Local tissue interactions across the dorsal midline of the forebrain establish CNS laterality

Brain asymmetry is encoded at the level of axon terminal morphology

BACKGROUND

Functional lateralization is a conserved feature of the central nervous system (CNS). However, underlying left-right asymmetries within neural circuitry and the mechanisms by which they develop are poorly described.

RESULTS

In this study, we use focal electroporation to examine the morphology and connectivity of individual neurons of the lateralized habenular nuclei. Habenular projection neurons on both sides of the brain share a stereotypical unipolar morphology and elaborate remarkable spiraling terminal arbors in their target interpeduncular nucleus, a morphology unlike that of any other class of neuron described to date. There are two quite distinct sub-types of axon arbor that differ both in branching morphology and in their localization within the target nucleus. Critically, both arbor morphologies are elaborated by both left and right-sided neurons, but at greatly differing frequencies. We show that these differences in cell type composition account for the gross connectional asymmetry displayed by the left and right habenulae. Analysis of the morphology and projections of individual post-synaptic neurons suggests that the target nucleus has the capacity to either integrate left and right inputs or to handle them independently, potentially relaying information from the left and right habenulae within distinct downstream pathways, thus preserving left-right coding. Furthermore, we find that signaling from the unilateral, left-sided parapineal nucleus is necessary for both left and right axons to develop arbors with appropriate morphology and targeting. However, following parapineal ablation, left and right habenular neurons continue to elaborate arbors with distinct, lateralized morphologies.

CONCLUSION

By taking the analysis of asymmetric neural circuitry to the level of single cells, we have resolved left-right differences in circuit microarchitecture and show that lateralization can be recognized at the level of the morphology and connectivity of single projection neuron axons. Crucially, the same circuitry components are specified on both sides of the brain, but differences in the ratios of different neuronal sub-types results in a lateralized neural architecture and gross connectional asymmetry. Although signaling from the parapineal is essential for the development of normal lateralization, additional factors clearly act during development to confer left-right identity upon neurons in this highly conserved circuit.

Nodal signaling is required for closure of the anterior neural tube in zebrafish

Background

Nodals are secreted signaling proteins with many roles in vertebrate development. Here, we identify a new role for Nodal signaling in regulating closure of the rostral neural tube of zebrafish.

Results

We find that the neural tube in the presumptive forebrain fails to close in zebrafish Nodal signaling mutants. For instance, the cells that will give rise to the pineal organ fail to move from the lateral edges of the neural plate to the midline of the diencephalon. The open neural tube in Nodal signaling mutants may be due in part to reduced function of N-cadherin, a cell adhesion molecule expressed in the neural tube and required for neural tube closure. N-cadherin expression and localization to the membrane are reduced in fish that lack Nodal signaling. Further, N-cadherin mutants and morphants have a pineal phenotype similar to that of mutants with deficiencies in the Nodal pathway. Overexpression of an activated form of the TGFβ Type I receptor Taram-A (Taram-A*) cell autonomously rescues mesendoderm formation in fish with a severe decrease in Nodal signaling. We find that overexpression of Taram-A* also corrects their open neural tube defect. This suggests that, as in mammals, the mesoderm and endoderm have an important role in regulating closure of the anterior neural tube of zebrafish.

Conclusion

This work helps establish a role for Nodal signals in neurulation, and suggests that defects in Nodal signaling could underlie human neural tube defects such as exencephaly, a fatal condition characterized by an open neural tube in the anterior brain.

Six3 represses nodal activity to establish early brain asymmetry in zebrafish

The vertebrate brain is anatomically and functionally asymmetric; however, the molecular mechanisms that establish left-right brain patterning are largely unknown. In zebrafish, asymmetric left-sided Nodal signaling within the developing dorsal diencephalon is required for determining the direction of epithalamic asymmetries. Here, we show that Six3, a transcription factor essential for forebrain formation and associated with holoprosencephaly in humans, regulates diencephalic Nodal activity during initial establishment of brain asymmetry. Reduction of Six3 function causes brain-specific deregulation of Nodal pathway activity, resulting in epithalamic laterality defects. Based on misexpression and genetic epistasis experiments, we propose that Six3 acts in the neuroectoderm to establish a prepattern of bilateral repression of Nodal activity. Subsequently, Nodal signaling from the left lateral plate mesoderm alleviates this repression ipsilaterally. Our data reveal a Six3-dependent mechanism for establishment of correct brain laterality and provide an entry point to understanding the genetic regulation of Nodal signaling in the brain.

