Follistatin interacts with Noggin in the development of the axial skeleton

Building a vertebra: Development of the amniote sclerotome

In embryonic development, the vertebral column arises from the sclerotomal compartment of the somites. The sclerotome is a mesenchymal cell mass which can be subdivided into several subpopulations specified by different regulatory mechanisms and giving rise to different parts of the vertebrae like vertebral body, vertebral arch, ribs, and vertebral joints. This review gives a short overview on the molecular and cellular basis of the formation of sclerotomal subdomains and the morphogenesis of their vertebral derivatives.

Significance of Premature Vertebral Mineralization in Zebrafish Models in Mechanistic and Pharmaceutical Research on Hereditary Multisystem Diseases

Zebrafish are increasingly becoming an important model organism for studying the pathophysiological mechanisms of human diseases and investigating how these mechanisms can be effectively targeted using compounds that may open avenues to novel treatments for patients. The zebrafish skeleton has been particularly instrumental in modeling bone diseases as-contrary to other model organisms-the lower load on the skeleton of an aquatic animal enables mutants to survive to early adulthood. In this respect, the axial skeletons of zebrafish have been a good read-out for congenital spinal deformities such as scoliosis and degenerative disorders such as osteoporosis and osteoarthritis, in which aberrant mineralization in humans is reflected in the respective zebrafish models. Interestingly, there have been several reports of hereditary multisystemic diseases that do not affect the vertebral column in human patients, while the corresponding zebrafish models systematically show anomalies in mineralization and morphology of the spine as their leading or, in some cases, only phenotype. In this review, we describe such examples, highlighting the underlying mechanisms, the already-used or potential power of these models to help us understand and amend the mineralization process, and the outstanding questions on how and why this specific axial type of aberrant mineralization occurs in these disease models.

MiR-9-1 controls osteoblastic regulation of lymphopoiesis

The highly conserved MicroRNA-9 (miR-9) family consists of three members. We discovered that miR-9-1 deletion reduced mature miR-9 expression, causing 43% of the mice to display smaller size and postweaning lethality. MiR-9-1-deficient mice with growth defects experienced severe lymphopenia, but other blood cells were unaffected. The lymphopenia wasn't due to defects in hematopoietic progenitors, as mutant bone marrow (BM) cells underwent normal lymphopoiesis after transplantation into wild-type recipients. Additionally, miR-9-1-deficient mice exhibited impaired osteoblastic bone formation, as mutant mesenchymal stem cells (MSCs) failed to differentiate into osteoblastic cells (OBs). RNA sequencing revealed reduced expression of master transcription factors for osteoblastic differentiation, Runt-related transcription factor 2 (Runx2) and Osterix (Osx), and genes related to collagen formation, extracellular matrix organization, and cell adhesion, in miR-9-1-deficient MSCs. Follistatin (Fst), an antagonist of bone morphogenetic proteins (BMPs), was found to be a direct target of miR-9-1. Its deficiency led to the up-regulation of Fst, inhibiting BMP signaling in MSCs, and reducing IL-7 and IGF-1. Thus, miR-9-1 controls osteoblastic regulation of lymphopoiesis by targeting the Fst/BMP/Smad signaling axis.

Sclerotome is compartmentalized by parallel Shh and Bmp signaling downstream of CaMKII

