The early topography of thalamocortical projections is shifted in Ebf1 and Dlx1/2 mutant mice

Development and Evolution of Thalamocortical Connectivity

Conscious perception in mammals depends on precise circuit connectivity between cerebral cortex and thalamus; the evolution and development of these structures are closely linked. During the wiring of reciprocal thalamus-cortex connections, thalamocortical axons (TCAs) first navigate forebrain regions that had undergone substantial evolutionary modifications. In particular, the organization of the pallial-subpallial boundary (PSPB) diverged significantly between mammals, reptiles, and birds. In mammals, transient cell populations in internal capsule and early corticofugal projections from subplate neurons closely interact with TCAs to guide pathfinding through ventral forebrain and PSPB crossing. Prior to thalamocortical axon arrival, cortical areas are initially patterned by intrinsic genetic factors. Thalamocortical axons then innervate cortex in a topographically organized manner to enable sensory input to refine cortical arealization. Here, we review the mechanisms underlying the guidance of thalamocortical axons across forebrain boundaries, the implications of PSPB evolution for thalamocortical axon pathfinding, and the reciprocal influence between thalamus and cortex during development.

The transcription factor EBF1 non-cell-autonomously regulates cardiac growth and differentiation

Reciprocal interactions between non-myocytes and cardiomyocytes regulate cardiac growth and differentiation. Here, we report that the transcription factor Ebf1 is highly expressed in non-myocytes and potently regulates heart development. Ebf1-deficient hearts display myocardial hypercellularity and reduced cardiomyocyte size, ventricular conduction system hypoplasia, and conduction system disease. Growth abnormalities in Ebf1 knockout hearts are observed as early as embryonic day 13.5. Transcriptional profiling of Ebf1-deficient embryonic cardiac non-myocytes demonstrates dysregulation of Polycomb repressive complex 2 targets, and ATAC-Seq reveals altered chromatin accessibility near many of these same genes. Gene set enrichment analysis of differentially expressed genes in cardiomyocytes isolated from E13.5 hearts of wild-type and mutant mice reveals significant enrichment of MYC targets and, consistent with this finding, we observe increased abundance of MYC in mutant hearts. EBF1-deficient non-myocytes, but not wild-type non-myocytes, are sufficient to induce excessive accumulation of MYC in co-cultured wild-type cardiomyocytes. Finally, we demonstrate that BMP signaling induces Ebf1 expression in embryonic heart cultures and controls a gene program enriched in EBF1 targets. These data reveal a previously unreported non-cell-autonomous pathway controlling cardiac growth and differentiation.

A Stay in Friedrich Bonhoeffer's Lab in Tubingen in the Mid-eighties

The main focus of research for which Friedrich Bonhoeffer's work is known in the Neuroscience community was pioneer experiments on how axonal projections could organize into "maps", what mechanisms are involved in axon guidance and involve gradients of guiding molecules, and isolation of the first such molecules, e.g. RAGS (ephrin A5) and RGM (repulsive guidance molecule). Other papers have described in detail these contributions as well as Friedrich Bonhoeffer's personality. In the mid-eighties, I made a 2-year stay in his lab and initiated a line of research on development of binocular connections in Mammals, particularly the guidance of retinal fibers to one or the other side of the brain. In this paper I recall these circumstances as they pertain to Neuroscience as it stood at the time, and explain as best as I can how his lab was a conducive setting for the discoveries made there and how Friedrich Bonhoeffer acted for me as a scientist and a tutor.

Nolz1 expression is required in dopaminergic axon guidance and striatal innervation

Midbrain dopaminergic (DA) axons make long longitudinal projections towards the striatum. Despite the importance of DA striatal innervation, processes involved in establishment of DA axonal connectivity remain largely unknown. Here we demonstrate a striatal-specific requirement of transcriptional regulator Nolz1 in establishing DA circuitry formation. DA projections are misguided and fail to innervate the striatum in both constitutive and striatal-specific Nolz1 mutant embryos. The lack of striatal Nolz1 expression results in nigral to pallidal lineage conversion of striatal projection neuron subtypes. This lineage switch alters the composition of secreted factors influencing DA axonal tract formation and renders the striatum non-permissive for dopaminergic and other forebrain tracts. Furthermore, transcriptomic analysis of wild-type and Nolz1^−/− mutant striatal tissue led to the identification of several secreted factors that underlie the observed guidance defects and proteins that promote DA axonal outgrowth. Together, our data demonstrate the involvement of the striatum in orchestrating dopaminergic circuitry formation.

The mechanisms regulating midbrain dopaminergic innervation during development are unclear. Here, the authors showed that Nolz1 is required for axonal guidance of dopaminergic neurons during embryonic development of the mouse brain.

A Combinatorial Cell and Drug Delivery Strategy for Huntington's Disease Using Pharmacologically Active Microcarriers and RNAi Neuronally-Committed Mesenchymal Stromal Cells

For Huntington's disease (HD) cell-based therapy, the transplanted cells are required to be committed to a neuronal cell lineage, survive and maintain this phenotype to ensure their safe transplantation in the brain. We first investigated the role of RE-1 silencing transcription factor (REST) inhibition using siRNA in the GABAergic differentiation of marrow-isolated adult multilineage inducible (MIAMI) cells, a subpopulation of MSCs. We further combined these cells to laminin-coated poly(lactic-co-glycolic acid) PLGA pharmacologically active microcarriers (PAMs) delivering BDNF in a controlled fashion to stimulate the survival and maintain the differentiation of the cells. The PAMs/cells complexes were then transplanted in an ex vivo model of HD. Using Sonic Hedgehog (SHH) and siREST, we obtained GABAergic progenitors/neuronal-like cells, which were able to secrete HGF, SDF1 VEGFa and BDNF, of importance for HD. GABA-like progenitors adhered to PAMs increased their mRNA expression of NGF/VEGFa as well as their secretion of PIGF-1, which can enhance reparative angiogenesis. In our ex vivo model of HD, they were successfully transplanted while attached to PAMs and were able to survive and maintain this GABAergic neuronal phenotype. Together, our results may pave the way for future research that could improve the success of cell-based therapy for HDs.

Development of the Thalamocortical Interactions: Past, Present and Future

For the past two decades, we have advanced in our understanding of the mechanisms implicated in the formation of brain circuits. The connection between the cortex and thalamus has deserved much attention, as thalamocortical connectivity is crucial for sensory processing and motor learning. Classical dye tracing studies in wild-type and knockout mice initially helped to characterize the developmental progression of this connectivity and revealed key transcription factors involved. With the recent advances in technical tools to specifically label subsets of projecting neurons, knock-down genes individually and/or modify their activity, the field has gained further understanding on the rules operating in thalamocortical circuit formation and plasticity. In this review, I will summarize the most relevant discoveries that have been made in this field, from development to early plasticity processes covering three major aspects: axon guidance, thalamic influence on sensory cortical specification, and the role of spontaneous thalamic activity. I will emphasize how the implementation of new tools has helped the field to progress and what I consider to be open questions and the perspective for the future.

