Genomic insights into the host specific adaptation of the Pneumocystis genus

Pneumocystis jirovecii, the fungal agent of human Pneumocystis pneumonia, is closely related to macaque Pneumocystis. Little is known about other Pneumocystis species in distantly related mammals, none of which are capable of establishing infection in humans. The molecular basis of host specificity in Pneumocystis remains unknown as experiments are limited due to an inability to culture any species in vitro. To explore Pneumocystis evolutionary adaptations, we have sequenced the genomes of species infecting macaques, rabbits, dogs and rats and compared them to available genomes of species infecting humans, mice and rats. Complete whole genome sequence data enables analysis and robust phylogeny, identification of important genetic features of the host adaptation, and estimation of speciation timing relative to the rise of their mammalian hosts. Our data reveals insights into the evolution of P. jirovecii, the sole member of the genus able to infect humans.

Cissé, Ma et al. utilize genomic data from Pneumocystis species infecting macaques, rabbit, dogs and rats to investigate the molecular basis of host specificity in Pneumocystis. Their analyses provide insight to the specific adaptations enabling the infection of humans by P. jirovecii.

Characterization of Pneumocystis murina Bgl2, an Endo-β-1,3-Glucanase and Glucanosyltransferase

Glucan is the major cell wall component of Pneumocystis cysts. In the current study, we have characterized Pneumocystis Bgl2 (EC 3.2.1.58), an enzyme with glucanosyltransferase and β-1,3 endoglucanase activity in other fungi. Pneumocystis murina, Pneumocystis carinii, and Pneumocystis jirovecii bgl2 complementary DNA sequences encode proteins of 437, 447, and 408 amino acids, respectively. Recombinant P. murina Bgl2 expressed in COS-1 cells demonstrated β-glucanase activity, as shown by degradation of the cell wall of Pneumocystis cysts. It also cleaved reduced laminaripentaose and transferred oligosaccharides, resulting in polymers of 6 and 7 glucan residues, demonstrating glucanosyltransferase activity. Surprisingly, confocal immunofluorescence analysis of P. murina-infected mouse lung sections using an antibody against recombinant Bgl2 showed that the native protein is localized primarily to the trophic form of Pneumocystis in both untreated mice and mice treated with caspofungin, an antifungal drug that inhibits β-1,3-glucan synthase. Thus, like other fungi, Bgl2 of Pneumocystis has both endoglucanase and glucanosyltransferase activities. Given that it is expressed primarily in trophic forms, further studies are needed to better understand its role in the biology of Pneumocystis.

Diversity and Complexity of the Large Surface Protein Family in the Compacted Genomes of Multiple Pneumocystis Species

Pneumocystis, a major opportunistic pathogen in patients with a broad range of immunodeficiencies, contains abundant surface proteins encoded by a multicopy gene family, termed the major surface glycoprotein (Msg) gene superfamily. This superfamily has been identified in all Pneumocystis species characterized to date, highlighting its important role in Pneumocystis biology. In this report, through a comprehensive and in-depth characterization of 459 msg genes from 7 Pneumocystis species, we demonstrate, for the first time, the phylogeny and evolution of conserved domains in Msg proteins and provide a detailed description of the classification, unique characteristics, and phylogenetic relatedness of five Msg families. We further describe, for the first time, the relative expression levels of individual msg families in two rodent Pneumocystis species, the substantial variability of the msg repertoires in P. carinii from laboratory and wild rats, and the distinct features of the expression site for the classic msg genes in Pneumocystis from 8 mammalian host species. Our analysis suggests multiple functions for this superfamily rather than just conferring antigenic variation to allow immune evasion as previously believed. This study provides a rich source of information that lays the foundation for the continued experimental exploration of the functions of the Msg superfamily in Pneumocystis biology.IMPORTANCEPneumocystis continues to be a major cause of disease in humans with immunodeficiency, especially those with HIV/AIDS and organ transplants, and is being seen with increasing frequency worldwide in patients treated with immunodepleting monoclonal antibodies. Annual health care associated with Pneumocystis pneumonia costs ∼$475 million dollars in the United States alone. In addition to causing overt disease in immunodeficient individuals, Pneumocystis can cause subclinical infection or colonization in healthy individuals, which may play an important role in species preservation and disease transmission. Our work sheds new light on the diversity and complexity of the msg superfamily and strongly suggests that the versatility of this superfamily reflects multiple functions, including antigenic variation to allow immune evasion and optimal adaptation to host environmental conditions to promote efficient infection and transmission. These findings are essential to consider in developing new diagnostic and therapeutic strategies.

MUC1 mediates Pneumocystis murina binding to airway epithelial cells

Previous studies have shown that Pneumocystis binds to pneumocytes, but the proteins responsible for binding have not been well defined. Mucins are the major glycoproteins present in mucus, which serves as the first line of defence during airway infection. MUC1 is the best characterised membrane-tethered mucin and is expressed on the surface of most airway epithelial cells. Although by electron microscopy Pneumocystis primarily binds to type I pneumocytes, it can also bind to type II pneumocytes. We hypothesized that Pneumocystis organisms can bind to MUC1 expressed by type II pneumocytes. Overexpression of MUC1 in human embryonic kidney HEK293 cells increased Pneumocystis binding, while knockdown of MUC1 expression by siRNA in A549 cells, a human adenocarcinoma-derived alveolar type II epithelial cell line, decreased Pneumocystis binding. Immunofluorescence labelling indicated that MUC1 and Pneumocystis were co-localised in infected mouse lung tissue. Incubation of A549 cells with Pneumocystis led to phosphorylation of ERK1/2 that increased with knockdown of MUC1 expression by siRNA. Pneumocystis caused increased IL-6 and IL-8 secretion by A549 cells, and knockdown of MUC1 further increased their secretion in A549 cells. Taken together, these results suggest that binding of Pneumocystis to MUC1 expressed by airway epithelial cells may facilitate establishment of productive infection.

The Major Surface Glycoprotein of Pneumocystis murina Does Not Activate Dendritic Cells

The major surface glycoprotein (Msg) is the most abundant surface protein among Pneumocystis species. Given that Msg is present on both the cyst and trophic forms of Pneumocystis and that dendritic cells play a critical role in initiating host immune responses, we undertook studies to examine activation of bone marrow-derived myeloid dendritic cells by Msg purified from Pneumocystis murina. Incubation of dendritic cells with Msg did not lead to increased expression of CD40, CD80, CD86, or major histocompatibility complex class II or to increased secretion of any of 10 cytokines. Microarray analysis identified very few differentially expressed genes. In contrast, lipopolysaccharide-activated dendritic cells had positive results of all of these assays. However, Msg did bind to mouse mannose macrophage receptor and human DC-SIGN, 2 C-type lectins expressed by dendritic cells that are important in recognition of pathogen-associated high-mannose glycoproteins. Deglycosylation of Msg demonstrated that this binding was dependent on glycosylation. These studies suggest that Pneumocystis has developed a mechanism to avoid activation of dendritic cells, potentially by the previously identified loss of genes that are responsible for the high level of protein mannosylation found in other fungi.

