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Evolution of cholinergic proteins in developing slow and fast skeletal muscles in chick embryo
Insights
Investigating chick muscle development, this study reveals that muscle activity influences the synthesis of acetylcholine receptors and acetylcholinesterase at the motor end-plate. Paralysis affects enzyme forms but not receptor localization.
Area of Science:
- Neuroscience
- Developmental Biology
- Muscle Physiology
Background:
- The differentiation of slow (anterior latissimus dorsi) and fast (posterior latissimus dorsi) chick muscles involves cholinergic pathways.
- Understanding the developmental patterns of acetylcholine receptors and acetylcholinesterase is crucial for motor end-plate formation.
Purpose of the Study:
- To investigate the cholinergic differentiation of two distinct chick muscles during embryonic development and post-hatching.
- To determine the role of muscle activity in the biochemical differentiation of the developing motor end-plate.
Main Methods:
- Autoradiographical and biochemical analyses were employed to study muscle differentiation.
- Chick embryos were paralyzed using Flaxedil to assess the impact of muscle activity on motor end-plate components.
Main Results:
- Acetylcholine receptor accumulation rates differ between anterior latissimus dorsi and posterior latissimus dorsi muscles post-embryonic day 15, suggesting varied synthesis rates.
- Muscle paralysis did not significantly alter total acetylcholinesterase but reduced specific enzyme forms and histochemical staining.
- Paralysis increased total acetylcholine receptor content without altering degradation rates, indicating preserved receptor localization.
Conclusions:
- Muscle activity plays a role in regulating the synthesis of cholinergic proteins, particularly acetylcholine receptors.
- Motor end-plate formation and cholinergic protein distribution are influenced by muscle activity, impacting multiply and focally innervated muscles differently.
Abstract:
1. The cholinergic differentiation of two phenotypically muscles of the chick, the slow multiply innervated anterior latissimus dorsi (a.l.d.) and the fast focally innervated posterior latissimus dorsi (p.l.d.), was investigated during embryonic life and after hatching using both autoradiographical and biochemical methods. 2. The contents in total protein and in acetylcholinesterase activity follow similar development patterns in both muscles, but, after the 15th day in ovo, the accumulation of choline acetyltransferase activity and of acetylcholine nicotinic receptor sites as determined by alpha-bungarotoxin binding occurs at a faster rate in a.l.d. than in p.l.d. 3. In muscle of the p.l.d., a rapid increase of the total number of acetylcholine receptor clusters takes place after the 11th day of embryonic life although some clusters could be observed on myofibres as soon as the 4th day in ovo. 4. The rate of degradation of cholinergic receptor sites in chick muscle is constant around 28 hr up to the 10th day after hatching; thus the different rates of accumulation of acetylcholine receptor in a.l.d. and p.l.d., respectively, after the 15th day of embryonic life must be due to different rates of receptor synthesis. 5. The role of muscle activity in the biochemical differentiation of the developing motor end-plate was investigated in chick embryos which had been paralysed by repeated injections into the yolk sac of a curare-like agent, Flaxedil (May & Baker). 6. The total content in acetylcholinesterase of both a.l.d. and p.l.d. muscles is not significantly modified by paralysis. However, the histochemical staining of end-plates for acetylcholinesterase as well as the heavy form of this enzyme (19 . 5 S) are consistently reduced after Flaxedil injection. 7. In muscles from Flaxedil-treated embryos, the total content in acetylcholine receptor sites as determined by alpha-bungarotoxin binding is higher than in those from control embryos, whereas the rate of degradation of these sites is not significantly altered. 8. The localization of the acetylcholine receptors under the motor nerve terminals is not prevented by blocking muscle activity at the postsynaptic level. Clusters of receptor are still present, and there is no significant change in the number and distribution of these clusters along the myofibres of a.l.d. and p.l.d. muscles. 9. These results are discussed with respect to motor end-plate formation in multiply and focally innervated embryo muscles, and in relation to the control of cholinergic proteins distribution and synthesis by muscle activity.