Selective Lutheran glycoprotein gene expression at the blood-brain barrier in normal brain and in human brain tumors

Dimerization and phosphorylation of Lutheran/basal cell adhesion molecule are critical for its function in cell migration on laminin

Tumor cell migration depends on the interactions of adhesion proteins with the extracellular matrix. Lutheran/basal cell adhesion molecule (Lu/BCAM) promotes tumor cell migration by binding to laminin α5 chain, a subunit of laminins 511 and 521. Lu/BCAM is a type I transmembrane protein with a cytoplasmic domain of 59 (Lu) or 19 (Lu(v13)) amino acids. Here, using an array of techniques, including site-directed mutagenesis, immunoblotting, FRET, and proximity-ligation assays, we show that both Lu and Lu(v13) form homodimers at the cell surface of epithelial cancer cells. We mapped two small-XXX-small motifs in the transmembrane domain as potential sites for monomers docking and identified three cysteines in the cytoplasmic domain as being critical for covalently stabilizing dimers. We further found that Lu dimerization and phosphorylation of its cytoplasmic domain were concomitantly needed to promote cell migration. We conclude that Lu is the critical isoform supporting tumor cell migration on laminin 521 and that the Lu:Lu(v13) ratio at the cell surface may control the balance between cellular firm adhesion and migration.

Association analysis of rare variants near the APOE region with CSF and neuroimaging biomarkers of Alzheimer's disease

BACKGROUND

The APOE ε4 allele is the most significant common genetic risk factor for late-onset Alzheimer's disease (LOAD). The region surrounding APOE on chromosome 19 has also shown consistent association with LOAD. However, no common variants in the region remain significant after adjusting for APOE genotype. We report a rare variant association analysis of genes in the vicinity of APOE with cerebrospinal fluid (CSF) and neuroimaging biomarkers of LOAD.

METHODS

Whole genome sequencing (WGS) was performed on 817 blood DNA samples from the Alzheimer's Disease Neuroimaging Initiative (ADNI). Sequence data from 757 non-Hispanic Caucasian participants was used in the present analysis. We extracted all rare variants (MAF (minor allele frequency) < 0.05) within a 312 kb window in APOE's vicinity encompassing 12 genes. We assessed CSF and neuroimaging (MRI and PET) biomarkers as LOAD-related quantitative endophenotypes. Gene-based analyses of rare variants were performed using the optimal Sequence Kernel Association Test (SKAT-O).

RESULTS

A total of 3,334 rare variants (MAF < 0.05) were found within the APOE region. Among them, 72 rare non-synonymous variants were observed. Eight genes spanning the APOE region were significantly associated with CSF Aβ_1-42 (p < 1.0 × 10^-3). After controlling for APOE genotype and adjusting for multiple comparisons, 4 genes (CBLC, BCAM, APOE, and RELB) remained significant. Whole-brain surface-based analysis identified highly significant clusters associated with rare variants of CBLC in the temporal lobe region including the entorhinal cortex, as well as frontal lobe regions. Whole-brain voxel-wise analysis of amyloid PET identified significant clusters in the bilateral frontal and parietal lobes showing associations of rare variants of RELB with cortical amyloid burden.

CONCLUSIONS

Rare variants within genes spanning the APOE region are significantly associated with LOAD-related CSF Aβ_1-42 and neuroimaging biomarkers after adjusting for APOE genotype. These findings warrant further investigation and illustrate the role of next generation sequencing and quantitative endophenotypes in assessing rare variants which may help explain missing heritability in AD and other complex diseases.

BCAM and LAMA5 Mediate the Recognition between Tumor Cells and the Endothelium in the Metastatic Spreading of KRAS-Mutant Colorectal Cancer

PURPOSE

KRAS mutations confer adverse prognosis to colorectal cancer, and no targeted therapies have shown efficacy in this patient subset. Paracrine, nongenetic events induced by KRAS-mutant tumor cells are expected to result in specific deregulation and/or relocation of tumor microenvironment (TME) proteins, which in principle can be exploited as alternative therapeutic targets.

EXPERIMENTAL DESIGN

A multimodal strategy combining ex vivo/in vitro phage display screens with deep-sequencing and bioinformatics was applied to uncover TME-specific targets in KRAS-mutant hepatic metastasis from colorectal cancer. Expression and localization of BCAM and LAMA5 were validated by immunohistochemistry in preclinical models of human hepatic metastasis and in a panel of human specimens (n = 71). The antimetastatic efficacy of two BCAM-mimic peptides was evaluated in mouse models. The role of BCAM in the interaction of KRAS-mutant colorectal cancer cells with TME cells was investigated by adhesion assays.

