The control of transitions along the main body axis

Although vertebrates display a large variety of forms and sizes, the mechanisms controlling the layout of the basic body plan are substantially conserved throughout the clade. Following gastrulation, head, trunk, and tail are sequentially generated through the continuous addition of tissue at the caudal embryonic end. Development of each of these major embryonic regions is regulated by a distinct genetic network. The transitions from head-to-trunk and from trunk-to-tail development thus involve major changes in regulatory mechanisms, requiring proper coordination to guarantee smooth progression of embryonic development. In this review, we will discuss the key cellular and embryological events associated with those transitions giving particular attention to their regulation, aiming to provide a cohesive outlook of this important component of vertebrate development.

Tgfbr1 regulates lateral plate mesoderm and endoderm reorganization during the trunk to tail transition

During the trunk to tail transition the mammalian embryo builds the outlets for the intestinal and urogenital tracts, lays down the primordia for the hindlimb and external genitalia, and switches from the epiblast/primitive streak to the tailbud as the driver of axial extension. Genetic and molecular data indicate that Tgfbr1 is a key regulator of the trunk to tail transition. Tgfbr1 has been shown to control the switch of the neuro mesodermal-competent cells from the epiblast to the chordo-neural hinge to generate the tail bud. We now show that Tgfbr1 signaling also controls the remodeling of the lateral plate mesoderm (LPM) and of the embryonic endoderm associated with the trunk to tail transition. In the absence of Tgfbr1 the two LPM layers do not converge at the end of the trunk, extending instead as separate layers enclosing the celomic cavity until the caudal embryonic extremity, and failing to activate markers of primordia for the hindlimb and external genitalia. However, this extended LPM, does not exhibit the molecular signatures characteristic of this tissue in the trunk. The vascular remodeling involving the dorsal aorta and the umbilical artery leading to the connection between embryonic and extraembryonic circulation was also affected in the Tgfbr1 mutant embryos. Similar alterations in the LPM and vascular system were also observed in Isl1 null mutants, indicating that this factor acts in the regulatory cascade downstream of Tgfbr1 in LPM-derived tissues. In addition, in the absence of Tgfbr1 the embryonic endoderm fails to expand to form the endodermal cloaca and to extend posteriorly to generate the tail gut. We present evidence suggesting that the remodeling activity of Tgfbr1 in the LPM and endoderm results from the control of the posterior primitive streak fate after its regression during the trunk to tail transition. Our data, together with previously reported observations, place Tgfbr1 at the top of the regulatory processes controlling the trunk to tail transition.

Endothelial cells require functional FLVCR1a during developmental and adult angiogenesis

The Feline Leukemia Virus Subgroup C Receptor 1a (FLVCR1a) is a transmembrane heme exporter essential for embryonic vascular development. However, the exact role of FLVCR1a during blood vessel development remains largely undefined. Here, we show that FLVCR1a is highly expressed in angiogenic endothelial cells (ECs) compared to quiescent ECs. Consistently, ECs lacking FLVCR1a give rise to structurally and functionally abnormal vascular networks in multiple models of developmental and pathologic angiogenesis. Firstly, zebrafish embryos without FLVCR1a displayed defective intersegmental vessels formation. Furthermore, endothelial-specific Flvcr1a targeting in mice led to a reduced radial expansion of the retinal vasculature associated to decreased EC proliferation. Moreover, Flvcr1a null retinas showed defective vascular organization and loose attachment of pericytes. Finally, adult neo-angiogenesis is severely affected in murine models of tumor angiogenesis. Tumor blood vessels lacking Flvcr1a were disorganized and dysfunctional. Collectively, our results demonstrate the critical role of FLVCR1a as a regulator of developmental and pathological angiogenesis identifying FLVCR1a as a potential therapeutic target in human diseases characterized by aberrant neovascularization.

Isolated Incudostapedial Cholesteatomas: Unique Radiologic and Surgical Features

OBJECTIVES

Congenital cholesteatomas originate from epithelial tissue present within the middle ear in patients with an intact tympanic membrane, no history of otologic surgery, otorrhea, or tympanic membrane perforation. They are diagnosed by a pearl-like lesion on otoscopy and computed tomography (CT) scan showing an expansile soft-tissue mass. We describe a series of patients with no prior otologic history presenting with progressive unilateral conductive hearing loss and normal otoscopy. The CT scans showed ossicular erosion without obvious soft-tissue mass. Surgery confirmed incudostapedial erosion found to be cholesteatoma. In this study, we characterize the clinical course of patients diagnosed with isolated incudostapedial cholesteatoma (IIC) and review possible pathologic mechanisms.

METHODS

Retrospective review of IIC cases treated by the Department of Pediatric Otolaryngology, Rady Children's Hospital, San Diego, 2014 to 2020. Data included patient demographics, clinical features, imaging, surgical findings, and audiologic data.

RESULTS

Five patients were diagnosed with IIC (3 [60%] female; mean age at presentation 10.7 years [range 5.5-16.0]). All patients presented with postlingual unilateral conductive hearing loss and normal otoscopy without any past otologic history; delay in diagnosis ranged from 4 months to several years. The CT scans showed ossicular chain erosion with an absent long process of the incus and/or stapes superstructure. All patients underwent middle ear exploration, revealing a thin layer of cholesteatoma in the incudostapedial region, confirmed by histopathology. Mean preoperative speech reception threshold was 55 dB and improved to a mean of 31 dB in the 4 patients who underwent ossicular chain reconstruction.

