Revealing MCL-1 Regulation of Mitochondrial Dynamics in Human Pluripotent Stem Cell-Derived Systems
Rasmussen, Megan Loraine
The B cell CLL/lymphoma-2 (BCL-2) family of proteins control the mitochondrial pathway of cell death, also known as intrinsic apoptosis. The ability of the cell to sense stress and translate it into a death signal has been a major theme of research for nearly four decades; however, other mechanisms by which the BCL-2 family coordinates cellular homeostasis beyond its role in initiating cell death are emerging. Our laboratory aims to understand how the BCL-2 family of proteins regulate development at the mitochondria using pluripotent stem cells as a model system. Human pluripotent stem cells (hPSCs) maintain a highly fragmented mitochondrial network, but the mechanisms regulating this phenotype remain unknown. In this dissertation, I describe a non-cell death function of the anti-apoptotic protein, Myeloid cell leukemia-1 (MCL-1), in regulating mitochondrial dynamics and metabolism in hPSCs and cardiomyocytes. MCL-1 is induced upon reprogramming, and its depletion in hPSCs induces changes to the mitochondrial network, as well as loss of key pluripotency transcription factors. Aside from localizing at the outer mitochondrial membrane as other BCL-2 family members, MCL-1 is unique in that it also resides at the mitochondrial matrix. Mechanistically, we find MCL-1 to interact with DRP-1 and OPA1, two GTPases responsible for remodeling the mitochondrial network. Depletion of MCL-1 compromised the levels and activity of these key regulators of mitochondrial dynamics. The interactions between MCL-1 and the mitochondrial dynamics regulators are also maintained upon differentiation of hPSCs to cardiomyocytes (hiPSC-CMs). In these cells, inhibition of MCL-1 by BH3 mimetics resulted in the disruption of mitochondrial morphology and dynamics as well as disorganization of the actin cytoskeleton. Interfering with MCL-1 function affects the homeostatic proximity of DRP-1 and MCL-1 at the outer mitochondrial membrane, resulting in decreased functionality of hiPSC-CMs. Cardiomyocytes display abnormal cardiac performance even after caspase inhibition, supporting a non-apoptotic activity of MCL-1 in hiPSC-CMs. BH3 mimetics targeting MCL-1 are promising anti-tumor therapeutics. Progression towards using MCL-1 inhibitors depends on understanding not only its canonical function in preventing apoptosis, but also in the maintenance of mitochondrial dynamics and function.