Nanoparticle Uptake Mechanisms in Human Corneal Epithelial Cells

Study Background and Research Question

Ocular drug delivery remains a significant challenge due to formidable anatomical and physiological barriers, particularly the tear film and corneal epithelium. While topical ophthalmic formulations dominate the market, their efficacy is hampered by rapid clearance and poor tissue penetration. The corneal epithelium, despite comprising only about 10% of corneal thickness, accounts for roughly 90% of its barrier function, thanks to tight junctions and multiple cell layers. To address these limitations, nanoparticle-based drug delivery systems have emerged as promising vehicles for enhancing ocular bioavailability, but critical questions remain about how physicochemical properties of nanoparticles—specifically size and surface chemistry—influence their interactions with, and uptake by, corneal epithelial cells. The reference study by Azadi and David (ACS Biomater. Sci. Eng. 2024, 10, 429–441) directly addresses this gap, aiming to unravel the cellular uptake mechanisms of engineered nanoparticles in a human corneal epithelial cell model.

Key Innovation from the Reference Study

The pivotal advance in this work lies in systematically dissecting how nanoparticle size and surface modification dictate their internalization by HCECs. The study leverages well-characterized poly(lactic-co-glycolic) acid (PLGA) nanoparticles, surface-modified for either mucoadhesion (alginate, chitosan) or mucopenetration (polyethylene glycol, PEG), to interrogate uptake pathways under controlled in vitro conditions. Notably, the authors combine quantitative uptake assays with the use of pharmacological inhibitors to parse the relative contributions of energy-dependent endocytic pathways—specifically macropinocytosis, caveolae-mediated, and clathrin-mediated endocytosis—to nanoparticle entry. This mechanistic clarity is essential, as it connects nanoparticle design parameters directly to cellular transport phenomena, providing a rational basis for engineering more effective ocular drug carriers.

Methods and Experimental Design Insights

PLGA nanoparticles were synthesized via the emulsion-solvent evaporation method, ensuring monodispersity (polydispersity index <0.2) and spherical morphology. The final nanoparticles, ranging from 100–250 nm and exhibiting zeta potentials from –25 to +15 mV, were surface-modified with alginate, chitosan, or PEG to modulate mucoadhesion and mucopenetration properties. Human corneal epithelial cells (HCECs) were cultured as confluent monolayers and exposed to simulated mucosal solutions to recapitulate the in vivo ocular microenvironment.

Cytotoxicity was assessed using the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay, confirming that nanoparticle concentrations up to 100 μg/mL resulted in minimal toxicity (70–100% cell viability) over 24 hours. Cellular uptake was quantified using fluorescence-based detection after incubation with nanoparticles, both in the presence and absence of endocytosis inhibitors. These included agents targeting macropinocytosis, caveolae-mediated, and clathrin-mediated endocytosis, enabling pathway-specific functional dissection.

Protocol Parameters

• Nanoparticle synthesis: Prepare PLGA nanoparticles (100–250 nm) via emulsion-solvent evaporation; ensure monodispersity (PDI < 0.2).
• Surface functionalization: Use alginate or chitosan for mucoadhesion; PEGylation for enhanced mucopenetration.
• Cell culture: Human corneal epithelial cells (HCECs), seeded as confluent monolayers.
• Simulated mucosal environment: Integrate mucosal solution to mimic the ocular surface.
• Cytotoxicity assessment: MTT assay post 24 h nanoparticle exposure, up to 100 μg/mL.
• Uptake quantification: Use fluorescence microscopy or flow cytometry after incubation (typically 24 h).
• Endocytosis pathway inhibition: Apply selective inhibitors for macropinocytosis, caveolae-mediated, and clathrin-mediated pathways prior to nanoparticle addition.

