The role of calcium in neuronal membrane tension and synaptic plasticity

Calcium is a primary second messenger that plays a role in cellular functions including growth, movement and responses to drugs. The role that calcium plays in mediating communication between neurons by synaptic vesicle release is well established. This review focuses on the dependence of the physical properties of neuronal plasma membranes on calcium levels. After describing the key features of synaptic plasticity, we summarize the general role of calcium in cell function and the signaling pathways responsible for intracellular increase in calcium levels. We then present findings showing that increases in intracellular calcium levels cause neurites to contract and break synaptic connections by changes in membrane tension.

Unraveling Hidden Cell Signaling Pathways Using Biophysical Methods: Application to the Gαq/Phospholipase Cβ Signaling System

The success of pharmaceutical therapies relies on how well cells respond to a particular drug, but accurately predicting responses can be difficult due to the complex and numerous potential molecular interactions that are possible in cells, and the responses of individuals can be variable due to cryptic and unexpected interactions. With the advancement of proteomics and fluorescence imaging methods, it is now possible to elucidate novel secondary signaling pathways and predict unexpected responses that might otherwise be missed, allowing for the development of better therapeutics. The Gαq/PLCβ signaling pathway is activated by agents that mediate allergic responses, neurotransmission, and heart rate, as well as other functions that are critical for survival. This Review describes the factors that must be considered in delineating signaling pathways and describes the novel translational role that we have uncovered for this signaling pathway.

Multiple functions of phospholipase Cβ1 at a glance

Phospholipase Cβ (PLCβ) is the main effector of the Gq family of heterotrimeric G proteins that transduces signals from hormones and neurotransmitters into Ca2+ signals. While PLCβ is critical for Ca2+ responses, recent studies have suggested that PLCβ has additional roles independent of its lipase activity. These novel functions are carried out by a cytosolic population of PLCβ that binds and inhibits the component 3 promoter of RNA-induced silencing complex (C3PO) to impact cytosolic RNA populations. Additionally, cytosolic PLCβ binds to stress granule proteins, keeping them dispersed and thus inhibiting stress granule formation. Upon activation of the Gα subunit of Gq (Gαq), cytosolic PLCβ relocalizes to the membrane, releasing C3PO and stress granule proteins, which in turn promotes activation of C3PO and RNA processing, as well as sequestration of specific transcripts into newly formed stress granules. As highlighted in this Cell Science at a Glance and the accompanying poster, the link between Gαq signaling, increased intracellular Ca2+ and changes in RNA processing impacts neuronal cell differentiation and may also affect neuronal development and dysfunction.

Deformation of caveolae impacts global transcription and translation processes through relocalization of cavin-1

Caveolae are invaginated membrane domains that provide mechanical strength to cells in addition to being focal points for the localization of signaling molecules. Caveolae are formed through the aggregation of caveolin-1 or -3 (Cav1/3), membrane proteins that assemble into multifunctional complexes with the help of caveola-associated protein cavin-1. In addition to its role in the formation of caveolae, cavin-1, also called polymerase I and transcript release factor, is further known to promote ribosomal RNA transcription in the nucleus. However, the mechanistic link between these functions is not clear. Here, we found that deforming caveolae by subjecting cells to mild osmotic stress (150–300 mOsm) changes levels of GAPDH, Hsp90, and Ras only when Cav1/cavin-1 levels are reduced, suggesting a link between caveola deformation and global protein expression. We show that this link may be due to relocalization of cavin-1 to the nucleus upon caveola deformation. Cavin-1 relocalization is also seen when Cav1-Gαq contacts change upon stimulation. Furthermore, Cav1 and cavin-1 levels have been shown to have profound effects on cytosolic RNA levels, which in turn impact the ability of cells to form stress granules and RNA-processing bodies (p-bodies) which sequester and degrade mRNAs, respectively. Our studies here using a cavin-1-knockout cell line indicate adaptive changes in cytosolic RNA levels but a reduced ability to form stress granules. Taken together, our findings suggest that caveolae, through release of cavin-1, communicate extracellular cues to the cell interior to impact transcriptional and translational.

