Chicken yolk contains bona fide high density lipoprotein particles

High Dietary Folic Acid Supplementation Reduced the Composition of Fatty Acids and Amino Acids in Fortified Eggs

AIMS

The study aimed to evaluate the effects of dietary folic acid (FA) on the production performance of laying hens, egg quality, and the nutritional differences between eggs fortified with FA and ordinary eggs.

METHODS

A total of 288 26-week-old Hy-Line Brown laying hens (initial body weights 1.65 ± 0.10 kg) with a similar weight and genetic background were used. A completely randomized design divided the birds into a control group and three treatment groups. Each group consisted of six replicates, with twelve chickens per replicate. Initially, all birds were fed a basal diet for 1 week. Subsequently, they were fed a basal diet supplemented with 0, 5, 10, or 15 mg/kg FA in a premix for a duration of 6 weeks.

RESULTS

Supplementation of FA could significantly (p < 0.05) enhance the FA content in egg yolks, particularly when 10 mg/kg was used, as it had the most effective enrichment effect. Compared to the control group, the Glu content in the 10 and 15 mg/kg FA groups showed a significant (p < 0.05) decrease. Additionally, the contents of Asp, Ile, Tyr, Phe, Cys, and Met in the 15 mg/kg FA group were significantly (p < 0.05) lower compared to the other groups. Adding FA did not have significant effects on the levels of vitamin A and vitamin E in egg yolk, but the vitamin D content in the 5 and 10 mg/kg FA groups showed a significant (p < 0.05) increase. Furthermore, the addition of FA did not have a significant effect on the levels of Cu, Fe, Mn, Se, and Zn in egg yolk. The dietary FA did not have a significant effect on the total saturated fatty acids (SFA) and polyunsaturated fatty acid (PUFA) content in egg yolk. However, the total monounsaturated fatty acid (MUFA) content in the 5 and 10 mg/kg groups significantly (p < 0.05) increased. These changes in nutritional content might be attributed to the increased very low-density lipoprotein (VLDL) protein content. The significant decrease in solute carrier family 1 Member 1 (SLC1A1), solute carrier family 1 Member 2 (SLC1A2), and solute carrier family 1 Member 3 (SLC1A3) gene expression compared to the control group appeared to be the reason for the decrease in amino acid content in egg yolk within the dietary FA group.

CONCLUSION

The findings suggest that the appropriate addition of FA can enhance the levels of MUFA and vitamin D in egg yolks, thereby improving their nutritional value. Excessive intake of FA can decrease the effectiveness of enriching FA in egg yolk and impact the enrichment of certain amino acids. The yolk of eggs produced by adding 10 mg/kg of FA to the feed contains the optimal amount of nutrients. This study informs consumers purchasing FA-fortified eggs.

An EAV-HP insertion in the promoter region of SLCO1B3 has pleiotropic effects on chicken liver metabolism based on the transcriptome and proteome analysis

Solute carrier organic anion transporter 1B3 (SLCO1B3) is an important liver primarily highly expressed gene, its encoded protein (OATP1B3) involved in the transport of multi-specific endogenous and exogenous substances. We previously reported that an EAV-HP inserted mutation (IM+) in the 5' flanking region of SLCO1B3 was the causative mutation of chicken blue eggs, and a further research showed that IM+ significantly reduced the expression of SLCO1B3 in liver. Herein, we confirmed a cholate response element (IR-1) played an important role in activating SLCO1B3 and in vitro experiments showed that the activation of IR-1 can be significantly reduced by the EAV-HP IM+ . We performed transcriptome and proteomic analysis using the same set of IM+ and IM- liver tissues from Yimeng hens (a Chinese indigenous breed) to study the effect of SLCO1B3 and OATP1B3 expression reduction on chicken liver function. The results showed that common differential expression pathways were screened out from both transcriptome and proteome, in which fatty acid metabolism and drug metabolism-cytochrome P450 were significantly enriched in the KEGG analysis. The lipid-related metabolism was weakened in IM+ group, which was validated by serum biochemical assay. We unexpectedly found that EAV-HP fragment was highly expressed in the liver of the IM+ chickens. We cloned the EAV-HP full-length transcript and obtained the complete open reading frame. It is worth noting that there was some immune related differential expressed genes, such as NFKBIZ, NFKBIA, and IL1RL1, which were higher expressed in the IM+ group, which may due to the high expression of EAV-HP. Our study showed that EAV-HP IM+ reduced the expression of SLCO1B3 in liver, resulting in the decrease of fatty metabolism and exogenous substance transport capacity. The mutation itself also expressed in the liver and may be involved in the immune process. The mechanism needs further study.