Genetic diversity of Pneumocystis jirovecii from a cluster of cases of pneumonia in renal transplant patients: Cross-sectional study

Pneumocystis jirovecii can cause severe potentially life-threatening pneumonia (PCP) in kidney transplant patients. Prophylaxis of patients against PCP in this setting is usually performed during 6 months after transplantation. The aim of this study is to describe the molecular epidemiology of a cluster of PCP in renal transplant recipients in Brazil. Renal transplant patients who developed PCP between May and December 2011 had their formalin-fixed paraffin-embedded (FFPE) lung biopsy samples analysed. Pneumocystis jirovecii 23S mitochondrial large subunit of ribosomal RNA (23S mtLSU-rRNA), 26S rRNA, and dihydropteroate synthase (DHPS) genes were amplified by polymerase chain reaction (PCR), sequenced, and analysed for genetic variation. During the study period, 17 patients developed PCP (only four infections were documented within the first year after transplantation) and six (35.3%) died. Thirty FFPE samples from 11 patients, including one external control HIV-infected patient, had fungal DNA successfully extracted for further amplification and sequencing for all three genes. A total of five genotypes were identified among the 10 infected patients. Of note, four patients were infected by more than one genotype and seven patients were infected by the same genotype. DNA extracted from FFPE samples can be used for genotyping; this approach allowed us to demonstrate that multiple P. jirovecii strains were responsible for this cluster, and one genotype was found infecting seven patients. The knowledge of the causative agents of PCP may help to develop new initiatives for control and prevention of PCP among patients undergoing renal transplant and improve routine PCP prophylaxis.

β-Glucans Are Masked but Contribute to Pulmonary Inflammation During Pneumocystis Pneumonia

β-glucans, which can activate innate immune responses, are a major component in the cell wall of the cyst form of Pneumocystis In the current study, we examined whether β-1,3-glucans are masked by surface proteins in Pneumocystis and what role β-glucans play in Pneumocystis-associated inflammation. For 3 species, including Pneumocystis jirovecii, which causes Pneumocystis pneumonia in humans, Pneumocystis carinii, and Pneumocystis murina, β-1,3-glucans were masked in most organisms, as demonstrated by increased exposure following trypsin treatment. Using quantitative polymerase chain reaction and microarray techniques, we demonstrated in a mouse model of Pneumocystis pneumonia that treatment with caspofungin, an inhibitor of β-1,3-glucan synthesis, for 21 days decreased expression of a broad panel of inflammatory markers, including interferon γ, tumor necrosis factor α, interleukin 1β, interleukin 6, and multiple chemokines/chemokine ligands. Thus, β-glucans in Pneumocystis cysts are largely masked, which likely decreases innate immune activation; this mechanism presumably was developed for interactions with immunocompetent hosts, in whom organism loads are substantially lower. In immunosuppressed hosts with a high organism burden, organism death and release of glucans appears to be an important contributor to deleterious host inflammatory responses.

Distinguishing highly similar gene isoforms with a clustering-based bioinformatics analysis of PacBio single-molecule long reads

Background

Gene isoforms are commonly found in both prokaryotes and eukaryotes. Since each isoform may perform a specific function in response to changing environmental conditions, studying the dynamics of gene isoforms is important in understanding biological processes and disease conditions. However, genome-wide identification of gene isoforms is technically challenging due to the high degree of sequence identity among isoforms. Traditional targeted sequencing approach, involving Sanger sequencing of plasmid-cloned PCR products, has low throughput and is very tedious and time-consuming. Next-generation sequencing technologies such as Illumina and 454 achieve high throughput but their short read lengths are a critical barrier to accurate assembly of highly similar gene isoforms, and may result in ambiguities and false joining during sequence assembly. More recently, the third generation sequencer represented by the PacBio platform offers sufficient throughput and long reads covering the full length of typical genes, thus providing a potential to reliably profile gene isoforms. However, the PacBio long reads are error-prone and cannot be effectively analyzed by traditional assembly programs.

Results

We present a clustering-based analysis pipeline integrated with PacBio sequencing data for profiling highly similar gene isoforms. This approach was first evaluated in comparison to de novo assembly of 454 reads using a benchmark admixture containing 10 known, cloned msg genes encoding the major surface glycoprotein of Pneumocystis jirovecii. All 10 msg isoforms were successfully reconstructed with the expected length (~1.5 kb) and correct sequence by the new approach, while 454 reads could not be correctly assembled using various assembly programs. When using an additional benchmark admixture containing 22 known P. jirovecii msg isoforms, this approach accurately reconstructed all but 4 these isoforms in their full-length (~3 kb); these 4 isoforms were present in low concentrations in the admixture. Finally, when applied to the original clinical sample from which the 22 known msg isoforms were cloned, this approach successfully identified not only all known isoforms accurately (~3 kb each) but also 48 novel isoforms.

Conclusions

PacBio sequencing integrated with the clustering-based analysis pipeline achieves high-throughput and high-resolution discrimination of highly similar sequences, and can serve as a new approach for genome-wide characterization of gene isoforms and other highly repetitive sequences.

Electronic supplementary material

The online version of this article (doi:10.1186/s13040-016-0090-8) contains supplementary material, which is available to authorized users.

Pneumocystis encodes a functional endo-β-1,3-glucanase that is expressed exclusively in cysts

β-1,3-glucan is a major cell wall component of Pneumocystis cysts. We have characterized endo-β-1,3-glucanase (Eng) from 3 species of Pneumocystis. The gene eng is a single-copy gene that encodes a protein containing 786 amino acids in P. carinii and P. murina, and 788 amino acids in P. jirovecii, including a signal peptide for the former 2 but not the latter. Recombinant Eng expressed in Escherichia coli was able to solubilize the major surface glycoprotein of Pneumocystis, thus potentially facilitating switching of the expressed major surface glycoprotein (Msg) variant. Confocal immunofluorescence analysis of P. murina-infected mouse lung sections localized Eng exclusively to the cyst form of Pneumocystis. No Eng was detected after mice were treated with caspofungin, a β-1,3-glucan synthase inhibitor that is known to reduce the number of cysts. Thus, Eng is a cyst-specific protein that may play a role in Msg variant expression in Pneumocystis.

