Glycoproteomics in neurodegenerative diseases

Abstract

Protein glycosylation regulates protein function and cellular distribution. Additionally, aberrant protein glycosylations have been recognized to play major roles in human disorders, including neurodegenerative diseases. Glycoproteomics, a branch of proteomics that catalogs and quantifies glycoproteins, provides a powerful means to systematically profile the glycopeptides or glycoproteins of a complex mixture that are highly enriched in body fluids, and therefore, carry great potential to be diagnostic and/or prognostic markers. Application of this mass spectrometry-based technology to the study of neurodegenerative disorders (e.g., Alzheimer's disease and Parkinson's disease) is relatively new, and is expected to provide insight into the biochemical pathogenesis of neurodegeneration, as well as biomarker discovery. In this review, we have summarized the current understanding of glycoproteins in biology and neurodegenerative disease, and have discussed existing proteomic technologies that are utilized to characterize glycoproteins. Some of the ongoing studies, where glycoproteins isolated from cerebrospinal fluid and human brain are being characterized in Parkinson's disease at different stages versus controls, are presented, along with future applications of targeted validation of brain specific glycoproteins in body fluids.

Figure 1

Cognitive impairment associated with PD progression is characterized pathologically by the spreading of α-synuclein aggregates, the main component of Lewy bodies, from brainstem to limbic system and eventually to the neocortex (Braak et al., 2003). The boxed area, the middle frontal gyrus, is the tissue source for a recent nonbiased profiling (Pan et al., 2007a; Shi et al., 2008) as well as characterization of glycoproteins to reveal proteins unique to PD and/or PD progression, particularly development of dementia.

Figure 2

GO analysis of glycoproteins identified in human brain (A) and CSF (B), to clearly emphasize the fact that a majority of the proteins are distributed to extracellular and membrane compartments.

Figure 3

The illustration of mass spectrometry-based targeted quantitative analysis to detect N-linked glycopeptides in body fluids. Synthetic peptides with stable isotope labeling are used as internal standards for the quantification of endogenous glycopeptides. As an example, N-linked glycopeptide AQLLQGLGFN*L#TER (Corticosteroid-binding globulin) was extracted from human serum with hydrazide chemistry-based solid-phase extraction and detected with an LC MALDI TOF/TOF platform with targeted approach. (Note: # indicates the amino acid that was stable isotope labeled (¹³C and ¹⁵N) in reference peptides; * indicates enzyme-catalyzed conversion of asparagines to aspartic acid at the site of carbohydrate attachment.)