Regulation of thalamic development by sonic hedgehog

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.

Gli2-Mediated Shh Signaling Is Required for Thalamocortical Projection Guidance

The thalamocortical projections are part of the most important higher level processing connections in the vertebrates and follow a highly ordered pathway from their origin in the thalamus to the cerebral cortex. Their functional complexities are not only due to an extremely elaborate axon guidance process but also due to activity-dependent mechanisms. Gli2 is an intermediary transcription factor in the Sonic hedgehog (Shh) pathway. During neural early development, Shh has an important role in dorsoventral patterning, diencephalic anteroposterior patterning, and many later developmental processes, such as axon guidance and cell migration. Using a Gli2 knockout mouse line, we have studied the role of Shh signaling mediated by Gli2 in the development of the thalamocortical projections during embryonic development. In wild-type brains, we have described the normal trajectory of the thalamocortical axons into the context of the prosomeric model. Then, we have compared it with the altered thalamocortical axons course in Gli2 homozygous embryos. The thalamocortical axons followed different trajectories and were misdirected to other territories probably due to alterations in the Robo/Slit signaling mechanism. In conclusion, the alteration of Gli2-mediated Shh signaling produces an erroneous specification of several territories related with the thalamocortical axons. This is translated into a huge modification in the pathfinding signaling mechanisms needed for the correct wiring of the thalamocortical axons.

Nkx2.9 Contributes to Mid-Hindbrain Patterning by Regulation of mdDA Neuronal Cell-Fate and Repression of a Hindbrain-Specific Cell-Fate

Nkx2.9 is a member of the NK homeobox family and resembles Nkx2.2 both in homology and expression pattern. However, while Nkx2.2 is required for development of serotonergic neurons, the role of Nkx2.9 in the mid-hindbrain region is still ill-defined. We have previously shown that Nkx2.9 expression is downregulated upon loss of En1 during development. Here, we determined whether mdDA neurons require Nkx2.9 during their development. We show that Nkx2.9 is strongly expressed in the IsO and in the VZ and SVZ of the embryonic midbrain, and the majority of mdDA neurons expressed Nkx2.9 during their development. Although the expression of Dat and Cck are slightly affected during development, the overall development and cytoarchitecture of TH-expressing neurons is not affected in the adult Nkx2.9-depleted midbrain. Transcriptome analysis at E14.5 indicated that genes involved in mid- and hindbrain development are affected by Nkx2.9-ablation, such as Wnt8b and Tph2. Although the expression of Tph2 extends more rostral into the isthmic area in the Nkx2.9 mutants, the establishment of the IsO is not affected. Taken together, these data point to a minor role for Nkx2.9 in mid-hindbrain patterning by repressing a hindbrain-specific cell-fate in the IsO and by subtle regulation of mdDA neuronal subset specification.

Cilia, ciliopathies and hedgehog-related forebrain developmental disorders

Development of the forebrain critically depends on the Sonic Hedgehog (Shh) signaling pathway, as illustrated in humans by the frequent perturbation of this pathway in holoprosencephaly, a condition defined as a defect in the formation of midline structures of the forebrain and face. The Shh pathway requires functional primary cilia, microtubule-based organelles present on virtually every cell and acting as cellular antennae to receive and transduce diverse chemical, mechanical or light signals. The dysfunction of cilia in humans leads to inherited diseases called ciliopathies, which often affect many organs and show diverse manifestations including forebrain malformations for the most severe forms. The purpose of this review is to provide the reader with a framework to understand the developmental origin of the forebrain defects observed in severe ciliopathies with respect to perturbations of the Shh pathway. We propose that many of these defects can be interpreted as an imbalance in the ratio of activator to repressor forms of the Gli transcription factors, which are effectors of the Shh pathway. We also discuss the complexity of ciliopathies and their relationships with forebrain disorders such as holoprosencephaly or malformations of cortical development, and emphasize the need for a closer examination of forebrain defects in ciliopathies, not only through the lens of animal models but also taking advantage of the increasing potential of the research on human tissues and organoids.

Amphibian thalamic nuclear organization during larval development and in the adult frog Xenopus laevis: Genoarchitecture and hodological analysis

