Enhancement of both long-term depression induction and optokinetic response adaptation in mice lacking delphilin

The endocytic adaptor AP-2 maintains Purkinje cell function by balancing cerebellar parallel and climbing fiber synapses

The loss of cerebellar Purkinje cells is a hallmark of neurodegenerative movement disorders, but the mechanisms remain enigmatic. We show that endocytic adaptor protein complex 2 (AP-2) is crucial for Purkinje cell survival. Using mouse genetics, viral tracing, calcium imaging, and kinematic analysis, we demonstrate that loss of the AP-2 μ subunit in Purkinje cells leads to early-onset ataxia and progressive degeneration. Synaptic dysfunction, marked by an overrepresentation of parallel fibers (PFs) over climbing fibers (CFs), precedes Purkinje cell loss. Mechanistically, AP-2 interacts with the PF-enriched protein GRID2IP, and its loss triggers GRID2IP degradation and glutamate δ2 receptor (GLURδ2) accumulation, leading to an excess of PFs while CFs are reduced. The overrepresentation of PFs increases Purkinje cell network activity, which is mitigated by enhancing glutamate clearance with ceftriaxone. These findings highlight the role of AP-2 in regulating GRID2IP levels in Purkinje cells to maintain PF-CF synaptic balance and prevent motor dysfunction.

Systemic pharmacological suppression of neural activity reverses learning impairment in a mouse model of Fragile X syndrome

The enhancement of associative synaptic plasticity often results in impaired rather than enhanced learning. Previously, we proposed that such learning impairments can result from saturation of the plasticity mechanism (Nguyen-Vu et al., 2017), or, more generally, from a history-dependent change in the threshold for plasticity. This hypothesis was based on experimental results from mice lacking two class I major histocompatibility molecules, MHCI H2-Kb and H2-Db (MHCI KbDb-/-), which have enhanced associative long-term depression at the parallel fiber-Purkinje cell synapses in the cerebellum (PF-Purkinje cell LTD). Here, we extend this work by testing predictions of the threshold metaplasticity hypothesis in a second mouse line with enhanced PF-Purkinje cell LTD, the Fmr1 knockout mouse model of Fragile X syndrome (FXS). Mice lacking Fmr1 gene expression in cerebellar Purkinje cells (L7-Fmr1 KO) were selectively impaired on two oculomotor learning tasks in which PF-Purkinje cell LTD has been implicated, with no impairment on LTD-independent oculomotor learning tasks. Consistent with the threshold metaplasticity hypothesis, behavioral pre-training designed to reverse LTD at the PF-Purkinje cell synapses eliminated the oculomotor learning deficit in the L7-Fmr1 KO mice, as previously reported in MHCI KbDb-/-mice. In addition, diazepam treatment to suppress neural activity and thereby limit the induction of associative LTD during the pre-training period also eliminated the learning deficits in L7-Fmr1 KO mice. These results support the hypothesis that cerebellar LTD-dependent learning is governed by an experience-dependent sliding threshold for plasticity. An increased threshold for LTD in response to elevated neural activity would tend to oppose firing rate stability, but could serve to stabilize synaptic weights and recently acquired memories. The metaplasticity perspective could inform the development of new clinical approaches for addressing learning impairments in autism and other disorders of the nervous system.

Artificial cerebellum on FPGA: realistic real-time cerebellar spiking neural network model capable of real-world adaptive motor control

The cerebellum plays a central role in motor control and learning. Its neuronal network architecture, firing characteristics of component neurons, and learning rules at their synapses have been well understood in terms of anatomy and physiology. A realistic artificial cerebellum with mimetic network architecture and synaptic plasticity mechanisms may allow us to analyze cerebellar information processing in the real world by applying it to adaptive control of actual machines. Several artificial cerebellums have previously been constructed, but they require high-performance hardware to run in real-time for real-world machine control. Presently, we implemented an artificial cerebellum with the size of 104 spiking neuron models on a field-programmable gate array (FPGA) which is compact, lightweight, portable, and low-power-consumption. In the implementation three novel techniques are employed: (1) 16-bit fixed-point operation and randomized rounding, (2) fully connected spike information transmission, and (3) alternative memory that uses pseudo-random number generators. We demonstrate that the FPGA artificial cerebellum runs in real-time, and its component neuron models behave as those in the corresponding artificial cerebellum configured on a personal computer in Python. We applied the FPGA artificial cerebellum to the adaptive control of a machine in the real world and demonstrated that the artificial cerebellum is capable of adaptively reducing control error after sudden load changes. This is the first implementation and demonstration of a spiking artificial cerebellum on an FPGA applicable to real-world adaptive control. The FPGA artificial cerebellum may provide neuroscientific insights into cerebellar information processing in adaptive motor control and may be applied to various neuro-devices to augment and extend human motor control capabilities.

