Perlecan is a component of cartilage matrix and promotes chondrocyte attachment

TLDR

Aggrecan is a major proteoglycan in cartilage, and perlecan, a heparan sulfate proteoglycan, is also present in cartilage matrices such as hyaline cartilage, articular surfaces, and growth plates. The study aimed to characterize synthesis and secretion of aggrecan and perlecan in primary rat chondrocyte cultures. Chondrocytes were pulse‑labeled with [35S]methionine, chased for up to six hours, and the radiolabeled perlecan and aggrecan were immunoprecipitated and analyzed by SDS‑PAGE. Chondrocytes produce precursor proteins for both aggrecan and perlecan, yet only aggrecan is secreted as a fully processed proteoglycan; perlecan is secreted with fewer post‑translational modifications, its ~13‑kb mRNA encodes a 400‑kDa core protein, it carries chondroitin and heparan sulfate chains, and purified perlecan and laminin enhance chondrocyte attachment, demonstrating perlecan’s role as a key cartilage matrix component.

Abstract

ABSTRACT Aggrecan, a chondroitin/keratan sulfate-containing pro teoglycan, is a major component of cartilaginous tissues. Immunolocalization studies, using antibodies directed to perlecan, a heparan sulfate proteoglycan first detected in basement membranes, and laminin (another major component of basement membranes), indicate that perlecan and laminin are also present in the matrices of hyaline cartilage in the nasal septum, the articular surface of the bone and the growth plate of the developing bone. Consequently, we used antibodies to both aggrecan and perlecan to characterize their synthesis and secretion by primary cultures of chondrocytes derived from the rat chondrosarcoma. Chondrocytes were pulsed for 20 minutes with [35S]methionine and then chased for up to six hours. The radiolabeled perlecan and aggrecan were immunoprecipitated and analyzed by SDS-PAGE. The results show that chondrocytes synthesize precursor proteins to both proteoglycans, but that only the aggrecan precursor protein is secreted as a proteoglycan. Perlecan was also secreted but with less posttranslational modifica tions than aggrecan. Northern blot analyses of the RNAs from immortalized rat chondrocytes indicated that the major mRNA encoding for perlecan was approximately 13 kb in length, similar in size to that expressed by other cell types, which synthesize 400 kDa core protein perlecan. Analyses of the proteoglycan fractions from the extracts of bovine articular surface indicated that perlecan in this tissue contains both chondroitin and heparan sulfate side chains. Purified perlecan and laminin were found to promote attachment of immortalized rat chondrocytes in vitro. These studies indicated that perlecan, once thought to be a unique component of the basement membranes, is more widely distributed and is an important component of the cartilage matrix, where it may provide for cell adhesion to the matrix.