Fig 2: Additional structural alterations:a shift of triads orientation from transverse to longitudinal in fast twitch fibers of EDL and sternomastoid (A, larger arrow) resulting in the development of large jSR plaques carrying multiple rows of feet (D and E). This occurs in fast twitch fibers of Tdn-null fibers, as previously reported, and in the double null mutants. B) The double null mutant fibers show a small number of quite large sacs always located in correspondence of the Z-line (not shown) and filled with a finely granular material similar to the CASQ content of the jSR (star) and some flat SR cisternae (A, B, white arrows, C and detail in F). The flat SR cisternae are separated by small densities that are clearly different from feet (compare D and F, at the same magnification). Bars: A, 0.5 µm; B, 0.5 µm; C, 0.1 µm; D–F, 0.1 µm.

Fig 3: Expression levels of muscle calcium-regulated proteins in mice after a high-fat diet. Protein levels in control mice (CONT) and high-fat diet-fed mice (HFD; upper panel: 4 weeks; lower panels: 12 weeks). Left: representative western blots of RyR, DHPR, CSQ, SERCA1 and PV proteins (A: 4 weeks; C: 12 weeks). A protein expression levels expressed relative to the value of CONT mice (B: 4 weeks; D: 12 weeks). Values are means ± SE (n = 7).

Fig 4: Images from thin sections at right angle to the T-tubule long axis (A) and parallel to it (B, C and inset) from WT EDL muscles.A) In WT muscles the two jSR cisternae (jSR) facing the central T-tubule (TT, white space) are relatively large, they contain electron dense polymer of CASQ and are joined to T-tubules by two feet. B, C and inset). A row of periodically disposed feet (RyRs, blue dots in C) fills the jSR-T tubule junctional gap and the profiles of small anchors (green in C) project into the jSR lumen (yellow in C) in a position alternate to that of feet (arrowheads in C, better seen in the inset). The distal ends of anchor connect to a thin linear density, particularly prominent in C, presumably a long CASQ polymer. Bars: A–C, 0.1 µm; inset, 0.05 µm.

Fig 5: Phosphorylated glycogen phosphorylase, GPa, localizes at SR.(A) Sections of a z-stack (corrected as in Figure 4) of human muscle co-stained for GPa and Casq1(I.D. #164, MHN). (B) 3-D renderings of the z-stacks that contained the sections in A. Both proteins delineate two terminal cisternae, TC, in every triad. The inset in Ac shows close proximity of the imaged proteins, without actual colocalization (documented with Figure 5—figure supplement 1). (C, D) Sections and stack from a human myofiber co-stained for GPa and STIM1 (I.D. #164, MHN). GPa delineates two TC in every triad (Ca, Da). STIM1 forms clusters in close proximity to the GPa bands, without actual colocalization. Minor quantities of both proteins are located outside triads. A large quantity of STIM1, unaccompanied by GPa, appears inside a nucleus (cf. (Lee et al., 2018). These images are the first to resolve TC with conventional optics (a resolution of ~100 nm is estimated with Figure 5—figure supplement 2).