up a specific, specialized subcellular compartment to alter function during differentiation and scale it down during disease.

Biochemical and functional characterization of the ROC domain of DAPK establishes a new paradigm of GTP regulation in ROCO proteins

DAPK (death-associated protein kinase) is a newly recognized member of the mammalian family of ROCO proteins, characterized by common ROC (Ras of complex proteins) and COR (C-terminal of ROC) domains. In the present paper, we review our recent work showing that DAPK is functionally a ROCO protein; its ROC domain binds and hydrolyses GTP. Furthermore, GTP binding regulates DAPK catalytic activity in a novel manner by enhancing autophosphorylation on inhibitory Ser308, thereby promoting the kinase 'off' state. This is a novel mechanism for in cis regulation of kinase activity by the distal ROC domain. The functional similarities between DAPK and the Parkinson's disease-associated protein LRRK2 (leucine-rich repeat protein kinase 2), another member of the ROCO family, are also discussed.

Tyrosine phosphorylation of a SNARE protein, syntaxin 17: implications for membrane trafficking in the early secretory pathway

The T-cell protein tyrosine phosphatase is expressed as two splice variants - TC45, a nuclear protein, and TC48, which is localized predominantly in the ER (endoplasmic reticulum). Yeast two-hybrid screening revealed direct interaction of TC48 with Syntaxin17, a SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein localized predominantly in the ER and to some extent in the ER-Golgi intermediate compartment. Syntaxin 17 did not interact with TC45. C-terminal 40 amino acids of TC48 were sufficient for interaction with syntaxin 17. Overexpressed syntaxin 17 was phosphorylated at tyrosine upon pervanadate treatment (a tyrosine phosphatase inhibitor/tyrosine kinase activator) of COS-1 cells. Mutational analysis identified Tyr156 in the cytoplasmic domain as the major site of phosphorylation. Endogenous syntaxin 17 was phosphorylated by pervanadate treatment in CHO and MIN6 cells but was not phosphorylated in a variety of other cell lines tested. c-Abl was identified as one of the kinases, which phosphorylates syntaxin 17 in MIN6 cells. Phosphorylation of endogenous and overexpressed syntaxin 17 was reduced in the presence of IGF receptor and EGF receptor kinase inhibitors. Serum depletion reduced pervanadate-induced phosphorylation of endogenous syntaxin 17. TC48 coexpression reduced phosphorylation of syntaxin 17 by pervanadate and purified TC48 directly dephosphorylated syntaxin 17. β-COP dispersal by overexpressed syntaxin 17 was reduced after pervanadate-induced phosphorylation. A phospho-mimicking mutant (Y156E) of syntaxin 17 showed reduced interaction with COPI vesicles. These results suggest that tyrosine phosphorylation of syntaxin 17 is likely to have a role in regulating syntaxin 17 dependent membrane trafficking in the early secretory pathway.

Characterizing synaptic vesicle proteins using synaptosomal fractions and cultured hippocampal neurons

Cloning and characterization of synaptic vesicle proteins and their binding counterparts on the presynaptic plasma membrane have greatly advanced our understanding of the molecular mechanisms involved in the synaptic vesicle cycle and neurotransmitter release. This unit discusses multidisciplinary approaches to characterize proteins from synaptosome-enriched subcellular fractions and localize them within cultured neurons. The first approach regroups methods used to isolate synaptic vesicles from rat brain synaptosomal preparations, allowing for specific biochemical investigation of synaptic vesicle proteins. The second is a detailed procedure for pre-embedding immunogold staining and electron microscopic observation, which permits the morphological identification of proteins in individual vesicles at intact synapses. Additionally, this chapter proposes methods for light microscopic examination of hippocampal neurons. It includes procedures for embryonic and postnatal hippocampal neuron culture and describes an immunocytochemical staining protocol used to investigate synaptic vesicle protein localization with respect to other proteins or subcellular structures.

