AKAP79 modulation of L-type channels involves disruption of intramolecular interactions in the CaV1.2 subunit

Spatial and temporal crosstalk between the cAMP and Ca2+ signaling systems

The ubiquitous secondary messengers, Ca²⁺ and cAMP, play a vital role in shaping a diverse array of physiological processes. More significantly, accumulating evidence over the past several decades underpin extensive crosstalk between these two canonical messengers in discrete sub-cellular nanodomains across various cell types. Within such specialized nanodomains, each messenger fine-tunes signaling to maintain homeostasis by manipulating the activities of cellular machinery accountable for the metabolism or activity of the complementary pathway. Interaction between these messengers is ensured by scaffolding proteins which tether components of the signaling machinery in close proximity. Disruption of dynamic communications between Ca²⁺ and cAMP at these loci consequently is linked to several pathological conditions. This review summarizes recent novel mechanisms underlying effective crosstalk between Ca²⁺ and cAMP in such nanodomains.

Rapid Turnover of the Cardiac L-Type CaV1.2 Channel by Endocytic Recycling Regulates Its Cell Surface Availability

Calcium entry through Ca_V1.2 L-type calcium channels regulates cardiac contractility. Here, we study the impact of exocytic and post-endocytic trafficking on cell surface channel abundance in cardiomyocytes. Single-molecule localization and confocal microscopy reveal an intracellular Ca_V1.2 pool tightly associated with microtubules from the perinuclear region to the cell periphery, and with actin filaments at the cell cortex. Channels newly inserted into the plasma membrane become internalized with an average time constant of 7.5 min and are sorted out to the Rab11a-recycling compartment. Ca_V1.2 recycling suffices for maintaining stable L-type current amplitudes over 20 hr independent of de novo channel transport along microtubules. Disruption of the actin cytoskeleton re-routes Ca_V1.2 from recycling toward lysosomal degradation. We identify endocytic recycling as essential for the homeostatic regulation of voltage-dependent calcium influx into cardiomyocytes. This mechanism provides the basis for a dynamic adjustment of the channel's surface availability and thus, of heart's contraction.

Molecular mimicking of C-terminal phosphorylation tunes the surface dynamics of CaV1.2 calcium channels in hippocampal neurons

L-type voltage-gated Ca_V1.2 calcium channels (Ca_V1.2) are key regulators of neuronal excitability, synaptic plasticity, and excitation-transcription coupling. Surface-exposed Ca_V1.2 distributes in clusters along the dendrites of hippocampal neurons. A permanent exchange between stably clustered and laterally diffusive extra-clustered channels maintains steady-state levels of Ca_V1.2 at dendritic signaling domains. A dynamic equilibrium between anchored and diffusive receptors is a common feature among ion channels and is crucial to modulate signaling transduction. Despite the importance of this fine regulatory system, the molecular mechanisms underlying the surface dynamics of Ca_V1.2 are completely unexplored. Here, we examined the dynamic states of Ca_V1.2 depending on phosphorylation on Ser-1700 and Ser-1928 at the channel C terminus. Phosphorylation at these sites is strongly involved in Ca_V1.2-mediated nuclear factor of activated T cells (NFAT) signaling, long-term potentiation, and responsiveness to adrenergic stimulation. We engineered Ca_V1.2 constructs mimicking phosphorylation at Ser-1700 and Ser-1928 and analyzed their behavior at the membrane by immunolabeling protocols, fluorescence recovery after photobleaching, and single particle tracking. We found that the phosphomimetic S1928E variant increases the mobility of Ca_V1.2 without altering the steady-state maintenance of cluster in young neurons and favors channel stabilization later in differentiation. Instead, mimicking phosphorylation at Ser-1700 promoted the diffusive state of Ca_V1.2 irrespective of the differentiation stage. Together, these results reveal that phosphorylation could contribute to the establishment of channel anchoring mechanisms depending on the neuronal differentiation state. Finally, our findings suggest a novel mechanism by which phosphorylation at the C terminus regulates calcium signaling by tuning the content of Ca_V1.2 at signaling complexes.

