Isolation of cDNA clones for mouse cytoskeletal gamma-actin and differential expression of cytoskeletal actin mRNAs in mouse cells

Cytoplasmic gamma-actin contributes to a compensatory remodeling response in dystrophin-deficient muscle

Dystrophin mechanically links the costameric cytoskeleton and sarcolemma, yet dystrophin-deficient muscle exhibits abnormalities in cell signaling, gene expression, and contractile function that are not clearly understood. We generated new antibodies specific for cytoplasmic gamma-actin and confirmed that gamma-actin most predominantly localized to the sarcolemma and in a faint reticular lattice within normal muscle cells. However, we observed that gamma-actin levels were increased 10-fold at the sarcolemma and within the cytoplasm of striated muscle cells from dystrophin-deficient mdx mice. Transgenic overexpression of the dystrophin homologue utrophin, or functional dystrophin constructs in mdx muscle, restored gamma-actin to normal levels, whereas gamma-actin remained elevated in mdx muscle expressing nonfunctional dystrophin constructs. We conclude that increased cytoplasmic gamma-actin in dystrophin-deficient muscle may be a compensatory response to fortify the weakened costameric lattice through recruitment of parallel mechanical linkages. However, the presence of excessive myoplasmic gamma-actin may also contribute to altered cell signaling or gene expression in dystrophin-deficient muscle.

Beta and gamma actin mRNAs are differentially located within myoblasts

Actin is of fundamental importance to all eukaryotic cells. Of the six mammalian actins, beta (beta) and gamma (gamma) cytoplasmic are the isoforms found in all nonmuscle cells and differ by only four amino acids at the amino-terminal region. Both genes are regulated temporally and spatially, though no differences in protein function have been described. Using fluorescent double in situ hybridization we describe the simultaneous intracellular localization of both beta and gamma actin mRNA. This study shows that myoblasts differentially segregate the beta and gamma actin mRNAs. The distribution of gamma actin mRNA, only to perinuclear and nearby cytoplasm, suggests a distribution based on diffusion or restriction to nearby cytoplasm. The distribution of beta actin mRNA, perinuclear and at the cell periphery, implicates a peripheral localizing signal which is unique to the beta isoform. The peripheral beta actin mRNA corresponded to cellular morphologies, extending processes, and ruffling edges that reflect cell movement. Total actin and gamma actin protein steady-state distributions were identified by specific antibodies. gamma actin protein was found in both stress fibers and at the cell plasma membrane and does not correspond to its mRNA distribution. We suggest that localized protein synthesis rather than steady-state distribution functionally differentiates between the actin isoforms.

The chicken skeletal muscle alpha-actin promoter is tissue specific in transgenic mice

We have generated transgenic mouse lines that carry the promoter region of the chicken skeletal muscle alpha (alpha sk) actin gene linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. In adult mice, the pattern of transgene expression resembled that of the endogenous alpha sk actin gene. In most of the transgenic lines, high levels of CAT activity were detected in striated muscle (skeletal and cardiac) but not in the other tissues tested. In striated muscle, transcription of the transgene was initiated at the normal transcriptional start site of the chicken alpha sk actin gene. The region from nucleotides -191 to +27 of the chicken alpha sk actin gene was sufficient to direct the expression of CAT in striated muscle of transgenic mice. These observations suggest that the mechanism of tissue-specific actin gene expression is well conserved in higher vertebrate species.

The chicken skeletal muscle alpha-actin promoter is tissue specific in transgenic mice

We have generated transgenic mouse lines that carry the promoter region of the chicken skeletal muscle alpha (alpha sk) actin gene linked to the bacterial chloramphenicol acetyltransferase (CAT) gene. In adult mice, the pattern of transgene expression resembled that of the endogenous alpha sk actin gene. In most of the transgenic lines, high levels of CAT activity were detected in striated muscle (skeletal and cardiac) but not in the other tissues tested. In striated muscle, transcription of the transgene was initiated at the normal transcriptional start site of the chicken alpha sk actin gene. The region from nucleotides -191 to +27 of the chicken alpha sk actin gene was sufficient to direct the expression of CAT in striated muscle of transgenic mice. These observations suggest that the mechanism of tissue-specific actin gene expression is well conserved in higher vertebrate species.

Isolation and characterization of a cDNA that encodes mouse fibroblast tropomyosin isoform 2

We isolated and characterized a cDNA clone encoding tropomyosin isoform 2 (TM2) from a mouse fibroblast cDNA library. TM2 was found to contain 284 amino acids and was closely related to the rat smooth and skeletal muscle alpha-TMs and the human fibroblast TM3. The amino acid sequence of TM2 showed a nearly complete match with that of human fibroblast TM3 except for the region from amino acids 189 to 213, the sequence of which was identical to those of rat smooth and skeletal muscle alpha-TMs. These results suggest that TM2 is expressed from the same gene that encodes the smooth muscle alpha-TM, the skeletal muscle alpha-TM, and TM3 via an alternative RNA-splicing mechanism. Comparison of the expression of TM2 mRNA in low-metastatic Lewis lung carcinoma P29 cells and high-metastatic D6 cells revealed that it was significantly less in D6 cells than in P29 cells, supporting our previous observations (K. Takenaga, Y. Nakamura, and S. Sakiyama, Mol. Cell. Biol. 8:3934-3937, 1988) at the protein level.

Isolation and characterization of a cDNA that encodes mouse fibroblast tropomyosin isoform 2

We isolated and characterized a cDNA clone encoding tropomyosin isoform 2 (TM2) from a mouse fibroblast cDNA library. TM2 was found to contain 284 amino acids and was closely related to the rat smooth and skeletal muscle alpha-TMs and the human fibroblast TM3. The amino acid sequence of TM2 showed a nearly complete match with that of human fibroblast TM3 except for the region from amino acids 189 to 213, the sequence of which was identical to those of rat smooth and skeletal muscle alpha-TMs. These results suggest that TM2 is expressed from the same gene that encodes the smooth muscle alpha-TM, the skeletal muscle alpha-TM, and TM3 via an alternative RNA-splicing mechanism. Comparison of the expression of TM2 mRNA in low-metastatic Lewis lung carcinoma P29 cells and high-metastatic D6 cells revealed that it was significantly less in D6 cells than in P29 cells, supporting our previous observations (K. Takenaga, Y. Nakamura, and S. Sakiyama, Mol. Cell. Biol. 8:3934-3937, 1988) at the protein level.