Mitochondrial homeostasis, dendritic remodeling and autophagy in neurodegeneration.
A. Julio Martinez (Endowed) Chair in Neuropathology
Dr. Chu's research focuses on mechanisms of neurodegeneration and neuroprotection in Parkinson's (PD) and related neurodegenerative diseases. A major focus is delineating why adaptive cellular mechanisms fail to protect neurons, with emphasis on alterations in kinase signaling, mitophagy and mitochondrial biogenesis. Molecular and biochemical studies in neuron culture and mouse models are integrated with studies of diseased human brain tissues. We are currently focused on potentially reversible mechanisms by which mutations in PD-linked genes affect mitochondrial distribution and dendritic extension/synaptogenesis.
Our ultrastructural study of human PD SNc neurons in 2003 was the first to implicate altered mitophagy in PD. We found that parkinsonian stressors act to promote degradation of mitochondria by autophagy, while suppressing mitochondrial biogenesis. ERK1/2, which is activated in human PD neurons, phosphorylates the mitochondrial transcription factor A to reduce its function in mtDNA transcription, and we are characterizing the functional effects of other phosphorylation sites we identified. Mutant LRRK2 and loss of the recessive parkinsonian mitochondrial kinase PINK1 result in dysregulated mitochondrial calcium handling, leading to increased mitophagy and dendrite retraction. Current efforts include live imaging in neurons and in the intact brains of transparent organisms to study the role of aging in PD pathogenesis, and continued study of novel mechanisms by which PINK1 promotes neuronal differentiation and dendritogenesis.
We are also interested in basic mechanisms by which mitochondria signal distress to the rest of the cell. We found that the heavily studied Parkin pathway is not as robust in neurons as in glycolytic cell types. We used mass spectrometry, lipid biochemistry, cell biology, computational modeling and site-directed mutagenesis to establish that cardiolipin, an ancient “bacterial” phospholipid, serves to recruit the autophagic machinery to damaged mitochondria. In response to rotenone or 6-OHDA, there is enzyme-dependent redistribution of cardiolipin from the inner mitochondrial membrane to the outer surface of mitochondria, and cardiolipin thus exposed is bound directly by the autophagy protein LC3. We also identified the first functional phosphorylation site on LC3, and found that PKA phosphorylates this site to reduce LC3 participation in autophagy, protecting against autophagic dendrite degeneration. Thus, autophagy and mitophagy are tightly regulated by brakes and accelerators. We are currently studying the effects of adult onset autophagy deficiency in Pink1 knockout mice.
Trainees in the laboratory will be exposed to imaging, biochemical/proteomic, immunochemical and molecular techniques as applied to primary neuron culture, iPSC-derived neurons and transgenic/knockout mouse models. In addition, we study diseased tissues from patients with PD and Lewy body dementia (LBD). The ability to test predictions in human disease samples has translated to some of our most significant experimental discoveries.
RK Dagda, SJ Cherra III, SM Kulich, A Tandon, D Park & CT Chu. (2009) Loss of PINK1 function promotes mitophagy through effects on oxidative stress and mitochondrial fission. J Biol Chem 284: 13843-13855. F1000 Recommended: 13 Apr 2010 http://f1000.com/prime/2890958#recommendations
SJ Cherra III, SM Kulich, G Uechi, M Balasubramani, J Mountzouris, BW Day & CT Chu. (2010) Regulation of the autophagy protein LC3 by phosphorylation. J Cell Biol 190: 533-539. F1000 Recommended: 21 Sep 2010 http://f1000.com/prime/5189958
CT Chu, J Ji, RK Dagda, JF Jiang, YY Tyurina, AA Kapralov, VA Tyurin, N Yanamala, IH Shrivastava, D Mohammadyani, KZQ Wang, J Zhu, J Klein-Seetharaman, K Balasubramanian, AA Amoscato, G Borisenko, Z Huang, AM Gusdon, A Cheikhi, EK Steer, R Wang, C Baty, S Watkins, I Bahar, H Bayir & VE Kagan (2013) Cardiolipin externalization to the outer mitochondrial membrane acts as an elimination signal for mitophagy in neuronal cells. Nature Cell Biol 15:1197-1205. Featured in Science magazine’s Editors’ Choice, 25 Oct 2013 (Science 342: 403, 2013) F1000 Recommended: 28 Oct 2013; 06 Dec 2013; 12 Dec 2013; 19 Dec 2013; 20 Jan 2014. http://f1000.com/prime/718109987
RK Dagda, I Pien, R Wang, J Zhu, KZQ Wang, J Callio, TD Banerjee, RY Dagda & CT Chu (2014) Beyond the mitochondrion: cytosolic PINK1 remodels dendrites through Protein Kinase A. J Neurochem 128: 864-877. Commentary: http://onlinelibrary.wiley.com/doi/10.1111/jnc.12529
J Neurochem 128: 787-789. 2013 Mark Smith Award Runner-up, International Society for Neurochemistry (ISN).
KZQ Wang, J Zhu, RK Dagda, G Uechi, SJ Cherra III, AM Gusdon, M Balasubramani & CT Chu. (2014) ERK-mediated phosphorylation of TFAM downregulates mitochondrial transcription. Mitochondrion 17: 132-140.
M Kostic, MHR Ludtmann, H Bading, M Hershfinkel, E Steer, CT Chu, AY Abramov & I Sekler. (2015) PKA phosphorylation of NCLX reverses mitochondrial calcium overload and depolarization, promoting survival of PINK1-deficient dopaminergic neurons. Cell Reports 13: 376-386. Featured in Neural Cell News 9.38, 7 Oct 2015.