Cellular compartment

About: Cellular compartment is a research topic. Over the lifetime, 1082 publications have been published within this topic receiving 53794 citations. The topic is also known as: cell compartmentation.

The time sequence of RNA and protein synthesis in cellular compartments following an acute dietary challenge with amino acid mixtures.

Abstract: The absorption of amino acids from the alimentary tract during digestion following a meal which contains protein is a rapid process with a maximum rate of absorption within the 1st hour (Dawson & Holdsworth, 1962; Chen, Rogers & Harper, 1962). Most of the amino acids absorbed at this early time are apparently derived from the exogenous protein, due to the differences in rate of digestion of exogenous and endogenous protein in the gut (Nasset & Ju, 1961). Thus, appreciable quantities of exogenous amino acids are present in the distal duodenum and proximal jejunum and are absorbed from the small intestine before amino acids derived from the more slowly digested endogenous protein alter the amino acid composition of the intestinal contents. Consequently, in the interval between one meal and the next, the supply of amino acids from the intestine to the portal blood will fluctuate in both quantity and quality. Free amino acids in portal blood are removed from circulation by the liver and incorporated into tissue protein in an extremely short time, a matter of minutes. The rate of incorporation of amino acids into cellular protein from portal blood is a function not only of the cellular rate of protein synthesis but also of the rate of penetration of the amino acids into the cell (Henriques, Henriques & Neuberger, 1955). T h e net uptake is related to both the extracellular and intracellular concentrations of amino acids, a relationship which is not a simple and direct one, and depends on the efficiency of transport mechanisms for the individual amino acids (Christensen, 1955). Nevertheless, Hanking & Roberts (1965) have shown by incubating liver slices in a medium containing various amounts of phenylalanine or threonine that the amino acid incorporating activity of ribosomes isolated from the liver slices is sensitive to the extracellular concentration of these amino acids. These observations suggest that protein synthesis in the liver cell is acutely responsive to variations in intracellular levels of amino acids induced by primary alterations in the extracellular concentration of a single essential amino acid. I n studying the pattern of absorption of individual amino acids from mixtures in the rat intestine in situ, Delhumeau, Velez Pratt & Gitler (1962) found that modifications of the ratios of amino acids from equimolar mixtures to those in hydrolysates of egg albumin, casein or zein did not change the relative order of absorption of the individual amino acids (based on the percentage absorbed), but the total amount absorbed was markedly higher for the amino acids present in the simulated egg albumin hydrolysate. Furthermore, they observed nearly simultaneous absorption of all amino acids from the simulated egg albumin mixture. It therefore seems likely that the amino acid content of portal blood during intestinal protein digestion can influence the pattern appearing in the tissues and modify the efficiency of utilization of the amino acids by the cells.

Differential transition metal uptake and fluorescent probe localization in hippocampal slices

Abstract: Metals are taken up by the combined action of metal transporters and ion channels. In this communication we have measured the uptake of the biologically important transition metals Mn, Fe, Co, Ni, Cu, Zn and Cd by rat and mouse hippocampal slices using the fluorescent probes FluoZin-3 (FZ3) and Newport Green (NPG), introduced by acetoxymethyl ester (AM) loading. The combination of metals and probes is also used to attempt to localize cellular sites into which metals translocate. We show that FZ3 and NPG partition into different cellular compartments; FZ3 into neuropil, whereas NPG localizes in neuropil and compartments within the cell bodies of neurons. Ni, Zn and Cd pass across the plasma membrane and then accumulate in intracellular vesicles and within intracellular membranes of cell bodies. The latter accumulate Cd, while synaptic vesicles take up Co. The passage of Mn, Cu and Fe into cells can be detected but there is some uncertainty about their disposition within the cell. All of our experiments are consistent with metals accumulating in intracellular compartments rather than the cytoplasm. Whether and to what extent there are transient elevations of free zinc levels in the cytoplasm remains unclear.

Origin and spatiotemporal dynamics of the peroxisomal endomembrane system.

