Profiling the phospho-status of the BKCa channel alpha subunit in rat brain reveals unexpected patterns and complexity

Mining recent brain proteomic databases for ion channel phosphosite nuggets

Voltage-gated ion channels underlie electrical activity of neurons and are dynamically regulated by diverse cell signaling pathways that alter their phosphorylation state. Recent global mass spectrometric–based analyses of the mouse brain phosphoproteome have yielded a treasure trove of new data as to the extent and nature of phosphorylation of numerous ion channel principal or α subunits in mammalian brain. Here we compile and review data on 347 phosphorylation sites (261 unique) on 42 different voltage-gated ion channel α subunits that were identified in these recent studies. Researchers in the ion channel field can now begin to explore the role of these novel in vivo phosphorylation sites in the dynamic regulation of the localization, activity, and expression of brain ion channels through multisite phosphorylation of their principal subunits.

Brain phosphoproteome obtained by a FASP-based method reveals plasma membrane protein topology

Taking advantage of the recently developed Filter Assisted Sample Preparation (FASP) method for sample preparation, we performed an in-depth analysis of phosphorylation sites in mouse brain. To maximize the number of detected phosphorylation sites, we fractionated proteins by size exclusion chromatography (SEC) or separated tryptic peptides on an anion exchanger (SAX) prior or after the TiO(2)-based phosphopeptide enrichment, respectively. SEC allowed analysis of minute tissue samples (1 mg total protein), and resulted in identification of more than 4000 sites in a single experiment, comprising eight fractions. SAX in a pipet tip format offered a convenient and rapid way to fractionate phosphopeptides and mapped more than 5000 sites in a single six fraction experiment. To enrich peptides containing phosphotyrosine residues, we describe a filter aided antibody capturing and elution (FACE) method that requires only the uncoupled instead of resin-immobilized capture reagent. In total, we identified 12,035 phosphorylation sites on 4579 brain proteins of which 8446 are novel. Gene Ontology annotation reveals that 23% of identified sites are located on plasma membrane proteins, including a large number of ion channels and transporters. Together with the glycosylation sites from a recent large-scale study, they can confirm or correct predicted membrane topologies of these proteins, as we show for the examples calcium channels and glutamate receptors.

Ca2+-activated K+ channels: from protein complexes to function

Molecular research on ion channels has demonstrated that many of these integral membrane proteins associate with partner proteins, often versatile in their function, or even assemble into stable macromolecular complexes that ensure specificity and proper rate of the channel-mediated signal transduction. Calcium-activated potassium (K(Ca)) channels that link excitability and intracellular calcium concentration are responsible for a wide variety of cellular processes ranging from regulation of smooth muscle tone to modulation of neurotransmission and control of neuronal firing pattern. Most of these functions are brought about by interaction of the channels' pore-forming subunits with distinct partner proteins. In this review we summarize recent insights into protein complexes associated with K(Ca) channels as revealed by proteomic research and discuss the results available on structure and function of these complexes and on the underlying protein-protein interactions. Finally, the results are related to their significance for the function of K(Ca) channels under cellular conditions.

Palmitoylation of the S0-S1 linker regulates cell surface expression of voltage- and calcium-activated potassium (BK) channels

S-palmitoylation is rapidly emerging as an important post-translational mechanism to regulate ion channels. We have previously demonstrated that large conductance calcium- and voltage-activated potassium (BK) channels are palmitoylated within an alternatively spliced (STREX) insert. However, these studies also revealed that additional site(s) for palmitoylation must exist outside of the STREX insert, although the identity or the functional significance of these palmitoylated cysteine residues are unknown. Here, we demonstrate that BK channels are palmitoylated at a cluster of evolutionary conserved cysteine residues (Cys-53, Cys-54, and Cys-56) within the intracellular linker between the S0 and S1 transmembrane domains. Mutation of Cys-53, Cys-54, and Cys-56 completely abolished palmitoylation of BK channels lacking the STREX insert (ZERO variant). Palmitoylation allows the S0-S1 linker to associate with the plasma membrane but has no effect on single channel conductance or the calcium/voltage sensitivity. Rather, S0-S1 linker palmitoylation is a critical determinant of cell surface expression of BK channels, as steady state surface expression levels are reduced by ∼55% in the C53:54:56A mutant. STREX variant channels that could not be palmitoylated in the S0-S1 linker also displayed significantly reduced cell surface expression even though STREX insert palmitoylation was unaffected. Thus our work reveals the functional independence of two distinct palmitoylation-dependent membrane interaction domains within the same channel protein and demonstrates the critical role of S0-S1 linker palmitoylation in the control of BK channel cell surface expression.

