Immunocytochemical localization of nuclear 3,5,3'-triiodothyronine (L-T3) receptors in astrocyte cultures

By means of a monoclonal antibody (mab) against the rat liver nuclear L-T3 receptor (NT3R) and a polyclonal anti-GFAp serum, it has been possible to demonstrate nuclear thyroid hormone receptors in astrocyte cultures. On day 3, 47% of GFAp+ cell nuclei were labeled by 2B3 mab. Between day 3 and day 15, the number of GFA+ cell nuclei stained by 2B3 mab increased from 47 to 75%. Thyroid hormone nuclear receptors were present in fibrous and protoplasmic astrocytes. However, they developed asynchronously in both types of astrocytes. Indeed, 60% of fibrous astrocytes were stained by 2B3 mab on day 3 and this percentage reached 77% after 8 days in vitro. In contrast, only 30% of protoplasmic astrocytes were immunoreactive for 2B3 mab on day 3 and this percentage increased slowly reaching 47% on day 8 and around 75-80% on day 15. By immunoblotting, the monoclonal antibody recognized two bands of proteins with a molecular weight of 57 and 45 kDa respectively. These proteins have the same electrophoretic mobility as [125I]bromoacetyl-LT3 rat liver nuclear L-T3 receptor. This paper presents the first immunocytochemical localization of nuclear L-T3 receptors in astrocyte cultures. Furthermore, we show that thyroid hormone receptors develop more rapidly in fibrous than in protoplasmic astrocytes.

Structure determination of Mengo virus

The structure of Mengo virus was determined to 3.0 A resolution using human rhinovirus 14 as an initial phasing model at 8.0 A resolution. Oscillation diffraction photographs were collected at the Cornell High Energy Synchrotron Source using orthorhombic Mengo virus crystals. The crystal space group was P2(1)2(1)2(1), a = 441.4, b = 427.3 and c = 421.9 A, with one icosahedral particle per asymmetric unit, giving 60-fold noncrystallographic redundancy. The orientations of the four viral particles in the unit cell were determined with a rotation function. Their positions relative to the crystallographic symmetry axes were found by a combination of Patterson-function analysis and a subsequent R-factor search using human rhinovirus 14 atomic coordinates as a model. The initial phases to 8.0 A resolution were then computed by placing human rhinovirus 14 particles in the orientations and positions of Mengo virus particles. These phases were improved by ten cycles of real-space molecular replacement averaging. Phases between 8.0 and 3.0 A resolution were obtained by molecular replacement phase extension. One or two reciprocal-space lattice points were used for each extension followed by two cycles of averaging.

Immunocytochemical localization of thyroid hormone nuclear receptors in cultured hypothalamic dopaminergic neurons

By means of a monoclonal antibody against the rat liver L-triiodothyronine nuclear receptor and a polyclonal anti-tyrosine hydroxylase serum, it has been possible to demonstrate thyroid hormone nuclear receptors in immunoreactive tyrosine hydroxylase cell nuclei in fetal rat hypothalamic cultures. After 8 days in vitro, the ratio of tyrosine hydroxylase cells that were immunoreactive for the thyroid hormone receptor to those not stained for this receptor (64% to 36% respectively) remains unchanged despite an increase in the number of tyrosine hydroxylase-positive cells with time (from day 8 to day 21) in culture. The presence of thyroid hormone nuclear receptor in dopaminergic neurons is correlated with a morphological effect of L-triiodothyronine in this neuronal population. Our results demonstrate, for the first time, the presence of triiodothyronine nuclear receptors in fetal rat dopaminergic neurons and the existence of a cellular heterogeneity in the distribution of the thyroid hormone receptor. The presence of these receptors in fetal hypothalamic dopaminergic neurons suggests that some effects of L-triiodothyronine on the maturation of DA neurons may result from a direct effect of this hormone through an interaction with its specific nuclear receptors.

Crystallization and preliminary X-ray analysis of type B influenza virus neuraminidase complexed with antibody Fab fragments

Fab fragments from four different monoclonal antibodies have been complexed with influenza B virus neuraminidase (B/Lee/40) and the complexes have been crystallized. Three of the complex crystals are, so far, not suitable for X-ray diffraction studies, but the fourth (B/Lee/40 NA-B1Fab) forms large crystals which diffract X-rays to 3.0 A resolution. The crystals have a space group of F432, a = 441.21 A. Vm calculations show that the asymmetric unit contains two monomeric complexes.

