The plasminogen activator system: involvement in central nervous system inflammation and a potential site for therapeutic intervention.

BACKGROUND: Extracellular proteases such as plasminogen activators (PAs) and matrix metalloproteinases modulate cell-cell and cell-matrix interactions. Components of the PA/plasmin system have been shown to be increased in areas of inflammation, and have been suggested to play a role in inflammatory neurologic disorders such as epilepsy, stroke, brain trauma, Alzheimer's' disease and multiple sclerosis (MS). In the present study, we evaluated the involvement of the PA system in the animal model of MS, experimental autoimmune encephalomyelitis (EAE). METHODS: EAE was induced by myelin oligodendrocyte glycoprotein (MOG) in mice deficient for the urokinase PA (uPA-/-), or the urokinase PA receptor (uPAR-/-). Mice were evaluated for EAE clinical signs and histopathologic parameters, and compared with wild-type (WT) EAE mice. Lymphocytes from the knockout (KO) and WT mice were analyzed for ex vivo restimulation, cytokine secretion, and antigen presentation. Finally, WT EAE mice were treated with PAI-1dp, an 18 amino acid peptide derived from the PA inhibitor protein (PAI-1). RESULTS: EAE was aggravated in uPA-/- and uPAR-/- mice, and this was accompanied by more severe histopathologic features and microglial activation. By contrast, specific T- cell reactivity towards the encephalitogenic antigen MOG was markedly reduced in the KO animals, as shown by a marked reduction in proliferation and pro-inflammatory cytokine secretion in these mice. Antigen presentation was also reduced in all the KO animals, raising an immunologic paradox. When the mice were treated with PAI-1, a peptide derived from the PA system, a marked and significant improvement in EAE was seen. The clinical improvement was linked to reduced T-cell reactivity, further emphasizing the importance of the PA system in immunomodulation during neuroinflammation. CONCLUSIONS: Cumulatively, our results suggest a role for uPA and uPAR in EAE pathogenesis, as exacerbation of disease was seen in their absence. Furthermore, the successful amelioration of EAE by PAI-1 treatment suggests that the PA system can be considered a potential site for therapeutic intervention in the treatment of neuroimmune diseases.

A synthetic heparin-mimicking polyanionic compound inhibits central nervous system inflammation.

The immunomodulating capacity of heparin led us to test the effect of the synthetic heparin-mimicking and low anticoagulant compound RG-13577 on the course of experimental autoimmune encephalomyelitis (EAE) and central nervous system (CNS) inflammation. EAE was induced in SJL mice by inoculation with whole mouse spinal cord homogenate. RG-13577, delivered intraperitoneally, inhibited the clinical signs of acute EAE and markedly ameliorated inflammation in the spinal cord, primarily by inhibiting heparanase activity in lymphocytes and astrocytes and thus impairing lymphocyte traffic. RG-13577 treatment was effective when started on day of disease induction or day 7 after induction. The low molecular weight heparin, enoxaparin, tested under the same conditions, exerted only a minor insignificant inhibitory effect. RG-13577 also inhibited the tyrosine phosphorylation of several proteins, particularly Erk1 and Erk2 of the MAP kinase signaling pathways associated with inflammation and cell proliferation. RG-13577 blocked the activity of sPLA(2) and inhibited CNS PGE(2) production both in vivo and in vitro.

Extracellular phospholipase A2 inhibitors suppress central nervous system inflammation.

Phospholipase A2 (PLA2) plays a key role in the production of proinflammatory mediators, namely the arachidonic acid-derived eicosanoids, lysophospholipids, and platelet-activating factor, and indirectly influences the generation of cytokines, nitric oxide (NO), and free radicals. Accordingly, regulation of its activity is important in the treatment of inflammation. Since the main site of PLA2 action in inflammatory processes is the cell membrane, we synthesized extracellular PLA2 inhibitors (ExPLIs) composed of N-derivatized phosphatidyl-ethanolamine linked to polymeric carriers. These membrane-anchored lipid conjugates do not penetrate the cell and interfere with vital phospholipid metabolism or cell viability. The ExPLIs markedly inhibited central nervous system inflammation. This was reflected by the suppressed production and secretion of lipopolysaccharide-induced sPLA2, prostaglandin E2, and NO by glial cells and by the amelioration of experimental autoimmune encephalomyelitis in rats and mice.