An osteocalcin-deficient mouse strain without endocrine abnormalities.

Osteocalcin (OCN), the most abundant noncollagenous protein in the bone matrix, is reported to be a bone-derived endocrine hormone with wide-ranging effects on many aspects of physiology, including glucose metabolism and male fertility. Many of these observations were made using an OCN-deficient mouse allele (Osc-) in which the 2 OCN-encoding genes in mice, Bglap and Bglap2, were deleted in ES cells by homologous recombination. Here we describe mice with a new Bglap and Bglap2 double-knockout (dko) allele (Bglap/2p.Pro25fs17Ter) that was generated by CRISPR/Cas9-mediated gene editing. Mice homozygous for this new allele do not express full-length Bglap or Bglap2 mRNA and have no immunodetectable OCN in their serum. FTIR imaging of cortical bone in these homozygous knockout animals finds alterations in the collagen maturity and carbonate to phosphate ratio in the cortical bone, compared with wild-type littermates. However, µCT and 3-point bending tests do not find differences from wild-type littermates with respect to bone mass and strength. In contrast to the previously reported OCN-deficient mice with the Osc-allele, serum glucose levels and male fertility in the OCN-deficient mice with the Bglap/2pPro25fs17Ter allele did not have significant differences from wild-type littermates. We cannot explain the absence of endocrine effects in mice with this new knockout allele. Possible explanations include the effects of each mutated allele on the transcription of neighboring genes, or differences in genetic background and environment. So that our findings can be confirmed and extended by other interested investigators, we are donating this new Bglap and Bglap2 double-knockout strain to the Jackson Laboratories for academic distribution.

Somatic Activating Mutations in GNAQ and GNA11 Are Associated with Congenital Hemangioma.

Congenital hemangioma is a rare vascular tumor that forms in utero. Postnatally, the tumor either involutes quickly (i.e., rapidly involuting congenital hemangioma [RICH]) or partially regresses and stabilizes (i.e., non-involuting congenital hemangioma [NICH]). We hypothesized that congenital hemangiomas arise due to somatic mutation and performed massively parallel mRNA sequencing on affected tissue from eight participants. We identified mutually exclusive, mosaic missense mutations that alter glutamine at amino acid 209 (Glu209) in GNAQ or GNA11 in all tested samples, at variant allele frequencies (VAF) ranging from 3% to 33%. We verified the presence of the mutations in genomic DNA using a combination of molecular inversion probe sequencing (MIP-seq) and digital droplet PCR (ddPCR). The Glu209 GNAQ and GNA11 missense variants we identified are common in uveal melanoma and have been shown to constitutively activate MAPK and/or YAP signaling. When we screened additional archival formalin-fixed paraffin-embedded (FFPE) congenital cutaneous and hepatic hemangiomas, 4/8 had GNAQ or GNA11 Glu209 variants. The same GNAQ or GNA11 mutation is found in both NICH and RICH, so other factors must account for these tumors' different postnatal behaviors.

RNA-seq in Skeletal Biology.

PURPOSE OF REVIEW: The goal of this paper is to review state-of-the-art transcriptome profiling methods and their recent applications in the field of skeletal biology. RECENT FINDINGS: Next-generation sequencing of mRNA (RNA-seq) methods have been established and routinely used in skeletal biology research. RNA-seq has led to the identification of novel genes and transcription factors involved in skeletal development and disease, through its application in small and large animal models, as well as human tissue and cells. With the availability of advanced techniques such as single-cell RNA-seq, novel cell types in skeletal tissues are being identified. As the sequencing technologies are rapidly evolving, the exciting discoveries supported by transcriptomics will continue to emerge and improve our understanding of the biology of the skeleton.

A somatic GNA11 mutation is associated with extremity capillary malformation and overgrowth.

