Activity-dependent gating of CaMKII autonomous activity by Drosophila CASK

Synaptic Mechanisms of Ethanol Tolerance and Neuroplasticity: Insights from Invertebrate Models

Alcohol tolerance is a neuroadaptive response that leads to a reduction in the effects of alcohol caused by previous exposure. Tolerance plays a critical role in the development of alcohol use disorder (AUD) because it leads to the escalation of drinking and dependence. Understanding the molecular mechanisms underlying alcohol tolerance is therefore important for the development of effective therapeutics and for understanding addiction in general. This review explores the molecular basis of alcohol tolerance in invertebrate models, Drosophila and C. elegans, focusing on synaptic transmission. Both organisms exhibit biphasic responses to ethanol and develop tolerance similar to that of mammals. Furthermore, the availability of several genetic tools makes them a great candidate to study the molecular basis of ethanol response. Studies in invertebrate models show that tolerance involves conserved changes in the neurotransmitter systems, ion channels, and synaptic proteins. These neuroadaptive changes lead to a change in neuronal excitability, most likely to compensate for the enhanced inhibition by ethanol.

A glutamate receptor C-tail recruits CaMKII to suppress retrograde homeostatic signaling

Presynaptic homeostatic plasticity (PHP) adaptively enhances neurotransmitter release following diminished postsynaptic glutamate receptor (GluR) functionality to maintain synaptic strength. While much is known about PHP expression mechanisms, postsynaptic induction remains enigmatic. For over 20 years, diminished postsynaptic Ca2+ influx was hypothesized to reduce CaMKII activity and enable retrograde PHP signaling at the Drosophila neuromuscular junction. Here, we have interrogated inductive signaling and find that active CaMKII colocalizes with and requires the GluRIIA receptor subunit. Next, we generated Ca2+-impermeable GluRs to reveal that both CaMKII activity and PHP induction are Ca2+-insensitive. Rather, a GluRIIA C-tail domain is necessary and sufficient to recruit active CaMKII. Finally, chimeric receptors demonstrate that the GluRIIA tail constitutively occludes retrograde homeostatic signaling by stabilizing active CaMKII. Thus, the physical loss of the GluRIIA tail is sensed, rather than reduced Ca2+, to enable retrograde PHP signaling, highlighting a unique, Ca2+-independent control mechanism for CaMKII in gating homeostatic plasticity.

CASK loss of function differentially regulates neuronal maturation and synaptic function in human induced cortical excitatory neurons

Loss-of-function (LOF) mutations in CASK cause severe developmental phenotypes, including microcephaly with pontine and cerebellar hypoplasia, X-linked intellectual disability, and autism. Unraveling the pathological mechanisms of CASK-related disorders has been challenging owing to limited human cellular models to study the dynamic roles of this molecule during neuronal maturation and synapse development. Here, we investigate cell-autonomous functions of CASK in cortical excitatory induced neurons (iNs) generated from CASK knockout (KO) isogenic human embryonic stem cells (hESCs) using gene expression, morphometrics, and electrophysiology. While immature CASK KO iNs show robust neuronal outgrowth, mature CASK KO iNs display severe defects in synaptic transmission and synchronized network activity without compromising neuronal morphology and synapse numbers. In the developing human cortical excitatory neurons, CASK functions to promote both structural integrity and establishment of cortical excitatory neuronal networks. These results lay the foundation for future studies identifying suppressors of such phenotypes relevant to human patients.

The Non-Linear Path from Gene Dysfunction to Genetic Disease: Lessons from the MICPCH Mouse Model

Most human disease manifests as a result of tissue pathology, due to an underlying disease process (pathogenesis), rather than the acute loss of specific molecular function(s). Successful therapeutic strategies thus may either target the correction of a specific molecular function or halt the disease process. For the vast majority of brain diseases, clear etiologic and pathogenic mechanisms are still elusive, impeding the discovery or design of effective disease-modifying drugs. The development of valid animal models and their proper characterization is thus critical for uncovering the molecular basis of the underlying pathobiological processes of brain disorders. MICPCH (microcephaly and pontocerebellar hypoplasia) is a monogenic condition that results from variants of an X-linked gene, CASK (calcium/calmodulin-dependent serine protein kinase). CASK variants are associated with a wide range of clinical presentations, from lethality and epileptic encephalopathies to intellectual disabilities, microcephaly, and autistic traits. We have examined CASK loss-of-function mutations in model organisms to simultaneously understand the pathogenesis of MICPCH and the molecular function/s of CASK. Our studies point to a highly complex relationship between the potential molecular function/s of CASK and the phenotypes observed in model organisms and humans. Here we discuss the implications of our observations from the pathogenesis of MICPCH as a cautionary narrative against oversimplifying molecular interpretations of data obtained from genetically modified animal models of human diseases.

Bisphenol a affects neurodevelopmental gene expression, cognitive function, and neuromuscular synaptic morphology in Drosophila melanogaster

Bisphenol A (BPA) is an environmentally prevalent endocrine disrupting chemical that can impact human health and may be an environmental risk factor for neurodevelopmental disorders. BPA has been associated with behavioral impairment in children and a variety of neurodevelopmental phenotypes in model organisms. We used Drosophila melanogaster to explore the consequences of developmental BPA exposure on gene expression, cognitive function, and synapse development. Our transcriptome analysis indicated neurodevelopmentally relevant genes were predominantly downregulated by BPA. Among the misregulated genes were those with roles in learning, memory, and synapse development, as well as orthologs of human genes associated with neurodevelopmental and neuropsychiatric disorders. To examine how gene expression data corresponded to behavioral and cellular phenotypes, we first used a predator-response behavioral paradigm and found that BPA disrupts visual perception. Further analysis using conditioned courtship suppression showed that BPA impairs associative learning. Finally, we examined synapse morphology within the larval neuromuscular junction and found that BPA significantly increased the number of axonal branches. Given that our findings align with studies of BPA in mammalian model organisms, this data indicates that BPA impairs neurodevelopmental pathways that are functionally conserved from invertebrates to mammals. Further, because Drosophila do not possess classic estrogen receptors or estrogen, this research suggests that BPA can impact neurodevelopment by molecular mechanisms distinct from its role as an estrogen mimic.

Loss of CASK Accelerates Heart Failure Development

[Figure: see text].

