The Evolution of Viruses - Zhdanov, V. M. 1990

Special Section
Rhabdoviruses

Rhabdoviruses represent the largest and most branched group among the five groups of negative-sense single-stranded RNA Viruses. They are extensive not only in the number of viruses they encompass, but also in the range of diverse host kingdoms: plants, Protozoa, and animals, the latter including mammals, birds, fish, and insects. Despite such A wide variety of ecological niches, rhabdoviruses form a fairly compact group in terms of Genome Size, virion architecture, and, naturally, Replication mechanisms [Kaverin N. V., 1982; Brown F. et al., 1979; Matthews R., 1982].

The family Rhabdoviridae comprises three genera: Vesiculovirus (vesicular stomatitis viruses), Lyssavirus (rabies viruses), and plant rhabdoviruses, which in turn are subdivided into three subgroups: A — lettuce necrotic yellows viruses, B — potato yellow dwarf viruses, and C — non-enveloped virions.

Animal rhabdovirus virions are bullet-shaped with dimensions of 50–95×130–380 nm, whereas plant rhabdovirus virions are more typically rod-shaped with dimensions of 45–95×135–180 nm. The nucleocapsid has a helical Symmetry, a diameter of 50 nm, and a helical filament size of 20×700 nm. The outer envelope bears spikes 5–10 nm in length and 3 nm in diameter; an internal membrane (matrix) is located between the envelope and the nucleoid, and deviations from the normal shape are possible. The Genome is a single-stranded negative-sense RNA with a Molecular Weight of 3.5×10⁶–4.6×10⁶. It sequentially harbors the genes 3'–N–NS–M–G–L–5' corresponding to transcribed (and non-transcribed) NUCLEOTIDES (Fig. 20). Excluding the leader I sequence, the Proteins of rhabdoviruses (vesicular stomatitis virus) have the following molecular weights: polymerase L — 150,000, nucleoprotein N — 50,000–62,000, matrix protein M — 20,000–30,000, surface glycoprotein G — 70,000–80,000, non-structural protein NS — 40,000–50,000.

Virions consist of an internal component, an inner membrane, and an outer envelope. The RNA, together with the nucleocapsid protein, forms a helix whose strands are coiled into a "spool," which also houses the Proteins of the polymerase-transcriptase complex. The diameter of the spool is 50 nm, and upon unwinding, it forms helical filaments (20×700 nm). The nucleocapsid is surrounded by a membrane composed of M protein, which is externally covered by a lipid layer into which envelope glycoprotein spikes are inserted. Virion dimensions are 130–380×50–95 nm, with animal rhabdoviruses being predominantly rod-shaped.

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Fig. 20. Structure OF THE genomes of standard and defective interfering particles of vesicular stomatitis virus. Letters indicate genes; numbers in parentheses indicate the number of nucleotides absent in METABOLISM/31.html">Transcription products; 1 — standard virus; 2 — defective interfering particle.

Rabies virus proteins have molecular weights of 190,000 (L), 58,000–62,000 (N), 22,000–25,000 (M1 or M2), 65,000–80,000 (G), 35,000–40,000 (NS or M1), and are antigenically distinct from the proteins of vesicular stomatitis virus. Group A plant rhabdoviruses are similar to vesicular stomatitis viruses in their protein profile (L transcriptase — 145,000–170,000 and M matrix protein — 18,000–25,000). Group B plant rhabdoviruses are more similar to lyssaviruses (M1 — 27,000–44,000, M2 — 21,000–39,000). The remaining proteins (G — 71,000–93,000, N — 55,000–60,000) are similar in both groups.

The genome strategy of rhabdoviruses is typical of other negative-sense RNA viruses. Following adsorption onto cellular receptors, viropexis, and the fusion of viral and cellular membranes, the un-enveloped ribonucleoprotein begins to function in the Cytoplasm. Each Gene is transcribed separately, generating positive-sense viral mRNAs that are processed using cellular Enzymes. In the process, they acquire cap structures and poly(A) tails, and in this form, they interact with Ribosomes to form polyribosomal complexes.

Virions attach via their Glycoproteins to Cell Membrane Receptors, penetrate via viropexis, and exchange Lipids with The Cell membrane. As a result of partial deproteinization, the nucleocapsid becomes functionally active. The products of transcription are 5 monocistronic mRNAs and a leader sequence. Transfection is initiated at the 3' end of the virion RNA, and cistrons are transcribed sequentially to produce individual mRNAs. All of them possess cap structures and poly(A) tails, the synthesis of which is catalyzed by cellular enzymes. These mRNAs encode the synthesis of their corresponding proteins. RNA replication is driven by the enzymatic activity of L + NS proteins and proceeds via The formation of a plus-strand and a replicative intermediate. Regulatory mechanisms exist whereby minus-sense RNA strands are synthesized much more frequently than plus-sense strands, and various proteins are synthesized in different quantities. During RNA Synthesis, various classes of DI particles are formed. Nucleocapsid assembly takes place in the cytoplasm, whereas virions are assembled at the cell membranes and exit the cell via budding.