Differentiation and neural integration of hippocampal neuronal progenitors: signaling pathways sequentially involved

In the context of their potential implication in regenerative strategies, we characterized cell mechanisms underlying the fate of embryonic rat hippocampal H19-7 progenitors in culture upon induction of their differentiation, and tested their capacities to integrate into a neuronal network in vitro. Without addition of growth factors, nearly 100% of cells expressed various neuronal markers, with a progressive rise of the expression of Synapsin I and II, suggesting that cells developed as mature neurons with synaptogenic capacities. Fully differentiated neurons were identified as glutamatergic and expressed the receptor-associated protein PSD-95. Quantification of ATP showed that 60% of cells died within 24 h after differentiation. Cell death was shown to imply Erk1/2-dependent intrinsic mitochondrial apoptosis signaling pathway, with activation of caspase-9 and -3, finally leading to single-strand DNA. Surviving neurons displayed high levels of Akt, phospho-Akt, and antiapoptotic proteins such as Bcl-2 and Bcl-XL, with decreased caspase activation. In the absence of trophic support, the proapoptotic death-associated protein (DAP) kinase was dramatically stimulated by 24 h postdifferentiation, along with increased levels of p38 and phospho-p38, and caspase reactivation. These findings show that different signaling pathways are sequentially triggered by differentiation, and highlight that ultimate cell death would involve p38 and DAP kinase activation. This was supported by the improvement of cell survival at 24-h postdifferentiation when cells were treated by PD169316, a specific inhibitor of p38. Finally, when seeded on rat hippocampal primary cultured neurons, a significant number of differentiated H19-7 cells were able to survive and to develop cell-cell communication.

Subcellular phosphoproteomics

Protein phosphorylation represents one of the most extensively studied post-translational modifications, primarily due to the emergence of sensitive methods enabling the detection of this modification both in vitro and in vivo. The availability of enrichment methods combined with sensitive mass spectrometry instrumentation has played a crucial role in uncovering the dynamic changes and the large expanding repertoire of this reversible modification. The structural changes imparted by the phosphorylation of specific residues afford exquisite mechanisms for the regulation of protein functions by modulating new binding sites on scaffold proteins or by abrogating protein-protein interactions. However, the dynamic interplay of protein phosphorylation is not occurring randomly within the cell but is rather finely orchestrated by specific kinases and phosphatases that are unevenly distributed across subcellular compartments. This spatial separation not only regulates protein phosphorylation but can also control the activity of other enzymes and the transfer of other post-translational modifications. While numerous large-scale phosphoproteomics studies highlighted the extent and diversity of phosphoproteins present in total cell lysates, the further understanding of their regulation and biological activities require a spatio-temporal resolution only achievable through subcellular fractionation. This review presents a first account of the emerging field of subcellular phosphoproteomics where cell fractionation approaches are combined with sensitive mass spectrometry methods to facilitate the identification of low abundance proteins and to unravel the intricate regulation of protein phosphorylation.

Death-associated protein kinase-mediated cell death modulated by interaction with DANGER

Death-associated protein kinase (DAPK) is a key player in multiple cell death signaling pathways. We report that DAPK is regulated by DANGER, a partial MAB-21 domain-containing protein. DANGER binds directly to DAPK and inhibits DAPK catalytic activity. DANGER-deficient mouse embryonic fibroblasts and neurons exhibit greater DAPK activity and increased sensitivity to cell death stimuli than do wild-type control cells. In addition, DANGER-deficient mice manifest more severe brain damage after acute excitotoxicity and transient cerebral ischemia than do control mice. Accordingly, DANGER may physiologically regulate the viability of neurons and represent a potential therapeutic target for stroke and neurodegenerative diseases.

Exocytosis mechanisms underlying insulin release and glucose uptake: conserved roles for Munc18c and syntaxin 4

