Kv4 Channel Modulation

When Kv4 channels are expressed in heterologous systems, the resulting channel biophysical characteristics do not match those measured in neurons. This observation has led many to ask whether Kv4 channels are the targets of post-translational modifications and interact with other proteins that affect the biophysical properties of the channel. In fact, when Kv4 channel mRNA was injected into oocytes, transient currents could be elicited, but these currents differed from native currents (Serodio et al. 1994). However, when rat brain mRNA that did not result in transient potassium currents when injected alone was co-injected with Kv4 mRNA, the currents more closely resembled native currents. This suggested that additional proteins may interact with the channel in ways the are important for native channel function. Since this time many proteins that interact with and modify Kv4 channels have been discovered.

Kvβ-subunits are cytoplasmic proteins that can interact with α-subunits that make up

potassium channels. There are 3 β-subunits known to modify potassium channel expression

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subfamilies, the β-subunit causes channels to inactivate more rapidly, even in channels that

do not normally inactivate by themselves (Birnbaum et al. 2004). However, when Kv4.3 and

Kvβ-subunits are expressed together in HEK298 cells, more Kv4.3 channels are trafficked to

the membrane, but the channels’ voltage sensitivity and kinetics are unaffected by the β-

subunit (Yang et al. 2001).

Another family of cytoplasmic proteins, the Ca2+ sensitive K channel interacting

proteins (KChIPs), on the other hand, interact with Kv4 α-subunits and alter the kinetics and

voltage sensitivity of Kv4 channels. When the N-terminus of Kv4.3 was used as bait in a yeast-two hybrid experiment, 3 proteins, KChIP 1-3, were found to interact with Kv4 channels in cells, increase surface expression of the channel, and change their biophysical properties (An et al. 2000; Shibata et al. 2003). For instance, KChIPs shifted the half- activation voltage from 28.2 mV to -12.1 mV, a value that matches measurements of native channels. However, this interaction did slow the inactivation time constant from 28.2 msec to 104.1 msec and in some cases eliminated it all together (Holmqvist et al. 2002). This value is much slower than reported in most neurons. Despite this, microscopy studies have

demonstrated that KChIPS colocalize with Kv4 channels in almost all areas of the brain where Kv4 channels are expressed and are now accepted as being a necessary component of Kv4 channels in neurons and ventricular myocytes (Rhodes et al. 2004; Strassle et al. 2005). The crystal structure of the interaction between these two types of proteins was solved and

shows that these proteins interact through the presence of hydrophobic α-helices on the N-

terminal region of the Kv4 channel and the C-terminal region of the KChIP (Scannevin et al. 2004; Zhou et al. 2004). This interaction is calcium dependent and acts to stabilize the

inhibit A-currents (Ramakers and Storm 2002) and this effect on Kv4 channels is dependent upon KChIPs (Holmqvist et al. 2001). Therefore, the KChIP interaction with Kv4 channels is essential for the expression of native A-type potassium currents.

Even after the discovery of KChIPs, a remaining puzzle of Kv4 A-type currents remained—fast inactivation. KChIPs co-expressed with Kv4 channels resulted in the opposite effect seen in native cells; these proteins slowed inactivation. However, Nadal and colleges reported a factor found in cerebellar mRNA could accelerate the inactivation time constant when expressed with Kv4 mRNA (Nadal et al. 2001). This factor was later

identified as DPPX (Nadal et al. 2003). DPPX is a transmembrane protein that interacts with the channel transmembrane domains. DPPX results in robust surface expression of Kv4 channels and changes the biophysical properties of the channels. Half-inactivation and half- activation voltages were shifted to more negative values, and the recovery time from

inactivation was decreased. These properties are quintessential characteristics of Kv4 channels and are dependent upon the interaction with DPPX.

Finally, multiple putative PKA, PKC, ERK and CAMKII phosphorylation sites have been found on intracellular portions of Kv4 channels (Adams et al. 2000; Anderson et al. 2000). Stimulation of PKA and PKC activation of MAPK results in the half-activation voltage shifting to the right, thereby inhibiting Kv4 channel activity in pyramidal neurons of the hippocampus (Hoffman and Johnston 1998; Yuan et al. 2002; Yuan et al. 2006).

However, direct evidence that phosphorylation of Kv4 channels affects the voltage sensitivity has yet to be demonstrated. Two studies have reported that while Kv4 channels are directly phosphorylated by MAPK, any biophysical effect by this modification of Kv4 channels was dependent on KChIPs interacting with the channel (Schrader et al. 2002; Schrader et al.

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2006). CAMKII also can directly phosphorylate Kv4 channels, but this modification only results in increased current density due to increased expression on the cell membrane. It does not result in any biophysical changes in channel kinetics or activation (Varga et al. 2004). With such diverse mechanisms to control Kv4 channel expression and physiology, it is not difficult to imagine how this channel can be modulated to change the physiology of the entire neuron.