Membrane trafficking in osteoclasts and implications for osteoporosis

Echinacoside promotes osteogenesis and angiogenesis and inhibits osteoclast formation

PURPOSE

The purpose of this research is to demonstrate echinacoside promotes osteogenesis and angiogenesis and inhibits osteoclast formation.

METHODS

We conducted a cell experiment in vitro to study how echinacoside affects angiogenesis, osteogenesis and osteoclast formation. We used polymerase chain reaction and Western blotting to detect the expression levels of proteins and genes related to angiogenesis, osteogenesis and osteoclast formation. We established a bone fracture model with rats to test angiogenesis, osteogenesis and osteoclast formation of echinacoside. We labelled osteogenic markers, blood vessels and osteoclastic markers in fracture sections of rats.

RESULTS

The in vitro cell experiments showed echinacoside improved the osteogenic activity of mouse embryo osteoblast precursor cells and promoted the migration and tube formation of human umbilical vein endothelial cells. In addition, it inhibited differentiation of mouse leukaemia cells of monocyte macrophage. Echinacoside increased the expression of related proteins and genes and improved angiogenesis and osteogenesis while inhibiting osteoclast formation by repressing the expression of related proteins and genes. From in vivo experiments, the results of IHC and HE experiments demonstrated echinacoside significantly decreased the content of MMP-9 and improved the content of VEGF and OCN. The fluorescence immunoassay showed echinacoside promoted the activities of RUNX2 and VEGF and inhibited CTSK. Echinacoside reduced the content of TNF-α, IL-1β and IL-6, thus demonstrating its anti-inflammatory activity.

CONCLUSION

Echinacoside improved angiogenesis and osteogenesis and inhibited osteoclast formation to promote fracture healing.

RUFY4 deletion prevents pathological bone loss by blocking endo-lysosomal trafficking of osteoclasts

Mature osteoclasts degrade bone matrix by exocytosis of active proteases from secretory lysosomes through a ruffled border. However, the molecular mechanisms underlying lysosomal trafficking and secretion in osteoclasts remain largely unknown. Here, we show with GeneChip analysis that RUN and FYVE domain-containing protein 4 (RUFY4) is strongly upregulated during osteoclastogenesis. Mice lacking Rufy4 exhibited a high trabecular bone mass phenotype with abnormalities in osteoclast function in vivo. Furthermore, deleting Rufy4 did not affect osteoclast differentiation, but inhibited bone-resorbing activity due to disruption in the acidic maturation of secondary lysosomes, their trafficking to the membrane, and their secretion of cathepsin K into the extracellular space. Mechanistically, RUFY4 promotes late endosome-lysosome fusion by acting as an adaptor protein between Rab7 on late endosomes and LAMP2 on primary lysosomes. Consequently, Rufy4-deficient mice were highly protected from lipopolysaccharide- and ovariectomy-induced bone loss. Thus, RUFY4 plays as a new regulator in osteoclast activity by mediating endo-lysosomal trafficking and have a potential to be specific target for therapies against bone-loss diseases such as osteoporosis.

Osteoclast Methods in Protein Phosphatase Research

Osteoclasts are specialized cells that degrade bone and are essential for bone formation and maintaining bone homeostasis. Excess or deficient activity of these cells can significantly alter bone mass, structure, and physical strength, leading to significant morbidity, as in osteoporosis or osteopetrosis, among many other diseases. Protein phosphorylation in osteoclasts plays critical roles in the signaling pathways that govern the production of osteoclasts and regulate their bone-resorbing activity. In this chapter, we describe the isolation of mouse splenocytes and their differentiation into mature osteoclasts on resorptive (e.g., bone) and non-resorptive (e.g., plastic or glass) surfaces, examining matrix resorption by osteoclasts, immunofluorescence staining of these cells, and knocking out genes by CRISPR in the mouse osteoclastogenic cell line RAW264.7.

Myo6 mediates osteoclast function and is essential for joint damage in collagen-induced arthritis

OBJECTIVES

To explore the novel function of MYO6 on Osteoclast differentiation and its joint destruction capacity in Rheumatoid arthritis mice model.

METHODS

We examined joint erosion in a collagen-induced arthritis (CIA) mouse model using micro-CT, with the mice having a MYO6 knockout background. Inflammatory cytokines were analyzed using an enzyme-linked immunosorbent assay (ELISA). In vitro, we investigated the osteoclastogenesis ability of bone marrow-derived macrophages isolated from MYO6-/- mice and their littermate controls, examining both morphological and functional differences. Furthermore, we explored podosome formation and endosome maturation using immunofluorescence staining.

RESULTS

We found that MYO6 deficiency attenuated arthritis development and bone destruction in CIA mice as well as impaired osteoclast differentiation by inhibiting NFATc1 induction. Our findings indicate that MYO6 is essential for the organization of podosomes by modulating the FAK/AKT and integrin-β3/Src pathways. MYO6 also mediates endosome transportation by regulating the expression of Rab5 and GM130. This may impact the maintenance and functionality of the ruffled border, as well as the regulation of autophagy in osteoclasts.

CONCLUSION

Our results demonstrated a critical function of MYO6 in osteoclast differentiation and its potential relevance in experimental arthritis.

Genomic association using principal components of morphometric traits in horses: identification of genes related to bone growth

The measurement of morphometric traits in horses is important for determining breed qualification and is one of the main selection criteria for the species. The development of an index (HPC) that consists of principal components weighted by additive genetic values allows to explore the most relevant relationships using a reduced number of variables that explain the greatest amount of variation in the data. Genome-wide association studies (GWAS) using HPC are a relatively new approach that permits to identify regions related to a set of traits. The aim of this study was to perform GWAS using HPC for 15 linear measurements as the explanatory variable in order to identify associated genomic regions and to elucidate the biological mechanisms linked to this index in Campolina horses. For GWAS, weighted single-step GBLUP was applied to HPC. The eight genomic windows that explained the highest proportion of additive genetic variance were identified. The sum of the additive variance explained by the eight windows was 95.89%. Genes involved in bone and cartilage development were identified (SPRY2, COL9A2, MIR30C, HEYL, BMP8B, LTBP1, FAM98A, and CRIM1). They represent potential positional candidates for the HPC of the linear measurements evaluated. The HPC is an efficient alternative to reduce the 15 usually measured traits in Campolina horses. Moreover, candidate genes inserted in region that explained high additive variance of the HPC were identified and might be fine-mapped for searching putative mutation/markers.

