Journal of Nanomaterials
Volume 2020 (2020), Article ID 2187169, 14 pages
https://doi.org/10.1155/2020/2187169

Review Article

Exosome: A Novel Nanocarrier Delivering Noncoding RNA for Bone Tissue Engineering

Keda Liu,¹ Nanjue Cao,² Yuhe Zhu,¹ and Wei Wang¹

¹School and Hospital of Stomatology, China Medical University, Liaoning Provincial Key Laboratory of Oral Diseases, Shenyang 110001, China
²The Fourth Affiliated Hospital, Zhejiang University School of Medicine, Yiwu 322000, China

Correspondence should be addressed to Wei Wang; wwang75@cmu.edu.cn

Received 9 July 2020; Accepted 31 July 2020; Published 14 August 2020

Academic Editor: Hui-Qi Xie

Copyright © 2020 Keda Liu et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Bone growth and metabolism are mainly regulated by a series of intracellular molecules and extracellular stimuli. Exosome, as a nanoscale substance secreted to the outside of the cells, plays an extensive role in intercellular communication. This review provides theoretical references and evidences for further exploration of exosomes as noncoding RNA carriers to regulate bone tissue recovery through the following aspects: (1) basic characteristics of exosomes, (2) research progress of exosomal noncoding RNA in bone tissue engineering, (3) current status and advantages of engineering exosomes as nanocarriers for noncoding RNA delivery, and (4) problems and application prospects of exosome therapy in the field of orthopedics.

1. Introduction

Destructive osteopathy is mainly due to the failure of the bone remodeling process, including enhanced osteoclast activity or reduced osteoblast generation [1, 2]. Osteoarthritis, osteoporosis, etc. will lead to a continuous decline in the quality of patients’ life [3, 4], deemed to be an important global health problem [5, 6]. The process of bone remodeling is to repair bone damage and restore aging bone tissue [7]. This is the prerequisite for maintaining mass and mechanical strength of bone [8]. Mutual regulation of osteoblasts and osteoclasts is essential for bone remodeling [9], dominated by molecular signals [10, 11].

Extracellular vesicles are natural nanoscale particles secreted by cells. They are spherical bilayer membrane vesicles with diameters ranging from 30 nm to 1000 nm, including exosomes of uniform size (30-150 nm), microvesicles of varying sizes (50-1000 nm), and apoptotic bodies [12]. Recent studies reported that the key factor for bone reconstruction was exosome [13]. These organelles play a crucial role in intercellular messengers, transferring bioactive molecules to nearby or distant cells under physiological and pathological conditions [14, 15]. Noncoding RNAs in exosomes supply a method that cells can straightly regulate the expression of protein in target cells. As a kind of nanomaterial, exosome has been explored in the field of diagnostic [16], therapeutics [17], and drug delivery [18] (Figure 1).

Figure 1: Exosome production, secretion, and cargo transfer from donor cells to recipient cells. The inset shows the clinical application of noncoding RNA exosomes [19].

Existing drug carriers, such as artificially manufactured liposomes, can cause accumulated toxicity and immunogenicity in the body, thus limiting the wide application of such drug carriers [20, 21]. As a natural drug delivery carrier, exosomes have significant advantages of biocompatibility, high stability, and targeting. In order to better understand the mechanism of exosomes as noncoding RNA vectors regulating bone remodeling, we reviewed the newest findings on the feature and role of exosomes in bone formation. In addition, we focused on elaborating the feasible clinical application of bone exosomes and the characteristics of exosomal noncoding RNAs in the regulation of bone reconstruction.

2. Biological Characteristics of Exosomes

Exosome was first discovered in 1981, considered cellular trash initially [22]. Subsequently, exosomes caught the interest of immunologists in the 1990s. Raposo et al. in 1996 reported that exosomes have the ability of antigen presentation, activating T cell immune response [23]. In addition, exosomes were found to carry miRNAs and mRNAs for intercellular communication in 2007. Most importantly, the mechanism of exosomal transport was cracked by three independent groups [24–26]. The history, biomarkers, and functions of exosomes are shown in Figure 2.

Figure 2: (a) Historical key developments in exosome research. (b) Biomarkers and functions of exosome.

