GLOBULAR PROTEINS. TYPES OF PROTEINS GLOBULAR PROTEINS FIBROUS PROTEINS.
Nanoparticles: Pushed off target with proteins
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NATURE NANOTECHNOLOGY | VOL 8 | FEBRUARY 2013 | www.nature.com/naturenanotechnology 79 news & views I n a biological environment proteins bind to the surface of nanoparticles and form what is known as a protein corona. is corona can alter the properties of the nanoparticles and therefore needs to be taken into account when characterizing the biological interactions of nanoparticles 1 and assessing the potential clinical relevance of new multifunctional nanomaterials. One intended application of such nanomaterials is to carry drugs or therapies to diseased cells and uses nanoparticles that are coated with biomolecules that can target specific receptors on cells. Writing in Nature Nanotechnology, Eugene Mahon, Kenneth Dawson and colleagues at University College Dublin have now shown that protein coronas can adversely affect the targeting ability of these nanoparticles 2 . Assessing the targeting efficiency of a nanoparticle is a complex process. Appropriate in vitro models can provide important information, and can identify and validate biomarkers that can be used as a surrogate for clinical activity. However, the targeting efficiency of a nanoparticle ultimately needs to be assessed in vivo and preferably in a clinically relevant model, although conclusive answers can be difficult to obtain because of the complexities of pathophysiology and disease progression. Furthermore, issues such as the tumour enhanced permeation effect 3,4 (in which nanoparticles accumulate in tumour tissue more than in normal tissue) and endocytosis mechanisms (the process by which materials are absorbed by cells) and their potential manipulation 5 , also need to be taken into account. Limitations in nanoparticle targeting strategies have previously been identified 6,7 . ese can, in part, be linked to the need to balance issues such as targeting efficiency, ligand density, and the size and surface properties of the nanoparticles depending on how localized the targeting has to be and, moreover, through which route the nanoparticles are administered 8 . Dawson and colleagues used fluorescent silica nanoparticles to examine the effect of the protein corona on the targeting efficiency of nanoparticles 2 . e silica nanoparticles were coated with polymer chains of polyethylene glycol, which could be used to attach the protein transferrin. Transferrin is known to be a potential targeting molecule because cancer cell metabolism of iron can lead to overexpression of the transferrin receptor protein. Using confocal imaging, the researchers found that when exposed to cells the transferrin-conjugated nanoparticles were localized with transferrin receptors. Furthermore, uptake in cells in which expression of transferrin receptors had been silenced was notably reduced in comparison with non-silenced cells. is confirmed that the nanoparticles were, in part, internalized by the cells via receptors. To examine whether this targeting behaviour is retained in a more realistic biological environment and one that is more representative of in vivo studies, Dawson and colleagues next explored the behaviour of the nanoparticles in fetal bovine serum. e researchers found that the addition of serum decreases the overall uptake of the nanoparticles in the cells and, moreover, the fraction of uptake that is due to transferrin receptors. Similar behaviour was also found to occur in human serum. ese results suggest that the nanoparticles lose their targeting capabilities when in biological fluid. is behaviour is the result of proteins in the serum forming a protein corona around the nanoparticles, which shields the transferrin and stops it from binding to the targeted receptors on the cells (Fig. 1). One important implication of these results is that experiments carried out in non-biological media, such as phosphate- buffer saline, cannot be used to provide a conclusive analysis of targeting efficiency. NANOPARTICLES Pushed off target with proteins The formation of a protein corona around the surface of a nanoparticle can alter its targeting capabilities. Rogério Gaspar Figure 1 | Schematic diagram of transferrin-conjugated nanoparticles targeting transferrin receptors in the presence of serum proteins 2 . The serum proteins (green, purple and blue structures) form a protein corona around the nanoparticles. As a result, the transferrin proteins (red structures) on the nanoparticles are hidden and cannot bind to the transferrin receptors (yellow structures) on the cells. © 2013 Macmillan Publishers Limited. All rights reserved
Transcript of Nanoparticles: Pushed off target with proteins
NATURE NANOTECHNOLOGY | VOL 8 | FEBRUARY 2013 | www.nature.com/naturenanotechnology 79
news & views
In a biological environment proteins bind to the surface of nanoparticles and form what is known as a protein corona.
