IRX3 and IRX5 Inhibit Adipogenic Differentiation of Hypertrophic Chondrocytes and Promote Osteogenesis

IRX5 Promoted SREBP1-Mediated de Novo Fatty Acid Synthesis via HMGN4 in Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC), a prevalent malignant tumour, ranks highly in both morbidity and mortality, and its prevention and treatment need further studies. The transcription factor iroquois homeobox 5 (IRX5) plays an essential role in HCC, whereas little is known about its exact functions and underlying mechanisms in tumour metabolism reprogramming. Besides, as a transcription factor that mainly locates in nuclei, IRX5 lacks a nuclear localisation sequence, which makes uncovering the mechanism of IRX5 translocating into the nuclei of great significance. Here, we first found that both IRX5 and HCC development are highly expressed; IRX5 accelerates de novo fatty acid synthesis and promotes cancer cell proliferation and progression. Moreover, the GST pull-down combined with GC/MS experiments identified an interaction between IRX5 and high-mobility group nucleosomal binding domain 4 (HMGN4). Immunofluorescence analysis showed that IRX5 and HMGN4 colocalised within the nucleus. Coimmunoprecipitation further confirmed their direct interaction. The elevated expression of HMGN4 enhanced the nuclear transport of IRX5. Taken together, our observations suggest that HMGN4 driving IRX5 nuclear translocation promotes HCC development via de novo fatty acid synthesis reprogramming.

Irx3/5 Null Deletion in Mice Blocks Cochlea-Saccule Segregation and Disrupts the Auditory Tonotopic Map

A gene cadre orchestrates the normal development of sensory and non-sensory cells in the inner ear, segregating the cochlea with a distinct tonotopic sound frequency map, similar brain projection, and five vestibular end-organs. However, the role of genes driving the ear development is largely unknown. Here, we show double deletion of the Iroquois homeobox 3 and 5 transcription factors (Irx3/5 DKO) leads to the fusion of the saccule and the cochlear base. The overlying otoconia and tectorial membranes are absent in the Irx3/5 DKO inner ear, and the primary auditory neurons project fibers to both the saccule and cochlear hair cells. The central neuronal projections from the cochlear apex-base contour are not fully segregated into a dorsal and ventral innervation in the Irx3/5 DKO cochlear nucleus, obliterating the characteristic tonotopic auditory map. Additionally, Irx3/5 deletion reveals a pronounced cochlear-apex-vestibular "vestibular-cochlear" nerve (VCN) bilateral connection that is less noticeable in wild-type control mice. Moreover, the incomplete segregation of apex and base projections that expands fibers to connect with vestibular nuclei. The results suggest the mammalian cochlear apex is a derived lagena reminiscent of sarcopterygians. Thus, Irx3 and 5 are potential evolutionary branch-point genes necessary for balance-sound segregation, which fused into a saccule-cochlea organization.

A novel genetic mouse model of osteoporosis with double heterozygosity of Irx3 and Irx5 characterizes sex-dependent phenotypes in bone homeostasis

Iroquois homeobox gene 3 (Irx3) and Irx5 encode transcription factors that play crucial roles in limb development and bone formation. Previous studies using knockout mice have revealed a role of Irx3 and Irx5 in osteogenesis in young adult mice. However, whether these genes are also essential for bone homeostasis in adulthood and contribute to bone diseases remain poorly understood. Osteoporosis is a disease characterized by lower bone mineral density and disrupted bone microarchitecture, typically occurs in postmenopausal women. Here, we demonstrate that Irx3/5dHet mice with a half-reduction of Irx3 and Irx5 dosage serve as a novel model of osteoporosis. By micro-computed tomography, we found that Irx3/5dHet mice exhibited sex-dependent bone loss patterns. While male Irx3/5dHet mice progressively lost trabecular microstructures with aging, female mutants exhibited lower bone mineral density (BMD) and bone volume fraction (BV/TV) at early adulthood (9-15 weeks old) but without further loss later at 1 year of age. Bone marrow adipocytes are known to be elevated at the expenses of lower osteogenesis in osteoporotic bone marrow. Surprisingly, we found sex-dependent changes in adipogenesis at the age of skeletal maturity that bone marrow adipocytes were reduced in female Irx3/5dHet mice along with deteriorated osteogenesis, while male mice exhibited elevated adipogenesis. In summary, we reported a novel genetic model for osteoporosis-like phenotypes, highlighting sex-dependent bone mineral density and bone marrow adipocyte characteristics.

