The secretory pathway Ca(2+)-ATPase 1 is associated with cholesterol-rich microdomains of human colon adenocarcinoma cells

Use of aequorin-based indicators for monitoring Ca2+ in acidic organelles

Over the last years, there is accumulating evidence that acidic organelles can accumulate and release Ca²⁺ upon cell activation. Hence, reliable recording of Ca²⁺ dynamics in these compartments is essential for understanding the physiopathological aspects of acidic organelles. Genetically encoded Ca²⁺ indicators (GECIs) are valuable tools to monitor Ca²⁺ in specific locations, although their use in acidic compartments is challenging due to the pH sensitivity of most available fluorescent GECIs. By contrast, bioluminescent GECIs have a combination of features (marginal pH sensitivity, low background, no phototoxicity, no photobleaching, high dynamic range and tunable affinity) that render them advantageous to achieve an enhanced signal-to-noise ratio in acidic compartments. This article reviews the use of bioluminescent aequorin-based GECIs targeted to acidic compartments. A need for more measurements in highly acidic compartments is identified.

Store-Operated Calcium Entry and Its Implications in Cancer Stem Cells

Tumors are composed by a heterogeneous population of cells. Among them, a sub-population of cells, termed cancer stem cells, exhibit stemness features, such as self-renewal capabilities, disposition to differentiate to a more proliferative state, and chemotherapy resistance, processes that are all mediated by Ca²⁺. Ca²⁺ homeostasis is vital for several physiological processes, and alterations in the patterns of expressions of the proteins and molecules that modulate it have recently become a cancer hallmark. Store-operated Ca²⁺ entry is a major mechanism for Ca²⁺ entry from the extracellular medium in non-excitable cells that leads to increases in the cytosolic Ca²⁺ concentration required for several processes, including cancer stem cell properties. Here, we focus on the participation of STIM, Orai, and TRPC proteins, the store-operated Ca²⁺ entry key components, in cancer stem cell biology and tumorigenesis.

Calcium levels in the Golgi complex regulate clustering and apical sorting of GPI-APs in polarized epithelial cells

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are lipid-associated luminal secretory cargoes selectively sorted to the apical surface of the epithelia where they reside and play diverse vital functions. Cholesterol-dependent clustering of GPI-APs in the Golgi is the key step driving their apical sorting and their further plasma membrane organization and activity; however, the specific machinery involved in this Golgi event is still poorly understood. In this study, we show that the formation of GPI-AP homoclusters (made of single GPI-AP species) in the Golgi relies directly on the levels of calcium within cisternae. We further demonstrate that the TGN calcium/manganese pump, SPCA1, which regulates the calcium concentration within the Golgi, and Cab45, a calcium-binding luminal Golgi resident protein, are essential for the formation of GPI-AP homoclusters in the Golgi and for their subsequent apical sorting. Down-regulation of SPCA1 or Cab45 in polarized epithelial cells impairs the oligomerization of GPI-APs in the Golgi complex and leads to their missorting to the basolateral surface. Overall, our data reveal an unexpected role for calcium in the mechanism of GPI-AP apical sorting in polarized epithelial cells and identify the molecular machinery involved in the clustering of GPI-APs in the Golgi.

Identification of Prognostic Dosage-Sensitive Genes in Colorectal Cancer Based on Multi-Omics

