Use of a phage display technique to identify potential osteoblast binding sites within osteoclast lacunae

Optimizing Ligand Valency to Maximize Tendon Accumulation of Peptide-Targeted Nanoparticles

In many tissues, including musculoskeletal tissues such as tendon, systemic delivery typically results in poor targeting of free drugs. Hence, we previously developed a targeted drug delivery nanoparticle (NP) system for tendon healing, leveraging a tartrate resistant acid phosphatase (TRAP) binding peptide (TBP) ligand. The greatest tendon targeting was observed with NPs functionalized with 30 000 TBP ligands per NP at day 7 during the proliferative healing phase, relative to the inflammatory (day 3) and early remodeling (day 14) phases of healing. Nevertheless, TRAP activity varies throughout healing and, therefore, may offer an opportunity for optimizing temporal therapeutic targeting through multivalent interactions. Hence, in this study, we hypothesized that the ligand density (9000-55,000 TBPs per NP) can optimize tendon accumulation on the basis of variable TRAP levels. The multivalent nanoparticles were loaded with three different fluorophores. In vitro, the ligand density and fluorophore had no effect on the physicochemical properties of the NPs, including size, charge, polydispersity index, or dye loading efficiency; however, the TRAP binding affinity correlated positively with the ligand density. In vivo, the ligand density correlated positively with NP homing and retention in the tendon, establishing opportunities to leverage ligand density for tendon targeting across the tendon healing cascade, during aging, and in other tendon pathologies, including tendinopathies.

Development of a nanoparticle-based tendon-targeting drug delivery system to pharmacologically modulate tendon healing

Satisfactory healing following acute tendon injury is marred by fibrosis. Despite the high frequency of tendon injuries and poor outcomes, there are no pharmacological therapies in use to enhance the healing process. Moreover, systemic treatments demonstrate poor tendon homing, limiting the beneficial effects of potential tendon therapeutics. To address this unmet need, we leveraged our existing tendon healing spatial transcriptomics dataset and identified an area enriched for expression of Acp5 (TRAP) and subsequently demonstrated robust TRAP activity in the healing tendon. This unexpected finding allowed us to refine and apply our existing TRAP binding peptide (TBP) functionalized nanoparticle (NP) drug delivery system (DDS) to facilitate improved delivery of systemic treatments to the healing tendon. To demonstrate the translational potential of this DDS, we delivered niclosamide (NEN), an S100a4 inhibitor. While systemic delivery of free NEN did not alter healing, TBP-NPNEN enhanced both functional and mechanical recovery, demonstrating the translational potential of this approach to enhance the tendon healing process.

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.

Bone-Targeted Nanoparticle Drug Delivery System-Mediated Macrophage Modulation for Enhanced Fracture Healing

Despite decades of progress, developing minimally invasive bone-specific drug delivery systems (DDS) to improve fracture healing remains a significant clinical challenge. To address this critical therapeutic need, nanoparticle (NP) DDS comprised of poly(styrene-alt-maleic anhydride)-b-poly(styrene) (PSMA-b-PS) functionalized with a peptide that targets tartrate-resistant acid phosphatase (TRAP) and achieves preferential fracture accumulation has been developed. The delivery of AR28, a glycogen synthase kinase-3 beta (GSK3β) inhibitor, via the TRAP binding peptide-NP (TBP-NP) expedites fracture healing. Interestingly, however, NPs are predominantly taken up by fracture-associated macrophages rather than cells typically associated with fracture healing. Therefore, the underlying mechanism of healing via TBP-NP is comprehensively investigated herein. TBP-NPAR28 promotes M2 macrophage polarization and enhances osteogenesis in preosteoblast-macrophage co-cultures in vitro. Longitudinal analysis of TBP-NPAR28 -mediated fracture healing reveals distinct spatial distributions of M2 macrophages, an increased M2/M1 ratio, and upregulation of anti-inflammatory and downregulated pro-inflammatory genes compared to controls. This work demonstrates the underlying therapeutic mechanism of bone-targeted NP DDS, which leverages macrophages as druggable targets and modulates M2 macrophage polarization to enhance fracture healing, highlighting the therapeutic benefit of this approach for fractures and bone-associated diseases.

Integrating osteoimmunology and nanoparticle-based drug delivery systems for enhanced fracture healing

Fracture healing is a complex interplay of molecular and cellular mechanisms lasting from days to weeks. The inflammatory phase is the first stage of fracture healing and is critical in setting the stage for successful healing. There has been growing interest in exploring the role of the immune system and novel therapeutic strategies, such as nanoparticle drug delivery systems in enhancing fracture healing. Advancements in nanotechnology have revolutionized drug delivery systems to the extent that they can modulate immune response during fracture healing by leveraging unique physiochemical properties. Therefore, understanding the intricate interactions between nanoparticle-based drug delivery systems and the immune response, specifically macrophages, is essential for therapeutic efficacy. This review provides a comprehensive overview of the relationship between the immune system and nanoparticles during fracture healing. Specifically, we highlight the influence of nanoparticle characteristics, such as size, surface properties, and composition, on macrophage activation, polarization, and subsequent immune responses. IMPACT STATEMENT: This review provides valuable insights into the interplay between fracture healing, the immune system, and nanoparticle-based drug delivery systems. Understanding nanoparticle-macrophage interactions can advance the development of innovative therapeutic approaches to enhance fracture healing, improve patient outcomes, and pave the way for advancements in regenerative medicine.