IQGAP1 scaffolding links phosphoinositide kinases to cytoskeletal reorganization

IQGAP1 is a multidomain scaffold protein that coordinates the direction and impact of multiple signaling pathways by scaffolding its various binding partners. However, the spatial and temporal resolution of IQGAP1 scaffolding remains unclear. Here, we use fluorescence imaging and correlation methods that allow for real-time live-cell changes in IQGAP1 localization and complex formation during signaling. We find that IQGAP1 and PIPKIγ interact on both the plasma membrane and in cytosol. Epidermal growth factor (EGF) stimulation, which can initiate cytoskeletal changes, drives the movement of the cytosolic pool toward the plasma membrane to promote cytoskeletal changes. We also observe that a significant population of cytosolic IQGAP1-PIPKIγ complexes localize to early endosomes, and in some instances form aggregated clusters which become highly mobile upon EGF stimulation. Our imaging studies show that PIPKIγ and PI3K bind simultaneously to IQGAP1, which may accelerate conversion of PI4P to PI(3,4,5)P₃ that is required for cytoskeletal changes. Additionally, we find that IQGAP1 is responsible for PIPKIγ association with two proteins associated with cytoskeletal changes, talin and Cdc42, during EGF stimulation. These results directly show that IQGAP1 provides a physical link between phosphoinositides (through PIPKIγ), focal adhesion formation (through talin), and cytoskeletal reorganization (through Cdc42) upon EGF stimulation. Taken together, our results support the importance of IQGAP1 in regulating cell migration by linking phosphoinositide lipid signaling with cytoskeletal reorganization.

Stimulation of phospholipase Cβ1 by Gαq promotes the assembly of stress granule proteins

[Figure: see text].

Cell shape regulates subcellular organelle location to control early Ca2+ signal dynamics in vascular smooth muscle cells

The shape of the cell is connected to its function; however, we do not fully understand underlying mechanisms by which global shape regulates a cell's functional capabilities. Using theory, experiments and simulation, we investigated how physiologically relevant cell shape changes affect subcellular organization, and consequently intracellular signaling, to control information flow needed for phenotypic function. Vascular smooth muscle cells going from a proliferative and motile circular shape to a contractile fusiform shape show changes in the location of the sarcoplasmic reticulum, inter-organelle distances, and differential distribution of receptors in the plasma membrane. These factors together lead to the modulation of signals transduced by the M3 muscarinic receptor/Gq/PLCβ pathway at the plasma membrane, amplifying Ca2+ dynamics in the cytoplasm, and the nucleus resulting in phenotypic changes, as determined by increased activity of myosin light chain kinase in the cytoplasm and enhanced nuclear localization of the transcription factor NFAT. Taken together, our observations show a systems level phenomenon whereby global cell shape affects subcellular organization to modulate signaling that enables phenotypic changes.

Stimulation of the Gαq/phospholipase Cβ1 signaling pathway returns differentiated cells to a stem-like state

Phospholipase Cβ1 is activated by Gαq to generate calcium signals in response to hormones and neurotransmitters. Besides carrying out this plasma membrane function, PLCβ1 has a cytosolic population that helps to drive the differentiation of PC12 cells by inhibiting a nuclease that promotes RNA-induced silencing (C3PO). Here, we show that down-regulating PLCβ1 or reducing its cytosolic population by activating Gαq to localize it to the plasma membrane returns differentiated PC12 and SK-N-SH cells to an undifferentiated state. In this state, PC12 cells have a spherical morphology, resume proliferation, and express the stem cell transcription factors nanog and Oct4. Similar changes are seen when C3PO is down-regulated. This return to a stem-like state is accompanied by shifts in multiple miR populations. Surprisingly, de-differentiation can be induced by extended stimulation of Gαq where cells return to a spherical morphology and levels of specific miRs return to their undifferentiated values. In complementary studies, we followed the real-time hydrolysis of a fluorescent-tagged miR in cells where PLCβ1 or C3PO were down-regulated in PC12 cells and find substantial differences in miR processing in the undifferentiated and differentiated states. Taken together, our studies suggest that PLCβ1, through its ability to regulate C3PO and endogenous miR populations, mediates the differentiation of two types of cultured neuronal cells.

The Gαq/phospholipase Cβ signaling system represses tau aggregation

Alzheimer's disease is typified by calcium dysfunction and neurofibrillary tangles of tau aggregates along with mitotic proteins. Using PC12 cells as a model system, we determined whether the Gαq/PLCβ/ calcium signaling pathway impacts the manifestation of Alzheimer's disease. Down-regulating PLCβ significantly increases tau protein expression and causes a large increase in tau aggregation. Stimulating Gαq to activate PLCβ results in a modest reduction in tau aggregation while inhibiting PLCβ activity results in a modest enhancement of tau aggregation. These results suggest that PLCβ may effect tau aggregation by an additional mechanism that is independent of its ability to transduce calcium signals. To this end, we found that a cytosolic population of PLCβ binds to a mitotic protein found in neurofibrillary tangles, CDK18, which promotes tau phosphorylation and aggregation. Taken together, our studies show that the loss of PLCβ1 can promote Alzheimer's disease by a combination of its catalytic activity and its interaction with mitotic proteins thus offering an orthogonal method to control tau aggregation.

