ABSTRACT
BACKGROUND: Proteolysis of huntingtin (Htt) plays a key role in the pathogenesis of Huntington's disease (HD). However, the environmental cues and signaling pathways that regulate Htt proteolysis are poorly understood. One stimulus may be the DNA damage that accumulates in neurons over time, and the subsequent activation of signaling pathways such as those regulated by IkappaB kinase (IKK), which can influence neurodegeneration in HD. METHODOLOGY/PRINCIPAL FINDINGS: We asked whether DNA damage induces the proteolysis of Htt and if activation of IKK plays a role. We report that treatment of neurons with the DNA damaging agent etoposide or gamma-irradiation promotes cleavage of wild type (WT) and mutant Htt, generating N-terminal fragments of 80-90 kDa. This event requires IKKbeta and is suppressed by IKKalpha. Elevated levels of IKKalpha, or inhibition of IKKbeta expression by a specific small hairpin RNA (shRNA) or its activity by sodium salicylate, prevents Htt proteolysis and increases neuronal resistance to DNA damage. Moreover, IKKbeta phosphorylates the anti-apoptotic protein Bcl-xL, a modification known to reduce Bcl-xL levels, and activates caspases that can cleave Htt. When IKKbeta expression is blocked, etoposide treatment does not decrease Bcl-xL and activation of caspases is diminished. Similar to silencing of IKKbeta, increasing the level of Bcl-xL in neurons prevents etoposide-induced caspase activation and Htt proteolysis. CONCLUSIONS/SIGNIFICANCE: These results indicate that DNA damage triggers cleavage of Htt and identify IKKbeta as a prominent regulator. Moreover, IKKbeta-dependent reduction of Bcl-xL is important in this process. Thus, inhibition of IKKbeta may promote neuronal survival in HD as well as other DNA damage-induced neurodegenerative disorders.
Subject(s)
DNA Damage , I-kappa B Kinase/metabolism , I-kappa B Kinase/physiology , Nerve Tissue Proteins/metabolism , Nuclear Proteins/metabolism , Animals , Caspases/metabolism , Etoposide/pharmacology , Humans , Huntingtin Protein , Mice , Models, Biological , Mutation , Neurodegenerative Diseases/pathology , Neurons/metabolism , Oligonucleotides/chemistry , Signal Transduction , bcl-X Protein/metabolismABSTRACT
Human embryonic stem cells (hESCs) have been proposed as a source of dopamine (DA) neurons for transplantation in Parkinson's disease (PD). We have investigated the effect of in vitro predifferentiation on in vivo survival and differentiation of hESCs implanted into the 6-OHDA (6-hydroxydopamine)-lesion rat model of PD. The hESCs were cocultured with PA6 cells for 16, 20, or 23 days, leading to the in vitro differentiation into DA neurons. Grafted hESC-derived cells survived well and expressed neuronal markers. However, very few exhibited a DA neuron phenotype. Reversal of lesion-induced motor deficits was not observed. Rats grafted with hESCs predifferentiated in vitro for 16 days developed severe teratomas, whereas most rats grafted with hESCs predifferentiated for 20 and 23 days remained healthy until the end of the experiment. This indicates that prolonged in vitro differentiation of hESCs is essential for preventing formation of teratomas.