Structure and function of the membrane microdomains in osteoclasts

The cell membrane structure is closely related to the occurrence and progression of many metabolic bone diseases observed in the clinic and is an important target to the development of therapeutic strategies for these diseases. Strong experimental evidence supports the existence of membrane microdomains in osteoclasts (OCs). However, the potential membrane microdomains and the crucial mechanisms underlying their roles in OCs have not been fully characterized. Membrane microdomain components, such as scaffolding proteins and the actin cytoskeleton, as well as the roles of individual membrane proteins, need to be elucidated. In this review, we discuss the compositions and critical functions of membrane microdomains that determine the biological behavior of OCs through the three main stages of the OC life cycle.

Characterization of Rab32- and Rab38-positive lysosome-related organelles in osteoclasts and macrophages

Both the biogenesis and functions of osteoclasts and macrophages involves dynamic membrane traffic. We screened transcript levels for Rab family small GTPases related to osteoclasts and identified Rab38. Rab38 expression is upregulated during osteoclast differentiation and maturation. In osteoclasts, both Rab38 and its paralog, Rab32, colocalize to lysosome-related organelles (LROs). In macrophages, Rab32 is also found in LROs. LROs are part of the endocytic pathway but are distinct from lysosomes. After receptor activator of NF-κB ligand stimulation, LROs contain cathepsin K and tartrate-resistant acid phosphatase inside and help both proteins to accumulate around bone resorption pits. After osteoclast maturation, these enzymes are hardly found within LROs. In macrophages derived from Rab32 and Rab38 double knockout mice, both acidification and V-ATPase a3 localization were severely compromised. Both the double knockout macrophage and bafilomycin-treated wildtype macrophage show an increase in Lamp1-positive organelles, implying that biogenesis of lysosomes and LROs are related. These results indicate that Rab32 and Rab38 both play a crucial role in LRO biogenesis in macrophages and in osteoclasts.

Sugar transporter Slc37a2 regulates bone metabolism in mice via a tubular lysosomal network in osteoclasts

Osteoclasts are giant bone-digesting cells that harbor specialized lysosome-related organelles termed secretory lysosomes (SLs). SLs store cathepsin K and serve as a membrane precursor to the ruffled border, the osteoclast's 'resorptive apparatus'. Yet, the molecular composition and spatiotemporal organization of SLs remains incompletely understood. Here, using organelle-resolution proteomics, we identify member a2 of the solute carrier 37 family (Slc37a2) as a SL sugar transporter. We demonstrate in mice that Slc37a2 localizes to the SL limiting membrane and that these organelles adopt a hitherto unnoticed but dynamic tubular network in living osteoclasts that is required for bone digestion. Accordingly, mice lacking Slc37a2 accrue high bone mass owing to uncoupled bone metabolism and disturbances in SL export of monosaccharide sugars, a prerequisite for SL delivery to the bone-lining osteoclast plasma membrane. Thus, Slc37a2 is a physiological component of the osteoclast's unique secretory organelle and a potential therapeutic target for metabolic bone diseases.

Osteoclasts, Master Sculptors of Bone

Osteoclasts are multinucleated cells with the unique ability to resorb bone matrix. Excessive production or activation of osteoclasts leads to skeletal pathologies that affect a significant portion of the population. Although therapies that effectively target osteoclasts have been developed, they are associated with sometimes severe side effects, and a fuller understanding of osteoclast biology may lead to more specific treatments. Along those lines, a rich body of work has defined essential signaling pathways required for osteoclast formation, function, and survival. Nonetheless, recent studies have cast new light on long-held views regarding the origin of these cells during development and homeostasis, their life span, and the cellular sources of factors that drive their production and activity during homeostasis and disease. In this review, we discuss these new findings in the context of existing work and highlight areas of ongoing and future investigation.

Alpinetin alleviates osteoporosis by promoting osteogenic differentiation in BMSCs by triggering autophagy via PKA/mTOR/ULK1 signaling

Osteoporosis, a systemic bone disease that is characterized by a reduction in bone mass and destruction of bone microstructure, is becoming a serious problem worldwide. Bone marrow mesenchymal stem cells (BMSCs) can differentiate into bone-forming osteoblasts, and play an important role in maintaining homeostasis of bone metabolism, thus being a potential therapeutic target for osteoporosis. Although the phytochemical alpinetin (APT) has been reported to possess a variety of pharmacological activities, it is still unclear whether APT can influence the osteogenic differentiation of on BMSCs and if it can improve osteoporosis. In this study, we found that APT treatment was able to enhance osteogenic differentiation levels of human BMSCs in vitro and mouse ones in vivo as revealed by multiple osteogenic markers including increased alkaline phosphatase activity and osteocalcin expression. Mechanistically, the protein kinase A (PKA)/mTOR/ULK1 signaling was involved in the action of APT to enhance the osteogenic differentiation of BMSCs. In addition, oral administration of APT significantly mitigated the bone loss in a dexamethasone-induced mouse model of osteoporosis through strengthening PKA signaling and autophagy. Altogether, these data demonstrate that APT promotes osteogenic differentiation in BMSCs by augmenting the PKA/mTOR/ULK1 autophagy signaling, highlighting its potential therapeutic application for treating osteoporotic diseases.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Epigallocatechin-3-gallate inhibits osteoclastic differentiation by modulating mitophagy and mitochondrial functions

A natural plant product, epigallocatechin-3-gallate (EGCG), was evaluated for its effectiveness in the regulation of osteoclastogenesis. We found that EGCG inhibited the osteoclast (OC) differentiation in vitro, and in primary bone marrow cells in a dose-dependent manner. Quantitative RT-PCR studies showed that the EGCG reduced the expression of OC differentiation markers. DCFDA, MitoSOX, and JC-1 staining revealed that the EGCG attenuated the reactive oxygen species (ROS), and mitochondrial membrane potential; and flux analysis corroborated the effect of EGCG. We further found that the EGCG inhibited mRNA and protein expressions of mitophagy-related molecules. We confirmed that the OC differentiation was inhibited by EGCG by modulating mitophagy through AKT and p38MAPK pathways. Furthermore, in silico analysis revealed that the binding of RANK and RANKL was blocked by EGCG. Overall, we defined the mechanisms of osteoclastogenesis during arthritis for developing a new therapy using a natural compound besides the existing therapeutics.