Re-track: Software to analyze the retraction and protrusion velocities of neurites, filopodia and other structures

We have developed new software, Re-track, that will quantify the rates of retraction and protrusion of structures emanating from the central core of a cell, such as neurites or filopodia. Re-Track, uses time-lapse images of cells in TIFF format and calculates the velocity of retraction or protrusion of a selected structure. The software uses a flexible moving boundary and has the ability to correct this boundary throughout analysis. Re-Track is fast, platform independent, and user friendly, and it can be used to follow biological events such as changes in neuronal connections, tip-growing cells such as moss, adaptive migration of cells, and similar behavior in non-biological systems.

Gαq-mediated calcium dynamics and membrane tension modulate neurite plasticity

The formation and disruption of synaptic connections during development are a fundamental step in neural circuit formation. Subneuronal structures such as neurites are known to be sensitive to the level of spontaneous neuronal activity, but the specifics of how neurotransmitter-induced calcium activity regulates neurite homeostasis are not yet fully understood. In response to stimulation by neurotransmitters such as acetylcholine, calcium responses in cells are mediated by the Gαq/phospholipase Cβ (PLCβ)/phosphatidylinositol 4,5-bisphosphate (PI(4,5)P2) signaling pathway. Here, we show that prolonged Gαq stimulation results in the retraction of neurites in PC12 cells and the rupture of neuronal synapses by modulating membrane tension. To understand the underlying cause, we dissected the behavior of individual components of the Gαq/PLCβ/PI(4,5)P2 pathway during retraction and correlated these with the retraction of the membrane and cytoskeletal elements impacted by calcium signaling. We developed a mathematical model that combines biochemical signaling with membrane tension and cytoskeletal mechanics to show how signaling events are coupled to retraction velocity, membrane tension, and actin dynamics. The coupling between calcium and neurite retraction is shown to be operative in the Caenorhabditis elegans nervous system. This study uncovers a novel mechanochemical connection between Gαq/PLCβ /PI(4,5)P2 that couples calcium responses with neural plasticity.

Regulation of bifunctional proteins in cells: Lessons from the phospholipase Cβ/G protein pathway

Some proteins can serve multiple functions depending on different cellular conditions. An example of a bifunctional protein is inositide-specific mammalian phospholipase Cβ (PLCβ). PLCβ is activated by G proteins in response to hormones and neurotransmitters to increase intracellular calcium. Recently, alternate cellular function(s) of PLCβ have become uncovered. However, the conditions that allow these different functions to be operative are unclear. Like many mammalian proteins, PLCβ has a conserved catalytic core along with several regulatory domains. These domains modulate the intensity and duration of calcium signals in response to external sensory information, and allow this enzyme to inhibit protein translation in a noncatalytic manner. In this review, we first describe PLCβ's cellular functions and regulation of the switching between these functions, and then discuss the thermodynamic considerations that offer insight into how cells manage multiple and competitive associations allowing them to rapidly shift between functional states.

The role of phospholipase Cβ on the plasma membrane and in the cytosol: How modular domains enable novel functions

Phospholipase Cβ (PLCβ) is a signaling enzyme activated by G proteins to generate calcium signals. The catalytic core of PLCβ is surrounded by modular domains that mediate the interaction of the enzyme with known protein partners on the plasma membrane. The C-terminal region PLCβ contains a novel coiled-coil domain that is required for Gαq binding and activation. Recent work has shown that this domain also binds a number of cytosolic proteins that regulate protein translation, and that these proteins compete with Gαq for PLCβ binding. The ability of PLCβ to shuttle between the cytosol to impact protein translation and the plasma membrane to mediate calcium signals puts PLCβ in a central role in cell function.

