Molecular Mechanisms Responsible for Therapeutic Potential of Mesenchymal Stem Cell-Derived Secretome

Abstract

Mesenchymal stem cell (MSC)-sourced secretome, defined as the set of MSC-derived bioactive factors (soluble proteins, nucleic acids, lipids and extracellular vesicles), showed therapeutic effects similar to those observed after transplantation of MSCs. MSC-derived secretome may bypass many side effects of MSC-based therapy, including unwanted differentiation of engrafted MSCs. In contrast to MSCs which had to be expanded in culture to reach optimal cell number for transplantation, MSC-sourced secretome is immediately available for treatment of acute conditions, including fulminant hepatitis, cerebral ischemia and myocardial infarction. Additionally, MSC-derived secretome could be massively produced from commercially available cell lines avoiding invasive cell collection procedure. In this review article we emphasized molecular and cellular mechanisms that were responsible for beneficial effects of MSC-derived secretomes in the treatment of degenerative and inflammatory diseases of hepatobiliary, respiratory, musculoskeletal, gastrointestinal, cardiovascular and nervous system. Results obtained in a large number of studies suggested that administration of MSC-derived secretomes represents a new, cell-free therapeutic approach for attenuation of inflammatory and degenerative diseases. Therapeutic effects of MSC-sourced secretomes relied on their capacity to deliver genetic material, growth and immunomodulatory factors to the target cells enabling activation of anti-apoptotic and pro-survival pathways that resulted in tissue repair and regeneration.

Keywords: degenerative diseases; inflammatory diseases; mesenchymal stem cells; secretome; therapy.

Figure 1

Molecular mechanisms responsible for beneficial effects of MSC-derived secretome in asthma. Administration of MSC-sourced secretome significantly reduced influx of circulating eosinophils, neutrophils, monocytes and lymphocytes in asthmatic lungs resulting in alleviation of on-going inflammation. MSC-CM or MSC-Exos reduced TGF-β production, decreased collagen deposition and attenuated fibrosis in the lungs. Additionally, MSC-derived secretome attenuated antigen-presenting function of DCs and suppressed Th2 and Th17 cell-driven inflammatory response in asthmatic lungs, but increased total number of lung-infiltrated IL-10-producing Tregs which created immunosuppressive microenvironment that allowed better functional recovery of asthmatic animals.

Figure 2

Herapeutic effects of MSC-derived secretome in cartilage regeneration. Chondrocytes cultured in the presence of MSC-derived secretome significantly reduce production of inflammatory cytokines which play detrimental role in cartilage degeneration during OA development (TNF-α, IL-1β, IL-6, nitric oxide (NO)) and increase production of immunosuppressive IL-10 which protects cartilage from inflammation-related injury. Accelerated neotissue filling and increased synthesis of type II collagen were noticed in osteoarthritic animals that received MSC-sourced secretome. MSC-derived extracellular vesicles (EVs) promoted endogenous cartilage repair and regeneration by delivering miR-320c and miR-92a-3p which restore homeostasis in bioenergetics and cell metabolism in proliferating chondrocytes.

Figure 3

Therapeutic effects of MSC-derived secretome in attenuation of experimental colitis. Administration of MSC-sourced extracellular vesicles (EVs), including MSC-derived exosomes (Exos), efficiently alleviated dextran sodium sulphate (DSS)-induced colitis. Intravenous injection of MSC-EVs significantly decreased activity of myeloperoxidase (MPO), malondialdehyde (MDA) and notably increased expression of superoxide dismutase (SOD) and glutathione (GSH) in inflamed colons, indicating that modulation of anti-oxidant/oxidant balance in inflamed gut had important role for MSC-EVs-based therapeutic effects. Down-regulation of ubiquitin and ubiquitin-associated molecules (K48, K63 and FK2) in inflamed gut were also responsible for MSC-Exo-based attenuation of colitis. Additionally, MSC-derived secretome attenuated production of inflammatory cytokines (TNF-α, IL-1β, IL-6, IL-7) in colon macrophages resulting in alleviation of on-going inflammation.

