Motoneuron Survival Is Enhanced in the Absence of Neuromuscular Junction Formation in Embryos

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Fig. 1.
Defective neuromuscular development in cranial muscles of rapsyn mutants. Whole masseter muscles (A,A′, B, D,D′, E) and sections of tongue muscle (C, F) from embryonic day (E) 18 control (A–C) and rapsyn^−/−mutant mice (D–F) were stained with fluorescein-conjugated anti-neurofilament and anti-SV2 to visualize nerves (A, B, D,E), and with rhodamine-α-bungarotoxin to visualize AChRs (A′, C, D′,F). In controls, axons form rudimentary terminal arbors on AChR-rich patches of myotube membrane. In mutants, motor axons grow excessively and fail to form terminal arbors, and no AChR-rich clusters are present on myotubes. Region marked bybracket in A is shown at higher magnification in B; arrows inB indicate rudimentary terminal arbors. Scale bar (shown in F): A, A′,C, D, D′,F, 100 μm; B, E, 25 μm.
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Fig. 2.
Micrographs of the trochlear nucleus (A, C, E) and trigeminal motor nucleus (B, D,F) of wild-type (A,B), MuSK^−/− (C,D), and rapsyn^−/−(E, F) mice. Note the normal morphological features of the motoneurons in the transgenic mice. Scale bar, 100 μm.

Table 1.

Number of motoneurons in agrin null mutant mice (agrin^−/−) and their wild-type and heterozygous (control) littermates

Control and agrin^−/− values are the average ± SEM. The values in parentheses indicate the number of animals examined (
↵F1-160 p < 0.01). Percentages (%) refer to the increase in the number of motoneurons with respect to the controls.

Table 2.

Number of motoneurons in z-specific agrin mutant mice (agrin^Δz/Δz) and their wild-type and heterozygous (control) littermates

Control and agrin^Δz/Δz values are the average ± SEM. The values in parentheses indicate the number of animals examined (
↵* p < 0.05,
↵F2-165 p < 0.001). Percentages (%) refer to the increase in the number of motoneurons with respect to the controls.
↵F2-a The control values were 1800 and 2128 hypoglossal motoneurons.

Table 3.

Number of motoneurons in MuSK mutant mice (MuSK^−/−) and their wild-type littermates

MuSK^+/+ and MuSK^−/− values are the average ± SEM. The values in parentheses indicate the number of animals examined (
↵F3-160 p < 0.01,
↵F3-165 p < 0.001). Percentages (%) refer to the increase in the number of motoneurons with respect to the controls (MuSK^+/+).

Table 4.

Number of motoneurons in rapsyn null mutant mice (rapsyn^−/−) and their wild-type littermates

Rapsyn^+/+ and rapsyn^−/− values are the average ± SEM. The values in parentheses indicate the number of animals examined (
↵F4-150 p < 0.05,
↵F4-165 p < 0.001). Percentages (%) refer to the increase in the number of motoneurons with respect to the controls (rapsyn^+/+).

Table 5.

Size of motoneurons in μm² in MuSK null mutant mice (MuSK^−/−) and their wild-type littermates

Motoneurons from three wild-type animals and three MuSK^−/− mice were analyzed for each of four cranial nuclei. MuSK^+/+ and MuSK^−/− values are the average ± SEM. The values in parentheses indicate the number of cells examined. Percentages (%) refer to the increase in the number of motoneurons with respect to the controls (MuSK^+/+).

Table 7.

Quantitative analysis of motoneuron populations in mutant mice compared with controls

	Trochlear	%	Trigeminal motor	%	Facial	%	Hypoglossal	%
WT and (+/−)	198 ± 7 (29)		1078 ± 27 (29)		4315 ± 111 (26)		1909 ± 109 (10)
Agrin^−/−	271 ± 27 (7)^7-165	37	1339 ± 58 (6)^7-165	24	4734 ± 89 (5)	10	2412 ± 98 (3)^7-150	26
Agrin^Δz/Δz	259 ± 13 (5)^7-160	30	1380 ± 64 (6)^7-165	28	4964 ± 169 (5)^7-150	15	2518 (1)	32
Rapsyn^−/−	223 ± 16 (7)	12	1511 ± 35 (8)^7-165	40	5238 ± 173 (8)^7-165	21	2567 ± 96 (3)^7-150	35
MuSK^−/−	297 ± 17 (8)^7-165	50	1523 ± 37 (6)^7-165	41	5251 ± 130 (6)^7-160	22	2282 ± 100 (5)^7-150	20
Mean of synaptic mutants	264 ± 9 (27)^7-165	33	1444 ± 28 (26)^7-165	34	5079 ± 110 (24)^7-165	18	2405 ± 60 (12)^7-165	26

Numbers are the average ± SEM. The values in parentheses indicate the number of animals examined (
↵F7-150 p< 0.05,
↵F7-160 p < 0.01,
↵F7-165 p < 0.001). Percentages (%) refer to the increase in the number of motoneurons with respect to the controls. WT, Wild type.