The Lung Elastin Matrix Undergoes Rapid Degradation Upon Adult Loss of Hox5 Function

Hox genes encode transcription factors that are critical for embryonic skeletal patterning and organogenesis. The Hoxa5, Hoxb5, and Hoxc5 paralogs are expressed in the lung mesenchyme and function redundantly during embryonic lung development. Conditional loss-of-function of these genes during postnatal stages leads to severe defects in alveologenesis, specifically in the generation of the elastin network, and animals display bronchopulmonary dysplasia (BPD) or BPD-like phenotype. Here we show the surprising results that mesenchyme-specific loss of Hox5 function at adult stages leads to rapid disruption of the mature elastin matrix, alveolar enlargement, and an emphysema-like phenotype. As the elastin matrix of the lung is considered highly stable, adult disruption of the matrix was not predicted. Just 2 weeks after deletion, adult Hox5 mutant animals show significant increases in alveolar space and changes in pulmonary function, including reduced elastance and increased compliance. Examination of the extracellular matrix (ECM) of adult Tbx4rtTA; TetOCre; Hox5afafbbcc lungs demonstrates a disruption of the elastin network although the underlying fibronectin, interstitial collagen and basement membrane appear unaffected. An influx of macrophages and increased matrix metalloproteinase 12 (MMP12) are observed in the distal lung 3 days after Hox5 deletion. In culture, fibroblasts from Hox5 mutant lungs exhibit reduced adhesion. These findings establish a novel role for Hox5 transcription factors as critical regulators of lung fibroblasts at adult homeostasis.

EphB3 as a potential mediator of developmental and reparative osteogenesis

The ephrin-B family of membrane-bound ligands is involved in skeletal patterning, osteogenesis, and bone homeostasis. Yet, despite the increasing collection of data affirming their importance in bone, the Eph tyrosine kinases that serve as the receptors for these ephrins in osteoblast stem cell niches remain unidentified. Here we report the expression of EphB3 at sites of bone growth in the embryo, especially at the calvaria suture fronts, periosteum, chondrocytes and trabeculae of developing long bones. Strong EphB3 expression persisted in the adult calvarial sutures and in the proliferative chondrocytes of long bones, both of which are documented niches for osteoblastic stem cells. We observed EphB3-positive cells in the tissue filling a created calvarial injury, further implying EphB3 involvement in bone healing. Genetic knockout of EphB3 caused an increase in the bone tissue volume as a fraction of total volume in six-week old calvaria and in femoral trabecular density, compared to wild type controls. This difference resolved by twelve weeks of age, when we instead observed an increase in the bone volume of femoral trabeculae and in trabecular thickness. Our data identify EphB3 as a candidate regulator of osteogenesis either alone or in combination with other bone-expressed Ephs, and indicate that it appears to function as a limiter of bone growth.

Maternal SMCHD1 regulates Hox gene expression and patterning in the mouse embryo

Parents transmit genetic and epigenetic information to their offspring. Maternal effect genes regulate the offspring epigenome to ensure normal development. Here we report that the epigenetic regulator SMCHD1 has a maternal effect on Hox gene expression and skeletal patterning. Maternal SMCHD1, present in the oocyte and preimplantation embryo, prevents precocious activation of Hox genes postimplantation. Without maternal SMCHD1, highly penetrant posterior homeotic transformations occur in the embryo. Hox genes are decorated with Polycomb marks H2AK119ub and H3K27me3 from the oocyte throughout early embryonic development; however, loss of maternal SMCHD1 does not alter these marks. Therefore, we propose maternal SMCHD1 acts downstream of Polycomb marks to establish a chromatin state necessary for persistent epigenetic silencing and appropriate Hox gene expression later in the developing embryo. This is a striking role for maternal SMCHD1 in long-lived epigenetic effects impacting offspring phenotype.

Identification of Jmjd3 as an Essential Epigenetic Regulator of Hox Gene Temporal Collinear Activation for Body Axial Patterning in Mice

Body axial patterning develops via a rostral-to-caudal sequence and relies on the temporal colinear activation of Hox genes. However, the underlying mechanism of Hox gene temporal colinear activation remains largely elusive. Here, with small-molecule inhibitors and conditional gene knockout mice, we identified Jmjd3, a subunit of TrxG, as an essential regulator of temporal colinear activation of Hox genes with its H3K27me3 demethylase activity. We demonstrated that Jmjd3 not only initiates but also maintains the temporal collinear expression of Hox genes. However, we detected no antagonistic roles between Jmjd3 and Ezh2, a core subunit of PcG repressive complex 2, during the processes of axial skeletal patterning. Our findings provide new insights into the regulation of Hox gene temporal collinear activation for body axial patterning in mice.

