Hepatosteatosis and estrogen increase apolipoprotein O production in the chicken

Apolipoprotein O modulates cholesterol metabolism via NRF2/CYB5R3 independent of LDL receptor

Apolipoprotein O (APOO) plays a critical intracellular role in regulating lipid metabolism. Here, we investigated the roles of APOO in metabolism and atherogenesis in mice. Hepatic APOO expression was increased in response to hyperlipidemia but was inhibited after simvastatin treatment. Using a novel APOO global knockout (Apoo-/-) model, it was found that APOO depletion aggravated diet-induced obesity and elevated plasma cholesterol levels. Upon crossing with low-density lipoprotein receptor (LDLR) and apolipoprotein E (APOE) knockout hyperlipidemic mouse models, Apoo-/- Apoe-/- and Apoo-/- Ldlr-/- mice exhibited elevated plasma cholesterol levels, with more severe atherosclerotic lesions than littermate controls. This indicated the effects of APOO on cholesterol metabolism independent of LDLR and APOE. Moreover, APOO deficiency reduced cholesterol excretion through bile and feces while decreasing phospholipid unsaturation by inhibiting NRF2 and CYB5R3. Restoration of CYB5R3 expression in vivo by adeno-associated virus (AAV) injection reversed the reduced degree of phospholipid unsaturation while decreasing blood cholesterol levels. This represents the first in vivo experimental validation of the role of APOO in plasma cholesterol metabolism independent of LDLR and elucidates a previously unrecognized cholesterol metabolism pathway involving NRF2/CYB5R3. APOO may be a metabolic regulator of total-body cholesterol homeostasis and a target for atherosclerosis management. Apolipoprotein O (APOO) regulates plasma cholesterol levels and atherosclerosis through a pathway involving CYB5R3 that regulates biliary and fecal cholesterol excretion, independently of the LDL receptor. In addition, down-regulation of APOO may lead to impaired mitochondrial function, which in turn aggravates diet-induced obesity and fat accumulation.

MIC26 and MIC27 are bona fide subunits of the MICOS complex in mitochondria and do not exist as glycosylated apolipoproteins

Impairments of mitochondrial functions are linked to human ageing and pathologies such as cancer, cardiomyopathy, neurodegeneration and diabetes. Specifically, aberrations in ultrastructure of mitochondrial inner membrane (IM) and factors regulating them are linked to diabetes. The development of diabetes is connected to the 'Mitochondrial Contact Site and Cristae Organising System' (MICOS) complex which is a large membrane protein complex defining the IM architecture. MIC26 and MIC27 are homologous apolipoproteins of the MICOS complex. MIC26 has been reported as a 22 kDa mitochondrial and a 55 kDa glycosylated and secreted protein. The molecular and functional relationship between these MIC26 isoforms has not been investigated. In order to understand their molecular roles, we depleted MIC26 using siRNA and further generated MIC26 and MIC27 knockouts (KOs) in four different human cell lines. In these KOs, we used four anti-MIC26 antibodies and consistently detected the loss of mitochondrial MIC26 (22 kDa) and MIC27 (30 kDa) but not the loss of intracellular or secreted 55 kDa protein. Thus, the protein assigned earlier as 55 kDa MIC26 is nonspecific. We further excluded the presence of a glycosylated, high-molecular weight MIC27 protein. Next, we probed GFP- and myc-tagged variants of MIC26 with antibodies against GFP and myc respectively. Again, only the mitochondrial versions of these tagged proteins were detected but not the corresponding high-molecular weight MIC26, suggesting that MIC26 is indeed not post-translationally modified. Mutagenesis of predicted glycosylation sites in MIC26 also did not affect the detection of the 55 kDa protein band. Mass spectrometry of a band excised from an SDS gel around 55 kDa could not confirm the presence of any peptides derived from MIC26. Taken together, we conclude that both MIC26 and MIC27 are exclusively localized in mitochondria and that the observed phenotypes reported previously are exclusively due to their mitochondrial function.

Selective footprints and genes relevant to cold adaptation and other phenotypic traits are unscrambled in the genomes of divergently selected chicken breeds

BACKGROUND

The genomes of worldwide poultry breeds divergently selected for performance and other phenotypic traits may also be affected by, and formed due to, past and current admixture events. Adaptation to diverse environments, including acclimation to harsh climatic conditions, has also left selection footprints in breed genomes.

