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

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.

Detection of Gene and Protein Expression in Mouse Embryos and Tissue Sections

Analysis of gene (mRNA and protein) expression patterns is central to the study of embryonic development. This chapter details methods for detecting mRNA and protein expression in whole-mouse embryos and in tissue sections, including mRNA in situ hybridization, immunohistochemistry, and detection of enzymatic and fluorescent protein reporters. We focus on histological methods; molecular methods of measuring gene expression (for example, RNAseq, PCR) are not included here.

Development and identification of Set transgenic mice

As a multifunctional protein involved in numerous biological processes, Set is expressed in several embryonic and adult organs. Furthermore, Set is overexpressed in numerous types of human cancers, including acute myeloid leukemia, breast cancer and pancreatic cancer. The expression of Set in germ cells is involved in gonad development, and the overexpression of Set has been observed in polycystic ovaries. In order to elucidate the physiological and pathological roles of Set, a Set transgenic mouse model was developed, in which the global overexpression of Set in adult tissues could be induced via the Cre/loxP system with the precise deletion of the Stop fragment in double-transgenic hybrids. This result was then confirmed by genotypical and protein analysis using polymerase chain reaction and bioluminescence imaging. In conclusion, the conditional Set transgenic mice carrying a reporter system were successfully generated. The transgenic mice open a new window for the further investigation of the function of Set using tissue-specific Cre mice and inducible Cre systems.

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.

New insights into the vertebral Hox code of archosaurs

Variation in axial formulae (i.e., number and identity of vertebrae) is an important feature in the evolution of vertebrates. Vertebrae at different axial positions exhibit a region-specific morphology. Key determinants for the establishment of particular vertebral shapes are the highly conserved Hox genes. Here, we analyzed Hox gene expression in the presacral vertebral column in the Nile crocodile in order to complement and extend a previous examination in the alligator and thus establish a Hox code for the axial skeleton of crocodilians in general. The newly determined expression of HoxA-4, C-5, B-7, and B-8 all revealed a crocodilian-specific pattern. HoxA-4 and HoxC-5 characterize cervical morphologies and the latter furthermore is associated with the position of the forelimb relative to the axial skeleton. HoxB-7 and HoxB-8 map exclusively to the dorsal vertebral region. The resulting expression patterns of these two Hox genes is the first description of their exact expression in the archosaurian embryo. Our comparative analyses of the Hox code in several amniote taxa provide new evidence that evolutionary differences in the axial skeleton correspond to changes in Hox gene expression domains. We detect two general processes: (i) expansion of a Hox gene's expression domain as well as (ii) a shift of gene expression. We infer that the ancestral archosaur Hox code may have resembled that of the crocodile. In association with the evolution of morphological traits, it may have been modified to patterns that can be observed in birds.