Implications for the domain arrangement of axonin-1 derived from the mapping of its NgCAM binding site.

The neuronal cell adhesion molecule axonin-1 is composed of six immunoglobulin and four fibronectin type III domains. Axonin-1 promotes neurite outgrowth, when presented as a substratum for neurons in vitro, via a neuronal receptor that has been identified as the neuron-glia cell adhesion molecule, NgCAM, based on the blocking effect of polyclonal antibodies directed to NgCAM. Here we report the identification of axonin-1 domains involved in NgCAM binding. NgCAM-conjugated microspheres were tested for binding to COS cells expressing domain deletion mutants of axonin-1. In addition, monoclonal antibodies directed to axonin-1 were assessed for their ability to block the axonin-1-NgCAM interaction, and their epitopes were mapped using the domain deletion mutants. The results suggest that the four amino-terminal immunoglobulin domains of axonin-1 form a domain conglomerate which is necessary and sufficient for NgCAM binding. Surprisingly, NgCAM binding to membrane-bound axonin-1 was increased strongly by deletion of the fifth or sixth immunoglobulin domains of axonin-1. Based on these results and on negative staining electron microscopy, we propose a horseshoe-shaped domain arrangement of axonin-1 that obscures the NgCAM binding site. Neurite outgrowth studies with truncated forms of axonin-1 show that axonin-1 is a neurite outgrowth-promoting substratum in the absence of the NgCAM binding site.     

In vivo metabolism of a mutant apolipoprotein, apoA-IIowa, associated with hypoalphalipoproteinemia and hereditary systemic amyloidosis.

Apolipoprotein (apo) A-I is the major protein constituent of plasma high density lipoproteins (HDL). A kindred has been identified in which a glycine to arginine mutation at residue 26 in apoA-I is associated with hypoalphalipoproteinemia and hereditary systemic amyloidosis. We isolated the mutant protein, termed apoA-IIowa, from the plasma of an affected subject and studied its in vivo metabolism compared to that of normal apoA-I in two heterozygous apoA-IIowa subjects and two normal controls. Normal and mutant apoA-I were radioiodinated with 131I and 125I, respectively, reassociated with autologous plasma lipoproteins, and simultaneously injected into all subjects. Kinetic analysis of the plasma radioactivity curves demonstrated that the mutant apoA-IIowa was rapidly cleared from plasma (mean fractional catabolic rate [FCR] 0.559 day-1) compared with normal apoA-I (mean FCR 0.244 day-1) in all four subjects. The FCR of normal apoA-I was also substantially faster in the heterozygous apoA-IIowa subjects (mean FCR 0.281 days-1) than in the normal controls (mean FCR 0.203 days-1). Despite the rapid removal from plasma of apoA-IIowa, the cumulative urinary excretion of its associated radioactivity after 2 weeks (44%) of the injected dose) was substantially less than that associated with normal apoA-I (78% of injected dose), indicating extravascular sequestration of radiolabeled apoA-IIowa.(ABSTRACT TRUNCATED AT 250 WORDS)     

Comparison of biodiversity and ground cover between a commercial rotationally grazed property and an adjacent nature reserve in semi‐arid rangeland

Continuous livestock grazing can have negative effects on biodiversity and landscape function in arid and semi‐arid rangelands. Alternative grazing management practices, such as rotational grazing, may be a viable option for broad‐scale biodiversity conservation and sustainable pastoral management. This study compared ground cover, plant species composition and floristic and functional diversity along gradients of grazing intensity between a pastoral property rotationally grazed by goats and an adjacent nature reserve (ungrazed by commercial livestock) in semi‐arid south‐eastern Australia. Understorey plant species composition differed significantly between the rotationally grazed property and the nature reserve, with a greater proportion and frequency of palatable species recorded in the nature reserve. Understorey plant species richness, diversity, functional biodiversity measures and ground cover declined with increasing grazing pressure close to water points under commercial rotational grazing management. However, at a whole‐paddock scale, there were few differences in plant biodiversity and ground cover between the rotationally grazed property and the nature reserve, despite differences in overall plant species composition. Flexible, adaptive, rotational grazing should be investigated further for its potential to achieve both socio‐economic and biodiversity conservation outcomes in semi‐arid rangelands to complement existing conservation reserves.     

