Multimodal Hox5 activity generates motor neuron diversity

Motor neurons (MNs) are the final output of circuits driving fundamental behaviors, such as respiration and locomotion. Hox proteins are essential in generating the MN diversity required for accomplishing these functions, but the transcriptional mechanisms that enable Hox paralogs to assign distinct MN subtype identities despite their promiscuous DNA binding motif are not well understood. Here we show that Hoxa5 controls chromatin accessibility in all mouse spinal cervical MN subtypes and engages TALE co-factors to directly bind and regulate subtype-specific genes. We identify a paralog-specific interaction of Hoxa5 with the phrenic MN-specific transcription factor Scip and show that heterologous expression of Hoxa5 and Scip is sufficient to suppress limb-innervating MN identity. We also demonstrate that phrenic MN identity is stable after Hoxa5 downregulation and identify Klf proteins as potential regulators of phrenic MN maintenance. Our data identify multiple modes of Hoxa5 action that converge to induce and maintain MN identity.

Hoxa5 Activity Across the Lateral Somitic Frontier Regulates Development of the Mouse Sternum

The skeletal system derives from multiple embryonic sources whose derivatives must develop in coordination to produce an integrated whole. In particular, interactions across the lateral somitic frontier, where derivatives of the somites and lateral plate mesoderm come into contact, are important for proper development. Many questions remain about genetic control of this coordination, and embryological information is incomplete for some structures that incorporate the frontier, including the sternum. Hox genes act in both tissues as regulators of skeletal pattern. Here, we used conditional deletion to characterize the tissue-specific contributions of Hoxa5 to skeletal patterning. We found that most aspects of the Hoxa5 skeletal phenotype are attributable to its activity in one or the other tissue, indicating largely additive roles. However, multiple roles are identified at the junction of the T1 ribs and the anterior portion of the sternum, or presternum. The embryology of the presternum has not been well described in mouse. We present a model for presternum development, and show that it arises from multiple, paired LPM-derived primordia. We show evidence that HOXA5 expression marks the embryonic precursor of a recently identified lateral presternum structure that is variably present in therians.

HOXA5 Participates in Brown Adipose Tissue and Epaxial Skeletal Muscle Patterning and in Brown Adipocyte Differentiation

Brown adipose tissue (BAT) plays critical thermogenic, metabolic and endocrine roles in mammals, and aberrant BAT function is associated with metabolic disorders including obesity and diabetes. The major BAT depots are clustered at the neck and forelimb levels, and arise largely within the dermomyotome of somites, from a common progenitor with skeletal muscle. However, many aspects of BAT embryonic development are not well understood. Hoxa5 patterns other tissues at the cervical and brachial levels, including skeletal, neural and respiratory structures. Here, we show that Hoxa5 also positively regulates BAT development, while negatively regulating formation of epaxial skeletal muscle. HOXA5 protein is expressed in embryonic preadipocytes and adipocytes as early as embryonic day 12.5. Hoxa5 null mutant embryos and rare, surviving adults show subtly reduced iBAT and sBAT formation, as well as aberrant marker expression, lower adipocyte density and altered lipid droplet morphology. Conversely, the epaxial muscles that arise from a common dermomyotome progenitor are expanded in Hoxa5 mutants. Conditional deletion of Hoxa5 with Myf5/Cre can reproduce both BAT and epaxial muscle phenotypes, indicating that HOXA5 is necessary within Myf5-positive cells for proper BAT and epaxial muscle development. However, recombinase-based lineage tracing shows that Hoxa5 does not act cell-autonomously to repress skeletal muscle fate. Interestingly, Hoxa5-dependent regulation of adipose-associated transcripts is conserved in lung and diaphragm, suggesting a shared molecular role for Hoxa5 in multiple tissues. Together, these findings establish a role for Hoxa5 in embryonic BAT development.

Deletion of Yy1 in mouse lung epithelium unveils molecular mechanisms governing pleuropulmonary blastoma pathogenesis

Pleuropulmonary blastoma (PPB) is a very rare pediatric lung disease. It can progress from abnormal epithelial cysts to an aggressive sarcoma with poor survival. PPB is difficult to diagnose as it can be confounded with other cystic lung disorders, such as congenital pulmonary airway malformation (CPAM). PPB is associated with mutations in DICER1 that perturb the microRNA (miRNA) profile in lung. How DICER1 and miRNAs act during PPB pathogenesis remains unsolved. Lung epithelial deletion of the Yin Yang1 (Yy1) gene in mice causes a phenotype mimicking the cystic form of PPB and affects the expression of key regulators of lung development. Similar changes in expression were observed in PPB but not in CPAM lung biopsies, revealing a distinctive PPB molecular signature. Deregulation of molecules promoting epithelial-mesenchymal transition (EMT) was detected in PPB specimens, suggesting that EMT might participate in tumor progression. Changes in miRNA expression also occurred in PPB lung biopsies. miR-125a-3p, a candidate to regulate YY1 expression and lung branching, was abnormally highly expressed in PPB samples. Together, these findings support the concept that reduced expression of YY1, due to the abnormal miRNA profile resulting from DICER1 mutations, contributes to PPB development via its impact on the expression of key lung developmental genes.This article has an associated First Person interview with the joint first authors of the paper.

Phrenic-specific transcriptional programs shape respiratory motor output

The precise pattern of motor neuron (MN) activation is essential for the execution of motor actions; however, the molecular mechanisms that give rise to specific patterns of MN activity are largely unknown. Phrenic MNs integrate multiple inputs to mediate inspiratory activity during breathing and are constrained to fire in a pattern that drives efficient diaphragm contraction. We show that Hox5 transcription factors shape phrenic MN output by connecting phrenic MNs to inhibitory premotor neurons. Hox5 genes establish phrenic MN organization and dendritic topography through the regulation of phrenic-specific cell adhesion programs. In the absence of Hox5 genes, phrenic MN firing becomes asynchronous and erratic due to loss of phrenic MN inhibition. Strikingly, mice lacking Hox5 genes in MNs exhibit abnormal respiratory behavior throughout their lifetime. Our findings support a model where MN-intrinsic transcriptional programs shape the pattern of motor output by orchestrating distinct aspects of MN connectivity.

In mammals, air is moved in and out of the lungs by a sheet of muscle called the diaphragm. When this muscle contracts air gets drawn into the lungs and as the muscle relaxes this pushes air back out. Movement of the diaphragm is controlled by a group of nerve cells called motor neurons which are part of the phrenic motor column (or PMC for short) that sits within the spinal cord. The neurons within this column work together with nerve cells in the brain to coordinate the speed and duration of each breath.

For the lungs to develop normally, the neurons that control how the diaphragm contracts need to start working before birth. During development, motor neurons in the PMC cluster together and connect with other nerve cells involved in breathing. But, despite their essential role, it is not yet clear how neurons in the PMC develop and join up with other nerve cells.

