Ca-dependent nonsecretory vesicle fusion in a secretory cell

Constitutive Plasma Membrane Turnover in T-REx293 cells via Ordered Membrane Domain Endocytosis under Mitochondrial Control

Clathrin/dynamin-independent endocytosis of ordered plasma membrane domains (ordered membrane domain endocytosis, OMDE) can become massive in response to cytoplasmic Ca elevations, G protein activation by non-hydrolyzable GTP analogs, and enhanced oxidative metabolism. In patch-clamped murine bone marrow macrophages (BMMs), cytoplasmic succinate and pyruvate, but not β-hydroxybutyrate, induce OMDE of 75% of the plasma membrane within 2 min. The responses require palmitoylation of membrane proteins, being decreased by 70% in BMMs lacking the acyltransferase, DHHC5, by treatment with carnitine to shift long-chain acyl groups from cytoplasmic to mitochondrial acyl-CoAs, by bromopalmitate/albumin complexes to block DHHCs, and by the mitochondria-specific cyclosporin, NIM811, to block permeability transition pores that may release mitochondrial coenzyme A into the cytoplasm. Using T-REx293 cells, OMDE amounts to 40% with succinate, pyruvate, or GTPγS, and it is inhibited by actin cytoskeleton disruption. Pyruvate-induced OMDE is blocked by the hydrophobic antioxidant, edaravone, which prevents permeability transition pore openings. Using fluorescent 3kD dextrans to monitor endocytosis, OMDE appears to be constitutively active in T-REx293 cells but not in BMMs. After 1 h without substrates or bicarbonate, pyruvate and hydroxybutyrate inhibit constitutive OMDE, as expected for a shift of CoA from long-chain acyl-CoAs to other CoA metabolites. In the presence of bicarbonate, pyruvate strongly enhances OMDE, which is then blocked by β-hydroxybutyrate, bromopalmitate/albumin complexes, cyclosporines, or edaravone. After pyruvate responses, T-REx293 cells grow normally with no evidence for apoptosis. Fatty acid-free albumin (15 μM) inhibits basal OMDE in T-REx293 cells, as do cyclosporines, carnitine, and RhoA blockade. Surprisingly, OMDE in the absence of substrates and bicarbonate is not inhibited by siRNA knockdown of the acyltransferases, DHHC5 or DHHC2, which are required for activated OMDE in patch clamp experiments. We verify biochemically that small CoA metabolites decrease long-chain acyl-CoAs. We verify also that palmitoylations of many PM-associated proteins decrease and increase when OMDE is inhibited and stimulated, respectively, by different metabolites. STED microscopy reveals that vesicles formed during constitutive OMDE in T-REX293 cells have 90 to 130 nm diameters. In summary, OMDE is likely a major G-protein-dependent endocytic mechanism that can be constitutively active in some cell types, albeit not BMMs. OMDE depends on different DHHC acyltransferases in different circumstances and can be limited by local supplies of fatty acids, CoA, and long-chain acyl-CoAs.

Longitudinal diffusion barriers imposed by myofilaments and mitochondria in murine cardiac myocytes

Using optical and electrical methods, we document that diffusion in the cytoplasm of BL6 murine cardiomyocytes becomes restricted >20-fold as molecular weight increases from 30 to 2,000, roughly as expected for pores with porin channel dimensions. Bodipy-FL ATP diffuses >40-fold slower than in free water at 25°C. From several fluorophores analyzed, bound fluorophore fractions range from 0.1 for a 2 kD FITC-labeled polyethylene glycol to 0.93 for sulforhodamine. Unbound fluorophores diffuse at 0.5-8 × 10-7 cm2/s (5-80 μm2/s). Analysis of Na/K pump and veratridine-modified Na channel currents suggests that Na diffusion is nearly unrestricted at 35°C (time constant for equilibration with the pipette tip, ∼20 s). Using multiple strategies, we estimate that at 35°C, ATP diffuses four to eight times slower than in free water. To address whether restrictions are caused more by protein or membrane networks, we verified first that a protein gel, 10 g% gelatin, restricts diffusion with strong dependence on molecular weight. Solute diffusion in membrane-extracted cardiac myofilaments, confined laterally by suction into large-diameter pipette tips, is less restricted than in intact myocytes. Notably, myofilaments extracted similarly from skeletal (diaphragm) myocytes are less restrictive. Solute diffusion in myocytes with sarcolemma permeabilized by β-escin (80 µM) is similar to diffusion in intact myocytes. Restrictions are strain-dependent, being twofold greater in BL6 myocytes than in CD1/J6/129svJ myocytes. Furthermore, longitudinal diffusion is 2.5-fold more restricted in CD1/J6/129svJ myocytes lacking the mitochondrial porin, VDAC1, than in WT CD1/J6/129svJ myocytes. Thus, mitochondria networks restrict long-range diffusion while presumably optimizing nucleotide transfer between myofilaments and mitochondria. We project that diffusion restrictions imposed by both myofilaments and the outer mitochondrial membrane are important determinants of total free cytoplasmic AMP and ADP (∼10 μM). However, the capacity of diffusion to deliver ATP to myofilaments remains ∼100-fold greater than ATP consumption.