Serum and acute phase protein changes in laying hens, infested with poultry red mite

The poultry red mite (PRM) is one of the most economically important ectoparasites of laying hens globally. This mite can have significant deleterious effects on its fowl host including distress, anemia, reduced egg production, and reduced egg quality. This study was conducted to evaluate the influence of PRM on the serum protein profile in laying hens and its effect on the acute phase proteins (APPs) to assess their potential as biomarkers for mite infestation. Three APPs: alpha-1 acid glycoprotein (AGP), serum amyloid-A (SAA), and ceruloplasmin (CP) were measured in serum samples collected from laying hens at 12 and 17 wk of age, and then for up to 4 mo after a challenge with PRM (starting at 18.5 wk of age). The serum protein profile (SDS-PAGE/nanoflow HPLC electrospray tandem mass spectrometry) and concentration of individual serum proteins (SDS-PAGE-band densitometry) were also compared. Post challenge there was a positive correlation (r = 0.489; P < 0.004) between the levels of SAA and the PRM numbers. The levels of SAA steadily increased after the PRM challenge and were significantly different than the pre-challenge levels at 28, 32, and 36 wk of age (P < 0.01). The PRM numbers also peaked around 31-33 wk of age. The results for AGP and CP in comparison were inconsistent. Proteomics revealed the presence of 2 high molecular weight proteins in the serum between 12 and 17 wk of age. These were identified as Apolipoprotein-B and Vitellogenin-2, and their increase was commensurate with the onset of lay. No other major differences were detected in the protein profiles of blood sera collected pre and post challenge. We conclude that SAA could be used as a useful biomarker to monitor PRM infestation in commercial poultry flocks and that PRM infestation does not disrupt the production of the major proteins in the serum that are associated with egg formation.

Staphylococcus aureus Utilizes Host-Derived Lipoprotein Particles as Sources of Fatty Acids

Methicillin-resistant Staphylococcus aureus (MRSA) is a threat to global health. Consequently, much effort has focused on the development of new antimicrobials that target novel aspects of S. aureus physiology. Fatty acids are required to maintain cell viability, and bacteria synthesize fatty acids using the type II fatty acid synthesis (FASII) pathway. FASII is significantly different from human fatty acid synthesis, underscoring the therapeutic potential of inhibiting this pathway. However, many Gram-positive pathogens incorporate exogenous fatty acids, bypassing FASII inhibition and leaving the clinical potential of FASII inhibitors uncertain. Importantly, the source(s) of fatty acids available to pathogens within the host environment remains unclear. Fatty acids are transported throughout the body by lipoprotein particles in the form of triglycerides and esterified cholesterol. Thus, lipoproteins, such as low-density lipoprotein (LDL), represent a potentially rich source of exogenous fatty acids for S. aureus during infection. We sought to test the ability of LDLs to serve as a fatty acid source for S. aureus and show that cells cultured in the presence of human LDLs demonstrate increased tolerance to the FASII inhibitor triclosan. Using mass spectrometry, we observed that host-derived fatty acids present in the LDLs are incorporated into the staphylococcal membrane and that tolerance to triclosan is facilitated by the fatty acid kinase A, FakA, and Geh, a triacylglycerol lipase. Finally, we demonstrate that human LDLs support the growth of S. aureus fatty acid auxotrophs. Together, these results suggest that human lipoprotein particles are a viable source of exogenous fatty acids for S. aureus during infection.IMPORTANCE Inhibition of bacterial fatty acid synthesis is a promising approach to combating infections caused by S. aureus and other human pathogens. However, S. aureus incorporates exogenous fatty acids into its phospholipid bilayer. Therefore, the clinical utility of targeting bacterial fatty acid synthesis is debated. Moreover, the fatty acid reservoir(s) exploited by S. aureus is not well understood. Human low-density lipoprotein particles represent a particularly abundant in vivo source of fatty acids and are present in tissues that S. aureus colonizes. Herein, we establish that S. aureus is capable of utilizing the fatty acids present in low-density lipoproteins to bypass both chemical and genetic inhibition of fatty acid synthesis. These findings imply that S. aureus targets LDLs as a source of fatty acids during pathogenesis.

