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Communicative & Integrative Biology logoLink to Communicative & Integrative Biology
. 2009 May-Jun;2(3):243–244. doi: 10.4161/cib.2.3.8165

Tunneling nanotubes (TNT)

A potential mechanism for intercellular HIV trafficking

Eliseo A Eugenin 1,, Peter J Gaskill 1, Joan W Berman 1,2
PMCID: PMC2717534  PMID: 19641744

Abstract

Cell-to-cell communication coordinates the development of multicellular systems, and is mediated by soluble factors, gap junctions and the recently described tunneling nanotubes (TNT). Both TNT and gap junctions facilitate the transfer of intracellular mediators between the cytoplasm of connected cells. We recently described that HIV induced the formation of TNT in human primary macrophages in correlation with viral replication. Based on these results we hypothesized that during HIV infection, TNTs are hijacked by HIV to spread infection. TNT like structures may be a novel mechanism of amplification of HIV infection. Our findings and those of others require further investigation to identify the specific mechanisms by which pathogens use TNT.

Key words: AIDS, filopodia, inflammation, HIV, connexin, communication, macrophages


Tunneling nanotubes (TNT) and gap junctions are the only two described communication systems that allow exchange of cytoplasmatic factors through direct contact between the cytoplasm of connected cells. These communication systems coordinate biological processes, including development, metabolism, homeostasis and the immune response.16 The major differences between TNT and gap junctions are the distances reached and the sizes of the molecules transferred. TNT mediate long-range communication through extended processes, while gap junctions facilitate close cell-to-cell communication. Gap junctions allow the trafficking of small molecules, up to 1.2 kDa,6 while TNT allow the exchange of small molecules, organelles and vesicles.2

Our recent report, using primary human macrophages and HIV, suggested that TNT could be “hijacked” for the virus to spread between connected cells during the periods of high viral replication. HIV infection of macrophages enhanced the numbers of TNT and more infected cells expressed TNT, suggesting that HIV induced the expression or stability of these processes to allow viral spread through this mechanism.1 In the past year it has been shown that the virus utilized TNT-like structures to spread infection between connected T cells7 and we demonstrated HIV-p24 in TNT of HIV infected macrophages.1 We and others proposed that HIV, by using this pathway of spread, will infect cells more efficiently without entering the extracellular compartment, reducing viral exposure to natural anti-viral activities as well as to potential antiviral drugs. In agreement with this, it has been demonstrated that viral infections, including HIV, are increased by several orders of magnitude when cell to cell contact is involved, suggesting that direct actin cytoskeleton interactions between connected cells allow efficient viral spread.8,9

Although there is a basal level of TNT expression by cells under normal tissue culture conditions, the signals that guide the formation of TNT are unknown. However, re-examination of reports in the existing literature show that there are published descriptions of increased formation of TNT like structures in inflammatory conditions. In in vitro pathological conditions the formation of TNT-like structures has been observed after infection with Listeria monocytogenes and mycobacterium bovis,1012 in astrocytes treated with H2O2,13 microglia activated with PMA and calcium ionophore,14 monocyte/macrophages treated with LPS plus IFNγ,15 mouse neuronal cells infected with exogenous prion protein (PrP)16 and more recently in lymphocytes infected with HIV7 and in human macrophages infected with HIV.1 In vivo, TNT like structures have been observed in Drosophila,17,18 between immune cells in lymph nodes (reviewed in refs. 2 and 3), in the dendritic cells (DC) of the gut19,20 and in the MHC class II+ cells in the mouse cornea.21 Interestingly, viruses, such as African swine fever, Ebola, Herpes Simplex, Marburg filoviruses and Poxvirus Vaccinia encode viral factors or alter cell activation to induce formation of filopodia structures to allow viral trafficking between the extracellular matrix or environment into cells,2228 suggesting that viruses are able to use filopodia and TNT like structures to improve viral spread.

