Signaling microdomains define the specificity of receptor-mediated InsP(3) pathways in neurons

Visualization of Na,K-ATPase interacting proteins using FRET technique

Signaling transduction mediated by protein aggregates within specific microdomains has been receiving increased attention. We previously showed that Na,K-ATPase, partially inhibited by ouabain, induces intracellular calcium (Ca(2+)) oscillations which involve Ca(2+) release from the endoplasmic reticulum (ER). Plasma membrane bound Na,K-ATPase and proteins in the ER are in close proximity to each other, and signal transduction may occur via a physical interaction or a microdomain. To study these signaling pathways and intricate microenvironments, sophisticated methods are required. One way to detect molecular interactions in the nanometer scale (1-10 nm) is fluorescence resonance energy transfer (FRET). Thus, FRET provides vital insight into the action of Na,K-ATPase to trigger intracellular signaling events.

IRBIT, a novel inositol 1,4,5-trisphosphate (IP3) receptor-binding protein, is released from the IP3 receptor upon IP3 binding to the receptor

The inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)Rs) are IP(3)-gated Ca(2+) channels on intracellular Ca(2+) stores. Herein, we report a novel protein, termed IRBIT (IP(3)R binding protein released with inositol 1,4,5-trisphosphate), which interacts with type 1 IP(3)R (IP(3)R1) and was released upon IP(3) binding to IP(3)R1. IRBIT was purified from a high salt extract of crude rat brain microsomes with IP(3) elution using an affinity column with the huge immobilized N-terminal cytoplasmic region of IP(3)R1 (residues 1-2217). IRBIT, consisting of 530 amino acids, has a domain homologous to S-adenosylhomocysteine hydrolase in the C-terminal and in the N-terminal, a 104 amino acid appendage containing multiple potential phosphorylation sites. In vitro binding experiments showed the N-terminal region of IRBIT to be essential for interaction, and the IRBIT binding region of IP(3)R1 was mapped to the IP(3) binding core. IP(3) dissociated IRBIT from IP(3)R1 with an EC(50) of approximately 0.5 microm, i.e. it was 50 times more potent than other inositol polyphosphates. Moreover, alkaline phosphatase treatment abolished the interaction, suggesting that the interaction was dualistically regulated by IP(3) and phosphorylation. Immunohistochemical studies and co-immunoprecipitation assays showed the relevance of the interaction in a physiological context. These results suggest that IRBIT is released from activated IP(3)R, raising the possibility that IRBIT acts as a signaling molecule downstream from IP(3)R.

Signalling specificity in GPCR-dependent Ca2+ signalling

Cells use signalling networks to translate with high fidelity extracellular signals into specific cellular functions. Signalling networks are often composed of multiple signalling pathways that act in concert to regulate a particular cellular function. In the centre of the networks are the receptors that receive and transduce the signals. A versatile family of receptors that detect a remarkable variety of signals are the G protein-coupled receptors (GPCRs). Virtually all cells express several GPCRs that use the same biochemical machinery to transduce their signals. Considering the specificity and fidelity of signal transduction, a central question in cell signalling is how signalling specificity is achieved, in particular among GPCRs that use the same biochemical machinery. Ca(2+) signalling is particularly suitable to address such questions, since [Ca(2+)](i) can be recorded with excellent spatial and temporal resolutions in living cells and tissues and now in living animals. Ca(2+) is a unique second messenger in that both biochemical and biophysical components form the Ca(2+) signalling complex to regulate its concentration. Both components act in concert to generate repetitive [Ca(2+)](i) oscillations that can be either localized or in the form of global, propagating Ca(2+) waves. Most of the key proteins that form Ca(2+) signalling complexes are known and their activities are reasonably well understood on the biochemical and biophysical levels. We review here the information gained from studying Ca(2+) signalling by GPCRs to gain further understanding of the mechanisms used to generate cellular signalling specificity.

