Influenza virus functions

The antiviral medications amantadine and rimantadine can help reduce severity of illness in individuals with influenza that begin utilizing the drugs within two days of the onset of symptoms. These drugs work by hindering the change in pH that is necessary for the flu virion to release its contents into the cytosol of a host cell. Two additional antiviral drugs, zanamavir and oseltamivir, are effective against both A and B types of influenza.

Instead of interfering with pH shifts, zanamavir and oseltamivir block the glycoprotein neuraminidase so that the release of new virus particles is inhibited and their spread is thwarted. It is important to note that antibiotics are not capable of fighting the influenza virus itself, but are sometimes given to patients with the flu to stem attacks of opportunistic microorganisms that are responsible for many influenza complications. Though widespread familiarity with the flu makes it seem relatively benign to much of the general population, the virus can be devastating.

In and , more than 20 million people died from a strain of the virus commonly known as the Spanish flu that circulated through almost all inhabited regions of the globe.

Many other outbreaks have occurred since that time, though none have been as deadly. Nevertheless, influenza together with complications of the virus is consistently among the top ten common causes of death in the United States, ranking higher than some other much more widely publicized killers, such as the HIV virus that causes AIDS. License Info. Image Use. Custom Photos.

Site Info. Contact Us. The Galleries:. Photo Gallery. Silicon Zoo. Chip Shots. DNA Gallery. Amino Acids. Religion Collection. Cocktail Collection. The virus that caused this particular outbreak is influenza A subtype H5N1. Since , H5N1 infections in birds have spread, initially throughout Asia.

Then as birds traveled along their migratory routes, H5N1 dispersed to Russia and Europe, and later to countries in the Middle East and on the African continent. Most human cases of H5N1 influenza have been traced to direct contact with infected poultry, but there have been a few cases of person-to-person transmission, particularly in clusters where multiple family members became infected. One reason why avian H5N1 is not readily transmissible among people has to do with the hemagglutinin, or HA, protein of the virus that determines which cell type the virus can enter.

As with other viruses, the influenza virus must attach to specific proteins called receptors on the outside of cells in order to gain entry into cells and cause an infection.

Unlike human influenza viruses, which infect cells high in the respiratory tract, the H5N1 HA protein attaches to cells much lower in the respiratory track. The virus is so deep within the respiratory tract that it is not coughed up or sneezed out, and so it does not easily infect other people. If the HA protein of H5N1 were to mutate so that it could infect cells higher in the respiratory tract, then it would more likely be able to pass from person to person.

As of July , there have been some laboratory-confirmed cases of H5N1 infections in humans, in 16 different countries, and close to deaths. The countries with the overall highest case numbers are Egypt, where almost all cases in have occurred, followed by Indonesia and Vietnam.

H5N1 continues to circulate in poultry, and small and sporadic clusters of human infections are still occurring. However, H5N1 currently does not transmit easily between people, so the risk of a large outbreak is low at this time. Highly pathogenic H5 avian virus infections were first reported in birds in the United States in December Over approximately the next six months, more than findings of infection with H5N2, H5N8, and H5N1 viruses were confirmed, mostly in poultry including backyard and commercial flocks.

More than 40 million birds in 20 states were either infected or exposed. No human infections by these H5 viruses have been reported in the United States, but their presence in birds makes it more likely than human H5 infections could occur in the United States.

Individuals having close contact with live infected poultry or surfaces contaminated with the avian influenza viruses are at highest risk of infection in places where the viruses circulate. There have been no reports of infection occurring from eating properly cooked poultry.

In addition to the H5 viral subtypes, other avian influenza strains have occasionally infected humans in recent years. These include the H7N2 strain which infected two individuals in the eastern United States in and , and the H9N2 strain which has caused illness in several people in Asia in and In March of , a new subtype of avian influenza was found to infect humans.

Influenza A H7N9 had previously been detected in birds, but this particular variant had never been seen before in humans or animals. The initial wave of H7N9 infections occurred in the spring of in China, followed by a larger, second wave in the first half of in China and a few neighboring countries. As of February , approximately cases and deaths have been reported to the WHO, mostly in China.

People in the majority of cases had exposure to infected poultry or contaminated environments. The H7N9 virus causes a severe respiratory illness in most infected people, but it currently does not appear to spread easily from person to person. Swine influenza, or swine flu, is a very contagious respiratory disease of pigs. Although pigs become ill, they generally do not die from swine flu viruses. In April of , an influenza virus originating in swine was discovered to be capable of infecting humans and spreading from person to person.

