Negative control of store-operated Ca2+ influx by B cell receptor cross-linking

Src-family tyrosine kinases and the Ca2+ signal

In this review, we shall describe the rich crosstalk between non-receptor Src-family kinases (SFKs) and the Ca²⁺ transient generated in activated cells by a variety of extracellular and intracellular stimuli, resulting in diverse signaling events. The exchange of information between SFKs and Ca²⁺ is reciprocal, as it flows in both directions. These kinases are main actors in pathways leading to the generation of the Ca²⁺ signal, and reciprocally, the Ca²⁺ signal modulates SFKs activity and functions. We will cover how SFKs participate in the generation of the cytosolic Ca²⁺ rise upon activation of a series of receptors and the mechanism of clearance of this Ca²⁺ signal. The role of SFKs modulating Ca²⁺-translocating channels participating in these events will be amply discussed. Finally, the role of the Ca²⁺ sensor protein calmodulin on the activity of c-Src, and potentially on other SFKs, will be outlined as well. This article is part of a Special Issue entitled: ECS Meeting edited by Claus Heizmann, Joachim Krebs and Jacques Haiech.

IP3 receptors: some lessons from DT40 cells

Inositol-1,4,5-trisphosphate receptors (IP3Rs) are intracellular Ca2+ channels that are regulated by IP3 and Ca2+ and are modulated by many additional signals. These properties allow them to initiate and, via Ca2+-induced Ca2+ release, regeneratively propagate Ca2+ signals evoked by receptors that stimulate formation of IP3. The ubiquitous expression of IP3R highlights their importance, but it also presents problems when attempting to resolve the behavior of defined IP3R. DT40 cells are a pre-B-lymphocyte cell line in which high rates of homologous recombination afford unrivalled opportunities to disrupt endogenous genes. DT40-knockout cells with both alleles of each of the three IP3R genes disrupted provide the only null-background for analysis of homogenous recombinant IP3R. We review the properties of DT40 cells and consider three areas where they have contributed to understanding IP3R behavior. Patch-clamp recording from the nuclear envelope and Ca2+ release from intracellular stores loaded with a low-affinity Ca2+ indicator address the mechanisms leading to activation of IP(3)R. We show that IP3 causes intracellular IP3R to cluster and re-tune their responses to IP3 and Ca2+, better equipping them to mediate regenerative Ca2+ signals. Finally, we show that DT40 cells reliably count very few IP3R into the plasma membrane, where they mediate about half the Ca2+ entry evoked by the B-cell antigen receptor.

CD19 signaling is impaired in murine peritoneal and splenic B-1 B lymphocytes

B-1 cells reside predominantly within the coelomic cavities, tonsils, Peyer's patches, spleen (a minor fraction - approximately 5%) and are absent in the lymph nodes. They are the primary sources of natural IgM in the body. B-1 cells express polyreactive B cell receptors (BCRs) that cross react with self-antigens and are thus implicated in auto-immune disorders. Previously, we reported that peritoneal B-1 cells are deficient in CD19-mediated intracellular signals leading to Ca(2+) mobilization. Here, we find that splenic B-1 cells, like peritoneal B-1 cells, are defective in Ca(2+) release upon B cell activation by co-cross-linking BCR and CD19. In the absence of extracellular sources of Ca(2+), intracellular Ca(2+) flux is similar between B-1 and B-2 cells. Moreover, the intracellular component of Ca(2+) release in both subsets of B cells is mostly PI3K dependent. BCR and CD19 co-cross-linking activates Akt, a key mediator of survival and proliferation signals downstream of PI3K in splenic B-2 cells. Splenic B-1 cells, on the other hand, do not phosphorylate Akt (S473) upon similar treatment. Furthermore, BCR+CD19 cross-linking induced phosphorylation of JNK is much reduced in splenic B-1 cells. In contrast, B-1 cells exhibited increased levels of constitutively active pLyn which appears to have an inhibitory role. The CD19 induced Ca(2+) response and BCR induced proliferation response were restored by a partial inhibition of pLyn with Src kinase specific inhibitors. These findings suggest a defect in CD19-mediated signals in both peritoneal and splenic B-1 B lymphocytes, which is in part, due to higher levels of constitutively active Lyn.

Membrane-delimited regulation of novel background K+ channels by MgATP in murine immature B cells

In WEHI-231, a representative immature B cell line, Ca(2+) entry is paradoxically augmented by treatment with 2-aminoethoxydiphenyl borate (2-APB), a blocker of inositol 1,4,5-trisphosphate receptor and of nonselective cation channels (Nam, J. H., Yun, S. S., Kim, T. J., Uhm, D.-Y., and Kim, S. J. (2003) FEBS Lett. 535, 113-118). The initial goal of the present study was to elucidate the effects of 2-APB on membrane currents, which revealed the presence of novel K(+) channels in WEHI-231 cells. Under whole-cell patch clamp conditions, 2-APB induced background K(+) current (I(K,bg)) and hyperpolarization in WEHI-231 cells. Lowering of intracellular MgATP also induced the I(K,bg). The I(K,bg) was blocked by micromolar concentrations of quinidine but not by tetraethylammonium. In a single channel study, two types of voltage-independent K(+) channels were found with large (346 picosiemens) and medium conductance (112 picosiemens), named BK(bg) and MK(bg), respectively. The excision of membrane patches (inside-out (i-o) patches) greatly increased the P(o) of BK(bg). In i-o patches, cytoplasmic MgATP (IC(50) = 0.18 mm) decreased the BK(bg) activity, although non-hydrolyzable adenosine 5'-(beta,gamma-imino)triphosphate had no effect. A pretreatment with Al(3+) or wortmannin (50 microm) blocked the inhibitory effects of MgATP. A direct application of phosphoinositide 4,5-bisphosphate (10 microm) inhibited the BK(bg) activity. Meanwhile, the activity of MK(bg) was unaffected by MgATP. In cell-attached conditions, the BK(bg) activity was largely increased by 2-APB. In i-o patches, however, the MgATP-induced inhibition of BK(bg) was weakly reversed by the addition of 2-APB. In summary, WEHI-231 cells express the unique background K(+) channels. The BK(bg)s are inhibited by membrane-delimited elevation of phosphoinositide 4,5-bisphosphate. The activation of BK(bg) would hyperpolarize the membrane, which augments the calcium influx in WEHI-231 cells.