Inflammatory activation of the FcγR and IFNγR pathways co-influences the differentiation and activity of osteoclasts

Osteoclasts are polykaryons formed by cell–cell fusion of highly motile progenitors of the myeloid lineage. Osteoclast activity can preserve skeletal strength and bone homeostasis. However, osteoclasts are responsible for bone destruction in rheumatoid arthritis (RA). Fc receptors activated by IgG immune complexes (IC) can boost osteoclast differentiation and bone loss in the course of RA. In contrast, interferon (IFN) γ secreted by immune cells blocks osteoclast activation. Despite their hypothetical importance in the regulation of osteoclast differentiation in RA, the interconnection between the two pathways has not been described so far. Here, we show by total internal reflection fluorescence (TIRF) microscopy that FcγR3 and IFNγ receptor (IFNγR) locate at close vicinity to each other on the human osteoclast surface. Moreover, the average distance increases during the differentiation process. Interestingly, FcγR and IFNγR activation shapes the position of both receptors to each other. Surprisingly, the inhibitory action of IFNγ on in-vitro human osteoclast differentiation depends on the osteoclast differentiation stage. Indeed, IFNγR activation in early osteoclast precursors completely inhibits the formation of polynucleated osteoclasts, while in premature osteoclasts, it further enhanced their fusion. In addition, gene expression analyses showed that IFNγR activation on early precursor cells but not on premature osteoclasts could induce FcγR expression, suggesting a co-regulation of both receptors on human osteoclast precursors. Phosphokinase array data of precursor cells demonstrate that the observed divergence of IFNγR signaling is dependent on the mitogen−activated protein kinase (MAPK) downstream signaling pathway. Overall, our data indicate that FcγR and IFNγR signaling pathways co-influence the differentiation and activity of osteoclasts dependent on the differentiation state, which might reflect the different stages in RA.

A Review of Signaling Transduction Mechanisms in Osteoclastogenesis Regulation by Autophagy, Inflammation, and Immunity

Osteoclastogenesis is an ongoing rigorous course that includes osteoclast precursors fusion and bone resorption executed by degradative enzymes. Osteoclastogenesis is controlled by endogenous signaling and/or regulators or affected by exogenous conditions and can also be controlled both internally and externally. More evidence indicates that autophagy, inflammation, and immunity are closely related to osteoclastogenesis and involve multiple intracellular organelles (e.g., lysosomes and autophagosomes) and certain inflammatory or immunological factors. Based on the literature on osteoclastogenesis induced by different regulatory aspects, emerging basic cross-studies have reported the emerging disquisitive orientation for osteoclast differentiation and function. In this review, we summarize the partial potential therapeutic targets for osteoclast differentiation and function, including the signaling pathways and various cellular processes.

Transcriptome Profiling of Osteoblasts in a Medaka (Oryzias latipes) Osteoporosis Model Identifies Mmp13b as Crucial for Osteoclast Activation

Matrix metalloproteases (MMPs) play crucial roles in extracellular matrix (ECM) modulation during osteoclast-driven bone remodeling. In the present study, we used transcriptome profiling of bone cells in a medaka model for osteoporosis and bone regeneration to identify factors critical for bone remodeling and homeostasis. This identified mmp13b, which was strongly expressed in osteoblast progenitors and upregulated under osteoporotic conditions and during regeneration of bony fin rays. To characterize the role of mmp13b in bone remodeling, we generated medaka mmp13b mutants by CRISPR/Cas9. We found that mmp13b mutants form normal numbers of osteoblasts and osteoclasts. However, osteoclast activity was severely impaired under osteoporotic conditions. In mmp13b mutants and embryos treated with the MMP13 inhibitor CL-82198, unmineralized collagens and mineralized bone matrix failed to be degraded. In addition, the dynamic migratory behavior of activated osteoclasts was severely affected in mmp13b mutants. Expression analysis showed that maturation genes were downregulated in mmp13b deficient osteoclasts suggesting that they remain in an immature and non-activated state. We also found that fin regeneration was delayed in mmp13b mutants with a concomitant alteration of the ECM and reduced numbers of osteoblast progenitors in regenerating joint regions. Together, our findings suggest that osteoblast-derived Mmp13b alters the bone ECM to allow the maturation and activation of osteoclasts during bone remodeling in a paracrine manner. Mmp13b-induced ECM alterations are also required to facilitate osteoblast progenitor recruitment and full regeneration of bony fin rays.

Improvement of polydopamine-loaded salidroside on osseointegration of titanium implants

Background

Microarc oxidation (MAO) on the surface of medical pure titanium can improve its histocompatibility, and loading drugs on the surface can resist excessive intimal hyperplasia.

Methods

In this study, salidroside (SAL) was loaded on the surface of porous titanium (Ti) with polydopamine (PDA) carrier. The effects of SAL on the osteogenesis and angiogenesis of Ti implants were studied by phalloidin staining, alizarin red staining, ALP staining, wound-healing assay, cell transwell assay, matrigel tube formation, and osteogenic and angiogenic genes and proteins expression detected by PCR and western blot in vitro. The bone defect model experiments in rats was established in vivo including X-ray, micro CT, hematoxylin and eosin staining (HE), immunohistochemistry (IHC), Goldner's trichrome analysis, Safranin O-fast green staining and determination of contents of TNF-α and IL-6 in serum.

Results

EDS and EDS mapping showed that SAL could be loaded on the surface of the MAO coating by PDA. A drug release experiment showed that SAL loaded on the Ti coating could release slowly and stably without sudden release risk. In vitro cell experiments showed that the SAL coating could promote the proliferation, morphology, calcification and alkaline phosphate activity of MC3T3-E1 cells. At the same time, it promoted the migration and tube formation of HUVEC cells. The SAL coating promoted osteogenesis and angiogenesis by promoting the expression of genes and proteins related to. In vivo experiments, HE and IHC showed that SAL significantly promoted the expression of COL-1 and CD31. Goldner's trichrome and Safranin O-fast green staining showed that SAL coating could increase the new bone tissue around the implantation site. The SAL coating had anti-inflammatory activity by reducing the levels of TNF-α and IL-6 in vivo.

Conclusion

Therefore, SAL could improve osteogenesis and angiogenesis in conjunction with the Ti-PDA coating.

