Genetic activation of BK currents in vivo generates bidirectional effects on neuronal excitability

Abstract

Large-conductance calcium-activated potassium channels (BK) are potent negative regulators of excitability in neurons and muscle, and increasing BK current is a novel therapeutic strategy for neuroand cardioprotection, disorders of smooth muscle hyperactivity, and several psychiatric diseases. However, in some neurons, enhanced BK current is linked with seizures and paradoxical increases in excitability, potentially complicating the clinical use of agonists. The mechanisms that switch BK influence from inhibitory to excitatory are not well defined. Here we investigate this dichotomy using a gain-of-function subunit (BK R207Q ) to enhance BK currents. Heterologous expression of BK R207Q generated currents that activated at physiologically relevant voltages in lower intracellular Ca 2+ , activated faster, and deactivated slower than wild-type currents. We then used BK R207Q expression to broadly augment endogenous BK currents in vivo, generating a transgenic mouse from a circadian clock-controlled Period1 gene fragment (Tg-BK R207Q ). The specific impact on excitability was assessed in neurons of the suprachiasmatic nucleus (SCN) in the hypothalamus, a cell type where BK currents regulate spontaneous firing under distinct day and night conditions that are defined by different complements of ionic currents. In the SCN, Tg-BK R207Q expression converted the endogenous BK current to fast-activating, while maintaining similar current-voltage properties between day and night. Alteration of BK currents in Tg-BK R207Q SCN neurons increased firing at night but decreased firing during the day, demonstrating that BK currents generate bidirectional effects on neuronal firing under distinct conditions.V oltage-gated K + channels generally oppose excitability by producing hyperpolarizing current in response to membrane depolarization (1). One distinctive member of this family is the BK channel (K Ca 1.1), encoded by the Kcnma1 gene (2, 3). BK channels are widely expressed in excitable and nonexcitable cells (4), suggesting that they are highly versatile players in membrane signaling. In neurons and muscle, BK currents are activated by simultaneous membrane depolarization and an increase in intracellular Ca 2+ (Ca 2+ i ) (1, 5), shaping the falling phase of the action potential, the afterhyperpolarization (AHP), and some Ca 2+ transients that underlie neurotransmitter release and secretion (6-9). Deletion of Kcnma1 leads to a constellation of defects related to hyperexcitability (2, 10). For this reason, BK agonists have been pursued as novel targets for treating stroke, seizure, cardiac ischemia, urinary incontinence, asthma, erectile dysfunction, and hypertension (11,12). Linkage analysis and expression profiling have also implicated Kcnma1 in schizophrenia, autism, mental retardation, and alcoholism (13-17).Whereas selectively activating BK currents that suppress excitability would be therapeutically useful, a paradoxical excitatory role for BK has also been uncovered in several tissues. BK antagonists can reduce heart rate (...

Knockout of the BK β4-subunit promotes a functional coupling of BK channels and ryanodine receptors that mediate a fAHP-induced increase in excitability

“…In some studies, BK channels have been shown to increase, rather than decrease, AP frequency (Brenner et al 2005;Gu et al 2007;Jaffe et al 2011;Lovell and McCobb 2001;MartinezEspinosa et al 2014;Montgomery et al 2012;Shruti et al 2008). Given that BK channels are activated by specific calcium sources, this suggests that some calcium channels' effects on excitability may be partly mediated by activation of proexcitatory BK channels.…”

Membrane Currents, Gene Expression, and Circadian Clocks

“…A study in which BK currents were inhibited with iberiotoxin suggests that this current may contribute 40% of the afterhyperpolarization that occurs after an action potential in the SCN (Cloues and Sather 2003). Blocking the BK current (iberiotoxin) can reduce daytime peak firing rate in at least some SCN neurons (Pitts et al 2006) and genetic manipulation of the BK channel also impacts daytime firing frequency (Montgomery and Meredith 2012;Montgomery et al 2013). Recent work has found that inactivating BK currents predominate during the day in the SCN.…”

Mis-expression of the BK K+ channel disrupts suprachiasmatic nucleus circuit rhythmicity and alters clock-controlled behavior

“…This increased sensitivity may also be influenced by other factors such as a change in light input to the SCN in Tg-BK R207Q mice, since Per1 is expressed in the retina (70), or developmental differences. The light-induced phase shifts normally transduced through Per1-mediated increases in firing (2,31,60,69) (45). However, those mice were expected to have an inverted circadian phase with strong diurnal behavioral activity, stemming from Tg-BK…”

“…5, B and E). In addition, although some neurons increased their firing (45) Decreased circadian amplitude in the SCN circuit of Tg-BK R207Q mice. To assess how these changes in daytime excitability impacted rhythmicity of the SCN circuit, we performed long-term multielectrode array recordings from organotypic SCN cultures (Fig.…”