The stop signal revised: immature cerebellar granule neurons in the external germinal layer arrest pontine mossy fiber growth

AraC interacts with p75NTR transmembrane domain to induce cell death of mature neurons

Cytosine arabinoside (AraC) is one of the main therapeutic treatments for several types of cancer, including acute myeloid leukaemia. However, after a high-dose AraC chemotherapy regime, patients develop severe neurotoxicity and cell death in the central nervous system leading to cerebellar ataxia, dysarthria, nystagmus, somnolence and drowsiness. AraC induces apoptosis in dividing cells. However, the mechanism by which it leads to neurite degeneration and cell death in mature neurons remains unclear. We hypothesise that the upregulation of the death receptor p75^NTR is responsible for AraC-mediated neurodegeneration and cell death in leukaemia patients undergoing AraC treatment. To determine the role of AraC-p75^NTR signalling in the cell death of mature neurons, we used mature cerebellar granule neurons' primary cultures from p75^NTR knockout and p75^NTRCys259 mice. Evaluation of neurite degeneration, cell death and p75^NTR signalling was done by immunohistochemistry and immunoblotting. To assess the interaction between AraC and p75^NTR, we performed cellular thermal shift and AraTM assays as well as Homo-FRET anisotropy imaging. We show that AraC induces neurite degeneration and programmed cell death of mature cerebellar granule neurons in a p75^NTR-dependent manner. Mechanistically, Proline 252 and Cysteine 256 residues facilitate AraC interaction with the transmembrane domain of p75^NTR resulting in uncoupling of p75^NTR from the NFκB survival pathway. This, in turn, exacerbates the activation of the cell death/JNK pathway by recruitment of TRAF6 to p75^NTR. Our findings identify p75^NTR as a novel molecular target to develop treatments for counteract AraC-mediated cell death of mature neurons.

The Primary Ciliary Deficits in Cerebellar Bergmann Glia of the Mouse Model of Fragile X Syndrome

Primary cilia are non-motile cilia that function as antennae for cells to sense signals. Deficits of primary cilia cause ciliopathies, leading to the pathogenesis of various developmental disorders; however, the contribution of primary cilia to neurodevelopmental disorders is largely unknown. Fragile X syndrome (FXS) is a genetically inherited disorder and is the most common known cause of autism spectrum disorders. FXS is caused by the silencing of the fragile X mental retardation 1 (FMR1) gene, which encodes for the fragile X mental retardation protein (FMRP). Here, we discovered a reduction in the number of primary cilia and the Sonic hedgehog (Shh) signaling in cerebellar Bergmann glia of Fmr1 KO mice. We further found reduced granule neuron precursor (GNP) proliferation and thickness of the external germinal layer (EGL) in Fmr1 KO mice, implicating that primary ciliary deficits in Bergmann glia may contribute to cerebellar developmental phenotypes in FXS, as Shh signaling through primary cilia in Bergmann glia is known to mediate proper GNP proliferation in the EGL. Taken together, our study demonstrates that FMRP loss leads to primary ciliary deficits in cerebellar Bergmann glia which may contribute to cerebellar deficits in FXS.

Components of Endocannabinoid Signaling System Are Expressed in the Perinatal Mouse Cerebellum and Required for Its Normal Development

Endocannabinoid (eCB) signaling system (ECS), encompassing cannabinoid receptors and enzymes involved in the synthesis and degradation of the endogenous cannabinoid signaling lipids, is highly expressed in the cerebellar cortex of adult humans and rodents. In addition to their well-established role in neuromodulation, eCBs have been shown to play key roles in aspects of neurodevelopment in the fore- and mid-brain, including neurogenesis, cell migration, and synapse specification. However, little is known about the role of ECS in cerebellar development. In this study, we conducted immunohistochemical characterization of ECS components through key stages of cerebellar development in mice using antibodies for 2-arachidonoylglycerol (2-AG) synthetizing and degrading enzymes and the major brain cannabinoid receptor, cannabinoid receptor 1 (CB1), in combination with cerebellar cell markers. Our results reveal a temporally, spatially, and cytologically dynamic pattern of expression. Production, receptor binding, and degradation of eCBs are tightly controlled, thus localization of eCB receptors and the complementary cannabinoid signaling machinery determines the direction, duration, and ultimately the outcome of eCB signaling. To gain insights into the role of eCB signaling in cerebellar development, we characterized gross anatomy of cerebellar midvermis in CB1 knock-out (CB1 KO) mice, as well as their performance in cerebellar-influenced motor tasks. Our results show persistent and selective anatomic and behavioral alterations in CB1 KOs. Consequently, the insights gained from this study lay down the foundation for investigating specific cellular and molecular mechanisms regulated by eCB signaling during cerebellar development.

Eph/ephrin Function Contributes to the Patterning of Spinocerebellar Mossy Fibers Into Parasagittal Zones

Purkinje cell microcircuits perform diverse functions using widespread inputs from the brain and spinal cord. The formation of these functional circuits depends on developmental programs and molecular pathways that organize mossy fiber afferents from different sources into a complex and precisely patterned map within the granular layer of the cerebellum. During development, Purkinje cell zonal patterns are thought to guide mossy fiber terminals into zones. However, the molecular mechanisms that mediate this process remain unclear. Here, we used knockout mice to test whether Eph/ephrin signaling controls Purkinje cell-mossy fiber interactions during cerebellar circuit formation. Loss of ephrin-A2 and ephrin-A5 disrupted the patterning of spinocerebellar terminals into discrete zones. Zone territories in the granular layer that normally have limited spinocerebellar input contained ectopic terminals in ephrin-A2 ^-/-;ephrin-A5 ^-/- double knockout mice. However, the overall morphology of the cerebellum, lobule position, and Purkinje cell zonal patterns developed normally in the ephrin-A2 ^-/-;ephrin-A5 ^-/- mutant mice. This work suggests that communication between Purkinje cell zones and mossy fibers during postnatal development allows contact-dependent molecular cues to sharpen the innervation of sensory afferents into functional zones.

