Targeting Orphan Receptors for Multiple Sclerosis

ROR-alpha ligand binding domain
Crystal structure of the ligand-binding domain of RORα complexed with cholesterol sulfate. PDB ID=1S0X
Scientists at the Scripps Research Institute have reported on compounds that are able to suppress severity and disease progression in animal models of multiple sclerosis. The compounds, exemplified by SR1001, act by selectively suppressing a subset of T-helper cells characterised by their production of interleukin-17 (TH17 cells). TH17 cells have been implicated in a variety of autoimmune diseases including rheumatoid arthritis, multiple sclerosis, inflammatory bowel disease and systemic lupus erythematosus.

SR1001 selectively binds to two orphan nuclear receptors: retinoic acid receptor-related orphan receptors α and γt (RORα and RORγt). These receptors have indispensible roles in the development and function of TH17 cells, providing a mechanism for modulating one component of the immune system without general immunosuppression. The team reports that SR1001 induces a conformational change in the receptors that results in their reduced affinity for co-activators and increased affinity for co-repressors. The net result is inhibition of the receptors’ transcriptional activity.

SR1001 blocked the development of murine TH17 cells and inhibited cytokine production by differentiated murine and human TH17 cells. Although a drug is some way off, the team suggests that the results demonstrate the feasibility of targeting TH17 cells and the potential of such an approach for the treatment of autoimmune diseases.

The study is published in Nature.

Barring Leukocytes May Reduce Multiple Sclerosis Symptoms

Image: Flickr - Darren D
Multiple sclerosis (MS), believed to be an immune-mediated disorder, is the most common disabling condition of the central nervous system (CNS) affecting young adults. Infiltration of leukocytes into the brain, helped by upregulation of matrix metalloproteinases (MMPs) which cleave components of the extracellular matrix, plays a significant role in causing the demyelination and axonal degeneration associated with MS.

Synthesis of MMPs is regulated by extracellular matrix metalloproteinase inducer (EMMPRIN, CD147), a multifunctional member of the immunoglobulin superfamily, and researchers at the Hotchkiss Brain Institute, University of Calgary have shown that EMMPRIN levels are significantly increased in the brain of MS patients, particularly in plaque-containing regions. The team also showed that EMMPRIN levels are increased in mice with experimental autoimmune encephalomyelitis (EAE), an animal model of MS. EMMPRIN was upregulated on peripheral leukocytes before the appearance of MS-like symptoms and on infiltrating leukocytes and resident cells within the CNS once clinical symptoms had developed. Treating the mice with an anti-EMMPRIN antibody reduced MMP activity, infiltration of leukocytes into the CNS, and the severity of MS-like symptoms. To be effective, the antibody had to be administered at the onset of clinical signs, a time that is typically associated with significant infiltration of leukocytes into the CNS.

The study, which is published in the Journal of Neuroscience, suggests that blocking the activity of EMMPRIN may be a novel way to treat MS.

Dual-receptor T-cells may Trigger MS

Image: Flickr - didier.bier
Multiple sclerosis (MS) is an autoimmune disorder in which T-cells attack and damage the fatty myelin sheaths around the axons of the brain and spinal cord, disrupting the conduction of electrical signals along the nerve fibres. Although both genetic factors and viral infections have been suggested to contribute to the development of MS, no single virus has been conclusively linked to the disease and other mechanisms could also play a role. Animal models induced by CD8+ T-cells show similarities to human MS, and researchers at the University of Washington investigating the causes of MS have engineered mice that over-express CD8+ T-cells that recognise myelin basic protein (MBP), a candidate autoantigen in MS.

When infected with vaccinia virus engineered to produce MBP, the infection should activate the CD8+ T-cells to attack virally infected cells and also other cells that produce MBP. As expected, mice infected with the engineered virus developed MS-like disease but, surprisingly, symptoms were also triggered by infection with wild-type virus. This suggested that the engineered CD8+ T cells expressed a second receptor that recognised wild-type virus and subsequent cross-breeding experiments confirmed that some of the CD8+ T cells did indeed have receptors for both MBP and wild-type virus. Once activated by the virus, the dual-receptor CD8+ T cells were than able to attack cells producing MBP.

