Persistent activation of protein kinase Calpha is not necessary for expression of cerebellar long-term depression

Celsr3 is required for Purkinje cell maturation and regulates cerebellar postsynaptic plasticity

Atypical cadherin Celsr3 is critical for brain embryonic development, and its role in the postnatal cerebellum remains unknown. Using Celsr3-GFP mice, Celsr3 shows high expression in postnatal Purkinje cells (PCs). Mice with conditional knockout (cKO) of Celsr3 in postnatal PCs exhibit deficit in motor coordination and learning, atrophic PC dendrites, and decreased synapses. Whole-PC recording in cerebellar slices discloses a reduction frequency of mEPSC and defective postsynaptic plasticity (LTP and LTD) in Celsr3 cKO mutants. Wnt5a perfusion enhances LTP formation, which could be occluded by cAMP agonist and diminished by cAMP antagonist in control, but not in Celsr3 cKO or Fzd3 cKO cerebellar slices. Celsr3 cKO resulted in the failure of mGluR1 agonist-induced LTD and paired stimulation-induced PKCα overexpression in PC dendrites, and downregulation of mGluR1 expression compvared to controls. In conclusion, Celsr3 is required for PCs maturation and regulates postsynaptic LTP and LTD through Wnt5a/cAMP and mGluR1/PKCα signaling respectively.

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Celsr3 cKO in postnatal PCs impairs mouse motor coordination and learning

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Celsr3 inactivation affects the maturation of PC dendrites and synapses

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Celsr3 is required for the cerebellar LTP induction via the Wnt5a/cAMP signaling

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Celsr3 regulates the cerebellar LTD induction through the mGluR1/PKCα pathway

DGKγ Knock-Out Mice Show Impairments in Cerebellar Motor Coordination, LTD, and the Dendritic Development of Purkinje Cells through the Activation of PKCγ

Diacylglycerol kinase γ (DGKγ) regulates protein kinase C (PKC) activity by converting DG to phosphatidic acid (PA). DGKγ directly interacts with PKCγ and is phosphorylated by PKCγ, resulting in the upregulation of lipid kinase activity. PKC dysfunction impairs motor coordination, indicating that the regulation of PKC activity is important for motor coordination. DGKγ and PKC are abundantly expressed in cerebellar Purkinje cells. However, the physiological role of DGKγ has not been elucidated. Therefore, we developed DGKγ knock-out (KO) mice and tested their cerebellar motor coordination. In DGKγ KO mice, cerebellar motor coordination and long-term depression (LTD) were impaired, and the dendrites of Purkinje cells from DGKγ KO mice were significantly retracted. Interestingly, treatment with the cPKC inhibitor Gö6976 (Gö) rescued the dendritic retraction of primary cultured Purkinje cells from DGKγ KO mice. In contrast, treatment with the PKC activator 12-o-tetradecanoylphorbol 13-acetate (TPA) reduced morphologic alterations in the dendrites of Purkinje cells from wild-type (WT) mice. In addition, we confirmed the upregulation of PKCγ activity in the cerebellum of DGKγ KO mice and rescued impaired LTD in DGKγ KO mice with a PKCγ-specific inhibitor. Furthermore, impairment of motor coordination observed in DGKγ KO mice was rescued in tm1c mice with DGKγ reexpression induced by the FLP-flippase recognition target (FRT) recombination system. These results indicate that DGKγ is involved in cerebellar LTD and the dendritic development of Purkinje cells through the regulation of PKCγ activity, and thus contributes to cerebellar motor coordination.

Switching On Depression and Potentiation in the Cerebellum

Long-term depression (LTD) and long-term potentiation (LTP) in the cerebellum are important for motor learning. However, the signaling mechanisms controlling whether LTD or LTP is induced in response to synaptic stimulation remain obscure. Using a unified model of LTD and LTP at the cerebellar parallel fiber-Purkinje cell (PF-PC) synapse, we delineate the coordinated pre- and postsynaptic signaling that determines the direction of plasticity. We show that LTP is the default response to PF stimulation above a well-defined frequency threshold. However, if the calcium signal surpasses the threshold for CaMKII activation, then an ultrasensitive "on switch" activates an extracellular signal-regulated kinase (ERK)-based positive feedback loop that triggers LTD instead. This postsynaptic feedback loop is sustained by another, trans-synaptic, feedback loop that maintains nitric oxide production throughout LTD induction. When full depression is achieved, an automatic "off switch" inactivates the feedback loops, returning the network to its basal state and demarcating the end of the early phase of LTD.