Expression of Pneumocystis jirovecii major surface glycoprotein in Saccharomyces cerevisiae

The major surface glycoprotein (Msg), which is the most abundant protein expressed on the cell surface of Pneumocystis organisms, plays an important role in the attachment of this organism to epithelial cells and macrophages. In the present study, we expressed Pneumocystis jirovecii Msg in Saccharomyces cerevisiae, a phylogenetically related organism. Full-length P. jirovecii Msg was expressed with a DNA construct that used codons optimized for expression in yeast. Unlike in Pneumocystis organisms, recombinant Msg localized to the plasma membrane of yeast rather than to the cell wall. Msg expression was targeted to the yeast cell wall by replacing its signal peptide, serine-threonine-rich region, and glycophosphatidylinositol anchor signal region with the signal peptide of cell wall protein α-agglutinin of S. cerevisiae, the serine-threonine-rich region of epithelial adhesin (Epa1) of Candida glabrata, and the carboxyl region of the cell wall protein (Cwp2) of S. cerevisiae, respectively. Immunofluorescence analysis and treatment with β-1,3 glucanase demonstrated that the expressed Msg fusion protein localized to the yeast cell wall. Surface expression of Msg protein resulted in increased adherence of yeast to A549 alveolar epithelial cells. Heterologous expression of Msg in yeast will facilitate studies of the biologic properties of Pneumocystis Msg.

Sequencing and characterization of the complete mitochondrial genomes of three Pneumocystis species provide new insights into divergence between human and rodent Pneumocystis

Pneumocystis jirovecii is an important opportunistic pathogen associated with AIDS and other immunodeficient conditions. Currently, very little is known about its nuclear and mitochondrial genomes. In this study, we sequenced the complete mitochondrial genome (mtDNA) of this organism and its closely related species Pneumocystis carinii and Pneumocystis murina by a combination of sequencing technologies. Our study shows that P. carinii and P. murina mtDNA share a nearly identical number and order of genes in a linear configuration, whereas P. jirovecii has a circular mtDNA containing nearly the same set of genes but in a different order. Detailed studies of the mtDNA terminal structures of P. murina and P. carinii suggest a unique replication mechanism for linear mtDNA. Phylogenetic analysis supports a close association of Pneumocystis species with Taphrina, Saitoella, and Schizosaccharomyces, and divergence within Pneumocystis species, with P. murina and P. carinii being more closely related to each other than either is to P. jirovecii. Comparative analysis of four complete P. jirovecii mtDNA sequences in this study and previously reported mtDNA sequences for diagnosing and genotyping suggests that the current diagnostic and typing methods can be improved using the complete mtDNA data. The availability of the complete P. jirovecii mtDNA also opens the possibility of identifying new therapeutic targets.

A Benchmark Study on Error Assessment and Quality Control of CCS Reads Derived from the PacBio RS

PacBio RS, a newly emerging third-generation DNA sequencing platform, is based on a real-time, single-molecule, nano-nitch sequencing technology that can generate very long reads (up to 20-kb) in contrast to the shorter reads produced by the first and second generation sequencing technologies. As a new platform, it is important to assess the sequencing error rate, as well as the quality control (QC) parameters associated with the PacBio sequence data. In this study, a mixture of 10 prior known, closely related DNA amplicons were sequenced using the PacBio RS sequencing platform. After aligning Circular Consensus Sequence (CCS) reads derived from the above sequencing experiment to the known reference sequences, we found that the median error rate was 2.5% without read QC, and improved to 1.3% with an SVM based multi-parameter QC method. In addition, a De Novo assembly was used as a downstream application to evaluate the effects of different QC approaches. This benchmark study indicates that even though CCS reads are post error-corrected it is still necessary to perform appropriate QC on CCS reads in order to produce successful downstream bioinformatics analytical results.

Outbreaks of Pneumocystis pneumonia in 2 renal transplant centers linked to a single strain of Pneumocystis: implications for transmission and virulence

BACKGROUND

There have been numerous reports of clustered outbreaks of Pneumocystis pneumonia (PCP) at renal transplant centers over the past 2 decades. It has been unclear whether these outbreaks were linked epidemiologically to 1 or several unique strains, which could have implications for transmission patterns or strain virulence.

METHODS

Restriction fragment length polymorphism (RFLP) analysis was used to compare Pneumocystis isolates from 3 outbreaks of PCP in renal transplant patients in Germany, Switzerland, and Japan, as well as nontransplant isolates from both human immunodeficiency virus (HIV)-infected and uninfected patients.

RESULTS

Based on RFLP analysis, a single Pneumocystis strain caused pneumonia in transplant patients in Switzerland (7 patients) and Germany (14 patients). This strain was different from the strain that caused an outbreak in transplant patients in Japan, as well as strains causing sporadic cases of PCP in nontransplant patients with or without HIV infection.

CONCLUSIONS

Two geographically distinct clusters of PCP in Europe were due to a single strain of Pneumocystis. This suggests either enhanced virulence of this strain in transplant patients or a common, but unidentified, source of transmission. Outbreaks of PCP can be better understood by enhanced knowledge of transmission patterns and strain variation.

Characterization of the meiosis-specific recombinase Dmc1 of pneumocystis

The life cycle of Pneumocystis, which causes life-threatening pneumonia in immunosuppressed patients, remains poorly defined. In the present study, we have identified and characterized an orthologue of dmc1, a gene specific for meiotic recombination in yeast, in 3 species of Pneumocystis. dmc1 is a single-copy gene that is transcribed as ∼1.2-kb messenger RNA, which encodes a protein of 336-337 amino acids. Pneumocystis Dmc1 was 61%-70% identical to those from yeast. Confocal microscopy results indicated that the expression of Dmc1 is primarily confined to the cyst form of Pneumocystis. By sequence analysis of 2 single-copy regions of the human Pneumocystis jirovecii genome, we can infer multiple recombination events, which are consistent with meiotic recombination in this primarily haploid organism. Taken together, these studies support the occurrence of a sexual phase in the life cycle of Pneumocystis.

Restriction fragment length polymorphism typing demonstrates substantial diversity among Pneumocystis jirovecii isolates

Better understanding of the epidemiology and transmission patterns of human Pneumocystis should lead to improved strategies for preventing Pneumocystis pneumonia (PCP). We have developed a typing method for Pneumocystis jirovecii that is based on restriction fragment length polymorphism (RFLP) analysis after polymerase chain reaction amplification of an approximately 1300 base-pair region of the msg gene family, which comprises an estimated 50-100 genes/genome. The RFLP pattern was reproducible in samples containing >1000 msg copies/reaction and was stable over time, based on analysis of serial samples from the same patient. In our initial analysis of 48 samples, we found that samples obtained from different individuals showed distinct banding patterns; only samples obtained from the same patient showed an identical RFLP pattern. Despite this substantial diversity, samples tended to cluster on the basis of country of origin. In an evaluation of samples obtained from an outbreak of PCP in kidney transplant recipients in Germany, RFLP analysis demonstrated identical patterns in samples that were from 12 patients previously linked to this outbreak, as well as from 2 additional patients. Our results highlight the presence of a remarkable diversity in human Pneumocystis strains. RFLP may be very useful for studying clusters of PCP in immunosuppressed patients, to determine whether there is a common source of infection.

