Rhes, a striatal specific protein, mediates mutant-huntingtin cytotoxicity

Probing the metabolic aberrations underlying mutant huntingtin toxicity in yeast and assessing their degree of preservation in humans and mice

Metabolomics is a powerful multiparameter tool for evaluating phenotypic traits associated with disease processes. We have used (1)H NMR metabolome profiling to characterize metabolic aberrations in a yeast model of Huntington's disease that are attributable to the mutant huntingtin protein's gain-of-toxic-function effects. A group of 11 metabolites (alanine, acetate, galactose, glutamine, glycerol, histidine, proline, succinate, threonine, trehalose, and valine) exhibited significant concentration changes in yeast expressing the N-terminal fragment of a mutant human huntingtin gene. Correspondence analysis was used to compare results from our yeast model to data reported from transgenic mice expressing a mutant huntingtin gene fragment and Huntington's disease patients. This technique enabled us to identify a variety of both model-specific (pertaining to a single species) and conserved (observed in multiple species) biomarkers related to mutant huntingtin's toxicity. Among the 59 metabolites identified, four compounds (alanine, glutamine, glycerol, and valine) changed significantly in concentration in all three Huntington's disease systems. We propose that alanine, glutamine, glycerol, and valine should be considered as promising biomarkers for evaluating new Huntington's disease therapies, as well as for providing unique insight into the mechanisms associated with mutant huntingtin toxicity.

Serines 13 and 16 are critical determinants of full-length human mutant huntingtin induced disease pathogenesis in HD mice

The N-terminal 17 amino acids of huntingtin (NT17) can be phosphorylated on serines 13 and 16; however, the significance of these modifications in Huntington's disease pathogenesis remains unknown. In this study, we developed BAC transgenic mice expressing full-length mutant huntingtin (fl-mhtt) with serines 13 and 16 mutated to either aspartate (phosphomimetic or SD) or alanine (phosphoresistant or SA). Both mutant proteins preserve the essential function of huntingtin in rescuing knockout mouse phenotypes. However, fl-mhtt-induced disease pathogenesis, including motor and psychiatric-like behavioral deficits, mhtt aggregation, and selective neurodegeneration are abolished in SD but preserved in SA mice. Moreover, modification of these serines in expanded repeat huntingtin peptides modulates aggregation and amyloid fibril formation in vitro. Together, our findings demonstrate that serines 13 and 16 are critical determinants of fl-mhtt-induced disease pathogenesis in vivo, supporting the targeting of huntingtin NT17 domain and its modifications in HD therapy.

Molecules of madness

My professional life over five decades meandered from a high school ambition to be a psychiatrist and understand the "mind" to biochemical studies of neurotransmitters and drugs. Hopefully, the tale of my quirky impatient curiosity about "too many" different areas will be useful for young scientists embarking on their own careers.

Balance between synaptic versus extrasynaptic NMDA receptor activity influences inclusions and neurotoxicity of mutant huntingtin

The neurodegenerative disorder Huntington disease (HD) is caused by an expanded CAG repeat in the huntingtin gene, resulting in loss of striatal and cortical neurons. Although, the gene product is widely expressed, it remains unclear why neurons are selectively targeted. Here, we demonstrate the relationship between synaptic and extrasynaptic activity, inclusion formation of mutant huntingtin protein (mtHtt), and neuronal survival. Synaptic NMDA receptor (NMDAR) activity induces mtHtt inclusions via a TCP1 ring complex (TRiC)-dependent mechanism, rendering neurons more resistant to mtHtt-mediated cell death. In contrast, stimulation of extrasynaptic NMDARs increases vulnerability of mtHtt-neurons to cell death by impairing a neuroprotective CREB—PGC-1α cascade and increasing the small guanine nucleotide-binding protein Rhes, which is known to sumoylate and disaggregate mtHtt. Treatment of transgenic YAC128 HD mice with low-dose memantine blocks extrasynaptic (but not synaptic) NMDARs and ameliorates neuropathological and behavioral manifestations. By contrast, high-dose memantine also blocks synaptic NMDAR activity, decreases neuronal inclusions, and worsens these outcomes. Our findings offer a rational therapeutic approach for protecting susceptible neurons in HD.