Tree functional traits as predictors of microburst-associated treefalls in tropical wet forests

On 19 May 2018, a microburst caused 600 isolated forest gaps in a Costa Rican tropical forest. We surveyed fallen and standing trees within gaps to determine whether certain variables are associated with treefalls. Our results highlight considerations for future research to understand the impacts of microbursts in tropical forests.     

Linking species functional roles to their network roles

Species roles in ecological networks combine to generate their architecture, which contributes to their stability. Species trait diversity also affects ecosystem functioning and resilience, yet it remains unknown whether species’ contributions to functional diversity relate to their network roles. Here, we use 21 empirical pollen transport networks to characterise this relationship. We found that, apart from a few abundant species, pollinators with original traits either had few interaction partners or interacted most frequently with a subset of these partners. This suggests that narrowing of interactions to a subset of the plant community accompanies pollinator niche specialisation, congruent with our hypothesised trade‐off between having unique traits vs. being able to interact with many mutualist partners. Conversely, these effects were not detected in plants, potentially because key aspects of their flowering traits are conserved at a family level. Relating functional and network roles can provide further insight into mechanisms underlying ecosystem functioning.     

Evidence that hepatic lipase and endothelial lipase have different substrate specificities for high-density lipoprotein phospholipids

Hepatic lipase (HL) and endothelial lipase (EL) are both members of the triglyceride lipase gene family. HL hydrolyzes phospholipids and triglycerides in triglyceride-rich lipoproteins and high-density lipoproteins (HDL). EL hydrolyzes HDL phospholipids and has low triglyceride lipase activity. The aim of this study was to determine if HL and EL hydrolyze different HDL phospholipids and whether HDL phospholipid composition regulates the interaction of EL and HL with the particle surface. Spherical, reconstituted HDL (rHDL) containing either 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC), 1-palmitoyl-2-arachidonylphosphatidylcholine (PAPC), or 1-palmitoyl-2-docosahexanoylphosphatidylcholine (PDPC) as the only phospholipid, apolipoprotein A-I as the only apolipoprotein, and either cholesteryl esters (CE) only or mixtures of CE and triolein (TO) in their core were prepared. The rHDL were similar in size and had comparable core lipid/apoA-I molar ratios. The CE-containing rHDL were used to determine the kinetics of HL- and EL-mediated phospholipid hydrolysis. For HL the V(max) of phospholipid hydrolysis for (POPC)rHDL > (PLPC)rHDL approximately (PDPC)rHDL > (PAPC)rHDL, while the K(m)(app) for (POPC)rHDL > (PDPC)rHDL > (PLPC)rHDL > (PAPC)rHDL. For EL the V(max) for (PDPC)rHDL > (PAPC)rHDL > (PLPC)rHDL approximately (POPC)rHDL, while the K(m)(app) for (PAPC)rHDL approximately (PLPC)rHDL > (POPC)rHDL > (PDPC)rHDL. The kinetics of EL- and HL-mediated TO hydrolysis was determined using rHDL that contained TO in their core. For HL the V(max) of TO hydrolysis for (PLPC)rHDL > (POPC)rHDL > (PAPC)rHDL > (PDPC)rHDL, while the K(m)(app) for (PLPC)rHDL > (POPC)rHDL approximately (PAPC)rHDL > (PDPC)rHDL. For EL the V(max) and K(m)(app) for (PAPC)rHDL > (PDPC)rHDL > (PLPC)rHDL > (POPC)rHDL. These results establish that EL and HL have different substrate specificities for rHDL phospholipids and that their interactions with the rHDL surface are regulated by phospholipids.     