Proteomic profiling reveals neuronal ion channel dysregulation and cellular responses to DNA damage-induced cell cycle arrest and senescence in human neuroblastoma SH-SY5Y cells exposed to cypermethrin

Cypermethrin (CYP), a synthetic pyrethroid of class II, is widely used as a pesticide worldwide. The primary target of cypermethrin is a voltage-gated sodium channel. The neurotoxicity of CYP has been extensively studied in terms of affecting neuronal development, increasing cellular oxidative stress, and apoptosis. However, little is known about how it affects the expression of channel proteins involved in synaptic transmission, as well as the effects of cypermethrin on DNA damage and cell cycle processes. We found that the ligand and voltage-gated calcium channels and proteins involved in synaptic transmission including NMDA 1 receptor subunit, alpha 1A-voltage-dependent calcium channel, synaptotagmin-17, and synaptojanin-2 were downregulated in CYP-treated cells. After 48h of CYP exposure, cell viability was reduced with flattened and enlarged morphology. The levels of 23 proteins regulating cell cycle processes were altered in CYP-treated cells, according to a proteomic study. The cell cycle analysis showed elevated G₀/G₁ cell cycle arrest and DNA fragmentation at the sub-G₀ stage after CYP exposure. CYP treatment also increased senescence-associated β-galactosidase positive cells, DNA damage, and apoptotic markers. Taken together, the current study showed that cypermethrin exposure caused DNA damage and hastened cellular senescence and apoptosis via disrupting cell cycle regulation. In addition, despite its primary target sodium channel, CYP might cause synaptic dysfunction via the downregulation of synaptic proteins and dysregulation of synapse-associated ion channels.

Immune-Triggered Forms of Plasticity Across Brain Regions

Immune cells play numerous roles in the host defense against the invasion of microorganisms and pathogens, which induces the release of inflammatory mediators (e.g., cytokines and chemokines). In the CNS, microglia is the major resident immune cell. Recent efforts have revealed the diversity of the cell types and the heterogeneity of their functions. The refinement of the synapse structure was a hallmark feature of the microglia, while they are also involved in the myelination and capillary dynamics. Another promising feature is the modulation of the synaptic transmission as synaptic plasticity and the intrinsic excitability of neurons as non-synaptic plasticity. Those modulations of physiological properties of neurons are considered induced by both transient and chronic exposures to inflammatory mediators, which cause behavioral disorders seen in mental illness. It is plausible for astrocytes and pericytes other than microglia and macrophage to induce the immune-triggered plasticity of neurons. However, current understanding has yet achieved to unveil what inflammatory mediators from what immune cells or glia induce a form of plasticity modulating pre-, post-synaptic functions and intrinsic excitability of neurons. It is still unclear what ion channels and intracellular signaling of what types of neurons in which brain regions of the CNS are involved. In this review, we introduce the ubiquitous modulation of the synaptic efficacy and the intrinsic excitability across the brain by immune cells and related inflammatory cytokines with the mechanism for induction. Specifically, we compare neuro-modulation mechanisms by microglia of the intrinsic excitability of cerebellar Purkinje neurons with cerebral pyramidal neurons, stressing the inverted directionality of the plasticity. We also discuss the suppression and augmentation of the extent of plasticity by inflammatory mediators, as the meta-plasticity by immunity. Lastly, we sum up forms of immune-triggered plasticity in the different brain regions with disease relevance. Together, brain immunity influences our cognition, sense, memory, and behavior via immune-triggered plasticity.

Structural biology and thermodynamics of GluD receptors

Glutamate delta (GluD) receptors are a functionally enigmatic subfamily of ionotropic glutamate receptors. Despite sharing similar sequences and structures with AMPA, NMDA, and kainate receptors, GluD receptors do not bind glutamate nor function as ligand-gated ion channels. Binding d-serine and engaging in transsynaptic protein-protein interactions, GluD receptors are thought to undergo complex conformational rearrangements for non-ionotropic signaling that regulates synaptic plasticity. Recent structural, biochemical, and computational studies have elucidated multiple conformational and thermodynamic factors governing the unique properties of GluD receptors. Here, we review advances in biophysical insights into GluD receptors and discuss the structural thermodynamic relationships that underpin their neurobiological functions.

Proteomics Analysis of CA1 Region of the Hippocampus in Pre-, Progression and Pathological Stages in a Mouse Model of the Alzheimer's Disease

BACKGROUND

CA1 subregion of the hippocampal formation is one of the primarily affected structures in AD, yet not much is known about proteome alterations in the extracellular milieu of this region.

OBJECTIVE

In this study, we aimed to identify the protein expression alterations throughout the pre-pathological, progression and pathological stages of AD mouse model.

METHODS

The CA1 region perfusates were collected by in-vivo intracerebral push-pull perfusion from transgenic 5XFAD mice and their non-transgenic littermates at 3, 6 and 12 wereβmonths of age. Morris water maze test and immunohistochemistry staining of A performed to determine the stages of the disease in this mouse model. The protein expression differences were analyzed by label-free shotgun proteomics analysis.

RESULTS

A total of 251, 213 and 238 proteins were identified in samples obtained from CA1 regions of mice at 3, 6 and 12 months of age, respectively. Of these, 68, 41 and 33 proteins showed statistical significance. Pathway analysis based on the unique and common proteins within the groups revealed that several pathways are dysregulated during different stages of AD. The alterations in glucose and lipid metabolisms respectively in pre-pathologic and progression stages of the disease, lead to imbalances in ROS production via diminished SOD level and impairment of neuronal integrity.

CONCLUSION

We conclude that CA1 region-specific proteomic analysis of hippocampal degeneration may be useful in identifying the earliest as well as progressional changes that are associated with Alzheimer's disease.

