Conditional deletion of the TGF-beta type II receptor in Col2a expressing cells results in defects in the axial skeleton without alterations in chondrocyte differentiation or embryonic development of long bones

Advance in candidate genes in mandibular retrognathism: A systematic review

OBJECTIVE

This research aims to dissect the polygenic nature of non-syndromic mandibular retrognathism (MR) and to better understand the genetic underpinnings of MR, with a particular focus on the role of ethnic diversity in influencing genetic predispositions.

METHODS

A comprehensive systematic review was conducted on MR. Electronic databases such as PubMed and Google Scholar were employed, utilizing terms like 'mandibular', 'retrognathism', 'gene', and 'genetic'. This study strictly adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) framework.

RESULTS

Ten genetic studies were identified that satisfied the eligibility criteria, involving 1010 participants. Variations in candidate genes were reported across different populations, including myosin 1 H (MYO1H), matrilin 1 (MATN1), a disintegrin and metalloproteinase with thrombospondin motifs 9 (ADAMTS9), bone morphogenetic protein 2 (BMP2), parathyroid hormone (PTH), the vitamin-D related genes: vitamin D receptor (VDR), cytochrome P450 family 24 subfamily A member 1 (CYP24A1), and cytochrome P450 family 27 subfamily B member 1 (CYP27B1), collagen type II alpha 1 chain (COL2A1), transforming growth factor-β (TGF-β), TGF-β receptor 2 (TGFBR2), epidermal growth factor (EGF), and EGF receptor gene (EGFR).

CONCLUSION

These findings shed light on the role of genetic factors in MR. Future studies should adopt a multicentric approach to expand sample sizes and enhance the analysis of genetic variants associated with MR.

Fine-Tuned Expression of Evolutionarily Conserved Signaling Molecules in the Ciona Notochord

The notochord is an axial structure required for the development of all chordate embryos, from sea squirts to humans. Over the course of more than half a billion years of chordate evolution, in addition to its structural function, the notochord has acquired increasingly relevant patterning roles for its surrounding tissues. This process has involved the co-option of signaling pathways and the acquisition of novel molecular mechanisms responsible for the precise timing and modalities of their deployment. To reconstruct this evolutionary route, we surveyed the expression of signaling molecules in the notochord of the tunicate Ciona, an experimentally amenable and informative chordate. We found that several genes encoding for candidate components of diverse signaling pathways are expressed during notochord development, and in some instances, display distinctive regionalized and/or lineage-specific patterns. We identified and deconstructed notochord enhancers associated with TGF-β and Ctgf, two evolutionarily conserved signaling genes that are expressed dishomogeneously in the Ciona notochord, and shed light on the cis-regulatory origins of their peculiar expression patterns.

The integral role of fibronectin in skeletal morphogenesis and pathogenesis

Fibronectin (FN) serves as a critical organizer of extracellular matrix networks in two principal isoforms, the plasma FN and the cellular FN. While FN's pivotal role in various organ systems, including the blood vasculature, is well-established, its contribution to the development of the skeletal system is much less explored. Furthermore, the pathomechanisms of spondyloepiphyseal dysplasia caused by FN mutations remain elusive. In this minireview, we discuss findings from our recent two studies using i) an iPSC-based cell culture model to explore how FN mutations in spondyloepiphyseal dysplasia impact mesenchymal cell differentiation into chondrocytes and ii) conditional FN knockout mouse models to determine the physiological roles of FN isoforms during postnatal skeletal development. The data revealed that FN mutations cause severe intracellular and matrix defects in mesenchymal cells and impair their ability to differentiate into chondrocytes. The findings further demonstrate the important roles of both FN isoforms in orchestrating regulated chondrogenesis during skeletal development. We critically discuss the findings in the context of the existing literature.

Impaired Tertiary Dentin Secretion after Shallow Injury in Tgfbr2-Deficient Dental Pulp Cells Is Rescued by Extended CGRP Signaling

The transforming growth factor β (TGFβ) superfamily is a master regulator of development, adult homeostasis, and wound repair. Dysregulated TGFβ signaling can lead to cancer, fibrosis, and musculoskeletal malformations. We previously demonstrated that TGFβ receptor 2 (Tgfbr2) signaling regulates odontoblast differentiation, dentin mineralization, root elongation, and sensory innervation during tooth development. Sensory innervation also modulates the homeostasis and repair response in adult teeth. We hypothesized that Tgfbr2 regulates the neuro-pulpal responses to dentin injury. To test this, we performed a shallow dentin injury with a timed deletion of Tgfbr2 in the dental pulp mesenchyme of mice and analyzed the levels of tertiary dentin and calcitonin gene-related peptide (CGRP) axon sprouting. Microcomputed tomography imaging and histology indicated lower dentin volume in Tgfbr2cko M1s compared to WT M1s 21 days post-injury, but the volume was comparable by day 56. Immunofluorescent imaging of peptidergic afferents demonstrated that the duration of axon sprouting was longer in injured Tgfbr2cko compared to WT M1s. Thus, CGRP+ sensory afferents may provide Tgfbr2-deficient odontoblasts with compensatory signals for healing. Harnessing these neuro-pulpal signals has the potential to guide the development of treatments for enhanced dental healing and to help patients with TGFβ-related diseases.

Transcriptome Analysis Elucidates the Potential Key Genes Involved in Rib Development in bmp6-Deficient Silver Carp (Hypophthalmichthys molitrix)

Bone morphogenetic protein 6 (BMP-6) is a constituent of the TGF-β superfamily, known for its ability to stimulate bone and cartilage formation. The investigation of bmp6's involvement in the formation of intermuscular bones in fish has garnered significant attention in recent years. The rib cage is an important skeletal structure that plays a protective function for internal organs in fish. However, there has been limited research conducted on the effects of the bmp6 gene on rib development. Silver carp is one of four major fish in China, favoured for its affordability and tender muscle. Nevertheless, the presence of numerous intermuscular bones in silver carp significantly hinders the advancement of its palatability and suitability for processing. This study showcases the effective utilisation of CRISPR/Cas9 technology for the purpose of disrupting the bmp6 gene in silver carp, leading to the creation of chimeras in the P0 generation, marking the first instance of such an achievement. The chimeras exhibited complete viability, normal appearance, and partial intermuscular bones loss, with approximately 30% of them displaying rib bifurcation or bending. Subsequently, a transcriptome analysis on ribs of P0 chimeras and wild-type silver carp was conducted, leading to the identification of 934 genes exhibiting differential expression, of which 483 were found to be up-regulated and 451 were found to be down-regulated. The results of the KEGG analysis revealed that the "NF-kappa B signalling pathway", "Hippo signalling pathway", "osteoclast differentiation", and "haematopoietic cell lineage" exhibited enrichment and displayed a significant correlation with bone development. The up-regulated genes such as tnfα, fos, and ctgf in pathways may facilitate the proliferation and differentiation of osteoclasts, whereas the down-regulation of genes such as tgfb2 and tgfbr1 in pathways may hinder the formation and specialisation of osteoblasts, ultimately resulting in rib abnormalities. This study presents novel findings on the impact of bmp6 gene deletion on the rib development of silver carp, while simultaneously investigating the previously unexplored molecular mechanisms underlying rib defects in fish.

β1-Integrin-Mediated Uptake of Chondrocyte Extracellular Vesicles Regulates Chondrocyte Homeostasis

Osteoarthritis (OA) is the most prevalent age-related degenerative disorder, which severely reduces the quality of life of those affected. Whilst management strategies exist, no cures are currently available. Virtually all joint resident cells generate extracellular vesicles (EVs), and alterations in chondrocyte EVs during OA have previously been reported. Herein, we investigated factors influencing chondrocyte EV release and the functional role that these EVs exhibit. Both 2D and 3D models of culturing C28I/2 chondrocytes were used for generating chondrocyte EVs. We assessed the effect of these EVs on chondrogenic gene expression as well as their uptake by chondrocytes. Collectively, the data demonstrated that chondrocyte EVs are sequestered within the cartilage ECM and that a bi-directional relationship exists between chondrocyte EV release and changes in chondrogenic differentiation. Finally, we demonstrated that the uptake of chondrocyte EVs is at least partially dependent on β1-integrin. These results indicate that chondrocyte EVs have an autocrine homeostatic role that maintains chondrocyte phenotype. How this role is perturbed under OA conditions remains the subject of future work.

