Purkinje cell spinogenesis during architectural rewiring in the mature cerebellum

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.

GluD2 Endows Parallel Fiber-Purkinje Cell Synapses with a High Regenerative Capacity

Although injured axons usually do not regenerate in the adult CNS, parallel fibers (PFs) regenerate synaptic connections onto cerebellar Purkinje cells (PCs). In this study, we investigated the role of GluD2 in this regenerative process after PF transection using GluD2-knock-out (KO) mice. All dendritic spines on distal dendrites were innervated by PFs in sham-operated wild-type controls, whereas one-third were devoid of innervation in GluD2-KO mice. In both genotypes, a steep drop in the number of PF synapses occurred with a reciprocal surge in the number of free spines on postlesion day 1, when the PF territory aberrantly expanded toward the proximal dendrites. In wild-type mice, the territory and number of PF synapses were nearly fully restored to normal on postlesion day 7, although PF density remained low. Moreover, presynaptic and postsynaptic elements were markedly enlarged, and the PF terminal-to-PC spine contact ratio increased from 1:1 to 1:2 at most synapses. On postlesion day 30, the size and contact ratio of PF synapses returned to sham-operated control values and PF density recovered through the sprouting and elongation of PF collaterals. However, GluD2-KO mice showed neither a hypertrophic response nor territorial restoration 7 d postlesion, nor the recovery of PF axons or synapses on postlesion day 30. This suggests that PF wiring regenerates initially by inducing hypertrophic responses in surviving synaptic elements (hypertrophic phase), followed by collateral formation by PF axons and retraction of PF synapses (remodeling phase). Without GluD2, no transition to these regenerative phases occurs.

SIGNIFICANCE STATEMENT

The glutamate receptor GluD2 expressed at parallel fiber (PF)-Purkinje cell (PC) synapses regulates the formation and maintenance of the synapses. To investigate the role of GluD2 in their extraordinarily high regenerative capacity, the process after surgical transection of PFs was compared between wild-type and GluD2-knock-out mice. We discovered that, in wild-type mice, PF synapses regenerate initially by inducing hypertrophic responses in surviving synaptic elements, and then by sprouting and elongation of PF collaterals. Subsequently, hypertrophied PF synapses remodel into compact synapses. In GluD2-knock-out mice, PF wiring remains in the degenerative phase, showing neither a hypertrophic response nor recovery of PF axons or synapses. Our finding thus highlights that synaptic connection in the adult brain can regenerate with aid of GluD2.

Lesion-induced and activity-dependent structural plasticity of Purkinje cell dendritic spines in cerebellar vermis and hemisphere

Neuroplasticity allows the brain to encode experience and learn behaviors, and also to re-acquire lost functions after damage. The cerebellum is a suitable structure to address this topic because of its strong involvement in learning processes and compensation of lesion-induced deficits. This study was aimed to characterize the effects of a hemicerebellectomy (HCb) combined or not with the exposition to environmental enrichment (EE) on dendritic spine density and size in Purkinje cell proximal and distal compartments of cerebellar vermian and hemispherical regions. Male Wistar rats were housed in enriched or standard environments from the 21st post-natal day (pnd) onwards. At the 75th pnd, rats were submitted to HCb or sham lesion. Neurological symptoms and spatial performance in the Morris water maze were evaluated. At the end of testing, morphological analyses assessed dendritic spine density, area, length, and head diameter on vermian and hemispherical Purkinje cells. All hemicerebellectomized (HCbed) rats showed motor compensation, but standard-reared HCbed animals exhibited cognitive impairment that was almost completely compensated in enriched HCbed rats. The standard-reared HCbed rats showed decreased density with augmented size of Purkinje cell spines in the vermis, and augmented both density and size in the hemisphere. Enriched HCbed rats almost completely maintained the spine density and size induced by EE. Both lesion-induced and activity-dependent cerebellar plastic changes may be interpreted as "beneficial" brain reactions, aimed to support behavioral performance rescuing.

