Nemaline myopathy rod bodies. Structure and composition.

Area of Science:

• Muscle ultrastructure
• Protein biochemistry
• Cellular biology

Background:

• Nemaline myopathy is characterized by the presence of abnormal protein aggregates known as nemaline rods.
• The precise composition and structure of nemaline rods and their relationship to Z-lines remain incompletely understood.
• Understanding these structures is crucial for elucidating the pathophysiology of nemaline myopathy.

Purpose of the Study:

• To elucidate the ultrastructural composition of nemaline rods in nemaline myopathy.
• To investigate the relationship between nemaline rods and Z-lines.
• To identify the key protein components of nemaline rods.

Main Methods:

• Glycerinated nemaline myopathy muscle was treated with calcium-activated protease (CAF).
• Electron microscopy was used to examine the structural changes and identify filament arrangements.
• Heavy meromyosin (HMM) labeling was employed to identify actin filaments.
• Gel electrophoresis was performed on CAF-treated muscle to analyze protein composition.

Main Results:

• CAF digestion exposed 6-7 nm backbone filaments within nemaline rods, identified as actin filaments via HMM decoration.
• CAF treatment removed cross-striated patterns and Z-filament-like connections, suggesting alpha-actinin's role in rod structure.
• Gel electrophoresis revealed that alpha-actinin was largely released, with actin and myosin remaining in the residue.
• Intact rods exhibited oblique filament patterns, thin (7 nm) and thick (11 nm) lines, and amorphous material, varying with sectioning.
• Cross-sections showed predominantly small square net (SS) and basket-weave (BW) forms, also present in Z-lines.

Conclusions:

• Nemaline rods are lateral polymers of Z-line units, primarily composed of actin filaments.
• Alpha-actinin likely plays a role in the formation and structural organization of nemaline rods.
• Rod length and Z-line width may be determined by the overlap of oppositely polarized actin filaments.
• Deregulation of actin filament length could initiate nemaline rod formation.