The Ca2+ channel beta subunit determines whether stimulation of Gq-coupled receptors enhances or inhibits N current.
In superior cervical ganglion (SCG) neurons, stimulation of M(1) receptors (M(1)Rs) produces a distinct pattern of modulation of N-type calcium (N-) channel activity, enhancing currents elicited with negative test potentials and inhibiting currents elicited with positive test potentials. Exogenously applied arachidonic acid (AA) reproduces this profile of modulation, suggesting AA functions as a downstream messenger of M(1)Rs. In addition, techniques that diminish AA's concentration during M(1)R stimulation minimize N-current modulation. However, other studies suggest depletion of phosphatidylinositol-4,5-bisphosphate during M(1)R stimulation suffices to elicit modulation. In this study, we used an expression system to examine the physiological mechanisms regulating modulation. We found the beta subunit (Ca(V)beta) acts as a molecular switch regulating whether modulation results in enhancement or inhibition. In human embryonic kidney 293 cells, stimulation of M(1)Rs or neurokinin-1 receptors (NK-1Rs) inhibited activity of N channels formed by Ca(V)2.2 and coexpressed with Ca(V)beta1b, Ca(V)beta3, or Ca(V)beta4 but enhanced activity of N channels containing Ca(V)beta2a. Exogenously applied AA produced the same pattern of modulation. Coexpression of Ca(V)beta2a, Ca(V)beta3, and Ca(V)beta4 recapitulated the modulatory response previously seen in SCG neurons, implying heterogeneous association of Ca(V)beta with Ca(V)2.2. Further experiments with mutated, chimeric Ca(V)beta subunits and free palmitic acid revealed that palmitoylation of Ca(V)beta2a is essential for loss of inhibition. The data presented here fit a model in which Ca(V)beta2a blocks inhibition, thus unmasking enhancement. Our discovery that the presence or absence of palmitoylated Ca(V)beta2a toggles M(1)R- or NK-1R-mediated modulation of N current between enhancement and inhibition identifies a novel role for palmitoylation. Moreover, these findings predict that at synapses, modulation of N-channel activity by M(1)Rs or NK-1Rs will fluctuate between enhancement and inhibition based on the presence of palmitoylated Ca(V)beta2a.
Heneghan
John F
JF
Department of Physiology, University of Massachusetts Medical School, Worcester, MA 01655, USA.
Mitra-Ganguli
Tora
T
Stanish
Lee F
LF
Liu
Liwang
L
Zhao
Rubing
R
Rittenhouse
Ann R
AR
D000818:Animals; D002118:Calcium / Q000378:metabolism; D020864:Calcium Channels, N-Type / Q000378:metabolism*; D002478:Cells, Cultured; D004553:Electric Conductivity; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000378:metabolism*; D006801:Humans; D008564:Membrane Potentials; D008954:Models, Biological; D021122:Protein Subunits / Q000378:metabolism*; D051381:Rats; D043643:Receptor, Muscarinic M1 / Q000378:metabolism*; D017783:Superior Cervical Ganglion / Q000378:metabolism
D020864:Calcium Channels, N-Type; D021122:Protein Subunits; D043643:Receptor, Muscarinic M1; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D002118:Calcium