Parizad M. Bilimoria

A Centrosomal Cdc20-APC Pathway Controls Dendrite Morphogenesis in Postmitotic Neurons

The ubiquitin ligase anaphase-promoting complex (APC) recruits the coactivator Cdc20 to drive mitosis in cycling cells. However, the nonmitotic functions of Cdc20-APC have remained unexplored. We report that Cdc20-APC plays an essential role in dendrite morphogenesis in postmitotic neurons. Knockdown of Cdc20 in cerebellar slices and in postnatal rats in vivo profoundly impairs the formation of granule neuron dendrite arbors in the cerebellar cortex. Remarkably, Cdc20 is enriched at the centrosome in neurons, and the centrosomal localization is critical for Cdc20-dependent dendrite development. We also find that the centrosome-associated protein histone deacetylase 6 (HDAC6) promotes the polyubiquitination of Cdc20, stimulates the activity of centrosomal Cdc20-APC, and drives the differentiation of dendrites. These findings define a postmitotic function for Cdc20-APC in the morphogenesis of dendrites in the mammalian brain. The identification of a centrosomal Cdc20-APC ubiquitin signaling pathway holds important implications for diverse biological processes, including neuronal connectivity and plasticity.

The Neuropeptide VGF Produces Antidepressant-Like Behavioral Effects and Enhances Proliferation in the Hippocampus

Brain-derived neurotrophic factor (BDNF) is upregulated in the hippocampus by antidepressant treatments, and BDNF produces antidepressant-like effects in behavioral models of depression. In our previous work, we identified genes induced by BDNF and defined their specific roles in hippocampal neuronal development and plasticity. To identify genes downstream of BDNF that may play roles in psychiatric disorders, we examined a subset of BDNF-induced genes also regulated by 5-HT (serotonin), which includes the neuropeptide VGF (nonacronymic). To explore the function of VGF in depression, we first investigated the expression of the neuropeptide in animal models of depression. VGF was downregulated in the hippocampus after both the learned helplessness and forced swim test (FST) paradigms. Conversely, VGF infusion in the hippocampus of mice subjected to FST reduced the time spent immobile for up to 6 d, thus demonstrating a novel role for VGF as an antidepressant-like agent. Recent evidence indicates that chronic treatment of rodents with antidepressants increases neurogenesis in the adult dentate gyrus and that neurogenesis is required for the behavioral effects of antidepressants. Our studies using [3H]thymidine and bromodeoxyuridine as markers of DNA synthesis indicate that chronic VGF treatment enhances proliferation of hippocampal progenitor cells both in vitro and in vivo with survival up to 21 d. By double immunocytochemical analysis of hippocampal neurons, we demonstrate that VGF increases the number of dividing cells that express neuronal markers in vitro. Thus, VGF may act downstream of BDNF and exert its effects as an antidepressant-like agent by enhancing neurogenesis in the hippocampus.

A Cdc20-APC Ubiquitin Signaling Pathway Regulates Presynaptic Differentiation

Cdc20-APC in Synapse Formation The E3 ubiquitin ligase Cdc20-anaphase promoting complex (Cdc20-APC) has important roles in the control of the cell division cycle. Yang et al. (p. 575) now show that Cdc20-APC also appears to be required for proper formation of synapses by developing neurons in the rat brain. When Cdc20-APC was depleted from cultured neurons or in the brains of developing rat pups, synapse formation was inhibited. The brain-enriched transcription factor NeuroD2 was shown to be a possible target of Cdc20-APC–stimulated degradation. NeuroD2 may act by promoting synthesis of Complexin II, a protein that regulates synaptic vesicle fusion. The ubiquitin ligase Cdc20-APC is required for proper synapse formation in the developing rat brain. Presynaptic axonal differentiation is essential for synapse formation and the establishment of neuronal circuits. However, the mechanisms that coordinate presynaptic development in the brain are largely unknown. We found that the major mitotic E3 ubiquitin ligase Cdc20-anaphase promoting complex (Cdc20-APC) regulates presynaptic differentiation in primary postmitotic mammalian neurons and in the rat cerebellar cortex. Cdc20-APC triggered the degradation of the transcription factor NeuroD2 and thereby promoted presynaptic differentiation. The NeuroD2 target gene encoding Complexin II, which acts locally at presynaptic sites, mediated the ability of NeuroD2 to suppress presynaptic differentiation. Thus, our findings define a Cdc20-APC ubiquitin signaling pathway that governs presynaptic development, which holds important implications for neuronal connectivity and plasticity in the brain.

A molecular basis for phosphorylation-dependent SUMO conjugation by the E2 UBC9.

Phosphorylation and small ubiquitin-like modifier (SUMO) conjugation contribute to the spatial and temporal regulation of substrates containing phosphorylation-dependent SUMO consensus motifs (PDSMs). Myocyte-enhancement factor 2 (MEF2) is a transcription factor and PDSM substrate whose modification by SUMO drives postsynaptic dendritic differentiation. NMR analysis revealed that the human SUMO E2 interacted with model substrates for phosphorylated and nonphosphorylated MEF2 in similar extended conformations. Mutational and biochemical analysis identified a basic E2 surface that enhanced SUMO conjugation to phosphorylated PDSM substrates MEF2 and heat-shock transcription factor 1 (HSF1), but not to nonphosphorylated MEF2 or HSF1, nor the non-PDSM substrate p53. Mutant ubiquitin-conjugating enzyme UBC9 isoforms defective in promoting SUMO conjugation to phosphorylated MEF2 in vitro and in vivo also impair postsynaptic differentiation in organotypic cerebellar slices. These data support an E2-dependent mechanism that underlies phosphorylation-dependent SUMO conjugation in pathways that range from the heat-shock response to nuclear hormone signaling to brain development.