Now, Vagnozzi et al. show that a set of genes which make the transcription factor Hox5 control the position and organization of motor neurons in the PMC. Transcription factors work as genetic switches, turning sets of genes on and off. Vagnozzi et al. showed that removing the Hox5 transcription factors from motor neurons in the PMC changed their activity and disordered their connections with other breathing-related nerve cells. Hox5 transcription factors regulate the production of proteins called cadherins which join together neighboring cells. Therefore, motor neurons lacking Hox5 were unable to make enough cadherins to securely stick together and connect with other nerve cells.

Further experiments showed that removing the genes that code for Hox5 caused mice to have breathing difficulties in the first two weeks after birth. Although half of these mutant mice were eventually able to breathe normally, the other half died within a week. These breathing defects are reminiscent of the symptoms observed in sudden infant death syndrome (also known as SIDS).

Abnormalities in breathing occur in many other diseases, including sleep apnea, muscular dystrophy and amyotrophic lateral sclerosis (ALS). A better understanding of how the connections between nerve cells involved in breathing are formed, and the role of Hox5 and cadherins, could lead to improved treatment options for these diseases.

A scientific journey in the garden of the Hox genes: an interview with Jacqueline Deschamps

For this Special Issue of The International Journal of Develomental Biology on Hox genes, the guest editors met Jacqueline Deschamps for an interview about her research career dedicated to understanding how Hox gene expression is initiated, maintained and functionally utilized in the mouse embryo. We describe here her journey through some of the main discoveries which led to our current knowledge about how Hox genes contribute to shaping the animal body plan. This journey was a human adventure also, of more than 30 years, in the light of which Jacqueline Deschamps delivers here messages to the younger generations of scientists.

HOXA5 protein expression and genetic fate mapping show lineage restriction in the developing musculoskeletal system

HOX proteins act during development to regulate musculoskeletal morphology. HOXA5 patterns skeletal structures surrounding the cervical-thoracic transition including the vertebrae, ribs, sternum and forelimb girdle. However, the tissue types in which it acts to pattern the skeleton, and the ultimate fates of embryonic cells that activate Hoxa5 expression are unknown. A detailed characterization of HOXA5 expression by immunofluorescence was combined with Cre/LoxP genetic lineage tracing to map the fate of Hoxa5 expressing cells in axial musculoskeletal tissues and in their precursors, the somites and lateral plate mesoderm. HOXA5 protein expression is dynamic and spatially restricted in derivatives of both the lateral plate mesoderm and somites, including a subset of the lateral sclerotome, suggesting a local role in regulating early skeletal patterning. HOXA5 expression persists from somite stages through late development in differentiating skeletal and connective tissues, pointing to a continuous and direct role in skeletal patterning. In contrast, HOXA5 expression is excluded from the skeletal muscle and muscle satellite cell lineages. Furthermore, the descendants of Hoxa5-expressing cells, even after HOXA5 expression has extinguished, never contribute to these lineages. Together, these findings suggest cell autonomous roles for HOXA5 in skeletal development, as well as non-cell autonomous functions in muscle through expression in surrounding connective tissues. They also support the notion that different Hox genes display diverse tissue specificities and locations to achieve their patterning activity.

Four decades of Hox gene investigation and many more to go

Forty years ago, Ed Lewis established for the first time the organization of homeotic genes along the chromosome and its importance in embryo patterning. To celebrate this seminal discovery, the International Journal of Developmental Biology decided to launch a Special Issue. It is with honor, pleasure, but also humility that we accepted the challenge of acting as guest editors for this Special Issue. We entitled the issue Hox genes: past, present and future of master regulator genes since despite four decades of amazing discoveries, numerous questions remain unanswered, which open new avenues of research. This is well-acknowledged by Robb Krumlauf and Jacqueline Deschamps in the Introductory articles. The high-level reviews and original research reports collected in this Special Issue also reflect the wide-range and important topics that are still in the spotlights including the origins of Hox genes, the regulatory events controlling their expression, the mechanisms driving the action of HOX proteins, and their multiple roles in normal development and pathogenesis.

Lung cancer susceptibility genetic variants modulate HOXB2 expression in the lung

The HOX genes are transcription factors that are expressed in coordinated spatiotemporal patterns to ensure normal development. Ectopic expression may instead lead to the development and progression of tumors. Genetic polymorphisms in the regions of four HOX gene clusters were tested for association with lung cancer in 420 cases and 3,151 controls. The effect of these variants on lung gene expression (expression quantitative trait loci, eQTL) was tested in a discovery set of 409 non-tumor lung samples and validated in two lung eQTL replication sets (n = 287 and 342). The expression levels of HOXB2 were evaluated at the mRNA and protein levels by quantitative real-time PCR and immunohistochemistry in paired tumor and non-tumor lung tissue samples. The most significant SNP associated with lung cancer in the HOXB cluster was rs10853100 located upstream of the HOXB cluster. HOXB2 was the top eQTL-regulated gene with several polymorphisms associated with its mRNA expression levels in lung tissue. This includes the lung cancer SNP rs10853100 that was significantly associated with HOXB2 expression (P=3.39E-7). In the lung eQTL discovery and replication sets, the lung cancer risk allele (T) for rs10853100 was associated with lower HOXB2 expression levels. In paired normal-tumor samples, HOXB2 mRNA and protein levels were significantly reduced in tumors when compared to non-tumor lung tissues. Genetic variants in the HOXB cluster may confer susceptibility to lung cancer by modulating the expression of HOXB2 in the lung.

Loss of the zona pellucida-binding protein 2 (Zpbp2) gene in mice impacts airway hypersensitivity and lung lipid metabolism in a sex-dependent fashion

The human chromosomal region 17q12-q21 is one of the best replicated genome-wide association study loci for childhood asthma. The associated SNPs span a large genomic interval that includes several protein-coding genes. Here, we tested the hypothesis that the zona pellucida-binding protein 2 (ZPBP2) gene residing in this region contributes to asthma pathogenesis using a mouse model. We tested the lung phenotypes of knock-out (KO) mice that carry a deletion of the Zpbp2 gene. The deletion attenuated airway hypersensitivity (AHR) in female, but not male, mice in the absence of allergic sensitization. Analysis of the lipid profiles of their lungs showed that female, but not male, KO mice had significantly lower levels of sphingosine-1-phosphate (S1P), very long-chain ceramides (VLCCs), and higher levels of long-chain ceramides compared to wild-type controls. Furthermore, in females, lung resistance following methacholine challenge correlated with lung S1P levels (Pearson correlation coefficient 0.57) suggesting a link between reduced AHR in KO females, Zpbp2 deletion, and S1P level regulation. In livers, spleens and blood plasma, however, VLCC, S1P, and sphingosine levels were reduced in both KO females and males. We also find that the Zpbp2 deletion was associated with gain of methylation in the adjacent DNA regions. Thus, we demonstrate that the mouse ortholog of ZPBP2 has a role in controlling AHR in female mice. Our data also suggest that Zpbp2 may act through regulation of ceramide metabolism. These findings highlight the importance of phospholipid metabolism for sexual dimorphism in AHR.

