Hoxa1 and Hoxb1 synergize in patterning the hindbrain, cranial nerves and second pharyngeal arch

Craniofacial development in marsupial mammals: Developmental origins of evolutionary change

Biologists have long studied the evolutionary consequences of the differences in reproductive and life history strategies of marsupial and eutherian mammals. Over the past few decades, the impact of these strategies on the development of the marsupial embryo and neonate has received attention. In this review, the differences in development in the craniofacial region in marsupial and eutherian mammals will be discussed. The review will highlight differences at the organogenic and cellular levels, and discuss hypotheses for shifts in the expression of important regulatory genes. The major difference in the organogenic period is a whole-scale shift in the relative timing of central nervous system structures, in particular those of the forebrain, which are delayed in marsupials, relative to the structures of the oral-facial apparatus. Correlated with the delay in development of nervous system structures, the ossification of the bones of the neurocranium are delayed, while those of the face are accelerated. This study will also review work showing that the neural crest, which provides much of the cellular material to the facial skeleton and may also carry important patterning information, is notably accelerated in its development in marsupials. Potential consequences of these observations for hypotheses on constraint, evolutionary integration, and the existence of developmental modules is discussed. Finally, the implications of these results for hypotheses on the genetic modulation of craniofacial patterning are presented.

Distinct roles for hindbrain and paraxial mesoderm in the induction and patterning of the inner ear revealed by a study of vitamin-A-deficient quail

The hindbrain and cranial paraxial mesoderm have been implicated in the induction and patterning of the inner ear, but the precise role of the two tissues in these processes is still not clear. We have addressed these questions using the vitamin-A-deficient (VAD) quail model, in which VAD embryos lack the posterior half of the hindbrain that normally lies next to the inner ear. Using a battery of molecular markers, we show that the anlagen of the inner ear, the otic placode, is induced in VAD embryos in the absence of the posterior hindbrain. By performing grafting and ablation experiments in chick embryos, we also show that cranial paraxial mesoderm which normally lies beneath the presumptive otic placode is necessary for otic placode induction and that paraxial mesoderm from other locations cannot induce the otic placode. Two members of the fibroblast growth factor family, FGF3 and FGF19, continue to be expressed in this mesodermal population in VAD embryos, and these may be responsible for otic placode induction in the absence of the posterior hindbrain. Although the posterior hindbrain is not required for otic placode induction in VAD embryos, the subsequent patterning of the inner ear is severely disrupted. Several regional markers of the inner ear, such as Pax2, EphA4, SOHo1 and Wnt3a, are incorrectly expressed in VAD otocysts, and the sensory patches and vestibulo-acoustic ganglia are either greatly reduced or absent. Exogenous application of retinoic acid prior to 30 h of development is able rescue the VAD phenotype. By performing such rescue experiments before and after 30 h of development, we show that the inner ear defects of VAD embryos correlate with the absence of the posterior hindbrain. These results show that induction and patterning of the inner ear are governed by separate developmental processes that can be experimentally uncoupled from each other.

A Hox Regulatory Network Establishes Motor Neuron Pool Identity and Target-Muscle Connectivity

Spinal motor neurons acquire specialized "pool" identities that determine their ability to form selective connections with target muscles in the limb, but the molecular basis of this striking example of neuronal specificity has remained unclear. We show here that a Hox transcriptional regulatory network specifies motor neuron pool identity and connectivity. Two interdependent sets of Hox regulatory interactions operate within motor neurons, one assigning rostrocaudal motor pool position and a second directing motor pool diversity at a single segmental level. This Hox regulatory network directs the downstream transcriptional identity of motor neuron pools and defines the pattern of target-muscle connectivity.

Dynamic expression of COUP-TFI and COUP-TFII during development and functional maturation of the mouse inner ear

COUP-TFs (chicken ovalbumin upstream promoter-transcription factors) are orphan members of the nuclear receptor superfamily of ligand-activated receptors for which their ligands have not been identified. The two mammalian proteins, COUP-TFI and COUP-TFII, share 80% sequence identity and regulate many aspects of mammalian development and differentiation. In this report, we systemically examined the temporal and spatial expression of COUP-TFI and COUP-TFII transcripts and, for the first time, their protein during development and functional maturation of the cochlea. Both COUP-TFI and COUP-TFII were expressed early in the developing otic vesicle. COUP-TFI expression correlated with the differentiation of hair cells and support cells in the organ of Corti, whereas COUP-TFII expression was down-regulated with differentiation of the organ of Corti. Furthermore, we report for the first time, that the generally nuclear COUP-TFI receptor protein was localized in the cytoplasm of maturing hair cells and pillar cells. Collectively, although COUP-TFI and COUP-TFII are homologues, the expression of each orphan receptor has a restricted and dynamic expression during cochlea development particularly during patterning and differentiation of the cochlear structures.

Homozygous HOXA1 mutations disrupt human brainstem, inner ear, cardiovascular and cognitive development

We identified homozygous truncating mutations in HOXA1 in three genetically isolated human populations. The resulting phenotype includes horizontal gaze abnormalities, deafness, facial weakness, hypoventilation, vascular malformations of the internal carotid arteries and cardiac outflow tract, mental retardation and autism spectrum disorder. This is the first report to our knowledge of viable homozygous truncating mutations in any human HOX gene and of a mendelian disorder resulting from mutations in a human HOX gene critical for development of the central nervous system.

Insights into the urbilaterian brain: conserved genetic patterning mechanisms in insect and vertebrate brain development

Recent molecular genetic analyses of Drosophila melanogaster and mouse central nervous system (CNS) development revealed strikingly similar genetic patterning mechanisms in the formation of the insect and vertebrate brain. Thus, in both insects and vertebrates, the correct regionalization and neuronal identity of the anterior brain anlage is controlled by the cephalic gap genes otd/Otx and ems/Emx, whereas members of the Hox genes are involved in patterning of the posterior brain. A third intermediate domain on the anteroposterior axis of the vertebrate and insect brain is characterized by the expression of the Pax2/5/8 orthologues, suggesting that the tripartite ground plans of the protostome and deuterostome brains share a common evolutionary origin. Furthermore, cross-phylum rescue experiments demonstrate that insect and mammalian members of the otd/Otx and ems/Emx gene families can functionally replace each other in embryonic brain patterning. Homologous genes involved in dorsoventral regionalization of the CNS in vertebrates and insects show remarkably similar patterning and orientation with respect to the neurogenic region (ventral in insects and dorsal in vertebrates). This supports the notion that a dorsoventral body axis inversion occurred after the separation of protostome and deuterostome lineages in evolution. Taken together, these findings demonstrate conserved genetic patterning mechanisms in insect and vertebrate brain development and suggest a monophyletic origin of the brain in protostome and deuterostome bilaterians.