Haploinsufficiency of X-linked intellectual disability gene CASK induces post-transcriptional changes in synaptic and cellular metabolic pathways

Heterozygous mutations in the X-linked gene CASK are associated with intellectual disability, microcephaly, pontocerebellar hypoplasia, optic nerve hypoplasia and partially penetrant seizures in girls. The Cask^+/- heterozygous knockout female mouse phenocopies the human disorder and exhibits postnatal microencephaly, cerebellar hypoplasia and optic nerve hypoplasia. It is not known if Cask^+/- mice also display seizures, nor is known the molecular mechanism by which CASK haploinsufficiency produces the numerous documented phenotypes. 24-h video electroencephalography demonstrates that despite sporadic seizure activity, the overall electrographic patterns remain unaltered in Cask^+/- mice. Additionally, seizure threshold to the commonly used kindling agent, pentylenetetrazol, remains unaltered in Cask^+/- mice, indicating that even in mice the seizure phenotype is only partially penetrant and may have an indirect mechanism. RNA sequencing experiments on Cask^+/- mouse brain uncovers a very limited number of changes, with most differences arising in the transcripts of extracellular matrix proteins and the transcripts of a group of nuclear proteins. In contrast to limited changes at the transcript level, quantitative whole-brain proteomics using iTRAQ quantitative mass-spectrometry reveals major changes in synaptic, metabolic/mitochondrial, cytoskeletal, and protein metabolic pathways. Unbiased protein-protein interaction mapping using affinity chromatography demonstrates that CASK may form complexes with proteins belonging to the same functional groups in which altered protein levels are observed. We discuss the mechanism of the observed changes in the context of known molecular function/s of CASK. Overall, our data indicate that the phenotypic spectrum of female Cask^+/- mice includes sporadic seizures and thus closely parallels that of CASK haploinsufficient girls; the Cask^+/- mouse is thus a face-validated model for CASK-related pathologies. We therefore surmise that CASK haploinsufficiency is likely to affect brain structure and function due to dysregulation of several cellular pathways including synaptic signaling and cellular metabolism.

Interactions between amyloid precursor protein-like (APPL) and MAGUK scaffolding proteins contribute to appetitive long-term memory in Drosophila melanogaster

Amyloid precursor protein (APP), the precursor of amyloid beta peptide, plays a central role in Alzheimer's disease (AD), a pathology characterized by memory decline and synaptic loss upon aging. Understanding the physiological role of APP is fundamental in deciphering the progression of AD, and several studies suggest a synaptic function via protein-protein interactions. Nevertheless, it remains unclear whether and how these interactions contribute to memory. In Drosophila, we previously showed that APP-like (APPL), the fly APP homolog, is required for aversive associative memory in the olfactory memory center, the mushroom body (MB). In the present study, we show that APPL is required for appetitive long-term memory (LTM), another form of associative memory, in a specific neuronal subpopulation of the MB, the α'/β' Kenyon cells. Using a biochemical approach, we identify the synaptic MAGUK (membrane-associated guanylate kinase) proteins X11, CASK, Dlgh2 and Dlgh4 as interactants of the APP intracellular domain (AICD). Next, we show that the Drosophila homologs CASK and Dlg are also required for appetitive LTM in the α'/β' neurons. Finally, using a double RNAi approach, we demonstrate that genetic interactions between APPL and CASK, as well as between APPL and Dlg, are critical for appetitive LTM. In summary, our results suggest that APPL contributes to associative long-term memory through its interactions with the main synaptic scaffolding proteins CASK and Dlg. This function should be conserved across species.

Synapse-specific and compartmentalized expression of presynaptic homeostatic potentiation

Postsynaptic compartments can be specifically modulated during various forms of synaptic plasticity, but it is unclear whether this precision is shared at presynaptic terminals. Presynaptic homeostatic plasticity (PHP) stabilizes neurotransmission at the Drosophila neuromuscular junction, where a retrograde enhancement of presynaptic neurotransmitter release compensates for diminished postsynaptic receptor functionality. To test the specificity of PHP induction and expression, we have developed a genetic manipulation to reduce postsynaptic receptor expression at one of the two muscles innervated by a single motor neuron. We find that PHP can be induced and expressed at a subset of synapses, over both acute and chronic time scales, without influencing transmission at adjacent release sites. Further, homeostatic modulations to CaMKII, vesicle pools, and functional release sites are compartmentalized and do not spread to neighboring pre- or post-synaptic structures. Thus, both PHP induction and expression mechanisms are locally transmitted and restricted to specific synaptic compartments.

Everything we think and do is the result of communication between neurons. This communication takes place at junctions called synapses. When two nerve cells or neurons communicate at a synapse, the output terminal of the first cell releases a chemical called a neurotransmitter. This binds to receiver proteins, or receptors, on the second cell. When this communication is interrupted, synapses can adapt to maintain a stable dialogue between them. This can occur in two ways. Either the first neuron starts to release more neurotransmitter from its output terminal, or the second neuron produces extra receptors with which to detect the neurotransmitter.

But how specific are these changes? The brain contains far more synapses than neurons because each neuron can form synapses with many other cells. Can a neuron adjust how much of the neurotransmitter it releases at some of its synapses while leaving the others unchanged?

Li et al. have now addressed this question by studying a special type of synapse that forms between neurons and muscles, known as a neuromuscular junction. At one particular neuromuscular junction in fruit flies, a single neuron splits into two output terminals, each of which forms a synapse with a different muscle. Li et al. show that when the number of neurotransmitter receptors in one of the muscles is artificially reduced, the associated output terminal compensates by increasing its neurotransmitter release. By contrast, the other output terminal remains unaffected.

This suggests that a neuron can induce remarkably specific changes in a subset of its synapses. This discovery paves the way towards identifying the smallest possible unit of change that can occur in the neurons’ ability to communicate. This unit may in turn be the smallest change that can support learning. Such knowledge will help us understand how the nervous system processes and stabilizes information transfer, both in health and after injury or disease.

The secret life of kinases: insights into non-catalytic signalling functions from pseudokinases

Over the past decade, our understanding of the mechanisms by which pseudokinases, which comprise ∼10% of the human and mouse kinomes, mediate signal transduction has advanced rapidly with increasing structural, biochemical, cellular and genetic studies. Pseudokinases are the catalytically defective counterparts of conventional, active protein kinases and have been attributed functions as protein interaction domains acting variously as allosteric modulators of conventional protein kinases and other enzymes, as regulators of protein trafficking or localisation, as hubs to nucleate assembly of signalling complexes, and as transmembrane effectors of such functions. Here, by categorising mammalian pseudokinases based on their known functions, we illustrate the mechanistic diversity among these proteins, which can be viewed as a window into understanding the non-catalytic functions that can be exerted by conventional protein kinases.