Variation in the major surface glycoprotein genes in Pneumocystis jirovecii

The genome of Pneumocystis, which causes life-threatening pneumonia in immunosuppressed patients, contains a multicopy gene family that encodes the major surface glycoprotein (Msg). Pneumocystis can vary the expressed Msg, presumably as a mechanism to avoid host immune responses. Analysis of 24 msg-gene sequences obtained from a single human isolate of Pneumocystis demonstrated that the sequences segregate into 2 branches. Results of a number of analyses suggest that recombination between msg genes is an important mechanism for generating msg diversity. Intrabranch recombination occurred more frequently than interbranch recombination. Restriction-fragment length polymorphism analysis of human isolates of Pneumocystis demonstrated substantial variation in the repertoire of the msg-gene family, variation that was not observed in laboratory isolates of Pneumocystis in rats or mice; this may be the result of examining outbred versus captive populations. Increased diversity in the Msg repertoire, generated in part by recombination, increases the potential for antigenic variation in this abundant surface protein.

Identification and characterization of rad51 of Pneumocystis

Rad51, a eukaryotic homolog of RecA, is an important protein involved in DNA recombination and repair. We have characterized rad51 of Pneumocystis carinii and Pneumocystis murina. rad51 is a single copy gene that encodes a 1.2 kb mRNA, which contains an open reading frame encoding 343 amino acids. Rad51 from Pneumocystis showed high homology to those from yeast. ATP binding motifs GEFRTGKS and LLIVD, similar to those of Saccharomyces cerevisiae and Schizosaccharomyces pombe, are conserved in Pneumocystis Rad51. The recombinant protein when expressed in E. coli showed DNA-dependent ATPase activity. Since Rad51 is a key enzyme in DNA repair and recombination, it potentially plays an important role in the recombination process leading to antigenic variation and thereby resistance to host immune responses in Pneumocystis.

Quantitative real-time polymerase chain-reaction assay allows characterization of pneumocystis infection in immunocompetent mice

Pneumocystis causes pneumonia in immunodeficient hosts but also likely causes infection in healthy hosts. To characterize infection in healthy mice, we developed and validated a real-time polymerase chain reaction assay for quantitation of Pneumocystis carinii f. sp. muris. In healthy mice exposed to Pneumocystis-infected animals, organisms were first detected at 2-3 weeks, peaked at 5-6 weeks, and were cleared by 7-9 weeks. The peak organism load in healthy animals was 2-3 logs lower than that in immunodeficient animals. This approach should facilitate studies of anti-Pneumocystis immune mechanisms in healthy hosts and provide insights into the development of Pneumocystis pneumonia in immunodeficient hosts.

Characterization of variants of the gene encoding the p55 antigen in Pneumocystis from rats and mice

Variants of the p55 gene in rat-derived Pneumocystis carinii have been identified and its counterpart in mouse-derived P. carinii f. sp. muris has been cloned. By PCR amplification of P. carinii genomic DNA, five variants were identified that differed from each other in size and sequence, primarily in the number and size of encoded amino acid repeats. For P. carinii f. sp. muris, a single PCR fragment (471 bp) was obtained, which contained an incomplete ORF encoding a 157 aa protein that was most similar to a p55 variant in P. carinii, with nucleotide and amino acid sequence identity of 79 and 68 %, respectively. Southern blot analysis revealed the presence of more than one copy of the p55 gene in both Pneumocystis species. Thus, like other Pneumocystis antigens, p55 exhibits polymorphism that could potentially benefit the organism in host interactions.

Characterization of thioredoxin reductase genes (trr1) from Pneumocystis carinii and Pneumocystis jiroveci

We have characterized the thioredoxin reductase (trr1) genes from Pneumocystis carinii and Pneumocystis jiroveci, and have demonstrated that multiple copies of an approximately 500 base pair fragment of the trr1 gene are present in P. carinii, but not in P. jiroveci. Thioredoxin reductases encoded by the full-length genes have predicted molecular weights of approximately 35,000 and show high homology to yeast Trr1. An NADPH-binding domain with a putative redox active site CAVC as well as an flavin-adenine dinucleotide-binding domain are highly conserved in both proteins, which were 85% identical. The multicopy trr1 gene fragments in P. carinii are not transcribed or expressed. Duplication of the gene fragment likely occurred in conjunction with duplication of the kexin homologue, protease-1, which is located immediately upstream of the trr1 gene. Thioredoxin reductase, an enzyme implicated in the growth, survival and pathogenicity of certain microbes, could be a potential target for therapeutic intervention in Pneumocystis infection.

A single-copy gene encodes Kex1, a serine endoprotease of Pneumocystis jiroveci

We have cloned and characterized the kex1 gene of Pneumocystis jiroveci. Unlike the case for Pneumocystis carinii, in which the homologous PRT-1 genes are multicopy, kex1 is a single-copy gene encoding a protein homologous to fungal serine endoproteases, which localize to the Golgi apparatus. Thus, substantial biological differences can be seen among Pneumocystis species.

Analysis of variation in tandem repeats in the intron of the major surface glycoprotein expression site of the human form of Pneumocystis carinii

Variation in the tandem repeats in the expression site of the human-derived Pneumocystis carinii major surface glycoprotein gene was characterized by denaturing gel electrophoresis. The number of repeats in 147 isolates ranged from 2 to 6, with 2, 3, and 4 repeats being the most common. Sequence analysis identified 3 types of repeat units that differed by 1 nucleotide, which suggests a hierarchy of evolution of human-derived P. carinii. Examination of sequential samples obtained from 6 patients at an interval of 10-90 days showed an identical repeat pattern in each patient. However, in 2 of 4 patients with 2-3 different samples obtained within 4 days, different repeat patterns were observed among the samples. Quantifying the number of repeats by denaturing gel electrophoresis is a simple and rapid-typing method that can be used alone or in combination with other methods to study the epidemiology of P. carinii.

Characterization of the expression site of the major surface glycoprotein of human-derived Pneumocystis carinii

The major surface glycoprotein (MSG) of Pneumocystis carinii, a pathogen responsible for pulmonary infection in AIDS and other immunocompromised patients, is an abundant surface protein that potentially allows the organism to evade host defences by antigenic variation. MSG is encoded by a multicopy gene family; in two specific forms of rat-derived P. carinii, regulation of MSG expression uses a single expression site, termed the upstream conserved sequence (UCS), through two related but distinct mechanisms. In the current study, the UCS of the MSG from human-derived P. carinii was obtained using an RNA ligase-mediated rapid amplification of cDNA ends technique. Southern blot analysis demonstrated that the UCS was present in a single copy per genome, whereas multiple copies of the downstream MSG gene were present. Sequencing and restriction fragment length polymorphism analysis of polymerase chain reaction products amplified from pulmonary samples of patients with P. carinii pneumonia demonstrated that multiple MSG genes were expressed in a given host, and that different patterns of MSG expression were seen among different patients. Tandem repeats present in the single intron occurred with varying frequency in different patient isolates, potentially providing a new method for typing human isolates. Thus, human-derived P. carinii regulates MSG expression in a manner similar to P. carinii f. sp. carinii and, in immunosuppressed patients, in whom immune pressures that probably drive antigenic variation are functioning inadequately, P. carinii can express a broad repertoire of MSG variants.

