Nebulin: the nebulous, multifunctional giant of striated muscle.

Nebulin is a giant, modular sarcomeric protein and although it was discovered over 2 decades ago, it remains one of the most nebulous components of striated muscle. Previously, several groups identified nebulin as the prime candidate molecule for functioning as a "ruler" to specify the precise lengths of the actin (thin) filaments in skeletal muscle, yet this proposal has never been proven. This article reviews the evidence implicating nebulin as a thin filament ruler, including the most recent studies highlighting its potentially extensive isoform diversity and exciting reports revealing its expression in cardiac tissue. Also examined are novel findings indicating that nebulin is actually a multifunctional filament system, perhaps playing roles in signal transduction, contractile regulation, and myofibril force generation; these ideas are especially intriguing given the growing number of mutations in this giant molecule that are associated with human myopathies.

The complete mouse nebulin gene sequence and the identification of cardiac nebulin.

Nebulin is a giant (M(r) 750-850kDa), modular sarcomeric protein proposed to regulate the assembly, and to specify the precise lengths of actin (thin) filaments in vertebrate skeletal muscles. Nebulin's potential role as a molecular template is based on its structural and biochemical properties. Its central approximately 700kDa portion associates with actin along the entire length of the thin filament, its N-terminal region extends to thin filament pointed ends, and approximately 80kDa of its C-terminal region integrates within the Z-line lattice. Here, we determined the exon/intron organization of the entire mouse nebulin gene, which contains 165 exons in a 202kb segment. We identified 16 novel exons, 15 of which encode nebulin-repeat motifs (12 from its central region and 3 from its Z-line region). One novel exon shares high sequence homology to the 20 residue repeats of the tight-junction protein, ZO-1. RT-PCR analyses revealed that all 16 novel exons are expressed in mouse skeletal muscle. Surprisingly, we also amplified mRNA transcripts from mouse and human heart cDNA using primers designed along the entire length of nebulin. The expression of cardiac-specific nebulin transcripts was confirmed by in situ hybridization in fetal rat cardiomyocytes and in embryonic Xenopus laevis (frog) heart. On the protein level, antibodies specific for skeletal muscle nebulin's N and C-terminal regions stained isolated rat cardiac myofibrils at the pointed and barbed ends of thin filaments, respectively. These data indicate a conserved molecular layout of the nebulin filament systems in both cardiac and skeletal myofibrils. We propose that thin filament length regulation in cardiac and skeletal muscles may share conserved nebulin-based mechanisms, and that nebulin isoform diversity may contribute to thin filament length differences in cardiac and skeletal muscle.