Conditional Loss of Hoxa5 Function Early after Birth Impacts on Expression of Genes with Synaptic Function

Hoxa5 is a member of the Hox gene family that plays critical roles in successive steps of the central nervous system formation during embryonic and fetal development. In the mouse, Hoxa5 was recently shown to be expressed in the medulla oblongata and the pons from fetal stages to adulthood. In these territories, Hoxa5 transcripts are enriched in many precerebellar neurons and several nuclei involved in autonomic functions, while the HOXA5 protein is detected mainly in glutamatergic and GABAergic neurons. However, whether HOXA5 is functionally required in these neurons after birth remains unknown. As a first approach to tackle this question, we aimed at determining the molecular programs downstream of the HOXA5 transcription factor in the context of the postnatal brainstem. A comparative transcriptomic analysis was performed in combination with gene expression localization, using a conditional postnatal Hoxa5 loss-of-function mouse model. After inactivation of Hoxa5 at postnatal days (P)1–P4, we established the transcriptome of the brainstem from P21 Hoxa5 conditional mutants using RNA-Seq analysis. One major finding was the downregulation of several genes associated with synaptic function in Hoxa5 mutant specimens including different actors involved in glutamatergic synapse, calcium signaling pathway, and GABAergic synapse. Data were confirmed and extended by reverse transcription quantitative polymerase chain reaction analysis, and the expression of several HOXA5 candidate targets was shown to co-localize with Hoxa5 transcripts in precerebellar nuclei. Together, these new results revealed that HOXA5, through the regulation of key actors of the glutamatergic/GABAergic synapses and calcium signaling, might be involved in synaptogenesis, synaptic transmission, and synaptic plasticity of the cortico-ponto-cerebellar circuitry in the postnatal brainstem.

Respiratory consequences of targeted losses of Hoxa5 gene function in mice

Fetal development of the respiratory tract and diaphragm requires strict coordination between genetically controlled signals and mechanical forces produced by the neural network that generates breathing. HOXA5, which is expressed in the mesenchyme of the trachea, lung and diaphragm, and in phrenic motor neurons, is a key transcription factor regulating lung development and function. Consequently, most Hoxa5^-/- mutants die at birth from respiratory failure. However, the extensive effect of the null mutation makes it difficult to identify the origins of respiratory dysfunction in newborns. To address the physiological impact of Hoxa5 tissue-specific roles, we used conditional gene targeting with the Dermo1^Cre and Olig2^Cre mouse lines to produce specific Hoxa5 deletions in the mesenchyme and motor neurons, respectively. Hoxa5 expression in the mesenchyme is critical for trachea development, whereas its expression in phrenic motor neurons is essential for diaphragm formation. Breathing measurements in adult mice with whole-body plethysmography demonstrated that, at rest, only the motor neuron deletion affects respiration, resulting in higher breathing frequency and decreased tidal volume. But subsequent exposure to a moderate hypoxic challenge (Fi_O2 =0.12; 10 min) revealed that both mutant mice hyperventilate more than controls. Hoxa5^flox/flox;Dermo1^+/Cre mice showed augmented tidal volume while Hoxa5^flox/flox;Olig2^+/Cre mice had the largest increase in breathing frequency. No significant differences were observed between medulla-spinal cord preparations from E18.5 control and Hoxa5^flox/flox;Olig2^+/Cre mouse embryos that could support a role for Hoxa5 in fetal inspiratory motor command. According to our data, Hoxa5 expression in the mesenchyme and phrenic motor neurons controls distinct aspects of respiratory development.

HOXA5 plays tissue-specific roles in the developing respiratory system

Hoxa5 is essential for development of several organs and tissues. In the respiratory system, loss of Hoxa5 function causes neonatal death due to respiratory distress. Expression of HOXA5 protein in mesenchyme of the respiratory tract and in phrenic motor neurons of the central nervous system led us to address the individual contribution of these Hoxa5 expression domains using a conditional gene targeting approach. Hoxa5 does not play a cell-autonomous role in lung epithelium, consistent with lack of HOXA5 expression in this cell layer. In contrast, ablation of Hoxa5 in mesenchyme perturbed trachea development, lung epithelial cell differentiation and lung growth. Further, deletion of Hoxa5 in motor neurons resulted in abnormal diaphragm innervation and musculature, and lung hypoplasia. It also reproduced the neonatal lethality observed in null mutants, indicating that the defective diaphragm is the main cause of impaired survival at birth. Thus, Hoxa5 possesses tissue-specific functions that differentially contribute to the morphogenesis of the respiratory tract.

Lung development requires an active ERK/MAPK pathway in the lung mesenchyme

BACKGROUND

Reciprocal epithelial-mesenchymal communications are critical throughout lung development, dictating branching morphogenesis and cell specification. Numerous signaling molecules are involved in these interactions, but the way epithelial-mesenchymal crosstalk is coordinated remains unclear. The ERK/MAPK pathway transduces several important signals in lung formation. Epithelial inactivation of both Mek genes, encoding ERK/MAPK kinases, causes lung agenesis and death. Conversely, Mek mutation in mesenchyme results in lung hypoplasia, trachea cartilage malformations, kyphosis, omphalocele, and death. Considering the negative impact of kyphosis and omphalocele on intrathoracic space and, consequently, on lung growth, the exact role of ERK/MAPK pathway in lung mesenchyme remains unresolved.

RESULTS

To address the role of the ERK/MAPK pathway in lung mesenchyme in absence of kyphosis and omphalocele, we used the Tbx4^Cre deleter mouse line, which acts specifically in lung mesenchyme. These Mek mutants did not develop kyphosis and omphalocele but they presented lung hypoplasia, tracheal defects, and neonatal death. Tracheal cartilage anomalies suggested a role for the ERK/MAPK pathway in the control of chondrocyte hypertrophy. Moreover, expression data indicated potential interactions between the ERK/MAPK and canonical Wnt pathways during lung formation.

CONCLUSIONS

Lung development necessitates a functional ERK/MAPK pathway in the lung mesenchymal layer in order to coordinate efficient epithelial-mesenchymal interactions. Developmental Dynamics 246:72-82, 2017. © 2016 Wiley Periodicals, Inc.

Pathomechanisms of Congenital Cystic Lung Diseases: Focus on Congenital Cystic Adenomatoid Malformation and Pleuropulmonary Blastoma

It is well established that a number of birth defects are associated with improper formation of the respiratory tract. Important progress has been made in the identification of components of the regulatory networks controlling lung morphogenesis. They comprise a variety of soluble factors, receptors, transcription factors, and miRNAs. However, the underlying molecular mechanisms remain unsolved and fundamental questions, such as those related to lung branching are still unanswered. Congenital cystic lung diseases consist of a heterogeneous group of rare lung diseases mainly detected prenatally and characterized by airway dilatation. Despite their apparent phenotypic heterogeneity, these malformations are proposed to be related to a common malformation sequence occurring during lung branching morphogenesis.

