Production and Functional Characterization of Murine Osteoclasts Differentiated from ER-Hoxb8-Immortalized Myeloid Progenitor Cells

Moesin activation controls bone resorption and tunneling nanotube-dependent osteoclast fusion

Osteoclasts are multinucleated cells unique in their ability to resorb bone. Osteoclastogenesis involves several steps of actin-driven rearrangements that participate not only in the cell-cell fusion process, but also in the formation of the sealing zone, the adhesive structure determining the resorption area. Despite the importance of these actin cytoskeleton-based processes, their precise mechanisms of regulation are still poorly characterized. Here, we found that moesin, a member of the Ezrin/Radixin/Moesin (ERM) protein family, is activated during osteoclast maturation and plays an instrumental role for both osteoclast fusion and function. In mouse and human osteoclast precursors, moesin is negatively regulated to potentiate their ability to fuse and degrade bone. Accordingly, we demonstrated that moesin depletion decreases membrane-to-cortex attachment and enhances formation of tunneling nanotubes (TNTs), F-actin-containing intercellular bridges that we revealed to trigger osteoclast fusion. In addition, via a β3-integrin/RhoA/SLK pathway and independently of its role in fusion, moesin regulates the number and organization of sealing zones in mature osteoclast, and thus participates in the control of bone resorption. Supporting these findings, we found that moesin-deficient mice are osteopenic with a reduced density of trabecular bones and increased osteoclast abundance and activity. These findings provide a better understanding of the regulation of osteoclast biology, and open new opportunities to specifically target osteoclast activity in bone disease therapy.

Advancing Bone-Targeted Drug Delivery: Leveraging Biological Factors and Nanoparticle Designs to Improve Therapeutic Efficacy

Designing targeted drug delivery systems to effectively treat bone diseases ranging from osteoporosis to nonunion bone defects remains a significant challenge. Previously, nanoparticles (NPs) self-assembled from diblock copolymers of poly(styrene-alt-maleic anhydride)-b-poly(styrene) (PSMA-b-PS) delivering a Wnt agonist were shown to effectively target bone and improve healing via the introduction of a peptide with high affinity to tartrate-resistant acid phosphatase (TRAP), an enzyme deposited by the osteoclasts during bone remodeling. Despite these promising results, the underlying biological factors governing targeting and subsequent drug delivery system (DDS) design parameters have not been examined to enable the rational design to improve bone selectivity. Therefore, this work investigated the effect of target ligand density, the treatment window after injury, specificity of TRAP binding peptide (TBP), the extent of TRAP deposition, and underlying genetic factors (e.g., mouse strain differences) on TBP-NP targeting. Data based on in vitro binding studies and in vivo biodistribution analyses using a murine femoral fracture model suggest that TBP-NP-TRAP interactions and TBP-NP bone accumulation were ligand-density-dependent; in vitro, TRAP affinity was correlated with ligand density up to the maximum of 200,000 TBP ligands/NP, while NPs with 80,000 TBP ligands showed 2-fold increase in fracture accumulation at day 21 post injury compared with that of untargeted or scrambled controls. While fracture accumulation exhibited similar trends when injected at day 3 compared to that at day 21 postfracture, there were no significant differences observed between TBP-functionalized and control NPs, possibly due to saturation of TRAP by NPs at day 3. Leveraging a calcium-depletion diet, TRAP deposition and TBP-NP bone accumulation were positively correlated, confirming that TRAP-TBP binding leads to TBP-NP bone accumulation in vivo. Furthermore, TBP-NP exhibited similar bone accumulation in both C57BL/6 and BALB/c mouse strains versus control NPs, suggesting the broad applicability of TBP-NP regardless of the underlying genetic differences. These studies provide insight into TBP-NP design, mechanism, and therapeutic windows, which inform NP design and treatment strategies for fractures and other bone-associated diseases that leverage TRAP, such as marrow-related hematologic diseases.

Characterization and CRISPR/Cas9-mediated genetic manipulation of neutrophils derived from Hoxb8-ER-immortalized myeloid progenitors

Neutrophils represent a first line of defense against a wide variety of microbial pathogens. Transduction with an estrogen receptor-Hoxb8 transcription factor fusion construct conditionally immortalizes myeloid progenitor cells (NeutPro) capable of differentiation into neutrophils. This system has been very useful for generating large numbers of murine neutrophils for in vitro and in vivo studies. However, some questions remain as to how closely neutrophils derived from these immortalized progenitors reflect primary neutrophils. Here we describe our experience with NeutPro-derived neutrophils as it relates to our studies of Yersinia pestis pathogenesis. NeutPro neutrophils have circular or multilobed nuclei, similar to primary bone marrow neutrophils. Differentiation of neutrophils from NeutPro cells leads to increased expression of CD11b, GR1, CD62L, and Ly6G. However, the NeutPro neutrophils expressed lower levels of Ly6G than bone marrow neutrophils. NeutPro neutrophils produced reactive oxygen species at slightly lower levels than bone marrow neutrophils, and the 2 cell types phagocytosed and killed Y. pestis in vitro to a similar degree. To further demonstrate their utility, we used a nonviral method for nuclear delivery of CRISPR/Cas9 guide RNA complexes to delete genes of interest in NeutPro cells. In summary, we have found these cells to be morphologically and functionally equivalent to primary neutrophils and useful for in vitro assays related to studies of bacterial pathogenesis.

