Bone-targeting AAV-mediated silencing of Schnurri-3 prevents bone loss in osteoporosis

Mechanisms and Delivery of tRNA Therapeutics

Transfer ribonucleic acid (tRNA) therapeutics will provide personalized and mutation specific medicines to treat human genetic diseases for which no cures currently exist. The tRNAs are a family of adaptor molecules that interpret the nucleic acid sequences in our genes into the amino acid sequences of proteins that dictate cell function. Humans encode more than 600 tRNA genes. Interestingly, even healthy individuals contain some mutant tRNAs that make mistakes. Missense suppressor tRNAs insert the wrong amino acid in proteins, and nonsense suppressor tRNAs read through premature stop signals to generate full length proteins. Mutations that underlie many human diseases, including neurodegenerative diseases, cancers, and diverse rare genetic disorders, result from missense or nonsense mutations. Thus, specific tRNA variants can be strategically deployed as therapeutic agents to correct genetic defects. We review the mechanisms of tRNA therapeutic activity, the nature of the therapeutic window for nonsense and missense suppression as well as wild-type tRNA supplementation. We discuss the challenges and promises of delivering tRNAs as synthetic RNAs or as gene therapies. Together, tRNA medicines will provide novel treatments for common and rare genetic diseases in humans.

Development of AAV-Mediated Gene Therapy Approaches to Treat Skeletal Diseases

Adeno-associated viral (AAV) vectors have emerged as crucial tools in advancing gene therapy for skeletal diseases, offering the potential for sustained expression with low postinfection immunogenicity and pathogenicity. Preclinical studies support both the therapeutic efficacy and safety of these vectors, illustrating the promise of AAV-mediated gene therapy. Emerging technologies and innovations in AAV-mediated gene therapy strategies, such as gene addition, gene replacement, gene silencing, and gene editing, offer new approaches to clinical application. Recently, the increasing preclinical applications of AAV to rare skeletal diseases, such as fibrodysplasia ossificans progressiva (FOP) and osteogenesis imperfecta (OI), and prevalent bone diseases, such as osteoporosis, bone fracture, critical-sized bone defects, and osteoarthritis, have been reported. Despite existing limitations in clinical use, such as high cost and safety, the AAV-mediated gene transfer platform is a promising approach to deliver therapeutic gene(s) to the skeleton to treat skeletal disorders, including those otherwise intractable by other therapeutic approaches. This review provides a comprehensive overview of the therapeutic advancements, challenges, limitations, and solutions within AAV-based gene therapy for prevalent and rare skeletal diseases.

AAV-based gene editing of type 1 collagen mutation to treat osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a genetic disorder characterized by bone fragility, low bone mass, fractures, and extraskeletal manifestations. Since OI is commonly caused by single-nucleotide mutation(s) in the COL1A1 or COL1A2 genes encoding type I collagens, we developed a genome-editing strategy to correct a Col1a2 mutation in an OIM mouse model resembling a severe dominant form of human type III OI. Using a recombinant adeno-associated virus (rAAV), we delivered CRISPR-Cas9 to bone-forming osteoblast-lineage cells in the skeleton. Homology-directed repair (HDR)-mediated gene editing efficiency in these cells was improved when CRISPR-Cas9 was coupled with a donor AAV vector containing a promoterless partial mouse Col1a2 complementary DNA sequence. This approach effectively reversed the dysregulation of osteogenic differentiation by a Col1a2 mutation in vitro. Furthermore, systemic administration of dual rAAVs in OIM mice lowered bone matrix turnover rates by reducing osteoblast and osteoclast development while improving the cellular network of mechano-sensing osteocytes embedded in the bone matrix. This strategy significantly improved bone architecture/mass/mineralization, skeletal deformities, grip strength, and spontaneous fractures. Our study is the first demonstration that HDR-mediated gene editing via AAV-mediated delivery effectively corrects a collagen mutation in OI osteoblasts and reverses skeletal phenotypes in OIM mice.

Osteocyte-derived sclerostin impairs cognitive function during ageing and Alzheimer's disease progression

Ageing increases susceptibility to neurodegenerative disorders, such as Alzheimer's disease (AD). Serum levels of sclerostin, an osteocyte-derived Wnt-β-catenin signalling antagonist, increase with age and inhibit osteoblastogenesis. As Wnt-β-catenin signalling acts as a protective mechanism for memory, we hypothesize that osteocyte-derived sclerostin can impact cognitive function under pathological conditions. Here we show that osteocyte-derived sclerostin can cross the blood-brain barrier of old mice, where it can dysregulate Wnt-β-catenin signalling. Gain-of-function and loss-of-function experiments show that abnormally elevated osteocyte-derived sclerostin impairs synaptic plasticity and memory in old mice of both sexes. Mechanistically, sclerostin increases amyloid β (Aβ) production through β-catenin-β-secretase 1 (BACE1) signalling, indicating a functional role for sclerostin in AD. Accordingly, high sclerostin levels in patients with AD of both sexes are associated with severe cognitive impairment, which is in line with the acceleration of Αβ production in an AD mouse model with bone-specific overexpression of sclerostin. Thus, we demonstrate osteocyte-derived sclerostin-mediated bone-brain crosstalk, which could serve as a target for developing therapeutic interventions against AD.

Circular RNA circ-3626 promotes bone formation by modulating the miR-338-3p/Runx2 axis

OBJECTIVE

Disorders of bone homeostasis are the key factors leading to metabolic bone disease, such as senile osteoporosis, which is characterized by age-related bone loss. Bone marrow stromal cells (BMSCs) possess high osteogenic capacity which has been regarded as a practical approach to preventing bone loss. Previous studies have shown that the osteogenic differentiation ability of BMSCs is significantly decreased in senile osteoporosis. Recently, circular RNAs (circRNAs) have been regarded as critical regulators in controlling the osteogenic differentiation of BMSCs by sponging microRNAs (miRNAs). Our study aimed to discover new and critical osteogenesis-related circRNAs that can promote bone formation in senile osteoporosis.

METHODS

We detected the dysregulated circRNAs of BMSCs upon osteogenic differentiation induction and identified the critical osteogenic circRNA (circ-3626). The relationship between circ-3626 and osteoporosis was further verified in clinical bone samples and aged mice by qPCR. Moreover, circ-3626 AAV was constructed to examine the osteogenic effect of circ-3626 on bone formation via using Micro-CT, double calcein labeling, and the three-point bending tests. Bioinformatics analysis, Luciferase report gene assays, FISH, RNA pull-down, qPCR, Western Blots, and alizarin red staining assay explore the effects and mechanisms of circ-3626 on osteogenic differentiation of BMSCs.

RESULTS

Circ-3626 was identified as a pivotal osteogenesis-related circRNA via RNA sequencing. The results of alizarin red staining, Western blots, and qPCR assays suggest that overexpressing circ-3626 dramatically accelerates the osteogenic capability of BMSCs. Furthermore, the bone repair capability of aging mice could be significantly improved by circ-3626 AAV treatment. Micro RNA miR-338-3p was identified as the downstream target of circ-3626. Overexpression of circ-3626 increases the expression of Runx2 by sponging miR-338-3p, thereby promoting the osteogenic differentiation of BMSCs by upregulating the expression of osteogenic genes. In addition, Western blots, and qPCR assays suggest circ-3626 AAV treatment promote the expression of Runx2 and osteogenic marker genes.

CONCLUSION

Thus, we demonstrate that circ-3626 plays a pivotal role in promoting bone formation through the miR-338-3p/Runx2 axis and may provide new strategies for preventing and treating the bone loss of senile osteoporosis.

Elimination of Senescent Osteocytes by Bone-Targeting Delivery of β-Galactose-Modified Maytansinoid Prevents Age-Related Bone Loss

The accumulation of senescent cells in bone during aging contributes to senile osteoporosis, and clearance of senescent cells by senolytics could effectively alleviate bone loss. However, the applications of senolytics are limited due to their potential toxicities. Herein, small extracellular vesicles (sEVs) have been modified by incorporating bone-targeting peptide, specifically (AspSerSer)6, to encapsulate galactose-modified Maytansinoids (DM1). These modified vesicles are referred to as (AspSerSer)6-sEVs/DM1-Gal, and they have been designed to specifically clear the senescent osteocytes in bone tissue. In addition, the elevated activity of lysosomal β-galactosidase in senescent osteocytes, but not normal cells in bone tissue, could break down DM1-Gal to release free DM1 for selective elimination of senescent osteocytes. Mechanically, DM1 could disrupt tubulin polymerization, subsequently inducing senescent osteocytes apoptosis. Further, administration of bone-targeting senolytics to aged mice could alleviate aged-related bone loss without non-obvious toxicity. Overall, this bone-targeting senolytics could act as a novel candidate for specific clearance of senescent osteocytes, ameliorating age-related bone loss, with a promising therapeutic potential for senile osteoporosis.