Norepinephrine Facilitates Induction of Long-term Depression through β-Adrenergic Receptor at Parallel Fiber-to-Purkinje Cell Synapses in the Flocculus

The cerebellum is involved in motor learning, and long-term depression (LTD) at parallel fiber-to-Purkinje cell (PF-PC) synapses has been considered to be a primary cellular mechanism for motor learning. In addition, the contribution of norepinephrine (NE) to cerebellum-dependent learning paradigms has been reported. Thus, the roles of LTD and of NE in motor learning have been studied separately, and the relationship between the effects of NE and LTD remains unclear. Here, we examined effects of β-adrenergic receptor (β-AR) activity on the synaptic transmission and LTD at PF-PC synapses in the cerebellar flocculus. The flocculus regulates adaptation of oculomotor reflexes, and we previously reported the involvement of both LTD and β-AR in adaptation of an oculomotor reflex. Here we found that specific agonists for β-AR or NE did not directly change synaptic transmission, but lowered the threshold for LTD induction at PF-PC synapses in the flocculus. In addition, protein kinase A (PKA), which is activated downstream of β-AR, facilitated the LTD induction. Altogether, these results suggest that NE facilitates LTD induction at PF-PC synapses in the flocculus by activating PKA through β-AR.

GluD1 knockout mice with a pure C57BL/6N background show impaired fear memory, social interaction, and enhanced depressive-like behavior

The GluD1 gene is associated with susceptibility for schizophrenia, autism, depression, and bipolar disorder. However, the function of GluD1 and how it is involved in these conditions remain elusive. In this study, we generated a Grid1 gene-knockout (GluD1-KO) mouse line with a pure C57BL/6N genetic background and performed several behavioral analyses. Compared to a control group, GluD1-KO mice showed no significant anxiety-related behavioral differences, evaluated using behavior in an open field, elevated plus maze, a light-dark transition test, the resident-intruder test of aggression and sensorimotor gating evaluated by the prepulse inhibition test. However, GluD1-KO mice showed (1) higher locomotor activity in the open field, (2) decreased sociability and social novelty preference in the three-chambered social interaction test, (3) impaired memory in contextual, but not cued fear conditioning tests, and (4) enhanced depressive-like behavior in a forced swim test. Pharmacological studies revealed that enhanced depressive-like behavior in GluD1-KO mice was restored by the serotonin reuptake inhibitors imipramine and fluoxetine, but not the norepinephrine transporter inhibitor desipramine. In addition, biochemical analysis revealed no significant difference in protein expression levels, such as other glutamate receptors in the synaptosome and postsynaptic densities prepared from the frontal cortex and the hippocampus. These results suggest that GluD1 plays critical roles in fear memory, sociability, and depressive-like behavior.

The Concept of Transmission Coefficient Among Different Cerebellar Layers: A Computational Tool for Analyzing Motor Learning

High-fidelity regulation of information transmission among cerebellar layers is mainly provided by synaptic plasticity. Therefore, determining the regulatory foundations of synaptic plasticity in the cerebellum and translating them to behavioral output are of great importance. To date, many experimental studies have been carried out in order to clarify the effect of synaptic defects, while targeting a specific signaling pathway in the cerebellar function. However, the contradictory results of these studies at the behavioral level further add to the ambiguity of the problem. Information transmission through firing rate changes in populations of interconnected neurons is one of the most widely accepted principles of neural coding. In this study, while considering the efficacy of synaptic interactions among the cerebellar layers, we propose a firing rate model to realize the concept of transmission coefficient. Thereafter, using a computational approach, we test the effect of different values of transmission coefficient on the gain adaptation of a cerebellar-dependent motor learning task. In conformity with the behavioral data, the proposed model can accurately predict that disruption in different forms of synaptic plasticity does not have the same effect on motor learning. Specifically, impairment in training mechanisms, like in the train-induced LTD in parallel fiber-Purkinje cell synapses, has a significant negative impact on all aspects of learning, including memory formation, transfer, and consolidation, although it does not disrupt basic motor performance. In this regard, the overinduction of parallel fiber-molecular layer interneuron LTP could not prevent motor learning impairment, despite its vital role in preserving the robustness of basic motor performance. In contrast, impairment in plasticity induced by interneurons and background activity of climbing fibers is partly compensable through overinduction of train-induced parallel fiber-Purkinje cell LTD. Additionally, blockade of climbing fiber signaling to the cerebellar cortex, referred to as olivary system lesion, shows the most destructive effect on both motor learning and basic motor performance. Overall, the obtained results from the proposed computational framework are used to provide a map from procedural motor memory formation in the cerebellum. Certainly, the generalization of this concept to other multi-layered networks of the brain requires more physiological and computational researches.

Occurrence of long-term depression in the cerebellar flocculus during adaptation of optokinetic response

Long-term depression (LTD) at parallel fiber (PF) to Purkinje cell (PC) synapses has been considered as a main cellular mechanism for motor learning. However, the necessity of LTD for motor learning was challenged by demonstration of normal motor learning in the LTD-defective animals. Here, we addressed possible involvement of LTD in motor learning by examining whether LTD occurs during motor learning in the wild-type mice. As a model of motor learning, adaptation of optokinetic response (OKR) was used. OKR is a type of reflex eye movement to suppress blur of visual image during animal motion. OKR shows adaptive change during continuous optokinetic stimulation, which is regulated by the cerebellar flocculus. After OKR adaptation, amplitudes of quantal excitatory postsynaptic currents at PF-PC synapses were decreased, and induction of LTD was suppressed in the flocculus. These results suggest that LTD occurs at PF-PC synapses during OKR adaptation.