Molecular characterization and developmental expression of NORPEG, a novel gene induced by retinoic acid

We have characterized NORPEG, a novel gene from human retinal pigment epithelial cells (ARPE-19), in which its expression is induced by all-trans-retinoic acid. Two transcripts ( approximately 3 and approximately 5 kilobases in size) have been detected for this gene, which is localized to chromosome band 5p13.2-13.3. Placenta and testis showed the highest level of expression among various human tissues tested. Six ankyrin repeats and a long coiled-coil domain are present in the predicted sequence of the NORPEG protein, which contains 980 amino acid residues. This approximately 110-kDa protein was transiently expressed in COS-7 cells as a FLAG fusion protein and immunolocalized to the cytoplasm. Confocal microscopic analysis of the NORPEG protein in ARPE-19 cells showed threadlike projections in the cytoplasm reminiscent of the cytoskeleton. Consistent with this localization, the expressed NORPEG protein showed resistance to solubilization by Triton X-100 and KCl. An ortholog of NORPEG characterized from mouse encoded a protein that showed 91% sequence similarity to the human NORPEG protein. The expression of Norpeg mRNA was detected in mouse embryo at embryonic day 9.5 by in situ hybridization, and the expression appears to be developmentally regulated. In adult mouse, the highest level of expression was detected in the seminiferous tubules of testis.

Heme-binding by Drosophila retinoid- and fatty acid-binding glycoprotein (RFABG), a member of the proapolipophorin gene family

We previously have cloned and characterized a retinoid- and fatty acid-binding glycoprotein (RFABG) isolated from the heads of Drosophila melanogaster. The protein is composed of two glycosylated subunits (Mr = >200,000 and 70,000) and is a member of the proapolipophorin gene family. Spectral analysis of purified RFABG revealed an absolute absorbance peak at 405 nm, which is typical for a heme-containing protein. The aim of the present study was to characterize the heme-binding properties of RFABG. Upon saturation of the protein solution with carbon monoxide followed by dithionite reduction, a red shift of the Soret peak to 424 nm and the characteristic alpha- and beta- bands at 567 and 539 nm were observed. Native RFABG contains approximately 0.175 moles of heme (mol/mol) indicating that purified RFABG is primarily the apoprotein. Hemin-agarose affinity chromatography of the native RFABG followed by Western blot analysis showed a single immunoreactive band at 70 kDa, indicating that the heme-binding domain resides in the 70 kDa subunit. Although retinoid and fatty acid also bind to the 70 kDa subunit, no competition was observed when an excess of heme was added to a solution of retinoid or fatty acid bound to RFABG. Heme added to a solution of purified RFABG bound in a saturable manner with an affinity of 3.8 x 10(-7) m.Thus, the current study clearly demonstrates that retinoid- and fatty acid-binding glycoprotein is a novel heme-binding protein, which may be involved in the transport and/or metabolism of heme in Drosophila.

Light history and age-related changes in retinal light damage

PURPOSE

To determine the effects of age and long-term light- or dark-rearing environments on acute, intense-light-mediated retinal degeneration.

METHODS

Male albino rats were maintained in a dim cyclic light environment or in darkness for as long as 1 year. When aged 2, 4, 8, and 12 months, some rats were given the synthetic antioxidant dimethylthiourea (DMTU) by intraperitoneal injection and were exposed to intense visible light for as long as 24 hours. Uninjected control rats were exposed to light at the same time. Other rats were treated with light of lower intensity for various periods. Two weeks after intense-light treatment, photoreceptor cell degeneration was estimated by determining the level of rhodopsin and by measuring the content of photoreceptor cell DNA. Light-induced changes in retinal DNA were analyzed immediately after exposure by neutral gel electrophoresis and by 8-hydroxy-deoxyguanosine measurements. Expression of the antioxidative stress protein heme oxygenase-1 (HO-1) was determined by northern blot analysis of mRNA in retinal extracts.

RESULTS

At all ages, rats reared in cyclic dim-light conditions had lower rhodopsin levels than did rats reared in darkness; photoreceptor cell DNA levels were unaffected by the rearing environment. Senescent losses in rhodopsin and retinal DNA were significant after rats were 12 months old. Dim-light-reared rats exhibited an age-related increase in retinal light damage susceptibility, whereas dark-reared rats were equally susceptible to damage at all ages. In both types of rats, the mechanism of light-induced cell death involved an apoptotic process, visualized by the pattern of DNA fragments on electrophoretic gels. The process also induced the expression of HO-1 mRNA. Photoreceptor cell loss determined by biochemical measurement, DNA fragmentation, and HO-1 induction were dramatically reduced by the administration of DMTU.

CONCLUSIONS

The age-related increase in susceptibility to retinal light damage in rats is influenced by their long-term daily light history. Decreasing retinal irradiance by dark-rearing eliminates the age-related increase in light damage, suggesting a correlation between light environment and retinal gene expression associated with damage. In all rats, retinal light damage resulted in a pattern of DNA fragmentation consistent with apoptotic cell death and in an increased expression of HO-1 mRNA. Antioxidant treatment greatly reduced apoptosis and HO-1 expression. This indicates that light damage involves an oxidative process that may also trigger apoptosis in the retina. The rat aging model may provide useful insights into the role of light environment associated with retinal degeneration in an aging human population.

Identification of a new member of transforming growth factor-beta superfamily in Drosophila: the first invertebrate activin gene

Activins, a subgroup of the transforming growth factor-beta (TGF-beta) superfamily, have been extensively studied in vertebrates for their roles in growth and development. However, activins are not thought to be expressed in invertebrates. The identification of the first invertebrate activin gene is reported here. A genomic clone representing 102 F region of the Drosophila chromosome 4 is found to encode a putative activin beta. The predicted protein sequence has a multibasic protease site that would generate a mature C-terminal peptide containing 113 amino acids showing > 60% similarity to the vertebrate activin beta B (inhibin beta B) sequences. A TGF-beta family signature as well as all 9 cysteine residues conserved in the vertebrate activins are also present in this mature peptide sequence. Northern blot and RT-PCR analyses indicated that the activin beta gene is expressed in embryo, larva and adult stages of Drosophila.

