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The role of alphaB-crystallin in preventing hypertonic stress was investigated through transfection studies. Two cell lines, the U-251 MG human glioma cell line, and NIH/3T3 fibroblasts were transfected in plasmids and analyzed for alphaB-crystallin expression. The transfected NIH/3T3 fibroblasts overexpressed the heat shock protein HSP27. This contrasts to normal NIH/3T3 fibroblasts which express little or no HSP. This suggests that alphaB-crystallin interacts with cell-specific mechanisms to increase survival during osmotic stress.

Abstract Background The mutation in Huntington's disease is a polyglutamine expansion near the N-terminus of huntingtin. Huntingtin expressed in immortalized neurons is cleaved near the N-terminus to form N-terminal polypeptides known as cleavage products A and B (cpA and cpB). CpA and cpB with polyglutamine expansion form inclusions in the nucleus and cytoplasm, respectively. The formation of cpA and cpB in primary neurons has not been established and the proteases involved in the formation of these fragments are unknown. Results Delivery of htt cDNA into the mouse striatum using adeno-associated virus or into primary cortical neurons using lentivirus generated cpA and cpB, indicating that neurons in brain and in vitro can form these fragments. A screen of small molecule protease inhibitors introduced to clonal striatal X57 cells and HeLa cells identified compounds that reduced levels of cpA and are inhibitors of the aspartyl proteases cathepsin D and cathepsin E. The most effective compound, P1-N031, is a transition state mimetic for aspartyl proteases. By western blot analysis, cathepsin D was easily detected in clonal striatal X57 cells, mouse brain and primary neurons, whereas cathepsin E was only detectible in clonal striatal X57 cells. In primary neurons, levels of cleavage product A were not changed by the same compounds that were effective in clonal striatal cells or by mRNA silencing to partially reduce levels of cathepsin D. Instead, treating primary neurons with compounds that are known to inhibit gamma secretase activity either indirectly (Imatinib mesylate, Gleevec) or selectively (LY-411,575 or DAPT) reduced levels of cpA. LY-411,575 or DAPT also increased survival of primary neurons expressing endogenous full-length mutant huntingtin. Conclusion We show that cpA and cpB are produced from a larger huntingtin fragment in vivo in mouse brain and in primary neuron cultures. The aspartyl protease involved in forming cpA has cathepsin-D like properties in immortalized neurons and gamma secretase-like properties in primary neurons, suggesting that cell type may be a critical factor that specifies the aspartyl protease responsible for cpA. Since gamma secretase inhibitors were also protective in primary neurons, further study of the role of gamma-secretase activity in HD neurons is justified.

Oxidative stress contributes to neurodegeneration in Huntington's disease (HD). However, the origins of oxidative stress in HD remain unclear. Studies in HD transgenic models suggest involvement of mitochondrial dysfunction, which would lead to overproduction of reactive oxygen species (ROS). Impaired mitochondria complexes occur in late stages of HD but not in presymptomatic or early-stageHD patients. Thus, other mechanisms may account for the earliest source of oxidative stress caused by endogenous mutant huntingtin. Here, we report that decreased levels of a major intracellular antioxidant glutathione coincide with accumulation of ROS in primary HD neurons prepared from embryos of HDknock-in mice (HD140Q/140Q), which have human huntingtin exon 1 with 140 CAG repeats inserted into the endogenous mouse huntingtin gene. Uptake of extracellular cysteine through the glutamate/cysteine transporter EAAC1 is required for de novo synthesis of glutathione in neurons. We found that, compared with wild-type neurons, HD neurons had lower cell surface levels of EAAC1 and were deficient in taking up cysteine. Constitutive trafficking of EAAC1 from recycling endosomes relies on Rab11 activity, which is defective in the brain of HD140Q/140Q mice. Enhancement of Rab11 activity by expression of a dominant-active Rab11 mutant in primary HD neurons ameliorated the deficit in cysteine uptake, increased levels of intracellular glutathione, normalized clearance of ROS, and improved neuronal survival. Our data support a novel mechanism for oxidative stress in HD: Rab11 dysfunction slows trafficking of EAAC1 to the cell surface and impairs cysteine uptake, thereby leading to deficient synthesis of glutathione. [ABSTRACT FROM AUTHOR]

Polyglutamine expansion in the N terminus of huntingtin (htt) causes selective neuronal dysfunction and cell death by unknown mechanisms. Truncated htt expressed in vitro produced htt immunoreactive cytoplasmic bodies (htt bodies). The fibrillar core of the mutant htt body resisted protease treatment and contained cathepsin D, ubiquitin, and heat shock protein (HSP) 40. The shell of the htt body was composed of globules 14-34 nm in diameter and was protease sensitive. HSP70, proteasome, dynamin, and the htt binding partners htt interacting protein 1 (HIP1), SH3-containing Grb2-like protein (SH3GL3), and 14.7K-interacting protein were reduced in their normal location and redistributed to the shell. Removal of a series of prolines adjacent to the polyglutamine region in htt blocked formation of the shell of the htt body and redistribution of dynamin, HIP1, SH3GL3, and proteasome to it. Internalization of transferrin was impaired in cells that formed htt bodies. In cortical neurons of Huntington's disease patients with early stage pathology, dynamin immunoreactivity accumulated in cytoplasmic bodies. Results suggest that accumulation of a nonfibrillar form of mutant htt in the cytoplasm contributes to neuronal dysfunction by sequestering proteins involved in vesicle trafficking. [ABSTRACT FROM AUTHOR]