Multi-omics analyses reveal aberrant differentiation trajectory with WNT1 loss-of-function in type XV osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a group of severe genetic bone disorders characterized by congenital low bone mass, deformity, and frequent fractures. Type XV OI is a moderate to severe form of skeletal dysplasia caused by WNT1 variants. In this cohort study from southern China, we summarized the clinical phenotypes of patients with WNT1 variants and found that the proportion of type XV patients was around 10.3% (25 out of 243) with a diverse spectrum of phenotypes. Functional assays indicated that variants of WNT1 significantly impaired its secretion and effective activity, leading to moderate to severe clinical manifestations, porous bone structure, and enhanced osteoclastic activities. Analysis of proteomic data from human skeleton indicated that the expression of SOST (sclerostin) was dramatically reduced in type XV patients compared to patients with COL1A1 quantitative variants. Single-cell transcriptome data generated from human tibia samples of patients diagnosed with type XV OI and leg-length discrepancy, respectively, revealed aberrant differentiation trajectories of skeletal progenitors and impaired maturation of osteocytes with loss of WNT1, resulting in excessive CXCL12+ progenitors, fewer mature osteocytes, and the existence of abnormal cell populations with adipogenic characteristics. The integration of multi-omics data from human skeleton delineates how WNT1 regulates the differentiation and maturation of skeletal progenitors, which will provide a new direction for the treatment strategy of type XV OI and relative low bone mass diseases such as early onset osteoporosis.

Skeletal stem and progenitor cells in bone development and repair

Bone development, growth, and repair are complex processes involving various cell types and interactions, with central roles played by skeletal stem and progenitor cells. Recent research brought new insights into the skeletal precursor populations that mediate intramembranous and endochondral bone development. Later in life, many of the cellular and molecular mechanisms determining development are reactivated upon fracture, with powerful trauma-induced signaling cues triggering a variety of postnatal skeletal stem/progenitor cells (SSPCs) residing near the bone defect. Interestingly, in this injury context, the current evidence suggests that the fates of both SSPCs and differentiated skeletal cells can be considerably flexible and dynamic, and that multiple cell sources can be activated to operate as functional progenitors generating chondrocytes and/or osteoblasts. The combined implementation of in vivo lineage tracing, cell surface marker-based cell selection, single-cell molecular analyses, and high-resolution in situ imaging has strongly improved our insights into the diversity and roles of developmental and reparative stem/progenitor subsets, while also unveiling the complexity of their dynamics, hierarchies, and relationships. Albeit incompletely understood at present, findings supporting lineage flexibility and possibly plasticity among sources of osteogenic cells challenge the classical dogma of a single primitive, self-renewing, multipotent stem cell driving bone tissue formation and regeneration from the apex of a hierarchical and strictly unidirectional differentiation tree. We here review the state of the field and the newest discoveries in the origin, identity, and fates of skeletal progenitor cells during bone development and growth, discuss the contributions of adult SSPC populations to fracture repair, and reflect on the dynamism and relationships among skeletal precursors and differentiated cell lineages. Further research directed at unraveling the heterogeneity and capacities of SSPCs, as well as the regulatory cues determining their fate and functioning, will offer vital new options for clinical translation toward compromised fracture healing and bone regenerative medicine.

Advancing bone regeneration: Unveiling the potential of 3D cell models in the evaluation of bone regenerative materials

Bone, a rigid yet regenerative tissue, has garnered extensive attention for its impressive healing abilities. Despite advancements in understanding bone repair and creating treatments for bone injuries, handling nonunions and large defects remains a major challenge in orthopedics. The rise of bone regenerative materials is transforming the approach to bone repair, offering innovative solutions for nonunions and significant defects, and thus reshaping orthopedic care. Evaluating these materials effectively is key to advancing bone tissue regeneration, especially in difficult healing scenarios, making it a critical research area. Traditional evaluation methods, including two-dimensional cell models and animal models, have limitations in predicting accurately. This has led to exploring alternative methods, like 3D cell models, which provide fresh perspectives for assessing bone materials' regenerative potential. This paper discusses various techniques for constructing 3D cell models, their pros and cons, and crucial factors to consider when using these models to evaluate bone regenerative materials. We also highlight the significance of 3D cell models in the in vitro assessments of these materials, discuss their current drawbacks and limitations, and suggest future research directions. STATEMENT OF SIGNIFICANCE: This work addresses the challenge of evaluating bone regenerative materials (BRMs) crucial for bone tissue engineering. It explores the emerging role of 3D cell models as superior alternatives to traditional methods for assessing these materials. By dissecting the construction, key factors of evaluating, advantages, limitations, and practical considerations of 3D cell models, the paper elucidates their significance in overcoming current evaluation method shortcomings. It highlights how these models offer a more physiologically relevant and ethically preferable platform for the precise assessment of BRMs. This contribution is particularly significant for "Acta Biomaterialia" readership, as it not only synthesizes current knowledge but also propels the discourse forward in the search for advanced solutions in bone tissue engineering and regeneration.

