β-Catenin signaling specifies progenitor cell identity in parallel with Shh signaling in the developing mammalian thalamus

β-Catenin and SOX2 Interaction Regulate Visual Experience-Dependent Cell Homeostasis in the Developing Xenopus Thalamus

In the vertebrate brain, sensory experience plays a crucial role in shaping thalamocortical connections for visual processing. However, it is still not clear how visual experience influences tissue homeostasis and neurogenesis in the developing thalamus. Here, we reported that the majority of SOX2-positive cells in the thalamus are differentiated neurons that receive visual inputs as early as stage 47 Xenopus. Visual deprivation (VD) for 2 days shifts the neurogenic balance toward proliferation at the expense of differentiation, which is accompanied by a reduction in nuclear-accumulated β-catenin in SOX2-positive neurons. The knockdown of β-catenin decreases the expression of SOX2 and increases the number of progenitor cells. Coimmunoprecipitation studies reveal the evolutionary conservation of strong interactions between β-catenin and SOX2. These findings indicate that β-catenin interacts with SOX2 to maintain homeostatic neurogenesis during thalamus development.

Roles of Fibroblast Growth Factors in the Axon Guidance

Fibroblast growth factors (FGFs) have been widely studied by virtue of their ability to regulate many essential cellular activities, including proliferation, survival, migration, differentiation and metabolism. Recently, these molecules have emerged as the key components in forming the intricate connections within the nervous system. FGF and FGF receptor (FGFR) signaling pathways play important roles in axon guidance as axons navigate toward their synaptic targets. This review offers a current account of axonal navigation functions performed by FGFs, which operate as chemoattractants and/or chemorepellents in different circumstances. Meanwhile, detailed mechanisms behind the axon guidance process are elaborated, which are related to intracellular signaling integration and cytoskeleton dynamics.

Developmental trajectories of thalamic progenitors revealed by single-cell transcriptome profiling and Shh perturbation

The thalamus is the principal information hub of the vertebrate brain, with essential roles in sensory and motor information processing, attention, and memory. The complex array of thalamic nuclei develops from a restricted pool of neural progenitors. We apply longitudinal single-cell RNA sequencing and regional abrogation of Sonic hedgehog (Shh) to map the developmental trajectories of thalamic progenitors, intermediate progenitors, and post-mitotic neurons as they coalesce into distinct thalamic nuclei. These data reveal that the complex architecture of the thalamus is established early during embryonic brain development through the coordinated action of four cell differentiation lineages derived from Shh-dependent and -independent progenitors. We systematically characterize the gene expression programs that define these thalamic lineages across time and demonstrate how their disruption upon Shh depletion causes pronounced locomotor impairment resembling infantile Parkinson's disease. These results reveal key principles of thalamic development and provide mechanistic insights into neurodevelopmental disorders resulting from thalamic dysfunction.

The bHLH Transcription Factors in Neural Development and Therapeutic Applications for Neurodegenerative Diseases

The development of functional neural circuits in the central nervous system (CNS) requires the production of sufficient numbers of various types of neurons and glial cells, such as astrocytes and oligodendrocytes, at the appropriate periods and regions. Hence, severe neuronal loss of the circuits can cause neurodegenerative diseases such as Huntington's disease (HD), Parkinson's disease (PD), Alzheimer's disease (AD), and Amyotrophic Lateral Sclerosis (ALS). Treatment of such neurodegenerative diseases caused by neuronal loss includes some strategies of cell therapy employing stem cells (such as neural progenitor cells (NPCs)) and gene therapy through cell fate conversion. In this report, we review how bHLH acts as a regulator in neuronal differentiation, reprogramming, and cell fate determination. Moreover, several different researchers are conducting studies to determine the importance of bHLH factors to direct neuronal and glial cell fate specification and differentiation. Therefore, we also investigated the limitations and future directions of conversion or transdifferentiation using bHLH factors.

Astrocytes and neurons share region-specific transcriptional signatures that confer regional identity to neuronal reprogramming

Neural cell diversity is essential to endow distinct brain regions with specific functions. During development, progenitors within these regions are characterized by specific gene expression programs, contributing to the generation of diversity in postmitotic neurons and astrocytes. While the region-specific molecular diversity of neurons and astrocytes is increasingly understood, whether these cells share region-specific programs remains unknown. Here, we show that in the neocortex and thalamus, neurons and astrocytes express shared region-specific transcriptional and epigenetic signatures. These signatures not only distinguish cells across these two brain regions but are also detected across substructures within regions, such as distinct thalamic nuclei, where clonal analysis reveals the existence of common nucleus-specific progenitors for neurons and astrocytes. Consistent with their shared molecular signature, regional specificity is maintained following astrocyte-to-neuron reprogramming. A detailed understanding of these regional-specific signatures may thus inform strategies for future cell-based brain repair.

Extrinsic Regulators of mRNA Translation in Developing Brain: Story of WNTs

Extrinsic molecules such as morphogens can regulate timed mRNA translation events in developing neurons. In particular, Wingless-type MMTV integration site family, member 3 (Wnt3), was shown to regulate the translation of Foxp2 mRNA encoding a Forkhead transcription factor P2 in the neocortex. However, the Wnt receptor that possibly mediates these translation events remains unknown. Here, we report Frizzled member 7 (Fzd7) as the Wnt3 receptor that lays downstream in Wnt3-regulated mRNA translation. Fzd7 proteins co-localize with Wnt3 ligands in developing neocortices. In addition, the Fzd7 proteins overlap in layer-specific neuronal subpopulations expressing different transcription factors, Foxp1 and Foxp2. When Fzd7 was silenced, we found decreased Foxp2 protein expression and increased Foxp1 protein expression, respectively. The Fzd7 silencing also disrupted the migration of neocortical glutamatergic neurons. In contrast, Fzd7 overexpression reversed the pattern of migratory defects and Foxp protein expression that we found in the Fzd7 silencing. We further discovered that Fzd7 is required for Wnt3-induced Foxp2 mRNA translation. Surprisingly, we also determined that the Fzd7 suppression of Foxp1 protein expression is not Wnt3 dependent. In conclusion, it is exhibited that the interaction between Wnt3 and Fzd7 regulates neuronal identity and the Fzd7 receptor functions as a downstream factor in ligand Wnt3 signaling for mRNA translation. In particular, the Wnt3-Fzd7 signaling axis determines the deep layer Foxp2-expressing neurons of developing neocortices. Our findings also suggest that Fzd7 controls the balance of the expression for Foxp transcription factors in developing neocortical neurons. These discoveries are presented in our manuscript within a larger framework of this review on the role of extrinsic factors in regulating mRNA translation.

