Skeletal and cardiac alpha-actin isoforms exhibit unanticipated temporal and tissue-specific gene expression patterns in developing avian limbs and embryos

Nebulin interacts with CapZ and regulates thin filament architecture within the Z-disc

The barbed ends of actin filaments in striated muscle are anchored within the Z-disc and capped by CapZ; this protein blocks actin polymerization and depolymerization in vitro. The mature lengths of the thin filaments are likely specified by the giant "molecular ruler" nebulin, which spans the length of the thin filament. Here, we report that CapZ specifically interacts with the C terminus of nebulin (modules 160-164) in blot overlay, solid-phase binding, tryptophan fluorescence, and SPOTs membrane assays. Binding of nebulin modules 160-164 to CapZ does not affect the ability of CapZ to cap actin filaments in vitro, consistent with our observation that neither of the two C-terminal actin binding regions of CapZ is necessary for its interaction with nebulin. Knockdown of nebulin in chick skeletal myotubes using small interfering RNA results in a reduction of assembled CapZ, and, strikingly, a loss of the uniform alignment of the barbed ends of the actin filaments. These data suggest that nebulin restricts the position of thin filament barbed ends to the Z-disc via a direct interaction with CapZ. We propose a novel molecular model of Z-disc architecture in which nebulin interacts with CapZ from a thin filament of an adjacent sarcomere, thus providing a structural link between sarcomeres.

Temperature and the expression of seven muscle-specific protein genes during embryogenesis in the Atlantic cod Gadus morhua L

Seven cDNA clones coding for different muscle-specific proteins (MSPs) were isolated from the fast muscle tissue of Atlantic cod Gadus morhua L. In situ hybridization using cRNA probes was used to characterize the temporal and spatial patterns of gene expression with respect to somite stage in embryos incubated at 4 degrees C, 7 degrees C and 10 degrees C. MyoD transcripts were first observed in the presomitic mesoderm prior to somite formation, and in the lateral compartment of the forming somites. MyoD expression was not observed in the adaxial cells that give rise to the slow muscle layer, and expression was undetectable by in situ hybridization in the lateral somitic mesoderm after the 35-somite stage, during development of the final approximately 15 somites. RT-PCR analysis, however, confirmed the presence of low levels of the transcript during these later stages. A phylogenetic comparison of the deduced aminoacid sequences of the full-length MyoD cDNA clone and those from other teleosts, and inference from the in situ expression pattern suggested homology with a second paralogue (MyoD2) recently isolated from the gilthead seabream Sparus aurata. Following MyoD expression, alpha-actin was the first structural gene to be switched on at the 16-somite stage, followed by myosin heavy chain, troponin T, troponin I and muscle creatine kinase. The final mRNA in the series to be expressed was troponin C. All genes were switched on prior to myofibril assembly. The troponin C sequence was unusual in that it showed the greatest sequence identity with the rainbow trout Oncorhynchus mykiss cardiac/slow form, but was expressed in the fast myotomal muscle and not in the heart. In addition, the third TnC calcium binding site showed a lower level of sequence conservation than the rest of the sequence. No differences were seen in the timing of appearance or rate of posterior progression (relative to somite stage) of any MSP transcripts between embryos raised at the different temperatures. It was concluded that myofibrillar genes are activated asynchronously in a distinct temporal order prior to myofibrillar assembly and that this process was highly canalized over the temperature range studied.

Fate restriction in limb muscle precursor cells precedes high-level expression of MyoD family member genes

The mechanisms by which pluripotent embryonic cells generate unipotent tissue progenitor cells during development are unknown. Molecular/genetic experiments in cultured cells have led to the hypothesis that the product of a single member of the MyoD gene family (MDF) is necessary and sufficient to establish the positive aspects of the determined state of myogenic precursor cells: i.e., the ability to initiate and maintain the differentiated state (Weintraub, H., Davis, R., Tapscott, S., Thayer, M., Krause, M., Benezra, R., Blackwell, T. K., Turner, D., Rupp, R., Hollenberg, S. et al. (1991) Science 251, 761–766). Embryonic cell type determination also involves negative regulation, such as the restriction of developmental potential for alternative cell types, that is not directly addressed by the MDF model. In the experiments reported here, phenotypic restriction in myogenic precursor cells is assayed by an in vivo ‘notochord challenge’ to evaluate their potential to ‘choose’ between two alternative cell fate endpoints: cartilage and muscle (Williams, B. A. and Ordahl, C. P. (1997) Development 124, 4983–4997). Two separate myogenic precursor cell populations were found to be phenotypically restricted while expressing the Pax3 gene and prior to MDF gene activation. Therefore, while MDF family members act positively during myogenic differentiation, phenotypic restriction, the negative aspect of cell specification, requires cellular and molecular events and interactions that precede MDF expression in myogenic precursor cells. The qualities of muscle formed by the determined myogenic precursor cells in these experiments further indicate that their developmental potential is intermediate between that of myoblastic stem cells taken from fetal or adult tissue (which lack mitotic and morphogenetic potential when tested in vivo) and embryonic stem cells (which are multipotent). We hypothesize that such embryonic myogenic progenitor cells represent a distinct class of determined embryonic cell, one that is responsible for both tissue growth and tissue morphogenesis.