MALDI imaging of glial scar-forming glycans in Alzheimers disease

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Alzheimer’s disease (AD) is a dementing disorder characterized by the pathological accumulation of beta- amyloid (Ab) and hyperphosphorylated tau neurofibrillary tangles (NFTs). Recent evidence suggests a close topographical relationship P-tau accumulation and neuronal loss with changes in gene expression mediating chondroitin sulfate (CS) and dermatan sulfate (DS) glycosaminoglycan (GAG) metabolism. CS/DS-GAGs are extracellular polyanionic polymers that have previously been shown to interact with P-tau and Ab, although the underlying molecular mechanisms and physiological consequences for these interactions remain unclear. Studies that investigate the relationship between changes in brain CS/DS-GAGs and AD-associated pathological endpoints are therefore necessary to elucidate a novel role for glycans in the clinical pathogenesis of AD. The biological functions of extracellular CS/DS-GAGs are highly influenced by the incorporation of sulfated disaccharide isomers (0S-, 4S-, 6S-, 2S6S-, 4S6S-CS and 2S4S-DS) into the glycan matrix lattices. Often referred to as the biological “sulfation code”, the relative abundance of non-, mono- and di-sulfated CS/DS isomers is believed to control ion buffering, protein-glycan interactions, and neurocircuit synapse stability in the brain. Using state-of-the-art liquid chromatography tandem mass spectrometry (LC-MS/MS), our Preliminary Data show a significant increase in the relative abundance of glial scar-associated isomers (6S- and 4S6S-CS) within the medial frontal gyrus of patients with AD compared to non-AD controls. Moreover, these glial scar- associated CS isomers positively correlate with the abundance of P-tau from the same brain tissue, suggesting that changes in the brain CS/DS-GAG sulfation code could serve as a novel and unexplored player in the progression of AD. Specifically, we hypothesize that the increase in glial-scarring CS/DS-GAGs may represent 1) a physiological CNS barrier to isolate P-tau neuropathology from the healthy brain tissue and 2) a repressor of neurogenesis and source of cellular toxicity, potentially exacerbating neurodegeneration in AD. Here, we propose to uncover the spatial distribution of CS/DS isomers with the underlying cellular (neuronal, glial) and pathological (P-tau, Ab) tissue pathology by coupling matrix assisted laser desorption ionization (MALDI) mass spectrometry imaging (IMS) of CS/DS isomers with histochemical protein labeling using advanced High Definition Imaging (HDI) overlay technology. Deciphering a relationship between maladaptive changes in the brain CS/DS-GAG sulfation code and development of AD pathology is a novel and unexplored area of glycan- based neuroscience with the potential to revolutionize the field of AD research.