Connexin43 Region 266-283, via Src Inhibition, Reduces Neural Progenitor Cell Proliferation Promoted by EGF and FGF-2 and Increases Astrocytic Differentiation

Neural progenitor cells (NPCs) are self-renewing cells that give rise to the major cells in the nervous system and are considered to be the possible cell of origin of glioblastoma. The gap junction protein connexin43 (Cx43) is expressed by NPCs, exerting channel-dependent and -independent roles. We focused on one property of Cx43—its ability to inhibit Src, a key protein in brain development and oncogenesis. Because Src inhibition is carried out by the sequence 266–283 of the intracellular C terminus in Cx43, we used a cell-penetrating peptide containing this sequence, TAT-Cx43_266–283, to explore its effects on postnatal subventricular zone NPCs. Our results show that TAT-Cx43_266–283 inhibited Src activity and reduced NPC proliferation and survival promoted by epidermal growth factor (EGF) and fibroblast growth factor-2 (FGF-2). In differentiation conditions, TAT-Cx43_266–283 increased astrocyte differentiation at the expense of neuronal differentiation, which coincided with a reduction in Src activity and β-catenin expression. We propose that Cx43, through the region 266–283, reduces Src activity, leading to disruption of EGF and FGF-2 signaling and to down-regulation of β-catenin with effects on proliferation and differentiation. Our data indicate that the inhibition of Src might contribute to the complex role of Cx43 in NPCs and open new opportunities for further research in gliomagenesis.

Dlp-mediated Hh and Wnt signaling interdependence is critical in the niche for germline stem cell progeny differentiation

Although multiple signaling pathways work synergistically in various niches to control stem cell self-renewal and differentiation, it remains poorly understood how they cooperate with one another molecularly. In the Drosophila ovary, Hh and Wnt pathways function in the niche to promote germline stem cell (GSC) progeny differentiation. Here, we show that glypican Dlp-mediated Hh and Wnt signaling interdependence operates in the niche to promote GSC progeny differentiation by preventing BMP signaling. Hh/Wnt-mediated dlp repression is essential for their signaling interdependence in niche cells and for GSC progeny differentiation by preventing BMP signaling. Mechanistically, Hh and Wnt downstream transcription factors directly bind to the same dlp regulatory region and recruit corepressors composed of transcription factor Croc and Egg/H3K9 trimethylase to repress Dlp expression. Therefore, our study reveals a novel mechanism for Hh/Wnt signaling-mediated direct dlp repression and a novel regulatory mechanism for Dlp-mediated Hh/Wnt signaling interdependence in the GSC differentiation niche.

Wnt/β-Catenin Signaling in Neural Stem Cell Homeostasis and Neurological Diseases

Neural stem/progenitor cells (NSCs) maintain the ability of self-renewal and differentiation and compose the complex nervous system. Wnt signaling is thought to control the balance of NSC proliferation and differentiation via the transcriptional coactivator β-catenin during brain development and adult tissue homeostasis. Disruption of Wnt signaling may result in developmental defects and neurological diseases. Here, we summarize recent findings of the roles of Wnt/β-catenin signaling components in NSC homeostasis for the regulation of functional brain circuits. We also suggest that the potential role of Wnt/β-catenin signaling might lead to new therapeutic strategies for neurological diseases, including, but not limited to, spinal cord injury, Alzheimer's disease, Parkinson's disease, and depression.

Development of the thalamus: From early patterning to regulation of cortical functions

The thalamus is a brain structure of the vertebrate diencephalon that plays a central role in regulating diverse functions of the cerebral cortex. In traditional view of vertebrate neuroanatomy, the thalamus includes three regions, dorsal thalamus, ventral thalamus, and epithalamus. Recent molecular embryological studies have redefined the thalamus and the associated axial nomenclature of the diencephalon in the context of forebrain patterning. This new view has provided a useful conceptual framework for studies on molecular mechanisms of patterning, neurogenesis and fate specification in the thalamus as well as the guidance mechanisms for thalamocortical axons. Additionally, the availability of genetic tools in mice has led to important findings on how thalamic development is linked to the development of other brain regions, particularly the cerebral cortex. This article will give an overview of the organization of the embryonic thalamus and how progenitor cells in the thalamus generate neurons that are organized into discrete nuclei. I will then discuss how thalamic development is orchestrated with the development of the cerebral cortex and other brain regions. This article is categorized under: Nervous System Development > Vertebrates: Regional Development Nervous System Development > Vertebrates: General Principles.

The Thalamus Regulates Retinoic Acid Signaling and Development of Parvalbumin Interneurons in Postnatal Mouse Prefrontal Cortex

GABAergic inhibitory neurons in the prefrontal cortex (PFC) play crucial roles in higher cognitive functions. Despite the link between aberrant development of PFC interneurons and a number of psychiatric disorders, mechanisms underlying the development of these neurons are poorly understood. Here we show that the retinoic acid (RA)-degrading enzyme CYP26B1 (cytochrome P450 family 26, subfamily B, member 1) is transiently expressed in the mouse frontal cortex during postnatal development, and that medial ganglionic eminence (MGE)-derived interneurons, particularly in parvalbumin (PV)-expressing neurons, are the main cell type that has active RA signaling during this period. We found that frontal cortex-specific Cyp26b1 knock-out mice had an increased density of PV-expressing, but not somatostatin-expressing, interneurons in medial PFC, indicating a novel role of RA signaling in controlling PV neuron development. The initiation of Cyp26b1 expression in neonatal PFC coincides with the establishment of connections between the thalamus and the PFC. We found that these connections are required for the postnatal expression of Cyp26b1 in medial PFC. In addition to this region-specific role in postnatal PFC that regulates RA signaling and PV neuron development, the thalamocortical connectivity had an earlier role in controlling radial dispersion of MGE-derived interneurons throughout embryonic neocortex. In summary, our results suggest that the thalamus plays multiple, temporally separate roles in interneuron development in the PFC.

