Definition of pre- and postsynaptic forms of the CT carbohydrate antigen at the neuromuscular junction: ubiquitous expression of the CT antigens and the CT GalNAc transferase in mouse tissues.

At the rodent neuromuscular junction, the synaptic expression of the CT carbohydrate antigens is defined by the binding of two monoclonal antibodies, CT1 and CT2. CT1 preferentially stains the presynaptic membrane, while CT2 preferentially stains the postsynaptic apparatus. Here we show that the differential subsynaptic distribution of these antigens is due to a preference of CT1 for structures containing N-acetyl neuraminic acid (NeuAc) and a preference of CT2 for structures containing N-glycolyl neuraminic acid (NeuGc). This was found to be the case both in binding to cultured myotubes, where NeuAc/NeuGc levels were manipulated by feeding acetylated N-acetyl mannosamine precursors, and in binding to purified GM2 ganglioside containing either NeuAc or NeuGc. At human neuromuscular junctions, where the enzymatic machinery to make NeuGc is absent [Proc. Natl. Acac. Sci. USA 95 (1998) 11751], CT1 and GM2(NeuAc) antibodies stained, while CT2 did not. Thus, the N-glycolyl modification of sialic acid helps to define the differential distribution of the CT antigens at the rodent neuromuscular junction, and this difference is lost in humans. In addition, sulfatase and 9-O-acetylesterase treatment of cells or tissues increased the amount of CT1 and CT2 antibody binding, with sulfatase differentially unmasking CT antigen expression on particular glycoproteins. Despite its uniquely synaptic localization in skeletal muscle, the CT antigens and the CT GalNAc transferase are ubiquitously expressed in other mouse tissues, including brain, spinal cord, and peripheral nerve. One of the proteins that can be co-purified with a CT-reactive glycoprotein is alpha dystroglycan. These data better define the sub-synaptic structures of the CT carbohydrate antigens at the neuromuscular junction and demonstrate their ubiquitous presence in mouse tissues, including the brain.