Proteomic analysis highlights the molecular complexities of native Kv4 channel macromolecular complexes

Voltage-gated K(+) (Kv) channels are key determinants of membrane excitability in the nervous and cardiovascular systems, functioning to control resting membrane potentials, shape action potential waveforms and influence the responses to neurotransmitters and neurohormones. Consistent with this functional diversity, multiple types of Kv currents, with distinct biophysical properties and cellular/subcellular distributions, have been identified. Rapidly activating and inactivating Kv currents, typically referred to as I(A) (A-type) in neurons, for example, regulate repetitive firing rates, action potential back-propagation (into dendrites) and modulate synaptic responses. Currents with similar properties, referred to as I(to,f) (fast transient outward), expressed in cardiomyocytes, control the early phase of myocardial action potential repolarization. A number of studies have demonstrated critical roles for pore-forming (α) subunits of the Kv4 subfamily in the generation of native neuronal I(A) and cardiac I(to,f) channels. Studies in heterologous cells have also suggested important roles for a number of Kv channel accessory and regulatory proteins in the generation of functional I(A) and I(to,f) channels. Quantitative mass spectrometry-based proteomic analysis is increasingly recognized as a rapid and, importantly, unbiased, approach to identify the components of native macromolecular protein complexes. The recent application of proteomic approaches to identify the components of native neuronal (and cardiac) Kv4 channel complexes has revealed even greater complexity than anticipated. The continued emphasis on development of improved biochemical and analytical proteomic methods seems certain to accelerate progress and to provide important new insights into the molecular determinants of native ion channel protein complexes.

Mass spectrometry-based phosphoproteomics reveals multisite phosphorylation on mammalian brain voltage-gated sodium and potassium channels

Voltage-gated sodium and potassium channels underlie electrical activity of neurons, and are dynamically regulated by diverse cell signaling pathways that ultimately exert their effects by altering the phosphorylation state of channel subunits. Recent mass spectrometric-based studies have led to a new appreciation of the extent and nature of phosphorylation of these ion channels in mammalian brain. This has allowed for new insights into how neurons dynamically regulate the localization, activity and expression through multisite ion channel phosphorylation.

Quantitative proteomics of the Cav2 channel nano-environments in the mammalian brain

Local Ca(2+) signaling occurring within nanometers of voltage-gated Ca(2+) (Cav) channels is crucial for CNS function, yet the molecular composition of Cav channel nano-environments is largely unresolved. Here, we used a proteomic strategy combining knockout-controlled multiepitope affinity purifications with high-resolution quantitative MS for comprehensive analysis of the molecular nano-environments of the Cav2 channel family in the whole rodent brain. The analysis shows that Cav2 channels, composed of pore-forming alpha1 and auxiliary beta subunits, are embedded into protein networks that may be assembled from a pool of approximately 200 proteins with distinct abundance, stability of assembly, and preference for the three Cav2 subtypes. The majority of these proteins have not previously been linked to Cav channels; about two-thirds are dedicated to the control of intracellular Ca(2+) concentration, including G protein-coupled receptor-mediated signaling, to activity-dependent cytoskeleton remodeling or Ca(2+)-dependent effector systems that comprise a high portion of the priming and release machinery of synaptic vesicles. The identified protein networks reflect the cellular processes that can be initiated by Cav2 channel activity and define the molecular framework for organization and operation of local Ca(2+) signaling by Cav2 channels in the brain.