Osteal macrophages support osteoclast-mediated resorption and contribute to bone pathology in a postmenopausal osteoporosis mouse model

Osteal macrophages (osteomacs) support osteoblast function and promote bone anabolism, but their contribution to osteoporosis has not been explored. Although mouse ovariectomy (OVX) models have been repeatedly used, variation in strain, experimental design and assessment modalities have contributed to no single model being confirmed as comprehensively replicating the full gamut of osteoporosis pathological manifestations. We validated an OVX model in adult C3H/HeJ mice and demonstrated that it presents with human postmenopausal osteoporosis features with reduced bone volume in axial and appendicular bone and bone loss in both trabecular and cortical bone including increased cortical porosity. Bone loss was associated with increased osteoclasts on trabecular and endocortical bone and decreased osteoblasts on trabecular bone. Importantly, this OVX model was characterized by delayed fracture healing. Using this validated model, we demonstrated that osteomacs are increased post-OVX on both trabecular and endocortical bone. Dual F4/80 (pan-macrophage marker) and tartrate-resistant acid phosphatase (TRAP) staining revealed osteomacs frequently located near TRAP⁺ osteoclasts and contained TRAP⁺ intracellular vesicles. Using an in vivo inducible macrophage depletion model that does not simultaneously deplete osteoclasts, we observed that osteomac loss was associated with elevated extracellular TRAP in bone marrow interstitium and increased serum TRAP. Using in vitro high-resolution confocal imaging of mixed osteoclast-macrophage cultures on bone substrate, we observed macrophages juxtaposed to osteoclast basolateral functional secretory domains scavenging degraded bone byproducts. These data demonstrate a role for osteomacs in supporting osteoclastic bone resorption through phagocytosis and sequestration of resorption byproducts. Overall, our data expose a novel role for osteomacs in supporting osteoclast function and provide the first evidence of their involvement in osteoporosis pathogenesis. © 2021 American Society for Bone and Mineral Research (ASBMR).

The Roles of Long Non-coding RNA in Osteoporosis

The Human Genome Project (HGP) announced in 2001 that it had sequenced the entire human genome, yielding nearly complete human DNA. About 98.5 percent of the human genome has been found to be non-coding sequences. Long non-coding RNA (lncRNA) is a non-coding RNA with a length between 200 and 100,000 nucleotide units. Because of shallow research on lncRNA, it was believed that it had no biological functions, but exists as a by-product of the transcription process. With the development of high-throughput sequencing technology, studies have shown that lncRNA plays important roles in many processes by participating in epigenetics, transcription, translation and protein modification. Current researches have shown that lncRNA also has an important part in the pathogenesis of osteoporosis. Osteoporosis is a common disorder of bone metabolism, also a major medical and socioeconomic challenge worldwide. It is characterized by a systemic reduction in bone mass and microstructure changes, which increases the risk of brittle fractures. It is more common in postmenopausal women and elderly men. However, the roles of lncRNA and relevant mechanisms in osteoporosis remain unclear. Based on this background, we hereby review the roles of lncRNA in osteoporosis, and how it influences the functions of osteoblasts and osteoclasts, providing reference to clinical diagnosis, treatment and prognosis of osteoporosis.

The V-ATPase a3 Subunit: Structure, Function and Therapeutic Potential of an Essential Biomolecule in Osteoclastic Bone Resorption

This review focuses on one of the 16 proteins composing the V-ATPase complex responsible for resorbing bone: the a3 subunit. The rationale for focusing on this biomolecule is that mutations in this one protein account for over 50% of osteopetrosis cases, highlighting its critical role in bone physiology. Despite its essential role in bone remodeling and its involvement in bone diseases, little is known about the way in which this subunit is targeted and regulated within osteoclasts. To this end, this review is broadened to include the three other mammalian paralogues (a1, a2 and a4) and the two yeast orthologs (Vph1p and Stv1p). By examining the literature on all of the paralogues/orthologs of the V-ATPase a subunit, we hope to provide insight into the molecular mechanisms and future research directions specific to a3. This review starts with an overview on bone, highlighting the role of V-ATPases in osteoclastic bone resorption. We then cover V-ATPases in other location/functions, highlighting the roles which the four mammalian a subunit paralogues might play in differential targeting and/or regulation. We review the ways in which the energy of ATP hydrolysis is converted into proton translocation, and go in depth into the diverse role of the a subunit, not only in proton translocation but also in lipid binding, cell signaling and human diseases. Finally, the therapeutic implication of targeting a3 specifically for bone diseases and cancer is discussed, with concluding remarks on future directions.

An Ammonia-Induced Calcium Phosphate Nanostructure: A Potential Assay for Studying Osteoporosis and Bone Metastasis

Osteoclastic resorption of bones plays a central role in both osteoporosis and bone metastasis. A reliable in vitro assay that simulates osteoclastic resorption in vivo would significantly speed up the process of developing effective therapeutic solutions for those diseases. Here, we reported the development of a novel and robust nanostructured calcium phosphate coating with unique functions on the track-etched porous membrane by using an ammonia-induced mineralization (AiM) technique. The calcium phosphate coating uniformly covers one side of the PET membrane, enabling testing for osteoclastic resorption. The track-etched pores in the PET membrane allow calcium phosphate mineral pins to grow inside, which, on the one hand, enhances coating integration with a membrane substrate and, on the other hand, provides diffusion channels for delivering drugs from the lower chamber of a double-chamber cell culture system. The applications of the processed calcium phosphate coating were first demonstrated as a drug screening device by using alendronate, a widely used drug for osteoporosis. It was confirmed that the delivery of alendronate significantly decreased both the number of monocyte-differentiated osteoclasts and coating resorption. To demonstrate the application in studying bone metastasis, we delivered a PC3 prostate cancer-conditioned medium and confirmed that both the differentiation of monocytes into osteoclasts and the osteoclastic resorption of the calcium phosphate coating were significantly enhanced. This novel assay thus provides a new platform for studying osteoclastic activities and assessing drug efficacy in vitro.

Skeletal Functions of Voltage Sensitive Calcium Channels

Voltage-sensitive calcium channels (VSCCs) are ubiquitous multimeric protein complexes that are necessary for the regulation of numerous physiological processes. VSCCs regulate calcium influx and various intracellular processes including muscle contraction, neurotransmission, hormone secretion, and gene transcription, with function specificity defined by the channel's subunits and tissue location. The functions of VSCCs in bone are often overlooked since bone is not considered an electrically excitable tissue. However, skeletal homeostasis and adaptation relies heavily on VSCCs. Inhibition or deletion of VSCCs decreases osteogenesis, impairs skeletal structure, and impedes anabolic responses to mechanical loading. RECENT FINDINGS: While the functions of VSCCs in osteoclasts are less clear, VSCCs have distinct but complementary functions in osteoblasts and osteocytes. PURPOSE OF REVIEW: This review details the structure, function, and nomenclature of VSCCs, followed by a comprehensive description of the known functions of VSCCs in bone cells and their regulation of bone development, bone formation, and mechanotransduction.

