FRET-based sensor analysis reveals caveolae are spatially distinct Ca2+ stores in endothelial cells

Genetically Encoded Fluorescent Indicators for Organellar Calcium Imaging

Optical Ca(2+) indicators are powerful tools for investigating intracellular Ca(2+) signals in living cells. Although a variety of Ca(2+) indicators have been developed, deciphering the physiological functions and spatiotemporal dynamics of Ca(2+) in intracellular organelles remains challenging. Genetically encoded Ca(2+) indicators (GECIs) using fluorescent proteins are promising tools for organellar Ca(2+) imaging, and much effort has been devoted to their development. In this review, we first discuss the key points of organellar Ca(2+) imaging and summarize the requirements for optimal organellar Ca(2+) indicators. Then, we highlight some of the recent advances in the engineering of fluorescent GECIs targeted to specific organelles. Finally, we discuss the limitations of currently available GECIs and the requirements for advancing the research on intraorganellar Ca(2+) signaling.

Peroxisome Proliferator-Activated Receptor γ-Mediated Inhibition on Hypoxia-Triggered Store-Operated Calcium Entry. A Caveolin-1-Dependent Mechanism

Our previous publication demonstrated that peroxisome proliferator-activated receptor γ (PPARγ) inhibits the pathogenesis of chronic hypoxia (CH)-induced pulmonary hypertension by targeting store-operated calcium entry (SOCE) in rat distal pulmonary arterial smooth muscle cells (PASMCs). In this study, we aim to determine the role of a membrane scaffolding protein, caveolin-1, during the suppressive process of PPARγ on SOCE. Adult (6-8 weeks) male Wistar rats (200-250 g) were exposed to CH (10% O2) for 21 days to establish CH-induced pulmonary hypertension. Primary cultured rat distal PASMCs were applied for the molecular biological experiments. First, hypoxic exposure led to 2.5-fold and 1-fold increases of caveolin-1 protein expression in the distal pulmonary arteries and PASMCs, respectively. Second, effective knockdown of caveolin-1 significantly reduced hypoxia-induced SOCE for 58.2% and 41.5%, measured by Mn(2+) quenching and extracellular Ca(2+) restoration experiments, respectively. These results suggested that caveolin-1 acts as a crucial regulator of SOCE, and hypoxia-up-regulated caveolin-1 largely accounts for hypoxia-elevated SOCE in PASMCs. Then, by using a high-potency PPARγ agonist, GW1929, we detected that PPARγ activation inhibited SOCE and caveolin-1 protein for 62.5% and 59.8% under hypoxia, respectively, suggesting that caveolin-1 also acts as a key target during the suppressive process of PPARγ on SOCE in PASMCs. Moreover, by using effective small interfering RNAs against PPARγ and caveolin-1, and PPARγ antagonist, T0070907, we observed that PPARγ plays an inhibitory role on caveolin-1 protein by promoting its lysosomal degradation, without affecting the messenger RNA level. PPARγ inhibits SOCE, at least partially, by suppressing cellular caveolin-1 protein in PASMCs.

A novel chimeric aequorin fused with caveolin-1 reveals a sphingosine kinase 1-regulated Ca²⁺ microdomain in the caveolar compartment

Caveolae are plasma membrane invaginations enriched in sterols and sphingolipids. Sphingosine kinase 1 (SK1) is an oncogenic protein that converts sphingosine to sphingosine 1-phosphate (S1P), which is a messenger molecule involved in calcium signaling. Caveolae contain calcium responsive proteins, but the effects of SK1 or S1P on caveolar calcium signaling have not been investigated. We generated a Caveolin-1-Aequorin fusion protein (Cav1-Aeq) that can be employed for monitoring the local calcium concentration at the caveolae ([Ca²⁺]cav). In HeLa cells, Cav1-Aeq reported different [Ca²⁺] as compared to the plasma membrane [Ca²⁺] in general (reported by SNAP25-Aeq) or as compared to the cytosolic [Ca²⁺] (reported by cyt-Aeq). The Ca²⁺ signals detected by Cav1-Aeq were significantly attenuated when the caveolar structures were disrupted by methyl-β-cyclodextrin, suggesting that the caveolae are specific targets for Ca²⁺ signaling. HeLa cells overexpressing SK1 showed increased [Ca²⁺]cav during histamine-induced Ca²⁺ mobilization in the absence of extracellular Ca²⁺ as well as during receptor-operated Ca²⁺ entry (ROCE). The SK1-induced increase in [Ca²⁺]cav during ROCE was reverted by S1P receptor antagonists. In accordance, pharmacologic inhibition of SK1 reduced the [Ca²⁺]cav during ROCE. S1P treatment stimulated the [Ca²⁺]cav upon ROCE. The Ca²⁺ responses at the plasma membrane in general were not affected by SK1 expression. In summary, our results show that SK1/S1P-signaling regulates Ca²⁺ signals at the caveolae. This article is part of a Special Issue entitled: 13th European Symposium on Calcium.

