Effect of intracellular lipid droplets on cytosolic Ca2+ and cell death during ischaemia-reperfusion injury in cardiomyocytes

Calcium ion delivery by microbubble-assisted sonoporation stimulates cell death in human gastrointestinal cancer cells

Ultrasound-mediated cell membrane permeabilization - sonoporation, enhances drug delivery directly to tumor sites while reducing systemic side effects. The potential of ultrasound to augment intracellular calcium uptake - a critical regulator of cell death and proliferation - offers innovative alternative to conventional chemotherapy. However, calcium therapeutic applications remain underexplored in sonoporation studies. This research provides a comprehensive analysis of calcium sonoporation (CaSP), which combines ultrasound treatment with calcium ions and SonoVue microbubbles, on gastrointestinal cancer cells LoVo and HPAF-II. Initially, optimal sonoporation parameters were determined: an acoustic wave of 1 MHz frequency with a 50 % duty cycle at intensity of 2 W/cm2. Subsequently, various cellular bioeffects, such as viability, oxidative stress, metabolism, mitochondrial function, proliferation, and cell death, were assessed following CaSP treatment. CaSP significantly impaired cancer cell function by inducing oxidative and metabolic stress, evidenced by increased mitochondrial depolarization, decreased ATP levels, and elevated glucose uptake in a Ca2+ dose-dependent manner, leading to activation of the intrinsic apoptotic pathway. Cellular response to CaSP depended on the TP53 gene's mutational status: colon cancer cells were more susceptible to CaSP-induced apoptosis and G1 phase cell cycle arrest, whereas pancreatic cancer cells showed a higher necrotic response and G2 cell cycle arrest. These promising results encourage future research to optimize sonoporation parameters for clinical use, investigate synergistic effects with existing treatments, and assess long-term safety and efficacy in vivo. Our study highlights CaSP's clinical potential for improved safety and efficacy in cancer therapy, offering significant implications for the pharmaceutical and biomedical fields.

Intracellular lipase and regulation of the lipid droplet

PURPOSE OF REVIEW

Lipid droplets are increasingly recognized as distinct intracellular organelles that have functions exclusive to the storage of energetic lipids. Lipid droplets modulate macrophage inflammatory phenotype, control the availability of energy for muscle function, store excess lipid, sequester toxic lipids, modulate mitochondrial activity, and allow transfer of fatty acids between tissues.

RECENT FINDINGS

There have been several major advances in our understanding of the formation, dissolution, and function of this organelle during the past two years. These include new information on movement and partition of amphipathic proteins between the cytosol and lipid droplet surface, molecular determinants of lipid droplet formation, and pathways leading to lipid droplet hydrophobic lipid formation. Rapid advances in mitochondrial biology have also begun to define differences in their function and partnering with lipid droplets to modulate lipid storage versus oxidation.

SUMMARY

This relationship of lipid droplets biology and cellular function provides new understanding of an important cellular organelle that influences muscle function, adipose lipid storage, and diseases of lipotoxicity.

Lipid droplets, autophagy, and ageing: A cell-specific tale

Lipid droplets are the essential organelle for storing lipids in a cell. Within the variety of the human body, different cells store, utilize and release lipids in different ways, depending on their intrinsic function. However, these differences are not well characterized and, especially in the context of ageing, represent a key factor for cardiometabolic diseases. Whole body lipid homeostasis is a central interest in the field of cardiometabolic diseases. In this review we characterize lipid droplets and their utilization via autophagy and describe their diverse fate in three cells types central in cardiometabolic dysfunctions: adipocytes, hepatocytes, and macrophages.

