Matriglycan: a novel polysaccharide that links dystroglycan to the basement membrane

Abstract

Associations between cells and the basement membrane are critical for a variety of biological events including cell proliferation, cell migration, cell differentiation and the maintenance of tissue integrity. Dystroglycan is a highly glycosylated basement membrane receptor, and is involved in physiological processes that maintain integrity of the skeletal muscle, as well as development and function of the central nervous system. Aberrant O-glycosylation of the α subunit of this protein, and a concomitant loss of dystroglycan's ability to function as a receptor for extracellular matrix (ECM) ligands that bear laminin globular (LG) domains, occurs in several congenital/limb-girdle muscular dystrophies (also referred to as dystroglycanopathies). Recent genetic studies revealed that mutations in DAG1 (which encodes dystroglycan) and at least 17 other genes disrupt the ECM receptor function of dystroglycan and cause disease. Here, we summarize recent advances in our understanding of the enzymatic functions of two of these disease genes: the like-glycosyltransferase (LARGE) and protein O-mannose kinase (POMK, previously referred to as SGK196). In addition, we discuss the structure of the glycan that directly binds the ECM ligands and the mechanisms by which this functional motif is linked to dystroglycan. In light of the fact that dystroglycan functions as a matrix receptor and the polysaccharide synthesized by LARGE is the binding motif for matrix proteins, we propose to name this novel polysaccharide structure matriglycan.

Keywords: LARGE; O-mannosyl glycan; POMK; dystroglycan; muscular dystrophy.

Fig. 1.

O-Mannosyl glycans identified on α-dystroglycan. The O-mannosyl glycan structures designated as cores M1, M2 and M3 are shown surrounded by dotted lines. The modifications outside each box are those known to enable extension of the respective core glycan. 3S represents 3-O-sulfation. The three domains of α-dystroglycan are indicated below the glycan structures. Green circles and yellow squares indicate O-mannose and O-GalNAc-initiated glycans on the mucin-like domains, respectively (the numbers and the order of glycosylation sites do not follow the published mapping studies precisely). Symbolic representations of monosaccharides and other molecules are described in the box at the bottom. DGN stands for the N-terminus of α-dystroglycan.

Fig. 2.

O-Linked glycosylation of α-dystroglycan's mucin-like domain, and enzymes in the protein processing pathway that are known or predicted to contribute. The left-hand box indicates sequential O-glycosylation steps that occur in the ER. The right-hand box indicates how the nascent dystroglycan can be modified by various glycosyltransferases (GTs) in the Golgi after being processed in the ER. Currently, it is unclear how a defect in POMGNT1 perturbs the post-phosphoryl modification on the phosphorylated core M3 glycan in the Golgi. Defects in DOLK, GMPPB, ISPD and DPM1, 2 and 3 are predicted to perturb POMT activity by reducing the availability of the donor substrate (Table I) or co-factor(s) (Willer et al. 2012). The structure of the moiety that links the phosphorylated core M3 glycan to matriglycan (indicated as [?]) has not yet been solved. Symbols are colored as in Figure 1.

Fig. 3.

The polysaccharide (matriglycan) synthesized by LARGE. (A) The chemical structure of matriglycan. (B) Solid-phase assay testing binding to laminin-111. ELISA plates were coated with biotinylated high-MW matriglycan (>13 disaccharide repeats, closed circles), low-MW matriglycan (<13 disaccharide repeats, open circles) or GlcA (negative control, closed square). (C) Quantitation of data from a solid-phase assay testing the binding of sugars that are described in (B) by mAb IIH6. (B) and (C) are modified from Figure 3 and extended data from Figure 7, respectively, in Goddeeris et al. (2013), and the error bars indicate s.e.m. (n = 3).