Journal of Neuroscience, Vol 14, 2809-2817, Copyright © 1994 by Society for Neuroscience
A novel epitope of entactin is present at the mammalian neuromuscular junction
AY Chiu and J Ko
Division of Neurosciences, Beckman Research Institute of the City of Hope, Duarte, California 91010.
The extracellular matrix (ECM) at the neuromuscular junction (NMJ) is
biochemically and functionally specialized, and bears molecules that can
regulate both the formation and function of this peripheral synapse. We
have previously purified one synaptic component of the muscle ECM--a unique
laminin isoform named s-laminin--from a rat schwannoma cell line (Chiu et
al., 1992). To develop new probes for the ECM, monoclonal antibodies were
generated against other components produced by this cell line. One of these
new antibodies, 9H6, binds selectively at the synaptic cleft of NMJs in
adult rats, but not at extrasynaptic sites on the muscle surface. On
Western blots, 9H6 recognizes a 150 kDa band that colocalizes, and
copurifies with the laminin-binding, ECM glycoprotein entactin under both
reducing and nonreducing conditions. N-terminal sequence analysis also
indicates that the 9H6 antigen is related to entactin. However, polyclonal
antibodies to entactin stain both synaptic and extrasynaptic sites. Thus,
9H6 appears to identify an entactin epitope with a very restricted
distribution. Treatment with N-glycanase reduces the molecular mass of
entactin and eliminates 9H6 binding, suggesting that the 9H6 epitope at
synapses is dependent on glycosylation. Recent studies have shown that
novel isoforms of laminin, collagen IV, agrin, and AChE are selectively
sequestered at the NMJ. Our results indicate that the entactin present at
the synaptic cleft also differs from entactin present outside the synapse.
The synaptic form of entactin may contribute to the unique functions of the
ECM at the neuromuscular synapse.