Classical microglial activation may contribute to neuronal injury occurring with HIV associated neurological disorders (HAND). Emerging evidence suggests that matrix metalloproteinases (MMPs) could play a critical role in stimulating such activation. For example, inhibition of MMP activity blocks microglial activation in response to stimuli including lipopolysaccharide. In addition, minocycline, a potent inhibitor of MMP expression and activity, abrogates microglial activation occurring with osmotic demyelination as well as with simian immunodeficiency viral (SIV) infection. The mechanisms by which MMPs might activate microglia are not completely understood. Of interest, however, is the potential for MMPs to generate ligands for integrins that are highly expressed on microglia. Integrin dependent signaling can in turn stimulate changes associated with an activated phenotype. That this mechanism could be important is underlined by data showing that inhibition of microglial integrin expression, or function, blocks microglial phagocytosis and migration. In addition, recent studies have shown that select integrin antagonists can substantially reduce microglial activation and associated neurotoxicity in more than one disease model. In previous publications we have shown that HIV proteins can increase MMP release from brain derived cells, and that MMP levels are increased in spinal fluid samples from patients with HAND. In the present application, we hypothesize that these MMPs generate specific cell adhesion molecule (CAM) fragments that will in turn engage microglial integrins. Our focus on CAM fragments is based on several considerations. CAMs are easily accessible by virtue of their proximity to the cell surface, an area where MMP activity may be concentrated. We and others have shown that MMPs stimulate ectodomain shedding of these molecules, and elevated levels of soluble forms can be detected in spinal fluid samples from patients with brain inflammation. Moreover, we have recently published data showing that the shed domain of at least one CAM can interact with a microglial integrin that has been well linked to an activated phenotype. In the present R21 proposal we plan to identify microglial integrin-binding ligands that may be increased in association with HIV, to investigate the hypothesis that these ligands stimulate classical, pro-inflammatory microglial activation, and to determine whether this activation is of sufficient magnitude to be inimical to vulnerable neurons. We will focus on CAMs that are widely expressed in the CNS, including the immunoglobulin (Ig) domain containing intercellular cell adhesion molecules (ICAMs) and synaptic CAMs (synCAMs), as well as the cadherins. We will also focus on integrins that mediate microglial activation in other disease models, such as LFA-1 and Mac- 1. This dual PI grant relies on expertise related to MMPs (KC), as well as microglia and dopaminergic neurons (KMZ). The possibility that MMPs contribute to microglial activation in the setting of HIV is yet untested and clinically relevant, in that it would allow MMP inhibitor to be used in an attempt to reduce such activation. Further study of the receptors that underlie MMP dependent effects is also clinically relevant. At least one soluble CAM can interact with LFA-1, a microglial integrin for which a clinically tolerable antagonist has been developed.
Microglial activation has been linked to neurotoxicity in varied disease models. Understanding whether proteases and integrins contribute to microglial activation in the setting of HIV disease may allow us to test specific antagonists for their abilit to reduce HIV associated neurological disease.
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