Detection, validation, and downstream analysis of allelic variation in gene expression

Common sequence variants within a gene often generate important differences in expression of corresponding mRNAs. This high level of local (allelic) control-or cis modulation-rivals that produced by gene targeting, but expression is titrated finely over a range of levels. We are interested in exploiting this allelic variation to study gene function and downstream consequences of differences in expression dosage. We have used several bioinformatics and molecular approaches to estimate error rates in the discovery of cis modulation and to analyze some of the biological and technical confounds that contribute to the variation in gene expression profiling. Our analysis of SNPs and alternative transcripts, combined with eQTL maps and selective gene resequencing, revealed that between 17 and 25% of apparent cis modulation is caused by SNPs that overlap probes rather than by genuine quantitative differences in mRNA levels. This estimate climbs to 40-50% when qualitative differences between isoform variants are included. We have developed an analytical approach to filter differences in expression and improve the yield of genuine cis-modulated transcripts to approximately 80%. This improvement is important because the resulting variation can be successfully used to study downstream consequences of altered expression on higher-order phenotypes. Using a systems genetics approach we show that two validated cis-modulated genes, Stk25 and Rasd2, are likely to control expression of downstream targets and affect disease susceptibility.

Rhes, a protein with selective expression in the striatum, plays a major role in Huntington’s disease pathogenesis

Evaluation of: Subramaniam S, Sixt KM, Barrow R, Snyder SH: Rhes, a striatal specific protein, mediates mutant-huntingtin cytotoxicity. Science 324, 1327–1330 (2009). Huntington’s disease (HD) is an autosomal dominant neurodegenerative disorder characterized by choreiform movements, cognitive deficits and psychiatric disturbances. The disease is caused by an abnormal expansion of a CAG repeat located in exon 1 of the gene encoding the huntingtin protein (Htt). The genetic defect encodes a polyglutamine tract in the N-terminal part of Htt that confers a toxic function to the protein. The most striking neuropathological hallmark in HD patients is the selective atrophy of the striatum. The mechanisms underlying the particular vulnerability of the striatum are unknown. Subramaniam and collaborators demonstrate that the cytotoxicity of mutant Htt is greatly enhanced in the presence of the small GTPase, Rhes, a protein of unclear function that has a preferential expression in the striatum. The study demonstrates that Rhes is an E3 ligase, interacts with mutant Htt and modifies it through SUMOylation, a post-transcriptional process that consists of the addition of the protein SUMO1 to mutant Htt. By contrast, the GTPase activity of Rhes does not seem to be involved in the toxicity of mutant Htt. The Rhes-mediated sumoylation of mutant Htt eventually leads to reduced levels of neuroprotective insoluble aggregates, and increased levels of the toxic soluble form of mutant Htt. These completely novel results shed new light on HD pathogenesis. The selective expression of Rhes in the striatum and its role in mutant Htt toxicity could explain why the striatum is so vulnerable in HD. This work may lead to new therapeutic strategies targeting Rhes.

SUMOylation of the mitochondrial fission protein Drp1 occurs at multiple nonconsensus sites within the B domain and is linked to its activity cycle

Dynamin-related protein (Drp) 1 is a key regulator of mitochondrial fission and is composed of GTP-binding, Middle, insert B, and C-terminal GTPase effector (GED) domains. Drp1 associates with mitochondrial fission sites and promotes membrane constriction through its intrinsic GTPase activity. The mechanisms that regulate Drp1 activity remain poorly understood but are likely to involve reversible post-translational modifications, such as conjugation of small ubiquitin-like modifier (SUMO) proteins. Through a detailed analysis, we find that Drp1 interacts with the SUMO-conjugating enzyme Ubc9 via multiple regions and demonstrate that Drp1 is a direct target of SUMO modification by all three SUMO isoforms. While Drp1 does not harbor consensus SUMOylation sequences, our analysis identified2 clusters of lysine residues within the B domain that serve as noncanonical conjugation sites. Although initial analysis indicates that mitochondrial recruitment of ectopically expressed Drp1 in response to staurosporine is unaffected by loss of SUMOylation, we find that Drp1 SUMOylation is enhanced in the context of the K38A mutation. This dominant-negative mutant, which is deficient in GTP binding and hydrolysis, does not associate with mitochondria and prevents normal mitochondrial fission. This finding suggests that SUMOylation of Drp1 is linked to its activity cycle and is influenced by Drp1 localization.

Science Signaling Podcast: 21 July 2009

A small G protein may limit the toxicity of mutant huntingtin to the brain.