Fast endocytic recycling determines TRPC1-STIM1 clustering in ER-PM junctions and plasma membrane function of the channel.

Stromal interaction molecule 1 (STIM1) senses depletion of ER-Ca2+ store and clusters in ER-PM junctions where it associates with and gates Ca2+ influx channels, Orai1 and TRPC1. Clustering of TRPC1 with STIM1 and Orai1 in these junctions is critical since Orai1-mediated Ca2+ entry triggers surface expression of TRPC1 while STIM1 gates the channel. Thus, plasma membrane function of TRPC1 depends on the delivery of the channel to the sites where STIM1 puncta are formed. This study examines intracellular trafficking mechanism(s) that determine plasma membrane expression and function of TRPC1 in cells where Orai1 and TRPC1 are endogenously expressed and contribute to Ca2+ entry. We report that TRPC1 is internalized by Arf6-dependent pathway, sorted to Rab5-containing early endosomes, and trafficked to ER-PM junctions by Rab4-dependent fast recycling. Overexpression of Arf6, or Rab5, but not the respective dominant negative mutants, induced retention of TRPC1 in early endosomes and suppressed TRPC1 function. Notably, cells expressing Arf6 or Rab5 displayed an inwardly rectifying ICRAC current that is mediated by Orai1 instead of TRPC1-associated ISOC, demonstrating that Orai1 function was not altered. Importantly, expression of Rab4, but not STIM1, with Rab5 rescued surface expression and function of TRPC1, restoring generation of ISOC. Together, these data demonstrate that trafficking via fast recycling endosomes determines TRPC1-STIM1 clustering within ER-PM junctions following ER-Ca2+ store depletion which is critical for the surface expression and function of the channel. Ca2+ influx mediated by TRPC1 modifies Ca2+-dependent physiological response of cells.

Role of TRPC Channels in Store-Operated Calcium Entry.

Store-operated calcium entry (SOCE) is a ubiquitous Ca(2+) entry pathway that is activated in response to depletion of Ca(2+) stores within the endoplasmic reticulum (ER) and contributes to the control of various physiological functions in a wide variety of cell types. The transient receptor potential canonical (TRPC) channels (TRPCs 1-7), that are activated by stimuli leading to PIP2 hydrolysis, were first identified as molecular components of SOCE channels. TRPC channels show a miscellany of tissue expression, physiological functions and channel properties. However, none of the TRPC members display currents that resemble I CRAC. Intensive search for the CRAC channel component led to identification of Orai1 and STIM1, now established as being the primary constituents of the CRAC channel. There is now considerable evidence that STIM1 activates both Orai1 and TRPC1 via distinct domains in its C-terminus. Intriguingly, TRPC1 function is not only dependent on STIM1 but also requires Orai1. The critical functional interaction between TRPC1 and Orai1, which determines the activation of TRPC1, has also been identified. In this review, we will discuss current concepts regarding the role of TRPC channels in SOCE, the physiological functions regulated by TRPC-mediated SOCE, and the complex mechanisms underlying the regulation of TRPCs, including the functional interactions with Orai1 and STIM1.

TRPC1 contributes to the Ca2+-dependent regulation of adenylate cyclases.

SOCE (store-operated Ca2+ entry) is mediated via specific plasma membrane channels in response to ER (endoplasmic reticulum) Ca2+ store depletion. This route of Ca2+ entry is central to the dynamic interplay between Ca2+ and cAMP signalling in regulating the activity of Ca2+-sensitive adenylate cyclase isoforms (AC1, AC5, AC6 and AC8). Two proteins have been identified as key components of SOCE: STIM1 (stromal interaction molecule 1), which senses ER Ca2+ store content and translocates to the plasma membrane upon store depletion, where it then activates Orai1, the pore-forming component of the CRAC (Ca2+ release-activated Ca2+) channel. Previous studies reported that co-expression of STIM1 and Orai1 in HEK-293 (human embryonic kidney 293) cells enhances Ca2+-stimulated AC8 activity and that AC8 and Orai1 directly interact to enhance this regulation. Nonetheless, the additional involvement of TRPC (transient receptor potential canonical) channels in SOCE has also been proposed. In the present study, we evaluate the contribution of TRPC1 to SOCE-mediated regulation of Ca2+-sensitive ACs in HEK-293 cells stably expressing AC8 (HEK-AC8) and HSG (human submandibular gland) cells expressing an endogenous Ca2+-inhibited AC6. We demonstrate a role for TRPC1 as an integral component of SOCE, alongside STIM1 and Orai1, in regulating Ca2+ fluxes within AC microdomains and influencing cAMP production.

PIP2 and septin control STIM1/Orai1 assembly by regulating cytoskeletal remodeling via a CDC42-WASP/WAVE-ARP2/3 protein complex.