Eed controls craniofacial osteoblast differentiation and mesenchymal proliferation from the neural crest

The histone methyltransferase Polycomb repressive complex 2 (PRC2) is required for specification of the neural crest, and mis-regulation of neural crest development can cause severe congenital malformations. PRC2 is necessary for neural crest induction, but the embryonic, cellular, and molecular consequences of PRC2 activity after neural crest induction are incompletely understood. Here we show that Eed, a core subunit of PRC2, is required for craniofacial osteoblast differentiation and mesenchymal proliferation after induction of the neural crest. Integrating mouse genetics with single-cell RNA sequencing, our results reveal that conditional knockout of Eed after neural crest cell induction causes severe craniofacial hypoplasia, impaired craniofacial osteogenesis, and attenuated craniofacial mesenchymal cell proliferation that is first evident in post-migratory neural crest cell populations. We show that Eed drives mesenchymal differentiation and proliferation in vivo and in primary craniofacial cell cultures by regulating diverse transcription factor programs that are required for specification of post-migratory neural crest cells. These data enhance understanding of epigenetic mechanisms that underlie craniofacial development, and shed light on the embryonic, cellular, and molecular drivers of rare congenital syndromes in humans.

IRX5 suppresses osteogenic differentiation of hBMSCs by inhibiting protein synthesis

In our previous study, IRX5 has been revealed a significant role in adipogenesis of hBMSCs. Considering the expansion of adipose tissue in bone marrow in aged and ovariectomy-related osteoporosis, the effect of IRX5 on the osteogenesis of BMSCs still needs to be elucidated. In vivo, models of aging-induced and ovariectomy-induced osteoporotic mice, and in vitro studies of IRX5 gene gain- and loss-of-function in hBMSCs were employed. Histology, immunofluorescence, qRT-PCR, and Western blot analysis were performed to detect the functions of IRX5 in hBMSCs osteogenic differentiation. RNA-seq, transmission electron microscopy, Seahorse mito-stress assay, and Surface Sensing of Translation assay were conducted to explore the effect of mammalian/mechanistic target of rapamycin (mTOR)-mediated ribosomal translation and mitochondrial functions in the regulation of hBMSCs differentiation by IRX5. As a result, elevated IRX5 protein expression levels were observed in the bone marrow of osteoporotic mice compared to normal mice. IRX5 overexpression attenuated osteogenic processes, whereas IRX5 knockdown resulted in enhanced osteogenesis in hBMSCs. RNA-seq and enrichment analysis unveiled that IRX5 overexpression exerted inhibitory effects on ribosomal translation and mitochondrial functions. Furthermore, the application of the mTOR activator, MHY1485, effectively reversed the inhibitory impact of IRX5 on osteogenesis and mitochondrial functions in hBMSCs. In summary, our findings suggest that IRX5 restricts mTOR-mediated ribosomal translation, consequently impairing mitochondrial OxPhos, which in turn results in osteogenic dysfunction of hBMSCs.

Deletion of NADPH oxidase 2 in chondrocytes exacerbates ethanol-mediated growth plate disruption in mice without major effects on bone architecture or gene expression

BACKGROUND

Excess reactive oxygen species generated by NADPH oxidase 2 (Nox2) in response to ethanol exposure mediate aspects of skeletal toxicity including increased osteoclast differentiation and activity. Because perturbation of chondrocyte differentiation in the growth plate by ethanol could be prevented by dietary antioxidants, we hypothesized that Nox2 in the growth plate was involved in ethanol-associated reductions in longitudinal bone growth.

METHODS

Nox2 conditional knockout mice were generated, where the essential catalytic subunit of Nox2, cytochrome B-245 beta chain (Cybb), is deleted in chondrocytes using a Cre-Lox model with Cre expressed from the collagen 2a1 promoter (Col2a1-Cre). Wild-type and Cre-Lox mice were fed an ethanol Lieber-DeCarli-based diet or pair-fed a control diet for 8 weeks.

RESULTS

Ethanol treatment significantly reduced the number of proliferating chondrocytes in the growth plate, enhanced bone marrow adiposity, shortened femurs, reduced body length, reduced cortical bone volume, and decreased mRNA levels of a number of osteoblast and chondrocyte genes. Conditional knockout of Nox2 enzymatic activity in chondrocytes did not consistently prevent any ethanol effects. Rather, knockout mice had fewer proliferating chondrocytes than wild-type mice in both the ethanol- and control-fed animals. Additional analysis of tibia samples from Nox4 knockout mice showed that loss of Nox4 activity also reduced the number of proliferating chondrocytes and altered chondrocyte size in the growth plate.

CONCLUSIONS

Although Nox enzymatic activity regulates growth plate development, ethanol-associated disruption of the growth plate morphology is independent of ethanol-mediated increases in Nox2 activity.

The Origin and Fate of Chondrocytes: Cell Plasticity in Physiological Setting

PURPOSE OF REVIEW

Here, we discuss the origin of chondrocytes, their destiny, and their plasticity in relationship to bone growth, articulation, and formation of the trabeculae. We also consider these processes from a biological, clinical, and evolutionary perspective.

