The effects of Tag-1 on the maturation of mouse cerebellar granule neurons

Genome binding properties of Zic transcription factors underlie their changing functions during neuronal maturation

Background

The Zic family of transcription factors (TFs) promote both proliferation and maturation of cerebellar granule neurons (CGNs), raising the question of how a single, constitutively expressed TF family can support distinct developmental processes. Here we use an integrative experimental and bioinformatic approach to discover the regulatory relationship between Zic TF binding and changing programs of gene transcription during CGN differentiation.

Results

We first established a bioinformatic pipeline to integrate Zic ChIP-seq data from the developing mouse cerebellum with other genomic datasets from the same tissue. In newborn CGNs, Zic TF binding predominates at active enhancers that are co-bound by developmentally-regulated TFs including Atoh1, whereas in mature CGNs, Zic TF binding consolidates toward promoters where it co-localizes with activity-regulated TFs. We then performed CUT&RUN-seq in differentiating CGNs to define both the time course of developmental shifts in Zic TF binding and their relationship to gene expression. Mapping Zic TF binding sites to genes using chromatin looping, we identified the set of Zic target genes that have altered expression in RNA-seq from Zic1 or Zic2 knockdown CGNs.

Conclusion

Our data show that Zic TFs are required for both induction and repression of distinct, developmentally regulated target genes through a mechanism that is largely independent of changes in Zic TF binding. We suggest that the differential collaboration of Zic TFs with other TF families underlies the shift in their biological functions across CGN development.

Contactin 2 homophilic adhesion structure and conformational plasticity

The cell-surface attached glycoprotein contactin 2 is ubiquitously expressed in the nervous system and mediates homotypic cell-cell interactions to organize cell guidance, differentiation, and adhesion. Contactin 2 consists of six Ig and four fibronectin type III domains (FnIII) of which the first four Ig domains form a horseshoe structure important for homodimerization and oligomerization. Here we report the crystal structure of the six-domain contactin 2Ig1-6 and show that the Ig5-Ig6 combination is oriented away from the horseshoe with flexion in interdomain connections. Two distinct dimer states, through Ig1-Ig2 and Ig3-Ig6 interactions, together allow formation of larger oligomers. Combined size exclusion chromatography with multiangle light scattering (SEC-MALS), small-angle X-ray scattering (SAXS) and native MS analysis indicates contactin 2Ig1-6 oligomerizes in a glycan dependent manner. SAXS and negative-stain electron microscopy reveals inherent plasticity of the contactin 2 full-ectodomain. The combination of intermolecular binding sites and ectodomain plasticity explains how contactin 2 can function as a homotypic adhesion molecule in diverse intercellular environments.

Excitatory granule neuron precursors orchestrate laminar localization and differentiation of cerebellar inhibitory interneuron subtypes

GABAergic interneurons migrate long distances through stereotyped migration programs toward specific laminar positions. During their migration, GABAergic interneurons are morphologically alike but then differentiate into a rich array of interneuron subtypes critical for brain function. How interneuron subtypes acquire their final phenotypic traits remains largely unknown. Here, we show that cerebellar molecular layer GABAergic interneurons, derived from the same progenitor pool, use separate migration paths to reach their laminar position and differentiate into distinct basket cell (BC) and stellate cell (SC) GABAergic interneuron subtypes. Using two-photon live imaging, we find that SC final laminar position requires an extra step of tangential migration supported by a subpopulation of glutamatergic granule cells (GCs). Conditional depletion of GCs affects SC differentiation but does not affect BCs. Our results reveal how timely feedforward control of inhibitory interneuron migration path regulates their terminal differentiation and, thus, establishment of the local inhibitory circuit assembly.

Using the Allen gene expression atlas of the adult mouse brain to gain further insight into the physiological significance of TAG-1/Contactin-2

The anatomic gene expression atlas (AGEA) of the adult mouse brain of the Allen Institute for Brain Science is a transcriptome-based atlas of the adult C57Bl/6 J mouse brain, based on the extensive in situ hybridization dataset of the Institute. This spatial mapping of the gene expression levels of mice under baseline conditions could assist in the formation of new, reasonable transcriptome-derived hypotheses on brain structure and underlying biochemistry, which could also have functional implications. The aim of this work is to use the data of the AGEA (in combination with Tabula Muris, a compendium of single cell transcriptome data collected from mice, enabling direct and controlled comparison of gene expression among cell types) to provide further insights into the physiology of TAG-1/Contactin-2 and its interactions, by presenting the expression of the corresponding gene across the adult mouse brain under baseline conditions and to investigate any spatial genomic correlations between TAG-1/Contactin-2 and its interacting proteins and markers of mature and immature oligodendrocytes, based on the pre-existing experimental or bibliographical evidence. The across-brain correlation analysis on the gene expression intensities showed a positive spatial correlation of TAG-1/Contactin-2 with the gene expression of Plp1, Myrf, Mbp, Mog, Cldn11, Bace1, Kcna1, Kcna2, App and Nfasc and a negative spatial correlation with the gene expression of Cspg4, Pdgfra, L1cam, Ncam1, Ncam2 and Ptprz1. Spatially correlated genes are mainly expressed by mature oligodendrocytes (like Cntn2), while spatially anticorrelated genes are mainly expressed by oligodendrocyte precursor cells. According to the data presented in this work, we propose that even though Contactin-2 expression during development correlates with high plasticity events, such as neuritogenesis, in adulthood it correlates with pathways characterized by low plasticity.

