APOA5 and triglyceride metabolism, lesson from human APOA5 deficiency

PURPOSE OF REVIEW: In this review we compare the phenotype and lipoprotein abnormalities of some patients who were found to carry mutations in the APOA5 gene predicted to result in apolipoprotein A-V deficiency. RECENT FINDINGS: The sequencing of the APOA5 gene in patients with primary hypertriglyceridemia, in whom mutations of the LPL and APOC2 genes had been excluded, led to the identification of four families with two different mutations in this gene predicted to result in truncated apolipoprotein A-V. The first mutation (Q148X) was found in a homozygous state in a child with severe type V hyperlipidemia, some clinical manifestations of chylomicronemia syndrome and a slight reduction in plasma postheparin lipoprotein lipase activity. Carriers of a different mutation (Q139X) were recently reported. Four Q139X heterozygotes had type V hyperlipidemia and markedly reduced plasma postheparin lipoprotein lipase activity. The hypertriglyceridemic Q139X heterozygote had other factors that could have contributed to hypertriglyceridemia. ApoB-100 kinetic studies in hypertriglyceridemic Q139X heterozygotes revealed an impairment of very low-density lipoprotein catabolism. SUMMARY: Mutations in the APOA5 gene, leading to truncated apolipoprotein A-V devoid of lipid-binding domains located in the carboxy-terminal end of the protein, if present in the homozygous state, are expected to cause severe type V hyperlipidemia in patients with no mutations in LPL or APOC2 genes. If present in the heterozygous state, these mutations predispose to hypertriglyceridemia in combination with other genetic factors or pathological conditions.     

Impairment of 8-iso-PGF2ALPHA isoprostane metabolism by dietary conjugated linoleic acid (CLA)

8-iso-PGF_2α isoprostane (IP) is one of the most-used markers of lipid peroxidation in experimental models and humans. After its formation, it is promptly metabolized to 2,3 dinor (DIN) in peroxisomes.Conjugated linoleic acid (CLA) is preferentially β-oxidized in peroxisomes which may compete with IP, and thereby may affect its metabolism.In order to verify whether CLA is able to influence IP formation and/or metabolism and to explain the mechanism, we challenged rats supplemented with CLA or with triolein (as a control fatty acid), with a single dose of carbon tetrachloride (CCl₄) or of bacterial lipopolysaccharide (LPS). The results showed that IP and its precursor arachidonic acid hydroperoxide, as well as malondialdheyde (MDA), increase significantly in the liver of rats challenged with CCl₄, irrespective of the diet, while in LPS-treated rats only nitrites in liver and isoprostane in plasma increase. On the other hand, the peroxisomal β-oxidation products of IP, the DIN, is significantly lower in the CLA group with respect to control and triolein groups.To further investigate whether this is due to competition between CLA and IP at the cellular level, we incubated human fibroblasts from healthy subjects or patients with adrenoleukodystrophy (ALD), with CLA and/or commercially available IP. The rationale of this approach is based on the deficient peroxisomal β-oxidation of fibroblasts from ALD patients, leading to a reduced formation of DIN. In both normal and ALD cells, the presence of CLA significantly inhibits the formation of DIN from IP.We may conclude that both in vitro and in vivo studies strongly suggest that CLA may impair IP catabolism in peroxisomes. Consequently an increase of IP, as a sole result of CLA intake, cannot be considered as a marker of lipid peroxidation.     

Hepatic retinol secretion and storage are altered by dietary CLA: common and distinct actions of CLA c9,t11 and t10,c12 isomers

Conjugated linoleic acid (CLA) is a polyunsaturated fatty acid obtained from ruminant products. Previous studies in rats and pigs showed that a dietary equimolar mixture of c9,t11 and t10,c12 CLA isomers induces changes in serum and tissue levels of retinoids (vitamin A derivatives). However, the mechanism(s) responsible for these actions remain(s) unexplored. Given the numerous crucial biological functions regulated by retinoids, it is key to establish whether the perturbations in retinoid metabolism induced by dietary CLA mediate some of the beneficial effects associated with intake of this fatty acid or, rather, have adverse consequences on health. To address this important biological question, we began to explore the mechanisms through which dietary CLA alters retinoid metabolism. By using enriched preparations of CLA c9,t11 or CLA t10,c12, we uncoupled the effects of these two CLA isomers on retinoid metabolism. Specifically, we show that both isomers induce hepatic retinyl ester accumulation. However, only CLA t10,c12 enhances hepatic retinol secretion, resulting in increased serum levels of retinol and its specific carrier, retinol-binding protein (RBP). Dietary CLA t10,c12 also redistributes retinoids from the hepatic stores toward the adipose tissue and possibly stimulates hepatic retinoid oxidation. Using mice lacking RBP, we also demonstrate that this key protein in retinoid metabolism mediates hepatic retinol secretion and its redistribution toward fat tissue induced by CLA t10,c12 supplementation.     

