Phospholipase cbeta4 is specifically involved in climbing fiber synapse elimination in the developing cerebellum

Direct and indirect pathways for heterosynaptic interaction underlying developmental synapse elimination in the mouse cerebellum

Developmental synapse elimination is crucial for shaping mature neural circuits. In the neonatal mouse cerebellum, Purkinje cells (PCs) receive excitatory synaptic inputs from multiple climbing fibers (CFs) and synapses from all but one CF are eliminated by around postnatal day 20. Heterosynaptic interaction between CFs and parallel fibers (PFs), the axons of cerebellar granule cells (GCs) forming excitatory synapses onto PCs and molecular layer interneurons (MLIs), is crucial for CF synapse elimination. However, mechanisms for this heterosynaptic interaction are largely unknown. Here we show that deletion of AMPA-type glutamate receptor functions in GCs impairs CF synapse elimination mediated by metabotropic glutamate receptor 1 (mGlu1) signaling in PCs. Furthermore, CF synapse elimination is impaired by deleting NMDA-type glutamate receptors from MLIs. We propose that PF activity is crucial for CF synapse elimination by directly activating mGlu1 in PCs and indirectly enhancing the inhibition of PCs through activating NMDA receptors in MLIs.

Protein kinase Cγ negatively regulates the intrinsic excitability in zebrin-negative cerebellar Purkinje cells

Protein kinase C γ (PKCγ), a neuronal isoform present exclusively in the central nervous system, is most abundantly expressed in cerebellar Purkinje cells (PCs). Targeted deletion of PKCγ causes a climbing fiber synapse elimination in developing PCs and motor deficit. However, physiological roles of PKCγ in adult mouse PCs are little understood. In this study, we aimed to unravel the roles of PKCγ in mature mouse PCs by deleting PKCγ from adult mouse PCs of PKCγfl/fl mice via cerebellar injection of adeno-associated virus (AAV) vectors expressing Cre recombinase under the control of the PC-specific L7-6 promoter. Whole cell patch-clamp recording of PCs showed higher intrinsic excitability in PCs virally lacking PKCγ [PKCγ-conditional knockout (PKCγ-cKO) PCs] than in wild-type (WT) mouse PCs in the zebrin-negative module, but not in the zebrin-positive module. AAV-mediated PKCγ re-expression in PKCγ-deficient mouse PCs in the zebrin-negative module restored the enhanced intrinsic excitability to a level comparable to that of wild-type mouse PCs. In parallel with higher intrinsic excitability, we found larger hyperpolarization-activated cyclic nucleotide-gated (HCN) channel currents in PKCγ-cKO PCs located in the zebrin-negative module, compared with those in WT mouse PCs in the same region. However, pharmacological inhibition of the HCN currents did not restore the enhanced intrinsic excitability in PKCγ-cKO PCs in the zebrin-negative module. These results suggested that PKCγ suppresses the intrinsic excitability in zebrin-negative PCs, which is likely independent of the HCN current inhibition.

A polypeptide derived from pilose antler ameliorates CUMS-induced depression-like behavior by SENP2-PLCβ4 signaling axis

Neurogenesis is known to be closely associated with depression. We aimed to investigate whether a polypeptide monomer derived from pilose antler (polypeptide sequence LSALEGVFYP, PAP) exerts an antidepressant effect by influencing neurogenesis, and to elucidate the mechanism of its antidepressant action. Behavioral tests were performed to observe the antidepressant effect of PAP. Neurogenesis in the dentate gyrus (DG) region of hippocampus was observed by immunofluorescence. The expression of key proteins of Sentrin/SUMO-specific proteases 2 (SENP2)- Phosphoinositide-specific phospholipase C beta 4 (PLCβ4) pathway was accessed by co-immunoprecipitation (Co-IP), and the calcium homeostasis associated proteins were observed via Western blot (WB). Subsequently, temozolomide (TMZ) pharmacologically blocked neurogenesis to verify the antidepressant effect of PAP on neurogenesis. The mechanism of PAP antidepressant effect was verified by constructing a sh-SENP2 virus vector to silence SENP2 protein. Finally, corticosterone (CORT)-induced PC12 cell model was used to verify whether PAP was involved in the process of deconjugated PLCβ4 SUMOylated. The results showed that PAP improved depression-like behavior and neurogenesis induced by chronic unpredictable mild stimulation (CUMS). In addition, PAP acted on SENP2-PLCβ4 pathway to deconjugate the SUMOylation of PLCβ4 and affect calcium homeostasis. Pharmacological blockade of neurogenesis by TMZ treatment impaired the antidepressant efficacy of PAP. Knockout of SENP2 in the CUMS model attenuated the antidepressant response of PAP, and the impaired neurogenesis was not ameliorated by PAP treatment. In summary, PAP acted on the SENP2-PLCβ4 signaling pathway to inhibit the SUMOylation of PLCβ4 and maintain calcium homeostasis, thereby protecting neurogenesis and playing an antidepressant role.

TCR Signals Controlling Adaptive Immunity against Toxoplasma and Cancer

T cells play a crucial role in adaptive immunity by recognizing and eliminating foreign pathogens and abnormal cells such as cancer cells. T cell receptor (TCR), which is expressed on the surface of T cells, recognizes and binds to specific antigens presented by major histocompatibility complex (MHC) molecules on antigen-presenting cells (APCs). This activation process leads to the proliferation and differentiation of T cells, allowing them to carry out their specific immune response functions. This chapter outlines the TCR signaling pathways that are common to different T cell subsets, as well as the recently elucidated TCR signaling pathway specific to CD8+ T cells and its role in controlling anti-Toxoplasma and anti-tumor immunity.

Regulation of cerebellar network development by granule cells and their molecules

The well-organized cerebellar structures and neuronal networks are likely crucial for their functions in motor coordination, motor learning, cognition, and emotion. Such cerebellar structures and neuronal networks are formed during developmental periods through orchestrated mechanisms, which include not only cell-autonomous programs but also interactions between the same or different types of neurons. Cerebellar granule cells (GCs) are the most numerous neurons in the brain and are generated through intensive cell division of GC precursors (GCPs) during postnatal developmental periods. While GCs go through their own developmental processes of proliferation, differentiation, migration, and maturation, they also play a crucial role in cerebellar development. One of the best-characterized contributions is the enlargement and foliation of the cerebellum through massive proliferation of GCPs. In addition to this contribution, studies have shown that immature GCs and GCPs regulate multiple factors in the developing cerebellum, such as the development of other types of cerebellar neurons or the establishment of afferent innervations. These studies have often found impairments of cerebellar development in animals lacking expression of certain molecules in GCs, suggesting that the regulations are mediated by molecules that are secreted from or present in GCs. Given the growing recognition of GCs as regulators of cerebellar development, this review will summarize our current understanding of cerebellar development regulated by GCs and molecules in GCs, based on accumulated studies and recent findings, and will discuss their potential further contributions.