RECENT FINDINGS

Chondrocytes, which provide the template for the formation of most bones, are responsible for skeletal growth and articulation during postnatal life. In recent years our understanding of the fate of these cells has changed dramatically. Current evidence indicates a paradoxical situation during skeletogenesis, with some cells of mesenchymal condensation differentiating directly into osteoblasts, whereas others of the same kind give rise to highly similar osteoblasts via a complex process of differentiation involving several chondrocyte intermediates. The situation becomes even more paradoxical during postnatal growth when stem cells in the growth plate produce differentiated, functional progenies, which thereafter presumably dedifferentiate into another type of stem cell. Such a remarkable transition from one cell type to another under postnatal physiological conditions provides a fascinating example of cellular plasticity that may have valuable clinical implications.

Hypertrophic chondrocytes at the junction of musculoskeletal structures

Hypertrophic chondrocytes are found at unique locations at the junction of skeletal tissues, cartilage growth plate, articular cartilage, enthesis and intervertebral discs. Their role in the skeleton is best understood in the process of endochondral ossification in development and bone fracture healing. Chondrocyte hypertrophy occurs in degenerative conditions such as osteoarthritis. Thus, the role of hypertrophic chondrocytes in skeletal biology and pathology is context dependent. This review will focus on hypertrophic chondrocytes in endochondral ossification, in which they exist in a transient state, but acting as a central regulator of differentiation, mineralization, vascularization and conversion to bone. The amazing journey of a chondrocyte from being entrapped in the extracellular matrix environment to becoming proliferative then hypertrophic will be discussed. Recent studies on the dynamic changes and plasticity of hypertrophic chondrocytes have provided new insights into how we view these cells, not as terminally differentiated but as cells that can dedifferentiate to more progenitor-like cells in a transition to osteoblasts and adipocytes, as well as a source of skeletal stem and progenitor cells residing in the bone marrow. This will provide a foundation for studies of hypertrophic chondrocytes at other skeletal sites in development, tissue maintenance, pathology and therapy.

Single-cell RNA sequencing analysis of the temporomandibular joint condyle in 3 and 4-month-old human embryos

BACKGROUND

The temporomandibular joint (TMJ) is a complex joint consisting of the condyle, the temporal articular surface, and the articular disc. Functions such as mastication, swallowing and articulation are accomplished by the movements of the TMJ. To date, the TMJ has been studied more extensively, but the types of TMJ cells, their differentiation, and their interrelationship during growth and development are still unclear and the study of the TMJ is limited. The aim of this study was to establish a molecular cellular atlas of the human embryonic temporomandibular joint condyle (TMJC) by single-cell RNA sequencing, which will contribute to understanding and solving clinical problems.

RESULTS

Human embryos at 3 and 4 months of age are an important stage of TMJC development. We performed a comprehensive transcriptome analysis of TMJC tissue from human embryos at 3 and 4 months of age using single-cell RNA sequencing. A total of 16,624 cells were captured and the gene expression profiles of 15 cell clusters in human embryonic TMJC were determined, including 14 known cell types and one previously unknown cell type, "transition state cells (TSCs)". Immunofluorescence assays confirmed that TSCs are not the same cell cluster as mesenchymal stem cells (MSCs). Pseudotime trajectory and RNA velocity analysis revealed that MSCs transformed into TSCs, which further differentiated into osteoblasts, hypertrophic chondrocytes and tenocytes. In addition, chondrocytes (CYTL1^high + THBS1^high) from secondary cartilage were detected only in 4-month-old human embryonic TMJC.

CONCLUSIONS

Our study provides an atlas of differentiation stages of human embryonic TMJC tissue cells, which will contribute to an in-depth understanding of the pathophysiology of the TMJC tissue repair process and ultimately help to solve clinical problems.

Decreased CRISPLD2 expression impairs osteogenic differentiation of human mesenchymal stem cells during in vitro expansion

Human mesenchymal stem cells (hMSCs) are the cornerstone of regenerative medicine; large quantities of hMSCs are required via in vitro expansion to meet therapeutic purposes. However, hMSCs quickly lose their osteogenic differentiation potential during in vitro expansion, which is a major roadblock to their clinical applications. In this study, we found that the osteogenic differentiation potential of human bone marrow stem cells (hBMSCs), dental pulp stem cells (hDPSCs), and adipose stem cells (hASCs) was severely impaired after in vitro expansion. To clarify the molecular mechanism underlying this in vitro expansion-related loss of osteogenic capacity in hMSCs, the transcriptome changes following in vitro expansion of these hMSCs were compared. Cysteine-rich secretory protein LCCL domain-containing 2 (CRISPLD2) was identified as the most downregulated gene shared by late passage hBMSCs, hDPSCs, and hASCs. Both the secreted and non-secreted CRISPLD2 proteins progressively declined in hMSCs during in vitro expansion when the cells gradually lost their osteogenic potential. We thus hypothesized that the expression of CRISPLD2 is critical for hMSCs to maintain their osteogenic differentiation potential during in vitro expansion. Our studies showed that the knockdown of CRISPLD2 in early passage hBMSCs inhibited the cells' osteogenic differentiation in a siRNA dose-dependent manner. Transcriptome analysis and immunoblotting indicated that the CRISPLD2 knockdown-induced osteogenesis suppression might be attributed to the downregulation of matrix metallopeptidase 1 (MMP1) and forkhead box Q1 (FOXQ1). Furthermore, adeno-associated virus (AAV)-mediated CRISPLD2 overexpression could somewhat rescue the impaired osteogenic differentiation of hBMSCs during in vitro expansion. These results revealed that the downregulation of CRISPLD2 contributes to the impaired osteogenic differentiation of hMSCs during in vitro expansion. Our findings shed light on understanding the loss of osteogenic differentiation in hMSCs and provide a potential therapeutic target gene for bone-related diseases.