CONCLUSION

Isolated incudostapedial cholesteatoma should be included as a possible etiology in pediatric patients with insidious onset of unilateral conductive hearing loss with normal otoscopy, unremarkable otologic history, and a CT scan showing ossicular abnormality/disruption without notable middle ear mass. These patients should be counseled preoperatively regarding the possibility of cholesteatoma and should undergo middle ear exploration with possible ossiculoplasty.

A Tgfbr1/Snai1-dependent developmental module at the core of vertebrate axial elongation

Formation of the vertebrate postcranial body axis follows two sequential but distinct phases. The first phase generates pre-sacral structures (the so-called primary body) through the activity of the primitive streak on axial progenitors within the epiblast. The embryo then switches to generate the secondary body (post-sacral structures), which depends on axial progenitors in the tail bud. Here we show that the mammalian tail bud is generated through an independent functional developmental module, concurrent but functionally different from that generating the primary body. This module is triggered by convergent Tgfbr1 and Snai1 activities that promote an incomplete epithelial to mesenchymal transition on a subset of epiblast axial progenitors. This EMT is functionally different from that coordinated by the primitive streak, as it does not lead to mesodermal differentiation but brings axial progenitors into a transitory state, keeping their progenitor activity to drive further axial body extension.

THU0071 CD5L IN RHEUMATOID ARTHRITIS: PROTECTIVE OR PROMOTER?

Background:

Rheumatoid Arthritis (RA) is an aggressive auto-immune disease characterized by synovial hyperplasia and chronic inflammation. The main players of RA pathogenesis are T-cell and B-cell dependent pathways and some myeloid cells are also abundant in the synovial tissue. However, how inflammation is initiated, propagated and maintained remains controversial. Unbiased proteomic reports revealed an enrichment in the scavenger receptor CD5L, a component of serum and synovial tissues of arthritic patients.1 Upon secretion, this blood circulating glycoprotein represses pathogenic Th17 cells, promotes M2 polarization and binds and aggregates Gram-negative and -positive bacteria.2-4 However, its mechanisms of action has not been established either in health or disease.

Objectives:

We intend to clarify whether CD5L is an immune component that helps resolving RA or a factor that aggravates the disease.

Methods:

We analyzed by ELISA the presence of CD5L in samples from RA patients covering different stages of the disease, and correlated with other markers of RA. In parallel, we experimentally induced collagen induced arthritis (CIA) in CD5L knockout (KO) mice to evaluate the incidence and severity of the disease. The differences between the cellular groups in circulation vs the composition on secondary lymph organs using flow cytometry were also investigated in KO and WT mice. The histopathology of the joints was examined, while cytokine concentrations at several timepoints and total Ig levels were measured by ELISA and cytometric bead assays, respectively.

Results:

The samples from RA patients showed increased CD5L levels concomitant with the severity of the disease and a direct correlation with Sharp RTG Score or IL-6 serum levels, and inversely correlated with COMP levels. However, these correlations did not clarify whether CD5L helps to resolve RA or is a component that aggravates the disease. To clarify this aspect, we provoked CIA in CD5L KO mice and observed a higher incidence of RA, higher severity and a much lower recovery rate when compared with WT mice. To corroborate these data, the H&E staining of sagittal section of fore- and hindpaws revealed histopathology consistency with RA, with notable inflammatory signs especially in KO mice. WT animals with RA also showed higher levels of CD5L when compared with the control group, which confirms the observations obtained for human samples. Total serum IgG levels did not correlate with the disease severity but KO mice presented higher quantities of IgG and IL-6 when compared with WT mice.

Conclusion:

Overall, these data imply that CD5L is not a promoter of the disease but rather a fundamental protective molecule against inflammation.

References:

[1]Balakrishnan L et al. (2014) Clin Proteom 11:1

[2]Wang C et al. (2015) Cell 163:1413-27

[3]Sanjurjo L et al. (2018) Front Immunol 9:480

[4]Martinez VG et al. (2014) Cell Mol Immunol 11:343–354

Acknowledgments:

This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No683356-FOLSMART

Disclosure of Interests:

None declared

Two CRISPR/Cas9-mediated methods for targeting complex insertions, deletions, or replacements in mouse

Genetically modified model organisms are valuable tools for probing gene function, dissecting complex signaling networks, studying human disease, and more. CRISPR/Cas9 technology has significantly democratized and reduced the time and cost of generating genetically modified models to the point that small gene edits are now routinely and efficiently generated in as little as two months. However, generation of larger and more sophisticated gene-modifications continues to be inefficient. Alternative ways to provide the replacement DNA sequence, method of Cas9 delivery, and tethering the template sequence to Cas9 or the guide RNA (gRNA) have all been tested in an effort to maximize homology-directed repair for precise modification of the genome. We present two CRISPR/Cas9 methods that have been used to successfully generate large and complex gene-edits in mouse. In the first method, the Cas9 enzyme is used in conjunction with two sgRNAs and a long single-stranded DNA (lssDNA) template prepared by an alternative protocol. The second method utilizes a tethering approach to couple a biotinylated, double-stranded DNA (dsDNA) template to a Cas9-streptavidin fusion protein.

