Cloning and sequence analysis of human butyrophilin reveals a potential receptor function

The extracellular region of bovine milk butyrophilin exhibits closer structural similarity to human myelin oligodendrocyte glycoprotein than to immunological BTN family receptors

Bovine butyrophilin (BTN1A1) is an abundant type I transmembrane glycoprotein exposed on the surface of milk fat globules. We have solved the crystal structure of its extracellular region via multiple wavelength anomalous dispersion after incorporation of selenomethionine into the bacterially produced protein. The butyrophilin ectodomain exhibits two subdomains with immunoglobulin fold, each comprising a β-sandwich with a central disulfide bridge as well as one N-linked glycosylation. The fifth Cys residue at position 193 is unpaired and prone to forming disulfide crosslinks. The apparent lack of a ligand-binding site or receptor activity suggests a function predominantly as hydrophilic coat protein to prevent coagulation of the milk fat droplets. While there is less structural resemblance to members of the human butyrophilin family such as BTN3A, which play a role as immune receptors, the N-terminal bovine butyrophilin subdomain shows surprising similarity to the human myelin oligodendrocyte glycoprotein, a protein exposed on the surface of myelin sheaths. Thus, our study lends structural support to earlier hypotheses of a correlation between the consumption of cow milk and prevalence of neurological autoimmune diseases and may offer guidance for the breeding of cattle strains that express modified butyrophilin showing less immunological cross-reactivity.

Butyrophilin Btn2a2 inhibits TCR activation and phosphatidylinositol 3-kinase/Akt pathway signaling and induces Foxp3 expression in T lymphocytes

The butyrophilin-related protein Btn2a2 was upregulated on murine APC including CD19(+) B cells, CD11b(+)F4/80(+) peritoneal macrophages, and CD11c(+) bone marrow-derived dendritic cells after activation with LPS or Pam3CysK4, suggesting a role in modulation of T lymphocytes. Consistent with this, binding of mouse Btn2a2-Fc to CD3(+) primary mouse T cells stimulated with anti-CD3 and anti-CD28 reduced the number of proliferating cells and entry of cells into the cell cycle. Binding of Btn2a2-Fc to anti-CD3-stimulated T cells inhibited CD3ε, Zap70, and subsequent Erk1/2 activation. It also interfered with activation of the regulatory subunit of PI3K, p85, and activation of Akt in T cells stimulated with both anti-CD3 and anti-CD28. Inhibition of Akt activation by Btn2a2-Fc was, in contrast to inhibition by programmed death ligand-1-Fc, not overcome by anti-CD28 costimulation. Using Foxp3-GFP-transgenic, naive T cells, Btn2a2-Fc induced de novo expression of Foxp3 in a dose-dependent manner, and Btn2a2-Fc-induced CD4(+)CD25(+)Foxp3(+) T cells had inhibitory properties. The data indicate an important physiological role for Btn2a2 in inhibiting T cell activation and inducing Foxp3(+) regulatory T cells.

Novel SNPs of butyrophilin (BTN1A1) and milk fat globule epidermal growth factor (EGF) 8 (MFG-E8) are associated with milk traits in dairy goat

Butyrophilin (BTN1A1) and milk fat globule epidermal growth factor (EGF) 8 (MFG-E8) genes are both milk fat globule membrane proteins. BTN1A1 plays a key role in the secretion of milk lipid and production which has effects on performance traits, while the MFG-E8 is vital for the development of the mammary gland and phagocytic clearance of apoptotic cells. Therefore, BTN1A1 and MFG-E8 gene are candidate genes for quantitative traits in mammalian animals with respect to milk performance traits. The objective of this study is to investigate variations in goat BTN1A1 and MFG-E8 gene and analyze their associations with growth trait and milk performance. In this study, the goat BTN1A1 gene showed a novel single-nucleotide polymorphism (SNP): XM_001494179:g.8659C>T, resulting in a missense mutation: CTT (Leu)>TTT (Phe) at position 377 aa of the BTN1A1 (526 aa); the goat MFG-E8 gene showed four novel SNPs: NC_007319: g.843delA, 6417delC, 14892T>C and 14996A>C, only the 14892T>C result in a synonymous mutation. The associations between genotypes and production traits were analyzed. Significant statistical results implied that HinfI locus of BTN1A1 gene is associated with milk fat yield (P=0.004), total solid (P=0.002), solid-non fat (P=0.018) and first milk yield (P=0.030). The DA and EcoRV loci of MFG-E8 gene are associated with milk fat yield (DA locus: P=0.000; EcoRV locus: P=0.033) and total solid (DA locus: P=0.002; EcoRV locus: P=0.015) in the Xinong Saanen dairy goat.

