Eric S. Tucker

Diminished dosage of 22q11 genes disrupts neurogenesis and cortical development in a mouse model of 22q11 deletion/DiGeorge syndrome

The 22q11 deletion (or DiGeorge) syndrome (22q11DS), the result of a 1.5- to 3-megabase hemizygous deletion on human chromosome 22, results in dramatically increased susceptibility for “diseases of cortical connectivity” thought to arise during development, including schizophrenia and autism. We show that diminished dosage of the genes deleted in the 1.5-megabase 22q11 minimal critical deleted region in a mouse model of 22q11DS specifically compromises neurogenesis and subsequent differentiation in the cerebral cortex. Proliferation of basal, but not apical, progenitors is disrupted, and subsequently, the frequency of layer 2/3, but not layer 5/6, projection neurons is altered. This change is paralleled by aberrant distribution of parvalbumin-labeled interneurons in upper and lower cortical layers. Deletion of Tbx1 or Prodh (22q11 genes independently associated with 22q11DS phenotypes) does not similarly disrupt basal progenitors. However, expression analysis implicates additional 22q11 genes that are selectively expressed in cortical precursors. Thus, diminished 22q11 gene dosage disrupts cortical neurogenesis and interneuron migration. Such developmental disruption may alter cortical circuitry and establish vulnerability for developmental disorders, including schizophrenia and autism.

Proliferative and transcriptional identity of distinct classes of neural precursors in the mammalian olfactory epithelium

Neural precursors in the developing olfactory epithelium (OE) give rise to three major neuronal classes – olfactory receptor (ORNs), vomeronasal (VRNs) and gonadotropin releasing hormone (GnRH) neurons. Nevertheless, the molecular and proliferative identities of these precursors are largely unknown. We characterized two precursor classes in the olfactory epithelium (OE) shortly after it becomes a distinct tissue at midgestation in the mouse: slowly dividing self-renewing precursors that express Meis1/2 at high levels, and rapidly dividing neurogenic precursors that express high levels of Sox2 and Ascl1. Precursors expressing high levels of Meis genes primarily reside in the lateral OE, whereas precursors expressing high levels of Sox2 and Ascl1 primarily reside in the medial OE. Fgf8 maintains these expression signatures and proliferative identities. Using electroporation in the wild-type embryonic OE in vitro as well as Fgf8, Sox2 and Ascl1 mutant mice in vivo, we found that Sox2 dose and Meis1 – independent of Pbx co-factors – regulate Ascl1 expression and the transition from lateral to medial precursor state. Thus, we have identified proliferative characteristics and a dose-dependent transcriptional network that define distinct OE precursors: medial precursors that are most probably transit amplifying neurogenic progenitors for ORNs, VRNs and GnRH neurons, and lateral precursors that include multi-potent self-renewing OE neural stem cells.

Cxcr4 regulation of interneuron migration is disrupted in 22q11.2 deletion syndrome

Interneurons are thought to be a primary pathogenic target for several behavioral disorders that arise during development, including schizophrenia and autism. It is not known, however, whether genetic lesions associated with these diseases disrupt established molecular mechanisms of interneuron development. We found that diminished 22q11.2 gene dosage—the primary genetic lesion in 22q11.2 deletion syndrome (22q11.2 DS)—specifically compromises the distribution of early-generated parvalbumin-expressing interneurons in the Large Deletion (LgDel) 22q11.2DS mouse model. This change reflects cell-autonomous disruption of interneuron migration caused by altered expression of the cytokine C-X-C chemokine receptor type 4 (Cxcr4), an established regulator of this process. Cxcr4 is specifically reduced in LgDel migrating interneurons, and genetic analysis confirms that diminished Cxcr4 alters interneuron migration in LgDel mice. Thus, diminished 22q11.2 gene dosage disrupts cortical circuit development by modifying a critical molecular signaling pathway via Cxcr4 that regulates cortical interneuron migration and placement.

Three phases of DiGeorge/22q11 deletion syndrome pathogenesis during brain development: patterning, proliferation, and mitochondrial functions of 22q11 genes.

