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Lee MC, Lee JS, Kim S, Jamaiyar A, Wu W, Gonzalez ML, Durán TCA, Madrigal-Salazar AD, Bassous N, Carvalho V, Choi C, Kim DS, Seo JW, Rodrigues N, Teixeira SFCF, Alkhateeb AF, Soto JAL, Hussain MA, Leijten J, Feinberg MW, Shin SR. Synergistic effect of Hypoxic Conditioning and Cell-Tethering Colloidal Gels enhanced Productivity of MSC Paracrine Factors and Accelerated Vessel Regeneration. ADVANCED MATERIALS (DEERFIELD BEACH, FLA.) 2024:e2408488. [PMID: 39380372 DOI: 10.1002/adma.202408488] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 06/14/2024] [Revised: 09/26/2024] [Indexed: 10/10/2024]
Abstract
Microporous hydrogels have been widely used for delivering therapeutic cells. However, several critical issues, such as the lack of control over the harsh environment they are subjected to under pathological conditions and rapid egression of cells from the hydrogels, have produced limited therapeutic outcomes. To address these critical challenges, cell-tethering and hypoxic conditioning colloidal hydrogels containing mesenchymal stem cells (MSCs) are introduced to increase the productivity of paracrine factors locally and in a long-term manner. Cell-tethering colloidal hydrogels that are composed of tyramine-conjugated gelatin prevent cells from egressing through on-cell oxidative phenolic crosslinks while providing mechanical stimulation and interconnected microporous networks to allow for host-implant interactions. Oxygenating microparticles encapsulated in tyramine-conjugated colloidal microgels continuously generated oxygen for 2 weeks with rapid diffusion, resulting in maintaining a mild hypoxic condition while MSCs consumed oxygen under severe hypoxia. Synergistically, local retention of MSCs within the mild hypoxic-conditioned and mechanically robust colloidal hydrogels significantly increased the secretion of various angiogenic cytokines and chemokines. The oxygenating colloidal hydrogels induced anti-inflammatory responses, reduced cellular apoptosis, and promoted numerous large blood vessels in vivo. Finally, mice injected with the MSC-tethered oxygenating colloidal hydrogels significantly improved blood flow restoration and muscle regeneration in a hindlimb ischemia (HLI) model.
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Affiliation(s)
- Myung Chul Lee
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
- Medicinal Materials Research Center, Biomedical Research Division, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea
| | - Jae Seo Lee
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
- Harvard Medical School and Wellman Center for Photomedicine, Massachusetts General Hospital, Cambridge, MA, 02139, USA
| | - Seongsoo Kim
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
- Biomaterials Research Center, Biomedical Research Division, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea
| | - Anurag Jamaiyar
- Department of Medicine, Cardiovascular Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, 02115, USA
| | - Winona Wu
- Department of Medicine, Cardiovascular Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, 02115, USA
- Division of Vascular and Endovascular Surgery, Beth Israel Deaconess Medical Center, Boston, MA, 02115, USA
| | - Montserrat Legorreta Gonzalez
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Tania Carolina Acevedo Durán
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Andrea Donaxi Madrigal-Salazar
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Nicole Bassous
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Violeta Carvalho
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
- MEtRICs, University of Minho, Campus de Azurém, Guimarães, 4800-058, Portugal
- ALGORITMI/LASI Center, University of Minho, Campus de Azurém, Guimarães, 4800-058, Portugal
- Center for MicroElectromechanical Systems (CMEMS-UMinho), University of Minho, Campus de Azurém, Guimarães, 4800-058, Portugal
- LABBELS-Associate Laboratory, Braga/Guimarães, 4710-057, Portugal
| | - Cholong Choi
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Da-Seul Kim
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Jeong Wook Seo
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Nelson Rodrigues
- MEtRICs, University of Minho, Campus de Azurém, Guimarães, 4800-058, Portugal
- COMEGI-Center for Research in Organizations, Markets and Industrial Management, Lusíada Norte University, Porto, 1349-001, Portugal
| | | | - Abdulhameed F Alkhateeb
- Department of Electrical and Computer Engineering, King Abdulaziz University, Jeddah, 21589, Saudi Arabia
| | - Javier Alejandro Lozano Soto
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
| | - Mohammad Asif Hussain
- Department of Electrical and Computer Engineering, King Abdulaziz University, Jeddah, 21589, Saudi Arabia
| | - Jeroen Leijten
- Leijten Lab, Department of BioEngineering Technologies, Faculty of Science and Technology, Technical Medical Centre, University of Twente, Drienerlolaan 5, Enschede, 7522 NB, The Netherlands
| | - Mark W Feinberg
- Department of Medicine, Cardiovascular Division, Brigham and Women's Hospital, Harvard Medical School, Boston, MA, 02115, USA