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Alternative method for generating long, single-stranded DNA templates for CRISPR/Cas9 editing.

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Demonstration that using two sgRNAs with Cas9-streptavidin/biotinylated-dsDNA is feasible for large DNA modifications.

A tissue-specific, Gata6-driven transcriptional program instructs remodeling of the mature arterial tree

Connection of the heart to the systemic circulation is a critical developmental event that requires selective preservation of embryonic vessels (aortic arches). However, why some aortic arches regress while others are incorporated into the mature aortic tree remains unclear. By microdissection and deep sequencing in mouse, we find that neural crest (NC) only differentiates into vascular smooth muscle cells (SMCs) around those aortic arches destined for survival and reorganization, and identify the transcription factor Gata6 as a crucial regulator of this process. Gata6 is expressed in SMCs and its target genes activation control SMC differentiation. Furthermore, Gata6 is sufficient to promote SMCs differentiation in vivo, and drive preservation of aortic arches that ought to regress. These findings identify Gata6-directed differentiation of NC to SMCs as an essential mechanism that specifies the aortic tree, and provide a new framework for how mutations in GATA6 lead to congenital heart disorders in humans.

Reorganisation of Hoxd regulatory landscapes during the evolution of a snake-like body plan

Within land vertebrate species, snakes display extreme variations in their body plan, characterized by the absence of limbs and an elongated morphology. Such a particular interpretation of the basic vertebrate body architecture has often been associated with changes in the function or regulation of Hox genes. Here, we use an interspecies comparative approach to investigate different regulatory aspects at the snake HoxD locus. We report that, unlike in other vertebrates, snake mesoderm-specific enhancers are mostly located within the HoxD cluster itself rather than outside. In addition, despite both the absence of limbs and an altered Hoxd gene regulation in external genitalia, the limb-associated bimodal HoxD chromatin structure is maintained at the snake locus. Finally, we show that snake and mouse orthologous enhancer sequences can display distinct expression specificities. These results show that vertebrate morphological evolution likely involved extensive reorganisation at Hox loci, yet within a generally conserved regulatory framework.

An ultra-conserved element in Hoxa2 is required for the normal expression of Hoxa3 and normal thymus development (HEM7P.223)

We analyzed thymus development in a Hoxa2 mutant mouse (Hoxa21) that carries a deletion of an ultraconserved element in the protein coding sequence of Hoxa2. Our phenotypic analysis revealed that the Hoxa21 mutants exhibit hypoplastic and ectopic thymii. Further analysis demonstrated delayed Foxn1 expression within the developing thymus of the mutants. These phenotypes resemble the alterations in thymus development found in Hoxa3 mutant embryos. Intriguingly, analysis of thymus development in mutants carrying a different Hoxa2 null allele that contains an intact ultraconserved element (Hoxa22) revealed no alteration in thymus development. This findings suggested that the abnormal thymus development seen in the Hoxa21 mutants may be due to alterations in Hoxa3 expression. Consistent with this hypothesis Hoxa21 mutants exhibited greatly reduced Hoxa3 expression in neural crest-derived mesenchyme and pharyngeal endoderm, embryonic cell types required for normal thymus development. In addition analysis of Hoxa21+/- Hoxa3 null/+ compound heterozygotes revealed thymus phenotypes that were very similar to Hoxa3-deficient mutants. These data suggest that Hoxa2, although expressed in thymic primordium, does not play a direct role in thymic organogenesis. Our results suggest that the ultraconserved enhancer element located the first exon of Hoxa2 is necessary for normal Hoxa3 expression in cell types necessary for normal thymus development.

The relationship of TP53 R72P polymorphism to disease outcome and TP53 mutation in myelodysplastic syndromes

Nonsynonymous TP53 exon 4 single-nucleotide polymorphism (SNP), R72P, is linked to cancer and mutagen susceptibility. R72P associations with specific cancer risk, particularly hematological malignancies, have been conflicting. Myelodysplastic syndrome (MDS) with chromosome 5q deletion is characterized by erythroid hypoplasia arising from lineage-specific p53 accumulation resulting from ribosomal insufficiency. We hypothesized that apoptotically diminished R72P C-allele may influence predisposition to del(5q) MDS. Bone marrow and blood DNA was sequenced from 705 MDS cases (333 del(5q), 372 non-del(5q)) and 157 controls. Genotype distribution did not significantly differ between del(5q) cases (12.6% CC, 38.1% CG, 49.2% GG), non-del(5q) cases (9.7% CC, 44.6% CG, 45.7% GG) and controls (7.6% CC, 37.6% CG, 54.8% GG) (P=0.13). Allele frequency did not differ between non-del(5q) and del(5q) cases (P=0.91) but trended towards increased C-allele frequency comparing non-del(5q) (P=0.08) and del(5q) (P=0.10) cases with controls. Median lenalidomide response duration increased proportionate to C-allele dosage in del(5q) patients (2.2 (CC), 1.3 (CG) and 0.89 years (GG)). Furthermore, C-allele homozygosity in del(5q) was associated with prolonged overall and progression-free survival and non-terminal interstitial deletions that excluded 5q34, whereas G-allele homozygozity was associated with inferior outcome and terminal deletions involving 5q34 (P=0.05). These findings comprise the largest MDS R72P SNP analysis.

