Sonia Garel

A requirement for the immediate early gene Zif268 in the expression of late LTP and long-term memories

The induction of long-term potentiation (LTP) in the dentate gyrus of the hippocampus is associated with a rapid and robust transcription of the immediate early gene Zif268. We used a mutant mouse with a targeted disruption of Zif268 to ask whether this gene, which encodes a zinc finger transcription factor, is required for the maintenance of late LTP and for the expression of long-term memory. We show that whereas mutant mice exhibit early LTP in the dentate gyrus, late LTP is absent when measured 24 and 48 hours after tetanus in the freely moving animal. In both spatial and non-spatial learning tasks, short-term memory remained intact, whereas performance was impaired in tests requiring long-term memory. Thus, Zif268 is essential for the transition from short- to long-term synaptic plasticity and for the expression of long-term memories.

Tangential neuronal migration controls axon guidance: a role for neuregulin-1 in thalamocortical axon navigation.

Neuronal migration and axon guidance constitute fundamental processes in brain development that are generally studied independently. Although both share common mechanisms of cell biology and biochemistry, little is known about their coordinated integration in the formation of neural circuits. Here we show that the development of the thalamocortical projection, one of the most prominent tracts in the mammalian brain, depends on the early tangential migration of a population of neurons derived from the ventral telencephalon. This tangential migration contributes to the establishment of a permissive corridor that is essential for thalamocortical axon pathfinding. Our results also demonstrate that in this process two different products of the Neuregulin-1 gene, CRD-NRG1 and Ig-NRG1, mediate the guidance of thalamocortical axons. These results show that neuronal tangential migration constitutes a novel mechanism to control the timely arrangement of guidance cues required for axonal tract formation in the mammalian brain.

Dose-dependent functions of Fgf8 in regulating telencephalic patterning centers

Mouse embryos bearing hypomorphic and conditional null Fgf8 mutations have small and abnormally patterned telencephalons. We provide evidence that the hypoplasia results from decreased Foxg1 expression, reduced cell proliferation and increased cell death. In addition, alterations in the expression of Bmp4, Wnt8b, Nkx2.1 and Shh are associated with abnormal development of dorsal and ventral structures. Furthermore, nonlinear effects of Fgf8 gene dose on the expression of a subset of genes, including Bmp4 and Msx1, correlate with a holoprosencephaly phenotype and with the nonlinear expression of transcription factors that regulate neocortical patterning. These data suggest that Fgf8 functions to coordinate multiple patterning centers, and that modifications in the relative strength of FGF signaling can have profound effects on the relative size and nature of telencephalic subdivisions.

Molecular regionalization of the neocortex is disrupted in Fgf8 hypomorphic mutants.

The neocortex is divided into multiple areas with specific architecture, molecular identity and pattern of connectivity with the dorsal thalamus. Gradients of transcription factor expression in the cortical primordium regulate molecular regionalization and potentially the patterning of thalamic projections. We show that reduction of Fgf8 levels in hypomorphic mouse mutants shifts early gradients of gene expression rostrally, thereby modifying the molecular identity of rostral cortical progenitors. This shift correlates with a reduction in the size of a molecularly defined rostral neocortical domain and a corresponding rostral expansion of more caudal regions. Despite these molecular changes, the topography of projections between the dorsal thalamus and rostral neocortex in mutant neonates appears the same as the topography of wild-type littermates. Overall, our study demonstrates the role of endogenous Fgf8 in regulating early gradients of transcription factors in cortical progenitor cells and in molecular regionalization of the cortical plate

Family of Ebf/Olf-1-related genes potentially involved in neuronal differentiation and regional specification in the central nervous system

Two novel mouse genes, Ebf2 and Ebf3, have been identified which show high similarity to the rodent Ebf/Olf‐1 and the Drosophila collier genes. The strong conservation of the protein regions corresponding to the DNA binding and dimerisation domains previously defined in Ebf strongly suggests that Ebf2 and Ebf3 also constitute DNA sequence‐specific transcription factors. Determination of the chromosomal locations of the two genes indicated that the different members of this novel mouse multigene family are not clustered. A detailed analysis of the expression of each of the three Ebf genes in the developing central nervous system revealed partially overlapping patterns with two salient features: 1) In the region extending from the midbrain to the spinal cord, the expression of the three genes correlated with neuronal maturation, with a general activation in early post‐mitotic cells, followed by specific patterns of extinction also consistent with the neurogenic gradient. 2) In the forebrain area, although the patterns of expression of the Ebf genes also reflected neuronal maturation, they appeared in addition to be region specific. These data suggest that Ebf genes may be involved in the control of neuronal differentiation in the CNS and in enforcing regional diversity in populations of post‐mitotic forebrain neurons. Dev. Dyn. 1997;210:191–205.

Microglia Modulate Wiring of the Embryonic Forebrain

Dysfunction of microglia, the tissue macrophages of the brain, has been associated with the etiology of several neuropsychiatric disorders. Consistently, microglia have been shown to regulate neurogenesis and synaptic maturation at perinatal and postnatal stages. However, microglia invade the brain during mid-embryogenesis and thus could play an earlier prenatal role. Here, we show that embryonic microglia, which display a transiently uneven distribution, regulate the wiring of forebrain circuits. Using multiple mouse models, including cell-depletion approaches and cx3cr1(-/-), CR3(-/-), and DAP12(-/-) mutants, we find that perturbing microglial activity affects the outgrowth of dopaminergic axons in the forebrain and the laminar positioning of subsets of neocortical interneurons. Since defects in both dopamine innervation and cortical networks have been linked to neuropsychiatric diseases, our study provides insights into how microglial dysfunction can impact forebrain connectivity and reveals roles for immune cells during normal assembly of brain circuits.