During transcription, the first 70 nucleotides are not transcribed, followed by the synthesis of a 48-nucleotide untranslated sequence, a 59-nucleotide sequence of the 3' region encoded by the L gene, and a 3-nucleotide spacer between the leader sequence and the N gene. Next, 4 dinucleotides—spacers between the 4th intercistronic junction—and an undecamer sequence between each gene junction are synthesized, followed by a decanucleotide at the 5' end of each gene. Thus, Translation does not copy the entire genome, but instead ensures the independent copying of each individual gene.

According to the most widely accepted (though not conclusively proven) scheme, transcriptase initiates reading from the 3' end of the genomic RNA molecule; after generating a 48-nucleotide leader sequence, it skips 3 nucleotides and transcribes the mRNA of the IV gene; an oligo(U) sequence is located at the end of this gene. A poly(A) sequence is synthesized at this oligo(U) site via a "slippage" mechanism. Then, the transcriptase skips the dinucleotide spacer and transcribes The sequence of the next gene, again up to the oligo(U) sequence and dinucleotide spacer, and so forth. At the end of the L gene, following polyadenylation, synthesis is terminated (the terminal 59 nucleotides are not read). Proteins encoded by the L and NS genes participate in transcription, with the L protein ensuring the initiation of RNA chain synthesis and the NS protein responsible for their elongation [De B., Banerjee A., 1984].

The gene encoding the Synthesis of the vesicular stomatitis virus phosphoprotein, when initiated from an internal site, also directs the synthesis of an additional protein (7,000) that appears to be non-structural [Horn T. et al., 1986]. Transcription of the entire genome results in the generation of a full-length plus-strand, which serves as a template for the synthesis of progeny virion RNAs. The nucleocapsid is formed via self-assembly, while virion formation occurs at the cell membrane, into which molecules of the envelope glycoprotein are embedded.

As previously mentioned, rhabdoviruses infect a wide range of animals and plants, with the primary mode of transmission being via vectors—aphids and other sap-sucking plant insects, as well as mosquitoes and other Blood-sucking animal insects. Along with this, some animal viruses have secondarily lost their vectors, as is partially the case with rabies, whereas others have established complex relationships with their insect hosts, as seen with the Drosophila sigma virus.

The rhabdovirus family is subdivided into several genera. Among animal viruses, the genera Vesiculovirus and Lyssavirus are distinguished. The former includes vesicular stomatitis virus, whose serotypes and related viruses are transmitted by insects. All of them share a serological relationship determined by the N protein and type-specific antigenic features conferred by the G protein. The rabies virus group, In addition to the aforementioned virus, includes immunologically related viruses. These have retained The ability to be transmitted via insects, whereas the rabies virus has lost this capacity. The genus Sigmavirus is represented by a single species that infects Drosophila, inducing increased sensitivity to carbon dioxide without other pathological manifestations. Curiously, this phenomenon can be reproduced upon infecting Drosophila with other rhabdoviruses, such as vesicular stomatitis virus. The virus is transmitted both horizontally and vertically, by both females and males.

Among unclassified rhabdoviruses, the group of fish rhabdoviruses stands out as deserving Classification into a separate genus. These viruses include the causative agents of hemorrhagic and necrotic diseases in carp, infectious hematopoietic necrosis in salmon, sockeye, and Chinook salmon, hemorrhagic septicemia in trout, grayling, and whitefish, and hemorrhagic necrosis in pike fry. Their genomic structure resembles that of the rabies virus and contains the polymerase (L) gene. During replication, they produce an envelope glycoprotein (G), two matrix proteins (M1 and M2), a nucleocapsid protein (N), and, additionally, a non-structural protein (NV). mRNAs encoding these proteins, as well as progeny virion RNA, have been isolated, demonstrating that they are synthesized in varying quantities [Kurath G., Leong J., 1985].

A comparison of the genes encoding the M protein of vesicular stomatitis virus and spring viremia of carp viruses revealed significant (28%) Homology, undoubtedly indicating a common origin for these viruses [Kiochi A., Ray P., 1984]. Rhabdoviruses also infect aquatic animals such as crabs.

Plant rhabdoviruses remain poorly classified. Among them, researchers distinguish the lettuce necrotic yellows virus group, the potato yellow dwarf virus group, as well as A number of other plant viruses with various vectors (aphids, bugs, etc.).

