Aptamer-functionalized lipid nanoparticles targeting osteoblasts as a novel RNA interference-based bone anabolic strategy

Fostering tissue engineering and regenerative medicine to treat musculoskeletal disorders in bone and muscle

The musculoskeletal system, which is vital for movement, support, and protection, can be impaired by disorders such as osteoporosis, osteoarthritis, and muscular dystrophy. This review focuses on the advances in tissue engineering and regenerative medicine, specifically aimed at alleviating these disorders. It explores the roles of cell therapy, particularly Mesenchymal Stem Cells (MSCs) and Adipose-Derived Stem Cells (ADSCs), biomaterials, and biomolecules/external stimulations in fostering bone and muscle regeneration. The current research underscores the potential of MSCs and ADSCs despite the persistent challenges of cell scarcity, inconsistent outcomes, and safety concerns. Moreover, integrating exogenous materials such as scaffolds and external stimuli like electrical stimulation and growth factors shows promise in enhancing musculoskeletal regeneration. This review emphasizes the need for comprehensive studies and adopting innovative techniques together to refine and advance these multi-therapeutic strategies, ultimately benefiting patients with musculoskeletal disorders.

The Discovery of a Specific CKIP-1 Ligand for the Potential Treatment of Disuse Osteoporosis

Bone homeostasis relies on the delicate balance between osteoblast-mediated bone formation and osteoclast-mediated bone resorption. The casein kinase 2 interacting protein-1 (CKIP-1), a specific CK2α subunit-interacting protein, has been documented as one of the crucial negative regulators of bone formation. CKIP-1 siRNA therapy has constraints that limit its use in clinical applications. Therefore, it is necessary to explore effective targeting strategies for CKIP-1. In this study, we observed an upregulation of CKIP-1 protein expression in the microgravity environment, while its ubiquitination levels decreased. We further investigated the interaction between CKIP-1 and VHL and found that VHL enhanced CKIP-1 degradation through the ubiquitylation-proteasome system (UPS). Additionally, we discovered a small molecule ligand, named C77, through DNA-encoded library (DEL) screening, which binds to CKIP-1 both in vivo and in vitro, as confirmed by Surface Plasmon Resonance (SPR) and the Cellular Thermal shift assay (CETSA), respectively. Our findings demonstrated the potential of VHL and C77 as guiding factors in the development of CKIP-1-based Proteolysis-Targeting Chimeras (PROTACs), which could be future therapeutic interventions in disuse osteoporosis.

Current advance of nanotechnology in diagnosis and treatment for malignant tumors

Cancer remains a significant risk to human health. Nanomedicine is a new multidisciplinary field that is garnering a lot of interest and investigation. Nanomedicine shows great potential for cancer diagnosis and treatment. Specifically engineered nanoparticles can be employed as contrast agents in cancer diagnostics to enable high sensitivity and high-resolution tumor detection by imaging examinations. Novel approaches for tumor labeling and detection are also made possible by the use of nanoprobes and nanobiosensors. The achievement of targeted medication delivery in cancer therapy can be accomplished through the rational design and manufacture of nanodrug carriers. Nanoparticles have the capability to effectively transport medications or gene fragments to tumor tissues via passive or active targeting processes, thus enhancing treatment outcomes while minimizing harm to healthy tissues. Simultaneously, nanoparticles can be employed in the context of radiation sensitization and photothermal therapy to enhance the therapeutic efficacy of malignant tumors. This review presents a literature overview and summary of how nanotechnology is used in the diagnosis and treatment of malignant tumors. According to oncological diseases originating from different systems of the body and combining the pathophysiological features of cancers at different sites, we review the most recent developments in nanotechnology applications. Finally, we briefly discuss the prospects and challenges of nanotechnology in cancer.

Potential Targeting Mechanisms for Bone-Directed Therapies

Bone development is characterized by complex regulation mechanisms, including signal transduction and transcription factor-related pathways, glycobiological processes, cellular interactions, transportation mechanisms, and, importantly, chemical formation resulting from hydroxyapatite. Any abnormal regulation in the bone development processes causes skeletal system-related problems. To some extent, the avascularity of cartilage and bone makes drug delivery more challenging than that of soft tissues. Recent studies have implemented many novel bone-targeting approaches to overcome drawbacks. However, none of these strategies fully corrects skeletal dysfunction, particularly in growth plate-related ones. Although direct recombinant enzymes (e.g., Vimizim for Morquio, Cerezyme for Gaucher, Elaprase for Hunter, Mepsevii for Sly diseases) or hormone infusions (estrogen for osteoporosis and osteoarthritis), traditional gene delivery (e.g., direct infusion of viral or non-viral vectors with no modifications on capsid, envelope, or nanoparticles), and cell therapy strategies (healthy bone marrow or hematopoietic stem cell transplantation) partially improve bone lesions, novel delivery methods must be addressed regarding target specificity, less immunogenicity, and duration in circulation. In addition to improvements in bone delivery, potential regulation of bone development mechanisms involving receptor-regulated pathways has also been utilized. Targeted drug delivery using organic and inorganic compounds is a promising approach in mostly preclinical settings and future clinical translation. This review comprehensively summarizes the current bone-targeting strategies based on bone structure and remodeling concepts while emphasizing potential approaches for future bone-targeting systems.

Repurposing AS1411 for constructing ANM-PROTACs

Proteolysis-targeting chimeras (PROTACs) are heterobifunctional molecules consisting of two ligands joined by a linker, enabling them to simultaneously bind with an E3 ligase and a protein of interest (POI) and trigger proteasomal degradation of the POI. Limitations of PROTAC include lack of potent E3 ligands, poor cell selectivity, and low permeability. AS1411 is an antitumor aptamer specifically recognizing a membrane-nucleus shuttling nucleolin (NCL). Here, we repurpose AS1411 as a ligand for an E3 ligase mouse double minute 2 homolog (MDM2) via anchoring the NCL-MDM2 complex. Then, we construct an AS1411-NCL-MDM2-based PROTAC (ANM-PROTAC) by conjugating AS1411 with large-molecular-weight ligands for "undruggable" oncogenic STAT3, c-Myc, p53-R175H, and AR-V7. We show that the ANM-PROTAC efficiently penetrates tumor cells, recruits MDM2 and degrades the POIs. The ANM-PROTAC achieves tumor-selective distribution and exhibits excellent antitumor activity with no systemic toxicity. This is a PROTAC with built-in tumor-targeting and cell-penetrating capacities.

