Kenji Shimamura

FGF and Shh Signals Control Dopaminergic and Serotonergic Cell Fate in the Anterior Neural Plate

During development, distinct classes of neurons are specified in precise locations along the dorso-ventral and anterior-posterior axes of the neural tube. We provide evidence that intersections of Shh, which is expressed along the ventral neural tube, and FGF8, which is locally produced at the mid/hindbrain boundary and in the rostral forebrain, create induction sites for dopaminergic neurons in the midbrain and forebrain. The same intersection, when preceded by a third signal, FGF4, which is expressed in the primitive streak, defines an inductive center for hindbrain 5-HT neurons. These findings illustrate that cell patterning in the neural plate is a multistep process in which early inducers, which initially divide the neural plate into crude compartments, are replaced by multiple local organizing centers, which specify distinct neuronal cell types within these compartments.

Pallial and subpallial derivatives in the embryonic chick and mouse telencephalon, traced by the expression of the genes Dlx-2, Emx-1, Nkx-2.1, Pax-6, and Tbr-1

Pallial and subpallial morphological subdivisions of the developing chicken telencephalon were examined by means of gene markers, compared with their expression pattern in the mouse. Nested expression domains of the genes Dlx‐2 and Nkx‐2.1, plus Pax‐6‐expressing migrated cells, are characteristic for the mouse subpallium. The genes Pax‐6, Tbr‐1, and Emx‐1 are expressed in the pallium. The pallio‐subpallial boundary lies at the interface between the Tbr‐1 and Dlx‐2 expression domains. Differences in the expression topography of Tbr‐1 and Emx‐1 suggest the existence of a novel “ventral pallium” subdivision, which is an Emx‐1‐negative pallial territory intercalated between the striatum and the lateral pallium. Its derivatives in the mouse belong to the claustroamygdaloid complex. Chicken genes homologous to these mouse genes are expressed in topologically comparable patterns during development. The avian subpallium, called “paleostriatum,” shows nested Dlx‐2 and Nkx‐2.1 domains and migrated Pax‐6‐positive neurons; the avian pallium expresses Pax‐6, Tbr‐1, and Emx‐1 and also contains a distinct Emx‐1‐negative ventral pallium, formed by the massive domain confusingly called “neostriatum.” These expression patterns extend into the septum and the archistriatum, as they do into the mouse septum and amygdala, suggesting that the concepts of pallium and subpallium can be extended to these areas. The similarity of such molecular profiles in the mouse and chicken pallium and subpallium points to common sets of causal determinants. These may underlie similar histogenetic specification processes and field homologies, including some comparable connectivity patterns. J. Comp. Neurol. 424:409–438, 2000.

Combinatorial Roles of Olig2 and Neurogenin2 in the Coordinated Induction of Pan-Neuronal and Subtype-Specific Properties of Motoneurons

Distinct classes of neurons are generated at defined times and positions during development of the nervous system. It remains elusive how specification of neuronal identity coordinates with acquisition of pan-neuronal properties. Here we show that basic helix-loop-helix (bHLH) transcription factors Olig2 and Neurogenin2 (Ngn2) play vital roles in the coordinated induction of pan-neuronal and subtype-specific properties of motoneurons. Olig2 and Ngn2 are specifically coexpressed in motoneuron progenitors. Misexpression studies in chick demonstrate the specific, combinatorial actions of Olig2 and Ngn2 in motoneuron generation. Our results further revealed crossregulatory interactions between bHLH and homeodomain transcription factors in the specification of motoneurons. We suggest that distinct classes of transcription factors collaborate to generate motoneurons in the ventral neural tube.

T-Brain-1: A homolog of Brachyury whose expression defines molecularly distinct domains within the cerebral cortex

The mechanisms that regulate regional specification and evolution of the cerebral cortex are obscure. To this end, we have identified and characterized a novel murine and human gene encoding a putative transcription factor related to the Brachyury (T) gene that is expressed only in postmitotic cells. T-brain-1 (Tbr-1) mRNA is largely restricted to the cerebral cortex, where during embryogenesis it distinguishes domains that we propose may give rise to paleocortex, limbic cortex, and neocortex. Tbr-1 and Id-2 expression in the neocortex have discontinuities that define molecularly distinct neocortical areas. Tbr-1 expression is analyzed in the context of the prosomeric model. Topological maps are proposed for the organization of the dorsal telencephalon.

Control of anteroposterior and dorsoventral domains of Nkx-6.1 gene expression relative to other Nkx genes during vertebrate CNS development

Here we report the isolation, sequence and developmental expression in the central nervous system of several members of the chicken and mouse Nkx gene family. These are among the earliest genes to be regionally expressed in the neural plate; they are expressed just above the axial mesendoderm (prechordal mesendoderm and notochord). Each Nkx gene has a distinct spatial pattern of expression along the anterior-posterior axis of the ventral central nervous system: Nkx-2. 2 is expressed along the entire axis, whereas Nkx-2.1 is restricted to the forebrain, and Nkx-6.1 and Nkx-6.2 are largely excluded from the forebrain. They are also expressed in distinct patterns along the dorsal-ventral axis. These genes are expressed in both the ventricular and mantle zones; in the mantle zone Nkx-6.1 is co-expressed with Islet-1 in a subset of motor neurons. Like other Nkx genes, expression of Nkx-6.1 is induced by the axial mesendoderm and by sonic hedgehog protein. BMP-7 represses Nkx-6.1 expression. While the notochord can induce Nkx-6.1 expression in the anterior neural plate, sonic hedgehog protein does not, suggesting that the notochord produces additional molecules that can regulate ventral patterning.

