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CHAPTER
8
Bone Matrix Proteoglycans in Skeletal
Function
Vardit Kram and Marian F. Young
National Institutes of Dental and Craniofacial Research, National Institutes of Health,
Bethesda, MD, USA
The extracellular matrix (ECM) of bone is a dynamic
network of cell-secreted molecules. During skeletal tis-
sue development and remodeling, there are extensive
variations in the types and amounts of extracellular
macromolecules that are synthesized and expressed.
The main manifestations of osteogenesis imperfecta
(OI), namely, weak brittle bones, short stature, sclera
opacity, loose joints and poor teeth, are reminiscent
of the manifestations of proteoglycan (PG) deficien-
cies. Interestingly, certain PGs are known to directly
bind to collagen fibrils and to influence collagen fibril
diameter, intermolecular crosslinking and aggregation
into thicker bundles.
1
Since the interaction of collagen
with non-collagenous proteins of the matrix is essen-
tial for bone formation, and being the most abundant
non-collagenous components of the ECM, it is logical
to hypothesize that recognizing a proteoglycan's role in
musculoskeletal biology may be crucial for understand-
ing some forms of OI. Several other experimental obser-
vations link OI and PGs; for example, pre-osteoblasts
isolated from OI patients synthesize reduced amounts
of proteoglycans indicating they may be downstream
mediators of defective collagen.
2-4
PGs are widely expressed macromolecules composed
of a protein core to which at least one glycosaminogly-
can (GAG) side chain is covalently bound via a serine
residue. GAG chains are linear, negatively charged
polysaccharides. The GAG chains can be divided into
two distinct groups based on the presence of the hexo-
samine isomers: glucosaminoglycans (such as heparin
and keratin) have D-glucosamine and the galactosami-
noglycans (chondroitin and dermatan) that are
composed of D-galactosamine. Further, GAG residues
may either be sulfated (e.g., chondroitin sulfate (CS),
dermatan sulfate (DS), keratan sulfate (KS), heparane
and heparan sulfate (HS)) or non-sulfated hyaluronan
(also called hyaluronic acid, HA). The GAG chain iden-
tity relays both structural and functional characteristics
to the PG. For example, GAGs rich in iduronic acid are
more flexible and have a higher affinity to collagen than
GAGs rich in glucuronic acids.
5-7
The synthesis of PG is a multi-step process. The core
protein is translated by ribosomes and the protein is
then translocated into the rough endoplasmic reticu-
lum (rER). Most
O-
glycosylation and all sulfation of
the proteoglycan take place in the Golgi apparatus
in multiple enzymatic steps, with different modulat-
ing enzymes localized in specific compartments of the
Golgi. For
O-
linked glycosylation a tetrasaccharide link
is built on to a serine within the core protein that serves
as a primer for polysaccharide growth. Two sugars are
then added alternatively by two glycosyl transferases.
8
In terms of the GAG side chains, CS chains are found
mostly on ECM-localized PGs, whereas membrane-
associated PGs usually contain HS chains.
9
Precursor
HS chains are synthesized in the Golgi as non-sulfated
copolymers attached to the HSPG core protein. The first
step of HS biosynthesis is polymerization, which is cat-
alyzed by members of the Ext family (Ext1 and Ext2).
Similar processes generate CS and KS PGs, but use dif-
ferent enzymes. Once the assembly of the initial chain
is completed, modifications occur - some amino groups
are subsequently sulfated.
10
After synthesis, PGs are
either translocated to the cell surface (i.e., membrane