AKAP95-mediated nuclear anchoring of PKA mediates cortisol-induced PTGS2 expression in human amnion fibroblasts

Phosphorylation of the transcription factors cyclic adenosine monophosphate response element-binding protein (CREB) and signal transducer and activator of transcription 3 (STAT3) by protein kinase A (PKA) is required for the cortisol-induced production of cyclooxygenase-2 (COX-2) and prostaglandin E₂ (PGE₂) in human amnion fibroblasts, which critically mediates human parturition (labor). We found that PKA was confined in the nucleus by A-kinase-anchoring protein 95 (AKAP95) in amnion fibroblasts and that this localization was key to the cortisol-induced expression of PTGS2, the gene encoding COX-2. Cortisol increased the abundance of nuclear PKA by stimulating the expression of the gene encoding AKAP95. Knockdown of AKAP95 not only reduced the amounts of nuclear PKA and phosphorylated CREB but also attenuated the induction of PTGS2 expression in primary human amnion fibroblasts treated with cortisol, whereas the phosphorylation of STAT3 in response to cortisol was not affected. The abundances of AKAP95, phosphorylated CREB, and COX-2 were markedly increased in human amnion tissue after labor compared to those in amnion tissues from cesarean sections without labor. These results highlight an essential role for PKA that is anchored in the nucleus by AKAP95 in the phosphorylation of CREB and the consequent induction of COX-2 expression by cortisol in amnion fibroblasts, which may be important in human parturition.

Modified sympathetic nerve regulation in AKAP5-null mice

Genetic analyses have revealed an important association between A-kinase anchoring proteins (AKAPs) and the intracellular calcium modulating system. AKAP5, also known as AKAP79/150, is an anchoring protein between PKA and voltage-dependent calcium channels, ryanodine receptor-2, phospholamban and other molecules. The aim of the present study was to elucidate the physiological importance of AKAP5 in the creation of cardiac rhythm using AKAP5-null mice. ECG analysis showed a normal sinus rhythm and a decreased responsiveness to isoproterenol in AKAP5-null mice compared with wild-type mice. Analysis of heart rate variability revealed that the R-R interval was unstable in AKAP5-null mutants and that the low-frequency components had decreased, indicating that the tonus of the sympathetic nervous system was affected. Furthermore, the atrium of the AKAP5-null mice showed a decreased positive inotropic response to isoproterenol, indicating the involvement of AKAP5 in a PKA-dependent pathway. Thus, our present study revealed that AKAP5 plays a significant role in the regulation of sympathetic nerve activities.

C-terminal modulatory domain controls coupling of voltage-sensing to pore opening in Cav1.3 L-type Ca(2+) channels

Activity of voltage-gated Ca_v1.3 L-type Ca²⁺ channels is required for proper hearing as well as sinoatrial node and brain function. This critically depends on their negative activation voltage range, which is further fine-tuned by alternative splicing. Shorter variants miss a C-terminal regulatory domain (CTM), which allows them to activate at even more negative potentials than C-terminally long-splice variants. It is at present unclear whether this is due to an increased voltage sensitivity of the Ca_v1.3 voltage-sensing domain, or an enhanced coupling of voltage-sensor conformational changes to the subsequent opening of the activation gate. We studied the voltage-dependence of voltage-sensor charge movement (Q_ON-V) and of current activation (I_Ca-V) of the long (Ca_v1.3_L) and a short Ca_v1.3 splice variant (Ca_v1.3_42A) expressed in tsA-201 cells using whole cell patch-clamp. Charge movement (Q_ON) of Ca_v1.3_L displayed a much steeper voltage-dependence and a more negative half-maximal activation voltage than Ca_v1.2 and Ca_v3.1. However, a significantly higher fraction of the total charge had to move for activation of Ca_v1.3 half-maximal conductance (Ca_v1.3: 68%; Ca_v1.2: 52%; Ca_v3.1: 22%). This indicated a weaker coupling of Ca_v1.3 voltage-sensor charge movement to pore opening. However, the coupling efficiency was strengthened in the absence of the CTM in Ca_v1.3_42A, thereby shifting I_Ca-V by 7.2 mV to potentials that were more negative without changing Q_ON-V. We independently show that the presence of intracellular organic cations (such as n-methyl-D-glucamine) induces a pronounced negative shift of Q_ON-V and a more negative activation of I_Ca-V of all three channels. These findings illustrate that the voltage sensors of Ca_v1.3 channels respond more sensitively to depolarization than those of Ca_v1.2 or Ca_v3.1. Weak coupling of voltage sensing to pore opening is enhanced in the absence of the CTM, allowing short Ca_v1.3_42A splice variants to activate at lower voltages without affecting Q_ON-V.