Eye-specific segregation and differential fasciculation of developing retinal ganglion cell axons in the mouse visual pathway

Prior to forming and refining synaptic connections, axons of projection neurons navigate long distances to their targets. While much is known about guidance cues for axon navigation through intermediate choice points, whether and how axons are organized within tracts is less clear. Here we analyze the organization of retinal ganglion cell (RGC) axons in the developing mouse retinogeniculate pathway. RGC axons are organized by both eye-specificity and topography in the optic nerve and tract: ipsilateral RGC axons are segregated from contralateral axons and are offset laterally in the tract relative to contralateral axon topographic position. To identify potential cell-autonomous factors contributing to the segregation of ipsilateral and contralateral RGC axons in the visual pathway, we assessed their fasciculation behavior in a retinal explant assay. Ipsilateral RGC neurites self-fasciculate more than contralateral neurites in vitro and maintain this difference in the presence of extrinsic chiasm cues. To further probe the role of axon self-association in circuit formation in vivo, we examined RGC axon organization and fasciculation in an EphB1^-/- mutant, in which a subset of ipsilateral RGC axons aberrantly crosses the midline but targets the ipsilateral zone in the dorsal lateral geniculate nucleus on the opposite side. Aberrantly crossing axons retain their association with ipsilateral axons in the contralateral tract, indicating that cohort-specific axon affinity is maintained independently of guidance signals present at the midline. Our results provide a comprehensive assessment of RGC axon organization in the retinogeniculate pathway and suggest that axon self-association contributes to pre-target axon organization.

Developmental interactions between thalamus and cortex: a true love reciprocal story

The developmental programs that control the specification of cortical and thalamic territories are maintained largely as independent processes. However, bulk of evidence demonstrates the requirement of the reciprocal interactions between cortical and thalamic neurons as key for the correct development of functional thalamocortical circuits. This reciprocal loop of connections is essential for sensory processing as well as for the execution of complex sensory-motor tasks. Here, we review recent advances in our understanding of how mutual collaborations between both brain regions define area patterning and cell differentiation in the thalamus and cortex.

Molecular guidance cues in the development of visual pathway

70%–80% of our sensory input comes from vision. Light hit the retina at the back of our eyes and the visual information is relayed into the dorsal lateral geniculate nuclei (dLGN) and primary visual cortex (V1) thereafter, constituting the image-forming visual circuit. Molecular cues are one of the key factors to guide the wiring and refinement of the image-forming visual circuit during pre- and post-embryonic stages. Distinct molecular cues are involved in different developmental stages and nucleus, suggesting diverse guidance mechanisms. In this review, we summarize molecular guidance cues throughout the image-forming visual circuit, including chiasm determination, eye-specific segregation and refinement in the dLGN, and at last the reciprocal connections between the dLGN and V1.

Guidance of retinal axons in mammals

In order to navigate through the surrounding environment many mammals, including humans, primarily rely on vision. The eye, composed of the choroid, sclera, retinal pigmented epithelium, cornea, lens, iris and retina, is the structure that receives the light and converts it into electrical impulses. The retina contains six major types of neurons involving in receiving and modifying visual information and passing it onto higher visual processing centres in the brain. Visual information is relayed to the brain via the axons of retinal ganglion cells (RGCs), a projection known as the optic pathway. The proper formation of this pathway during development is essential for normal vision in the adult individual. Along this pathway there are several points where visual axons face 'choices' in their direction of growth. Understanding how these choices are made has advanced significantly our knowledge of axon guidance mechanisms. Thus, the development of the visual pathway has served as an extremely useful model to reveal general principles of axon pathfinding throughout the nervous system. However, due to its particularities, some cellular and molecular mechanisms are specific for the visual circuit. Here we review both general and specific mechanisms involved in the guidance of mammalian RGC axons when they are traveling from the retina to the brain to establish precise and stereotyped connections that will sustain vision.

Genetic Labeling of Nuclei-Specific Thalamocortical Neurons Reveals Putative Sensory-Modality Specific Genes

The thalamus is a central brain structure with topographically ordered long-range axonal projections that convey sensory information to the cortex via distinct nuclei. Although there is an increasing knowledge about genes important for thalamocortical (TC) development, the identification of genetic landmarks of the distinct thalamic nuclei during the embryonic development has not been addressed systematically. Indeed, a more comprehensive understanding of how the axons from the individual nuclei find their way and connect to their corresponding cortical area is called for. Here, we used a genetic dual labeling strategy in mice to purify distinct principal sensory thalamic neurons. Subsequent genome-wide transcriptome profiling revealed genes specifically expressed in each nucleus during embryonic development. Analysis of regulatory regions of the identified genes revealed key transcription factors and networks that likely underlie the specification of individual sensory-modality TC connections. Finally, the importance of correct axon targeting for the specific sensory-modality population transcriptome was evidenced in a Sema6A mutant, in which visual TC axons are derailed at embryonic life. In sum, our data determined the developmental transcriptional profile of the TC neurons that will eventually support sensory processing.

A data-driven modeling approach to identify disease-specific multi-organ networks driving physiological dysregulation

Multiple physiological systems interact throughout the development of a complex disease. Knowledge of the dynamics and connectivity of interactions across physiological systems could facilitate the prevention or mitigation of organ damage underlying complex diseases, many of which are currently refractory to available therapeutics (e.g., hypertension). We studied the regulatory interactions operating within and across organs throughout disease development by integrating in vivo analysis of gene expression dynamics with a reverse engineering approach to infer data-driven dynamic network models of multi-organ gene regulatory influences. We obtained experimental data on the expression of 22 genes across five organs, over a time span that encompassed the development of autonomic nervous system dysfunction and hypertension. We pursued a unique approach for identification of continuous-time models that jointly described the dynamics and structure of multi-organ networks by estimating a sparse subset of ∼12,000 possible gene regulatory interactions. Our analyses revealed that an autonomic dysfunction-specific multi-organ sequence of gene expression activation patterns was associated with a distinct gene regulatory network. We analyzed the model structures for adaptation motifs, and identified disease-specific network motifs involving genes that exhibited aberrant temporal dynamics. Bioinformatic analyses identified disease-specific single nucleotide variants within or near transcription factor binding sites upstream of key genes implicated in maintaining physiological homeostasis. Our approach illustrates a novel framework for investigating the pathogenesis through model-based analysis of multi-organ system dynamics and network properties. Our results yielded novel candidate molecular targets driving the development of cardiovascular disease, metabolic syndrome, and immune dysfunction.