Abstract: The peroxisome is an organelle with essential roles in lipid metabolism, maintenance of reactive oxygen species homeostasis, and anaplerotic replenishment of tricarboxylic acid cycle intermediates destined for mitochondria (Islinger et al., 2012; Beach and Titorenko, 2013; Wanders, 2014). Peroxisomes constitute a dynamic endomembrane system. The homeostatic state of this system is upheld via two pathways for assembling and maintaining the diverse peroxisomal compartments constituting it; the relative contribution of each pathway to preserving such system may vary in different organisms and under various physiological conditions. One pathway begins with the targeting of certain peroxisomal membrane proteins to an endoplasmic reticulum (ER) template and their exit from the template via pre-peroxisomal carriers; these carriers mature into metabolically active peroxisomes containing the entire complement of membrane and matrix proteins (Titorenko and Rachubinski, 2009; Hu et al., 2012; Tabak et al., 2013). Another pathway operates via growth and maturation of pre-existing peroxisomal precursors that do not originate from the ER; mature peroxisomes proliferate by undergoing fission (Nuttall et al., 2011; Hettema et al., 2014; Knoops et al., 2014). Recent studies have uncovered new roles for the peroxisomal endomembrane system in orchestrating important developmental decisions and defining organismal longevity (Titorenko and Rachubinski, 2004; Dixit et al., 2010; Beach et al., 2012). This Frontiers Research Topic is focused on the advances in our understanding of how evolutionarily distant organisms coordinate the formation, maturation, proliferation, maintenance, inheritance, and quality control of the peroxisomal endomembrane system and how peroxisomal endomembranes communicate with other cellular compartments to orchestrate complex biological processes and various developmental programs from inside the cell. Veenhuis and van der Klei (2014) provide insights into the mechanisms underlying the biogenesis of early peroxisomal precursors that do not arise from the ER. In the yeast Hansenula polymorpha, these vesicular precursors undergo multistep maturation into metabolically active peroxisomes only after acquiring the peroxin Pex3; the subsequent import of membrane and matrix proteins into such Pex3-containing peroxisomal precursors drives their multistep conversion into mature peroxisomes. Agrawal and Subramani (2013) discuss the relationship between two alternative routes of peroxisome biogenesis. One route involves the de novo formation of peroxisomes from an ER template, while the second involves the growth and division of pre-existing peroxisomes. The authors suggest that both routes operate simultaneously in organisms across phyla and propose a model that integrates the two routes into a single pathway for peroxisome assembly and maintenance. The model posits that a balance between progression rates of the two routes is modulated by various intracellular and extracellular signals; such modulation enables to preserve the peroxisomal endomembrane system under various physiological conditions. Kim and Mullen (2013) review the diverse ways through which the peroxin Pex16 can function to preserve the peroxisomal endomembrane system in such evolutionarily distant organisms as the yeast Yarrowia lipolytica, the plant Arabidopsis thaliana, and mammals. They dissect the mechanisms by which this peroxin orchestrates both routes of peroxisome biogenesis, i.e., the route of de novo formation of peroxisomes from an ER template and the route of growth and division of pre-existing peroxisomes, in these organisms. To explore the relationship between peroxisomes and the ER, Barton et al. (2013) concurrently visualized both organelles in living A. thaliana plants expressing differently colored peroxisome- and ER-targeted fluorescent proteins. The authors provide evidence that, although peroxisomes can be found in close contact with the ER, no luminal continuity exists between the two organelles. Mohanty and McBride (2013) evaluate evidence that the recently discovered vesicular flow from mitochondria to peroxisomes plays important roles in the assembly, maintenance, metabolic functions, and signaling activities of the peroxisomal endomembrane system in mammalian cells. They outline the molecular mechanisms underlying such a vesicular coupling of mitochondria to different compartments of the peroxisomal endomembrane system. Hasan et al. (2013) explore the molecular dynamics of a multistep process for protein import into the matrix of peroxisomes. They discuss the recent advances in our understanding of the mechanisms underlying the formation of a receptor/cargo-complex in the cytosol and its subsequent docking at the peroxisomal membrane, the translocation of the cargo protein across the membrane and its release into the peroxisomal matrix, and the recycling of receptor molecules. Ast et al. (2013) discuss the molecular mechanisms by which isoforms of proteins known to be peroxisomal can also be actively sorted to the cytosol, mitochondrion, nucleus, or plastid. The authors hypothesize that such dual sorting of peroxisomal proteins within a cell could have an important role in extending the metabolic capacity of peroxisomes in response to specific changes in cell physiology and/or environmental conditions. Kunze and Hartig (2013) explore how different intermediates of the glyoxylate cycle are transported across the peroxisomal membrane and how individual enzymes of this cycle are distributed along both sides of the membrane. They suggest that the efficient and selective transport of glyoxylate and other metabolites across the peroxisomal membrane may be essential for the high functional adaptability of peroxisomes. Fujiki et al. (2014) discuss the multicomponent protein machineries that in mammalian cells orchestrate the assembly of membrane proteins in the peroxisomal membrane and the import of matrix proteins across this membrane. Nordgren et al. (2013) examine the mechanisms by which a selective degradation of dysfunctional or excessive peroxisomes in a mammalian cell enables it to maintain a healthy population of peroxisomes. They discuss the evidence supporting the essential contribution of defects in peroxisome degradation to human disease. Van Veldhoven and Baes (2013) review how mutations impairing peroxisome biogenesis affect organismal size, development, and longevity in various invertebrate and vertebrate models, including nematodes, fruit fly, zebrafish, and mouse. Their analysis implies that a reduced size at birth, delay in development, and shortened lifespan are the most common features of different peroxisome biogenesis deficiencies. Maruyama and Kitamoto (2013), as well as Peraza-Reyes and Berteaux-Lecellier (2013), explore mechanisms underlying the essential roles of peroxisomes in the morphogenetic program initiated by physical damage to hyphae, biosynthesis of biotin, and sexual development in fungi.

Transport-exclusion pharmacology to localize lactate dehydrogenase activity within cells

Abstract: Recent in vitro and in vivo work has shown that lactate provides an important source of carbon for metabolic reactions in cancer cell mitochondria. An interesting question is whether lactate is oxidized by lactate dehydrogenase (LDH) in the cytosol and/or in mitochondria. Since metabolic processes in the cytosol and mitochondria are affected by redox balance, the location of LDH may have important regulatory implications in cancer metabolism. Within most mammalian cells, metabolic processes are physically separated by membrane-bound compartments. Our general understanding of this spatial organization and its role in cellular function, however, suffers from the limited number of techniques to localize enzymatic activities within a cell. Here, we describe an approach to assess metabolic compartmentalization by monitoring the activity of pharmacological inhibitors that cannot be transported into specific cellular compartments. Oxamate, which chemically resembles pyruvate, is transported into mitochondria and inhibits LDH activity in purified mitochondria. GSK-2837808A, in contrast, is a competitive inhibitor of NAD, which cannot cross the inner mitochondrial membrane. GSK-2837808A did not inhibit the LDH activity of intact mitochondria, but GSK-2837808A did inhibit LDH activity after the inner mitochondrial membrane was disrupted. Our results are consistent with some mitochondrial LDH that is accessible to oxamate, but inaccessible to GSK-2837808A until mitochondria are homogenized. This strategy of using inhibitors with selective access to subcellular compartments, which we refer to as transport-exclusion pharmacology, is broadly applicable to localize other metabolic reactions within cells.