The sclerotome in vertebrates comprises an embryonic population of cellular progenitors that give rise to diverse adult tissues including the axial skeleton, ribs, intervertebral discs, connective tissue, and vascular smooth muscle. In the thorax, this cell population arises in the ventromedial region of each of the segmented tissue blocks known as somites. How and when sclerotome adult tissue fates are specified and how the gene signatures that predate those fates are regulated has not been well studied. We have identified a previously unknown role for Ca ²⁺ /calmodulin-dependent protein kinase II (CaMKII) in regulating sclerotome patterning in zebrafish. Mechanistically, CaMKII regulates the activity of parallel signaling inputs that pattern sclerotome gene expression. In one downstream arm, CaMKII regulates distribution of the established sclerotome-inductive morphogen sonic hedgehog (Shh), and thus Shh-dependent sclerotome genes. In the second downstream arm, we show a previously unappreciated inductive requirement for Bmp signaling, where CaMKII activates expression of bmp4 and consequently Bmp activity. Bmp activates expression of a second subset of stereotypical sclerotome genes, while simultaneously repressing Shh-dependent markers. Our work demonstrates that CaMKII promotes parallel Bmp and Shh signaling as a mechanism to first promote global sclerotome specification, and that these pathways subsequently regionally activate and refine discrete compartmental genetic programs. Our work establishes how the earliest unique gene signatures that likely drive distinct cell behaviors and adult fates arise within the sclerotome.

Premature Vertebral Mineralization in hmx1-Mutant Zebrafish

H6 family homeobox 1 (HMX1) regulates multiple aspects of craniofacial development, and mutations in HMX1 are linked to an ocular defect termed oculoauricular syndrome of Schorderet–Munier–Franceschetti (OAS) (MIM #612109). Recently, additional altered orofacial features have been reported, including short mandibular rami, asymmetry of the jaws, and altered premaxilla. We found that in two mutant zebrafish lines termed hmx1mut10 and hmx1mut150, precocious mineralization of the proximal vertebrae occurred. Zebrafish hmx1mut10 and hmx1mut150 report mutations in the SD1 and HD domains, which are essential for dimerization and activity of hmx1. In hmx1mut10, the bone morphogenetic protein (BMP) antagonists chordin and noggin1 were downregulated, while bmp2b and bmp4 were highly expressed and specifically localized to the dorsal region prior to the initiation of the osteogenic process. The osteogenic promoters runx2b and spp1 were also upregulated. Supplementation with DMH1—an inhibitor of the BMP signaling pathway—at the specific stage in which bmp2b and bmp4 are highly expressed resulted in reduced vertebral mineralization, resembling the wildtype mineralization progress of the axial skeleton. These results point to a possible role of hmx1 as part of a complex gene network that inhibits bmp2b and bmp4 in the dorsal region, thus regulating early axial skeleton development.

Development, Pathogenesis, and Regeneration of the Intervertebral Disc: Current and Future Insights Spanning Traditional to Omics Methods

The intervertebral disc (IVD) is the fibrocartilaginous joint located between each vertebral body that confers flexibility and weight bearing capabilities to the spine. The IVD plays an important role in absorbing shock and stress applied to the spine, which helps to protect not only the vertebral bones, but also the brain and the rest of the central nervous system. Degeneration of the IVD is correlated with back pain, which can be debilitating and severely affects quality of life. Indeed, back pain results in substantial socioeconomic losses and healthcare costs globally each year, with about 85% of the world population experiencing back pain at some point in their lifetimes. Currently, therapeutic strategies for treating IVD degeneration are limited, and as such, there is great interest in advancing treatments for back pain. Ideally, treatments for back pain would restore native structure and thereby function to the degenerated IVD. However, the complex developmental origin and tissue composition of the IVD along with the avascular nature of the mature disc makes regeneration of the IVD a uniquely challenging task. Investigators across the field of IVD research have been working to elucidate the mechanisms behind the formation of this multifaceted structure, which may identify new therapeutic targets and inform development of novel regenerative strategies. This review summarizes current knowledge base on IVD development, degeneration, and regenerative strategies taken from traditional genetic approaches and omics studies and discusses the future landscape of investigations in IVD research and advancement of clinical therapies.