This corona can alter the properties of the nanoparticles and therefore needs to be taken into account when characterizing the biological interactions of nanoparticles1 and assessing the potential clinical relevance of new multifunctional nanomaterials. One intended application of such nanomaterials is to carry drugs or therapies to diseased cells and uses nanoparticles that are coated with biomolecules that can target specific receptors on cells. Writing in Nature Nanotechnology, Eugene Mahon, Kenneth Dawson and colleagues at University College Dublin have now shown that protein coronas can adversely affect the targeting ability of these nanoparticles2.
Assessing the targeting efficiency of a nanoparticle is a complex process. Appropriate in vitro models can provide important information, and can identify and validate biomarkers that can be used as a surrogate for clinical activity. However, the targeting efficiency of a nanoparticle ultimately needs to be assessed in vivo and preferably in a clinically relevant model, although conclusive answers can be difficult to obtain because of the complexities of pathophysiology and disease progression. Furthermore, issues such as the tumour enhanced permeation effect3,4 (in which nanoparticles accumulate in tumour tissue more than in normal tissue) and endocytosis mechanisms (the process by which materials are absorbed by cells) and their potential manipulation5, also need to be taken into account.
Limitations in nanoparticle targeting strategies have previously been identified6,7. These can, in part, be linked to the need to balance issues such as targeting efficiency, ligand density, and the size and surface properties of the nanoparticles depending on how localized the targeting has to be and, moreover, through which route the nanoparticles are administered8.
Dawson and colleagues used fluorescent silica nanoparticles to examine the effect of the protein corona on the targeting efficiency of nanoparticles2. The silica nanoparticles were coated with polymer chains of
polyethylene glycol, which could be used to attach the protein transferrin. Transferrin is known to be a potential targeting molecule because cancer cell metabolism of iron can lead to overexpression of the transferrin receptor protein.
Using confocal imaging, the researchers found that when exposed to cells the transferrin-conjugated nanoparticles were localized with transferrin receptors. Furthermore, uptake in cells in which expression of transferrin receptors had been silenced was notably reduced in comparison with non-silenced cells. This confirmed that the nanoparticles were, in part, internalized by the cells via receptors.
To examine whether this targeting behaviour is retained in a more realistic biological environment and one that is more representative of in vivo studies, Dawson
and colleagues next explored the behaviour of the nanoparticles in fetal bovine serum. The researchers found that the addition of serum decreases the overall uptake of the nanoparticles in the cells and, moreover, the fraction of uptake that is due to transferrin receptors. Similar behaviour was also found to occur in human serum. These results suggest that the nanoparticles lose their targeting capabilities when in biological fluid. This behaviour is the result of proteins in the serum forming a protein corona around the nanoparticles, which shields the transferrin and stops it from binding to the targeted receptors on the cells (Fig. 1).
One important implication of these results is that experiments carried out in non-biological media, such as phosphate-buffer saline, cannot be used to provide a conclusive analysis of targeting efficiency.
NANOPARTICLES
Pushed off target with proteinsThe formation of a protein corona around the surface of a nanoparticle can alter its targeting capabilities.
Rogério Gaspar
Figure 1 | Schematic diagram of transferrin-conjugated nanoparticles targeting transferrin receptors in the presence of serum proteins2. The serum proteins (green, purple and blue structures) form a protein corona around the nanoparticles. As a result, the transferrin proteins (red structures) on the nanoparticles are hidden and cannot bind to the transferrin receptors (yellow structures) on the cells.
© 2013 Macmillan Publishers Limited. All rights reserved
80 NATURE NANOTECHNOLOGY | VOL 8 | FEBRUARY 2013 | www.nature.com/naturenanotechnology
news & views
Therefore, certain previous measurements of targeting efficiency may be inaccurate.
The complexity of achieving targeting in a living organism, be it a relevant animal model or in man, cannot be overstated. The work of Dawson and colleagues provides important information that could in the future be used to develop improved targeting strategies. However, such results need to be considered together with a range of additional data. For example, as well as problems related to proteins in circulation and the corona, the targeting nanoparticles also have to overcome problems related to the route of administration, organs, tissues and cellular internalization and trafficking.
