Disruption of Fuz in mouse embryos generates hypoplastic hindbrain development and reduced cranial nerve ganglia.

BACKGROUND: The brain and spinal cord formation is initiated in the earliest stages of mammalian pregnancy in a highly organized process known as neurulation. Environmental or genetic interferences can impair neurulation, resulting in clinically significant birth defects known collectively as neural tube defects. The Fuz gene encodes a subunit of the CPLANE complex, a macromolecular planar polarity effector required for ciliogenesis. Ablation of Fuz in mouse embryos results in exencephaly and spina bifida, including dysmorphic craniofacial structures due to defective cilia formation and impaired Sonic Hedgehog signaling. RESULTS: We demonstrate that knocking Fuz out during embryonic mouse development results in a hypoplastic hindbrain phenotype, displaying abnormal rhombomeres with reduced length and width. This phenotype is associated with persistent reduction of ventral neuroepithelial stiffness in a notochord adjacent area at the level of the rhombomere 5. The formation of cranial and paravertebral ganglia is also impaired in these embryos. CONCLUSIONS: This study reveals that hypoplastic hindbrain development, identified by abnormal rhombomere morphology and persistent loss of ventral neuroepithelial stiffness, precedes exencephaly in Fuz ablated murine mutants, indicating that the gene Fuz has a critical function sustaining normal neural tube development and neuronal differentiation.

The multiple functions of hindbrain boundary cells: Tinkering boundaries?

Embryonic boundaries were first described in Drosophila, and then in vertebrate embryos, as cellular interfaces between compartments. They display signaling properties and in vertebrates might allocate cells fated to different anatomical structures, or cells that will play different functions over time. One of the vertebrate embryonic structures with boundaries is the hindbrain, the posterior brain vesicle, which is transitory segmented upon morphogenesis. The hindbrain is formed by iterative units called rhombomeres that constitute units of gene expression and cell-lineage compartments. Rhombomeric cells are segregated by interhombomeric boundaries, which are prefigured by sharp gene expression borders. Hindbrain boundaries were first described as static groups of cells. However, later discoveries demonstrated the dynamic behavior of this specific cell population. They play distinct functional properties during brain morphogenesis that partially overlap on time, starting as a mechanical barrier to prevent cell intermingling, becoming a signaling hub, to finally constitute a group of proliferating progenitors providing differentiated neurons to the system. In this review, I try to give a more functional overview of this segmentation process and in particular of hindbrain boundaries. I will discuss the new challenges in the field on how to integrate cell fate specification and morphogenesis during brain embryonic development.

Evolutionary emergence of the rac3b/rfng/sgca regulatory cluster refined mechanisms for hindbrain boundaries formation.

Developmental programs often rely on parallel morphogenetic mechanisms that guarantee precise tissue architecture. While redundancy constitutes an obvious selective advantage, little is known on how novel morphogenetic mechanisms emerge during evolution. In zebrafish, rhombomeric boundaries behave as an elastic barrier, preventing cell intermingling between adjacent compartments. Here, we identify the fundamental role of the small-GTPase Rac3b in actomyosin cable assembly at hindbrain boundaries. We show that the novel rac3b/rfng/sgca regulatory cluster, which is specifically expressed at the boundaries, emerged in the Ostariophysi superorder by chromosomal rearrangement that generated new cis-regulatory interactions. By combining 4C-seq, ATAC-seq, transgenesis, and CRISPR-induced deletions, we characterized this regulatory domain, identifying hindbrain boundary-specific cis-regulatory elements. Our results suggest that the capacity of boundaries to act as an elastic mesh for segregating rhombomeric cells evolved by cooption of critical genes to a novel regulatory block, refining the mechanisms for hindbrain segmentation.

Inter-rhombomeric interactions reveal roles for fibroblast growth factors signaling in segmental regulation of EphA4 expression.

