Transthyretin gene expression in choroid plexus first evolved in reptiles

Abstract

The presence of transthyretin in mammals and birds, but not amphibia, suggested that transthyretin expression first appeared in stem reptiles. Therefore, transthyretin synthesis was studied in a lizard. Transthyretin synthesis in choroid plexus pieces from Tiliqua rugosa was demonstrated by incorporation of radiactive amino acids. Oligonucleotides corresponding to conserved regions of transthyretin were used as primers in polymerase chain reaction with lizard choroid plexus cDNA. Amplified DNA was used to screen a lizard choroid plexus cDNA library. A full-length transthyretin cDNA clone was isolated and sequenced. A three-dimensional model of lizard transthyretin was obtained by homology modeling. The central channel of transthyretin, containing the thyroxine-binding site, was found to be completely conserved between reptiles and mammals. Transthyretin expression was not detected in lizard liver. These data suggest that transthyretin first evolved in the choroid plexus of the brain. Due to a change in tissue distribution of gene expression, occurring much later during evolution, transthyretin also became a plasma protein, synthesized in the liver.

Evolution of the Thyroid Hormone-Binding Protein, Transthyretin

“…Multiple sequence alignment carried out using Clustal X (Gibson et al, 1994) of transthyretin with the prepeptide from representative species of all vertebrate groups: human (Sasaki et al, 1985), pig (Sus scrofa, Duan et al, 1995), rat (Rattus norvegicus, , dunnart (Sminthopsis macroura, Duan et al, 1995), wallaby (Macropus eugenii, Brack et al, 1995), opossum (Monodelphis domestica, Duan et al, 1995), chicken (Gallus gallus, Duan et al, 1991), skink (Tiliqua rugosa, Achen et al, 1993), frog (Rana catesbeiana, Yamauchi et al, 1998), and sea bream (Sparus aurata, Santos and Power, 1999). Coloured blocks refer to the physiochemical properties of the amino acids according to the key.…”

“…Sequence Identity (First Line of Each Row, %) and Similarity (Second Line of Each Row, %) Based on Clustal X (Gibson et al, 1994) Alignments between Transthyretins from Eutherian Mammals-Human (Sasaki et al, 1985), Pig (Sus scrofa, Duan et al, 1995), and Rat (Rattus norvegicus, ; Marsupial Mammals-Dunnart (Sminthopsis macroura, Duan et al, 1995) (Australian Polyprotodonta), Wallaby (Macropus eugenii, Brack et al, 1995) (Australian Dyprotodonta), and Opossum (Monodelphis domestica, Duan et al, 1995) (American Polyprotodonta); Birds-Chicken (Gallus gallus, Duan et al, 1991); Reptiles-Skink (Tiliqua rugosa, Achen et al, 1993); Amphibians-Frog (Rana catesbeiana, Yamauchi et al, 1998) chain conformations were modelled using alternative rotamer-based techniques (Mendes et al, 1999a,b). The structure of sea bream TTR is compared with the corresponding structures from human, rat, and chicken in Fig.…”

“…The cDNA generally consists of a 5Ј untranslated region (14 -30 nucleotides), a coding region that corresponds to 127-130 amino acids, and a 3Ј untranslated region (115-181 nucleotides) preceding the poly(A) tail. The size of the mature protein is 130 amino acids in the pig ; 127 amino acids in the rat , human (Mita et al, 1984;Soprano et al, 1985;Wallace et al, 1986), mouse (Wakasugi et al, 1985), and wallaby (Macropus eugenii, Brack et al, 1995); 129 amino acids in short-tailed grey opposum (Monodelphis domestica), stripe-faced dunnart (Sminthopsis macroura), and sugar glider ; and 130 amino acids in the chicken , lizard (Achen et al, 1993), bullfrog (Yamauchi et al, 1998), and teleost fish (Santos and Power, 1999). Comparison of the amino acid sequence of TTR from mammals and marsupials with TTR from birds, amphibians, and fish demonstrates the presence in the latter species of 3 additional amino acids at the N-terminus, Val-Ser-His in chicken and lizard; Gly-Thr-His in frog, and Asp-Lys-His in fish.…”

“…In two reptiles, the turtle (Trachemys scripta) and lizard (Tiliqua rugosa), TTR is present in the brain but little or no expression has been detected in the liver (Achen et al, 1993;Richardson et al, 1997). The protein synthesised in the liver is secreted into the blood, whilst from the choroid plexus epithelium it is secreted into the cerebrospinal fluid, presumably to carry essential TH to the brain (Dickson et al, 1987).…”

“…In common with other plasma proteins TTR is expressed and secreted by the liver (Schreiber, 1987) and its expression at this site in homeotherms is proposed to have occurred independently in birds, eutherians, and diprotodont marsupials (Richardson et al, 1994;Schreiber et al, 1993Schreiber et al, , 1995. The other main site of TTR gene expression in reptiles, birds, eutherians is the choroid plexus (Achen et al, 1993) where it is the only thyroid hormone-binding protein synthesised (Schreiber, 1987), and TTR is presumed to facilitate the transport of TH into the brain.…”