Cerebral cortical specification by early potential restriction of progenitor cells and later phenotype control of postmitotic neurons

Somato-dendritic decoupling as a novel mechanism for protracted cortical maturation

BACKGROUND

Both human and animal data indicate that disruption of the endogenously slow maturation of temporal association cortical (TeA) networks is associated with abnormal higher order cognitive development. However, the neuronal mechanisms underlying the endogenous maturation delay of the TeA are poorly understood.

RESULTS

Here we report a novel form of developmental plasticity that is present in the TeA. It was found that deep layer TeA neurons, but not hippocampal or primary visual neurons, exist in a protracted 'embryonic-like' state through a mechanism involving reduced somato-dendritic communication and a non-excitable somatic membrane. This mechanism of neural inactivity is present in intact tissue and shows a remarkable transition into an active somato-dendritically coupled state. The quantity of decoupled cells diminishes in a protracted and age-dependent manner, continuing into adolescence.

CONCLUSIONS

Based on our data, we propose a model of neural plasticity through which protracted compartmentalization and decoupling in somato-dendritic signalling plays a key role in controlling how excitable neurons are incorporated into recurrent cortical networks independent of neurogenesis.

The double inhibition of endogenously produced BMP and Wnt factors synergistically triggers dorsal telencephalic differentiation of mouse ES cells

Embryonic stem (ES) cells are becoming a popular model of in vitro neurogenesis, as they display intrinsic capability to generate neural progenitors that undergo the known steps of in vivo neural development. These include the acquisition of distinct regional fates, which depend on growth factors and signals that are present in the culture medium. The control of the intracellular signaling that is active at different steps of ES cell neuralization, even when cells are cultured in chemically defined medium, is complicated by the endogenous production of growth factors. However, this endogenous production has been poorly investigated so far. To address this point, we performed a high-throughput analysis of the expression of morphogens during mouse ES cell neuralization in minimal medium. We found that during their neuralization, ES cells increased the expression of members of Wnt, Fibroblast Growth Factor (FGF), and BMP families. Conversely, the expression of Activin/Nodal and Shh ligands was low in early steps of neuralization. In this experimental condition, neural progenitors and neurons generated by ES cells expressed a gene expression profile that was consistent with a midbrain identity. We found that endogenous BMP and Wnt signaling, but not FGF signaling, synergistically affected ES cell neural patterning, by turning off a profile of dorsal/telencephalic gene expression. Double BMP and Wnt inhibition allowed neuralized ES cells to sequentially activate key genes of cortical differentiation. Our findings are consistent with a novel synergistic effect of Wnt and BMP endogenous signaling of ES cells in inhibiting a cortical differentiation program.

The control of precerebellar neuron migration by RNA-binding protein Csde1

Neuronal migration during brain development sets the position of neurons for the subsequent wiring of neural circuits. To understand the molecular mechanism regulating the migrating process, we considered the migration of mouse precerebellar neurons. Precerebellar neurons originate in the rhombic lip of the hindbrain and show stereotypic, long-distance tangential migration along the circumference of the hindbrain to form precerebellar nuclei at discrete locations. To identify the molecular components underlying this navigation, we screened for genes expressed in the migrating precerebellar neurons. As a result, we identified the following three genes through the screening; Calm1, Septin 11, and Csde1. We report here functional analysis of one of these genes, Csde1, an RNA-binding protein implicated in the post-transcriptional regulation of a subset of cellular mRNA, by examining its participation in precerebellar neuronal migration. We found that shRNA-mediated inhibition of Csde1 expression resulted in a failure of precerebellar neurons to complete their migration into their prospective target regions, with many neurons remaining in migratory paths. Furthermore, those that did reach their destination failed to invade the depth of the hindbrain via radial migration. These results have uncovered a crucial role of Csde1 in the proper control of both radial and tangential migration of precerebellar neurons.

Gene expression in the rat brain: high similarity but unique differences between frontomedial-, temporal- and occipital cortex

BACKGROUND

The six-layered neocortex of the mammalian brain may appear largely homologous, but is in reality a modular structure of anatomically and functionally distinct areas. However, global gene expression seems to be almost identical across the cerebral cortex and only a few genes have so far been reported to show regional enrichment in specific cortical areas.

RESULTS

In the present study on adult rat brain, we have corroborated the strikingly similar gene expression among cortical areas. However, differential expression analysis has allowed for the identification of 30, 24 and 11 genes enriched in frontomedial -, temporal- or occipital cortex, respectively. A large proportion of these 65 genes appear to be involved in signal transduction, including the ion channel Fxyd6, the neuropeptide Grp and the nuclear receptor Rorb. We also find that the majority of these genes display increased expression levels around birth and show distinct preferences for certain cortical layers and cell types in rodents.