CaMKII knockdown affects both early and late phases of olfactory long-term memory in the honeybee

Honeybees are able to solve complex learning tasks and memorize learned information for long time periods. The molecular mechanisms mediating long-term memory (LTM) in the honeybee Apis mellifera are, to a large part, still unknown. We approached this question by investigating the potential function of the calcium/calmodulin-dependent protein kinase II (CaMKII), an enzyme known as a 'molecular memory switch' in vertebrates. CaMKII is able to switch to a calcium-independent constitutively active state, providing a mechanism for a molecular memory and has further been shown to play an essential role in structural synaptic plasticity. Using a combination of knockdown by RNA interference and pharmacological manipulation, we disrupted the function of CaMKII during olfactory learning and memory formation. We found that learning, memory acquisition and mid-term memory were not affected, but all manipulations consistently resulted in an impaired LTM. Both early LTM (24 h after learning) and late LTM (72 h after learning) were significantly disrupted, indicating the necessity of CaMKII in two successive stages of LTM formation in the honeybee.

PKA and cAMP/CNG Channels Independently Regulate the Cholinergic Ca(2+)-Response of Drosophila Mushroom Body Neurons

The mushroom bodies (MBs) are the most prominent structures in adult Drosophila brain. They have been involved in several crucial functions, such as learning and memory, sleep, locomotor activity, and decision making.

The mushroom bodies (MBs), one of the main structures in the adult insect brain, play a critical role in olfactory learning and memory. Though historical genes such as dunce and rutabaga, which regulate the level of cAMP, were identified more than 30 years ago, their in vivo effects on cellular and physiological mechanisms and particularly on the Ca²⁺-responses still remain largely unknown. In this work, performed in Drosophila, we took advantage of in vivo bioluminescence imaging, which allowed real-time monitoring of the entire MBs (both the calyx/cell-bodies and the lobes) simultaneously. We imaged neuronal Ca²⁺-activity continuously, over a long time period, and characterized the nicotine-evoked Ca²⁺-response. Using both genetics and pharmacological approaches to interfere with different components of the cAMP signaling pathway, we first show that the Ca²⁺-response is proportional to the levels of cAMP. Second, we reveal that an acute change in cAMP levels is sufficient to trigger a Ca²⁺-response. Third, genetic manipulation of protein kinase A (PKA), a direct effector of cAMP, suggests that cAMP also has PKA-independent effects through the cyclic nucleotide-gated Ca²⁺-channel (CNG). Finally, the disruption of calmodulin, one of the main regulators of the rutabaga adenylate cyclase (AC), yields different effects in the calyx/cell-bodies and in the lobes, suggesting a differential and regionalized regulation of AC. Our results provide insights into the complex Ca²⁺-response in the MBs, leading to the conclusion that cAMP modulates the Ca²⁺-responses through both PKA-dependent and -independent mechanisms, the latter through CNG-channels.

Intracellular translocation of histone deacetylase 5 regulates neuronal cell apoptosis

Histone deacetylase 5 (HDAC5) undergoes signal-dependent shuttling between the nucleus and cytoplasm, which is regulated in part by calcium/calmodulin-dependent kinase (CaMK)-mediated phosphorylation. Here, we report that HDAC5 regulates the survival of cortical neurons in pathological conditions. HDAC5 was evenly localized to the nucleus and cytoplasm in cultured cortical neurons. However, in response to 50μM NMDA conditions that induced neuronal cell apoptosis, nuclear-distributed HDAC5 was rapidly phosphorylated and translocated into cytoplasm of the cultured cortical neurons. Treatment with a CaMKII inhibitor KN93 suppressed HDAC5 phosphorylation and nuclear translocation induced by NMDA, whereas constitutively active CaMKIIα (T286D) stimulated HDAC5 nuclear export. Importantly, we showed that ectopic expression of nuclear-localized HDAC5 in cortical neurons suppressed NMDA-induced apoptosis. Finally, inactivation of HDAC5 by treatment with the class II-specific HDAC inhibitor trichostatin A (TSA) promoted NMDA-induced neuronal cell apoptosis. Altogether, these data identify HDAC5 and its intracellular translocation as key effectors of multiple pathways that regulate neuronal cell apoptosis.

Neuron-specific protein interactions of Drosophila CASK-β are revealed by mass spectrometry

Modular scaffolding proteins are designed to have multiple interactors. CASK, a member of the membrane-associated guanylate kinase (MAGUK) superfamily, has been shown to have roles in many tissues, including neurons and epithelia. It is likely that the set of proteins it interacts with is different in each of these diverse tissues. In this study we asked if within the Drosophila central nervous system, there were neuron-specific sets of CASK-interacting proteins. A YFP-tagged CASK-β transgene was expressed in genetically defined subsets of neurons in the Drosophila brain known to be important for CASK function, and proteins present in an anti-GFP immunoprecipitation were identified by mass spectrometry. Each subset of neurons had a distinct set of interacting proteins, suggesting that CASK participates in multiple protein networks and that these networks may be different in different neuronal circuits. One common set of proteins was associated with mitochondria, and we show here that endogenous CASK-β co-purifies with mitochondria. We also determined CASK-β posttranslational modifications for one cell type, supporting the idea that this technique can be used to assess cell- and circuit-specific protein modifications as well as protein interaction networks.

CASK and CaMKII function in Drosophila memory

Calcium (Ca²⁺) and Calmodulin (CaM)-dependent serine/threonine kinase II (CaMKII) plays a central role in synaptic plasticity and memory due to its ability to phosphorylate itself and regulate its own kinase activity. Autophosphorylation at threonine 287 (T287) switches CaMKII to a Ca²⁺ independent and constitutively active state replicated by overexpression of a phosphomimetic CaMKII-T287D transgene or blocked by expression of a T287A transgene. A second pair of sites, T306 T307 in the CaM binding region once autophosphorylated, prevents CaM binding and inactivates the kinase during synaptic plasticity and memory, and can be blocked by a TT306/7AA transgene. Recently the synaptic scaffolding molecule called CASK (Ca²⁺/CaM-associated serine kinase) has been shown to control both sets of CaMKII autophosphorylation events during neuronal growth, Ca²⁺ signaling and memory in Drosophila. Deletion of either full length CASK or just its CaMK-like and L27 domains removed middle-term memory (MTM) and long-term memory (LTM), with CASK function in the α′/ß′ mushroom body neurons being required for memory. In a similar manner directly changing the levels of CaMKII autophosphorylation (T287D, T287A, or TT306/7AA) in the α′/ß′ neurons also removed MTM and LTM. In the CASK null mutant expression of either the Drosophila or human CASK transgene in the α′/ß′ neurons was found to completely rescue memory, confirming that CASK signaling in α′/β′ neurons is necessary and sufficient for Drosophila memory formation and that the neuronal function of CASK is conserved between Drosophila and human. Expression of human CASK in Drosophila also rescued the effect of CASK deletion on the activity state of CaMKII, suggesting that human CASK may also regulate CaMKII autophosphorylation. Mutations in human CASK have recently been shown to result in intellectual disability and neurological defects suggesting a role in plasticity and learning possibly via regulation of CaMKII autophosphorylation.