Hello from the other side: Membrane contact of lipid droplets with other organelles and subsequent functional implications

Lipid droplets (LDs) are ubiquitous organelles that play crucial roles in response to physiological and environmental cues. The identification of several neutral lipid synthesizing and regulatory protein complexes have propelled significant advance on the mechanisms of LD biogenesis in the endoplasmic reticulum (ER). Increasing evidence suggests that distinct proteins and regulatory factors, which localize to membrane contact sites (MCS), are involved not only in interorganellar lipid exchange and transport, but also function in other important cellular processes, including autophagy, mitochondrial dynamics and inheritance, ion signaling and inter-regulation of these MCS. More and more tethers and molecular determinants are associated to MCS and to a diversity of cellular and pathophysiological processes, demonstrating the dynamics and importance of these junctions in health and disease. The conjugation of lipids with proteins in supramolecular complexes is known to be paramount for many biological processes, namely membrane biosynthesis, cell homeostasis, regulation of organelle division and biogenesis, and cell growth. Ultimately, this physical organization allows the contact sites to function as crucial metabolic hubs that control the occurrence of chemical reactions. This leads to biochemical and metabolite compartmentalization for the purposes of energetic efficiency and cellular homeostasis. In this review, we will focus on the structural and functional aspects of LD-organelle interactions and how they ensure signaling exchange and metabolites transfer between organelles.

Modulation of Cardiac Arrhythmogenesis by Epicardial Adipose Tissue: JACC State-of-the-Art Review

Obesity is a significant risk factor for arrhythmic cardiovascular death. Interactions between epicardial adipose tissue (EAT) and myocytes are thought to play a key role in the development of arrhythmias. In this review, the authors investigate the influence of EAT on arrhythmogenesis. First, they summarize electrocardiographic evidence showing the association between increased EAT volume and atrial and ventricular conduction delay. Second, they detail the structural cross talk between EAT and the heart and its arrhythmogenicity. Adipose tissue infiltration within the myocardium constitutes an anatomical obstacle to cardiac excitation. It causes activation delay and increases the risk of arrhythmias. Intercellular electrical coupling between cardiomyocytes and EAT can further slow conduction and increase the risk of block, favoring re-entry and arrhythmias. Finally, EAT secretes multiple substances that influence cardiomyocyte electrophysiology either by modulating ion currents and electrical coupling or by stimulating fibrosis. Thus, structural and paracrine cross talk between EAT and cardiomyocytes facilitates arrhythmias.

Rediscovering potential molecular targets for glioma therapy through the analysis of the cell of origin, microenvironment, and metabolism

Gliomas are the most common type of malignant brain tumors. Despite significant medical advances, gliomas remain incurable and are associated with high mortality. Although numerous biomarkers of diagnostic value have been identified and significant progress in the prognosis of the outcome has been made, the treatment has not been parallelly improved during the last three decades. This review summarizes and discusses three aspects of recent discoveries related to glioma, with the objective to highlight the advantages of glioma-specific drugs targeting the cell of origin, microenvironment, and metabolism. Given the heterogeneous nature of gliomas, various cell populations have been implicated as likely sources of the tumor. Depending on the mutation(s) acquired by the cells, it is believed that neuronal stem/progenitor cells, oligodendrocyte progenitor cells, mature neurons, and glial cells can initiate cell transformation into a malignant phenotype. The level of tumorigenicity appears to be inversely correlated with the maturation of a given cell population. The microenvironment of gliomas includes non-cancer cells such as immune cells, fibroblasts, and cells of blood vessels, as well as secreted molecules and the extracellular matrix, and all these components play a vital role during tumor initiation and progression. We will discuss in detail how the tumor microenvironment can stimulate and drive the transformation of non-tumor cell populations into tumor-supporting cells or glioma cells. Metabolic reprogramming is a key feature of gliomas and is thought to reflect the adaptation to the increased nutritional requirements of tumor cell proliferation, growth, and survival. Mutations in the IDH gene can shape metabolic reprogramming and may generate some vulnerabilities in glioma cells, such as abnormal lipid metabolism and sensitivity to endoplasmic reticulum stress (ERS). We will analyze the prominent metabolic features of malignant gliomas and the key pathways regulating glioma metabolism. This review is intended to provide a conceptual background for the development of glioma therapies based on the properties of tumor cell populations, microenvironment, and metabolism.