BACKGROUND: The basic ground plan of vertebrate hindbrain is established through a process of segmentation, which generates eight transient lineage-restricted cellular compartments called rhombomeres (r). The segments adopt distinct individual identities in response to axial patterning signals. It is unclear whether signaling between rhombomeres plays a conserved role in regulating segmental patterning during hindbrain development. RESULTS: Using tissue manipulations of rhombomeres in chicken embryos, we have uncovered roles for r2 and r4 in regulating the expression of EphA4 in r3 and r5. Perturbations of signaling pathways reveal that these regulatory inputs from r2 and r4 into EphA4 expression are mediated independent of inputs from Krox20 through cues involving fibroblast growth factor (FGF) signaling. These interactions are stage dependent and are set up in embryos with <10 somites. CONCLUSIONS: We show that r2 and r4 function as temporally dynamic signaling centers in the early patterning of adjacent hindbrain segments and this activity is dependent upon the FGF pathway. These results reveal that inter-rhombomeric signaling is a conserved feature of the regulatory networks that control the specification of individual rhombomere identities in vertebrate hindbrain segmentation. However, the timing of when restricted domains of FGF signaling are coupled to formation of r4 may vary between the species.

An atlas of anterior hox gene expression in the embryonic sea lamprey head: Hox-code evolution in vertebrates.

In the hindbrain and the adjacent cranial neural crest (NC) cells of jawed vertebrates (gnathostomes), nested and segmentally-restricted domains of Hox gene expression provide a combinatorial Hox-code for specifying regional properties during head development. Extant jawless vertebrates, such as the sea lamprey (Petromyzon marinus), can provide insights into the evolution and diversification of this Hox-code in vertebrates. There is evidence for gnathostome-like spatial patterns of Hox expression in lamprey; however, the expression domains of the majority of lamprey hox genes from paralogy groups (PG) 1-4 are yet to be characterized, so it is unknown whether they are coupled to hindbrain segments (rhombomeres) and NC. In this study, we systematically describe the spatiotemporal expression of all 14 sea lamprey hox genes from PG1-PG4 in the developing hindbrain and pharynx to investigate the extent to which their expression conforms to the archetypal gnathostome hindbrain and pharyngeal hox-codes. We find many similarities in Hox expression between lamprey and gnathostome species, particularly in rhombomeric domains during hindbrain segmentation and in the cranial neural crest, enabling inference of aspects of Hox expression in the ancestral vertebrate embryonic head. These data are consistent with the idea that a Hox regulatory network underlying hindbrain segmentation is a pan vertebrate trait. We also reveal differences in hindbrain domains at later stages, as well as expression in the endostyle and in pharyngeal arch (PA) 1 mesoderm. Our analysis suggests that many Hox expression domains that are observed in extant gnathostomes were present in ancestral vertebrates but have been partitioned differently across Hox clusters in gnathostome and cyclostome lineages after duplication.

An exercise in brain genoarchitectonics: Analysis of AZIN2-Lacz expressing neuronal populations in the mouse hindbrain.

We examined in detail the distribution of AZIN2 (antizyme inhibitor 2) expression in the adult mouse hindbrain and neighboring spinal cord. AZIN2, similar to previously known AZIN1, is a recently-discovered, a functional paralog of ornithine decarboxylase (ODC). Due to their structural similarity to ODC, both AZIN1 and AZIN2 counteract the inhibitory action of 3 known antizymes (AZ1-3) on the ODC synthesis of polyamines, thus increasing intracytoplasmic levels of polyamines. AZIN2 is strongly, but heterogeneously, expressed in the brain. Our study uses a mouse line carrying an AZIN2-LacZ construct, and, in our topographic analysis of AZIN2-positive structures, we intend to share new knowledge about the rhombomeric segmentation of the hindbrain (a function of Hox paralogs and other genes). The observed labeled cell populations predominantly coincide with known cholinergic and glutamatergic cells, but occasionally also correspond to GABAergic, and possibly glycinergic cells. Some imperfectly known hindbrain populations stood out in unprecedented detail, and some axonal tracts were also differentially stained. © 2017 Wiley Periodicals, Inc.

The vertebrate Hox gene regulatory network for hindbrain segmentation: Evolution and diversification: Coupling of a Hox gene regulatory network to hindbrain segmentation is an ancient trait originating at the base of vertebrates.

