Asymmetric Inhibition of Ulk2 Causes Left-Right Differences in Habenular Neuropil Formation

Department of Biological Sciences, Vanderbilt University, Nashville, Tennessee 37205, USA.
The Journal of Neuroscience : The Official Journal of the Society for Neuroscience (Impact Factor: 6.34). 07/2011; 31(27):9869-78. DOI: 10.1523/JNEUROSCI.0435-11.2011
Source: PubMed


Studies of the zebrafish epithalamus have provided recent insights into the development of left-right brain asymmetry, which is crucial to normal human brain function. The habenular nuclei of zebrafish are robustly asymmetric, with dense elaboration of neuropil only in the left lateral subnucleus. Because this feature is tightly correlated with asymmetric expression of K(+) channel tetramerization domain-containing proteins 12.1 and 12.2 (Kctd12.1/12.2), we screened for Kctd12.1-interacting proteins to identify molecular mechanisms leading to neuropil asymmetry, and uncovered a novel interaction between Kctd12.1 and Unc-51-like kinase 2 (Ulk2). We show here that knockdown of Ulk2 or overexpression of Kctd12 proteins reduces asymmetric neuropil elaboration. Conversely, overexpression of Ulk2 or mutation of kctd12 genes causes excess neuropil elaboration. We conclude that Ulk2 activity promotes neuropil elaboration while Kctd12 proteins limit Ulk2 activity asymmetrically. This work describes a regulatory mechanism for neuronal process extension that may be conserved in other developmental contexts in addition to the epithalamus.