Modeling human skeletal development using human pluripotent stem cells

Chondrocytes and osteoblasts differentiated from induced pluripotent stem cells (iPSCs) will provide insights into skeletal development and genetic skeletal disorders and will generate cells for regenerative medicine applications. Here, we describe a method that directs iPSC-derived sclerotome to chondroprogenitors in 3D pellet culture then to articular chondrocytes or, alternatively, along the growth plate cartilage pathway to become hypertrophic chondrocytes that can transition to osteoblasts. Osteogenic organoids deposit and mineralize a collagen I extracellular matrix (ECM), mirroring in vivo endochondral bone formation. We have identified gene expression signatures at key developmental stages including chondrocyte maturation, hypertrophy, and transition to osteoblasts and show that this system can be used to model genetic cartilage and bone disorders.

Iroquois homeobox 3 regulates odontoblast proliferation and differentiation mediated by Wnt5a expression

Iroquois homeobox (Irx) genes are TALE-class homeobox genes that are evolutionarily conserved across species and have multiple critical cellular functions in fundamental tissue development processes. Previous studies have shown that Irxs genes are expressed during tooth development. However, the precise roles of genes in teeth remain unclear. Here, we demonstrated for the first time that Irx3 is an essential molecule for the proliferation and differentiation of odontoblasts. Using cDNA synthesized from postnatal day 1 (P1) tooth germs, we examined the expression of all Irx genes (Irx1-Irx6) by RT-PCR and found that all genes except Irx4 were expressed in the tooth tissue. Irx1-Irx3 a were expressed in the dental epithelial cell line M3H1 cells, while Irx3 and Irx5 were expressed in the dental mesenchymal cell line mDP cells. Only Irx3 was expressed in both undifferentiated cell lines. Immunostaining also revealed the presence of IRX3 in the dental epithelial cells and mesenchymal condensation. Inhibition of endogenous Irx3 by siRNA blocks the proliferation and differentiation of mDP cells. Wnt3a, Wnt5a, and Bmp4 are factors involved in odontoblast differentiation and were highly expressed in mDP cells by quantitative PCR analysis. Interestingly, the expression of Wnt5a (but not Wnt3a or Bmp4) was suppressed by Irx3 siRNA. These results suggest that Irx3 plays an essential role in part through the regulation of Wnt5a expression during odontoblast proliferation and differentiation.

Investigating neural differentiation of mouse P19 embryonic stem cells in a time-dependent manner by bioinformatic, microscopic and transcriptional analyses

BACKGROUND

As an available cell line, mouse pluripotent P19 has been widely employed for neuronal differentiation studies. In this research, by applying the in vitro differentiation of this cell line into neuron-like cells through retinoic acid (RA) treatment, the roles of some genes including DNMT3B, ICAM1, IRX3, JAK2, LHX1, SOX9, TBX3 and THY1 in neural differentiation was investigated.

METHODS AND RESULTS

Bioinformatics, microscopic, and transcriptional studies were conducted in a time-dependent manner after RA-induced neural differentiation. According to bioinformatics studies, we determined the engagement of the metabolic and developmental super-pathways and pathways in neural cell differentiation, particularly focusing on the considered genes. According to our qRT-PCR analyses, JAK2, SOX9, TBX3, LHX1 and IRX3 genes were found to be significantly overexpressed in a time-dependent manner (p < 0.05). In addition, the significant downregulation of THY1, DNMT3B and ICAM1 genes was observed during the experiment (p < 0.05). The optical microscopic investigation showed that the specialized extensions of the neuron-like cells were revealed on day 8 after RA treatment.

CONCLUSION

Accordingly, the neural differentiation of P19 cell line and the role of the considered genes during the differentiation were proved. However, our results warrant further in vivo studies.