Long bone development requires a threshold of Hox function

The Hoxd(Del(11-13)) mutant is one of the animal models for human synpolydactyly, characterized by short and syndactylous digits. Here we have characterized in detail the cartilage and bone defects in these mutants. We report two distinct phenotypes: (i) a delay and change in pattern of chondrocyte maturation of metacarpals/metatarsals and (ii) formation of a poor and not centrally positioned primary ossification center in the proximal-intermediate phalanx. In the metacarpals of Hoxd(Del(11-13)) mutants, ossification occurs postnataly, in the absence of significant Ihh expression and without the establishment of growth plates, following patterns similar to those of short bones. The strong downregulation in Ihh expression is associated with a corresponding increase of the repressor form of Gli3. To evaluate the contribution of this alteration to the phenotype, we generated double Hoxd(Del(11-13));Gli3 homozygous mutants. Intriguingly, these double mutants showed a complete rescue of the phenotype in metatarsals but only partial phenotypic rescue in metacarpals. Our results support Hox genes being required in a dose-dependent manner for long bone cartilage maturation and suggest that and excess of Gli3R mediates a significant part of the Hoxd(Del(11-13)) chondrogenic phenotype.

Monosomal karyotype in MDS: explaining the poor prognosis?

Monosomal karyotype (MK) is associated with an adverse prognosis in patients in acute myeloid leukemia (AML). This study analyzes the prognostic impact of MK in a cohort of primary, untreated patients with myelodysplastic syndromes (MDS). A total of 431 patients were extracted from an international database. To analyze whether MK is an independent prognostic marker in MDS, cytogenetic and clinical data were explored in uni- and multivariate models regarding overall survival (OS) as well as AML-free survival. In all, 204/431 (47.3%) patients with MK were identified. Regarding OS, MK was prognostically significant in patients with ≤ 4 abnormalities only. In highly complex karyotypes (≥ 5 abnormalities), MK did not separate prognostic subgroups (median OS 4.9 months in MK+ vs 5.6 months in patients without MK, P=0.832). Based on the number of abnormalities, MK-positive karyotypes (MK+) split into different prognostic subgroups (MK+ and 2 abnormalities: OS 13.4 months, MK+ and 3 abnormalities: 8.0 months, MK+ and 4 abnormalities: 7.9 months and MK+ and ≥ 5 abnormalities: 4.9 months; P<0.01). In multivariate analyses, MK was not an independent prognostic factor. Our data support the hypothesis that a high number of complex abnormalities, associated with an instable clone, define the subgroup with the worst prognosis in MDS, independent of MK.

Concerted involvement of Cdx/Hox genes and Wnt signaling in morphogenesis of the caudal neural tube and cloacal derivatives from the posterior growth zone

Decrease in Cdx dosage in an allelic series of mouse Cdx mutants leads to progressively more severe posterior vertebral defects. These defects are corrected by posterior gain of function of the Wnt effector Lef1. Precocious expression of Hox paralogous 13 genes also induces vertebral axis truncation by antagonizing Cdx function. We report here that the phenotypic similarity also applies to patterning of the caudal neural tube and uro-rectal tracts in Cdx and Wnt3a mutants, and in embryos precociously expressing Hox13 genes. Cdx2 inactivation after placentation leads to posterior defects, including incomplete uro-rectal septation. Compound mutants carrying one active Cdx2 allele in the Cdx4-null background (Cdx2/4), transgenic embryos precociously expressing Hox13 genes and a novel Wnt3a hypomorph mutant all manifest a comparable phenotype with similar uro-rectal defects. Phenotype and transcriptome analysis in early Cdx mutants, genetic rescue experiments and gene expression studies lead us to propose that Cdx transcription factors act via Wnt signaling during the laying down of uro-rectal mesoderm, and that they are operative in an early phase of these events, at the site of tissue progenitors in the posterior growth zone of the embryo. Cdx and Wnt mutations and premature Hox13 expression also cause similar neural dysmorphology, including ectopic neural structures that sometimes lead to neural tube splitting at caudal axial levels. These findings involve the Cdx genes, canonical Wnt signaling and the temporal control of posterior Hox gene expression in posterior morphogenesis in the different embryonic germ layers. They shed a new light on the etiology of the caudal dysplasia or caudal regression range of human congenital defects.

Hox genes and regional patterning of the vertebrate body plan

Several decades have passed since the discovery of Hox genes in the fruit fly Drosophila melanogaster. Their unique ability to regulate morphologies along the anteroposterior (AP) axis (Lewis, 1978) earned them well-deserved attention as important regulators of embryonic development. Phenotypes due to loss- and gain-of-function mutations in mouse Hox genes have revealed that the spatio-temporally controlled expression of these genes is critical for the correct morphogenesis of embryonic axial structures. Here, we review recent novel insight into the modalities of Hox protein function in imparting specific identity to anatomical regions of the vertebral column, and in controlling the emergence of these tissues concomitantly with providing them with axial identity. The control of these functions must have been intimately linked to the shaping of the body plan during evolution.

