Heparan sulphate and HB-GAM (heparin-binding growth-associated molecule) in the development of the thalamocortical pathway of rat brain

Extracellular matrix (ECM) molecules, such as laminin, tenascin, chondroiti n sulphate proteoglycans and heparan sulphate proteoglycans have been sugge sted to have 'signpost' and directing roles in the formation of axonal proj ections in cortical development. We show here that the expression of the ne urite outgrowth-promoting protein heparin-binding growth-associated molecul e (HB-GAM) and N-syndecan, a transmembrane heparan sulphate proteoglycan pr eviously isolated as a receptor for HB-GAM, is spatiotemporally associated with the developing thalamocortical pathway in the rat brain. Using in situ hybridization, thalamic neurons were shown to express mRNA for N-syndecan, and in vitro, thalamic neurons grew more neurites on HB-GAM than on lamini n. The HB-GAM-induced neurite outgrowth in thalamic neurons was inhibited b y heparitinase, heparin, soluble N-syndecan and by an excess of soluble HB- GAM in the culture medium. In a pathway assay, thalamic neurons selectively preferred attaching and growing neurites on matrices containing HB-GAM tha n on those containing fibronectin or laminin alone, suggesting that HB-GAM may modulate the effect of other ECM proteins. On an unfixed brain slice pr eparation, thalamic neurons repeatedly showed a typical neurite outgrowth a nd attachment pattern resembling the expression pattern of HB-GAM. On the b rain slices, the neurite outgrowth was significantly inhibited by heparitin ase, heparin and soluble HB-GAM, thus displaying features of neurite outgro wth on matrix-bound HB-GAM. Our results suggest that HB-GAM is important fo r the neurite outgrowth of thalamic neurons and it may function as an ECM-b ound guidance cue for thalamic neurons that possess HB-GAM-binding heparan sulphates on their cell membrane.