Phospholipase Cβ1 regulates proliferation of neuronal cells

Cells have developed lineage-specific mechanisms to control proliferation and drive morphologic changes upon differentiation. A hallmark of differentiation is the assembly of signaling molecules that transduce extracellular signals, such as the production of the G protein-regulated enzyme phospholipase Cβ (PLCβ), which generates calcium signals from sensory stimuli. We found that in most cancerous cell lines there is positive correlation between PLCβ1 levels and cell proliferation. In cells of neuronal lineage, however, reducing PLCβ1 levels increases the rate of proliferation. Using a combination of biochemical and biophysical methods, we find that, in the G₁ phase, a cytosolic population of PLCβ1 associates with cyclin-dependent kinase 16 (CDK16), a neuron-specific enzyme that is activated by cyclin Y to inactivate the antioncogenic protein p27^Kip1. Binding of PLCβ1 directly inhibits CDK16 activity and in turn reduces the ability of cells to enter the S phase. Activation of Gα_q by carbachol causes movement of PLCβ from the cytosol to the plasma membrane, reducing its association with CDK16. Similarly, the overexpression of activated Gα_q moves PLCβ1 to the membrane, reverses G₁ arrest, and promotes proliferation, thereby connecting external stimuli with cell proliferation. Our results present a model in which the transient high expression of PLCβ1 that occurs at the onset of differentiation arrests cells in the G₁ phase through its association with CDK16 and allows CDK16 to transition to its postmitotic function of neurite outgrowth and trafficking of synaptic vesicles. The novel role of PLCβ1 in neuronal cell proliferation offers a unique interaction that can be manipulated to guide cells into a neuronal phenotype or to develop therapies for neuroblastomas.-Garwain, O., Valla, K., Scarlata, S. Phospholipase Cβ1 regulates proliferation of neuronal cells.

Phospholipase Cβ interacts with cytosolic partners to regulate cell proliferation

Phospholipase Cβ (PLCβ) is the main effector of the Gαq signaling pathway relaying different extracellular sensory information to generate intracellular calcium signals. Besides this classic function, we have found that PLCβ plays an important but unknown role in regulating PC12 cell differentiation by interacting with components in the RNA-induced silencing machinery. In trying to understand the role of PLCβ in PC12 cell differentiation, we find that over-expressing PLCβ reduces PC12 cell proliferation while down-regulating PLCβ increases the rate of cell proliferation. However, this behavior is not seen in other cancerous cell lines. To determine the underlying mechanism, we carried out mass spectrometry analysis of PLCβ complexes in PC12 cells. We find that in unsynchronized cells, PLCβ primarily binds cyclin-dependent kinase (CDK)16 whose activity plays a key role in cell proliferation. In vitro studies show a direct association between the two proteins that result in loss in CDK16 activity. When cells are arrested in the G2/M phase, a large population of PLCβ is bound to Ago2 in a complex that contains C3PO and proteins commonly found in stress granules. Additionally, another population of PLCβ complexes with CDK18 and cyclin B1. Fluorescence lifetime imaging microscopy (FLIM) confirms cell cycle dependent associations between PLCβ and these other protein binding partners. Taken together, our studies suggest that PLCβ may play an active role in mediating interactions required to move through the cell cycle.

Regulation of the activity of the promoter of RNA-induced silencing, C3PO

RNA-induced silencing is a process which allows cells to regulate the synthesis of specific proteins. RNA silencing is promoted by the protein C3PO (component 3 of RISC). We have previously found that phospholipase Cβ, which increases intracellular calcium levels in response to specific G protein signals, inhibits C3PO activity towards certain genes. Understanding the parameters that control C3PO activity and which genes are impacted by G protein activation would help predict which genes are more vulnerable to downregulation. Here, using a library of 10¹⁸ oligonucleotides, we show that C3PO binds oligonucleotides with structural specificity but little sequence specificity. Alternately, C3PO hydrolyzes oligonucleotides with a rate that is sensitive to substrate stability. Importantly, we find that oligonucleotides with higher Tm values are inhibited by bound PLCβ. This finding is supported by microarray analysis in cells over-expressing PLCβ1. Taken together, this study allows predictions of the genes whose post-transcriptional regulation is responsive to the G protein/phospholipase Cβ/calcium signaling pathway.

Super-resolution Visualization of Caveola Deformation in Response to Osmotic Stress

Caveolae are protein-dense plasma membrane domains structurally composed of caveolin-1 or -3 along with other proteins. Our previous studies have shown that caveolae enhance calcium signals generated through the Gα_q/phospholipase Cβ signaling pathway and that subjecting cells to hypo-osmotic stress reverses this enhancement. In this study, we have used super-resolution fluorescence microscopy supplemented by fluorescence correlation studies to determine the structural factors that underlie this behavior. We find similar and significant population of Gα_q and one of its receptors, bradykinin type 2 receptor (B2R), as well as a significant population of Gα_i and its coupled β2-adrenergic receptor (βAR), are localized to caveola domains. Although mild osmotic stress deforms caveolae and alters interactions between the caveolae and these proteins, the general structure and the localization of caveola components remain largely unchanged. This deformation eliminates the ability of caveolae to stabilize calcium signals mediated through Gα_q-B2R, but does not affect cAMP signals mediated through Gα_i and βAR. Structurally, we find that mild osmotic stress corresponding roughly to a pressure of 3.82 newtons/m² increases the domain diameter by ∼30% and increases the fluorescence intensity in the center of the domain mouth suggesting a flattening of the invagination. Approximate calculations show that caveolae in muscle tissue have the strength to handle the stress of muscle movement.

