Brevican, a chondroitin sulfate proteoglycan of rat brain, occurs as secreted and cell surface glycosylphosphatidylinositol-anchored isoforms

Abstract

cDNA clones encoding proteins related to the aggrecan versican family of proteoglycan core proteins have been isolated with antisera against rat brain synaptic junctions. Two sets of overlapping cDNAs have been characterized that differ in their 3'-terminal regions. Northern analyses with probes derived from unique regions of each set were found to hybridize with two brain-specific transcripts of 3.3 and 3.6 kilobases (kb). The 3.6-kb transcript encodes a polypeptide that exhibits 82% sequence identity with bovine brevican and is thought to be the rat ortholog of brevican. Interestingly, the polypeptide deduced from the open reading frame of the 3.3-kb transcript is truncated just carboxyl-terminal of the central domain of brevican and instead contains a putative glypiation signal. Antibodies raised against a bacterially expressed glutathione S-transferase-brevican fusion protein have been used to show that both soluble and membrane-bound brevican isoforms exist. Treatment of the crude membrane fraction and purified synaptic plasma membranes with phosphatidylinositol-specific phospholipase C revealed that isoforms of brevican are indeed glycosylphosphatidylinositol-anchored to the plasma membrane. Moreover, digestions with chondroitinase ABC have indicated that rat brevican, like its bovine ortholog, is a conditional chondroitin sulfate proteoglycan. Immunohistochemical studies have shown that brevican is widely distributed in the brain and is localized extracellularly. During postnatal development, amounts of both soluble and phosphatidyl-inositol-specific phospholipase C- sensitive isoforms increase, suggesting a role for brevican in the terminally differentiating and the adult nervous system.