Exosomes are membrane vesicles with diameters between 30 and 150 nm [27, 28] (Figure 3). They are derived from body cells including tumor cells [29, 30]. Studies revealed that exosomes are also secreted from bone-related cells (osteoclasts, osteoclast precursors, osteoblasts, and MSCs) [31–33]. Exosomes reflect the complex molecular processing that occurs inside the parent cells and are more appropriate as a potential replacement for parent cell in body fluids. Multiple studies have confirmed the loyalty of exosomal cargo to parental cells [34, 35]. Exosomes can encapsulate fragments of genomic and different kinds of RNA, such as miRNA, lncRNA, circRNA, and ribosomal RNA [36–39].

(a)

(b)

(c)

(d)

(e)

Figure 3: Characterization and cellular uptake of exosomes. As shown in (a) and (b), the morphology of exosomes was observed under TEM and SEM. In (c) and (d), exosome is a round membranous vesicle with a diameter of about 30-150 nm and the results of western blot showed that exosome was able to express marker proteins CD9, CD81, and CD63. In (e), to verify whether exosome interacts with cell, exosome labeled by Dil and receptor cell were cocultured in vitro for 6 h. Under a fluorescence microscope, over 85% of cell showed positive result of Dil, indicating that the Dil-labeled exosome has been efficiently transferred to receptor cells [30, 35, 53]. (a–d) Copyright 2020 BBRC published by Elsevier Ltd. (b) Copyright 2019 EXCLI J published by University of Mainz. (c) Copyright 2020 Int Immunopharmacology published by Elsevier Ltd.

3. Exosomes from Osteoclasts and Osteoblasts

3.1. Osteoclast Exosomes

Exosomes secreted by osteoclasts may provide clues on how to coordinate bone absorption and construction. In calcitriol-stimulated mouse bone marrow (C-SMM), osteoclasts and osteoblasts differentiated synergistically within six days. Osteoclast precursor-derived exosomes stimulated the proliferation of osteoclasts in C-SMM, while exosomes of mature osteoclasts inhibited osteoclast formation [13, 40]. Osteoclast exosomes stimulate the osteoblasts and other nonosteoclasts in a paracrine manner and produce factors that regulate osteoclast differentiation.

miRNA-214, involved in bone remodeling, adjusts Osterix and activates transcription factor (ATF) 4, which is an essential transcription factor in osteogenesis process [41, 42]. Higher levels of miRNA-214 were noted in the plasma of fractured elderly women and ovariectomized mouse. In vitro, by using transwell membranes (that only permitted exosomes through), exosomal miRNA-214 was transmitted from osteoclasts to osteoblasts, inhibiting proliferation and mineralizing activity of osteoblast in vitro and vivo [43, 44].

3.2. Osteoblast Exosomes

Osteoblasts release exosomes containing receptor activator RANKL. These exosomes induce osteoclast precursors to differentiate to osteoclasts [31]. Osteoblasts release RANKL exosomes and osteoclasts release RANK exosomes. Studies have shown that the RANKL-RANK-osteoprotectin signal network is the essence of bone biology. RANKL exosome from osteoblasts may be capable of promoting osteoclast formation and activity without direct intercellular contact. RANK exosome may inhibit RANKL in a method the same as osteoprotectin. Both RANKL and RANK-containing exosome may be able to fuse in their target cells to release regulatory molecules, which could assist to reprogram target cells [45].

4. The Roles of Exosomal Noncoding RNAs in Bone Growth

Noncoding RNA refers to RNA that does not encode protein. They have biological functions at the RNA level. Noncoding RNAs are generally divided into 3 categories according to their size: less than 50 nt, including microRNA (miRNA), small interfering RNA (siRNA), and piwi-interacting RNA (piRNA); 50 nt to 500 nt, including nuclear small RNA (snRNA) and small nucleolar RNA (snoRNA); and greater than 500 nt, long noncoding RNA (lncRNA) and circular RNA (circRNA) [46, 47]. This article reviews current research hotspots of osteoarthritis: miRNA, lncRNA, and circRNA.

