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Full length mutant huntingtin is required for altered Ca2+ signaling and apoptosis of striatal neurons in the YAC mouse model of Huntington's disease.

ABSTRACT: Huntington's disease (HD) is caused by a progressive loss of striatal medium spiny neurons (MSN). The molecular trigger of HD is a polyglutamine expansion in the Huntingtin protein (Htt). The mutant Htt protein forms insoluble nuclear aggregates which have been proposed to play a key role in causing neuronal cell death in HD. Other lines of investigation suggest that expression of mutant Htt facilitates activity of the NR2B subtype of NMDA receptors and the type 1 inositol 1,4,5-trisphosphate receptors (InsP(3)R1), and that disturbed calcium (Ca(2+)) signaling causes apoptosis of MSNs in HD. The YAC128 transgenic HD mouse model expresses the full-length human Htt protein with 120Q CAG repeat expansion and displays an age-dependent loss of striatal neurons as seen in human HD brain. In contrast, the shortstop mice express an amino-terminal fragment of the mutant Htt protein (exons 1 and 2) and display no behavioral abnormalities or striatal neurodegeneration despite widespread formation of neuronal inclusions. Here we compared Ca(2+) signals in primary MSN neuronal cultures derived from YAC128 and shortstop mice to their wild-type non-transgenic littermates. Repetitive application of glutamate results in supranormal Ca(2+) responses in YAC128 MSNs, but not in shortstop MSNs. In addition, while currents mediated by the NR2B subtype of NMDA receptors were increased in YAC128 MSNs, currents in SS MSNs were found to be similar to WT. Furthermore, YAC128 MSNs were sensitized to glutamate-induced apoptosis. Consistent with these findings, we found that application of glutamate induced rapid loss of mitochondrial membrane potential in YAC128 MSNs. In contrast, SS MSNs do not show increased cell death postglutamate treatment nor accelerated loss of mitochondrial membrane potential following glutamate stimulation. Glutamate-induced loss of mitochondrial membrane potential in YAC128 MSNs could be prevented by inhibitors of NR2B NMDA receptors and mGluR1/5 receptors. Our results are consistent with the hypothesis that disturbed neuronal Ca(2+) signaling plays a significant role in the degeneration of MSN containing full-length mutant Htt(exp). Furthermore, the results obtained with neurons from shortstop mice provide additional evidence that not all fragments of mutant Htt(exp) are toxic to neurons.

SUBMITTER: Zhang H

PROVIDER: S-EPMC2528878 | biostudies-literature | 2008 Jul

REPOSITORIES: biostudies-literature

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Full length mutant huntingtin is required for altered Ca2+ signaling and apoptosis of striatal neurons in the YAC mouse model of Huntington's disease.

Zhang Hua H Li Qin Q Graham Rona K RK Slow Elizabeth E Hayden Michael R MR Bezprozvanny Ilya I

Neurobiology of disease 20080416 1

Huntington's disease (HD) is caused by a progressive loss of striatal medium spiny neurons (MSN). The molecular trigger of HD is a polyglutamine expansion in the Huntingtin protein (Htt). The mutant Htt protein forms insoluble nuclear aggregates which have been proposed to play a key role in causing neuronal cell death in HD. Other lines of investigation suggest that expression of mutant Htt facilitates activity of the NR2B subtype of NMDA receptors and the type 1 inositol 1,4,5-trisphosphate re ...[more]

PMID: 18502655

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Project description:Huntington disease (HD) is an inherited neurodegenerative disease resulting from an abnormal expansion of polyglutamine in huntingtin (Htt). Compromised oxidative stress defense systems have emerged as a contributing factor to the pathogenesis of HD. Indeed activation of the Nrf2 pathway, which plays a prominent role in mediating antioxidant responses, has been considered as a therapeutic strategy for the treatment of HD. Given the fact that there is an interrelationship between impairments in mitochondrial dynamics and increased oxidative stress, in this present study we examined the effect of mutant Htt (mHtt) on these two parameters. STHdh(Q111/Q111) cells, striatal cells expressing mHtt, display more fragmented mitochondria compared to STHdh(Q7/Q7) cells, striatal cells expressing wild type Htt, concurrent with alterations in the expression levels of Drp1 and Opa1, key regulators of mitochondrial fission and fusion, respectively. Studies of mitochondrial dynamics using cell fusion and mitochondrial targeted photo-switchable Dendra revealed that mitochondrial fusion is significantly decreased in STHdh(Q111/Q111) cells. Oxidative stress leads to dramatic increases in the number of STHdh(Q111/Q111) cells containing swollen mitochondria, while STHdh(Q7/Q7) cells just show increases in the number of fragmented mitochondria. mHtt expression results in reduced activity of Nrf2, and activation of the Nrf2 pathway by the oxidant tBHQ is significantly impaired in STHdh(Q111/Q111) cells. Nrf2 expression does not differ between the two cell types, but STHdh(Q111/Q111) cells show reduced expression of Keap1 and p62, key modulators of Nrf2 signaling. In addition, STHdh(Q111/Q111) cells exhibit increases in autophagy, whereas the basal level of autophagy activation is low in STHdh(Q7/Q7) cells. These results suggest that mHtt disrupts Nrf2 signaling which contributes to impaired mitochondrial dynamics and may enhance susceptibility to oxidative stress in STHdh(Q111/Q111) cells.

Project description:The neurobiological functions of a number of kinases expressed in the brain are unknown. Here, we report new findings on DCLK3 (doublecortin like kinase 3), which is preferentially expressed in neurons in the striatum and dentate gyrus. Its function has never been investigated. DCLK3 expression is markedly reduced in Huntington's disease. Recent data obtained in studies related to cancer suggest DCLK3 could have an anti-apoptotic effect. Thus, we hypothesized that early loss of DCLK3 in Huntington's disease may render striatal neurons more susceptible to mutant huntingtin (mHtt). We discovered that DCLK3 silencing in the striatum of mice exacerbated the toxicity of an N-terminal fragment of mHtt. Conversely, overexpression of DCLK3 reduced neurodegeneration produced by mHtt. DCLK3 also produced beneficial effects on motor symptoms in a knock-in mouse model of Huntington's disease. Using different mutants of DCLK3, we found that the kinase activity of the protein plays a key role in neuroprotection. To investigate the potential mechanisms underlying DCLK3 effects, we studied the transcriptional changes produced by the kinase domain in human striatal neurons in culture. Results show that DCLK3 regulates in a kinase-dependent manner the expression of many genes involved in transcription regulation and nucleosome/chromatin remodelling. Consistent with this, histological evaluation showed DCLK3 is present in the nucleus of striatal neurons and, protein-protein interaction experiments suggested that the kinase domain interacts with zinc finger proteins, including the transcriptional activator adaptor TADA3, a core component of the Spt-ada-Gcn5 acetyltransferase (SAGA) complex which links histone acetylation to the transcription machinery. Our novel findings suggest that the presence of DCLK3 in striatal neurons may play a key role in transcription regulation and chromatin remodelling in these brain cells, and show that reduced expression of the kinase in Huntington's disease could render the striatum highly vulnerable to neurodegeneration.

Dataset Information

Full length mutant huntingtin is required for altered Ca2+ signaling and apoptosis of striatal neurons in the YAC mouse model of Huntington's disease.

Publications

Full length mutant huntingtin is required for altered Ca2+ signaling and apoptosis of striatal neurons in the YAC mouse model of Huntington's disease.

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