Altered serum cytokines in patients with symptomatic disk herniation and depressive symptoms

Purpose

An increasing number of studies have indicated the important role of cytokines in the development of depressive disturbances (DD). In medically ill patients, cytokines can provoked sickness behavior, the signs of which resemble DD. This results in alterations in behavior to limit energy expenditure and redirect it to cope with particular diseases. The aim of our study was to investigate the role of pro-inflammatory IL-6, TNF-α, and IL-1β and anti-inflammatory IL-10 and TGF-β in DD observed in patients suffering from pain caused by disk herniation (DH) qualified for surgery.

Patients and methods

The intensity of DD assessed by using Beck Depression Inventory, pain intensity, and functional impairment were evaluated in 70 patients with DH who were qualified for surgery. Pro-inflammatory serum levels of TNF-α, IL-1, IL-6, anti-inflammatory TGF-β, and IL-10 were measured.

Results

Elevated serum levels of TGF-β, IL-10, and IL-6 were found in the group with moderate and severe depressive symptoms (SD) compared with the groups with mild (MD) or no depressive symptoms (ND). TGF-β levels were negatively correlated with pain intensity, as assessed using the Present Pain Intensity scale in SD. Functional impairment measured using the Oswestry Disability Index was the most advanced in SD group.

Conclusion

Results of our study can suggest association between depressive disturbances and anti-inflammatory cytokines TGF-β and IL-10. Functional impairment of SD group is more severe but serum levels of TGF-β and IL-10, which are involved in the healing processes, are increased.

Developmental morphogens direct human induced pluripotent stem cells toward an annulus fibrosus-like cell phenotype

Introduction

Therapeutic interventions for intervertebral disc herniation remain scarce due to the inability of endogenous annulus fibrosus (AF) cells to respond to injury and drive tissue regeneration. Unlike other orthopedic tissues, such as cartilage, delivery of exogenous cells to the site of annular injury remains underdeveloped, largely due to a lack of an ideal cell source and the invasive nature of cell isolation. Human induced pluripotent stem cells (iPSCs) can be differentiated to specific cell fates using biochemical factors and are, therefore, an invaluable tool for cell therapy approaches. While differentiation protocols have been developed for cartilage and fibrous connective tissues (e.g., tendon), the signals that regulate the induction and differentiation of human iPSCs toward the AF fate remain unknown.

Methods

iPSC-derived sclerotome cells were treated with various combinations of developmental signals including transforming growth factor beta 3 (TGF-β3), connective tissue growth factor (CTGF), platelet derived growth factor BB (PDGF-BB), insulin-like growth factor 1 (IGF-1), or the Hedgehog pathway activator, Purmorphamine, and gene expression changes in major AF-associated ECM genes were assessed. The top performing combination treatments were further validated by using three distinct iPSC lines and by assessing the production of upregulated ECM proteins of interest. To conduct a broader analysis of the transcriptomic shifts elicited by each factor combination, and to compare genetic profiles of treated cells to mature human AF cells, a 96.96 Fluidigm gene expression array was applied, and principal component analysis was employed to identify the transcriptional signatures of each cell population and treatment group in comparison to native AF cells.

Results

TGF-β3, in combination with PDGF-BB, CTGF, or IGF-1, induced an upregulation of key AF ECM genes in iPSC-derived sclerotome cells. In particular, treatment with a combination of TGF-β3 with PDGF-BB for 14 days significantly increased gene expression of collagen II and aggrecan and increased protein deposition of collagen I and elastin compared to other treatment groups. Assessment of genes uniquely highly expressed by AF cells or SCL cells, respectively, revealed a shift toward the genetic profile of AF cells with the addition of TGF-β3 and PDGF-BB for 14 days.

Discussion

These findings represent an initial approach to guide human induced pluripotent stem cells toward an AF-like fate for cellular delivery strategies.

The roles and regulatory mechanisms of TGF-β and BMP signaling in bone and cartilage development, homeostasis and disease

Transforming growth factor-βs (TGF-βs) and bone morphometric proteins (BMPs) belong to the TGF-β superfamily and perform essential functions during osteoblast and chondrocyte lineage commitment and differentiation, skeletal development, and homeostasis. TGF-βs and BMPs transduce signals through SMAD-dependent and -independent pathways; specifically, they recruit different receptor heterotetramers and R-Smad complexes, resulting in unique biological readouts. BMPs promote osteogenesis, osteoclastogenesis, and chondrogenesis at all differentiation stages, while TGF-βs play different roles in a stage-dependent manner. BMPs and TGF-β have opposite functions in articular cartilage homeostasis. Moreover, TGF-β has a specific role in maintaining the osteocyte network. The precise activation of BMP and TGF-β signaling requires regulatory machinery at multiple levels, including latency control in the matrix, extracellular antagonists, ubiquitination and phosphorylation in the cytoplasm, nucleus-cytoplasm transportation, and transcriptional co-regulation in the nuclei. This review weaves the background information with the latest advances in the signaling facilitated by TGF-βs and BMPs, and the advanced understanding of their diverse physiological functions and regulations. This review also summarizes the human diseases and mouse models associated with disordered TGF-β and BMP signaling. A more precise understanding of the BMP and TGF-β signaling could facilitate the development of bona fide clinical applications in treating bone and cartilage disorders.

Identification of a novel, MSC-induced macrophage subtype via single-cell sequencing: implications for intervertebral disc degeneration therapy

Intervertebral disc (IVD) degeneration is a common pathological condition associated with low back pain. Recent evidence suggests that mesenchymal signaling cells (MSCs) promote IVD regeneration, but underlying mechanisms remain poorly defined. One postulated mechanism is via modulation of macrophage phenotypes. In this manuscript, we tested the hypothesis that MSCs produce trophic factors that alter macrophage subsets. To this end, we collected conditioned medium from human, bone marrow-derived STRO3+ MSCs. We then cultured human bone marrow-derived macrophages in MSC conditioned medium (CM) and performed single cell RNA-sequencing. Comparative analyses between macrophages cultured in hypoxic and normoxic MSC CM showed large overlap between macrophage subsets; however, we identified a unique hypoxic MSC CM-induced macrophage cluster. To determine if factors from MSC CM simulated effects of the anti-inflammatory cytokine IL-4, we integrated the data from macrophages cultured in hypoxic MSC CM with and without IL-4 addition. Integration of these data sets showed considerable overlap, demonstrating that hypoxic MSC CM simulates the effects of IL-4. Interestingly, macrophages cultured in normoxic MSC CM in the absence of IL-4 did not significantly contribute to the unique cluster within our comparison analyses and showed differential TGF-β signaling; thus, normoxic conditions did not approximate IL-4. In addition, TGF-β neutralization partially limited the effects of MSC CM. In conclusion, our study identified a unique macrophage subset induced by MSCs within hypoxic conditions and supports that MSCs alter macrophage phenotypes through TGF-β-dependent mechanisms.

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.

Single cell RNA sequencing reveals emergent notochord-derived cell subpopulations in the postnatal nucleus pulposus

Intervertebral disc degeneration is a leading cause of chronic low back pain. Cell-based strategies that seek to treat disc degeneration by regenerating the central nucleus pulposus (NP) hold significant promise, but key challenges remain. One of these is the inability of therapeutic cells to effectively mimic the performance of native NP cells, which are unique amongst skeletal cell types in that they arise from the embryonic notochord. In this study, we use single cell RNA sequencing to demonstrate emergent heterogeneity amongst notochord-derived NP cells in the postnatal mouse disc. Specifically, we established the existence of progenitor and mature NP cells, corresponding to notochordal and chondrocyte-like cells, respectively. Mature NP cells exhibited significantly higher expression levels of extracellular matrix (ECM) genes including aggrecan, and collagens II and VI, along with elevated transforming growth factor-beta and phosphoinositide 3 kinase-protein kinase B signaling. Additionally, we identified Cd9 as a novel surface marker of mature NP cells, and demonstrated that these cells were localized to the NP periphery, increased in numbers with increasing postnatal age, and co-localized with emerging glycosaminoglycan-rich matrix. Finally, we used a goat model to show that Cd9+ NP cell numbers decrease with moderate severity disc degeneration, suggesting that these cells are associated with maintenance of the healthy NP ECM. Improved understanding of the developmental mechanisms underlying regulation of ECM deposition in the postnatal NP may inform improved regenerative strategies for disc degeneration and associated low back pain.