Protein Profiles for Muscle Development and Intramuscular Fat Accumulation at Different Post-Hatching Ages in Chickens

Muscle development and growth influences the efficiency of poultry meat production, and is closely related to deposition of intramuscular fat (IMF), which is crucial in meat quality. To clarify the molecular mechanisms underlying muscle development and IMF deposition in chickens, protein expression profiles were examined in the breast muscle of Beijing-You chickens at ages 1, 56, 98 and 140 days, using isobaric tags for relative and absolute quantification (iTRAQ). Two hundred and four of 494 proteins were expressed differentially. The expression profile at day 1 differed greatly from those at day 56, 98 and 140. KEGG pathway analysis of differential protein expression from pair-wise comparisons (day 1 vs. 56; 56 vs. 98; 98 vs. 140), showed that the fatty acid degradation pathway was more active during the stage from day 1 to 56 than at other periods. This was consistent with the change in IMF content, which was highest at day 1 and declined dramatically thereafter. When muscle growth was most rapid (days 56-98), pathways involved in muscle development were dominant, including hypertrophic cardiomyopathy, dilated cardiomyopathy, cardiac muscle contraction, tight junctions and focal adhesion. In contrast with hatchlings, the fatty acid degradation pathway was downregulated from day 98 to 140, which was consistent with the period for IMF deposition following rapid muscle growth. Changes in some key specific proteins, including fast skeletal muscle troponin T isoform, aldehyde dehydrogenase 1A1 and apolipoprotein A1, were verified by Western blotting, and could be potential biomarkers for IMF deposition in chickens. Protein-protein interaction networks showed that ribosome-related functional modules were clustered in all three stages. However, the functional module involved in the metabolic pathway was only clustered in the first stage (day 1 vs. 56). This study improves our understanding of the molecular mechanisms underlying muscle development and IMF deposition in chickens.

Increasing Levels of Dietary Hempseed Products Leads to Differential Responses in the Fatty Acid Profiles of Egg Yolk, Liver and Plasma of Laying Hens

The limited efficiency with which dietary alpha-linolenic acid (ALA) is converted by hens into docosahexaenoic acid (DHA) for egg deposition is not clearly understood. In this study, dietary ALA levels were increased via the inclusion of hempseed (HS) and hempseed oil (HO) in hen diets, with the goal of assessing the effects on the fatty acid (FA) profiles of total lipids and lipid classes in yolk, liver and plasma. Forty-eight hens were individually caged and fed one of six diets containing either HS:10, 20 or 30, HO:4.5 or 9.0 (%, diet) or a control (containing corn oil), providing a range (0.1-1.28 %, diet) of ALA. Fatty acid methyl esters of total lipids and lipid classes, including phosphatidyl choline (PtdCho) and ethanolamine (PtdEtn) in yolk, plasma and liver were then determined. Levels of n-3 FAs in both total lipids and lipid classes increased in all tissues. ALA and eicosapentaenoic acid (EPA) increased linearly, while docosapentaenoic acid and DHA increased quadratically. The FA profiles of yolk closely reflected levels in both plasma and liver. While ALA was highly concentrated in the triacylglycerol, it was low but equally distributed between PtdCho and PtdEtn in all tissues; however, the net accumulation was lower (P < 0.0001) in liver compared to yolk and plasma. Levels of EPA and ALA in yolk-PtdEtn were linearly (P < 0.0001; R (2) = 0.93) associated, and reflected those in liver-PtdEtn (P < 0.0001; R (2) = 0.90). In the liver, a strong inverse correlation (P < 0.0001; r = -0.94) between PL-DHA and ALA-to-EPA ratio in PtdEtn supports theories of low substrate availability, possibly limiting the conversion of ALA into DHA for egg enrichment.

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.

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

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Apo-IV expression is suppressed by estrogen.

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

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

Sex differences in the accumulation of chlorinated dioxins in the cormorant (Phalacrocorax carbo): implication of hepatic sequestration in the maternal transfer

This study focuses on gender-specific accumulation features of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/DFs) and dioxin-like polychlorinated biphenyls (DL-PCBs) in common (great) cormorants (Phalacrocorax carbo) collected from Lake Biwa, Japan. Male cormorants showed a tendency to have higher levels of PCDD/DFs and DL-PCBs than did females. For PCDDs, the male-to-female concentration ratio (M/F ratio) for each congener significantly decreased with the liver-to-muscle concentration ratio (Li/Mu ratio). For PCDFs, there also was a weak negative relationship between Li/Mu ratio and M/F ratio. In contrast, for DL-PCBs, M/F ratios showed no correlation with Li/Mu ratios, but increased with an increase in lipophilicity. These results, together with observations from prior studies, suggest that congener-specific hepatic sequestration may be a factor limiting the maternal transfer of PCDDs and, to a lesser extent, PCDFs. Maternal transfer of DL-PCBs seems to favor more lipophilic congeners in the cormorants.