CONCLUSIONS

Since specific patterns of expression often are linked to equally specialised biological functions, we propose that these cortex sub-region enriched genes are important for proper functioning of the cortical regions in question.

Natural genetic diversity as a means to uncover stem cell regulatory pathways

Natural genetic diversity is a largely untapped reservoir for use in the discovery of stem cell regulatory pathways. Here we explore the means by which phenotypic diversity in mice can lead to the discovery of novel genes affecting stem cell regulation. We use as an example the discovery that latexin is a regulator of the natural size of the hematopoietic stem cell population in mice. The fact that it is a negative regulator of stem cell numbers, and thus served as a brake on stem cell expansion, led us to consider the possibility that it acts as a tumor suppressor. Experimental evidence supporting this hypothesis is reviewed.

Osteogenic properties of human myogenic progenitor cells

Here, we identified human myogenic progenitor cells coexpressing Pax7, a marker of muscle satellite cells and bone-specific alkaline phosphatase, a marker of osteoblasts, in regenerating muscle. To determine whether human myogenic progenitor cells are able to act as osteoprogenitor cells, we cultured both primary and immortalized progenitor cells derived from the healthy muscle of a nondystrophic woman. The undifferentiated myogenic progenitors spontaneously expressed two osteoblast-specific proteins, bone-specific alkaline phosphatase and Runx2, and were able to undergo terminal osteogenic differentiation without exposure to an exogenous inductive agent such as bone morphogenetic proteins. They also expressed the muscle lineage-specific proteins Pax7 and MyoD, and lost their osteogenic characteristics in association with terminal muscle differentiation. Both myoblastic and osteoblastic properties are thus simultaneously expressed in the human myogenic cell lineage prior to commitment to muscle differentiation. In addition, C3 transferase, a specific inhibitor of Rho GTPase, blocked myogenic but not osteogenic differentiation of human myogenic progenitor cells. These data suggest that human myogenic progenitor cells retain the capacity to act as osteoprogenitor cells that form ectopic bone spontaneously, and that Rho signaling is involved in a critical switch between myogenesis and osteogenesis in the human myogenic cell lineage.

Immortalization of human myogenic progenitor cell clone retaining multipotentiality

Human myogenic cells have limited ability to proliferate in culture. Although forced expression of telomerase can immortalize some cell types, telomerase alone delays senescence of human primary cultured myogenic cells, but fails to immortalize them. In contrast, constitutive expression of both telomerase and the E7 gene from human papillomavirus type 16 immortalizes primary human myogenic cells. We have established an immortalized primary human myogenic cell line preserving multipotentiality by ectopic expression of telomerase and E7. The immortalized human myogenic cells exhibit the phenotypic characteristics of their primary parent, including an ability to undergo myogenic, osteogenic, and adipogenic terminal differentiation under appropriate culture conditions. The immortalized cells will be useful for both basic and applied studies aimed at human muscle disorders. Furthermore, immortalization by transduction of telomerase and E7 represents a useful method by which to expand human myogenic cells in vitro without compromising their ability to differentiate.

Early regionalisation of the neocortex and the medial ganglionic eminence

The two major functional classes of neurons that build the cerebral cortex are generated in two distinct parts of the telencephalon. Excitatory long distance projecting neurons are produced dorsally in the pallium, whereas local inhibitory interneurons are mainly produced in the medial ridge of the ventral telencephalon. These two parts of the telencephalon are molecularly regionalized from early embryonic stages, but cellular indices of regionalisation are observed only at later stages of development. We have looked for cellular indices of regionalisation in the cortical anlage at early embryonic stages, when the first efferent cortical neurons are generated. Similarly, we have looked for functional regionalisation of the medial ganglionic eminence at the same stages, when the future cortical interneurones are generated. Here, we summarize data showing that two regions in the mouse cortex embryo, the lateral and dorsal cortex, differ strongly in their early neurogenesis. Moreover, the two domains differ in their capacity to produce GABAergic neurons in vitro; this capacity is only observed in the dorsal cortex. The differentiation of the two domains appears to be independent of the laterorostral to mediocaudal gradient of maturation of the cortex. In the basal telencephalon too, the capacity to differentiate GABAergic neurons is not uniformly distributed across the medial ganglionic eminence. The neurogenesis of future cortical interneurons is seen to be highly active in a small area located in the rostral MGE, at mid dorso-ventral level.