Undoubtedly, rhabdoviruses share a common origin, as it is difficult to imagine the independent, repeated emergence of subgroups within such a compact and distinctive group of viruses. The possible origin of all five negative-sense RNA virus families has already been discussed. Much like Bunyaviruses, rhabdoviruses likely originated as arthropod-borne (insect-borne) pathogens. However, it remains unclear how fish rhabdovirus diseases arose: whether they initially involved ectoparasites that were subsequently lost due to the simplicity and reliability of waterborne transmission, or whether they represent an evolutionarily older branch of rhabdoviruses. All these questions require dedicated research.

The historical relationships between PLANT AND ANIMAL rhabdoviruses are more or less understood. Plant rhabdoviruses were likely primary, given that sap-sucking plant insects evolved before blood-sucking vectors and may have served as the evolutionary source for the latter.

The classification of fish and plant rhabdoviruses is far from straightforward. For instance, studies on fish rhabdoviruses showed that the proteins of two salmonid viruses are more similar in electrophoretic mobility to the proteins of rabies virus and potato yellow dwarf virus, whereas the proteins of two carp and pike viruses more closely resemble those of vesicular stomatitis virus. Naturally, no serological relationship exists among mammalian, fish, insect, and plant viruses.

The similarities in Morphology, ultrastructure, chemical composition, and genome strategy among all these viruses suggest a common origin. It has been established that goose erythrocytes are agglutinated not only by vesicular stomatitis and rabies viruses, but also by plant rhabdoviruses. This points to a strong similarity in surface structures among such disparate viruses. Nevertheless, these facts provide no definitive clues regarding the exact origin of these viruses.

The evolution of rhabdoviruses proved to be more complex than a straightforward pathway from plants to animals, as different groups of plant viruses could have given rise to different groups of animal viruses—namely, the vesicular stomatitis and rabies virus groups, respectively. From this perspective, It is interesting that the fish hematopoietic necrosis virus contains a Cistron between the G and L genes that encodes a non-structural NV protein (12,000), which is absent in vesicular stomatitis virus and reduced in rabies virus [Tordo N. et al., 1986].

Fig. 21. Model showing the homology between the rabies virus glycoprotein and the "toxic" loop of neurotoxins. A single letter in a circle indicates identity between the glycoprotein and the toxin; a thin line denotes highly conserved neurotoxin residues; 10 residues within the glycoprotein that are absent in the neurotoxin are enclosed in a box.

Another unexpected finding is the structural similarity between the rabies virus glycoprotein and snake venom neurotoxins, extending even to amino acid homology (Fig.¹ 21). Is this an evolutionary relationship or a case of convergent molecular evolution? A striking example of molecular convergence is the similarity in Amino acid sequences between the rabies virus glycoprotein and snake venom neurotoxins [Suzuki A. et al., 1984]. The rabies virus glycoprotein molecule consists of 505 amino acid residues. After multiplying in Muscles, the virus reaches peripheral nerves and spreads along Neurons, selectively infecting specific populations of them. The host receptor for rabies virus is the Acetylcholine Receptor, which also serves as the Ligand for curare-like snake venom neurotoxins. Comparative studies revealed significant correspondence between a fragment of the rabies virus glycoprotein and snake venom neurotoxins—specifically, in the Regions of the venom molecules responsible for neurotoxicity and those interacting with acetylcholine receptors. It is likely that the corresponding region of the glycoprotein molecule also recognizes the receptor, thereby determining the neurotropism of the virus.

To understand the potential evolutionary pathways of rhabdoviruses, it should be recalled that vesicular stomatitis viruses are transmitted by mosquitoes and other blood-sucking insects, whereas plant rhabdoviruses are transmitted by aphids; in both cases, the viruses replicate within the insect vectors. Therefore, it is quite possible that insects were the primary "hosts" of rhabdoviruses, although the direct precursors of currently existing rhabdoviruses have not survived. Later, with The Emergence of hematophagous insects, rhabdoviruses could evolve toward adapting to parasitism in warm-blooded organisms, potentially giving rise to viruses such as the vesicular stomatitis agent, whereas the appearance of aphids enabled rhabdoviruses to evolve toward plant parasitism. Given that the diversification of angiosperms and insects occurred in the Late Mesozoic to Early Cenozoic, the emergence of these rhabdovirus groups should be dated to no more than 50–80 million years ago. As for rhabdoviruses infecting fish, they may have been introduced by arthropod parasites, with subsequent evolution proceeding independently of them, since in the aquatic environment, the transmission of fish disease- вызывающих viruses is reliably ensured even without vectors.

