Glycobiology of ion transport in the nervous system.

The nervous system is richly endowed with large transmembrane proteins that mediate ion transport, including gated ion channels as well as energy-consuming pumps and transporters. Transport proteins undergo N-linked glycosylation which can affect expression, location, stability, and function. The N-linked glycans of ion channels are large, contributing between 5 and 50 % of their molecular weight. Many contain a high density of negatively charged sialic acid residues which modulate voltage-dependent gating of ion channels. Changes in the size and chemical composition of glycans are responsible for developmental and cell-specific variability in the biophysical and functional properties of many ion channels. Glycolipids, principally gangliosides, exert considerable influence on some forms of ion transport, either through direct association with ion transport proteins or indirectly through association with proteins that activate transport through appropriate signaling. Examples of both pumps and ion channels have been revealed which depend on ganglioside regulation. While some of these processes are localized in the plasma membrane, ganglioside-regulated ion transport can also occur at various loci within the cell including the nucleus. This chapter will describe ion channel and ion pump structures with a focus on the functional effects of glycosylation on ion channel availability and function, and effects of alterations in glycosylation on nervous system function. It will also summarize highlights of the research on glycolipid/ganglioside-mediated regulation of ion transport.

Ablation of phospholamban and sarcolipin results in cardiac hypertrophy and decreased cardiac contractility.

AIMS: Improving the sarco(endo)plasmic reticulum (SR) Ca(2+)-ATPase (SERCA) function has clinical implications in treating heart failure. The present study aimed to determine the effect of constitutive activation of the SERCA pump on cardiac contractility in normal mice and during pressure-overload-induced cardiac hypertrophy. METHODS AND RESULTS: The SERCA pump was constitutively activated in both atrial and ventricular chambers of the mouse heart by ablating its key regulators, phospholamban (PLN) and sarcolipin (SLN). The double-knockout (dKO) mice for PLN and SLN showed increased SERCA pump activity, Ca(2+) transients and SR Ca(2+) load, and developed cardiac hypertrophy. Echocardiographic measurements showed that the basal cardiac function was not affected in the young dKO mice. However, the cardiac function worsened upon ageing and when subjected to pressure overload. CONCLUSION: Our studies suggest that the constitutive activation of the SERCA pump is detrimental to cardiac function. Our findings also emphasize the need for dynamic regulation of the SERCA pump by PLN and/or SLN to maintain cardiac contractility in normal conditions and during pathophysiological states.

Exchange protein directly activated by cyclic AMP increases melanoma cell migration by a Ca2+-dependent mechanism.

Melanoma has a poor prognosis due to its strong metastatic ability. Although Ca(2+) plays a major role in cell migration, little is known about the role of Ca(2+) in melanoma cell migration. We recently found that the exchange protein directly activated by cyclic AMP (Epac) increases melanoma cell migration via a heparan sulfate-related mechanism. In addition to this mechanism, we also found that Epac regulates melanoma cell migration by a Ca(2+)-dependent mechanism. An Epac agonist increased Ca(2+) in several different melanoma cell lines but not in melanocytes. Ablation of Epac1 with short hairpin RNA inhibited the Epac agonist-induced Ca(2+) elevation, suggesting the critical role of Epac1 in Ca(2+) homeostasis in melanoma cells. Epac-induced Ca(2+) elevation was negated by the inhibition of phospholipase C (PLC) and inositol triphosphate (IP(3)) receptor. Furthermore, Epac-induced cell migration was reduced by the inhibition of PLC or IP(3) receptor. These data suggest that Epac activates Ca(2+) release from the endoplasmic reticulum via the PLC/IP(3) receptor pathway, and this Ca(2+) elevation is involved in Epac-induced cell migration. Actin assembly was increased by Epac-induced Ca(2+), suggesting the involvement of actin in Epac-induced cell migration. In human melanoma specimens, mRNA expression of Epac1 was higher in metastatic melanoma than in primary melanoma, suggesting a role for Epac1 in melanoma metastasis. In conclusion, our findings reveal that Epac is a potential target for the suppression of melanoma cell migration, and, thus, the development of metastasis.