Hoxa5: A Key Player in Development and Disease

A critical position in the developmental hierarchy is occupied by the Hox genes, which encode transcription factors. Hox genes are crucial in specifying regional identity along the embryonic axes and in regulating morphogenesis. In mouse, targeted mutations of Hox genes cause skeletal transformations and organ defects that can impair viability. Here, we present the current knowledge about the Hoxa5 gene, a paradigm for the function and the regulation of Hox genes. The phenotypic survey of Hoxa5-/- mice has unveiled its critical role in the regional specification of the skeleton and in organogenesis. Most Hoxa5-/- mice die at birth from respiratory distress due to tracheal and lung dysmorphogenesis and impaired diaphragm innervation. The severity of the phenotype establishes that Hoxa5 plays a predominant role in lung organogenesis versus other Hox genes. Hoxa5 also governs digestive tract morphogenesis, thyroid and mammary glands development, and ovary homeostasis. Deregulated Hoxa5 expression is reported in cancers, indicating Hoxa5 involvement in tumor predisposition and progression. The dynamic Hoxa5 expression profile is under the transcriptional control of multiple cis-acting sequences and trans-acting regulators. It is also modulated by epigenetic mechanisms, implicating chromatin modifications and microRNAs. Finally, lncRNAs originating from alternative splicing and distal promoters encompass the Hoxa5 locus.

Crucial requirement of ERK/MAPK signaling in respiratory tract development

There was an error published in Development 141, 3197-3211. In the key for Fig. 3C, the grey bars were labelled with the incorrect genotype name. The correct genotype is Mek1+/flox;Mek2−/−; Dermo1+/cre. This error does not affect the conclusions of the paper. The authors apologise to readers for this mistake.

Epithelial inactivation of Yy1 abrogates lung branching morphogenesis

Yin Yang 1 (YY1) is a multifunctional zinc-finger-containing transcription factor that plays crucial roles in numerous biological processes by selectively activating or repressing transcription, depending upon promoter contextual differences and specific protein interactions. In mice, Yy1 null mutants die early in gestation whereas Yy1 hypomorphs die at birth from lung defects. We studied how the epithelial-specific inactivation of Yy1 impacts on lung development. The Yy1 mutation in lung epithelium resulted in neonatal death due to respiratory failure. It impaired tracheal cartilage formation, altered cell differentiation, abrogated lung branching and caused airway dilation similar to that seen in human congenital cystic lung diseases. The cystic lung phenotype in Yy1 mutants can be partly explained by the reduced expression of Shh, a transcriptional target of YY1, in lung endoderm, and the subsequent derepression of mesenchymal Fgf10 expression. Accordingly, SHH supplementation partially rescued the lung phenotype in vitro. Analysis of human lung tissues revealed decreased YY1 expression in children with pleuropulmonary blastoma (PPB), a rare pediatric lung tumor arising during fetal development and associated with DICER1 mutations. No evidence for a potential genetic interplay between murine Dicer and Yy1 genes during lung morphogenesis was observed. However, the cystic lung phenotype resulting from the epithelial inactivation of Dicer function mimics the Yy1 lung malformations with similar changes in Shh and Fgf10 expression. Together, our data demonstrate the crucial requirement for YY1 in lung morphogenesis and identify Yy1 mutant mice as a potential model for studying the genetic basis of PPB.

Perinatal induction of Cre recombination with tamoxifen

Temporal control of site-specific recombination is commonly achieved by using a tamoxifen-inducible form of Cre or Flp recombinases. Although powerful protocols of induction have been developed for gene inactivation at adult stages or during embryonic development, induction of recombination at late gestational or early postnatal stages is still difficult to achieve. In this context, using the ubiquitous CMV-CreER(T2) transgenic mice, we have tested and validated two procedures to achieve recombination just before and just after birth. The efficiency of recombination was evaluated in the brain, which is known to be more problematic to target. For the late gestation treatment with tamoxifen, different protocols of complementary administration of progesterone and estrogen were tested. However, delayed delivery and/or mortality of pups due to difficult delivery were always observed. To circumvent this problem, pups were collected from tamoxifen-treated pregnant dams by caesarian section at E18.5 and given to foster mothers. For postnatal treatment, different dosages of tamoxifen were administered by intragastric injection to the pups during 3 or 4 days after birth. The efficiency of these treatments was analyzed at P7 using a transgenic reporter line. They were also validated with the Hoxa5 conditional allele. In conclusion, we have developed efficient procedures that allow achieving efficient recombination of floxed alleles at perinatal stages. These protocols will allow investigating the late/adult functions of many developmental genes, whose characterization has been so far restricted to embryonic development.

Crucial requirement of ERK/MAPK signaling in respiratory tract development

The mammalian genome contains two ERK/MAP kinase genes, Mek1 and Mek2, which encode dual-specificity kinases responsible for ERK/MAP kinase activation. In order to define the function of the ERK/MAPK pathway in the lung development in mice, we performed tissue-specific deletions of Mek1 function on a Mek2 null background. Inactivation of both Mek genes in mesenchyme resulted in several phenotypes, including giant omphalocele, kyphosis, pulmonary hypoplasia, defective tracheal cartilage and death at birth. The absence of tracheal cartilage rings establishes the crucial role of intracellular signaling molecules in tracheal chondrogenesis and provides a putative mouse model for tracheomalacia. In vitro, the loss of Mek function in lung mesenchyme did not interfere with lung growth and branching, suggesting that both the reduced intrathoracic space due to the dysmorphic rib cage and the omphalocele impaired lung development in vivo. Conversely, Mek mutation in the respiratory epithelium caused lung agenesis, a phenotype resulting from the direct impact of the ERK/MAPK pathway on cell proliferation and survival. No tracheal epithelial cell differentiation occurred and no SOX2-positive progenitor cells were detected in mutants, implying a role for the ERK/MAPK pathway in trachea progenitor cell maintenance and differentiation. Moreover, these anomalies were phenocopied when the Erk1 and Erk2 genes were mutated in airway epithelium. Thus, the ERK/MAPK pathway is required for the integration of mesenchymal and epithelial signals essential for the development of the entire respiratory tract.

YY1 acts as a transcriptional activator of Hoxa5 gene expression in mouse organogenesis

The Hox gene family encodes homeodomain-containing transcriptional regulators that confer positional information to axial and paraxial tissues in the developing embryo. The dynamic Hox gene expression pattern requires mechanisms that differentially control Hox transcription in a precise spatio-temporal fashion. This implies an integrated regulation of neighbouring Hox genes achieved through the sharing and the selective use of defined enhancer sequences. The Hoxa5 gene plays a crucial role in lung and gut organogenesis. To position Hoxa5 in the regulatory hierarchy that drives organ morphogenesis, we searched for cis-acting regulatory sequences and associated trans-acting factors required for Hoxa5 expression in the developing lung and gut. Using mouse transgenesis, we identified two DNA regions included in a 1.5-kb XbaI-XbaI fragment located in the Hoxa4-Hoxa5 intergenic domain and known to control Hoxa4 organ expression. The multifunctional YY1 transcription factor binds the two regulatory sequences in vitro and in vivo. Moreover, the mesenchymal deletion of the Yy1 gene function in mice results in a Hoxa5-like lung phenotype with decreased Hoxa5 and Hoxa4 gene expression. Thus, YY1 acts as a positive regulator of Hoxa5 expression in the developing lung and gut. Our data also support a role for YY1 in the coordinated expression of Hox genes for correct organogenesis.