Several studies have already identified the prognostic markers in colorectal cancer (CRC) based on somatic copy number alteration (SCNA). However, very little information is available regarding their value as a prognostic marker. Gene dosage effect is one important mechanism of copy number and dosage-sensitive genes are more likely to behave like driver genes. In this work, we propose a new pipeline to identify the dosage-sensitive prognostic genes in CRC. The RNAseq data, the somatic copy number of CRC from TCGA were assayed to screen out the SCNAs. Wilcoxon rank-sum test was used to identify the differentially expressed genes in alteration samples with |SCNA| > 0.3. Cox-regression was used to find the candidate prognostic genes. An iterative algorithm was built to identify the stable prognostic genes. Finally, the Pearson correlation coefficient was calculated between gene expression and SCNA as the dosage effect score. The cell line data from CCLE was used to test the consistency of the dosage effect. The differential co-expression network was built to discover their function in CRC. A total of six amplified genes (NDUFB4, WDR5B, IQCB1, KPNA1, GTF2E1, and SEC22A) were found to be associated with poor prognosis. They demonstrate a stable prognostic classification in more than 50% threshold of SCNA. The average dosage effect score was 0.5918 ± 0.066, 0.5978 ± 0.082 in TCGA and CCLE, respectively. They also show great stability in different data sets. In the differential co-expression network, these six genes have the top degree and are connected to the driver and tumor suppressor genes. Function enrichment analysis revealed that gene NDUFB4 and GTF2E1 affect cancer-related functions such as transmembrane transport and transformation factors. In conclusion, the pipeline for identifying the prognostic dosage-sensitive genes in CRC was proved to be stable and reliable.

Lipid-dependent coupling of secretory cargo sorting and trafficking at the trans-Golgi network

In eukaryotic cells, the trans-Golgi network (TGN) serves as a platform for secretory cargo sorting and trafficking. In recent years, it has become evident that a complex network of lipid–lipid and lipid–protein interactions contributes to these key functions. This review addresses the role of lipids at the TGN with a particular emphasis on sphingolipids and diacylglycerol. We further highlight how these lipids couple secretory cargo sorting and trafficking for spatiotemporal coordination of protein transport to the plasma membrane.

Exploring new routes for secretory protein export from the trans-Golgi network

Sorting of soluble proteins for transport to intracellular compartments and for secretion from cells is essential for cell and tissue homeostasis. The trans-Golgi network (TGN) is a major sorting station that sorts secretory proteins into specific carriers to transport them to their final destinations. The sorting of lysosomal hydrolases at the TGN by the mannose 6-phosphate receptor is well understood. The recent discovery of a Ca²⁺-based sorting of secretory cargo at the TGN is beginning to uncover the mechanism by which cells sort secretory cargoes from Golgi residents and cargoes destined to the other cellular compartments. This Ca²⁺-based sorting involves the cytoplasmic actin cytoskeleton, which through membrane anchored Ca²⁺ ATPase SPCA1 and the luminal Ca²⁺ binding protein Cab45 sorts of a subset of secretory proteins at the TGN. We present this discovery and highlight important challenges that remain unaddressed in the overall pathway of cargo sorting at the TGN.

Store-independent Orai1-mediated Ca2+ entry and cancer

Ca²⁺ channels play an important role in the development of different types of cancer, and considerable progress has been made to understand the pathophysiological mechanisms underlying the role of Ca²⁺ influx in the development of different cancer hallmarks. Orai1 is among the most ubiquitous and multifunctional Ca²⁺ channels. Orai1 mediates the highly Ca²⁺-selective Ca²⁺ release-activated current (I_CRAC) and participates in the less Ca²⁺-selective store-operated current (I_SOC), along with STIM1 or STIM1 and TRPC1, respectively. Furthermore, Orai1 contributes to a variety of store-independent Ca²⁺ influx mechanisms, including the arachidonate-regulated Ca²⁺ current, together with Orai3 and the plasma membrane resident pool of STIM1, as well as the constitutive Ca²⁺ influx processes activated by the secretory pathway Ca²⁺-ATPase-2 (SPCA2) or supported by physical and functional interaction with the small conductance Ca²⁺-activated K⁺ channel 3 (SK3) or the voltage-dependent K_v10.1 channel. This review summarizes the current knowledge concerning the store-independent mechanisms of Ca²⁺ influx activation through Orai1 channels and their role in the development of different cancer features.

