Expression and conservation of apolipoprotein AIV in an avian species

From worms to humans: Understanding intestinal lipid metabolism via model organisms

The intestine is responsible for efficient absorption and packaging of dietary lipids before they enter the circulatory system. This review provides a comprehensive overview of how intestinal enterocytes from diverse model organisms absorb dietary lipid and subsequently secrete the largest class of lipoproteins (chylomicrons) to meet the unique needs of each animal. We discuss the putative relationship between diet and metabolic disease progression, specifically Type 2 Diabetes Mellitus. Understanding the molecular response of intestinal cells to dietary lipid has the potential to undercover novel therapies to combat metabolic syndrome.

The effect of microbial challenge on the intestinal proteome of broiler chickens

Background

In poultry production intestinal health and function is paramount to achieving efficient feed utilisation and growth. Uncovering the localised molecular mechanisms that occur during the early and important periods of growth that allow birds to grow optimally is important for this species. The exposure of young chicks to used litter from older flocks, containing mixed microbial populations, is a widely utilised model in poultry research. It rarely causes mortality but effects an immunogenic stimulation sufficient enough to cause reduced and uneven growth that is reflective of a challenging growing environment.

Methods

A mixed microbial challenge was delivered as used litter containing Campylobacter jejuni and coccidial oocysts to 120 male Ross 308 broiler chicks, randomly divided into two groups: control and challenged. On day 12, 15, 18 and 22 (pre- and 3, 6 and 10 days post-addition of the used litter) the proximal jejunum was recovered from 6 replicates per group and differentially abundant proteins identified between groups and over time using 2D DiGE.

Results

The abundance of cytoskeletal proteins of the chicken small intestinal proteome, particularly actin and actin associated proteins, increased over time in both challenged and control birds. Villin-1, an actin associated anti-apoptotic protein, was reduced in abundance in the challenged birds indicating that many of the changes in cytoskeletal protein abundance in the challenged birds were as a result of an increased rate of apoptosis. A number of heat shock proteins decreased in abundance over time in the intestine and this was more pronounced in the challenged birds.

Conclusions

The small intestinal proteome sampled from 12 to 22 days of age showed considerable developmental change, comparable to other species indicating that many of the changes in protein abundance in the small intestine are conserved among vertebrates. Identifying and distinguishing the changes in proteins abundance and molecular pathways that occur as a result of normal growth from those that occur as a result of a challenging microbial environment is important in this major food producing animal.

A novel estrogen-regulated avian apolipoprotein

In search for yet uncharacterized proteins involved in lipid metabolism of the chicken, we have isolated a hitherto unknown protein from the serum lipoprotein fraction with a buoyant density of ≤1.063 g/ml. Data obtained by protein microsequencing and molecular cloning of cDNA defined a 537 bp cDNA encoding a precursor molecule of 178 residues. As determined by SDS-PAGE, the major circulating form of the protein, which we designate apolipoprotein-VLDL-IV (Apo-IV), has an apparent M_r of approximately 17 kDa. Northern Blot analysis of different tissues of laying hens revealed Apo-IV expression mainly in the liver and small intestine, compatible with an involvement of the protein in lipoprotein metabolism. To further investigate the biology of Apo-IV, we raised an antibody against a GST-Apo-IV fusion protein, which allowed the detection of the 17-kDa protein in rooster plasma, whereas in laying hens it was detectable only in the isolated ≤1.063 g/ml density lipoprotein fraction. Interestingly, estrogen treatment of roosters caused a reduction of Apo-IV in the liver and in the circulation to levels similar to those in mature hens. Furthermore, the antibody crossreacted with a 17-kDa protein in quail plasma, indicating conservation of Apo-IV in avian species. In search for mammalian counterparts of Apo-IV, alignment of the sequence of the novel chicken protein with those of different mammalian apolipoproteins revealed stretches with limited similarity to regions of ApoC-IV and possibly with ApoE from various mammalian species. These data suggest that Apo-IV is a newly identified avian apolipoprotein.