Hindbrain development is orchestrated by a vertebrate gene regulatory network that generates segmental patterning along the anterior-posterior axis via Hox genes. Here, we review analyses of vertebrate and invertebrate chordate models that inform upon the evolutionary origin and diversification of this network. Evidence from the sea lamprey reveals that the hindbrain regulatory network generates rhombomeric compartments with segmental Hox expression and an underlying Hox code. We infer that this basal feature was present in ancestral vertebrates and, as an evolutionarily constrained developmental state, is fundamentally important for patterning of the vertebrate hindbrain across diverse lineages. Despite the common ground plan, vertebrates exhibit neuroanatomical diversity in lineage-specific patterns, with different vertebrates revealing variations of Hox expression in the hindbrain that could underlie this diversification. Invertebrate chordates lack hindbrain segmentation but exhibit some conserved aspects of this network, with retinoic acid signaling playing a role in establishing nested domains of Hox expression.

Hox proteins drive cell segregation and non-autonomous apical remodelling during hindbrain segmentation.

Hox genes encode a conserved family of homeodomain transcription factors regulating development along the major body axis. During embryogenesis, Hox proteins are expressed in segment-specific patterns and control numerous different segment-specific cell fates. It has been unclear, however, whether Hox proteins drive the epithelial cell segregation mechanism that is thought to initiate the segmentation process. Here, we investigate the role of vertebrate Hox proteins during the partitioning of the developing hindbrain into lineage-restricted units called rhombomeres. Loss-of-function mutants and ectopic expression assays reveal that Hoxb4 and its paralogue Hoxd4 are necessary and sufficient for cell segregation, and for the most caudal rhombomere boundary (r6/r7). Hox4 proteins regulate Eph/ephrins and other cell-surface proteins, and can function in a non-cell-autonomous manner to induce apical cell enlargement on both sides of their expression border. Similarly, other Hox proteins expressed at more rostral rhombomere interfaces can also regulate Eph/ephrins, induce apical remodelling and drive cell segregation in ectopic expression assays. However, Krox20, a key segmentation factor expressed in odd rhombomeres (r3 and r5), can largely override Hox proteins at the level of regulation of a cell surface target, Epha4. This study suggests that most, if not all, Hox proteins share a common potential to induce cell segregation but in some contexts this is masked or modulated by other transcription factors.

Hindbrain rhombomere centers harbor a heterogenous population of dividing progenitors which rely on Notch signaling.

Tissue growth and morphogenesis are interrelated processes, whose tight coordination is essential for the production of different cell fates and the timely precise allocation of stem cell capacities. The zebrafish embryonic brainstem, the hindbrain, exemplifies such coupling between spatiotemporal cell diversity acquisition and tissue growth as the neurogenic commitment is differentially distributed over time. Here, we combined cell lineage and in vivo imaging approaches to reveal the emergence of specific cell population properties within the rhombomeres. We studied the molecular identity of hindbrain rhombomere centers and showed that they harbor different progenitor capacities that change over time. By clonal analysis, we revealed that cells within the center of rhombomeres decrease the proliferative capacity to remain mainly in the G1 phase. Proliferating progenitors give rise to neurons by asymmetric and symmetric neurogenic divisions while maintaining the pool of progenitors. The proliferative capacity of these cells differs from their neighbors, and they are delayed in the onset of Notch activity. Through functional studies, we demonstrated that they rely on Notch3 signaling to be maintained as non-committed progenitors. In this study, we show that cells in rhombomere centers, despite the neurogenic asynchrony, might share steps of a similar program with the rhombomere counterparts, to ensure proper tissue growth.

Molecular Segmentation of the Spinal Trigeminal Nucleus in the Adult Mouse Brain.

The trigeminal column is a hindbrain structure formed by second order sensory neurons that receive afferences from trigeminal primary (ganglionic) nerve fibers. Classical studies subdivide it into the principal sensory trigeminal nucleus located next to the pontine nerve root, and the spinal trigeminal nucleus which in turn consists of oral, interpolar and caudal subnuclei. On the other hand, according to the prosomeric model, this column would be subdivided into segmental units derived from respective rhombomeres. Experimental studies have mapped the principal sensory trigeminal nucleus to pontine rhombomeres (r) r2-r3 in the mouse. The spinal trigeminal nucleus emerges as a plurisegmental formation covering several rhombomeres (r4 to r11 in mice) across pontine, retropontine and medullary hindbrain regions. In the present work we reexamined the issue of rhombomeric vs. classical subdivisions of this column. To this end, we analyzed its subdivisions in an AZIN2-lacZ transgenic mouse, known as a reference model for hindbrain topography, together with transgenic reporter lines for trigeminal fibers. We screened as well for genes differentially expressed along the axial dimension of this structure in the adult and juvenile mouse brain. This analysis yielded genes from multiple functional families that display transverse domains fitting the mentioned rhombomeric map. The spinal trigeminal nucleus thus represents a plurisegmental structure with a series of distinct neuromeric units having unique combinatorial molecular profiles.

