Col2CreER(T2), a mouse model for a chondrocyte-specific and inducible gene deletion

Chondrocyte fatty acid oxidation drives osteoarthritis via SOX9 degradation and epigenetic regulation

Osteoarthritis is the most prevalent age-related degenerative joint disease and is closely linked to obesity. However, the underlying mechanisms remain unclear. Here we show that altered lipid metabolism in chondrocytes, particularly enhanced fatty acid oxidation (FAO), contributes to osteoarthritis progression. Excessive FAO causes acetyl-CoA accumulation, thereby altering protein-acetylation profiles, where the core FAO enzyme HADHA is hyperacetylated and activated, reciprocally boosting FAO activity and exacerbating OA progression. Mechanistically, elevated FAO reduces AMPK activity, impairs SOX9 phosphorylation, and ultimately promotes its ubiquitination-mediated degradation. Additionally, acetyl-CoA orchestrates epigenetic modulation, affecting multiple cellular processes critical for osteoarthritis pathogenesis, including the transcriptional activation of MMP13 and ADAMTS7. Cartilage-targeted delivery of trimetazidine, an FAO inhibitor and AMPK activator, demonstrates superior efficacy in a mouse model of metabolism-associated post-traumatic osteoarthritis. These findings suggest that targeting chondrocyte-lipid metabolism may offer new therapeutic strategies for osteoarthritis.

A census of the lung: CellCards from LungMAP

The human lung plays vital roles in respiration, host defense, and basic physiology. Recent technological advancements such as single-cell RNA sequencing and genetic lineage tracing have revealed novel cell types and enriched functional properties of existing cell types in lung. The time has come to take a new census. Initiated by members of the NHLBI-funded LungMAP Consortium and aided by experts in the lung biology community, we synthesized current data into a comprehensive and practical cellular census of the lung. Identities of cell types in the normal lung are captured in individual cell cards with delineation of function, markers, developmental lineages, heterogeneity, regenerative potential, disease links, and key experimental tools. This publication will serve as the starting point of a live, up-to-date guide for lung research at https://www.lungmap.net/cell-cards/. We hope that Lung CellCards will promote the community-wide effort to establish, maintain, and restore respiratory health.

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).

Aging aggravates intervertebral disc degeneration by regulating transcription factors toward chondrogenesis

Osterix is a critical transcription factor of mesenchymal stem cell fate, where its loss or loss of Wnt signaling diverts differentiation to a chondrocytic lineage. Intervertebral disc (IVD) degeneration activates the differentiation of prehypertrophic chondrocyte-like cells and inactivates Wnt signaling, but its interactive role with osterix is unclear. First, compared to young-adult (5 mo), mechanical compression of old (18 mo) IVD induced greater IVD degeneration. Aging (5 vs 12 mo) and/or compression reduced the transcription of osterix and notochordal marker T by 40-75%. Compression elevated the transcription of hypertrophic chondrocyte marker MMP13 and pre-osterix transcription factor RUNX2, but less so in 12 mo IVD. Next, using an Ai9/td reporter and immunohistochemical staining, annulus fibrosus and nucleus pulposus cells of young-adult IVD expressed osterix, but aging and compression reduced its expression. Lastly, in vivo LRP5-deficiency in osterix-expressing cells inactivated Wnt signaling in the nucleus pulposus by 95%, degenerated the IVD to levels similar to aging and compression, reduced the biomechanical properties by 45-70%, and reduced the transcription of osterix, notochordal markers and chondrocytic markers by 60-80%. Overall, these data indicate that age-related inactivation of Wnt signaling in osterix-expressing cells may limit regeneration by depleting the progenitors and attenuating the expansion of chondrocyte-like cells.

Spatial distribution of type II collagen gene expression in the mouse intervertebral disc