Hoxa5/Cre transgenic mice: novel tools for regional deletion along the anterior-posterior axis

The Hoxa5 homeobox gene encodes a transcription factor that plays a critical role in specifying the identity of the cervico-thoracic region along the anterior-posterior embryo axis and in orchestrating organ morphogenesis. The loss of Hoxa5 function results in skeletal transformations, lethality at birth due to lung defects, and organ anomalies affecting the digestive tract, the mammary gland and the ovary. Study of Hoxa5 gene regulation has revealed the interplay of several control regions that direct Hoxa5 developmental expression. Enhancers targeting expression in the CNS, the paraxial and lateral plate mesoderm at the cervico-thoracic level, and in the mesenchymal compartment of the respiratory and digestive tracts have been identified. Using these molecular tools, we have generated two Hoxa5/Cre transgenic mouse lines carrying different combinations of Hoxa5 regulatory enhancers and allowing site-specific recombination in subsets of Hoxa5 expression sites as tested with the Rosa26/lacZ reporter mice. Further validation of the recombination efficiency of the Hoxa5/Cre transgenic lines was performed with mice carrying a Hoxa5 conditional allele. Hoxa5 deletion with the Hoxa5/Cre mouse lines recapitulates Hoxa5 mutant phenotypes, such as skeletal defects, neonatal lethality, and lung malformations. Hoxa5/Cre mouse lines provide novel genetic tools for gene function analysis in defined tissues along the anterior-posterior axis.

Partial functional redundancy between Hoxa5 and Hoxb5 paralog genes during lung morphogenesis

Hox genes encode transcription factors governing complex developmental processes in several organs. A subset of Hox genes are expressed in the developing lung. Except for Hoxa5, the lack of overt lung phenotype in single mutants suggests that Hox genes may not play a predominant role in lung ontogeny or that functional redundancy may mask anomalies. In the Hox5 paralog group, both Hoxa5 and Hoxb5 genes are expressed in the lung mesenchyme whereas Hoxa5 is also expressed in the tracheal mesenchyme. Herein, we generated Hoxa5;Hoxb5 compound mutant mice to evaluate the relative contribution of each gene to lung development. Hoxa5;Hoxb5 mutants carrying the four mutated alleles displayed an aggravated lung phenotype, resulting in the death of the mutant pups at birth. Characterization of the phenotype highlighted the role of Hoxb5 in lung formation, the latter being involved in branching morphogenesis, goblet cell specification, and postnatal air space structure, revealing partial functional redundancy with Hoxa5. However, the Hoxb5 lung phenotypes were less severe than those seen in Hoxa5 mutants, likely because of Hoxa5 compensation. New specific roles for Hoxa5 were also unveiled, demonstrating the extensive contribution of Hoxa5 to the developing respiratory system. The exclusive expression of Hoxa5 in the trachea and the phrenic motor column likely underlies the Hoxa5-specific trachea and diaphragm phenotypes. Altogether, our observations establish that the Hoxa5 and Hoxb5 paralog genes shared some functions during lung morphogenesis, Hoxa5 playing a predominant role.

The loss of Hoxa5 function promotes Notch-dependent goblet cell metaplasia in lung airways

Hox genes encode transcription factors controlling complex developmental processes in various organs. Little is known, however, about how HOX proteins control cell fate. Herein, we demonstrate that the goblet cell metaplasia observed in lung airways from Hoxa5^−/− mice originates from the transdifferentiation of Clara cells. Reduced CC10 expression in Hoxa5^−/− embryos indicates that altered cell specification occurs prior to birth. The loss of Hoxa5 function does not preclude airway repair after naphthalene exposure, but the regenerated epithelium presents goblet cell metaplasia and less CC10-positive cells, demonstrating the essential role of Hoxa5 for correct differentiation. Goblet cell metaplasia in Hoxa5^−/− mice is a FOXA2-independent process. However, it is associated with increased Notch signaling activity. Consistent with these findings, expression levels of activated NOTCH1 and the effector gene HEY2 are enhanced in patients with chronic obstructive pulmonary disease. In vivo administration of a γ-secretase inhibitor attenuates goblet cell metaplasia in Hoxa5^−/− mice, highlighting the contribution of Notch signaling to the phenotype and suggesting a potential therapeutic strategy to inhibit goblet cell differentiation and mucus overproduction in airway diseases. In summary, the loss of Hoxa5 function in lung mesenchyme impacts on epithelial cell fate by modulating Notch signaling.

The loss of Hoxa5 function causes estrous acyclicity and ovarian epithelial inclusion cysts

Hox genes encode transcription factors that play essential roles during embryo morphogenesis and organogenesis. Expression of several Hox members persists at the adult age, indicating a wide spectrum of action from embryonic to postnatal life. In the present study, we reported that in adult mice, the Hoxa5 gene shows a dynamic expression profile in the ovary that depends on the estrous cycle, the gestational status, and the age of the female, suggesting that Hoxa5 may have distinct physiological functions in the ovary. Consistent with a role for Hoxa5 in ovarian function, Hoxa5(-/-) nulliparous females exhibit precocious puberty and an early onset of estrous acyclicity. They show a prolonged estrous cycle with increased metestrus-diestrus length, a phenotype that worsens with age. Older mutant females also develop ovarian epithelial inclusion cysts reminiscent of human endosalpingiosis. Immunolabeling studies suggest that these cysts originate from the ovarian surface epithelium, a source of epithelial ovarian carcinomas. Staining of the Hoxa5(-/-) ovarian cysts by the ovarian cancer markers paired box gene 8 (PAX8) and Wilms' tumor 1 (WT1) further strengthens the notion that these cysts may constitute preneoplastic lesions. Moreover, the deregulation of the estrous cycle and the presence of ovarian epithelial cysts in Hoxa5(-/-) older females correlate with a reduced expression of specific epidermal growth factor receptor signaling components, namely Egfr, Areg, and Btc. Altogether, our data unveil that Hoxa5, a stroma-specific gene, plays a significant role in ovarian biology and may be involved in ovarian cancer predisposition.

Multiple promoters and alternative splicing: Hoxa5 transcriptional complexity in the mouse embryo

Background

The genomic organization of Hox clusters is fundamental for the precise spatio-temporal regulation and the function of each Hox gene, and hence for correct embryo patterning. Multiple overlapping transcriptional units exist at the Hoxa5 locus reflecting the complexity of Hox clustering: a major form of 1.8 kb corresponding to the two characterized exons of the gene and polyadenylated RNA species of 5.0, 9.5 and 11.0 kb. This transcriptional intricacy raises the question of the involvement of the larger transcripts in Hox function and regulation.