•

Apo-VLDL-IV (Apo-IV) is a newly identified avian apolipoprotein.

•

Apo-IV expression is suppressed by estrogen.

•

Apo-IV containing VLDL particles are excluded from uptake into yolk.

•

Apo-IV has limited similarity to mammalian ApoC-IV.

Specific sequences in N termini of apolipoprotein A-IV modulate its anorectic effect

Rodent apoA-IV is expressed predominantly in small intestine and also expressed to a small extent in liver and hypothalamus. ApoA-IV has been shown to inhibit food intake in rats when injected centrally. In the current study, we hypothesize that a specific sequence within rat apoA-IV is responsible for mediating the anorectic effect. We use a bacterial expression system to generate truncation mutants (Δ249-371, Δ117-371 and Δ1-61) of rat apoA-IV and assess the ability of various regions of the molecule to inhibit food intake. The results indicate that a responsible sequence exists within the N-terminal 61 amino acids of rat apoA-IV. Synthetic peptides (1-30 EVTSDQVANVMWDYFTQLSNNAKEAVEQLQ, 1-15 EVTSDQVANVMWDYF and 17-30 QLSNNAKEAVEQLQ) were used to specify the region in between residues 1 and 30. A 14-mer peptide (17-30) encompassing this sequence was capable of reducing food intake in a dose-dependent manner whereas a peptide designed on a more C-terminal region (211-232) of apoA-IV (QEKLNHQMEGLAFQMKKNAEEL) failed to exhibit the dose-dependent anorectic effect. The isolation of this sequence provides a valuable tool for future work directed at identifying apoA-IV binding proteins and is a key step for exploring the potential of therapeutic manipulation of food intake via this pathway.

Isolation and characterization of some novel genes of the apolipoprotein A-I family in Japanese eel, Anguilla japonica

Apolipoproteins such as apolipoprotein (apo) A-I, apoA-IV, and apoE are lipid binding proteins synthesized mainly in the liver and the intestine and play an important role in the transfer of exogenous or endogenous lipids through the circulatory system. To investigate the mechanism of lipid transport in fish, we have isolated some novel genes of the apoA-I family, apoIA-I (apoA-I isoform) 1–11, from Japanese eel by PCR amplification. Some of the isolated genes of apoIA-I corresponded to 28kDa-1 cDNAs which had already been deposited into the database and encoded an apolipoprotein with molecular weight of 28 kDa in the LDL, whereas others seemed to be novel genes. The structural organization of all apoIA-Is consisted of four exons separated by three introns. ApoIA-I10 had a total length of 3232 bp, whereas other genes except for apoIA-I9 ranged from 1280 to 1441 bp. The sequences of apoIA-Is at the exon-intron junctions were mostly consistent with the consensus sequence (GT/AG) at exon-intron boundaries, whereas the sequences of 3′ splice acceptor in intron 1 of apoIA-I1-7 were (AC) but not (AG). The deduced amino acid sequences of all apoIA-Is contained a putative signal peptide and a propeptide of 17 and 5 amino acid residues, respectively. The mature proteins of apoIA-I1-3, 7, and 8 consisted of 237 amino acids, whereas those of apoIA-I4-6 consisted of 239 amino acids. The mature apoIA-I10 sequence showed 65% identity to amino acid sequence of apoIA-I11 which was associated with an apolipoprotein with molecular weight of 23 kDa in the VLDL. All these mature apoIA-I sequences satisfied the common structural features depicted for the exchangeable apolipoproteins such as apoA-I, apoA-IV, and apoE but apoIA-I11 lacked internal repeats 7, 8, and 9 when compared with other members of apoA-I family. Phylogenetic analysis showed that these novel apoIA-Is isolated from Japanese eel were much closer to apoA-I than apoA-IV and apoE, suggesting new members of the apoA-I family.

