Systematic review of extracellular vesicle-derived microRNAs involved in organ fibrosis: implications for arthrofibrosis therapy.

A search was devised in October 2023 to locate articles relevant to this study according to the Preferred Reporting Items for Systematic Review and Meta-Analysis (PRISMA) guideline (Fig.  1 ) [ 33 ]. To identify records with concepts of common interest in Embase, PubMed, and Scopus databases, the “AND” function was used to combine the following subject terms: fibrosis, exosomes, and miRNAs. Two authors (V.G. and D.S.) independently performed data extraction and a third author (J.A.M.) involved discrepancy resolution. Following the removal of duplicates, all three databases together yielded 315 records. These records were further refined by applying the following exclusion criteria: (i) non-miRNA exosomal cargo; (ii) theoretical studies without experimental validation; and (iii) editorial or review articles. miRNAs with fewer than 3 publications documenting involvement in fibrotic conditions were excluded from consideration ( n  = 101) in order to limit the search to a more tractable set consisting of the most promising candidates for AF therapy. Applying these criteria resulted in fifty-six articles describing miRNAs that were further categorized as having either anti-fibrotic or pro-fibrotic characteristics. Fig. 1 Modified PRISMA flow diagram for systemic reviews [ 33 ]. miRNA: micro ribonucleic acids, IncRNA: long non-coding RNA, tRNA: transfer RNA. Special Exclusion Criteria: exosomal miRNAs with less than 3 publications supporting association with fibrotic pathologies were excluded Modified PRISMA flow diagram for systemic reviews [ 33 ]. miRNA: micro ribonucleic acids, IncRNA: long non-coding RNA, tRNA: transfer RNA. Special Exclusion Criteria: exosomal miRNAs with less than 3 publications supporting association with fibrotic pathologies were excluded

Based on our search criteria, 8 miRNAs were identified: 7 for their anti-fibrotic potential (Sect.  3.1 ; Table 1 ) and 1 for their profibrotic potential (Sect.  3.2 ; Table 2 ). miRNAs having both anti- and pro-fibrotic effects are listed in Sect.  3.3 (Table 3 ). Below we summarize each of these miRNAs in alpha-numeric order. Table 1 Candidates of anti-fibrotic Exosomal MicroRNAs (miRs) for arthrofibrosis treatment. miRNAs Target Source of exosomes Model Pro/Anti -fibrosis Outcomes Ref. In vitro In vivo [let-7 family] let-7d Lung Mouse BALF from PF TGF-β1-induced mouse lung pericytes Mouse PF model via bleomycin Anti- Suppressed Col1A and α-SMA by inhibiting TGF-βR1/FoxM1/Smad3/β-catenin [ 36 ] let-7 Lung Human MenSCs TGF-β1-induced MLE-12 cells Mouse PF model via bleomycin Anti- Reduced fibrosis score, collagen deposition, and GSH-Px by inhibiting LOX1/NLRP3/caspase 3 [ 37 ] let-7 Liver Human liver stem cells TNF-α- or TGF-β1-induced cholangiocytes (H69) MDR2 KO mouse Anti- Reduced ductular reaction and biliary fibrosis by inhibiting IL-13 and NF-κB [ 94 ] let-7c Kidney Human BMSCs transfected with let-7c TGF-β1-induced NRK-52E cells Mouse RF model via UUO Anti- Attenuated kidney injury with downregulated Col4α1, MMP9, and TGF-β1/TGF-βR1 [ 95 ] let-7i-5p Kidney BMSCs trans-fected with let-7i-5p antagomir TGF-β1-induced NRK-52E cells Mouse RF model via UUO Pro- Reduced renal fibrosis and improved kidney function by activating TSC1/mTOR [ 38 ] [miR-26 family] miR-26a-5p Lung HUMSCs transfected with miR-26a-5p TGF-β1-induced MLE-12 cells Mouse PF model via silica Anti- Suppressed EMT by inhibiting ADAM17/Notch [ 41 ] miR-26a Kidney HEK293 transfected with miR-26a mTECs Mouse RF model via alderstone Anti- Alleviated lipocalin 2, α-SMA, Col1, and fibronectin by inhibiting CTGF/Smad3 [ 42 ] miR-26a Kidney HEK293 trans-fected with miR-26a precursor MSMSCs Mouse RF model via UUO Anti- Decreased CTGF and TGF-β1 by inhibiting FoxO1 [ 45 ] miR-26a-5p Heart MSMSCs transfected with miR-26a-5p MSMSCs and H9C2 Mouse chronic kidney disease model Anti- Depressed cardiac fibrosis with low expression of FoxO1, CTGF, fibronectin, and Col1A1 [ 96 ] [miR-29 family] miR-29b-3p Lung Human BMSCs Pulmonary fibroblasts (LL29) Mouse PF model via bleomycin Anti- Suppressed fibroblast proliferation by downregulating FZD6 [ 46 ] miR-29a/b/c-3p Kidney Mouse satellite cells transduced with Ad-miR29 NA Mouce RF model via UUO Anti- Reduced TGF-β, α-SMA, fibronectin, and Col1A1 by inhibiting YY1 and TGF-β3. [ 49 ] miR-29b-3p Kidney Human liver stem cells mTECs and mkCFs Mouse AAN model via AA Anti- Prevented fibroblast activation by interfering β-catenin [ 47 ] miR-29b-3p Heart Rat BMSCs NA Rat MF model via MI Anti- Reduced MF and collagen volume fraction by inhibiting ADAMTS16 [ 48 ] miR-29b Heart HUMSCs loaded with miR-29b TGF-β1-induced cardiac fibroblasts Mouse MF model via MI Anti- Alleviated inflammation and fibrosis, and improved cardiac function [ 51 ] miR-29a Uterus Human BMSCs Mouse endometrial epithelial cells Mouse IUA model Anti- Endometrium repair with reduced α-SMA, Col1, and Smad2/3 [ 97 ] miR-29a Skin Human ADSCs transfected with miR-29a human hypertrophic scar fibroblasts Mouse thermal model Anti- Reduced excessive scar formation by inhibiting TGF-β2/Smad3 [ 50 ] [miR-146 family] miR-146a-5p Liver Human and mouse serum NA Mouse schisto-somiasis model Anti- Low level of miR-146-5p in LF mice and patients. [ 52 ] miR-146a Heart Rat ADSCs transfected with miR-146a hypoxia-induced cardiomyoblasts (H9c2) Rat MF model via MI Anti- Suppressed apoptosis, inflammatory response, and fibrosis by downregulation of EGR1 [ 53 ] miR-146a-5p Heart Human CPCs Rat neonatal cardiomyocytes Rat MF model via Dox/Trz Anti- Prevented MF, inflammatory cell infiltrate, and ventricular dysfunction [ 54 ] miR-146a Urethra HUMSCs treated with TNF-α TGF-β1-induced urethral fibroblasts Rat urethral fibrosis model via TGF-β1 Anti- Suppressed urethral fibrosis, stricture, fibroblast activation, and