Metabolic State Determines Sensitivity to Cellular Stress in Huntington Disease: Normalization by Activation of PPARγ

Impairments in mitochondria and transcription are important factors in the pathogenesis of Huntington disease (HD), a neurodegenerative disease caused by a polyglutamine expansion in the huntingtin protein. This study investigated the effect of different metabolic states and peroxisome proliferator-activated receptor γ (PPARγ) activation on sensitivity to cellular stressors such as H2O2 or thapsigargin in HD. Striatal precursor cells expressing wild type (STHdhQ7) or mutant huntingtin (STHdhQ111) were prepared in different metabolic conditions (glucose vs. pyruvate). Due to the fact that STHdhQ111 cells exhibit mitochondrial deficits, we expected that in the pyruvate condition, where ATP is generated primarily by the mitochondria, there would be greater differences in cell death between the two cell types compared to the glucose condition. Intriguingly, it was the glucose condition that gave rise to greater differences in cell death. In the glucose condition, thapsigargin treatment resulted in a more rapid loss of mitochondrial membrane potential (ΔΨm), a greater activation of caspases (3, 8, and 9), and a significant increase in superoxide/reactive oxygen species (ROS) in STHdhQ111 compared to STHdhQ7, while both cell types showed similar kinetics of ΔΨm-loss and similar levels of superoxide/ROS in the pyruvate condition. This suggests that bioenergetic deficiencies are not the primary contributor to the enhanced sensitivity of STHdhQ111 cells to stressors compared to the STHdhQ7 cells. PPARγ activation significantly attenuated thapsigargin-induced cell death, concomitant with an inhibition of caspase activation, a delay in ΔΨm loss, and a reduction of superoxide/ROS generation in STHdhQ111 cells. Expression of mutant huntingtin in primary neurons induced superoxide/ROS, an effect that was significantly reduced by constitutively active PPARγ. These results provide significant insight into the bioenergetic disturbances in HD with PPARγ being a potential therapeutic target for HD

DISC1 and Huntington's disease-overlapping pathways of vulnerability to neurological disorder?

We re-annotated the interacting partners of the neuronal scaffold protein DISC1 using a knowledge-based approach that incorporated recent protein interaction data and published literature to. This revealed two highly connected networks. These networks feature cellular function and maintenance, and cell signaling. Of potentially greatest interest was the novel finding of a high degree of connectivity between the DISC1 scaffold protein, linked to psychiatric illness, and huntingtin, the protein which is mutated in Huntington's disease. The potential link between DISC1, huntingtin and their interacting partners may open new areas of research into the effects of pathway dysregulation in severe neurological disorders

Immunohistochemical localization of TRPC6 in the rat substantia nigra.

Transient receptor potential channels (TRPC) are plasma membrane, nonselective cationic channels and have been proposed as candidates involved in the regulation of cellular Ca2+ influx [D.E. Clapham, L.W. Runnels, C., Strubing, The TRP ion channel family, Nat. Rev. Neurosci. 2 (2001) 387-396; A. Martorana, C. Giampa, Z. DeMarch, M.T. Viscomi, S. Patassini, G. Sancesario, G. Bernardi, F.R. Fusco, Distribution of TRPC1 receptors in dendrites of rat substantia nigra: a confocal and electron microscopy study, Eur. J. Neurosci. 24 (2006) 732-738]. Studies on regional localization patterns of TRPCs are necessary to provide helpful guidelines for correlating current types with particular channels. In this study, we examined the distribution of one particular member of TRPC superfamily, namely, TRPC6, in the substantia nigra of normal rat brain. Single and double label immunohistochemistry were employed to perform both light and confocal microscopy observations. Our single label studies showed that, in the substantia nigra, TRPC6 labeled the perikarya with a diffuse and intense immunoreaction product distributed throughout cell cytoplasm whereas only a light immunostaining was observed in the cell nuclei. No labeling of axon or terminals was observed, although TRPC6 was evenly distributed in the neuropil. Our dual label studies showed a TRPC6 immunoreactivity pattern that was localized into the proximal dendrites and axon hillock of the large dopaminergic neurons identified by TH immunoreaction. Furthermore, our double label immunofluorescence study for TRPC6 and mGluR1 showed a complete co-localization of the two markers in the substantia nigra. Moreover, TRPC6 did not co-localize with synaptophysin. Thus, our study shows the postsynaptic localization of TRPC6 and its association with mGluR1 in the midbrain dopamine neurons

Striatal modulation of cAMP-response-element-binding protein (CREB) after excitotoxic lesions: implications with neuronal vulnerability in Huntington's disease

Recent evidence has shown that the activity of cAMP responsive element-binding protein (CREB) and of CREB-binding protein (CBP) is decreased in Huntington's disease (HD) [Steffan et al. (2000)Proc. Natl Acad. Sci. USA, 97, 6763-6768; Gines et al. (2003)Hum. Mol. Genet., 12, 497-508; Rouaux et al. (2004) Biochem. Pharmacol., 68, 1157-1164; Sugars et al. (2004)J. Biol. Chem., 279, 4988-4999]. Such decrease is thought to reflect the impaired energy metabolism observed in a HD mouse model, where a decline in striatum cAMP levels has been observed [Gines et al. (2003)Hum. Mol. Genet., 12, 497-508]. Increased levels of CREB have also been demonstrated to exert neuroprotective functions [Lonze & Ginty (2002)Neuron, 35, 605-623; Lonze et al. (2002)Neuron, 34, 371-385]. Our study aimed to investigate the distribution of CREB in the neuronal subpopulations of the striatum in normal rats compared to the HD model of quinolinic acid lesion. Twenty-five Wistar rats were administered quinolinic acid 100 mm into the right striatum, and killed after 24 h, 48 h, 1 week, 2 weeks, and six weeks, respectively. The contralateral striata were used as controls. Dual-label immunofluorescence was employed using antibodies against phosphorylated CREB and each of the different neuronal subpopulations markers. Our results show that activated CREB levels decrease progressively in projection neurons and parvalbumin (PARV) and calretinin (CALR) interneurons, whereas such levels remain stable in cholinergic and somatostatin interneurons. Thus, we speculate that the ability of cholinergic interneurons to maintain their levels of CREB after excitotoxic lesions is one of the factors determining their protection in Huntington's disease

Distribution of TRPC1 receptors in dendrites of rat substantia nigra: a confocal and electron microscopy study.

