Multiple opsin mRNA species in bovine retina

Scallop genome reveals molecular adaptations to semi-sessile life and neurotoxins

Bivalve molluscs are descendants of an early-Cambrian lineage superbly adapted to benthic filter feeding. Adaptations in form and behavior are well recognized, but the underlying molecular mechanisms are largely unknown. Here, we investigate the genome, various transcriptomes, and proteomes of the scallop Chlamys farreri, a semi-sessile bivalve with well-developed adductor muscle, sophisticated eyes, and remarkable neurotoxin resistance. The scallop's large striated muscle is energy-dynamic but not fully differentiated from smooth muscle. Its eyes are supported by highly diverse, intronless opsins expanded by retroposition for broadened spectral sensitivity. Rapid byssal secretion is enabled by a specialized foot and multiple proteins including expanded tyrosinases. The scallop uses hepatopancreas to accumulate neurotoxins and kidney to transform to high-toxicity forms through expanded sulfotransferases, probably as deterrence against predation, while it achieves neurotoxin resistance through point mutations in sodium channels. These findings suggest that expansion and mutation of those genes may have profound effects on scallop's phenotype and adaptation.

Intrauterine growth retardation alters mitochondrial gene expression and function in fetal and juvenile rat skeletal muscle

Uteroplacental insufficiency alters the anabolic metabolism of the fetus, resulting in intrauterine growth retardation (IUGR). The metabolic and physiologic factors that cause IUGR have long standing consequences after birth. Postnatal growth and glucose metabolism are altered in the IUGR infant. Skeletal muscle is an important component of growth and metabolizes up to 70% of i.v. glucose. The ability of skeletal muscle to metabolize glucose is affected by ATP availability. We hypothesized that gene expression and function of proteins involved in mitochondrial ATP production and distribution would be altered in juvenile IUGR muscle. To test this hypothesis, we used a model of IUGR, induced by bilateral uterine artery ligation in the pregnant rat, that mimics uteroplacental insufficiency in the human. RT-PCR was used to measure the mRNA levels of three important mitochondrial proteins; NADH-ubiquinone-oxireductase subunit 4L(ND-4L), subunit C of the F1F0-ATP synthase (SUC), and adenine nucleotide translocator 1 (ANT1) in IUGR and control rats in fetal and juvenile life. In the fetus, mRNA levels of all three proteins were significantly increased in IUGR skeletal muscle. In contrast, in juvenile animals, mRNA levels of all three proteins were significantly decreased. mRNA levels of other metabolically important proteins, glucose-6-phosphate dehydrogenase and carnitine-palmitoyl-transferase II, were not significantly altered in IUGR juvenile animals. To assess if decreased gene expression is associated with altered mitochondrial function, we measured the mitochondrial NAD+/NADH ratio in d 21 juvenile control and IUGR muscle. At d 21, decreased gene expression if ND-4L, SUC, and ANTI is associated with a decreased mitochondrial NAD+/NADH ratio. The results of our study suggest that the metabolic alterations associated with uteroplacental insufficiency in the rat result in altered fetal and postnatal muscle mitochondrial mRNA expression as well as altered postnatal mitochondrial function.

Membrane topology and retention of microsomal aldehyde dehydrogenase in the endoplasmic reticulum

Microsomal aldehyde dehydrogenase (msALDH) is anchored to the endoplasmic reticulum (ER) membrane by the hydrophobic domain at its carboxyl terminus, and most of the molecule is exposed to the cytoplasm (Masaki, R., Yamamoto, A., and Tashiro, Y.(1994) J. Cell Biol. 126, 1407-1420). To determine the membrane topology and the intracellular localization of msALDH, the amino-terminal region of bovine opsin containing N-glycosylation sites was fused to the carboxyl terminus of msALDH, and three chimeric proteins with extensions of different sizes were expressed in COS cells. Indirect immunofluorescence microscopy showed the ER localization of all of the chimeric proteins similar to wild-type msALDH. Immunoblotting revealed that the two chimeric proteins containing longer extensions, those with the N-glycosylation site at distances of 13 and 21 amino acids from the membrane anchor, respectively, were glycosylated. These results indicate that the membrane binding domain of msALDH spans the bilayer of the ER. The carbohydrate chain of the chimeras was sensitive to endoglycosidase H but resistant to endoglycosidase D. Upon treatment of transfected COS cells with brefeldin A, the carbohydrate chain was processed to an endoglycosidase H-resistant form, presumably by cis/medial Golgi-specific enzymes redistributed in the ER. These biochemical results in addition to immunofluorescence microscopic observations suggest that msALDH is retained in the ER by blockading of the exit from the ER.

