Fatty acid transport across the cell membrane: Regulation by fatty acid transporters

Transport of long-chain fatty acids across the cell membrane has long been thought to occur by passive diffusion. However, in recent years there has been a fundamental shift in understanding, and it is now generally recognized that fatty acids cross the cell membrane via a protein-mediated mechanism. Membrane-associated fatty acid-binding proteins (‘fatty acid transporters’) not only facilitate but also regulate cellular fatty acid uptake, for instance through their inducible rapid (and reversible) translocation from intracellular storage pools to the cell membrane. A number of fatty acid transporters have been identified, including CD36, plasma membrane-associated fatty acid-binding protein (FABP_pm), and a family of fatty acid transport proteins (FATP1–6). Fatty acid transporters are also implicated in metabolic disease, such as insulin resistance and type-2 diabetes. In this report we briefly review current understanding of the mechanism of transmembrane fatty acid transport, and the function of fatty acid transporters in healthy cardiac and skeletal muscle, and in insulin resistance/type-2 diabetes. Fatty acid transporters hold promise as a future target to rectify lipid fluxes in the body and regain metabolic homeostasis.     

Protein kinase D1 is essential for contraction-induced glucose uptake but is not involved in fatty acid uptake into cardiomyocytes

Increased contraction enhances substrate uptake into cardiomyocytes via translocation of the glucose transporter GLUT4 and the long chain fatty acid (LCFA) transporter CD36 from intracellular stores to the sarcolemma. Additionally, contraction activates the signaling enzymes AMP-activated protein kinase (AMPK) and protein kinase D1 (PKD1). Although AMPK has been implicated in contraction-induced GLUT4 and CD36 translocation in cardiomyocytes, the precise role of PKD1 in these processes is not known. To study this, we triggered contractions in cardiomyocytes by electric field stimulation (EFS). First, the role of PKD1 in GLUT4 and CD36 translocation was defined. In PKD1 siRNA-treated cardiomyocytes as well as cardiomyocytes from PKD1 knock-out mice, EFS-induced translocation of GLUT4, but not CD36, was abolished. In AMPK siRNA-treated cardiomyocytes and cardiomyocytes from AMPKα2 knock-out mice, both GLUT4 and CD36 translocation were abrogated. Hence, unlike AMPK, PKD1 is selectively involved in glucose uptake. Second, we analyzed upstream factors in PKD1 activation. Cardiomyocyte contractions enhanced reactive oxygen species (ROS) production. Using ROS scavengers, we found that PKD1 signaling and glucose uptake are more sensitive to changes in intracellular ROS than AMPK signaling or LCFA uptake. Furthermore, silencing of death-activated protein kinase (DAPK) abrogated EFS-induced GLUT4 but not CD36 translocation. Finally, possible links between PKD1 and AMPK signaling were investigated. PKD1 silencing did not affect AMPK activation. Reciprocally, AMPK silencing did not alter PKD1 activation. In conclusion, we present a novel contraction-induced ROS-DAPK-PKD1 pathway in cardiomyocytes. This pathway is activated separately from AMPK and mediates GLUT4 translocation/glucose uptake, but not CD36 translocation/LCFA uptake.     

Creatine uptake in brain and skeletal muscle of mice lacking guanidinoacetate methyltransferase assessed by magnetic resonance spectroscopy.

Creatine (Cr) levels in skeletal muscle and brain of a mouse model of Cr deficiency caused by guanidinoacetate methyltransferase absence (GAMT-/-) were studied after Cr supplementation with 2 g.kg body wt-1.day-1 Cr for 35 days. Localized 1H magnetic resonance spectroscopy (MRS) was performed in brain (cerebellum and thalamus/hippocampus) and in hind leg muscle of GAMT-/- mice before and after Cr supplementation and in control (Con) mice. As expected, a signal for Cr was hardly detectable in MR spectra of GAMT-/- mice before Cr supplementation. In the thalamus/hippocampus region of these mice, an increase in N-acetylasparate (NAA) was observed. During Cr administration, Cr levels increased faster in skeletal muscle compared with brain, but this occurred only during the first day of supplementation. Thereafter, Cr levels increased by 0.8 mM/day in all studied locations. After 35 days of Cr supplementation, Cr levels in all locations were higher compared with Con mice on a Cr-free diet and NAA levels normalized. Only because of the repeated MRS measurements performed in this longitudinal Cr supplementation study on GAMT-/- mice were we able to discover the initial faster uptake of Cr in skeletal muscle compared with brain, which may represent muscular Cr uptake independent of Cr transporter expression. Our results can provide the basis for additional experiments to optimize Cr supplementation in GAMT deficiency, as increases in brain Cr are slow in patients after Cr supplementation.     

