Seeing CO in a New Light: Enhancing Autophagy in Mesenchymal Stromal Cells and Implications for Cell Therapy

This year’s Nobel Prize in Physiology or Medicine was awarded to Dr. Yoshinori Ohsumi from the Tokyo Institute of Technology's Institute of Innovative Research in Japan. Dr. Ohsumi “discovered and elucidated mechanisms underlying autophagy, a fundamental process for degrading and recycling cellular components” (1). Cells use autophagy as a quality control mechanism to deliver and eliminate damaged proteins and organelles, a process critical for counteracting the negative consequences of cell stress and aging.

The central role of mesenchymal stromal cells (MSCs) in mitigating proinflammatory responses and magnifying antiinflammatory and antioxidant networks in the lung has been confirmed by numerous studies. MSCs provide/use a variety of intercellular/systematic functions, including control of immune cell proliferation and differentiation, secretion of effector molecules with antiinflammatory capacity, protective effector functions for the epithelium and endothelium, secretion of antibacterial peptides, and many more. Consequently, the potential use of MSCs as a cell-based therapy in acute respiratory distress syndrome (ARDS) has been supported increasingly. Currently, according to clinicaltrials.gov, there are seven ongoing clinical trials using MSCs to treat patient cohorts affected by ARDS. In ARDS, lung injury has the increased potential to result in acute epithelial cell death of lung cells, leading to pulmonary edema, inflammation, and a decline in pulmonary function. Epithelial cell death has been linked to mitochondrial damage and oxidative stress that, when paired with the release of damage factors, drives the pathophysiology of ARDS. During cell-based therapy, MSCs will reach the injured lung at this stage.

Autophagy is one of the potential mechanisms that MSCs use to protect themselves against apoptosis in this highly inflammatory and prooxidant microenvironment. This protection increases the delivery and degradation of damaged proteins and organelles, including mitochondria, to maintain healthy and functional pools of proteins. Autophagy’s pro- and con- apoptotic effects should continue to be explored. It is of paramount importance that research identify and clearly define the potential mechanisms that improve the survival and therapeutic effects of MSCs in ARDS and other disorders with high inflammatory and oxidative stress conditions.

In this issue of the Journal, Ghanta and coauthors (pp. 300–309) describe the consequences of defective autophagy for mitochondrial function and MSC survival (2). Autophagy consists of a tightly controlled cascade of multiple proteins and protein complexes that regulate the distinct stage of autophagosome initiation and formation. Mechanisms driving the delivery of proteins and organelles into the lysosome in MSCs are complex and are not completely understood. These authors examine two important paths via a series of in vitro experiments. LC3B, a microtubule-associated protein, and Beclin 1 have been designated as central hubs for autophagy because of their multiple interactions with autophagy-related proteins. Both proteins have been demonstrated to be important in the assembly of the complex pathway of autophagy.

As expected, a deficiency in these two proteins has important consequences for MSC survival after exogenous exposure to reactive oxygen species (ROS). Cell death was mediated mostly by mitochondrial dysfunction, with a consequential increase in mitochondrial ROS and a decrease in ATP production. This outcome is apparently caused by an inability of the cell to eliminate ROS-induced damaged mitochondria. The most intriguing observation of the work is the effect of carbon monoxide (CO) exposure on the modulation of autophagy, rescuing the normal cells from oxidative stress and increasing the ability of MSCs to respond to oxidative stress. Because the protective effect was observed only in normal cells, the authors concluded that autophagy is the main target of CO. Cytoprotective effects of low doses of CO, particularly in sepsis, have been reported previously in rodent and large-animal models, but not in the context of MSCs. The beneficial effects of CO include mitigation of inflammatory responses, promotion of bacterial killing, and the inhibition of macrophage cell death by improving mitochondrial function (3). In lung cells, exposure to CO increased the expression and activation of LC3B and autophagosome formation via a mitochondrial ROS-dependent process (4).

Two important concepts are generated by these observations. The first is the possibility that CO therapy could enhance autophagy as a mechanism of repair not only in MSCs, but also in other cells of the lung. In addition, a key issue is that with age, MSCs, like other types of cells, are susceptible to losses in autophagic and mitochondrial function. This causes a decrease in their function and in their ability to repair damaged organs and properly respond to injury. Exhaustion of MSCs has been considered a potential cause of increased vulnerability to ARDS with age. Hence, potential therapies that improve the capacity of MSCs have important implications.

However, disagreement persists in the scientific community regarding the importance of autophagy in the function of MSCs. For example, in contrast to Ghanta and colleagues, (2) Dang and colleagues reported that knockdown of Beclin 1 expression increased the survival of MSCs and improved their therapeutic effect in septic mice (5). In view of these divergent observations, it is important to conduct additional studies to reconcile these conflicting results. It should be noted that the current studies have mainly involved in vitro observations. Future in vivo experiments using models of acute lung injury are required to confirm the importance of autophagy in the survival and mitochondrial function of MSCs, as well as the therapeutic effect of CO, which thus far has remained a speculative but highly provocative observation.