Role of calmodulin-calmodulin kinase II, cAMP/protein kinase A and ERK 1/2 on Aeromonas hydrophila-induced apoptosis of head kidney macrophages

The role of calcium (Ca²⁺) and its dependent protease calpain in Aeromonas hydrophila-induced head kidney macrophage (HKM) apoptosis has been reported. Here, we report the pro-apoptotic involvement of calmodulin (CaM) and calmodulin kinase II gamma (CaMKIIg) in the process. We observed significant increase in CaM levels in A. hydrophila-infected HKM and the inhibitory role of BAPTA/AM, EGTA, nifedipine and verapamil suggested CaM elevation to be Ca²⁺-dependent. Our studies with CaM-specific siRNA and the CaM inhibitor calmidazolium chloride demonstrated CaM to be pro-apoptotic that initiated the downstream expression of CaMKIIg. Using the CaMKIIg-targeted siRNA, specific inhibitor KN-93 and its inactive structural analogue KN-92 we report CaM-CaMKIIg signalling to be critical for apoptosis of A. hydrophila-infected HKM. Inhibitor studies further suggested the role of calpain-2 in CaMKIIg expression. CaMK Kinase (CaMKK), the other CaM dependent kinase exhibited no role in A. hydrophila-induced HKM apoptosis. We report increased production of intracellular cAMP in infected HKM and our results with KN-93 or KN-92 implicate the role of CaMKIIg in cAMP production. Using siRNA to PKACA, the catalytic subunit of PKA, anti-PKACA antibody and H-89, the specific inhibitor for PKA we prove the pro-apoptotic involvement of cAMP/PKA pathway in the pathogenicity of A. hydrophila. Our inhibitor studies coupled with siRNA approach further implicated the role of cAMP/PKA in activation of extracellular signal-regulated kinase 1 and 2 (ERK 1/2). We conclude that the alteration in intracellular Ca²⁺ levels initiated by A. hydrophila activates CaM and calpain-2; both pathways converge on CaMKIIg which in turn induces cAMP/PKA mediated ERK 1/2 phosphorylation leading to caspase-3 mediated apoptosis of infected HKM.

Aeromonas hydrophila is a natural fish pathogen and is known to induce apoptosis of HKM. Head kidney is an important immune-organ in fish and HKM are critical for immunity against the invading pathogen. The mechanisms of cell death induced by A. hydrophila are incompletely characterized. We have studied the role of Ca²⁺-dependent signalling pathways in the induction of A. hydrophila-induced HKM apoptosis. We observed that A. hydrophila infection led to increased CaM expression in infected HKM which was Ca²⁺-dependent. The inhibitor and siRNA studies suggested CaM to be pro-apoptotic and triggered CaMKIIg expression in the infected HKM. Calpain-2 appeared to influence CaMKIIg expression. However, further studies are needed to understand the process. We report that the CaM-CaMKIIg pathway is important for initiating cAMP production within the infected HKM. The pro-apoptotic activation of cAMP dependent PKA was quite evident. The activation of ERK 1/2 was observed in the HKM and results clearly suggested the pro-active role of cAMP/PKA in the process. Thus we conclude that CaM-CaMKIIg initiates the cAMP/PKA pathway that induces ERK 1/2 phosphorylation to promote caspase-3 mediated apoptosis of the A. hydrophila-infected HKM.

CaMKII: claiming center stage in postsynaptic function and organization

While CaMKII has long been known to be essential for synaptic plasticity and learning, recent work points to new dimensions of CaMKII function in the nervous system, revealing that CaMKII also plays an important role in synaptic organization. Ca(2+)-triggered autophosphorylation of CaMKII not only provides molecular memory by prolonging CaMKII activity during long-term plasticity (LTP) and learning but also represents a mechanism for autoactivation of CaMKII's multifaceted protein-docking functions. New details are also emerging about the distinct roles of CaMKIIα and CaMKIIβ in synaptic homeostasis, further illustrating the multilayered and complex nature of CaMKII's involvement in synaptic regulation. Here, I review novel molecular and functional insight into how CaMKII supports synaptic function.

ATP P2X3 receptors and neuronal sensitization

Increasing evidence indicates the importance of extracellular adenosine triphosphate (ATP) in the modulation of neuronal function. In particular, fine control of ATP release and the selective and discrete ATP receptor operation are crucial elements of the crosstalk between neuronal and non-neuronal cells in the peripheral and central nervous systems. In peripheral neurons, ATP signaling gives an important contribution to neuronal sensitization, especially that involved in neuropathic pain. Among other subtypes, P2X3 receptors expressed on sensory neurons are sensitive even to nanomolar concentrations of extracellular ATP, and therefore are important transducers of pain stimuli. P2X3 receptor function is highly sensitive to soluble factors like neuropeptides and neurotrophins, and is controlled by transduction mechanisms, protein-protein interactions and discrete membrane compartmentalization. More recent findings have demonstrated that P2X3 receptors interact with the synaptic scaffold protein calcium/calmodulin-dependent serine protein kinase (CASK) in a state dependent fashion, indicating that CASK plays a crucial role in the modulation of P2X3 receptor stability and efficiency. Activation of P2X3 receptors within CASK/P2X3 complex has important consequences for neuronal plasticity and possibly for the release of neuromodulators and neurotransmitters. Better understanding of the interactome machinery of P2X3 receptors and their integration with other receptors and channels on neuronal surface membranes, is proposed to be essential to unveil the process of neuronal sensitization and related, abnormal pain signaling.

CASK regulates CaMKII autophosphorylation in neuronal growth, calcium signaling, and learning

Calcium (Ca²⁺)/calmodulin (CaM)-dependent kinase II (CaMKII) activity plays a fundamental role in learning and memory. A key feature of CaMKII in memory formation is its ability to be regulated by autophosphorylation, which switches its activity on and off during synaptic plasticity. The synaptic scaffolding protein CASK (calcium (Ca²⁺)/calmodulin (CaM) associated serine kinase) is also important for learning and memory, as mutations in CASK result in intellectual disability and neurological defects in humans. We show that in Drosophila larvae, CASK interacts with CaMKII to control neuronal growth and calcium signaling. Furthermore, deletion of the CaMK-like and L27 domains of CASK (CASK β null) or expression of overactive CaMKII (T287D) produced similar effects on synaptic growth and Ca²⁺ signaling. CASK overexpression rescues the effects of CaMKII overactivity, consistent with the notion that CASK and CaMKII act in a common pathway that controls these neuronal processes. The reduction in Ca²⁺ signaling observed in the CASK β null mutant caused a decrease in vesicle trafficking at synapses. In addition, the decrease in Ca²⁺ signaling in CASK mutants was associated with an increase in Ether-à-go-go (EAG) potassium (K⁺) channel localization to synapses. Reducing EAG restored the decrease in Ca²⁺ signaling observed in CASK mutants to the level of wildtype, suggesting that CASK regulates Ca²⁺ signaling via EAG. CASK knockdown reduced both appetitive associative learning and odor evoked Ca²⁺ responses in Drosophila mushroom bodies, which are the learning centers of Drosophila. Expression of human CASK in Drosophila rescued the effect of CASK deletion on the activity state of CaMKII, suggesting that human CASK may also regulate CaMKII autophosphorylation.