HOXB4's road map to stem cell expansion

Homeodomain-containing transcription factors are important regulators of stem cell behavior. HOXB4 mediates expansion of adult and embryo-derived hematopoietic stem cells (HSCs) when expressed ectopically. To define the underlying molecular mechanisms, we performed gene expression profiling in combination with subsequent functional analysis with enriched adult HSCs and embryonic derivatives expressing inducible HOXB4. Thereby, we identified a set of overlapping genes that likely represent "universal" targets of HOXB4. A substantial number of loci are involved in signaling pathways important for controlling self-renewal, maintenance, and differentiation of stem cells. Functional assays performed on selected pathways confirmed the biological coherence of the array results. HOXB4 activity protected adult HSCs from the detrimental effects mediated by the proinflammatory cytokine TNF-alpha. This protection likely contributes to the competitive repopulation advantage of HOXB4-expressing HSCs observed in vivo. The concept of TNF-alpha inhibition may also prove beneficial for patients undergoing bone marrow transplantation. Furthermore, we demonstrate that HOXB4 activity and FGF signaling are intertwined. HOXB4-mediated expansion of adult and ES cell-derived HSCs was enhanced by specific and complete inhibition of FGF receptors. In contrast, the expanding activity of HOXB4 on hematopoietic progenitors in day 4-6 embryoid bodies was blunted in the presence of basic FGF (FGF2), indicating a dominant negative effect of FGF signaling on the earliest hematopoietic cells. In summary, our results strongly suggest that HOXB4 modulates the response of HSCs to multiple extrinsic signals in a concerted manner, thereby shifting the balance toward stem cell self-renewal.

Sludge bed development in denitrifying reactors using different inocula-performance and microbiological aspects

Aerobic and methanogenic consortia were evaluated as inocula for laboratory scale denitrifying reactors, fed with a synthetic wastewater with acetate as the main electron donor. The denitrifying microflora of inocula and reactors was evaluated by specific denitrifying activity, enumeration and isolation of denitrifiers, which were screened by amplified ribosomal DNA restriction analysis. Reactor performance was monitored by COD and nitrate removal efficiencies and granule size. The aerobic sludge failed to form granules, probably due to the development of a filamentous, nitrate-reducing organism which was characterised by 16SrDNA sequencing as Bacillus cereus. The methanogenic sludge showed denitrifying activity and adapted very rapidly to denitrifying conditions in the two reactors seeded with granules of different sizes. Denitrifiers grew around the granules, increasing the specific denitrifying activity of the sludge over 10-fold. Exopolymer-forming organisms, belonging to the same species, were isolated from both reactors. Granule size increased during operation, but flotation of the aggregates, related to gas retention was observed.

Different levels of Hoxa2 are required for particular developmental processes

Hoxa2 is required for a variety of developmental processes in the branchial arches and in the hindbrain. We have created a Hoxa2 allele that is about 45% as active in transcription as its wild-type counterpart. This allele, together with the Hoxa2 null and wild-type alleles, allowed the generation of embryos developing in the presence of different levels of Hoxa2 activity. Analysis of these embryos indicates that in general the hindbrain is more resistant to Hoxa2 deficiencies than the second branchial arch. Also, within the second arch, proximo-caudal areas are more sensitive than the rostro-distal. In the hindbrain, basic segmentation and patterning processes seem to occur normally at Hoxa2 levels as low as 20% of the normal. In addition, specific neuronal markers along the dorso-ventral axis of the hindbrain seem differentially affected by reduced Hoxa2 levels. These results provide new clues to understand the role of Hoxa2 in the different embryonic areas where it is required.

Formation of the middle ear: recent progress on the developmental and molecular mechanisms

The middle ear allows animals to hear while moving in an aerial medium. It is composed of a cavity harbouring a chain of three ossicles that transmit vibrations produced by airborne sound in the tympanic membrane into the inner ear, where they are converted into neural impulses. The middle ear develops in the branchial arches, and this requires sequential interactions between the epithelia and the underlying mesenchyme. Gene-inactivation experiments have identified genes required for the formation of different middle ear components. Some encode for signalling molecules, including Endothelin1 and Fgf8, probable mediators of epithelial-mesenchymal interactions. Other genes, including Eya1, Prx1, Hoxa1, Hoxa2, Dlx1, Dlx2, Dlx5, and Gsc, are most likely involved in patterning and morphogenetic processes in the neural crest-derived mesenchyme. Mechanisms controlling formation of a functional tympanic membrane are also discussed. Basically, the tympanic ring, which serves as support for the tympanic membrane, directs invagination of the first pharyngeal cleft ectoderm to form the external acoustic meatus (EAM), which provides the outer layer of the membrane. Gsc and Prx1 are essential for tympanic ring development. While invaginating, the EAM controls skeletogenesis in the underlying mesenchyme to form the manubrium of the malleus, the link between the membrane and the middle ear ossicles.