Development of a Nanoparticle-Based Tendon-Targeting Drug Delivery System to Pharmacologically Modulate Tendon Healing

Tendon regeneration following acute injury is marred by a fibrotic healing response that prevents complete functional recovery. Despite the high frequency of tendon injuries and the poor outcomes, including functional deficits and elevated risk of re-injury, there are currently no pharmacological therapies in clinical use to enhance the healing process. Several promising pharmacotherapies have been identified; however, systemic treatments lack tendon specificity, resulting in poor tendon biodistribution and perhaps explaining the largely limited beneficial effects of these treatments on the tendon healing process. To address this major unmet need, we leveraged our existing spatial transcriptomics dataset of the tendon healing process to identify an area of the healing tendon that is enriched for expression of Acp5. Acp5 encodes tartrate-resistant acid phosphatase (TRAP), and we demonstrate robust TRAP activity in the healing tendon. This unexpected finding allowed us to refine and apply our existing TRAP binding peptide (TBP) functionalized nanoparticle (NP) drug delivery system (DDS) to facilitate improved delivery of systemic treatments to the healing tendon. To demonstrate the translational potential of this drug delivery system, we delivered the S100a4 inhibitor, Niclosamide to the healing tendon. We have previously shown that genetic knockdown of S100a4 enhances tendon healing. While systemic delivery of Niclosamide did not affect the healing process, relative to controls, TBP-NP delivery of Niclosamide enhanced both functional and mechanical outcome measures. Collectively, these data identify a novel tendon-targeting drug delivery system and demonstrate the translational potential of this approach to enhance the tendon healing process.

Drug Delivery Approaches to Improve Tendon Healing

Tendon injuries disrupt the transmission of forces from muscle to bone, leading to chronic pain, disability, and a large socioeconomic burden. Tendon injuries are prevalent; there are over 300,000 tendon repair procedures a year in the United States to address acute trauma or chronic tendinopathy. Successful restoration of function after tendon injury remains challenging clinically. Despite improvements in surgical and physical therapy techniques, the high complication rate of tendon repair procedures motivates the use of therapeutic interventions to augment healing. While many biological and tissue engineering approaches have attempted to promote scarless tendon healing, there is currently no standard clinical treatment to improve tendon healing. Moreover, the limited efficacy of systemic delivery of several promising therapeutic candidates highlights the need for tendon-specific drug delivery approaches to facilitate translation. This review article will synthesize the current state-of-the-art methods that have been used for tendon-targeted delivery through both systemic and local treatments, highlight emerging technologies used for tissue-specific drug delivery in other tissue systems, and outline future challenges and opportunities to enhance tendon healing through targeted drug delivery.

Targeting Agents in Biomaterial-Mediated Bone Regeneration

Bone diseases are a global public concern that affect millions of people. Even though current treatments present high efficacy, they also show several side effects. In this sense, the development of biocompatible nanoparticles and macroscopic scaffolds has been shown to improve bone regeneration while diminishing side effects. In this review, we present a new trend in these materials, reporting several examples of materials that specifically recognize several agents of the bone microenvironment. Briefly, we provide a subtle introduction to the bone microenvironment. Then, the different targeting agents are exposed. Afterward, several examples of nanoparticles and scaffolds modified with these agents are shown. Finally, we provide some future perspectives and conclusions. Overall, this topic presents high potential to create promising translational strategies for the treatment of bone-related diseases. We expect this review to provide a comprehensive description of the incipient state-of-the-art of bone-targeting agents in bone regeneration.

A novel nano delivery system targeting different stages of osteoclasts

Osteoclast (OC) abnormalities represent osteoporosis's critical mechanism (OP). OCs undergo multiple processes that range from monocytic to functional. Different drugs target OCs at different developmental stages; however, almost no Suitable drug-targeted delivery systems exist. Therefore, we designed two dual-targeting nanoparticles to target OCs at different functional stages. Using the calcitonin gene-related peptide receptor (CGRPR), which OC precursors highly express, and specific TRAPpeptides screened in the bone resorption lacuna, where mature OCs function, respectively, two types of dual-targeted nanoparticles were constructed. Afterwards, nanoparticles were grafted with hyaluronic acid (HA), which specifically binds to CD44 on the surface of the OCs. In vivo and in vitro experiments show that both nanoparticles have noticeable targeting effects on OCs. This suggests that dual-targeting nanoparticles designed for different functional periods of OC can be well targeted to the corresponding OC, and further promote the more precise delivery of drugs used to treat OP.

Reduction of leukemic burden via bone-targeted nanoparticle delivery of an inhibitor of C-chemokine (C-C motif) ligand 3 (CCL3) signaling

Leukemias are challenging diseases to treat due, in part, to interactions between leukemia cells and the bone marrow microenvironment (BMME) that contribute significantly to disease progression. Studies have shown that leukemic cells secrete C-chemokine (C-C motif) ligand 3 (CCL3), to disrupt the BMME resulting in loss of hematopoiesis and support of leukemic cell survival and proliferation. In this study, a murine model of blast crisis chronic myelogenous leukemia (bcCML) that expresses the translocation products BCR/ABL and Nup98/HoxA9 was used to determine the role of CCL3 in BMME regulation. Leukemic cells derived from CCL3-/- mice were shown to minimally engraft in a normal BMME, thereby demonstrating that CCL3 signaling was necessary to recapitulate bcCML disease. Further analysis showed disruption in hematopoiesis within the BMME in the bcCML model. To rescue the altered BMME, therapeutic inhibition of CCL3 signaling was investigated using bone-targeted nanoparticles (NP) to deliver Maraviroc, an inhibitor of C-C chemokine receptor type 5 (CCR5), a CCL3 receptor. NP-mediated Maraviroc delivery partially restored the BMME, significantly reduced leukemic burden, and improved survival. Overall, our results demonstrate that inhibiting CCL3 via CCR5 antagonism is a potential therapeutic approach to restore normal hematopoiesis as well as reduce leukemic burden within the BMME.

