Westendorf, Jennifer J.

Abstract Osteogenesis imperfecta (OI) is a heterogeneous group of inherited connective tissue disorders primarily caused by dominant mutations in COL1A1 or COL1A2 that impair type I procollagen folding and secretion. Misfolded collagen accumulates in the endoplasmic reticulum (ER), triggering ER stress and osteoblast dysfunction and bone fragility. Current pharmacologic therapy focuses on inhibiting bone resorption but has limited efficacy and does not address the underlying biology of the disease. The epigenetic regulator Polycomb-repressive complex 2 (PRC2) has emerged as an important regulator of bone formation. Genetic and pharmacologic disruption of PRC2 enhanced osteogenic differentiation in wild-type cells. Here we demonstrate that inhibition of the PRC2 through targeting its essential component embryonic ectoderm development (EED) enhances osteogenic differentiation, improves bone architecture in male Col1a2+/G610C OI mouse models, modulates the integrated stress response (ISR), and improves ER morphology in OI cells. These findings identify EED inhibition as a novel epigenetic strategy to restore collagen homeostasis and improve skeletal integrity in OI.

Abstract Germline and osteoblast-directed deletion of G protein-gated inwardly rectifying K+ channel 3 (Girk3) was recently shown to increase bone mass after 18 wk of age in male mice. Here, we show that germline Girk3 deletion also increases trabecular and cortical bone mass and increases the mechanical strength of the femur in female mice after 18 wk of age. Unlike male mice, however, osteoblast-directed Girk3 deletion using 2.3 kb-Col1a1-Cre does not increase bone mass in adult female mice. To discover mechanisms underlying high bone mass in female Girk3−/− mice, bulk RNA-sequencing was performed on 2-d-old calvarial bone, revealing lower expression of proinflammatory cytokines such as IL-1β and IL-6 in Girk3−/− mice. Accordingly, cytokines and chemokines are largely suppressed in the circulation of adult Girk3−/− mice compared to WT littermates. The cytokines GM-CSF, IL-1β, IL-2, and IL-9 are reduced in the serum of both male and female Girk3−/− mice, while eotaxin, IFNγ, MIP-1α, and others are sexually dimorphic. Histomorphometry reveals that osteoclast activity is modestly reduced in Girk3−/− bone, which is supported by in vitro osteoclast resorption assays. However, deletion of Girk3 in myeloid-lineage cells with LysM-Cre is not sufficient to recapitulate high bone mass in either male or female mice. Moreover, female Girk3−/− mice are not protected from ovariectomy-induced bone loss. Finally, single-cell screening using cytometry by time-of-flight in the BM revealed no differences in immune cell abundances due to global Girk3 deletion. Taken together, while Girk3 regulates inflammatory cytokine expression in the bone and serum, deletion of Girk3 in myeloid-lineage cells does not affect bone mass.

Abstract Studies on human and animal models have demonstrated a complex molecular regulatory network between the dental mesenchyme and epithelium governing tooth development. However, epigenetic regulation of tooth development is largely unexplored. This study aimed to elucidate the relationship between epigenetic modifiers and dental root development using mice deficient in histone deacetylase 3 (Hdac3) under the control of the osterix promoter (Osx-Cre/Hdac3fl/fl or Hdac3-CKOosx). We observed tooth root size and histology in Hdac3-CKOosx mice. Dental pulp progenitor cells (DPCs) were isolated from lower incisors, and calcification and gene expression were assessed. Hdac3 depletion in osterix-expressing dental pulp stem cells, including odontoblasts, caused a progressive postnatal obstruction, resulting in relatively short roots and small root apices of the first molar. Mild degeneration was observed during the development of dentin and cementum structures. Dentin and cementum had uneven borders and showed disordered H&E staining in Hdac3-CKOosx mice that had a thin cementum compared to that of WT mice. Hdac3 inhibition/deletion in dental pulp stem cells probably influenced Msh homeobox 1 (Msx1) and Col1a1 expression in the early developmental stage, thereby driving differentiation in DPCs. Subsequently, Msx1, Col1a1, and osteocalcin expression were remarkably downregulated during calcification. Deletion or inhibition of Hdac3 in conditional KO dental pulp stem cells cultured in mineralization medium resulted in aberrant cell cycle control, and the early stages of maturation of DPCs and odontoblasts were inhibited. Inhibition of Hdac3 in cementocytes also restricted their proliferation and calcification. These results suggested that the deletion or inhibition of Hdac3 in the dental mesenchyme may cause development and maturation deficiency of tooth root.