BACKGROUND: Capillary malformation is a cutaneous vascular anomaly that is present at birth, darkens over time, and can cause overgrowth of tissues beneath the stain. The lesion is caused by a somatic activating mutation in GNAQ. In a previous study, we were unable to identify a GNAQ mutation in patients with a capillary malformation involving an overgrown lower extremity. We hypothesized that mutations in GNA11 or GNA14, genes closely related to GNAQ, also may cause capillary malformations. METHODS: Human capillary malformation tissue obtained from 8 patients that had tested negative for GNAQ mutations were studied. Lesions involved an extremity (n = 7) or trunk (n = 1). Droplet digital PCR (ddPCR) was used to detect GNA11 or GNA14 mutant cells (p.Arg183) in the specimens. Single molecule molecular inversion probe sequencing (smMIP-seq) was performed to search for other mutations in GNA11. Mutations were validated by subcloning and sequencing amplimers. RESULTS: We found a somatic GNA11 missense mutation (c.547C > T; p.Arg183Cys) in 3 patients with a diffuse capillary malformation of an extremity. Mutant allelic frequencies ranged from 0.3 to 5.0%. GNA11 or GNA14 mutations were not found in 5 affected tissues or in unaffected tissues (white blood cell DNA). CONCULSIONS: GNA11 mutations are associated with extremity capillary malformations causing overgrowth. Pharmacotherapy that affects GNA11 signaling may prevent the progression of capillary malformations.

SHP2 regulates chondrocyte terminal differentiation, growth plate architecture and skeletal cell fates.

Loss of PTPN11/SHP2 in mice or in human metachondromatosis (MC) patients causes benign cartilage tumors on the bone surface (exostoses) and within bones (enchondromas). To elucidate the mechanisms underlying cartilage tumor formation, we investigated the role of SHP2 in the specification, maturation and organization of chondrocytes. Firstly, we studied chondrocyte maturation by performing RNA-seq on primary chondrocyte pellet cultures. We found that SHP2 depletion, or inhibition of the ERK1/2 pathway, delays the terminal differentiation of chondrocytes from the early-hypertrophic to the late-hypertrophic stage. Secondly, we studied chondrocyte maturation and organization in mice with a mosaic postnatal inactivation of Ptpn11 in chondrocytes. We found that the vertebral growth plates of these mice have expanded domains of early-hypertrophic chondrocytes that have not yet terminally differentiated, and their enchondroma-like lesions arise from chondrocytes displaced from the growth plate due to a disruption in the organization of maturation and ossification zones. Furthermore, we observed that lesions from human MC patients also display disorganized chondrocyte maturation zones. Next, we found that inactivation of Ptpn11 in Fsp1-Cre-expressing fibroblasts induces exostosis-like outgrowths, suggesting that loss of SHP2 in cells on the bone surface and at bone-ligament attachment sites induces ectopic chondrogenesis. Finally, we performed lineage tracing to show that exostoses and enchondromas in mice likely contain mixtures of wild-type and SHP2-deficient chondrocytes. Together, these data indicate that in patients with MC, who are heterozygous for inherited PTPN11 loss-of-function mutations, second-hit mutations in PTPN11 can induce enchondromas by disrupting the organization and delaying the terminal differentiation of growth plate chondrocytes, and can induce exostoses by causing ectopic chondrogenesis of cells on the bone surface. Furthermore, the data are consistent with paracrine signaling from SHP2-deficient cells causing SHP2-sufficient cells to be incorporated into the lesions.

Somatic mosaic activating mutations in PIK3CA cause CLOVES syndrome.

Congenital lipomatous overgrowth with vascular, epidermal, and skeletal anomalies (CLOVES) is a sporadically occurring, nonhereditary disorder characterized by asymmetric somatic hypertrophy and anomalies in multiple organs. We hypothesized that CLOVES syndrome would be caused by a somatic mutation arising during early embryonic development. Therefore, we employed massively parallel sequencing to search for somatic mosaic mutations in fresh, frozen, or fixed archival tissue from six affected individuals. We identified mutations in PIK3CA in all six individuals, and mutant allele frequencies ranged from 3% to 30% in affected tissue from multiple embryonic lineages. Interestingly, these same mutations have been identified in cancer cells, in which they increase phosphoinositide-3-kinase activity. We conclude that CLOVES is caused by postzygotic activating mutations in PIK3CA. The application of similar sequencing strategies will probably identify additional genetic causes for sporadically occurring, nonheritable malformations.

A normative study of the synovial fluid proteome from healthy porcine knee joints.

