Deborah J. Baro

An improved parameter estimation method for Hodgkin-Huxley models.

We consider whole-cell voltage-clamp data of isolated currents characterized by the Hodgkin-Huxley paradigm. We examine the errors associated with the typical parameter estimation method for these data and show them to be unsatisfactorally large especially if the time constants of activation and inactivation are not sufficiently separated. The size of these errors is due to the fact that the steady-state and kinetic properties of the current are estimated disjointly. We present an improved parameter estimation method that utilizes all of the information in the voltage-clamp conductance data to estimate steady-state and kinetic properties simultaneously and illustrate its success compared to the standard method using simulated data and data from P. interruptus shal channels expressed in oocytes.

Genes and channels: patch/voltage-clamp analysis and single-cell RT-PCR

Abstract. Technological advances in electrophysiology and molecular biology in the last two decades have led to great progress in ion channel research. The invention of the patch-clamp recording technique has enabled the characterization of the biophysical and pharmacological properties of single channels. Rapid progress in the development of molecular biology techniques and their application to ion channel research led to the cloning, in the 1980s, of genes encoding all major classes of voltage- and ligand-gated ionic channels. It has become clear that operationally defined channel types represent extended families of ionic channels. Several experimental approaches have been developed to test whether there is a correlation between the detection of particular ion channel subunit mRNAs and the electrophysiological response to a pharmacological or electrical stimulus in a cell. In one method, whole-cell patch-clamp recording is performed on a cell in culture or tissue-slice preparation. The biophysical and pharmacological properties of the ionic channels of interest are characterized and the cytoplasmic contents of the recorded cell are then harvested into the patch pipette. In a variant of this method, the physiological properties of a cell are characterized with a two-electrode voltage clamp and, following the recording, the entire cell is harvested for its RNA. In both methods, the RNA from a single cell is reverse-transcribed into cDNA by a reverse transcriptase and subsequently amplified by the polymerase chain reaction, i.e. by the so-called single-cell/reverse transcription/polymerase chain reaction method (SC-RT-PCR). This review presents an analysis of the results of work obtained by using a combination of whole-cell patch-clamp recording or two-electrode voltage clamp and SC-RT-PCR with emphasis on its potential and limitations for quantitative analysis.

Alternative Splicing in the Pore-Forming Region of shaker Potassium Channels

We have cloned cDNAs for the shaker potassium channel gene from the spiny lobster Panulirus interruptus. As previously found in Drosophila, there is alternative splicing at the 5′ and 3′ ends of the coding region. However, in Panulirus shaker, alternative splicing also occurs within the pore-forming region of the protein. Three different splice variants were found within the P region, two of which bestow unique electrophysiological characteristics to channel function. Pore I and pore II variants differ in voltage dependence for activation, kinetics of inactivation, current rectification, and drug resistance. The pore 0 variant lacks a P region exon and does not produce a functional channel. This is the first example of alternative splicing within the pore-forming region of a voltage-dependent ion channel. We used a recently identified potassium channel blocker, κ-conotoxin PVIIA, to study the physiological role of the two pore forms. The toxin selectively blocked one pore form, whereas the other form, heteromers between the two pore forms, and Panulirus shal were not blocked. When it was tested in thePanulirus stomatogastric ganglion, the toxin produced no effects on transient K+ currents or synaptic transmission between neurons.

Alternate splicing of the shal gene and the origin of IA diversity among neurons in a dynamic motor network

The pyloric motor system, in the crustacean stomatogastric ganglion, produces a continuously adaptive behavior. Each cell type in the neural circuit possesses a distinct yet dynamic electrical phenotype that is essential for normal network function. We previously demonstrated that the transient potassium current (I(A)) in the different component neurons is unique and modulatable, despite the fact that the shal gene encodes the alpha-subunits that mediate I(A) in every cell. We now examine the hypothesis that alternate splicing of shal is responsible for pyloric I(A) diversity. We found that alternate splicing generates at least 14 isoforms. Nine of the isoforms were expressed in Xenopus oocytes and each produced a transient potassium current with highly variable properties. While the voltage dependence and inactivation kinetics of I(A) vary significantly between pyloric cell types, there are few significant differences between different shal isoforms expressed in oocytes. Pyloric I(A) diversity cannot be reproduced in oocytes by any combination of shal splice variants. While the function of alternate splicing of shal is not yet understood, our studies show that it does not by itself explain the biophysical diversity of I(A) seen in pyloric neurons.

Differential Expression and Targeting of K+ Channel Genes in the Lobster Pyloric Central Pattern Generatora

Abstract: A molecular analysis of motor pattern generation is an essential complement to electrophysiological and computational investigations. In arthropods, A‐channels are posttranslationally modified multimeric proteins containing Shaker family α‐subunits that may interact with β‐subunits, γ‐subunits, and other auxiliary proteins. One consequence of A‐channel structure is that several mechanisms could underlie the cell‐specific differences in pyloric IAs including differential gene expression, alternate splicing, and posttranslational modifications. Oocyte expression studies, single‐cell RT‐PCR, and immunocytochemistry suggest that differential α‐subunit gene expression is not a mechanism for creating pyloric IA heterogeneity, and that the same gene, shal, encodes the a‐subnuits for the entire family of somatic IAs in the pyloric network. Changes in the level of shal gene expression alter A‐channel density between cells, but cannot account for the differences in the biophysical properties of the six pyloric IAs. Preliminary data suggest that the shal gene also encodes the A‐channel α‐subunits for the coarse and fine neuropil but not for most axons. A second gene, shaker, encodes the A‐channel α‐subunits in the majority of axons and at the neuromuscular junction. The distinct properties of the two types of A‐channels are consistent with the different roles of IA at the different locations. Both the shaker and shal genes are alternately spliced, and investigations are under way to determine whether alternate splicing is a mechanism for generating pyloric IA heterogeneity.

The lobster shaw gene: cloning, sequence analysis and comparison to fly shaw.

We cloned and sequenced the cDNA for the shaw gene, encoding a voltage-dependent potassium (K+) channel, from the spiny lobster, Panulirus interruptus. The deduced amino acid sequence has a high degree of homology to the Drosophila melanogaster Shaw protein. In addition, lobster Shaw has several putative sites for post-translational modifications.