The Functions, Methods, and Mobility of Mitochondrial Transfer Between Cells

Abstract

Mitochondria are vital organelles in cells, regulating energy metabolism and apoptosis. Mitochondrial transcellular transfer plays a crucial role during physiological and pathological conditions, such as rescuing recipient cells from bioenergetic deficit and tumorigenesis. Studies have shown several structures that conduct transcellular transfer of mitochondria, including tunneling nanotubes (TNTs), extracellular vesicles (EVs), and Cx43 gap junctions (GJs). The intra- and intercellular transfer of mitochondria is driven by a transport complex. Mitochondrial Rho small GTPase (MIRO) may be the adaptor that connects the transport complex with mitochondria, and myosin XIX is the motor protein of the transport complex, which participates in the transcellular transport of mitochondria through TNTs. In this review, the roles of TNTs, EVs, GJs, and related transport complexes in mitochondrial transcellular transfer are discussed in detail, as well as the formation mechanisms of TNTs and EVs. This review provides the basis for the development of potential clinical therapies targeting the structures of mitochondrial transcellular transfer.

Keywords: Cx43 gap junction; Miro; extracellular vesicles; mitochondria; myosin XIX; transcellular transport; tunneling nanotubes.

Figure 1

Three Main Forms of Intercellular Communication Related to Transcellular Transfer of Mitochondria. Under the stimulation of energy stress, inflammatory stimulation, or DNA damage, intracellular ROS levels increase and three forms of intercellular communication related to mitochondrial transcellular transfer are formed: TNTs, EVs, and GJs. (A) TNTs connects the cytoplasm of two cells, whose main framework is F-actin. Mitochondria are anchored to the actin skeleton by specific transport complex and driven by them to move from one cell to another via TNTS. (B) EV endocytosis is mediated by the NAD+/CD38/cADPR/Ca2+ pathway: Intracellular NAD+ increases and transfers to the extracellular environment under stress conditions. Then, NAD+ is catalyzed by activated transmembrane protein CD38 to generate cADPR, a second messenger controlling the release of the intracellular Ca²⁺ pool. cADPR acts on Ryanodine receptors (RyRs) on the endoplasmic reticulum, which leads to an increase in the intracellular Ca²⁺ concentration, following the remodeling of the actin cytoskeleton and invagination of cell membrane, thereby completing the endocytosis of EVs. The release of EVs and the formation of TNTs might also be mediated by this pathway. Through transcellular transfer of mitochondria, based on the actin cytoskeleton, the OXPHOS, the ATP level, and the viability of the recipient cells are all improved. (C) Cx43 gap junction channel (GJCs) also take part in the transcellular transfer of mitochondria. There are three possible pathways: Ca2+ or ROS exchange via Cx43 GJCs to modulate the formation of channels transferring mitochondria, or the direct transfer of mitochondria. ERMES: a complex that anchors mitochondria to endoplasmic reticulum.

Figure 2

Three Stages of TNT Formation. (A) Under the control of M-sec and exocyst complexes, cells needing healthy mitochondria emit a membrane protrusion containing F-actin; (B) Exocyst complexes induce F-actin remodeling, resulting in membrane protrusion prolongation that forms a filopodium-like membrane structure; (C) The prolongated membrane protrusion contacts the target cell (mitochondrial donor cell) and fuses with the target cell membrane to form a membrane channel connecting the cytoplasm of the two cells, which is termed a TNs. However, the fusion mechanism of the phospholipid bilayers of the donor and recipient cells remains unclear.

Figure 3

Formation of TNTs Mediated by M-Sec and the Exocyst Complex. The exocyst complex consists of eight proteins: Sec3, Sec35, Sec36, Sec38, Sec310, Sec315, Exo70, and Exo84. During the interactions of M-sec with the exocyst complex, small GTPase RalA and Cdc42 promote the assembly of the exocyst complex and lead to the remodeling of actin. Active RalA (RalA-GTP) interacts with two components of the exocyst complex, Sec5 and Exo84, which bind to RalA competitively. This interaction leads directly to the aggregation of G-actin and the subsequent formation of the membrane protrusion. The combination of Cdc42 and Sec3 plays a role in the prolongation stage of the membrane protrusion. In addition, LST1 recruits RalA to the submembrane region, and promotes its interaction with the exocyst complex and recruits filamin, an actin cross-linked protein. Moreover, LST1 can interact with M-sec, myosin, and myoferlin, which might also participate in the subsequent process of mitochondrial anchoring and transfer. Overall, a variety of molecules are involved in the formation of the membrane protrusion, including some of the molecules mentioned later. Together, they form a multi-molecular complex that regulates the formation of TNTs. Dotted arrows indicate interactions between proteins.

Figure 4

MIRO Mediates the Intracellular and Intercellular Distribution of Mitochondria. Mitochondria move to adapt the ATP demand of different parts of cells. Long-distance transport of mitochondria is usually mediated by microtubules, while short-distance transport of mitochondria is mediated by actin. MIRO might be a key protein involved in mitochondrial transport. MIRO participates in the formation of various mitochondria transport complexes. MIRO/TRAK/kinesin and MIRO/TRAK/dynein transport complexes mediate mitochondrial movement to the plus (+) and minus (−) ends of microtubules, respectively. The ERMES complex, containing MIRO, tethers mitochondria to the endoplasmic reticulum. This might be related to Ca²⁺ exchange. Under the stimulation of nerve growth factor (NGF), PI3K, and RHOA signals, mitochondrial transport changes from microtubule-based transport to microfilament-based transport, completing the actin−based docking of mitochondria. Actin-based docking of mitochondria might represent the transitional stage of mitochondria entering TNTs, while the mitochondrial movement in TNTs is probably mediated by the MIRO/myosin XIX transport complex. These movements are regulated by the local ATP/ADP ratio. However, the transitional mechanism of mitochondria entering TNTs remains unclear. The dotted box: Two calcium ions bind to the EF-hand Ca²⁺-binding domains of MIRO, resulting in the change of kinesin configuration: The microtubule binding site of kinesin binds to MIRO, leading to the separation of mitochondria from the microtubule.