The new virus is named influenza A H1N1 , although it is commonly referred to as swine flu. Although it is called swine flu, the new H1N1 virus is transmitted from person to person, and not through contact with pigs or pork products. The new H1N1 virus is made up of a novel combination of segments from four different influenza virus strains - a Eurasian swine virus, a North American swine virus, and avian and human influenza virus segments.

Reassortment of segments from these different viruses produced a unique virus that had not been seen before by the human population. When novel viruses like this emerge, natural immunity is usually limited or nonexistent in humans. The H1N1 influenza virus outbreak originated in Mexico in early , and then spread rapidly throughout North America. Within a few weeks, the novel swine-origin H1N1 virus extended its reach around the globe.

In June , as a result of the global spread of the H1N1 virus, the WHO issued its first pandemic declaration of the 21st century - the first since the flu pandemic of The pandemic declaration acknowledged the inability to contain the virus and recognized its inevitable further spread within affected countries and into new countries.

The new H1N1 virus became the dominant influenza strain in most parts of the world, including the United States. Like other influenza pandemics, the H1N1 outbreak occurred in waves.

The first wave took place in the spring of , with a second wave commencing in late August as children and college students returned to classes. The outbreak peaked in October of , with flu activity reported in all 50 states, as well as numerous other countries and territories.

By January , flu activity had returned to below baseline levels. The H1N1 virus continues to circulate at low levels, but it is no longer the dominant influenza strain, and its behavior more closely resembles a seasonal influenza virus than a pandemic flu.

From the time the outbreak began in April through April , the CDC estimated that about 60 million Americans became infected with the H1N1 virus, , Americans were hospitalized and 12, deaths occurred as a consequence of the H1N1 flu.

The highest hospitalization rates occurred in young children. Exact numbers are not known due to the widespread nature of the outbreak and because most patients, especially those with mild cases, were not tested. The large majority of infections in the United States and most other countries were mild, although pregnant women and individuals with certain underlying medical conditions had an increased risk of severe and fatal illness.

There were some differences between the pandemic H1N1 flu and regular, seasonal flu. First, the H1N1 flu continued to spread during the summer months, which is uncommon for seasonal flu. Second, a much larger percentage of H1N1 patients exhibited symptoms of vomiting and diarrhea than is common with regular seasonal flu. There were also more reports of severe respiratory disease, especially in young and otherwise healthy people, infected with the new H1N1 virus than with seasonal flu viruses.

Significantly, the majority of cases of H1N1 infection, including severe and fatal cases, occurred in young and otherwise healthy individuals generally between the ages of 5 and 50, with relatively few deaths among the elderly. This is in contrast to the situation with seasonal flu which primarily afflicts the very young and the elderly, and where 90 percent of severe and lethal cases occur in people over the age of Deaths among the elderly accounted for only 11 percent of H1N1 deaths.

Proper use of these drugs can shorten the duration and lessen the severity of the sickness and reduce the chance of spreading the disease. The drugs reduce the risk of pneumonia - a major cause of death from influenza - and the need for hospitalization. To be most effective, the antiviral drugs should be administered as soon as possible after the onset of symptoms.

A vaccine to protect against the H1N1 virus was developed, tested, and approved and became available in October Due to the fact that the virus used to prepare the vaccine grew more slowly than most seasonal flu viruses do, production of the vaccine lagged and widespread distribution of the vaccine occurred later than anticipated.

Priority for the vaccine was initially given to health care and emergency workers and individuals at high risk for severe disease, but by the winter of availability was extended to the general population.

Later, some doses went unused. Although some had concerns about the safety of the H1N1 vaccine, flu vaccines have a very good safety profile. While mild side effects, such as soreness at the site of injection, aches, and low-grade fever, may occur as a result of receiving a flu shot, it is not possible to get the flu H1N1 or seasonal from the vaccine. The flu shot, or inactivated vaccine, is made from only a portion of the virus — a purified protein that makes our immune system develop protection.

Likewise, the nasal spray version of the flu vaccine contains attenuated or weakened virus that is not able to cause the flu. Given the potential serious health outcomes from the flu, especially for high-risk population groups, the benefits of vaccination as the best way to prevent influenza infection and its complications far outweigh the risk of relatively minor side effects from the vaccination.

