Mitochondrial Transfer and Regulators of Mesenchymal Stromal Cell Function and Therapeutic Efficacy

Abstract

Mesenchymal stromal cell (MSC) metabolism plays a crucial role in the surrounding microenvironment in both normal physiology and pathological conditions. While MSCs predominantly utilize glycolysis in their native hypoxic niche within the bone marrow, new evidence reveals the importance of upregulation in mitochondrial activity in MSC function and differentiation. Mitochondria and mitochondrial regulators such as sirtuins play key roles in MSC homeostasis and differentiation into mature lineages of the bone and hematopoietic niche, including osteoblasts and adipocytes. The metabolic state of MSCs represents a fine balance between the intrinsic needs of the cellular state and constraints imposed by extrinsic conditions. In the context of injury and inflammation, MSCs respond to reactive oxygen species (ROS) and damage-associated molecular patterns (DAMPs), such as damaged mitochondria and mitochondrial products, by donation of their mitochondria to injured cells. Through intercellular mitochondria trafficking, modulation of ROS, and modification of nutrient utilization, endogenous MSCs and MSC therapies are believed to exert protective effects by regulation of cellular metabolism in injured tissues. Similarly, these same mechanisms can be hijacked in malignancy whereby transfer of mitochondria and/or mitochondrial DNA (mtDNA) to cancer cells increases mitochondrial content and enhances oxidative phosphorylation (OXPHOS) to favor proliferation and invasion. The role of MSCs in tumor initiation, growth, and resistance to treatment is debated, but their ability to modify cancer cell metabolism and the metabolic environment suggests that MSCs are centrally poised to alter malignancy. In this review, we describe emerging evidence for adaptations in MSC bioenergetics that orchestrate developmental fate decisions and contribute to cancer progression. We discuss evidence and potential strategies for therapeutic targeting of MSC mitochondria in regenerative medicine and tissue repair. Lastly, we highlight recent progress in understanding the contribution of MSCs to metabolic reprogramming of malignancies and how these alterations can promote immunosuppression and chemoresistance. Better understanding the role of metabolic reprogramming by MSCs in tissue repair and cancer progression promises to broaden treatment options in regenerative medicine and clinical oncology.

Keywords: MSC differentiation; cancer metabolism; hematological malignancy; mesenchymal stromal cells (MSCs); metabolic reprogramming; mitochondrial biogenesis; mitochondrial dynamics; mitochondrial transfer.

Figure 1

A simplified schematic model of mitochondrial dynamics in MSCs during self-renewal and differentiation. (A) In the undifferentiated state, mitochondrial morphology is fragmented with few cristae localized around the nucleus. Fission proteins maintain a fragmented mitochondrial network where glycolysis is a major source of energy production. OXPHOS, intracellular ATP, and ROS levels are low. (B) During differentiation, mitochondria are generally distributed throughout the cytoplasm and are reorganized. Distinct differences exist dependent upon cell fate such that osteoblasts and adipocytes favor development of an interconnected tubular network with higher cristae density. In these cell types, mitochondrial biogenesis is activated and fusion proteins aid in maintenance of elongated mitochondria. Osteoblasts and adipocytes also rely more upon OXPHOS and generally produce more ATP. Conversely, chondrocytes possess spherical mitochondria and produce less energy by OXPHOS. (C) Upon differentiation, mitochondrial capacity is modified by several mechanisms to ensure that the demands of adipogenic, chondrogenic, and osteogenic lineages are met.

Figure 2

Schematic representation of the various mitochondrial transfer mechanisms utilized between MSCs and damaged cells with dysfunctional mitochondria. (A) Tunneling nanotubes (TNTs) are actin-dependent cytoskeletal protrusions that serve as cytoplasmic bridges between cells. Miro1 regulates transport of mitochondria across TNTs. Cx43-mediated gap junctions also serve at cell-cell junctions to enable mitochondrial transfer. (B) Extracellular vesicles (EVs) can convey mtDNA or fragments of mitochondria and, though less well-documented, macrovesicles are suggested to contain entire mitochondria. (C) Cell fusion enables sharing of cytoplasmic contents during either transient or permanent fusion of the plasma membrane of two cells. Upon transfer of healthy mitochondria from MSCs, respiration increases in the recipient cell and restores cell function, cell survival, and tissue repair.