Hox-driven conditional immortalization of myeloid and lymphoid progenitors: Uses, advantages, and future potential

Those who study macrophage biology struggle with the decision whether to utilize primary macrophages derived directly from mice or opt for the convenience and genetic tractability of immortalized macrophage-like cell lines in in vitro studies. Particularly when it comes to studying phagocytosis and phagosomal maturation-a signature cellular process of the macrophage-many commonly used cell lines are not representative of what occurs in primary macrophages. A system developed by Mark Kamps' group, that utilizes conditionally constitutive activity of Hox transcription factors (Hoxb8 and Hoxa9) to immortalize differentiation-competent myeloid cell progenitors of mice, offers an alternative to the macrophage/macrophage-like dichotomy. In this resource, we will review the use of Hoxb8 and Hoxa9 as hematopoietic regulators to conditionally immortalize murine hematopoietic progenitor cells which retain their ability to differentiate into many functional immune cell types including macrophages, neutrophils, basophils, osteoclasts, eosinophils, dendritic cells, as well as limited potential for the generation of lymphocytes. We further demonstrate that the use of macrophages derived from Hoxb8/Hoxa9 immortalized progenitors and their similarities to bone marrow-derived macrophages. To supplement the existing data, mass spectrometry-based proteomics, flow cytometry, cytology, and in vitro phagosomal assays were conducted on macrophages derived from Hoxb8 immortalized progenitors and compared to bone marrow-derived macrophages and the macrophage-like cell line J774. We additionally propose the use of a standardized nomenclature to describe cells derived from the Hoxb8/Hoxa9 system in anticipation of their expanded use in the study of leukocyte cell biology.

A Practical Procedure for the In Vitro Generation of Human Osteoclasts and Their Characterization

Osteoclasts are multinucleated cells derived from the hematopoietic monocyte/macrophage lineage that possess the unique capacity to resorb bone. Due to the crucial role of osteoclasts in maintaining bone homeostasis and pathologies, this cell type is pivotal in multiple research areas dedicated to bone physiology in health and disease. Although numerous methods for generation of human osteoclasts are already available, those rely either on cell labeling-based purification or an intermediate adhesion step after which cells are directly differentiated toward osteoclasts. While the former requires additional reagents and equipment, the latter harbors the risk of variable osteoclast formation due to varying numbers of osteoclast precursors available for different donors. In this study, we report a facile and reliable three-step method for the generation of human osteoclasts from blood-derived precursor cells. Monocytes were obtained after adhering peripheral blood-derived mononuclear cells to plastic substrates followed by macrophage induction and proliferation resulting in a homogeneous population of osteoclast precursors. Finally, macrophages were seeded into suitable culture vessels and differentiated toward osteoclasts. Osteoclastogenesis was monitored longitudinally using nondestructive techniques, while the functionality of mature osteoclasts was confirmed after 14 days of culture by analysis of functional (e.g., elevated tartrate-resistant acid phosphatase [TRAP]-activity, resorption) and morphological (e.g., presence of TRAP, actin ring, and integrin β₃) characteristics. Furthermore, we propose to use combinatory staining of three morphological osteoclast markers, rather than previously reported staining of a single or maximal two markers, to clearly distinguish osteoclasts from undifferentiated mononuclear cells. Impact statement Research related to bone biology requires a standardized and reliable method for in vitro generation of human osteoclasts. We here describe such a procedure which avoids shortcomings of previously published protocols. Further, we report on nondestructive methods to qualitatively and quantitatively monitor osteoclastogenesis longitudinally, and on analysis of osteoclast generation and functionality after 14 days. Specifically, we recommend assessment of morphological human osteoclast characteristics using combinatory staining of three markers to confirm successful osteoclast generation.