The Mechanism Switching the Osteoclast From Short to Long Duration Bone Resorption

The current models of osteoclastic bone resorption focus on immobile osteoclasts sitting on the bone surface and drilling a pit into the bone matrix. It recently appeared that many osteoclasts also enlarge their pit by moving across the bone surface while resorbing. Drilling a pit thus represents only the start of a resorption event of much larger amplitude. This prolonged resorption activity significantly contributes to pathological bone destruction, but the mechanism whereby the osteoclast engages in this process does not have an answer within the standard bone resorption models. Herein, we review observations that lead to envision how prolonged resorption is possible through simultaneous resorption and migration. According to the standard pit model, the “sealing zone” which surrounds the ruffled border (i.e., the actual resorption apparatus), “anchors” the ruffled border against the bone surface to be resorbed. Herein, we highlight that continuation of resorption demands that the sealing zone “glides” inside the cavity. Thereby, the sealing zone emerges as the structure responsible for orienting and displacing the ruffled border, e.g., directing resorption against the cavity wall. Importantly, sealing zone displacement stringently requires thorough collagen removal from the cavity wall - which renders strong cathepsin K collagenolysis indispensable for engagement of osteoclasts in cavity-enlargement. Furthermore, the sealing zone is associated with generation of new ruffled border at the leading edge, thereby allowing the ruffled border to move ahead. The sealing zone and ruffled border displacements are coordinated with the migration of the cell body, shown to be under control of lamellipodia at the leading edge and of the release of resorption products at the rear. We propose that bone resorption demands more attention to osteoclastic models integrating resorption and migration activities into just one cell phenotype.

Coenzyme Q10 inhibits RANKL-induced osteoclastogenesis by regulation of mitochondrial apoptosis and oxidative stress in RAW264.7 cells

Coenzyme Q10 (CoQ10) has been reported to improve bone density and the number of trabeculae in postmenopausal osteoporosis, but the mechanism remains to be elucidated. We aimed to investigate the effects of CoQ10 on receptor activator of NF-κB ligand (RANKL)-induced osteoclastogenesis and the underlying molecular mechanisms. RAW264.7 cells were treated with different concentrations of RANKL to differentiate into osteoclasts, and then these cells were treated with different concentrations of CoQ10 with or without H₂ O₂ . Tartrate-resistant acid phosphatase staining was performed to detect osteoclasts. Cell viability was tested by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, cell apoptosis was examined by flow cytometry, and the effects of CoQ10 on protein and messenger RNA expression of mitochondrial apoptosis-associated proteins and osteoclast marker proteins were measured by quantitative reverse transcription polymerase chain reaction and western blot, respectively. Furthermore, enzyme-linked immunosorbent assay was conducted to analyze the activities of malondialdehyde (MDA), superoxide dismutase (SOD), and catalase (CAT). RANKL significantly induced osteoclastogenesis in RAW264.7 cells, with the greatest efficiency at 50 ng/ml. CoQ10 had no significant effects on cell viability but it significantly increased the percentages of cell apoptosis. Mechanically, CoQ10 statistically decreased the levels of Bcl-2 and cytochrome C in mitochondria and upregulated the levels of Bax, cleaved caspase 3, and cytochrome C in the cytoplasm. Moreover, CoQ10 significantly decreased RANKL-induced osteoclastogenesis regardless of exposure to H₂ O₂ . In addition, CoQ10 statistically reduced MDA activity and elevated the activities of SOD and CAT, as well as the expression of oxidative stress-related proteins. CoQ10 may inhibit RANKL-induced osteoclastogenesis by regulation of mitochondrial apoptosis and oxidative stress in RAW264.7 cells.

Membrane Transport Proteins in Osteoclasts: The Ins and Outs

During bone resorption, the osteoclast must sustain an extraordinarily low pH environment, withstand immense ionic pressures, and coordinate nutrient and waste exchange across its membrane to sustain its unique structural and functional polarity. To achieve this, osteoclasts are equipped with an elaborate set of membrane transport proteins (pumps, transporters and channels) that serve as molecular ‘gatekeepers’ to regulate the bilateral exchange of ions, amino acids, metabolites and macromolecules across the ruffled border and basolateral domains. Whereas the importance of the vacuolar-ATPase proton pump and chloride voltage-gated channel 7 in osteoclasts has long been established, comparatively little is known about the contributions of other membrane transport proteins, including those categorized as secondary active transporters. In this Special Issue review, we provide a contemporary update on the ‘ins and outs’ of membrane transport proteins implicated in osteoclast differentiation, function and bone homeostasis and discuss their therapeutic potential for the treatment of metabolic bone diseases.

Effects of varying gelatin coating concentrations on RANKL induced osteoclastogenesis

Here, we assessed the effects of varying concentrations of gelatin coating on Receptor Activator of Nuclear Factor κ-B Ligand (RANKL)-induced RAW264.7 murine macrophage differentiation into osteoclast (OC) via osteoclastogenesis. The microstructures of coating surfaces with different concentrations of gelatin were examined by scanning electron microscopy and atomic force microscopy. Increased gelatin coating concentrations led to decreased gel rigidity but increased surface adhesion force attenuated OC differentiation and the decreased actin ring formation in RANKL-induced osteoclastogenesis. The decreased actin ring formation is associated with decreased lysosomal-associated membrane protein 1 (LAMP1) activity and bone resorption in the differentiated OCs with different gelatin coating concentrations as compared to the cells differentiated without gelatin coatings. In addition, increasing concentrations of gelatin coating attenuated the medium TGF-β1 protein levels and the expression levels of TGF-β and type-I (R1) and type-II (R2) TGF-β receptors in OCs, suggesting the gelatin-induced suppression of TGF-β signaling for the regulation of RNAKL-induced OC differentiation. Taken together, these findings showed that changes in gelatin coating concentrations, which were associated with altered gel thickness and substrate rigidity, might attenuate TGF-β signaling events to modulate OC differentiation and concomitant actin ring formation and bone matrix resorption in RANKL-induced osteoclastogenesis.