inflammation [ 55 ] [miR-148 / -152 family] miR-148a Liver HUMSCs BMDM and RAW264.7 Mouse LF model via CCI4 Anti- Alleviated LF with macrophages by inhibiting KLF6/STAT3 [ 59 ] miR-148a-5p Liver Mice BMSCs transfected with miR-148a-5p TGF-β1-induced HSCs Mouse LF model via thioacetamide Anti- Reduced TGF-β1, TIMP-1, Col1, and α-SMA levels by inhibiting Smad4 [ 58 ] miR‑152‑5p Heart Human venous plasma from AMI patients H9C2 transfected with miR-152-5p and its inhibitor NA Anti- Increased fibrosis, apoptotic proteins when used miR-152-5p inhibitor by activating ARHGAP6/ROCK [ 60 ] [miR-214 family] miR-214 Liver Mouse HSCs transfected with miR-21 Mouse hepatocytes, human HSCs and hepatocytes NA Anti- Loss of miR-214 expression promotes CTGF-driven fibrosis (ColA1, α-SMA) [ 62 ] miR-214 Uterus Human ESCs transfected with miR-214 TGF-β1-induced ESCs and epithelial cells Mouse endometriosis model Anti- Reduced CTGF and fibrotic proteins [ 63 , 64 ] [miR-223 family] miR-223-3p Lung HUMSCs Silica-induced RAW264.7 and NIH/3T3 cell lines Mouse PF model via silica Anti- Attenuated inflammatory and fibrosis factors by suppressing NLRP3 and circPWWP2A [ 66 ] miR-223 Liver IL-6/PA-treated myeloid cells Mouse hepatocytes (AML12) Mouse IL-6RA KO with high-fat diet Anti- Reduced expression of anti-fibrotic miR-223 in the LF model [ 68 ] miR-223 Heart HUMSCs transfected with miR-223 TGF-β1-induced human cardiac fibroblasts Mouse MF model via MI Anti- Relieved MF and inflammation via P53/S100A9 axis [ 67 ] (Abbreviation) AA: aristolochic acid, AAN: aristolochic acid nephropathy, Ad: adenovirus, ADAMTS: a disintegrin and metalloproteinase (ADAM) with thrombospondin motifs, ADSCs: adipose-derived mesenchymal stem cells, AMI: acute myocardial infarction, ARHGAP: Rho GTPase-activating protein, BALF: bronchoalveolar lavage fluid, BMDM: bone morrow-derived macrophages, BMSCs: bone marrow-derived mesenchymal stem cells, CCI4: carbon tetrachloride, circ: circular RNA, Col: collagen, CPCs: cardiac-resident mesenchymal progenitor cells, CTGF: connective tissue growth factor, Dox: doxorubicin, EGR: early growth response, EMT: epithelial to mesenchymal transition, ESCs: endometrial stromal cells, Fox: forkhead box, FZD: frizzled, GSH-Px: glutathione peroxidase, H9C2: cardiac myoblasts, HEK: human embryonic kidney, HSCs: hepatic stellate cells, HUMSCs: human umbilical cord mesenchymal stem cells, IL: interleukin, IUA: intrauterine adhesion, KLF: Krüppel-like factor, KO: knockout, Let: lethal, LF: liver fibrosis, LOX: lectin-like oxidized low-density lipoprotein receptor, MDR: multidrug resistance protein, MenSCs: menstrual blood-derived stem cells, MF: myocardial fibrosis, MI: myocardial infarction, mkCFs: mouse kidney cortical fibroblasts, MLE: murine lung epithelial, MMP: matrix metalloproteinase, MSMSCs: mouse skeletal muscle satellite cells, mTECs: mouse tubular epithelial cells, mTOR: mammalian target of rapamycin, NA: not applicable, NF: nuclear factor, NIH/3T3: murine fibroblast cell line, NLRP: nucleotide-binding and oligomerization domain-like receptor family pyrin domain-containing, Notch: neurogenic locus notch homolog protein, NRK: neighboring rat kidney tubular epithelial, PA: palmitic acid, PF: pulmonary fibrosis, RA: receptor A, RAW264.7: mouse macrophages, RF: renal fibrosis, ROCK: Rho-associated coiled-coil containing kinase, SMA: smooth muscle actin, Smad: suppressor of mothers against decapentaplegic, STAT: signal transducer and activator of transcription, TGF: transforming growth factor, TIMP: tissue inhibitor of metalloproteinase, TNF: tumor necrosis factor, Trz: trastuzumab, TSC: tuberous sclerosis complex, UUO: unilateral ureteral obstruction, YY: transcription factor Yin Yang Candidates of anti-fibrotic Exosomal MicroRNAs (miRs) for arthrofibrosis treatment. (Abbreviation) AA: aristolochic acid, AAN: aristolochic acid nephropathy, Ad: adenovirus, ADAMTS: a disintegrin and metalloproteinase (ADAM) with thrombospondin motifs, ADSCs: adipose-derived mesenchymal stem cells, AMI: acute myocardial infarction, ARHGAP: Rho GTPase-activating protein, BALF: bronchoalveolar lavage fluid, BMDM: bone morrow-derived macrophages, BMSCs: bone marrow-derived mesenchymal stem cells, CCI4: carbon tetrachloride, circ: circular RNA, Col: collagen, CPCs: cardiac-resident mesenchymal progenitor cells, CTGF: connective tissue growth factor, Dox: doxorubicin, EGR: early growth response, EMT: epithelial to mesenchymal transition, ESCs: endometrial stromal cells, Fox: forkhead box, FZD: frizzled, GSH-Px: glutathione peroxidase, H9C2: cardiac myoblasts, HEK: human embryonic kidney, HSCs: hepatic stellate cells, HUMSCs: human umbilical cord mesenchymal stem cells, IL: interleukin, IUA: intrauterine adhesion, KLF: Krüppel-like factor, KO: knockout, Let: lethal, LF: liver fibrosis, LOX: lectin-like oxidized low-density lipoprotein receptor, MDR: multidrug resistance protein, MenSCs: menstrual blood-derived stem cells, MF: myocardial fibrosis, MI: myocardial infarction, mkCFs: mouse kidney cortical fibroblasts, MLE: murine lung epithelial, MMP: matrix metalloproteinase, MSMSCs: mouse skeletal muscle satellite cells, mTECs: mouse tubular epithelial cells, mTOR: mammalian target of rapamycin, NA: not applicable, NF: nuclear factor, NIH/3T3: murine fibroblast cell line, NLRP: nucleotide-binding and oligomerization domain-like receptor family pyrin domain-containing, Notch: neurogenic locus notch homolog protein, NRK: neighboring rat kidney tubular epithelial, PA: palmitic acid, PF: pulmonary fibrosis, RA: receptor A, RAW264.7: mouse macrophages, RF: renal fibrosis, ROCK: Rho-associated coiled-coil containing kinase, SMA: smooth muscle actin, Smad: suppressor of mothers against decapentaplegic, STAT: signal transducer and activator of transcription, TGF: transforming growth factor, TIMP: tissue inhibitor of metalloproteinase, TNF: tumor necrosis factor, Trz: trastuzumab, TSC: tuberous