Complex diseases such as hypertension often involve maladaptive autonomic nervous system control over the cardiovascular, renal, hepatic, immune, and endocrine systems. We studied the pathogenesis of physiological homeostasis by examining the temporal dynamics of gene expression levels from multiple organs in an animal model of autonomic dysfunction characterized by cardiovascular disease, metabolic dysregulation, and immune system aberrations. We employed a data-driven modeling approach to jointly predict continuous gene expression dynamics and gene regulatory interactions across organs in the disease and control phenotypes. We combined our analyses of multi-organ gene regulatory network dynamics and connectivity with bioinformatic analyses of genetic mutations that could regulate gene expression. Our multi-organ modeling approach to investigate the mechanisms of complex disease pathogenesis revealed novel candidates for therapeutic interventions against the development and progression of complex diseases involving autonomic nervous system dysfunction.

Semaphorin-Plexin signaling influences early ventral telencephalic development and thalamocortical axon guidance

Background

Sensory processing relies on projections from the thalamus to the neocortex being established during development. Information from different sensory modalities reaching the thalamus is segregated into specialized nuclei, whose neurons then send inputs to cognate cortical areas through topographically defined axonal connections.

Developing thalamocortical axons (TCAs) normally approach the cortex by extending through the subpallium; here, axonal navigation is aided by distributed guidance cues and discrete cell populations, such as the corridor neurons and the internal capsule (IC) guidepost cells. In mice lacking Semaphorin-6A, axons from the dorsal lateral geniculate nucleus (dLGN) bypass the IC and extend aberrantly in the ventral subpallium. The functions normally mediated by Semaphorin-6A in this system remain unknown, but might depend on interactions with Plexin-A2 and Plexin-A4, which have been implicated in other neurodevelopmental processes.

Methods

We performed immunohistochemical and neuroanatomical analyses of thalamocortical wiring and subpallial development in Sema6a and Plxna2; Plxna4 null mutant mice and analyzed the expression of these genes in relevant structures.

Results

In Plxna2; Plxna4 double mutants we discovered TCA pathfinding defects that mirrored those observed in Sema6a mutants, suggesting that Semaphorin-6A − Plexin-A2/Plexin-A4 signaling might mediate dLGN axon guidance at subpallial level.

In order to understand where and when Semaphorin-6A, Plexin-A2 and Plexin-A4 may be required for proper subpallial TCA guidance, we then characterized their spatiotemporal expression dynamics during early TCA development. We observed that the thalamic neurons whose axons are misrouted in these mutants normally express Semaphorin-6A but not Plexin-A2 or Plexin-A4. By contrast, all three proteins are expressed in corridor cells and other structures in the developing basal ganglia.

This finding could be consistent with an hypothetical action of Plexins as guidance signals through Sema6A as a receptor on dLGN axons, and/or with their indirect effect on TCA guidance due to functions in the morphogenesis of subpallial intermediate targets. In support of the latter possibility, we observed that in both Plxna2; Plxna4 and Sema6a mutants some IC guidepost cells abnormally localize in correspondence of the ventral path misrouted TCAs elongate into.

Conclusions

These findings implicate Semaphorin-6A − Plexin-A2/Plexin-A4 interactions in dLGN axon guidance and in the spatiotemporal organization of guidepost cell populations in the mammalian subpallium.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0083-4) contains supplementary material, which is available to authorized users.

Thalamic neuronal specification and early circuit formation

The thalamus is a central structure of the brain, primarily recognized for the relay of incoming sensory and motor information to the cerebral cortex but also key in high order intracortical communication. It consists of glutamatergic projection neurons organized in several distinct nuclei, each having a stereotype connectivity pattern and functional roles. In the adult, these nuclei can be appreciated by architectural boundaries, although their developmental origin and specification is only recently beginning to be revealed. Here, we summarize the current knowledge on the specification of the distinct thalamic neurons and nuclei, starting from early embryonic patterning until the postnatal days when active sensory experience is initiated and the overall system connectivity is already established. We also include an overview of the guidance processes important for establishing thalamocortical connections, with emphasis on the early topographical specification. The extensively studied thalamocortical axon branching in the cortex is briefly mentioned; however, the maturation and plasticity of this connection are beyond the scope of this review. In separate chapters, additional mechanisms and/or features that influence the specification and development of thalamic neurons and their circuits are also discussed. Finally, an outlook of future directions is given. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 830-843, 2017.

Development of Cortical Maps: Perspectives From the Barrel Cortex

One approach to examining how higher sensory, motor, and cognitive faculties emerge in the neocortex is to elucidate the underlying wiring principles of the brain during development. The mammalian neocortex is a layered structure generated from a sheet of proliferating ventricular cells that progressively divide to form specific functional areas, such as the primary somatosensory (S1) and motor (M1) cortices. The basic wiring pattern in each of these functional areas is based on a similar framework, but is distinct in detail. Functional specialization in each area derives from a combination of molecular cues within the cortex and neuronal activity-dependent cues provided by innervating axons from the thalamus. One salient feature of neocortical development is the establishment of topographic maps in which neighboring neurons receive input relayed from neighboring sensory afferents. Barrels, which are prominent sensory units in the somatosensory cortex of rodents, have been examined in detail, and data suggest that the initial, gross formation of the barrel map relies on molecular cues, but the refinement of this topography depends on neuronal activity. Several excellent reviews have been published on the patterning and plasticity of the barrel cortex and the precise targeting of ventrobasal thalamic axons. In this review, the authors will focus on the formation and functional maturation of synapses between thalamocortical axons and cortical neurons, an event that coincides with the formation of the barrel map. They will briefly review cortical patterning and the initial targeting of thalamic axons, with an emphasis on recent findings. The rest of the review will be devoted to summarizing their understanding of the cellular and molecular mechanisms underlying thalamocortical synapse maturation and its role in barrel map formation.

New loci for body fat percentage reveal link between adiposity and cardiometabolic disease risk

To increase our understanding of the genetic basis of adiposity and its links to cardiometabolic disease risk, we conducted a genome-wide association meta-analysis of body fat percentage (BF%) in up to 100,716 individuals. Twelve loci reached genome-wide significance (P<5 × 10(-8)), of which eight were previously associated with increased overall adiposity (BMI, BF%) and four (in or near COBLL1/GRB14, IGF2BP1, PLA2G6, CRTC1) were novel associations with BF%. Seven loci showed a larger effect on BF% than on BMI, suggestive of a primary association with adiposity, while five loci showed larger effects on BMI than on BF%, suggesting association with both fat and lean mass. In particular, the loci more strongly associated with BF% showed distinct cross-phenotype association signatures with a range of cardiometabolic traits revealing new insights in the link between adiposity and disease risk.