TGFβ as a gatekeeper of BMP action in the developing growth plate

The ligands that comprise the Transforming Growth Factor β superfamily highly govern the development of the embryonic growth plate. Members of this superfamily activate canonical TGFβ and/or BMP (Bone Morphogenetic Protein) signaling pathways. How these pathways interact with one another is an area of active investigation. These two signaling pathways have been described to negatively regulate one another through crosstalk involving Smad proteins, the primary intracellular effectors of canonical signaling. More recently, a mechanism for regulation of the BMP pathway through TGFβ and BMP receptor interactions has been described. Here in this review, we demonstrate examples of how TGFβ is a gatekeeper of BMP action in the developing growth plate at both the receptor and transcriptional levels.

Local modulation of the Wnt/β-catenin and bone morphogenic protein (BMP) pathways recapitulates rib defects analogous to cerebro-costo-mandibular syndrome

Ribs are seldom affected by developmental disorders, however, multiple defects in rib structure are observed in the spliceosomal disease cerebro-costo-mandibular syndrome (CCMS). These defects include rib gaps, found in the posterior part of the costal shaft in multiple ribs, as well as missing ribs, shortened ribs and abnormal costotransverse articulations, which result in inadequate ventilation at birth and high perinatal mortality. The genetic mechanism of CCMS is a loss-of-function mutation in SNRPB, a component of the major spliceosome, and knockdown of this gene in vitro affects the activity of the Wnt/β-catenin and bone morphogenic protein (BMP) pathways. The aim of the present study was to investigate whether altering these pathways in vivo can recapitulate rib gaps and other rib abnormalities in the model animal. Chick embryos were implanted with beads soaked in Wnt/β-catenin and BMP pathway modulators during somitogenesis, and incubated until the ribs were formed. Some embryos were harvested in the preceding days for analysis of the chondrogenic marker Sox9, to determine whether pathway modulation affected somite patterning or chondrogenesis. Wnt/β-catenin inhibition manifested characteristic rib phenotypes seen in CCMS, including rib gaps (P < 0.05) and missing ribs (P < 0.05). BMP pathway activation did not cause rib gaps but yielded missing rib (P < 0.01) and shortened rib phenotypes (P < 0.05). A strong association with vertebral phenotypes was also noted with BMP4 (P < 0.001), including scoliosis (P < 0.05), a feature associated with CCMS. Reduced expression of Sox9 was detected with Wnt/β-catenin inhibition, indicating that inhibition of chondrogenesis precipitated the rib defects in the presence of Wnt/β-catenin inhibitors. BMP pathway activators also reduced Sox9 expression, indicating an interruption of somite patterning in the manifestation of rib defects with BMP4. The present study demonstrates that local inhibition of the Wnt/β-catenin and activation of the BMP pathway can recapitulate rib defects, such as those observed in CCMS. The balance of Wnt/β-catenin and BMP in the somite is vital for correct rib morphogenesis, and alteration of the activity of these two pathways in CCMS may perturb this balance during somite patterning, leading to the observed rib defects.

The role of chick Ebf genes in the mediolateral patterning of the somites

This study was conducted to check whether the three chick Early B-cell Factor (Ebf) genes, particularly cEbf1, would be targets for Shh and Bmp signals during somites mediolateral (ML) patterning. Tissue manipulations and gain and loss of function experiments for Shh and Bmp4 were performed and the results revealed that cEbf1 expression was initiated in the cranial presomitic mesoderm by low dose of Bmp4 from the lateral mesoderm and maintained in the ventromedial part of the epithelial somite and the medial sclerotome by Shh from the notochord; while cEbf2/3 expression was induced and maintained by Bmp4 and inhibited by high dose of Shh. To determine whether Ebf1 plays a role in somite patterning, transfection of a dominant-negative construct was carried out; this showed suppression of cPax1 expression in the medial sclerotome and upregulation and medial expansion of cEbf3 and cPax3 expression in sclerotome and dermomyotome, respectively, suggesting that Ebf1 is important for ML patterning. Thus, it is possible that low doses of Bmp4 set up Ebf1 expression which, together with Shh from the notochord, leads to establishment of the medial sclerotome and suppression of lateral identities. These data also conclude that Bmp4 is required in both the medial and lateral domain of the somitic mesoderm to keep the ML identity of the sclerotome through maintenance of cEbf gene expression. These striking findings are novel and give a new insight on the role of Bmp4 on mediolateral patterning of somites.