A pilot study of proteomic profiles of human hepatocellular carcinoma in the United States

Human hepatocellular carcinoma (HCC) is one of the commonest causes of mortality among solid organ malignancies. The incidence of HCC in the United States is rising. Few proteomic biomarker studies have been done in U.S. populations. Tumor and nonmalignant tissue from three American patients with hepatitis and non-hepatitis-associated HCC were analyzed to find common differences in protein expression. Proteins were separated by 2D electrophoresis (isoelectric focusing followed by 10% SDS-PAGE). Gels were fixed and then stained with Coomassie brilliant blue. Digitization and processing were performed using the PDQuest software. The Student's t-test was used to detect quantitative protein changes between tumor and nonmalignant liver consistent in all sample pairs with a cutoff made at P < 0.01. This yielded a total of 20 spots with significant (>2 fold) abundance changes. Matrix-assisted laser desorption ionization mass spectrometry analysis was performed using Waters Micomass M@LDI SYSTEM. The proteins were then identified using manual ProFound. Among the 20 spots, 10 showed overexpression and 10 showed underexpression in tumor. Overexpressed proteins included beta-5-tubulin, beta-actin, vimentin, hypermethylated in cancer 2 protein, heat-shock 70-kDa protein 9B, serum albumin, 39S ribosomal protein L45, butyrophilin, autoimmune regulator, and transcription factor ETV7. Underexpressed proteins included BiP protein, superoxide dismutase, peroxiredoxin 2, inoraganic pyrophosphatase, keratin 8, carbonic anhydrase 1, repulsive guidance molecule, catalase, C-1-tetrahydrofolate synthase, and hemoglobin alpha-2. Of particular interest, the protein autoimmune regulator was expressed 14-fold higher in tumor tissue, suggesting it may have a role in HCC. Validation and further investigation of these protein changes may lead to the discovery of new molecular targets for therapy, biomarkers for early detection, and new endpoints for therapeutic efficacy and toxicity.

Structural and functional aspects of three major glycoproteins of the human milk fat globule membrane

The MUC1 mucin, lactadherin, and butyrophilin are 3 major components of the human milk fat globule membrane. The mucin inhibits binding of S-fimbriated Escherichia coli to buccal epithelial cells, and lactadherin prevents symptomatic rotavirus infection in breast-fed infants. Butyrophilin has been suggested to be a structural component of the human milk fat globule (HMFG) membrane and to have receptor functions, but has no known anti-infective activity. These HMFG glycoproteins also are present in skimmed milk, possibly associated with phospholipid micelles, while mucin is also in a soluble form. Mucin and lactadherin resist digestion in the stomach of milk-fed infants, while butyrophilin is rapidly degraded. The MUC1 mucin is an extended rod-like structure forming part of the glycocalyx on the surface of many epithelial cells and membranes of milk, and may act as a decoy for binding of infective agents. The extracellular segment of butyrophilin has homology to Ig superfamily receptors and an intracellular domain with homology to developmentally regulated proteins. Lactadherin is a laterally mobile cell adhesion molecule that interacts with integrins and has a novel means of membrane-association involving specific binding to phosphatidylserine. The structural and functional aspects of these glycoproteins are discussed with regard to their role in human milk for breast-fed infants.

The B7 family of ligands and its receptors: new pathways for costimulation and inhibition of immune responses

T cell activation is dependent upon signals delivered through the antigen-specific T cell receptor and accessory receptors on the T cell. A primary costimulatory signal is delivered through the CD28 receptor after engagement of its ligands, B7-1 (CD80) or B7-2 (CD86). Engagement of CTLA-4 (CD152) by the same B7-1 or B7-2 ligands results in attenuation of T cells responses. Recently, molecular homologs of CD28 and CTLA-4 receptors and their B7-like ligands have been identified. ICOS is a CD28-like costimulatory receptor with a unique B7-like ligand. PD-1 is an inhibitory receptor, with two B7-like ligands. Additional members of B7 and CD28 gene families have been proposed. Integration of signals through this family of costimulatory and inhibitory receptors and their ligands is critical for activation of immune responses and tolerance. Understanding these pathways will allow development of new strategies for therapeutic intervention in immune-mediated diseases.