The early patterning of the thalamus during embryonic development defines rostral and caudal progenitor domains, which are conserved from fishes to mammals. However, the subsequent developmental mechanisms that lead to the adult thalamic configuration have only been investigated for mammals and other amniotes. In this study, we have analyzed in the anuran amphibian Xenopus laevis (an anamniote vertebrate), through larval and postmetamorphic development, the progressive regional expression of specific markers for the rostral (GABA, GAD67, Lhx1, and Nkx2.2) and caudal (Gbx2, VGlut2, Lhx2, Lhx9, and Sox2) domains. In addition, the regional distributions at different developmental stages of other markers such as calcium binding proteins and neuropeptides, helped the identification of thalamic nuclei. It was observed that the two embryonic domains were progressively specified and compartmentalized during premetamorphosis, and cell subpopulations characterized by particular gene expression combinations were located in periventricular, intermediate and superficial strata. During prometamorphosis, three dorsoventral tiers formed from the caudal domain and most pronuclei were defined, which were modified into the definitive nuclear configuration through the metamorphic climax. Mixed cell populations originated from the rostral and caudal domains constitute most of the final nuclei and allowed us to propose additional subdivisions in the adult thalamus, whose main afferent and efferent connections were assessed by tracing techniques under in vitro conditions. This study corroborates shared features of early gene expression patterns in the thalamus between Xenopus and mouse, however, the dynamic changes in gene expression observed at later stages in the amphibian support mechanisms different from those of mammals.

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.

Diencephalic Size Is Restricted by a Novel Interplay Between GCN5 Acetyltransferase Activity and Retinoic Acid Signaling

Diencephalic defects underlie an array of neurological diseases. Previous studies have suggested that retinoic acid (RA) signaling is involved in diencephalic development at late stages of embryonic development, but its roles and mechanisms of action during early neural development are still unclear. Here we demonstrate that mice lacking enzymatic activity of the acetyltransferase GCN5 ((Gcn5^hat/hat )), which were previously characterized with respect to their exencephalic phenotype, exhibit significant diencephalic expansion, decreased diencephalic RA signaling, and increased diencephalic WNT and SHH signaling. Using a variety of molecular biology techniques in both cultured neuroepithelial cells treated with a GCN5 inhibitor and forebrain tissue from (Gcn5^hat/hat ) embryos, we demonstrate that GCN5, RARα/γ, and the poorly characterized protein TACC1 form a complex in the nucleus that binds specific retinoic acid response elements in the absence of RA. Furthermore, RA triggers GCN5-mediated acetylation of TACC1, which results in dissociation of TACC1 from retinoic acid response elements and leads to transcriptional activation of RA target genes. Intriguingly, RA signaling defects caused by in vitro inhibition of GCN5 can be rescued through RA-dependent mechanisms that require RARβ. Last, we demonstrate that the diencephalic expansion and transcriptional defects seen in (Gcn5^hat/hat ) mutants can be rescued with gestational RA supplementation, supporting a direct link between GCN5, TACC1, and RA signaling in the developing diencephalon. Together, our studies identify a novel, nonhistone substrate for GCN5 whose modification regulates a previously undescribed, tissue-specific mechanism of RA signaling that is required to restrict diencephalic size during early forebrain development.SIGNIFICANCE STATEMENT Changes in diencephalic size and shape, as well as SNPs associated with retinoic acid (RA) signaling-associated genes, have been linked to neuropsychiatric disorders. However, the mechanisms that regulate diencephalic morphogenesis and the involvement of RA signaling in this process are poorly understood. Here we demonstrate a novel role of the acetyltransferase GCN5 in a previously undescribed mechanism of RA signaling in the developing forebrain that is required to maintain the appropriate size of the diencephalon. Together, our experiments identify a novel nonhistone substrate of GCN5, highlight an essential role for both GCN5 and RA signaling in early diencephalic development, and elucidate a novel molecular regulatory mechanism for RA signaling that is specific to the developing forebrain.

An Evolutionarily Conserved Network Mediates Development of the zona limitans intrathalamica, a Sonic Hedgehog-Secreting Caudal Forebrain Signaling Center

Recent studies revealed new insights into the development of a unique caudal forebrain-signaling center: the zona limitans intrathalamica (zli). The zli is the last brain signaling center to form and the first forebrain compartment to be established. It is the only part of the dorsal neural tube expressing the morphogen Sonic Hedgehog (Shh) whose activity participates in the survival, growth and patterning of neuronal progenitor subpopulations within the thalamic complex. Here, we review the gene regulatory network of transcription factors and cis-regulatory elements that underlies formation of a shh-expressing delimitated domain in the anterior brain. We discuss evidence that this network predates the origin of chordates. We highlight the contribution of Shh, Wnt and Notch signaling to zli development and discuss implications for the fact that the morphogen Shh relies on primary cilia for signal transduction. The network that underlies zli development also contributes to thalamus induction, and to its patterning once the zli has been set up. We present an overview of the brain malformations possibly associated with developmental defects in this gene regulatory network (GRN).