Calcium response after stimulation by Substance P of U373 MG cells: inhibition of store-operated calcium entry by protein kinase C

In this paper we investigate the Ca2+ response after Substance P (SP) stimulation of U373 MG cells. SP is a tachykinin and physiologically acts as a neurotransmitter and neuromodulator in the nervous system, but pathologically triggers malignant glial cells, such as U373 MG, to release cytokines and increase proliferation rate. In this paper we show that SP increases the proliferation rate of U373 MG cells and the intracellular Ca2+ concentration by mobilizing Ca2+ only from thapsigargin-sensitive stores. In fact, Ca2+ entry through store-operated calcium entry (SOCE) channels, which was observed after thapsigargin treatment, was not detected after stimulation by SP. The inhibition of SOCE after SP stimulation must be mediated by protein kinase C (PKC), because it was not observed in the presence of calphostin C (an inhibitor of PKC). Moreover, stimulation by SP-induced membrane potential hyperpolarization. Our results are consistent with the following sequence of events: (i) SP interacts with NK(1) receptors; (ii) fast homologous receptor desensitization occurs; (iii) reuptake by endoplasmic reticulum Ca(2+)-ATPase quantitatively overwhelms the extrusion by plasma membrane Ca2+-ATPase. These results have two important consequences. In U373 MG cells the SOCE does not contribute to the Ca2+ response after SP, and is not necessarily involved in promoting cell proliferation.

Role of phospholipase C-L2, a novel phospholipase C-like protein that lacks lipase activity, in B-cell receptor signaling

Phospholipase C (PLC) plays important roles in phosphoinositide turnover by regulating the calcium-protein kinase C signaling pathway. PLC-L2 is a novel PLC-like protein which lacks PLC activity, although it is very homologous with PLC delta. PLC-L2 is expressed in hematopoietic cells, but its physiological roles and intracellular functions in the immune system have not yet been clarified. To elucidate the physiological function of PLC-L2, we generated mice which had a genetic PLC-L2 deficiency. PLC-L2-deficient mice grew with no apparent abnormalities. However, mature B cells from PLC-L2-deficient mice were hyperproliferative in response to B-cell receptor (BCR) cross-linking, although B2 cell development appeared to be normal. Molecular biological analysis revealed that calcium influx and NFATc accumulation in nuclei were increased in PLC-L2-deficient B cells. Extracellular signal-regulated kinase activity was also enhanced in PLC-L2-deficient B cells. These mice had a stronger T-cell-independent antigen response. These results indicate that PLC-L2 is a novel negative regulator of BCR signaling and immune responses.

An inositol 1,4,5-trisphosphate receptor-dependent cation entry pathway in DT40 B lymphocytes

We examined the roles of inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) in calcium signaling using DT40 B lymphocytes, and a variant lacking the three IP3R isoforms (IP3R-KO). In wild-type cells, B cell receptor (BCR) stimulation activates a cation entry route that exhibits significantly greater permeability to Ba2+ than does capacitative calcium entry. This cation entry is absent in IP3R-KO cells. Expression of the type-3 IP3R (IP3R-3) in the IP3R-KO cells rescued not only agonist-dependent release of intracellular Ca2+, but also Ba2+ influx following receptor stimulation. Similar results were obtained with an IP3R-3 mutant carrying a conservative point mutation in the selectivity filter region of the channel (D2477E); however, an IP3R-3 mutant in which this same aspartate was replaced by alanine (D2477A) failed to restore either BCR-induced Ca2+ release or receptor-dependent Ba2+ entry. These results suggest that in DT40 B lymphocytes, BCR stimulation activates a novel cation entry across the plasma membrane that depends upon, or is mediated by, fully functional IP3R.

Transient receptor potential 1 regulates capacitative Ca(2+) entry and Ca(2+) release from endoplasmic reticulum in B lymphocytes

Capacitative Ca(2+) entry (CCE) activated by release/depletion of Ca(2+) from internal stores represents a major Ca(2+) influx mechanism in lymphocytes and other nonexcitable cells. Despite the importance of CCE in antigen-mediated lymphocyte activation, molecular components constituting this mechanism remain elusive. Here we demonstrate that genetic disruption of transient receptor potential (TRP)1 significantly attenuates both Ca(2+) release-activated Ca(2+) currents and inositol 1,4,5-trisphosphate (IP(3))-mediated Ca(2+) release from endoplasmic reticulum (ER) in DT40 B cells. As a consequence, B cell antigen receptor-mediated Ca(2+) oscillations and NF-AT activation are reduced in TRP1-deficient cells. Thus, our results suggest that CCE channels, whose formation involves TRP1 as an important component, modulate IP(3) receptor function, thereby enhancing functional coupling between the ER and plasma membrane in transduction of intracellular Ca(2+) signaling in B lymphocytes.