A three-dimensional model of interferon-tau

Interferon-tau: current applications and potential in antiviral therapy

Interferon-tau (IFN-tau) was initially identified as an ovine pregnancy protein. Produced by the trophoblast, it is important in preventing degradation of the corpus luteum and has been used as an early marker for ovine pregnancy. As a member of the family of type I interferons, IFN-tau has demonstrated promising antiviral activity against human viral infections in vitro. Additionally, it displays high species cross-reactivity despite its absence in humans. To date, IFN-tau has shown efficacy in reducing replication of human immunodeficiency virus, feline immunodeficiency virus, and human papillomavirus. While IFN-tau shares similar antiviral activity to IFN-alpha, the current interferon of choice for treatment of viral infections, it lacks the associated toxicity. This may make IFN-tau an attractive alternative to IFN-alpha for the treatment of viral infections.

Molecular characterization of novel variants of interferon-tau (IFNT) gene in Garole breed of sheep (Ovis aries)

The survivability of embryo, especially during the early embryonic life is dependent on the effective maternal recognition of pregnancy. Interferon-tau (IFNT), secreted from the elongating blastocyst, acts as the primary signal for maternal recognition of pregnancy in ruminant ungulates. IFNT has been studied extensively in many domesticated and wild ruminant species. In the present study, we have cloned and characterized the IFNT gene of Garole sheep, a popular Indian micro-sheep breed, which is known across the world for its high prolificacy and fecundity. The 588 bp sequences of two variants of IFNT gene described in this study are novel variants, compared to the variants reported previously in sheep. It exhibited more than 96% identity with other ovine IFNT variants and phylogenetically placed in a single clad containing the ovine, caprine and musk ox IFNT variants. The IFNT of Garole sheep demonstrated the highest identity with the genomic derived and highly expressed ovine IFNT variants.

Effect of buffer species on the thermally induced aggregation of interferon-tau

It is now becoming apparent that a common pathway of protein aggregation involves the unimolecular structural rearrangement from the native state to a slightly expanded aggregation-competent species. It is the goal of this study to understand the aggregation and the effects of buffer on the stability of IFN-tau. In this study, the thermally-induced aggregation of interferon-tau (IFN-tau) is described. By monitoring the aggregation rate in the presence of increasing amounts of sucrose, the relative change in surface area (Deltas) for conversion to the aggregation-competent state can be determined. Under conditions of pH 7 and in 20 mM buffer, the protein displays different aggregation rates depending on the nature of the buffer species. The protein aggregates mostly quickly in phosphate buffer, slower in the presence of Tris and slowest in the presence of histidine. The largest value for Deltas occurs for the histidine-containing samples, where aggregation proceeds via a slightly expanded aggregation competent state with a surface area increase of 7.6%. Furthermore, it appears that histidine binds to the native state of IFN-tau, thereby stabilizing the native state and retarding aggregation. Measurement of the second virial coefficient, B(22), for different formulations indicates that inclusion of histidine has only a small effect on repulsion between protein molecules, suggesting that colloidal stabilization is not the dominant mechanism for stabilization of IFN-tau. This study represents the first detailed biophysical study of specific buffer-induced stabilization, resulting in shifting the equilibrium towards the native state and away form the expanded aggregation-competent species.

Solution behavior of a novel type 1 interferon, interferon-τ

Interferon-tau (IFN-tau) is a novel cytokine that appears during fetal development of mammals. It is currently being investigated for treatment of viral infections and autoimmune diseases. In order to develop a commercial product, a stable formulation will need to be identified. In this study, the solution behavior of IFN-tau was studied using a variety of biophysical methods. The overall structure of IFN-tau is well defined, with the polypeptide chain folding into a four-helix bundle structure, much like other type 1 interferons. However, its solution behavior has not been characterized. The globular structure has a free energy of unfolding of approximately 4 kcal/mole at room temperature. IFN-tau was found to remain monomeric upon increasing the protein concentration, even up to 60 mg/mL. The overall structure of IFN-tau is maintained across a pH range of 2-8, but is significantly altered in the presence of nonaqueous solvents. However, IFN-tau appears to refold efficiently when diluted into an aqueous medium from a nonaqueous solution. This behavior allows the protein to be formulated in low water content formulations suitable for use in capsules.

