Muscarinic Acetylcholine Receptors from Avian Retina and Heart Undergo Different Patterns of Molecular Maturation

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Abstract
Muscarinic acetylcholine receptors (mAChRs) from the avian CNS exist in two molecualr weight forms whose concentrations change during development. Here, we have compared the development of mAChRs from embryonic hearts with those of the CNS. Analysis of [3H]-propylbenzilylcholine mustard (PrBCM)-labeled retina and heart mAChRs by sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed two atropine-sensitive peaks for each tissue. Apparent molecular masses of retina mAChRs, 86 .+-. 0.7 kilodaltons (kDa) and 72 .+-. 0.7 kDa, were different from those of heart mAChRs, 77 .+-.1.0 kDa and 52 .+-. 0.9 kDa. During retina development, the major receptor type changed from 86 kDa to 72 kDa. No such change occurred during heart development. Furthermore, the 52-kDa species appeared to be generated by endogenous proteolysis, as prolonged incubation of heart membranes at 37.degree. C increased the amount of 52-kDa peptide with a decrease of 77-kDa peptide. Protease inhibitors blocked this conversion. Incubation of retina membranes at 37.degree. C did not result in a conversion of the 86-kDa peptide into the 72-kDa peptide, but it did not cause the appearance of a minor amount of 52-kDa peptide. The proteolysis of retina mAChRs was not enhanced by cohomoenizing them with heart tissue, arguing against the presence of releasable proteases in heart. Membrane-bound retina and heart mAchRs displayed similar sensitivity to exogenous (Staphylococcus aureus V8) protease, indicating that heart receptors were not unusually susceptible to proteolytic attack; analysis of the labeled polypeptides with the V8 protease showed different patterns of digestion for the retina and heart receptors. Chronic treatment of embryos with a muscarinic agonist reduced the concentration of receptors, but the Mr profile remained unaffected. The fact that muscle mAChRs, unlike neural mAChRs, did not change molecular forms during development may reflect different mechanisms of synaptogenesis in CN2 and PNS.