Synovial fluid in an articulating joint contains proteins derived from the blood plasma and proteins that are produced by cells within the joint tissues, such as synovium, cartilage, ligament, and meniscus. The proteome composition of healthy synovial fluid and the cellular origins of many synovial fluid components are not fully understood. Here, we present a normative proteomics study using porcine synovial fluid. Using our optimized method, we identified 267 proteins with high confidence in healthy synovial fluid. We also evaluated mRNA expression data from tissues that can contribute to the synovial fluid proteome, including synovium, cartilage, blood, and liver, to better estimate the relative contributions from these sources to specific synovial fluid components. We identified 113 proteins in healthy synovial fluid that appear to be primarily derived from plasma transudates, 37 proteins primarily derived from synovium, and 11 proteins primarily derived from cartilage. Finally, we compared the identified synovial fluid proteome to the proteome of human plasma, and we found that the two body fluids share many similarities, underlining the detected plasma derived nature of many synovial fluid components. Knowing the synovial fluid proteome of a healthy joint will help to identify mechanisms that cause joint disease and pathways involved in disease progression.

Modeling degenerative disk disease in the lumbar spine: a combined experimental, constitutive, and computational approach.

Using a continuum approach for modeling the constitutive mechanical behavior of the intervertebral disk's annulus fibrosus holds the potential for facilitating the correlation of morphology and biomechanics of this clinically important tissue. Implementation of a continuum representation of the disk's tissues into computational models would yield a particularly valuable tool for investigating the effects of degenerative disease. However, to date, relevant efforts in the literature towards this goal have been limited due to the lack of a computationally tractable and implementable constitutive function. In order to address this, annular specimens harvested from a total of 15 healthy and degenerated intervertebral disks were tested under planar biaxial tension. Predictions of a strain energy function, which was previously shown to be unconditionally convex, were fit to the experimental data, and the optimized coefficients were used to modify a previously validated finite element model of the L4/L5 functional spinal unit. Optimization of material coefficients based on experimental results indicated increases in the micro-level orientation dispersion of the collagen fibers and the mechanical nonlinearity of these fibers due to degeneration. On the other hand, the finite element model predicted a progressive increase in the stress generation in annulus fibrosus due to stepwise degeneration of initially the nucleus and then the entire disk. Range of motion was predicted to initially increase with the degeneration of the nucleus and then decrease with the degeneration of the annulus in all rotational loading directions, except for axial rotation. Overall, degeneration was observed to specifically impact the functional effectiveness of the collagen fiber network of the annulus, leading to changes in the biomechanical behavior at both the tissue level and the motion-segment level.

The role of Meteorin-like in skeletal development and bone fracture healing.

Meteorin-like protein (Metrnl), homologous to the initially identified neurotrophic factor Meteorin, is a secreted, multifunctional protein. Here we used mouse models to investigate Metrnl's role in skeletal development and bone fracture healing. During development Metrnl was expressed in the perichondrium and primary ossification center. In neonates, single cell RNA-seq of diaphyseal bone demonstrated strongest expression of Metrnl transcript by osteoblasts. In vitro, Metrnl was osteoinductive, increasing osteoblast differentiation and mineralization in tissue culture models. In vivo, loss of Metrnl expression resulted in no change in skeletal metrics in utero, at birth, or during postnatal growth. Six-week-old Metrnl-null mice displayed similar body length, body weight, tibial length, femoral length, BV/TV, trabecular number, trabecular thickness, and cortical thickness as littermate controls. In 4-month-old mice, lack of Metrnl expression did not change structural stiffness, ultimate force, or energy to fracture of femora under 3-point-bending. Last, we investigated the role of Metrnl in bone fracture healing. Metrnl expression increased in response to tibial injury, however, loss of Metrnl expression did not affect the amount of bone deposited within the healing tissue nor did it change the structural parameters of healing tissue. This work identifies Metrnl as a dispensable molecule for skeletal development. However, the osteoinductive capabilities of Metrnl may be utilized to modulate osteoblast differentiation in cell-based orthopedic therapies.

Cartilage thickness distribution affects computational model predictions of cervical spine facet contact parameters.