Tangential migration of glutamatergic neurons and cortical patterning during development: Lessons from Cajal-Retzius cells

Tangential migration is a mode of cell movement, which in the developing cerebral cortex, is defined by displacement parallel to the ventricular surface and orthogonal to the radial glial fibers. This mode of long-range migration is a strategy by which distinct neuronal classes generated from spatially and molecularly distinct origins can integrate to form appropriate neural circuits within the cortical plate. While it was previously believed that only GABAergic cortical interneurons migrate tangentially from their origins in the subpallial ganglionic eminences to integrate in the cortical plate, it is now known that transient populations of glutamatergic neurons also adopt this mode of migration. These include Cajal-Retzius cells (CRs), subplate neurons (SPs), and cortical plate transient neurons (CPTs), which have crucial roles in orchestrating the radial and tangential development of the embryonic cerebral cortex in a noncell-autonomous manner. While CRs have been extensively studied, it is only in the last decade that the molecular mechanisms governing their tangential migration have begun to be elucidated. To date, the mechanisms of SPs and CPTs tangential migration remain unknown. We therefore review the known signaling pathways, which regulate parameters of CRs migration including their motility, contact-redistribution and adhesion to the pial surface, and discuss this in the context of how CR migration may regulate their signaling activity in a spatial and temporal manner. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 847-881, 2016.

Cross-modal plasticity in sensory deprived animal models: From the thalamocortical development point of view

Over recent decades, our understanding of the plasticity of the central nervous system has expanded enormously. Accordingly, it is now widely accepted that the brain can adapt to changes by reorganizing its circuitry, both in response to external stimuli and experience, as well as through intrinsic mechanisms. A clear example of this is the activation of a deprived sensory area and the expansion of spared sensory cortical regions in individuals who suffered peripheral sensory loss. Despite the efforts to understand these neuroplastic changes, the mechanisms underlying such adaptive remodeling remains poorly understood. Progress in understanding these events may be hindered by the highly varied data obtained from the distinct experimental paradigms analyzed, which include different animal models and neuronal systems, as well as studies into the onset of sensory loss. Here, we will establish the current state-of-the-art describing the principal observations made according to the time of sensory deprivation with respect to the development of the thalamocortical connectivity. We will review the experimental data obtained from animal models where sensory deprivation has been induced either before or after thalamocortical axons reach and invade their target cortical areas. The anatomical and functional effects of sensory loss on the primary sensory areas of the cortex will be presented. Indeed, we consider that the comparative approach of this review is a necessary step in order to help deciphering the processes that underlie sensory neuroplasticity, for which studies in animal models have been indispensable. Understanding these mechanisms will then help to develop restorative strategies and prostheses that will overcome the functional loss.

The C. elegans COE transcription factor UNC-3 activates lineage-specific apoptosis and affects neurite growth in the RID lineage

Mechanisms that regulate apoptosis in a temporal and lineage-specific manner remain poorly understood. The COE (Collier/Olf/EBF) transcription factors have been implicated in the development of many cell types, including neurons. Here, we show that the sole Caenorhabditis elegans COE protein, UNC-3, together with a histone acetyltransferase, CBP-1/P300, specifies lineage-specific apoptosis and certain aspects of neurite trajectory. During embryogenesis, the RID progenitor cell gives rise to the RID neuron and RID sister cell; the latter undergoes apoptosis shortly after cell division upon expression of the pro-apoptotic gene egl-1. We observe UNC-3 expression in the RID progenitor, and the absence of UNC-3 results in the failure of the RID lineage to express a Pegl-1::GFP reporter and in the survival of the RID sister cell. Lastly, UNC-3 interacts with CBP-1, and cbp-1 mutants exhibit a similar RID phenotype to unc-3. Thus, in addition to playing a role in neuronal terminal differentiation, UNC-3 is a cell lineage-specific regulator of apoptosis.

Map transfer from the thalamus to the neocortex: inputs from the barrel field

Sensory perception relies on the formation of stereotyped maps inside the brain. This feature is particularly well illustrated in the mammalian neocortex, which is subdivided into distinct cortical sensory areas that comprise topological maps, such as the somatosensory homunculus in humans or the barrel field of the large whiskers in rodents. How somatosensory maps are formed and relayed into the neocortex remain essential questions in developmental neuroscience. Here, we will present our current knowledge on whisker map transfer in the mouse model, with the goal of linking embryonic and postnatal studies into a comprehensive framework.

Development of the prethalamus is crucial for thalamocortical projection formation and is regulated by Olig2

Thalamocortical axons (TCAs) pass through the prethalamus in the first step of their neural circuit formation. Although it has been supposed that the prethalamus is an intermediate target for thalamocortical projection formation, much less is known about the molecular mechanisms of this targeting. Here, we demonstrated the functional implications of the prethalamus in the formation of this neural circuit. We show that Olig2 transcription factor, which is expressed in the ventricular zone (VZ) of prosomere 3, regulates prethalamus formation, and loss of Olig2 results in reduced prethalamus size in early development, which is accompanied by expansion of the thalamic eminence (TE). Extension of TCAs is disorganized in the Olig2-KO dorsal thalamus, and initial elongation of TCAs is retarded in the Olig2-KO forebrain. Microarray analysis demonstrated upregulation of several axon guidance molecules, including Epha3 and Epha5, in the Olig2-KO basal forebrain. In situ hybridization showed that the prethalamus in the wild type excluded the expression of Epha3 and Epha5, whereas loss of Olig2 resulted in reduction of this Ephas-negative area and the corresponding expansion of the Ephas-positive TE. Dissociated cultures of thalamic progenitor cells demonstrated that substrate-bound EphA3 suppresses neurite extension from dorsal thalamic neurons. These results indicate that Olig2 is involved in correct formation of the prethalamus, which leads to exclusion of the EphA3-expressing region and is crucial for proper TCA formation. Our observation is the first report showing the molecular mechanisms underlying how the prethalamus acts on initial thalamocortical projection formation.

Regulation of chick early B-cell factor-1 gene expression in feather development

The chick Ebf1 (early B-cell factor-1) gene is a member of a novel family of helix loop helix transcription factors. The expression profile, regulation and significance of this gene have been extensively studied in lymphatic, nervous, adipose and muscular tissues. However, cEbf1 expression, regulation and function in the feather of chick embryo have not yet been investigated. cEbf1 expression was first detected throughout the mesenchymal core of some few feather placodes (D7-D7.5). After feathers became mature and grew distally (D9 and D10), the mesenchymal expression of cEbf1 became confined to the caudal margin of the proximal half of all formed feather buds. Because this dynamic pattern of expression resembles that of Sonic Hedgehog (Shh) protein and bone morphogenetic protein (Bmp4) plus the crucial role of these two major signals in feather development, we hypothesized that cEbf1 expression in the feather may be regulated by Shh and Bmp4. In a feather explant culture system, Shh signals are necessary to initiate and maintain cEbf1 expression in the posterior half of the feather bud, while Bmp4 is crucial for the initial cEbf1 expression in the anterior half of the feather bud. Inhibition of Shh, not only down-regulates cEbf1, but also changes the morphology of feather buds, which become irregular and fused. This is the first study to demonstrate that cEbf1 expression in the feather bud is under the control of Shh and Bmp4 signals and that expression may play a role in the normal development of feathers.

