Huntingtin protein is mutated in Huntington disease. We previously reported that wild-type but not mutant huntingtin stimulates transcription of the gene encoding brain-derived neurotrophic factor (BDNF; ref. 2). Here we show that the neuron restrictive silencer element (NRSE) is the target of wild-type huntingtin activity on BDNF promoter II. Wild-type huntingtin inhibits the silencing activity of NRSE, increasing transcription of BDNF. We show that this effect occurs through cytoplasmic sequestering of repressor element-1 transcription factor/neuron restrictive silencer factor (REST/NRSF), the transcription factor that binds to NRSE. In contrast, aberrant accumulation of REST/NRSF in the nucleus is present in Huntington disease. We show that wild-type huntingtin coimmunoprecipitates with REST/NRSF and that less immunoprecipitated material is found in brain tissue with Huntington disease. We also report that wild-type huntingtin acts as a positive transcriptional regulator for other NRSE-containing genes involved in the maintenance of the neuronal phenotype. Consistently, loss of expression of NRSE-controlled neuronal genes is shown in cells, mice and human brain with Huntington disease. We conclude that wild-type huntingtin acts in the cytoplasm of neurons to regulate the availability of REST/NRSF to its nuclear NRSE-binding site and that this control is lost in the pathology of Huntington disease. These data identify a new mechanism by which mutation of huntingtin causes loss of transcription of neuronal genes.

Several neurological diseases are characterized by the altered activity of one or a few ubiquitously expressed cell proteins, but it is not known how these normal proteins turn into harmful executors of selective neuronal cell death. We selected huntingtin in Huntington's disease to explore this question because the dominant inheritance pattern of the disease seems to exclude the possibility that the wild-type protein has a role in the natural history of this condition. However, even in this extreme case, there is considerable evidence that normal huntingtin is important for neuronal function and that the activity of some of its downstream effectors, such as brain-derived neurotrophic factor, is reduced in Huntington's disease.

Huntingtin is a protein that is mutated in Huntington's disease (HD), a dominant inherited neurodegenerative disorder. We previously proposed that, in addition to the gained toxic activity of the mutant protein, selective molecular dysfunctions in HD may represent the consequences of the loss of wild-type protein activity. We first reported that wild-type huntingtin positively affects the transcription of the brain-derived neurotrophic factor (

The expansion of a polyglutamine tract in the ubiquitously expressed huntingtin protein causes Huntington's disease (HD), a dominantly inherited neurodegenerative disease. We show that the activity of the cholesterol biosynthetic pathway is altered in HD. In particular, the transcription of key genes of the cholesterol biosynthetic pathway is severely affected in vivo in brain tissue from HD mice and in human postmortem striatal and cortical tissue; this molecular dysfunction is biologically relevant because cholesterol biosynthesis is reduced in cultured human HD cells, and total cholesterol mass is significantly decreased in the CNS of HD mice and in brain-derived ST14A cells in which the expression of mutant huntingtin has been turned on. The transcription of the genes of the cholesterol biosynthetic pathway is regulated via the activity of sterol regulatory element-binding proteins (SREBPs), and we found an ϳ50% reduction in the amount of the active nuclear form of SREBP in HD cells and mouse brain tissue. As a consequence, mutant huntingtin reduces the transactivation of an SRE-luciferase construct even under conditions of SREBP overexpression or in the presence of an exogenous N-terminal active form of SREBP. Finally, the addition of exogenous cholesterol to striatal neurons expressing mutant huntingtin prevents their death in a dosedependent manner. We conclude that the cholesterol biosynthetic pathway is impaired in HD cells, mice, and human subjects, and that the search for HD therapies should also consider cholesterol levels as both a potential target and disease biomarker.

One cardinal feature of Huntington's disease (HD) is the degeneration of striatal neurons, whose survival greatly depends on the binding of cortical brain-derived neurotrophic factor (BDNF) with high-affinity (TrkB) and low-affinity neurotrophin receptors [p75 panneurotrophin receptor (p75 NTR )]. With a few exceptions, results obtained in HD mouse models demonstrate a reduction in cortical BDNF mRNA and protein, although autopsy data from a limited number of human HD cortices are conflicting. These studies indicate the presence of defects in cortical BDNF gene transcription and transport to striatum. We provide new evidence indicating a significant reduction in BDNF mRNA and protein in the cortex of 20 HD subjects in comparison with 17 controls, which supports the hypothesis of impaired BDNF production in human HD cortex. Analyses of the BDNF isoforms show that transcription from BDNF promoter II and IV is down-regulated in human HD cortex from an early symptomatic stage. We also found that TrkB mRNA levels are reduced in caudate tissue but not in the cortex, whereas the mRNA levels of T-Shc (a truncated TrkB isoform) and p75NTR are increased in the caudate. This indicates that, in addition to the reduction in BDNF mRNA, there is also unbalanced neurotrophic receptor signaling in HD.