Brain-derived neurotrophic factor facilitates maturation of mesenchymal stem cell-derived dopamine progenitors to functional neurons.

The generation of dopamine (DA) neurons from stem cells holds great promise in the treatment of Parkinson's disease and other neural disease associated with dysfunction of DA neurons. Mesenchymal stem cells (MSCs) derived from the adult bone marrow show plasticity with regards to generating cells of other germ layers. In addition to reduced ethical concerns, MSCs could be transplanted across allogeneic barriers, making them desirable stem cells for clinical applications. We have reported on the generation of DA cells from human MSCs using sonic hedgehog (SHH), fibroblast growth factor 8 and basic fibroblast growth factor. Despite the secretion of DA, the cells did not show evidence of functional neurons, and were therefore designated DA progenitors. Here, we report on the role of brain-derived neurotrophic factor (BDNF) in the maturation of the MSC-derived DA progenitors. 9-day induced MSCs show significant tropomyosin-receptor-kinase B expression, which correlate with its ligand, BDNF, being able to induce functional maturation. The latter was based on Ca2+ imaging analyses and electrophysiology. BDNF-treated cells showed the following: increases in intracellular Ca2+ upon depolarization and after stimulation with the neurotransmitters acetylcholine and GABA and, post-synaptic currents by electrophysiological analyses. In addition, BDNF induced increased DA release upon depolarization. Taken together, these results demonstrate the crucial role for BDNF in the functional maturation of MSC-derived DA progenitors.

Reciprocal amplification of ROS and Ca(2+) signals in stressed mdx dystrophic skeletal muscle fibers.

Muscular dystrophies are among the most severe inherited muscle diseases. The genetic defect is a mutation in the gene for dystrophin, a cytoskeletal protein which protects muscle cells from mechanical damage. Mechanical stress, applied as osmotic shock, elicits an abnormal surge of Ca(2+) spark-like events in skeletal muscle fibers from dystrophin deficient (mdx) mice. Previous studies suggested a link between changes in the intracellular redox environment and appearance of Ca(2+) sparks in normal mammalian skeletal muscle. Here, we tested whether the exaggerated Ca(2+) responses in mdx fibers are related to oxidative stress. Localized intracellular and mitochondrial Ca(2+) transients, as well as ROS production, were assessed with confocal microscopy. The rate of basal cellular but not mitochondrial ROS generation was significantly higher in mdx cells. This difference was abolished by pre-incubation of mdx fibers with an inhibitor of NAD(P)H oxidase. In addition, immunoblotting showed a significantly stronger expression of NAD(P)H oxidase in mdx muscle, suggesting a major contribution of this enzyme to oxidative stress in mdx fibers. Osmotic shock produced an abnormal and persistent Ca(2+) spark activity, which was suppressed by ROS-reducing agents and by inhibitors of NAD(P)H oxidase. These Ca(2+) signals resulted in mitochondrial Ca(2+) accumulation in mdx fibers and an additional boost in cellular and mitochondrial ROS production. Taken together, our results indicate that the excessive ROS production and the simultaneous activation of abnormal Ca(2+) signals amplify each other, finally culminating in a vicious cycle of damaging events, which may contribute to the abnormal stress sensitivity in dystrophic skeletal muscle.

TRPC5 channels undergo changes in gating properties during the activation-deactivation cycle.

TRPC5 are non-specific cation channels activated through phospholipase C-dependent pathways, although the precise gating mechanism is unknown. TRPC5 current-voltage relationships (I-Vs) change systematically during the activation-deactivation cycle, shifting between outwardly rectifying and doubly rectifying shapes. Since several TRP family members exhibit voltage-dependent properties, we investigated whether the various I-V relationships were due to changes in gating. Using patch-clamp recordings of rat TRPC5 transfected HEK293 cells, we found that TRPC5 currents had distinct biophysical characteristics correlated with individual I-V shapes, a phenomenon we call 'phases.' At rest, channels were closed at most potentials, although strong depolarizations (>+80 mV) stimulated small outward currents (Phase 0). For 10-15 sec after activation, voltage steps evoked small inward and large outward currents with time- and voltage-dependent kinetics (Phase 1, outwardly-rectifying I-Vs). At maximal inward amplitude, currents were voltage-independent at all potentials (Phase 2, doubly-rectifying I-Vs owing to Mg2+ block). During desensitization (Phase 3), currents reverted to a Phase 1-like voltage-dependence. La3+ ions potentiated inward TRPC5 currents by promoting a reversible transition from Phase 3 to Phase 2. Single channel recordings revealed asymmetric conductance properties with values of approximately 40 pS at negative potentials and approximately 130 pS at >+60 mV. Mutation of D633, a cytoplasmic residue that mediates Mg2+ block, decreased channel activity at negative potentials during Phase 2. We conclude that TRPC5 gating properties can switch reversibly between voltage-dependent and voltage-independent states. The modulation of phase transitions by external agents such as La3+ and EBP50, a scaffolding protein, may constitute a novel mechanism for regulation of channel activity.