Assembling a functional tympanic membrane: signals from the external acoustic meatus coordinate development of the malleal manubrium

In terrestrial mammals, hearing starts with the perception of acoustic pressure by the tympanic membrane. Vibrations in this membrane are then transduced into the inner ear by the ossicle chain of the middle ear, composed of the malleus, incus and stapes. The proper connection of the ossicle chain with the tympanic membrane, provided by the insertion of the manubrium of the malleus into the eardrum, is essential for the functionality of the hearing apparatus. We describe here the mechanisms regulating the development of the manubrium and its integration into the tympanic membrane. We show that the external acoustic meatus (EAM), which eventually forms the outer epithelium of the tympanic membrane, plays an essential role in this developmental process. Histological and expression analyses indicate that the manubrium develops close to the EAM with a similar temporal sequence. In addition, when the middle ear ossicles are allowed to develop in vitro under conditions that do not support further EAM development, the manubrium develops only up to the stage of its induction at the time of explantation. Moreover, genetically or teratogenically derived alterations in the EAM also have an effect on manubrial development. Finally, we show that the EAM is the source of two quite opposite activities, one that induces chondrogenesis and another that represses it. The combination of these two activities results in the proper positioning of the manubrium.

BMP signaling is essential for development of skeletogenic and neurogenic cranial neural crest

BMP signaling is essential for a wide variety of developmental processes. To evaluate the role of Bmp2/4 in cranial neural crest (CNC) formation or differentiation after its migration into the branchial arches, we used Xnoggin to block their activities in specific areas of the CNC in transgenic mice. This resulted in depletion of CNC cells from the targeted areas. As a consequence, the branchial arches normally populated by the affected neural crest cells were hypomorphic and their skeletal and neural derivatives failed to develop. In further analyses, we have identified Bmp2 as the factor required for production of migratory cranial neural crest. Its spatial and temporal expression patterns mirror CNC emergence and Bmp2 mutant embryos lack both branchial arches and detectable migratory CNC cells. Our results provide functional evidence for an essential role of BMP signaling in CNC development.

Hoxa-2 restricts the chondrogenic domain and inhibits bone formation during development of the branchial area

In Hoxa-2(−/−)embryos, the normal skeletal elements of the second branchial arch are replaced by a duplicated set of first arch elements. We show here that Hoxa-2 directs proper skeletal formation in the second arch by preventing chondrogenesis and intramembranous ossification. In normal embryos, Hoxa-2 is expressed throughout the second arch mesenchyme, but is excluded from the chondrogenic condensations. In the absence of Hoxa-2, chondrogenesis is activated ectopically within the rostral Hoxa-2 expression domain to form the mutant set of cartilages. In Hoxa-2(−/−)embryos the Sox9 expression domain is shifted into the normal Hoxa-2 domain. Misexpression of Sox9 in this area produces a phenotype resembling that of the Hoxa-2 mutants. These results indicate that Hoxa-2 acts at early stages of the chondrogenic pathway, upstream of Sox9 induction. We also show that Hoxa-2 inhibits dermal bone formation when misexpressed in its precursors. Furthermore, molecular analyses indicate that Cbfa1 is upregulated in the second branchial arches of Hoxa-2 mutant embryos suggesting that prevention of Cbfa1 induction might mediate Hoxa-2 inhibition of dermal bone formation during normal second arch development. The implications of these results on the patterning of the branchial area are discussed.

Murine macrophages secrete interferon gamma upon combined stimulation with interleukin (IL)-12 and IL-18: A novel pathway of autocrine macrophage activation

Interferon (IFN)-gamma, a key immunoregulatory cytokine, has been thought to be produced solely by activated T cells and natural killer cells. In this study, we show that murine bone marrow- derived macrophages (BMMPhi) secrete large amounts of IFN-gamma upon appropriate stimulation. Although interleukin (IL)-12 and IL-18 alone induce low levels of IFN-gamma mRNA transcripts, the combined stimulation of BMMPhi with both cytokines leads to the efficient production of IFN-gamma protein. The macrophage-derived IFN-gamma is biologically active as shown by induction of inducible nitric oxide synthase as well as upregulation of CD40 in macrophages. Our findings uncover a novel pathway of autocrine macrophage activation by demonstrating that the macrophage is not only a key cell type responding to IFN-gamma but also a potent IFN-gamma-producing cell.

Segmental identity can change independently in the hindbrain and rhombencephalic neural crest

In this study we tested whether the segmental identities of the hindbrain and its derived neural crest are necessarily linked or, instead, if they can be altered independently. Using morphological criteria, we show that the hindbrains of Hoxa-2 mutant mice, in which the second arch skeletal derivatives assume first arch characteristics (Gendron-Maguire et al. [1993] Cell 75:1317-1331; Rijli et al. [1993] Cell 75:1333-1349), retain normal segmental identities. Also, by phenotypic analysis, we show that, with retinoic acid, changes can be induced in the identity of the preotic hindbrain without effects in its derived neural crest. Our data thus indicate that identity changes in the hindbrain and branchial arch neural crest can occur independently. Moreover, if Hoxa-2 is concomitantly induced by retinoic acid in the first branchial arch, the proximal derivatives of this arch are also affected. We propose a model for the patterning of the branchial region, according to which the segmental identity in this area is provided mainly by the branchial arches.

