Presynaptic kainate receptors are localized close to release sites in rat hippocampal synapses

A comparative analysis of kainate receptor GluK2 and GluK5 knockout mice in a pure genetic background

Kainate receptors (KARs) are members of the glutamate receptor family that regulate synaptic function in the brain. Although they are known to be associated with psychiatric disorders, how they are involved in these disorders remains unclear. KARs are tetrameric channels assembled from a combination of GluK1-5 subunits. Among these, GluK2 and GluK5 subunits are the major heteromeric subunits in the brain. To determine the functional similarities and differences between GluK2 and GluK5 subunits, we generated GluK2 KO and GluK5 KO mice on a C57BL/6N background, a well-characterized inbred strain, and compared their behavioral phenotypes. We found that GluK2 KO and GluK5 KO mice exhibited the same phenotypes in many tests, such as reduced locomotor activity, impaired motor function, and enhanced depressive-like behavior. No change was observed in motor learning, anxiety-like behavior, or sociability. Additionally, we identified subunit-specific phenotypes, such as reduced motivation toward their environment in GluK2 KO mice and an enhancement in the contextual memory in GluK5 KO mice. These results revealed that GluK2 and GluK5 subunits not only function in a coordinated manner but also have a subunit-specific role in regulating behavior. To summarize, we demonstrated subunit-specific and common behavioral effects of GluK2 and GluK5 subunits for the first time. Moreover, to the best of our knowledge, this is the first evidence of the involvement of the GluK5 subunit in the expression of depressive-like behavior and contextual memory, which strongly indicates its role in psychiatric disorders.

Presynaptic c-Jun N-terminal Kinase 2 regulates NMDA receptor-dependent glutamate release

Activation of c-Jun N-terminal kinase (JNK) signaling pathway is a critical step for neuronal death occurring in several neurological conditions. JNKs can be activated via receptor tyrosine kinases, cytokine receptors, G-protein coupled receptors and ligand-gated ion channels, including the NMDA glutamate receptors. While JNK has been generally associated with postsynaptic NMDA receptors, its presynaptic role remains largely unexplored. Here, by means of biochemical, morphological and functional approaches, we demonstrate that JNK and its scaffold protein JIP1 are also expressed at the presynaptic level and that the NMDA-evoked glutamate release is controlled by presynaptic JNK-JIP1 interaction. Moreover, using knockout mice for single JNK isoforms, we proved that JNK2 is the essential isoform in mediating this presynaptic event. Overall the present findings unveil a novel JNK2 localization and function, which is likely to play a role in different physiological and pathological conditions.

Presynaptic NMDA receptors - dynamics and distribution in developing axons in vitro and in vivo

During cortical development, N-methyl-D-aspartate (NMDA) receptors (NMDARs) facilitate presynaptic terminal formation, enhance neurotransmitter release and are required in presynaptic neurons for spike-timing-dependent long-term depression (tLTD). However, the extent to which NMDARs are found within cortical presynaptic terminals has remained controversial, and the sub-synaptic localization and dynamics of axonal NMDARs are unknown. Here, using live confocal imaging and biochemical purification of presynaptic membranes, we provide strong evidence that NMDARs localize to presynaptic terminals in vitro and in vivo in a developmentally regulated manner. The NR1 and NR2B subunits (also known as GRIN1 and GRIN2B, respectively) were found within the active zone membrane, where they could respond to synaptic glutamate release. Surprisingly, NR1 also appeared in glutamatergic and GABAergic synaptic vesicles. During synaptogenesis, NR1 was mobile throughout axons - including growth cones and filopodia, structures that are involved in synaptogenesis. Upon synaptogenic contact, NMDA receptors were quickly recruited to terminals by neuroligin-1 signaling. Unlike dendrites, the trafficking and distribution of axonal NR1 were insensitive to activity changes, including NMDA exposure, local glutamate uncaging or action potential blockade. These results support the idea that presynaptic NMDARs play an early role in presynaptic development.

Neuropeptide Y inhibits interleukin-1β-induced phagocytosis by microglial cells

Background

Neuropeptide Y (NPY) is emerging as a modulator of communication between the brain and the immune system. However, in spite of increasing evidence that supports a role for NPY in the modulation of microglial cell responses to inflammatory conditions, there is no consistent information regarding the action of NPY on microglial phagocytic activity, a vital component of the inflammatory response in brain injury. Taking this into consideration, we sought to assess a potential new role for NPY as a modulator of phagocytosis by microglial cells.