| | - Su Ryon Shin
- Division of Engineering in Medicine, Department of Medicine, Brigham and Women's Hospital, Harvard Medical School, Cambridge, MA, 02139, USA
- Center for MicroElectromechanical Systems (CMEMS-UMinho), University of Minho, Campus de Azurém, Guimarães, 4800-058, Portugal
- LABBELS-Associate Laboratory, Braga/Guimarães, 4710-057, Portugal
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Qian W, Huang L, Xu Y, Lu W, Wen W, Guo Z, Zhu W, Li Y. Hypoxic ASCs-derived Exosomes Attenuate Colitis by Regulating Macrophage Polarization via miR-216a-5p/HMGB1 Axis. Inflamm Bowel Dis 2022; 29:602-619. [PMID: 36287066 DOI: 10.1093/ibd/izac225] [Citation(s) in RCA: 13] [Impact Index Per Article: 6.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 07/10/2022] [Indexed: 02/06/2023]
Abstract
BACKGROUND Exosomes derived from mesenchymal stem cells have shown therapeutic effects for colitis. As a more clinically accessible resource, the therapeutic potential of exosomes from adipose-derived stem cells (ASCs) has not been fully elucidated, and whether hypoxia precondition could improve the therapeutic effect of ASC-derived exosomes in colitis remains elusive. METHODS In this study, exosomes were derived from ASCs under normoxia (NExos) and hypoxia (HExos) and were identified by detecting their morphology, size distribution, and exosome surface markers. The concentration of inflammation-related cytokines was detected by ELISA, and macrophage phenotype-related genes were determined by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), western blot, and immunofluorescence. A miRNA microarray sequencing analysis was conducted to confirm the differentially expressed miRNAs. Dextran sulfate sodium-induced colitis was employed as an in vivo assay. RESULTS Administration of NExos alleviated inflammation by modulating the balance of macrophages both in cellular assays and in vivo experiments, and HExos showed higher therapeutic efficiency than NExos. The miR-216a-5p in HExos was significantly enriched and promoted macrophage M2 polarization through transfer to macrophages by exosomes. The miR-216a-5p was confirmed to target the 3'-UTR of HMGB1. Mechanistically, hypoxia-induced ASCs release miR-216a-5p in an exosomal way that induced macrophage M2 polarization by regulating the HMGB1/TLR4/NF-κB signaling pathway. CONCLUSIONS Exosomal miR-216a-5p released from hypoxia-prime ASCs showed higher therapeutic efficiency than NExos in experimental colitis by promoting the M2 macrophage phenotype, which indicated that hypoxia prime may represent a promising approach to optimizing the function of ASC-derived exosomes.
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Affiliation(s)
- Wenwei Qian
- Department of General Surgery, Jinling Hospital, Medical School of Southeast University, Nanjing, PR China
| | - Liangyu Huang
- Department of Colorectal Surgery, The First Affiliated Hospital, Zhejiang University School of Medicine, Hangzhou, PR China
| | - Yihan Xu
- Department of General Surgery, Jinling Hospital, Medical School of Nanjing University, Nanjing, PR China
| | - Wen Lu
- Department of Endocrinology and Metabolism, the First Affiliated Hospital of Soochow University, Suzhou, PR China
| | - Weiwei Wen
- Department of General Surgery, Jinling Hospital, Medical School of Southeast University, Nanjing, PR China
| | - Zhen Guo
- Department of General Surgery, Jinling Hospital, Medical School of Nanjing University, Nanjing, PR China
| | - Weiming Zhu
- Department of General Surgery, Jinling Hospital, Medical School of Southeast University, Nanjing, PR China.,Department of General Surgery, Jinling Hospital, Medical School of Nanjing University, Nanjing, PR China
| | - Yi Li
- Department of General Surgery, Jinling Hospital, Medical School of Southeast University, Nanjing, PR China.,Department of General Surgery, Jinling Hospital, Medical School of Nanjing University, Nanjing, PR China
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Liu J, Qiu P, Qin J, Wu X, Wang X, Yang X, Li B, Zhang W, Ye K, Peng Z, Lu X. Allogeneic adipose-derived stem cells promote ischemic muscle repair by inducing M2 macrophage polarization via the HIF-1α/IL-10 pathway. Stem Cells 2020; 38:1307-1320. [PMID: 32627897 PMCID: PMC7590195 DOI: 10.1002/stem.3250] [Citation(s) in RCA: 24] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/14/2020] [Revised: 05/19/2020] [Accepted: 06/02/2020] [Indexed: 12/12/2022]
Abstract