The developing chicken yolk sac acquires nutrient transport competence by an orchestrated differentiation process of its endodermal epithelial cells

During chicken yolk sac (YS) growth, mesodermal cells in the area vasculosa follow the migrating endodermal epithelial cell (EEC) layer in the area vitellina. Ultimately, these cells form the vascularized YS that functions in nutrient transfer to the embryo. How and when EECs, with their apical aspect directly contacting the oocytic yolk, acquire the ability to take up yolk macromolecules during the vitellina-to-vasculosa transition has not been investigated. In addressing these questions, we found that with progressive vascularization, the expression level in EECs of the nutrient receptor triad, LRP2-cubilin-amnionless, changes significantly. The receptor complex, competent for uptake of yolk proteins, is produced by EECs in the area vasculosa but not in the area vitellina. Yolk components endocytosed by LRP2-cubilin-amnionless, preformed and newly formed lipid droplets, and yolk-derived very low density lipoprotein, shown to be efficiently endocytosed and lysosomally processed by EECs, probably provide substrates for resynthesis and secretion of nutrients, such as lipoproteins. In fact, as directly demonstrated by pulse-chase experiments, EECs in the vascularized, but not in the avascular, region efficiently produce and secrete lipoproteins containing apolipoprotein A-I (apoA-I), apoB, and/or apoA-V. In contrast, perilipin 2, a lipid droplet-stabilizing protein, is produced exclusively by the EECs of the area vitellina. These data suggest a differentiation process that orchestrates the vascularization of the developing YS with the induction of yolk uptake and lipoprotein secretion by EECs to ensure embryo nutrition.

High-density lipoprotein metabolism and the human embryo

BACKGROUND

High-density lipoprotein (HDL) appears to be the dominant lipoprotein particle in human follicular fluid (FF). The reported anti-atherogenic properties of HDL have been attributed in part to reverse cholesterol transport. The discoveries of the scavenger receptor class B type I (SR-BI) and the ATP-binding cassette A1 lipid (ABCA1) transporter have generated studies aimed at unraveling the pathways of HDL biogenesis, remodeling and catabolism. The production of SR-BI and ABCA1 knockout mice as well as other lipoprotein metabolism-associated mutants has resulted in reduced or absent fertility, leading us to postulate the existence of a human hepatic-ovarian HDL-associated axis of fertility. Here, we review an evolving literature on the role of HDL metabolism on mammalian fertility and oocyte development.

METHODS

An extensive online search was conducted of published articles relevant to the section topics discussed. All relevant English language articles contained in Pubmed/Medline, with no specific time frame for publication, were considered for this narrative review. Cardiovascular literature was highly cited due to the wealth of relevant knowledge on HDL metabolism, and the dearth thereof in the reproductive field.

RESULTS

Various vertebrate models demonstrate a role for HDL in embryo development and fertility. In our clinical studies, FF levels of HDL cholesterol and apolipoprotein AI levels were negatively associated with embryo fragmentation, but not with embryo cell cleavage rate. However, the HDL component, paraoxonase 1 arylesterase activity, was positively associated with embryo cell cleavage rate.

CONCLUSIONS

HDL contributes to intra-follicular cholesterol homeostasis which appears to be important for successful oocyte and embryo development.

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.

Association of serum protein levels with egg productivity in Taiwan red-feathered country chickens

This study investigated the relationship between serum protein levels and egg productivity in Taiwan red-feathered country chickens (TRFCC). The total egg numbers of TRFCC (n=157) were recorded from 25 to 48 weeks of age. Serum samples were collected at 24 and 35 weeks of age, then classified by total egg number into four groups according to mean+/-1S.D. Serum X protein levels were evaluated by protein chip technology and with an insulin-like growth factor-I (IGF-I) immunoassay. Other serum proteins (apolipoprotein A-I, ovotransferrin and vitellogenin) were found at different levels between the most productive and the least productive groups of TRFCC, were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and were quantified by age. The results showed that levels of vitellogenin were positively correlated with total egg number at 24 and at 35 weeks old (p<0.01). Levels of apolipoprotein A-I and an IGF-1-related marker (termed X protein) in serum at 35 weeks old were correlated with total egg number (p<0.01). Serum ovotransferrin levels remained constant regardless of total egg number. To investigate the concentration differences among the four groups of TRFCC, serum protein levels of each group were analyzed by one-way ANOVA. The results showed that those protein levels, except for ovotransferrin, in the least productive group significantly differed from the other three groups (p<0.05). Although the correlation between those protein levels and the total egg number was not significant at the time of initial egg production, the selection strategy of those protein levels at later stages of egg production should provide a screening model to improve selection.

