Modulation of delayed rectifier potassium channel by protein kinase C zeta-containing signaling complex in pheochromocytoma cells

Circadian control of p75 neurotrophin receptor leads to alternate activation of Nrf2 and c-Rel to reset energy metabolism in astrocytes via brain-derived neurotrophic factor

Circadian clock genes regulate energy metabolism partly through neurotrophins in the body. The low affinity neurotrophin receptor p75^NTR is a clock component directly regulated by the transcriptional factor Clock:Bmal1 complex. Brain-derived neurotrophic factor (BDNF) is expressed in the brain and plays a key role in coordinating metabolic interactions between neurons and astrocytes. BDNF transduces signals through TrkB and p75^NTR receptors. This review highlights a novel molecular mechanism by which BDNF via circadian control of p75^NTR leads to daily resetting of glucose and glycogen metabolism in brain astrocytes to accommodate their functional interaction with neurons. Astrocytes store glycogen as an energy reservoir to provide active neurons with the glycolytic metabolite lactate. Astrocytes predominantly express the truncated receptor TrkB.T1 which lacks an intracellular receptor tyrosine kinase domain. TrkB.T1 retains the capacity to regulate cell morphology through regulation of Rho GTPases. In contrast, p75^NTR mediates generation of the bioactive lipid ceramide upon stimulation with BDNF and inhibits PKA activation. As ceramide directly activates PKCζ, we discuss the importance of the TrkB.T1-p75^NTR-ceramide-PKCζ signaling axis in the stimulation of glycogen and lipid synthesis and activation of RhoA. Ceramide-PKCζ-casein kinase 2 signaling activates Nrf2 to support oxidative phosphorylation via upregulation of antioxidant enzymes. In the absence of p75^NTR, TrkB.T1 functionally interacts with adenosine A_2AR and dopamine D1R receptors to enhance cAMP-PKA signaling and activate Rac1 and NF-κB c-Rel, favoring glycogen hydrolysis, gluconeogenesis and aerobic glycolysis. Thus, diurnal changes in p75^NTR levels in astrocytes resets energy metabolism via BDNF to accommodate their metabolic interaction with neurons.

Sequestosome1/p62: a regulator of redox-sensitive voltage-activated potassium channels, arterial remodeling, inflammation, and neurite outgrowth

Sequestosome1/p62 (SQSTM1) is an oxidative stress-inducible protein regulated by the redox-sensitive transcription factor Nrf2. It is not an antioxidant but known as a multifunctional regulator of cell signaling with an ability to modulate targeted or selective degradation of proteins through autophagy. SQSTM1 implements these functions through physical interactions with different types of proteins including atypical PKCs, nonreceptor-type tyrosine kinase p56(Lck) (Lck), polyubiquitin, and autophagosomal factor LC3. One of the notable physiological functions of SQSTM1 is the regulation of redox-sensitive voltage-gated potassium (Kv) channels which are composed of α and β subunits: (Kvα)4 (Kvβ)4. Previous studies have established that SQSTM1 scaffolds PKCζ, enhancing phosphorylation of Kvβ which induces inhibition of pulmonary arterial Kv1.5 channels under acute hypoxia. Recent studies reveal that Lck indirectly interacts with Kv1.3 α subunits and plays a key role in acute hypoxia-induced Kv1.3 channel inhibition in T lymphocytes. Kv1.3 channels provide a signaling platform to modulate the migration and proliferation of arterial smooth muscle cells and activation of T lymphocytes, and hence have been recognized as a therapeutic target for treatment of restenosis and autoimmune diseases. In this review, we focus on the functional interactions of SQSTM1 with Kv channels through two key partners aPKCs and Lck. Furthermore, we provide molecular insights into the functions of SQSTM1 in suppression of proliferation of arterial smooth muscle cells and neointimal hyperplasia following carotid artery ligation, in T lymphocyte differentiation and activation, and in NGF-induced neurite outgrowth in PC12 cells.