RESULTS

Using the Chicken 50K_CobbCons SNP chip, we genotyped four divergently selected breeds: two aboriginal, cold tolerant Ushanka and Orloff Mille Fleur, one egg-type Russian White subjected to artificial selection for cold tolerance, and one meat-type White Cornish. Signals of selective sweeps were determined in the studied breeds using three methods: (1) assessment of runs of homozygosity islands, (2) F_ST based population differential analysis, and (3) haplotype differentiation analysis. Genomic regions of true selection signatures were identified by two or more methods or in two or more breeds. In these regions, we detected 540 prioritized candidate genes supplemented them with those that occurred in one breed using one statistic and were suggested in other studies. Amongst them, SOX5, ME3, ZNF536, WWP1, RIPK2, OSGIN2, DECR1, TPO, PPARGC1A, BDNF, MSTN, and beta-keratin genes can be especially mentioned as candidates for cold adaptation. Epigenetic factors may be involved in regulating some of these important genes (e.g., TPO and BDNF).

CONCLUSION

Based on a genome-wide scan, our findings can help dissect the genetic architecture underlying various phenotypic traits in chicken breeds. These include genes representing the sine qua non for adaptation to harsh environments. Cold tolerance in acclimated chicken breeds may be developed following one of few specific gene expression mechanisms or more than one overlapping response known in cold-exposed individuals, and this warrants further investigation.

Comparison of Selection Signatures between Korean Native and Commercial Chickens Using 600K SNP Array Data

Korean native chickens (KNCs) comprise an indigenous chicken breed of South Korea that was restored through a government project in the 1990s. The KNC population has not been developed well and has mostly been used to maintain purebred populations in the government research institution. We investigated the genetic features of the KNC population in a selection signal study for the efficient improvement of this breed. We used 600K single nucleotide polymorphism data sampled from 191 KNCs (NG, 38; NL, 29; NR, 52; NW, 39; and NY, 33) and 54 commercial chickens (Hy-line Brown, 10; Lohmann Brown, 10; Arbor Acres, 10; Cobb, 12; and Ross, 12). Haplotype phasing was performed using EAGLE software as the initial step for the primary data analysis. Pre-processed data were analyzed to detect selection signals using the ‘rehh’ package in R software. A few common signatures of selection were identified in KNCs. Most quantitative trait locus regions identified as candidate regions were associated with traits related to reproductive organs, eggshell characteristics, immunity, and organ development. Block patterns with high linkage disequilibrium values were observed for LPP, IGF11, LMNB2, ERBB4, GABRB2, NTM, APOO, PLOA1, CNTN1, NTSR1, DEF3, CELF1, and MEF2D genes, among regions with confirmed selection signals. NL and NW lines contained a considerable number of selective sweep regions related to broilers and layers, respectively. We recommend focusing on improving the egg and meat traits of KNC NL and NW lines, respectively, while improving multiple traits for the other lines.

The exchangeable apolipoproteins in lipid metabolism and obesity

Dyslipidemia, characterized by increased plasma levels of low-density lipoprotein cholesterol (LDL-C), very low-density lipoprotein cholesterol (VLDL-C), triglyceride (TG), and reduced plasma levels of high-density lipoprotein cholesterol (HDL-C), is confirmed as a hallmark of obesity and cardiovascular diseases (CVD), posing serious risks to the future health of humans. Thus, it is important to understand the molecular metabolism of dyslipidemia, which could help reduce the morbidity and mortality of obesity and CVD. Currently, several exchangeable apolipoproteins, such as apolipoprotein A1 (ApoA1), apolipoprotein A5 (ApoA5), apolipoprotein E (ApoE), and apolipoprotein C3 (ApoC3), have been verified to exert vital effects on modulating lipid metabolism and homeostasis both in plasma and in cells, which consequently affect dyslipidemia. In the present review, we summarize the findings of the effect of exchangeable apolipoproteins on affecting lipid metabolism in adipocytes and hepatocytes. Furthermore, we also provide new insights into the mechanisms by which the exchangeable apolipoproteins influence the pathogenesis of dyslipidemia and its related cardio-metabolic disorders.

Genome-wide association analysis of egg production performance in chickens across the whole laying period

BACKGROUND

Egg production is the most economically-important trait in layers as it directly influences benefits of the poultry industry. To better understand the genetic architecture of egg production, we measured traits including age at first egg (AFE), weekly egg number (EN) from onset of laying eggs to 80 weeks which was divided into five stage (EN1: from onset of laying eggs to 23 weeks, EN2: from 23 to 37 weeks, EN3: from 37 to 50 weeks, EN4: from 50 to 61 weeks, EN5: from 61 to 80 weeks) based on egg production curve and total egg number across the whole laying period (Total-EN). Then we performed genome-wide association studies (GWAS) in 1078 Rhode Island Red hens using a linear mixed model.

