Developmental dynamics of Purkinje cells and dendritic spines in rat cerebellar cortex

Histomorphometry of the cortical layers and the dentate nucleus of the human fetal cerebellum

Objectives

This study was aimed at determining the histomorphometry of the cerebellar cortical laminae and the dentate nucleus of the human fetal cerebellum; the number and shape of the neurons; and the gestational age of appearance of the cerebellar folia, white matter and arbor vitae cerebelli.

Methods

Microscopic sections of the human fetal cerebellum stained with hematoxylin and eosin and Bielschowsky silver stain were studied.

Results

The thickness of the cortical laminae of the human fetal cerebellum varied among gestational weeks as follows: external granular layer: 36.06 ± 9.36-50.05 ± 34.06 μm, molecular layer: 32.76 ± 17.16-52 ± 28.6 μm, Purkinje cell layer: 9.36 ± 6.8-15.6 ± 4.68 μm and internal granular layer: 66.65 ± 24.42-146.63 ± 47.79 μm. Similarly, the number of neurons per field of view at 1000X under a compound microscope varied among gestational weeks as follows: external granular layer: 89.92 ± 42-142.84 ± 50, molecular layer: 15 ± 12.5-25 ± 8.25, Purkinje cell layer: 3.5 ± 1-5 ± 2.5 and internal granular layer: 98.5 ± 69.75-224 ± 47.White matter in the fetal cerebellum was already present at the age of 12th gestational week, whereas cerebellar folia appeared at 16-20 gestational weeks. Arbor vitae cerebelli and the dentate nucleus became conspicuous after the 20th gestational week. Fetal neurons were round except for Purkinje cells.

Conclusions

The thickness and neuronal counts of the human fetal cerebellar cortical layers and the measurements of the dentate nucleus along with other histomorphological features varied with gestational age from the 12th week of gestation until birth.

Myocyte Enhancer Factor 2c Regulates Dendritic Complexity and Connectivity of Cerebellar Purkinje Cells

Mef2c haploinsufficiency is implicated in behavioral deficits related to autism, schizophrenia, and intellectual disability. Although perturbations in the cerebellum, notably Purkinje cells, have been linked to these neurological disorders, the underlying mechanisms remain poorly understood. In this study, we investigated the roles of Mef2c in cerebellar Purkinje cells during the first three weeks of postnatal development. Our analysis revealed that in comparison to other members of the Mef2 family, Mef2c expression is limited to postnatal Purkinje cells. Because the role of Mef2c has not been assessed in GABAergic neurons, we set out to determine the functional significance of Mef2c by knocking down the expression of Mef2c selectively in Purkinje cells. We found that the loss of Mef2c expression during the first and second postnatal week results in an increase in dendritic arborization without impact on the general growth and migration of Purkinje cells. The influence of Mef2c on dendritic arborization persists throughout the first three weeks, but is most prominent during the first postnatal week suggesting a critical period of Mef2c activity. Additionally, the loss of Mef2c expression results in an increase in the number of spines accompanied by an increase in Gad67 and vGluT1 puncta and decrease in vGluT2 puncta. Thus, our results reveal the specific expression and functional relevance of Mef2c in developing Purkinje cells and offer insight to how disruption of the expression of Mef2c in a GABAergic neuronal subtype may lead to pathogenesis of cerebellar-associated disorders.

Antioxidant supplementation upregulates calbindin expression in cerebellar Purkinje cells of rat pups subjected to post natal exposure to sodium arsenite

Optimal cytoplasmic calcium (Ca²⁺) levels have been associated with adequate cell functioning and neuronal survival. Altered intracellular Ca²⁺ levels following impaired Ca²⁺ homeostasis could induce neuronal degeneration or even cell death. There are reports of arsenite induced oxidative stress and the associated disturbances in intracellular calcium homeostasis. The present study focused on determining the strategies that would modulate tissue redox status and calcium binding protein (CaBP) (Calbindin D28k-CB) expression affected adversely by sodium arsenite (NaAsO₂) exposure (postnatal) of rat pups. NaAsO₂ alone or along with antioxidants (AOXs) (alpha lipoic acid or curcumin) was administered by intraperitoneal (i.p.) route from postnatal day (PND) 1-21 (covering rapid brain growth period - RBGP) to experimental groups and animals receiving sterile water by the same route served as the controls. At the end of the experimental period, the animals were subjected to euthanasia and the cerebellar tissue obtained therefrom was processed for immunohistochemical localization and western blot analysis of CB protein. CB was diffusely expressed in cell body as well as dendritic processes of Purkinje cells (PCs) along the PC Layer (PCL) in all cerebellar folia of the control and the experimental animals. The multilayered pattern of CB +ve cells along with their downregulated expression and low packing density was significantly evident in the arsenic (iAs) alone exposed group as against the controls and AOX supplemented groups. The observations are suggestive of AOX induced restoration of CaBP expression in rat cerebellum following early postnatal exposure to NaAsO₂.

