Osteoclast-derived IGF1 is required for pagetic lesion formation in vivo

Tissue macrophages: origin, heterogenity, biological functions, diseases and therapeutic targets

Macrophages are immune cells belonging to the mononuclear phagocyte system. They play crucial roles in immune defense, surveillance, and homeostasis. This review systematically discusses the types of hematopoietic progenitors that give rise to macrophages, including primitive hematopoietic progenitors, erythro-myeloid progenitors, and hematopoietic stem cells. These progenitors have distinct genetic backgrounds and developmental processes. Accordingly, macrophages exhibit complex and diverse functions in the body, including phagocytosis and clearance of cellular debris, antigen presentation, and immune response, regulation of inflammation and cytokine production, tissue remodeling and repair, and multi-level regulatory signaling pathways/crosstalk involved in homeostasis and physiology. Besides, tumor-associated macrophages are a key component of the TME, exhibiting both anti-tumor and pro-tumor properties. Furthermore, the functional status of macrophages is closely linked to the development of various diseases, including cancer, autoimmune disorders, cardiovascular disease, neurodegenerative diseases, metabolic conditions, and trauma. Targeting macrophages has emerged as a promising therapeutic strategy in these contexts. Clinical trials of macrophage-based targeted drugs, macrophage-based immunotherapies, and nanoparticle-based therapy were comprehensively summarized. Potential challenges and future directions in targeting macrophages have also been discussed. Overall, our review highlights the significance of this versatile immune cell in human health and disease, which is expected to inform future research and clinical practice.

Synchronized long-term delivery of growth hormone and insulin-like growth factor 1 through poly (lactic-co-glycolic acid) nanoparticles on polycaprolactone scaffolds for enhanced osteochondral regeneration

The regeneration of osteochondral defects is challenging due to the complex structure of the osteochondral unit. This study aimed to develop a biomimetic scaffold by loading growth hormone (GH) and insulin-like growth factor-1 (IGF-1) into poly (lactic-co-glycolic acid) (PLGA) nanoparticles and incorporating them into polycaprolactone (PCL) scaffolds to promote synchronized osteochondral regeneration. The nanoparticles were successfully immobilized onto PCL scaffolds pre-modified with polydopamine (PDA) to enhance cell adhesion and proliferation. The scaffolds exhibited a sustained release of GH and IGF-1 over 30 days. In vitro studies using rabbit adipose-derived stem cells (ADSCs) showed that the GH/IGF-1 nanoparticle-loaded scaffolds (PCL/PDA/M-PLGA) significantly promoted cell proliferation, chondrogenic differentiation, and osteogenic differentiation compared to control PCL/PDA scaffolds. In vivo experiments using a rabbit osteochondral defect model revealed that the PCL/PDA/M-PLGA scaffolds facilitated superior osteochondral regeneration, evidenced by increased subchondral bone formation and cartilage matrix deposition. Overall, this study demonstrates the potential of GH/IGF-1 nanoparticle-loaded PCL scaffolds for synchronized osteochondral regeneration and provides a promising strategy for treating osteochondral defects.

IGF signaling pathway in bone and cartilage development, homeostasis, and disease

The skeleton plays a fundamental role in the maintenance of organ function and daily activities. The insulin-like growth factor (IGF) family is a group of polypeptide substances with a pronounced role in osteoblast differentiation, bone development, and metabolism. Disturbance of the IGFs and the IGF signaling pathway is inextricably linked with assorted developmental defects, growth irregularities, and jeopardized skeletal structure. Recent findings have illustrated the significance of the action of the IGF signaling pathway via growth factors and receptors and its interactions with dissimilar signaling pathways (Wnt/β-catenin, BMP, TGF-β, and Hh/PTH signaling pathways) in promoting the growth, survival, and differentiation of osteoblasts. IGF signaling also exhibits profound influences on cartilage and bone development and skeletal homeostasis via versatile cell-cell interactions in an autocrine, paracrine, and endocrine manner systemically and locally. Our review summarizes the role and regulatory function as well as a potentially integrated gene network of the IGF signaling pathway with other signaling pathways in bone and cartilage development and skeletal homeostasis, which in turn provides an enlightening insight into visualizing bright molecular targets to be eligible for designing effective drugs to handle bone diseases and maladies, such as osteoporosis, osteoarthritis, and dwarfism.