RESULTS

Estimates of pedigree and SNP-based genetic parameter showed that AFE and EN1 exhibited high heritability (0.51 ± 0.09, 0.53 ± 0.08), while the h2 for EN in other stages varied from low (0.07 ± 0.04) to moderate (0.24 ± 0.07) magnitude. Subsequently, seven univariate GWAS for AFE and ENs were carried out independently, from which a total of 161 candidate SNPs located on GGA1, GGA2, GGA5, GGA6, GGA9 and GGA24 were identified. Thirteen SNP located on GGA6 were associated with AFE and an interesting gene PRLHR that may affect AFE through regulating oxytocin secretion in chickens. Sixteen genome-wide significant SNPs associated with EN3 were in a strong linkage disequilibrium (LD) region spanning from 117.87 Mb to 118.36 Mb on GGA1 and the most significant SNP (rs315777735) accounted for 3.57% of phenotypic variance. Genes POLA1, PDK3, PRDX4 and APOO identified by annotating sixteen genome-wide significant SNPs can be considered as candidates associated with EN3. Unfortunately, our study did not find any candidate gene for the total egg number.

CONCLUSIONS

Findings in our study could provide promising genes and SNP markers to improve egg production performance based on marker-assisted breeding selection, while further functional validation is still needed in other populations.

Oleic acid induces the novel apolipoprotein O and reduces mitochondrial membrane potential in chicken and human hepatoma cells

Non-alcoholic fatty liver disease (NAFLD) is marked by hepatic fat accumulation and reflects a spectrum of chronic liver diseases associated with obesity, impaired insulin sensitivity and dyslipidemia. Apolipoprotein O (ApoO) is a new member of the plasma apolipoprotein family that may play a role in lipid metabolism and electron transport activity of the mitochondrium. However, its physiological functions have not been elucidated yet. Based on our previous data in a non-mammalian experimental system [1], we hypothesized that hepatic expression of ApoO is tightly linked not only to diet-induced hepatosteatosis, but also to increased lipoprotein-production induced by, e.g., hormones and oxidative stress. To gain insight into a mammalian experimental system, we compared the effects of lipid loading on ApoO regulation in chicken hepatoma LMH cells with those in the human hepatoma cell line HepG2. Incubation of the cells with BSA-complexed oleic acid (OA-Alb) induced triglyceride accumulation, but did not affect cell viability. qPCR using specific primer pairs and Western blot analysis with in-house produced rabbit anti-ApoO antisera demonstrated significant increase in ApoO transcript and protein levels in both cell lines. ROS formation due to OA-Alb treatment was only slightly altered in LMH cells, indicating an intact antioxidant defense system of the cells. Oxidative stress applied by addition of H2O2 revealed induction of ApoO transcript and protein level in the same or even higher extent as monitored in the presence of OA-Alb. Upon treatment with estrogen for 24 h quantitative analysis of ApoO transcript and protein revealed increases of ApoO expression supporting the assumption that estrogen affects lipoprotein metabolism at various points. Furthermore, both cell lines showed a significant decrease of the mitochondrial membrane potential upon incubation with OA-Alb. Therefore, we assume that our findings support a role of ApoO as an effector of compromised mitochondrial function that likely accompanies the onset of non-alcoholic fatty liver disease.

Estrogen-enhanced apical and basolateral secretion of apolipoprotein B-100 by polarized trophoblast-derived BeWo cells

Cholesterol is an important nutrient for fetal development and transplacental transport occurs at all stages of human pregnancy. Furthermore, cholesterol is required for membrane building as well as steroid hormone synthesis. Therefore, all placental cell types require cholesterol for proper function. In human term placenta, the syncytiotrophoblast (STB) faces the maternal circulation. Uptake of maternal-derived cholesterol at the apical membrane of the STB is well understood, but the route by which cholesterol exits at the basal side for subsequent transfer across the fetal endothelial cells (FEC) or to other placental cell types remains not well characterized. Our aim was to provide evidence for basal secretion of apolipoprotein B-100 (apoB) containing lipoproteins. Furthermore, we investigated the placental localization of apolipoprotein receptors (LRP2, LDLR and LRP1) to identify cell targets of lipoprotein particles secreted in a polarized fashion by the STB. In trophoblast-derived BeWo cells grown on permeable filter supports, we demonstrate by immunoprecipitation apical as well as basolateral apoB secretion, which was significantly upregulated by estrogen-treatment for 24 or 48 h. Furthermore, we showed by immunofluorescence microscopy apoB and microsomal triglyceride transfer protein subunits localization in the STB and placental stromal cells in situ. All investigated receptors were detected by RT-qPCR and western blot in BeWo cells, but only expression of LRP2 was estrogen-inducible. In situ, the multi-ligand receptor LRP2 was expressed exclusively in the cytotrophoblast (CTB), the STB precursor cell type. LDLR and LRP1 localized to trophoblasts as well as stromal cells in situ. In summary, basal apoB secretion by BeWo cells supports the concept of basal lipoprotein particle secretion by placental STB. These lipoprotein particles may serve as cholesterol source for STB precursor cells, the CTBs, as well as all stromal cells of the chorionic villi including FECs, which were herein demonstrated to express apoB receptors, LRP2 and LDLR, respectively.