Morphological development and maturation of the GABAergic synapses in the mouse cerebellar granular layer

Prolidase enzyme is required for extracellular matrix integrity and impacts on postnatal cerebellar cortex development

The extracellular matrix is essential for brain development, lamination, and synaptogenesis. In particular, the basement membrane below the pial meninx (pBM) is required for correct cortical development. The last step in the catabolism of the most abundant protein in pBM, collagen Type IV, requires prolidase, an exopeptidase cleaving the imidodipeptides containing pro or hyp at the C-terminal end. Mutations impairing prolidase activity lead in humans to the rare disease prolidase deficiency characterized by severe skin ulcers and mental impairment. Thus, the dark-like (dal) mouse, in which the prolidase is knocked-out, was used to investigate whether the deficiency of prolidase affects the neuronal maturation during development of a brain cortex area. Focusing on the cerebellar cortex, thinner collagen fibers and disorganized pBM were found. Aberrant cortical granule cell proliferation and migration occurred, associated to defects in brain lamination, and in particular in maturation of Purkinje neurons and formation of synaptic contacts. This study deeply elucidates a link between prolidase activity and neuronal maturation shedding new light on the molecular basis of functional aspects in the prolidase deficiency.

Malformation of the Posterior Cerebellar Vermis Is a Common Neuroanatomical Phenotype of Genetically Engineered Mice on the C57BL/6 Background

C57BL/6 mice exhibit spontaneous cerebellar malformations consisting of heterotopic neurons and glia in the molecular layer of the posterior vermis, indicative of neuronal migration defect during cerebellar development. Recognizing that many genetically engineered (GE) mouse lines are produced from C57BL/6 ES cells or backcrossed to this strain, we performed histological analyses and found that cerebellar heterotopia were a common feature present in the majority of GE lines on this background. Furthermore, we identify GE mouse lines that will be valuable in the study of cerebellar malformations including diverse driver, reporter, and optogenetic lines. Finally, we discuss the implications that these data have on the use of C57BL/6 mice and GE mice on this background in studies of cerebellar development or as models of disease.

Embryonic development of GABAergic terminals in the mouse hypothalamic nuclei involved in feeding behavior

The inhibitory neurotransmitter gamma-amino butyric acid (GABA) plays important roles in energy balance and feeding behavior in the hypothalamus. To reveal the time course of GABAergic network formation, we examined the immunohistochemical localization of glutamic acid decarboxylase (GAD), a GABAergic neuron marker, vesicular GABA transporter (VGAT), a marker of inhibitory terminals, and K+-Cl--cotransporter2 (KCC2), which shifts GABA action from excitation to inhibition, in the developing mouse hypothalamus. GABAergic terminals, seen as GAD- and VGAT-positive dots, increased in density during embryonic development. Moreover, the onset of KCC2 localization was almost concomitant with GABAergic terminal formation, and KCC2-positive profiles increased in density during development. This suggested that after the formation of GABAergic terminals, GABAergic action may change to inhibition in the hypothalamus. This maturation appears to proceed as follows: the lateral hypothalamus (LH) matures first, followed by the paraventricular nucleus (PVN) by the time of birth, while the ventromedial hypothalamus (VMH) and the arcuate nucleus (Arc) are not fully mature at the time of birth. Our findings suggest that GABAergic networks in the "feeding center" (LH) and the "exit" (PVN) may mature before birth, while those in the "satiety center" (VMH) and "higher control center" (Arc) may mature after birth.

Methylmercury induces an initial increase in GABA-evoked currents in Xenopus oocytes expressing α1 and α6 subunit-containing GABAA receptors

Early onset effects of methylmercury (MeHg) on recombinant α₁β₂γ_2S or α₆β₂γ_2S subunit-containing GABA_A receptors were examined. These are two of the most prevalent receptor types found in cerebellum-a consistent target of MeHg-induced neurotoxicity. Heterologously expressed receptors were used in order to: (1) isolate receptor-mediated events from extraneous effects of MeHg due to stimulation of the receptor secondary to increased release of GABA seen with MeHg in neurons in situ and (2) limit the phenotypes of GABA_A receptors present at one time. Initial changes in I_GABA in Xenopus laevis oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors were compared during continuous bath application of MeHg. A time-dependent increase in I_GABA mediated by both receptor subtypes occurred following the first 25-30min of MeHg (5μM) exposure. In α₆β₂γ_2S containing receptors, the MeHg-induced increase in I_GABA was less pronounced compared to that mediated by α₁β₂γ_2S containing receptors, although the pattern of effects was generally similar. Washing with MeHg-free solution reversed the increase in current amplitude. Application of bicuculline at the time of peak potentiation of I_GABA rapidly and completely reversed the MeHg-induced currents. Therefore these MeHg-increased inward currents are mediated specifically by the two subtypes of GABA_A receptors and appear to entail direct actions of MeHg on the receptor. However bicuculline did not affect stimulation by MeHg of oocyte endogenous Cl^- -mediated current, which presumably results from increased [Ca²⁺]_i. Thus, MeHg initially potentiates I_GABA in oocytes expressing either α₁β₂γ_2S or α₆β₂γ_2S receptors prior to its more defined later effects, suggesting that MeHg may initially interact directly with GABA_A receptors in a reversible manner to cause this potentiation.