Type 2 diabetes has been coined "a two-hit disease," as it involves specific defects of glucose-stimulated insulin secretion from the pancreatic beta cells in addition to defects in peripheral tissue insulin action required for glucose uptake. Both of these processes, insulin secretion and glucose uptake, are mediated by SNARE (soluble N-ethylmaleimide-sensitive factor attachment protein receptor) protein core complexes composed of syntaxin, SNAP-23/25, and VAMP proteins. The SNARE core complex is regulated by the Sec1/Munc18 (SM) family of proteins, which selectively bind to their cognate syntaxin isoforms with high affinity. The process of insulin secretion uses multiple Munc18-syntaxin isoform pairs, whereas insulin action in the peripheral tissues appears to use only the Munc18c-syntaxin 4 pair. Importantly, recent reports have linked obesity and Type 2 diabetes in humans with changes in protein levels and single nucleotide polymorphisms (SNPs) of Munc18 and syntaxin isoforms relevant to these exocytotic processes, although the molecular mechanisms underlying the observed phenotypes remain incomplete (5, 104, 144). Given the conservation of these proteins in two seemingly disparate processes and the need to design and implement novel and more effective clinical interventions, it will be vitally important to delineate the mechanisms governing these conserved SNARE-mediated exocytosis events. Thus, we provide here an up-to-date historical review of advancements in defining the roles and molecular mechanisms of Munc18-syntaxin complexes in the pathophysiology of Type 2 diabetes.

Targeting protein kinases in central nervous system disorders

Protein kinases are a growing drug target class in disorders in peripheral tissues, but the development of kinase-targeted therapies for central nervous system (CNS) diseases remains a challenge, largely owing to issues associated specifically with CNS drug discovery. However, several candidate therapeutics that target CNS protein kinases are now in various stages of preclinical and clinical development. We review candidate compounds and discuss selected CNS protein kinases that are emerging as important therapeutic targets. In addition, we analyse trends in small-molecule properties that correlate with key challenges in CNS drug discovery, such as blood-brain barrier penetrance and cytochrome P450-mediated metabolism, and discuss the potential of future approaches that will integrate molecular-fragment expansion with pharmacoinformatics to address these challenges.

Negative regulation of Caenorhabditis elegans epidermal damage responses by death-associated protein kinase

Wounding of epidermal layers triggers multiple coordinated responses to damage. We show here that the Caenorhabditis elegans ortholog of the tumor suppressor death-associated protein kinase, dapk-1, acts as a previously undescribed negative regulator of barrier repair and innate immune responses to wounding. Loss of DAPK-1 function results in constitutive formation of scar-like structures in the cuticle, and up-regulation of innate immune responses to damage. Overexpression of DAPK-1 represses innate immune responses to needle wounding. Up-regulation of innate immune responses in dapk-1 requires the TIR-1/p38 signal transduction pathway; loss of function in this pathway synergizes with dapk-1 to drastically reduce adult lifespan. Our results reveal a previously undescribed function for the DAPK tumor suppressor family in regulation of epithelial damage responses.

DAP-kinase is a mediator of endoplasmic reticulum stress-induced caspase activation and autophagic cell death

Damage to endoplasmic reticulum (ER) homeostasis that cannot be corrected by the unfolded protein response activates cell death. Here, we identified death-associated protein kinase (DAPk) as an important component in the ER stress-induced cell death pathway. DAPk-/- mice are protected from kidney damage caused by injection of the ER stress-inducer tunicamycin. Likewise, the cell death response to ER stress-inducers is reduced in DAPk-/- primary fibroblasts. Both caspase activation and autophagy induction, events that are activated by ER stress and precede cell death, are significantly attenuated in the DAPk null cells. Notably, in this cellular setting, autophagy serves as a second cell killing mechanism that acts in concert with apoptosis, as the depletion of Atg5 or Beclin1 from fibroblasts significantly protected from ER stress-induced death when combined with caspase-3 depletion. We further show that ER stress promotes the catalytic activity of DAPk by causing dephosphorylation of an inhibitory autophosphorylation on Ser(308) by a PP2A-like phosphatase. Thus, DAPk constitutes a critical integration point in ER stress signaling, transmitting these signals into two distinct directions, caspase activation and autophagy, leading to cell death.

Multidisciplinary approaches for characterizing synaptic vesicle proteins

Investigation of synaptic vesicle membrane proteins using multidisciplinary approaches, particularly to characterize synaptic vesicle proteins in synapses, can greatly advance our knowledge of the molecular mechanisms involved in synaptic vesicle exocytosis and neurotransmission. Three approaches are presented in this unit to identify and characterize synaptic vesicle proteins. The first is a subcellular fractionation method used to isolate synaptic vesicles from rat brain synaptosomal preparations, which can then be used in a variety of biochemical studies on synaptic vesicle proteins. The second is a detailed procedure for pre-embedding immunogold staining and electron microscopic observation, techniques that permit the morphological identification of synaptic vesicle proteins in individual vesicles at the intact synapse. Finally, a protocol for immunocytochemical staining of cultured hippocampal neurons for light microscopic examination is provided, which allows one to stain multisynaptic vesicle proteins and determine their localization in relation to other proteins or subcellular structures in synapses.

