Microglia are implicated in the initiation and progression of Alzheimer?s disease (AD), making their regulation a therapeutic target. As positive effectors, microglia phagocytose and clear toxic proteins; as negative effectors they release inflammatory mediators. Imbalance of microglial function is believed to contribute to AD progression. Among microglial regulatory proteins linked to AD susceptibility are immune inhibitory members of the Siglec family, sialic acid binding immunoglobulin-like lectins. Siglec overexpression results in increased susceptibility to AD; depletion decreases susceptibility, supporting the hypothesis that Siglec-mediated inhibition restricts microglial phagocytosis and exacerbates AD proteinopathy. Multiple inhibitory Siglecs are expressed on human (and mouse) microglia. They bind to complementary ligands ? sialic acid-terminated glycan chains on glycoproteins in the local brain milieu ? to trigger immune suppression. We discovered a sialoglycan ligand for microglial inhibitory Siglecs in human cerebral cortex. We propose that this ligand is responsible for Siglec- mediated microglial inhibition. Knowledge of the structure of this ligand, its expression and its function may provide new opportunities for therapeutic microglial modulation.
Three aims leverage the diverse expertise of three principal investigators to test the hypothesis that Siglecs and Siglec ligands function in microglial regulation and AD progression in vivo. We will also develop drug-like Siglec inhibitors to impact Siglec function in the brain. Completion of these aims will help define the roles of immunomodulatory Siglecs and Siglec ligands in AD, and may provide novel lead molecules for AD therapy.
Aim 1 : Identify microglial Siglecs and Siglec ligands in human and mouse brains. Several immune inhibitory Siglecs are expressed on human microglia. Using expressed Siglecs as probes, we discovered a Siglec ligand in human brain cortex and identified it as a sialylated keratan sulfate proteoglycan. The ligand appears to be overexpressed in human AD, and mice express a similar ligand. We will characterize the expression of Siglecs and Siglec ligands in human AD and mouse models of AD, identify the protein carrier of Siglec ligands in human and mouse, and determine the structures of immune regulatory sialoglycans. Microglia from mutant mouse models will be used to test the roles of Siglecs, Siglec ligands, and Siglec ligation on microglial function.
Aim 2 : Test the role of Siglecs and Siglec ligands in AD progression in vivo. We will create mouse genetic models lacking Siglecs and Siglec ligands crossed with human A?- and tau-expressing models of AD to test the hypothesis that the Siglec regulatory pathway contributes to AD initiation and progression. Subsequently, Siglec- humanized mice will be used to validate human Siglecs as therapeutic targets.
Aim 3 : Develop brain-deliverable Siglec inhibitors. Novel and expandable small molecule sialomimetic libraries will be synthesized and screened for pan- and selective Siglec inhibition to discover brain-deliverable sialomimetic Siglec inhibitors for pre-clinical testing.
Alzheimer?s disease is thought to be caused by misfolded proteins that accumulate as toxic debris in the brain. We now understand that microglia, brain cells that normally clean up debris, may not function correctly in Alzheimer?s disease. This project studies a set of natural molecules that regulate microglial function, Siglecs and Siglec ligands, how they change in Alzheimer?s disease, and how we can intervene to alter microglial function and improve disease outcomes.