Protein kinase C activity is a protective modifier of Purkinje neuron degeneration in cerebellar ataxia

Among the many types of neurons expressing protein kinase C (PKC) enzymes, cerebellar Purkinje neurons are particularly reliant on appropriate PKC activity for maintaining homeostasis. The importance of PKC enzymes in Purkinje neuron health is apparent as mutations in PRKCG (encoding PKCγ) cause cerebellar ataxia. PRKCG has also been identified as an important node in ataxia gene networks more broadly, but the functional role of PKC in other forms of ataxia remains unexplored, and the mechanisms by which PKC isozymes regulate Purkinje neuron health are not well understood. Here, we investigated how PKC activity influences neurodegeneration in inherited ataxia. Using mouse models of spinocerebellar ataxia type 1 (SCA1) and 2 (SCA2) we identify an increase in PKC-mediated substrate phosphorylation in two different forms of inherited cerebellar ataxia. Normalizing PKC substrate phosphorylation in SCA1 and SCA2 mice accelerates degeneration, suggesting that the increased activity observed in these models is neuroprotective. We also find that increased phosphorylation of PKC targets limits Purkinje neuron membrane excitability, suggesting that PKC activity may support Purkinje neuron health by moderating excitability. These data suggest a functional role for PKC enzymes in ataxia gene networks, and demonstrate that increased PKC activity is a protective modifier of degeneration in inherited cerebellar ataxia.

Protein kinase C isoforms at the neuromuscular junction: localization and specific roles in neurotransmission and development

The protein kinase C family (PKC) regulates a variety of neural functions including neurotransmitter release. The selective activation of a wide range of PKC isoforms in different cells and domains is likely to contribute to the functional diversity of PKC phosphorylating activity. In this review, we describe the isoform localization, phosphorylation function, regulation and signalling of the PKC family at the neuromuscular junction. Data show the involvement of the PKC family in several important functions at the neuromuscular junction and in particular in the maturation of the synapse and the modulation of neurotransmission in the adult.

A common framework of signal processing in the induction of cerebellar LTD and cortical STDP

Cerebellar long-term depression (LTD) and cortical spike-timing-dependent synaptic plasticity (STDP) are two well-known and well-characterized types of synaptic plasticity. Induction of both types of synaptic plasticity depends on the spike timing, pairing frequency, and pairing numbers of two different sources of spiking. This implies that the induction of synaptic plasticity may share common frameworks in terms of signal processing regardless of the different signaling pathways involved in the two types of synaptic plasticity. Here we propose that both types share common frameworks of signal processing for spike-timing, pairing-frequency, and pairing-numbers detection. We developed system models of both types of synaptic plasticity and analyzed signal processing in the induction of synaptic plasticity. We found that both systems have upstream subsystems for spike-timing detection and downstream subsystems for pairing-frequency and pairing-numbers detection. The upstream systems used multiplication of signals from the feedback filters and nonlinear functions for spike-timing detection. The downstream subsystems used temporal filters with longer time constants for pairing-frequency detection and nonlinear switch-like functions for pairing-numbers detection, indicating that the downstream subsystems serve as a leaky integrate-and-fire system. Thus, our findings suggest that a common conceptual framework for the induction of synaptic plasticity exists despite the differences in molecular species and pathways.

Gating of long-term depression by Ca2+/calmodulin-dependent protein kinase II through enhanced cGMP signalling in cerebellar Purkinje cells

