Nanoparticle Uptake by Corneal Cells: Size and Surface Effects

Study Background and Research Question

Ocular diseases remain a major clinical challenge, partly due to the formidable barriers limiting drug penetration into the eye. Standard topical ophthalmic formulations, though widely employed, often suffer from poor bioavailability—primarily because of rapid clearance by the tear film and the highly selective corneal epithelium. The need for improved delivery technologies has prompted a surge of interest in nanoparticle-based systems, which offer the potential to prolong drug retention, enhance tissue penetration, and enable controlled release. However, significant gaps persist in understanding how the physicochemical properties of nanoparticles—namely, size and surface chemistry—affect their interactions with the corneal barrier.

The reference study by Azadi and David (ACS Biomater. Sci. Eng. 2024, 10, 429−441) addresses this knowledge gap, focusing on human cornea epithelial cells (HCECs) as a model system to dissect the mechanisms governing nanoparticle uptake.

Key Innovation from the Reference Study

The central innovation of this work lies in its systematic investigation of how both the size and surface modifications of poly(lactic-co-glycolic acid) (PLGA) nanoparticles modulate their internalization by HCECs. The study not only characterizes nanoparticle physicochemical parameters with precision but also employs a sophisticated in vitro corneal model combining HCEC monolayers and simulated mucosal solution. By interrogating uptake pathways with a panel of endocytosis inhibitors, the authors reveal that energy-dependent endocytosis—specifically, macropinocytosis and caveolae-mediated mechanisms—predominates under physiologically relevant conditions. This nuanced mechanistic insight moves beyond earlier, more descriptive studies, providing actionable guidance for the design of ocular nanotherapeutics.

Methods and Experimental Design Insights

The study deployed a robust suite of experimental approaches:

• PLGA nanoparticles were synthesized via the emulsion-solvent evaporation method, then further surface-modified with mucoadhesive polymers (alginate, chitosan) or a mucopenetrative polymer (polyethylene glycol, PEG).
• Particle characterization included size (100–250 nm), zeta potential (−25 to +15 mV), and polydispersity index (PDI < 0.2), ensuring monodispersity and reproducibility.
• Cellular toxicity was assessed via the MTT assay, confirming that concentrations up to 100 μg/mL produced only mild toxicity (cell viability 70–100%).
• The in vitro uptake model incorporated HCEC monolayers overlaid with simulated mucosal fluid, recapitulating the complex ocular surface environment.
• Pharmacological inhibitors targeting specific endocytic pathways (e.g., macropinocytosis, caveolae-mediated, clathrin-mediated, and phagocytosis) enabled mechanistic dissection of nanoparticle internalization.

This integrative methodology allowed the authors to confidently interpret uptake data in the context of relevant biological and physicochemical variables.

Core Findings and Why They Matter

The reference study's principal findings can be summarized as follows:

• Nanoparticle size and surface chemistry are dominant determinants of cellular uptake. Among various formulations, 100 nm PLGA nanoparticles, especially those surface-modified with PEG (PEG-PLGA-150), demonstrated the highest internalization by HCECs.
• Energy-dependent endocytosis, particularly macropinocytosis and caveolae-mediated pathways, constitute the main routes of nanoparticle entry. This was verified by selective inhibition experiments, which showed marked reductions in uptake upon blockade of these pathways, while phagocytosis inhibitors had no effect for the studied size range.
• Clathrin-mediated endocytosis contributed partially to uptake, but was not the dominant route.
• Surface modification with PEG enhanced mucopenetration and cellular uptake relative to mucoadhesive coatings (alginate, chitosan).
• Minimal cytotoxicity was observed, supporting the safety profile of these nanoparticle formulations at relevant concentrations.

These results collectively provide strong evidence that rational engineering of nanoparticle size and surface chemistry can substantially improve corneal cell uptake, a critical step toward effective ocular drug delivery. The emphasis on energy-dependent mechanisms further suggests that uptake can be modulated by manipulating cellular endocytic activity, offering additional levers for therapeutic optimization.

For a complementary overview of how nanoparticle uptake mechanisms are modulated by physicochemical properties in ocular systems, the internal article "Nanoparticle Uptake in Corneal Cells: Role of Physicochemical Properties" corroborates and expands on these mechanistic insights.