Activity of the SPCA1 Calcium Pump Couples Sphingomyelin Synthesis to Sorting of Secretory Proteins in the Trans-Golgi Network

How the principal functions of the Golgi apparatus-protein processing, lipid synthesis, and sorting of macromolecules-are integrated to constitute cargo-specific trafficking pathways originating from the trans-Golgi network (TGN) is unknown. Here, we show that the activity of the Golgi localized SPCA1 calcium pump couples sorting and export of secreted proteins to synthesis of new lipid in the TGN membrane. A secreted Ca2+-binding protein, Cab45, constitutes the core component of a Ca2+-dependent, oligomerization-driven sorting mechanism whereby secreted proteins bound to Cab45 are packaged into a TGN-derived vesicular carrier whose membrane is enriched in sphingomyelin, a lipid implicated in TGN-to-cell surface transport. SPCA1 activity is controlled by the sphingomyelin content of the TGN membrane, such that local sphingomyelin synthesis promotes Ca2+ flux into the lumen of the TGN, which drives secretory protein sorting and export, thereby establishing a protein- and lipid-specific secretion pathway.

Parkinson disease related ATP13A2 evolved early in animal evolution

Several human P5-type transport ATPases are implicated in neurological disorders, but little is known about their physiological function and properties. Here, we investigated the relationship between the five mammalian P5 isoforms ATP13A1-5 in a comparative study. We demonstrated that ATP13A1-4 isoforms undergo autophosphorylation, which is a hallmark P-type ATPase property that is required for substrate transport. A phylogenetic analysis of P5 sequences revealed that ATP13A1 represents clade P5A, which is highly conserved between fungi and animals with one member in each investigated species. The ATP13A2-5 isoforms belong to clade P5B and diversified from one isoform in fungi and primitive animals to a maximum of four in mammals by successive gene duplication events in vertebrate evolution. We revealed that ATP13A1 localizes in the endoplasmic reticulum (ER) and experimentally demonstrate that ATP13A1 likely contains 12 transmembrane helices. Conversely, ATP13A2-5 isoforms reside in overlapping compartments of the endosomal system and likely contain 10 transmembrane helices, similar to what was demonstrated earlier for ATP13A2. ATP13A1 complemented a deletion of the yeast P5A ATPase SPF1, while none of ATP13A2-5 could complement either the loss of SPF1 or that of the single P5B ATPase YPK9 in yeast. Thus, ATP13A1 carries out a basic ER function similar to its yeast counterpart Spf1p that plays a role in ER related processes like protein folding and processing. ATP13A2-5 isoforms diversified in mammals and are expressed in the endosomal system where they may have evolved novel complementary or partially redundant functions. While most P5-type ATPases are widely expressed, some P5B-type ATPases (ATP13A4 and ATP13A5) display a more limited tissue distribution in the brain and epithelial glandular cells, where they may exert specialized functions. At least some P5B isoforms are of vital importance for the nervous system, since ATP13A2 and ATP13A4 are linked to respectively Parkinson disease and autism spectrum disorders.

Development of a Whole Organism Platform for Phenotype-Based Analysis of IGF1R-PI3K-Akt-Tor Action

Aberrant regulation of the insulin-like growth factor (IGF)/insulin (IIS)-PI3K-AKT-TOR signaling pathway is linked to major human diseases, and key components of this pathway are targets for therapeutic intervention. Current assays are molecular target- or cell culture-based platforms. Due to the great in vivo complexities inherited in this pathway, there is an unmet need for whole organism based assays. Here we report the development of a zebrafish transgenic line, Tg(igfbp5a:GFP), which faithfully reports the mitotic action of IGF1R-PI3K-Akt-Tor signaling in epithelial cells in real-time. This platform is well suited for high-throughput assays and real-time cell cycle analysis. Using this platform, the dynamics of epithelial cell proliferation in response to low [Ca²⁺] stress and the distinct roles of Torc1 and Torc2 were elucidated. The availability of Tg(igfbp5a:GFP) line provides a whole organism platform for phenotype-based discovery of novel players and inhibitors in the IIS-PI3K-Akt-Tor signaling pathway.