Differential activities of Sonic hedgehog mediated by Gli transcription factors define distinct neuronal subtypes in the dorsal thalamus

The dorsal thalamus (DT) is a pivotal region in the vertebrate brain that relays inputs from the peripheral sensory organs to higher cognitive centers. It consists of clusters of neurons with relevant functions, called brain nuclei. However, the mechanisms underlying development of the DT, including specification of the neuronal subtypes and morphogenesis of the nuclear structures, remain largely unknown. As a first step to this end, we focused on two transcription factors Sox14 and Gbx2 that are expressed in the specific brain nuclei in the chick DT. The onset of their expression was found in distinct populations of the postmitotic cells in the prosomere 2, which was regulated by the differential activities of Sonic hedgehog (Shh) in a manner consistent with the action as a morphogen. Furthermore, both gain- and loss-of-function results strongly suggest that such distinct inductive activities are mediated selectively by different Gli factors. These results suggest that cooperation of the differential expression of Gli factors and the activity gradient of Shh signaling generates the distinct thalamic neurons at the specific locations.

Patterns of gene expression in the neural plate and neural tube subdivide the embryonic forebrain into transverse and longitudinal domains

The analysis of gene expression patterns in the neural plate and neural tube of the embryonic mouse forebrain shows that, at its earliest stages, the neuroepithelium is subdivided into molecularly distinct domains. Here, and in previous publications, we provide evidence that the patterns of gene expression can be related to primary morphogenetic processes that organize the histological primordia of the embryonic central nervous system into longitudinal and transverse domains. Longitudinal domains are generated by dorsoventral patterning signals produced by the axial mesendoderm and nonneural ectoderm. Transverse domains (proneuromeres and neuromeres) expressing distinct combinations of genes are present in the neural plate and neural tube.

Domains of Regulatory Gene Expression and the Developing Optic Chiasm: Correspondence With Retinal Axon Paths and Candidate Signaling Cells

In mammals, some axons from each retina cross at the optic chiasm, whereas others do not. Although several loci have been identified within the chiasmatic region that appear to provide guidance cues to the retinal axons, the underlying molecular mechanisms that regulate this process are poorly understood. Here we investigate whether the earliest retinal axon trajectories and a cellular population (CD44 and stage‐specific embryonic antigen 1 [SSEA] neurons), previously implicated in directing axon growth in the developing chiasm (reviewed in Mason and Sretavan [1997] Curr. Op. Neurobiol. 7:647–653), correlate with the expression patterns of several regulatory genes (BF‐1, BF‐2, Dlx‐2, Nkx‐2.1, Nkx‐2.2, and Shh). These studies demonstrate that gene expression patterns in the chiasmatic region reflect the longitudinal subdivisions of the forebrain in that axon tracts in this region generally are aligned parallel to these subdivisions. Moreover, zones defined by overlapping domains of regulatory gene expression coincide with sites implicated in providing guidance information for retinal axon growth in the developing optic chiasm. Together, these data support the hypothesis that molecularly distinct, longitudinally aligned domains in the forebrain regulate the pattern of retinal axon projections in the developing hypothalamus. J. Comp. Neurol. 1999:346–358, 1999.

Antioxidant enzyme, 3-mercaptopyruvate sulfurtransferase-knockout mice exhibit increased anxiety-like behaviors: A model for human mercaptolactate- cysteine disulfiduria

Human mercaptolactate-cysteine disulfiduria (MCDU) was first recognized and reported in 1968. Most cases of MCDU are associated with mental retardation, while the pathogenesis remains unknown. To investigate it, we generated homozygous 3-mercaptopyruvate sulfurtransferase (MST: EC 2.8.1.2) knockout (KO) mice using C57BL/6 embryonic stem cells as an animal model. The MST-KO mice showed significantly increased anxiety-like behaviors with an increase in serotonin level in the prefrontal cortex (PFC), but not with abnormal morphological changes in the brain. MCDU can be caused by loss in the functional diversity of MST; first, MST functions as an antioxidant protein. MST possessing 2 redox-sensing molecular switches maintains cellular redox homeostasis. Second, MST can produce H2S (or HS−). Third, MST can also produce SOx. It is concluded that behavioral abnormality in MST-KO mice is caused by MST function defects such as an antioxidant insufficiency or a new transducer, H2S (or HS−) and/or SOx deficiency.

Cadherin-based adhesions in the apical endfoot are required for active Notch signaling to control neurogenesis in vertebrates

The development of the vertebrate brain requires an exquisite balance between proliferation and differentiation of neural progenitors. Notch signaling plays a pivotal role in regulating this balance, yet the interaction between signaling and receiving cells remains poorly understood. We have found that numerous nascent neurons and/or intermediate neurogenic progenitors expressing the ligand of Notch retain apical endfeet transiently at the ventricular lumen that form adherens junctions (AJs) with the endfeet of progenitors. Forced detachment of the apical endfeet of those differentiating cells by disrupting AJs resulted in precocious neurogenesis that was preceded by the downregulation of Notch signaling. Both Notch1 and its ligand Dll1 are distributed around AJs in the apical endfeet, and these proteins physically interact with ZO-1, a constituent of the AJ. Furthermore, live imaging of a fluorescently tagged Notch1 demonstrated its trafficking from the apical endfoot to the nucleus upon cleavage. Our results identified the apical endfoot as the central site of active Notch signaling to securely prohibit inappropriate differentiation of neural progenitors.