Sequential steps in synaptic targeting of sensory afferents are mediated by constitutive and developmentally regulated glycosylations of CAMs

Sensory afferents in the leech are labeled with both constitutive and devel opmentally regulated glycosylations (markers) of their cell adhesion molecu les (CAMs). Their constitutive mannose marker, recognized by Lan3-2 monoclo nal antibody (mAb), mediates the formation of their diffuse central arbors. We show that, at the ultrastructural level, these arbors consist of large, loosely organized axons rich with filopodia and synaptic vesicles. Perturb ing the mannose-specific adhesion of this first targeting step leads to a g ain in cell-cell contact but a loss of filopodia and synaptic vesicles. Dur ing the second targeting step, galactose markers divide afferents into diff erent subsets, We focus on the subset labeled by the marker recognized by L az2-369 mAb. Initially, the galactose marker appears where afferents contac t central neurons. Subsequently it spreads proximally and distally, coverin g the entire afferent surface. Afferents now gain cell-cell contact, with c entral neurons and self-similar afferents, but lose filopodia and synaptic vesicles. Extant synaptic vesicles prevail where afferents are apposed to c entral neurons, These neurons develop postsynaptic densities and en passant synapses are forming. Perturbing the galactose-specific adhesion of this s econd targeting step causes a loss of cell-cell contact but a gain in filop odia and synaptic vesicles, essentially returning afferents to the first ta rgeting step. The transformation of afferent growth, progressing from manno se- to galactose-specific adhesion, is consistent with a change from cell-m atrix to cell-cell adhesion. By performing opposing functions in a temporal sequence, constitutive and developmentally regulated glycosylations of CAM s collaborate in the synaptogenesis of afferents and the consolidation of s elf-similar afferents. (C) 1999 Academic Press.