KCNQ channels regulate age-related memory impairment

In humans KCNQ2/3 heteromeric channels form an M-current that acts as a brake on neuronal excitability, with mutations causing a form of epilepsy. The M-current has been shown to be a key regulator of neuronal plasticity underlying associative memory and ethanol response in mammals. Previous work has shown that many of the molecules and plasticity mechanisms underlying changes in alcohol behaviour and addiction are shared with those of memory. We show that the single KCNQ channel in Drosophila (dKCNQ) when mutated show decrements in associative short- and long-term memory, with KCNQ function in the mushroom body α/βneurons being required for short-term memory. Ethanol disrupts memory in wildtype flies, but not in a KCNQ null mutant background suggesting KCNQ maybe a direct target of ethanol, the blockade of which interferes with the plasticity machinery required for memory formation. We show that as in humans, Drosophila display age-related memory impairment with the KCNQ mutant memory defect mimicking the effect of age on memory. Expression of KCNQ normally decreases in aging brains and KCNQ overexpression in the mushroom body neurons of KCNQ mutants restores age-related memory impairment. Therefore KCNQ is a central plasticity molecule that regulates age dependent memory impairment.

CASK and CaMKII function in the mushroom body α'/β' neurons during Drosophila memory formation

Ca(2+)/CaM serine/threonine kinase II (CaMKII) is a central molecule in mechanisms of synaptic plasticity and memory. A vital feature of CaMKII in plasticity is its ability to switch to a calcium (Ca(2+)) independent constitutively active state after autophosphorylation at threonine 287 (T287). A second pair of sites, T306 T307 in the calmodulin (CaM) binding region once autophosphorylated, prevent subsequent CaM binding and inactivates the kinase during synaptic plasticity and memory. Recently a synaptic molecule called Ca(2+)/CaM-dependent serine protein kinase (CASK) has been shown to control both sets of CaMKII autophosphorylation events and hence is well poised to be a key regulator of memory. We show deletion of full length CASK or just its CaMK-like and L27 domains disrupts middle-term memory (MTM) and long-term memory (LTM), with CASK function in the α'/β' subset of mushroom body neurons being required for memory. Likewise directly changing the levels of CaMKII autophosphorylation in these neurons removed MTM and LTM. The requirement of CASK and CaMKII autophosphorylation was not developmental as their manipulation just in the adult α'/β' neurons was sufficient to remove memory. Overexpression of CASK or CaMKII in the α'/β' neurons also occluded MTM and LTM. Overexpression of either Drosophila or human CASK in the α'/β' neurons of the CASK mutant completely rescued memory, confirming that CASK signaling in α'/β' neurons is necessary and sufficient for Drosophila memory formation and that the neuronal function of CASK is conserved between Drosophila and human. At the cellular level CaMKII overexpression in the α'/β' neurons increased activity dependent Ca(2+) responses while reduction of CaMKII decreased it. Likewise reducing CASK or directly expressing a phosphomimetic CaMKII T287D transgene in the α'/β' similarly decreased Ca(2+) signaling. Our results are consistent with CASK regulating CaMKII autophosphorylation in a pathway required for memory formation that involves activity dependent changes in Ca(2+) signaling in the α'/β' neurons.

Is alcoholism learned? Insights from the fruit fly

Alcohol addiction is a complex, unique human disease. Breaking addiction down into contributing endophenotypes enables its study in a variety of model systems. The Drosophila model system has been most often used to study alcohol sensitivity, tolerance, and physiological dependence. However, none of these endophenotypes can account for the near-permanent quality of the addicted state. It has been recently discussed that addictive drugs may hijack the learning-and-memory machinery to produce persistent behavioral changes. Learning and memory is amenable to experimental study, and provides us with a window into how alcohol affects higher-order mental functions that are likely to contribute compulsive drug use. Here, we review the Drosophila literature that links alcohol-related behaviors to learning and memory.

KCNQ channels show conserved ethanol block and function in ethanol behaviour

In humans, KCNQ2/3 channels form an M-current that regulates neuronal excitability, with mutations in these channels causing benign neonatal familial convulsions. The M-current is important in mechanisms of neural plasticity underlying associative memory and in the response to ethanol, with KCNQ controlling the release of dopamine after ethanol exposure. We show that dKCNQ is broadly expressed in the nervous system, with targeted reduction in neuronal KCNQ increasing neural excitability and KCNQ overexpression decreasing excitability and calcium signalling, consistent with KCNQ regulating the resting membrane potential and neural release as in mammalian neurons. We show that the single KCNQ channel in Drosophila (dKCNQ) has similar electrophysiological properties to neuronal KCNQ2/3, including conserved acute sensitivity to ethanol block, with the fly channel (IC(50) = 19.8 mM) being more sensitive than its mammalian ortholog (IC(50) = 42.1 mM). This suggests that the role of KCNQ in alcohol behaviour can be determined for the first time by using Drosophila. We present evidence that loss of KCNQ function in Drosophila increased sensitivity and tolerance to the sedative effects of ethanol. Acute activation of dopaminergic neurons by heat-activated TRP channel or KCNQ-RNAi expression produced ethanol hypersensitivity, suggesting that both act via a common mechanism involving membrane depolarisation and increased dopamine signalling leading to ethanol sedation.

DlgS97/SAP97, a neuronal isoform of discs large, regulates ethanol tolerance

From a genetic screen for Drosophila melanogaster mutants with altered ethanol tolerance, we identified intolerant (intol), a novel allele of discs large 1 (dlg1). Dlg1 encodes Discs Large 1, a MAGUK (Membrane Associated Guanylate Kinase) family member that is the highly conserved homolog of mammalian PSD-95 and SAP97. The intol mutation disrupted specifically the expression of DlgS97, a SAP97 homolog, and one of two major protein isoforms encoded by dlg1 via alternative splicing. Expression of the major isoform, DlgA, a PSD-95 homolog, appeared unaffected. Ethanol tolerance in the intol mutant could be partially restored by transgenic expression of DlgS97, but not DlgA, in specific neurons of the fly's brain. Based on co-immunoprecipitation, DlgS97 forms a complex with N-methyl-D-aspartate (NMDA) receptors, a known target of ethanol. Consistent with these observations, flies expressing reduced levels of the essential NMDA receptor subunit dNR1 also showed reduced ethanol tolerance, as did mutants in the gene calcium/calmodulin-dependent protein kinase (caki), encoding the fly homolog of mammalian CASK, a known binding partner of DlgS97. Lastly, mice in which SAP97, the mammalian homolog of DlgS97, was conditionally deleted in adults failed to develop rapid tolerance to ethanol's sedative/hypnotic effects. We propose that DlgS97/SAP97 plays an important and conserved role in the development of tolerance to ethanol via NMDA receptor-mediated synaptic plasticity.