HIV-1 Nucleocapsid Mimics the Membrane Adaptor Syntenin PDZ to Gain Access to ESCRTs and Promote Virus Budding

HIV-1 recruits cellular endosomal sorting complexes required for transport (ESCRTs) to bud virions from the membrane. Disruption of the viral nucleocapsid (NC) domain integrity affects HIV-1 budding. However, the molecular mechanisms of NC's involvement in HIV budding remain unclear. We find that NC mimics the PDZ domains of syntenin, a membrane-binding adaptor involved in cell-to-cell contact/communication, to capture the Bro1 domain of ALIX, which is an ESCRTs recruiting cellular adaptor. NC binds membranes via basic residues in either the distal or proximal zinc fingers, and NC-membrane binding is essential for Bro1 capture and HIV-1 budding. Removal of RNA enhances NC membrane binding, suggesting a dynamic competition between membrane lipids and RNA for the same binding sites in NC. Remarkably, syntenin PDZ can substitute for NC function in HIV-1 budding. Thus, NC mimics syntenin PDZs to function as a membrane-binding adaptor critical for HIV-1 budding at specific microdomains of the membrane.

Phospholipase Cβ-TRAX Association Is Required for PC12 Cell Differentiation

When treated with nerve growth factor, PC12 cells will differentiate over the course of several days. Here, we have followed changes during differentiation in the cellular levels of phosphoinositide-specific phospholipase Cβ (PLCβ) and its activator, Gα_q, which together mediate Ca²⁺ release. We also followed changes in the level of the novel PLCβ binding partner TRAX (translin-associated factor X), which promotes RNA-induced gene silencing. We find that the level of PLCβ increases 4-fold within 24 h, whereas Gα_q increases only 1.4-fold, and this increase occurs ∼24 h later than PLCβ. Alternately, the level of TRAX remains constant over the 72 h tested. When PLCβ1 or TRAX is down-regulated, differentiation does not occur. The impact of PLCβ on differentiation appears independent of Gα_q as down-regulating Gα_q at constant PLCβ does not affect differentiation. Förster resonance energy transfer studies after PLCβ association with its partners indicate that PLCβ induced soon after nerve growth factor treatment associates with TRAX rather than Gα_q Functional measurements of Ca²⁺ signals to assess the activity of PLCβ-Gα_q complexes and measurements of the reversal of siRNA(GAPDH) to assess the activity of PLCβ-TRAX complexes additionally suggest that the newly synthesized PLCβ associates with TRAX to impact RNA-induced silencing. Taken together, our studies show that PLCβ, through its ability to bind TRAX and reverse RNA silencing of specific genes, plays a key role in switching PC12 cells to their differentiated state.

RNA-induced silencing attenuates G protein-mediated calcium signals

Phospholipase Cβ (PLCβ) is activated by G protein subunits in response to environmental stimuli to increase intracellular calcium. In cells, a significant portion of PLCβ is cytosolic, where it binds a protein complex required for efficient RNA-induced silencing called C3PO (component 3 promoter of RISC). Binding between C3PO and PLCβ raises the possibility that RNA silencing activity can affect the ability of PLCβ to mediate calcium signals. By use of human and rat neuronal cell lines (SK-N-SH and PC12), we show that overexpression of one of the main components of C3PO diminishes Ca(2+) release in response to Gαq/PLCβ stimulation by 30 to 40%. In untransfected SK-N-SH or PC12 cells, the introduction of siRNA(GAPDH) [small interfering RNA(glyceraldehyde 3-phosphate dehydrogenase)] reduces PLCβ-mediated calcium signals by ∼30%, but addition of siRNA(Hsp90) (heat shock protein 90) had little effect. Fluorescence imaging studies suggest an increase in PLCβ-C3PO association in cells treated with siRNA(GAPDH) but not siRNA(Hsp90). Taken together, our studies raise the possibility that Ca(2+) responses to extracellular stimuli can be modulated by components of the RNA silencing machinery.-Philip, F., Sahu, S., Golebiewska, U., Scarlata, S. RNA-induced silencing attenuates G protein-mediated calcium signals.