Variable Meis-dependence among paralog group-1 Hox proteins

Optimal function of Hox transcription factors may require Meis and Pbx cofactors. Here we test the in vivo Meis-dependence of two zebrafish paralog group-1 (PG1) Hox proteins. Misexpression of Hoxb1a induces ectopic gene expression throughout the anterior nervous system, while Hoxb1b induces ectopic expression primarily in hindbrain rhombomere 2. These activities are drastically reduced when endogenous Meis function is disrupted, demonstrating that both proteins are Meis-dependent. Upon addition of Meis3, Hoxb1b mimics the more severe Hoxb1a phenotype, indicating that Hoxb1b requires higher Meis levels than Hoxb1a. Using chimeric proteins we map this difference to the N-terminus, which contains the transcription activation domain. Lastly, we demonstrate strong genetic interactions between meis and PG1 hox genes, as well as between meis and pbx genes, in vivo. Our results are consistent with PG1 hox genes requiring pbx and meis function in vivo and reveal that different Hox proteins have distinct Meis requirements.

Hox genes and their candidate downstream targets in the developing central nervous system

1. Homeobox (Hox) genes were originally discovered in the fruit fly Drosophila, where they function through a conserved homeodomain as transcriptional regulators to control embryonic morphogenesis. Since then over 1000 homeodomain proteins have been identified in several species. In vertebrates, 39 Hox genes have been identified as homologs of the original Drosophila complex, and like their Drosophila counterparts they are organized within chromosomal clusters. Vertebrate Hox genes have also been shown to play a critical role in embryonic development as transcriptional regulators. 2. Both the Drosophila and vertebrate Hox genes have been shown to interact with various cofactors, such as the TALE homeodomain proteins, in recognition of consensus sequences within regulatory elements of their target genes. These protein-protein interactions are believed to contribute to enhancing the specificity of target gene recognition in a cell-type or tissue- dependent manner. The regulatory activity of a particular Hox protein on a specific regulatory element is highly variable and dependent on its interacting partners within the transcriptional complex. 3. In vertebrates, Hox genes display spatially restricted patterns of expression within the developing CNS, both along the anterioposterior and dorsoventral axis of the embryo. Their restricted gene expression is suggestive of a regulatory role in patterning of the CNS, as well as in cell specification. Determining the precise function of individual Hox genes in CNS morphogenesis through classical mutational analyses is complicated due to functional redundancy between Hox genes. 4. Understanding the precise mechanisms through which Hox genes mediate embryonic morphogenesis requires the identification of their downstream target genes. Although Hox genes have been implicated in the regulation of several pathways, few target genes have been shown to be under their direct regulatory control. Development of methodologies used for the isolation of target genes and for the analysis of putative targets will be beneficial in establishing the genetic pathways controlled by Hox factors. 5. Within the developing CNS various cell adhesion molecules and signaling molecules have been identified as candidate downstream target genes of Hox proteins. These targets play a role in processes such as cell migration and differentiation, and are implicated in contributing to neuronal processes such as plasticity and/or specification. Hence, Hox genes not only play a role in patterning of the CNS during early development, but may also contribute to cell specification and identity.

Hmx homeobox gene function in inner ear and nervous system cell-type specification and development

The Hmx homeobox gene family is comprised of three members in mammals, Hmx1, Hmx2, and Hmx3, which are conserved across the animal kingdom and are part of the larger NKL clustered family of homeobox genes. Expression domains of Hmx genes in distantly related species such as Drosophila and mouse suggest an ancestral function in rostral central nervous system development. During vertebrate evolution, the Hmx genes appear to have been recruited into additional roles in inner ear morphogenesis and specification of vestibular inner ear sensory and supporting cell types. Being derived from a common ancestor, the vertebrate Hmx gene family is thus a strong candidate to investigate functional overlap versus the unique roles played by multiple genes belonging to the same family. The functions of Hmx2 and Hmx3 were investigated via directed gene mutagenesis and the primary regions where Hmx2 and Hmx3 exert their individual functions are consistent with their expression domains, such as the vestibule and uterus. Meanwhile, it is notable that some tissues where both Hmx2 and Hmx3 are extensively expressed were not severely affected in either of the Hmx2 or Hmx3 single mutant mice, suggesting a possible functional overlap existing between these two genes. Compound Hmx2 and Hmx3 double mutant mice showed more severe defects in the inner ear than those displayed by either single knockout. Furthermore, novel abnormalities in the hypothalamic-neuroendocrine system, which were never observed in either of the single mutant mice, confirmed a hypothesis that Hmx2 and Hmx3 also function redundantly to control embryonic development of the central nervous system.

Evolution of the brain developmental plan: Insights from agnathans

In vertebrate evolution, the brain exhibits both conserved and unique morphological features in each animal group. Thus, the molecular program of nervous system development is expected to have experienced various changes through evolution. In this review, we discuss recent data from the agnathan lamprey (jawless vertebrate) together with available information from amphioxus and speculate the sequence of changes during chordate evolution that have been brought into the brain developmental plan to yield the current variety of the gnathostome (jawed vertebrate) brains.

Crk-associated substrate (Cas) family member, NEDD9, is regulated in human neuroblastoma cells and in the embryonic hindbrain by all-trans retinoic acid

The vitamin A metabolite, all-trans retinoic acid (atRA), plays an essential role in vertebrate embryogenesis, including development of the nervous system. In the human neuroblastoma cell line, SH-SY5Y, atRA rapidly induces (within 4 hr) the expression of the Crk-associated substrate (Cas) family member, neural precursor cell-expressed, developmentally down-regulated gene 9 (NEDD9) also called the human enhancer of filamentation (HEF1). NEDD9 is expressed in the developing hindbrain (5-somite stage) in the presumptive rhombomeres 2, 3, and 5 before the onset of overt segmentation. Exposure of rat embryos to excess atRA at times ranging from E9.25 to E12 leads to altered NEDD9 expression in the developing hindbrain within 6 hr. NEDD9 expression is also perturbed in vitamin A-deficient embryos. A putative retinoic acid response element in the 5' region of the NEDD9 promoter binds specifically to a RXR/RAR heterodimer and forms a higher molecular weight complex upon addition of a retinoic acid receptor-specific antibody. Regulation of NEDD9 may be an important means whereby atRA promotes cell spreading and neurite outgrowth in SH-SY5Y human neuroblastoma cells, and NEDD9 represents a new downstream target of atRA and its receptors in the developing hindbrain.

