Glial- and fat-specific expression of the rat glycerol phosphate dehydrogenase-luciferase fusion gene in transgenic mice

Noggin-mediated effects on metabolite profiles of microglia and oligodendrocytes after ischemic insult

Recent studies show that shifts in energy metabolism in activated microglia are linked to their functions and immune responses in the ischemic brain. We previously reported that an antagonist of the bone morphogenetic protein, noggin, enhanced myelination in the ischemic brain during the chronic phase, and conditioned media (CM) from activated BV2 microglia treated with noggin after ischemia/reperfusion (I/R) increased the expression of myelin basic protein (MBP) in oligodendrocytes (MO3.13). To determine whether noggin induced changes in cell metabolism, metabolite profiles in BV2 and MO3.13 cells were analyzed by untargeted metabolomics using ¹H nuclear magnetic resonance spectroscopy. Compared to vehicle-treated BV2 cells, noggin treatment (100 ng/mL for 3 h after I/R) suppressed the I/R-induced increase in intracellular glucose and lactate levels but increased extracellular levels of glucose and several amino acids. When MO3.13 cells were exposed to noggin CM from BV2 cells, most of the vehicle CM-induced changes in the levels of metabolites such as choline, formate, and intermediates of oxidative phosphorylation were reversed, while the glycerol level was markedly increased. An increase in glycerol level was also observed in the noggin-treated ischemic brain and was further supported by the expression of glycerol-3-phosphate dehydrogenase 1 (required for glycerol synthesis) in the cytoplasm of MBP-positive oligodendrocytes in the ischemic brains treated with noggin. These results suggest that noggin-induced changes in the metabolism of microglia provide a favorable environment for myelin synthesis in oligodendrocytes during the recovery phase after ischemic stroke.

Seasonal freeze resistance of rainbow smelt (Osmerus mordax) is generated by differential expression of glycerol-3-phosphate dehydrogenase, phosphoenolpyruvate carboxykinase, and antifreeze protein genes

In winter, rainbow smelt (Osmerus mordax) accumulate glycerol and produce an antifreeze protein (AFP), which both contribute to freeze resistance. The role of differential gene expression in the seasonal pattern of these adaptations was investigated. First, cDNAs encoding smelt and Atlantic salmon (Salmo salar) phosphoenolpyruvate carboxykinase (PEPCK) and smelt glyceraldehyde-3-phosphate dehydrogenase (GAPDH) were cloned so that all sequences required for expression analysis would be available. Using quantitative PCR, expression of beta actin in rainbow smelt liver was compared with that of GAPDH in order to determine its validity as a reference gene. Then, levels of glycerol-3-phosphate dehydrogenase (GPDH), PEPCK, and AFP relative to beta actin were measured in smelt liver over a fall-winter-spring interval. Levels of GPDH mRNA increased in the fall just before plasma glycerol accumulation, implying a driving role in glycerol synthesis. GPDH mRNA levels then declined during winter, well in advance of serum glycerol, suggesting the possibility of GPDH enzyme or glycerol conservation in smelt during the winter months. PEPCK mRNA levels rose in parallel with serum glycerol in the fall, consistent with an increasing requirement for amino acids as metabolic precursors, remained elevated for much of the winter, and then declined in advance of the decline in plasma glycerol. AFP mRNA was elevated at the onset of fall sampling in October and remained elevated until April, implying separate regulation from GPDH and PEPCK. Thus, winter freezing point depression in smelt appears to result from a seasonal cycle of GPDH gene expression, with an ensuing increase in the expression of PEPCK, and a similar but independent cycle of AFP gene expression.

