Regulation of mitochondrial fission and fusion in neuronal injury

Abstract

Mitochondria are dynamic organelles meaning that they constantly fuse with each other, divide and move. The creation of such dynamic nature of mitochondria seems to be very purposeful. For example mitochondrial fusion is identified as a repair mechanism to dilate damaged mitochondria while mitochondrial fission results in the mitochondrial biogenesis and generates smaller mass mitochondria that can move faster to strategic locations within cellular compartments. In mammals, Mitofusin 2 (Mfn2) an outer mitochondrial membranes (OMM) protein and Optic atrophy 1 (Opal), found in the inter mitochondrial space in association with the inner mitochondrial membrane (IMM), regulate OMM and IMM fusion, respectively. In non neuronal cells, the key components of mitochondrial dynamics have been linked to the regulation of cell death induced by apoptotic signaling. Neurons possess unique morphological complexities and undergo cell death by distinct mechanisms which are far more complicated than just apoptosis. A fundamental question that has arisen from the previous studies is whether mitochondrial fission and fusion machineries impact neuronal survival and function. The first goal of my PhD thesis has been to tackle the important questions of whether mitochondrial morphology defects are associated with neuronal demise and whether components of mitochondrial fusion can rescue neuronal loss in physiologically relevant models. The results of my studies have culminated in a number of key findings. First, mitochondrial morphological defects have been identified as early events following different modes of neuronal injury such as DNA damage (induced by camptothecin), oxidative stress (induced by H2O2) and calcium toxicity (induced by overactivation of glutamate receptors). While mitochondrial fission contributes to the dramatic mitochondrial fragmentation following neuronal death, a defective mitochondrial fusion and loss of IMM integrity are identified as two of the major mechanisms contributing to the mitochondrial dysfunction and neuronal demise. Second, the fusion proteins Mfn2 and Opal are shown to confer neuroprotection in response to multiple cell death stimuli. Mfn2 was identified as an antiapoptotic protein that functions upstream of cytochrome c release to attenuate neuronal loss, whereas Opal functions at the IMM level to maintain mitochondrial cristae morphology. Third, mitochondrial remodeling as a result of loss of Opa1 oligomers is identified as hallmarks of excitotoxic neuronal injury. Opa1 is essential for neuroprotection by inhibition of calpain (a calcium activated protease) as calpastatin, an endogenous inhibitor of calpains, fails to protect against excitotoxicity following Opa1 knockdown. Our findings are the first to identify Opa1 as a key regulator of neuronal fate following calcium deregulation associated with excitotoxic neuronal injury. The second goal of my PhD thesis has been to study the regulation of mitochondrial fission in post mitotic neurons. In mammals, Dynamin related protein 1 (Drp1), a cytosolic protein, is recruited to the mitochondrial OMM to induce mitochondrial fission. Here, a novel mechanism has been identified for the regulation of Drp1 recruitment and mitochondrial fission. A physical and functional interaction has been documented between Drp1 and cyclin dependent kinase 5 (Cdk5), an important regulator of neuronal plasticity and neuronal loss. Cdk5 phosphorylates Drp1 at a conserved serine residue (Ser585) and results in Drp1 recruitment from the cytoplasm to the mitochondria to induce its fission. These findings suggest a regulatory mechanism through which Cdk5 performs its multiple functions during neuronal development and disease through modulation of mitochondrial shape. In conclusion, this research identifies a missing link between mitochondrial fission and neuronal development and disease through a Drp1-Cdk5 cross talk. These findings have broad implications for reassessment of fundamental concepts in neurobiology such as synaptic plasticity and strength, axonal growth, neuronal demise and injury associated mitochondrial dysfunction from a different angle. A the same time this research offers the components of the mitochondrial fusion machinery as promising targets in rescuing neuronal loss associated with a wide range of neurological disorder.