Molecular Structure of Sarcomere-to-Membrane Attachment at M-Lines in C. elegans Muscle

Figure 1

The body wall muscle of C. elegans. The color drawing on the right depicts a cross-section through an adult nematode emphasizing that the body wall muscle is organized into four quadrants. Each quadrant consists of interlocking pairs of mononuclear spindle-shaped cells (23 or 24 per quadrant). In the enlargement, notice that the myofilament lattice is restricted to one side of the cell, rather than filling the entire cross-sectional area as in a vertebrate striated muscle cell. Several planes of section are depicted; one of which emphasizes that fundamentally this is striated muscle with typical A-bands containing thick filaments organized around M-lines, and overlapping thin filaments attached to the Z-disk analogs called dense bodies. Note the plane parallel to the paper; this is the plane viewed when an animal, lying on a glass slide, is examined by light microscopy. At the bottom right is a typical image obtained with polarizing optics showing that bright A-bands alternate with dark I-bands; in the enlargement (right), the cross-sections of the dense bodies can be seen in the I-bands. At the bottom left is a typical image obtained by immunofluorescence microscopy using an antibody to UNC-95 [6] (enlargement is shown at the top left); notice that UNC-95 localizes to both rows of dense bodies (arrow) and M-lines (arrowhead). In the large drawing, the plane of section shown on the left side is a true cross-section through the nematode and through a body wall muscle cell; at the top of the figure is a typical transmission EM view of two sarcomeres; an arrow marks a dense body and an arrowhead marks an M-line. In the EM, at the bottom is located the thick cuticle and thin hypodermal cell and basement membrane; the cross-sections of thick filaments in the A-bands and the cross sections of the thin filaments in the I-bands (surrounding the dense bodies) can be seen. Notice that in the drawing and in the EM, all of the dense bodies and the M-lines are anchored to the muscle cell membrane. Indeed, the EM reveals that there is electron dense material at the base of each dense body and M-line that is likely to be responsible for this anchorage. This is in contrast to vertebrate striated muscle in which only the peripherally-located sarcomeres are anchored to the muscle cell membrane through costameres at the level of the Z-disks and possibly the M-lines.