Wnt/Axin1/beta-catenin signaling regulates asymmetric nodal activation, elaboration, and concordance of CNS asymmetries

Nodal activity in the left lateral plate mesoderm (LPM) is required to activate left-sided Nodal signaling in the epithalamic region of the zebrafish forebrain. Epithalamic Nodal signaling subsequently determines the laterality of neuroanatomical asymmetries. We show that overactivation of Wnt/Axin1/beta-catenin signaling during late gastrulation leads to bilateral epithalamic expression of Nodal pathway genes independently of LPM Nodal signaling. This is consistent with a model whereby epithalamic Nodal signaling is normally bilaterally repressed, with Nodal signaling from the LPM unilaterally alleviating repression. We suggest that Wnt signaling regulates the establishment of the bilateral repression. We identify a second role for the Wnt pathway in the left/right regulation of LPM Nodal pathway gene expression, and finally, we show that at later stages Axin1 is required for the elaboration of concordant neuroanatomical asymmetries.

Three Ways to Make Two Sides: Genetic Models of Asymmetric Nervous System Development

Many anatomical and functional features of nervous systems are asymmetric about the left-right axis. These asymmetries can exhibit either random or invariant laterality at the population level. Recent studies in fish and worms provide insight into the developmental mechanisms used to create both types of asymmetry. These studies reveal diverse and molecularly complex strategies for developing asymmetric nervous systems.

Asymmetric innervation of the habenula in zebrafish

The habenular complex is a paired structure found in the diencephalon of all vertebrates, linking the forebrain and midbrain. Habenulae are asymmetrical and may contribute to lateralized behavior. Recent studies in zebrafish have characterized molecular pathways that give rise to the habenular asymmetry and the distinct projections of the left and right habenula to the midbrain. However, it is unclear whether there are asymmetries in habenula afferents from the forebrain. By lipophilic dye tracing, we find that axons innervating the habenula derive primarily from a region in the lateral diencephalon containing migrated neurons of the eminentia thalami (EmT). EmT neurons terminate in neuropils in both ipsilateral and contralateral habenula. These axons, together with axons from migrated neurons of the posterior tuberculum and pallial neurons, cross the midline via the habenular commissure. Subsets of pallial neurons terminate only in the medial right habenula, regardless of which side of the brain they originate from. These include an unusual type of forebrain projection: axons that cross the midline twice, at both the anterior and habenular commissures. Our data establish that there is asymmetric innervation of the habenula from the telencephalon, suggesting a mechanism by which habenula asymmetry might contribute to lateralized behavior.

Temporally regulated asymmetric neurogenesis causes left-right difference in the zebrafish habenular structures

The habenular neurons on both sides of the zebrafish diencephalon show an asymmetric (laterotopic) axonal projection pattern into the interpeduncular nucleus. We previously revealed that the habenula could be subdivided into medial and lateral subnuclei, and a prominent left-right difference in the size ratio of these subnuclei accounts for the asymmetry in its neural connectivity. In the present study, birth date analysis showed that neural precursors for the lateral subnuclei were born at earlier stages than those for the medial subnuclei. More neurons for the early-born lateral subnuclei were generated on the left side, while more neurons for the late-born medial subnuclei were generated on the right side. Genetic hyperactivation and repression of Notch signaling revealed that differential timing determines both specificity and asymmetry in the neurogenesis of neural precursors for the habenular subnuclei.

