Novel hexad repeats conserved in a putative transporter with restricted expression in cell types associated with growth, calcium exchange and homeostasis

Export of heme by the feline leukemia virus C receptor regulates mitochondrial biogenesis and redox balance in the hematophagous insect Rhodnius prolixus

Heme is a prosthetic group of proteins involved in vital physiological processes. It participates, for example, in redox reactions crucial for cell metabolism due to the variable oxidation state of its central iron atom. However, excessive heme can be cytotoxic due to its prooxidant properties. Therefore, the control of intracellular heme levels ensures the survival of organisms, especially those that deal with high concentrations of heme during their lives, such as hematophagous insects. The export of heme initially attributed to the feline leukemia virus C receptor (FLVCR) has recently been called into question, following the discovery of choline uptake by the same receptor in mammals. Here, we found that RpFLVCR is a heme exporter in the midgut of the hematophagous insect Rhodnius prolixus, a vector for Chagas disease. Silencing RpFLVCR decreased hemolymphatic heme levels and increased the levels of intracellular dicysteinyl-biliverdin, indicating heme retention inside midgut cells. FLVCR silencing led to increased expression of heme oxygenase (HO), ferritin, and mitoferrin mRNAs while downregulating the iron importers Malvolio 1 and 2. In contrast, HO gene silencing increased FLVCR and Malvolio expression and downregulated ferritin, revealing crosstalk between heme degradation/export and iron transport/storage pathways. Furthermore, RpFLVCR silencing strongly increased oxidant production and lipid peroxidation, reduced cytochrome c oxidase activity, and activated mitochondrial biogenesis, effects not observed in RpHO-silenced insects. These data support FLVCR function as a heme exporter, playing a pivotal role in heme/iron metabolism and maintenance of redox balance, especially in an organism adapted to face extremely high concentrations of heme.

The prognostic marker FLVCR2 associated with tumor progression and immune infiltration for acute myeloid leukemia

Acute myeloid leukemia (AML) is one of the most common hematopoietic malignancies in adults. The tumor microenvironment (TME) has a critical effect on AML occurrence, recurrence, and progression. The gene feline leukemia virus subgroup C cellular receptor family member 2 (FLVCR2) belongs to the major facilitator superfamily of transporter protein members, which is primarily involved in transporting small molecules. The potential role of FLVCR2 in the TME in AML has not been investigated. To clarify the expression and role of FLVCR2 in AML, we analyzed the Gene Expression Omnibus and The Cancer Genome Atlas databases and found that FLVCR2 mRNA expression significantly increased among patients with AML. Furthermore, based on an analysis of the Gene Expression Profiling Interactive Analysis database, FLVCR2 upregulation predicted dismal overall survival of patients with AML. Our validation analysis revealed the significant upregulation of FLVCR2 within the bone marrow of AML relative to healthy controls by western blotting and qPCR assays. Gene set enrichment analysis was conducted to explore FLVCR2’s related mechanism in AML. We found that high FLVCR2 expression was related to infiltration degrees of immune cells and immune scores among AML cases, indicating that FLVCR2 possibly had a crucial effect on AML progression through the immune response. Specifically, FLVCR2 upregulation was negatively related to the immune infiltration degrees of activated natural killer cells, activated memory CD4+ T cells, activated dendritic cells, and CD8+ T cells using CIBERSORT analysis. According to the in vitro research, FLVCR2 silencing suppressed AML cell growth and promoted their apoptosis. This study provides insights into FLVCR2’s effect on tumor immunity, indicating that it might serve as an independent prognostic biomarker and was related to immune infiltration within AML.

MFSD7C switches mitochondrial ATP synthesis to thermogenesis in response to heme

ATP synthesis and thermogenesis are two critical outputs of mitochondrial respiration. How these outputs are regulated to balance the cellular requirement for energy and heat is largely unknown. Here we show that major facilitator superfamily domain containing 7C (MFSD7C) uncouples mitochondrial respiration to switch ATP synthesis to thermogenesis in response to heme. When heme levels are low, MSFD7C promotes ATP synthesis by interacting with components of the electron transport chain (ETC) complexes III, IV, and V, and destabilizing sarcoendoplasmic reticulum Ca2+-ATPase 2b (SERCA2b). Upon heme binding to the N-terminal domain, MFSD7C dissociates from ETC components and SERCA2b, resulting in SERCA2b stabilization and thermogenesis. The heme-regulated switch between ATP synthesis and thermogenesis enables cells to match outputs of mitochondrial respiration to their metabolic state and nutrient supply, and represents a cell intrinsic mechanism to regulate mitochondrial energy metabolism.