PIASx Is a MEF2 SUMO E3 Ligase That Promotes Postsynaptic Dendritic Morphogenesis

Postsynaptic morphogenesis of dendrites is essential for the establishment of neural connectivity in the brain, but the mechanisms that govern postsynaptic dendritic differentiation remain poorly understood. Sumoylation of the transcription factor myocyte enhancer factor 2A (MEF2A) promotes the differentiation of postsynaptic granule neuron dendritic claws in the cerebellar cortex. Here, we identify the protein PIASx as a MEF2 SUMO E3 ligase that represses MEF2-dependent transcription in neurons. Gain-of-function and genetic knockdown experiments in rat cerebellar slices and in the postnatal cerebellum in vivo reveal that PIASx drives the differentiation of granule neuron dendritic claws in the cerebellar cortex. MEF2A knockdown suppresses PIASx-induced dendritic claw differentiation, and expression of sumoylated MEF2A reverses PIASx knockdown-induced loss of dendritic claws. These findings define the PIASx-MEF2 sumoylation signaling link as a key mechanism that orchestrates postsynaptic dendritic claw morphogenesis in the cerebellar cortex and suggest novel functions for SUMO E3 ligases in brain development and plasticity.

Cultures of Cerebellar Granule Neurons

INTRODUCTIONPrimary cultures of granule neurons from the post-natal rat cerebellum provide an excellent model system for molecular and cell biological studies of neuronal development and function. The cerebellar cortex, with its highly organized structure and few neuronal subtypes, offers a well-characterized neural circuitry. Many fundamental insights into the processes of neuronal apoptosis, migration, and differentiation in the mammalian central nervous system have come from investigating granule neurons in vitro. Granule neurons are the most abundant type of neurons in the brain. In addition to the sheer volume of granule neurons, the homogeneity of the population and the fact that they can be transfected with ease render them ideal for elucidating the molecular basis of neuronal development. This protocol for isolating granule neurons from post-natal rats is relatively straightforward and quick, making use of standard enzymatic and mechanical dissociation methods. In a serum-based medium containing an inhibitor of mitosis, cerebellar granule neurons can be maintained with high purity. Axons and dendrites can be clearly distinguished on the basis of morphology and markers. For even greater versatility, this protocol for culturing granule neurons can be combined with knockout or transgenic technologies, or used in cerebellar slice overlay assays.

A JIP3-Regulated GSK3β/DCX Signaling Pathway Restricts Axon Branching

Axon branching plays a critical role in establishing the accurate patterning of neuronal circuits in the brain. However, the mechanisms that control axon branching remain poorly understood. Here we report that knockdown of the brain-enriched signaling protein JNK-interacting protein 3 (JIP3) triggers exuberant axon branching and self-contact in primary granule neurons of the rat cerebellar cortex. JIP3 knockdown in cerebellar slices and in postnatal rat pups in vivo leads to the formation of ectopic branches in granule neuron parallel fiber axons in the cerebellar cortex. We also find that JIP3 restriction of axon branching is mediated by the protein kinase glycogen synthase kinase 3β (GSK3β). JIP3 knockdown induces the downregulation of GSK3β in neurons, and GSK3β knockdown phenocopies the effect of JIP3 knockdown on axon branching and self-contact. Finally, we establish doublecortin (DCX) as a novel substrate of GSK3β in the control of axon branching and self-contact. GSK3β phosphorylates DCX at the distinct site of Ser327 and thereby contributes to DCX function in the restriction of axon branching. Together, our data define a JIP3-regulated GSK3β/DCX signaling pathway that restricts axon branching in the mammalian brain. These findings may have important implications for our understanding of neuronal circuitry during development, as well as the pathogenesis of neurodevelopmental disorders of cognition.

Molecular Control of Axon Branching

Axon branching is a complex morphological process, the regulation of which we are just beginning to understand. Many factors known to be important for axon growth and guidance have emerged as key regulators of axon branching. The extrinsic factors implicated in axon branching include traditional axon guidance cues such as the slits, semaphorins, and ephrins; neurotrophins such as BDNF; the secreted glycoprotein Wnt; the extracellular matrix protein anosmin-1; and certain transmembrane cell adhesion molecules—as well as sensory experience and neuronal activity. Although less is known about the intracellular control of axon branching, in recent years significant advances have been made in this area. Kinases and their regulators, Rho GTPases and their regulators, transcription factors, ubiquitin ligases, and several microtubule and actin-binding proteins are now implicated in the control of axon branching. It is likely that many more branching regulators remain to be discovered, as do the links between extrinsic cues and intracellular signaling proteins in the control of axon branching.

FLEXIQinase, a mass spectrometry-based assay, to unveil multikinase mechanisms

We introduce a mass spectrometry–based method that provides residue-resolved quantitative information about protein phosphorylation. In this assay we combined our full-length expressed stable isotope–labeled protein for quantification strategy (FLEXIQuant) with a traditional kinase assay to determine the mechanisms of multikinase substrate phosphorylation such as priming-dependent kinase activities. The assay monitors the decrease in signal intensity of the substrate peptides and the concomitant increase in the (n × 80 Da)-shifted phosphorylated peptide. We analyzed the c-Jun N-terminal kinase (JNK)-dependent glycogen synthase kinase 3β (GSK3β) activity on doublecortin (DCX) revealing mechanistic details about the role of phosphorylation cross-talk in GSK3β activity and permitting an advanced model for GSK3β-mediated signaling.

A mouse model for too much TV

In a new study published in Scientific Reports, Christakis and colleagues investigate a mouse model for technology-induced overstimulation. We review their findings, discuss the challenges of defining overstimulation, and consider the resemblance of the phenotypes observed in Christakis et al. to those noted in genetic models of attention deficit hyperactivity disorder (ADHD).