Transthyretin (prealbumin) gene expression in choroid plexus is strongly conserved during evolution of vertebrates

1. The major protein synthesized and secreted by the choroid plexus from mammals, birds, reptiles and probably amphibians is similar in subunit structure to transthyretin. 2. In mammals and birds the proportion of transthyretin mRNA is much higher in choroid plexus RNA than in liver RNA. No transthyretin mRNA is found in brain outside the choroid plexus. 3. Transthyretin-like protein, such as that secreted by the choroid plexus, was not detected in amphibian serum and was present in very low levels in reptile serum. 4. It is proposed that transthyretin synthesis and secretion arose earlier in evolution in the choroid plexus than in the liver.     

The distribution of cerebral expression of the transferrin gene is species specific.

Various plasma proteins, for example, transferrin, are synthesized not only in the liver, but also in the brain. The proportion of transferrin mRNA in total RNA from different regions of brains from various mammalian species was studied by Northern blot analysis. Absolute amounts of transferrin mRNA were determined in brain, choroid plexus, and liver from rats, sheep, and pigs by hybridization in solution followed by ribonuclease protection assay. Corrections for differences in yields of RNA were made using internal RNA standards. Large proportions of transferrin mRNA in total RNA and high absolute levels of transferrin mRNA in choroid plexus were found only in rats. Small proportions of transferrin mRNA were observed in RNA from choroid plexus from mice, dogs, and rabbits, while no transferrin mRNA at all was detected in choroid plexus from humans, sheep, pigs, cows, and guinea pigs. In further analysis of sheep and pigs, various amounts of transferrin mRNA were found in many parts of the brain, in contrast to the absence of transferrin mRNA from choroid plexus. In conclusion, a striking species specificity was observed for the pattern of cerebral expression of the transferrin gene.     

Species specificity and developmental patterns of expression of the beta amyloid precursor protein (APP) gene in brain, liver and choroid plexus in birds.

1. Human APP cDNA hybridized to a 3.5 kb mRNA in liver and brain RNA from chickens, pigeons, quail and ducks as well as in RNA from choroid plexus of chicken and quail. In contrast to all other species hitherto examined a 1.6 kb mRNA hybridizing to APP cDNA was found in abundant amounts in RNA from chicken and quail livers. 2. In the chicken, before hatching, the levels of APP mRNA in total RNA from liver and choroid plexus were higher than those in RNA from liver and choroid plexus of adults. However, RNA from the rest of the brain of chicken embryos contained less APP mRNA than RNA from brain of adults. 3. In the chicken, between 10 and 40 days after hatching, APP mRNA levels in RNA from liver were higher than adult levels, APP mRNA levels in RNA from choroid plexus were similar to adult levels and APP mRNA levels in RNA from the rest of brain were below the adult levels.     

Localization of immunoreactive transthyretin (prealbumin) and of transthyretin mRNA in fetal and adult rat brain

We used a combination of immunohistochemical and molecular-biological techniques to investigate the localization of transthyretin (TTR) in the brains of adult and fetal rats. The immunohistochemical studies employed antibodies purified by immunosorbent affinity chromatography, permitting the specific staining and localization of TTR using the unlabeled peroxidase-antiperoxidase method. TTR mRNA levels were measured by Northern-blot analysis of poly (A+) RNA, followed by hybridization to 32P-labeled TTR cDNA; TTR mRNA was localized in brain tissue sections by in situ hybridization. Immunoreactive TTR was found to be specifically localized in the choroid plexus epithelial cells of adult rat brain. High levels of TTR mRNA were found in poly (A+) RNA samples obtained from the choroid plexus. In addition, the specific localization of TTR mRNA in the epithelial cells of the choroid plexus was demonstrated by in situ hybridization. Neither immunoreactive TTR nor TTR mRNA were found in other regions of adult rat brains. The levels of TTR mRNA in the choroid plexus were at least 30 times higher than those observed in the adult liver. Immunoreactive TTR was observed in the brains of fetal rats on as early as the 11th day of gestation. This immunoreactive TTR was localized in the tela choroidea, the developmental forerunner of the choroid plexus. Immunoreactive TTR was also observed in the fetal choroid plexus as it began to form (14th day of gestation) as well as in the more completely developed choroid plexus (18th day of gestation).(ABSTRACT TRUNCATED AT 250 WORDS)     

Ontogenesis of transthyretin gene expression in chicken choroid plexus and liver.