We still have a limited understanding of TNT function and turnover. It is unclear whether all TNTs are similar in length, internal size, permeability, transport capability (internal and external transport), and signaling properties. Our studies in human macrophages identified two distinct TNT morphologies that are altered by HIV infection, referred to as short- and long-range TNT. Both types of TNT, as well as filopodia, can coexist independently in different regions of the same cell, suggesting a cellular compartmentalization for the formation and transport through these processes in each cell.1 However, the specific function of each type of process, and whether these processes have differential permeability or transport properties remains unclear. In our system, filopodia did not connect cells. A recent report supports the idea of different types of TNT on different cells, showing that TNT in lymphocytes are almost impermeable to calcium,7 in contrast to the high permeability to calcium of TNT observed in dendritic and THP-1 cells.29 TNT in other systems allows transport of mitochondria and vesicles, suggesting that the internal pore size is large enough for the trafficking of these organelles (reviewed in refs. 2 and 30). The point that mitochondria can be exchanged between TNT connected cells is extremely important because this could be one of the first demonstrations of transfer of genetic material between non-dividing cells, suggesting that at least mitochondrial DNA is not cell type exclusive and can be shared between several cell types connected by TNT. Although it is still unclear whether multiple types of TNT exist or represent different maturation stages of the same processes, the potential for the transfer of organelles and changes in signaling opens a new era in understanding the cell as a unique entity.

Several groups have proposed at least two models to explain the varied types of TNT, based on cell type, permeability, signaling capabilities, length and function. The first model proposes a tube generated from one or both cells involved, resulting in fusion of membranes, leaving a continuous tube between the connected cells, allowing the transfer of molecules between the connected cells. The second model proposes that the processes do not form a tube, but rather are composed of adhesion molecules and other molecules involved in signaling that aggregate in the tip of the TNT, like a synapse, to coordinate intercellular communication. Based on the multiple lengths, cell types and potential function of these TNT in normal and in pathologic conditions, we believe that both proposed TNT systems may exist and require extensive investigation to determine the function of these TNT structures.

In conclusion, we propose that TNT processes may help HIV infection and other pathogens to spread more efficiently while avoiding extracellular anti-viral responses, increasing the chance that small populations of infected macrophages or T-cells will spread infection to a large number of other cells. An understanding of the role of this new communication system in normal and pathological conditions may open new potential therapeutic opportunities to target HIV infection and replication.

Acknowledgements

This work was supported by the National Institutes of Mental Health grants (MH075679, MH083497 and MH070297 to Joan W. Berman), NIH Centers for AIDS Research (CFAR) Grant AI-051519, the National Institute of Health Experimental Neuropathology Training Grant (NS07098 to Peter J. Gaskill), the National Institutes of Drug Abuse (Ruth L. Kirschstein National Research Service Award, F32DA024965 to Peter J. Gaskill) and by a KO1 grant from the National Institute of Mental Health (MH076679 to Eliseo A. Eugenin).

Addendum to: Eugenin EA, Gaskill PJ, Berman JW. Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages: a potential mechanism for intercellular HIV trafficking. Cell Immunol. 2009;254:142–148. doi: 10.1016/j.cellimm.2008.08.005.

Footnotes

Previously published online as a Communicative & Integrative Biology E-publication: http://www.landesbioscience.com/journals/cib/article/8165