Contralateral efferent neurons can be detected in the hindbrain outside of rhombomere 4

A group of efferent neurons whose bodies are located contralaterally and extend projections across the ventral midline of the hindbrain is considered as a rhombomere 4-specific characteristic. These neurons contribute to the vestibulo-acoustic nerve. At the level of rhombomere 2, a similar kind of efferents have only been described as a result of several experimental manipulations and have been interpreted as being due to rhombomere 2 acquiring rhombomere 4 identity. Here is shown that contralateral efferents can also be detected in rhombomere 2 of normal mouse and chicken hindbrains. These findings indicate that neural processes crossing the midline should not be considered as a rhombomere 4-specific characteristic. They also imply that the formation of the contralateral efferents at different rostro-caudal levels might be under different genetic controls, because Hoxb-1, which is not expressed in rhombomere 2, seems to be essential for their proper formation in rhombomere 4.

Retinoic acid disturbs mouse middle ear development in a stage-dependent fashion

The mammalian middle ear contains a chain of three ossicles, the malleus, incus, and stapes, that transmit into the inner ear the vibrations produced in the tympanic membrane by aerial sound. I show here that retinoic acid interferes with the formation of the middle ear in a stage-specific fashion. The malformations produced are derived from a retinoic acid-induced inhibition of the formation and/or migration of the cranial neural crest that generates the middle ear skeletal elements and not from a respecification of neural crest identity to more posterior fates. I have used these effects of retinoic acid to analyze the temporal sequence of events involved in the morphogenesis of the middle ear. I show that the middle ear bones develop from several primordia that originate in a typical temporal sequence from Day 8 plus 4.5 hr to Day 8 plus 7.5 hr of pregnancy. Moreover, interactions between adjacent bones are not required for their proper formation. My results also suggest a Hoxa-2-mediated patterning of the otic capsule in the region where the oval window is located.

Intracellular posttranslational modifications of S1133 avian reovirus proteins

Avian reovirus S1133 specifies at least 10 primary translation products, eight of which are present in the viral particle and two of which are nonstructural proteins. In the work presented here, we studied the covalent modifications undergone by these translation products in the infected cell. The structural polypeptide mu2 was shown to be intracellularly modified by both myristoylation and proteolysis. The site-specific cleavage of mu2 yielded a large carboxy-terminal fragment and a myristoylated approximately 5,500-Mr peptide corresponding to the amino terminus. Both mu2 and its cleavage products were found to be structural components of the reovirion. Most avian reovirus proteins were found to be glycosylated and to have a blocking group at the amino terminus. In contrast to the mammalian reovirus system, none of the avian reovirus polypeptides was found to incorporate phosphorus during infection. Our results add to current understanding of the similarities and differences between avian and mammalian reoviruses.

Development of the mammalian ear: coordinate regulation of formation of the tympanic ring and the external acoustic meatus

The tympanic membrane in mammals is a trilaminar structure formed by the apposition of two epithelial cell layers, along with an intervening layer of cells derived from pharyngeal arch mesenchyme. One epithelial layer is contributed by the external acoustic meatus, a derivative of the first pharyngeal cleft. The other epithelial layer is contributed by the tubotympanic recess, a derivative of the first pharyngeal pouch. We demonstrate here an absolute correlation between formation of the external acoustic meatus and formation of the tympanic ring, a first arch-derived membrane bone that anchors the tympanic membrane. Experimental loss of the tympanic ring by retinoic acid treatment, or duplication of the ring in Hoxa-2 null mutant embryos, resulted in corresponding alterations in formation of the external acoustic meatus. We suggest that the tympanic ring primordium induces formation and morphogenesis of the external acoustic meatus, and that expression of the Hoxa-2 and goosecoid genes may be involved in regulating the formation and morphogenesis of these structures.

Protein characterization and targeted disruption of Grg, a mouse gene related to the groucho transcript of the Drosophila Enhancer of split complex

The Grg gene encodes a 197 amino acid protein homologous to the amino-terminal domain of the product of the groucho gene of the Drosophila Enhancer of split complex. Analysis with a polyclonal antisera specific for the Grg protein revealed that Grg is a 25 kd nuclear protein that can participate in specific protein-protein interactions. A null mutation of the Grg gene was constructed by gene targeting. Mice homozygous for this mutation completed embryogenesis and were born, but exhibited varying degrees of post-natal growth deficiency. No dosage-sensitive genetic interaction was detected between the Notch1 and Grg genes in mice heterozygous for a Notch1 mutant allele and homozygous for the Grg null mutation.

Goosecoid is not an essential component of the mouse gastrula organizer but is required for craniofacial and rib development

Goosecoid (gsc) is an evolutionarily conserved homeobox gene expressed in the gastrula organizer region of a variety of vertebrate embryos, including zebrafish, Xenopus, chicken and mouse. To understand the role of gsc during mouse embryogenesis, we generated gsc-null mice by gene targeting in embryonic stem cells. Surprisingly, gsc-null embryos gastrulated and formed the primary body axes; gsc-null mice were born alive but died soon after birth with numerous craniofacial defects. In addition, rib fusions and sternum abnormalities were detected that varied depending upon the genetic background. Transplantation experiments suggest that the ovary does not provide gsc function to rescue gastrulation defects. These results demonstrate that gsc is not essential for organizer activity in the mouse but is required later during embryogenesis for craniofacial and rib cage development.