Comparison with Existing Internal Articles

Recent internal resources have explored similar themes:

• "Nanoparticle Uptake Mechanisms in Human Corneal Epithelial Cells" and "Nanoparticle Uptake in Corneal Cells: Size and Surface Effects" both highlight macropinocytosis and caveolae-mediated endocytosis as dominant uptake routes, in alignment with the reference study. The discussion of energy dependence and the partial role of clathrin-mediated pathways is consistent across these works.
• "Cytochalasin D in Applied Actin Polymerization Inhibition Workflows" provides methodological context on the use of actin polymerization inhibitors (such as Cytochalasin D) to dissect cytoskeletal contributions to endocytosis. Such inhibitors are instrumental in distinguishing actin-dependent uptake mechanisms from other pathways, further supporting mechanistic clarity in nanoparticle-uptake studies.
• For broader context on the role of Cytochalasin D as a selective actin polymerization inhibitor—relevant for modulating cell cycle arrest at G1-S transition, tumor cell proliferation inhibition, and viral transcription inhibition—see "Cytochalasin D: Selective Actin Polymerization Inhibitor Benchmarks".

Taken together, these resources reinforce the centrality of particle size, surface chemistry, and cytoskeletal dynamics in governing nanoparticle uptake by corneal epithelial cells.

Limitations and Transferability

While this reference study offers valuable mechanistic insights, several limitations should be noted:

• In vitro system: The HCEC monolayer with simulated mucosal fluid, though advanced, cannot fully replicate the dynamic and multicellular environment of the intact ocular surface. In vivo factors such as tear turnover, immune surveillance, and tissue remodeling may alter nanoparticle behavior.
• Size and chemistry range: The findings are most robust for PLGA nanoparticles within the 100–250 nm size window and for the specific surface modifications tested (PEG, alginate, chitosan). Extrapolation to other polymers or particulate systems should be approached with caution.
• Endocytosis specificity: Although the major uptake routes were identified, the interplay between these pathways and potential crosstalk with other cell types (e.g., immune cells) in the cornea remains to be explored.

Nonetheless, the mechanistic conclusions are well-supported within the defined experimental space and provide a strong platform for further translational work.

Protocol Parameters

• Nanoparticle formulation: PLGA nanoparticles, 100–250 nm in diameter, surface-modified with PEG, alginate, or chitosan.
• Uptake evaluation: Incubation with HCEC monolayers in simulated mucosal solution for up to 24 h, with nanoparticle concentrations ≤100 μg/mL to minimize cytotoxicity.
• Endocytosis pathway interrogation: Use of pathway-specific inhibitors (e.g., amiloride for macropinocytosis, filipin for caveolae-mediated endocytosis, chlorpromazine for clathrin-mediated endocytosis) at literature-backed concentrations and preincubation times.
• Cytotoxicity assessment: MTT assay post-incubation to confirm cell viability (target ≥70%).
• Data analysis: Quantify nanoparticle uptake via fluorescence or other suitable labeling, normalized to cell number and viability.

Why this cross-domain matters, maturity, and limitations

The mechanistic insights derived from the study of nanoparticle uptake in ocular epithelial cells have broader implications for drug delivery research. Understanding the cytoskeletal and endocytic machinery underlying nanoparticle internalization enables the rational design of carriers not only for ophthalmology but also for other mucosal or barrier-rich tissues. However, the transferability of these findings is contingent on similarities in tissue architecture and physiological barriers. Maturity of this approach is high within preclinical ocular models, but further validation in vivo and in other tissue systems remains necessary.

Research Support Resources

Researchers seeking to dissect actin-dependent endocytic pathways or to optimize nanoparticle uptake protocols can utilize Cytochalasin D (SKU B6645), a potent and selective actin polymerization inhibitor, as described on the APExBIO platform. This compound is widely used to modulate cytoskeletal function in studies of endocytosis, cell cycle arrest at the G1-S transition, tumor cell proliferation inhibition, and viral transcription inhibition. When applying Cytochalasin D in ocular nanoparticle uptake workflows, standard concentrations (0.2–0.5 μg/mL for cell culture) are recommended, with prompt use of freshly prepared solutions for reproducibility.