Localization of phospholipid-related signal molecules in salivary glands of rodents: A review

BACKGROUND

In the 1950s, Hokin conducted initial studies on phosphoinositide turnover/cycle in salivary glandular cells. From these studies, the idea emerged that receptor-mediated changes in intramembranous levels of phosphoinositides represent an early step in the stimulus-response pathway. Based on this idea and the general view that knowledge of the exact localization of a given endogenous molecule in cells in situ is important for understanding its functional significance, we have reviewed available information about the localization of several representative phosphoinositide-signaling molecules in the salivary glands in situ in mice.

HIGHLIGHT

We focused on phosphatidylinositol 4-kinase, phosphatidylinositol 4 phosphate 5-kinase α, β, γ, phospholipase Cβ, muscarinic cholinoceptors 1 and 3, diacylglycerol kinase ζ, phospholipase D1 and 2, ADP-ribosylation factor 6 and its exchange factors for Arf6, and cannabinoid receptors. These molecules individually exhibit differential localization in a spatiotemporal manner in the exocrine glands, making it possible to deduce their functional significance, such as their involvement in secretion and cell differentiation.

CONCLUSION

Although phosphoinositide-signaling molecules whose in situ localization in glandular cells has been clarified are still limited, the obtained information on their localization suggests that their functional significance is more valuable in glandular ducts than in acini. It thus suggests the necessity of greater attention to the ducts in their physio-pharmacological analyses. The purpose of this review is to encourage more in situ localization studies of phosphoinositide-signaling molecules with an aim to further understand their possible involvement in the pathogenesis of salivary gland diseases.

Pore-like diffusion barriers in murine cardiac myocytes

Using both optical and electrical methods, we document that solute diffusion in the cytoplasm of BL6 murine cardiac myocytes becomes restricted >30-fold as molecular weight increases from 30 to 2000, roughly as expected for pores with dimensions of cardiac porin channels. The Bodipy-FL ATP analogue diffuses ∼50-fold slower in BL6 cardiac cytoplasm than in free water. From several fluorophores analyzed, our estimates of bound fluorophore fractions range from 0.1 for a 2 kD FITC-labeled polyethylene glycol to 0.93 for sulforhodamine. We estimate that diffusion coefficients of unbound fluorophores range from 0.5 to 8 x 10 -7 cm 2 /s. Analysis of Na/K pump and veratridine-modified Na channel currents confirms that Na diffusion is nearly unrestricted (time constant for equilibration with the pipette tip, ∼20 s). Using three different approaches, we estimate that ATP diffuses 8 to 10-times slower in the cytoplasm of BL6 myocytes than in free water. To address whether restrictions are caused more by cytoplasmic protein or membrane networks, we verified first that a protein gel, 10 gram% gelatin, restricts solute diffusion with strong dependence on molecular weight. Solute diffusion in membrane-extracted cardiac myofilaments, confined laterally by suction into large-diameter pipette tips, is however less restricted than in intact myocytes. Notably, myofilaments from equivalently extracted skeletal (diaphragm) myocytes restrict diffusion less than cardiac myofilaments. Solute diffusion in myocytes with sarcolemma permeabilized by β-escin (80 µM) is similarly restricted as in intact myocytes. Diffusion restriction in cardiac myocytes is strain-dependent, being about two-fold greater in BL6 myocytes than in myocytes with a CD1/J6/129svJ background. Furthermore, diffusion is 2.5-fold more restricted in CD1/J6/129svJ myocytes lacking the mitochondrial porin, Vdac1, than in WT CD1/J6/129svJ myocytes. We conclude that both myofilaments and mitochondria networks restrict diffusion in cardiac myocytes. As a result, long-range solute diffusion may preferentially occur via passage through porin channels and intramembrane mitochondrial spaces, where diffusion is less restricted than in myofilament spaces.

Structural basis for gating mechanism of the human sodium-potassium pump

P2-type ATPase sodium-potassium pumps (Na⁺/K⁺-ATPases) are ion-transporting enzymes that use ATP to transport Na⁺ and K⁺ on opposite sides of the lipid bilayer against their electrochemical gradients to maintain ion concentration gradients across the membranes in all animal cells. Despite the available molecular architecture of the Na⁺/K⁺-ATPases, a complete molecular mechanism by which the Na⁺ and K⁺ ions access into and are released from the pump remains unknown. Here we report five cryo-electron microscopy (cryo-EM) structures of the human alpha3 Na⁺/K⁺-ATPase in its cytoplasmic side-open (E1), ATP-bound cytoplasmic side-open (E1•ATP), ADP-AlF₄⁻ trapped Na⁺-occluded (E1•P-ADP), BeF₃⁻ trapped exoplasmic side-open (E2P) and MgF₄²⁻ trapped K⁺-occluded (E2•P_i) states. Our work reveals the atomically resolved structural detail of the cytoplasmic gating mechanism of the Na⁺/K⁺-ATPase.