Huntingtin (Htt) localizes to endosomes, but its role in the endocytic pathway is not established. Recently, we found that Htt is important for the activation of Rab11, a GTPase involved in endosomal recycling. Here we studied fibroblasts of healthy individuals and patients with Huntington's disease (HD), which is a movement disorder caused by polyglutamine expansion in Htt. The formation of endocytic vesicles containing transferrin at plasma membranes was the same in control and HD patient fibroblasts.However, HD fibroblasts were delayed in recycling biotin-transferrin back to the plasma membrane. Membranes of HD fibroblasts supported less nucleotide exchange on Rab11 than did control membranes. Rab11-positive vesicular and tubular structures in HD fibroblasts were abnormally large, suggesting that they were impaired in forming vesicles. We used total internal reflection fluorescence imaging of living fibroblasts to monitor fluorescence-labeled transferrin-carrying transport intermediates that emerged from recycling endosomes. HD fibroblasts had fewer small vesicles and more large vesicles and long tubules than did control fibroblasts. Dominant active Rab11 expressed in HD fibroblasts normalized the recycling of biotin-transferrin. We propose a novel mechanism for cellular dysfunction by the HD mutation arising from the inhibition of Rab11 activity and a deficit in vesicle formation at recycling endosomes. [ABSTRACT FROM AUTHOR]

Oxidative stress contributes to neurodegeneration in Huntington's disease (HD). However, the origins of oxidative stress in HD remain unclear. Studies in HD transgenic models suggest involvement of mitochondrial dysfunction, which would lead to overproduction of reactive oxygen species (ROS). Impaired mitochondria complexes occur in late stages of HD but not in presymptomatic or early-stage HD patients. Thus, other mechanisms may account for the earliest source of oxidative stress caused by endogenous mutant huntingtin. Here, we report that decreased levels of a major intracellular antioxidant glutathione coincide with accumulation of ROS in primary HD neurons prepared from embryos of HD knock-in mice (HD(140Q/140Q)), which have human huntingtin exon 1 with 140 CAG repeats inserted into the endogenous mouse huntingtin gene. Uptake of extracellular cysteine through the glutamate/cysteine transporter EAAC1 is required for de novo synthesis of glutathione in neurons. We found that, compared with wild-type neurons, HD neurons had lower cell surface levels of EAAC1 and were deficient in taking up cysteine. Constitutive trafficking of EAAC1 from recycling endosomes relies on Rab11 activity, which is defective in the brain of HD(140Q/140Q) mice. Enhancement of Rab11 activity by expression of a dominant-active Rab11 mutant in primary HD neurons ameliorated the deficit in cysteine uptake, increased levels of intracellular glutathione, normalized clearance of ROS, and improved neuronal survival. Our data support a novel mechanism for oxidative stress in HD: Rab11 dysfunction slows trafficking of EAAC1 to the cell surface and impairs cysteine uptake, thereby leading to deficient synthesis of glutathione.

Patients with Huntington's disease have an expanded polyglutamine tract in huntingtin and suffer severe brain atrophy and neurodegeneration. Because membrane dysfunction can occur in Huntington's disease, we addressed whether mutant huntingtin in brain and primary neurons is present in lipid rafts, which are cholesterol-enriched membrane domains that mediate growth and survival signals. Biochemical analysis of detergent-resistant membranes from brains and primary neurons of wild-type and presymptomatic Huntington's disease knock-in mice showed that wild-type and mutant huntingtin were recovered in lipid raft-enriched detergent-resistant membranes. The association with lipid rafts was stronger for mutant huntingtin than wild-type huntingtin. Lipid rafts extracted from Huntington's disease mice had normal levels of lipid raft markers (G(alphaq), Ras, and flotillin) but significantly more glycogen synthase kinase 3-beta. Increases in glycogen synthase kinase 3-beta have been associated with apoptotic cell death. Treating Huntington's disease primary neurons with inhibitors of glycogen synthase kinase 3-beta reduced neuronal death. We speculate that accumulation of mutant huntingtin and glycogen synthase kinase 3-beta in lipid rafts of presymptomatic Huntington's disease mouse neurons contributes to neurodegeneration in Huntington's disease.

Polyglutamine expansion in the N terminus of huntingtin (htt) causes selective neuronal dysfunction and cell death by unknown mechanisms. Truncated htt expressed in vitro produced htt immunoreactive cytoplasmic bodies (htt bodies). The fibrillar core of the mutant htt body resisted protease treatment and contained cathepsin D, ubiquitin, and heat shock protein (HSP) 40. The shell of the htt body was composed of globules 14-34 nm in diameter and was protease sensitive. HSP70, proteasome, dynamin, and the htt binding partners htt interacting protein 1 (HIP1), SH3-containing Grb2-like protein (SH3GL3), and 14.7K-interacting protein were reduced in their normal location and redistributed to the shell. Removal of a series of prolines adjacent to the polyglutamine region in htt blocked formation of the shell of the htt body and redistribution of dynamin, HIP1, SH3GL3, and proteasome to it. Internalization of transferrin was impaired in cells that formed htt bodies. In cortical neurons of Huntington's disease patients with early stage pathology, dynamin immunoreactivity accumulated in cytoplasmic bodies. Results suggest that accumulation of a nonfibrillar form of mutant htt in the cytoplasm contributes to neuronal dysfunction by sequestering proteins involved in vesicle trafficking.