A high throughput proteomics screen identifies novel substrates of death-associated protein kinase

Death-associated protein kinase (DAPk) is a Ser/Thr kinase whose activity is necessary for different cell death phenotypes. Although its contribution to cell death is well established, only a handful of direct substrates have been identified; these do not fully account for the multiple cellular effects of DAPk. To identify such substrates on a large scale, we developed an in vitro, unbiased, proteomics-based assay to search for novel DAPk substrates. Biochemical fractionation and mass spectrometric analysis were used to purify and identify several potential substrates from HeLa cell lysate. Here we report the identification of two such candidate substrates, the ribosomal protein L5 and MCM3, a replication licensing factor. Although L5 proved to be a weak substrate, MCM3 was efficiently and specifically phosphorylated by DAPk on a unique site, Ser160. Significantly DAPk phosphorylated this site in vivo upon overexpression in 293T cells. Activation of endogenous DAPk by increasing intracellular Ca2+ also led to increased phosphorylation of MCM3. Importantly short hairpin RNA-mediated knockdown of endogenous DAPk blocked both basal phosphorylation and Ca2+-induced phosphorylation, indicating that DAPk is both necessary and sufficient for MCM3 Ser160 phosphorylation in vivo. Identification of MCM3 as an in vivo DAPk substrate indicates the usefulness of this approach for identification of physiologically relevant substrates that may shed light on novel functions of the kinase.

Autophagy and tumor suppression: recent advances in understanding the link between autophagic cell death pathways and tumor development

Autophagy is a process by which the cell recycles its components through self-consumption of cellular organelles and bulk cytoplasm. In times of stress, it serves to generate much needed nutrients. When overactivated, however, the orderly destruction of organelles can lead to cell death. At times, autophagic cell death is used as an alternative to apoptosis to eliminate unwanted, damaged, or transformed cells. Consistent with this, tumorigenesis is associated with a downregulation in autophagy, and genes that mediate the execution of the process have been shown to be tumor suppressors. At the same time, basal autophagy has been harnessed by some tumor cells as a survival mechanism to protect against ischemia and signals that induce apoptosis. Thus, the relationship between autophagy and tumor development is complex. Here, we discuss the basic machinery of mammalian autophagy and its regulators, with specific emphasis on those genes that have been linked to cancer. Research supporting the divergent nature of autophagy in both tumor suppression and tumor progression is presented. We conclude with a survey of recent approaches to treating cancer with strategies that modulate autophagy.

Prolactin modulates the association and phosphorylation of SNARE and kinesin/MAP-2 proteins in neonatal pancreatic rat islets

Prolactin induces maturation of insulin secretion in cultured neonatal rat islets. In this study, we investigated whether the improved secretory response to glucose caused by prolactin involves alteration in the expression, association and phosphorylation of several proteins that participate in these processes. Messenger RNA was extracted from neonatal rat islets cultured for 5 days in the presence of prolactin and reverse transcribed. Gene expression was analyzed by semi-quantitative RT-PCR and by Western blotting for proteins. The gene transcription and protein expression of kinesin and MAP-2 were increased in prolactin-treated islets compared to the controls. The association and phosphorylation of proteins was analyzed by immunoprecipitation followed by Western blotting, after acute exposure to prolactin. Prolactin increased the association between SNARE proteins and kinesin/MAP-2 while the association of munc-18/syntaxin 1A was decreased. Serine phosphorylation of SNAP-25, syntaxin 1A, munc-18, MAP-2 was significantly higher whereas kinesin phosphorylation was decreased in prolactin-treated islets. There was an increase in SNARE complex formation in islets stimulated with prolactin, 22 mM glucose, 40 mM K(+), 200 microM carbachol and 1 microM PMA. The prolactin-induced increase in the formation of SNARE complex and syntaxin 1A phosphorylation was inhibited by PD098059 and U0126, inhibitors of the MAPK pathway. These findings indicate that prolactin primes pancreatic beta-cells to release insulin by increasing the expression and phosphorylation/association of proteins implicated in the secretory machinery and the MAPK/PKC pathway is important for this effect.