Small-Volume Effect Enables Robust, Sensitive, and Efficient Information Transfer in the Spine

Why is the spine of a neuron so small that it can contain only small numbers of molecules and reactions inevitably become stochastic? We previously showed that, despite such noisy conditions, the spine exhibits robust, sensitive, and efficient features of information transfer using the probability of Ca²⁺ increase; however, the mechanisms are unknown. In this study, we show that the small volume effect enables robust, sensitive, and efficient information transfer in the spine volume, but not in the cell volume. In the spine volume, the intrinsic noise in reactions becomes larger than the extrinsic noise of input, resulting in robust information transfer despite input fluctuation. In the spine volume, stochasticity makes the Ca²⁺ increase occur with a lower intensity of input, causing higher sensitivity to lower intensity of input. The volume-dependency of information transfer increases its efficiency in the spine volume. Thus, we propose that the small-volume effect is the functional reason why the spine has to be so small.

Expression of mutant DISC1 in Purkinje cells increases their spontaneous activity and impairs cognitive and social behaviors in mice

In addition to motor function, the cerebellum has been implicated in cognitive and social behaviors. Various structural and functional abnormalities of Purkinje cells (PCs) have been observed in schizophrenia and autism. As PCs express the gene Disrupted-In-Schizophrenia-1 (DISC1), and DISC1 variants have been associated with neurodevelopmental disorders, we evaluated the role of DISC1 in cerebellar physiology and associated behaviors using a mouse model of inducible and selective expression of a dominant-negative, C-terminus truncated human DISC1 (mutant DISC1) in PCs. Mutant DISC1 male mice demonstrated impaired social and novel placement recognition. No group differences were found in novelty-induced hyperactivity, elevated plus maze test, spontaneous alternation, spatial recognition in Y maze, sociability or accelerated rotarod. Expression of mutant DISC1 was associated with a decreased number of large somata PCs (volume: 3000-5000μm³) and an increased number of smaller somata PCs (volume: 750-1000μm³) without affecting the total number of PCs or the volume of the cerebellum. Compared to control mice, attached loose patch recordings of PCs in mutant DISC1 mice revealed increased spontaneous firing of PCs; and whole cell recordings showed increased amplitude and frequency of mEPSCs without significant changes in either R_input or parallel fiber EPSC paired-pulse ratio. Our findings indicate that mutant DISC1 alters the physiology of PCs, possibly leading to abnormal recognition memory in mice.

Sexual dimorphism of the cerebellar vermis in schizophrenia

Converging lines of evidence implicate structural and functional abnormalities in the cerebellum in schizophrenia (SCZ). The cerebellar vermis is of particular interest given its association with clinical symptoms and cognitive deficits in SCZ and its known connections with cortical regions such as the prefrontal cortex. Prior neuroimaging studies have shown structural and functional abnormalities in the vermis in SCZ. In this study, we examined the cerebellar vermis in 50 individuals with SCZ and 54 healthy controls (HC) using a quantitative volumetric approach. All participants underwent high-resolution structural magnetic resonance imaging (MRI). The vermis was manually traced for each participant, and vermis volumes were computed using semiautomated methods. Volumes for total vermis and vermis subregions (anterior and posterior vermis) were analyzed in the SCZ and HC groups. Significant diagnosis-by-sex interaction effects were found in total vermis and vermis subregion analyses. These effects appeared to be driven by significantly decreased posterior vermis volumes in males with SCZ. Exploratory analyses did not reveal significant effects of clinical variables (FEP status, illness duration, and BPRS total score and subscores) on vermis volumes. The findings herein highlight the presence of neural sex differences in SCZ and the need for considering sex-related factors in studying the disorder.

Rearing conditions differently affect the motor performance and cerebellar morphology of prenatally stressed juvenile rats

The cerebellum is one of the most vulnerable parts of the brain to environmental changes. In this study, the effect of diverse environmental rearing conditions on the motor performances of prenatally stressed juvenile rats and its reflection to the cerebellar morphology were investigated. Prenatally stressed Wistar rats were grouped according to different rearing conditions (Enriched=EC, Standard=SC and Isolated=IC) after weaning. Six weeks later, male and female offspring from different litters were tested behaviorally. In rotarod and string suspension tests, females gained better scores than males. Significant gender and housing effects were observed especially on the motor functions requiring fine skills with the best performance by enriched females, but the worst by enriched males. The susceptibility of cerebellar macro- and micro-neurons to environmental conditions was compared using stereological methods. In female groups, no differences were observed in the volume proportions of cerebellar layers, soma sizes and the numerical densities of granule or Purkinje cells. However, a significant interaction between housing and gender was observed in the granule to Purkinje cell ratio of males, due to the increased numerical densities of the granule cells in enriched males. These data imply that proper functioning of the cerebellum relies on its well organized and evolutionarily conserved structure and circuitry. Although early life stress leads to long term behavioral and neurobiological consequences in the offspring, diverse rearing conditions can alter the motor skills of animals and synaptic connectivity between Purkinje and granular cells in a gender dependent manner.