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    • "Indeed, Beretta et al. [11] have shown that dorsal and ventral habenular neurons arise from distinct progenitor populations. While progress has been made in describing habenular neurogenesis, differentiation and elaboration of processes, there are no known markers for habenular progenitors [12-16]. Therefore, finding marker genes that label dorsal habenular progenitors will be fundamental to studying how the diverse set of dorsal habenular neurons are generated and integrated into neural circuits underpinning aversive behavior as well as pathological addictive and depressive behaviors. "
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    ABSTRACT: Background The conserved habenular nuclei function as a relay system connecting the forebrain with the brain stem. They play crucial roles in various cognitive behaviors by modulating cholinergic, dopaminergic and serotonergic activities. Despite the renewed interest in this conserved forebrain region because of its importance in regulating aversion and reward behaviors, the formation of the habenular nuclei during embryogenesis is poorly understood due to their small size and deep location in the brain, as well as the lack of known markers for habenular progenitors. In zebrafish, the bilateral habenular nuclei are subdivided into dorsal and ventral compartments, are particularly large and found on the dorsal surface of the brain, which facilitates the study of their development. Results Here we examine the expression of a homeodomain transcription factor, dbx1b, and its potential to serve as an early molecular marker of dorsal habenular progenitors. Detailed spatiotemporal expression profiles demonstrate that the expression domain of dbx1b correlates with the presumptive habenular region, and dbx1b-expressing cells are proliferative along the ventricle. A lineage-tracing experiment using the Cre-lox system confirms that all or almost all dorsal habenular neurons are derived from dbx1b-expressing cells. In addition, mutant analysis and pharmacological treatments demonstrate that both initiation and maintenance of dbx1b expression requires precise regulation by fibroblast growth factor (FGF) signaling. Conclusions We provide clear evidence in support of dbx1b marking the progenitor populations that give rise to the dorsal habenulae. In addition, the expression of dbx1b in the dorsal diencephalon is tightly controlled by FGF signaling.
    Full-text · Article · Sep 2014 · Neural Development
    • "These results appear to be consistent with a recently proposed model of asymmetric local modulation of habenular neuropil growth mediated by Kctd proteins in zebrafish (Taylor et al., 2011). In this model, Kctd12.1 and Kctd12.2 are expressed primarily in the L and R Hb, respectively, and exert a stronger inhibition of neuropil growth on the R than the L Hb through interacting with Ulk2, a kinase that is expressed bilaterally in the Hb from 48 hpf (Taylor et al., 2011). Whether Kctd proteins inhibit the ability of Daam1a to promote neuropil growth in the R Hb is yet to be determined. "
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    ABSTRACT: Although progress has been made in resolving the genetic pathways that specify neuronal asymmetries in the brain, little is known about genes that mediate the development of structural asymmetries between neurons on left and right. In this study, we identify daam1a as an asymmetric component of the signalling pathways leading to asymmetric morphogenesis of the habenulae in zebrafish. Daam1a is a member of the Formin family of actin-binding proteins and the extent of Daam1a expression in habenular neuron dendrites mirrors the asymmetric growth of habenular neuropil between left and right. Local loss and gain of Daam1a function affects neither cell number nor subtype organisation but leads to a decrease or increase of neuropil, respectively. Daam1a therefore plays a key role in the asymmetric growth of habenular neuropil downstream of the pathways that specify asymmetric cellular domains in the habenulae. In addition, Daam1a mediates the development of habenular efferent connectivity as local loss and gain of Daam1a function impairs or enhances, respectively, the growth of habenular neuron terminals in the interpeduncular nucleus. Abrogation of Daam1a disrupts the growth of both dendritic and axonal processes and results in disorganised filamentous actin and α-tubulin. Our results indicate that Daam1a plays a key role in asymmetric habenular morphogenesis mediating the growth of dendritic and axonal processes in dorsal habenular neurons.
    No preview · Article · Oct 2013 · Development
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    • "Accumulations of neuropil differ between the left and right dorsal habenulae of larval zebrafish, as visualized by immunolabeling with antibodies against acetylated α-Tubulin (Concha et al., 2000; Taylor et al., 2011) or Synaptic Vesicle Protein 2 (SV2) (Hendricks and Jesuthasan, 2007; Miyasaka et al., 2009), and by labeling of membrane-tagged GFP in live Tg(gng8:nfsB- CAAX-GFP) c375 larvae. The left side has an expanded neuropil that extends the width of the dorsal habenula, while the right dorsal habenula has three more distinct, small clusters (Figure 1E). "
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    ABSTRACT: The dorsal habenular nuclei of the zebrafish epithalamus have become a valuable model for studying the development of left-right (L-R) asymmetry and its function in the vertebrate brain. The bilaterally paired dorsal habenulae exhibit striking differences in size, neuroanatomical organization, and molecular properties. They also display differences in their efferent connections with the interpeduncular nucleus (IPN) and in their afferent input, with a subset of mitral cells distributed on both sides of the olfactory bulb innervating only the right habenula. Previous studies have implicated the dorsal habenulae in modulating fear/anxiety responses in juvenile and adult zebrafish. It has been suggested that the asymmetric olfactory-habenula pathway (OB-Ha), revealed by selective labeling from an lhx2a:YFP transgene, mediates fear behaviors elicited by alarm pheromone. Here we show that expression of the fam84b gene demarcates a unique region of the right habenula that is the site of innervation by lhx2a:YFP-labeled olfactory axons. Upon ablation of the parapineal, which normally promotes left habenular identity; the fam84b domain is present in both dorsal habenulae and lhx2a:YFP-labeled olfactory bulb neurons form synapses on the left and the right side. To explore the relevance of the asymmetric olfactory projection and how it might influence habenular function, we tested activation of this pathway using odorants known to evoke behaviors. We find that alarm substance or other aversive odors, and attractive cues, activate fos expression in subsets of cells in the olfactory bulb but not in the lhx2a:YFP expressing population. Moreover, neither alarm pheromone nor chondroitin sulfate elicited fos activation in the dorsal habenulae. The results indicate that L-R asymmetry of the epithalamus sets the directionality of olfactory innervation, however, the lhx2a:YFP OB-Ha pathway does not appear to mediate fear responses to aversive odorants.
    Full-text · Article · May 2013 · Frontiers in Neural Circuits
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