Colocalization of aldolase and FBPase in cytoplasm and nucleus of cardiomyocytes

Intracellular energy production and distribution in hypoxia

The hydrolysis of ATP is the primary source of metabolic energy for eukaryotic cells. Under physiological conditions, cells generally produce more than sufficient levels of ATP to fuel the active biological processes necessary to maintain homeostasis. However, mechanisms underpinning the distribution of ATP to subcellular microenvironments with high local demand remain poorly understood. Intracellular distribution of ATP in normal physiological conditions has been proposed to rely on passive diffusion across concentration gradients generated by ATP producing systems such as the mitochondria and the glycolytic pathway. However, subcellular microenvironments can develop with ATP deficiency due to increases in local ATP consumption. Alternatively, ATP production can be reduced during bioenergetic stress during hypoxia. Mammalian cells therefore need to have the capacity to alter their metabolism and energy distribution strategies to compensate for local ATP deficits while also controlling ATP production. It is highly likely that satisfying the bioenergetic requirements of the cell involves the regulated distribution of ATP producing systems to areas of high ATP demand within the cell. Recently, the distribution (both spatially and temporally) of ATP-producing systems has become an area of intense investigation. Here, we review what is known (and unknown) about intracellular energy production and distribution and explore potential mechanisms through which this targeted distribution can be altered in hypoxia, with the aim of stimulating investigation in this important, yet poorly understood field of research.

Association of α-1-Antichymotrypsin Expression with the Development of Conformational Changes of Tau Protein in Alzheimer's Disease Brain

In Alzheimer's disease (AD), two mutually exclusive amino-terminal-dependent conformations have been reported to occur during the aggregation of Tau protein into neurofibrillary tangles (NFTs). An early conformation of full-length Tau, involving the bending of the amino terminus over the third repeated domain, is recognized by the Alz-50 antibody, followed by a second conformation recognized by Tau-66 antibody that depends on the folding of the proline-rich region over the third repeated domain in a molecule partially truncated at the amino- and carboxyl-termini. α-1-antichymotrypsin (ACT) is an acute phase serum glycoprotein that accumulates abnormally in the brain of AD patients, and since it is considered to promote the in vitro and in vivo aggregation of amyloid-β, we here seek further evidence that ACT may also contribute to the abnormal aggregation of Tau in AD. By analyzing brain samples from a population of AD cases under immunofluorescence and high-resolution confocal microscopy, we demonstrate here the abundant expression of ACT in hippocampal neurons, visualized as a granular diffuse accumulation, frequently reaching the nuclear compartment. In a significant number of these neurons, intracellular NFTs composed of abnormally phosphorylated and truncated Tau at Asp⁴²¹ were also observed to coexist in separated regions of the cytoplasm. However, we found strong colocalization between ACT and diffuse aggregates of Tau-66-positive granules, which was not observed with Alz-50 antibody. These results suggest that ACT may play a role during the development of Tau conformational changes facilitating its aggregation during the formation of the neurofibrillary pathology in AD.

Nuclear dihydroxyacetone phosphate signals nutrient sufficiency and cell cycle phase to global histone acetylation

Global histone acetylation varies with changes in the nutrient and cell cycle phases; however, the mechanisms connecting these variations are not fully understood. Herein, we report that nutrient-related and cell-cycle-regulated nuclear acetate regulates global histone acetylation. Histone deacetylation-generated acetate accumulates in the nucleus and induces histone hyperacetylation. The nuclear acetate levels were controlled by glycolytic enzyme triosephosphate isomerase 1 (TPI1). Cyclin-dependent kinase 2 (CDK2), which is phosphorylated and activated by nutrient-activated mTORC1, phosphorylates TPI1 Ser 117 and promotes nuclear translocation of TPI1, decreases nuclear dihydroxyacetone phosphate (DHAP) and induces nuclear acetate accumulation because DHAP scavenges acetate via the formation of 1-acetyl-DHAP. CDK2 accumulates in the cytosol during the late G1/S phases. Inactivation or blockade of nuclear translocation of TPI1 abrogates nutrient-dependent and cell-cycle-dependent global histone acetylation, chromatin condensation, gene transcription and DNA replication. These results identify the mechanism of maintaining global histone acetylation by nutrient and cell cycle signals.