The Ciliopathy Gene Ftm/Rpgrip1l Controls Mouse Forebrain Patterning via Region-Specific Modulation of Hedgehog/Gli Signaling

Primary cilia are essential for CNS development. In the mouse, they play a critical role in patterning the spinal cord and telencephalon via the regulation of Hedgehog/Gli signaling. However, despite the frequent disruption of this signaling pathway in human forebrain malformations, the role of primary cilia in forebrain morphogenesis has been little investigated outside the telencephalon. Here we studied development of the diencephalon, hypothalamus and eyes in mutant mice in which the Ftm/Rpgrip1l ciliopathy gene is disrupted. At the end of gestation, Ftm^−/− fetuses displayed anophthalmia, a reduction of the ventral hypothalamus and a disorganization of diencephalic nuclei and axonal tracts. In Ftm^−/− embryos, we found that the ventral forebrain structures and the rostral thalamus were missing. Optic vesicles formed but lacked the optic cups. In Ftm^−/− embryos, Sonic hedgehog (Shh) expression was virtually lost in the ventral forebrain but maintained in the zona limitans intrathalamica (ZLI), the mid-diencephalic organizer. Gli activity was severely downregulated but not lost in the ventral forebrain and in regions adjacent to the Shh-expressing ZLI. Reintroduction of the repressor form of Gli3 into the Ftm^−/− background restored optic cup formation. Our data thus uncover a complex role of cilia in development of the diencephalon, hypothalamus and eyes via the region-specific control of the ratio of activator and repressor forms of the Gli transcription factors. They call for a closer examination of forebrain defects in severe ciliopathies and for a search for ciliopathy genes as modifiers in other human conditions with forebrain defects.

SIGNIFICANCE STATEMENT The Hedgehog (Hh) signaling pathway is essential for proper forebrain development as illustrated by a human condition called holoprosencephaly. The Hh pathway relies on primary cilia, cellular organelles that receive and transduce extracellular signals and whose dysfunctions lead to rare inherited diseases called ciliopathies. To date, the role of cilia in the forebrain has been poorly studied outside the telencephalon. In this paper we study the role of the Ftm/Rpgrip1l ciliopathy gene in mouse forebrain development. We uncover complex functions of primary cilia in forebrain morphogenesis through region-specific modulation of the Hh pathway. Our data call for further examination of forebrain defects in ciliopathies and for a search for ciliopathy genes as modifiers in human conditions affecting forebrain development.

Radial glia fibers translate Fgf8 morphogenetic signals to generate a thalamic nuclear complex protomap in the mantle layer

Thalamic neurons are distributed between different nuclear groups of the thalamic multinuclear complex; they develop topologically ordered specific projections that convey information on voluntary motor programs and sensory modalities to functional areas in the cerebral cortex. Since thalamic neurons present a homogeneous morphology, their functional specificity is derived from their afferent and efferent connectivity. Adequate development of thalamic afferent and efferent connections depends on guide signals that bind receptors in nuclear neuropils and axonal growth cones, respectively. These are finally regulated by regionalization processes in the thalamic neurons, codifying topological information. In this work, we studied the role of Fgf8 morphogenetic signaling in establishing the molecular thalamic protomap, which was revealed by Igsf21, Pde10a and Btbd3 gene expression in the thalamic mantle layer. Fgf8 signaling activity was evidenced by pERK expression in radial glia cells and fibers, which may represent a scaffold that translates neuroepithelial positional information to the mantle layer. In this work, we describe the fact that Fgf8-hypomorphic mice did not express pERK in radial glia cells and fibers and presented disorganized thalamic regionalization, increasing neuronal death in the ventro-lateral thalamus and strong disruption of thalamocortical projections. In conclusion, Fgf8 encodes the positional information required for thalamic nuclear regionalization and the development of thalamocortical projections.

Canonical Wnt signaling regulates patterning, differentiation and nucleogenesis in mouse hypothalamus and prethalamus

The hypothalamus is a small, but anatomically and functionally complex region of the brain whose development is poorly understood. In this study, we have explored its development by studying the canonical Wnt signaling pathway, generating gain and loss of function mutations of beta-catenin (Ctnnb1) in both hypothalamic and prethalamic neuroepithelium. Deletion of Ctnnb1 resulted in an anteriorized and hypoplastic hypothalamus. Posterior structures were lost or reduced, and anterior structures were expanded. In contrast, overexpression of a constitutively active mutant form of Ctnnb1 resulted in severe hyperplasia of prethalamus and hypothalamus, and expanded expression of a subset of posterior and premamillary hypothalamic markers. Moderate defects in differentiation of Arx-positive GABAergic neural precursors were observed in both prethalamus and hypothalamus of Ctnnb1 loss of function mutants, while in gain of function mutants, their differentiation was completely suppressed, although markers of prethalamic progenitors were preserved. Multiple other region-specific markers, including several specific posterior hypothalamic structures, were also suppressed in Ctnnb1 gain of function mutations. Severe, region-specific defects in hypothalamic nucleogenesis were also observed in both gain and loss of function mutations of Ctnnb1. Finally, both gain and loss of function of Ctnnb1 also produced severe, non-cell autonomous disruptions of pituitary development. These findings demonstrate a central and multifaceted role for canonical Wnt signaling in regulating growth, patterning, differentiation and nucleogenesis in multiple diencephalic regions.

