Cholesterol depletion modulates basal L-type Ca2+ current and abolishes its -adrenergic enhancement in ventricular myocytes

Evaluation of chronic drug-induced electrophysiological and cytotoxic effects using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs)

Introduction: Cardiotoxicity is one of the leading causes of compound attrition during drug development. Most in vitro screening platforms aim at detecting acute cardio-electrophysiological changes and drug-induced chronic functional alterations are often not studied in the early stage of drug development. Therefore, we developed an assay using human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) that evaluates both drug-induced acute and delayed electrophysiological and cytotoxic effects of reference compounds with clinically known cardiac outcomes. Methods: hiPSC-CMs were seeded in 48-well multielectrode array (MEA) plates and were treated with four doses of reference compounds (covering and exceeding clinical free plasma peak concentrations -fC_max values) and MEA recordings were conducted for 4 days. Functional-electrophysiological (field-potentials) and viability (impedance) parameters were recorded with a MEA machine. Results: To assess this platform, we tested tyrosine-kinase inhibitors with high-cardiac risk profile (sunitinib, vandetanib and nilotinib) and low-cardiac risk (erlotinib), as well as known classic cardiac toxic drugs (doxorubicin and BMS-986094), ion-channel trafficking inhibitors (pentamidine, probucol and arsenic trioxide) and compounds without known clinical cardiotoxicity (amoxicillin, cetirizine, captopril and aspirin). By evaluating the effects of these compounds on MEA parameters, the assay was mostly able to recapitulate different drug-induced cardiotoxicities, represented by a prolongation of the field potential, changes in beating rate and presence of arrhythmic events in acute (<2 h) or delayed phase ≥24 h, and/or reduction of impedance during the delayed phase (≥24 h). Furthermore, a few reference compounds were tested in hiPSC-CMs using fluorescence- and luminescence-based plate reader assays, confirming the presence or absence of cytotoxic effects, linked to changes of the impedance parameters measured in the MEA assay. Of note, some cardiotoxic effects could not be identified at acute time points (<2 h) but were clearly detected after 24 h, reinforcing the importance of chronic drug evaluation. Discussion: In conclusion, the evaluation of chronic drug-induced cardiotoxicity using a hiPSC-CMs in vitro assay can contribute to the early de-risking of compounds and help optimize the drug development process.

Förster Resonance Energy Transfer-Based Single-Cell Imaging Reveals Piezo1-Induced Ca2+ Flux Mediates Membrane Ruffling and Cell Survival

A mechanosensitive ion channel, Piezo1 induces non-selective cation flux in response to various mechanical stresses. However, the biological interpretation and underlying mechanisms of cells resulting from Piezo1 activation remain elusive. This study elucidates Piezo1-mediated Ca²⁺ influx driven by channel activation and cellular behavior using novel Förster Resonance Energy Transfer (FRET)-based biosensors and single-cell imaging analysis. Results reveal that extracellular Ca²⁺ influx via Piezo1 requires intact caveolin, cholesterol, and cytoskeletal support. Increased cytoplasmic Ca²⁺ levels enhance PKA, ERK, Rac1, and ROCK activity, which have the potential to promote cancer cell survival and migration. Furthermore, we demonstrate that Piezo1-mediated Ca²⁺ influx upregulates membrane ruffling, a characteristic feature of cancer cell metastasis, using spatiotemporal image correlation spectroscopy. Thus, our findings provide new insights into the function of Piezo1, suggesting that Piezo1 plays a significant role in the behavior of cancer cells.

Methyl-β-Cyclodextrin Impairs the Phosphorylation of the β₂ Subunit of L-Type Calcium Channels and Cytosolic Calcium Homeostasis in Mature Cerebellar Granule Neurons

The activation of L-type calcium channels (LTCCs) prevents cerebellar granule neurons (CGNs) from entering low-K⁺-induced apoptosis. In previous works, we showed that LTCCs are largely associated with caveolin-1-rich lipid rafts in the CGN plasma membrane. In this work, we show that protein kinase A (PKA) and calmodulin-dependent protein kinase II (CaMK-II) are associated with caveolin-1-rich lipid rafts of mature CGNs, and we further show that treatment with the cholesterol-trapping and lipid raft-disrupting agent methyl-β-cyclodextrin decreases the phosphorylation level of the LTCC β₂ subunit and the steady-state calcium concentration in neuronal somas ([Ca²⁺]_i) to values close to those measured in 5 mM KCl proapoptotic conditions. These effects correlate with the effects produced by a short (15 min) treatment of CGNs with H-89 and KN-93—inhibitors of PKA and CaMK-II, respectively—in 25 mM KCl medium. Moreover, only a 15 min incubation of CGNs with H-89 produces about a 90% inhibition of the calcium entry that would normally occur through LTCCs to increase [Ca²⁺]_i upon raising the extracellular K⁺ from 5 to 25 mM, i.e., from proapoptotic to survival conditions. In conclusion, the results of this work suggest that caveolin-1-rich lipid rafts play a major role in the control of the PKA- and CaMK-II-induced phosphorylation level of the LTCC β₂ subunit, thus preventing CGNs from entering apoptosis.