Mitotic granule cell precursors undergo highly dynamic morphological transitions throughout the external germinal layer of the chick cerebellum

The developing cerebellum of amniotes is characterised by a unique, transient, secondary proliferation zone: the external germinal layer (EGL). The EGL is comprised solely of granule cell precursors, whose progeny migrate inwardly to form the internal granule cell layer. While a range of cell morphologies in the EGL has long been known, how they reflect the cells' differentiation status has previously only been inferred. Observations have suggested a deterministic maturation from outer to inner EGL that we wished to test experimentally. To do this, we electroporated granule cell precursors in chick with plasmids encoding fluorescent proteins and probed labelled cells with markers of both proliferation (phosphohistone H3) and differentiation (Axonin1/TAG1 and NeuroD1). We show that granule cell precursors can display a range of complex forms throughout the EGL while mitotically active. Overexpression of full length NeuroD1 within granule cell precursors does not abolish proliferation, but biases granule cells towards precocious differentiation, alters their migration path and results in a smaller and less foliated cerebellum. Our results show that granule cells show a greater flexibility in differentiation than previously assumed. We speculate that this allows the EGL to regulate its proliferative activity in response to overall patterns of cerebellar growth.

A newly generated neuronal cell model of CLN7 disease reveals aberrant lysosome motility and impaired cell survival

Mutations in the CLN7/MFSD8 gene encoding the lysosomal membrane protein CLN7 are causative of CLN7 disease, an inherited neurodegenerative disorder that typically affects children. To gain insight into the pathomechanisms of CLN7 disease, we established an immortalized cell line based on cerebellar (Cb) granule neuron precursors isolated from Cln7^-/- mice. Here, we demonstrate that Cln7-deficient neuron-derived Cb cells display an abnormal phenotype that includes increased size and defective outward movement of late endosomes and lysosomes as well as impaired lysosomal exocytosis. Whereas Cln7^-/- Cb cells appeared to be autophagy-competent, loss of Cln7 resulted in enhanced cell death under prolonged nutrient deprivation. Furthermore, reduced cell survival of Cln7-deficient cells was accompanied by a significantly impaired protein kinase B/Akt phosphorylation at Ser473 during long-term starvation. In summary, our data demonstrate for the first time that the putative lysosomal transporter CLN7 is relevant for lysosome motility and plays an important role for neuronal cell survival under conditions of starvation.

Recent advances in understanding the mechanisms of cerebellar granule cell development and function and their contribution to behavior

The cerebellum is the focus of an emergent series of debates because its circuitry is now thought to encode an unexpected level of functional diversity. The flexibility that is built into the cerebellar circuit allows it to participate not only in motor behaviors involving coordination, learning, and balance but also in non-motor behaviors such as cognition, emotion, and spatial navigation. In accordance with the cerebellum’s diverse functional roles, when these circuits are altered because of disease or injury, the behavioral outcomes range from neurological conditions such as ataxia, dystonia, and tremor to neuropsychiatric conditions, including autism spectrum disorders, schizophrenia, and attention-deficit/hyperactivity disorder. Two major questions arise: what types of cells mediate these normal and abnormal processes, and how might they accomplish these seemingly disparate functions? The tiny but numerous cerebellar granule cells may hold answers to these questions. Here, we discuss recent advances in understanding how the granule cell lineage arises in the embryo and how a stem cell niche that replenishes granule cells influences wiring when the postnatal cerebellum is injured. We discuss how precisely coordinated developmental programs, gene expression patterns, and epigenetic mechanisms determine the formation of synapses that integrate multi-modal inputs onto single granule cells. These data lead us to consider how granule cell synaptic heterogeneity promotes sensorimotor and non-sensorimotor signals in behaving animals. We discuss evidence that granule cells use ultrafast neurotransmission that can operate at kilohertz frequencies. Together, these data inspire an emerging view for how granule cells contribute to the shaping of complex animal behaviors.

Glucocorticoid Effects on Cerebellar Development in a Chicken Embryo Model: Exploring Changes in PAX6 and Metalloproteinase-9 After Exposure to Dexamethasone

The developing cerebellum is vulnerable to effects of glucocorticoids and cerebellar dysfunction is associated with neurodevelopmental disorders (e.g. autism). Transcription factor PAX6 and matrix metalloproteinase-9 (MMP-9) are critical for normal cerebellar development and are highly expressed in migrating neurones. Alterations in MMP-9 and PAX6 are associated with altered cerebellar development. In the present study, we characterised the growth rate and development of the cortical layers, and further investigated how the levels of PAX6 and MMP-9, as well as glucocorticoid receptor (GR) and proliferating cell nuclear antigen (PCNA), change in the cerebellum during the foetal period [embryonic day (E)12-21] in chicken, which corresponds to the human perinatal period. Dexamethasone (DEX) was administered in ovo at E13 and E16, aiming to investigate how prenatal exposure to glucocorticoids interferes with normal development. DEX reduced foetal and cerebellar weight at E17 in a dose-dependent manner linked to a reduced level of PCNA and, over time, down-regulation of GR. We report that promoter activity of PAX6 and MMP-9 increased as a result of GR-stimulation in vitro. Prenatal DEX increased the protein level of PAX6 in a transient manner. PAX6 is reduced in mature granule neurones, and this occurred earlier in embryos exposed to DEX than in non-exposed controls. DEX exposure also led to a slow-onset down-regulation of MMP-9. Taken together, these findings indicate that excess prenatal glucocorticoid stimulation disturbs normal development of the cerebellum through mechanisms associated with reduced proliferation and accelerated maturation where PAX6 and MMP-9 play important roles.

