Developmental regulation of a glycolipid epitope on actively extending growth cones and central and peripheral projections in the medicinal leech

The monoclonal antibody, A2B5, that recognizes vertebrate gangliosides also recognizes embryonic cells in the medicinal leech (Hirudo medicinalis) in a spatially and temporally regulated way. Furthermore, A2B5-positive glycolipids could be isolated from embryonic leeches. Early in development A2B5 labeled axon tracts within the central nervous system coincident with the initial growth of axons, while later in embryogenesis, A2B5 also labeled the forming peripheral nerves. This central and peripheral staining disappeared during the latter third of embryogenesis. A2B5 also labeled the growing tips of an identified myo-organizing cell. This cell, the C-cell, projects an array of parallel processes and exhibits a discrete transition in the rate of growth cone extension (J. Jellies, and W. B. Kristan, Jr. 1991. Dev. Biol. 148, 334-354.). A2B5 failed to recognize the relatively non motile growth cones of the C-cell during early embryogenesis. The C-cell growth cones began to exhibit A2B5 labeling as they became more rapidly extending and this labeling persisted throughout the later motile phase of C-cell growth. In addition to its widespread distribution on embryonic (but not mature) cellular processes, A2B5 also labeled the mitotic profiles of dividing cells in all tissues. Thus, the A2B5 epitope may be presented intracellularly, or both intra- and extracellularly. When glycolipids were extracted from embryonic leeches, partitioned by elution from a silicic acid column, and analyzed using high-performance thin-layer chromatography, at least one of the major glycolipid bands was resorcinol-positive, consistent with presentation of sialic acid residues. The fraction enriched for the resorcinol-positive band was recognized by A2B5 on dot blots, as was ganglioside GQ1b. While potential mechanisms remain to be examined, on the basis of the distinct distribution of the A2B5 epitope and our finding of A2B5-positive glycolipids in leech embryos, we suggest that complex polar glycolipids may play a role in the extension of cellular projections in the medicinal leech.