Identification of interferon-tau isoforms expressed by the peri-implantation goat (Capra hircus) conceptus

Interferon-tau (IFN-tau) is the maternal recognition of pregnancy factor in pecoran ruminants. The aims of this study were to identify the various IFN-tau transcripts in the peri-implantation caprine (ca) conceptus and to compare these nucleotide sequences phylogenetically with established mRNA sequences from the goat. Conceptuses (n = 5) were collected from Boer crossbred and Angora female goats by laparotomy at days 17 and 18 of pregnancy. Total cellular RNA was extracted and RT-PCR was performed by standard procedures using a DNA polymerase with proofreading activity and gene-specific primers complimentary to non-coding regions of the published caIFN-tau sequence. Nine distinct nucleotide sequences were isolated that encode five distinct caIFN-tau proteins. These caIFN-tau have greater sequence homology with ovine IFN-tau (94-96% nucleotide identity; 90-93% amino acid identity) than with bovine IFN-tau (<92% nucleotide identity; <85% amino acid identity). The novel caIFN-tau isoforms contained pronounced nucleotide and amino acid sequence identity with one another (97-99% nucleotide identity; 94-99% amino acid identity) but only moderate sequence identity with the previously identified caIFN-tau (94-96% nucleotide identity; 87-90% amino acid identity). In conclusion, multiple caIFN-tau mRNA species are expressed during peri-implantation conceptus development and distinct clusters of caIFN-tau genes appear to have evolved in this species.

Induction of blood 2',5'-oligoadenylate synthetase activity in mice by gastric administration of ovine IFN-tau

Interferon-tau (IFN-tau) is a member of the type I IFN family. Although its distribution is restricted to ruminants, IFN-tau is active against cells of humans and mice. It has been reported that oral administration of ovine IFN-tau (OvIFN-tau) prevents both acute and chronic relapsing forms of murine experimental allergic encephalomyelitis (EAE). Here, we examined the effect in mice of peroral gastric administration of OvIFN-tau on the induction of 2',5'-oligoadenylate synthetase (OAS) activity in whole blood. Before administration, mice were deprived of food and drink for 6 h, and IFN was given by either intraperitoneal (i.p.) injection or per-oral gastric administration. When administered gastrically, IFN was introduced directly into the upper part of the stomach using an oral feeding needle. OAS activity in whole blood increased dependent on dose (0-105 U) and time (0-24 h), with no significant difference in the level of activity between the two routes. An increase in the activity of OAS in blood following administration of OvIFN-tau was observed in ICR, BALB/c, C57BL/6, NZW/N, and SJL/J mice, although the extent of the increase varied with the strain. Blood OAS levels also increased on administration of murine IFN-alpha (MuIFN-alpha). However, higher levels were detected after i.p. injection than after gastric administration.

Type I interferons and limitin: a comparison of structures, receptors, and functions

The type I interferon (IFN) family includes IFN-alpha, IFN-beta, IFN-pi, and IFN-tau. These molecules are clustered according to sequence homologies, use of the same cell surface receptor, and similar functions. IFN-alpha and IFN-beta have a globular structure composed of five a-helices. Their receptors, IFNAR1 and IFNAR2, belong to the class II cytokine receptor family for a-helical cytokines. Information about structure-function relationships between these and other IFNs is being provided by comparative sequence analysis, reference to a prototypic three-dimensional structure, analysis with monoclonal antibodies, construction of hybrid molecules and site directed mutagenesis. While much remains to be done, it should someday be possible to understand differences among IFNs in terms of how they interact with their corresponding receptors. Our recently identified IFN-like molecule, limitin, has weak sequence homology to IFN-alpha, IFN-beta, and IFN-omega and displays its biological functions through the same IFN-alpha/beta receptors. While limitin has antiproliferative, immunomodulatory, and antiviral effects like IFN-alpha and IFN-beta, it is unique in lacking influence on myeloid and erythroid progenitors. Further analysis of this functionally unique cytokine should be informative about complex IFN-receptor interactions. Furthermore, a human homologue or synthetic variant might be superior for clinical applications as an IFN without myelosuppressive properties.

IFN-tau exhibits potent suppression of human papillomavirus E6/E7 oncoprotein expression

The effects of interferon-tau (IFN-tau) on tumor suppressor factors and virus oncoprotein expression were compared with two other type I IFN in human papillomavirus (HPV-16)-transformed cells. Nontumorigenic human keratinocytes, HuKc/HPV-16d-2C (d-2C), treated with recombinant human IFN-alpha2a (Roferon), a human recombinant alpha IFN hybrid, alpha B/D (IFN-alphaB/D), or ovine IFN-tau were evaluated for their effects on the levels of E6 and E7 expression. IFN-tau was comparable to IFN-alpha2a in decreasing intracellular levels of E6 and E7, and IFN-alphaB/D was more effective than IFN-a2a in suppressing E7 levels. All three IFN were capable of increasing the cellular concentration of wild-type p53 tumor suppressor with the magnitude IFN-tau > IFN-alpha2a > IFN-alphaB/D. Increases in p53 concentrations correlated with the observed decreases in E6 mRNA and protein levels. The antiviral effects observed in this study reveal that IFN-tau has potent antipapillomavirus activity. Sequences/structures unique to IFN-tau could allow for alternative IFN/receptor interactions and may explain the differences in biologic function.

