Mitochondrial calcium overload and necrosis in tauopathies caused by inhibition of Mfn2 and NCLX

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Abstract Tau aggregates (also known as neurofibrillary tangles or NFTs) are hallmarks of the neurodegenerative diseases collectively known as tauopathies. These diseases are a broad class of a dozen or more dementias that include the frontotemporal dementias, Pick’s disease, and chronic traumatic encephalopathy. Alzheimer’s is also a tauopathy, even though this disease is best known for the formation of amyloid plaques. In this case, the extracellular amyloid plaques induce intracellular tau aggregation. The formation of tau aggregates is preceded by tau oligomerization, which is considered the primary cause of toxicity, ultimately inducing cell death. Several recent papers suggest that tau oligomers or aggregates inhibit calcium release from mitochondria, which then triggers cell death through necrosis by opening of the mitochondrial permeability transition pore (MPTP). The target of this inhibition is the Na+/Ca2+ exchanger NCLX, which is localized to the mitochondrial inner membrane. It is unclear how cytosolic tau oligomers and aggregates interfere with NCLX function because of their different cellular locations. Data from the parent grant of this supplement provides a potential mechanism for inhibition. These data show that NCLX on the mitochondrial inner membrane is controlled by Mfn2 on the outer membrane. Mfn2 is one of two mitochondrial outer membrane fusion proteins, but it also plays a role in tethering of mitochondria to the endoplasmic reticulum where calcium exchange is important for regulating mitochondrial functions. The parent grant focuses on the effects that the connection between Mfn2 and NCLX have on a peripheral neuropathy (the Charcot Marie Tooth disease CMT2A), and investigates the possibility that CMT2A is caused by excessive Ca2+ release into the cytosol through overactive NCLX. Here, we hypothesize that cytosolic tau oligomers or tau aggregates have the opposite effect, causing cell death by loss of NCLX function, and that this toxicity is mediated by interference with Mfn2 function. The resulting Ca2+ overload in mitochondria can then cause necrosis by opening MPTP. In this scenario, Mfn2 would provide a bridge between the cytosol, which is where tau aggregates are localized, and NCLX on the inner membrane. The aim of this supplement is therefore to determine whether the toxicity of tau oligomers or aggregates is due to calcium overload in mitochondria and if this overload is caused by interference with Mfn2 and its downstream effector NCLX. The effects of tau oligomers and aggregates on Mfn2/NCLX-mediated Ca2+ release and cell death will first be investigated using an existing cultured cell model for tauopathies (HEK293 sensor cells) and then with increasingly more complex systems (differentiated neurons and Zebrafish) in preparation for submitting an R01 with mouse models. The discovery of this novel pathway for tau-induced toxicity also provides new targets for drug treatment (Mfn2 and NCLX). Pilot experiments with several candidate molecules will be conducted as part of this supplement in preparation for large scale screens for other molecules as this project progresses.