Depletion of SMN protein in mesenchymal progenitors impairs the development of bone and neuromuscular junction in spinal muscular atrophy

Spinal muscular atrophy (SMA) is a neuromuscular disorder characterized by the deficiency of the survival motor neuron (SMN) protein, which leads to motor neuron dysfunction and muscle atrophy. In addition to the requirement for SMN in motor neurons, recent studies suggest that SMN deficiency in peripheral tissues plays a key role in the pathogenesis of SMA. Using limb mesenchymal progenitor cell (MPC)-specific SMN-depleted mouse models, we reveal that SMN reduction in limb MPCs causes defects in the development of bone and neuromuscular junction (NMJ). Specifically, these mice exhibited impaired growth plate homeostasis and reduced insulin-like growth factor (IGF) signaling from chondrocytes, rather than from the liver. Furthermore, the reduction of SMN in fibro-adipogenic progenitors (FAPs) resulted in abnormal NMJ maturation, altered release of neurotransmitters, and NMJ morphological defects. Transplantation of healthy FAPs rescued the morphological deterioration. Our findings highlight the significance of mesenchymal SMN in neuromusculoskeletal pathogenesis of SMA and provide insights into potential therapeutic strategies targeting mesenchymal cells for the treatment of SMA.

Bone-Targeting Peptide and RNF146 Modified Apoptotic Extracellular Vesicles Alleviate Osteoporosis

Background

Osteoporosis is a highly prevalent disease that causes fractures and loss of motor function. Current drugs targeted for osteoporosis often have inevitable side effects. Bone marrow mesenchymal stem cell (BMSCs)-derived apoptotic extracellular vesicles (ApoEVs) are nanoscale extracellular vesicles, which has been shown to promote bone regeneration with low immunogenicity and high biological compatibility. However, natural ApoEVs cannot inherently target bones, and are often eliminated by macrophages in the liver and spleen. Thus, our study aimed to reconstruct ApoEVs to enhance their bone-targeting capabilities and bone-promoting function and to provide a new method for osteoporosis treatment.

Methods

We conjugated a bone-targeting peptide, (Asp-Ser-Ser)6 ((DSS)6), onto the surface of ApoEVs using standard carbodiimide chemistry with DSPE-PEG-COOH serving as the linker. The bone-targeting ability of (DSS)6-ApoEVs was determined using an in vivo imaging system and confocal laser scanning microscopy (CLSM). We then loaded ubiquitin ligase RING finger protein146 (RNF146) into BMSCs via adenovirus transduction to obtain functional ApoEVs. The bone-promoting abilities of (DSS)6-ApoEVs and (DSS)6-ApoEVsRNF146 were measured in vitro and in vivo.

Results

Our study successfully synthesized bone-targeting and gained functional (DSS)6-ApoEVsRNF146 and found that engineered ApoEVs could promote osteogenesis in vitro and exert significant bone-targeting and osteogenesis-promoting effects to alleviate osteoporosis in a mouse model.

Conclusion

To promote the bone-targeting ability of natural ApoEVs, we successfully synthesized engineered ApoEVs, (DSS)6-ApoEVsRNF146 and found that they could significantly promote osteogenesis and alleviate osteoporosis compared with natural ApoEVs, which holds great promise for the treatment of osteoporosis.

Design of Nanodrug Delivery Systems for Tumor Bone Metastasis

Tumor metastasis is a complex process that is controlled at the molecular level by numerous cytokines. Primary breast and prostate tumors most commonly metastasize to bone, and the development of increasingly accurate targeted nanocarrier systems has become a research focus for more effective anti-bone metastasis therapy. This review summarizes the molecular mechanisms of bone metastasis and the principles and methods for designing bone-targeted nanocarriers and then provides an in-depth review of bone-targeted nanocarriers for the treatment of bone metastasis in the context of chemotherapy, photothermal therapy, gene therapy, and combination therapy. Furthermore, this review also discusses the treatment of metastatic and primary bone tumors, providing directions for the design of nanodelivery systems and future research.

Impaired Efferocytosis Enables Apoptotic Osteoblasts to Escape Osteoimmune Surveillance During Aging

Macrophage efferocytosis of apoptotic osteoblasts (apoOBs) is a key osteoimmune process for bone homeostasis. However, apoOBs frequently accumulate in aged bone marrow, where they may mount proinflammatory responses and progressive bone loss. The reason why apoOBs are not cleared during aging remains unclear. In this study, it is demonstrated that aged apoOBs upregulate the immune checkpoint molecule CD47, which is controlled by SIRT6-regulated transcriptional pausing, to evade clearance by macrophages. Using osteoblast- and myeloid-specific gene knockout mice, SIRT6 is further revealed to be a critical modulator for apoOBs clearance via targeting CD47-SIRPα checkpoint. Moreover, apoOBs activate SIRT6-mediated chemotaxis to recruit macrophages by releasing apoptotic vesicles. Two targeting delivery strategies are developed to enhance SIRT6 activity, resulting in rejuvenated apoOBs clearance and delayed age-related bone loss. Collectively, the findings reveal a previously unknown linkage between immune surveillance and bone homeostasis and targeting the SIRT6-regulated mechanism can be a promising therapeutic strategy for age-related bone diseases.

Characterization and biodistribution of under-employed gene therapy vector AAV7

IMPORTANCE

The use of adeno-associated viruses (AAVs) as gene delivery vectors has vast potential for the treatment of many severe human diseases. Over one hundred naturally existing AAV capsid variants have been described and classified into phylogenetic clades based on their sequences. AAV8, AAV9, AAVrh.10, and other intensively studied capsids have been propelled into pre-clinical and clinical use, and more recently, marketed products; however, less-studied capsids may also have desirable properties (e.g., potency differences, tissue tropism, reduced immunogenicity, etc.) that have yet to be thoroughly described. These data will help build a broader structure-function knowledge base in the field, present capsid engineering opportunities, and enable the use of novel capsids with unique properties.

AAV-Mediated Targeting of the Activin A-ACVR1R206H Signaling in Fibrodysplasia Ossificans Progressiva

Fibrodysplasia ossificans progressiva (FOP) is an ultra-rare genetic disorder characterized by progressive disabling heterotopic ossification (HO) at extra-skeletal sites. Here, we developed adeno-associated virus (AAV)-based gene therapy that suppresses trauma-induced HO in FOP mice harboring a heterozygous allele of human ACVR1R206H (Acvr1R206H/+) while limiting the expression in non-skeletal organs such as the brain, heart, lung, liver, and kidney. AAV gene therapy carrying the combination of codon-optimized human ACVR1 (ACVR1opt) and artificial miRNAs targeting Activin A and its receptor ACVR1R206H ablated the aberrant activation of BMP-Smad1/5 signaling and the osteogenic differentiation of Acvr1R206H/+ skeletal progenitors. The local delivery of AAV gene therapy to HO-causing cells in the skeletal muscle resulted in a significant decrease in endochondral bone formation in Acvr1R206H/+ mice. These mice showed little to no expression in a major AAV-targeted organ, the liver, due to liver-abundant miR-122-mediated repression. Thus, AAV gene therapy is a promising therapeutic strategy to explore in suppressing HO in FOP.

CircFam190a: a critical positive regulator of osteoclast differentiation via enhancement of the AKT1/HSP90β complex

The identification of key regulatory factors that control osteoclastogenesis is important. Accumulating evidence indicates that circular RNAs (circRNAs) are discrete functional entities. However, the complexities of circRNA expression as well as the extent of their regulatory functions during osteoclastogenesis have yet to be revealed. Here, based on circular RNA sequencing data, we identified a circular RNA, circFam190a, as a critical regulator of osteoclast differentiation and function. During osteoclastogenesis, circFam190a is significantly upregulated. In vitro, circFam190a enhanced osteoclast formation and function. In vivo, overexpression of circFam190a induced significant bone loss, while knockdown of circFam190a prevented pathological bone loss in an ovariectomized (OVX) mouse osteoporosis model. Mechanistically, our data suggest that circFam90a enhances the binding of AKT1 and HSP90β, promoting AKT1 stability. Altogether, our findings highlight the critical role of circFam190a as a positive regulator of osteoclastogenesis, and targeting circFam190a might be a promising therapeutic strategy for treating pathological bone loss.