Methylmercury-Dependent Increases in Fluo4 Fluorescence in Neonatal Rat Cerebellar Slices Depend on Granule Cell Migrational Stage and GABAA Receptor Modulation

Methylmercury (MeHg) disrupts cerebellar function, especially during development. Cerebellar granule cells (CGC), which are particularly susceptible to MeHg by unknown mechanisms, migrate during this process. Transient changes in intracellular Ca(2+) (Ca(2+) i) are crucial to proper migration, and MeHg is well known to disrupt CGC Ca(2+) i regulation. Acutely prepared slices of neonatal rat cerebellum in conjunction with confocal microscopy and fluo4 epifluorescence were used to track changes induced by MeHg in CGC Ca(2+) i regulation in the external (EGL) and internal granule cell layers (IGL) as well as the molecular layer (ML). MeHg caused no cytotoxicity but did cause a time-dependent increase in fluo4 fluorescence that depended on the stage of CGC development. CGCs in the EGL were most susceptible to MeHg-induced increases in fluo4 fluorescence. MeHg increased fluorescence in CGC processes but only diffusely; Purkinje cells rarely fluoresced in these slices. Neither muscimol nor bicuculline alone altered baseline fluo4 fluorescence in any CGC layer, but each delayed the onset and reduced the magnitude of effect of MeHg on fluo4 fluorescence in the EGL and ML. In the IGL, both muscimol and bicuculline delayed the onset of MeHg-induced increases in fluo4 fluorescence but did not affect fluorescence magnitude. Thus, acute exposure to MeHg causes developmental stage-dependent increases in Ca(2+) i in CGCs. Effects are most prominent in CGCs during development or early stages of migration. GABAA receptors participate in an as yet unclear manner to MeHg-induced Ca(2+) i dysregulation of CGCs.

Motor dysfunction in cerebellar Purkinje cell-specific vesicular GABA transporter knockout mice

γ-Aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the adult mammalian central nervous system and plays modulatory roles in neural development. The vesicular GABA transporter (VGAT) is an essential molecule for GABAergic neurotransmission due to its role in vesicular GABA release. Cerebellar Purkinje cells (PCs) are GABAergic projection neurons that are indispensable for cerebellar function. To elucidate the significance of VGAT in cerebellar PCs, we generated and characterized PC-specific VGAT knockout (L7-VGAT) mice. VGAT mRNAs and proteins were specifically absent in the 40-week-old L7-VGAT PCs. The morphological characteristics, such as lamination and foliation of the cerebellar cortex, of the L7-VGAT mice were similar to those of the control littermate mice. Moreover, the protein expression levels and patterns of pre- (calbindin and parvalbumin) and postsynaptic (GABA-A receptor α1 subunit and gephyrin) molecules between the L7-VGAT and control mice were similar in the deep cerebellar nuclei that receive PC projections. However, the L7-VGAT mice performed poorly in the accelerating rotarod test and displayed ataxic gait in the footprint test. The L7-VGAT mice also exhibited severer ataxia as VGAT deficits progressed. These results suggest that VGAT in cerebellar PCs is not essential for the rough maintenance of cerebellar structure, but does play an important role in motor coordination. The L7-VGAT mice are a novel model of ataxia without PC degeneration, and would also be useful for studying the role of PCs in cognition and emotion.

