Bidirectional signaling between calcium channels of skeletal muscle requires multiple direct and indirect interactions

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Abstract
We have defined regions of the skeletal muscle ryanodine receptor (RyR1) essential for bidirectional signaling with dihydropyridine receptors (DHPRs) and for the organization of DHPR into tetrad arrays by expressing RyR1–RyR3 chimerae in dyspedic myotubes. RyR1–RyR3 constructs bearing RyR1 residues 1–1681 restored wild-type DHPR tetrad arrays and, in part, skeletal-type excitation–contraction (EC) coupling (orthograde signaling) but failed to enhance DHPR Ca 2+ currents (retrograde signaling) to WT RyR1 levels. Within this region, the D2 domain (amino acids 1272–1455), although ineffective on its own, dramatically enhanced the formation of tetrads and EC coupling rescue by constructs that otherwise are only partially effective. These findings suggest that the orthograde signal and DHPR tetrad formation require the contributions of numerous RyR regions. Surprisingly, we found that RyR3, although incapable of supporting EC coupling or tetrad formation, restored a significant level of Ca 2+ current, revealing a functional interaction with the skeletal muscle DHPR. Thus, our data support the hypotheses that ( i ) the structural/functional link between RyR1 and the skeletal muscle DHPR requires multiple interacting regions, ( ii ) the D2 domain of RyR1 plays a key role in stabilizing this interaction, and ( iii ) a form of retrograde signaling from RyR3 to the DHPR occurs in the absence of direct protein–protein interactions.

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