Deciphering postnatal limb development at single-cell resolution

The early postnatal limb developmental progression bridges embryonic and mature stages and mirrors the pathological remodeling of articular cartilage. However, compared with multitudinous research on embryonic limb development, the early postnatal stage seems relatively unnoticed. Here, a systematic work to portray the postnatal limb developmental landscape was carried out by characterization of 19,952 single cells from murine hindlimbs at 4 postnatal stages using single-cell RNA sequencing technique. By delineation of cell heterogeneity, the candidate progenitor sub-clusters marked by Cd34 and Ly6e were discovered in articular cartilage and enthesis, and three cellular developmental branches marked by Col10a1, Spp1, and Tnni2 were reflected in growth plate. The representative transcriptomes and developmental patterns were intensively explored, and the key regulation mechanisms as well as evolvement in osteoarthritis were discussed. Above all, these results expand horizons of postnatal limb developmental biology and reach the interconnections between limb development, remodeling, and regeneration.

Hnrnpk maintains chondrocytes survival and function during growth plate development via regulating Hif1α-glycolysis axis

The harmonious functioning of growth plate chondrocytes is crucial for skeletogenesis. These cells rely on an appropriate intensity of glycolysis to maintain survival and function in an avascular environment, but the underlying mechanism is poorly understood. Here we show that Hnrnpk orchestrates growth plate development by maintaining the appropriate intensity of glycolysis in chondrocytes. Ablating Hnrnpk causes the occurrence of dwarfism, exhibiting damaged survival and premature differentiation of growth plate chondrocytes. Furthermore, Hnrnpk deficiency results in enhanced transdifferentiation of hypertrophic chondrocytes and increased bone mass. In terms of mechanism, Hnrnpk binds to Hif1a mRNA and promotes its degradation. Deleting Hnrnpk upregulates the expression of Hif1α, leading to the increased expression of downstream glycolytic enzymes and then exorbitant glycolysis. Our study establishes an essential role of Hnrnpk in orchestrating the survival and differentiation of chondrocytes, regulating the Hif1α-glycolysis axis through a post-transcriptional mechanism during growth plate development.

Hedgehog signaling orchestrates cartilage-to-bone transition independently of Smoothened

Although recent lineage studies strongly support a chondrocyte-to-osteoblast differentiation continuum, the biological significance and molecular basis remain undetermined. In silico analysis at a single-cell level indicates a transient shutdown of Hedgehog-related transcriptome during simulated cartilage-to-bone transition. Prompted by this, we genetically induce gain- and loss-of function to probe the role of Hedgehog signaling in cartilage-to-bone transition. Ablating Smo in hypertrophic chondrocytes (HCs) does not result in any phenotypic outcome, whereas deleting Ptch1 in HCs leads to disrupted formation of primary spongiosa and actively proliferating HCs-derived osteogenic cells that contribute to bony bulges seen in adult mutant mice. In HCs-derived osteoblasts, constitutive activation of Hedgehog signaling blocks their further differentiation to osteocytes. Moreover, ablation of both Smo and Ptch1 in HCs reverses neither persistent Hedgehog signaling nor bone overgrowths. These results establish a functional contribution of extended chondrocyte lineage to bone homeostasis and diseases, governed by an unanticipated mode of regulation for Hedgehog signaling independently of Smo.

IRX5 promotes adipogenesis of hMSCs by repressing glycolysis

Iroquois homeobox transcription factor 5 (IRX5) plays a pivotal role in extramedullary adipogenesis, but little is known about the effects of IRX5 on adipogenesis of human bone marrow-derived mesenchymal stem cells (hMSCs). In this study, we aimed to determine the effect of IRX5 on hMSCs adipogenesis. By means of qPCR analysis, we determined that IRX5 expression was elevated during adipogenic commitment of hMSCs. The biologic role of IRX5 was further investigated by employing a gain/loss-of-function strategy using an in vitro lentivirus-based system. IRX5 overexpression promoted adipogenesis whereas IRX5 knockdown reduced the adipogenic phenotype. RNA-seq and metabolomics revealed that IRX5 overexpression repressed glycolysis. Dual-luciferase assay results showed that IRX5 overexpression transcriptionally activates peroxisome proliferator-activated receptor gamma coactivator (PGC-1α). Metformin and PGC-1α inhibitor reversed IRX5-induced adipogenesis and glycolytic inhibition. Collectively, IRX5 facilitates adipogenic differentiation of hMSCs by transcriptionally regulating PGC-1α and inhibiting glycolysis, revealing a potential target to control bone marrow-derived mesenchymal stem cells (BMSCs) fate decision and bone homeostasis.

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.