Repeated intermittent alcohol exposure during the third trimester-equivalent increases expression of the GABA(A) receptor δ subunit in cerebellar granule neurons and delays motor development in rats

Exposure to ethanol (EtOH) during fetal development can lead to long-lasting alterations, including deficits in fine motor skills and motor learning. Studies suggest that these are, in part, a consequence of cerebellar damage. Cerebellar granule neurons (CGNs) are the gateway of information into the cerebellar cortex. Functionally, CGNs are heavily regulated by phasic and tonic GABAergic inhibition from Golgi cell interneurons; however, the effect of EtOH exposure on the development of GABAergic transmission in immature CGNs has not been investigated. To model EtOH exposure during the 3rd trimester-equivalent of human pregnancy, neonatal pups were exposed intermittently to high levels of vaporized EtOH from postnatal day (P) 2 to P12. This exposure gradually increased pup serum EtOH concentrations (SECs) to ∼60 mM (∼0.28 g/dl) during the 4 h of exposure. EtOH levels gradually decreased to baseline 8 h after the end of exposure. Surprisingly, basal tonic and phasic GABAergic currents in CGNs were not significantly affected by postnatal alcohol exposure (PAE). However, PAE increased δ subunit expression at P28 as detected by immunohistochemical and western blot analyses. Also, electrophysiological studies with an agonist that is highly selective for δ-containing GABA(A) receptors, 4,5,6,7-tetrahydroisoxazolo[4,5-c]pyridine-3-ol (THIP), showed an increase in THIP-induced tonic current. Behavioral studies of PAE rats did not reveal any deficits in motor coordination, except for a delay in the acquisition of the mid-air righting reflex that was apparent at P15 to P18. These findings demonstrate that repeated intermittent exposure to high levels of EtOH during the equivalent of the last trimester of human pregnancy has significant but relatively subtle effects on motor coordination and GABAergic transmission in CGNs in rats.

GABA/glycine signaling during degeneration and regeneration of mouse hypoglossal nerves

In the adult central nervous system (CNS), GABA and glycine (Gly) are predominant inhibitory neurotransmitters, negatively regulating glutamatergic transmission. In the immature CNS, on the other hand, they act as trophic factors, mediating morphogenesis. In the present study, to investigate their involvement in axonal regeneration, we morphologically examined changes in their signaling in mouse hypoglossal nuclei during degeneration and regeneration of hypoglossal nerves. We found that (1) expression and localization of presynaptic elements were not changed, (2) localization of gephyrin, which anchors GABA and Gly receptors, was spread on the surface of motor neuron cell bodies and dendrites, (3) KCC2-expression markedly decreased, (4) choline acetyltransferase, which mediates acetylcholine-synthesis, immediately disappeared from the motor neurons, and (5) the synaptic cleft of both excitatory and inhibitory synapses became irregularly wider, in the hypoglossal nuclei of the sutured side after the operation. These changes gradually normalized during regeneration. These results suggested that synthesis of acetylcholine may be stopped in the motor neuron after axotomy. GABA/Gly may be normally released from presynaptic terminals, be spilled over the original synaptic cleft, be diffused into the neighboring space, bind to extrasynaptically localized receptors, and mediate depolarization of the membrane potential of motor neurons during degeneration and regeneration. Furthermore, it was suggested that GABA/Gly signaling in postsynaptic motor neurons went back to being immature after axotomy, and may play an important role in axonal regeneration.

Glutamatergic neurons induce expression of functional glutamatergic synapses in primary myotubes

Background

The functioning of the nervous system depends upon the specificity of its synaptic contacts. The mechanisms triggering the expression of the appropriate receptors on postsynaptic membrane and the role of the presynaptic partner in the differentiation of postsynaptic structures are little known.

Methods and Findings

To address these questions we cocultured murine primary muscle cells with several glutamatergic neurons, either cortical, cerebellar or hippocampal. Immunofluorescence and electrophysiology analyses revealed that functional excitatory synaptic contacts were formed between glutamatergic neurons and muscle cells. Moreover, immunoprecipitation and immunofluorescence experiments showed that typical anchoring proteins of central excitatory synapses coimmunoprecipitate and colocalize with rapsyn, the acetylcholine receptor anchoring protein at the neuromuscular junction.

Conclusions

These results support an important role of the presynaptic partner in the induction and differentiation of the postsynaptic structures.

Maturation of the GABAergic transmission in normal and pathologic motoneurons

γ-aminobutyric acid (GABA) acting on Cl⁻-permeable ionotropic type A (GABA_A) receptors (GABA_AR) is the major inhibitory neurotransmitter in the adult central nervous system of vertebrates. In immature brain structures, GABA exerts depolarizing effects mostly contributing to the expression of spontaneous activities that are instructive for the construction of neural networks but GABA also acts as a potent trophic factor. In the present paper, we concentrate on brainstem and spinal motoneurons that are largely targeted by GABAergic interneurons, and we bring together data on the switch from excitatory to inhibitory effects of GABA, on the maturation of the GABAergic system and GABA_AR subunits. We finally discuss the role of GABA and its GABA_AR in immature hypoglossal motoneurons of the spastic (SPA) mouse, a model of human hyperekplexic syndrome.