RESULTS

BCAM and LAMA5 were identified as molecular targets within both tumor cells and TME of KRAS-mutant hepatic metastasis from colorectal cancer, where they were specifically overexpressed. Two BCAM-mimic peptides inhibited KRAS-mutant hepatic metastasis in preclinical models. Genetic suppression and biochemical inhibition of either BCAM or LAMA5 impaired adhesion of KRAS-mutant colorectal cancer cells specifically to endothelial cells, whereas adhesion to pericytes and hepatocytes was unaffected.

CONCLUSIONS

These data show that the BCAM/LAMA5 system plays a functional role in the metastatic spreading of KRAS-mutant colorectal cancer by mediating tumor-TME interactions and as such represents a valuable therapeutic candidate for this large, currently untreatable patient group. Clin Cancer Res; 22(19); 4923-33. ©2016 AACR.

A preliminary sketch of horn cancer transcriptome in Indian zebu cattle

Horn cancer, a type of squamous cell carcinoma, in zebu cattle is an expensive affair in Indian agriculture sector, which accounts for 83.34% of total tumors found. In general, cancer tissue confirms considerably different expression patterns when compared to a normal stage. This includes not only up/down regulation, but also, the aberrant gene expression, the presence of different non-coding RNAs (ncRNAs), pseudogenes expression and genes involved in unusual pathways. We employed Roche 454 next generation sequencing platform to sequence Bos indicus cancerous and normal horn tissue transcripts. This resulted into a total of 909,345 high-confidence deep sequencing reads and detected a range of unusual transcriptional events including tumor associated genes. We also validated expression of two of the four tested genes in five other similar tissue samples by RT-qPCR. Further, seven cancer specific non-coding transcripts were accessed and a few of them have been suggested as cancer specific markers. This study for the first time provides primary transcriptome sketch of Bos indicus horn cancer tissue, and also demonstrates the suitability of the 454 sequencer for transcriptome analysis, which supports the concept of varied gene expression in cancerous condition.

Identification and expression profiling of blood-brain barrier membrane proteins

Blood-brain barrier (BBB) membrane proteins play crucial roles in the proper functioning of the BBB as well as in disease progression. Previously, we developed a novel approach for identifying membrane proteins expressed at the BBB, which we referred to as multiplex expression cloning. In this study, the proteome coverage of the multiplex expression cloning approach was expanded to allow the identification of a total of 30 BBB membrane proteins that are diverse in function and abundance. To unveil those membrane proteins that are enriched at the BBB and hence partially responsible for some of its unique characteristics, the transcript abundance levels for all 30 BBB membrane proteins were compared with those found in microvessels derived from lung, liver, heart, and kidney. Such quantitative PCR profiling of RNA samples from laser capture microdissected microvessels revealed that the transcripts for five membrane proteins, namely Lutheran glycoprotein, carbonic anhydrase IV, uncoupling protein 2, podocalyxin, and solute carrier family 38, member 5, were BBB selective, in that expression was elevated in brain microvessels when compared with all of the vascular beds tested. Many other membrane protein transcripts, whereas not as BBB-restricted, showed selective expression within subsets of tissues indicating other potential parallels and contrasts between vascular beds in the body. The identification of BBB membrane proteins could help better understand the molecular mechanisms responsible for BBB function and those with selective expression may have utility for BBB-targeted therapies.

Multiplex expression cloning of blood-brain barrier membrane proteins

The blood-brain barrier (BBB) is a vascular endothelial interface that separates the brain interior from the bloodstream. Membrane proteins resident at the BBB play important functional and regulatory roles. The current study describes the development and successful implementation of a multiplex expression cloning (MEC) method to allow facile identification of BBB membrane proteins. The overriding goal of the MEC approach was to mine a BBB cDNA library and selectively isolate membrane protein-encoding cDNAs. This selection process was achieved via fluorescence-activated cell sorting (FACS) of cDNA-expressing mammalian host cells for those cells that were immunolabeled with a BBB membrane protein-specific polyclonal antiserum (BMSPA). After optimization of the host cell expression system, four selection rounds allowed the isolation of a panel of 15 unique cDNAs that encoded BBB membrane proteins. The identified proteins display significant diversity in structure, function and in vivo expression levels. The MEC approach thus proved effective for conducting moderate throughput membrane proteome analyses of the BBB while limiting any biases caused by membrane protein insolubility or low in vivo expression levels that can complicate other proteomic approaches.

Erythrocyte adhesion receptors: blood group antigens and related molecules

During the second half of the 20th century, blood bankers quickly expanded our knowledge of human erythrocyte blood group antigens. By the dawn of the 21st century, several hundred blood group antigen polymorphisms had been identified. Hot on the heels of the serologists, membrane biochemists and molecular geneticists defined both the biochemical and genetic bases of most of these antigens. Perhaps to their surprise, this work has led to the discovery of functionally diverse and important membrane proteins expressed on the surface of red cells, including numerous adhesion molecules. Red cells express an unexpected number of such adhesion receptors, some of which contribute to human disease, as well as to normal red cell development. And perhaps most interestingly, study of these molecules has elucidated ways in which even mature red cells respond to external stimuli, such as adrenergic hormones.