MiR133b-mediated inhibition of EGFR-PTK pathway promotes rAAV2 transduction by facilitating intracellular trafficking and augmenting second-strand synthesis

Recombinant adeno-associated virus (rAAV) is an extremely attractive vector in the in vivo delivery of gene therapy as it is safe and its genome is simple. However, challenges including low permissiveness to specific cells and restricted tissue specificity have hindered its clinical application. Based on the previous studies, epidermal growth factor receptor-protein tyrosine kinase (EGFR-PTK) negatively regulated rAAV transduction, and EGFR-positive cells were hardly permissive to rAAV transduction. We constructed a novel rAAV-miRNA133b vector, which co-expressed miRNA133b and transgene, and investigated its in vivo and in vitro transduction efficiency. Confocal microscopy, live-cell imaging, pharmacological reagents and labelled virion tracking were used to analyse the effect of miRNA133b on rAAV2 transduction and the underlying mechanisms. The results demonstrated that miRNA133b could promote rAAV2 transduction and the effects were limited to EGFR-positive cells. The increased transduction was found to be a direct result of decreased rAAV particles degradation in the cytoplasm and enhanced second-strand synthesis. ss-rAAV2-miRNA133b vector specifically increased rAAV2 transduction in EGFR-positive cells or tissues, while ss-rAAV2-Fluc-miRNA133b exerted an antitumor effect. rAAV-miRNA133b vector might emerge as a promising platform for delivering various transgene to treat EGFR-positive cell-related diseases, such as non-small-cell lung cancer.

Schnurri-3 inhibition rescues skeletal fragility and vascular skeletal stem cell niche pathology in a mouse model of osteogenesis imperfecta

Osteogenesis imperfecta (OI) is a disorder of low bone mass and increased fracture risk due to a range of genetic variants that prominently include mutations in genes encoding type collagen. While it is well known that OI reflects defects in the activity of bone-forming osteoblasts, it is currently unclear whether OI also reflects defects in the many other cell types comprising bone, including defects in skeletal vascular endothelium or the skeletal stem cell populations that give rise to osteoblasts and whether correcting these broader defects could have therapeutic utility. Here, we find that numbers of skeletal stem cells (SSCs) and skeletal arterial endothelial cells (AECs) are augmented in Col1a2oim/oim mice, a well-studied animal model of moderate to severe OI, suggesting that disruption of a vascular SSC niche is a feature of OI pathogenesis. Moreover, crossing Col1a2oim/oim mice to mice lacking a negative regulator of skeletal angiogenesis and bone formation, Schnurri 3 (SHN3), not only corrected the SSC and AEC phenotypes but moreover robustly corrected the bone mass and spontaneous fracture phenotypes. As this finding suggested a strong therapeutic utility of SHN3 inhibition for the treatment of OI, a bone-targeting AAV was used to mediate Shn3 knockdown, rescuing the Col1a2oim/oim phenotype and providing therapeutic proof-of-concept for targeting SHN3 for the treatment of OI. Overall, this work both provides proof-of-concept for inhibition of the SHN3 pathway and more broadly addressing defects in the stem/osteoprogentior niche as is a strategy to treat OI.

The intricate mechanism of PLS3 in bone homeostasis and disease

Since our discovery in 2013 that genetic defects in PLS3 lead to bone fragility, the mechanistic details of this process have remained obscure. It has been established that PLS3 variants cause syndromic and nonsyndromic osteoporosis as well as osteoarthritis. PLS3 codes for an actin-bundling protein with a broad pattern of expression. As such, it is puzzling how PLS3 specifically leads to bone-related disease presentation. Our review aims to summarize the current state of knowledge regarding the function of PLS3 in the predominant cell types in the bone tissue, the osteocytes, osteoblasts and osteoclasts. This is related to the role of PLS3 in regulating mechanotransduction, calcium regulation, vesicle trafficking, cell differentiation and mineralization as part of the complex bone pathology presented by PLS3 defects. Considering the consequences of PLS3 defects on multiple aspects of bone tissue metabolism, our review motivates the study of its mechanism in bone diseases which can potentially help in the design of suitable therapy.

An angiogenic approach to osteoanabolic therapy targeting the SHN3-SLIT3 pathway

Often, disorders of impaired bone formation involve not only a cell intrinsic defect in the ability of osteoblasts to form bone, but moreover a broader dysfunction of the skeletal microenvironment that limits osteoblast activity. Developing approaches to osteoanabolic therapy that not only augment osteoblast activity but moreover correct this microenvironmental dysfunction may enable both more effective osteoanabolic therapies and also addressing a broader set of indications where vasculopathy or other forms microenvironment dysfunction feature prominently. We here review evidence that SHN3 acts as a suppressor of not only the cell intrinsic bone formation activity of osteoblasts, but moreover of the creation of a local osteoanabolic microenvironment. Mice lacking Schnurri3 (SHN3, HIVEP3) display a very robust increase in bone formation, that is due to de-repression of ERK pathway signaling in osteoblasts. In addition to loss of SHN3 augmenting the differentiation and bone formation activity of osteoblasts, loss of SHN3 increases secretion of SLIT3 by osteoblasts, which in a skeletal context acts as an angiogenic factor. Through this angiogenic activity, SLIT3 creates an osteoanabolic microenvironment, and accordingly treatment with SLIT3 can increase bone formation and enhance fracture healing. These features both validate vascular endothelial cells as a therapeutic target for disorders of low bone mass alongside the traditionally targeted osteoblasts and osteoclasts and indicate that targeting the SHN3/SLIT3 pathway provides a new mechanism to induce therapeutic osteoanabolic responses.

Super enhancers targeting ZBTB16 in osteogenesis protect against osteoporosis

As the major cell precursors in osteogenesis, mesenchymal stem cells (MSCs) are indispensable for bone homeostasis and development. However, the primary mechanisms regulating osteogenic differentiation are controversial. Composed of multiple constituent enhancers, super enhancers (SEs) are powerful cis-regulatory elements that identify genes that ensure sequential differentiation. The present study demonstrated that SEs were indispensable for MSC osteogenesis and involved in osteoporosis development. Through integrated analysis, we identified the most common SE-targeted and osteoporosis-related osteogenic gene, ZBTB16. ZBTB16, positively regulated by SEs, promoted MSC osteogenesis but was expressed at lower levels in osteoporosis. Mechanistically, SEs recruited bromodomain containing 4 (BRD4) at the site of ZBTB16, which then bound to RNA polymerase II-associated protein 2 (RPAP2) that transported RNA polymerase II (POL II) into the nucleus. The subsequent synergistic regulation of POL II carboxyterminal domain (CTD) phosphorylation by BRD4 and RPAP2 initiated ZBTB16 transcriptional elongation, which facilitated MSC osteogenesis via the key osteogenic transcription factor SP7. Bone-targeting ZBTB16 overexpression had a therapeutic effect on the decreased bone density and remodeling capacity of Brd4^fl/fl Prx1-cre mice and osteoporosis (OP) models. Therefore, our study shows that SEs orchestrate the osteogenesis of MSCs by targeting ZBTB16 expression, which provides an attractive focus and therapeutic target for osteoporosis. Without SEs located on osteogenic genes, BRD4 is not able to bind to osteogenic identity genes due to its closed structure before osteogenesis. During osteogenesis, histones on osteogenic identity genes are acetylated, and OB-gain SEs appear, enabling the binding of BRD4 to the osteogenic identity gene ZBTB16. RPAP2 transports RNA Pol II from the cytoplasm to the nucleus and guides Pol II to target ZBTB16 via recognition of the navigator BRD4 on SEs. After the binding of the RPAP2-Pol II complex to BRD4 on SEs, RPAP2 dephosphorylates Ser5 at the Pol II CTD to terminate the transcriptional pause, and BRD4 phosphorylates Ser2 at the Pol II CTD to initiate transcriptional elongation, which synergistically drives efficient transcription of ZBTB16, ensuring proper osteogenesis. Dysregulation of SE-mediated ZBTB16 expression leads to osteoporosis, and bone-targeting ZBTB16 overexpression is efficient in accelerating bone repair and treating osteoporosis.

Novel one-pot strategy for fabrication of a pH-Responsive bone-targeted drug self-frame delivery system for treatment of osteoporosis

Osteoporosis (OP) is a systemic metabolic orthopedic disorder prevalent in elderly people, that is characterized by a decrease in bone mass. Although many therapeutics have been adopted for OP treatment, many of them are still not well satisfied clinical requirements and therefore development of novel therapeutics is of great significance. In this work, a novel bone-targeting drug self-frame delivery system (DSFDS) with high drug loading efficiency and pH responsive drug release was fabricated by condensation of curcumin (Cur), amino group terminated polyethylene glycol (NH₂-PEG), and alendronate (ALN) using hexachlorocyclotriphosphonitrile (HCCP) as the linker. The final product named as HCCP-Cur-PEG-ALN (HCPA NPs) displayed excellent water dispersity with small size (181.9 ​± ​25.9 ​nm). Furthermore, the drug loading capacity of Cur can reach 25.8%, and Cur can be released from HCPA NPs under acidic environment. Owing to the introduction of ALN, HCPA NPs exhibited strong binding to HAp in vitro and excellent bone-targeting effect in vivo. Results from cellular and biochemical analyses revealed that HCPA NPs could effectively inhibit the formation and differentiation function of osteoclasts. More importantly, we also demonstrated that HCPA NPs could effectively reduce bone loss in OVX mice with low toxicity to major organs. The above results clearly demonstrated that HCPA NPs are promising for OP treatment. Given the simplicity and well designability of fabrication strategy, explicit therapy efficacy and low toxicity of HCPA NPs, we believe that this work should be of great interest for fabrication of various DSFDS to deal with many diseases.