Anandamide Revisited: How Cholesterol and Ceramides Control Receptor-Dependent and Receptor-Independent Signal Transmission Pathways of a Lipid Neurotransmitter

Anandamide is a lipid neurotransmitter derived from arachidonic acid, a polyunsaturated fatty acid. The chemical differences between anandamide and arachidonic acid result in a slightly enhanced solubility in water and absence of an ionisable group for the neurotransmitter compared with the fatty acid. In this review, we first analyze the conformational flexibility of anandamide in aqueous and membrane phases. We next study the interaction of the neurotransmitter with membrane lipids and discuss the molecular basis of the unexpected selectivity of anandamide for cholesterol and ceramide from among other membrane lipids. We show that cholesterol behaves as a binding partner for anandamide, and that following an initial interaction mediated by the establishment of a hydrogen bond, anandamide is attracted towards the membrane interior, where it forms a molecular complex with cholesterol after a functional conformation adaptation to the apolar membrane milieu. The complex is then directed to the anandamide cannabinoid receptor (CB1) which displays a high affinity binding pocket for anandamide. We propose that cholesterol may regulate the entry and exit of anandamide in and out of CB1 by interacting with low affinity cholesterol recognition sites (CARC and CRAC) located in transmembrane helices. The mirror topology of cholesterol binding sites in the seventh transmembrane domain is consistent with the delivery, extraction and flip-flop of anandamide through a coordinated cholesterol-dependent mechanism. The binding of anandamide to ceramide illustrates another key function of membrane lipids which may occur independently of protein receptors. Interestingly, ceramide forms a tight complex with anandamide which blocks the degradation pathway of both lipids and could be exploited for anti-cancer therapies.

Casein kinase 2 inhibition impairs spontaneous and oxytocin-induced contractions in late pregnant mouse uterus

NEW FINDINGS

What is the central question of this study? Does the inhibition of the protein kinase casein kinase 2 (CK2) alter the uterine contractility? What is the main finding and its importance? Inhibition of CK2 impaired the spontaneous and oxytocin-induced contractility in late pregnant mouse uterus. This finding suggests that CK2 is a novel pathway mediating oxytocin-induced contractility in the uterus and thus opens up the possibility for this class of drugs to be developed as a new class of tocolytics.

ABSTRACT

The protein kinase casein kinase 2 (CK2) is a ubiquitously expressed serine or threonine kinase known to phosphorylate a number of substrates. The aim of this study was to assess the effect of CK2 inhibition on spontaneous and oxytocin-induced uterine contractions in 19 day pregnant mice. The CK2 inhibitor CX-4945 elicited a concentration-dependent relaxation in late pregnant mouse uterus. CX-4945 and another selective CK2 inhibitor, apigenin, also inhibited the oxytocin-induced contractile response in late pregnant uterine tissue. Apigenin also blunted the prostaglandin F_2α response, but CX-4945 did not. Casein kinase 2 was located in the lipid raft fractions of the cell membrane, and disruption of lipid rafts was found to reverse its effect. The results of the present study suggest that CK2, located in lipid rafts of the cell membrane, is an active regulator of spontaneous and oxytocin-induced uterine contractions in the late pregnant mouse.

Cholesterol modulates the cellular localization of Orai1 channels and its disposition among membrane domains

Store Operated Calcium Entry (SOCE) is one of the most important mechanisms for calcium mobilization in to the cell. Two main proteins sustain SOCE: STIM1 that acts as the calcium sensor in the endoplasmic reticulum (ER) and Orai1 responsible for calcium influx upon depletion of ER. There are many studies indicating that SOCE is modulated by the cholesterol content of the plasma membrane (PM). However, a myriad of questions remain unanswered concerning the precise molecular mechanism by which cholesterol modulates SOCE. In the present study we found that reducing PM cholesterol results in the internalization of Orai1 channels, which can be prevented by overexpressing caveolin 1 (Cav1). Furthermore, Cav1 and Orai1 associate upon SOCE activation as revealed by FRET and coimmunoprecipitation assays. The effects of reducing cholesterol were not limited to an increased rate of Orai1 internalization, but also, affects the lateral movement of Orai1, inducing movement in a linear pattern (unobstructed diffusion) opposite to basal cholesterol conditions were most of Orai1 channels moves in a confined space, as assessed by Fluorescence Correlation Spectroscopy, Cav1 overexpression inhibited these alterations maintaining Orai1 into a confined and partially confined movement. These results not only highlight the complex effect of cholesterol regulation on SOCE, but also indicate a direct regulatory effect on Orai1 localization and compartmentalization by this lipid.

