Protective Coating of Elastin Fibers

Recent data make it possible to picture HA and proteoglycans as part of the covering of elastic fibers. It has been known for a long time that amorphous elastin is covered with microfibrils. In early embryonic development, which can be as early as 23 h in the chick, elastin colocalized with fibrillin-1 and with fibulin-1 (122). Fibrillin-2 was also expressed at that stage and all three are known to be associated with cross-linked elastin. The elastic fiber assembly process involves association with fibrillin and the hydrophobic sequence in exon 30 is a major element (123). Versican variant V3, when over expressed in smooth muscle cells, increases expression of tropoelastin and the formation of elastic fibers (124) By electron microscopic immunolocalization of antiversican antibodies the C-terminal region of the proteoglycan versican was shown to bind to fibrillin microfibrils (42). The localization appeared to be on or near the beads on the microfibrils. Other data suggested that the bond was covalent. The direct interactions of versican with HA (38,39) and link protein (39) and the fact that versican has been shown to form intermolecular disulfide bonds (43) have already been mentioned. A model for a protective coating of elastin fibers consisting of a proteoglycan and its bound HA, both of which have been detected within normal elastic fibers of human dermis (125), can now be outlined (Fig. 3). A network of fibrillin surrounds the elastin fiber. Versican binds to the fibrillin through its C-terminal region and to HA through its amino terminal region. HA can align itself to another HA molecule which can bind proteoglycans, possibly heparan sulfate proteoglycans (not shown), and this can lead to the formation of larger networks. This presents HA in a strategic structural position.

Heparan Sulfate Elastin

Figure 3 Proposed model for HA association with an elastin fiber. Versican binds to the beaded string fibrillin microfibrils and to HA. A second molecule of HA aligns itself onto the HA. Visualize the network as surrounding the fiber on all sides and forming networks with other components, e.g., heparan sulfate proteoglycans, that can bind to the second molecule of HA.

Figure 3 Proposed model for HA association with an elastin fiber. Versican binds to the beaded string fibrillin microfibrils and to HA. A second molecule of HA aligns itself onto the HA. Visualize the network as surrounding the fiber on all sides and forming networks with other components, e.g., heparan sulfate proteoglycans, that can bind to the second molecule of HA.

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