Selective interleukin-12 synthesis defect in 12/15-lipoxygenase-deficient macrophages associated with reduced atherosclerosis in a mouse model of familial hypercholesterolemia

Targeted gene disruption or overexpression of 12/15-lipoxygenase in mice on the genetic background of apolipoprotein E or low density lipoprotein-receptor (LDL-R) deficiency has implicated 12/15-lipoxygenase in atherogenesis. The data support indirectly a role for 12/15-lipoxygenase in the oxidative modification of low density lipoprotein. In this study we set out to explore other potential mechanisms for 12/15-lipoxygenase in atherosclerosis using apolipoprotein B mRNA editing catalytic polypeptide-1/LDL-R double-deficient mice, a model highly related to the human condition of familial hypercholesterolemia. 12/15-Lipoxygenase deficiency in this strain led to approximately 50% decrease in aortic lesions in male and female mice at 8 months on a chow diet in the absence of cholesterol differences. While studying 12/15-lipoxygenase-deficient macrophages in culture, we discovered a remarkable selective defect (75-90% decrease) in interleukin-12 production but not in tumor necrosis factor-alpha or nitric oxide release, in response to lipopolysaccharide in the presence or absence of interferon-gamma priming. The lipopolysaccharide/interferon-gamma response was associated with a 33-50% decrease in nuclear interferon consensus sequence-binding protein, which is consistent with interferon consensus sequence-binding protein containing protein complex-dependent regulation of the interleukin-12 p40 gene. The decrease in interleukin-12 production was recapitulated in vivo in mouse aortas of the triple knockout group and was reflected in a marked decrease in interferon-gamma expression. The data provide support for a novel mechanism linking the 12/15-lipoxygenase pathway to a known immunomodulatory Th1 cytokine in atherogenesis.     

Acute inflammation increases selective uptake of HDL cholesteryl esters into adrenals of mice overexpressing human sPLA2

The acute-phase protein secretory phospholipase A2 (sPLA2) influences the metabolism of high-density lipoproteins (HDL). The adrenals are known to utilize HDL cholesterol as a source of sterols. The aim of the present study was to test the hypothesis that sPLA2 enhances the selective uptake of HDL into the adrenals in response to acute inflammation as a possible physiological role for the sPLA2-HDL interaction. Human sPLA2-transgenic mice, in which sPLA2 expression is upregulated by inflammatory stimuli, were used. Ten hours after induction of the acute-phase response (APR) by injection of bacterial lipopolysaccharide (LPS), plasma levels of HDL cholesterol decreased significantly in sPLA2-transgenic mice (-18%, P < 0.05) but remained unchanged in wild-type mice. The fractional catabolic rates of both 125I-labeled tyraminecellobiose (TC)-HDL and [3H]cholesteryl ether increased significantly in the sPLA2-transgenic mice after induction of the APR (0.18 +/- 0.01 vs. 0.21 +/- 0.01 pool/h, P < 0.05, and 0.31 +/- 0.02 vs. 0.42 +/- 0.05 pool/h, P < 0.05, respectively) but remained unchanged in the wild-type mice (0.10 +/- 0.01 vs. 0.22 +/- 0.02 pool/h, respectively). After induction of the APR, in both groups HDL holoparticle uptake by the liver was increased (P < 0.001). sPLA2-transgenic mice had 2.4-fold higher selective uptake into the adrenals after induction of the APR than wild-type mice (156 +/- 6 vs. 65 +/- 5%/ micro g tissue protein, P < 0.001). In summary, upregulation of sPLA2 expression during the APR specifically increases the selective uptake of HDL cholesteryl ester into the adrenals. These data suggest a novel metabolic role for sPLA2: modification of HDL during the APR to promote increased adrenal uptake of HDL cholesteryl ester to serve as source for steroid hormone synthesis.