PTPδ is a presynaptic organizer for the formation and maintenance of climbing fiber to Purkinje cell synapses in the developing cerebellum

Functionally mature neural circuits are shaped during postnatal development by eliminating redundant synapses formed during the perinatal period. In the cerebellum of neonatal rodents, each Purkinje cell (PC) receives synaptic inputs from multiple (more than 4) climbing fibers (CFs). During the first 3 postnatal weeks, synaptic inputs from a single CF become markedly larger and those from the other CFs are eliminated in each PC, leading to mono-innervation of each PC by a strong CF in adulthood. While molecules involved in the strengthening and elimination of CF synapses during postnatal development are being elucidated, much less is known about the molecular mechanisms underlying CF synapse formation during the early postnatal period. Here, we show experimental evidence that suggests that a synapse organizer, PTPδ, is required for early postnatal CF synapse formation and the subsequent establishment of CF to PC synaptic wiring. We showed that PTPδ was localized at CF-PC synapses from postnatal day 0 (P0) irrespective of the expression of Aldolase C (Aldoc), a major marker of PC that distinguishes the cerebellar compartments. We found that the extension of a single strong CF along PC dendrites (CF translocation) was impaired in global PTPδ knockout (KO) mice from P12 to P29-31 predominantly in PCs that did not express Aldoc [Aldoc (-) PCs]. We also demonstrated via morphological and electrophysiological analyses that the number of CFs innervating individual PCs in PTPδ KO mice were fewer than in wild-type (WT) mice from P3 to P13 with a significant decrease in the strength of CF synaptic inputs in cerebellar anterior lobules where most PCs are Aldoc (-). Furthermore, CF-specific PTPδ-knockdown (KD) caused a reduction in the number of CFs innervating PCs with decreased CF synaptic inputs at P10-13 in anterior lobules. We found a mild impairment of motor performance in adult PTPδ KO mice. These results indicate that PTPδ acts as a presynaptic organizer for CF-PC formation and is required for normal CF-PC synaptic transmission, CF translocation, and presumably CF synapse maintenance predominantly in Aldoc (-) PCs. Furthermore, this study suggests that the impaired CF-PC synapse formation and development by the lack of PTPδ causes mild impairment of motor performance.

The molecular signals that regulate activity-dependent synapse refinement in the brain

The formation of appropriate synaptic connections is critical for the proper functioning of the brain. Early in development, neurons form a surplus of immature synapses. To establish efficient, functional neural networks, neurons selectively stabilize active synapses and eliminate less active ones. This process is known as activity-dependent synapse refinement. Defects in this process have been implicated in neuropsychiatric disorders such as schizophrenia and autism. Here we review the manner and mechanisms by which synapse elimination is regulated through activity-dependent competition. We propose a theoretical framework for the molecular mechanisms of synapse refinement, in which three types of signals regulate the refinement. We then describe the identity of these signals and discuss how multiple molecular signals interact to achieve appropriate synapse refinement in the brain.

The Emerging Key Role of the mGluR1-PKCγ Signaling Pathway in the Pathogenesis of Spinocerebellar Ataxias: A Neurodevelopmental Viewpoint

Spinocerebellar ataxias (SCAs) are a heterogeneous group of autosomal dominantly inherited progressive disorders with degeneration and dysfunction of the cerebellum. Although different subtypes of SCAs are classified according to the disease-associated causative genes, the clinical syndrome of the ataxia is shared, pointing towards a possible convergent pathogenic pathway among SCAs. In this review, we summarize the role of SCA-associated gene function during cerebellar Purkinje cell development and discuss the relationship between SCA pathogenesis and neurodevelopment. We will summarize recent studies on molecules involved in SCA pathogenesis and will focus on the mGluR1-PKCγ signaling pathway evaluating the possibility that this might be a common pathway which contributes to these diseases.

SENP2-PLCβ4 signaling regulates neurogenesis through the maintenance of calcium homeostasis

Neurogenesis plays a critical role in brain physiology and behavioral performance, and defective neurogenesis leads to neurological and psychiatric disorders. Here, we show that PLCβ4 expression is markedly reduced in SENP2-deficient cells and mice, resulting in decreased IP₃ formation and altered intracellular calcium homeostasis. PLCβ4 stability is regulated by the SUMO-dependent ubiquitin-mediated proteolytic pathway, which is catalyzed by PIAS2α and RNF4. SUMOylated PLCβ4 is transported to the nucleus through Nup205- and RanBP2-dependent pathways and regulates nuclear signaling. Furthermore, dysregulated calcium homeostasis induced defects in neurogenesis and neuronal viability in SENP2-deficient mice. Finally, SENP2 and PLCβ4 are stimulated by starvation and oxidative stress, which maintain calcium homeostasis regulated neurogenesis. Our findings provide mechanistic insight into the critical roles of SENP2 in the regulation of PLCβ4 SUMOylation, and the involvement of SENP2-PLCβ4 axis in calcium homeostasis regulated neurogenesis under stress.

Mechanisms governing activity-dependent synaptic pruning in the developing mammalian CNS

Almost 60 years have passed since the initial discovery by Hubel and Wiesel that changes in neuronal activity can elicit developmental rewiring of the central nervous system (CNS). Over this period, we have gained a more comprehensive picture of how both spontaneous neural activity and sensory experience-induced changes in neuronal activity guide CNS circuit development. Here we review activity-dependent synaptic pruning in the mammalian CNS, which we define as the removal of a subset of synapses, while others are maintained, in response to changes in neural activity in the developing nervous system. We discuss the mounting evidence that immune and cell-death molecules are important mechanistic links by which changes in neural activity guide the pruning of specific synapses, emphasizing the role of glial cells in this process. Finally, we discuss how these developmental pruning programmes may go awry in neurodevelopmental disorders of the human CNS, focusing on autism spectrum disorder and schizophrenia. Together, our aim is to give an overview of how the field of activity-dependent pruning research has evolved, led to exciting new questions and guided the identification of new, therapeutically relevant mechanisms that result in aberrant circuit development in neurodevelopmental disorders.

mGluR1 signaling in cerebellar Purkinje cells: Subcellular organization and involvement in cerebellar function and disease

The cerebellum is essential for the control, coordination, and learning of movements, and for certain aspects of cognitive function. Purkinje cells are the sole output neurons in the cerebellar cortex and therefore play crucial roles in the diverse functions of the cerebellum. The type 1 metabotropic glutamate receptor (mGluR1) is prominently enriched in Purkinje cells and triggers downstream signaling pathways that are required for functional and structural plasticity, and for synaptic responses. To understand how mGluR1 contributes to cerebellar functions, it is important to consider not only the operational properties of this receptor, but also its spatial organization and the molecular interactions that enable its proper functioning. In this review, we highlight how mGluR1 and its related signaling molecules are organized into tightly coupled microdomains to fulfill physiological functions. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunction in ataxias of human patients and mouse models.

