Connective Tissue Growth Factor reporter mice label a subpopulation of mesenchymal progenitor cells that reside in the trabecular bone region

Connective tissue growth factor promotes cementogenesis and cementum repair via Cx43/β-catenin axis

Background

Orthodontic tooth movement inevitably induces cementum resorption, which is an urgent problem for orthodontists to confront. Human periodontal ligament stem cells (hPDLSCs) exert an important role in the orthodontic tooth movement and exhibit multidirectional differentiation ability in cementum regeneration. Connective tissue growth factor (CTGF) is an important extracellular matrix protein for bone homeostasis and cell differentiation. The purpose of our study was to explore the role of CTGF in cementum repair and cementogenesis and to elucidate its underlying mechanism.

Methods

A cementum defect model was established by tooth movement with heavy forces, and the cementum repair effect of CTGF was observed via micro-CT, HE staining and immunohistochemical staining. RT‒qPCR, western blotting (WB), alizarin red staining and ALP activity experiments verified the mineralization ability of hPDLSCs stimulated with CTGF. The expression of Cx43 in periodontal ligament cells was detected by WB and immunofluorescence (IF) experiments after CTGF stimulation in vivo and in vitro. Subsequently, the mineralization ability of hPDLSCs was observed after application of CTGF and the small interfering RNA Si-Cx43. Additionally, co-intervention via application of the small interfering RNA Si-CTGF and the Cx43 agonist ATRA in hPDLSCs was performed to deepen the mechanistic study. Next, WB, IF experiments and co-immunoprecipitation were conducted to confirm whether CTGF triggers the Cx43/β-catenin axis to regulate cementoblast differentiation of hPDLSCs.

Results

Local oral administration of CTGF to the cementum defects in vivo facilitated cementum repair. CTGF facilitated the cementogenesis of hPDLSCs in a concentration-dependent manner. Cx43 acted as a downstream effector of CTGF to regulate cementoblast differentiation. Si-Cx43 reduced CTGF-induced cementoblast differentiation. The Cx43 agonist ATRA restored the low differentiation capacity induced by Si-CTGF. Further mechanistic studies showed that CTGF triggered the activation of β-catenin in a dose-dependent manner. In addition, co-localization IF analysis and co-immunoprecipitation demonstrated that Cx43 interacted with β-catenin at cell‒cell connections. Si-Cx43 attenuated the substantial expression of β-catenin induced by CTGF. The Cx43 agonist reversed the inhibition of β-catenin induced by Si-CTGF. IF demonstrated that the nuclear importation of β-catenin was related to the immense expression of Cx43 at cell‒cell junctions.

Conclusions

Taken together, these data demonstrate that CTGF promotes cementum repair and cementogenesis through activation of the Cx43/β-catenin signalling axis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13287-022-03149-8.

DNMT1 is a negative regulator of osteogenesis

The role and underlying mechanisms of DNA methylation in osteogenesis/chondrogenesis remain poorly understood. We here reveal DNA methyltransferase 1 (DNMT1), which is responsible for copying DNA methylation onto the newly synthesized DNA strand after DNA replication, is overexpressed in sponge bone of people and mice with senile osteoporosis and required for suppression of osteoblast (OB) differentiation of mesenchymal stem cells (MSCs) and osteoprogenitors. Depletion of DNMT1 results in demethylation at the promoters of key osteogenic genes such as RORA and Fgfr2, and consequent upregulation of their transcription in vitro. Mechanistically, DNMT1 binds exactly to the promoters of these genes and are responsible for their 5-mc methylation. Conversely, simultaneous depletion of RORA or Fgfr2 blunts the effects of DNMT1 silencing on OB differentiation, suggesting RORA or Fgfr2 may be crucial for modulating osteogenic differentiation downstream of DNMT1. Collectively, these results reveal DNMT1 as a key repressor of OB differentiation and bone formation while providing us a new rationale for specific inhibition of DNMT1 as a potential therapeutic strategy to treat age-related bone loss.

Summary: DNMT1 is overexpressed in sponge bone of people and mice with senile osteoporosis and required for suppression of osteoblast (OB) differentiation of mesenchymal stem cells (MSCs) and osteoprogenitors.

Vangl2 limits chaperone-mediated autophagy to balance osteogenic differentiation in mesenchymal stem cells

Lysosomes are the recycling center and nutrient signaling hub of the cell. Here, we show that lysosomes also control mesenchymal stem cell (MSC) differentiation by proteomic reprogramming. The chaperone-mediated autophagy (CMA) lysosome subgroup promotes osteogenesis, while suppressing adipogenesis, by selectively removing osteogenesis-deterring factors, especially master transcriptional factors, such as adipogenic TLE3, ZNF423, and chondrogenic SOX9. The activity of the CMA-committed lysosomes in MSCs are controlled by Van-Gogh-like 2 (Vangl2) at lysosomes. Vangl2 directly binds to lysosome-associated membrane protein 2A (LAMP-2A) and targets it for degradation. MSC-specific Vangl2 ablation in mice increases LAMP-2A expression and CMA-lysosome numbers, promoting bone formation while reducing marrow fat. The Vangl2:LAMP-2A ratio in MSCs correlates inversely with the capacity of the cells for osteoblastic differentiation in humans and mice. These findings demonstrate a critical role for lysosomes in MSC lineage acquisition and establish Vangl2-LAMP-2A signaling as a critical control mechanism.