sclerosis complex, UUO: unilateral ureteral obstruction, YY: transcription factor Yin Yang Table 2 A candidate of pro-fibrotic Exosomal microRNA-21 (miR-21) for arthrofibrosis treatment. miRNAs Target Source of exosomes Model Pro/Anti -fibrosis Outcomes Ref. In vitro In vivo [miR-21 family] miR-21 Lung Arsenic-treated pulmonary epithelial cells MRC-5 cells miR-21 KO mouse model Pro- Blocked PF by inhibiting Akt activation and glycolysis in miR-21 KO mouse [ 70 ] miR-21 Lung HBE HBE and MRC-5 cells Mouse PF model via CS Pro- Up-regulated miR-21 by activating TGF-β1/Smad3 in CS-induced PF [ 71 ] miR-21-5p Lung Fibrocytes from fibrotic lungs MRC-5 cells and fibrocytes Rat RF model via TGF-β1 Pro- Increased miR-21-5p expression in fibrotic cells and patients [ 73 ] miR-21-5p Lung Human and mouse serum NA Mouse PF model via bleomycin Pro- Elevated miR-21 expression in serum exosomes [ 72 ] miR-21-5p Lung Rat BALF 293 T cell line Rat RF model via coal dust particles Pro- Over-expressed miR-21-5p in rat BALF by suppressing Smad7 [ 98 ] miR-21 Kidney HPTCs HPTCs Rat calorific restriction model Pro- Prevented the occurrence of EMT by inhibiting miR-21 [ 74 ] miR-21 Kidney TGF-β1-stimulated rat NRK-52E cells Rat NRK-49 F cells Mouse RF model via UUO and Rab27a KO model Pro- Activated fibroblasts and aggravated RF by PTEN/Akt in RF model and vice versa in KO model [ 75 ] miR-21a-5p Kidney Mouse BMSCs Tubular epithelial cells Mouse RF model via UUO Anti- Alleviated RF by attenuating glycolysis and PFKM [ 76 ] miR-21 Heart HPB transfected with miR-21 hypoxia-induced cardiac muscle cells Mouse MF model via MI Pro- Enhanced MF when treated with miR-21 mimic-loaded exosomes [ 99 ] miR-21-3p Heart Rat cardiac fibroblasts Rat cardiomyocytes Mouse CH model via Angiotensin II Pro- Attenuated CH by miR-21 inhibitor [ 100 ] miR-21 Pancreas Mouse PSCs transfected with miR-21 Rat SAM-K PSCs Mouse chronic pancreatitis model via cerulein Pro- Enhanced expression of miR-21 and Col1A [ 101 ] miR-21a-3p Tendon HUMSCs TGF-β1-induced rat fibroblast cells Rat Achilles tendon injury model Pro- Inhibited tendon adhesion when treated with low miR-21-3p-expressed HUMSC exosomes [ 78 ] (Abbreviation) Akt: protein kinase B, BALF bronchoalveolar lavage fluid, BMSCs: bone marrow-derived mesenchymal stem cells, CH: cardiac hypertrophy, Col: collagen, CS: cigarette smoke, EMT: epithelial to mesenchymal transition, HBE: human bronchial epithelial, HPB: human peripheral blood, HPTCs: human proximal tubular cells, HUMSCs: human umbilical cord mesenchymal stem cells, KO: knockout, MF: myocardial fibrosis, MI: myocardial infarction, MRC-5: human embryonic lung fibroblast, NA: not applicable, NRK: neighboring rat kidney tubular epithelial, PF: pulmonary fibrosis, PFKM: phosphofructokinase muscle isoform, PSCs: pancreatic stellate cells, PTEN: phosphatase and tensin homolog, RF: renal fibrosis, Smad: suppressor of mothers against decapentaplegic, TGF: transforming growth factor, UUO: unilateral ureteral obstruction A candidate of pro-fibrotic Exosomal microRNA-21 (miR-21) for arthrofibrosis treatment. Mouse RF model via UUO and Rab27a KO model (Abbreviation) Akt: protein kinase B, BALF bronchoalveolar lavage fluid, BMSCs: bone marrow-derived mesenchymal stem cells, CH: cardiac hypertrophy, Col: collagen, CS: cigarette smoke, EMT: epithelial to mesenchymal transition, HBE: human bronchial epithelial, HPB: human peripheral blood, HPTCs: human proximal tubular cells, HUMSCs: human umbilical cord mesenchymal stem cells, KO: knockout, MF: myocardial fibrosis, MI: myocardial infarction, MRC-5: human embryonic lung fibroblast, NA: not applicable, NRK: neighboring rat kidney tubular epithelial, PF: pulmonary fibrosis, PFKM: phosphofructokinase muscle isoform, PSCs: pancreatic stellate cells, PTEN: phosphatase and tensin homolog, RF: renal fibrosis, Smad: suppressor of mothers against decapentaplegic, TGF: transforming growth factor, UUO: unilateral ureteral obstruction Table 3 Other candidates of Exosomal MicroRNAs (miRs) for arthrofibrosis treatment. miRNAs Target Source of exosomes Model Pro/Anti -fibrosis Outcomes Ref. In vitro In vivo [miR-122 family] miR-122 Liver ADSCs transfected with miR-122 Human HSCs Mouse LF model via CCI4 Anti- Suppressed the activation of HSCs and collagen deposition [ 82 ] miR-122 Liver Rat hepatic and plasma from LF NA Rat LF model via CCI4 Anti- Suppressed pro-fibrotic miRNAs by PDE5 inhibitor [ 102 ] miR-122 Liver PA-treated Huh-7 and serum from NAFLD patients Human HSCs NA Pro- Enhanced expression of fibrosis markers including Col1A1, α-SMA, and TGF-β1 [ 88 ] miR-122a Kidney Human BMSCs transfected with miR-122a TGF-β1-induced HK-2 Rat RF model via UUO Anti- Reduced expansion of renal tubule and interstitial expansion by inhibiting mTOR and autophagy [ 83 ] [miR-192 family] miR-192 Liver JFH-1 and Huh-7 cells transfected with miR-192 Human HSCs NA Pro- Upregulated fibrogenic markers and activated trans-differentiation of HSCs [ 103 ] miR-192 Liver PA-treated Huh-7 and serum from NAFLD patients Human HSCs NA Pro- Enhanced expression of fibrosis markers including Col1A1, α-SMA, and TGF-β1 [ 88 ] miR-192-5p Liver Human ADSCs Human hypertrophic scar fibroblasts Mouse full-thickness skin defect model Anti- Attenuated hypertrophic scar formation and trans-differentiation by inhibiting IL-17RA/Smad [ 79 ] miR-192-5p Liver Human and rat serum from NAFLD NA Rat with high-fat/-cholesterol diet Pro- Activated proinflammatory macrophages and NAFLD by inhibiting Rictor/Akt/FoxO1 axis [ 89 ] [miR-150 family] miR-150-5p Liver Mouse ADSCs TGF-β1-induced mouse HSCs Mouse LF model via CCI4 Anti- Attenuated hepatic fibrosis by inhibiting CXCL1 [ 80 ] miR-150 Kidney Hypoxia mTECs (NRK-52E) Mouse NRK-49 F cells Mouse RF model via IR Pro- Developed more pro-fibrotic manifestations by inhibiting SOCS1 [ 86 , 87 ] [miR-182 family] miR-181-5p Liver Mouse ADSCs transfected with miR-181-5p