Ceramide inhibits Kv currents and contributes to TP-receptor-induced vasoconstriction in rat and human pulmonary arteries

Neutral sphingomyelinase (nSMase)-derived ceramide has been proposed as a mediator of hypoxic pulmonary vasoconstriction (HPV), a specific response of the pulmonary circulation. Voltage-gated K+ (Kv) channels are modulated by numerous vasoactive factors, including hypoxia, and their inhibition has been involved in HPV. Herein, we have analyzed the effects of ceramide on Kv currents and contractility in rat pulmonary arteries (PA) and in mesenteric arteries (MA). The ceramide analog C6-ceramide inhibited Kv currents in PA smooth muscle cells (PASMC). Similar effects were obtained after the addition of bacterial sphingomyelinase (SMase), indicating a role for endogenous ceramide in Kv channel regulation. Kv current was reduced by stromatoxin and diphenylphosphine oxide-1 (DPO-1), selective inhibitors of Kv2.1 and Kv1.5 channels, respectively. The inhibitory effect of ceramide was still present in the presence of stromatoxin or DPO-1, suggesting that this sphingolipid inhibited both components of the native Kv current. Accordingly, ceramide inhibited Kv1.5 and Kv2.1 channels expressed in Ltk− cells. Ceramide-induced effects were reduced in human embryonic kidney 293 cells expressing Kv1.5 channels but not the regulatory subunit Kvβ2.1. The nSMase inhibitor GW4869 reduced the thromboxane-endoperoxide receptor agonist U46619-induced, but not endothelin-1-induced pulmonary vasoconstriction that was partly restored after addition of exogenous ceramide. The PKC-ζ pseudosubstrate inhibitor (PKCζ-PI) inhibited the Kv inhibitory and contractile effects of ceramide. In MA ceramide had no effect on Kv currents and GW4869 did not affect U46619-induced contraction. The effects of SMase were also observed in human PA. These results suggest that ceramide represents a crucial signaling mediator in the pulmonary vasculature.

Role of Protein Kinase Cζ and Its Adaptor Protein p62 in Voltage-Gated Potassium Channel Modulation in Pulmonary Arteries

Voltage-gated potassium (K(V)) channels play an essential role in regulating pulmonary artery function, and they underpin the phenomenon of hypoxic pulmonary vasoconstriction. Pulmonary hypertension is characterized by inappropriate vasoconstriction, vascular remodeling, and dysfunctional K(V) channels. In the current study, we aimed to elucidate the role of PKCzeta and its adaptor protein p62 in the modulation of K(V) channels. We report that the thromboxane A(2) analog 9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F(2alpha) methyl acetate (U46619) inhibited K(V) currents in isolated mice pulmonary artery myocytes and the K(V) current carried by human cloned K(V)1.5 channels expressed in Ltk(-) cells. Using protein kinase C (PKC)zeta(-/-) and p62(-/-) mice, we demonstrate that these two proteins are involved in the K(V) channel inhibition. PKCzeta coimmunoprecipitated with K(V)1.5, and this interaction was markedly reduced in p62(-/-) mice. Pulmonary arteries from PKCzeta(-/-) mice also showed a diminished [Ca(2+)](i) and contractile response, whereas genetic inactivation of p62(-/-) resulted in an absent [Ca(2+)](i) response, but it preserved contractile response to U46619. These data demonstrate that PKCzeta and its adaptor protein p62 play a key role in the modulation of K(V) channel function in pulmonary arteries. These observations identify PKCzeta and/or p62 as potential therapeutic targets for the treatment of pulmonary hypertension.

Nerve growth factor enhances voltage-gated Na+ channel activity and Transwell migration in Mat-LyLu rat prostate cancer cell line

The highly dynamic nature of voltage-gated Na+ channel (VGSC) expression and its controlling mechanism(s) are not well understood. In this study, we investigated the possible involvement of nerve growth factor (NGF) in regulating VGSC activity in the strongly metastatic Mat-LyLu cell model of rat prostate cancer (PCa). NGF increased peak VGSC current density in a time- and dose-dependent manner. NGF also shifted voltage to peak and the half-activation voltage to more positive potentials, and produced currents with faster kinetics of activation; sensitivity to the VGSC blocker tetrodotoxin (TTX) was not affected. The NGF-induced increase in peak VGSC current density was suppressed by both the pan-trk antagonist K252a, and the protein kinase A (PKA) inhibitor KT5720. NGF did not affect the Nav1.7 mRNA level, but the total VGSC alpha-subunit protein level was upregulated. NGF potentiated the cells' migration in Transwell assays, and this was not affected by TTX. We concluded that NGF upregulated functional VGSC expression in Mat-LyLu cells, with PKA as a signaling intermediate, but enhancement of migration by NGF was independent of VGSC activity.

