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University of Maryland School of Medicine Research Illuminates Key Aspects of How We Fall Asleep and Wake Up
Brain Initiative Grant Funded: MULTIPARAMETRIC BIOSENSOR IMAGING IN BRAIN SLICES
Our research addresses the mechanisms by which ion channels regulate the key aspects of membrane excitability that underlie information coding. In my lab, we combine the genetic manipulation of ion channels with electrophysiology and in vivo physiology. Our long term interests are to identify the critical biophysical properties of specific channels that ultimately influence the physiology of the whole animal.
BK Channels (KCa1.1)
Our current studies focus on a unique potassium channel, BK (Big K+), the large conductance voltage and Ca2+-activated K+ channel. The BK channel pore-forming alpha subunit is encoded by a single gene (Kcnma1 in mouse and Slowpoke in Drosophila) that can be extensively alternately spliced. Like other Kv family members, BK channels are comprised of a tetramer of alpha subunits, modulatory beta (β1-4) and gamma subunits (ɣ1-4 or LRRC26, 52, 55, and 38), and are closely localized with intracellular Ca2+ sources such as voltage-gated Ca2+ channels and RyRs.
BK channels serve as an excellent substrate for linking channel properties to cellular signaling due to several exceptional features, including the unusually large unitary conductance, allosteric voltage and Ca2+-dependent gating, and well-characterized biophysical properties. Several years ago, I made a deletion of the BK channel in mouse (Kcnma1–/– or Slo–/–). We and our collaborators showed that BK channels regulate heart rate, urination, locomotion, sexual function, circadian rhythms, neurovascular coupling, aspects of hearing, and are the targets of a fungal neurotoxin that causes Ryegrass Staggers.
Suprachiasmatic Nucleus (SCN)
Circadian physiology is an ideal model system for studying information coding. Daily behavioral and physiological rhythms (~ 24 hrs) are a universal trait of animals, vital for adaptation to their environment and overall fitness. In mammals, lesion and transplantation studies have localized the principal circadian pacemaker to the suprachiasmatic nucleus (SCN) of the hypothalamus, identifying a discrete neural substrate for a complex behavior. We identified a novel role for the BK channel in the daily patterning of neural activity in the SCN. Kcnma1–/– mice have degraded circadian behavioral and physiological rhythms, and their SCN neurons exhibit aberrant daily action potential rhythms in the SCN circuit.
We are currently studying the circadian regulation of BK current properties in SCN neurons and how specific properties of the BK current influence the neural representation of circadian time.
BK Channels in Cardiovascular Function
BK channels are directly implicated in cardiovascular function through their regulation of vascular tone. However, a role for BK channels in the heart itself has been mostly discounted based on the weak relative expression. We recently found that several selective blockers of BK channels caused a counter-intuitive decrease in heart rate (bradycardia). BK antagonist-induced bradycardia was not observed in mice lacking BK channels (Kcnma1–/–), supporting a role for the channels in controlling heart rate and the confirming specificity of this effect. The aims of this project are to determine the mechanism and integrated contribution of BK channels in the vasculature, neurons, and the heart itself to cardiovascular function.[/expand]
Smooth muscle (bladder)
Who wants to get up in the middle of the night to pee? It turns out very little is known about circadian regulation of bladder function. Nocturia, excessive urination at night, is a persistent disorder affecting > 50% of people in some age groups and significantly decreasing quality of life. However, existing treatments have no specific mechanism to address the circadian component of nocturia. The goal of this project is to develop a mechanistic explanation for the poorly understood processes that govern the normal circadian pattern of urination and how these mechanisms may go awry in nocturia.
We have shown that BK channels are potent regulators of urinary bladder smooth muscle (UBSM) contractility. Kcnma1–/– knockout mice, lacking the BK channel, exhibit increased UBSM tone, hyperactive contractions, and unstable bladder pressure. Importantly, Kcnma1–/– mice manifest frank incontinence during the sleep period, generating a novel rodent model of nocturia involving UBSM dysfunction. Based on BK function in the brain linking clock genes and the daily patterning of membrane excitability, this project addresses whether bladder, like the brain, possesses intrinsic mechanisms for regulating excitability over the circadian cycle. We are interested in understanding if peripheral rhythms, such as day-night differences in bladder smooth muscle contractility, are controlled at the level of the brain (SCN) or are intrinsic to the muscle, or both.
Alternate Splicing of Kcnma1
BK channels are physiologically activated by voltage and Ca2+, modulating a diversity of membrane signals in different cells types. Even in excitable cells, such as neurons and muscle where they play prominent roles, their influence encompasses diverse roles in action potential repolarization, afterhyperpolarizations, repetitive firing, spontaneous firing, neurotransmitter release, plateau potentials, and baseline membrane potentials. Although BK channels have been extensively studied at the biophysical level, the impact of tissue-specific variation in BK current properties has only been addressed in a few specialized systems, such as the non-mammalian cochlea.
By combining genetic manipulation of BK channels and cloning of native splice variants with cellular, circuit, and physiological recordings, we are identifying novel systems, such as the suprachiasmatic nucleus, where direct links between BK properties and excitability can be established.
Novel Roles for BK Channels
BK channels are widely, but specifically, expressed in both excitable and non-excitable tissues. Overall, less is known about their roles in non-excitable cell types or intact physiological systems. Unlike the voltage-gated K+ channel family, there is only one gene that encodes the BK channel, and Kcnma1–/– mice display a surprising number of phenotypes at the cellular and systems levels. This lack of redundancy has enabled us to use the BK channel deletion mouse as a selective mechanism for perturbing signaling in a variety of pathways. To identify new systems in which BK channels play dominant roles, we are conducting phenotypic screens in Kcnma1–/– mice with global and tissue-specific conditional deletions of the BK channel.
The lab is supported by grants from The National Heart, Lung, and Blood Institute, The National Institute of Diabetes and Digestive and Kidney Diseases, The National Science Foundation, The American Heart Association, and The S&R Foundation Ryuji Ueno Award for Ion Channels or Barrier Function Research (The American Physiological Society).