Robert Brenner, Ph.D.
Our Research Interest
The focus of our studies has been the large conductance (BK-type) calcium-activated potassium channels. BK channels are gated to open by both micromolar calcium concentrations and voltage. This regulates many voltage-dependent processes that occur coincident with calcium increases, such as regulation of some action potentials, regulation of hormone and neurotransmitter release, and regulation of smooth muscle contractility. While BK channels are encoded by a single gene, they are broadly expressed and have diverse biophysical properties in native cells. This makes these channels an ideal protein for elucidating how ion channel function is regulated to tune the unique electrical properties of individual cells.
Our work has focused on a family of tissue-specific accessory β subunits that interact with the pore-forming α subunit (Figure above), and dramatically alter BK channel biophysical properties in a manner apparently reminiscent of BK channels in native tissues. Our experimental approach is to study these channels in transfected cells using site-directed mutagenesis and patch clamp electrophysiology to understand the biophysical mechanisms that underlies their modulation of BK channels. We use mouse gene knockouts and physiological studies to understand the relevance of BK β subunit interactions.
Potassium Channel Biophysical Studies
Measuring BK channel open-probability at very negative voltage in the absence and presence of calcium has revealed a Yin-Yang effect of β subunit on BK channel open probability (Figure to right). BK channel β1 and β4 subunits promote channel opening by adjusting the voltage range where the voltage-sensor is activated to more negative potentials (Yin). However, in the absence of voltage-sensor activation β1 and β4 subunits inhibit channel opening by increasing the energetic barrier between the open to closed transition (Yang). As well, β subunits also appear to increase the energetic barrier of open and closed transition states to slow channel opening and closing. Our ongoing studies have been to elucidate that actual molecular interactions between the pore-forming α subunit and the modulatory β subunit that mediate effects on channel open probability and kinetics.
Potassium Channel Regulation of Smooth Muscle Function
Our study of BK β1 knockout mice demonstrated that BK channels are tuned by the smooth muscle specific β1 subunit to sense calcium signals from ryanodine receptors in the sarcoplasmic reticulum (SR). Calcium release from ryanodine receptors is an important feedback signal that mediates relaxation of smooth muscle. BK β1 knockout mice have BK channels that do not adequately respond to calcium release, and we found that the knockouts have increased vascular tone and blood pressure, increases airway smooth muscle constriction and abnormally constricted bladder muscle. Our ongoing studies in airway smooth muscle has allowed us to understand the role of BK and other potassium channels in regulation of airway smooth muscle excitation-contraction coupling, constriction and control of airflow. Although voltage-dependent calcium channel blockers have long lost favor as bronchodilators for asthma, we have found that human potassium channel polymorphisms do affect asthma severity in individuals, and some potassium channel modulators show promise as bronchodilators. Indeed, an in vitro contraction study of airway shows that the commonly used beta-agonist only partly relaxes airway while a novel potassium channel modulator (Compound X, Figure above) causes a complete and sustained relaxation.
Potassium Channel Regulation of Neuronal Action Potentials
Our studies of BK channels in neurons have focused on the neuron-specific BK β4 subunit. A major finding from our lab is that the slow gating conferred by β4 subunit reduces BK channel contribution to action potential repolarization in hippocampus dentate gyrus granule neurons. Interestingly, knockout β4 appears to result in a faster-gated BK channel and BK gain-of-function. Functional studies indicate that BK channel gain of function paradoxically increases excitability of these neurons and cause seizures. This is consistent with computational modeling studies that indicate that BK sharpening of action potentials, and larger fAHP can reduce activation of other potassium channels that otherwise moderate action potential firing rates. In ongoing studies we are identifying the calcium channels that feed BK channel activation and contribute to seizures. We are also characterizing the adaptive and maladaptive changes of BK channel that follow seizures.
Herlihy JT, Semenov I, Brenner R, Assessment of airway hyperresponsiveness in murine tracheal rings. Methods Mol Biol, 2013, 1032: 257-269.
Evseev A, Semenov I, Archer CR, Medina JL, Dube PH, Shapiro MS, Brenner R, Functional effects of KCNQ K+ channels in airway smooth muscle, Frontiers Physiol, 2013, in press.
Semenov, I, Herlihy J T, Brenner R. In vitro measurements of tracheal constriction using mice. J Vis Exp, 2012, doi:3703