Expanding the clinical spectrum of Fowler syndrome: Three siblings with survival into adulthood and systematic review of the literature

Proliferative vasculopathy and hydranencephaly-hydrocephaly syndrome (PVHH, OMIM 225790), also known as Fowler syndrome, is a rare autosomal recessive disorder of brain angiogenesis. PVHH has long been considered to be prenatally lethal. We evaluated the phenotypes of the first three siblings with survival into adulthood, performed a systematic review of the Fowler syndrome literature and delineated genotype-phenotype correlations using a scoring system to rate the severity of the disease. Thirty articles were included, describing 69 individual patients. To date, including our clinical reports, 72 patients have been described with Fowler syndrome. Only 6/72 (8%) survived beyond birth. Although our three patients carry the same mutations (c.327T>A-p.Asn109Lys and c.887C>T-p.Ser296Leu) in FLVCR2, only two of them presented with the same cerebral features, ventriculomegaly and cerebral calcifications, as affected fetuses. The third sibling has a surprisingly milder clinical and radiological phenotype, suggesting intrafamilial variability. Although no clear phenotype-genotype correlation exists, some variants appear to be associated with a less severe phenotype compatible with life. As such, it is important to consider Fowler syndrome in patients with gross ventriculomegaly, cortical malformations and/or cerebral calcifications on brain imaging.

Major surgery induces acute changes in measured DNA methylation associated with immune response pathways

Surgery is an invasive procedure evoking acute inflammatory and immune responses that can influence risk for postoperative complications including cognitive dysfunction and delirium. Although the specific mechanisms driving these responses have not been well-characterized, they are hypothesized to involve the epigenetic regulation of gene expression. We quantified genome-wide levels of DNA methylation in peripheral blood mononuclear cells (PBMCs) longitudinally collected from a cohort of elderly patients undergoing major surgery, comparing samples collected at baseline to those collected immediately post-operatively and at discharge from hospital. We identified acute changes in measured DNA methylation at sites annotated to immune system genes, paralleling changes in serum-levels of markers including C-reactive protein (CRP) and Interleukin 6 (IL-6) measured in the same individuals. Many of the observed changes in measured DNA methylation were consistent across different types of major surgery, although there was notable heterogeneity between surgery types at certain loci. The acute changes in measured DNA methylation induced by surgery are relatively stable in the post-operative period, generally persisting until discharge from hospital. Our results highlight the dramatic alterations in gene regulation induced by invasive surgery, primarily reflecting upregulation of the immune system in response to trauma, wound healing and anaesthesia.

Investigation of Gene Expression and DNA Methylation From Seven Different Brain Regions of a Crab-Eating Monkey as Determined by RNA-Seq and Whole-Genome Bisulfite Sequencing

The crab-eating monkey is widely used in biomedical research for pharmacological experiments. Epigenetic regulation in the brain regions of primates involves complex patterns of DNA methylation. Previous studies of methylated CpG-binding domains using microarray technology or peak identification of sequence reads mostly focused on developmental stages or disease, rather than normal brains. To identify correlations between gene expression and DNA methylation levels that may be related to transcriptional regulation, we generated RNA-seq and whole-genome bisulfite sequencing data from seven different brain regions from a single crab-eating monkey. We identified 92 genes whose expression levels were significantly correlated, positively or negatively, with DNA methylation levels. Among them, 11 genes exhibited brain region-specific characteristics, and their expression patterns were strongly correlated with DNA methylation level. Nine genes (SLC2A5, MCM5, DRAM1, TTC12, DHX40, COR01A, LRAT, FLVCR2, and PTER) had effects on brain and eye function and development, and two (LHX6 and MEST) were previously identified as genes in which DNA methylation levels change significantly in the promoter region and are therefore considered brain epigenetic markers. Furthermore, we characterized DNA methylation of repetitive elements at the whole genome through repeat annotation at single-base resolution. Our results reveal the diverse roles of DNA methylation at single-base resolution throughout the genome and reflect the epigenetic variations in adult brain tissues.