Developmental localization of potassium chloride co-transporter 2 (KCC2), GABA and vesicular GABA transporter (VGAT) in the postnatal mouse somatosensory cortex

Gamma-amino butyric acid (GABA) mediates the hyperpolarization of membrane potential, negatively regulating glutamatergic activity in the adult brain, whereas, mediates depolarization in the immature brain. This developmental shift in GABA actions is induced by the expression of potassium chloride co-transporter 2 (KCC2). In this study, we focused on the developing mouse somatosensory cortex, where the barrel structure in layer 4 is altered by the whisker-lesion during the critical period, before postnatal day 4 (P4). First, to clarify the time-course of postnatal changes in GABA actions, we investigated the developmental localization of KCC2. Second, to reveal its spatial and temporal relationship with GABA synapse formation, we examined the developmental localization of GABA and vesicular GABA transporter. KCC2 was localized within the pyramidal cells in layer 5 after P3, granule cells in layer 4 after P5 and neurons in layers 2 and 3 after P7, indicating that KCC2 was expressed in the chronological order of neuronal settling at the destination. The onset of KCC2 localization was almost concomitant with the formation of GABA synapses, suggesting that GABA was inhibitory after GABA synapse formation. Furthermore, extrasynaptically released GABA might be involved in the maintenance of activity-dependent plasticity as an excitatory transmitter during the critical period.

SAT1, A Glutamine Transporter, is Preferentially Expressed in GABAergic Neurons

Subsets of GABAergic neurons are able to maintain high frequency discharge patterns, which requires efficient replenishment of the releasable pool of GABA. Although glutamine is considered a preferred precursor of GABA, the identity of transporters involved in glutamine uptake by GABAergic neurons remains elusive. Molecular analyses revealed that SAT1 (Slc38a1) features system A characteristics with a preferential affinity for glutamine, and that SAT1 mRNA expression is associated with GABAergic neurons. By generating specific antibodies against SAT1 we show that this glutamine carrier is particularly enriched in GABAergic neurons. Cellular SAT1 distribution resembles that of GAD67, an essential GABA synthesis enzyme, suggesting that SAT1 can be involved in translocating glutamine into GABAergic neurons to facilitate inhibitory neurotransmitter generation.

Implications of activity-dependent neurotransmitter-receptor matching

Electrical activity has numerous roles in early neuronal development. Calcium transients generated at low frequencies regulate neural induction and neuronal proliferation, migration and differentiation. Recent work demonstrates that these signals participate in specification of the transmitters expressed in different classes of neurons. Matching of postsynaptic receptor expression with the novel expression of transmitters ensues. These findings have intriguing implications for development, mature function and evolution of the nervous system.

Development of the cerebellar body in sharks: spatiotemporal relations of Pax6 expression, cell proliferation and differentiation

We have studied the patterns of cell proliferation, regional organization and differentiation in the cerebellar body of embryos and juveniles of two shark species by immunohistochemistry with antibodies against proliferating cell nuclear antigen (PCNA), Pax6, reelin (RELN), GABA, glutamic acid decarboxylase (GAD) and calretinin (CR). The organization of Pax6-expressing cells was also studied by in situ hybridization. Our results reveal that a transient secondary matrix zone, the external germinal layer, is formed in sharks at early stages of cerebellar development and is the source of the earliest Pax6-expressing (granule) cells. Later in development, new granule Pax6-expressing cells arise from medial proliferation zones and accumulate medially in the granular eminences. The GABAergic components appear very early, and show clear regional differences. The medial proliferation zones remain active even in adults. Taken together, the proliferation and differentiation markers used in the present study highlight striking similarities during development between the cerebellar body of elasmobranchs and the cerebella of tetrapods. These results show the importance of elasmobranch models to reconstruct the evolutionary developmental history of the vertebrate cerebellum.

Developmental expression and differentiation-related neuron-specific splicing of metastasis suppressor 1 (Mtss1) in normal and transformed cerebellar cells

Background

Mtss1 encodes an actin-binding protein, dysregulated in a variety of tumors, that interacts with sonic hedgehog/Gli signaling in epidermal cells. Given the prime importance of this pathway for cerebellar development and tumorigenesis, we assessed expression of Mtss1 in the developing murine cerebellum and human medulloblastoma specimens.

