Gremlin 1 identifies a skeletal stem cell with bone, cartilage, and reticular stromal potential

Pluripotent stem cells for skeletal tissue engineering

Here, we review the use of human pluripotent stem cells for skeletal tissue engineering. A number of approaches have been used for generating cartilage and bone from both human embryonic stem cells and induced pluripotent stem cells. These range from protocols relying on intrinsic cell interactions and signals from co-cultured cells to those attempting to recapitulate the series of steps occurring during mammalian skeletal development. The importance of generating authentic tissues rather than just differentiated cells is emphasized and enabling technologies for doing this are reported. We also review the different methods for characterization of skeletal cells and constructs at the tissue and single-cell level, and indicate newer resources not yet fully utilized in this field. There have been many challenges in this research area but the technologies to overcome these are beginning to appear, often adopted from related fields. This makes it more likely that cost-effective and efficacious human pluripotent stem cell-engineered constructs may become available for skeletal repair in the near future.

Skeletal Stem/Progenitor Cells in Periosteum and Skeletal Muscle Share a Common Molecular Response to Bone Injury

Bone regeneration involves skeletal stem/progenitor cells (SSPCs) recruited from bone marrow, periosteum, and adjacent skeletal muscle. To achieve bone reconstitution after injury, a coordinated cellular and molecular response is required from these cell populations. Here, we show that SSPCs from periosteum and skeletal muscle are enriched in osteochondral progenitors, and more efficiently contribute to endochondral ossification during fracture repair as compared to bone-marrow stromal cells. Single-cell RNA sequencing (RNAseq) analyses of periosteal cells reveal the cellular heterogeneity of periosteum at steady state and in response to bone fracture. Upon fracture, both periosteal and skeletal muscle SSPCs transition from a stem/progenitor to a fibrogenic state prior to chondrogenesis. This common activation pattern in periosteum and skeletal muscle SSPCs is mediated by bone morphogenetic protein (BMP) signaling. Functionally, Bmpr1a gene inactivation in platelet-derived growth factor receptor alpha (Pdgfra)-derived SSPCs impairs bone healing and decreases SSPC proliferation, migration, and osteochondral differentiation. These results uncover a coordinated molecular program driving SSPC activation in periosteum and skeletal muscle toward endochondral ossification during bone regeneration. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Alternatively Spliced Isoforms of the P2X7 Receptor: Structure, Function and Disease Associations

The P2X7 receptor (P2X7R) is an ATP-gated membrane ion channel that is expressed by multiple cell types. Following activation by extracellular ATP, the P2X7R mediates a broad range of cellular responses including cytokine and chemokine release, cell survival and differentiation, the activation of transcription factors, and apoptosis. The P2X7R is made up of three P2X7 subunits that contain specific domains essential for the receptor’s varied functions. Alternative splicing produces P2X7 isoforms that exclude one or more of these domains and assemble in combinations that alter P2X7R function. The modification of the structure and function of the P2X7R may adversely affect cellular responses to carcinogens and pathogens, and alternatively spliced (AS) P2X7 isoforms have been associated with several cancers. This review summarizes recent advances in understanding the structure and function of AS P2X7 isoforms and their associations with cancer and potential role in modulating the inflammatory response.

Cebpb Regulates Skeletal Stem Cell Osteogenic Differentiation and Fracture Healing via the WNT/β-Catenin Pathway

Fracture is the most common traumatic organ injury, and fracture nonunion is a critical clinical challenge. The research on the mechanisms of skeletal stem cell (SSC) differentiation and fracture healing may help develop new treatment strategies and improve the prognosis of patients at high risk of nonunion. Bioinformatic analysis of scRNA-seq data of mouse SSCs and mouse osteoprogenitors was applied to discover major transcription factors for the regulation of SSC differentiation. FACS was used to isolate SSCs prospectively. The expression of Cebpb, osteogenesis-related genes (Runx2, Sp7, and Bglap2), and markers for Notch, Hedgehog, MAPK, BMP2/SMAD, and WNT/β-catenin signaling pathways (Hes1, Gli1, p-Erk1/2, p-Smad1/5/9, and β-catenin) were detected in SSCs with qPCR or western blot, respectively. Alkaline phosphatase assay and alizarin red S staining were used to illustrate the osteogenic differentiation ability of SSCs in vitro. A WNT inhibitor, IWR-1, was further used to explore the mechanism of WNT signaling in the differentiation of SSCs. Micro-CT, mechanical testing, and immunohistochemistry of osteogenic and chondrogenic proteins (Sp7 and Col2α1) were used to demonstrate the capacity of Cebpb knockdown in promoting fracture healing in a monocortical defect model. We found that Cebpb was the crucial transcription factor regulating SSC differentiation. Inhibiting Cebpb in SSCs enhanced the expression of active β-catenin to promote the expression of WNT target genes, thus facilitating the osteogenic differentiation of SSCs. Bone mass, mechanical properties, and osteogenic protein expression were also increased in the Cebpb inhibition group compared to the group without Cebpb inhibition. Collectively, our results proved that Cebpb knockdown promotes SSC osteogenic differentiation and fracture healing via the WNT/β-catenin signaling pathway.

Cathepsin K+ Non-Osteoclast Cells in the Skeletal System: Function, Models, Identity, and Therapeutic Implications

Cathepsin K (Ctsk) is a cysteine protease of the papain superfamily initially identified in differentiated osteoclasts; it plays a critical role in degrading the bone matrix. However, subsequent in vivo and in vitro studies based on animal models elucidate novel subpopulations of Ctsk-expressing cells, which display markers and properties of mesenchymal stem/progenitor cells. This review introduces the function, identity, and role of Ctsk⁺ cells and their therapeutic implications in related preclinical osseous disorder models. It also summarizes the available in vivo models for studying Ctsk⁺ cells and their progeny. Further investigations of detailed properties and mechanisms of Ctsk⁺ cells in transgenic models are required to guide potential therapeutic targets in multiple diseases in the future.

A novel lineage of osteoprogenitor cells with dual epithelial and mesenchymal properties govern maxillofacial bone homeostasis and regeneration after MSFL

Bone regeneration originates from proliferation and differentiation of osteoprogenitors via either endochondral or intramembranous ossification; and the regeneration capacities decline with age and estrogen loss. Maxillary sinus floor lifting (MSFL) is a commonly used surgical procedure for guiding bone regeneration in maxilla. Radiographic analysis of 1210 clinical cases of maxilla bone regeneration after MSFL revealed that the intrasinus osteogenic efficacy was independent of age and gender, however; and this might be related to the Schneiderian membrane that lines the sinus cavity. In view of the particularity of this biological process, our present study aimed to elucidate the underlying mechanism of MSFL-induced bone regeneration. We first established a murine model to simulate the clinical MSFL. By single-cell RNA-sequencing and flow cytometry-based bulk RNA-sequencing, we identified a novel Krt14⁺Ctsk⁺ subset of cells that display both epithelial and mesenchymal properties and the transcriptomic feature of osteoprogenitors. Dual recombinases-mediated lineage tracing and loss-of-function analyses showed that these Krt14⁺Ctsk⁺ progenitors contribute to both MSFL-induced osteogenesis and physiological bone homeostasis by differentiating into Krt14^-Ctsk⁺ descendants which show robust osteogenic capacity. In addition, we detected a similar population of Krt14⁺Ctsk⁺ cells in human samples of Schneiderian membrane, which show a highly similar osteogenic potential and transcriptomic feature to the corresponding cells in mice. The identification of this Krt14⁺Ctsk⁺ population, featured by osteoprogenitor characteristics and dual epithelial-mesenchymal properties, provides new insight into the understanding of bone regeneration and may open more possibilities for clinical applications.

