NCX and EAAT transporters in ischemia: At the crossroad between glutamate metabolism and cell survival

Energy metabolism impairment is a central event in the pathophysiology of ischemia. The limited availability of glucose and oxygen strongly affects mitochondrial activity, thus leading to ATP depletion. In this setting, the switch to alternative energy sources could ameliorate cells survival by enhancing ATP production, thus representing an attractive strategy for ischemic treatment. In this regard, some studies have recently re-evaluated the metabolic role of glutamate and its potential to promote cell survival under pathological conditions. In the present review, we discuss the ability of glutamate to exert an “energizing role” in cardiac and neuronal models of hypoxia/reoxygenation (H/R) injury, focusing on the Na⁺/Ca²⁺ exchanger (NCX) and the Na⁺-dependent excitatory amino acid transporters (EAATs) as key players in this metabolic pathway.     

Arsenic and weight loss: At a crossroad between lipogenesis and lipolysis

Arsenic is found in soil, food, water and earth crust. Arsenic exposure is associated with chronic diseases such as cancer, cardiovascular disease as well as diabetes. One of complex effects of arsenic is on weight gain or loss. Involvement of arsenic in both weight loss and gain signaling pathways has previously been reported; however, too little attention has been paid to its weight reducing effect. Animal studies exhibited a role of arsenic in weight loss. In this regard, arsenic interference with endocrine system, leptin and adiponectin hormones as well as thermogenesis is more evidence. Apparently, arsenic-induced weight lossis generally meditated by its interaction with thermogenesis. In this review we have discussed the irregularities in metabolic pathways induced by arsenic that can lead to weight loss.     

UVRAG: At the crossroad of autophagy and genomic stability

UVRAG is a promoter of the autophagy pathway, and its deficiency may fuel the development of cancers. Intriguingly, our recent study has demonstrated that this protein also mediates the repair of damaged DNA and patrols centrosome stability, mechanisms that commonly prevent cancer progression, in a manner independent of its role in autophagy signaling. Given the central role of UVRAG in genomic stability and autophagic cleaning, it is speculated that UVRAG is a bona fide genome protector and that the decrease in UVRAG seen in some cancers may render these cells vulnerable to chromosomal damage, making UVRAG an appealing target for cancer therapy.     

The role of Sirt1: At the crossroad between promotion of longevity and protection against Alzheimer's disease neuropathology

Sirt1, a mammalian member of the sirtuin gene family, holds great potential for promoting longevity, preventing against disease and increasing cell survival. For example, studies suggest that the beneficial impact of caloric restriction in promoting longevity and cellular function may be mediated, in part, by Sirt1 through mechanisms involving PGC-1α, which plays important role in the regulation of cellular metabolism and inflammatory and antioxidant responses. Sirt1 may also interfere with mechanisms implicated in pathological disorders. We will present recent evidence indicating that Sirt1 may protect against Alzheimer's disease by interfering with the generation of β-amyloid peptides. We will discuss Sirt1 as a potential novel target, in addition to the development of Sirt1 activators for the prevention and treatment of Alzheimer's disease.     

At the crossroad of lifespan,calorie restriction,chromatin and disease: Meeting on sirtuins

Longevity, lifespan, cancer, cellullar transformation, energy, calorie restriction, diabetes…what can tie together such a diversity of hot topics in biomedical research? Emerging findings suggest that the answer lies in understanding the functions of the recently discovered family of proteins known as Sirtuins. Barcelona hosted the first scientific meeting completely focused on these evolutionary conserved protein deacetylases, bringing together experts in the biochemistry to cellular biology, mice models, drug targeting and pathophysiology of these molecules. Their work, summarized here, establishes the Sirtuins as major players in cellular homeostasis and human diseases that act through a whole range of biochemical substrates and physiological processes. Undoubtedly, this is an increasingly expanding field that it is here to stay and growth.     

Iron and glutathione at the crossroad of redox metabolism in neurons

Neurons, as non-dividing cells, encounter a myriad of stressful conditions throughout their lifespan. In particular, there is increasing evidence that iron progressively accumulates in the brain with age and that iron induced oxidative stress is the cause of several forms of neurodegeneration. Here, we review recent evidence that gives support to the following notions: 1) neuronal iron accumulation leads to oxidative stress and cell death; 2) neuronal survival to iron accumulation associates with decreased expression of the iron import transporter DMT1 and increased expression of the efflux transporter IREG1; and 3) the adaptive process of neurons towards iron-induced oxidative stress includes a marked increase in both the expression of the catalytic subunit of gamma glutamate-cysteine ligase and glutathione. These findings may help to understand aging-related neurodegeneration hallmarks: oxidative damage, functional impairment and cell death.     