Prediction of three-dimensional models for foot-and-mouth disease virus and hepatitis A virus

Atomic models of foot-and-mouth disease virus and hepatitis A virus have been predicted using amino acid sequence alignments with the known structures of Mengo virus and human rhinovirus 14. The structural models are consistent with results of biochemical and immunological studies. The two viruses appear to have surface features exceedingly different than those of other picornaviruses. They also have large hydrophobic cavities within VP1 suggesting that it may be possible to inhibit their infectivity with suitably designed antiviral agents that block uncoating.

Image plane detector spectrophotometer: application to O(2)atmospheric band nightglow

A new variety of low resolution spectrometer is described. This device, an image plane detector spectrophotometer, has high sensitivity and modest resolution sufficient to determine the rotational temperature and brightness of molecular band emissions. It uses an interference filter as a dispersive element and a multichannel image plane detector as the photon collecting device. The data analysis technique used to recover the temperature of the emitter and the emission brightness is presented. The atmospheric band of molecular oxygen is used to illustrate the use of the device.

A monoclonal antibody to the rat nuclear triiodothyronine receptor: production and characterization

The nuclear T3 receptor (NTR) was affinity-labeled with bromoacetyl-[125I]T3, purified by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and used to immunize BALB/c mice. Spleen cells from one strongly immunoreactive mouse were fused with Sp2 mouse myeloma cells, and 328 hybridomas were screened by a dot-blot immunoassay using as antigen, a preparation of NTR partially purified by diethylaminoethyl-Sephadex chromatography. Four positive cultures were thus found; three of which were confirmed by comparing Western blotting patterns with the electrophoretic mobility of the affinity-labeled NTR. One of these 3 hybridomas was further subcloned by limiting dilution and gave rise to the 2B3 clone, which produces an immunoglobulin of the immunoglobulin G1 subclass. Several lines of evidence indicated that the 2B3 monoclonal antibody was indeed directed against the NTR. The antibody recognized a protein with the same electrophoretic mobility as the affinity-labeled receptor. Thus, Western blotting revealed a predominant protein with a mol wt of 57,000 and a less abundant 45,000 component on sodium dodecyl sulfate gels, and multiple isoelectric variants of the 57,000 protein, with a predominant form at pI 6.2, were detected on two-dimensional gels. Incubation of the 2B3 antibody with the NTR labeled with [125I]T3 resulted in the formation of an antibody-receptor complex, as indicated by a shift of the radioactivity peak upon gel filtration on Sephacryl S-300. In contrast, control ascitic fluid did not change the elution profile of the labeled NTR. The 2B3 antibody is able to remove the T3-binding activity from rat liver nuclear extracts. Finally, in accordance with previous T3-binding experiments, expected amounts of NTR were found in pituitary, liver, brain, kidney, spleen, and testis with the use of the Western blotting technique and immunohistochemistry on frozen tissue sections. This antibody should prove useful in the characterization and purification of the NTR and also in the study of its distribution in different tissues and cell types.

Three-dimensional model of the capsid proteins of two biologically different Theiler virus strains: clustering of amino acid difference identifies possible locations of immunogenic sites on the virion

To explore structural features of the Theiler murine encephalomyelitis virion, we have constructed a three-dimensional model of the capsid proteins (VP1, VP2, and VP3) of the BeAn strain based on the atomic coordinates of the closely related Mengo virus. By superimposition of amino acid differences between BeAn virus and another Theiler virus strain, GDVII, on the three-dimensional model, clusters of differences were found in four distinct sites; the VP1 third corner, the VP2 "puff," and the VP3 first corner and "knob." These clusters, which are found on the surface of the virion, may represent neutralizing immunogenic sites that have come under selective pressure from neutralizing antibodies. Furthermore, the putative viral receptor binding site ("pit") of the two Theiler virus strains was found to be markedly conserved.

Structural analysis of a series of antiviral agents complexed with human rhinovirus 14

The binding to human rhinovirus 14 of a series of eight antiviral agents that inhibit picornaviral uncoating after entry into host cells has been characterized crystallographically. All of these bind into the same hydrophobic pocket within the viral protein VP1 beta-barrel structure, although the orientation and position of each compound within the pocket was found to differ. The compounds cause the protein shell to be less flexible, thereby inhibiting disassembly. Although the antiviral potency of these compounds varies by 120-fold, they all induce the same conformational changes on the virion. The interactions of these compounds with the viral capsid are consistent with their observed antiviral activities against human rhinovirus 14 drug-resistant mutants and other rhinovirus serotypes. Crystallographic studies of one of these mutants confirm the partial sequencing data and support the finding that this is a single mutation that occurs within the binding pocket.

