Altered fucosyltransferase expression in the superior temporal gyrus of elderly patients with schizophrenia

Abstract

Glycosylation is a post-translational modification that is an essential element in cell signaling and neurodevelopmental pathway regulation. Glycan attachment can influence the tertiary structure and molecular interactions of glycosylated substrates, adding an additional layer of regulatory complexity to functional mechanisms underlying central cell biological processes. One type of enzyme-mediated glycan attachment, fucosylation, can mediate glycoprotein and glycolipid cell surface expression, trafficking, secretion, and quality control to modulate a variety of inter- and intracellular signaling cascades. Building on prior reports of glycosylation abnormalities and evidence of dysregulated glycosylation enzyme expression in schizophrenia, we examined the protein expression of 5 key fucose-modifying enzymes: GDP-fucose:protein O-fucosyltransferase 1 (POFUT1), GDP-fucose:protein O-fucosyltransferase 2 (POFUT2), fucosyltransferase 8 (FUT8), fucosyltransferase 11 (FUT11), and plasma α-L-fucosidase (FUCA2) in postmortem superior temporal gyrus of schizophrenia (N = 16) and comparison (N = 14) subjects. We also used the fucose binding protein, Aleuria aurantia lectin (AAL), to assess α-1,6-fucosylated N-glycoprotein abundance in the same subjects. In schizophrenia we found increased expression of POFUT2, a fucosyltransferase uniquely responsible for O-fucosylation of thrombospondin-like repeat domains that is involved in a non-canonical endoplasmic reticulum quality control pathway. We also found decreased expression of FUT8 in schizophrenia. Given that FUT8 is the only α-1,6-fucosyltransferase expressed in mammals, the concurrent decrease in AAL binding in schizophrenia, particularly evident for N-glycoproteins in the ~52–58kDa and ~60–70kDa molecular mass ranges, likely reflects a consequence of abnormal FUT8 expression in the disorder. Dysregulated FUT8 and POFUT2 expression could potentially explain a variety of molecular abnormalities in schizophrenia.

Glycosylation enzyme mRNA expression in dorsolateral prefrontal cortex of elderly patients with schizophrenia: Evidence for dysregulation of multiple glycosylation pathways

“…This may suggest abnormalities of brain aging in schizophrenia that are consistent with atypical neurodevelopment in earlier stages of the disorder. As with the fucosyltransferase POFUT2, protein levels of FUT8 were found decreased in schizophrenia STG (Mueller et al, 2017), supporting the notion that glycan processing enzyme expression protein and/or mRNA levels demonstrate brain region-specific abnormalities in schizophrenia.…”

“…B3GNT8, MGAT4A, and FUT8 are primarily localized to the Golgi apparatus, thus the opposite valence of protein and mRNA expression changes may be an indication that glycosylation alterations in schizophrenia stem from abnormalities of glycan or glycoprotein processing in the ER which the cell attempts to compensate for further along the processing pathway in the Golgi apparatus. While the expression of some other GlcNAcTases found to be abnormally transcribed in this study were found unchanged at the protein level (Kim et al, 2018;Kippe et al, 2015;Mueller et al, 2017), this does not preclude the possibility that PTMs or functional properties of normally-expressed enzymes may be impaired. The majority of enzymes examined in this report are themselves glycosylated; hence, the altered expression of these enzymes may not only affect the function of a variety of substrate proteins and lipids, but also the efficacy and kinetics of the enzyme-mediated glycosylation pathways in which they participate.…”

“…We have speculated on potential consequences of abnormal gene expression of glycosylation enzymes in schizophrenia; however, inconsistencies between mRNA abundance and protein levels of the corresponding gene product from samples of postmortem human brain are not uncommon (Harrison and Weinberger, 2005;Kristiansen et al, 2007;Meador-Woodruff and Healy, 2000). We have recently reported decreased protein expression of the GlcNAcTases B3GNT8 and MGAT4A in the DLPFC of these same subjects (Kippe et al, 2015), and decreased protein expression of the fucosyltransferases FUT8 and POFUT2 in the STG in a different subject cohort (Mueller et al, 2017), while both mRNA and protein levels of EDEM2 and UGGT2 are increased in schizophrenia DLPFC. The data presented herein do not contradict our prior reports, but instead suggest that increased mRNA expression may be cellular compensation for insufficient protein expression of these enzymes in schizophrenia.…”

“…B3GALTL in the ER is responsible for elongating O-fucosylproteins following POFUT2-mediated fucosylation of thrombospondin-like repeat (TSR) domains (Sato et al, 2006) and for some ADAMTS (a disintegrin and metalloproteinase with thrombospondin motifs) proteins, the O-linked disaccharide (produced from sequential glycan addition by POFUT2 and B3GALTL) has been shown to be necessary for secretion into the extracellular space (Ricketts et al, 2007;Wang et al, 2007). We have recently reported that the protein expression levels of POFUT2 in the superior temporal gyrus (STG; Brodmann area 22) is decreased in schizophrenia (Mueller et al, 2017). While we did not identify mRNA expression changes of POFUT2 in the current study, this may reflect a brain-region specific alteration in POFUT2 protein levels or the increased transcript levels of B3GALTL identified in DLPFC may arise from inadequate or abnormal TSR-domain O-fucosylation.…”

Altered protein expression of galactose and N-acetylgalactosamine transferases in schizophrenia superior temporal gyrus

“…The investigator who performed western blots was blind to subject diagnosis until study completion, and the investigator who performed statistical analyses was blind to diagnosis during data collection. Sample sizes were determined a priori using a power calculation (β = 0.2, π = 0.8) based on prior assessments of glycosylation enzyme protein expression in our lab (Kippe et al, 2015;Mueller et al, 2017). Data analysis was performed using Prism 6.07 (GraphPad Software Inc., La Jolla, CA) and STATISTICA 7.1 (StatSoft Inc., Tulsa, OK).…”