Autonomous and nonautonomous functions for Hox/Pbx in branchiomotor neuron development

Hox genes in development and beyond

Hox genes encode evolutionarily conserved transcription factors that are essential for the proper development of bilaterian organisms. Hox genes are unique because they are spatially and temporally regulated during development in a manner that is dictated by their tightly linked genomic organization. Although their genetic function during embryonic development has been interrogated, less is known about how these transcription factors regulate downstream genes to direct morphogenetic events. Moreover, the continued expression and function of Hox genes at postnatal and adult stages highlights crucial roles for these genes throughout the life of an organism. Here, we provide an overview of Hox genes, highlighting their evolutionary history, their unique genomic organization and how this impacts the regulation of their expression, what is known about their protein structure, and their deployment in development and beyond.

Identifying the Potential Roles of PBX4 in Human Cancers Based on Integrative Analysis

PBX4 belongs to the pre-B-cell leukemia homeobox (PBX) transcription factors family and acts as a transcriptional cofactor of HOX proteins participating in several pathophysiological processes. Recent studies have revealed that the dysregulation of PBX4 is closely related to multiple diseases, especially cancers. However, the research on PBX4’s potential roles in 33 cancers from the Cancer Genome Atlas (TCGA) is still insufficient. Therefore, we performed a comprehensive pan-cancer analysis to explore the roles of PBX4with multiple public databases. Our results showed that PBX4 was differentially expressed in 17 types of human cancer and significantly correlated to the pathological stage, tumor grade, and immune and molecular subtypes. We used the Kaplan–Meier plotter and PrognoScan databases to find the significant associations between PBX4 expression and prognostic values of multiple cancers. It was also found that PBX4 expression was statistically related to mutation status, DNA methylation, immune infiltration, drug sensitivity, and immune checkpoint blockade (ICB) therapy. Additionally, we found that PBX4 was involved in different functional states of multiple cancers from the single-cell resolution perspective. Enrichment analysis results showed that PBX4-related genes were enriched in the cell cycle process, MAPK cascade, ncRNA metabolic process, positive regulation of GTPase activity, and regulation of lipase activity and mainly participated in the pathways of cholesterol metabolism, base excision repair, herpes simplex virus 1 infection, transcriptional misregulation in cancer, and Epstein–Barr virus infection. Altogether, our integrative analysis could help in better understanding the potential roles of PBX4 in different human cancers.

The Hox protein conundrum: The "specifics" of DNA binding for Hox proteins and their partners

Homeotic genes (Hox genes) are homeodomain-transcription factors involved in conferring segmental identity along the anterior-posterior body axis. Molecular characterization of HOX protein function raises some interesting questions regarding the source of the binding specificity of the HOX proteins. How do HOX proteins regulate common and unique target specificity across space and time? This review attempts to summarize and interpret findings in this area, largely focused on results from in vitro and in vivo studies in Drosophila and mouse systems. Recent studies related to HOX protein binding specificity compel us to reconsider some of our current models for transcription factor-DNA interactions. It is crucial to study transcription factor binding by incorporating components of more complex, multi-protein interactions in concert with small changes in binding motifs that can significantly impact DNA binding specificity and subsequent alterations in gene expression. To incorporate the multiple elements that can determine HOX protein binding specificity, we propose a more integrative Cooperative Binding model.

Synergistic effects of Pb and repeated heat pulse on developmental neurotoxicity in zebrafish

Pollutant discharges to the aquatic environment often contain multiple environmental stressors, affecting aquatic organisms. To mimic the discharges from nuclear and industry facilities, the combined effects of two independent types of stressors, heavy metal Pb and repeated heat pulse were addressed in this study. We investigated the developmental toxicity of combined treatment, especially its toxic effects on zebrafish neurodevelopment. The normal embryos at 4 hpf were exposed to 0.2 mM of Pb dissolved in the bathing medium with different temperatures (30, 32, and 34 °C) and then maintained in an incubator at 28 °C. After performing above treatment once every 24 h for 6 days, we found that combined treatment significantly affected neural development, including loss of dopaminergic (DA) neurons and brain vasculature, disruption of locomotor activity and neurodevelopmental genes expression in a temperature-dependent manner as compared to the Pb alone exposure group, indicating that repeated heat pulse enhances these negative impacts induced by Pb. In contrast, no apparent toxicity was observed in repeated heat pulse alone groups, suggesting that Pb treatment reduces thermal tolerance in zebrafish, which emphasized the importance to evaluate synergistic effects of Pb and repeated heat pulse. Moreover, repeated heat pulse aggravated Pb-induced apoptosis in the zebrafish brain. Further study of the underlying mechanism suggested that Caspase 3 regulated apoptosis was involved in this process. Taken together, our findings shed light on the full understanding of toxic effects of discharges from industrial applications on living organisms and its environmental impact.

Coordinate regulation of retinoic acid synthesis by pbx genes and fibroblast growth factor signaling by hoxb1b is required for hindbrain patterning and development

The vertebrate hindbrain is composed of a series of lineage-restricted segments termed rhombomeres. Segment-specific gene expression drives unique programs of neuronal differentiation. Two critical embryonic signaling pathways, Fibroblast Growth Factor (FGF) and Retinoic Acid (RA), regulate early embryonic rhombomere patterning. The earliest expressed hox genes, hoxb1b and hoxb1a in zebrafish, are logical candidates for establishing signaling networks that specify segmental identity. We sought to determine the mechanism by which hox genes regulate hindbrain patterning in zebrafish. We demonstrate that hoxb1a regulates r4-specific patterning, while hoxb1b regulates rhombomere segmentation and size. Hoxb1a and hoxb1b redundantly regulate vhnf1 expression. Loss of hoxb1b together with pbx4 reverts the hindbrain to a groundstate identity, demonstrating the importance of hox genes in patterning nearly the entire hindbrain, and a key requirement for Pbx in this process. Additionally, we provide evidence that while pbx genes regulate RA signaling, hoxb1b regulates hindbrain identity through complex regulation of FGF signaling.

Parallel Pbx-Dependent Pathways Govern the Coalescence and Fate of Motor Columns

The clustering of neurons sharing similar functional properties and connectivity is a common organizational feature of vertebrate nervous systems. Within motor networks, spinal motor neurons (MNs) segregate into longitudinally arrayed subtypes, establishing a central somatotopic map of peripheral target innervation. MN organization and connectivity relies on Hox transcription factors expressed along the rostrocaudal axis; however, the developmental mechanisms governing the orderly arrangement of MNs are largely unknown. We show that Pbx genes, which encode Hox cofactors, are essential for the segregation and clustering of neurons within motor columns. In the absence of Pbx1 and Pbx3 function, Hox-dependent programs are lost and the remaining MN subtypes are unclustered and disordered. Identification of Pbx gene targets revealed an unexpected and apparently Hox-independent role in defining molecular features of dorsally projecting medial motor column (MMC) neurons. These results indicate Pbx genes act in parallel genetic pathways to orchestrate neuronal subtype differentiation, connectivity, and organization.