Uncovering a novel role of PLCβ4 in selectively mediating TCR signaling in CD8+ but not CD4+ T cells

Because of their common signaling molecules, the main T cell receptor (TCR) signaling cascades in CD4⁺ and CD8⁺ T cells are considered qualitatively identical. Herein, we show that TCR signaling in CD8⁺ T cells is qualitatively different from that in CD4⁺ T cells, since CD8α ignites another cardinal signaling cascade involving phospholipase C β4 (PLCβ4). TCR-mediated responses were severely impaired in PLCβ4-deficient CD8⁺ T cells, whereas those in CD4⁺ T cells were intact. PLCβ4-deficient CD8⁺ T cells showed perturbed activation of peripheral TCR signaling pathways downstream of IP₃ generation. Binding of PLCβ4 to the cytoplasmic tail of CD8α was important for CD8⁺ T cell activation. Furthermore, GNAQ interacted with PLCβ4, mediated double phosphorylation on threonine 886 and serine 890 positions of PLCβ4, and activated CD8⁺ T cells in a PLCβ4-dependent fashion. PLCβ4-deficient mice exhibited defective antiparasitic host defense and antitumor immune responses. Altogether, PLCβ4 differentiates TCR signaling in CD4⁺ and CD8⁺ T cells and selectively promotes CD8⁺ T cell–dependent adaptive immunity.

Purkinje Neurons with Loss of STIM1 Exhibit Age-Dependent Changes in Gene Expression and Synaptic Components

The stromal interaction molecule 1 (STIM1) is an ER-Ca²⁺ sensor and an essential component of ER-Ca²⁺ store operated Ca²⁺ entry. Loss of STIM1 affects metabotropic glutamate receptor 1 (mGluR1)-mediated synaptic transmission, neuronal Ca²⁺ homeostasis, and intrinsic plasticity in Purkinje neurons (PNs). Long-term changes of intracellular Ca²⁺ signaling in PNs led to neurodegenerative conditions, as evident in individuals with mutations of the ER-Ca²⁺ channel, the inositol 1,4,5-triphosphate receptor. Here, we asked whether changes in such intrinsic neuronal properties, because of loss of STIM1, have an age-dependent impact on PNs. Consequently, we analyzed mRNA expression profiles and cerebellar morphology in PN-specific STIM1 KO mice (STIM1^PKO ) of both sexes across ages. Our study identified a requirement for STIM1-mediated Ca²⁺ signaling in maintaining the expression of genes belonging to key biological networks of synaptic function and neurite development among others. Gene expression changes correlated with altered patterns of dendritic morphology and greater innervation of PN dendrites by climbing fibers, in aging STIM1^PKO mice. Together, our data identify STIM1 as an important regulator of Ca²⁺ homeostasis and neuronal excitability in turn required for maintaining the optimal transcriptional profile of PNs with age. Our findings are significant in the context of understanding how dysregulated calcium signals impact cellular mechanisms in multiple neurodegenerative disorders.SIGNIFICANCE STATEMENT In Purkinje neurons (PNs), the stromal interaction molecule 1 (STIM1) is required for mGluR1-dependent synaptic transmission, refilling of ER Ca²⁺ stores, regulation of spike frequency, and cerebellar memory consolidation. Here, we provide evidence for a novel role of STIM1 in maintaining the gene expression profile and optimal synaptic connectivity of PNs. Expression of genes related to neurite development and synaptic organization networks is altered in PNs with persistent loss of STIM1. In agreement with these findings the dendritic morphology of PNs and climbing fiber innervations on PNs also undergo significant changes with age. These findings identify a new role for dysregulated intracellular calcium signaling in neurodegenerative disorders and provide novel therapeutic insights.

Cellular expression and subcellular localization of diacylglycerol kinase γ in rat brain

Gq protein-coupled receptors lead to activation of phospholipase C, which triggers phosphoinositide signaling. Diacylglycerol (DG) is one of the phosphoinositide metabolites and serves as a second messenger. Diacylglycerol kinase (DGK) phosphorylates DG to produce another second messenger phosphatidic acid. Of the DGK family, DGKγ is predominantly expressed in the brain at the mRNA level. Recent studies have shown the expression of DGKγ in vascular endothelial cells and adrenal medullary cells at the protein level, although its detailed cellular expression pattern and subcellular localization in the brain remain to be determined. In the present study, we addressed this point using specific DGKγ antibody. DGKγ was expressed in both projection neurons and interneurons in the cerebral cortex, hippocampal formation, and cerebellum. In cerebellar Purkinje cells, DGKγ was distributed to the soma and dendrites. Fractionation study revealed that DGKγ was enriched in the internal membranes containing the endoplasmic reticulum and Golgi complex. In immunoelectron microscopy, DGKγ was localized throughout the smooth endoplasmic reticulum system. These findings suggest that DGKγ shows unique cellular expression pattern in the brain and distinct subcellular localization different from other DGK isozymes.

Location-dependent role of phospholipase C signaling in the brain: Physiology and pathology

Phosphoinositide-specific phospholipases C (PI-PLCs) are a class of enzymes involved in the phosphatidylinositol metabolism, which is implicated in the activation of several signaling pathways and which controls several cellular processes. The scientific community has long accepted the existence of a nuclear phosphoinositide (PI) metabolism, independent from the cytoplasmic one, critical in nuclear function control. Indeed, nuclear PIs are involved in many activities, such as cell cycle regulation, cell proliferation, cell differentiation, membrane transport, gene expression and cytoskeletal dynamics. There are several types of PIs and enzymes implicated in brain activities and among these enzymes, PI-PLCs contribute to a specific and complex network in the developing nervous system. Moreover, considering the abundant presence of PI-PLCβ1, PI-PLCγ1 and PI-PLCβ4 in the brain, a specific role for each PLC subtype has been suggested in the control of neuronal activity, which is important for synapse function, development and other mechanisms. The focus of this review is to describe the latest research about the involvement of PI-PLC signaling in the nervous system, both physiologically and in pathological conditions. Indeed, PI-PLC signaling imbalance seems to be also linked to several brain disorders including epilepsy, movement and behavior disorders, neurodegenerative diseases and, in addition, some PI-PLC subtypes could become potential novel signature genes for high-grade gliomas.