The study suggests a role for dual-receptor cells in autoimmune diseases and could explain how infection with a common virus triggers MS in genetically predisposed people, whilst having no lasting effects in most of the population. In the ‘dual-receptor model’, autoimmune activation could be triggered by a chance event leading to T-cells that recognise both MBP and a viral antigen. The prevalence of dual-receptor T cells is presently unclear and the team plan to assess whether they are more common in MS patients.

The study is published in Nature Immunology.

Pathway in MS May Drive Treatment Outcome

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Although interferon-β (IFN-β) is used to reduce the number and severity of relapses in patients with relapsing remitting multiple sclerosis (RRMS), a frustration for both patients and physicians is that the treatment is more effective in some patients than others. Studies by researchers at Stanford University and colleagues have now suggested that there may be two distinct versions of multiple sclerosis and that the effectiveness of β-interferon may depend on the type of disease that the patient has.

The team found that superficially similar experimental autoimmune encephalitis (EAE), an animal model of multiple sclerosis, can be induced either by T helper type 1 (TH1) cells or by TH17 cells. IFN-β was effective in treating TH1-induced EAE but exacerbated disease caused by TH17 cells. In TH1-induced EAE, treatment was accompanied by increased interleukin-10 (IL-10) production whereas in TH17-induced EAE, IL-10 levels were unaffected by treatment, although IL-17 levels were reduced. Both induction of IL-10 production and suppression of IL-17 levels were dependent on IFN-γ; in the absence of IFN-γ signalling, IFN-β did not reduce the symptoms of EAE.

The team then measured IL-17 levels in blood samples from 26 RRMS patients taken before and about 2 years after starting treatment with IFN-β. When treatment responses were matched to IL-17 levels a clear pattern emerged: patients with a good response had very low levels of the IL-17 family member, IL-17F, whereas those who responded poorly – about 30% of the patients – had high levels of IL-17F. Although the team caution that the results need to be confirmed in larger groups of patients, the study has the potential to transform treatment for MS by stratifying patients into likely IFN-β responders and non-responders. Eventually, a simple blood test could improve the response rate to IFN-β and spare non-responders from the known flu-like side effects of a treatment that could even worsen their disease.

The study is published in the journal Nature Medicine.

Lipidomics Reveals New Target for Multiple Sclerosis

Arachidonic acid (AA) has important physiological roles and is a key mediator of inflammation. Released from membrane glycerophospholipids by cyctosolic phospolipase-A2α (cPLA2α), AA may be further converted to prostaglandins, leukotrienes, lipoxins, and hydroxy-eicosatetraenoic acids by cyclooxygenases (COXs), lipoxygenases (LOs), and terminal enzymes.

The Arachidonic Acid Pathway Image: Wikipedia
The Arachidonic Acid Pathway

Image: Wikipedia

It has already been established that cPLA2α plays an important role in experimental autoimmune encephalomyelitis (EAE), an animal model of multiple sclerosis (MS), but the downstream effectors have remained elusive. Collaborators from Tokyo, Kyushu and Osaka Universities have now conducted a targeted analysis of the AA cascade in the spinal cords of naïve and EAE mice using transcriptomics and lipidomics. The lipidomics study identified constitutive generation of eicosanoids in the spinal cords of naïve mice, predominantly via the cyclooxygenase (COX) pathway, with prostaglandin-D2 (PGD2) and metabolites favoured. In EAE lesions, however, levels of prostaglandin-E2 (PGE2) and metabolites were increased while PGD2 decreased. The researchers then examined microsomal PGE synthase-1 (mPGES-1), a key enzyme involved in the production of PGE2 in inflammation. Their findings suggest that PGE2 levels in the spinal cords of EAE mice are dependent on mPGES-1 expressed in macrophages/microglia. In addition, the clinical course of EAE was less severe in mPGES-1-/- mice.

Although there are currently no known, effective inhibitors of rodent mPGES-1, a potent and selective inhibitor of the human enzyme, MF63, was reported last year. In that study MF63 suppressed PGE2 production, pyresis and inflammatory pain in a knock-in mouse expressing human mPGES-1.

