The mystery of Huntington disease (HD) is how a toxic protein that is expressed in all the cells in the body kills only a select subset of neurons in the brain. In the disease, the mutant huntingtin protein (mHtt) affects mainly neurons in the striatum, leading to the movement disorders characteristic of HD. To a lesser extent mHtt affects the cortex, leading to dementia. The secret to these limits may lie in the selective expression of a partner protein that cooperates with huntingtin to elicit toxicity, according to a paper out in the June 4 Science. The work, from Solomon Snyder’s lab at the Johns Hopkins University School of Medicine, suggests that a G protein that is expressed mainly in the striatum, and somewhat in the cortex, is required for the mutant huntingtin protein to kill cells, likely through its effects on protein solubility. The work supports the idea that soluble huntingtin, not aggregates, is responsible for cell death, and could lead to a new target for reducing the protein’s toxicity.

The G protein, Rhes (Ras homolog enriched in striatum) was discovered over a decade ago by differential cloning of striatal transcripts (Falk et al., 1999), and has mainly been studied for its role in dopamine neurotransmission. Using coimmunoprecipitation approaches, first author Srinivasa Subramaniam was able to demonstrate that mHtt, but not wild-type Htt, formed a complex with Rhes in cells overexpressing the two proteins. The researchers found the same complex in striatal cells from transgenic mice expressing mHtt. In cultured HEK293 or striatal cells, neither Rhes nor mHtt alone caused significant cell death, but the combination did. Finally, the researchers showed that shRNA knockdown of Rhes in PC12 cells prevented cell death induced by mHtt.

How does Rhes mediate huntingtin toxicity? mHtt forms aggregates in cells, but similar to the amyloid peptide and other disease-causing proteins, the toxic culprit seems to be soluble forms of the peptides. In the case of mHtt, post-translational modification with the small, ubiquitin-like modifier SUMO has been shown to decrease aggregation and increase neurotoxicity in a fly model of the disease (see ARF related news story on Steffan et al., 2004). Subramaniam and colleagues found that in HEK293 cells, expression of Rhes increased mHtt SUMOylation and reduced aggregation. As in the fly studies, SUMOylation of mHtt correlated with cell death: mutation of the mHtt lysine acceptor residues that link to the SUMO peptide enhanced aggregation and rendered mHtt non-toxic in cells that also expressed Rhes and the SUMOylating enzyme SUMO1.

The results suggest that Rhes controls SUMOylation of mHtt, and further studies showed that Rhes associates with the ubiquitin-conjugating enzyme Ubc9, a cellular E2 ligase known to function in SUMOylation. Furthermore, Rhes stimulated SUMOylation of mHtt in test tube reactions in the presence of SUMO1. Rhes was not exclusive to mHtt, and it could stimulate SUMOylation of other cellular substrates, though not wild-type Htt. Finally, the investigators showed that the SUMOylation, disaggregation, and enhancement of Htt toxicity by Rhes relied on the farnesylation of Rhes and its membrane localization, but not on its GTPase activity.

These in vitro results imply that the toxicity of huntingtin may be circumscribed by the expression pattern of Rhes. That fits with what is known of Rhes distribution. “Rhes is very selectively concentrated in the corpus striatum, but there is also a good amount in the cerebral cortex. There is no Rhes in the cerebellum. Interestingly in HD, the primary initial symptoms are all motor, and then you get dementia. You never ever get cerebellar symptoms. It all fits with the localization of Rhes,” Snyder told ARF. The caveat is that all of the studies looking at Rhes expression have been in rodents, although Snyder reports that they have preliminary evidence that the pattern holds up in human brain.

The closest relative of Rhes, the dexamethasone-induced Ras protein 1 (Dexras1) is widely expressed throughout the brain, and recently has been implicated in regulating the signaling pathways emanating from amyloid precursor protein (Lau et al., 2008). In fact, Snyder says his lab first became interested in Rhes because they had spent 10 years studying Dexras1 function. Despite the similarity of the two proteins, Dexras1 does not promote Htt toxicity, he says. That role is unique to Rhes.