1. Chicken liver transthyretin cDNA hybridizes strongly with choroid plexus transthyretin mRNA from chickens, pigeons, quails and ducks. 2. In the chicken at hatching the choroid plexus has reached 70%, total brain 30%, and liver 5.8% of their organ masses in adults. 3. The proportion of transthyretin mRNA in total RNA is 0.45-times the adult value in the choroid plexus of the chicken at hatching. 4. In the liver at hatching, the proportion of transthyretin mRNA in total RNA is 1.1-times the value in adult chickens. 5. The pattern of maturation of transthyretin gene expression in chicken liver is comparable to that in precocial, but differs from that in altricial mammals.     

High prealbumin and transferrin mRNA levels in the choroid plexus of rat brain

Expression of plasma protein genes in various parts of the rat brain was studied by hybridizing radioactive cDNA to RNA in cytoplasmic extracts. No mRNA could be detected in brain for the beta subunit of fibrinogen, major acute phase alpha 1-protein, alpha 1-acid glycoprotein and albumin. However, per g tissue, the choroid plexus contained at least 100 times larger amounts of prealbumin mRNA than the liver and about the same amount of transferrin mRNA as liver. No prealbumin mRNA was found in other areas of the brain. The results obtained suggest very active synthesis of prealbumin in choroid plexus, which would be an important link in the transport of thyroid hormones from the blood to the brain via the cerebrospinal fluid.     

Rat choroid plexus specializes in the synthesis and the secretion of transthyretin (prealbumin). Regulation of transthyretin synthesis in choroid plexus is independent from that in liver

Synthesis of total protein and of transthyretin in rat choroid plexus was studied by measuring the incorporation of radioactive leucine into proteins in choroid plexus tissue incubated in vitro. About 20% of the protein newly synthesized in choroid plexus and about 50% of the newly synthesized protein secreted into the medium was transthyretin. Evidently, the choroid plexus is very active in the biosynthesis of this carrier protein for thyroid hormones and could be an important link in the chemical communication between the body and the central nervous system. Acute inflammation, which leads to a profound rearrangement of the pattern of plasma protein synthesis rates in the liver, produced distinct changes in the levels for plasma protein mRNAs in the liver. The levels of the mRNAs for alpha 1-acid glycoprotein and major acute phase alpha 1-protein increased more than 30-fold, those for transthyretin and albumin decreased to 27 and 57% of normal, respectively. The pattern of the observed changes in the levels of mRNAs for plasma proteins in the liver was independent of whether the acute inflammation was produced by subcutaneous injection of turpentine or intraperitoneal injection of a suspension of talcum. However, levels of transthyretin mRNA in choroid plexus were affected only very slightly, or not at all. Apparently, transthyretin synthesis in liver and choroid plexus is regulated independently during the acute phase response. No mRNA was detected in choroid plexus for albumin, alpha 1-acid glycoprotein, and major acute phase alpha 1-protein under any conditions.     

Effects of the extracellular matrix on fetal choroid plexus epithelial cells: changes in morphology and multicellular organization do not affect gene expression.

We have developed a primary culture system for fetal mouse choroid plexus epithelial cells which maintains their differentiated phenotype. When grown on a reconstituted basement membrane substrate (Matrigel) epithelial cells formed aggregates which became embedded in the matrix and developed into characteristic and highly reproducible multicellular vesicular structures. These vesicles consisted of a squamous layer of epithelial cells with extensive attachment to the matrix substrate, surrounding a fluid-filled lumen. Electron microscopy showed that cells comprising these vesicles had a high degree of membrane specialization and polarized morphology which in many respects mimicked the in vivo morphology. Biochemical analyses demonstrated that under these culture conditions the tissue-specific pattern of gene expression of fetal choroid plexus epithelium was maintained. After 6 days in culture these cells contained approximately the same amount of transthyretin mRNA as the 12.5-day choroid plexus in vivo, and the level of total RNA per cell, which is proportional to the protein synthetic capability of the cells, was also maintained. The pattern of protein secretion was also very similar to that generated by fetal mouse choroid plexus cells in vivo. In contrast choroid plexus epithelial cells attached poorly to collagen I gels. Heterogeneous aggregates were formed in which cell-cell interactions were more extensive than cell-substrate interactions, and in no cases was a central lumen observed. Cells on the surface of large aggregates showed some evidence of membrane polarization, while the majority of cells in the cultures exhibited little evidence of polarized morphology. Despite the striking difference in morphology and multicellular organization these cells still expressed high levels of transthyretin mRNA and maintained the same pattern of protein synthesis as cells cultured on Matrigel. These results indicate that the basement membrane is important for the organization of choroid plexus epithelial cells into a functional epithelium in vitro and thus presumably the maintenance of the integrity of the blood-brain barrier in vivo. In contrast to several other epithelial systems which have been studied, the type of extracellular matrix does not appear to directly influence tissue-specific gene expression by choroid plexus epithelial cells. Thus the level of gene expression is not dependent on the cytoarchitecture and multicellular organization of this cell type.     