Segmentation and compartition in the early avian hindbrain

For the comparative embryologists of the early 20th century, the segment-like bulges that appear transiently during the early stages of vertebrate hindbrain development were both the object of fascination and the subject of vigorous dispute. Conflicting views were held as to the significance of these 'rhombomeres' to brain development and their more general relevance to head evolution. Whether rhombomeres are inconsequential bumps in the embryonic brain or true segments-iterative or metameric units-has only recently been resolved. A number of studies using more modern techniques (such as immunohistochemistry, in situ hybridisation, axonal tracing, single cell labelling, heterotopic and orthotopic grafting, and the manipulation of gene expression by electroporation) have shown that the hindbrain has a truly metameric cellular organisation. The avian embryo has played a particularly prominent role in such studies by virtue of its large size and accessibility, its amenability to microsurgery, and its well-described anatomy. Furthermore, electrophysiological studies, also on avian embryos, have shown that segmentation of the parent neuroepithelium into rhombomeres plays a crucial part in establishing the functional organization of the hindbrain. Segmentation suggests the early allocation of defined sets of precursor cells and is therefore presumed to allow a specific identity for each successive segment to emerge from a common ground plan. This short review will focus on the contribution the avian embryo has made to our understanding of this fly-like region of the vertebrate brain, in respect of its morphology and neuronal architecture, the cellular and molecular mechanisms involved in establishing and maintaining the segments, and the molecular controls of segmental identity.

Differences in gene expression between wild type and Hoxa1 knockout embryonic stem cells after retinoic acid treatment or leukemia inhibitory factor (LIF) removal

Homeobox (Hox) genes encode a family of transcription factors that regulate embryonic patterning and organogenesis. In embryos, alterations of the normal pattern of Hox gene expression result in homeotic transformations and malformations. Disruption of the Hoxa1 gene, the most 3' member of the Hoxa cluster and a retinoic acid (RA) direct target gene, results in abnormal ossification of the skull, hindbrain, and inner ear deficiencies, and neonatal death. We have generated Hoxa1(-/-) embryonic stem (ES) cells (named Hoxa1-15) from Hoxa1(-/-) mutant blastocysts to study the Hoxa1 signaling pathway. We have characterized in detail these Hoxa1(-/-) ES cells by performing microarray analyses, and by this technique we have identified a number of putative Hoxa-1 target genes, including genes involved in bone development (e.g. Col1a1, Postn/Osf2, and the bone sialoprotein gene or BSP), genes that are expressed in the developing brain (e.g. Nnat, Wnt3a, BDNF, RhoB, and Gbx2), and genes involved in various cellular processes (e.g. M-RAS, Sox17, Cdkn2b, LamA1, Col4a1, Foxa2, Foxq1, Klf5, and Igf2). Cell proliferation assays and Northern blot analyses of a number of ES cell markers (e.g. Rex1, Oct3/4, Fgf4, and Bmp4) suggest that the Hoxa1 protein plays a role in the inhibition of cell proliferation by RA in ES cells. Additionally, Hoxa1(-/-) ES cells express high levels of various endodermal markers, including Gata4 and Dab2, and express much less Fgf5 after leukemia inhibitory factor (LIF) withdrawal. Finally, we propose a model in which the Hoxa1 protein mediates repression of endodermal differentiation while promoting expression of ectodermal and mesodermal characteristics.

Hush puppy: a new mouse mutant with pinna, ossicle, and inner ear defects

OBJECTIVES/HYPOTHESIS

Deafness can be associated with abnormalities of the pinna, ossicles, and cochlea. The authors studied a newly generated mouse mutant with pinna defects and asked whether these defects are associated with peripheral auditory or facial skeletal abnormalities, or both. Furthermore, the authors investigated where the mutation responsible for these defects was located in the mouse genome.

METHODS

The hearing of hush puppy mutants was assessed by Preyer reflex and electrophysiological measurement. The morphological features of their middle and inner ears were investigated by microdissection, paint-filling of the labyrinth, and scanning electron microscopy. Skeletal staining of skulls was performed to assess the craniofacial dimensions. Genome scanning was performed using microsatellite markers to localize the mutation to a chromosomal region.

RESULTS

Some hush puppy mutants showed early onset of hearing impairment. They had small, bat-like pinnae and normal malleus but abnormal incus and stapes. Some mutants had asymmetrical defects and showed reduced penetrance of the ear abnormalities. Paint-filling of newborns' inner ears revealed no morphological abnormality, although half of the mice studied were expected to carry the mutation. Reduced numbers of outer hair cells were demonstrated in mutants' cochlea on scanning electron microscopy. Skeletal staining showed that the mutants have significantly shorter snouts and mandibles. Genome scan revealed that the mutation lies on chromosome 8 between markers D8Mit58 and D8Mit289.

CONCLUSION

The study results indicate developmental problems of the first and second branchial arches and otocyst as a result of a single gene mutation. Similar defects are found in humans, and hush puppy provides a mouse model for investigation of such defects.

Hox code in embryos of Chinese soft-shelled turtlePelodiscus sinensis correlates with the evolutionary innovation in the turtle

Turtles have the most unusual body plan of the amniotes, with a dorsal shell consisting of modified ribs. Because this morphological change in the ribs can be described as an axial-level specific alteration, the evolution of the turtle carapace should depend on changes in the Hox code. To identify turtle-specific changes in developmental patterns, we cloned several Hox genes from the Chinese soft-shelled turtle, Pelodiscus sinensis, examined their expression patterns during embryogenesis, and compared them with those of chicken and mouse embryos. We detected possibly turtle-specific derived traits in Hoxc-6 expression, which is restricted to the paraxial part of the embryo; in the expression of Hoxa-5 and Hoxb-5, the transcripts of which were detected only at the cervical level; and in Hoxc-8 and Hoxa-7 expression, which is shifted anteriorly relative to that of the other two amniote groups. From the known functions of the Hox orthologs in model animals, these P. sinensis-specific changes apparently correlate with specializations in the turtle-specific body plan.

Specification and patterning of neural crest cells during craniofacial development

Craniofacial evolution is considered fundamental to the origin of vertebrates and central to this process was the formation of a migratory, multipotent cell population known as the neural crest. The number of cell types that arise from the neural crest is truly astonishing as is the number of tissues and organs to which the neural crest contributes. In addition to forming melanocytes as well as many neurons and glia in the peripheral nervous system, neural crest cells also contribute much of the cartilage, bone and connective tissue of the face. These multipotent migrating cells are capable of self renewing decisions and based upon these criteria are often considered stem cells or stem cell-like. Rapid advances in our understanding of neural crest cell patterning continue to shape our appreciation of the evolution of neural crest cells and their impact on vertebrate craniofacial morphogenesis.