Low doses of uranium and osteoclastic bone resorption: key reciprocal effects evidenced using new in vitro biomimetic models of bone matrix

Uranium is widely spread in the environment due to its natural and anthropogenic occurrences, hence the importance of understanding its impact on human health. The skeleton is the main site of long-term accumulation of this actinide. However, interactions of this metal with biological processes involving the mineralized extracellular matrix and bone cells are still poorly understood. To get a better insight into these interactions, we developed new biomimetic bone matrices containing low doses of natural uranium (up to 0.85 µg of uranium per cm²). These models were characterized by spectroscopic and microscopic approaches before being used as a support for the culture and differentiation of pre-osteoclastic cells. In doing so, we demonstrate that uranium can exert opposite effects on osteoclast resorption depending on its concentration in the bone microenvironment. Our results also provide evidence for the first time that resorption contributes to the remobilization of bone matrix-bound uranium. In agreement with this, we identified, by HRTEM, uranium phosphate internalized in vesicles of resorbing osteoclasts. Thanks to the biomimetic matrices we developed, this study highlights the complex mutual effects between osteoclasts and uranium. This demonstrates the relevance of these 3D models to further study the cellular mechanisms at play in response to uranium storage in bone tissue, and thus better understand the impact of environmental exposure to uranium on human bone health.

The Regulatory Role of Rho GTPases and their Substrates in Osteoclastogenesis

Pathological bone loss diseases (osteolysis, Paget's diseases) are commonly caused by the excessive differentiation and activity of osteoclasts. The Rho GTPases family members Rac1/2 (Rac1 and Rac2) have been reported for their special role in exerting multiple cellular functions during osteoclastic differentiation, which includes the most prominent function on dynamic actin cytoskeleton rearranging. Besides that, the increasing studies demonstrated that the regulating effects of Rac1/2 on the osteoclastic cytoskeletal organization are through the GEFs member Dock5. Although the amount of relevant studies on this topic is still limited, several excellent studies have been reported that extensively explored the molecular mechanisms involved in Rac1/2 and Dock5 during the osteoclastogenesis regulation, as well as their role as the therapeutic target in bone loss diseases. Herein, in this review, we aim to focus on recent advances studies for extensively understanding the role of Rho GTPases Rac1/2 and Dock5 in osteoclastogenesis, as well as their role as a potential therapeutic target in regulating osteoclastogenesis.

The molecular structure and function of sorting nexin 10 in skeletal disorders, cancers, and other pathological conditions

SNX10 is a member of the phox homology domain-containing family of phosphoinositide-binding proteins. Intracellularly, SNX10 localizes to endosomes where it mediates intracellular trafficking, endosome organization, and protein localization to the centrosome and cilium. It is highly expressed in bone and the gut where it participates in bone mineral and calcium homeostasis through the regulation of osteoclastic bone resorption and gastric acid secretion, respectively. Not surprisingly, patients harboring mutations in SNX10 mutation manifest a phenotype of autosomal recessive osteopetrosis or malignant infantile osteopetrosis, which is clinically characterized by dense bones with increased cortical bone into the medullary space with bone marrow occlusion or depletion, bone marrow failure, and anemia. Accordingly, SNX10 mutant osteoclasts exhibit impaired bone resorptive capacity. Beyond the skeleton, there is emerging evidence implicating SNX10 in cancer development, metabolic disorders, inflammation, and chaperone-mediated autophagy. Understanding the structural basis through which SNX10 exerts its diverse biological functions in both cell and tissue-specific manners may therefore inform new therapeutic opportunities toward the treatment and management of SNX10-related diseases.

Aesculetin Inhibits Osteoclastic Bone Resorption through Blocking Ruffled Border Formation and Lysosomal Trafficking

For the optimal resorption of mineralized bone matrix, osteoclasts require the generation of the ruffled border and acidic resorption lacuna through lysosomal trafficking and exocytosis. Coumarin-type aesculetin is a naturally occurring compound with anti-inflammatory and antibacterial effects. However, the direct effects of aesculetin on osteoclastogenesis remain to be elucidated. This study found that aesculetin inhibited osteoclast activation and bone resorption through blocking formation and exocytosis of lysosomes. Raw 264.7 cells were differentiated in the presence of 50 ng/mL receptor activator of nuclear factor-κB ligand (RANKL) and treated with 1–10 μM aesculetin. Differentiation, bone resorption, and lysosome biogenesis of osteoclasts were determined by tartrate-resistance acid phosphatase (TRAP) staining, bone resorption assay, Western blotting, immunocytochemical analysis, and LysoTracker staining. Aesculetin inhibited RANKL-induced formation of multinucleated osteoclasts with a reduction of TRAP activity. Micromolar aesculetin deterred the actin ring formation through inhibition of induction of αvβ3 integrin and Cdc42 but not cluster of differentiation 44 (CD44) in RANKL-exposed osteoclasts. Administering aesculetin to RANKL-exposed osteoclasts attenuated the induction of autophagy-related proteins, microtubule-associated protein light chain 3, and small GTPase Rab7, hampering the lysosomal trafficking onto ruffled border crucial for bone resorption. In addition, aesculetin curtailed cellular induction of Pleckstrin homology domain-containing protein family member 1 and lissencephaly-1 involved in lysosome positioning to microtubules involved in the lysosomal transport within mature osteoclasts. These results demonstrate that aesculetin retarded osteoclast differentiation and impaired lysosomal trafficking and exocytosis for the formation of the putative ruffled border. Therefore, aesculetin may be a potential osteoprotective agent targeting RANKL-induced osteoclastic born resorption for medicinal use.

Osteoclast Fusion: Physiological Regulation of Multinucleation through Heterogeneity-Potential Implications for Drug Sensitivity

Classically, osteoclast fusion consists of four basic steps: (1) attraction/migration, (2) recognition, (3) cell-cell adhesion, and (4) membrane fusion. In theory, this sounds like a straightforward simple linear process. However, it is not. Osteoclast fusion has to take place in a well-coordinated manner-something that is not simple. In vivo, the complex regulation of osteoclast formation takes place within the bone marrow-in time and space. The present review will focus on considering osteoclast fusion in the context of physiology and pathology. Special attention is given to: (1) regulation of osteoclast fusion in vivo, (2) heterogeneity of osteoclast fusion partners, (3) regulation of multi-nucleation, (4) implications for physiology and pathology, and (5) implications for drug sensitivity and side effects. The review will emphasize that more attention should be given to the human in vivo reality when interpreting the impact of in vitro and animal studies. This should be done in order to improve our understanding of human physiology and pathology, as well as to improve anti-resorptive treatment and reduce side effects.

