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Cohen, 1977). In these experiments, the clustering of
AChRs occurs normally at the site of neurite contact in
the absence of cholinergic transmission.
Motor axons
Basal Lamina
remove muscle
Muscle cells
The clustering of AChRs is a result of both migra-
tion within the muscle membrane and, after several
hours, the insertion of newly synthesized protein. The
studies discussed above strongly suggest that nerve
terminals produce a signal that initiates receptor clus-
tering at the postsynaptic cell. Interestingly, AChR
clustering can also be produced by the basal lamina,
an extracellular matrix that ensheathes each muscle
cell. When muscle cells are damaged in adult frogs,
they degenerate, leaving behind the basal lamina
(Figure 8.17). New myofibers then form beneath this
basal lamina, and AChR clusters form at the original
synaptic sites along the basal lamina, even if motor
nerve terminals are absent (Burden et al., 1979). These
results motivated a search for a “clustering” signal
among the extracellular matrix molecules. A proteo-
glycan that is able to mimic the clustering ability of
nerve terminals or the basal lamina, named Agrin, was
subsequently isolated from the electric organ of the
marine ray Torpedo californica , a site rich in cholinergic
synapses (Godfrey et al., 1984; Nitkin et al., 1987).
Monoclonal antibodies directed against Agrin have
been used to localize this protein to motor neuron cell
bodies, the synaptic basal lamina, and muscle cells
(Reist et al., 1987; Magill-Solc and McMahon, 1988;
Fallon and Gelfman, 1989).
To test whether release of neuronal Agrin (called z-
agrin in mammals) is responsible for AChR cluster for-
mation, polyclonal antibodies were used to block its
function in vitro (Reist et al., 1992). The polyclonal
antibodies are raised against Torpedo Agrin, and they
bind selectively to chick Agrin, blocking cluster for-
mation in chick nerve-muscle cultures. However,
muscle cells also produce Agrin, and they could also
induce clustering. Since the polyclonal antiserum does
not block rat Agrin, a co-culture experiment was
designed that made use of tissue from chicks and rats
(Figure 8.18). The antiserum does prevent clustering
on rat muscle cells that are innervated by chick
neurons (Figure 8.18). However, it does not block
AChR cluster formation on chick muscle cells that are
innervated by rat neurons. Thus, rat neurons must be
releasing the Agrin that elicits cluster formation. A
neuron-specific isoform of Agrin, generated by alter-
native splicing of the mRNA, has since been shown to
muscle regenerates
AChR clusters form
under basal lamina
FIGURE 8.17 The extracellular matrix contains a factor that
induces AChR clustering. A. In the adult frog cutaneous pectoris
muscle, the motor axons were cut and the muscle cells were
destroyed, leaving only the basal lamina which contains extracellu-
lar matrix molecules. B. New myofibers are generated as a result of
cell division. C. AChR clusters (green) form on the regenerated
muscle fibers, directly beneath the synaptic portion (red) of the basal
lamina. (Adapted from Burden et al., 1979)
have greater cluster-inducing activity than that found
in muscles (Ruegg et al., 1992; Ferns et al., 1993).
The results suggest that neuron-derived Agrin is
necessary for AChR cluster formation in vivo, and its
contribution to neuromuscular formation in situ was
confirmed with homologous recombination. Agrin
knockout mice demonstrate that this signal is not
required for the embryonic aggregation of AChRs to
the central region of the muscle (Figure 8.15) but is nec-
essary for their alignment with the motor nerve termi-
nal. Furthermore, postsynaptic sites appear to be less
mature than normal, suggesting that this signaling
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