Single-cell transcriptomics profiling the compatibility mechanism of As2O3-indigo naturalis formula based on bone marrow stroma cells

Compound realgar natural indigo tablet is the only oral arsenic agent widely used in acute promyelocytic leukemia (APL) treatment. However, as a therapeutic drug for diseases of the blood system, the scientific knowledge of As₂O₃-indigo naturalis formula compatibility has not been studied in bone marrow stromal cells (BMSCs). We chose arsenic trioxide (As₂O₃: A), tanshinone IIA (T) and indirubin (I) as representative active compounds of realgar, indigo naturalis, and Salvia miltiorrhiza, respectively, to evaluated the pharmaceutical mechanism and the compatibility of ATI (drug combination) using single-cell RNA sequencing (scRNA-seq). The overlapped genes associated with both disease and drug were selected in BMSCs for in-depth analysis. Results show that joint applications of ATI had the strongest therapeutic efficacy in a murine APL model. Lepr-MSCs, OLCs and BMECs were the sensitive cell groups targeted by ATI in the murine APL model. ATI could regulate the related genes of osteogenic differentiation, adipogenic differentiation, and endothelial cell migration in bone marrow mesenchymal lineage cells in murine APL model and improve normal hematopoiesis-related gene expression and poor prognosis of Lepr-MSCs, OLCs and BMECs in mice with leukemia according to scRNA-seq data. The strongest regulatory effects were found in the joint applications of ATI. ATI combination had the potential mechanism to maintain the stability of the hematopoietic microenvironment and promote hematopoiesis to assist in the treatment of APL. This study illustrated the potential mechanism of ATI in regulating BMSCs from the overall perspective of the hematopoietic microenvironment, and broadened the scientific understanding of ATI compatibility in BMSCs.

Aged Callus Skeletal Stem/Progenitor Cells Contain an Inflammatory Osteogenic Population With Increased IRF and NF-κB Pathways and Reduced Osteogenic Potential

Skeletal stem/progenitor cells (SSPCs) are critical for fracture repair by providing osteo-chondro precursors in the callus, which is impaired in aging. However, the molecular signatures of callus SSPCs during aging are not known. Herein, we performed single-cell RNA sequencing on 11,957 CD45-CD31-Ter119- SSPCs isolated from young and aged mouse calluses. Combining unsupervised clustering, putative makers, and DEGs/pathway analyses, major SSPC clusters were annotated as osteogenic, proliferating, and adipogenic populations. The proliferating cluster had a differentiating potential into osteogenic and adipogenic lineages by trajectory analysis. The osteoblastic/adipogenic/proliferating potential of individual clusters was further evidenced by elevated expression of genes related to osteoblasts, adipocytes, or proliferation. The osteogenic cluster was sub-clustered into house-keeping and inflammatory osteogenic populations that were decreased and increased in aged callus, respectively. The majority of master regulators for the inflammatory osteogenic population belong to IRF and NF-κB families, which was confirmed by immunostaining, RT-qPCR, and Western blot analysis. Furthermore, cells in the inflammatory osteogenic sub-cluster had reduced osteoblast differentiation capacity. In conclusion, we identified 3 major clusters in callus SSPCs, confirming their heterogeneity and, importantly, increased IRF/NF-κB-mediated inflammatory osteogenic population with decreased osteogenic potential in aged cells.

Psoralen alleviates radiation-induced bone injury by rescuing skeletal stem cell stemness through AKT-mediated upregulation of GSK-3β and NRF2

BACKGROUND

Repairing radiation-induced bone injuries remains a significant challenge in the clinic, and few effective medicines are currently available. Psoralen is a principal bioactive component of Cullen corylifolium (L.) Medik and has been reported to have antitumor, anti-inflammatory, and pro-osteogenesis activities. However, less information is available regarding the role of psoralen in the treatment of radiation-induced bone injury. In this study, we explored the modulatory effects of psoralen on skeletal stem cells and their protective effects on radiation-induced bone injuries.

METHODS

The protective effects of psoralen on radiation-induced osteoporosis and irradiated bone defects were evaluated by microCT and pathological analysis. In addition, the cell proliferation, osteogenesis, and self-renewal of SSCs were explored. Further, the underlying mechanisms of the protective of psoralen were investigated by using RNA sequencing and functional gain and loss experiments in vitro and in vivo. Statistical significance was analyzed using Student's t test. The one-way ANOVA was used in multiple group data analysis.

RESULTS

Here, we demonstrated that psoralen, a natural herbal extract, mitigated radiation-induced bone injury (irradiation-induced osteoporosis and irradiated bone defects) in mice partially by rescuing the stemness of irradiated skeletal stem cells. Mechanistically, psoralen restored the stemness of skeletal stem cells by alleviating the radiation-induced suppression of AKT/GSK-3β and elevating NRF2 expression in skeletal stem cells. Furthermore, the expression of KEAP1 in skeletal stem cells did not significantly change in the presence of psoralen. Moreover, blockade of NRF2 in vivo partially abolished the promising effects of psoralen in a murine model of irradiation-induced osteoporosis and irradiated bone regeneration.

CONCLUSIONS

In summary, our findings identified psoralen as a potential medicine to mitigate bone radiation injury. In addition, skeletal stem cells and AKT-GSK-3β and NRF2 may thus represent therapeutic targets for treating radiation-induced bone injury.

Stem Cell Biomarkers and Tumorigenesis in Gastric Cancer

Stomach cancer has a high mortality, which is partially caused by an absence of suitable biomarkers to allow detection of the initiation stages of cancer progression. Thus, identification of critical biomarkers associated with gastric cancer (GC) is required to advance its clinical diagnoses and treatment. Recent studies using tracing models for lineage analysis of GC stem cells indicate that the cell fate decision of the gastric stem cells might be an important issue for stem cell plasticity. They include leucine-rich repeat-containing G-protein-coupled receptor 5 (Lgr5⁺), Cholecystokinin receptor 2 (Cckr2⁺), and axis inhibition protein 2 (Axin2⁺) as the stem cell markers in the antrum, Trefoil Factor 2 (TFF2⁺), Mist1⁺ stem cells, and Troy+ chief cells in the corpus. By contrast, Estrogen receptor 1 (eR1), Leucine-rich repeats and immunoglobulin-like domains 1 (Lrig1), SRY (sex determining region Y)-box 2 (Sox2), and B lymphoma Mo-MLV insertion region 1 homolog (Bmi1) are rich in both the antrum and corpus regions. These markers might help to identify the cell-lineage identity and analyze the plasticity of each stem cell population. Thus, identification of marker genes for the development of GC and its environment is critical for the clinical application of cancer stem cells in the prevention of stomach cancers.

Inhibiting Hh Signaling in Gli1+ Osteogenic Progenitors Alleviates TMJOA

The increased prevalence of temporomandibular joint osteoarthritis (TMJOA) in children and adolescents has drawn considerable attention as it may interfere with mandibular condyle growth, resulting in dento-maxillofacial deformities. However, treatments for osteoarthritis have been ineffective at restoring the damaged bone and cartilage structures due to poor understanding of the underlying degenerative mechanism. In this study, we demonstrate that Gli1+ cells residing in the subchondral bone contribute to bone formation and homeostasis in the mandibular condyle, identifying them as osteogenic progenitors in vivo. Furthermore, we show that, in a TMJOA mouse model, derivatives of Gli1+ cells undergo excessive expansion along with increased but uneven distribution of osteogenic differentiation in the subchondral bone, which leads to abnormal subchondral bone remodeling via Hedgehog (Hh) signaling activation and to the development of TMJOA. The selective pharmacological inhibition and specific genetic inhibition of Hh signaling in Gli1+ osteogenic progenitors result in improved subchondral bone microstructure, attenuated local immune inflammatory response in the subchondral bone, and reduced degeneration of the articular cartilage, providing in vivo functional evidence that targeting Hh signaling in Gli1+ osteogenic progenitors can modulate bone homeostasis in osteoarthritis and provide a potential approach for treating TMJOA.

p16INK4A-dependent senescence in the bone marrow niche drives age-related metabolic changes of hematopoietic progenitors

Rapid and effective leukocyte response to infection is a fundamental function of the bone marrow (BM). However, with increasing age, this response becomes impaired, resulting in an increased burden of infectious diseases. Here, we investigate how aging changes the metabolism and function of hematopoietic progenitor cells (HPCs) and the impact of the BM niche on this phenotype. We found that, in response to lipopolysaccharide-induced stress, HPC mitochondrial function is impaired, and there is a failure to upregulate the TCA cycle in progenitor populations in aged animals compared with young animals. Furthermore, aged mesenchymal stromal cells (MSCs) of the BM niche, but not HPCs, exhibit a senescent phenotype, and selective depletion of senescent cells from the BM niche, as well as treatment with the senolytic drug ABT-263, improves mitochondrial function of HPCs when stressed with lipopolysaccharide. In summary, age-related HPC metabolic dysfunction occurs indirectly as a "bystander phenomenon" in the aging BM niche and can be restored by targeting senescent MSCs.