Rest represses maturation within migrating facial branchiomotor neurons

The vertebrate brain arises from the complex organization of millions of neurons. Neurogenesis encompasses not only cell fate specification from neural stem cells, but also the terminal molecular and morphological maturation of neurons at correct positions within the brain. RE1-silencing transcription factor (Rest) is expressed in non-neural tissues and neuronal progenitors where it inhibits the terminal maturation of neurons by repressing hundreds of neuron-specific genes. Here we show that Rest repression of maturation is intimately linked with the migratory capability of zebrafish facial branchiomotor neurons (FBMNs), which undergo a characteristic tangential migration from hindbrain rhombomere (r) 4 to r6/r7 during development. We establish that FBMN migration is increasingly disrupted as Rest is depleted in zebrafish rest mutant embryos, such that around two-thirds of FBMNs fail to complete migration in mutants depleted of both maternal and zygotic Rest. Although Rest is broadly expressed, we show that de-repression or activation of Rest target genes only within FBMNs is sufficient to disrupt their migration. We demonstrate that this migration defect is due to precocious maturation of FBMNs, based on both morphological and molecular criteria. We further show that the Rest target gene and alternative splicing factor srrm4 is a key downstream regulator of maturation; Srrm4 knockdown partially restores the ability of FBMNs to migrate in rest mutants while preventing their precocious morphological maturation. Rest must localize to the nucleus to repress its targets, and its subcellular localization is highly regulated: we show that targeting Rest specifically to FBMN nuclei rescues FBMN migration in Rest-deficient embryos. We conclude that Rest functions in FBMN nuclei to inhibit maturation until the neurons complete their migration.

Generating spinal motor neuron diversity: a long quest for neuronal identity

Understanding how thousands of different neuronal types are generated in the CNS constitutes a major challenge for developmental neurobiologists and is a prerequisite before considering cell or gene therapies of nervous lesions or pathologies. During embryonic development, spinal motor neurons (MNs) segregate into distinct subpopulations that display specific characteristics and properties including molecular identity, migration pattern, allocation to specific motor columns, and innervation of defined target. Because of the facility to correlate these different characteristics, the diversification of spinal MNs has become the model of choice for studying the molecular and cellular mechanisms underlying the generation of multiple neuronal populations in the developing CNS. Therefore, how spinal motor neuron subpopulations are produced during development has been extensively studied during the last two decades. In this review article, we will provide a comprehensive overview of the genetic and molecular mechanisms that contribute to the diversification of spinal MNs.

Glimpse into Hox and tale regulation of cell differentiation and reprogramming

During embryonic development, cells become gradually restricted in their developmental potential and start elaborating lineage-specific transcriptional networks to ultimately acquire a unique differentiated state. Hox genes play a central role in specifying regional identities, thereby providing the cell with critical information on positional value along its differentiation path. The exquisite DNA-binding specificity of the Hox proteins is frequently dependent upon their interaction with members of the TALE family of homeodomain proteins. In addition to their function as Hox-cofactors, TALE homeoproteins control multiple crucial developmental processes through Hox-independent mechanisms. Here, we will review recent findings on the function of both Hox and TALE proteins in cell differentiation, referring mostly to vertebrate species. In addition, we will discuss the direct implications of this knowledge on cell plasticity and cell reprogramming.

Hox genes: choreographers in neural development, architects of circuit organization

The neural circuits governing vital behaviors, such as respiration and locomotion, are comprised of discrete neuronal populations residing within the brainstem and spinal cord. Work over the past decade has provided a fairly comprehensive understanding of the developmental pathways that determine the identity of major neuronal classes within the neural tube. However, the steps through which neurons acquire the subtype diversities necessary for their incorporation into a particular circuit are still poorly defined. Studies on the specification of motor neurons indicate that the large family of Hox transcription factors has a key role in generating the subtypes required for selective muscle innervation. There is also emerging evidence that Hox genes function in multiple neuronal classes to shape synaptic specificity during development, suggesting a broader role in circuit assembly. This Review highlights the functions and mechanisms of Hox gene networks and their multifaceted roles during neuronal specification and connectivity.

A Hox gene controls lateral line cell migration by regulating chemokine receptor expression downstream of Wnt signaling

The posterior lateral line primordium in zebrafish provides an amenable model to study mechanisms of collective cell migration. The directed migration of the cell cluster along the path of Sdf1a chemokine requires two receptors, Cxcr4b and Cxcr7b, which are expressed in the leading and trailing part of the primordium, respectively. The polarized expression of receptors is regulated by Wnt signaling, but downstream players mediating this control remain to be found. Here, we show that the Hox homeobox gene Hoxb8a is a critical component that acts downstream of the Wnt pathway to coordinate the expression of both chemokine receptors. We find that Hoxb8a is expressed in the leading part of the primordium and is required for the correct speed and extent of migration. Hoxb8a expression is dependent upon Wnt activity and needed both for cxcr4b expression and to repress and thus restrict cxcr7b expression to the trailing zone of the primordium. In the absence of Wnt activity, overexpressed Hoxb8a is able to repress cxcr7b but not up-regulate cxcr4b expression. Together with results from expressing dominant activator and repressor constructs, these findings suggest that Hoxb8a is induced by and cooperates with Wnt signaling to up-regulate cxcr4b, and acts through multiple mechanisms to repress cxcr7b expression.

Identification of multiple subsets of ventral interneurons and differential distribution along the rostrocaudal axis of the developing spinal cord

The spinal cord contains neuronal circuits termed Central Pattern Generators (CPGs) that coordinate rhythmic motor activities. CPG circuits consist of motor neurons and multiple interneuron cell types, many of which are derived from four distinct cardinal classes of ventral interneurons, called V0, V1, V2 and V3. While significant progress has been made on elucidating the molecular and genetic mechanisms that control ventral interneuron differentiation, little is known about their distribution along the antero-posterior axis of the spinal cord and their diversification. Here, we report that V0, V1 and V2 interneurons exhibit distinct organizational patterns at brachial, thoracic and lumbar levels of the developing spinal cord. In addition, we demonstrate that each cardinal class of ventral interneurons can be subdivided into several subsets according to the combinatorial expression of different sets of transcription factors, and that these subsets are differentially distributed along the rostrocaudal axis of the spinal cord. This comprehensive molecular profiling of ventral interneurons provides an important resource for investigating neuronal diversification in the developing spinal cord and for understanding the contribution of specific interneuron subsets on CPG circuits and motor control.

The role of Zic transcription factors in regulating hindbrain retinoic acid signaling

Background

The reiterated architecture of cranial motor neurons aligns with the segmented structure of the embryonic vertebrate hindbrain. Anterior-posterior identity of cranial motor neurons depends, in part, on retinoic acid signaling levels. The early vertebrate embryo maintains a balance between retinoic acid synthetic and degradative zones on the basis of reciprocal expression domains of the retinoic acid synthesis gene aldhehyde dehydrogenase 1a2 (aldh1a2) posteriorly and the oxidative gene cytochrome p450 type 26a1 (cyp26a1) in the forebrain, midbrain, and anterior hindbrain.