Through cryo-EM analysis, here authors reveal conformational rearrangements that are critical for the gating mechanism of the human alpha3 Na+/K+−ATPase

TMEM16F and dynamins control expansive plasma membrane reservoirs

Cells can expand their plasma membrane laterally by unfolding membrane undulations and by exocytosis. Here, we describe a third mechanism involving invaginations held shut by the membrane adapter, dynamin. Compartments open when Ca activates the lipid scramblase, TMEM16F, anionic phospholipids escape from the cytoplasmic monolayer in exchange for neutral lipids, and dynamins relax. Deletion of TMEM16F or dynamins blocks expansion, with loss of dynamin expression generating a maximally expanded basal plasma membrane state. Re-expression of dynamin2 or its GTPase-inactivated mutant, but not a lipid binding mutant, regenerates reserve compartments and rescues expansion. Dynamin2-GFP fusion proteins form punctae that rapidly dissipate from these compartments during TMEM16F activation. Newly exposed compartments extend deeply into the cytoplasm, lack numerous organellar markers, and remain closure-competent for many seconds. Without Ca, compartments open slowly when dynamins are sequestered by cytoplasmic dynamin antibodies or when scrambling is mimicked by neutralizing anionic phospholipids and supplementing neutral lipids. Activation of Ca-permeable mechanosensitive channels via cell swelling or channel agonists opens the compartments in parallel with phospholipid scrambling. Thus, dynamins and TMEM16F control large plasma membrane reserves that open in response to lateral membrane stress and Ca influx.

Atomic insights into ML-SI3 mediated human TRPML1 inhibition

Transient receptor potential mucolipin 1 (TRPML1) regulates lysosomal calcium signaling, lipid trafficking, and autophagy-related processes. This channel is regulated by phosphoinositides and the low pH environment of the lysosome, maintaining calcium levels essential for proper lysosomal function. Recently, several small molecules specifically targeting the TRPML family have been demonstrated to modulate channel activity. One of these, a synthetic antagonist ML-SI3, can prevent lysosomal calcium efflux and has been reported to block downstream TRPML1-mediated induction of autophagy. Here, we report a cryo-electron microscopy structure of human TRPML1 with ML-SI3 at 2.9-Å resolution. ML-SI3 binds to the hydrophobic cavity created by S5, S6, and PH1, the same cavity where the synthetic agonist ML-SA1 binds. Electrophysiological characterizations show that ML-SI3 can compete with ML-SA1, blocking channel activation yet does not inhibit PI(3,5)P₂-dependent activation of the channel. Consequently, this work provides molecular insight into how ML-SI3 and native lipids regulate TRPML1 activity.

Cracking the code of sodium/calcium exchanger (NCX) gating: Old and new complexities surfacing from the deep web of secondary regulations

Cell membranes spatially define gradients that drive the complexity of biological signals. To guarantee movements and exchanges of solutes between compartments, membrane transporters negotiate the passages of ions and other important molecules through lipid bilayers. The Na⁺/Ca²⁺ exchangers (NCXs) in particular play central roles in balancing Na⁺ and Ca²⁺ fluxes across diverse proteolipid borders in all eukaryotic cells, influencing cellular functions and fate by multiple means. To prevent progression from balance to disease, redundant regulatory mechanisms cooperate at multiple levels (transcriptional, translational, and post-translational) and guarantee that the activities of NCXs are finely-tuned to cell homeostatic requirements. When this regulatory network is disturbed by pathological forces, cells may approach the end of life. In this review, we will discuss the main findings, controversies and open questions about regulatory mechanisms that control NCX functions in health and disease.

Localization of phosphatidylinositol 4-phosphate 5-kinase (PIP5K) α, β, γ in the three major salivary glands in situ of mice and their response to β-adrenoceptor stimulation