Proliferative vasculopathy and hydranencephaly-hydrocephaly syndrome or Fowler syndrome: Report of a family and insight into the disease's mechanism

Background

Fowler syndrome is a rare autosomal recessive disorder characterized by hydranencephaly–hydrocephaly and multiple pterygium due to fetal akinesia. To date, around 45 cases from 27 families have been reported, and the pathogenic bi‐allelic mutations in FLVCR2 gene described in 15 families. The pathogenesis of this condition has not been fully elucidated so far.

Methods

We report on an additional family with two affected fetuses carrying a novel homozygous mutation in FLVCR2 gene, and describe the impact of known mutants on the protein structural and functional impairment.

Results

The present report confirms the genetic homogeneity of Fowler syndrome and describes a new FLVCR2 mutation affecting the protein function. The structural analysis of the present and previously published FLVCR2 mutations supports the hypothesis of a reduced heme import as the underlying disease's mechanism due to the stabilization of the occluded conformation or a protein misfolding.

Conclusion

Our data suggest the hypothesis of heme deficiency as the major pathogenic mechanism of Fowler syndrome.

Do Mammalian Cells Really Need to Export and Import Heme?

Heme is a cofactor that is essential to almost all forms of life. The production of heme is a balancing act between the generation of the requisite levels of the end-product and protection of the cell and/or organism against any toxic substrates, intermediates and, in this case, end-product. In this review, we provide an overview of our understanding of the formation and regulation of this metallocofactor and discuss new research on the cell biology of heme homeostasis, with a focus on putative transmembrane transporters now proposed to be important regulators of heme distribution. The main text is complemented by a discussion dedicated to the intricate chemistry and biochemistry of heme, which is often overlooked when new pathways of heme transport are conceived.

Genetics and molecular biology of brain calcification

Brain calcification is a common neuroimaging finding in patients with neurological, metabolic, or developmental disorders, mitochondrial diseases, infectious diseases, traumatic or toxic history, as well as in otherwise normal older people. Patients with brain calcification may exhibit movement disorders, seizures, cognitive impairment, and a variety of other neurologic and psychiatric symptoms. Brain calcification may also present as a single, isolated neuroimaging finding. When no specific cause is evident, a genetic etiology should be considered. The aim of the review is to highlight clinical disorders associated with brain calcification and provide summary of current knowledge of diagnosis, genetics, and pathogenesis of brain calcification.

Maternal age effects on myometrial expression of contractile proteins, uterine gene expression, and contractile activity during labor in the rat

Advanced maternal age of first time pregnant mothers is associated with prolonged and dysfunctional labor and significant risk of emergency cesarean section. We investigated the influence of maternal age on myometrial contractility, expression of contractile associated proteins (CAPs), and global gene expression in the parturient uterus. Female Wistar rats either 8 (YOUNG n = 10) or 24 (OLDER n = 10) weeks old were fed laboratory chow, mated, and killed during parturition. Myometrial strips were dissected to determine contractile activity, cholesterol (CHOL) and triglycerides (TAG) content, protein expression of connexin-43 (GJA1), prostaglandin-endoperoxide synthase 2 (PTGS2), and caveolin 1 (CAV-1). Maternal plasma concentrations of prostaglandins PGE2, PGF2α, and progesterone were determined by RIA. Global gene expression in uterine samples was compared using Affymetrix Genechip Gene 2.0 ST arrays and Ingenuity Pathway analysis (IPA). Spontaneous contractility in myometrium exhibited by YOUNG rats was threefold greater than OLDER animals (P < 0.027) but maternal age had no significant effect on myometrial CAP expression, lipid profiles, or pregnancy-related hormones. OLDER myometrium increased contractile activity in response to PGF2α, phenylephrine, and carbachol, a response absent in YOUNG rats (all P < 0.002). Microarray analysis identified that maternal age affected expression of genes related to immune and inflammatory responses, lipid transport and metabolism, steroid metabolism, tissue remodeling, and smooth muscle contraction. In conclusion YOUNG laboring rat myometrium seems primed to contract maximally, whereas activity is blunted in OLDER animals and requires stimulation to meet contractile potential. Further work investigating maternal age effects on myometrial function is required with focus on lipid metabolism and inflammatory pathways.