Chemical modification of AAV9 capsid with N-ethyl maleimide alters vector tissue tropism

Although more adeno-associated virus AAV-based drugs enter the clinic, vector tissue tropism remains an unresolved challenge that limits its full potential despite that the tissue tropism of naturally occurring AAV serotypes can be altered by genetic engineering capsid vie DNA shuffling, or molecular evolution. To further expand the tropism and thus potential applications of AAV vectors, we utilized an alternative approach that employs chemical modifications to covalently link small molecules to reactive exposed Lysine residues of AAV capsids. We demonstrated that AAV9 capsid modified with N-ethyl Maleimide (NEM) increased its tropism more towards murine bone marrow (osteoblast lineage) while decreased transduction of liver tissue compared to the unmodified capsid. In the bone marrow, AAV9-NEM transduced Cd31, Cd34, and Cd90 expressing cells at a higher percentage than unmodified AAV9. Moreover, AAV9-NEM localized strongly in vivo to cells lining the calcified trabecular bone and transduced primary murine osteoblasts in culture, while WT AAV9 transduced undifferentiated bone marrow stromal cells as well as osteoblasts. Our approach could provide a promising platform for expanding clinical AAV development to treat bone pathologies such as cancer and osteoporosis. Thus, chemical engineering the AAV capsid holds great potential for development of future generations of AAV vectors.

Targeting strategies for bone diseases: signaling pathways and clinical studies

Since the proposal of Paul Ehrlich's magic bullet concept over 100 years ago, tremendous advances have occurred in targeted therapy. From the initial selective antibody, antitoxin to targeted drug delivery that emerged in the past decades, more precise therapeutic efficacy is realized in specific pathological sites of clinical diseases. As a highly pyknotic mineralized tissue with lessened blood flow, bone is characterized by a complex remodeling and homeostatic regulation mechanism, which makes drug therapy for skeletal diseases more challenging than other tissues. Bone-targeted therapy has been considered a promising therapeutic approach for handling such drawbacks. With the deepening understanding of bone biology, improvements in some established bone-targeted drugs and novel therapeutic targets for drugs and deliveries have emerged on the horizon. In this review, we provide a panoramic summary of recent advances in therapeutic strategies based on bone targeting. We highlight targeting strategies based on bone structure and remodeling biology. For bone-targeted therapeutic agents, in addition to improvements of the classic denosumab, romosozumab, and PTH1R ligands, potential regulation of the remodeling process targeting other key membrane expressions, cellular crosstalk, and gene expression, of all bone cells has been exploited. For bone-targeted drug delivery, different delivery strategies targeting bone matrix, bone marrow, and specific bone cells are summarized with a comparison between different targeting ligands. Ultimately, this review will summarize recent advances in the clinical translation of bone-targeted therapies and provide a perspective on the challenges for the application of bone-targeted therapy in the clinic and future trends in this area.

Schnurri-3 inhibition suppresses bone and joint damage in models of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic inflammatory disease that leads to systemic and articular bone loss by activating bone resorption and suppressing bone formation. Despite current therapeutic agents, inflammation-induced bone loss in RA continues to be a significant clinical problem due to joint deformity and lack of articular and systemic bone repair. Here, we identify the suppressor of bone formation, Schnurri-3 (SHN3), as a potential target to prevent bone loss in RA. SHN3 expression in osteoblast-lineage cells is induced by proinflammatory cytokines. Germline deletion or conditional deletion of Shn3 in osteoblasts limits articular bone erosion and systemic bone loss in mouse models of RA. Similarly, silencing of SHN3 expression in these RA models using systemic delivery of a bone-targeting recombinant adenoassociated virus protects against inflammation-induced bone loss. In osteoblasts, TNF activates SHN3 via ERK MAPK-mediated phosphorylation and, in turn, phosphorylated SHN3 inhibits WNT/β-catenin signaling and up-regulates RANKL expression. Accordingly, knock-in of a mutation in Shn3 that fails to bind ERK MAPK promotes bone formation in mice overexpressing human TNF due to augmented WNT/β-catenin signaling. Remarkably, Shn3-deficient osteoblasts are not only resistant to TNF-induced suppression of osteogenesis, but also down-regulate osteoclast development. Collectively, these findings demonstrate SHN3 inhibition as a promising approach to limit bone loss and promote bone repair in RA.

Downregulation of the RNA-binding protein PUM2 facilitates MSC-driven bone regeneration and prevents OVX-induced bone loss

BACKGROUND

Although mRNA dysregulation can induce changes in mesenchymal stem cell (MSC) homeostasis, the mechanisms by which post-transcriptional regulation influences MSC differentiation potential remain understudied. PUMILIO2 (PUM2) represses translation by binding target mRNAs in a sequence-specific manner.

METHODS

In vitro osteogenic differentiation assays were conducted using human bone marrow-derived MSCs. Alkaline phosphatase and alizarin red S staining were used to evaluate the osteogenic potential of MSCs. A rat xenograft model featuring a calvarial defect to examine effects of MSC-driven bone regeneration. RNA-immunoprecipitation (RNA-IP) assay was used to determine the interaction between PUM2 protein and Distal-Less Homeobox 5 (DLX5) mRNA. Ovariectomized (OVX) mice were employed to evaluate the effect of gene therapy for postmenopausal osteoporosis.

RESULTS

Here, we elucidated the molecular mechanism of PUM2 in MSC osteogenesis and evaluated the applicability of PUM2 knockdown (KD) as a potential cell-based or gene therapy. PUM2 level was downregulated during MSC osteogenic differentiation, and PUM2 KD enhanced MSC osteogenic potential. Following PUM2 KD, MSCs were transplanted onto calvarial defects in 12-week-old rats; after 8 weeks, transplanted MSCs promoted bone regeneration. PUM2 KD upregulated the expression of DLX5 mRNA and protein and the reporter activity of its 3'-untranslated region. RNA-IP revealed direct binding of PUM2 to DLX5 mRNA. We then evaluated the potential of adeno-associated virus serotype 9 (AAV9)-siPum2 as a gene therapy for osteoporosis in OVX mice.

CONCLUSION

Our findings suggest a novel role for PUM2 in MSC osteogenesis and highlight the potential of PUM2 KD-MSCs in bone regeneration. Additionally, we showed that AAV9-siPum2 is a potential gene therapy for osteoporosis.

Obesity, but not high-fat diet, is associated with bone loss that is reversed via CD4+CD25+Foxp3+ Tregs-mediated gut microbiome of non-obese mice

Osteoporosis is characterized by decreased bone mass, microarchitectural deterioration, and increased bone fragility. High-fat diet (HFD)-induced obesity also results in bone loss, which is associated with an imbalanced gut microbiome. However, whether HFD-induced obesity or HFD itself promotes osteoclastogenesis and consequent bone loss remains unclear. In this study, we developed HFD-induced obesity (HIO) and non-obesity (NO) mouse models to evaluate the effect of HFD on bone loss. NO mice were defined as body weight within 5% of higher or lower than that of chow diet fed mice after 10 weeks HFD feeding. NO was protected from HIO-induced bone loss by the RANKL /OPG system, with associated increases in the tibia tenacity, cortical bone mean density, bone volume of cancellous bone, and trabecular number. This led to increased bone strength and improved bone microstructure via the microbiome-short-chain fatty acids (SCFAs) regulation. Additionally, endogenous gut-SCFAs produced by the NO mice activated free fatty acid receptor 2 and inhibited histone deacetylases, resulting in the promotion of Treg cell proliferation in the HFD-fed NO mice; thereby, inhibiting osteoclastogenesis, which can be transplanted by fecal microbiome. Furthermore, T cells from NO mice retain differentiation of osteoclast precursors of RAW 264.7 macrophages ex vivo. Our data reveal that HFD is not a deleterious diet; however, the induction of obesity serves as a key trigger of bone loss that can be blocked by a NO mouse-specific gut microbiome.