Role of the Protomap and Target-derived Signals in the Development of Intrahemispheric Connections

Mechanisms intrinsic to the early cerebral cortex have been implicated in the establishment of cortical area identity. However, the extent to which the cortical protomap contributes to the formation of highly complex intrahemispheric connections remains obscure. Mechanisms by which postmitotic neurons establish correct corticocortical connections later in corticogenesis also remain to be elucidated. Here, we used a new transplantation method, employing donor tissue harvested from enhanced green fluorescent protein-expressing rats, to show that cortical progenitors are regionally specified for connectional potential and that this controls the development of specific intrahemispheric projections. The acquisition of connectional capacity relies on positional cues within the cortical primordium, but is independent of thalamic inputs. In addition, since cortical neurons developing in organotypic slice culture extended axons more prominently into their normal cortical target tissues than into non-target tissues, we suggest that cortical neurons respond to specific signals derived from their cortical targets.

Organization and development of corticocortical associative neurons expressing the orphan nuclear receptor Nurr1

The developmental mechanism that contributes to the highly organized axonal connections within the cerebral cortex is not well understood. This is partly due to the lack of molecular markers specifically expressed in corticocortical associative neurons during the period of circuit formation. We have shown previously that latexin, a carboxypeptidase A inhibitor, is expressed in intrahemispheric corticocortical neurons from the second postnatal week in the rat (Arimatsu et al. [1999] Cereb. Cortex 9:569-576). In the present study, we first demonstrate in the adult rat that the orphan nuclear receptor Nurr1 is coexpressed in latexin-expressing neurons located in layer V, sublayer VIa, and the white matter of the lateral sector of the neocortex, and also in latexin-negative early born neurons in sublayer VIb of the entire neocortex. Virtually all Nurr1-expressing neurons exhibit immunoreactivity for phosphate-activated glutaminase but not for gamma-aminobutyric acid, suggesting that they are glutamatergic-excitatory neurons. By combining Nurr1 immunohistochemistry and 5-bromo-2'-deoxyuridine-birthdating, we then show that Nurr1 is expressed in (early born) subplate neurons and (later born) presumptive latexin-expressing neurons from embryonic day 18 onward. Finally, by combination of Nurr1 immunohistochemistry and retrograde tracing, we show that Nurr1-expressing neurons, including those in sublayer VIb, contribute predominantly to long-range intrahemispheric corticocortical projections. These results raise the possibility that Nurr1 plays a role in the establishment and maintenance of normal corticocortical circuitry and function.

Emx2 patterns the neocortex by regulating FGF positional signaling

Molecular genetic studies implicate fibroblast growth factor 8 (FGF8), and the transcription factor Emx2, in development of the neocortical area map. Both are proposed to specify area position along the anterior-to-posterior axis of the cortical primordium. Whether FGF8 and Emx2 act independently or coordinately, or whether one controls the other, has not been determined. Here we report that Emx2, by regulating FGF8, has an indirect but vital role in area-map development. Using electroporation-mediated gene transfer in living mouse embryos, we found that overexpressing Emx2 altered the area map, but only when ectopic Emx2 overlapped the FGF8 source. Furthermore, we found that FGF8 levels were decreased by excess Emx2, and increased in mice lacking Emx2. Finally, cortical domain shifts that characterize Emx2 mutants were rescued by sequestering excess FGF8 with a truncated FGF receptor construct. These findings begin to clarify the signaling network that patterns the neocortical area map.

Constructing the mammalian neocortex: the role of intrinsic factors

The mammalian neocortex is subdivided into regions that are specialised for the processing of particular forms of information. These regions are distinct in terms of their cytoarchitecture, electrophysiology, and connectivity. How this regional diversity is generated through development is currently a topic of considerable interest and has centered upon two main issues. First, to what extent are these regions prespecified by intrinsic genetic mechanisms? Second, what is the influence of extrinsic activity in transmitting signals that ultimately shape functional regions? Historically, experimental evidence has tended to emphasise the role of extrinsic influences, but the identification and analysis of several genes that are expressed asymmetrically in the developing neocortex have tempered this viewpoint. We review current literature from the standpoint that intrinsic influences act early in neocortical development to generate molecular patterning whose main role is the guidance of long-range projections from the dorsal thalamus. Extrinsic influences appear to generate receptive fields for peripheral input, the summation of which determines the areal extent of particular neocortical region.