Long-term depression (LTD) at parallel fibre synapses on a cerebellar Purkinje cell has been regarded as a cellular basis for motor learning. Although Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) has been implicated in the LTD induction as an important Ca(2+)-sensing molecule, the underlying signalling mechanism remains unclear. Here, we attempted to explore the potential signalling pathway underlying the CaMKII involvement in LTD using a systems biology approach, combined with validation by electrophysiological and FRET imaging experiments on a rat cultured Purkinje cell. Model simulation predicted the following cascade as a candidate mechanism for the CaMKII contribution to LTD: CaMKII negatively regulates phosphodiesterase 1 (PDE1), subsequently facilitates the cGMP/protein kinase G (PKG) signalling pathway and down-regulates protein phosphatase 2A (PP-2A), thus supporting the LTD-inducing positive feedback loop consisting of mutual activation of protein kinase C (PKC) and mitogen-activated protein kinase (MAPK). This model suggestion was corroborated by whole-cell patch clamp recording experiments. In addition, FRET measurement of intracellular cGMP concentration revealed that CaMKII activation causes sustained increase of cGMP, supporting the signalling mechanism of LTD induction by CaMKII. Furthermore, we found that activation of the cGMP/PKG pathway by nitric oxide (NO) can support LTD induction without activation of CaMKII. Thus, this study clarified interaction between NO and Ca(2+)/CaMKII, two important factors required for LTD.

TNFα-induced IP3R1 expression through TNFR1/PC-PLC/PKCα and TNFR2 signalling pathways in human mesangial cell

BACKGROUND

Little information is available regarding the mechanisms involved in cytokine-induced type 1 inositol 1,4,5-trisphosphate receptor (IP(3)R1) expression in human mesangial cells (HMCs) in the occurrence of hepatorenal syndrome (HRS). Over-expression of IP(3)R1 would enhance both IP(3)-binding activity and sensitivity. We hypothesize that it is possible that increased IP(3)R1, induced by TNFα, would lead to increased IP(3) sensitivity in response to a variety of vasoconstrictors, and promote HMC contraction and thus lead to reduced GFP, promoting HRS occurrence and development.

METHODS

Quantitative real-time polymerase chain reaction and immunoblot assay were used to examine the effects of TNFα on IP(3)R1 mRNA and protein expression. Several inhibitors of kinases, depletion PKC, over-expression of dominant-negative mutant of PKC and non-radioactive PKC assay were used to examine the mechanism of signal transduction of TNFα-regulated IP(3)R1 in HMCs.

RESULTS

TNFα increased IP(3)R1 mRNA and protein expression in HMCs, an effect that was blocked by prolonged incubated chronic PMA, D609, safingol and also by transfection with domain-negative PKCα construct. TNFα activated and promoted autophosphorylation of the PKCα. In addition, both anti-TNFR1 and anti-TNFR2 antibodies blocked TNFα-induced IP(3)R1 protein expression, while only anti-TNFR1 antibodies but not anti-TNFR2 antibodies attenuated TNFα-induced PKCα activity.

CONCLUSIONS

TNFα increased the expression of IP(3)R1, and this was mediated, at least in part, through the TNFR1/PC-PLC/PKCα and TNFR2 signalling pathways in HMCs.

Synaptic activity-related classical protein kinase C isoform localization in the adult rat neuromuscular synapse

Protein kinase C (PKC) is essential for signal transduction in a variety of cells, including neurons and myocytes, and is involved in both acetylcholine release and muscle fiber contraction. Here, we demonstrate that the increases in synaptic activity by nerve stimulation couple PKC to transmitter release in the rat neuromuscular junction and increase the level of alpha, betaI, and betaII isoforms in the membrane when muscle contraction follows the stimulation. The phosphorylation activity of these classical PKCs also increases. It seems that the muscle has to contract in order to maintain or increase classical PKCs in the membrane. We use immunohistochemistry to show that PKCalpha and PKCbetaI were located in the nerve terminals, whereas PKCalpha and PKCbetaII were located in the postsynaptic and the Schwann cells. Stimulation and contraction do not change these cellular distributions, but our results show that the localization of classical PKC isoforms in the membrane is affected by synaptic activity.

A positive feedback signal transduction loop determines timing of cerebellar long-term depression

Synaptic activity produces short-lived second messengers that ultimately yield a long-term depression (LTD) of cerebellar Purkinje cells. Here, we test the hypothesis that these brief second messenger signals are translated into long-lasting biochemical signals by a positive feedback loop that includes protein kinase C (PKC) and mitogen-activated protein kinase. Histochemical "epistasis" experiments demonstrate the reciprocal activation of these kinases, and physiological experiments--including the use of a light-activated protein kinase--demonstrate that such reciprocal activation is required for LTD. Timed application of enzyme inhibitors reveals that this positive feedback loop causes PKC to be active for more than 20 min, allowing sufficient time for LTD expression. Such regenerative mechanisms may sustain other long-lasting forms of synaptic plasticity and could be a general mechanism for prolonging signal transduction networks.