High ATP2A2 expression correlates with better prognosis of diffuse astrocytic tumor patients

Novel molecular markers are required for defining subsets of diffuse astrocytic tumor patients with differing prognoses. Here, we examined ATP2A2 expression in 109 human diffuse astrocytic tumor samples (39 grade II diffuse astrocytoma (DA), 19 grade III anaplastic astrocytoma (AA), 51 grade IV glioblastoma) and its correlation with patient clinicopathologic characteristics. ATP2A2 expression significantly correlated with tumor grade and survival (P<0.05). High ATP2A2 expression was detected in 35.3% (18/51) of glioblastoma patients, compared to 61.5% (24/39) in grade II, and 52.6% (10/19) in grade III astrocytoma patients (P=0.043). The median survival was 45±5.3 (95% CI, 34.7-55.3) months in patients with high ATP2A2 expression and 16±5.0 (95% CI, 6.3-25.7) months in patients with low ATP2A2 expression (P<0.0001). Additionally, high grade astrocytoma patients with high ATP2A2 expression showed longer survival (median, 31.0±4.9 months, 95% CI, 21.4-40.7) than those with low ATP2A2 expression (median: 13.0±1.6 months, 95% CI, 9.9-16.1; P=0.027). Furthermore, both ATP2A2 overexpression and IDH1 mutation were detected in secondary glioblastoma, AA developed from DA and oligodendrogiomas with IDH1 mutation. The MTT assays showed that lentiviral ATP2A2 overexpression significantly suppressed the clonogenic growth of glioblastoma U251MG cells (P<0.05). Xenografts stably overexpressing ATP2A2 were markedly smaller in size 4 weeks post inoculation (P<0.05). Our findings identified high ATP2A2 expression in a subset of astrocytoma patients that was associated with better prognosis and ATP2A2 suppressed astrocytoma growth.

Structure/activity relationship of thapsigargin inhibition on the purified Golgi/secretory pathway Ca2+/Mn2+-transport ATPase (SPCA1a)

The Golgi/secretory pathway Ca²⁺/Mn²⁺-transport ATPase (SPCA1a) is implicated in breast cancer and Hailey-Hailey disease. Here, we purified recombinant human SPCA1a from Saccharomyces cerevisiae and measured Ca²⁺-dependent ATPase activity following reconstitution in proteoliposomes. The purified SPCA1a displays a higher apparent Ca²⁺ affinity and a lower maximal turnover rate than the purified sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA1a). The lipids cholesteryl hemisuccinate, linoleamide/oleamide, and phosphatidylethanolamine inhibit and phosphatidic acid and sphingomyelin enhance SPCA1a activity. Moreover, SPCA1a is blocked by micromolar concentrations of the commonly used SERCA1a inhibitors thapsigargin (Tg), cyclopiazonic acid, and 2,5-di-tert-butylhydroquinone. Because tissue-specific targeting of SERCA2b by Tg analogues is considered for prostate cancer therapy, the inhibition of SPCA1a by Tg might represent an off-target risk. We assessed the structure-activity relationship (SAR) of Tg for SPCA1a by in silico modeling, site-directed mutagenesis, and measuring the potency of a series of Tg analogues. These indicate that Tg and the analogues are bound via the Tg scaffold but with lower affinity to the same homologous cavity as on the membrane surface of SERCA1a. The lower Tg affinity may depend on a more flexible binding cavity in SPCA1a, with low contributions of the Tg O-3, O-8, and O-10 chains to the binding energy. Conversely, the protein interaction of the Tg O-2 side chain with SPCA1a appears comparable with that of SERCA1a. These differences define a SAR of Tg for SPCA1a distinct from that of SERCA1a, indicating that Tg analogues with a higher specificity for SPCA1a can probably be developed.

