ROLE OF HOXA2 IN MOUSE EXTERNAL EAR MORPHOGENESIS: A MODEL TO DECIPHER HUMAN CRANIOFACIAL GENETIC DISORDERS

Oral Communication presentad at the ";;Forum des Jeunes Chercheurs";; Brest (France) October 2011. Substituting abstract

Hox gene and development of the auditory circuit

Sound vibration is sensed by hair cells in the inner ear. The information is transmitted to the cochlear nucleus in the brainstem via spiral ganglion neurons. The information is further transmitted to higher relaying centers in the brain such as superior olivary complex and inferior colliculus. The connectivity between these components is topographically organized in a frequency-specific manner. It is known that the organization is well-established from the beginning of the circuit development. However, little is still known about the molecular mechanisms underlying the development of connectivity in the auditory circuit. Homeobox transcription factors of the Hox gene family are known for their involvement in early anterior-posterior axis patterning of neuronal progenitors in the hindbrain. Recent evidence indicates that they also play important roles in late aspects of neuronal development and establishment of topographic circuitry. Moreover, a mutation in the HOXA2 gene has been recently shown to be responsible for hearing deficits in humans. By means of spatiotemporally controlled Hoxa2 and Hoxb2 conditional mutations in the mouse we analyzed the involvement of these factors in auditory circuit development and connectivity

Assembly of the Auditory Circuitry by a Hox Genetic Network in the Mouse Brainstem

Rhombomeres (r) contribute to brainstem auditory nuclei during development. Hox genes are determinants of rhombomere-derived fate and neuronal connectivity. Little is known about the contribution of individual rhombomeres and their associated Hox codes to auditory sensorimotor circuitry. Here, we show that r4 contributes to functionally linked sensory and motor components, including the ventral nucleus of lateral lemniscus, posterior ventral cochlear nuclei (VCN), and motor olivocochlear neurons. Assembly of the r4-derived auditory components is involved in sound perception and depends on regulatory interactions between Hoxb1 and Hoxb2. Indeed, in Hoxb1 and Hoxb2 mutant mice the transmission of low-level auditory stimuli is lost, resulting in hearing impairments. On the other hand, Hoxa2 regulates the Rig1 axon guidance receptor and controls contralateral projections from the anterior VCN to the medial nucleus of the trapezoid body, a circuit involved in sound localization. Thus, individual rhombomeres and their associated Hox codes control the assembly of distinct functionally segregated sub-circuits in the developing auditory brainstem

Jaw transformation with gain of symmetry after Dlx5/Dlx6 inactivation: mirror of the past ?

Summary: In modern vertebrates upper and lower jaws are morphologically different. Both develop from the mandibular arch, which is colonized mostly by Hox-free neural crest cells. Here we show that simultaneous inactivation of the murine homeobox genes Dlx5 and Dlx6 results in the transformation of the lower jaw into an upper jaw and in symmetry of the snout. This is the first homeotic-like transformation found in this Hox-free region after gene inactivation. A suggestive parallel comes from the paleontological record, which shows that in primitive vertebrates both jaws are essentially mirror images of each other. Our finding supports the notion that Dlx genes are homeotic genes associated with morphological novelty in the vertebrate lineage

Human teneurin-1 is a direct target of the homeobox transcription factor EMX2 at a novel alternate promoter

<p>Abstract</p> <p>Background</p> <p>Teneurin-1 is a member of a family of type II transmembrane proteins conserved from <it>C.elegans </it>to vertebrates. Teneurin expression in vertebrates is best studied in mouse and chicken, where the four members teneurin-1 to -4 are predominantly expressed in the developing nervous system in area specific patterns. Based on their distinct, complementary expression a possible function in the establishment of proper connectivity in the brain was postulated. However, the transcription factors contributing to these distinctive expression patterns are largely unknown. Emx2 is a homeobox transcription factor, known to be important for area specification in the developing cortex. A study of Emx2 knock-out mice suggested a role of Emx2 in regulating patterned teneurin expression.</p> <p>Results</p> <p>5'RACE of human teneurin-1 revealed new alternative untranslated exons that are conserved in mouse and chicken. Closer analysis of the conserved region around the newly identified transcription start revealed promoter activity that was induced by EMX2. Mutation of a predicted homeobox binding site decreased the promoter activity in different reporter assays <it>in vitro </it>and <it>in vivo </it>in electroporated chick embryos. We show direct <it>in vivo </it>binding of EMX2 to the newly identified promoter element and finally confirm that the endogenous alternate transcript is specifically upregulated by EMX2.</p> <p>Conclusions</p> <p>We found that human teneurin-1 is directly regulated by EMX2 at a newly identified and conserved promoter region upstream of the published transcription start site, establishing teneurin-1 as the first human EMX2 target gene. We identify and characterize the EMX2 dependent promoter element of human teneurin-1.</p