Barhl2 Determines the Early Patterning of the Diencephalon by Regulating Shh

The diencephalon is the primary relay network transmitting sensory information to the anterior forebrain. During development, distinct progenitor domains in the diencephalon give rise to the pretectum (p1), the thalamus and epithalamus (p2), and the prethalamus (p3), respectively. Shh plays a significant role in establishing the progenitor domains. However, the upstream events influencing the expression of Shh are largely unknown. Here, we show that Barhl2 homeobox gene is expressed in the p1 and p2 progenitor domains and the in zona limitans intrathalamica (ZLI) and regulates the acquisition of identity of progenitor cells in the developing diencephalon. Targeted deletion of Barhl2 results in the ablation of Shh expression in the dorsal portion of ZLI and causes thalamic p2 progenitors to take the fate of p1 progenitors and form pretectal neurons. Moreover, loss of Barhl2 leads to the absence of thalamocortical axon projections, the loss of habenular afferents and efferents, and a gross diminution of the pineal gland. Thus, by acting upstream of Shh signaling pathway, Barhl2 plays a crucial role in patterning the progenitor domains and establishing the positional identities of progenitor cells in the diencephalon.

Development of the Neuroendocrine Hypothalamus

The neuroendocrine hypothalamus is composed of the tuberal and anterodorsal hypothalamus, together with the median eminence/neurohypophysis. It centrally governs wide-ranging physiological processes, including homeostasis of energy balance, circadian rhythms and stress responses, as well as growth and reproductive behaviours. Homeostasis is maintained by integrating sensory inputs and effecting responses via autonomic, endocrine and behavioural outputs, over diverse time-scales and throughout the lifecourse of an individual. Here, we summarize studies that begin to reveal how different territories and cell types within the neuroendocrine hypothalamus are assembled in an integrated manner to enable function, thus supporting the organism's ability to survive and thrive. We discuss how signaling pathways and transcription factors dictate the appearance and regionalization of the hypothalamic primordium, the maintenance of progenitor cells, and their specification and differentiation into neurons. We comment on recent studies that harness such programmes for the directed differentiation of human ES/iPS cells. We summarize how developmental plasticity is maintained even into adulthood and how integration between the hypothalamus and peripheral body is established in the median eminence and neurohypophysis. Analysis of model organisms, including mouse, chick and zebrafish, provides a picture of how complex, yet elegantly coordinated, developmental programmes build glial and neuronal cells around the third ventricle of the brain. Such conserved processes enable the hypothalamus to mediate its function as a central integrating and response-control mediator for the homeostatic processes that are critical to life. Early indications suggest that deregulation of these events may underlie multifaceted pathological conditions and dysfunctional physiology in humans, such as obesity.

ProNodal acts via FGFR3 to govern duration of Shh expression in the prechordal mesoderm

The secreted glycoprotein sonic hedgehog (Shh) is expressed in the prechordal mesoderm, where it plays a crucial role in induction and patterning of the ventral forebrain. Currently little is known about how Shh is regulated in prechordal tissue. Here we show that in the embryonic chick, Shh is expressed transiently in prechordal mesoderm, and is governed by unprocessed Nodal. Exposure of prechordal mesoderm microcultures to Nodal-conditioned medium, the Nodal inhibitor CerS, or to an ALK4/5/7 inhibitor reveals that Nodal is required to maintain both Shh and Gsc expression, but whereas Gsc is largely maintained through canonical signalling, Nodal signals through a non-canonical route to maintain Shh. Further, Shh expression can be maintained by a recombinant Nodal cleavage mutant, proNodal, but not by purified mature Nodal. A number of lines of evidence suggest that proNodal acts via FGFR3. ProNodal and FGFR3 co-immunoprecipitate and proNodal increases FGFR3 tyrosine phosphorylation. In microcultures, soluble FGFR3 abolishes Shh without affecting Gsc expression. Further, prechordal mesoderm cells in which Fgfr3 expression is reduced by Fgfr3 siRNA fail to bind to proNodal. Finally, targeted electroporation of Fgfr3 siRNA to prechordal mesoderm in vivo results in premature Shh downregulation without affecting Gsc. We report an inverse correlation between proNodal-FGFR3 signalling and pSmad1/5/8, and show that proNodal-FGFR3 signalling antagonises BMP-mediated pSmad1/5/8 signalling, which is poised to downregulate Shh. Our studies suggest that proNodal/FGFR3 signalling governs Shh duration by repressing canonical BMP signalling, and that local BMPs rapidly silence Shh once endogenous Nodal-FGFR3 signalling is downregulated.