Re-thinking the bone remodeling cycle mechanism and the origin of bone loss

Proper bone remodeling necessarily requires that osteoblasts reconstruct the bone that osteoclasts have resorbed. However, the cellular events connecting resorption to reconstruction have remained poorly known. The consequence is a fragmentary understanding of the remodeling cycle where only the resorption and formation steps are taken into account. New tools have recently made possible to elucidate how resorption shifts to formation, thereby allowing to comprehend the remodeling cycle as a whole. This new knowledge is reviewed herein. It shows how teams of osteoclasts and osteoblast lineage cells are progressively established and how they are subjected therein to reciprocal interactions. Contrary to the common view, osteoclasts and osteoprogenitors are intermingled on the eroded surfaces. The analysis of the resorption and cell population dynamics shows that osteoprogenitor cell expansion and resorption proceed as an integrated mechanism; that a threshold cell density of osteoprogenitors on the eroded surface is mandatory for onset of bone formation; that the cell initiating osteoprogenitor cell expansion is the osteoclast; and that the osteoclast therefore triggers putative osteoprogenitor reservoirs positioned at proximity of the eroded bone surface (bone lining cells, canopy cells, pericytes). The interplay between magnitude of resorption and rate of cell expansion governs how soon bone reconstruction is initiated and may determine uncoupling and permanent bone loss if a threshold cell density is not reached. The clinical perspectives opened by these findings are discussed.

Osteoclasts secrete osteopontin into resorption lacunae during bone resorption

Osteopontin (OPN) is a non-collagenous extracellular sialylated glycoprotein located in bone. It is believed to be one of the key components in osteoclast attachment to bone during resorption. In this study, we characterized OPN and other glycoproteins found in the resorption lacunae to confirm the role of osteoclasts in OPN secretion using electron microscopy and mass spectrometry. Additionally, we examined the glycan epitopes of resorption pits and the effects of different glycan epitopes on the differentiation and function of osteoclasts. Osteoarthritic femoral heads were examined by immunohistochemistry to reveal the presence of OPN in areas of increased bone metabolism in vivo. Our results demonstrate that human osteoclasts secrete OPN into resorption lacunae on native human bone and on carbonated hydroxyapatite devoid of natural OPN. OPN is associated with an elevated bone turnover in osteoarthritic bone under experimental conditions. Our data further confirm that osteoclasts secrete OPN into the resorption pit where it may function as a chemokine for subsequent bone formation. We show that α2,3- and α2,6-linked sialic acids have a role in the process of osteoclast differentiation. OPN is one of the proteins that has both of the above sialic residues, hence we propose that de-sialylation can effect osteoclast differentiation in bone.

In Vivo Translation of Peptide-Targeted Drug Delivery Systems Discovered by Phage Display

Therapeutic compounds with narrow therapeutic windows and significant systemic side effects benefit from targeted drug delivery strategies. Peptide-protein interactions are often exploited for targeting, with phage display a primary method to identify high-affinity peptide ligands that bind cell surface and matrix bound receptors preferentially expressed in target tissues. After isolating and sequencing high-binding phages, peptides are easily synthesized and chemically modified for incorporation into drug delivery systems, including peptide-drug conjugates, polymers, and nanoparticles. This review describes the phage display methodology to identify targeting peptide sequences, strategies to functionalize drug carriers with phage-derived peptides, specific examples of drug carriers with in vivo translation, and limitations and future applications of phage display to drug delivery.

Multivalent Presentation of Peptide Targeting Groups Alters Polymer Biodistribution to Target Tissues

Drug delivery to bone is challenging, whereby drug distribution is commonly <1% of injected dose, despite development of several bone-targeted drug delivery systems specific to hydroxyapatite. These bone-targeted drug delivery systems still suffer from poor target cell localization within bone, as at any given time overall bone volume is far greater than acutely remodeling bone volume, which harbors relevant cell targets (osteoclasts or osteoblasts). Thus, there exists a need to target bone-acting drugs specifically to sites of bone remodeling. To address this need, this study synthesized oligo(ethylene glycol) copolymers based on a peptide with high affinity to tartrate-resistant acid phosphatase (TRAP), an enzyme deposited by osteoclasts during the bone resorption phase of bone remodeling, which provides greater specificity relevant for bone cell drugging. Gradient and random peptide orientations, as well as polymer molecular weights, were investigated. TRAP-targeted, high molecular weight (M_n) random copolymers exhibited superior accumulation in remodeling bone, where fracture accumulation was observed for at least 1 week and accounted for 14% of tissue distribution. Intermediate and low M_n random copolymer accumulation was lower, indicating residence time depends on M_n. High M_n gradient polymers were cleared, with only 2% persisting at fractures after 1 week, suggesting TRAP binding depends on peptide density. Peptide density and M_n are easily modified in this versatile targeting platform, which can be applied to a range of bone drug delivery applications.

Fracture-Targeted Delivery of β-Catenin Agonists via Peptide-Functionalized Nanoparticles Augments Fracture Healing

Despite several decades of progress, bone-specific drug delivery is still a major challenge. Current bone-acting drugs require high-dose systemic administration which decreases therapeutic efficacy and increases off-target tissue effects. Here, a bone-targeted nanoparticle (NP) delivery system for a β-catenin agonist, 3-amino-6-(4-((4-methylpiperazin-1-yl)sulfonyl)phenyl)-N-(pyridin-3-yl)pyrazine-2-carboxamide, a glycogen synthase kinase 3 beta (GSK-3β) inhibitor, was developed to enhance fracture healing. The GSK-3β inhibitor loading capacity was found to be 15 wt % within highly stable poly(styrene-alt-maleic anhydride)-b-poly(styrene) NPs, resulting in ∼50 nm particles with ∼ -30 mV surface charge. A peptide with high affinity for tartrate-resistant acid phosphatase (TRAP), a protein deposited by osteoclasts on bone resorptive surfaces, was introduced to the NP corona to achieve preferential delivery to fractured bone. Targeted NPs showed improved pharmacokinetic profiles with greater accumulation at fractured bone, accompanied by significant uptake in regenerative cell types (mesenchymal stem cells (MSCs) and osteoblasts). MSCs treated with drug-loaded NPs in vitro exhibited 2-fold greater β-catenin signaling than free drug that was sustained for 5 days. To verify similar activity in vivo, TOPGAL reporter mice bearing fractures were treated with targeted GSK-3β inhibitor-loaded NPs. Robust β-galactosidase activity was observed in fracture callus and periosteum treated with targeted carriers versus controls, indicating potent β-catenin activation during the healing process. Enhanced bone formation and microarchitecture were observed in mice treated with GSK-3β inhibitor delivered via TRAP-binding peptide-targeted NPs. Specifically, increased bone bridging, ∼4-fold greater torsional rigidity, and greater volumes of newly deposited bone were observed 28 days after treatment, indicating expedited fracture healing.