Neuropilin asymmetry mediates a left-right difference in habenular connectivity

The medial habenular nuclei of the zebrafish diencephalon, which lie bilateral to the pineal complex, exhibit left-right differences in their neuroanatomy, gene expression profiles and axonal projections to the unpaired midbrain target--the interpeduncular nucleus (IPN). Efferents from the left habenula terminate along the entire dorsoventral extent of the IPN, whereas axons from the right habenula project only to the ventral IPN. How this left-right difference in connectivity is established and the factors involved in differential target recognition are unknown. Prior to IPN innervation, we find that only the left habenula expresses the zebrafish homologue of Neuropilin1a (Nrp1a), a receptor for class III Semaphorins (Sema3s). Directional asymmetry of nrp1a expression relies on Nodal signaling and the presence of the left-sided parapineal organ. Loss of Nrp1a, through parapineal ablation or depletion by antisense morpholinos, prevents left habenular neurons from projecting to the dorsal IPN. Selective depletion of Sema3D, but not of other Sema family members, similarly disrupts innervation of the dorsal IPN. Conversely, Sema3D overexpression results in left habenular projections that extend to the dorsal IPN, as well as beyond the target. The results indicate that Sema3D acts in concert with Nrp1a to guide neurons on the left side of the brain to innervate the target nucleus differently than those on the right side.

Selective asymmetry in a conserved forebrain to midbrain projection

How the left and right sides of the brain acquire anatomical and functional specializations is not well understood. The zebrafish has proven to be a useful model to explore the genetic basis of neuroanatomical asymmetry in the developing forebrain. The dorsal diencephalon or epithalamus consists of the asymmetric pineal complex and adjacent paired nuclei, the left and right medial habenulae, which in zebrafish larvae, exhibit differences in their size, neuropil density and patterns of gene expression. In all vertebrates, axons from the medial habenular nuclei project within a prominent fiber bundle, the fasciculus retroflexus, to a shared midbrain target, the interpeduncular nucleus of the ventral tegmentum. However, in zebrafish, projections from the left habenula innervate the dorsal and ventral regions of the target nucleus, whereas right habenular efferents project only to the ventral region. A similar dorsoventral difference in habenular connectivity is found in another teleost species, the highly derived southern flounder, Paralichthys lethostima. In this flatfish, directional asymmetry of the habenular projection appears to be independent of the left-right morphology and orientation that an individual adopts post-metamorphosis. Comparative anterograde labeling of the brains of salamanders, frogs and mice reveals that axons emanating from the left and right medial habenulae do not project to different domains, but rather, they traverse the target nucleus in a complementary mirror image pattern. Thus, although the habenulo-interpeduncular conduction system is highly conserved in the vertebrate brain, the stereotypic dorsoventral topography of left-right connections appears to be a feature that is specific to teleosts.

Is the early left-right axis like a plant, a kidney, or a neuron? The integration of physiological signals in embryonic asymmetry

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well-characterized, the left-right (LR) axis has only relatively recently begun to be understood at the molecular level. The mechanisms that ensure invariant LR asymmetry of the heart, viscera, and brain involve fundamental aspects of cell biology, biophysics, and evolutionary biology, and are important not only for basic science but also for the biomedicine of a wide range of birth defects and human genetic syndromes. The LR axis links biomolecular chirality to embryonic development and ultimately to behavior and cognition, revealing feedback loops and conserved functional modules occurring as widely as plants and mammals. This review focuses on the unique and fascinating physiological aspects of LR patterning in a number of vertebrate and invertebrate species, discusses several profound mechanistic analogies between biological regulation in diverse systems (specifically proposing a nonciliary parallel between kidney cells and the LR axis based on subcellular regulation of ion transporter targeting), highlights the possible importance of early, highly-conserved intracellular events that are magnified to embryo-wide scales, and lays out the most important open questions about the function, evolutionary origin, and conservation of mechanisms underlying embryonic asymmetry.

The interplay between the pineal complex and the habenular nuclei in lower vertebrates in the context of the evolution of cerebral asymmetry

This paper presents an overview on the epithalamus of vertebrates, with particular reference to the pineal and to the asymmetrical organization of the habenular nuclei in lower vertebrates. The relationship between the pineal and the habenulae in the course of phylogenesis is here emphasized, taking data in the frog as example. Altogether the data support the hypothesis, put forward also in earlier studies, of a correlation of habenular asymmetry in lower vertebrates with phylogenetic modification of the pineal complex. The present re-visitation was also stimulated by recent data on the asymmetrical expression of Nodal genes, which involves the pineal and habenular structures in zebrafish. The comparative analysis of data, from cyclostomes to mammals, suggests that transformation of epithalamic structures may play an important role in brain evolution. In addition, in mammals, including rodents, a remarkable complexity has evolved in the organization of the habenulae and their functional interactions with the pineal gland. The evolution of these two epithalamic structures seems to open also new perspectives of knowledge on their implication in the regulation of biological rhythms.