The Spontaneous Ataxic Mouse Mutant Tippy is Characterized by a Novel Purkinje Cell Morphogenesis and Degeneration Phenotype

This study represents the first detailed analysis of the spontaneous neurological mouse mutant, tippy, uncovering its unique cerebellar phenotype. Homozygous tippy mutant mice are small, ataxic, and die around weaning. Although the cerebellum shows grossly normal foliation, tippy mutants display a complex cerebellar Purkinje cell phenotype consisting of abnormal dendritic branching with immature spine features and patchy, non-apoptotic cell death that is associated with widespread dystrophy and degeneration of the Purkinje cell axons throughout the white matter, the cerebellar nuclei, and the vestibular nuclei. Moderate anatomical abnormalities of climbing fiber innervation of tippy mutant Purkinje cells were not associated with changes in climbing fiber-EPSC amplitudes. However, decreased ESPC amplitudes were observed in response to parallel fiber stimulation and correlated well with anatomical evidence for patchy dark cell degeneration of Purkinje cell dendrites in the molecular layer. The data suggest that the Purkinje neurons are a primary target of the tippy mutation. Furthermore, we hypothesize that the Purkinje cell axonal pathology together with disruptions in the balance of climbing fiber and parallel fiber-Purkinje cell input in the cerebellar cortex underlie the ataxic phenotype in these mice. The constellation of Purkinje cell dendritic malformation and degeneration phenotypes in tippy mutants is unique and has not been reported in any other neurologic mutant. Fine mapping of the tippy mutation to a 2.1 MB region of distal chromosome 9, which does not encompass any gene previously implicated in cerebellar development or neuronal degeneration, confirms that the tippy mutation identifies novel biology and gene function.

Long-term in vivo time-lapse imaging of synapse development and plasticity in the cerebellum

Synapses are continuously formed and eliminated throughout life in the mammalian brain, and emerging evidence suggests that this structural plasticity underlies experience-dependent changes of brain functions such as learning and long-term memory formation. However, it is generally difficult to understand how the rewiring of synaptic circuitry observed in vivo eventually relates to changes in animal's behavior. This is because afferent/efferent connections and local synaptic circuitries are very complicated in most brain regions, hence it is largely unclear how sensorimotor information is conveyed, integrated, and processed through a brain region that is imaged. The cerebellar cortex provides a particularly useful model to challenge this problem because of its simple and well-defined synaptic circuitry. However, owing to the technical difficulty of chronic in vivo imaging in the cerebellum, it remains unclear how cerebellar neurons dynamically change their structures over a long period of time. Here, we showed that the commonly used method for neocortical in vivo imaging was not ideal for long-term imaging of cerebellar neurons, but simple optimization of the procedure significantly improved the success rate and the maximum time window of chronic imaging. The optimized method can be used in both neonatal and adult mice and allows time-lapse imaging of cerebellar neurons for more than 5 mo in ∼80% of animals. This method allows vital observation of dynamic cellular processes such as developmental refinement of synaptic circuitry as well as long-term changes of neuronal structures in adult cerebellum under longitudinal behavioral manipulations.

Eph receptors are involved in the activity-dependent synaptic wiring in the mouse cerebellar cortex

Eph receptor tyrosine kinases are involved in many cellular processes. In the developing brain, they act as migratory and cell adhesive cues while in the adult brain they regulate dendritic spine plasticity. Here we show a new role for Eph receptor signalling in the cerebellar cortex. Cerebellar Purkinje cells are innervated by two different excitatory inputs. The climbing fibres contact the proximal dendritic domain of Purkinje cells, where synapse and spine density is low; the parallel fibres contact the distal dendritic domain, where synapse and spine density is high. Interestingly, Purkinje cells have the intrinsic ability to generate a high number of spines over their entire dendritic arborisations, which can be innervated by the parallel fibres. However, the climbing fibre input continuously exerts an activity-dependent repression on parallel fibre synapses, thus confining them to the distal Purkinje cell dendritic domain. Such repression persists after Eph receptor activation, but is overridden by Eph receptor inhibition with EphA4/Fc in neonatal cultured cerebellar slices as well as mature acute cerebellar slices, following in vivo infusion of the EphA4/Fc inhibitor and in EphB receptor-deficient mice. When electrical activity is blocked in vivo by tetrodotoxin leading to a high spine density in Purkinje cell proximal dendrites, stimulation of Eph receptor activation recapitulates the spine repressive effects of climbing fibres. These results suggest that Eph receptor signalling mediates the repression of spine proliferation induced by climbing fibre activity in Purkinje cell proximal dendrites. Such repression is necessary to maintain the correct architecture of the cerebellar cortex.