Expression, purification, and characterization of interferon-tau produced in Pichia pastoris grown in a minimal medium

Interferon-tau (IFN-tau) is a novel type I IFN that was originally identified as a pregnancy recognition hormone. IFN-tau shares all of the biological properties of other type I IFNs including antiviral activity and antiproliferative activity through induction of the cell cycle inhibitor gene product p21WAF1. It is a promising therapy for cancers, viral infections, and for autoimmune disorders such as multiple sclerosis, without the adverse side effects associated with IFN-alpha and IFN-beta. Here, we describe novel growth and induction conditions for the expression of functionally active and uniformly 15N-labeled IFN-tau from Pichia pastoris in a minimal media for use in initial 2D- and 3D-NMR studies in solution. Purified 15N-IFN-tau was homogenous, as determined by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and MALDI-TOF mass spectrometer (MS), and retained full biological activity. MS analysis confirmed uniform isotopic labeling of IFN-tau with 15N incorporation exceeding 99%. Circular dichroism (CD) as well as 1D-NMR and 15N-1H heteronuclear single quantum coherence (HSQC) spectra confirmed that purified 15N-labeled IFN-tau has a stable secondary structure. Besides providing a route for isotope labeling of IFN-tau, our procedure may be useful for the expression and purification of other proteins that are difficult to obtain in Pichia pastoris grown in minimal media.

Crystal structure of ovine interferon-tau at 2.1 A resolution

Ovine interferon-tau (ovIFN-tau) is a pregnancy recognition hormone required for normal embryonic development in sheep. In addition to its novel role in reproductive physiology, ovIFN-tau displays antiviral and antiproliferative activities similar to the IFN-alpha subtypes. To probe the structural basis for its unique activity profile, the crystal structure of ovIFN-tau has been determined at 2.1 A resolution. The fold of ovIFN-tau is similar to the previously determined crystal structures of human IFN-alpha2b and human and murine IFN-beta, which each contain five alpha-helices. Comparison of ovIFN-tau with huIFN-alpha2b, huIFN-beta, and muIFN-beta reveals unexpected structural differences that occur in regions of considerable sequence identity. Specifically, main-chain differences up to 11 A occur for residues in helix A, the AB loop, helix B, and the BC loop. Furthermore, these regions are known to be important for receptor binding and biological activity. Of particular interest, a buried ion pair is observed in ovIFN-tau between Glu71 and Arg145 which displaces a conserved tryptophan residue (Trp77) from the helical bundle core. This ion pair represents a major change in the core of ovIFN-tau compared to huIFN-alpha2b. Based on amino acid sequence comparisons, these ovIFN-tau structural features may be conserved in several human IFN-alpha subtypes and IFN-omega. The structure identifies potential problems in interpreting site-directed mutagenesis data on the human IFN-alpha family that consists of 12 proteins.

The evolution of the type I interferons

There are five recognized subtypes within the type I interferons (IFN), IFN-alpha, IFN-beta, IFN-delta, IFN-omega, and IFN-tau, although others may remain to be described, and the IFN-omega may have to be subdivided further because of their evident structural complexity. Together, they constitute an ancient family of intronless genes, possibly present in all vertebrates. THe IFNA/IFNB genes originated by duplication of a progenitor after the divergence of birds, most probably about 250 million years ago (MYA). The avian gene itself proceeded to duplicate to form a series of independent subtypes. The IFND, to date described only in the pig, arose from the IFNA lineage before the emergence of mammals about 180 MYA and might, therefore, be generally distributed in present day species. The IFNB, which occurs as a single gene in primates and rodents, have been duplicated in some other orders. Recent events have produced 10 or more genes in bovid species. The IFNA, which are clustered with the IFNW in humans and cattle, exist as multiple genes in all mammals so far examined as a result of a series of duplication events, some of which occurred recently and, therefore, independently in separate mammalian lineages. The IFNW diverged from the IFNA approximately 130 MYA, just prior to the emergence of mammals, and have continued to duplicate since then. The IFNT, which play a role in reproduction of ruminants, arose from an IFNW within the Artiodactyla suborder about 36 MYA and are found only in the suborder Ruminantia. These genes have also continued to duplicate to form an extensive family. Consequently, their involvement in early pregnancy is a feature of ruminants and not of other mammalian species.

