Nebulin is a giant (Mr 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 ∼700kDa portion associates with actin along the entire length of the thin filament, its N-terminal region extends to thin filament pointed ends, and ∼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. © 2003 Elsevier Science Ltd. All rights reserved.
The Krüppel-like transcription factors (KLF) are zinc finger proteins that activate and suppress target gene transcription. Although KLF factors have been implicated in regulating many developmental processes, a comprehensive gene expression analysis has not been reported. Here we present the chicken KLF gene family and expression during the first five days of embryonic development. Fourteen chicken KLF genes or expressed sequences have been previously identified. Through synteny analysis and cDNA mapping, we have identified the KLF9 gene and determined that the gene presently named KLF1 is the true ortholog of KLF17 in other species. In situ hybridization expression analyses show that in general KLFs are broadly expressed in multiple cell and tissue types. Expression of KLFs 3, 7, 8, and 9, is widespread at all stages examined. KLFs 2, 4, 5, 6, 10, 11, 15, and 17 show more restricted patterns that suggest multiple functions during early stages of embryonic development.
Arsenic is a naturally occurring metalloid and environmental contaminant. Arsenic exposure in drinking water is reported to cause cancer of the liver, kidneys, lung, bladder, and skin as well as birth defects, including neural tube, facial, and vasculogenic defects. The early embryonic period most sensitive to arsenic includes a variety of cellular processes. One key cellular process is epithelial-mesenchymal transition (EMT) where epithelial sheets develop into three-dimensional structures. An embryonic prototype of EMT is found in the atrioventricular (AV) canal of the developing heart, where endothelia differentiate to form heart valves. Effects of arsenic on this cellular process were examined by collagen gel invasion assay (EMT assay) using explanted AV canals from chicken embryo hearts. AV canals treated with 12.5-500 ppb arsenic showed a loss of mesenchyme at 12.5 ppb, and mesenchyme formation was completely inhibited at 500 ppb. Altered gene expression in arsenic-treated explants was investigated by microarray analysis. Genes whose expression was altered consistently at exposure levels of 10, 25, and 100 ppb were identified, and results showed that 25 ppb in vitro was particularly effective. Three hundred and eighty two genes were significantly altered at this exposure level. Cytoscape analysis of the microarray data using the chicken interactome identified four clusters of altered genes based on published relationships and pathways. This analysis identified cytoskeleton and cell adhesion-related genes whose disruption is consistent with an altered ability to undergo EMT. These studies show that EMT is sensitive to arsenic and that an interactome-based approach can be useful in identifying targets.