In vivo clonal analysis reveals spatiotemporal regulation of thalamic nucleogenesis

The thalamus, a crucial regulator of cortical functions, is composed of many nuclei arranged in a spatially complex pattern. Thalamic neurogenesis occurs over a short period during mammalian embryonic development. These features have hampered the effort to understand how regionalization, cell divisions, and fate specification are coordinated and produce a wide array of nuclei that exhibit distinct patterns of gene expression and functions. Here, we performed in vivo clonal analysis to track the divisions of individual progenitor cells and spatial allocation of their progeny in the developing mouse thalamus. Quantitative analysis of clone compositions revealed evidence for sequential generation of distinct sets of thalamic nuclei based on the location of the founder progenitor cells. Furthermore, we identified intermediate progenitor cells that produced neurons populating more than one thalamic nuclei, indicating a prolonged specification of nuclear fate. Our study reveals an organizational principle that governs the spatial and temporal progression of cell divisions and fate specification and provides a framework for studying cellular heterogeneity and connectivity in the mammalian thalamus.

The thalamus—a brain structure commonly associated with relaying sensory information between cortex and other regions—is organized into many cell clusters called nuclei. Each thalamic nucleus is populated by neurons with distinct patterns of gene expression and connections to other brain regions and plays a distinct role in cortical functions. In this study, we performed an analysis of developing cells in the thalamus, using the mosaic analysis with double markers (MADM) method in mice, a technique that allows the labeling of descendants of dividing cells. Using 3 different transgenic mouse lines allowed us to determine the cell lineage of thalamic progenitor cells at different locations and stages of differentiation. By genetically labeling single progenitor cells, we measured how cell division and maturation occurs during the brief time span when neurons are generated. Our data also show how neurons eventually contribute to multiple nuclei across the thalamus. The organizational principles that we found in the thalamus might apply to the development of other brain structures that are composed of multiple nuclei.

bHLH transcription factors in neural development, disease, and reprogramming

The formation of functional neural circuits in the vertebrate central nervous system (CNS) requires that appropriate numbers of the correct types of neuronal and glial cells are generated in their proper places and times during development. In the embryonic CNS, multipotent progenitor cells first acquire regional identities, and then undergo precisely choreographed temporal identity transitions (i.e. time-dependent changes in their identity) that determine how many neuronal and glial cells of each type they will generate. Transcription factors of the basic-helix-loop-helix (bHLH) family have emerged as key determinants of neural cell fate specification and differentiation, ensuring that appropriate numbers of specific neuronal and glial cell types are produced. Recent studies have further revealed that the functions of these bHLH factors are strictly regulated. Given their essential developmental roles, it is not surprising that bHLH mutations and de-regulated expression are associated with various neurological diseases and cancers. Moreover, the powerful ability of bHLH factors to direct neuronal and glial cell fate specification and differentiation has been exploited in the relatively new field of cellular reprogramming, in which pluripotent stem cells or somatic stem cells are converted to neural lineages, often with a transcription factor-based lineage conversion strategy that includes one or more of the bHLH genes. These concepts are reviewed herein.

Specification of neurotransmitter identity by Tal1 in thalamic nuclei

BACKGROUND

The neurons contributing to thalamic nuclei are derived from at least two distinct progenitor domains: the caudal (cTH) and rostral (rTH) populations of thalamic progenitors. These neural compartments exhibit unique neurogenic patterns, and the molecular mechanisms underlying the acquisition of neurotransmitter identity remain largely unclear.

RESULTS

T-cell acute lymphocytic leukemia protein 1 (Tal1) was expressed in the early postmitotic cells in the rTH domain, and its expression was maintained in mature thalamic neurons in the ventrolateral geniculate nucleus (vLG) and the intergeniculate leaflet (IGL). To investigate a role of Tal1 in thalamic development, we used a newly generated mouse line driving Cre-mediated recombination in the rTH domain. Conditional deletion of Tal1 did not alter regional patterning in the developing diencephalon. However, in the absence of Tal1, rTH-derived thalamic neurons failed to maintain their postmitotic neuronal features, including neurotransmitter profile. Tal1-deficient thalamic neurons lost their GABAergic markers such as Gad1, Npy, and Penk in IGL/vLG. These defects may be associated at least in part with down-regulation of Nkx2.2, which is known as a critical regulator of rTH-derived GABAergic neurons.

CONCLUSIONS

Our results demonstrate that Tal1 plays an essential role in regulating neurotransmitter phenotype in the developing thalamic nuclei. Developmental Dynamics 246:749-758, 2017. © 2017 Wiley Periodicals, Inc.

β-catenin is required for taste bud cell renewal and behavioral taste perception in adult mice

Taste stimuli are transduced by taste buds and transmitted to the brain via afferent gustatory fibers. Renewal of taste receptor cells from actively dividing progenitors is finely tuned to maintain taste sensitivity throughout life. We show that conditional β-catenin deletion in mouse taste progenitors leads to rapid depletion of progenitors and Shh⁺ precursors, which in turn causes taste bud loss, followed by loss of gustatory nerve fibers. In addition, our data suggest LEF1, TCF7 and Wnt3 are involved in a Wnt pathway regulatory feedback loop that controls taste cell renewal in the circumvallate papilla epithelium. Unexpectedly, taste bud decline is greater in the anterior tongue and palate than in the posterior tongue. Mutant mice with this regional pattern of taste bud loss were unable to discern sweet at any concentration, but could distinguish bitter stimuli, albeit with reduced sensitivity. Our findings are consistent with published reports wherein anterior taste buds have higher sweet sensitivity while posterior taste buds are better tuned to bitter, and suggest β-catenin plays a greater role in renewal of anterior versus posterior taste buds.

By remaining relatively constant throughout adult life, the sense of taste helps keep the body healthy. However, taste perception can be disrupted by various environmental factors, including cancer therapies. Here, we show that Wnt/β-catenin signaling, a pathway known to control normal tissue maintenance and associated with the development of cancers, is required for taste cell renewal and behavioral taste sensitivity in mice. Our findings are significant as they suggest that chemotherapies targeting the Wnt pathway in cancerous tissues may cause taste dysfunction and further diminish the quality of life of patients.