Results

During development, Mtss1 is transiently expressed in granule cells, from the time point they cease to proliferate to their synaptic integration. It is also expressed by granule cell precursor-derived medulloblastomas. In the adult CNS, Mtss1 is found exclusively in cerebellar Purkinje cells. Neuronal differentiation is accompanied by a switch in Mtss1 splicing. Whereas immature granule cells express a Mtss1 variant observed also in peripheral tissues and comprising exon 12, this exon is replaced by a CNS-specific exon, 12a, in more mature granule cells and in adult Purkinje cells. Bioinformatic analysis of Mtss1 suggests that differential exon usage may affect interaction with Fyn and Src, two tyrosine kinases previously recognized as critical for cerebellar cell migration and histogenesis. Further, this approach led to the identification of two evolutionary conserved nuclear localization sequences. These overlap with the actin filament binding site of Mtss1, and one also harbors a potential PKA and PKC phosphorylation site.

Conclusion

Both the pattern of expression and splicing of Mtss1 is developmentally regulated in the murine cerebellum. These findings are discussed with a view on the potential role of Mtss1 for cytoskeletal dynamics in developing and mature cerebellar neurons.

GABAergic synaptogenesis marks the onset of differentiation of basket and stellate cells in mouse cerebellum

Type 2 glycine transporter (GlyT2) mediates intracellular glycine transport and is expressed selectively in glycinergic neurons. Expression of GlyT2 gene promoter-driven enhanced green fluorescent protein (eGFP) in BAC transgenic mice allows selective visualization of glycinergic neurons by fluorescence microscopy. Here, we show that cerebellar interneuron precursors identified by the transcription factor Pax2, including gamma-aminobutyric acid (GABA)ergic interneurons of the molecular layer (ML; basket and stellate cells), transiently express GlyT2-eGFP during development. In contrast, expression of endogenous GlyT2 is restricted to glycinergic Golgi cells. Comparison with knock-in mice expressing eGFP in GABAergic neurons [glutamic acid decarboxylase (GAD)67-eGFP] revealed that GlyT2-eGFP expression often precedes GAD67-eGFP and is therefore a marker of immature GABAergic interneurons. In the internal granule cell layer, GABAergic Golgi cells differentiated shortly after birth, prior to glycinergic Golgi cells. In the ML, GlyT2-eGFP-positive precursor cells migrated until the boundary with the external granule cell layer, forming an inside-out maturation gradient that determined the final position of interneurons in the ML. After migration, GlyT2-eGFP gradually disappeared, while interneurons differentiated morphologically and became immunoreactive for parvalbumin, the GABA(A) receptor alpha1 subunit, and the K(+)Cl(-) exchanger KCC2 (K(+)Cl(-) cotransporter type 2). Numerous presumptive GABAergic synaptic terminals were seen on immature ML interneurons as early as P4, preceding the expression of these neurochemical markers. These results suggest that GABAergic synaptogenesis marks the onset of differentiation of basket and stellate cells in the mouse cerebellum, and that GABAergic synaptic function might contribute to the differentiation of interneurons in the cerebellar cortex.

Developmental localization of potassium chloride co-transporter 2 (KCC2) in the Purkinje cells of embryonic mouse cerebellum

Developmental shift in GABA actions from depolarization to hyperpolarization occurs as a result of decreasing the intracellular Cl(-) concentration regulated by K(+)-Cl(-) co-transporter 2 (KCC2). To clarify the time-course of the developmental shift on the Purkinje cells, we examined KCC2-localization in the embryonic mouse cerebellum. The KCC2 was first detected within the Purkinje cells in the Purkinje cell layer of the hemisphere at embryonic day 15 (E15) and the vermis at E17, but the ventricular and intermediate zones were negative. These results suggest that GABA might become inhibitory on the Purkinje cells after their settling in the Purkinje cell layer.

Molecular and synaptic organization of GABAA receptors in the cerebellum: Effects of targeted subunit gene deletions

GABAA receptors form heteromeric GABA-gated chloride channels assembled from a large family of subunit genes. In cerebellum, distinct GABAA receptor subtypes, differing in subunit composition, are segregated between cell types and synaptic circuits. The cerebellum therefore represents a useful system to investigate the significance of GABAA receptor heterogeneity. For instance, studies of mice carrying targeted deletion of major GABAA receptor subunit genes revealed the role of alpha subunit variants for receptor assembly, synaptic targeting, and functional properties. In addition, these studies unraveled mandatory association between certain subunits and demonstrated distinct pharmacology of receptors mediating phasic and tonic inhibition. Although some of these mutants have a profound loss of GABAA receptors, they exhibit only minor impairment of motor function, suggesting activation of compensatory mechanisms to preserve inhibitory networks in the cerebellum. These adaptations include an altered balance between phasic and tonic inhibition, activation of voltage-independent K+ conductances, and upregulation of GABAA receptors in interneurons that are not affected directly by the mutation. Deletion of the alpha1 subunit gene leads to complete loss of GABAA receptors in Purkinje cells. A striking alteration occurs in these mice, whereby presynaptic GABAergic terminals are preserved in the molecular layer but make heterologous synapses with spines, characterized by a glutamatergic-like postsynaptic density. During development of alpha1(0/0) mice, GABAergic synapses are initially formed but are replaced upon spine maturation. These findings suggest that functional GABAA receptors are required for long-term maintenance of GABAergic synapses in Purkinje cells.