Molecular characterization and developmental expression of a retinoid- and fatty acid-binding glycoprotein from Drosophila. A putative lipophorin

A detailed understanding of the mechanism of lipid transport in insects has been hampered by the inability to identify the proapolipophorin gene that encodes apolipophorins I and II, the principal protein components of lipophorin, the lipid transport vehicle. Here we provide the first molecular description of the Drosophila gene encoding a retinoid- and fatty acid-binding glycoprotein (RFABG) and present evidence that it is a member of the proapolipophorin gene family. The gene, localized to the chromosome 4 (102 F region), encodes a 3351-amino acid protein that could serve as the precursor for the approximately 70-kDa and >200-kDa polypeptides associated with RFABG. The N-terminal sequence of the approximately 70-kDa polypeptide and that predicted for the >200-kDa polypeptide showed high sequence similarity to blowfly apolipophorin II and apolipophorin I, respectively. The RFABG precursor contains a signal peptide and exhibits a multidomain mosaic protein structure, which is typical of extracellular proteins. It has structural domains similar to lipid-binding proteins, namely vitellogenins and apolipoprotein B. The protein also contains a domain similar to the D domain of von Willebrand factor and mucin. The gene is expressed in the Drosophila embryo during development in cells that make up the amnioserosa and fat bodies. Immunolocalizations using specific antibodies against RFABG reveal that the protein is initially dispersed through the embryonic amnioserosa sac and latter concentrated at skeletal muscle-epidermis apodemeal contact junctions during larval development. This novel gene may play an important role in the transport of lipids, including retinoids and fatty acids, in insects.

Transforming growth factor-beta inhibits the cytokine-mediated expression of the inducible nitric oxide synthase mRNA in human retinal pigment epithelial cells

Human retinal pigment epithelial (RPE) cells in culture respond to a mixture of cytokines (IFN-gamma, IL-1 beta, TNF-alpha) by producing large amounts of nitric oxide. Transforming growth factor-beta, unlike other growth factors, was found to inhibit this response by more than 75%. The expression of mRNA for the inducible form of nitric oxide synthase in RPE cells treated with cytokines was demonstrated by reverse transcription-polymerase chain reaction, sequencing of the PCR product and northern blotting. Transforming growth factor-beta was highly effective in inhibiting (by 75%) the cytokine-induced nitric oxide synthase mRNA expression. This response by RPE may play an important role in the etiology of infectious and inflammatory diseases affecting retina.

Photoreceptor cells in the vitiligo mouse die by apoptosis. TRPM-2/clusterin expression is increased in the neural retina and in the retinal pigment epithelium

PURPOSE

To determine the mechanism of photoreceptor cell death in the vitiligo mouse, a model of retinal degeneration in which the genetic defect is not retina specific but is instead caused by single point mutation in the microphthalmia (mi) gene that codes for a basic helix-loop-helix DNA transcription factor.

METHODS

Detection of apoptotic cells was performed in fixed retinal tissue using the TUNEL assay in animals 1, 2, 4, 6, 8, 16, 32, 40, and 52 weeks. Electron microscopic analysis was used to confirm the morphologic hallmarks of apoptosis, and Southern blot analysis was used to detect internucleosomal DNA fragmentation. Additionally, the expression of a gene associated with apoptosis, TRPM-2/clusterin, was examined.

RESULTS

At ages beyond the time of normal retinal programmed cell death, vitiligo retinas had significantly more TUNEL-positive photoreceptor cells and more photoreceptor cells with condensed chromatin than controls. DNA internucleosomal fragmentation ladders were present in vitiligo retinas even as late as 15 weeks, a time well beyond developmental apoptosis in controls. TRPM-2/clusterin mRNA levels in vitiligo neural retinas were similar to controls initially but were two times greater than controls by 12 weeks. Surprisingly, TRPM-2/clusterin mRNA levels were elevated in the retinal pigment epithelium in the mutant; the expression at one week was two times greater than normals and remained elevated for many months, even though retinal pigment epithelial cells showed no morphologic evidence of apoptosis.

CONCLUSIONS

The morphologic and biochemical data suggest that photoreceptor cells die by apoptosis in vitiligo mice. The increased retinal TRPM-2/clusterin mRNA levels may be a direct response to these events. The increased expression of this gene in the retinal pigment epithelium, however, may reflect its role in tissue regression and membrane remodeling. Mechanisms by which the mi gene defect might result in the vitiligo retinopathy are proposed.

Induction of heme oxygenase 1 in the retina by intense visible light: suppression by the antioxidant dimethylthiourea

The effect of intense visible light (light damage) on the expression of heme oxygenase 1 (HO-1), a protein induced by oxidative stress, was investigated in the rat retina. A sensitive reverse transcription-PCR assay demonstrated the expression of mRNA for HO-1 as well as HO-2, the noninducible HO form, in the normal retina. As analyzed by Northern blotting, however, HO-1 mRNA was barely detectable under normal circumstances. After exposure to intense visible light, retinas had markedly higher HO-1 mRNA levels than unexposed controls, with increases up to 52- and 98-fold at 12 and 24 hr of exposure, respectively. Intense light exposure also resulted in an increase in HO-1 protein. In contrast, no appreciable change in HO-2 mRNA or protein was observed. The increase in HO-1 message was more pronounced in rats previously reared in the dark than in those reared in a weak cyclic-light environment. A marked decrease from the high level of HO-1 mRNA induced by light insult was observed when the animals were allowed to recover in the dark for 24 hr after light exposure. Most important, treatment of animals with 1,3-dimethylthiourea, a synthetic antioxidant, prior to light exposure effectively blocked the increase in HO-1 mRNA. Thus, HO-1 is a sensitive marker for assessing light-induced insult in the retina. Since increased expression of HO-1 is thought to be a cellular defense against oxidative damage, its expression may play an important role in protecting the retina against light damage.

An early decrease in interphotoreceptor retinoid-binding protein gene expression in Abyssinian cats homozygous for hereditary rod-cone degeneration

Levels of interphotoreceptor retinoid-binding protein (IRBP) protein and message in retinas of Abyssinian cats homozygous for progressive rod-cone degeneration were determined at early ages, well before the onset of clinical retinal degeneration. IRBP gene expression was assessed by immunochemical quantitation of IRBP protein, and by Northern blotting and slot-blotting of total RNA using a human IRBP cDNA probe. Morphology was assessed by electron microscopy and immunocytochemistry. Levels of both IRBP protein and message in affected Abyssinian cat retinas were significantly reduced below normal as early as 4 weeks of age at the earliest stage of retinal disorientation. Opsin mRNA was more abundant in affected Abyssinian cat retinas than in control retinas. This was at least 1 year before the onset of clinical symptoms. The reduction in IRBP gene expression to levels significantly below normal well before the onset of retinal degeneration in affected Abyssinian cat retinas indicates that this represents a primary defect or at least an early problem that could itself cause adverse effects.