A systems biology analysis of brain microvascular endothelial cell lipotoxicity

Background

Neurovascular inflammation is associated with a number of neurological diseases including vascular dementia and Alzheimer’s disease, which are increasingly important causes of morbidity and mortality around the world. Lipotoxicity is a metabolic disorder that results from accumulation of lipids, particularly fatty acids, in non-adipose tissue leading to cellular dysfunction, lipid droplet formation, and cell death.

Results

Our studies indicate for the first time that the neurovascular circulation also can manifest lipotoxicity, which could have major effects on cognitive function. The penetration of integrative systems biology approaches is limited in this area of research, which reduces our capacity to gain an objective insight into the signal transduction and regulation dynamics at a systems level. To address this question, we treated human microvascular endothelial cells with triglyceride-rich lipoprotein (TGRL) lipolysis products and then we used genome-wide transcriptional profiling to obtain transcript abundances over four conditions. We then identified regulatory genes and their targets that have been differentially expressed through analysis of the datasets with various statistical methods. We created a functional gene network by exploiting co-expression observations through a guilt-by-association assumption. Concomitantly, we used various network inference algorithms to identify putative regulatory interactions and we integrated all predictions to construct a consensus gene regulatory network that is TGRL lipolysis product specific.

Conclusion

System biology analysis has led to the validation of putative lipid-related targets and the discovery of several genes that may be implicated in lipotoxic-related brain microvascular endothelial cell responses. Here, we report that activating transcription factors 3 (ATF3) is a principal regulator of TGRL lipolysis products-induced gene expression in human brain microvascular endothelial cell.

L-type Ca2+ channels in heart and brain

L-type calcium channels (Cav1) represent one of the three major classes (Cav1–3) of voltage-gated calcium channels. They were identified as the target of clinically used calcium channel blockers (CCBs; so-called calcium antagonists) and were the first class accessible to biochemical characterization. Four of the 10 known α1 subunits (Cav1.1–Cav1.4) form the pore of L-type calcium channels (LTCCs) and contain the high-affinity drug-binding sites for dihydropyridines and other chemical classes of organic CCBs. In essentially all electrically excitable cells one or more of these LTCC isoforms is expressed, and therefore it is not surprising that many body functions including muscle, brain, endocrine, and sensory function depend on proper LTCC activity. Gene knockouts and inherited human diseases have allowed detailed insight into the physiological and pathophysiological role of these channels. Genome-wide association studies and analysis of human genomes are currently providing even more hints that even small changes of channel expression or activity may be associated with disease, such as psychiatric disease or cardiac arrhythmias. Therefore, it is important to understand the structure–function relationship of LTCC isoforms, their differential contribution to physiological function, as well as their fine-tuning by modulatory cellular processes.