4PBA reduces growth deficiency in osteogenesis imperfecta by enhancing transition of hypertrophic chondrocytes to osteoblasts

Short stature is a major skeletal phenotype in osteogenesis imperfecta (OI), a genetic disorder mainly caused by mutations in genes encoding type I collagen. However, the underlying mechanism is poorly understood, and no effective treatment is available. In OI mice that carry a G610C mutation in COL1A2, we previously found that mature hypertrophic chondrocytes (HCs) are exposed to cell stress due to accumulation of misfolded mutant type I procollagen in the endoplasmic reticulum (ER). By fate mapping analysis of HCs in G610C OI mice, we found that HCs stagnate in the growth plate, inhibiting translocation of HC descendants to the trabecular area and their differentiation to osteoblasts. Treatment with 4-phenylbutyric acid (4PBA), a chemical chaperone, restored HC ER structure and rescued this inhibition, resulting in enhanced longitudinal bone growth in G610C OI mice. Interestingly, the effects of 4PBA on ER dilation were limited in osteoblasts, and the bone fragility was not ameliorated. These results highlight the importance of targeting HCs to treat growth deficiency in OI. Our findings demonstrate that HC dysfunction induced by ER disruption plays a critical role in the pathogenesis of OI growth deficiency, which lays the foundation for developing new therapies for OI.

Clinical and Molecular Update on the Fourth Reported Family with Hamamy Syndrome

We report on 2 cousins, a girl and a boy, born to first-cousin Lebanese parents with Hamamy syndrome, exhibiting developmental delay, intellectual disability, severe telecanthus, abnormal ears, dentinogenesis imperfecta, and bone fragility. Whole-exome sequencing studies performed on the 2 affected individuals and one obligate carrier revealed the presence of a homozygous c.503G>A (p.Arg168His) missense mutation in IRX5 in both sibs, not reported in any other family. Review of the literature and differential diagnoses are discussed.

Irx3 and Irx5 - Novel Regulatory Factors of Postnatal Hypothalamic Neurogenesis

The hypothalamus is a brain region that exhibits highly conserved anatomy across vertebrate species and functions as a central regulatory hub for many physiological processes such as energy homeostasis and circadian rhythm. Neurons in the arcuate nucleus of the hypothalamus are largely responsible for sensing of peripheral signals such as leptin and insulin, and are critical for the regulation of food intake and energy expenditure. While these neurons are mainly born during embryogenesis, accumulating evidence have demonstrated that neurogenesis also occurs in postnatal-adult mouse hypothalamus, particularly in the first two postnatal weeks. This second wave of active neurogenesis contributes to the remodeling of hypothalamic neuronal populations and regulation of energy homeostasis including hypothalamic leptin sensing. Radial glia cell types, such as tanycytes, are known to act as neuronal progenitors in the postnatal mouse hypothalamus. Our recent study unveiled a previously unreported radial glia-like neural stem cell (RGL-NSC) population that actively contributes to neurogenesis in the postnatal mouse hypothalamus. We also identified Irx3 and Irx5, which encode Iroquois homeodomain-containing transcription factors, as genetic determinants regulating the neurogenic property of these RGL-NSCs. These findings are significant as IRX3 and IRX5 have been implicated in FTO-associated obesity in humans, illustrating the importance of postnatal hypothalamic neurogenesis in energy homeostasis and obesity. In this review, we summarize current knowledge regarding postnatal-adult hypothalamic neurogenesis and highlight recent findings on the radial glia-like cells that contribute to the remodeling of postnatal mouse hypothalamus. We will discuss characteristics of the RGL-NSCs and potential actions of Irx3 and Irx5 in the regulation of neural stem cells in the postnatal-adult mouse brain. Understanding the behavior and regulation of neural stem cells in the postnatal-adult hypothalamus will provide novel mechanistic insights in the control of hypothalamic remodeling and energy homeostasis.

Only the Co-Transcriptional Activity of β-Catenin Is Required for the Local Regulatory Effects in Hypertrophic Chondrocytes on Developmental Bone Modeling

In hypertrophic chondrocytes, β-catenin has two roles. First, it locally suppresses the differentiation of osteoclasts at the chondro-osseous junction by maintaining the pro-osteoclastic factor receptor activator of NF-κB ligand (RANKL) at low levels. Second, it promotes the differentiation of osteoblast-precursors from chondrocytes. Yet, β-catenin is a dual-function protein, which can either participate in cell-cell adherens junctions or serve as a transcriptional co-activator in canonical Wnt signaling interacting with T-cell factor/lymphoid enhancer-binding factor (TCF/LEF) transcription factors. Hence, whenever studying tissue-specific requirements of β-catenin using a conventional conditional knockout approach, the functional mechanisms underlying the defects in the conditional mutants remain ambiguous. To decipher mechanistically which of the two molecular functions of β-catenin is required in hypertrophic chondrocytes, we used different approaches. We analyzed the long bones of newborn mice carrying either the null-alleles of Lef1 or Tcf7, or mice in which Tcf7l2 was conditionally deleted in the hypertrophic chondrocytes, as well as double mutants for Lef1 and Tcf7l2, and Tcf7 and Tcf7l2. Furthermore, we analyzed Ctnnb1 mutant newborns expressing a signaling-defective allele that retains the cell adhesion function in hypertrophic chondrocytes. None of the analyzed Tcf/Lef single or double mutants recapitulated the previously published phenotype upon loss of β-catenin in hypertrophic chondrocytes. However, using this particular Ctnnb1 allele, maintaining cell adhesion function, we show that it is the co-transcriptional activity of β-catenin, which is required in hypertrophic chondrocytes to suppress osteoclastogenesis and to promote chondrocyte-derived osteoblast differentiation. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

