Bovine Foamy Virus: Shared and Unique Molecular Features In Vitro and In Vivo

Abstract

The retroviral subfamily of Spumaretrovirinae consists of five genera of foamy (spuma) viruses (FVs) that are endemic in some mammalian hosts [1]. Closely related species may be susceptible to the same or highly related FVs. FVs are not known to induce overt disease and thus do not pose medical problems to humans and livestock or companion animals. A robust lab animal model is not available or is a lab animal a natural host of a FV. Due to this, research is limited and often focused on the simian FVs with their well-established zoonotic potential. The authors of this review and their groups have conducted several studies on bovine FV (BFV) in the past with the intention of (i) exploring the risk of zoonotic infection via beef and raw cattle products, (ii) studying a co-factorial role of BFV in different cattle diseases with unclear etiology, (iii) exploring unique features of FV molecular biology and replication strategies in non-simian FVs, and (iv) conducting animal studies and functional virology in BFV-infected calves as a model for corresponding studies in primates or small lab animals. These studies gained new insights into FV-host interactions, mechanisms of gene expression, and transcriptional regulation, including miRNA biology, host-directed restriction of FV replication, spread and distribution in the infected animal, and at the population level. The current review attempts to summarize these findings in BFV and tries to connect them to findings from other FVs.

Keywords: BFV; animal experiment; animal model; antiviral host restriction; bovine foamy virus; foamy virus; gene expression; miRNA function; model system; spuma virus.

Figure 1

Phylogenetic tree of known exogenous and endogenous foamy viruses (FVs) (blue branches) and members of the Orthoretrovirinae. A fasta file with the conserved regions of the Pol proteins (supplement from ref. [2] and prototype FV (PFV, U21247.1) was used for alignment with ClustalW (http://www.clustal.org/). From the alignment, an ML tree was created using fastml (https://fastml.tau.ac.il, default parameters). The resulting newick tree was displayed by Itol (https://itol.embl.de/).

Figure 2

Genetic structure and schematic illustration of bovine foamy virus (BFV) gene expression and the BFV primary miRNA. (A) The BFV provirus DNA genome is shown on top schematically and out of scale with the terminal long terminal repeats (LTRs) consisting of the U3, R, and U5 regions. The position of the miRNA cassette in the U3 regions is indicated in color. BFV genes are shown as overlapping open boxes sub-divided into the mature protein domains. Proteolytic processing is marked by dotted lines. The spliced bet gene is separately shown below the genome. Broken arrows indicate the transcriptional start sited and direction of LTR- and internal promoter- (IP) directed gene expression and the Tas-mediated transactivation of the 5’LTR and the IP is indicated in red. Below, a selection of the major early and late BFV transcripts starting at the IP and LTR are shown with spliced-out areas indicated by broken lines. Only the major BFV IP-directed Tas mRNA is shown (*). The shift between early and late transcription is marked by a boxed arrow at the right-hand margin. (B) The predicted folding and secondary structure of the BFV dumbbell-shaped miRNA precursor (BFV pri-miRNA) is given, for additional information, and the sequence of the mature and stable miRNA, see below and Whisnant et al., 2014 [22].

Figure 3

BFV100-infected canine fetal thymus Cf2Th cells: (A) Giemsa stained syncytia; (B) detection of BFV Gag proteins (red) by indirect immunofluorescence, nuclei were stained in blue; BFV particles budding from the (C) plasma membrane (magnification is 60,000-fold) and (D) accumulating intracellularly in the endoplasmic reticulum (magnification is 32,000-fold) as visualized by transmission electron microscopy. Scale bars in (A,B) are 250 µm and in (C,D) 500 nm.