Preliminary X-ray crystallographic analysis of canine parvovirus crystals

The first diffraction pattern of a crystalline single-stranded DNA virus has been obtained. Canine parvovirus was crystallized in a monoclinic P21 unit cell with a = 264.4 A, b = 350.3 A, c = 267.8 A and beta = 90.86 degrees (1 A = 0.1 nm). The diffraction pattern extends to at least 2.8 A resolution. Packing of the particles suggests that they have a diameter around 257 A, in excellent agreement with the reported molecular weight of 5.5 x 10(6).

The atomic structure of Mengo virus at 3.0 A resolution

The structure of Mengo virus, a representative member of the cardio picornaviruses, is substantially different from the structures of rhino- and polioviruses. The structure of Mengo virus was solved with the use of human rhinovirus 14 as an 8 A resolution structural approximation. Phase information was then extended to 3 A resolution by use of the icosahedral symmetry. This procedure gives promise that many other virus structures also can be determined without the use of the isomorphous replacement technique. Although the organization of the major capsid proteins VP1, VP2, and VP3 of Mengo virus is essentially the same as in rhino- and polioviruses, large insertions and deletions, mostly in VP1, radically alter the surface features. In particular, the putative receptor binding "canyon" of human rhinovirus 14 becomes a deep "pit" in Mengo virus because of polypeptide insertions in VP1 that fill part of the canyon. The minor capsid peptide, VP4, is completely internal in Mengo virus, but its association with the other capsid proteins is substantially different from that in rhino- or poliovirus. However, its carboxyl terminus is located at a position similar to that in human rhinovirus 14 and poliovirus, suggesting the same autocatalytic cleavage of VP0 to VP4 and VP2 takes place during assembly in all these picornaviruses.

Implications of the picornavirus capsid structure for polyprotein processing

Mature picornaviral proteins are derived by progressive, posttranslational cleavage of a precursor polyprotein. These cleavages play a role in the control of virus functions. Although the processed termini are separated by as much as 75 A in the native virus capsid, the fold and arrangement of polypeptide chains in a protomer before proteolysis are likely to be similar to that found in the mature virus. The three-dimensional structures of rhinovirus and Mengo virus suggest that the cleavage sites within the protomeric precursor are in structurally flexible regions. The final proteolytic processing event, maturation of the virion peptide VP0 (also called peptide 1AB) appears to occur by an unusual autocatalytic serine protease-type mechanism possibly involving viral RNA basic groups that would serve as proton-abstractors during the cleavage reaction.

Comparison of the three-dimensional structure of two human rhinoviruses (HRV2 and HRV14)

An attempt has been made to build a model of human rhinovirus 2 (HRV2) based on the known human rhinovirus 14 (HRV14) structure. HRV2 was selected because its amino acid sequence is known and because it belongs to the minor rhinovirus receptor class as compared to HRV14, which belongs to the major class. Initial alignment of HRV2 with HRV14 based on the primary sequence and the knowledge of the three-dimensional structure of HRV14 showed that the most probable position of the majority of insertions and deletions occurred in the vicinity of the neutralizing immunogenic sites (NIm). Out of a total of 855 amino acids present in one copy of each of the capsid proteins VP1 through VP4 of HRV14, 411 are different between the two viruses. There are also 6 amino acid residues inserted and 14 residues deleted in HRV2 relative to HRV14. Examination of amino acid interactions showed several cases of conservation of function, e.g., salt bridges or the filling of restricted space. The largest variation amongst the residues lining the canyon, the putative receptor binding site, was in the carboxy-terminal residues of VP1.

The site of attachment in human rhinovirus 14 for antiviral agents that inhibit uncoating

WIN 51711 and WIN 52084 are structurally related, antiviral compounds that inhibit the replication of rhino (common cold) viruses and related picornaviruses. They prevent the pH-mediated uncoating of the viral RNA. The compounds consist of a 3-methylisoxazole group that inserts itself into the hydrophobic interior of the VP1 beta-barrel, a connecting seven-membered aliphatic chain, and a 4-oxazolinylphenoxy group (OP) that covers the entrance to an ion channel in the floor of the "canyon." Viral disassembly may be inhibited by preventing the collapse of the VP1 hydrophobic pocket or by blocking the flow of ions into the virus interior.