Results

This manuscript investigates the role of zinc finger of the cerebellum (zic) transcription factors in regulating levels of retinoic acid and differentiation of cranial motor neurons. Depletion of zebrafish Zic2a and Zic2b results in a strong downregulation of aldh1a2 expression and a concomitant reduction in activity of a retinoid-dependent transgene. The vagal motor neuron phenotype caused by loss of Zic2a/2b mimics a depletion of Aldh1a2 and is rescued by exogenously supplied retinoic acid.

Conclusion

Zic transcription factors function in patterning hindbrain motor neurons through their regulation of embryonic retinoic acid signaling.

Hox transcription factors influence motoneuron identity through the integrated actions of both homeodomain and non-homeodomain regions

BACKGROUND

Hox transcription factors play a critical role in the specification of motoneuron subtypes within the spinal cord. Our previous work showed that two orthologous members of this family, Hoxd10 and Hoxd11, exert opposing effects on motoneuron development in the lumbosacral (LS) spinal cord of the embryonic chick: Hoxd10 promotes the development of lateral motoneuron subtypes that project to dorsal limb muscles, while Hoxd11 represses the development of lateral subtypes in favor of medial subtypes that innervate ventral limb muscles and axial muscles. The striking degree of homology between the DNA-binding homeodomains of Hoxd10 and Hoxd11 suggested that non-homeodomain regions mediate their divergent effects. In the present study, we investigate the relative contributions of homeodomain and non-homeodomain regions of Hoxd10 and Hoxd11 to motoneuron specification.

RESULTS

Using in ovo electroporation to express chimeric and mutant constructs in LS motoneurons, we find that both the homeodomain and non-homeodomain regions of Hoxd10 are necessary to specify lateral motoneurons. In contrast, non-homeodomain regions of Hoxd11 are sufficient to repress lateral motoneuron fates in favor of medial fates.

CONCLUSIONS

Together, our data demonstrate that even closely related Hox orthologues rely on distinct combinations of homeodomain-dependent and -independent mechanisms to specify motoneuron identity.

olig2-Expressing hindbrain cells are required for migrating facial motor neurons

The complicated trajectory of facial motor neuron migration requires coordination of intrinsic signals and cues from the surrounding environment. Migration begins in rhombomere (r) 4 where facial motor neurons are born and proceeds in a caudal direction. Once facial motor neurons reach their target rhombomeres, they migrate laterally and radially from the ventral neural tube. In zebrafish, as facial motor neurons migrate through r5/r6, they pass near cells that express olig2, which encodes a bHLH transcription factor. In this study, we found that olig2 function is required for facial motor neurons to complete their caudal migration into r6 and r7 and form stereotypical clusters. Additionally, embryos that lack mafba function, in which facial motor neurons also fail to complete caudal migration, lack olig2 expression in r5 and r6. Our data raise the possibility that cells expressing olig2 are intermediate targets that help guide facial motor neuron migration.

A Gal4/UAS system for conditional transgene expression in rhombomere 4 of the zebrafish hindbrain

BACKGROUND

The zebrafish is well established as a model organism for the study of vertebrate embryogenesis, but transgenic lines enabling restricted gene expression are still lacking for many tissues.

RESULTS

We first generated the hoxb1a(β-globin):eGFP(um8) line that expresses eGFP in hindbrain rhombomere 4 (r4), as well as in facial motor neurons migrating caudally from r4. Second, we generated the hoxb1a(β-globin) Gal4VP16(um60) line to express the exogenous Gal4VP16 transcription factor in r4. Lastly, we prepared the UAS(β-actin):hoxa3a(um61) line where the hoxa3a gene, which is normally expressed in r5 and r6, is under control of Gal4-regulated UAS elements. Crossing the hoxb1a(β-globin):Gal4VP16(um60) line to the UAS(β-actin):hoxa3a(um61) line drives robust hoxa3a expression in r4. We find that transgenic expression of hoxa3a in r4 does not affect hoxb1a expression, but has variable effects on migration of facial motorneurons and formation of Mauthner neurons. While cases of somatic transgene silencing have been reported in zebrafish, we have not observed such silencing to date, possibly because of our efforts to minimize repetitive sequences in the transgenic constructs.

CONCLUSION

We have generated three transgenic lines that will be useful for future studies by permitting the labeling of r4-derived cells, as well as by enabling r4-specific expression of various transgenes.

Pbx-dependent regulation of lbx gene expression in developing zebrafish embryos

Ladybird (Lbx) homeodomain transcription factors function in neural and muscle development--roles conserved from Drosophila to vertebrates. Lbx expression in mice specifies neural cell types, including dorsally located interneurons and association neurons, within the neural tube. Little, however, is known about the regulation of vertebrate lbx family genes. Here we describe the expression pattern of three zebrafish ladybird genes via mRNA in situ hybridization. Zebrafish lbx genes are expressed in distinct but overlapping regions within the developing neural tube, with strong expression within the hindbrain and spinal cord. The Hox family of transcription factors, in cooperation with cofactors such as Pbx and Meis, regulate hindbrain segmentation during embryogenesis. We have identified a novel regulatory interaction in which lbx1 genes are strongly downregulated in Pbx-depleted embryos. Further, we have produced a transgenic zebrafish line expressing dTomato and EGFP under the control of an lbx1b enhancer--a useful tool to acertain neuron location, migration, and morphology. Using this transgenic strain, we have identified a minimal neural lbx1b enhancer that contains key regulatory elements for expression of this transcription factor.

Planar polarity pathway and Nance-Horan syndrome-like 1b have essential cell-autonomous functions in neuronal migration

Components of the planar cell polarity (PCP) pathway are required for the caudal tangential migration of facial branchiomotor (FBM) neurons, but how PCP signaling regulates this migration is not understood. In a forward genetic screen, we identified a new gene, nhsl1b, required for FBM neuron migration. nhsl1b encodes a WAVE-homology domain-containing protein related to human Nance-Horan syndrome (NHS) protein and Drosophila GUK-holder (Gukh), which have been shown to interact with components of the WAVE regulatory complex that controls cytoskeletal dynamics and with the polarity protein Scribble, respectively. Nhsl1b localizes to FBM neuron membrane protrusions and interacts physically and genetically with Scrib to control FBM neuron migration. Using chimeric analysis, we show that FBM neurons have two modes of migration: one involving interactions between the neurons and their planar-polarized environment, and an alternative, collective mode involving interactions between the neurons themselves. We demonstrate that the first mode of migration requires the cell-autonomous functions of Nhsl1b and the PCP components Scrib and Vangl2 in addition to the non-autonomous functions of Scrib and Vangl2, which serve to polarize the epithelial cells in the environment of the migrating neurons. These results define a role for Nhsl1b as a neuronal effector of PCP signaling and indicate that proper FBM neuron migration is directly controlled by PCP signaling between the epithelium and the migrating neurons.