Adipose-derived mesenchymal stem cells (ASCs) are multipotent stromal cells that possess considerable therapeutic potential for tissue remodeling. However, their protective mechanism in critical limb ischemia has not been fully defined. After the occlusion of blood vessels, hypoxia becomes a prominent feature of the ischemic limb. This study investigated the immunomodulatory effect of ASCs on ischemic muscle repair and explored the specific mechanism. We found that the ability of RAW264.7 cells to migrate was impaired in hypoxia, whereas coculturing with ASCs could enhance the migration capacity. In addition, under hypoxic conditions, the paracrine effect of ASCs was enhanced and ASCs could induce RAW264.7 macrophages toward the anti-inflammatory M2 phenotype. We further demonstrated that ASCs-derived interleukin 10 (IL-10), mediated by hypoxia inducible factor-1α (HIF-1α), played a crucial role in the induction of M2 macrophages by activating the signal transducer and activator of transcription 3 (STAT3)/Arginase (Arg-1) pathway. Our in vivo experiments revealed that transplanted ASCs exhibited an immunomodulatory effect by recruiting macrophages to ischemic muscle and increasing the density of M2 macrophages. The transplantation of ASCs into ischemic limbs induced increased blood flow reperfusion and limb salvage rate, whereas the depletion of tissue macrophages or transplanting HIF-1α-silenced ASCs inhibited the therapeutic effect. These findings elucidated the critical role of macrophages in ASCs-mediated ischemic muscle repair and proved that allogeneic ASCs could exert the protective effect by enhancing the recruitment of macrophages and inducing macrophages toward M2 phenotype through HIF-1α/IL-10 pathway.
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Affiliation(s)
- Junchao Liu
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Peng Qiu
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Jinbao Qin
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Xiaoyu Wu
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Xin Wang
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Xinrui Yang
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Bo Li
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Wenjie Zhang
- Shanghai Key Laboratory of Tissue Engineering, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Kaichuang Ye
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Zhiyou Peng
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China
| | - Xinwu Lu
- Department of Vascular Surgery, Shanghai Ninth People's Hospital, Shanghai JiaoTong University School of Medicine, Shanghai, People's Republic of China.,Vascular Center of Shanghai JiaoTong University, Shanghai, People's Republic of China
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Bao Y, Huang S, Zhao Z. Comparison of Different Culture Conditions for Mesenchymal Stem Cells from Human Umbilical Cord Wharton’s Jelly for Stem Cell Therapy. Turk J Haematol 2019; 37:67-69. [PMID: 31718116 PMCID: PMC7057755 DOI: 10.4274/tjh.galenos.2019.2019.0439] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Affiliation(s)
- Yu Bao
- Zhejiang University Faculty of Medicine, Children’s Hospital, Department of Nephrology, Zhejiang, China
| | - Shumin Huang
- Zhejiang University Faculty of Medicine, Children’s Hospital, Clinic of Division of Child Health Care, Zhejiang, China
| | - Zhengyan Zhao
- Zhejiang University Faculty of Medicine, Children’s Hospital, Clinic of Division of Child Health Care, Zhejiang, China
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Dhillon J, Young SA, Sherman SE, Bell GI, Amsden BG, Hess DA, Flynn LE. Peptide-modified methacrylated glycol chitosan hydrogels as a cell-viability supporting pro-angiogenic cell delivery platform for human adipose-derived stem/stromal cells. J Biomed Mater Res A 2018; 107:571-585. [PMID: 30390406 DOI: 10.1002/jbm.a.36573] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/01/2018] [Revised: 09/26/2018] [Accepted: 10/27/2018] [Indexed: 12/18/2022]
Abstract
Cell-based therapies involving the injection of adipose-derived stem/stromal cells (ASCs) within rationally designed biomaterials are a promising approach for stimulating angiogenesis. With this focus, the current work explored the effects of incorporating integrin-binding RGD or IKVAV peptides within in situ-gelling N-methacrylate glycol chitosan (MGC) hydrogels on the response of encapsulated human ASCs. Initial studies focused on hydrogel characterization to validate that the MGC, MGC-RGD, and MGC-IKVAV hydrogels had similar biomechanical properties. ASC viability following encapsulation and culture under 2% O2 was significantly impaired in the MGC-IKVAV group relative to the MGC and MGC-RGD groups. In contrast, sustained viability, along with enhanced cell spreading and metabolic activity were observed in the MGC-RGD group. Investigation of angiogenic transcription suggested that the incorporation of the peptide groups did not substantially alter the pro-angiogenic gene expression profile of the encapsulated ASCs after 7 days of culture under 2% O2. Consistent with the in vitro findings, preliminary in vivo characterization following subcutaneous implantation into NOD/SCID mice showed that ASC retention was enhanced in the MGC-RGD hydrogels relative to the MGC-IKVAV group at 14 days. Further, the encapsulated ASCs in the MGC and MGC-RGD groups promoted murine CD31+ endothelial cell recruitment to the peri-implant region. Overall, the results indicate that the MGC-RGD and MGC hydrogels are promising platforms for ASC delivery, and suggest that strategies that support long-term ASC viability can augment in vivo angiogenesis through paracrine mechanisms. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 571-585, 2019.
