There is a need in autoimmune diseases to uncover the mechanisms involved in the natural resolution of inflammation. In this article, we demonstrate that granulocytic myeloid-derived suppressor cells (G-MDSCs) abundantly accumulate within the peripheral lymphoid compartments and target organs of mice with experimental autoimmune encephalomyelitis prior to disease remission. In vivo transfer of G-MDSCs ameliorated experimental autoimmune encephalomyelitis, significantly decreased demyelination, and delayed disease onset through inhibition of encephalitogenic Th1 and Th17 immune responses. Exposure of G-MDSCs to the autoimmune milieu led to up-regulation of the programmed death 1 ligand that was required for the G-MDSC–mediated suppressive function both in vitro and in vivo. Importantly, myeloid-derived suppressor cells were enriched in the periphery of subjects with active multiple sclerosis and suppressed the activation and proliferation of autologous CD4+ T cells ex vivo. Collectively, this study revealed a pivotal role for myeloid-derived suppressor cells in the regulation of multiple sclerosis, which could be exploited for therapeutic purposes.

MS incidence rose markedly over 3 decades in a genetically stable population in tandem with a transition from rural to urban living, thus possibly implicating environmental factors introduced by urbanization.

Glutamate dehydrogenase (GDH), an enzyme central to glutamate metabolism, is located in the mitochondria although there is evidence for extramitochondrial localization of GDH. In the human, housekeeping and nerve tissue-specific isoforms, encoded by the GLUD1 and GLUD2 genes, have been identified. The two isoenzymes differ markedly in their baseline activities, allosteric regulation, and thermal stability. GTP potently inhibits GLUD1-derived GDH (IC(50) = 0.2 muM), whereas the GLUD2-derived isoenzyme is resistant to this compound. The GLUD2-derived GDH shows low basal activity and has the capacity to be activated fully by ADP or L-leucine. We used molecular biological tools to study the subcellular localization of GLUD1-derived GDH in cultured cells and the molecular basis of its regulation. COS7 cells, transfected with a GLUD1-pEGFP-N3 vector, revealed a GFP fluorescence pattern nearly identical to that of the mitochondrial marker pDsRed2-Mito. Site-directed mutagenesis of GLUD1 gene showed that replacement of Gly456 by Ala made the enzyme resistant to GTP (IC(50) = 2.8 +/- 0.15 microM) without altering its regulation by ADP. Substitution of Ser for Arg443 rendered the enzyme virtually inactive at its basal state, but fully responsive to ADP activation. The Arg443Ser mutant was more active at pH 7.0 than at pH 8.0. The Gly456Ala change therefore dissociated GLUD2-derived GDH function from GTP, whereas the Arg443Ser change made enzyme regulation possible without this inhibitor. These properties may allow the brain isoenzyme to function well under conditions of intracellular acidification and increased turnover of ATP to ADP, as occurs in synaptic astrocytes during excitatory transmission.

Glutamate dehydrogenase (GDH) is an enzyme central to the metabolism of glutamate that also plays a role in cellular energetics. In the human, GDH exists in a housekeeping isoenzyme (hGDH1) encoded by the GLUD1 gene and a neural and testicular tissue-specific isoform (hGDH2) encoded by the GLUD2 gene. There is evolutionary evidence that the GLUD1 was retroposed to the X chromosome in the ape ancestor (>23 million years ago), where it gave rise to GLUD2 through random mutations and directional selection. In the human, the two mature GDH isoproteins are highly homologous, differing in only 16 of their 505 amino acid residues. Functional analyses of highly purified recombinant wild-type hGDH2 revealed that this adaptive evolution dissociated the enzyme from GTP control, permitted regulation almost entirely by ADP and/or L-leucine, and fine-tuned its activity to the relatively low cellular pH that occurs in synaptic astrocytes during excitatory transmission. Study of structure-function relationships, using site-directed mutagenesis of GLUD1 at single sites differing from GLUD2, showed that the Arg443Ser and the Gly456Ala change reproduced some, but not all, of the properties of hGDH2. In addition, we created a double hGDH1 mutant that had both Arg443Ser and Gly456Ala in the same polypeptide chain. Functional analyses revealed that the doubly mutated enzyme did not acquire all the characteristics of the wild-type hGDH2. Hence, additional amino acid changes, acting in concert with Arg443Ser and Gly456Ala, ought to be responsible the unique properties of the brain-specific human isoenzyme.

Parkinson's disease (PD), a common neurodegenerative disorder characterized by progressive loss of dopaminergic neurons and their terminations in the basal ganglia, is thought to be related to genetic and environmental factors. Although the pathophysiology of PD neurodegeneration remains unclear, protein misfolding, mitochondrial abnormalities, glutamate dysfunction and/or oxidative stress have been implicated. In this study, we report that a rare T1492G variant in GLUD2, an X-linked gene encoding a glutamate dehydrogenase (a mitochondrial enzyme central to glutamate metabolism) that is expressed in brain (hGDH2), interacted significantly with age at PD onset in Caucasian populations. Individuals hemizygous for this GLUD2 coding change that results in substitution of Ala for Ser445 in the regulatory domain of hGDH2 developed PD 6-13 years earlier than did subjects with other genotypes in two independent Greek PD groups and one North American PD cohort. However, this effect was not present in female PD patients who were heterozygous for the DNA change. The variant enzyme, obtained by substitution of Ala for Ser445, showed an enhanced basal activity that was resistant to GTP inhibition but markedly sensitive to modification by estrogens. Thus, a gain-of-function rare polymorphism in hGDH2 hastens the onset of PD in hemizygous subjects, probably by damaging nigral cells through enhanced glutamate oxidative dehydrogenation. The lack of effect in female heterozygous PD patients could be related to a modification of the overactive variant enzyme by estrogens. INTRODUCTIONParkinson's disease (PD) affects about 1.8% of people over the age of 65 years. 1 It is clinically characterized by tremor, rigidity and bradykinesia, which often occur along with disturbances of posture and gait. 2 About 80% of PD cases are sporadic, with males being more frequently affected than females (ratio¼1.5:1).The etiology of PD remains largely unknown, although interplay of environmental toxins with genetic susceptibility may be operational. The genetic hypothesis is supported by community-based studies showing that a substantial number of PD patients have similarly affected relatives. 3,4 This was also revealed by a PD study conducted by us on Crete, 5 an island of about 0.6 million people sharing the same genetic and cultural background and a common environment. Moreover, this study suggested an oligogenic model for PD, according to which disease-predisposing alleles from two or more genes need to be present in the same individual for the disorder to be expressed.With regard to the pathophysiology of PD, several hypotheses have been proposed in an attempt to explain the degeneration of dopaminergic neurons in this disorder. One of these suggests that mitochondrial dysfunction is involved, 6 a contention supported by the recent discovery of monogenetic forms of PD resulting from mutations of