A comprehensive series of Irx cluster mutants reveals diverse roles in facial cartilage development

Proper function of the vertebrate skeleton requires the development of distinct articulating embryonic cartilages. Irx transcription factors are arranged in co-regulated clusters that are expressed in the developing skeletons of the face and appendages. IrxB cluster genes are required for the separation of toes in mice and formation of the hyoid joint in zebrafish, yet whether Irx genes have broader roles in skeletal development remains unclear. Here, we perform a comprehensive loss-of-function analysis of all 11 Irx genes in zebrafish. We uncover conserved requirements for IrxB genes in formation of the fish and mouse scapula. In the face, we find a requirement for IrxAb genes and irx7 in formation of anterior neural crest precursors of the jaw, and for IrxBa genes in formation of endodermal pouches and gill cartilages. We also observe extensive joint loss and cartilage fusions in animals with combinatorial losses of Irx clusters, with in vivo imaging revealing that at least some of these fusions arise through inappropriate chondrogenesis. Our analysis reveals diverse roles for Irx genes in the formation and later segmentation of the facial skeleton.

Distinct skeletal stem cell types orchestrate long bone skeletogenesis

Skeletal stem and progenitor cell populations are crucial for bone physiology. Characterization of these cell types remains restricted to heterogenous bulk populations with limited information on whether they are unique or overlap with previously characterized cell types. Here we show, through comprehensive functional and single-cell transcriptomic analyses, that postnatal long bones of mice contain at least two types of bone progenitors with bona fide skeletal stem cell (SSC) characteristics. An early osteochondral SSC (ocSSC) facilitates long bone growth and repair, while a second type, a perivascular SSC (pvSSC), co-emerges with long bone marrow and contributes to shape the hematopoietic stem cell niche and regenerative demand. We establish that pvSSCs, but not ocSSCs, are the origin of bone marrow adipose tissue. Lastly, we also provide insight into residual SSC heterogeneity as well as potential crosstalk between the two spatially distinct cell populations. These findings comprehensively address previously unappreciated shortcomings of SSC research.

Regulation and Role of Transcription Factors in Osteogenesis

Bone is a dynamic tissue constantly responding to environmental changes such as nutritional and mechanical stress. Bone homeostasis in adult life is maintained through bone remodeling, a controlled and balanced process between bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoblasts secrete matrix, with some being buried within the newly formed bone, and differentiate to osteocytes. During embryogenesis, bones are formed through intramembraneous or endochondral ossification. The former involves a direct differentiation of mesenchymal progenitor to osteoblasts, and the latter is through a cartilage template that is subsequently converted to bone. Advances in lineage tracing, cell sorting, and single-cell transcriptome studies have enabled new discoveries of gene regulation, and new populations of skeletal stem cells in multiple niches, including the cartilage growth plate, chondro-osseous junction, bone, and bone marrow, in embryonic development and postnatal life. Osteoblast differentiation is regulated by a master transcription factor RUNX2 and other factors such as OSX/SP7 and ATF4. Developmental and environmental cues affect the transcriptional activities of osteoblasts from lineage commitment to differentiation at multiple levels, fine-tuned with the involvement of co-factors, microRNAs, epigenetics, systemic factors, circadian rhythm, and the microenvironments. In this review, we will discuss these topics in relation to transcriptional controls in osteogenesis.

Irx3 and Irx5 in Ins2-Cre+ cells regulate hypothalamic postnatal neurogenesis and leptin response

Obesity is mainly due to excessive food intake. IRX3 and IRX5 have been suggested as determinants of obesity in connection with the intronic variants of FTO, but how these genes contribute to obesity via changes in food intake remains unclear. Here, we show that mice doubly heterozygous for Irx3 and Irx5 mutations exhibit lower food intake with enhanced hypothalamic leptin response. By lineage tracing and single-cell RNA sequencing using the Ins2-Cre system, we identify a previously unreported radial glia-like neural stem cell population with high Irx3 and Irx5 expression in early postnatal hypothalamus and demonstrate that reduced dosage of Irx3 and Irx5 promotes neurogenesis in postnatal hypothalamus leading to elevated numbers of leptin-sensing arcuate neurons. Furthermore, we find that mice with deletion of Irx3 in these cells also exhibit a similar food intake and hypothalamic phenotype. Our results illustrate that Irx3 and Irx5 play a regulatory role in hypothalamic postnatal neurogenesis and leptin response.