Highlights d MLKL expression is induced in the Schwann cells of sciatic nerves following injury d MLKL is phosphorylated at serine 441 site in the Schwann cells of injured nerves d The serine 441 phospho-mimic MLKL breaks down myelin sheath in non-injured nerves d MLKL-mediated myelin breakdown promotes functional axon regeneration
Necroptosis is a form of regulated necrosis that is implicated in various human diseases including Alzheimer’s disease. Necroptosis requires the translocation of the pseudokinase MLKL from the cytosol to the plasma membrane after its phosphorylation by the kinase RIPK3. Using protein cross-linking followed by affinity purification, we detected the lipid raft–associated proteins flotillin-1 and flotillin-2 and the ESCRT-associated proteins ALIX and syntenin-1 in membrane-localized MLKL immunoprecipitates. Phosphorylated MLKL was removed from membranes through either flotillin-mediated endocytosis followed by lysosomal degradation or ALIX–syntenin-1–mediated exocytosis. Thus, cells undergoing necroptosis need to overcome these independent suppressive mechanisms before plasma membrane disruption can occur.
In response to apoptotic stimuli, mitochondria in mammalian cells release cytochrome c and other apoptogenic proteins, leading to the subsequent activation of caspases and apoptotic cell death. This process is promoted by the pro-apoptotic members of the Bcl-2 family of proteins, such as Bim and Bax, which, respectively, initiate and execute cytochrome c release from the mitochondria. Here we report the discovery of a small molecule that efficiently blocks Bim-induced apoptosis after Bax is activated on the mitochondria. The cellular target of this small molecule was identified to be the succinate dehydrogenase subunit B (SDHB) protein of complex II of the mitochondrial electron transfer chain (ETC). The molecule protects the integrity of the ETC and allows treated cells to continue to proliferate after apoptosis induction. Moreover, this molecule blocked dopaminergic neuron death and reversed Parkinson-like behavior in a rat model of Parkinson's disease.
Demyelination in the central nervous system (CNS) underlies many human diseases, including multiple sclerosis (MS). We report here the findings of our study of the CNS demyelination process using immune-induced [experimental autoimmune encephalomyelitis (EAE)] and chemical-induced [cuprizone (CPZ)] mouse models of demyelination. We found that necroptosis, a receptor-interacting protein 3 (RIP3) kinase and its substrate mixed lineage kinase domainlike protein (MLKL)-dependent cell death program, played no role in the demyelination process, whereas the MLKL-dependent, RIP3independent function of MLKL in the demyelination process initially discovered in the peripheral nervous system in response to nerve injury, also functions in demyelination in the CNS in these models. Moreover, a receptor-interacting protein 1 (RIP1) kinase inhibitor, RIPA-56, blocked disease progression in the EAE-induced model but showed no effect in the CPZ-induced model. It does so most likely at a step of monocyte elevation downstream of T cell activation and myelin-specific antibody generation, although upstream of breakdown of the blood-brain barrier. RIP1-kinase dead knock-in mice shared a similar result as mice treated with the RIP1 inhibitor. These results indicate that RIP1 kinase inhibitor is a potential therapeutic agent for immune-mediated demyelination diseases that works by prevention of monocyte elevation, a function previously unknown for RIP1 kinase.RIP1 kinase | myelin | demyelination | multiple sclerosis | MLKL M yelin is a lipid-rich (fatty) substance formed by glial cells called oligodendrocytes in the central nervous system (CNS) and by Schwann cells in the peripheral nervous system (PNS) that protects and nourishes neuron axons by wrapping the axons (1). Physiologically, the myelin structure speeds the transmission of electrical impulses called action potentials along myelinated axons by insulating the axon and reducing axonal membrane capacitance (2). The demyelination process occurs during nerve injury and happens in many human diseases, including, for example, in the PNS, Charcot-Marie-Tooth disease (3) and abdominal numbness brought about by diabetes (4), and in the CNS, multiple sclerosis (MS), during which the insulating covers of nerve cells in the brain and spinal cord are damaged by infiltrated immune cells, such as T cells and monocytes (5,6). Patients with MS are paralyzed gradually from the lower body to the upper body and eventually die; most of these patients are young adults ranging in age from their 20s-40s (7).The molecular mechanisms that underlie these demyelination diseases are largely unknown. A recent study indicated that preventing oligodendrocytes from necroptosis, a regulated form of necrotic cell death induced by ligands of the tumor necrosis factor (TNF) receptor family or Toll-like receptors, as well as certain viral infections, could decrease disease symptoms in MS disease models of experimental autoimmune encephalomyelitis (EAE) and a cuprizone (CPZ)-induced demyelination model (8). The TNF re...
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