Association of the transcriptional corepressor TIF1beta with heterochromatin protein 1 (HP1): an essential role for progression through differentiation

The transcriptional intermediary factor 1beta (TIF1beta) is a corepressor for KRAB-domain-containing zinc finger proteins and is believed to play essential roles in cell physiology by regulating chromatin organization at specific loci through association with chromatin remodeling and histone-modifying activities and recruitment of heterochromatin protein 1 (HP1) proteins. In this study, we have engineered a modified embryonal carcinoma F9 cell line (TIF1beta(HP1box/-)) expressing a mutated TIF1beta protein (TIF1beta(HP1box)) unable to interact with HP1 proteins. Phenotypic analysis of TIF1beta(HP1box/-) and TIF1beta(+/-) cells shows that TIF1beta-HP1 interaction is not required for differentiation of F9 cells into primitive endoderm-like (PrE) cells on retinoic acid (RA) treatment but is essential for further differentiation into parietal endoderm-like (PE) cells on addition of cAMP and for differentiation into visceral endoderm-like cells on treatment of vesicles with RA. Complementation experiments reveal that TIF1beta-HP1 interaction is essential only during a short window of time within early differentiating PrE cells to establish a selective transmittable competence to terminally differentiate on further cAMP inducing signal. Moreover, the expression of three endoderm-specific genes, GATA6, HNF4, and Dab2, is down-regulated in TIF1beta(HP1box/-) cells compared with wild-type cells during PrE differentiation. Collectively, these data demonstrate that the interaction between TIF1beta and HP1 proteins is essential for progression through differentiation by regulating the expression of endoderm differentiation master players.

Loss of function but no gain of function caused by amino acid substitutions in the hexapeptide of Hoxa1 in vivo

Homeodomain containing transcription factors of the Hox family play critical roles in patterning the anteroposterior embryonic body axis, as well as in controlling several steps of organogenesis. Several Hox proteins have been shown to cooperate with members of the Pbx family for the recognition and activation of identified target enhancers. Hox proteins contact Pbx via a conserved hexapeptide motif. Previous biochemical studies provided evidence that critical amino acid substitutions in the hexapeptide sequence of Hoxa1 abolish its interaction with Pbx. As a result, these substitutions also abolish Hoxa1 activity on known target enhancers in cellular models, suggesting that Hoxa1 activity relies on its capacity to interact with Pbx. Here, we show that mice with mutations in the Hoxa1 hexapeptide display hindbrain, cranial nerve, and skeletal defects highly reminiscent of those reported for the Hoxa1 loss of function. Since similar hexapeptide mutations in the mouse Hoxb8 and the Drosophila AbdA proteins result in activity modulation and gain of function, our data demonstrate that the functional importance of the hexapeptide in vivo differs according to the Hox proteins.

Paralog group 1 hox genes regulate rhombomere 5/6 expression of vhnf1, a repressor of rostral hindbrain fates, in a meis-dependent manner

The vertebrate hindbrain is segmented into an array of rhombomeres (r), but it remains to be fully understood how segmentation is achieved. Here we report that reducing meis function transforms the caudal hindbrain to an r4-like fate, and we exploit this experimental state to explore how r4 versus r5-r6 segments are set aside. We demonstrate that r4 transformation of the caudal hindbrain is mediated by paralog group 1 (PG1) hox genes and can be repressed by vhnf1, a gene expressed in r5-r6. We further find that vhnf1 expression is regulated by PG1 hox genes in a meis-dependent manner. This implies that PG1 hox genes not only induce r4 fates throughout the caudal hindbrain, but also induce expression of vhnf1, which then represses r4 fates in the future r5-r6. Our results further indicate that r4 transformation of the caudal hindbrain occurs at intermediate levels of meis function, while extensive removal of meis function produces a hindbrain completely devoid of segments, suggesting that different hox-dependent processes may have distinct meis requirements. Notably, reductions in the function of another Hox cofactor, pbx, have not been reported to transform the caudal hindbrain, suggesting that Meis and Pbx proteins may also function differently in their roles as Hox cofactors.

Transgenic mice ectopically expressing HOXA5 in the dorsal spinal cord show structural defects of the cervical spinal cord along with sensory and motor defects of the forelimb

Mutation of murine Hoxa5 has shown that HOXA5 controls lung, gastrointestinal tract and vertebrae development. Hoxa5 is also expressed in the spinal cord, yet no central nervous system phenotype has been described in Hoxa5 knockouts. To identify the role of Hoxa5 in spinal cord development, we developed transgenic mice that express HOXA5 in the dorsal spinal cord in the brachial region. Using HOXA5-specific antibodies, we show this expression pattern is ectopic as the endogenous protein is expressed only in the ventral spinal cord at this anterio-posterior level. This transgenic line (Hoxa5SV2) also displays forelimb-specific motor and sensory defects. Hoxa5SV2 transgenic mice cannot support their body weight in a forelimb hang, and forelimb strength is decreased. However, Rotarod performance was not impaired in Hoxa5SV2 mice. Hoxa5SV2 mice also show a delayed forelimb response to noxious heat, although hindlimb response time was normal. Administration of an analgesic significantly reduced the hang test defect and decreased the transgene effect on forelimb strength, indicating that pain pathways may be affected. The morphology of transgenic cervical (but not lumbar) spinal cord is highly aberrant. Nissl staining indicates superficial laminae of the dorsal horn are severely disrupted. The distribution of cells and axons immunoreactive for substance P, neurokinin-B, and their primary receptors were aberrant only in transgenic cervical spinal cord. Further, we see increased levels of apoptosis in transgenic spinal cord at embryonic day 13.5. Our evidence suggests apoptosis due to HOXA5 misexpression is a major cause of loss of superficial lamina cells in Hoxa5SV2 mice.

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.

Association between the HOXA1 A218G polymorphism and increased head circumference in patients with autism

BACKGROUND

The HOXA1 gene plays a major role in brainstem and cranial morphogenesis. The G allele of the HOXA1 A218G polymorphism has been previously found associated with autism.

METHODS

We performed case-control and family-based association analyses, contrasting 127 autistic patients with 174 ethnically matched controls, and assessing for allelic transmission disequilibrium in 189 complete trios.

RESULTS

A, and not G, alleles were associated with autism using both case-control (chi(2) = 8.96 and 5.71, 1 df, p <.005 and <.025 for genotypes and alleles, respectively), and family-based (transmission/disequilibrium test chi(2) = 8.80, 1 df, p <.005) association analyses. The head circumference of 31 patients carrying one or two copies of the G allele displayed significantly larger median values (95.0th vs. 82.5th percentile, p <.05) and dramatically reduced interindividual variability (p <.0001), compared with 166 patients carrying the A/A genotype.