Methods

The N9 murine microglial cell line was used to evaluate the role of NPY in phagocytosis. For that purpose, an IgG-opsonized latex bead assay was performed in the presence of lipopolysaccharide (LPS) and an interleukin-1β (IL-1β) challenge, and upon NPY treatment. A pharmacological approach using NPY receptor agonists and antagonists followed to uncover which NPY receptor was involved. Moreover, western blotting and immunocytochemical studies were performed to evaluate expression of p38 mitogen-activated protein kinase (MAPK) and heat shock protein 27 (HSP27), in an inflammatory context, upon NPY treatment.

Results

Here, we show that NPY inhibits phagocytosis of opsonized latex beads and inhibits actin cytoskeleton reorganization triggered by LPS stimulation. Co-stimulation of microglia with LPS and adenosine triphosphate also resulted in increased phagocytosis, an effect inhibited by an interleukin-1 receptor antagonist, suggesting involvement of IL-1β signaling. Furthermore, direct application of LPS or IL-1β activated downstream signaling molecules, including p38 MAPK and HSP27, and these effects were inhibited by NPY. Moreover, we also observed that the inhibitory effect of NPY on phagocytosis was mediated via Y₁ receptor activation.

Conclusions

Altogether, we have identified a novel role for NPY in the regulation of microglial phagocytic properties, in an inflammatory context.

Controlling the neuronal differentiation of stem cells by the intracellular delivery of retinoic acid-loaded nanoparticles

The manipulation of endogenous stem cell populations from the subventricular zone (SVZ), a neurogenic niche, creates an opportunity to induce neurogenesis and influence brain regenerative capacities in the adult brain. Herein, we demonstrate the ability of polyelectrolyte nanoparticles to induce neurogenesis exclusively after being internalized by SVZ stem cells. The nanoparticles are not cytotoxic for concentrations equal or below 10 μg/mL. The internalization process is rapid, and nanoparticles escape endosomal fate in a few hours. Retinoic acid-loaded nanoparticles increase the number of neuronal nuclear protein (NeuN)-positive neurons and functional neurons responding to depolarization with KCl and expressing NMDA receptor subunit type 1 (NR1). These nanoparticles offer an opportunity for in vivo delivery of proneurogenic factors and neurodegenerative disease treatment.

Neuropeptide Y modulation of interleukin-1{beta} (IL-1{beta})-induced nitric oxide production in microglia

Given the modulatory role of neuropeptide Y (NPY) in the immune system, we investigated the effect of NPY on the production of NO and IL-1β in microglia. Upon LPS stimulation, NPY treatment inhibited NO production as well as the expression of inducible nitric-oxide synthase (iNOS). Pharmacological studies with a selective Y(1) receptor agonist and selective antagonists for Y(1), Y(2), and Y(5) receptors demonstrated that inhibition of NO production and iNOS expression was mediated exclusively through Y(1) receptor activation. Microglial cells stimulated with LPS and ATP responded with a massive release of IL-1β, as measured by ELISA. NPY inhibited this effect, suggesting that it can strongly impair the release of IL-1β. Furthermore, we observed that IL-1β stimulation induced NO production and that the use of a selective IL-1 receptor antagonist prevented NO production upon LPS stimulation. Moreover, NPY acting through Y(1) receptor inhibited LPS-stimulated release of IL-1β, inhibiting NO synthesis. IL-1β activation of NF-κB was inhibited by NPY treatment, as observed by confocal microscopy and Western blotting analysis of nuclear translocation of NF-κB p65 subunit, leading to the decrease of NO synthesis. Our results showed that upon LPS challenge, microglial cells release IL-1β, promoting the production of NO through a NF-κB-dependent pathway. Also, NPY was able to strongly inhibit NO synthesis through Y(1) receptor activation, which prevents IL-1β release and thus inhibits nuclear translocation of NF-κB. The role of NPY in key inflammatory events may contribute to unravel novel gateways to modulate inflammation associated with brain pathology.

GluR5-mediated glutamate signaling regulates hypothalamo-pituitary-adrenocortical stress responses at the paraventricular nucleus and median eminence

Glutamate is the main excitatory neurotransmitter in the central nervous system, and plays an excitatory role in generation of hypothalamic-pituitary-adrenocortical (HPA) axis responses to stress. The current study assesses the role of kainate-preferring receptors in glutamatergic excitation of the HPA axis. In situ hybridization and immunohistochemical analyses confirmed the existence of the GluR5 kainate subunit in the paraventricular nucleus of the hypothalamus (PVN). Importantly, GluR5 immunoreactivity was enriched in the external lamina of the median eminence, where it is co-localized with corticotropin releasing hormone (CRH). Intra-PVN infusion of LY382884 increased plasma adrenocorticotropin (ACTH), corticosterone and PVN c-Fos immunoreactivity. Infusions of LY382884 into the median eminence region, on the other hand, reduced restraint induced ACTH release without altering c-Fos expression. Together, these findings provide evidence for glutamate-mediated signaling in control of CRH release at the PVN and median eminence, mediated by way of kainate-preferring receptor complexes.