The neuron-specific formin Delphilin nucleates nonmuscle actin but does not enhance elongation

The formin Delphilin binds the glutamate receptor, GluRδ2, in dendritic spines of Purkinje cells. Both proteins play a role in learning. To understand how Delphilin functions in neurons, we studied the actin assembly properties of this formin. Formins have a conserved formin homology 2 domain, which nucleates and associates with the fast-growing end of actin filaments, influencing filament growth together with the formin homology 1 (FH1) domain. The strength of nucleation and elongation varies widely across formins. Additionally, most formins have conserved domains that regulate actin assembly through an intramolecular interaction. Delphilin is distinct from other formins in several ways: its expression is limited to Purkinje cells, it lacks classical autoinhibitory domains, and its FH1 domain has minimal proline-rich sequence. We found that Delphilin is an actin nucleator that does not accelerate elongation, although it binds to the barbed end of filaments. In addition, Delphilin exhibits a preference for actin isoforms, nucleating nonmuscle actin but not muscle actin, which has not been described or systematically studied in other formins. Finally, Delphilin is the first formin studied that is not regulated by intramolecular interactions. We speculate how the activity we observe is consistent with its localization in the small dendritic spines.

Differential regulations of vestibulo-ocular reflex and optokinetic response by β- and α2-adrenergic receptors in the cerebellar flocculus

Norepinephrine modulates synaptic plasticity in various brain regions and is implicated in memory formation, consolidation and retrieval. The cerebellum is involved in motor learning, and adaptations of the vestibulo-ocular reflex (VOR) and optokinetic response (OKR) have been studied as models of cerebellum-dependent motor learning. Previous studies showed the involvement of adrenergic systems in the regulation of VOR, OKR and cerebellar synaptic functions. Here, we show differential contributions of β- and α2-adrenergic receptors in the mouse cerebellar flocculus to VOR and OKR control. Effects of application of β- or α2-adrenergic agonist or antagonist into the flocculus suggest that the β-adrenergic receptor activity maintains the VOR gain at high levels and contributes to adaptation of OKR, and that α2-adrenergic receptor counteracts the β-receptor activity in VOR and OKR control. We also examined effects of norepinephrine application, and the results suggest that norepinephrine regulates VOR and OKR through β-adrenergic receptor at low concentrations and through α2-receptor at high concentrations.

Non diaphanous formin delphilin acts as a barbed end capping protein

Formins are multi domain proteins present ubiquitously in all eukaryotes from lower fungi to higher vertebrates. Formins are characterized by the presence of formin homology domain-2 (FH2) and formin homology domain-1 (FH1). There are fifteen different formins present in mouse and human. Among these metazoan formins, Delphilin is a unique formin having two PDZ domains at the N-terminus and FH1, FH2 domain at the C-terminus respectively. In this study we observed that Delphilin binds to actin filaments, and Delphilin inhibits actin filament elongation like barbed end capping protein CapZ. In vitro, Delphilin stabilized actin filaments by inhibiting actin filament depolymerisation. Therefore, our study demonstrates Delphilin as an actin-filament capping protein.

A saturation hypothesis to explain both enhanced and impaired learning with enhanced plasticity

Across many studies, animals with enhanced synaptic plasticity exhibit either enhanced or impaired learning, raising a conceptual puzzle: how enhanced plasticity can yield opposite learning outcomes? Here, we show that the recent history of experience can determine whether mice with enhanced plasticity exhibit enhanced or impaired learning in response to the same training. Mice with enhanced cerebellar LTD, due to double knockout (DKO) of MHCI H2-K^b/H2-D^b (K^bD^b−/−), exhibited oculomotor learning deficits. However, the same mice exhibited enhanced learning after appropriate pre-training. Theoretical analysis revealed that synapses with history-dependent learning rules could recapitulate the data, and suggested that saturation may be a key factor limiting the ability of enhanced plasticity to enhance learning. Optogenetic stimulation designed to saturate LTD produced the same impairment in WT as observed in DKO mice. Overall, our results suggest that the recent history of activity and the threshold for synaptic plasticity conspire to effect divergent learning outcomes.

DOI:http://dx.doi.org/10.7554/eLife.20147.001

All animals can learn from their experiences. One of the main ideas for how learning occurs is that it involves changes in the strength of the connections between neurons, known as synapses. The ability of synapses to become stronger or weaker is referred to as synaptic plasticity. High levels of synaptic plasticity are generally thought to be good for learning, while low levels of synaptic plasticity make learning more difficult.

Nevertheless, studies have also reported that high levels of synaptic plasticity can sometimes impair learning. To explain these mixed results, Nguyen-Vu, Zhao, Lahiri et al. studied mice that had been genetically modified to show greater synaptic plasticity than normal mice. The same individual mutant animals were sometimes less able to learn an eye-movement task than unmodified mice, and at other times better able to learn exactly the same task. The main factor that determined how well the mice could learn was what the mice had experienced shortly before they began the training.

Nguyen-Vu et al. propose that some experiences change the strength of synapses so much that they temporarily prevent those synapses from undergoing any further changes. Animals with these “saturated” synapses will struggle to learn a new task, even if their brains are normally capable of high levels of synaptic plasticity. Notably, even normal activity appears to be able to put the synapses of the mutant mice into a saturated state, whereas this saturation would only occur in normal mice under a restricted set of circumstances. Consistent with this idea, Nguyen-Vu et al. showed that a specific type of pre-training that desaturates synapses improved the ability of the modified mice to learn the eye-movement task. Conversely, a different procedure that is known to saturate synapses impaired the learning ability of the unmodified mice.

A future challenge is to test these predictions experimentally by measuring changes in synaptic plasticity directly, both in brain slices and in living animals. The results could ultimately help to develop treatments that improve the ability to learn and so could provide benefits to a wide range of individuals, including people who have suffered a brain injury or stroke.

DOI:http://dx.doi.org/10.7554/eLife.20147.002

A formin-g role during development and disease

Several different protein families were shown to be involved in the regulation of actin filament formation and have been studied extensively in processes such as cell migration. Among them are members of the formin family, which tend to promote the formation of linear actin filaments. Studies in recent years, often using loss of function animal models, have indicated that formin family members play roles beyond cell motility in vitro and are involved in processes ranging from tissue morphogenesis and cell differentiation to diseases such as cancer and cardiomyopathy. Therefore the aim of this review is to discuss these findings and to start putting them into a subcellular context.