SPCA2 couples Ca2+ influx via Orai1 to Ca2+ uptake into the Golgi/secretory pathway

Dysregulation of the Golgi/Secretory Pathway Ca²⁺ transport ATPase SPCA2 is implicated in breast cancer. During lactation and in luminal breast cancer types, SPCA2 interacts with the plasma membrane Ca²⁺ channel Orai1, promoting constitutive Ca²⁺ influx, which is termed store independent Ca²⁺ entry (SICE). The mechanism of SPCA2/Orai1 interaction depends on the N- and C-termini of SPCA2. These extensions may play a dual role in activating not only Orai1, but also Ca²⁺ transport into the Golgi/secretory pathway, which we tested by investigating the impact of various SPCA2 N- and/or C-terminal truncations on SICE and Ca²⁺ transport activity of SPCA2. C-terminal truncations impair SICE and SPCA2 activity, but also affect targeting, whereas N-terminal truncations affect targeting and inactivate SPCA2, but remarkably, SICE activation remains unaffected. Importantly, overexpression of SPCA2 increases the Ca²⁺ content of non-ER stores, which depends on Orai1 and SPCA2 activity. Thus, Orai1-mediated Ca²⁺-influx and SPCA2-mediated Ca²⁺ uptake activity into the Golgi/secretory pathway might be coupled possibly in a microdomain. This channel/pump complex may efficiently transfer Ca²⁺ into the secretory pathway, which might play a role in SPCA2-expressing secretory cells, such as mammary gland during lactation.

Secretory pathway Ca(2+)-ATPase isoform 1 knockdown promotes Golgi apparatus stress injury in a mouse model of focal cerebral ischemia-reperfusion: In vivo and in vitro study

The present study was designed to investigate the potential role of secretory pathway Ca(2+)-ATPase isoform 1(SPCA1) in experimental focal cerebral ischemia-reperfusion injury. Cerebral ischemia-reperfusion was induced by transient middle cerebral artery occlusion (MCAO) for 2h s in Sprague-Dawley rats, and then the expression levels of SPAC1 mRNA and protein were determined. Results showed that SPCA1 level was transiently increased 1 day after reperfusion in peri-infarction area, while markedly increased in infarction core on 3day and 7 day after reperfusion. Then a SPCA1 lentivirus was used to achieve knockdown of SPCA1 gene: Ca(2+) transporting type 2C, member 1 (ATP2C1) gene. It has been observed that SPCA1 knockdown by lentivirus markedly increased cerebral infarction volume in vivo. Meanwhile, SPCA1 knockdown also facilitated per-oxidative production, including nitric oxide (NO) and 3-nitrotyrosine (3-NT) and decreased the expression of total superoxide dismutase (SOD) and manganese superoxide dismutase (MnSOD). Moreover, in vitro study showed that SPCA1 knockdown increased hydrogen peroxide (H2O2)-induced lactate dehydrogenase (LDH) leakage dose-dependently, and elevated caspase3 level in neuro-2a (N2a) cells. In addition, SPCA1 knockdown increased H2O2-induced production of nitric oxide and 3-NT dose-dependently, and reversed the increased activity of total SOD and MnSOD in neuro-2a cells. In conclusion, the present study indicated that SPCA1 could suppress over active Golgi apparatus (GA) stress thus attenuate cerebral ischemia-reperfusion injury.