Macrophage membrane-reversibly camouflaged nanotherapeutics accelerate fracture healing by fostering MSCs recruitment and osteogenic differentiation

The fracture healing outcome is largely dependent on the quantities as well as osteogenic differentiation capacities of mesenchymal stem cells (MSCs) at the lesion site. Herein, macrophage membrane (MM)-reversibly cloaked nanocomplexes (NCs) are engineered for the lesion-targeted and hierarchical co-delivery of short stromal derived factor-1α peptide (sSDF-1α) and Ckip-1 small interfering RNA (Ckip-1 siRNA, siCkip-1) to promote bone repair by concurrently fostering recruitment and osteogenic differentiation of endogenous MSCs. To construct the NCs, a membrane-penetrating α-helical polypeptide first assembles with siCkip-1, and the cationic NCs are sequentially coated with catalase and an outer shell of sSDF-1α-anchored MM. Due to MM-assisted inflammation homing, intravenously injected NCs could efficiently accumulate at the fractured femur, where catalase decomposes the local hydrogen peroxide to generate oxygen bubbles that drives the shedding of sSDF-1α-anchored MM in the extracellular compartment. The exposed, cationic inner core thus enables robust trans-membrane delivery into MSCs to induce Ckip-1 silencing. Consequently, sSDF-1α-guided MSCs recruitment cooperates with siCkip-1-mediated osteogenic differentiation to facilitate bone formation and accelerate bone fracture healing. This study provides an enlightened strategy for the hierarchical co-delivery of macromolecular drugs into different cellular compartments, and it also renders a promising modality for the management of fracture healing.

Osteoblast-specific down-regulation of NLRP3 inflammasome by aptamer-functionalized liposome nanoparticles improves bone quality in postmenopausal osteoporosis rats

Rationale: NLRP3 inflammasome is critical in the development and progression of many metabolic diseases driven by chronic inflammation, but its effect on the pathology of postmenopausal osteoporosis (PMOP) remains poorly understood. Methods: We here firstly examined the levels of NLRP3 inflammasome in PMOP patients by ELISA. Then we investigated the possible mechanisms underlying the effect of NLRP3 inflammasome on PMOP by RNA sequencing of osteoblasts treated with NLRP3 siRNA and qPCR. Lastly, we accessed the effect of decreased NLRP3 levels on ovariectomized (OVX) rats. To specifically deliver NLRP3 siRNA to osteoblasts, we constructed NLRP3 siRNA wrapping osteoblast-specific aptamer (CH6)-functionalized lipid nanoparticles (termed as CH6-LNPs-siNLRP3). Results: We found that the levels of NLRP3 inflammasome were significantly increased in patients with PMOP, and were negatively correlated with estradiol levels. NLRP3 knock-down influenced signal pathways including immune system process, interferon signal pathway. Notably, of the top ten up-regulated genes in NLRP3-reduced osteoblasts, nine genes (except Mx2) were enriched in immune system process, and five genes were related to interferon signal pathway. The in vitro results showed that CH6-LNPs-siNLRP3 was relatively uniform with a dimeter of 96.64 ± 16.83 nm and zeta potential of 38.37 ± 1.86 mV. CH6-LNPs-siNLRP3 did not show obvious cytotoxicity and selectively delivered siRNA to bone tissue. Moreover, CH6-LNPs-siNLRP3 stimulated osteoblast differentiation by activating ALP and enhancing osteoblast matrix mineralization. When administrated to OVX rats, CH6-LNPs-siNLRP3 promoted bone formation and bone mass, improved bone microarchitecture and mechanical properties by decreasing the levels of NLRP3, IL-1β and IL-18 and increasing the levels of OCN and Runx2. Conclusion: NLRP3 inflammasome may be a new biomarker for PMOP diagnosis and plays a key role in the pathology of PMOP. CH6-LNPs-siNLRP3 has potential application for the treatment of PMOP.

Lipid nanoparticle-based strategies for extrahepatic delivery of nucleic acid therapies - challenges and opportunities

The advent of lipid nanoparticles (LNPs) containing ionizable cationic lipids has enabled the encapsulation, stabilization, and intracellular delivery of nucleic acid payloads, leading to FDA-approved siRNA-based therapy and mRNA-based vaccines. Other nucleic acid-based therapeutic modalities, including protein replacement and CRISPR-mediated gene knockout and editing, are being tested in clinical trials, in many cases, for the treatment of liver-related diseases. However, to fully exploit these therapies beyond the liver, improvements in their delivery to extrahepatic targets are needed. Towards this end, both active targeting strategies based on targeting ligands grafted onto LNPs and passive targeting relying on physicochemical LNP parameters such as surface composition, charge, and size are being evaluated. Often, the latter strategy depends on the interaction of LNPs with blood components, forming what is known as the biomolecular corona. Here, I discuss potential challenges related to current LNP-based targeting strategies and the studies of the biomolecular corona on LNPs. I propose potential solutions to overcome some of these obstacles and present approaches currently being tested in preclinical and clinical studies, which face fewer biological barriers than traditional organ-targeting approaches.

Biomimetic Vesicles with Designer Phospholipids Can Sense Environmental Redox Cues

Cell-like materials that sense environmental cues can serve as next-generation biosensors and help advance the understanding of intercellular communication. Currently, bottom-up engineering of protocell models from molecular building blocks remains a grand challenge chemists face. Herein, we describe giant unilamellar vesicles (GUVs) with biomimetic lipid membranes capable of sensing environmental redox cues. The GUVs employ activity-based sensing through designer phospholipids that are fluorescently activated in response to specific reductive (hydrogen sulfide) or oxidative (hydrogen peroxide) conditions. These synthetic phospholipids are derived from 1,2-dipalmitoyl-rac-glycero-3-phosphocholine and they possess a headgroup with heterocyclic aromatic motifs. Despite their structural deviation from the phosphocholine headgroup, the designer phospholipids (0.5-1.0 mol %) mixed with natural lipids can vesiculate, and the resulting GUVs (7-20 μm in diameter) remain intact over the course of redox sensing. All-atom molecular dynamics simulations gave insight into how these lipids are positioned within the hydrophobic core of the membrane bilayer and at the membrane-water interface. This work provides a purely chemical method to investigate potential redox signaling and opens up new design opportunities for soft materials that mimic protocells.

Engineering aspects of lipid-based delivery systems: In vivo gene delivery, safety criteria, and translation strategies

Defects in the genome cause genetic diseases and can be treated with gene therapy. Due to the limitations encountered in gene delivery, lipid-based supramolecular colloidal materials have emerged as promising gene carrier systems. In their non-functionalized form, lipid nanoparticles often demonstrate lower transgene expression efficiency, leading to suboptimal therapeutic outcomes, specifically through reduced percentages of cells expressing the transgene. Due to chemically active substituents, the engineering of delivery systems for genetic drugs with specific chemical ligands steps forward as an innovative strategy to tackle the drawbacks and enhance their therapeutic efficacy. Despite intense investigations into functionalization strategies, the clinical outcome of such therapies still needs to be improved. Here, we highlight and comprehensively review engineering aspects for functionalizing lipid-based delivery systems and their therapeutic efficacy for developing novel genetic cargoes to provide a full snapshot of the translation from the bench to the clinics. We outline existing challenges in the delivery and internalization processes and narrate recent advances in the functionalization of lipid-based delivery systems for nucleic acids to enhance their therapeutic efficacy and safety. Moreover, we address clinical trials using these vectors to expand their clinical use and principal safety concerns.