Brucella adaptation and survival at the crossroad of metabolism and virulence

"In vivo" bacterial nutrition, i.e. the nature of the metabolic network and substrate(s) used by bacteria within their host, is a fundamental aspect of pathogenic or symbiotic lifestyles. A typical example are the Brucella spp., facultative intracellular pathogens responsible for chronic infections of animals and humans. Their virulence relies on their ability to modulate immune response and the physiology of host cells, but the fine-tuning of their metabolism in the host during infection appears increasingly crucial. Here we review new insights on the links between Brucella virulence and metabolism, pointing out the need to investigate both aspects to decipher Brucella infectious strategies.     

GSK-3b inhibition: At the crossroad between Akt and mTOR constitutive activation to enhance cyclin D1 protein stability in mantle cell lymphoma

Strategies able to down-regulate the aberrant expression of cyclin D1 may prove of therapeutic relevance in cancer patients. This is particularly true for mantle cell lymphoma (MCL) in which cyclin D1 is overexpressed as a consequence of the t(11;14)(q13;q32) translocation. We have recently demonstrated that an increased cyclin D1 stability also contributes to the high levels of this protein observed in MCL cells. This effect is mediated by a constitutive activation of PI3-K/Akt, which keeps GSK-3b inhibited. Here we show that inhibition of PI3-K/Akt induces a 40% decrease of cyclin D1 half-life as a result of accumulation of the dephosphorylated/active form of GSK-3b within the nucleus, where this kinase can phosphorylate cyclin D1 on Thr286 thereby promoting its nuclear export. Translocation of cyclin D1 into the cytoplasm is mediated by the nuclear exportin CRM1, whose association with cyclin D1 increases following PI3-K/Akt inhibition. Notably, rapamycin down-regulated GSK-3b Ser9 phosphorylation with concurrent nuclear export of cyclin D1 only in MCL cells in which GSK-3b is under the control of mTOR. These findings suggest that the ability to down-regulate cyclin D1 through GSK-3b may identify subsets of MCL patients who may benefit from the treatment with mTOR inhibitors and stimulate further studies to assess whether the inability to affect GSK-3b activity may constitute a clinically relevant resistance factor to mTOR inhibitors.     

The chloroplast Rieske iron-sulfur protein. At the crossroad of electron transport and signal transduction

We have addressed the functional and structural roles of three domains of the chloroplast Rieske iron-sulfur protein; that is, the flexible hinge that connects the transmembrane helix to the soluble cluster-bearing domain, the N-terminal stromal protruding domain, and the transmembrane helix. To this aim mutants were generated in the green alga Chlamydomonas reinhardtii. Their capacities to assemble the cytochrome b6f complex, perform plastoquinol oxidation, and signal redox-induced activation of the light-harvesting complex II kinase during state transition were tested in vivo. Deletion of one residue and extensions of up to five residues in the flexible hinge had no significant effect on complex accumulation or electron transfer efficiency. Deletion of three residues (Delta3G) dramatically decreased reaction rates by a factor of approximately 10. These data indicate that the chloroplast iron-sulfur protein-linking domain is much more flexible than that of its counterpart in mitochondria. Despite greatly slowed catalysis in the Delta3G mutant, there was no apparent delay in light-harvesting complex II kinase activation or state transitions. This indicates that conformational changes occurring in the Rieske protein did not represent a limiting step for kinase activation within the time scale tested. No phenotype could be associated with mutations in the N-terminal stromal-exposed domain. In contrast, the N17V mutation in the Rieske protein transmembrane helix resulted in a large decrease in the cytochrome f synthesis rate. This reveals that the Rieske protein transmembrane helix plays an active role in assembly-mediated control of cytochrome f synthesis. We propose a structural model to interpret this phenomenon based on the C. reinhardtii cytochrome b6f structure.     