Local and targeted drug delivery for bone regeneration

While experimental bone regeneration approaches commonly employ cells, technological hurdles prevent translation of these therapies. Alternatively, emulating the spatiotemporal cascade of endogenous factors through controlled drug delivery may provide superior bone regenerative approaches. Surgically placed drug depots have clinical indications. Additionally, noninvasive systemic delivery can be used as needed for poorly healing bone injuries. However, a major hurdle for systemic delivery is poor bone biodistribution of drugs. Thus, peptides, aptamers, and phosphate-rich compounds with specificity toward proteins, cells, and molecules within the regenerative bone microenvironment may enable the design of targeted carriers with bone biodistribution greater than that achieved by drug alone. These carriers, combined with osteoregenerative drugs and/or stimuli-sensitive linkers, may enhance bone regeneration while minimizing off-target tissue effects.

Early reversal cells in adult human bone remodeling: osteoblastic nature, catabolic functions and interactions with osteoclasts

The mechanism coupling bone resorption and formation is a burning question that remains incompletely answered through the current investigations on osteoclasts and osteoblasts. An attractive hypothesis is that the reversal cells are likely mediators of this coupling. Their nature is a big matter of debate. The present study performed on human cancellous bone is the first one combining in situ hybridization and immunohistochemistry to demonstrate their osteoblastic nature. It shows that the Runx2 and CD56 immunoreactive reversal cells appear to take up TRAcP released by neighboring osteoclasts. Earlier preclinical studies indicate that reversal cells degrade the organic matrix left behind by the osteoclasts and that this degradation is crucial for the initiation of the subsequent bone formation. To our knowledge, this study is the first addressing these catabolic activities in adult human bone through electron microscopy and analysis of molecular markers. Periosteoclastic reversal cells show direct contacts with the osteoclasts and with the demineralized resorption debris. These early reversal cells show (1) ¾-collagen fragments typically generated by extracellular collagenases of the MMP family, (2) MMP-13 (collagenase-3) and (3) the endocytic collagen receptor uPARAP/Endo180. The prevalence of these markers was lower in the later reversal cells, which are located near the osteoid surfaces and morphologically resemble mature bone-forming osteoblasts. In conclusion, this study demonstrates that reversal cells colonizing bone surfaces right after resorption are osteoblast-lineage cells, and extends to adult human bone remodeling their role in rendering eroded surfaces osteogenic.

The growth factor-like adipokine tartrate-resistant acid phosphatase 5a interacts with the rod G3 domain of adipocyte-produced nidogen-2

The adipokine tartrate resistant acid phosphatase (TRAP) 5a isoform exerts a growth factor-effect on pre-adipocytes. This study aimed to identify potential TRAP 5a interacting proteins in pre-adipocytes using pull down assays in combination with mass spectrometry. Nidogen-2, a protein shown to be expressed intracellularly and for secretion by pre-adipocytes, was shown to interact, through its globular G3 domain, with TRAP 5a in vitro. In vivo, TRAP 5a interacted with nidogen-2 in cultured 3T3-L1 mouse pre-adipocytes, as well as with transforming growth factor-β (TGF-β) interacting protein (TRIP-1), which is a protein that has previously been suggested to interact with TRAP in bone. In addition, TRAP 5a and nidogen-2 co-localized in adipose tissue cells in situ. These results indicate that TRAP 5a interacts with nidogen-2 and TRIP-1 in pre-adipocytic cells.

The reversal phase of the bone-remodeling cycle: cellular prerequisites for coupling resorption and formation

The reversal phase couples bone resorption to bone formation by generating an osteogenic environment at remodeling sites. The coupling mechanism remains poorly understood, despite the identification of a number of ‘coupling' osteogenic molecules. A possible reason is the poor attention for the cells leading to osteogenesis during the reversal phase. This review aims at creating awareness of these cells and their activities in adult cancellous bone. It relates cell events (i) on the bone surface, (ii) in the mesenchymal envelope surrounding the bone marrow and appearing as a canopy above remodeling surfaces and (iii) in the bone marrow itself within a 50-μm distance of this canopy. When bone remodeling is initiated, osteoprogenitors at these three different levels are activated, likely as a result of a rearrangement of cell–cell and cell–matrix interactions. Notably, canopies are brought under the osteogenic influence of capillaries and osteoclasts, whereas bone surface cells become exposed to the eroded matrix and other osteoclast products. In several diverse pathophysiological situations, including osteoporosis, a decreased availability of osteoprogenitors from these local reservoirs coincides with decreased osteoblast recruitment and impaired initiation of bone formation, that is, uncoupling. Overall, this review stresses that coupling does not only depend on molecules able to activate osteogenesis, but that it also demands the presence of osteoprogenitors and ordered cell rearrangements at the remodeling site. It points to protection of local osteoprogenitors as a critical strategy to prevent bone loss.