Foxp2 Regulates Identities and Projection Patterns of Thalamic Nuclei During Development

The molecular mechanisms underlying the formation of the thalamus during development have been investigated intensively. Although transcription factors distinguishing the thalamic primordium from adjacent brain structures have been uncovered, those involved in patterning inside the thalamus are largely unclear. Here, we show that Foxp2, a member of the forkhead transcription factor family, regulates thalamic patterning during development. We found a graded expression pattern of Foxp2 in the thalamic primordium of the mouse embryo. The expression levels of Foxp2 were high in the posterior region and low in the anterior region of the thalamic primordium. In Foxp2 (R552H) knockin mice, which have a missense loss-of-function mutation in the forkhead domain of Foxp2, thalamic nuclei of the posterior region of the thalamus were shrunken, while those of the intermediate region were expanded. Consistently, Foxp2 (R552H) knockin mice showed changes in thalamocortical projection patterns. Our results uncovered important roles of Foxp2 in thalamic patterning and thalamocortical projections during development.

Generation of thalamic neurons from mouse embryonic stem cells

The thalamus is a diencephalic structure that plays crucial roles in relaying and modulating sensory and motor information to the neocortex. The thalamus develops in the dorsal part of the neural tube at the level of the caudal forebrain. However, the molecular mechanisms that are essential for thalamic differentiation are still unknown. Here, we have succeeded in generating thalamic neurons from mouse embryonic stem cells (mESCs) by modifying the default method that induces the most-anterior neural type in self-organizing culture. A low concentration of the caudalizing factor insulin and a MAPK/ERK kinase inhibitor enhanced the expression of the caudal forebrain markers Otx2 and Pax6. BMP7 promoted an increase in thalamic precursors such as Tcf7l2⁺/Gbx2⁺ and Tcf7l2⁺/Olig3⁺ cells. mESC thalamic precursors began to express the glutamate transporter vGlut2 and the axon-specific marker VGF, similar to mature projection neurons. The mESC thalamic neurons extended their axons to cortical layers in both organotypic culture and subcortical transplantation. Thus, we have identified the minimum elements sufficient for in vitro generation of thalamic neurons. These findings expand our knowledge of thalamic development.

The Many Hats of Sonic Hedgehog Signaling in Nervous System Development and Disease

Sonic hedgehog (Shh) signaling occurs concurrently with the many processes that constitute nervous system development. Although Shh is mostly known for its proliferative and morphogenic action through its effects on neural stem cells and progenitors, it also contributes to neuronal differentiation, axonal pathfinding and synapse formation and function. To participate in these diverse events, Shh signaling manifests differently depending on the maturational state of the responsive cell, on the other signaling pathways regulating neural cell function and the environmental cues that surround target cells. Shh signaling is particularly dynamic in the nervous system, ranging from canonical transcription-dependent, to non-canonical and localized to axonal growth cones. Here, we review the variety of Shh functions in the developing nervous system and their consequences for neurodevelopmental diseases and neural regeneration, with particular emphasis on the signaling mechanisms underlying Shh action.

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.

Wnt Signaling Specifies Anteroposterior Progenitor Zone Identity in the Drosophila Visual Center

During brain development, various types of neuronal populations are produced from different progenitor pools to produce neuronal diversity that is sufficient to establish functional neuronal circuits. However, the molecular mechanisms that specify the identity of each progenitor pool remain obscure. Here, we show that Wnt signaling is essential for the specification of the identity of posterior progenitor pools in the Drosophila visual center. In the medulla, the largest component of the visual center, different types of neurons are produced from two progenitor pools: the outer proliferation center (OPC) and glial precursor cells (GPCs; also known as tips of the OPC). We found that OPC-type neurons are produced from the GPCs at the expense of GPC-type neurons when Wnt signaling is suppressed in the GPCs. In contrast, GPC-type neurons are ectopically induced when Wnt signaling is ectopically activated in the OPC. These results suggest that Wnt signaling is necessary and sufficient for the specification of the progenitor pool identity. We also found that Homothorax (Hth), which is temporally expressed in the OPC, is ectopically induced in the GPCs by suppression of Wnt signaling and that ectopic induction of Hth phenocopies the suppression of Wnt signaling in the GPCs. Thus, Wnt signaling is involved in regionalization of the fly visual center through the specification of the progenitor pool located posterior to the medulla by suppressing Hth expression.

SIGNIFICANCE STATEMENT

Brain consists of considerably diverse neurons of different origins. In mammalian brain, excitatory and inhibitory neurons derive from the dorsal and ventral telencephalon, respectively. Multiple progenitor pools also contribute to the neuronal diversity in fly brain. However, it has been unclear how differences between these progenitor pools are established. Here, we show that Wnt signaling, an evolutionarily conserved signaling, is involved in the process that establishes the differences between these progenitor pools. Because β-catenin signaling, which is under the control of Wnt ligands, specifies progenitor pool identity in the developing mammalian thalamus, Wnt signaling-mediated specification of progenitor pool identity may be conserved in insect and mammalian brains.

Function of Armcx3 and Armc10/SVH Genes in the Regulation of Progenitor Proliferation and Neural Differentiation in the Chicken Spinal Cord

The eutherian X-chromosome specific family of Armcx genes has been described as originating by retrotransposition from Armc10/SVH, a single Arm-containing somatic gene. Armcx3 and Armc10/SVH are characterized by high expression in the central nervous system and they play an important role in the regulation of mitochondrial distribution and transport in neurons. In addition, Armcx/Arm10 genes have several Armadillo repeats in their sequence. In this study we address the potential role of this gene family in neural development by using the chick neural tube as a model. We show that Armc10/SVH is expressed in the chicken spinal cord, and knocking-down Armc10/SVH by sh-RNAi electroporation in spinal cord reduces proliferation of neural precursor cells (NPCs). Moreover, we analyzed the effects of murine Armcx3 and Armc10 overexpression, showing that both proteins regulate progenitor proliferation, while Armcx3 overexpression also specifically controls neural maturation. We show that the phenotypes found following Armcx3 overexpression require its mitochondrial localization, suggesting a novel link between mitochondrial dynamics and regulation of neural development. Furthermore, we found that both Armcx3 and Armc10 may act as inhibitors of Wnt-β-catenin signaling. Our results highlight both common and differential functions of Armcx/Armc10 genes in neural development in the spinal cord.

