Contribution of LEV-8 subunit to the kinetics of activation and desensitization of C. elegans muscle levamisole-sensitive nicotinic receptors

HERNANDO, G.; RAYES, D.; BOUZAT, C.

San Diego, CA. U.S.A

Congreso; 40th annual meeting of Society for Neuroscience; 2010

Society for Neuroscience. SFN.

Nicotinic acetylcholine receptors (AChRs) are pentameric ligand-gatedion channels that mediate fast synaptic transmission. In nematodes, themuscle levamisole-sensitive AChR (L-AChR) is a target of anthelminticdrugs. L-AChR from C. elegans muscle appears to be composed ofthree essential (UNC-63, UNC-38, and UNC-29) and two accessorysubunits (LEV-1 and LEV-8). We explored the contribution of α-typeLEV-8 subunit to the kinetics of activation and desensitization ofL-AChRs from C. elegans muscle cultured cells. Single-channelactivity of L-AChRs can be readily detected from muscle cells derivedfrom LEV-8 null mutant strain (lev-8(x-15)), thus confirming thatLEV-8 is not an essential subunit. Channel conductance is similar tothat of wild-type L-AChRs (~36 pS). In contrast, the duration of theslowest open component differs from that of wild-type L-AChRs, beingabout 2.5-fold more prolonged (0.32±0.04 and 0.70±0.11 ms forwild-type and mutant, respectively). A dramatic difference betweenrecordings from LEV-8 null mutant and wild-type muscle cells is atime-dependent reduction in the frequency of opening events. Thus,whereas channel activity remains constant during the course of therecording in wild-type cells, it disappears within the first three minutesin the null mutant. Such a reduction is compatible with enhanceddesensitization. To investigate this, we recorded macroscopic currentselicited by rapid application of levamisole or ACh in the whole-cellconfiguration. Our results show that the mean amplitude of currentsfrom the null mutant is similar to that from wild-type strain. However,the decay time constant is ~4-fold reduced in the mutant, indicatingfaster desensitization. In addition, the steady state current, whichrepresents receptors that remain active during the agonist-pulse, issmaller in the null mutant (41±10% and 19±10% of the peak currentfor wild-type and mutant, respectively). Taken together, our resultsreveal that L-AChRs lacking LEV-8 show increased open channellifetime and enhanced desensitization. Thus, the properties ofL-AChRs-mediated responses in muscle cells can be substantiallychanged whether or not accessory LEV-8 subunit is incorporated intothe pentameric receptor.