Properties of soleus muscle Z‐lines and induced Z‐line analogs revealed by dissection with Ca²⁺‐activated neutral protease

Abstract

Rat soleus muscle Z-lines and Z-line anomalies induced by neostigmine methyl sulfate (NMS) and cat soleus muscle Z-lines and Z-line anomalies induced by tenotomy were examined by electron microscopy before and after dissection of muscle fibers with Ca2+-activated neutral protease (CAF) to elucidate structural properties of Z-lines and related Z-line-type structures. In both normal and treated muscles, interdigitation of thin (6-7 nm) filaments, which were continuous with I-filaments (actin) from adjacent sarcomeres, was observed at the Z-line in longitudinal section. Both neostigmine methyl sulfate and tenotomy treatments induced muscle atrophy associated with Z-line degradation, streaming, and irregular distribution and accumulation of Z-line material and Z-rod formation. Tenotomized muscle also was characterized by the presence of N-line-like bands and I-Z-I brushes. CAF digestion removed the electron-dense covering material from Z-rods and revealed a backbone of actin filaments. The origin of Z-rods, their structural similarity to Z-lines in longitudinal and cross section, and their susceptibility to CAF indicate that Z-rods are directly related to native Z-lines and are probably lateral polymers of a basic Z-line unit. The regular square net alignment (22 nm) of I-filaments (actin) in cross sections of I-Z-I brushes which contain no N-lines suggests that the I-square net arrangement near the Z-line is determined by Z-filament-actin filament interaction rather than by the N-line or other factors. The results suggest that I-filaments (actin) penetrate the mammalian Z-line and are Z-line constituents and that the width of Z-lines and the length of Z-rods are determined by the amount of overlap of actin filaments. The perpendicular periodicity of Z-rods and the zigzag-oblique arrowheadlike appearance seen in longitudinal sections of Z-lines are attributed to alpha-actinin.

Evidence for a dimeric assembly of two titin/telethonin complexes induced by the telethonin C-terminus

“…To satisfy minimal symmetry constrains of the twofold axial symmetry of actin (Liversage et al, 2001), the 2:1 complex of titin suffices. This stoichiometry is also compatible with the tetragonal lattice viewed along the filament axis in the Z-disk (Liversage et al, 2001;Luther, 2000;Yamaguchi et al, 1983) and the orthogonal a-actinin crosslinks at 195 Å intervals (Atkinson et al, 2001;Luther and Squire, 2002). Hence, the observed titin/telethonin dimeric assembly does not provide direct answers to previously reported titin:actin 6:2 or 2:1 stoichiometries (Liversage et al, 2001;Luther and Squire, 2002;Young et al, 1998).…”

The vertebrate muscle Z-disc: sarcomere anchor for structure and signalling

“…Indeed, this is how nemaline rods appear to form spontaneously following mutations or physiological insult e.g. following tenotomy (Yamaguchi et al 1983). What is special about the striated muscle Z-disc is its fixed width in particular types of muscles, so it is essential to have markers that define the limits of Z-disc assembly.…”

The muscle Z band: lessons in stress management

“…These anomalous rod bodies, which appear in cardiac muscle with aging, in diseased skeletal muscle or can also be induced experimentally, are closely related to Z bands (Yamaguchi et al, 1982(Yamaguchi et al, , 1983b. Z rods…”

“…Despite the lack of evidence that 0~-actinin directly cross-links actin filaments in the Z band, there is little doubt that 0c-actinin is one of the principal components of Z filaments, the internal Z band filamentous structures that link actin filaments (Chowrashi & Pepe, 1982;Yamaguchi et al, 1983b;Takahashi & Hattori, 1989). Other possible roles for Z band 0~-actinin may be to anchor nebulin filaments, titin filaments or other cytoskeletal components (discussed below).…”