The role of macrophage plasticity in neurodegenerative diseases

Tissue-resident macrophages and recruited macrophages play pivotal roles in innate immunity and the maintenance of brain homeostasis. Investigating the involvement of these macrophage populations in eliciting pathological changes associated with neurodegenerative diseases has been a focal point of research. Dysregulated states of macrophages can compromise clearance mechanisms for pathological proteins such as amyloid-β (Aβ) in Alzheimer's disease (AD) and TDP-43 in Amyotrophic lateral sclerosis (ALS). Additionally, recent evidence suggests that abnormalities in the peripheral clearance of pathological proteins are implicated in the pathogenesis and progression of neurodegenerative diseases. Furthermore, numerous genome-wide association studies have linked genetic risk factors, which alter the functionality of various immune cells, to the accumulation of pathological proteins. This review aims to unravel the intricacies of macrophage biology in both homeostatic conditions and neurodegenerative disorders. To this end, we initially provide an overview of the modifications in receptor and gene expression observed in diverse macrophage subsets throughout development. Subsequently, we outlined the roles of resident macrophages and recruited macrophages in neurodegenerative diseases and the progress of targeted therapy. Finally, we describe the latest advances in macrophage imaging methods and measurement of inflammation, which may provide information and related treatment strategies that hold promise for informing the design of future investigations and therapeutic interventions.

Regulation of bone homeostasis: signaling pathways and therapeutic targets

As a highly dynamic tissue, bone is continuously rebuilt throughout life. Both bone formation by osteoblasts and bone resorption by osteoclasts constitute bone reconstruction homeostasis. The equilibrium of bone homeostasis is governed by many complicated signaling pathways that weave together to form an intricate network. These pathways coordinate the meticulous processes of bone formation and resorption, ensuring the structural integrity and dynamic vitality of the skeletal system. Dysregulation of the bone homeostatic regulatory signaling network contributes to the development and progression of many skeletal diseases. Significantly, imbalanced bone homeostasis further disrupts the signaling network and triggers a cascade reaction that exacerbates disease progression and engenders a deleterious cycle. Here, we summarize the influence of signaling pathways on bone homeostasis, elucidating the interplay and crosstalk among them. Additionally, we review the mechanisms underpinning bone homeostatic imbalances across diverse disease landscapes, highlighting current and prospective therapeutic targets and clinical drugs. We hope that this review will contribute to a holistic understanding of the signaling pathways and molecular mechanisms sustaining bone homeostasis, which are promising to contribute to further research on bone homeostasis and shed light on the development of targeted drugs.

A Multifunctional Therapeutic Strategy Using P7C3 as A Countermeasure Against Bone Loss and Fragility in An Ovariectomized Rat Model of Postmenopausal Osteoporosis

By 2060, an estimated one in four Americans will be elderly. Consequently, the prevalence of osteoporosis and fragility fractures will also increase. Presently, no available intervention definitively prevents or manages osteoporosis. This study explores whether Pool 7 Compound 3 (P7C3) reduces progressive bone loss and fragility following the onset of ovariectomy (OVX)-induced osteoporosis. Results confirm OVX-induced weakened, osteoporotic bone together with a significant gain in adipogenic body weight. Treatment with P7C3 significantly reduced osteoclastic activity, bone marrow adiposity, whole-body weight gain, and preserved bone area, architecture, and mechanical strength. Analyses reveal significantly upregulated platelet derived growth factor-BB and leukemia inhibitory factor, with downregulation of interleukin-1 R6, and receptor activator of nuclear factor kappa-B (RANK). Together, proteomic data suggest the targeting of several key regulators of inflammation, bone, and adipose turnover, via transforming growth factor-beta/SMAD, and Wingless-related integration site/be-catenin signaling pathways. To the best of the knowledge, this is first evidence of an intervention that drives against bone loss via RANK. Metatranscriptomic analyses of the gut microbiota show P7C3 increased Porphyromonadaceae bacterium, Candidatus Melainabacteria, and Ruminococcaceae bacterium abundance, potentially contributing to the favorable inflammatory, and adipo-osteogenic metabolic regulation observed. The results reveal an undiscovered, and multifunctional therapeutic strategy to prevent the pathological progression of OVX-induced bone loss.

