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Dihydropyridine receptor gene expression in skeletal muscle from mdx and control mice
Y Péréon1, C Dettbarn, J Navarro
1Department of Physiology and Biophysics, University of Texas Medical Branch, Galveston 77555-0641, USA.
Abstract:
The expression of isoform-specific dihydropyrine receptor-calcium channel (DHPR) alpha 1-subunit genes was investigated in mdx and control mouse diaphragm (DIA) and tibialis anterior (TA). RNase protection assays were carried out with a rat DHPR cDNA probe specific for skeletal muscle and a mouse DHPR cDNA probe specific for cardiac muscle. The level of expression of the gene encoding the cardiac DHPR was very weak in TA muscle from both control and mdx mice. Compared to TA, DIA expressed mRNA for the cardiac isoform at significantly higher levels, but mdx and control mouse DIA levels were similar to one another. In contrast, mRNA expression levels for the DHPR skeletal muscle isoform were lower in control DIA than TA. However, there was a dramatic increase in the expression for the DHPR skeletal muscle isoform in mdx DIA compared with control DIA, reaching the TA expression level, whereas dystrophy did not affect TA expression. [3H]-PN200-110 binding was used to further assess DIA DHPR expression at the protein level. The density of binding sites for the probe was not significantly affected in DIA muscles of mdx vs. control mice, but it was reduced in older mdx and control mice. The increase in DHPR mRNA levels without a consequent increase in DHPR protein expression could be secondary to possible enhanced protein degradation which occurs in mdx DIA. The altered DHPR expression levels found here do not appear to be responsible for the severe deficits in contractile function of the mdx DIA.
Insights
Duchenne muscular dystrophy (DMD) in mdx mice shows increased skeletal muscle dihydropyridine receptor (DHPR) mRNA but not protein in the diaphragm. This altered DHPR expression is unlikely to cause the severe muscle function deficits observed in mdx mice.
Area of Science:
- Molecular biology
- Genetics
- Physiology
Background:
- Dihydropyridine receptor (DHPR) calcium channels are crucial for muscle excitation-contraction coupling.
- The mdx mouse model exhibits Duchenne muscular dystrophy, characterized by muscle degeneration and functional deficits.
Purpose of the Study:
- To investigate the expression patterns of DHPR alpha 1-subunit isoforms in the diaphragm (DIA) and tibialis anterior (TA) muscles of mdx and control mice.
- To determine if altered DHPR expression contributes to the contractile dysfunction in mdx mouse diaphragm.
Main Methods:
- RNase protection assays were used to quantify mRNA levels of cardiac and skeletal muscle DHPR isoforms.
- Radioligand binding assays ([3H]-PN200-110) assessed DHPR protein expression at the binding site level.
Main Results:
- Skeletal muscle DHPR mRNA expression was significantly increased in the diaphragm of mdx mice compared to controls, reaching levels seen in TA muscle.
- Cardiac DHPR mRNA levels were higher in the diaphragm than TA but similar between mdx and control mice.
- DHPR protein binding site density in the diaphragm was not significantly different between mdx and control mice, though it decreased with age in both groups.
Conclusions:
- The observed increase in DHPR mRNA in mdx diaphragm does not correlate with increased protein levels, suggesting post-transcriptional regulation or enhanced protein degradation.
- Altered DHPR expression is unlikely to be the primary cause of the severe contractile dysfunction in the mdx mouse diaphragm.