Self-Renewal and Cancers of the Gastric Epithelium: An Update and the Role of the Lectin TFF1 as an Antral Tumor Suppressor

In 2020, gastric cancer was the fourth leading cause of cancer deaths globally. About 90% of gastric cancers are sporadic and the vast majority are correlated with Helicobacter pylori infection; whereas familial clustering is observed in about 10% of cases. Gastric cancer is now considered to be a disease originating from dysregulated self-renewal of the gastric glands in the setting of an inflammatory environment. The human stomach contains two types of gastric units, which show bi-directional self-renewal from a complex variety of stem cells. This review focuses on recent progress concerning the characterization of the different stem cell populations and the mainly mesenchymal signals triggering their stepwise differentiation as well as the genesis of pre-cancerous lesions and carcinogenesis. Furthermore, a model is presented (Lectin-triggered Receptor Blocking Hypothesis) explaining the role of the lectin TFF1 as an antral tumor suppressor possibly regulating Lgr5⁺ antral stem cells in a paracrine or maybe autocrine fashion, with neighboring antral gland cells having a role as niche cells.

MDS cells impair osteolineage differentiation of MSCs via extracellular vesicles to suppress normal hematopoiesis

Myelodysplastic syndrome (MDS) is a clonal disorder of hematopoietic stem cells (HSCs), characterized by ineffective hematopoiesis and frequent progression to leukemia. It has long remained unresolved how MDS cells, which are less proliferative, inhibit normal hematopoiesis and eventually dominate the bone marrow space. Despite several studies implicating mesenchymal stromal or stem cells (MSCs), a principal component of the HSC niche, in the inhibition of normal hematopoiesis, the molecular mechanisms underlying this process remain unclear. Here, we demonstrate that both human and mouse MDS cells perturb bone metabolism by suppressing the osteolineage differentiation of MSCs, which impairs the ability of MSCs to support normal HSCs. Enforced MSC differentiation rescues the suppressed normal hematopoiesis in both in vivo and in vitro MDS models. Intriguingly, the suppression effect is reversible and mediated by extracellular vesicles (EVs) derived from MDS cells. These findings shed light on the novel MDS EV-MSC axis in ineffective hematopoiesis.

SM22α-lineage niche cells regulate intramembranous bone regeneration via PDGFRβ-triggered hydrogen sulfide production

Bone stromal cells are critical for bone homeostasis and regeneration. Growing evidence suggests that non-stem bone niche cells support bone homeostasis and regeneration via paracrine mechanisms, which remain to be elucidated. Here, we show that physiologically quiescent SM22α-lineage stromal cells expand after bone injury to regulate diverse processes of intramembranous bone regeneration. The majority of SM22α-lineage cells neither act as stem cells in vivo nor show their expression patterns. Dysfunction of SM22α-lineage niche cells induced by loss of platelet-derived growth factor receptor β (PDGFRβ) impairs bone repair. We further show that PDGFRβ-triggered hydrogen sulfide (H₂S) generation in SM22α-lineage niche cells facilitates osteogenesis and angiogenesis and suppresses overactive osteoclastogenesis. Collectively, these data demonstrate that non-stem SM22α-lineage niche cells support the niche for bone regeneration with a PDGFRβ/H₂S-dependent regulatory mechanism. Our findings provide further insight into non-stem bone stromal niche cell populations and niche-regulation strategy for bone repair.

Construction of tissue-engineered nucleus pulposus by stimulation with periodic mechanical stress and BMP-2

Intervertebral disc (IVD) degeneration, which is common among elderly individuals, mainly manifests as low back pain and is caused by structural deterioration of the nucleus pulposus (NP) due to physiological mechanical stress. NP mesenchymal stem cells (NPMSCs) around the IVD endplate have multidirectional differentiation potential and can be used for tissue repair. To define favorable conditions for NPMSC proliferation and differentiation into chondroid cells for NP repair, the present study simulated periodic mechanical stress (PMS) of the NP under physiological conditions using MSC chondrogenic differentiation medium and recombinant human BMP-2 (rhBMP-2). rhBMP-2 effectively promoted NPMSC proliferation and differentiation. To clarify the mechanism of action of rhBMP-2, integrin alpha 1 (ITG A1) and BMP-2 were inhibited. PMS regulated the BMP-2/Smad1/RUNX2 pathway through ITG A1 and promoted NPMSC proliferation and differentiation. During tissue-engineered NP construction, PMS can effectively reduce osteogenic differentiation and promote extracellular matrix protein synthesis to enhance structural NP recovery.

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Extraction of NPMSCs from degenerated nucleus pulposus

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NPMSCs cultured in vitro by simulating physiological mechanical stress

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ITG A1 to promote proliferation and differentiation of NPMSCs through BMP-2/Smad1/RUNX2

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Injectable tissue-engineered nucleus pulposus

Gremlin1 is a therapeutically targetable FGFR1 ligand that regulates lineage plasticity and castration resistance in prostate cancer

Among the greatest hurdles in clinical management of prostate cancer (PCa) are the progression to lethal castration-resistant prostate cancer (CRPC) and the lack of suitable targeted therapies for advanced disease. Here we identify Gremlin1 as a ligand for fibroblast growth factor receptor 1 (FGFR1), which promotes lineage plasticity and drives castration resistance. Importantly, we generate a specific anti-Gremlin1 therapeutic antibody and demonstrate synergistic effect with androgen deprivation therapy (ADT) in CRPC. GREM1 transcription is suppressed by androgen receptor (AR) and released following ADT. We show that Gremlin1 binds to FGFR1 and activates downstream MAPK signaling. Gremlin1 interacts with FGFR1 differently to its canonical ligand FGF1, as revealed through protein structure docking and mutagenesis experiments. Altogether, our data indicate Gremlin1 as a promising candidate therapeutic target for CRPC.

The long and winding road: homeostatic and disordered haematopoietic microenvironmental niches: a narrative review

Haematopoietic microenvironmental niches have been described as the 'gatekeepers' for the blood and immune systems. These niches change during ontogeny, with the bone marrow becoming the predominant site of haematopoiesis in post-natal life under steady state conditions. To determine the structure and function of different haematopoietic microenvironmental niches, it is essential to clearly define specific haematopoietic stem and progenitor cell subsets during ontogeny and to understand their temporal appearance and anatomical positioning. A variety of haematopoietic and non-haematopoietic cells contribute to haematopoietic stem and progenitor cell niches. The latter is reported to include endothelial cells and mesenchymal stromal cells (MSCs), skeletal stem cells and/or C-X-C motif chemokine ligand 12-abundant-reticular cell populations, which form crucial components of these microenvironments under homeostatic conditions. Dysregulation or deterioration of such cells contributes to significant clinical disorders and diseases worldwide and is associated with the ageing process. A critical appraisal of these issues and of the roles of MSC/C-X-C motif chemokine ligand 12-abundant-reticular cells and the more recently identified skeletal stem cell subsets in bone marrow haematopoietic niche function under homeostatic conditions and during ageing will form the basis of this research review. In the context of haematopoiesis, clinical translation will deal with lessons learned from the vast experience garnered from the development and use of MSC therapies to treat graft versus host disease in the context of allogeneic haematopoietic transplants, the recent application of these MSC therapies to treating emerging and severe coronavirus disease 2019 (COVID-19) infections, and, given that skeletal stem cell ageing is one proposed driver for haematopoietic ageing, the potential contributions of these stem cells to haematopoiesis in healthy bone marrow and the benefits and challenges of using this knowledge for rejuvenating the age-compromised bone marrow haematopoietic niches and restoring haematopoiesis.