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Affiliation(s)
- Jobanpreet Dhillon
- Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, N6A 3K7, Canada.,Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, N6A 5B7, Canada
| | - Stuart A Young
- Department of Chemical Engineering, Queen's University, Kingston, Ontario, K7L 3N6, Canada.,Human Mobility Research Centre, Queen's University, Kingston, Ontario, K7L 3N6, Canada
| | - Stephen E Sherman
- Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, N6A 5B7, Canada.,Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
| | - Gillian I Bell
- Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, N6A 5B7, Canada.,Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
| | - Brian G Amsden
- Department of Chemical Engineering, Queen's University, Kingston, Ontario, K7L 3N6, Canada.,Human Mobility Research Centre, Queen's University, Kingston, Ontario, K7L 3N6, Canada
| | - David A Hess
- Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, N6A 5B7, Canada.,Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
| | - Lauren E Flynn
- Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, N6A 3K7, Canada.,Department of Chemical and Biochemical Engineering, Thompson Engineering Building, The University of Western Ontario, London, Ontario, N6A 5B9, Canada
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6
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Young SA, Sherman SE, Cooper TT, Brown C, Anjum F, Hess DA, Flynn LE, Amsden BG. Mechanically resilient injectable scaffolds for intramuscular stem cell delivery and cytokine release. Biomaterials 2018; 159:146-160. [PMID: 29324306 DOI: 10.1016/j.biomaterials.2018.01.008] [Citation(s) in RCA: 35] [Impact Index Per Article: 5.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2017] [Revised: 11/24/2017] [Accepted: 01/03/2018] [Indexed: 12/17/2022]
Abstract
A promising strategy for treating peripheral ischemia involves the delivery of stem cells to promote angiogenesis through paracrine signaling. Treatment success depends on cell localization, retention, and survival within the mechanically dynamic intramuscular (IM) environment. Herein we describe an injectable, in situ-gelling hydrogel for the IM delivery of adipose-derived stem/stromal cells (ASCs), specifically designed to withstand the dynamic loading conditions of the lower limb and facilitate cytokine release from encapsulated cells. Copolymers of poly(trimethylene carbonate)-b-poly(ethylene glycol)-b-poly(trimethylene carbonate) diacrylate were used to modulate the properties of methacrylated glycol chitosan hydrogels crosslinked by thermally-initiated polymerization using ammonium persulfate and N,N,N',N'-tetramethylethylenediamine. The scaffolds had an ultimate compressive strain over 75% and maintained mechanical properties during compressive fatigue testing at physiological levels. Rapid crosslinking (<3 min) was achieved at low initiator concentration (5 mM). Following injection and crosslinking within the scaffolds, human ASCs demonstrated high viability (>90%) over two weeks in culture under both normoxia and hypoxia. Release of angiogenic and chemotactic cytokines was enhanced from encapsulated cells under sustained hypoxia, in comparison to normoxic and tissue culture polystyrene controls. When delivered by IM injection in a mouse model of hindlimb ischemia, human ASCs were well retained in the scaffold over 28 days and significantly increased the IM vascular density compared to untreated controls.
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Affiliation(s)
- Stuart A Young
- Department of Chemical Engineering, Queen's University, Kingston, Ontario, K7L 3N6, Canada; Human Mobility Research Centre, Queen's University, Kingston, Ontario, K7L 3N6, Canada
| | - Stephen E Sherman
- Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, Canada; Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
| | - Tyler T Cooper
- Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, Canada; Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
| | - Cody Brown
- Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, N6A 3K7, Canada
| | - Fraz Anjum
- Pharmaceutical Production Research Facility, University of Calgary, Calgary, Alberta, T2N 1N4, Canada
| | - David A Hess
- Krembil Centre for Stem Cell Biology, Molecular Medicine Research Laboratories, Robarts Research Institute, London, Ontario, Canada; Department of Physiology and Pharmacology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, Canada
| | - Lauren E Flynn
- Department of Anatomy and Cell Biology, Schulich School of Medicine and Dentistry, The University of Western Ontario, London, Ontario, N6A 3K7, Canada; Department of Chemical and Biochemical Engineering, Thompson Engineering Building, The University of Western Ontario, London, Ontario, N6A 5B9, Canada.
| | - Brian G Amsden
- Department of Chemical Engineering, Queen's University, Kingston, Ontario, K7L 3N6, Canada; Human Mobility Research Centre, Queen's University, Kingston, Ontario, K7L 3N6, Canada.
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