Hair follicle-resident progenitor cells are a major cellular contributor to heterotopic subcutaneous ossifications in a mouse model of Albright hereditary osteodystrophy.

Heterotopic ossifications (HOs) are the pathologic process by which bone inappropriately forms outside of the skeletal system. Despite HOs being a persistent clinical problem in the general population, there are no definitive strategies for their prevention and treatment due to a limited understanding of the cellular and molecular mechanisms contributing to lesion development. One disease in which the development of heterotopic subcutaneous ossifications (SCOs) leads to morbidity is Albright hereditary osteodystrophy (AHO). AHO is caused by heterozygous inactivation of GNAS, the gene that encodes the α-stimulatory subunit (Gαs) of G proteins. Previously, we had shown using our laboratory's AHO mouse model that SCOs develop around hair follicles (HFs). Here we show that SCO formation occurs due to inappropriate expansion and differentiation of HF-resident stem cells into osteoblasts. We also show in AHO patients and mice that Secreted Frizzled Related Protein 2 (SFRP2) expression is upregulated in regions of SCO formation and that elimination of Sfrp2 in male AHO mice exacerbates SCO development. These studies provide key insights into the cellular and molecular mechanisms contributing to SCO development and have implications for potential therapeutic modalities not only for AHO patients but also for patients suffering from HOs with other etiologies.

Endothelial to mesenchymal Notch signaling regulates skeletal repair.

We present a transcriptomic analysis that provides a better understanding of regulatory mechanisms within the healthy and injured periosteum. The focus of this work is on characterizing early events controlling bone healing during formation of periosteal callus on day 3 after fracture. Building on our previous findings showing that induced Notch1 signaling in osteoprogenitors leads to better healing, we compared samples in which the Notch 1 intracellular domain is overexpressed by periosteal stem/progenitor cells, with control intact and fractured periosteum. Molecular mechanisms and changes in skeletal stem/progenitor cells (SSPCs) and other cell populations within the callus, including hematopoietic lineages, were determined. Notably, Notch ligands were differentially expressed in endothelial and mesenchymal populations, with Dll4 restricted to endothelial cells, whereas Jag1 was expressed by mesenchymal populations. Targeted deletion of Dll4 in endothelial cells using Cdh5CreER resulted in negative effects on early fracture healing, while deletion in SSPCs using α-smooth muscle actin-CreER did not impact bone healing. Translating these observations into a clinically relevant model of bone healing revealed the beneficial effects of delivering Notch ligands alongside the osteogenic inducer, BMP2. These findings provide insights into the regulatory mechanisms within the healthy and injured periosteum, paving the way for novel translational approaches to bone healing.

Hematopoietic and stromal DMP1-Cre labeled cells form a unique niche in the bone marrow.

Skeletogenesis and hematopoiesis are interdependent. Niches form between cells of both lineages where microenvironmental cues support specific lineage commitment. Because of the complex topography of bone marrow (BM), the identity and function of cells within specialized niches has not been fully elucidated. Dentin Matrix Protein 1 (DMP1)-Cre mice have been utilized in bone studies as mature osteoblasts and osteocytes express DMP1. DMP1 has been identified in CXCL12+ cells and an undefined CD45+ population. We crossed DMP1-Cre with Ai9 reporter mice and analyzed the tdTomato+ (tdT+) population in BM and secondary hematopoietic organs. CD45+tdT+ express myeloid markers including CD11b and are established early in ontogeny. CD45+tdT+ cells phagocytose, respond to LPS and are radioresistant. Depletion of macrophages caused a significant decrease in tdT+CD11b+ myeloid populations. A subset of CD45+tdT+ cells may be erythroid island macrophages (EIM) which are depleted after G-CSF treatment. tdT+CXCL12+ cells are in direct contact with F4/80 macrophages, express RANKL and form a niche with B220+ B cells. A population of resident cells within the thymus are tdT+ and express myeloid markers and RANKL. In conclusion, in addition to targeting osteoblast/osteocytes, DMP1-Cre labels unique cell populations of macrophage and stromal cells within BM and thymus niches and expresses key microenvironmental factors.

Lineage Tracing of RGS5-CreER-Labeled Cells in Long Bones During Homeostasis and Injury.

Regulator of G protein signaling 5 (RGS5) is a GTPase activator for heterotrimeric G-protein α-subunits, shown to be a marker of pericytes. Bone marrow stromal cell population (BMSCs) is heterogeneous. Populations of mesenchymal progenitors, cells supportive of hematopoiesis, and stromal cells regulating bone remodeling have been recently identified. Periosteal and bone marrow mesenchymal stem cells (MSCs) are participating in fracture healing, but it is difficult to distinguish the source of cells within the callus. Considering that perivascular cells exert osteoprogenitor potential, we generated an RGS5 transgenic mouse model (Rgs5-CreER) which when crossed with Ai9 reporter animals (Rgs5/Tomato), is suitable for lineage tracing during growth and post-injury. Flow cytometry analysis and histology confirmed the presence of Rgs5/Tomato+ cells within CD31+ endothelial, CD45+ hematopoietic, and CD31-CD45- mesenchymal/perivascular cells. A tamoxifen chase showed expansion of Rgs5/Tomato+ cells expressing osterix within the trabeculae positioned between mineralized matrix and vasculature. Long-term chase showed proportion of Rgs5/Tomato+ cells contributes to mature osteoblasts expressing osteocalcin. Following femoral fracture, Rgs5/Tomato+ cells are observed around newly formed bone within the BM cavity and expressed osterix and osteocalcin, while contribution within periosteum was low and limited to fibroblastic callus with very few positive chondrocytes. In addition, BM injury model confirmed that RGS5-Cre labels population of BMSCs expands during injury and participates in osteogenesis. Under homeostatic conditions, lineage-traced RGS5 cells within the trabecular area demonstrate osteoprogenitor capacity that in an injury model contributes to new bone formation primarily within the BM niche.