Abstract Osteoporosis and other metabolic bone diseases are prevalent in the aging population. While bone has the capacity to regenerate throughout life, bone formation rates decline with age and contribute to reduced bone density and strength. Identifying mechanisms and pathways that increase bone accrual in adults could prevent fractures and accelerate healing. G protein-gated inwardly rectifying K+ (GIRK) channels are key effectors of G protein-coupled receptor signaling. Girk3 was recently shown to regulate endochondral ossification. Here, we demonstrate that deletion of Girk3 increases bone mass after 18 weeks of age. Male 24-week-old Girk3-/- mice have greater trabecular bone mineral density and bone volume fraction than wildtype (WT) mice. Osteoblast activity is moderately increased in 24-week-old Girk3-/- mice compared to WT mice. In vitro, Girk3-/- bone marrow stromal cells (BMSCs) are more proliferative than WT BMSCs. Calvarial osteoblasts and BMSCs from Girk3-/- mice are also more osteogenic than WT cells, with altered expression of genes that regulate the wingless-related integration site (Wnt) family. Wnt inhibition via Dickkopf-1 (Dkk1) or β-catenin inhibition via XAV939 prevents enhanced mineralization, but not proliferation, in Girk3-/- BMSCs and slows these processes in WT cells. Finally, selective ablation of Girk3 from cells expressing Cre recombinase from the 2.3 kb-Col1a1 promoter, including osteoblasts and osteocytes, is sufficient to increase bone mass and bone strength in male mice at 24 weeks of age. Taken together, these data demonstrate that Girk3 regulates progenitor cell proliferation, osteoblast differentiation, and bone mass accrual in adult male mice.

Abstract Histone deacetylase 3 (Hdac3) is an epigenetic regulator of gene expression and interacts with skeletal transcription factors such as Runx2. We previously reported that conditional deletion of Hdac3 in Osterix-Cre recombinase-expressing osteoprogenitor cells (Hdac3 CKOOsx) caused osteopenia and increased marrow adiposity, both hallmarks of skeletal aging. We also showed that Runx2+ cells within osteogenic cultures of Hdac3-depleted bone marrow stromal cells (BMSCs) contain lipid droplets (LDs). Cellular senescence, a nonproliferative metabolically active state, is associated with increased marrow adiposity, bone loss, and aging. In this study, we sought to determine if Hdac3 depleted Runx2+ pre-osteoblasts from young mice exhibit chromatin changes associated with early cellular senescence and how these events correlate with the appearance of LDs. We first confirmed that BMSCs from Hdac3 CKOOsx mice have more Runx2 + LD+ cells compared with controls under osteogenic conditions. We then measured senescence-associated distention of satellite (SADS) DNA and telomere-associated foci (TAFs) in Hdac3 CKOOsx and control BMSCs. In situ, Runx2+ cells contained more SADS per nuclei in Hdac3 CKOOsx femora than in controls. Runx2+ BMSCs from Hdac3 CKOOsx mice also contained more SADS and TAFs per nuclei than Runx2+ cells from age-matched control mice in vitro. SADs and TAFs were present at similar levels in Runx2 + LD+ cells and Runx2 + LD− cells from Hdac3 CKOOsx mice. Hdac inhibitors also increased the number of SADS in Runx2 + LD+ and Runx2 + LD− WT BMSCs. Senolytics reduced viable cell numbers in Hdac3 CKOOsx BMSC cultures. These data demonstrate that the depletion of Hdac3 in osteochondral progenitor cells triggers LD formation and early events in cellular senescence in Runx2+ BMSCs through mutually exclusive mechanisms.

Abstract Osteoarthritis (OA) is a degenerative joint disease characterized by pain, stiffness, and limited joint motion affecting 7% of the world’s population. PH domain leucine-rich repeat protein phosphates (PHLPPs) have been identified as modifiable targets for cartilage regeneration and joint pain. PHLPP1 and PHLPP2 are abnormally expressed in osteoarthritic cartilage and sensory neurons. PHLPP1-/- mice are protected from cartilage degradation and pain-related behaviors (allodynia and reduced mobility) after surgery that destabilizes the medial meniscus (DMM). Here, we investigate the effects of PHLPP inhibitors (i) on human chondrocytes and the effect of Phlpp1 deletion on cell populations within lumbar dorsal root ganglia (DRGs). Immunohistochemistry was performed on mouse DRGs by cryosectioning fixed L3-5 DRGs and using primary antibodies against Fabp7, Anti tubulin beta 3, and PGP9.5/UCHL1. Human chondrocytes were subjected to Cycloheximide chase experiments and treated with PHLPPi (NSC117079 or NSC45586), followed by Western blot analysis to assess protein levels including Phlpp1, Phlpp2, Sox9, p-AKT (Ser473), total AKT, and B-actin, at various time points. We found that PHLPPi prolongs the phosphorylated AKT response when compared to controls and does not affect degradation of Phlpp1 or Phlpp2; additionally, NSC117079 increased production of cartilage extracellular matrix components (glycosaminoglycans and aggrecan) in over 90% of human articular cartilage explants from OA patients. These data advance our understanding of how PHLPPi function within chondrocytes and provide a basis toward understanding their activity and safety in vivo. Our results indicate that PHLPPi NSC117079 is a novel osteoarthritis disease-modifying drug candidate that may have palliative effects.