Isolated Plin5-deficient cardiomyocytes store less lipid droplets than normal, but without increased sensitivity to hypoxia

Plin5 is abundantly expressed in the heart where it binds to lipid droplets (LDs) and facilitates physical interaction between LDs and mitochondria. We isolated cardiomyocytes from adult Plin5^+/+ and Plin5^-/- mice to study the role of Plin5 for fatty acid uptake, LD accumulation, fatty acid oxidation, and tolerance to hypoxia. Cardiomyocytes isolated from Plin5^-/- mice cultured with oleic acid stored less LDs than Plin5^+/+, but comparable levels to Plin5^+/+ cardiomyocytes when adipose triglyceride lipase activity was inhibited. The ability to oxidize fatty acids into CO₂ was similar between Plin5^+/+ and Plin5^-/- cardiomyocytes, but Plin5^-/- cardiomyocytes had a transient increase in intracellular fatty acid oxidation intermediates. After pre-incubation with oleic acids, Plin5^-/- cardiomyocytes retained a higher content of glycogen and showed improved tolerance to hypoxia compared to Plin5^+/+. In isolated, perfused hearts, deletion of Plin5 had no important effect on ventricular pressures or infarct size after ischemia. Old Plin5^-/- mice had reduced levels of cardiac triacylglycerides, increased heart weight, and apart from modest elevated expression of mRNAs for beta myosin heavy chain Myh7 and the fatty acid transporter Cd36, other genes involved in fatty acid oxidation, glycogen metabolism and glucose utilization were essentially unchanged by removal of Plin5. Plin5 seems to facilitate cardiac LD storage primarily by repressing adipose triglyceride lipase activity without altering cardiac fatty acid oxidation capacity. Expression of Plin5 and cardiac LD content of isolated cardiomyocytes has little importance for tolerance to acute hypoxia and ischemia, which contrasts the protective role for Plin5 in mouse models during myocardial ischemia.

Enhancing Glycolysis Protects against Ischemia-Reperfusion Injury by Reducing ROS Production

After myocardial ischemia-reperfusion, fatty acid oxidation shows fast recovery while glucose oxidation rates remain depressed. A metabolic shift aimed at increasing glucose oxidation has shown to be beneficial in models of myocardial ischemia-reperfusion. However, strategies aimed at increasing glucose consumption in the clinic have provided mixed results and have not yet reached routine clinical practice. A better understanding of the mechanisms underlying the protection afforded by increased glucose oxidation may facilitate the transfer to the clinic. The purpose of this study was to evaluate if the modulation of reactive oxygen species (ROS) was involved in the protection afforded by increased glucose oxidation. Firstly, we characterized an H9C2 cellular model in which the use of glucose or galactose as substrates can modulate glycolysis and oxidative phosphorylation pathways. In this model, there were no differences in morphology, cell number, or ATP and PCr levels. However, galactose-grown cells consumed more oxygen and had an increased Krebs cycle turnover, while cells grown in glucose had increased aerobic glycolysis rate as demonstrated by higher lactate and alanine production. Increased aerobic glycolysis was associated with reduced ROS levels and protected the cells against simulated ischemia-reperfusion injury. Furthermore, ROS scavenger N-acetyl cysteine (NAC) was able to reduce the amount of ROS and to prevent cell death. Lastly, cells grown in galactose showed higher activation of mTOR/Akt signaling pathways. In conclusion, our results provide evidence indicating that metabolic shift towards increased glycolysis reduces mitochondrial ROS production and prevents cell death during ischemia-reperfusion injury.