Colored motifs reveal computational building blocks in the C. elegans brain

Background

Complex networks can often be decomposed into less complex sub-networks whose structures can give hints about the functional organization of the network as a whole. However, these structural motifs can only tell one part of the functional story because in this analysis each node and edge is treated on an equal footing. In real networks, two motifs that are topologically identical but whose nodes perform very different functions will play very different roles in the network.

Methodology/Principal Findings

Here, we combine structural information derived from the topology of the neuronal network of the nematode C. elegans with information about the biological function of these nodes, thus coloring nodes by function. We discover that particular colorations of motifs are significantly more abundant in the worm brain than expected by chance, and have particular computational functions that emphasize the feed-forward structure of information processing in the network, while evading feedback loops. Interneurons are strongly over-represented among the common motifs, supporting the notion that these motifs process and transduce the information from the sensor neurons towards the muscles. Some of the most common motifs identified in the search for significant colored motifs play a crucial role in the system of neurons controlling the worm''s locomotion.

Conclusions/Significance

The analysis of complex networks in terms of colored motifs combines two independent data sets to generate insight about these networks that cannot be obtained with either data set alone. The method is general and should allow a decomposition of any complex networks into its functional (rather than topological) motifs as long as both wiring and functional information is available.     

31P saturation transfer spectroscopy predicts differential intracellular macromolecular association of ATP and ADP in skeletal muscle

The kinetics of phosphoryl exchange involving ATP and ADP have been investigated successfully by in vivo ³¹P magnetic resonance spectroscopy using magnetization transfer. However, magnetization transfer effects seen on the signals of ATP also could arise from intramolecular cross-relaxation. This relaxation process carries information on the association state of ATP in the cell. To disentangle contributions of chemical exchange and cross-relaxation to magnetization transfer effects seen in ³¹P magnetic resonance spectroscopy of skeletal muscle, we performed saturation transfer experiments on wild type and double-mutant mice lacking the cytosolic muscle creatine kinase and adenylate kinase isoforms. We find that cross-relaxation, observed as nuclear Overhauser effects (NOEs), is responsible for magnetization transfer between ATP phosphates both in wild type and in mutant mice. Analysis of ³¹P relaxation properties identifies these effects as transferred NOEs, i.e. underlying this process is an exchange between free cellular ATP and ATP bound to slowly rotating macromolecules. This explains the β-ATP signal decrease upon saturation of the γ-ATP resonance. Although this usually is attributed to β-ADP ↔ β-ATP phosphoryl exchange, we did not detect an effect of this exchange on the β-ATP signal as expected for free [ADP], derived from the creatine kinase equilibrium reaction. This indicates that in resting muscle, conditions prevail that prevent saturation of β-ADP spins and puts into question the derivation of free [ADP] from the creatine kinase equilibrium. We present a model, matching the experimental result, for ADP ↔ ATP exchange, in which ADP is only transiently present in the cytosol.     

Polar-localised poplar K<Superscript>+</Superscript> channel capable of controlling electrical properties of wood-forming cells

In previous studies, we have shown that annual expression profiles of cambial and wood tissue with respect to the Shaker K⁺ channel PTORK correlate with cambial activity. To follow PTORK-gene activity on the cellular level, we isolated the respective promoter regions and generated transgenic Arabidopsis plants expressing the GUS gene under the control of the PTORK promoter. Cross-sections of petioles showed PTORK-driven signals predominantly in the xylem parenchyma surrounding the vessels and in the phloem. Antibodies raised against a unique N-terminal region of PTORK in histo-immunochemical analyses recognised this K⁺-release channel in growth-active poplar plants only. PTORK labelling was found in differentiating xylem cells (young fibres) and mature xylem (vessel-associated cells of the ray parenchyma). Patch-clamp measurements on fibre cell protoplasts, derived from young poplar twigs, identified outward-rectifying K⁺ channels as the major K⁺ conductance of this cell type, which resembled the biophysical properties of PTORK when expressed in Xenopus oocytes.Electronic Supplementary Material Supplementary material is available for this article at Matthias Arend and Andrea Stinzing contributed equally to this work     

Monocarboxylate transporters in subsarcolemmal and intermyofibrillar mitochondria