Species-specific deployment of Runx2 isoforms and differential regulation of target genes during avian jaw development and evolution

Developmental regulation of bone formation in the jaw skeleton is essential to species-specific adaptation. The jaws are derived from neural crest mesenchyme (NCM), a progenitor population that directs skeletal patterning by exerting temporal and spatial control over molecular and cellular programs for osteogenesis. One important NCM-mediated gene is Runx2, which is a transcription factor required for osteoblast differentiation. RUNX2 protein binds many target genes involved in the deposition and resorption of bone. To determine the extent to which changes in Runx2 structure, function, and expression underlie the evolution of the jaw skeleton, we compare Runx2 across vertebrates and within birds. Runx2 contains two alternative promoters, tandem repeats of glutamine and alanine with variable lengths in different species, a conserved DNA-binding domain, an exon that is alternatively spliced, as well as two possible C-termini. Such alternative splicing produces eight potential isoforms that show distinct stage- and species-specific patterns in the jaw primordia of chick, quail and duck embryos. We also find that certain isoforms are strongly induced by TGFβ signaling whereas others are not. Overexpressing Runx2 isoforms in NCM reveals that some are transcriptionally activating, while others are repressive. But context appears to be relevant since species-specific polymorphisms in the promoter of target genes like Mmp13, can modulate the effects of different isoforms. Overall, our study indicates that the structure and species-specific deployment of Runx2 isoforms affect the transcriptional activity of target genes in ways that may have played a generative and regulatory role in the evolution of the avian jaw skeleton.

Enhancer of Zeste Homolog 2 (Ezh2) is essential for patterning of multiple musculoskeletal tissues but dispensable for tendon differentiation

An efficient musculoskeletal system depends on the precise assembly and coordinated growth and function of muscles, skeleton and tendons. However, the mechanisms that drive integrated musculoskeletal development and coordinated growth and differentiation of each of these tissues are still being uncovered. Epigenetic modifiers have emerged as critical regulators of cell fate differentiation, but so far almost nothing is known about their roles in tendon biology. Previous studies have shown that epigenetic modifications driven by Enhancer of zeste homolog 2 (EZH2), a major histone methyltransferase, have significant roles in vertebrate development including skeletal patterning and bone formation. We now find that targeting Ezh2 through the limb mesenchyme also has significant effects on tendon and muscle patterning, likely reflecting the essential roles of early mesenchymal cues mediated by Ezh2 for coordinated patterning and development of all tissues of the musculoskeletal system. Conversely, loss of Ezh2 in the tendon cells did not disrupt the tendon cell fate suggesting that tenocyte differentiation and tendon maturation are independent of Ezh2 signaling.

Similar sequences but dissimilar biological functions of GDF11 and myostatin

Growth differentiation factor 11 (GDF11) and myostatin (MSTN) are closely related TGFβ family members that are often believed to serve similar functions due to their high homology. However, genetic studies in animals provide clear evidence that they perform distinct roles. While the loss of Mstn leads to hypermuscularity, the deletion of Gdf11 results in abnormal skeletal patterning and organ development. The perinatal lethality of Gdf11-null mice, which contrasts with the long-term viability of Mstn-null mice, has led most research to focus on utilizing recombinant GDF11 proteins to investigate the postnatal functions of GDF11. However, the reported outcomes of the exogenous application of recombinant GDF11 proteins are controversial partly because of the different sources and qualities of recombinant GDF11 used and because recombinant GDF11 and MSTN proteins are nearly indistinguishable due to their similar structural and biochemical properties. Here, we analyze the similarities and differences between GDF11 and MSTN from an evolutionary point of view and summarize the current understanding of the biological processing, signaling, and physiological functions of GDF11 and MSTN. Finally, we discuss the potential use of recombinant GDF11 as a therapeutic option for a wide range of medical conditions and the possible adverse effects of GDF11 inhibition mediated by MSTN inhibitors.

Thyroid hormone regulates proximodistal identity in the fin skeleton

Across the ∼30,000 species of ray-finned fish, fins show incredible diversity in overall shape and in the patterning of the supportive bony rays. Fin length mutant zebrafish have provided critical insights into the developmental pathways that regulate relative fin size. However, the processes that govern skeletal patterning along the proximodistal axis of the fin have remained less well understood. Here, we show that thyroid hormone regulates proximodistal identity of fin rays, distalizing gene expression profiles, morphogenetic processes during outgrowth, and ultimate morphology of the fin. This role for thyroid hormone in specifying proximodistal identity appears conserved between development and regeneration, in all the fins, and between species. We demonstrate that proximodistal identity is regulated independently from pathways that determine size, and we show that modulating proximodistal patterning relative to growth can recapitulate the spectrum of fin ray diversity found in nature.

Novel insights into the structure and transport mechanisms of TAPT1

Transmembrane anterior-posterior transformation protein 1 (TAPT1), encoded by the TAPT1 gene expressed in the basal ciliary body, plays a crucial role in cilia formation as well as axial skeletal patterning. Mutations in this gene have been reported to cause several ciliopathies and osteo-related diseases. Unfortunately, the cellular and molecular pathogenic mechanisms are still unclear also due to the lack of X-ray crystallographic structure and further characterization of TAPT1 protein. In this study, we attempted to characterize this protein by in silico techniques. A 3D structure of TAPT1 was generated by the ab initio method, which was further used for the analysis of the substrate-binding site, to determine pore size and for the prediction of the possible substrate(s). Validation by using different software packages revealed a reliable 3D model of TAPT1. Topology modeling revealed that TAPT1 has eight transmembrane helices with a total number of 27 helices in secondary structure. The amino acid residues H235, R323, K443, N446, S447, L450, K453, S454, Y457, K511, N513, D533, K535, D536, and T538 were found to form the pore surface as well as involved in the binding interaction with the substrate(s). This study predicted flavonoids as the possible substrate for TAPT1, which could further be confirmed by ingenuity pathway analysis. Moreover, our analysis indicated that TAPT1 might localize in the mitochondrial membrane in addition to the ciliary basal body. Our study gives novel insights for TAPT1 structure and its function.