“The report by Subramanian and colleagues presents evidence for a potentially important new mechanism elucidating a pathway for striatal neuronal degeneration in Huntington’s disease,” says Michael Levine of the University of California at Los Angeles, who was not involved with the study. “This mechanism has the potential for uncovering a novel therapeutic target but still needs validation in a mammalian organism in an in vivo preparation where neurons exist in an environment in which both cell autonomous and cell-cell interactions have important roles.”

Snyder reports that those in vivo experiments are underway. His lab is now crossing Rhes knockout mice (Spano et al., 2004) with Huntington mice, and if the model is correct, the loss of Rhes should alleviate symptoms. The researchers will have results within a year, and an affirmative would boost the idea that Rhes itself or the Rhes/huntingtin interaction could be a promising target for neuroprotection in HD.—Pat McCaffrey

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  1. Huntington disease (HD) is an ultimately fatal genetic neurodegenerative disease with a triad of cognitive, neuropsychiatric, and motor symptoms caused by the polyglutamine expansion in the coding region of the Huntingtin gene. Despite intense research in the field, there are currently no disease-modifying treatments for HD and treatment remains limited to management of symptoms. Since the discovery of the gene, the search for anatomical or molecular explanations for the preferential loss of striatal medium spiny neurons in HD has be an active area of research. Mutant huntingtin is expressed throughout the body, so the reason that it should preferentially lead to loss of a small subset of neurons is not obvious.

    Several hypotheses regarding the preferential involvement of the striatum in the course of HD pathogenesis have emerged over the past decade. In 2001, Zuccato et al. suggested that a lack of neurotrophic support from BDNF, normally made in the cortex and transported to the striatum, is at least in part responsible for this preferential susceptibility. In 2002, Zeron et al. provided evidence for a role for NR2B-subtype NMDAR activation as a trigger for selective neuronal degeneration in HD. More recent studies (Benchoua, 2008; Chavrin, 2005) have suggested that the high expression of dopamine exacerbates mutant huntingtin toxicity in the striatum.

    In this study, Subramaniam and colleagues describe a series of elegant biochemical experiments showing that a recently identified, striatally-enriched small G protein, Rhes (ras homolog enriched in the striatum), directly interacts with Huntingtin, induces SUMOylation of the mutant protein, and thereby confers mutant Huntingtin toxicity to cells expressing it. Previous studies have implicated Rhes in dopamine signaling and striatal function, with conflicting reports on the effect of Rhes knockout on locomotion in different genetic backgrounds (Errico, 2008; Spano, 2004). Their findings also suggest that Rhes might cause toxicity, partly, by preventing neurons from forming the intracellular deposits of mutant Htt called inclusion bodies. These results are consistent with previous findings indicating that inclusion body formation may serve as a protective response by neurons against more soluble and toxic forms of the protein (Arrasate et al., 2004).

    The current study provides an interesting insight into the selectivity of cell death in Huntington disease. Preventing farnesylation of Rhes may be a therapeutic target to investigate, especially since farnesyl transferase inhibitors have already been developed for the clinic. The absence of drastic phenotypes in the knockouts is encouraging, if Rhes is to be pursued as a therapeutic target.

    However, further studies are needed to truly establish Rhes as a striatal-specific mediator of mutant Huntington toxicity and as a therapeutic target for HD therapy. First and foremost, it will be essential to verify that Rhes mediates toxicity of mutant Huntingtin in primary neurons, in vivo, as well as in the cell line models used in this study. Will the HD phenotype be significantly reduced in a Rhes knockout background? Will ectopic expression of Rhes in primary cortical and hippocampal neurons confer the same degree of toxicity of mutant Huntington as in primary striatal neurons? Second, while Rhes is highly enriched in the striatum, its mRNA is also expressed in various other regions, notably the CA1/CA3 layers of the hippocampus, the granular layer of the cerebellum, and anterior thalamus (Vargiu 2004, Harrison 2008). Whether the Rhes protein is also expressed in these other neuronal population needs further exploration to determine whether Rhes is a truly striatal-specific factor. Third, both Rhes and dopamine are suggested to contribute to the preferential loss of striatal neurons, and Rhes is suggested to have roles in dopamine signaling in the striatum. Finally, it will be interesting to see whether Rhes can also explain the differential susceptibilities of medium spiny neurons and interneurons within the striatum, as interneurons are largely spared in HD.