Whether subsarcolemmal (SS) and intermyofibrillar (IMF) mitochondria contain monocarboxylate transporters (MCTs) is controversial. We have examined the presence of MCT1, 2, and 4 in highly purified SS and IMF mitochondria. These mitochondria were not contaminated with plasma membrane, sarcoplasmic reticulum or endosomal compartments, as the marker proteins for these sub-cellular compartments (Na+-K+-ATPase, Ca2+-ATPase, and the transferrin receptor) were not present in SS or IMF mitochondria. MCT1, MCT2, and MCT4 were all present at the plasma membrane. However, MCT1 and MCT4 were associated with SS mitochondria. In contrast, the IMF mitochondria were completely devoid of MCT1 and MCT4. However, MCT2 was associated with both SS and IMF mitochondria. These observations suggest that SS and IMF mitochondria have different capacities for metabolizing monocarboxylates. Thus, the controversy as to whether mitochondria can take up and oxidize lactate will need to take account of the different distribution of MCTs between SS and IMF mitochondria.     

Monokine induced by interferon gamma and IFN-gamma response to a fusion protein of Mycobacterium tuberculosis ESAT-6 and CFP-10 in Brazilian tuberculosis patients

IFN-gamma responses to Mycobacterium tuberculosis antigens ESAT-6 and CFP-10 have been proposed as specific markers of M. tuberculosis infection. Monokine induced by gamma interferon (MIG/CXCL9) has been shown to be expressed by IFN-gamma stimulated mononuclear cells and to attract activated T-cells through the chemokine receptor CXCR3. Since MIG is induced early in the response to IFN-gamma, measuring MIG may provide an interesting marker to assess downstream IFN-gamma induced responses, in contrast to assays that mainly focus on quantifying production of IFN-gamma per se. We, therefore, investigated MIG and IFN-gamma responses to a fusion protein of ESAT-6 and CFP-10, and compared responses to the conserved mycobacterial antigen 85B (Ag85B) and purified protein derivative (PPD) of M. tuberculosis, in 29 BCG vaccine controls and 24 TB patients. IFN-gamma secreting cells were determined by ELISPOT, and MIG production was measured by ELISA and flow cytometry. Production of MIG in response to ESAT-6/CFP-10, Ag85B and PPD correlated overall with increased numbers of IFN-gamma secreting cells (r=0.55, P<0.0001). A significant increase was noted among patients compared to controls in the secretion of IFN-gamma and MIG following stimulation with ESAT-6/CFP-10 or PPD (P<0.05). Moreover, MIG intracellular expression was higher in TB patients compared to BCG vaccines (P<0.05) in response to ESAT-6/CFP-10 or PPD. We conclude that MIG production correlates significantly with enhanced T-cell IFN-gamma production induced by M. tuberculosis-specific antigens ESAT-6/CFP-10. These results point to MIG as a potential novel biomarker that may be helpful in assessing downstream responses induced by IFN-gamma in TB.     

Overexpression of Vesicle-associated Membrane Protein (VAMP) 3, but Not VAMP2, Protects Glucose Transporter (GLUT) 4 Protein Translocation in an in Vitro Model of Cardiac Insulin Resistance

Cardiac glucose utilization is regulated by reversible translocation of the glucose transporter GLUT4 from intracellular stores to the plasma membrane. During the onset of diet-induced insulin resistance, elevated lipid levels in the circulation interfere with insulin-stimulated GLUT4 translocation, leading to impaired glucose utilization. Recently, we identified vesicle-associated membrane protein (VAMP) 2 and 3 to be required for insulin- and contraction-stimulated GLUT4 translocation, respectively, in cardiomyocytes. Here, we investigated whether overexpression of VAMP2 and/or VAMP3 could protect insulin-stimulated GLUT4 translocation under conditions of insulin resistance. HL-1 atrial cardiomyocytes transiently overexpressing either VAMP2 or VAMP3 were cultured for 16 h with elevated concentrations of palmitate and insulin. Upon subsequent acute stimulation with insulin, we measured GLUT4 translocation, plasmalemmal presence of the fatty acid transporter CD36, and myocellular lipid accumulation. Overexpression of VAMP3, but not VAMP2, completely prevented lipid-induced inhibition of insulin-stimulated GLUT4 translocation. Furthermore, the plasmalemmal presence of CD36 and intracellular lipid levels remained normal in cells overexpressing VAMP3. However, insulin signaling was not retained, indicating an effect of VAMP3 overexpression downstream of PKB/Akt. Furthermore, we revealed that endogenous VAMP3 is bound by the contraction-activated protein kinase D (PKD), and contraction and VAMP3 overexpression protect insulin-stimulated GLUT4 translocation via a common mechanism. These observations indicate that PKD activates GLUT4 translocation via a VAMP3-dependent trafficking step, which pathway might be valuable to rescue constrained glucose utilization in the insulin-resistant heart.