A lipid switch unlocks Parkinson's disease-associated ATP13A2

ATP13A2 is a lysosomal P-type transport ATPase that has been implicated in Kufor-Rakeb syndrome and Parkinson's disease (PD), providing protection against α-synuclein, Mn(2+), and Zn(2+) toxicity in various model systems. So far, the molecular function and regulation of ATP13A2 remains undetermined. Here, we demonstrate that ATP13A2 contains a unique N-terminal hydrophobic extension that lies on the cytosolic membrane surface of the lysosome, where it interacts with the lysosomal signaling lipids phosphatidic acid (PA) and phosphatidylinositol(3,5)bisphosphate [PI(3,5)P2]. We further demonstrate that ATP13A2 accumulates in an inactive autophosphorylated state and that PA and PI(3,5)P2 stimulate the autophosphorylation of ATP13A2. In a cellular model of PD, only catalytically active ATP13A2 offers cellular protection against rotenone-induced mitochondrial stress, which relies on the availability of PA and PI(3,5)P2. Thus, the N-terminal binding of PA and PI(3,5)P2 emerges as a key to unlock the activity of ATP13A2, which may offer a therapeutic strategy to activate ATP13A2 and thereby reduce α-synuclein toxicity or mitochondrial stress in PD or related disorders.

Cofilin recruits F-actin to SPCA1 and promotes Ca2+-mediated secretory cargo sorting

The cofilin CFL-1 recruits actin to the P-type calcium ATPase SPCA1 at the trans-Golgi network, thereby activating the ATPase, promoting Ca²⁺ influx, and driving secretory cargo sorting.

The actin filament severing protein cofilin-1 (CFL-1) is required for actin and P-type ATPase secretory pathway calcium ATPase (SPCA)-dependent sorting of secretory proteins at the trans-Golgi network (TGN). How these proteins interact and activate the pump to facilitate cargo sorting, however, is not known. We used purified proteins to assess interaction of the cytoplasmic domains of SPCA1 with actin and CFL-1. A 132–amino acid portion of the SPCA1 phosphorylation domain (P-domain) interacted with actin in a CFL-1–dependent manner. This domain, coupled to nickel nitrilotriacetic acid (Ni-NTA) agarose beads, specifically recruited F-actin in the presence of CFL-1 and, when expressed in HeLa cells, inhibited Ca²⁺ entry into the TGN and secretory cargo sorting. Mutagenesis of four amino acids in SPCA1 that represent the CFL-1 binding site also affected Ca²⁺ import into the TGN and secretory cargo sorting. Altogether, our findings reveal the mechanism of CFL-1–dependent recruitment of actin to SPCA1 and the significance of this interaction for Ca²⁺ influx and secretory cargo sorting.

Changes in secretory pathway Ca(2+)-ATPase 2 following focal cerebral ischemia/reperfusion injury

This study aimed to investigate changes in secretory pathway Ca²⁺-ATPase 2 expression following cerebral ischemia/reperfusion injury, and to define the role of Ca²⁺-ATPases in oxidative stress. A rat model of cerebral ischemia/reperfusion injury was established using the unilateral middle cerebral artery occlusion method. Immunohistochemistry and reverse transcription-PCR assay results showed that compared with the control group, the expression of secretory pathway Ca²⁺-ATPase 2 protein and mRNA in the cerebral cortex and hippocampus of male rats did not significantly change during the ischemic period. However, secretory pathway Ca²⁺-ATPase 2 protein and mRNA expression reduced gradually at 1, 3, and 24 hours during the reperfusion period. Our experimental findings indicate that levels of secretory pathway Ca²⁺-ATPase 2 protein and mRNA expression in brain tissue change in response to cerebral ischemia/reperfusion injury.

Overlapping ATP2C1 and ASTE1 genes in human genome: implications for SPCA1 expression?

The ATP2C1 gene encodes for the secretory pathway calcium (Ca²⁺)-ATPase pump (SPCA1), which localizes along the secretory pathway, mainly in the trans-Golgi. The loss of one ATP2C1 allele causes Hailey-Hailey disease in humans but not mice. Examining differences in genomic organization between mouse and human we speculate that the overlap between ATP2C1 and ASTE1 genes only in humans could explain this different response to ATP2C1 dysregulation. We propose that ASTE1, overlapping with ATP2C1 in humans, affects alternative splicing, and potentially protein expression of the latter. If dysregulated, the composition of the SPCA1 isoform pool could diverge from the physiological status, affecting cytosolic Ca²⁺-signaling, and in turn perturbing cell division, leading to cell death or to neoplastic transformation.