Role of the MAGUK protein family in synapse formation and function

Synaptic function is crucially dependent on the spatial organization of the presynaptic and postsynaptic apparatuses and the juxtaposition of both membrane compartments. This precise arrangement is achieved by a protein network at the submembrane region of each cell that is built around scaffold proteins. The membrane-associated guanylate kinase (MAGUK) family of proteins is a widely expressed and well-conserved group of proteins that plays an essential role in the formation and regulation of this scaffolding. Here, we review general features of this protein family, focusing on the discs large and calcium/calmodulin-dependent serine protein kinase subfamilies of MAGUKs in the formation, function, and plasticity of synapses.

Pre and postsynaptic roles for Drosophila CASK

CASK ('calcium/calmodulin-dependent serine protein kinase'), also known in Drosophila as 'Caki' or 'Camguk/CMG', and in C. elegans as 'Lin-2', is thought to play an important role in cell-cell junction formation and at synapses in particular. To understand the role of CASK in synapse formation and function, we functionally and morphologically analyzed Drosophila embryonic and larval glutamatergic neuromuscular junctions (NMJs) after pan-cellular and tissue-specific manipulation of CASK expression. Our results show that Drosophila CASK is associated with both pre and postsynaptic membranes. Loss of presynaptic CASK led to less evoked synaptic transmission, fewer spontaneous synaptic events, and reduced synaptic vesicle cycling. These changes were accompanied by a reduction in the number of synapses but no change in overall NMJ size. Loss of postsynaptic CASK, on the other hand, caused reduced spontaneous synaptic current amplitudes and smaller glutamate-gated currents. These changes were accompanied by loss of postsynaptic glutamate receptors, but the receptor loss was subtype-specific: Only receptors containing GluRIIA subunits were lost in CASK mutants. Receptors containing GluRIIB were unaffected.

A novel form of presynaptic CaMKII-dependent short-term potentiation between Lymnaea neurons

Short-term plasticity is thought to form the basis for working memory, the cellular mechanisms of which are the least understood in the nervous system. In this study, using in vitro reconstructed synapses between the identified Lymnaea neuron visceral dorsal 4 (VD4) and left pedal dorsal 1 (LPeD1), we demonstrate a novel form of short-term potentiation (STP) which is 'use'- but not time-dependent, unlike most previously defined forms of short-term synaptic plasticity. Using a triple-cell configuration we demonstrate for the first time that a single presynaptic neuron can reliably potentiate both inhibitory and excitatory synapses. We further demonstrate that, unlike previously described forms of STP, the synaptic potentiation between Lymnaea neurons does not involve postsynaptic receptor sensitization or presynaptic residual calcium. Finally, we provide evidence that STP at the VD4-LPeD1 synapse requires presynaptic calcium/calmodulin dependent kinase II (CaMKII). Taken together, our study identifies a novel form of STP which may provide the basis for both short- and long-term potentiation, in the absence of any protein synthesis-dependent steps, and involve CaMKII activity exclusively in the presynaptic cell.

Altered gene regulation and synaptic morphology in Drosophila learning and memory mutants

Genetic studies in Drosophila have revealed two separable long-term memory pathways defined as anesthesia-resistant memory (ARM) and long-lasting long-term memory (LLTM). ARM is disrupted in radish (rsh) mutants, whereas LLTM requires CREB-dependent protein synthesis. Although the downstream effectors of ARM and LLTM are distinct, pathways leading to these forms of memory may share the cAMP cascade critical for associative learning. Dunce, which encodes a cAMP-specific phosphodiesterase, and rutabaga, which encodes an adenylyl cyclase, both disrupt short-term memory. Amnesiac encodes a pituitary adenylyl cyclase-activating peptide homolog and is required for middle-term memory. Here, we demonstrate that the Radish protein localizes to the cytoplasm and nucleus and is a PKA phosphorylation target in vitro. To characterize how these plasticity pathways may manifest at the synaptic level, we assayed synaptic connectivity and performed an expression analysis to detect altered transcriptional networks in rutabaga, dunce, amnesiac, and radish mutants. All four mutants disrupt specific aspects of synaptic connectivity at larval neuromuscular junctions (NMJs). Genome-wide DNA microarray analysis revealed ∼375 transcripts that are altered in these mutants, suggesting defects in multiple neuronal signaling pathways. In particular, the transcriptional target Lapsyn, which encodes a leucine-rich repeat cell adhesion protein, localizes to synapses and regulates synaptic growth. This analysis provides insights into the Radish-dependent ARM pathway and novel transcriptional targets that may contribute to memory processing in Drosophila.

Peptide-induced modulation of synaptic transmission and escape response in Drosophila requires two G-protein-coupled receptors

Neuropeptides are found in both mammals and invertebrates and can modulate neural function through activation of G-protein-coupled receptors (GPCRS). The precise mechanisms by which many of these GPCRs modulate specific signaling cascades to regulate neural function are not well defined. We used Drosophila melanogaster as a model to examine both the cellular and behavioral effects of DPKQDFMRFamide, the most abundant peptide encoded by the dFMRF gene. We show that DPKQDFMRFamide enhanced synaptic transmission through activation of two G-protein-coupled receptors, Fmrf Receptor (FR) and Dromyosupressin Receptor-2 (DmsR-2). The peptide increased both the presynaptic Ca(2+) response and the quantal content of released transmitter. Peptide-induced modulation of synaptic function could be abrogated by depleting intracellular Ca(2+) stores or by interfering with Ca(2+) release from the endoplasmic reticulum through disruption of either the ryanodine receptor or the inositol 1,4,5-trisphosphate receptor. The peptide also altered behavior. Exogenous DPKQDFMRFamide enhanced fictive locomotion; this required both the FR and DmsR-2. Likewise, both receptors were required for an escape response to intense light exposure. Thus, coincident detection of a peptide by two GPCRs modulates synaptic function through effects of Ca(2+)-induced Ca(2+) release, and we hypothesize that these mechanisms are involved in behavioral responses to environmental stress.