STING-dependent interferon signatures restrict osteoclast differentiation and bone loss in mice

Stimulator of interferon genes (STING) is a key mediator of type-I interferon (IFN-I) signaling in response to a variety of stimuli, but the contribution of STING to homeostatic processes is not fully characterized. Previous studies showed that ligand activation of STING limits osteoclast differentiation in vitro through the induction of IFNβ and IFN-I interferon-stimulated genes (ISGs). In a disease model (SAVI) driven by the V154M gain-of-function mutation in STING, fewer osteoclasts form from SAVI precursors in response to receptor activator of NF-kappaB ligand (RANKL) in an IFN-I-dependent manner. Due to the described role of STING-mediated regulation of osteoclastogenesis in activation settings, we sought to determine whether basal STING signaling contributes to bone homeostasis, an unexplored area. Using whole-body and myeloid-specific deficiency, we show that STING signaling prevents trabecular bone loss in mice over time and that myeloid-restricted STING activity is sufficient for this effect. STING-deficient osteoclast precursors differentiate with greater efficiency than wild types. RNA sequencing of wild-type and STING-deficient osteoclast precursor cells and differentiating osteoclasts reveals unique clusters of ISGs including a previously undescribed ISG set expressed in RANKL naïve precursors (tonic expression) and down-regulated during differentiation. We identify a 50 gene tonic ISG signature that is STING dependent and shapes osteoclast differentiation. From this list, we identify interferon-stimulated gene 15 (ISG15) as a tonic STING-regulated ISG that limits osteoclast formation. Thus, STING is an important upstream regulator of tonic IFN-I signatures shaping the commitment to osteoclast fates, providing evidence for a nuanced and unique role for this pathway in bone homeostasis.

OTUB1 promotes osteoblastic bone formation through stabilizing FGFR2

Bone homeostasis is maintained by the balance between osteoblastic bone formation and osteoclastic bone resorption. Dysregulation of this process leads to multiple diseases, including osteoporosis. However, the underlying molecular mechanisms are not fully understood. Here, we show that the global and conditional osteoblast knockout of a deubiquitinase Otub1 result in low bone mass and poor bone strength due to defects in osteogenic differentiation and mineralization. Mechanistically, the stability of FGFR2, a crucial regulator of osteogenesis, is maintained by OTUB1. OTUB1 attenuates the E3 ligase SMURF1-mediated FGFR2 ubiquitination by inhibiting SMURF1's E2 binding. In the absence of OTUB1, FGFR2 is ubiquitinated excessively by SMURF1, followed by lysosomal degradation. Consistently, adeno-associated virus serotype 9 (AAV9)-delivered FGFR2 in knee joints rescued the bone mass loss in osteoblast-specific Otub1-deleted mice. Moreover, Otub1 mRNA level was significantly downregulated in bones from osteoporotic mice, and restoring OTUB1 levels through an AAV9-delivered system in ovariectomy-induced osteoporotic mice attenuated osteopenia. Taken together, our results suggest that OTUB1 positively regulates osteogenic differentiation and mineralization in bone homeostasis by controlling FGFR2 stability, which provides an optical therapeutic strategy to alleviate osteoporosis.

BMAL1-TTK-H2Bub1 loop deficiency contributes to impaired BM-MSC-mediated bone formation in senile osteoporosis

During the aging process, the reduced osteogenic differentiation of bone marrow mesenchymal stem cells (BM-MSCs) results in decreased bone formation, which contributes to senile osteoporosis. Previous studies have confirmed that interrupted circadian rhythm plays an indispensable role in age-related disease. However, the mechanism underlying the impaired osteogenic differentiation of BM-MSCs during aging and its relationship with interrupted circadian rhythm remains unclear. In this study, we confirmed that the circadian rhythm was interrupted in aging mouse skeletal systems. The level of the core rhythm component BMAL1 but not that of CLOCK in the osteoblast lineage was decreased in senile osteoporotic specimens from both human and mouse. BMAL1 targeted TTK as a circadian-controlled gene to phosphorylate MDM2 and regulate H2Bub1 level, while H2Bub1 in turn regulated the expression of BMAL1. The osteogenic capacity of BM-MSCs was maintained by a positive loop formed by BMAL1-TTK-MDM2-H2Bub1. Furthermore, we demonstrated that using bone-targeting recombinant adeno-associated virus 9 (rAAV9) to enhance Bmal1 or Ttk might have a therapeutic effect on senile osteoporosis and delays bone repair in aging mice. In summary, our study indicated that targeting the BMAL1-TTK-MDM2-H2Bub1 axis via bone-targeting rAAV9 might be a promising strategy for the treatment of senile osteoporosis.

Targeted postnatal knockout of Sclerostin using a bone-targeted adeno-associated viral vector increases bone anabolism and decreases canalicular density

PURPOSE

The creation of murine gene knockout models to study bone gene functions often requires the resource intensive crossbreeding of Cre transgenic and gene-floxed strains. The developmental versus postnatal roles of genes can be difficult to discern in such models. For example, embryonic deletion of the Sclerostin (Sost) gene establishes a high-bone mass phenotype in neonatal mice that may impact on future bone growth. To generate a postnatal skeletal knockout of Sost in adult mice, this study used a single injection of a bone-targeted recombinant adeno-associated virus (rAAV) vector.

METHODS

8-week-old Sost^flox/flox mice were injected with saline (control) or a single injection containing 5 × 10¹¹ vg AAV8-Sp7-Cre vector. Ai9 fluorescent Cre reporter mice were dosed in parallel to confirm targeting efficiency. After 6 weeks, detailed bone analysis was performed via microCT, biomechanical testing, and bone histology on vertebral and long bone specimens.

RESULTS

The AAV8-Sp7-Cre vector induced widespread persistent recombination in the bone compartment. Regional microCT analyses revealed significant increases in bone with vector treatment. In the L3 vertebrae, Sost^flox/flox:AAV-Cre showed a 22 % increase in bone volume and 21 % in trabecular bone fraction compared to controls; this translated to a 17 % increase in compressive strength. In the tibiae, Sost^flox/flox:AAV-Cre led to small but statistically significant increases in cortical bone volume and thickness. These were consistent with a 25 % increase in mineral apposition rate, but this did not translate into increased four-point bending strength. Ploton silver nitrate stain on histological sections revealed an unexpected increase in canalicular density associated with Sost ablation.

CONCLUSION

This report demonstrates a proof-of-concept that the AAV8-Sp7-Cre vector can efficiently produce postnatal skeletal knockout mice using gene-floxed strains. This technology has the potential for broad utility in the bone field with existing conditional lines. These data also confirm an important postnatal role for Sost in regulating bone homeostasis, consistent with prior studies using neutralizing Sclerostin antibodies, and highlights a novel role of Sost in canalicular remodeling.

WNT-modulating gene silencers as a gene therapy for osteoporosis, bone fracture, and critical-sized bone defects

Treating osteoporosis and associated bone fractures remains challenging for drug development in part due to potential off-target side effects and the requirement for long-term treatment. Here, we identify recombinant adeno-associated virus (rAAV)-mediated gene therapy as a complementary approach to existing osteoporosis therapies, offering long-lasting targeting of multiple targets and/or previously undruggable intracellular non-enzymatic targets. Treatment with a bone-targeted rAAV carrying artificial microRNAs (miRNAs) silenced the expression of WNT antagonists, schnurri-3 (SHN3), and sclerostin (SOST), and enhanced WNT/β-catenin signaling, osteoblast function, and bone formation. A single systemic administration of rAAVs effectively reversed bone loss in both postmenopausal and senile osteoporosis. Moreover, the healing of bone fracture and critical-sized bone defects was also markedly improved by systemic injection or transplantation of AAV-bound allograft bone to the osteotomy sites. Collectively, our data demonstrate the clinical potential of bone-specific gene silencers to treat skeletal disorders of low bone mass and impaired fracture repair.

A bioactive poly(ether-ether-ketone) nanocomposite scaffold regulates osteoblast/osteoclast activity for the regeneration of osteoporotic bone

Due to the lower regeneration capacity of the osteoporotic bone, the treatment of osteoporotic defects is extremely challenging in clinics. In this study, strontium-doped bioactive glass nanoparticles loaded with sodium alendronate (ALN), namely A-SrBG, were incorporated into the poly(ether-ether-ketone) matrix to fabricate a bioactive composite scaffold (ASP), which was expected to both inhibit bone resorption and promote bone regeneration. The results showed that such a composite scaffold with interconnected macropores (200-400 μm) could release Ca²⁺, Sr²⁺, and ALN in vitro. The proliferation, alkaline phosphatase (ALP) activity, expression of osteogenesis-related genes, and formation of calcified nodules of rat bone marrow stromal cells (rBMSCs) were clearly evidenced, and the reduction in the proliferation, tartrate-resistant acid phosphatase (TRAP) activity, cell fusion, and expression of osteoclastogenesis-related genes of osteoclasts was observed as well. In the presence of the ASP scaffold, enhanced osteogenesis along with inhibiting osteoclastogenesis was observed by modulating the osteoprotegerin (OPG)/receptor activator for nuclear factor κB ligand (RANKL) ratio. The efficacy of the composite scaffold in the regeneration of osteoporotic critical-sized cranial defect in a rat model was evaluated. Therefore, the bioactive composite scaffold with excellent biocompatibility and osteogenic potential could be a promising material for the repair of osteoporotic bone defects.