Sarcolemmal dependence of cardiac protection and stress-resistance: roles in aged or diseased hearts

Disruption of the sarcolemmal membrane is a defining feature of oncotic death in cardiac ischaemia-reperfusion (I-R), and its molecular makeup not only fundamentally governs this process but also affects multiple determinants of both myocardial I-R injury and responsiveness to cardioprotective stimuli. Beyond the influences of membrane lipids on the cytoprotective (and death) receptors intimately embedded within this bilayer, myocardial ionic homeostasis, substrate metabolism, intercellular communication and electrical conduction are all sensitive to sarcolemmal makeup, and critical to outcomes from I-R. As will be outlined in this review, these crucial sarcolemmal dependencies may underlie not only the negative effects of age and common co-morbidities on myocardial ischaemic tolerance but also the on-going challenge of implementing efficacious cardioprotection in patients suffering accidental or surgically induced I-R. We review evidence for the involvement of sarcolemmal makeup changes in the impairment of stress-resistance and cardioprotection observed with ageing and highly prevalent co-morbid conditions including diabetes and hypercholesterolaemia. A greater understanding of membrane changes with age/disease, and the inter-dependences of ischaemic tolerance and cardioprotection on sarcolemmal makeup, can facilitate the development of strategies to preserve membrane integrity and cell viability, and advance the challenging goal of implementing efficacious 'cardioprotection' in clinically relevant patient cohorts. Linked Articles This article is part of a themed section on Molecular Pharmacology of G Protein-Coupled Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.20/issuetoc.

Cholesterol Depletion Alters Cardiomyocyte Subcellular Signaling and Increases Contractility

Membrane cholesterol levels play an important factor in regulating cell function. Sarcolemmal cholesterol is concentrated in lipid rafts and caveolae, which are flask-shaped invaginations of the plasma membrane. The scaffolding protein caveolin permits the enrichment of cholesterol in caveolae, and caveolin interactions with numerous proteins regulate their function. The purpose of this study was to determine whether acute reductions in cardiomyocyte cholesterol levels alter subcellular protein kinase activation, intracellular Ca²⁺ and contractility. Methods: Ventricular myocytes, isolated from adult Sprague Dawley rats, were treated with the cholesterol reducing agent methyl-β-cyclodextrin (MβCD, 5 mM, 1 hr, room temperature). Total cellular cholesterol levels, caveolin-3 localization, subcellular, ERK and p38 mitogen activated protein kinase (MAPK) signaling, contractility, and [Ca²⁺]i were assessed. Results: Treatment with MβCD reduced cholesterol levels by ~45 and shifted caveolin-3 from cytoskeleton and triton-insoluble fractions to the triton-soluble fraction, and increased ERK isoform phosphorylation in cytoskeletal, cytosolic, triton-soluble and triton-insoluble membrane fractions without altering their subcellular distributions. In contrast the primary effect of MβCD was on p38 subcellular distribution of p38α with little effect on p38 phosphorylation. Cholesterol depletion increased cardiomyocyte twitch amplitude and the rates of shortening and relaxation in conjunction with increased diastolic and systolic [Ca²⁺]i. Conclusions: These results indicate that acute reductions in membrane cholesterol levels differentially modulate basal cardiomyocyte subcellular MAPK signaling, as well as increasing [Ca²⁺]_i and contractility.

Cholesterol is required for maintaining T-tubule integrity and intercellular connections at intercalated discs in cardiomyocytes

BACKGROUNDS

Low serum cholesterol levels are associated with cardiac arrhythmias and poor prognosis in patients with chronic heart failure. However, the underlying mechanisms by which decreases in cholesterol content lead to cardiac dysfunction remain unclear. Multiple studies have implicated damage to cardiac transverse (T)-tubules as a key mediator of excitation-contraction (E-C) coupling dysfunction and heart failure. Since the T-tubule membrane system is enriched in cholesterol, we hypothesized that depletion of membrane cholesterol promotes T-tubule remodeling and E-C coupling dysfunction.