Gli1+ Osteogenic Progenitors Contribute to Condylar Development and Fracture Repair

The condyle plays a pivotal role in mandible development, which is regulated by various signaling molecules. The hedgehog (Hh) signaling pathway is known to modulate several processes during bone formation. However, the role of Gli1, as the read-out of Hh signaling activity, in condylar development and fracture healing has not been clarified. In this study, we discovered that a population of Gli1+ cells residing immediately below the cartilage functions as osteogenic progenitors by using Gli1-Cre ^ERT2 ;tdTomato mice. These Gli1+ cells contributed to nearly all osteoblasts in the subchondral bone during condyle postnatal development. Interestingly, Gli1-lineage cells could differentiate into osteoblasts and chondrocytes during fracture healing. Inhibiting Wnt/β-catenin signaling downregulated the proliferation and differentiation of Gli1+ cells in vitro. These findings suggest that Gli1+ progenitor cells participate in not only normal bone formation but also fracture healing; moreover, these cells may provide a potential target for promoting bone regeneration of the mandible.

Periosteal Skeletal Stem Cells and Their Response to Bone Injury

Bone exhibits remarkable self-repair ability without fibrous scars. It is believed that the robust regenerative capacity comes from tissue-resident stem cells, such as skeletal stem cells (SSCs). Roughly, SSC has two niches: bone marrow (BM) and periosteum. BM-SSCs have been extensively studied for years. In contrast, our knowledge about periosteal SSCs (P-SSCs) is quite limited. There is abundant clinical evidence for the presence of stem cell populations within the periosteum. Researchers have even successfully cultured “stem-like” cells from the periosteum in vitro. However, due to the lack of effective markers, it is difficult to evaluate the stemness of real P-SSCs in vivo. Recently, several research teams have developed strategies for the successful identification of P-SSCs. For the first time, we can assess the stemness of P-SSCs from visual evidence. BM-SSCs and P-SSCs not only have much in common but also share distinct properties. Here, we provide an updated review of P-SSCs and their particular responses to bone injury.

Identification of Dental Stem Cells Similar to Skeletal Stem Cells

Stem and progenitor cells play important roles in the development and maintenance of teeth and bone. Surface markers expressed in bone marrow–derived mesenchymal stem cells are also expressed in dental tissue–derived stem cells. Mouse skeletal stem cells (mSSCs, CD45−Ter119−Tie2−CD51+Thy−6C3−CD105−CD200+) and human skeletal stem cells (hSSCs, CD45−CD235a−TIE2−CD31−CD146−PDPN+CD73+CD164+) have been identified in bone and shown to play important roles in skeletal development and regeneration. However, it is unclear whether dental tissues also harbor mSSC or hSSC populations. Here, we employed rainbow tracers and found that clonal expansion occurred in mouse dental tissues similar to that in bone. We sorted the mSSC population from mouse periodontal ligament (mPDL) tissue and mouse dental pulp (mDP) tissue in the lower incisors by fluorescence-activated cell sorting (FACS). In addition, we demonstrated that mPDL-derived skeletal stem cells (mPDL-SSCs) and mDP-derived skeletal stem cells (mDP-SSCs) have similar clonogenic capacity, as well as cementogenic and odontogenic potential, but not adipogenic potential, similar to the characteristics of mSSCs. Moreover, we found that the dental tissue–derived mSSC population plays an important role in repairing clipped incisors. Importantly, we sorted the hSSC population from human periodontal ligament (hPDL) and human dental pulp (hDP) tissue in molars and identified its stem cell characteristics. Finally, hPDL-like and hDP-like structures were generated after transplanting hPDL-SSCs and hDP-SSCs beneath the renal capsules. In conclusion, we demonstrated that mouse and human PDL and DP tissues harbor dental stem cells similar to mSSCs and hSSCs, respectively, providing a precise stem cell population for the exploration of dental diseases.

Periosteum-derived podoplanin-expressing stromal cells regulate nascent vascularization during epiphyseal marrow development

Bone marrow development and endochondral bone formation occur simultaneously. During endochondral ossification, periosteal vasculatures and stromal progenitors invade the primary avascular cartilaginous anlage, which induces primitive marrow development. We previously determined that bone marrow podoplanin (PDPN)-expressing stromal cells exist in the perivascular microenvironment and promote megakaryopoiesis and erythropoiesis. In this study, we aimed to examine the involvement of PDPN-expressing stromal cells in postnatal bone marrow generation. Using histological analysis, we observed that periosteum-derived PDPN-expressing stromal cells infiltrated the cartilaginous anlage of the postnatal epiphysis and populated on the primitive vasculature of secondary ossification center. Furthermore, immunophenotyping and cellular characteristic analyses indicated that the PDPN-expressing stromal cells constituted a subpopulation of the skeletal stem cell lineage. In vitro xenovascular model cocultured with human umbilical vein endothelial cells and PDPN-expressing skeletal stem cell progenies showed that PDPN-expressing stromal cells maintained vascular integrity via the release of angiogenic factors and vascular basement membrane-related extracellular matrices. We show that in this process, Notch signal activation committed the PDPN-expressing stromal cells into a dominant state with basement membrane-related extracellular matrices, especially type IV collagens. Our findings suggest that the PDPN-expressing stromal cells regulate the integrity of the primitive vasculatures in the epiphyseal nascent marrow. To the best of our knowledge, this is the first study to comprehensively examine how PDPN-expressing stromal cells contribute to marrow development and homeostasis.

Hedgehog Signaling Controls Bone Homeostasis by Regulating Osteogenic/Adipogenic Fate of Skeletal Stem/Progenitor Cells in Mice

Skeletal stem/progenitor cells (SSPCs) can differentiate into osteogenic or adipogenic lineage. The mechanism governing lineage allocation of SSPCs is still not completely understood. Hedgehog (Hh) signaling plays an essential role in specifying osteogenic fate of mesenchymal progenitors during embryogenesis. However, it is still unclear whether Hh signaling is required for lineage allocation of SSPCs in postnatal skeleton, and whether its dysregulation is related to age-related osteoporosis. Here, we demonstrated that Hh signaling was activated in metaphyseal SSPCs during osteogenic differentiation in the adult skeleton, and its activity decreased with aging. Inactivation of Hh signaling by genetic ablation of Smo, a key molecule in Hh signaling, in Osx-Cre-targeted SSPCs and hypertrophic chondrocytes led to decreased bone formation and increased bone marrow adiposity, two key pathological features of age-related osteoporosis. Moreover, we found that the bone-fat imbalance phenotype caused by Smo deletion mainly resulted from aberrant allocation of SSPCs toward adipogenic lineage at the expense of osteogenic differentiation, but not due to accelerated transdifferentiation of chondrocytes into adipocytes. Mechanistically, we found that Hh signaling regulated osteoblast versus adipocyte fate of SSPCs partly through upregulating Wnt signaling. Thus, our results indicate that Hh signaling regulates bone homeostasis and age-related osteoporosis by acting as a critical switch of cell fate decisions of Osx-Cre-targeted SSPCs in mice and suggest that Hh signaling may serve as a potential therapeutic target for the treatment of osteoporosis and other metabolic bone diseases. © 2021 American Society for Bone and Mineral Research (ASBMR).

The Origin and Contribution of Cancer-Associated Fibroblasts in Colorectal Carcinogenesis

BACKGROUND & AIMS

Cancer-associated fibroblasts (CAFs) play an important role in colorectal cancer (CRC) progression and predict poor prognosis in CRC patients. However, the cellular origins of CAFs remain unknown, making it challenging to therapeutically target these cells. Here, we aimed to identify the origins and contribution of colorectal CAFs associated with poor prognosis.