Localization and DNA sequence of a replication origin in the rhodopsin gene locus of Chinese hamster cells

A chromosomal origin of DNA replication has been localized within the single-copy rhodopsin gene locus in Chinese hamster (line CHO) cells using two methods. In the first method, single-copy segments were identified at 3 to 15 kb intervals within approximately 75 kb (kb = 10(3) bases) of cloned genomic DNA containing the early-replicating rhodopsin gene near its middle. The cloned single-copy segments were then used as hybridization probes to quantify the replication of their corresponding genomic segments as synchronized cells progressed into S phase. In the second method, genomic DNA synthesized in vivo or in permeabilized early S phase cells was hybridized with slot-blots of the cloned single-copy DNA segments to identify the earliest replicating part of the 75 kb mapped region. The first method indicates that the earliest replicating DNA is located within a 10 kb region beginning 4 kb upstream from and extending 1 kb beyond the rhodopsin gene. The second method confirms the location in the vicinity of the rhodopsin gene and indicates that the earliest replicating region is located within or very near the 4.5 kb rhodopsin gene itself. An extended region of 12 kb that encompasses the entire early-replicating region has been sequenced for analysis and comparison with currently characterized origin regions associated with the CHO dihydrofolate reductase (dhfr) and human c-myc genes. There are several sequence similarities between the dhfr rhodopsin origin regions, including common transcription promoter consensus sequences, rodent Alu repeats with their 3'-A+T rich flanking sequences, A+T-rich yeast ARS and Drosophila SAR consensus sequences, and simple (GA)n repeats, but there are no extended regions of direct similarity. The rhodopsin gene locus is the second sequenced CHO origin region.

Isolation and analysis of the human MEKA gene encoding a retina-specific protein

We have isolated and sequenced a human gene encoding photoreceptor cell-specific MEKA protein. The protein coding region was separated into three exons, which encoded 246 amino acid residues with a molecular weight of 28,311. The coding region of the human gene exhibited the homologies of 90.7% nucleotides and 88.5% amino acids with those of the bovine cDNA. Southern blot analysis revealed that the human MEKA gene has a single copy number. However, the anti-bovine MEKA stained both rod and cone photoreceptor cells in the human retina.

Isolation of a specific cellular mRNA by subtractive hybridization in Theiler's virus persistent infection

Viruses change the mRNA repertoire of the tissues they infect. They add viral mRNAs and they specifically alter the expression of some host genes. These events can play important parts in pathogenesis. In principle, it should be possible to isolate viral mRNAs and to identify changes in host gene expression using subtractive hybridization. We tested this approach in the persistent infection of mouse central nervous system by Theiler's virus. A cDNA library was constructed with poly A+ RNA from infected mouse spinal cords. The library was screened with a subtracted probe. We identified one mitochondrial gene, coding for subunit 1 of cytochrome oxidase, which is overexpressed in infected tissues whereas another mitochondrial gene, URF 2, is not. Subtractive hybridization should prove to be invaluable in studying the pathogenesis of chronic human central nervous system diseases of unknown etiology.

Visinin: a novel calcium binding protein expressed in retinal cone cells

Visinin is a retinal cone cell-specific protein (molecular weight 24,000, pI 5.1). To investigate its function, visinin cDNA was isolated from a chick retinal lambda gt11 cDNA library, using anti-visinin serum. The beta-galactosidase-visinin fusion protein was used for purifying epitope-selected antibody. The purified visinin antibody reacted only with a 24 kd protein in retinal cone cells. Visinin mRNA was expressed only in the retinal photoreceptor layer. The nucleotide sequence of the cDNA revealed that visinin has three E-F hand structures and is a Ca2+ binding protein. Visinin protein expressed in E. coli exhibited Ca2+ binding activity. These results suggest that visinin is a photoreceptor-specific Ca2+ binding protein and may be involved in phototransduction in the cone cells.

