Gs and I have had several discussions about the possible advantages of the filamentous shape over the spherical shape found in influenza. Here are some snips of interest I found while researching the subject; I'll have more to add to this post as time permits and welcome input from others:
Shapes of virions:
1. They are generally spherical or elliptical in shape, ranging from approximately 80–120nm in diameter and are occasionally filamentous, reaching more than 20μm in length.
1. Whether spherical, elliptical or filamentous, the lipid-enveloped virions are regular in shape and covered with HA and NA glycoproteins and M2 proteins. Abundant HA proteins are distributed over the virion surface.
1. Recent electron microscopic analyses of influenza virions clearly show that the native virion morphology is uniformly spherical, elliptical, or filamentous. Irregularly shaped virus particles are mainly artifacts caused during sample processing, indicating that such virus particles unlikely reflect the true virion structure.
2. Certain strains of influenza virus, such as A/Udorn/72, are capable of producing filamentous as well as spherical virions.
5. Yet, despite their high degree of genetic similarity, we found the morphology of virions produced by the pH1N1 isolate, A/California/04/09 (ACal-04/09), to be predominantly spherical, whereas TR strains were observed to be mostly filamentous. In addition, nine clinical pH1N1 samples collected from nasal swab specimens showed similar spherical morphology as the ACal-04/09 strain. (also spherical in human lung and swine kidney epithelial cells)
Methods of entry into cells
2. Influenza virus is known to trigger clathrin-mediated endocytosis to facilitate entry into host cells. Interestingly, influenza virus remains capable of infecting host cells, with little to no loss of infectivity, even when clathrin-mediated endocytosis is blocked. Recent work has shown that influenza virus is one of a growing list of viruses that are capable of inducing and utilizing macropinocytosis for cellular entry.
2. Additionally, it is completely unclear how a 100-nm by 1- to 50-μm filament would be able to fit into a standard clathrin-coated pit. In examining the entry of the filamentous form of influenza virus, we have validated previous results showing that influenza virus can utilize macropinocytosis as an alternate entry pathway
Amino Acid changes found to determine shapes
3. Whereas A/WSN/33 (H1N1) virus generally forms particles that are predominantly spherical, observations made by negative stain electron microscopy showed that several of the mutant virions, such as K95A, K98A, R101A, and K102A, display a wide range of shapes and sizes that varied in a temperature-dependent manner. The K102A mutant is particularly interesting in that it can form extended filamentous particles. These results support the proposition that the helix six domain is involved in the process of virus assembly.
4. A plasmid-based reverse genetics system was used to transfer the M segment of influenza A/Udorn/72 (H3N2) virus into influenza A/WSN/33 (H1N1) virus. While WSN virions display spherical morphology, recombinant WSN-Mud virus acquired the ability of the parental Udorn strain to form filamentous virus particles. ...Characterization of these recombinant viruses revealed that amino acid residues 95 and 204 are critical in determining filamentous virus particle formation.
5. Sequence analysis between TR and pH1N1 viruses revealed four amino acid differences in the viral matrix protein (M1), a known determinant of influenza morphology, at positions 30, 142, 207, and 209. To test the role of these amino acids in virus morphology, we rescued mutant pH1N1 viruses in which each of the four M1 residues were replaced with the corresponding TR residue.
5. pH1N1 containing substitutions at positions 30, 207 and 209 exhibited a switch to filamentous morphology, indicating a role for these residues in virion morphology. These data suggest that a few mutations present in pH1N1 contribute to morphological change and efficient transmission of influenza viruses.
