Versican

Versican is a large chondroitin sulfate proteoglycan with an apparent molecular mass of more than 1000kDa. In 1989, Zimmermann and Ruoslahti cloned and sequenced the core protein of fibroblast chondroitin sulfate proteoglycan. They designated it versican in recognition of its versatile modular structure. Versican belongs to the lectican protein family, with Aggrecan (abundant in Cartilage), Brevican and Neurocan (nervous system proteoglycans) as other members. Versican is also known as Chondroitin sulfate proteoglycan core protein 2 (CSPG-2), PG-M, and Chondroitin sulfate proteoglycan 2.

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Structure

These proteoglycans share a homologous globular N-terminal, C-terminal, and glycosaminoglycan (GAG) binding regions.

The N-terminal (G1) globular domain consists of Ig-like loop and two link modules, and has Hyaluronan (HA) binding properties.

The C-terminal (G3) globular domain consists of one or two Epidermal growth factor (EGF) repeats, a C-type lectin domain and complement regulatory protein (CRP)-like domain. The C-terminal domain binds a variety of ligands in ECM which contribute significantly to the functions of lecticans.

The central domain of versican V0 contains both the GAG-α and GAG-β domains. V1 isoforms has the GAG-β domain, V2 has the GAG-α domain, and V3 is void of any GAG attachment domains. The GAGs, being composed of repeating disaccharide units, contribute to the negative charge and many other properties of proteoglycans.

Function

The role of versican in cell adhesion, migration, and proliferation is extensively studied. Versican is often considered an anti-adhesive molecule. Considering the large size (>1000 kDa) and hydration capability of versican, it is possible that the interaction of integrins (large family of cell adhesion molecules) with their cell surface receptors is sterically hindered.

N-terminus

The N-terminal of Versican has an important role in maintaining the integrity of the ECM by interacting with hyaluronan. Its interactions with link protein has also been studied.

C-terminus

The C-terminal of Versican interacts with a variety of molecules in the matrix. One important family of ligands is the tenascin family. For example, The C-lectin domain of versican interacts with Tenascin R through its fibronectin type III (FnIII) repeat 3-5 domain in a calcium dependent manner, in vivo. Different tenascin domains interact with a wide range of cellular receptors, including integrins, cell adhesion molecules and members of the syndecan and glypican proteoglycan families. Versican’s C-terminal domain interacts with Fibulin-2, a protein whose expression is associated with that of versican in the developing heart. The EGF domain of the C-terminal of versican also binds the EGF-receptor molecule, in vivo.

Glycosaminoglycan binding region

The central domain of Versican is decorated with glycosaminoglycans. The structural and functional diversity of Versican is increased by variations in GAG sulfation patterns and the type of GAG chains bound to the core protein. There is a single versican gene, however alternative splicing of its mRNA produces 4 distinct versican isoforms that differ in their potential number of GAG chains. All isoforms have homologous N-terminal (HA binding) and C-terminal (lectin-like) domains. The central domain of versican V0 contains both the GAG-α and GAG-β domains. V1 isoforms has the GAG-β domain, V2 has the GAG-α domain, and V3 is void of any GAG attachment domains, and only consists of the N-terminal and C-terminal globular domains. It is known that the isoforms are differentially expressed in different tissue types. The biological significance of alternative splicing is yet to be determined.

Because of their negatively charged sulfates or carboxyl groups, chondroitin sulfate chains are attracted to various positively charged molecules such as certain growth factors, cytokines, and chemokines. This interaction in the extracellular matrix or on the cell surface is important in the formation of immobilized gradients of these factors, their protection from proteolytic cleavage, and their presentation to specific cell-surface receptors. The binding of versican with leukocyte adhesion molecules L-selectin, P-selectin, and CD44 is also mediated by the interaction of CS chains of versican with the carbohydrate-binding domain of these molecules. Both CD44 and L-selectin have been implicated in leukocyte trafficking. The ability of versican to bind a large panel of chemokines and the biological consequences of such binding has also been examined. Versican can bind specific chemokines through its CS chains and this interaction down-regulates the chemokines function. Recently, in light of results that V1 and V2 isoforms of versican have opposite effects on cell proliferation, glycosaminoglycan domain GAG-β has been implicated in versican-enhanced cell proliferation and versican-induced reduction of cell apoptosis.

Expression

Expression of versican is observed in various adult tissues such as blood vessels, skin, and developing heart. Smooth muscle cells of blood vessels, epithelial cells of skin, and the cells of central and peripheral nervous system are a few examples of cell types that express versican physiologically. Versican is involved in development, guiding embryonic cell migration important in the formation of the heart and outlining the path for neural crest cell migration.

Versican is a key factor in inflammation through interactions with adhesion molecules on the surfaces of inflammatory leukocytes and interactions with chemokines that are involved in recruiting inflammatory cells. Increased versican expression is often observed in tumor growth in tissues such as breast, brain, ovary, gastrointestinal tract, prostate, and melanoma, Sarcoma, and mesothelioma. In the adult central nervous system, versican is found in perineuronal nets, where it may stabilize synaptic connections. Versican can also inhibit nervous system regeneration and axonal growth following an injury to the central nervous system.