Caveolae-mediated endocytosis of the glucosaminoglycan-interacting adipokine tartrate resistant acid phosphatase 5a in adipocyte progenitor lineage cells

Adipogenesis depends on growth factors controlling proliferation/differentiation of mesenchymal stem cells (MSCs). Membrane binding and endocytosis of growth factors are often coupled to receptor activation and downstream signaling leading to specific cellular responses. The novel adipokine tartrate-resistant acid phosphatase (TRAP) 5a exhibits a growth factor-like effect on MSCs and pre-adipocytes and induces hyperplastic obesity in vivo. However its molecular interaction with pre-adipocytes remains unknown. Therefore, this study aimed to investigate membrane interaction of TRAP and its endocytosis routes in pre-adipocytes. Confocal and/or electron microscopy were used to detect TRAP in untreated or TRAP 5a/b treated pre-adipocytes under conditions that allow or inhibit endocytosis in combination with co-staining of endocytotic vesicles. TRAP interaction with heparin/heparan sulfate was verified by gel filtration. It could be shown that TRAP 5a, but not 5b, binds to the membrane of pre-adipocytes where it co-localizes with heparin-sulfate proteoglycan glypican-4. Also in vitro, TRAP 5a exhibited affinity for both heparin and heparan sulfate with heparin inhibiting its enzyme activity. Upon caveolae-mediated endocytosis of saturating levels of TRAP 5a, TRAP 5a co-localized intracellularly with glypican-4 and late endosomal marker Rab-7 positive vesicles. The protein was also located in multivesicular bodies (MVBs) but did not co-localize with lysosomal marker LAMP-1. TRAP 5a endocytosis was also detectable in pre-osteoblasts, but not fibroblasts, embryonic MSCs or mature adipocytes. These results indicate that TRAP 5a exhibits binding to cell surface, endocytosis and affinity to glucosaminoglycans (GAGs) in pre-adipocyte and pre-osteoblast lineage cells in a manner similar to other heparin-binding growth factors.

Autosomal dominant osteopetrosis revisited: lessons from recent studies

Systematic studies of autosomal dominant osteopetrosis (ADO) were followed by the identification of underlying mutations giving unique possibilities to perform translational studies. What was previously designated ADO1 turned out to be a high bone mass phenotype caused by a missense mutation in the first propeller of LRP5, a region of importance for binding inhibitory proteins. Thereby, ADO1 cannot be regarded as a classical form of osteopetrosis but must now be considered a disease of LRP5 activation. ADO (Albers-Schönberg disease, or previously ADO2) is characterized by increased number of osteoclasts and a defect in the chloride transport system (ClC-7) of importance for acidification of the resorption lacuna (a form of Chloride Channel 7 Deficiency Osteopetrosis). Ex vivo studies of osteoclasts from ADO have shown that cells do form normally but have reduced resorption capacity and an expanded life span. Bone formation seems normal despite decreased osteoclast function. Uncoupling of formation from resorption makes ADO of interest for new strategies for treatment of osteoporosis. Recent studies have integrated bone metabolism in whole-body energy homeostasis. Patients with ADO may have decreased insulin levels indicating importance beyond bone metabolism. There seems to be a paradigm shift in the treatment of osteoporosis. Targeting ClC-7 might introduce a new principle of dual action. Drugs affecting ClC-7 could be antiresorptive, still allowing ongoing bone formation. Inversely, drugs affecting the inhibitory site of LRP5 might stimulate bone formation and inhibit resorption. Thereby, these studies have highlighted several intriguing treatment possibilities, employing novel modes of action, which could provide benefits to the treatment of osteoporosis.

TRIP-1: a regulator of osteoblast function

Transforming growth factor β (TGFβ) receptor interacting protein-1 (TRIP-1) is an intracellular protein expressed in osteoblasts with high affinity for type 5b tartrate resistant acid phosphatase (TRAP). It is suggested that through this interaction, TRIP-1 serves as a positive regulator of TGFβ signaling and osteoblast differentiation during bone remodeling. We show here that TRIP-1 is abundant in osteoblasts in vivo and in vitro. TRIP-1 mRNA and protein expression were increased at early stages and decreased at later stages during osteoblast differentiation, suggesting a predominant role during early maturation. To investigate a role during bone remodeling, primary osteoblasts were treated with different hormones and factors that are known to affect remodeling. TRIP-1 levels were decreased with dexamethasone and increased with vitamin D(3) , dihydrotestosterone (DHT), TGFβ1, and bone morphogenic protein 2 (BMP-2). Treatment with parathyroid hormone (PTH) and β-estradiol did not affect TRIP-1 levels. Transfected small interfering RNA (siRNA) against TRIP-1 inhibited osteoblast differentiation as characterized by a decrease in alkaline phosphatase staining and enzyme activity, and decrease in the expression of collagen I, alkaline phosphatase, Runx2, osteopontin, and osteocalcin. The proliferation of osteoblasts was also affected by TRIP-1 siRNA. This particular effect was defined by decreased cell number, marked reduction of cyclin D1, a 38% decrease of cells in S phase (p < 0.001) and a 97% increase of cells in the G2/M phase (p < 0.01) of the cell cycle. However, TRIP-1 siRNA did not induce an effect in apoptosis. Using a TGFβ luciferase reporter we found that knocking down TRIP-1 decreased the activation of TGFβ signaling by 40% percent (p < 0.001). In conclusion, our characterization of TRIP-1 in osteoblasts provides the first evidence of its key role as a positive regulator of osteoblast function.

A molecular description of acid phosphatase

Acid phosphatase is ubiquitous in distribution in various organisms. Although it catalyzes simple hydrolytic reactions, it is considered as an interesting enzyme in biological systems due to its involvement in different physiological activities. However, earlier reviews on acid phosphatase reveal some fragmentary information and do not give a holistic view on this enzyme. So, the present review summarizes studies on biochemical properties, structure, catalytic mechanism, and applications of acid phosphatase. Recent advancement of acid phosphatase in agricultural and clinical fields is emphasized where it is presented as potent agent for sustainable agricultural practices and diagnostic marker in bone metabolic disorders. Also, its significance in prostate cancer therapies as a therapeutic target has been discussed. At the end, current studies and prospects of immobilized acid phosphatase are included.

