Cells utilize voltage-activated channels that conduct sodium and calcium ions to transmit a variety intracellular signals, such as action potentials propogations along an axon towards a nerve terminal, or calcium influx that mediates neurotransmitter release or muscle contraction. Voltage-dependent potassium channels, by repolarizing the membrane, generally serve to deactivate sodium and calcium channels, thereby shaping the strength and duration of such signals. An extensive variety of potassium channels and their regulation underlies much of the rich diversity in electrical properties observed among cells types serving different physiological functions. My research interests are to understand how ion channels are regulated to contribute to the unique membrane and signaling properties of electrically excitable cells. Our experimental approach is to understand ion channel biophysical interactions in heterologous expression systems utilizing patch clamp recording techniques. The functional relevence of these interactions are ascertained by complementary studies with transgenic and gene knockout studies in mice. This integrative approach allows us the opportunity to understand ion channel modulation, not only at the molecular and cellular level, but also in the context of the sytem physiology and intact organism.

The focus of my studies have 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 vascular smooth muscle tone. BK channels are encoded by a single gene, however 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 excitable cells.

Our work has focused on a family of tissue-specific accessory β subunits that interact with the pore-forming α subunit, and dramatically alter BK channel biophysical properties in a manner apparently reminiscent of BK channels in native tissues. For example, the beta2 subunit confers BK channels with a increased calcium sensitivity, and inactivation properties observed in adrenal chromaffin BK channels. The beta4 subunit confers the slow gating and pharmacology oberved in many neuronal cells. Our studies have focused on determining what role the various BK/beta subunit channels serve in physiology of various cell types.

Our initial studies have focused on the role of a smooth muscle specific accessory subunit, the β1 subunit. In expression systems, this subunit enhances the response of BK channels to calcium. To understand the role of this protein, we generated knockout mice for the β1 subunit and studied its role in regulating vascular tone. In cerebral vascular smooth muscle, BK channels have an established role in mediating vasodilation following intravascular pressure or by a number of second messenger signals.

Our study of BK β1 knockout mice demonstrated that BK channels are tuned by the 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 vasodilation. 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 as a result. This work is important because it demonstrates that BK potassium channels must be regulated to respond to the local calcium environment of a vascular smooth muscle, and are likely, in other tissues to be regulated to sense calcium in a manner specific for those cells.

More recently, we have cloned a related accessory subunit, the β4 subunit. The BK β4 subunit also modulates BK channel calcium sensitivity, but in a unique manner. Unlike the smooth muscle β1 subunit or the β2 subunits, that increases BK channel openings at all calcium concentration, the β4 subunit reduces BK channel openings at low calcium but increases openings at high calcium concentrations. Our hypothesis is that the β4 subunit allows the BK potassium channel to function as a threshhold detector of calcium, silencing BK channels during graded depolorizations and global calcium increases but activating BK channels and effectively reducing excitability where the calcium concentration is high (such as synaptic termini) or during an action potential. Currently, we are studying the role of the β4 subunit in regulating electrical excitability in neurons.