Neurogenin1 is a determinant of zebrafish basal forebrain dopaminergic neurons and is regulated by the conserved zinc finger protein Tof/Fezl

The development of vertebrate basal forebrain dopaminergic (DA) neurons requires the conserved zinc finger protein Too Few (Tof/Fezl) in zebrafish. However, how Tof/Fezl regulates the commitment and differentiation of these DA neurons is not known. Proneural genes encoding basic helix-loop-helix transcription factors regulate the development of multiple neuronal lineages, but their involvement in vertebrate DA neuron determination is unclear. Here we show that neurogenin 1 (ngn1), a vertebrate proneural gene related to the Drosophila atonal, is expressed in and required for specification of DA progenitor cells, and when overexpressed leads to supernumerary DA neurons in the forebrain of zebrafish. Overexpression of ngn1 is also sufficient to induce tyrosine hydroxylase expression in addition to the pan-neuronal marker Hu in nonneural ectoderm. We further show that Tof/Fezl is required to establish basal forebrain ngn1-expressing DA progenitor domains. These findings identify Ngn1 as a determinant of brain DA neurons and provide insights into how Tof/Fezl regulates the development of these clinically important neuronal types.

A novel homeobox mutation in the PITX2 gene in a family with Axenfeld-Rieger syndrome associated with brain, ocular, and dental phenotypes

Axenfeld-Rieger Syndrome (ARS) is a genetically heterogeneous birth defect characterized by malformation of the anterior segment of the eye associated with glaucoma. Mutation of the PITX2 homeobox gene has been identified as a cause of ARS. We report a novel Arg5Trp missense mutation in the PITX2 homeodomain, which is associated with brain abnormalities. One patient had a small sella turcica likely to reflect hypoplasia of the pituitary gland and consistent with the critical role identified for Pitx2 in pituitary development in mice. Two patients had an enlarged cisterna magna, one with a malformed cerebellum, and two had executive skills deficits one in isolation and one in association with a below average intellectual capacity. The mutation caused a typical ARS ocular phenotype. All affected had iris hypoplasia, anterior iris to corneal adhesions, and corectopia. The ocular phenotype varied significantly in severity and showed some asymmetry. All affected also had redundant peri-umbilical skin, a hypoplastic maxilla, microdontia, and hypodontia missing between 20 and 27 teeth with an unusual pattern of tooth loss. Dental phenotypes were documented as they are often poorly characterized in ARS patients. All affected individuals showed an absence of first permanent molars with variable absence of other rarely absent teeth: the permanent upper central incisors, maxillary and mandibular first and second molars, and the mandibular canines. Based on the distinctive dental anomalies, we suggest that the dental phenotype can assist in predicting the presence of a PITX2 mutation and the possibility of brain abnormalities.

CyNodal, the Japanese newt nodal-related gene, is expressed in the left side of the lateral plate mesoderm and diencephalon

The nodal and nodal-related genes play fundamental roles during deuterostome left-right axis formation. Several of these genes show left-sided expression in the lateral plate mesoderm and brain region. We have isolated the nodal-related gene, CyNodal, from Cynops pyrrhogaster. CyNodal mRNA is detected at the marginal zone and left side of several tissues. The left-sideness of CyNodal mRNA expression is highly conserved throughout vertebrate evolution. However, CyNodal mRNA expression shows little variation from the Xenopus nodal-related gene 1, in that CyNodal gene expression in the left lateral plate mesoderm shifts from posterior to anterior at least twice.