Cognition and hippocampal plasticity in the mouse is altered by monosomy of a genomic region implicated in Down syndrome

Down syndrome (DS) is due to increased copy number of human chromosome 21. The contribution of different genetic regions has been tested using mouse models. As shown previously, the Abcg1-U2af1 genetic region contributes to cognitive defects in working and short-term recognition memory in Down syndrome mouse models. Here we analyzed the impact of monosomy of the same genetic interval, using a new mouse model, named Ms2Yah. We used several cognitive paradigms and did not detect defects in the object recognition or the Morris water maze tests. However, surprisingly, Ms2Yah mice displayed increased associative memory in a pure contextual fear-conditioning test and decreased social novelty interaction along with a larger long-term potentiation recorded in the CA1 area following stimulation of Schaffer collaterals. Whole-genome expression studies carried out on hippocampus showed that the transcription of only a small number of genes is affected, mainly from the genetic interval (Cbs, Rsph1, Wdr4), with a few additional ones, including the postsynaptic Gabrr2, Gabbr1, Grid2p, Park2, and Dlg1 and the components of the Ubiquitin-mediated proteolysis (Anapc1, Rnf7, Huwe1, Park2). The Abcg1–U2af1 region is undeniably encompassing dosage-sensitive genes or elements whose change in copy number directly affects learning and memory, synaptic function, and autistic related behavior.

Whole Transcriptome Analysis (RNA Sequencing) of Peripheral Blood Mononuclear Cells of Vitiligo Patients

Vitiligo is an idiopathic disorder characterized by depigmented patches on the skin due to a loss of melanocytes. The cause of melanocyte destruction is not fully understood. The aim of this study was to detect the potential pathways involved in the vitiligo pathogenesis to further understand the causes and entity of vitiligo. For that the transcriptome of peripheral blood mononuclear cells of 4 vitiligo patients and 4 control subjects was analyzed using the SOLiD System platform and whole transcriptome RNA sequencing application. Altogether 2,470 genes were expressed differently and GRID2IP showed the highest deviation in patients compared to controls. Using functional analysis, altogether 993 associations between the gene groups and diseases were found. The analysis revealed associations between vitiligo and diseases such as lichen planus, limb-girdle muscular dystrophy type 2B, and facioscapulohumeral muscular dystrophy. Additionally, the gene groups with an altered expression pattern are participating in processes such as cell death, survival and signaling, inflammation, and oxidative stress. In conclusion, vitiligo is rather a systemic than a local skin disease; the findings from an enormous amount of RNA sequencing data support the previous findings about vitiligo and should be further analyzed.

Long-term potentiation of inhibitory synaptic transmission onto cerebellar Purkinje neurons contributes to adaptation of vestibulo-ocular reflex

Synaptic plasticity in the cerebellum is thought to contribute to motor learning. In particular, long-term depression (LTD) at parallel fiber (PF) to Purkinje neuron (PN) excitatory synapses has attracted much attention of neuroscientists as a primary cellular mechanism for motor learning. In contrast, roles of plasticity at cerebellar inhibitory synapses in vivo remain unknown. Here, we have investigated the roles of long-lasting enhancement of transmission at GABAergic synapses on a PN that is known as rebound potentiation (RP). Previous studies demonstrated that binding of GABAA receptor with GABAA receptor-associated protein (GABARAP) is required for RP, and that a peptide that blocks this binding suppresses RP induction. To address the functional roles of RP, we generated transgenic mice that express this peptide fused to a fluorescent protein selectively in PNs using the PN-specific L7 promoter. These mice failed to show RP, although they showed no changes in the basal amplitude or frequency of miniature IPSCs. The transgenic mice also showed no abnormality in gross cerebellar morphology, LTD, or other excitatory synaptic properties, or intrinsic excitability of PNs. Next, we attempted to evaluate their motor control and learning ability by examining reflex eye movements. The basal dynamic properties of the vestibulo-ocular reflex and optokinetic response, and adaptation of the latter, were normal in the transgenic mice. In contrast, the transgenic mice showed defects in the adaptation of vestibulo-ocular reflex, a model paradigm of cerebellum-dependent motor learning. These results together suggest that RP contributes to a certain type of motor learning.

A simple machine vision-driven system for measuring optokinetic reflex in small animals

The optokinetic reflex (OKR) is useful to monitor the function of the visual and motor nervous systems. However, OKR measurement is not open to all because dedicated commercial equipment or detailed instructions for building in-house equipment is rarely offered. Here we describe the design of an easy-to-install/use yet reliable OKR measuring system including a computer program to visually locate the pupil and a mathematical procedure to estimate the pupil azimuth from the location data. The pupil locating program was created on a low-cost machine vision development platform, whose graphical user interface allows one to compose and operate the program without programming expertise. Our system located mouse pupils at a high success rate (~90 %), estimated their azimuth precisely (~94 %), and detected changes in OKR gain due to the pharmacological modulation of the cerebellar flocculi. The system would promote behavioral assessment in physiology, pharmacology, and genetics.