Context-dependent TGFβ family signalling in cell fate regulation

The transforming growth factor-β (TGFβ) family are a large group of evolutionarily conserved cytokines whose signalling modulates cell fate decision-making across varying cellular contexts at different stages of life. Here we discuss new findings in early embryos that reveal how, in contrast to our original understanding of morphogen interpretation, robust cell fate specification can originate from a noisy combination of signalling inputs and a broad range of signalling levels. We compare this evidence with novel findings on the roles of TGFβ family signalling in tissue maintenance and homeostasis during juvenile and adult life, spanning the skeletal, haemopoietic and immune systems. From these comparisons, it emerges that in contrast to robust developing systems, relatively small perturbations in TGFβ family signalling have detrimental effects at later stages in life, leading to aberrant cell fate specification and disease, for example in cancer or congenital disorders. Finally, we highlight novel strategies to target and amend dysfunction in signalling and discuss how gleaning knowledge from different fields of biology can help in the development of therapeutics for aberrant TGFβ family signalling in disease.

Single Cell RNA Sequencing Reveals Emergent Notochord-Derived Cell Subpopulations in the Postnatal Nucleus Pulposus

Intervertebral disc degeneration is a leading cause of chronic low back pain. Cell-based strategies that seek to treat disc degeneration by regenerating the central nucleus pulposus hold significant promise, but key challenges remain. One of these is the inability of therapeutic cells to effectively mimic the performance of native nucleus pulposus cells, which are unique amongst skeletal cell types in that they arise from the embryonic notochord. In this study we use single cell RNA sequencing to demonstrate emergent heterogeneity amongst notochord-derived nucleus pulposus cells in the postnatal mouse disc. Specifically, we established the existence of early and late stage nucleus pulposus cells, corresponding to notochordal progenitor and mature cells, respectively. Late stage cells exhibited significantly higher expression levels of extracellular matrix genes including aggrecan, and collagens II and VI, along with elevated TGF-β and PI3K-Akt signaling. Additionally, we identified Cd9 as a novel surface marker of late stage nucleus pulposus cells, and demonstrated that these cells were localized to the nucleus pulposus periphery, increased in numbers with increasing postnatal age, and co-localized with emerging glycosaminoglycan-rich matrix. Finally, we used a goat model to show the Cd9+ nucleus pulposus cell numbers decrease with moderate severity disc degeneration, suggesting that these cells are associated with maintenance of the healthy nucleus pulposus extracellular matrix. Improved understanding of the developmental mechanisms underlying regulation of ECM deposition in the postnatal NP may inform improved regenerative strategies for disc degeneration and associated low back pain.

Spatiotemporal Characterization of Human Early Intervertebral Disc Formation at Single-Cell Resolution

The intervertebral disc (IVD) acts as a fibrocartilaginous joint to anchor adjacent vertebrae. Although several studies have demonstrated the cellular heterogeneity of adult mature IVDs, a single-cell transcriptomic atlas mapping early IVD formation is still lacking. Here, the authors generate a spatiotemporal and single cell-based transcriptomic atlas of human IVD formation at the embryonic stage and a comparative mouse transcript landscape. They identify two novel human notochord (NC)/nucleus pulposus (NP) clusters, SRY-box transcription factor 10 (SOX10)⁺ and cathepsin K (CTSK)⁺ , that are distributed in the early and late stages of IVD formation and they are validated by lineage tracing experiments in mice. Matrisome NC/NP clusters, T-box transcription factor T (TBXT)⁺ and CTSK⁺ , are responsible for the extracellular matrix homeostasis. The IVD atlas suggests that a subcluster of the vertebral chondrocyte subcluster might give rise to an inner annulus fibrosus of chondrogenic origin, while the fibroblastic outer annulus fibrosus preferentially expresseds transgelin and fibromodulin . Through analyzing intercellular crosstalk, the authors further find that notochordal secreted phosphoprotein 1 (SPP1) is a novel cue in the IVD microenvironment, and it is associated with IVD development and degeneration. In conclusion, the single-cell transcriptomic atlas will be leveraged to develop preventative and regenerative strategies for IVD degeneration.

Biomaterials delivery strategies to repair degenerated intervertebral discs by regulating the inflammatory microenvironment

Intervertebral disc degeneration (IVDD) is one of the leading causes of lower back pain. Although IVDD cannot directly cause death, it can cause pain, psychological burdens, and economic burdens to patients. Current conservative treatments for IVDD can relieve pain but cannot reverse the disease. Patients who cannot tolerate pain usually resort to a strategy of surgical resection of the degenerated disc. However, the surgical removal of IVDD can affect the stability of adjacent discs. Furthermore, the probability of the reherniation of the intervertebral disc (IVD) after surgery is as high as 21.2%. Strategies based on tissue engineering to deliver stem cells for the regeneration of nucleus purposes (NP) and annulus fibrosus (AF) have been extensively studied. The developed biomaterials not only locally withstand the pressure of the IVD but also lay the foundation for the survival of stem cells. However, the structure of IVDs does not provide sufficient nutrients for delivered stem cells. The role of immune mechanisms in IVDD has recently become clear. In IVDD, the IVD that was originally in immune privilege prevents the attack of immune cells (mainly effector T cells and macrophages) and aggravates the disease. Immune regulatory and inflammatory factors released by effector T cells, macrophages, and the IVD further aggravate IVDD. Reversing IVDD by regulating the inflammatory microenvironment is a potential approach for the treatment of the disease. However, the biological factors modulating the inflammatory microenvironment easily degrade in vivo. It makes it possible for different biomaterials to modulate the inflammatory microenvironment to repair IVDD. In this review, we have discussed the structures of IVDs and the immune mechanisms underlying IVDD. We have described the immune mechanisms elicited by different biological factors, including tumor necrosis factors, interleukins, transforming growth factors, hypoxia-inducible factors, and reactive oxygen species in IVDs. Finally, we have discussed the biomaterials used to modulate the inflammatory microenvironment to repair IVDD and their development.

Conditional Deletion of HIF-2α in Mouse Nucleus Pulposus Reduces Fibrosis and Provides Mild and Transient Protection From Age-Dependent Structural Changes in Intervertebral Disc

Hypoxia-inducible factors (HIFs) are critical to the development and homeostasis of hypoxic tissues. Although HIF-2α, one of the main HIF-α isoforms, is expressed in nucleus pulposus (NP) cells, its functions remain unknown. We deleted HIF-2α in the NP tissue using a notochord-specific FoxA2Cre allele to study HIF-2α function in the adult intervertebral disc. Unlike observations in HIF-1αcKO mice, fate mapping studies using Rosa26-mTmG reporter showed that HIF-2α loss in NP did not negatively impact cell survival or affect compartment development. Rather, loss of HIF-2α resulted in slightly better attributes of NP morphology in 14-month-old HIF-2αcKO mice as evident from lower scores of degeneration. These 14-month-old HIF-2αcKO mice also exhibited significant reduction in NP tissue fibrosis and lower collagen turnover in the annulus fibrosis (AF) compartment. Imaging-Fourier transform-infrared (FTIR) analyses showed decreased collagen and protein content in the NP and maintained chondroitin sulfate levels in 14-month-old HIF-2αcKO . Mechanistically, global transcriptomic analysis showed enrichment of differentially expressed genes with Gene Ontology (GO) terms related to metabolic processes and cell development, molecular functions concerned with histone and protein binding, and associated pathways, including oxidative stress. Noteworthy, these morphological differences were not apparent in 24-month-old HIF-2αcKO , indicating that aging is the dominant factor in governing disc health. Together these data suggest that loss of HIF-2α in the NP compartment is not detrimental to the intervertebral disc development but rather mitigates NP tissue fibrosis and offers mild but transient protection from age-dependent early degenerative changes. © 2022 American Society for Bone and Mineral Research (ASBMR).

