Rapidly emerging convergent reports and our preliminary data in the field of Glycoscience provide potentially powerful and unanticipated opportunities to target glycans to alter the course of Alzheimer's disease (AD). Glycan-protein interactions are implicated in both the amelioration and progression of AD, but a detailed understanding of the role of glycosylation in AD is just emerging. One convergent theme is the role of sialic acid, a common glycan terminus of many glycans in the brain. Understanding sialoglycans that modulate A? and tau accumulation, particularly via microglial regulation, could represent an important advance in AD knowledge. Microglia maintain brain health by clearing debris and misfolded proteins and are responsible, in part, for effective phagocytic clearance of A? and tau. Modulating natural mechanisms that regulate microglial proliferation, survival and function may impact AD progression. Relevant to this goal are findings that human and mouse microglia express unusual sets of cell surface immune inhibitory molecules of the Siglec family ? sialic acid binding Ig-like lectins. Among these, Siglec-8 is a prominent marker of human microglia and its paralog, mouse Siglec-F, is highly expressed on mouse microglia in proteinopathy-mediated neuronal degeneration. We discovered that both of these immune inhibitory Siglecs bind to particular brain glycoproteins of the sialylated keratan sulfate (KS) class. This is consistent with the recent finding that an enzyme required for KS biosynthesis is up-regulated in human AD, and that knocking out this enzyme in an AD mouse model results in increased microglial clearance of A?. The findings suggest that sialylated sulfated KS chains engage Siglecs that are abundant on microglia in pathological lesions, blocking their functions. The above observations suggest that there is an urgent need to define the roles of sulfation and sialylation in AD, as well as opportunities to design glycomimetic inhibitors that target the endogenous enzymes and immune regulatory Siglecs that may be associated with AD. We have assembled a team of three PIs (Schnaar, Johns Hopkins, and Moremen and Woods, UGA) and gifted co-investigators who together have all of the necessary expertise in AD disease pathology, enzymology, glycan structure, synthesis and biophysics, and have developed the following specific aims to define in more complete detail the roles of glycan sialylation and sulfation in AD progression:
Aim 1 ? We will quantify and characterize microglial Siglecs and brain Siglec ligands in human AD and in an advanced mouse model of AD;
Aim 2 ? We will disrupt biosynthesis of microglial Siglec ligands in mice and cross them with AD mice to clarify the relationship between sialoglycan-mediated microglial inhibition and AD progression;
Aim 3 ?We will synthesize sialylated and sulfated glycans as ligands for highly relevant microglial and related human Siglecs;
and Aim 4 ? We will develop small molecule inhibitors of the enzymes that are responsible for assembling the relevant sulfated sialoglycan sequence for testing in translational models of glycomimetic treatment of AD.
Alzheimer's disease is elicited by toxic proteins, amyloid-beta and tau, in the brain, which build up over time and cause the death of nerve cells. Microglia, the brain's immune cells, are meant to clean up toxic proteins, but fall short of this goal in Alzheimer's disease. We study the natural control mechanisms of microglia with the goal of releasing the brain's immune cells to remove toxic proteins and halt or reverse disease progression.
