Glycoproteomic profile of human tissue-nonspecific alkaline phosphatase expressed in osteoblasts

Diana Atanasova; Ekaterina Mirgorodskaya; Lavanya Moparthi; Stefan Koch; Mathias Haarhaus; Sonoko Narisawa; José Luis Millán; Eva Landberg; Per Magnusson

doi:10.1093/jbmrpl/ziae006

Glycoproteomic profile of human tissue-nonspecific alkaline phosphatase expressed in osteoblasts

JBMR Plus. 2024 Jan 22;8(2):ziae006. doi: 10.1093/jbmrpl/ziae006. eCollection 2024 Feb.

Authors

Affiliations

¹ Department of Clinical Chemistry, and Department of Biomedical and Clinical Sciences, Linköping University, Linköping SE-58185, Sweden.
² Proteomics Core Facility, Sahlgrenska Academy, University of Gothenburg, Gothenburg SE-41346, Sweden.
³ Wallenberg Centre for Molecular Medicine, Linköping University, Linköping SE-58185, Sweden.
⁴ Department of Biomedical and Clinical Sciences, Linköping University, Linköping SE-58185, Sweden.
⁵ Division of Renal Medicine, Department of Clinical Science, Intervention and Technology, Karolinska Institutet, Karolinska University Hospital, Stockholm SE-14186, Sweden.
⁶ Sanford Children's Health Research Center, Sanford Burnham Prebys Medical Discovery Institute, La Jolla, CA 92037, United States.

Abstract

Tissue-nonspecific alkaline phosphatase (TNALP) is a glycoprotein expressed by osteoblasts that promotes bone mineralization. TNALP catalyzes the hydrolysis of the mineralization inhibitor inorganic pyrophosphate and ATP to provide inorganic phosphate, thus controlling the inorganic pyrophosphate/inorganic phosphate ratio to enable the growth of hydroxyapatite crystals. N-linked glycosylation of TNALP is essential for protein stability and enzymatic activity and is responsible for the presence of different bone isoforms of TNALP associated with functional and clinical differences. The site-specific glycosylation profiles of TNALP are, however, elusive. TNALP has 5 potential N-glycosylation sites located at the asparagine (N) residues 140, 230, 271, 303, and 430. The objective of this study was to reveal the presence and structure of site-specific glycosylation in TNALP expressed in osteoblasts. Calvarial osteoblasts derived from Alpl^+/- expressing SV40 Large T antigen were transfected with soluble epitope-tagged human TNALP. Purified TNALP was analyzed with a lectin microarray, matrix-assisted laser desorption/ionization-time of flight mass spectrometry, and liquid chromatography with tandem mass spectrometry. The results showed that all sites (n = 5) were fully occupied predominantly with complex-type N-glycans. High abundance of galactosylated biantennary N-glycans with various degrees of sialylation was observed on all sites, as well as glycans with no terminal galactose and sialic acid. Furthermore, all sites had core fucosylation except site N271. Modelling of TNALP, with the protein structure prediction software ColabFold, showed possible steric hindrance by the adjacent side chain of W270, which could explain the absence of core fucosylation at N271. These novel findings provide evidence for N-linked glycosylation on all 5 sites of TNALP, as well as core fucosylation on 4 out of 5 sites. We anticipate that this new knowledge can aid in the development of functional and clinical assays specific for the TNALP bone isoforms.

Keywords: N-linked glycosylation; alkaline phosphatase; biomineralization; bone formation; glycoprotein.