Timing and plasticity of specification of CaM-Kinase II alpha expression by neocortical neurons

In this work, the differential expression of a chemical marker, the alpha-isoform of the calcium/calmodulin-dependent protein kinase II (CaM-Kinase II alpha) and the development of the spinal cord projection were used to determine in vivo the embryonic stages at which different aspects of the phenotype of neocortical cells are specified. We first performed a quantitative, immunocytochemical study on the levels of CaM-Kinase II alpha expression in the frontal, parietal and occipital cortical areas of control adult rats. We found that the levels of expression of CaM-Kinase II alpha were larger in the frontal and parietal areas than in the occipital areas. In addition, all layer V neurons identified as projecting to the spinal cord were CaM-Kinase II alpha immunopositive. We then grafted embryonic day (E) 12 or 14 cells from the presumptive frontal or occipital cortex of donor fetuses into the frontal or occipital cortex of newborn hosts. Cortical cells grafted at E12 differentiate neurons with molecular (CaM-Kinase II alpha) and connectivity (spinal cord projection) phenotypes appropriate to the cortical area where they complete their development whereas cells taken at E14 differentiate neurons with molecular and connectivity phenotypes appropriate to their cortical locus of origin. These findings suggest that E12 progenitors destined to generate layer V neurons are multipotent. The final phenotype of their progeny depends on regionalizing signals expressed in the environment. Later in corticogenesis, committed progenitors become unable to respond to regionalizing signals and generate neurons whose phenotype is appropriate to the initial cortical position of the precursor.

Reduction of early thalamic input alters adult corticocortical connectivity

The functional specificity of mammalian isocortex requires that precise connections be established between cortical areas and their targets. While recent studies of cortical development have focused on intrinsic specification, the role of extrinsic factors has received considerably less attention. In the present study, we examined how early removal of thalamic input affects the development of visual corticocortical connections. Hamster pups received ablations of visual thalamic nuclei on the day of birth. At 30 days of age, an injection of horseradish peroxidase (HRP) was placed into the area of cortex deafferented by the early thalamic ablation to retrogradely label adult corticocortical connections. Ablated animals displayed a significant increase in the number of corticocortical connections compared to control animals. The increased connectivity in ablated animals was primarily due to a significant increase in the number of corticocortical projections arising from non-visual areas. These results demonstrate that an intact thalamocortical projection is necessary for the development of normal cortical connectivity.

In vitro clonal analysis of rat cerebral cortical neurons expressing latexin, a subtype-specific molecular marker of glutamatergic neurons

Latexin is expressed in a subset of glutamatergic projection neurons located in the lateral but not the dorsomedial neocortex in rat. In the present study, we performed clonal analysis of embryonic cortical neurons in vitro using latexin as a subtype-specific molecular marker of glutamatergic neurons to address whether certain precursors in early cortical anlage are already committed to a single molecular phenotype. Dissociated cells from lateral cortex at embryonic day 12 were labelled with a replication-deficient retroviral vector containing the bacterial lacZ gene, and cultured in a monolayer for 3 weeks. Double-immunofluorescence staining was used to simultaneously visualize latexin- and beta-galactosidase-immunopositive neurons. Out of 572 beta-galactosidase-immunopositive clones examined, 27 clones contained latexin-expressing neuron(s). Of these 27 clones, 25 clones that contained three or more neurons were mixed clones containing both latexin-immunopositive and -immunonegative neurons. These results suggest that committed precursors to producing solely latexin-expressing neurons are not exist in the early cerebral cortical anlage.

Development and plasticity of cortical areas and networks

The development of cortical layers, areas and networks is mediated by a combination of factors that are present in the cortex and are influenced by thalamic input. Electrical activity of thalamocortical afferents has a progressive role in shaping cortex. For early thalamic innervation and patterning, the presence of activity might be sufficient; for features that develop later, such as intracortical networks that mediate emergent responses of cortex, the spatiotemporal pattern of activity often has an instructive role. Experiments that route projections from the retina to the auditory pathway alter the pattern of activity in auditory thalamocortical afferents at a very early stage and reveal the progressive influence of activity on cortical development. Thus, cortical features such as layers and thalamocortical innervation are unaffected, whereas features that develop later, such as intracortical connections, are affected significantly. Surprisingly, the behavioural role of 'rewired' cortex is also influenced profoundly, indicating the importance of patterned activity for this key aspect of cortical function.