Targeting a moonlighting function of aldolase induces apoptosis in cancer cells

Muscle fructose-1,6-bisphosphate aldolase (ALDOA) is among the most abundant glycolytic enzymes in all cancer cells. Here, we show that the enzyme plays a previously unknown and critical role in a cancer cell survival. Simultaneous inhibition of ALDOA activity and interaction with F-actin cytoskeleton using ALDOA slow-binding inhibitor UM0112176 leads to a rapid cofilin-dependent loss of F-actin stress fibers which is associated with elevated ROS production, inhibition of ATP synthesis, increase in calcium levels, caspase activation and arrested cellular proliferation. These effects can be reproduced by silencing of ALDOA. The mechanism of pharmacological action is, however, independent of the catalytic function of the enzyme, specific to cancer cells, and is most deleterious to cells undergoing the epithelial–mesenchymal transition, a process facilitating cancer cell invasion. Our results demonstrate that the overabundance of ALDOA in cancer cells is associated with its moonlighting rather than catalytic functions. This may have significant implications for development of novel broad-based anti-cancer therapies.

Fragmentation of the Golgi Apparatus in Neuroblastoma Cells Is Associated with Tau-Induced Ring-Shaped Microtubule Bundles

Abnormal fibrillary aggregation of tau protein is a pathological condition observed in Alzheimer's disease and other tauopathies; however, the presence and pathological significance of early non-fibrillary aggregates of tau remain under investigation. In cell and animal models expressing normal or modified tau, toxic effects altering the structure and function of several membranous organelles have also been reported in the absence of fibrillary structures; however, how these abnormalities are produced is an issue yet to be addressed. In order to obtain more insights into the mechanisms by which tau may disturb intracellular membranous elements, we transiently overexpressed human full-length tau and several truncated tau variants in cultured neuroblastoma cells. After 48 h of transfection, either full-length or truncated tau forms produced significant fragmentation of the Golgi apparatus (GA) with no changes in cell viability. Noteworthy is that in the majority of cells exhibiting dispersion of the GA, a ring-shaped array of cortical or perinuclear microtubule (Mt) bundles was also generated under the expression of either variant of tau. In contrast, Taxol treatment of non-transfected cells increased the amount of Mt bundles but not sufficiently to produce fragmentation of the GA. Tau-induced ring-shaped Mt bundles appeared to be well-organized and stable structures because they were resistant to Nocodazole post-treatment and displayed a high level of tubulin acetylation. These results further indicate that a mechanical force generated by tau-induced Mt-bundling may be responsible for Golgi fragmentation and that the repeated domain region of tau may be the main promoter of this effect.

Identification of a multienzyme complex for glucose metabolism in living cells

Sequential metabolic enzymes in glucose metabolism have long been hypothesized to form multienzyme complexes that regulate glucose flux in living cells. However, it has been challenging to directly observe these complexes and their functional roles in living systems. In this work, we have used wide-field and confocal fluorescence microscopy to investigate the spatial organization of metabolic enzymes participating in glucose metabolism in human cells. We provide compelling evidence that human liver-type phosphofructokinase 1 (PFKL), which catalyzes a bottleneck step of glycolysis, forms various sizes of cytoplasmic clusters in human cancer cells, independent of protein expression levels and of the choice of fluorescent tags. We also report that these PFKL clusters colocalize with other rate-limiting enzymes in both glycolysis and gluconeogenesis, supporting the formation of multienzyme complexes. Subsequent biophysical characterizations with fluorescence recovery after photobleaching and FRET corroborate the formation of multienzyme metabolic complexes in living cells, which appears to be controlled by post-translational acetylation on PFKL. Importantly, quantitative high-content imaging assays indicated that the direction of glucose flux between glycolysis, the pentose phosphate pathway, and serine biosynthesis seems to be spatially regulated by the multienzyme complexes in a cluster-size-dependent manner. Collectively, our results reveal a functionally relevant, multienzyme metabolic complex for glucose metabolism in living human cells.

