Healthcare and Medicine Reference
In-Depth Information
A
Controls
AChR
MuSK
A
Chick motor neuron
Rat motor neuron
t = 0
Agrin added
t = 1 hour
Chick
muscle
Rat
muscle
P
P
P
P
P
P
MuSK
Anti-Agrin antibody
t = 4 hours
Chick motor neuron
Rat motor neuron
P
P
P
P
P
P
t = 24 hours
P
P
P
Chick
muscle
Rat
muscle
P
P
B
M uSK phosphorylation
100
100
Rat neural Agrin
Chick neural Agrin
Anti-chick Agrin antibody
Rat muscle Agrin
Chick muscle Agrin
80
80
B
Chicken neuron - rat muscle
Rat neuron - chick muscle
60
60
140
140
120
100
120
100
80
60
40
20
0
40
40
80
60
40
20
0
20
20
0
0
0
2
4
6
8
24
Time (hours)
Control
Anti-Agrin
Control
Anti-Agrin
FIGURE 8.19 Agrin induces ACh receptor phosphorylation
prior to clustering. A. When cultures are exposed to Agrin, the Agrin
receptor (MuSK) is phosphorylated within minutes (blue line), and
the AChRs are maximally phosphorylated within an hour (red line).
Receptor aggregation occurs over the next few hours (green line),
but the level of phosphorylation begins to decline. B. The plot shows
a gradual decline in phosphorylation over 24 hours. In contrast,
AChR clustering is maximal by six hours after Agrin exposure and
declines slightly by 24 hours. (Adapted from Ferns et al., 1996; Glass
et al., 1996)
FIGURE 8.18 Neural Agrin induces ACh receptor clusters. A. To
test whether neural Agrin or muscle Agrin induced AChR clusters,
mixed chick-rat cultures were produced. This strategy was used
because a function-blocking antibody (black pentagon) exists for
chick Agrin. Normal AChR aggregation was observed in control het-
erospecific cultures (Controls, top). The antibody blocked AChR
clustering when the motor neuron was from chick (Anti-Agrin anti-
body, left) but not when the muscle cells were from chick (right).
MuSK is the Agrin receptor. B. These results were quantified for each
type of culture. The anti-Agrin antibody decreased the number of
AChR aggregates to 25% in chick motor neuron cultures. (Adapted
from Reist et al., 1992)
phosphorylation, and the AChR b- and d-subunits are
sites of action in chick and mouse muscle cells (Wallace
et al., 1991; Qu and Huganir, 1994). In fact, an inhibitor
of tyrosine kinase can prevent AChR clustering in
response to Agrin (Wallace, 1994; Ferns et al., 1996). The
temporal relationship between b-subunit phosphoryla-
tion and receptor clustering is quite close. Receptor
phosphorylation reaches a peak within one hour, and
receptor clustering then occurs over the next 6 hours.
By the time that clustering has reached a maximum,
phosphorylation is in steep decline (Figure 8.19). The
amount or location of receptor subunit phosphoryla-
tion may also interfere with cluster formation. For
pathway regulates more than just receptor clustering
(Gautam et al., 1996).
PO STSYNAPTIC RESPONSE TO AGR IN
Although there is strong evidence that neural Agrin
initiates AChR clustering at the developing NMJ, the
signal transduction mechanism is not fully resolved. It
is known that Agrin induces postsynaptic tyrosine
 
 
 
 
 
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