Retinal-specific ATP-binding cassette transporter (ABCR/ABCA4) is expressed at the choroid plexus in rat brain

ATP-binding cassette (ABC) transporter A4 is a member of the ABC transporter subfamily A which has been reported to be exclusively expressed in the retina. In contrast, a previous report has suggested a possible relationship between ABCA4 and CNS function. The purpose of the present study was to investigate the localization of ABCA4 mRNA and protein in rat brain. In situ hybridization analysis revealed that ABCA4 mRNA was localized in the lateral ventricles. RT-PCR analysis detected ABCA4 mRNA in isolated rat choroid plexus and conditionally immortalized rat choroid plexus epithelial cells (TR-CSFB). Furthermore, ABCA4 protein was also detected in the isolated rat choroid plexus at about 250 kDa by western blot analysis, and its apparent molecular size was reduced by N-glycosidase F treatment. These results suggest that glycosylated ABCA4 protein is expressed in rat choroid plexus epithelial cells. ABCA4 may play a role in the function of the blood-cerebrospinal fluid barrier and affect CSF conditions.     

Production and Secretion of Adrenomedullin by Cultured Choroid Plexus Carcinoma Cells

Abstract: Adrenomedullin is a potent vasodilator peptide that was originally isolated from pheochromocytoma. The production and secretion of adrenomedullin by cultured choroid plexus carcinoma cells were studied by radioimmunoassay and northern blot hybridization. Choroid plexus carcinoma is a rare malignant tumor derived from the epithelium of the choroid plexus. Immunoreactive adrenomedullin was detected in the conditioned medium of choroid plexus carcinoma cells (40.8 ± 7.5 fmol/10⁵ cells/24 h; mean ± SEM, n = 5). Reverse-phase HPLC of the conditioned medium showed one major peak of the immunoreactive peptide eluting in the position of synthetic human adrenomedullin and two smaller peaks eluting earlier. Addition of interleukin-1β (10 ng/ml) alone or in combination with three cytokines, interferon-γ (100 U/ml), tumor necrosis factor-α (20 ng/ml), and interleukin-1β (10 ng/ml), caused significant increases in the immunoreactive adrenomedullin concentrations in the medium (∼175 and 293% of the control level, respectively). Northern blot analysis showed the expression of 1.6-kb adrenomedullin mRNA in the total RNA sample prepared from cultured choroid plexus carcinoma cells. Treatment with either interleukin-1β or the combination of three cytokines caused significant increases in levels of adrenomedullin mRNA in parallel with those in immunoreactive adrenomedullin concentrations in the conditioned medium. These findings raise a possibility that adrenomedullin is secreted from the choroid plexus and has physiological roles in the CNS via the CSF. In addition, adrenomedullin secreted from choroid plexus carcinoma may be related to the pathophysiology of the tumor.     

The cDNA structure and expression analysis of the genes for the cysteine proteinase inhibitor cystatin C and for beta 2-microglobulin in rat brain

Tissue patterns of gene expression were analyzed by measuring mRNA levels and incorporation of radioactive amino acids for cystatin C and beta 2-microglobulin, the two extracellular proteins in the brain with the highest ratio of concentration in cerebrospinal fluid over that in blood plasma. The primary structure of rat cystatin C mRNA from choroid plexus was determined by nucleotide sequencing of cloned cDNA and the tissue patterns of gene expression were analysed by RNA blot analysis and in situ hybridization. Cystatin C was found to be composed of 120 amino acids and to contain a potential site for N-linked glycosylation. The tissue with the highest cystatin C mRNA level was the choroid plexus of the brain. Cystatin C mRNA was also detected in lower levels in other areas of the brain, testis, epididymis, seminal vesicles, prostate, ovary, submandibular gland, and, in trace amounts, in liver. Choroid plexus pieces in culture secreted radioactive cystatin C when incubated with radioactive leucine. Rat beta 2-microglobulin cDNA was cloned and identified by nucleotide sequencing and comparison of the obtained sequence with that of mouse and human beta 2-microglobulin cDNA. Tissue levels of beta 2-microglobulin mRNA in the rat were measured by hybridization to rat beta 2-microglobulin cDNA. The highest levels of beta 2-microglobulin mRNA were observed in liver and choroid plexus. Other parts of the brain and testis contained lower levels of beta 2-microglobulin mRNA.     

