ScienceWeek

BIOENGINEERING: ON VIRUSES AS NANOPLATFORMS

The following points are made by T. Douglas and M. Young (Science 2006 312:873):

1) The essential nature of all viruses is to infect a host cell, replicate, package its nucleic acid, and exit the cell. In the process, viruses have evolved to move through a broad range of chemical environments. In their journey, viruses demonstrate a remarkable plasticity in their metastable structure and dynamics, including coordinated assembly and disassembly and site-specific delivery of cargo molecules. Viruses have emerged as platforms for synthetic manipulation with a range of applications from materials to medicine. Chemical or genetic manipulation makes it possible to impart new functions to protein cage architectures, combining the best of evolution and human design. Characterized viruses represent only a fraction of the predicted viral diversity present in the biosphere. Recent revelations suggest that viruses are the most abundant biological entities on the planet and are second only to prokaryotes in terms of biomass [1].

2) If we view viruses as molecular containers, there are three important interfaces that can be exploited: the exterior, the interior, and the interface between protein subunits making up the container (or capsid). Typically, viruses are assembled from repeating subunits to form highly symmetrical and homogeneous architectures [2,3]. Viruses occur in a range of shapes and sizes, from 18 to 500 nm for icosahedral structures and greater than 2 µm in length for filamentous or rod-shaped viruses. This variety provides a library of platforms for tailored applications where size, shape, and stability are required. All viruses encode, package, and transport viral nucleic acid. However, many will assemble (either naturally or through genetic manipulation) into noninfectious containers devoid of genetic material. Conceptually, this allows one to replace the natural viral cargo with a wide range of synthetic cargos. The plasticity of the structural building blocks (subunits) to both chemical and genetic modifications, without affecting the overall architecture, gives rise to a rich resource for materials and pharmaceutical applications.

3) The interior interface of the viral capsid architecture has been used for directing encapsulation and synthesis of both organic and inorganic materials. All viruses package their viral nucleic acid within their capsid architecture, and the principles governing the packaging of this cargo have been exploited to package nonviral cargoes [4]. For example, the native positive-charge density on the interior interface of empty (nucleic acid free) cowpea chlorotic mottle virus (CCMV) capsid was used for nucleating inorganic mineralization reactions to form spatially constrained nanoparticles of polyoxometalate salts (tungstates H2W12O4210 , molybdates, and vanadates V10O286 ) [4]. In addition, through protein design and genetic engineering, the charge on the interior surface of the CCMV capsid has been altered, from positive to negative, without disrupting the ability to assemble. This negative-charge density was effective at directing the surface nucleation of transition metal oxides (Fe2O3, Fe3O4, and Co2O3), which proceed though cationic precursors stabilized at the highly anionic capsid interior interface [5].

4) Spatially resolved elemental imaging of these materials provides a view of the hard-soft interface, an important aspect of biomaterials, and one that is experimentally difficult to probe. The anisotropic rod-shaped tobacco mosaic virus (TMV) has also been used as a template for formation of metal nanowires using the interior cavity of the virus as a constraining environment. The interior interface of the capsid architectures also provides a rich, highly symmetric, and repetitive surface for encapsulation of cargo molecules through covalent attachment to site-specifically engineered residues on the interior surface. Thus, a cysteine residue genetically introduced into a subunit presents a reactive thiol group in the assembled protein cage architecture at all symmetry-related sites. Medically relevant small molecules such as therapeutics and imaging agents can be chemically attached to these reactive functional groups. The utility of this approach has been demonstrated with the use of a viruslike protein cage architecture to attach and selectively release the anticancer drug doxorubicin. This approach is medically advantageous because the protein cage acts to sequester its cargo (either natural or synthetic) until directed to be released. During administration and clearance, the encapsulated drug or imaging agent potentially remains invisible to the exterior environment and is therefore inert and biologically unavailable en route to its targeted cell.

References (abridged):

1. C. A. Suttle, Nature 437, 356 (2005)

2. C. M. Shepherd et al., Nucleic Acids Res. 34, D386 (2006)

3. Y. Zhu, B. Carragher, D. J. Kriegman, R. A. Milligan, C. S. Potter, J. Struct. Biol. 135, 302 (2001)

4. T. Douglas, M. J. Young, Nature 393, 152 (1998)

5. T. Douglas et al., Adv. Mater. 14, 415 (2002)

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