** Notes from GS regarding positions:
most viruses have D30,V142,S207,A209 in M1
including the bird index, seasonal H3N2, old seasonal H1N1,
old classical swine H1N1, triple reassortant US-swine(1998) [filamentous] ,
early European swine H1N1 (1978)
then in 1998 we have Italian swine with the mutations
that are now also in ******=pH1N1
some H5N1 from ~2001 had N207 and T209
The Roles of the Shapes
5. Though it is known that influenza morphology affects virus production, its role in viral transmission is still unclear. Early reports showed that most influenza strains isolated from humans are predominantly filamentous and, upon continual passage in egg or tissue culture, adopt a more uniformly spherical morphology. This switch in morphology correlates with an increase in virus titer. Therefore, it is likely that the increased levels of virus production by spherical influenza strains results in more efficient viral transmission.
5. Substitutions at these residues resulted in lower viral titers, reduced growth kinetics, and small plaque phenotypes compared to wild-type, suggesting a correlation between influenza morphology and efficient cell-to-cell spread in vitro.
5. Furthermore, we observed efficient virus-like particle production from cells expressing wild-type pH1N1 M1, but not M1 containing substitutions at positions 30, 207, and 209, or M1 from other strains. These data suggest a direct role for pH1N1 specific M1 residues in the production and release of spherical progeny, which may contribute to the rapid spread of the pandemic virus.
********************
1. Native Morphology of Influenza Virions
Takeshi Noda
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/
2. Filamentous Influenza Virus Enters Cells via Macropinocytosis
Jeremy S. Rossman,a,b,* George P. Leser,a,b and Robert A. Lamb
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3457176/
3. Influenza A Viruses with Mutations in the M1 Helix Six Domain Display a Wide Variety of Morphological Phenotypes 2005
Laura M. Burleigh1,2, Lesley J. Calder2, John J. Skehel2, and David A. Steinhauer1,2,*
http://jvi.asm.org/content/79/2/1262.full
4. Reverse genetics studies on the filamentous morphology of influenza A virus 2002
Svetlana V. Bourmakina and Adolfo García-Sastre
http://vir.sgmjournals.org/content/84/3/517.full
5. Specific Residues in the 2009 H1N1 Swine-Origin Influenza Matrix Protein Influence Virion Morphology and Efficiency of Viral Spread In Vitro
Kristy M. Bialas, Emily A. Desmet,¤ and Toru Takimoto
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3507794/
Shapes of virions:
1. They are generally spherical or elliptical in shape, ranging from approximately 80–120nm in diameter and are occasionally filamentous, reaching more than 20μm in length.
1. Whether spherical, elliptical or filamentous, the lipid-enveloped virions are regular in shape and covered with HA and NA glycoproteins and M2 proteins. Abundant HA proteins are distributed over the virion surface.
1. Recent electron microscopic analyses of influenza virions clearly show that the native virion morphology is uniformly spherical, elliptical, or filamentous. Irregularly shaped virus particles are mainly artifacts caused during sample processing, indicating that such virus particles unlikely reflect the true virion structure.
2. Certain strains of influenza virus, such as A/Udorn/72, are capable of producing filamentous as well as spherical virions.
5. Yet, despite their high degree of genetic similarity, we found the morphology of virions produced by the pH1N1 isolate, A/California/04/09 (ACal-04/09), to be predominantly spherical, whereas TR strains were observed to be mostly filamentous. In addition, nine clinical pH1N1 samples collected from nasal swab specimens showed similar spherical morphology as the ACal-04/09 strain. (also spherical in human lung and swine kidney epithelial cells)
Methods of entry into cells
2. Influenza virus is known to trigger clathrin-mediated endocytosis to facilitate entry into host cells. Interestingly, influenza virus remains capable of infecting host cells, with little to no loss of infectivity, even when clathrin-mediated endocytosis is blocked. Recent work has shown that influenza virus is one of a growing list of viruses that are capable of inducing and utilizing macropinocytosis for cellular entry.