Rab GTPases in Osteoclastic Bone Resorption and Autophagy

Small guanosine triphosphate hydrolases (GTPases) of the Rab family are involved in plasma membrane delivery, fusion events, and lysosomal and autophagic degradation pathways, thereby regulating signaling pathways and cell differentiation and function. Osteoclasts are bone-resorbing cells that maintain bone homeostasis. Polarized vesicular trafficking pathways result in the formation of the ruffled border, the osteoclast’s resorptive organelle, which also assists in transcytosis. Here, we reviewed the different roles of Rab GTPases in the endomembrane machinery of osteoclasts and in bone diseases caused by the dysfunction of these proteins, with a particular focus on autophagy and bone resorption. Understanding the molecular mechanisms underlying osteoclast-related bone disease development is critical for developing and improving therapies.

Learning from Monocyte-Macrophage Fusion and Multinucleation: Potential Therapeutic Targets for Osteoporosis and Rheumatoid Arthritis

Excessive bone resorption by osteoclasts (OCs) covers an essential role in developing bone diseases, such as osteoporosis (OP) and rheumatoid arthritis (RA). Monocytes or macrophages fusion and multinucleation (M-FM) are key processes for generating multinucleated mature cells with essential roles in bone remodelling. Depending on the phenotypic heterogeneity of monocyte/macrophage precursors and the extracellular milieu, two distinct morphological and functional cell types can arise mature OCs and giant cells (GCs). Despite their biological relevance in several physiological and pathological responses, many gaps exist in our understanding of their formation and role in bone, including the molecular determinants of cell fusion and multinucleation. Here, we outline fusogenic molecules during M-FM involved in OCs and GCs formation in healthy conditions and during OP and RA. Moreover, we discuss the impact of the inflammatory milieu on modulating macrophages phenotype and their differentiation towards mature cells. Methodological approach envisaged searches on Scopus, Web of Science Core Collection, and EMBASE databases to select relevant studies on M-FM, osteoclastogenesis, inflammation, OP, and RA. This review intends to give a state-of-the-art description of mechanisms beyond osteoclastogenesis and M-FM, with a focus on OP and RA, and to highlight potential biological therapeutic targets to prevent extreme bone loss.

Osteoclasts' Ability to Generate Trenches Rather Than Pits Depends on High Levels of Active Cathepsin K and Efficient Clearance of Resorption Products

Until recently, it was well-accepted that osteoclasts resorb bone according to the resorption cycle model. This model is based on the assumption that osteoclasts are immobile during bone erosion, allowing the actin ring to be firmly attached and thereby provide an effective seal encircling the resorptive compartment. However, through time-lapse, it was recently documented that osteoclasts making elongated resorption cavities and trenches move across the bone surface while efficiently resorbing bone. However, it was also shown that osteoclasts making rounded cavities and pits indeed resorb bone while they are immobile. Only little is known about what distinguishes these two different resorption modes. This is of both basic and clinical interest because these resorption modes are differently sensitive to drugs and are affected by the gender as well as age of the donor. In the present manuscript we show that: 1. levels of active cathepsin K determine the switch from pit to trench mode; 2. pit and trench mode depend on clathrin-mediated endocytosis; and 3. a mechanism integrating release of resorption products and membrane/integrin recycling is required for prolongation of trench mode. Our study therefore contributes to an improved understanding of the molecular and cellular determinants for the two osteoclastic bone resorption modes.

Sotrastaurin, a PKC inhibitor, attenuates RANKL-induced bone resorption and attenuates osteochondral pathologies associated with the development of OA

Osteoarthritis (OA) is a common degenerative disease that affects the musculoskeletal structure of the whole joint, which is characterized by progressive destruction of both articular cartilage and subchondral bone. Treatment of the bone pathologies, particularly osteoclast‐mediated subchondral bone loss in the early stages of OA, could prevent subsequent cartilage degeneration and progression of OA. In the present study, the PKC inhibitor, Sotrastaurin, was found to inhibit RANKL‐induced osteoclast formation in vitro in a dose‐ and time‐dependent manner. In particular, SO exerted its anti‐osteoclastic effect predominantly at the early stages of RANKL stimulation, suggesting inhibitory effects on precursor cell fusion. Using mature osteoclasts cultured on bovine bone discs, we showed that SO also exerts anti‐resorptive effects on mature osteoclasts bone resorptive function. Mechanistically, SO attenuates the early activation of the p38, ERK and JNK signalling pathways, leeding to impaired induction of crucial osteoclast transcription factors c‐Jun, c‐Fos and NFATc1. We also showed that SO treatment significantly inhibited the phosphorylation of PKCδ and MARCKS, an upstream regulator of cathepsin K secretion. Finally, in animal studies, SO significantly alleviates the osteochondral pathologies of subchondral bone destruction as well as articular cartilage degeneration following DMM‐induced OA, markedly improving OARSI scores. The reduced subchondral bone loss was associated with marked reductions in TRAP(+) osteoclasts in the subchondral bone tissue. Collectively, our data provide evidence for the protective effects of SO against OA by preventing aberrant subchondral bone and articular cartilage changes. Thus, SO demonstrates potential for further development as an alternative therapeutic option against OA.

The osteoclast cytoskeleton - current understanding and therapeutic perspectives for osteoporosis

Osteoclasts are giant multinucleated myeloid cells specialized for bone resorption, which is essential for the preservation of bone health throughout life. The activity of osteoclasts relies on the typical organization of osteoclast cytoskeleton components into a highly complex structure comprising actin, microtubules and other cytoskeletal proteins that constitutes the backbone of the bone resorption apparatus. The development of methods to differentiate osteoclasts in culture and manipulate them genetically, as well as improvements in cell imaging technologies, has shed light onto the molecular mechanisms that control the structure and dynamics of the osteoclast cytoskeleton, and thus the mechanism of bone resorption. Although essential for normal bone physiology, abnormal osteoclast activity can cause bone defects, in particular their hyper-activation is commonly associated with many pathologies, hormonal imbalance and medical treatments. Increased bone degradation by osteoclasts provokes progressive bone loss, leading to osteoporosis, with the resulting bone frailty leading to fractures, loss of autonomy and premature death. In this context, the osteoclast cytoskeleton has recently proven to be a relevant therapeutic target for controlling pathological bone resorption levels. Here, we review the present knowledge on the regulatory mechanisms of the osteoclast cytoskeleton that control their bone resorption activity in normal and pathological conditions.