METHODS AND RESULTS

We first examined the impact of membrane cholesterol depletion on T-tubule architecture by treating isolated C57BL/6 murine cardiomyocytes with methyl-β-cyclodextrin (MβCD). T-tubule structural integrity was progressively decreased by MβCD in a concentration- and time-dependent manner. Membrane cholesterol depletion also promoted a severe decrease in the amplitude of Ca(2+) transients and an increase in Ca(2+) release dyssynchrony as well as a significant increase in the frequency of spontaneous Ca(2+) sparks. Reintroduction of cholesterol restored T-tubule integrity and partially restored Ca(2+) handling properties in acutely-treated myocytes and slowed T-tubule deterioration in response to chronic MβCD exposure. Studies were extended to determine the impact of membrane cholesterol depletion on T-tubule structure in intact hearts. In addition to T-tubule remodeling, Langendorff perfusion of MβCD resulted in rapid and severe disruption of the intercellular connections between cardiomyocytes, in particular at intercalated disc regions in intact hearts.

CONCLUSIONS

These data provide the first evidence that cholesterol plays a critical role in maintaining cardiac T-tubule structure as well as the integrity of intercalated discs.

Cholesterol influences potassium currents in inner hair cells isolated from guinea pig cochlea

OBJECTIVE

There is a correlation between serum hyperlipidemia and hearing loss. Cholesterol is an integral component of the cell membrane and regulates the activity of ion channels in the lipid bilayer. The aim of this study was to investigate the effects of cholesterol on the potassium currents in IHCs by using the cholesterol-depleting drug, MβCD, and water-soluble cholesterol.

METHODS

IHCs were acutely isolated from a mature guinea-pig cochlea and potassium currents were recorded. MβCD and water-soluble cholesterol were applied to IHCs under pressure puff pipettes.

RESULTS

IHCs showed outwardly rectifying currents (I_K,f and I_K,s) in response to depolarizing voltage pulses, with only a slight inward current (I_K,n) when hyperpolarized. In 10mM MβCD solutions, the amplitude of outward K currents reversely decreased; however, fast activation kinetics was preserved. In contrast, in solution of 1mM water-soluble cholesterol, the amplitude of outward K currents reversely increased. At the membrane potential of +110mV, relative conductances were 0.87±0.07 and 1.18±0.11 in MβCD solutions and cholesterol solutions, respectively.

CONCLUSION

The amplitude of K currents in isolated IHCs was reversely changed by cholesterol-depleting drug and water-soluble cholesterol. These results demonstrated the possibility of the involvement of IHC function in hyperlipidemia-induced inner ear disorders.

Cholesterol regulates contractility and inotropic response to β2-adrenoceptor agonist in the mouse atria: Involvement of Gi-protein-Akt-NO-pathway

Majority of cardiac β2-adrenoceptors is located in cholesterol-rich microdomains. Here, we have investigated the underlying mechanisms by which a slight to moderate cholesterol depletion with methyl-β-cyclodextrin (MβCD, 1 and 5mM) interferes with contractility and inotropic effect of β2-adrenergic agonist (fenoterol, 50μM) in the mouse atria. Treatment with MβCD itself increased amplitude of Ca²⁺ transient but did not change the contraction amplitude due to a clamping action of elevated NO. Cholesterol depletion significantly attenuated the positive inotropic response to fenoterol which is accompanied by increase in NO generation and decrease in Ca²⁺ transient. Influence of 1mM MβCD on the fenoterol-driven changes in both contractility and NO level was strongly attenuated by inhibition of G_i-protein (pertussis toxin), Akt (Akt 1/2 kinase inhibitor) or NO-synthase (L-NAME). After exposure to 5mM MβCD, pertussis toxin or Akt inhibitor could recover the β2-agonist effects on contractility, NO production and Ca²⁺ transient, while L-NAME only reduced NO level. An adenylyl cyclase activator (forskolin, 50nM) had no influence on the MβCD-induced changes in the β2-agonist effects. Obtained results suggest that slight cholesterol depletion upregulates G_i-protein/Akt/NO-synthase signaling that attenuates the positive inotropic response to β2-adrenergic stimulation without altering the Ca²⁺ transient. Whilst moderate cholesterol depletion additionally could suppress the enhancement of the Ca²⁺ transient amplitude caused by the β2-adrenergic agonist administration in G_i-protein/Akt-dependent but NO-independent manner.

