N-type glycosylated proteomics overview of human Alzheimer’s disease brain

A group from Department of Surgery, Beth Israel Deaconess Medical Center, Harvard Medical School, USA, etc. has reported about N-type glycosylated proteomics overview of normal, asymptomatic and symptomatic human Alzheimer’s disease brain.

The neuropathological hallmark of AD is the formation of β-amyloid (Aβ)-containing extracellular deposits and progression of intraneuronal neurofibrillary tangles from hyperphosphorylated tau.

To investigate the larger glycoprotein landscape, a qualitative N-type glycosylated proteomics analysis of brain samples was performed from 30 humans including those with normal, asymptomatic AD, and symptomatic AD. A combination of lectin affinity isolation and hydrophilic interaction chromatography (HILIC) was optimized and employed to selectively enrich N-glycoproteins prior to LC-MS/MS analysis. A combination of ConA, RCA, SNA, WGA, VVA, and AAL could target the major monosaccharide components of N-glycans in the brains. As a result, a total of 303 glycoproteins were identified from 2,035 unique glycopeptides, 580 N-linked glycosylation sites, and 124 glycans, making up a total of 1,901 glycoforms (remark: if a single glycosylation site was identified with three different glycans, they are counted as three glycoforms) that were identified from all samples.

The most abundant glycans range within the 1201-1250 MW range, which contribute to over 20% of the total glycoforms detected from the samples based on the glycan composition. The majority of glycans in this MW range are from Man5GlcNAc2 (Man5) structure. The second most frequent glycan is assigned as a biantennary glycan with a bisecting GlcNAc and core fucosylation. Other common glycans include several high-mannose, complex, and hybrid N-glycans with different degrees of fucosylation on glycopeptides were identified across the tissues. When considering the distribution based on the overall glycoforms, there is no statistical difference in the glycan molecular weight or glycan distribution among the different sample types.

To gain more information about the glycosylation pattern in different sample groups, it was analyzed at glycosylation site level, When comparing different sample types (normal, asymptomatic AD, and symptomatic AD) for the collective changes in glycosylation across different glycosylation sites, it was observed that a decrease in galactosylation, fucosylation, bisection, and the number of antennary glycans in asymptomatic and symptomatic AD samples, compared with normal brain samples. For asymptomatic compared to symptomatic AD samples, there are generally higher levels of galactosylation, fucosylation, bisection, and the number of antennary and hybrid glycans.

A definitive N-type glycan marker unique to Alzheimer’s disease has yet to be found.