Ceramide levels regulated by carnitine palmitoyltransferase 1C control dendritic spine maturation and cognition.

The brain-specific isoform carnitine palmitoyltransferase 1C (CPT1C) has been implicated in the hypothalamic regulation of food intake and energy homeostasis. Nevertheless, its molecular function is not completely understood, and its role in other brain areas is unknown. We demonstrate that CPT1C is expressed in pyramidal neurons of the hippocampus and is located in the endoplasmic reticulum throughout the neuron, even inside dendritic spines. We used molecular, cellular, and behavioral approaches to determine CPT1C function. First, we analyzed the implication of CPT1C in ceramide metabolism. CPT1C overexpression in primary hippocampal cultured neurons increased ceramide levels, whereas in CPT1C-deficient neurons, ceramide levels were diminished. Correspondingly, CPT1C knock-out (KO) mice showed reduced ceramide levels in the hippocampus. At the cellular level, CPT1C deficiency altered dendritic spine morphology by increasing immature filopodia and reducing mature mushroom and stubby spines. Total protrusion density and spine head area in mature spines were unaffected. Treatment of cultured neurons with exogenous ceramide reverted the KO phenotype, as did ectopic overexpression of CPT1C, indicating that CPT1C regulation of spine maturation is mediated by ceramide. To study the repercussions of the KO phenotype on cognition, we performed the hippocampus-dependent Morris water maze test on mice. Results show that CPT1C deficiency strongly impairs spatial learning. All of these results demonstrate that CPT1C regulates the levels of ceramide in the endoplasmic reticulum of hippocampal neurons, and this is a relevant mechanism for the correct maturation of dendritic spines and for proper spatial learning.

Ten novel HMGCL mutations in 24 patients of different origin with 3-hydroxy-3-methyl-glutaric aciduria.

3-Hydroxy-3-methylglutaric aciduria is a rare autosomal recessive genetic disorder that affects ketogenesis and L-leucine catabolism. The clinical acute symptoms include vomiting, convulsions, metabolic acidosis, hypoketotic hypoglycaemia and lethargy. To date, 33 mutations in 100 patients have been reported in the HMGCL gene. In this study 10 new mutations in 24 patients are described. They include: 5 missense mutations: c.109G>A, c.425C>T, c.521G>A, c.575T>C and c.598A>T, 2 nonsense mutations: c.242G>A and c.559G>T, one small deletion: c.853delC, and 2 mutations in intron regions: c.497+4A>G and c.750+1G>A. Two prevalent mutations were detected, 109G>T (E37X) in 38% of disease alleles analyzed and c.504_505delCT in 10% of them. Although patients are mainly of European origin (71%) and mostly Spanish (54%), the group is ethnically diverse and includes, for the first time, patients from Pakistan, Palestine and Ecuador. We also present a simple, efficient method to express the enzyme and we analyze the possible functional effects of missense mutations. The finding that all identified missense mutations cause a >95% decrease in the enzyme activity, indicates that the disease appears only in very severe genotypes."

CPT1c is localized in endoplasmic reticulum of neurons and has carnitine palmitoyltransferase activity.

CPT1c is a carnitine palmitoyltransferase 1 (CPT1) isoform that is expressed only in the brain. The enzyme has recently been localized in neuron mitochondria. Although it has high sequence identity with the other two CPT1 isoenzymes (a and b), no CPT activity has been detected to date. Our results indicate that CPT1c is expressed in neurons but not in astrocytes of mouse brain sections. Overexpression of CPT1c fused to the green fluorescent protein in cultured cells demonstrates that CPT1c is localized in the endoplasmic reticulum rather than mitochondria and that the N-terminal region of CPT1c is responsible for endoplasmic reticulum protein localization. Western blot experiments with cell fractions from adult mouse brain corroborate these results. In addition, overexpression studies demonstrate that CPT1c does not participate in mitochondrial fatty acid oxidation, as would be expected from its subcellular localization. To identify the substrate of CPT1c enzyme, rat cDNA was overexpressed in neuronal PC-12 cells, and the levels of acylcarnitines were measured by high-performance liquid chromatography-mass spectrometry. Palmitoylcarnitine was the only acylcarnitine to increase in transfected cells, which indicates that palmitoyl-CoA is the enzyme substrate and that CPT1c has CPT1 activity. Microsomal fractions of PC-12 and HEK293T cells overexpressing CPT1c protein showed a significant increase in CPT1 activity of 0.57 and 0.13 nmol.mg(-1).min(-1), respectively, which is approximately 50% higher than endogenous CPT1 activity. Kinetic studies demonstrate that CPT1c has similar affinity to CPT1a for both substrates but 20-300 times lower catalytic efficiency.

A new LC-ESI-MS/MS method to measure long-chain acylcarnitine levels in cultured cells.

The quantitative evaluation of long-chain acylcarnitines in lipid extracts from cultured cells or tissues is a prerequisite to study carnitine palmitoyltransferase (CPT) activity. There is thus a need for the accurate measurement of the concentration of long-chain acylcarnitines at the lowest concentration present in lipid extracts. Here we report a fast and reliable quantitative method based on the use of weak acid extraction and liquid chromatography-electrospray ionization tandem mass spectrometry (LC-ESI-MS/MS) to quantify acylcarnitines through hydrophilic interaction chromatography. The method was validated using isotopic dilution and the results allow the analysis of a large number of samples at low concentration levels (down to 0.35 nmol L(-1) for palmitoylcarnitine) with good inter- and intra-day precision. The method was used for the quantitative study of changes in concentration of palmitoylcarnitine and other acylcarnitines in PC-12 cells over-expressing CPT1a gene. It was also used to measure CPT1 activity in mitochondria isolated from transfected cells, giving similar results to the more common radiometric method, but with higher sensitivity.

C-terminal end and aminoacid Lys48 in HMG-CoA lyase are involved in substrate binding and enzyme activity.

3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) lyase adopts a (betaalpha)(8) TIM barrel structure with an additional beta9, alpha11 and alpha12 helices. Location of HMG part of the substrate has been suggested but the binding mode for the CoA moiety remains to be resolved. As mutation F305 fs(-2), which involves the last 21 residues of the protein, and mutation K48N caused 3-hydroxy-3-methylglutaric aciduria in two patients, we examined the role of the C-terminal end and Lys(48) in enzyme activity. Expression studies of various C-terminal-end-deleted and K48N-mutated proteins revealed that residues 311-313 (localized in the loop between alpha11 and alpha12 helices) and Lys(48) are essential for enzyme activity. An in silico docking model locating HMG-CoA on the surface of the enzyme implicates Asn(311) and Lys(313) in substrate binding by establishing multiple polar contacts with phosphate and ribose groups of adenosine, and Lys(48) by contacting the carboxyl group of the panthotenic acid moiety.