Lysosome repositioning as an autophagy escape mechanism by Mycobacterium tuberculosis Beijing strain

Induction of host cell autophagy by starvation was shown to enhance lysosomal delivery to mycobacterial phagosomes, resulting in the restriction of Mycobacterium tuberculosis reference strain H37Rv. Our previous study showed that strains belonging to M. tuberculosis Beijing genotype resisted starvation-induced autophagic elimination but the factors involved remained unclear. Here, we conducted RNA-Seq of macrophages infected with the autophagy-resistant Beijing strain (BJN) compared to macrophages infected with H37Rv upon autophagy induction by starvation. Results identified several genes uniquely upregulated in BJN-infected macrophages but not in H37Rv-infected cells, including those encoding Kxd1 and Plekhm2, which function in lysosome positioning towards the cell periphery. Unlike H37Rv, BJN suppressed enhanced lysosome positioning towards the perinuclear region and lysosomal delivery to its phagosome upon autophagy induction by starvation, while depletion of Kxd1 and Plekhm2 reverted such effects, resulting in restriction of BJN intracellular survival upon autophagy induction by starvation. Taken together, these data indicated that Kxd1 and Plekhm2 are important for the BJN strain to suppress lysosome positioning towards the perinuclear region and lysosomal delivery into its phagosome during autophagy induction by starvation to evade starvation-induced autophagic restriction.

Production of Murine Macrophages from Hoxb8-Immortalized Myeloblasts: Utility and Use in the Context of Salmonella Infection

Salmonella enterica is a Gram-negative intracellular pathogen that causes a range of life-threatening diseases in humans and animals worldwide. In a systemic infection, the ability of Salmonella to survive/replicate in macrophages, particularly in the liver and spleen, is crucial for virulence. Transformed macrophage cell lines and primary macrophages prepared from mouse bone marrow are commonly used models for the study of Salmonella infection. However, these models raise technical or ethical issues that highlight the need for alternative methods. This chapter describes a technique for immortalizing early hematopoietic progenitor cells derived from wild-type or transgenic mice and using them to produce macrophages. It validates, through a specific example, the interest of this cellular approach for the study of Salmonella infection.

In Vivo Functions of Mouse Neutrophils Derived from HoxB8-Transduced Conditionally Immortalized Myeloid Progenitors

Although neutrophils play important roles in immunity and inflammation, their analysis is strongly hindered by their short-lived and terminally differentiated nature. Prior studies reported conditional immortalization of myeloid progenitors using retroviral expression of an estrogen-dependent fusion protein of the HoxB8 transcription factor. This approach allowed the long-term culture of mouse myeloid progenitors (HoxB8 progenitors) in estrogen-containing media, followed by differentiation toward neutrophils upon estrogen withdrawal. Although several reports confirmed the in vitro functional responsiveness of the resulting differentiated cells (HoxB8 neutrophils), little is known about their capacity to perform in vivo neutrophil functions. We have addressed this issue by an in vivo transplantation approach. In vitro-generated HoxB8 neutrophils showed a neutrophil-like phenotype and were able to perform conventional neutrophil functions, like respiratory burst, chemotaxis, and phagocytosis. The i.v. injection of HoxB8 progenitors into lethally irradiated recipients resulted in the appearance of circulating donor-derived HoxB8 neutrophils. In vivo-differentiated HoxB8 neutrophils were able to migrate to the inflamed peritoneum and to phagocytose heat-killed Candida particles. The reverse passive Arthus reaction could be induced in HoxB8 chimeras but not in irradiated, nontransplanted control animals. Repeated injection of HoxB8 progenitors also allowed us to maintain stable circulating HoxB8 neutrophil counts for several days. Injection of arthritogenic K/B×N serum triggered robust arthritis in HoxB8 chimeras, but not in irradiated, nontransplanted control mice. Taken together, our results indicate that HoxB8 progenitor-derived neutrophils are capable of performing various in vivo neutrophil functions, providing a framework for using the HoxB8 system for the in vivo analysis of neutrophil function.

Authentication of Primary Murine Cell Lines by a Microfluidics-Based Lab-On-Chip System

The reliable authentication of cell lines is a prerequisite for the reproducibility and replicability of experiments. A common method of cell line authentication is the fragment length analysis (FLA) of short-tandem repeats (STR) by capillary electrophoresis. However, this technique is not always accessible and is often costly. Using a microfluidic electrophoresis system, we analyzed the quality and integrity of different murine cell lines by STR profiling. As a proof of concept, we isolated and immortalized hematopoietic progenitor cells (HPC) of various genotypes through retroviral transduction of the fusion of the estrogen receptor hormone-binding domain with the coding sequence of HoxB8. Cell lines were maintained in the HPC state with Flt3 ligand (FL) and estrogen treatment and could be characterized upon differentiation. In a validation cohort, we applied this technique on primary mutant Kras-driven pancreatic cancer cell lines, which again allowed for clear discrimination. In summary, our study provides evidence that FLA of STR-amplicons by microfluidic electrophoresis allows for stringent quality control and the tracking of cross-contaminations in both genetically stable HPC lines and cancer cell lines, making it a simple and cost-efficient alternative to traditional capillary electrophoresis.