References

  • 1.Eugenin EA, Gaskill PJ, Berman JW. Tunneling nanotubes (TNT) are induced by HIV-infection of macrophages: a potential mechanism for intercellular HIV trafficking. Cell Immunol. 2009;254:142–148. doi: 10.1016/j.cellimm.2008.08.005. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Gerdes HH, Bukoreshtliev NV, Barroso JF. Tunneling nanotubes: a new route for the exchange of components between animal cells. FEBS Lett. 2007;581:2194–2201. doi: 10.1016/j.febslet.2007.03.071. [DOI] [PubMed] [Google Scholar]
  • 3.Onfelt B, Nedvetzki S, Yanagi K, Davis DM. Cutting edge: Membrane nanotubes connect immune cells. J Immunol. 2004;173:1511–1513. doi: 10.4049/jimmunol.173.3.1511. [DOI] [PubMed] [Google Scholar]
  • 4.Onfelt B, Purbhoo MA, Nedvetzki S, Sowinski S, Davis DM. Long-distance calls between cells connected by tunneling nanotubules. Sci STKE. 2005;2005:55. doi: 10.1126/stke.3132005pe55. [DOI] [PubMed] [Google Scholar]
  • 5.Rustom A, Saffrich R, Markovic I, Walther P, Gerdes HH. Nanotubular highways for intercellular organelle transport. Science. 2004;303:1007–1010. doi: 10.1126/science.1093133. [DOI] [PubMed] [Google Scholar]
  • 6.Saez JC, Berthoud VM, Branes MC, Martinez AD, Beyer EC. Plasma membrane channels formed by connexins: their regulation and functions. Physiol Rev. 2003;83:1359–1400. doi: 10.1152/physrev.00007.2003. [DOI] [PubMed] [Google Scholar]
  • 7.Sowinski S, Jolly C, Berninghausen O, Purbhoo MA, Chauveau A, Köhler K, et al. Membrane nanotubes physically connect T cells over long distances presenting a novel route for HIV-1 transmission. Nat Cell Biol. 2008;10:211–219. doi: 10.1038/ncb1682. [DOI] [PubMed] [Google Scholar]
  • 8.Chen P, Hübner W, Spinelli MA, Chen BK. Predominant mode of human immunodeficiency virus transfer between T cells is mediated by sustained Env-dependent neutralization-resistant virological synapses. J Virol. 2007;81:12582–12595. doi: 10.1128/JVI.00381-07. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 9.Dimitrov DS, Willey RL, Sato H, Chang LJ, Blumenthal R, Martin MA. Quantitation of human immunodeficiency virus type 1 infection kinetics. J Virol. 1993;67:2182–2190. doi: 10.1128/jvi.67.4.2182-2190.1993. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 10.Dramsi S, Cossart P. Intracellular pathogens and the actin cytoskeleton. Annu Rev Cell Dev Biol. 1998;14:137–166. doi: 10.1146/annurev.cellbio.14.1.137. [DOI] [PubMed] [Google Scholar]
  • 11.Onfelt B, Nedvetzki S, Benninger RK, Purbhoo MA, Sowinski S, Hume AN, et al. Structurally distinct membrane nanotubes between human macrophages support long-distance vesicular traffic or surfing of bacteria. J Immunol. 2006;177:8476–8483. doi: 10.4049/jimmunol.177.12.8476. [DOI] [PubMed] [Google Scholar]
  • 12.Wehland J, Carl UD. The sophisticated survival strategies of the pathogen Listeria monocytogenes. Int Microbiol. 1998;1:11–18. [PubMed] [Google Scholar]
  • 13.Zhu D, Tan KS, Zhang X, Sun AY, Sun GY, Lee JC. Hydrogen peroxide alters membrane and cytoskeleton properties and increases intercellular connections in astrocytes. J Cell Sci. 2005;118:3695–3703. doi: 10.1242/jcs.02507. [DOI] [PubMed] [Google Scholar]
  • 14.Martínez AD, Eugenín EA, Brañes MC, Bennett MV, Sáez JC. Identification of second messengers that induce expression of functional gap junctions in microglia cultured from newborn rats. Brain Res. 2002;943:191–201. doi: 10.1016/s0006-8993(02)02621-5. [DOI] [PubMed] [Google Scholar]
  • 15.Eugenín EA, Brañes MC, Berman JW, Sáez JC. TNFalpha plus IFNgamma induce connexin43 expression and formation of gap junctions between human monocytes/macrophages that enhance physiological responses. J Immunol. 2003;170:1320–1328. doi: 10.4049/jimmunol.170.3.1320. [DOI] [PubMed] [Google Scholar]