Promotion of differentiation in developing mouse cerebellar granule cells by a cell adhesion molecule BT-IgSF

Brain- and testis-specific immunoglobulin superfamily (BT-IgSF) (also known as IgSF11), one of the immunoglobulin superfamily proteins, is a cell adhesion molecule, expressed in the developing cerebellum. We hypothesized that BT-IgSF might have some function in the development of cerebellum, although the physiological roles of BT-IgSF in the cerebellum remain unclear. To investigate the role of BT-IgSF in the development of mouse cerebellum, we first determined the presence of BT-IgSF in the newborn mouse cerebellum; its expression level was found to be much higher than that in the adults. BT-IgSF was abundantly expressed in the molecular layer, where cerebellar granule cell precursors (CGCPs) are in the differentiation stage during migration. We subsequently analyzed the effects of BT-IgSF-knockdown and -overexpression on the proliferation and differentiation of primary cultured CGCPs. BT-IgSF suppressed the proliferation of CGCPs, and promoted their differentiation into cerebellar granule cells. Taken together, our results suggested that BT-IgSF is one of the important cell adhesion molecules that regulate the developmentof mouse cerebellum.

BDNF activates an NFI-dependent neurodevelopmental timing program by sequestering NFATc4

We show that BDNF regulates the timing of neurodevelopment via a novel mechanism of extranuclear sequestration of NFATc4 in Golgi. This leads to accelerated derepression of an NFI temporal occupancy gene program in cerebellar granule cells that includes Bdnf itself, revealing an autoregulatory loop within the program driven by BDNF and NFATc4.

Glycosylphosphatidylinositol-Anchored Immunoglobulin Superfamily Cell Adhesion Molecules and Their Role in Neuronal Development and Synapse Regulation

Immunoglobulin superfamily (IgSF) cell adhesion molecules (CAMs) are cell surface glycoproteins that not only mediate interactions between neurons but also between neurons and other cells in the nervous system. While typical IgSF CAMs are transmembrane molecules, this superfamily also includes CAMs, which do not possess transmembrane and intracellular domains and are instead attached to the plasma membrane via a glycosylphosphatidylinositol (GPI) anchor. In this review, we focus on the role GPI-anchored IgSF CAMs have as signal transducers and ligands in neurons, and discuss their functions in regulation of neuronal development, synapse formation, synaptic plasticity, learning, and behavior. We also review the links between GPI-anchored IgSF CAMs and brain disorders.

Manduca Contactin Regulates Amyloid Precursor Protein-Dependent Neuronal Migration

Amyloid precursor protein (APP) was originally identified as the source of β-amyloid peptides that accumulate in Alzheimer's disease (AD), but it also has been implicated in the control of multiple aspects of neuronal motility. APP belongs to an evolutionarily conserved family of transmembrane proteins that can interact with a variety of adapter and signaling molecules. Recently, we showed that both APP and its insect ortholog [APPL (APP-Like)] directly bind the heterotrimeric G-protein Goα, supporting the model that APP can function as an unconventional Goα-coupled receptor. We also adapted a well characterized assay of neuronal migration in the hawkmoth, Manduca sexta, to show that APPL-Goα signaling restricts ectopic growth within the developing nervous system, analogous to the role postulated for APP family proteins in controlling migration within the mammalian cortex. Using this assay, we have now identified Manduca Contactin (MsContactin) as an endogenous ligand for APPL, consistent with previous work showing that Contactins interact with APP family proteins in other systems. Using antisense-based knockdown protocols and fusion proteins targeting both proteins, we have shown that MsContactin is selectively expressed by glial cells that ensheath the migratory neurons (expressing APPL), and that MsContactin-APPL interactions normally prevent inappropriate migration and outgrowth. These results provide new evidence that Contactins can function as authentic ligands for APP family proteins that regulate APP-dependent responses in the developing nervous system. They also support the model that misregulated Contactin-APP interactions might provoke aberrant activation of Goα and its effectors, thereby contributing to the neurodegenerative sequelae that typify AD.

