Fig. 4

Distribution of the dystrophin glycoprotein complex in mouse intrafusal fibers. Panel a shows two intrafusal fibers labeled by anti-dystrophin antibodies (red channel) and by antibodies against the vesicular glutamate transporter 1 (vGluT1; white channel). Panels b–d show the boxed area in panel c at a higher magnification. Note that dystrophin is concentrated in the intrafusal fiber plasma membrane in areas that are not in contact with the sensory neuron. The blue color represents nuclei stained with 4′,6-diamidin-2-phenylindol (DAPI). Panels e–j show the distribution of utrophin (red channel) in the central region of muscle spindles from wildtype (e–g) and from DMD^mdx mice (h–j). Anti-vGluT1 antibodies (green channel in panels e–j) were used to label the sensory nerve terminal. Panels d, g and j show the merged channels. Utrophin is not detectable in the equatorial region of muscle fibers from wildtype mice (e) but severely upregulated in intrafusal fibers from DMD^mdx mice (h). Note the absence of utrophin in the contact area between intrafusal fiber and sensory nerve terminal. Asterisks mark corresponding positions in all panels. Panel k shows a single confocal section of a muscle spindle stained with antibodies against vGluT1 (magenta) and against dystrobrevin (green) to indicate that other components of the DGC have a similar distribution as dystrophin, i.e. are concentrated in areas of the intrafusal fiber that are not in contact with the sensory nerve terminal