Stochasticity in Ca2+ increase in spines enables robust and sensitive information coding

A dendritic spine is a very small structure (∼0.1 µm³) of a neuron that processes input timing information. Why are spines so small? Here, we provide functional reasons; the size of spines is optimal for information coding. Spines code input timing information by the probability of Ca²⁺ increases, which makes robust and sensitive information coding possible. We created a stochastic simulation model of input timing-dependent Ca²⁺ increases in a cerebellar Purkinje cell's spine. Spines used probability coding of Ca²⁺ increases rather than amplitude coding for input timing detection via stochastic facilitation by utilizing the small number of molecules in a spine volume, where information per volume appeared optimal. Probability coding of Ca²⁺ increases in a spine volume was more robust against input fluctuation and more sensitive to input numbers than amplitude coding of Ca²⁺ increases in a cell volume. Thus, stochasticity is a strategy by which neurons robustly and sensitively code information.

MMP-2 mediates Purkinje cell morphogenesis and spine development in the mouse cerebellum

Matrix metalloproteinase-2 (MMP-2) is a highly studied proteolytic enzyme, involved in many detrimental and beneficial functions throughout the body, and also active in the central nervous system (CNS). MMP-2 is profoundly expressed in the developing cerebellum and was recently reported to modulate granule cell proliferation by affecting cell cycle kinetics in cerebella of postnatal day 3 mouse pups. In this report, a two-dimensional difference gel electrophoresis proteomics study was implemented at this postnatal stage and revealed 16 differentially expressed proteins between MMP-2-deficient (MMP-2(-/-)) and wild-type cerebella. Among those, collapsin response mediator protein 1 (CRMP1) could be identified as the most significant differential protein between the two genotypes. Western blot experiments confirmed this finding and further disclosed a significant increase in phosphorylated CRMP1 expression in MMP-2(-/-) cerebella. Strikingly, subsequent immunohistochemical and microscopic analyses revealed an aberrant Purkinje cell (PC) dendritogenesis, possibly related to upregulated (phospho-) CRMP1 levels in these neonatal MMP-2(-/-) animals. Further, detailed morphometric analyses showed persistent PC morphological changes in MMP-2(-/-) mice, from the neonatal stage until adulthood. These were characterized by a reduced growth of PC somata, reduced dendritic tree sizes, and a decreased dendritic arborization. During development, the observed defects were accompanied by a temporarily disturbed parallel fiber and climbing fiber synaptic input on the PCs, while in adult MMP-2(-/-) animals, an increased PC spine density and reduced spine lengths were noted. The observed PC abnormalities might contribute to the mild defects in motor performance, i.e. balance and coordination, detected in adult MMP-2(-/-) mice. Overall, these findings indicate the importance of MMP-2 in CNS development and dendritogenesis, and highlight the importance of a correct developmental wiring for adult brain morphology and function.

Synaptic changes in the brain of subjects with schizophrenia

Clinical, epidemiological, neuroimaging and postmortem data all suggest schizophrenia is a neurodevelopmental disorder, and that synaptic disturbances might play a critical role in developing the disease. In 1982, Feinberg proposed that the schizophrenia might arise as a result of abnormal synaptic pruning. His hypothesis has survived 40years of accumulated data, and we review the critical findings related to synaptic dysfunction of schizophrenia. While it is clear that synaptic disturbances are integral and important for understanding the pathophysiology of schizophrenia, it has also become obvious that synaptic disturbances cannot be studied and understood as an independent disease hallmark, but only as a part of a complex network of homeostatic events. Development, glial-neural interaction, changes in energy homeostasis, diverse genetic predisposition, neuroimmune processes and environmental influences all can tip the delicate homeostatic balance of the synaptic morphology and connectivity in a uniquely individual fashion, thus contributing to the emergence of the various symptoms of this devastating disorder. Finally, we argue that based on a predominant change in gene expression pattern we can broadly sub-stratify schizophrenia into "synaptic" "oligodendroglial", "metabolic" and "inflammatory" subclasses.

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.

Model of very fast (> 75 Hz) network oscillations generated by electrical coupling between the proximal axons of cerebellar Purkinje cells