Phosphatidylinositol 4-phosphate 5-kinase (PIP5K), which is composed of three isozymes (α, β and γ), catalyzes the production of phosphatidylinositol bisphosphate (PIP2). This phospholipid functions in membrane trafficking, as an anchor for actin cytoskeletons and as a regulator of intramembranous channels/transporters. It is also a precursor of such second messengers as diacylglycerol, inositol triphosphate and phosphatidylinositol (3,4,5)-triphosphate. In the present study, the expression and localization of endogenous PIP5Ks were examined in the three major salivary glands of young adult mice in situ. In western blotting of normal control glands, immunoreactive bands for individual PIP5Ks were detectable, with the highest density in the parotid gland and the weakest density in the submandibular gland. In immuno-light microscopy under non-stimulated condition, weak immunoreactivity for PIP5Kα was confined to the apical plasmalemma in parotid, but not sublingual or submandibular, acinar cells. Immunoreactivity for PIP5Kβ was weak to moderate and confined to ductal cells but not acinar cells, whereas that for PIP5Kγ was selectively and intensely detected in myoepithelial cells but not acinar cells, and it was weak in ductal cells in the three glands. In western blot of the parotid gland stimulated by isoproterenol, a β-adrenoceptor agonist, no changes were seen in the intensity of immunoreactive bands for any of the PIP5Ks. In contrast, in immuno-light microscopy, the apical immunoreactivity for PIP5Kα in parotid acinar cells was transiently and distinctly increased after the stimulation. The increased immunoreactivity was ultrastructurally localized on most apical microvilli and along contiguous plasma membrane, where membranous invaginations of various shapes and small vesicles were frequently found. It was thus suggested that PIP5Kα is involved in post-exocytotic membrane dynamics via microvillous membranes. The present finding further suggests that each of the three isoforms of PIP5K functions through its product PIP2 discretely in different cells of the glands to regulate saliva secretion.

Effects of ticagrelor on the sodium/calcium exchanger 1 (NCX1) in cardiac derived H9c2 cells

Ticagrelor is a direct acting and reversibly binding P2Y₁₂ antagonist approved for the prevention of thromboembolic events. Clinical effects of ticagrelor cannot be simply accounted for by pure platelet inhibition, and off-target mechanisms can potentially play a role. In particular, recent evidence suggests that ticagrelor may also influence heart function and improve the evolution of myocardial ischemic injury by more direct effects on myocytes. The cardiac sodium/calcium exchanger 1 (NCX1) is a critical player in the generation and control of calcium (Ca²⁺) signals, which orchestrate multiple myocyte activities in health and disease. Altered expression and/or activity of NCX1 can have profound consequences for the function and fate of myocytes. Whether ticagrelor affects cardiac NCX1 has not been investigated yet. To explore this hypothesis, we analyzed the expression, localization and activity of NCX1 in the heart derived H9c2-NCX1 cells following ticagrelor exposure. We found that ticagrelor concentration- and time-dependently reduced the activity of the cardiac NCX1 in H9c2 cells. In particular, the inhibitory effect of ticagrelor on the Ca²⁺-influx mode of NCX1 was evident within 1 h and further developed after 24 h, when NCX1 activity was suppressed by about 55% in cells treated with 1 μM ticagrelor. Ticagrelor-induced inhibition of exchanger activity was reached at clinically relevant concentrations, without affecting the expression levels and subcellular distribution of NCX1. Collectively, these findings suggest that cardiac NCX1 is a new downstream target of ticagrelor, which may contribute to the therapeutic profile of ticagrelor in clinical practice.

TMEM16F activation by Ca2+ triggers plasma membrane expansion and directs PD-1 trafficking

TMEM16F is a Ca2+ -gated ion channel that is required for Ca2+ -activated phosphatidylserine exposure on the surface of many eukaryotic cells. TMEM16F is widely expressed and has roles in platelet activation during blood clotting, bone formation and T cell activation. By combining microscopy and patch clamp recording we demonstrate that activation of TMEM16F by Ca2+ ionophores in Jurkat T cells triggers large-scale surface membrane expansion in parallel with phospholipid scrambling. With continued ionophore application,TMEM16F-expressing cells then undergo extensive shedding of ectosomes. The T cell co-receptor PD-1 is selectively incorporated into ectosomes. This selectivity depends on its transmembrane sequence. Surprisingly, cells lacking TMEM16F not only fail to expand surface membrane in response to elevated cytoplasmic Ca2+, but instead undergo rapid massive endocytosis with PD-1 internalisation. These results establish a new role for TMEM16F as a regulator of Ca2+ activated membrane trafficking.

Structural basis for PtdInsP2-mediated human TRPML1 regulation

Transient receptor potential mucolipin 1 (TRPML1), a lysosomal channel, maintains the low pH and calcium levels for lysosomal function. Several small molecules modulate TRPML1 activity. ML-SA1, a synthetic agonist, binds to the pore region and phosphatidylinositol-3,5-bisphosphate (PtdIns(3,5)P2), a natural lipid, stimulates channel activity to a lesser extent than ML-SA1; moreover, PtdIns(4,5)P2, another natural lipid, prevents TRPML1-mediated calcium release. Notably, PtdIns(3,5)P2 and ML-SA1 cooperate further increasing calcium efflux. Here we report the structures of human TRPML1 at pH 5.0 with PtdIns(3,5)P2, PtdIns(4,5)P2, or ML-SA1 and PtdIns(3,5)P2, revealing a unique lipid-binding site. PtdIns(3,5)P2 and PtdIns(4,5)P2 bind to the extended helices of S1, S2, and S3. The phosphate group of PtdIns(3,5)P2 induces Y355 to form a π-cation interaction with R403, moving the S4-S5 linker, thus allosterically activating the channel. Our structures and electrophysiological characterizations reveal an allosteric site and provide molecular insight into how lipids regulate TRP channels.