Heterogeneity of murine periosteum progenitors involved in fracture healing

The periosteum is the major source of cells involved in fracture healing. We sought to characterize progenitor cells and their contribution to bone fracture healing. The periosteum is highly enriched with progenitor cells, including Sca1+ cells, fibroblast colony-forming units, and label-retaining cells compared to the endosteum and bone marrow. Using lineage tracing, we demonstrate that alpha smooth muscle actin (αSMA) identifies long-term, slow-cycling, self-renewing osteochondroprogenitors in the adult periosteum that are functionally important for bone formation during fracture healing. In addition, Col2.3CreER-labeled osteoblast cells contribute around 10% of osteoblasts but no chondrocytes in fracture calluses. Most periosteal osteochondroprogenitors following fracture can be targeted by αSMACreER. Previously identified skeletal stem cell populations were common in periosteum but contained high proportions of mature osteoblasts. We have demonstrated that the periosteum is highly enriched with skeletal progenitor cells, and there is heterogeneity in the populations of cells that contribute to mature lineages during periosteal fracture healing.

Markers for Identification of Postnatal Skeletal Stem Cells In Vivo

PURPOSE OF REVIEW

The adult skeleton contains stem cells involved in growth, homeostasis, and healing. Mesenchymal or skeletal stem cells are proposed to provide precursors to osteoblasts, chondrocytes, marrow adipocytes, and stromal cells. We review the evidence for existence and functionality of different skeletal stem cell pools, and the tools available for identifying or targeting these populations in mouse and human tissues.

RECENT FINDINGS

Lineage tracing and single cell-based techniques in mouse models indicate that multiple pools of stem cells exist in postnatal bone. These include growth plate stem cells, stem and progenitor cells in the diaphysis, reticular cells that only form bone in response to injury, and injury-responsive periosteal stem cells. New staining protocols have also been described for prospective isolation of human skeletal stem cells. Several populations of postnatal skeletal stem and progenitor cells have been identified in mice, and we have an increasing array of tools to target these cells. Most Cre models lack a high degree of specificity to define single populations. Human studies are less advanced and require further efforts to refine methods for identifying stem and progenitor cells in adult bone.

Stem and progenitor cells in skeletal development

Accumulating evidence supports the idea that stem and progenitor cells play important roles in skeletal development. Over the last decade, the definition of skeletal stem and progenitor cells has evolved from cells simply defined by their in vitro behaviors to cells fully defined by a combination of sophisticated approaches, including serial transplantation assays and in vivo lineage-tracing experiments. These approaches have led to better identification of the characteristics of skeletal stem cells residing in multiple sites, including the perichondrium of the fetal bone, the resting zone of the postnatal growth plate, the bone marrow space and the periosteum in adulthood. These diverse groups of skeletal stem cells appear to closely collaborate and achieve a number of important biological functions of bones, including not only bone development and growth, but also bone maintenance and repair. Although these are important findings, we are only beginning to understand the diversity and the nature of skeletal stem and progenitor cells, and how they actually behave in vivo.

Bone Marrow-Harvesting Technique Influences Functional Heterogeneity of Mesenchymal Stem/Stromal Cells and Cartilage Regeneration

BACKGROUND

Connective tissue progenitors (CTPs) from native bone marrow (BM) or their culture-expanded progeny, often referred to as mesenchymal stem/stromal cells, represents a promising strategy for treatment of cartilage injuries. But the cartilage regeneration capacity of these cells remains unpredictable because of cell heterogeneity.

HYPOTHESIS

The harvest technique of BM may highly influence stem cell heterogeneity and, thus, cartilage formation because these cells have distinct spatial localization within BM from the same bone.

STUDY DESIGN

Controlled laboratory study.

METHODS

CTPs obtained from the femur of patients undergoing total hip replacement by 2 harvest techniques-BM aspiration and BM collection-after bone rasping were immunophenotyped by flow cytometry and evaluated for chondrogenic ability. The spatial localization of different CTP subsets in BM was verified by immunohistochemistry.

RESULTS

Cells from the BM after rasping were significantly more chondrogenic than the donor-matched aspirate, whereas no notable difference in their osteogenic or adipogenic potential was observed. The authors then assessed whether distinct immunophenotypically defined CTP subsets were responsible for the different chondrogenic capacity. Cells directly isolated from BM after rasping contained a higher percentage (mean, 7.2-fold) of CD45-CD271+CD56+ CTPs as compared with BM aspirates. The presence of this subset in the harvested BM strongly correlated with chondrogenic ability, showing that CD271+CD56+ cells are enriched in chondroprogenitors. Furthermore, evaluation of these CTP subsets in BM revealed that CD271+CD56+ cells were localized in the bone-lining regions whereas CD271+CD56- cells were found in the perivascular regions. Since the iliac crest remains a frequent site of BM harvest for musculoskeletal regeneration, the authors also compared the spatial distribution of these subsets in trabeculae of femoral head and iliac crest and found CD271+CD56+ bone-lining cells in both tissues.