Zebrafish Tshz3b negatively regulates Hox function in the developing hindbrain

In flies, the zinc-finger protein Teashirt promotes trunk segmental identities, in part, by repressing the expression and function of anterior hox paralog group (PG) 1-4 genes that specify head fates. Anterior-posterior patterning of the vertebrate hindbrain also requires Hox PG 1-4 function, but the role of vertebrate teashirt-related genes in this process has not been investigated. In this work, we use overexpression and structure-function analyses to show that zebrafish tshz3b antagonizes Hox-dependent hindbrain segmentation. Ectopic Tshz3b perturbs the specification of rhombomere identities and leads to the caudal expansion of r1, the only rhombomere whose identity is specified independently of Hox function. This overexpression phenotype does not require the homeodomain and C-terminal zinc fingers that are unique to vertebrate Teashirt-related proteins, but does require that Tshz3b function as a repressor. Together, these results argue that the negative regulation of Hox PG 1-4 function is a conserved characteristic of Teashirt-related proteins.

Zebrafish Prickle1b mediates facial branchiomotor neuron migration via a farnesylation-dependent nuclear activity

The facial branchiomotor neurons (FBMNs) undergo a characteristic tangential migration in the vertebrate hindbrain. We previously used a morpholino knockdown approach to reveal that zebrafish prickle1b (pk1b) is required for this migration. Here we report that FBMN migration is also blocked in a pk1b mutant with a disruption in the consensus farnesylation motif. We confirmed that this lipid modification is required during FBMN migration by disrupting the function of farnesyl biosynthetic enzymes. Furthermore, farnesylation of a tagged Pk1b is required for its nuclear localization. Using a unique rescue approach, we have demonstrated that Pk1b nuclear localization and farnesylation are required during FBMN migration. Our data suggest that Pk1b acts at least partially independently of core planar cell polarity molecules at the plasma membrane, and might instead be acting at the nucleus. We also found that the neuronal transcriptional silencer REST is necessary for FBMN migration, and we provide evidence that interaction between Pk1b and REST is required during this process. Finally, we demonstrate that REST protein, which is normally localized in the nuclei of migrating FBMNs, is depleted from the nuclei of Pk1b-deficient neurons. We conclude that farnesylation-dependent nuclear localization of Pk1b is required to regulate REST localization and thus FBMN migration.

Multiple mechanisms mediate motor neuron migration in the zebrafish hindbrain

The transmembrane protein Van gogh-like 2 (Vangl2) is a component of the noncanonical Wnt/Planar Cell Polarity (PCP) signaling pathway, and is required for tangential migration of facial branchiomotor neurons (FBMNs) from rhombomere 4 (r4) to r5-r7 in the vertebrate hindbrain. Since vangl2 is expressed throughout the zebrafish hindbrain, it might also regulate motor neuron migration in other rhombomeres. We tested this hypothesis by examining whether migration of motor neurons out of r2 following ectopic hoxb1b expression was affected in vangl2(-) (trilobite) mutants. Hoxb1b specifies r4 identity, and when ectopically expressed transforms r2 to an "r4-like" compartment. Using time-lapse imaging, we show that GFP-expressing motor neurons in the r2/r3 region of a hoxb1b-overexpressing wild-type embryo migrate along the anterior-posterior (AP) axis. Furthermore, these cells express prickle1b (pk1b), a Wnt/PCP gene that is specifically expressed in FBMNs and is essential for their migration. Importantly, GFP-expressing motor neurons in the r2/r3 region of hoxb1b-overexpressing trilobite mutants and pk1b morphants often migrate, even though FBMNs in r4 of the same embryos fail to migrate longitudinally (tangentially) into r6 and r7. These observations suggest that tangentially migrating motor neurons in the anterior hindbrain (r1-r3) can use mechanisms that are independent of vangl2 and pk1b functions. Interestingly, analysis of tri; val double mutants also suggests a role for vangl2-independent factors in neuronal migration, since the valentino mutation partially suppresses the trilobite mutant migration defect. Together, the hoxb1b and val experiments suggest that multiple mechanisms regulate motor neuron migration along the AP axis of the zebrafish hindbrain.

The Hox cofactors Meis1 and Pbx act upstream of gata1 to regulate primitive hematopoiesis

During vertebrate development, the initial wave of hematopoiesis produces cells that help to shape the developing circulatory system and oxygenate the early embryo. The differentiation of primitive erythroid and myeloid cells occurs within a short transitory period, and is subject to precise molecular regulation by a hierarchical cascade of transcription factors. The TALE-class homeodomain transcription factors Meis and Pbx function to regulate embryonic hematopoiesis, but it is not known where Meis and Pbx proteins participate in the hematopoietic transcription factor cascade. To address these questions, we have ablated Meis1 and Pbx proteins in zebrafish, and characterized their molecular effects on known markers of primitive hematopoiesis. Embryos lacking Meis1 and Pbx exhibit a severe reduction in the expression of gata1, the earliest marker of erythroid cell fate, and fail to produce visible circulating blood cells. Concomitant with a loss of gata1, Meis1- and Pbx-depleted embryos exhibit downregulated embryonic hemoglobin (hbae3) expression, and possess increased numbers of pu.1-positive myeloid cells. gata1-overexpression rescues hbae3 expression in Pbx-depleted; meis1-morphant embryos, placing Pbx and Meis1 upstream of gata1 in the erythropoietic transcription factor hierarchy. Our study conclusively demonstrates that Meis1 and Pbx act to specify the erythropoietic cell lineage and inhibit myelopoiesis.

Roles of noncanonical Wnt/PCP pathway genes in neuronal migration and neurulation in zebrafish

Noncanonical Wnt/planar cell polarity (PCP) pathways regulate oriented cell movement during development in both Drosophila and vertebrates. Recent studies have revealed similarities and differences between these pathways in the tissues on which they act. In zebrafish, PCP pathway genes regulate the directional migration of a specific population of motor neurons in the hindbrain, as well as morphogenesis of the neuroepithelium. In the present review, neuronal and neuroepithelial defects in zebrafish PCP pathway mutants are compared, and the possible cellular and molecular mechanisms underlying these phenotypes are discussed. Future analyses of zebrafish PCP mutants will reveal the general mechanisms underlying the development of the neuroepithelium and provide novel insights into both conserved and diverse functions of PCP pathway genes in vertebrate development.

Chick Lrrn2, a novel downstream effector of Hoxb1 and Shh, functions in the selective targeting of rhombomere 4 motor neurons

BACKGROUND

Capricious is a Drosophila adhesion molecule that regulates specific targeting of a subset of motor neurons to their muscle target. We set out to identify whether one of its vertebrate homologues, Lrrn2, might play an analogous role in the chick.