Undisturbed climbing fiber pruning in the cerebellar cortex of CX3 CR1-deficient mice

Pruning, the elimination of excess synapses is a phenomenon of fundamental importance for correct wiring of the central nervous system. The establishment of the cerebellar climbing fiber (CF)-to-Purkinje cell (PC) synapse provides a suitable model to study pruning and pruning-relevant processes during early postnatal development. Until now, the role of microglia in pruning remains under intense investigation. Here, we analyzed migration of microglia into the cerebellar cortex during early postnatal development and their possible contribution to the elimination of CF-to-PC synapses. Microglia enrich in the PC layer at pruning-relevant time points giving rise to the possibility that microglia are actively involved in synaptic pruning. We investigated the contribution of microglial fractalkine (CX₃ CR1) signaling during postnatal development using genetic ablation of the CX₃ CR1 receptor and an in-depth histological analysis of the cerebellar cortex. We found an aberrant migration of microglia into the granule and the molecular layer. By electrophysiological analysis, we show that defective fractalkine signaling and the associated migration deficits neither affect the pruning of excess CFs nor the development of functional parallel fiber and inhibitory synapses with PCs. These findings indicate that CX₃ CR1 signaling is not mandatory for correct cerebellar circuit formation. MAIN POINTS: Ablation of CX₃ CR1 results in a transient migration defect in cerebellar microglia. CX₃ CR1 is not required for functional pruning of cerebellar climbing fibers. Functional inhibitory and parallel fiber synapse development with Purkinje cells is undisturbed in CX₃ CR1-deficient mice.

Refinement of Cerebellar Network Organization by Extracellular Signaling During Development

The cerebellum forms regular neural network structures consisting of a few major types of neurons, such as Purkinje cells, granule cells, and molecular layer interneurons, and receives two major inputs from climbing fibers and mossy fibers. Its regular structures consist of three well-defined layers, with each type of neuron designated to a specific location and forming specific synaptic connections. During the first few weeks of postnatal development in rodents, the cerebellum goes through dynamic changes via proliferation, migration, differentiation, synaptogenesis, and maturation, to create such a network structure. The development of this organized network structure presumably relies on the communication between developing elements in the network, including not only individual neurons, but also their dendrites, axons, and synapses. Therefore, it is reasonable that extracellular signaling via synaptic transmission, secreted molecules, and cell adhesion molecules, plays important roles in cerebellar network development. Although it is not yet clear as to how overall cerebellar development is orchestrated, there is indeed accumulating lines of evidence that extracellular signaling acts toward the development of individual elements in the cerebellar networks. In this article, we introduce what we have learned from many studies regarding the extracellular signaling required for cerebellar network development, including our recent study suggesting the importance of unbiased synaptic inputs from parallel fibers.

Phospholipase C β3 is Required for Climbing Fiber Synapse Elimination in Aldolase C-positive Compartments of the Developing Mouse Cerebellum

In the cerebellum of neonatal mice, multiple climbing fibers (CFs) form excitatory synapses on each Purkinje cell (PC). Only one CF is strengthened in each PC from postnatal day 3 (P3) to P7, whereas the other weaker CFs are eliminated progressively from ∼P7 to ∼P11 (early phase of CF elimination) and from ∼P12 to ∼P17 (late phase of CF elimination). Type 1 metabotropic glutamate receptor (mGluR1) triggers a canonical pathway in PCs for the late phase of CF elimination. Among downstream signaling molecules of mGluR1, phospholipase C β3 (PLCβ3) and β4 (PLCβ4) are expressed complementarily in PCs of aldolase C (Aldoc)-positive (+) and Aldoc-negative (-) cerebellar compartments, respectively. PLCβ4 is reported to mediate the late phase of CF elimination in the anterior half of the cerebellar vermis which corresponds to the Aldoc (-) region. However, roles of PLCβ3 and Aldoc in CF synapse elimination are unknown. Here, we investigated CF innervation of PCs in Aldoc-tdTomato knock-in mice that underwent lentivirus-mediated knockdown (KD) of PLCβ3 in PCs during postnatal development. By recording CF-mediated excitatory postsynaptic currents from PCs and immunostaining CF synaptic terminals, we found that significantly higher percentage of PCs with PLCβ3-KD remained multiply innervated by CFs in Aldoc (+) compartments after P12, which was accompanied by impaired elimination of somatic CF synapses and reduced dendritic CF translocation. In contrast, deletion of Aldoc had no effect on CF synapse elimination. These results suggest that PLCβ3 is required for the late phase of CF elimination in Aldoc (+) PCs.

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.

Developmental synapse remodeling in the cerebellum and visual thalamus

Functional neural circuits of mature animals are shaped during postnatal development by eliminating early-formed redundant synapses and strengthening of necessary connections. In the nervous system of newborn animals, redundant synapses are only transient features of the circuit. During subsequent postnatal development, some synapses are strengthened whereas other redundant connections are weakened and eventually eliminated. In this review, we introduce recent studies on the mechanisms of developmental remodeling of climbing fiber-to-Purkinje cell synapses in the cerebellum and synapses from the retina to neurons in the dorsal lateral geniculate nucleus of the visual thalamus (retinogeniculate synapses). These are the two representative models of developmental synapse remodeling in the brain and they share basic principles, including dependency on neural activity. However, recent studies have disclosed that, in several respects, the two models use different molecules and strategies to establish mature synaptic connectivity. We describe similarities and differences between the two models and discuss remaining issues to be tackled in the future in order to understand the general schemes of developmental synapse remodeling.

Towards an Understanding of Synapse Formation

Synapses are intercellular junctions specialized for fast, point-to-point information transfer from a presynaptic neuron to a postsynaptic cell. At a synapse, a presynaptic terminal secretes neurotransmitters via a canonical release machinery, while a postsynaptic specialization senses neurotransmitters via diverse receptors. Synaptic junctions are likely organized by trans-synaptic cell-adhesion molecules (CAMs) that bidirectionally orchestrate synapse formation, restructuring, and elimination. Many candidate synaptic CAMs were described, but which CAMs are central actors and which are bystanders remains unclear. Moreover, multiple genes encoding synaptic CAMs were linked to neuropsychiatric disorders, but the mechanisms involved are unresolved. Here, I propose that engagement of multifarious synaptic CAMs produces parallel trans-synaptic signals that mediate the establishment, organization, and plasticity of synapses, thereby controlling information processing by neural circuits. Among others, this hypothesis implies that synapse formation can be understood in terms of inter- and intracellular signaling, and that neuropsychiatric disorders involve an impairment in such signaling.