Osteocytes and Paget's Disease of Bone

PURPOSE OF REVIEW

To describe the contributions of osteocytes to the lesions in Paget's disease, which are characterized by locally overactive bone resorption and formation.

RECENT FINDINGS

Osteocytes, the most abundant cells in bone, are altered in Paget's disease lesions, displaying increased size, decreased canalicular length, incomplete differentiation, and less sclerostin expression compared to controls in both patients and mouse models. Pagetic lesions show increased senescent osteocytes that express RANK ligand, which drives osteoclastic bone resorption. Abnormal osteoclasts in Paget's disease secrete abundant IGF1, which enhances osteocyte senescence, contributing to lesion formation. Recent data suggest that osteocytes contribute to lesion formation in Paget's disease by responding to high local IGF1 released from abnormal osteoclasts. Here we describe the characteristics of osteocytes in Paget's disease and their role in bone lesion formation based on recent results with mouse models and supported by patient data.

Rehabilitation: Neurogenic Bone Loss after Spinal Cord Injury

Osteoporosis is a common skeletal disorder which can severely limit one's ability to complete daily tasks due to the increased risk of bone fractures, reducing quality of life. Spinal cord injury (SCI) can also result in osteoporosis and sarcopenia. Most individuals experience sarcopenia and osteoporosis due to advancing age; however, individuals with SCI experience more rapid and debilitating levels of muscle and bone loss due to neurogenic factors, musculoskeletal disuse, and cellular/molecular events. Thus, preserving and maintaining bone mass after SCI is crucial to decreasing the risk of fragility and fracture in vulnerable SCI populations. Recent studies have provided an improved understanding of the pathophysiology and risk factors related to musculoskeletal loss after SCI. Pharmacological and non-pharmacological therapies have also provided for the reduction in or elimination of neurogenic bone loss after SCI. This review article will discuss the pathophysiology and risk factors of muscle and bone loss after SCI, including the mechanisms that may lead to muscle and bone loss after SCI. This review will also focus on current and future pharmacological and non-pharmacological therapies for reducing or eliminating neurogenic bone loss following SCI.

Osteoclast-derived IGF1 induces RANKL production in osteocytes and contributes to pagetic lesion formation

We previously reported that measles virus nucleocapsid protein (MVNP) expression in osteoclasts (OCLs) of patients with Paget disease (PD) or targeted to the OCL lineage in MVNP-transgenic mice (MVNP mice) increases IGF1 production in osteoclasts (OCL-IGF1) and leads to development of PD OCLs and pagetic bone lesions (PDLs). Conditional deletion of Igf1 in OCLs of MVNP mice fully blocked development of PDLs. In this study, we examined whether osteocytes (OCys), key regulators of normal bone remodeling, contribute to PD. OCys in PDLs of patients and of MVNP mice expressed less sclerostin, and had increased RANKL expression compared with OCys in bones from WT mice or normal patients. To test whether increased OCL-IGF1 is sufficient to induce PDLs and PD phenotypes, we generated TRAP-Igf1 (T-Igf1) transgenic mice to determine whether increased IGF1 expression in the absence of MVNP in OCLs is sufficient to induce PDLs and pagetic OCLs. We found that T-Igf1 mice at 16 months of age developed PD OCLs, PDLs, and OCys, with decreased sclerostin and increased RANKL, similar to MVNP mice. Thus, pagetic phenotypes could be induced by OCLs expressing increased IGF1. OCL-IGF1 in turn increased RANKL production in OCys to induce PD OCLs and PDLs.