Suppression of heterotopic ossification in fibrodysplasia ossificans progressiva using AAV gene delivery

Heterotopic ossification is the most disabling feature of fibrodysplasia ossificans progressiva, an ultra-rare genetic disorder for which there is currently no prevention or treatment. Most patients with this disease harbor a heterozygous activating mutation (c.617 G > A;p.R206H) in ACVR1. Here, we identify recombinant AAV9 as the most effective serotype for transduction of the major cells-of-origin of heterotopic ossification. We use AAV9 delivery for gene replacement by expression of codon-optimized human ACVR1, ACVR1R206H allele-specific silencing by AAV-compatible artificial miRNA and a combination of gene replacement and silencing. In mouse skeletal cells harboring a conditional knock-in allele of human mutant ACVR1 and in patient-derived induced pluripotent stem cells, AAV gene therapy ablated aberrant Activin A signaling and chondrogenic and osteogenic differentiation. In Acvr1(R206H) knock-in mice treated locally in early adulthood or systemically at birth, trauma-induced endochondral bone formation was markedly reduced, while inflammation and fibroproliferative responses remained largely intact in the injured muscle. Remarkably, spontaneous heterotopic ossification also substantially decreased in in Acvr1(R206H) knock-in mice treated systemically at birth or in early adulthood. Collectively, we develop promising gene therapeutics that can prevent disabling heterotopic ossification in mice, supporting clinical translation to patients with fibrodysplasia ossificans progressiva.

Translating musculoskeletal bioengineering into tissue regeneration therapies

Musculoskeletal injuries and disorders are the leading cause of physical disability worldwide and a considerable socioeconomic burden. The lack of effective therapies has driven the development of novel bioengineering approaches that have recently started to gain clinical approvals. In this review, we first discuss the self-repair capacity of the musculoskeletal tissues and describe causes of musculoskeletal dysfunction. We then review the development of novel biomaterial, immunomodulatory, cellular, and gene therapies to treat musculoskeletal disorders. Last, we consider the recent regulatory changes and future areas of technological progress that can accelerate translation of these therapies to clinical practice.

miR-210-3p protects against osteoarthritis through inhibiting subchondral angiogenesis by targeting the expression of TGFBR1 and ID4

Excessive subchondral angiogenesis is a key pathological feature of osteoarthritis (OA), as it alters the balance of subchondral bone remodeling and causes progressive cartilage degradation. We previously found that miR-210-3p correlates negatively with angiogenesis, though the specific mechanism of miR-210-3p-related angiogenesis in subchondral bone during OA progression remains unclear. This study was conducted to identify the miR-210-3p-modulating subchondral angiogenesis mechanism in OA and investigate its therapeutic effect. We found that miR-210-3p expression correlated negatively with subchondral endomucin positive (Emcn+) vasculature in the knee joints of OA mice. miR-210-3p overexpression regulated the angiogenic ability of endothelial cells (ECs) under hypoxic conditions in vitro. Mechanistically, miR-210-3p inhibited ECs angiogenesis by suppressing transforming growth factor beta receptor 1 (TGFBR1) mRNA translation and degrading DNA-binding inhibitor 4 (ID4) mRNA. In addition, TGFBR1 downregulated the expression of ID4. Reduced ID4 levels led to a negative feedback regulation of TGFBR1, enhancing the inhibitory effect of miR-210-3p on angiogenesis. In OA mice, miR-210-3p overexpression in ECs via adeno-associated virus (AAV) alleviated cartilage degradation, suppressed the type 17 immune response and relieved symptoms by attenuating subchondral Emcn+ vasculature and subchondral bone remodeling. In conclusion, we identified a miR-210-3p/TGFBR1/ID4 axis in subchondral ECs that modulates OA progression via subchondral angiogenesis, representing a potential OA therapy target.

AAV-mediated delivery of osteoblast/osteoclast-regulating miRNAs for osteoporosis therapy

Osteoporosis occurs due to a dysregulation in bone remodeling, a process requiring both bone-forming osteoblasts and bone-resorbing osteoclasts. Current leading osteoporosis therapies suppress osteoclast-mediated bone resorption but show limited therapeutic effects because osteoblast-mediated bone formation decreases concurrently. We developed a gene therapy strategy for osteoporosis that simultaneously promotes bone formation and suppresses bone resorption by targeting two microRNAs (miRNAs)-miR-214-3p and miR-34a-5p. We modulated the expression of these miRNAs using systemically delivered recombinant adeno-associated viral (rAAV) vectors targeting the bone. rAAV-mediated overexpression of miR-214-3p or inhibition of miR-34a-5p in the skeleton resulted in bone loss in adult mice, resembling osteoporotic bones. Conversely, rAAV-mediated inhibition of miR-214-3p or overexpression of miR-34a-5p reversed bone loss in mouse models for postmenopausal and senile osteoporosis by increasing osteoblast-mediated bone formation and decreasing osteoclast-mediated bone resorption. Notably, these mice did not show any apparent pathological phenotypes in non-skeletal tissues. Mechanistically, inhibiting miR-214-3p upregulated activating transcription factor 4 in osteoblasts and phatase and tensin homolog in osteoclasts, while overexpressing miR-34a-5p downregulated Notch1 in osteoblasts and TGF-β-induced factor homeobox 2 in osteoclasts. In summary, bone-targeting rAAV-mediated regulation of miR-214-3p or miR-34a-5p is a promising new approach to treat osteoporosis, while limiting adverse effects in non-skeletal tissues.

Bioinspired Nanovesicles Convert the Skeletal Endothelium-Associated Secretory Phenotype to Treat Osteoporosis

Recently, bone marrow endothelial cells (BMECs) were found to play an important role in regulating bone homeostasis. However, few studies utilized BMECs to treat bone metabolic diseases including osteoporosis. Here, we reported bioinspired nanovesicles (BNVs) prepared from human induced pluripotent stem cells-derived endothelial cells under hypoxia culture through an extrusion approach. Abundant membrane C-X-C motif chemokine receptor 4 conferred these BNVs bone-targeting ability and the endothelial homology facilitated the BMEC tropism. Due to their unique endogenous miRNA cargos, these BNVs re-educated BMECs to secret cytokines favoring osteogenesis and anti-inflammation. Owing to the conversion of secretory phenotype, the osteogenic differentiation of bone mesenchymal stem cells was facilitated, and the M1-macrophage-dominant pro-inflammatory microenvironment was ameliorated in osteoporotic bones. Taken together, this study proposed BMEC-targeting nanovesicles treating osteoporosis via converting the skeletal endothelium-associated secretory phenotype.

Curative Cell and Gene Therapy for Osteogenesis Imperfecta

Osteogenesis imperfecta (OI) describes a series of genetic bone fragility disorders that can have a substantive impact on patient quality of life. The multidisciplinary approach to management of children and adults with OI primarily involves the administration of antiresorptive medication, allied health (physiotherapy and occupational therapy), and orthopedic surgery. However, advances in gene editing technology and gene therapy vectors bring with them the promise of gene-targeted interventions to provide an enduring or perhaps permanent cure for OI. This review describes emergent technologies for cell- and gene-targeted therapies, major hurdles to their implementation, and the prospects of their future success with a focus on bone disorders. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

STARD3NL inhibits the osteogenic differentiation by inactivating the Wnt/β‐catenin pathway via binding to Annexin A2 in osteoporosis

Osteoporosis is one of the leading forms of systemic diseases related to bone metabolism in the world. STARD3 N‐terminal like (STARD3NL) showed robust association with osteoporosis‐related traits. Yet, the molecular functional mechanisms of STARD3NL in osteoblasts is still obscure. In this study, we demonstrated a high level of STARD3NL expression in the bone tissues from the patients with low bone mass and ovariectomized (OVX)‐induced osteoporotic mice. We identified Stard3nl as a potent factor that negatively and positively regulates osteoblast differentiation and cell proliferation, respectively. Furthermore, inhibition of Stard3nl induced β‐catenin gene expression and the nuclear translocation of β‐catenin, as well as Wnt signalling activities, contributing to the activation of Wnt/β‐catenin signalling. Mechanistic studies revealed that Stard3nl bound with Annexin A2 (Anxa2) to suppress β‐catenin expression, resulting into the suppression of Wnt signalling and downstream osteogenic differentiation. Moreover, adeno‐associated virus 9 (AAV9)‐mediated silencing of Stard3nl reversed bone loss in OVX‐induced osteoporotic mice by the injection into the knee joints. Collectively, our study revealed that Stard3nl suppressed osteogenesis via binding with Anxa2, resulting into the inactivation of Wnt signalling. It also highlights the preventive and therapeutic potential of STARD3NL as a specific and novel target for osteoporotic patients.