Changes in clusterin expression associated with light-induced retinal damage in rats

Prolonged periods of high-intensity visible light exposure lead to photoreceptor cell degeneration, but the mechanism of damage is not understood. Increased clusterin mRNA levels have been found in several models of apoptosis, as well as in neurodegeneration. We report here that changes in clusterin mRNA levels are also associated with light-induced retinal damage in adult male albino rats. Animals previously maintained in darkness or a weak cyclic light environment were exposed to intense visible green light for up to 24 h. Some rats were pretreated with a synthetic antioxidant, dimethylthiourea (DMTU), which reduces photoreceptor cell degeneration. Clusterin mRNA steady-state levels increased with the duration of light exposure in both cyclic light and dark reared animals, suggesting that an apoptotic mechanism may be involved. Animals pretreated with DMTU showed a delay in the initial increase in clusterin mRNA levels, suggesting that oxidative damage is involved in the damage mechanism. However, the incomplete suppression of increasing steady-state clusterin mRNA levels by DMTU suggests that either oxidative damage triggers a second pathway or multiple damage mechanisms are induced in the retina by light exposure.

Expression of retina-specific genes by mouse retinoblastoma cells

PURPOSE

Two cell lines derived from ocular tumors of a transgenic mouse expressing the SV40 large T antigen have been established as models of human retinoblastoma. One line, TM, originated from a metastasis, and the other, TE, originated from the primary tumor. The authors compared these two lines with the normal adult mouse eye by analysis of the expression of five photoreceptor cell-specific proteins: IRBP, opsin, rod- and cone-specific transducins, and S-antigen. The authors sought to determine which of these proteins was expressed qualitatively and to examine semi-quantitatively for changes in the levels of expression in the cell lines.

METHOD

Western blot analysis was used to detect photoreceptor-specific intracellular or secreted proteins. Total RNA was prepared from cultured cells or from mouse adult whole eye. Specific messenger levels in total RNA were determined either by northern hybridization analysis or by a semi-quantitative polymerase chain reaction (PCR), coupled to complementary DNA (cDNA) substrates prepared from total RNA.

RESULTS

IRBP was present in the retinoblastoma cell lines and secreted into the medium. Neither S-antigen nor opsin were detectable by immunoblotting. IRBP and cone transducin mRNA were present in both cell lines. In contrast, opsin, rod transducin, and S-Antigen mRNAs were not detectable by PCR. beta-actin was present in the mRNA populations of whole eye and retinoblastoma. SV40 large T antigen mRNA was present only in retinoblastoma cells.

CONCLUSIONS

IRBP and cone transducin expression in mouse retinoblastoma cells is independent of signaling provided directly or indirectly through large T antigen or Rb105 regulatory cascades. The pattern of photoreceptor-specific gene expression is similar to that seen in human retinoblastoma cell lines. These murine-derived cell lines may be useful as a tool to study IRBP and cone transducin expression in vitro and to determine early retinoblast expression patterns in the mouse.

A glycoprotein binding retinoids and fatty acids is present in Drosophila

In the search for a possible Drosophila melanogaster homolog of interphotoreceptor retinoid-binding protein (IRBP), a approximately 140-kDa retinoid- and fatty acid-binding glycoprotein found in vertebrates, the 110,000 g supernatant fraction prepared from homogenates of fly heads was analyzed for the presence of proteins capable of binding radiolabeled retinol and palmitic acid. A soluble protein, which binds concanavalin A and has a retention time on size-exclusion high-performance liquid chromatography identical to that of purified bovine IRBP, was identified as binding both ligands. As assessed by fluorescence titration, the protein fraction obtained by concanavalin A-Sepharose affinity chromatography and size-exclusion chromatography of fly head supernatant had apparent dissociation constants of 2.9 x 10(-7) +/- 0.6 M for all-trans retinol, with the number (n) of independent ligand binding sites per protein molecule = 2, and 3.5 x 10(-7) +/- 0.1 M for 16-[9-anthroyloxy] palmitic acid with n = 7. High-performance liquid chromatography of hexane extracts of this protein fraction resolved several peaks with polarity and relative retention times similar, but not identical to all-trans retinol and retinal and their 9-, 11-, and 13-cis isomers. Gas chromatography/mass spectrometry analysis of fatty acid methyl esters prepared following lipid extraction of the protein identified lauric, myristic, palmitic, palmitoleic, and oleic acids as being covalently bound. Laurate, myristate, palmitate, and stearate were noncovalently bound. The apparent molecular mass of the Drosophila protein as assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and silver staining of the retinoid- and fatty acid-binding peak obtained by hydrophobic interaction chromatography of the size-exclusion fraction was approximately 70 kDa.

Increase in retinyl palmitate concentration in eyes and livers and the concentration of interphotoreceptor retinoid-binding protein in eyes of vitiligo mutant mice

Retinyl esters play an important role in the visual cycle because they are involved in regeneration of 11-cis-retinal for use in rhodopsin formation. In the present study, retinyl ester concentrations were significantly elevated in eyes and livers of mice homozygous for the vitiligo mutation (mivit/mivit). Vitiligo mice demonstrate a slowly progressing retinal degeneration characterized by gradual loss of photoreceptor cells and rhodopsin as well as uneven pigmentation of the retinal pigment epithelium (RPE). Analysis of retinoids by h.p.l.c. indicated that the retinyl palmitate level was increased fivefold in eyes of affected mice at 10 weeks postnatally and was threefold higher at 22 weeks of age. Accumulation of retinyl palmitate occurred in the RPE rather than the neural retina. Furthermore, the concentration of all-trans-retinol was elevated in the RPE of vitiligo mice. Levels of interphotoreceptor retinoid binding protein (IRBP) were increased in vitiligo mice between ages 4 and 14 weeks, but returned to normal by 16 weeks. Increased IRBP levels were not due to increased protein synthesis because IRBP mRNA levels did not differ significantly between control and affected animals. To examine possible systemic involvement in vitiligo mice, retinoids were evaluated in liver and plasma. Mean hepatic total vitamin A levels in affected mice were approximately 1.7 times higher than controls. Analysis of esterified and non-esterified retinoids in liver showed that the concentration of retinyl palmitate was elevated. Plasma retinol levels were normal. This study provides the first evidence of altered systemic retinoid metabolism in vitiligo mice, which occurs, significantly, under normal dietary conditions.

Increased expression of heme oxygenase-1 in human retinal pigment epithelial cells by transforming growth factor-beta

Antibodies specific for heme oxygenase-1 (HO-1) were produced in rabbits, using the multiple antigen peptide (MAP) technique, and were employed to investigate the ability of transforming growth factor-beta 1 (TGF-beta 1) to induce the HO-1 protein in cultured human retinal pigment epithelial (RPE) cells. Western blot analyses showed that the cytokine induced HO-1 in these cells in a time- and dose-dependent manner. TGF-beta 1 also increased the mRNA for HO-1 in treated cells prior to the increase in HO-1 protein. The induction was effectively blocked by a neutralizing antibody preparation against TGF-beta 1. When tested under similar conditions, other growth factors such as basic fibroblast growth factor-I, platelet-derived growth factor, insulin-like growth factor, transforming growth factor-alpha, and epidermal growth factor did not show appreciable induction of HO-1. Lipopolysaccharide, tumor necrosis factor-alpha, and interferon-gamma were also not inducers, although TGF-beta 2 effectively induced HO-1. Heavy metal ions and thiol reagents were also highly potent inducers of HO-1 in human RPE cells. The induction of HO-1 by TGF-beta 1 was also observed in bovine choroid fibroblasts, but not in HELA, HEL or bovine corneal fibroblasts. Our results demonstrate for the first time that HO-1 can be induced by an important cytokine, TGF-beta 1, causing an increase in the expression of both HO-1 message and protein in specific neuroepithelial and fibroblast cells.