Rotenone-stimulated superoxide release from mitochondrial complex I acutely augments L-type Ca2+ current in A7r5 aortic smooth muscle cells

Voltage-gated L-type Ca(2+) current (ICa,L) induces contraction of arterial smooth muscle cells (ASMCs), and ICa,L is increased by H2O2 in ASMCs. Superoxide released from the mitochondrial respiratory chain (MRC) is dismutated to H2O2 We studied whether superoxide per se acutely modulates ICa,L in ASMCs using cultured A7r5 cells derived from rat aorta. Rotenone is a toxin that inhibits complex I of the MRC and increases mitochondrial superoxide release. The superoxide content of mitochondria was estimated using mitochondrial-specific MitoSOX and HPLC methods, and was shown to be increased by a brief exposure to 10 μM rotenone. ICa,L was recorded with 5 mM BAPTA in the pipette solution. Rotenone administration (10 nM to 10 μM) resulted in a greater ICa,L increase in a dose-dependent manner to a maximum of 22.1% at 10 μM for 1 min, which gradually decreased to 9% after 5 min. The rotenone-induced ICa,L increase was associated with a shift in the current-voltage relationship (I-V) to a hyperpolarizing direction. DTT administration resulted in a 17.9% increase in ICa,L without a negative shift in I-V, and rotenone produced an additional increase with a shift. H2O2 (0.3 mM) inhibited ICa,L by 13%, and additional rotenone induced an increase with a negative shift. Sustained treatment with Tempol (4-hydroxy tempo) led to a significant ICa,L increase but it inhibited the rotenone-induced increase. Staurosporine, a broad-spectrum protein kinase inhibitor, partially inhibited ICa,L and completely suppressed the rotenone-induced increase. Superoxide released from mitochondria affected protein kinases and resulted in stronger ICa,L preceding its dismutation to H2O2 The removal of nitric oxide is a likely mechanism for the increase in ICa,L.

Ac‑hE‑18A‑NH2, a novel dual‑domain apolipoprotein mimetic peptide, inhibits apoptosis in macrophages by promoting cholesterol efflux

A novel synthetic dual-domain apolipoprotein (apo)-mimetic peptide, Ac-hE-18A-NH2, has been proposed to possess several apo A-I- and apo E-mimetic properties. This study investigated the protective effect of this peptide on oxidized low-density lipoprotein (ox-LDL)-induced apoptosis in RAW264.7 cells. For this purpose, RAW264.7 cells were exposed to 50 µg/ml ox-LDL for 48 h, and then incubated with the peptide Ac-hE-18A-NH2 at various concentrations. Apoptosis was detected using annexin V-fluorescein isothiocyanate staining and flow cytometric analysis. The study revealed that the peptide Ac-hE-18A-NH2 (1, 10 and 50 µg/ml) inhibited ox-LDL-mediated apoptosis, and this was accompanied by an increased rate of intracellular cholesterol efflux, and decreased total cholesterol levels in the cells in a concentration-dependent manner. The peptide also decreased caspase-3 activity and increased B-cell lymphoma 2 protein (Bcl-2) expression in macrophages in a dose-dependent manner. Moreover, blockage of cholesterol efflux by brefeldin A decreased the protective effect of Ac-hE-18A-NH2 against ox-LDL induced apoptosis, while increasing the cholesterol efflux by β-cyclodextrin administration led to a marked decrease in the rate of apoptosis of the cells. These findings demonstrate that the apo-mimetic peptide Ac-hE-18A-NH2 exerts a protective effect against apoptosis by reducing the accumulation of cholesterol.

Glucose-6-phosphate dehydrogenase and NADPH redox regulates cardiac myocyte L-type calcium channel activity and myocardial contractile function

We recently demonstrated that a 17-ketosteroid, epiandrosterone, attenuates L-type Ca²⁺ currents (I_Ca-L) in cardiac myocytes and inhibits myocardial contractility. Because 17-ketosteroids are known to inhibit glucose-6-phosphate dehydrogenase (G6PD), the rate-limiting enzyme in the pentose phosphate pathway, and to reduce intracellular NADPH levels, we hypothesized that inhibition of G6PD could be a novel signaling mechanism which inhibit I_Ca-L and, therefore, cardiac contractile function. We tested this idea by examining myocardial function in isolated hearts and Ca²⁺ channel activity in isolated cardiac myocytes. Myocardial function was tested in Langendorff perfused hearts and I_Ca-L were recorded in the whole-cell patch configuration by applying double pulses from a holding potential of −80 mV and then normalized to the peak amplitudes of control currents. 6-Aminonicotinamide, a competitive inhibitor of G6PD, increased pCO₂ and decreased pH. Additionally, 6-aminonicotinamide inhibited G6PD activity, reduced NADPH levels, attenuated peak I_Ca-L amplitudes, and decreased left ventricular developed pressure and ±dp/dt. Finally, dialyzing NADPH into cells from the patch pipette solution attenuated the suppression of I_Ca-L by 6-aminonicotinamide. Likewise, in G6PD-deficient mice, G6PD insufficiency in the heart decreased GSH-to-GSSG ratio, superoxide, cholesterol and acetyl CoA. In these mice, M-mode echocardiographic findings showed increased diastolic volume and end-diastolic diameter without changes in the fraction shortening. Taken together, these findings suggest that inhibiting G6PD activity and reducing NADPH levels alters metabolism and leads to inhibition of L-type Ca²⁺ channel activity. Notably, this pathway may be involved in modulating myocardial contractility under physiological and pathophysiological conditions during which the pentose phosphate pathway-derived NADPH redox is modulated (e.g., ischemia-reperfusion and heart failure).