Emx1 and Emx2 cooperate to regulate cortical size, lamination, neuronal differentiation, development of cortical efferents, and thalamocortical pathfinding.

The homeobox transcription factors Emx1 and Emx2 are expressed in overlapping patterns that include cortical progenitors in the dorsal telencephalic neuroepithelium. We have addressed cooperation of Emx1 and Emx2 in cortical development by comparing phenotypes in Emx1; Emx2 double mutant mice with wild‐type and Emx1 and Emx2 single mutants. Emx double mutant cortex is greatly reduced compared with wild types and Emx single mutants; the hippocampus and dentate gyrus are absent, and growth and lamination of the olfactory bulbs are defective. Cell proliferation and death are relatively normal early in cortical neurogenesis, suggesting that hypoplasia of the double mutant cortex is primarily due to earlier patterning defects. Expression of cortical markers persists in the reduced double mutant neocortex, but the laminar patterns exhibited are less sharp than normal, consistent with deficient cytoarchitecture, probably due in part to reduced numbers of preplate and Reelin‐positive Cajal‐Retzius neurons. Subplate neurons also exhibit abnormal differentiation in double mutants. Cortical efferent axons fail to exit the double mutant cortex, and TCAs pass through the striatum and approach the cortex but do not enter it. This TCA pathfinding defect appears to be non‐cell autonomous and supports the hypothesis that cortical efferents are required scaffolds to guide TCAs into cortex. In double mutants, some TCAs fail to turn into ventral telencephalon and take an aberrant ventral trajectory; this pathfinding defect correlates with an Emx2 expression domain in ventral telencephalon. The more severe phenotypes in Emx double mutants suggest that Emx1 and Emx2 cooperate to regulate multiple features of cortical development. J. Comp. Neurol. 457:345–360, 2003.

Patterning of the lateral ganglionic eminence by the Gsh1 and Gsh2 homeobox genes regulates striatal and olfactory bulb histogenesis and the growth of axons through the basal ganglia.

The function of the Gsh1 and Gsh2 homeobox transcription factors during development of the mouse telencephalon was studied using loss of function mutations. No telencephalic phenotype was observed in Gsh1 mutants, whereas Gsh2 and Gsh1/2 mutants showed progressively more severe defects in development of neurons derived from the lateral ganglionic eminence (LGE). These defects arise from abnormal dorsoventral specification of LGE progenitor cells. Mice lacking both Gsh1 and Gsh2 have severe hypoplasia of the striatum, olfactory tubercle, and interneurons that migrate from the dorsal LGE to the olfactory bulb. In addition, Gsh function is linked to the development of telencephalic dopaminergic neurons. These observations show that Gsh1 and Gsh2 have early roles in defining the identity of LGE progenitor cells. As a result of the basal ganglia defects in the Gsh1/2 mutants, there are pallial heterotopia near the cortical/subcortical limit and defects in the pathfinding of corticofugal and thalamocortical fibers. These findings highlight the developmental interdependence of adjacent telencephalic structures. J. Comp. Neurol. 461:151–165, 2003.

Dlx-dependent and -independent regulation of olfactory bulb interneuron differentiation

Olfactory bulb interneuron development is a complex multistep process that involves cell specification in the ventral telencephalon, tangential migration into the olfactory bulb, and local neuronal maturation. Although several transcription factors have been implicated in this process, how or when they act remains to be elucidated. Here we explore the mechanisms that result in olfactory bulb interneuron defects in Dlx1&2−/− (distal-less homeobox 1 and 2) and Mash1−/− (mammalian achaete-schute homolog 1) mutants. We provide evidence that Dlx1&2 and Mash1 regulate parallel molecular pathways that are required for the generation of these cells, thereby providing new insights into the mechanisms underlying olfactory bulb development. The analysis also defined distinct anatomical zones related to olfactory bulb development. Finally we show that Dlx1&2 are required for promoting tangential migration to the olfactory bulb, potentially via regulating the expression of ErbB4 (v-erb-a erythroblastic leukemia viral oncogene homolog 4), Robo2 (roundabout homolog 2), Slit1 (slit homolog 1), and PK2 (prokineticin 2), which have all been shown to play essential roles in this migration.

Ebf gene function is required for coupling neuronal differentiation and cell cycle exit.

Helix-loop-helix transcription factors of the Ebf/Olf1 family have previously been implicated in the control of neurogenesis in the central nervous system in both Xenopus laevis and the mouse, but their precise roles have remained unclear. We have characterised two family members in the chick, and have performed a functional analysis by gain- and loss-of-function experiments. This study revealed several specific roles for Ebf genes in the spinal cord and hindbrain regions of higher vertebrates, and enabled their precise positioning along the neurogenic cascade. During neurogenesis, cell cycle exit appears to be tightly coupled to migration to the mantle layer and to neuronal differentiation. We show that antagonizing Ebf gene activity allows the uncoupling of these processes. Ebf gene function is necessary to initiate neuronal differentiation and migration toward the mantle layer in neuroepithelial progenitors, but it is not required for cell cycle exit. Ebf genes therefore appear to be master controllers of neuronal differentiation and migration, coupling them to cell cycle exit and earlier steps of neurogenesis. Mutual activation between proneural and Ebf genes suggests that besides their involvement in the engagement of differentiation, Ebf genes may also participate in the stabilisation of the committed state. Finally, gain-of-function data raise the possibility that, in addition to these general roles, Ebf genes may be involved in neuronal subtype specification in particular regions of the CNS.