The authors of the present study, published in PNAS, also examined autopsy brain tissues obtained from MS patients. In agreement with the data in murine EAE, immunohistochemistry on MS lesions revealed the overt expression of mPGES-1 protein in CD68+ macrophages. These data suggest that human MS pathology also appears to be influenced by the mPGES-1/PGE2 axis of the AA cascade and that inhibition of mPGES-1 may have utility in the treatment of MS.

Role for Galanin in Multiple Sclerosis

spineThe neuropeptide galanin is widely distributed in the nervous system and levels are known to increase dramatically in response to injury. Galanin and galanin receptors have recently been shown to be overexpressed in some brain areas of people with Alzheimer’s disease and researchers at the University of Bristol have now shown that galanin is also upregulated in microglia from lesions and shadow plaques in multiple sclerosis sufferers as well as in oligodendrocytes from mice with EAE, an experimental form of the disease. To investigate whether the increased levels of galanin were modulating disease activity, the team monitored the development of EAE in wild type (WT) mice, galanin knockout (Gal-KO) mice, mice over-expressing galanin (Gal-OE) and mice expressing a mutated form of the galanin receptor-2 (GalR2-Mut). It was found that Gal-OE mice were completely resistant to the development of clinical symptoms whilst Gal-KO mice developed clinical disease earlier than WT mice and GalR2-Mut mice developed more severe disease than WT mice and at an earlier time point. The study clearly shows the importance of galanin in limiting disease severity in mice with EAE and suggests that GalR2 agonists, if these could be identified, may be of benefit to MS patients. The study is published in the August 26th online edition of PNAS.

The Renin-Angiotensin System and MS

The renin-angiotensin system (RAS) was first studied for its role in regulation of the cardiovascular system and drugs that modulate the RAS are now widely used to treat high blood pressure, myocardial infarction and stroke. More recently, it has become apparent that components of the RAS also mediate inflammatory processes and two recently published studies have now expanded on the link between the RAS and multiple sclerosis (MS). A team led by researchers at Stanford University School of Medicine found that multiple sclerosis lesions from brains of MS patients had elevated levels of both the angiotensin I receptor (AT1R) and angiotensin converting enzyme (ACE). The team then showed that treatment with the ACE inhibitor, lisinopril, or the AT1R antagonist, candesartan, could prevent the development of experimental autoimmune encephalomyelitis (EAE) in mice and, perhaps more importantly, reverse the symptoms of established disease. Reduced activation of AT1R was shown to increase the number of Treg cells in the CNS and suppress TH1/TH17-mediated immune responses to autoantigens.
renin angiotensin system scheme
The study is published in the online early edition of PNAS.

The second study, by researchers in Germany and also published in the online early edition of PNAS, showed that renin, ACE and AT1R were all up-regulated in the inflamed spinal cord and immune system, including antigen presenting cells (APC), of mice with EAE. Pretreatment with the renin inhibitor, aliskiren; the ACE inhibitor, enalapril; or the AT1R antagonist, losartan, reduced the severity of EAE symptoms and losartan was also found to ameliorate the course of established disease. Blockade of AT1R was found not to have a direct effect on T-cell responses but to significantly reduce APC in the spinal cord and immune organs, and to reduce cytokine-induced APC migration.

Since drugs that modulate the RAS have been used in millions of people around the world and have few side effects, the researchers hope that clinical trials to test their effectiveness in MS patients should be straightforward to carry out.

New Therapeutic Target for Multiple Sclerosis

Multiple sclerosis (MS), an autoimmune disease that results in damage to oligodendrocytes which maintain insulating myelin sheaths around nerve fibres in the central nervous system, is the most common disabling neurological disorder affecting young adults. The inflammatory process in MS is initiated by T-cells which recognise myelin as ‘foreign’ and attack it as if it were an invading virus. Disease-modifying agents are now available to treat MS but these are not effective for all patients and advanced forms of MS remain difficult to treat. Analysis of tissue samples from MS patients has revealed that two pathways more usually associated with blood pressure regulation – the renin-angiotensin system and the kallikrein-kinin system – are altered compared with samples from normal subjects.