The Breast Cancer Susceptibility Gene Product (γ-Synuclein) Alters Cell Behavior through its [corrected] Interaction with Phospholipase Cβ

The breast cancer susceptibility gene protein, also known as γ-synuclein, is highly expressed in human breast cancer in a stage-specific manner, with highest expression in late stage cancer. In model systems, γ-synuclein binds phospholipase Cβ2 which is regulated by Gαq to generate intracellular Ca(2+) signals. PLCβ2, which is also absent in normal tissue but highly expressed in breast cancer, is additionally regulated by Rac to promote migration pathways. We have found that γ-synuclein binds to the same region of PLCβ2 as Gαq. Using cells that mimic stage 4 breast cancer (MDA MB 231), we show that down-regulation of γ-synuclein reduces the protein level of PLCβ but increases the transcript level over 40 fold. γ-Synuclein down-regulation also promotes the interaction between Gαq and PLCβ resulting in a stronger Ca(2+) response to Gαq agonists. The ability of γ-synuclein to interfere with Gαq-PLCβ interactions allows more PLCβ to colocalize with Rac impacting Rac-mediated pathways that may give rise to cancerous phenotypes.

High pressure promotes alpha-synuclein aggregation in cultured neuronal cells

α-Synuclein is found in plaques associated with Parkinson's and other neurodegenerative diseases. Changes in α-synuclein oligomerization are thought to give rise to nucleation of neurodegenerative plaques. Here, we investigated the effect of hydrostatic pressure on the aggregation of α-synuclein in cultured neuronal cells. We found that hydrostatic pressure is associated with a transition from monomeric to higher order α-synuclein aggregates. We then tested whether this aggregation is associated with the loss of binding partners, such as phospholipase Cβ. We found that increased pressure reduces the level of PLCβ1 and the amount of α-synuclein/PLCβ1 complexes. These studies suggest that pressure promotes release of α-synuclein from protein partners promoting its oligomerization.

Osmotic Stress Reduces Ca2+ Signals through Deformation of Caveolae

Caveolae are membrane invaginations that can sequester various signaling proteins. Caveolae have been shown to provide mechanical strength to cells by flattening to accommodate increased volume when cells are subjected to hypo-osmotic stress. We have previously found that caveolin, the main structural component of caveolae, specifically binds Gαq and stabilizes its activation state resulting in an enhanced Ca(2+) signal upon activation. Here, we show that osmotic stress caused by decreasing the osmolarity in half reversibly changes the configuration of caveolae without releasing a significant portion of caveolin molecules. This change in configuration due to flattening leads to a loss in Cav1-Gαq association. This loss in Gαq/Cav1 association due to osmotic stress results in a significant reduction of Gαq/phospholipase Cβ-mediated Ca(2+) signals. This reduced Ca(2+) response is also seen when caveolae are reduced by treatment with siRNA(Cav1) or by dissolving them by methyl-β-cyclodextran. No change in Ca(2+) release with osmotic swelling can be seen when growth factor pathways are activated. Taken together, these results connect the mechanical deformation of caveolae to Gαq-mediated Ca(2+) signals.

Development of a universal RNA beacon for exogenous gene detection

Stem cell therapy requires a nontoxic and high-throughput method to achieve a pure cell population to prevent teratomas that can occur if even one cell in the implant has not been transformed. A promising method to detect and separate cells expressing a particular gene is RNA beacon technology. However, developing a successful, specific beacon to a particular transfected gene can take months to develop and in some cases is impossible. Here, we report on an off-the-shelf universal beacon that decreases the time and cost of applying beacon technology to select any living cell population transfected with an exogenous gene.

Linking alpha-synuclein properties with oxidation: a hypothesis on a mechanism underling cellular aggregation

α-Synuclein is a small, natively unstructured protein with propensity to aggregate. α-Synuclein fibrils are major components of Lewy bodies that are hallmarks of many neurodegenerative diseases. The solution properties and aggregation behavior of α-synuclein has been well characterized, but despite numerous studies that address the role of α-synuclein in cells, a clear physiological function of this protein remains a mystery. Over a hundred review articles of α-synuclein have been written in the last decade, making it difficult to list all of the important studies that have added to our insight of α-synuclein physiology. Instead, we briefly review the status of α-synuclein research and propose a model based on the idea that α-synuclein may not have an intrinsic activity in cells but rather, it modifies the function of a group of protein partners that in turn affect cell processes. We propose that it is the loss of its cellular partners under oxidative conditions that promotes α-synuclein aggregation accelerating neuronal death.