DiGeorge, or 22q11 deletion syndrome (22q11DS), the most common survivable human genetic deletion disorder, is caused by deletion of a minimum of 32 contiguous genes on human chromosome 22, and presumably results from diminished dosage of one, some, or all of these genes—particularly during development. Nevertheless, the normal functions of 22q11 genes in the embryo or neonate, and their contribution to developmental pathogenesis that must underlie 22q11DS are not well understood. Our data suggests that a substantial number of 22q11 genes act specifically and in concert to mediate early morphogenetic interactions and subsequent cellular differentiation at phenotypically compromised sites—the limbs, heart, face and forebrain. When dosage of a broad set of these genes is diminished, early morphogenesis is altered, and initial 22q11DS phenotypes are established. Thereafter, functionally similar subsets of 22q11 genes—especially those that influence the cell cycle or mitochondrial function—remain expressed, particularly in the developing cerebral cortex, to regulate neurogenesis and synaptic development. When dosage of these genes is diminished, numbers, placement and connectivity of neurons and circuits essential for normal behavior may be disrupted. Such disruptions likely contribute to vulnerability for schizophrenia, autism, or attention deficit/hyperactivity disorder seen in most 22q11DS patients.

Molecular Specification and Patterning of Progenitor Cells in the Lateral and Medial Ganglionic Eminences

We characterized intrinsic and extrinsic specification of progenitors in the lateral and medial ganglionic eminences (LGE and MGE). We identified seven genes whose expression is enriched or restricted in either the LGE [biregional cell adhesion molecule-related/downregulated by oncogenes binding protein (Boc), Frizzled homolog 8 (Fzd8), Ankrd43 (ankyrin repeat domain-containing protein 43), and Ikzf1 (Ikaros family zinc finger 1)] or MGE [Map3k12 binding inhibitory protein 1 (Mbip); zinc-finger, SWIM domain containing 5 (Zswim5); and Adamts5 [a disintegrin-like and metallopeptidase (reprolysin type) with thrombospondin type 1 motif, 5]]. Boc, Fzd8, Mbip, and Zswim5 are apparently expressed in LGE or MGE progenitors, whereas the remaining three are seen in the postmitotic mantle zone. Relative expression levels are altered and regional distinctions are lost for each gene in LGE or MGE cells propagated as neurospheres, indicating that these newly identified molecular characteristics of LGE or MGE progenitors depend on forebrain signals not available in the neurosphere assay. Analyses of Pax6Sey/Sey, Shh−/−, and Gli3XtJ/XtJ mutants suggests that LGE and MGE progenitor identity does not rely exclusively on previously established forebrain-intrinsic patterning mechanisms. Among a limited number of additional potential patterning mechanisms, we found that extrinsic signals from the frontonasal mesenchyme are essential for Shh- and Fgf8-dependent regulation of LGE and MGE genes. Thus, extrinsic and intrinsic forebrain patterning mechanisms cooperate to establish LGE and MGE progenitor identity, and presumably their capacities to generate distinct classes of neuronal progeny.

Traumatic brain injury and epilepsy: Underlying mechanisms leading to seizure.

Post-traumatic epilepsy continues to be a major concern for those experiencing traumatic brain injury. Post-traumatic epilepsy accounts for 10-20% of epilepsy cases in the general population. While seizure prophylaxis can prevent early onset seizures, no available treatments effectively prevent late-onset seizure. Little is known about the progression of neural injury over time and how this injury progression contributes to late onset seizure development. In this comprehensive review, we discuss the epidemiology and risk factors for post-traumatic epilepsy and the current pharmacologic agents used for treatment. We highlight limitations with the current approach and offer suggestions for remedying the knowledge gap. Critical to this pursuit is the design of pre-clinical models to investigate important mechanistic factors responsible for post-traumatic epilepsy development. We discuss what the current models have provided in terms of understanding acute injury and what is needed to advance understanding regarding late onset seizure. New model designs will be used to investigate novel pathways linking acute injury to chronic changes within the brain. Important components of this transition are likely mediated by toll-like receptors, neuroinflammation, and tauopathy. In the final section, we highlight current experimental therapies that may prove promising in preventing and treating post-traumatic epilepsy. By increasing understanding about post-traumatic epilepsy and injury expansion over time, it will be possible to design better treatments with specific molecular targets to prevent late-onset seizure occurrence following traumatic brain injury.