CONCLUSIONS

The HOXA1 A218G polymorphism explains approximately 5% of the variance in the head circumference of autistic patients and represents to our knowledge the first known gene variant providing sizable contributions to cranial morphology. The disease specificity of this finding is currently being investigated. Nonreplications in genetic linkage/association studies could partly stem from the dyshomogeneous distribution of an endophenotype morphologically defined by cranial circumference.

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.

Neuroblast formation and patterning during early brain development inDrosophila

The Drosophila embryo provides a useful model system to study the mechanisms that lead to pattern and cell diversity in the central nervous system (CNS). The Drosophila CNS, which encompasses the brain and the ventral nerve cord, develops from a bilaterally symmetrical neuroectoderm, which gives rise to neural stem cells, called neuroblasts. The structure of the embryonic ventral nerve cord is relatively simple, consisting of a sequence of repeated segmental units (neuromeres), and the mechanisms controlling the formation and specification of the neuroblasts that form these neuromeres are quite well understood. Owing to the much higher complexity and hidden segmental organization of the brain, our understanding of its development is still rudimentary. Recent investigations on the expression and function of proneural genes, segmentation genes, dorsoventral-patterning genes and a number of other genes have provided new insight into the principles of neuroblast formation and patterning during embryonic development of the fly brain. Comparisons with the same processes in the trunk help us to understand what makes the brain different from the ventral nerve cord. Several parallels in early brain patterning between the fly and the vertebrate systems have become evident.

Molecular Genetic Advances in Semicircular Canal Abnormalities and Sensorineural Hearing Loss: A Report of 16 Cases

OBJECTIVES: The study goals were (1) to determine if the degree and pattern of semicircular canal dysmorphology and the presence or absence of a cochlea in patients with congenital sensorineural hearing loss predict audiologic outcome, severity, or the frequencies involved and (2) to review the recent advances in molecular genetics of the semicircular canals and correlate this information with audiologic and anatomic patterns seen in our series of patients

DESIGN AND SETTING: We conducted a retrospective study at a tertiary care center with a large otologic and cochlear implant service.

PATIENTS AND METHODS: The study population consisted of 16 patients with congenital sensorineural hearing loss in 28 congenitally malformed inner ears consisting of semicircular canal dysplasia or aplasia, with or without cochlear malformation. History, physical examination, computed tomography scans, and serial audiograms were reviewed. Factors analyzed included other phenotypic dysmorphology characteristic of syndromes, audiometric configuration, severity and type of hearing loss, and the presence of associated inner ear anomalies other than the vestibular system. An extensive review of the literature regarding molecular genetic factors in semicircular canal anomalies, with or without cochlear abnormalities, was performed.

RESULTS: Sixteen patients (31 ears) were identified with profound sensorineural hearing loss and semicircular canal abnormalities. Only 3 patients had known syndromes, although 4 patients had other congenital anomalies. Most radiographic detectable abnormalities were bilateral. Audiograms of the patients demonstrated pure tone averages between 90 and 100 dB in the affected ears with few exceptions. No correlation was found between type and severity of malformation of either the cochlea or semicircular canals with the severity of hearing loss. There was no stepwise progression of hearing loss increasing malformation severity. Seven of the 16 patients received cochlear implants. Of these 7, 3 patients had cochlear hypoplasia and 1 patient had a common cavity deformity. Audiologic follow-up on all 7 patients revealed improvement in both speech assessment threshold and pure tone average. Presence or absence of the cochlea was not a factor in outcome after cochlear implantation.

CONCLUSION: We have assembled the largest series of patients with semicircular canal dysmorphology, with or without various cochlear abnormalities. Our study failed to correlate the type and severity of semicircular canal malformation with any specific audiologic outcome. The variation in hearing loss severity and pattern even in patients with similar bony radiographic findings must be explained by other non-radiologically detectable defects, likely abnormalities in membranous labyrinthine development. New molecular genetic discoveries have linked specific genes to the development of certain inner ear structures in mice studies. The independent development of the individual semicircular canals in relation to the cochlea and vestibule and the variability in hearing loss suggest a more complex embryologic process than merely an arrest in development as previously thought. As genetic studies are extended into humans, we will likely be able to stratify these patients by molecular defect and severity of hearing loss. (Otolaryngol Head Neck Surg 2003;129:637-46.)

Hoxa10 and Hoxd10 coordinately regulate lumbar motor neuron patterning

The paralogous Hox genes Hoxa10 and Hoxd10 are expressed in overlapping domains in the developing lumbar spinal cord and surrounding mesoderm. Independent inactivation of these two genes alters the trajectory of spinal nerves and decreases the complement of motor neurons present in the lumbar spinal cord, whereas dual inactivation of these two genes has been shown to alter peripheral nerve growth and development in the mouse hindlimb. We have examined the organization and distribution of lumbar motor neurons in the spinal cords of Hoxa10/Hoxd10 double mutant animals. Double mutant animals have decreased numbers of lumbar motor neurons in both the medial and lateral motor columns. The anteroposterior position of the lumbar motor column is shifted caudally in double mutant animals, and the distribution of motor neurons is altered across individual spinal segments. Distinctions between classes of motor neurons based on positional specificity appear disrupted in double mutants. Double mutants also demonstrate abnormal spinal cord vasculature and altered kidney placement and size. Our observations suggest that Hoxa10 and Hoxd10 activity is required to specify the position of the lumbar motor column and to provide segmental specification and identity for the lumbar motor neurons.

The urbilaterian brain: developmental insights into the evolutionary origin of the brain in insects and vertebrates

Classical phylogenetic, neuroanatomical and neuroembryological studies propose an independent evolutionary origin of the brains of insects and vertebrates. Contrasting with this, data from three sets of molecular and genetic analyses indicate that the developmental program of brains of insects and vertebrates might be highly conserved and suggest a monophyletic origin of the brain of protostomes and deuterostomes. First, recent results of molecular phylogeny imply that none of the currently living animals correspond to evolutionary intermediates between protostomes and deuterostomes, thus making it impossible to infer the morphological organization of an ancestral bilaterian brain from living specimens. Second, recent molecular genetic evidence provides support for the body axis inversion hypothesis, which implies that a dorsoventral inversion of the body axis occurred in protostomes versus deuterostomes, leading to the inverted location of neurogenic regions in these animal groups. Third, recent developmental genetic analyses are uncovering the existence of structurally and functionally homologous genes that have comparable and interchangeable functions in early brain development in insect and vertebrate model systems. Thus, development of the anteriormost part of the embryonic brain in both insects and vertebrates depends upon the otd/Otx and ems/Emx genes; development of the posterior part of the embryonic brain in both insects and vertebrates involves homologous control genes of the Hox cluster. These findings, which demonstrate the conserved expression and function of key patterning genes involved in embryonic brain development in insects and vertebrates support the hypothesis that the brains of protostomes and deuterostomes are of monophyletic, urbilaterian origin.