Ontogenesis of nuclear T3 receptors in primary cultured astrocytes and neurons

Nuclear T3 receptor (NTR) have been characterized in separated cultures of neurons and astrocytes. Scatchard analysis indicated the presence of a single class of high-affinity sites in both cell lines. The apparent equilibrium association constant ranged from 1.80 +/- 0.41 X 10(10) M-1 to 3.27 +/- 0.74 +/- 10(10) M-1 in neurons and from 1.01 +/- 0.09 to 1.80 +/- 0.73 X 10(10) M-1 in astrocytes depending on the time in culture. In neurons, the maximal binding capacity (MBC) increased from 0.049 +/- 0.008 ng T3/mg DNA to 0.328 +/- 0.052 ng T3/mg DNA between 3 and 12 days of culture. In astrocytes, the changes in MBC were less pronounced ranging from a minimum of 0.095 +/- 0.024 ng T3/mg DNA at the 7th day of culture to a maximum of 0.198 +/- 0.048 ng T3/mg DNA at the 21st day. The relative binding affinity of the receptor for thyroid hormone analogs was in the order TRIAC greater than L-T3 greater than D-T3 greater than L-T4 in both cell lines. These results show that nuclear T3 receptors similar to those found in vivo are present in primary cultures of both astrocytes and neurons.

Regulation of protein phosphorylation by triiodothyronine (T3) in neural cell cultures. Part I: Astrocytes

Dissociated cells from 2-day-old rat cerebral hemispheres were cultured for 17 days in absence of thyroid hormones using conditions yielding mainly glial cells. Triiodothyronine (10(-8) M) was added for 0-72 h before the end of the incubation and [32P]phosphate was added for the last 4 h. Soluble (105,000 X g supernatant), particulate (105,000 X g pellet) and HMG (high mobility group; 0.75 M perchloric acid-soluble proteins) fractions were prepared and phosphorylated proteins in each fraction were analyzed by polyacrylamide gel electrophoresis. In the soluble fraction a protein (Mr = 19,000) incorporates less [32P]phosphate after only 4 h of T3 treatment. The maximal effect is attained after 7 h (-42%) and remains unchanged up until 72 h. In this fraction, the phosphorylation of some other proteins is increased but the maximal effect is observed 48 and 72 h after T3 administration. In the particulate fraction, exposure to T3 rapidly (4 h) increases the amount of a protein (Mr = 45,000) identified as beta-actin. Protein phosphorylation in this fraction is slightly, or not at all, affected by T3. In contrast, a rapid (between 4 and 7 h) increased phosphorylation of a 17 kDa protein in the HMG fraction is observed following T3 stimulation. This nuclear protein was further characterized as HMG 14. These results show that thyroid hormones can produce direct effects (not mediated by neurons) on the phosphorylation of specific proteins in cultured glial cells. Possible functional implications of the observed protein changes are discussed in this paper.

Regulation of protein phosphorylation by triiodothyronine (T3) in neural cell cultures. Part II: Neurons

Cerebral hemisphere from 16- to 18-day-old rat fetuses were dissociated and cells were cultured in absence of thyroid hormones. Neuron-enriched cultures were obtained either by using cells after 6 days of culture (before extensive glial cell proliferation) or by adding cytosine arabinoside for 48 h after 4 days of culture and using cells on day 9. Cells were incubated with T3 (10(-8) M) for 0-72 h and [32P]phosphate was added for the last 4 h of incubation. HMG (high mobility group; 0.75 M perchloric acid-soluble proteins) were prepared and phosphorylated proteins were analyzed by polyacrylamide gel electrophoresis. T3 rapidly (4-7 h) increased the phosphorylation of histone H1 and of a protein with apparent molecular mass of 17000 Da identified as HMG 14. In addition, in cells not treated with cytosine arabinoside, histone H1 was resolved into 3 subfractions and each of these responded to the hormone with a different time course. These results indicate that thyroid hormones act on the phosphorylation of specific nuclear proteins and therefore may influence chromatin structure and gene expression in primary neuronal cell cultures.