Developmental changes of MEKA protein and opsin in normal and rd mice

Antisera raised against photoreceptor-specific MEKA and opsin proteins were provided for immunohistochemical studies on the retinas of normal (BJ57BL/6) and retinal degeneration (rd, C3H/He) mice. The expression of MEKA protein began at postnatal day 6 (P6) in the photoreceptor cells and gradually increased until P10 in both normal and rd mice. Thereafter the MEKA proteins in the photoreceptor cells of rd mice gradually decreased and disappeared at P18, whereas those of normal mice were increasing until P18. The time course of MEKA proteins in the photoreceptor cells of normal and rd mice was almost similar to that of opsin protein, except that the MEKA protein disappeared earlier than opsin.

Identification of a retina-specific MEKA protein as a 33 K protein

A photoreceptor-specific MEKA protein was purified from bovine retinal soluble fraction. The purified sample was eluted as a single peak of 74 kDa protein from a Superose column, which was dissolved into three components, MEKA protein (32 kDa), beta-(36 kDa) and gamma-(10 kDa) subunits of transducin on a SDS-PAGE. From several lines of evidence, we concluded that MEKA protein is identical with a 33k phosphoprotein reported by Lee et al (1).

Isolation of a novel retina-specific clone (MEKA cDNA) encoding a photoreceptor soluble protein

We have reported the isolation of clones which are candidates for retina-specific cDNAs. One of the cDNA clones, pCR-470, was further characterized. We found that mRNA corresponding to the pCR-470 was expressed only in the retina and encodes an unknown soluble protein whose molecular weight and pI are calculated to be 26,935 and 5.35, respectively. We designated it as a MEKA protein, because its amino acid sequence starts from M-E-K-A. It was found by in situ hybridization that MEKA mRNA was transcribed only in the photoreceptor cells and accumulated in the inner segments just like opsin mRNA. The MEKA cDNA was ligated with expression vector PEX 1, and a MEKA-fusion protein synthesized in E. coli was purified and used as an antigen. By the Western blot analysis anti-MEKA protein serum reacted with a soluble 32 kDa protein from bovine retina and 33 kDa for chick, but not with proteins from other tissues. Immunohistochemical study showed that anti-MEKA stained only the photoreceptor cells in bovine, chick, rat and mouse retinas.

Expression of a bovine rhodopsin gene in Xenopus oocytes: demonstration of light-dependent ionic currents

Xenopus oocytes express a gene encoding bovine rhodopsin as well as its SP6 RNA polymerase-derived transcripts and total retinal mRNA. The opsin produced is in unglycosylated (30 kDa) and two glycosylated (35 kDa and 41 kDa) forms. Incubation of the cells expressing the above proteins with 11-cis-retinal generates rhodopsin, which was purified by immunoaffinity chromatography. The purified protein shows the expected UV/visible absorption spectrum and characteristic light-dependent activation of the rod outer segment GTP binding protein. Oocytes expressing rhodopsin exhibit light-dependent ionic currents that are detected by voltage-clamp techniques.

Two types of lamprey retina photoreceptors immunoreactive to rod- or cone-specific antibodies

The localization of structures immunoreactive to antisera against opsin (OPS; a specific marker for rods) and against visinin (VIS; a specific marker for cones in the vertebrate retina) was investigated in the retina of the river lamprey (Lampetra japonica) by means of fluorescence microscopy. Different immunoreactive structures were found with the two antisera. A vitreadly located structure in the outer nuclear layer displayed OPS-like immunoreactivity, while a scleradly located one exhibited VIS-like reactivity. Thus, the two antisera applied to the retina of the river lamprey permit the distinguishing of rod and cone immunoreactive photoreceptors such as occurs typically in all vertebrate classes. The former appears in the outer segments of a type of rod cells, and the latter in the cell bodies and axons of cones.