Historically, influenza pandemics arise about three to four times each century. The most recent pandemic , and the first of the 21st century, occurred in , some 40 years after the previous pandemic.

The H1N1 flu, commonly known as swine flu, spread around the globe faster than any virus in history, largely due to air travel. Pandemic flu strains are of deep concern because there is no or only limited natural immunity to novel flu strains, and therefore nearly everyone is susceptible to infection.

A high percentage of the population could become ill at any one time and overwhelm public health systems, and a large number of deaths could occur. We were very fortunate in the case of the H1N1 pandemic. Most people suffered only a mild illness. H1N1 was not an especially virulent virus. Further, the virus remained stable and did not mutate to a more deadly form or to a drug resistant form.

Other influenza strains have been far more lethal. Currently, there is concern about the new avian H7N9 virus. Most patients have experienced severe respiratory illness, with about one-third of the cases resulting in death.

Although the virus does not appear to pass easily from person to person, there is always the worry that it could mutate into a form that is more transmissible. There are drugs that are effective against influenza, but the possibility that a virus could acquire resistance to the drugs is a serious issue.

There are four different antiviral drugs, of two different classes, that are effective against influenza. However, influenza viruses can and do develop resistance to these drugs - as one of the main circulating seasonal viruses did during a recent flu season - so that the drugs can no longer be used to treat or prevent infections.

There is a need to develop additional drugs that can prevent or alleviate flu symptoms. Vaccines can be developed to protect humans from influenza viruses. However, as was strikingly obvious during the H1N1 pandemic, vaccine production takes many months. By the time a vaccine was developed, tested, produced, and distributed, many individuals had already been infected.

Clearly, a more rapid method of vaccine development is needed. The goal of developing a universal flu vaccine, one that would provide durable protection against multiple flu strains, remains a challenging feat. The greatest fear is that a new pandemic influenza virus could emerge that could pass from person to person as easily as the H1N1 virus, but be as deadly as the H5N1 virus. Additional concerns are that an influenza virus could mutate into a form that would be resistant to anti-influenza drugs, such as Tamiflu, or that the virus could change so that a vaccine no longer afforded protection.

Even though the H1N1 pandemic was relatively mild, knowing how lethal and unpredictable influenza viruses can be, we must continue to remain alert and prepare for future pandemics. There are eight conservative disulfide bonds in the NA structure, and one additional bond in the N2, N8, and N9 subtypes.

The invariance of disulfide bonds confirms their importance in the formation of a stable NA structure. It is assumed that, because of its proximity to the symmetry axis of the tetramer, the uncoupled Cys of N1 NA takes part in the coupling of subunits. The tetramer assembly mechanism is not universal: for instance, in influenza virus B neuraminidases disulfide bonds are formed by Cys54, whereas Cys78 takes part in polypeptide chains binding N2 numbering [ 2 ].

The Neu5Ac binding site is located above the first strands of the third and the fourth motifs in a big loop on the NA surface.

The active site is located at the N-terminal end of central parallel strands [ 2 ]; Fig. This site is surrounded by twelve flexible loops, which go upwards from that axis [ 6 ].

Functional a. Recent X-ray studies of neuraminidases from the first phylogenic group have shown that, in comparison with neuraminidases from the second phylogenic group, they have a slightly different structure of the polypeptide chain around the enzyme active centre. In particular, there is a cavity in close proximity to the active site, which is formed by a change in the dimensional orientation of "loop The NA's reaction mechanism Scheme 1 was proposed based on the results of structural studies of the crystallized protein [ 7 ].

Mechanism of substrate desialylation by influenza virus neuraminidase according to [ 7 ], [ 15 ], and [ 16 ]. After the introduction of the Neu5Ac residue into the active centre, Neu5Ac conformation changes from chair to half-chair, i.

The molecule of aglycone leaves the enzyme active site with glycosidic oxygen, protonated by the solvent. Multiple contacts between the intermediate product and the a. Neu5Ac2en, in which the C2 atom is in sp2-form, mimics the intermediate reaction product in planar conformation [ 6 ]. At this stage of the reaction, the neuraminic acid residue is covalently bound to the hydroxyl group of Tyr, which is characteristic of all exosialidases [ 15 , 17 ].

