Who needs a current? Functional remodelling in the Kv4.2-/-mouse

Potassium channels are key sculptors of neuronal excitability. In contrast to the wealth of knowledge about these channels in expression systems, it has proven exceedingly difficult to pinpoint the actions of specific subunits in native membranes. This is due to inadequate pharmacological tools, as well as expression of many α subunits in individual cells, heteromultimeric channels, auxiliary subunits, and overlap in channel biophysical properties. It is widely accepted that genetic manipulations are ultimately required to unravel this knot, but this approach has its own limitations. One concern is the potential compensation for loss of one subunit by altered expression of existing subunits or de novo expression of novel subunits. In this issue of The Journal of Physiology, Nerbonne and colleagues (Nerbonne et al. 2008) provide a compelling example of functional compensation after knock out of the Kv4.2 α subunit in visual cortical pyramidal neurons. In an earlier paper, this group transfected neocortical pyramidal neurons with a dominant negative (DN) construct that eliminated expression of Kv4.2 subunits. They showed that the fast, transient A-type current was mediated by Kv4.2 and that down-regulation of Kv4.2 broadened action potentials (APs), slowed AP repolarization, and altered repetitive firing (Yuan et al. 2005). In the present work, the authors compared somatic currents, action potentials and repetitive firing of pyramidal cells from wild-type (WT) and Kv4.2 null mice (knock out: KO). In most cells, the rapidly inactivating A current was abolished in the KO mice and the component that inactivated at an intermediate rate was unchanged in amplitude. Surprisingly, the slowly- and non-inactivating components were larger in KO animals. Thus, pyramidal cells compensate for the loss of Kv4.2 by remodelling expression of the remaining channels. Further, this response to chronic absence of Kv4.2 subunits during development differed dramatically from the response to the relatively acute reduction of Kv4.2 with the DN construct. Even more remarkably, despite loss of one of the major regulators of AP width and threshold in the KO animals, APs were unchanged and repetitive firing preserved, suggesting homeostatic remodelling of function as well as current densities. Another intriguing finding was that a subset of KO animals expressed a fast transient current that differed from the A current of WT mice in pharmacology (insensitive to heteropodatoxin) and its recovery from inactivation (slower). This finding suggests that the form of compensation can be different among pyramidal neurons. Nerbonne et al. (2008) convincingly demonstrate that pyramidal cells can dramatically alter K+ channel expression and yet maintain AP form and the ability to fire repetitively. It is unclear how rapidly this reorganization can occur, whether remodelling is restricted to somatic membrane or also includes dendrites and/or axons, or whether remodelling remains stable over time. While repetitive firing is maintained in the KO animals, and maximal firing rates appear similar in WT and KO, the slopes of the frequency versus current relationships are quite different. As with behavioural phenotypes of many transgenic animals, perhaps additional tests will reveal limits to reconstitution of function? The molecular identity of the up-regulated channel subunits is an important remaining question, the answer to which will be necessary to discern the mechanisms underlying channel remodelling. For the majority of cells where the slowly and non-inactivating currents are enhanced, several possibilities exist for the up-regulated subunits. Tests with relatively specific pharmacological agents (e.g. α-dendrotoxin: blocks Kv1 channels) may provide insights into which families of subunits are remodelled. The finding that action potential form was unaltered in the KO animals suggests that at least some of the up-regulated potassium channels must have rapid kinetics (similar to Kv4-mediated current). Recent work suggests contributions of Kv1 and Kv2 channels to the slowly inactivating current in neocortical pyramidal neurons (Guan et al. 2006, 2007) and these subunits would be prime suspects for up-regulation. Although the Kv1 current is usually small in neocortical pyramidal cells, it activates rapidly and at subthreshold voltages, and the inactivation and recovery from inactivation kinetics match the up-regulated current (Guan et al. 2006). Kv2 channels are slowly activating but probably contribute to AP repolarization. For the minority of cells where a smaller A-like current remains, Kv1 channels are again prime candidates. Notably, α-dendrotoxin-sensitive currents have a significant transient component in some rat neocortical pyramidal cells (Guan et al. 2006). The findings in this report invite caution when using KO animals to determine ion channel function and are reminiscent of remodelling of slowly inactivating K+ currents (and other channels) to maintain network homeostasis in response to prolonged changes in activity levels of pyramidal cells in culture (Desai et al. 1999). It is tempting to hypothesize that similar mechanisms are at play in these very different manipulations, although it is unclear how activity is altered in the cells from the Kv4.2 KO. While homeostatic changes in intrinsic excitability are well documented, it is not known how neurons sense changes in neural activity (mean firing rate, [Ca2+]i, Ca2+-dependent proteins?) or how deviations from a set point are transduced into changes in specific aspects of excitability. Nerbonne and colleagues have provided incentive for further studies into mechanisms regulating neuronal excitability in response to altered activity and in response to injury and disease.