Lipid Droplets and the Management of Cellular Stress

Lipid droplets are cytosolic fat storage organelles present in most eukaryotic cells. Long regarded merely as inert fat reservoirs, they are now emerging as major regulators of cellular metabolism. They act as hubs that coordinate the pathways of lipid uptake, distribution, storage, and use in the cell. Recent studies have revealed that they are also essential components of the cellular stress response. One of the hallmark characteristics of lipid droplets is their capacity to buffer excess lipids and to finely tune their subsequent release based on specific cellular requirements. This simple feature of lipid droplet biology, buffering and delayed release of lipids, forms the basis for their pleiotropic roles in the cellular stress response. In stressed cells, lipid droplets maintain energy and redox homeostasis and protect against lipotoxicity by sequestering toxic lipids into their neutral lipid core. Their mobility and dynamic interactions with mitochondria enable an efficient delivery of fatty acids for optimal energy production. Lipid droplets are also involved in the maintenance of membrane and organelle homeostasis by regulating membrane composition, preventing lipid peroxidation and removing damaged proteins and lipids. Finally, they also engage in a symbiotic relationship with autophagy and act as reservoirs of bioactive lipids that regulate inflammation and immunity. Thus, lipid droplets are central managers of lipid metabolism that function as safeguards against various types of cellular stress.

Empagliflozin normalizes the size and number of mitochondria and prevents reduction in mitochondrial size after myocardial infarction in diabetic hearts

To explore mechanisms by which SGLT2 inhibitors protect diabetic hearts from heart failure, we examined the effect of empagliflozin (Empa) on the ultrastructure of cardiomyocytes in the noninfarcted region of the diabetic heart after myocardial infarction (MI). OLETF, a rat model of type 2 diabetes, and its nondiabetic control, LETO, received a sham operation or left coronary artery ligation 12 h before tissue sampling. Tissues were sampled from the posterior ventricle (i.e., the remote noninfarcted region in rats with MI). The number of mitochondria was larger and small mitochondria were more prevalent in OLETF than in LETO. Fis1 expression level was higher in OLETF than in LETO, while phospho‐Ser637‐Drp1, total Drp1, Mfn1/2, and OPA1 levels were comparable. MI further reduced the size of mitochondria with increased Drp1‐Ser616 phosphorylation in OLETF. The number of autophagic vacuoles was unchanged after MI in LETO but was decreased in OLETF. Lipid droplets in cardiomyocytes and tissue triglycerides were increased in OLETF. Empa administration (10 mg/kg per day) reduced blood glucose and triglycerides and paradoxically increased lipid droplets in cardiomyocytes in OLETF. Empa suppressed Fis1 upregulation, increased Bnip3 expression, and prevented reduction in both mitochondrial size and autophagic vacuole number after MI in OLETF. Together with the results of our parallel study showing upregulation of SOD2 and catalase by Empa, the results indicate that Empa normalizes the size and number of mitochondria in diabetic hearts and that diabetes‐induced excessive reduction in mitochondrial size after MI was prevented by Empa via suppression of ROS and restoration of autophagy.

IL-6: A Potential Role in Cardiac Metabolic Homeostasis

Interleukin-6 (IL-6) is implicated in multiple biological functions including immunity, neural development, and haematopoiesis. Recently, mounting evidence indicates that IL-6 plays a key role in metabolism, especially lipid metabolic homeostasis. A working heart requires a high and constant energy input which is largely generated by fatty acid (FA) β-oxidation. Under pathological conditions, the precise balance between cardiac FA uptake and metabolism is perturbed so that excessive FA is accumulated, thereby predisposing to myocardial dysfunction (cardiac lipotoxicity). In this review, we summarize the current evidence that suggests the involvement of IL-6 in lipid metabolism. Cardiac metabolic features and consequences of myocardial lipotoxicity are also briefly analyzed. Finally, the roles of IL-6 in cardiac FA uptake (i.e., serum lipid profile and myocardial FA transporters) and FA metabolism (namely, β-oxidation, mitochondrial function, biogenesis, and FA de novo synthesis) are discussed. Overall, understanding how IL-6 transmits signals to affect lipid metabolism in the heart might allow for development of better clinical therapies for obesity-associated cardiac lipotoxicity.