Hydroxylation of the oxocarbonium ion with the solvent and product leaving the enzyme active site in the form of Neu5Ac are the limiting stages of the catalytic reaction. It is worth mentioning that there are no significant changes in the coordinates of the NA active site during the reaction [ 18 ]. The presence of invariant residues in the active site, the similarity of the structural organization, and the architecture of complexes with Neu5Ac and with Neu5Ac2en allow to assume that the mechanism of NA functioning for the A and B influenza viruses is identical [ 6 ].

The structure of the neuraminidase active site is strictly conservative not only between subtypes, but also between the types of the enzyme, which points to the importance of all its components and the evolutionary stabilized functioning of this system.

This observation has allowed to design an NA inhibitor for the influenza virus which mimics the transition state of the hydrolysis reaction, and Neu5Ac2en Fig. The success of this drug has initiated a number of studies aimed at designing new NA inhibitors. The main structural elements of the new class of inhibitors without the oxygen atom in the cycle are cyclohexane and cyclopentane. One of those structures is the 3S,4R,5R aminoacetamido 1-ethylpropoxy cyclohexenecarboxylic acid oseltamivir or Tamiflu Fig.

The structure of this molecule is adjusted to coordinates of the amino acids, which interact with the glycerol chain of Neu5Ac2en [ 20 ].

Successful use of this drug has stimulated the development of new NA inhibitors with hydrophobic groups [ 21 ]. Besides, a NA inhibitor on the base of a cyclopentane structure has been developed; it has all the functionally important parts of zanamivir carboxyl, acetamide, C4-hydroxyl which fit into the NA active centre.

BCX preamivir Fig. At present, preamivir analogs are at the development stage. Zanamivir and oseltamivir are already used as drug products, whereas BCX has entered the last phase of clinical trials.

Until recently, it was considered that active uncontrolled use of zanamivir and oseltamivir would not have a significant influence on the development of resistance in influenza virus strains. That is, even if resistant strains emerge, they would not be able to replicate in the absence of the inhibitor [ 24 ]. The mutation HisTyr had been spotted in studies of resistance in vitro and in vivo, as well as in clinical isolates [ 26 ].

This leaves us with hope that the strain of the influenza virus that will cause the next pandemic might be susceptible to this NA inhibitor. There is data indicating that NA is relevant at different stages of infection. Firstly, it is considered that it helps the virus approach the target cells by cleavage of sialic acids from respiratory tract mucins [ 26 ].

Secondly, it may take part in the fusion of viral and cell membranes [ 27 ]. Thirdly, it facilitates budding of new virions by preventing their aggregation, caused by the interaction of the HA of the first virus with the sialylated glycans of the second one [ 27 ].

In addition, there is data suggesting that NA amplifies HA haemagglutinating activity by cleavage of the terminal neuraminic acid residues of the oligosaccharides surrounding the receptor-binding site of HA [ 28 ].

One of the most interesting features of the influenza virus is the coexistence of two proteins whose functions are to some extent contradictory, namely: haemagglutinin, which has a receptor-binding function; and neuraminidase, which has a receptor destroying function. Studies of the viruses resistant to NA inhibitors, artificial viral reassortants which have HA and NA of different origins , and virus particles designed by means of reverse genetics, which lack NA or HA activity, show that the NA and HA of the influenza virus act in concert and their evolution proceeds interdependently [ 29 - 35 ].

Also, it raises a question as to their oligosaccharide specificity, because Neu5Ac-terminated oligosaccharide chains in viral hosts are quite diverse. The method based on the use of this substrate was proposed [ 36 ] first as an alternative to colorimetric or radioactive methods.

After cleavage of the neuraminic acid, MU-Neu5Ac forms a fluorophore which is activated by light at a wavelength of nm, and its fluorescence maximum is achieved at pH High fluorescence intensity fold higher than for MUNeu5Ac is useful in studies of low-activity neuraminidases [ 37 ].

The main disadvantage of this method is the short lifetime of the product of chemiluminescent hydrolysis, which has to be recorded within 5 minutes. The amount of free neuraminic acid is usually determined after cleavage [ 39 ]; the most convenient procedure for assay of Neu5Ac allows for conducting measurements in the presence of the sialylated substrate [ 40 ].

An alternative procedure is based on assay of the second product of the hydrolysis, the desialylated glycoprotein, with the help of lectin for example, Peanut agglutinin , which is specific for the unmasked terminal galactose [ 41 , 42 ].

