Mitochondrial Stress Disassembles Nuclear Architecture through Proteolytic Activation of PKCδ and Lamin B1 Phosphorylation in Neuronal Cells: Implications for Pathogenesis of Age-related Neurodegenerative Diseases

Mitochondrial dysfunction and oxidative stress are hallmarks of pathophysiological processes in age-related neurodegenerative diseases including Parkinson’s, Alzheimer’s and Huntington’s diseases. Neuronal cells are highly vulnerable to mitochondrial stress, however, the cellular and molecular mechanisms underlying the enhanced vulnerability are not well understood. Previously, we demonstrated that the novel PKC isoform PKCδ is highly expressed in dopamin(DA)ergic neurons and plays a key role in inducing apoptotic cell death during neurotoxic stress via caspase-3-mediated proteolytic activation. Herein, we further uncovered a key downstream molecular event of PKCδ signaling following mitochondrial dysfunction that governs neuronal cell death by dissembling nuclear architecture. Exposing N27 DAergic cell line to the mitochondrial complex-1 inhibitor tebufenpyrad induced PKCδ phosphorylation at the T505 activation loop accompanied by caspase-3-dependent proteolytic activation of the kinase. Subcellular analysis using high-resolution 3D confocal microscopy revealed that proteolytically activated cleaved PKCδ translocates to the nuclear compartment, colocalizing with Lamin B1. Electron microscopy also enabled the visualization of nuclear membrane damage triggered by subjecting the DAergic neuronal cells by Tebufenpyrad (Tebu) toxicity. In silico analyses identified that the threonine site on Lamin B1 (T575) is likely phosphorylated by PKCδ, suggesting that Lamin B1 serves as a key downstream target of the kinase. Interestingly, N27 DAergic cells stably expressing the PKCδ proteolytic cleavage site-resistant mutant failed to induce nuclear damage, PKCδ activation, and Lamin B1 phosphorylation. Furthermore, CRISPR/Cas9-based stable knockdown of PKCδ greatly attenuated Tebu-induced Lamin B1 phosphorylation. Also, studies using Lamin B1 ^T575G mutated at phosphorylation and PKCδ-ΔNLS-overexpressing N27 cells showed that PKCδ activation and translocation to the nuclear membrane are critically required for phosphorylating Lamin B1 at T575 to induce nuclear membrane damage during Tebu insult. Additionally, Tebu failed to induce Lamin B1 damage and Lamin B1 phosphorylation in organotypic midbrain slices cultured from PKCδ ^-/- mouse pups. More importantly, we observed higher PKCδ activation, Lamin B1 phosphorylation and Lamin B1 loss in nigral DAergic neurons from the postmortem brains of PD patients as compared to age-matched healthy control brains, thus providing translational relevance of our finding. Collectively, our data reveal that PKCδ functions as a Lamin B1 kinase to disassemble the nuclear membrane during the neuronal cell death process triggered by mitochondrial stress. This mechanistic insight may have important implications for the etiology of age-related neurodegenerative diseases resulting from mitochondrial dysfunction as well as for the development of novel treatment strategies.