SIGNIFICANCE STATEMENT

Members of the amyloid precursor protein (APP) family participate in many aspects of neuronal development, but the ligands that normally activate APP signaling have remained controversial. This research provides new evidence that members of the Contactin family function as authentic ligands for APP and its orthologs, and that this evolutionarily conserved class of membrane-attached proteins regulates key aspects of APP-dependent migration and outgrowth in the embryonic nervous system. By defining the normal role of Contactin-APP signaling during development, these studies also provide the framework for investigating how the misregulation of Contactin-APP interactions might contribute to neuronal dysfunction in the context of both normal aging and neurodegenerative conditions, including Alzheimer's disease.

The role of Gpi-anchored axonal glycoproteins in neural development and neurological disorders

This review article focuses on the Contactin (CNTN) subset of the Immunoglobulin supergene family (IgC2/FNIII molecules), whose components share structural properties (the association of Immunoglobulin type C2 with Fibronectin type III domains), as well as a general role in cell contact formation and axonal growth control. IgC2/FNIII molecules include 6 highly related components (CNTN 1-6), associated with the cell membrane via a Glycosyl Phosphatidyl Inositol (GPI)-containing lipid tail. Contactin 1 and Contactin 2 share ~50 (49.38)% identity at the aminoacid level. They are components of the cell surface, from which they may be released in soluble forms. They bind heterophilically to multiple partners in cis and in trans, including members of the related L1CAM family and of the Neurexin family Contactin-associated proteins (CNTNAPs or Casprs). Such interactions are important for organising the neuronal membrane, as well as for modulating the growth and pathfinding of axon tracts. In addition, they also mediate the functional maturation of axons by promoting their interactions with myelinating cells at the nodal, paranodal and juxtaparanodal regions. Such interactions also mediate differential ionic channels (both Na⁺ and K⁺) distribution, which is of critical relevance in the generation of the peak-shaped action potential. Indeed, thanks to their interactions with Ankyrin G, Na⁺ channels map within the nodal regions, where they drive axonal depolarization. However, no ionic channels are found in the flanking Contactin1-containing paranodal regions, where CNTN1 interactions with Caspr1 and with the Ig superfamily component Neurofascin 155 in cis and in trans, respectively, build a molecular barrier between the node and the juxtaparanode. In this region K⁺ channels are clustered, depending upon molecular interactions with Contactin 2 and with Caspr2. In addition to these functions, the Contactins appear to have also a role in degenerative and inflammatory disorders: indeed Contactin 2 is involved in neurodegenerative disorders with a special reference to the Alzheimer disease, given its ability to work as a ligand of the Alzheimer Precursor Protein (APP), which results in increased Alzheimer Intracellular Domain (AICD) release in a γ-secretase-dependent manner. On the other hand Contactin 1 drives Notch signalling activation via the Hes pathway, which could be consistent with its ability to modulate neuroinflammation events, and with the possibility that Contactin 1-dependent interactions may participate to the pathogenesis of the Multiple Sclerosis and of other inflammatory disorders.

The immunoglobulin-like superfamily member IGSF3 is a developmentally regulated protein that controls neuronal morphogenesis

The establishment of a functional brain depends on the fine regulation and coordination of many processes, including neurogenesis, differentiation, dendritogenesis, axonogenesis, and synaptogenesis. Proteins of the immunoglobulin-like superfamily (IGSF) are major regulators during this sequence of events. Different members of this class of proteins play nonoverlapping functions at specific developmental time-points, as shown in particular by studies of the cerebellum. We have identified a member of the little studied EWI subfamily of IGSF, the protein IGSF3, as a membrane protein expressed in a neuron specific- and time-dependent manner during brain development. In the cerebellum, it is transiently found in membranes of differentiating granule cells, and is particularly concentrated at axon terminals. There it co-localizes with other IGSF proteins with well-known functions in cerebellar development: TAG-1 and L1. Functional analysis shows that IGSF3 controls the differentiation of granule cells, more precisely axonal growth and branching. Biochemical experiments demonstrate that, in the developing brain, IGSF3 is in a complex with the tetraspanin TSPAN7, a membrane protein mutated in several forms of X-linked intellectual disabilities. In cerebellar granule cells, TSPAN7 promotes axonal branching and the size of TSPAN7 clusters is increased by downregulation of IGSF3. Thus IGSF3 is a novel regulator of neuronal morphogenesis that might function through interactions with multiple partners including the tetraspanin TSPAN7. This developmentally regulated protein might thus be at the center of a new signaling pathway controlling brain development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 75-92, 2017.