Physiological Barriers and Strategies of Lipid-Based Nanoparticles for Nucleic Acid Drug Delivery

Lipid-based nanoparticles (LBNPs) are currently the most promising vehicles for nucleic acid drug (NAD) delivery. Although their clinical applications have achieved success, the NAD delivery efficiency and safety are still unsatisfactory, which are, to a large extent, due to the existence of multi-level physiological barriers in vivo. It is important to elucidate the interactions between these barriers and LBNPs, which will guide more rational design of efficient NAD vehicles with low adverse effects and facilitate broader applications of nucleic acid therapeutics. This review describes the obstacles and challenges of biological barriers to NAD delivery at systemic, organ, sub-organ, cellular, and subcellular levels. The strategies to overcome these barriers are comprehensively reviewed, mainly including physically/chemically engineering LBNPs and directly modifying physiological barriers by auxiliary treatments. Then the potentials and challenges for successful translation of these preclinical studies into the clinic are discussed. In the end, a forward look at the strategies on manipulating protein corona (PC) is addressed, which may pull off the trick of overcoming those physiological barriers and significantly improve the efficacy and safety of LBNP-based NADs delivery.

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.

Strategies for non-viral vectors targeting organs beyond the liver

In recent years, nanoparticles have evolved to a clinical modality to deliver diverse nucleic acids. Rising interest in nanomedicines comes from proven safety and efficacy profiles established by continuous efforts to optimize physicochemical properties and endosomal escape. However, despite their transformative impact on the pharmaceutical industry, the clinical use of non-viral nucleic acid delivery is limited to hepatic diseases and vaccines due to liver accumulation. Overcoming liver tropism of nanoparticles is vital to meet clinical needs in other organs. Understanding the anatomical structure and physiological features of various organs would help to identify potential strategies for fine-tuning nanoparticle characteristics. In this Review, we discuss the source of liver tropism of non-viral vectors, present a brief overview of biological structure, processes and barriers in select organs, highlight approaches available to reach non-liver targets, and discuss techniques to accelerate the discovery of non-hepatic therapies.

Leveraging selective knockdown of Sost gene by polyethyleneimine-siRNA-chitosan reduced gold nanoparticles to promote osteogenesis in MC3T3-E1 & MEF cells

Aim: Osteoporosis is a systemic skeletal disorder characterized by reduced osteoblast differentiation, predominantly by overexpression of the Sost gene. A layer-by-layer approach enabled encapsulation of Sost siRNA to enhance the short half-life and poor transfection capacity of siRNA. Materials & methods: Polyethyleneimine and siRNA on chitosan-coated gold nanoparticles (PEI/siRNA/Cs-AuNPs) were engineered using chitosan-reduced gold nanoparticles. They were characterized by dynamic light scattering, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared and gel-mobility assays. Detailed in vitro experiments, gene silencing and western blots were performed. Results: A total of 80% knockdown of the target sclerostin protein was observed by PEI/siRNA/Cs-AuNPs, q-PCR showed threefold downregulation of the Sost gene. Osteogenic markers RunX2 and Alp were significantly upregulated. Conclusion: We report a safe, biocompatible nanotherapeutic strategy to enhance siRNA protection and subsequent silencing to augment bone formation.

Persistent Targeting DNA Nanocarrier Made of 3D Structural Unit Assembled from Only One Basic Multi-Palindromic Oligonucleotide for Precise Gene Cancer Therapy

Construction of a simple, reconfigurable, and stimuli-responsive DNA nanocarrier remains a technical challenge. In this contribution, by designing three palindromic fragments, a simplest four-sticky end-contained 3D structural unit (PS-unit) made of two same DNA components is proposed. Via regulating the rotation angle of central longitudinal axis of PS-unit, the oriented assembly of one-component spherical architecture is accomplished with high efficiency. Introduction of an aptamer and sticky tail warehouse into one component creates a size-change-reversible targeted siRNA delivery nanovehicle. Volume swelling of 20 nm allows one carrier to load 1987 siPLK1s. Once entering cancer cells and responding to glutathione (GSH) stimuli, siPLK1s are almost 100% released and original size of nanovehicle is restored, inhibiting the expression of PLK1 protein and substantially suppressing tumor growth (superior to commercial transfection agents) in tumor-bearing mice without systemic toxicity.

Bone Targeting Nanoparticles for the Treatment of Osteoporosis

Osteoporosis (OP) affects millions of people worldwide, especially postmenopausal women and the elderly. Although current available anti-OP agents can show promise in slowing down bone resorption, most are not specifically delivered to the hard tissue, causing significant toxicity. A bone-targeted nanodrug delivery system can reduce side effects and precisely deliver drug candidates to the bone. This review focuses on the progress of bone-targeted nanoparticles in OP therapy. We enumerate the existing OP medications, types of bone-targeted nanoparticles and categorize pairs of the most common bone-targeting functional groups. Finally, we summarize the potential use of bone-targeted nanoparticles in OP treatment. Ongoing research into the development of targeted ligands and nanocarriers will continue to expand the possibilities of OP-targeted therapies into clinical application.

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.

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.

Surface modification of lipid nanoparticles for gene therapy

Gene therapies have the potential to target and effectively treat a variety of diseases including cancer as well as genetic, neurological, and autoimmune disorders. Although we have made significant advances in identifying non-viral strategies to deliver genetic cargo, certain limitations remain. In general, gene delivery is challenging for several reasons including the instabilities of nucleic acids to enzymatic and chemical degradation and the presence of restrictive biological barriers such as cell, endosomal and nuclear membranes. The emergence of lipid nanoparticles (LNPs) helped overcome many of these challenges. Despite its success, further optimization is required for LNPs to yield efficient gene delivery to extrahepatic tissues, as LNPs favor accumulation in the liver after systemic administration. In this mini-review, we provide an overview of current preclinical approaches in that LNP surface modification was leveraged for cell and tissue targeting by conjugating aptamers, antibodies, and peptides among others. In addition to their cell uptake and efficiency-enhancing effects, we outline the (dis-)advantages of the different targeting moieties and commonly used conjugation strategies.