BK channels mediate a novel ionic mechanism that regulates glucose-dependent electrical activity and insulin secretion in mouse pancreatic β-cells

BK channels are large unitary conductance K(+) channels cooperatively activated by intracellular calcium and membrane depolarisation. We show that BK channels regulate electrical activity in β-cells of mouse pancreatic islets exposed to elevated glucose. In 11.1 mM glucose, the non-peptidyl BK channel blocker paxilline increased the height of β-cell action potentials (APs) by 21 mV without affecting burst- or silent-period durations. In isolated β-cells, paxilline increased AP height by 16 mV without affecting resting membrane potential. In voltage clamp, paxilline blocked a transient component of outward current activated by a short depolarisation, which accounted for at least 90% of the initial outward K(+) current. This BK current (I(BK)) was blocked by the Ca(2+) channel blockers Cd(2+) (200 μM) or nimodipine (1 μM), and potentiated by FPL-64176 (1 μM). I(BK) was also 56% blocked by the BK channel blocker iberiotoxin (100 nM). I(BK) activated more than 10-fold faster than the delayed rectifier I(Kv) over the physiological voltage range, and partially inactivated. An AP-like command revealed that I(BK) activated and deactivated faster than I(Kv) and accounted for 86% of peak I(K), explaining why I(BK) block increased AP height. A higher amplitude AP-like command, patterned on an AP recorded in 11.1 mM glucose plus paxilline, activated 4-fold more I(Kv) and significantly increased Ca(2+) entry. Paxilline increased insulin secretion in islets exposed to 11.1 mM glucose by 67%, but did not affect basal secretion in 2.8 mM glucose. These data suggest a modified model of β-cell AP generation where I(BK) and I(Kv) coordinate the AP repolarisation.

LRRC26 auxiliary protein allows BK channel activation at resting voltage without calcium

Large-conductance, voltage- and calcium-activated potassium (BK, or K(Ca)1.1) channels are ubiquitously expressed in electrically excitable and non-excitable cells, either as alpha-subunit (BKalpha) tetramers or together with tissue specific auxiliary beta-subunits (beta1-beta4). Activation of BK channels typically requires coincident membrane depolarization and elevation in free cytosolic Ca(2+) concentration ([Ca(2+)](i)), which are not physiological conditions for most non-excitable cells. Here we present evidence that in non-excitable LNCaP prostate cancer cells, BK channels can be activated at negative voltages without rises in [Ca(2+)](i) through their complex with an auxiliary protein, leucine-rich repeat (LRR)-containing protein 26 (LRRC26). LRRC26 modulates the gating of a BK channel by enhancing the allosteric coupling between voltage-sensor activation and the channel's closed-open transition. This finding reveals a novel auxiliary protein of a voltage-gated ion channel that gives an unprecedentedly large negative shift ( approximately -140 mV) in voltage dependence and provides a molecular basis for activation of BK channels at physiological voltages and calcium levels in non-excitable cells.

Multisite phosphorylation of voltage-gated sodium channel alpha subunits from rat brain

Reversible phosphorylation of ion channels underlies cellular plasticity in mammalian neurons. Voltage-gated sodium or Nav channels underlie action potential initiation and propagation, dendritic excitability, and many other aspects of neuronal excitability. Various protein kinases have been suggested to phosphorylate the primary or alpha subunit of Nav channels, affecting diverse aspects of channel function. Previous studies of Nav alpha subunit phosphorylation have led to the identification of a small set of phosphorylation sites important in mediating diverse aspects of Nav channel function. Here we use nanoflow liquid chromatography tandem mass spectrometry (nano-LC MS/MS) on Nav alpha subunits affinity-purified from rat brain with two distinct monoclonal antibodies to identify 15 phosphorylation sites on Nav1.2, 12 of which have not been previously reported. We also found 3 novel phosphorylation sites on Nav1.1. In general, commonly used phosphorylation site prediction algorithms did not accurately predict these novel in vivo phosphorylation sites. Our results demonstrate that specific Nav alpha subunits isolated from rat brain are highly phosphorylated, and suggest extensive modulation of Nav channel activity in mammalian brain. Identification of phosphorylation sites using monoclonal antibody-based immunopurification and mass spectrometry is an effective approach to define the phosphorylation status of Nav channels and other important membrane proteins in mammalian brain.