Very fast oscillations (VFO; > 75 Hz) occur transiently in vivo, in the cerebellum of mice genetically modified to model Angelman syndrome, and in a mouse model of fetal alcohol syndrome. We recently reported VFO in slices of mouse cerebellar cortex (Crus I and II of ansiform and paramedian lobules), either in association with gamma oscillations (approximately 40 Hz, evoked by nicotine) or in isolation [evoked by nicotine in combination with gamma-aminobutyric acid (GABA)(A) receptor blockade]. The experimental data suggest a role for electrical coupling between Purkinje cells (blockade of VFO by drugs reducing gap junction conductance and spikelets in some Purkinje cells); and the data suggest the specific involvement of Purkinje cell axons (because of field oscillation maxima in the granular layer). We show here that a detailed network model (1000 multicompartment Purkinje cells) replicates the experimental data when gap junctions are located on the proximal axons of Purkinje cells, provided sufficient spontaneous firing is present. Unlike other VFO models, most somatic spikelets do not correspond to axonal spikes in the parent axon, but reflect spikes in electrically coupled axons. The model predicts gating of VFO frequency by g(Na) inactivation, and experiments prolonging this inactivation time constant, with beta-pompilidotoxin, are consistent with this prediction. The model also predicts that cerebellar VFO can be explained as an electrically coupled system of axons that are not intrinsic oscillators: the electrically uncoupled cells do not individually oscillate (in the model) and axonal firing rates are much lower in the uncoupled state than in the coupled state.

Expression and adhesion profiles of SynCAM molecules indicate distinct neuronal functions

Cell-cell interactions through adhesion molecules play key roles in the development of the nervous system. Synaptic cell adhesion molecules (SynCAMs) comprise a group of four immunoglobulin (Ig) superfamily members that mediate adhesion and are prominently expressed in the brain. Although SynCAMs have been implicated in the differentiation of neurons, there has been no comprehensive analysis of their expression patterns. Here we examine the spatiotemporal expression patterns of SynCAMs by using reverse transcriptase-polymerase chain reaction, in situ hybridization, and immunohistological techniques. SynCAMs 1-4 are widely expressed throughout the developing and adult central nervous system. They are prominently expressed in neurons throughout the brain and are present in both excitatory and inhibitory neurons. Investigation of different brain regions in the developing and mature mouse brain indicates that each SynCAM exhibits a distinct spatiotemporal expression pattern. This is observed in all regions analyzed and is particularly notable in the cerebellum, where SynCAMs display highly distinct expression in cerebellar granule and Purkinje cells. These unique expression profiles are complemented by specific heterophilic adhesion patterns of SynCAM family members, as shown by cell overlay experiments. Three prominent interactions are observed, mediated by the extracellular domains of SynCAMs 1/2, 2/4, and 3/4. These expression and adhesion profiles of SynCAMs together with their previously reported functions in synapse organization indicate that SynCAM proteins contribute importantly to the synaptic circuitry of the central nervous system.

Critical dynamics of randomly assembled and diluted threshold networks

The dynamical behavior of a class of randomly assembled networks of binary threshold units subject to random deletion of connections is studied based on the annealed approximation suitable in the thermodynamic limit. The dynamical phase diagram is constructed for several forms of the probability density distribution of nonvanishing connection strengths. The family of power-law distribution functions rho0(x)=(1-alpha)/(2|x|alpha) is found to play a special role in expanding the domain of stable, ordered dynamics at the expense of the disordered, "chaotic" phase. Relationships with other recent studies of the dynamics of complex networks allowing for variable in-degree of the units are explored. The relevance of the pruning of network connections to neural modeling and developmental neurobiology is discussed.

Ontogenetic alterations in molecular and structural correlates of dendritic growth after developmental exposure to polychlorinated biphenyls

OBJECTIVE

Perinatal exposure to polychlorinated biphenyls (PCBs) is associated with decreased IQ scores, impaired learning and memory, psychomotor difficulties, and attentional deficits in children. It is postulated that these neuropsychological deficits reflect altered patterns of neuronal connectivity. To test this hypothesis, we examined the effects of developmental PCB exposure on dendritic growth.

METHODS

Rat dams were gavaged from gestational day 6 through postnatal day (PND) 21 with vehicle (corn oil) or the commercial PCB mixture Aroclor 1254 (6 mg/kg/day). Dendritic growth and molecular markers were examined in pups during development.

RESULTS

Golgi analyses of CA1 hippocampal pyramidal neurons and cerebellar Purkinje cells indicated that developmental exposure to PCBs caused a pronounced age-related increase in dendritic growth. Thus, even though dendritic lengths were significantly attenuated in PCB-treated animals at PND22, the rate of growth was accelerated at later ages such that by PND60, dendritic growth was comparable to or even exceeded that observed in vehicle controls. Quantitative reverse transcriptase polymerase chain reaction analyses demonstrated that from PND4 through PND21, PCBs generally increased expression of both spinophilin and RC3/neurogranin mRNA in the hippocampus, cerebellum, and cortex with the most significant increases observed in the cortex.

CONCLUSIONS

This study demonstrates that developmental PCB exposure alters the ontogenetic profile of dendritogenesis in critical brain regions, supporting the hypothesis that disruption of neuronal connectivity contributes to neuropsychological deficits seen in exposed children.