TGF-β1-induced HST-T6 cells Mouse LF model via CCI4 Anti- Reduced Col1, vimentin, α-SMA, and fibronectin by inhibiting STAT3 and Bcl-2 [ 84 ] miR-181a-2-3p Liver L-02 cells treated with citreoviridin HSCs ICR mouse model Pro- Reduced mitochondrial calcium accumulation and HSC activation by miR-181a-2-3p-antagomir [ 90 ] miR-181d Kidney Human BMSCs TGF-β1-induced HK-2 cells Rat RF model via UUO Anti- Restricted RF, Col4 α1/1, α-SMA, and TGF-βR1 by inhibiting KLF6 and NF-κB [ 81 ] miR-181a Heart Hypoxia-induced iCMs Hypoxia-induced iCMs Rat MF model via MI Pro- Attenuated MF and hypertrophy by miR-181a antagomir [ 85 ] (Abbreviation) ADSCs: adipose-derived mesenchymal stem cells, Akt: protein kinase B, Bcl-2: B-cell lymphoma 2, BMSCs: bone marrow-derived mesenchymal stem cells, CCI4: carbon tetrachloride, Col: collagen, CXCL: C-X-C motif chemokine ligand, Fox: forkhead box, HK: human renal tubular epithelial cells, HSCs: hepatic stellate cells, HST-T6: hepatic stellate, Huh-7: human hepatoma cells, iCMs: human induced pluripotent stem cell-derived cardiomyocytes, ICR: Institute of Cancer Research, IL: interleukin, IR: ischemia-reperfusion, JFH-1: hepatitis C virus genotype 2a, KLF: Krüppel-like factor, L-02: normal hepatocytes, LF: liver fibrosis, MF: myocardial fibrosis, MI: myocardial infarction, mTOR: mammalian target of rapamycin, NA: not applicable, NAFLD: non-alcoholic fatty liver disease, NF: nuclear factor, NRK: neighboring rat kidney tubular epithelial, PA: palmitic acid, PDE: phosphodiesterase, RA: receptor A, RF: renal fibrosis, Rictor: rapamycin-insensitive companion of mammalian target of rapamycin, SMA: smooth muscle actin, Smad: suppressor of mothers against decapentaplegic, SOCS: suppressor of cytokine signaling, STAT: signal transducer and activator of transcription, TGF: transforming growth factor, UUO: unilateral ureteral obstruction Other candidates of Exosomal MicroRNAs (miRs) for arthrofibrosis treatment. (Abbreviation) ADSCs: adipose-derived mesenchymal stem cells, Akt: protein kinase B, Bcl-2: B-cell lymphoma 2, BMSCs: bone marrow-derived mesenchymal stem cells, CCI4: carbon tetrachloride, Col: collagen, CXCL: C-X-C motif chemokine ligand, Fox: forkhead box, HK: human renal tubular epithelial cells, HSCs: hepatic stellate cells, HST-T6: hepatic stellate, Huh-7: human hepatoma cells, iCMs: human induced pluripotent stem cell-derived cardiomyocytes, ICR: Institute of Cancer Research, IL: interleukin, IR: ischemia-reperfusion, JFH-1: hepatitis C virus genotype 2a, KLF: Krüppel-like factor, L-02: normal hepatocytes, LF: liver fibrosis, MF: myocardial fibrosis, MI: myocardial infarction, mTOR: mammalian target of rapamycin, NA: not applicable, NAFLD: non-alcoholic fatty liver disease, NF: nuclear factor, NRK: neighboring rat kidney tubular epithelial, PA: palmitic acid, PDE: phosphodiesterase, RA: receptor A, RF: renal fibrosis, Rictor: rapamycin-insensitive companion of mammalian target of rapamycin, SMA: smooth muscle actin, Smad: suppressor of mothers against decapentaplegic, SOCS: suppressor of cytokine signaling, STAT: signal transducer and activator of transcription, TGF: transforming growth factor, UUO: unilateral ureteral obstruction Let-7 is one of the largest miRNA families with anti-fibrotic potential, as all but one of its five members suppress TGF-β1-driven induction of myofibroblast differentiation [ 34 , 35 ]. For example, it was found that exosomes from bronchoalveolar lavage fluid (BALF) with low let-7d enhanced lung pericyte differentiation into myofibroblasts, as evidenced by increases in collagen type 1 (COL1) and alpha smooth muscle actin (α-SMA) expression via a TGF-β receptor 1 (TGF-βR1)-dependent pathway [ 36 ]. Exosomal let-7 from menstrual blood-derived stem cells alleviated lung fibrosis in mice treated with bleomycin by suppressing reactive oxygen species (ROS) production and mitochondrial deoxyribonucleic acid (DNA) damage [ 37 ]. In multidrug resistance 2 gene knockout (MDR2 KO) mice, liver stem cell exosomes carrying let-7a/c improved liver scarring by decreasing Lin28a/b, nuclear factor kappa B (NF-κB), and nuclear receptor subfamily 1 group H member 4 (NR1H4) activation in cholangiocytes. In a unilateral ureteral obstruction (UUO) model, an antagomir targeting let-7i-5p reduced renal fibrosis by limiting collagen and fibronectin secretion of cells undergoing epithelial-mesenchymal transition (EMT) via inhibition of tuberous sclerosis complex 1 (TSC1) and mammalian target of rapamycin (mTOR) pathways [ 38 ]. This finding suggests that unlike other members of the let-7 family, let-7i-5p is pro-fibrotic. miR-26 is another family of anti-fibrotic miRNAs that appear to oppose myofibroblast differentiation and inflammatory responses that precede or accompany scar establishment [ 39 , 40 ]. In a silica/TGF-induced lung fibrosis model, exosomal miR-26a-5p from human umbilical cord mesenchymal stem cells (HUMSCs) disrupted a disintegrin and metalloproteinase domain-containing protein 17 (ADAM17)/neurogenic locus notch homolog protein (Notch) axis of EMT to lessen scarring in vivo (Fig.  2 A-D) [ 41 ]. miR-26a also inhibited suppressor of mothers against decapentaplegic homolog 3 (Smad3) activation and reduced EMT in aldosterone-induced tubulointerstitial fibrosis [ 42 ]. Here, connective tissue growth factor (CTGF) expression, a prominent downstream effector of TGF-β1 signaling known to promote myofibroblast survival [ 43 , 44 ], was suppressed by exosomal miR-26a to limit muscle atrophy via forkhead box protein O1 (FoxO1)/glycogen synthase kinase 3β (GSK-3β) in an obstructive kidney disease model [ 45 ]. Clearly, the miRNA-26 family has potential as an anti-fibrotic treatment. Fig. 2 Anti-fibrotic effects of exosomal micro ribonucleic acids (miRs/miRNAs) in fibrous tissues. ( A-D ) EVs-miR-26a-5p(+) improved the respiratory function damaged by silica in mice. ( A ) Fluorescent signals of DiR-labeled