4.1. miRNA

The latest two years of exosome miRNA in the regulation of bone growth and development is listed in Table 1. Exosome miR-128-3p regulates osteogenesis and fracture healing by targeting Smad5 [48]. Decreasing miR-221-3p in exosomes can significantly reduce chondrocyte proliferation and migration in vitro [49]. Exosome miR-17 secreted by keratinocytes can induce osteoclast differentiation [30]. By freeze and thaw method, researchers combined exosomes and miR-140 to promote cartilage differentiation of bone marrow stem cells, thereby enhancing cartilage repair and regeneration [50]. Human umbilical cord mesenchymal stem cell exosomes can effectively inhibit bone marrow mesenchymal stem cell apoptosis and prevent osteoporosis in rats. The mechanism is mediated by the miR-1263/Mob1/Hippo signaling pathway [51]. Exosome miR-8485 promotes cartilage differentiation of bone marrow mesenchymal stem cells by regulating the Wnt/β-catenin pathway [52].

Table 1: List of miRNA deliveries by exosome in both basic and clinical research of bone formation.

miRNA
Target gene
Resource
Recipient cells
Mechanism and result
Ref.

miR-128-3P
Smad5
MSCs
MSCs
miR-128-3p was highly expressed in aged exosome, inhibiting fracture healing by targeting Smad5.
[48]

miR-221-3p

CPCs
Murine chondrocytes
In vivo experiments confirmed that miR-221-3p was highly enriched in CPCs-Exos, which can stimulate chondrocyte proliferation and migration.
[49]

miR-140

Freeze and thaw method
BMSCs
miR-140 was fused into exosomes by freezing and thawing, promoting the differentiation of BMSCs into chondrocytes.[
[50]

miR-1263
Mob1
HUC-MSCs
BMSCs
Exosomes inhibit BMSC apoptosis through miR-1263/Mob1/Hippo signaling pathway.
[51]

miR-8485
DACT1, GSK3B
Chondrocytes
BMSCs
Exosomal miR-8485 regulated the Wnt/β-catenin pathways to promote chondrogenic differentiation of BMSCs.
[52]

miR-192-5p
RAC2
BMSCs

The exosomal miR-192-5p of BMSCs can delay the inflammatory response of rheumatoid arthritis by regulating the expression of proinflammatory factors.
[53]

miR-375
IGFBP3
hASCs
BMSCs
Exosomes derived from hASCs overexpressing miR-375 promoted bone regeneration by binding to IGFBP3.
[84]

miR-26a-5p
PTGS2
BMSC
Synovial fibroblasts
The hBMSC-derived exosome- miR-26a-5p inhibited the damage of synovial fibroblasts by targeting PTGS2.
[85]

miR-155

VECs
Macrophages
VEC-Exos was rich in miR-155, which indirectly inhibited osteoclast activity by interacting with macrophages to improve the symptoms of osteoporosis.
[86]

miR-129-5p
Sp1
MMs
hMSCs
Multiple myeloma (MM) exosome miR-129-5p reduced alkaline phosphatase (ALPL) activity by inhibiting the expression of transcription factor Sp1 in hMSCs.
[87]

miR-17
RANKL
Keratinocytes
Osteoclasts
The exosomal miR-17 secreted by keratinocytes with middle ear cholesteatoma can upregulate RANKL in fibroblasts and induce osteoclast differentiation.
[30]

MSCs: mesenchymal stem cells; BMSCs: bone marrow mesenchymal stem cells; HUC-MSCs: human umbilical cord mesenchymal stem cells; hASCs: human adipose mesenchymal stem cells; VECs: vascular endothelial cells; CPCs: chondrogenic progenitor cells; MMs: multiple myelomas.

Studies on miRNAs have demonstrated that the regulation of the target genes affects the expression of regulators upstream and downstream of each signaling pathway, to regulate the osteogenic differentiation process [54]. Runx2 is a transcription factor that plays an important role in osteoblast differentiation [55], which is precisely or indirectly regulated by many miRNAs, including miR-221 [56, 57], miR-133a-5p [58], miR-467g [59], miR-218 [60, 61], and miR-210 [62]. The expression of Osx can be downregulated by some miRNAs, inhibiting osteogenic differentiation, such as miR-145 [63] and miR-143 [64, 65]. Some miRNAs that regulate the expression of bone-related gene have not been studied in exosomes, providing clues to the research of exosomes in the field of orthopedics (Table 2).