METHODS

To elucidate CAF origins, we used a colitis-associated CRC mouse model in 5 different fate-mapping mouse lines with 5-bromodeoxyuridine dosing. RNA sequencing of fluorescence-activated cell sorting-purified CRC CAFs was performed to identify a potential therapeutic target in CAFs. To examine the prognostic significance of the stromal target, CRC patient RNA sequencing data and tissue microarray were used. CRC organoids were injected into the colons of knockout mice to assess the mechanism by which the stromal gene contributes to colorectal tumorigenesis.

RESULTS

Our lineage-tracing studies revealed that in CRC, many ACTA2⁺ CAFs emerge through proliferation from intestinal pericryptal leptin receptor (Lepr)⁺ cells. These Lepr-lineage CAFs, in turn, express melanoma cell adhesion molecule (MCAM), a CRC stroma-specific marker that we identified with the use of RNA sequencing. High MCAM expression induced by transforming growth factor β was inversely associated with patient survival in human CRC. In mice, stromal Mcam knockout attenuated orthotopically injected colorectal tumoroid growth and improved survival through decreased tumor-associated macrophage recruitment. Mechanistically, fibroblast MCAM interacted with interleukin-1 receptor 1 to augment nuclear factor κB-IL34/CCL8 signaling that promotes macrophage chemotaxis.

CONCLUSIONS

In colorectal carcinogenesis, pericryptal Lepr-lineage cells proliferate to generate MCAM⁺ CAFs that shape the tumor-promoting immune microenvironment. Preventing the expansion/differentiation of Lepr-lineage CAFs or inhibiting MCAM activity could be effective therapeutic approaches for CRC.

Hypertrophic chondrocytes serve as a reservoir for marrow-associated skeletal stem and progenitor cells, osteoblasts, and adipocytes during skeletal development

Hypertrophic chondrocytes give rise to osteoblasts during skeletal development; however, the process by which these non-mitotic cells make this transition is not well understood. Prior studies have also suggested that skeletal stem and progenitor cells (SSPCs) localize to the surrounding periosteum and serve as a major source of marrow-associated SSPCs, osteoblasts, osteocytes, and adipocytes during skeletal development. To further understand the cell transition process by which hypertrophic chondrocytes contribute to osteoblasts or other marrow associated cells, we utilized inducible and constitutive hypertrophic chondrocyte lineage tracing and reporter mouse models (Col10a1CreERT2; Rosa26fs-tdTomato and Col10a1Cre; Rosa26fs-tdTomato) in combination with a PDGFRaH2B-GFP transgenic line, single-cell RNA-sequencing, bulk RNA-sequencing, immunofluorescence staining, and cell transplantation assays. Our data demonstrate that hypertrophic chondrocytes undergo a process of dedifferentiation to generate marrow-associated SSPCs that serve as a primary source of osteoblasts during skeletal development. These hypertrophic chondrocyte-derived SSPCs commit to a CXCL12-abundant reticular (CAR) cell phenotype during skeletal development and demonstrate unique abilities to recruit vasculature and promote bone marrow establishment, while also contributing to the adipogenic lineage.

On the horizon: Hedgehog signaling to heal broken bones

Uncovering the molecular pathways that drive skeletal repair has been an ongoing challenge. Initial efforts have relied on in vitro assays to identify the key signaling pathways that drive cartilage and bone differentiation. While these assays can provide some clues, assessing specific pathways in animal models is critical. Furthermore, definitive proof that a pathway is required for skeletal repair is best provided using genetic tests. Stimulating the Hh (Hedgehog) pathway can promote cartilage and bone differentiation in cell culture assays. In addition, the application of HH protein or various pathway agonists in vivo has a positive influence on bone healing. Until recently, however, genetic proof that the Hh pathway is involved in bone repair has been lacking. Here, we consider both in vitro and in vivo studies that examine the role of Hh in repair and discuss some of the challenges inherent in their interpretation. We also identify needed areas of study considering a new appreciation for the role of cartilage during repair, the variety of cell types that may have differing roles in repair, and the recent availability of powerful lineage tracing techniques. We are optimistic that emerging genetic tools will make it possible to precisely define when and in which cells promoting Hh signaling can best promote skeletal repair, and thus, the clinical potential for targeting the Hh pathway can be realized.

Single-cell transcriptomics of LepR-positive skeletal cells reveals heterogeneous stress-dependent stem and progenitor pools

Leptin receptor (LepR)-positive cells are key components of the bone marrow hematopoietic microenvironment, and highly enrich skeletal stem and progenitor cells that maintain homeostasis of the adult skeleton. However, the heterogeneity and lineage hierarchy within this population has been elusive. Using genetic lineage tracing and single-cell RNA sequencing, we found that Lepr-Cre labels most bone marrow stromal cells and osteogenic lineage cells in adult long bones. Integrated analysis of Lepr-Cre-traced cells under homeostatic and stress conditions revealed dynamic changes of the adipogenic, osteogenic, and periosteal lineages. Importantly, we discovered a Notch3+ bone marrow sub-population that is slow-cycling and closely associated with the vasculatures, as well as key transcriptional networks promoting osteo-chondrogenic differentiation. We also identified a Sca-1+ periosteal sub-population with high clonogenic activity but limited osteo-chondrogenic potential. Together, we mapped the transcriptomic landscape of adult LepR+ stem and progenitor cells and uncovered cellular and molecular mechanisms underlying their maintenance and lineage specification.

Recent Advances in Single-Cell View of Mesenchymal Stem Cell in Osteogenesis

Osteoblasts continuously replenished by osteoblast progenitor cells form the basis of bone development, maintenance, and regeneration. Mesenchymal stem cells (MSCs) from various tissues can differentiate into the progenitor cell of osteogenic lineage and serve as the main source of osteoblasts. They also respond flexibly to regenerative and anabolic signals emitted by the surrounding microenvironment, thereby maintaining bone homeostasis and participating in bone remodeling. However, MSCs exhibit heterogeneity at multiple levels including different tissue sources and subpopulations which exhibit diversified gene expression and differentiation capacity, and surface markers used to predict cell differentiation potential remain to be further elucidated. The rapid advancement of lineage tracing methods and single-cell technology has made substantial progress in the characterization of osteogenic stem/progenitor cell populations in MSCs. Here, we reviewed the research progress of scRNA-seq technology in the identification of osteogenic markers and differentiation pathways, MSC-related new insights drawn from single-cell technology combined with experimental technology, and recent findings regarding the interaction between stem cell fate and niche in homeostasis and pathological process.

Regeneration of Jaw Joint Cartilage in Adult Zebrafish

The poor intrinsic repair capacity of mammalian joint cartilage likely contributes to the high incidence of arthritis worldwide. Adult zebrafish can regenerate many structures that show limited or no healing capacity in mammals, including the jawbone. To test whether zebrafish can also regenerate damaged joints, we developed a surgical injury model in which the zebrafish jaw joint is destabilized via transection of the major jaw joint ligament, the interopercular-mandibular (IOM). Unilateral transection of the IOM ligament in 1-year-old fish resulted in an initial reduction of jaw joint cartilage by 14 days, with full regeneration of joint cartilage by 28 days. Joint cartilage regeneration involves the re-entry of articular chondrocytes into the cell cycle and the upregulated expression of sox10, a marker of developing chondrocytes in the embryo that becomes restricted to a subset of joint chondrocytes in adults. Genetic ablation of these sox10-expressing chondrocytes shows that they are essential for joint cartilage regeneration. To uncover the potential source of new chondrocytes during joint regeneration, we performed single-cell RNA sequencing of the uninjured adult jaw joint and identified multiple skeletal, connective tissue, and fibroblast subtypes. In particular, we uncovered a joint-specific periosteal population expressing coch and grem1a, with the jaw joint chondrocytes marked by grem1a expression during regeneration. Our findings demonstrate the capacity of zebrafish to regenerate adult joint cartilage and identify candidate cell types that can be tested for their roles in regenerative response.