Heme in pathophysiology: a matter of scavenging, metabolism and trafficking across cell membranes

Heme (iron-protoporphyrin IX) is an essential co-factor involved in multiple biological processes: oxygen transport and storage, electron transfer, drug and steroid metabolism, signal transduction, and micro RNA processing. However, excess free-heme is highly toxic due to its ability to promote oxidative stress and lipid peroxidation, thus leading to membrane injury and, ultimately, apoptosis. Thus, heme metabolism needs to be finely regulated. Intracellular heme amount is controlled at multiple levels: synthesis, utilization by hemoproteins, degradation and both intracellular and intercellular trafficking. This review focuses on recent findings highlighting the importance of controlling intracellular heme levels to counteract heme-induced oxidative stress. The contributions of heme scavenging from the extracellular environment, heme synthesis and incorporation into hemoproteins, heme catabolism and heme transport in maintaining adequate intracellular heme content are discussed. Particular attention is put on the recently described mechanisms of heme trafficking through the plasma membrane mediated by specific heme importers and exporters. Finally, the involvement of genes orchestrating heme metabolism in several pathological conditions is illustrated and new therapeutic approaches aimed at controlling heme metabolism are discussed.

Heme and FLVCR-related transporter families SLC48 and SLC49

Heme is critical for a variety of cellular processes, but excess intracellular heme may result in oxidative stress and membrane injury. Feline leukemia virus subgroup C receptor (FLVCR1), a member of the SLC49 family of four paralogous genes, is a cell surface heme exporter, essential for erythropoiesis and systemic iron homeostasis. Disruption of FLVCR1 function blocks development of erythroid progenitors, likely due to heme toxicity. Mutations of SLC49A1 encoding FLVCR1 are noted in patients with a rare neurodegenerative disorder: posterior column ataxia with retinitis pigmentosa. FLVCR2 is highly homologous to FLVCR1 and may function as a cellular heme importer. Mutations of SLC49A2 encoding FLVCR2 are observed in Fowler syndrome, a rare proliferative vascular disorder of the brain. The functions of the remaining members of the SLC49 family, MFSD7 and DIRC2 (encoded by the SLC49A3 and SLC49A4 genes), are unknown, although the latter is implicated in hereditary renal carcinomas. SLC48A1 (heme responsive gene-1, HRG-1), the sole member of the SLC48 family, is associated with the endosome and appears to transport heme from the endosome into the cytosol.

A novel population of α-smooth muscle actin-positive cells activated in a rat model of stroke: an analysis of the spatio-temporal distribution in response to ischemia

In a rat model of stroke, the spatio-temporal distribution of α-smooth muscle actin-positive, (αSMA+) cells was investigated in the infarcted hemisphere (ipsilateral) and compared with the contralateral hemisphere. At day 3 postischemia, αSMA+ cells were concentrated in two main loci within the ipsilateral hemisphere (Area A) in the medial corpus callosum and (Area B) midway through the striatum adjacent to the lateral ventricle. By day 7 and further by day 14, fewer αSMA+ cells remained in Areas A and B but a steady increase in the peri-infarct was observed. αSMA+ cells also expressed glial acidic fibrillary protein [GFAP: αSMA+/GFAP+ (29%); αSMA+/GFAP- (71%) phenotypes] and feline leukemia virus C receptor 2 (FLVCR2), but not ED1(microglia) and established markers of pericytes normally located in vascular wall. αSMA+ cells were also located close to the subventricular zones (SVZ) adjacent to Areas A and B. In conclusion, αSMA+ cells have been identified in a spatial and temporal sequence from the SVZ, at intermediate loci and in the vicinity of the peri-infarct. It is hypothesized that novel populations of αSMA+ precursors of pericytes are born on the SVZ, migrate into the peri-infarct region and are incorporated into new vessels of the peri-infarct regions.