Elimination of all redundant climbing fiber synapses requires granule cells in the postnatal cerebellum

Different afferent synapse populations interact to control the specificity of connections during neuronal circuit maturation. The elimination of all but one climbing-fiber onto each Purkinje cell during the development of the cerebellar cortex is a particularly well studied example of synaptic refinement. The suppression of granule cell precursors by X irradiation during postnatal days 4 to 7 prevents this synaptic refinement, indicating a critical role for granule cells. Several studies of cerebellar development have suggested that synapse elimination has a first phase which is granule cell-independent and a second phase which is granule cell-dependent. In this study, we show that sufficiently-strong irradiation restricted to postnatal days 5 or 6 completely abolishes climbing fiber synaptic refinement, leaving the olivo-cerebellar circuit in its immature configuration in the adult, with up to 5 climbing fibers innervating the Purkinje cell in some cases. This implies that the putative early phase of climbing fiber synapse elimination can be blocked by irradiation-induced granule cell loss if this loss is sufficiently large, and thus indicates that the entire process of climbing fiber synapse elimination requires the presence of an adequate number of granule cells. The specific critical period for this effect appears to be directly related to the timing of Purkinje cell and granule cell development in different cerebellar lobules, indicating a close, spatiotemporal synchrony between granule-cell development and olivo-cerebellar synaptic maturation.

Roles for neuronal and glial autophagy in synaptic pruning during development

The dendritic protrusions known as spines represent the primary postsynaptic location for excitatory synapses. Dendritic spines are critical for many synaptic functions, and their formation, modification, and turnover are thought to be important for mechanisms of learning and memory. At many excitatory synapses, dendritic spines form during the early postnatal period, and while many spines are likely being formed and removed throughout life, the net number are often gradually "pruned" during adolescence to reach a stable level in the adult. In neurodevelopmental disorders, spine pruning is disrupted, emphasizing the importance of understanding its governing processes. Autophagy, a process through which cytosolic components and organelles are degraded, has recently been shown to control spine pruning in the mouse cortex, but the mechanisms through which autophagy acts remain obscure. Here, we draw on three widely studied prototypical synaptic pruning events to focus on two governing principles of spine pruning: 1) activity-dependent synaptic competition and 2) non-neuronal contributions. We briefly review what is known about autophagy in the central nervous system and its regulation by metabolic kinases. We propose a model in which autophagy in both neurons and non-neuronal cells contributes to spine pruning, and how other processes that regulate spine pruning could intersect with autophagy. We further outline future research directions to address outstanding questions on the role of autophagy in synaptic pruning.

Purkinje Cell Signaling Deficits in Animal Models of Ataxia

Purkinje cell (PC) dysfunction or degeneration is the most frequent finding in animal models with ataxic symptoms. Mutations affecting intrinsic membrane properties can lead to ataxia by altering the firing rate of PCs or their firing pattern. However, the relationship between specific firing alterations and motor symptoms is not yet clear, and in some cases PC dysfunction precedes the onset of ataxic signs. Moreover, a great variety of ionic and synaptic mechanisms can affect PC signaling, resulting in different features of motor dysfunction. Mutations affecting Na⁺ channels (Na_V1.1, Na_V1.6, Na_Vβ4, Fgf14 or Rer1) reduce the firing rate of PCs, mainly via an impairment of the Na⁺ resurgent current. Mutations that reduce Kv3 currents limit the firing rate frequency range. Mutations of Kv1 channels act mainly on inhibitory interneurons, generating excessive GABAergic signaling onto PCs, resulting in episodic ataxia. Kv4.3 mutations are responsible for a complex syndrome with several neurologic dysfunctions including ataxia. Mutations of either Cav or BK channels have similar consequences, consisting in a disruption of the firing pattern of PCs, with loss of precision, leading to ataxia. Another category of pathogenic mechanisms of ataxia regards alterations of synaptic signals arriving at the PC. At the parallel fiber (PF)-PC synapse, mutations of glutamate delta-2 (GluD2) or its ligand Crbl1 are responsible for the loss of synaptic contacts, abolishment of long-term depression (LTD) and motor deficits. At the same synapse, a correct function of metabotropic glutamate receptor 1 (mGlu1) receptors is necessary to avoid ataxia. Failure of climbing fiber (CF) maturation and establishment of PC mono-innervation occurs in a great number of mutant mice, including mGlu1 and its transduction pathway, GluD2, semaphorins and their receptors. All these models have in common the alteration of PC output signals, due to a variety of mechanisms affecting incoming synaptic signals or the way they are processed by the repertoire of ionic channels responsible for intrinsic membrane properties. Although the PC is a final common pathway of ataxia, the link between specific firing alterations and neurologic symptoms has not yet been systematically studied and the alterations of the cerebellar contribution to motor signals are still unknown.

Maturation, Refinement, and Serotonergic Modulation of Cerebellar Cortical Circuits in Normal Development and in Murine Models of Autism

The formation of the complex cerebellar cortical circuits follows different phases, with initial synaptogenesis and subsequent processes of refinement guided by a variety of mechanisms. The regularity of the cellular and synaptic organization of the cerebellar cortex allowed detailed studies of the structural plasticity mechanisms underlying the formation of new synapses and retraction of redundant ones. For the attainment of the monoinnervation of the Purkinje cell by a single climbing fiber, several signals are involved, including electrical activity, contact signals, homosynaptic and heterosynaptic interaction, calcium transients, postsynaptic receptors, and transduction pathways. An important role in this developmental program is played by serotonergic projections that, acting on temporally and spatially regulated postsynaptic receptors, induce and modulate the phases of synaptic formation and maturation. In the adult cerebellar cortex, many developmental mechanisms persist but play different roles, such as supporting synaptic plasticity during learning and formation of cerebellar memory traces. A dysfunction at any stage of this process can lead to disorders of cerebellar origin, which include autism spectrum disorders but are not limited to motor deficits. Recent evidence in animal models links impairment of Purkinje cell function with autism-like symptoms including sociability deficits, stereotyped movements, and interspecific communication by vocalization.

Retrograde BDNF to TrkB signaling promotes synapse elimination in the developing cerebellum

Elimination of early-formed redundant synapses during postnatal development is essential for functional neural circuit formation. Purkinje cells (PCs) in the neonatal cerebellum are innervated by multiple climbing fibers (CFs). A single CF is strengthened whereas the other CFs are eliminated in each PC dependent on postsynaptic activity in PC, but the underlying mechanisms are largely unknown. Here, we report that brain-derived neurotrophic factor (BDNF) from PC facilitates CF synapse elimination. By PC-specific deletion of BDNF combined with knockdown of BDNF receptors in CF, we show that BDNF acts retrogradely on TrkB in CFs, and facilitates elimination of CF synapses from PC somata during the third postnatal week. We also show that BDNF shares signaling pathway with metabotropic glutamate receptor 1, a key molecule that triggers a canonical pathway for CF synapse elimination. These results indicate that unlike other synapses, BDNF mediates punishment signal for synapse elimination in the developing cerebellum.

During development, synapses are selectively strengthened or eliminated by activity-dependent competition. Here, the authors show that BDNF-TrkB retrograde signaling is a “punishment” signal that leads to elimination of climbing fiber-onto-Purkinje cell synapses in the developing cerebellum.