Physiology and diseases of tissue-resident macrophages

Embryo-derived tissue-resident macrophages are the first representatives of the haematopoietic lineage to emerge in metazoans. In mammals, resident macrophages originate from early yolk sac progenitors and are specified into tissue-specific subsets during organogenesis-establishing stable spatial and functional relationships with specialized tissue cells-and persist in adults. Resident macrophages are an integral part of tissues together with specialized cells: for instance, microglia reside with neurons in brain, osteoclasts reside with osteoblasts in bone, and fat-associated macrophages reside with white adipocytes in adipose tissue. This ancillary cell type, which is developmentally and functionally distinct from haematopoietic stem cell and monocyte-derived macrophages, senses and integrates local and systemic information to provide specialized tissue cells with the growth factors, nutrient recycling and waste removal that are critical for tissue growth, homeostasis and repair. Resident macrophages contribute to organogenesis, promote tissue regeneration following damage and contribute to tissue metabolism and defence against infectious disease. A correlate is that genetic or environment-driven resident macrophage dysfunction is a cause of degenerative, metabolic and possibly inflammatory and tumoural diseases. In this Review, we aim to provide a conceptual outline of our current understanding of macrophage physiology and its importance in human diseases, which may inform and serve the design of future studies.

Osteoclasts: Other functions

Osteoclasts are the only cells that can efficiently resorb bone. They do so by sealing themselves on to bone and removing the mineral and organic components. Osteoclasts are essential for bone homeostasis and are involved in the development of diseases associated with decreased bone mass, like osteoporosis, or abnormal bone turnover, like Paget's disease of bone. In addition, compromise of their development or resorbing machinery is pathogenic in multiple types of osteopetrosis. However, osteoclasts also have functions other than bone resorption. Like cells of the innate immune system, they are derived from myeloid precursors and retain multiple immune cell properties. In addition, there is now strong evidence that osteoclasts regulate osteoblasts through a process known as coupling, which coordinates rates of bone resorption and bone formation during bone remodeling. In this article we review the non-resorbing functions of osteoclasts and highlight their importance in health and disease.

Update on the pathogenesis and genetics of Paget’s disease of bone

Studies over the past two decades have led to major advances in the pathogenesis of Paget’s disease of bone (PDB) and particularly on the role of genetic factors. Germline mutations of different genes have been identified, as a possible cause of this disorder, and most of the underlying pathways are implicated in the regulation of osteoclast differentiation and function, whereas other are involved in cell autophagy mechanisms. In particular, about 30 different germline mutations of the Sequestosome 1 gene (SQSTM1) have been described in a significant proportion of familial and sporadic PDB cases. The majority of SQSTM1 mutations affect the ubiquitin-binding domain of the protein and are associated to a more severe clinical expression of the disease. Also, germline mutations in the ZNF687 and PFN1 genes have been associated to severe, early onset, polyostotic PDB with increased susceptibly to neoplastic degeneration, particularly giant cell tumor. Mutations in the VCP (Valosin Containing Protein) gene cause the autosomal dominant syndrome “Inclusion Body Myopathy, PDB, Fronto-temporal Dementia,” characterized by pagetic manifestations, associated with myopathy, amyotrophic lateral sclerosis and fronto-temporal dementia. Moreover, germline mutations in the TNFRSF11A gene, which encodes for RANK, were associated with rare syndromes showing some histopathological, radiological, and clinical overlap with PDB and in two cases of early onset PDB-like disease. Likewise, genome wide association studies performed in unrelated PDB cases identified other potential predisposition genes and/or susceptibility loci. Thus, it is likely that polygenic factors are involved in the PDB pathogenesis in many individuals and that modifying genes may contribute in refining the clinical phenotype. Moreover, the contribution of somatic mutations of SQSTM1 gene and/or epigenetic mechanisms in the pathogenesis of skeletal pagetic abnormalities and eventually neoplastic degeneration, cannot be excluded. Indeed, clinical and experimental observations indicate that genetic susceptibility might not be a sufficient condition for the clinical development of PDB without the concomitant intervention of viral infection, in primis paramixoviruses, and/or other environmental factors (e.g., pesticides, heavy metals or tobacco exposure), at least in a subset of cases. This review summarizes the most important advances that have been made in the field of cellular and molecular biology PDB over the past decades.

