Nanoparticle Uptake in Corneal Epithelial Cells: Mechanistic Insights

Study Background and Research Question

Ocular diseases remain a significant challenge for clinicians and drug delivery scientists due to the complex anatomical and physiological barriers of the eye. Despite the widespread use of topical ophthalmic formulations, their therapeutic efficacy is limited by factors such as rapid tear turnover, the multifaceted structure of the tear film, and the layered architecture of the cornea. The corneal epithelium, comprising only 10% of corneal thickness but responsible for 90% of its barrier function, is a major impediment to drug penetration. To overcome these barriers and improve bioavailability, researchers have turned to polymeric nanoparticles as vehicles for ocular drug delivery. However, a crucial knowledge gap persists regarding how nanoparticle size and surface chemistry modulate their interactions with corneal tissue and govern their internalization by epithelial cells. The reference study by Azadi and David addresses this by systematically investigating the uptake mechanisms of poly(lactic-co-glycolic acid) (PLGA) nanoparticles in human corneal epithelial cells (HCECs) (ACS Biomater. Sci. Eng. 2024, 10, 429–441).

Key Innovation from the Reference Study

The central innovation of the study lies in its rigorous evaluation of how nanoparticle physicochemical parameters—specifically size (100–250 nm) and surface chemistry (alginate, chitosan, PEG modifications)—affect cellular uptake routes in HCECs. By parsing the roles of macropinocytosis, caveolae-mediated, and clathrin-mediated endocytosis using pharmacological inhibitors, the authors provide direct mechanistic insights essential for the rational design of ocular nanoparticle formulations. This level of mechanistic dissection represents a substantial advance over prior studies, which often focused on uptake efficiency without clarifying the underlying pathways or the impact of particle surface engineering.

Methods and Experimental Design Insights

Azadi and David synthesized monodisperse PLGA nanoparticles via the emulsion-solvent evaporation method, followed by surface modification with mucoadhesive (alginate [ALG], chitosan [CHS]) and mucopenetrative (polyethylene glycol [PEG]) polymers. The resulting nanoparticles exhibited controlled diameters (100–250 nm), spherical morphology, and zeta potentials ranging from −25 to +15 mV. Cytotoxicity was assessed using the MTT assay, confirming high cell viability (70–100%) at concentrations up to 100 μg/mL after 24 hours of incubation. For uptake studies, a physiologically relevant in vitro corneal epithelium model was employed, integrating an HCEC monolayer with simulated mucosal solution to mimic the ocular surface environment. The researchers then used specific inhibitors to block distinct endocytic pathways, enabling quantification of the contribution of macropinocytosis, caveolae-mediated, and clathrin-mediated endocytosis to nanoparticle internalization.

Core Findings and Why They Matter

The investigation demonstrated that energy-dependent endocytosis is the principal route for nanoparticle uptake in HCECs, with macropinocytosis and caveolae-mediated endocytosis emerging as the dominant pathways. Notably, 100 nm PLGA and PEG-PLGA-150 nanoparticles exhibited the highest cellular internalization rates. While clathrin-mediated uptake also contributed, phagocytosis was not involved within the studied size and surface chemistry ranges. These findings are pivotal, as they highlight the possibility of fine-tuning nanoparticle design to exploit specific endocytic mechanisms, thus enhancing delivery efficiency and therapeutic outcomes for ocular drug formulations. Moreover, the demonstration that PEGylated and smaller-sized nanoparticles are more efficiently internalized provides actionable parameters for developing next-generation ocular nanomedicines. The study also reinforces the relative biocompatibility of PLGA-based nanoparticles for ocular applications, addressing a common translational concern regarding cytotoxicity (see internal summary).

Comparison with Existing Internal Articles

Several recent analyses echo and extend the mechanistic findings of Azadi and David. As summarized by a recent review, energy-dependent uptake—particularly macropinocytosis and caveolae-mediated endocytosis—dominates the internalization of nanoparticles by HCECs, corroborating the reference paper's conclusions. Furthermore, the in-depth technical report on Cytochalasin D elucidates the molecular underpinnings of actin dynamics and their disruption, which is directly relevant since actin polymerization is a central component of macropinocytosis. This intersection underscores the value of actin polymerization inhibitors, such as Cytochalasin D, for dissecting uptake pathways and validating mechanistic hypotheses in nanoparticle-cell interaction studies. Internal laboratory-focused articles further highlight Cytochalasin D (SKU B6645) as a robust tool for reproducibly inhibiting actin-mediated processes in both viability and uptake assays, reinforcing its utility for protocol optimization (see scenario-driven guidance).

Limitations and Transferability

The study's in vitro design, while highly controlled and physiologically informed, does not fully capture the complexities of the in vivo ocular environment, such as dynamic tear turnover and immune surveillance. The nanoparticle size range (100–250 nm) and surface chemistries studied, although clinically relevant, may not encompass all candidate nanoparticle formulations. Additionally, while the inhibition studies clarify endocytic pathways, off-target effects of pharmacological inhibitors and potential compensatory cellular responses remain possible confounders. The transferability of results to in vivo systems and to other epithelial tissues should be approached with caution, and further validation in animal models is warranted for translational applications.

Protocol Parameters

• Nanoparticle preparation: PLGA nanoparticles synthesized using emulsion-solvent evaporation; surface modified with alginate, chitosan, or PEG for mucoadhesive/mucopenetrative properties.
• Size and surface charge: 100–250 nm diameter; zeta potential from −25 to +15 mV.
• Cell viability assay: MTT assay after 24 h incubation with up to 100 μg/mL nanoparticles.
• Uptake assay: HCEC monolayer co-incubated with nanoparticles in simulated mucosal solution; uptake measured via fluorescence quantification.
• Pathway inhibition: Use of specific pharmacologic inhibitors to dissect roles of macropinocytosis, caveolae-mediated, and clathrin-mediated endocytosis.
• Actin polymerization inhibition: For mechanistic studies of endocytosis, Cytochalasin D can be used at 0.2–0.5 μg/mL in cell culture, as reported in the product information.

Why this cross-domain matters, maturity, and limitations

The interplay between nanoparticle physicochemical properties and cellular uptake pathways is critical not only for ocular drug delivery, but also for broader applications in targeted nanomedicine and nanotoxicology. The current findings are mature in the context of in vitro corneal models, but their extension to other epithelial barriers or in vivo systems requires further investigation. Notably, the mechanistic approaches validated in this study—such as the use of actin polymerization inhibitors—are broadly transferable to studies of drug delivery, viral entry, and cellular trafficking in diverse tissues, provided that tissue-specific differences are considered.

Outlook and Future Directions

The mechanistic clarity provided by this study establishes a rational framework for engineering nanoparticle-based ocular therapeutics with improved bioavailability and reduced irritation. Future work should extend these findings to animal models and clinical samples, exploring the influence of additional surface chemistries and dynamic ocular conditions. The demonstrated importance of actin-mediated uptake further suggests that modulation of cytoskeletal dynamics could be strategically leveraged to enhance or restrict nanoparticle internalization, according to therapeutic goals.

Research Support Resources

For researchers seeking to probe endocytic pathways in nanoparticle uptake or to validate mechanisms involving actin cytoskeleton disruption, Cytochalasin D (SKU B6645) from APExBIO offers a well-characterized, potent actin polymerization inhibitor suitable for cell-based assays. Its established efficacy in inhibiting actin-driven processes makes it a reliable reagent for mechanistic studies aligned with the approaches outlined above. Consult product guidelines for optimal concentrations and storage conditions when integrating Cytochalasin D into your experimental workflow.