Transient receptor potential channels (TRPC) are plasma membrane, non-selective cationic channels and have been proposed as candidates involved in the regulation of cellular Ca2+ influx. The expression, at mRNA level, of several TRPCs has been demonstrated recently in dopaminergic neurons of the substantia nigra (SN). The aim of the present study was to characterize the expression of TRPC1, at a protein level, in the substantia nigra neurons and non-excitable cells of Wistar rats. Single-label immunohistochemistry and double-label immunofluorescence were used to study the expression of TRPC1 among substantia nigra dopamine neurons and cellular processes using antibodies against tyrosine hydroxylase (TH), substance P (SP), enkephalin, synaptophysin, vesicular glutamate transporter-2 (Vglut-2), microtubule associated protein-2 and metabotropic glutamate receptor 1 (mGluR1). Moreover, the ultrastructural localization of TRPC1 was investigated by means of electron microscopy. A set of dual label experiments was also performed to investigate the presence of TRPC1 among glial cells. Our results showed that TRPC1 is localized mainly in dendritic processes of dopamine neurons, whereas a relatively small percentage of neuronal somata display a light TRPC1 immunoreactivity. Such results were confirmed by our electron microscopy observations. Our study demonstrates, for the first time, a coexpression of TRPC1 and mGluR1 receptors in dendrites of the substantia nigra dopaminergic neurons. Such observation reinforces the concept of an involvement of TRPC1 in mGluR1-mediated excitatory inputs in rat dopamine neurons

Striatal modulation of cAMP-response-element-binding protein (CREB) after excitotoxic lesions: implications with neuronal vulnerability in Huntington's disease.

Recent evidence has shown that the activity of cAMP responsive element-binding protein (CREB) and of CREB-binding protein (CBP) is decreased in Huntington's disease (HD) [Steffan et al. (2000)Proc. Natl Acad. Sci. USA, 97, 6763-6768; Gines et al. (2003)Hum. Mol. Genet., 12, 497-508; Rouaux et al. (2004) Biochem. Pharmacol., 68, 1157-1164; Sugars et al. (2004)J. Biol. Chem., 279, 4988-4999]. Such decrease is thought to reflect the impaired energy metabolism observed in a HD mouse model, where a decline in striatum cAMP levels has been observed [Gines et al. (2003)Hum. Mol. Genet., 12, 497-508]. Increased levels of CREB have also been demonstrated to exert neuroprotective functions [Lonze & Ginty (2002)Neuron, 35, 605-623; Lonze et al. (2002)Neuron, 34, 371-385]. Our study aimed to investigate the distribution of CREB in the neuronal subpopulations of the striatum in normal rats compared to the HD model of quinolinic acid lesion. Twenty-five Wistar rats were administered quinolinic acid 100 mm into the right striatum, and killed after 24 h, 48 h, 1 week, 2 weeks, and six weeks, respectively. The contralateral striata were used as controls. Dual-label immunofluorescence was employed using antibodies against phosphorylated CREB and each of the different neuronal subpopulations markers. Our results show that activated CREB levels decrease progressively in projection neurons and parvalbumin (PARV) and calretinin (CALR) interneurons, whereas such levels remain stable in cholinergic and somatostatin interneurons. Thus, we speculate that the ability of cholinergic interneurons to maintain their levels of CREB after excitotoxic lesions is one of the factors determining their protection in Huntington's disease

Distribution of TRPC1 receptors in dendrites of rat substantia nigra: a confocal and electron microscopy study.

Transient receptor potential channels (TRPC) are plasma membrane, non-selective cationic channels and have been proposed as candidates involved in the regulation of cellular Ca2+ influx. The expression, at mRNA level, of several TRPCs has been demonstrated recently in dopaminergic neurons of the substantia nigra (SN). The aim of the present study was to characterize the expression of TRPC1, at a protein level, in the substantia nigra neurons and non-excitable cells of Wistar rats. Single-label immunohistochemistry and double-label immunofluorescence were used to study the expression of TRPC1 among substantia nigra dopamine neurons and cellular processes using antibodies against tyrosine hydroxylase (TH), substance P (SP), enkephalin, synaptophysin, vesicular glutamate transporter-2 (Vglut-2), microtubule associated protein-2 and metabotropic glutamate receptor 1 (mGluR1). Moreover, the ultrastructural localization of TRPC1 was investigated by means of electron microscopy. A set of dual label experiments was also performed to investigate the presence of TRPC1 among glial cells. Our results showed that TRPC1 is localized mainly in dendritic processes of dopamine neurons, whereas a relatively small percentage of neuronal somata display a light TRPC1 immunoreactivity. Such results were confirmed by our electron microscopy observations. Our study demonstrates, for the first time, a coexpression of TRPC1 and mGluR1 receptors in dendrites of the substantia nigra dopaminergic neurons. Such observation reinforces the concept of an involvement of TRPC1 in mGluR1-mediated excitatory inputs in rat dopamine neurons