EVs-miR-26a-5p(+). ( B ) A schematic diagram for experimental design. ( C ) Immunofluorescence staining in lung tissue. ( D ) Western blotting analysis of a disintegrin and metalloproteinase domain-containing protein 17 (Adam17), neurogenic locus notch homolog protein 1 (Notch1), hairy and enhancer of split 1 (Hes1), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) [ 41 ]. Copyright 2023, Elsevier Inc. ( E and F ) Bone marrow-derived mesenchymal stem cells-secreted extracellular vesicles (BMSC-EVs) containing miR-29b-3p inhibit the expression of Frizzled 6 (FZD6) and improve pulmonary fibrosis (PF). ( E ) Sirius Red staining of mouse pulmonary tissues. ( F ) Summary of interactions between BMSC-EVs containing miR-29b-3p and PF. PBS: phosphate-buffered saline, NC: negative control [ 46 ]. Copyright 2020, Wiley Periodicals LLC Anti-fibrotic effects of exosomal micro ribonucleic acids (miRs/miRNAs) in fibrous tissues. ( A-D ) EVs-miR-26a-5p(+) improved the respiratory function damaged by silica in mice. ( A ) Fluorescent signals of DiR-labeled EVs-miR-26a-5p(+). ( B ) A schematic diagram for experimental design. ( C ) Immunofluorescence staining in lung tissue. ( D ) Western blotting analysis of a disintegrin and metalloproteinase domain-containing protein 17 (Adam17), neurogenic locus notch homolog protein 1 (Notch1), hairy and enhancer of split 1 (Hes1), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) [ 41 ]. Copyright 2023, Elsevier Inc. ( E and F ) Bone marrow-derived mesenchymal stem cells-secreted extracellular vesicles (BMSC-EVs) containing miR-29b-3p inhibit the expression of Frizzled 6 (FZD6) and improve pulmonary fibrosis (PF). ( E ) Sirius Red staining of mouse pulmonary tissues. ( F ) Summary of interactions between BMSC-EVs containing miR-29b-3p and PF. PBS: phosphate-buffered saline, NC: negative control [ 46 ]. Copyright 2020, Wiley Periodicals LLC In organs like the heart, lung, kidney, uterus, and skin, miR-29 members are vital in modulating fibrotic gene expressions from pathologic TGF-β signaling. Bone marrow-derived mesenchymal stem cell (BMSC) exosomes loaded with miR-29b-3p inhibited fibroblast activation by targeting frizzled-6 (FDZ6), a receptor of the wingless-type mouse mammary tumor virus integration site family member 4 (Wnt4) ligand critical for cell differentiation in the mouse idiopathic pulmonary fibrosis (IPF) model (Fig.  2 E and F) [ 46 ]. Liver stem cell-derived exosomes with miR-29a/b/c interfered with Wnt signaling and α-SMA expression in TGF-β1-treated fibroblasts [ 47 ]. These anti-fibrotic properties also were observed in a rat myocardial infarction model where miR-29b-3 targeted ADAM with thrombospondin motifs 16 (ADAMTS16), a promoter of cardiac myofibroblasts that regulates angiogenic ventricular remodeling [ 48 ]. Bioengineering exosomes permits one to not only specify the cargo but also to modify the surface recognition site for more efficient delivery to target cells. In renal fibrosis, surface decoration with a rabies viral glycoprotein peptide was able to selectively deliver exosomal miR-29 to prevent muscle wastage by inhibiting the transcription factor Yin Yang 1 (YY1)/TGF-β3 axis [ 49 ]. Alternatively, human adipose-derived mesenchymal stem cells (ADSCs) overexpressing miR-29a inhibited TGF-β2/Smad3 signaling and reduced excessive scarring in mice after thermal injury to the skin [ 50 ]. HUMSCs-derived exosomes loaded with a miR-29b mimic suppressed expression of interleukin 1β (IL-1β)/-6, tumor necrosis factor alpha (TNF-α), and inducible nitric oxide synthase (iNOS) and reduced myocardial fibrosis when delivered through a microneedle patch to a myocardial infarction [ 51 ]. Taken together, these findings strongly suggest that members of the miR-29 family merit further consideration for therapeutic development in AF. Another candidate is miR-146 due to its role as a regulator of fibrotic TGF signals in disease progression. A study on Schistosoma japonicum infection-induced fibrosis concluded that the expression of exosomal miR-146a-5p in mouse and human serum negatively correlated with the severity of liver fibrosis [ 52 ]. Post-myocardial infarction, exosomal miR-146a from transfected ADSCs directly suppressed early growth response factor 1 (EGR1) activation and IL-1β/-6 and TNF-α expression due to inhibition of the toll-like receptor 4 (TLR4)/NF-κB pathway [ 53 ]. Similarly, exosomes secreted by cardiac-resident mesenchymal progenitor cells (CPCs) with miR-146a-5p prevented drug-induced cardiotoxicity, left ventricular scarring, and cardiac dysfunction [ 54 ]. In a rat urethral fibrosis model induced by TGF-β1, miR-146a-enriched exosomes derived from TNF-α-treated HUMSC suppressed urethral stricture by reduction of IL-1β/-6, IL-1 receptor-associated kinase 1 (IRAK1), TNF receptor-associated factor 6 (TRAF6), and NF-κB levels in target cells [ 55 ]. Thus, the broad anti-fibrotic effects of miR-146a in liver, heart, and urethral fibrosis, make it a promising candidate for AF therapy. The members of the miR-148/miR-152 family share the same stem-loop structure prior to scission and target pathways responsible for cell inflammation, proliferation, differentiation, and survival [ 56 , 57 ]. Exosomes from BMSCs with miR-148a-5p mitigated liver fibrosis in a mouse model by downregulating Smad4 in stellate cells, as confirmed by reduced TGF-β1 and tissue inhibitor of metalloproteinase 1 (TIMP-1) [ 58 ]. BMSCs-derived exosomes carrying miR-148a reduced liver scarring by affecting Kruppel-like factor 6 (KLF6) to suppress pro-inflammatory macrophages and promote anti-inflammatory macrophages via inhibition of the signal transducer and activator of transcription 3 (STAT3) axis [ 59 ]. In patients with acute myocardial infarction, the downregulated exosomal miR-152-5p from serum correlates strongly as a biomarker for cardiac fibrosis [ 60 ]. Inhibiting inflammatory pathways using