Myocardial triacylglycerol metabolism

Myocardial triacylglycerol (TAG) constitutes a highly dynamic fatty acid (FA) storage pool that can be used for an energy reserve in the cardiomyocyte. However, derangements in myocardial TAG metabolism and accumulation are commonly associated with cardiac disease, suggesting an important role of intramyocardial TAG turnover in the regulation of cardiac function. In cardiomyocytes, TAG is synthesized by acyltransferases and phosphatases at the sarcoplasmic reticulum and mitochondrial membrane and then packaged into cytosolic lipid droplets for temporary storage or into lipoproteins for secretion. A complex interplay among lipases, lipase regulatory proteins, and lipid droplet scaffold proteins leads to the controlled release of FAs from the cardiac TAG pool for subsequent mitochondrial β-oxidation and energy production. With the identification and characterization of proteins involved in myocardial TAG metabolism as well as the identification of the importance of cardiac TAG turnover, it is now evident that adequate regulation of myocardial TAG metabolism is critical for both cardiac energy metabolism and function. In this article, we review the current understanding of myocardial TAG metabolism and discuss the potential role of myocardial TAG turnover in cardiac health and disease. This article is part of a Special Issue entitled "Focus on Cardiac Metabolism".

Tissue cholesterol content alterations in streptozotocin-induced diabetic rats

AIM

Diabetes is associated with elevated serum total cholesterol level and disrupted lipoprotein subfractions. The aim of this study was to examine alterations in the tissue cholesterol contents closely related to diabetic complications.

METHODS

Intraperitoneal injection of streptozotocin was used to induce type 1 diabetes in adult male Sprague-Dawley rats. On d 35 after the injection, liver, heart, intestine, kidney, pancreas, cerebral cortex and hippocampus were isolated from the rats. The content of total and free cholesterol in the tissues was determined using HPLC. The ATP-binding cassette protein A1 (ABCA1) protein and ApoE mRNA were measured using Western blot and QT-PCR analyses, respectively.

RESULTS

In diabetic rats, the level of free cholesterol was significantly decreased in the peripheral tissues, but significantly elevated in hippocampus, as compared with those in the control rats. Diabetic rats showed a trend of decreasing the total cholesterol level in the peripheral tissues, but significant change was only found in kidney and liver. In diabetic rats, the level of the ABCA1 protein was significantly increased in the peripheral tissues and cerebral cortex; the expression of ApoE mRNA was slightly decreased in hippocampus and cerebral cortex, but the change had no statistical significance.

CONCLUSION

Type 1 diabetes decreases the free cholesterol content in the peripheral tissues and increases the free cholesterol content in hippocampus. The decreased free cholesterol level in the peripheral tissues may be partly due to the increased expression of the ABCA1 protein.

Down-regulation of replication factor C-40 (RFC40) causes chromosomal missegregation in neonatal and hypertrophic adult rat cardiac myocytes

Background

Adult mammalian cardiac myocytes are generally assumed to be terminally differentiated; nonetheless, a small fraction of cardiac myocytes have been shown to replicate during ventricular remodeling. However, the expression of Replication Factor C (RFC; RFC140/40/38/37/36) and DNA polymerase δ (Pol δ) proteins, which are required for DNA synthesis and cell proliferation, in the adult normal and hypertrophied hearts has been rarely studied.

Methods

We performed qRT-PCR and Western blot analysis to determine the levels of RFC and Pol δ message and proteins in the adult normal cardiac myocytes and cardiac fibroblasts, as well as in adult normal and pulmonary arterial hypertension induced right ventricular hypertrophied hearts. Immunohistochemical analyses were performed to determine the localization of the re-expressed DNA replication and cell cycle proteins in adult normal (control) and hypertrophied right ventricle. We determined right ventricular cardiac myocyte polyploidy and chromosomal missegregation/aneuploidy using Fluorescent in situ hybridization (FISH) for rat chromosome 12.