The death-associated protein kinases: structure, function, and beyond

Death-associated protein kinase (DAPk) is the founding member of a newly classified family of Ser/Thr kinases, whose members not only possess significant homology in their catalytic domains, but also share cell death-associated functions. The realization that DAPk is a tumor suppressor gene, whose expression is lost in multiple tumor types, has spurred a flurry of interest in the kinase family and produced an impressive body of literature concerning its function, regulation, and connection to disease. The DAPk family has been linked to several cell death-related signaling pathways, and functions other than cell death have also been proposed. This review presents a thorough structural analysis of the kinases, discusses methods of regulation, clarifies their cellular targets and functions, and shows how these functions are integrated. Although many gaps in our knowledge still remain, the data generated to date can be combined to delineate a place for the DAPk family within the general cell death-signaling network.

Death-associated protein kinase phosphorylates mammalian ribosomal protein S6 and reduces protein synthesis

Death-associated protein kinase (DAPK) is a pro-apoptotic, calcium/calmodulin-regulated protein kinase that is a drug discovery target for neurodegenerative disorders. Despite the potential profound physiological role of DAPK in neuronal function and pathophysiology, the endogenous substrate(s) of this kinase and the mechanisms via which DAPK elicits its biological action remain largely unknown. We report here that the mammalian 40S ribosomal protein S6 is a DAPK substrate. Results from immunoprecipitation experiments are consistent with endogenous DAPK being associated with endogenous S6 in rat brain. When S6 is a component of the 40S ribosomal subunit complex, DAPK selectively phosphorylates it at serine 235, one of the five sites in S6 that are phosphorylated by the S6 kinase family of proteins. The amino acid sequence flanking serine 235 matches the established pattern for DAPK peptide and protein substrates. Kinetic analyses using purified 40S subunits revealed a K(m) value of 9 microM, consistent with S6 being a potential physiological substrate of DAPK. This enzyme-substrate relationship has functional significance. DAPK suppresses translation in rabbit reticulocyte lysate, and treatment of neuroblastoma cells with a stimulator of DAPK reduces protein synthesis. In both cases, suppression of translation correlates with increased phosphorylation of S6 at serine 235. These results demonstrate that DAPK is a S6 kinase and provide evidence for a novel role of DAPK in the regulation of translation.

Calcium-dependent interactions of the human norepinephrine transporter with syntaxin 1A

The norepinephrine (NE) transporter (NET) terminates noradrenergic signaling by clearing released NE at synapses. The activity of NET can be rapidly regulated by depolarization and receptor activation via Ca2+ and kinase/phosphatase-linked pathways. The SNARE protein syntaxin 1A (SYN1A) interacts with NET and influences transporter surface trafficking and catalytic activity. In this study, we establish a link between changes in intracellular Ca2+ and SYN1A/NET interactions. SYN1A influenced NE transport only in the presence of Ca2+ in brain cortical synaptosomes. Although NET/SYN1A associations were sensitive to manipulations of Ca2+ in CHO cells, in vitro binding experiments using purified NET and SYN1A fusion proteins demonstrated a lack of direct Ca2+ sensitivity. Disruption of NET/SYN1A interaction abolished inhibition of NE transport by phorbol ester (PMA) to activate protein kinase C (PKC), but had no effect on transport inhibition by the Ca2+ calmodulin kinase (CaMK) inhibitor KN93. Furthermore, PMA enhanced Ca2+-dependent modulation of NE transport in synaptosomes. Our data reveal roles for SYN1A in the Ca2+-dependent regulation of NET, likely reliant on regulation by PKC signaling, but independent of CaMK.