Expression and function of bradykinin receptors in microglia

Expression of bradykinin (BK) receptors and their cellular function were investigated in microglia. Microglial cells were isolated from mixed cultures of cerebrocortical cells from postnatal day 3 Wistar rats. Reverse transcription-PCR (RT-PCR) showed that rat primary microglia express mRNAs for the type 2 bradykinin (B(2)) receptor subtype but not the type 1 (B(1)) receptor subtype under our experimental condition. However, the expression of B(1) receptor was greatly up-regulated after the treatment of microglia with BK for 24 hours. The expression of B(2) receptor in microglia was further confirmed by immunocytochemistry. Membrane currents were measured using whole-cell recording under voltage-clamp conditions. In 14% of patched cells (12/85 cells), BK (100-200 nM) induced an outward current at the holding potential of -20 mV, with oscillations in 2 cases. The BK-induced outward current was transient and desensitized rapidly. TEA inhibited the BK-induced outward current in a dose-dependent manner. These results suggest that microglia express B(2) receptors and presumably increase the intracellular Ca(2+) concentration via inositol trisphosphate with the subsequent activation of Ca(2+)-dependent K(+) channels. Our data provide the first evidence that microglia express functional BK receptors and support the idea that microglia play an important role in CNS inflammatory responses.

Lanthanides potentiate TRPC5 currents by an action at extracellular sites close to the pore mouth

Mammalian members of the classical transient receptor potential channel (TRPC) subfamily (TRPC1-7) are Ca(2+)-permeable cation channels involved in receptor-mediated increases in intracellular Ca(2+). Unlike most other TRP-related channels, which are inhibited by La(3+) and Gd(3+), currents through TRPC4 and TRPC5 are potentiated by La(3+). Because these differential effects of lanthanides on TRPC subtypes may be useful for clarifying the role of different TRPCs in native tissues, we characterized the potentiating effect in detail and localized the molecular determinants of potentiation by mutagenesis. Whole cell currents through TRPC5 were reversibly potentiated by micromolar concentrations of La(3+) or Gd(3+), whereas millimolar concentrations were inhibitory. By comparison, TRPC6 was blocked to a similar extent by La(3+) or Gd(3+) at micromolar concentrations and showed no potentiation. Dual effects of lanthanides on TRPC5 were also observed in outside-out patches. Even at micromolar concentrations, the single channel conductance was reduced by La(3+), but reduction in conductance was accompanied by a dramatic increase in channel open probability, leading to larger integral currents. Neutralization of the negatively charged amino acids Glu(543) and Glu(595)/Glu(598), situated close to the extracellular mouth of the channel pore, resulted in a loss of potentiation, and, for Glu(595)/Glu(598) in a modification of channel inhibition. We conclude that in the micromolar range, the lanthanide ions La(3+) and Gd(3+) have opposite effects on whole cell currents through TRPC5 and TRPC6 channels. The potentiation of TRPC4 and TRPC5 by micromolar La(3+) at extracellular sites close to the pore mouth is a promising tool for identifying the involvement of these isoforms in receptor-operated cation conductances of native cells.

Group I metabotropic glutamate receptors activate a calcium-sensitive transient receptor potential-like conductance in rat hippocampus

In CA3 pyramidal neurons from organotypic slice cultures, activation of G(q)-coupled group I metabotropic glutamate receptors (mGluRs) induces a non-selective cationic conductance that enhances excitability. We have found that this response shares several properties with conductances that are mediated by the transient receptor potential (TRP) family of ion channels, including inhibition by La(3+), 2-aminoethoxydiphenylborane (2APB), cis-N-(2-phenylcyclopentyl)azacyclotridec-1-en-2-amine (MDL 12,330A) and a doubly rectifying current-voltage relationship. Stimulation of mGluR1 and mGluR5 converged to activate the TRP-like conductance in a synergistic manner, and activation of either subtype alone produced only a fraction of the normal response. Activation of the cationic current required elevated intracellular Ca(2+). Chelating intracellular Ca(2+) or blocking Ca(2+) entry through voltage-gated Ca(2+) channels attenuated responses to the activation of mGluRs. Conversely, raising intracellular Ca(2+) potentiated mGluR activation of the TRP-like conductance. Under control conditions, blocking G protein activation using intracellular GDPbetaS with or without N-(2, 6-dimethylphenylcarbamoylmethyl) triethylammonium chloride (QX-314) prevented mGluR-mediated activation of the TRP-like conductance. Following G protein blockade, however, the coupling between mGluRs 1 and/or 5 and the TRP-like conductance was rescued by increasing intracellular Ca(2+). This suggests that a G protein-independent signalling pathway is also activated by group I mGluRs. Such a pathway may represent an alternative transduction mechanism to maintain metabotropic responses under conditions where G proteins are functionally uncoupled from their cognate receptors.