Table 2: List of miRNAs in bone formation.

miRNA
Target genes
Cell type
Mechanism and result
Ref.

miR-221
ZFPM2, Runx2
Osteoblasts
miR-221 inhibited osteoblast differentiation by targeting ZFPM2 and Runx2 to reduce the expression levels of osteocalcin (OC), alkaline phosphatase (ALP), and collagen, type I, α1 (COL1A1).
[56, 57]

miR-218
Runx2
Osteoclasts
miR-218 participated in the negative regulation of osteoclastogenesis and bone resorption by inhibiting the p38MAPK-c-Fos-NFATc1 signaling pathway.
[60, 61]

miR-210
Runx2
MSCs
Overexpression of miR-210 in MSC led to the differentiation of MSC into osteoblasts by upregulating Runx2, ALP, and osteocalcin.
[62]

miR-145
Smad3, SEMA3A
BMSCs
MicroRNA-145 suppressed osteogenic differentiation of hBMSCs partially via targeting Smad3 and SEMA3A.
[63]

miR-143-3p
BMPR2
BMSCs
miR-143-3p regulated the early chondrogenic differentiation of BMSCs and promoted cartilage damage repair by targeting BMPR2.
[64, 65]

miR-542-3p
SFRP1
Osteoblasts
miR-542-3p prevented ovariectomy-induced osteoporosis in rats by targeting SFRP1.
[88]

miR-433-3p
DDK1
Osteoblasts
MicroRNA-433-3p promoted osteoblast differentiation through targeting DKK1.
[89]

miR-133a-5p
Runx2
Osteoblasts
miR-133a-5p inhibited the expression of osteoblast differentiation-associated markers by targeting the 3’ UTR of RUNX2.
[58]

miR-467g
Ihh/Runx-2
Osteoblasts
miR-467g inhibited new bone regeneration by targeting Ihh/Runx-2 signaling.
[59]

miR-208a-3p
ACVR1
Osteoblasts
miR-208a-3p suppresses osteoblast differentiation and inhibits bone formation by targeting ACVR1.
[90]

MSCs: mesenchymal stem cells; BMSCs: bone marrow mesenchymal stem cells.

4.2. lncRNA

The lncRNA-miRNA-mRNA regulation model plays a critical role in osteogenic differentiation [66]. lncRNA can serve as a competitive endogenous RNA, competing for miRNA binding sites to reduce its direct impact on mRNA. lncRNA MALAT1 can competitively bind to miR-30, inhibiting the interaction between miR-30 and Runx2 to upregulate the transcription level of Runx2 and strengthen osteogenic differentiation of adipose mesenchymal stem cells [67]. Study revealed that lncRNA TUG1 [68], lncRNA PCAT1 [69], and lncRNA HIF1A-AS2 also indirectly regulate the activity of osteogenesis-related signaling molecules by adsorbing miRNA [70]. Downregulated lncRNA MEG3 promotes osteogenic differentiation of human dental follicle stem cells by epigenetically regulating the Wnt pathway [71]. Researchers have indicated that exosome lncRNA-MALAT secreted by bone marrow mesenchymal stem cells enhanced osteoblast activity in osteoporotic mice [72]. Exosomal lncRNA-RUNX2-AS1 secreted by multiple myeloma cells can reduce osteogenic differentiation of MSC [73].

