Complex alteration of NMDA receptors in transgenic Huntington's disease mouse brain: analysis of mRNA and protein expression, plasma membrane association, interacting proteins, and phosphorylation.

We analyzed NMDA receptor subunit mRNAs, proteins, and anchoring proteins in mice transgenic for exon 1 of the HD gene. R6/2 mice had decreased levels of mRNAs encoding epsilon1 and epsilon2 NMDA receptor subunits (mouse orthologs of rat NR2A and NR2B subunits), but not the zeta1 subunit (mouse ortholog of NR1), as assessed by gene expression profiling and Northern blotting. In situ hybridization resolved mRNA decreases spatially to the CA1 field of hippocampus. Western blotting revealed decreases in plasma membrane-associated epsilon1 and epsilon2 subunits in hippocampus, and decreases in plasma membrane-associated zeta1 subunit in cortex and hippocampus. In addition, PSD-95 and alpha-actinin-2, proteins essential for anchoring NMDA receptors, were decreased. Finally, we found a decreased level of tyrosine-phosphorylated epsilon1 subunit, another determinant of NMDA receptor trafficking, in R6/2 hippocampus. Taken together, these data demonstrate multiple levels of NMDA receptor dysregulation, including abnormalities in mRNA expression levels, receptor stoichiometry, protein phosphorylation, and receptor trafficking.

Analysis of cellular, transgenic and human models of Huntington's disease reveals tyrosine hydroxylase alterations and substantia nigra neuropathology.

Huntington's disease (HD) is a progressive, autosomal dominant neurodegenerative disorder that is pathologically characterized by a striatal-specific degeneration. Aberrant dopamine neurotransmission has been proposed as a mechanism underlying the movement disorder of HD. We report that the enzymatic activity of tyrosine hydroxylase (TH), the rate-limiting enzyme for dopamine biosynthesis, is decreased in a transgenic mouse model of HD. In addition, mutant huntingtin was found to disrupt transcription of TH and dopamine beta-hydroxylase (DbetaH) promoter reporter constructs. In situ hybridization revealed extensive loss of TH mRNA and decreased dopaminergic cell size in human HD substantia nigra. TH-immunoreactive protein was reduced in human grade 4 HD substantia nigra by 32% compared to age-matched controls. These findings implicate abnormalities in dopamine neurotransmission in HD and may provide new insights into targets for pharmacotherapy.

Inhibition of tryptophan hydroxylase activity and decreased 5-HT1A receptor binding in a mouse model of Huntington's disease.

The pathogenic mechanisms of the mutant huntingtin protein that cause Huntington's disease (HD) are unknown. Previous studies have reported significant decreases in the levels of serotonin (5-HT) and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the brains of the R6/2 transgenic mouse model of HD. In an attempt to elucidate the cause of these neurochemical perturbations in HD, the protein levels and enzymatic activity of tryptophan hydroxylase (TPH), the rate-limiting enzyme in 5-HT biosynthesis, were determined. Enzyme activity was measured in brainstem homogenates from 4-, 8-, and 12-week-old R6/2 mice and compared with aged-matched wild-type control mice. We observed a 62% decrease in brainstem TPH activity (p = 0.009) in 4-week-old R6/2 mice, well before the onset of behavioral symptoms. In addition, significant decreases in TPH activity were also observed at 8 and 12 weeks of age (61%, p = 0.02 and 86%, p = 0.005, respectively). In the 12-week-old-mice, no change in immunoreactive TPH was observed. In vitro binding showed that TPH does not bind to exon 1 of huntingtin in a polyglutamine-dependent manner. Specifically, glutathione-S-transferase huntingtin exon 1 proteins with 20, 32 or 53 polyglutamines did not interact with radiolabeled tryptophan hydroxylase. Therefore, the inhibition of TPH activity does not appear to result from a direct huntingtin/TPH interaction. Receptor binding analyses for the 5-HT1A receptor in 12-week-old R6/2 mice revealed significant reductions in 8-OH-[3H]DPAT binding in several hippocampal and cortical regions. These results demonstrate that the serotonergic system in the R6/2 mice is severely disrupted in both presymptomatic and symptomatic mice. The presymptomatic inhibition of TPH activity in the R6/2 mice may help explain the functional consequences of HD and provide insights into new targets for pharmacotherapy.

Increased huntingtin protein length reduces the number of polyglutamine-induced gene expression changes in mouse models of Huntington's disease.

Both transcriptional dysregulation and proteolysis of mutant huntingtin (htt) are postulated to be important components of Huntington's disease (HD) pathogenesis. In previous studies, we demonstrated that transgenic mice that express short mutant htt fragments containing 171 or fewer N-terminal residues (R6/2 and N171-82Q mice) recapitulate many of the mRNA changes observed in human HD brain. To examine whether htt protein length influences the ability of its expanded polyglutamine domain to alter gene expression, we conducted mRNA profiling analyses of mice that express an extended N-terminal fragment (HD46, HD100; 964 amino acids) or full-length (YAC72; 3144 amino acids) mutant htt transprotein. Oligonucleotide microarray analyses of HD46 and YAC72 mice identified fewer differentially expressed mRNAs than were seen in transgenic mice expressing short N-terminal mutant htt fragments. Histologic analyses also detected limited changes in these mice (small decreases in adenosine A2a receptor mRNA and dopamine D2 receptor binding in HD100 animals; small increases in dopamine D1 receptor binding in HD46 and HD100 mice). Neither HD46 nor YAC72 mice exhibited altered mRNA levels similar to those observed previously in R6/2 mice, N171-82Q mice or human HD patients. These findings suggest that htt protein length influences the ability of an expanded polyglutamine domain to alter gene expression. Furthermore, our findings suggest that short N-terminal fragments of mutant htt might be responsible for the gene expression alterations observed in human HD brain.