Morphology and Transverse Stiffness of Drosophila Myofibrils Measured by Atomic Force Microscopy

Abstract

Atomic force microscopy was used to investigate the surface morphology and transverse stiffness of myofibrils from Drosophila indirect flight muscle exposed to different physiologic solutions. I- and A-bands were clearly observed, and thick filaments were resolved along the periphery of the myofibril. Interfilament spacings correlated well with estimates from previous x-ray diffraction studies. Transverse stiffness was measured by using a blunt tip to indent a small section of the myofibrillar surface in the region of myofilament overlap. At 10 nm indention, the effective transverse stiffness (K( perpendicular)) of myofibrils in rigor solution (ATP-free, pCa 4.5) was 10.3 +/- 5.0 pN nm(-1) (mean +/- SEM, n = 8); in activating solution (pCa 4.5), 5.9 +/- 3.1 pN nm(-1); and in relaxing solution (pCa 8), 4.4 +/- 2.0 pN nm(-1). The apparent transverse Young's modulus (E( perpendicular)) was 94 +/- 41 kPa in the rigor state and 40 +/- 17 kPa in the relaxed state. The value of E( perpendicular) for calcium-activated myofibrils (55 +/- 29 kPa) was approximately a tenth that of Young's modulus in the longitudinal direction, a difference that at least partly reflects the transverse flexibility of the myosin molecule.

Transverse Stiffness of Myofibrils of Skeletal and Cardiac Muscles Studied by Atomic Force Microscopy

“…2. In accord with the results of previous AFM studies of muscle myofibrils [9,10], rigid Z-bands could clearly be located in the transverse stiffness distributions of rigor myofibrils, but sarcomere structures were hardly distinguishable in the transverse stiffness distributions of relaxed myofibrils. For skeletal as well as cardiac myofibrils, the myofibrils in a rigor state were laterally much stiffer than those in a relaxed state, and cardiac myofibrils were significantly stiffer than skeletal myofibrils in each state.…”

“…In skeletal and cardiac myofibrils, the actin and myosin heavy chain (MHC) remained almost undigested by calpain and trypsin treatments [20]. Consistent with the results of previous AFM studies [9,10], the AFM images of skeletal and cardiac myofibrils in a rigor state clearly showed striation patterns characteristic of striated muscle fibers. Similarly, the transverse stiffness distributions along rigor myofibrils (Fig.…”

“…In cardiac muscle fibers, however, the extra series elastic components further increase longitudinal stiffness [5,6]. The transverse stiffness of skeletal muscle has been studied by osmotic compressions [7,8] and that of myofibrils by the use of atomic force microscope (AFM) technology [9,10]. And it has been clarified that the structure of sarcomeres was substantially less stiff in the transverse direction than in the longitudinal direction.…”

Invertebrate muscles: Thin and thick filament structure; molecular basis of contraction and its regulation, catch and asynchronous muscle

“…What determines thick filament spacing is unknown. Transverse stiffness varies as a function of contraction state (rigor > contracting > relaxed) (Nyland and Maughan, 2000), and thus one possibility is that interaction with the surrounding cage of thin filaments is a sufficient mechanism. Thick filament spacing (56 nm in Drosophila) does not vary during contraction (Irving and Maughan, 2000).…”

Mechanical Strength of Sarcomere Structures of Skeletal Myofibrils Studied by Submicromanipulation

“…Recently it has become possible to examine the mechanical properties of cellular components at the molecular level by employing submicromanipulation techniques Miyata et al, 1996;Yoshikawa et al, 1999;Nishizaka et al, 2000;Nyland and Maughan, 2000;Li et al, 2002). In the present studies, the mechanical strength of sarcomere structures of skeletal muscle was studied by rupturing peripheral structures of single myofibrils by use of optical tweezers (Block, 1990) and atomic force microscope (AFM) (Heckl and Marti, 1998).…”