There are at present more than 40 nanopharmaceuticals in routine clinical use9, but the development of targeted nanoparticles with clinical applications would provide a new and innovative approach to personalized medicine. To achieve this, the design of new nanoparticles needs to be based on a sound understanding of the problems that can arise due to interactions with biomolecules. Dawson and colleagues have taken an important step along this road, but future studies will have to consider both mechanistic approaches and demonstrate targeting specificity and efficiency with translational models. ❐
Rogério Gaspar is at the Faculty of Pharmacy, University of Lisbon, 1649-003 Lisbon, Portugal. e-mail: [email protected]
References1. Cedervall, T. et al. Proc. Natl Acad. Sci. USA 104, 2050–2055 (2007).2. Salvati, A. et al. Nature Nanotech. 8, 137–143 (2013).3. Maeda, H. et al. Adv. Drug Deliv. Rev. http://dx.doi.org/10.1016/
j.addr.2012.10.002 (2012).4. Jain, R. K. Adv. Drug Deliv. Rev. 64, 353–365 (2012).5. Duncan, R. & Richardson, S. Mol. Pharmaceutics
9, 2380–2402 (2012).6. Ruoslahti, E. et al. J. Cell Biol. 188, 759–768 (2010).7. Allen, T. M. & Cullis, P. R. Adv. Drug Deliv. Rev. http://dx.doi.
org/10.1016/j.addr.2012.09.037 (2012).8. Koshkaryev, A. et al. Adv. Drug Deliv. Rev. http://dx.doi.
org/10.1016/j.addr.2012.08.009 (2012).9. Duncan, R. & Gaspar, R. Mol. Pharmaceutics
8, 2101–2141 (2011).
The ultimate level of miniaturization for an electronic device consists of a molecule controlling its electronic
functions. In this context, the use of a one-molecule-thick layer self-assembled on top of an electrode’s surface provides vital information en route to actual molecular electronics, for which individual molecules would serve as the active component. To this end, self-assembled monolayers (SAMs) of organic molecules have been incorporated in a wide range of electronic devices1–3, but if on one hand the flexibility of organic synthesis opens up the possibility of tuning the electronic properties of individual molecular circuits by rational design4, on the other hand, controlling the monolayer geometry (that is, packing density, stiffness, orientation of individual atoms and so on) remains a crucial — and still challenging — factor for the realization of circuit elements using SAM structures5. Moreover, a statistically robust method to measure the electronic performance of these types of devices is also required. Now, reporting in Nature Nanotechnology, Christian Nijhuis, Damien Thompson and colleagues6 address both these issues by showing that subtle changes in the structure of the molecules making up the SAM have a profound effect on the supramolecular geometry of the monolayer, and directly link this effect to the performance of their electronic devices.
In their work, Nijhuis and Thompson — who are based at the National University of Singapore and University College Cork (Ireland), respectively — construct molecular diodes made of alkanethiolate SAMs on a flat gold or silver substrate7, which terminate with an electrochemically active ferrocene head group (red, Fig. 1). They use a soft liquid-metal eutectic alloy of gallium and indium as the top electrode that contacts with the ferrocene head group, a method previously developed by one of the authors8, which offers a very convenient and reliable way for
measuring the properties of molecular diodes for a statistically meaningful number of junctions (that is, 500–700 of them in the present work). The authors examine the rectification ratio — the ratio of the currents flowing through the device as the voltage is applied in the forward and reverse directions — of the diode junctions, its reproducibility over successive measurements and the overall yield of working devices that can be fabricated from a given sample preparation procedure as the number of CH2 groups in the hydrocarbon chain of the alkanethiolate
MOLECULAR ELECTRONICS
Van der Waals rectifiersSubtle effects in the way self-assembled monolayers pack can be harnessed to produce dramatic enhancements in the performance of organic diodes.
Steven L. Bernasek
Ferrocene group
Alkyl chain
Sulphur atom
n even n odd
Rect
ifica
tion
ratio
8 9 10 11 12 13 14n
a b c
Ag
EGaInGa2O3
Figure 1 | Effect of van der Waals interactions on the supramolecular structure and performance of organic diodes. a–c, Schematic representation of a SAM attached on a silver substrate by a sulphur atom (yellow) and composed of an even (a) and odd (b) number, n, of CH2 groups (blue). If the number is even the molecular packing is less efficient than if the number is odd. This is due to a different tilt angle of the ferrocene head group (red) and an enhanced van der Waals interaction between the chains. As a result of this subtle difference, the rectification ratio of devices when n is odd is about ten times better than when n is even (c). EGaIn, eutectic alloy of gallium and indium.
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