4.3. circRNA

Increasing evidence indicates the various functions of circRNA in bone marrow mesenchymal stem cell osteogenesis, which proves to be a valuable checkpoint for the treatment of bone diseases [74]. CircRNA_436 might be a part of the critical regulators of periodontal ligament stem cell differentiation by coordinating with miR-107 and miR-335 to affect the Wnt/β-catenin pathway [75]. CircRNA_0127781 may serve as one of the essential regulators in the inhibition of osteoblast differentiation by interacting with miR-210 and miR-335 [76–78]. CircRNA_33287 would block miR-214-3p to intensify the osteogenesis process and active the construction of ectopic bone [79]. Both circRNA_19142 and circRNA_5846 target miR-7067-5p to regulate osteoblast differentiation [80]. Lipopolysaccharide (LPS) is indicated to promote bone resorption by activating TLRs [81]. GO analysis showed that circRNA_3140 is related to the TLR signaling pathway [75]. The mechanism of exosomal circRNA in the regulation of osteogenic differentiation is seldom studied. However, the circular structure of circRNA makes it more stable than linear RNA and difficult to be degraded [82]. It has great research potential in exosome engineering. Studies have shown that during the osteogenic differentiation of periodontal ligament stem cells, differentially expressed circRNAs are rich in membrane-bound vesicles [83].

5. Engineering Exosomes as Nanocarrier for Noncoding RNA Delivery

5.1. Methods of Exosome Extraction

The separation and purification of exosomes have always been a concern of researchers. It is crucial to obtain high-purity exosomes for subsequent research. Researchers currently use ultracentrifugation [91], immunomagnetic beads [92], ultrafiltration [93], size-exclusion chromatography [94], or kits to achieve exosomes (Figure 4). In Table 3, we summarize several methods for extracting exosomes and list their characteristics.

Figure 4: Methods of exosome extraction and engineering.

Table 3: Comparison of methods about exosome extraction.

Name
Method
Advantage
Disadvantage
Ref.

Ultracentrifuge
(1) Low speed to remove cells and apoptotic debris
(2) High speed to eliminate larger vesicles
(3) Higher speed to precipitate exosomes
(1) The gold standard for exosome extraction
(2) Simple operation
(1) A lot of materials need to be prepared in the early stage
(2) The exosome yield is low
(3) Repeated centrifugation causes damage to exosome vesicles and reduces its quality
(4) Soluble proteins can form clumps with exosomes and cause pollution
[78]

Concentration gradient centrifugation
In different liquid density intervals, each particle will eventually stay at the same position as its own density
Compared with ultracentrifugation, the recovery rate and purity are higher
Same as above
[78, 82]

Ultrafiltration
Using ultrafiltration membranes with different molecular weight (MWCO) for separation, small molecular substances will be filtered to the other side of the membrane
Simple and efficient without affecting the biological activity of exosomes
(1) The adhesion of the filter membrane may lose exosomes
(2) The pressure and shearing force during filtration may cause of the damage of exosomes
[80]

Immunomagnetic bead
By incubating with magnetic beads coated with antimarker antibodies, exosomes can be adsorbed and separated
(1) High specificity
(2) Easy to operate
(3) Not affecting the exosomal morphological integrity
(1) Low efficiency
(2) Biological activity is easily affected by pH and salt concentration
(3) Not conducive to downstream experiments
[79]

Size-exclusion chromatography
With columns filled with porous polymer microspheres, molecules pass through the microspheres according to their diameter. Molecules with a small radius take longer to migrate through the pores of the column
Exosomes are not affected by shear forces
(1) Time-consuming
(2) Not suitable for large sample processing
(3) Special equipment required
[81]

5.2. Merging Therapeutic RNA into Exosomes

Exosome cargo with therapeutic activity is not restricted to naturally occurring cell-derived biomolecules. Instead, exosomes can also carry exogenous therapeutic molecules. Exosomes have been engineered to bind therapeutic molecules, including protein [95], small molecule drugs [96], peptide ligands [97], and therapeutic RNA [98, 99]. Researchers have used multiple methods to engineer exosomes for cargo delivery, including incubation with saponin, electrical stimulation, sonication [100], extrusion, freeze/thaw [101], click chemistry [102, 103], and antibody binding [104] (Figure 4).

We concentrate on methods and applications of fusing exogenous RNA into exosomes. One mean is through electroporation of exosomes. Researchers electroplated siRNA into dendritic cell-derived exosomes. Up to 60% GAPDH RNA and protein were knocked down in the mouse cortex, midbrain, and striatum by intravenous injection of electroplated exosomes [98]. Electroplating of siRNA against MAPK-1 into exosomes derived from peripheral blood monocytes reduced the expression of MAPK-1 in donor lymphocytes and monocytes [105]. In a conclusion, these results indicated that electroporation is wildly used in various kinds of exosomes and recipient cells. However, this method is inappropriate for all types of RNA cargo. For instance, researchers reported that miRNA cannot be electroplated into HEK 293-derived exosomes, indicating that some dimension or structure of RNA may be less suitable for this method [106].