miR-148/152 offers another novel alternative to attenuate the overproduction of ECM components during scar development. Another critical regulator of fibrosis is miR-214. It is a recognized modulator of inflammation-mediated excessive accumulation of ECM proteins due to overexpression of CTGF [ 61 ]. CTGF expression in activated hepatocytes from a liver fibrosis model was inhibited by hepatic stellate cell (HSC)-derived exosomes with enhanced levels of miR-214 [ 62 ]. Endometrial lesions were cured by transfected ectopic endometrial stromal cells (ESCs) to enrich miR-214 levels, resulting in decreased expressions of fibrosis-associated proteins including collagen and CTGF [ 63 ]. Notably, another study recorded lower circulating levels of miR-214 serving as a pathologic marker in endometriosis [ 64 ]. These findings indicate that the anti-fibrotic effects of miR-214 are mediated by the down-regulation of CTGF-activated pathways that play a fibrogenic role in multiple organ systems. miR-223 exerts anti-inflammatory and anti-fibrotic effects on ECM-producing genes as a part of resident cell activation during failed tissue repair [ 65 ]. HUMSCs-derived exosomes alleviated silica-induced lung fibrosis via local enhancement of miR-223-3p to repress macrophage-directed nucleotide-binding domain, leucine-rich repeat, and pyrin domain-containing protein 3 (NLRP3)-related inflammation [ 66 ]. Exosomes from miR-223 mimic transfected HUMSCs protected against cardiomyocyte inflammation and apoptosis via the TGF/P53/S100A9 axis to alleviate scarring post-infarction [ 67 ]. The correlation with inflammation was confirmed by the transfer of biochemically primed macrophage exosomes to hepatocytes to inhibit miR-223 resulting in activation of transcriptional activator with PDZ-binding motif (TAZ), NLRP3, and C-X-C motif chemokine ligand 10 (CXCL10) [ 68 ]. miR-21 plays an important role in fibrosis by modulating multiple fibrotic genes involved in activating resident cell and immunomodulated ECM secretion [ 69 ]. Arsenic exposure in human bronchial epithelial (HBE) cells caused secretion of miR-21-rich exosomes that in fibroblasts activated the phosphatase and tensin homolog (PTEN)/protein kinase B (Akt) signal to promote glycolytic-myofibroblast differentiation [ 70 ]. Pulmonary fibrosis (PF) from cigarette smoke is partly due to subsequent enrichment of HBE exosomes with miR-21, which, when taken up by fibroblasts, modulates the TGF-β1/Smad3 pathway to increase collagen and α-SMA expression (Fig.  3 A) [ 71 ]. Further, serum exosomal miR-21-5p was reported as a strong indicator of IPF [ 72 ]. The fibrotic character of miR-21-5p was recorded as both a biomarker in pneumonia patients’ BALF fibrocytes and a therapeutic agent in controlling TGF-β1 downstream genes [ 73 ]. Fig. 3 Pro-fibrotic effects of exosomal micro ribonucleic acid 21 (miR-21) in fibrous tissues. ( A ) A schematic diagram showing that circRNA-0026344 induces aberrant epithelium-fibroblast cross-talk in airway epithelial cells exposed to cigarette smoke (CS) by regulating miR-21 and causing pulmonary fibrosis. circRNA: circular ribonucleic acid, Smad: suppressor of mothers against decapentaplegic homolog, TGF-β1: transforming growth factor beta 1, ECM: extracellular matrix [ 71 ]. Copyright 2021, Elsevier B.V. ( B ) Inhibition of tendon adhesion by delivering low-abundance miR-21a-3p via human umbilical cord mesenchymal stem cell-derived exosomes (HUMSC-Exos). PBS: phosphate buffered saline, Col3A1: collagen type 3A1, α-SMA: alpha smooth muscle actin, COX2: cyclooxygenase-2 [ 78 ]. Copyright 2020, Yao et al Pro-fibrotic effects of exosomal micro ribonucleic acid 21 (miR-21) in fibrous tissues. ( A ) A schematic diagram showing that circRNA-0026344 induces aberrant epithelium-fibroblast cross-talk in airway epithelial cells exposed to cigarette smoke (CS) by regulating miR-21 and causing pulmonary fibrosis. circRNA: circular ribonucleic acid, Smad: suppressor of mothers against decapentaplegic homolog, TGF-β1: transforming growth factor beta 1, ECM: extracellular matrix [ 71 ]. Copyright 2021, Elsevier B.V. ( B ) Inhibition of tendon adhesion by delivering low-abundance miR-21a-3p via human umbilical cord mesenchymal stem cell-derived exosomes (HUMSC-Exos). PBS: phosphate buffered saline, Col3A1: collagen type 3A1, α-SMA: alpha smooth muscle actin, COX2: cyclooxygenase-2 [ 78 ]. Copyright 2020, Yao et al Senescence from a high-glucose diet maintained higher levels of miR-21 in tubular cells-derived exosomes that facilitated EMT and the peroxisome proliferator-activated receptor alpha (PPARα)/hypoxia-inducible factor 1 alpha (HIF-1α) pathway in kidney fibrosis [ 74 ]. In UUO-induced renal fibrosis, exosomes from TGF-β1-treated neighboring rat kidney tubular epithelial (NRK)-52E cells stimulated miR-21/PTEN/Akt in fibroblasts, leading to scar formation [ 75 ]. Notably, abolishing miR-21-5p packaging in mesenchymal stem cells (MSCs)-derived exosomes attenuated its metabolic reprogramming toward aerobic glycolysis via the inhibition of phosphofructokinase muscle isoform (PFKM), a rate-limiting enzyme of glycolysis in tubular epithelial cells (TECs) [ 76 ]. Besides organ fibrosis, the pro-fibrotic effects of miR-21 could be observed in tendon and epidural fibrosis [ 77 ]. For example, exosomes secreted by HUMSCs with low miR-21a-3p avoided p65 activation and subsequent myofibroblast differentiation causing tendon adhesion (Fig.  3 B) [ 78 ]. Thus, miR-21 plays an essential regulator of fibrosis across various tissues, and inhibiting its activity has the potential to lessen the severity of fibrotic conditions. Table 3 is an additional list of exosomal miRNAs demonstrating both anti- and pro-fibrotic effects. It is increasingly apparent that exosomal miRNAs secreted by MSCs either naturally or under metabolic priming, pack pharmaceutically beneficial miRNAs. For example, miR-192-5p [ 79 ], miR-150-5p [ 