Hoxb1 neural crest preferentially form glia of the PNS

The vertebrate cranial neural crest cells give rise to many complex derivatives of the head, neck, and face, including neuronal and glial cells that act in concert for proper development of the anterior-peripheral nervous system. Several genes have been implicated in the processes of neural crest specification, migration, and differentiation; among these are the hox gene clusters. To determine the fates of hox-expressing cranial neural crest, we describe the results of a genetic lineage analysis by using the Cre/loxP system to drive the activation of different ROSA26 reporter alleles under the regulation of the hoxb1 locus. By targeting the 3' untranslated region of the hoxb1 gene, we have preserved endogenous gene activity and have been able to accurately follow the fates of the cells derived from the hoxb1 expression domain. Emphasis was placed on identifying the cell and tissue types that arise from the rhombomere 4-derived neural crest. Our results demonstrate that, in addition to forming much of the cartilage, bones, and muscle of the ears and neck, a significant population of rhombomere 4-derived neural crest is fated to generate the glial component of the seventh cranial nerve.

Development of vestibular afferent projections into the hindbrain and their central targets

In contrast to most other sensory systems, hardly anything is known about the neuroanatomical development of central projections of primary vestibular neurons and how their second order target neurons develop. Recent data suggest that afferent projections may develop not unlike other sensory systems, forming first the overall projection by molecular means followed by an as yet unspecified phase of activity mediated refinement. The latter aspect has not been tested critically and most molecules that guide the initial projection are unknown. The molecular and topological origin of the vestibular and cochlear nucleus neurons is also only partially understood. Auditory and vestibular nuclei form from several rhombomeres and a given rhombomere can contribute to two or more auditory or vestibular nuclei. Rhombomere compartments develop as functional subdivisions from a single column that extends from the hindbrain to the spinal cord. Suggestions are provided for the molecular origin of these columns but data on specific mutants testing these proposals are not yet available. Overall, the functional significance of both overlapping and segregated projections are not yet fully experimentally explored in mammals. Such lack of details of the adult organization compromises future developmental analysis.

Decreased embryonic retinoic acid synthesis results in a DiGeorge syndrome phenotype in newborn mice

Retinoic acid (RA), the active derivative of vitamin A, is involved in various developmental and homeostatic processes. To define whether certain developmental events are particularly sensitive to a decrease in embryonic RA levels, we generated mice bearing a hypomorphic allele of the RA-synthesizing enzyme Raldh2. The resulting mutant mice, which die perinatally, exhibit the features of the human DiGeorge syndrome (DGS) with heart outflow tract septation defects and anomalies of the aortic arch-derived head and neck arteries, laryngeal-tracheal cartilage defects, and thymus/parathyroid aplasia or hypoplasia. Analysis of Raldh2 hypomorph embryos reveal selective defects of the posterior (third to sixth) branchial arches, including absence or hypoplasia of the corresponding aortic arches and pharyngeal pouches, and local down-regulation of RA-target genes. Thus, a decreased level of embryonic RA (through genetic and/or nutritional causes) could represent a major modifier of the expressivity of human 22q11del-associated DiGeorge/velocardiofacial syndromes and, if severe enough, could on its own lead to the clinical features of the DiGeorge syndrome.

Fgfr1 regulates patterning of the pharyngeal region

Development of the pharyngeal region depends on the interaction and integration of different cell populations, including surface ectoderm, foregut endoderm, paraxial mesoderm, and neural crest. Mice homozygous for a hypomorphic allele of Fgfr1 have craniofacial defects, some of which appeared to result from a failure in the early development of the second branchial arch. A stream of neural crest cells was found to originate from the rhombomere 4 region and migrate toward the second branchial arch in the mutants. Neural crest cells mostly failed to enter the second arch, however, but accumulated in a region proximal to it. Both rescue of the hypomorphic Fgfr1 allele and inactivation of a conditional Fgfr1 allele specifically in neural crest cells indicated that Fgfr1 regulates the entry of neural crest cells into the second branchial arch non-cell-autonomously. Gene expression in the pharyngeal ectoderm overlying the developing second branchial arch was affected in the hypomorphic Fgfr1 mutants at a stage prior to neural crest entry. Our results indicate that Fgfr1 patterns the pharyngeal region to create a permissive environment for neural crest cell migration.

Developmental neurogenetics and neuro-ophthalmology

The field of developmental neurogenetics has burgeoned over the past decade. Through the combined efforts of developmental biologists, geneticists, and clinicians, genetic defects resulting in neuro-ophthalmic disorders such as holoprosencephaly, microphthalmia, dominant optic atrophy, and optic nerve colobomas have been identified and characterized at the molecular level. Experimental studies in model organisms are continuing to identify novel genes critical for ocular and central nervous system development. Mutations in some of these genes have revealed a spectrum of pathology similar to that observed in septo-optic dysplasia, Möebius syndrome, and Duane retraction syndrome. This review examines our current knowledge of the molecular genetics of neuro-ophthalmic disease and focuses on several candidate genes for afferent and efferent visual system disorders.

The role of vitamin A in mammalian reproduction and embryonic development

Since the late 1980s, there has been an explosion of information on the molecular mechanisms and functions of vitamin A. This review focuses on the essential role of vitamin A in female reproduction and embryonic development and the metabolism of vitamin A (retinol) that results in these functions. Evidence strongly supports that in situ-generated all-trans retinoic acid (atRA) is the functional form of vitamin A in female reproduction and embryonic development. This is supported by the ability to reverse most reproductive and developmental blocks found in vitamin A deficiency with atRA, the block in embryonic development that occurs in retinaldehyde dehydrogenase type 2 null mutant mice, and the essential roles of the retinoic acid receptors, at least in embryogenesis. Early studies of embryos from marginally vitamin A-deficient (VAD) pregnant rats revealed a collection of defects called the vitamin A-deficiency syndrome. The manipulation of all-trans retinoic acid (atRA) levels in the diet of VAD female rats undergoing a reproduction cycle has proved to be an important new tool in deciphering the points of atRA function in early embryos and has provided a means to generate large numbers of embryos at later stages of development with the vitamin A-deficiency syndrome. The essentiality of the retinoid receptors in mediating the activity of atRA is exemplified by the many compound null mutant embryos that now recapitulate both the original vitamin A-deficiency syndrome and exhibit a host of new defects, many of which can also be observed in the VAD-atRA-supported rat embryo model and in retinaldehyde dehydrogenase type 2 (RALDH2) mutant mice. A major task for the future is to elucidate the atRA-dependent pathways that are normally operational in vitamin A-sufficient animals and that are perturbed in deficiency, thus leading to the characteristic VAD phenotypes described above.