A histocytological and radiological overview of the natural history of intervertebral disk: from embryonic formation to age-related degeneration

PURPOSE

To elucidate the natural history of intervertebral disk (IVD) and characterize its embryonic beginnings and age-related degeneration.

METHODS

Coronal sections of embryonic (E13.5-neonatal) and postnatal (4-60-week-old) Sprague-Dawley rat IVD were stained by a series of histological stainings (hematoxylin and eosin, Alcian blue, Picrosirius red, Masson, Periodic acid-Schiff). Growth kinetics within embryonic IVD were evaluated by immunohistochemical staining of Ki67 and proliferating cell nuclear antigen. Postnatal maturation and degeneration of IVD were visualized on radiology by X-ray, CT, and MR imaging.

RESULTS

During the formation of rat IVD, inner annulus fibrosus (AF) and cartilaginous endplate (CEP) shared similar cell density, extracellular matrix, and potential of growth kinetics; notochord provided increased and enlarged cytoplasmic vacuoles to generate nucleus pulposus (NP), part of which was retained within CEP. Postnatally, vacuolated notochord cells were reduced by devacuolation, while chondrocytic NP cells increased; cartilaginous layers of CEP were narrowed by vertebrae growth and secondary ossification; fibrotic portion of AF decreased as cartilaginous matrix accumulated and infiltrated outward. In aged and degenerated IVD, large longitudinal fissures were detected near the boundaries between inner and outer AF, whereas both reduced cellularity and accumulated cell clusters were evident within the dehydrated NP; only part of these histocytological changes could be reported on radiology.

CONCLUSIONS

By showing that the natural history of IVD is orchestrated by a dynamic histocytological regulation, our study may facilitate better understanding of the developmental defects, cellular heterogeneity, age-related degenerative mechanisms, and biological regeneration of IVD. These slides can be retrieved under Electronic Supplementary Material.

Development of the axial skeleton and intervertebral disc

Development of the axial skeleton is a complex, stepwise process that relies on intricate signaling and coordinated cellular differentiation. Disruptions to this process can result in a myriad of skeletal malformations that range in severity. The notochord and the sclerotome are embryonic tissues that give rise to the major components of the intervertebral discs and the vertebral bodies of the spinal column. Through a number of mouse models and characterization of congenital abnormalities in human patients, various growth factors, transcription factors, and other signaling proteins have been demonstrated to have critical roles in the development of the axial skeleton. Balance between opposing growth factors as well as other environmental cues allows for cell fate specification and divergence of tissue types during development. Furthermore, characterization of progenitor cells for specific cell lineages has furthered the understanding of specific spatiotemporal cues that cells need in order to initiate and complete development of distinct tissues. Identifying specific marker genes that can distinguish between the various embryonic and mature cell types is also of importance. Clinically, understanding developmental clues can aid in the generation of therapeutics for musculoskeletal disease through the process of developmental engineering. Studies into potential stem cell therapies are based on knowledge of the normal processes that occur in the embryo, which can then be applied to stepwise tissue engineering strategies.

Assessment of noggin level in pulmonary arterial hypertension patients

Noggin (NOG) is a protein that is involved in the development of many body tissues, including nerve tissue, muscles, and bones. The NOG protein plays a role in germ layer-specific derivation of specialized cells. Via NOG, the formation of neural tissues, the notochord, hair follicles, and eye structures arise from the ectoderm germ layer, while noggin activity in the mesoderm gives way to the formation of cartilage, bone and muscle growth. In the endoderm, NOG is involved in the development of the lungs.