Developmental increase of total cell numbers in the murine cerebellum

The cerebellum has been widely used as a paradigm to study basic mechanisms of brain development and cortical histogenesis. Its highly regular structure has always made it particularly attractive to approaches relying on, and yielding, quantitative information, which provide a cornerstone of systems-oriented integrative analyses. Astonishingly, though, a systematic quantification of cell generation during cerebellar development has so far not been provided. Here, we use the isotropic fractionator (i.e., cell counts based on tissue homogenates from anatomically defined regions; cf. Herculano-Houzel S, Lent R., J Neurosci. 2005;25:2518-21) to assess the developmental increase of total cell numbers in the murine cerebellum from embryonic day 17 into early adulthood. Our data show that the quantitative increase of cerebellar cell numbers follows a classical, S-shaped growth curve as described by the Hill-equation. The adult murine cerebellum was found to comprise a total of (44.03+/-0.42) * 10(6) cells, half of which are generated before postnatal day 12+/-0.18. Consistent results were obtained by using two approaches to cell counting, one based on manual assessment, the other on flow cytometry. These data provide a reliable quantitative description of cerebellar growth in the mouse and define a predictive model that should allow their integration with quantitative and qualitative descriptions of cerebellar development.

Developmental localization of potassium chloride co-transporter 2 in granule cells of the early postnatal mouse cerebellum with special reference to the synapse formation

In the adult CNS, GABA is the predominant inhibitory neurotransmitter, mediating the hyperpolarization of membrane potential and regulating the glutamatergic activity. In the immature CNS, on the other hand, GABA mediates depolarization and is involved in controlling morphogenesis. This developmental shift in GABA actions from depolarization to hyperpolarization occurs as a result of decreasing the intracellular chloride ion (Cl(-)) concentration ([Cl(-)](i)) which is regulated by the potassium (K(+))-Cl(-) co-transporter 2 (KCC2). To clarify the time-course of changes in the GABA actions during development, we examined the developmental localization of the KCC2 in the granule cells of the postnatal mouse cerebellum using specific antibodies against KCC2. The granule cell precursors and migrating granule cells were devoid of immunoreactivity against KCC2 antibodies. At postnatal day 3 (P3), the KCC2-immunolabeling was negative in the internal granular layer, although synaptophysin-positive mossy fiber terminals were detected. At P5, we first detected the KCC2-immunolabeling at the somata of granule cells and their dendrites before granule cells received inhibitory input from Golgi cells. Almost all KCC2-positive dendrites (more than 98%) attached to and formed synapses with mossy fiber terminals. As development proceeded, the number of KCC2-positive granule cells increased, and all granule cells became positive by P21. These results suggested that GABAergic transmission on granule cells might shift from excitation to inhibition after the synapse formation, and the excitatory synapse-formation and related factors might be the triggers for the expression and localization of the KCC2 in the granule cells. Furthermore, it was also suggested that formation of the GABAergic synapses and GABAergic transmission were not necessary for the KCC2-expression in the mouse cerebellar granule cells in vivo.

Transcriptional profiling of depolarization-dependent phenotypic alterations in primary cultures of developing granule neurons

Rat cerebellar granule neurons cultured in medium supplemented with elevated KCl are extensively used as a model to examine the coupling between neural activity and Ca(2+)-dependent gene expression. Elevated (25 mM) KCl is believed to mimic endogenous neural activity because it promotes depolarization and Ca(+2)-dependent survival and some aspects of maturation. By comparison, at least half of the granule neurons grown in standard medium containing 5 mM KCl undergo apoptosis beginning approximately 4 days in vitro. However, accumulating evidence suggests that chronic depolarization induces phenotypic abnormalities whereas growth in chemically defined medium containing 5 mM KCl more closely resembles the constitutive phenotype. To examine this, oligonucleotide microarrays and RT-PCR of selected mRNAs were used to compare transcription profiles of cultures grown in 5 mM and 25 mM KCl. In some cases, N-methyl-D-aspartate (NMDA) which, like elevated KCl, promotes long-term survival was also tested. Robust changes in several gene groups were observed and indicated that growth in elevated KCl: induces expression of mRNAs that are not normally observed; represses expression of mRNAs that should be present; maintains expression of mRNAs that are markers of immature neurons. Supplementation of the growth medium with NMDA instead of elevated KCl produces similar abnormalities. Altogether, these data indicate that growth in 5 mM KCl more closely mimics survival and maturation of granule neurons in vivo and should therefore be adopted in future studies.