Aptamer-Assisted Blockade of the Immune Suppressor Sialic Acid-Binding Immunoglobulin-Like Lectin-15 for Cancer Immunotherapy

The percentage of low response and adaptive resistance to current antibody-based immune checkpoint blockade (ICB) therapy requires the development of novel immunotherapy strategies. Here, we developed an aptamer-assisted immune checkpoint blockade (Ap-ICB) against sialic acid-binding immunoglobulin-like lectin-15 (Siglec-15), a novel immune suppressor broadly upregulated on cancer cells and tumor infiltrating myeloid cells, which is mutually exclusive of programmed cell death ligand 1 (PD-L1). Using protein aptamer selection, we identified WXY3 aptamer with high affinity against Siglec-15 protein/Siglec-15 positive cells. We demonstrated that WXY3 aptamer rescued antigen-specific T cell responses in vitro and in vivo. Importantly, the WXY3 Ap-ICB against Siglec-15 amplified anti-tumor immunity in the tumor microenvironment and inhibited tumor growth/metastasis in syngeneic mouse model, which may result from enhanced macrophage and T cell functionality. In addition, by using aptamer-based spherical nucleic acids, we developed a synergetic ICB strategy of multivalent binding and steric hindrance, which further improves the in vivo anti-tumor effect. Taken together, our results support Ap-ICB targeted Siglec-15 as a potential strategy for normalization cancer immunotherapy.

Amplified Targeted Drug Delivery Independent of Target Number through Alternative Administration of Two Matched Nanoparticles

Targeting nanoparticles (NPs) based on the specific binding of ligands with molecular targets provides a promising tool for tissue-selective drug delivery. However, the number of molecular targets on the cell surface is limited, hindering the number of NPs that can bind and, thus, limiting the therapeutic outcome. Although several strategies have been developed to enhance drug delivery, such as enhancing drug loading and circulation time or increasing the enhanced permeability and retention effect of nanocarriers, none have resolved this issue. Herein, we designed a simple method for amplified and targeted drug delivery using two matched NPs. One NP was aptamer-functionalized to specifically bind to target cells, while the other was aptamer-complementary DNA-functionalized to specifically bind to aptamer-NPs. Alternate administration of the two matched NPs enables their continuous accumulation in the disease site despite their limited molecular targets. As a proof of concept, the method was tested in a breast cancer model and significantly enhanced chemotherapy of tumor cells in vitro and in vivo. The potential applications of this method in a brain injury model were also demonstrated. Overall, the study describes a method for amplified targeted drug delivery independent of the target number.

Advantages of Material Biofunctionalization Using Nucleic Acid Aptamers in Tissue Engineering and Regenerative Medicine

Material engineering is a fundamental issue in the applications of materials in the medical field. One of the aspects of material engineering is incorporating recognition sites on the surface of biomaterials, which plays an essential role in increasing the efficiency of tissue engineering scaffolds in various aspects. The application of peptides and antibodies to establish the recognition and adhesion sites has limitations, such as fragility and instability under physical and chemical processes. Therefore, synthetic ligands such as nucleic acid aptamers have received much attention for easy synthesis, minimal immunogenicity, high specificity, and stability under processing. Due to the effective role of these ligands in increasing the efficiency of engineered constructs in this study, the advantages of nucleic acid aptamers in tissue engineering will be reviewed. Aptamer-functionalized biomaterials can attract endogenous stem cells to wounded areas and organize their actions to facilitate tissue regeneration. This approach harnesses the body's inherent regeneration potential to treat many diseases. Also, increased efficacy in controlled release, slow and targeted drug delivery are important issues in drug delivery for tissue engineering approaches which can be achieved by incorporating aptamers in drug delivery systems. Aptamer-functionalized scaffolds have very applications, such as diagnosis of cancer, hematological infections, narcotics, heavy metals, toxins, controlled release from the scaffolds, and in vivo cell tracing. Aptasensors, as a result of many advantages over other traditional assay methods, can replace older methods. Furthermore, their unique targeting mechanism also targets compounds with no particular receptors. Targeting cell homing, local and targeted drug delivery, cell adhesion efficacy, cytocompatibility and bioactivity of scaffolds, aptamer-based biosensor, and aptamer-functionalized scaffolds are the topics that will be examined in this review study.

Surface engineering of lipid nanoparticles: targeted nucleic acid delivery and beyond

Harnessing surface engineering strategies to functionalize nucleic acid-lipid nanoparticles (LNPs) for improved performance has been a hot research topic since the approval of the first siRNA drug, patisiran, and two mRNA-based COVID-19 vaccines, BNT162b2 and mRNA-1273. Currently, efforts have been mainly made to construct targeted LNPs for organ- or cell-type-specific delivery of nucleic acid drugs by conjugation with various types of ligands. In this review, we describe the surface engineering strategies for nucleic acid-LNPs, considering ligand types, conjugation chemistries, and incorporation methods. We then outline the general purification and characterization techniques that are frequently used following the engineering step and emphasize the specific techniques for certain types of ligands. Next, we comprehensively summarize the currently accessible organs and cell types, as well as the other applications of the engineered LNPs. Finally, we provide considerations for formulating targeted LNPs and discuss the challenges of successfully translating the "proof of concept" from the laboratory into the clinic. We believe that addressing these challenges could accelerate the development of surface-engineered LNPs for targeted nucleic acid delivery and beyond.

The use of peptides, aptamers, and variable domains of heavy chain only antibodies in tissue engineering and regenerative medicine

Over the years, much research has been focused on the use of small molecules such as peptides or aptamers or more recently on the use of variable antigen-binding domain of heavy chain only antibodies in the field of tissue engineering and regenerative medicine. The use of these molecules originated as an alternative for the larger conventional antibodies, of which most drawbacks are derived from their size and complex structure. In the field of tissue engineering and regenerative medicine, biological functionalities are often conjugated to biomaterials in order to (re-)create an in vivo like situation, especially when bioinert biomaterials are used. Those biomaterials are functionalized with these functionalities for instance for the purpose of cell attachment or cell targeting for targeted drug delivery but also for local enrichment or blocking of ligands such as growth factors or cytokines on the biomaterial surface. In this review, we further refer to peptides, aptamers, and variable antigen-binding domain of heavy chain only antibodies as biological functionalities. Here, we compare these biological functionalities within the field of tissue engineering and regenerative medicine and give an overview of recent work in which these biological functionalities have been explored. We focus on the previously mentioned purposes of the biological functionalities. We will compare structural differences, possible modifications and (chemical) conjugation strategies. In addition, we will provide an overview of biologicals that are, or have been, involved in clinical trials. Finally, we will highlight the challenges of each of these biologicals. STATEMENT OF SIGNIFICANCE: In the field of tissue engineering there is broad application of functionalized biomaterials for cell attachment, targeted drug delivery and local enrichment or blocking of growth factors. This was previously mostly done via conventional antibodies, but their large size and complex structure impose various challenges with respect of retaining biological functionality. Peptides, aptamers and VHHs may provide an alternative solution for the use of conventional antibodies. This review discusses the use of these molecules for biological functionalization of biomaterials. For each of the molecules, their characteristics, conjugation possibilities and current use in research and clinical trials is described. Furthermore, this review sets out the benefits and challenges of using these types of molecules for different fields of application.