The shape of the human language-ready brain

Our core hypothesis is that the emergence of our species-specific language-ready brain ought to be understood in light of the developmental changes expressed at the levels of brain morphology and neural connectivity that occurred in our species after the split from Neanderthals–Denisovans and that gave us a more globular braincase configuration. In addition to changes at the cortical level, we hypothesize that the anatomical shift that led to globularity also entailed significant changes at the subcortical level. We claim that the functional consequences of such changes must also be taken into account to gain a fuller understanding of our linguistic capacity. Here we focus on the thalamus, which we argue is central to language and human cognition, as it modulates fronto-parietal activity. With this new neurobiological perspective in place, we examine its possible molecular basis. We construct a candidate gene set whose members are involved in the development and connectivity of the thalamus, in the evolution of the human head, and are known to give rise to language-associated cognitive disorders. We submit that the new gene candidate set opens up new windows into our understanding of the genetic basis of our linguistic capacity. Thus, our hypothesis aims at generating new testing grounds concerning core aspects of language ontogeny and phylogeny.

Soluble guanylate cyclase generation of cGMP regulates migration of MGE neurons

Here we have provided evidence that nitric oxide-cyclic GMP (NO-cGMP) signaling regulates neurite length and migration of immature neurons derived from the medial ganglionic eminence (MGE). Dlx1/2(-/-) and Lhx6(-/-) mouse mutants, which exhibit MGE interneuron migration defects, have reduced expression of the gene encoding the α subunit of a soluble guanylate cyclase (Gucy1A3). Furthermore, Dlx1/2(-/-) mouse mutants have reduced expression of NO synthase 1 (NOS1). Gucy1A3(-/-) mice have a transient reduction in cortical interneuron number. Pharmacological inhibition of soluble guanylate cyclase and NOS activity rapidly induces neurite retraction of MGE cells in vitro and in slice culture and robustly inhibits cell migration from the MGE and caudal ganglionic eminence. We provide evidence that these cellular phenotypes are mediated by activation of the Rho signaling pathway and inhibition of myosin light chain phosphatase activity.

An RNAi-based approach to down-regulate a gene family in vivo

Genetic redundancy poses a major problem to the analysis of gene function. RNA interference allows the down-regulation of several genes simultaneously, offering the possibility to overcome genetic redundancy, something not easily achieved with traditional genetic approaches. Previously we have used a polycistronic miR155-based framework to knockdown expression of three genes of the early B cell factor family in cultured cells. Here we develop the system further by generating transgenic mice expressing the RNAi construct in vivo in an inducible manner. Expression of the transgene from the strong CAG promoter is compatible with a normal function of the basal miRNA/RNAi machinery, and the miR155 framework readily allows inducible expression from the Rosa26 locus as shown by Gfp. However, expression of the transgene in hematopoietic cells does not lead to changes in B cell development and neuronal expression does not affect cerebellar architecture as predicted from genetic deletion studies. Protein as well as mRNA levels generated from Ebf genes in hetero- and homozygous animals are comparable to wild-type levels. A likely explanation for the discrepancy in the effectiveness of the RNAi construct between cultured cells and transgenic animals lies in the efficiency of the sequences used, possibly together with the complexity of the transgene. Since new approaches allow to overcome efficiency problems of RNAi sequences, the data lay the foundation for future work on the simultaneous knockdown of several genes in vivo.

In and out from the cortex: development of major forebrain connections

In this review we discuss recent advances in the understanding of the development of forebrain projections attending to their origin, fate determination, and axon guidance. Major forebrain connections include callosal, corticospinal, corticothalamic and thalamocortical projections. Although distinct transcriptional programs specify these subpopulations of projecting neurons, the mechanisms involved in their axonal development are similar. Guidance by short- and long-range molecular cues, interaction with intermediate target populations and activity-dependent mechanisms contribute to their development. Moreover, some of these connections interact with each other showing that the development of these axonal tracts is a well-orchestrated event. Finally, we will recapitulate recent discoveries that challenge the field of neural wiring that show that these forebrain connections can be changed once formed. The field of reprogramming has arrived to postmitotic cortical neurons and has showed us that forebrain connectivity is not immutable and might be changed by manipulations in the transcriptional program of matured cells.

Thalamic control of neocortical area formation in mice

The mammalian neocortex undergoes dramatic transformation during development, from a seemingly homogenous sheet of neuroepithelial cells into a complex structure that is tangentially divided into discrete areas. This process is thought to be controlled by a combination of intrinsic patterning mechanisms within the cortex and afferent axonal projections from the thalamus. However, roles of thalamic afferents in the formation of areas are still poorly understood. In this study, we show that genetically increasing or decreasing the size of the lateral geniculate nucleus of the mouse thalamus resulted in a corresponding change in the size of the primary visual area. Furthermore, elimination of most thalamocortical projections from the outset of their development resulted in altered areal gene expression patterns, particularly in the primary visual and somatosensory areas, where they lost sharp boundaries with adjacent areas. Together, these results demonstrate the critical roles of thalamic afferents in the establishment of neocortical areas.

Mapping remodeling of thalamocortical projections in the living reeler mouse brain by diffusion tractography

A major challenge in neuroscience is to accurately decipher in vivo the entire brain circuitry (connectome) at a microscopic level. Currently, the only methodology providing a global noninvasive window into structural brain connectivity is diffusion tractography. The extent to which the reconstructed pathways reflect realistic neuronal networks depends, however, on data acquisition and postprocessing factors. Through a unique combination of approaches, we designed and evaluated herein a framework for reliable fiber tracking and mapping of the living mouse brain connectome. One important wiring scheme, connecting gray matter regions and passing fiber-crossing areas, was closely examined: the lemniscal thalamocortical (TC) pathway. We quantitatively validated the TC projections inferred from in vivo tractography with correlative histological axonal tracing in the same wild-type and reeler mutant mice. We demonstrated noninvasively that changes in patterning of the cortical sheet, such as highly disorganized cortical lamination in reeler, led to spectacular compensatory remodeling of the TC pathway.

Sensory map transfer to the neocortex relies on pretarget ordering of thalamic axons

Sensory maps, such as the representation of mouse facial whiskers, are conveyed throughout the nervous system by topographic axonal projections that preserve neighboring relationships between adjacent neurons. In particular, the map transfer to the neocortex is ensured by thalamocortical axons (TCAs), whose terminals are topographically organized in response to intrinsic cortical signals. However, TCAs already show a topographic order early in development, as they navigate toward their target. Here, we show that this preordering of TCAs is required for the transfer of the whisker map to the neocortex. Using Ebf1 conditional inactivation that specifically perturbs the development of an intermediate target, the basal ganglia, we scrambled TCA topography en route to the neocortex without affecting the thalamus or neocortex. Notably, embryonic somatosensory TCAs were shifted toward the visual cortex and showed a substantial intermixing along their trajectory. Somatosensory TCAs rewired postnatally to reach the somatosensory cortex but failed to form a topographic anatomical or functional map. Our study reveals that sensory map transfer relies not only on positional information in the projecting and target structures but also on preordering of axons along their trajectory, thereby opening novel perspectives on brain wiring.