Netrin-1/DCC Signaling Differentially Regulates the Migration of Pax7, Nkx6.1, Irx2, Otp, and Otx2 Cell Populations in the Developing Interpeduncular Nucleus.

The interpeduncular nucleus (IPN) is a hindbrain structure formed by three main subdivisions, the prodromal (Pro) domain located at the isthmus (Ist), and the rostral and caudal interpeduncular domains (IPR, IPC) within rhombomere 1 (r1). Various cell populations can be detected in the IPN through the expression of the Nkx6.1, Otp, Otx2, Pax7, and/or Irx2 transcription factors. These cell populations follow independent dorsoventral tangential and radial migratory routes targeting the ventral paramedian region of Ist and r1. Here we set out to examine the influence of the Netrin-1/DCC pathway on these migrations, since it is known to regulate other processes of neuronal migration in the brain. To this end, we analyzed IPN development in late gestational wild-type and DCC-/- mice, using mainly in situ hybridization (ISH) to identify the cells expressing each of the aforementioned genes. We found that the migration of Nkx6.1 + and Irx2 + cells into the Pro domain was strongly disrupted by the loss of DCC, as occurred with the migration of Pax7 +, Irx2 +, and Otp + cells that would normally form the IPR. In addition, there was mild impairment of the migration of the Pax7 + and Otx2 + cells that form the IPC. These results demonstrate that the Netrin-1/DCC signaling pathway is involved in the migration of most of the IPN populations, mainly affecting those of the Pro and IPR domains of this nucleus. There are psychiatric disorders that involve the medial habenula (mHb)-IPN system, so that this experimental model could provide a basis to study their neurodevelopmental etiology.

Time for Radical Changes in Brain Stem Nomenclature-Applying the Lessons From Developmental Gene Patterns.

The traditional subdivision of the brain stem into midbrain, pons, and medulla oblongata is based purely on the external appearance of the human brain stem. There is an urgent need to update the names of brain stem structures to be consistent with the discovery of rhomobomeric segmentation based on gene expression. The most important mistakes are the belief that the pons occupies the upper half of the hindbrain, the failure to recognize the isthmus as the first segment of the hindbrain, and the mistaken inclusion of diencephalic structures in the midbrain. The new nomenclature will apply to all mammals. This essay recommends a new brain stem nomenclature based on developmental gene expression, progeny analysis, and fate mapping. In addition, we have made comment on the names given to a number of internal brain stem structures and have offered alternatives where necessary.

The Serotonin Brainstem Hypothesis for the Sudden Infant Death Syndrome.

The sudden infant death syndrome (SIDS) is the leading cause of postneonatal infant mortality in the United States today, with an overall rate of 0.39/1000 live births. It is defined as the sudden and unexpected death of an infant <12 months of age that remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. The serotonin brainstem hypothesis has been a leading hypothesis for SIDS over the last 2 decades. Our laboratory has studied this hypothesis over time with a variety of tissue techniques, including tissue receptor autoradiography, high performance liquid chromatography, Western blot analysis, immunocytochemistry, and proteomics. The purpose of this article is to review the progress in our laboratory toward supporting this hypothesis. We conclude that an important subset of SIDS infants has serotonergic abnormalities resulting from a "core lesion" in the medullary reticular formation comprised of nuclei that contain serotonin neurons. This lesion could lead to a failure of protective brainstem responses to homeostatic challenges during sleep in a critical developmental period which cause sleep-related sudden death.

Cerebellar networks and neuropathology of cerebellar developmental disorders.