Core Findings and Why They Matter

The reference study demonstrates that energy-dependent endocytosis is the predominant mechanism for nanoparticle uptake in HCECs. Among the various formulations, 100 nm PLGA nanoparticles and PEGylated PLGA (PEG-PLGA-150) exhibited the highest cellular uptake. Detailed inhibitor studies revealed that macropinocytosis and caveolae-mediated endocytosis are the major internalization routes for these nanosystems. Clathrin-mediated endocytosis contributed partially, while phagocytosis was not implicated for the nanoparticle sizes and surface chemistries evaluated. Importantly, these findings argue against a one-size-fits-all model for nanoparticle uptake, instead indicating that optimal drug delivery to the cornea may require tailoring both size and surface chemistry to exploit dominant cellular pathways. This mechanistic insight is crucial for designing next-generation ocular therapeutics with enhanced penetration, controlled release, and reduced off-target effects (Azadi & David, 2024).

Comparison with Existing Internal Articles

Internal resources provide complementary perspectives on cellular uptake mechanisms, particularly in the context of actin dynamics and pharmacological manipulation. Articles such as “Cytochalasin D: Mechanistic Insights and Translational Potential” and “Cytochalasin D in Cell Biology: Protocols & Troubleshooting Tips” discuss how actin polymerization inhibitors like Cytochalasin D are instrumental in dissecting endocytic pathways, including macropinocytosis and caveolae-mediated uptake. These articles detail practical protocols for employing Cytochalasin D to modulate actin-dependent processes in cellular uptake assays, offering troubleshooting strategies and highlighting the compound’s role in elucidating the interplay between cytoskeletal dynamics and nanoparticle internalization. The reference study’s reliance on pathway-specific pharmacological inhibitors echoes these experimental strategies, underscoring the translational relevance of actin-targeting agents in nanoparticle uptake research. Additionally, “Cytochalasin D: Advanced Insights into Actin Polymerization Inhibition” bridges fundamental cell biology and practical assay design, reinforcing the value of actin polymerization inhibitors in mechanistic studies.

Limitations and Transferability

While the in vitro HCEC monolayer model with simulated mucosal environment approximates physiological conditions, it cannot fully recapitulate the complexity of the in vivo ocular surface, including tear film dynamics, immune surveillance, and stromal-epithelial interactions. Additionally, the nanoparticle sizes and surface chemistries investigated, though representative, do not encompass the full spectrum of clinically relevant formulations. The exclusive focus on PLGA-based systems may limit transferability to other polymeric or lipid nanoparticle platforms. Furthermore, inhibitor studies, while informative, are subject to off-target effects and may not entirely discriminate between overlapping endocytic pathways. Thus, while the findings provide a strong mechanistic foundation, their direct translation to clinical settings or other nanoparticle classes should be approached with appropriate caution.

Why this cross-domain matters, maturity, and limitations

The mechanistic insights into nanoparticle uptake pathways revealed by this study have broader implications for the rational design of drug delivery systems targeting not only ocular tissues but potentially other mucosal barriers. The elucidation of actin-dependent endocytosis routes connects directly to established cellular biology research, where actin polymerization inhibitors like Cytochalasin D have long served as tools for pathway dissection. However, clinical translation will require further validation in more complex models, including ex vivo or in vivo systems, and a nuanced understanding of how these pathways are modulated in disease states or in response to therapeutic interventions.

Research Support Resources

For researchers aiming to dissect endocytic pathways or validate uptake mechanisms in similar ocular or epithelial models, well-characterized actin polymerization inhibitors are essential. Cytochalasin D (SKU B6645, APExBIO) is a potent and selective tool for inhibiting actin polymerization, supporting detailed mechanistic studies in cellular uptake, cell cycle arrest at the G1-S transition, and related applications. Its established use in workflow protocols and its documented effects on actin microfilaments make it suitable for probing the roles of macropinocytosis and other actin-dependent processes in nanoparticle internalization. Researchers can refer to APExBIO for technical specifications, recommended concentrations, and storage guidelines.