Ablation of glutamate receptor GluRδ2 in adult Purkinje cells causes multiple innervation of climbing fibers by inducing aberrant invasion to parallel fiber innervation territory

Glutamate receptor GluRδ2 is exclusively expressed in Purkinje cells (PCs) from early development and plays key roles in parallel fiber (PF) synapse formation, elimination of surplus climbing fibers (CFs), long-term depression, motor coordination, and motor learning. To address its role in adulthood, we previously developed a mouse model of drug-induced GluRδ2 ablation in adult PCs (Takeuchi et al., 2005). In that study, we demonstrated an essential role to maintain the connectivity of PF-PC synapses, based on the observation that both mismatching of presynaptic and postsynaptic specializations and disconnection of PF-PC synapses are progressively increased after GluRδ2 ablation. Here, we pursued its role for CF wiring in adult cerebellum. In parallel with the disconnection of PF-PC synapses, ascending CF branches exhibited distal extension to innervate distal dendrites of the target and neighboring PCs. Furthermore, transverse CF branches, a short motile collateral rarely forming synapses in wild-type animals, displayed aberrant mediolateral extension to innervate distal dendrites of neighboring and remote PCs. Consequently, many PCs were wired by single main CF and other surplus CFs innervating a small part of distal dendrites. Electrophysiological recording further revealed that surplus CF-EPSCs characterized with slow rise time and small amplitude emerged after GluRδ2 ablation, and increased progressively both in number and amplitude. Therefore, GluRδ2 is essential for maintaining CF monoinnervation in adult cerebellum by suppressing aberrant invasion of CF branches to the territory of PF innervation. Thus, GluRδ2 fuels heterosynaptic competition and gives PFs the competitive advantages over CFs throughout the animal's life.

Dendritic spine dynamics

Dendritic spines are the postsynaptic components of most excitatory synapses in the mammalian brain. Spines accumulate rapidly during early postnatal development and undergo a substantial loss as animals mature into adulthood. In past decades, studies have revealed that the number and size of dendritic spines are regulated by a variety of gene products and environmental factors, underscoring the dynamic nature of spines and their importance to brain plasticity. Recently, in vivo time-lapse imaging of dendritic spines in the cerebral cortex suggests that, although spines are highly plastic during development, they are remarkably stable in adulthood, and most of them last throughout life. Therefore, dendritic spines may provide a structural basis for lifelong information storage, in addition to their well-established role in brain plasticity. Because dendritic spines are the key elements for information acquisition and retention, understanding how spines are formed and maintained, particularly in the intact brain, will likely provide fundamental insights into how the brain possesses the extraordinary capacity to learn and to remember.

Axonal competition in the synaptic wiring of the cerebellar cortex during development and in the mature cerebellum

Purkinje cell (PC) dendrites are made by a proximal dendritic domain, which is provided with scattered clusters of spines innervated by a single climbing fiber (CF) and by a distal domain with a high density of spines innervated by parallel fibers (PFs). Following block of electrical activity a spine increase occurs in the proximal domain and the new spines are innervated by the PFs while the number of synaptic contacts formed by the CF is reduced. Also the GABAergic input expands its territory of innervation on the proximal domain, which undergoes a profound restructuring of the glutamate and GABA receptors. Excitatory-like postsynaptic assemblies appear not only on the new spines, but also on the smooth region of the dendrite and both of them may be innervated by GABAergic terminals. In this case GABA receptors coexist with the glutamate receptors leading to the formation of hybrid synapses. In contrast, PF synapses contain solely glutamate receptors. Thus, the expression of glutamate receptors appears to be an intrinsic property of the PC, while the expression of the GABA receptors is induced by the presence of GABAergic terminals. The data highlight an important feature of the CF input; its electrical activity, in addition to inducing a powerful phasic excitation and a tonic inhibition, controls the finer architecture of the cerebellar cortex.

Activity-dependent plasticity of developing climbing fiber-Purkinje cell synapses

Elimination of redundant synapses and strengthening of the surviving ones are crucial steps in the development of the nervous system. Both processes can be readily followed at the climbing fiber to Purkinje cell synapse in the cerebellum. Shortly after birth, around five equally strong climbing fiber synapses are established. Subsequently, one of these five synaptic connections starts to grow in size and synaptic strength, while the others degenerate and eventually disappear. Both the elimination of the redundant climbing fiber synapses and the strengthening of the surviving one depend on a combination of a genetically coded blueprint and synaptic activity. Recently, it has been shown that synaptic activity affects the synaptic strength of developing climbing fibers. Remarkably, the same pattern of paired activity of the presynaptic climbing fiber and the postsynaptic Purkinje cell resulted in strengthening of already "large" climbing fibers and weakening of already "weak" climbing fibers. In this review, we will integrate the current knowledge of synaptic plasticity of climbing fibers with that of other processes affecting climbing fiber development.