80 ], and miR-181d [ 81 ] were highly expressed in therapeutically relevant stem cells-derived exosomes. Injection of these exosomes reduced fibrotic scars in vital organs, presumably by dampening inflammatory signals on resident cells. It should be noted that the desired anti-fibrotic effects were also achieved by bioengineering exosomal miR-122 [ 82 , 83 ] and miR-181-5p [ 84 ]. Qu and colleagues claim that approximately 3.5 times more engineered exosomal miR-181 was transported into recipient HSCs than with naturally occurring exosomes. These exosomes prevented myofibroblast activity via STAT3/B-cell lymphoma 2 (Bcl-2) [ 84 ]. Interestingly, hypoxic cues mimicking the molecular dysfunction observed during infarction delivered to induced pluripotent cardiomyocytes increased the levels of exosomal miR-181a, which suggests that a therapy involving antagomirs to miR-181a might alleviate cardiac hypertrophy [ 85 ]. Similarly, hypoxic stress in renal fibrosis triggered epithelial cells to pack more miR-150 in exosomes that eventually were taken up by mouse fibroblasts to sustain soft tissue scarring [ 86 , 87 ]. Exosomal miR-122 and miR-192 from non-alcoholic fatty liver disease (NAFLD) patient blood mimicked the palmitic acid treatment response on the Huh-7 cell exosomes, reportedly increasing COL1A1, α-SMA, and TGF-β1 levels in recipient fibroblasts [ 88 , 89 ]. Citreoviridin, a mycotoxin that triggers liver damage, influences the exposed hepatocytes to encapsulate extra miR-181a-2-3p to activate HSCs by mitochondrial calcium overloading [ 90 ]. Unlike the candidate miRNAs in Tables  1 and 2 , these miRNAs do not produce outcomes that can be categorized as either pro-fibrosis or anti-fibrosis. Therefore, these are not viewed as priority candidates for anti-AF therapy. The limited variety in primary miRNA literature and variation in parent and recipient cells studied mandate additional research prior to any inference on its global role in soft tissue fibrosis. The risk of bias for animal intervention studies was assessed using a Systemic Review Centre for Laboratory Animal Experimentation (SYRCLE’s) tool comprising 10 checklist items: (1) selection bias/sequence generation, (2) selection bias/baseline characteristics, (3) selection bias/allocation concealment, (4) performance bias/random housing, (5) performance bias/blinding, (6) detection bias/random outcome assessment, (7) detection bias/blinding, (8) attrition bias/incomplete outcome data, (9) reporting bias/selective outcome reporting, and (10) other sources of bias [ 91 ]. A total of 51 references were processed and summarized in Fig.  4 . Overall, most of the references were verified by low risk of bias in baseline characteristics (96.1%), allocation concealment (49.0%), and selective outcome reporting (100%) or unclear risk. Fig. 4 Systemic Review Centre for Laboratory Animal Experimentation (SYRCLE’s) risk of bias tool for animal studies: (1) selection bias/sequence generation, (2) selection bias/baseline characteristics, (3) selection bias/allocation concealment, (4) performance bias/random housing, (5) performance bias/blinding, (6) detection bias/random outcome assessment, (7) detection bias/blinding, (8) attrition bias/incomplete outcome data, (9) reporting bias/selective outcome reporting, and (10) other sources of bias Systemic Review Centre for Laboratory Animal Experimentation (SYRCLE’s) risk of bias tool for animal studies: (1) selection bias/sequence generation, (2) selection bias/baseline characteristics, (3) selection bias/allocation concealment, (4) performance bias/random housing, (5) performance bias/blinding, (6) detection bias/random outcome assessment, (7) detection bias/blinding, (8) attrition bias/incomplete outcome data, (9) reporting bias/selective outcome reporting, and (10) other sources of bias

In this comprehensive review, we summarized the literature on the roles of exosomal miRNAs in fibrosis to identify candidates for intraarticular AF therapy (Fig.  5 ). The common mechanisms such as excessive ECM deposition, myofibroblast activation, exaggerated inflammatory response, traumatic tissue injury, and tissue stiffness underlying fibrogenesis in multiple organ systems and articular joints can allow for predicting the translatability of established antifibrotic miRNAs in AF. From this survey, we can conclude that the most eligible candidates include members of the let-7, miR-26, miR-29, miR-146, miR-148/-152, miR-214, and miR-223 families that oppose fibrogenic pathways driven by dysregulated TGF/CTGF expression. These sequences are generally enriched in exosomes derived from pluripotent cells (e.g., BMSCs), which could serve as a source of exosomes for AF therapy. Another option is to introduce single or multiple synthetic miRNAs into joint cells using manufactured exosomes (e.g., lipid nanoparticles) as carriers [ 92 ]. Delivery can be targeted to myofibroblasts or immune cells by incorporating ligands or antibodies to cell surface receptors into lipid membranes [ 93 ]. A similar approach could be used to deliver synthetic antagomirs to pro-fibrotic miRs (e.g., miR-21). Fig. 5 Summarized candidate micro ribonucleic acids (miRs/miRNAs) for arthrofibrosis treatment. PRO: pro-fibrosis, ANTI: anti-fibrosis, ECM: extracellular matrix Summarized candidate micro ribonucleic acids (miRs/miRNAs) for arthrofibrosis treatment. PRO: pro-fibrosis, ANTI: anti-fibrosis, ECM: extracellular matrix Thus, there appear to be a number of miR-based strategies that can be pursued to improve AF outcomes. In vitro and in vivo studies are needed to sort out which of these options to pursue for advanced development and clinical testing. These should include evaluation of excipients (e.g., hydrogels) to facilitate intraarticular retention of exosomes, thereby minimizing the need for multiple joint injections that tend to promote synovial inflammation and increase risks for infection.