Gli1+ Mesenchymal Stem Cells in Bone and Teeth

Mesenchymal stem cells (MSCs) are remarkable and noteworthy. Identification of markers for MSCs enables the study of their niche in vivo. It has been identified that glioma-associated oncogene 1 positive (Gli1+) cells are mesenchymal stem cells supporting homeostasis and injury repair, especially in the skeletal system and teeth. This review outlines the role of Gli1+ cells as an MSC subpopulation in both bones and teeth, suggesting the prospects of Gli1+ cells in stem cell-based tissue engineering.

Embryonic requirements for Tcf12 in the development of the mouse coronal suture

A major feature of Saethre-Chotzen syndrome is coronal craniosynostosis, the fusion of the frontal and parietal bones at the coronal suture. It is caused by heterozygous loss-of-function mutations in either of the bHLH transcription factors TWIST1 and TCF12. Although compound heterozygous Tcf12; Twist1 mice display severe coronal synostosis, the individual role of Tcf12 had remained unexplored. Here, we show that Tcf12 controls several key processes in calvarial development, including the rate of frontal and parietal bone growth, and the boundary between sutural and osteogenic cells. Genetic analysis supports an embryonic requirement for Tcf12 in suture formation, as combined deletion of Tcf12 in embryonic neural crest and mesoderm, but not in postnatal suture mesenchyme, disrupts the coronal suture. We also detected asymmetric distribution of mesenchymal cells on opposing sides of the wild-type frontal and parietal bones, which prefigures later bone overlap at the sutures. In Tcf12 mutants, reduced asymmetry is associated with bones meeting end-on-end, possibly contributing to synostosis. Our results support embryonic requirements of Tcf12 in proper formation of the overlapping coronal suture.

Isolation and in vitro characterization of murine young-adult long bone skeletal progenitors

Skeletal stem and progenitor cells (SSPCs) constitute a reservoir of bone-forming cells necessary for bone development, modeling and remodeling, as well as for fracture healing. Recent advances in tools to identify and isolate SSPCs have revealed that cells with multipotent properties are present not only in neonatal bone, but also in adult bone marrow and periosteum. The long bone metaphysis and endosteum have been proposed as an additional SSPC niche, although in vitro approaches to study their cellular and molecular characteristics are still limited. Here, we describe a comprehensive procedure to isolate and culture SSPCs derived from the metaphysis and endosteum of young-adult mice. Based on flow cytometry analysis of known SSPC markers, we found the presence of putative multipotent SSPCs, similar to neonatal bone tissue. In vitro, metaphyseal/endosteal SSPCs possess self-renewing capacity, and their multipotency is underscored by the ability to differentiate into the osteogenic and adipogenic lineage, while chondrogenic potential is limited. Expansion of metaphyseal/endosteal SSPCs under low oxygen conditions increases their proliferation capacity, while progenitor properties are maintained, likely reflecting their hypoxic niche in vivo. Collectively, we propose a validated isolation and culture protocol to study metaphyseal/endosteal SSPC biology in vitro.

Colon stroma mediates an inflammation-driven fibroblastic response controlling matrix remodeling and healing

Chronic inflammation is often associated with the development of tissue fibrosis, but how mesenchymal cell responses dictate pathological fibrosis versus resolution and healing remains unclear. Defining stromal heterogeneity and identifying molecular circuits driving extracellular matrix deposition and remodeling stands to illuminate the relationship between inflammation, fibrosis, and healing. We performed single-cell RNA-sequencing of colon-derived stromal cells and identified distinct classes of fibroblasts with gene signatures that are differentially regulated by chronic inflammation, including IL-11-producing inflammatory fibroblasts. We further identify a transcriptional program associated with trans-differentiation of mucosa-associated fibroblasts and define a functional gene signature associated with matrix deposition and remodeling in the inflamed colon. Our analysis supports a critical role for the metalloprotease Adamdec1 at the interface between tissue remodeling and healing during colitis, demonstrating its requirement for colon epithelial integrity. These findings provide mechanistic insight into how inflammation perturbs stromal cell behaviors to drive fibroblastic responses controlling mucosal matrix remodeling and healing.

Mechanically-regulated bone repair

Fracture healing is a complex, multistep process that is highly sensitive to mechanical signaling. To optimize repair, surgeons prescribe immediate weight-bearing as-tolerated within 24 hours after surgical fixation; however, this recommendation is based on anecdotal evidence and assessment of bulk healing outcomes (e.g., callus size, bone volume, etc.). Given challenges in accurately characterizing the mechanical environment and the ever-changing properties of the regenerate, the principles governing mechanical regulation of repair, including their cell and molecular basis, are not yet well defined. However, the use of mechanobiological rodent models, and their relatively large genetic toolbox, combined with recent advances in imaging approaches and single-cell analyses is improving our understanding of the bone microenvironment in response to loading. This review describes the identification and characterization of distinct cell populations involved in bone healing and highlights the most recent findings on mechanical regulation of bone homeostasis and repair with an emphasis on osteo-angio coupling. A discussion on aging and its impact on bone mechanoresponsiveness emphasizes the need for novel mechanotherapeutics that can re-sensitize skeletal stem and progenitor cells to physical rehabilitation protocols.

Pancreatic stromal Gremlin 1 expression during pancreatic tumorigenesis

Chronic pancreatitis (CP) is a major risk factor of pancreatic ductal adenocarcinoma (PDAC). How CP promotes pancreatic oncogenesis remains unclear. A characteristic feature of PDAC is its prominent desmoplasia in the tumor microenvironment, composed of activated fibroblasts and macrophages. Macrophages can be characterized as M1 or M2, with tumor-inhibiting or -promoting functions, respectively. We reported that Gremlin 1 (GREM1), a key pro-fibrogenic factor, is upregulated in the stroma of CP. The current study aimed to investigate the expression of GREM1 and correlation between GREM1 and macrophages within the pancreas during chronic inflammation and the development of PDAC. By mRNA in situ hybridization, we detected GREM1 mRNA expression within α-smooth muscle actin (SMA)-positive fibroblasts of the pancreatic stroma. These designated FibroblastsGrem1+ marginally increased from CP to pancreatic intraepithelial neoplasia (PanIN) and PDAC. Within PDAC, FibroblastsGrem1+ increased with higher pathological tumor stages and in a majority of PDAC subtypes screened. Additionally, FibroblastsGrem1+ positively correlated with total macrophages (MacCD68+) and M2 macrophages (M2CD163+) in PDAC. To begin exploring potential molecular links between FibroblastsGrem1+ and macrophages in PDAC, we examined the expression of macrophage migration inhibitory factor (MIF), an endogenous counteracting molecule of GREM1 and an M1 macrophage promoting factor. By IHC staining of MIF, we found MIF to be expressed by tumor cells, positively correlated with GREM1; by IHC co-staining, we found MIF to be negatively correlated with M2CD163+ expression. Our findings suggest that GREM1 expression by activated fibroblasts may promote PDAC development, and GREM1/MIF may play an important role in macrophage phenotype.

Col6a1+/CD201+ mesenchymal cells regulate intestinal morphogenesis and homeostasis

Intestinal mesenchymal cells encompass multiple subsets, whose origins, functions, and pathophysiological importance are still not clear. Here, we used the Col6a1Cre mouse, which targets distinct fibroblast subsets and perivascular cells that can be further distinguished by the combination of the CD201, PDGFRα and αSMA markers. Developmental studies revealed that the Col6a1Cre mouse also targets mesenchymal aggregates that are crucial for intestinal morphogenesis and patterning, suggesting an ontogenic relationship between them and homeostatic PDGFRαhi telocytes. Cell depletion experiments in adulthood showed that Col6a1+/CD201+ mesenchymal cells regulate homeostatic enteroendocrine cell differentiation and epithelial proliferation. During acute colitis, they expressed an inflammatory and extracellular matrix remodelling gene signature, but they also retained their properties and topology. Notably, both in homeostasis and tissue regeneration, they were dispensable for normal organ architecture, while CD34+ mesenchymal cells expanded, localised at the top of the crypts, and showed increased expression of villous-associated morphogenetic factors, providing thus evidence for the plasticity potential of intestinal mesenchymal cells. Our results provide a comprehensive analysis of the identities, origin, and functional significance of distinct mesenchymal populations in the intestine.