Highlighted article: In the chick prechordal mesoderm, the Nodal precursor proNodal acts via a non-canonical route to inhibit BMP signalling and thus maintain Shh expression

Fgf15 regulates thalamic development by controlling the expression of proneural genes

The establishment of the brain structural complexity requires a precisely orchestrated interplay between extrinsic and intrinsic signals modulating cellular mechanisms to guide neuronal differentiation. However, little is known about the nature of these signals in the diencephalon, a complex brain region that processes and relays sensory and motor information to and from the cerebral cortex and subcortical structures. Morphogenetic signals from brain organizers regulate histogenetic processes such as cellular proliferation, migration, and differentiation. Sonic hedgehog (Shh) in the key signal of the ZLI, identified as the diencephalic organizer. Fgf15, the mouse gene orthologous of human, chick, and zebrafish Fgf19, is induced by Shh signal and expressed in the diencephalic alar plate progenitors during histogenetic developmental stages. This work investigates the role of Fgf15 signal in diencephalic development. In the absence of Fgf15, the complementary expression pattern of proneural genes: Ascl1 and Nng2, is disrupted and the GABAergic thalamic cells do not differentiate; in addition dorsal thalamic progenitors failed to exit from the mitotic cycle and to differentiate into neurons. Therefore, our findings indicate that Fgf15 is the Shh downstream signal to control thalamic regionalization, neurogenesis, and neuronal differentiation by regulating the expression and mutual segregation of neurogenic and proneural regulatory genes.

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 conserved barH-like homeobox-2 gene barhl2 acts downstream of orthodentricle-2 and together with iroquois-3 in establishment of the caudal forebrain signaling center induced by Sonic Hedgehog

In this study, we investigated the gene regulatory network that governs formation of the Zona limitans intrathalamica (ZLI), a signaling center that secretes Sonic Hedgehog (Shh) to control the growth and regionalization of the caudal forebrain. Using loss- and gain-of-function, explants and grafting experiments in amphibians, we demonstrate that barhl2 acts downstream of otx2 and together with the iroquois (irx)-3 gene in establishment of the ZLI compartment initiated by Shh influence. We find that the presumptive (pre)-ZLI domain expresses barhl2, otx2 and irx3, whereas the thalamus territory caudally bordering the pre-ZLI expresses barhl2, otx2 and irx1/2 and early on irx3. We demonstrate that Barhl2 activity is required for determination of the ZLI and thalamus fates and that within the p2 alar plate the ratio of Irx3 to Irx1/2 contributes to ZLI specification and size determination. We show that when continuously exposed to Shh, neuroepithelial cells coexpressing barhl2, otx2 and irx3 acquire two characteristics of the ZLI compartment-the competence to express shh and the ability to segregate from anterior neural plate cells. In contrast, neuroepithelial cells expressing barhl2, otx2 and irx1/2, are not competent to express shh. Noteworthy in explants, under Shh influence, ZLI-like cells segregate from thalamic-like cells. Our study establishes that Barhl2 activity plays a key role in p2 alar plate patterning, specifically ZLI formation, and provides new insights on establishment of the signaling center of the caudal forebrain.

Hedgehog: Multiple Paths for Multiple Roles in Shaping the Brain and Spinal Cord

Since the discovery of the segment polarity gene Hedgehog in Drosophila three decades ago, our knowledge of Hedgehog signaling pathway has considerably improved and paved the way to a wide field of investigations in the developing and adult central nervous system. Its peculiar transduction mechanism together with its implication in tissue patterning, neural stem cell biology, and neural tissue homeostasis make Hedgehog pathway of interest in a high number of normal or pathological contexts. Consistent with its role during brain development, misregulation of Hedgehog signaling is associated with congenital diseases and tumorigenic processes while its recruitment in damaged neural tissue may be part of the repairing process. This review focuses on the most recent data regarding the Hedgehog pathway in the developing and adult central nervous system and also its relevance as a therapeutic target in brain and spinal cord diseases.

Myofibroblasts

PURPOSE OF REVIEW

Interest in the myofibroblast as a key player in propagation of chronic progressive fibrosis continues to elicit many publications, with focus on its cellular origins and the mechanisms underpinning their differentiation and/or transition. The objective of the review is to highlight this recent progress.

RECENT FINDINGS

The epithelial origin of the myofibroblast in fibrosis has been challenged by recent studies, with the pericyte suggested as a possible precursor instead. Additional signaling pathways, including Notch, Wnt, and hedgehog, are implicated in myofibroblast differentiation. The importance of NADPH oxidase 4 was highlighted recently to suggest a potential link between cellular/oxidative stress and the genesis of the myofibroblast. Recent observations on the importance of lysophosphatidic acid in fibrosis suggest that this may be due, in part, to its ability to regulate myofibroblast differentiation. Finally, there is increasing evidence for the role of epigenetic mechanisms in regulating myofibroblast differentiation, including DNA methylation and miRNA regulation of gene expression.

SUMMARY

These recent discoveries open up a whole new array of potential targets for novel antifibrotic therapies. This is of special importance given the current bleak outlook for chronic progressive fibrotic diseases, such as scleroderma, due to lack of effective therapies.