BK ZERO isoform HEK293 stably transfected cell lines differing 3'UTRs to assess miR-9 regulation

Research has identified the large conductance voltage- and calcium-activated potassium channel (BK) as a key regulator of neuronal excitability genetically associated to behavioral alcohol tolerance. Sensitivity to ethanol at the molecular level is characterized by acute potentiation of channel activity. BK isoforms show variations in alcohol sensitivity and are differentially distributed on the plasma membrane surface in response to prolonged exposure. MicroRNA (MiRNA) targeting of alcohol-sensitive isoforms coupled with active internalization of BK channels in response to ethanol are believed to be key in establishing homeostatic adaptations that produce persistent changes within the plasma membrane of neurons. In fact, microRNA 9 (miR-9) upregulated expression is a key event in persistent alcohol tolerance mediating acute EtOH desensitization of BK channels. The exact nature of these interactions remains a current topic of discussion. To further study the effects of miR-9 on the expression and distribution of BK channel isoforms we designed an experimental model by transfecting human BK channel isoforms ZERO heterologous constructs in human embryonic kidney cells 293 (HEK293) cells respectively expressing 2.1 (miR-9 responsive), 2.2 (unresponsive) and control (no sequence) 3'untranslated region (3'UTR) miRNA recognition sites. We used imaging techniques to characterize the stably transfected monoclonal cell lines, and electrophysiology to validate channel activity. Finally, we used immunocytochemistry to validate isoform responsiveness to miR-9. Our findings suggest the cell lines were successfully transfected to express either the 2.1 or 2.2 version of ZERO. Patch clamp recordings confirm that these channels retain their functionality and immunohistochemistry shows differential responses to miR-9, making these cells viable for use in future alcohol dependence studies.

Voltage-induced Ca2+ release by ryanodine receptors causes neuropeptide secretion from nerve terminals

Depolarisation-secretion coupling is assumed to be dependent only on extracellular calcium ([Ca²⁺ ]_o ). Ryanodine receptor (RyR)-sensitive stores in hypothalamic neurohypophysial system (HNS) terminals produce sparks of intracellular calcium ([Ca²⁺ ]_i ) that are voltage-dependent. We hypothesised that voltage-elicited increases in intraterminal calcium are crucial for neuropeptide secretion from presynaptic terminals, whether from influx through voltage-gated calcium channels and/or from such voltage-sensitive ryanodine-mediated calcium stores. Increases in [Ca²⁺ ]_i upon depolarisation in the presence of voltage-gated calcium channel blockers, or in the absence of [Ca²⁺ ]_o , still give rise to neuropeptide secretion from HNS terminals. Even in 0 [Ca²⁺ ]_o , there was nonetheless an increase in capacitance suggesting exocytosis upon depolarisation. This was blocked by antagonist concentrations of ryanodine, as was peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Furthermore, such depolarisations lead to increases in [Ca²⁺ ]_i . Pre-incubation with BAPTA-AM resulted in > 50% inhibition of peptide secretion elicited by high K⁺ in 0 [Ca²⁺ ]_o . Nifedipine but not nicardipine inhibited both the high K⁺ response for neuropeptide secretion and intraterminal calcium, suggesting the involvement of CaV1.1 type channels as sensors in voltage-induced calcium release. Importantly, RyR antagonists also modulate neuropeptide release under normal physiological conditions. In conclusion, our results indicate that depolarisation-induced neuropeptide secretion is present in the absence of external calcium, and calcium release from ryanodine-sensitive internal stores is a significant physiological contributor to neuropeptide secretion from HNS terminals.

Time-Dependent Effects of Ethanol on BK Channel Expression and Trafficking in Hippocampal Neurons

BACKGROUND

The large conductance Ca(2+) - and voltage-activated K(+) channel (BK) is an important player in molecular and behavioral alcohol tolerance. Trafficking and surface expression of ion channels contribute to the development of addictive behaviors. We have previously reported that internalization of the BK channel is a component of molecular tolerance to ethanol (EtOH).

METHODS

Using primary cultures of hippocampal neurons, we combine total internal reflection fluorescence microscopy, electrophysiology, and biochemical techniques to explore how exposure to EtOH affects the expression and subcellular localization of endogenous BK channels over time.

RESULTS

Exposure to EtOH changed the expression of endogenous BK channels in a time-dependent manner at the perimembrane area (plasma membrane and/or the area adjacent to it), while total protein levels of BK remain unchanged. These results suggest a redistribution of the channel within the neurons rather than changes in synthesis or degradation rates. Our results showed a temporally nonlinear effect of EtOH on perimembrane expression of BK. First, there was an increase in BK perimembrane expression after 10 minutes of EtOH exposure that remained evident after 3 hours, although not correlated to increases in functional channel expression. In contrast, after 6 hours of EtOH exposure, we observed a significant decrease in both BK perimembrane expression and functional channel expression. Furthermore, after 24 hours of EtOH exposure, perimembrane levels of BK had returned to baseline.

CONCLUSIONS

We report a complex time-dependent pattern in the effect of EtOH on BK channel trafficking, including successive increases and decreases in perimembrane expression and a reduction in active BK channels after 3 and 6 hours of EtOH exposure. Possible mechanisms underlying this multiphasic trafficking are discussed. As molecular tolerance necessarily underlies behavioral tolerance, the time-dependent alterations we see at the level of the channel may be relevant to the influence of drinking patterns on the development of behavioral tolerance.

