Ganglioside GM2 modulates the erythrocyte Ca2+-ATPase through its binding to the calmodulin-binding domain and its 'receptor'

The PLEKHA7-PDZD11 complex regulates the localization of the calcium pump PMCA and calcium handling in cultured cells

The plasma membrane calcium ATPase (PMCA) extrudes calcium from the cytosol to the extracellular space to terminate calcium-dependent signaling. Although the distribution of PMCA is crucial for its function, the molecular mechanisms that regulate the localization of PMCA isoforms are not well understood. PLEKHA7 is implicated by genetic studies in hypertension and the regulation of calcium handling. PLEKHA7 recruits the small adapter protein PDZD11 to adherens junctions, and together they control the trafficking and localization of plasma membrane associated proteins, including the Menkes copper ATPase. Since PDZD11 binds to the C-terminal domain of b-isoforms of PMCA, PDZD11 and its interactor PLEKHA7 could control the localization and activity of PMCA. Here, we test this hypothesis using cultured cell model systems. We show using immunofluorescence microscopy and a surface biotinylation assay that KO of either PLEKHA7 or PDZD11 in mouse kidney collecting duct epithelial cells results in increased accumulation of endogenous PMCA at lateral cell–cell contacts and PDZ-dependent ectopic apical localization of exogenous PMCA4x/b isoform. In HeLa cells, coexpression of PDZD11 reduces membrane accumulation of overexpressed PMCA4x/b, and analysis of cytosolic calcium transients shows that PDZD11 counteracts calcium extrusion activity of overexpressed PMCA4x/b, but not PMCA4x/a, which lacks the PDZ-binding motif. Moreover, KO of PDZD11 in either endothelial (bEnd.3) or epithelial (mouse kidney collecting duct) cells increases the rate of calcium extrusion. Collectively, these results suggest that the PLEKHA7–PDZD11 complex modulates calcium homeostasis by regulating the localization of PMCA.

Plasma Membrane Calcium ATPase-Neuroplastin Complexes Are Selectively Stabilized in GM1-Containing Lipid Rafts

The recent identification of plasma membrane (Ca²⁺)-ATPase (PMCA)-Neuroplastin (Np) complexes has renewed attention on cell regulation of cytosolic calcium extrusion, which is of particular relevance in neurons. Here, we tested the hypothesis that PMCA-Neuroplastin complexes exist in specific ganglioside-containing rafts, which could affect calcium homeostasis. We analyzed the abundance of all four PMCA paralogs (PMCA1-4) and Neuroplastin isoforms (Np65 and Np55) in lipid rafts and bulk membrane fractions from GM2/GD2 synthase-deficient mouse brains. In these fractions, we found altered distribution of Np65/Np55 and selected PMCA isoforms, namely PMCA1 and 2. Cell surface staining and confocal microscopy identified GM1 as the main complex ganglioside co-localizing with Neuroplastin in cultured hippocampal neurons. Furthermore, blocking GM1 with a specific antibody resulted in delayed calcium restoration of electrically evoked calcium transients in the soma of hippocampal neurons. The content and composition of all ganglioside species were unchanged in Neuroplastin-deficient mouse brains. Therefore, we conclude that altered composition or disorganization of ganglioside-containing rafts results in changed regulation of calcium signals in neurons. We propose that GM1 could be a key sphingolipid for ensuring proper location of the PMCA-Neuroplastin complexes into rafts in order to participate in the regulation of neuronal calcium homeostasis.

Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein-lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

Glycosphingolipid-Protein Interaction in Signal Transduction

Glycosphingolipids (GSLs) are a class of ceramide-based glycolipids essential for embryo development in mammals. The synthesis of specific GSLs depends on the expression of distinctive sets of GSL synthesizing enzymes that is tightly regulated during development. Several reports have described how cell surface receptors can be kept in a resting state or activate alternative signalling events as a consequence of their interaction with GSLs. Specific GSLs, indeed, interface with specific protein domains that are found in signalling molecules and which act as GSL sensors to modify signalling responses. The regulation exerted by GSLs on signal transduction is orthogonal to the ligand–receptor axis, as it usually does not directly interfere with the ligand binding to receptors. Due to their properties of adjustable production and orthogonal action on receptors, GSLs add a new dimension to the control of the signalling in development. GSLs can, indeed, dynamically influence progenitor cell response to morphogenetic stimuli, resulting in alternative differentiation fates. Here, we review the available literature on GSL–protein interactions and their effects on cell signalling and development.