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

Mechanisms controlling the guidance of thalamocortical axons through the embryonic forebrain

Thalamocortical axons must cross a complex cellular terrain through the developing forebrain, and this terrain has to be understood for us to learn how thalamocortical axons reach their destinations. Selective fasciculation, guidepost cells and various diencephalic and telencephalic gradients have been implicated in thalamocortical guidance. As our understanding of the relevant forebrain patterns has increased, so has our knowledge of the guidance mechanisms. Our aim here is to review recent observations of cellular and molecular mechanisms related to: the growth of thalamofugal projections to the ventral telencephalon, thalamic axon avoidance of the hypothalamus and extension into the telencephalon to form the internal capsule, the crossing of the pallial-subpallial boundary, and the growth towards the cerebral cortex. We shall review current theories for the explanation of the maintenance and alteration of topographic order in the thalamocortical projections to the cortex. It is now increasingly clear that several mechanisms are involved at different stages of thalamocortical development, and each contributes substantially to the eventual outcome. Revealing the molecular and cellular mechanisms can help to link specific genes to details of actual developmental mechanisms.

The lhx2 transcription factor controls thalamocortical axonal guidance by specific regulation of robo1 and robo2 receptors

The assembly of neural circuits is dependent upon the generation of specific neuronal subtypes, each subtype displaying unique properties that direct the formation of selective connections with appropriate target cells. Actions of transcription factors in neural progenitors and postmitotic cells are key regulators in this process. LIM-homeodomain transcription factors control crucial aspects of neuronal differentiation, including subtype identity and axon guidance. Nonetheless, their regulation during development is poorly understood and the identity of the downstream molecular effectors of their activity remains largely unknown. Here, we demonstrate that the Lhx2 transcription factor is dynamically regulated in distinct pools of thalamic neurons during the development of thalamocortical connectivity in mice. Indeed, overexpression of Lhx2 provokes defective thalamocortical axon guidance in vivo, while specific conditional deletion of Lhx2 in the thalamus produces topographic defects that alter projections from the medial geniculate nucleus and from the caudal ventrobasal nucleus in particular. Moreover, we demonstrate that Lhx2 influences axon guidance and the topographical sorting of axons by regulating the expression of Robo1 and Robo2 guidance receptors, which are essential for these axons to establish correct connections in the cerebral cortex. Finally, augmenting Robo1 function restores normal axon guidance in Lhx2-overexpressing neurons. By regulating axon guidance receptors, such as Robo1 and Robo2, Lhx2 differentially regulates the axon guidance program of distinct populations of thalamic neurons, thus enabling the establishment of specific neural connections.

The importance of combinatorial gene expression in early Mammalian thalamic patterning and thalamocortical axonal guidance

The thalamus is essential for sensory perception. In mammals, work on the mouse has taught us most of what we know about how it develops and connects to the cortex. The mature thalamus of all mammalian species comprises numerous anatomically distinct collections of neurons called nuclei that differ in function, connectivity, and molecular constitution. At the time of its initial appearance as a distinct structure following neural tube closure, the thalamus is already patterned by the regional expression of numerous regulatory genes. This patterning, which lays down the blueprint for later development of thalamic nuclei, predates the development of thalamocortical projections. In this review we apply novel analytical methods to gene expression data available in the Allen Developing Mouse Brain Atlas to highlight the complex organized molecular heterogeneity already present among cells in the thalamus from the earliest stages at which it contains differentiating neurons. This early patterning is likely to invest in axons growing from different parts of the thalamus the ability to navigate in an ordered way to their appropriate area in the cerebral cortex. We review the mechanisms and cues that thalamic axons use, encounter, and interpret to attain the cortex. Mechanisms include guidance by previously generated guidepost cells, such as those in the subpallium that maintain thalamic axonal order and direction, and axons such as those of reciprocal projections from intermediate structures or from the cortex itself back toward the thalamus. We show how thalamocortical pathfinding involves numerous guidance cues operating at a series of steps along their route. We stress the importance of the combinatorial actions of multiple genes for the development of the numerous specific identities and functions of cells in this exquisitely complex system and their orderly innervation of the cortex.

Early B-cell factors 2 and 3 (EBF2/3) regulate early migration of Cajal-Retzius cells from the cortical hem

Cajal–Retzius (CR) cells play a crucial role in the formation of the cerebral cortex, yet the molecules that control their development are largely unknown. Here, we show that Ebf transcription factors are expressed in forebrain signalling centres—the septum, cortical hem and the pallial–subpallial boundary—known to generate CR cells. We identified Ebf2, through fate mapping studies, as a novel marker for cortical hem- and septum-derived CR cells. Loss of Ebf2 in vivo causes a transient decrease in CR cell numbers on the cortical surface due to a migratory defect in the cortical hem, and is accompanied by upregulation of Ebf3 in this and other forebrain territories that produce CR cells, without affecting proper cortical lamination. Accordingly, using in vitro preparations, we demonstrated that both Ebf2 and Ebf3, singly or together, control the migration of CR cells arising in the cortical hem. These findings provide evidence that Ebfs directly regulate CR cell development.

Emergent growth cone responses to combinations of Slit1 and Netrin 1 in thalamocortical axon topography

How guidance cues are integrated during the formation of complex axonal tracts remains largely unknown. Thalamocortical axons (TCAs), which convey sensory and motor information to the neocortex, have a rostrocaudal topographic organization initially established within the ventral telencephalon [1-3]. Here, we show that this topography is set in a small hub, the corridor, which contains matching rostrocaudal gradients of Slit1 and Netrin 1. Using in vitro and in vivo experiments, we show that Slit1 is a rostral repellent that positions intermediate axons. For rostral axons, although Slit1 is also repulsive and Netrin 1 has no chemotactic activity, the two factors combined generate attraction. These results show that Slit1 has a dual context-dependent role in TCA pathfinding and furthermore reveal that a combination of cues produces an emergent activity that neither of them has alone. Our study thus provides a novel framework to explain how a limited set of guidance cues can generate a vast diversity of axonal responses necessary for proper wiring of the nervous system.

Ebf1 deficiency causes increase of Müller cells in the retina and abnormal topographic projection at the optic chiasm

The Ebf transcription factors play important roles in the developmental processes of many tissues. We have shown previously that four members of the Ebf family are expressed during mouse retinal development and are both necessary and sufficient to specify multiple retinal cell fates. Here we describe the changes in cell differentiation and retinal ganglion cell (RGC) projection in Ebf1 knockout mice. Analysis of marker expression in Ebf1 null mutant retinas reveals that loss of Ebf1 function causes a significant increase of Müller cells. Moreover, there is an obvious decrease of ipsilateral and retinoretinal projections of RGC axons at the optic chiasm, whereas the contralateral projection significantly increases in the mutant mice. These data together suggests that Ebf1 is required for suppressing the Müller cell fate during retinogenesis and important for the correct topographic projection of RGC axons at the optic chiasm.