L-type Ca2+ channel function during Timothy syndrome

Voltage-gated, dihydropyridine-sensitive L-type Ca(2+) channels are multimeric proteins composed of a pore-forming transmembrane α(1) subunit (Ca(v)1.2) and accessory β, α(2)δ, and γ subunits. Ca(2+) entry via Ca(v)1.2 channels shapes the action potential (AP) of cardiac myocytes and is required for excitation-contraction coupling. Two de novo point mutations of Ca(v)1.2 glycine residues, G406R and G402S, cause a rare multisystem disorder called Timothy syndrome (TS). Here, we discuss recent work on the mechanisms by which Ca(v)1.2 channels bearing TS mutations display slowed inactivation that leads to increased Ca(2+) influx, prolonging the cardiac AP and promoting lethal arrhythmias. Based on these studies, we propose a model in which the scaffolding protein AKAP79/150 stabilizes the open conformation of Ca(v)1.2-TS channels and facilitates physical interactions among adjacent channels via their C-tails, increasing the activity of adjoining channels and amplifying Ca(2+) influx.

GPCR Retreat 2012: timing is everything

In London, Ontario, the 13th Annual Joint meeting of the Great Lakes GPCR Retreat and the Club des Récepteurs à Sept Domaines Transmembranaires (known simply as the GPCR Retreat) was held on 17-19 October 2012, organized by Steve Ferguson and Peter Chidiac. This meeting gathered together a core group of investigators from Michigan, Ontario and Québec and has steadily increased its attendance in both the eastern (Europe) and western (USA, Canada) directions. This year's buzz naturally centered around the Nobel Prize in Chemistry, which was won the week before by Brian Kobilka and Robert Lefkowitz for their work on receptor structure and function. Michel Bouvier provided a heartfelt tribute to one of the attendees, Marc Caron, a pioneer in the GPCR field, has made many contributions to the work that led to this year's Nobel Prize. The meeting featured interesting sessions on the physiological roles of GPCRs in the nervous system, circadian biology and cancer, dealing at the cellular and molecular level with GPCR, G protein and effector structure and function, regulation and trafficking--with an overall focus on how to move molecular pharmacology in vivo.

What can naturally occurring mutations tell us about Ca(v)1.x channel function?

Voltage-gated Ca^2 + channels allow for Ca^2 +-dependent intracellular signaling by directly mediating Ca^2 + ion influx, by physical coupling to intracellular Ca^2 + release channels or functional coupling to other ion channels such as Ca^2 + activated potassium channels. L-type Ca^2 + channels that comprise the family of Ca_v1 channels are expressed in many electrically excitable tissues and are characterized by their unique sensitivity to dihydropyridines. In this issue, we summarize genetic defects in L-type Ca^2 + channels and analyze their role in human diseases (Ca^2 + channelopathies); e.g. mutations in Ca_v1.2 α1 cause Timothy and Brugada syndrome, mutations in Ca_v1.3 α1 are linked to sinoatrial node dysfunction and deafness while mutations in Ca_v1.4 α1 are associated with X-linked retinal disorders such as an incomplete form of congenital stationary night blindness. Herein, we also put the mutations underlying the channel's dysfunction into the structural context of the pore-forming α1 subunit. This analysis highlights the importance of combining functional data with structural analysis to gain a deeper understanding for the disease pathophysiology as well as for physiological channel function. This article is part of a Special Issue entitled: Calcium channels.

Regulation of Ca(V)2 calcium channels by G protein coupled receptors

Voltage gated calcium channels (Ca²⁺ channels) are key mediators of depolarization induced calcium influx into excitable cells, and thereby play pivotal roles in a wide array of physiological responses. This review focuses on the inhibition of Ca(V)2 (N- and P/Q-type) Ca²⁺-channels by G protein coupled receptors (GPCRs), which exerts important autocrine/paracrine control over synaptic transmission and neuroendocrine secretion. Voltage-dependent inhibition is the most widespread mechanism, and involves direct binding of the G protein βγ dimer (Gβγ) to the α1 subunit of Ca(V)2 channels. GPCRs can also recruit several other distinct mechanisms including phosphorylation, lipid signaling pathways, and channel trafficking that result in voltage-independent inhibition. Current knowledge of Gβγ-mediated inhibition is reviewed, including the molecular interactions involved, determinants of voltage-dependence, and crosstalk with other cell signaling pathways. A summary of recent developments in understanding the voltage-independent mechanisms prominent in sympathetic and sensory neurons is also included. This article is part of a Special Issue entitled: Calcium channels.