Patterning the mammalian cerebral cortex

When and how is the area map of the cerebral cortex set up during development? Recent studies indicate that regional pattern emerges early in cortical neurogenesis, and that this pattern does not require cues from extrinsic innervation. Studies of mutant mice indicate a role for embryonic signaling centers and for specific transcription factors in regionalizing the cortex. Thus, it is increasingly probable that the cortex is partitioned using the same types of mechanisms--and in some cases, the same gene families--that are used in patterning other parts of the embryo. This emerging model is likely to be the basis for many future studies. However, new evidence also confirms the special nature of the cerebral cortex, in that cues from developing connections appear to modify and refine the final area map.

Stage of specification of the spinal cord and tectal projections from cortical grafts

In order to determine the embryonic age at which the hodological phenotype developed by neocortical cells is specified, we have examined the spinal or tectal projections developed by embryonic (E) grafts of presumptive frontal or occipital neocortex placed into the frontal or occipital neocortex of newborn host rats. Grafts of E13, E14 and E16 cells of the frontal cortex transplanted into the occipital cortex of newborns are capable of developing and maintaining in adulthood a spinal cord axon. Grafts of E12 cells do not project to the spinal cord but send fibres to the superficial layers of the tectum. In addition, following transplantation into the frontal cortex, early embryonic (E12) cells from the presumptive occipital cortex are capable of differentiating into neurons with spinal cord projection but are practically incapable of developing a tectal projection. When grafted at E14 into the frontal cortex, occipital cells lose the capacity to project to the spinal cord but become able to send fibres to the tectum. Taken together, these findings indicate that young (E12) embryonic frontal and occipital cortical cells are competent to subsequently differentiate into neurons projecting to the spinal cord or tectum according to instructive signals available in the cortical territory where they complete their development. By E13/E14, some cortical cells are specified and their capacity to contact targets that are not appropriate to their embryonic origin is much reduced. These findings are consistent with the notion that cortical specification involves progressive restriction in cell multipotentiality and fate specification toward region-specific phenotypes.

A role for neural determination genes in specifying the dorsoventral identity of telencephalic neurons

Neurogenin1 (Ngn1), Neurogenin2(Ngn2), and Mash1 encode bHLH transcription factors with neuronal determination functions. In the telencephalon, theNgns and Mash1 are expressed at high levels in complementary dorsal and ventral domains, respectively. We found thatNgn function is required to maintain these two separate expression domains, as Mash1 expression is up-regulated in the dorsal telencephalon of Ngn mutant embryos. We have taken advantage of the replacement of the Ngns by Mash1 in dorsal progenitors to address the role of the neural determination genes in neuronal-type specification in the telencephalon. InNgn2 single and Ngn1; Ngn2 double mutants, a population of early born cortical neurons lose expression of dorsal-specific markers and ectopically express a subset of ventral telencephalic-specific markers. Analysis of Mash1; Ngn2double mutant embryos and of embryos carrying a Ngn2 toMash1 replacement mutation demonstrated that ectopic expression of Mash1 is required and sufficient to confer these ventral characteristics to cortical neurons. Our results indicate that in addition to acting as neuronal determinants, Mash1 andNgns play a role in the specification of dorsal-ventral neuronal identity, directly linking pathways of neurogenesis and regional patterning in the forebrain.

Genomic organization and regulatory elements of the rat latexin gene, which is expressed in a cell type-specific manner in both central and peripheral nervous systems

Latexin, a carboxypeptidase A inhibitor, is expressed in a cell type-specific manner in both central and peripheral nervous systems in the rat. In the neocortex, a specific subpopulation of neurons in layers V and VI expresses latexin. In the primary sensory ganglia, the expression is restricted to smaller diameter neurons. As a first step to clarify regulatory mechanisms underlying cell type-specific expression of latexin, we have determined the organization of the rat latexin gene and analyzed its regulatory elements. The latexin gene spans approximately 5.8 kb, and consists of six exons and five introns. Three transcription initiation sites were mapped. The upstream region lacks typical TATA or CAAT boxes but has several GC-rich sites. To assess promoter activity, the luciferase reporter gene fused to the 5'-flanking region (6.4 kb) of the latexin gene was transiently transfected into several cell lines. Luciferase activity was 2-8 times higher in latexin-expressing cells (PC12) than non-expressing cells (NS20 and L6). Deletion analysis with PC12 cells revealed that a core promoter is located between nucleotide positions -261 and -201 relative to the A of the initiation codon. Nerve growth factor (NGF)-responsive element(s) is located between positions -518 and -262, in which AP-1, AP-2 and NF-kappaB binding sites are found. Furthermore, we demonstrate that a 1.3 kb genomic fragment containing the first intron has transcriptional enhancing activity in PC12 cells. These results suggest that up and downstream regulatory elements are involved in the control of cell type-specific expression of latexin.