Appropriate Bmp7 levels are required for the differentiation of midline guidepost cells involved in corpus callosum formation

Guidepost cells are essential structures for the establishment of major axonal tracts. How these structures are specified and acquire their axon guidance properties is still poorly understood. Here, we show that in mouse embryos appropriate levels of Bone Morphogenetic Protein 7 (Bmp7), a member of the TGF-β superfamily of secreted proteins, are required for the correct development of the glial wedge, the indusium griseum, and the subcallosal sling, three groups of cells that act as guidepost cells for growing callosal axons. Bmp7 is expressed in the region occupied by these structures and its genetic inactivation in mouse embryos caused a marked reduction and disorganization of these cell populations. On the contrary, infusion of recombinant Bmp7 in the developing forebrain induced their premature differentiation. In both cases, changes were associated with the disruption of callosal axon growth and, in most animals fibers did not cross the midline forming typical Probst bundles. Addition of Bmp7 to cortical explants did not modify the extent of their outgrowth nor their directionality, when explants were exposed to a focalized source of the protein. Together, these results indicate that Bmp7 is indirectly required for corpus callosum formation by controlling the timely differentiation of its guidepost cells.

Coe genes are expressed in differentiating neurons in the central nervous system of protostomes

Genes of the coe (collier/olfactory/early B-cell factor) family encode Helix-Loop-Helix transcription factors that are widely conserved in metazoans and involved in many developmental processes, neurogenesis in particular. Whereas their functions during vertebrate neural tube formation have been well documented, very little is known about their expression and role during central nervous system (CNS) development in protostomes. Here we characterized the CNS expression of coe genes in the insect Drosophila melanogaster and the polychaete annelid Platynereis dumerilii, which belong to different subgroups of protostomes and show strikingly different modes of development. In the Drosophila ventral nerve cord, we found that the Collier-expressing cells form a subpopulation of interneurons with diverse molecular identities and neurotransmitter phenotypes. We also demonstrate that collier is required for the proper differentiation of some interneurons belonging to the Eve-Lateral cluster. In Platynereis dumerilii, we cloned a single coe gene, Pdu-coe, and found that it is exclusively expressed in post mitotic neural cells. Using an original technique of in silico 3D registration, we show that Pdu-coe is co-expressed with many different neuronal markers and therefore that, like in Drosophila, its expression defines a heterogeneous population of neurons with diverse molecular identities. Our detailed characterization and comparison of coe gene expression in the CNS of two distantly-related protostomes suggest conserved roles of coe genes in neuronal differentiation in this clade. As similar roles have also been observed in vertebrates, this function was probably already established in the last common ancestor of all bilaterians.

EBF factors drive expression of multiple classes of target genes governing neuronal development

Background

Early B cell factor (EBF) family members are transcription factors known to have important roles in several aspects of vertebrate neurogenesis, including commitment, migration and differentiation. Knowledge of how EBF family members contribute to neurogenesis is limited by a lack of detailed understanding of genes that are transcriptionally regulated by these factors.

Results

We performed a microarray screen in Xenopus animal caps to search for targets of EBF transcriptional activity, and identified candidate targets with multiple roles, including transcription factors of several classes. We determined that, among the most upregulated candidate genes with expected neuronal functions, most require EBF activity for some or all of their expression, and most have overlapping expression with ebf genes. We also found that the candidate target genes that had the most strongly overlapping expression patterns with ebf genes were predicted to be direct transcriptional targets of EBF transcriptional activity.

Conclusions

The identification of candidate targets that are transcription factor genes, including nscl-1, emx1 and aml1, improves our understanding of how EBF proteins participate in the hierarchy of transcription control during neuronal development, and suggests novel mechanisms by which EBF activity promotes migration and differentiation. Other candidate targets, including pcdh8 and kcnk5, expand our knowledge of the types of terminal differentiated neuronal functions that EBF proteins regulate.

Thalamocortical pathfinding defects precede degeneration of the reticular thalamic nucleus in polysialic acid-deficient mice

The modification of the neural cell adhesion molecule (NCAM) with polysialic acid (polySia) is tightly linked to neural development. Genetic ablation of the polySia-synthesizing enzymes ST8SiaII and ST8SiaIV generates polySia-negative but NCAM-positive (II(-/-)IV(-/-)) mice characterized by severe defects of major brain axon tracts, including internal capsule hypoplasia. Here, we demonstrate that misguidance of thalamocortical fibers and deficiencies of corticothalamic connections contribute to internal capsule defects in II(-/-)IV(-/-) mice. Thalamocortical fibers cross the primordium of the reticular thalamic nucleus (Rt) at embryonic day 14.5, before they fail to turn into the ventral telencephalon, thus deviating from their normal trajectory without passing through the internal capsule. At postnatal day 1, a reduction and massive disorganization of fibers traversing the Rt was observed, whereas terminal deoxynucleotidyl transferase dUTP nick end labeling and cleaved caspase-3 staining indicated abundant apoptotic cell death of Rt neurons at postnatal day 5. Furthermore, during postnatal development, the number of Rt neurons was drastically reduced in 4-week-old II(-/-)IV(-/-) mice, but not in the NCAM-deficient N(-/-) or II(-/-)IV(-/-)N(-/-) triple knock-out animals displaying no internal capsule defects. Thus, degeneration of the Rt in II(-/-)IV(-/-) mice may be a consequence of malformation of thalamocortical and corticothalamic fibers providing major excitatory input into the Rt. Indeed, apoptotic death of Rt neurons could be induced by lesioning corticothalamic fibers on whole-brain slice cultures. We therefore propose that anterograde transneuronal degeneration of the Rt in polysialylation-deficient, NCAM-positive mice is caused by defective afferent innervation attributable to thalamocortical pathfinding defects.

A forward genetic screen with a thalamocortical axon reporter mouse yields novel neurodevelopment mutants and a distinct emx2 mutant phenotype

Background

The dorsal thalamus acts as a gateway and modulator for information going to and from the cerebral cortex. This activity requires the formation of reciprocal topographic axon connections between thalamus and cortex. The axons grow along a complex multistep pathway, making sharp turns, crossing expression boundaries, and encountering intermediate targets. However, the cellular and molecular components mediating these steps remain poorly understood.

Results

To further elucidate the development of the thalamocortical system, we first created a thalamocortical axon reporter line to use as a genetic tool for sensitive analysis of mutant mouse phenotypes. The TCA-tau-lacZ reporter mouse shows specific, robust, and reproducible labeling of thalamocortical axons (TCAs), but not the overlapping corticothalamic axons, during development. Moreover, it readily reveals TCA pathfinding abnormalities in known cortical mutants such as reeler. Next, we performed an unbiased screen for genes involved in thalamocortical development using random mutagenesis with the TCA reporter. Six independent mutant lines show aberrant TCA phenotypes at different steps of the pathway. These include ventral misrouting, overfasciculation, stalling at the corticostriatal boundary, and invasion of ectopic cortical cell clusters. An outcross breeding strategy coupled with a genomic panel of single nucleotide polymorphisms facilitated genetic mapping with small numbers of mutant mice. We mapped a ventral misrouting mutant to the Emx2 gene, and discovered that some TCAs extend to the olfactory bulbs in this mutant. Mapping data suggest that other lines carry mutations in genes not previously known for roles in thalamocortical development.