A bone-targeted engineered exosome platform delivering siRNA to treat osteoporosis

The complex pathogenesis of osteoporosis includes excessive bone resorption, insufficient bone formation and inadequate vascularization, a combination which is difficult to completely address with conventional therapies. Engineered exosomes carrying curative molecules show promise as alternative osteoporosis therapies, but depend on specifically-functionalized vesicles and appropriate engineering strategies. Here, we developed an exosome delivery system based on exosomes secreted by mesenchymal stem cells (MSCs) derived from human induced pluripotent stem cells (iPSCs). The engineered exosomes BT-Exo-siShn3, took advantage of the intrinsic anti-osteoporosis function of these special MSC-derived exosomes and collaborated with the loaded siRNA of the Shn3 gene to enhance the therapeutic effects. Modification of a bone-targeting peptide endowed the BT-Exo-siShn3 an ability to deliver siRNA to osteoblasts specifically. Silencing of the osteoblastic Shn3 gene enhanced osteogenic differentiation, decreased autologous RANKL expression and thereby inhibited osteoclast formation. Furthermore, Shn3 gene silencing increased production of SLIT3 and consequently facilitated vascularization, especially formation of type H vessels. Our study demonstrated that BT-Exo-siShn3 could serve as a promising therapy to kill three birds with one stone and implement comprehensive anti-osteoporosis effects.

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A bone-targeted engineered exosome platform BT-Exo-siShn3 could deliver siRNA to osteoblasts specifically.

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Comprehensive anti-osteoporosis effects were implemented based on the synchronization of exosome-carrier and siRNA-cargo.

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The BT-Exo-siShn3 platform was the first drug delivery system using exosomes produced by iPSC-derivatives.

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This study proposed a versatile paradigm of targeted therapies for different diseases based on iPSC-derivatives.

Real-time MR tracking of AAV gene therapy with βgal-responsive MR probe in a murine model of GM1-gangliosidosis

Transformative results of adeno-associated virus (AAV) gene therapy in patients with spinal muscular atrophy and Leber's congenital amaurosis led to approval of the first two AAV products in the United States to treat these diseases. These extraordinary results led to a dramatic increase in the number and type of AAV gene-therapy programs. However, the field lacks non-invasive means to assess levels and duration of therapeutic protein function in patients. Here, we describe a new magnetic resonance imaging (MRI) technology for real-time reporting of gene-therapy products in the living animal in the form of an MRI probe that is activated in the presence of therapeutic protein expression. For the first time, we show reliable tracking of enzyme expression after a now in-human clinical trial AAV gene therapy (ClinicalTrials.gov: NTC03952637) encoding lysosomal acid beta-galactosidase (βgal) using a self-immolative βgal-responsive MRI probe. MRI enhancement in AAV-treated enzyme-deficient mice (GLB-1^-/-) correlates with βgal activity in central nervous system and peripheral organs after intracranial or intravenous AAV gene therapy, respectively. With >1,800 gene therapies in phase I/II clinical trials (ClinicalTrials.gov), development of a non-invasive method to track gene expression over time in patients is crucial to the future of the gene-therapy field.

A novel therapeutic strategy for skeletal disorders: Proof of concept of gene therapy for X-linked hypophosphatemia

Adeno-associated virus (AAV) vectors are a well-established gene transfer approach for rare genetic diseases. Nonetheless, some tissues, such as bone, remain refractory to AAV. X-linked hypophosphatemia (XLH) is a rare skeletal disorder associated with increased levels of fibroblast growth factor 23 (FGF23), resulting in skeletal deformities and short stature. The conventional treatment for XLH, lifelong phosphate and active vitamin D analogs supplementation, partially improves quality of life and is associated with severe long-term side effects. Recently, a monoclonal antibody against FGF23 has been approved for XLH but remains a high-cost lifelong therapy. We developed a liver-targeting AAV vector to inhibit FGF23 signaling. We showed that hepatic expression of the C-terminal tail of FGF23 corrected skeletal manifestations and osteomalacia in a XLH mouse model. Our data provide proof of concept for AAV gene transfer to treat XLH, a prototypical bone disease, further expanding the use of this modality to treat skeletal disorders.

γ-Aminobutyric Acid Promotes Osteogenic Differentiation of Mesenchymal Stem Cells by Inducing TNFAIP3

BACKGROUND

Osteoporosis is the most common metabolic bone disease. There is still an unmet need for novel therapeutic agents that could be beneficial as osteoporosis treatments. It has been reported that the neurotransmitter γ-aminobutyric acid (GABA) might be associated with human bone formation. However, the precise mechanism remains unclear.

OBJECTIVE

To investigate the effect of GABA on bone metabolism and explore the possible role of TNFAIP3 in this process.

METHODS

GABA had little effect on the proliferation of human mesenchymal stem cells (hMSCs) and RAW 264.7 cells, as indicated by the cell counting kit-8 (CCK-8) assay. The results showed that GABA enhanced the intensity of ALP staining, ALP activity, and accumulation of Ca2+ mineralized nodules in hMSCs during osteogenic induction.

RESULTS

The qRT-PCR results indicated that GABA treatment significantly increased the mRNA expression of osteogenic genes in hMSCs. In RAW 264.7 cells, TRAP staining showed that GABA did not alter the number or size of osteoclasts or the expression of osteoclastic genes, which suggests that GABA does not affect osteoclastic differentiation. Mechanistically, GABA treatment significantly induced the sustained expression of TNFAIP3. Furthermore, by knocking down TNFAIP3, the osteogenic effect of GABA was antagonized, which suggests that TNFAIP3 mediates the effects of GABA in hMSCs.

CONCLUSION

Our results suggested that GABA treatment positively regulated osteogenic differentiation by upregulating TNFAIP3, while no obvious effect on osteoclastic differentiation was detected. Therefore, our results provide a potential gene therapy for the treatment of osteoporosis and low bone mineral density.

The identification and structural analysis of potential 14-3-3 interaction sites on the bone regulator protein Schnurri-3

14-3-3 proteins regulate many intracellular processes and their ability to bind in subtly different fashions to their numerous partner proteins provides attractive drug-targeting points for a range of diseases. Schnurri-3 is a suppressor of mouse bone formation and a candidate target for novel osteoporosis therapeutics, and thus it is of interest to determine whether it interacts with 14-3-3. In this work, potential 14-3-3 interaction sites on mammalian Schnurri-3 were identified by an in silico analysis of its protein sequence. Using fluorescence polarization, isothermal titration calorimetry and X-ray crystallography, it is shown that synthetic peptides containing either phosphorylated Thr869 or Ser542 can indeed interact with 14-3-3, with the latter capable of forming an interprotein disulfide bond with 14-3-3σ: a hitherto unreported phenomenon.

Cellular and Tissue Selectivity of AAV Serotypes for Gene Delivery to Chondrocytes and Cartilage

Background: Despite several studies on the effect of adeno-associated virus (AAV)-based therapeutics on osteoarthritis (OA), information on the transduction efficiency and applicable profiles of different AAV serotypes to chondrocytes in hard cartilage tissue is still limited. Moreover, the recent discovery of additional AAV serotypes makes it necessary to screen for more suitable AAV serotypes for specific tissues. Here, we compared the transduction efficiencies of 14 conventional AAV serotypes in human chondrocytes, mouse OA models, and human cartilage explants obtained from OA patients. Methods: To compare the transduction efficiency of individual AAV serotypes, green fluorescent protein (GFP) expression was detected by fluorescence microscopy or western blotting. Likewise, to compare the transduction efficiencies of individual AAV serotypes in cartilage tissues, GFP expression was determined using fluorescence microscopy or immunohistochemistry, and GFP-positive cells were counted. Results: Only AAV2, 5, 6, and 6.2 exhibited substantial transduction efficiencies in both normal and OA chondrocytes. All AAV serotypes except AAV6 and rh43 could effectively transduce human bone marrow mesenchymal stem cells. In human and mouse OA cartilage tissues, AAV2, AAV5, AAV6.2, AAV8, and AAV rh39 showed excellent tissue specificity based on transduction efficiency. These results indicate the differences in transduction efficiencies of AAV serotypes between cellular and tissue models. Conclusions: Our findings indicate that AAV2 and AAV6.2 may be the best choices for AAV-mediated gene delivery into intra-articular cartilage tissue. These AAV vectors hold the potential to be of use in clinical applications to prevent OA progression if appropriate therapeutic genes are inserted into the vector.

Recombinant sclerostin inhibits bone formation in vitro and in a mouse model of sclerosteosis

Background

Sclerosteosis, a severe autosomal recessive sclerosing skeletal dysplasia characterised by excessive bone formation, is caused by absence of sclerostin, a negative regulator of bone formation that binds LRP5/6 Wnt co-receptors. Current treatment is limited to surgical management of symptoms arising from bone overgrowth. This study investigated the effectiveness of sclerostin replacement therapy in a mouse model of sclerosteosis.

