Spatial Configuration and Composition of Charge Modulates Transport into a Mucin Hydrogel Barrier

Abstract

The mucus barrier is selectively permeable to a wide variety of molecules, proteins, and cells, and establishes gradients of these particulates to influence the uptake of nutrients, the defense against pathogens, and the delivery of drugs. Despite its importance for health and disease, the criteria that govern transport through the mucus barrier are largely unknown. Studies with uniformly functionalized nanoparticles have provided critical information about the relevance of particle size and net charge for mucus transport. However, these particles lack the detailed spatial arrangements of charge found in natural mucus-interacting substrates, such as certain viruses, which may have important consequences for transport through the mucus barrier. Using a novel, to our knowledge, microfluidic design that enables us to measure real-time transport gradients inside a hydrogel of mucins, the gel-forming glycoprotein component of mucus, we show that two peptides with the same net charge, but different charge arrangements, exhibit fundamentally different transport behaviors. Specifically, we show that certain configurations of positive and negative charges result in enhanced uptake into a mucin barrier, a remarkable effect that is not observed with either charge alone. Moreover, we show that the ionic strength within the mucin barrier strongly influences transport specificity, and that this effect depends on the detailed spatial arrangement of charge. These findings suggest that spatial charge distribution is a critical parameter to modulate transport through mucin-based barriers, and have concrete implications for the prediction of mucosal passage, and the design of drug delivery vehicles with tunable transport properties.

Methods to determine the interactions of micro- and nanoparticles with mucus

“…G' and G'') to porcine intestinal mucus, the charge on the gel, its interactions and size filtering properties will be different. In order to isolate native mucins to use in permeation studies several purification methods have been used Li et al [11] have partially purified the mucin from pig stomach however, they used a method without steps taken to prevent proteolysis which would degrade the mucin leading to loss of gel forming properties.…”

Mucus as a Barrier to Drug Delivery – Understanding and Mimicking the Barrier Properties

“…Mucus from rinsed porcine small intestine None Groo et al [40] Custom-made microfluidic device Purified mucins from porcine stomach None Li et al [29] the overestimation of the mucus barrier compared to the in vivo situation. For example, the mucus layers were 939 AE 110 lm [28] and 446 lm [40] in the models where native mucus was applied to filters, whereas a mucus layer of 250 lm was tested in the microfluidic device [29].…”

“…Most commonly, gastrointestinal mucus is isolated from pigs and used as obtained [27] or following purification [28]. Furthermore, mucins can be isolated from native mucus and reconstituted to form a gel [9,29].…”

“…The models composed of mucus on a filter insert allow the assessment and comparison of drug permeation through mucus without considering the epithelial barrier. In addition, a microfluidic system with controllable flow has been used to detect the distribution of fluorescent peptides in a mucin gel of 250 lm thickness [29] (table 1), whereas single or multiple particle tracking is a widespread technique for the assessment of particle diffusion in different biologically relevant media such as mucus [41][42][43].…”

“…Purifying mucins decreases the biological variation associated with using native mucus, but does not eliminate it; thus, the reproducibility of experiments might be compromised in such a model. Purified mucins are employed in the microfluidic system published by Li et al [29] and being custom built; this system is more difficult to implement in different laboratories as compared to the filter insert-based models. The advantages of the microfluidic system include controllable flow and the possibility of testing thin layers of mucus, whereas the minimum amount of mucus needed for full coverage of the filters will result in thicker mucus layers, which potentially can result in Table 1.…”