Directional asymmetry of the zebrafish epithalamus guides dorsoventral innervation of the midbrain target

The zebrafish epithalamus, consisting of the pineal complex and flanking dorsal habenular nuclei, provides a valuable model for exploring how left-right differences could arise in the vertebrate brain. The parapineal lies to the left of the pineal and the left habenula is larger, has expanded dense neuropil, and distinct patterns of gene expression from the right habenula. Under the influence of Nodal signaling, positioning of the parapineal sets the direction of habenular asymmetry and thereby determines the left-right origin of habenular projections onto the midbrain target, the interpeduncular nucleus (IPN). In zebrafish with parapineal reversal, neurons from the left habenula project to a more limited ventral IPN region where right habenular axons would normally project. Conversely, efferents from the right habenula adopt a more extensive dorsoventral IPN projection pattern typical of left habenular neurons. Three members of the leftover-related KCTD (potassium channel tetramerization domain containing) gene family are expressed differently by the left and right habenula, in patterns that define asymmetric subnuclei. Molecular asymmetry extends to protein levels in habenular efferents, providing additional evidence that left and right axons terminate within different dorsoventral regions of the midbrain target. Laser-mediated ablation of the parapineal disrupts habenular asymmetry and consequently alters the dorsoventral distribution of innervating axons. The results demonstrate that laterality of the dorsal forebrain influences the formation of midbrain connections and their molecular properties.

Quantum dots are powerful multipurpose vital labeling agents in zebrafish embryos

Recently, inorganic fluorescent contrast agents composed of semiconductor materials have been introduced to biological imaging approaches. These so-called quantum dots provide unique and promising properties unreached by organic fluorophores, but their use as contrast agents within live organisms has been limited, probably due in part to concerns about their in vivo tolerance. Using transparent zebrafish embryos, we challenged quantum dots with a series of intravital imaging problems. We show that quantum dots provide a high fluorescent yield within targeted tissues, possess immense photostability, can be targeted to specific subcellular compartments, remain within targeted cells as lineage tracers, are easily separable from conventional organic fluorescent dyes, and are fixable, allowing them to be used in combination with immunohistochemistry after live recordings. Thus, quantum dots combine the specific advantages of different organic fluorescent contrast agents and promise to become the first fluorophore feasible for long-lasting intravital time-lapse studies. Finally, we show by co-labeling blood vessels of the vasculature and major axon tracts of the nervous system that, for establishing these networks, the same guidance cues might be used in some body parts, whereas in others, both networks appear to develop independently from one another. Thus, the bright fluorescence of quantum dots will help to unravel many open questions in the fields of embryology, cell biology, as well as phenotyping and disease diagnosis.

Left-right asymmetry in embryonic development: a comprehensive review

Embryonic morphogenesis occurs along three orthogonal axes. While the patterning of the anterior-posterior and dorsal-ventral axes has been increasingly well characterized, the left-right (LR) axis has only recently begun to be understood at the molecular level. The mechanisms which ensure invariant LR asymmetry of the heart, viscera, and brain represent a thread connecting biomolecular chirality to human cognition, along the way involving fundamental aspects of cell biology, biophysics, and evolutionary biology. An understanding of LR asymmetry is important not only for basic science, but also for the biomedicine of a wide range of birth defects and human genetic syndromes. This review summarizes the current knowledge regarding LR patterning in a number of vertebrate and invertebrate species, discusses several poorly understood but important phenomena, and highlights some important open questions about the evolutionary origin and conservation of mechanisms underlying embryonic asymmetry.

fsi zebrafish show concordant reversal of laterality of viscera, neuroanatomy, and a subset of behavioral responses

Asymmetries in CNS neuroanatomy are assumed to underlie the widespread cognitive and behavioral asymmetries in vertebrates. Studies in humans have shown that the laterality of some cognitive asymmetries is independent of the laterality of the viscera; discrete mechanisms may therefore regulate visceral and neural lateralization. However, through analysis of visceral, neuroanatomical, and behavioral asymmetries in the frequent-situs-inversus (fsi) line of zebrafish, we show that the principal left-right body asymmetries are coupled to certain brain asymmetries and lateralized behaviors. fsi fish with asymmetry defects show concordant reversal of heart, gut, and neuroanatomical asymmetries in the diencephalon. Moreover, the neuroanatomical reversals in reversed fsi fish correlate with reversal of some behavioral responses in both fry and adult fsi fish. Surprisingly, two behavioral asymmetries do not reverse, suggesting that at least two separable mechanisms must influence functional lateralization in the CNS. Partial reversal of CNS asymmetries may generate new behavioral phenotypes; supporting this idea, reversed fsi fry differ markedly from their normally lateralized siblings in their behavioral response to a novel visual feature. Revealing a link between visceral and brain asymmetry and lateralized behavior, our studies help to explain the complexity of the relationship between the lateralities of visceral and neural asymmetries.