Enterophilins, a new family of leucine zipper proteins bearing a b30.2 domain and associated with enterocyte differentiation

Enterocyte terminal differentiation occurs at the crypt-villus junction through the transcriptional activation of cell-specific genes, many of which code for proteins of the brush border membrane such as intestinal alkaline phosphatase, sucrase-isomaltase, or the microvillar structural protein villin. Several studies have shown that this sharp increase in specific mRNA levels is intimately associated with arrest of cell proliferation. We isolated several clones from a guinea pig intestine cDNA library. They encode new proteins characterized by an original structure associating a carboxyl-terminal B30.2/RFP-like domain and a long leucine zipper at the amino terminus. The first member of this novel gene family codes for a 65-kDa protein termed enterophilin-1, which is specifically expressed in enterocytes before their final differentiation. Enterophilin-1 is the most abundant in the small intestine but is still present in significant amounts in colonic enterocytes. In Caco-2 cells, a similar 65-kDa protein was recognized by a specific anti-enterophilin-1 antibody, and its expression was positively correlated with cell differentiation status. In addition, transfection of HT-29 cells with enterophilin-1 full-length cDNA slightly inhibited cell growth and promoted an increase in alkaline phosphatase activity. Taken together, these data identify enterophilins as a new family of proteins associated with enterocyte differentiation.

The cluster of BTN genes in the extended major histocompatibility complex

We sequenced the 170-kb cluster of BTN genes in the extended major histocompatibility complex region, 4 Mb telomeric of human leukocyte antigen class I genes, at 6p22.1. The cluster consists of seven genes belonging to the expanding B7/butyrophilin-like group, a subset of the immunoglobulin gene superfamily. The main complex is composed of six genes, from two subfamilies, BTN2 and BTN3, arranged in pairs. This alternating pattern must have evolved by duplications of an original block of two genes, one from each subfamily. The sequences from the two subfamilies share approximately 50% amino acid identity. By analysis of repeat elements within each block, these duplications may be dated to approximately 100 million years ago, at about the time of the branching of the Rodentia and Primate lineages. The single BTN1A1 (butyrophilin) gene was positioned approximately 25 kb centromeric to the cluster. Each gene covers approximately 12 kb and consists of seven (BTN2 subfamily) or nine (BTN3 subfamily) coding exons. The predicted leader sequence, immunoglobulin-like IgV (variable)/IgC (constant) ectodomains, and the predicted transmembrane domain are encoded on separate exons and are separated from a B30.2 domain by a variable number of very short exons, 21 and 27 nucleotides in length. BTN transcripts were detected in all tissues examined. Alternative splicing, involving particularly the carboxyl-terminal B30.2 domain, was a notable feature. Most transcripts of BTN2 subfamily genes contained this domain, whereas BTN3 genes did not. Using immunofluorescence, we showed surface expression of BTN-green fluorescent protein fusions in mammalian cell transfectants.

A review and proposed nomenclature for major proteins of the milk-fat globule membrane

The characteristics and possible functions of the most abundant proteins associated with the bovine milk-fat globule membrane are reviewed. Under the auspices of the Milk Protein Nomenclature Committee of the ADSA, a revised nomenclature for the major membrane proteins is proposed and discussed in relation to earlier schemes. We recommend that proteins be assigned specific names as they are identified by molecular cloning and sequencing techniques. The practice of identifying proteins according to their Mr, electrophoretic mobility, or staining characteristics should be discontinued, except for uncharacterized proteins. The properties and amino acid sequences of the following proteins are discussed in detail: MUC1, xanthine dehydrogenase/oxidase, CD36, butyrophilin, adipophilin, periodic acid Schiff 6/7 (PAS 6/7), and fatty acid binding protein. In addition, a compilation of less abundant proteins associated with the bovine milk-fat globule membrane is presented.

