Understanding the interaction between nanoparticles and the lung lining is an ever-increasing concern as the amount of airborne particulate matter introduced into the environment by human activity grows.1
Nanoparticles have been implicated in a number of acute and chronic medical conditions, particularly those associated with inflammation and cardiovascular disease.2–4
A better understanding of the mechanisms by which nanoparticles penetrate the lung lining and enter the lungs would allow for better treatment and prevention of diseases.
Survanta is an excellent model system for investigating the mechanisms of nanoparticle transport and toxicity in the lungs because it mimics the composition and function of the alveolar lining while being a relatively well-defined experimental system. Additionally, Survanta is a widely used clinical surfactant for the treatment of Respiratory Distress Syndrome (RDS) making its choice particularly relevant.5,6
Survanta's ability to form stable monolayers at the alveoli-air interface that spread with alveoli expansion and contraction to maintain a low surface tension are critical for proper lung function, which is compromised in RDS due to an incomplete surfactant layer. Both mechanistic and in vivo studies have been performed addressing these issues, making Survanta one of the best-characterized lung surfactants available.7–9
The two major lipid components in Survanta, 1,2-dipalmitoyl-sn
-glycero-3-phosphocholine (DPPC) and 1-palmitoyl-2-oleoyl-sn
-(1-glycerol)] (POPG), have melting temperatures significantly above and below room temperature, 41 and −2 °C respectively.10
When deposited onto mica, this difference in transition temperature leads to distinct fluid and gel domains that exhibit little structural or phase changes over time at room temperature. These distinct domains allow for direct comparison between nanoparticle interaction with fluid and gel phases. Second, the presence of fatty acids (palmitic) and proteins, observable with atomic force microscopy (AFM),11,12
brings us closer to the level of complexity seen in actual cell membranes.
Nanoscale disruption of lipid membranes by charged polymer-based nanoparticles is well documented in the literature.13–19
These studies demonstrate that charged nanoparticles disrupt biological membranes; however, they leave many open questions regarding the mechanism of disruption, including the role of headgroup charge,13,16
the influence of lipid phase,13,15
and the effects of cholesterol, proteins and other membrane components. In 2005, Mecke et al. addressed the question of phase by observing that only lipid in the fluid phase was removed by amine terminated poly(amidoamine) (PAMAM) dendrimers.15
A number of mechanisms for membrane disruption have been suggested. In one such model, the lipids encapsulate the polymer in a lipid vesicle.16
More recently, Gewirth and colleagues suggest the possibility that disruption of the electrostatic interactions between the charged mica substrate and the lipid bilayer lead to lipid removal.18,19
PAMAM dendrimers were chosen as the model nanoparticles for interaction with Survanta because of the well-controlled size, excellent polydispersity (1.01), and well-defined surface chemistry.20–22
Under AFM imaging conditions, unbuffered and neutral pH, all primary amine groups of the PAMAM dendrimers are expected to be protonated.23,24
Space filling models of G5 and G7 dendrimers are shown in .
Figure 1 Space filling models of equilibrated generation five (G5) and generation seven (G7) poly(amidoamine) PAMAM dendrimers. The G5 dendrimer has 128 surface amines and an approximate diameter of 5 nm. The G7 dendrimer has 512 surface amines and an approximate (more ...)
There are three key results reported in this paper. First, the fluid domain is removed more than an order of magnitude faster than the gel domain. Second, dendrimer accumulation on lipid edges and terraces preceding lipid removal has been directly imaged for both fluid and gel domains and all tested dendrimer generations for the first time. Third, immediately following lipid removal the mica surface is clean. This indicates that lipid defects are not induced by dendrimers binding to mica and displacing the lipid.