Calcium- and ATP-dependent regulation of Na/Ca exchange function in BHK cells: Comparison of NCX1 and NCX3 exchangers

Na⁺/Ca²⁺ exchangers (NCX) mediate bidirectional Ca²⁺ fluxes across cell membranes and contribute to Ca²⁺ homeostasis in many cell types. Exchangers are regulated by gating reactions that depend on Na⁺ and Ca²⁺ binding to transport and regulatory sites. A Na⁺_i-dependent inactivation is prominent in all isoforms, whereas Ca²⁺_i-dependent regulation varies among isoforms. Here we characterize new details of NCX operation and describe differences and similarities between NCX3 and NCX1 regulation by intracellular Ca²⁺ and ATP. To compare isoforms, we employed BHK cells expressing NCX3 or NCX1 constitutively and exchange activity was analysed in whole-cell and excised patch recordings under "zero-trans" conditions (i.e., with only one transported ion species on each side). Using BHK cells with low cytoplasmic Ca²⁺ buffering, outward (reverse) currents, reflecting Ca²⁺ influx, are activated by applying extracellular Ca²⁺ (Ca_o) in the presence of Na⁺ on the cytoplasmic side. When firstly activated, peak outward NCX3 currents rapidly decay over seconds and then typically develop a secondary transient peak with slower kinetics, until Ca_o removal abolishes all outward current. The delayed rise of outward current is the signature of an activating process since peak outward NCX3 currents elicited at subsequent Ca_o bouts remain stimulated for minutes and slower decline towards a non-zero level during continued Ca_o application. Secondary transient peaks and current stimulation are suppressed by increasing the intracellular Ca²⁺ buffer capacity or by replacing cytoplasmic ATP with the analogues AMP-PNP or ATPγS. In BHK cells expressing NCX1, outward currents activated under identical settings decay to a steady-state level during single Ca_o application and are significantly larger, causing strong and long-lived run down of subsequent outward currents. NCX1 current run down is not prevented by increasing cytoplasmic Ca²⁺ buffering but secondary transient peaks in the outward current profile can be resolved in the presence of ATP. Finally, inward currents recorded in patches excised from NCX3-expressing cells reveal a proteolysis-sensitive, Ca-dependent inactivation process that is unusual for NCX1 forward activity. Together, our results suggest that NCX function is regulated more richly than appreciated heretofore, possibly including processes that are lost in excised membrane patches.

Application of Electrophysiology Measurement to Study the Activity of Electro-Neutral Transporters

The transport of ions through cell membranes ensures the fine control of ion content within and outside the cell that is indispensable for cell survival. These transport mechanisms are mediated by the activities of specialized transporter proteins. Specifically,pH dynamics are finely controlled by plasma membrane proton (H⁺) extrusion systems, such as the Na⁺/H⁺ exchanger (NHE) protein family. Despite extensive efforts to study the mechanisms underlying NHE regulation, our current understanding of the biophysical and molecular properties of the NHE family is inadequate because of the limited availability of methods to effectively measure NHE activity. In this manuscript, we used H⁺-selective electrodes during whole-cell patch clamping recording to measure NHE-induced H⁺ flux. We proposed this approach to overcome some limitations of typically used methods to measure NHE activity, such as radioactive uptake and fluorescent membrane permeants. Measurement of NHE activity using the described method enables high sensitivity and time resolution and more efficient control of intracellular H⁺ concentrations. H⁺-selective electrodes are based on the fact that transporter activity creates an ion gradient in close proximity to the cell membrane. An H⁺-selective electrode moving up to and away from the cell membrane in a repetitive, oscillatory fashion records a voltage difference that is dependent on H⁺ flux. While H⁺-selective electrodes are used to detect H⁺ flux moving out of the cell, the patch clamp method in the whole-cell configuration is used to control the intracellular ion composition. Moreover, application of the giant patch clamp technique allows modification of the intracellular composition of not only ions but also lipids. The transporter activity of NHE isoform 3 (NHE3) was measured using this technical approach to study the molecular basis of NHE3 regulation by phosphoinositides.

Lipid signaling to membrane proteins: From second messengers to membrane domains and adapter-free endocytosis

Hilgemann et al. explain how lipid signaling to membrane proteins involves a hierarchy of mechanisms from lipid binding to membrane domain coalescence.

Lipids influence powerfully the function of ion channels and transporters in two well-documented ways. A few lipids act as bona fide second messengers by binding to specific sites that control channel and transporter gating. Other lipids act nonspecifically by modifying the physical environment of channels and transporters, in particular the protein–membrane interface. In this short review, we first consider lipid signaling from this traditional viewpoint, highlighting innumerable Journal of General Physiology publications that have contributed to our present understanding. We then switch to our own emerging view that much important lipid signaling occurs via the formation of membrane domains that influence the function of channels and transporters within them, promote selected protein–protein interactions, and control the turnover of surface membrane.