Developmental dissociation of presynaptic inhibitory neurotransmitter and postsynaptic receptor clustering in the hypoglossal nucleus

At postsynaptic densities of mouse hypoglossal motoneurons, the proportion of glycine receptors co-clustered with GABAA receptors increases from neonatal to adult animals, suggesting that mixed synapses might play a greater role in adult synaptic inhibition. We visualized the presynaptic correlates of these developmental changes using immunocytochemistry. At P5, presynaptic terminals contained glycine and GlyT2 and/or GABA and GAD65, but at P15, the majority of inhibitory terminals contained glycine and GlyT2 only. The GABAergic component of evoked inhibitory postsynaptic currents in HMs decreased strongly between P5 and P15. Similarly, miniature inhibitory postsynaptic currents evolved from mainly glycinergic and mixed glycinergic/GABAergic events at P3-5 to predominantly glycinergic currents at P15. These results indicate that the decrease in the proportion of functional mixed inhibitory synapses with maturation results from a loss of the ability of presynaptic terminals to release both neurotransmitters during development while co-aggregation of GlyRs + GABAARs at postsynaptic loci remained.

Formation of GABAergic synapses in the cerebellum

In the adult central nervous system (CNS), gamma-amino butyric acid (GABA) is a predominant inhibitory neurotransmitter, and is involved in the expression of various higher brain functions. In the cerebellum, formation of GABAergic synapses is crucial for cerebellar functions. However, it is not fully understood how GABAergic synapses and networks are formed. We are morphologically investigating the developmental changes in GABAergic signaling and the mechanisms underlying the assembly of GABAergic synapses using the cerebellum, which provides an ideal system for the investigation of brain development. The anatomy and development of GABAergic synapses and networks in the cerebellar cortex are reviewed, the key factors for the formation of GABAergic synapses are addressed, and the mechanisms underlying the formation of cerebellar GABAergic networks are discussed.

Developmental expression of GABA transporter-1 and 3 during formation of the GABAergic synapses in the mouse cerebellar cortex

In the brain, gamma-amino butyric acid (GABA), released extrasynaptically and synaptically from GABAergic neurons, plays important roles in morphogenesis, expression of higher functions and so on. In the GABAergic transmission system, plasma membrane GABA transporters (GATs) mediate GABA-uptake from the synaptic cleft in the mature brain and are thought to mediate diacrine of cytosolic GABA in the immature brain. In the present study, we focused on two GATs (GAT-1 and GAT-3) in the mouse cerebellar cortex, which are widely localized in neural and glial cells. Firstly, we examined the localization of GATs in the dendrites and cell bodies of developing GABAergic neurons, where GABA is extrasynaptically distributed, to clarify the GABA-diacrine before synaptogenesis. Secondly, we examined the developmental changes in the localization of GATs to reveal the development of the GABA-uptake system. Neither transporter was detected within the dendrites and cell bodies of GABAergic neurons, including Purkinje, stellate, basket and Golgi cells, in the immature cerebellar cortex. GAT-1 was observed within the Golgi cell axon terminals after postnatal day 5 (P5) and presynaptic axons of stellate and basket cells after P7. GAT-3 was localized within the astrocyte processes, sealing the GABAergic synapses in the Purkinje cell and granular layers after P10. These results indicated that GABA-diacrine did not work in the mouse cerebellar cortex. The onset of GAT-1-expression was prior to that of GAT-3. GAT-1 started to be localized within the GABAergic axon terminals during synapse formation. GAT-3 started to be localized within astrocyte processes when they sealed the synapses.