Lysophosphatidylcholine acyltransferase 2-mediated lipid droplet production supports colorectal cancer chemoresistance

Lipid droplet (LD) accumulation is a now well-recognised hallmark of cancer. However, the significance of LD accumulation in colorectal cancer (CRC) biology is incompletely understood under chemotherapeutic conditions. Since drug resistance is a major obstacle to treatment success, we sought to determine the contribution of LD accumulation to chemotherapy resistance in CRC. Here we show that LD content of CRC cells positively correlates with the expression of lysophosphatidylcholine acyltransferase 2 (LPCAT2), an LD-localised enzyme supporting phosphatidylcholine synthesis. We also demonstrate that LD accumulation drives cell-death resistance to 5-fluorouracil and oxaliplatin treatments both in vitro and in vivo. Mechanistically, LD accumulation impairs caspase cascade activation and ER stress responses. Notably, droplet accumulation is associated with a reduction in immunogenic cell death and CD8⁺ T cell infiltration in mouse tumour grafts and metastatic tumours of CRC patients. Collectively our findings highlight LPCAT2-mediated LD accumulation as a druggable mechanism to restore CRC cell sensitivity.

Lipid droplets (LD) accumulation correlates with colorectal cancer (CRC) relapse. Here the authors show that chemotherapy induces LD synthesis via acyltransferase LPCAT2 which, in turn, promotes chemoresistance via LD accumulation both in vitro and in vivo by blocking chemotherapy-induced ER stress.

Anaesthetic-induced cardioprotection in an experimental model of the Takotsubo syndrome - isoflurane vs. propofol

BACKGROUND

Takotsubo syndrome (TS) is an acute cardiac condition with a substantial mortality for which no specific treatment is available. We have previously shown that isoflurane attenuates the development of left ventricular (LV) dysfunction in an experimental TS-model. We compared the effects of equi-anaesthetic doses of isoflurane, propofol and ketamine+midazolam on haemodynamics, global and regional LV systolic function and the activation of intracellular metabolic pathways in experimental TS. We hypothesized that cardioprotection in experimental TS is specific for isoflurane.

METHODS

Forty-five rats were randomized to isoflurane (0.6 MAC, n = 15), propofol (bolus 200 mg/kg+360 mg/kg/h, n = 15) or ketamine (100 mg/kg)+midazolam (10 mg/kg, n = 15) anaesthesia. Arterial pressure, heart rate and body temperature were continuously measured and arterial blood gas analysis was performed intermittently. TS was induced by intraperitoneal injection of isoprenaline, 50 mg/kg. LV echocardiography was performed 90 min after isoprenaline injection. Apical cardiac tissue was analysed by global discovery proteomics and pathway analysis.

RESULTS

Isoprenaline-induced changes in arterial blood pressure, heart rate or body temperature did not differ between groups. LV ejection fraction was higher and extent of LV akinesia was lower with isoflurane, when compared with the propofol and the ketamine+midazolam groups. In this TS-model, the proteomic analysis revealed an up-regulation of pathways involved in inflammation, coagulation, endocytosis and lipid metabolism. This up-regulation was clearly attenuated with isoflurane compared to propofol.

CONCLUSION

In an experimental model of TS, isoflurane, but not propofol, exerts a cardioprotective effect. The proteomic analysis suggests that inflammation might be involved in pathogenesis of TS.