Short- and long-term depression of rat cerebellar parallel fibre synaptic transmission mediated by synaptic crosstalk

Cerebellar granule cell to Purkinje cell synapses have been reported to show plasticity when stimulating the parallel fibres, but not when granule cell axons are stimulated in the granular layer. The latter absence of plasticity has been attributed either to the synapses made by ascending granule cell axons lacking some feature needed to evoke plasticity, such as metabotropic glutamate receptors, or to spillover of glutamate between adjacent active synapses being essential for plasticity to occur and having a greater effect for parallel fibre stimulation than for granular layer stimulation. Here we show that both long-term depression (LTD) and endocannabinoid plasticity can depend on interaction between adjacent synapses. These results focus attention on the need to characterize the spatial pattern of parallel fibre activity evoked by physiological stimuli, in order to assess the conditions under which synaptic plasticity will occur in vivo.

Dynamic imaging of cerebellar Purkinje cells reveals a population of filopodia which cross-link dendrites during early postnatal development

Two-photon microscopy was used to image dye-loaded filopodia of Purkinje cells in acute rat cerebellar slices. In the process of examining filopodia in Purkinje cells from a period of rapid dendritic growth (P10-21), we observed a small subset of filopodia which appeared to form connections between two dendrites of the same cell, usually between the tips of two adjacent dendrites or the tip of a dendrite and the shaft of another. There were fewer of these 'filopodial bridges' present at P18-21 than at an earlier stage in development (P10-12) and they were absent in mature Purkinje cells. Filopodial bridges do not appear to be an artifact of living brain slice preparation as they may also be seen by dye-loading Purkinje cells in slices prepared from perfusion-fixed brain. They have varied morphologies which are mostly similar to conventional, unattached filopodia. However, when measured over tens of minutes, filopodial bridges were observed to be less motile than conventional filopodia as indicated by a reduced index of expansion. While the functions of these novel structures are unknown it is attractive to speculate that they play an instructive role in Purkinje cell dendritic development.

Don't get too excited: mechanisms of glutamate-mediated Purkinje cell death

Purkinje cells (PCs) present a unique cellular profile in both the cerebellum and the brain. Because they represent the only output cell of the cerebellar cortex, they play a vital role in the normal function of the cerebellum. Interestingly, PCs are highly susceptible to a variety of pathological conditions that may involve glutamate-mediated 'excitotoxicity', a term coined to describe an excessive release of glutamate, and a subsequent over-activation of excitatory amino acid (NMDA, AMPA, and kainite) receptors. Mature PCs, however, lack functional NMDA receptors, the means by which Ca(2+) enters the cell in classic hippocampal and cortical models of excitotoxicity. In PCs, glutamate predominantly mediates its effects, first via a rapid influx of Ca(2+)through voltage-gated calcium channels, caused by the depolarization of the membrane after AMPA receptor activation (and through Ca(2+)-permeable AMPA receptors themselves), and second, via a delayed release of Ca(2+) from intracellular stores. Although physiological levels of intracellular free Ca(2+) initiate vital second messenger signaling pathways in PCs, excessive Ca(2+) influx can detrimentally alter dendritic spine morphology via interactions with the neuronal cytoskeleton, and thus can perturb normal synaptic function. PCs possess various calcium-binding proteins, such as calbindin-D28K and parvalbumin, and glutamate transporters, in order to prevent glutamate from exerting deleterious effects. Bergmann glia are gaining recognition as key players in the clearance of extracellular glutamate; these cells are also high in S-100beta, a protein with both neurodegenerative and neuroprotective abilities. In this review, we discuss PC-specific mechanisms of glutamate-mediated excitotoxic cell death, the relationship between Ca(2+) and cytoskeleton, and the implications of glutamate, and S-100beta for pathological conditions, such as traumatic brain injury.

Long-term NR2B expression in the cerebellum alters granule cell development and leads to NR2A down-regulation and motor deficits

N-methyl-D-aspartate receptor (NMDAR) composition in granule cells changes characteristically during cerebellar development. To analyze the importance of NR2B replacement by NR2C and NR2A subunits until the end of the first month of age, we generated mice with lasting NR2B expression but deficiency for NR2C (NR2C-2B mice). Mutant phenotype was different from NR2C knock-out mice as loss of granule cells and morphological changes in NR2C/2B cerebellar architecture were already evident from the second postnatal week. Increased NR2B subunit levels led also to a gradual down-regulation of cerebellar NR2A levels, preceding the development of motor impairment in adult animals. Therefore, cerebellar NR2A is important for proper motor coordination and cannot be replaced by long-term expression of NR2B. Consequently, the physiological exchange of NMDA receptor subunits during cerebellar granule cell maturation is important for accurate postnatal development and function.