Human TRPML1 channel structures in open and closed conformations

Transient receptor potential mucolipin 1 (TRPML1) is a Ca²⁺-releasing cation channel that mediates the calcium signalling and homeostasis of lysosomes. Mutations in TRPML1 lead to mucolipidosis type IV, a severe lysosomal storage disorder. Here we report two electron cryo-microscopy structures of full-length human TRPML1: a 3.72-Å apo structure at pH 7.0 in the closed state, and a 3.49-Å agonist-bound structure at pH 6.0 in an open state. Several aromatic and hydrophobic residues in pore helix 1, helices S5 and S6, and helix S6 of a neighbouring subunit, form a hydrophobic cavity to house the agonist, suggesting a distinct agonist-binding site from that found in TRPV1, a TRP channel from a different subfamily. The opening of TRPML1 is associated with distinct dilations of its lower gate together with a slight structural movement of pore helix 1. Our work reveals the regulatory mechanism of TRPML channels, facilitates better understanding of TRP channel activation, and provides insights into the molecular basis of mucolipidosis type IV pathogenesis.

Na/K pump inactivation, subsarcolemmal Na measurements, and cytoplasmic ion turnover kinetics contradict restricted Na spaces in murine cardiac myocytes

The Na/K pump exports cytoplasmic Na ions while importing K ions, and its activity is thought to be affected by restricted intracellular Na diffusion in cardiac myocytes. Lu and Hilgemann find instead that the pump can enter an inactivated state and that inactivation can be relieved by cytoplasmic Na.

Decades ago, it was proposed that Na transport in cardiac myocytes is modulated by large changes in cytoplasmic Na concentration within restricted subsarcolemmal spaces. Here, we probe this hypothesis for Na/K pumps by generating constitutive transsarcolemmal Na flux with the Na channel opener veratridine in whole-cell patch-clamp recordings. Using 25 mM Na in the patch pipette, pump currents decay strongly during continuous activation by extracellular K (τ, ∼2 s). In contradiction to depletion hypotheses, the decay becomes stronger when pump currents are decreased by hyperpolarization. Na channel currents are nearly unchanged by pump activity in these conditions, and conversely, continuous Na currents up to 0.5 nA in magnitude have negligible effects on pump currents. These outcomes are even more pronounced using 50 mM Li as a cytoplasmic Na congener. Thus, the Na/K pump current decay reflects mostly an inactivation mechanism that immobilizes Na/K pump charge movements, not cytoplasmic Na depletion. When channel currents are increased beyond 1 nA, models with unrestricted subsarcolemmal diffusion accurately predict current decay (τ ∼15 s) and reversal potential shifts observed for Na, Li, and K currents through Na channels opened by veratridine, as well as for Na, K, Cs, Li, and Cl currents recorded in nystatin-permeabilized myocytes. Ion concentrations in the pipette tip (i.e., access conductance) track without appreciable delay the current changes caused by sarcolemmal ion flux. Importantly, cytoplasmic mixing volumes, calculated from current decay kinetics, increase and decrease as expected with osmolarity changes (τ >30 s). Na/K pump current run-down over 20 min reflects a failure of pumps to recover from inactivation. Simulations reveal that pump inactivation coupled with Na-activated recovery enhances the rapidity and effectivity of Na homeostasis in cardiac myocytes. In conclusion, an autoregulatory mechanism enhances cardiac Na/K pump activity when cytoplasmic Na rises and suppresses pump activity when cytoplasmic Na declines.

The biophysical and molecular basis of intracellular pH sensing by Na+/H+ exchanger-3

Epithelial Na(+)/H(+) exchanger-3 (NHE3) transport is fundamental for renal and intestinal sodium reabsorption. Cytoplasmic protons are thought to serve as allosteric modifiers of the exchanger and to trigger its transport through protein conformational change. This effect presupposes an intracellular pH (pHi) dependence of NHE3 activity, although the biophysical and molecular basis of NHE3 pHi sensitivity have not been defined. NHE3, when complexed with the calcineurin homologous protein-1 (CHP1), had a shift in pHi sensitivity (0.4 units) toward the acidic side in comparison with NHE3 alone, as measured by oscillating pH electrodes combined with whole-cell patch clamping. Indeed, CHP1 interaction with NHE3 inhibited NHE3 transport in a pHi -dependent manner. CHP1 binding to NHE3 also affected its acute regulation. Intracellular perfusion of peptide from the CHP1 binding region (or pHi modification to reduce the CHP1 amount bound to NHE3) was permissive and cooperative for dopamine inhibition of NHE3 but reversed that of adenosine. Thus, CHP1 interaction with NHE3 apparently establishes the exchanger set point for pHi, and modification in this set point is effective in the hormonal stimuli-mediated regulation of NHE3. CHP1 may serve as a regulatory cofactor for NHE3 conformational change, dependent on intracellular protonation.