Inhibition of presynaptic release-facilitatory kainate autoreceptors by extracellular cyclic GMP

We found that both alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainate autoreceptors were present on the glutamate-releasing terminals of cerebellar parallel/climbing fibers and that they functioned as facilitatory autoreceptors. Extracellular cGMP inhibited the neurotransmitter release evoked by presynaptic kainate receptor activation; the inhibitory effect of extracellular cGMP was selective for the kainate autoreceptor-mediated response and did not affect the AMPA autoreceptor-mediated response. Endogenously synthesized cGMP might be the physiological source for the extracellular cGMP modulating the response to kainate autoreceptor activation.

Presynaptic glutamate receptors: physiological functions and mechanisms of action

Glutamate acts on postsynaptic glutamate receptors to mediate excitatory communication between neurons. The discovery that additional presynaptic glutamate receptors can modulate neurotransmitter release has added complexity to the way we view glutamatergic synaptic transmission. Here we review evidence of a physiological role for presynaptic glutamate receptors in neurotransmitter release. We compare the physiological roles of ionotropic and metabotropic glutamate receptors in short- and long-term regulation of synaptic transmission. Furthermore, we discuss the physiological conditions that are necessary for their activation, the source of the glutamate that activates them, their mechanisms of action and their involvement in higher brain function.

Pre-synaptic and post-synaptic localization of EphA4 and EphB2 in adult mouse forebrain

The ephrin receptors EphA4 and EphB2 have been implicated in synaptogenesis and long-term potentiation in the cerebral cortex and hippocampus, where they are generally viewed as post-synaptic receptors. To determine the precise distribution of EphA4 and EphB2 in mature brain synapses, we used subcellular fractionation and electron microscopy to examine the adult mouse forebrain/midbrain. EphA4 and EphB2 were both enriched in microsomes and synaptosomes. In synaptosomes, they were present in the membrane and the synaptic vesicle fractions. While EphA4 was tightly associated with PSD-95-enriched post-synaptic density fractions, EphB2 was easily extracted with detergents. In contrast, both receptors were found in the pre-synaptic active zone fraction. By electron microscopy, EphA4 was mainly detected in axon terminals, whereas EphB2 was more frequently detected in large dendritic shafts, in the hippocampus and cerebral cortex. However, in the ventrobasal thalamus, EphB2 was detected most frequently in axon terminals and thin dendritic shafts. The localization of EphA4 and EphB2 in multiple compartments of neurons and synaptic junctions suggests that they interact with several distinct scaffolding proteins and play diverse roles at synapses.

Molecular determinants of kainate receptor trafficking

Glutamate receptors of the kainate subtype are ionotropic receptors that play a key role in the modulation of neuronal network activity. The role of kainate receptors depends on their precise membrane and subcellular localization in presynaptic, extrasynaptic and postsynaptic domains. These receptors are composed of the combination of five subunits, three of them having several splice variants. The subunits and splice variants show great divergence in their C-terminal cytoplasmic tail domains, which have been implicated in intracellular trafficking of homomeric and heteromeric receptors. Differential trafficking of kainate receptors to specific neuronal compartments likely relies on interactions between the different kainate receptor subunits with distinct subsets of protein partners that interact with C-terminal domains. These C-terminal domains have also been implicated in the degradation of kainate receptors. Finally, the phosphorylation of the C-terminal domain regulates receptor trafficking and function. This review summarizes our knowledge on the regulation of membrane delivery and trafficking of kainate receptors implicating C-terminal domains of the different isoforms and focuses on the identification and characterization of the function of interacting partners.