The cerebellar system is a series of axonal projections and synaptic circuits as networks, similar to those of the limbic system and those subserving the propagation and spread of seizures. Three principal cerebellar networks are identified and cerebellar disease often affects components of the networks other than just the cerebellar cortex. Contemporary developmental neuropathology of the cerebellum is best considered in the context of alterations of developmental processes: embryonic segmentation and genetic gradients along the three axes of the neural tube, individual neuronal and glial cell differentiation, migration, synaptogenesis, and myelination. Precisely timed developmental processes may be delayed or precocious rhombencephalosynapsis and pontocerebellar hypoplasia exemplify opposite gradients in the horizontal axis. Chiari II malformation may be reconsidered as a disorder of segmentation rather than simply due to mechanical forces upon normally developing hindbrain structures. Cellular nodules in the roof of the fourth ventricle are heterotopia of histologically differentiated but architecturally disoriented and disorganized neurons and glial cells; they often are less mature immunocytochemically than similar cells in adjacent normal folia. Cell rests are nodules of undifferentiated neuroepithelial cells. Both are frequent in human fetuses and neonates. Axonal projections from heterotopia to adjacent cerebellar folia or nuclei are few or absent, hence these nodules are clinically silent despite neuronal differentiation.

Possibilities and limitations of three-dimensional reconstruction and simulation techniques to identify patterns, rhythms and functions of apoptosis in the early developing neural tube.

The now classical idea that programmed cell death (apoptosis) contributes to a plethora of developmental processes still has lost nothing of its impact. It is, therefore, important to establish effective three-dimensional (3D) reconstruction as well as simulation techniques to decipher the exact patterns and functions of such apoptotic events. The present study focuses on the question whether and how apoptosis promotes neurulation-associated processes in the spinal cord of Tupaia belangeri (Tupaiidae, Scandentia, Mammalia). Our 3D reconstructions demonstrate that at least two craniocaudal waves of apoptosis consecutively pass through the dorsal spinal cord. The first wave appears to be involved in neural fold fusion and/or in selection processes among premigratory neural crest cells. The second one seems to assist in establishing the dorsal signaling center known as the roof plate. In the hindbrain, in contrast, apoptosis among premigratory neural crest cells progresses craniocaudally but discontinuously, in a segment-specific manner. Unlike apoptosis in the spinal cord, these segment-specific apoptotic events, however, precede later ones that seemingly support neural fold fusion and/or postfusion remodeling. Arguing with Whitehead that biological patterns and rhythms differ in that biological rhythms depend "upon the differences involved in each exhibition of the pattern" (Whitehead in An enquiry concerning the principles of natural knowledge. Cambridge University Press, London, 1919, p. 198) we show that 3D reconstruction and simulation techniques can contribute to distinguish between (static) patterns and (dynamic) rhythms of apoptosis. By deciphering novel patterns and rhythms of developmental apoptosis, our reconstructions help to reconcile seemingly inconsistent earlier findings in chick and mouse embryos, and to create rules for computer simulations.

Cellular and Molecular Underpinnings of Neuronal Assembly in the Central Auditory System during Mouse Development.

During development, the organization of the auditory system into distinct functional subcircuits depends on the spatially and temporally ordered sequence of neuronal specification, differentiation, migration and connectivity. Regional patterning along the antero-posterior axis and neuronal subtype specification along the dorso-ventral axis intersect to determine proper neuronal fate and assembly of rhombomere-specific auditory subcircuits. By taking advantage of the increasing number of transgenic mouse lines, recent studies have expanded the knowledge of developmental mechanisms involved in the formation and refinement of the auditory system. Here, we summarize several findings dealing with the molecular and cellular mechanisms that underlie the assembly of central auditory subcircuits during mouse development, focusing primarily on the rhombomeric and dorso-ventral origin of auditory nuclei and their associated molecular genetic pathways.

Two Subpopulations of Noradrenergic Neurons in the Locus Coeruleus Complex Distinguished by Expression of the Dorsal Neural Tube Marker Pax7.