Calcium-induced apoptosis of developing cerebellar granule neurons depends causally on NGFI-B

Immediate early gene nerve growth factor-induced clone B (NGFI-B), a nuclear receptor important for differentiation and apoptosis, is expressed in mice and rat cerebellum from an early stage of postnatal development. Following apoptotic stimuli NGFI-B translocates to mitochondria to initiate cell death processes. Controlled cell death is critical for correct cerebellar development. Immunohistochemical analysis of NGFI-B in sections of mice cerebella showed NGFI-B to be expressed in granule neurons in vivo at a time (P8-11) when apoptosis is known to occur. The importance of NGFI-B for apoptosis of cultured rat cerebellar granule neurons was investigated by inducing apoptosis with calcium ionophore A23187 (CaI, 0.1μM). Imaging studies of gfp-tagged NGFI-B confirmed that mitochondrial translocation of NGFI-B occurred following treatment with CaI and was reduced by addition of 9-cis-retinoic acid (1μM), a retinoid X receptor (RXR) agonist that prevents dimerization of RXR and NGFI-B that is known to occur before translocation. Consequently, 9-cis-retinoic acid partly reduced cell death. To address the causality of NGFI-B in apoptosis further, knock-down by siRNA was performed and it removed 85% of the NGFI-B protein. This resulted in a complete inhibition of apoptosis after CaI exposure. Together these findings suggest that NGFI-B plays a role in controlling correct cerebellar development.

Spontaneous activity and functional connectivity in the developing cerebellorubral system

The development of the cerebellar system depends in part on the emergence of functional connectivity in its input and output pathways. Characterization of spontaneous activity within these pathways provides insight into their functional status in early development. In the present study we recorded extracellular activity from the interpositus nucleus (IP) and its primary downstream target, the red nucleus (RN), in unanesthetized rats at postnatal days 8 (P8) and P12, a period of dramatic change in cerebellar circuitry. The two structures exhibited state-dependent activity patterns and age-related changes in rhythmicity and overall firing rate. Importantly, sensory feedback (i.e., reafference) from myoclonic twitches (spontaneous, self-generated movements that are produced exclusively during active sleep) drove neural activity in the IP and RN at both ages. Additionally, anatomic tracing confirmed the presence of cerebellorubral connections as early as P8. Finally, inactivation of the IP and adjacent nuclei using the GABAA receptor agonist muscimol caused a substantial decrease in neural activity in the contralateral RN at both ages, as well as the disappearance of rhythmicity; twitch-related activity in the RN, however, was preserved after IP inactivation, indicating that twitch-related reafference activates the two structures in parallel. Overall, the present findings point to the contributions of sleep-related spontaneous activity to the development of cerebellar networks.

Twitch-related and rhythmic activation of the developing cerebellar cortex

The cerebellum is a critical sensorimotor structure that exhibits protracted postnatal development in mammals. Many aspects of cerebellar circuit development are activity dependent, but little is known about the nature and sources of the activity. Based on previous findings in 6-day-old rats, we proposed that myoclonic twitches, the spontaneous movements that occur exclusively during active sleep (AS), provide generalized as well as topographically precise activity to the developing cerebellum. Taking advantage of known stages of cerebellar cortical development, we examined the relationship between Purkinje cell activity (including complex and simple spikes), nuchal and hindlimb EMG activity, and behavioral state in unanesthetized 4-, 8-, and 12-day-old rats. AS-dependent increases in complex and simple spike activity peaked at 8 days of age, with 60% of units exhibiting significantly more activity during AS than wakefulness. Also, at all three ages, approximately one-third of complex and simple spikes significantly increased their activity within 100 ms of twitches in one of the two muscles from which we recorded. Finally, we observed rhythmicity of complex and simple spikes that was especially prominent at 8 days of age and was greatly diminished by 12 days of age, likely due to developmental changes in climbing fiber and mossy fiber innervation patterns. All together, these results indicate that the neurophysiological activity of the developing cerebellum can be used to make inferences about changes in its microcircuitry. They also support the hypothesis that sleep-related twitches are a prominent source of discrete climbing and mossy fiber activity that could contribute to the activity-dependent development of this critical sensorimotor structure.

Polarity transitions during neurogenesis and germinal zone exit in the developing central nervous system

During neural development, billions of neurons differentiate, polarize, migrate and form synapses in a precisely choreographed sequence. These precise developmental events are accompanied by discreet transitions in cellular polarity. While radial glial neural stem cells are highly polarized, transiently amplifying neural progenitors are less polarized after delaminating from their parental stem cell. Moreover, preceding their radial migration to a final laminar position neural progenitors re-adopt a polarized morphology before they embarking on their journey along a glial guide to the destination where they will fully mature. In this review, we will compare and contrast the key polarity transitions of cells derived from a neuroepithelium to the well-characterized polarity transitions that occur in true epithelia. We will highlight recent advances in the field that shows that neuronal progenitor delamination from germinal zone (GZ) niche shares similarities to an epithelial-mesenchymal transition. Moreover, studies in the cerebellum suggest the acquisition of radial migration and polarity in transiently amplifying neural progenitors share similarities to mesenchymal-epithelial transitions. Where applicable, we will compare and contrast the precise molecular mechanisms used by epithelial cells and neuronal progenitors to control plasticity in cell polarity during their distinct developmental programs.

In vivo analysis of Purkinje cell firing properties during postnatal mouse development

Purkinje cell activity is essential for controlling motor behavior. During motor behavior Purkinje cells fire two types of action potentials: simple spikes that are generated intrinsically and complex spikes that are induced by climbing fiber inputs. Although the functions of these spikes are becoming clear, how they are established is still poorly understood. Here, we used in vivo electrophysiology approaches conducted in anesthetized and awake mice to record Purkinje cell activity starting from the second postnatal week of development through to adulthood. We found that the rate of complex spike firing increases sharply at 3 wk of age whereas the rate of simple spike firing gradually increases until 4 wk of age. We also found that compared with adult, the pattern of simple spike firing during development is more irregular as the cells tend to fire in bursts that are interrupted by long pauses. The regularity in simple spike firing only reached maturity at 4 wk of age. In contrast, the adult complex spike pattern was already evident by the second week of life, remaining consistent across all ages. Analyses of Purkinje cells in alert behaving mice suggested that the adult patterns are attained more than a week after the completion of key morphogenetic processes such as migration, lamination, and foliation. Purkinje cell activity is therefore dynamically sculpted throughout postnatal development, traversing several critical events that are required for circuit formation. Overall, we show that simple spike and complex spike firing develop with unique developmental trajectories.

