Mesenchymal stem cell-mediated transfer of mitochondria: mechanisms and functional impact

Abstract

There is a steadily growing interest in the use of mitochondria as therapeutic agents. The use of mitochondria derived from mesenchymal stem/stromal cells (MSCs) for therapeutic purposes represents an innovative approach to treat many diseases (immune deregulation, inflammation-related disorders, wound healing, ischemic events, and aging) with an increasing amount of promising evidence, ranging from preclinical to clinical research. Furthermore, the eventual reversal, induced by the intercellular mitochondrial transfer, of the metabolic and pro-inflammatory profile, opens new avenues to the understanding of diseases' etiology, their relation to both systemic and local risk factors, and also leads to new therapeutic tools for the control of inflammatory and degenerative diseases. To this end, we illustrate in this review, the triggers and mechanisms behind the transfer of mitochondria employed by MSCs and the underlying benefits as well as the possible adverse effects of MSCs mitochondrial exchange. We relay the rationale and opportunities for the use of these organelles in the clinic as cell-based product.

Keywords: Cell-based therapy; Mesenchymal stem/stromal cells; Mitochondria; Mitochondrial transfer.

Fig. 1

Comparative biological impact between MSCs and MSC-derived mitochondria on target cells. Immunomodulation by MSC, involves a paracrine secretion of factors such as indoleamine 2,3-dioxygenase (IDO), transforming growth factor beta (TGF-β) and histocompatibility locus antigen G (HLA-G5), promoting inhibition of T cell proliferation. In addition, transfer or MSC-derived mitochondria has shown to induce regulatory T cells (Treg) differentiation, the involved mechanisms and pathways are still to be unraveled. MSCs exert anti-inflammatory effects also by the secretion of factors, such as prostaglandin E2 (PGE2) and tumor necrosis factor-stimulated gene 6 (TSG-6). PGE2 secreted by MSCs bind to EP2 and EP4 receptors expressed by macrophage inducing cyclic AMP upregulation (cAMP) promoting polarization to M2 phenotype. TSG-6 secreted by MSC inhibits activation of M1 macrophages. Mechanisms and pathways of anti-inflammatory effects from mitochondrial transfer are yet to be confirmed. The anti-apoptotic effect of MSCs is due to the enhancement of B cell lymphoma 2 (Bcl-2) expression. Likewise mitochondrial transfer also has anti-apoptotic effects in target cells, by increased expression of Bcl-2 and reduced activity of Bcl-2-associated X (Bax) and Caspase-3 (Casp3) (unpublished data). MSCs have shown the ability to regulate oxidative stress by increasing levels of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px). Similarly mitochondrial transfer exhibits oxidative stress regulation by the enhanced expression of mitochondrial superoxide dismutase 2 (SOD-2)

Fig. 2

MSCs as cellular therapy. MSCs can be isolated and expanded from several tissues, including extraembryonic and adult tissues. In addition, to cellular therapy, MSCs can be used as expanded or modified cells, intact or minimally modified cells, or subcellular components as mitochondria

Fig. 3

Cellular mechanism of mesenchymal stem cell-mediated transfer of mitochondria. Overview of mitochondrial transfer mechanisms has been shown in different cellular models; these include the transfer of mitochondria via microvesicles, cell fusion, gap junctions (GJs) and intracellular nanotubes. Extracellular vesicles (EVs) and apoptotic bodies are a group of heterogenous vesicles, ranging from 400 to 1000 nm, that are secreted by different types of cells to the extracellular medium. EVs are formed by three different group of vesicles, classified depending on varying sizes, origin and composition. Larger EVs, such as microvesicles and apoptotic bodies, can be loaded with partial or entire mitochondrial particles and mitochondrial genome. While smaller EVs, such as exosomes, can only contain genetic material including mitochondrial DNA. EVs can participate in intercellular mitochondrial exchanges, as seen between MSCs and macrophages and astrocytes via arrestin domain-containing protein 1-mediated MVs [73]. Cell Fusion, where two cells transitorily or completely fuse their membranes and share cytosolic compounds, such as organelles, while the nucleus remains intact. For instance, myeloid and lymphoid cells fuse in low in response to injury or inflammation [145]. Tunneling nanotubes (TNTs) are small membranous thin cytoplasmic extensions bordered by a plasma membrane and connecting cells of 50–1000 nm in diameter, containing both F-actin and microtubules. M-sec, a mammalian protein, induces formation of TNTs that only contain actin filaments, but without microtubules. Rho GTPases play an important role in mitochondrial motility through TNTs. For instance, Miro1 and microtubules are involved in the regulation of organelle transfer. While Track and Myosin aid to move the mitochondria through the filament [32, 153]. Gap Junctions are a processed by which MSCs attached to cells in regions of high connexin expression to exchange compounds, from organelles to molecules. BMSCs formed Connexin43-mediated gap junctional channels with the alveolar epithelium, which allowed the transfer of mitochondria that led to cellular protection upon infection [24]

Fig. 4

Tissue-specific MSCs’ mitochondrial transfer and its mechanism to receptor cells of different origins in various pathophysiological conditions, such as: Mitochondrial transfer in lung injury, such as acute lung injury, asthma, chronic obstructive pulmonary disease, and rotenone-induced lung injury. MSCs-derived mitochondrial transfer to pulmonary alveoli and lung epithelia contributed to protection from acute lung injury, mitochondrial bioenergetics, and ATP production, while reducing cytokine production, programmed cell death, and animal mortality [24, 71, 167, 168]. Mitochondrial transfer in stroke models can alleviate the pathological symptoms of stroke, provoke a restoration of neurological activity, reduce brain lesion volume, inflammatory response, apoptosis, and restore the bioenergetics of the recipient cells and stimulate their proliferation [–179]. Mitochondrial transfer in heart injury has shown to restore damaged cells’ functions, prevent cell death, increase the mitochondria potential, and rescue aerobic respiration and cells from apoptosis [, –175]. Mitochondrial transfer in other tissues have demonstrated to have the ability to combat infection by enhancing macrophage bacterial phagocytosis in the harmed tissue [18, 25]. Furthermore, mitochondrial ATP generation appeared to control the insulin secretion from islet β-cells in response to high extracellular glucose level, improving function of insulin cells and other cells in the diabetic niche [169], while mitochondrial transfer in HSCs and the bone marrow niche have been used and tested in Pearson Syndrome (PS) and Kearns–Sayre syndrome (KSS). To improve HSCs’ function, aerobic capacity and mitochondrial membrane potential [62, 63]