The intracellular Ca²⁺ channels of membrane traffic

Regulation of organellar fusion and fission by Ca ( 2+) has emerged as a central paradigm in intracellular membrane traffic. Originally formulated for Ca ( 2+) -driven SNARE-mediated exocytosis in the presynaptic terminals, it was later expanded to explain membrane traffic in other exocytic events within the endo-lysosomal system. The list of processes and conditions that depend on the intracellular membrane traffic includes aging, antigen and lipid processing, growth factor signaling and enzyme secretion. Characterization of the ion channels that regulate intracellular membrane fusion and fission promises novel pharmacological approaches in these processes when their function becomes aberrant. The recent identification of Ca ( 2+) permeability through the intracellular ion channels comprising the mucolipin (TRPMLs) and the two-pore channels (TPCs) families pinpoints the candidates for the Ca ( 2+) channel that drive intracellular membrane traffic. The present review summarizes the recent developments and the current questions relevant to this topic.

Do annexins participate in lipid messenger mediated intracellular signaling? A question revisited

Annexins are physiologically important proteins that play a role in calcium buffering but also influence membrane structure, participate in Ca²⁺-dependent membrane repair events and in remodelling of the cytoskeleton. Thirty years ago several peptides isolated from lung perfusates, peritoneal leukocytes, neutrophiles and renal cells were proven inhibitory to the activity of phospholipase A₂. Those peptides were found to derive from structurally related proteins: annexins AnxA1 and AnxA2. These findings raised the question whether annexins may participate in regulation of the production of lipid second messengers and, therefore, modulate numerous lipid mediated signaling pathways in the cell. Recent advances in the field of annexins made also with the use of knock-out animal models revealed that these proteins are indeed important constituents of specific signaling pathways. In this review we provide evidence supporting the hypothesis that annexins, as membrane-binding proteins and organizers of the membrane lateral heterogeneity, may participate in lipid mediated signaling pathways by affecting the distribution and activity of lipid metabolizing enzymes (most of the reports point to phospholipase A₂) and of protein kinases regulating activity of these enzymes. Moreover, some experimental data suggest that annexins may directly interact with lipid metabolizing enzymes and, in a calcium-dependent or independent manner, with some of their substrates and products. On the basis of these observations, many investigators suggest that annexins are capable of linking Ca²⁺, redox and lipid signaling to coordinate vital cellular responses to the environmental stimuli.

Annexins as organizers of cholesterol- and sphingomyelin-enriched membrane microdomains in Niemann-Pick type C disease

Growing evidence suggests that membrane microdomains enriched in cholesterol and sphingomyelin are sites for numerous cellular processes, including signaling, vesicular transport, interaction with pathogens, and viral infection, etc. Recently some members of the annexin family of conserved calcium and membrane-binding proteins have been recognized as cholesterol-interacting molecules and suggested to play a role in the formation, stabilization, and dynamics of membrane microdomains to affect membrane lateral organization and to attract other proteins and signaling molecules onto their territory. Furthermore, annexins were implicated in the interactions between cytosolic and membrane molecules, in the turnover and storage of cholesterol and in various signaling pathways. In this review, we focus on the mechanisms of interaction of annexins with lipid microdomains and the role of annexins in membrane microdomains dynamics including possible participation of the domain-associated forms of annexins in the etiology of human lysosomal storage disease called Niemann-Pick type C disease, related to the abnormal storage of cholesterol in the lysosome-like intracellular compartment. The involvement of annexins and cholesterol/sphingomyelin-enriched membrane microdomains in other pathologies including cardiac dysfunctions, neurodegenerative diseases, obesity, diabetes mellitus, and cancer is likely, but is not supported by substantial experimental observations, and therefore awaits further clarification.