Determination of the embryonic inner ear

Inner ear induction, like induction of other tissues examined in recent years, is likely to be comprised of several stages. The process begins during gastrulation when the ectoderm is competent to respond to induction. It appears that a signal from the endomesoderm underlying the otic area during gastrulation initiates induction complemented by a signal from presumptive neural tissue. By the neural plate stage, a region of ectoderm outside the neural plate is "biased" toward ear formation; this process may be part of a more general "placodal" bias shared by several sensory tissues. Induction continues during neurulation when a signal from neural tissue (possibly augmented by mesoderm underlying the otic area) results in ectoderm committed to otic vesicle formation at the time of neural tube closure. Studies on several gene families implicate them in the ear determination process. Fibroblast Growth Factor (FGF) family members are clearly involved in induction: FGFs are appropriately expressed for such a role, and have been shown to be essential for inner ear development. FGFs also have inductive activity, although it is not clear if they are sufficient for ear induction. Activation of transcription factors in the otic ectoderm, for example, by Pax gene family members, provides evidence for important changes in the responding ectoderm beginning during gastrulation and continuing through specification at the end of neurulation, although few functional tests have defined the role of these genes in determination. The challenge remains to merge embryologic data with gene function studies to develop a clear model for the molecular basis of inner ear induction.

Targeted insertion results in a rhombomere 2-specific Hoxa2 knockdown and ectopic activation of Hoxa1 expression

Recent studies indicated that retention of selectable marker cassettes in targeted Hox loci may cause unexpected phenotypes in mutant mice, due to neighborhood effects. However, the molecular mechanisms have been poorly investigated. Here, we analysed the effects of the targeted insertion of a PGK-neo cassette in the 3' untranslated region of Hoxa2. Even at this 3' position, the insertion resulted in homozygous mutants that unexpectedly did not survive beyond 3 weeks of age. Molecular analysis of the targeted allele revealed a selective "knockdown" of Hoxa2 expression in rhombomere 2 and associated patterning abnormalities. Moreover, Hoxa1 was ectopically expressed in the hindbrain and branchial arches of mutant embryos. Of interest, we demonstrated that the ectopic expression was due to the generation of neo-Hoxa1 fusion transcripts, resulting from aberrant alternative splicing. These defects could be rescued after removal of the PGK-neo cassette by Flp-mediated recombination. These results underscore the complexity of transcriptional regulation at Hox loci and provide insights into the in vivo regulation of Hoxa2 segmental expression. They also provide a molecular basis for the interpretation of unexpected Hox knockout phenotypes in which the targeted selectable marker is retained in the locus.

A compendium of mouse knockouts with inner ear defects

Genetically engineered strains of mice, modified by gene targeting (knockouts), are increasingly being employed as alternative effective research tools in elucidating the genetic basis of human deafness. An impressive array of auditory and vestibular mouse knockouts is already available as a valuable resource for studying the ontogenesis, morphogenesis and function of the mammalian inner ear. This article provides a current catalog of mouse knockouts with inner ear morphogenetic malformations and hearing or balance deficits resulting from ablation of genes that are regionally expressed in the inner ear and/or within surrounding tissues, such as the hindbrain, neural crest and mesenchyme.

Specification of the mammalian cochlea is dependent on Sonic hedgehog

Organization of the inner ear into auditory and vestibular components is dependent on localized patterns of gene expression within the otic vesicle. Surrounding tissues are known to influence compartmentalization of the otic vesicle, yet the participating signals remain unclear. This study identifies Sonic hedgehog (Shh) secreted by the notochord and/or floor plate as a primary regulator of auditory cell fates within the mouse inner ear. Whereas otic induction proceeds normally in Shh(-/-) embryos, morphogenesis of the inner ear is greatly perturbed by midgestation. Ventral otic derivatives including the cochlear duct and cochleovestibular ganglia failed to develop in the absence of Shh. The origin of the inner ear defects in Shh(-/-) embryos could be traced back to alterations in the expression of a number of genes involved in cell fate specification including Pax2, Otx1, Otx2, Tbx1, and Ngn1. We further show that several of these genes are targets of Shh signaling given their ectopic activation in transgenic mice that misexpress Shh in the inner ear. Taken together, our data support a model whereby auditory cell fates in the otic vesicle are established by the direct action of Shh.

Coincident iterated gene expression in the amphioxus neural tube

The segmental patterning of the vertebrate hindbrain has been intensely investigated, yet the evolutionary origin of hindbrain segmentation remains unclear. In the vertebrate sister group, amphioxus (Cephalochordata), the embryonic neural tube lacks obvious morphological segmentation, but comparative Hox gene expression analysis has suggested the presence of a region homologous to the vertebrate hindbrain. Does this region contain ancient segmental features shared with the vertebrate hindbrain? To help address this question we cloned the paired-like amphioxus homeodomain gene shox and found that its expression is segmental in the amphioxus neural tube. We also uncovered a previously uncharacterized iterated neural tube expression pattern of the zinc-finger gene AmphiKrox. We propose that these genes, along with amphioxus islet and AmphiMnx, share a one-somite width periodicity of expression in the neural tube, the coincidence of which may reflect an underlying segmental organization. We hypothesize that the segmental patterning of neurons in the neural tube was present in the amphioxus/vertebrate ancestor, but the establishment of a bona fide segmented hindbrain may indeed have arisen in the vertebrate lineage.

The Hox Paradox: More complex(es) than imagined

An understanding of the origin of different body plans requires knowledge of how the genes and genetic pathways that control embryonic development have evolved. The Hox genes provide an appealing starting point for such studies because they play a well-understood causal role in the regionalization of the body plan of all bilaterally symmetric animals. Vertebrate evolution has been characterized by gene, and possibly genome, duplication events, which are believed to have provided raw genetic material for selection to act upon. It has recently been established that the Hox gene organization of ray-finned fishes, such as the zebrafish, differs dramatically from that of their lobe-finned relatives, a group that includes humans and all the other widely used vertebrate model systems. This unusual Hox gene organization of zebrafish is the result of a duplication event within the ray-finned fish lineage. Thus, teleosts, such as zebrafish, have more Hox genes arrayed over more clusters (or "complexes") than do tetrapod vertebrates. Here, I review our understanding of Hox cluster architecture in different vertebrates and consider the implications of gene duplication for Hox gene regulation and function and the evolution of different body plans.