Inhibition of CGRP signaling impairs fracture healing in mice.

Calcitonin gene-related peptide (CGRP) is a neuropeptide produced by sensory nerves and functions as a pain sensor. It acts by binding to the calcitonin-like receptor (CLR, protein; Calcrl, gene). CGRP inhibition has been recently introduced as therapeutic treatment of migraine-associated pain. Previous studies have shown that CGRP stimulates bone formation. The aim of our study is to determine whether the inhibition of CGRP signaling negatively impacted fracture healing. Using α-smooth muscle actin (αSMA) Cre animals crossed with Ai9 reporter mice, we showed that CGRP-expressing nerves are near αSMA + cells in the periosteum. In vitro experiments revealed that periosteal cells express Calcrl and receptor activity modifying protein 1; and CGRP stimulation increased periosteal cell proliferation. Using a tamoxifen-inducible model αSMACre/CLRfl/fl , we targeted the deletion of CLR to periosteal progenitor cells and examined fracture healing. Microcomputed tomography of fractured femurs showed a reduction in bone mass in αSMACre+/CLRfl/fl female mice relative to controls and callus volume in males. Pharmacological CGRP-CLR inhibition was achieved by subcutaneous delivery of customized pellets with small molecule inhibitor olcegepant (BIBN-4096) at a dose of 10 µg/day. BIBN-4096-treated C57BL/6J mice had a higher latency toward thermal nociception than placebo-treated mice, indicating impaired sensory function through CGRP inhibition. CGRP inhibition also resulted in reduced callus volume, bone mass, and bone strength compared to placebo controls. These results indicate that inhibiting CGRP by deleting CLR or by using BIBN-4096, contributes to delayed bone healing.

Novel population of human monocyte and osteoclast progenitors from pluripotent stem cells and peripheral blood.

Osteoclasts are multinuclear cells of monocytic lineage, with the ability to resorb bone. Studies in mouse have identified bone marrow clonal progenitors able to generate mature osteoclast cells (OCs) in vitro and in vivo. These osteoclast progenitors (OCPs) can also generate macrophages and dendritic cells. Interestingly, cells with equivalent potential can be detected in periphery. In humans, cells with OCP activity have been identified in bone marrow and periphery; however, their characterization has not been as extensive. We have developed reproducible methods to derive, from human pluripotent stem cells, a population containing monocyte progenitors able to generate functional OCs. Within this population, we have identified cells with monocyte and osteoclast progenitor activity based on CD11b and CD14 expression. A population double positive for CD11b and CD14 contains cells with expected osteoclastic potential. However, the double negative (DN) population, containing most of the hematopoietic progenitor activity, also presents a very high osteoclastic potential. These progenitor cells can also be differentiated to macrophage and dendritic cells. Further dissection within the DN population identified cells bearing the phenotype CD15-CD115+ as the population with highest monocytic progenitor and osteoclastic potential. When similar methodology was used to identify OCPs from human peripheral blood, we confirmed a published OCP population with the phenotype CD11b+CD14+. In addition, we identified a second population (CD14-CD11bloCD115+) with high monocytic progenitor activity that was also able to form osteoclast like cells, similar to the 2 populations identified from pluripotent stem cells.

Perivascular osteoprogenitors are associated with transcortical channels of long bones.

Bone remodeling and regeneration are dependent on resident stem/progenitor cells with the ability to replenish mature osteoblasts and repair the skeleton. Using lineage tracing approaches, we identified a population of Dmp1+ cells that reside within cortical bone and are distinct from osteocytes. Our aims were to characterize this stromal population of transcortical perivascular cells (TPCs) in their resident niche and evaluate their osteogenic potential. To distinguish this population from osteoblasts/osteocytes, we crossed mice containing inducible DMP1CreERT2/Ai9 Tomato reporter (iDMP/T) with Col2.3GFP reporter (ColGFP), a marker of osteoblasts and osteocytes. We observed iDMP/T+;ColGFP- TPCs within cortical bone following tamoxifen injection. These cells were perivascular and located within transcortical channels. Ex vivo bone outgrowth cultures showed TPCs migrated out of the channels onto the plate and expressed stem cell markers such as Sca1, platelet derived growth factor receptor beta (PDGFRß), and leptin receptor. In a cortical bone transplantation model, TPCs migrate from their vascular niche within cortical bone and contribute to new osteoblast formation and bone tube closure. Treatment with intermittent parathyroid hormone increased TPC number and differentiation. TPCs were unable to differentiate into adipocytes in the presence of rosiglitazone in vitro or in vivo. Altogether, we have identified and characterized a novel stromal lineage-restricted osteoprogenitor that is associated with transcortical vessels of long bones. Functionally, we have demonstrated that this population can migrate out of cortical bone channels, expand, and differentiate into osteoblasts, therefore serving as a source of progenitors contributing to new bone formation.