Blood-brain barrier genomics, proteomics, and new transporter discovery

The blood-brain barrier (BBB) is an impermeable cellular interface that physically separates the blood from the interstices of the brain. The endothelial cells lining the brain blood vessels form the principle barrier, and their unique phenotype is a consequence of dynamic interactions with several perivascular cell types present in the brain parenchyma. In addition, BBB dysfunction has been observed in the large majority of neurological diseases, but the causes of aberrant vascular behavior are generally unknown. Because of its barrier phenotype, drug delivery to the brain has also proven to be a very difficult task. Global genomics and proteomics analyses are currently being used to examine BBB function in healthy and diseased brain to better characterize this dynamic interface. It is becoming increasingly evident that these approaches have the potential to clarify the unique attributes of a healthy BBB, to identify therapeutic targets in diseased brain, and to identify novel conduits for noninvasive delivery of drugs against these targets. This review will discuss the application of genomics and proteomics to blood-brain barrier research and will offer views on the prospects of such approaches.

Hypoxia induces de-stabilization of the LAT1 large neutral amino acid transporter mRNA in brain capillary endothelial cells

Blood-brain barrier (BBB) transport of large neutral amino acids is mediated by the large neutral amino acid transporter type 1 (LAT1 transporter). Although the gene encoding the Glut1 glucose transporter is up-regulated in hypoxia, the response of the LAT1 gene to hypoxia is not known. The present study investigates the changes in the LAT1 mRNA in cultured bovine brain capillary endothelial cells exposed to 1% O2 for 24-48 h. The LAT1 mRNA was initially down-regulated in hypoxia with reciprocal changes in the Glut1 mRNA. No changes in the 4F2hc mRNA in hypoxia were observed. Hypoxia caused an initial de-stabilization of the LAT1 mRNA, and the t1/2 of the LAT1 mRNA in control and hypoxic cells was 6.4 +/- 0.5 and 2.4 +/- 0.1 h, respectively. To further explore post-transcriptional regulation of LAT1 gene expression, the polysome and cytosol fractions of the control and hypoxic endothelial cells were isolated, and LAT1 mRNA binding proteins were detected by ultraviolet light cross-linking. Whereas the cytosol contained no LAT1 mRNA binding proteins, the cell polysome fraction expressed several LAT1 mRNA binding proteins, including principal 40-, 70- and 80-kDa proteins. These studies are consistent with post-transcriptional de-stabilization of the LAT1 large neutral amino acid transporter in hypoxia.

Differences in multidrug resistance phenotype and matrix metalloproteinases activity between endothelial cells from normal brain and glioma

Endothelial cells (ECs) are new targets for tumor therapy. In this work, we purified endothelial cells from intracerebral and subcutaneous experimental gliomas as well as from normal brain in order to define some of the phenotypical differences between angiogenic and quiescent brain vasculature. We show that the multidrug resistance genes encoding drug efflux pumps at the brain endothelium are expressed differently in normal and tumoral vasculature. We also show that ECs from gliomas present increased activity of gelatinase B (MMP9), key enzyme in the angiogenic process. Importantly, we observe a different phenotype between ECs in the intracerebral and subcutaneous models. Our results provide molecular evidence of phenotypic distinction between tumoral and normal brain vasculature and indicate that the EC phenotype depends on interactions both with tumor cells and also with the microenvironment.

Vascular proteomics and subtractive antibody expression cloning

The cloning of genes expressing proteins that are differentially expressed in the organ microvasculature has the potential to address a variety of problems ranging from the analysis of disease pathogenesis to drug targeting for particular tissues. This study describes a methodology designed to analyze differential protein expression in the brain microvasculature. The method can be applied to other organs and is particularly suited to the cloning of cDNAs encoding membrane proteins. The technology merges a tissue-specific polyclonal antiserum with a cDNA library expression cloning system. The tissue-specific antiserum is subtracted with protein extracts from control tissues to remove those antibodies that recognize common antigenic proteins. Then, the depleted antiserum is used to expression clone tissue-specific proteins from a cDNA library expressed in mammalian cells. The methodology was evaluated with a rabbit polyclonal antiserum prepared against purified bovine brain capillaries. The antiserum was absorbed with acetone powders of liver and kidney and then used to screen a bovine brain capillary cDNA library in COS cells. The initial clone detected with this expression methodology was the Lutheran membrane glycoprotein, which is specifically expressed at the brain microvasculature compared with liver and kidney tissues. This subtractive expression cloning methodology provides a new approach to "vascular proteomics" and to the detection of proteins specifically expressed at the microvasculature, including membrane proteins.