With motion-sparing disk replacement implants gaining popularity as an alternative to anterior cervical discectomy and fusion (ACDF) for the treatment of certain spinal degenerative disorders, recent laboratory investigations have studied the effects of disk replacement and implant design on spinal kinematics and kinetics. Particularly relevant to cervical disk replacement implant design are any postoperative changes in solid stresses or contact conditions in the articular cartilage of the posterior facets, which are hypothesized to lead to adjacent-level degeneration. Such changes are commonly investigated using finite element methods, but significant simplification of the articular geometry is generally employed. The impact of such geometric representations has not been thoroughly investigated. In order to assess the effects of different models of cartilage geometry on load transfer and contact pressures in the lower cervical spine, a finite element model was generated using cadaver-based computed tomography imagery. Mesh resolution was varied in order to establish model convergence, and cadaveric testing was undertaken to validate model predictions. The validated model was altered to include four different geometric representations of the articular cartilage. Model predictions indicate that the two most common representations of articular cartilage geometry result in significant reductions in the predictive accuracy of the models. The two anatomically based geometric models exhibited less computational artifact, and relatively minor differences between them indicate that contact condition predictions of spatially varying thickness models are robust to anatomic variations in cartilage thickness and articular curvature. The results of this work indicate that finite element modeling efforts in the lower cervical spine should include anatomically based and spatially varying articular cartilage thickness models. Failure to do so may result in loss of fidelity of model predictions relevant to investigations of physiological import.

RNAseq and RNA molecular barcoding reveal differential gene expression in cortical bone following hindlimb unloading in female mice.

Disuse-induced bone loss is seen following spinal cord injury, prolonged bed rest, and exposure to microgravity. We performed whole transcriptomic profiling of cortical bone using RNA sequencing (RNAseq) and RNA molecular barcoding (NanoString) on a hindlimb unloading (HLU) mouse model to identify genes whose mRNA transcript abundances change in response to disuse. Eleven-week old female C57BL/6 mice were exposed to ambulatory loading or HLU for 7 days (n = 8/group). Total RNA from marrow-flushed femoral cortical bone was analyzed on HiSeq and NanoString platforms. The expression of several previously reported genes associated with Wnt signaling and metabolism was altered by HLU. Furthermore, the increased abundance of transcripts, such as Pfkfb3 and Mss51, after HLU imply these genes also have roles in the cortical bone's response to altered mechanical loading. Our study demonstrates that an unbiased approach to assess the whole transcriptomic profile of cortical bone can reveal previously unidentified mechanosensitive genes and may eventually lead to novel targets to prevent disuse-induced osteoporosis.

RNA-seq Analysis of Peri-Implant Tissue Shows Differences in Immune, Notch, Wnt, and Angiogenesis Pathways in Aged Versus Young Mice.

The number of total joint replacements (TJRs) in the United States is increasing annually. Cementless implants are intended to improve upon traditional cemented implants by allowing bone growth directly on the surface to improve implant longevity. One major complication of TJR is implant loosening, which is related to deficient osseointegration in cementless TJRs. Although poor osseointegration in aged patients is typically attributed to decreased basal bone mass, little is known about the molecular pathways that compromise the growth of bone onto porous titanium implants. To identify the pathways important for osseointegration that are compromised by aging, we developed an approach for transcriptomic profiling of peri-implant tissue in young and aged mice using our murine model of osseointegration. Based on previous findings of changes of bone quality associated with aging, we hypothesized that aged mice have impaired activation of bone anabolic pathways at the bone-implant interface. We found that pathways most significantly downregulated in aged mice relative to young mice are related to angiogenic, Notch, and Wnt signaling. Downregulation of these pathways is associated with markedly increased expression of inflammatory and immune genes at the bone-implant interface in aged mice. These results identify osseointegration pathways affected by aging and suggest that an increased inflammatory response in aged mice may compromise peri-implant bone healing. Targeting the Notch and Wnt pathways, promoting angiogenesis, or modulating the immune response at the peri-implant site may enhance osseointegration and improve the outcome of joint replacement in older patients. © 2021 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

Immune and repair responses in joint tissues and lymph nodes after knee arthroplasty surgery in mice.