Association of phospholamban with a cGMP kinase signaling complex

The cGMP kinase signaling complex identified previously in tracheal smooth muscle membranes contains a number of cGMP kinase substrates termed G0 through G4. G0, G1, and G2 were identified as IP(3) receptor I (IP(3)RI), IRAG, and cGMP kinase I. Sequencing of purified G3 and G4 showed that these proteins were proteolytic cleavage products of IRAG. However, the purified cGMP kinase signaling complex contained following additional proteins: alpha-actin, calponin H1, and phospholamban (PLB) as verified by MALDI-TOF as well as MS/MS sequencing and immune detection. The complex of these six proteins was immune precipitated by antibodies to each protein. The proteins were phosphorylated by the endogenous cGMP kinase I with the exception of alpha-actin and calponin H1. The complex did not contain the Ca(2+)-ATPase SERCA II. PLB, IP(3)RI, and cGMP kinase Ibeta were co-immune precipitated after expression in COS-7 cells. These results suggest that PLB may have additional functions to regulate the activity of SERCA II.

Regulatory pathways in lacrimal gland epithelium

Tears are a complex fluid that continuously cover the exposed surface of the eye, namely the cornea and conjunctiva. Tears are secreted in response to the multitude of environmental stresses that can harm the ocular surface such as cold, mechanical stimulation, physical injury, noxious chemicals, as well as infections from various organisms. Tears also provide nutrients and remove waste from cells of the ocular surface. Because of the varied function of tears, tears are complex and are secreted by several different tissues. Tear secretion is under tight neural control allowing tears to respond rapidly to changing environmental conditions. The lacrimal gland is the main contributor to the aqueous portion of the tear film and the regulation of secretion from this gland has been well studied. Despite multiple redundencies in pathways to stimulate secretion from the lacrimal gland, defects can occur resulting in dry eye syndromes. These diseases can have deleterious effects on vision. In this review, we summarize the latest information regarding the regulatory pathways, which control secretion from the lacrimal gland, and their roles in the pathogenesis of dry eye syndromes.

Selective association of TRPC channel subunits in rat brain synaptosomes

TRPC genes encode a ubiquitous family of ion channel proteins responsible for Ca(2+) influx following stimulation of G-protein-coupled membrane receptors linked to phospholipase C. These channels may be localized to large multimeric signaling complexes via association with PDZ-containing scaffolding proteins. Based on sequence homology, the TRPC channel family can be divided into two major subgroups: TRPC1, -C4, and -C5 and TRPC3, -C6, and -C7. Although TRPC channels are thought to be tetramers, the actual subunit composition remains unknown. To determine subunit arrangement, individual TRPC channel pairs were heterologously expressed in Sf9 insect cells and immunoprecipitated using affinity-purified rabbit polyclonal antibodies specific for each channel subtype. Reciprocal co-immunoprecipitations showed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate but that cross-association between the two major subgroups does not occur. Additionally, the interaction between each TRPC channel and the PDZ-containing protein, INAD (protein responsible for the inactivation-no-after-potential Drosophila mutant), was examined. TRPC1, -C4, and -C5 co-immunoprecipitated with INAD, whereas TRPC3, -C6, and -C7 did not. To define channel subunit interactions in vivo, immunoprecipitations were performed from isolated rat brain synaptosomal preparations. The results revealed that TRPC1, -C4, and -C5 co-associate and that TRPC3, -C6, and -C7 co-associate in both cortex and cerebellum but that cross-association between the two major subgroups does not occur. These results demonstrate that TRPC channels are present in nerve terminals and provide the first direct evidence for selective assembly of channel subunits in vivo.

Junctional signaling microdomains: bridging the gap between the neuronal cell surface and Ca2+ stores

Growing evidence suggests that plasma membranes are locally differentiated into microdomains that are important interaction sites for organization of signaling molecules. These signaling microdomains create local conditions that enhance molecular interactions, excluding others, thereby ensuring speed, spatial localization, and specificity of signal transduction. With the special emphasis on InsP(3) and Ca(2+) signaling pathways, we will discuss here the evolving concept of signaling microdomains that provide a key framework for understanding the differential regulation of many cellular target proteins.