Conclusions

These data demonstrate the feasibility of a forward genetic approach to understanding mammalian brain morphogenesis and wiring. A robust axonal reporter enabled sensitive analysis of a specific axon tract inside the mouse brain, identifying mutant phenotypes at multiple steps of the pathway, and revealing a new aspect of the Emx2 mutant. The phenotypes highlight vulnerable choice points and latent tendencies of TCAs, and will lead to a refined understanding of the elements and interactions required to form the thalamocortical system.

See Commentary: http://www.biomedcentral.com/1741-7007/9/1

Time-lapse imaging and cell-specific expression profiling reveal dynamic branching and molecular determinants of a multi-dendritic nociceptor in C. elegans

Nociceptive neurons innervate the skin with complex dendritic arbors that respond to pain-evoking stimuli such as harsh mechanical force or extreme temperatures. Here we describe the structure and development of a model nociceptor, the PVD neuron of C. elegans, and identify transcription factors that control morphogenesis of the PVD dendritic arbor. The two PVD neuron cell bodies occupy positions on either the right (PVDR) or left (PVDL) sides of the animal in posterior-lateral locations. Imaging with a GFP reporter revealed a single axon projecting from the PVD soma to the ventral cord and an elaborate, highly branched arbor of dendritic processes that envelop the animal with a web-like array directly beneath the skin. Dendritic branches emerge in a step-wise fashion during larval development and may use an existing network of peripheral nerve cords as guideposts for key branching decisions. Time-lapse imaging revealed that branching is highly dynamic with active extension and withdrawal and that PVD branch overlap is prevented by a contact-dependent self-avoidance, a mechanism that is also employed by sensory neurons in other organisms. With the goal of identifying genes that regulate dendritic morphogenesis, we used the mRNA-tagging method to produce a gene expression profile of PVD during late larval development. This microarray experiment identified>2,000 genes that are 1.5X elevated relative to all larval cells. The enriched transcripts encode a wide range of proteins with potential roles in PVD function (e.g., DEG/ENaC and Trp channels) or development (e.g., UNC-5 and LIN-17/frizzled receptors). We used RNAi and genetic tests to screen 86 transcription factors from this list and identified eleven genes that specify PVD dendritic structure. These transcription factors appear to control discrete steps in PVD morphogenesis and may either promote or limit PVD branching at specific developmental stages. For example, time-lapse imaging revealed that MEC-3 (LIM homeodomain) is required for branch initiation in early larval development whereas EGL-44 (TEAD domain) prevents ectopic PVD branching in the adult. A comparison of PVD-enriched transcripts to a microarray profile of mammalian nociceptors revealed homologous genes with potentially shared nociceptive functions. We conclude that PVD neurons display striking structural, functional and molecular similarities to nociceptive neurons from more complex organisms and can thus provide a useful model system in which to identify evolutionarily conserved determinants of nociceptor fate.

Specificity and plasticity of thalamocortical connections in Sema6A mutant mice

The establishment of connectivity between specific thalamic nuclei and cortical areas involves a dynamic interplay between the guidance of thalamocortical axons and the elaboration of cortical areas in response to appropriate innervation. We show here that Sema6A mutants provide a unique model to test current ideas on the interactions between subcortical and cortical guidance mechanisms and cortical regionalization. In these mutants, axons from the dorsal lateral geniculate nucleus (dLGN) are misrouted in the ventral telencephalon. This leads to invasion of presumptive visual cortex by somatosensory thalamic axons at embryonic stages. Remarkably, the misrouted dLGN axons are able to find their way to the visual cortex via alternate routes at postnatal stages and reestablish a normal pattern of thalamocortical connectivity. These findings emphasize the importance and specificity of cortical cues in establishing thalamocortical connectivity and the spectacular capacity of the early postnatal cortex for remapping initial sensory representations.

During brain development, the emergence of distinct areas in the cerebral cortex involves an interplay between patterning of the cortical sheet in the early embryo and later influences of incoming connections made from other brain areas, namely the thalamus. Connectivity between the thalamus and the cortex is initially smooth and graded, and a prominent model for how thalamocortical connectivity is established proposes thalamic axons are topographically sorted as they course through subcortical regions and then passively delivered to appropriate areas of the cortical sheet. We have used mutant mice lacking the guidance molecule Semaphorin-6A to test this model. In these mutants, Semaphorin-6A axons from the visual part of the thalamus are subcortically misrouted and fail to innervate the presumptive visual cortex, which is instead invaded by somatosensory thalamic axons. Despite this major disruption in initial connectivity, many visual thalamic axons find their way specifically to visual cortex, arriving several days later than usual. These late-arriving axons often follow alternate routes, and upon arrival are able to out-compete earlier-arriving somatosensory axons to reestablish grossly normal thalamocortical connectivity. These results argue strongly against an essential role for early subcortical targeting in the establishment of thalamocortical connectivity patterns and suggest instead the existence of highly specific target-selection mechanisms that match thalamic axons with appropriate cortical areas.

Initial misrouting and subsequent recovery of thalamocortical axon projections in Semaphorin-6A mutant mice highlights the importance of specific target-selection mechanisms that match thalamic axons with appropriate cortical areas.

The genetic control of neocortex volume and covariation with neocortical gene expression in mice

Background

The size of the cerebral cortex varies widely within human populations, and a large portion of this variance is modulated by genetic factors. The discovery and characterization of these genes and their variants can contribute to an understanding of individual differences in brain development, behavior, and disease susceptibility. Here we use unbiased stereological techniques to map quantitative trait loci (QTLs) that modulate the volume of neocortex.

Results

We estimated volumes bilaterally in an expanded set of BXD recombinant inbred strains (n = 56 strains and 223 animals) taken from the Mouse Brain Library . We generated matched microarray data for the cerebral cortex in the same large panel of strains and in parental neonates to efficiently nominate and evaluate candidate genes. Volume of the neocortex varies widely, and is a heritable trait. Genome-wide mapping of this trait revealed two QTLs – one on chromosome (Chr) 6 at 88 ± 5 Mb and another at Chr 11 (41 ± 8 Mb). We generated both neonatal and adult neocortical gene expression databases using microarray technology. Using these databases in combination with other bioinformatic tools we have identified positional candidates on these QTL intervals.

Conclusion

This study is the first to use the expanded set of BXD strains to map neocortical volume, and we found that normal variation of this trait is, at least in part, genetically modulated. These results provide a baseline from which to assess the genetic contribution to regional variation in neocortical volume, as well as other neuroanatomic phenotypes that may contribute to variation in regional volume, such as proliferation, death, and number and packing density of neurons