Duplication and MHC linkage of the CTX family of genes in Xenopus and in mammals

The effects of whole genome duplications that characterize the evolution of vertebrates have been studied on the gene of the Xenopus thymocyte molecule CTX and its mammalian relatives. CTX, with an extracellular part consisting of one V and one C2 external domain, defines a new subset of the immunoglobulin superfamily and is conserved from amphibians to mammals. The number of CTX loci, their polymorphism, and their genetic linkages have been studied in several Xenopus species and in humans. In the genetically simplest species, X. tropicalis (2n = 20), the unique CTX locus is linked to the MHC. In the polyploid species, all CTX genes, unlike many other immune system genes, have remained in the genome; i.e. there are two CTX loci in the tetraploid species X. laevis (2n = 6) and six CTX loci in the dodecaploid species X. ruwenzoriensis (2n = 108). In X. laevis, one CTX gene is linked to the MHC and the other not, presumably because one set of MHC class I and II has been deleted from the corresponding linkage group. The various mammalian homologues are less related to each other than are the Xenopus CTX genes among each other, and they do not cross-hybridize with each other because they stem from the ancient polyploidization. Some human CTX homologies are on chromosomes 11 and 21, but others are on chromosomes 1, 6 and 19, which contain MHC paralogous regions; this suggests that a very ancient linkage group has been preserved.

Genomics of the major histocompatibility complex: haplotypes, duplication, retroviruses and disease

The genomic region encompassing the Major Histocompatibility Complex (MHC) contains polymorphic frozen blocks which have developed by local imperfect sequential duplication associated with insertion and deletion (indels). In the alpha block surrounding HLA-A, there are ten duplication units or beads on the 62.1 ancestral haplotype. Each bead contains or contained sequences representing Class I, PERB11 (MHC Class I chain related (MIC) and human endogenous retrovirus (HERV) 16. Here we consider explanations for co-occurrence of genomic polymorphism, duplication and HERVs and we ask how these features encode susceptibility to numerous and very diverse diseases. Ancestral haplotypes differ in their copy number and indels in addition to their coding regions. Disease susceptibility could be a function of all of these differences. We propose a model of the evolution of the human MHC. Population-specific integration of retroviral sequences could explain rapid diversification through duplication and differential disease susceptibility. If HERV sequences can be protective, there are exciting prospects for manipulation. In the meanwhile, it will be necessary to understand the function of MHC genes such as PERB11 (MIC) and many others discovered by genomic sequencing.

CTX, a Xenopus thymocyte receptor, defines a molecular family conserved throughout vertebrates

CTX, a cortical thymocyte marker in Xenopus, is an immunoglobulin superfamily (Igsf) member comprising one variable and one constant C2-type Igsf domain, a transmembrane segment and a cytoplasmic tail. Although resembling that of the TCR and immunoglobulins, the variable domain is not encoded by somatic rearrangement of the gene but by splicing of two half-domain exons. The C2 domain, also encoded by two exons, has an extra pair of cysteines. The transmembrane segment is free of charged residues, and the cytoplasmic tail (70 amino acids) contains one tyrosine and many glutamic acid residues. ChT1, a chicken homologue of CTX, has the same structural and genetic features, and both molecules are expressed on the thymocyte surface. We cloned new mouse (CTM) and human (CTH) cDNA and genes which are highly homologous to CTX/ChT1 but not lymphocyte specific. Similarity with recently described human cell surface molecules, A33 antigen and CAR (coxsackie and adenovirus 5 receptor), and a number of expressed sequence tags leads us to propose that CTX defines a novel subset of the Igsf, conserved throughout vertebrates and extending beyond the immune system. Strong homologies within vertebrate sequences suggest that the V and C2 CTX domains are scions of a very ancient lineage.