Genetic engineering of Hoxb8-immortalized hematopoietic progenitors - a potent tool to study macrophage tissue migration

Tumor-associated macrophages (TAMs) are detrimental in most cancers. Controlling their recruitment is thus potentially therapeutic. We previously found that TAMs perform protease-dependent mesenchymal migration in cancer, while macrophages perform amoeboid migration in other tissues. Inhibition of mesenchymal migration correlates with decreased TAM infiltration and tumor growth, providing rationale for a new cancer immunotherapy specifically targeting TAM motility. To identify new effectors of mesenchymal migration, we produced ER-Hoxb8-immortalized hematopoietic progenitors (cells with estrogen receptor-regulated Hoxb8 expression), which show unlimited proliferative ability in the presence of estrogen. The functionality of macrophages differentiated from ER-Hoxb8 progenitors was compared to bone marrow-derived macrophages (BMDMs). They polarized into M1- and M2-orientated macrophages, generated reactive oxygen species (ROS), ingested particles, formed podosomes, degraded the extracellular matrix, adopted amoeboid and mesenchymal migration in 3D, and infiltrated tumor explants ex vivo using mesenchymal migration. We also used the CRISPR/Cas9 system to disrupt gene expression of a known effector of mesenchymal migration, WASP (also known as WAS), to provide a proof of concept. We observed impaired podosome formation and mesenchymal migration capacity, thus recapitulating the phenotype of BMDM isolated from Wasp-knockout mice. Thus, we validate the use of ER-Hoxb8-immortalized macrophages as a potent tool to investigate macrophage functionalities.

A role for G protein-coupled receptor 137b in bone remodeling in mouse and zebrafish

G protein-coupled receptor 137b (GPR137b) is an orphan seven-pass transmembrane receptor of unknown function. In mouse, Gpr137b is highly expressed in osteoclasts in vivo and is upregulated during in vitro differentiation. To elucidate the role that GPR137b plays in osteoclasts, we tested the effect of GPR137b deficiency on osteoclast maturation and resorbing activity. We used CRISPR/Cas9 gene editing in mouse-derived ER-Hoxb8 immortalized myeloid progenitors to generate GPR137b-deficient osteoclast precursors. Decreasing Gpr137b in these precursors led to increased osteoclast differentiation and bone resorption activity. To explore the role of GPR137b during skeletal development, we generated zebrafish deficient for the ortholog gpr137ba. Gpr137ba-deficient zebrafish are viable and fertile and do not display overt morphological defects as adults. However, analysis of osteoclast function in gpr137ba^-/- mutants demonstrated increased bone resorption. Micro-computed tomography evaluation of vertebral bone mass and morphology demonstrated that gpr137ba-deficiency altered the angle of the neural arch, a skeletal site with high osteoclast activity. Vital staining of gpr137ba^-/- fish with calcein and alizarin red indicated that bone formation in the mutants is also increased, suggesting high bone turnover. These results identify GPR137b as a conserved negative regulator of osteoclast activity essential for normal resorption and patterning of the skeleton. Further, these data suggest that coordination of osteoclast and osteoblast activity is a conserved process among vertebrates and may have similar regulation.

p62/sequestosome 1 deficiency accelerates osteoclastogenesis in vitro and leads to Paget's disease-like bone phenotypes in mice

The sequestosome 1 gene encodes the p62 protein and is the major genetic risk factor associated with Paget's disease of bone. In 2004, p62 was reported to up-regulate osteoclast differentiation by activating the transcription factors Nfatc1 and NF-κB. Here, we characterized the osteoclastogenic potential of murine p62^-/--derived cells compared with WT cells. Our data confirmed previous findings indicating that p62 is induced during murine osteoclast differentiation. Surprisingly, an indispensable role for p62 in in vitro osteoclast differentiation was not reproducible because p62-deficient osteoclasts exhibited robust activation of Nfatc1, NF-κB, and osteoclast marker enzymes. Thus, we concluded that in vitro osteoclast differentiation is not negatively influenced by knocking out p62. On the contrary, our results revealed that p62 deficiency accelerates osteoclastogenesis. Differentiation potential, multinucleation status, and soluble receptor activator of NF-κB ligand (sRANKL) sensitivity were significantly elevated in p62-deficient, murine bone marrow-derived stem cells. Moreover, femur ultrastructures visualized by micro-computed tomography revealed pronounced accumulation of adipocytes and trabecular bone material in distal femora of obese p62^-/- mice. Increased tartrate-resistant acid phosphatase activity, along with increased trabecular bone and accumulation of adipocytes, was confirmed in both paraffin-embedded decalcified and methyl methacrylate-embedded nondecalcified bones from p62^-/- mice. Of note, Paget's disease-like osteolytic lesions and increased levels of the bone turnover markers CTX-I and PINP were also observed in the p62^-/- mice. Our results indicate that p62 predominantly suppresses murine in vitro osteoclast differentiation and highlight previously undetected Paget's disease-like phenotypes in p62^-/- mice in vivo.