RESULTS

We have shown that Lrrn2 is expressed from early development in the prospective rhombomere 4 (r4) of the chick hindbrain. Subsequently, its expression in the hindbrain becomes restricted to a specific group of motor neurons, the branchiomotor neurons of r4, and their pre-muscle target, the second branchial arch (BA2), along with other sites outside the hindbrain. Misexpression of the signalling molecule Sonic hedgehog (Shh) via in ovo electroporation results in upregulation of Lrrn2 exclusively in r4, while the combined expression of Hoxb1 and Shh is sufficient to induce ectopic Lrrn2 in r1/2. Misexpression of Lrrn2 in r2/3 results in axonal rerouting from the r2 exit point to the r4 exit point and BA2, suggesting a direct role in motor axon guidance.

CONCLUSION

Lrrn2 acts downstream of Hoxb1 and plays a role in the selective targeting of r4 motor neurons to BA2.

Restricted patterns of Hoxd10 and Hoxd11 set segmental differences in motoneuron subtype complement in the lumbosacral spinal cord

During normal vertebrate development, Hoxd10 and Hoxd11 are expressed by differentiating motoneurons in restricted patterns along the rostrocaudal axis of the lumbosacral (LS) spinal cord. To assess the roles of these genes in the attainment of motoneuron subtypes characteristic of LS subdomains, we examined subtype complement after overexpression of Hoxd10 or Hoxd11 in the embryonic chick LS cord and in a Hoxd10 loss-of-function mouse embryo. Data presented here provide evidence that Hoxd10 defines the position of the lateral motor column (LMC) as a whole and, in rostral LS segments, specifically promotes the development of motoneurons of the lateral subdivision of the lateral motor column (LMCl). In contrast, Hoxd11 appears to impart a caudal and medial LMC (LMCm) identity to some motoneurons and molecular profiles suggestive of a suppression of LMC development in others. We also provide evidence that Hoxd11 suppresses the expression of Hoxd10 and the retinoic acid synthetic enzyme, retinaldehyde dehydrogenase 2 (RALDH2). In a normal chick embryo, Hoxd10 and RALDH2 are expressed throughout the LS region at early stages of motoneuron differentiation but their levels decline in Hoxd11-expressing caudal LS segments that ultimately contain few LMCl motoneurons. We hypothesize that one of the roles played by Hoxd11 is to modulate Hoxd10 and local retinoic acid levels and thus, perhaps define the caudal boundaries of the LMC and its subtype complement.

Pbx3 is required for normal locomotion and dorsal horn development

The transcription cofactor Pbx3 is critical for the function of hindbrain circuits controlling respiration in mammals, but the perinatal lethality caused by constitutively null mutations has hampered investigation of other roles it may play in neural development and function. Here we report that the conditional loss of Pbx3 function in most tissues caudal to the hindbrain resulted in progressive deficits of posture, locomotion, and sensation that became apparent during adolescence. In adult mutants, the size of the dorsal horn of the spinal cord and the numbers of calbindin-, PKC-gamma, and calretinin-expressing neurons in laminae I-III were markedly reduced, but the ventral cord and peripheral nervous system appeared normal. In the embryonic dorsal horn, Pbx3 expression was restricted to a subset of glutamatergic neurons, but its absence did not affect the initial balance of excitatory and inhibitory interneuron phenotypes. By embryonic day 15 a subset of Meis(+) glutamatergic neurons assumed abnormally superficial positions and the number of calbindin(+) neurons was increased three-fold in the mutants. Loss of Pbx3 function thus leads to the incorrect specification of some glutamatergic neurons in the dorsal horn and alters the integration of peripheral sensation into the spinal circuitry regulating locomotion.

Zebrafish Hoxb1a regulates multiple downstream genes including prickle1b

Despite 30 years of Hox gene study, we have a remarkably limited knowledge of the downstream target genes that Hox transcription factors regulate to confer regional identity. Here, we have used a microarray approach to identify genes that function downstream of a single vertebrate Hox gene, zebrafish hoxb1a. This gene plays a critical and conserved role in vertebrate hindbrain development, conferring identity to hindbrain rhombomere (r) 4. For example, zebrafish Hoxb1a, similar to mouse Hoxb1, is required for the migration of r4-derived facial branchiomotor neurons into the posterior hindbrain. We have screened microarrays carrying more than 16,000 expressed sequence tags (ESTs) for genes that are differentially regulated in normal versus Hoxb1a-deficient r4 tissue. Using this approach, we have identified both positively and negatively regulated candidate Hoxb1a target genes. We have used in situ hybridization to validate twelve positively regulated Hoxb1a targets. These downstream targets are expressed in a variety of subdomains within r4, with one gene, a novel prickle homolog (pk1b), expressed specifically within the facial branchiomotor neurons. Using morpholino knock-down and cell transplantation, we demonstrate that the Hoxb1a target Prickle1b functions cell-autonomously to control facial neuron migration, a single aspect of r4 identity.

Pbx homeodomain proteins pattern both the zebrafish retina and tectum

BACKGROUND

Pbx genes encode TALE class homeodomain transcription factors that pattern the developing neural tube, pancreas, and blood. Within the hindbrain, Pbx cooperates with Hox proteins to regulate rhombomere segment identity. Pbx cooperates with Eng to regulate midbrain-hindbrain boundary maintenance, and with MyoD to control fast muscle cell differentiation. Although previous results have demonstrated that Pbx is required for proper eye size, functions in regulating retinal cell identity and patterning have not yet been examined.

RESULTS

Analysis of retinal ganglion cell axon pathfinding and outgrowth in pbx2/4 null embryos demonstrated a key role for pbx genes in regulating neural cell behavior. To identify Pbx-dependent genes involved in regulating retino-tectal pathfinding, we conducted a microarray screen for Pbx-dependent transcripts in zebrafish, and detected genes that are specifically expressed in the eye and tectum. A subset of Pbx-dependent retinal transcripts delineate specific domains in the dorso-temporal lobe of the developing retina. Furthermore, we determined that some Pbx-dependent transcripts also require Meis1 and Gdf6a function. Since gdf6a expression is also dependent on Pbx, we propose a model in which Pbx proteins regulate expression of the growth factor gdf6a, which in turn regulates patterning of the dorso-temporal lobe of the retina. This, in concert with aberrant tectal patterning in pbx2/4 null embryos, may lead to the observed defects in RGC outgrowth.

CONCLUSION

These data define a novel role for Pbx in patterning the vertebrate retina and tectum in a manner required for proper retinal ganglion cell axon outgrowth.

Modern mosaic analysis in the zebrafish

One of the most powerful tools used to gain insight into complex developmental processes is the analysis of mosaic embryos. A mosaic is defined as an organism that contains cells of more than one genotype, usually wild-type and mutant. It is the interplay between wild-type and mutant cells in the mosaic that reveals information about the normal function of the mutated gene. Mosaic analysis has been utilized extensively in Caenorhabditis elegans, Drosophila, mice, and zebrafish to elucidate when, where, and how a gene acts during development. In the zebrafish, mosaic analysis has been used to dissect a number of different developmental processes, including gastrulation movements, mesoderm and endoderm specification, neuronal patterning and migration, axon pathfinding, angiogenesis, and cardiac, retinal, and neural crest development. Mosaic analysis is a particularly effective method for understanding gene function in the zebrafish, a model organism particularly suited to forward genetic, molecular, and classical embryological approaches. These attributes, when combined with the accessibility and optical clarity of the zebrafish embryo, facilitate the real time observation of individual cell behaviors and interactions within mosaic embryos.