Relationship between phenotypic features in Loeys-Dietz syndrome and the presence of intracranial aneurysms

OBJECTIVE

Loeys-Dietz syndrome (LDS) is a rare autosomal dominant condition characterized by aneurysms of the aorta, aortic branches, and intracranial arteries; skeletal and cutaneous abnormalities; and craniofacial malformations. Previous authors have reported that higher craniofacial severity index (CFI) scores, which indicate more severe craniofacial abnormalities, correlate with the severity of aortic aneurysm pathology. However, the association between syndromic features and the formation of intracranial aneurysms in LDS patients has yet to be determined. In this study, the authors evaluate the incidence of phenotypic abnormalities, craniofacial features, and Chiari malformation type I (CM-I) in a large LDS cohort and explore possible risk factors for the development of intracranial aneurysms.

METHODS

This was a retrospective cohort study of all patients with LDS who had been seen at the Marfan Syndrome and Aortopathy Center at Washington University School of Medicine in St. Louis in 2010–2022. Medical records were reviewed to obtain demographic, clinical, and radiographic data. The prevalence of craniofacial, skeletal, and cutaneous pathologies was determined. Bivariate logistic regression was performed to identify possible risk factors for the formation of an intracranial aneurysm.

RESULTS

Eighty-one patients with complete medical records and intracranial vascular imaging were included in the analysis, and 18 patients (22.2%) had at least 1 intracranial aneurysm. Patients frequently demonstrated the thin or translucent skin, doughy skin texture, hypertelorism, uvular abnormalities, and joint hypermobility typical of LDS. CM-I was common, occurring in 7.4% of the patients. Importantly, the patients with intracranial aneurysms were more likely to have CM-I (22.2%) than those without intracranial aneurysms (3.2%). The mean CFI score in the cohort with available data was 1.81, with higher means in the patients with the TGFBR1 or TGFBR2 disease-causing variants (2.05 and 3.30, respectively) and lower in the patients with the SMAD3, TGFB2, or TGFB3 pathogenic variants (CFI < 1). No significant CFI difference was observed in patients with or without intracranial aneurysms (2.06 vs 1.74, p = 0.61).

CONCLUSIONS

CM-I, and not the CFI, is significantly associated with the presence of intracranial aneurysms in patients with LDS. Surveillance for intracranial aneurysms is essential in all patients with LDS and should not be limited to those with severe phenotypes. Long-term monitoring studies will be necessary to determine whether a correlation between craniofacial abnormalities and adverse outcomes from intracranial aneurysms (growth, intervention, or rupture) exists.

TGFβ signaling is required for sclerotome resegmentation during development of the spinal column in Gallus gallus

We previously showed the importance of TGFβ signaling in development of the mouse axial skeleton. Here, we provide the first direct evidence that TGFβ signaling is required for resegmentation of the sclerotome using chick embryos. Lipophilic fluorescent tracers, DiO and DiD, were microinjected into adjacent somites of embryos treated with or without TGFβRI inhibitors, SB431542, SB525334 or SD208, at developmental day E2.5 (HH16). Lineage tracing of labeled cells was observed over the course of 4 days until the completion of resegmentation at E6.5 (HH32). Vertebrae were malformed and intervertebral discs were small and misshapen in inhibitor injected embryos. Hypaxial myofibers were also increased in thickness after treatment with the inhibitor. Inhibition of TGFβ signaling resulted in alterations in resegmentation that ranged between full, partial, and slanted shifts in distribution of DiO or DiD labeled cells within vertebrae. Patterning of rostro-caudal markers within sclerotome was disrupted at E3.5 after treatment with TGFβRI inhibitor with rostral domains expressing both rostral and caudal markers. We propose that TGFβ signaling regulates rostro-caudal polarity and subsequent resegmentation in sclerotome during spinal column development.

Transforming Growth Factor Beta Receptor 2 (TGFBR2) Promoter Region Polymorphisms May Be Involved in Mandibular Retrognathism

Skeletal malocclusions are common phenotypes in humans and have a strong influence on genetic factors. Transforming growth factor beta (TGFβ) controls numerous functions of the human body, including cell proliferation, differentiation, and migration. Thus, this study is aimed at evaluating whether genetic polymorphisms in TGFB1 and its receptor TGFBR2 are associated with mandibular retrognathism in German children and adolescents. Children and teenagers older than 8 years in the mixed or permanent dentition were included in this study. Patients with syndromes and facial trauma and patients with congenital alterations were excluded. Digital cephalometric tracings were performed using the anatomical landmarks point A, point B, sella (S), and nasion (N). Patients that have a retrognathic mandible (SNB < 78°) were selected as case group, and the patients with an orthognathic mandible (SNB = 78°– 82°) were selected as the control group. Genomic deoxyribonucleic acid (DNA) from saliva was used to evaluate four genetic polymorphisms in TGFB1 (rs1800469 and rs4803455) and TGBR2 (rs3087465 and rs764522) using real-time PCR. Chi-square or Fisher exact tests were used to compare gender, genotype, and allele distribution among groups. Genotype distribution was calculated in an additive and recessive model. Haplotype analysis was also performed. The established alpha of this study was 5%. A total of 146 patients (age ranging from 8 to 18 years) were included in this epidemiological genetic study. The genetic polymorphism rs3087465 in TGFBR2 was associated with mandibular retrognathism. Carrying the AA genotype in the rs3087465 polymorphism decreased the chance of having mandibular retrognathism (odds ratio = 0.25, confidence interval 95% = 0.06 to 0.94, p = 0.045). None of the haplotypes was associated with mandibular retrognathism (p > 0.05). In conclusion, we found that the genetic polymorphism rs3087465 in the promoter region of the TGFBR2 was associated with mandibular retrognathism in Germans.

Fibrillin-1 deficiency in the outer perichondrium causes longitudinal bone overgrowth in mice with Marfan syndrome

A disproportionate tall stature is the most evident manifestation in Marfan syndrome (MFS), a multisystem condition caused by mutations in the extracellular protein and TGFβ modulator, fibrillin-1. Unlike cardiovascular manifestations, there has been little effort devoted to unravel the molecular mechanism responsible for long bone overgrowth in MFS. By combining the Cre-LoxP recombination system with metatarsal bone cultures, here we identify the outer layer of the perichondrium as the tissue responsible for long bone overgrowth in MFS mice. Analyses of differentially expressed genes in the fibrillin-1 deficient perichondrium predicted that loss of TGFβ signaling may influence chondrogenesis in the neighboring epiphyseal growth plate (GP). Immunohistochemistry revealed that fibrillin-1 deficiency in the outer perichondrium is associated with decreased accumulation of latent TGFβ-binding proteins (LTBPs)-3 and - 4, and reduced levels of phosphorylated (activated) Smad2. Consistent with these findings, mutant metatarsal bones grown in vitro were longer and released less TGFβ than the wild type counterparts. Moreover, addition of recombinant TGFβ1 normalized linear growth of mutant metatarsal bones. We conclude that longitudinal bone overgrowth in MFS is accounted for by diminished sequestration of LTBP-3 and LTBP-4 into the fibrillin-1 deficient matrix of the outer perichondrium, which results in less TGFβ signaling locally and improper GP differentiation distally.

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.