Milk fat globule glycoproteins in human milk and in gastric aspirates of mother's milk-fed preterm infants

Human milk fat globule (HMFG) glycoproteins can prevent infections by microorganisms in breast-fed infants; the MUC-1 mucin inhibits binding of S-fimbriated Escherichia coli to buccal mucosa, and lactadherin may prevent symptomatic rotavirus infections. In this study, the survival of these HMFG glycoproteins in the stomach of human milk-fed preterm infants (gestational age = 27.5 +/- 0.4 wk) was assessed, and levels in their mothers' milk determined, using specific RIAs. Butyrophilin, a major component of HMFG membrane that has no demonstrated antimicrobial activity, was studied for comparison. The levels of mucin, lactadherin, and butyrophilin in 41 milk samples of 20 mothers were 729 +/- 75, 93 +/- 10, and 41 +/- 3 microg/mL, respectively. Mucin and lactadherin were significantly higher in early milk samples (<15 d postpartum) than in later milk samples (15-90 d postpartum), whereas butyrophilin showed no such difference. Significant amounts of mucin and lactadherin were found in almost all gastric aspirates of human milk-fed infants, even 4 h after feeding (mucin, 270 +/- 30 microg/mL; lactadherin, 23.2 +/- 4.4 microg/mL), whereas butyrophilin was rapidly degraded in the majority of aspirates. Western blot analysis demonstrated that the immunoreactive mucin, lactadherin, and butyrophilin in the milk-fed gastric aspirates had the expected native molecular weights. Mucin and lactadherin survived at all gastric pH values, whereas butyrophilin was found only at pH > 4. Neither lactadherin nor butyrophilin were detected in gastric aspirates of formula-fed infants (gestational age = 27.8 +/- 0.5 wk), whereas the very low level of mucin (9.1 +/- 1.1 microg/mL) in this group is presumably cross-reacting gastric mucin. These results demonstrate that two HMFG glycoproteins implicated in prevention of infection, MUC-1 mucin and lactadherin, survive and maintain their integrity in the stomachs of human milk-fed preterm infants.

Structure and sequence of the bovine butyrophilin gene

The complete sequence of the bovine butyrophilin gene (BTN) is described and compared with the mouse gene (Btn). Both genes contain seven exons separated by six introns, and the organisation of exons is closely associated with structural domains of the protein. Individual exons of BTN and Btn are 68-87% similar in sequence. There are no canonical TATA or CCAAT boxes associated with the transcription initiation sites in the genes of either species. However, a number of potential binding sites for transcription factors were identified in the 5'-flanking DNA, some of which may function in regulating expression of the gene in mammary tissue. Conservation of a 110-bp region in the promoters of BTN and Btn may have some functional significance. Cloning and sequencing of BTN provides an additional mammary-specific gene promoter that may be used for driving the expression of transgenes in the lactating mammary gland, and for determining the basis for tissue-specific gene expression. In addition, the sequence of BTN may be used to map intragenic polymorphisms and identify quantitative trait loci in commercial livestock.

A 1.1-Mb transcript map of the hereditary hemochromatosis locus

In the process of positionally cloning a candidate gene responsible for hereditary hemochromatosis (HH), we constructed a 1.1-Mb transcript map of the region of human chromosome 6p that lies 4.5 Mb telomeric to HLA-A. A combination of three gene-finding techniques, direct cDNA selection, exon trapping, and sample sequencing, were used initially for a saturation screening of the 1.1-Mb region for expressed sequence fragments. As genetic analysis further narrowed the HH candidate locus, we sequenced completely 0.25 Mb of genomic DNA as a final measure to identify all genes. Besides the novel MHC class 1-like HH candidate gene HLA-H, we identified a family of five butyrophilin-related sequences, two genes with structural similarity to a type 1 sodium phosphate transporter, 12 novel histone genes, and a gene we named RoRet based on its strong similarity to the 52-kD Ro/SSA lupus and Sjogren's syndrome auto-antigen and the RET finger protein. Several members of the butyrophilin family and the RoRet gene share an exon of common evolutionary origin called B30-2. The B30-2 exon was originally isolated from the HLA class 1 region, yet has apparently "shuffled" into several genes along the chromosome telomeric to the MHC. The conservation of the B30-2 exon in several novel genes and the previously described amino acid homology of HLA-H to MHC class 1 molecules provide further support that this gene-rich region of 6p21.3 is related to the MHC. Finally, we performed an analysis of the four approaches for gene finding and conclude that direct selection provides the most effective probes for cDNA screening, and that as much as 30% of ESTs in this 1.1-Mb region may be derived from noncoding genomic DNA.