Co-factors and co-repressors of Engrailed: expression in the central nervous system and cerci of the cockroach, Periplaneta americana

In the larval cockroach (Periplaneta americana), knockout of Engrailed (En) in the medial sensory neurons of the cercal sensory system changes their axonal arborization and synaptic specificity. Immunocytochemistry has been used to investigate whether the co-repressor Groucho (Gro; vertebrate homolog: TLE) and the co-factor Extradenticle (Exd; vertebrate homolog: Pbx) are expressed in the cercal system. Gro/TLE is expressed ubiquitously in cell nuclei in the embryo, except for the distal pleuropodia. Gro is expressed in all nuclei of the thoracic and abdominal central nervous system (CNS) of first instar larva, although some neurons express less Gro than others. Cercal sensory neurons express Gro protein, which might therefore act as a co-repressor with En. Exd/Pbx is expressed in the proximal portion of all segmental appendages in the embryo, with the exception of the cerci. In the first instar CNS, Exd protein is expressed in subsets of neurons (including dorsal unpaired medial neurons) in the thoracic ganglia, in the first two abdominal ganglia, and in neuromeres A8-A11 of the terminal ganglion. Exd is absent from the cerci. Because Ultrabithorax/Abdominal-A (Ubx/Abd-A) can substitute for Exd as En co-factors in Drosophila, Ubx/Abd-A immunoreactivity has also been investigated. Ubx/Abd-A immunostaining is present in abdominal segments of the embryo and first instar CNS as far caudal as A7 and faintly in the T3 segment. However, Ubx/Abd-A is absent in the cerci and their neurons. Thus, in contrast to its role in Drosophila segmentation, En does not require the co-factors Exd or Ubx/Abd-A in order to control the synaptic specificity of cockroach sensory neurons.

Conserved co-regulation and promoter sharing of hoxb3a and hoxb4a in zebrafish

The expression of zebrafish hoxb3a and hoxb4a has been found to be mediated through five transcripts, hoxb3a transcripts I-III and hoxb4a transcripts I-II, driven by four promoters. A "master" promoter, located about 2 kb downstream of hoxb5a, controls transcription of a pre-mRNA comprising exon sequences of both genes. This unique gene structure is proposed to provide a novel mechanism to ensure overlapping, tissue-specific expression of both genes in the posterior hindbrain and spinal cord. Transgenic approaches were used to analyze the functions of zebrafish hoxb3a/hoxb4a promoters and enhancer sequences containing regions of homology that were previously identified by comparative genomics. Two neural enhancers were shown to establish specific anterior expression borders within the hindbrain and mediate expression in defined neuronal populations derived from hindbrain rhombomeres (r) 5 to 8, suggesting a late role of the genes in neuronal cell lineage specification. Species comparison showed that the zebrafish hoxb3a r5 and r6 enhancer corresponded to a sequence within the mouse HoxA cluster controlling activity of Hoxa3 in r5 and r6, whereas a homologous region within the HoxB cluster activated Hoxb3 expression but limited to r5. We conclude that the similarity of hoxb3a/Hoxa3 regulatory mechanisms reflect the shared descent of both genes from a single ancestral paralog group 3 gene.

Hox cofactors in vertebrate development

Hox genes encode homeodomain-containing transcription factors that pattern the body axes of animal embryos. It is well established that the exquisite DNA-binding specificity that allows different Hox proteins to specify distinct structures along the body axis is frequently dependent on interactions with other DNA-binding proteins which act as Hox cofactors. These include the PBC and MEIS classes of TALE (Three Amino acid Loop Extension) homeodomain proteins. The PBC class comprises fly Extradenticle (Exd) and vertebrate Pbx homeoproteins, whereas the MEIS class includes fly Homothorax (Hth) and vertebrate Meis and Prep homeoproteins. Exd was first implicated as a Hox cofactor based on mutant phenotypes in the fly. In vertebrates, PBC and MEIS homeobox proteins play important roles in development and disease. In this review, we describe the evidence that these functions reflect a requirement for Pbx and Meis/Prep proteins as Hox cofactors. However, there is mounting evidence that, like in the fly, Pbx and Meis/Prep proteins function more broadly, and we also discuss how "Hox cofactors" function as partners for other, non-Hox transcription factors during development. Conversely, we review the evidence that Hox proteins have functions that are independent of Pbx and Meis/Prep cofactors and discuss the possibility that other proteins may participate in the DNA-bound Hox complex, contributing to DNA-binding specificity in the absence of, or in addition to, Pbx and Meis/Prep.

Zebrafish foggy/spt 5 is required for migration of facial branchiomotor neurons but not for their survival

Transcript elongation is a critical step in the production of mature messenger RNAs. Many factors have been identified that are required for transcript elongation, including Spt 5. Studies in yeast determined that spt 5 is required for cell viability, and analyses in Drosophila indicate Spt 5 is localized to sites of active transcription, suggesting it is required generally for transcription. However, the requirement for spt 5 for cell viability in a metazoan organism has not been addressed. We determined that zebrafish foggy/spt 5 is required cell-autonomously for the posterior migration of facial branchiomotor neurons from rhombomere 4 (r4) into r6 and r7 of the hindbrain. These genetic mosaics also give us the unique opportunity to determine whether spt 5 is required for mRNA transcription equivalently at all loci by addressing two processes within the same cell-neuronal migration and cell viability. In a wild-type host, spt 5 null facial branchiomotor neurons survive to at least 5 days postfertilization while failing to migrate posteriorly. This finding indicates that spt 5-dependent transcript elongation is required cell-autonomously for a complex cell migration but not for the survival of these same cells. This work provides evidence that transcript elongation is not a global mechanism equivalently required by all loci and may actually be under more strict developmental regulation.

Neuronal development and migration in zebrafish hindbrain explants

The zebrafish embryo is an excellent system for studying dynamic processes such as cell migration during vertebrate development. Dynamic analysis of neuronal migration in the zebrafish hindbrain has been hampered by morphogenetic movements in vivo, and by the impermeability of embryos. We have applied a recently reported technique of embryo explant culture to the analysis of neuronal development and migration in the zebrafish hindbrain. We show that hindbrain explants prepared at the somitogenesis stage undergo normal morphogenesis for at least 14 h in culture. Importantly, several aspects of hindbrain development such as patterning, neurogenesis, axon guidance, and neuronal migration are largely unaffected, inspite of increased cell death in explanted tissue. These results suggest that hindbrain explant culture can be employed effectively in zebrafish to analyze neuronal migration and other dynamic processes using pharmacological and imaging techniques.

