9% ± 10.0% in control to 22.9% ± 15.2% at HSA-LRP4−/− NMJs (Figure S2D), although every AChR cluster was positive for synaptophysin. In agreement, mEPP frequency was reduced by 53.3% at HSA-LRP4−/− NMJs (0.42 ± 0.09/min in HSA-LRP4−/− versus 0.90 ± 0.11/min in control littermates) (Figures 2A, 2D, and 2E), indicating compromised vesicle release in the absence of muscle LRP4. Amplitudes of endplate potentials (EPPs) were decreased from 22.2mV ± 2.17mV in control to 6.75mV ± 2.22mV in HSA-LRP4−/− mice (p < 0.01, control n = 5, and HSA-LRP4−/− n = 4), suggesting a reduction

in quantum contents. During development in wild-type mice, AG-014699 nmr mEPP amplitudes decreased from 2.30mV ± 0.23mV at P0 to 1.63mV ± 0.10mV at P15 NMJs (Figures 2C and 2H), whereas the frequencies increased during this time (from 0.90 ± 0.11/min at P0 to 27.0 ± 3.1/min at P15) (Figures 2E and 2J), in agreement with previous reports (Kelly, 1978). In HSA-LRP4−/− mice, however, mEPP frequencies were unable to increase during this time and were only 2.8% of control littermate at P15 when most mutant mice died selleckchem (Figures 2E and 2J). Notice that mEPP amplitudes in HSA-LRP4−/− mice between P0 and P15 were similar (Figures 2C and 2H). These results indicate that in the absence

of muscle LRP4, the development or maturation of presynaptic terminals is more severely impaired than postsynaptic counterparts. Next, we examined NMJ structures in control and HSA-LRP4−/− mice by electron microscopic analysis. In control or LRP4loxp/+ NMJs, axon terminals were filled with synaptic vesicles, some of which were docked on electron-dense active zones (Figure 3A). Quantitatively, nerve terminal numbers at P0 decreased from 3.16 ± 0.75 in control to 1.5 ± 0.54 in HSA-LRP4−/− mice and at P15 from 4.0 ± 0.89 in control to 1.40 ± 0.55 in HSA-LRP4−/− mice, respectively (p < 0.01, n = 6) (Figure 3B). The number of active zones was decreased from 2.17 ± 0.75 and 3.0 ± 0.89 in control to 1.0 ± 0.63 Dipeptidyl peptidase and 0.33 ± 0.52 in mutant mice at P0 and P15, respectively (p < 0.05 in P0 and p < 0.01 in P15, n = 6) (Figure 3C).

The density of synaptic vesicles within 0.04 μm2 areas surrounding active zones or adjacent to terminals was decreased in both P0 and P15 HSA-LRP4−/− mice (5.67 ± 1.0 in control to 1.67 ± 1.73 in mutants and 9.78 ± 1.20 in control to 3.67 ± 2.12 in mutants, respectively) (p < 0.01, n = 9) (Figure 3D), although the sizes or diameters of synaptic vesicles were similar between control and HSA-LRP4−/− mice (Figure 3E). Muscle LRP4 mutation seems to have little effect on the synaptic cleft width (Figure 3F); however, postjunctional folds, which were occasional at P0 and prominent at P15, were decreased (Figure 3A), as indicated by reduction in postsynaptic membrane length from 9.93 ± 2.57 μm in control to 6.19 ± 1.26 μm in HSA-LRP4−/− mice at P0 (p < 0.05, n = 6) and from 30.2 ± 8.51 μm in control to 15.9 ± 6.90 μm in HSA-LRP4−/− mice at P15 (p < 0.