Lipid body accumulation alters calcium signaling dynamics in immune cells

There is well-established variability in the numbers of lipid bodies (LB) in macrophages, eosinophils, and neutrophils. Similarly to the steatosis observed in adipocytes and hepatocytes during hyperinsulinemia and nutrient overload, immune cell LB hyper-accumulate in response to bacterial and parasitic infection and inflammatory presentations. Recently we described that hyperinsulinemia, both in vitro and in vivo, drives steatosis and phenotypic changes in primary and transformed mast cells and basophils. LB reach high numbers in these steatotic cytosols, and here we propose that they could dramatically impact the transcytoplasmic signaling pathways. We compared calcium release and influx responses at the population and single cell level in normal and steatotic model mast cells. At the population level, all aspects of FcɛRI-dependent calcium mobilization, as well as activation of calcium-dependent downstream signaling targets such as NFATC1 phosphorylation are suppressed. At the single cell level, we demonstrate that LB are both sources and sinks of calcium following FcɛRI cross-linking. Unbiased analysis of the impact of the presence of LB on the rate of trans-cytoplasmic calcium signals suggest that LB enrichment accelerates calcium propagation, which may reflect a Bernoulli effect. LB abundance thus impacts this fundamental signaling pathway and its downstream targets.

Activation of Liver X receptors in the heart leads to accumulation of intracellular lipids and attenuation of ischemia-reperfusion injury

Liver X receptor (LXR)-α and -β play a major role in lipid and glucose homeostasis. Their expression and function in the heart is not well characterized. Our aim was to describe the expression of LXRs in the murine heart, and to determine effects of cardiac LXR activation on target gene expression, lipid homeostasis and ischemia. Both LXRα and -β were expressed in heart tissues, HL-1 cells and isolated cardiomyocytes as determined by qRT-PCR. Elevated cardiac expression of LXR target genes and LXRβ was observed 24 h after in vivo permanent coronary artery ligation. The synthetic LXR agonist GW3965 induced mRNA expression of the LXR target genes in HL-1 cells and isolated cardiomyocytes. This was associated with a buildup of intracellular triglycerides and expanding lipid droplets as quantified by confocal microscopy. Mice injected with GW3965 had cardiac LXR activation as judged by increased target gene expression and lipid droplet accumulation. GW3965 in vivo and in vitro increased expression of genes inducing triglyceride synthesis, and altered expression of lipid droplet-binding protein genes. GW3965 protected HL-1 cells against hypoxia-reoxygenation induced apoptosis. LXR activation by GW3965 in vivo prior to heart isolation and perfusion with induced global ischemia and reperfusion improved left ventricular contractile function and decreased infarct size. In conclusion, LXRs are expressed in the murine heart in the basal state, and are activated by myocardial infarction. Activation of LXR by the synthetic agonist GW3965 is associated with intracardiac accumulation of lipid droplets and protection against myocardial ischemia-reperfusion injury.

Low density lipoprotein receptor-related protein 1 expression correlates with cholesteryl ester accumulation in the myocardium of ischemic cardiomyopathy patients

Our hypothesis was that overexpression of certain lipoprotein receptors might be related to lipid accumulation in the human ischemic myocardium. Intramyocardial lipid overload contributes to contractile dysfunction and arrhythmias in cardiomyopathy. Thus, the purpose of this study was to assess the effect of hypercholesterolemic LDL and hypertrigliceridemic VLDL dose on LRP1 expression in cardiomyocytes, as well as the potential correlation between LRP1 expression and neutral lipid accumulation in the left ventricle tissue from ischemic cardiomyopathy patients. Cell culture experiments include control and LRP1-deficient cardiomyocytes exposed to lipoproteins under normoxic and hypoxic conditions. Explanted hearts from 18 ICM patients and eight non-diseased hearts (CNT) were included. Low density lipoprotein receptor-related protein 1 (LRP1), very low density lipoprotein receptor (VLDLR) and low density lipoprotein receptor (LDLR) expression was analyzed by real time PCR and Western blotting. Cholesteryl ester (CE), triglyceride (TG) and free cholesterol (FC) content was assess by thin layer chromatography following lipid extraction. Western blotting experiments showed that protein levels of LRP1, VLDLR and HIF-1α were significantly upregulated in ischemic hearts. Immunohistochemistry and confocal microscopy analysis showed that LRP1 and HIF-1α were upregulated in cardiomyocytes of ICM patients. In vitro studies showed that VLDL, LDL and hypoxia exerted an upregulatory effect on LRP1 expression and that LRP1 played a major role in cholesteryl ester accumulation from lipoproteins in cardiomyocytes. Myocardial CE accumulation strongly correlated with LRP1 levels in ischemic hearts. Taken together, our results suggest that LRP1 upregulation is key for myocardial cholesterol ester accumulation in ischemic human hearts and that LRP1 may be a target to prevent the deleterious effects of myocardial cholesterol accumulation in ischemic cardiomyopathy.