Comparison of the role of metabotropic glutamate receptor subtype 1 in developmental refinement of neuronal connectivity between the cerebellum and the sensory thalamus

Developmental refinement of neuronal connectivity is crucial for proper brain function. In the early phase of development, input fibers arrive at their target areas guided by specific molecular cues and form abundant immature synapses. Then, functionally important synapses are preserved and strengthened by neural activity while unnecessary synapses are eliminated. Afferent synapses in the sensory thalamus, such as from retina to lateral geniculate nucleus, and climbing fiber (CF)-Purkinje cell (PC) synapses in the cerebellum are valuable models for studying this developmental refinement of synaptic connectivity because only a limited number of input fibers innervate a given postsynaptic thalamocortical (TC) neuron or PC. The metabotropic glutamate receptor subtype 1 (mGluR1) is required for the refinement of both afferent-TC neuron and CF-PC synapses. However, mGluR1 functions differently at these synapses. While mGluR1 is critical for elimination of surplus CF-PC synapses in the cerebellum, retinogeniculate synapses require mGluR1 for maintenance of mature connectivity.

A positive feedback loop linking enhanced mGluR function and basal calcium in spinocerebellar ataxia type 2

Metabotropic glutamate receptor 1 (mGluR1) function in Purkinje neurons (PNs) is essential for cerebellar development and for motor learning and altered mGluR1 signaling causes ataxia. Downstream of mGluR1, dysregulation of calcium homeostasis has been hypothesized as a key pathological event in genetic forms of ataxia but the underlying mechanisms remain unclear. We find in a spinocerebellar ataxia type 2 (SCA2) mouse model that calcium homeostasis in PNs is disturbed across a broad range of physiological conditions. At parallel fiber synapses, mGluR1-mediated excitatory postsynaptic currents (EPSCs) and associated calcium transients are increased and prolonged in SCA2 PNs. In SCA2 PNs, enhanced mGluR1 function is prevented by buffering [Ca²⁺] at normal resting levels while in wildtype PNs mGluR1 EPSCs are enhanced by elevated [Ca²⁺]. These findings demonstrate a deleterious positive feedback loop involving elevated intracellular calcium and enhanced mGluR1 function, a mechanism likely to contribute to PN dysfunction and loss in SCA2.

DOI:http://dx.doi.org/10.7554/eLife.26377.001

Presynaptic Membrane Receptors Modulate ACh Release, Axonal Competition and Synapse Elimination during Neuromuscular Junction Development

During the histogenesis of the nervous system a lush production of neurons, which establish an excessive number of synapses, is followed by a drop in both neurons and synaptic contacts as maturation proceeds. Hebbian competition between axons with different activities leads to the loss of roughly half of the neurons initially produced so connectivity is refined and specificity gained. The skeletal muscle fibers in the newborn neuromuscular junction (NMJ) are polyinnervated but by the end of the competition, 2 weeks later, the NMJ are innervated by only one axon. This peripheral synapse has long been used as a convenient model for synapse development. In the last few years, we have studied transmitter release and the local involvement of the presynaptic muscarinic acetylcholine autoreceptors (mAChR), adenosine autoreceptors (AR) and trophic factor receptors (TFR, for neurotrophins and trophic cytokines) during the development of NMJ and in the adult. This review article brings together previously published data and proposes a molecular background for developmental axonal competition and loss. At the end of the first week postnatal, these receptors modulate transmitter release in the various nerve terminals on polyinnervated NMJ and contribute to axonal competition and synapse elimination.

Type-1 metabotropic glutamate receptor signaling in cerebellar Purkinje cells in health and disease

The cerebellum is a brain structure involved in coordination, control, and learning of movements, as well as certain aspects of cognitive function. Purkinje cells are the sole output neurons from the cerebellar cortex and therefore play crucial roles in the overall function of the cerebellum. The type-1 metabotropic glutamate receptor (mGluR1) is a key “hub” molecule that is critically involved in the regulation of synaptic wiring, excitability, synaptic response, and synaptic plasticity of Purkinje cells. In this review, we aim to highlight how mGluR1 controls these events in Purkinje cells. We also describe emerging evidence that altered mGluR1 signaling in Purkinje cells underlies cerebellar dysfunctions in several clinically relevant mouse models of human ataxias.

LTD-like molecular pathways in developmental synaptic pruning

In long-term depression (LTD) at synapses in the adult brain, synaptic strength is reduced in an experience-dependent manner. LTD thus provides a cellular mechanism for information storage in some forms of learning. A similar activity-dependent reduction in synaptic strength also occurs in the developing brain and there provides an essential step in synaptic pruning and the postnatal development of neural circuits. Here we review evidence suggesting that LTD and synaptic pruning share components of their underlying molecular machinery and may thus represent two developmental stages of the same type of synaptic modulation that serve different, but related, functions in neural circuit plasticity. We also assess the relationship between LTD and synaptic pruning in the context of recent findings of LTD dysregulation in several mouse models of autism spectrum disorder (ASD) and discuss whether LTD deficits can indicate impaired pruning processes that are required for proper brain development.

Cellular and circuit mechanisms underlying spinocerebellar ataxias

Degenerative ataxias are a common form of neurodegenerative disease that affect about 20 individuals per 100,000. The autosomal dominant spinocerebellar ataxias (SCAs) are caused by a variety of protein coding mutations (single nucleotide changes, deletions and expansions) in single genes. Affected genes encode plasma membrane and intracellular ion channels, membrane receptors, protein kinases, protein phosphatases and proteins of unknown function. Although SCA-linked genes are quite diverse they share two key features: first, they are highly, although not exclusively, expressed in cerebellar Purkinje neurons (PNs), and second, when mutated they lead ultimately to the degeneration of PNs. In this review we summarize ataxia-related changes in PN neurophysiology that have been observed in various mouse knockout lines and in transgenic models of human SCA. We also highlight emerging evidence that altered metabotropic glutamate receptor signalling and disrupted calcium homeostasis in PNs form a common, early pathophysiological mechanism in SCAs. Together these findings indicate that aberrant calcium signalling and profound changes in PN neurophysiology precede PN cell loss and are likely to lead to cerebellar circuit dysfunction that explains behavioural signs of ataxia characteristic of the disease.

Cerebellar plasticity and motor learning deficits in a copy-number variation mouse model of autism

A common feature of autism spectrum disorder (ASD) is the impairment of motor control and learning, occurring in a majority of children with autism, consistent with perturbation in cerebellar function. Here we report alterations in motor behaviour and cerebellar synaptic plasticity in a mouse model (patDp/+) for the human 15q11-13 duplication, one of the most frequently observed genetic aberrations in autism. These mice show ASD-resembling social behaviour deficits. We find that in patDp/+ mice delay eyeblink conditioning--a form of cerebellum-dependent motor learning--is impaired, and observe deregulation of a putative cellular mechanism for motor learning, long-term depression (LTD) at parallel fibre-Purkinje cell synapses. Moreover, developmental elimination of surplus climbing fibres--a model for activity-dependent synaptic pruning--is impaired. These findings point to deficits in synaptic plasticity and pruning as potential causes for motor problems and abnormal circuit development in autism.