Cadherin-7 regulates mossy fiber connectivity in the cerebellum

To establish highly precise patterns of neural connectivity, developing axons must stop growing at their appropriate destinations and specifically synapse with target cells. However, the molecular mechanisms governing these sequential steps remain poorly understood. Here, we demonstrate that cadherin-7 (Cdh7) plays a dual role in axonal growth termination and specific synapse formation during the development of the cerebellar mossy fiber circuit. Cdh7 is expressed in mossy fiber pontine nucleus (PN) neurons and their target cerebellar granule neurons during synaptogenesis and selectively mediates synapse formation between those neurons. Additionally, Cdh7 presented by mature granule neurons diminishes the growth potential of PN axons. Furthermore, knockdown of Cdh7 in PN neurons in vivo severely impairs the connectivity of PN axons in the developing cerebellum. These findings reveal a mechanism by which a single bifunctional cell-surface receptor orchestrates precise wiring by regulating axonal growth potential and synaptic specificity.

Cerebellar zonal patterning relies on Purkinje cell neurotransmission

Cerebellar circuits are patterned into an array of topographic parasagittal domains called zones. The proper connectivity of zones is critical for motor coordination and motor learning, and in several neurological diseases cerebellar circuits degenerate in zonal patterns. Despite recent advances in understanding zone function, we still have a limited understanding of how zones are formed. Here, we focused our attention on Purkinje cells to gain a better understanding of their specific role in establishing zonal circuits. We used conditional mouse genetics to test the hypothesis that Purkinje cell neurotransmission is essential for refining prefunctional developmental zones into sharp functional zones. Our results show that inhibitory synaptic transmission in Purkinje cells is necessary for the precise patterning of Purkinje cell zones and the topographic targeting of mossy fiber afferents. As expected, blocking Purkinje cell neurotransmission caused ataxia. Using in vivo electrophysiology, we demonstrate that loss of Purkinje cell communication altered the firing rate and pattern of their target cerebellar nuclear neurons. Analysis of Purkinje cell complex spike firing revealed that feedback in the cerebellar nuclei to inferior olive to Purkinje cell loop is obstructed. Loss of Purkinje neurotransmission also caused ectopic zonal expression of tyrosine hydroxylase, which is only expressed in adult Purkinje cells when calcium is dysregulated and if excitability is altered. Our results suggest that Purkinje cell inhibitory neurotransmission establishes the functional circuitry of the cerebellum by patterning the molecular zones, fine-tuning afferent circuitry, and shaping neuronal activity.

Temporal regulation of nuclear factor one occupancy by calcineurin/NFAT governs a voltage-sensitive developmental switch in late maturing neurons

Dendrite and synapse development are critical for establishing appropriate neuronal circuits, and disrupted timing of these events can alter neural connectivity. Using microarrays, we have identified a nuclear factor I (NFI)-regulated temporal switch program linked to dendrite formation in developing mouse cerebellar granule neurons (CGNs). NFI function was required for upregulation of many synapse-related genes as well as downregulation of genes expressed in immature CGNs. Chromatin immunoprecipitation analysis revealed that a central feature of this program was temporally regulated NFI occupancy of late-expressed gene promoters. Developing CGNs undergo a hyperpolarizing shift in membrane potential, and depolarization inhibits their dendritic and synaptic maturation via activation of calcineurin (CaN) (Okazawa et al., 2009). Maintaining immature CGNs in a depolarized state blocked NFI temporal occupancy of late-expressed genes and the NFI switch program via activation of the CaN/nuclear factor of activated T-cells, cytoplasmic (NFATc) pathway and promotion of late-gene occupancy by NFATc4, and these mechanisms inhibited dendritogenesis. Conversely, inhibition of the CaN/NFATc pathway in CGNs maturing under physiological nondepolarizing conditions upregulated the NFI switch program, NFI temporal occupancy, and dendrite formation. NFATc4 occupied the promoters of late-expressed NFI program genes in immature mouse cerebellum, and its binding was temporally downregulated with development. Further, NFI temporal binding and switch gene expression were upregulated in the developing cerebellum of Nfatc4 (-/-) mice. These findings define a novel NFI switch and temporal occupancy program that forms a critical link between membrane potential/CaN and dendritic maturation in CGNs. CaN inhibits the program and NFI occupancy in immature CGNs by promoting NFATc4 binding to late-expressed genes. As maturing CGNs become more hyperpolarized, NFATc4 binding declines leading to onset of NFI temporal binding and the NFI switch program.

The effects of target skeletal muscle cells on dorsal root ganglion neuronal outgrowth and migration in vitro

Targets of neuronal innervations play a vital role in regulating the survival and differentiation of innervating neurotrophin-responsive neurons. During development, neurons extend axons to their targets, and then their survival become dependent on the trophic substances secreted by their target cells. Sensory endings were present on myoblasts, myotubes, and myofibers in all intrafusal bundles regardless of age. The interdependence of sensory neurons and skeletal muscle (SKM) cells during both embryonic development and the maintenance of the mature functional state has not been fully understood. In the present study, neuromuscular cocultures of organotypic dorsal root ganglion (DRG) explants and dissociate SKM cells were established. Using this culture system, the morphological relationship between DRG neurons and SKM cells, neurites growth and neuronal migration were investigated. The migrating neurons were determined by fluorescent labeling of microtubule-associated protein-2 (MAP-2) and neurofilament 200 (NF-200) or growth-associated protein 43 (GAP-43). The expression of NF-200 and GAP-43 and their mRNAs was evaluated by Western blot assay and real time-PCR analysis. The results reveal that DRG explants showed more dense neurites outgrowth in neuromuscular cocultures as compared with that in the culture of DRG explants alone. The number of total migrating neurons (the MAP-2-expressing neurons) and the percentage NF-200-immunoreactive (IR) and GAP-43-IR neurons increased significantly in the presence of SKM cells. The levels of NF-200 and GAP-43 and their mRNAs increased significantly in neuromuscular cocultures as compared with that in the culture of DRG explants alone. These results suggested that target SKM cells play an important role in regulating neuronal protein synthesis, promoting neuritis outgrowth and neuronal migration of DRG explants in vitro. These results not only provide new clues for a better understanding of the association of SKM cells with DRG sensory neurons during development, they may also have implications for axonal regeneration after nerve injury.

