L–DOPA–induced dyskinesia, the rate–limiting side–effect in the therapy of Parkinson’s Disease, is mediated by activation of mTOR signaling in the striatum. We show that Rhes, a striatal–specific protein, binds to and activates mTOR. Moreover, Rhes deleted mice manifest reduced striatal mTOR signaling and diminished dyskinesia but maintain motor improvement upon L–DOPA treatment, implying therapeutic benefit for Rhes–binding drugs.

Background:The striatal-specific protein Rhes is implicated in the selective pathology of HD. Results: Rhes binds Beclin-1 and activates autophagy, a lysosomal degradation pathway critical in aging and neurodegeneration. Conclusion: Rhes-induced autophagy occurs independent of mTOR and JNK-1 signaling and is inhibited by huntingtin. Significance: The restricted expression of Rhes and its effect on autophagy may explain the selective striatal pathology and delayed onset of HD.

We recently reported that the small G-protein Rhes has the properties of a SUMO-E3 ligase and mediates mutant huntingtin (mHtt) cytotoxicity. We now demonstrate that Rhes is a physiologic regulator of sumoylation, which is markedly reduced in the corpus striatum of Rhes-deleted mice. Sumoylation involves activation and transfer of small ubiquitin-like modifier (SUMO) from the thioester of E1 to the thioester of Ubc9 (E2) and final transfer to lysines on target proteins, which is enhanced by E3s. We show that E1 transfers SUMO from its thioester directly to lysine residues on Ubc9, forming isopeptide linkages. Conversely, sumoylation on E1 requires transfer of SUMO from the thioester of Ubc9. Thus, the process regarded as "autosumoylation" reflects intermolecular transfer between E1 and Ubc9, which we designate "cross-sumoylation." Rhes binds directly to both E1 and Ubc9, enhancing cross-sumoylation as well as thioester transfer from E1 to Ubc9. Huntington disease (HD)3 is an autosomal dominant disorder caused by an expansion of glutamine residues in the gene encoding the protein huntingtin (Htt) (1). Htt and mutant Htt (mHtt) are ubiquitously expressed throughout the brain and peripheral tissues, yet HD is associated with highly selective degradation of the striatum with no notable alterations in peripheral tissues. Rhes, a member of the Ras family of small G-proteins, was identified on the basis of its selective localization to the corpus striatum of the brain, hence its designation as Ras homolog enriched in striatum (2). We recently reported that Rhes binds mHtt selectively and with high avidity to enhance mHtt sumoylation, with Rhes acting as an apparent E3 ligase for mHtt as well as other substrates such as RanGAP and SP100 (3). Sumoylation of mHtt leads to its disaggregation and augmented neurotoxicity (3, 4). Thus, selective striatal neuronal loss in HD may reflect the tissue-specific influence of Rhes on mHtt sumoylation and cytotoxicity.Sumoylation involves a multistep enzymatic cascade analogous to ubiquitination, beginning with the activation and attachment of SUMO as a thioester bond to E1, the Aos1/Uba2 heterodimer. The SUMO group is then transferred from E1 to Ubc9, the E2-conjugating enzyme, as a thioester bond. In the final step, SUMO is transferred to lysine residues of target proteins forming an isopeptide bond, a process enhanced by E3 ligases (5).Sumoylation can regulate protein function through changes in localization, stability, and activity. Because the sumoylation cascade employs only a single E1 (Aos1/Uba2) and a single E2 (Ubc9), regulation of these enzymes could have substantial effects upon sumoylation. SUMO E1 and Ubc9 are themselves sumoylated in yeast and mammalian cells (6, 7). Recently, it was reported that attachment of SUMO at lysine 14 on Ubc9 acts as a molecular switch to modulate sumoylation of target proteins (8). Molecular mechanisms underlying "autosumoylation" of SUMO-E1 and Ubc9 have not hitherto been elucidated.We now report that mice with deletion of Rhes manifest a p...

Glycosylation is essential to brain development and function, but prior studies have often been limited to a single analytical technique and excluded region- and sex-specific analyses. Here, using several methodologies, we analyze Asn-linked and Ser/Thr/Tyr-linked protein glycosylation between brain regions and sexes in mice. Brain N-glycans are less complex in sequence and variety compared to other tissues, consisting predominantly of high-mannose and fucosylated/bisected structures. Most brain O-glycans are unbranched, sialylated O-GalNAc and O-mannose structures. A consistent pattern is observed between regions, and sex differences are minimal compared to those in plasma. Brain glycans correlate with RNA expression of their synthetic enzymes, and analysis of glycosylation genes in humans show a global downregulation in the brain compared to other tissues. We hypothesize that this restricted repertoire of protein glycans arises from their tight regulation in the brain. These results provide a roadmap for future studies of glycosylation in neurodevelopment and disease.

Although the mutated protein causing Huntington's disease (HD) is expressed throughout the body, the major pathology of HD is localized to the striatum of the brain. We previously reported that the striatal-enriched protein Rhes binds the mutated huntingtin protein and enhances its cytotoxicity. We now demonstrate that Rhes-deleted mice are dramatically protected from neurotoxicity and motor dysfunction in a striatal-specific model of HD elicited by 3-nitropropionic acid. This finding suggests that Rhes may, in part, determine the striatal selectivity of HD.

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A common missense variant (rs13107325 (C->T), A391T) in SLC39A8, a gene encoding a transporter of divalent cations including manganese (Mn), is convincingly associated with schizophrenia and has pleiotropic effects on several additional brain-related phenotypes.Homozygous loss-of-function mutations in SLC39A8 result in undetectable serum Mn and a Congenital Disorder of Glycosylation (CDG) due to the exquisite sensitivity of glycosyltransferases to Mn concentration. Here, we identified Mn-related changes in human carriers of the SLC39A8 missense allele. Analysis of structural brain MRI scans from the UK Biobank showed a dose-dependent change in the ratio of T2w to T1w signal in several brain regions, presumably from altered transport of paramagnetic cations including Mn. We confirmed a specific reduction of serum Mn and showed through comprehensive profiling reduced complexity and branching of the plasma protein N-glycome in both heterozygous and homozygous minor allele carriers. N-glycome profiling of two individuals with SLC39A8-CDG showed similar but more severe alterations in branching that improved with Mn supplementation, suggesting that hypofunction of the common missense variant exists on a spectrum with potential for reversibility.Characterizing the functional impact of this variant may enhance our understanding of schizophrenia pathogenesis and identify novel therapeutic targets and biomarkers of disease.