Storage Capacity Diverges With Synaptic Efficiency in an Associative Memory Model With Synaptic Delay and Pruning

It is known that storage capacity per synapse increases by synaptic pruning in the case of a correlation-type associative memory model. However, the storage capacity of the entire network then decreases. To overcome this difficulty, we propose decreasing the connectivity while keeping the total number of synapses constant by introducing delayed synapses. In this paper, a discrete synchronous-type model with both delayed synapses and their prunings is discussed as a concrete example of the proposal. First, we explain the Yanai-Kim theory by employing statistical neurodynamics. This theory involves macrodynamical equations for the dynamics of a network with serial delay elements. Next, considering the translational symmetry of the explained equations, we rederive macroscopic steady-state equations of the model by using the discrete Fourier transformation. The storage capacities are analyzed quantitatively. Furthermore, two types of synaptic prunings are treated analytically: random pruning and systematic pruning. As a result, it becomes clear that in both prunings, the storage capacity increases as the length of delay increases and the connectivity of the synapses decreases when the total number of synapses is constant. Moreover, an interesting fact becomes clear: the storage capacity asymptotically approaches 2/pi due to random pruning. In contrast, the storage capacity diverges in proportion to the logarithm of the length of delay by systematic pruning and the proportion constant is 4/pi. These results theoretically support the significance of pruning following an overgrowth of synapses in the brain and may suggest that the brain prefers to store dynamic attractors such as sequences and limit cycles rather than equilibrium states.

Synapse efficiency diverges due to synaptic pruning following overgrowth

In the development of the brain, it is known that synapses are pruned following overgrowth. This pruning following overgrowth seems to be a universal phenomenon that occurs in almost all areas-visual cortex, motor area, association area, and so on. It has been shown numerically that the synapse efficiency is increased by systematic deletion. We discuss the synapse efficiency to evaluate the effect of pruning following overgrowth, and analytically show that the synapse efficiency diverges as O(|ln c|) at the limit where connecting rate c is extremely small. Under a fixed synapse number criterion, the optimal connecting rate, which maximizes memory performance, exists.

Subtype switching of vesicular glutamate transporters at parallel fibre-Purkinje cell synapses in developing mouse cerebellum

Two subtypes of the vesicular glutamate transporter are expressed differentially in two excitatory afferents synapsing on to Purkinje cells: VGluT1 (BNPI) in axon terminals of cerebellar granule cells (i.e. parallel fibres; PFs) and VGluT2 (DNPI) in those of the inferior olivary neurons (climbing fibres; CFs). In the present study, we examined their expression in the developing mouse cerebellum. By in situ hybridization, the inferior olivary nucleus selectively expressed VGluT2 mRNA through postnatal life. In the cerebellum, both subtypes were transcribed in the external and internal granular layers during the first postnatal week. Thereafter, VGluT1 mRNA showed marked upregulation in the internal granular layer, whereas VGluT2 mRNA disappeared from the external and internal granular layers by the end of the third postnatal week. By immunohistochemistry, CF terminals consistently exhibited VGluT2 immunoreactivity in the postnatal cerebellum. By contrast, in the first 10 days of postnatal life, VGluT2 predominated in PF terminals, despite the transcription of both transporters in developing granule cells. During the second 10 days, VGluT2 in PF terminals was replaced with VGluT1 from deep regions of the molecular layer upwards, correlating with dendritic translocation of CFs. This replacement was accomplished by postnatal day 30. Taking that late-borne PFs are laid down successively on earlier ones in the molecular layer, the deep-to-superficial replacement represents maturation-linked switching from VGluT2 to VGluT1 in individual PFs, and is likely to be regulated at both the transcription and translation levels.

Impaired development of the cerebellum in transgenic mice expressing the immediate-early protein IE180 of pseudorabies virus

Pseudorabies virus (PRV) infection in animals other than its natural host almost always gives rise to fatal diseases in the central nervous system as a result of infection of peripheral neurons and subsequently to the brain. PRV immediate-early protein (IE180) activates transcription of the PRV early and late genes, and other viral and cellular genes, and represses its own transcription. To examine specific effects of IE180 in neuropathogenicity, we have generated four transgenic mouse lines expressing IE180 in a tetracycline-regulated system. In the transgenic mouse lines, cerebellar symptoms such as ataxic gait, tremor and motor discoordination were observed. Histopathology of the cerebella in the transgenic mouse lines showing severe symptoms was remarkable for a failure of layer formation and a reduction in cerebellar size. These findings suggest that IE180 affects the cascade of gene expression for development of the murine cerebellum, resulting in the impairment of the cerebellar development and differentiation.

Morphological study of organotypic cerebellar cultures

Organotypic cerebellar cultures from 8-days-old (P8) mouse pups were studied following 11 days of in vitro (I IDIV) culturing. The cerebellar cytoarchitectonic structure was maintained in most parasagittal cerebellar cortical slice cultures (also containing the deep cerebellar nuclei). The two main extrinsic excitatory inputs (the climbing and the mossy fibers) seem to be replaced by other axonal types: in the molecular layer mostly by parallel fibers (for climbing fibers) and in the granular layer by intrinsic mossy fiber collaterals of local excitatory interneurons, the unipolar brush cells. However, in a few organotypic cultures, which (although preserving the trilaminar cerebellar cortical structure) were "granuloprival" but also contained some of the deep cerebellar nuclei, the participation of extracortical axons from the deep cerebellar nuclei in the replacement of the missing afferents is suggested.