Possible involvement of the mouse Grg protein in transcription

The mouse Grg gene encodes a 197 amino acid nuclear protein homologous to the amino-terminal domain of the product of the groucho (gro) gene of the Drosophila Enhancer of split complex. Recent work has suggested that the gro protein functions as a transcriptional corepressor during Drosophila development. We therefore examined possible roles of the mouse Grg protein in DNA binding and in vitro transcription. No sequence-specific DNA binding activity was detected by polymerase chain reaction-DNA binding site selection nor was the glutamine-rich Grg protein capable of acting as an activation domain in an in vivo transactivation assay. However, depletion of Grg protein from HeLa nuclear extracts inhibited the in vitro transcription activity of the extracts. We suggest that Grg protein may interact with components of the basal transcription machinery.

Genomic organization, alternative polyadenylation, and chromosomal localization of Grg, a mouse gene related to the groucho transcript of the Drosophila Enhancer of split complex

The Grg gene encodes a 197-amino-acid protein homologous to the amino-terminal domain of the product of the groucho gene of the Drosophila Enhancer of split complex. We describe here the genomic organization of the mouse Grg gene. It spans approximately 7 kb on chromosome 10 and consists of seven exons. The 3' region of the Grg gene contains two functional polyadenylation sites that give rise to two transcripts that are differentially expressed among adult mouse tissues. The promoter region is very GC rich and lacks TATA box and "initiator" sequences. Primer extension analysis and ribonuclease protection assays show that Grg has a major transcription start site situated down-stream of putative binding motifs for the transcription factors Sp1, E2A, and PuF.

Hoxa-2 mutant mice exhibit homeotic transformation of skeletal elements derived from cranial neural crest

Mice homozygous for a targeted mutation of the Hoxa-2 (Hox 1.11) gene are born with cleft palates and die within 24 hr of birth. Analysis of stained skeletons revealed that homozygous mutant animals contained multiple cranial skeletal defects, including a duplication of the ossification centers of the bones of the middle ear. Histological analysis suggested that this duplication resulted from the transformation of skeletal elements derived from the second branchial arch into more anterior structures, resulting in a duplication of Meckel's cartilage adjacent to the otic capsule. Skeletal elements normally derived from the second arch were absent in the mutants. These data provide direct experimental evidence for the existence of a branchial Hox code.

Cloning and developmental expression of Grg, a mouse gene related to the groucho transcript of the Drosophila Enhancer of split complex

Genes of the Enhancer of split complex are involved in neural-epidermal cell fate decisions during early embryogenesis in Drosophila. One of these genes, the product of the Enhancer of split m9/10 or groucho transcript, encodes a ubiquitous nuclear protein with homology at the carboxy-terminus to G-protein beta-subunits. Here we describe the cloning and RNA expression analysis of a mouse gene, designated Grg, that is homologous to just the amino-terminal region of the groucho product. Grg encodes a 197 amino acid protein that shares 53% amino acid identity with the corresponding region of the product of the Drosophila groucho gene. However, the mouse Grg protein does not contain the region homologous to G-protein beta-subunits. An analysis by in situ hybridization of the spatial and temporal localization of Grg RNA expression revealed that, while the initial pattern of Grg expression was quite restricted, by midgestation Grg RNA was ubiquitously expressed in the developing embryo. Widespread Grg RNA expression was maintained in adult mice. The implications of these results for the existence of separable functional domains of the Drosophila groucho product, and possible roles of the Grg gene during mouse development, are discussed.

Avian reovirus S1133 can replicate in mouse L cells: effect of pH and cell attachment status on viral infection

Previous reports have suggested that avian reovirus S1133 fails to replicate in mouse L cells. In this article, we report that replication does occur under certain culture conditions. The avian reovirus was found to grow in mouse L cells at pH 6.4 and 7.2 but not at pH 8.2. Culture medium with a basic pH directly inhibited viral transcription and genome replication. As a result, viral protein synthesis was also affected. At permissive pH levels, avian reovirus grew better in monolayers than in suspension cultures of L cells because of the influence of cell attachment status on viral macromolecular synthesis. Our results not only show that avian reovirus can replicate in mouse L cells but also help to explain why it did not in previous studies.

The stimulatory effect of actinomycin D on avian reovirus replication in L cells suggests that translational competition dictates the fate of the infection

Indirect immunostaining of avian reovirus S1133-infected L-cell monolayers showed that most of the cells can support viral replication. However, the number of cells in which the virus was actually replicating depended on the multiplicity of virus infection. The presence of actinomycin D during infection increased viral protein synthesis, viral growth, and the number of actively infected cells at late infection times. The antibiotic elicited these effects by triggering viral replication in cells that already contained unproductive cytoplasmic virus but that would not get productively infected in the absence of the drug. From these results, we propose a model for the interaction between L cells and avian reovirus S1133 in which viral versus host mRNA competition for the translational machinery determines the fate of the virus infection.