Analysis and functional implications of phosphorylation of neuronal voltage-gated potassium channels

Phosphorylation is the most common and abundant post-translational modification to eukaryotic proteins, regulating a plethora of dynamic cellular processes. Here, we review and discuss recent advances in our knowledge of the breadth and importance of reversible phosphorylation in regulating the expression, localization and function of mammalian neuronal voltage-gated potassium (Kv) channels, key regulators of neuronal function. We highlight the role of modern mass spectrometric techniques and phosphospecific antibodies in revealing the extent and nature of phosphorylation at specific sites in Kv channels. We also emphasize the role of reversible phosphorylation in dynamically regulating diverse aspects of Kv channel biology. Finally, we discuss as important future directions the determination of the mechanistic basis for how altering phosphorylation state affects Kv channel expression, localization and function, the nature of macromolecular signaling complexes containing Kv channels and enzymes regulating their phosphorylation state, and the specific role of Kv channel phosphorylation in regulating neuronal function during physiological and pathophysiological events.

Large conductance, Ca2+-activated K+ channels (BKCa) and arteriolar myogenic signaling

Myogenic, or pressure-induced, vasoconstriction is critical for local blood flow autoregulation. Underlying this vascular smooth muscle (VSM) response are events including membrane depolarization, Ca(2+) entry and mobilization, and activation of contractile proteins. Large conductance, Ca(2+)-activated K(+) channel (BK(Ca)) has been implicated in several of these steps including, (1) channel closure causing membrane depolarization, and (2) channel opening causing hyperpolarization to oppose excessive pressure-induced vasoconstriction. As multiple mechanisms regulate BK(Ca) activity (subunit composition, membrane potential (Em) and Ca(2+) levels, post-translational modification) tissue level diversity is predicted. Importantly, heterogeneity in BK(Ca) channel activity may contribute to tissue-specific differences in regulation of myogenic vasoconstriction, allowing local hemodynamics to be matched to metabolic requirements. Knowledge of such variability will be important to exploiting the BK(Ca) channel as a therapeutic target and understanding systemic effects of its pharmacological manipulation.

Proteomic analyses of native brain K(V)4.2 channel complexes

Somatodendritic A-type (I(A)) voltage-gated K(+) (K(V)) channels are key regulators of neuronal excitability, functioning to control action potential waveforms, repetitive firing and the responses to synaptic inputs. Rapidly activating and inactivating somatodendritic I(A) channels are encoded by K(V)4 alpha subunits and accumulating evidence suggests that these channels function as components of macromolecular protein complexes. Mass spectrometry (MS)-based proteomic approaches were developed and exploited here to identify potential components and regulators of native brain K(V)4.2-encoded I(A) channel complexes. Using anti-K(V)4.2 specific antibodies, K(V)4.2 channel complexes were immunoprecipitated from adult wild type mouse brain. Parallel control experiments were performed on brain samples isolated from (K(V)4.2(-/-)) mice harboring a targeted disruption of the KCND2 (K(V)4.2) locus. Three proteomic strategies were employed: an in-gel approach, coupled to one-dimensional liquid chromatography-tandem MS (1D-LC-MS/MS), and two in-solution approaches, followed by 1D- or 2D-LC-MS/MS. The targeted in-gel 1D-LC-MS/MS analyses demonstrated the presence of the K(V)4 alpha subunits (K(V)4.2, K(V)4.3 and K(V)4.1) and the K(V)4 accessory, KChIP (KChIP1-4) and DPP (DPP6 and 10), proteins in native brain K(V)4.2 channel complexes. The more comprehensive, in-solution approach, coupled to 2D-LC-MS/MS, also called Multidimensional Protein Identification Technology (MudPIT), revealed that additional regulatory proteins, including the K(V) channel accessory subunit K(V)beta1, are also components of native brain K(V)4.2 channel complexes. Additional biochemical and functional approaches will be required to elucidate the physiological roles of these newly identified K(V)4 interacting proteins.