The substrate specificity of NA is its ability to discriminate between sialic acids for example, Neu5Ac and Neu5Gc and linkage type with the next residue , or , as well as the ability to identify internal regions of the oligosaccharide chain. In particular, the following structures have been used for the determination of NA substrate specificity:. Methods based on the use of those substrates achieve only one of the listed goals; in particular, they allow to study specificity at the level of SiaGal or SiaGal.

More broad specificity can be studied with the use of an analytical procedure which employs a number of synthetic substrates. In [ 42 ], a panel of three oligosaccharides was used: 3'SiaLac, 6'SiaLac and 6'SiaLacNAc, in the form of polyacrylamide conjugates; and neuraminidase activity was measured by lectin, specific for galactose residues, which appear as the result of NA action see above. A new simple and sensitive method for NA specificity determination has been developed recently [ 48 ].

Stability, relative hydrophility, electroneutrality, small size, and ability to use standard fluorescent filter for detection are the advantages of this label.

The method is based on a quantitative separation of the electroneutral product of the reaction and the negatively charged substrate, separation is performed either on a microcartrige with an anion-exchange sorbent or microplates, the semipermeable bottom of which consists of an anion-exchange material. For greater reliability one may quantify the amount of the reaction substrate, along with the quantity of the reaction product.

The high sensitivity of the method makes it possible to work with low substrate concentrations mol , as well as with low virus concentrations. Studies of desialylation kinetics, in particular the reaction velocity and its dependence on substrate and enzyme concentration, is important for understanding the reaction mechanism, as well as for the choice of the correct concentration range.

In turn, the correct range allows to study desialylation specificity in cases when the NA quantity in the test sample is unknown [ 49 ]. It is worth mentioning that only this approach allows to study many aspects of NA substrate specificity see above , namely to study the influence of the sialic acid type, the type of linkage between the sialic acid and the next sugar, and the influence of the inner glycan sugars.

As already mentioned above, high molecular weight substrates, along with low molecular weight substrates, can be used for studying NA activity and specificity. Low molecular weight substrates allow to study the reaction mechanism and desialylation kinetics without the complications of multivalent interactions NA is a tetramer and the possible influence of HA, which interacts with multivalent conjugate 3 — 5 orders of magnitude better than with the monomeric one [ 50 ].

High molecular weight substrates appear to be a more accurate model for studying natural interactions; that is when there is a necessity to account the NA tetrameric organization, the clustering of NA molecules on the cell surface, and the involvement of the second surface glycoprotein, HA, which is present on the viral surface in larger amount.

Investigation of the evolution of the influenza virus NA substrate specificity for viruses isolated from humans, and its comparison with the substrate specificity of influenza virus NAs isolated from different hosts, such as ducks and pigs, is of great importance.

The first undertaking could shed light on the question of the unique character of pandemic strains, while the second could help detect in advance the properties of the enzyme which facilitate the crossing of the interspecies barrier. Hydrolytic activity towards 6'SiaLac was identified only for viruses isolated in and further, and starting from isolates an increase of activity towards this substrate was registered [ 46 ].

It has been shown recently that N2 influenza viruses are highly active towards 3'SiaLac, while their activity towards 6'SiaLac varies from extremely low avian and early human isolates to high swine and latter human isolates.

It has been shown that NA activity towards 6'SiaLac depends also on the host type and, for human viruses, on the year of isolation [ 45 ]. For N1 strains isolated in the s [ 43 , 44 ], it was shown that their neuraminidase equally recognizes 3'SiaLac and 6'SiaLac. Data on the substrate specificity of N1 and the N2 NAs of several duck, swine and human influenza virus isolates were obtained with the use of BODIPY-labeled synthetic oligosaccharides [ 48 - 51 ].

In summary, the nature of the host cell line used for virus accumulation influences NA substrate specificity [ 42 ]. The reason for this effect remains unknown. It is difficult to compare the results of substrate specificity studies conducted by different authors due to the use of both different influenza virus strains and different substrates in varying concentrations.

It is also worth mentioning that studies of the influenza virus with the simultaneous use of high- and low-molecular weight substrates of a defined structure have yet to be conducted. Despite the limited amount of data published to date, it is already possible to discuss some features. Firstly, the NA substrate specificity of human isolates differs from that of avian isolates.