Genetic tools such as the Cre-Lox reporter system are powerful aids for tissue-specific cell tracking. For example, it would be useful in examining intervertebral disc (IVD) cell populations in normal and diseased states. A Cre recombinase and its recognition site, loxP have been adapted from the bacteriophage for use in genetic manipulation. The reporter mice used here express the red fluorescent protein, tdTomato with flanking LoxP sites (Rosa26 TdTomato mice). We compared two different Collagen type II (Col2) promoter constructs that drive Cre-recombinase expression in mice: (a) Col2-Cre, which allows constitutive Cre-recombinase expression under the control of the Col2 promoter/enhancer and (b) Col2-CreER, which contains a shorter promoter/enhancer region than Col2-Cre, but has human estrogen binding elements that bind tamoxifen, resulting in Cre-recombinase expression. The goal of the study is to characterize Cre-recombinase distribution pattern in Col2-Cre and Col2-CreER mice using tdTomato as reporter in the spine. The expression patterns of these two mice were further compared with Col2 gene expression in the native mouse NP and AF tissues by real-time PCR. We crossed Col2-Cre mice or Col2-CreER mice with the tdTomato reporter mice, and compared the tdTomato expression patterns. Col2-CreER/tdTomato mice were injected with tamoxifen at postnatal day 7 to activate the Cre-recombinase. TdTomato in the constitutively active Col2-Cre mice was detected in the nucleus pulposus (NP), the entire annulus fibrosus (AF), and in cartilaginous endplate and growth plate cells in the lower lumbar and coccygeal spine. In contrast, when Col2-CreER activity was induced by tamoxifen at P7, tdTomato was limited to the inner AF, and was absent from the NP. We have described the differences in Col2 reporter gene expression, in Col2-Cre/tdTomato and Col2-Cre-ER/tdTomato mouse IVD. The information provided here will help to guide future investigations of IVD biology.

Characterization of Cre recombinase mouse lines enabling cell type-specific targeting of postnatal intervertebral discs

Cre/loxP technology is an important tool for studying cell type-specific gene functions. Cre recombinase mouse lines, including Agc1-CreER^T2 , Col2a1-Cre; Col2a1-CreER^T2 , Shh-Cre, Shh-CreER^T2 , and Osx-Cre, have been proven to be valuable tools to elucidate the biology of long bones, yet the information for their activity in postnatal intervertebral disc (IVD) tissues was very limited. In this study, we used R26-mTmG fluorescent reporter to systematically analyze cell specificity and targeting efficiency of these six mouse lines in IVD tissues at postnatal growing and adult stages. We found that Agc1-CreER^T2 is effective to direct recombination in all components of IVDs, including annulus fibrosus (AF), nucleus pulposus (NP), and cartilaginous endplate (CEP), upon tamoxifen induction at either 2 weeks or 2 months of ages. Moreover, Col2a1-Cre targets most of the cells in IVDs, except for some cells in the outer AF (OAF) and NP. In contrast, the activity of Col2a1-CreER^T2 is mainly limited to the IAF of IVD tissues at either stage of tamoxifen injection. Similarly, Shh-Cre directs recombination specifically in all NP cells, whereas Shh-CreER^T2 is active only in a few NP cells when tamoxifen is administered at either stage. Finally, Osx-Cre targets cells in the CEP, but not in the NP or AF of IVDs tissues at these two stages. Thus, our data demonstrated that all these Cre lines can direct recombination in IVD tissues at postnatal stages with different cell type specificity and/or targeting efficiency, and can, therefore, serve as valuable tools to dissect cell type-specific gene functions in IVD development and homeostasis.

Postnatal deletion of Alk5 gene in meniscal cartilage accelerates age-dependent meniscal degeneration in mice

Activation of transforming growth factor-β (TGF-β) signaling has been used to enhance healing of meniscal degeneration in several models. However, the exact role and molecular mechanism of TGF-β signaling in meniscus maintenance and degeneration are still not understood due to the absence of in vivo evidence. In this study, we found that the expression of activin receptor-like kinases 5 (ALK5) in the meniscus was decreased with the progression of age and/or osteoarthritis induced meniscal degeneration. Col2α1 positive cells were found to be specifically distributed in the superficial and inner zones of the anterior horn, as well as the inner zone of the posterior horn in mice, indicating that Col2α1-CreER^T2 mice can be a used for studying gene function in menisci. Furthermore, we deleted Alk5 in Col2α1 positive cells in meniscus by administering tamoxifen. Alterations in the menisci structure were evaluated histologically. The expression levels of genes and proteins associated with meniscus homeostasis and TGF-β signaling were analyzed by quantitative real-time PCR analysis (qRT-PCR) and immunohistochemistry (IHC). Our results revealed severe and progressive meniscal degeneration phenotype in 3- and 6-month-old Alk5 cKO mice compared with Cre-negative control, including aberrantly increased hypertrophic meniscal cells, severe fibrillation, and structure disruption of meniscus. qRT-PCR and IHC results showed that disruption of anabolic and catabolic homeostasis of chondrocytes may contribute to the meniscal degeneration phenotype observed in Alk5 cKO mice. Thus, TGF-β/ALK5 signaling plays a chondro-protective role in menisci homeostasis, in part, by inhibiting matrix degradation and maintaining extracellular matrix proteins levels in meniscal tissues.