Global scaling down of excitatory postsynaptic responses in cerebellar Purkinje cells impairs developmental synapse elimination

Synapse elimination is crucial for precise neural circuit formation during postnatal development. We examined how relative differences in synaptic strengths among competing inputs and/or absolute synaptic strengths contribute to climbing fiber (CF) to Purkinje cell (PC) synapse elimination in the cerebellum. We generated mice with PC-selective deletion of stargazin (TARP γ-2), the major AMPA receptor auxiliary subunit in PCs (γ-2 PC-KO mice). Whereas relative differences between "strong" and "weak" CF-mediated postsynaptic response are preserved, absolute strengths of CF inputs are scaled down globally in PCs of γ-2 PC-KO mice. Although the early phase of CF elimination is normal, dendritic translocation of the strongest CF and the late phase of CF elimination that requires Ca(2+)-dependent activation of Arc/Arg3.1 in PCs are both impaired in γ-2 PC-KO mice. We conclude that, although relative differences in CF synaptic inputs are initially essential, proper synaptic scaling is crucial for accomplishing CF synapse elimination.

The synaptic targeting of mGluR1 by its carboxyl-terminal domain is crucial for cerebellar function

The metabotropic glutamate receptor subtype 1 (mGluR1, Grm1) in cerebellar Purkinje cells (PCs) is essential for motor coordination and motor learning. At the synaptic level, mGluR1 has a critical role in long-term synaptic depression (LTD) at parallel fiber (PF)-PC synapses, and in developmental elimination of climbing fiber (CF)-PC synapses. mGluR1a, a predominant splice variant in PCs, has a long carboxyl (C)-terminal domain that interacts with Homer scaffolding proteins. Cerebellar roles of the C-terminal domain at both synaptic and behavior levels remain poorly understood. To address this question, we introduced a short variant, mGluR1b, which lacks this domain into PCs of mGluR1-knock-out (KO) mice (mGluR1b-rescue mice). In mGluR1b-rescue mice, mGluR1b showed dispersed perisynaptic distribution in PC spines. Importantly, mGluR1b-rescue mice exhibited impairments in inositol 1,4,5-trisphosphate receptor (IP3R)-mediated Ca(2+) release, CF synapse elimination, LTD induction, and delay eyeblink conditioning: they showed normal transient receptor potential canonical (TRPC) currents and normal motor coordination. In contrast, PC-specific rescue of mGluR1a restored all cerebellar defects of mGluR1-KO mice. We conclude that the long C-terminal domain of mGluR1a is required for the proper perisynaptic targeting of mGluR1, IP3R-mediated Ca(2+) release, CF synapse elimination, LTD, and motor learning, but not for TRPC currents and motor coordination.

Type 1 inositol trisphosphate receptor regulates cerebellar circuits by maintaining the spine morphology of purkinje cells in adult mice

The structural maintenance of neural circuits is critical for higher brain functions in adulthood. Although several molecules have been identified as regulators for spine maintenance in hippocampal and cortical neurons, it is poorly understood how Purkinje cell (PC) spines are maintained in the mature cerebellum. Here we show that the calcium channel type 1 inositol trisphosphate receptor (IP3R1) in PCs plays a crucial role in controlling the maintenance of parallel fiber (PF)-PC synaptic circuits in the mature cerebellum in vivo. Significantly, adult mice lacking IP3R1 specifically in PCs (L7-Cre;Itpr1(flox/flox)) showed dramatic increase in spine density and spine length of PCs, despite having normal spines during development. In addition, the abnormally rearranged PF-PC synaptic circuits in mature cerebellum caused unexpectedly severe ataxia in adult L7-Cre;Itpr1(flox/flox) mice. Our findings reveal a specific role for IP3R1 in PCs not only as an intracellular mediator of cerebellar synaptic plasticity induction, but also as a critical regulator of PF-PC synaptic circuit maintenance in the mature cerebellum in vivo; this mechanism may underlie motor coordination and learning in adults.

Arc/Arg3.1 is a postsynaptic mediator of activity-dependent synapse elimination in the developing cerebellum

Neural circuits are shaped by activity-dependent elimination of redundant synapses during postnatal development. In many systems, postsynaptic activity is known to be crucial, but the precise mechanisms remain elusive. Here, we report that the immediate early gene Arc/Arg3.1 mediates elimination of surplus climbing fiber (CF) to Purkinje cell (PC) synapses in the developing cerebellum. CF synapse elimination was accelerated when activity of channelrhodopsin-2-expressing PCs was elevated by 2-day photostimulation. This acceleration was suppressed by PC-specific knockdown of either the P/Q-type voltage-dependent Ca(2+) channels (VDCCs) or Arc. PC-specific Arc knockdown had no appreciable effect until around postnatal day 11 but significantly impaired CF synapse elimination thereafter, leaving redundant CF terminals on PC somata. The effect of Arc knockdown was occluded by simultaneous knockdown of P/Q-type VDCCs in PCs. We conclude that Arc mediates the final stage of CF synapse elimination downstream of P/Q-type VDCCs by removing CF synapses from PC somata.

Disruption of cerebellar microzonal organization in GluD2 (GluRδ2) knockout mouse

Cerebellar cortex has an elaborate rostrocaudal organization comprised of numerous microzones. Purkinje cells (PCs) in the same microzone show synchronous activity of complex spikes (CSs) evoked by excitatory inputs from climbing fibers (CFs) that arise from neurons in the inferior olive (IO). The synchronous CS activity is considered to depend on electrical coupling among IO neurons and anatomical organization of the olivo-cerebellar projection. To determine how the CF–PC wiring contributes to the formation of microzone, we examined the synchronous CS activities between neighboring PCs in the glutamate receptor δ2 knockout (GluD2 KO) mouse in which exuberant surplus CFs make ectopic innervations onto distal dendrites of PCs. We performed in vivo two-photon calcium imaging for PC populations to detect CF inputs. Neighboring PCs in GluD2 KO mice showed higher synchrony of calcium transients than those in wild-type (control) mice. Moreover, the synchrony in GluD2 KO mice hardly declined with mediolateral separation between PCs up to ~200 μm, which was in marked contrast to the falloff of the synchrony in control mice. The enhanced synchrony was only partially affected by the blockade of gap junctional coupling. On the other hand, transverse CF collaterals in GluD2 KO mice extended beyond the border of microzone and formed locally clustered ectopic synapses onto dendrites of neighboring PCs. Furthermore, PCs in GluD2 KO mice exhibited clustered firing (Cf), the characteristic CF response that was not found in PCs of wild-type mice. Importantly, Cf was often associated with localized calcium transients in distal dendrites of PCs, which are likely to contribute to the enhanced synchrony of calcium signals in GluD2 KO mice. Thus, our results indicate that CF signals in GluD2 KO mice propagate across multiple microzones, and that proper formation of longitudinal olivo-cerebellar projection is essential for the spatiotemporal organization of CS activity in the cerebellum.