Brain Asymmetry: Switching from Left to Right

The relationship between structural and functional asymmetries in the brain remains unclear. A recent report describes a zebrafish mutant that provides us with some enticing clues about this relationship.

Laterotopic representation of left-right information onto the dorso-ventral axis of a zebrafish midbrain target nucleus

The habenulae are part of an evolutionarily highly conserved limbic-system conduction pathway that connects telencephalic nuclei to the interpeduncular nucleus (IPN) of the midbrain . In zebrafish, unilateral activation of the Nodal signaling pathway in the left brain specifies the laterality of the asymmetry of habenular size . We show "laterotopy" in the habenulo-interpeduncular projection in zebrafish, i.e., the stereotypic, topographic projection of left-sided habenular axons to the dorsal region of the IPN and of right-sided habenular axons to the ventral IPN. This asymmetric projection is accounted for by a prominent left-right (LR) difference in the size ratio of the medial and lateral habenular sub-nuclei, each of which specifically projects either to ventral or dorsal IPN targets. Asymmetric Nodal signaling directs the orientation of laterotopy but is dispensable for the establishment of laterotopy itself. Our results reveal a mechanism by which information distributed between left and right sides of the brain can be transmitted bilaterally without loss of LR coding, which may play a crucial role in functional lateralization of the vertebrate brain .

The dorsal diencephalic conduction system of zebrafish as a model of vertebrate brain lateralisation

Lateralisation is an attractive and intriguing feature of the vertebrate CNS studied for decades in the different disciplines of the neurosciences. Due to the complexity of the phenomena and intrinsic limitations of the approaches used to date, it has been difficult to establish useful links between the different, and usually distant, levels of lateralisation e.g. between genetics, morphology, physiology and behaviour. Recently, the dorsal diencephalon of the teleost zebrafish has emerged as a valuable model to begin addressing this issue and as a result unravel the role of vertebrate CNS lateralisation. Zebrafish is a well-established genetic system that allows a 'bottom up' ('gene to behaviour') approach to the study of lateralisation. In fact, it is the single vertebrate system to date in which asymmetric gene expression in the brain has been directly linked to asymmetric morphology. Zebrafish offers several experimental advantages that allow the study of brain lateralisation using a wide range of experimental tools, from study of gene function through in vivo analysis of morphology and physiology to behavioural assessments. Altogether, these features will allow the establishment of operational links between lower (genetics and morphology) and upper (physiology and behaviour) levels of brain lateralisation.

Parapineal specific expression of gfi1 in the zebrafish epithalamus

We describe the isolation of zebrafish growth factor independent 1 (gfi1) and present an analysis of its pattern of expression during early development. As with its murine homologue, gfi1 expression is detected in the ganglion cells of the neural retina and in developing hair cells of the ear. In keeping with a role in the development of sensory hair cells, gfi1 is also expressed in neuromasts of the anterior and posterior lateral line system. Finally, gfi1 is expressed in the developing epithalamus in the dorsal diencephalon where its transcription is restricted to the parapineal.

gemini encodes a zebrafish L-type calcium channel that localizes at sensory hair cell ribbon synapses