Mechanobiological regulation of the remodelling cycle in trabecular bone and possible biomechanical pathways for osteoporosis

BACKGROUND

The rapid loss of trabeculae as observed during osteoporosis is attributed to pathological changes in the bone remodelling process. In this study, it is proposed that osteoporosis is due to altered signals resulting from either (i) a decrease in the mechanosensitivity of the sensor cells or (ii) an increase in the bone tissue elastic modulus.

METHODS

To test these hypotheses, a mechanobiological algorithm was developed and applied to simulate the remodelling cycle in a realistic trabecular strut. The model is based on the supposition that bone resorption is initiated either to remove damaged tissue or when strains fall below a lower threshold; bone formation is triggered when strains exceed an upper threshold.

FINDINGS

Applying this algorithm to a realistic trabecula, resorption and subsequent refilling of a cavity was simulated. Results showed that decreases in the mechanosensitivity (simulated by increasing the upper strain threshold) led to under-refilling of cavities. A critical sensitivity was found to exist, above which perforation of the strut due to osteoclastic resorption occurred. It was also found that increases in the bone tissue elastic modulus lead to an increased propensity for trabecular perforation.

INTERPRETATION

It may be concluded that if cells become less mechanosensitive, or if increases in the elastic modulus of trabecular bone tissue occurs, the possibility of trabecular perforation and therefore the rapid loss of bone mass increases. If this is true, the preservation of the bone mineral content or maintenance of bone cell mechanosensitivity are potential therapeutic strategies for the prevention of osteoporosis.

Biology and clinical significance of tartrate-resistant acid phosphatases: new perspectives on an old enzyme

Type 5 tartrate-resistant acid phosphatase (TRAP) has been a clinically relevant biomarker for about 50 years. It has always been a reliable and specific cytochemical marker for hairy cell leukemia and for differentiated cells of monocytic lineage. Only recently has the test for serum TRAP activity been accepted as sensitive and specific enough for clinical use as a marker of osteoclasts and bone resorption. This has come about through steady advances in knowledge about TRAP enzymology, structure, function, and molecular regulation and a consequent appreciation that TRAP isoforms 5a and 5b have very different clinical significance. As a measure of osteoclast number and bone resorption, TRAP 5b has diagnostic and prognostic applications in osteoporosis, cancers with bone metastasis, chronic renal failure, and perhaps other metabolic and pathologic bone diseases. Serum TRAP 5a, on the other hand, has no relationship to bone metabolism but seems instead to be a measure of activated macrophages and chronic inflammation. Exploration of the real clinical usefulness of serum TRAP 5a for diagnosis and disease management in a wide variety of chronic inflammatory diseases is only now beginning. This perspective traces the important basic scientific developments that have led up to the refinement of serum TRAP isoform immunoassays and their validation as biomarkers of disease. Many unanswered questions remain, providing a wealth of opportunity for continued research of this multifaceted enzyme.

Local communication on and within bone controls bone remodeling

Bone remodeling is required for healthy calcium homeostasis and for repair of damage occurring with stress and age. Osteoclasts resorb bone and osteoblasts form bone. These processes normally occur in a tightly regulated sequence of events, where the amount of formed bone equals the amount of resorbed bone, thereby restoring the removed bone completely. Osteocytes are the third cell type playing an essential role in bone turnover. They appear to regulate activation of bone remodeling, and they exert both positive and negative regulation on both osteoclasts and osteoblasts. In this review, we consider the intricate communication between these bone cells in relation to bone remodeling, reviewing novel data from patients with mutations rendering different cell populations inactive, which have shown that these interactions are more complex than originally thought. We highlight the high probability that a detailed understanding of these processes will aid in the development of novel treatments for bone metabolic disorders, i.e. we discuss the possibility that bone resorption can be attenuated pharmacologically without a secondary reduction in bone formation.

Loss of trabeculae by mechano-biological means may explain rapid bone loss in osteoporosis

Osteoporosis is characterized by rapid and irreversible loss of trabecular bone tissue leading to increased bone fragility. In this study, we hypothesize two causes for rapid loss of bone trabeculae; firstly, the perforation of trabeculae is caused by osteoclasts resorbing a cavity so deep that it cannot be refilled and, secondly, the increases in bone tissue elastic modulus lead to increased propensity for trabecular perforation. These hypotheses were tested using an algorithm that was based on two premises: (i) bone remodelling is a turnover process that repairs damaged bone tissue by resorbing and returning it to a homeostatic strain level and (ii) osteoblast attachment is under biochemical control. It was found that a mechano-biological algorithm based on these premises can simulate the remodelling cycle in a trabecular strut where damaged bone is resorbed to form a pit that is subsequently refilled with new bone. Furthermore, the simulation predicts that there is a depth of resorption cavity deeper than which refilling of the resorption pits is impossible and perforation inevitably occurs. However, perforation does not occur by a single fracture event but by continual removal of microdamage after it forms beneath the resorption pit. The simulation also predicts that perforations would occur more easily in trabeculae that are more highly mineralized (stiffer). Since both increased osteoclast activation rates and increased mineralization have been measured in osteoporotic bone, either or both may contribute to the rapid loss of trabecular bone mass observed in osteoporotic patients.