Tgfbr2 in Dental Pulp Cells Guides Neurite Outgrowth in Developing Teeth

Transforming growth factor β (TGFβ) plays an important role in tooth morphogenesis and mineralization. During postnatal development, the dental pulp (DP) mesenchyme secretes neurotrophic factors that guide trigeminal nerve fibers into and throughout the DP. This process is tightly linked with dentin formation and mineralization. Our laboratory established a mouse model in which Tgfbr2 was conditionally deleted in DP mesenchyme using an Osterix promoter-driven Cre recombinase (Tgfbr2 ^cko ). These mice survived postnatally with significant defects in bones and teeth, including reduced mineralization and short roots. Hematoxylin and eosin staining revealed reduced axon-like structures in the mutant mice. Reporter imaging demonstrated that Osterix-Cre activity within the tooth was active in the DP and derivatives, but not in neuronal afferents. Immunofluorescence staining for β3 tubulin (neuronal marker) was performed on serial cryosections from control and mutant molars on postnatal days 7 and 24 (P7, P24). Confocal imaging and pixel quantification demonstrated reduced innervation in Tgfbr2 ^cko first molars at both stages compared to controls, indicating that signals necessary to promote neurite outgrowth were disrupted by Tgfbr2 deletion. We performed mRNA-Sequence (RNA-Seq) and gene onotology analyses using RNA from the DP of P7 control and mutant mice to investigate the pathways involved in Tgfbr2-mediated tooth development. These analyses identified downregulation of several mineralization-related and neuronal genes in the Tgfbr2 ^cko DP compared to controls. Select gene expression patterns were confirmed by quantitative real-time PCR and immunofluorescence imaging. Lastly, trigeminal neurons were co-cultured atop Transwell filters overlying primary Tgfbr2 ^f/f DP cells. Tgfbr2 in the DP was deleted via Adenovirus-expressed Cre recombinase. Confocal imaging of axons through the filter pores showed increased axonal sprouting from neurons cultured with Tgfbr2-positive DP cells compared to neurons cultured alone. Axon sprouting was reduced when Tgfbr2 was knocked down in the DP cells. Immunofluorescence of dentin sialophosphoprotein in co-cultured DP cells confirmed reduced mineralization potential in cells with Tgfbr2 deletion. Both our proteomics and RNA-Seq analyses indicate that axonal guidance cues, particularly semaphorin signaling, were disrupted by Tgfbr2 deletion. Thus, Tgfbr2 in the DP mesenchyme appears to regulate differentiation and the cells' ability to guide neurite outgrowth during tooth mineralization and innervation.

TMEM165 a new player in proteoglycan synthesis: loss of TMEM165 impairs elongation of chondroitin- and heparan-sulfate glycosaminoglycan chains of proteoglycans and triggers early chondrocyte differentiation and hypertrophy

TMEM165 deficiency leads to skeletal disorder characterized by major skeletal dysplasia and pronounced dwarfism. However, the molecular mechanisms involved have not been fully understood. Here, we uncover that TMEM165 deficiency impairs the synthesis of proteoglycans by producing a blockage in the elongation of chondroitin-and heparan-sulfate glycosaminoglycan chains leading to the synthesis of proteoglycans with shorter glycosaminoglycan chains. We demonstrated that the blockage in elongation of glycosaminoglycan chains is not due to defect in the Golgi elongating enzymes but rather to availability of the co-factor Mn²⁺. Supplementation of cell with Mn²⁺ rescue the elongation process, confirming a role of TMEM165 in Mn²⁺ Golgi homeostasis. Additionally, we showed that TMEM165 deficiency functionally impairs TGFβ and BMP signaling pathways in chondrocytes and in fibroblast cells of TMEM165 deficient patients. Finally, we found that loss of TMEM165 impairs chondrogenic differentiation by accelerating the timing of Ihh expression and promoting early chondrocyte maturation and hypertrophy. Collectively, our results indicate that TMEM165 plays an important role in proteoglycan synthesis and underline the critical role of glycosaminoglycan chains structure in the regulation of chondrogenesis. Our data also suggest that Mn²⁺ supplementation may be a promising therapeutic strategy in the treatment of TMEM165 deficient patients.

Divergence in chondrogenic potential between in vitro and in vivo of adipose- and synovial-stem cells from mouse and human

BACKGROUND

Somatic stem cell transplantation has been performed for cartilage injury, but the reparative mechanisms are still conflicting. The chondrogenic potential of stem cells are thought as promising features for cartilage therapy; however, the correlation between their potential for chondrogenesis in vitro and in vivo remains undefined. The purpose of this study was to investigate the intrinsic chondrogenic condition depends on cell types and explore an indicator to select useful stem cells for cartilage regeneration.

METHODS

The chondrogenic potential of two different stem cell types derived from adipose tissue (ASCs) and synovium (SSCs) of mice and humans was assessed using bone morphogenic protein-2 (BMP2) and transforming growth factor-β1 (TGFβ1). Their in vivo chondrogenic potential was validated through transplantation into a mouse osteochondral defect model.

RESULTS

All cell types showed apparent chondrogenesis under the combination of BMP2 and TGFβ1 in vitro, as assessed by the formation of proteoglycan- and type 2 collagen (COL2)-rich tissues. However, our results vastly differed with those observed following single stimulation among species and cell types; apparent chondrogenesis of mouse SSCs was observed with supplementation of BMP2 or TGFβ1, whereas chondrogenesis of mouse ASCs and human SSCs was observed with supplementation of BMP2 not TGFβ1. Human ASCs showed no obvious chondrogenesis following single stimulation. Mouse SSCs showed the formation of hyaline-like cartilage which had less fibrous components (COL1/3) with supplementation of TGFβ1. However, human cells developed COL1/3+ tissues with all treatments. Transcriptomic analysis for TGFβ receptors and ligands of cells prior to chondrogenic induction did not indicate their distinct reactivity to the TGFβ1 or BMP2. In the transplanted site in vivo, mouse SSCs formed hyaline-like cartilage (proteoglycan+/COL2+/COL1-/COL3-) but other cell types mainly formed COL1/3-positive fibrous tissues in line with in vitro reactivity to TGFβ1.

CONCLUSION

Optimal chondrogenic factors driving chondrogenesis from somatic stem cells are intrinsically distinct among cell types and species. Among them, the response to TGFβ1 may possibly represent the fate of stem cells when locally transplanted into cartilage defects.

Skeletal Deformities in Osterix-Cre;Tgfbr2f/f Mice May Cause Postnatal Death

Transforming growth factor β (TGFβ) signaling plays an important role in skeletal development. We previously demonstrated that the loss of TGFβ receptor II (Tgfbr2) in Osterix-Cre-expressing mesenchyme results in defects in bones and teeth due to reduced proliferation and differentiation in pre-osteoblasts and pre-odontoblasts. These Osterix-Cre;Tgfbr2^f/f mice typically die within approximately four weeks for unknown reasons. To investigate the cause of death, we performed extensive pathological analysis on Osterix-Cre- (Cre-), Osterix-Cre+;Tgfbr2^f/wt (HET), and Osterix-Cre+;Tgfbr2^f/f (CKO) mice. We also crossed Osterix-Cre mice with the ROSA26mTmG reporter line to identify potential off-target Cre expression. The findings recapitulated published skeletal and tooth abnormalities and revealed previously unreported osteochondral dysplasia throughout both the appendicular and axial skeletons in the CKO mice, including the calvaria. Alterations to the nasal area and teeth suggest a potentially reduced capacity to sense and process food, while off-target Cre expression in the gastrointestinal tract may indicate an inability to absorb nutrients. Additionally, altered nasal passages and unexplained changes in diaphragmatic muscle support the possibility of hypoxia. We conclude that these mice likely died due to a combination of breathing difficulties, malnutrition, and starvation resulting primarily from skeletal deformities that decreased their ability to sense, gather, and process food.

Canonical and noncanonical TGF-β signaling regulate fibrous tissue differentiation in the axial skeleton

Previously, we showed that embryonic deletion of TGF-β type 2 receptor in mouse sclerotome resulted in defects in fibrous connective tissues in the spine. Here we investigated how TGF-β regulates expression of fibrous markers: Scleraxis, Fibromodulin and Adamtsl2. We showed that TGF-β stimulated expression of Scleraxis mRNA by 2 h and Fibromodulin and Adamtsl2 mRNAs by 8 h of treatment. Regulation of Scleraxis by TGF-β did not require new protein synthesis; however, protein synthesis was required for expression of Fibromodulin and Adamtsl2 indicating the necessity of an intermediate. We subsequently showed Scleraxis was a potential intermediate for TGF-β-regulated expression of Fibromodulin and Adamtsl2. The canonical effector Smad3 was not necessary for TGF-β-mediated regulation of Scleraxis. Smad3 was necessary for regulation of Fibromodulin and Adamtsl2, but not sufficient to super-induce expression with TGF-β treatment. Next, the role of several noncanonical TGF-β pathways were tested. We found that ERK1/2 was activated by TGF-β and required to regulate expression of Scleraxis, Fibromodulin, and Adamtsl2. Based on these results, we propose a model in which TGF-β regulates Scleraxis via ERK1/2 and then Scleraxis and Smad3 cooperate to regulate Fibromodulin and Adamtsl2. These results define a novel signaling mechanism for TGFβ-mediated fibrous differentiation in sclerotome.