The importance of a local tissue immune response in healing injured tissues such as skin and lung is well established. Little is known about whether sterile wounds elicit lymph node (LN) responses and inflammatory responses after injury of musculoskeletal tissues that are mechanically loaded during the repair response. We investigated LN and tissue immune responses in a tibial implant model of joint replacement surgery where wounded tissue is subjected to movement and mechanical loading postoperatively. Draining inguinal and iliac LNs expanded postoperatively, including increases in regulatory T cells and activation of a subset of T cells. Thus, tissue injury was actively sensed in secondary lymphoid organs, with the potential to activate adaptive immunity. Joint tissues exhibited three temporally distinct immune response components, including a novel interferon (IFN) response with activation of signal transducer and activator of transcription (STAT) and interferon regulatory factor (IRF) pathways. Fibrovascular tissue formation was not associated with a macrophage type 2 (M2) reparative immune response, but instead with delayed induction of interleukin-1 family (IL-1ß, IL-33, IL-36), IL-17, and prostaglandin pathway genes concomitant with transforming growth factor (TGF)-ß and growth factor signaling, fibroblast activation, and tissue formation. Tissue remodeling was associated with activity of the HOX antisense intergenic RNA (HOTAIR) pathway. These results provide insights into immune responses and regulation of tissue healing after knee arthroplasty that potentially can be used to develop therapeutic strategies to improve healing, prevent arthrofibrosis, and improve surgical outcomes. © 2021 American Society for Bone and Mineral Research (ASBMR).

Tmem100- and Acta2-Lineage Cells Contribute to Implant Osseointegration in a Mouse Model.

Metal implants are commonly used in orthopedic surgery. The mechanical stability and longevity of implants depend on adequate bone deposition along the implant surface. The cellular and molecular mechanisms underlying peri-implant bone formation (ie, osseointegration) are incompletely understood. Herein, our goal was to determine the specific bone marrow stromal cell populations that contribute to bone formation around metal implants. To do this, we utilized a mouse tibial implant model that is clinically representative of human joint replacement procedures. Using a lineage-tracing approach, we found that both Acta2.creERT2 and Tmem100.creERT2 lineage cells are involved in peri-implant bone formation, and Pdgfra- and Ly6a/Sca1-expressing stromal cells (PαS cells) are highly enriched in both lineages. Single-cell RNA-seq analysis indicated that PαS cells are quiescent in uninjured bone tissue; however, they express markers of proliferation and osteogenic differentiation shortly after implantation surgery. Our findings indicate that PαS cells are mobilized to repair bone tissue and participate in implant osseointegration after surgery. Biologic therapies targeting PαS cells might improve osseointegration in patients undergoing orthopedic procedures. © 2021 American Society for Bone and Mineral Research (ASBMR).

The micromechanical role of the annulus fibrosus components under physiological loading of the lumbar spine.

To date, studies that have investigated the kinematics of spinal motion segments have largely focused on the contributions that the spinal ligaments play in the resultant motion patterns. However, the specific roles played by intervertebral disk components, in particular the annulus fibrosus, with respect to global motion is not well understood in spite of the relatively large literature base with respect to the local ex vivo mechanical properties of the tissue. The primary objective of this study was to implement the nonlinear and orthotropic mechanical behavior of the annulus fibrosus in a finite element model of an L4/L5 functional spinal unit in the form of a strain energy potential where the individual mechanical contributions of the ground substance and fibers were explicitly defined. The model was validated biomechanically under pure moment loading to ensure that the individual role of each soft tissue structure during load bearing was consistent throughout the physiologically relevant loading range. The fibrous network of the annulus was found to play critical roles in limiting the magnitude of the neutral zone and determining the stiffness of the elastic zone. Under flexion, lateral bending, and axial rotation, the collagen fibers were observed to bear the majority of the load applied to the annulus fibrosus, especially in radially peripheral regions where disk bulging occurred. For the first time, our data explicitly demonstrate that the exact fiber recruitment sequence is critically important for establishing the range of motion and neutral zone magnitudes of lumbar spinal motion segments.

Proteolysis and cartilage development are activated in the synovium after surgical induction of post traumatic osteoarthritis.

