Biomedical Engineering Reference
In-Depth Information
27.3 TISSUE-SPECIFIC EXPRESSION OF HSPGs
AND THE ENZYMES THAT MODIFY THEM
and the level of the expression determine the extent of
N-sulfation of HS.
Diversity of cell-specific HS expression has been shown
in an elegant study that generated HS saccharide-specific
monoclonal antibodies [27]. Each of the monoclonal anti-
bodies generated against a specific HS structure showed a
spatially restricted pattern of expression, demonstrating that
certain HS structures are tissue-specific. Since commonly
occurring saccharide structures are not likely to elicit an
immune response in the host, a phage display technique,
which does not rely on the host immune response, was used
for the generation of these antibodies that revealed the
segregation of distinct HS structures in specific cellular
locations.
The expression and activity of enzymes that process
either the core proteins or the HS of HSPGs are often closely
coupled to biological processes. Under normal conditions,
growth factors and other proteins bound to HSPGs are kept
in an inactive, sequestered state. Following a pathological
condition, such as tissue injury or microbial infection,
metalloproteinases are activated that cleave the ectodomains
of the core proteins, resulting in their shedding from the cell
surface as well as the release of their bound factors that can
then become functionally active [18]. Enzymes that specifi-
cally degrade HS, such as heparanases, have long been
correlated with tumor metastasis in various cancers [19].
Cleavage of HS leads to enhanced cell mobility as a conse-
quence of HS remodeling in ECM of epithelial and endo-
thelial cells.
Thus, in addition to their immense diversity in structure,
the biologically driven synthesis and degradation of HSPGs
can result in spatially restricted signaling that is consistent
and reproducible. It is not surprising that both growth factors
as well as viruses have evolved to fully exploit these
properties of HSPGs and therefore, it seems a natural
next step to exploit these natural “barcodes” for enhanced
drug efficacy and targeted drug delivery.
In order for HSPGs to be useful for protein targeting, they
need to be expressed in a cell- and tissue-type-specific
manner that is regulated by distinct physiological and/or
pathological signals. For example, three of the four synde-
cans are distributed in tissue-specific patterns [10,21]. Syn-
decan-1 is common to many epithelia, developing
mesenchyme, and some leukocytes. Syndecan-2 is enriched
not only in many mesenchymal cell types, but in developing
neural tissues as well. Syndecan-3 is mostly present in neural
tissue and in the musculoskeletal system. In contrast, syn-
decan-4 is widely distributed across the organism. Similarly,
the HSPG agrin is present on most cell types, but only
neurons produce an alternatively spliced isoform, Z
agrin,
that produces an 8-19 amino acid domain that increase its
acetylcholine receptor clustering activity dramatically and
also encodes a heparin-binding domain (HBD) that acts
synergistically with another growth factor called neuregu-
lin1 [22]. Specific tissue distribution can also be regulated by
environmental stress. Syndecan-2 is expressed in amoeboid
microglial cells of the corpus callosum of the brain in
neonatal rats during development and increases significantly
upon a hypoxic exposure [23].
However, simply expressing a given core protein, does
not necessarily mean that the structure of the HS chain is the
same. The size, structure, and ligand affinity of the HS
chains on a single proteoglycan species may differ signifi-
cantly when expressed by different cell types [24]. For
example, syndecan-1 from different cell types shows varied
lengths of the HS chains as well as highly reproducible
differences in the amount of certain sugar modifications that
include the number of highly sulfated domains. Moreover,
while knocking out the syndecan-1 gene in mice produces
enhanced leukocyte adhesion under inflammatory condi-
tions and delayed skin and cornea wound healing, it does
not affect the cell-specific composition of HS in the affected
tissue [25]. This finding suggests that much of the cell-
specificity of HS structures depends mostly on the regulation
and expression of the biosynthetic enzymes that determine
the specific HS structures. Consistently, enzymes that
polymerize and modify HS chains are expressed in a tis-
sue-specific and developmental stage-dependent manner to
produce distinct HS sequences. For example, NDSTs are the
first enzymes engaged in the modification of the sugar
backbone and are believed to create substrates for further
enzymatic activity along the polymer [26]. There are four
isozymes in the vertebrate family of NDSTs. Although they
share the same overall structure and possess a high degree of
protein identity, the expression profiles are different. NDST1
and NDST2 are expressed abundantly and ubiquitously in all
tissues, whereas NDST2 and NDST4 are restricted to adult
brain and fetal tissues. Both the type of enzyme expressed
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27.4 HEPARIN-BINDING PROTEINS
AND GROWTH FACTORS
Many cells are constantly poised to respond to a large variety
of growth and differentiation factors through cell-surface
receptors. In maintaining biological homeostasis, however,
it is important that these potent signaling factors are only
presented to such a cell at the right place and time. One way
that this can be achieved is through the localized release and
selective binding of ligands to the ECM through highly
specific interactions with HSPGs. While these are generally
fairly low-affinity interactions (micromolar to high nano-
molar range), they serve to concentrate the ligand near its
high-affinity cell-surface receptor as well as protect it from
proteolysis [28]. Not surprisingly, protein-HS interactions
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