Central noradrenergic neurons, collectively defined by synthesis of the neurotransmitter norepinephrine, are a diverse collection of cells in the hindbrain, differing in their anatomy, physiological and behavioral functions, and susceptibility to disease and environmental insult. To investigate the developmental basis of this heterogeneity, we have used an intersectional genetic fate mapping strategy in mice to study the dorsoventral origins of the En1-derived locus coeruleus (LC) complex which encompasses virtually all of the anatomically defined LC proper, as well as a portion of the A7 and subcoeruleus (SubC) noradrenergic nuclei. We show that the noradrenergic neurons of the LC complex originate in two different territories of the En1 expression domain in the embryonic hindbrain. Consistent with prior studies, we confirm that the majority of the LC proper arises from the alar plate, the dorsal domain of the neural tube, as defined by expression of Pax7Cre . In addition, our analysis shows that a large proportion of the En1-derived A7 and SubC nuclei also originate in the Pax7Cre -defined alar plate. Surprisingly, however, we identify a smaller subpopulation of the LC complex that arises from outside the Pax7Cre expression domain. We characterize the distribution of these neurons within the LC complex, their cell morphology, and their axonal projection pattern. Compared to the broader LC complex, the newly identified Pax7Cre -negative noradrenergic subpopulation has very sparse projections to thalamic nuclei, suggestive of distinct functions. This developmental genetic analysis opens new avenues of investigation into the functional diversity of the LC complex.

Morphogenesis of the cerebellum and cerebellum-related structures in the shark Scyliorhinus canicula: insights on the ground pattern of the cerebellar ontogeny.

Because the cerebellum emerged at the agnathan-gnathostome transition and cartilaginous fishes are at the base of the gnathostome lineage, this group is crucial to determine the basic developmental pattern of the cerebellum and to gain insights into its origin. We have systematically analyzed key events in the development of cerebellum and cerebellum-related structures of the shark Scyliorhinus canicula. Three developmental periods are distinguished based on anatomical observations combined with molecular analysis. We present neurochemical and genoarchitectonic evidence on the onset of cerebellar development, the rostral and caudal cerebellar boundaries, the compartmentalization of the cerebellum, and correspondence of cerebellar domains to rhombomeric segmentation of the rostral hindbrain. Our observations, mainly based on the expression pattern of ScHoxA2, support the origin of both the upper and lower auricular leaves from r1 and exclude any cerebellar origin from r2. Correlation between subrhombomeres r1a/r1b and cerebellar domains is proposed based on the ScEn2 expression. The ScEn2 and ScOtx2 expression patterns revealed an antero-posterior cerebellar compartmentalization similar to that of mammals, and supported certain fissures (commonly used to define cerebellar domains) as reliable anatomical landmarks. At difference from mammals, the expression of ScEn2 along the cerebellar median-lateral axis does not reveal a multiple-banded pattern. The present study provides an atlas of cerebellar development in one of the most basal extant gnathostome lineages and emphasizes the importance of combining classic descriptive with modern molecular studies to gain knowledge on the ancestral condition of cerebellar developmental processes and the origins and evolution of the cerebellum.

Genoarchitecture of the rostral hindbrain of a shark: basis for understanding the emergence of the cerebellum at the agnathan-gnathostome transition.

The cerebellum is present in all extant gnathostomes or jawed vertebrates, of which cartilaginous fishes represent the most ancient radiation. Since the isthmic organizer induces the formation of the cerebellum, comparative genoarchitectonic analysis on the meso-isthmo-cerebellar region of cartilaginous fishes with respect to that of jawless vertebrates could reveal why the isthmic organizer acquires the ability to induce the formation of the cerebellum in gnathostomes. In the present work we analyzed the expression pattern of a variety of genes related to the cerebellar formation and patterning (ScOtx2, ScGbx2, ScFgf8, ScLmx1b, ScIrx1, ScIrx3, ScEn2, ScPax6 and ScLhx9) by in situ hybridization, and the distribution of Pax6 protein in the developing hindbrain of the shark Scyliorhinus canicula. The genoarchitectonic code in this species revealed high degree of conservation with respect to that of other gnathostomes. This resemblance may reveal the features of the ancestral condition of the gene network operating for specification of the rostral hindbrain patterning. Accordingly, the main subdivisions of the rostral hindbrain of S. canicula could be recognized. Our results support the existence of a rhombomere 0, identified as the ScFgf8/ScGbx2/ScEn2-positive and mainly negative ScIrx3 domain just caudal to the midbrain ScIrx1/ScOtx2/ScLmx1b-positive domain. The differential ScEn2 and Pax6 expression in the rhombomere 1 revealed anterior and posterior subdivisions. Interestingly, dissimilarities between S. canicula and lampreys (jawless vertebrates) were noted in the expression of Irx, Lhx and Pax genes, which could be part of significant gene network changes through evolution that caused the emergence of the cerebellum.