CONCLUSION

Chondrogenically distinct CTP subsets have distinct spatial localization in BM; hence, the harvest technique of BM determines the efficiency of cartilage formation.

CLINICAL RELEVANCE

The harvest technique of BM may be of major importance in determining the clinical success of BM mesenchymal stem/stromal cells in cartilage repair.

Vascular Endothelial Cell-Specific Connective Tissue Growth Factor (CTGF) Is Necessary for Development of Chronic Hypoxia-Induced Pulmonary Hypertension

Chronic hypoxia frequently complicates the care of patients with interstitial lung disease, contributing to the development of pulmonary hypertension (PH), and premature death. Connective tissue growth factor (CTGF), a matricellular protein of the Cyr61/CTGF/Nov (CCN) family, is known to exacerbate vascular remodeling within the lung. We have previously demonstrated that vascular endothelial-cell specific down-regulation of CTGF is associated with protection against the development of PH associated with hypoxia, though the mechanism for this effect is unknown. In this study, we generated a transgenic mouse line in which the Ctgf gene was floxed and deleted in vascular endothelial cells that expressed Cre recombinase under the control of VE-Cadherin promoter (eCTGF KO mice). Lack of vascular endothelial-derived CTGF protected against the development of PH secondary to chronic hypoxia, as well as in another model of bleomycin-induced pulmonary hypertension. Importantly, attenuation of PH was associated with a decrease in infiltrating inflammatory cells expressing CD11b or integrin α_M (ITGAM), a known adhesion receptor for CTGF, in the lungs of hypoxia-exposed eCTGF KO mice. Moreover, these pathological changes were associated with activation of—Rho GTPase family member—cell division control protein 42 homolog (Cdc42) signaling, known to be associated with alteration in endothelial barrier function. These data indicate that endothelial-specific deletion of CTGF results in protection against development of chronic-hypoxia induced PH. This protection is conferred by both a decrease in inflammatory cell recruitment to the lung, and a reduction in lung Cdc42 activity. Based on our studies, CTGF inhibitor treatment should be investigated in patients with PH associated with chronic hypoxia secondary to chronic lung disease.

Visual reporters for study of the osteoblast lineage

Advancing our understanding of osteoblast biology and differentiation is critical to elucidate the pathological mechanisms responsible for skeletal diseases such as osteoporosis. Histology and histomorphometry, the classical methods to study osteoblast biology, identify osteoblasts based on their location and morphology and ability to mineralize matrix, but do not clearly define their stage of differentiation. Introduction of visual transgenes into the cells of osteoblast lineage has revolutionized the field and resulted in a paradigm shift that allowed for specific identification and isolation of subpopulations within the osteoblast lineage. Knowledge acquired from the studies based on GFP transgenes has allowed for more precise interpretation of studies analyzing targeted overexpression or deletion of genes in the osteoblast lineage. Here, we provide a condensed overview of the currently available promoter-fluorescent reporter transgenic mice that have been generated and evaluated to varying extents. We cover different stages of the lineage as transgenes have been utilized to identify osteoprogenitors, pre-osteoblasts, osteoblasts, or osteocytes. We show that each of these promoters present with advantages and disadvantages. The studies based on the use of these reporter mice have improved our understanding of bone biology. They constitute attractive models to target osteoblasts and help to understand their cell biology.

Mesenchymal progenitor cells for the osteogenic lineage

Mesenchymal progenitors of the osteogenic lineage provide the flexibility for bone to grow, maintain its function and homeostasis. Traditionally, colony-forming-unit fibroblasts (CFU-Fs) have been regarded as surrogates for mesenchymal progenitors; however, this definition cannot address the function of these progenitors in their native setting. Transgenic murine models including lineage-tracing technologies based on the cre-lox system have proven to be useful in delineating mesenchymal progenitors in their native environment. Although heterogeneity of cell populations of interest marked by a promoter-based approach complicates overall interpretation, an emerging complexity of mesenchymal progenitors has been revealed. Current literatures suggest two distinct types of bone progenitor cells; growth-associated mesenchymal progenitors contribute to explosive growth of bone in early life, whereas bone marrow mesenchymal progenitors contribute to the much slower remodeling process and response to injury that occurs mainly in adulthood. More detailed relationships of these progenitors need to be studied through further experimentation.

The role of CCN family genes in haematological malignancies

Haematological malignancies, although a broad range of specific disease types, continue to show considerable overlap in classification, and patients are treated using similar chemotherapy regimes. In this review we look at the role of the CCN family of matricellular proteins and indicate their role in nine haematological malignancies including both myeloid and lymphoid neoplasms. The potential for further haematological neoplasms with CCN family associations is argued by summarising the demonstrated role of CCN family genes in the differentiation of haematopoietic stem cells (HSC) and mesenchymal stem cells. The expanding field of knowledge encompassing CCN family genes and cancers of the HSC-lineage highlights the importance of extracellular matrix-interactions in both normal physiology and tumorigenesis of the blood, bone marrow and lymph nodes.