Dynamic visualization of calcium-dependent signaling in cellular microdomains

Cells rely on the coordinated action of diverse signaling molecules to sense, interpret, and respond to their highly dynamic external environment. To ensure the specific and robust flow of information, signaling molecules are often spatially organized to form distinct signaling compartments, and our understanding of the molecular mechanisms that guide intracellular signaling hinges on the ability to directly probe signaling events within these cellular microdomains. Ca(2+) signaling in particular owes much of its functional versatility to this type of exquisite spatial regulation. As discussed below, a number of methods have been developed to investigate the mechanistic and functional implications of microdomains of Ca(2+) signaling, ranging from the application of Ca(2+) buffers to the direct and targeted visualization of Ca(2+) signaling microdomains using genetically encoded fluorescent reporters.

Acidic intracellular Ca(2+) stores and caveolae in Ca(2+) signaling and diabetes

Acidic Ca(2+) stores, particularly lysosomes, are newly discovered players in the well-orchestrated arena of Ca(2+) signaling and we are at the verge of understanding how lysosomes accumulate Ca(2+) and how they release it in response to different chemical, such as NAADP, and physical signals. Additionally, it is now clear that lysosomes play a key role in autophagy, a process that allows cells to recycle components or to eliminate damaged structures to ensure cellular well-being. Moreover, lysosomes are being unraveled as hubs that coordinate both anabolism via insulin signaling and catabolism via AMPK. These acidic vesicles have close contact with the ER and there is a bidirectional movement of information between these two organelles that exquisitely regulates cell survival. Lysosomes also connect with plasma membrane where caveolae are located as specialized regions involved in Ca(2+) and insulin signaling. Alterations of all these signaling pathways are at the core of insulin resistance and diabetes.

Compartmentalisation of second messenger signalling pathways

The ability of a cell to transform an extracellular stimulus into a downstream event that directs specific physiological outcomes, requires the orchestrated, spatial and temporal response of many signalling proteins. The notion of compartmentalised signalling pathways was popularised in the 1980s by Brunton and colleagues, with their discovery that spatially segregated cAMP directs a variety of signalling responses in cardiomyocytes. It is now understood that compartmentalisation is a common mechanism used by all cells to ensure the interaction of signalling 'second messenger' molecules with localised 'pools' of appropriate effector proteins. In this way, the cell can elicit differential cellular responses by using a single, freely diffusible, molecular species. Recently, the compartmentalisation schemes employed by signalling systems involving cyclic nucleotides, calcium and nitric oxide have been elucidated and as a result, the varied range of functional consequences underpinned by such strategies can be better appreciated.

Measuring baseline Ca(2+) levels in subcellular compartments using genetically engineered fluorescent indicators

Intracellular Ca(2+) signaling is involved in a series of physiological and pathological processes. In particular, an intimate crosstalk between bioenergetic metabolism and Ca(2+) homeostasis has been shown to determine cell fate in resting conditions as well as in response to stress. The endoplasmic reticulum and mitochondria represent key hubs of cellular metabolism and Ca(2+) signaling. However, it has been challenging to specifically detect highly localized Ca(2+) fluxes such as those bridging these two organelles. To circumvent this issue, various recombinant Ca(2+) indicators that can be targeted to specific subcellular compartments have been developed over the past two decades. While the use of these probes for measuring agonist-induced Ca(2+) signals in various organelles has been extensively described, the assessment of basal Ca(2+) concentrations within specific organelles is often disregarded, in spite of the fact that this parameter is vital for several metabolic functions, including the enzymatic activity of mitochondrial dehydrogenases of the Krebs cycle and protein folding in the endoplasmic reticulum. Here, we provide an overview on genetically engineered, organelle-targeted fluorescent Ca(2+) probes and outline their evolution. Moreover, we describe recently developed protocols to quantify baseline Ca(2+) concentrations in specific subcellular compartments. Among several applications, this method is suitable for assessing how changes in basal Ca(2+) levels affect the metabolic profile of cancer cells.