Regulation of Latent Sensory Hair Cell Precursors by Glia in the Zebrafish Lateral Line

The lateral line is a placodally derived mechanosensory organ in anamniotes that detects the movement of water. In zebrafish embryos, a migrating primordium deposits seven to nine clusters of sensory hair cells, or neuromasts, at intervals along the trunk. Postembryonically, neuromasts continue to be added. We show that some secondary neuromasts arise from a pool of latent precursors that are deposited by the primordium between primary neuromasts. Interneuromast cells lie adjacent to the lateral line nerve and associated glia. These cells remain quiescent while they are juxtaposed with the glia; however, when they move away from the nerve they increase proliferation and form neuromasts. If glia are manually removed or genetically ablated by mutations in cls/sox10, hypersensitive (hps), or rowgain (rog), neuromasts precociously differentiate. Transplantation of wt glia into mutants rescues the appropriate temporal differentiation of interneuromast cells. Our studies reveal a role for glia in regulating sensory hair cell precursors.

moz regulates Hox expression and pharyngeal segmental identity in zebrafish

In vertebrate embryos, streams of cranial neural crest (CNC) cells migrate to form segmental pharyngeal arches and differentiate into segment-specific parts of the facial skeleton. To identify genes involved in specifying segmental identity in the vertebrate head, we screened for mutations affecting cartilage patterning in the zebrafish larval pharynx. We present the positional cloning and initial phenotypic characterization of a homeotic locus discovered in this screen. We show that a zebrafish ortholog of the human oncogenic histone acetyltransferase MOZ (monocytic leukemia zinc finger) is required for specifying segmental identity in the second through fourth pharyngeal arches. In moz mutant zebrafish, the second pharyngeal arch is dramatically transformed into a mirror-image duplicated jaw. This phenotype resembles a similar but stronger transformation than that seen in hox2 morpholino oligo (hox2-MO) injected animals. In addition, mild anterior homeotic transformations are seen in the third and fourth pharyngeal arches of moz mutants. moz is required for maintenance of most hox1-4 expression domains and this requirement probably at least partially accounts for the moz mutant homeotic phenotypes. Homeosis and defective Hox gene expression in moz mutants is rescued by inhibiting histone deacetylase activity with Trichostatin A. Although we find early patterning of the moz mutant hindbrain to be normal, we find a late defect in facial motoneuron migration in moz mutants. Pharyngeal musculature is transformed late, but not early, in moz mutants. We detect relatively minor defects in arch epithelia of moz mutants. Vital labeling of arch development reveals no detectable changes in CNC generation in moz mutants, but later prechondrogenic condensations are mispositioned and misshapen. Mirror-image hox2-dependent gene expression changes in postmigratory CNC prefigure the homeotic phenotype in moz mutants. Early second arch ventral expression of goosecoid (gsc) in moz mutants and in animals injected with hox2-MOs shifts from lateral to medial, mirroring the first arch pattern. bapx1, which is normally expressed in first arch postmigratory CNC prefiguring the jaw joint, is ectopically expressed in second arch CNC of moz mutants and hox2-MO injected animals. Reduction of bapx1 function in wild types causes loss of the jaw joint. Reduction of bapx1 function in moz mutants causes loss of both first and second arch joints, providing functional genetic evidence that bapx1 contributes to the moz-deficient homeotic pattern. Together, our results reveal an essential embryonic role and a crucial histone acetyltransferase activity for Moz in regulating Hox expression and segmental identity, and provide two early targets, bapx1 and gsc, of moz and hox2 signaling in the second pharyngeal arch.

Integration of anteroposterior and dorsoventral regulation of Phox2b transcription in cranial motoneuron progenitors by homeodomain proteins

Little is known about the molecular mechanisms that integrate anteroposterior (AP) and dorsoventral (DV) positional information in neural progenitors that specify distinct neuronal types within the vertebrate neural tube. We have previously shown that in ventral rhombomere (r)4 of Hoxb1 and Hoxb2 mutant mouse embryos, Phox2bexpression is not properly maintained in the visceral motoneuron progenitor domain (pMNv), resulting in a switch to serotonergic fate. Here, we show that Phox2b is a direct target of Hoxb1 and Hoxb2. We found a highly conserved Phox2b proximal enhancer that mediates rhombomere-restricted expression and contains separate Pbx-Hox (PH) and Prep/Meis (P/M) binding sites. We further show that both the PH and P/M sites are essential for Hox-Pbx-Prep ternary complex formation and regulation of the Phox2b enhancer activity in ventral r4. Moreover, the DV factor Nkx2.2 enhances Hox-mediated transactivation via a derepression mechanism. Finally, we show that induction of ectopic Phox2b-expressing visceral motoneurons in the chick hindbrain requires the combined activities of Hox and Nkx2 homeodomain proteins. This study takes an important first step to understand how activators and repressors, induced along the AP and DV axes in response to signaling pathways, interact to regulate specific target gene promoters, leading to neuronal fate specification in the appropriate developmental context.

Retinoic acid influences anteroposterior positioning of epidermal sensory neurons and their gene expression in a developing chordate (amphioxus)

In developing chordates, retinoic acid (RA) signaling patterns the rostrocaudal body axis globally and affects gene expression locally in some differentiating cell populations. Here we focus on development of epidermal sensory neurons in an invertebrate chordate (amphioxus) to determine how RA signaling influences their rostrocaudal distribution and gene expression (for AmphiCoe, a neural precursor gene; for amphioxus islet and AmphiERR, two neural differentiation genes; and for AmphiHox1, -3, -4, and -6). Treatments with RA or an RA antagonist (BMS009) shift the distribution of developing epidermal neurons anteriorly or posteriorly, respectively. These treatments also affect gene expression patterns in the epidermal neurons, suggesting that RA levels may influence specification of neuronal subtypes. Although colinear expression of Hox genes is well known for the amphioxus central nervous system, we find an unexpected comparable colinearity for AmphiHox1, -3, -4, and -6 in the developing epidermis; moreover, RA levels affect the anteroposterior extent of these Hox expression domains, suggesting that RA signaling controls a colinear Hox code for anteroposterior patterning of the amphioxus epidermis. Thus, in amphioxus, the developing peripheral nervous system appears to be structured by mechanisms parallel to those that structure the central nervous system. One can speculate that, during evolution, an ancestral deuterostome that structured its panepidermal nervous system with an RA-influenced Hox code gave rise to chordates in which this patterning mechanism persisted within the epidermal elements of the peripheral nervous system and was transferred to the neuroectoderm as the central nervous system condensed dorsally.