Lessons from joint development for cartilage repair in the clinic

More than 250 years ago, William Hunter stated that when cartilage is destroyed it never recovers. In the last 20 years, the understanding of the mechanisms that lead to joint formation and the knowledge that some of these mechanisms are reactivated in the homeostatic responses of cartilage to injury has offered an unprecedented therapeutic opportunity to achieve cartilage regeneration. Very large investments in ambitious clinical trials are finally revealing that, although we do not have perfect medicines yet, disease modification is a feasible possibility for human osteoarthritis.

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.

Folic acid rescues corticosteroid-induced vertebral malformations in chick embryos through targeting TGF-β signaling

Folic acid (FA) is routinely supplemented in the food of pregnant women or women planning a pregnancy, but whether FA exerts a positive effect on preventing fetal bone malformation remains obscure. In this study, we first exposed chick embryos with different concentrations of FA (1-10,000 pmol/egg) and studied vertebral mineralization and ossification through alcian blue and alizarin red as well as hematoxylin and eosin staining. Morphological measurements of the thoracic vertebral bodies demonstrated that 100 pmol/egg FA exhibited the tendency of shortening the growth plate, extended the ossification center, and increased the amount of Type I collagen. Second, we suggested that FA treatment promotes osteogenesis by demonstrating increased RUNX family transcription factor 2 (Runx2) and Osterix expressions in MC3T3-E1 and ATDC5 cells. Transforming growth factor-β (TGF-β) signaling was also upregulated by FA exposure, and addition of smad2/3 small interfering RNA knocks down FA-induced increased p-smad2/3, Runx2, and Osterix expression in vitro during chondrogenesis induction. Third, we employed dexamethasone (Dex), exposed chick embryos as an animal model of skeletal developmental retardation, to explore whether FA could rescue the loss of embryonic bone mass. Micro-computed tomography imaging showed that the addition of FA improved the reduction of bone mass in our model. Histological analysis of the vertebral bodies revealed that FA dramatically improved the delayed turnover of the zones of growth plate caused by Dex exposure. Immunofluorescence on the chick embryonic vertebrae and chondrocytes showed that FA supplementation upregulated the expression of TGF-β1, p-smad2/3, and improved Runx2 as well as Osterix expression in the Dex + FA group compared with the Dex group. Lastly, we found that supplementation with TGF-β1 (1 ng/egg) rescued bone mass loss caused by Dex as was also seen in FA exposure. Taken together these results, our data revealed that FA supplementation was able to rescue Dex exposure-induced inhibitive osteogenesis through targeting on the TGF-β signaling pathway.

Role of fibrillin-2 in the control of TGF-β activation in tumor angiogenesis and connective tissue disorders

Fibrillins constitute a family of large extracellular glycoproteins which multimerize to form microfibrils, an important structure in the extracellular matrix. It has long been assumed that fibrillin-2 was barely present during postnatal life, but it is now clear that fibrillin-2 molecules form the structural core of microfibrils, and are masked by an outer layer of fibrillin-1. Mutations in fibrillins give rise to heritable connective tissue disorders, including Marfan syndrome and congenital contractural arachnodactyly. Fibrillins also play an important role in matrix sequestering of members of the transforming growth factor-β family, and in context of Marfan syndrome excessive TGF-β activation has been observed. TGF-β activation is highly dependent on integrin binding, including integrin αvβ8 and αvβ6, which are upregulated upon TGF-β exposure. TGF-β is also involved in tumor progression, metastasis, epithelial-to-mesenchymal transition and tumor angiogenesis. In several highly vascularized types of cancer such as hepatocellular carcinoma, a positive correlation was found between increased TGF-β plasma concentrations and tumor vascularity. Interestingly, fibrillin-1 has a higher affinity to TGF-β and, therefore, has a higher capacity to sequester TGF-β compared to fibrillin-2. The previously reported downregulation of fibrillin-1 in tumor endothelium affects the fibrillin-1/fibrillin-2 ratio in the microfibrils, exposing the normally hidden fibrillin-2. We postulate that fibrillin-2 exposure in the tumor endothelium directly stimulates tumor angiogenesis by influencing TGF-β sequestering by microfibrils, leading to a locally higher active TGF-β concentration in the tumor microenvironment. From a therapeutic perspective, fibrillin-2 might serve as a potential target for future anti-cancer therapies.

Transforming Growth Factor β Enhances Tissue Formation by Passaged Nucleus Pulposus Cells In Vitro

The nucleus pulposus (NP) is composed of NP and notochord cell. It is a paucicellular tissue and if it is to be used as a source of cells for tissue engineering the cell number will have to be expanded by cell passaging. The hypothesis of this study is that passaged NP and notochordal cells grown in three-dimensional (3D) culture in the presence of transforming growth factor β (TGFβ) will show enhanced NP tissue formation compared with cells grown in the absence of this growth factor. Bovine NP cells isolated by sequential enzymatic digestion from caudal intervertebral discs were either placed directly in 3D culture (P0) or serially passaged up to passage 3 (P3) prior to placement in 3D culture. Serial cell passage in monolayer culture led to de-differentiation, increased senescence and oxidative stress and decreases in the gene expression of NP and notochordal associated markers and increases in de-differentiation markers. The NP tissue regeneration capacity of cells in 3D culture decreases with passaging as indicated by diminished tissue thickness and total collagen content when compared with tissues formed by P0 cells. Immunohistochemical studies showed that type II collagen accumulation appeared to decrease. TGFβ1 or TGFβ3 treatment enhanced the ability of cells at each passage to form tissue, in part by decreasing cell death. However, neither TGFβ1 nor TGFβ3 were able to restore the notochordal phenotype. Although TGFβ1/3 recovered NP tissue formation by passaged cells, to generate NP in vitro that resembles the native tissue will require identification of conditions facilitating retention of notochordal cell differentiation. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 38:438-449, 2020.

Inactivation of FAM20B causes cell fate changes in annulus fibrosus of mouse intervertebral disc and disc defects via the alterations of TGF-β and MAPK signaling pathways

Intervertebral disc (IVD) disorder is often caused by the defect of annulus fibrosus (AF), especially that of the outer AF. Studies about the mechanisms governing the development of the outer AF are needed for a better understanding of pathogenesis of IVD defects. Glycosaminoglycans (GAGs) are essential components of extracellular matrix (ECM) in AF. FAM20B is a newly identified xylose kinase that catalyzes the biosynthesis of GAGs. In this study, we created Fam20B conditional knockout (cKO) mice in which FAM20B was inactivated in type I collagen-expressing cells, the main type of cells in the outer AF of IVD. The cKO mice showed severe spine deformity and remarkable IVD defects associated with AF malformation. The AF of cKO mice had a lower level of chondroitin sulfate and heparan sulfate, and the outer AF cells lost their normal fibroblast-like morphology and acquired chondrocyte phenotypes, expressing a higher level of Sox 9 and type II collagen along with a reduced level of type I collagen. The level of phospho-Smad 2 and phospho-Smad 3, and that of scleraxis, a downstream target molecule of canonical TGF-β signaling pathway were significantly lower in the AF of cKO mice. The AF in cKO mice also manifested altered levels in the molecules associated with the activations of MAPK pathway; the changes included the increase of phospho-P38 and phospho-ERK and a decrease of phospho-JNK. These results indicate that FAM20B plays an essential role in the development of AF by regulating the TGF-β signaling and MAPK signaling pathways.

TGF-β signaling in intervertebral disc health and disease

OBJECTIVE

This paper aims to provide a comprehensive review of the changing role of transforming growth factor-β (TGF-β) signaling in intervertebral disc (IVD) health and disease.

METHODS

A comprehensive literature search was performed using PubMed terms 'TGF-β' and 'IVD'.

RESULTS

TGF-β signaling is necessary for the development and growth of IVD, and can play a protective role in the restoration of IVD tissues by stimulating matrix synthesis, inhibiting matrix catabolism, inflammatory response and cell loss. However, excessive activation of TGF-β signaling is detrimental to the IVD, and inhibition of the aberrant TGF-β signaling can delay IVD degeneration.

CONCLUSIONS

Activation of TGF-β signaling has a promising treatment prospect for IVD degeneration, while excessive activation of TGF-β signaling may contribute to the progression of IVD degeneration. Studies aimed at elucidating the changing role of TGF-β signaling in IVD at different pathophysiological stages and its specific molecular mechanisms are needed, and these studies will contribute to safe and effective TGF-β signaling-based treatments for IVD degeneration.