Activation of β-catenin signaling in aggrecan-expressing cells in temporomandibular joint causes osteoarthritis-like defects

β-Catenin plays a critical role in cartilage formation and development. To further understand the role of β-catenin in osteoarthritis (OA) development in temporomandibular joint (TMJ), we have generated β-catenin conditional activation mice (β-cat(ex3)^Agc1CreER) by breeding Agc1-CreER mice with β-catenin^flox(ex3)/+ mice. Results of histologic analysis showed the progressive TMJ defects in 3- and 6-month-old β-cat(ex3)^Agc1CreER mice (tamoxifen induction was performed at 2 weeks of age), including decreased chondrocyte numbers in the superficial layer associated with less Alcian blue staining, increased numbers of hypertrophic chondrocytes in deep layers, and rough articular surface. Compared to the TMJ phenotype of β-cat(ex3)^Col2CreER mice, β-cat(ex3)^Agc1CreER mice showed much severe morphological defects in the superficial layer of TMJ. This may reflect that Agc1-CreER mice could efficiently target cells in the superficial layer of TMJ. Results of immunostaining showed significantly increased expression of MMP13, Col-X, Adamts4, and Adamts5 in TMJ of β-cat(ex3)^Agc1CreER mice. Results of proliferating cell nuclear antigen (PCNA), Ki67, and terminal deoxinucleotidyl transferase-mediated dUTP-fluorescein nick end labeling (TUNEL) staining further demonstrated that cell proliferation was decreased and cell apoptosis was increased in condylar cartilage of β-cat(ex3)^Agc1CreER mice. Our findings indicate that abnormal upregulation of β-catenin in TMJ leads to defects assembling to OA-like phenotype, further demonstrating that β-catenin plays a critical role in TMJ pathogenesis.

Therapies targeting a cartilage-regulating signaling protein could mitigate painful damage inflicted on the jaw by age-related osteoarthritis. Many older individuals experience degeneration of the temporomandibular joint (TMJ), where the upper and lower jaw connect. Researchers led by Di Chen of Rush University Medical School in Chicago, USA, have developed a genetically-modified mouse model that reveals a likely molecular driver for TMJ osteoarthritis. Previous studies have implicated a protein called β-catenin in this process, and Chen’s team generated mice in which β-catenin levels can be selectively boosted in cartilage-forming cells at skeletal joints. This increased β-catenin markedly altered the organization of TMJ cartilage, with decreased cell proliferation and increased cell death. The effects were strikingly similar to human osteoarthritis, and the researchers hypothesize that compounds that counter β-catenin could offer useful treatments for this condition.

Therapeutic effects of gefitinib-encapsulated thermosensitive injectable hydrogel in intervertebral disc degeneration

Intervertebral disc (IVD) degeneration is one of the most widespread musculoskeletal diseases worldwide, which remains an intractable clinical challenge. The aim of this study is to investigate the therapeutic potential of the small molecule gefitinib (an epidermal growth factor receptor (EGFR) inhibitor) in ameliorating IVD degeneration. Aberrant EGFR activation levels were detected in both human and rat degenerative IVDs, which prompted us to investigate the functional roles of EGFR by utilizing inducible cartilage-specific EGFR-deficient mice. We demonstrated that conditional EGFR deletion in mice increased nucleus pulposus (NP) extracellular matrix (ECM) production and autophagy marker activation while MMP13 expression decreased. These outcomes are comparable to the use of a controlled-release injectable thermosensitive hydrogel of gefitinib to block EGFR activity in a puncture-induced rat model. We also conducted a case series study involving patients with non-small cell lung cancer and IVD degeneration who received gefitinib treatment from 2010 to 2015. Gefitinib-treated patients displayed a relative slower disc degenerating progression, in contrast to control subjects. These findings thus provide evidence that suppression of EGFR by the FDA-approved drug gefitinib can protect IVD degeneration in rats, implying the potential application of gefitinib as a small molecule drug for treating IVD degeneration.

Specific Deletion of β-Catenin in Col2-Expressing Cells Leads to Defects in Epiphyseal Bone

The role of canonical Wnt/β-catenin signaling in postnatal bone growth has not been fully defined. In the present studies, we generated β-catenin conditional knockout (KO) mice and deleted β-catenin in Col2-expressing chondrocytes and mesenchymal progenitor cells. Findings from analyzing the β-cateninCol2CreER KO mice revealed severe bone destruction and bone loss phenotype in epiphyseal bone, probably due to the increase in osteoclast formation and the accumulation of adipocytes in this area. In addition, we also found bone destruction and bone loss phenotype in vertebral bone in β-cateninCol2CreER KO mice. These findings indicate that β-catenin signaling plays a critical role in postnatal bone remodeling. Our study provides new insights into the regulation of epiphyseal bone homeostasis at postnatal stage.