Calcium around the Golgi apparatus: implications for intracellular membrane trafficking

As with other complex cellular functions, intracellular membrane transport involves the coordinated engagement of a series of organelles and machineries; in the last couple of decades more importance has been given to the role of calcium (Ca(2+)) in the regulation of membrane trafficking, which is directly involved in coordinating the endoplasmic reticulum-to-Golgi-to-plasma membrane delivery of cargo. Consequently, the Golgi apparatus (GA) is now considered not just the place proteins mature in as they move to their final destination(s), but it is increasingly viewed as an intracellular Ca(2+) store. In the last few years the mechanisms regulating the homeostasis of Ca(2+) in the GA and its role in membrane trafficking have begun to be elucidated. Here, these recent discoveries that shed light on the role Ca(2+) plays as of trigger of different steps during membrane trafficking has been reviewed. This includes recruitment of proteins and SNARE cofactors to the Golgi membranes, which are both fundamental for the membrane remodeling and the regulation of fusion/fission events occurring during the passage of cargo across the GA. I conclude by focusing attention on Ca(2+) homeostasis dysfunctions in the GA and their related pathological implications.

Proteomic analysis of lipid raft-enriched membranes isolated from internal organelles

The mitochondria-associated membrane (MAM) is a sub-region of the endoplasmic reticulum (ER) that facilitates crosstalk between the ER and mitochondria. The MAM actively influences vital cellular processes including Ca(2+) signaling and protein folding. Detergent-resistant microdomains (DRMs) may localize proteins to the mitochondria/MAM interface to coordinate these events. However, the protein composition of DRMs isolated from this region is not known. Lipid-raft enriched DRMs were isolated from a combined mitochondria/MAM sample and analyzed using two-dimensional reversed-phased tandem mass spectrometry. Strict post-acquisition filtering of the acquired data led to the confident identification 250 DRM proteins. The majority (58%) of the identified proteins are bona fide mitochondrial or ER proteins according to Gene Ontology annotation. Additionally, 74% of the proteins have previously been noted as MAM-resident or -associated proteins. Furthermore, ∼20% of the identified proteins have a documented association with lipid rafts. Most importantly, known internal LR marker proteins (inositol 1,4,5-trisphosphate receptor type 3, erlin-2, and voltage-dependent anion channel 1) were detected as well as most of the components of the mitochondrial/MAM-localized Ca(2+) signaling complex. Our study provides the basis for future work probing how the protein activities at the mitochondrion/MAM interface are dependent upon the integrity of these internal lipid-raft-like domains.

Reduced levels of folate transporters (PCFT and RFC) in membrane lipid rafts result in colonic folate malabsorption in chronic alcoholism

We studied the effect of chronic ethanol ingestion on folate transport across the colonic apical membranes (CAM) in rats. Male Wistar rats were fed 1 g/kg body weight/day ethanol (20%) solution orally for 3 months and folate transport was studied in the isolated colon apical membrane vesicles. The folate transport was found to be carrier mediated, saturable, with pH optima at 5.0. Chronic ethanol ingestion reduced the folate transport across the CAM by decreasing the affinity of transporters (high Km) for the substrate and by decreasing the number of transporter molecules (low Vmax) on the colon luminal surface. The decreased transport activity at the CAM was associated with down-regulation of the proton-coupled folate transporter (PCFT) and the reduced folate carrier (RFC) which resulted in decreased PCFT and RFC protein levels in the colon of rats fed alcohol chronically. Moreover, the PCFT and the RFC were found to be distributed in detergent insoluble fraction of the CAM in rats. Floatation experiments on Optiprep density gradients demonstrated the association of the PCFT and the RFC protein with lipid rafts (LR). Chronic alcoholism decreased the PCFT and the RFC protein levels in the CAM LR in accordance with the decreased synthesis. Hence, we propose that downregulation in the expression of the PCFT and the RFC in colon results in reduced levels of these transporters in colon apical membrane LR as a mechanism of folate malabsorption during chronic alcoholism.