The TGFβ type I receptor TGFβRI functions as an inhibitor of BMP signaling in cartilage

The TGFβ signaling pathway is activated when TGFβ ligands induce formation of TGFβRI and TGFβRII receptor complexes. However, loss of TGFβRI in mouse cartilage led to more severe defects than did loss of TGFβRII. Most of the defects were rescued by deletion of the BMP receptor ACVRL1, suggesting that a major role of TGFβRI in cartilage development is to suppress BMP signaling by ACVRL1. TGFβRI prevents the formation of ACVRL1/ACTRIIB complexes, which have high affinity for BMP9, the most abundant BMP in circulation. These results demonstrate a form of cross talk between BMP and TGFβ signaling pathways in cartilage that may also operate in other tissues where the relative output of these 2 pathways is required.

The type I TGFβ receptor TGFβRI (encoded by Tgfbr1) was ablated in cartilage. The resulting Tgfbr1^Col2 mice exhibited lethal chondrodysplasia. Similar defects were not seen in mice lacking the type II TGFβ receptor or SMADs 2 and 3, the intracellular mediators of canonical TGFβ signaling. However, we detected elevated BMP activity in Tgfbr1^Col2 mice. As previous studies showed that TGFβRI can physically interact with ACVRL1, a type I BMP receptor, we generated cartilage-specific Acvrl1 (Acvrl1^Col2) and Acvrl1/Tgfbr1 (Acvrl1/Tgfbr1^Col2) knockouts. Loss of ACVRL1 alone had no effect, but Acvrl1/Tgfbr1^Col2 mice exhibited a striking reversal of the chondrodysplasia seen in Tgfbr1^Col2 mice. Loss of TGFβRI led to a redistribution of the type II receptor ACTRIIB into ACVRL1/ACTRIIB complexes, which have high affinity for BMP9. Although BMP9 is not produced in cartilage, we detected BMP9 in the growth plate, most likely derived from the circulation. These findings demonstrate that the major function of TGFβRI in cartilage is not to transduce TGFβ signaling, but rather to antagonize BMP signaling mediated by ACVRL1.

Antagonism of BMP signaling is insufficient to induce fibrous differentiation in primary sclerotome

Sclerotome is the embryonic progenitor of the axial skeleton. It was previously shown that Tgfbr2 is required in sclerotome for differentiation of fibrous skeletal tissues including the annulus fibrosus of the intervertebral disc. Alternatively, BMP signaling is required to form the vertebral body through chondrogenesis. In addition, TGFβ added to sclerotome cultures induces expression of markers for fibrous tissue differentiation but not cartilage or bone. The mechanism of how TGFβ signaling regulates this lineage decision in sclerotome is not known and could be due to the production of instructive or inhibitory signals or a combination of the two. Here we show that TGFβ antagonizes BMP/ Smad1/5 signaling in primary sclerotome likely through regulation of Noggin, an extracellular BMP antagonist, to prevent chondrogenesis. We then tested whether inhibition of BMP signaling, and inhibition of chondrogenesis, is sufficient to push cells toward the fibrous cell fate. While Noggin inhibited BMP/ Smad1/5 signaling and the formation of chondrogenic nodules in sclerotome cultures; Noggin and inhibition of BMP signaling through Gremlin or DMH2 were insufficient to induce fibrous tissue differentiation. The results suggest inhibition of BMP signaling is not sufficient to stimulate fibrous tissue differentiation and additional signals are likely required. We propose that TGFβ has a dual role in regulating sclerotome fate. First, it inhibits BMP signaling potentially through Noggin to prevent chondrogenesis and, second, it provides an unknown instructive signal to promote fibrous tissue differentiation in sclerotome. The results have implications for the design of stem cell-based therapies for skeletal diseases.

BMPs, TGFβ, and border security at the interzone

Synovial joints enable movement and protect the integrity of the articular cartilage. Joints form within skeletal condensations destined to undergo chondrogenesis. The suppression of this chondrogenic program in the interzone is the first morphological sign of joint formation. While we have a fairly good understanding of the essential roles of BMP and TGFβ family members in promoting chondrogenic differentiation in developing skeletal elements, we know very little about how BMP activity is suppressed specifically within the interzone, a crucial step in joint development. The function of the BMP ligand Gdf5 has been especially difficult to decipher. On the one hand, Gdf5 is required to promote chondrogenesis of articular elements. On the other hand, Gdf5 is highly expressed in the joint interzone where chondrogenesis must be suppressed for the formation of many joints. Here we review the evidence that BMP signaling must be suppressed within the joint interzone for joint morphogenesis to progress, and consider how Gdf5 exerts its divergent effects on chondrogenesis and joint formation. We also consider how TGFβ signaling impacts formation of the interzone. Finally, we propose a model whereby Gdf5 exerts distinct effects in the interzone vs. surrounding cartilage based on the repertoire of BMP receptors available in these tissues. Understanding how BMP antagonists and counteracting TGFβ signals intersect with Gdf5 to sculpt the joint interzone is essential for understanding the origin of osteoarthritis and other diseases of joint tissues.

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.

Latest advances in intervertebral disc development and progenitor cells

This paper is a concise review aiming to assemble the most relevant topics presented by the authors at ORS-Philadelphia Spine Research Society Fourth International Spine Research Symposium. It centers on the latest advances in disc development, its main structural entities, and the populating cells, with emphasis on the advances in pivotal molecular pathways responsible for forming the intervertebral discs (IVD). The objective of finding and emphasizing pathways and mechanisms that function to control tissue formation is to identify and to explore modifications occurring during normal aging, disease, and tissue repair. Thus, to comprehend that the cellular and molecular basis of tissue degeneration are crucial in the study of the dynamic interplay that includes cell-cell communication, gene regulation, and growth factors required to form a healthy and functional tissue during normal development.

TGF-β Initiates β-Catenin-Mediated CTGF Secretory Pathway in Old Bovine Nucleus Pulposus Cells: A Potential Mechanism for Intervertebral Disc Degeneration

We have recently demonstrated that overexpression of Smurf2 under the control of type II collagen alpha 1 (Col2a1) promoter induces an intervertebral disc degeneration phenotype in Col2a1‐Smurf2 transgenic mice. The chondrocyte‐like cells that express type II collagen and Smurf2 in the transgenic mouse discs are prone to degenerate. However, how the chondrocyte‐like cells contribute to disc degeneration is not known. Here, we utilized primary old bovine nucleus pulposus (NP) cells as substitutes for the chondrocyte‐like cells in Col2a1‐Smurf2 transgenic mouse discs to identify mechanism. We found that 35% of the cells were senescent; TGF‐β treatment of the cells induced a rapid moderate accumulation of β‐catenin, which interacted with connective tissue growth factor (CTGF/CCN2) in the cytoplasm and recruited it to the membrane for secretion. The TGF‐β‐initiated β‐catenin‐mediated CTGF secretory cascade did not occur in primary young bovine NP cells; however, when Smurf2 was overexpressed in young bovine NP cells, the cells became senescent and allowed this cascade to occur. These results suggest that Smurf2‐induced disc degeneration in Col2a1‐Smurf2 transgenic mice occurs through activation of CTGF secretory pathway in senescent disc cells. © 2018 The Authors JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Mechanotransduction and cell biomechanics of the intervertebral disc

Mechanical loading of the intervertebral disc (IVD) initiates cell-mediated remodeling events that contribute to disc degeneration. Cells of the IVD, nucleus pulposus (NP) and anulus fibrosus (AF), will exhibit various responses to different mechanical stimuli which appear to be highly dependent on loading type, magnitude, duration, and anatomic zone of cell origin. Cells of the NP, the innermost region of the disc, exhibit an anabolic response to low-moderate magnitudes of static compression, osmotic pressure, or hydrostatic pressure, while higher magnitudes promote a catabolic response marked by increased protease expression and activity. Cells of the outer AF are responsive to physical forces in a manner that depends on frequency and magnitude, as are cells of the NP, though they experience different forces, deformations, pressure, and osmotic pressure in vivo. Much remains to be understood of the mechanotransduction pathways that regulate IVD cell responses to loading, including responses to specific stimuli and also differences among cell types. There is evidence that cytoskeletal remodeling and receptor-mediated signaling are important mechanotransduction events that can regulate downstream effects like gene expression and posttranslational biosynthesis, all of which may influence phenotype and bioactivity. These and other mechanotransduction events will be regulated by known and to-be-discovered cell-matrix and cell-cell interactions, and depend on composition of extracellular matrix ligands for cell interaction, matrix stiffness, and the phenotype of the cells themselves. Here, we present a review of the current knowledge of the role of mechanical stimuli and the impact upon the cellular response to loading and changes that occur with aging and degeneration of the IVD.