Generation and characterization of Rac3 knockout mice

Rac proteins are members of the Rho family of GTPases involved in the regulation of actin dynamics. The three highly homologous Rac proteins in mammals are the ubiquitous Rac1, the hematopoiesis-specific Rac2, and the least-characterized Rac3. We show here that Rac3 mRNA is widely and specifically expressed in the developing nervous system, with highest concentration at embryonic day 13 in the dorsal root ganglia and ventral spinal cord. At postnatal day 7 Rac3 appears particularly abundant in populations of projection neurons in several regions of the brain, including the fifth layer of the cortex and the CA1-CA3 region of the hippocampus. We generated mice deleted for the Rac3 gene with the aim of analyzing the function of this GTPase in vivo. Rac3 knockout animals survive embryogenesis and show no obvious developmental defects. Interestingly, specific behavioral differences were detected in the Rac3-deficient animals, since motor coordination and motor learning on the rotarod was superior to that of their wild-type littermates. No obvious histological or immunohistological differences were observed at major sites of Rac3 expression. Our results indicate that, in vivo, Rac3 activity is not strictly required for normal development in utero but may be relevant to later events in the development of a functional nervous system.

A bipotent neural progenitor cell line cloned from a cerebellum of an adultp53-deficient mouse generates both neurons and oligodendrocytes

Here we report developmental characteristics of a clonal cell line 2Y-3t established from a multifocal neoplasm that arose in a cerebellum of an adult p53-deficient mouse. The tumorigenicity of the line was not observed in soft agar assay or in nude mouse assay. In serum-containing medium, 2Y-3t cells were epithelial-like in morphology and were mitotic. When they were cultured in serum-free medium, the expressions of neural stem and/or progenitor cell markers were decreased. Concomitantly, the expressions of neuronal and oligodendrocyte markers were increased in concert with morphological differentiation, and DNA synthesis ceased. None of astrocyte markers were detected under these culture conditions. Double-labelling studies revealed that two cell populations coexisted, expressing neuronal or oligodendrocyte markers. Triiodothyronine (T3) increased the oligodendrocyte population when 2Y-3t cells were cultured in serum-free medium. Recloning of the line gave rise to three types of subclones. Sixteen subclones were capable of generating both neurons and oligodendrocytes, four subclones were capable of generating only neurons and one subclone was capable of generating only oligodendrocytes. Thus, 2Y-3t cells have characteristics of bipotent neural progenitor cells capable of generating both neurons and oligodendrocytes. In addition, the line expressed mRNA for Pax-2 and had GAD67-positive cells when cultured in serum-free medium. However, none of the mRNAs for Zic-1, Math1, zebrin or Calbindin-D28k were detected, suggesting that the 2Y-3t line might generate the GABAergic interneuron lineage of the mouse cerebellum.

GABAergic signaling in the developing cerebellum

In the adult central nervous system (CNS), GABA is a predominant inhibitory neurotransmitter that regulates glutamatergic activity. Recent studies have revealed that GABA serves as an excitatory transmitter in the immature CNS and acts as a trophic factor for brain development. Furthermore, synaptic transmission by GABA is also involved in the expression of higher brain functions, such as memory, learning and anxiety. These results indicate that GABA plays various roles in the expression of brain functions and GABAergic roles change developmentally in accordance with alterations in GABAergic transmission and signaling. We have investigated morphologically the developmental changes in the GABAergic transmission system and the key factors important for the formation of GABAergic synapses and networks using the mouse cerebellum, which provides an ideal system for the investigation of brain development. Here, we focus on GABA and GABA(A) receptors in the developing cerebellum and address the processes of how GABA exerts its effect on developing neurons and the mechanisms underlying the formation of functional GABAergic synapses.

Extrasynaptic localization of GABA in the developing mouse cerebellum

In the adult brain, gamma-amino butyric acid (GABA) is synaptically released and mediates inhibitory transmission. Recent studies have revealed that GABA is a trophic factor for brain development. To reveal the distribution of GABA and its secretion mechanisms during brain development, we investigated the immunohistochemical localization of two molecules, GABA and vesicular GABA transporter (VGAT), which is a GABAergic vesicle protein, in the developing mouse cerebellum by means of newly developed antibodies. Furthermore, we tested the relationship between developmental changes in distribution of above two molecules in the presynapses and ontogeny of GABAergic synapses. GABAergic synapses were detected by immunohistochemistry for the GABA(A) receptor alpha1 subunit, which is an essential subunit for inhibitory synaptic transmission in the mature cerebellar cortex. Until postnatal day 7 (P7), GABA was localized throughout the GABAergic neurons, and VGAT accumulated at axon varicosities and growth cones, where the alpha1 subunit did not accumulate. After P10, both GABA and VGAT became confined to the terminal sites where the alpha1 subunit was localized. These results suggested that GABA was extrasynaptically released from axon varicosities and growth cones by vesicular secretion 'exocytosis' and from all parts of GABAergic neurons during the cerebellar development by non-vesicular secretion 'diacrine'.