Effects of GH/IGF axis on bone and cartilage

Growth hormone (GH) and its mediator, the insulin-like growth factor-1 (IGF-1) regulate somatic growth, metabolism and many aspects of aging. As such, actions of GH/IGF have been studied in many tissues and organs over decades. GH and IGF-1 are part of the hypothalamic/pituitary somatotrophic axis that consists of many other regulatory hormones, receptors, binding proteins, and proteases. In humans, GH/IGF actions peak during pubertal growth and regulate skeletal acquisition through stimulation of extracellular matrix production and increases in bone mineral density. During aging the activity of these hormones declines, a state called somatopaguss, which associates with deleterious effects on the musculoskeletal system. In this review, we will focus on GH/IGF-1 action in bone and cartilage. We will cover many studies that have utilized congenital ablation or overexpression of members of this axis, as well as cell-specific gene-targeting approaches used to unravel the nature of the GH/IGF-1 actions in the skeleton in vivo.

Increased S1P expression in osteoclasts enhances bone formation in an animal model of Paget's disease

Paget's disease (PD) is characterized by increased numbers of abnormal osteoclasts (OCLs) that drive exuberant bone formation, but the mechanisms responsible for the increased bone formation remain unclear. We previously reported that OCLs from 70% of PD patients express measles virus nucleocapsid protein (MVNP), and that transgenic mice with targeted expression of MVNP in OCLs (MVNP mice) develop bone lesions and abnormal OCLs characteristic of PD. In this report, we examined if OCL-derived sphingosine-1-phosphate (S1P) contributed to the abnormal bone formation in PD, since OCL-derived S1P can act as a coupling factor to increase normal bone formation via binding S1P-receptor-3 (S1PR3) on osteoblasts (OBs). We report that OCLs from MVNP mice and PD patients expressed high levels of sphingosine kinase-1 (SphK-1) compared with wild-type (WT) mouse and normal donor OCLs. SphK-1 production by MVNP-OCLs was interleukin-6 (IL-6)-dependent since OCLs from MVNP/IL-6^-/- mice expressed lower levels of SphK-1. Immunohistochemistry of bone biopsies from a normal donor, a PD patient, WT and MVNP mice confirmed increased expression levels of SphK-1 in OCLs and S1PR3 in OBs of the PD patient and MVNP mice compared with normal donor and WT mice. Further, MVNP-OCLs cocultured with OBs from MVNP or WT mice increased OB-S1PR3 expression and enhanced expression of OB differentiation markers in MVNP-OBs precursors compared with WT-OBs, which was mediated by IL-6 and insulin-like growth factor 1 secreted by MVNP-OCLs. Finally, the addition of an S1PR3 antagonist (VPC23019) to WT or MVNP-OBs treated with WT and MVNP-OCL-conditioned media (CM) blocked enhanced OB differentiation of MVNP-OBs treated with MVNP-OCL-CM. In contrast, the addition of the SIPR3 agonist, VPC24191, to the cultures enhanced osterix and Col-1A expression in MVNP-OBs treated with MVNP-OCL-CM compared with WT-OBs treated with WT-OCL-CM. These results suggest that IL-6 produced by PD-OCLs increases S1P in OCLs and S1PR3 on OBs, to increase bone formation in PD.