Nanoparticle Uptake in Corneal Cells: Mechanistic Insights from Size and Surface Chemistry

Study Background and Research Question

Ocular drug delivery remains a formidable challenge due to the eye’s complex protective barriers, particularly the tear film and the multilayered cornea. Topical ophthalmic formulations, although widely used, are hindered by rapid clearance and limited penetration, resulting in low bioavailability and suboptimal therapeutic outcomes. Recent advances in nanoparticle-based delivery systems promise to address these shortcomings by prolonging surface retention and enhancing tissue penetration. However, the precise relationship between nanoparticle physicochemical properties and their cellular uptake mechanisms in the corneal epithelium has not been fully elucidated. The reference study by Azadi and David (ACS Biomater. Sci. Eng. 2024, 10, 429−441) directly addresses this gap, seeking to map how nanoparticle size and surface chemistry influence internalization by human corneal epithelial cells (HCECs).

Key Innovation from the Reference Study

The principal innovation lies in the systematic evaluation of poly(lactic-co-glycolic acid) (PLGA) nanoparticles, engineered with distinct surface chemistries (mucoadhesive and mucopenetrative coatings), across a controlled size spectrum. This approach enabled the authors to dissect not only the efficiency but also the mechanistic pathways of nanoparticle uptake by HCECs. The study’s use of an in vitro corneal model integrating simulated mucosal fluid further strengthens the physiological relevance of their findings. The nuanced insight into endocytic routes—specifically distinguishing between macropinocytosis, caveolae-mediated, and clathrin-mediated pathways—marks a significant advance for rational drug delivery system design.

Methods and Experimental Design Insights

PLGA nanoparticles were synthesized via emulsion-solvent evaporation, then surface-modified with mucoadhesive (alginate, chitosan) or mucopenetrative (polyethylene glycol, PEG) polymers. The resulting particles were monodisperse (PDI < 0.2), spherical, and ranged from 100–250 nm in diameter, with zeta potentials spanning −25 to +15 mV. Cytotoxicity was quantified using the MTT assay after 24-hour incubations up to 100 μg/mL, revealing minimal toxicity (70–100% cell viability), supporting their suitability for cellular uptake studies.

The cellular uptake experiments utilized a monolayer of HCECs exposed to the nanoparticle formulations in the presence of simulated mucosal solution, mimicking the ocular surface environment. A suite of endocytosis pathway inhibitors—each targeting distinct internalization mechanisms—was employed to parse the relative contributions of macropinocytosis, caveolae-mediated endocytosis, and clathrin-mediated endocytosis, while phagocytosis was ruled out as a significant pathway under the tested conditions.

Protocol Parameters

• Nanoparticle preparation: Emulsion-solvent evaporation; PLGA base with surface functionalization using alginate, chitosan, or PEG depending on target mucoadhesive or mucopenetrative properties.
• Particle size selection: 100–250 nm diameter; monodispersity confirmed (PDI < 0.2).
• Cell incubation: 24 hours with nanoparticle concentrations up to 100 μg/mL for cytotoxicity and uptake studies.
• Inhibitor studies: Use of pathway-specific inhibitors (e.g., amiloride for macropinocytosis, filipin for caveolae-mediated uptake, chlorpromazine for clathrin-mediated uptake) to delineate mechanism.
• Assessment endpoints: MTT assay for viability; fluorescence or microscopy-based quantification for nanoparticle uptake.

Core Findings and Why They Matter

The study provides several pivotal findings. First, energy-dependent endocytosis—rather than passive diffusion—was established as the dominant mechanism for nanoparticle entry into HCECs. Notably, both macropinocytosis and caveolae-mediated endocytosis were identified as primary uptake routes, with clathrin-mediated endocytosis playing a lesser, yet discernible, role (reference). Phagocytosis was not implicated, aligning with the absence of professional phagocytes in the corneal epithelium. Among the formulations, 100 nm PLGA and PEG-PLGA-150 nanoparticles achieved the highest uptake, emphasizing the importance of both nanoscale size and surface PEGylation for efficient cellular internalization.

These findings are significant for the rational design of ocular drug delivery systems. By pinpointing the most effective nanoparticle sizes and surface chemistries for corneal cell uptake, researchers can better tailor formulations to maximize bioavailability, minimize off-target effects, and reduce irritation—key obstacles in ocular pharmacotherapy.

Comparison with Existing Internal Articles

Several internal resources support and expand upon these findings. For instance, "Nanoparticle Uptake in Corneal Cells: Role of Physicochemical Properties" corroborates that nanoparticle size and surface chemistry are decisive for endocytic pathway selection, highlighting the dominance of energy-dependent mechanisms in HCECs. Moreover, the internal article "Cytochalasin D as an Actin Polymerization Inhibitor: Applied Workflows" addresses how actin cytoskeleton disruption—achieved through specific inhibitors—can modulate uptake pathways, offering practical protocols for distinguishing between macropinocytosis and other routes. These resources reinforce the methodological and mechanistic rigor of the reference study and provide additional context for experimental setup and troubleshooting.

Limitations and Transferability

While the in vitro HCEC model with simulated mucosal fluid advances physiological relevance, it cannot fully replicate the dynamic environment of the human ocular surface in vivo, where tear flow, blinking, and immune surveillance may impact nanoparticle fate. The study’s focus on PLGA-based systems is justified by their biocompatibility and established use, but further research is needed to generalize these findings to alternative polymers or hybrid nanoparticles. Additionally, the inhibitor-based pathway dissection, while robust, may not entirely separate overlapping endocytic processes—a common challenge in cellular uptake studies. Finally, translating these insights into clinical formulations will require validation in ex vivo or in vivo models, considering inter-individual variability and long-term safety.

Research Support Resources

To support investigations into nanoparticle internalization mechanisms or to validate the role of actin-dependent pathways in cellular uptake, researchers often employ actin polymerization inhibitors such as Cytochalasin D (SKU B6645). As a selective inhibitor of actin polymerization with an IC₅₀ of 25 nM, Cytochalasin D is widely used to block macropinocytosis and related endocytic processes, providing a direct means to probe cytoskeletal contributions to nanoparticle uptake. Typical experimental concentrations for cell culture applications range from 0.2 to 0.5 μg/mL, as reported in the product information. When designing mechanistic uptake studies, referencing well-established protocols—such as those detailed in recent internal articles on Cytochalasin D—can help ensure robust, reproducible results.