Avian phospholipid transfer protein causes HDL conversion without affecting cholesterol efflux from macrophages

Circulatory phospholipid transfer protein (PLTP) has two major functions: 1) transfer of phospholipids towards HDL particles; and 2) modulation of HDL size and composition via the HDL conversion process. In the laying hen (Gallus gallus), the massive oocyte-targeted lipid flow is achieved through the concerted actions of lipases, lipid transfer proteins, and relatives of the LDL receptor family. The aim of the study was to gain insights into the structure and functions of chicken PLTP. The results demonstrate that PLTP is highly conserved from chicken to mammals, as (i) chicken PLTP is associated with plasma HDL; (ii) it clearly possesses phospholipid transfer activity; (iii) it is inactivated at +58 degrees C; and (iv) it mediates conversion of avian and human HDL into small prebeta-mobile HDL and large fused alpha-mobile HDL particles. Our data show that HDL from different chicken models is similar in chemical and physical properties to that of man based on PLTP activity, cholesterol efflux, and HDL conversion assays. In contrast to mammals, PLTP-facilitated HDL remodeling did not enhance cholesterol efflux efficiency of chicken HDL particles.

Avian apolipoprotein A-V binds to LDL receptor gene family members

Apolipoprotein A-V (apoA-V) affects plasma triglyceride (TG) levels; however, the properties of apoA-V that mediate its action(s) are still incompletely understood. It is unclear how apoA-V, whose plasma concentration is extremely low, can affect the pronounced TG differences observed in individuals with various apoA-V dysfunctions. To gain novel insights into apoA-V biology, we expanded our previous studies in the chicken to this apolipoprotein. First, we characterized the first avian apoA-V, revealing its expression not only in liver and small intestine but also in brain, kidney, and ovarian follicles and showing its presence in the circulation. Second, we demonstrate directly that galline apoA-V binds to the major LDL receptor family member (LR) of the laying hen and that this interaction does not depend on the association of the apolipoprotein with lipid or lipoproteins. We propose that a direct interaction with LRs may represent a novel, additional mechanism for the modulation of TG levels by apoA-V.

Analysis of chicken serum proteome and differential protein expression during development in single-comb White Leghorn hens

Serum is believed to harbor thousands of distinct proteins that are either actively secreted or leak from various blood cells or tissues. Exploring protein composition in serum may accelerate the discovery of novel protein biomarkers for specific economic traits in livestock species. This study analyzed serum protein composition to establish a 2-DE reference map, and monitored protein dynamics of single-comb White Leghorn hens at 8, 19 and 23 weeks after hatching. A total of 119 CBB-stained and 315 silver-stained serum protein spots were analyzed by MALDI-TOF MS. Of these, 98 CBB-stained and 94 silver-stained protein spots were significantly matched to existing chicken proteins. The identified spots represented 30 distinctive proteins in the serum of laying hens. To compare protein expression during development, expression levels of 47 protein spots were quantified by relative spot volume with Melanie 3 software. Ten protein spots increased and 3 protein spots decreased as hen age increased. Previous research has suggested that some of these proteins play critical roles in egg production. The differentially expressed proteins with unknown identities will be valuable candidates for further explorations of their roles in egg production of laying hens.

Overexpression of apolipoprotein A-IV enhances lipid secretion in IPEC-1 cells by increasing chylomicron size

