Released online: 14 February 2000
Of all the examples of transmitter regulation of voltage-dependent channels, probably none has been more thoroughly studied than inhibition of calcium current in neurons, originally described by
Now, just when many might have thought that the subject was nearly exhausted, -S to activate G proteins (
The beauty of experiments on G-protein modulation of neuronal calcium channels is the directness with which mental pictures of molecular mechanisms can be built from traces on the oscilloscope. As a result of much work, there is now good evidence that the rapid, membrane-delimited G-protein inhibition of calcium channels is mediated by the ß subunits of G-proteins binding directly to the calcium channel (
The molecular picture inferred from the facilitation protocol has turned out to fit extremely well with direct studies of the molecules involved. Binding of G-protein ß subunits to fusion proteins from the calcium channel has been observed in direct biochemical studies (
subunits are bound to the channel to start with. An additional point of close correspondence between studies on cloned and native channels concerns a likely interaction between G-proteins and the ß subunit of the calcium channel in regulating gating of the channel. The ß subunit of the calcium channel is a cytoplasmic protein that alters the voltage dependence and kinetics of gating and binds to a region of the main calcium channel subunit (the cytoplasmic loop between the first and second pseudo-subunit domains) at which G-protein ß
subunits also interact. In both native neurons and cloned channels, depleting the channel ß subunit enhances G-protein inhibition, suggesting functional competition between the two proteins (
What is the physiological significance of all of this? So far, the clearest physiological role for G-protein inhibition of calcium channels is in mediating neurotransmitter inhibition of synaptic transmission, by reducing calcium entry into the presynaptic terminal. There are many parallels between the actions of transmitters on calcium channels (usually studied in cell bodies) and on synaptic transmission (although G-proteins also can act more directly on transmitter release by inhibiting still-unknown steps in exocytotosis). In fact, it has been observed that phorbol esters can reduce or eliminate transmitter inhibition of synaptic transmission at some synapses (-S, and it remains to be seen whether physiological activation of G-proteins by transmitters can be equally effective. Perhaps the effect of more physiological activation of G-proteins will be to slow the development of kinase C effects.
An even more fundamental unanswered question is how and when protein kinase C is activated at particular cells or synapses during normal functioning of the nervous system. In studies on isolated cells, phorbol esters were very effective in blocking G-protein inhibition of channels, but it has proven difficult to find equivalent effects of natural transmitters like acetylcholine and glutamate that would be expected to activate protein kinase C (K.J. Swartz, personal communication). As for many other aspects of ion channel function, our increasing understanding of complex molecular mechanisms has surpassed our knowledge of how those mechanisms are actually used.
Submitted: 2 February 2000
Revised: 2 February 2000
Accepted: 2 February 2000
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