Store-operated calcium entry (SOCE) is triggered by assembly of Orai1 with STIM proteins in ER-PM junctions. Plasma membrane PIP2 as well as PIP2-binding protein, SEPT4, significantly impact Orai1-STIM1 interaction. While septins and PIP2 can organize the actin cytoskeleton, it is unclear whether the status of actin within the junctions contributes to SOCE. We report herein that actin remodeling modulates STIM1 clustering. Our findings show that a PIP2- and SEPT4-dependent mechanism involving CDC42, WASP/WAVE, and ARP2 regulates actin remodeling into a ring-like structure around STIM1 puncta. CDC42 localization in the ER-plasma membrane region is enhanced following ER-Ca2+ store depletion. PIP2 depletion or knockdown of SEPT4 attenuate the recruitment of CDC42 to the ER-PM region. Importantly, knockdown of SEPT4, or CDC42+ARP2, disrupts the organization of actin as well as STIM1 clustering. Consequently, Orai1 recruitment to STIM1 puncta, SOCE, and NFAT translocation to the nucleus are all attenuated. Ca2+ influx induced by STIM1-C terminus is not affected by CDC42 knockdown. In aggregate, our findings reveal that PIP2 and SEPT4 affect Orai1/STIM1 clustering by coordinating actin remodeling within ER-PM junctions. This dynamic reorganization of actin has an important role in regulation of SOCE and downstream Ca2+-dependent effector functions.

Cytotoxic effects of different concentrations of chlorhexidine.

PURPOSE: To evaluate the cytotoxic effects of different concentrations of Chlorhexidine (Chx) to the odontoblast cell line MDPC-23. METHODS: The odontoblast-like cells were seeded (30,000 cells/cm2) in 60 wells of 24-well dishes and then incubated in contact with the following experimental and control solutions: Group 1: 0.0024% Chx; Group 2: 0.004% Chx; Group 3: 0.02% Chx; Group 4: Phosphate buffer saline solution (PBS, negative control); and Group 5: 0.06% H2O2 (positive control). Cell metabolic activity was measured by MTT assay and the cell morphology was analyzed by SEM. RESULTS: The cytotoxic effects of Chx are dose-dependent. The reduction in the cell metabolism for Groups 1, 2, and 3 was 24.8%, 29.9% and 70.8%, respectively. No statistical difference was observed between the Groups 1 and 2 in which no significant cell morphology changes occurred. Consequently, it was concluded that 0.02% Chx solution presents high cytotoxicity to the odontoblast-like cells MDPC-23. On the other hand, 0.0024% and 0.004% Chx causes slight cytopathic effects to the cultured cells.

TRPC1, Orai1, and STIM1 in SOCE: Friends in tight spaces.

Store-operated calcium entry (SOCE) is a ubiquitous Ca2+ entry pathway that is activated in response to depletion of ER-Ca2+ stores and critically controls the regulation of physiological functions in miscellaneous cell types. The transient receptor potential canonical 1 (TRPC1) is the first member of the TRPC channel subfamily to be identified as a molecular component of SOCE. While TRPC1 has been shown to contribute to SOCE and regulate various functions in many cells, none of the reported TRPC1-mediated currents resembled ICRAC, the highly Ca2+-selective store-dependent current first identified in lymphocytes and mast cells. Almost a decade after the cloning of TRPC1 two proteins were identified as the primary components of the CRAC channel. The first, STIM1, is an ER-Ca2+ sensor protein involved in activating SOCE. The second, Orai1 is the pore-forming component of the CRAC channel. Co-expression of STIM1 and Orai1 generated robust ICRAC. Importantly, STIM1 was shown to also activate TRPC1 via its C-terminal polybasic domain, which is distinct from its Orai1-activating domain, SOAR. In addition, TRPC1 function critically depends on Orai1-mediated Ca2+ entry which triggers recruitment of TRPC1 into the plasma membrane where it is then activated by STIM1. TRPC1 and Orai1 form discrete STIM1-gated channels that generate distinct Ca2+ signals and regulate specific cellular functions. Surface expression of TRPC1 can be modulated by trafficking of the channel to and from the plasma membrane, resulting in changes to the phenotype of TRPC1-mediated current and [Ca2+]i signals. Thus, TRPC1 is activated downstream of Orai1 and modifies the initial [Ca2+]i signal generated by Orai1 following store depletion. This review will summarize the important findings that underlie the current concepts for activation and regulation of TRPC1, as well as its impact on cell function.

Radiation inhibits salivary gland function by promoting STIM1 cleavage by caspase-3 and loss of SOCE through a TRPM2-dependent pathway.

