Inhibitory mechanism by polysialic acid for lamina-specific branch formation of thalamocortical axons

During development, thalamocortical axons form arbors primarily in layer 4 of the neocortex. This lamina-specific branch formation was studied in cult ures of rat thalamic explants grown next to chemically fixed cortical slice s. After a week in vitro, thalamic axons formed branches specifically in th e target layer of fixed cortical slices, regardless of the orientation of t he ingrowth. This in vitro system permits a direct assessment of contributi ons of membrane-associated molecules to thalamic axon branch formation. To this end, the present study uses three enzymatic perturbations: chondroitin ase, phosphatidylinositol phospholipase C, or the polysialic acid (PSA)-spe cific endoneuraminidase (endo N). With endo N pretreatment of cortex, the n umber of branch points was increased significantly, whereas branch tip leng th was decreased. In addition, the localization of branch points to the tar get layer was weakened considerably. These features of branch formation wer e not altered by the other two enzymatic treatments, except that branch tip s were shortened by chondroitinase treatment to the same extent as in endo N treatment. These results suggest that membrane-bound components are invol ved in lamina-specific branch formation of thalamocortical axons, and in pa rticular that PSA moieties contribute to laminar specificity by inhibiting branch emergence in inappropriate layers.