Hoxb1 functions in both motoneurons and in tissues of the periphery to establish and maintain the proper neuronal circuitry

Formation of neuronal circuits in the head requires the coordinated development of neurons within the central nervous system (CNS) and neural crest-derived peripheral target tissues. Hoxb1, which is expressed throughout rhombomere 4 (r4), has been shown to be required for the specification of facial branchiomotor neuron progenitors that are programmed to innervate the muscles of facial expression. In this study, we have uncovered additional roles for Hoxb1-expressing cells in the formation and maintenance of the VIIth cranial nerve circuitry. By conditionally deleting the Hoxb1 locus in neural crest, we demonstrate that Hoxb1 is also required in r4-derived neural crest to facilitate and maintain formation of the VIIth nerve circuitry. Genetic lineage analysis revealed that a significant population of r4-derived neural crest is fated to generate glia that myelinate the VIIth cranial nerve. Neural crest cultures show that the absence of Hoxb1 function does not appear to affect overall glial progenitor specification, suggesting that a later glial function is critical for maintenance of the VIIth nerve. Taken together, these results suggest that the molecular program governing the development and maintenance of the VIIth cranial nerve is dependent upon Hoxb1, both in the neural crest-derived glia and in the facial branchiomotor neurons.

Comparative genomic analysis of vertebrate Hox3 and Hox4 genes

We used a comparative genomic approach to identify putative cis-acting regulatory sequences of the zebrafish hoxb3a and hoxb4a genes. We aligned genomic sequences spanning the clustered Hoxb1 to Hoxb5 genes from pufferfish, mice, and humans with the zebrafish hoxba and hoxbb cluster sequences. We identified multiple blocks of conserved sequences in non-coding regions within and surrounding the Hoxb3/b4 gene locus; a subset of these blocks are conserved in the zebrafish hoxbb cluster, despite loss of hoxb3/b4 genes. Overall, we find that the architecture of the Hoxb3/b4 loci and of the conserved sequence elements is very similar in teleosts and mammals. Our analyses also revealed two alternative transcripts of the zebrafish hoxb3a gene and an exon sequence unusually located 10 kb upstream of adjacent hoxb4a; an equivalent murine Hoxb3 exon has not yet been confirmed. We show that many of the Hoxb3/b4 conserved non-coding sequences correlate with functional neural enhancers previously described in the mouse. Further, within the conserved non-coding sequences we have identified binding sites for transcription factors, including Kreisler/Valentino, Krox20, Hox, and Pbx, some of which had not been previously described for the mouse. Finally, we demonstrate that the regulatory sequences of zebrafish hoxa3a are divergent with respect to the mouse ortholog Hoxa3, or the paralog hoxb3a. Despite limited conservation of regulatory sequences, zebrafish hoxa3a and hoxb3a genes share very similar expression profiles.

Prep1.1 has essential genetic functions in hindbrain development and cranial neural crest cell differentiation

In this study we analysed the function of the Meinox gene prep1.1 during zebrafish development. Meinox proteins form heterotrimeric complexes with Hox and Pbx members, increasing the DNA binding specificity of Hox proteins in vitro and in vivo. However, a role for a specific Meinox protein in the regulation of Hox activity in vivo has not been demonstrated. In situ hybridization showed that prep1.1 is expressed maternally and ubiquitously up to 24 hours post-fertilization (hpf), and restricted to the head from 48 hpf onwards. Morpholino-induced prep1.1 loss-of-function caused significant apoptosis in the CNS. Hindbrain segmentation and patterning was affected severely, as revealed by either loss or defective expression of several hindbrain markers (foxb1.2/mariposa, krox20, pax2.1 and pax6.1), including anteriorly expressed Hox genes (hoxb1a, hoxa2 and hoxb2), the impaired migration of facial nerve motor neurons, and the lack of reticulospinal neurons (RSNs) except Mauthner cells. Furthermore, the heads of prep1.1 morphants lacked all pharyngeal cartilages. This was not caused by the absence of neural crest cells or their impaired migration into the pharyngeal arches, as shown by expression of dlx2 and snail1, but by the inability of these cells to differentiate into chondroblasts. Our results indicate that prep1.1 has a unique genetic function in craniofacial chondrogenesis and, acting as a member of Meinox-Pbc-Hox trimers, it plays an essential role in hindbrain development.

Turning heads: development of vertebrate branchiomotor neurons

The cranial motor neurons innervate muscles that control eye, jaw, and facial movements of the vertebrate head and parasympathetic neurons that innervate certain glands and organs. These efferent neurons develop at characteristic locations in the brainstem, and their axons exit the neural tube in well-defined trajectories to innervate target tissues. This review is focused on a subset of cranial motor neurons called the branchiomotor neurons, which innervate muscles derived from the branchial (pharyngeal) arches. First, the organization of the branchiomotor pathways in zebrafish, chick, and mouse embryos will be compared, and the underlying axon guidance mechanisms will be addressed. Next, the molecular mechanisms that generate branchiomotor neurons and specify their identities will be discussed. Finally, the caudally directed or tangential migration of facial branchiomotor neurons will be examined. Given the advances in the characterization and analysis of vertebrate genomes, we can expect rapid progress in elucidating the cellular and molecular mechanisms underlying the development of these vital neuronal networks. Developmental Dynamics 229:143-161, 2004.

Motor neuron columnar fate imposed by sequential phases of Hox-c activity

The organization of neurons into columns is a prominent feature of central nervous system structure and function. In many regions of the central nervous system the grouping of neurons into columns links cell-body position to axonal trajectory, thus contributing to the establishment of topographic neural maps. This link is prominent in the developing spinal cord, where columnar sets of motor neurons innervate distinct targets in the periphery. We show here that sequential phases of Hox-c protein expression and activity control the columnar differentiation of spinal motor neurons. Hox expression in neural progenitors is established by graded fibroblast growth factor signalling and translated into a distinct motor neuron Hox pattern. Motor neuron columnar fate then emerges through cell autonomous repressor and activator functions of Hox proteins. Hox proteins also direct the expression of genes that establish motor topographic projections, thus implicating Hox proteins as critical determinants of spinal motor neuron identity and organization.

Nkx6.1 controls migration and axon pathfinding of cranial branchio-motoneurons

As many studies have focused on the mechanisms of motoneuron specification,little is known about the factors that control the subsequent development of postmitotic motoneurons. Previously, we showed that the transcription factor Nkx6.1 is required for the early specification of somatic motoneuron progenitors in the spinal cord. Our present analysis of hindbrain motoneuron development in Nkx6.1-deficient mouse embryos reveals that the early specification of branchio-motoneurons is independent of Nkx6.1 function, but that it is required for their subsequent development. In Nkx6.1mutant mice, we observed defects in the migration, as well as in the axon projections of branchio-motoneurons. A detailed analysis of the migratory defect in facial branchio-motoneurons reveals ectopic expression of the cell surface receptors Ret and Unc5h3 in premigratory neurons,but no changes in the rhombomeric environment. Taken together, our findings demonstrate a requirement for Nkx6.1 in the development of postmitotic motoneurons, and suggest a cell-autonomous function in the control of branchio-motoneuron migration.

Cranial skeletal biology

To artists, the face is a mirror of the soul. To biologists, the face reflects remarkable structural diversity--think of bulldogs and wolfhounds or galapagos finches. How do such variations in skeletal form arise? Do the same mechanisms control skeletogenesis elsewhere in the body? The answers lie in the molecular machinery that generates neural crest cells, controls their migration, and guides their differentiation to cartilage and bone.