Vitronectin promotes the progress of the initial differentiation stage in cerebellar granule cells

Vitronectin (VN), which is an extracellular matrix protein, is known to be involved in the proliferation and differentiation of primary cultured cerebellar granule cell precursors (CGCPs); however, the effect of VN is not fully understood. In this study, we analyzed the effects of VN loss on the proliferation and differentiation of CGCPs in VN knockout (VNKO) mice in vivo. First, immunohistochemistry showed that VN was distributed in the region from the inner external granule layer (iEGL) through the internal granule layer (IGL) in wild-type (WT) mice. Next, we observed the formation of the cerebellar cortex using sagittal sections of VNKO mice at postnatal days (P) 5, 8 and 11. Loss of VN suppressed the ratio of NeuN, a neuronal differentiation marker, to positive cerebellar granule cells (CGCs) in the external granule layer (EGL) and the ratio of CGCs in the IGL at P8, indicating that the loss of VN suppresses the differentiation into CGCs. However, the loss of VN did not significantly affect the proliferation of CGCPs. Next, the effect of VN loss on the initial differentiation stage of CGCPs was examined. The loss of VN increased the expression levels of Transient axonal glycoprotein 1 (TAG1), a marker of neurons in the initial differentiation stage, in the cerebella of VNKO mice at P5 and 8 and increased the ratio of TAG1-positive cells in the primary culture of VNKO-derived CGCPs, indicating that the loss of VN accumulates the CGCPs in the initial differentiation stage. Taken together, these results demonstrate that VN promotes the progress of the initial differentiation stage of CGCPs.

Deficits in cerebellar granule cell development and social interactions in CD47 knockout mice

CD47 is involved in neurite differentiation in cultured neurons, but the function of CD47 in brain development is largely unknown. We determined that CD47 mRNA was robustly expressed in the developing cerebellum, especially in granule cells. CD47 protein was mainly expressed in the inner layer of the external granule layer (EGL), molecular layer, and internal granule layer (IGL), where granule cells individually become postmitotic and migrate, leading to neurite fasciculation. At postnatal day 8 (P8), CD47 knockout mice exhibited an increased number of proliferating granule cells in the EGL, whereas the CD47 agonist peptide 4N1K increased the number of postmitotic cells in primary granule cells. Knocking out the CD47 gene and anti-CD47 antibody impaired the radial migration of granule cells from the EGL to the IGL individually in mice and slice cultures. In primary granule cells, knocking out CD47 reduced the number of axonal collaterals and dendritic branches; by contrast, overexpressing CD47 or 4N1K treatment increased the axonal length and numbers of axonal collaterals and dendritic branches. Furthermore, the length of the fissure between Lobules VI and VII was decreased in CD47 knockout mice at P21 and at 14 wk after birth. Lastly, CD47 knockout mice exhibited increased social interaction at P21 and depressive-like behaviors at 10 wk after birth. Our study revealed that the cell adhesion molecule CD47 participates in multiple phases of granule cell development, including proliferation, migration, and neurite differentiation implying that aberrations of CD47 are risk factors that cause abnormalities in cerebellar development and atypical behaviors.

Pioneering axons regulate neuronal polarization in the developing cerebral cortex

The polarization of neurons, which mainly includes the differentiation of axons and dendrites, is regulated by cell-autonomous and non-cell-autonomous factors. In the developing central nervous system, neuronal development occurs in a heterogeneous environment that also comprises extracellular matrices, radial glial cells, and neurons. Although many cell-autonomous factors that affect neuronal polarization have been identified, the microenvironmental cues involved in neuronal polarization remain largely unknown. Here, we show that neuronal polarization occurs in a microenvironment in the lower intermediate zone, where the cell adhesion molecule transient axonal glycoprotein-1 (TAG-1) is expressed in cortical efferent axons. The immature neurites of multipolar cells closely contact TAG-1-positive axons and generate axons. Inhibition of TAG-1-mediated cell-to-cell interaction or its downstream kinase Lyn impairs neuronal polarization. These results show that the TAG-1-mediated cell-to-cell interaction between the unpolarized multipolar cells and the pioneering axons regulates the polarization of multipolar cells partly through Lyn kinase and Rac1.

New insights into the roles of the contactin cell adhesion molecules in neural development

In vertebrates, the contactin (CNTN) family of neural cell recognition molecules includes six related cell adhesion molecules that play non-overlapping roles in the formation and maintenance of the nervous system. CNTN1 and CNTN2 are the prototypical members of the family and have been involved, through cis- and trans-interactions with distinct cell adhesion molecules, in neural cell migration, axon guidance, and the organization of myelin subdomains. In contrast, the roles of CNTN3-6 are less well characterized although the generation of null mice and the recent identification of a common extracellular binding partner have considerably advanced our grasp of their physiological roles in particular as they relate to the wiring of sensory tissues. In this review, we aim to present a summary of our current understanding of CNTN functions and give an overview of the challenges that lie ahead in understanding the roles these proteins play in nervous system development and maintenance.