Neuronal and astrocytic shuttle mechanisms for cytosolic-mitochondrial transfer of reducing equivalents: current evidence and pharmacological tools

The malate-aspartate shuttle and the glycerol phosphate shuttle act to transfer reducing equivalents from NADH in the cytosol to the mitochondria since the inner mitochondrial membrane is impermeable to NADH and NAD+. This transfer of reducing equivalents is essential for maintaining a favorable NAD+/NADH ratio required for the oxidative metabolism of glucose and synthesis of neurotransmitters in brain. There is evidence that both the malate-aspartate shuttle and glycerol phosphate shuttle function in brain; however, there is controversy about the relative importance and cellular localization of these shuttles. The malate-aspartate shuttle is considered the most important shuttle in brain. It is particularly important in neurons and may be extremely low, or even non-existent in brain astrocytes. Several studies provide evidence of glycerol phosphate shuttle activity in brain cells; however, the activity of this shuttle in brain has been questioned. A number of pharmacological tools, including aminooxyacetic acid, beta-methyleneaspartate, phenylsuccinate, and 3-nitropropionic acid, have been used to inhibit the four enzymes and two carrier proteins that participate in the malate-aspartate shuttle. Although no drugs completely inhibit the glycerol phosphate shuttle, evidence for the existence of this shuttle is provided by studies using drugs to inhibit the malate-aspartate shuttle. This report evaluates the evidence for each shuttle in brain cells and the drugs that can be used as pharmacological tools to study these shuttles.

Potential roles for presenilin-1 in oxygen sensing and in glial-specific gene expression

The integral membrane glycoprotein presenilin-1 (PS1), in concert with beta-secretase and beta amyloid precursor protein (betaAPP), orchestrate cleavage of betaAPP into amyloidogenic Abeta peptides. To gain further insight into PS1 function, we undertook a gene expression profiling study that interrogated the expression of 12,000 genes in the forebrain of PS1-hypomorphic mice that exhibit highly attenuated PS1 activity. Using stringent RNA screening, DNA array and Northern assay, we report significant down-regulation in the expression of betaAPP and hypoxia inducible factor-1 alpha (HIF-1alpha), and a marked up-regulation in the expression of glial-specific markers that include S100beta protein, glycerol phosphate dehydrogenase, and glial fibrillary acidic protein. These data suggest potential roles for PS1 in the cellular response to hypoxia and glial-specific gene expression.

Oligodendrocytes as glucocorticoids target cells: functional analysis of the glycerol phosphate dehydrogenase gene

Previous research has established that the development and function of oligodendrocytes are influenced by glucocorticoids. The enzyme glycerol phosphate dehydrogenase (E.C.1.1.1.8) has been used as a model to study glucocorticoid regulation of gene expression in oligodendrocytes and the C6 glial cell line. In the rat brain this enzyme is exclusively localized to oligodendrocytes. The sequence of the 5' flanking region for the rat gene encoding Glycerol Phosphate Dehydrogenase (GPDH; EC 1.1.1.8) was determined. 4 kb of sequence from the 5' flanking region, exon 1, and part of intron 1 of the rat GPDH gene was compared to the corresponding mouse sequence. Dotplot matrix comparison revealed that the rat sequence is more than 80% similar to the mouse sequence, but differs from the mouse sequence in two regions: the rat sequence is devoid of 200 bp of B1 repeat sequence that is present in the mouse, and the rat sequence has an excess 700 bp of B2 repeat sequence inserted between -0.7 kb and -1. 4 kb that is absent in the mouse. To determine the regulatory activity of the rat GPDH 5' flanking region, various portions of the rat GPDH 5' flanking region were placed in luciferase reporter constructs and tested for transcriptional activity. Transient transfection of reporter constructs into the C6 glial cell line revealed that the distal end of the 5' flanking region was glucocorticoid-inducible. A 385 bp Glucocorticoid Response Unit (GRU) was identified whose glucocorticoid induction was enhanced by dibutyryl-cAMP and reduced by phorbol esters. Sequence analysis of the GRU revealed the presence of four consensus GRE sequences and other putative consensus elements. Results here suggest that the 5' flanking region of the GPDH gene mediates the ligand-inducible regulation of GPDH, and that multiple signaling pathways converge at the 5' regulatory sequence to modulate GPDH gene expression in oligodendrocytes.