Central regulation of locomotor behavior of Drosophila melanogaster depends on a CASK isoform containing CaMK-like and L27 domains

Genetic causes for disturbances of locomotor behavior can be due to muscle, peripheral neuron, or central nervous system pathologies. The Drosophila melanogaster homolog of human CASK (also known as caki or camguk) is a molecular scaffold that has been postulated to have roles in both locomotion and plasticity. These conclusions are based on studies using overlapping deficiencies that largely eliminate the entire CASK locus, but contain additional chromosomal aberrations as well. More importantly, analysis of the sequenced Drosophila genome suggests the existence of multiple protein variants from the CASK locus, further complicating the interpretation of experiments using deficiency strains. In this study, we generated small deletions within the CASK gene that eliminate gene products containing the CaMK-like and L27 domains (CASK-β), but do not affect transcripts encoding the smaller forms (CASK-α), which are structurally homologous to vertebrate MPP1. These mutants have normal olfactory habituation, but exhibit a striking array of locomotor problems that includes both initiation and motor maintenance defects. Previous studies had suggested that presynaptic release defects at the neuromuscular junction in the multigene deficiency strain were the likely basis of its locomotor phenotype. The locomotor phenotype of the CASK-β mutant, however, cannot be rescued by expression of a CASK-β transgene in motor neurons. Expression in a subset of central neurons that does not include the ellipsoid body, a well-known pre-motor neuropil, provides complete rescue. Full-length CASK-β, while widely expressed in the nervous system, appears to have a unique role within central circuits that control motor output.

Structure of the CaMKIIdelta/calmodulin complex reveals the molecular mechanism of CaMKII kinase activation

Structural and biophysical studies reveal how CaMKII kinases, which are important for cellular learning and memory, are switched on by binding of Ca²⁺/calmodulin.

Long-term potentiation (LTP), a long-lasting enhancement in communication between neurons, is considered to be the major cellular mechanism underlying learning and memory. LTP triggers high-frequency calcium pulses that result in the activation of Calcium/Calmodulin (CaM)-dependent kinase II (CaMKII). CaMKII acts as a molecular switch because it remains active for a long time after the return to basal calcium levels, which is a unique property required for CaMKII function. Here we describe the crystal structure of the human CaMKIIδ/Ca²⁺/CaM complex, structures of all four human CaMKII catalytic domains in their autoinhibited states, as well as structures of human CaMKII oligomerization domains in their tetradecameric and physiological dodecameric states. All four autoinhibited human CaMKIIs were monomeric in the determined crystal structures but associated weakly in solution. In the CaMKIIδ/Ca²⁺/CaM complex, the inhibitory region adopted an extended conformation and interacted with an adjacent catalytic domain positioning T287 into the active site of the interacting protomer. Comparisons with autoinhibited CaMKII structures showed that binding of calmodulin leads to the rearrangement of residues in the active site to a conformation suitable for ATP binding and to the closure of the binding groove for the autoinhibitory helix by helix αD. The structural data, together with biophysical interaction studies, reveals the mechanism of CaMKII activation by calmodulin and explains many of the unique regulatory properties of these two essential signaling molecules.

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CaMKII enzymes transmit calcium ion (Ca²⁺) signals released inside the cell by regulating signal transduction pathways through phosphorylation: Ca²⁺ first binds to the small regulatory protein CaM; this Ca²⁺/CaM complex then binds to and activates the kinase, which phosphorylates other proteins in the cell. Since CaMKs remain active long after rapid Ca²⁺ pulses have dropped they function as molecular switches that turn on or off crucial cell functions in response to Ca²⁺ levels. The multifunctional CaMKII forms of this enzyme – of which there are four in human – are important in many processes including signaling in neurons and controlling of the heart rate. They are particularly abundant in the brain where they probably play a role in memory. CaMKII forms an exceptionally large, dodecameric complex. Here, we describe the crystal structure of this complex for each of the four human CaMKII catalytic domains in their autoinhibited states, a complex of CaMKII with Ca²⁺/CaM, as well as the structure of the oligomerization domain (the part of the protein that mediates complex formation) in its physiological dodecameric state and in a tetradecameric state. Detailed comparison of this large body of structural data together with biophysical studies has allowed us to better understand the structural mechanisms of CaMKII activation by CaM and to explain many of the complex regulatory features of these essential enzymes.

Ion channels to inactivate neurons in Drosophila

Ion channels are the determinants of excitability; therefore, manipulation of their levels and properties provides an opportunity for the investigator to modulate neuronal and circuit function. There are a number of ways to suppress electrical activity in Drosophila neurons, for instance, over-expression of potassium channels (i.e. Shaker Kv1, Shaw Kv3, Kir2.1 and DORK) that are open at resting membrane potential. This will result in increased potassium efflux and membrane hyperpolarisation setting resting membrane potential below the threshold required to fire action potentials. Alternatively over-expression of other channels, pumps or co-transporters that result in a hyperpolarised membrane potential will also prevent firing. Lastly, neurons can be inactivated by, disrupting or reducing the level of functional voltage-gated sodium (Nav1 paralytic) or calcium (Cav2 cacophony) channels that mediate the depolarisation phase of action potentials. Similarly, strategies involving the opposite channel manipulation should allow net depolarisation and hyperexcitation in a given neuron. These changes in ion channel expression can be brought about by the versatile transgenic (i.e. Gal4/UAS based) systems available in Drosophila allowing fine temporal and spatial control of (channel) transgene expression. These systems are making it possible to electrically inactivate (or hyperexcite) any neuron or neural circuit in the fly brain, and much like an exquisite lesion experiment, potentially elucidate whatever interesting behaviour or phenotype each network mediates. These techniques are now being used in Drosophila to reprogram electrical activity of well-defined circuits and bring about robust and easily quantifiable changes in behaviour, allowing different models and hypotheses to be rapidly tested.

PDF cells are a GABA-responsive wake-promoting component of the Drosophila sleep circuit

Daily sleep cycles in humans are driven by a complex circuit within which GABAergic sleep-promoting neurons oppose arousal. Drosophila sleep has recently been shown to be controlled by GABA, which acts on unknown cells expressing the Rdl GABAA receptor. We identify here the relevant Rdl-containing cells as PDF-expressing small and large ventral lateral neurons (LNvs) of the circadian clock. LNv activity regulates total sleep as well as the rate of sleep onset; both large and small LNvs are part of the sleep circuit. Flies mutant for pdf or its receptor are hypersomnolent, and PDF acts on the LNvs themselves to control sleep. These features of the Drosophila sleep circuit, GABAergic control of onset and maintenance as well as peptidergic control of arousal, support the idea that features of sleep-circuit architecture as well as the mechanisms governing the behavioral transitions between sleep and wake are conserved between mammals and insects.