The bone marrow microenvironment acts as a protective niche for malignant cells. Within the bone marrow microenvironment is the endosteal niche, which is the area where the ossified surface of the bone and marrow cavity interact. The endosteal niche provides the opportunity for hematological malignancies such as acute myeloid leukemia (AML) of the bone marrow to interact with osteoblasts from the bone. To study the interaction between AML cells and osteoblasts, our lab developed a co‐culture model of osteoblasts (MC3T3 osteoblast cell line) and AML cells (KG1a cell line or patient samples from bone marrow aspirates). AML cells expressing high levels of CXCR4 were cultured alone or with osteoblasts and then challenged with SDF‐1, the ligand to CXCR4 and a chemokine readily abundant in the bone marrow microenvironment that our lab has previously shown actually induces apoptosis in AML cells expressing high levels of CXCR4 and that osteoblasts are able to protect AML cells from SDF‐1 induced apoptosis. Our lab now shows that treating osteoblasts with cyclosporine (CSA) prevents their ability to effectively protect AML cells from SDF‐1 induced cell death as assayed by annexin‐V staining of AML cells by flow cytometry. CSA treatment of osteoblasts decreased protein levels of tissue non‐specific alkaline phosphatase (ALPL), a phosphatase necessary for osteoblast mineralization, compared to vehicle control. As such, osteoblasts were treated with control siRNA or ALPL siRNA. CXCR4 expressing AML cells were then co‐cultured with the transfected osteoblasts and challenged with SDF‐1. siRNA knockdown of ALPL in osteoblasts was sufficient to prevent protection of AML cells from the apoptotic stimuli. These studies indicate that osteoblast differentiation plays a role in mediating the protection of AML cells. Therefore, targeting specific differentiation factors in osteoblasts may help to make to bone marrow microenvironment a more hostile environment for AML cells and aid in better malignancy eradication. Support or Funding Information This study was supported by NIH R21 CA194217 (to K.E.H.). R.M.S. is supported by the Mayo Clinic Medical Scientist Training Program Robert L. Howell Physician‐Scientist Scholarship.

AbstractBackgroundSkeletal development requires precise extrinsic and intrinsic signals to regulate processes that form and maintain bone and cartilage. Notch1 is a highly conserved signaling receptor that regulates cell fate decisions by controlling the duration of transcriptional bursts. Epigenetic molecular events reversibly modify DNA and histone tails by influencing the spatial organization of chromatin and can fine‐tune the outcome of a Notch1 transcriptional response. Histone deacetylase 1 and 2 (HDAC1 and HDAC2) are chromatin modifying enzymes that mediate osteoblast differentiation. While an HDAC1‐Notch interaction has been studied in vitro and in Drosophila, its role in mammalian skeletal development and disorders is unclear. Osteosclerosis is a bone disorder with an abnormal increase in the number of osteoblasts and excessive bone formation.MethodsHere, we tested whether Hdac1/2 contribute to the pathogenesis of osteosclerosis in a murine model of the disease owing to conditionally cre‐activated expression of the Notch1 intracellular domain in immature osteoblasts.ResultsImportantly, selective homozygous deletions of Hdac1/2 in osteoblasts partially alleviate osteosclerotic phenotypes (Col2.3kb‐Cre; TGRosaN1ICD/+; Hdac1flox/flox; Hdac2flox/flox) with a 40% decrease in bone volume and a 22% decrease in trabecular thickness in 4 weeks old when compared to male mice with heterozygous deletions of Hdac1/2 (Col2.3 kb‐Cre; TGRosaN1ICD/+; Hdac1flox/+; Hdac2flox/+). Osteoblast‐specific deletion of Hdac1/2 in male and female mice results in no overt bone phenotype in the absence of the Notch1 gain‐of‐function (GOF) allele.ConclusionsThese results provide evidence that Hdac1/2 contribute to Notch1 pathogenic signaling in the mammalian skeleton. Our study on epigenetic regulation of Notch1 GOF‐induced osteosclerosis may facilitate further mechanistic studies of skeletal birth defects caused by Notch‐related GOF mutations in human patients, such as Adams‐Oliver disease, congenital heart disease, and lateral meningocele syndrome.