Cellular gene expression in papillomas of the choroid plexus from transgenic mice that express the simian virus 40 large T antigen.

Transgenic mice that contain the simian virus 40 (SV40) enhancer-promoter and large tumor (T) antigen gene develop papillomas of the choroid plexus. The tumors remain well differentiated on histological examination and express normal levels of tissue-specific mRNAs for transthyretin (TTR) and the 5-HT1C serotonin receptor, two differentiated cell markers. Both Northern (RNA) blot analysis and in situ cytohybridization have been used to monitor the steady-state levels of the mRNAs from the viral oncogene (T antigen) and from several cellular oncogenes. In situ hybridization demonstrated, in serial sections, increased levels of both T antigen mRNA and p53 mRNA localized in the tumor tissue but not in the normal brain tissue. The ratios of the steady-state levels of mRNA for p53/TTR and p53/L32, a ribosomal protein gene, were 2- to 20-fold higher in the tumor tissue than in the normal choroid plexus tissue. Several other oncogenes did not show elevated levels of mRNA in these tumors. p53 protein levels were not detectable in normal brain tissue, but p53 levels were very high in tumor tissue in which all of the p53 was found in a complex with the SV40 large T antigen. These data continue to show a close relationship between SV40 T-antigen-mediated tumorigenesis and the role of p53 in these tumors.     

Regional Distribution and Relative Abundance of Serotonin2c Receptors in Human Brain: Effect of Suicide

Abnormalities in serotonin receptor subtypes have been observed in the postmortem brain of suicide victims. We examined the regional distribution of serotonin (5HT)(2C) receptor mRNA in several areas of the human brain and also compared its protein and mRNA expression in the prefrontal cortex (PFC), hippocampus, and choroid plexus between suicide victims and normal control subjects. 5HT(2C) receptors were found to be distributed in several areas of the human brain (in order of abundance): highly concentrated and richest in choroid plexus; hypothalamus; nucleus accumbens; with the lowest abundance in PFC and cerebellum. Comparison of 5HT(2C) receptors between suicide victims and control subjects showed higher protein levels in the PFC but not the hippocampus or choroid plexus of suicide victims. However, there were no significant differences in mRNA levels between suicide victims and control subjects in these brain areas. These results suggest that 5HT(2C) receptors are richly distributed throughout the brain with the highest level in the choroid plexus and that abnormalities in protein expression of 5HT(2C) receptors in the PFC may be associated with suicide.     

Rat transthyretin (prealbumin). Molecular cloning, nucleotide sequence, and gene expression in liver and brain

Transthyretin cDNA was isolated from a rat liver cDNA library. Analysis of the nucleotide sequence revealed a signal peptide-like sequence preceding a section coding for a full length subunit and an untranslated sequence at the 3' end. The deduced primary structure of rat transthyretin was compared with that of human transthyretin. It was highly conserved at the binding sites for thyroxine and the interfaces and core regions of the subunits. The cDNA for transthyretin was used to measure mRNA levels by hybridization. During acute inflammation, the amount of transthyretin mRNA in liver decreased (reaching a minimum of 25% of the normal level 36 h after inducing inflammation), suggesting regulation of transthyretin synthesis at the mRNA level. Transthyretin mRNA was found only in the liver and in the choroid plexus, but not in other parts of the central nervous system nor in the adrenal glands, kidney, spleen, testes, heart, lung, intestine, and ovaries. One gram of choroid plexus contained about 25 times larger amounts of transthyretin mRNA than 1 g of liver. By synthesizing an important hormone carrier protein, the choroid plexus may be an important link in the chemical communication between the central nervous system and the bloodstream.     