Delivery of epidermal growth factor receptor inhibitor via a customized collagen scaffold promotes meniscal defect regeneration in a rabbit model

Meniscal injury is one of the most common knee joint injuries, which remains an intractable challenge in clinical practice to date. Aberrant epidermal growth factor receptor (EGFR) activation levels in both human and mice menisci following injury, prompted us to investigate the functional role of EGFR by utilizing an inducible cartilage-specific EGFR-deficient mouse model. We demonstrated that conditional EGFR deletion in mice resulted in increased partial meniscectomy-induced ECM production within the meniscus, which is comparable to utilization of the small molecule EGFR inhibitor, gefitinib, to block EGFR activity. Here, we combined intra-articular delivery of gefitinib with an implanted customized collagen scaffold to substitute for lost meniscal tissue, as well as to promote meniscal regeneration and prevent osteoarthritis (OA) progression in a rabbit meniscectomy model.

STATEMENT OF SIGNIFICANCE

The main novelty of this study is the finding of a new application for small molecule EGFR inhibitor in meniscal injury therapy. This study also highlights the importance of using a customized collagen scaffold to provide robust mechanical strength and effectively promote meniscus regeneration. In summary, our study finds that intra-articular delivery of gefitinib together with implantation of a customized, multi-layer collagen scaffold not only enhanced meniscal regeneration, but also protected articular cartilage from degeneration in rabbit model. These results provide valuable insight for meniscal tissue engineering studies and clinical practice.

Fibroblast Growth Factor Receptor 3 Inhibits Osteoarthritis Progression in the Knee Joints of Adult Mice

OBJECTIVE

Fibroblast growth factor (FGF) signaling is involved in articular cartilage homeostasis. This study was undertaken to investigate the role and mechanisms of FGF receptor 3 (FGFR-3) in the pathogenesis of osteoarthritis (OA) caused by surgery and aging in mice.

METHODS

FGFR-3 was conditionally deleted or activated in articular chondrocytes in adult mice subjected to surgical destabilization of the medial meniscus (DMM). A mouse model of human achondroplasia was also used to assess the role of FGFR-3 in age-associated spontaneous OA. Knee joint cartilage was histologically evaluated and scored using the Osteoarthritis Research Society International system. The expression of genes associated with articular cartilage maintenance was quantitatively evaluated in hip cartilage explants. The effect of inhibiting Indian hedgehog (IHH) signaling in Fgfr3-deficient explants was analyzed.

RESULTS

Conditional Fgfr3 deletion in mice aggravated DMM-induced cartilage degeneration. Matrix metalloproteinase 13 and type X collagen levels were up-regulated, while type II collagen levels were down-regulated, in the articular cartilage of these mice. Conversely, FGFR-3 activation attenuated cartilage degeneration induced by DMM surgery and age. IHH signaling and runt-related transcription factor 2 levels in mouse articular chondrocytes were up-regulated in the absence of Fgfr3, while inhibition of IHH signaling suppressed the increases in the expression of Runx2, Mmp13, and other factors in Fgfr3-deficient mouse cartilage explants.

CONCLUSION

Our findings indicate that FGFR-3 delays OA progression in mouse knee joints at least in part via down-regulation of IHH signaling in articular chondrocytes.

Deletion of Runx2 in Articular Chondrocytes Decelerates the Progression of DMM-Induced Osteoarthritis in Adult Mice

Runx2 may play an important role in development of osteoarthritis (OA). However, the specific role of Runx2 in articular chondrocyte function and in OA development in adult mice has not been fully defined. In this study, we performed the destabilization of the medial meniscus (DMM) surgery at 12-week-old mice to induce OA in adult Runx2^Agc1CreER mice, in which Runx2 was specifically deleted in Aggrecan-expressing chondrocytes by administering tamoxifen at 8-weeks of age. Knee joint samples were collected 8- and 12-weeks post-surgery and analyzed through histology, histomorphometry and micro-computed tomography (μCT). Our results showed that severe OA-like defects were observed after DMM surgery in Cre-negative control mice, including articular cartilage degradation and subchondral sclerosis, while the defects were significantly ameliorated in Runx2^Agc1CreER KO mice. Immunohistochemical (IHC) results showed significantly reduced expression of MMP13 in Runx2^Agc1CreER KO mice compared to that in Cre-negative control mice. Results of quantitative reverse-transcription PCR (qRT-PCR) demonstrated that expression of the genes encoding for matrix degradation enzymes was significantly decreased in Runx2^Agc1CreER KO mice. Thus, our findings suggest that inhibition of Runx2 in chondrocytes could at least partially rescue DMM-induced OA-like defects in adult mice.