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.

Regulation by estrogen of synthesis and secretion of apolipoprotein A-I in the chicken hepatoma cell line, LMH-2A

The synthesis and secretion of apolipoprotein A-I (apoA-I) in response to the treatment with estrogen were investigated in the chicken hepatoma cell line, LMH-2A. Exposure of these cells to exogenous estrogen for up to 48 h results in a decrease of apoA-I production, as evident from Western blotting, immunoprecipitation, and immunofluorescence experiments. Likewise, the secretion of apoA-I is also decreased in estrogen-treated cells when compared to controls. However, under both conditions, the disappearance of the apoprotein from the cells occurs very rapidly and with similar kinetics. The bulk of apoA-I secreted from LMH-2A cells is recovered on lipoprotein particles with a buoyant density of > or =1.10 g/ml, corresponding to HDL and heavy LDL. Interestingly, apoA-I is detectable on apoB-containing lipoproteins by sequential immunoprecipitation, suggesting that the two apoproteins co-reside at least on a subfraction of the secreted particles, or that apoB- and apoA-I-containing particles interact. These interactions are more pronounced in estrogen-treated cells, most likely due to the dramatic estrogen-mediated induction of apoB synthesis and secretion.

Comparative 17beta-estradiol response and lipoprotein interactions of an avian apolipoprotein

Apolipoprotein D (apo D), a member of the lipocalin protein family, has been identified and cloned in several mammalian species; its physiological functions, however, remain poorly understood. As with other lipocalins, apo D can bind small hydrophobic ligands. Lipids and hormones, such as cholesterol, arachidonic acid, and progesterone can bind to apo D; but the physiological significance of these interactions is not clear. We previously reported the existence of an avian (Gallus domesticus) apo D-like protein, and indicated a possible role for it in reproduction. This report provides a further comparative characterization of this avian protein. Evidence is presented that the putative avian apo D, like some (e.g., human) but not other (e.g., rat) mammalian apo Ds, preferentially associates with high density lipoproteins (HDL) in the circulation. These results confirm the apolipoprotein nature of the avian apo D-like protein, and indicate that it has conserved the HDL-interaction property of some mammalian apo Ds. The response of circulatory levels of the avian protein to 17beta-estradiol treatment is also examined. Large estrogen-dependent increases are known to occur in the circulatory levels of some avian apolipoproteins, such as apo B and vitellogenins, that represent major yolk precursors and nutrient sources for the embryo. Although the avian apo D-like protein is also a known yolk precursor, the minor estrogen-dependent increase observed for this apolipoprotein (less than 7% that of apo B) distinguishes it from the major yolk-precursor apolipoproteins. The response of the avian apo D-like protein to 17beta-estradiol is more like that of other yolk precursor proteins that transport regulatory molecules such as vitamin A and thyroid hormones.

Chicken coagulation factor XIIIA is produced by the theca externa and stabilizes the ovarian follicular wall

Development of the follicle in egg-laying species such as the chicken is regulated by systemic factors as well as by the highly orchestrated interplay of differentially expressed genes within this organ. Differential mRNA display analysis of defined phases of follicle development resulted in the characterization of coagulation factor XIIIA. It is expressed and produced by cells of the theca externa in a highly regulated manner during distinct growth phases of the follicle. Transcripts for factor XIIIA are already detectable at the beginning of follicle development and peak at the end of phase 2. Protein levels, however, still increase during phase 3, peak shortly after ovulation, and persist until the postovulatory tissue is completely resorbed. Factor XIIIA is secreted as a monomer into the extracellular matrix of the theca externa and is not associated with factor XIIIB as is the case in plasma. Our data suggest that, due to its transglutaminase activity, factor XIIIA stabilizes the follicular wall by cross-linking matrix components. Thus, coagulation factor XIIIA might play a key role in coping with the massive mechanical stress exerted by the large amount of yolk accumulating during the rapid growth phase of the oocyte.