Results

RFC40-mRNA and protein was undetectable, whereas Pol δ message was detectable in the cardiac myocytes isolated from control adult hearts. Although RFC40 and Pol δ message and protein significantly increased in hypertrophied hearts as compared to the control hearts; however, this increase was marginal as compared to the fetal hearts. Immunohistochemical analyses revealed that in addition to RFC40, proliferative and mitotic markers such as cyclin A, phospho-Aurora A/B/C kinase and phospho-histone 3 were also re-expressed/up-regulated simultaneously in the cardiac myocytes. Interestingly, FISH analyses demonstrated cardiac myocytes polyploidy and chromosomal missegregation/aneuploidy in these hearts. Knock-down of endogenous RFC40 caused chromosomal missegregation/aneuploidy and decrease in the rat neonatal cardiac myocyte numbers.

Conclusion

Our novel findings suggest that transcription of RFC40 is suppressed in the normal adult cardiac myocytes and its insufficient re-expression may be responsible for causing chromosomal missegregation/aneuploidy and in cardiac myocytes during right ventricular hypertrophy.

Cholesterol influences voltage-gated calcium channels and BK-type potassium channels in auditory hair cells

The influence of membrane cholesterol content on a variety of ion channel conductances in numerous cell models has been shown, but studies exploring its role in auditory hair cell physiology are scarce. Recent evidence shows that cholesterol depletion affects outer hair cell electromotility and the voltage-gated potassium currents underlying tall hair cell development, but the effects of cholesterol on the major ionic currents governing auditory hair cell excitability are unknown. We investigated the effects of a cholesterol-depleting agent (methyl beta cyclodextrin, MβCD) on ion channels necessary for the early stages of sound processing. Large-conductance BK-type potassium channels underlie temporal processing and open in a voltage- and calcium-dependent manner. Voltage-gated calcium channels (VGCCs) are responsible for calcium-dependent exocytosis and synaptic transmission to the auditory nerve. Our results demonstrate that cholesterol depletion reduced peak steady-state calcium-sensitive (BK-type) potassium current by 50% in chick cochlear hair cells. In contrast, MβCD treatment increased peak inward calcium current (~30%), ruling out loss of calcium channel expression or function as a cause of reduced calcium-sensitive outward current. Changes in maximal conductance indicated a direct impact of cholesterol on channel number or unitary conductance. Immunoblotting following sucrose-gradient ultracentrifugation revealed BK expression in cholesterol-enriched microdomains. Both direct impacts of cholesterol on channel biophysics, as well as channel localization in the membrane, may contribute to the influence of cholesterol on hair cell physiology. Our results reveal a new role for cholesterol in the regulation of auditory calcium and calcium-activated potassium channels and add to the growing evidence that cholesterol is a key determinant in auditory physiology.

Cholesterol and ion channels

A variety of ion channels, including members of all major ion channel families, have been shown to be regulated by changes in the level of membrane cholesterol and partition into cholesterol-rich membrane domains. In general, several types of cholesterol effects have been described. The most common effect is suppression of channel activity by an increase in membrane cholesterol, an effect that was described for several types of inwardly-rectifying K(+) channels, voltage-gated K(+) channels, Ca(+2) sensitive K(+) channels, voltage-gated Na(+) channels, N-type voltage-gated Ca(+2) channels and volume-regulated anion channels. In contrast, several types of ion channels, such as epithelial amiloride-sensitive Na(+) channels and Transient Receptor Potential channels, as well as some of the types of inwardly-rectifying and voltage-gated K(+) channels were shown to be inhibited by cholesterol depletion. Cholesterol was also shown to alter the kinetic properties and current-voltage dependence of several voltage-gated channels. Finally, maintaining membrane cholesterol level is required for coupling ion channels to signalling cascades. In terms of the mechanisms, three general mechanisms have been proposed: (i) specific interactions between cholesterol and the channel protein, (ii) changes in the physical properties of the membrane bilayer and (iii) maintaining the scaffolds for protein-protein interactions. The goal of this review is to describe systematically the role of cholesterol in regulation of the major types of ion channels and to discuss these effects in the context of the models proposed.

