Alterations in the expression of muscle-specific genes mediated by troponin C antisense oligodeoxynucleotide

Overexpression of sTnC polypeptide in muscle cells is controlled by its rapid degradation

The check-points that maintain stoichiometric synthesis of muscle proteins were examined by misexpression of slow troponin C (sTnC) in mouse C2 myotubes. The sTnC mRNA was overexpressed in myotubes by transfecting these cells with a plasmid construct containing the constitutive CMV promoter-driven sTnC cDNA. An approximately four-fold increase of sTnC mRNA level in the transfected cells was observed. However, the increased mRNA level did not produce a corresponding increase of the sTnC polypeptide level in transfected cells. Only a modest 1.5-fold increase of the sTnC polypeptide level in the transfected cells was observed. The excess sTnC polypeptide in transfected cells was found in the soluble form which was not complexed with other thin filament proteins. The difference between the increase of sTnC mRNA and the polypeptide levels in transfected cells was not due to inefficient translation of the overexpressed sTnC mRNA. Analyses of the stability of the sTnC polypeptide in the thin filament and in the unassembled soluble forms showed that the excess soluble sTnC polypeptide was degraded more rapidly than the sTnC polypeptide of the thin filament. Analyses of the mRNA and polypeptide levels of several thin filament complements showed no effect of overexpression of the sTnC mRNA.

Uncoordinated inhibition of gene expression for muscle proteins by a troponin T antisense oligodeoxynucleotide

The effect of antisense oligodeoxynucleotide to rat troponin T (TnT) mRNA on its expression in differentiated rat L6 myotubes in culture was examined. The target sequence following the initiation codon was between nucleotides 83 and 97 and is found in all mRNAs produced from the f-TNT gene. Our studies showed that chimeric oligomer with one phosphorothioate linkage at the 3'-end was considerably more resistant to nucleases than was a phosphodiester oligomer. The chimeric oligomer produced >50% inhibition of TnT polypeptide synthesis. Synthesis of myosin heavy chain (MHC), troponin I (TnI), and alpha and beta tropomyosins (Tm) was not inhibited by the anti-TnT oligomer. However, synthesis of alpha-actin and troponin C (TnC) was somewhat affected by this treatment. Furthermore, compared with the untreated control myotubes, the steady-state level of TnT mRNA was reduced by approximately 40%-50% in anti-TnT oligomer-treated myotubes. Cellular levels of three other muscle mRNAs, alpha-Tm, s-TnI, and alpha-actin were also reduced by approximately 30%-40%. In contrast, fast TnI, beta-Tm, and TnC mRNA levels were not significantly affected by this treatment. Therefore, inhibition of TnT synthesis in differentiated myotubes uncoupled the coordinated expression of muscle proteins.

Inhibition of troponin C production without affecting other muscle protein synthesis by the antisense oligodeoxynucleotide

The effect of blocking expression of a specific gene with antisense phosphodiester oligodeoxynucleotides on the coordinate regulation of myogenesis was studied. Different regions of both fast and slow troponin C (TnC) mRNAs were targeted for binding of the antisense oligomer. The 5'-cap region of both mRNAs was found to be the most effective target for inhibiting the expression of these genes. Approximately 40%-60% inhibition of expression of a specific isoform of TnC was achieved. However, inhibition of the TnC expression did not appreciably alter the pattern of myogenesis of mouse C2C12 cells. The differentiated murine muscle cells were able to cope with this reduced level of the target gene expression by antisense phosphodiester oligomers. We have also used a phosphorothioate oligomer targeted against a common sequence within the coding region of both fast and slow TnC mRNAs. This oligomer was found to be ineffective in blocking TnC gene expression.

Inhibition of myogenesis in mouse C2 cells by double-stranded phosphorothioate oligodeoxynucleotides containing mef-1 sequence

Phosphorothioate oligonucleotides containing the muscle creatinine kinase enhancer sequence (mef-1) and a mutant of the enhancer sequence (mmef-1) were tested for their ability to block muscle differentiation in mouse C2 cells in culture. Maximum inhibition of fusion of myoblasts was observed at 10 microM concentration of mef-1 oligomer. No appreciable inhibition of fusion with the mmef-1 oligomer at the same concentration was observed. Synthesis of myogenin, muscle creatinine kinase, and myosin heavy chain polypeptides were reduced in mef-1 oligomer-treated cells. In contrast, no significant reduction in the synthesis of these polypeptides in mmef-1-treated cells was detected. The overall protein synthesis was not affected. These results suggest that muscle differentiation may be disrupted by competition of the oligomer with the endogenous promoter for specific transcription factor(s).

Detection of ribonuclease H-generated mRNA fragments in human leukemia cells following reversible membrane permeabilization in the presence of antisense oligodeoxynucleotides

The involvement of ribonuclease H (RNase H) in antisense phenomena in intact cells has, to date, only been adequately demonstrated for microinjected Xenopus systems. The significance of RNase H for the antisense effects of oligodeoxynucleotides observed in human and other mammalian cell cultures has remained obscure, in part because of inadequate analytic methods. In this report we show that the "reverse ligation-mediated PCR" (RL-PCR) procedure permits amplification of RNA fragments produced by oligodeoxynucleotide-directed RNase H activity. We have used this procedure to demonstrate RNase H-dependent antisense effects in irreversibly permeabilized (dead) cells and reversibly permeabilized (live) cells.

Antisense oligonucleotides as therapeutic agents--is the bullet really magical?

Because of the specificity of Watson-Crick base pairing, attempts are now being made to use oligodeoxynucleotides (oligos) in the therapy of human disease. However, for a successful outcome, the oligo must meet at least six criteria: (i) the oligos can be synthesized easily and in bulk; (ii) the oligos must be stable in vivo; (iii) the oligos must be able to enter the target cell; (iv) the oligos must be retained by the target cell; (v) the oligos must be able to interact with their cellular targets; and (vi) the oligos should not interact in a non-sequence-specific manner with other macromolecules. Phosphorothioate oligos are examples of oligos that are being considered for clinical therapeutic trials and meet some, but not all, of these criteria. The potential use of phosphorothioate oligos as inhibitors of viral replication is highlighted.

Expression of c-jun/AP-1 during myogenic differentiation in mouse C2C12 myoblasts

Mitogen withdrawal triggers myogenic differentiation in skeletal myoblasts in culture. We have examined the expression of the proto-oncogene c-jun during this process in mouse C2C12 myoblasts. c-jun belongs to a family of immediate early genes whose expression is activated in cultured cells in response to the addition of serum growth factors. Interestingly, expression of c-jun was maintained in mouse C2C12 and rat L6 myoblasts undergoing myogenic differentiation under low-serum conditions. Previously it has been reported that expression of c-jun is downregulated during differentiation of C2 cells. However, our results using C2C12 cells, a subclone of the C2 line, show that c-jun mRNA, protein and the activator-protein 1 (AP-1) DNA-binding activity were easily detected in proliferating myoblasts and differentiated myotubes. Although overexpression of c-jun has been shown to block myogenic differentiation in C2 cells, results presented here suggest that expression of c-jun at physiological levels may not interfere with skeletal myogenesis.