Reciprocal actions of ATF5 and Shh in proliferation of cerebellar granule neuron progenitor cells

Precise regulation of neuroprogenitor cell proliferation and differentiation is required for successful brain development, but the factors that contribute to this are only incompletely understood. The transcription factor ATF5 promotes proliferation of cerebral cortical neuroprogenitor cells and its down regulation permits their differentiation. Here, we examine the expression and regulation of ATF5 in cerebellar granule neuron progenitor cells (CGNPs) as well as the role of ATF5 in the transition of CGNPs to postmitotic cerebellar granule neurons (GCNs). We find that ATF5 is expressed by proliferating CGNPs in both the embryonic and postnatal cerebellar external granule layer (EGL) and in the rhombic lip, the embryonic structure from which the EGL arises. In contrast, ATF5 is undetectable in postmitotic GCNs. In highly enriched dissociated cultures of CGNPs and CGNs, ATF5 is expressed only in CGNPs. Constitutive ATF5 expression in CGNPs does not affect their proliferation or exit from the cell cycle. In contrast, in presence of sonic hedgehog (Shh), a mitogen for CGNPs, constitutively expressed ATF5 promotes CGNP proliferation and delays their cell cycle exit and differentiation. Conversely, ATF5 loss-of-function conferred by a dominant-negative form of ATF5 significantly diminishes Shh-stimulated CGNP proliferation and promotes differentiation. In parallel with its stimulation of CGNP proliferation, Shh enhances ATF5 expression by what appeared to be a posttranscriptional mechanism involving protein stabilization. These findings indicate a reciprocal interaction between ATF5 and Shh in which Shh stimulates ATF5 expression and in which ATF5 contributes to Shh-stimulated CGNP expansion.

Cut-like homeobox 1 and nuclear factor I/B mediate ENGRAILED2 autism spectrum disorder-associated haplotype function

Both common and rare variants contribute to autism spectrum disorder (ASD) risk, but few variants have been established as functional. Previously we demonstrated that an intronic haplotype (rs1861972-rs1861973 A-C) in the homeobox transcription factor ENGRAILED2 (EN2) is significantly associated with ASD. Positive association has also been reported in six additional data sets, suggesting EN2 is an ASD susceptibility gene. Additional support for this possibility requires identification of functional variants that affect EN2 regulation or activity. In this study, we demonstrate that the A-C haplotype is a transcriptional activator. Luciferase (luc) assays in mouse neuronal cultures determined that the A-C haplotype increases expression levels (50%, P < 0.01, 24 h; 250%, P < 0.0001, 72 h). Mutational analysis indicates that the A-C haplotype activator function requires both associated A and C alleles. A minimal 202-bp element is sufficient for function and also specifically binds a protein complex. Mass spectrometry identified these proteins as the transcription factors, Cut-like homeobox 1 (Cux1) and nuclear factor I/B (Nfib). Subsequent antibody supershifts and chromatin immunoprecipitations demonstrated that human CUX1 and NFIB bind the A-C haplotype. Co-transfection and knock-down experiments determined that both CUX1 and NFIB are required for the A-C haplotype activator function. These data demonstrate that the ASD-associated A-C haplotype is a transcriptional activator, and both CUX1 and NFIB mediate this activity. These results provide biochemical evidence that the ASD-associated A-C haplotype is functional, further supporting EN2 as an ASD susceptibility gene.

VEGF modulates NMDA receptors activity in cerebellar granule cells through Src-family kinases before synapse formation

NMDA type glutamate receptors (NMDARs) are best known for their role in synaptogenesis and synaptic plasticity. Much less is known about their developmental role before neurons form synapses. We report here that VEGF, which promotes migration of granule cells (GCs) during postnatal cerebellar development, enhances NMDAR-mediated currents and Ca(2+) influx in immature GCs before synapse formation. The VEGF receptor Flk1 forms a complex with the NMDAR subunits NR1 and NR2B. In response to VEGF, the number of Flk1/NR2B coclusters on the cell surface increases. Stimulation of Flk1 by VEGF activates Src-family kinases, which increases tyrosine phosphorylation of NR2B. Inhibition of Src-family kinases abolishes the VEGF-dependent NR2B phosphorylation and amplification of NMDAR-mediated currents and Ca(2+) influx in GCs. These findings identify VEGF as a modulator of NMDARs before synapse formation and highlight a link between an activity-independent neurovascular guidance cue (VEGF) and an activity-regulated neurotransmitter receptor (NMDAR).

Development of axon-target specificity of ponto-cerebellar afferents

The function of neuronal networks relies on selective assembly of synaptic connections during development. We examined how synaptic specificity emerges in the pontocerebellar projection. Analysis of axon-target interactions with correlated light-electron microscopy revealed that developing pontine mossy fibers elaborate extensive cell-cell contacts and synaptic connections with Purkinje cells, an inappropriate target. Subsequently, mossy fiber-Purkinje cell connections are eliminated resulting in granule cell-specific mossy fiber connectivity as observed in mature cerebellar circuits. Formation of mossy fiber-Purkinje cell contacts is negatively regulated by Purkinje cell-derived BMP4. BMP4 limits mossy fiber growth in vitro and Purkinje cell-specific ablation of BMP4 in mice results in exuberant mossy fiber-Purkinje cell interactions. These findings demonstrate that synaptic specificity in the pontocerebellar projection is achieved through a stepwise mechanism that entails transient innervation of Purkinje cells, followed by synapse elimination. Moreover, this work establishes BMP4 as a retrograde signal that regulates the axon-target interactions during development.