Defining the oligomerization state of γ-synuclein in solution and in cells

γ-Synuclein is expressed at high levels in neuronal cells and in multiple invasive cancers. Like its family member α-synuclein, γ-synuclein is thought to be natively unfolded but does not readily form fibrils. The function of γ-synuclein is unknown, but we have found that it interacts strongly with the enzyme phospholipase Cβ (PLCβ), altering its interaction with G proteins. As a first step in determining its role, we have characterized its oligomerization using fluorescence homotransfer, photon-counting histogram analysis, and native gel electrophoresis. We found that when its expressed in Escherichia coli and purified, γ-synuclein appears monomeric on chromatographs under denaturing conditions, but under native conditions, it appears as oligomers of varying sizes. We followed the monomer-to-tetramer association by labeling the protein with fluorescein and following the concentration-dependent loss in fluorescence anisotropy resulting from fluorescence homotransfer. We also performed photon-counting histogram analysis at increasing concentrations of fluorescein-labeled γ-synuclein and found concentration-dependent oligomerization. Addition of PLCβ2, a strong γ-synuclein binding partner whose cellular expression is correlated with γ-synuclein, results in disruption of γ-synuclein oligomers. Similarly, its binding to lipid membranes promotes the monomer form. When we exogenously express γ-synuclein or microinject purified protein into cells, the protein appears monomeric. Our studies show that even though purified γ-synuclein form oligomers, when binding partners are present, as in cells, it dissociates to a monomer to bind these partners, which in turn may modify protein function and integrity.

Hydrolysis rates of different small interfering RNAs (siRNAs) by the RNA silencing promoter complex, C3PO, determines their regulation by phospholipase Cβ

C3PO plays a key role in promoting RNA-induced gene silencing. C3PO consists of two subunits of the endonuclease translin-associated factor X (TRAX) and six subunits of the nucleotide-binding protein translin. We have found that TRAX binds strongly to phospholipase Cβ (PLCβ), which transmits G protein signals from many hormones and sensory inputs. The association between PLCβ and TRAX is thought to underlie the ability of PLCβ to reverse gene silencing by small interfering RNAs. However, this reversal only occurs for some genes (e.g. GAPDH and LDH) but not others (e.g. Hsp90 and cyclophilin A). To understand this specificity, we carried out studies using fluorescence-based methods. In cells, we find that PLCβ, TRAX, and their complexes are identically distributed through the cytosol suggesting that selectivity is not due to large scale sequestration of either the free or complexed proteins. Using purified proteins, we find that PLCβ binds ∼5-fold more weakly to translin than to TRAX but ∼2-fold more strongly to C3PO. PLCβ does not alter TRAX-translin assembly to C3PO, and brightness studies suggest one PLCβ binds to one C3PO octamer without a change in the number of TRAX/translin molecules suggesting that PLCβ binds to an external site. Functionally, we find that C3PO hydrolyzes siRNA(GAPDH) at a faster rate than siRNA(Hsp90). However, when PLCβ is bound to C3PO, the hydrolysis rate of siRNA(GAPDH) becomes comparable with siRNA(Hsp90). Our results show that the selectivity of PLCβ toward certain genes lies in the rate at which the RNA is hydrolyzed by C3PO.

Decoding information in cell shape

Shape is an indicator of cell health. But how is the information in shape decoded? We hypothesize that decoding occurs by modulation of signaling through changes in plasma membrane curvature. Using analytical approaches and numerical simulations, we studied how elongation of cell shape affects plasma membrane signaling. Mathematical analyses reveal transient accumulation of activated receptors at regions of higher curvature with increasing cell eccentricity. This distribution of activated receptors is periodic, following the Mathieu function, and it arises from local imbalance between reaction and diffusion of soluble ligands and receptors in the plane of the membrane. Numerical simulations show that transient microdomains of activated receptors amplify signals to downstream protein kinases. For growth factor receptor pathways, increasing cell eccentricity elevates the levels of activated cytoplasmic Src and nuclear MAPK1,2. These predictions were experimentally validated by changing cellular eccentricity, showing that shape is a locus of retrievable information storage in cells.