The Mechanotransduction Signaling Pathways in the Regulation of Osteogenesis

Bones are constantly exposed to mechanical forces from both muscles and Earth's gravity to maintain bone homeostasis by stimulating bone formation. Mechanotransduction transforms external mechanical signals such as force, fluid flow shear, and gravity into intracellular responses to achieve force adaptation. However, the underlying molecular mechanisms on the conversion from mechanical signals into bone formation has not been completely defined yet. In the present review, we provide a comprehensive and systematic description of the mechanotransduction signaling pathways induced by mechanical stimuli during osteogenesis and address the different layers of interconnections between different signaling pathways. Further exploration of mechanotransduction would benefit patients with osteoporosis, including the aging population and postmenopausal women.

Lipid nanoparticles (LNPs) for in vivo RNA delivery and their breakthrough technology for future applications

RNA therapeutics show a significant breakthrough for the treatment of otherwise incurable diseases and genetic disorders by regulating disease-related gene expression. The successful development of COVID-19 mRNA vaccines further emphasizes the potential of RNA therapeutics in the prevention of infectious diseases as well as in the treatment of chronic diseases. However, the efficient delivery of RNA into cells remains a challenge, and nanoparticle delivery systems such as lipid nanoparticles (LNPs) are necessary to fully realize the potential of RNA therapeutics. While LNPs provide a highly efficient platform for the in vivo delivery of RNA by overcoming various biological barriers, several challenges remain to be resolved for further development and regulatory approval. These include a lack of targeted delivery to extrahepatic organs and a gradual loss of therapeutic potency with repeated doses. In this review, we highlight the fundamental aspects of LNPs and their uses in the development of novel RNA therapeutics. Recent advances in LNP-based therapeutics and preclinical/clinical studies are overviewed. Lastly, we discuss the current limitations of LNPs and introduce breakthrough technologies that might overcome these challenges in future applications.

Harnessing Nucleic Acids Nanotechnology for Bone/Cartilage Regeneration

The effective regeneration of weight-bearing bone defects and critical-sized cartilage defects remains a significant clinical challenge. Traditional treatments such as autologous and allograft bone grafting have not been successful in achieving the desired outcomes, necessitating the need for innovative therapeutic approaches. Nucleic acids have attracted significant attention due to their ability to be designed to form discrete structures and programmed to perform specific functions at the nanoscale. The advantages of nucleic acid nanotechnology offer numerous opportunities for in-cell and in vivo applications, and hold great promise for advancing the field of biomaterials. In this review, the current abilities of nucleic acid nanotechnology to be applied in bone and cartilage regeneration are summarized and insights into the challenges and future directions for the development of this technology are provided.

Noncanonical Condensation of Nucleic Acid Chains by Hydrophobic Gold Nanocrystals

Nucleic acid condensates are essential for various biological processes and have numerous applications in nucleic acid nanotechnology, gene therapy, and mRNA vaccines. However, unlike the in vivo condensation that is dependent on motor proteins, the in vitro condensation efficiency remains to be improved. Here, we proposed a hydrophobic interaction-driven mechanism for condensing long nucleic acid chains using atomically precise hydrophobic gold nanoclusters (Au NCs). We found that hydrophobic Au NCs could condense long single-stranded DNA or RNA to form composites of spherical nanostructures, which further assembled into bead-shaped suprastructures in the presence of excessive Au NCs. Thus, suprastructures displayed gel-like behaviors, and Au NCs could diffuse freely inside the condensates, which resemble the collective motions of condensin complexes inside chromosomes. The dynamic hydrophobic interactions between Au NCs and bases allow for the reversible release of nucleic acids in the presence of mild triggering agents. Our method represents a significant advancement toward the development of more efficient and versatile nucleic acid condensation techniques.

Cell unit-inspired natural nano-based biomaterials as versatile building blocks for bone/cartilage regeneration

The regeneration of weight-bearing bone defects and critical-sized cartilage defects remains a significant challenge. A wide range of nano-biomaterials are available for the treatment of bone/cartilage defects. However, their poor compatibility and biodegradability pose challenges to the practical applications of these nano-based biomaterials. Natural biomaterials inspired by the cell units (e.g., nucleic acids and proteins), have gained increasing attention in recent decades due to their versatile functionality, compatibility, biodegradability, and great potential for modification, combination, and hybridization. In the field of bone/cartilage regeneration, natural nano-based biomaterials have presented an unparalleled role in providing optimal cues and microenvironments for cell growth and differentiation. In this review, we systematically summarize the versatile building blocks inspired by the cell unit used as natural nano-based biomaterials in bone/cartilage regeneration, including nucleic acids, proteins, carbohydrates, lipids, and membranes. In addition, the opportunities and challenges of natural nano-based biomaterials for the future use of bone/cartilage regeneration are discussed.

Noninvasive and Microinvasive Nanoscale Drug Delivery Platforms for Hard Tissue Engineering

Bone tissue plays a crucial role in protecting internal organs and providing structural support and locomotion of the body. Treatment of hard tissue defects and medical conditions due to physical injuries, genetic disorders, aging, metabolic syndromes, and infections is more often a complex and drawn out process. Presently, dealing with hard-tissue-based clinical problems is still mostly conducted via surgical interventions. However, advances in nanotechnology over the last decades have led to shifting trends in clinical practice toward noninvasive and microinvasive methods. In this review article, recent advances in the development of nanoscale platforms for bone tissue engineering have been reviewed and critically discussed to provide a comprehensive understanding of the advantages and disadvantages of noninvasive and microinvasive methods for treating medical conditions related to hard tissue regeneration and repair.

A potential therapeutic drug for osteoporosis: prospect for osteogenic LncRNAs

Long non-coding RNAs (LncRNAs) play essential roles in multiple physiological processes including bone formation. Investigators have revealed that LncRNAs regulated bone formation through various signaling pathways and micro RNAs (miRNAs). However, several problems exist in current research studies on osteogenic LncRNAs, including sophisticated techniques, high cost for in vivo experiment, as well as low homology of LncRNAs between animal model and human, which hindered translational medicine research. Moreover, compared with gene editing, LncRNAs would only lead to inhibition of target genes rather than completely knocking them out. As the studies on osteogenic LncRNA gradually proceed, some of these problems have turned osteogenic LncRNA research studies into slump. This review described some new techniques and innovative ideas to address these problems. Although investigations on osteogenic LncRNAs still have obtacles to overcome, LncRNA will work as a promising therapeutic drug for osteoporosis in the near future.