The molecular and cellular signatures of the mouse eminentia thalami support its role as a signalling centre in the developing forebrain

The mammalian eminentia thalami (EmT) (or thalamic eminence) is an embryonic forebrain structure of unknown function. Here, we examined the molecular and cellular properties of the mouse EmT. We first studied mRNA expression of signalling molecules and found that the EmT is a structure, rich in expression of secreted factors, with Wnts being the most abundantly detected. We then examined whether EmT tissue could induce cell fate changes when grafted ectopically. For this, we transplanted EmT tissue from a tau-GFP mouse to the ventral telencephalon of a wild type host, a telencephalic region where Wnt signalling is not normally active but which we showed in culture experiments is competent to respond to Wnts. We observed that the EmT was able to induce in adjacent ventral telencephalic cells ectopic expression of Lef1, a transcriptional activator and a target gene of the Wnt/β-catenin pathway. These Lef1-positive;GFP-negative cells expressed the telencephalic marker Foxg1 but not Ascl1, which is normally expressed by ventral telencephalic cells. These results suggest that the EmT has the capacity to activate Wnt/β-catenin signalling in the ventral telencephalon and to suppress ventral telencephalic gene expression. Altogether, our data support a role of the EmT as a signalling centre in the developing mouse forebrain.

Gbx2 is essential for maintaining thalamic neuron identity and repressing habenular characters in the developing thalamus

The thalamus and habenula, two important nodes of the forebrain circuitry, are derived from a single developmental compartment, called prosomere 2, in the diencephalon. Habenular and thalamic neurons display distinct molecular identity, neurochemistry, and connectivity. Furthermore, their progenitors exhibit distinctive neurogenic patterns with a marked delay in the onset of neurogenesis in the thalamus. However, the progenitors in prosomere 2 express many common developmental regulators and the mechanism underlying the specification and differentiation of these two populations of neurons remains unknown. Gbx2, coding for a homeodomain transcription factor, is initially expressed in thalamic neuronal precursors that have just exited the cell cycle, and its expression is maintained in many mature thalamic neurons in adults. Deletion of Gbx2 severely disrupts histogenesis of the thalamus and abolishes thalamocortical projections in mice. Here, by using genome-wide transcriptional profiling, we show that Gbx2 promotes thalamic but inhibits habenular molecular characters. Remarkably, although Gbx2 is expressed in postmitotic neuronal precursors, deletion of Gbx2 changes gene expression and cell proliferation in dividing progenitors in the developing thalamus. These defects are partially rescued by the mosaic presence of wild-type cells, demonstrating a cell non-autonomous role of Gbx2 in regulating the development of thalamic progenitors. Our results suggest that Gbx2 is essential for the acquisition of the thalamic neuronal identity by repressing habenular identity through a feedback signaling from postmitotic neurons to progenitors.

Prepatterning and patterning of the thalamus along embryonic development of Xenopus laevis

Previous developmental studies of the thalamus (alar part of the diencephalic prosomere p2) have defined the molecular basis for the acquisition of the thalamic competence (preparttening), the subsequent formation of the secondary organizer in the zona limitans intrathalamica, and the early specification of two anteroposterior domains (rostral and caudal progenitor domains) in response to inducing activities and that are shared in birds and mammals. In the present study we have analyzed the embryonic development of the thalamus in the anuran Xenopus laevis to determine conserved or specific features in the amphibian diencephalon. From early embryonic stages to the beginning of the larval period, the expression patterns of 22 markers were analyzed by means of combined In situ hybridization (ISH) and immunohistochemical techniques. The early genoarchitecture observed in the diencephalon allowed us to discern the boundaries of the thalamus with the prethalamus, pretectum, and epithalamus. Common molecular features were observed in the thalamic prepatterning among vertebrates in which Wnt3a, Fez, Pax6 and Xiro1 expression were of particular importance in Xenopus. The formation of the zona limitans intrathalamica was observed, as in other vertebrates, by the progressive expression of Shh. The largely conserved expressions of Nkx2.2 in the rostral thalamic domain vs. Gbx2 and Ngn2 (among others) in the caudal domain strongly suggest the role of Shh as morphogen in the amphibian thalamus. All these data showed that the molecular characteristics observed during preparttening and patterning in the thalamus of the anuran Xenopus (anamniote) share many features with those described during thalamic development in amniotes (common patterns in tetrapods) but also with zebrafish, strengthening the idea of a basic organization of this diencephalic region across vertebrates.

The ciliogenic transcription factor Rfx3 is required for the formation of the thalamocortical tract by regulating the patterning of prethalamus and ventral telencephalon

Primary cilia are complex subcellular structures that play key roles during embryogenesis by controlling the cellular response to several signaling pathways. Defects in the function and/or structure of primary cilia underlie a large number of human syndromes collectively referred to as ciliopathies. Often, ciliopathies are associated with mental retardation (MR) and malformation of the corpus callosum. However, the possibility of defects in other forebrain axon tracts, which could contribute to the cognitive disorders of these patients, has not been explored. Here, we investigate the formation of the corticothalamic/thalamocortical tracts in mice mutant for Rfx3, which regulates the expression of many genes involved in ciliogenesis and cilia function. Using DiI axon tracing and immunohistochemistry experiments, we show that some Rfx3(-/-) corticothalamic axons abnormally migrate toward the pial surface of the ventral telencephalon (VT). Some thalamocortical axons (TCAs) also fail to leave the diencephalon or abnormally project toward the amygdala. Moreover, the Rfx3(-/-) VT displays heterotopias containing attractive guidance cues and expressing the guidance molecules Slit1 and Netrin1. Finally, the abnormal projection of TCAs toward the amygdala is also present in mice carrying a mutation in the Inpp5e gene, which is mutated in Joubert Syndrome and which controls cilia signaling and stability. The presence of identical thalamocortical malformations in two independent ciliary mutants indicates a novel role for primary cilia in the formation of the corticothalamic/thalamocortical tracts by establishing the correct cellular environment necessary for its development.

Transcriptional control of GABAergic neuronal subtype identity in the thalamus

Background

The thalamus is often defined as the ‘gateway to consciousness’, a feature that is supported by the specific connectivity and electrophysiological properties of its neurons. Inhibitory GABAergic neurons are required for the dynamic gating of information passing through the thalamus. The high degree of heterogeneity among thalamic GABA neurons suggests that, during embryonic development, alternative differentiation programmes exist to guide the acquisition of inhibitory neuron subtype identity.

