Nanoparticle Uptake in Corneal Cells: Impact of Size and Surface Chemistry

Study Background and Research Question

Ocular diseases continue to pose therapeutic challenges due to the unique anatomical and physiological barriers of the eye. The corneal epithelium, in particular, is a formidable obstacle for topically applied drugs, limiting bioavailability and therapeutic efficacy. Traditional approaches—such as viscosity enhancers, ointments, and in situ gels—have been deployed to address these challenges but often introduce side effects like irritation or blurred vision. In this context, nanoparticle-based drug-delivery systems have emerged as promising alternatives, offering enhanced retention, controlled release, and improved tissue penetration. However, a fundamental question remains: How do the physicochemical properties of nanoparticles, specifically size and surface chemistry, determine their interactions and uptake by corneal epithelial cells?

Key Innovation from the Reference Study

The reference study by Azadi and David (ACS Biomater. Sci. Eng. 2024) provides a systematic evaluation of how engineered variations in nanoparticle design affect their uptake by human cornea epithelial cells (HCECs). Importantly, it bridges a critical knowledge gap by dissecting the relative contributions of nanoparticle size and surface functionalization to the mechanisms of cellular internalization. Rather than assuming a single dominant route of entry, the study employs a nuanced investigation of multiple endocytic pathways, differentiating between macropinocytosis, caveolae-mediated, and clathrin-mediated endocytosis. This mechanistic clarity is essential for rational nanoparticle design in ocular drug delivery.

Methods and Experimental Design Insights

The researchers synthesized poly(lactic-co-glycolic acid) (PLGA) nanoparticles using the emulsion-solvent evaporation method—a widely adopted approach for generating biocompatible, biodegradable drug carriers. Three surface modifications were explored:

• Mucoadhesive coatings: alginate (ALG) and chitosan (CHS)
• Mucopenetrative coating: polyethylene glycol (PEG)

Particles were engineered to be monodisperse (PDI < 0.2) and spherical, with diameters ranging from 100 to 250 nm and zeta potentials from −25 to +15 mV. The cytotoxicity of these nanoparticles was evaluated in HCECs using the MTT assay, confirming low toxicity (≥70% cell viability at ≤100 μg/mL after 24 hours). To mimic the ocular surface, the uptake studies utilized a monolayer of HCECs overlaid with a simulated mucosal solution, a design that more accurately reflects the in vivo mucous environment than simple cell cultures. Cellular uptake was quantified using fluorescence-based assays, and mechanistic insights were derived from the use of pharmacological inhibitors targeting specific endocytic pathways. This allowed for dissection of the contributions of macropinocytosis, caveolae-mediated, clathrin-mediated endocytosis, and phagocytosis under controlled conditions.

Core Findings and Why They Matter

Key results from the study are as follows:

• Particle size and surface chemistry are critical: Among all formulations, 100 nm PLGA nanoparticles and PEG-coated PLGA (PEG-PLGA-150) nanoparticles demonstrated the highest uptake by HCECs.
• Dominant uptake mechanisms: Uptake is primarily energy-dependent, with macropinocytosis and caveolae-mediated endocytosis representing the dominant internalization routes. Clathrin-mediated endocytosis contributes partially, but phagocytosis does not play a role within the tested size and surface chemistry ranges.
• Low toxicity at relevant concentrations: All tested nanoparticles exhibited mild cytotoxicity, supporting their suitability for ocular drug delivery applications.

These findings are significant for several reasons. First, they provide empirical evidence that tuning nanoparticle size and surface chemistry can selectively enhance cellular uptake, which is essential for delivering therapeutics across the corneal barrier. Second, the elucidation of endocytic mechanisms enables more rational design of nanoparticle systems, potentially leading to improved clinical formulations with better efficacy and safety profiles. Finally, the study's in vitro model, incorporating a simulated mucosal layer, adds translational relevance by more closely recapitulating the in vivo ocular environment.

Comparison with Existing Internal Articles

The mechanisms highlighted in this study—particularly the roles of macropinocytosis and clathrin-mediated endocytosis—resonate with findings from related research in cell biology and pharmacology. For example, the internal article "Mechanisms of Spiroplasma eriocheiris Entry into Drosophila S2 Cells" also identifies macropinocytosis and clathrin-mediated endocytosis as key entry routes for pathogens, underscoring the generalizability of these pathways across cell types and contexts. Furthermore, "Rottlerin: Advancing Translational Research with PKCδ Inhibition" discusses how selective PKCδ inhibitors like Rottlerin can help dissect the roles of specific signaling pathways in endocytosis and apoptosis, providing a mechanistic toolkit that complements the current study's findings. Such cross-validation highlights the relevance of endocytic pathway analysis not only in drug delivery but also in understanding host-pathogen interactions and cell signaling. Importantly, the application of PKC inhibitors has been shown to influence processes such as cell proliferation inhibition and apoptosis induction, which are also affected during nanoparticle uptake and intracellular trafficking.

Limitations and Transferability

While the study's in vitro model advances the field by simulating key aspects of the ocular surface, several limitations merit consideration:

• In vitro vs. in vivo: The model, despite its sophistication, cannot fully replicate the dynamic, multi-layered structure of the human cornea and the continuous turnover of the tear film.
• Cell line specificity: Results obtained using HCECs may not be directly translatable to other ocular cell types or to the complex tissue architecture found in vivo.
• Payload generalizability: While the study focused on fluorescently labeled nanoparticles, therapeutic payloads may alter physicochemical properties and interact differently with the mucosal and epithelial barriers.

Despite these caveats, the mechanistic insights into endocytic pathways are broadly informative for the design of ocular drug-delivery systems and can guide further in vivo validation.

Protocol Parameters

• Nanoparticle synthesis: Emulsion-solvent evaporation method for PLGA; adjust polymer ratio for target size (100–250 nm).
• Surface modification: Add alginate or chitosan for mucoadhesive properties; use PEG for mucopenetrative coatings.
• Cytotoxicity testing: MTT assay after 24 h incubation at ≤100 μg/mL nanoparticle concentration in HCECs.
• Uptake assessment: Use HCEC monolayers with simulated mucosal solution; quantify uptake with fluorescence assays and confirm pathway involvement with pharmacological inhibitors (targeting macropinocytosis, caveolae-mediated, clathrin-mediated endocytosis).

Why this cross-domain matters, maturity, and limitations

The convergence of findings between nanoparticle uptake in ocular cells and research on endocytic mechanisms in other cell systems (e.g., Drosophila S2 cells, cancer lines) underscores the evolutionary conservation of these pathways. For researchers investigating cell proliferation inhibition, apoptosis induction, or targeted delivery of PKC inhibitors, understanding endocytosis is critical for maximizing cellular uptake and therapeutic efficacy. However, differences among cell types and tissue environments mean that results from one system may not universally apply, reinforcing the need for context-specific validation.

Research Support Resources

Researchers aiming to explore endocytic mechanisms, cell signaling, or apoptosis in ocular and other cell models may benefit from modulators such as PKC inhibitors. Rottlerin (SKU B6803) from APExBIO is a selective PKCδ inhibitor frequently used to probe signaling pathways involved in cell proliferation inhibition, apoptosis induction, caspase-3 activation, and PARP cleavage. When integrated into nanoparticle studies or endocytic pathway assays, such tools can help elucidate the molecular underpinnings of cellular uptake and response. For detailed protocols and scenario-driven advice, see additional guidance in internal resources.