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

Untangling the wires: development of sparse, distributed connectivity in the mushroom body calyx

Appropriate perception and representation of sensory stimuli pose an everyday challenge to the brain. In order to represent the wide and unpredictable array of environmental stimuli, principle neurons of associative learning regions receive sparse, combinatorial sensory inputs. Despite the broad role of such networks in sensory neural circuits, the developmental mechanisms underlying their emergence are not well understood. As mammalian sensory coding regions are numerically complex and lack the accessibility of simpler invertebrate systems, we chose to focus this review on the numerically simpler, yet functionally similar, Drosophila mushroom body calyx. We bring together current knowledge about the cellular and molecular mechanisms orchestrating calyx development, in addition to drawing insights from literature regarding construction of sparse wiring in the mammalian cerebellum. From this, we formulate hypotheses to guide our future understanding of the development of this critical perceptual center.

Developmental Rewiring between Cerebellar Climbing Fibers and Purkinje Cells Begins with Positive Feedback Synapse Addition

During postnatal development, cerebellar climbing fibers alter their innervation strengths onto supernumerary Purkinje cell targets, generating a one-to-few connectivity pattern in adulthood. To get insight about the processes responsible for this remapping, we reconstructed serial electron microscopy datasets from mice during the first postnatal week. Between days 3 and 7, individual climbing fibers selectively add many synapses onto a subset of Purkinje targets in a positive-feedback manner, without pruning synapses from other targets. Active zone sizes of synapses associated with powerful versus weak inputs are indistinguishable. Changes in synapse number are thus the predominant form of early developmental plasticity. Finally, the numbers of climbing fibers and Purkinje cells in a local region nearly match. Initial over-innervation of Purkinje cells by climbing fibers is therefore economical: the number of axons entering a region is enough to assure that each ultimately retains a postsynaptic target and that none branched there in vain.

Wilson et al. use electron microscopy to reveal that developmental rewiring in the cerebellum begins with significant synapse addition by climbing fibers onto a few preferred Purkinje cells. They also find that rewiring is economical: all climbing fibers initially entering a cerebellar region play a role in final connectivity there.

ADP Ribosylation Factor 6 Regulates Neuronal Migration in the Developing Cerebral Cortex through FIP3/Arfophilin-1-dependent Endosomal Trafficking of N-cadherin

During neural development, endosomal trafficking controls cell shape and motility through the polarized transport of membrane proteins related to cell–cell and cell–extracellular matrix interactions. ADP ribosylation factor 6 (Arf6) is a critical small GTPase that regulates membrane trafficking between the plasma membrane and endosomes. We herein demonstrated that the knockdown of endogenous Arf6 in mouse cerebral cortices led to impaired neuronal migration in the intermediate zone and cytoplasmic retention of N-cadherin and syntaxin12 in migrating neurons. Rescue experiments with separation-of-function Arf6 mutants identified Rab11 family-interacting protein 3 (FIP3)/Arfophilin-1, a dual effector for Arf6 and Rab11, as a downstream effector of Arf6 in migrating neurons. The knockdown of FIP3 led to impaired neuronal migration in the intermediate zone and cytoplasmic retention of N-cadherin in migrating neurons, similar to that of Arf6, which could be rescued by the coexpression of wild-type FIP3 but not FIP3 mutants lacking the binding site for Arf6 or Rab11. These results suggest that Arf6 regulates cortical neuronal migration in the intermediate zone through the FIP3-dependent endosomal trafficking.

ROS produced by NOX2 control in vitro development of cerebellar granule neurons development

Reactive oxygen species (ROS) act as signaling molecules that regulate nervous system physiology. ROS have been related to neural differentiation, neuritogenesis, and programmed cell death. Nevertheless, little is known about the mechanisms involved in the regulation of ROS during neuronal development. In this study, we evaluated the mechanisms by which ROS are regulated during neuronal development and the implications of these molecules in this process. Primary cultures of cerebellar granule neurons (CGN) were used to address these issues. Our results show that during the first 3 days of CGN development in vitro (days in vitro; DIV), the levels of ROS increased, reaching a peak at 2 and 3 DIV under depolarizing (25 mM KCl) and nondepolarizing (5 mM KCl) conditions. Subsequently, under depolarizing conditions, the ROS levels markedly decreased, but in nondepolarizing conditions, the ROS levels increased gradually. This correlated with the extent of CGN maturation. Also, antioxidants and NADPH-oxidases (NOX) inhibitors reduced the expression of Tau and MAP2. On the other hand, the levels of glutathione markedly increased at 1 DIV. We inferred that the ROS increase at this time is critical for cell survival because glutathione depletion leads to axonal degeneration and CGN death only at 2 DIV. During the first 3 DIV, NOX2 was upregulated and expressed in filopodia and growth cones, which correlated with the hydrogen peroxide (H2O2) distribution in the cell. Finally, NOX2 KO CGN showed shorter neurites than wild-type CGN. Taken together, these results suggest that the regulation of ROS is critical during the early stages of CGN development.

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.

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.

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.

Morphogenesis in Kyoto: a confluence of cell and developmental biology

Understanding morphogenesis is the ultimate multidisciplinary (ad)venture. Three-dimensional tissues are generated from the actions of genes, biochemical pathways, and cells that form multicellular networks and interact with their biomechanical environment. A comprehensive explanation of morphogenetic processes must encompass these different levels of analysis. A recent meeting in Kyoto on “Building the Body Plan: How Cell Adhesion, Signaling, and Cytoskeletal Regulation Shape Morphogenesis” highlighted recent advances in tackling this challenging problem.