Strategies for targeted gene delivery using lipid nanoparticles and cell-derived nanovesicles

Gene therapy is a promising approach for the treatment of many diseases. However, the effective delivery of the cargo without degradation in vivo is one of the major hurdles. With the advent of lipid nanoparticles (LNPs) and cell-derived nanovesicles (CDNs), gene delivery holds a very promising future. The targeting of these nanosystems is a prerequisite for effective transfection with minimal side-effects. In this review, we highlight the emerging strategies utilized for the effective targeting of LNPs and CDNs, and we summarize the preparation methodologies for LNPs and CDNs. We have also highlighted the non-ligand targeting of LNPs toward certain organs based on their composition. It is highly expected that continuing the developments in the targeting approaches of LNPs and CDNs for the delivery system will further promote them in clinical translation.

The complex pathophysiology of bone fragility in obesity and type 2 diabetes mellitus: therapeutic targets to promote osteogenesis

Fractures associated with Type2 diabetes (T2DM) are major public health concerns in an increasingly obese and aging population. Patients with obesity or T2DM have normal or better than normal bone mineral density but at an increased risk for fractures. Hence it is crucial to understand the pathophysiology and mechanism of how T2DM and obesity result in altered bone physiology leading to increased fracture risk. Although enhanced osteoclast mediated bone resorption has been reported for these patients, the most notable observation among patients with T2DM is the reduction in bone formation from mostly dysfunction in osteoblast differentiation and survival. Studies have shown that obesity and T2DM are associated with increased adipogenesis which is most likely at the expense of reduced osteogenesis and myogenesis considering that adipocytes, osteoblasts, and myoblasts originate from the same progenitor cells. Furthermore, emerging data point to an inter-relationship between bone and metabolic homeostasis suggesting that these physiologic processes could be under the control of common regulatory pathways. Thus, this review aims to explore the complex mechanisms involved in lineage differentiation and their effect on bone pathophysiology in patients with obesity and T2DM along with an examination of potential novel pharmacological targets or a re-evaluation of existing drugs to improve bone homeostasis.

Smart Strategies to Overcome Drug Delivery Challenges in the Musculoskeletal System

The musculoskeletal system (MSKS) is composed of specialized connective tissues including bone, muscle, cartilage, tendon, ligament, and their subtypes. The primary function of the MSKS is to provide protection, structure, mobility, and mechanical properties to the body. In the process of fulfilling these functions, the MSKS is subject to wear and tear during aging and after injury and requires subsequent repair. MSKS diseases are a growing burden due to the increasing population age. The World Health Organization estimates that 1.71 billon people suffer from MSKS diseases worldwide. MSKS diseases usually involve various dysfunctions in bones, muscles, and joints, which often result in pain, disability, and a decrease in quality of life. The most common MSKS diseases are osteoporosis (loss of bone), osteoarthritis (loss of cartilage), and sarcopenia (loss of skeletal muscle). Because of the disease burden and the need for treatment, regenerative drug therapies for MSKS disorders are increasingly in demand. However, the difficulty of effective drug delivery in the MSKS has become a bottleneck for developing MSKS therapeutics. The abundance of extracellular matrix and its small pore size in the MSKS present a formidable barrier to drug delivery. Differences of vascularity among various MSKS tissues pose complications for drug delivery. Novel strategies are necessary to achieve successful drug delivery in different tissues composing the MSKS. Those considerations include the route of administration, mechanics of surrounding fluids, and biomolecular interactions, such as the size and charge of the particles and targeting motifs. This review focuses on recent advances in challenges to deliver drugs to each tissue of the MSKS, current strategies of drug delivery, and future ideas of how to overcome drug delivery challenges in the MSKS.

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.

Optimizing Delivery of Therapeutic Growth Factors for Bone and Cartilage Regeneration

Bone- and cartilage-related diseases, such as osteoporosis and osteoarthritis, affect millions of people worldwide, impairing their quality of life and increasing mortality. Osteoporosis significantly increases the bone fracture risk of the spine, hip, and wrist. For successful fracture treatment and to facilitate proper healing in the most complicated cases, one of the most promising methods is to deliver a therapeutic protein to accelerate bone regeneration. Similarly, in the setting of osteoarthritis, where degraded cartilage does not regenerate, therapeutic proteins hold great promise to promote new cartilage formation. For both osteoporosis and osteoarthritis treatments, targeted delivery of therapeutic growth factors, with the aid of hydrogels, to bone and cartilage is a key to advance the field of regenerative medicine. In this review article, we propose five important aspects of therapeutic growth factor delivery for bone and cartilage regeneration: (1) protection of protein growth factors from physical and enzymatic degradation, (2) targeted growth factor delivery, (3) controlling GF release kinetics, (4) long-term stability of regenerated tissues, and (5) osteoimmunomodulatory effects of therapeutic growth factors and carriers/scaffolds.

Bioinspired Mild Photothermal Effect-Reinforced Multifunctional Fiber Scaffolds Promote Bone Regeneration

Bone fractures are often companied with poor bone healing and high rates of infection. Early recruitment of mesenchymal stem cells (MSCs) is critical for initiating efficient bone repair, and mild thermal stimulation can accelerate the recovery of chronic diseases. Here, a bioinspired, staged photothermal effect-reinforced multifunctional scaffold was fabricated for bone repair. Uniaxially aligned electrospun polycaprolactone nanofibers were doped with black phosphorus nanosheets (BP NSs) to endow the scaffold with excellent near-infrared (NIR) responsive capability. Apt19S was then decorated on the surface of the scaffold to selectively recruit MSCs toward the injured site. Afterward, microparticles of phase change materials loaded with antibacterial drugs were also deposited on the surface of the scaffold, which could undergo a solid-to-liquid phase transition above 39 °C, triggering the release of payload to eliminate bacteria and prevent infection. Under NIR irradiation, photothermal-mediated up-regulation of heat shock proteins and accelerated biodegradation of BP NSs could promote the osteogenic differentiation of MSCs and biomineralization. Overall, this strategy shows the ability of bacteria elimination, MSCs recruitment, and bone regeneration promotion with the assistance of photothermal effect in vitro and in vivo, which emphasizes the design of a bioinspired scaffold and its potential for a mild photothermal effect in bone tissue engineering.