Results

Taking advantage of the accessibility of the developing chick embryo, we have used in ovo manipulations of gene expression to test the role of candidate transcription factors in controlling GABAergic neuronal subtype identity in the developing thalamus.

Conclusions

In this study, we describe two alternative differentiation programmes for GABAergic neurogenesis in the thalamus and identify Helt and Dlx2 as key transcription factors that are sufficient to direct neuronal progenitors along a specific differentiation pathway at the expense of alternative lineage choices. Furthermore, we identify Calb2, a gene encoding for the GABA subtype marker calretinin as a target of the transcription factor Sox14. This work is a step forward in our understanding of how GABA neuron diversity in the thalamus is achieved during development and will help future investigation of the molecular mechanisms that lead up to the acquisition of different synaptic targets and electrophysiological features of mature thalamic inhibitory neurons.

Wnt signaling activates Shh signaling in early postnatal intervertebral discs, and re-activates Shh signaling in old discs in the mouse

Intervertebral discs (IVDs) are strong fibrocartilaginous joints that connect adjacent vertebrae of the spine. As discs age they become prone to failure, with neurological consequences that are often severe. Surgical repair of discs treats the result of the disease, which affects as many as one in seven people, rather than its cause. An ideal solution would be to repair degenerating discs using the mechanisms of their normal differentiation. However, these mechanisms are poorly understood. Using the mouse as a model, we previously showed that Shh signaling produced by nucleus pulposus cells activates the expression of differentiation markers, and cell proliferation, in the postnatal IVD. In the present study, we show that canonical Wnt signaling is required for the expression of Shh signaling targets in the IVD. We also show that Shh and canonical Wnt signaling pathways are down-regulated in adult IVDs. Furthermore, this down-regulation is reversible, since re-activation of the Wnt or Shh pathways in older discs can re-activate molecular markers of the IVD that are lost with age. These data suggest that biological treatments targeting Wnt and Shh signaling pathways may be feasible as a therapeutic for degenerative disc disease.

Zrf1 is required to establish and maintain neural progenitor identity

The molecular mechanisms underlying specification from embryonic stem cells (ESCs) and maintenance of neural progenitor cells (NPCs) are largely unknown. Recently, we reported that the Zuotin-related factor 1 (Zrf1) is necessary for chromatin displacement of the Polycomb-repressive complex 1 (PRC1). We found that Zrf1 is required for NPC specification from ESCs and that it promotes the expression of NPC markers, including the key regulator Pax6. Moreover, Zrf1 is essential to establish and maintain Wnt ligand expression levels, which are necessary for NPC self-renewal. Reactivation of proper Wnt signaling in Zrf1-depleted NPCs restores Pax6 expression and the self-renewal capacity. ESC-derived NPCs in vitro resemble most of the characteristics of the self-renewing NPCs located in the developing embryonic cortex, which are termed radial glial cells (RGCs). Depletion of Zrf1 in vivo impairs the expression of key self-renewal regulators and Wnt ligand genes in RGCs. Thus, we demonstrate that Zrf1 plays an essential role in NPC generation and maintenance.

Cross-regulation of Connexin43 and β-catenin influences differentiation of human neural progenitor cells

Connexin43 (Cx43) is the most widely and abundantly expressed gap junction (GJ) protein and it is strongly associated with the regulation of cell cycle progression. Emerging roles for Cx43 in cell adhesion and migration during neural differentiation have also been recently recognized, and this has emphasized the involvement of Cx43 in different physiological process beyond its role as a GJ protein. In this study, we explore the function of Cx43 in the differentiation of human neural progenitor cells (hNPCs) using viral vectors that mediate the overexpression or knockdown of the protein. Results showed that in the absence of this protein fetal cortex-derived hNPCs differentiated toward a neuronal phenotype at expenses of a glial phenotype. Furthermore, the silencing of Cx43 did not affect hNPC proliferation rate or numbers of apoptotic cells. The increase in the number of neurons was not recapitulated when GJ intercellular communications were pharmacologically blocked, and this suggested that Cx43 was influencing hNPCs differentiation with a GJ-independent effect. In addition, Cx43 knockdown significantly increased β-catenin signaling, which has been shown to regulate the transcription of pro-neuronal genes during embryonic neural development. Our results add further support to the hypothesis that Cx43 protein itself regulates key signaling pathways during development and neurogenesis beyond its role as GJ protein.

Parcellation of the thalamus into distinct nuclei reflects EphA expression and function

Intercellular signaling via the Eph receptor tyrosine kinases and their ligands, the ephrins, acts to shape many regions of the developing brain. One intriguing consequence of Eph signaling is the control of mixing between discrete cell populations in the developing hindbrain, contributing to the formation of segregated rhombomeres. Since the thalamus is also a parcellated structure comprised of discrete nuclei, might Eph signaling play a parallel role in cell segregation in this brain structure? Analyses of expression reveal that several Eph family members are expressed in the forming thalamus and that cells expressing particular receptors form cellular groupings as development proceeds. Specifically, expression of receptors EphA4 or EphA7 and ligand ephrin-A5 is localized to distinct thalamic domains. EphA4 and EphA7 are often coexpressed in regions of the forming thalamus, with each receptor marking discrete thalamic domains. In contrast, ephrin-A5 is expressed by a limited group of thalamic cells. Within the ventral thalamus, EphA4 is present broadly, occasionally overlapping with ephrin-A5 expression. EphA7 is more restricted in its expression and is largely nonoverlapping with ephrin-A5. In mutant mice lacking one or both receptors or ephrin-A5, the appearance of the venteroposterolateral (VPL) and venteroposteromedial (VPM) nuclear complex is altered compared to wild type mice. These in vivo results support a role for Eph family members in the definition of the thalamic nuclei. In parallel, in vitro analysis reveals a hierarchy of mixing among cells expressing ephrin-A5 with cells expressing EphA4 alone, EphA4 and EphA7 together, or EphA7 alone. Together, these data support a model in which EphA molecules promote the parcellation of discrete thalamic nuclei by limiting the extent of cell mixing.