Abnormal axonal guidance and brain anatomy in mouse mutants for the cell recognition molecules close homolog of L1 and NgCAM-related cell adhesion molecule

Cell recognition molecules of the L1 family serve important functions in the developing and the mature nervous system. Mutations in genes encoding the L1 family members close homolog of L1 (CHL1) and NgCAM-related cell adhesion molecule (NrCAM) have been found to alter connectivity and morphology of several brain regions. In order to emphasize similarities and differences of these two structurally related molecules, null mutants for CHL1 and NrCAM were directly compared with respect to axonal guidance in the hippocampus and the olfactory bulb and the sizes of the ventricular system and the cerebellar vermis using a combined structural magnetic resonance imaging (MRI) and histological approach. The results demonstrate that the absence of CHL1 leads to aberrant hippocampal mossy fiber projections whereas in both mutants, CHL1 and NrCAM, the guidance of the olfactory nerve projections is disturbed. Both mutations also alter the size of the ventricular system and the vermis with a specific profile of changes and partially opposite effects in each of the mutants. CHL1/NrCAM double-mutant mice do not show any enhancement of the single mutant's phenotype but balance the opposing effects on the ventricular system. In summary, the results show that CHL1 and NrCAM both affect axonal guidance and the anatomy of the ventricular system and the cerebellar vermis but act differently on these processes.

Phocein: A potential actor in vesicular trafficking at Purkinje cell dendritic spines

Phocein is an intracellular protein highly expressed in neurons. It is the major partner of the striatin family members which are scaffolding proteins involved in signaling and trafficking. Due to its association with dynamin via direct interactions with nucleotide diphosphate kinase (NDPK) and EPS15, phocein has been implicated in vesicular trafficking, acting in particular in the endocytic process. This review focuses on immuno-cytochemical studies showing the strict localization of phocein in Purkinje cell dendritic spines involved in excitatory transmission in the cerebellum of postnatal and adult rodents. Immunogold labeling sometimes detects phocein in close vicinity with endocytic-like membrane profiles suggesting that phocein plays a role in endocytosis. Furthermore, co-localization of phocein and SG2NA within spines suggests that their interactions have a functional significance in the molecular cascades that underly membrane trafficking in post-synaptic structures. As the striatin family members are highly concentrated in dendritic spines, their interactions with phocein might be involved in mediating synaptic plasticity through spine remodeling by endocytosis.

Reinnervation of late postnatal Purkinje cells by climbing fibers: neosynaptogenesis without transient multi-innervation

Synaptic partner selection and refinement of projections are important in the development of precise and functional neuronal connections. We investigated the formation of new synaptic connections in a relatively mature system to test whether developmental events can be recapitulated at later stages (i.e., after the mature synaptic organization has been established), using a model of postlesional reinnervation in the olivo-cerebellar pathway. During the development of this pathway, synaptic connections between climbing fibers (CFs) and Purkinje cells (PCs) are diffuse and redundant before synapse elimination refines the pattern. The regression of CFs during the first 2 postnatal weeks in the rat leads to mono-innervation of each PC. After unilateral transection of the rat olivo-cerebellar pathway and intracerebellar injection of BDNF 24 h after lesion, axons from the remaining inferior olive can sprout into the deafferented hemicerebellum and establish new contacts with denervated PCs at later developmental stages. We found that these contacts are first established on somatic thorns before the CFs translocate to the PC dendrites, recapitulating the morphological steps of normal CF-PC synaptogenesis, but on a relatively mature PC. However, electrophysiology of PC reinnervation by transcommissural CFs in these animals showed that each PC is reinnervated by only one CF. This mono-innervation contrasts with the reinnervation of grafted immature PCs in the same cerebellum. Our results provide evidence that relatively mature PCs do not receive several olivary afferents during late reinnervation, suggesting a critical role of the target cell state in the control of CF-PC synaptogenesis. Thus, synapse exuberance and subsequent elimination are not a prerequisite to reach a mature relationship between synaptic partners.