Intestinal apolipoprotein A-IV expression is highly regulated by dietary lipid in newborn swine, suggesting a role in lipid absorption. Constitutive overexpression of apoA-IV in newborn swine enterocytes enhances basolateral secretion of triacylglycerol (TG) in TG-rich lipoproteins 4.9-fold (Lu, S., Yao, Y., Meng, S., Cheng, X., and Black, D. D. (2002) J. Biol. Chem. 277, 31929-31937). To investigate the mechanism of this enhancement, IPEC-1 cells were transfected with a tetracycline-regulatable expression system (Tet-On). In cells incubated with oleic acid, a dose response relationship was observed between medium doxycycline concentration and basolateral apoA-IV and TG secretion. Similarly regulated expression of apoA-I did not enhance lipid secretion. The mean diameter of TG-rich lipoproteins secreted from doxycycline-treated cells was larger than from untreated cells (87.0 nm versus 53.4 nm). Basolateral apoB secretion decreased. Using the same expression system, full-length human apoA-IV (376 amino acids); a "pig-like" human apoA-IV, lacking the C-terminal EQQQ repeats (361 amino acids); and a "chicken-like" apoA-IV, further truncated to 343 amino acids, were expressed in IPEC-1 cells. With increasing protein secretion, cells expressing the full-length human apoA-IV displayed a 2-fold increase in TG secretion; in sharp contrast, cells expressing the pig-like human apoA-IV displayed a 25-fold increase in TG secretion and a 27-fold increase in lipoprotein diameter. When human apoA-IV was further truncated to yield a chicken-like protein, TG secretion was inhibited. We conclude that overexpression of swine apoA-IV enhances basolateral TG secretion in a dose-dependent manner by increasing the size of secreted lipoproteins. These data suggest that the region in the human apoA-IV protein from residues 344 to 354 is critical to its ability to enhance lipid secretion, perhaps by enabling the packaging of additional core TG into chylomicron particles. The EQQQ-rich region may play an inhibitory or modulatory role in chylomicron packaging in humans.

Cloning, characterization and comparative analysis of pig plasma apolipoprotein A-IV

Pig apolipoprotein (apo) A-IV cDNA was cloned, characterized and compared to the human ortholog. Mature porcine apo A-IV consists of 362 amino acids and displays a 75.6% sequence identity with human protein. Pig apo A-IV is the smallest reported mammalian apo A-IV because it lacks the repeated motifs of glutamine and glutamic acid at the carboxyl terminus. A phylogenic tree of apo A-IV mammalian proteins reveals that porcine apo A-IV is more closely related to humans and primates than to rodents. This protein is highly hydrophobic and is mainly associated with lipoproteins.

Phylogenetic distribution of apolipoproteins A-I and E in vertebrates as determined by Western blot analysis

A putative apolipoprotein E (apoE) has been identified in the HDL and VHDL fractions of the turtle. This observation is of particular interest considering apoE has been reported absent in the domestic hen (Hermier et al., '95; Biochim Biophys Acta: 105-118, 1995) and thus presumed absent in nonmammalian vertebrates altogether. As a result, partial amino acid sequencing of this protein was performed and revealed that one fragment shared 41% sequence identity to human apoE. Western blot analysis using antisera to apoE demonstrated cross-reactivity to a 34-kDa protein (putative apoE) in turtle plasma. Further investigation using anti-apoE antibody in Western blot analysis detected immunoreactive apoE in the plasma of lamprey, spiny dogfish, skate, and alligator, but not in flounder, newt or python; its absence in several species of birds was confirmed. Using anti-apoA-I antibody, apoA-I was detected in all vertebrate groups except a representative teleost (flounder). Apo-A-I antibody cross-reacted weakly with some putative apoE proteins (chicken, spiny dogfish and skate) and the reverse was true for anti-apoE, which cross-reacted with putative apoA-I in birds, reptiles, and elasmobranchs, confirming the molecular similarity and phylogenetic relatedness of these two proteins.