Another strategy for fusing RNA into exosomes is overexpressing the cargo RNA in the exosome donor cells. The specificity of this method is poor. This kind of cargo RNA includes chemically modified 3benzopyridine miRNA [99], mRNA [107, 108], and shRNA [109]. Researchers incubated exosomes carrying these RNAs with recipient cells. These mRNAs were translated into protein. Target genes were knocked down by shRNA and miRNA. This strategy is applicable to various RNA cargos and recipient cells.

5.3. Targeting Exosomes to Recipient Cells

The safe application of exosome therapy requires the assessment of potential exosomes targeting cell and subcellular regions. Although the pharmacokinetics of exosomes used by IV has not been elaborated, the application of purified exosomes in mice can cause exosomes to accumulate in the kidney, liver, and spleen [98, 106]. This distribution is similar to most nanoparticle delivery vehicles, usually through bile excretion, kidney clearance, or macrophage clearance [110, 111]. Strategies for exosomes to target specific recipient cells have been reported.

One strategy is to utilize proteins and peptides from the virus with the ability of targeted delivery. For example, researchers used EBV glycoprotein 350 modifying exosomes to target CD19+ B cells, but not other peripheral blood mononuclear cells (PBMCs) [112, 113]. Ligands of viruses can also enhance exosome-mediated transport to target cells in vivo. RVG-labeled exosomes transferred siRNA to the mouse brain, while unlabeled exosomes convey siRNA to the liver, spleen, and kidney [98]. Viral ligands can not only enhance the targeting ability of exosomes but also enhance the ability of exosomes to be integrated into the recipient cells. Exosomes labeled with VSV-G and OVA peptides are absorbed by DC at a higher rate than exosomes labeled with OVA alone [114, 115]. Although viruses can be used to improve the ability of exosome to target cells, the strategy is limited to the known interactions. The side effects of this strategy are still unknown.

6. Exosome-Based Strategies for Restoration of Bone Defect

Bone-derived exosome containing noncoding RNA is considered significant in regulating bone formation and absorption (Figure 5). Various key factors, regulating osteoclasts and osteoblasts (such as RUNX2, BMP, and sclerostin), are adjusted by specific bone-derived exosomes embodying noncoding RNA [116]. Studies showed that intravenously, exosomes tend to their original place [117].

(a)

(b)

(c)

Figure 5: Effect of exosome on the differentiation of cells in vivo and vitro. In (a), the production of proteoglycan in BMSCs was induced by chondrocyte exosomes, but such effect was counteracted by chondrocyte exosomes with miR-8485 inhibitor. As is shown in (b) and (c), the results of Van Gieson staining of undecalcified specimens showed that bone regeneration was markedly increased in the scaffold+200 μg Exos group (%), and the percentage of new bone formation area was significantly greater than that in the scaffold group (%) or the scaffold+100 μg Exos group (%) () [33, 52]. (a) Copyright 2020 BBRC published by Elsevier Ltd. (b, c) Copyright 2016 Int J Biol Sci published by Ivyspring International Publisher.

However, the same noncoding exosomes may have opposite effect on the differentiation and proliferation of osteoclasts and osteoblasts. Therefore, during the process of bone remodeling, bone-derived exosomes may not have functions completely coherent with the parental cells. The abundance of noncoding RNA in recipient cells and exosomes does not match, indicating that in addition to noncoding RNA, other components of exosomes also have regulatory effects [118]. Researchers have utilized exosomes from other tissues to enhance tissue healing efficacy [119] and to reduce joint injury and osteoarthritis by restoring matrix homeostasis and decreasing inflammation [120]. Efforts were being made to test the effects of engineered exosomes on orthodontic tooth movement models in mouse [121].