Store-operated Ca2+ entry (SOCE) is critical for salivary gland fluid secretion. We report that radiation treatment caused persistent salivary gland dysfunction by activating a TRPM2-dependent mitochondrial pathway, leading to caspase-3-mediated cleavage of stromal interaction molecule 1 (STIM1) and loss of SOCE. After irradiation, acinar cells from the submandibular glands of TRPM2+/+ , but not those from TRPM2-/- mice, displayed an increase in the concentrations of mitochondrial Ca2+ and reactive oxygen species, a decrease in mitochondrial membrane potential, and activation of caspase-3, which was associated with a sustained decrease in STIM1 abundance and attenuation of SOCE. In a salivary gland cell line, silencing the mitochondrial Ca2+ uniporter or caspase-3 or treatment with inhibitors of TRPM2 or caspase-3 prevented irradiation-induced loss of STIM1 and SOCE. Expression of exogenous STIM1 in the salivary glands of irradiated mice increased SOCE and fluid secretion. We suggest that targeting the mechanisms underlying the loss of STIM1 would be a potentially useful approach for preserving salivary gland function after radiation therapy.

STIM2 enhances receptor-stimulated Ca²âº signaling by promoting recruitment of STIM1 to the endoplasmic reticulum-plasma membrane junctions.

A central component of receptor-evoked Ca(2+) signaling is store-operated Ca(2+) entry (SOCE), which is activated by the assembly of STIM1-Orai1 channels in endoplasmic reticulum (ER) and plasma membrane (PM) (ER-PM) junctions in response to depletion of ER Ca(2+). We report that STIM2 enhances agonist-mediated activation of SOCE by promoting STIM1 clustering in ER-PM junctions at low stimulus intensities. Targeted deletion of STIM2 in mouse salivary glands diminished fluid secretion in vivo and SOCE activation in dispersed salivary acinar cells stimulated with low concentrations of muscarinic receptor agonists. STIM2 knockdown in human embryonic kidney (HEK) 293 cells diminished agonist-induced Ca(2+) signaling and nuclear translocation of NFAT (nuclear factor of activated T cells). STIM2 lacking five carboxyl-terminal amino acid residues did not promote formation of STIM1 puncta at low concentrations of agonist, whereas coexpression of STIM2 with STIM1 mutant lacking the polybasic region STIM1ΔK resulted in co-clustering of both proteins. Together, our findings suggest that STIM2 recruits STIM1 to ER-PM junctions at low stimulus intensities when ER Ca(2+) stores are mildly depleted, thus increasing the sensitivity of Ca(2+) signaling to agonists.

Trafficking mechanisms and regulation of TRPC channels.

TRPC channels are Ca(2+)-permeable cation channels which are regulated downstream from receptor-coupled PIP2 hydrolysis. These channels contribute to a wide variety of cellular functions. Loss or gain of channel function has been associated with dysfunction and aberrant physiology. TRPC channel functions are influenced by their physical and functional interactions with numerous proteins that determine their regulation, scaffolding, trafficking, as well as their effects on the downstream cellular processes. Such interactions also compartmentalize the Ca(2+) signals arising from TRPC channels. A large number of studies demonstrate that trafficking is a critical mode by which plasma membrane localization and surface expression of TRPC channels are regulated. This review will provide an overview of intracellular trafficking pathways as well as discuss the current state of knowledge regarding the mechanisms and components involved in trafficking of the seven members of the TRPC family (TRPC1-TRPC7).

Phospholipase D2: a pivotal player modulating RBL-2H3 mast cell structure.

The current study examined the role of PLD2 in the maintenance of mast cell structure. Phospholipase D (PLD) catalyzes hydrolysis of phosphatidylcholine to produce choline and phosphatidic acid (PA). PLD has two isoforms, PLD1 and PLD2, which vary in expression and localization depending on the cell type. The mast cell line RBL-2H3 was transfected to overexpress catalytically active (PLD2CA) and inactive (PLD2CI) forms of PLD2. The results of this study show that PLD2CI cells have a distinct star-shaped morphology, whereas PLD2CA and RBL-2H3 cells are spindle shaped. In PLD2CI cells, the Golgi complex was also disorganized with dilated cisternae, and more Golgi-associated vesicles were present as compared with the PLD2CA and RBL-2H3 cells. Treatment with exogenous PA led to the restoration of the wild-type Golgi complex phenotype in PLD2CI cells. Conversely, treatment of RBL-2H3 and PLD2CA cells with 1% 1-Butanol led to a disruption of the Golgi complex. The distribution of acidic compartments, including secretory granules and lysosomes, was also modified in PLD2CI cells, where they concentrated in the perinuclear region. These results suggest that the PA produced by PLD2 plays an important role in regulating cell morphology in mast cells.