Mass spectrometric analysis reveals a functionally important PKA phosphorylation site in a Kir3 channel subunit

Phosphorylation of the Kir3 channel by cAMP-dependent protein kinase (PKA) potentiates activity and strengthens channel-PIP(2) interactions, whereas phosphorylation by protein kinase C (PKC) exerts the opposite effects (Keselman et al., Channels 1:113-123, 2007; Lopes et al., Channels 1:124-134, 2007). Unequivocal identification of phosphorylated residues in ion channel proteins has been difficult, but recent advances in mass spectrometry techniques have allowed precise identification of phosphorylation sites (Park et al., Science 313:976-979, 2006). In this study, we utilized mass spectrometry to identify phosphorylation sites within the Kir3.1 channel subunit. We focused on the Kir3.1 C-terminal cytosolic domain that has been reported to be regulated by several modulators. In vitro phosphorylation by PKA exhibited a convincing signal upon treatment with a phosphoprotein stain. The phosphorylated C terminus was subjected to mass spectrometric analysis using matrix-assisted lased desorption/ionization-time of flight mass spectroscopy (MS). Peptides whose mass underwent a shift corresponding to addition of a phosphate group were then subjected to tandem MS (MS/MS) in order to confirm the modification and determine its precise location. Using this approach, we identified S385 as an in vitro phosphorylation site. Mutation of this residue to alanine resulted in a reduced sensitivity of Kir3.1* currents to H89 and Forskolin, confirming an in vivo role for this novel site of the Kir3.1 channel subunit in its regulation by PKA.

Palmitoylation gates phosphorylation-dependent regulation of BK potassium channels

Large conductance calcium- and voltage-gated potassium (BK) channels are important regulators of physiological homeostasis and their function is potently modulated by protein kinase A (PKA) phosphorylation. PKA regulates the channel through phosphorylation of residues within the intracellular C terminus of the pore-forming alpha-subunits. However, the molecular mechanism(s) by which phosphorylation of the alpha-subunit effects changes in channel activity are unknown. Inhibition of BK channels by PKA depends on phosphorylation of only a single alpha-subunit in the channel tetramer containing an alternatively spliced insert (STREX) suggesting that phosphorylation results in major conformational rearrangements of the C terminus. Here, we define the mechanism of PKA inhibition of BK channels and demonstrate that this regulation is conditional on the palmitoylation status of the channel. We show that the cytosolic C terminus of the STREX BK channel uniquely interacts with the plasma membrane via palmitoylation of evolutionarily conserved cysteine residues in the STREX insert. PKA phosphorylation of the serine residue immediately upstream of the conserved palmitoylated cysteine residues within STREX dissociates the C terminus from the plasma membrane, inhibiting STREX channel activity. Abolition of STREX palmitoylation by site-directed mutagenesis or pharmacological inhibition of palmitoyl transferases prevents PKA-mediated inhibition of BK channels. Thus, palmitoylation gates BK channel regulation by PKA phosphorylation. Palmitoylation and phosphorylation are both dynamically regulated; thus, cross-talk between these 2 major posttranslational signaling cascades provides a mechanism for conditional regulation of BK channels. Interplay of these distinct signaling cascades has important implications for the dynamic regulation of BK channels and physiological homeostasis.

Control of K(Ca) channels by calcium nano/microdomains

Transient elevations in cytoplasmic Ca(2+) trigger a multitude of Ca(2+)-dependent processes in CNS neurons and many other cell types. The specificity, speed, and reliability of these processes is achieved and ensured by tightly restricting Ca(2+) signals to very local spatiotemporal domains, "Ca(2+) nano- and microdomains," that are centered around Ca(2+)-permeable channels. This arrangement requires that the Ca(2+)-dependent effectors reside within these spatial boundaries where the properties of the Ca(2+) domain and the Ca(2+) sensor of the effector determine the channel-effector activity. We use Ca(2+)-activated K(+) channels (K(Ca)) with either micromolar (BK(Ca) channels) or submicromolar (SK(Ca) channels) affinity for Ca(2+) ions to provide distance constraints for Ca(2+)-effector coupling in local Ca(2+) domains and review their significance for the cell physiology of K(Ca) channels in the CNS. The results may serve as a model for other processes operated by local Ca(2+) domains.