Calcium control of neurotransmitter release

Upon entering a presynaptic terminal, an action potential opens Ca(2+) channels, and transiently increases the local Ca(2+) concentration at the presynaptic active zone. Ca(2+) then triggers neurotransmitter release within a few hundred microseconds by activating synaptotagmins Ca(2+). Synaptotagmins bind Ca(2+) via two C2-domains, and transduce the Ca(2+) signal into a nanomechanical activation of the membrane fusion machinery; this activation is mediated by the Ca(2+)-dependent interaction of the synaptotagmin C2-domains with phospholipids and SNARE proteins. In triggering exocytosis, synaptotagmins do not act alone, but require an obligatory cofactor called complexin, a small protein that binds to SNARE complexes and simultaneously activates and clamps the SNARE complexes, thereby positioning the SNARE complexes for subsequent synaptotagmin action. The conserved function of synaptotagmins and complexins operates generally in most, if not all, Ca(2+)-regulated forms of exocytosis throughout the body in addition to synaptic vesicle exocytosis, including in the degranulation of mast cells, acrosome exocytosis in sperm cells, hormone secretion from endocrine cells, and neuropeptide release.

Evidence for sustained ATP release from liver cells that is not mediated by vesicular exocytosis

Extracellular ATP regulates many important cellular functions in the liver by stimulating purinergic receptors. Recent studies have shown that rapid exocytosis of ATP-enriched vesicles contributes to ATP release from liver cells. However, this rapid ATP release is transient, and ceases in ~30 s after the exposure to hypotonic solution. The purpose of these studies was to assess the role of vesicular exocytosis in sustained ATP release. An exposure to hypotonic solution evoked sustained ATP release that persisted for more than 15 min after the exposure. Using FM1-43 (N-(3-triethylammoniumpropyl)-4-(4-(dibutylamino)styryl)pyridinium dibromide) fluorescence to measure exocytosis, we found that hypotonic solution stimulated a transient increase in FM1-43 fluorescence that lasted ~2 min. Notably, the rate of FM1-43 fluorescence and the magnitude of ATP release were not correlated, indicating that vesicular exocytosis may not mediate sustained ATP release from liver cells. Interestingly, mefloquine potently inhibited sustained ATP release, but did not inhibit an increase in FM1-43 fluorescence evoked by hypotonic solution. Consistent with these findings, when exocytosis of ATP-enriched vesicles was specifically stimulated by 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), mefloquine failed to inhibit ATP release evoked by NPPB. Thus, mefloquine can pharmacologically dissociate sustained ATP release and vesicular exocytosis. These results suggest that a distinct mefloquine-sensitive membrane ATP transport may contribute to sustained ATP release from liver cells. This novel mechanism of membrane ATP transport may play an important role in the regulation of purinergic signaling in liver cells.

Massive calcium-activated endocytosis without involvement of classical endocytic proteins

We describe rapid massive endocytosis (MEND) of >50% of the plasmalemma in baby hamster kidney (BHK) and HEK293 cells in response to large Ca transients. Constitutively expressed Na/Ca exchangers (NCX1) are used to generate Ca transients, whereas capacitance recording and a membrane tracer dye, FM 4–64, are used to monitor endocytosis. With high cytoplasmic adenosine triphosphate (ATP; >5 mM), Ca influx causes exocytosis followed by MEND. Without ATP, Ca transients cause only exocytosis. MEND can then be initiated by pipette perfusion of ATP, and multiple results indicate that ATP acts via phosphatidylinositol-bis 4,5-phosphate (PIP₂) synthesis: PIP₂ substitutes for ATP to induce MEND. ATP-activated MEND is blocked by an inositol 5-phosphatase and by guanosine 5′-[γ-thio]triphosphate (GTPγS). Block by GTPγS is overcome by the phospholipase C inhibitor, U73122, and PIP₂ induces MEND in the presence of GTPγS. MEND can occur in the absence of ATP and PIP₂ when cytoplasmic free Ca is clamped to 10 µM or more by Ca-buffered solutions. ATP-independent MEND occurs within seconds during Ca transients when cytoplasmic solutions contain polyamines (e.g., spermidine) or the membrane is enriched in cholesterol. Although PIP₂ and cholesterol can induce MEND minutes after Ca transients have subsided, polyamines must be present during Ca transients. MEND can reverse over minutes in an ATP-dependent fashion. It is blocked by brief β-methylcyclodextrin treatments, and tests for involvement of clathrin, dynamins, calcineurin, and actin cytoskeleton were negative. Therefore, we turned to the roles of lipids. Bacterial sphingomyelinases (SMases) cause similar MEND responses within seconds, suggesting that ceramide may be important. However, Ca-activated MEND is not blocked by reagents that inhibit SMases. MEND is abolished by the alkylating phospholipase A₂ inhibitor, bromoenol lactone, whereas exocytosis remains robust, and Ca influx causes MEND in cardiac myocytes without preceding exocytosis. Thus, exocytosis is not prerequisite for MEND. From these results and two companion studies, we suggest that Ca promotes the formation of membrane domains that spontaneously vesiculate to the cytoplasmic side.

