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Full Abstracts – PHYSIOLOGY AND AGING
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Drosophila Lipoprotein Metabolism.
Wilhelm Palm, Julio Sampaio, Suzanne Eaton. Max Planck Institute of Molecular Cell Biology , Dresden, Germany.
Lipoproteins transport dietary and endogenously synthesized lipids between different organs, and their dysfunction is associated with many pathological
conditions. However, it is not well understood how lipoproteins influence tissue lipid composition. Here, we identify the major Drosophila lipoproteins and
characterize their assembly and lipidation. To probe for lipoprotein function, we genetically perturb interorgan lipid transport, and use lipid mass
spectrometry to quantitatively describe the resultant changes in the lipidomes of individual organs. The apoB family lipoprotein lipophorin (Lpp) is the
major lipid carrier in the larval hemolymph. Lpp is produced in the fat body in a process that depends on Microsomal Triglyceride Transfer Protein (MTP).
Subsequently, Lpp is recruited to the intestine, where it is further loaded with lipids in a process that depends on Lipid Transfer Particle (LTP), a second
apoB family lipoprotein originating from the fat body. The fat body exports phospholipids bearing long-chain fatty acid residues to Lpp. The intestine loads
Lpp with sterols, and with diacylglycerols bearing medium-chain fatty acid residues that are derived partly from dietary sources and partly from endogenous
synthesis in enterocytes. Lpp delivers lipids from fat body and intestine to imaginal discs, which utilize them to build their fat stores. In addition, Lpp
provides a significant fraction of membrane phospholipids to imaginal disc cells. In contrast, fat storage in the brain relies only on Lpp lipids derived directly
from the fat body, and Lpp is not a major source of brain phospholipids. These studies define the major routes of interorgan lipid transport in Drosophila,
and uncover surprising tissue-specific differences in lipoprotein lipid utilization.
72
MPC1 plays an essential role in pyruvate metabolism in yeast,
Drosophila
and human disease.
Daniel K. Bricker
1
, Thomas Orsak
2
, John Schell
2
, Yu-
Chan Chen
2
, Eric Taylor
2
, Michele Brivet
3
, Audrey Boutron
3
, Jared Rutter
2
, Carl S. Thummel
1
. 1) Dept of Human Genetics, University of Utah School of
Medicine, Salt Lake City, UT; 2) Dept of Biochemistry, University of Utah School of Medicine, Salt Lake City, UT; 3) Laboratoire de Biochimie, AP-HP
Hôpital de Bicêtre, Le Kremlin Bicêtre, France.
ATP production through oxidative metabolism is critical to cellular function and survival. The ability of pyruvate to enter mitochondria is central to this
process, as pyruvate connects glycolysis in the cytosol to the mitochondrial TCA cycle. Despite extensive biochemical characterization of mitochondrial
pyruvate uptake, the genetic identity of the transporter responsible is unknown. Through our work in fly and yeast systems, we have identified a candidate
for the mitochondrial pyruvate transporter and linked this function to human disease. Our current effort is focused on the Mitochondrial Pyruvate Carrier
(MPC) proteins MPC1 and MPC2, which associate with each other at the inner mitochondrial membrane. Deletion mutants of
Drosophila MPC1
(dMPC1)
are viable, but die rapidly on a diet of only sugar due to an inability to generate ATP. Metabolomic profiling revealed that
dMPC1
mutants accumulate
pyruvate and are depleted of TCA cycle intermediates, consistent with a defect in pyruvate metabolism. Studies in yeast have demonstrated that MPC1 is
required for pyruvate transport across the mitochondrial membrane. A human genetic analysis has identified three families with children displaying lactic
acidosis and hyperpyruvatemia. Biochemical characterization of cells isolated from affected individuals revealed no defects in pyruvate dehydrogenase, but
an inability of mitochondria to take up pyruvate for oxidative metabolism. A causal locus was identified through linkage analysis to mutations that change
conserved amino acids in MPC1. Taken together, our data supports the model that the MPC proteins act as a pyruvate transporter, providing an essential link
between glycolysis and mitochondrial oxidative metabolism.
73
Mitochondrial genotype alters the nuclear transcriptional response to varied levels of hypoxia in Drosophila.
David M. Rand, Patrick A. Flight,
Nicholas Jourjine, Lei Zhu. Ecology & Evolutionary Biol, Brown Univ, Providence, RI.
Exposure to reduced oxygen tension, or hypoxia, is a common occurrence in a wide array of environmental conditions ranging from cancer to stressed
microhabitats in nature. One response to hypoxia is to reduce cellular demand for oxygen by down-regulating mitochondrial functions. Despite the central
role mitochondria play in oxygen consumption, the effect of alternative mitochondrial genotypes on the hypoxic response has not been examined in flies.
Here we use mtDNA introgression strains of Drosophila to examine the effects of alternative mtDNA-encoded genes on the nuclear transcriptional response
to varied hypoxia. Flies carrying mtDNA from either D. melanogaster OreR, D. melanogaster Zimbabwe, D. simulans siI, or D. simulans siII on a D.
melanogaster OreR nuclear chromosomal background were constructed using balancer substitutions and maternal cytoplasm from these four genotypes.
Replicate cultures of adult males of each genotype were exposed to four different oxygen tensions (normoxia, 6%;, 3%;, and 1.5%) for 2 hours, and flash
frozen. Expression profiles were determined using Affymetrix 2.0 arrays. The mtDNA genotype design allows for partitioning effects to alternative mtDNAs
within a species, or fixed differences between Dmel and Dsim mtDNAs. MtDNA has subtle effects on gene expression under normoxia (~10 genes altered)
and strong hypoxia (1.5%; ~25 genes altered), but had pronounced effects at 3% (>200 genes altered) and 6% (>500 genes altered). These results provide
strong evidence for mitochondrial retrograde signaling in the nuclear transcriptional response to different levels of hypoxia. Gene ontology analyses reveal
that alternative mtDNAs within species alter genes associated with ‘oxidation-reduction’ and ‘mitochondrion’, while the effects of Dmel vs. Dsim mtDNAs
alter very different classes of genes (‘ribonuclear protein’; ‘ribosome biogenesis’). These studies offer the first evidence that genes in mtDNA play a critical
role in modulating the nuclear transcriptional response to hypoxia, and do so with varying degree under different levels of hypoxic stress.