Reactive oxygen species contribute to Ca2+ signals produced by osmotic stress in mouse skeletal muscle fibres.

Ca(2+) sparks, localized elevations in cytosolic [Ca(2+)], are rarely detected in intact adult mammalian skeletal muscle under physiological conditions. However, they have been observed in permeabilized cells and in intact fibres subjected to stresses, such as osmotic shock and strenuous exercise. Our previous studies indicated that an excess in cellular reactive oxygen species (ROS) generation over the ROS scavenging capabilities could be one of the up-stream causes of Ca(2+) spark appearance in permeabilized muscle fibres. Here we tested whether the cytosolic ROS balance is compromised in intact skeletal muscle fibres that underwent osmotic shock and whether this misbalance contributes to unmasking Ca(2+) sparks. Spontaneous Ca(2+) sparks and the rate of ROS generation were assessed with single photon confocal microscopy and fluorescent indicators fluo-4, CM-H(2)DCFDA and MitoSOX Red. Osmotic shock produced spontaneous Ca(2+) sparks and a concomitant significant increase in ROS production. Preincubation of muscle cells with ROS scavengers (e.g. MnTBAP, Mn-cpx 3, TIRON) nearly eliminated Ca(2+) sparks. In addition, inhibitors of NAD(P)H oxidase (DPI and apocynin) significantly reduced ROS production and suppressed the appearance of Ca(2+) sparks. Taken together, the data suggest that ROS contribute to the abnormal Ca(2+) spark activity in mammalian skeletal muscle subjected to osmotic stress and also indicate that NAD(P)H oxidase is a possible source of ROS. We propose that ROS-dependent Ca(2+) sparks are an important component of adaptive/maladaptive muscle responses under various pathological conditions such as eccentric stretch, osmotic changes during ischaemia and reperfusion, and some muscle diseases.

Induction of calcium influx through TRPC5 channels by cross-linking of GM1 ganglioside associated with alpha5beta1 integrin initiates neurite outgrowth.

Previous studies demonstrated that cross-linking of GM1 ganglioside with multivalent ligands, such as B subunit of cholera toxin (CtxB), induced Ca2+ influx through an unidentified, voltage-independent channel in several cell types. Application of CtxB to undifferentiated NG108-15 cells resulted in outgrowth of axon-like neurites in a Ca2+ influx-dependent manner. In this study, we demonstrate that CtxB-induced Ca2+ influx is mediated by TRPC5 channels, naturally expressed in these cells and primary neurons. Both Ca2+ influx and neurite induction were blocked by TRPC5 small interfering RNA (siRNA). Pretreatment of NG108-15 cells with neuraminidase increased cell-surface GM1 and greatly enhanced the signal. GM1 was not directly associated with TRPC5 but rather with alpha5beta1 integrin, which opened the channel through a signaling sequence after cross-linking of the GM1/integrin complex. This cascade included autophosphorylation of focal adhesion kinase and subsequent activation of phospholipase Cgamma (PLCgamma) and phosphoinositide-3 kinase [PI(3)K]. Pharmacological blockers that inhibited tyrosine kinase, PLC, and PI(3)K suppressed both CtxB-induced Ca2+ influx and neurite outgrowth. These were also suppressed by SK&F96365, a nonspecific transient receptor potential channel blocker. Confocal immunocytochemistry revealed that GM1 cross-linking induced colocalization of GM1 with these signaling elements in sprouting regions of plasma membrane. In primary cerebellar granular neurons (CGNs), TRPC5 was detected at 2 d in vitro (2 DIV), a stage corresponding to CtxB-stimulated Ca2+ influx. Neurite outgrowth in CGNs, determined at 3 DIV, was accelerated by CtxB and suppressed by TRPC5 siRNA and the above blockers. The crucial role of GM1 was indicated with CGNs from ganglio-series null mice, in which growth of axons was significantly retarded.