The antiproliferative and antiviral activities of IFN-tau variants in human cells

The IFN-tau are type I IFN expressed by the early trophoblast of cattle and sheep but have activity on human cells and have been predicted to have potential therapeutic value. We have compared a series of mutant bovine and ovine IFN-tau with regard to their ability to inhibit the proliferation of Daudi cells and to evoke an antiviral (AV) response in WISH cells. Whereas Daudi cell growth was inhibited by Bo-IFN-tau1 in the 1 nM range, WISH cells were much less responsive, requiring exposure to 150 nM for protection against vesicular stomatitis virus. Replacement of lysines at positions 34, 107, 121, and 132 in Bo-IFN-tau, which are in regions predicted to interact with the type I receptor, led to modest but significant alterations in antiproliferative (AP) and AV activities. Replacement of the lysine residues at 160 and 164 had marked effects on biopotency, with K160 being particularly important. The different IFN-tau were able to activate the transcription factors ISGF3 and AAF (GAF) in Daudi cells at concentrations that correlated reasonably well with their AP potencies. Stat activation occurred in WISH cells in response to approximately 2 nM Bo-IFN-tau1, but ISGF3 formation could not be demonstrated even at the 100-fold higher IFN-tau concentrations that gave viral protection. Pretreatment of WISH cells with Hu-IFN-gamma allowed ISGF3 formation to be observed in response to subsequent treatment with Bo-IFN-tau1 or type I human IFN but did not increase the AV responsiveness of the cells. No evidence was found that IFN-tau elicit uniquely different responses on human cells than type I Hu-IFN, except they are much less potent. The data emphasize the importance of a region near the carboxyl terminus for the functional activity of type I IFN, and that although ISFG3 formation may be necessary, its mere presence is not sufficient to provide an antiviral response.

Elucidation of the basic three-dimensional structure of type I interferons and its functional and evolutionary implications

The scientific and personal backgrounds of the crystallographic elucidation of the three-dimensional structure of murine interferon-beta (Mu-IFN-beta) are described. This structure, elucidated in 1990, is still the only experimentally determined structure for type I IFNs. Model-building studies for various type I IFNs based on the Mu-IFN-beta structure and the arguments on the receptor-binding epitopes appearing since then are reviewed. An updated set of a table and a figure demonstrating a strong correlation between the degree of amino acid sequence variation in various cytokine proteins and that in their cognate receptor proteins is given. The origin of a remarkably larger rate of evolutionary change in amino acid sequences of cytokine proteins despite their physiologic significance is discussed in view of the cytokine network and the neutral theory of evolution.

Interferon tau: a novel pregnancy recognition signal

PROBLEM

Trophectoderm of ruminant conceptuses (embryo and associated membranes) secretes tau interferons (IFNtau) as the pregnancy recognition signal. How does it act?

METHOD

Review of current data.

RESULTS

IFNtau acts on uterine epithelium to suppress transcription of the genes for estrogen receptor and oxytocin receptor. This blocks development of the uterine luteolytic mechanism and, therefore, release of luteolytic pulses of prostaglandin F2alpha, but it has no effect on expression of the progesterone receptor. Maintenance of progesterone secretion by the corpus luteum ensures establishment and maintenance of pregnancy. Secretion of IFNtau on days 12-15 for sheep and days 14-17 for cows and goats is essential for pregnancy recognition.

CONCLUSION

We propose that IFNtau affects endometrial gene expression by activating the Jak/Stat pathway, which results in formation of the ISGF3alpha transcription factor complex. ISGF3alpha binds to interferon-stimulated response elements and activates transcription of interferon-responsive genes such as interferon regulatory factor-1 (IRF-1) which, in turn, activates expression of the negative-acting transcription factor IRF-2. Pregnancy (or intrauterine injection of roIFNtau) results in a transient increase in endometrial IRF-1 expression followed 36-48 hr later by a sustained increase in IRF-2. We propose that IRF-2, or an IFNtau-induced negative regulatory factor like IRF-2, suppresses expression of the estrogen receptor gene and directly or indirectly blocks expression of the gene for oxytocin receptor to abrogate the uterine luteolytic mechanism and ensure the establishment of pregnancy.

The interferon-tau genes of the giraffe, a nonbovid species

Genes for interferon-tau (IFNT) have been cloned from several Bovidae, and the IFNT lineage has been predicted to have arisen from the IFNW, as the divergence of the Ruminantia from other artiodactyls. Southern genomic blotting with probes specific for IFNT identified a gene in the giraffe, a nonbovid member of the Ruminantia. Here this gene has been cloned, sequenced, and expressed. It encodes a 172-amino acid mature protein with antiviral activity on bovine cells. Giraffe IFN-tau lacks the normally conserved Cys99 but has a pair of other cysteines (Cys64 and Cys86) that could provide a disulfide bridge. The giraffe IFNT gene possesses the highly conserved promoter region that distinguishes IFNT from IFNW. It seems likely that IFNT emerged before the divergence of the Giraffidae and Bovidae, which occurred some 24 million years ago.