  • 16.Gousset K, Schiff E, Langevin C, Marijanovic Z, Caputo A, Browman DT, et al. Prions hijack tunnelling nanotubes for intercellular spread. Nat Cell Biol Nat Cell Biol. 2009 doi: 10.1038/ncb1841. In press. Epub ahead of print. [DOI] [PubMed] [Google Scholar]
  • 17.Hsiung F, Ramirez-Weber FA, Iwaki DD, Kornberg TB. Dependence of Drosophila wing imaginal disc cytonemes on Decapentaplegic. Nature. 2005;437:560–563. doi: 10.1038/nature03951. [DOI] [PubMed] [Google Scholar]
  • 18.Kornberg T. Pictures in cell biology. Cytonemes. Trends Cell Biol. 1999;9:434. doi: 10.1016/s0962-8924(99)01653-0. [DOI] [PubMed] [Google Scholar]
  • 19.Niess JH, Brand S, Gu X, Landsman L, Jung S, McCormick BA, et al. CX3CR1-mediated dendritic cell access to the intestinal lumen and bacterial clearance. Science. 2005;307:254–258. doi: 10.1126/science.1102901. [DOI] [PubMed] [Google Scholar]
  • 20.Rescigno M, Urbano M, Valzasina B, Francolini M, Rotta G, Bonasio R, et al. Dendritic cells express tight junction proteins and penetrate gut epithelial monolayers to sample bacteria. Nat Immunol. 2001;2:361–367. doi: 10.1038/86373. [DOI] [PubMed] [Google Scholar]
  • 21.Chinnery HR, Pearlman E, McMenamin PG. Cutting edge: membrane nanotubes in vivo: a feature of MHC class II+ cells in the mouse cornea. J Immunol. 2008;180:5779–5783. doi: 10.4049/jimmunol.180.9.5779. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 22.Cudmore S, Cossart P, Griffiths G, Way M. Actin-based motility of vaccinia virus. Nature. 1995;378:636–638. doi: 10.1038/378636a0. [DOI] [PubMed] [Google Scholar]
  • 23.Favoreel HW, Van Minnebruggen G, Adriaensen D, Nauwynck HJ. Cytoskeletal rearrangements and cell extensions induced by the US3 kinase of an alphaherpesvirus are associated with enhanced spread. Proc Natl Acad Sci USA. 2005;102:8990–8995. doi: 10.1073/pnas.0409099102. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 24.Gill MB, Edgar R, May JS, Stevenson PG. A gamma-herpesvirus glycoprotein complex manipulates actin to promote viral spread. PLoS ONE. 2008;3:1808. doi: 10.1371/journal.pone.0001808. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 25.Hartlieb B, Weissenhorn W. Filovirus assembly and budding. Virology. 2006;344:64–70. doi: 10.1016/j.virol.2005.09.018. [DOI] [PubMed] [Google Scholar]
  • 26.Jouvenet N, Windsor M, Rietdorf J, Hawes P, Monaghan P, Way M, et al. African swine fever virus induces filopodia-like projections at the plasma membrane. Cell Microbiol. 2006;8:1803–1811. doi: 10.1111/j.1462-5822.2006.00750.x. [DOI] [PubMed] [Google Scholar]
  • 27.Kolesnikova L, Bohil AB, Cheney RE, Becker S. Budding of Marburgvirus is associated with filopodia. Cell Microbiol. 2007;9:939–951. doi: 10.1111/j.1462-5822.2006.00842.x. [DOI] [PubMed] [Google Scholar]
  • 28.Noda T, Ebihara H, Muramoto Y, Fujii K, Takada A, Sagara H, et al. Assembly and budding of Ebolavirus. PLoS Pathog. 2006;2:99. doi: 10.1371/journal.ppat.0020099. [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 29.Watkins SC, Salter RD. Functional connectivity between immune cells mediated by tunneling nanotubules. Immunity. 2005;23:309–318. doi: 10.1016/j.immuni.2005.08.009. [DOI] [PubMed] [Google Scholar]
  • 30.Sherer NM, Lehmann MJ, Jimenez-Soto LF, Horensavitz C, Pypaert M, Mothes W. Retroviruses can establish filopodial bridges for efficient cell-to-cell transmission. Nat Cell Biol. 2007;9:310–315. doi: 10.1038/ncb1544. [DOI] [PMC free article] [PubMed] [Google Scholar]

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