Targeting the annulus fibrosus of the intervertebral disc: Col1a2-Cre(ER)T mice show specific activity of Cre recombinase in the outer annulus fibrosus

Degeneration of the intervertebral disc (IVD) is a major underlying contributor to back pain-the single leading cause of disability worldwide. However, we possess a limited understanding of the etiology underlying IVD degeneration. To date, there are a limited number of mouse models that have been used to target proteins in specific compartments of the IVD to explore their functions in disc development, homeostasis and disease. Furthermore, the majority of reports exploring the composition and function of the outer encapsulating annulus fibrosus (AF) of the IVD have considered it as one tissue, without considering the numerous structural and functional differences existing between the inner and outer AF. In addition, no mouse models have yet been reported that enable specific targeting of genes within the outer AF. In the current report, we discuss these issues and demonstrate the localized activity of Cre recombinase in the IVD of Col1a2-Cre(ER)T;ROSA26mTmG mice possessing a tamoxifen-dependent Cre recombinase driven by a Cola2 promoter and distal enhancer and the mTmG fluorescent reporter. Following tamoxifen injection of 3-week-old Col1a2-Cre(ER)T;ROSA26mTmG mice, we show Cre activity specifically in the outer AF of the IVD, as indicated by expression of the GFP reporter. Thus, Col1a2-Cre(ER)T;ROSA26mTmG mice may prove to be a valuable tool in delineating the function of proteins in this unique compartment of the IVD, and in further exploring the compositional differences between the inner and outer AF in disc homeostasis, aging and disease.

Conditional Deletion of Fgfr3 in Chondrocytes leads to Osteoarthritis-like Defects in Temporomandibular Joint of Adult Mice

Osteoarthritis (OA) in the temporomandibular joint (TMJ) is a common degenerative disease in adult, which is characterized by progressive destruction of the articular cartilage. To investigate the role of FGFR3 in the homeostasis of TMJ cartilage during adult stage, we generated Fgfr3^f/f; Col2a1-CreER^T2 (Fgfr3 cKO) mice, in which Fgfr3 was deleted in chondrocytes at 2 months of age. OA-like defects were observed in Fgfr3 cKO TMJ cartilage. Immunohistochemical staining and quantitative real-time PCR analyses revealed a significant increase in expressions of COL10, MMP13 and AMAMTS5. In addition, there was a sharp increase in chondrocyte apoptosis at the Fgfr3 cKO articular surface, which was accompanied by a down-regulation of lubricin expression. Importantly, the expressions of RUNX2 and Indian hedgehog (IHH) were up-regulated in Fgfr3 cKO TMJ. Primary Fgfr3 cKO chondrocytes were treated with IHH signaling inhibitor, which significantly reduced expressions of Runx2, Col10, Mmp13 and Adamts5. Furthermore, the IHH signaling inhibitor partially alleviated OA-like defects in the TMJ of Fgfr3 cKO mice, including restoration of lubricin expression and improvement of the integrity of the articular surface. In conclusion, our study proposes that FGFR3/IHH signaling pathway plays a critical role in maintaining the homeostasis of TMJ articular cartilage during adult stage.

Chondrocytes-Specific Expression of Osteoprotegerin Modulates Osteoclast Formation in Metaphyseal Bone

Bone marrow stromal cells/osteoblasts were originally thought to be the major player in regulating osteoclast differentiation through expressing RANKL/OPG cytokines. Recent studies have established that chondrocytes also express RANKL/OPG and support osteoclast formation. Till now, the in vivo function of chondrocyte-produced OPG in osteoclast formation and postnatal bone growth has not been directly investigated. In this study, chondrocyte-specific Opg transgenic mice were generated by using type II collagen promoter. The Col2-Opg transgenic mice showed delayed formation of secondary ossification center and localized increase of bone mass in proximal metaphysis of tibiae. TRAP staining showed that osteoclast numbers were reduced in both secondary ossification center and proximal metaphysis. This finding was further confirmed by in vitro chondrocyte/spleen cell co-culture assay. In contrast, the mineral apposition rates were not changed in Col2-Opg transgenic mice. TUNEL staining revealed more apoptotic hypertrophic chondrocytes in the growth plate of Col2-Opg mice. Flow cytometry analysis showed fewer RANK-expressing cells in the marrow of Col2a1-Opg mice, suggesting the role of OPG in blocking the differentiation of early mesenchymal progenitors into RANK-expressing pre-osteoclasts. Our results demonstrated that OPG expression in chondrocyte increases bone mass in the proximal metaphysis of tibiae through negative regulation of osteoclast formation.