Modulation of DRAK2 autophosphorylation by antigen receptor signaling in primary lymphocytes

Death-associated protein-related apoptotic kinase-2 (DRAK2), a member of the death-associated protein-like family of serine/threonine kinases, is highly expressed in lymphoid organs and is a negative regulator of T cell activation. To investigate the regulation of DRAK2 activity in primary lymphocytes, we employed mass spectrometry to identify sites of autophosphorylation on DRAK2. These studies have revealed a key site of autophosphorylation on serine 12. Using a phospho-specific antibody to detect Ser(12) phosphorylation, we found that autophosphorylation is induced by antigen receptor stimulation in T and B cells. In Jurkat T cells, resting B cells and thymocytes, DRAK2 was hypophosphorylated on Ser(12) but rapidly phosphorylated with antigen receptor ligation. This increase in phosphorylation was dependent on intracellular calcium mobilization, because BAPTA-AM blocked DRAK2 kinase activity, whereas the SERCA inhibitor thapsigargin promoted Ser(12) phosphorylation. Our results show that DRAK2 kinase activity is regulated in a calcium-dependent manner and that Ser(12) phosphorylation is necessary for optimal suppression of T cell activation by this kinase, suggesting a potential feedback loop may act to modulate the activity of this kinase following antigen receptor signaling.

Protein kinase D 3 is localized in vesicular structures and interacts with vesicle-associated membrane protein 2

Protein kinase D localizes in the Golgi and regulates protein transport from the Golgi to the plasma membrane. In the present study, we found that PKD3, a novel member of the PKD family, and its fluorescent protein fusions localized in the Golgi and in the vesicular structures that are in part marked by endosome markers. Fluorescent recovery after photobleaching (FRAP) showed that the PKD3-associated vesicular structures were constantly forming and dissolving, reflecting active subcellular structures. FRAP on plasma membrane-located PKD3 indicated a slower recovery of PKD3 fluorescent signal compared to those of PKC isoforms, implying a different targeting mechanism at the plasma membrane. VAMP2, the vesicle-localized v-SNARE, was later identified as a novel binding partner of PKD3 through yeast two-hybrid screening. PKD3 directly interacted with VAMP2 in vitro and in vivo, and colocalized in part with VAMP2 vesicles in cells. PKD3 did not phosphorylate VAMP-GFP and the purified GST-VAMP2 protein in in vitro phosphorylation assays. Rather, PKD3 was found to promote the recruitment of VAMP2 vesicles to the plasma membrane in response to PMA, while the kinase dead PKD3 abolished this effect. Thus, the kinase activity of PKD3 was required for PMA-induced plasma membrane trafficking of VAMP2. In summary, our findings suggest that PKD3 localizes to vesicular structures that are part of the endocytic compartment. The vesicular distribution may be attributed in part to the direct interaction between PKD3 and vesicle-associated membrane protein VAMP2, through which PKD3 may regulate VAMP2 vesicle trafficking by facilitating its recruitment to the target membrane.

DAPK1 variants are associated with Alzheimer's disease and allele-specific expression

Genetic factors play an important role in the etiology of late-onset Alzheimer's disease (LOAD). We tested gene-centric single nucleotide polymorphisms (SNPs) on chromosome 9 and identified two SNPs in the death-associated protein kinase, DAPK1, that show significant association with LOAD. SNP rs4878104 was significantly associated with LOAD in our discovery case-control sample set (WU) and replicated in each of two initial validation case-control sample sets (P<0.05, UK1, SD). The risk-allele frequency of this SNP showed a similar direction in three other case-control sample sets. A meta-analysis of the six sample sets combined, totaling 2012 cases and 2336 controls, showed an allelic P-value of 0.0016 and an odds ratio (OR) of 0.87 (95%CI: 0.79-0.95). Minor allele homozygotes had a consistently lower risk than major allele homozygotes in the discovery and initial two replication sample sets, which remained significant in the meta-analysis of all six sample sets (OR=0.7, 95%CI: 0.58-0.85), whereas the risk for heterozygous subjects was not significantly different from that of major allele homozygotes. A second SNP, rs4877365, which is in high linkage disequilibrium with rs4878104 (r2=0.64), was also significantly associated with LOAD (meta P=0.0017 in the initial three sample sets). Furthermore, DAPK1 transcripts show differential allelic gene expression, and both rs4878104 and rs4877365 were significantly associated with DAPK1 allele-specific expression (P=0.015 to <0.0001). These data suggest that genetic variation in DAPK1 modulates susceptibility to LOAD.