Cytosolic PLA2(alpha) activation in Purkinje neurons and its role in AMPA-receptor trafficking

Cytosolic phospholipase A(2)alpha (cPLA(2)alpha) selectively releases arachidonic acid from membrane phospholipids and has been proposed to be involved in the induction of long-term depression (LTD), a form of synaptic plasticity in the cerebellum. This enzyme requires two events for its full activation: Ca(2+)-dependent translocation from the cytosol to organelle membranes in order to access phospholipids as substrates, and phosphorylation by several kinases. However, the subcellular distribution and activation of cPLA(2)alpha in Purkinje cells and the role of arachidonic acid in cerebellar LTD have not been fully elucidated. In cultured Purkinje cells, stimulation of AMPA receptors, but not metabotropic glutamate receptors, triggered translocation of cPLA(2)alpha to the somatic and dendritic Golgi compartments. This translocation required Ca(2+) influx through P-type Ca(2+) channels. AMPA plus PMA, a chemical method for inducing LTD, released arachidonic acid via phosphorylation of cPLA(2)alpha. AMPA plus PMA induced a decrease in surface GluR2 for more than 2 hours. Interestingly, this reduction was occluded by a cPLA(2)alpha-specific inhibitor. Furthermore, PMA plus arachidonic acid caused the prolonged internalization of GluR2 without activating AMPA receptors. These results suggest that cPLA(2)alpha regulates the persistent decrease in the expression of AMPA receptors, underscoring the role of cPLA(2)alpha in cerebellar LTD.

Src-family protein tyrosine kinase negatively regulates cerebellar long-term depression

Protein phosphorylation is a major mechanism for the regulation of synaptic transmission. Previous studies have shown that several serine/threonine kinases are involved in the induction of long-term depression (LTD) at excitatory synapses on a Purkinje neuron (PN) in the cerebellum. Here, we show that Src-family protein tyrosine kinases (SFKs) are involved in the regulation of the LTD induction. Intracellular application of c-Src suppressed LTD. We also show that application of a SFK-selective inhibitor PP2 recovered LTD from the suppression caused by the inhibition of mGluR1 activity. These results indicate that SFKs negatively regulate the LTD induction at excitatory synapses on a cerebellar PN.

mGluR1-mediated facilitation of long-term potentiation at inhibitory synapses on a cerebellar Purkinje neuron

Synaptic plasticity has been studied extensively at excitatory synapses, whereas studies on plasticity at GABAergic inhibitory synapses have been limited. In the rat cerebellar cortex, postsynaptic depolarization of a Purkinje neuron (PN) induces long-term potentiation of GABA(A) receptor (GABA(A)R) responsiveness (termed rebound potentiation; RP). Induction of RP requires an increase in intracellular Ca(2+) concentration and resultant activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). We previously reported that GABA(B) receptor (GABA(B)R) activation coupled with depolarization suppresses RP induction by facilitating protein phosphatase 1 (PP-1)-mediated inhibition of CaMKII through down-regulation of cAMP-dependent protein kinase A (PKA) activity. Here, we examined the involvement of metabotropic glutamate receptor type 1 (mGluR1) in RP regulation. RP was monitored with the amplitudes of either the current responses to GABA or miniature inhibitory postsynaptic currents recorded from a PN in a primary culture or in a cerebellar slice. Inhibition of mGluR1 by an antagonist, 7(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate-ethyl-ester (CPCCOEt), prevented RP induction, which was abolished either by activation of adenylyl cyclase or by inhibition of PP-1. Furthermore, mGluR1 inhibition impaired depolarization-induced CaMKII activation. By contrast, activation of mGluR1 by the agonist (R,S)3,5-dihydroxyphenylglycine (DHPG) rescued RP induction from its suppression by GABA(B)R activation. The rescue was impaired either by inhibition of PKA or by facilitation of PP-1 activity. In addition, mGluR1 activation counteracted the GABA(B)R-mediated CaMKII inhibition. Taken together, these results suggest that mGluR1 activity counteracts GABA(B)R activity and contributes to RP induction through PKA activation, down-regulation of PP-1 and up-regulation of CaMKII.