Lipids modulate the increase of BK channel calcium sensitivity by the β1 subunit

Co-expression of the auxiliary β1 subunit with the pore forming α subunit of BK dramatically alters apparent calcium sensitivity. Investigation of the mechanism underlying the increase in calcium sensitivity of BK in smooth muscle has concentrated on the energetic effect of β1's interaction with α. We take a novel approach, exploring whether β1 modification of calcium sensitivity reflects altered interaction between the channel protein and surrounding lipids. We reconstituted hSlo BK α and BK α+β1 channels into two sets of bilayers. One set contained POPE with POPS, POPG, POPA and POPC, where the length of acyl chains is constant, but surface charge differs. The second set is a series of neutral bilayers formed from DOPE with phosphatidylcholines (PCs) of varying acyl chain lengths: C (14:1), C (18:1), C (22:1) and C (24:1), and with brain sphingomyelin (SPM), in which surface charge is constant, but bilayer thickness varies. The increase in calcium sensitivity caused by the β1 subunit was preserved in negatively charged lipid bilayers but not in neutral bilayers, indicating that modification of apparent Ca(2+) sensitivity by β1 is modulated by membrane lipids, requiring negatively charged lipids in the membrane. Moreover, the presence of β1 reduces BK activity in thin bilayers of PC 14:1 and thick bilayers containing SPM, but has no significant effect on activity of BK in PC 18:1, PC 22:1 and PC 24:1 bilayers. These data suggest that auxiliary β1 subunits fine-tune channel gating not only through direct subunit-subunit interactions but also by modulating lipid-protein interactions.

Large conductance voltage- and Ca2+-gated potassium (BK) channel β4 subunit influences sensitivity and tolerance to alcohol by altering its response to kinases

Tolerance is a well described component of alcohol abuse and addiction. The large conductance voltage- and Ca(2+)-gated potassium channel (BK) has been very useful for studying molecular tolerance. The influence of association with the β4 subunit can be observed at the level of individual channels, action potentials in brain slices, and finally, drinking behavior in the mouse. Previously, we showed that 50 mm alcohol increases both α and αβ4 BK channel open probability, but only α BK develops acute tolerance to this effect. Currently, we explore the possibility that the influence of the β4 subunit on tolerance may result from a striking effect of β4 on kinase modulation of the BK channel. We examine the influence of the β4 subunit on PKA, CaMKII, and phosphatase modulation of channel activity, and on molecular tolerance to alcohol. We record from human BK channels heterologously expressed in HEK 293 cells composed of its core subunit, α alone (Insertless), or co-expressed with the β4 BK auxiliary subunit, as well as, acutely dissociated nucleus accumbens neurons using the cell-attached patch clamp configuration. Our results indicate that BK channels are strongly modulated by activation of specific kinases (PKA and CaMKII) and phosphatases. The presence of the β4 subunit greatly influences this modulation, allowing a variety of outcomes for BK channel activity in response to acute alcohol.

ATP elicits inward currents in isolated vasopressinergic neurohypophysial terminals via P2X2 and P2X3 receptors

Effects of extracellular adenosine tri-phosphate (ATP) on ionic currents were investigated using the perforated-patch whole-cell recording technique on isolated terminals of the Hypothalamic Neurohypophysial System (HNS). ATP induced a current response in 70% of these isolated terminals. This inwardly-rectifying, inactivating current had an apparent reversal near 0 mV and was dose-dependent on ATP with an EC50=9.6+/-1.0 microM. In addition, current amplitudes measured at maximal ATP concentrations and optimum holding potentials had a current density of 70.8 pA pF(-1) and were greatly inhibited by suramin and PPADS. Different purinergic receptor agonists were tested, with the following efficacy: ATP > or = 2-methylthioATP > ATP-gamma-S > Bz-Bz-ATP > alpha,beta-methylene-ATP > beta,gamma-methylene-ATP. However, UTP and ADP were ineffective. These data suggest the involvement of a P2X purinergic receptor in the ATP-induced responses. Immunocytochemical labeling in vasopressinergic terminals indicates the existence of P2X(2,3,4, and 7), but not P2X6 receptors. Additionally, P2X(2 and 3) were not found in terminals which labeled for oxytocin. In summary, the EC50, decay, inactivation, and pharmacology indicate that a functional mixture of P2X(2 and 3) homomeric receptors mediate the majority of the ATP responses in vasopressinergic HNS terminals. We speculate that the characteristics of these types of receptors reflect the function of co-released ATP in the terminal compartment of these and other CNS neurons.

Ca2+ syntillas, miniature Ca2+ release events in terminals of hypothalamic neurons, are increased in frequency by depolarization in the absence of Ca2+ influx

Localized, brief Ca2+ transients (Ca2+ syntillas) caused by release from intracellular stores were found in isolated nerve terminals from magnocellular hypothalamic neurons and examined quantitatively using a signal mass approach to Ca2+ imaging. Ca2+ syntillas (scintilla, L., spark, from a synaptic structure, a nerve terminal) are caused by release of approximately 250,000 Ca ions on average by a Ca2+ flux lasting on the order of tens of milliseconds and occur spontaneously at a membrane potential of -80 mV. Syntillas are unaffected by removal of extracellular Ca2+, are mediated by ryanodine receptors (RyRs) and are increased in frequency, in the absence of extracellular Ca2+, by physiological levels of depolarization. This represents the first direct demonstration of mobilization of Ca2+ from intracellular stores in neurons by depolarization without Ca2+ influx. The regulation of syntillas by depolarization provides a new link between neuronal activity and cytosolic [Ca2+] in nerve terminals.