Activity-dependent presynaptic and postsynaptic structural plasticity in the mature cerebellum

Two models of spine formation have been proposed. Spines can derive from emerging dendritic filopodia that have encountered presynaptic partners, or presynaptic molecules may induce the spine maturation event directly from the dendritic shaft. The first model applies better to the Purkinje cell (PC), because numerous free spines have been described in several conditions, particularly when granule cells degenerate before parallel fiber (PF) synapses are formed. A large number of new spines, many of them being free, appear in the proximal dendritic domain after blockage of electrical activity by tetrodotoxin (TTX). A complete blockage of the AMPA receptors by NBQX (2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzoquinoxaline-7-sulfonamide), leading to a complete absence of PF- and climbing fiber (CF)-evoked EPSCs and of spontaneous glutamatergic quantal events, mimics the TTX effect. In contrast, metabotropic glutamate receptor blockage by MCPG [(S)-alpha-methyl-4-carboxyphenylglycine] is ineffective. In normal conditions, in the proximal dendritic domain of the PC, clusters of a few spines are present only under each CF varicosity. It has been proposed that the active CF is responsible for spine pruning in the territory surrounding the CF synapses. Here, we show that such a pruning is mediated by AMPA but not by metabotropic receptors. Finally, after AMPA receptor blockage, there is a reduced number of spines in each spine cluster underlying CF varicosity. In conclusion, PCs tend to express spines over the entire dendritic territory. CF activity reinforces the CF synaptic contacts and actively suppresses spines in the surrounding territory, which is an effect mediated by AMPA receptors.

A theoretical network model to analyse neurogenesis and synaptogenesis in the dentate gyrus

We describe a strongly biologically motivated artificial neural network approach to model neurogenesis and synaptic turnover as it naturally occurs for example in the hippocampal dentate gyrus (DG) of the developing and adult mammalian and human brain. The results suggest that cell proliferation (CP) has not only a functional meaning for computational tasks and learning but is also relevant for maintaining homeostatic stability of the neural activity. Moderate rates of CP buffer disturbances in input activity more effectively than networks without or very high CP. Up to a critical mark an increase of CP enhances synaptogenesis which might be beneficial for learning. However, higher rates of CP are rather ineffective as they destabilize the network: high CP rates and a disturbing input activity effect a reduced cell survival. By these results the simulation model sheds light on the recurrent interdependence of structure and function in biological neural networks especially in hippocampal circuits and the interacting morphogenetic effects of neurogenesis and synaptogenesis.

Nitric oxide-evoked cGMP production in Purkinje cells in rat cerebellum: an immunocytochemical and pharmacological study

The cerebellar cells that account for glutamate-dependent cyclic GMP (cGMP) production, involving activation of the ionotropic glutamate receptors/nitric oxide synthase/soluble guanylyl cyclase pathway, are not fully established. In the present paper we have searched for the localisation of the cGMP response to the nitric oxide (NO) donor S-nitroso-penicillamine (SNAP 1muM), expected to generate local NO concentrations in the low nanomolar physiological range and evoking a cGMP response dependent on glutamate release and on the consequent activation of ionotropic glutamate NMDA/non-NMDA receptors, in cerebellar slices from adult rat. We have found that low concentration of exogenous NO evoked cGMP accumulation in Purkinje cells in an ionotropic glutamate receptor-dependent and tetrodotoxin-sensitive manner. Such immunocytochemical localisation appears consistent with functional evidence for physiologically relevant glutamate-dependent cGMP production in Purkinje cells in rat cerebellar cortex.

Immunogold localization of phocein in dendritic spines

Phocein, a widely expressed intracellular protein involved in clathrin- and dynamin-dependent membrane dynamics, has been shown to interact with members of the striatin family of proteins, striatin, SG2NA, and zinedin. Immunogold labeling was performed to assess subcellular localization of phocein in neurons of the rodent cerebellar cortex and hippocampal Ammon's horn. Most of the phocein-bound gold particles were located within dendritic thorns and spines of the cerebellar Purkinje cells and hippocampal pyramidal neurons, as observed previously for striatin in striatal neurons. The postsynaptic profiles containing phocein were engaged in asymmetric synapses with the main types of afferents in the cerebellum and in the hippocampus. In the cerebellum, phocein-bound immunogold particle numbers ranged from 1-20 in approximately 50% of the Purkinje cell spines. In these spines most of the immunogold particles were found in the neuroplasm ( approximately 70%) and on nonsynaptic plasma membrane domains and related structures such as endocytic-like profiles ( approximately 18%). As soon as the first postnatal week, phocein was detected in the Purkinje cell somatic and dendritic thorns making asymmetric synapses with climbing fibers. During the following weeks the protein was located in the dendritic spines, as observed in the adult molecular layer. Finally, double immunogold labeling revealed a distribution of phocein and SG2NA suggesting that the two proteins could interact in the Purkinje cell spines. The early postnatal expression of phocein, a protein involved in membrane dynamics, suggests that it may have functional relevance in dendritic remodeling during development and potentially in spine plasticity during adulthood.