2. Additionally, it is completely unclear how a 100-nm by 1- to 50-μm filament would be able to fit into a standard clathrin-coated pit. In examining the entry of the filamentous form of influenza virus, we have validated previous results showing that influenza virus can utilize macropinocytosis as an alternate entry pathway
Amino Acid changes found to determine shapes
3. Whereas A/WSN/33 (H1N1) virus generally forms particles that are predominantly spherical, observations made by negative stain electron microscopy showed that several of the mutant virions, such as K95A, K98A, R101A, and K102A, display a wide range of shapes and sizes that varied in a temperature-dependent manner. The K102A mutant is particularly interesting in that it can form extended filamentous particles. These results support the proposition that the helix six domain is involved in the process of virus assembly.
4. A plasmid-based reverse genetics system was used to transfer the M segment of influenza A/Udorn/72 (H3N2) virus into influenza A/WSN/33 (H1N1) virus. While WSN virions display spherical morphology, recombinant WSN-Mud virus acquired the ability of the parental Udorn strain to form filamentous virus particles. ...Characterization of these recombinant viruses revealed that amino acid residues 95 and 204 are critical in determining filamentous virus particle formation.
5. Sequence analysis between TR and pH1N1 viruses revealed four amino acid differences in the viral matrix protein (M1), a known determinant of influenza morphology, at positions 30, 142, 207, and 209. To test the role of these amino acids in virus morphology, we rescued mutant pH1N1 viruses in which each of the four M1 residues were replaced with the corresponding TR residue.
5. pH1N1 containing substitutions at positions 30, 207 and 209 exhibited a switch to filamentous morphology, indicating a role for these residues in virion morphology. These data suggest that a few mutations present in pH1N1 contribute to morphological change and efficient transmission of influenza viruses.
** Notes from GS regarding positions:
most viruses have D30,V142,S207,A209 in M1
including the bird index, seasonal H3N2, old seasonal H1N1,
old classical swine H1N1, triple reassortant US-swine(1998) [filamentous] ,
early European swine H1N1 (1978)
then in 1998 we have Italian swine with the mutations
that are now also in ******=pH1N1
some H5N1 from ~2001 had N207 and T209
The Roles of the Shapes
5. Though it is known that influenza morphology affects virus production, its role in viral transmission is still unclear. Early reports showed that most influenza strains isolated from humans are predominantly filamentous and, upon continual passage in egg or tissue culture, adopt a more uniformly spherical morphology. This switch in morphology correlates with an increase in virus titer. Therefore, it is likely that the increased levels of virus production by spherical influenza strains results in more efficient viral transmission.
5. Substitutions at these residues resulted in lower viral titers, reduced growth kinetics, and small plaque phenotypes compared to wild-type, suggesting a correlation between influenza morphology and efficient cell-to-cell spread in vitro.
5. Furthermore, we observed efficient virus-like particle production from cells expressing wild-type pH1N1 M1, but not M1 containing substitutions at positions 30, 207, and 209, or M1 from other strains. These data suggest a direct role for pH1N1 specific M1 residues in the production and release of spherical progeny, which may contribute to the rapid spread of the pandemic virus.
********************
1. Native Morphology of Influenza Virions
Takeshi Noda
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3249889/
2. Filamentous Influenza Virus Enters Cells via Macropinocytosis
Jeremy S. Rossman,a,b,* George P. Leser,a,b and Robert A. Lamb
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3457176/
3. Influenza A Viruses with Mutations in the M1 Helix Six Domain Display a Wide Variety of Morphological Phenotypes 2005
Laura M. Burleigh1,2, Lesley J. Calder2, John J. Skehel2, and David A. Steinhauer1,2,*
http://jvi.asm.org/content/79/2/1262.full
4. Reverse genetics studies on the filamentous morphology of influenza A virus 2002
Svetlana V. Bourmakina and Adolfo García-Sastre
http://vir.sgmjournals.org/content/84/3/517.full
5. Specific Residues in the 2009 H1N1 Swine-Origin Influenza Matrix Protein Influence Virion Morphology and Efficiency of Viral Spread In Vitro
Kristy M. Bialas, Emily A. Desmet,¤ and Toru Takimoto
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3507794/