Impaired presynaptic function and elimination of synapses at premature stages during postnatal development of the cerebellum in the absence of CALEB (CSPG5/neuroglycan C)

Chicken acidic leucine-rich EGF-like domain-containing brain protein (CALEB), also known as chondroitin sulfate proteoglycan (CSPG)5 or neuroglycan C, is a neural chondroitin sulfate-containing and epidermal growth factor (EGF)-domain-containing transmembrane protein that is implicated in synaptic maturation. Here, we studied the role of CALEB within the developing cerebellum. Adult CALEB-deficient mice displayed impaired motor coordination in Rota-Rod experiments. Analysis of the neuronal connectivity of Purkinje cells by patch-clamp recordings demonstrated impairments of presynaptic maturation of inhibitory synapses. GABAergic synapses on Purkinje cells revealed decreased evoked amplitudes, altered paired-pulse facilitation and reduced depression after repetitive stimulation at early postnatal but not at mature stages. Furthermore, the elimination of supernumerary climbing fiber synapses on Purkinje cells was found to occur at earlier developmental stages in the absence of CALEB. For example, at postnatal day 8 in wild-type mice, 54% of Purkinje cells had three or more climbing fiber synapses in contrast to mutants where this number was decreased to less than 25%. The basic properties of the climbing fiber Purkinje cell synapse remained unaffected. Using Sholl analysis of dye-injected Purkinje cells we revealed that the branching pattern of the dendritic tree of Purkinje cells was not impaired in CALEB-deficient mice. The alterations observed by patch-clamp recordings correlated with a specific pattern and timing of expression of CALEB in Purkinje cells, i.e. it is dynamically regulated during development from a high chondroitin sulfate-containing form to a non-chondroitin sulfate-containing form. Thus, our results demonstrated an involvement of CALEB in the presynaptic differentiation of cerebellar GABAergic synapses and revealed a new role for CALEB in synapse elimination in Purkinje cells.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

Synapse elimination in the developing cerebellum

Neural circuits in neonatal animals contain numerous redundant synapses that are functionally immature. During the postnatal period, unnecessary synapses are eliminated while functionally important synapses become stronger and mature. The climbing fiber (CF) to the Purkinje cell (PC) synapse is a representative model for the analysis of postnatal refinement of neuronal circuits in the central nervous system. PCs are initially innervated by multiple CFs with similar strengths around postnatal day 3 (P3). Only a single CF is selectively strengthened during P3–P7 (functional differentiation), and the strengthened CF undergoes translocation from soma to dendrites of PCs from P9 on (dendritic translocation). Following the functional differentiation, supernumerary CF synapses on the soma are eliminated, which proceeds in two distinct phases: the early phase from P7 to around P11 and the late phase from around P12 to P17. Here, we review our current understanding of cellular and molecular mechanisms of CF synapse elimination in the developing cerebellum.

Serotonergic control of Purkinje cell maturation and climbing fibre elimination by 5-HT3 receptors in the juvenile mouse cerebellum

Functional serotonin 3 (5-HT3) receptors are transiently expressed by cerebellar granule cells during early postnatal development, where they modulate short-term synaptic plasticity at the parallel fibre-Purkinje cell synapse. Here, we show that serotonin controls maturation of Purkinje cells in the mouse cerebellum. The 5-HT3 receptors regulate morphological maturation of Purkinje cells during early postnatal development, and this effect is mediated by the glycoprotein reelin. Using whole-cell patch-clamp recordings we also investigated physiological development of Purkinje cells in 5-HT3A receptor knockout mice during early postnatal development, and found abnormal physiological maturation, characterized by a more depolarized resting membrane potential, an increased input resistance and the ability to fire action potentials upon injection of a depolarizing current at an earlier age. Furthermore, short-term synaptic plasticity was impaired at both the parallel fibre-Purkinje cell and the climbing fibre-Purkinje cell synapses, and both the amplitude and the frequency of spontaneous miniature events recorded from Purkinje cells were increased. The expedited morphological and physiological maturation affects the whole cerebellar cortical network, as indicated by delayed climbing fibre elimination in 5-HT3A receptor knockout mice. There was no difference between wild-type and 5-HT3A receptor knockout mice in any of the morphological or physiological properties described above at later ages, indicating a specific time window during which serotonin regulates postnatal development of the cerebellum via 5-HT3 receptors expressed by granule cells.

Architecture and development of olivocerebellar circuit topography

The cerebellum has a simple tri-laminar structure that is comprised of relatively few cell types. Yet, its internal micro-circuitry is anatomically, biochemically, and functionally complex. The most striking feature of cerebellar circuit complexity is its compartmentalized topography. Each cell type within the cerebellar cortex is organized into an exquisite map; molecular expression patterns, dendrite projections, and axon terminal fields divide the medial-lateral axis of the cerebellum into topographic sagittal zones. Here, we discuss the mechanisms that establish zones and highlight how gene expression and neural activity contribute to cerebellar pattern formation. We focus on the olivocerebellar system because its developmental mechanisms are becoming clear, its topographic termination patterns are very precise, and its contribution to zonal function is debated. This review deconstructs the architecture and development of the olivocerebellar pathway to provide an update on how brain circuit maps form and function.

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.

Organotypic coculture preparation for the study of developmental synapse elimination in mammalian brain

We developed an organotypic coculture preparation allowing fast and efficient identification of molecules that regulate developmental synapse elimination in the mammalian brain. This coculture consists of a cerebellar slice obtained from rat or mouse at postnatal day 9 (P9) or P10 and a medullary explant containing the inferior olive dissected from rat at embryonic day 15. We verified that climbing fibers (CFs), the axons of inferior olivary neurons, formed functional synapses onto Purkinje cells (PCs) in the cerebellum of cocultures. PCs were initially reinnervated by multiple CFs with similar strengths. Surplus CFs were eliminated subsequently, and the remaining CFs became stronger. These changes are similar to those occurring in developing cerebellum in vivo. Importantly, the changes in CF innervations in cocultures involved the same molecules required for CF synapse elimination in vivo, including NMDA receptor, type 1 metabotropic glutamate receptor and glutamate receptor δ2 (GluRδ2). We demonstrate that gain- and loss-of-function analyses can be efficiently performed by lentiviral-mediated overexpression and RNAi-induced knockdown of GluRδ2. Using this approach, we identified neuroligin-2 as a novel molecule that promotes CF synapse elimination in postsynaptic PCs. Thus, our coculture preparation will greatly facilitate the elucidation of molecular mechanisms of synapse elimination.