Massive endocytosis driven by lipidic forces originating in the outer plasmalemmal monolayer: a new approach to membrane recycling and lipid domains

The roles that lipids play in endocytosis are the subject of debate. Using electrical and imaging methods, we describe massive endocytosis (MEND) in baby hamster kidney (BHK) and HEK293 cells when the outer plasma membrane monolayer is perturbed by the nonionic detergents, Triton X-100 (TX100) and NP-40. Some alkane detergents, the amphipathic drugs, edelfosine and tamoxifen, and the phospholipase inhibitor, U73122, are also effective. Uptake of the membrane tracer, FM 4–64, into vesicles and loss of reversible FM 4–64 binding confirm that 40–75% of the cell surface is internalized. Ongoing MEND stops in 2–4 s when amphipaths are removed, and amphipaths are without effect from the cytoplasmic side. Thus, expansion of the outer monolayer is critical. As found for Ca-activated MEND, vesicles formed are <100 nm in diameter, membrane ruffles are lost, and β-cyclodextrin treatments are inhibitory. However, amphipath-activated MEND does not require Ca transients, adenosine triphosphate (ATP) hydrolysis, G protein cycling, dynamins, or actin cytoskeleton remodeling. With elevated cytoplasmic ATP (>5 mM), MEND can reverse completely and be repeated multiple times in BHK and HEK293 cells, but not cardiac myocytes. Reversal is blocked by N-ethylmaleimide and a nitric oxide donor, nitroprusside. Constitutively expressed Na/Ca exchangers internalize roughly in proportion to surface membrane, whereas Na/K pump activities decrease over-proportionally. Sodium dodecyl sulfate and dodecylglucoside do not cause MEND during their application, but MEND occurs rapidly when they are removed. As monitored capacitively, the binding of these detergents decreases with MEND, whereas TX100 binding does not decrease. In summary, nonionic detergents can fractionate the plasma membrane in vivo, and vesicles formed connect immediately to physiological membrane-trafficking mechanisms. We suggest that lateral and transbilayer inhomogeneities of the plasma membrane provide potential energies that, when unbridled by triggers, can drive endocytosis by lipidic forces.

Massive Ca-induced membrane fusion and phospholipid changes triggered by reverse Na/Ca exchange in BHK fibroblasts

Baby hamster kidney (BHK) fibroblasts increase their cell capacitance by 25-100% within 5 s upon activating maximal Ca influx via constitutively expressed cardiac Na/Ca exchangers (NCX1). Free Ca, measured with fluo-5N, transiently exceeds 0.2 mM with total Ca influx amounting to approximately 5 mmol/liter cell volume. Capacitance responses are half-maximal when NCX1 promotes a free cytoplasmic Ca of 0.12 mM (Hill coefficient approximately 2). Capacitance can return to baseline in 1-3 min, and responses can be repeated several times. The membrane tracer, FM 4-64, is taken up during recovery and can be released at a subsequent Ca influx episode. Given recent interest in signaling lipids in membrane fusion, we used green fluorescent protein (GFP) fusions with phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)) and diacylglycerol (DAG) binding domains to analyze phospholipid changes in relation to these responses. PI(4,5)P(2) is rapidly cleaved upon activating Ca influx and recovers within 2 min. However, PI(4,5)P(2) depletion by activation of overexpressed hM1 muscarinic receptors causes only little membrane fusion, and subsequent fusion in response to Ca influx remains massive. Two results suggest that DAG may be generated from sources other than PI(4,5)P in these protocols. First, acylglycerols are generated in response to elevated Ca, even when PI(4,5)P(2) is metabolically depleted. Second, DAG-binding C1A-GFP domains, which are brought to the cell surface by exogenous ligands, translocate rapidly back to the cytoplasm in response to Ca influx. Nevertheless, inhibitors of PLCs and cPLA2, PI(4,5)P(2)-binding peptides, and PLD modification by butanol do not block membrane fusion. The cationic agents, FM 4-64 and heptalysine, bind profusely to the extracellular cell surface during membrane fusion. While this binding might reflect phosphatidylserine (PS) "scrambling" between monolayers, it is unaffected by a PS-binding protein, lactadherin, and by polylysine from the cytoplasmic side. Furthermore, the PS indicator, annexin-V, binds only slowly after fusion. Therefore, we suggest that the luminal surfaces of membrane vesicles that fuse to the plasmalemma may be rather anionic. In summary, our results provide no support for any regulatory or modulatory role of phospholipids in Ca-induced membrane fusion in fibroblasts.