New insights into the function and regulation of vitamin D target proteins.

Calbindin-D(28k) has been reported to be a facilitator of calcium diffusion and to protect against apoptotic cell death. Most recently, we found that the presence of calbindin-D(28k) results in reduced calcium influx through voltage-dependent L-type Ca(2+) channels and enhanced sensitivity of the channels to calcium dependent inactivation. Co-immunoprecipitation and GST pull down assays indicate that calbindin-D(28k) interacts with the C-terminus of the L-type calcium channel alpha(1c) subunit (Ca(v)1.2). This is the first report of the binding of calbindin to a calcium channel and provides new insight concerning mechanisms by which calbindin acts to modulate intracellular calcium. Besides calbindin, another major target of 1,25(OH)(2)D(3) is 24(OH)ase, which is involved in the catabolism of 1,25(OH)(2)D(3). We reported that C/EBPbeta is a major transcriptional activator of 24(OH)ase that cooperates with CBP/p300 in regulating VDR mediated 24(OH)ase transcription. Recently, we found, in addition to p160 coactivators, that SWI/SNF complexes (that facilitate transcription by remodeling chromatin using the energy of ATP hydrolysis) are also involved in VDR mediated 24(OH)ase transcription and functionally cooperate with C/EBPbeta in regulating 24(OH)ase. These findings define novel mechanisms that may be of fundamental importance in understanding how 1,25(OH)(2)D(3) mediates its multiple biological effects.

Calbindin-D28k decreases L-type calcium channel activity and modulates intracellular calcium homeostasis in response to K+ depolarization in a rat beta cell line RINr1046-38.

Calbindin-D(28k), acts as a modulator of depolarization induced calcium transients in the pancreatic beta cell. However, specific mechanisms have not been defined. Here we show for the first time that the calcium binding protein calbindin-D(28k) acts by affecting calcium influx through voltage-dependent calcium channels in RIN pancreatic beta cells. Whole-cell patch-clamp recordings revealed that Ca(2+) current amplitudes of calbindin-D(28k) expressing RINr1046-38 beta cells were smaller than the Ca(2+) current amplitudes in control cells in response to depolarizing pulses. The peak current was observed at +20mV and the average amplitude was approximately 50pA in the calbindin expressing cells compared to approximately 250pA in control cells. In calbindin-D(28k) expressing cells, the channels had enhanced sensitivity to Ca(2+) dependent inactivation and currents decayed much more rapidly than in control cells. The Ca(2+) channels affected by calbindin were found to have biophysical properties consistent with dihydropyridine-sensitive L-type calcium channels. In response to depolarizing concentrations of K(+), calbindin expression caused a five-fold decrease in the rate of rise of [Ca(2+)](i) and decay was slower in the calbindin expressing cells. Application of verapamil resulted in a drop in the [Ca(2+)](i) signal to pre-stimulation levels indicating that the Ca(2+) channel responsible for the depolarization evoked Ca(2+) entry, modulated by calbindin, is the L-type. Co-immunoprecipitation and GST pull-down assays indicate that calbindin-D(28k) can interact with the alpha(1) subunit of Ca(v)1.2. We thus conclude that calbindin-D(28k) can regulate calcium influx via L-type calcium channels. Our findings suggest a role for calbindin-D(28k) in the beta cell in modulating Ca(2+) influx via L-type voltage-dependent calcium channels.

A cytosolic residue mediates Mg2+ block and regulates inward current amplitude of a transient receptor potential channel.

