Changes in the Brain Extracellular Matrix Sulfation Code in Alzheimer's Disease

Abstract

Alzheimer's Disease (AD) is a neurodegenerative disorder associated with synapse dysfunction and neurocircuit hyperactivation. Neural networks involved in memory and cognition are regulated by extracellular matrices known as perineuronal nets (PNNs), and these unique matrix structures are reduced in the human AD brain. PNNs are comprised of chondroitin sulfate glycosaminoglycan (CS-GAG) lattices that consist of five differentially-sulfated CS isomers (CS-A, -C, -D, -E and -O), with their relative abundance comprising a “sulfation code” that controls key biological functions including neuroplasticity (CS-A), protein-glycan binding (CS-O, CS-D), and neuroinflammation (CS-C, CS-E). Although changes in CS-GAGs promote PNN dysfunction, synapse reorganization, and neurocircuit hyperactivation in rodents, it is unknown if such changes are features in human AD brain. Using LC-MS/MS analysis, we report a decrease in the nonsulfated ΔO (p=0.0002) in the middle frontal gyrus (MFG) of human AD brain compared to controls. This effect was offset by an increase in the monosulfated ΔC (p=0.024) and trending increase in the disulfated ΔE (p=0.067), which yielded an increase in overall CS-GAG sulfation (p<0.0001). We next sought to determine whether changes in CS-GAGs in AD also associate with Braak stage and topographic distribution of neurofibrillary tangles including phosphorylated tau (P-tau). We found that more advanced Braak stage associated with a decrease in the non-sulfated ΔO (p<0.0001), increases in the sulfated ΔC (p=0.011) and ΔE (p=0.047), and a trending increase in the ΔD (p=0.093), which yielded a Braak stage-dependent increase in overall CS-GAG sulfation (p<0.0001). Antibody labeling against P-tau S396 showed robust accumulation within layer V of the MFG neocortex of AD patients, and whereas P-tau burden was inversely correlated with ΔO (p=0.0017), it was positively correlated with ΔC (p=0.0037), ΔD (p=0.0189), ΔE (p=0.0014), as well as with overall CS sulfation (p<0.0001). Finally, as the increase in Braak stage and P-tau accumulation associate with cognitive decline in AD, we correlated cognitive function (assessed using the Mini-Mental State Examination-MMSE) with changes in CS-GAG sulfation patterns within the MFG. We report that MMSE scoring correlated positively with ΔO (p<0.0006) and negatively with ΔE (p=0.0062) and CS sulfation (p<0.0001) but did not associate with changes in ΔC, ΔD, or ΔA isomer abundance. Since the changes in CS-GAG sulfation reported herein were present prior to the regional onset of classical AD pathology (e.g. Braak III/IV, prior to tangle formation in the MFG), they may play an active role to initiate and/or progress underlying degenerative processes and thus represents a novel target for the prevention and treatment of AD.