Chromosomal localization of the human heme oxygenase genes: heme oxygenase-1 (HMOX1) maps to chromosome 22q12 and heme oxygenase-2 (HMOX2) maps to chromosome 16p13.3

Heme oxygenase catalyzes the oxidation of heme to biliverdin, the precursor of the bile pigment bilirubin, and carbon monoxide, a putative neurotransmitter. We have employed polymerase chain reaction and fluorescence in situ hybridization to determine the chromosome localization of the genes coding for the two known heme oxygenase isozymes. Heme oxygenase-1 (HMOX1), the inducible form, was localized to human chromosome 22q12, while heme oxygenase-2 (HMOX2), the constitutive form, was localized to chromosome 16p13.3.

Light deprivation profoundly affects gene expression of interphotoreceptor retinoid-binding protein in the mouse eye

Ambient light appears to play a role in regulating gene and protein expression of interphotoreceptor retinoid-binding protein (IRBP), a protein that facilitates the transport of retinoids between the neural retina and pigment epithelium in the visual cycle. Pregnant CD-1 mice were placed in the dark approximately 48 hr before parturition, and the pups were reared for 14 days under these conditions. Control animals were reared on a 12 hr light/12 hr dark cycle. Northern blotting of total RNA isolated from whole mouse eyes at post-natal days 7-14 (P7-P14) showed a marked reduction in IRBP message in the light-deprived animals to 10-20% of levels in control animals. Reprobing of the blots for opsin and S-antigen message showed a significant decrease of about 80-90% in opsin message at 5.1 kb but no change in S-antigen message in the eyes of light-deprived mice. Light microscopic examination of the light-deprived mouse retinas showed no apparent abnormalities in morphological development and immunocytochemistry demonstrated normal distribution and levels of IRBP protein. Immunochemical quantitation of IRBP protein confirmed that there was no reduction in light-deprived as compared to normal mouse eyes. Similarly, when adult mice were light-deprived for 14 days, a marked reduction in IRBP message was also observed with no decrease in the amount of IRBP protein. Thus, light deprivation causes a large decrease in IRBP message in the mouse eye, but IRBP protein is not decreased. The dramatic effect of light deprivation on IRBP mRNA and some opsin mRNAs, but not on S-antigen message and the fact that IRBP protein levels are relatively unaffected, suggest a complex pathway of light regulation of photoreceptor function previously not encountered. This may involve regulatory controls at levels including gene transcription, mRNA stability or protein degradation that may make use of a feedback control mechanism involving light- or dark-dependent signal transduction.

Heme oxygenase: expression in human retina and modulation by stress agents in a human retinoblastoma cell model system

PCR and Southern blot analyses demonstrate that mRNA for heme oxygenase (HO), a well known "stress protein" in a number of tissues, is present in human retina. Western and northern blots show that the protein and mRNA are also expressed in human Y-79 retinoblastoma cells in culture and that the HO enzyme is rapidly induced by its substrate, heme. Moreover, HO is also induced by two chemicals, sodium arsenite and menadione, that act as agents of oxidative stress. HO is the regulatory enzyme in the heme degradative pathway and an increase in its activity could lead to the accumulation of bilirubin, an antioxidant, in the cell at the expense of heme, a prooxidant. The HO pathway may thus be of importance in protecting the retina against oxidative stress in vivo. Moreover, the Y-79 culture system should provide an excellent model for use in examining stress mechanisms in retinal cells at a molecular level.

Redistribution and reduction of interphotoreceptor retinoid-binding protein during ocular coronavirus infection

Inoculation of the neurotropic coronavirus mouse hepatitis virus strain JHM intravitreally or into the anterior chamber causes acute infection of the retinal pigment epithelium (RPE) and neural retina. Weeks later, many retinas have foci of moderate to severe atrophy. The effect of coronavirus infection (after intravitreal inoculation) was examined on interphotoreceptor retinoid-binding protein (IRBP), the glycolipoprotein in the interphotoreceptor matrix (IPM) thought to transport retinoids between the photoreceptors and the RPE. Changes in IRBP distribution accompanied virus-associated retinal pathology, including photoreceptor loss and RPE abnormalities. Immunohistochemistry on days 3 and 6 showed that IRBP had diffused into the neural retina away from the IPM. The IRBP became localized abnormally in the same areas as virus-induced lesions, shown by staining adjacent sections with a monoclonal antibody specific for the viral nucleocapsid protein. Moreover, the level of IRBP in isolated retinas, measured in an immunoslot-blot assay, decreased significantly by day 3 and remained low through day 23. This decrease was confirmed in eyecups isolated on day 6. It may be caused in part by loss of photoreceptors and diffusion of IRBP through the retina into the vitreous. These studies show that a virus may induce an acute, limited infection in the retina that can be cleared by the host. However, the infection initiated a series of events resulting in long-term reduction and redistribution of a critical photoreceptor protein.

Interphotoreceptor retinoid-binding protein (IRBP) in progressive rod-cone degeneration (prcd)--biochemical, immunocytochemical and immunologic studies

Interphotoreceptor retinoid-binding protein (IRBP) is synthesized and secreted by photoreceptor cells and is thought to facilitate the transport of retinoids during the visual cycle as well as fatty acids essential to the maintenance of normal outer segment membranes. Proteins such as IRBP, which are unique to the photoreceptor cells in the retina, are prime candidates in the consideration of biochemical defects which could contribute to photoreceptor cell degeneration in man and animals. In this study, the association between IRBP and retinal degeneration was examined using the progressive rod-cone degeneration (prcd) mutant retina in dogs as an animal model. This study shows that loss of IRBP is not an early occurrence in prcd. IRBP is present in relatively normal amounts and distribution even at 1.7 years of age, a time when there is extensive visual cell disease and degeneration. By 2.7-3.0 years of age, IRBP loss correlates with the severity of the disease and concomitant loss of photoreceptor cells. IRBP immunoreactivity was present in the interphotoreceptor matrix (IPM) as long as inner segments were present to a significant degree. The late loss of IRBP immunoreactivity seems to be, therefore, the result of advanced degeneration and end-stage atrophy of the retina. In addition, immunological studies were carried out in order to examine the possible role of an autoimmune response against IRBP in the disease cascade. Normal, heterozygote and prcd-affected dogs had measurable antibody titers to IRBP, but there was no correlation between disease state and antibody levels.