Cholesterol modulates ion channels via down-regulation of phosphatidylinositol 4,5-bisphosphate

Ubiquitously expressed Mg(2+)-inhibitory cation (MIC) channels are permeable to Ca2+ and Mg2+ and are essential for cell viability. When membrane cholesterol level was increased by pre-incubating cells with a water-soluble form of cholesterol, the endogenous MIC current in HEK293 cells was negatively regulated. The application of phosphatidylinositol 4,5-bisphosphate (PIP2) recovered MIC current from cholesterol effect. As PIP2 is the direct modulator for MIC channels, high cholesterol content may cause down-regulation of PIP2. To test this possibility, we examined the effect of cholesterol on two exogenously expressed PIP2-sensitive K+ channels: human Ether-a-go-go related gene (HERG) and KCNQ. Enrichment with cholesterol inhibited HERG currents, while inclusion of PIP2 in the pipette solution blocked the cholesterol effect. KCNQ channel was also inhibited by cholesterol. The effects of cholesterol on these channels were blocked by pre-incubating cells with inhibitors for phospholipase C, which may indicate that cholesterol enrichment induces the depletion of PIP2 via phospholipase C activation. Lipid analysis showed that cholesterol enrichment reduced gamma-(32)P incorporation into PIP2 by approximately 35%. Our results suggest that cholesterol may modulate ion channels by changing the levels of PIP2. Thus, an important cross-talk exists among two plasma membrane-enriched lipids, cholesterol and PIP2.

Lipid lateral mobility in cochlear outer hair cells: regional differences and regulation by cholesterol

The outer hair cell (OHC) lateral plasma membrane houses the transmembrane protein prestin, a necessary component of the yet unknown molecular mechanism(s) underlying electromotility and the exquisite sensitivity and frequency selectivity of mammalian hearing. The importance of the plasma membrane environment in modulating OHC electromotility has been substantiated by recent studies demonstrating that membrane cholesterol alters prestin activity in a manner consistent with cholesterol-induced changes in auditory function. Cholesterol is known to affect membrane material properties, and measurements of lipid lateral mobility provide a method to asses these changes in living OHCs. Using fluorescence recovery after photobleaching (FRAP), we characterized regional differences in the lateral diffusion of the lipid analog di-8-ANEPPS in OHCs and investigated whether lipid mobility, which reflects membrane fluidity, is sensitive to membrane cholesterol. FRAP experiments revealed quantitative differences in lipid lateral mobility among the apical, lateral, and basal regions of the OHC and demonstrated that diffusion in individual regions is uniquely sensitive to cholesterol manipulations. Interestingly, in the lateral region, both cholesterol depletion and loading significantly reduced the effective diffusion coefficient from control values. Thus, the fluidity of the OHC lateral plasma membrane is regulated by cholesterol levels in a non-monotonic manner, suggesting that the overall material properties of the lateral plasma membrane are optimally tuned for OHC function in the native state. These results support the idea that the cholesterol-dependent regulation of prestin function and electromotility correlates with changes in the properties of the lipid environment that surrounds and supports prestin.

The intracellular II-III loops of Cav1.2 and Cav1.3 uncouple L-type voltage-gated Ca2+ channels from glucagon-like peptide-1 potentiation of insulin secretion in INS-1 cells via displacement from lipid rafts

L-type Ca(2+) channels play a key role in the integration of physiological signals regulating insulin secretion that probably requires their localization to specific subdomains of the plasma membrane. We investigated the role of the intracellular II-III loop domains of the L-type channels Ca(v)1.2 and 1.3 in coupling of Ca(2+) influx with glucose-stimulated insulin secretion (GSIS) potentiated by the incretin hormone glucagon-like peptide (GLP)-1. In INS-1 cell lines expressing the Ca(v)1.2/II-III or Ca(v)1.3/II-III peptides, GLP-1 potentiation of GSIS was inhibited markedly, coincident with a decrease in GLP-1-stimulated cAMP accumulation and the redistribution of Ca(v)1.2 and Ca(v)1.3 out of lipid rafts. Neither the Ca(v)1.2/II-III nor the Ca(v)1.3/II-III peptide decreased L-type current density compared with untransfected INS-1 cells. GLP-1 potentiation of GSIS was restored by the L-type channel agonist 2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methyl ester (FPL-64176). In contrast, potentiation of GSIS by 8-bromo-cAMP (8-Br-cAMP) was inhibited in Ca(v)1.2/II-III but not Ca(v)1.3/II-III cells. These differences may involve unique protein-protein interactions because the Ca(v)1.2/II-III peptide, but not the Ca(v)1.3/II-III peptide, immunoprecipitates Rab3-interacting molecule (RIM) 2 from INS-1 cell lysates. RIM2, and its binding partner Piccolo, localize to lipid rafts, and they may serve as anchors for Ca(v)1.2 localization to lipid rafts in INS-1 cells. These findings suggest that the II-III interdomain loops of Ca(v)1.2, and possibly Ca(v)1.3, direct these channels to membrane microdomains in which the proteins that mediate potentiation of GSIS by GLP-1 and 8-Br-cAMP assemble.