Distal extension of climbing fiber territory and multiple innervation caused by aberrant wiring to adjacent spiny branchlets in cerebellar Purkinje cells lacking glutamate receptor delta 2

Organized synapse formation on to Purkinje cell (PC) dendrites by parallel fibers (PFs) and climbing fibers (CFs) is crucial for cerebellar function. In PCs lacking glutamate receptor delta2 (GluRdelta2), PF synapses are reduced in number, numerous free spines emerge, and multiple CF innervation persists to adulthood. In the present study, we conducted anterograde and immunohistochemical labelings to investigate how CFs innervate PC dendrites under weakened synaptogenesis by PFs. In the GluRdelta2 knock-out mouse, CFs were distributed in the molecular layer more closely to the pial surface compared with the wild-type mouse. Serial electron microscopy demonstrated that CFs in the knock-out mouse innervated all spines protruding from proximal dendrites of PCs, as did those in the wild-type mouse. In the knock-out mouse, however, CF innervation extended distally to spiny branchlets, where nearly half of the spines were free of innervation in contrast to complete synapse formation by PFs in the wild-type mouse. Furthermore, from the end point of innervation, CFs aberrantly jumped to form ectopic synapses on adjacent spiny branchlets, whose proximal portions were often innervated by different CFs. Without GluRdelta2, CFs are thus able to expand their territory along and beyond dendritic trees of the target PC, resulting in persistent surplus CFs by innervating the distal dendritic segment. We conclude that GluRdelta2 is essential to restrict CF innervation to the proximal dendritic segment, by which territorized innervation by PFs and CFs is properly structured and the formation of excess CF wiring to adjacent PCs is suppressed.

Developmental changes in the localisation of the mGluR1alpha subtype of metabotropic glutamate receptors in Purkinje cells

The regulation of neurotransmitter receptors during synapse formation has been studied extensively at the neuromuscular junction, but little is known about the development of excitatory neurotransmitter receptors during synaptogenesis in central synapses. In this study we show qualitatively and quantitatively that a receptor undergoes changes in localisation on the surface of rat Purkinje cells during development in association with its excitatory synapses. The presence of mGluR1alpha at parallel and climbing fibre synapses on developing Purkinje cells was studied using high-resolution immunoelectron microscopy. Immunoreactivity for mGluR1alpha was detected from embryonic day 18 in Purkinje cells, and showed dramatic changes in its localisation with age. At early postnatal ages (P0 and P3), mGluR1alpha was found both in somata and stem dendrites but was not usually associated with synaptic contacts. At P7, mGluR1alpha became concentrated in somatic spines associated with climbing fibres and in the growing dendritic arborisation even before innervation by parallel fibres. During the second and third postnatal week, when spines and parallel fibre synapses were generated, mGluR1alpha became progressively concentrated in the molecular layer, particularly in the synaptic specialisations. As a result, during the fourth postnatal week, the pattern and level of mGluR1alpha expression became similar to the adult and mGluR1alpha appeared in high density in perisynaptic sites. Our results indicate that mGluR1alpha is present in the developing Purkinje cells prior to their innervation by climbing and parallel fibres and demonstrate that this receptor undergoes a dynamic and specific regulation during postnatal development in association with the establishment of synaptic inputs to Purkinje cell.

Atypical mouse cerebellar development is caused by ectopic expression of the forkhead box transcription factor HNF-3beta

To assess the role of hepatocyte nuclear factor-3beta (HNF-3beta) in hepatocyte-specific gene transcription, we reported the characterization of the liver phenotype with transgenic mice in which the -3-kb transthyretin (TTR) promoter functioned to increase HNF-3beta expression. During breeding of the TTR-HNF-3beta transgenic mice we noticed that they displayed severe ataxia. In this study, we describe the analysis of our transgenic cerebellar phenotype and demonstrate that ectopic expression of HNF-3beta disrupted cerebellar morphogenesis and caused reduction in cerebellar size. In postnatal cerebellum, the HNF-3beta transgene expression pattern is colocalized to glial fibrillary acidic protein-positive cerebellar astrocytes and Bergmann glial cells. As a result of protracted expression, the transgenic cerebella are impaired in terms of astrocyte dispersal and formation of Bergmann glial cell processes. This caused a disruption in neuronal cell migration to the cortical laminar layers and Purkinje dendritic arbor maturation, thus leading to diminished foliation. Differential hybridization of cDNA arrays was used to identify altered expression of cerebellar genes, which is consistent with the observed defect in transgenic cerebellar morphogenesis and size as well as glial maturation. These include diminished expression of the brain lipid-binding protein, which is required for glial morphological differentiation, and the basic helix-loop-helix NeuroD/Beta2 and homeodomain Engrailed-2 transcription factors, which are required for normal cerebellar morphogenesis and foliation. Undetectable levels of ataxia telangiectasia (ATM), which is required for proper development of the Purkinje dendritic arbor, were found in postnatal transgenic cerebella. Furthermore, the transgenic cerebella displayed levels of insulin-like growth factor binding protein-1 elevated to 22 times greater than those measured for wild-type cerebella, an elevation consistent with the reduction in transgenic cerebellar size.