NOG dimerizes by a core body, while two pairs of strands extend from it preceding by an N-terminal segment (called a clip segment) with approximately 20 amino acids. This clip twists around the BMP ligand and obstructs the growth factor surfaces from binding to both BMP receptors type I and type II. NOG binding to some BMPs inhibits these from combining and thus activating receptors of BMP, therefore, blocking non-Smad and Smad-dependent signaling.

The anti-proliferative noggin has particular effects in pulmonary arterial smooth muscle cells (PASMCs) that are exposed to specifically down regulated hypoxia. This occurs together with the BMP4 up-regulation levels of protein, and this imbalance between NOG and BMP4 consequence results in the activation and development of PAH disease. Our study consists of numerous examinations so as to explore new biomarkers in order to determine onset of PAH, and to discover the relationship between NOG serum level and gender, age, body mass index (BMI), waist circumferences (WC), smoking, types of PAH primaries and secondaries, as well as their grade.

TGF-β Family Signaling in Early Vertebrate Development

TGF-β family ligands function in inducing and patterning many tissues of the early vertebrate embryonic body plan. Nodal signaling is essential for the specification of mesendodermal tissues and the concurrent cellular movements of gastrulation. Bone morphogenetic protein (BMP) signaling patterns tissues along the dorsal-ventral axis and simultaneously directs the cell movements of convergence and extension. After gastrulation, a second wave of Nodal signaling breaks the symmetry between the left and right sides of the embryo. During these processes, elaborate regulatory feedback between TGF-β ligands and their antagonists direct the proper specification and patterning of embryonic tissues. In this review, we summarize the current knowledge of the function and regulation of TGF-β family signaling in these processes. Although we cover principles that are involved in the development of all vertebrate embryos, we focus specifically on three popular model organisms: the mouse Mus musculus, the African clawed frog of the genus Xenopus, and the zebrafish Danio rerio, highlighting the similarities and differences between these species.

Deletion of the sclerotome-enriched lncRNA PEAT augments ribosomal protein expression

To define a complete catalog of the genes that are activated during mouse sclerotome formation, we sequenced RNA from embryonic mouse tissue directed to form sclerotome in culture. In addition to well-known early markers of sclerotome, such as Pax1, Pax9, and the Bapx2/Nkx3-2 homolog Nkx3-1, the long-noncoding RNA PEAT (Pax1 enhancer antisense transcript) was induced in sclerotome-directed samples. Strikingly, PEAT is located just upstream of the Pax1 gene. Using CRISPR/Cas9, we generated a mouse line bearing a complete deletion of the PEAT-transcribed unit. RNA-seq on PEAT mutant embryos showed that loss of PEAT modestly increases bone morphogenetic protein target gene expression and also elevates the expression of a large subset of ribosomal protein mRNAs.

Early molecular events during retinoic acid induced differentiation of neuromesodermal progenitors

Bipotent neuromesodermal progenitors (NMPs) residing in the caudal epiblast drive coordinated body axis extension by generating both posterior neuroectoderm and presomitic mesoderm. Retinoic acid (RA) is required for body axis extension, however the early molecular response to RA signaling is poorly defined, as is its relationship to NMP biology. As endogenous RA is first seen near the time when NMPs appear, we used WNT/FGF agonists to differentiate embryonic stem cells to NMPs which were then treated with a short 2-h pulse of 25 nM RA or 1 µM RA followed by RNA-seq transcriptome analysis. Differential expression analysis of this dataset indicated that treatment with 25 nM RA, but not 1 µM RA, provided physiologically relevant findings. The 25 nM RA dataset yielded a cohort of previously known caudal RA target genes including Fgf8 (repressed) and Sox2 (activated), plus novel early RA signaling targets with nearby conserved RA response elements. Importantly, validation of top-ranked genes in vivo using RA-deficient Raldh2^-/- embryos identified novel examples of RA activation (Nkx1-2, Zfp503, Zfp703, Gbx2, Fgf15, Nt5e) or RA repression (Id1) of genes expressed in the NMP niche or progeny. These findings provide evidence for early instructive and permissive roles of RA in controlling differentiation of NMPs to neural and mesodermal lineages.

