THE NANOMELIC MUTATION IN THE AGGRECAN GENE IS EXPRESSED IN CHICK CHONDROCYTES AND NEURONS

We have established the presence of at least two large chondroitin sul fate proteoglycans in the developing chick brain, one that reacts excl usively with HNK-1, a carbohydrate epitope found on several neural spe cific molecules, and one that reacts with S103L, a defined peptide epi tope in the CS-2 domain of the cartilage-specific chondroitin sulfate proteoglycan (CSPG), aggrecan. In order to determine the relationships between the two distinct S103L-reactive CSPGs from cartilage (chondro cytes) and brain (neurons), as well as among the three large CSPGs exp ressed in brain, S103L, HNK-1 and versican, we studied the expression of these multiple proteoglycan species in the brain of nanomelic chick s. We have previously shown that homozygous embryos expressing the nan omelic phenotype exhibit a single point mutation in the aggrecan gene. In the present study, the S103L CSPG is not accumulated or synthesize d by embryonic chick CNS tissue or E8CH neuronal cultures derived from nanomelic chick embryo cerebral hemispheres. In contrast, expression of both versican and the HNK-1 CSPG was normal in the mutant embryo CN S. Pulse chase experiments demonstrated the presence of the 380 kDa pr ecursor in normal neurons and the 300 kDa truncated precursor in nanom elic neurons. Northern blot analysis revealed normal-sized mRNA but re duced levels of expression of the S103L CSPG message in nanomelic neur ons, while expression of the versican message was comparable in normal and nanomelic neurons. Most conclusively, the point mutation previous ly identified in nanomelic cartilage mRNA was also identified in nanom elic brain mRNA. Together these results provide evidence that a single aggrecan gene is expressed in both cartilage and CNS tissue leading t o the production of identical core proteins which then undergo differe ntial and tissue-specific post-translation processing, resulting in th e characteristic tissue-specific proteoglycans. Furthermore, versican and the HNK-1 CSPG, although structurally and chemically similar to th e S103L CSPG, are the products of separate genes. Copyright (C) 1996 I SDN.