Methodology/Principal Findings

We have undertaken the molecular characterization of the Hoxa5 larger transcripts. They initiate from two highly conserved distal promoters, one corresponding to the putative Hoxa6 promoter, and a second located nearby Hoxa7. Alternative splicing is also involved in the generation of the different transcripts. No functional polyadenylation sequence was found at the Hoxa6 locus and all larger transcripts use the polyadenylation site of the Hoxa5 gene. Some larger transcripts are potential Hoxa6/Hoxa5 bicistronic units. However, even though all transcripts could produce the genuine 270 a.a. HOXA5 protein, only the 1.8 kb form is translated into the protein, indicative of its essential role in Hoxa5 gene function. The Hoxa6 mutation disrupts the larger transcripts without major phenotypic impact on axial specification in their expression domain. However, Hoxa5-like skeletal anomalies are observed in Hoxa6 mutants and these defects can be explained by the loss of expression of the 1.8 kb transcript. Our data raise the possibility that the larger transcripts may be involved in Hoxa5 gene regulation.

Significance

Our observation that the Hoxa5 larger transcripts possess a developmentally-regulated expression combined to the increasing sum of data on the role of long noncoding RNAs in transcriptional regulation suggest that the Hoxa5 larger transcripts may participate in the control of Hox gene expression.

Influence of Hoxa5 on p53 tumorigenic outcome in mice

Hox genes encode transcription factors of crucial importance in the pattern formation of a large spectrum of species. Several studies have now proposed a role for these developmental genes in cancer biology. It has been suggested that HOXA5 possesses growth-suppressive properties through activation of p53 expression in human breast tissue. To assess the genetic cooperation that may exist between Hoxa5 and p53 in tumorigenesis, we generated Hoxa5/p53 compound mutant mice. The presence of Hoxa5 null alleles increased the susceptibility of p53(-/-) mice to develop tumors with a high prevalence for thymic lymphoma, suggesting that the loss of function of the two genes collaborate in tumor formation. To extend our analysis to mammary tumorigenesis, we performed Hoxa5/p53 whole mammary gland transplantations into wild-type hosts. In the p53(-/-) background, the presence of one Hoxa5 mutant allele had no impact on mammary tumor formation. In contrast, the complete loss of Hoxa5 function influenced the tumorigenic outcome of p53(+/-) mammary glands. However, the collaborative nature of this interaction did not depend on the transcriptional regulation of p53 by Hoxa5. Altogether, our data establish that Hoxa5 and p53 cooperate in mammary tumorigenesis in vivo.

[Hoxa5: a master gene with multifaceted roles]

The Hox gene family occupies a central position in the control of body patterning by regulating the transcription of downstream effectors that, in turn, direct the morphogenetic events leading to the complex body forms along the axes. Analysis of Hox mutant mouse lines has revealed a panoply of phenotypes indicative of the broad range of Hox genes action throughout embryonic and postnatal life. Although Hox genes have been the subject of extensive research in the last two decades, the comprehension of the mechanisms involved in their regulation and function still remains elusive. Here, we present an overview of our current knowledge about one Hox gene family member, Hoxa5. The phenotypic survey of Hoxa5 mutant mice has unveiled the crucial role of this gene in regulating morphogenesis and specifying regional identity along the embryo. A majority of Hoxa5 mutant pups die at birth from defective respiratory tract. Surviving mutants present deficient alveolar septation revealing the importance of Hoxa5 during formation and maturation of the lung. Hoxa5 also participates in the morphogenesis of the digestive tract as well as that of the thyroid and mammary glands. Hoxa5 expression is restricted to the mesenchyme, and its action appears to be mediated through the control of mesenchymal-epithelial interactions during organogenesis. The implication of Hoxa5 in tumorigenesis has also been documented. In breast cancer, Hoxa5 down-regulation may impact on p53 gene expression, contributing to the oncogenic process. In contrast, the loss of Hoxa5 function limits leukaemia associated with specific chromosomal translocations. Thus, inappropriate Hoxa5 gene expression may disrupt normal growth and differentiation programs causing neoplasia. Hox gene function is intimately linked to its correct expression. Regulation of Hoxa5 expression requires multiple cis-acting regions, some encompassing coding sequences from neighboring genes. Moreover, it is complicated by the presence of several transcription units. Together these data enlighten the importance of Hox cluster organization in Hoxa5 function.

Functional resolution of duplicated hoxb5 genes in teleosts

The duplication-degeneration-complementation (DDC) model predicts that subfunctionalization of duplicated genes is a common mechanism for their preservation. The additional Hox complexes of teleost fish constitute a good system in which to test this hypothesis. Zebrafish have two hoxb complexes, with two hoxb5 genes, hoxb5a and hoxb5b, the expression patterns of which suggest subfunctionalization of an ancestral hoxb5 gene. We characterized conserved non-coding elements (CNEs) near the zebrafish hoxb5 genes. One CNE, J3, is only retained in the hoxb5a locus, whereas the others, J1 and J2, are present in both hoxb5 loci. When tested individually, the enhancer activity of individual CNEs, including J3, extensively overlapped and did not support a role in subfunctionalization. By contrast, reporter transgene constructs encompassing multiple CNEs were able to target reporter gene expression to unique domains of hoxb5a and hoxb5b expression. The deletion of J3 from the hoxb5a locus resulted in expression that approached that of hoxb5b, whereas its insertion in the hoxb5b locus increased reporter expression and rendered it more similar to that of hoxb5a. Our results highlight the importance of interactions between CNEs in the execution of complementary subfunctions of duplicated genes.

Hox patterning of the vertebrate rib cage

Unlike the rest of the axial skeleton, which develops solely from somitic mesoderm, patterning of the rib cage is complicated by its derivation from two distinct tissues. The thoracic skeleton is derived from both somitic mesoderm,which forms the vertebral bodies and ribs, and from lateral plate mesoderm,which forms the sternum. By generating mouse mutants in Hox5, Hox6and Hox9 paralogous group genes, along with a dissection of the Hox10 and Hox11 group mutants, several important conclusions regarding the nature of the `Hox code' in rib cage and axial skeleton development are revealed. First, axial patterning is consistently coded by the unique and redundant functions of Hox paralogous groups throughout the axial skeleton. Loss of paralogous function leads to anterior homeotic transformations of colinear regions throughout the somite-derived axial skeleton. In the thoracic region, Hox genes pattern the lateral plate-derived sternum in a non-colinear manner, independent from the patterning of the somite-derived vertebrae and vertebral ribs. Finally, between adjacent sets of paralogous mutants, the regions of vertebral phenotypes overlap considerably;however, each paralogous group imparts unique morphologies within these regions. In all cases examined, the next-most posterior Hox paralogous group does not prevent the function of the more-anterior Hox group in axial patterning. Thus, the `Hox code' in somitic mesoderm is the result of the distinct, graded effects of two or more Hox paralogous groups functioning in any anteroposterior location.