Methods

Recombinant wild type mouse sclerostin (mScl) and novel mScl fusion proteins containing a C-terminal human Fc (mScl hFc), or C-terminal human Fc with a poly-aspartate motif (mScl hFc PD), were produced and purified using mammalian expression and standard chromatography methods. In vitro functionality and efficacy of the recombinant proteins were evaluated using three independent biophysical techniques and an in vitro bone nodule formation assay. Pharmacokinetic properties of the proteins were investigated in vivo following a single administration to young female wild type (WT) or SOST knock out (SOST^-/-) mice. In a six week proof-of-concept in vivo study, young female WT or SOST^-/- mice were treated with 10 mg/kg mScl hFc or mScl hFc PD (weekly), or 4.4 mg/kg mScl (daily). The effect of recombinant sclerostin on femoral cortical and trabecular bone parameters were assessed by micro computed tomography (μCT).

Results

Recombinant mScl proteins bound to the extracellular domain of the Wnt co-receptor LRP6 with high affinity (nM range) and completely inhibited matrix mineralisation in vitro. Pharmacokinetic assessment following a single dose administered to WT or SOST^-/- mice indicated the presence of hFc increased protein half-life from less than 5 min to at least 1.5 days. Treatment with mScl hFc PD over a six week period resulted in modest but significant reductions in trabecular volumetric bone mineral density (vBMD) and bone volume fraction (BV/TV), of 20% and 15%, respectively.

Conclusion

Administration of recombinant mScl hFc PD partially corrected the high bone mass phenotype in SOST^-/- mice, suggesting that bone-targeting of sclerostin engineered to improve half-life was able to negatively regulate bone formation in the SOST^-/- mouse model of sclerosteosis.

The translational potential of this article

These findings support the concept that exogenous sclerostin can reduce bone mass, however the modest efficacy suggests that sclerostin replacement may not be an optimal strategy to mitigate excessive bone formation in sclerosteosis, hence alternative approaches should be explored.

TRIM33 protects osteoblasts from oxidative stress-induced apoptosis in osteoporosis by inhibiting FOXO3a ubiquitylation and degradation

This study aimed to probe into the effect of TRIM33 on oxidative stress‐induced apoptosis of osteoblasts in osteoporosis and to probe into the underlying mechanism. The apoptosis of osteoblasts was induced by H₂O₂ treatment and tested by flow cytometry. A mouse osteoporosis model was conducted by ovariectomy (OVX). The function of TRIM33 was assessed by in vitro and in vivo experiments. The mechanism of TRIM33 was determined by immunoprecipitation, immunofluorescent staining and co‐transfection experiments. Here, we found that TRIM33 expression was lessened in the osteoblasts of patients with osteoporosis and was positively correlated with the bone mineral density of these patients. FOXO3a and TRIM33 were co‐localized in the osteoblasts nuclei. TRIM33 silence boosted FOXO3a degradation in normal osteoblasts, while TRIM33 overexpression restrained FOXO3a degradation in H₂O₂‐treated osteoblasts. The binding of TRIM33 to CBP and its overexpression restrained CBP‐mediated FOXO3a acetylation, thereby attenuating FOXO3a ubiquitylation. The H₂O₂‐induced apoptosis of osteoblasts was restrained by TRIM33 overexpression, while the FOXO3a knockdown reversed this trend. The in vivo experiments corroborated that TRIM33 overexpression attenuated the OVX‐driven impacts in mice. In general, our findings expounded that TRIM33 protected osteoblasts against oxidative stress‐induced apoptosis in osteoporosis and that the underlying mechanism was the restraint of FOXO3a ubiquitylation and degradation.

ZEB1 Mediates Bone Marrow Mesenchymal Stem Cell Osteogenic Differentiation Partly via Wnt/β-Catenin Signaling

Zinc finger E-box-binding homebox 1 (ZEB1) is a zinc-finger transcription factor best known for its role in promoting the epithelial-mesenchymal transition, which is also related to osteogenesis. Here, ZEB1 was investigated for its role in the commitment of bone marrow mesenchymal stem cells (BMSCs) to osteoblasts. In vitro, ZEB1 expression decreased following osteogenic differentiation. Furthermore, silencing of ZEB1 in BMSCs promoted osteogenic activity and mineralization. The increase in osteogenic differentiation induced by si-ZEB1 could be partly rescued by the inhibition of Wnt/β-catenin (si-β-catenin). In vivo, knockdown of ZEB1 in BMSCs inhibited the rapid bone loss of ovariectomized (OVX) mice. ZEB1 expression has also been negatively associated with bone mass and bone formation in postmenopausal women. In conclusion, ZEB1 is an essential transcription factor in BMSC differentiation and may serve as a potential anabolic strategy for treating and preventing postmenopausal osteoporosis (PMOP).

Bone targeting nanocarrier-assisted delivery of adenosine to combat osteoporotic bone loss

Extracellular adenosine has been shown to play a key role in maintaining bone health and could potentially be used to treat bone loss. However, systemic administration of exogenous adenosine to treat bone disorders remains a challenge due to the ubiquitous presence of adenosine receptors in different organs and the short half-life of adenosine in circulation. Towards this, we have developed a bone-targeting nanocarrier and determined its potential for systemic administration of adenosine. The nanocarrier, synthesized via emulsion suspension photopolymerization, is comprised of hyaluronic acid (HA) copolymerized with phenylboronic acid (PBA), a moiety that can form reversible bonds with adenosine. The bone binding affinity of the nanocarrier was achieved by alendronate (Aln) conjugation. Nanocarriers functionalized with the alendronate (Aln-NC) showed a 45% higher accumulation in the mice vertebrae in vivo compared to those lacking alendronate molecules (NCs). Systemic administration of adenosine via bone-targeting nanocarriers (Aln-NC) attenuated bone loss in ovariectomized (OVX) mice. Furthermore, bone tissue of mice treated with adenosine-loaded Aln-NC displayed trabecular bone characteristics comparable to healthy controls as shown by microcomputed tomography, histochemical staining, bone labeling, and mechanical strength. Overall, our results demonstrate the use of a bone-targeting nanocarrier towards systemic administration of adenosine and its application in treating bone degenerative diseases such as osteoporosis.

Review of a new bone tumor therapy strategy based on bifunctional biomaterials

Bone tumors, especially those in osteosarcoma, usually occur in adolescents. The standard clinical treatment includes chemotherapy, surgical therapy, and radiation therapy. Unfortunately, surgical resection often fails to completely remove the tumor, which is the main cause of postoperative recurrence and metastasis, resulting in a high mortality rate. Moreover, bone tumors often invade large areas of bone, which cannot repair itself, and causes a serious effect on the quality of life of patients. Thus, bone tumor therapy and bone regeneration are challenging in the clinic. Herein, this review presents the recent developments in bifunctional biomaterials to achieve a new strategy for bone tumor therapy. The selected bifunctional materials include 3D-printed scaffolds, nano/microparticle-containing scaffolds, hydrogels, and bone-targeting nanomaterials. Numerous related studies on bifunctional biomaterials combining tumor photothermal therapy with enhanced bone regeneration were reviewed. Finally, a perspective on the future development of biomaterials for tumor therapy and bone tissue engineering is discussed. This review will provide a useful reference for bone tumor-related disease and the field of complex diseases to combine tumor therapy and tissue engineering.

Ubiquitin-specific protease 53 promotes osteogenic differentiation of human bone marrow-derived mesenchymal stem cells

The ubiquitin protease pathway plays important role in human bone marrow-derived mesenchymal stem cell (hBMSC) differentiation, including osteogenesis. However, the function of deubiquitinating enzymes in osteogenic differentiation of hBMSCs remains poorly understood. In this study, we aimed to investigate the role of ubiquitin-specific protease 53 (USP53) in the osteogenic differentiation of hBMSCs. Based on re-analysis of the Gene Expression Omnibus database, USP53 was selected as a positive regulator of osteogenic differentiation in hBMSCs. Overexpression of USP53 by lentivirus enhanced osteogenesis in hBMSCs, whereas knockdown of USP53 by lentivirus inhibited osteogenesis in hBMSCs. In addition, USP53 overexpression increased the level of active β-catenin and enhanced the osteogenic differentiation of hBMSCs. This effect was reversed by the Wnt/β-catenin inhibitor DKK1. Mass spectrometry showed that USP53 interacted with F-box only protein 31 (FBXO31) to promote proteasomal degradation of β-catenin. Inhibition of the osteogenic differentiation of hBMSCs by FBXO31 was partially rescued by USP53 overexpression. Animal studies showed that hBMSCs with USP53 overexpression significantly promoted bone regeneration in mice with calvarial defects. These results suggested that USP53 may be a target for gene therapy for bone regeneration.