Temporal control of a dendritogenesis-linked gene via REST-dependent regulation of nuclear factor I occupancy

How the timing of gene expression is controlled during neuronal development is largely unknown. Here we describe a temporal mechanism of gene regulation in differentiating postmitotic neurons involving delayed promoter site occupancy by nuclear factor I and the control of its initial onset by the trans-repressor REST.

 Developing neurons undergo a series of maturational stages, and the timing of these events is critical for formation of synaptic circuitry. Here we addressed temporal regulation of the Gabra6 gene, which is expressed in a delayed manner during dendritogenesis in maturing cerebellar granule neurons (CGNs). Developmental up-regulation of Gabra6 transcription required a binding site for nuclear factor I (NFI) proteins. The amounts and DNA binding activities of NFI proteins were similar in immature and mature CGNs; however, NFI occupancy of the Gabra6 promoter in native chromatin was temporally delayed in parallel with Gabra6 gene expression, both in vivo and in culture. The trans-repressor RE1 silencing transcription factor (REST) occupied the Gabra6 proximal promoter in CGN progenitors and early postmitotic CGNs, and its departure mirrored the initial onset of NFI binding as CGNs differentiated. Furthermore constitutive REST expression blocked both Gabra6 expression and NFI occupancy in mature CGNs, whereas REST knockdown in immature CGNs accelerated the initiation of both events. These studies identify a novel mechanism for controlling the timing of dendritogenesis-associated gene expression in maturing neurons through delayed binding of NFI proteins to chromatin. They also establish a temporal function for REST in preventing premature promoter occupancy by NFI proteins in early-stage postmitotic neurons.

Engrailed homeobox genes regulate establishment of the cerebellar afferent circuit map

The spatial organization of the cerebellar afferent map has remarkable correspondence to two aspects of intrinsic patterning within the cerebellum embodied by a series of lobules and Purkinje cell (PC)-striped gene expression. Using male and female mice, we tested whether the Engrailed (En) homeobox genes are a common genetic substrate regulating all three systems, since they are expressed in spatially restricted domains within the cerebellum and are critical for patterning PC gene expression and foliation. Indeed, we discovered that En1/2 are necessary for the precise targeting of mossy fibers to distinct lobules, as well as their subsequent resolution into discrete parasagittal bands. Moreover, each En gene coordinately regulates afferent targeting and the striped pattern of PC protein expression (e.g., ZebrinII/AldolaseC) independent of regulating foliation. We further found that En1/2, rather than the presence of a full complement of lobules, are critical for generating PC protein stripes and mossy fiber bands, and that PC striped gene expression is determined before afferent banding. Thus, the En transcription factors not only regulate cerebellum circuit topography, but they also link afferent and efferent neurons precisely enough that alterations in PC protein expression can be used as a read out for underlying defects in circuitry. In summary, our data suggest that En1/2 are master regulators of three-dimensional organization of the cerebellum and coordinately regulate morphology, patterned gene expression, and afferent topography.

Knockdown of the neuronal nitric oxide synthase gene retard the development of the cerebellar granule neurons in vitro

The role of endogenous neuronal nitric oxide synthase (nNOS) gene in the development of cerebellar granule neurons (CGNs) is conflicting. Here, we tested the effect of antisense oligos (AS-ODN) on the endogenous nNOS gene and the development of the CGNs in vitro. The expression of nNOS increased in a development-dependent pattern both in terms of mRNA and protein. AS-ODN down-regulated nNOS gene, but in a posttranscriptional manner. Knockdown of nNOS protein decreased the viability of the CGNs from 7 to 13 days in culture (DIC). This activity of AS-ODN was mimicked by nNOS inhibitor I. The antagonist (nNOSi, MK-801, or ODQ) -induced decrease of cell viability was normalized by the provision of the sodium nitroprusside, an NO donor. This study provides direct evidence that endogenous nNOS, mainly by means of its principal product NO, plays an active role in sustaining the survival of developing CGNs at transition from differentiation to maturation.

A novel somatostatin-immunoreactive mossy fiber pathway associated with HSP25-immunoreactive purkinje cell stripes in the mouse cerebellum

Somatostatin 28 immunoreactivity (Sst28-ir) identifies a specific subset of mossy fiber terminals in the adult mouse cerebellum. By using double-labeling immunohistochemistry, we determined that Sst28-ir is associated with presynaptic mossy fiber terminal rosettes, and not Purkinje cells, Golgi cells, or unipolar brush cells. Sst28-ir mossy fibers are restricted to the central zone (lobules VI/VII) and nodular zone (lobules IX, X) of the vermis, and the paraflocculus and flocculus. Within each transverse zone the mossy fiber terminal fields form a reproducible array of parasagittal stripes. The boundaries of Sst28-ir stripes align with a specific array of Purkinje cell stripes revealed by using immunocytochemistry for the small heat shock protein HSP25. In the cerebellum of the homozygous weaver mouse, in which a subpopulation of HSP25-ir Purkinje cells are located ectopically, the corresponding Sst28-ir mossy fiber projection is also ectopic, suggesting a role for a specific Purkinje cell subset in afferent pattern formation. Likewise, in the scrambler mutant mouse, Sst28-ir mossy fibers show a very close association with HSP25-ir Purkinje cell clusters. HSP25 itself does not appear to be critical for normal patterning, however: in the KJR mouse, which does not express cerebellar HSP25, Sst28 expression appears to be normal. Likewise, the Purkinje cell patterning antigens zebrin II and HSP25 are expressed normally in both Sst- and Sst-receptor knockout mice, suggesting that somatostatinergic transmission is not necessary for Purkinje cell stripe formation.

