Silencing the cardiac potassium channel Kv4.3 by RNA interference in a CHO expression system

Silencing of Cav1.2 gene in neonatal cardiomyocytes by lentiviral delivered shRNA

Cav1.2 (alpha1C) and Cav1.3 (alpha1D) L-type Ca channels are co-expressed in the heart. To date, there are no pharmacological or biophysical tools to separate alpha1D from alpha1C Ca currents (I(Ca-L)) in cardiomyocytes. Here, we established a physiological model to study alpha1D I(Ca-L) in native myocytes using RNA interference. Transfection of rat neonatal cardiomyocytes (RNC) with alpha1C specific siRNA resulted in low silencing efficiency (50-60%) at the mRNA and protein levels. The use of lentivirus shRNA resulted in 100% transfection efficiency and 92% silencing of the alpha1C gene by real-time PCR and Western blot. Electrophysiological experiments showed that the total I(Ca-L) was similarly reduced by 80% in lentivirus transfected cells. Both biochemical and functional data demonstrated high transfection and silencing efficiency in the cardiomyocytes using lentiviral shRNA. This novel approach allows for the assessments of the roles of alpha1C and alpha1D Ca channels in native myocytes and could be used to examine their roles in physiological and pathological settings.

Post-transcriptional gene silencing of KChIP2 and Navbeta1 in neonatal rat cardiac myocytes reveals a functional association between Na and Ito currents

The Ca(2+)-independent transient outward potassium current (I(to)) encoded by the Kv4 family of potassium channels, is central to normal repolarization of cardiac myocytes. KChIPs are a group of Ca(2+)-binding accessory subunits that modulate Kv4-encoded currents. However, the biophysical effects of KChIP2 on Kv4 currents raise questions about the role that KChIP2 plays in forming the native I(to). Previous heterologous expression studies demonstrated that the Na channel beta1 subunit modulates the gating properties of Kv4.3 to closely recapitulate native I(to) suggesting that Na(v)beta1 may modulate the function of Kv4-encoded channels in native cardiomyocytes. Therefore we hypothesized the existence of a structural or functional complex between subunits of I(to) and I(Na). In co-immunoprecipitation of proteins from neonatal rat ventricular myocardium (NRVM), Na(v)beta1 was pulled-down by Kv4.x antibodies suggesting a structural association between subunits that comprise I(to) and I(Na). Remarkably, post-transcriptional gene silencing of KChIP2 in NRVM, using small interfering RNAs specific to KChIP2, suppressed both cardiac I(to) and I(Na) consistent with a functional coupling of these channels. KChIP2 silencing suppressed Na channel alpha and beta1 subunit mRNA levels, leaving Kv4.x mRNAs unaltered, but reducing levels of immunoreactive proteins. Post-transcriptional gene silencing of Na(v)beta1 reduced its protein expression. Silencing of Na(v)beta1 also reduced mRNA and protein levels of its alpha-subunit, Na(v)1.5. Surprisingly, silencing of Na(v)beta1 also produced a reduction in KChIP2 mRNA and protein as well as Kv4.x proteins resulting in remarkably decreased I(Na) and I(to). These data are consistent with a novel structural and functional association of I(Na) and I(to) in NRVMs.

Manipulation of the potassium channel Kv1.1 and its effect on neuronal excitability in rat sensory neurons

Potassium channels play a critical role in regulating many aspects of action potential (AP) firing. To establish the contribution of the voltage-dependent potassium channel Kv1.1 in regulating excitability, we used the selective blocker dendrotoxin-K (DTX-K) and small interfering RNA (siRNA) targeted to Kv1.1 to determine their effects on AP firing in small-diameter capsaicin-sensitive sensory neurons. A 5-min exposure to 10 nM DTX-K suppressed the total potassium current (I(K)) measured at +40 mV by about 33%. DTX-K produced a twofold increase in the number of APs evoked by a ramp of depolarizing current. Associated with increased firing was a decrease in firing threshold and rheobase. DTX-K did not alter the resting membrane potential or the AP duration. A 48-h treatment with siRNA targeted to Kv1.1 reduced the expression of this channel protein by about 60% as measured in Western blots. After treatment with siRNA, I(K) was no longer sensitive to DTX-K, indicating a loss of functional protein. Similarly, after siRNA treatment exposure to DTX-K had no effect on the number of evoked APs, firing threshold, or rheobase. However, after siRNA treatment, the firing threshold had values similar to those obtained after acute exposure to DTX-K, suggesting that the loss of Kv1.1 plays a critical role in setting this parameter of excitability. These results demonstrate that Kv1.1 plays an important role in limiting AP firing and that siRNA may be a useful approach to establish the role of specific ion channels in the absence of selective antagonists.

Kv4.3-mediated A-type K+ currents underlie rhythmic activity in hippocampal interneurons

Hippocampal-dependent learning and memory processes are associated with theta frequency rhythmic activity. Interneuron and pyramidal cell network interactions underlie this activity, but contributions of interneuron voltage-gated membrane conductances remain unclear. We show that interneurons at the CA1 lacunosum-moleculare (LM) and radiatum (RAD) junction (LM/RAD) display voltage-dependent subthreshold membrane potential oscillations (MPOs) generated by voltage-gated tetrodotoxin-sensitive Na+ and 4-aminopyridine (4-AP)-sensitive K+ currents. They also exhibit prominent 4-AP-sensitive A-type K+ currents, with gating properties showing activation at subthreshold membrane potentials. We found that LM/RAD cells are part of specific interneuron subpopulations expressing the K+ channel subunit Kv4.3 and their transfection with Kv4.3 small interfering RNA selectively impaired A-type K+ currents and MPOs. Thus, our findings reveal a novel function of Kv4.3-mediated A-type K+ currents in the generation of intrinsic MPOs in specific subpopulations of interneurons that may participate in hippocampal theta-related rhythmic activity.

Mammalian cell factories for efficient and stable protein expression

As the commercial market for therapeutic protein production from mammalian cells has expanded, so has the requirement for improved efficiency and stability of production. Rapid developments have been made in understanding the molecular environment of transgenes in chromatin, including elucidation of the contribution of epigenetic modifications to expression, and this understanding is being used to enhance expression from host cells. Technical advances surrounding the 'omics' revolution are enabling the rational identification of complex control factors that define the flow of information from transgene to desired protein. Using information from 'omics' interrogations, directed cell engineering has been employed to enhance the translational and secretory capacity of host cells. Taken together, these recent advances are likely to lead to improved routes for protein production in the future.