Specific mesenchymal/epithelial induction of olfactory receptor, vomeronasal, and gonadotropin-releasing hormone (GnRH) neurons.

We asked whether specific mesenchymal/epithelial (M/E) induction generates olfactory receptor neurons (ORNs), vomeronasal neurons (VRNs), and gonadotropin‐releasing hormone (GnRH) neurons, the major neuron classes associated with the olfactory epithelium (OE). To assess specificity of M/E‐mediated neurogenesis, we compared the influence of frontonasal mesenchyme on frontonasal epithelium, which becomes the OE, with that of the forelimb bud. Despite differences in position, morphogenetic and cytogenic capacity, both mesenchymal tissues support neurogenesis, expression of several signaling molecules and neurogenic transcription factors in the frontonasal epithelium. Only frontonasal mesenchyme, however, supports OE‐specific patterning and activity of a subset of signals and factors associated with OE differentiation. Moreover, only appropriate pairing of frontonasal epithelial and mesenchymal partners yields ORNs, VRNs, and GnRH neurons. Accordingly, the position and molecular identity of specialized frontonasal epithelia and mesenchyme early in gestation and subsequent inductive interactions specify the genesis and differentiation of peripheral chemosensory and neuroendocrine neurons. Developmental Dynamics 239:1723–1738, 2010.

Limited influence of olanzapine on adult forebrain neural precursors in vitro

We evaluated the activity of the atypical antipsychotic drug olanzapine on differentiation and gene expression in adult neural precursor cells in vitro. Neural precursors obtained from forebrain subventricular zone (SVZ)-derived neurospheres express a subset (13/24) of receptors known to bind olanzapine at high to intermediate affinities; in contrast, all 24 are expressed in the SVZ. In the presence of 10 nM, 100 nM or 1 microM olanzapine, there is no significant change in the frequency of oligodendrocytes, neurons, GABAergic neurons and astrocytes generated from neurosphere precursors. In parallel, there is no apparent change in cell proliferation in response to olanzapine, based upon bromodeoxyuridine incorporation. There are no major changes in cytological differentiation in response to the drug; however, at one concentration (10 nM) there is a small but statistically significant increase in the size of glial fibrillary acidic protein-labeled astrocytes derived from neurosphere precursors. In addition, olanzapine apparently modulates expression of one serotonin receptor -- 5HT2A -- in differentiating neurosphere cultures; however, it does not modify expression of several other receptors or schizophrenia vulnerability genes. Thus, olanzapine has a limited influence on differentiation and gene expression in adult neural precursor cells in vitro.

Cortical Interneurons Require Jnk1 to Enter and Navigate the Developing Cerebral Cortex

Proper assembly of cortical circuitry relies on the correct migration of cortical interneurons from their place of birth in the ganglionic eminences to their place of terminal differentiation in the cerebral cortex. Although molecular mechanisms mediating cortical interneuron migration have been well studied, intracellular signals directing their migration are largely unknown. Here we illustrate a novel and essential role for c-Jun N-terminal kinase (JNK) signaling in guiding the pioneering population of cortical interneurons into the mouse cerebral cortex. Migrating cortical interneurons express Jnk proteins at the entrance to the cortical rudiment and have enriched expression of Jnk1 relative to noninterneuronal cortical cells. Pharmacological blockade of JNK signaling in ex vivo slice cultures resulted in dose-dependent and highly specific disruption of interneuron migration into the nascent cortex. Time-lapse imaging revealed that JNK-inhibited cortical interneurons advanced slowly and assumed aberrant migratory trajectories while traversing the cortical entry zone. In vivo analyses of JNK-deficient embryos supported our ex vivo pharmacological data. Deficits in interneuron migration were observed in Jnk1 but not Jnk2 single nulls, and those migratory deficits were further exacerbated when homozygous loss of Jnk1 was combined with heterozygous reduction of Jnk2. Finally, genetic ablation of Jnk1 and Jnk2 from cortical interneurons significantly perturbed migration in vivo, but not in vitro, suggesting JNK activity functions to direct their guidance rather than enhance their motility. These data suggest JNK signaling, predominantly mediated by interneuron expressed Jnk1, is required for guiding migration of cortical interneurons into and within the developing cerebral cortex.