High-throughput sequencing of a 4.1 Mb linkage interval reveals FLVCR2 deletions and mutations in lethal cerebral vasculopathy

Rare lethal disease gene identification remains a challenging issue, but it is amenable to new techniques in high-throughput sequencing (HTS). Cerebral proliferative glomeruloid vasculopathy (PGV), or Fowler syndrome, is a severe autosomal recessive disorder of brain angiogenesis, resulting in abnormally thickened and aberrant perforating vessels leading to hydranencephaly. In three multiplex consanguineous families, genome-wide SNP analysis identified a locus of 14 Mb on chromosome 14. In addition, 280 consecutive SNPs were identical in two Turkish families unknown to be related, suggesting a founder mutation reducing the interval to 4.1 Mb. To identify the causative gene, we then specifically enriched for this region with sequence capture and performed HTS in a proband of seven families. Due to technical constraints related to the disease, the average coverage was only 7×. Nonetheless, iterative bioinformatic analyses of the sequence data identified mutations and a large deletion in the FLVCR2 gene, encoding a 12 transmembrane domain-containing putative transporter. A striking absence of alpha-smooth muscle actin immunostaining in abnormal vessels in fetal PGV brains, suggests a deficit in pericytes, cells essential for capillary stabilization and remodeling during brain angiogenesis. This is the first lethal disease-causing gene to be identified by comprehensive HTS of an entire linkage interval.

The Fowler syndrome-associated protein FLVCR2 is an importer of heme

Mutations in FLVCR2, a cell surface protein related by homology and membrane topology to the heme exporter/retroviral receptor FLVCR1, have recently been associated with Fowler syndrome, a vascular disorder of the brain. We previously identified FLVCR2 to function as a receptor for FY981 feline leukemia virus (FeLV). However, the cellular function of FLVCR2 remains unresolved. Here, we report the cellular function of FLVCR2 as an importer of heme, based on the following observations. First, FLVCR2 binds to hemin-conjugated agarose, and binding is competed by free hemin. Second, mammalian cells and Xenopus laevis oocytes expressing FLVCR2 display enhanced heme uptake. Third, heme import is reduced after the expression of FLVCR2-specific small interfering RNA (siRNA) or after the binding of the FY981 FeLV envelope protein to the FLVCR2 receptor. Finally, cells overexpressing FLVCR2 are more sensitive to heme toxicity, a finding most likely attributable to enhanced heme uptake. Tissue expression analysis indicates that FLVCR2 is expressed in a broad range of human tissues, including liver, placenta, brain, and kidney. The identification of a cellular function for FLVCR2 will have important implications in elucidating the pathogenic mechanisms of Fowler syndrome and of phenotypically associated disorders.

Prednisolone-induced differential gene expression in mouse liver carrying wild type or a dimerization-defective glucocorticoid receptor

BACKGROUND

Glucocorticoids (GCs) control expression of a large number of genes via binding to the GC receptor (GR). Transcription may be regulated either by binding of the GR dimer to DNA regulatory elements or by protein-protein interactions of GR monomers with other transcription factors. Although the type of regulation for a number of individual target genes is known, the relative contribution of both mechanisms to the regulation of the entire transcriptional program remains elusive. To study the importance of GR dimerization in the regulation of gene expression, we performed gene expression profiling of livers of prednisolone-treated wild type (WT) and mice that have lost the ability to form GR dimers (GRdim).

RESULTS

The GR target genes identified in WT mice were predominantly related to glucose metabolism, the cell cycle, apoptosis and inflammation. In GRdim mice, the level of prednisolone-induced gene expression was significantly reduced compared to WT, but not completely absent. Interestingly, for a set of genes, involved in cell cycle and apoptosis processes and strongly related to Foxo3a and p53, induction by prednisolone was completely abolished in GRdim mice. In contrast, glucose metabolism-related genes were still modestly upregulated in GRdim mice upon prednisolone treatment. Finally, we identified several novel GC-inducible genes from which Fam107a, a putative histone acetyltransferase complex interacting protein, was most strongly dependent on GR dimerization.

CONCLUSIONS

This study on prednisolone-induced effects in livers of WT and GRdim mice identified a number of interesting candidate genes and pathways regulated by GR dimers and sheds new light onto the complex transcriptional regulation of liver function by GCs.