Dendrites of cerebellar granule cells correctly recognize their target axons for synaptogenesis in vitro

Neural circuits are generated by precisely ordered synaptic connections among neurons, and this process is thought to rely on the ability of neurons to recognize specific partners. However, it is also known that neurons promiscuously form synapses with nonspecific partners, in particular when cultured in vitro, causing controversies about neural recognition mechanisms. Here we reexamined whether neurons can or cannot select particular partners in vitro. In the cerebellum, granule cell (GC) dendrites form synaptic connections specifically with mossy fibers, but not with climbing fibers. We cocultured GC neurons with pontine or inferior olivary axons, the major sources for mossy and climbing fibers, respectively, as well as with hippocampal axons as a control. The GC neurons formed synapses with pontine axons predominantly at the distal ends of their dendrites, reproducing the characteristic morphology of their synapses observed in vivo, whereas they failed to do so when combined with other axons. In the latter case, synaptic proteins could accumulate between axons and dendrites, but these synapses were randomly distributed throughout the contact sites, and also their synaptic vesicle recycling was anomalous. These observations suggest that GC dendrites can select their authentic partners for synaptogenesis even in vitro, forming the synapses with a GC-specific nature only with them.

Selective vulnerability of neurons in primary cultures and in neurodegenerative diseases

Primary neuronal cultures are commonly used to dissect the molecular and cellular mechanisms that underlie human brain diseases. Neurons dissociated from an embryonic brain and grown in culture dishes are almost by definition different from those residing inside a living brain. Not only are the individual cells stripped of their normal chemical and physical contacts, but the cellular composition of the cultures (the ratio of cell types) can be affected by many intrinsic and extrinsic factors, including brain region, neuronal birthday, gender, genetic background and in vitro age. Changes in any of these factors may have a strong impact on the outcome of the experiment. In a recent study, Romito-DiGiacomo et al. /54/ demonstrated that when neurons were harvested from murine embryonic cortex, the typical protocol favored cells that were just finishing cell division at the time of harvest. By taking advantage of the fact that the date of the final cell division (birthday) of a neuron correlates with its final position in the cortical plate they were able to assay deeper layer neurons (layers V-VI) separately from the more superficial layers (layers II-III). They reported that while the superficial cells were sensitive to the toxic effect of beta-amyloid, the deeper layer neurons were virtually resistant to death in its presence. The findings recapitulate selective vulnerability in the neocortex of Alzheimer's disease. This is a beautiful example of how to turn the apparent weakness of primary cultures into strength through experimental design and data interpretation. Selective vulnerability is a common feature of neurodegenerative disease, thus it is critical to use the right primary culture. Do you know what is in your culture?

Isolation and culture of post-natal mouse cerebellar granule neuron progenitor cells and neurons

The cerebellar cortex is a well described structure that provides unique opportunities for studying neuronal properties and development^1,2. Of the cerebellar neuronal types (granule cells, Purkinje cells and inhibitory interneurons), granule neurons are by far the most numerous and are the most abundant type of neurons in the mammalian brain. In rodents, cerebellar granule neurons are generated during the first two post-natal weeks from progenitor cells in the outermost layer of the cerebellar cortex, the external granule layer (EGL). The protocol presented here describes techniques to enrich and culture granule neurons and their progenitor cells from post-natal mouse cerebellum. We will describe procedures to obtain cultures of increasing purity^3,4, which can be used to study the differentiation of proliferating progenitor cells into granule neurons^5,6. Once the progenitor cells differentiate, the cultures also provide a homogenous population of granule neurons for experimental manipulation and characterization of phenomena such as synaptogenesis, glutamate receptor function⁷, interaction with other purified cerebellar cells^8,9 or cell death⁷.

Neurite outgrowth and in vivo sensory innervation mediated by a Ca(V)2.2-laminin beta 2 stop signal

Axons and dendrites of developing neurons establish distributed innervation patterns enabling precise discrimination in sensory systems. We describe the role of the extracellular matrix molecule, laminin beta2, interacting with the Ca(V)2.2 calcium channel in establishing appropriate sensory innervation. In vivo, Ca(V)2.2 is expressed on the growth cones of Xenopus laevis sensory neurites and laminin beta2 is expressed in the skin. Culturing neurons on a laminin beta2 substrate inhibits neurite outgrowth in a specific and calcium-dependent manner. Blocking signaling between laminin beta2 and Ca(V)2.2 leads to increased numbers of sensory terminals in vivo. These findings suggest that interactions between extracellular matrix molecules and calcium channels regulate connectivity in the developing nervous system.

Target-dependent inhibition of sympathetic neuron growth via modulation of a BMP signaling pathway

Target-derived factors modulate many aspects of peripheral neuron development including neuronal growth, survival, and maturation. Less is known about how initial target contact regulates changes in gene expression associated with these developmental processes. One early consequence of contact between growing sympathetic neurons and their cardiac myocyte targets is the inhibition of neuronal outgrowth. Analysis of neuronal gene expression following this contact revealed coordinate regulation of a bone morphogenetic protein (BMP)-dependent growth pathway in which basic helix-loop-helix transcription factors and downstream neurofilament expression contribute to the growth dynamics of developing sympathetic neurons. BMP2 had dose-dependent growth-promoting effects on sympathetic neurons cultured in the absence, but not the presence, of myocyte targets, suggesting that target contact alters neuronal responses to BMP signaling. Target contact also induced the expression of matrix Gla protein (MGP), a regulator of BMP function in the vascular system. Increased MGP expression inhibited BMP-dependent neuronal growth and MGP expression increased in sympathetic neurons during the period of target contact in vivo. These experiments establish MGP as a novel regulator of BMP function in the nervous system, and define developmental transitions in BMP responses during sympathetic development.