Aptamer-Based Targeted Protein Degradation

The selective removal of misfolded, aggregated, or aberrantly overexpressed protein plays an essential role in maintaining protein-dominated biological processes. In parallel, the precise knockout of abnormal proteins is inseparable from the accurate identification of proteins within complex environments. Guided by these precepts, small molecules, or antibodies, are commonly used as protein recognition tools for developing targeted protein degradation (TPD) technology. Indeed, TPD has shown tremendous prospects in chronic diseases, rare diseases, cancer research, and other fields. Meanwhile, aptamers are short RNA or DNA oligonucleotides that can bind to target proteins with high specificity and strong affinity. Accordingly, aptamers are actively used in designing and constructing TPD technology. In this perspective, we provide a brief introduction to TPD technology in its current progress, and we summarize its application challenges. Recent advances in aptamer-based TPD technology are reviewed, together with corresponding challenges and outlooks.

Understanding drug nanocarrier and blood-brain barrier interaction based on a microfluidic microphysiological model

As many nanoparticles (NPs) have been exploited as drug carriers to overcome the resistance of the blood-brain barrier (BBB), reliable in vitro BBB models are urgently needed to help researchers to comprehensively understand drug nanocarrier-BBB interaction during penetration, which can prompt pre-clinical nanodrug exploitation. Herein, we developed a microfluidic microphysiological model, allowing the analysis of BBB homeostasis and NP penetration. We found that the BBB penetrability of gold nanoparticles (AuNPs) was size- and modification-dependent, which might be caused by a distinct transendocytosis pathway. Notably, transferrin-modified 13 nm AuNPs held the strongest BBB penetrability and induced the slightest BBB dysfunction, while bare 80 nm and 120 nm AuNPs showed opposite results. Moreover, further analysis of the protein corona showed that PEGylation reduced the protein absorption, and some proteins facilitated the BBB penetration of NPs. The developed microphysiological model provides a powerful tool for understanding the drug nanocarrier-BBB interaction, which is vital for exploiting high-efficiency and biocompatible nanodrugs.

Optimal delivery strategies for nanoparticle-mediated mRNA delivery

Messenger RNA (mRNA) has emerged as a new and efficient agent for the treatment of various diseases. The success of lipid nanoparticle-mRNA against the novel coronavirus (SARS-CoV-2) pneumonia epidemic has proved the clinical potential of nanoparticle-mRNA formulations. However, the deficiency in the effective biological distribution, high transfection efficiency and good biosafety are still the major challenges in clinical translation of nanomedicine for mRNA delivery. To date, a variety of promising nanoparticles have been constructed and then gradually optimized to facilitate the effective biodistribution of carriers and efficient mRNA delivery. In this review, we describe the design of nanoparticles with an emphasis on lipid nanoparticles, and discuss the manipulation strategies for nanoparticle-biology (nano-bio) interactions for mRNA delivery to overcome the biological barriers and improve the delivery efficiency, because the specific nano-bio interaction of nanoparticles usually remoulds the biomedical and physiological properties of the nanoparticles especially the biodistribution, mechanism of cellular internalization and immune response. Finally, we give a perspective for the future applications of this promising technology. We believe that the regulation of nano-bio interactions would be a significant breakthrough to improve the mRNA delivery efficiency and cross biological barriers. This review may provide a new direction for the design of nanoparticle-mediated mRNA delivery systems.

Aptamer-modified M cell targeting liposomes for oral delivery of macromolecules

There is an urgent demand for non-invasive and high compliance delivery systems of macromolecules for long-term therapy. However, oral administration of macromolecules is hindered by low permeability and instability in the gastrointestinal (GI) tract. Therefore, we developed a novel aptamer-modified liposomes (Apt-Lip) with M cell targeting for oral delivery of exenatide (EXT). Firstly, we optimized aptamers to M cells by Cell-SELEX and aptamer truncations. The selected aptamer T-M3 (Apt-T-M3) with high binding affinity (K_d = 176 ± 108 nM) and specificity was modified on the surface of liposomes for targeting M cells. Liposomes were formulated by microfluidics system and characterized in terms of morphology, hydrodynamic diameter, zeta potential, and the efficiency of encapsulation. In comparison with non-targeting liposomes, cell uptake in M cells was significantly enhanced by Apt-Lip. Similarly, the transport efficiency of EXT was 2-fold increase using Apt-Lip in M cells. Additionally, the transepithelial electrical resistance (TEER) of M cell monolayers is significantly reduced. In ex vivo intestinal absorption study, Apt-Lip was proved to possess significantly high intestinal absorption in Peyer's patches (PPs) and M cells-specific targeting capacity. Consequently, Apt-Lip promoted the EXT transport could base not only on M cell mediated transport, but also on enhancement of paracellular permeability. In conclusion, the present study supported Apt-Lip as a promising M cell targeted delivery system for oral delivery of macromolecules.

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.

Aptamer Technology and Its Applications in Bone Diseases

Aptamers are single-stranded nucleic acids (DNA, short RNA, or other artificial molecules) produced by the Systematic Evolution of Ligands by Exponential Enrichment (SELEX) technology, which can be tightly and specifically combined with desired targets. As a comparable alternative to antibodies, aptamers have many advantages over traditional antibodies such as a strong chemical stability and rapid bulk production. In addition, aptamers can bind targets in various ways, and are not limited like the antigen-antibody combination. Studies have shown that aptamers have tremendous potential to diagnose and treat clinical diseases. However, only a few aptamer-based drugs have been used because of limitations of the aptamers and SELEX technology. To promote the development and applications of aptamers, we present a review of the methods optimizing the SELEX technology and modifying aptamers to boost the selection success rate and improve aptamer characteristics. In addition, we review the application of aptamers to treat bone diseases.

Nanomaterial-assisted theranosis of bone diseases

Bone-related diseases refer to a group of skeletal disorders that are characterized by bone and cartilage destruction. Conventional approaches can regulate bone homeostasis to a certain extent. However, these therapies are still associated with some undesirable problems. Fortunately, recent advances in nanomaterials have provided unprecedented opportunities for diagnosis and therapy of bone-related diseases. This review provides a comprehensive and up-to-date overview of current advanced theranostic nanomaterials in bone-related diseases. First, the potential utility of nanomaterials for biological imaging and biomarker detection is illustrated. Second, nanomaterials serve as therapeutic delivery platforms with special functions for bone homeostasis regulation and cellular modulation are highlighted. Finally, perspectives in this field are offered, including current key bottlenecks and future directions, which may be helpful for exploiting nanomaterials with novel properties and unique functions. This review will provide scientific guidance to enhance the development of advanced nanomaterials for the diagnosis and therapy of bone-related diseases.