Role of phospholipase C-β in RNA interference

Phospholipase C-β (PLCβ) enzymes are activated by G proteins in response to agents such as hormones and neurotransmitters, and have been implicated in leukemias and neurological disorders. PLCβ activity causes an increase in intracellular calcium which ultimately leads to profound changes in the cell. PLCβ localizes to three cellular compartments: the plasma membrane, the cytosol and the nucleus. Under most cell conditions, the majority of PLCβ localizes to the plasma membrane where it interacts with G proteins. In trying to determine the factors that localize PLCβ to the cytosol and nucleus, we have recently identified the binding partner, TRAX. TRAX is a nuclease and part of the machinery involved in RNA interference. This review discusses the interaction between PLCβ and TRAX, and its repercussions in G protein signaling and RNA silencing.

The Effect of Intra-Articular Hyaluronic Acid (Sinovial® One) on Knee Osteoarthritis: A Preliminary Study

Intra-articular injections of hyaluronic acid are a valid treatment option for patients with osteoarthritis. Differences in purity, origin, and molecular weight may influence the efficacy and safety of hyaluronic acid products, therefore, we evaluated the safety, efficacy, and duration of improvements following a single intra-articular injection of a low-medium molecular weight hyaluronic acid product of bacterial synthesis, Sinovial® One, on patients with osteoarthritis of the knee. The double-blind study enrolled 21 patients (24 knees) with symptomatic knee osteoarthritis, classified into moderate, severe and very severe osteoarthritis using the Western Ontario and McMaster Universities Osteoarthritis (WOMAC) pain functional Index and the Kellgren and Lawrence scales. At four months there was improvement in measured clinical parameters in 77.6% of the 24 treated knees, particularly in patients with moderate and severe osteoarthritis (improvement in 100% and 66.7%, respectively). No local or systemic adverse events were observed. These preliminary findings suggest that Sinovial® One is safe and effective for patients with knee osteoarthritis, providing long-lasting improvement in clinical parameters.

A loss in cellular protein partners promotes α-synuclein aggregation in cells resulting from oxidative stress

There is a consensus that oxidative stress promotes neurodegeneration and may be linked to plaque formation. α-Synuclein is the main component of neurodegenerative plaques. We have found that α-synuclein binds strongly to the enzyme phospholipase Cβ1 (PLCβ1) in vitro and in cells affecting both its G protein activation and its degradation. Because PLCβ1 binds to α-synuclein in cells, we tested whether decreasing its level would promote α-synuclein aggregation and whether overproducing PLCβ1 would inhibit aggregation. By imaging fluorescent α-synuclein in living HEK293, PC12, and SK-H-SH cells, we find that α-synuclein aggregation is directly related to the level of PLCβ1. Importantly, we found that oxidative stress does not affect the cellular levels of α-synuclein but results in the down-regulation of PLCβ1 thereby promoting α-synuclein aggregation. A peptide that mimics part of the α-synuclein binding site to PLCβ prevents aggregation. Our studies indicate that PLCβ1 can reduce cell damage under oxidative stress and offers a potential site that might be exploited to prevent α-synuclein aggregation.

Phospholipase Cβ1 is linked to RNA interference of specific genes through translin-associated factor X

Phospholipase Cβ1 (PLCβ1) is a G-protein-regulated enzyme whose activity results in proliferative and mitogenic changes in the cell. We have previously found that in solution PLCβ1 binds to the RNA processing protein translin-associated factor X (TRAX) with nanomolar affinity and that this binding competes with G proteins. Here, we show that endogenous PLCβ1 and TRAX interact in SK-N-SH cells and also in HEK293 cells induced to overexpress PLCβ1. In HEK293 cells, TRAX overexpression ablates Ca(2+) signals generated by G protein-PLCβ1 activation. TRAX plays a key role in down-regulation of proteins by small, interfering RNA, and PLCβ1 overexpression completely reverses the 2- to 4-fold down-regulation of GAPDH by siRNA in HEK293 and HeLa cells as seen by an ∼4-fold recovery in both the transcript and protein levels. Also, down-regulation of endogenous PLCβ1 in HEK293 and HeLa cells allows for an ∼20% increase in siRNA(GAPDH) silencing. While PLCβ1 overexpression results in a 50% reversal of cell death caused by siRNA(LDH), it does not affect cell survival or silencing of other genes (e.g., cyclophilin, Hsp90, translin). PLCβ1 overexpression in HEK293 and HeLa cells causes a 30% reduction in the total amount of small RNAs. LDH and GAPDH are part of a complex that promotes H2B synthesis that allows cells to progress through the S phase. We find that PLCβ1 reverses the cell death and completely rescues H2B levels caused by siRNA knockdown of LDH or GAPDH. Taken together, our study shows a novel role of PLCβ1 in gene regulation through TRAX association.