Postnatal isoform switch and protein localization of LEF1 and TCF7L2 transcription factors in cortical, thalamic, and mesencephalic regions of the adult mouse brain

β-Catenin signaling, leading to the activation of lymphoid enhancer-binding factor 1/T cell factor (LEF1/TCF) transcription factors, plays a well-established role in transcription regulation during development and tissue homeostasis. In the adult organism, the activity of this pathway has been found in stem cell niches and postmitotic thalamic neurons. Recently, studies show that mutations in components of β-catenin signaling networks have been associated with several psychiatric disorders, indicating the involvement of β-catenin and LEF1/TCF proteins in the proper functioning of the brain. Here, we report a comprehensive analysis of LEF1/TCF protein localization and the expression profile of their isoforms in cortical, thalamic, and midbrain regions in mice. We detected LEF1 and TCF7L2 proteins in neurons of the thalamus and dorsal midbrain, i.e., subcortical regions specialized in the integration of diverse sources of sensory information. These neurons also exhibited nuclear localization of β-catenin, suggesting the involvement of β-catenin/LEF1/TCF7L2 in the regulation of gene expression in these regions. Analysis of alternative splicing and promoter usage identified brain-specific TCF7L2 isoforms and revealed a developmentally coordinated transition in the composition of LEF1 and TCF7L2 isoforms. In the case of TCF7L2, the typical brain isoforms lack the so-called C clamp; in addition, the dominant-negative isoforms are predominant in the embryonic thalamus but disappear postnatally. The present study provides a necessary framework to understand the role of LEF1/TCF factors in thalamic and midbrain development until adulthood and predicts that the regulatory role of these proteins in the adult brain is significantly different from their role in the embryonic brain or other non-neural tissues.

Irx3 and Pax6 establish differential competence for Shh-mediated induction of GABAergic and glutamatergic neurons of the thalamus

During embryonic development, the presumptive GABAergic rostral thalamus (rTh) and glutamatergic caudal thalamus (cTh) are induced by Sonic hedgehog (Shh) signaling from the zona limitans intrathalamica (ZLI) at the rostral border of the thalamic primordium. We found that these inductions are limited to the neuroepithelium between the ZLI and the forebrain-midbrain boundary, suggesting a prepattern that limits thalamic competence. We hypothesized that this prepattern is established by the overlapping expression of two transcription factors: Iroquois-related homeobox gene 3 (Irx3) posterior to the ZLI, and paired box gene 6 (Pax6) anterior to the forebrain-midbrain boundary. Consistent with this assumption, we show that misexpression of Irx3 in the prethalamus or telencephalon results in ectopic induction of thalamic markers in response to Shh, that it functions as a transcriptional repressor in this context, and that antagonizing its function in the diencephalon attenuates thalamic specification. Similarly, misexpression of Pax6 in the midbrain together with Shh pathway activation results in ectopic induction of cTh markers in clusters of cells that fail to integrate into tectal layers and of atypical long-range projections, whereas antagonizing Pax6 function in the thalamus disrupts cTh formation. However, rTh markers are negatively regulated by Pax6, which itself is down-regulated by Shh from the ZLI in this area. Our results demonstrate that the combinatorial expression of Irx3 and Pax6 endows cells with the competence for cTh formation, whereas Shh-mediated down-regulation of Pax6 is required for rTh formation. Thus, thalamus induction and patterning depends both on a prepattern of Irx3 and Pax6 expression that establishes differential cellular competence and on Shh signaling from the ZLI organizer.

Proneural genes in neocortical development

Neurons, astrocytes and oligodendrocytes arise from CNS progenitor cells at defined times and locations during development, with transcription factors serving as key determinants of these different neural cell fates. An emerging theme is that the transcription factors that specify CNS cell fates function in a context-dependent manner, regulated by post-translational modifications and epigenetic alterations that partition the genome (and hence target genes) into active or silent domains. Here we profile the critical roles of the proneural genes, which encode basic-helix-loop-helix (bHLH) transcription factors, in specifying neural cell identities in the developing neocortex. In particular, we focus on the proneural genes Neurogenin 1 (Neurog1), Neurog2 and Achaete scute-like 1 (Ascl1), which are each expressed in a distinct fashion in the progenitor cell pools that give rise to all of the neuronal and glial cell types of the mature neocortex. Notably, while the basic functions of these proneural genes have been elucidated, it is becoming increasingly evident that tight regulatory controls dictate when, where and how they function. Current efforts to better understand how proneural gene function is regulated will not only improve our understanding of neocortical development, but are also critical to the future development of regenerative therapies for the treatment of neuronal degeneration or disease.

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.

Disruption of Wnt/β-catenin signaling in odontoblasts and cementoblasts arrests tooth root development in postnatal mouse teeth

Tooth development undergoes a series of complex reciprocal interactions between dental epithelium and the underlying mesenchymal cells. Compared with the study in tooth crown formation, little is known about the molecular mechanism underlying the development of tooth roots. In the present study, we conditionally knock out β-catenin gene (Ctnnb1) within developing odontoblasts and cementoblasts during the development of tooth roots, and observed rootless molars as well as incomplete incisors. Histological analyses revealed intact structure of molar crown and labial side of incisor, however, as for the molar roots and the lingual portion of incisor, the formation of dentin and periodontal tissues were greatly hampered. In situ hybridization experiments using probes of odontoblastic marker genes collagen type I, alpha 1 (Col1a1), osteocalcin (OC) and dentin sialophosphoprotein (Dspp) manifested striking undifferentiation of root odontoblasts in which Ctnnb1 was eliminated. Bromodeoxyuridine (BrdU) labeling and proliferating cell nuclear antigen (PCNA) immunohistochemical experiments also showed retarded proliferation of pre-odontoblasts in mutant mice. However, cell apoptosis was not affected. Additionally, a disrupted formation of cementoblasts, suggested by the absence of transcripts of bone sialoprotein (Bsp) in follicle mesenchyme, was also evident in mutant mice. Our study provides strong in vivo evidence to confirm that Wnt/β-catenin signaling is functionally significant to root odontogenesis and cementogenesis during the tooth root development.