Structural alterations in cardiac calcium release units resulting from overexpression of junctin

Junctin is a 26 kDa membrane protein that binds to calsequestrin, triadin, and ryanodine receptors (RyRs) within the junctional sarcoplasmic reticulum of calcium release units. The sequence of junctin includes a short N-termi nal cytoplasmic domain a single transmembrane domain, acid a highly charged C-terminal domain located in the sarcoplasmic reticulum lumen. Dog and mou se junctins are highly conserved at the transmembrane domains, but the lumi nal domains are more divergent. To probe the contribution of junctin to the architecture of calcium release units in heart, we engineered transgenic m ice overexpressing canine junctin and examined the left ventricular myocard ium by electron microscopy. Overall architecture of calcium release units i s similar in control myocardium and in myocardium overexpressing junctin by 5-10-fold. In both myocardia: junctional SR cisternae are closely associat ed with exterior membranes (plasmalemma and transverse tubules). The cister nae are flat; they contain a string of calsequestrin beads and are lined by a row of feet, or RyRs, on the side facing the exterior membranes. T tubul e surface density, measured as the perimeter of T tubule profiles nu area o f section, is the same in transgenic and control myocardia (305 nu 289 nm/n m(2)), Three changes affecting the junctional SR architecture are apparent in the myocardium overexpressing junctin, One is a more tightly zippered ap pearance of the junctional SR cisternae. The width of the junctional SR is narrower and less variable in overexpressing than in control myocardium and the calsequestrin content is more compact, A second change is the extensio n of zippered junctional SR domains to non-junctional regions, which we ter m "frustrated" junctional SR. A third change is an increase in the extent o f association between SR and T tubules, In junctin overexpressing myocardiu m junctional SR cisternae cover similar to 45% of the surface of all T tubu le profiles, while in control myocardium the coverage is similar to 30%. Ju nctional associations between SR and T tubules are increased in size, We co nclude that the increase in junctin expression affects the packing of calse questrin in the junctional SR and facilitates the association or SR and T t ubules, (C) 2001 Academic Press.