Members of the transient receptor potential (TRP) cation channel family control a wide variety of cellular functions by regulating calcium influx. In neurons, TRP channels may also modulate cell excitability. TRPC5 is a neuronal TRP channel that plays a role in controlling neurite extension in the hippocampus. Transiently expressed TRPC5 exhibits a doubly rectifying current-voltage relationship characterized by relatively large inward currents and a unique outwardly rectifying current with a "flat" segment between +10 and +40 mV that may be attributable to Mg2+ block. We find that intracellular Mg2+ blocks the outward current through TRPC5 with an IC50 of 457 microM. The block is mediated by a cytosolic aspartate residue, D633, situated between the termination of the sixth transmembrane domain and the "TRP box." The substitution of noncharged or positively charged residues for the negatively charged D633 resulted in a channel with markedly reduced inward currents, in addition to decreased Mg2+ block. This suggests that electrostatic attraction of cations by D633 may contribute to inward current amplitude in TRPC5. We propose that cytosolic negatively charged residues can modulate the conductivity of TRP cation channels.

TRPC5 activation kinetics are modulated by the scaffolding protein ezrin/radixin/moesin-binding phosphoprotein-50 (EBP50).

TRPC1-7 proteins are members of a family of mammalian non-specific cation channels that mediate receptor-operated, phospholipase Cbeta/Cgamma dependent Ca(2+) influx in various cell types. TRPC4 and TRPC5 form a subfamily within TRPCs. Uniquely in the TRPC family, these channels possess a C-terminal "VTTRL" motif that binds to PDZ-domains of the scaffolding protein, EBP50 (NHERF1; Tang et al., J Biol Chem 275:37559-37564). The functional effects of EBP50 on TRPC4/5 activity have not been investigated. We have cloned rat TRPC5 (rTRPC5), functionally expressed it in HEK293 cell, and studied channel regulation with patch-clamp techniques. Both rTRPC5 and its VTTRL deletion mutant (r5dV) were localized to the plasma membrane. rTRPC5 did not display any significant basal activity in unstimulated HEK293 cells. In cells co-expressing rTRPC5 and H1 histamine receptor, rTRPC5 current evoked by GTPgammaS or histamine developed in two phases: a slowly developing, small inward current was followed by a rapidly developing, transient, large inward current. Each phase had a characteristic non-linear current-voltage (I-V) relationship. Deletion of the VTTRL motif had no detectable effect on the biophysical properties of the channel. Co-expression of EBP50 with rTRPC5 caused a significant delay in the time-to-peak of the histamine-evoked, transient large inward current. EBP50 did not modify the activation kinetics of the VTTRL-deletion mutant. We conclude that the VTTRL motif is not necessary for activation of TRPC5, but may mediate the modulatory effect of EBP50 on TRPC5 activation kinetics.

TRPC4 can be activated by G-protein-coupled receptors and provides sufficient Ca(2+) to trigger exocytosis in neuroendocrine cells.

Transient receptor potential (TRP) channels form a large family of plasma membrane cation channels. Mammalian members of the "short" TRP family (TRP channel (TRPC) 1-7 are Ca(2+)-permeant, non-selective cation channels that are widely expressed in various cell types, including neurons. TRPC activity is linked through unknown mechanisms to G-protein-coupled receptors or receptor tyrosine kinases that activate phospholipase C. To investigate the properties and function of TRPC4 in neuronally derived cells, we transiently expressed mouse TRPC4 and histamine H(1) receptor in mouse adrenal chromaffin cells and PC12 cells. Histamine, but not thapsigargin, stimulated Mn(2+) influx in transfected cells. In the whole-cell patch clamp mode, histamine triggered a transient current in TRPC4-expressing cells. No current was evoked by perfusion with inositol 1,4,5-trisphosphate. When exocytosis was monitored with the capacitance detection technique, the magnitude of the membrane capacitance increase (Delta C(m)) on application of histamine in H(1) receptor/TRPC4-expressing chromaffin cells was comparable with that triggered by a train of depolarizing pulses. Our results indicate that TRPC4 channels behave as receptor, but not store-operated, channels in neuronally derived cells. TRPC4 channels can provide sufficient Ca(2+) influx to trigger a robust secretory response in voltage-clamped neurosecretory cells. Similar mechanisms may modulate exocytosis in other neuronal systems.