The actin nucleator Cobl is crucial for Purkinje cell development and works in close conjunction with the F-actin binding protein Abp1

Cortical actin dynamics shapes cells. To generate actin filaments, cells rely on actin nucleators. Cobl is a novel, brain-enriched, WH2 domain-based actin nucleator, yet, its functions remained largely elusive. Here, we reveal that Cobl plays a crucial role in Purkinje cell development using gene gun transfections within intact murine cerebellar contexts. Cobl deficiency impaired proper dendritic arborization of Purkinje cells and led to low-complexity arbors. Branch point numbers and density and especially higher order branching were strongly affected. Our efforts to reveal how Cobl is physically and functionally integrated into the cortical actin cytoskeleton showed that all Cobl loss-of-function phenotypes were exactly mirrored by knockdown of the F-actin-binding protein Abp1. By subcellular fractionations, protein interaction analyses, subcellular reconstitutions of protein complexes, colocalization studies in cells and tissues, and by functional analyses in neuronal morphogenesis we demonstrate that both proteins associate and work with each other closely. Cobl-mediated dendritic branch induction in hippocampal neurons critically relied on Abp1. Our study highlights that the functions of Abp1 are distinct from those of the Cobl-binding protein syndapin I. The importance of Cobl/Abp1 complex formation and of Abp1-mediated F-actin association was highlighted by functional rescue experiments demonstrating that a Cobl mutant deficient for Abp1 binding and an Abp1 mutant supporting Cobl association but lacking the F-actin binding ability failed to rescue the respective loss-of-function phenotypes. Thus, F-actin-anchored Cobl/Abp1 complexes seem crucial for neuromorphogenesis processes, particularly for the postnatal arborization of Purkinje cells representing the source for all motor coordination in the cerebellar cortex.

The δ2 glutamate receptor gates long-term depression by coordinating interactions between two AMPA receptor phosphorylation sites

Long-term depression (LTD) commonly affects learning and memory in various brain regions. Although cerebellar LTD absolutely requires the δ2 glutamate receptor (GluD2) that is expressed in Purkinje cells, LTD in other brain regions does not; why and how cerebellar LTD is regulated by GluD2 remains unelucidated. Here, we show that the activity-dependent phosphorylation of serine 880 (S880) in GluA2 AMPA receptor subunit, which is an essential step for AMPA receptor endocytosis during LTD induction, was impaired in GluD2-null cerebellum. In contrast, the basal phosphorylation levels of tyrosine 876 (Y876) in GluA2 were increased in GluD2-null cerebellum. An in vitro phosphorylation assay revealed that Y876 phosphorylation inhibited subsequent S880 phosphorylation. Conversely, Y876 dephosphorylation was sufficient to restore S880 phosphorylation and LTD induction in GluD2-null Purkinje cells. Furthermore, megakaryocyte protein tyrosine phosphatase (PTPMEG), which binds to the C terminus of GluD2, directly dephosphorylated Y876. These data indicate that GluD2 gates LTD by coordinating interactions between the two phosphorylation sites of the GluA2.

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.

Long-term depression and other synaptic plasticity in the cerebellum

Cerebellar long-term depression (LTD) is a type of synaptic plasticity and has been considered as a critical cellular mechanism for motor learning. LTD occurs at excitatory synapses between parallel fibers and a Purkinje cell in the cerebellar cortex, and is expressed as reduced responsiveness to transmitter glutamate. Molecular induction mechanism of LTD has been intensively studied using culture and slice preparations, which has revealed critical roles of Ca(2+), protein kinase C and endocytosis of AMPA-type glutamate receptors. Involvement of a large number of additional molecules has also been demonstrated, and their interactions relevant to LTD mechanisms have been studied. In vivo experiments including those on mutant mice, have reported good correlation of LTD and motor learning. However, motor learning could occur with impaired LTD. A possibility that cerebellar synaptic plasticity other than LTD compensates for the defective LTD has been proposed.

Glutamate-receptor-like molecule GluRδ2 involved in synapse formation at parallel fiber-Purkinje neuron synapses

Glutamate-receptor-like molecule δ2 (GluRδ2, GluD2) has been classified as an ionotropic glutamate receptor subunit. It is selectively expressed on the postsynaptic membrane at parallel fiber-Purkinje neuron synapses in the cerebellum. Mutant mice deficient in GluRδ2 show impaired synaptic plasticity, the decrease in the number of parallel fiber-Purkinje neuron synapses, multiple innervation of climbing fibers on a Purkinje neuron, and defects in motor control and learning. Thus, GluRδ2 plays crucial roles in the cerebellar function. Recent studies on GluRδ2 have shown that it has synaptogenic activity. GluRδ2 expressed in a non-neuronal cell induces presynaptic differentiation of granule neurons in a co-culture preparation. This synaptogenic activity depends on an extracellular N-terminal leucine/isoleucine/valine binding protein-like domain of GluRδ2. GluRδ2 plays critical roles in formation, maturation, and/or maintenance of granule neuron-Purkinje neuron synapses.

Molecular mechanism of parallel fiber-Purkinje cell synapse formation

The cerebellum receives two excitatory afferents, the climbing fiber (CF) and the mossy fiber-parallel fiber (PF) pathway, both converging onto Purkinje cells (PCs) that are the sole neurons sending outputs from the cerebellar cortex. Glutamate receptor δ2 (GluRδ2) is expressed selectively in cerebellar PCs and localized exclusively at the PF-PC synapses. We found that a significant number of PC spines lack synaptic contacts with PF terminals and some of residual PF-PC synapses show mismatching between pre- and postsynaptic specializations in conventional and conditional GluRδ2 knockout mice. Studies with mutant mice revealed that in addition to PF-PC synapse formation, GluRδ2 is essential for synaptic plasticity, motor learning, and the restriction of CF territory. GluRδ2 regulates synapse formation through the amino-terminal domain, while the control of synaptic plasticity, motor learning, and CF territory is mediated through the carboxyl-terminal domain. Thus, GluRδ2 is the molecule that bridges synapse formation and motor learning. We found that the trans-synaptic interaction of postsynaptic GluRδ2 and presynaptic neurexins (NRXNs) through cerebellin 1 (Cbln1) mediates PF-PC synapse formation. The synaptogenic triad is composed of one molecule of tetrameric GluRδ2, two molecules of hexameric Cbln1 and four molecules of monomeric NRXN. Thus, GluRδ2 triggers synapse formation by clustering four NRXNs. These findings provide a molecular insight into the mechanism of synapse formation in the brain.