The novel sequences of major plasma apolipoproteins in the eel Anguilla japonica

cDNAs encoding major plasma apolipoproteins (apo) were cloned from the eel Anguilla japonica liver and their nucleotide sequences determined. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis revealed that eel lipoproteins contain apolipoproteins of 28 kDa and 14 kDa as major components. Each of the two apolipoproteins showed two isoforms having different isoelectric points as demonstrated by two-dimensional electrophoresis. The two 28 kDa components had different N-terminal amino acid sequences, whereas the two 14 kDa components had an identical one. Then cDNA clones encoding these apolipoproteins were isolated from a cDNA library constructed from the eel liver. An acidic 28 kDa component (28 kDa-1) consisted of 259 amino acids including a putative signal peptide of 27 residues, whereas a basic 28 kDa component (28 kDa-2) was composed of 260 amino acids containing a putative signal peptide of 23 residues. The tandem repeating units, which are characteristic of apolipoproteins, for 28 kDa-1 showed 27.8% identity to that of porcine apoA-IV, although mammalian apoA-IV is about 40 kDa and much larger than 28 kDa-1. However, the repeating units of 28 kDa-2 showed 52.5% identity to that of Atlantic salmon apoA-I. The 14 kDa apolipoprotein consisted of 142 amino acids containing a putative signal peptide of 20 residues. It has a novel sequence differing from apolipoproteins of other vertebrates. The transcriptional expressions of 28 kDa-1, 28 kDa-2, and 14 kDa components were all restricted to the liver, except for the transcripts of 28 kDa-2 which were also slightly expressed in the intestine.

Conserved protein motifs and structural organization of a fish gene homologous to mammalian apolipoprotein E

Apolipoprotein E (apoE) plays a central role in lipid metabolism from its ability to interact with lipoprotein receptors. Besides its role in cardiovascular diseases, apoE polymorphism contributes to susceptibility to neurodegenerative diseases, such as Alzheimer's disease. The statistical significance of the combined match scores obtained after apoE motif-based protein sequence database searches, the structural features of the deduced protein, and the phylogenetic analysis, support the evidence that a homologue to mammalian apoE can be found in teleost fish. Isolation and characterization of the first nonmammalian APOE revealed that the zebrafish gene spans 2555/2692 bp instead of 3597 bp in human and has the same splice junctions and exon/intron organization as found in mammals, except that there is an additional intron that splits the last exon (exon 4) into two exons (exons 4 and 5). Enlargement of APOE size in the mammalian lineage occurs mainly by Alu repeats insertion. The additional intron found in zebrafish gene was also identified at the same splicing site in trout APOE and is located in the corresponding linker region following the conserved low density lipoprotein receptor binding domain. Primer extension and reverse transcriptase PCR (RT-PCR) assays demonstrated that two transcription start sites are located 26 and 28 bp upstream of the first intron and 22 or 24 bp downstream from a canonical TATA box. Sequence inspection of the 5'-flanking region upstream of the TATA box revealed potential regulatory DNA elements. These results will serve as a basis for comparative studies on transcriptional and post-transcriptional mechanisms of APOE regulation in vertebrates.

The -700/-310 fragment of the apolipoprotein A-IV gene combined with the -890/-500 apolipoprotein C-III enhancer is sufficient to direct a pattern of gene expression similar to that for the endogenous apolipoprotein A-IV gene

Spatial gene expression in the intestine is mediated by specific regulatory sequences. The three genes of the apoA-I/C-III/A-IV cluster are expressed in the intestine following cephalocaudal and crypt-to-villus axes. Previous studies have shown that the -780/-520 enhancer region of the apoC-III gene directs the expression of the apoA-I gene in both small intestinal villi and crypts, implying that other unidentified elements are necessary for a normal intestinal pattern of apoA-I gene expression. In this study, we have characterized transgenic mice expressing the chloramphenicol acetyltransferase gene under the control of different regions of the apoC-III and apoA-IV promoters. We found that the -890/+24 apoC-III promoter directed the expression of the reporter gene in crypts and villi and did not follow a cephalocaudal gradient of expression. In contrast, the -700/+10 apoA-IV promoter linked to the -500/-890 apoC-III enhancer directed the expression of the reporter gene in enterocytes with a pattern of expression similar to that of the endogenous apoA-IV gene. Furthermore, linkage of the -700/-310 apoA-IV distal promoter region to the -890/+24 apoC-III promoter was sufficient to restore the appropriate pattern of intestinal expression of the reporter gene. These findings demonstrate that the -700/-310 distal region of the apoA-IV promoter contains regulatory elements that, in combination with proximal promoter elements and the -500/-890 enhancer, are necessary and sufficient to restrict apoC-III and apoA-IV gene expression to villus enterocytes of the small intestine along the cephalocaudal axis.