L-type Ca2+ channels (LTCCs) drive the bulk of voltage-gated Ca2+ entry in vertebrate inner ear hair cells (HCs) and are essential for mammalian auditory processing. LTCC currents have been implicated in neurotransmitter release at the HC afferent active zone, the ribbon synapse. It is likely that LTCCs play a direct role in vesicle fusion; however, the subcellular localization of the channels in HCs has not been fully resolved. Via positional cloning, we show that mutations in a zebrafish LTCC encoding gene, cav1.3a, underlie the auditory-vestibular defects of gemini (gem) circler mutants. gem homozygous receptor mutant HCs display normal cell viability, afferent synaptogenesis, and peripheral innervation, yet exhibit strongly reduced extracellular potentials (approximately 50% of wild-type potentials). Apical FM1-43 uptake, however, is unaffected in gem mutant HCs, suggesting that mechanotransduction channels are functional. Using a Gem-specific antibody, we show that the bulk of Gem/Ca(v)1.3a immunoreactivity in HCs is restricted to basally located focal spots. The number and location of focal spots relative to nerve terminals, and their remarkable ring-shaped structure, which is reminiscent of synaptic dense bodies, are consistent with Gem/Ca(v)1.3a channels clustering at HC ribbon synapses.

Wnt genes define distinct boundaries in the developing human brain: implications for human forebrain patterning

Understanding the factors that govern human forebrain regionalization along the dorsal-ventral and left-right (L-R) axes is likely to be relevant to a wide variety of neurodevelopmental and neuropsychiatric conditions. Recent work in lower vertebrates has identified several critical signaling molecules involved in embryonic patterning along these axes. Among these are the Wingless-Int (WNT) proteins, involved in the formation of dorsal central nervous system (CNS) structures, as well as in visceral L-R asymmetry. We examined the expression of WNT2b and WNT7b in the human brain, because these genes have highly distinctive expression patterns in the embryonic mouse forebrain. In the human fetal telencephalon, WNT2b expression appears to define the cortical hem, a dorsal signaling center previously characterized in mouse, which is also confirmed by BMP7 expression. In diencephalon, WNT2b expression is restricted to medial dorsal structures, including the developing pineal gland and habenular nucleus, both implicated in CNS L-R asymmetry in lower organisms. At 5 weeks gestation, WNT7b is expressed in cerebral cortical and diencephalic progenitor cells. As the cortical plate develops, WNT7b expression shifts, demarcating deep layer neurons of the neocortex and the hippocampal formation. Spatial and temporal expression patterns show startling similarity between human and mouse, suggesting that the developmental roles of these WNT genes may be highly conserved, despite the far greater size and complexity of the human forebrain.

A Family of Acid-sensing Ion Channels from the Zebrafish

Acid-sensing ion channels (ASICs) are excitatory receptors for extracellular H(+). Proposed functions include synaptic transmission, peripheral perception of pain, and mechanosensation. Despite the physiological importance of these functions, the precise role of ASICs has not yet been established. In order to increase our understanding of the physiological role and basic structure-function relationships of ASICs, we report here the cloning of six new ASICs from the zebrafish (zASICs). zASICs possess the basic functional properties of mammalian ASICs: activation by extracellular H(+), Na(+) selectivity, and block by micromolar concentrations of amiloride. The zasic genes are broadly expressed in the central nervous system, whereas expression in the peripheral nervous system is scarce. This pattern suggests a predominant role for zASICs in neuronal communication. Our results suggest a conserved function for receptors of extracellular H(+) in the central nervous system of vertebrates.

Early steps in the development of the forebrain

The tremendous complexity of the adult forebrain makes it a challenging task to elucidate how this structure forms during embryonic development. Nevertheless, we are beginning to understand how a simple epithelial sheet of ectoderm gives rise to the labyrinthine network of cells that constitutes the functional forebrain. Here, we discuss early events in forebrain development--those that lead to the establishment of the anterior neural plate and the regional subdivision of this territory into the different domains of the prospective forebrain.

Time-lapse microscopy of brain development

Zebrafish embryos represent an ideal vertebrate model organism for noninvasive intravital imaging because of their optical clarity, external embryogenesis, and fast development. Many different labeling techniques have been adopted from other model organisms or newly developed to address a wealth of different developmental questions directly inside the living organism. The parallel advancements in the field of optical imaging let us now observe dynamic processes at the cellular and subcellular resolution. Combined with the repertoire of available surgical and genetic manipulations, zebrafish embryos provide the powerful and almost unique possibility to observe the interplay of molecular signals with cellular, morphological, and behavioral changes directly within a living and developing vertebrate organism. A bright future for zebrafish is yet to come, let there be light.