Comparative analysis of Hoxa5 allelic series

Analysis of the Hoxa5(-/-) mutants has revealed the critical role of Hoxa5 in survival, specification of axial identity, and ontogeny of organs, including the respiratory tract. The presence of the selection cassette in the original Hoxa5(-/-) mutation may interfere with the interpretation of the phenotypes. To circumvent this aspect and to bypass the lethality of the Hoxa5 mutation, we have designed a conditional approach and generated Hoxa5 allelic variants. The conditional allele (Hoxa5(floxed)) behaves as a wild-type allele. In contrast, both the Hoxa5(Delta) and the Hoxa5(floxneo) alleles are characterized by the loss of the functional transcript and protein, the lethality due to lung defects and the skeletal homeotic transformations similar to those of the Hoxa5(-/-) mutants. Analysis of neighboring Hox gene expression patterns in the Hoxa5 mutants produced further confirmed that the Hoxa5 allelic variants are true null alleles.

Impact of the loss of Hoxa5 function on lung alveogenesis

The involvement of genes controlling embryonic processes in the etiology of diseases often escapes attention because of the focus given to their inherent developmental role. Hoxa5 belongs to the Hox gene family encoding transcription factors known for their role in skeletal patterning. Hoxa5 is required for embryonic respiratory tract morphogenesis. We now show that the loss of Hoxa5 function has severe repercussions on postnatal lung development. Hoxa5-/- lungs present an emphysema-like morphology because of impaired alveogenesis. Chronic inflammation characteristics, including goblet cell hyperplasia, mucus hypersecretion, and recruitment of inflammatory cells, were also observed. Altered cell specification during lung morphogenesis triggered goblet cell anomalies. In addition, the defective motility of alveolar myofibroblast precursors in the embryonic lung led to the mispositioning of the alveolar myofibroblasts and to abnormal elastin deposition postnatally. Both goblet cell hyperplasia and elastic fiber abnormalities contributed to the chronic physiopathological features of Hoxa5-/- lungs. They constituted an attractive stimulus to recruit activated macrophages that in turn generated a positive feedback loop that perpetuated macrophage accumulation in the lung. The present work corroborates the notion that altered Hox gene expression may predispose to lung pathologies.

Stromal Hoxa5 function controls the growth and differentiation of mammary alveolar epithelium

Recent progress has enlightened the involvement of Hox genes in organogenesis. Several Hox genes are expressed in normal and neoplastic mammary glands. Using Hoxa5 mutant mice, we showed that Hoxa5-/- females present nursing defects. Characterization of the Hoxa5-/- mammary gland phenotype reveals changes in proliferation and differentiation of the epithelium of nulliparous and pregnant Hoxa5-/- females that precede the abnormal secretory activity at parturition. These defects likely underlie the incapacity of Hoxa5-/- dams to properly feed their pups. Hoxa5 expression is restricted to the mammary stroma at specific stages of mammary gland development. The loss of Hoxa5 function causes accelerated lobuloalveolar epithelium development, a phenotype that can be rescued upon grafting of mutant mammary epithelium into wild-type fat pads. Conversely, reciprocal grafting experiments demonstrate that Hoxa5-/- stroma cannot support normal proliferation of wild-type mammary epithelium. These data establish the essential contribution of Hoxa5 to mammary epithelium instruction by means of mesenchymal-epithelial crosstalk.

Identification of Notch target genes in uncommitted T-cell progenitors: No direct induction of a T-cell specific gene program

Deregulated Notch signaling occurs in the majority of human T-ALL. During normal lymphoid development, activation of the Notch signaling pathway poses a T-cell fate on hematopoietic progenitors. However, the transcriptional targets of the Notch pathway are largely unknown. We sought to identify Notch target genes by inducing Notch signaling in human hematopoietic progenitors using two different methods: an intracellular signal through transfection of activated Notch and a Notch-receptor dependent signal by interaction with its ligand Delta1. Gene expression profiles were generated and evaluated with respect to expression profiles of immature thymic subpopulations. We confirmed HES1, NOTCH1 and NRARP as Notch target genes, but other reported Notch targets, including the genes for Deltex1, pre-T-cell receptor alpha and E2A, were not found to be differentially expressed. Remarkably, no induction of T-cell receptor gene rearrangements or transcription of known T-cell specific genes was found after activation of the Notch pathway. A number of novel Notch target genes, including the transcription factor TCFL5 and the HOXA cluster, were identified and functionally tested. Apparently, Notch signaling is essential to open the T-cell pathway, but does not initiate the T-cell program itself.

Leukaemic transformation by CALM-AF10 involves upregulation of Hoxa5 by hDOT1L

Chromosomal translocation is a common cause of leukaemia and the most common chromosome translocations found in leukaemia patients involve the mixed lineage leukaemia (MLL) gene. AF10 is one of more than 30 MLL fusion partners in leukaemia. We have recently demonstrated that the H3K79 methyltransferase hDOT1L contributes to MLL-AF10-mediated leukaemogenesis through its interaction with AF10 (ref. 5). In addition to MLL, AF10 has also been reported to fuse to CALM (clathrin-assembly protein-like lymphoid-myeloid) in patients with T-cell acute lymphoblastic leukaemia (T-ALL) and acute myeloid leukaemia (AML). Here, we analysed the molecular mechanism of leukaemogenesis by CALM-AF10. We demonstrate that CALM-AF10 fusion is both necessary and sufficient for leukaemic transformation. Additionally, we provide evidence that hDOT1L has an important role in the transformation process. hDOT1L contributes to CALM-AF10-mediated leukaemic transformation by preventing nuclear export of CALM-AF10 and by upregulating the Hoxa5 gene through H3K79 methylation. Thus, our study establishes CALM-AF10 fusion as a cause of leukaemia and reveals that mistargeting of hDOT1L and upregulation of Hoxa5 through H3K79 methylation is the underlying mechanism behind leukaemia caused by CALM-AF10 fusion.

Dynamic expression of murine HOXA5 protein in the central nervous system

The Hox genes encode transcription factors that are indispensable for proper spatio-temporal patterning of the vertebrate body axes. As for other Hox genes, region-specific expression of Hoxa5 appears to be important for correct function during development. In mouse, Hoxa5 transcripts are differentially expressed in specific mesoderm-derived structures and in the most anterior domain of expression in the central nervous system (CNS), in contrast to indistinct patterns seen in the posterior CNS. However, the functional significance of any pattern of protein-coding RNAs must be verified by correlating the presence of the protein(s) and RNAs. Here, we describe the dynamic pattern of HOXA5 protein during mouse embryogenesis. The HOXA5 protein is detected as early as embryonic day (E) 9.0, and is found, as development proceeds, in several mesoderm-derived structures such as prevertebrae (pv), proximal forelimb bud, scapula, lung, trachea, and gut. In addition, the protein shows a strikingly restricted and dynamic expression pattern in the developing CNS, and is detected in both motor neurons and interneurons between E10.5 and E13.5. Moreover, this CNS region-specific HOXA5 protein pattern is more restricted than the pattern observed for the Hoxa5 transcripts. In many mesoderm-derived tissues affected by the Hoxa5 mutation, the expression pattern of HOXA5 protein corresponds to that of the putative functional Hoxa5 transcript. However, in the CNS, this correlation is exclusively demonstrated in the most anterior domain of expression. Overall, the HOXA5 protein pattern is consistent with its proposed role in positional specification in mesodermal structures, as well as in the embryonic neuraxis.