Ganglioside contained in the neuronal tissue-enriched acidic protein of 22 kDa (NAP-22) fraction prepared from the detergent-resistant membrane microdomain of rat brain inhibits the phosphatase activity of calcineurin

Neurons have well-developed membrane microdomains called "rafts" that are recovered as a detergent-resistant membrane microdomain fraction (DRM). Neuronal tissue-enriched acidic protein of 22 kDa (NAP-22) is one of the major protein components of neuronal DRM. To determine the cellular function of NAP-22, interacting proteins were screened with an immunoprecipitation assay, and calcineurin (CaN) was detected. Further studies with NAP-22 prepared from DRM and CaN expressed in bacteria showed the binding of these proteins and a dose-dependent inhibitory effect of the NAP-22 fraction on the phosphatase activity of CaN. On the other hand, NAP-22 expressed in bacteria showed low binding to CaN and a weak inhibitory effect on phosphatase activity. To solve this discrepancy, identification of a nonprotein component that modulates CaN activity in the DRM-derived NAP-22 fraction was attempted. After lyophilization, a lipid fraction was extracted with chloroform/methanol. The lipid fraction showed an inhibitory effect on CaN without NAP-22, and further fractionation of the extract with thin-layer chromatography showed the presence of several lipid bands having an inhibitory effect on CaN. The mobility of these bands coincided with that of authentic ganglioside (GM1a, GD1a, GD1b, and GT1b), and authentic ganglioside showed an inhibitory effect on CaN. Treatment of lipid with endoglycoceramidase, which degrades ganglioside to glycochain and ceramide, caused a diminution of the inhibitory effect. These results show that DRM-derived NAP-22 binds several lipids, including ganglioside, and that ganglioside inhibits the phosphatase activity of CaN.

Glycosphingolipids: synthesis and functions

Glycosphingolipids (GSLs) comprise a heterogeneous group of membrane lipids formed by a ceramide backbone covalently linked to a glycan moiety. Hundreds of different glycans can be linked to tens of different ceramide molecules, giving rise to an astonishing variety of structurally different compounds, each of which has the potential for a specific biological function. GSLs have been suggested to modulate membrane-protein function and to contribute to cell-cell communication. Although GSLs are dispensable for cellular life, they are indeed collectively required for the development of multicellular organisms, and are thus considered to be key molecules in 'cell sociology'. Consequently, the GSL make-up of individual cells is highly dynamic and is strictly linked to the cellular developmental and environmental state. In the present review, we discuss some of the available knowledge, open questions and future perspectives relating to the study of GSL biology.

Identification of lysosomal sialidase NEU1 and plasma membrane sialidase NEU3 in human erythrocytes

The sialylation level of molecules, sialoglycoproteins and gangliosides, protruding from plasma membranes regulates multiple facets of erythrocyte function, from interaction with endothelium to cell lifespan. Our results demonstrate that: (a) Both sialidases NEU1 and NEU3 are present on erythrocyte plasma membrane; (b) NEU1 is kept on the plasma membrane in absence of the protective protein/cathepsin A (PPCA); (c) NEU1 and NEU3 are retained on the plasma membrane, as peripheral proteins, associated to the external leaflet and released by alkaline treatments; (d) NEU1 and NEU3 are segregated in Triton X-100 detergent-resistant membrane domains (DRMs); (e) NEU3 shows activity also at neutral pH; and (f) NEU1 and NEU3 are progressively lost during erythrocyte life. Interestingly, sialidase activity released from erythrocyte membranes after an alkaline treatment preserves its functionality and recognizes sialoglycoproteins and gangliosides. On the other hand, the weak anchorage of sialidases to the plasma membrane and their loss during erythrocyte life could be a tool to preserve the cellular sialic acid content in order to avoid the early ageing of erythrocyte and processes of cell aggregation in the capillaries.

Decreases in plasma membrane Ca²⁺-ATPase in brain synaptic membrane rafts from aged rats

Precise regulation of free intracellular Ca(2+) concentrations [Ca(2+) ](i) is critical for normal neuronal function, and alterations in Ca(2+) homeostasis are associated with brain aging and neurodegenerative diseases. One of the most important proteins controlling [Ca(2+) ](i) is the plasma membrane Ca(2+) -ATPase (PMCA), the high-affinity transporter that fine tunes the cytosolic nanomolar levels of Ca(2+) . We previously found that PMCA protein in synaptic plasma membranes (SPMs) is decreased with advancing age and the decrease in enzyme activity is much greater than that in protein levels. In this study, we isolated raft and non-raft fractions from rat brain SPMs and used quantitative mass spectrometry to show that the specialized lipid microdomains in SPMs, the rafts, contain 60% of total PMCA, comprised all four isoforms. The raft PMCA pool had the highest specific activity and this decreased progressively with age. The reduction in PMCA protein could not account for the dramatic activity loss. Addition of excess calmodulin to the assay did not restore PMCA activity to that in young brains. Analysis of the major raft lipids revealed a slight age-related increase in cholesterol levels and such increases might enhance membrane lipid order and prevent further loss of PMCA activity.