A new level of regulation in gluconeogenesis: metabolic state modulates the intracellular localization of aldolase B and its interaction with liver fructose-1,6-bisphosphatase

Understanding how glucose metabolism is finely regulated at molecular and cellular levels in the liver is critical for knowing its relationship to related pathologies, such as diabetes. In order to gain insight into the regulation of glucose metabolism, we studied the liver-expressed isoforms aldolase B and fructose-1,6-bisphosphatase-1 (FBPase-1), key enzymes in gluconeogenesis, analysing their cellular localization in hepatocytes under different metabolic conditions and their protein-protein interaction in vitro and in vivo. We observed that glucose, insulin, glucagon and adrenaline differentially modulate the intracellular distribution of aldolase B and FBPase-1. Interestingly, the in vitro protein-protein interaction analysis between aldolase B and FBPase-1 showed a specific and regulable interaction between them, whereas aldolase A (muscle isozyme) and FBPase-1 showed no interaction. The affinity of the aldolase B and FBPase-1 complex was modulated by intermediate metabolites, but only in the presence of K(+). We observed a decreased association constant in the presence of adenosine monophosphate, fructose-2,6-bisphosphate, fructose-6-phosphate and inhibitory concentrations of fructose-1,6-bisphosphate. Conversely, the association constant of the complex increased in the presence of dihydroxyacetone phosphate (DHAP) and non-inhibitory concentrations of fructose-1,6-bisphosphate. Notably, in vivo FRET studies confirmed the interaction between aldolase B and FBPase-1. Also, the co-expression of aldolase B and FBPase-1 in cultured cells suggested that FBPase-1 guides the cellular localization of aldolase B. Our results provide further evidence that metabolic conditions modulate aldolase B and FBPase-1 activity at the cellular level through the regulation of their interaction, suggesting that their association confers a catalytic advantage for both enzymes.

Morphometric Analysis of Human Embryonic Stem Cell-Derived Ventricular Cardiomyocytes: Determining the Maturation State of a Population by Quantifying Parameters in Individual Cells

Quantitative methods were established to determine the level of maturation of human embryonic stem cell-derived ventricular cardiomyocytes (hESC-vCMs) that were treated with different metabolic stimulants (i.e., isoproterenol and oleic acid) during early differentiation. Cells were double-immunolabeled with α-actinin and COX IV antibodies, to label the myofibrils and mitochondria, respectively, after which images were acquired via confocal microscopy. In order to determine the extent of differentiation, image analysis protocols were then used to quantify cell shape and area, as well as the degree of myofibrillar organization and intercalation of mitochondria between the myofibrils within the cells. We demonstrated that oleic acid or isoproterenol alone, or a combination of the two, induced a more elongated hESC-vCM phenotype than the untreated controls. In addition, cells treated with isoproterenol alone exhibited a similar level of myofibrillar organization as the controls, but those treated with oleic acid with/without isoproterenol exhibited a more organized (parallel) orientation of myofibrils. The combined isoproterenol/oleic acid treatment also resulted in enhanced intercalation of mitochondria between the myofibrils. We suggest that these quantitative morphometric methods might serve as simple and effective tools that can be utilized in the determination of the level of structural maturation of hESC-vCMs.

Ubiquitous presence of gluconeogenic regulatory enzyme, fructose-1,6-bisphosphatase, within layers of rat retina

To shed some light on gluconeogenesis in mammalian retina, we have focused on fructose-1,6-bisphosphatase (FBPase), a regulatory enzyme of the process. The abundance of the enzyme within the layers of the rat retina suggests that, in mammals in contrast to amphibia, gluconeogenesis is not restricted to one specific cell of the retina. We propose that FBPase, in addition to its gluconeogenic role, participates in the protection of the retina against reactive oxygen species. Additionally, the nuclear localization of FBPase and of its binding partner, aldolase, in the retinal cells expressing the proliferation marker Ki-67 indicates that these two gluconeogenic enzymes are involved in non-enzymatic nuclear processes.

Are biological sensors modulated by their structural scaffolds? The role of the structural muscle proteins alpha-actinin-2 and alpha-actinin-3 as modulators of biological sensors

Biological sensors and their ability to detect and respond to change in the cellular environment can be modulated by protein scaffolds acting within their interaction network. The skeletal muscle alpha-actinins have been considered as primarily structural scaffold proteins. However, deficiency of alpha-actinin-3 due to a common null polymorphism results in predominantly metabolic changes in skeletal muscle function. In this review, we explore the range of phenotypes associated with alpha-actinin-3 deficiency, and draw supporting evidence from known interaction partners for its role as a scaffold which acts to modulate biological sensors that result in changes in muscle metabolism and structure.