Cdx protein interaction with Hoxa5 regulatory sequences contributes to Hoxa5 regional expression along the axial skeleton

Hox gene functions are intimately linked to correct developmental expression of the genes. The identification of cis-acting regulatory sequences and their associated trans-acting factors constitutes a key step in deciphering the mechanisms underlying the correct positioning of the functional domain of Hox genes along the anterior-posterior axis. We have identified DNA elements driving Hoxa5 regionalized expression in mice, using the 2.1-kb mesodermal enhancer (MES) localized in Hoxa5 3' flanking sequences as a starting point. The MES sequence comprises regulatory elements targeting Hoxa5 expression in the limbs, the urogenital and gastrointestinal tracts, and the cervical-upper thoracic region of the prevertebral column. A 164-bp DNA fragment within the MES caudally restricts Hoxa5 expression at the level of prevertebra 10, corresponding to the posterior limit of its functional domain. Cdx proteins directly bind to this element in vitro via two conserved sites. Preventing Cdx binding by mutating the sites causes caudal expansion of the transgene expression domain. Of all three murine Cdx proteins that bind this element in vitro, Cdx4 has emerged as a potential regional posterior repressor of Hoxa5 expression. The restrictive control provided by Cdx interactions with Hoxa5 regulatory sequences may be one of the critical events in cervicothoracic axial specification.

Respiratory adaptations to lung morphological defects in adult mice lacking Hoxa5 gene function

The Hoxa5 mutation is associated with a high perinatal mortality rate caused by a severe obstruction of the laryngotracheal airways, pulmonary dysmorphogenesis, and a decreased production of surfactant proteins. Surviving Hoxa5(-/-) mutant mice also display lung anomalies with deficient alveolar septation and areas of collapsed tissue, thus demonstrating the importance of Hoxa5 throughout lung development and maturation. Here, we address the functional consequences of the Hoxa5 mutation on respiration and chemoreflexes by comparing the breathing pattern of Hoxa5(-/-) mice to that of wild-type animals under resting conditions and during exposure to moderate ventilatory stimuli such as hypoxia and hypercapnia. Resting Hoxa5(-/-) mice present a higher breathing frequency and overall minute ventilation that likely compensate for their reduced lung alveolar surface available for gas exchange and their increased upper airway resistance. When exposed to ventilatory stimuli, Hoxa5(-/-) mice maintain the higher minute ventilation by adapting the tidal volume and/or the breathing frequency. The minute ventilation increase seen during hypoxia was similar for both groups of mice; however, the dynamics of the frequency response was genotype-dependent. The hypercapnic ventilatory response did not differ between genotypes. These findings reveal the strategies allowing survival of Hoxa5(-/-) mice facing morphologic anomalies leading to a significant deficit in gas exchange capacity.

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.

Reduced fertility in male mice deficient in the zinc metallopeptidase NL1

Members of the M13 family of zinc metalloendopeptidases have been shown to play critical roles in the metabolism of various neuropeptides and peptide hormones, and they have been identified as important therapeutic targets. Recently, a mouse NL1 protein, a novel member of the family, was identified and shown to be expressed mainly in the testis as a secreted protein. To define its physiological role(s), we used a gene targeting strategy to disrupt the endogenous murine Nl1 gene by homologous recombination and generate Nl1 mutant mice. The Nl1(-/-) mice were viable and developed normally, suggesting that zygotic expression of Nl1 is not required for development. However, Nl1(-/-) males produced smaller litters than their wild-type siblings, indicating specific male fertility problems. Reduced fertility may be explained by two impaired processes, decreased egg fertilization and perturbed early development of fertilized eggs. These two phenotypes did not result from gross anatomical modifications of the testis or from impaired spermatogenesis. Basic sperm parameters were also normal. Thus, our findings suggest that one of the roles of NL1 in mice is related to sperm function and that NL1 modulates the processes of fertilization and early embryonic development in vivo.

Accelerated Induction of Bladder Cancer inPatchedHeterozygous Mutant Mice

The PATCHED (PTC) gene is recognized as a tumor suppressor in basal cell carcinoma. Mapping of a minimal region of deletion at 9q22.3 and observation of a decreased PTC expression in superficial papillary bladder tumors led us to hypothesize that it could also be involved in this cancer. To further investigate this hypothesis, we submitted Ptc(+/-) heterozygous mutant mice and their wild-type littermates to chemical carcinogenesis by adding N-butyl-N-(4-hydroxybutyl) nitrosamine to their drinking water. Preneoplastic and neoplastic changes were observed significantly earlier in the Ptc(+/-) than in the wild-type mice. Our data support the hypothesis of Ptc acting as a tumor suppressor gene in bladder cancer.

[The PATCHED/Sonic Hedgehog signalling pathway in superficial bladder cancer]

Superficial bladder cancer shows a high frequency of total or partial chromosome 9 losses. Loss of heterozygosity at position 9q22.3 is one of the most frequent and is associated with highly recurrent tumours. The PATCHED gene, ortholog of a gene first described in the drosophila as a segment polarity gene, is located at 9q22.3. It is a member of a signal transduction pathway and a tumour suppressor gene (TSG), involved in basal cell carcinoma. We propose PATCHED as a TSG candidate in superficial bladder cancer.

Perturbed thyroid morphology and transient hypothyroidism symptoms in Hoxa5 mutant mice

The Hox family of transcriptional regulators has been extensively studied for their role in axial and appendicular patterning. Genetic analyses have also unveiled Hox gene function in organogenesis and postnatal development. A phenotypical survey of the Hoxa5(-/-) mutant mice shows that the surviving mutants display symptoms of hypothyroidism, including transient growth retardation, and delayed eye opening and ear elevation. Thyroid gland morphogenesis initiates normally, but follicle formation and thyroglobulin processing are abnormal at late gestation. The expression of several molecular markers essential for thyroid gland formation and function, namely Nkx2.1, Pax8, and Titf2, is affected in the developing thyroid gland of Hoxa5(-/-) mutants. As a consequence, the expression of thyroid effector genes, including the thyroglobulin and thyroperoxidase genes, is perturbed. Our characterization reveals that the loss of Hoxa5 function transiently affects thyroid development in a non-cell autonomous manner.