Cerebellar granule cells transplanted in vivo can follow physiological and unusual migratory routes to integrate into the recipient cortex

CNS repair by cell transplantation requires new neurons to integrate into complex recipient networks. We assessed how the migratory route of transplanted granule neurons and the developmental stage of the host rat cerebellum influence engraftment. In both embryonic and postnatal hosts, granule cells can enter the cerebellar cortex and achieve correct placement along their natural migratory pathway. Donor neurons can also reach the internal granular layer from the white matter and integrate following an unusual developmental pattern. Although the frequency of correct positioning declines in parallel with cortical development, in mature recipients correct homing is more frequent through the unusual path. Following depletion of granule cell precursors in the host, more granule neurons engraft, but their ability for achieving correct placement is unchanged. Therefore, while the cerebellar environment remains receptive for granule cells even after the end of development, their full integration is partially hindered by the mature cortical architecture.

Permissive and repulsive cues and signalling pathways of axonal outgrowth and regeneration

Successful axonal outgrowth in the adult central nervous system (CNS) is central to the process of nerve regeneration and brain repair. To date, much of the knowledge on axonal guidance and outgrowth comes from studies on neuritogenesis and patterning during development where distal growth cones constantly sample the local environment and respond to specific physical and trophic influences. Opposing permissive (e.g., growth factors) and hostile signals (e.g., repulsive cues) are processed, leading to growth cone remodelling, and a concomitant restructuring of the cytoskeleton, thereby permitting pioneering extension and a potential for establishing synaptic connections. Repulsive cues, such as semaphorins, ephrins and myelin-secreted inhibitory glycoproteins, act through their respective receptors to affect the collapsing or turning of growth cones via several pathways, such as the Rho GTPases signalling which precipitates the cytoskeletal changes. One of the direct modulators of microtubules is the family of brain-specific proteins, collapsin response mediator protein (CRMP). Exciting evidence emerged recently that cleavage of CRMPs in response to injury-activated proteases, such as calpain, signals axonal retraction and neuronal death in adult post-mitotic neurons, while blocking this signal transduction prevents axonal retraction and death following excitotoxic insult and cerebral ischemia. Regeneration is minimal in injured postnatal CNS, albeit the occurrence of some limited remodelling in areas where synaptic plasticity is prevalent. Frequently in the absence of axonal regeneration, there is not only an inevitable loss of functional connections, but also a loss of neurons, such as through the actions of dependence receptors. Deciphering the cues and signalling pathways of axonal guidance and outgrowth may hold the key to fully understanding nerve regeneration and brain repair, thereby opening the way for developing potential therapeutics.

Morphology, molecular codes, and circuitry produce the three-dimensional complexity of the cerebellum

The most noticeable morphological feature of the cerebellum is its folded appearance, whereby fissures separate its anterior-posterior extent into lobules. Each lobule is molecularly coded along the medial-lateral axis by parasagittal stripes of gene expression in one cell type, the Purkinje cells (PCs). Additionally, within each lobule distinct combinations of afferents terminate and supply the cerebellum with synchronized sensory and motor information. Strikingly, afferent terminal fields are organized into parasagittal domains, and this pattern bears a close relationship to PC molecular coding. Thus, cerebellum three-dimensional complexity obeys a basic coordinate system that can be broken down into morphology and molecular coding. In this review, we summarize the sequential stages of cerebellum development that produce its laminar structure, foliation, and molecular organization. We also introduce genes that regulate morphology and molecular coding, and discuss the establishment of topographical circuits within the context of the two coordinate systems. Finally, we discuss how abnormal cerebellar organization may result in neurological disorders like autism.

A novel role of the Mad family member Mad3 in cerebellar granule neuron precursor proliferation

During development, Sonic hedgehog (Shh) regulates the proliferation of cerebellar granule neuron precursors (GNPs) in part via expression of Nmyc. We present evidence supporting a novel role for the Mad family member Mad3 in the Shh pathway to regulate Nmyc expression and GNP proliferation. Mad3 mRNA is transiently expressed in GNPs during proliferation. Cultured GNPs express Mad3 in response to Shh stimulation in a cyclopamine-dependent manner. Mad3 is necessary for Shh-dependent GNP proliferation as measured by bromodeoxyuridine incorporation and Nmyc expression. Furthermore, Mad3 overexpression, but not that of other Mad proteins, is sufficient to induce GNP proliferation in the absence of Shh. Structure-function analysis revealed that Max dimerization and recruitment of the mSin3 corepressor are required for Mad3-mediated GNP proliferation. Surprisingly, basic-domain-dependent DNA binding of Mad3 is not required, suggesting that Mad3 interacts with other DNA binding proteins to repress transcription. Interestingly, cerebellar tumors and pretumor cells derived from patched heterozygous mice express high levels of Mad3 compared with adjacent normal cerebellar tissue. Our studies support a novel role for Mad3 in cerebellar GNP proliferation and possibly tumorigenesis, and they challenge the current paradigm that Mad3 should antagonize Nmyc by competition for direct DNA binding via Max dimerization.

Differential effects of AMPA receptor activation on survival and neurite integrity during neuronal development

While neuronal cultures are an established model for analyzing excitotoxic brain injury in the adult, in vitro systems have not been extensively employed to study how developing neurons respond to levels of excitatory compounds that are lethal to mature neurons. Recently, we reported that the in vivo differentiation programs of cerebellar granule cells (CGNs) are recapitulated in purified CGN cultures [Manzini M.C., Ward M.S., Zhang Q., Lieberman M.D., Mason C.A. (2006) The stop-signal revised: immature cerebellar granule neurons in the external germinal layer arrest pontine mossy fiber growth. J. Neurosci. 26:6040-6051]. Here, we have used this model system to compare the response of immature and mature neurons to excitotoxic compounds. We found that immature CGNs are less sensitive to AMPA receptor (AMPA-R) activation than mature cells and that levels of AMPA-R expression on the plasma membrane are critical in regulating the balance between death and survival during maturation of these neurons. However, the majority of immature cells that survive excitotoxic treatment bear a degenerating neurite, suggesting that AMPA-R activation can still cause damage in the absence of cell death.