Trans-synaptic interaction of GluRdelta2 and Neurexin through Cbln1 mediates synapse formation in the cerebellum

Elucidation of molecular mechanisms that regulate synapse formation is required for the understanding of neural wiring, higher brain functions, and mental disorders. Despite the wealth of in vitro information, fundamental questions about how glutamatergic synapses are formed in the mammalian brain remain unanswered. Glutamate receptor (GluR) delta2 is essential for cerebellar synapse formation in vivo. Here, we show that the N-terminal domain (NTD) of GluRdelta2 interacts with presynaptic neurexins (NRXNs) through cerebellin 1 precursor protein (Cbln1). The synaptogenic activity of GluRdelta2 is abolished in cerebellar primary cultures from Cbln1 knockout mice and is restored by recombinant Cbln1. Knockdown of NRXNs in cerebellar granule cells also hinders the synaptogenic activity of GluRdelta2. Both the NTD of GluRdelta2 and the extracellular domain of NRXN1beta suppressed the synaptogenic activity of Cbln1 in cerebellar primary cultures and in vivo. These results suggest that GluRdelta2 mediates cerebellar synapse formation by interacting with presynaptic NRXNs through Cbln1.

Human ataxias: a genetic dissection of inositol triphosphate receptor (ITPR1)-dependent signaling

A persistent mystery about the ataxias has been why mutations in genes--many of which are expressed widely in the brain--primarily cause ataxia, and not, for example, epilepsy or dementia. Why should a polyglutamine stretch in the TATA-binding protein (that is important in all cells) particularly disrupt cerebellar coordination? We propose that advances in the genetics of cerebellar ataxias suggest a rational hypothesis for how so many different genes lead to predominantly cerebellar defects. We argue that the unifying feature of many genes involved in cerebellar ataxias is their impact on the signaling protein ITPR1 (inositiol 1,4,5-triphosphate receptor type 1), that underlies coincidence detection in Purkinje cells and could play an important role in cerebellar coordination.

A nucleator arms race: cellular control of actin assembly

For over a decade, the actin-related protein 2/3 (ARP2/3) complex, a handful of nucleation-promoting factors and formins were the only molecules known to directly nucleate actin filament formation de novo. However, the past several years have seen a surge in the discovery of mammalian proteins with roles in actin nucleation and dynamics. Newly recognized nucleation-promoting factors, such as WASP and SCAR homologue (WASH), WASP homologue associated with actin, membranes and microtubules (WHAMM), and junction-mediating regulatory protein (JMY), stimulate ARP2/3 activity at distinct cellular locations. Formin nucleators with additional biochemical and cellular activities have also been uncovered. Finally, the Spire, cordon-bleu and leiomodin nucleators have revealed new ways of overcoming the kinetic barriers to actin polymerization.

Lipid signaling in cytosolic phospholipase A2alpha-cyclooxygenase-2 cascade mediates cerebellar long-term depression and motor learning

In this study, we show the crucial roles of lipid signaling in long-term depression (LTD), that is, synaptic plasticity prevailing in cerebellar Purkinje cells. In mouse brain slices, we found that cPLA(2)alpha knockout blocked LTD induction, which was rescued by replenishing arachidonic acid (AA) or prostaglandin (PG) D(2) or E(2). Moreover, cyclooxygenase (COX)-2 inhibitors block LTD, which is rescued by supplementing PGD(2)/E(2). The blockade or rescue occurs when these reagents are applied within a time window of 5-15 min following the onset of LTD-inducing stimulation. Furthermore, PGD(2)/E(2) facilitates the chemical induction of LTD by a PKC activator but is unable to rescue the LTD blocked by a PKC inhibitor. We conclude that PGD(2)/E(2) mediates LTD jointly with PKC, and suggest possible pathways for their interaction. Finally, we demonstrate in awake mice that cPLA(2)alpha deficiency or COX-2 inhibition attenuates short-term adaptation of optokinetic eye movements, supporting the view that LTD underlies motor learning.

Fifteen formins for an actin filament: a molecular view on the regulation of human formins

The regulation of the actin cytoskeleton is a key process for the stability and motility of eukaryotic cells. Besides the Arp2/3 complex and its nucleation promoting factors, WH2 domain-containing proteins and a diverse family of formin proteins have recently been recognized as actin nucleators and potent polymerization factors of actin filaments. Formins are defined by the presence of a catalytic formin homology 2 (FH2) domain, yet, the modular domain architecture appears significantly different for the eight formin families identified in humans. A diverse picture of protein localization, interaction partners and cell specific regulation emerged, suggesting various functions of formins in the building and maintenance of actin filaments. This review focuses on the domain architecture of human formins, the regulation mechanisms of their activation and the diversity in formin cellular functions.