Figure 4

Therapeutic potential of MSC-sourced secretome in myocardial regeneration. MSC-derived secretome promote myocardial regeneration by preventing apoptosis of cardiomyocytes, by inducing neo-angiogenesis in ischemic regions and by promoting survival, angiogenic potency and capacity for self-renewal of cardiac stem cells (CSCs). MSC-derived extracellular vesicles (EVs) increased survival of cardiomyocytes in ischemic lesions by preventing apoptosis and by inducing autophagy via modulation of AMPK/mTOR, Akt/mTOR and Wnt/β-catenin pathways. Administration of MSC-derived exosomes (Exos) resulted in up-regulation of anti-apoptotic Bcl-2, down-regulation of pro-apoptotic Bax and suppressed activity of caspase-3 in cardiomyocytes. MSC-Exos-mediated delivery of miR-210 and miR-125b-5p increased survival of cardiomyocytes by preventing p53 and Bak1-driven apoptosis. MSC-Exo-mediated delivery of stromal cell-derived factor-1 (SDF-1) and miR-132 resulted in enhanced tube formation and increased angiogenic capacity of endothelial cells (ECs). MSC-Exos-mediated modulation of CSCs function has been attributed to the delivery of miR-15, miR-21, miR-22, miR-126, miR-146a, miR-210 which prevented apoptosis and promoted survival of CSCs.

Figure 5

Molecular mechanisms responsible for beneficial effects of MSC-derived secretome in retinal regeneration. MSC-derived exosomes (Exos) promote regeneration of injured retina by supplying retinal ganglion cells (RGCs) with miR-17-92, miR-21, miR146a and neurotrophins (brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF) and platelet-derived growth factor (PDGF)). MSC-sourced secretome suppress detrimental immune response in the eye through the inhibition of antigen-presenting cells (macrophages and dendritic cells (DCs)) which results in attenuated activation of Th1 and Th17 cells and alleviation of retinal injury and inflammation.

Figure 6

Molecular mechanisms responsible for beneficial effects of MSC-derived secretomes in the therapy of ischemic brain damage and spinal cord injury. Administration of MSC-sourced extracellular vesicles (EVs), including MSC-derived exosomes (Exos), promoted neural regeneration in animal models of ischemic brain damage and spinal cord injury (SCI). MSC-Exos-based therapy improved neurogenesis, promoted axonal growth, increased presence of neuroblasts and endothelial cells (ECs) in ischemic regions of the brain. MSCs-Exo regulated neurogenesis by supplying neurons with miR-133b which promoted neurite outgrowth by targeting Ras homolog gene family member A (RhoA). Similarly, systemic injection of miR-133b-bearing MSC-Exos promoted recovery from SCI by promoting regeneration of axons through the activation of survival Erk1/2 and Stat-3 signaling pathways in regenerating neurons. After intravenous administration, MSC-Exos accumulated at the site of SCI and promoted generation of immunosuppressive M2 macrophages which, in IL-10-dependent manner, suppressed activation of neurotoxic A1 astrocytes through the inhibition of NF-kB. In similar manner, via down-regulation of NF-κB p65 signaling, MSC-EVs reduced migratory capacities of pericytes and maintained structural integrity of blood-spinal cord barrier (BSCB).

Figure 7

Molecular mechanisms responsible for beneficial effects of MSC-derived secretome in tissue repair and regeneration. Results obtained in experimental studies suggest that MSC-derived secretome represents a promising therapeutic tool for the treatment of degenerative and inflammatory diseases. Beneficial effects of MSC-sourced secretomes rely on their capacity to deliver neurotrophins (brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), hepatocyte growth factor (HGF), miR-17-92, miR-21, miR-124, miR-133b, miR146a which enable regeneration of injured liver, brain, spinal cord and eye. MSC-derived secretomes contain immunomodulatory factors which inhibit proliferation and activation of inflammatory immune cells and promote expansion of immunosuppressive cells resulting in alleviation of inflammation-related tissue injury. MSC-sourced secretomes are enriched with angiomodulatory factors (stromal cell derived factor-1 (SDF-1), miR-132, platelet-derived growth factor (PDGF), vascular endothelial growth factor (VEGF)) that promote angiogenesis and neo-vascularization in ischemic regions of brain and heart enhancing survival of injured neurons and cardiomyocytes.