Biogenesis of transverse tubules and triads: immunolocalization of the 1,4-dihydropyridine receptor, TS28, and the ryanodine receptor in rabbit skeletal muscle developing in situ.

Abstract

Abstract. Our previous immunofluorescence studies support the conclusion that the temporal appearance and subcellular distribution of TS28 (a marker of transverse (T) tubules and caveolae in adult skeletal muscle [Jorgensen, A. O., W. Arnold, A. C.-Y. Shen, S. Yuan, M. Gover, and K. P. . J. Cell Biol. 110:1173-1185), correspond very closely to those of T-tubules forming de novo in developing rabbit skeletal muscle (Yuan, S., W. Arnold, and A. O. . J. Ceil Biol. 110:1187-1198.To extend our morphological studies of the biogenesis of T-tubules and triads, the temporal appearance and subcellular distribution of the o~t-subunit of the 1,4-dihydropyridine receptor (a marker of the T-tubules and caveolae) was compared to (a) that of TS28; and (b) that of the ryanodine receptor (a marker of the junctional sarcoplasmic reticulum) in rabbit skeletal muscle cells developing in situ (day 19 of gestation to 10 d newborn) by double immunofluorescence labeling.The results presented show that the temporal appearance and relative subcellular distribution of the ct~-subunit of the 1,4-dihydropyridine receptor (otI-DHPR) are distinct from those of TS28 at the onset of the biogenesis of T-tubules. Thus, in a particular developing myotube the al-DHPR appeared before TS28 (secondary myotubes; day 19-24 of gestation). Furthermore, the otI-DHPR was distributed in discrete foci at the outer zone of the cytosol, while TS28 was confined to foci and rod-like structures at the cell periphery. As development proceeded (primary myotubes; day 24 of gestation) ,050% of the foci were positively labeled for both TS28 and the o~-DHPR, while ,020 and 30% of the foci were uniquely labeled for TS28 and the otl-DHPR, respectively. The foci labeled for both TS28 and the t~1-DHPR and the foci uniquely labeled for TS28 were generally confined to the cell periphery, while the foci uniquely labeled for the aI-DHPR were mostly confined to the outer zone of the cytosol.

Formation of junctions involved in excitation-contraction coupling in skeletal and cardiac muscle.

“…This idea received support from the finding that during myogenesis in vivo and in vitro (39,42,43) and upon differentiation of a myogenic cell line (F. Protasi, C.F.-A., and B.E.F., unpublished observations), DHPRs and RyRs appear simultaneously and form colocalized clusters in their respective membrane compartments. It now turns out that junction formation is much more complex and that although DHPRs and RyRs are clearly the functionally significant components of the junction, other, as of now unidentified proteins are necessary for the development of junctions between SR and surface membranes.…”

The endoplasmic reticulum-sarcoplasmic reticulum connection: distribution of endoplasmic reticulum markers in the sarcoplasmic reticulum of skeletal muscle fibers.

“…Ca2+-ATPase appears to reach the SR rapidly after synthesis by membrane-bound ribosomes, probably by simple diffusion along ER-SR continuities (39). In contrast, developmental studies have suggested that the ryanodine receptor first concentrates in specific vesicles that then fuse with the SR and adhere to the sarcolemma transverse tubules to yield the triads (40). This alternative pathway could be followed by the membrane proteins concentrated in the TC, in particular the 64-, 58-, and 28-kDa proteins that seem to coexist with the ryanodine receptor in the JFM.…”

Endoplasmic reticulum: a dynamic patchwork of specialized subregions.

“…The Ca" channel works as a voltage sensor that triggers activation of the RyR as soon as the sarcolemma is depolarized (Rios et al, 1991). This interaction is established soon during development and appears to play a key role also in the biogenesis of the SR (Yuan et al, 1991).…”

“…The same might be true for Ca21 stores of nonmuscle cells and for the ER degradation subcompartment, for which no direct evidence is available yet, however. As to their general disposition, the SR can only be arranged in parallel, as indicated by developmental results, documenting its origin from the fusion of multiple initially discrete subcompartments (Yuan et al, 1991). However, if the ER gradient concept proves correct, one can envisage the serial emerging of other subcompartments (for example, the luminal bodies) along the major axis of flow.…”