Prediction and validation of a mechanism to control the threshold for inhibitory synaptic plasticity

Synaptic plasticity, neuronal activity-dependent sustained alteration of the efficacy of synaptic transmission, underlies learning and memory. Activation of positive-feedback signaling pathways by an increase in intracellular Ca²⁺ concentration ([Ca²⁺]_i) has been implicated in synaptic plasticity. However, the mechanism that determines the [Ca²⁺]_i threshold for inducing synaptic plasticity is elusive. Here, we developed a kinetic simulation model of inhibitory synaptic plasticity in the cerebellum, and systematically analyzed the behavior of intricate molecular networks composed of protein kinases, phosphatases, etc. The simulation showed that Ca²⁺/calmodulin-dependent protein kinase II (CaMKII), which is essential for the induction of synaptic plasticity, was persistently activated or suppressed in response to different combinations of stimuli. The sustained CaMKII activation depended on synergistic actions of two positive-feedback reactions, CaMKII autophosphorylation and CaMKII-mediated inhibition of a CaM-dependent phosphodiesterase, PDE1. The simulation predicted that PDE1-mediated feedforward inhibition of CaMKII predominantly controls the Ca²⁺ threshold, which was confirmed by electrophysiological experiments in primary cerebellar cultures. Thus, combined application of simulation and experiments revealed that the Ca²⁺ threshold for the cerebellar inhibitory synaptic plasticity is primarily determined by PDE1.

Dual involvement of G-substrate in motor learning revealed by gene deletion

In this study, we generated mice lacking the gene for G-substrate, a specific substrate for cGMP-dependent protein kinase uniquely located in cerebellar Purkinje cells, and explored their specific functional deficits. G-substrate-deficient Purkinje cells in slices obtained at postnatal weeks (PWs) 10-15 maintained electrophysiological properties essentially similar to those from WT littermates. Conjunction of parallel fiber stimulation and depolarizing pulses induced long-term depression (LTD) normally. At younger ages, however, LTD attenuated temporarily at PW6 and recovered thereafter. In parallel with LTD, short-term (1 h) adaptation of optokinetic eye movement response (OKR) temporarily diminished at PW6. Young adult G-substrate knockout mice tested at PW12 exhibited no significant differences from their WT littermates in terms of brain structure, general behavior, locomotor behavior on a rotor rod or treadmill, eyeblink conditioning, dynamic characteristics of OKR, or short-term OKR adaptation. One unique change detected was a modest but significant attenuation in the long-term (5 days) adaptation of OKR. The present results support the concept that LTD is causal to short-term adaptation and reveal the dual functional involvement of G-substrate in neuronal mechanisms of the cerebellum for both short-term and long-term adaptation.

Cbln1 accumulates and colocalizes with Cbln3 and GluRdelta2 at parallel fiber-Purkinje cell synapses in the mouse cerebellum

Cbln1 (a.k.a. precerebellin) is secreted from cerebellar granule cells as homohexamer or in heteromeric complexes with Cbln3. Cbln1 plays crucial roles in regulating morphological integrity of parallel fiber (PF)-Purkinje cell (PC) synapses and synaptic plasticity. Cbln1-knockout mice display severe cerebellar phenotypes that are essentially indistinguishable from those in glutamate receptor GluRdelta2-null mice, and include severe reduction in the number of PF-PC synapses and loss of long-term depression of synaptic transmission. To understand better the relationship between Cbln1, Cbln3 and GluRdelta2, we performed light and electron microscopic immunohistochemical analyses using highly specific antibodies and antigen-exposing methods, i.e. pepsin pretreatment for light microscopy and postembedding immunogold for electron microscopy. In conventional immunohistochemistry, Cbln1 was preferentially associated with non-terminal portions of PF axons in the molecular layer but rarely overlapped with Cbln3. In contrast, antigen-exposing methods not only greatly intensified Cbln1 immunoreactivity in the molecular layer, but also revealed its high accumulation in the synaptic cleft of PF-PC synapses. No such synaptic accumulation was evident at other PC synapses. Furthermore, Cbln1 now came to overlap almost completely with Cbln3 and GluRdelta2 at PF-PC synapses. Therefore, the convergence of all three molecules provides the anatomical basis for a common signaling pathway regulating circuit development and synaptic plasticity in the cerebellum.

New (but old) molecules regulating synapse integrity and plasticity: Cbln1 and the delta2 glutamate receptor

The delta2 glutamate receptor (GluRdelta2) is predominantly expressed in cerebellar Purkinje cells and plays crucial roles in cerebellar functions: GluRdelta2-null mice display ataxia and impaired motor learning. Interestingly, the contact state of synapses between parallel fibers (PFs) and Purkinje cells is specifically and severely affected, and the number of normal PF synapses is markedly reduced in GluRdelta2-null Purkinje cells. Furthermore, long-term depression at PF-Purkinje cell synapses is abrogated. Cbln1, a member of the C1q/tumor necrosis factor (TNF) superfamily, is predominantly expressed and released from cerebellar granule cells. Unexpectedly, the behavioral, physiological and anatomical phenotypes of cbln1-null mice precisely mimic those of GluRdelta2-null mice. Thus, we propose that Cbln1, which is released from granule cells, and GluRdelta2, which is predominantly expressed in Purkinje cells, are involved in a common signaling pathway crucial for synapse formation/maintenance and plasticity in the cerebellum. Since molecules related to Cbln1 are expressed in various brain regions other than the cerebellum, other C1q/TNF superfamily proteins may also regulate various aspects of synapses in the CNS. Therefore, an understanding of the signaling mechanisms underlying Cbln1 and GluRdelta2 in the cerebellum will provide new insights into the roles of C1q/TNF superfamily proteins as new cytokines that regulate normal and abnormal brain functions.