A large chondroitin sulfate proteoglycan has the characteristics of a general extracellular matrix component of adult brain

Abstract

Extracellular matrix (ECM) is a secreted extracellular network. Few components of adult brain ECM are known. We have identified a new, large chondroitin sulfate proteoglycan (T1 antigen) that acts like a general ECM protein of brain. First, it is present throughout the brain; second, it has the properties of an extracellular protein; and third, it is extracted only under denaturing conditions. Immunocytochemical localization of the T1 antigen by light microscope shows it to be present throughout the rat brain in both white and gray matter. The T1 antigen outlines Purkinje and other large cells. No antigenicity is seen inside these cells. Biochemical evidence suggests that the T1 antigen is extracellular rather than cytosolic or intravesicular. The T1 antigen is disulfide-linked to two other proteins. Disulfide bonds are found only in extracellular or intravesicular proteins, not in intracellular cytosolic proteins. Moreover, the T1 antigen is probably not intravesicular. Unlike intravesicular proteins, only a small amount of T1 antigen is solubilized by nondenaturing detergents. While nondenaturing detergents extract but a small amount of T1 antigen from rat brain, the majority is solubilized by denaturing conditions (6 M guanidine-HCl). This behavior is similar to that of ECM components in other tissues and is unlike that of membrane proteins, even those linked to the cytoskeleton. We hypothesize that the insolubility of the T1 antigen in brain is due to its presence in an extracellular aggregate. The T1 antigen is a proteoglycan with a highly glycosylated protein core of 300 kDa. It does not appear to be related to the large, heavily glycosylated chondroitin sulfate proteoglycans aggrecan and versican, which were discovered in non-neural tissues. Antibodies to a 15 residue peptide present in both aggrecan and versican do not react with the T1 antigen.