Function of the Shaw potassium channel within the Drosophila circadian clock

Background

In addition to the molecular feedback loops, electrical activity has been shown to be important for the function of networks of clock neurons in generating rhythmic behavior. Most studies have used over-expression of foreign channels or pharmacological manipulations that alter membrane excitability. In order to determine the cellular mechanisms that regulate resting membrane potential (RMP) in the native clock of Drosophila we modulated the function of Shaw, a widely expressed neuronal potassium (K⁺) channel known to regulate RMP in Drosophila central neurons.

Methodology/Principal Findings

We show that Shaw is endogenously expressed in clock neurons. Differential use of clock gene promoters was employed to express a range of transgenes that either increase or decrease Shaw function in different clusters of clock neurons. Under LD conditions, increasing Shaw levels in all clock neurons (LNv, LNd, DN₁, DN₂ and DN₃), or in subsets of clock neurons (LNd and DNs or DNs alone) increases locomotor activity at night. In free-running conditions these manipulations result in arrhythmic locomotor activity without disruption of the molecular clock. Reducing Shaw in the DN alone caused a dramatic lengthening of the behavioral period. Changing Shaw levels in all clock neurons also disrupts the rhythmic accumulation and levels of Pigment Dispersing Factor (PDF) in the dorsal projections of LNv neurons. However, changing Shaw levels solely in LNv neurons had little effect on locomotor activity or rhythmic accumulation of PDF.

Conclusions/Significance

Based on our results it is likely that Shaw modulates pacemaker and output neuronal electrical activity that controls circadian locomotor behavior by affecting rhythmic release of PDF. The results support an important role of the DN clock neurons in Shaw-mediated control of circadian behavior. In conclusion, we have demonstrated a central role of Shaw for coordinated and rhythmic output from clock neurons.

CASK Functions as a Mg2+-independent neurexin kinase

CASK is a unique MAGUK protein that contains an N-terminal CaM-kinase domain besides the typical MAGUK domains. The CASK CaM-kinase domain is presumed to be a catalytically inactive pseudokinase because it lacks the canonical DFG motif required for Mg2+ binding that is thought to be indispensable for kinase activity. Here we show, however, that CASK functions as an active protein kinase even without Mg2+ binding. High-resolution crystal structures reveal that the CASK CaM-kinase domain adopts a constitutively active conformation that binds ATP and catalyzes phosphotransfer without Mg2+. The CASK CaM-kinase domain phosphorylates itself and at least one physiological interactor, the synaptic protein neurexin-1, to which CASK is recruited via its PDZ domain. Thus, our data indicate that CASK combines the scaffolding activity of MAGUKs with an unusual kinase activity that phosphorylates substrates recuited by the scaffolding activity. Moreover, our study suggests that other pseudokinases (10% of the kinome) could also be catalytically active.

CaMKIIT287 and T305 regulate history-dependent increases in alpha agonist-induced vascular tone

CaMKII is a calcium and calmodulin-activated kinase that has been shown to regulate learning and memory in the brain, and contractility in blood vessels. Following Ca activation, CaMKII autophosphorylates, gaining a calcium-independent autonomous activity that reflects a molecular memory of having previously come into contact with calcium. The present study addresses whether the molecular memory properties of CaMKII are involved in the modulation of sustained vascular tone. We demonstrate a history-dependence of α agonist-induced vascular tone and show that CaMKII activation in vascular cells is also history dependent. Autophosphorylation of Thr287, which is classically associated with autonomous activity, does not persist during tone maintenance after transient increases in intracellular calcium levels. However, we have found that another site, Thr305, known from in vitro studies to be inhibitory, is regulated by α agonists in that the inhibitory action is removed, thus leading to a delayed reactivation of CaMKII as measured by Thr287 phosphorylation. By the use of a small molecule CaMKII inhibitor (KN93) as well as a decoy peptide (autoinhibitory peptide; AIP) we show a cause and effect relationship between CaMKII reactivation and sustained vascular tone maintenance. Thus, it appears that a complex interplay between the regulation of Thr305 and Thr287 provides a novel mechanism by which a history-dependence is developed and contributes to a new facet of molecular memory for CaMKII of relevance to vascular tone maintenance.

Presynaptic ryanodine receptor-activated calmodulin kinase II increases vesicle mobility and potentiates neuropeptide release

Although it has been postulated that vesicle mobility is increased to enhance release of transmitters and neuropeptides, the mechanism responsible for increasing vesicle motion in nerve terminals and the effect of perturbing this mobilization on synaptic plasticity are unknown. Here, green fluorescent protein-tagged dense-core vesicles (DCVs) are imaged in Drosophila motor neuron terminals, where DCV mobility is increased for minutes after seconds of activity. Ca2+-induced Ca2+ release from presynaptic endoplasmic reticulum (ER) is shown to be necessary and sufficient for sustained DCV mobilization. However, this ryanodine receptor (RyR)-mediated effect is short-lived and only initiates signaling. Calmodulin kinase II (CaMKII), which is not activated directly by external Ca2+ influx, then acts as a downstream effector of released ER Ca2+. RyR and CaMKII are essential for post-tetanic potentiation of neuropeptide secretion. Therefore, the presynaptic signaling pathway for increasing DCV mobility is identified and shown to be required for synaptic plasticity.

Deletion of CASK in mice is lethal and impairs synaptic function

CASK is an evolutionarily conserved multidomain protein composed of an N-terminal Ca2+/calmodulin-kinase domain, central PDZ and SH3 domains, and a C-terminal guanylate kinase domain. Many potential activities for CASK have been suggested, including functions in scaffolding the synapse, in organizing ion channels, and in regulating neuronal gene transcription. To better define the physiological importance of CASK, we have now analyzed CASK "knockdown" mice in which CASK expression was suppressed by approximately 70%, and CASK knockout (KO) mice, in which CASK expression was abolished. CASK knockdown mice are viable but smaller than WT mice, whereas CASK KO mice die at first day after birth. CASK KO mice exhibit no major developmental abnormalities apart from a partially penetrant cleft palate syndrome. In CASK-deficient neurons, the levels of the CASK-interacting proteins Mints, Veli/Mals, and neurexins are decreased, whereas the level of neuroligin 1 (which binds to neurexins that in turn bind to CASK) is increased. Neurons lacking CASK display overall normal electrical properties and form ultrastructurally normal synapses. However, glutamatergic spontaneous synaptic release events are increased, and GABAergic synaptic release events are decreased in CASK-deficient neurons. In contrast to spontaneous neurotransmitter release, evoked release exhibited no major changes. Our data suggest that CASK, the only member of the membrane-associated guanylate kinase protein family that contains a Ca2+/calmodulin-dependent kinase domain, is required for mouse survival and performs a selectively essential function without being in itself required for core activities of neurons, such as membrane excitability, Ca2+-triggered presynaptic release, or postsynaptic receptor functions.