Connexins: a junctional crossroad to breast cancer

The mammary gland presents a valuable model for developmental studies, spanning the embryonic stage through menarche to menopause. The dynamic remodeling of this gland is orchestrated by cellular heterogeneity, integrating mammogenic, systemic and local cues. Gap junctional intercellular communication provides pivotal cross talk of mammary epithelial cells with the surrounding cells and their local microenvironment. Connexins are involved in regulating normal and pathological mammary gland development, through channel-dependent and channel-independent roles. Modulation of the isoforms of connexins expressed, as well as their differential assembly into connexons and recruitment of a variety of associated partners, contributes to the complexity of signaling relayed at the membrane. This confers context-dependent functions of connexins at different stages of development and carcinogenesis. This review will summarize available knowledge about the functional dynamics of connexins and gap junctions in regulating normal mammary gland development and its pathophysiology.     

The insulin receptor substrate (IRS) proteins: At the intersection of metabolism and cancer

Increasing evidence supports a connection between cancer and metabolism and emphasizes the need to understand how tumors respond to the metabolic microenvironment and how tumor cell metabolism is regulated. The insulin receptor (IR) and its close family member the insulin-like growth factor-1 receptor (IGF-1R) mediate the cellular response to insulin in normal cells and their function is tightly regulated to maintain metabolic homeostasis. These receptors are also expressed on tumor cells and their expression correlates with tumor progression and poor prognosis. Understanding how the IR/IGF-1R pathway functions in tumors is increasing in importance as the efficacy of drugs that target metabolic pathways, such as metformin, are investigated in prospective clinical trials. This review will focus on key signaling intermediates of the IR and IGF-1R, the Insulin Receptor Substrate (IRS) proteins, with an emphasis on IRS-2, and discuss how these adaptor proteins play a pivotal role at the intersection of metabolism and cancer.Key words: IRS proteins, insulin receptor, IGF-1 receptor, metabolism, cancer, metformin     

STK38 at the crossroad between autophagy and apoptosis

We describe the STK38 protein kinase as a conserved regulator of autophagy. We discovered STK38 as a novel binding partner of Beclin1, a key regulator of autophagy. By combining molecular, cell biological and genetic approaches, we show that STK38 promotes autophagosome formation in human cells and in Drosophila. Furthermore, we also provide evidence demonstrating that STK38 with the small GTPase RalB, assist the co-ordination between autophagic and apoptotic events upon autophagy induction, hence proposing a role for STK38 in determining cellular fate in response to autophagic conditions.     

E2F1 at the crossroad of proliferation and oxidative metabolism

Comment on: Blanchet E, et. Nat Cell Biol 2011; 13:1146-52.     

Oxidative Stress in Caenorhabditis elegans: Protective Effects of Spartin

Troyer syndrome is caused by a mutation in the SPG20 gene, which results in complete loss of expression of the protein spartin. We generated a genetic model of Troyer syndrome in worms to explore the locomotor consequences of a null mutation of the Caenorhabditis elegans SPG20 orthologue, F57B10.9, also known as spg-20. Spg-20 mutants showed decreased length, crawling speed, and thrashing frequency, and had a shorter lifespan than wild-type animals. These results suggest an age-dependent decline in motor function in mutant animals. The drug paraquat was used to induce oxidative stress for 4 days in the animals. We measured survival rate and examined locomotion by measuring crawling speed and thrashing frequency. After 4 days of paraquat exposure, 77% of wild-type animals survived, but only 38% of spg-20 mutant animals survived. Conversely, animals overexpressing spg-20 had a survival rate of 95%. We also tested lifespan after a 1 hour exposure to sodium azide. After a 24 hour recovery period, 87% of wild type animals survived, 57% of spg-20 mutant animals survived, and 82% of animals overexpressing spg-20 survived. In the behavioral assays, spg-20 mutant animals showed a significant decrease in both crawling speed and thrashing frequency compared with wild-type animals. Importantly, the locomotor phenotype for both crawling and thrashing was rescued in animals overexpressing spg-20. The animals overexpressing spg-20 had crawling speeds and thrashing frequencies similar to those of wild-type animals. These data suggest that the protein F57B10.9/SPG-20 might have a protective role against oxidative stress.     

Lipid rafts: at a crossroad between cell biology and physics

Membrane lateral heterogeneity is accepted as a requirement for the function of biological membranes and the notion of lipid rafts gives specificity to this broad concept. However, the lipid raft field is now at a technical impasse because the physical tools to study biological membranes as a liquid that is ordered in space and time are still being developed. This has lead to a disconnection between the concept of lipid rafts as derived from biochemical and biophysical assays and their existence in the cell. Here, we compare the concept of lipid rafts as it has emerged from the study of synthetic membranes with the reality of lateral heterogeneity in biological membranes. Further application of existing tools and the development of new tools are needed to understand the dynamic heterogeneity of biological membranes.