Osteoclast-osteoblast communication

Cells in osteoclast and osteoblast lineages communicate with each other through cell-cell contact, diffusible paracrine factors and cell-bone matrix interaction. Osteoclast-osteoblast communication occurs in a basic multicellular unit (BMU) at the initiation, transition and termination phases of bone remodeling. At the initiation phase, hematopoietic precursors are recruited to the BMU. These precursors express cell surface receptors including c-Fms, RANK and costimulatory molecules, such as osteoclast-associated receptor (OSCAR), and differentiate into osteoclasts following cell-cell contact with osteoblasts, which express ligands. Subsequently, the transition from bone resorption to formation is mediated by osteoclast-derived 'coupling factors', which direct the differentiation and activation of osteoblasts in resorbed lacunae to refill it with new bone. Bidirectional signaling generated by interaction between ephrinB2 on osteoclasts and EphB4 on osteoblast precursors facilitates the transition. Such interaction is likely to occur between osteoclasts and lining cells in the bone remodeling compartment (BRC). At the termination phase, bone remodeling is completed by osteoblastic bone formation and mineralization of bone matrix. Here, we describe molecular communication between osteoclasts and osteoblasts at distinct phases of bone remodeling.

Expression and synthesis of bone morphogenetic proteins by osteoclasts: a possible path to anabolic bone remodeling

Skeletal remodeling is a finely orchestrated process coupling bone formation to bone resorption. The dynamics of coupling is regulated by the microenvironment at the bone remodeling site, which in turn is influenced by the intercellular communication between cells like osteoclasts and osteoblasts. Understanding the dynamics of coupling is important in devising new therapeutic approaches to the treatment of skeletal diseases characterized by disturbances in the bone remodeling process. In this study, we report the localization of bone morphogenetic proteins (BMPs) in osteoclasts generated from primary cocultures of bone marrow cells from mouse femur and tibia with mouse calvarial osteoblasts, using immunocytochemistry and in situ hybridization. Positive staining was seen in osteoclasts for BMP-2, -4, -6, and -7. Real-time PCR was used to quantitatively confirm the expression of transcripts for BMP-2, BMP-4, and BMP-6 mRNA in murine osteoclasts. Finally, the presence of BMP-2, -4, -6, and-7 proteins was confirmed in osteoclast lysates by Western blotting. Overall, our data suggest a possible direct role for osteoclasts in promoting bone formation via expression and synthesis of BMPs, which then would play an important role in promoting the recruitment, proliferation, and differentiation of osteoblasts at bone resorption sites.

Are nonresorbing osteoclasts sources of bone anabolic activity?

Some osteopetrotic mutations lead to low resorption, increased numbers of osteoclasts, and increased bone formation, whereas other osteopetrotic mutations lead to low resorption, low numbers of osteoclasts, and decreased bone formation. Elaborating on these findings, we discuss the possibility that osteoclasts are the source of anabolic signals for osteoblasts. In normal healthy individuals, bone formation is coupled to bone resorption in a tight equilibrium. When this delicate balance is disturbed, the net result is pathological situations, such as osteopetrosis or osteoporosis. Human osteopetrosis, caused by mutations in proteins involved in the acidification of the resorption lacuna (ClC-7 or the a3-V-ATPase), is characterized by decreased resorption in face of normal or even increased bone formation. Mouse mutations leading to ablation of osteoclasts (e.g., loss of macrophage-colony stimulating factor [M-CSF] or c-fos) lead to secondary negative effects on bone formation, in contrast to mutations where bone resorption is abrogated with sustained osteoclast numbers, such as the c-src mice. These data indicate a central role for osteoclasts, and not necessarily their resorptive activity, in the control of bone formation. In this review, we consider the balance between bone resorption and bone formation, reviewing novel data that have shown that this principle is more complex than originally thought. We highlight the distinct possibility that osteoclast function can be divided into two more or less separate functions, namely bone resorption and stimulation of bone formation. Finally, we describe the likely possibility that bone resorption can be attenuated pharmacologically without the undesirable reduction in bone formation.

Osteoblastic Response to Resorbed Ceramic Surfaces; The Role of the Osteoclast in Osteoconduction

The mechanism by which carbonate substitution within the hydroxyapatite (HA) lattice improves osteoconduction is unclear. Discs of dense, sintered, phase-pure HA and carbonate substituted hydroxyapatite (CHA) were cultured with human CD14+ cells in the presence of macrophage-colony stimulating factor (M-CSF) and soluble receptor activator of nuclear factor (NF)-κB (sRANKL), during which time osteoclasts developed and resorbed the ceramic surface. Discs were then seeded with human osteoblasts (HOBs), and proliferation and collagen synthesis measured. Proliferation was increased on resorbed compared to control (unresorbed) surfaces on both materials. Collagen synthesis was increased on CHA compared to HA, an increase accelerated on a previously resorbed surface. The results suggest that osteoclasts can condition synthetic bioceramic surfaces and alter the responses of osteoblasts which subsequently populate them. Carbonate substitution may enhance osteoconduction via effects on enhanced bioresorption.

Differential regulation of osteoblasts by substrate microstructural features

Microtextured titanium implant surfaces enhance bone formation in vivo and osteoblast phenotypic expression in vitro, but the mechanisms are not understood. To determine the roles of specific microarchitectural features in modulating osteoblast behavior, we used Ti surfaces prepared by electrochemical micromachining as substrates for MG63 osteoblast-like cell culture. Cell response was compared to tissue culture plastic, a sand-blasted with large grit and acid-etched surface with defined mixed microtopography (SLA), polished Ti surfaces, and polished surfaces electrochemically machined through a photoresist pattern to produce cavities with 100, 30 and 10 microm diameters arranged so that the ratio of the microscopic-scale area of the cavities versus the microscopic-scale area of the flat region between the cavities was equal to 1 or 6. Microstructured disks were acid-etched, producing overall sub-micron-scale roughness (Ra=0.7 microm). Cell number, differentiation (alkaline phosphatase; osteocalcin) and local factor levels (TGF-beta1; PGE(2)) varied with microarchitecture. 100 microm cavities favored osteoblast attachment and growth, the sub-micron-scale etch enhanced differentiation and TGF-beta1 production, whereas PGE(2) depended on cavity dimensions but not the sub-micron-scale roughness.