The Expression Pattern of the Mouse Receptor Tyrosine Kinase GeneMDK1Is Conserved through Evolution and RequiresHoxa-2for Rhombomere-Specific Expression in Mouse Embryos

AbstractSegmentation of the hindbrain has been conserved throughout the vertebrate species and results in the transient formation of rhombomeres, which are lineage-restricted compartments. Studies on the molecular mechanisms underlying the segmentation process have revealed that rhombomeric boundaries coincide with the expression limits of several evolutionary conserved genes such as the zinc-finger transcription factorKrox-20and homeobox genes which are expressed in a specific spatial and temporal order and have been shown to be important regulators of segmental identity. In addition toKrox-20and Hox genes, several members of the Eph subfamily of receptor protein tyrosine kinase (RTK) genes are also expressed in a segment-restricted manner in the hindbrain, suggesting that these receptors may act in concert with Hox genes to establish regional identity. In the cascade of regulatory interactions leading to segmental identity,Krox-20appears to act “upstream” of Hox genes, but the identity of the “downstream” effectors has not yet been identified. We report here the isolation of the zebrafish orthologue of the mouse RTK geneMDK1which belongs to the Eph receptor subfamily and show that the major expression domains of the mouse and the zebrafish genes have been conserved through evolution. Since the coincident spatial and temporal expression ofHoxa-2andMDK1in the mouse hindbrain suggested a possible regulatory link between them, we analyzed the expression of theMDK1inHoxa-2null mutant embryos. A selective lack ofMDK1expression in rhombomere 3 ofHoxa-2mutant hindbrains together with an overall altered expression pattern in the other rhombomeres was observed, thus demonstrating thatMDK1lies downstream ofHoxa-2in the morphogenetic signaling cascade

Distinct roles of Hoxa2 and Krox20 in the development of rhythmic neural networks controlling inspiratory depth, respiratory frequency, and jaw opening

<p>Abstract</p> <p>Background</p> <p>Little is known about the involvement of molecular determinants of segmental patterning of rhombomeres (r) in the development of rhythmic neural networks in the mouse hindbrain. Here, we compare the phenotypes of mice carrying targeted inactivations of <it>Hoxa2</it>, the only <it>Hox </it>gene expressed up to r2, and of <it>Krox20</it>, expressed in r3 and r5. We investigated the impact of such mutations on the neural circuits controlling jaw opening and breathing in newborn mice, compatible with Hoxa2-dependent trigeminal defects and direct regulation of <it>Hoxa2 </it>by Krox20 in r3.</p> <p>Results</p> <p>We found that <it>Hoxa2 </it>mutants displayed an impaired oro-buccal reflex, similarly to <it>Krox20 </it>mutants. In contrast, while <it>Krox20 </it>is required for the development of the rhythm-promoting parafacial respiratory group (pFRG) modulating respiratory frequency,<it> Hoxa2 </it>inactivation did not affect neonatal breathing frequency. Instead, we found that <it>Hoxa2</it>^-/-but not <it>Krox20</it>^-/-mutation leads to the elimination of a transient control of the inspiratory amplitude normally occurring during the first hours following birth. Tracing of r2-specific progenies of <it>Hoxa2 </it>expressing cells indicated that the control of inspiratory activity resides in rostral pontine areas and required an intact r2-derived territory.</p> <p>Conclusion</p> <p>Thus, inspiratory shaping and respiratory frequency are under the control of distinct <it>Hox</it>-dependent segmental cues in the mammalian brain. Moreover, these data point to the importance of rhombomere-specific genetic control in the development of modular neural networks in the mammalian hindbrain.</p

Rostral and caudal pharyngeal arches share a common neural crest ground pattern

In vertebrates, face and throat structures, such as jaw, hyoid and thyroid cartilages develop from a rostrocaudal metameric series of pharyngeal arches, colonized by cranial neural crest cells (NCCs). Colinear Hox gene expression patterns underlie arch specific morphologies, with the exception of the first (mandibular) arch, which is devoid of any Hox gene activity. We have previously shown that the first and second (hyoid) arches share a common, Hox-free, patterning program. However, whether or not more posterior pharyngeal arch neural crest derivatives are also patterned on the top of the same ground-state remained an unanswered question. Here, we show that the simultaneous inactivation of all Hoxa cluster genes in NCCs leads to multiple jaw and first arch-like structures, partially replacing second, third and fourth arch derivatives, suggesting that rostral and caudal arches share the same mandibular arch-like ground patterning program. The additional inactivation of the Hoxd cluster did not significantly enhance such a homeotic phenotype, thus indicating a preponderant role of Hoxa genes in patterning skeletogenic NCCs. Moreover, we found that Hoxa2 and Hoxa3 act synergistically to pattern third and fourth arch derivatives. These results provide insights into how facial and throat structures are assembled during development, and have implications for the evolution of the pharyngeal region of the vertebrate head