The VLDL receptor: an LDL receptor relative with eight ligand binding repeats, LR8

The discovery in 1992 of a member of the low density lipoprotein receptor (LDLR) family with eight ligand binding repeats (LR8) has raised more questions than have been answered to date. Here, we summarize the current status of knowledge about this intriguing molecule, generally termed VLDL receptor, at the molecular biological, cell biological, and physiological levels. On one hand, the wealth of reports concerning the role(s) of this receptor in lipoprotein metabolism in mammalian systems has revealed partially conflicting details, particularly in regards to its natural ligand(s) and site of action. On the other hand, molecular genetic and biochemical studies in the chicken have clearly demonstrated the multiple roles of LR8 in the physiology and reproduction of egg-laying species, and have generated insights into the evolutionary aspects of the LDLR gene family.

Apolipoprotein A-I production by chicken granulosa cells

In avian species such as the chicken, development of the oocyte is associated with massive deposition of yolk in this cell. Oocytes grow within the follicle, a compartment consisting of a very specialized set of cells and acellular structures. The oocyte is surrounded by the perivitelline layer and granulosa cells, which are separated from the thecae by a pronounced basement membrane. In addition to the production of yolk precursors in the liver, we have long implied that cells within the follicle make a direct contribution to the growth of the oocyte. Here we show that chicken granulosa cells express and actively secrete apolipoprotein A-I (apoA-I) as a part of particles with very high density. The granulosa cell-derived, apoA-I-containing material is different from the small portion of yolk high density lipoprotein that arises via transfer from the peripheral circulation. We propose that the ApoA-I-containing particles secreted by granulosa cells 1) support the growth of the rapidly growing germ cell, possibly by direct lipid transfer to the plasma membrane of the oocyte, and/or 2) deliver cholesteryl esters to the steroid-producing cells of the theca layer. These findings are discussed with respect to the proposed functions of apoE (an apolipoprotein not found in chicken) within the mammalian follicle.

Low density lipoprotein receptor gene family members mediate yolk deposition

Yolk represents the last growth stage of a single cell, the oocyte, which contains, besides bona fide cytoplasm, endocytosed serum-derived lipoproteins and minor components essential for normal embryo development. Transport of bulk lipoproteins, micronutrients, and morphogens to oocytes in parallel with maintenance of somatic homeostasis is achieved by ligand targeting via cell-specific expression of receptors and subtle differences in ligand structure. Lipoprotein metabolism is the prime example of these regulatory principles, in which receptors belonging to the low density lipoprotein receptor gene family play key roles. Here, we present the laying hen's features that make it an attractive model system to dissect macromolecular transport processes at the molecular level. In addition to the characterization of a family of yolk precursor receptors, studies on systemic vs. oocyte-directed transport have uncovered new aspects of the biological rationale for simultaneous expression of closely related genes in a single organism.

Chicken lecithin-cholesterol acyltransferase. Molecular characterization reveals unusual structure and expression pattern

Rapidly growing oocytes in the laying hen are, in addition to the liver, targets of the so-called "reverse cholesterol transport" (RCT) (Vieira, P.M., Vieira, A.V., Sanders, E.J., Steyrer, E., Nimpf, J., and Schneider, W.J. (1995) J. Lipid Res. 36, 601-610), pointing to the importance of this process in nonplacental reproduction. We have begun to delineate the details of this unique transport pathway branch by molecular characterization of the first nonmammalian lecithin-cholesterol acyltransferase (LCAT), the enzyme that catalyzes an early step in RCT. The biological significance of the enzyme is underscored by the high degree of protein sequence identity (73%) maintained from chicken to man. Interestingly, the conservation extends much less to the cysteine residues; in fact, two of the cysteines thought to be important in mammalian enzymes (residues 31 and 184 in man) are absent from the chicken enzyme, providing proof of their dispensability for enzymatic activity. Antibodies prepared against a chicken LCAT fusion protein cross-react with human LCAT and identify a 64-kDa protein present in enzymatically active fractions obtained by hydrophobic chromatography of chicken serum. The developmental and tissue distribution pattern of LCAT in females is striking; during embryogenesis and adolescence, LCAT expression is extremely high in liver but undetectable in brain. Upon onset of laying, however, brain LCAT mRNA increases suddenly and is maintained at levels 5 times higher than in liver, in stark contrast to most mammals. In adult roosters, the levels of LCAT transcripts in brain are lower than in liver. Together with the molecular characterization of chicken LCAT, these newly discovered developmental changes and gender differences in its expression establish the avian oocyte/liver system as a powerful model to delineate in vivo regulatory elements of RCT.