Down-regulated expression of glutamate transporter GLAST in Purkinje cell-associated astrocytes of reeler and weaver mutant cerebella

The glutamate transporter plays an important role in rapid removal of glutamate from the synaptic cleft. Glutamate transporter GLAST is highly expressed in the Bergmann glia (BG), a unipolar cerebellar astrocyte associated structurally and functionally with Purkinje cells (PCs). Here we investigated the expression and localization in the reeler and weaver mutant cerebella with disorganized cytoarchitecture and disrupted synaptic circuitry. In the cortex of both cerebella, GLAST-expressing cells were astrocytes associating PCs; they were located around PC somata and primary dendrites, and extended glial fibrillary acidic protein (GFAP)-immunopositive processes surrounding PC somata and dendrites. Additional signals were detected in astrocytes of the reeler subcortex; they were dispersed among ectopic PCs and had GFAP-positive processes apposing to PC somata and stunted dendrites. Therefore, GLAST expression in PC-associated astrocytes was conserved in these mutants. Compared to the wild-type BG, however, the transcription level in individual mutant astrocytes was significantly reduced to about one-third level in the reeler and weaver cortex or one-sixth level in the reeler subcortex. Taking previous results on remarkable up-regulation during dendritogenic/synaptogenic stages and down-regulation following experimental glutamatergic denervation, it is suggested that GLAST expression in cerebellar astrocytes is regulated correlatively with cytological and/or synaptic differentiation of neighboring PCs.

Synaptic pruning in development: a computational account

Research with humans and primates shows that the developmental course of the brain involves synaptic overgrowth followed by marked selective pruning. Previous explanations have suggested that this intriguing, seemingly wasteful phenomenon is utilized to remove, "erroneous" synapses. We prove that this interpretation is wrong if synapses are Hebbian. Under limited metabolic energy resources restricting the amount and strength of synapses, we show that memory performance is maximized if synapses are first overgrown and then pruned following optimal "minimal-value" deletion. This optimal strategy leads to interesting insights concerning childhood amnesia.

Impaired parallel fiber-->Purkinje cell synapse stabilization during cerebellar development of mutant mice lacking the glutamate receptor delta2 subunit

The glutamate receptor delta2 subunit (GluRdelta2) is specifically expressed in cerebellar Purkinje cells (PCs) from early developmental stages and is selectively localized at dendritic spines forming synapses with parallel fibers (PFs). Targeted disruption of the GluRdelta2 gene leads to a significant reduction of PF-->PC synapses. To address its role in the synaptogenesis, the morphology and electrophysiology of PF-->PC synapses were comparatively examined in developing GluRdelta2 mutant and wild-type cerebella. PCs in GluRdelta2 mutant mice were normally produced, migrated, and formed spines, as did those in wild-type mice. At the end of the first postnatal week, 74-78% of PC spines in both mice formed immature synapses, which were characterized by small synaptic contact, few synaptic vesicles, and incomplete surrounding by astroglial processes, eliciting little electrophysiological response. During the second and third postnatal weeks when spines and terminals are actively generated, the percentage of PC spines forming synapses attained 98-99% in wild type but remained as low as 55-60% in mutants, and the rest were unattached to any nerve terminals. As a result, the number of PF synapses per single-mutant PCs was reduced to nearly a half-level of wild-type PCs. Parallelly, PF stimulation less effectively elicited EPSCs in mutant PCs than in wild-type PCs during and after the second postnatal week. These results suggest that the GluRdelta2 is involved in the stabilization and strengthening of synaptic connectivity between PFs and PCs, leading to the association of all PC spines with PF terminals to form functionally mature synapses.

Expression of mRNA for a neuronal differentiation factor, TA2O, in developing rat brains

In our previous study, a novel factor, TA2O, was isolated from NGlO8-l5 cells. The TA2O mRNA was increased by stimulation which also induced neuronal differentiation. Neuronal cells overexpressed with TA2O extended long neurites and stopped cell growth (Tohda et al., 1995, Neurosci. Res., 23: 21-27). We investigated the expression pattern of TA2O mRNA in developing rat brains to predict physiological roles of TA2O. TA2O mRNA began to increase between embryonic days 13 and 16. TA2O mRNA was observed mainly in neocortical, hippocampal and precerebellar neuroepithelium on embryonic day 16. Although the level of TA2O mRNA in the cerebral cortex was higher before birth than after birth, the level in cerebellar Purkinje cells increased gradually even after birth. The high expression level of TA2O mRNA in the hippocampus was maintained before and after birth. Thus, TA2O was expressed highly in brain regions in which neurons were changing morphologically and qualitatively, suggesting that TA2O may be involved in neuronal formation in vivo.