Tgfbr2 is required in Acan-expressing cells for maintenance of the intervertebral and sternocostal joints

BACKGROUND

Members of the transforming growth factor beta (TGF-β) family are secreted proteins that regulate skeletal development. TGF-β signaling is critical in embryonic development of the annulus fibrosus (AF) of the intervertebral disc (IVD). To address the question of the role of TGF-β signaling in postnatal development and maintenance of the skeleton, we generated mice in which Tgfbr2 was deleted at 2-weeks of age in Aggrecan (Acan)-expressing cells using inducible Cre/LoxP recombination.

METHODS

Localization of Cre recombination was visualized by crossing Acan^{tm1(cre/ERT2)Crm} mice to fluorescent mTmG reporter mice. Acan^{tm1(cre/ERT2)Crm} mice were mated to Tgfbr2 ^LoxP/LoxP mice and Cre recombinase was activated by tamoxifen injection at 2-weeks postnatally. Following tamoxifen injection, mice were aged to 3, 6, and 12-months and control mice were compared to the experimental (cKO) group. Mice were initially analyzed using X-ray and skeletal preparations. Sternocostal joints and IVD tissues were further analyzed histologically by hematoxylin and eosin (H&E), Safranin O, and Picrosirius Red staining as well as Col10 immunostaining.

RESULTS

Cre recombination was observed in the IVD and sternocostal joints. X-ray analysis revealed osteophyte formation within the disc space of 12-month-old cKO mice. Skeletal preparations confirmed calcification within the IVD and the sternocostal joints in cKO mice. H&E staining of cKO IVD revealed disorganized growth plates, delay in the formation of the bony endplate, and Col10 staining in the AF indicative of ectopic endochondral bone formation. Furthermore, proteoglycan loss was observed and collagen bundles within the inner AF were thinner and less organized. Alterations in the IVD were apparent beginning at 3 months and were progressively more visible at 6 and 12 months. Similarly, histological analysis of cKO sternocostal joints revealed joint calcification, proteoglycan loss, and disorganization of the collagen architecture at 12 months of age.

CONCLUSIONS

TGF-β signaling is important for postnatal development and maintenance of fibrocartilaginous IVD and sternocostal joints.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

Genetic and Molecular Insights Into Genotype-Phenotype Relationships in Osteopathia Striata With Cranial Sclerosis (OSCS) Through the Analysis of Novel Mouse Wtx Mutant Alleles

The X-linked WTX/AMER1 protein constitutes an important component of the β-catenin destruction complex that can both enhance and suppress canonical β-catenin signaling. Somatic mutations in WTX/AMER1 have been found in a proportion of the pediatric kidney cancer Wilms' tumor. By contrast, germline mutations cause the severe sclerosing bone dysplasia osteopathia striata congenita with cranial sclerosis (OSCS), a condition usually associated with fetal or perinatal lethality in male patients. Here we address the developmental and molecular function of WTX by generating two novel mouse alleles. We show that in addition to the previously reported skeletal abnormalities, loss of Wtx causes severe midline fusion defects including cleft palate and ectopic synostosis at the base of the skull. By contrast, deletion of the C-terminal part of the protein results in only mild developmental abnormalities permitting survival beyond birth. Adult analysis, however, revealed skeletal defects including changed skull morphology and an increased whole-body bone density, resembling a subgroup of male patients carrying a milder, survivable phenotype. Molecular analysis in vitro showed that while β-catenin fails to co-immunoprecipitate with the truncated protein, partial recruitment appears to be achieved in an indirect manner using AXIN/AXIN2 as a molecular bridge. Taken together our analysis provides a novel model for WTX-caused bone diseases and explains on the molecular level how truncation mutations in this gene may retain some of WTX-protein functions. © 2018 American Society for Bone and Mineral Research.

The primary cilium as a signaling nexus for growth plate function and subsequent skeletal development

The primary cilium is a solitary, antenna-like sensory organelle with many important roles in cartilage and bone development, maintenance, and function. The primary cilium's potential role as a signaling nexus in the growth plate makes it an attractive therapeutic target for diseases and disorders associated with bone development and maintenance. Many signaling pathways that are mediated by the cilium-such as Hh, Wnt, Ihh/PTHrP, TGFβ, BMP, FGF, and Notch-are also known to influence endochondral ossification, primarily by directing growth plate formation and chondrocyte behavior. Although a few studies have demonstrated that these signaling pathways can be directly tied to the primary cilium, many pathways have yet to be evaluated in context of the cilium. This review serves to bridge this knowledge gap in the literature, as well as discuss the cilium's importance in the growth plate's ability to sense and respond to chemical and mechanical stimuli. Furthermore, we explore the importance of using the appropriate mechanism to study the cilium in vivo and suggest IFT88 deletion is the best available technique. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 36:533-545, 2018.

TGF-β Family Signaling in Connective Tissue and Skeletal Diseases

The transforming growth factor β (TGF-β) family of signaling molecules, which includes TGF-βs, activins, inhibins, and numerous bone morphogenetic proteins (BMPs) and growth and differentiation factors (GDFs), has important functions in all cells and tissues, including soft connective tissues and the skeleton. Specific TGF-β family members play different roles in these tissues, and their activities are often balanced with those of other TGF-β family members and by interactions with other signaling pathways. Perturbations in TGF-β family pathways are associated with numerous human diseases with prominent involvement of the skeletal and cardiovascular systems. This review focuses on the role of this family of signaling molecules in the pathologies of connective tissues that manifest in rare genetic syndromes (e.g., syndromic presentations of thoracic aortic aneurysm), as well as in more common disorders (e.g., osteoarthritis and osteoporosis). Many of these diseases are caused by or result in pathological alterations of the complex relationship between the TGF-β family of signaling mediators and the extracellular matrix in connective tissues.

Patterning spinal nerves and vertebral bones

A prominent anatomical feature of the peripheral nervous system is the segmentation of mixed (motor, sensory and autonomic) spinal nerves alongside the spinal cord. During early development their axon growth cones avoid the developing vertebral elements by traversing the anterior/cranial half of each somite-derived sclerotome, so ensuring the separation of spinal nerves from vertebral bones as axons extend towards their peripheral targets. A glycoprotein expressed on the surface of posterior half-sclerotome cells confines growth cones to the anterior half-sclerotomes by contact repulsion. A closely similar glycoprotein is expressed in avian and mammalian grey matter, where we hypothesize it may have evolved to regulate neural plasticity in birds and mammals.

Whole Transcriptome Analysis of Notochord-Derived Cells during Embryonic Formation of the Nucleus Pulposus

Recapitulation of developmental signals represents a promising strategy for treating intervertebral disc degeneration. During development, embryonic notochord-derived cells (NDCs) are the direct progenitors of cells that populate the adult nucleus pulposus (NP) and are an important source of secreted signaling molecules. The objective of this study was to define global gene expression profiles of NDCs at key stages of embryonic disc formation. NDCs were isolated from Shh-cre;ROSA:YFP mice at embryonic day 12.5 and postnatal day 0, representing opposite ends of the notochord to NP transformation. Differences in global mRNA abundance across this developmental window were established using RNA-Seq. Protein expression of selected molecules was confirmed using immunohistochemistry. Principal component analysis revealed clustering of gene expression at each developmental stage with more than 5000 genes significantly differentially expressed between E12.5 and P0. There was significantly lower mRNA abundance of sonic hedgehog pathway elements at P0 vs E12.5, while abundance of elements of the transforming growth factor-beta and insulin-like growth factors pathways, and extracellular matrix components including collagen 6 and aggrecan, were significantly higher at P0. This study represents the first transcriptome-wide analysis of embryonic NDCs. Results suggest signaling and biosynthesis of NDCs change dramatically as a function of developmental stage.