Osteoclast deficiency results in disorganized matrix, reduced mineralization, and abnormal osteoblast behavior in developing bone

Studies of the influence of the osteoclast on bone development, in particular on mineralization and the formation of the highly organized lamellar architecture of cortical bone by osteoblasts, have not been reported. We therefore examined the micro- and ultrastructure of the developing bones of osteoclast-deficient CSF-1R-nullizygous mice (Csf1r(-/-) mice).

INTRODUCTION

Colony-stimulating factor-1 receptor (CSF-1R)-mediated signaling is critical for osteoclastogenesis. Consequently, the primary defect in osteopetrotic Csf1r(-/-) mice is severe osteoclast deficiency. Csf1r(-/-) mice therefore represent an ideal model system in which to investigate regulation by the osteoclast of osteoblast-mediated bone formation during development.

MATERIALS AND METHODS

Bones of developing Csf1r(-/-) mice and their littermate controls were subjected to X-ray analysis, histological examination by light microscopy and transmission electron microscopy, and a three-point bending assay to test their biomechanical strength. Bone mineralization in embryonic and postnatal bones was visualized by double staining with alcian blue and alizarin red. Bone formation by osteoblasts in these mice was also examined by double-calcein labeling and in femoral anlagen transplantation experiments.

RESULTS AND CONCLUSIONS

Frequent spontaneous fractures and decreased strength parameters (ultimate load, yield load, and stiffness) in a three-point bending assay showed the biomechanical weakness of long bones in Csf1r(-/-) mice. Histologically, these bones have an expanded epiphyseal chondrocyte region, a poorly formed cortex with disorganized collagen fibrils, and a severely disturbed matrix structure. The mineralization of their bone matrix at secondary sites of ossification is significantly reduced. While individual osteoblasts in Csf1r(-/-) mice have preserved their typical ultrastructure and matrix depositing activity, the layered organization of osteoblasts on the bone-forming surface and the direction of their matrix deposition toward the bone surface have been lost, resulting in their abnormal entrapment by matrix. Moreover, we also found that (1) osteoblasts do not express CSF-1R, (2) the bone defects in Csf1r(-/-) embryos develop later than the development of osteoclasts in normal embryos, and (3) the transplanted Csf1r(-/-) femoral anlagen develop normally in the presence of wildtype osteoclasts. These results suggest that the dramatic bone defects in Csf1r(-/-) mice are caused by a deficiency of the osteoclast-mediated regulation of osteoblasts and that the osteoclast plays an important role in regulating osteoblastic bone formation during development, in particular, in the formation of lamellar bone.

Pretreatment of bone with osteoclasts affects phenotypic expression of osteoblast-like cells

Implant surface morphology regulates osteoblast phenotypic expression. Osteoblast sensitivity to non-biologic surfaces suggests that native bone surface features may also affect osteoblast response. To test this, MG63 osteoblast-like cells were grown for 7 days on bovine cortical bone wafers pretreated with rat bone marrow osteoclasts for 0, 10 or 20 days. Response to osteoclast-treated surfaces was compared to the response of MG63 cells to titanium surfaces with smooth and rough microtopographies. Cell number, differentiation (alkaline phosphatase activity and osteocalcin levels), and local factors (PGE(2) and TGF-beta1) were measured in confluent cultures. Compared to culture on plastic, cell number was reduced on all three types of bone wafers; this effect was dose-dependent with increasing resorption of the surface. Alkaline phosphatase specific activity was increased (P<or=0.05) on all three surfaces compared with plastic, but this increase was not dependent on resorption time, indicating this parameter was sensitive to the surface (bovine bone vs. plastic) but not to osteoclast-resorption. There was a direct correlation between the area of the bone surface resorbed and the amount of osteocalcin, TGF-beta1 and PGE(2) (R(2)=0.8025, 0.8689, 0.8896, respectively). With 20 days of osteoclast pretreatment, there was a 20-fold increase in osteocalcin over plastic and a 7-fold increase over cultures on untreated bone wafers. Similar increases were found for TGF-beta1 and PGE(2). Thus, surface changes resulting from osteoclast pretreatment have a strong effect on osteoblast phenotypic expression, and suggest that microtopography may play a role.

A phage display technique identifies a novel regulator of cell differentiation

The formation of new bone during the process of bone remodeling occurs almost exclusively at sites of prior bone resorption. In an attempt to discover what regulatory pathways are utilized by osteoblasts to effect this site-specific formation event we probed components of an active bone resorption surface with an osteoblast phage expression library. In these experiments primary cultures of rat osteoblasts were used to construct a phage display library in T7 phage. Tartrate-resistant acid phosphatase (type V) (TRAP) was used as the bait in a biopanning procedure. 40 phage clones with very high affinity for TRAP were sequenced, and of the clones with multiple consensus sequences we identified a regulatory protein that modulates osteoblast differentiation. This protein is the TGFbeta receptor-interacting protein (TRIP-1). Our data demonstrate that TRAP activation of TRIP-1 evokes a TGFbeta-like differentiation process. Specifically, TRIP-1 activation increases the activity and expression of osteoblast alkaline phosphatase, osteoprotegerin, collagen, and Runx2. Moreover, we show that TRAP interacts with TRIP intracellularly, that activation of the TGFbeta type II receptor by TRIP-1 occurs in the presence of TRAP and that the differentiation process is mediated through the Smad2/3 pathway. A final experiment demonstrates that osteoblasts, when cultured in osteoclast lacunae containing TRAP, rapidly and specifically differentiate into a mature bone-forming phenotype. We hypothesize that binding to TRAP may be one mechanism by which the full osteoblast phenotype is expressed during the process of bone remodeling.