Protease-activated receptor-2 promotes osteogenesis in skeletal mesenchymal stem cells at the expense of adipogenesis: Involvement of interleukin-6

Bone marrow mesenchymal stem cells (MSCs) give rise to osteoblasts and adipocytes, with an inverse relationship between the two. The MSCs from protease-activated receptor-2 knockout (PAR2 KO) mice have a reduced capacity to generate osteoblasts. Here we describe the observation that PAR2 KO osteoblastic cultures generate more adipocytes than wildtype (WT) cultures. Osteoblasts from PAR2 KO mice expressed lower levels of osteoblastic genes (Runx2, Col1a1 and Bglap), and higher levels of the adipocytic gene Pparg than WT osteoblasts. Bone marrow stromal cells from PAR2 KO mice generated fewer osteoblastic colonies (assessed by staining for alkaline phosphatase activity and mineral deposition) and more adipocytic (Oil Red-O positive) colonies than cultures from WT mice. Similarly, cultures of the bone marrow stromal cell line (Kusa 4b10) in which PAR2 was knocked down (F2rl1 KD), were less osteoblastic and more adipocytic than vector control cells. Putative regulators of PAR2-mediated osteogenesis and suppression of adipogenesis were identified in an RNA-sequencing (RNA-seq) investigation; these include C1qtnf3, Gpr35, Grem1, Snorc and Tcea3, which were more highly expressed, and Cnr1, Enpep, Hmgn5, Il6 and Ramp3 which were expressed at lower levels, in control than in F2rl1 KD cells. Interleukin-6 (IL-6) levels were higher in medium harvested from F2rl1 KD cells than from control cells, and a neutralising anti-IL-6 antibody reduced the number of adipocytes in F2rl1 KD cultures to that of control cultures. Thus, PAR2 appears to be a mediator of the reciprocal relationship between osteogenesis and adipogenesis, with IL-6 having a regulatory role in these PAR2-mediated effects.

Single-Cell RNA Sequencing of In Vitro Expanded Chondrocytes: MSC-Like Cells With No Evidence of Distinct Subsets

OBJECTIVE

To investigate the heterogeneity of in vitro expanded chondrocytes used for autologous chondrocyte implantation.

METHODS

Human articular chondrocytes were expanded in vitro for 14 days, sorted into 86 single cells using fluorescence-activated cell sorting and subjected to single-cell RNA sequencing. Principal component, Cross R² hierarchical clustering, and differential gene expression analyses were used for data evaluation. Flow cytometry and single-cell RT-qPCR (reverse transcriptase quantitative polymerase chain reaction) was used to validate the results of the RNA sequencing data Polyclonal chondrocyte populations from the same donor were differentiated in vitro toward the osteogenic and adipogenic lineages.

RESULTS

There was considerable variation in gene expression between individual cells, but we found no evidence for separate cell subpopulations based on principal component, hierarchical clustering, and differential gene expression analysis. Most of the cells expressed all the markers defining mesenchymal stem cells, and as polyclonal chondrocyte populations from the same donor were shown to differentiate into osteocytes and adipocytes in vitro, these cells formally qualify as mesenchymal stem cells.

CONCLUSIONS

In vitro expanded chondrocytes consist of one single population of cells with heterogeneity in gene expression between the cells. Dedifferentiated chondrocytes qualify as mesenchymal stem cells as they fulfill all the criteria suggested by the International Society for Cellular Therapy.

Roles of mesenchymal stromal cells in the head and neck cancer microenvironment

Head and neck cancer (HNC), a common malignancy worldwide, is associated with high morbidity and mortality rates. Squamous cell carcinoma is the most common HNC type, followed by salivary gland carcinomas, head and neck sarcomas, and lymphomas. The microenvironment of HNCs comprises various cells that regulate tumor development. Recent studies have reported that the tumor microenvironment, which modulates cancer progression, regulates cancer treatment response. However, the presence of different types of stromal cells in cancers is a major challenge to elucidate the role of individual cells in tumor progression. The role of mesenchymal stromal cells (MSCs), which are a component of the tumor microenvironment, in HNC is unclear. The major impediment for characterizing the role of MSCs in cancer progression is the lack of MSC-specific markers and their phenotypic similarity with stromal cells. This review aimed to summarize the latest findings on the role of MSCs in the progression of HNC to improve our understanding of HNC pathophysiology.

Skeletal stem cell fate defects caused by Pdgfrb activating mutation

Autosomal dominant PDGFRβ gain-of-function mutations in mice and humans cause a spectrum of wasting and overgrowth disorders afflicting the skeleton and other connective tissues, but the cellular origin of these disorders remains unknown. We demonstrate that skeletal stem cells (SSCs) isolated from mice with a gain-of-function D849V point mutation in PDGFRβ exhibit colony formation defects that parallel the wasting or overgrowth phenotypes of the mice. Single-cell RNA transcriptomics with SSC-derived polyclonal colonies demonstrates alterations in osteogenic and chondrogenic precursors caused by PDGFRβD849V. Mutant cells undergo poor osteogenesis in vitro with increased expression of Sox9 and other chondrogenic markers. Mice with PDGFRβD849V exhibit osteopenia. Increased STAT5 phosphorylation and overexpression of Igf1 and Socs2 in PDGFRβD849V cells suggests that overgrowth in mice involves PDGFRβD849V activating the STAT5-IGF1 axis locally in the skeleton. Our study establishes that PDGFRβD849V causes osteopenic skeletal phenotypes that are associated with intrinsic changes in SSCs, promoting chondrogenesis over osteogenesis.

Guidelines for the use of flow cytometry and cell sorting in immunological studies (third edition)

The third edition of Flow Cytometry Guidelines provides the key aspects to consider when performing flow cytometry experiments and includes comprehensive sections describing phenotypes and functional assays of all major human and murine immune cell subsets. Notably, the Guidelines contain helpful tables highlighting phenotypes and key differences between human and murine cells. Another useful feature of this edition is the flow cytometry analysis of clinical samples with examples of flow cytometry applications in the context of autoimmune diseases, cancers as well as acute and chronic infectious diseases. Furthermore, there are sections detailing tips, tricks and pitfalls to avoid. All sections are written and peer-reviewed by leading flow cytometry experts and immunologists, making this edition an essential and state-of-the-art handbook for basic and clinical researchers.

Gremlin: a complex molecule regulating wound healing and fibrosis

Gremlin-1 is part of the TGF-β superfamily and is a BMP antagonist that blocks BMP signalling to precisely control BMP gradients. Gremlin-1 is primarily involved in organogenesis and limb patterning however, has recently been described as being involved in fibrotic diseases. Initially described as a key factor involved in diabetic kidney fibrosis due to being induced by high glucose, it has now been described as being associated with lung, liver, eye, and skin fibrosis. This suggests that it is a key conserved molecule mediating fibrotic events irrespective of organ. It appears that Gremlin-1 may have effects mediated by BMP-dependent and independent pathways. The aim of this review is to evaluate the role of Gremlin-1 in fibrosis, its mechanisms and if this can be targeted therapeutically in fibrotic diseases, which currently have very limited treatment options and are highly prevalent.