Transcriptional regulation of IL-15 expression during hematopoiesis.

Dendritic cells (DCs) are the most commonly studied source of the cytokine IL-15. Using an IL-15 reporter transgenic mouse, we have recently shown previously unappreciated differences in the levels of IL-15 expressed by subsets of conventional DCs (CD8⁺ and CD8⁻). In this study, we show that IL-15 promoter activity was differentially regulated in subsets of hematopoietically derived cells with IL-15 expression largely limited to myeloid lineages. In contrast, mature cells of the lymphoid lineages expressed little to no IL-15 activity. Surprisingly, we discovered that hematopoietic stem cells (lineage⁻Sca-1⁺c-Kit⁺) expressed high levels of IL-15, suggesting that IL-15 expression was extinguished during lymphoid development. In the case of T cells, this downregulation was Notch-dependent and occurred in a stepwise pattern coincident with increasing maturation and commitment to a T cell fate. Finally, we further demonstrate that IL-15 expression was also controlled throughout DC development, with key regulatory activity of IL-15 production occurring at the pre-DC branch point, leading to the generation of both IL-15⁺CD8⁺ and IL-15(⁻/low)CD8⁻ DC subsets. Thus, IL-15 expression is coordinated with cellular fate in myeloid versus lymphoid immune cells.

A novel population of cells expressing both hematopoietic and mesenchymal markers is present in the normal adult bone marrow and is augmented in a murine model of marrow fibrosis.

Bone marrow (BM) fibrosis is a feature of severe hyperparathyroidism. Consistent with this observation, mice expressing constitutively active parathyroid hormone (PTH)/PTH-related peptide receptors (PPR) in osteoblasts (PPR*Tg) display BM fibrosis. To obtain insight into the nature of BM fibrosis in such a model, a double-mutant mouse expressing constitutively active PPR and green fluorescent protein (GFP) under the control of the type I collagen promoter (PPR*Tg/GFP) was generated. Confocal microscopy and flow cytometry revealed the presence of a cell population expressing GFP (GFP(+)) that was also positive for the hematopoietic marker CD45 in the BM of both PPR*Tg/GFP and control animals. This cell population was expanded in PPR*Tg/GFP. The existence of cells expressing both type I collagen and CD45 in the adult BM was confirmed by IHC and fluorescence-activated cell sorting. An analysis of total RNA extracted from sorted GFP(+)CD45(+) cells showed that these cells produced type I collagen and PTH/PTH-related peptide receptor and receptor activator for NF-κB mRNAs, further supporting their features of being both mesenchymal and hematopoietic lineages. Similar cells, known as fibrocytes, are also present in pathological fibroses. Our findings, thus, indicate that the BM is a permissive microenvironment for the differentiation of fibrocyte-like cells and raise the possibility that these cells could contribute to the pathogenesis of BM fibrosis.

cPLA2 is protective against COX inhibitor-induced intestinal damage.

Cytosolic phospholipase A(2) (cPLA(2)) is the rate-limiting enzyme responsible for the generation of prostaglandins (PGs), which are bioactive lipids that play critical roles in maintaining gastrointestinal (GI) homeostasis. There has been a long-standing association between administration of cyclooxygenase (COX) inhibitors and GI toxicity. GI injury is thought to be induced by suppressed production of GI-protective PGs as well as direct injury to enterocytes. The present study sought to determine how pan-suppression of PG production via a genetic deletion of cPLA(2) impacts the susceptibility to COX inhibitor-induced GI injury. A panel of COX inhibitors including celecoxib, rofecoxib, sulindac, and aspirin were administered via diet to cPLA(2)(-/-) and cPLA(2)(+/+) littermates. Administration of celecoxib, rofecoxib, and sulindac, but not aspirin, resulted in acute lethality (within 2 weeks) in cPLA(2)(-/-) mice, but not in wild-type littermates. Histomorphological analysis revealed severe GI damage following celecoxib exposure associated with acute bacteremia and sepsis. Intestinal PG levels were reduced equivalently in both genotypes following celecoxib exposure, indicating that PG production was not likely responsible for the differential sensitivity. Gene expression profiling in the small intestines of mice identified drug-related changes among a panel of genes including those involved in mitochondrial function in cPLA(2)(-/-) mice. Further analysis of enterocytic mitochondria showed abnormal morphology as well as impaired ATP production in the intestines from celecoxib-exposed cPLA(2)(-/-) mice. Our data demonstrate that cPLA(2) appears to be an important component in conferring protection against COX inhibitor-induced enteropathy, which may be mediated through affects on enterocytic mitochondria.