No Entry SignA team led by researchers in Berlin have now shown that the bradykinin B1 receptor specifically controls infiltration of immune cells into the CNS. In the mouse EAE model of MS, activating the B1 receptor resulted in reduced disease severity whereas blocking the receptor led to more rapid disease onset. The protective effect of the B1 receptor was found to be mediated by its expression on T-cells. In an in vitro model of CNS penetration, a B1 receptor agonist, R838, considerably reduced the number of migrated T-cells and additional application of a B1 antagonist, R715, restored migration. Analysis of immune cells from the CNS of mice with EAE revealed that, in mice lacking the B1 receptor, the proportion of infiltrating CD4+ T-cells was increased and, significantly, the proportion of TH17 cells – believed to be crucial for autoimmune neuroinflammation – was also greater. The study suggests that the B1 receptor acts as an endogenous modulator of recruitment of pathogenic lymphocytes into the CNS and limits harmful immune responses. The authors hope that selective B1 receptor agonists could one day play a role in the management of chronic inflammatory diseases such as MS.

The study is published in the journal Nature Medicine.

mTOR Linked to Myelin Repair

MS lesionsThe axons of nerve cells are sheathed by an insulating layer of myelin which is made up of about 80% lipid and 20% protein. Demyelination, leading to impaired or lost conduction of signals along the nerve, is a hallmark of multiple sclerosis (MS). In relapsing-remitting MS myelin can be replaced but, after repeated attacks, the repair system becomes less efficient. Researchers at the University of Medicine and Dentistry of New Jersey have now identified a key pathway which regulates the production of new oligodendrocytes – the myelin-producing cells of the CNS – and the production of myelin. They found that activation of the mammalian target of rapamycin (mTOR) is essential for oligodendrocyte differentiation at the late progenitor to immature oligodendrocyte transition. The effects were found to be mediated via two distinct signalling complexes, mTORC1 and mTORC2. mTORC2 was found to control myelin gene expression at the mRNA level whereas mTORC1 influenced expression of myelin basic protein via an alternative mechanism.

Although it remains to be determined whether stimulation of the mTOR pathway or removal of some inhibitory mechanism would be most appropriate, allowing the pathway to function normally could provide new treatments for MS and other demyelinating diseases.

The study is published in the May 13th online edition of the Journal of Neuroscience.

More on the Mechanism of Glatiramer Acetate in Multiple Sclerosis

Approved in 1995, glatiramer acetate (Copaxone®, copolymer-1) is a disease-modifying drug that has been demonstrated to reduce the relapse rate and progression of disability in relapsing-remitting multiple sclerosis (RRMS) patients. The compound is a mixture of synthetic peptides (50-90 amino acids) composed of alanine, glutamic acid, lysine and tyrosine. Originally developed to mimic myelin basic protein, a major component of the neuronal myelin sheath, it was intended for use as an inducer of experimental autoimmune encephalitis (EAE). The unexpected inhibition of EAE that was observed with glatiramer acetate led to clinical trials and subsequent approval for RRMS.

The efficacy of glatiramer acetate has been ascribed to an effect on the adaptive immune response, shifting towards a Th2 polarisation of myelin-specific T-cells. Further studies have demonstrated an immunomodulatory effect on monocytes, macrophages and dendritic cells. However, the full mechanistic picture is still unclear.
Collaborating scientists from the University of Geneva, Technische Universität München and University of California, San Francisco, have now demonstrated an effect of glatiramer acetate on the IL-1 system. Their research has shown that treatment with the polymer increases blood levels of secreted IL-1 receptor antagonist (sIL-1Ra), a natural inhibitor of IL-1β, both in RRMS patients and in EAE mice. In the same subjects, levels of IL-1β were undetectable. Additional in vitro experiments with T-cell contact-activated monocytes, a model relevant to chronic inflammation, showed that glatiramer acetate strongly reduced expression of IL-1β, whilst enhancing expression of sIL-1Ra. This is in contrast to effects in monocytes subjected to acute inflammatory conditions (stimulation with LPS), where glatiramer acetate increased production of both sIL-1Ra and IL-1β. The authors conclude that the effects on the IL-1 system in chronic inflammatory conditions contribute to the therapeutic effects of glatiramer acetate in RRMS.

The study is published in the online early edition of the journal PNAS.