Identifying key regulators of the intestinal stem cell niche

The intestinal tract is lined by a single layer of epithelium that is one of the fastest regenerating tissues in the body and which therefore requires a very active and exquisitely controlled stem cell population. Rapid renewal of the epithelium is necessary to provide a continuous physical barrier from the intestinal luminal microenvironment that contains abundant microorganisms, whilst also ensuring an efficient surface for the absorption of dietary components. Specialised epithelial cell populations are important for the maintenance of intestinal homeostasis and are derived from adult intestinal stem cells (ISCs). Actively cycling ISCs divide by a neutral drift mechanism yielding either ISCs or transit-amplifying epithelial cells, the latter of which differentiate to become either absorptive lineages or to produce secretory factors that contribute further to intestinal barrier maintenance or signal to other cellular compartments. The mechanisms controlling ISC abundance, longevity and activity are regulated by several different cell populations and signalling pathways in the intestinal lamina propria which together form the ISC niche. However, the complexity of the ISC niche and communication mechanisms between its different components are only now starting to be unravelled with the assistance of intestinal organoid/enteroid/colonoid and single-cell imaging and sequencing technologies. This review explores the interaction between well-established and emerging ISC niche components, their impact on the intestinal epithelium in health and in the context of intestinal injury and highlights future directions and implications for this rapidly developing field.

Roles of the Mesenchymal Stromal/Stem Cell Marker Meflin/Islr in Cancer Fibrosis

Fibroblasts synthesise the extracellular matrix (ECM) such as collagen and elastin, the excessive accumulation of which can lead to fibrosis and organ dysfunction under pathological conditions. Cancer-associated fibroblasts (CAFs) are major constituents of the tumour microenvironment (TME) that accompany the desmoplastic reaction responsible for anti-cancer treatment resistance. Thus, it is important to dissect the roles of CAFs in the TME to develop new therapeutic strategies for refractory cancers. Recent progress in the studies of CAF biology suggests that the functions of CAFs are complicated and that they are composed of functionally distinct populations, including cancer-promoting CAFs (pCAFs) and cancer-restraining CAFs (rCAFs). We recently identified a new cell surface marker for rCAFs in pancreatic and colon cancers, designated as Meflin (mesenchymal stromal cell- and fibroblast-expressing Linx paralogue)/Islr (immunoglobulin super family containing leucine-rich repeat). Based on the distribution of Meflin/Islr-positive cells, we also considered it a specific candidate marker for mesenchymal stroma/stem cells. Meflin/Islr-positive CAFs have been shown to suppress cancer progression by being involved in regulating collagen structures and BMP signalling in the TME. This review describes the function of Meflin/Islr in cancer fibrosis as well as in cardiac and lung fibrosis and its potential in the development of new cancer therapeutics.

Multipotent stromal cells: One name, multiple identities

Multipotent stromal cells (MSCs) are vital for development, maintenance, function, and regeneration of most tissues. They can differentiate along multiple connective lineages, but unlike most other stem/progenitor cells, they carry out various other functions while maintaining their developmental potential. MSCs function as damage sensors, respond to injury by fostering regeneration through secretion of trophic factors as well as extracellular matrix (ECM) molecules, and contribute to fibrotic reparative processes when regeneration fails. Tissue-specific MSC identity, fate(s), and function(s) are being resolved through fate mapping coupled with single cell "omics," providing unparalleled insights into the secret lives of tissue-resident MSCs.

Intestinal mesenchymal cells regulate immune responses and promote epithelial regeneration in vitro and in dextran sulfate sodium-induced experimental colitis in mice

AIM

Disruption of the intestinal mucosal tolerance, that is, the immunological unresponsiveness to innocuous food antigens and the commensal microbiota, in the colon is associated with several chronic diseases including inflammatory bowel disease (IBD). Understanding the mechanisms responsible for intestinal mucosal tolerance has potential translational value for its therapy and management. Human intestinal mesenchymal cells (iMCs) play important roles in colonic mucosal tolerance, but further studies on their tissue regenerative and immunomodulatory capacities are necessary in order to fully understand their function in health and disease.

METHODS

In this study, we have isolated and analysed the capacity of human iMCs to promote wound healing and modulate immune responses in vitro and in vivo, using the dextran sulfate sodium (DSS)-induced colitis model.

RESULTS

Cultured iMCs were CD45^- CD73⁺ CD90⁺ CD105⁺ and accelerated the wound closure in a normal colon mucosa (NCM) 356 human epithelial cell wound healing assay. Furthermore, iMCs blocked the LPS-mediated induction of TNF-α in THP-1 macrophages and inhibited the proliferation of peripheral blood mononuclear cells, partly through the induction of indoleamine-2,3-dioxygenase. In DSS colitic mice, iMCs administration reduced the disease activity index and ameliorated intestinal tissue damage and permeability. Furthermore, iMCs reduced intestinal inflammation, evidenced by a decreased mRNA expression of pro-inflammatory cytokines, reduced IL-1β secretion by intestinal explants and inhibited colonic iNOS protein expression.

CONCLUSIONS

Our data show that human iMCs isolated from the noninflamed intestine possess tissue-regenerative and immunomodulatory capacities that could potentially be harnessed/restored in order to reduce IBD severity.

Tracing the skeletal progenitor transition during postnatal bone formation

Multiple distinct types of skeletal progenitors have been shown to contribute to endochondral bone development and maintenance. However, the division of labor and hierarchical relationship between different progenitor populations remain undetermined. Here we developed dual-recombinase fate-mapping systems to capture the skeletal progenitor transition during postnatal bone formation. We showed that postnatal osteoblasts arose primarily from chondrocytes before adolescence and from Lepr⁺ bone marrow stromal cells (BMSCs) after adolescence. This transition occurred in the diaphysis during adolescence and progressively spread to the metaphysis. The osteoblast-forming Lepr⁺ BMSCs derived primarily from fetal Col2⁺ cells. Conditional deletion of Runx2 from perinatal chondrocytes and adult Lepr⁺ BMSCs impaired bone lengthening and thickening, respectively. Forced running increased osteoblast formation by perinatal chondrocytes but not by adult Lepr⁺ BMSCs. Thus, the short-term developmental skeletal progenitors generated the long-term adult skeletal progenitors. They sequentially control the growth and maintenance of endochondral bones.

Spatially organized multicellular immune hubs in human colorectal cancer

Immune responses to cancer are highly variable, with mismatch repair-deficient (MMRd) tumors exhibiting more anti-tumor immunity than mismatch repair-proficient (MMRp) tumors. To understand the rules governing these varied responses, we transcriptionally profiled 371,223 cells from colorectal tumors and adjacent normal tissues of 28 MMRp and 34 MMRd individuals. Analysis of 88 cell subsets and their 204 associated gene expression programs revealed extensive transcriptional and spatial remodeling across tumors. To discover hubs of interacting malignant and immune cells, we identified expression programs in different cell types that co-varied across tumors from affected individuals and used spatial profiling to localize coordinated programs. We discovered a myeloid cell-attracting hub at the tumor-luminal interface associated with tissue damage and an MMRd-enriched immune hub within the tumor, with activated T cells together with malignant and myeloid cells expressing T cell-attracting chemokines. By identifying interacting cellular programs, we reveal the logic underlying spatially organized immune-malignant cell networks.

Sites of Cre-recombinase activity in mouse lines targeting skeletal cells

The Cre/Lox system is a powerful tool in the biologist's toolbox, allowing loss-of-function and gain-of-function studies, as well as lineage tracing, through gene recombination in a tissue-specific and inducible manner. Evidence indicates, however, that Cre transgenic lines have a far more nuanced and broader pattern of Cre activity than initially thought, exhibiting "off-target" activity in tissues/cells other than the ones they were originally designed to target. With the goal of facilitating the comparison and selection of optimal Cre lines to be used for the study of gene function, we have summarized in a single manuscript the major sites and timing of Cre activity of the main Cre lines available to target bone mesenchymal stem cells, chondrocytes, osteoblasts, osteocytes, tenocytes, and osteoclasts, along with their reported sites of "off-target" Cre activity. We also discuss characteristics, advantages, and limitations of these Cre lines for users to avoid common risks related to overinterpretation or misinterpretation based on the assumption of strict cell-type specificity or unaccounted effect of the Cre transgene or Cre inducers. © 2021 American Society for Bone and Mineral Research (ASBMR).