NOX4-Dependent Hydrogen Peroxide Overproduction in Human Atrial Fibrillation and HL-1 Atrial Cells: Relationship to Hypertension

Background/Objectives: Atrial fibrillation (AF) is the most common type of cardiac arrhythmia with patients dying frequently of stroke. In view of the unclear etiologies of AF and a potential role of oxidative stress, the present study examined cardiac reactive oxygen species production and NADPH oxidase (NOX) expression in AF patients. Methods and Results: Patients with AF were older than those without (58.8 ± 11.7 vs. 47.8 ± 19.2, p = 0.047). Whereas total O2∙- production (determined by electron spin resonance) was similar in patients with and without AF, H₂O₂ production was more than doubled in AF patients (149.8 ± 26.28 vs. 66.9 ± 7.14 pmol/mg/min, p = 0.0055), which correlated well with a doubling in NOX isoform 4 (NOX4) expression. AF patients with co-existing hypertension had three-fold higher H₂O₂ production compared to those without (239.0 ± 125.1 vs. 83.6 ± 51.3 pmol/mg/min, p = 0.003). Treatment of HL-1 atrial cells with angiotensin II, a known modulator of atrial structural remodeling, resulted in upregulation of NOX4 and H₂O₂ production, further implicating a potential role of NOX4 in atrial remodeling. Conclusion: Our data represent the first implication that NOX4-derived H₂O₂ may play an important role in the etiologies of AF.

Is fat so bad? Modulation of endoplasmic reticulum stress by lipid droplet formation

LDs (lipid droplets) have long been considered as inert particles used by the cells to store fatty acids and sterols as esterified non-toxic lipid species (i.e. triacylglycerols and steryl esters). However, accumulating evidence suggests that LDs behave as a dynamic compartment, which is involved in the regulation of several aspects of the homoeostasis of their originating organelle, namely the ER (endoplasmic reticulum). The ER is particularly sensitive to physiological/pathological stimuli, which can ultimately induce ER stress. In the present review, after considering the basic mechanisms of LD formation and the signal cascades leading to ER stress, we focus on the connections between these two pathways. Taking into consideration recent data from the literature, we will try to draw possible mechanisms for the role of LDs in the regulation of ER homoeostasis and in ER-stress-related diseases.

Not just fat: the structure and function of the lipid droplet

Lipid droplets (LDs) are independent organelles that are composed of a lipid ester core and a surface phospholipid monolayer. Recent studies have revealed many new proteins, functions, and phenomena associated with LDs. In addition, a number of diseases related to LDs are beginning to be understood at the molecular level. It is now clear that LDs are not an inert store of excess lipids but are dynamically engaged in various cellular functions, some of which are not directly related to lipid metabolism. Compared to conventional membrane organelles, there are still many uncertainties concerning the molecular architecture of LDs and how each function is placed in a structural context. Recent findings and remaining questions are discussed.

Loss of the NHE2 Na+/H+ exchanger in mice results in dilation of folliculo-stellate cell canaliculi

Genetic ablation of the NHE2 Na⁺/H⁺ exchanger causes gastric achlorhydria, absorptive defects in kidney and colon, and low fertility. Here we show that NHE2 is expressed in the pituitary, with the highest mRNA expression in pars distalis and lower expression in pars intermedia. In pars distalis of NHE2-null mice, prominent cyst-like dilatations of folliculo-stellate (FS) cell canaliculi developed with age, and there were increased FS cell area, accumulation of lipid in FS cell cytoplasm, redundancies in FS cell basement membrane, and other changes. The expansion of the canaliculi indicates that NHE2 is a major absorptive Na⁺/H⁺ exchanger in the luminal membranes lining the extensive network of channels formed by FS cells, which may provide a means of intrapituitary communication. The results suggest that NHE2 contributes to homeostatic regulation of the volume and composition of the canalicular fluid and may counter the secretory activity of the CFTR Cl⁻ channel, which is known to be expressed in pituitary.