A tracheomycosis as a tool for studying the impact of stem xylem dysfunction on leaf water status and gas exchange in Citrus aurantium L.

Permanent xylem blockage is a common result of attacks by herbivores and fungi. The mitosporic fungus Phoma tracheiphila (Petri) Kantschaveli et Gikachvili, is the agent of a Citrus tracheomycosis (“malsecco disease”) causing xylem impairment and leading to leaf shedding and plant dieback. In the present study, this pathogen was used for monitoring the effects of increasing levels of stem hydraulic resistance (R _stem) on leaf water status and gas exchange. In this view, measurements are reported of changes in the hydraulic resistance of infected stems (R _stem) of C. aurantium (sour orange) during progressive and irreversible xylem blockage with parallel measurements of leaf water potential and conductance to water vapour. Leaves were highly responsive to increasing R _stem as due to fungal infection, with substantial stomatal closure and drop in water potential.     

Hydraulic connections of leaves and fruit to the parent plant in Capsicum frutescens (hot pepper) during fruit ripening

Background and Aims

The hydraulic architecture and water relations of fruits and leaves of Capsicum frutescens were measured before and during the fruiting phase in order to estimate the eventual impact of xylem cavitation and embolism on the hydraulic isolation of fruits and leaves before maturation/abscission.

Methods

Measurements were performed at three different growth stages: (1) actively growing plants with some flowers before anthesis (GS1), (2) plants with about 50 % fully expanded leaves and immature fruits (GS2) and (3) plants with mature fruits and senescing basal leaves (GS3). Leaf conductance to water vapour as well as leaf and fruit water potential were measured. Hydraulic measurements were made using both the high-pressure flow meter (HPFM) and the vacuum chamber (VC) technique.

Key Results

The hydraulic architecture of hot pepper plants during the fruiting phase was clearly addressed to favour water supply to growing fruits. Hydraulic measurements revealed that leaves of GS1 plants as well as leaves and fruit peduncles of GS2 plants were free from significant xylem embolism. Substantial increases in leaf petiole and fruit peduncle resistivity were recorded in GS3 plants irrespective of the hydraulic technique used. The higher fraction of resistivity measured using the VC technique compared with the HPFM technique was apparently due to conduit embolism.

Conclusions

The present study is the first to look at the hydraulics of leaves and fruits during growth and maturation through direct, simultaneous measurements of water status and xylem efficiency of both plant regions at different hours of the day.     

Molecular Basis for the Dual Function of Eps8 on Actin Dynamics: Bundling and Capping

Actin capping and cross-linking proteins regulate the dynamics and architectures of different cellular protrusions. Eps8 is the founding member of a unique family of capping proteins capable of side-binding and bundling actin filaments. However, the structural basis through which Eps8 exerts these functions remains elusive. Here, we combined biochemical, molecular, and genetic approaches with electron microscopy and image analysis to dissect the molecular mechanism responsible for the distinct activities of Eps8. We propose that bundling activity of Eps8 is mainly mediated by a compact four helix bundle, which is contacting three actin subunits along the filament. The capping activity is mainly mediated by a amphipathic helix that binds within the hydrophobic pocket at the barbed ends of actin blocking further addition of actin monomers. Single-point mutagenesis validated these modes of binding, permitting us to dissect Eps8 capping from bundling activity in vitro. We further showed that the capping and bundling activities of Eps8 can be fully dissected in vivo, demonstrating the physiological relevance of the identified Eps8 structural/functional modules. Eps8 controls actin-based motility through its capping activity, while, as a bundler, is essential for proper intestinal morphogenesis of developing Caenorhabditis elegans.