Loss of Sarcomere-associated Formins Disrupts Z-line Organization, but does not Prevent Thin Filament Assembly in Caenorhabditis elegans Muscle.

Lei Mi Mi David Pruyne

doi:10.4172/2157-7099.1000318

Abstract

Members of the formin family of actin filament nucleation factors have been implicated in sarcomere formation, but precisely how these proteins affect sarcomere structure remains poorly understood. Of six formins in the simple nematode Caenorhabditis elegans, only FHOD-1 and CYK-1 contribute to sarcomere assembly in the worm's obliquely striated body-wall muscles. We analyze here the ultrastructure of body-wall muscle sarcomeres in worms with putative null fhod-1 and cyk-1 gene mutations. Contrary to a simple model that formins nucleate actin for thin filament assembly, formin mutant sarcomeres contain thin filaments. Rather, formin mutant sarcomeres are narrower and have deformed thin filament-anchoring Z-line structures. Thus, formins affect multiple aspects of sarcomere structure.

Highlights

Contraction of striated muscles depends on the sliding of parallel thin filaments and thick filaments past each other in repeating cytoskeletal units called sarcomeres [1]
All Z-line and M-line structures of body-wall muscles (BWMs) cells are firmly anchored to the plasma membrane and the adjacent basement membrane by integrin-containing complexes, and resemble vertebrate costameres that decorate only the most peripheral sarcomeres in striated muscle cells [16,17]
While any given cross section of vertebrate muscle viewed by Transmission electron microscopy (TEM) will reveal the same longitudinal position for all sarcomeres in that section, a similar cross section of BWM reveals the entire spectrum of sarcomere positions as it intersects adjacent sarcomeres (Figure 1, x/y-sections)

Summary

Introduction

Contraction of striated muscles depends on the sliding of parallel thin filaments and thick filaments past each other in repeating cytoskeletal units called sarcomeres [1]. The sarcomere can be further subdivided into a broad central A-band occupied by thick filaments and centered on the M-line, and I-bands that mark thick filament-free zones. Thin filaments emanate from the Z-lines, stretching through the I-bands to overlap with thick filaments in the A-band. The degree of overlap depends on the degree of muscle contraction. Myosin motor activity pulls the thin filaments toward the sarcomere center, increasing the amount of overlap between the thick and thin filaments. The overlap of thin and thick filaments is minimized and I-bands are widened in stretched muscle

Methods

Results

Conclusion