The evolution of rhabdoviral diseases via vector-borne transmission has led to the establishment of several nosological forms with distinct natural foci, exemplified by rhabdoviruses isolated from bats and mosquitoes. However, human activity—specifically the domestication and breeding of farm animals—resulted in the emergence of domestic animal anthroponoses, notably bovine ephemeral fever, which has remained an infection transmitted by biting midges and mosquitoes. Currently, the disease is prevalent in South Africa, Australia, and Japan, with viral Circulation maintained by Culicoides midges and Anopheles and Culex mosquitoes widely distributed in subtropical and temperate climates. Another agricultural zoonosis, vesicular stomatitis, has become even more "detached" from its natural foci in South America, where these reservoirs are associated with rodents and the disease is transmitted by numerous insect species (sandflies, mosquitoes, horseflies, and biting flies). However, because the disease involves lesions of the Oral Cavity, Nose and Lips, udder teats, and even the interdigital spaces, an additional alimentary transmission mechanism arises via feed (grass) and Water, which can become predominant under conditions of livestock overcrowding. Moreover, unlike the previous virus, the vesicular stomatitis virus is polypathogenic, affecting horses, mules, and cattle [Syurin V. N., Fomina N. V., 1979]. Thus, a typical domestic animal zoonosis is formed, "detached" from its natural foci. Both zoonotic viruses are non-pathogenic to humans.

The rabies virus has followed a rather unique evolutionary pathway. It is a polypathogenic virus that infects all mammalian species, causing a fatal infection. At the same time, regarding its transmission mechanism, it is a specialized virus spread through the bites of infected animals. Being a neurotropic virus, it is excreted into the external environment via saliva. Its neurotropism is specific, as it spreads centrifugally or centripetally along nerve fibers and selectively affects the Hypothalamus. This is likely responsible for the behavioral and emotional changes observed in infected animals (such as aggressiveness and the drive to bite). Due to its prolonged incubation period, the epizootic process appears protracted; although it is maintained primarily through viral circulation among wolves, dogs, and foxes, it also involves rodents, cats, horses, cattle, small ruminants, skunks, and bats. By contracting the infection from animals—predominantly dogs—humans act as a "dead-end" host in this process.

In general, the classical rabies virus isolated in various regions of the world appears to be quite homogeneous. However, similar viruses have been isolated from warm-blooded animals in different countries: the Nigerian bat virus, the Mokola virus isolated from a shrew and a human, and others. Serologically related viruses have also been isolated from mosquitoes and midges in Sudan (Obodhiang virus) and Nigeria (Kotonkan virus).

In the northern regions of the USSR, a rabies-like virus known as wild-rodent rabies (dikovanie) has been isolated, which causes a disease with a more sluggish progression. It is considered a geographical variant circulating in the wild among arctic foxes. In the USSR, viruses similar to the rabies agent were isolated in cases of bovine encephalomyelitis and human multiple sclerosis. It is hypothesized that the source of these viruses was not sick animals or humans, but rather white mice used as laboratory animals. It is important to note that these viruses proved distinct from laboratory strains of the rabies virus. In our opinion, all these facts point to a vector-borne transmission pathway for the ancestral rabies virus. Its prototypes are viruses serologically related to the rabies virus but possessing distinct antigenic profiles. These viruses (some designate them as rabies serotypes 2, 3, and 4), or at least some of them, still retain a vector-borne transmission mechanism today. The "detachment" of the rabies virus from hematophagous vectors and its transition to transmission via bites of infected animals led to the establishment of a zoonosis less dependent on tropical regions, facilitating its expansion into temperate countries. This likely occurred in the relatively late Cenozoic era in association with glaciation and global cooling.

Further Evolution of the rabies virus was driven by The impact of human activity. In populated areas, dogs rather than wolves became the primary reservoir of the pathogen, while the extermination of wolves and foxes led to the eradication of rabies in several economically developed countries, most notably in Great Britain. However, foci of "sylvatic" rabies still persist in Europe and Asia. Their eradication is currently unfeasible, as it would require the elimination of multiple predator species (wolves, foxes, jackals), which could have detrimental effects on natural biocenoses. In recent years, there has been a resurgence of natural foci associated with foxes.

It is also appropriate to mention a small group of hemorrhagic fever agents—filoviruses. The family Filoviridae and its eponymous genus (Filovirus) comprise the immunologically related Marburg and Ebola viruses, which are the causative agents of severe hemorrhagic fevers. Their virions are highly pleomorphic, appearing in native preparations as long filaments or irregularly shaped forms. In cultures, the virions are spherical with a diameter of 80 nm. The nucleocapsid is helical with a diameter of 50 nm, formed by filaments 20 nm thick, while the outer envelope is derived from a modified cell membrane. The RNA is of negative polarity with a molecular weight of 4.2x106. The virions contain 5 proteins: VP0—polymerase (190,000); VP1—glycoprotein of outer spikes (125,000–140,000); VP2 (98,000–104,000) and VP3 (38,000–40,000)—nucleocapsid proteins; VP4 (22,000–26,000)—a protein of unknown function.

The diseases caused by these viruses are characterized by natural focality. The ecology of these viruses remains insufficiently studied, making speculations about their evolution impractical.