Mutations in FLVCR2 are associated with proliferative vasculopathy and hydranencephaly-hydrocephaly syndrome (Fowler syndrome)

Proliferative vasculopathy and hydranencephaly-hydrocephaly syndrome (PVHH), also known as Fowler syndrome, is an autosomal-recessively inherited prenatal lethal disorder characterized by hydranencephaly; brain stem, basal ganglia, and spinal cord diffuse clastic ischemic lesions with calcifications; glomeruloid vasculopathy of the central nervous system and retinal vessels; and a fetal akinesia deformation sequence (FADS) with muscular neurogenic atrophy. To identify the molecular basis for Fowler syndrome, we performed autozygosity mapping studies in three consanguineous families. The results of SNP microarrays and microsatellite marker genotyping demonstrated linkage to chromosome 14q24.3. Direct sequencing of candidate genes within the target interval revealed five different germline mutations in FLVCR2 in five families with Fowler syndrome. FLVCR2 encodes a transmembrane transporter of the major facilitator superfamily (MFS) hypothesized to be involved in regulation of growth, calcium exchange, and homeostasis. This is the first gene to be associated with Fowler syndrome, and this finding provides a basis for further studies to elucidate the pathogenetic mechanisms and phenotypic spectrum of associated disorders.

Placental expression of secreted frizzled related protein-4 in the rat and the impact of glucocorticoid-induced fetal and placental growth restriction

Wnt genes regulate a diverse range of developmental processes, including placental formation. Activation of the WNT pathway results in translocation of beta-catenin (CTNNB1) into the nucleus and the subsequent activation of transcription factors that promote proliferation. The secreted frizzled related proteins (SFRPs) are thought to inhibit WNT signaling by binding to the WNT ligand or its frizzled receptor. In this study, we compared the expression patterns of one of these secreted molecules, SFRP4, in the two morphologically and functionally distinct regions of the rat placenta during the last third of pregnancy. In addition, we assessed whether placental SFRP4 expression is altered in a model of glucocorticoid-induced placental growth restriction. Temporal analyses of the rat placenta by quantitative RT-PCR, in situ hybridization, and immunohistochemistry during the final third of pregnancy demonstrated elevated levels of Sfrp4 mRNA and SFRP4 protein near term, specifically in trophoblast cells of the basal zone. This increase in expression of SFRP4 in basal zone trophoblasts was associated with a reduction in CTNNB1 nuclear translocation, consistent with inhibition of the WNT pathway. Maternal dexamethasone treatment (1 microg/ml of drinking water, Days 13-22), which has previously been shown to reduce placental growth, further increased the expression of Sfrp4 mRNA in both the basal and labyrinth zones of the placenta at Day 22. Collectively, these data demonstrate that increased expression of SFRP4 is associated with reduced growth of placental regions in normal pregnancy and after glucocorticoid-induced growth retardation. These observations, together with associated changes in CTNNB1 localization, support the hypothesis that increased placental expression of SFRP4 inhibits the WNT pathway and thereby influences placental growth via effects on cell fate signaling.

Comprehensive mapping of receptor-functioning domains in feline leukemia virus subgroup C receptor FLVCR1

Infection of cells by the highly anemogenic feline leukemia virus subgroup C (FeLV-C) is mediated by the heme exporter FLVCR1, a cell surface protein containing 12 potential transmembrane segments with six presumptive extracellular loops (ECLs). To identify FLVCR1 residues critical for mediating FeLV-C infection, we first independently isolated a human cDNA encoding the FLVCR2 protein that shares 52% identity to human FLVCR1, and we show that FLVCR2 does not function as a receptor for FeLV-C. Then, by generating specific hybrids between FLVCR1 and FLVCR2 and testing susceptibility of mouse cells expressing these hybrids to beta-galactosidase encoding FeLV-C, we identify FLVCR1 ECLs 1 and 6 as critical for mediating FeLV-C infection. Mouse cells expressing a hybrid protein containing FLVCR2 backbone with the ECL6 sequence from FLVCR1 were highly susceptible to FeLV-C infection. Using site-directed mutagenesis, we show that a single mutation of Asn463 in FLVCR2 ECL6 to an acidic Asp residue (a residue present in the corresponding position 487 in FLVCR1 ECL6) is sufficient to render FLVCR2 functional as an FeLV-C receptor. However, an Asp487Asn mutation in FLVCR1 ECL6 or substitution of the entire FLVCR1 ECL6 sequence for FLVCR2 ECL6 sequence does not disrupt receptor function. Subsequent substitutions show that residues within FLVCR1 ECL1 also contribute to mediating FeLV-C infection. Furthermore, our results suggest that FLVCR1 regions that mediate FeLV-C surface unit binding are distinct from ECL1 and ECL6. Our results are consistent with previous conclusions that infection of cells by gammaretroviruses involves interaction of virus with multiple receptor regions.