Why does GM1 induce a potent beneficial response to experimental Chagas disease?

Being one of the world's neglected diseases, Chagas has neither a vaccine nor a satisfactory therapy. Inoculation of murine models with the ganglioside GM1 has shown a strikingly nonlinear effect, leading to a strong decrease in parasite load at low doses but reverting to a load increase at high doses. Cardiocyte destruction concomitant with the disease is also significantly reduced by a moderate application of GM1. A mathematical model for the interaction between the parasite and the immune system is shown to explain these effects and is used to predict an optimal dosage that maximizes parasite removal with minimal cardiocyte destruction.

Simultaneous quantification of GM1 and GM2 gangliosides by isotope dilution tandem mass spectrometry

OBJECTIVES

Gangliosides (GGs) are considered as diagnostic biomarkers and therapeutic targets and agents. The goal of this study was to develop a tandem mass spectrometry (MS/MS) method for the simultaneous measurement of both GM1 and GM2 gangliosides in human cerebrospinal fluid (CSF) samples in order to be able to determine their concentrations in patients with Tay-Sachs and Sandhoff disease and assess whether drugs or transplantation affect their concentrations.

DESIGN AND METHODS

An API-4000 tandem mass spectrometer equipped with TurboIonSpray source and Shimadzu HPLC system was employed to perform the analysis using isotope dilution with deuterium labeled internal standards. To a 1.5 mL conical plastic Eppendorf centrifuge tube, 40 microL of human CSF sample was added and mixed with 400 microL of internal standard solution for deproteinization. After centrifugation, 100 microL of supernatant was injected onto a C-18 column. After a 2.5 min wash, the switching valve was activated and the analytes were eluted from the column with a water/methanol gradient into the MS/MS system. Quantification by multiple reaction-monitoring (MRM) analysis was performed in the negative mode.

RESULTS

The within-day coefficients of variation were <3% for GM1 and <2% for GM2 and the between-day coefficients of variation were <5% for both GM1 and GM2 at all concentrations tested. Accuracy ranged between 98% and 102% for both analytes. Good linearity was also obtained within the concentration range of 10-200 ng/mL (6.5-129.3 nmol/L) for GM1 and 5-100 ng/mL (3.6-72.3 nmol/L) for GM2 (r> or =0.995).

CONCLUSIONS

A new simple, accurate, and fast isotope dilution tandem mass spectrometry method was developed for the simultaneous quantification of GM1 and GM2 gangliosides in a small amount of human CSF. Concentrations were measured in "normal" CSF and in CSF from patients with Tay-Sachs disease.

Partitioning of the plasma membrane Ca2+-ATPase into lipid rafts in primary neurons: effects of cholesterol depletion

Spatial and temporal alterations in intracellular calcium [Ca(2+)](i) play a pivotal role in a wide array of neuronal functions. Disruption in Ca(2+) homeostasis has been implicated in the decline in neuronal function in brain aging and in neurodegenerative disorders. The plasma membrane Ca(2+)-ATPase (PMCA) is a high affinity Ca(2+) transporter that plays a crucial role in the termination of [Ca(2+)](i) signals and in the maintenance of low [Ca(2+)](i) essential for signaling. Recent evidence indicates that PMCA is uniquely sensitive to its lipid environment and is stimulated by lipids with ordered acyl chains. Here we show that both PMCA and its activator calmodulin (CaM) are partitioned into liquid-ordered, cholesterol-rich plasma membrane microdomains or 'lipid rafts' in primary cultured neurons. Association of PMCA with rafts was demonstrated in preparations isolated by sucrose density gradient centrifugation and in intact neurons by confocal microscopy. Total raft-associated PMCA activity was much higher than the PMCA activity excluded from these microdomains. Depletion of cellular cholesterol dramatically inhibited the activity of the raft-associated PMCA with no effect on the activity of the non-raft pool. We propose that association of PMCA with rafts represents a novel mechanism for its regulation and, consequently, of Ca(2+) signaling in the central nervous system.