Potassium channel dysfunction and depolarized resting membrane potential in a cell model of SCA3

Spinocerebellar ataxia type 3 (SCA3) is an autosomal dominant inherited neurodegenerative disease caused by the expansion of a polyglutamine repeat within the disease protein, ataxin-3. There is growing evidence that neuronal electrophysiological properties are altered in a variety of polyglutamine diseases such as Huntington's disease and SCA1 and that these alterations may contribute to disturbances of neuronal function prior to neurodegeneration. To elucidate possible electrophysiological changes in SCA3, we generated a stable PC12 cell model with inducible expression of normal and mutant human full-length ataxin-3 and analyzed the electrophysiological properties after induction of the recombinant ataxin-3 expression. Neuronally differentiated PC12 cells expressing the expanded form of ataxin-3 showed significantly decreased viabilities and developed ultrastructural changes resembling human SCA3. Prior to neuronal cell death, we found a significant reduction of the resting membrane potential and a hyperpolarizing shift of the activation curve of the delayed rectifier potassium current. These findings indicate that electrophysiological properties are altered in mutant ataxin-3 expressing neuronal cells and may contribute to neuronal dysfunction in SCA3.

Modulation of delayed rectifier potassium channels by alpha1-adrenergic activation via protein kinase C zeta and p62 in PC12 cells

When PC12 cells are exposed to nerve growth factor (NGF), they extend neurites and express autonomic ganglion cell properties. We have previously shown that NGF is capable of inducing p62 expression, enabling the formation of the protein kinase C zeta (PKCzeta)-p62-Kvbeta (beta-subunit of delayed rectifier K+ channel) complex, a Kv channel-modulating complex. The formation of this complex results in the shifting of the Kv channel activation curve to the left via PKCzeta activity. During the experiments, we noted that PC12 cells in a high-density culture exhibited a Kv channel activation curve shift similar to that observed in the NGF-treated cells. Therefore, we hypothesized that catecholamines released from PC12 cells may induce p62 expression. In order to test this idea, cells in a low-density culture were treated for 24h with norepinephrine (NE). In these cells, we noted a leftward shift of the activation curve. The presence of the alpha1-adrenergic antagonist specifically prevented the effects of NE. Pre-treatment of the low-density cells with alpha1-agonists induced changes similar to those associated with NE, confirming that NE modulates Kv channels via the alpha1-adrenergic receptor. NE's effects were blocked by treatment with PKCzeta specific inhibitors. Using Western blotting, we observed increased levels of p62 expression in both the high-density cells and the NE-treated low-density cells. These results suggest that locally secreted NE induces an increase in p62 expression, and also exerts a modulatory effect on Kv channels via the PKCzeta-p62-Kvbeta channel modulating complex.

A potassium channel blocker induces a long-lasting enhancement of corticostriatal responses

Disruptions in synaptic plasticity in the dorsal striatum may contribute to the pathophysiology underlying Parkinson's disease. Here we report a novel, chemically-induced form of plasticity induced by application of the potassium channel blocker tetraethylammonium (TEA) in the dorsolateral striatum of the adult rat. Transient application of TEA persistently increased synaptically-evoked extracellularly-recorded corticostriatal responses in an activity-, concentration- and time-dependent manner. Pharmacological experiments suggest that this plasticity is dependent on L-type calcium channel and protein kinase C (PKC) activation. Striatal dopamine depletion induced by nigrostriatal dopamine lesions with 6-hydroxydopamine significantly reduced, but did not abolish, TEA-mediated enhancement of the corticostriatal response. Intracellular recordings demonstrate that this TEA-mediated plasticity is associated with an increase in EPSP size and slope, as well as input resistance. Collectively, these findings demonstrate a novel form of L-type calcium channel-dependent plasticity in the adult dorsal striatum that is induced in the absence of dopaminergic input.