Controlling Calcium Entry

Ca(2+) enters cells through an assortment of Ca(2+)-permeable channels that respond to different stimuli and couple to different cellular responses. Several different Ca(2+) entry pathways can be activated by receptors that stimulate phospholipase C (PLC). Both limbs of this signaling pathway (IP(3) and diacylglycerol), PLC itself, and its substrate (PIP(2)) contribute to the coordinate regulation of these Ca(2+) entry pathways.

Phospholipase C-gamma is required for agonist-induced Ca2+ entry

We report here that PLC-gamma isoforms are required for agonist-induced Ca2+ entry (ACE). Overexpressed wild-type PLC-gamma1 or a lipase-inactive mutant PLC-gamma1 each augmented ACE in PC12 cells, while a deletion mutant lacking the region containing the SH3 domain of PLC-gamma1 was ineffective. RNA interference to deplete either PLC-gamma1 or PLC-gamma2 in PC12 and A7r5 cells inhibited ACE. In DT40 B lymphocytes expressing only PLC-gamma2, overexpressed muscarinic M5 receptors (M5R) activated ACE. Using DT40 PLC-gamma2 knockout cells, M5R stimulation of ER Ca2+ store release was unaffected, but ACE was abolished. Normal ACE was restored by transient expression of PLC-gamma2 or a lipase-inactive PLC-gamma2 mutant. The results indicate a lipase-independent role of PLC-gamma in the physiological agonist-induced activation of Ca2+ entry.

Group I metabotropic glutamate receptor signaling via Galpha q/Galpha 11 secures the induction of long-term potentiation in the hippocampal area CA1

Heterotromeric G-proteins of the Gq family are thought to transduce signals from group I metabotropic glutamate receptors (mGluRs) in central neurons. We investigated roles of this cascade in hippocampal long-term potentiation (LTP) by using null-mutant mice lacking the alpha subunit of Gq (Galphaq) or G11 (Galpha11). We found no obvious abnormalities in the morphology, layer structure, expression of NMDA receptors, and basic parameters of excitatory synaptic transmission in the hippocampus of Galphaq mutant mice. We used theta burst stimulation (TBS) (3-10 burst trains at 5 Hz; each train consisted of five stimuli at 100 Hz) to induce LTP at Schaffer collateral to CA1 pyramidal cell synapses. Conventional TBS with 10 burst trains induced robust LTP in wild-type, Galphaq mutant, and Galpha11 mutant mice. Weak TBS with three burst trains consistently induced LTP in wild-type mice. In contrast, the same weak TBS was insufficient to induce LTP in Galphaq and Galpha11 mutant mice. In wild-type mice, the LTP by weak TBS was abolished by inhibiting group I mGluR or protein kinase C (PKC) but not by blocking muscarinic acetylcholine receptors. Prior activation of group I mGluR by an agonist significantly enhanced the LTP by weak TBS in wild-type mice. However, this priming effect was absent in Galphaq mutant mice. These results indicate that the signaling from group I mGluR to PKC involving Galphaq/Galpha11 does not constitute the main pathway for LTP, but it secures LTP induction by lowering its threshold in the hippocampal area CA1.

Recovery from muscarinic modulation of M current channels requires phosphatidylinositol 4,5-bisphosphate synthesis

Suppression of M current channels by muscarinic receptors enhances neuronal excitability. Little is known about the molecular mechanism of this inhibition except the requirement for a specific G protein and the involvement of an unidentified diffusible second messenger. We demonstrate here that intracellular ATP is required for recovery of KCNQ2/KCNQ3 current from muscarinic suppression, with an EC(50) of approximately 0.5 mM. Substitution of nonhydrolyzable ATP analogs for ATP slowed or prevented recovery. ADPbetaS but not ADP also prevented the recovery. Receptor-mediated inhibition was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP(2)) was blocked by lipid kinase inhibitors. Lipid phosphorylation by PI 4-kinase is required for recovery from muscarinic modulation of M current.

Signaling microdomains: InsP(3) receptor localization takes on new meaning

A fundamental question in cell biology is how different receptor-mediated signaling cascades, despite utilizing many of the same intracellular components, can generate specific cellular responses. Delmas and colleagues (in this issue of Neuron) address this question in relation to the muscarinic acetylcholine receptor (M(1)AchR) and the B(2) bradykinin receptor (B(2)R). Using Trp channel isoforms as biosensors for PLC stimulation in response to agonist activation, they demonstrate a role for signaling microdomains in the induction of such selective responses.