TGF-β and BMP signaling in osteoblast, skeletal development, and bone formation, homeostasis and disease

Transforming growth factor-beta (TGF-β) and bone morphogenic protein (BMP) signaling has fundamental roles in both embryonic skeletal development and postnatal bone homeostasis. TGF-βs and BMPs, acting on a tetrameric receptor complex, transduce signals to both the canonical Smad-dependent signaling pathway (that is, TGF-β/BMP ligands, receptors, and Smads) and the non-canonical-Smad-independent signaling pathway (that is, p38 mitogen-activated protein kinase/p38 MAPK) to regulate mesenchymal stem cell differentiation during skeletal development, bone formation and bone homeostasis. Both the Smad and p38 MAPK signaling pathways converge at transcription factors, for example, Runx2 to promote osteoblast differentiation and chondrocyte differentiation from mesenchymal precursor cells. TGF-β and BMP signaling is controlled by multiple factors, including the ubiquitin–proteasome system, epigenetic factors, and microRNA. Dysregulated TGF-β and BMP signaling result in a number of bone disorders in humans. Knockout or mutation of TGF-β and BMP signaling-related genes in mice leads to bone abnormalities of varying severity, which enable a better understanding of TGF-β/BMP signaling in bone and the signaling networks underlying osteoblast differentiation and bone formation. There is also crosstalk between TGF-β/BMP signaling and several critical cytokines’ signaling pathways (for example, Wnt, Hedgehog, Notch, PTHrP, and FGF) to coordinate osteogenesis, skeletal development, and bone homeostasis. This review summarizes the recent advances in our understanding of TGF-β/BMP signaling in osteoblast differentiation, chondrocyte differentiation, skeletal development, cartilage formation, bone formation, bone homeostasis, and related human bone diseases caused by the disruption of TGF-β/BMP signaling.

Genome-wide microarray analysis of Atlantic cod (Gadus morhua) oocyte and embryo

Background

Regulation of gene expression plays a central role in embryonic development. Early stages are controlled by gametic transcripts, which are subsequently substituted with transcripts from the genome of the zygote. Transcriptomic analyses provide an efficient approach to explore the temporal gene expression profiles in embryos and to search for the developmental regulators. We report a study of early Atlantic cod development that used a genome-wide oligonucleotide microarray to examine the composition and putative roles of polyadenylated transcripts.

Results

The analyses were carried out in unfertilized oocytes, newly fertilized oocytes and embryos at the stages of mid-blastula transition and segmentation. Numerous genes transcribed in oocytes are involved in multiple aspects of cell maintenance and protection, including metabolism, signal perception and transduction, RNA processing, cell cycle, defense against pathogens and DNA damage. Transcripts found in unfertilized oocytes also encoded a large number of proteins implicated in cell adherence, tight junction and focal adhesion, suggesting high complexity in terms of structure and cellular interactions in embryos prior to midblastula transition (MBT). Prezygotic transcripts included multiple regulators that are most likely involved in developmental processes that take place long after fertilization, such as components of ErbB, hedgehog, notch, retinoid, TGFb, VEGF and Wnt signaling pathways, as well as transcripts involved in the development of nervous system. The major event of MBT was the activation of a large group of histones and other genes that modify chromatin structure preceding massive gene expression changes. A hallmark of events observed during segmentation was the induction of multiple transcription factors, including a large group of homeobox proteins in pace with decay of a large fraction of maternal transcripts. Microarray analyses detected a suite of master developmental regulators that control differentiation and maintenance of diverse cell lineages.

Conclusions

Transcriptome profiling of the early stages in Atlantic cod revealed the presence of transcripts involved in patterning and development of tissues and organs long before activation of the zygotic genome. The switch from maternal to zygotic developmental programs is associated with large-scale modification of chromosomes.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2164-15-594) contains supplementary material, which is available to authorized users.