7. The Advantages of Exosomes as Noncoding RNA Delivery System

7.1. Biocompatibility

The advantage of exosomes as drug carriers over existing artificial liposomes lies in their good biocompatibility. Currently, liposomes are the main delivery for siRNA and other RNAs. Liposomes can cause accumulated toxicity and immune response in the human body, without good expected effects. Kamerkar et al. used exosomes to deliver siRNA to prevent the production of KRAS mutant proteins. Research results showed that compared with liposomes, intravenous injection of siRNA-loaded exosomes can better inhibit the expression of KRAS protein, without immunogenicity in vivo [122]. Usman et al. used electroporation technology to introduce nucleotides into red blood cell-derived exosomes. The results showed that the nucleotides loaded with exosomes have a significant inhibitory effect on breast cancer cells and there is no immunity in the body [123]. Compared with other RNA drug delivery vehicles (such as adenovirus, lentivirus, retrovirus, and liposome), exosomes are not immunogenic and cytotoxic, showing good biocompatibility.

7.2. Biological Stability

Exosomes derived from antigen-presenting cells can express membrane-bound complement regulators CD55 and CD59 to enhance the stability of circulation in the body [122]. Studies have shown that even if exposed to an inflammatory environment, exosomes still have a longer circulation time [124]. A large number of studies have proved that due to their small size (≤100 nm), nanoparticles can achieve targeted aggregation of tissues through enhanced penetration and retention effects [125]. In addition, the circulation time of polyethylene glycol-modified exosomes can reach more than 60 minutes in vivo [126]. Polyethylene glycol-modified exosomes can significantly improve the biological stability of exosomes in vivo by prolonging the clearance time, making the study of exosomes as drug carriers more promising.

7.3. Targeting

Multiple studies have confirmed the loyalty of exosomal cargo to parental cells. Exosomes derived from central nerve cells can pass through the blood-brain barrier and target specific neurons. Exosomes derived from hypoxic tumor cells tend to recruit into hypoxic tumor tissues [127, 128]. The results of biodistribution studies also showed that the accumulation of exosomes in tumor tissues depended on the type of embryonic cells. Therefore, when studying exosomes targeting specific tissues or cells, it is necessary to consider the targeting efficiency [129].

8. Conclusions and Perspective

Exosomes are increasingly being studied in the field of bone reconstruction. From the initial research on tumor diseases and related mechanisms to studies on drug delivery and engineering exosomes, there is still a lot of research space.

(1) How to select cells with strong ability to secrete exosomes? The scaffold used with MSCs provides an ideal environment for the osteogenic differentiation of MSCs [130][131]. However, there is no research on the inducing factors involved in this mechanism. Scaffold may enhance the paracrine function of MSCs, thereby secreting exosomes to induce the osteogenic differentiation. (2) The loading of exosomes on the scaffold to promote bone repair has been widely studied, but how to make the scaffold play a controlled or slow-release effect is another problem that needs to be solved. Maybe the solution is to control the pore size and degradation rate of the scaffold material. (3) Another direction is to improve their targeting ability by modifying the exosomal membrane molecules and preventing unwanted derivatives from entering the exosomes

Further research is needed to evaluate the biological efficacy of exosomes in treating bone defects in vivo and vitro. As therapeutic delivery vectors, exosomes have engineering potential, and they are easy to design and well tolerated in vivo. Overcoming each of these miseries will turn the surprise discovery of exosomes as a drug delivery system into a viable mature technology.

Conflicts of Interest

All the authors declare no conflict of interest.

Authors’ Contributions

Keda Liu and Nanjue Cao contributed equally to this work.

Acknowledgments

This study was supported by the National Natural Science Foundation of China (No. 81970980); Liaoning Province, Colleges and Universities Basic Research Project (No. LFWK201717); Liaoning Provincial Key Research Plan Guidance Project (No. 2018225078); Liaoning Provincial Natural Science Foundation Guidance Project (No. 2019-ZD-0749); Shenyang Major Scientific and Technological Innovation Research and Development Plan (No. 19-112-4-027); Shenyang Young and Middle-aged Technological Innovation Talent Plan (No. RC200060); and the Second Batch of Medical Education Scientific Research Projects of the 13th Five-Year Plan of China Medical University (No. YDJK2018017).

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