Lycopene mitigates β-amyloid induced inflammatory response and inhibits NF-κB signaling at the choroid plexus in early stages of Alzheimer’s disease rats

The choroid plexus is able to modulate the cognitive function, through changes in the neuroinflammatory response and in brain immune surveillance. However, whether lycopene is involved in inflammatory responses at the choroid plexus in the early stages of Alzheimer's disease, and its molecular underpinnings are elusive. In this rat study, lycopene was used to investigate its protective effects on inflammation caused by β-amyloid. We characterized the learning and memory abilities, cytokine profiles of circulating TNF-α, IL-1β and IL-6β in the serum and the expressions of Toll like receptor 4 and nuclear factor-κB p65 mRNA and protein at the choroid plexus. The results showed that functional deficits of learning and memory in lycopene treatment groups were significantly improved compared to the control group without lycopene treatment in water maze test. The levels of serum TNF-α, IL-1β and IL-6β were significantly increased, and the expressions of TLR4 and NF-κB p65 mRNA and protein at the choroid plexus were up-regulated, indicating inflammation response was initiated following administration of Aβ_1–42. After intragastric pretreatment with lycopene, inflammatory cytokines were significantly reduced and lycopene also reversed the Aβ_1–42 induced up-regulation of TLR4 and NF-κB p65 mRNA and protein expressions at the choroid plexus. These results provided a novel evidence that lycopene significantly improved cognitive deficits and were accompanied by the attenuation of inflammatory injury via blocking the activation of NF-κB p65 and TLR4 expressions and production of cytokines, thereby endorsing its usefulness for diminishing β-amyloid deposition in the hippocampus tissues.     

The fetal rat binding protein for insulin-like growth factors is expressed in the choroid plexus and cerebrospinal fluid of adult rats

The biological effects of the insulin-like growth factors, IGF-I and IGF-II, on their receptors are modulated by IGF-binding proteins. Recently, we isolated a cDNA clone for one member of the family of IGF-binding proteins, BP-3A, a 30 kilodalton (kDa) protein synthesized by the BRL-3A rat liver cell line. BP-3A is related to but distinct from two other cloned IGF-binding proteins, the human amniotic fluid binding protein and the glycosylated binding subunit of the 150 kDa IGF-binding protein complex in serum. It is expressed in multiple nonneural tissues and in serum in the fetal rat and decreases after birth, similar to the developmental pattern of IGF-II expression. IGF-I, IGF-II, and their receptors are expressed in brain. The present study examines the expression of BP-3A in the rat central nervous system. By Northern blot analysis, BP-3A mRNA is present at high levels in brain stem, cerebral cortex, and hypothalamus from 21-day gestation rats and, like IGF-II mRNA, persists in adult rat brain. The site of BP-3A mRNA synthesis was localized by in situ hybridization to coronal sections of adult rat brain using 35S-labeled oligonucleotides, 48 bases in length, complementary and anticomplementary to the coding region of BP-3A. Specific hybridization of the BP-3A probe was observed exclusively to the choroid plexus extending from the level of the medial preoptic nucleus to the arcuate nucleus of the hypothalamus, similar to the previously reported preferential localization of IGF-II mRNA to the choroid plexus. Synthesis of BP-3A mRNA by choroid plexus suggested that BP-3A might be secreted into the cerebrospinal fluid. A 30 kDa IGF-binding protein was demonstrated in rat cerebrospinal fluid that is recognized by antibodies to BP-3A and, like purified BP-3A, has equal affinity for IGF-I and IGF-II. By analogy with other transport proteins synthesized by the choroid plexus, BP-3A may facilitate the secretion of IGF-II to the cerebrospinal fluid and modulate its biological actions at distant sites within the brain.     

Distribution of transferrin synthesis in brain and other tissues in the rat

Levels of transferrin mRNA were measured by hybridization to transferrin cDNA in extracts from various areas of rat brain and other tissues. The highest concentrations of transferrin mRNA were found in the liver and the choroid plexus of the lateral and third ventricles. Lower concentrations were observed in the medulla and thalamus, choroid plexus of the fourth ventricle, cortex, hypothalamus, cerebellum, pituitary, testis, placenta, stomach, spleen, kidney, muscle, and heart. Yolk sac, small intestine, and adrenal glands did not contain detectable transferrin mRNA levels. The size of transferrin mRNA was the same in liver, brain, and testis. Upon incubation of choroid plexus pieces with [14C]leucine in vitro, about 4% of the radioactive protein secreted into the medium was found to be transferrin. Together with previous data (Dickson, P.W., Howlett, G.J., and Schreiber, G. (1985) J. Biol. Chem. 260, 8214-8219; Dickson, P.W., Aldred, A.R., Marley, P.D., Bannister, D., and Schreiber (1986) J. Biol. Chem. 261, 3475-3478) the obtained data suggest that the choroid plexus plays a role in maintenance of homeostasis in the microenvironment of the central nervous system by synthesizing and secreting plasma proteins.