Targeted inhibition of osteoclastogenesis reveals the pathogenesis and therapeutics of bone loss under sympathetic neurostress

Sympathetic cues via the adrenergic signaling critically regulate bone homeostasis and contribute to neurostress-induced bone loss, but the mechanisms and therapeutics remain incompletely elucidated. Here, we reveal an osteoclastogenesis-centered functionally important osteopenic pathogenesis under sympatho-adrenergic activation with characterized microRNA response and efficient therapeutics. We discovered that osteoclastic miR-21 was tightly regulated by sympatho-adrenergic cues downstream the β2-adrenergic receptor (β₂AR) signaling, critically modulated osteoclastogenesis in vivo by inhibiting programmed cell death 4 (Pdcd4), and mediated detrimental effects of both isoproterenol (ISO) and chronic variable stress (CVS) on bone. Intriguingly, without affecting osteoblastic bone formation, bone protection against ISO and CVS was sufficiently achieved by a (D-Asp₈)-lipid nanoparticle-mediated targeted inhibition of osteoclastic miR-21 or by clinically relevant drugs to suppress osteoclastogenesis. Collectively, these results unravel a previously underdetermined molecular and functional paradigm that osteoclastogenesis crucially contributes to sympatho-adrenergic regulation of bone and establish multiple targeted therapeutic strategies to counteract osteopenias under stresses.

Targeting loop3 of sclerostin preserves its cardiovascular protective action and promotes bone formation

Sclerostin negatively regulates bone formation by antagonizing Wnt signalling. An antibody targeting sclerostin for the treatment of postmenopausal osteoporosis was approved by the U.S. Food and Drug Administration, with a boxed warning for cardiovascular risk. Here we demonstrate that sclerostin participates in protecting cardiovascular system and inhibiting bone formation via different loops. Loop3 deficiency by genetic truncation could maintain sclerostin’s protective effect on the cardiovascular system while attenuating its inhibitory effect on bone formation. We identify an aptamer, named aptscl56, which specifically targets sclerostin loop3 and use a modified aptscl56 version, called Apc001PE, as specific in vivo pharmacologic tool to validate the above effect of loop3. Apc001PE has no effect on aortic aneurysm and atherosclerotic development in ApoE−/− mice and hSOSTki.ApoE−/− mice with angiotensin II infusion. Apc001PE can promote bone formation in hSOSTki mice and ovariectomy-induced osteoporotic rats. In summary, sclerostin loop3 cannot participate in protecting the cardiovascular system, but participates in inhibiting bone formation.

Nanoparticle-mediated selective Sfrp-1 silencing enhances bone density in osteoporotic mice

Osteoporosis (OP) is characterized by a loss in bone mass and mineral density. The stimulation of the canonical Wnt/β-catenin pathway has been reported to promote bone formation, this pathway is controlled by several regulators as secreted frizzled-related protein-1 (Sfrp-1), antagonist of the pathway. Thus, Sfrp-1 silencing therapies could be suitable for enhancing bone growth. However, the systemic stimulation of Wnt/β-catenin has been correlated with side effects. This work hypothesizes the administration of lipid-polymer NPs (LPNPs) functionalized with a MSC specific aptamer (Apt) and carrying a SFRP1 silencing GapmeR, could favor bone formation in OP with minimal undesired effects. Suitable SFRP1 GapmeR-loaded Apt-LPNPs (Apt-LPNPs-SFRP1) were administered in osteoporotic mice and their biodistribution, toxicity and bone induction capacity were evaluated. The aptamer functionalization of the NPs modified their biodistribution profile showing a four-fold increase in the bone accumulation and a ten-fold decrease in the hepatic accumulation compared to naked LPNPs. Moreover, the histological evaluation revealed evident changes in bone structure observing a more compact trabecular bone and a cortical bone thickness increase in the Apt-LPNPs-SFRP1 treated mice with no toxic effects. Therefore, these LPNPs showed suitable properties and biodistribution profiles leading to an enhancement on the bone density of osteoporotic mice.

Peptide modified geniposidic acid targets bone and effectively promotes osteogenesis

Background

Geniposidic acid (GPA), one of the active components of Eucommia ulmoides, promote bone formation and treat osteoporosis by activating farnesoid X receptor (FXR). However, GPA has low oral availability and lack of bone targeting in the treatment of bone related diseases. With the development of modern technology, small molecules, amino acids, or aptamers are used for biological modification of drugs and target cells in bone tissue, which has become the trend of bone targeted research.

Methods

In this study, SDSSD (an osteoblast-targeting peptide) were modified in GPA using Fmoc solid-phase synthesis technique to form a new SDSSD-GPA conjugate (SGPA). The bone targeting of SGPA was evaluated using in vivo imaging and cell co-culture. In vitro, the effect of SGPA on cytotoxicity, osteoblastic activity, and mineralization ability were studied in mouse primary osteoblasts (OBs). In vivo, the therapeutic effect of SGPA on osteoporosis using an ovariectomized (OVX) mouse model. The bone mass, histomorphometry, serum biochemical parameters, and the molecular mechanism were evaluated.

Results

SGPA was enriched in OBs and tends to accumulate in bone tissue. In vitro, SGPA significantly enhanced the osteogenic activity and mineralization of OBs compared with GPA. In vivo, SGPA enhanced serum BALP and P1NP levels, increased the trabecular bone mass of the mice, and SGPA administration have a higher bone mineralization deposition rate than the GPA-treated mice. Moreover, SGPA significantly activated FXR and Runt-related transcription factor 2 (RUNX2).

Conclusions

Collectively, SGPA is enriched into OBs, and promotes bone formation by activating FXR-RUNX2 signalling, effectively treating osteoporosis at relatively low doses.

The translational potential of this article

This study demonstrates a more efficient and safe application of GPA in treating osteoporosis, provide a new concept for the bone targeted application of natural compounds.

Lipid nanoparticle-mediated silencing of osteogenic suppressor GNAS leads to osteogenic differentiation of mesenchymal stem cells in vivo

Approved drugs for the treatment of osteoporosis can prevent further bone loss but do not stimulate bone formation. Approaches that improve bone density in metabolic diseases are needed. Therapies that take advantage of the ability of mesenchymal stem cells (MSCs) to differentiate into various osteogenic lineages to treat bone disorders are of particular interest. Here we examine the ability of small interfering RNA (siRNA) to enhance osteoblast differentiation and bone formation by silencing the negative suppressor gene GNAS in bone MSCs. Using clinically validated lipid nanoparticle (LNP) siRNA delivery systems, we show that silencing the suppressor gene GNAS in vitro in MSCs leads to molecular and phenotypic changes similar to those seen in osteoblasts. Further, we demonstrate that these LNP-siRNAs can transfect a large proportion of mice MSCs in the compact bone following intravenous injection. Transfection of MSCs in various animal models led to silencing of GNAS and enhanced differentiation of MSCs into osteoblasts. These data demonstrate the potential for LNP delivery of siRNA to enhance the differentiation of MSCs into osteoblasts, and suggests that they are a promising approach for the treatment of osteoporosis and other bone diseases.