    As with other aspects of biology, a complex image is developing regarding the factors that regulate the preferential loss of striatal neurons in HD. Whether these anatomical and molecular regulators of toxicity, described here or yet to be discovered, act in a concerted fashion or independent of each other remains to be explored. Rhes is already suggested to have roles in dopamine signaling, and both Rhes and dopamine are suggested to contribute to the selective vulnerability of striatal neurons. How these signaling pathways are regulated and fine-tuned within a complex neural network will give us a better understanding of the pathological basis in Huntington disease.

    References:

    . Inclusion body formation reduces levels of mutant huntingtin and the risk of neuronal death. Nature. 2004 Oct 14;431(7010):805-10. PubMed.

    . Dopamine determines the vulnerability of striatal neurons to the N-terminal fragment of mutant huntingtin through the regulation of mitochondrial complex II. Hum Mol Genet. 2008 May 15;17(10):1446-56. PubMed.

    . Unraveling a role for dopamine in Huntington's disease: the dual role of reactive oxygen species and D2 receptor stimulation. Proc Natl Acad Sci U S A. 2005 Aug 23;102(34):12218-23. PubMed.

    . The GTP-binding protein Rhes modulates dopamine signalling in striatal medium spiny neurons. Mol Cell Neurosci. 2008 Feb;37(2):335-45. PubMed.

    . Rhes: A striatal-specific Ras homolog related to Dexras1. J Neurosci Res. 1999 Sep 15;57(6):782-8. PubMed.

    . Ontogeny and dopaminergic regulation in brain of Ras homolog enriched in striatum (Rhes). Brain Res. 2008 Dec 15;1245:16-25. PubMed.

    . Rhes is involved in striatal function. Mol Cell Biol. 2004 Jul;24(13):5788-96. PubMed.

    . Widespread expression of Huntington's disease gene (IT15) protein product. Neuron. 1995 May;14(5):1065-74. PubMed.

    . The small GTP-binding protein, Rhes, regulates signal transduction from G protein-coupled receptors. Oncogene. 2004 Jan 15;23(2):559-68. PubMed.

    . Increased sensitivity to N-methyl-D-aspartate receptor-mediated excitotoxicity in a mouse model of Huntington's disease. Neuron. 2002 Mar 14;33(6):849-60. PubMed.

    . Loss of huntingtin-mediated BDNF gene transcription in Huntington's disease. Science. 2001 Jul 20;293(5529):493-8. PubMed.

    View all comments by Hengameh Zahed
  2. This paper by Subramaniam and colleagues presents some intriguing findings. For one thing, they identify the small G protein, Rhes, as defining a potential new class of non-traditional SUMO E3 ligases, thus opening a potential new window on the SUMOylation machinery. In addition, their study raises the possibility that Rhes activity may exacerbate the pathology of mutant Htt by preferentially causing its SUMOylation with consequences similar to those observed in Drosophila and cells. It will be interesting to see whether Rhes knockout mutations will suppress pathogenesis in mouse models of HD.

References

News Citations

  1. SUMO versus Ubiquitin: A Fight for Huntington’s Disease?

Paper Citations

  1. . Rhes: A striatal-specific Ras homolog related to Dexras1. J Neurosci Res. 1999 Sep 15;57(6):782-8. PubMed.
  2. . SUMO modification of Huntingtin and Huntington's disease pathology. Science. 2004 Apr 2;304(5667):100-4. PubMed.
  3. . Dexras1 interacts with FE65 to regulate FE65-amyloid precursor protein-dependent transcription. J Biol Chem. 2008 Dec 12;283(50):34728-37. PubMed.
  4. . Rhes is involved in striatal function. Mol Cell Biol. 2004 Jul;24(13):5788-96. PubMed.

Further Reading

Primary Papers

  1. . Rhes, a striatal specific protein, mediates mutant-huntingtin cytotoxicity. Science. 2009 Jun 5;324(5932):1327-30. PubMed.