Osteoclast formation at the bone marrow/bone surface interface: Importance of structural elements, matrix, and intercellular communication

Osteoclasts, the multinucleated cells responsible for bone resorption, have an enormous destructive power which demands to be kept under tight control. Accordingly, the identification of molecular signals directing osteoclastogenesis and switching on their resorptive activity have received much attention. Mandatory factors were identified, but a very essential aspect of the control mechanism of osteoclastic resorption, i.e. its spatial control, remains poorly understood. Under physiological conditions, multinucleated osteoclasts are only detected on the bone surface, while their mono-nucleated precursors are only in the bone marrow. How are pre-osteoclasts targeted to the bone surface? How is their progressive differentiation coordinated with their approach to the bone surface sites to be resorbed, which is where they finally fuse? Here we review the information on the bone marrow distribution of differentiating pre-osteoclasts relative to the position of the mandatory factors for their differentiation as well as relative to physical entities that may affect their access to the remodelling sites. This info allows recognizing an "osteoclastogenesis route" through the bone marrow and leading to the coincident fusion/resorption site - but also points to what still remains to be clarified regarding this route and regarding the restriction of fusion at the resorption site. Finally, we discuss the mechanism responsible for the start of resorption and its spatial extension. This review underscores that fully understanding the control of bone resorption requires to consider it in both space and time - which demands taking into account the context of bone tissue.

Investigation of long non-coding RNA expression profiles in patients with post-menopausal osteoporosis by RNA sequencing

The present study aimed to investigate the implication of long non-coding RNA (lncRNA) expression profiles in post-menopausal osteoporosis (PMOP). A total of 10 patients with PMOP and 10 age-matched healthy post-menopausal females as controls were consecutively enrolled. Their peripheral blood mononuclear cells were obtained and lncRNA as well as mRNA expression profiles were detected by RNA sequencing, followed by bioinformatics analyses. The lncRNA expression profiles were able to distinguish patients with PMOP from controls based on principal component analysis and heatmap analysis. In total, 254 upregulated lncRNAs and 359 downregulated lncRNAs were identified in patients with PMOP vs. controls. The top 5 upregulated lncRNAs were RP11-704M14.1, RP11-754N21.1, RP11-408E5.5, ANKRD26P3 and TPTEP1. The top 5 downregulated lncRNAs were RP11-310E22.4, RP11-326K13.4, FABP5P1, SERPINB9P1 and RPL13P2. Based on the interaction of dysregulated lncRNAs and mRNAs by RNA sequencing, functional annotations were then performed. Gene Ontology enrichment analysis revealed that the dysregulated lncRNAs were enriched in terms including apoptotic process and positive regulation of NF-κB transaction, and Kyoto Encyclopedia of Genes and Genomes analysis suggested enrichment in PMOP-associated signaling pathways, including osteoclast differentiation, tumor necrosis factor signaling pathway and mitogen-activated protein kinase signaling pathway. In addition, the regulatory network and circos graph further indicated the implication of lncRNA expression profiles in PMOP via interactions with mRNAs. In conclusion, the present study suggested that aberrant lncRNA expression is deeply involved in the pathogenesis of PMOP by affecting osteoclast differentiation, inflammation and apoptotic processes.

Intracellular organization in cell polarity - placing organelles into the polarity loop

Many studies have investigated the processes that support polarity establishment and maintenance in cells. On the one hand, polarity complexes at the cell cortex and their downstream signaling pathways have been assigned as major regulators of polarity. On the other hand, intracellular organelles and their polarized trafficking routes have emerged as important components of polarity. In this Review, we argue that rather than trying to identify the prime 'culprit', now it is time to consider all these players as a collective. We highlight that understanding the intimate coordination between the polarized cell cortex and the intracellular compass that is defined by organelle positioning is essential to capture the concept of polarity. After briefly reviewing how polarity emerges from a dynamic maintenance of cellular asymmetries, we highlight how intracellular organelles and their associated trafficking routes provide diverse feedback for dynamic cell polarity maintenance. We argue that the asymmetric organelle compass is an indispensable element of the polarity network.

Probing the mechanical landscape – new insights into podosome architecture and mechanics

Podosomes are dynamic adhesion structures formed constitutively by macrophages, dendritic cells and osteoclasts and transiently in a wide variety of cells, such as endothelial cells and megakaryocytes. They mediate numerous functions, including cell–matrix adhesion, extracellular matrix degradation, mechanosensing and cell migration. Podosomes present as micron-sized F-actin cores surrounded by an adhesive ring of integrins and integrin–actin linkers, such as talin and vinculin. In this Review, we highlight recent research that has considerably advanced our understanding of the complex architecture–function relationship of podosomes by demonstrating that the podosome ring actually consists of discontinuous nano-clusters and that the actin network in between podosomes comprises two subsets of unbranched actin filaments, lateral and dorsal podosome-connecting filaments. These lateral and dorsal podosome-connecting filaments connect the core and ring of individual podosomes and adjacent podosomes, respectively. We also highlight recent insights into the podosome cap as a novel regulatory module of actomyosin-based contractility. We propose that these newly identified features are instrumental for the ability of podosomes to generate protrusion forces and to mechanically probe their environment. Furthermore, these new results point to an increasing complexity of podosome architecture and have led to our current view of podosomes as autonomous force generators that drive cell migration.

Medical Treatment for Osteoporosis: From Molecular to Clinical Opinions

Osteoporosis is a major concern all over the world. With aging, a gradual loss of bone mass results in osteopenia and osteoporosis. Heritable factors account for 60–80% of optimal bone mineralization. Modifiable factors, such as weight-bearing exercise, nutrition, body mass, and hormonal milieu, play an important role in the development of osteopenia and osteoporosis in adulthood. Currently, anti-resorptive agents, including estrogen, bisphosphonates, and selective estrogen receptor modulators (SERMs), are the drugs of choice for osteoporosis. Other treatments include parathyroid hormone (PTH) as well as the nutritional support of calcium and vitamin D. New treatments such as tissue-selective estrogen receptor complexes (TSECs) are currently in use too. This review, which is based on a systematic appraisal of the current literature, provides current molecular and genetic opinions on osteoporosis and its medical treatment. It offers evidence-based information to help researchers and clinicians with osteoporosis assessment. However, many issues regarding osteoporosis and its treatment remain unknown or controversial and warrant future investigation.

Coordination between Rac1 and Rab Proteins: Functional Implications in Health and Disease

The small GTPases of the Rho family regulate many aspects of actin dynamics, but are functionally connected to many other cellular processes. Rac1, a member of this family, besides its known function in the regulation of actin cytoskeleton, plays a key role in the production of reactive oxygen species, in gene transcription, in DNA repair, and also has been proven to have specific roles in neurons. This review focuses on the cooperation between Rac1 and Rab proteins, analyzing how the coordination between these GTPases impact on cells and how alterations of their functions lead to disease.