Kainate modulates presynaptic GABA release from two vesicle pools

Inhibitory control of local neuronal circuits is critical for prefrontal cortical functioning. Modulation of inhibitory circuits by several neuromodulators has been demonstrated, but the underlying mechanisms are unclear. Neuromodulator effects on synaptic vesicle recycling have received little attention. Controversy also exists whether different pools of synaptic vesicles underlie spontaneous and activity-dependent vesicle recycling. We therefore investigated the effects of kainate receptor activation on GABA release in rat prefrontal neocortex using electrophysiological and styryl dye imaging techniques in acute neocortical slices. Electrophysiological studies demonstrated that activation of kainate receptors increased the frequency, but not the amplitude of miniature IPSCs, suggesting a presynaptic action. Using styryl dye staining and multiphoton excitation microscopy, we visualized vesicular release from inhibitory GABAergic terminals in prefrontal cortical slices and demonstrate that kainate facilitates GABA release from presynaptic terminals. Our findings also indicate the presence of two pools of GABA-containing vesicles within inhibitory terminals. Kainate modulates both pools but only when vesicles are endocytosed and exocytosed by matching protocols of dye loading, i.e., spontaneous or evoked afferent activity.

Protein kinase C activity blocks neuropeptide Y-mediated inhibition of glutamate release and contributes to excitability of the hippocampus in status epilepticus

The unbalanced excitatory/inhibitory neurotransmitter function in the neuronal network afflicted by seizures is the main biochemical and biophysical hallmark of epilepsy. The aim of this work was to identify changes in the signaling mechanisms associated with neuropeptide Y (NPY)-mediated inhibition of glutamate release that may contribute to hyperexcitability. Using isolated rat hippocampal nerve terminals, we showed that the KCl-evoked glutamate release is inhibited by NPY Y2 receptor activation and is potentiated by the stimulation of protein kinase C (PKC). Moreover, we observed that immediately after status epilepticus (6 h postinjection with kainate, 10 mg/kg), the functional inhibition of glutamate release by NPY Y2 receptors was transiently blocked concomitantly with PKC hyperactivation. The pharmacological blockade of seizure-activated PKC revealed again the Y2 receptor-mediated inhibition of glutamate release. The functional activity of PKC immediately after status epilepticus was assessed by evaluating phosphorylation of the AMPA receptor subunit GluR1 (Ser-831), a substrate for PKC. Moreover, NPY-stimulated [35S]GTPgammaS autoradiographic binding studies indicated that the common target for Y2 receptor and PKC on the inhibition/potentiation of glutamate release was located downstream of the Y2 receptor, or its interacting G-protein, and involves voltage-gated calcium channels.

Ultrastructural localisation and differential agonist-induced regulation of AMPA and kainate receptors present at the presynaptic active zone and postsynaptic density

Activity-dependent changes in ionotropic glutamate receptors at the postsynaptic membrane are well established and this regulation plays a central role in the expression of synaptic plasticity. However, very little is known about the distributions and regulation of ionotropic receptors at presynaptic sites. To determine if presynaptic receptors are subject to similar regulatory processes we investigated the localisation and modulation of AMPA (GluR1, GluR2, GluR3) and kainate (GluR6/7, KA2) receptor subunits by ultrasynaptic separation and immunoblot analysis of rat brain synaptosomes. All of the subunits were enriched in the postsynaptic fraction but were also present in the presynaptic and non-synaptic synaptosome fractions. AMPA stimulation resulted in a marked decrease in postsynaptic GluR2 and GluR3 subunits, but an increase in GluR6/7. Conversely, GluR2 and GluR3 increased in the presynaptic fraction whereas GluR6/7 decreased. There were no significant changes in any of the compartments for GluR1. NMDA treatment decreased postsynaptic GluR1, GluR2 and GluR6/7 but increased presynaptic levels of these subunits. NMDA treatment did not evoke changes in GluR3 localisation. Our results demonstrate that presynaptic and postsynaptic subunits are regulated in opposite directions by AMPA and NMDA stimulation.

Kainate receptors

Kainate receptors form a family of ionotropic glutamate receptors that appear to play a special role in the regulation of the activity of synaptic networks. This review first describes briefly the molecular and pharmacological properties of native and recombinant kainate receptors. It then attempts to outline the general principles that appear to govern the function of kainate receptors in the activity of synaptic networks under physiological conditions. It subsequently describes the way that kainate receptors are involved in synaptic integration, synaptic plasticity, the regulation of neurotransmitter release and the control of neuronal excitability, and the manner in which they might play an important role in synaptogenesis and synaptic maturation. These functions require the proper subcellular localization of kainate receptors in specific functional domains of the neuron, necessitating complex cellular and molecular trafficking events. We show that our comprehension of these mechanisms is just starting to emerge. Finally, this review presents evidence that implicates kainate receptors in pathophysiological conditions such as epilepsy, excitotoxicity and pain, and that shows that these receptors represent promising therapeutic targets.