Duplication of the Hoxd11 gene causes alterations in the axial and appendicular skeleton of the mouse

The Hox genes encode a group of transcription factors essential for proper development of the mouse. Targeted mutation of the Hoxd11 gene causes reduced male fertility, vertebral transformation, carpal bone fusions, and reductions in digit length. A duplication of the Hoxd11 gene was created with the expectation that the consequences of restricted overexpression in the appropriate cells would provide further insight into the function of the Hoxd11 gene product. Genetic assays demonstrated that two tandem copies of Hoxd11 were functionally indistinguishable from the normal two copies of the gene on separate chromosomes with respect to formation of the axial and appendicular skeleton. Extra copies of Hoxd11 caused an increase in the lengths of some bones of the forelimb autopod and a decrease in the number of lumbar vertebrae. Further, analysis of the Hoxd11 duplication demonstrated that the Hoxd11 protein can perform some functions supplied by its paralogue Hoxa11. For example, the defects in forelimb bones are corrected when extra copies of Hoxd11 are present in the Hoxa11 homozygous mutant background. Thus, it appears that Hoxd11 can quantitatively compensate for the absence of Hoxa11 protein, and therefore Hoxa11 and Hoxd11 are functionally equivalent in the zeugopod. However, extra copies of Hoxd11 did not improve male or female fertility in Hoxa11 mutants. Interestingly, the insertion of an additional Hoxd11 locus into the HoxD complex does not appear to affect the expression patterns of the neighboring Hoxd10, -d12, or -d13 genes.

Early development of respiratory rhythm generation in mouse and chick

We are investigating neuronal circuits resulting from conservative developmental mechanisms orchestrating the segmentation of the vertebrates hindbrain into compartments called rhombomeres (r). Segmentation transcription factors Hoxa1, Krox20 and kreisler are expressed in the future rhombomeres r4-r5, r3 and r5, r5-r6, respectively. In mice, the in vivo and in vitro analysis of neuronal groups after inactivation of these three genes revealed distinct postnatal respiratory phenotypes associated with defects of central respiratory controls resulting from deletion, neoformation or reconfiguration of modular circuits. In chick and mice, we have found neuronal rhythm generators that conform to the rhombomeric anatomical pattern as early as at the end of the segmentation. By isolating chick hindbrain segments in vitro, we have also identified rhombomeric motifs allowing the formation or deletion of a specific (GABAergic) rhythm-promoting module. Therefore, primordial rhombomeric organization of the hindbrain seems to determine a modular organization of the rhythmogenic network, thereby influencing later function of brainstem respiratory control networks.

Conservation and diversity in the cis-regulatory networks that integrate information controlling expression of Hoxa2 in hindbrain and cranial neural crest cells in vertebrates

The Hoxa2 and Hoxb2 genes are members of paralogy group II and display segmental patterns of expression in the developing vertebrate hindbrain and cranial neural crest cells. Functional analyses have demonstrated that these genes play critical roles in regulating morphogenetic pathways that direct the regional identity and anteroposterior character of hindbrain rhombomeres and neural crest-derived structures. Transgenic regulatory studies have also begun to characterize enhancers and cis-elements for those mouse and chicken genes that direct restricted patterns of expression in the hindbrain and neural crest. In light of the conserved role of Hoxa2 in neural crest patterning in vertebrates and the similarities between paralogs, it is important to understand the extent to which common regulatory networks and elements have been preserved between species and between paralogs. To investigate this problem, we have cloned and sequenced the intergenic region between Hoxa2 and Hoxa3 in the chick HoxA complex and used it for making comparative analyses with the respective human, mouse, and horn shark regions. We have also used transgenic assays in mouse and chick embryos to test the functional activity of Hoxa2 enhancers in heterologous species. Our analysis reveals that three of the critical individual components of the Hoxa2 enhancer region from mouse necessary for hindbrain expression (Krox20, BoxA, and TCT motifs) have been partially conserved. However, their number and organization are highly varied for the same gene in different species and between paralogs within a species. Other essential mouse elements appear to have diverged or are absent in chick and shark. We find the mouse r3/r5 enhancer fails to work in chick embryos and the chick enhancer works poorly in mice. This implies that new motifs have been recruited or utilized to mediate restricted activity of the enhancer in other species. With respect to neural crest regulation, cis-components are embedded among the hindbrain control elements and are highly diverged between species. Hence, there has been no widespread conservation of sequence identity over the entire enhancer domain from shark to humans, despite the common function of these genes in head patterning. This provides insight into how apparently equivalent regulatory regions from the same gene in different species have evolved different components to potentiate their activity in combination with a selection of core components.

Analysis of two distinct retinoic acid response elements in the homeobox gene Hoxb1 in transgenic mice

Expression of vertebrate Hox genes is regulated by retinoids such as retinoic acid (RA) in cell culture and in early embryonic development. Retinoic acid response elements (RAREs) have been identified in Hox gene regulatory regions, suggesting that endogenous retinoids may be involved in the direct control of Hox gene patterning functions. Previously, two RAREs located 3' of the murine Hoxb1 gene, a DR(2) RARE and a DR(5) RARE, have been shown to regulate Hoxb1 mRNA expression in the neural epithelium and the foregut region, respectively; the foregut develops into the esophagus, liver, pancreas, lungs, and stomach. We have now examined the functional roles of these two types of 3' RAREs in regulating Hoxb1 expression at different stages of gestation, from embryonic day 7.5 to 13.5, in transgenic mice carrying specific RARE mutations. We demonstrate that the DR(5) RARE is required for the regulation of Hoxb-1 transgene region-specific expression in the gut and extraembryonic tissues, as well as for the RA-induced anteriorization of Hoxb-1 transgene expression in the gut. In contrast, expression of the Hoxb1 transgene in the neural epithelium requires only the DR(2) RARE. By in situ hybridization, we have identified a new site of Hoxb1 expression in the developing forelimbs at approximately day 12.5, and we show that, in transgenic embryos, expression in the forelimb buds requires that either the DR(2) or the DR(5) RARE is functional. Attainment of a high level of Hoxb1 transgene expression in other regions, such as in rhombomere 4 (r4) and in the somites, requires that both the DR(2) and DR(5) RAREs are functional. In addition, our transgenic data indicate that the Hoxb1 gene is expressed in other tissues such as the hernia gut, genital eminence, and lung. Our analysis shows that endogenous retinoids act through individual DR(2) and DR(5) RAREs to regulate Hoxb1 expression in different regions of the embryo and that functional redundancy between these DR(2) and DR(5) RAREs does not exist with respect to neural epithelium and the gut Hoxb1 expression.