Arthrofibrosis (AF) is the excessive deposition of fibrous tissue by intra- and peri-articular tissue cells, leading to joint pain and restricted range of motion (ROM). Annually, an alarming average of three million patients in the United States report abnormal stiffness after a joint insult from traumatic injury, surgery, or prolonged immobilization [ 1 ]. Conservative management practices like guided physiotherapy, corticosteroids, and splinting/bracing are often prescribed to limit relatively mild AF symptoms [ 2 , 3 ]. Surgery to remove adhesions or to release capsular contractures is employed in more severe cases, but these interventions often do not fully restore normal ROM, and fibrosis tends to redevelop in the months following surgery [ 4 , 5 ]. Consequently, there is an urgent need to develop pharmaceutical approaches to augment surgical and managerial approaches that improve ROM gains and overall patient quality of life. The characteristic thickening of the joint capsule associated with AF is thought to occur as part of the same aberrant wound-healing response that underlies the basic mechanisms of soft tissue fibrosis. This idea is further motivated by reports on the abundance of myofibroblasts in the contracted tissue. The characteristic of collagen-rich extracellular matrix (ECM) under high mechanical tension common in fibrotic soft tissues is often attributed in part to the presence of myofibroblast-like cell phenotypes that exhibit high levels of contractile activity. Myofibroblasts derive from multiple precursors, including but not limited to fibroblasts, pericytes, fibrocytes, epithelial, and endothelial cells [ 6 ]. While they are often transiently involved in normal wound healing, fibrosis is characterized by their abnormal persistence post-injury. As a result, many promising strategies to restore normal ECM architecture often target the differentiation and survival of these cells [ 7 , 8 ]. Myofibroblast persistence in fibrotic tissues is driven in part by chronic inflammation, which promotes and maintains the myofibroblast phenotype [ 9 , 10 ]. This explains why corticosteroids and nonsteroidal anti-inflammatory drugs (NSAIDs) are often prescribed as anti-fibrotic drugs. Likewise, drugs that directly target myofibroblast contractility by relaxing cytoskeletal tension, and drugs that minimize myofibroblast differentiation and survival show great promise as anti-fibrotic therapies [ 11 – 13 ]. Exosomes are nanosized lipid membrane-bound extracellular vesicles (30–150 nm) secreted by all living cells. They are now recognized to play a central role in intercellular communication [ 14 ]. Exosomes carry complex cargoes of bioactive molecules such as proteins, lipids, and nucleic acids including miRNAs, a class of short (~ 22 nucleotides) non-coding regulatory RNAs that modulate over 60% of protein-encoding gene expression when internalized by recipient cells [ 15 , 16 ]. These miRNAs regulate a multitude of cellular pathways related to inflammation [ 17 , 18 ], differentiation [ 19 ], ECM composition [ 20 ], and apoptosis [ 21 ]. miRNAs act by binding to specific messenger RNAs (mRNAs) via sequence homology. The formation of such miRNA/mRNA complexes can accelerate mRNA decay or repress translation [ 22 – 24 ]. The lipid membrane surrounding exosomes protects miRNAs from degradation by extracellular nucleases and greatly enhances the efficiency of transfer from donor to recipient cells [ 25 ]. miRNAs originate inside the nucleus as transcribed precursor miRNAs (pre-miRNAs) and reach maturity outside in the cytosol [ 26 , 27 ]. miRNAs are commonly cataloged into respective families based on sequence similarity. Genes encoding miRNA family members are often organized in clusters and are co-transcribed [ 28 , 29 ]. Precursor miRNAs are initially transcribed in the nucleus and undergo site-specific cleavage to produce mature miRNAs in the cytoplasm. The latter process can generate subtle differences in nucleotide sequences among family members that affect mRNA binding. This can result in multiple miRNA sequences acting in concert to target the same mRNA. In contrast, a single miRNA can target multiple mRNAs [ 30 ], as well as circular RNAs (circRNAs) [ 31 ], and transfer RNAs (tRNAs) [ 32 ]. Difficulties in understanding how miRNAs regulate their various targets are further compounded when one also considers that they are delivered via exosomes in both an autocrine and paracrine fashion along with multiple other bioactive species. This review focuses on miRNAs reported for substantial activity in organ fibrosis, with the goal of identifying candidate miRNAs for AF treatment. Despite mounting evidence that some miRNAs have anti-fibrotic properties in other organs, their therapeutic potential for the treatment of AF remains largely unexplored. Moreover, hundreds of peer-reviewed primary journals over the past few years have reported the effectiveness of miRNAs in soft tissue scarring of various organs. However, no comprehensive summaries of this unique niche currently exist. To address this issue, we systematically reviewed studies involving exosomal miRNA to identify sequences that target inflammation and excessive ECM deposition, the two classical indicators of fibrogenesis. Tabulating preclinical outcomes of exosomal miRNA treatment in scarring should delineate the molecular pattern of action, and this compendium is intended to serve as a guide for selecting a repertoire of miRNAs encapsulated in exosomes for novel anti-AF therapies.