Lipid droplets: size matters

The lipid droplet (LD), an organelle that exists ubiquitously in various organisms, from bacteria to mammals, has attracted much attention from both medical and cell biology fields. The LD in white adipocytes is often treated as the prototype LD, but is rather a special example, considering that its size, intracellular localization and molecular composition are vastly different from those of non-adipocyte LDs. These differences confer distinct properties on adipocyte and non-adipocyte LDs. In this article, we address the current understanding of LDs by discussing the differences between adipocyte and non-adipocyte LDs.

Disruption of Rac1 signaling reduces ischemia-reperfusion injury in the diabetic heart by inhibiting calpain

Diabetes increases myocardial ischemia/reperfusion (I/R) injury. However, the underlying mechanisms remain incompletely understood. This study investigated the role of Rac1 signaling and calpain in exacerbated I/R injury in diabetic hearts. Mice with cardiac-specific deletion of Rac1 (Rac1-ko) and transgenic mice with cardiac-specific superoxide dismutase-2 (SOD2) or calpastatin overexpression were rendered diabetic with streptozotocin. Isolated perfused hearts were subjected to global I/R. After I/R, Rac1 activity was significantly enhanced in diabetic compared with nondiabetic hearts. Diabetic hearts displayed more severe I/R injury than nondiabetic hearts, as evidenced by more lactate dehydrogenase release and apoptosis and decreased cardiac function. These adverse impacts of diabetes were abrogated in Rac1-ko hearts or by perfusion with the Rac1 inhibitor NSC23766. In an in vivo I/R mouse model, infarct size was much smaller in diabetic Rac1-ko compared with wild-type mice. Inhibition of Rac1 signaling prevented NADPH oxidase activation, reactive oxygen species production, and protein carbonyl accumulation, leading to inhibition of calpain activation. Furthermore, SOD2 or calpastatin overexpression significantly reduced I/R injury in diabetic hearts and improved cardiac function after I/R. In summary, Rac1 activation increases I/R injury in diabetic hearts and the role of Rac1 signaling is mediated, at least in part, through calpain activation.

High-precision recording of the action potential in isolated cardiomyocytes using the near-infrared fluorescent dye di-4-ANBDQBS

The use of voltage-sensitive fluorescent dyes (VSD) for noninvasive measurement of the action potential (AP) in isolated cells has been hindered by low-photon yield of the preparation, dye toxicity, and photodynamic damage. Here we used a new red-shifted VSD, di-4-ANBDQBS, and a fast electron-multiplied charge-coupled device camera for optical AP (OAP) recording in guinea pig cardiac myocytes. Loading di-4-ANBDQBS did not alter APs recorded with micropipette. With short laser exposures (just enough to record one OAP every 1-5 min), di-4-ANBDQBS yielded fluorescent signals with very high signal-to-background ratios (change in fluorescence on depolarization/fluorescence at resting potential: 19.2 ± 4.1%) and signal-to-noise ratios (40 ± 13.2). Quantum chemical calculations comparing the ANBDQ chromophore to the conventional ANEP chromophore showed that the higher wavelength and the greater voltage sensitivity of the former have the same electro-optical origin: a longer path for electron redistribution in the excited state. OAP closely tracked simultaneously recorded electrical APs, permitting measurement of AP duration within 1% error. Prolonged laser exposure caused progressive AP duration prolongation and instability. However, these effects were alleviated or abolished by reducing the dye concentration and by perfusion with antioxidants. Thus the presented technique provides a unique opportunity for noninvasive AP recording in single cardiomyocytes.