Anterior cruciate ligament (ACL) transection surgery in the minipig induces post-traumatic osteoarthritis (PTOA) in a pattern similar to that seen in human patients after ACL injury. Prior studies have reported the presence of cartilage matrix-degrading proteases, such as Matrix metalloproteinase-1 (MMP-1) and A disintegrin and metalloproteinase with thrombospondin motifs 4 (ADAMTS-4), in the synovial fluid of injured or arthritic joints; however, the tissue origin of these proteases is unknown. The objective of this study was to identify transcriptional processes activated in the synovium after surgical induction of PTOA with ACL transection, and to determine if processes associated with proteolysis were enriched in the synovium after ACL transection. Unilateral ACL transection was performed in adolescent Yucatan minipigs and synovium samples were collected at 1, 5, 9, and 14 days post-injury. Transcriptome-wide gene expression levels were determined using bulk RNA-Sequencing in the surgical animals and control animals with healthy knees. The greatest number of transcripts with significant changes was observed 1 day after injury. These changes were primarily associated with cellular proliferation, consistent with measurements of increased cellularity of the synovium at the two-week time point. At five to 14 days, the expression of transcripts relating to proteolysis and cartilage development was significantly enriched. While protease inhibitor-encoding transcripts (TIMP2, TIMP3) represented the largest fraction of protease-associated transcripts in the uninjured synovium, protease-encoding transcripts (including MMP1, MMP2, ADAMTS4) predominated after surgery. Cartilage development-associated transcripts that are typically not expressed by synovial cells, such as ACAN and COMP, were enriched in the synovium following ACL-transection. The upregulation in both catabolic processes (proteolysis) and anabolic processes (cartilage development) suggests that the synovium plays a complex, balancing role in the early response to PTOA induction.

Single-Cell RNA Sequencing of Calvarial and Long-Bone Endocortical Cells.

Single-cell RNA sequencing (scRNA-Seq) is emerging as a powerful technology to examine transcriptomes of individual cells. We determined whether scRNA-Seq could be used to detect the effect of environmental and pharmacologic perturbations on osteoblasts. We began with a commonly used in vitro system in which freshly isolated neonatal mouse calvarial cells are expanded and induced to produce a mineralized matrix. We used scRNA-Seq to compare the relative cell type abundances and the transcriptomes of freshly isolated cells to those that had been cultured for 12 days in vitro. We observed that the percentage of macrophage-like cells increased from 6% in freshly isolated calvarial cells to 34% in cultured cells. We also found that Bglap transcripts were abundant in freshly isolated osteoblasts but nearly undetectable in the cultured calvarial cells. Thus, scRNA-Seq revealed significant differences between heterogeneity of cells in vivo and in vitro. We next performed scRNA-Seq on freshly recovered long bone endocortical cells from mice that received either vehicle or sclerostin-neutralizing antibody for 1 week. We were unable to detect significant changes in bone anabolism-associated transcripts in immature and mature osteoblasts recovered from mice treated with sclerostin-neutralizing antibody; this might be a consequence of being underpowered to detect modest changes in gene expression, because only 7% of the sequenced endocortical cells were osteoblasts and a limited portion of their transcriptomes were sampled. We conclude that scRNA-Seq can detect changes in cell abundance, identity, and gene expression in skeletally derived cells. In order to detect modest changes in osteoblast gene expression at the single-cell level in the appendicular skeleton, larger numbers of osteoblasts from endocortical bone are required. © 2020 American Society for Bone and Mineral Research.

The Unmixing Problem: A Guide to Applying Single-Cell RNA Sequencing to Bone.

Bone is composed of a complex mixture of many dynamic cell types. Flow cytometry and in vivo lineage tracing have offered early progress toward deconvoluting this heterogeneous mixture of cells into functionally well-defined populations suitable for further studies. Single-cell sequencing is poised as a key complementary technique to better understand the cellular basis of bone metabolism and development. However, single-cell sequencing approaches still have important limitations, including transcriptional effects of cell isolation and sparse sampling of the transcriptome, that must be considered during experimental design and analysis to harness the power of this approach. Accounting for these limitations requires a deep knowledge of the tissue under study. Therefore, with the emergence of accessible tools for conducting and analyzing single-cell RNA sequencing (scRNA-seq) experiments, bone biologists will be ideal leaders in the application of scRNA-seq to the skeleton. Here we provide an overview of the steps involved with a single-cell sequencing analysis of bone, focusing on practical considerations needed for a successful study. © 2019 American Society for Bone and Mineral Research.